arcee-ai/nuclear_patents · Datasets at Hugging Face

patent_number stringlengths 0 9	section stringclasses 4 values	raw_text stringlengths 0 954k
046577270	abstract	The present system uses measurement inputs which reveal fission product barrier functional characteristics for emergency event classification in nuclear generating stations. The process determines the functional status of barriers to fission product release under transient or emergency conditions. The first part of the present process specifies which key indicators within a nuclear plant provide data indicative of fission product barrier status, and develops a logic table or diagram relating specific key symptoms to barrier status such that a computer identifies the functional status of each fission product barrier. In the second part of the present process nuclear power plant operators utilize the computer to determine emergency event classification. The computer utilizes the present process, using the installed nuclear generating station instrumentation which indicates flow rates in fluid and air handling systems, pressures in pipes and vessels, temperatures in pipes and vessels, radiation levels and other indications of equipment status to determine whether the fission product barriers are functional at a given point in time according to the criteria established during the first part of the process. The implentation is an orderly process in which nuclear power plant operator uses the process as programmed on the computer to determine and correctly characterize the condition or status of each fission product barrier at any time. The process results in direct indication of the status of fission product barriers and of the categorization of the emergency event according to established classes of emergencies. If the computer is non-functional, the nuclear plant operator uses the process as presented in implementing procedures to determine the status of each fission product barrier and to determine the proper event clasification. The operator relies upon installed plant instrumentation to determine the functional status of the barriers according to the process.
051724030	description	DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, denoted at 50 is an X-ray source accommodating chamber for accommodating an X-ray source therein. Barrel 5 is coupled to the X-ray source accommodating chamber and, to this barrel 5, a stage accommodating chamber 19 is coupled. The barrel 5 is equipped with a beryllium blocking window 6 through which the X-rays produced by the X-ray source are introduced into the stage accommodating chamber 19 along the z-axis direction. The stage accommodating chamber 19 accommodates therein a mask 13, a mask chuck 14 for holding the mask 13, a semiconductor wafer 15, a wafer chuck 16 for holding the wafer 15, a wafer stage 18 which is movable along the x-axis, y-axis and z-axis directions as well as in rotational directions about these axes, respectively, and an optical system 17 for the alignment of the mask and the wafer with respect to each of the x-axis, y-axis and z-axis directions. More particularly, the optical system 17 is operable to detect any positional deviation between alignment marks provided on the mask and the wafer. The wafer stage 18 can be moved stepwise in each of the x-axis and y-axis directions, for printing a circuit pattern formed on the mask 13 upon different shot areas on the wafer 15. By exposing the wafer 15 with the x-rays through the mask 13, the pattern of the mask 13 can be printed on a particular shot area of the wafer 15. Thus, the above-described structure constitutes what can be called an "X-ray stepper". To the barrel 5, a high vacuum pump 51 such as a turbo molecular pump, for example, is coupled, for vacuum-evacuation thereof. To the stage accommodating chamber 19, a low vacuum pump 11 such as an oil rotation pump, for example, is coupled by way of a discharging port 12 and a variable valve 10. The discharging port 12 is coupled to the same side of the stage accommodating chamber 19, to which the barrel 5 is coupled. However, the discharging port may be coupled to the wall of the stage accommodating chamber 19 on the opposite side remote from the barrel 5, as in the FIG. 6 example. The variable valve 10 is adapted to change the opening thereof automatically in response to a signal from a controller 9. The stage accommodating chamber 19 is equipped with an opening 31 for passage of X-rays, on the side of which opening a pressure detecting port 7 is provided to detect the pressure in an X-ray projection path 30 between the beryllium blocking window 6 and the mask 13. On the basis of the pressure detected by the pressure sensor 8, the controller 9 controls the opening of the variable valve 10. By this, the pressure in the stage accommodating chamber 19, more particularly, the pressure in the X-ray projection path (passageway) 30, can be controlled and maintained constant. The flow of helium gas from a tank 1 is adjusted by a manual adjusting valve 2 to a predetermined flow rate and, through a helium supply port 3 disposed just after (wafer 15 side) the beryllium blocking window 6 of the barrel 5, the helium gas is supplied into the stage accommodating chamber 19 through the X-ray projection path 30. The mask chuck 14 is provided with a helium discharging port 4 for discharging the supplied helium gas in the X-ray projection path 30 into the stage accommodating chamber 19. The helium discharging port 4 is arranged to avoid vibration or flexure of the mask 13 due to the gas flow of helium. FIG. 2 is a sectional view of the helium supply port 3, taken on line A--A in FIG. 1. FIG. 3 is a sectional view of the helium discharging port 4, taken on line B--B in FIG. 1. As seen in these drawings, in the neighborhood of the portion of the barrel 5 to which the helium supply port 3 is coupled, an inner cylinder 40 is provided in a concentric relationship with the barrel 5. The inner cylinder 40 has twelve (12) bores 41 formed equidistantly along the circumference of the cylinder 40. The helium introduced into the barrel 5 from the helium supply port 3, is then introduced into the X-ray projection path 30 through these bores 41 of the inner cylinder 40. Also, as best seen in FIG. 3, the mask chuck 14 has eight (8) helium discharging ports 4 which are formed equidistantly around the X-ray projection path 30, as passageways extending radially outwardly from the center of the X-ray projection path 30. The helium introduced into the X-ray projection path 30, is then introduced into the stage accommodating chamber 19 through these discharging ports 4. By introducing and discharging the helium gas radially inwardly and outwardly in the manner described above, it is possible to obtain a uniform or homogeneous helium ambience. For the exposure process in the X-ray exposure apparatus of the structure described above, first the inside atmosphere in the stage accommodating chamber 19 is replaced by a helium gas of a predetermined pressure. Then, in accordance with the quantity of impure gas introduction (leakage) into the stage accommodating chamber 19 (for example, the introduction of air through seal means), a constant amount of helium necessary for retaining the purity of helium in the stage accommodating chamber 19 is supplied continuously from the helium supply port 3. Additionally, the opening of the variable valve 10 coupled to the vacuum pump 10 is controlled continuously on the basis of the output of the pressure sensor 8. By this, the pressure in the X-ray projection path 30 can be controlled and maintained constant. Referring to FIG. 4 showing a second embodiment of the present invention, helium is supplied through a helium supply port 3 which is provided just before (light source side) the mask 13 held on the mask chuck 14. Additionally, a communication port 20 is formed to provide communication between the stage accommodating chamber 19 and the barrel 5, more particularly, a part of the barrel 5 just after (mask 13 side) of the blocking window 6. The helium gas from the tank 1 flowing through the manual valve 2 flows through the X-ray projection path 30 in a direction from the mask 13 to the blocking window 6 and, thereafter, through the communication port 20 as the helium gas introduced into the stage accommodating chamber 19. As in the first embodiment, the pressure detecting port 7 is provided on the side of the opening 31 of the stage accommodating chamber 19, for passage of X-rays, and it is operable to detect the pressure in the X-ray projection path 30. The remaining structure and operation of this embodiment are substantially the same as those of the first embodiment. Referring to FIG. 5 showing a third embodiment of the present invention, in this embodiment, the first embodiment is modified such that a thin film 21 is added between the mask 13 and the helium discharging port 4 to intercept them. Also, a port 22 is added to provide communication between the stage accommodating chamber 19 and the space which is defined between the thin film 21 and the mask 13. With this arrangement, it is possible to effectively suppress or minimize the vibration or flexure of the mask 13 due to the flow of helium, substantially without decreasing the amount of exposure of the wafer 16 with the X-rays. In FIG. 5, the helium from the tank 1 flowing through the valve 2, is supplied into the stage accommodating chamber 19 from the helium supply port 3 provided just after the blocking window 6 of the barrel 5. More particularly, the helium gas introduced from the helium supply port 3 into the X-ray projection path 30 flows to the discharging ports 4 which are provided just before (blocking window 6 side) the thin film 21, into the stage accommodating chamber. Into the space between the thin film 21 and the mask 13, helium is introduced from the stage accommodating chamber 19 side, through the port 22. The thin film 21 is disposed adjacent the helium discharging ports 4 which are formed in the mask chuck 14 for introduction of the helium gas supplied to the X-ray projection path 30, into the stage accommodating chamber 19. More particularly, it is disposed at one side of the ports 4 facing the mask 13. The communication port 22 provided between the thin film 21 and the mask 13 functions also to prevent the possible flexure or vibration of the thin film 21, due to the gas flow of helium, from adversely affecting the mask 13. Since substantially no differential pressure is produced on the opposite sides of the thin film 21, the thickness thereof may be very small on an order of a few microns. Examples of the material thereof are: an organic material such as polypropylene, polyethylene, polyamide, polycarbonate, vinyl chloride, fluorine plastic or the like; or an inorganic material such as Si.sub.3 N.sub.4, SiC, Be, SiO.sub.2 or the like. As in the first embodiment, the pressure detecting port 7 is provided on the side of the opening 31 of the stage accommodating chamber 19, for passage of X-rays and it is operable to detect the pressure in the X-ray projection path 30. The remaining structure and operation are similar to those of the first embodiment. Also, in the third embodiment, like the second embodiment, the supply port 3 may be provided on the mask chuck side, at a position between the blocking window 6 and the thin film 21, and the communication port 20 (see FIG. 4) may be provided on the barrel 5 side. Whether the structure without a thin film 21 as in the first and second embodiments or the structure with a thin film as in the third embodiment should be selected, may be determined on the basis of various conditions such as, for example, the mechanical structure of the components (such as the barrel 5, the optical system 17, the mask 14, etc.) disposed around the X-ray projection path 30, the flow rate of helium gas as supplied from the helium supply port 3, the material and thickness of the thin film 21, while taking into account the quantity of X-ray attenuation, the effect of vibration or flexure of the mask, and the like. In the preceding three embodiments, the supply and discharge of helium are executed through the ports 3 and 4 provided in the barrel 5 and the mask chuck 14, and the pressure detection is executed through the port 7 provided in the stage accommodating chamber 19. However, the present invention is not limited to such a form. By way of example, the supply port 3 may be provided in the stage accommodating chamber 19 so as to supply helium into the X-ray projection path 30, or the pressure detecting port 7 may be provided in the barrel 5 or the mask chuck 14 so as to detect the pressure in the X-ray projection path 30. Further, the position for the provision of the thin film 21 is not limited to the mask chuck 14. According to the present invention, as described hereinbefore, it is possible to control the helium ambience in the X-ray projection path mainly, to which the control of purity, pressure and the like of the helium gas should actually be executed. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.
	claims	1. An isotope production target rod for a light water nuclear power reactor, said isotope production target rod comprising:at least one rod central body including an outer shell that defines an internal cavity, the at least one rod central body configured to be inserted into the light water nuclear power reactor;a plurality of irradiation targets within the internal cavity, the irradiation targets not being nuclear fuel; anda low nuclear cross-section separating medium separating each of the irradiation targets from each other and positioning each of the irradiation targets in a spatial arrangement, the low nuclear cross-section separating medium having a neutron absorption cross-section less than about 1 barn for neutrons produced through nuclear fission in the light water nuclear power reactor and incident on the separating medium; and whereinthe irradiation targets and the low nuclear cross-section separating medium are not disposed in a container within the rod central body; andthe separating medium comprises at least one irradiation target receptacle within the internal cavity of each respective rod central body, each irradiation target receptacle including a plurality of target reservoirs in a pattern within at least a portion of an outer surface of each irradiation target receptacle. 2. The isotope production target rod of claim 1, wherein each target reservoir is sized to retain a single irradiation target. 3. The isotope production target rod of claim 1, wherein each target reservoir is sized to retain the respective target in a particular orientation. 4. The isotope production target rod of claim 2, wherein each target reservoir is sized to retain the respective target in a random orientation. 5. The isotope production target rod of claim 1, wherein each irradiation target receptacle is fabricated of at least one of zirconium and aluminum. 6. The isotope production target rod of claim 1, wherein each irradiation target receptacle comprises a solid body having the target reservoirs in the pattern along the at least a portion of an outer surface of the solid body. 7. The isotope production target rod of claim 6, wherein the target reservoirs are equally spaced around the body outer surface between opposing axial end portions of the body outer surface such that each target reservoir is a substantially equal distance from each adjacent target reservoir. 8. The isotope production target rod of claim 6, wherein the target reservoirs are equally spaced about at least one of opposing axial end portions of the body outer surface such that each target reservoir is a substantially equal distance for each adjacent target reservoir. 9. The isotope production target rod of claim 6, wherein each irradiation target receptacle comprises an axially threaded bore in at least one axial end portion. 10. The isotope production target rod of claim 1, wherein each irradiation target receptacle comprises a tubular body having the target reservoirs in the pattern equally spaced around an outer surface. 11. The isotope production target rod of claim 10, wherein the tubular body includes a longitudinal center cavity therethrough, the axial bore having threads in at least one axial end portion. 12. The isotope production target rod of claim 1, wherein each irradiation target receptacle comprises:a perforated tubular sleeve having a plurality of holes therein in the pattern; anda solid core fitted within the sleeve. 13. An isotope production target rod for a light water nuclear power reactor, comprising:at least one rod body including an outer shell that defines an internal cavity, the at least one rod body configured to be inserted into the light water nuclear power reactor;a separating structure disposed within the cavity that defines separate target spaces;a plurality of irradiation targets, the plurality of irradiation targets not being nuclear fuel, each of the plurality of irradiation targets being disposed in a different one of the target spaces such that the plurality of irradiation targets are separated from each other by the separating structure, and the irradiation targets not being disposed in a separate container within the rod body. 14. An isotope production target rod for a light water nuclear power reactor, comprising:at least one rod body including an outer shell that defines an internal cavity, the at least one rod body configured to be inserted into the light water nuclear power reactor;a cylinder disposed in the cavity of at least one of the rod bodies, the cylinder having a plurality of reservoirs formed in an outer lateral surface thereof;a plurality of irradiation targets, each irradiation target disposed in a different one of the plurality of reservoirs such that the plurality of irradiation targets are separated from one another, and each irradiation target not being nuclear fuel, wherein at least one of the plurality of reservoirs has an elliptical cross-section. 15. The isotope production target rod of claim 14, further comprising:a target container within the cavity of the at least one of the rod bodies, and the cylinder being disposed in the target container. 16. A fuel bundle for a light water nuclear power reactor, comprising:a plurality of fuel rods;at least one isotope production rod as recited in claim 1;a plurality of spacer grids maintaining the plurality of fuel rods and the isotope production rod in a spaced relationship in the light water nuclear power reactor; andan outer channel housing the plurality of fuel rods, the isotope production rod and the plurality of spacer grids. 17. The fuel bundle of claim 16, further comprising:an upper tie plate;a lower tie plate; and whereinthe plurality of fuel rods and the isotope production rod are disposed between the upper and lower tie plates.
	description	The invention relates to the manufacture of a glovebox-type chamber, as schematically represented in FIG. 1, used in particular for radioactive substance handling. Such a box 1 comprises a structure or frame 2 carrying transparent panels 3 provided with openings receiving glove ports 4 in which gloves are mounted. The invention relates to the localisation of the positions of the glove ports 4 to make the openings at the proper locations in the panels of such a glove box. In practice, the position of the glove ports on a panel conditions the accessibility to the inner space of the chamber. One solution consists in approximately assessing the positions of these openings before making them, hoping that they will enable the operator to access the entire inner volume of the chamber. Since the shapes of the glovebox change from box to box, this solution can result in placing the glove ports into positions not enabling the manipulator to access the entire inner volume when his/her arms are engaged in the glove ports. Consequently, it is necessary to manufacture again the panel with better defined locations, which is disadvantageous considering the high cost of such panels, which are made of lead-loaded Plexiglas and the supply delay of which is generally long. Another solution consists in making a prototyping to attach to the structure itself free of panels the glove ports at the locations foreseen, and checking that these locations enable the entire inner space to be accessed. The prototype can then be modified to correct the locations up to enable the operator to access the entire inner space with his/her arms engaged in the glove ports. In practice, the prototyping is made with angle beams or other profiles, such that it is boring and complex to implement. This complexity is further increased when the goal is to place several glove ports on the prototyping. The invention relates to a device for identifying a position of at least one glove port intended to equip a panel carried by a glovebox structure delimiting a closed chamber, this device comprising a base, means for attaching this base to the glovebox structure, at least one template comprising an opening able to receive a glove port, each template being carried by the base while being movable with respect to this base, and means for locking each template in position with respect to the base. The invention thus makes it possible to very significantly reduce the time required to determine the positions of the glove ports. When the panel in question is withdrawn, the device is attached to the structure, at the location of this panel: the operator can pass his/her hands through the glove ports in order to determine whether the positions of these glove ports are adapted or not. If necessary, the operator unlocks the elements to replace one or more glove ports as he/she wants before locking them into their new position in order to make a new test. By trial and error, the operator thus succeeds in identifying the optimum configuration. The invention also relates to a device thus defined, comprising a main post carrying a base, this main post being provided at the ends thereof with means for attaching to two cross members of the glovebox structure intended to receive the panel. The invention also relates to a device thus defined, wherein the base is slidably mounted on this main post, and comprising means for locking the base with respect to the main post. The invention also relates to a device thus defined, wherein each template is carried by the base while being movable with respect to this base perpendicularly to the main post, and comprising means for locking each template with respect to the base. The invention also relates to a device thus defined, comprising a system of graduation marks along the main post and on each means for attaching the template to a base, to locate the position of each template by visual reading. The idea underlying the invention is a device carrying one or more glove ports the positions of which are adjustable, which device is intended to be mounted on a glovebox structure free of panel. An operator can thus make tests enabling him/her to determine simply the positions of the glove ports which are best adapted to the task that has to be made in the box, and best adapted to access the entire inner space of the chamber. The device which is schematically represented in FIG. 2, wherein it is marked as 6, includes a main post 7 in the form of a slider forming profile equipped at one end thereof with a fixed latching tab 8 and at the other end thereof with an adjustable latching tab 9. The fixed tab 8 is rigidly integral with the end of the post 7 whereas the adjustable tab 9 is able to slide along this post. In use, the main post 7 extends vertically by having its fixed tab 8 latched to an upper horizontal cross member 12 of the glovebox structure 11, and its movable tab latched to a lower horizontal cross member 13 of the structure of this box 11. These cross members 12 and 13 correspond to the upper and lower edges of an opening of the structure intended to receive a panel when the box is completed. This main post 7 carries several identical assemblies 14, 16, 17 each including two opening templates each able to receive a glove port and possibly a glove. As can be seen in FIG. 2, the assembly 14 includes a slider forming cross member 18 which projects on either side from the post 7 by extending perpendicularly to the same, that is horizontally in position of use. This slider forming cross member 18 is carried by a lockable fastener 19 which is slidably mounted on the slider forming post 7 and which is lockable in position on this post 7, to adjust vertically the position of the assembly 14 at any desired height. The slider forming cross member 18 is itself transversally movable in the fastener 19 and lockable in position in the same, which enables the position of this cross member 18 to be transversally adjusted by offsetting more or less to left or right with respect to the post 7. The cross member 18 thus includes a first portion 21 projecting from a side of the post 7 and a second portion 22 projecting from the other side of the post 7. The first portion 21 carries a first template 23 and the second portion carries a second template 24. Each template 23, 24 is slidably mounted on the slider forming cross member 18 and it can be locked at a given position along this cross member. As can be seen in FIG. 2, the template 23 includes a plate 27 comprising an upper region equipped with means for nesting and locking in the slider forming cross member 18, and a lower region including an opening 28 on which a glove port is mounted. The template 24 is identical to the template 23. The template 23 and the template 24 are thus adjustable in position on the cross member 18 and lockable in position along this cross member. The fastener 19 includes a base having a part which engages in the post 7 and which is equipped with a locking screw enabling it to be immobilised along the post 7 by tightening this screw, and another part in which the cross member 18 is engaged with another locking screw to lock this cross member in position. Analogously, the top part of each template is equipped with a base engaging in or about the cross member 18 to slide along the same and it is provided with a tightening screw enabling the template to be locked in position along this cross member 18. Thus, both templates 23 and 24 are located at a same height which is adjustable by virtue of the movable fastener 19, and the side positions of both these templates are adjustable by moving sideways the cross member 18 with respect to the base of the lockable fastener 19 which carries it, and/or by moving sideways each template with respect to this cross member and by tightening the different locking screws to fix the transverse position of these templates. In the configuration of FIG. 2, the assembly 14 is located at the top part of the post 7, and the other two assemblies, namely the assemblies 16 and 17 which are identical to the assembly 14 are located below the assembly 14 along the post 7. In use, the operator starts with placing the post 7 by suspending it to the upper cross member 12 by means of the fixed fastener 8, and by attaching its lower region to the lower cross member 13 by means of the movable fastener 9. Once the device is thus placed, the operator adjusts the height of each assembly 14, 16 and 17 along the post 7, and he/she tightens the corresponding locking screws. The operator can then adjusts sideways the positions of each of both templates of each assembly before tightening the corresponding locking screws. When this prepositioning has been made, the operator which is marked as 26 on FIG. 3 can make tests. At this time, he/she passes his/her arms in the different glove ports to check that he/she can access the different inner regions of the box, and he/she can make feeling minimum disturbance the task that has to be made in this box. Optionally, the operator readjusts the positions of each glove port of the device until optimum position settings are achieved. When the operator considers that the glove ports carried by the templates are immobilized at the optimum positions, he/she can note down these positions after he/she has measured them. These measurements can be manually made with a tape measure, or simply by reading the graduation marks 29 intended to that end on the main post 7 and on the cross members enabling the position of each glove port to be identified. Another possibility can consist in using a laser measuring system for identifying the positions of each glove port. The position data thus collected are then transferred on the panel that has to be placed in the box opening delimited by the cross members 12 and 13, before passing to the operation of cutting these openings in the panel. The invention thus enables the position of each glove port to be accurately determined by making real tests for the ergonomics of handlings to be made. By the way, it is possible to equip each glove port with a glove to make the tests still more realistic. The invention further enables the positions identified to be accurately measured. The invention which has been set forth within the context of radioactive substance handling is also applicable to other fields such as for example chemistry, biology and bacteriology. It can also be applied to the hospital environment for the case of incubators, and of works on viruses. The invention is further applicable to the field of quick-assembly movable chambers on theatres of civil or military operations.
055639259	claims	1. A method for adjusting the radiation output delivered to an object from a radiation source, comprising the following steps: generating a radiation beam having a variable radiation output and a substantially lossless beam path from a radiation source to the object; delimiting the beam path by moving at least one beam-shielding device; defining an irradiated field of the object; varying the radiation output of the beam as a predetermined function of the position of the beam-shielding device, a wedge factor of the radiation output thereby varying according to a predetermined profile, in which the wedge factor is defined as the ratio between a reference radiation output along a reference axis of the beam with a predetermined physical wedge in the beam path and an actual radiation output of the beam in a substantially lossless beam path; and varying the radiation output such that the wedge factor is constant regardless of the size of the irradiated field. a radiation source generating a radiation beam having a variable radiation output; an irradiated field of the object; beam-shielding means for delimiting the output beam to at least one predetermined irradiation field of the object; a dose controller for varying a degree of shielding of the beam and the radiation output; and processing means for generating and applying to the dose controller set dose signals, comprising nominal dose signals and wedge correction factors, and for thereby varying the radiation output such that a wedge factor is constant regardless of the degree of shielding, where the output factor is defined as the ratio between a reference radiation output along a reference axis of the beam with a predetermined physical wedge in the beam path and an actual radiation output of the beam in a substantially lossless beam path. 2. A method as in claim 1, in which the wedge factor is equal to unity. 3. A system for adjusting the radiation output delivered to an object from a radiation source, comprising:
	description	This application is a National Stage Application of PCT/US2018/025216, filed Mar. 29, 2018, which claims the benefit of priority to U.S. Provisional patent application Ser. No. 62/478,419, filed Mar. 29, 2017, the entire disclosures of which are incorporated by reference in their entireties. To the extent appropriate, a claim of priority is made to each of the above disclosed applications. Nuclear fission reactors include breed-and-burn fast reactors (also referred to as traveling wave reactors, or TWRs). TWR means a reactor that would be designed to operate indefinitely using natural uranium, depleted uranium, spent light water reactor fuel, or thorium as a reload fuel after start up, and in which waves that breed and then burn would travel relative to the fuel. Thus, in some aspects, the TWR is a once-through fast reactor that runs on subcritical reload fuel which is bred up to a useful state and burned in situ. In a TWR, a wave of breeding and fissioning (a “breed-burn wave”) is originated in the core of the reactor and moves relative to the fuel. In cases where the fuel is stationary, the breed and burn wave expands outward from the ignition point. In some cases, the fuel is moved so that the breed and burn wave stays stationary relative to the core (e.g., a standing wave) but moves relative to the fuel; a standing wave is considered a type of TWR. Movement of fuel assemblies is referred to as “fuel shuffling” and is used to maintain the standing wave and for adjustment of reactor characteristics (heat, flux, power, fuel burn up, etc.). Nuclear fissioning occurs in the core in which the fuel assemblies are shuffled is housed in a reactor vessel. The fuel assemblies include fissile nuclear fuel assemblies and fertile nuclear fuel assemblies. Reactivity control is primarily accomplished by control rod assemblies also located in the core for adjustment of reactor characteristics. Fission energy developed by the standing wave creates thermal energy which is transferred in series through one or more heat transport loops to steam generators to produce electricity, and low temperature heat is rejected through a set of water-cooled vacuum condensers. The separation of coolant systems into both primary and intermediate coolant loops helps maintain the integrity of the core and the primary coolant loops. In the TWR, both the primary coolant and intermediate coolant loop utilizes liquid sodium. In one aspect, the technology relates to a method of replacing a cesium trap, the method includes: freezing a first cesium trap at least partially containing cesium therein, wherein the first cesium trap is located within a shielded cell; decoupling the first cesium trap from the shielded cell; removing the first cesium trap from the shielded cell; inserting a second cesium trap into the shielded cell; and attaching the second cesium trap to the shielded cell. In an example, the first cesium trap includes at least one of: remotely decoupling at least one lateral support anchor extending between the first cesium trap and the shielded cell; remotely decoupling the first cesium trap from a sodium processing circuit; and remotely disconnecting electrical power and instrument control attachments extending between the first cesium trap and the shielded cell. In another example, remotely decoupling the at least one lateral support anchor includes removing a connection member that couples a cell anchor extending from the shielded cell to a trap anchor extending from the first cesium trap. In yet another example, the connection member is at least one of a pin and a bolt. In still another example, remotely decoupling the first cesium trap from a sodium processing circuit includes: crimping at least one sodium line extending from the first cesium trap; and cutting the at least one sodium line adjacent the crimped portion such that a first portion of the at least one sodium line extends from a top portion of the first cesium trap and a second portion of the at least one sodium line remains part of the sodium processing circuit. In another example of the above aspect, remotely disconnecting electrical power and instrument control attachments includes unplugging at least one of an electrical power attachment and an instrument control attachment from a corresponding receiver disposed on a top portion of the first cesium trap. In an example, the first cesium trap includes: releasably coupling the first cesium trap to a lifting tool; and lifting, via the lifting tool, the first cesium trap out of the shielded cell such that a base of the first cesium trap slidably disengages with at least one locating pin extending from a bottom of the shielded cell. In another example, the method includes decoupling a cooling line inlet extending from the first cesium trap from a fixed cooling line extending from the shielded cell via the lifting operation. In yet another example, releasably coupling the first cesium trap to a lifting tool includes rotating and lifting at least one hook of the lifting tool into a corresponding lifting eye disposed on the first cesium trap. In still another example, inserting the second cesium trap includes: releasably coupling the second cesium trap to a lifting tool, wherein the lifting tool includes at least one hook and the second cesium trap includes at least one corresponding lifting eye; placing, via the lifting tool, the second cesium trap into the shielded cell; and simultaneously aligning a base of the second cesium trap with at least one locating pin extending from a bottom of the shielded cell. In another example of the above aspect, the method includes coupling, via a slidable engagement, a cooling line inlet extending from the second cesium trap to a fixed cooling line extending from the shielded cell. In an example, attaching the second cesium trap includes at least one of: coupling a cell anchor extending from the shielded cell to a trap anchor extending from the second cesium trap forming at least one lateral support anchor extending between the first cesium trap and the shielded cell; welding at least one first sodium line extending from the second cesium trap to at least one second sodium line extending from a sodium processing circuit; and connecting electrical power and instrument control attachments via plugging the attachments into a corresponding receiver disposed on a top portion of the second cesium trap. In another example, the first cesium trap contains a predetermined amount of cesium and the second cesium trap contains no amount of cesium. In yet another example, the shielded cell is an individualized shielded cell. In another aspect, the technology relates to a cesium trap having: a body having a top portion, the body enclosing a filter and an active material configured to remove cesium from a sodium stream; a cooling jacket disposed substantially around the body; and at least one first sodium line extending from the top portion, wherein the at least one first sodium line is configured to couple in flow communication with a corresponding at least one second sodium line of a sodium processing circuit. In an example, the cesium trap further includes at least one trap anchor extending from the cooling jacket, wherein the trap anchor is releasably couplable to at least one corresponding cell anchor extending from the shielded cell so as to form at least one lateral support anchor. In another example, the cesium trap includes at least one lifting eye disposed on the top portion, the at least one lifting eye configured to receive a lifting tool for placing the cesium trap into and removing the cesium trap from a shielded cell. In still another example, the cesium trap an inlet cooling line extending from a bottom portion of the cooling jacket, wherein the inlet cooling line has a free end that is releasably couplable in flow communication to a corresponding fixed cooling line within the shielded cell. In still another example, the cesium trap includes at least one receiver disposed on the top portion, wherein electrical power and instrument control attachments are configured to releasably plug into the at least one receiver. In another example, the cesium trap includes a base coupled to a bottom portion of the body, wherein at least one opening is defined within the base such that the base is positionable within a bottom of the shielded cell via at least one corresponding locating pin. In another aspect, the technology relates to a cesium trap having: a body having a top portion, the body enclosing a filter and an active material configured to remove cesium from a sodium stream containing cesium and argon; and at least one first sodium line extending from the top portion, wherein the at least one first sodium line is configured to couple in flow communication with a corresponding at least one second sodium line of a sodium processing circuit; wherein the filter is located to prevent the active material from leaving the cesium trap while allowing the sodium and argon to pass and the filter has a mean pore size of from 40 to 160 μm. In an example, the lower limit of the mean port size of the filter is selected from 40, 50, 60, 70, 80, and 90 μm. In another example, the upper limit of the mean port size of the filter is selected from 100, 110, 120, 130, 140, 150, and 160 μm. In yet another example, the filter is a sintered metal filter. In still another example, the sintered metal filter is sintered from one or more of stainless steel, Hastelloy®, Monel®, Inconel®, nickel, HT-9, and titanium. FIG. 1 illustrates, in a block diagram form, some of the basic components of a travelling wave reactor (TWR) 100. In general, the TWR 100 includes a reactor core 102 containing a plurality of fuel assemblies (not shown). The core 102 is disposed within a pool 104 holding a volume of liquid sodium coolant 106. The pool 104 is referred to as a hot pool and has a sodium temperature higher than that of a surrounding cold pool 108 (due to the energy generated by the fuel assemblies in the reactor core 102), which also contains liquid sodium coolant 106. The hot pool 104 is separated from the cold pool 108 by a redan 110. A headspace 112 above the level of the sodium coolant 106 is filled with an inert cover gas, such as argon. The reactor vessel 114 surrounds the reactor core 102, hot pool 104, and cold pool 108, and is sealed with a reactor head 116. The reactor head 116 provides various access points into the interior of the reactor vessel 114. The size of the reactor core 102 is selected based on a number of factors, including the characteristics of the fuel, desired power generation, available reactor 100 space, and so on. Various embodiments of a TWR may be used in low power (around 300 MWe—around 500 MWe), medium power (around 500 MWe—around 1000 MWe), and high power (around 1000 MWe and above) applications, as required or desired. The performance of the reactor 100 may be improved by providing one or more reflectors, not shown, around the core 102 to reflect neutrons back into the core 102. Additionally, fertile and fissile nuclear assemblies are moved (or “shuffled”) within and about the core 102 to control the nuclear reaction occurring therein. The sodium coolant 106 is circulated within the vessel 114 via a primary sodium coolant pump 118. The primary coolant pump 118 draws sodium coolant 106 from the cold pool 108 and injects it into a plenum below the reactor core 102. The coolant 106 is forced upward through the core and is heated due to the reactions taking place within the reactor core 102. Heated coolant 106 enters an intermediate heat exchanger(s) 120 from the hot pool 104, and exits the intermediate heat exchanger 120 and re-enters the cold pool 108. This primary coolant loop 122 thus circulates sodium coolant 106 entirely within the reactor vessel 114. The intermediate heat exchanger 120 incorporates a segment of a closed liquid sodium loop that is physically separated from the primary sodium pools 104 and 108 at all times (i.e., intermediate and primary sodium are never co-mingled). The intermediate heat exchanger 120 transfers heat from the primary coolant loop 122 (fully contained within the vessel 114) to an intermediate coolant loop 124 (that is only partially located within the vessel 114). The intermediate heat exchanger 120 passes through the redan 110, thus bridging the hot pool 104 and the cold pool 108 (so as to allow flow of sodium 106 in the primary coolant loop 122 therebetween). In an embodiment, four intermediate heat exchangers 120 are distributed within the vessel 114. Alternatively, two or six intermediate heat exchangers 120 are distributed within the vessel 114. The intermediate coolant loop 124 circulates sodium coolant 126 that passes through pipes into and out of the vessel 114, via the reactor head 116. An intermediate sodium pump 128 located outside of the reactor vessel 114 circulates the sodium coolant 126, for example, through a power generation system 129. Heat is transferred from the sodium coolant 106 of the primary coolant loop 122 to the sodium coolant 126 of the intermediate coolant loop 124 in the intermediate heat exchanger 120. The sodium coolant 126 of the intermediate coolant loop 124 passes through a plurality of tubes 130 within the intermediate heat exchanger 120. These tubes 130 keep separate the sodium coolant 106 of the primary coolant loop 122 from the sodium coolant 126 of the intermediate coolant loop 124, while transferring heat energy therebetween. A direct heat exchanger 132 extends into the hot pool 104 and provides cooling to the sodium coolant 106 within the primary coolant loop 122, usually in case of emergency. The direct heat exchanger 132 is configured to allow sodium coolant 106 to enter and exit the heat exchanger 132 from the hot pool 104. The direct heat exchanger 132 has a similar construction to the intermediate heat exchanger 120, where tubes 134 keep separate the NaK (Sodium-Potassium) of the primary coolant loop 122 from the direct heat exchanger coolant (NaK) 136 of a direct reactor coolant loop 138, while transferring heat energy therebetween. Additionally, TWR reactor 100 includes sodium processing circuit 140 extending into the hot pool 104. The sodium processing circuit 140 facilitates channeling sodium (Na) coolant 106 therethrough for receipt, storage, purification, and/or sampling and analysis of one or more subsystems. For example, the sodium processing circuit 140 may include, but is not limited to, any number of on-site storage tanks, cold traps (e.g., devices used to remove impurities from sodium), cesium traps (e.g., devices for trapping cesium (Cs), which are discussed in further detail below), electromagnetic pumps, heat exchangers, pipelines, testing equipment, and/or control valves. Other ancillary reactor components (both within and outside of the reactor vessel 114) include, but are not limited to, pumps, check valves, shutoff valves, flanges, drain tanks, etc., that are not depicted but would be apparent to a person of skill in the art. Additional penetrations through the reactor head 116 (e.g., a port for the primary coolant pump 118, inert cover gas and inspection ports, sodium processing, and cover gas ports, etc.) are not depicted. A control system 142 is utilized to control and monitor the various components and systems which make up the reactor 100. Broadly speaking, this disclosure describes configurations that improve the performance of the reactor 100 described in FIG. 1. Specifically, embodiments, configurations, and arrangements of a remotely removable cesium trap assembly are shown and described in more detail below with reference to FIGS. 2-5. In general, during operation of the reactor, radioactive cesium will enter the primary sodium coolant. To reduce the adverse radiological impact of this radionuclide, cesium traps are installed as part of the sodium processing circuit. The cesium trap assembly described below enables a cesium trap loaded with cesium to be removed from the sodium processing circuit and its shielded cell, and be replaced with another new empty cesium trap to facilitate the continued absorption and filtering of radioactive cesium from the primary sodium coolant. For example, the cesium trap assembly includes lifting eyes, lateral support anchors, sodium lines, and power and control attachments all positioned at a top portion of the cesium trap to increase remote access and ease of removal. Additionally, remotely controlled tools and lifts enable the loaded cesium trap to be removed from its shielded cell for transport to another location without direct access to the radioactive cesium. Installation of the new cesium trap may be performed with direct access to the shielded cell once the radioactive cesium is removed, thus decreasing installation time. By enabling the cesium traps to be removable and replaceable, the loaded cesium traps are no longer stored in place at the plant and adjacent the reactor. The loaded cesium traps may be transported to an off-plant facility for storage and/or preparation for disposal. Thus, the number of shielded cells adjacent the reactor may be reduced, thus increasing plant space around the reactor. Moreover, through remote access to the loaded cesium trap and direct access to the new cesium trap, removal and replacement time may be decreased, enabling replacement of a cesium trap to be performed while the reactor continues to operate and in a short amount to time. Additionally, the highly radioactive cesium trap is replaced with reducing exposure to the personnel performing the replacement. As, such risk to plant personnel and plant operations are reduced and overall reactor efficiency and viability may be increased. FIG. 2 is a schematic view of an example cesium trap assembly 200 and FIG. 3 is a top view of the cesium trap assembly 200 taken along section line 3-3. Referring to both FIGS. 2 and 3, the cesium trap assembly 200 includes a removable cesium trap 202 disposed within an interior chamber 204 of an individual shielded cell 206 that is configured to contain radioactive material therein. The cell 206 may be coverable via a removable cover 208 depicted in dashed lines in FIG. 2. In the example, the cesium trap 202 includes a body 210 at least partially surrounded by a cooling jacket 212 as discussed in further detail below. The cesium trap 202 is coupled in flow communication with a sodium line 214 that forms at least a portion of the sodium processing circuit 140 (shown in FIG. 1), such that a stream of sodium 216 may be channeled through the body 210. Sodium stream 216 may contain, but is not limited to, sodium, cesium, and argon. For example, an inlet sodium line 218 extends from the body 210 at a top portion 220 of the cesium trap 202, and an outlet sodium line 222 also extends from the body 210 at the top portion 220. Additionally, the cesium trap 202 is coupled in flow communication with a fixed cooling line 224 such that a stream of coolant 226 may be channeled through the cooling jacket 212. An inlet cooling line 228 extends from the cooling jacket 212 proximate a bottom portion 230 of the cesium trap 202 and includes a free end 232. The free end 232 is configured to releasably couple in flow communication with the fixed cooling line 224. In one example, the free end 232 has an enlarged diameter such that the free end 232 may be slidably engaged S over and around the fixed cooling line 224. In alternate examples, the inlet cooling line 228 may be releasably coupled to the fixed cooling line 224 via any other connection that enables the cesium trap 202 to function as described herein. An outlet cooling line 234 also extends from the cooling jacket 212 such that the coolant stream 226 may be exhausted into the interior chamber 204. The cesium trap 202 is coupled to the cell 206 within the interior chamber 204. A base 236 is coupled to the bottom portion 230 and includes at least one opening 238 defined therein. The openings 238 correspond to and are configured to slidably engage with locating pins 240 extending from a floor 242 of the cell 206. The locating pins 240 include a bullet nose 244 or other tapered tip to facilitate locating of and engagement with the openings 238. Additionally, the locating pins 240 reduce excessive lower cesium trap 202 movement during seismic events. In the example, the base 236 is sized larger than the cesium trap 202 to increase stability within the cell 206. The cesium trap 202 is also coupled within the interior chamber 204 via support anchors 246 configured to support lateral and vertical loads, such as seismic loads. The lateral support anchor 246 includes a cell anchor 248 coupled to and extending from a wall 250 of cell and a trap anchor 252 coupled to and extending from cooling jacket 212. The cell anchor 248 is releasably coupled to and on top of the trap anchor 252 via a connection member 254. The connection member 254 is a captive male fastener located on the cell anchor 248 that has a bullet nose entrance to allow a remote rotational device to be used to unthread it from a corresponding female capture nut located on the trap anchor 252. In alternative examples, the connection member 254 may be a pin, a bolt, or any other connection member that enables the lateral support anchor 246 to function as described herein. The cesium trap 202 is communicatively coupled to a control system, for example, control system 142, via electrical power attachment 256 and instrument control attachment 258 extending from cell 206. Electrical power and instrument control attachments 256 and 258 plug into the cesium trap 202 via at least one receiver 260 disposed at the top portion 220. Additionally, the cesium trap 202 includes at least one lifting eye 262, for example two or four for redundancy, extending from the top portion 220. In one example, lifting eyes 262 are welded to body 210, while in other examples lifting eyes 262 are unitarily formed with body 210. The lifting eyes 262 may be configured to facilitate rotation of the cesium trap 202 once lifted. Positioned above the cell 206 is a moveable removal cask 264 that facilitates replacing the cesium trap 202 within cell 206. The removal cask 264 defines an interior chamber 266 that is configured to contain radioactive material therein. Additionally, a floor valve 268 is positioned above the cell 206 that may be closed after removing the cover 208. The removal cask 264 includes a remote lifting tool 270 that releasably couples to the cesium trap 202 and facilities removing the cesium trap 202 from the cell 206. For example, lifting tool 270 includes an arm 272 with a plate 274 connected thereto. The plate 274 includes at least one hook 276 extending therefrom. The hooks 276 may be substantially “J”-shaped and correspond to the lifting eyes 262 disposed on cesium trap 202. In alternative examples, lifting tool 270 has any other configuration that enables function as described herein. Within interior chamber 266, removal cask 264 also includes a remote sodium line tool 278 that facilitates crimping and cutting the sodium line 214 as described further below, a remote anchor tool 280 that facilitates decoupling the lateral support anchors as described further below, and a remote attachment tool 282 that facilitates unplugging the electrical power and instrument control attachments 256 and 258 as described further below. In the example, each tool 270, 278, 280, and 282 are operatively coupled to a control system, for example, control system 142, for remote operation. Additionally, one or more cameras (not shown) are provided to assist remote operation. In alternative examples, removal cask 264 may include a single tool that combines all of the functions of tools 270, 278, 280, and 282 such that removal cask 264 may function as described herein. In operation, sodium coolant 106 is circulated through reactor 100 as described above in reference to FIG. 1. As the sodium coolant 106 is channeled through the reactor core 102, fissionable fuel transfers heat to the sodium coolant 106, in addition, volatile fission products such as cesium enter the sodium coolant 106. A portion 216 of the sodium coolant 106 is channeled through sodium processing circuit 140, including the inlet and outlet sodium lines 218 and 222 and the cesium trap 202, to, in part, remove the volatile fission products contained therein and reduce any adverse radiological impact. As the sodium stream 216 is channeled through cesium trap 202, the cesium contained therein is absorbed and filtered out of the sodium stream 216 before being channeled back into the reactor 100. The operation of cesium trap 202 will be discussed in further detail below. In some known reactors, the cesium traps are fixed within the shielded cells, and as such, once the traps are loaded with the extracted cesium, the entire cesium trap and extracted radioactive cesium are stored in place. However, long term operation of such reactors may produce a number of cesium traps loaded with radioactive cesium, with a number of radioactive shielded cells located in close proximity to the reactor core. In contrast, the above described cesium trap assembly 200 facilitates removal and replacement of the cesium traps 202 that are loaded with radioactive cesium. As such, the cesium trap 202 and radioactive cesium therein may be transferred to a more suitable long-term storage location either on-site and/or at a remote facility. Additionally, some of the reactor area surrounding the reactor core may be used for other reactor systems and processes. Once the cesium trap 202 contains a predetermined amount of cesium, the sodium stream 216 is redirected away from the cesium trap 202 and the remaining sodium within the sodium lines 214 and the cesium trap 202 is allowed to freeze. For example, the sodium stream 216 freezes at 208° Fahrenheit, such that turning off electrical heaters of the circuit (not shown) and stopping flow of the sodium stream 216 facilitates freezing the cesium trap 202 over time. In alternative examples, cooling gas may be circulated through the cooling jacket 212 to increase freezing time. To remove the cesium trap 202 from the shielded cell 206, the floor valve 268 is installed over the cell 206 and the cover 208 removed from the cell 206 and moved to a temporary storage location on-site. The floor valve 268 is closable such that reactor personnel are shielded from the high radiation (gamma) fields from the cesium trap 202 once the cover 208 is removed. The removal cask 264 is mated to the floor valve 268 and the floor valve 268 opened to provide access into interior chamber 204 of cell 206 while maintaining containment thereof. For example, the floor valve 268, cover 208, and removal cask 264 may all be positionable via a crane (not shown) within the reactor 100 and remotely operable. In alternative examples, the floor valve 268, cover 208, and removal cask 264 are movable via any other system that enables the cesium trap 202 to be removed and replaced as described herein. When the removal cask 264 is mated to the floor valve 268, the cesium trap 202 may be remotely decoupled from the shielded cell 206. To decouple the cesium trap 202, the sodium lines 214, the lateral support anchors 246, and the power and control attachments 256 and 258 extending to the cesium trap 202 are removed. For example, sodium line tool 278 is remotely operated to extend into interior chamber 204 and to decouple the cesium trap 202 from the sodium processing circuit 140. The sodium line tool 278 crimps both the inlet and outlet sodium lines 218 and 222 extending from the cesium trap 202 and cuts the sodium lines 218 and 222 adjacent to a crimped portion 284. As such, each sodium line 218 and 222 now includes two portions, a first portion 286 that remains extending from cesium trap 202 and a second portion 288 that remains part of the sodium processing circuit 140. Anchor tool 280 is remotely operated to extend into interior chamber 204 and decouple the lateral support anchor 246 extending between the cesium trap 202 and the cell 206. The anchor tool 280 removes connection member 254, for example, a pin or a bolt, such that trap anchor 252 is decoupled from cell anchor 248. Attachment tool 282 is remotely operated to extend into interior chamber 204 and disconnect the electrical power and instrument control attachments 256 and 258 from the cesium trap 202. The attachment tool 282 unplugs attachments 256 and 258 from the associated receivers 260. By remotely decoupling the cesium trap 202, direct access to the radioactive cell 206 is reduced. Additionally, by locating each of the sodium line 214, lateral support anchor 246, and attachments 256 and 258, proximate the top portion 220 ease of remote access increases, thus reducing replacement time. To remove the cesium trap 202 from the shielded cell 206, the lifting tool 270 extends into interior chamber 204 and is releasably coupled to the cesium trap 202. For example, lifting tool 270 rotates and lifts hooks 276 into the corresponding lifting eyes 262 such that cesium trap 202 is movable. The lifting tool 270 vertically lifts the cesium trap 202 away from the cell 206 and into the removal cask 264 such that the base 236 slidably disengages with the locating pins 240. As the lifting tool 270 vertically lifts the cesium trap 202, the inlet cooling line 228 is also decoupled from the fixed cooling line 224 as the free end 232 automatically slidably disengages with the fixed cooling line 224. In alternative examples, the inlet cooling line 228 may be cut to decouple the cesium trap 202, similar to the procedure with the sodium lines 214. Additionally, the trap anchor 252 is positioned above the cell anchor 248 so that the cesium trap 202 may be vertically lifted. Once the cesium trap 202 is within the removal cask 264, the cesium trap 202 with cesium therein and the removal cask 264 may be moved to another location, either on-site or to a remote site, for further processing and/or long-term storage. Additionally, once the radioactive cesium is removed, the cell 206 may be directly accessed for installing a new cesium trap 202. With the interior chamber 204 of cell 206 empty, a new replacement cesium trap, such as cesium trap 202 that is similar to the cesium trap removed but without any cesium therein, may be inserted into the shielded cell 206 using typical personnel and rigging methods on the overhead crane. The replacement cesium trap 202 is releasably coupled to a lifting tool, for example, the lifting tool 270 via corresponding hooks 276 and lifting eyes 262 such that the replacement cesium trap 202 may be placed into the interior chamber 204 of the cell 206. In alternative examples, the replacement cesium trap 202 is coupled to a different lifting tool, such as the crane within the reactor 100. As the replacement cesium trap 202 is lowered into the cell 206, the base 236 is simultaneously aligned with the locating pins 240 to position the replacement cesium trap 202 along the floor 242. By aligning the locating pins 240, the free end 232 of the inlet cooling line 228 is automatically slidably engaged with the fixed cooling line 224. Once the replacement cesium trap 202 is placed within the cell 206, the replacement cesium trap 202 is attached to the shielded cell 206. This attachment may be performed with direct access to the cell because no radioactive cesium is present therein. To attach the replacement cesium trap 202, the sodium lines 214, the lateral support anchors 246, and the power and control attachments 256 and 258 are extended to the cesium trap 202. For example, the replacement cesium trap 202 is coupled to the sodium processing circuit 140. The first portion 286 of the sodium line 214 extending from the cesium trap 202 is aligned and welded to the second portion 288 of the sodium line extending from the sodium processing circuit, thus forming the inlet and outlet sodium lines 218 and 222. The cell anchor 248 is coupled to the trap anchor 252 via a connection member 254, such as a pin or a bolt, forming the lateral support anchor 246. The electrical power and instrument control attachments 256 and 258 are connected, via plugging, to corresponding receivers 260. Once the replacement cesium trap 202 is attached to the shielded cell 206, the sodium processing circuit 140, and other piping and electrical connections, the cover 208 is replaced, and the replacement cesium trap 202 may begin to receive the sodium stream 216 and facilitate filtering and extracting cesium out of the sodium stream 216 as discussed further below. FIG. 4 is a perspective view of an example cesium trap 300 that includes a body 302 at least partially surrounded by a cooling jacket 304, similar to the cesium trap described above. The body 302 includes an inlet line 306 and an outlet line 308 such that a stream of sodium 310 is channeled therethrough. Within body 302, the cesium trap 300 includes a column of active material 312. For example, the active material is Reticulated Vitreous Carbon (RVC) that adsorbs cesium and removes the cesium from the sodium stream 310. Body 302 also includes a filter 314 that filters RVC particles that may become entrained within the sodium stream 310. Additionally, the cooling jacket 304 includes an inlet line 316 and an outlet line 318 such that a stream of coolant 320 is channeled therethrough. For example, the coolant 320 is nitrogen that reduces the operating temperature of the cesium trap 300. In alternative examples, the inlet line 306 and/or the outlet line 308 may extend into the cesium trap 300 such that the level of sodium therein to be lowered in preparation for disposal. In further alternative examples, the cesium trap 300 may include a capped line (not shown) that facilitates draining the sodium therein in a purpose built hot cell preparation facility and inserting material to reduce the chemical reactivity of sodium, such as lead, in preparation for disposal. In an example of the cesium trap 300, the filter 314 that prevents RVC from exiting the trap has a mean pore size of from 40 to 160 μm. In particular, a filter 314 having a mean pore size from a lower limit of any of 40, 50, 60, 70, 80, or even 90 μm to an upper limit of any of 100, 110, 120, 130, 140, 150 or even 160 μm are contemplated. Through testing, the above ranges of mean pore size where determined to be more effective in maintaining adequate filtration and throughput of the anticipated sodium and entrained argon stream to be passed through the cesium trap 300. Mean pore sizes of 25 or less for the filter 314 have been determined to have reduced throughput when there is significant amounts of argon entrained in the sodium to be passed through the cesium trap 300. This limits the power output of the reactor 100. The filter 314 may be any appropriate filter media. In an example, the filter 314 is a sintered metal filter. The sintered metal filter may be made of one or more of stainless steel, Hastelloy®, Monel®, Inconel®, nickel, HT-9, and titanium to name but a few possible metals. Alternatively, other filter types and filter media may be used having the mean pore size as described above. FIG. 5 is a flowchart illustrating a method 400 of replacing a cesium trap. The method 400 includes a first cesium trap at least partially containing cesium therein being frozen 402. The first cesium trap is decoupled 404 from a shielded cell and then removed 406 from the shielded cell. A second cesium trap is inserted 408 into the shielded cell and then attached 410 to the shielded cell. Decoupling 404 the first cesium trap includes removing lateral support anchors, sodium lines, and power and electrical connections extending thereto. For example, the first cesium trap may be remotely decoupled 412 from a lateral support anchor. As such, a connection member may be removed 414 between a cell anchor and a trap anchor. The first cesium trap may be remotely decoupled 416 from a sodium processing circuit. As such, a sodium line is crimped 418 and cut 420. The first cesium trap may be also remotely disconnected 422 from electrical power and instrument control attachments. As such, the electrical power and instrument control attachments are unplugged 424 from a corresponding receiver. Removing 406 the first cesium trap includes releasably coupling 426 the first cesium trap to a lifting tool. For example, the lifting tool may include a hook that is rotated and lifted 428 into a corresponding lifting eye of the cesium trap. The first cesium trap is then lifted 430 by the lifting tool out of the shielded shell. The lifting operation may also simultaneously decouple 432 a cooling line inlet from a fixed cooling line. Inserting 408 the second cesium trap includes coupling 434 the second cesium trap to a lifting tool, for example, via a hook extending from the lifting tool and a corresponding lifting eye of the cesium trap. The second cesium trap is then placed 436 into the shielded cell by the lifting tool, and simultaneously a base of the second cesium trap is aligned 438 with at least one locating pin extending from the floor of the shielded cell. Attaching 410 the second cesium trap includes re-coupling lateral support anchors, sodium lines, and power and electrical connections thereto. For example, a cell anchor may be coupled 440 to a trap anchor forming a lateral support anchor within the cell. A first sodium line of the second cesium trap may be welded 442 to a second sodium line of the sodium processing circuit forming the sodium lines of the second cesium trap. Additionally, the electrical power and instrument control attachments may be connected 444 into a corresponding receiver.
	abstract	Methods for producing Tc-99m radioisotope by proton irradiation of a fluid target matrix. A method of producing Tc-99m includes irradiating a fluid target matrix comprising Mo-100 with a proton beam to transform at least a portion of Mo-100 to Tc-99m. Optionally, the fluid target matrix further includes at least one of O-18, O-16, or N-14, which upon exposure to the proton beam concurrently transform at least a portion of O-18 to F-18, at least a portion of O-16 to N-13, at least a portion of the O-16 to O-15, or at least a portion of N-14 to C-11. The method further includes isolating Tc-99m and optionally at least one of F-18, N-13, O-15, or C-11 from the irradiated fluid target matrix. An additional source of Tc-99m is available from the decay of Mo-99 that is co-produced from the Mo-100 during irradiation with the proton beam.
	description	A detailed description will be presented below for a nuclear fuel assembly designed to be employable either for a UO2/light water reactor or for a MOX nuclear fuel/light water reactor in accordance with three independent embodiments of this invention. Referring to FIG. 4, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 fuel or MOX nuclear fuel in accordance with the first embodiment of this invention, will be described below. A thermal reactor currently in operation in any country in the world is required to observe the laws and regulations regarding the length of operation cycle thereof and the burn-up thereof, effective in the specific country in which the specific reactor is operating. Thus, possibilities exist that the entire requirements of this invention may not be allowed. In the first embodiment, however, a serious attention is paid to realize the feature and advantages of this invention as much as possible independently from the current legal restriction. In other words, the MOX enrichment grade of the only one kind of MOX nuclear fuel rods is selected as high as 14 weight % and the ratio of the quantity of the MOX rods with respect to the total quantity of the nuclear fuel rods, is selected to be as low as 33%. Referring to FIG. 4 illustrating a horizontal cross-section of a nuclear fuel assembly in accordance with the first embodiment of this invention, the assembly having MOX nuclear fuel rods and U O2 rods and having a burn-up capacity of 70 GWd/ton (heavy metal mass ton), symbol 1 shows highly enriched MOX nuclear fuel rods each of which contains U235 in 0.225 weight % and the fissionable Pu-s or Pu239 and Pu241 in approximately 14 weight %. The quantity employed is 24. This is the only one kind of MOX nuclear fuel rods employed for this assembly. Symbols 2 and 3 show UO2 rods each of which kinds contains U235 in 4.9 weight % and 4.5 weight % respectively. The quantity employed is 24 and 4 respectively. Symbol G shows gadolinium rods each of which contains U235 in 3.5 weight % and gadolinium in 3.5 weight % respectively. The quantity employed is 20. Thus, the MOX nuclear fuel rods accounts for 33% of the total quantity of the nuclear fuel rods contained in the nuclear fuel assembly. Each rod is approximately 4 m in length and approximately 11 mm in the external diameter. The nuclear fuel assembly is a square of which the length of each side is approximately 15 cm. The production process of the foregoing nuclear fuel assembly in accordance with this invention is nearly identical to that which is presently available. Firstly, Purex process or some other dry or wet spent nuclear fuel reprocessing processes are employed to separate Pu-s out of a spent fuel. The quantity of fissionable Pu-s or Pu239 and Pu241 contained in the separated Pu-s which usually is determined by a mass analysis process is 60 through 70%. An oxidation process is conducted to produce Pu-oxides. Secondly, the powder of PuO2 and UO2 are mixed to make the resultant grade of enrichment to a desired value. Thirdly, MINAS process or SBR process is conducted to well commingle the powder of PuO2 and UO2. Molding and sintering process are conducted to convert the powder of PuO2 and UO2 to sintered pellets of PuO2 and UO2. The product pellets are charged in a zircaloy sheath and the both ends thereof are sealed to produce fuel rods. MOX fuel rods, UO2 rods and other parts are fabricated to finish nuclear fuel assemblies. As is illustrated in FIG. 4, only one kind of MOX fuel rods 1 of which the enrichment grade is as high as 14 weight % are arranged at the area at which the effects of the moderator are less. Since the rods represented by symbols 2 and 3 are UO2 rods and since the rods represented by symbol 1 alone are MOX fuel rods, the ratio of the quantity of MOX fuel rods with respect to the total quantity of the nuclear fuel rods is as high as 33%. As a result, the grade of enrichment of 14 weight % is remarkably higher than that of the prior art of 5 weight %. On the other hand, the ratio (33%) of the quantity of the MOX fuel rods with respect to the total quantity of the nuclear fuel rods is remarkably less in comparison with that of the prior art or 80%. The function of the gadolinium fuel rods is identical to that of the prior art. Namely, it is to restrict fission to occur at the beginning of a reactor operation period. In other words, the gadolinium fuel rods are effective to reduce the possibility of fission to occur at the beginning of the reactor operation period but thereafter they lose the function and transit themselves to a fissionable fuel. In conclusion, the nuclear fuel assembly in accordance with this embodiment is provided with only one kind of MOX nuclear fuel rods each of which has remarkably large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, and the quantity of the MOX nuclear fuel rods is remarkably small. As was described earlier, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. Referring to FIG. 5, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 fuel alone or MOX nuclear fuel in accordance with the second embodiment of this invention, will be described below. As was described earlier, a thermal reactor currently in operation in any country in the world is required to observe the laws and regulations regarding the length of operation cycle thereof and the burn-up thereof, effective in the specific country in which the specific thermal reactor is operating. In the second embodiment as well, however, a serious attention is paid to realize the feature and advantages of this invention as much as possible within the limitation to observe the legal restriction presently effective generally in the world. In other words, the MOX enrichment grade is selected to be 6 weight % and the ratio of the quantity of the MOX rods with respect to the total quantity of the nuclear fuel rods, is selected to be 25%. Referring to FIG. 5 illustrating a horizontal cross-section of a nuclear fuel assembly in accordance with the second embodiment of this invention, the assembly having MOX nuclear fuel rods and UO2 nuclear fuel rods and a burn-up capacity of 45 GWd/ton (heavy metal mass ton), symbol 1 shows highly enriched MOX nuclear fuel rods each of which kinds contains U235 in 0.225 weight % and the fissionable Pu-s or Pu239 and Pu241 in 6 weight %. The quantity employed is 16. This is the only one kind of MOX nuclear fuel rods employed for this assembly. Symbols 2, 3 and 4 show UO2 fuel rods each of which kinds contains U235 in 4.0 weight %, 3.5 weight % and 3 weight % respectively. The quantity employed is 28, 8 and 4 respectively. Symbol G shows gadolinium rods each of which contains U235 in 2 weight % and gadolinium in 2 weight % respectively. The quantity employed is 16. Thus, the MOX nuclear fuel rods accounts for 25% of the total quantity of the nuclear fuel rods employed in the nuclear fuel assembly. The production process of the foregoing nuclear fuel assembly in accordance with this embodiment is entirely identical to that for the first embodiment. As is illustrated in FIG. 5, only one kind of MOX fuel rods 1 of which kind the enrichment grade is 6 weight % are arranged at the area at which the effects of the moderator are less. Since the rods represented by symbols 2, 3 and 4 are UO2 fuel rods and since the rods represented by symbol 1 alone are MOX fuel rods, the ratio of the quantity of MOX fuel rods with respect to the total quantity of the nuclear fuel rods is 25%. As a result, the grade of enrichment of 6 weight % is higher than that of the prior art or 5%. On the other hand, the ratio (25%) of the quantity of the MOX fuel rods with respect to the total quantity of the nuclear fuel rods is remarkably less than that of the prior art or 80%. The function of the gadolinium fuel rods is identical to that of the prior art. Namely, it is to restrict fission to occur at the beginning of the reactor operation period. In conclusion, the nuclear fuel assembly in accordance with this embodiment is provided with only one kind of MOX nuclear fuel rods each of which only one kind has large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, and the quantity of the MOX nuclear fuel rods is small. As was described earlier, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. Referring to FIG. 6, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 fuel alone or MOX nuclear fuel in accordance with the third embodiment of this invention, will be described below. As was described earlier, a thermal reactor currently in operation in any country in the world is required to observe the laws and regulations regarding the length of operation cycle thereof and the burn-up thereof, effective in the specific country in which the specific thermal reactor is presently operating. In the third embodiment, the best efforts are used to realize the feature and advantage of this invention as much as possible within the limitation of the design of the presently operating reactor. Referring to FIG. 6 illustrating a horizontal cross-section of a nuclear fuel assembly in accordance with the third embodiment of this invention, the assembly having MOX fuel rods and UO2 fuel rods and having a burn-up capacity of 45 GWd/ton (heavy metal mass ton), symbol 1 shows highly enriched MOX nuclear fuel rods each of which contains U235 in 0.225 weight % and the fissionable Pu-s or Pu239 and Pu241 in 6 weight %. The quantity employed is 24. This is the only one kind of the MOX nuclear fuel rods employed for this assembly. Symbols 2, 3 and 4 show UO2 fuel rods each of which kinds contains U235 in 4 weight %, 3.5 weight % and 3 weight % respectively. The quantity employed is 20, 8 and 4 respectively. Symbol G shows gadolinium rods each of which contains U235 in 2 weight % and gadolinium in 2.2 weight % respectively. The quantity employed is 16. Thus, the quantity of the MOX nuclear fuel rods accounts for 33% of the total quantity of the nuclear fuel rods employed in the assembly. The production process of the foregoing nuclear fuel assembly in accordance with this embodiment is entirely identical to that for the first and second embodiments. As is illustrated in FIG. 6, only one kind of MOX fuel rods 1 of which only one kind the enrichment grade is 6 weight % are arranged at the area at which the effects of the moderator are less. Since the rods represented by symbols 2, 3 and 4 are UO2 fuel rods and since the rods represented by symbol 1 alone are the MOX fuel rods, the ratio of the quantity of the MOX fuel rods with respect to the total quantity of nuclear fuel rods is 33%. As a result, the grade of enrichment of 6 weight % is higher than that of the prior art or 5%. On the other hand, the ratio (33%) of the quantity of the MOX fuel rods with respect to the total quantity of the nuclear fuel rods is remarkably less than that of the prior art or 80%. The function of the gadolinium fuel rods is identical to that of the prior art. As is identical to the first and second embodiments, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. In conclusion, the nuclear fuel assembly in accordance with this embodiment is provided with only one kind of MOX nuclear fuel rods each of which only one kind has relatively large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, and the quantity of the MOX nuclear fuel rods is small. As was described earlier, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. The above description has clarified that this invention has successfully provided an improvement applicable to a nuclear fuel assembly employable either for a thermal neutron reactor employing UO2 as the nuclear fuel and light water as the moderator/coolant or for a thermal neutron reactor employing the MOX nuclear fuel as the nuclear fuel and light water as the moderator/coolant, wherein the production cost is much less and the value of the spent fuel thereof is much larger than that of the nuclear fuel assembly available in the prior art.
	description	A series of cylindrical extruded uranium rods about 8 inches long and 1.360 inches in diameter, after cleaning by a nitric acid pickling treatment, washing and drying as pre-viously described, were subjected to the following canning procedure: Each rod was immediately dipped through a xc2xdinch depth of potassium-sodium lithium chloride flux of the composition 53% potassiwm chloride, 42% lithium chloride and 5% sodium chloride, by weight, into a molten bronze bath consisting of 53 parts of tin and 47 parts of copper, by weight, at a tem-perature of 720xc2x0 C. for 45 seconds. Upon removal from the bronze bath each rod, which was uniformly coated with a bronze coating, was immersed in a bath of molten tin at 640xc2x0 C. for 20 seconds. Immersion in the tin bath was effected by dropping the rod onto the high end of a sloping wire rack sunk in the tin bath, allowing the rod to roll to the lower end, and removing it by a pair of tongs from the molten metal where the surface had just been carefully scraped to remove all traces of flux. Upon removal from the tin bath the rod was centrifuged at 640 rpm, in a centrifuge supporting the rod at about 6 inches from the axis of rotation, for 5 seconds to remove excess tin and then immersed in a bath of 0.1% sodium-modified, degassed 13X aluminum-silicon alloy (Federal specification AN-QQ-A-366, Amendment 4 Al-13X) at 600xc2x0 C. for 6 seconds. While the rod was being coated in this manner a 2S aluminum can which had an inside diameter 15 mils greater than the rod and had been cleaned by washing the can first with trichlorethene, then with an aqueous 0.1% soap solution containing 0.1% sodium pyrophosphate at 80xc2x0 C., and finally with aqueous 20% o-phosphoric acid solution for 5 minutes at 20xc2x0 C. was heated in a chromium steel supporting sleeve to a temperature of 640xc2x0 C. and 80 grams of 13X aluminum-silicon alloy and an aluminum plug xc2xdinch thick, with a hole through its center, preheated to about 630xc2x0 C. and about 640xc2x0 C., respectivey, were added to the can just before the metallic uranium rod was withdrawn from the aluminum-silicon bath. The plug was used to space the rod from the end of the can and provide good heat conduct-ivity at this location. It was inserted on the molten 13X alloy in the can by means of a rod having a tapered end fitting the hole in the plug snugly and having a sleeve for forcing the plug off the end. The coated uranium rod was passed from the bath immediately into the can and a slightly tapered aluminum cap about 5/16 inch thick, which had been preheated to a temperature of about 600xc2x0 C., was inserted into the molten aluminum-silicon alloy filling the open end of the can. The complete assembly was then immediately quenched by immersion in water and the canned rod was removed from its supporting sleeve. In this manner a firm uniform bond between the uranium rod and the protective metallic can was obtained. The total elapsed time from immersion of the uranium rods in the bronze bath to their immersion in the quench tank was about 80-90 seconds. FIG. 1 is photomicrograph at 500 magnifications show-ing a section of the bond between one of the uranium rods of this series and its protective can. Another series of uranium rods of the same size as employed in EXAMPLE 1 was canned under similar conditions with the following changes. After the rods were centrifuged to remove excess tin, they were dipped into a molten 1.0% sodium-modified, degassed aluminum-silicon alloy containing 88% aluminum and 12% silicon (before modifying) at 600xc2x0xc2x15xc2x0 C. for 6 seconds. They were then inserted in aluminum cans, containing 70 grams of 0.1% sodium-modified, degassed aluminum-silicon alloy of the same composition, maintained at 590xc2x0 C. The cans were capped and quenched as in the preceding example. A 1.360xe2x80x3xc3x978xe2x80x3 metallic uranium rod having its ends machined down to accommodate ferrules was coated as described in EXAMPLE 1. Upon removal of the coated rod from the alum-inum-silicon bath it was immediately capped by aluminum caps pressed over the machine ends, redipped in the aluminum-silicon bath for b 2 seconds, then placed in the valley formed by a pair of smooth xe2x80x9cTransitexe2x80x9d (asbestos cement) rollers rotating at about 200 ft. per minute peripheral velocity. Sixty-five grams of the modified degassed 88/12 aluminum silicon alloy at a temperature of 640xc2x0 C. was poured into the trough formed between the uranium rod and one of the xe2x80x9cTransitexe2x80x9d rollers. When all of the aluminum-silicon had solidified, the rod was removed from the rollers and quenched by immersing it in water. It was then machined down to a 1.42 inch diameter to provide a smooth, even 30-mil coating. A small metallic uranium rod, prepared for coating by pickling in nitric acid solution as previously described, was immersed for 45 seconds in a speculum metal bath (67% copper and 33% tin) at 810xc2x0 C. During this period the bath cooled to 795xc2x0 C. The rod was withdrawn from the bath and immersed in a eutectic bronze bath (47% copper and 35% tin) for about 1 minute at 710xc2x0 C. It was withdrawn from this bath and centrifuged for 8 seconds at about 640 rpm. It was then quenched by immersion in water. The bronze coated rod was dried and than dipped for 30 seconds in an aluminum-silicon bath comprising 88 parts of aluminum and 12 parts silicon at a temperature of 593xc2x0 C. The coated rod was rolled until the coating solidified, then washed with water. A firm over coating was obtained. A cross-section of the coated rod, showing the bond between the rod and the coating is illustrated in the photomicrograph, FIG. 2 of the drawing, at 100 magni-fications. A metallic uranium rod prepared for coating as previously described was immersed for 40 seconds in a bronze bath con-sisting of 47 parts of copper and 53 parts of tin at 730xc2x0 C. During the period of immersion the bath cooled from 730xc2x0 C. to 720xc2x0 C. The coated rod was then immersed in a tin bath at 420xc2x0 C. for 10 seconds. The tin bath was covered with a 53% KCl, 42% LiCl, 5% NaCl flux. Upon withdrawal from the tin bath the rod was centrifuged for 5 seconds at about 640 rpm. It was then dipped quickly into an aluminum-silicon alloy bath containing 88% aluminum and 12% silicon and quickly withdrawn. The rod was dipped seven times in this bath, which was held at a temperature of 615xc2x0 C. After the seventh dip it was withdrawn and laid on xe2x80x9cTransitexe2x80x9d rollers and 35 grams of additional aluminum-silicon alloy of the same composition was poured into the trough between the rod and the roller. The rod was rotated for 10 seconds until the aluminum-silicon had solidified and was then quenched by immersion in water. FIG. 3 of the drawing is a photomicrograph of a cross-section of the coated rod at 100 magnification. A small uranium rod, after pickling in nitric acid, washing and drying, as previously described, was immersed in a speculum metal bath at 850xc2x0 C. for 1xc2xd minutes during which the bath cooled to 840xc2x0 C. The rod, upon withdrawal from this bath, was immersed in peritectic bronze at 700xc2x0 C. for xc2xd minute, centrifuged for 15 seconds at 640 rpm, then quenched in water. The rod was coated with a continuous uniform protective bronze coating. Rods coated in this manner are afforded a substantial degree of corrosion resistance by the bronze coating. A bronze-coated rod prepared as described in EXAMPLE 6 was dipped in 88/12 almuminum-silicon alloy at 595-600xc2x0 C. for 30 seconds. Upon withdrawal from the aluminum-silicon alloy bath it was rotated at about 200 ft. per minute on xe2x80x9cTransitexe2x80x9d rollers for 15 seconds then quenched in water. The rod was coated with an aluminum-silicon coating over bronze. An extruded uranium rod was pickled for one minute in 50% HNO3 at room temperature, washed, dried and then dipped through a top flux of 50% NaCl and 50% KCl into a bronze bath containing 65% copper, 33% tin, and 2% nickel. After immersion in this alloy for 60 seconds, the rod was removed, dipped in a 50% LiCl, 40% KCl, 10% NaCl bath at about 550xc2x0 C. for about xc2xd minute then cooled. The rod was completely coated with a continuous corrosion-resistant bronze coating. An extruded uranium rod was dipped through a flux contain-ing 66% CaCl2, 29% NaCl, and 5% KCl into a molten bronze con-sisting of 53% copper and 47% tin to which 5% of nickel (based on the weight of the bronze) had been added. The bath was at 840xc2x0 C. After three minutes in the bath the rod was withdrawn, cooled and cleaned anodically in 98% H2SO4 solution. A polished protective bronze coating was thus obtained on the metal. A metallic uranium rod 1.1 inches in diameter and 4 inches long was first dipped through a chloride flux contain-ing 53% potassium chloride, 42% lithium chloride, and 5% sodium chloride, into a 47% copper 53% tin bath at a tempera-ture between 740xc2x0 and 760xc2x0 C. for 20 seconds. Upon withdrawal from the bronze bath the rod was centrifuged for 5 seconds to remove excess metal, it was then immersed in a salt bath having the same composition as the flux on the bronze bath and main-tained at 605xc2x0 C. After 20 seconds in this salt bath the rod was withdrawn and immersed in an unmodified 88% aluminum 12% silicon bath at 635xc2x0 C. for 20 seconds. The rod was then placed on xe2x80x9cTransitexe2x80x9d rollers and rolled slowly while 22 grams of the aluminum-silicon alloy was poured into the trough be-tween one of the rollers and the rod. Thirty-two grams of 88/12 aluminum-siliccn alloy modified by the addition of 1% of zinc and 0.02% of sodium was then poured on and the roll-ing was continued until the coating had solidified. The rod was then quenched by immersion in water. When cool the coat-ing was machined to a uniform 30 mil thickness. The resulting coating formed a protection for tle metal, which was resistant to the corrosive action of hot aqueous hydrogen peroxide solution. It will be understood that we intend to include variations and modification of the invention and that the preceding examples are illustrations only and in no wise to be construed as limitations upon the invention, the scope of which is defineed in the appended claims, wherein
047117583	summary	BACKGROUND OF THE INVENTION The present invention is related to the long-term storage of spent fuel that has been removed from a nuclear reactor, and more particularly, to a spent fuel storage cask having a basket which supports the spent fuel and which dissipates heat generated by the spent fuel. The basket includes a plurality of grid assemblies which provide storage slots for the spent fuel and which conduct heat to the walls of the cask. FIG. 1 illustrates a typical fuel assembly 20 for supplying nuclear fuel to a reactor. Assembly 20 includes a bottom nozzle 22 and a top nozzle 24, between which are disposed elongated fuel rods 26. Each fuel rod 26 includes a cylindrical housing made of a zirconium alloy such as commercially available "Zircalloy-4", and is filled with pellets of fissionable fuel enriched with U-235. Within the assembly of fuel rods 26, tubular guides (not shown) are disposed between nozzles 22 and 24 to accommodate movably mounted control rods (not illustrated) and measuring instruments (not illustrated). The ends of these tubular guides are attached to nozzles 22 and 24 to form a skeletal support for fuel rods 26, which are not permanently attached to nozzles 22 and 24. Grid members 28 have apertures through which fuel rods 26 and the tubular guides extend to bundle these elements together. Commercially available fuel assemblies for pressurized water reactors include between 179 and 264 fuel rods, depending upon the particular design. A typical fuel assembly is about 4.1 meters long, about 19.7 cm wide, and has a mass of about 585 kg, but it will be understood that the precise dimensions vary from one fuel assembly design to another. After a service life of about 3 years in a pressurized water reactor, the U-235 enrichment of a fuel assembly 20 is depleted. Furthermore, a variety of fission products, having various half-lives, are present in rods 26. These fission products generate intense radioactivity and heat when assemblies 20 are removed from the reactor, and accordingly the assemblies 20 are moved to a pool containing boron salts dissolved in water (hereinafter "borated water") for short-term storage. Such a pool is designated by reference number 30 in FIG. 2. Pool 30 is typically 12.2 meters deep. A number of spent fuel racks 32 positioned at the bottom of pool 30 are provided with storage slots 34 to vertically accommodate fuel assemblies 20. A cask pad 36 is located at the bottom of pool 30. During the period when fuel assemblies 20 are stored in pool 30, the composition of the spent fuel in rods 26 changes. Isotopes with short half-lives decay, and consequently the proportion of fission products having relatively long half-lives increases. Accordingly, the level of radioactivity and heat generated by a fuel assembly 20 decreases relatively rapidly for a period and eventually reaches a state wherein the heat and radioactivity decrease very slowly. Even at this reduced level, however, rods 26 must be reliably isolated from the environment for the indefinite future. Dry storage casks provide one form of long-term storage for the spent fuel. After the heat generated by each fuel assembly 20 falls to a predetermined amount--such as 0.5 to 1.0 kilowatt per assembly, after perhaps 10 years of storage in pool 30--an opened cask is lowered to pad 36. By remote control the spent fuel is transferred to the cask, which is then sealed and drained of borated water. The cask can then be removed from pool 30 and transported to an above-ground storage area for long-term storage. The requirements which must be imposed on such a cask are rather severe. The cask must be immune from chemical attack during long-term storage. Furthermore, it must be sufficiently rugged mechanically to avoid even tiny ruptures or fractures during long-term storage and during transportation, when the cask might be subjected to rough treatment or accidents such as drops. Moreover, the cask must be able to transmit heat generated by the spent fuel to the environment while nevertheless shielding the environment from radiation generated by the spent fuel. The temperature of the rods 26 must be kept below a maximum temperature, such as 375.degree. C., to prevent deterioration of the zirconium alloy housing. Provisions must also be made to ensure that a chain reaction cannot be sustained within the cask; that is, that the effective criticality factor K.sub.eff remains less than one so that a self-sustaining reaction does not occur. These requirements impose stringent demands upon the cask, which must fulfill its storage function in an utterly reliable manner. SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a mechanically rugged storage cask which prevents fission products and radiation from escaping into the environment while dissipating heat generated by spent fuel. Another object of the present invention is to provide a storage cask having a basket with a plurality of grid assemblies for supporting spent fuel, either in the form of fuel assemblies or consolidated fuel, or both, and for conducting heat generated thereby to the walls of the cask. Another object of the present invention is to provide a storage cask having a basket with disk-shaped grid assemblies which are spaced apart from one another in a column and which expand, after the basket is inserted into a container to form the cask, so that they come into contact with corresponding rings on the interior walls of the container in order to transmit heat from the basket to the rings, the diameters of the grid assemblies in the column and their corresponding rings decreasing slightly from the top of the column to the bottom in order to facilitate insertion of the basket into the container. Another object of the present invention is to provide a basket having legs which support grid assemblies at spaced-apart positions, the grid assemblies being movably mounted on the legs by top and bottom rings which are affixed to the legs and which are spaced further apart than the thickness of the grid assemblies. Another object of the present invention is to provide heat-transmitting wedges for positioning cells within the basket while permitting differential expansion of the cells with respect to the basket, the cells in turn enclosing spent fuel assemblies. These and other objects can be attained by providing a container having a cavity defined by substantially cylindrical walls. A basket disposed in the cavity has a plurality of substantially disk-shaped grid assemblies which are mounted at different vertical positions above the container floor. Each grid assembly includes plates which are joined together to provide a matrix of apertures and metal elements which are affixed to the plates to provide a substantially circular periphery. The apertures provided by the matrixes of the grid assemblies are aligned to provide storage slots for accepting spent fuel. When the cask elements are fabricated at normal shop temperatures, the diameters of the grid assemblies are slightly less than the diameter of the inner cask wall, so that the basket can be inserted into the container. When spent fuel is loaded and stored, however, the temperature within the cask rises and the grid assemblies expand with respect to the container so that the peripheries of the grid assemblies come into heat-conducting contact with the container walls. In accordance with one aspect of the invention, the container walls include vertically spaced rings which project slightly into the cavity. Each ring corresponds to a grid assembly and is positioned adjacent the periphery of the corresponding grid assembly. At the time of fabrication the diameter of a ring is slightly (e.g., about 0.3 cm) greater than the diameter of the corresponding grid assembly, and in order to alleviate the risk that the basket might jam while it is being inserted into the container, the diameter of each grid assembly and the diameter and of its corresponding ring are slightly less than the diameters of the grid assembly and ring above it. The changing diameters not only afford a greater latitude for error during the initial stages of insertion, they also provide visual guides for correcting the alignment as the insertion process progresses. In accordance with another aspect of the invention, the basket includes legs to which the grid assemblies are mounted at spaced-apart positions. Four of the metal elements of each grid assembly are provided with holes through which the legs extend. The grid assemblies are confined between top and bottom rings affixed to the legs on either side of the holes. The distance between the rings is slightly greater than the thickness of the grid assemblies in order to permit differential expansion of the elements as the temperature within the cask rises. In accordance with another aspect of the invention, open-ended cells for enclosing fuel assemblies can be inserted into the storage slots of the basket. Each cell has four walls which have dimensions corresponding to those of the fuel assembly to be enclosed and which support "neutron poison." The cells are positioned in the storage slots by heat-conducting wedges which are welded to the plates of the grid assemblies and which are spaced apart slightly from two of the cell walls. This slight spacing allows differential expansion of the cells with respect to the basket but permits transfer of heat to the basket, particularly if the cask is flooded with helium. In accordance with yet another aspect of the invention, the rings on the container wall are wider than the thicknesses of the corresponding grid assemblies, and moreover beveled surfaces are provided on both the rings and the peripheries of the grid assemblies in order to facilitate insertion and/or removal of the basket.
	description	Pursuant to 35 U.S.C. § 119(a), this application claims the benefit of the earlier filing date and the right of priority to Korean Patent Applications No. 10-2017-0059996, filed on May 15, 2017, the contents of which are incorporated by reference herein in their entirety. The present invention relates to a nuclear reactor vessel cooling method, and more particularly, a power generation using heat of a reactor vessel during a normal operation, an emergency power generation using heat of the reactor vessel during an accident, and cooling of the reactor vessel. Nuclear reactors are divided according to installation positions of major components (steam generator, pressurizer, pump, etc.) into loop type reactors (e.g., commercial reactors: domestic) in which such major components are installed outside a reactor vessel and integral reactors (e.g., SMART reactors: domestic) in which the major components are installed inside a reactor vessel. Nuclear power plants are also divided into active plants and passive plants depending on a way of implementing a safety system. An active plant is a plant using an active component such as a pump operated by electric power of an emergency diesel generator (EDG) or the like to drive a safety system, and a passive plant is a plant using a passive component operated by passive power such as gravity, gas pressure or the like to drive the safety system. A passive safety system in a passive plant may maintain the reactor in a safe manner only with a natural force built in the system without an operator action or an AC power source of safety class such as an emergency diesel generator for more than a period of time (72 hours) required by regulatory requirements in the event of an accident, After 72 hours, using an operator action and a non-safety systems might be allowed to maintain the function of the safety systems and an emergency DC power source(battery). Unlike a general thermal power plant where heat generation is stopped when fuel supply is stopped, a reactor in a nuclear power plant generates residual heat from a reactor core for a significant period of time by a fission product produced and accumulated during a normal operation even when a fission reaction is stopped in the reactor core. Accordingly, a variety of safety systems for removing the residual heat of the core during an accident are installed in the nuclear power plant. In case of an active nuclear power plant (Conventional Nuclear Power Plant of Korea), a plurality of emergency diesel generators are provided in preparation for a case of interruption of electric power supply from the inside or outside at the time of an accident, and most active nuclear power plants use a pump to circulate cooling water, and thus a large-capacity emergency AC power source (a diesel generator) is provided due to the high power requirements of those active components. An operator action allowance time in an active nuclear power plant is estimated about 30 minutes. In order to exclude active components such as a pump and the like that require a large amount of electricity, a passive force such as gas pressure or gravity is introduced in a passive nuclear reactor (U.S. Westinghouse AP1000, Korean SMART) that has been developed or is being developed to enhance the safety of the nuclear power plant, and thus a large amount of power is not required other than small components such as a valve, which is essentially required for the operation of a passive safety system. However, to enhance the safety in a passive nuclear power plant, an operator action allowance time is drastically extended from 30 minutes to 72 hours or longer, and an emergency active power source (diesel generator) is excluded, and an emergency DC power source (battery) is adopted. And thus the emergency DC power source should be maintained for more than 72 hours. Therefore, the emergency power source capacity required per unit time in a passive nuclear power plant is relatively small compared to an active nuclear power plant, but it is very large in terms of the battery capacity because the emergency power should be supplied for 72 hours or more. In the other hand, a residual heat removal system (auxiliary feed water system or passive residual heat removal system) is employed as a system for removing the heat of a reactor coolant system (the sensible heat of the reactor coolant system and the residual heat of the core) using a residual heat removal heat exchanger connected to a primary system or secondary system when an accident occurs in various nuclear power plants including an integral reactor. (AP1000: U.S. Westinghouse, commercial loop type nuclear power plant and SMART reactor: Korea) Furthermore, a safety injection system is employed as a system for directly injecting cooling water into the reactor coolant system in case of a loss-of-coolant accident to maintain a water level of the reactor core and removing the heat of the reactor coolant system (the sensible heat of the reactor coolant system and the residual heat of the core) using the injected cooling water. (AP1000: U.S. Westinghouse, commercial loop type and SMART reactor: Korea) In addition, a reactor containment cooling system or spray system is a system for condensing steam using cooling or spraying to suppress a pressure rise when a pressure inside the reactor containment rises due to an accident such as a loss-of-coolant accident or a steam-line-break accident. Additionally, there are a method of directly spraying cooling water into the reactor containment (commercial loop type reactor: Korea), a method of inducing steam discharged in the reactor containment into a suppression tank (commercial boiling water reactor), a method of using a heat exchanger installed inside or outside the reactor containment (reinforced concrete containment building) (APR+: Korea), a method of using a surface of the steel containment vessel as a heat exchanger (AP1000: U.S. Westinghouse), or the like. As such, the nuclear power plant is provided with various safety systems, each of which consists of a plurality of trains of two or more trains, such as the residual heat removal system and the safety injection system for protecting the reactor core by cooling the reactor coolant system (including the reactor vessel) during an accident. However, in recent years, there has been a growing demand for safety enhancement of nuclear power plants due to the impact of Fukushima nuclear power plant (boiling water reactor) accident and the like, and thus there is a rising demand for safety facilities against a severe accident such as an external reactor vessel cooling system even in a pressurized water reactor (PWR) with a very low risk of leakage of large amounts of radioactive materials due to employing a very large-internal-volume nuclear reactor containment. In detail, the nuclear power plant is provided with various safety facilities for relieving accidents upon an occurrence of an accident. In addition, each safety facility consists of redundant trains, and probability that all trains fail simultaneously is very low. However, as public demands for safety of nuclear power plants increase, safety facilities have been enhanced in preparation of severe accidents even with very low probability of occurrence. The external reactor vessel cooling system is a system provided to cool the outside of reactor vessel during core meltdown to prevent damage of the reactor vessel, assuming that a serious damage occurs in the core cooling function and a severe accident that the core is melted occurs since various safety facilities do not adequately perform functions due to multiple failure causes at the time of an accident. (AP1000 Westinghouse of USA) When the reactor vessel is damaged, a large amount of radioactive materials may be discharged into a reactor building, and internal pressure of the reactor building may rise due to an large amount of steam generated by corium(melted core)-water reaction and gas generated by the corium-concrete reaction. The reactor building serves as a final barrier to prevent the radioactive material from being discharged into an external environment during an accident. If the reactor building is damaged due to an increase in internal pressure, a large amount of radioactive materials may be released to the external environment. Therefore, the external reactor vessel cooling system performs a very important function of suppressing radioactive materials from being discharged into the reactor containment and the increase of the internal pressure during a severe accident to prevent radioactive materials from being discharged into an external environment. The external reactor vessel cooling system which is adopted in many countries is a system in which cooling water is filled in the reactor cavity located at a lower part of the reactor vessel and the cooling water is introduced into the cooling flow path in a space between the thermal insulation material and the reactor vessel and then steam is discharged to an upper part of the cooling flow path. In addition, a method of injecting a liquid metal at the time of an accident to mitigate the critical heat flux phenomenon, a method of pressurized cooling water to induce single phase heat transfer, a method of modifying a surface of the external reactor vessel to increase the heat transfer efficiency, a method of forming a forced flow, and the like, may be taken into consideration. In the related art external reactor vessel cooling system, since a thermal insulation material has to perform an appropriate thermal insulation function during a normal operation of the nuclear power plant, a flow path is sealed such that inlet and outlet flow paths formed in the thermal insulating material must be properly opened in a timely manner at the time of an accident. Also, a time delay occurs to fill the reactor cavity, and the heat removal ability may be reduced due to a critical heat flux phenomenon or the like while evaporating cooling water to form a steam layer on the external reactor vessel. In addition, there is also a research on an external reactor cooling method using a liquid metal, but the liquid metal method has difficulties in the maintenance of the liquid metal. In addition, an external reactor cooling method in a pressurization manner has difficulties in the application of a natural circulation flow, and a method of modifying a surface of a reactor vessel has difficulties in the fabrication and maintenance of the surface, and a forced flow method has a disadvantage in that it must be supplied with electric power. In addition, since the external reactor vessel cooling system is operated by an operator action at the time of an accident, various instruments and components for monitoring the accident are required for the operation, and probability that a system in a standby mode fails to operate at the time of an accident is higher than probability that a system being operated is stopped to operate at the time of an accident. Thus, the present invention proposes an external reactor vessel cooling and electric power generation system, in which a large-scale turbine power generation facility in the related art is maintained almost same design, and a small-scale power generation facility is additionally installed to receive heat discharged from the reactor vessel during a normal operation or during an accident of the nuclear power plant and thus produce electricity. One aspect of the present invention is to provide an external reactor vessel cooling and electric power generation system having improved system reliability in which safety class or seismic design is easily applicable, and the reactor vessel cooling is carried out while continuously operating during an accident as well as during a normal operation to produce emergency power. Another aspect of the present invention is to provide an external reactor vessel cooling and electric power generation system having improved safety by removing residual heat of a predetermined scale or more during an accident as well as a normal operation. Another aspect of the present invention is to provide a nuclear power plant with improved economic efficiency and safety due to downsizing and reliability enhancement of an emergency power system. An external reactor vessel cooling and electric power generation system according to the present invention may include a reactor vessel, an external reactor vessel cooling section formed to enclose at least part of the reactor vessel so as to cool heat discharged from the reactor vessel, a power production section including a small turbine and a small generator to generate electric energy using a fluid that receives heat from the external reactor vessel cooling section, a condensation heat exchange section to perform a heat exchange of the fluid discharged after operating the small turbine, and condense the fluid to generate condensed water, and a condensed water storage section to collect therein the condensed water generated in the condensation heat exchange section, wherein the fluid receiving the heat from the reactor vessel may be circulated. In an embodiment, the condensed water in the condensed water storage section may be circulated through the external reactor vessel cooling section, the power production section, and the condensation heat exchange section, and the fluid may be phase-changed into gas by the heat received from the reactor vessel. In an embodiment, the system may further include an evaporation section connected to the external reactor vessel cooling section, to cause a heat exchange between the fluid inside the external reactor vessel cooling section and the condensed water of the condensed water storage section. The system may further include a first circulation part defined between the external reactor vessel cooling section and the evaporation section such that a fluid flows therealong, and a second circulation part defined sequentially along the evaporation section, the power production section, the condensation heat exchange section, and the condensed water storage section, such that a fluid flows therealong. In an embodiment, the first circulation part may be circulated by a single-phase fluid. In an embodiment, the power generation system may be operated during a normal operation of a nuclear power plant and during an accident of the nuclear power plant to produce electric power. The electric power generated during the normal operation of the nuclear power plant may be charged in an internal/external electric power system and an emergency battery. Also, the electric power charged in the emergency battery may be supplied as an emergency power source during the accident of the nuclear power plant. In an embodiment, the electric power generated during the accident of the nuclear power plant may be supplied as an emergency power source of the nuclear power plant. In an embodiment, the emergency power source may be used as power for operating a safety system of the nuclear power plant during the accident of the nuclear power plant, opening and closing a valve for the operation of the safety system, monitoring the safety system, or operating the external reactor vessel cooling and electric power generation system. In an embodiment, seismic design of seismic categories I, II or III may be applied, and safety classes 1, 2 or 3 may be applied. In an embodiment, the external reactor vessel cooling section may be provided with a discharge pipe that connects the external reactor vessel cooling section and the power production section to each other such that the fluid of the external reactor vessel cooling section is applied to the power production section. In an embodiment, the discharge pipe may be provided with a first discharge portion through which at least part of the fluid excessively supplied to the power production section bypasses the small turbine and the small generator. In an embodiment, the discharge pipe may further be provided with a liquid-gas separator that is connected to the discharge pipe such that only gas of the fluid is transferred to the power production section. In an embodiment, the condensation heat exchange section may be provided with a motor or pump that supplies a cooling fluid to the condensation heat exchange section to exchange heat with the fluid. The cooling fluid may include air, pure water, seawater, or a mixture thereof. In an embodiment, the condensed water storage section may be disposed below the condensation heat exchange section to collect the condensed water generated in the condensation heat exchange section. In an embodiment, the condensed water storage section may be connected to the external reactor vessel cooling section through a pipe so that the condensed water is supplied to the external reactor vessel cooling section. In an embodiment, the condensation heat exchange section or the condensed water storage section may be provided with an vent portion through which non-condensable gas accumulated in the condensation heat exchange section or in the condensed water storage section. In an embodiment, the vent portion may remove the non-condensable gas by pressure drop of the Venturi using a fan or a steam flow rate. In an embodiment, a shape of the external reactor vessel cooling section may include a cylindrical shape, a hemispherical shape, and a double vessel shape, or a combination thereof. In an embodiment, a pipe may be connected to an in-containment refueling water storage tank (IRWST) such that refueling water is supplied to the external reactor vessel cooling section. In an embodiment, the external reactor vessel cooling section may be provided with a second discharge portion through which the refueling water supplied from the IRWST is discharged. In an embodiment, a coating member may further be provided to prevent corrosion of the reactor vessel, and a surface of the coating member may be chemically processed to increase a surface area. In an embodiment, the system may further include a heat transfer member to smoothly transfer heat discharged from the reactor vessel and a surface of the heat transfer member may be chemically processed to increase a surface area. In an embodiment, the system may further include a core catcher provided inside the external reactor vessel cooling section to receive and cool corium when the reactor vessel is damaged. A loop or integral type nuclear power plant according to the present invention may include a reactor vessel, an external reactor vessel cooling section formed to enclose at least part of the reactor vessel so as to cool heat discharged from the reactor vessel, a power production section including a small turbine and a small generator to generate electric energy using a fluid that receives heat from the external reactor vessel cooling section, a condensation heat exchange section 140 to perform a heat exchange of the fluid discharged after operating the small turbine and condense the fluid to generate condensed water, and a condensed water storage section to collect therein the condensed water generated in the condensation heat exchange section, wherein the fluid receiving the heat from the reactor vessel may be circulated. The reactor external wall cooling and electric power generation system according to the present invention is configured to enclose the reactor vessel with small-scale facilities so as to produce electric power using heat transferred while cooling the reactor vessel. A phase of fluid is changed from liquid to gas by the transferred heat, and the power production section is driven using the gas. The external reactor vessel cooling section, the power production section, and the condensation heat exchange section of the present invention may continuously operate even during an accident as well as during a normal operation, to cool the reactor vessel and produce emergency power, thereby improving system reliability. The system may easily employ a safety class or seismic design using small scale facilities, which may result in improving reliability of the nuclear power plant including cooling of an external wall of the reactor vessel. The external reactor vessel cooling and electric power generation system according to the present invention can be designed to remove residual heat of a predetermined scale or more, which is generated in the reactor, by the external reactor vessel cooling section, and can continuously operate not only during a normal operation but also during an accident. This may result in lowering probability of operation failure during the accident, thereby improving safety of the nuclear power plant. The nuclear power plant according to the present invention can have improved economic efficiency by way of reducing a size of an emergency power system using the external reactor vessel cooling and electric power generation system. Description will now be given in detail according to exemplary embodiments disclosed herein, with reference to the accompanying drawings. For the sake of brief description with reference to the drawings, the same or equivalent components may be provided with the same or similar reference numbers, and description thereof will not be repeated. In describing the present invention, moreover, the detailed description will be omitted when a specific description for publicly known technologies to which the invention pertains is judged to obscure the gist of the present invention. The accompanying drawings are used to help easily understand the technical idea of the present disclosure and it should be understood that the idea of the present disclosure is not limited by the accompanying drawings. The idea of the present disclosure should be construed to extend to any alterations, equivalents and substitutes besides the accompanying drawings. It will be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are generally only used to distinguish one element from another. A singular representation may include a plural representation unless it represents a definitely different meaning from the context. Terms such as “include” or “has” are used herein and should be understood that they are intended to indicate an existence of several components, functions or steps, disclosed in the specification, and it is also understood that greater or fewer components, functions, or steps may likewise be utilized. FIG. 1A is a conceptual view of an external reactor vessel cooling and electric power generation system 100 according to an embodiment of the present invention. In the embodiment of the present invention, an insulating material 111 may be provided to surround a part of a reactor vessel 110, and a core 114 may be provided inside the reactor vessel 110. The core 114 refers to nuclear fuel. Electric power is produced by heat which is generated while nuclear fission is performed in the core 114. When an accident occurs in a nuclear power plant, residual heat may be generated for a considerable period of time even when the core 114 is stopped because a control rod is inserted in the core 114. If it is assumed that various safety and non-safety systems do not work at the time of an accident of a nuclear power plant, cooling water inside the reactor vessel 110 may be lost to increase the temperature of the nuclear fuel, thereby causing a core meltdown phenomenon. On the other hand, during a normal operation of the nuclear power plant, steam may be produced in the steam generator 113 by receiving heat from the reactor coolant system. The steam generator 113 may be a pressurized light water reactor. Further, the steam produced by the steam generator 113 may be steam that is phase-changed by receiving water through a main feedwater line 11 connected to a feedwater system 10 and an isolation valve 12. The steam produced by the steam generator 113 may be passed through a main steam line 14 connected to an isolation valve 13 and supplied to a large turbine 15 and a large generator (not shown) to produce electric power while fluid energy of the steam is converted into electric energy through mechanical energy. In addition, a reactor coolant pump 112 may circulate a coolant filled in the reactor vessel 110. A pressurizer 115 provided inside the reactor vessel 110 may control pressure of the reactor coolant system. In addition, a passive residual heat removal system including an emergency cooling water storage section 20 and a heat exchanger 21 may be provided therein, so that heat can also be discharged to the emergency cooling water storage section 20 by natural circulation due to a two-phase flow and opening and closing of a valve 24 during an accident. Further, when steam is generated while emergency cooling water is evaporated by heat transferred to the emergency cooling water storage section 20, the steam may be discharged through a steam discharge unit 25 such that the transferred heat can be discharged to the atmosphere. The external reactor vessel cooling and electric power generation system 100 is in an operating state even during a normal operation. And then the external reactor vessel cooling system continues to operate during an accident. Heat is continuously transferred to the reactor vessel 110 due to residual heat generated in the core 114 until the temperature of the reactor vessel 110 is remarkably reduced to reach a safe state of the reactor vessel 110 during an accident. Accordingly, an operator action for the operation of the external reactor vessel cooling system, various measuring instruments and control systems, valve operation or pump start and opening and closing of a thermal insulation material which are needed in the related art may not be required, and thus the probability of operation failure of the external reactor vessel cooling and electric power generation system 100 is greatly reduced to improve the safety of the nuclear power plant. In addition, since emergency electric power can be safely produced by the external reactor vessel cooling and electric power generation system 100 until the temperature of the reactor vessel is reduced to reach a safe state during an accident, the capacity of an emergency DC battery(emergency electric power source) can be decreased to improve the economic efficiency of the nuclear power plant and improve the reliability of an emergency power system of the nuclear power plant by securing an emergency power supply source of a safety system, thereby improving the safety of the nuclear power plant. In detail, in case of a passive nuclear power plant, emergency power required during an accident is less than about 0.05% compared to the power generation capacity generated from the nuclear power plant during a normal operation. However, it is designed to use a battery for 72 hours or more, and thus a very large battery is required, having a disadvantage of increasing the cost. However, the external reactor vessel cooling and electric power generation system 100 may be configured to produce an appropriate level of emergency power using residual heat continuously generated from the core (an amount of residual heat generated is several % (initial shutdown) to 1/several % (after 72 hours subsequent to shutdown) compared to a normal amount of thermal power) even after the reactor is shut down upon an occurrence of an accident. In addition, when power is produced using the external reactor vessel cooling and electric power generation system 100, the power production amount is about several tens kWe to several MWe, and the capacity is less than 1/several % compared to the feedwater system 10 and the large turbine 15 for a normal operation of the nuclear power plant. This system 100 has almost no influence on the operation of the nuclear power plant, and therefore, even when this system 100 fails during a normal operation. That is to say, this system 100 has a capacity less than 1/several %, so it has little effect on a nuclear power plant operation. In addition, when power is produced using the external reactor vessel cooling and electric power generation system 100, it may be constructed in a small scale compared to the large-capacity feedwater system 10 and the large turbine 15 for producing normal power. Therefore, it is easy to apply seismic design and safety class, and cost increase is not so great due to small facilities even when the seismic design and safety class are applied. Even in the event of an accident, the external reactor vessel cooling and electric power generation system can continue to operate as a normal operation without any additional valve operation, and therefore, during an accident, operation failure of valves, pumps, and etc. for operating the related art external reactor vessel cooling and electric power generation system, and probability of operation failure or malfunction due to errors of measuring instruments and control signals may be significantly reduced. Further, when a severe accident occurs and the external reactor vessel cooling section 120 and a power(electric power) production section 130 may do not work during the occurrence of the severe accident, a flow path through an in-containment refueling water storage tank (hereinafter, referred to as IRWST) 180 and a second discharge portion 127 is already formed, and therefore, smooth supply and discharge of cooling water may be enabled by a simple operation such as opening/closing of valves according to an operator action, such that the cooling water can be supplied well for cooling the reactor vessel 110. Particularly, since an integral type reactor in which a connecting line for a measuring instrument or the like is not installed at a lower portion of the reactor vessel has a simple structure, it is facilitated to apply the external reactor vessel cooling and electric power generation system 100. In addition, the external reactor vessel cooling and electric power generation system 100 may be utilized as additional residual heat removal means that plays a role of removing residual heat of the reactor core 114 during an accident. Hereinafter, the external reactor vessel cooling and electric power generation system 100 according to the present invention will be described in detail. A reactor vessel 110, an external reactor vessel cooling section 120 and an IRWST 180 may be provided inside a reactor building boundary 1. An insulating material 111 may additionally be provided on an external wall of the reactor vessel 110. The external reactor vessel cooling section 120 may be formed to enclose at least part of the reactor vessel 110. In addition, the external reactor vessel cooling section 120 may be formed to receive heat discharged from the reactor vessel 110 and cool the external wall of the reactor vessel 110. On the other hand, a power production section 130, a condensation heat exchange section 140, and a condensed water storage section 150 are provided outside the reactor building boundary 1. The power production section 130 may supply power by being connected to motors 141, 151 and 157, an internal/external electric power system 171, a charger 172, an emergency power consuming device(emergency safety systems, main control room, and etc.) 174 and an emergency battery 173. However, some of those components illustrated as being installed outside the reactor building boundary 1 may alternatively be disposed inside the reactor building boundary 1 depending on arrangement characteristics of nuclear power plants. The reactor vessel 110 may be a pressure vessel that circulates a reactor coolant therein, is provided with the core 114 therein, and is designed to withstand high pressure. In addition, the external reactor vessel cooling section 120 may be formed to enclose the reactor vessel 110, and receive heat discharged from the reactor vessel 110 so as to cool the external wall of the reactor vessel 110 using a fluid which is subjected to phase change from liquid to gas. In detail, the external reactor vessel cooling section 120 may be formed in a cylindrical shape. However, the shape of the external reactor vessel cooling section 120 is not limited to the cylindrical shape, but may alternatively include a hemispherical shape, a dual-container shape, or a combined shape thereof. In addition, the external reactor vessel cooling section 120 may further include a coating member 121 for preventing corrosion thereof. In this embodiment, the coating member 121 may have a surface modified in various ways and may be processed into an uneven shape (cooling fins) to increase a heat transfer surface area. Further, the coating member 121 may further include a heat transfer member (not shown) whose surface is chemically processed to increase a surface area so as to improve heat transfer efficiency. That is, the surfaces of the coating member 121 and the heat transfer member may be chemically processed to increase the surface areas thereof, such that the heat transfer can be efficiently carried out. The external reactor vessel cooling section 120 may be provided with a discharge pipe 122. The discharge pipe 122 may be connected to the external reactor vessel cooling section 120 and the power production section 130, respectively, such that the fluid of the external reactor vessel cooling section 120 can be supplied to the power production section 130 therethrough. The discharge pipe 122 may be branched to the pipe 124 to be connected to the power production section 130 through a valve 123. The discharge pipe 122 may be provided with a first discharge portion 126 connected to a valve 125. The fluid which could be excessively supplied to the power production section 130 may be discharged through the first discharge portion 126. In detail, the first discharge portion 126 is a pipe through which a fluid (gas, steam) is discharged from the external reactor vessel cooling section 120 to the outside of the reactor building (not shown). Accordingly, a part of the fluid (gas, steam) can be discharged through the first discharge portion 126 when pressure of the system is increased or the fluid (liquid) is excessively supplied. The present invention has illustrated that the fluid is discharged to the outside of the reactor building (not shown) through the first discharge portion 126. However, the fluid to be discharged may alternatively be bypassed the power production section 130 and supplied and condensed in a condensation heat exchange section 140 to be reused according to characteristics of the nuclear power plants. Further, the external reactor vessel cooling section 120 may be connected to the IRWST 180 such that refueling water is supplied through the pipe 183. In detail, the IRWST 180 may be connected to a valve 181 and a check valve 182. In addition, the external reactor vessel cooling section 120 may be provided with a second discharge portion 127 connected to a valve 127′. The second discharge portion 127 may be formed to discharge the refueling water supplied from the IRWST 180 therethrough. In detail, the second discharge portion 127 is a pipe through which a fluid (gas/steam or liquid/high temperature water) is discharged from the external reactor vessel cooling section 120 to the reactor building (not shown). Accordingly, the second discharge portion 127 can allow the reactor vessel to be cooled even when the external reactor vessel cooling section 120 and the power production section 130 are unable to perform cooling and electric power generation due to their failure or the like, which is caused by multiple failures of safety system, and may worsen to a severe accident and the like. Meanwhile, the power production section 130 may be configured such that the fluid moving from the external reactor vessel cooling section 120 is injected therein. As the moved fluid actuates the small turbine 131, fluid energy of the fluid may be converted into mechanical energy (rotational force), and the mechanical energy may be converted into electric energy through the small generator 132 connected to the small turbine 131 with a shaft, thereby producing electric power. The small turbine 131 may generate electric power by receiving heat of a preset scale from the reactor vessel 110 in consideration of characteristics during a normal operation and during an accident. In this embodiment, the present invention may have a configuration that power is produced in a variable manner in consideration of a rate of change in a steam flow rate which is caused due to a variation of a heat generated in the core 114 supplied during an accident, and may adjust a load of the power production section 130. Also, the small turbine 131 of the power production section 130 may be a small-capacity turbine, which may make it easy to apply seismic design or safety class to be described below. The electric power that can be generated by the power production section 130 has a capacity of several tens of kWe to several MWe, which is less than 1% compared to the large-capacity feedwater system 10 and the large turbine 15 for producing normal power of the nuclear power plant, and even when the facility operates or fails during a normal operation of the nuclear power plant, there is little influence on the operation of the large capacity feedwater system 10 and the large turbine 15 for producing the normal nuclear power. That is, the large-capacity feedwater system 10 and the large turbine 15 for producing the normal power are one of the biggest large-scale facilities of the nuclear power plant, and applying the seismic design and safety class above the whole facilities is very uneconomical because it causes a huge cost increase. Meanwhile, in case of the external reactor vessel cooling and electric power generation system 100 in which the small turbine 131 and the small generator 132 are provided, the size of the system 100 is much smaller than that of the feedwater system 10 and the large turbine 15. Thus, it is easy to apply seismic design or safety class thereto, and the increased cost due to applying the seismic design or the safety class is not so great. The small turbine 131 and the small generator 132 can be continuously operated to supply emergency electric power even when it is difficult to supply electric power due to an occurrence of an earthquake in conventional plants since seismic design is applied to the external reactor vessel cooling and electric power generation system 100. Also, the small turbine 131 and the small generator 132 can be continuously operated to supply emergency power even when various accidents occur since safety class is applied to the external reactor vessel cooling and electric power generation system 100 to secure system reliability. Considering that electric power required in case of a passive nuclear power plant during an accident is several tens kWe although the emergency power has a difference according to characteristics of nuclear power plants, sufficient power can be supplied with only electric power produced by the small turbine 131 and the small generator 132. Besides, since the emergency DC battery capacity of the passive nuclear power plant is not greater than the emergency power required in an active nuclear power plant, the DC battery may be recharged by electric power produced by the small turbine 131 and the small generator 132. The external reactor vessel cooling and electric power generation system 100 may be configured to have seismic design of seismic category I, II or seismic category III which are specified by ASME (American Society of Mechanical Engineers). In detail, seismic category I is applied to structures, systems and components which are classified as safety items, and should be designed to maintain an inherent ‘safety function’ in case of a safe shutdown earthquake (SSE). Also, seismic category I is designed such that the safety function is maintained even under an operating basis earthquake (OBE) in synchronization with a normal operation load, and appropriate allowable stresses and changes are within limits. Though not requiring nuclear safety or continuous functions, seismic category II is applied to items whose structural damages or interaction may lower the safety functions of the structures, systems and components of the seismic category I or cause damage to an operator located within a main control room. In detail, the structures, systems and components belonging to the seismic category II are not required to have functional integrity for the SSE, but required only to have structural integrity. In addition, the structures, systems and components of the seismic category II should be designed and arranged so as not to impair safety-related operations of the items belonging to the seismic category I. Seismic category III is designed according to uniform building codes (UBCs) or general industrial standards depending on individual design functions. The external reactor vessel cooling and electric power generation system 100 may be configured to have a safety class of safety classes 1, 2 or 3 of the nuclear power plant specified by the American Society of Mechanical Engineers (ASME). In detail, the safety class of the nuclear power plant is typically divided into safety classes 1 to 3. Safety class 1 is a grade assigned to a RCS(reactor coolant system) pressure-boundary portion of a facility and its support that constitute a reactor coolant pressure boundary (a portion that may cause a loss of coolant beyond a normal make-up capacity of the reactor coolant in the event of a failure). Safety class 2 may be assigned to a pressure-boundary portion of the reactor containment building and its support, and assigned to a facility and its support that perform the following safety functions while not belonging to safety class 1. A function of preventing the release of fission products or containing or isolating radioactive materials in the containment building A function of removing heat or radioactive materials generated in the containment building in case of an emergency (e.g., containment building spray system), a function of increasing negative reactivity to make the reactor in a subcritical state in case of an emergency or suppressing an increase of positive reactivity (e.g., boric acid injection system) A function of supplying coolant directly to the core during an emergency to ensure core cooling (e.g., residual heat removal, emergency core cooling system) and a function of supplying or maintaining sufficient reactor coolant for cooling the reactor core during an emergency (e.g., refueling water storage tank) Safety class 3 is not included in safety classes 1 and 2, and may be assigned to a facility that performs one of the following safety functions: A function of controlling concentration of hydrogen in the reactor containment building within an allowable limit A function of removing radioactive materials from a space (e.g., main control room, nuclear fuel building) outside the reactor containment building with safety class 1, 2 or 3 facilities A function of increasing negative reactivity to make or maintain the reactor in a subcritical state (e.g., boric acid make-up) A function of supplying or maintaining sufficient reactor coolant for core cooling (e.g., Reactor coolant replenishment system) A function of maintaining a geometric structure inside the reactor to ensure core reactivity control or core cooling capability (e.g., core support structure) A function of supporting or protecting the load for safety class 1, 2 or 3 facilities (concrete steel structures not included in KEPIC-MN, ASME sec. III). A function of shielding for radiation for people outside the reactor control room or nuclear power plant A cooling function of spent fuel (e.g., spent fuel pool and cooling system) A function of ensuring safety functions performed by safety class 1, 2 or 3 facilities (e.g., a function of removing heat from safety class 1, 2 or 3 heat exchangers, a safety class 2 or 3 pump lubrication function, a fuel supplying function of emergency diesel generator) A function of supplying electric power or motive power to safety class 1, 2 or 3 facilities A function of providing informations or controlling informations for safety class 1, 2 or 3 facilities to perform the safety functions automatically or manually A function of supplying or receiving signals for safety class 1, 2 or 3 facilities to perform the safety functions Manual or automatic interlocking function for safety class 1, 2 or 3 facilities to perform the safety functions A function of providing appropriate environmental conditions for safety class 1, 2 or 3 facilities and an operator A function corresponding to safety class 2 to which standards for the design and manufacture of pressure vessels, KEPIC-MN, ASME Sec. III, are not applied The condensation heat exchange section 140 is configured to perform heat exchange with the fluid discharged from the power production section 130 including the small turbine 131 and the small generator 132 subsequent to producing electric energy, and shrink the fluid to generate condensed water. In detail, the condensation heat exchange section 140 includes a heat exchanger to condense the fluid so as to recover the fluid in the form of condensed water. The heat exchanger of the condensation heat exchange section 140 may be a shell-and-tube type heat exchanger or a plate type heat exchanger. However, the heat exchanger disclosed herein is not limited to this and may be any heat exchanger capable of condensing the fluid to generate condensed water. The condensing heat exchange section 140 is provided with a motor 141 or a pump (not shown), and the motor 141 or the pump may supply a cooling fluid to the condensation heat exchange section 140 so as to exchange heat with the fluid. The cooling fluid may be air, pure water, seawater or a mixture thereof. The motor 141 may provide rotational power to a fan 142 or to a pump. The fan 142 may be a cooling fan when an air-cooling type heat exchanger is applied, and the condensation heat exchange section 140 may be downsized using the cooling fan. In addition, a discharge fan (not shown) may selectively be provided in an vent portion 143. Accordingly, when non-condensable gas is accumulated in the condensation heat exchange section 140, the discharge fan may remove the non-condensable gas, thereby improving the heat exchange of the condensation heat exchange section 140 and lowering pressure thereof. In addition, there may be various methods of discharging gas, such as using pressure drop of the Venturi by a steam flow rate, and thus the present invention is not limited to a specific form. The motor 141 may be supplied with electric power produced by the power production section 130 itself through a connecting line 133. The fan 142 connected to the motor 141 may supply cooling air to the condensation heat exchange section 140 to efficiently perform the heat exchange in the condensation heat exchange section 140. In addition, the motor 141 may be provided to charge electric power produced by the power production section 130 to an emergency battery 173 and receive electric power from the emergency battery 173. A pipe 144 may be provided between the condensation heat exchange section 140 and the condensed water storage section 150 such that the fluid flows therealong. The condensed water generated in the condensation heat exchange section 140 is transferred to the condensed water storage section 150 along the pipe 144. In detail, the condensed water storage section 150 may be disposed below the condensation heat exchange section 140 to collect the condensed water generated in the condensation heat exchange section 140. In the embodiment of the present invention, the condensed water in the condensation heat exchange section 140 is transferred to the condensed water storage section 150 by gravity. However, depending on characteristics of nuclear power plants, a pump may be installed in the connected pipe between the condensation heat exchange section 140 and the condensed water storage section 150 so that the condensed water is forcibly transferred. In addition, a pipe 144′ for transferring steam or non-condensable gas, which may be accumulated in the condensation heat exchange section 140, to the condensed water storage section 150 may be additionally provided. The condensed water collected in the condensed water storage section 150 may circulate through the external reactor vessel cooling section 120, the power production section 130, and the condensation heat exchange section 140. Further, the condensed water storage section 150 may be connected to the external reactor vessel cooling section 120 through a pipe 160 such that the condensed water is supplied to the external reactor vessel cooling section 120. Specifically, the condensed water of the condensed water storage section 150 may be supplied into a pipe 160 connected to the external reactor vessel cooling section 120 by gravity through a valve 154 and a check valve 155 connected to the pipe 153. The condensed water of the condensed water storage section 150 may be passed through a valve 161 and a check valve 162 by a motor 157 and a small feedwater pump 158 connected to a pipe 156 to be supplied into the pipe 160 connected to the external reactor vessel cooling section 120. Similar to the condensation heat exchange section 140 described above, the condensed water storage section 150 may include a motor 151 and a fan 152, and the motor 151 is capable of providing rotation power to the fan 152. The fan 152 may discharge non-condensable gas to lower pressure of the condensed water storage section 150 when the non-condensable gas is accumulated in the condensed water storage section 150. However, various methods of discharging gas including the method of using the pressure drop of the Venturi by the steam flow rate may be applied. Therefore, the method of discharging the non-condensing gas is not limited to a specific form. In the present invention, the motor 151 and the fan 152 are provided above the condensed water storage section 150, but the motor 151 and the fan 152 may alternatively be provided in the condensation heat exchange section 140. The motor 151 described above may receive power produced by the small turbine 131 itself through the connecting line 134. In addition, the motor 151 may be provided to charge electric power produced by the power production section 130 to the emergency battery 173 and receive electric power from the emergency battery 173. The power system 170 may be configured to use the power produced during the normal operation of the nuclear power plant as power of the internal/external electric power system 171. In detail, the internal/external electric power system 171 may be a system for processing electricity supplied from an on-site large turbine generator, the power production section 130, an on-site diesel generator, and an external power grid. In addition, electric energy may be stored in the emergency battery 173 through the charger 172, which is a facility for storing alternating current (AC) electricity supplied from the on-site, the outside, or the power production section 130 or the like. The emergency battery 173 may be a battery provided in an on-site to supply emergency DC power used during an accident. Further, the electric energy stored in the emergency battery 173 may be supplied to the emergency power consuming device 174 and used as emergency power. The emergency power may be used as power for operating the nuclear power plant safety system, opening or closing a valve for the operation of the nuclear power plant safety system, or monitoring the nuclear safety system during an accident of the nuclear power plant. Moreover, the electric power produced by the power production section 130 during an accident of the nuclear power plant may also be supplied as the emergency power of the nuclear power plant. Moreover, when the heat exchange section 120 and the power production section 130 fail due to multiple failures, a flow path through the IRWST 180 and the first discharge portion 127 is already formed, and therefore, smooth supply and discharge of cooling water may be enabled by a simple operation such as opening/closing valves according to an operator action, such that the cooling water can be supplied effectively for cooling the reactor vessel 110. FIG. 1B is a conceptual view illustrating a normal operation of the external reactor vessel cooling and electric power generation system 100 according to the embodiment of the present invention. Referring to FIG. 1B, it is a conceptual view illustrating system arrangement and a flow of fluid during a normal operation of a nuclear power plant. During the normal operation of the nuclear power plant, main feedwater (water) is supplied from the feedwater system 10 to the steam generator 113, and heat received from the core 114 by the reactor coolant circulation is transferred to a secondary system through the steam generator 113 so as to increase a temperature of the main feedwater and produce steam. The steam of the main feedwater produced from the steam generator 113 is supplied to the large turbine 15 along the main steam line 14 to rotate the large turbine 150 and rotate the large generator (not shown) connected through a shaft so as to produce electric power. The electric power produced through the large generator may be supplied to an on-site or off-site electric power system. On the other hand, feedwater supplied from the small feedwater pump 158 to the external reactor vessel cooling section 120 through the pipe 160 receives heat while rising along the external wall of the reactor vessel 110, thereby generating steam. The steam is supplied to the power production section 130 including the small turbine 131 and the small generator 132 along the discharge pipe 122 disposed at an upper part of the external reactor vessel cooling section 120. Fluid energy of the steam is converted into mechanical energy while rotating the small turbine 131, and the mechanical energy is converted into electric energy by the small generator 132 connected to the small turbine 131 by the shaft, thereby producing electric power. Further, the electric power produced by the power production section 130 may be supplied as electric power of the internal/external electric power system 171 through the electric power system 170. In addition, electric energy may be stored in the emergency battery 173 through the charger 172, which is a facility for storing alternating current (AC) electricity supplied from the on-site, the outside, or the power production section 130 or the like. The emergency battery 173 may be a battery provided in the on-site to supply emergency DC power used during an accident. Further, the electric power may be supplied to the emergency power consuming device 174 and used as emergency power. FIG. 1C is a conceptual view illustrating a design basis accident (DBA) operation of an external reactor vessel cooling and electric power generation system according to an embodiment of the present invention. Referring to FIG. 1C, it is a conceptual view illustrating the operation of the external reactor vessel cooling and electric power generation system 100 including the small feedwater pump 158, the power production section 130 and the like, during a design basis accident (DBA) of a nuclear power plant. Specifically, when an accident occurs in a nuclear power plant due to various causes, safety systems, such as a passive residual heat removal system, a passive safety injection system and a passive containment cooling system, including the emergency cooling water storage section 20, which are installed in a plurality of trains, may be automatically operated in response to related signals. Further, steam generated by the operation of the safety system may be discharged from a steam discharge portion 25 of the emergency cooling water storage section 20. The operation of the safety system may remove residual heat generated in the reactor coolant system 111 and the core 114. In addition, safety injection water may be supplied to the reactor coolant system 111 to lower pressure and temperature of the reactor coolant system 111 and lower the temperature of the core 114. Also, a pressure increase inside the reactor containment (not shown) may be suppressed by the operation of the passive containment cooling system, so as to protect the reactor containment. On the other hand, while the isolation valves 12, 13 provided in the main feedwater line 11 and the main steam line 14 are closed, the operation of the large turbine 15 is stopped. However, even when the reactor core 114 is stopped, residual heat is generated in the core 114 for a considerable period of time, and a lot of sensible heat is present in the reactor coolant system 110 and the reactor vessel 110. As a result, the temperature of the reactor coolant system 111 does not decrease rapidly. Accordingly, even when an accident occurs, the external reactor vessel cooling section 120 and the power production section 130 may be operated in the same state as a normal operation. Therefore, the power production section 130 may cool the reactor vessel 110 while continuously producing electric power. As time elapses, the residual heat generated in the core 114 may decrease and the temperature of the reactor vessel 110 may decrease while the reactor vessel 110 is cooled by the safety system. In this case, the external reactor vessel cooling and electric power generation system 100 may be operated in substantially the same manner as the normal operation while reducing the amount of electric power generated by the power production section 130 due to the reduction in an amount of heat transferred. FIG. 1D is a conceptual view illustrating a design basis accident (DBA) operation of an external reactor vessel cooling and electric power generation system 100 according to an embodiment of the present invention. Referring to FIG. 1D, it is a conceptual view of a case where the operation of the small feedwater pump 158 is disabled following the design basis accident and operation of the external reactor vessel cooling and electric power generation system 100 through the path 153 is activated. As illustrated in the foregoing case of FIG. 1C, the safety systems, such as the passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to relative signals. Accordingly, the reactor coolant system may be cooled, the residual heat of the core 114 may be removed and safety injection water may be supplied to the reactor coolant system so as to lower pressure and temperature of the reactor coolant system and lower the temperature of the core 114. Also, a pressure increase inside the reactor containment (not shown) may be suppressed so as to protect the reactor containment. On the other hand, while the isolation valves 12, 13 provided in the main feedwater line 11 and the main steam line 14 are closed, the operation of the large turbine 15 is stopped. In detail, when feedwater supplied from the small feedwater pump is interrupted for various reasons, the pipe 153 connected to the condensed water storage section 150 may be opened in response to a related signal or by an operator action to supply feedwater from the condensed water storage section 150. At this time, the feedwater may be supplied by natural circulation by gravity. The gravity may be applied to the condensed water in the condensed water storage section 150 so that the condensed water can be supplied in the natural circulation manner. Accordingly, the operation state of the external reactor vessel cooling section 120 and the power production section 130 may be similar to that during a normal operation except for the feedwater pump 158. As time elapses, when a steam generation amount is reduced due to a gradual reduction of the residual heat of the core 114, the operation state may become similar to that during the normal operation while adjusting the power production amount of the power production section 130. FIG. 1E is a conceptual view illustrating a severe accident operation of an external reactor vessel cooling and electric power generation system 100 in accordance with an embodiment of the present invention. Referring to FIG. 1E, it is a conceptual view in which the operation of the external reactor vessel cooling and electric power generation system 100 is disabled due to multiple failures including the failures of the system 100, and severe accidents(core melting) occur. As illustrated in the foregoing case of FIG. 1C, the safety systems, such as a passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to relative signals. However, when it is assumed that various safety systems do not operate although it is rarely likely to occur, an accident in which the core is melted down due to a temperature rise of the core may occur. For example, in order to block the discharge of radioactive materials to the outside of the reactor containment during a severe accident such as a generation of corium of nuclear accidents, the operation of the external reactor vessel cooling section 120 and the power production section 130 may be stopped. Accordingly, the pipe 183 connected to the IRWST 180 may be opened in response to a related signal or by an operator action to supply feedwater from the IRWST 180 so as to cool the lower part of the reactor vessel 110, and the valve 127′ installed on the second discharge portion 127 may be opened to discharge generated steam. Depending on characteristics of nuclear power plants, the feedwater may be forcibly injected by installing a pump (not shown) in the pipe 183 connected to the IRWST 180 or may be injected using gravity. Furthermore, even when a severe accident such as damage to the reactor vessel or exposure of the reactor core 114 occurs of nuclear accidents, in addition to the generation of the corium 114′ in the reactor, the operation of the external reactor vessel cooling section 120 and the power production section 130 may be stopped to allow the injection of feedwater through the IRWST 180 and the opening of the valve 127′ connected to the first discharge portion 127 in terms of protection. Furthermore, according to another embodiment described below, the same or similar reference numerals are designated to the same or similar configurations to the foregoing example, and the description thereof will be substituted by the earlier description. FIG. 2A is a conceptual view of an external reactor vessel cooling and electric power generation system in accordance with another embodiment of the present invention. An evaporation section 290 connected to an external reactor vessel cooling section 220 may further be provided. The evaporation section 290 may be configured to perform heat exchange between a fluid inside the external reactor vessel cooling section 220 and condensed water of a condensed water storage section 250. In detail, a first circulation part may be formed from the external reactor vessel cooling section 220 to the evaporation section 290 such that a fluid flow therealong. On the other hand, a second circulation part may be formed sequentially along the evaporation section 290, a power production section 230, a condensation heat exchange section 240, and the condensed water storage section 250 such that the fluid flows therealong. That is, the first circulation part and the second circulation part may have a dual circulation loop. The evaporation section 290 may be formed to be a boundary between the first circulation part and the second circulation part. The first circulation part may be configured such that a single-phase fluid flows therealong. In detail, the single-phase fluid of the first circulation part may be compressed gas. In addition, a compressor 293 and a blower (not shown) may be provided to allow the circulation of the single-phase fluid of the first circulation part, and activate the external reactor vessel cooling section 220 to transfer heat to the evaporation section 290. Further, the external reactor vessel cooling section 220 may be formed in a hemispherical shape and may also cool an external wall of a reactor vessel 210 without a coating member or a heat transfer enhancement member. FIG. 2B is a conceptual view illustrating a normal operation of the external reactor vessel cooling and electric power generation system according to the another embodiment of the present invention. Referring to FIG. 2B, it is a conceptual view illustrating system arrangement and a flow of fluid during a normal operation of a nuclear power plant. During a normal operation of a nuclear power plant, main feedwater (water) is supplied from the feedwater system 10 to a steam generator 213, and heat received from a core 214 by a reactor coolant circulation is transferred to a secondary system through the steam generator 213 to increase a temperature of the main feedwater and produce steam. The steam produced from the steam generator 213 is supplied to the large turbine 15 along the main steam line 14 to rotate the large turbine 150 and rotate the large generator (not shown) connected through the shaft so as to produce electric power. The electric power produced through the large generator may be supplied to an on-site or off-site from the power system. The single-phase fluid inside the external reactor vessel cooling section 220 provided along the external wall of the reactor vessel 210 is moved to the evaporation section 290 by receiving heat from the external wall of the reactor vessel 210. The single-phase fluid moved to the evaporation section 290 may exchange heat with a fluid, which is to be supplied to the power production section 230 including a small turbine 231 and a small generator 232, and circulate the first circulation part formed between the external reactor vessel cooling section 220 and the evaporation section 290. Further, a compressor 293 and a blower (not shown) connected to a motor 296 may be configured to allow the circulation of the single-phase fluid of the first circulation part. On the other hand, feedwater supplied from a small feedwater pump 258 to the evaporation section 290 through a pipe 260′ is supplied to the power production section 230 including a small turbine 231 and a small generator 232 while circulating the second circulation part. Fluid energy of the steam may be converted into mechanical energy while circulating the small turbine 231 and the mechanical energy may be converted into electric energy while rotating the small generator 232 connected through the shaft, thereby producing electric power. Further, the electric power produced by the power production section 230 may be utilized by the power system 270. FIG. 2C is a conceptual view illustrating a design basis accident operation of the external reactor vessel cooling and electric power generation system 200 according to the another embodiment of the present invention. Referring to FIG. 2C, it is a conceptual view illustrating an operation of the external reactor vessel cooling and electric power generation system 200 in case where the small feedwater pump 258, the power production section 230, and the like can be operated during a design basis accident of a nuclear power plant. Specifically, when an accident occurs in a nuclear power plant due to various causes, the safety systems, such as the passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to related signals. Further, steam generated by the operation of the safety system may be discharged from the steam discharge portion 25 of the emergency cooling water storage section 20. The operation of the safety system may remove residual heat generated in the reactor coolant system and the core 214. In addition, safety injection water may be supplied to the reactor coolant system to lower pressure and temperature of the reactor coolant system and lower temperature of the core 214. Also, a pressure increase in the reactor containment (not shown) may be suppressed by the operation of the passive containment cooling system so as to protect the reactor containment. On the other hand, while the isolation valves 12, 13 provided in the main feedwater line 11 and the main steam line 14 are closed, the operation of the large turbine 15 is stopped. However, since the temperature of the reactor vessel 210 is similar in the early stage of the accident, the external reactor vessel cooling section 220 and the power production section 230 connected to the evaporation section 290, respectively, may be operated in the same state as a normal operation. As time elapses, when the temperature of the reactor vessel 210 decreases in response residual heat generated in the core 214 being reduced and the reactor vessel 210 being cooled by the safety system, the power production section 230 may be operated similar to the normal operation while adjusting a power production amount according to an amount of heat transferred thereto. FIG. 2D is a conceptual view illustrating a design basis accident (DBA) operation of the external reactor vessel cooling and electric power generation system 200 according to the another embodiment of the present invention. Referring to FIG. 2D, it is a conceptual view illustrating the operation of the external reactor vessel cooling and electric power generation system 200 when the operation of the small feedwater pump 258 is disabled during a design basis accident of a nuclear power plant. As illustrated in the foregoing case of FIG. 2C, the safety systems, such as the passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to relative signals. Accordingly, the reactor coolant system may be cooled, the residual heat of the core 214 may be removed and safety injection water may be supplied to the reactor coolant system so as to lower pressure and temperature of the reactor coolant system and lower the temperature of the core 214. Also, a pressure increase in the reactor containment (not shown) may be suppressed so as to protect the reactor containment. On the other hand, while the isolation valves 12, 13 provided in the main feedwater line 11 and the main steam line 14 are closed, the operation of the large turbine 15 is stopped. In detail, when feedwater supplied from the small water supply pump is interrupted for various reasons, the pipe 253 connected to the condensed water storage section 250 may be opened in response to a related signal or by an operator action to supply water from the condensed water storage section 250. At this time, the feedwater may be supplied by natural circulation by gravity. The gravity may be applied to the condensed water in the condensed water storage section 250 so that the condensed water can be supplied in the natural circulation manner. Accordingly, the reactor vessel external wall cooling unit 220 and the power production section 230 can be operated in an operating state similar to that during normal operation. However, when the residual heat of the core 214 gradually decreases and the heat transferred to the evaporator 290 decreases, the power production section 230 may be operated in a similar manner to the normal operation while adjusting the power production amount. FIG. 2E is a conceptual view illustrating a severe accident operation of the external reactor vessel cooling and electric power generation system 200 in accordance with the another embodiment of the present invention. Referring to FIG. 2E, it is a conceptual view illustrating the operation of the external reactor vessel cooling and electric power generation system 200 when the power generation system 200 is shut down during a severe accident of a nuclear power plant. As illustrated in the foregoing case of FIG. 2C, the safety systems, such as the passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to relative signals. For example, when a severe accident such as a generation of corium 214′ occurs of nuclear accidents, the external reactor vessel cooling section 220 and the power production section 230, which are respectively connected to the evaporation section 290, may be interrupted. Accordingly, the pipe 283 connected to the IRWST 280 may be opened in response to a related signal or by an operator action to supply feedwater from the IRWST 280 so as to cool the lower part of the reactor vessel 210, and the valve 226 installed on the second discharge portion 227 may be opened to discharge generated steam. Depending on characteristics of nuclear power plants, the feedwater may be forcibly injected by installing a pump (not shown) in the pipe 283 connected to the IRWST 280 or may be injected using gravity. Furthermore, even in case where a severe accident such as damage to the reactor vessel or exposure of the reactor core 214 has occurred of nuclear accidents, in addition to the generation of the corium 214′ in the reactor, when the operation of the external reactor vessel cooling section 220 and the power production section 230 is stopped, the injection of feedwater through the IRWST 280 and the opening of the valve 226 connected to the second discharge portion 127 may be enabled in terms of protection. FIGS. 3A through 3E are conceptual views of an external reactor vessel cooling and electric power generation system in accordance with another embodiment of the present invention. Referring to FIG. 3A, an external reactor vessel cooling section 320a of an external reactor vessel cooling and electric power generation system 300a may have a hemispherical shape. In addition, the external reactor vessel cooling section 320a may further include a coating member 321a for preventing corrosion thereof. In this embodiment, the coating member 321a may have a surface modified in various ways and may be processed into an uneven shape (cooling fins) to increase a heat transfer surface area. Further, the coating member 321 may further include a heat transfer member (not shown) whose surface is chemically processed to increase a surface area so as to improve heat transfer efficiency. That is, the surfaces of the coating member 321 and the heat transfer member may be chemically processed to increase the surface areas thereof, such that the heat transfer can be efficiently carried out. Referring to FIG. 3B, an external reactor vessel cooling section 320b of an external reactor vessel cooling and electric power generation system 300b may have a mixed shape of a hemispherical shape and a cylindrical shape. Referring to FIG. 3C, an external reactor vessel cooling and electric power generation system 300c may further include a core catcher 328 provided inside an external reactor vessel cooling section 320c. The core catcher 328 may receive and cool corium when a reactor vessel 310 is damaged. Further, the external reactor vessel cooling section 320c may be connected to an IRWST 380′ through a pipe 383′ so that nuclear refueling water is supplied. In detail, the IRWST 380′ may be connected to a valve 381′ and a check valve 382′. In addition, the external reactor vessel cooling section 320c may be further provided with a coating member 321c for preventing corrosion thereof. Referring to FIG. 3D, an external reactor vessel cooling section 320d in an external reactor vessel cooling and electric power generation system 300d may have a shape of a dual vessel such that a cooling vessel (container) covers the entire reactor vessel. The external reactor vessel cooling and electric power generation system 300d may further include an evaporation section 390 connected to the external reactor vessel cooling section 320d, similar to the external reactor vessel cooling section 220 of FIG. 2A. The evaporation section 390 may be configured such that a fluid inside the external reactor vessel cooling section 320d exchanges heat with condensed water of a condensed water storage section 350. That is, the external reactor vessel cooling and electric power generation system 300d may be formed to have a dual circulation loop of a first circulation part and a second circulation part. Referring to FIG. 3E, an external reactor vessel cooling and electric power generation system 300e may further include a liquid-gas(water-steam) separator 329 connected to a discharge pipe 322. The liquid-gas separator 329 may be configured to transfer only gas in a fluid circulating in the external reactor vessel cooling section 320e to a power production section 330. Further, the system 300e may further include a cooling water recovery pipe 359 and a pump 339 by which a liquid separated from the liquid-gas separator 329 is transferred into a condensed water storage section 350. Although those various embodiments of the external reactor vessel cooling and electric power generation system of the present invention have been described above, the present invention may not be limited to the external reactor vessel cooling and electric power generation system and may include a loop or integral type nuclear power plant. In detail, the loop or integral type nuclear power plant according to the present invention may include a reactor vessel, and an external reactor vessel cooling section that is formed to enclose the reactor vessel to cool an external wall of the reactor vessel using a fluid, which is phase-changed from liquid to gas by receiving heat discharged from the reactor vessel. The nuclear power plant may also include a power production section provided with a small turbine that allows the fluid to move in the external reactor vessel cooling section and generates electric energy using the moved fluid. Further, the nuclear power plant may further include a condensation heat exchange section that causes a heat exchange of the fluid, which is discharged from the small turbine after the generation of the electric energy, and condenses the fluid to generate condensed water. The nuclear power plant may further include a condensed water storage section that collects the condensed water generated in the condensation heat exchange section and allows the condensed water of the condensed water storage section to circulate sequentially through the external reactor vessel cooling section, the power production section and the condensation heat exchange section. It is obvious to those skilled in the art that the present disclosure can be embodied in other specific forms without departing from the concept and essential characteristics thereof. The above detailed description should not be limitedly construed and should be considered illustrative in all aspects. The scope of the present invention should be determined by rational interpretation of the appended claims, and all changes within the scope of equivalents of the present invention are included in the scope of the present invention.
	claims	1. An x-ray tube, comprising: (a) a vacuum enclosure having an electron source and a target anode disposed therein, said target anode having a target surface positioned to receive electrons emitted by said electron source; and (b) a window mounted in said vacuum enclosure proximate to at least said target anode so that at least some x-rays produced as a result of said electrons striking said target surface pass through said window, and said window having a plurality of extended surfaces so that a coolant contacting said plurality of extended surfaces absorbs at least some heat dissipated by at least said window. 2. The x-ray tube as recited in claim 1 , wherein said vacuum enclosure comprises a plurality of extended surfaces. claim 1 3. The x-ray tube as recited in claim 1 , further comprising a cooling plenum joined to said vacuum enclosure and having mounted therein a compensating window, said compensating window cooperating with said extended surfaces of said window to define at least one flow passage capable of admitting at least a portion of said coolant so that said coolant absorbs at least some heat dissipated by at least said window. claim 1 4. The x-ray tube as recited in claim 1 , further comprising a cooling plenum joined to said vacuum enclosure and having mounted therein a compensating window disposed substantially proximate to said window and having an x-ray absorption coefficient value substantially equal to an x-ray absorption coefficient value of said window so that x-rays emitted from said compensating window are of substantially uniform intensity. claim 1 5. The x-ray tube as recited in claim 1 , wherein said plurality of extended surfaces comprise a plurality of fins. claim 1 6. The x-ray tube as recited in claim 1 , wherein said plurality of extended surfaces are disposed in a plane substantially parallel to a computerized tomography slice produced by the x-ray tube. claim 1 7. The x-ray tube as recited in claim 1 , wherein said vacuum enclosure substantially comprises copper. claim 1 8. A cooling system for use in conjunction with an x-ray tube and an x-ray tube vacuum enclosure, comprising: (a) a reservoir containing coolant in which the vacuum enclosure of the x-ray tube is at least partially immersed; (b) an external cooling unit continuously circulating said coolant through said reservoir; and (b) a window mounted in said vacuum enclosure, said window having a plurality of extended surfaces in contact with said coolant so that said coolant absorbs at least some heat dissipated by at least said window. 9. The cooling system as recited in claim 8 , further comprising a cooling plenum joined to said vacuum enclosure and having mounted therein a compensating window, said compensating window cooperating with said extended surfaces of said window to define at least one fluid passageway capable of admitting at least a portion of said coolant so that said coolant absorbs at least some heat dissipated by at least said window. claim 8 10. The cooling system as recited in claim 9 , wherein at least a portion of coolant exiting said external cooling unit is diverted initially to said fluid passageway. claim 9 11. The cooling system as recited in claim 9 , further comprising a plurality of extended surfaces disposed on said compensating window, said plurality of extended surfaces disposed on said compensating window cooperating with said extended surfaces of said window to define said fluid passageway. claim 9 12. The cooling system as recited in claim 9 , further comprising a fluid passageway defined by said cooling plenum, said fluid passageway defined by said cooling plenum being in fluid communication with said fluid passageway cooperatively defined by said compensating window and said extended surfaces of said window. claim 9 13. The cooling system as recited in claim 9 , wherein said coolant comprises a dielectric oil. claim 9 14. A method for cooling an x-ray tube, comprising the steps of: (a) providing a plurality of extended surfaces in a window of the x-ray tube; (b) placing a coolant in contact with said plurality of extended surfaces so that said coolant absorbs at least some heat dissipated by said window; (c) removing at least some heat from said coolant; and (d) repeating steps (b) and (c). 15. The method according to claim 14 , wherein said step of placing said coolant in contact with said extended surfaces comprises passing a flow of said coolant into contact with said extended surfaces. claim 14 16. The method according to claim 15 , further comprising the step of defining a fluid passageway substantially proximate to said extended surfaces of said window, said fluid passageway directing said flow of said coolant into contact with said extended surfaces. claim 15 17. The method according to claim 16 , wherein said step of removing at least some heat from said coolant occurs after said flow of coolant is discharged from said fluid passageway. claim 16 18. The method according to claim 16 , further comprising the step of placing at least a portion of said coolant discharged from said flow passageway in contact with the x-ray tube so that said at least a portion of said coolant absorbs at least some heat dissipated by the x-ray tube. claim 16 19. A window suitable for use in an x-ray device that includes an x-ray tube in contact with a coolant, the x-ray tube comprising a vacuum enclosure having an electron source and target anode disposed therein, and the target anode having a target surface positioned to receive electrons from the electron source so as to produce x-rays, wherein the window is configured to be mounted in the vacuum enclosure proximate to the target surface so that at least some of the x-rays pass through the window, the window comprising: (a) a body; and (b) a plurality of extended surfaces attached to said body, at least one of which is arranged for contact with the coolant. 20. The window as recited in claim 19 , wherein said plurality of extended surfaces are integral with said body. claim 19 21. The window as recited in claim 19 , wherein said plurality of extended surfaces comprise a plurality of fins, a slot being interposed between succeeding fins. claim 19 22. The window as recited in claim 21 , wherein a sum comprising a width of two fins added to a width of two slots is less than a minimum resolving power of the x-ray device. claim 21 23. The window as recited in claim 21 , wherein a width of each of said plurality of slots is substantially equal to a width of each of said plurality of said fins. claim 21 24. The window as recited in claim 21 , wherein each of said plurality of fins and said plurality of slots has a width greater than a minimum resolving power of the x-ray device. claim 21 25. The window as recited in claim 19 , wherein said window comprises beryllium. claim 19 26. The window as recited in claim 19 , further comprising an attenuator disposed at an end of each extended surface so that x-rays emitted through said window are of substantially uniform intensity. claim 19 27. The window as recited in claim 26 , wherein said attenuators comprise copper and said window comprises beryllium. claim 26 28. The window as recited in claim 19 , further comprising an attenuator disposed in a bottom of each slot defined between adjacent extended surfaces so that x-rays emitted through said window are of substantially uniform intensity. claim 19 29. The window as recited in claim 19 , wherein said extended surfaces are disposed in a plane substantially parallel to a computerized tomography slice produced by the x-ray device. claim 19 30. The window as recited in claim 19 , further comprising a clear area through which a useful beam portion of x-rays produced by the x-ray device pass. claim 19 31. The x-ray tube as recited in claim 1 , wherein said window substantially comprises a material selected from the group consisting of: beryllium, titanium, nickel, carbon, silicon, and aluminum. claim 1 32. The x-ray tube as recited in claim 1 , wherein at least one of said plurality of extended surfaces includes first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 1 33. The x-ray device as recited in claim 1 , further comprising a cooling plenum joined to said vacuum enclosure and including a compensating window, said compensating window having a plurality of extended surfaces, at least one of which is at least partially received within a corresponding recess defined by said plurality of extended surfaces of said window. claim 1 34. The window as recited in claim 19 , wherein said window comprises at least first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 19 35. The window as recited in claim 34 , wherein said first portion comprises a bottom of a slot cooperatively defined by a pair of adjacent extended surfaces, and said second portion comprises a part of at least one of said pair of adjacent extended surfaces. claim 34 36. The window as recited in claim 19 , wherein said window substantially comprises a material selected from the group consisting of: beryllium, titanium, nickel, carbon, silicon, and aluminum. claim 19 37. The window as recited in claim 19 , wherein at least one of said plurality of extended surfaces includes first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 19 38. An x-ray device, comprising: (a) a reservoir containing a volume of coolant; (b) a vacuum enclosure at least partially immersed in said volume of coolant, said vacuum enclosure having an electron source and a target anode disposed therein, said target anode including a target surface positioned to receive electrons emitted by said electron source; and (c) a window mounted in said vacuum enclosure proximate to said target anode, said window including a plurality of extended surfaces, at least one of which is in contact with said coolant. 39. The x-ray device as recited in claim 38 , wherein said vacuum enclosure includes a plurality of extended surfaces. claim 38 40. The x-ray device as recited in claim 39 , wherein at least one of said plurality of extended surfaces of said vacuum enclosure is in contact with said coolant. claim 39 41. The x-ray device as recited in claim 38 , further comprising a cooling plenum joined to said vacuum enclosure and including a compensating window, said compensating window cooperating with said extended surfaces of said window to define at least one flow passage in fluid communication with said reservoir. claim 38 42. The x-ray device as recited in claim 38 , further comprising a cooling plenum joined to said vacuum enclosure and including a compensating window disposed proximate to said window, and said compensating window having an x-ray absorption coefficient value substantially equal to an x-ray absorption coefficient value of said window. claim 38 43. The x-ray device as recited in claim 38 , further comprising a cooling plenum joined to said vacuum enclosure and including a compensating window, said compensating window including a plurality of extended surfaces, at least one of which is at least partially received within a corresponding recess defined by said plurality of extended surfaces of said window. claim 38 44. The x-ray device as recited in claim 38 , wherein said window includes at least first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 38 45. The x-ray device as recited in claim 38 , wherein at least one of said plurality of extended surfaces includes first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 38 46. A cooling system suitable for use in conjunction with an x-ray device that includes an x-ray tube vacuum enclosure, the cooling system comprising: (a) a reservoir containing a volume of coolant so that when the vacuum enclosure is received in said reservoir, the vacuum enclosure is at least partially immersed in said coolant; (b) an external cooling unit in fluid communication with said reservoir; and (c) a window mounted in said vacuum enclosure, said window having a plurality of extended surfaces, at least one of which is in contact with said coolant when the vacuum enclosure is received in said reservoir. 47. The cooling system as recited in claim 46 , further comprising a cooling plenum joined to said vacuum enclosure and cooperating with said window to define at least one fluid passageway configured for fluid communication with said external cooling unit. claim 46 48. The cooling system as recited in claim 47 , wherein at least a portion of said cooling plenum is in contact with said coolant contained in said reservoir when the vacuum enclosure is received in said reservoir. claim 47 49. The cooling system as recited in claim 47 , wherein said cooling plenum includes a compensating window disposed proximate said window and cooperating with said window to define said at least one fluid passageway. claim 47 50. The cooling system as recited in claim 49 , wherein said compensating window includes a plurality of extended surfaces, at least one of which is in contact with said coolant. claim 49 51. The cooling system as recited in claim 50 , wherein at least one of said plurality of extended surfaces of said compensating window is at least partially received within a corresponding recess defined by said plurality of extended surfaces of said window. claim 50
	abstract	One embodiment relates to a method of controllably reflecting electrons from an array of electron reflectors. An incident electron beam is formed from an electron source, and the incident beam is directed to the array of electron reflectors. A first plurality of the reflectors is configured to reflect electrons in a first reflective mode such that the reflected electrons exiting the reflector form a focused beam. A second plurality of the reflectors is configured to reflect electrons in a second reflective mode such that the reflected electrons exiting the reflector are defocused. Another embodiment relates to an apparatus of a dynamic pattern generator for reflection electron beam lithography. Other embodiments, aspects and features are also disclosed.
	description	1. Field of the Invention The present invention relates generally to an emergency core cooling system (ECCS) for a pressurized light water reactor, which directly injects emergency core cooling water, which is supplied from a high-pressure safety injection pump or a safety injection tank, into the downcomer of a reactor vessel, and, more particularly, to a downcomer injection extension duct technology for interrupting an emergency core cooling water direct bypass discharge phenomenon in which emergency core cooling water is guided by a strong cross-flow of the downcomer in the event of a large break loss of coolant accident (LBLOCA), and is then discharged out of the reactor. 2. Description of the Related Art A pressurized light water reactor can encounter unexpected safety problems even though it has been designed in consideration of a sufficient safety margin. If sufficient emergency core cooling water is not supplied when a safety problem in which a large quantity of cooling water leaks occurs, a core can overheat, resulting in damage to the reactor. In order to cool the core when the cooling water leaks, the pressurized light water reactor is equipped with a high-pressure safety injection pump and a safety injection tank such that the emergency core cooling water is exhausted externally. The supply of the emergency core cooling water is divided into two types according to the position of the injection nozzle end. Among the two types, one is a cold leg injection type, in which the injection nozzle is located at a cold leg, and the other is a direct vessel injection type, in which the injection nozzle is located at a reactor vessel. The cold leg injection type means that the emergency core cooling water is supplied to a reactor system through an injection line, which is connected to a cold leg corresponding to a pipe supplying cold water from the circulating pump of a reactor coolant circulatory system into the reactor vessel, and has a drawback in that, when the emergency core cooling water is supplied to a broken cold leg, the emergency core cooling water completely leaks out of the broken cold leg, and thus the reactor core cooling effect cannot be expected. As such, the direct vessel injection type is currently configured to include a direct vessel injection (DVI) nozzle, which supplies the emergency core cooling water to the reactor vessel, and to directly supply the emergency core cooling water to a downcomer between the reactor vessel and a core support barrel. However, the direct vessel injection type has a problem in that there is an increase in the emergency core cooling water direct bypass phenomenon, in which, when the cold leg is broken, the emergency core cooling water is headed to the broken cold leg by strong cross-flow of the downcomer, and is discharged out of the reactor vessel. As illustrated in FIGS. 1A through 1D, a conventional technology for preventing the emergency core cooling water direct bypass phenomenon is designed such that an injection extension duct 110 or 110′ is installed on the outer surface of a core barrel 100 of the downcomer 140 of FIG. 1A or on a baffle region in a core barrel 100 of FIG. 1B, and such that the injection extension duct 110 or 110′ is connected with the DVI nozzle 120 across the downcomer 140 using a pipe 130. Further, the conventional technology is designed such that an injection extension duct 210 is installed on the outer surface of a core barrel 200 of the downcomer 240 of FIG. 1C, and such that the injection extension duct 210 is connected with the DVI nozzle 220 across the downcomer 240 using projection nozzles 230 and 230′. As disclosed in U.S. Pat. Nos. 5,377,242 (James D. Carlton, et al.) and 5,135,708 (James D. Carlton, et al.), illustrated in FIGS. 1A and 1B respectively, the conventional method of connecting the DVI nozzle 120 with the injection extension duct 110 or 110′ in the downcomer 140 using the pipe 130 entails difficulty in the installation of the connecting portion because the gap in the downcomer 140 is narrow. When the reactor vessel is assembled with the core barrel, interference between the projections occurs. Further, according to this prior art, when a large cold leg 150 is broken, the emergency core cooling water can be effectively injected up to a lower portion or a core inlet of the downcomer 140. However, when a DVI line itself is broken, an outlet of the injection extension duct 110 or 110′ functions as an inlet of a break flow due to a siphon effect, and the level of the cooling water in the reactor vessel is lowered by the length of the injection extension duct 110 or 110′, so that the core cladding temperature is abnormally increased. This leads to a problem of noncompliance with safety regulations. As illustrated in FIG. 1C, another conventional technology similar to the aforementioned technology is adapted to directly connect the DVI nozzle 220 and the injection extension duct 210 in the downcomer 240 using a pipe, to position the protruding nozzles 230 and 230′ so as to be opposite to each other, and form a slight gap (Korean Patent Application Publication No. 10-2000-0074521). However, this conventional technology also has a problem in that, when the reactor vessel is assembled with the core barrel 200, interference between the nozzles 230 and 230′ protruding to the downcomer 240 occurs, thus making an assembly difficult, and thus a hole, which is used for a periodical withdrawal checkup of a neutron monitoring capsule installed at a lower portion of the reactor vessel, overlaps with the protruding nozzles, so that work becomes impossible. Further, when the DVI pipe line is broken, the gap between the upper connection nozzles of the injection extension duct 210 is narrow, and thus an inlet-outlet reverse phenomenon, in which the lowest outlet of the injection extension duct 210, located at the lowest position of the injection extension duct 210, functions as an inlet, is caused, although the quantity of intake of the break flow is not much. As such, there is a problem in that the level of the cooling water in the reactor vessel is significantly lowered down to the lowest outlet of the injection extension duct 210, and then the lowest outlet of the injection extension duct 210 functions as an inlet. There is a simpler technology in which an outlet of the DVI nozzle is vertically positioned at a right angle using an elbow 320 (Korean Patent Application Publication No. 10-2003-0064634). However, since the space occupied by the elbow 320 is similar to the gap in the downcomer 330, the reactor vessel 300 cannot be assembled with the core barrel 310. As a result of performing an emergency core cooling water bypass test, it was found that this simple vertical injection has little thermal hydraulic effect, because the direct bypass rate of the emergency core cooling water is very high (NED Vol. 225, “Effect of the yaw injection angle on the ECC bypass in comparison with the horizontal injection,” T. S. Kwon et al., 2003). According to the aforementioned conventional technology, the DVI line for the emergency core cooling water is broken, and thus the lowest outlet of the injection extension duct functions as an inlet for the break flow. In this case, the level of the cooling water in the reactor vessel is gradually lowered to reach a position equal to or lower than the lowest outlet of the injection extension duct, which is located at the lowest position of the injection extension duct. When the level of the cooling water is lowered, the reactor core is exposed. This has a lethal result when the reactor core is cooled. As described above, the conventional common technical problem is mostly attributable to a connection structure in which the DVI nozzle and the injection extension duct are connected to each other in the downcomer. Thus, in order to improve the assemblability between the reactor vessel and the core barrel, avoid an interference between the structures within a checkup work area during the operation, interrupt the inlet-outlet reverse phenomenon of an injection extension duct when the DVI line is broken, and avoid interference between the withdrawal inlet of the neutron monitoring capsule and the injection extension duct or the protruding nozzles, the concept of an injection extension duct having a new structure is required. An emergency core cooling water direct vessel injection system, which directly injects emergency core cooling water into the downcomer of a reactor vessel in a pressurized light water reactor complies with the following design requirements. First, the emergency core cooling water direct vessel injection system should be able to supply a larger quantity of emergency core cooling water to a core inlet through a lower portion of the downcomer by interrupting a phenomenon in which the emergency core cooling water is bypassed and discharged by a high-speed steam cross flow in the downcomer occurring in the event of a large break loss of coolant accident (LBLOCA). Second, a phenomenon in which the level of the cooling water in the reactor vessel is significantly reduced should not occur because an emergency core cooling water outlet of the lowest position of the injection extension duct functions as an inlet for a break flow so as to be able to be applied when the pipe of a direct vessel injection system is broken. Third, due to the injection extension duct installed in the downcomer, the cross flow resistance should not be excessively increased, and the flow induced vibration should not be excessively increased. Fourth, a connector of the injection extension duct and the direct vessel injection nozzle should not cause an interference when the reactor vessel is assembled with the core barrel in the downcomer or an interference with the withdrawal hole of a neutron monitoring capsule. Thereby, a practical application is possible, and the design can be certified, and continuous checkup during an operation of the reactor is possible. Accordingly, the present invention has been made by keeping in mind the above problems occurring in the related art. The present invention is directed to provide an emergency core cooling system having an injection extension duct for a pressurized light water reactor, in which the injection extension duct can be used in the event of a large break loss of coolant accident (LBLOCA) or a breakage accident of a direct vessel injection system pipe by interrupting the phenomenon in which emergency core cooling water is bypassed and discharged by a high-speed steam cross flow in a downcomer when a cold leg is broken, and preventing an inlet-outlet reverse phenomenon of the injection extension duct, thereby not interfering with the reactor assembly work and the periodical checkup work of the neutron monitoring capsule and reducing the cross flow resistance. According to one aspect of the present invention, there is provided an emergency core cooling system having core barrel injection extension ducts, which directly injects emergency core cooling water, which is supplied from a high-pressure safety injection pump or a safety injection tank for a pressurized light water reactor, into a reactor vessel downcomer, and in which the core barrel injection extension ducts interrupt an emergency core cooling water direct bypass discharge phenomenon, in which emergency core cooling water is controlled by a strong cross-flow of the downcomer in the event of a large break loss of coolant accident (LBLOCA) and is thereby discharged out of the reactor. According to the present invention, both a reactor assembly interference problem and an inlet-outlet reverse phenomenon occurring when any pipe of a direct vessel injection system is broken, which occurs in the prior art, are prevented. Thus, a concept of an emergency core cooling water injection extension duct of a downcomer having a new structure that is capable of preventing the level of the cooling water from being excessively lowered in the reactor vessel is realized. In the present invention, without a mechanical connection between the direct vessel injection nozzle and the injection extension duct using a pipe or a protrusion nozzle, the emergency core cooling water inlet and outlet, which are opposite each other, are designed to open in the downcomer. As illustrated in FIG. 2, although the direct vessel injection nozzle is not mechanically connected with the injection extension duct by means of a pipe, an emergency core cooling water intake port, through which the emergency core cooling water is injected from the direct vessel injection nozzle, is formed in the injection extension duct, located on an axis of the direct vessel injection nozzle, and a jet of the emergency core cooling water flows into the injection extension duct in a hydraulic connection fashion. The speed of the jet formed when the emergency core cooling water, supplied from a high-pressure safety injection pump or a safety injection tank, is injected into the reactor vessel downcomer through the direct vessel injection nozzle is high, and a jet stream capable of flowing across the downcomer between the direct vessel injection nozzle and the injection extension duct is formed by the momentum of the injected emergency core cooling water although the direct vessel injection nozzle is mechanically connected with the injection extension duct using a separate connection pipe or a separate protrusion nozzle. Thus, the direct vessel injection nozzle is hydraulically connected with the injection extension duct. Further, the emergency core cooling water does not yet flow into the injection extension duct, and thus part of the emergency core cooling water flows down into the downcomer, which is then stored in the downcomer of the reactor vessel, and contributes to the cooling of the reactor core. As a result, in a normal operation state, in which the emergency core cooling water is not injected, the direct vessel injection nozzle and the injection extension duct are open to the downcomer, and are connected by the water jet only when the emergency core cooling water is injected. Due to the connecting structure in the downcomer, interference, occurring when the reactor vessel is assembled with the core barrel, can be avoided. Further, interference between the withdrawal hole of a neutron monitoring capsule, which is attached to the downcomer of the reactor vessel and is periodically withdrawn and checked, and the injection extension duct can be fundamentally prevented. In the emergency core cooling system having core barrel injection extension ducts as illustrated in FIG. 3, the emergency core cooling water does not form water jets even in the event of an accident in which the direct vessel injection system pipe itself is broken, and thus the hydraulic connection between the direct vessel injection nozzle and the injection extension duct is automatically disconnected. As a result, only the cooling water in the hydraulically disconnected downcomer is discharged out of the reactor vessel through the direct vessel injection nozzle. Thus, in the prior art, in which the direct vessel injection nozzle and the injection extension duct are mechanically connected by a pipe and a nozzle, the inlet of the cooling water discharged out of the reactor vessel is located at the lowest position of the injection extension duct. However, in the present invention, the outlet of the cooling water is limited to the direct vessel injection nozzle, and thus the height of the inlet is further increased by the length of the injection extension duct compared to the prior art. The configuration in which the height of the inlet is further increased has a structural advantage in that it greatly contributes to preventing the level of the cooling water from being excessively lowered in the reactor vessel. Further, in the emergency core cooling system having core barrel injection extension ducts as illustrated in FIG. 5, lateral sides of the conventional injection extension duct are inclined as in a well known quadrilateral cross-sectional injection extension duct, so that the cross flow resistance of the downcomer is reduced. When the cross flow resistance against the jet of the cold leg is reduced, the magnitude of a flow vibration disturbance element is reduced. Thus, the present invention is provided to an emergency core cooling system having injection extension ducts for a pressurized light water reactor, in which the injection extension ducts can be used in the event of both a LBLOCA and a direct vessel injection line break. According to the present invention, the emergency core cooling system having core barrel injection extension ducts prevents the phenomenon in which the emergency core cooling water is directly bypassed and discharged in the event of the LBLOCA and thus makes a larger quantity of emergency core cooling water contribute to the cooling of the core. Further, in the event of a direct vessel injection pipe line break, the emergency core cooling system having core barrel injection extension ducts has an effect of preventing the level of the cooling water from being lowered in the downcomer due to an inlet-outlet reverse phenomenon at the lowest position of the outlet of the injection extension duct. Further, because the present invention features inclined lateral faces of the injection extension duct, which has a quadrilateral cross section in the prior art, the cross flow resistance of the downcomer is reduced. As a result of removing the connection structure between the direct vessel injection nozzle installed in the reactor vessel and the injection extension duct installed on the downcomer-side core barrel, the injection extension duct technology of the emergency core cooling water direct vessel injection system, which prevents an interference between the reactor vessel and the core barrel and a withdrawal interference for the neutron monitoring capsule, is provided, so that the safety and the safety regulatory requirements of the reactor can be sufficiently met. Reference will now be made in greater detail to an exemplary embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and a description to refer to the same or like parts. The detailed descriptions of known functions and constructions that might needlessly obscure the subject matter of the present invention will be avoided herein. As illustrated in FIG. 4A, an emergency core cooling system having injection extension ducts 26 of a core barrel 12 adopts a system of directly injecting emergency core cooling water into the downcomer 16 of a reactor vessel 10. Here, a pressurized light water reactor generally comprises an outer reactor vessel 10 and a core barrel 12, which has a diameter smaller than that of the reactor vessel 10 and is installed at the center of the reactor vessel 10. Further, a core 14 into which nuclear fuel rods are charged is located in the core barrel 12. The downcomer 16, which has an annular gap space due to the diameter difference between the core barrel 12 and the reactor vessel 10, is formed between the core barrel 12 and the reactor vessel 10. The reactor vessel 10 includes a plurality of cold legs 20, which function as inlets into a reactor cooling water circulatory path in the event of a normal operation, and a plurality of hot legs 22, which function as outlets through which cooling water, heated while sequentially flowing through the cold legs 20, the downcomer 16, and the core 14, flows to a steam generator. In the embodiment of the present invention, as illustrated in FIGS. 4A and 4B, four cold legs 20 and two hot legs 22 are installed. As illustrated in FIGS. 4A and 6, the emergency core cooling system of the pressurized light water reactor according to the present invention includes a plurality of direct vessel injection (DVI) nozzles 24 attached to an upper portion of the reactor vessel 10 constituting a nuclear reactor, and a plurality of injection extension ducts 26 for emergency core cooling water, which are installed on the outer surface of the core barrel 12 so as to face the DVI nozzles 24 with the downcomer 16 in between. Each injection extension duct 26 includes an emergency core cooling water intake port 28 in an outer surface thereof, wherein the emergency core cooling water intake port 28 passes through the outer surface of the injection extension duct 26, adopting a point intersecting an axis of the DVI nozzle 24 as the central point thereof, and has a diameter about twice the inner diameter DDVI of the DVI nozzle. According to this configuration of the present invention, no connection structure is installed between the DVI nozzle 24 and the injection extension duct 26 (particularly, the emergency core cooling water intake port). In other words, the DVI nozzle 24 is completely mechanically separated from the injection extension duct 26. In this manner, with the emergency core cooling system having the core barrel 12 and the injection extension duct 26 according to the present invention, the DVI nozzle 24 is separated from the injection extension duct 26 from a thermal hydraulic aspect when the pressurized light water reactor is normally operated without supply of the emergency core cooling water from the DVI nozzle 24, and no connection structure exists in the downcomer 16 between the DVI nozzle 24 and the injection extension duct 26 in the structural aspect. Thus, no interference occurs when the reactor vessel 10 and the core barrel 12 are assembled and when the neutron monitoring capsule is withdrawn. However, if a cold leg 20 is completely broken, the water jet speed of the emergency core cooling water injected through the DVI nozzle 24 amounts to about 22 m/sec while the safety injection tank injects the emergency core cooling water, and about 1.6 m/sec while the high-pressure safety injection pump injects the emergency core cooling water. The horizontal inertial force of the emergency core cooling water having such a water jet speed is sufficient for the emergency core cooling water to flow from the DVI nozzle 24 into the emergency core cooling water intake port 28 of the injection extension duct 26 facing the DVI nozzle 24 across the downcomer 16. Thus, the water jet formed while only the emergency core cooling water is injected causes the DVI nozzle 24 to be connected with the injection extension duct 26 in a thermal hydraulic fashion. The emergency core cooling water injected into the injection extension duct 26 can flow down to the lower portion of the downcomer 16 by means of gravity and flow momentum of the emergency core cooling water. The emergency core cooling water, which flows to the lower portion of the downcomer 16 in the injection extension duct 26 in the event of a large break loss of coolant accident (LBLOCA), is protected from a high-speed cross flow of the downcomer 16 by a wall of the injection extension duct. Further, although not all of the emergency core cooling water has flown into the injection extension duct 26, i.e. although part of the emergency core cooling water flows down to the downcomer 16, the emergency core cooling water that has not flown into the injection extension duct 26 is collected in the downcomer 16 of the reactor vessel 10, and thus helps cool the reactor core. In the case in which the pipe connected to the DVI nozzle 24 is broken, the emergency core cooling water is not injected into the broken DVI nozzle 24. Thus, the water jet of the emergency core cooling water is not formed. However, since the DVI nozzle 24 is separated from the injection extension duct 26 in a thermal hydraulic fashion, only the cooling water of the downcomer 16 around the DVI nozzle 24 leads to the formation of a break flow. In other words, when the DVI nozzle 24 is still connected with the injection extension duct 26 in spite of an accident in which the DVI line itself is broken, the lowest outlet of the injection extension duct 26 functions instead as an inlet for the break flow due to a siphon effect, so that the level of the cooling water in the reactor vessel 10 is remarkably lowered by the length of the injection extension duct 26. Thus, the break flow leaks out at a position where the DVI nozzle 24 faces the injection extension duct 26, i.e. at a position where the level of the cooling water is raised by the length of the injection extension duct 26 without the phenomenon in which the lowest outlet of the injection extension duct 26 functions as an inlet. Accordingly, the level of the cooling water of the reactor vessel 10 in the present invention can be maintained in a manner such that the level of the cooling water of the downcomer is much higher than in the structure in which the DVI nozzle 24 is mechanically connected with the injection extension duct 26 using a pipe, as in the prior art. Thus, the quantity of the cooling water that flows from the downcomer to the core due to the water level difference between the downcomer and the core is increased, so that the reactor core is more effectively cooled. As illustrated in FIG. 4A, the emergency core cooling system of the present invention is designed on a plane in a manner such that the angle α between the DVI nozzle 24 and the cold leg 20 is smaller than that between the DVI nozzle 24 and the hot leg 22. As also illustrated in FIG. 4A, the emergency core cooling system of the present invention is designed on a plane in a manner such that an angle β between the DVI nozzle 24 and the hot leg 22 is smaller than that between the DVI nozzle 24 and the cold leg 20. In the present invention, the angle between an axial line, which connects the DVI nozzle 24 and the injection extension duct 26, and the cold leg 20, and the angle between an axial line, which connects the DVI nozzle 24 and the injection extension duct 26, and the hot leg 22 are preferably smaller than the angle α between the DVI nozzle 24 and the cold leg 20 and the angle β between the DVI nozzle 24 and the hot leg 22, respectively. The embodiment of the present invention will be described in conjunction with the case in which an angle between the cold leg 20 and the hot leg 22 is 60°. An area between the angle α of FIG. 4A and the angle β of FIG. 4B is the movement path of an excore monitor that moves outside the reactor vessel in a vertical direction (in upward and downward directions). Thus, in consideration of the width of the injection extension duct 26 and the distance between the injection extension duct 26 and the neighboring hot leg 22, the maximum angle α is preferably less than 15°, and the maximum angle β is preferably less than 35°. The configuration of the injection extension duct 26 of the present invention will be described in greater detail with reference to FIG. 5A. The lowest outlet of the injection extension duct 26 is open, and the highest cap of the injection extension duct 26 is closed, and it includes at least one air vent 30 such that gas can be discharged when the nuclear reactor is filled with water. The injection extension duct 26 includes the emergency core cooling water intake port 28, which has about twice the inner diameter of the DVI nozzle 24 facing the injection extension duct 26, and is located on the axial line of the DVI nozzle 24. As illustrated in FIG. 6, the diameter DDUCT of the emergency core cooling water intake port 28 is about twice the inner diameter DDVI of the DVI nozzle 24 in consideration of a deflection of the jet by means of gravity in the case in which the jet of the DVI nozzle 24 has a small spreading and flow rate. Thereby, the emergency core cooling water is more easily introduced into the injection extension duct 26. As seen from FIG. 5A, the injection extension duct 26 includes lateral faces 32 inclined in a transverse direction at an angle of about 45° on opposite sides thereof, so that it can reduce the resistance of the cross flow of the downcomer 16 compared to an existing injection extension duct, the lateral faces of which are formed at an angle of 90°. The injection extension duct 26, which protrudes from the core barrel 12 toward the reactor vessel 10, preferably has a radial distance (h) limited to a range from about 3/25 to about 7/25 of the width of the diametrical gap of the downcomer 16 of FIG. 6. This is because, in the case in which the radial distance (h) must be smaller than the minimum inner diameter RKEY of the upper alignment key portion 34 of the reactor vessel 10 and the inner diameter RHL of the hot leg 22 of FIG. 4A, no interference occurs when the reactor vessel 10 and the core barrel 12 are assembled or when the neutron monitoring capsule is withdrawn. Thus, as for the cross-sectional shape of the injection extension duct 26 of the present invention, the radius of the curvature of the outer surface of the injection extension duct 26 protruding to the downcomer 16 is equal to the sum of the radius R of the core barrel 12 and the radial distance h of the injection extension duct 26, and the opposite lateral faces of the injection extension duct 26 are similar to the non-parallel opposite sides of an isosceles trapezoid. As in FIG. 5B, the injection extension duct 26 of the present invention has a vertical cross-sectional area defined by multiplying the average of the dimensions of the two circumferential parallel faces in the circumferential direction of the core barrel 12 by the radial distance, and is preferably equal to or greater than the effective cross-sectional area of the DVI nozzle 24. Here, considering all of the aforementioned conditions, including the condition that the radial distance h of the injection extension duct 26 be limited to a range from about 3/25 to about 7/25 of the width of the diametrical gap of the downcomer 16, a central angle Φ, subtending the longer circumferential face of the injection extension duct 26 in the circumferential direction of the core barrel 12, appropriately ranges from a minimum of 20° to a maximum of 35°. The length of the injection extension duct 26 installed on the core barrel 12 starts from the emergency core cooling water intake port 28 facing the DVI nozzle 24 of the reactor vessel 10, and then extends to a lower portion of the downcomer 16 below the positions of the cold leg 20 and the hot leg 22 along the outer surface of the core barrel 12. Further, the lowest position (length B of FIG. 6) of the outlet of the injection extension duct 26 is within a range from the central axis of the cold leg 20 to a lower portion of the downcomer 16. Considering that the surrounding cooling water is swept out by a strong intake break flow formed in the downcomer 16 around the broken cold leg 20 in the event of a LBLOCA, the lowest position of the outlet of the injection extension duct 26 must exist at a lower portion of the downcomer 16, which is lower than the position of the cold leg 20, and preferably extend toward a lower portion of the downcomer 16 from the central axis of the cold leg 20 within a range from about two to four times the inner diameter DCL of the cold leg, thereby being effective in preventing a direct bypass of the emergency core cooling water. Although the exemplary embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.
	claims	1. A method of installing an inlet mixer clamp assembly on a transition piece of a Boiling Water Reactor (BWR) jet pump assembly, the method comprising:forming a clamp plate, the clamp plate being formed to fit between lifting eyelets of the inlet mixer and cover upper surfaces of the transition piece,attaching the clamp plate to the transition piece by mounting a pair of mounting blocks onto a bottom surface of the clamp plate, each mounting block forming an aperture that is configured to allow the mounting block to cradle a bridge of the transition piece,attaching a jacking bolt to the clamp plate by penetrating the jacking bolt through the clamp plate and having the jacking bolt contact a top surface of the inlet mixer, andapplying a force to the top surface of the inlet mixer by tightening the jacking bolt. 2. The method of claim 1, further comprising:forming two beam bolt holes in the clamp plate, the beam bolt holes being configured to allow a pair of beam bolts of the inlet mixer to protrude through the clamp plate when the clamp plate is attached to the transition piece. 3. The method of claim 1, further comprising:forming first and second opposing lobes on the clamp plate, the first and second lobes being configured to extend away from the clamp plate at locations directly above the bridges of the transition piece when the clamp assembly is attached to the transition piece. 4. The method of claim 3, wherein the forming of the clamp plate includes bisecting the clamp plate into approximately two halves with a centerline, the centerline running through the middle of the first and second lobes, wherein the attaching of the jacking bolt includes penetrating at least one jacking bolt through the clamp plate on either side of the centerline. 5. The method of claim 4, further comprising:forming third and fourth opposing lobes on the clamp plate, the third and fourth lobes being configured to extend away from the centerline of the clamp plate, wherein the at least one jacking bolt on either side of the centerline penetrates the third and fourth lobes, respectively. 6. The method of claim 3, further comprising:attaching each mounting block to the first and second lobes, respectively.
	description	The present invention relates to a container having thick walls (“thick container”) such as a cask for housing, transporting and storing used nuclear fuel aggregate and substance contaminated by radioactivity, for example. Particularly this invention relates to a thick container in which a body section and a bottom section are formed integrally, or a thick drum which can be used as a cylinder of a large size pressing machine, or a canister for storing substance contaminated by radioactivity. The invention relates to a container or a drum which requires less troublesome steps for manufacturing and has an excellent end surface form, a manufacturing apparatus thereof and a manufacturing method thereof. As a cylinder or the like to be used in a cask or a large size pressing machine for containing and transferring and temporarily storing used nuclear fuel generated from a nuclear reactor, a container in which height, diameter and the like of its drum reach several meters is used. Containers having a wall that is several dozens centimeter in thickness have been suggested from a viewpoint of shielding from γ rays or high pressure resistance. A cask for containing and transporting and temporarily storing used nuclear fuel will be exemplified and there will be explained below a conventional container which has been used for these applications. FIG. 29 is a sectional view showing one example of the conventional cask. The cask 500 is composed of a container 501 which is formed with a body section 501a and a bottom section 501b made of stainless or carbon steel, a basket 502 for used containing nuclear fuel aggregate which is arranged in the container 501, and a neutron shielding body 503 provided on an outer periphery of the container 501. The neutron shielding body 503 is charged into a space between an outer drum 504 and the container 501, and a plurality of heat transfer fins (not shown) are provided between the container 501 and the outer drum 504. As the basket 502, a material to which boron having neutron absorbing ability is added is used. The bottom plate 501b made of stainless or carbon steel is welded with tungsten-inert gas (TIG welding) or welded with submerged-arc (SAW welding) to the container 501. A neutron shielding material 506 is sealed into the bottom plate 501b. Moreover, a primary cover 507 and a secondary cover 508 are attached to an upper section of the container 501 by bolts. A neutron shielding material 509 is sealed into the secondary cover 508. γ rays generated from used nuclear fuel aggregate are shielded by the body section 501, the bottom plate 501b, the primary cover 507 and the secondary cover 508. Moreover, neutron is shielded by the neutron shielding material 503 provided on the outer periphery of the container 501, the bottom plate 501b, the neutron shielding material 506 sealed into the secondary cover 508, and the secondary cover 508. A degradation heat of the used fuel aggregate is transmitted from the container 501 via the heat transfer fins to the outer drum 504 and is radiated to the outside therefrom. Next, how a container having a bottom (“bottomed container”) for the cask shown in FIG. 29 is manufactured will be explained below. FIGS. 30(a) through 30(e) are explanatory diagrams showing one example of the method of manufacturing the bottomed container of the cask shown in FIG. 29. As shown in FIG. 30(a), a metal billet 61 which is cogged into a predetermined dimension is upset onto an anvil with bore and is bored by a punch 63. As shown in FIGS. 30(b) and 30(c), a mandrel 65 is inserted through a hole 64 of the metal billet 61 and while it is being rotated, the hole 64 is widened by a hammer 66. As shown in FIG. 30(d), the mandrel 65 is replaced by a large-diameter mandrel 67 and hollow cogging is carried out by a hammer 68. As a result, the metal billet 61 is thinned so that a cylindrical body is formed (FIG. 30(e)). FIGS. 31(a), 31(a′), 31(b) and 31(b′) are explanatory diagrams showing the method of manufacturing the bottomed container according to an Erhardt boring method. This method is for pushing a punch 410 into a metal billet 200 put into a container so as to form the metal billet 200 into a cylindrical shape. This metal billet 200 has a rectangular section, and its diagonal length is equal with an inner diameter of a body section 300 of the container. Moreover, since the section of the metal billet 200 is rectangular, spaces 350 exist between the metal billet 200 and the container 200 (FIG. 31(a′)). When the metal billet 200 is upset into the body section 300 of the container and the punch 410 is pushed into a center axis of the metal billet 200, metal flow occurs due to metal swelling function of the punch 410. While this metal flow is filling the spaces 350 and a part of the metal flow is rising in the body section 300 of the container, and the metal billet 200 is formed into the cylindrical form (FIG. 31(b)). In addition, the bottomed container of the cask can be manufactured also by a backward extrusion pressing method (not shown). In the backward extrusion pressing method, after the metal billet having a circular section substantially equal with the inner diameter of the container is upset into the container, metal flow is generated between the punch and the container by a compressive force of the punch pressurizing along the center axis of the metal billet. While the metal is being raised to a backward direction, the metal billet is formed into a long cylindrical form. After the cylindrical body section 501a is formed by one of the above-mentioned methods, the bottom plate 501b is welded to its lower section. Further, in order to remove a thermal stress due to the welding, the container 501 is subject to heating treatment. However, in order to obtain the bottomed container of the conventional cask 500, since the bottom plate 501b is jointed to the cylindrical body section 501a by welding, the container should be subject to the heating treatment after the welding. For this reason, there arose a problem that the manufacturing requires troublesome steps. Moreover in the Erhardt boring method, as shown in FIG. 31, a drop of a temperature in a metal forward end portion rising in the spaces 350 causes scratches and wave-shaped defects. Further, as shown in the diagrams, since a faulty form portion (FIG. 31(b)) is inevitably generated in the cylindrical end section, this portion should be eliminated by a constant amount, and thus yield is greatly lowered. In addition, in the backward extrusion pressing method, the metal billet is formed while high friction is generated between the container and metal billet. For this reason, a lot of defects such as pockmarks and ribs are generated on an outer surface of the metal billet, and it takes a long time to remove these defects. Further, in both the Erhardt boring method and the backward extrusion pressing method, when a dimension and a thickness of the container to be formed become large, a pressure which is required for pressing becomes extremely large. Therefore, in these methods, it was difficult to manufacture a container having large dimension and thickness. Therefore, the present invention is devised in order to solve the above-mentioned problems, and its object is to provide a container which requires less troublesome manufacturing steps or a container in which defects generated on its cylindrical end portion and its surface can be suppressed. The radioactive substance container according to this invention comprises a thick bottomed container in which a bottom section and a body section are formed integrally by hot-dilating a metal billet in a container for forming. The radioactive substance container according to the next invention comprises a thick bottomed container in which when a metal billet is hot-dilated in a container for forming and its body section is worked, a boring uncompleted section remains on one end side of the body section so as to be a bottom section and the bottom section and the body section are formed integrally. As the above-mentioned radioactive substance container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required, and heating treatment of the welding can be omitted. Moreover, since the bottomed container is formed by the hot dilation, only a press pressure, which is lower than that at the time of hot backward extrusion forming, for example, is required. Here, at the time of the hot press working and the upsetting drawing, punching and drawing which are normally known can be combined, and they are not limited to detailed working methods described below. Moreover, the radioactive substance container according to the present invention can store not only used nuclear fuel but also substances and the like contaminated by radioactive rays. The bottomed container provided to the radioactive substance container includes a so-called thick container in which a thickness is thick with respect to its radius like a container to be used for a cask for transporting and storing used nuclear fuel. Here, the thick container is such that a ratio of a difference between an outer radius R0 and an inner radius Ri, namely, a thickness t=R0−Ri to an average radius R=(Ro−Ri)/2 is (t/R)> 1/10. When the section of the container is not circular, an equivalent diameter de=s/π may be used for calculating the outer radius R0, the inner radius Ri and the average radius. Here, s is a peripheral length of the section, and when a length of one side is a, s=4×a in the case of quadrate section. In addition, the bottomed container of the invention is suitable for a container in which a ratio (L/Di) of an axial length L to an inner diameter Di is not less than 1 like a cask for storing used nuclear fuel in which an axial length reaches a several meters. Moreover, the present invention may be applied to a canister as a radioactive substance container. When (L/Di) is less than 1, a certain effect is produced, but as (L/D1) becomes larger than 1, the effect according to this invention is produced more remarkably. The radioactive substance container according to the next invention is the radioactive substance container described above in which a section of the metal billet vertical to an axial direction is polygonal and a shape in a section of the container for forming vertical to an axial direction is circular. As this radioactive substance container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required. Moreover, as for the bottomed container is hot-dilated by setting the metal billet having a polygonal section vertical to the axial direction into the container for forming having a circular internal shape of the section vertical to the axial direction. At this forming step, since the container is dilated to be formed by a function which bends each side of the polygon, the bottomed container in which the bottom section and the body section are integral can be formed with a lower pressure than conventional one. The radioactive substance container according to the next invention is the radioactive substance container described above in which a section of the metal billet vertical to an axial direction is polygonal and a section of the container for forming vertical to an axial direction is polygonal. As this radioactive substance container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required. Moreover, the bottomed container is hot-dilated by setting the metal billet having a polygonal section vertical to the axial direction into the container for forming having a polygonal internal shape of the section vertical to the axial direction. At this forming step, since the container is dilated to be formed by a function which bends each side of the polygon, the bottomed container in which the bottom section and the body section are integral can be formed with a lower pressure than conventional one. Moreover, an internal shape of the container for forming is changed so that bottomed containers having external forms according to various radioactive substance containers can be formed easily. The radioactive substance container according to the next invention comprises a bottomed container for storing a basket for used nuclear fuel aggregate in which a bottom section and a body section is integral by hot dilation forming in a container for forming. Here, the basket is constituted by collecting angular pipes, for example, its section vertical to the axial direction has like an internal shape of a section of the bottomed container shown in FIG. 15(d). Moreover, its outer diameter reaches about 2 to 2.5 m. As this radioactive substance container, the thick bottomed container for containing used nuclear fuel aggregate, in which a dimension in the axial direction reaches a several meters and inner diameter reaches 2 to 2.5 meters, is used so that the conventional welding of a bottom plate is not required and the heat treatment after the welding can be omitted. Particularly in the bottomed container whose thickness is thick and dimension in the axial direction is a several meters and inner diameter reaches 2 to 2.5 meters, the effect which can omit the steps is extremely great. In this container having such a size, the bottomed container of the present invention can be manufactured about one month earlier than a conventional container in which a bottom plate is welded. This difference depends upon time required for the welding itself and time required for the post-welding heat treatment and cooling. Since the bottomed container according to this invention does not require theses processes, the manufacturing time can be greatly shortened. The radioactive substance container according to the next invention is the radioactive substance container described above in which a section of the boring punch has a dimension and a shape which approach to the section of the basket for used nuclear fuel aggregate. Since this radioactive substance container has a dimension such that the section of the boring punch is approximate to the section of the basket for used nuclear fuel aggregate, an operation for cutting the inside of the container becomes easy after the hot dilation forming, and the manufacturing does not require the troublesome steps. Here, “dimension such that the section of the boring punch is approximate to the section of the basket for used nuclear fuel aggregate” means a relationship that the dimension of the section of the boring punch is substantially equal with a difference between the dimension of the basket for used nuclear fuel aggregate and a cutting allowance of the inner side of the bottomed container to be formed. As the boring punch of the present invention, a boring punch 27c or 27d shown in FIGS. 27(c) and 27(d), for example, can be used. Moreover, the sectional shape of the basket is like the internal shape of the section of the bottomed container shown in FIGS. 15(c) and 15(d), for example, the sectional shape of the boring punch of the invention can be approximate to the these shapes. Further, it is considered that when the sectional shape of the basket for used nuclear fuel aggregate is the internal shape of the section of the bottomed container shown in FIG. 15(d), for example, the inner side of the bottomed container having the inner shape of the section shown in FIG. 15(c) is cut so as to be formed into the internal shape of the section shown in FIG. 15(d). In this case, the section of the boring punch where the internal shape of the section shown in FIG. 15(c) is formed has a dimension that it is approximate to the section of the basket. The radioactive substance container according to the next invention comprises a bottomed container, in which a dosage equivalent factor of γ rays on an outer wall surface of a substantially center portion of a side surface of the body is not more than 200 μSv/h in the case where radioactive substance is contained in a bottomed container in which its bottom section and body section are formed integrally by hot dilation forming in a container for forming. Since the bottomed container to be used as this radioactive substance container transports and stores used nuclear fuel, it requires the function for shielding the γ rays radiated from the used nuclear fuel. It is desirable that the dosage equivalent factor of the γ rays on the outer wall surface of the substantially center portion of the sides surface of the radioactive substance container is smaller, not more than 2000 μSv/h confirms to the standard of transportation and storing based on “The rules relating to transportation of nuclear fuel substance and the like outside factory and business establishment (dated on Dec. 28, 1978, Prime Minister's Office Statute No. 57) (Final Amendment dated Nov. 28, 1990, Prime Minister's Office Statute No. 56)”, “The notice for defining details relating the technical standards relating to transportation of nuclear fuel substance and the like outside factory and business establishment (dated Dec. 28, 1978 Science and Technology Agency Notice No. 11) (Final Amendment dated Nov. 28, 1990, Science and Technology Agency Notice No. 5)” and “The technical study relating to storing of used fuel in a dry type cask (July, 1992, Agency of Natural Resources and Energy)”. The bottomed container to be used as the radioactive substance container of the present invention takes the safety into consideration, and the bottomed container is formed into a thick container made of stainless steel or carbon steel where the thickness reaches several dozens cm so that the dosage equivalent factor can be lowered to about 1/10 of this value. In this radioactive substance container, since the body section and the bottom section of the thick container are formed integrally, the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Particularly in such a thick bottomed container, the effect which can omit the above-mentioned steps is extremely great. The radioactive substance container according to the next invention is the radioactive substance container described above in which an outer diameter of the bottomed container is not less than 1000 mm to not more than 3000 mm and its thickness is not less than 150 mm to not more than 300 mm. As this radioactive substance container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Particularly in such a thick bottomed container, the effect which can omit the above-mentioned steps is extremely great. The radioactive substance container comprises a bottomed container where a metal billet, in which at least a section vertical to an axial direction on a forward side with respect to the pressing direction (“pressing forward side”) is formed into a polygonal shape, is set into a container for forming and a boring punch is pushed into the metal billet and the metal billet is hot-dilated so that a bottom section and a body section are formed integrally. This radioactive substance container has the bottomed container where the body section and the bottom section are formed integrally. When such a bottomed container is used, the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Moreover, since this bottomed container can be formed with lower press pressure than the conventional one and the movement of the metal to the axial direction of the container is minimum, defects generated on an end portion and a container surface is less. For this reason, the adjustment of these defects requires less troublesome steps. The bottomed container of the radioactive substance container is suitable for a container in which a ratio (L/Di) of an axial length L to an inner diameter Di is not less than 1 like a cask for storing used nuclear fuel in which an axial length reaches a several meters. When such a thick bottomed container which is long with respect to the axial direction is tried to be formed by the conventional hot working method, the press pressure of several dozen-thousand ton is required, and a lot of defects occur on the end portion or the surface of the formed bottomed container. Therefore, such a thick container was conventionally manufactured by welding the bottom to a thick cylinder which was manufactured by the roll forging method or the like. On the contrary, the bottomed container according to this invention can be manufactured by one-time working as a thick bottomed container which is long with respect to the axial direction and can store used nuclear fuel aggregate. Moreover, since the press pressure is about ten-thousand tons, an existing large pressing machine can be used. Moreover, since defects do not occur on the end portion or the surface of the container, adjustment after the forming is seldom required. In addition, in the case where the container does not have such a large dimension, even if the thick container in which (t/R) exceeds 1/10 is formed by the hot press forming, high press pressure is not required. However, a lot of defects occur on the end portion or the surface of the formed bottomed container. For this reason, it is difficult to form such a container according to hot press forming, but this thick bottomed container in which (t/R) exceeds 1/10 can be formed by one-time working, and defects seldom occur on the end portion or the surface. The radioactive substance container according to the next invention comprises: a bottomed container where a bottom section and a body section are formed integrally by hot press pressure and γ rays generated from radioactive substance such as used fuel is shielded; a neutron shielding member which is provided around the bottomed container and shields neutron generated from the radioactive substance; and a cover for covering an opening of the bottomed container. The radioactive substance container according to the next invention comprises: a bottomed container which contains a radioactive substance such as used fuel into a body section with a bottom section and shields γ rays generated from the radioactive substance; a neutron shielding material which is arranged around the bottomed container and shields neutron generated from the radioactive substance, wherein a metal billet is heated and is upset and drawn so that the bottom section and the body section are formed integrally. In this radioactive substance container, the bottomed container, where the body section and the bottom section are integral, is formed. When such a bottomed container is used, the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. At the time of the hot press working and upsetting draw forming, the punching and drawing which are normally known can be combined, the forming is not limited to the detailed working method mentioned below. This radioactive substance container can contain not only used fuel but also substances contaminated by radioactive rays. The radioactive substance container according to the next invention is the radioactive substance container described above in which a spot facing section is further formed integrally with the bottom section at the time of forming the bottomed container. This radioactive substance container is a bottomed container in which the metal billet is hot-dilated and simultaneously the spot facing section is also provided on the bottomed section. In a cask, since the bottomed section has a neutron absorbing member, the spot facing section is provided on the bottomed section of the container. Since the spot facing section has been conventionally provided by the cutting work or by welding a bottom plate previously provided with a spot facing section to the body section, the manufacturing requires the troublesome steps. In the bottomed container, since the spot facing section is formed integrally at the time of the hot-dilation forming, the step of forming the spot facing section can be omitted. The radioactive substance container according to the next invention is the radioactive substance container described above in which a flange is further provided integrally with the body section of the bottomed container. In the conventional radioactive substance container, since the flange section is separately manufactured and is welded to the body section so as to be attached, the heat treatment after the welding is required, namely, the manufacturing requires the troublesome steps. Moreover, since the radioactive substance container itself requires sealing property and strength, the welded portion requires high solidity. According to the bottomed container of the present invention, since the flange and the body section are formed integrally, the welding and post-welding heat treatment steps are omitted, and simultaneously the sealing property and the strength of the container itself can be secured. The radioactive substance container according to the next invention is the radioactive substance container described above in which at least one of an external section and an internal section of the bottomed container vertical to the axial direction is polygonal. Since a basket is contained in the bottomed container to be used particularly for a cask as the radioactive substance container, it is preferable that the sectional shape of the inside of the bottomed container is formed according to the shape the basket. In the conventional method, after the section of the inside of the bottomed container is formed into a circular shape, it is formed into a shape according to the basket by the cutting or the like. When this radioactive substance container is hot-dilated, the section of the inside of the container can be formed into a shape according to the basket. For this reason, the cutting step which was conventionally required can be omitted. The polygonal shape of the inner section of the body section includes so-called polygonal shapes such as triangular and tetragonal shapes and also shapes shown in FIGS. 15(c) and 15(d). Thereafter, this is applied also to the following inventions. The hot dilation forming-use metal billet according to the next invention in which at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape. Since this hot dilation forming-use metal billet is formed so that at least the section vertical to the axial direction on the pressing forward side is formed into a polygonal shape, the metal billet is dilated towards the inner wall of the container by a function for bending each side of the polygon at the time of the hot dilation forming. Since the metal billet is dilated in a space between the pressing forward side and the body of the container at the time of the hot dilation forming, the phenomenon that the metal flows to the opposite direction to the pressing direction is suppressed. Due to these functions, with this hot dilation forming-use metal billet, a thick container where a ratio of the axial length to the diameter is not less than 1 can be formed with a press pressure which is a several part of the conventional press pressure. Moreover, defects which occurs on the end portion or the surface of the container after the forming can be suppressed. The hot dilation forming-use metal billet according to the next invention in which at least one plane is provided on at least one of a side surface on a pressing forward side and a side surface on a backward side with respect to the pressing direction (“pressing backward side”). In this hot dilation forming-use metal billet, since its side surface has at least one plane and the metal billet is hot-dilated by the function for bending the plane towards the inner wall of the container for forming, the press pressure required at the time of the hot dilation forming is lower than the case where the side surface is a curved surface. Therefore, the thick container which is long in the axial direction can be formed with the press pressure smaller than the conventional pressure. Moreover, internal defects such as cracks can be reduced in comparison with the case where the side surface is a bent surface. The hot dilation forming-use metal billet according to the next invention is the hot dilation forming-use metal billet described above in which a taper which becomes thinner towards the pressing direction is provided on the pressing forward side of the metal billet. The hot dilation forming-use metal billet according to the next invention is the hot dilation forming-use metal billet described above in which at least one or more stepped sections are provided so that the pressing forward side of the metal billet becomes thinner gradationally towards the pressing direction. In the above-mentioned hot dilation forming-use metal billet, since the timing at which the metal fills the vicinity of the bottom of the container for forming can be delayed at the final stage of the hot dilation forming, the upsetting of the metal billet can be suppressed at the final stage of the hot dilation forming. As a result, thus the press pressure can be reduced at the time of the hot dilation forming. The hot dilation forming-use metal billet according to the next invention is the one in which at least one plane is provided on a side surface and an extended section which engages with an end portion of an inlet of a container for forming is provided on an end portion on a pressing backward side. Since this hot dilation forming-use metal billet is provided with the extended section at the end portion on the pressing backward side, the metal billet is engaged with the end portion of the container by the extended section at the time of the hot dilation forming. With this function, constraint of the container on the metal billet becomes stronger so that the upsetting of the metal billet on the pressing forward side can be suppressed. Moreover, since the side surface is provided with at least one plane, the function for bending this plane and the function for suppressing the upsetting of the metal billet accrue. Therefore, due to their interaction, the press pressure can be suppressed to be small. Moreover, the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing. For this reason, since the step of forming the extended section which is extended to above the body section of the container on the pressing backward side is not required before the hot dilation forming step, the container manufacturing step can be simplified. Here, in this metal billet, the sectional shape is uniform along the axial direction. The hot dilation forming-use metal billet according to the next invention is the one in which at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape, and an extended section which engages with an end portion of an inlet of a container for forming is provided on a pressing backward side. Since this hot dilation forming-use metal billet is provided with the extended section on the pressing backward side, this extended section engages the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting of the metal billet on the pressing forward side can be suppressed. Moreover, since at least the section vertical to the axial direction on the pressing forward side was formed into a polygonal shape, the function for bending each side of the polygonal section and the function for suppressing the upsetting of the metal billet accrue. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing. For this reason, since the step of forming the extended section which is extended to above the body section of the container on the pressing backward side is not required before the hot dilation forming step, the steps of manufacturing the container can be simplified. The hot dilation forming-use metal billet according to the next invention is the one in which at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape, and at least one or more stepped sections are provided so that the pressing forward side becomes thinner gradationally towards a pressing direction, and an extended section which engages with an end portion of an inlet of a container for forming is provided on a pressing backward side. Since this hot dilation forming-use metal billet is provided with the extended section on the pressing backward side, the extended section latches the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least the section vertical to the axial direction on the pressing forward side was formed into a polygonal shape, the function for bending each side of the polygonal section and the function for suppressing the flow of the metal accrue. Further, since the pressing forward side becomes thinner gradationally towards the pressing direction, the timing at which the metal fills the bottom section of the container for forming can be delayed. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing. For this reason, since the step of forming the extended section which is extended to above the body section of the container on the pressing backward side is not required before the hot dilation forming, the steps of manufacturing the container can be simplified. Here, since the pressing forward side becomes thinner gradationally, the forming becomes comparatively easy. The hot dilation forming-use metal billet according to the next invention is the one in which at least one plane is provided on at least one of a side surface on a pressing forward side and a side surface on a pressing backward side, and at least one or more stepped sections are provided so that the pressing forward side becomes thinner gradationally towards the pressing direction, and an extended section which engages with an end portion of an inlet of a container for forming is provided on the pressing backward side. Since this hot dilation forming-use metal billet is provided with the extended section on the pressing backward side, the extended section latches the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least one of the side surfaces of the metal billet has at least one plane, the function for bending the plane and the function for suppressing the flow of the metal accrue. Further, since the pressing forward side becomes thinner gradationally towards the pressing direction, the timing at which the metal fills the bottom section of the container for forming can be delayed. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing. For this reason, since the step of forming the extended section which is extended to above the body section of the container on the pressing backward side is not required before the hot dilation forming, the steps of manufacturing the container can be simplified. Here, since the pressing forward side becomes thinner gradationally, the forming becomes comparatively easy. The container according to the next invention is the one in which a metal billet is hot-dilated in a container for forming, and a bottom section and body section are formed integrally and a thick bottomed container is obtained. The container according to the next invention is the one in which, when a metal billet is hot-dilated in a container for forming and a body section is worked, a boring uncompleted section is allowed to remain on one end side of the body section so as to be a bottom section, and an integrally thick bottomed container is obtained. As the container of the above-mentioned invention, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required, and heat treatment after that can be omitted. Moreover, since the bottomed container is formed by the hot dilation, only a press pressure, which is lower than that at the time of hot backward extrusion forming, for example, is required. Here, at the time of the hot press working and the upsetting drawing, punching and drawing which are normally known can be combined, and they are not limited to detailed working methods described below. In addition, like a cylinder or the like for a large pressing machine, the above-mentioned container includes a so-called thick container in which a thickness is thick with respect to its radius. Here, the thick container is such that a ratio of a difference between an outer radius R0 and an inner radius Ri, namely, a thickness t=R0−Ri to an average radius R=(Ro−Ri)/2 is (t/R)> 1/10. When the section of the container is not circular, an equivalent diameter de=s/π may be used for calculating the outer radius R0, the inner radius Ri and the average radius. Here, s is a peripheral length of the section, and when a length of one side is a, s=4×a in the case of quadrate section. The container according to this invention is suitable for a container in which a ratio (L/Di) of an axial length L to an inner diameter Di is not less than 1. Another container of the present invention includes a comparatively thin container like a boiler as a pressure container. Further, another container of the invention includes a container for chemical plant, a reactor container for petroleum refining plant, an ammonia synthetic cell, a heat exchange container, a pressure container such as a boiler, a casing for a large rotational equipment for containing a hydroelectric water turbine, a container to be used as a body of submarine and ship. The container according to the next invention is a container described above in which a section of the metal billet vertical to an axial direction is polygonal and an internal shape of a section of the container for forming vertical to the axial direction is circular. As this container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required. Moreover, since the metal billet, where a section vertical to the axial direction is polygonal, is set into the container for forming, where an internal shape of the section vertical to the axial direction is circular so that the bottomed container is hot-dilated. At this forming step, since the metal billet is dilated by the function for bending one side of the polygonal section, the bottomed container can be formed with a press pressure which is lower than the conventional pressure. The container according to the next invention is a container described above in which a section of the metal billet vertical to an axial direction is polygonal and an internal shape of a section of the container for forming vertical to the axial direction is polygonal. As this container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required. Moreover, since the metal billet, where a section vertical to the axial direction is polygonal, is set into the container for forming, where an internal shape of the section vertical to the axial direction is circular so that the bottomed container is hot-dilated. At this forming step, since the metal billet is dilated by the function for bending each side of the polygonal section, the bottomed container where the bottom section and the body section are integral can be formed with a press pressure which is lower than the conventional pressure. Moreover, the internal shape of the container for forming is changed so that bottomed containers having external shapes according to appellations can be obtained. The container according to the next invention is a container described above in which an outer diameter of the bottomed container is not less than 200 mm to not more than 4000 mm, and a thickness is not less than 20 mm to not more than 400 mm. As this container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Particularly in the thick bottomed container, the effect that the above-mentioned steps can be omitted is great. The container according to the next invention is the one in which a metal billet, where at least a section vertical to an axial direction on a pressing forward side is polygonal, is set into a container for forming, and a boring punch is pushed into the metal billet and the metal billet is hot-dilated to be formed into a bottomed container where a bottom section and a body section are integral. This container is a thick container and includes both a drum without a bottom and a bottomed container where a bottom section and a body section are integral. Particularly in the case of the thick bottomed container, the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Moreover, since the pressure required for pressing is lower than the conventional pressure, even in the case of a container which is thick and whose axial dimension reaches a several meters particularly like a container to be used as a cylinder or the like for a large pressing machine, it can be manufactured by the conventional facilities. Moreover, since a number of defects which occur on the end portion of the container surface is small, only less steps of correcting these defects after the forming are required. The container according to the next invention is a container described above in which the bottomed container is constituted so that at least one of an external section and an internal section of the bottomed container vertical to the axial direction is polygonal. Since a basket is contained in the bottomed container to be used particularly for a cask as the radioactive substance container, it is preferable that the inner section of the bottomed container is formed into a shape according to the basket. In the conventional method, after the inner section of the bottomed container is formed into a circular shape, and it is formed into the shape according to the basket by the cutting or the like. Since the inner section of this container can be formed into the shape according to the basket when the bottomed container is dilated, the cutting step which was required conventionally can be omitted. The bottomed container manufacturing apparatus according to the next invention comprises: a container for forming having at least a container body section and a container bottom section in which the container body section and the container bottom section can move relatively with respect to an axial direction of the container body section; and a boring punch which is mounted to a pressing machine and pressurizes a metal billet for hot dilation forming set into the container for forming. This bottomed container manufacturing apparatus has the container in which the bottomed section and the body section can move relatively. For this reason, when the body section of the container is tried to moved to the opposite direction to the pressing direction at the time of the hot dilation forming, the body section of the container moves to the opposite direction to the pressing direction together with the metal billet. Namely, since the body section of the container and the metal billet to be formed seldom move relatively, an increase in the press pressure at the time of the hot dilation forming can be suppressed. The bottomed container manufacturing apparatus according to the next invention comprises: a container for forming having at least container body sections and container bottom sections divided in an axial direction in which the container body section and the container bottom section can move relatively with respect to an axial direction of the container body section; and a boring punch which is mounted to a pressing machine and pressurizes a metal billet for hot dilation forming set into the container for forming. In this bottomed container, since the body section of the container extends along the whole axial direction, even in the case where the metal billet which is long in the axial direction, deformation of the metal billet in the axial direction at the time of the hot dilation forming can be absorbed by the whole container. Therefore, even in the case of the container which is long in the axial direction, an increase in the press pressure can be suppressed. The radioactive substance method of manufacturing a container according to the next invention comprises: the step of rounding a drum-shaped bottomed container where a bottom section and a body section are formed integrally by hot dilation and setting a tool so as to cut an external side of the bottomed container; and the step of cutting an internal section of the bottomed container into a shape according to at least one portion of an outer peripheral shape of a basket for containing used nuclear fuel aggregate. In this radioactive substance method of manufacturing a container, the outer side of the bottomed container where the bottom section and the body section are formed integral is finished by cutting, and the inner side is cut into a stepped shape so that a portion for containing a basket for used nuclear fuel aggregate is provided, or the inner side is finished by cutting so that the radioactive substance container is manufactured. The radioactive substance method of manufacturing a container according to the next invention comprises: the step of hot-dilating a bottomed container so that its bottom section and body section are integral; and the step of cutting an internal section of the bottomed container into a shape according to at least one portion of an outer peripheral shape of a basket for containing used nuclear fuel aggregate. In this radioactive substance method of manufacturing a container, the bottomed container where the bottom section and the body section are integral is formed by the hot dilation forming, and the outer side of the bottomed container is finished by cutting, and the inner side is cut into a stepped shape so that a portion for containing a basket for used nuclear fuel aggregate is provided, or the inner side is finished by cutting so that the radioactive substance container is manufactured. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet having at least one plane on a side surface into a container for forming with a gap from an inner wall; and the step of pushing a boring punch into the metal billet and bending the plane towards the inner wall so as to hot-dilate the metal billet. In this method of manufacturing a container, the metal billet is extended towards the inner wall of the container for forming by the function for bending the plane on the side surface of the metal billet. Moreover, since the metal billet is extended in space between the metal billet and the inner wall of the container for forming, the upsetting phenomenon of the metal billet can be suppressed. With these function, the method of manufacturing a container requires the press pressure which is lower than the conventional pressure, and defects which occurs on the end portion or the surface of the container after the forming can be suppressed. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet, which has at least one plane on a side surface and an extended section engaging with an end portion of an inlet of a container for forming on an end portion of a pressing backward side, into the container for forming with a gap from an inner wall; and the step of pushing a boring punch into the metal billet and bending the plane towards the inner wall so as to hot-dilate the metal billet. In this method of manufacturing a container, since the metal billet provided with the extended section engaging with the end portion of the opening of the container for forming is used for the end portion of the pressing backward side, the extended section engages the metal billet with the end portion of the container at the time of hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting of the pressing forward side can be suppressed. Moreover, since at least one plane is provided on the side surface, the bending function and the function for suppressing the phenomenon that the metal flows to the opposite side to the pressing direction accrue. Therefore, the press pressure can be suppressed small by their interaction, and deterioration of the end surface shape can be also suppressed. The method of manufacturing a container according to the next invention is characterized in that a metal billet, where at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape, is set into a container for forming, and a boring punch is pushed into the metal billet and the metal billet is hot-dilated. In this method of manufacturing a container, on the pressing forward side, since the metal billet is extended in the space between the pressing forward side and the container body, the flow of the metal directing to the opposite side of the pressing direction can be suppressed. For this reason, the phenomenon of the metal billet can be suppressed. For this reason, the method of manufacturing a container requires the press pressure which is lower than the conventional one, and defects which occur on the end portion or the surface of the container after the forming can be suppressed. The method of manufacturing a container according to the next invention is characterized in that a metal billet having at least one plane on at least one of a side surface on a pressing forward side and a side surface on a pressing backward side is set into a container for forming, and a boring punch is pushed into the metal billet and the metal billet is hot-dilated. In this method of manufacturing a container, since the metal billet having at least one plane on the side surface is hot-dilated, a force required for the hot dilation forming is weaker than the case where the side surface is a curved surface. Therefore, the press pressure is lower than the conventional method of manufacturing a containers, and internal defects such as cracks can be reduced. The hot pressing method of manufacturing a thick metal-made drum or a cylindrical container according to the next invention is characterized in that a metal billet having different diameter sections without joint, where its pressing forward side is composed of a member having a section with an outer diameter smaller than an inner diameter of a container or an outer diameter of a diagonal length or a member having a section with an outer diameter of a diagonal length equal with the inner diameter of the container and its backward side is composed of a member having a section with an outer diameter or a diagonal length equal with the inner diameter of the container, is set into the container for press forming which was heated to a press working temperature, and while a center of a workpiece of the metal billet without joint is being bored by a punch, the metal billet is press-worked. In this pressing method, the metal on the thick portion on the pressing backward side fills the container and simultaneously is worked, the constraint force is heightened, and the upsetting phenomenon of the metal billet without joint is suppressed, and the shape of the end surface is made to be satisfactory. Moreover, since the metal is supplied from the pressing backward side to the pressing forward side and the pressing forward side is pushed to be spread sideways and is simultaneously worked according to the effect of satisfactory plastic working of the steel heated to high temperature, it fills the space of the container to be formed. For this reason, a pressed product having a predetermined shape is manufactured from the metal billet without joint. As a result, the metal billet without joint reduces a pressing forming load and improves yield of the product, and a pressed product having excellent end surface shape can be obtained. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section having an outer diameter with a diagonal length of not more than an inner diameter of the container and its backward side has a section having an outer diameter substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; and the step of boring a center of a workpiece of the metal billet by means of the boring punch and simultaneously press-working the metal billet. In this method, since the metal billet, where the pressing forward side has a quadrate section whose the diagonal length of the pressing forward side is smaller than the inner diameter of the container, is used, the press pressure can be small by the function for bending the side surface composing the plane on the pressing forward side. Moreover, since the pressing backward side of the metal billet suppresses the upsetting of the pressing forward side, defects on the end portion and the container surface can be suppressed, and the press pressure can be reduced. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section having an outer diameter with a diagonal length of smaller than an inner diameter of the container and its backward side has a section having a diagonal length substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; and the step of boring a center of a workpiece of the metal billet by means of the punch and simultaneously press-working the metal billet. In this method, since the metal billet, where the pressing forward side has a quadrate section whose the diagonal length of the pressing forward side is smaller than the inner diameter of the container, is used, the press pressure can be small by the function for bending the side surface composing the plane on the pressing forward side. Moreover, the metal billet to be used in this method can be worked comparatively easier than the case of a circular sectional shape because the pressing forward side and backward side have angular sectional shape. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section with an outer diameter smaller than an inner diameter of the container and its backward side has a section with an outer diameter substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; and the step of boring a center of a workpiece of the metal billet by means of the punch and simultaneously press-working the metal billet. In the above-mentioned method of manufacturing a container, since the pressing forward side has a circular section whose diagonal length is smaller than the inner diameter of the container, the metal billet is extended to the space between the metal billet and the inner wall of the container for forming. For this reason, the press pressure can be lower than the conventional pressure. Moreover, since the pressing backward side of the metal billet suppresses the upsetting of the pressing forward side, defects on the end portion and the container surface can be also suppressed, and the press pressure can be also reduced. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet having at least one plane on a side surface into a container for forming with a gap from an inner wall; the step of pushing the metal billet so as to extend a pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of pushing a boring punch into the metal billet and bending the plane towards the inner wall so as to hot-dilate the metal billet. This method of manufacturing a container includes the step of extending the pressing backward side of the metal billet to above the body section of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, the metal billet is extended by the function for bending the plane of the metal billet towards the inner wall of the container for forming. With their interaction, this method of manufacturing a container can form a thick container by the lower press pressure than the backward extrusion method or the like. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet, where at least one plane is provided on a side surface and an extended section engaging with an end portion of an inlet of a container for forming is provided on a pressing backward side, into a container for forming with a gap from an inner wall; and the step of pushing a boring punch into the metal billet and bending the plane towards the inner wall so as to hot-dilate the metal billet. Since the metal billet to be used in this method of manufacturing a container is previously provided with the extended section engaging with the end portion of the inlet of the container for forming on the end portion on the pressing forward side. For this reason, since the step of extending the pressing backward side of the metal billet to above the body section of the container before the hot dilation forming, time required for the hot dilation is shortened. As a result, since the forming can be ended until the temperature of the metal billet is lowered, the end portion shape becomes more satisfactory. Moreover, since the extending step can be also omitted, the manufacturing does not require troublesome steps. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet, where at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape, into a container for forming; the step of pushing the metal billet so as to extend a pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of pushing a boring punch into the metal billet so as to hot-dilate the metal billet. This method of manufacturing a container includes the step includes the step of extending the pressing backward side of the metal billet to above the body section of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which at least the section vertical to the axial direction on the pressing forward side is formed into a polygonal shape, is dilated to be formed, the function for bending each side of the polygon towards the inner wall of the container for forming acts. With their interaction, the thick container can be formed with lower press pressure than the backward extrusion method or the like. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet, where at least one plane is provided on at least one of a side surface of a pressing forward side and a side surface of a pressing backward side, into a container for forming; the step of pushing the metal billet so as to extend a pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of pushing a boring punch into the metal billet so as to hot-dilate the metal billet. This method of manufacturing a container includes the step includes the step of extending the pressing backward side of the metal billet to above the body section of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, which is provided at least one plane on at least one side, is dilated to be formed, the function for bending the plane of the metal billet towards the inner wall of the container for forming acts. With the interaction, this method of manufacturing a container can form the thick container with lower press pressure than the backward extrusion method or the like. The method of hot pressing a thick metal-made cylinder or a cylindrical container according to the next invention is characterized in that a metal billet having different diameter sections without joint, where its pressing forward side is composed of a member having a section with an outer diameter smaller than an inner diameter of a container or an outer diameter of a diagonal length or a member having a section with an outer diameter of a diagonal length equal with the inner diameter of the container and its backward side is composed of a member having a section with an outer diameter or a diagonal length equal with the inner diameter of the container, is set into the container for press forming which was heated to a press working temperature, and the metal billet is pushed so that the pressing backward side of the metal billet is extended to an end portion of an inlet of the container for forming, and while a center of a work piece of the metal billet without joint is being bored by a punch, the metal billet is press-worked. This thick metal made cylinder or cylindrical container hot pressing method includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet wit the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal is supplied from the pressing backward side to the pressing forward side and the pressing forward side is pushed to be spread sideways and is simultaneously worked according to the effect of satisfactory plastic working of the steel heated to high temperature, it fills the space of the container to be formed. For this reason, a pressed product having a predetermined shape is manufactured from the metal billet without joint. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section having an outer diameter with a diagonal length of not more than an inner diameter of the container and its backward side has a section having an outer diameter substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; the step of pushing the metal billet so as to extend the pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of boring a center of a workpiece of the metal billet by means of the boring punch and simultaneously press-working the metal billet. This method of manufacturing a drum or a container includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet wit the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which the pressing forward side is a tetragonal section with its diagonal length is smaller than the inner diameter of the container, is used, the metal billet is dilated by the function for bending each side of the tetragonal section. Moreover, the pressing backward side of the metal billet suppresses the upsetting on the pressing forward side. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section having an outer diameter with a diagonal length of smaller than an inner diameter of the container and its backward side has a section having a diagonal length substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; the step of pushing the metal billet so as to extend the pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of boring a center of a workpiece of the metal billet by means of the punch and simultaneously press-working the metal billet. This method of manufacturing a drum or a container includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of hot dilation forming, the constraint of the container on the metal billet becomes stronger, and upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which the pressing forward side is a tetragonal section with a sectional length smaller than the internal diameter of the container, is used, the metal billet is dilated to be formed by the function for bending each side of the tetragonal section. Moreover, the pressing backward side of the metal billet suppresses upsetting on the pressing forward side. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. Further, since the metal billet to be used in this method has the pressing forward side and backward side whose sections are angular shape, the metal billet is worked comparatively easier than the metal billet having one round section. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section with an outer diameter smaller than an inner diameter of the container and its backward side has a section with an outer diameter substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; the step of pushing the metal billet so as to extend the pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of boring a center of a workpiece of the metal billet by means of the punch and simultaneously press-working the metal billet. This method of manufacturing a drum or a container includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of hot dilation forming, the constraint of the container on the metal billet becomes stronger, and upsetting on the pressing forward side can be suppressed. Moreover, since the pressing backward side of the metal billet has a diameter substantially equal with the inner diameter of the container for forming, upsetting on the pressing forward side can be suppressed. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. Further, since the metal billet to be used in this method has the pressing forward side and backward side whose sectional shapes are circular, the metal billet can be worked comparatively easier than the metal billet having different sectional shapes. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container comprises the step of forming the metal billet by means of a forging step and forming at least the pressing forward side of the metal billet into an angular section. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container is characterized in that the forging step includes the step of providing a taper which becomes thinner towards the pressing direction on the pressing forward side of the metal billet. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container is characterized in that the forging step includes the step of providing at least one stepped section so that the pressing forward side of the metal billet becomes thinner gradationally towards the pressing direction. In this method of manufacturing a container, at the final stage of the hot dilation forming, timing at which the metal fills the vicinity of the bottom of the container for forming can be delayed. With this function, since the upsetting phenomenon of the metal billet can be suppressed, an increase in the press pressure at the final stage of the hot dilation forming can be suppressed. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container comprises: the step of providing a drum-shaped member between the metal billet and the bottom of the container for forming and setting the metal billet into the container for forming; the step of pushing the boring punch into the metal billet and hot-dilating the metal billet so as to form the bottomed container where the bottom section and the body section are integral; the step of removing the drum-shaped member from the bottom section of the bottomed container after the forming; and the step of removing a pillar-shaped portion formed on the bottom section of the bottomed container by means of the drum-shaped member. In this manufacturing method, the bottomed container is formed by a drum-shaped member provided at the bottom of the metal billet, and simultaneously a spot facing section is formed on the bottom of the bottomed container. Since the spot facing section has been conventionally provided by cutting, the working requires troublesome steps. However, according to this method, since a pillar-shaped section which remains on the bottom of the container is only removed after the dilation forming, the working does not require less troublesome steps than the conventional method. Here, the drum-shaped member includes polygonal members whose section vertical to the axial direction is triangular, tetragonal or the like, a polygonal member whose angle of the polygon is rounded off and an elliptic member, so the shape is not limited to the drum. The method of manufacturing a container according to the next invention comprises: the step of providing a pillar-shaped member between the metal billet and the bottom of the container for forming and setting the metal billet into the container for forming; the step of pushing the boring punch into the metal billet and hot-dilating the metal billet so as to form the bottomed container where the bottom section and the body section are integral; and the step of removing the pillar-shaped member from the bottom section of the bottomed container after the forming. In this method, the bottomed container is formed by the pillar-shaped member provided at the bottom of the metal billet, and simultaneously the spot facing section is formed on the bottomed container. Since the spot facing section has been conventionally provided by the cutting, the working requires troublesome steps. According to this method, since the spot facing section can be formed simultaneously with the dilation forming, the working requires less troublesome steps than the conventional method. Moreover, since the step of removing the pillar-shaped member can be omitted, troublesome steps are not required for forming the spot facing section in comparison with the method where the spot facing section is formed. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container is characterized in that the body section of the container for forming can move relatively with respect to the bottom section of the container for forming. In this method of manufacturing a container, the metal billet is set into the container where the body section and the bottom section can move relatively so as to be hot-dilated. For this reason, when the metal billet tries to move the body section of the container to the opposite direction to the pressing direction at the time of the hot dilation forming, the body section of the container moves together with the metal billet to the opposite direction to the pressing direction. Namely, since the body of the container and the metal billet to be formed hardly move relatively, an increase in the press pressure can be suppressed at the time of the hot dilation forming. The method of manufacturing a container according to the next invention is characterized in that the body section of the container for forming is divided in an axial direction. In this method of manufacturing a container, since the body section of the container extends along the whole axial direction, even in the case where the metal billet which is long in the axial direction is formed, deformation of the metal billet with respect to the axial direction can be absorbed by the whole container at the time of the hot dilation forming. Therefore, even in the case where the thick container which is long along the axial direction is formed, an increase in the press pressure can be suppressed. The method of manufacturing a container according to the next invention comprises: the upsetting step of placing a pressurizing platform into a ring-shaped die formed with an opening at its inner end portion and putting a metal billet into a mold composed of the die and the pressurizing platform so as to pressurize the metal billet by means of a boring punch; and the metal billet drawing step of supporting the die by means of a drum-shaped spacer and pushing the metal billet by means of the boring punch. At the upsetting step, the metal billet is pressurized by the boring punch so that a material flows to between the opening portion of the die and the boring punch, and the metal billet is deformed into a dish shape. At this time, since the boring punch is held by the metal billet, the boring punch and the die including the metal billet are once allowed to recede. At the drawing step, the spacer is arranged below or above the die so as to support the die, and the boring punch is pushed into so that the metal billet is drawn by the die. As a result, the metal billet is deformed into a cup shape. These two steps are repeated plural times as the need arises so that the metal billet is formed into a final shape. Here, since these steps are heat working, it is necessary to heat the metal billet before the forming. This invention includes the case where the above-mentioned two steps are carried out once so that the metal billet is formed into a final shape. Moreover, the metal billet may be pressurized from above (see FIGS. 21 through 27), or pressurized from below (see FIG. 28). In such a manner the upsetting and the drawing are combined to be used so that an excessive pressure which is required in the backward extrusion forming is not required. For this reason, since the bottomed container can be formed by a normal large-size pressing machine, it is easily manufactured. The method of manufacturing a container according to the next invention comprises: the upsetting preparation step of stacking a plurality of ring-shaped dies formed with an opening on its inner end portion and stacking a plurality of pressurizing platforms respectively in the dies and putting a metal billet into a mold composed of the die and the pressurizing platform; the upsetting step of pressurizing the metal billet from above the mold using a boring punch to be operated by a pressing machine; the receding step of allowing the boring punch and the whole metal billet including and the upper die to recede; the drawing preparation step of removing the used pressurizing platform and placing a drum-shaped spacer onto the next die and placing the receded whole metal billet including the die onto the spacer; the drawing step of pushing the metal billet by means of the boring punch and drawing the metal billet by means of the die; and the repeating step of repeating the above-mentioned steps on the next pressurizing platform and die using a spacer of a length according to deformation of the metal billet. At the upsetting step, the metal billet is pressurized by the boring punch so that a material flows between the opening portion and the boring punch, and the metal billet is deformed with the boring punch being held. At the drawing step, the space is provided below the die and the metal billet is pressurized so that the die is allowed to pass and the metal billet is subject to the draw working. As a result, the metal billet is deformed into a cup shape. Here, the spacer may have any shape as long as it has a drum shape into which the drawn metal billet can be put. The upsetting preparation step through the drawing step are ended, these are further repeated. At this time, as for the die and pressurizing platform, the second ones from the top of the stacked dies and platforms are used. Since the spacer should be longer as the metal billet deforms, they are prepared for each step. In this manufacturing method, since the upsetting and the drawing are combined to be used, the pressurizing force can be suppressed small in comparison with the backward extrusion forming. For this reason, the container can be formed by a normal large-size pressing machine. The present invention will be detailed below with reference to the diagrams. However, the present invention is not limited to the embodiments described below. Moreover, a manufacturing method of a container or a drum of the present invention is not limited to methods to be disclosed below. Further, components of the following embodiments include ones which can be assumed by person skilled in the art. FIGS. 1(a) and 1(b) are explanatory diagrams showing one example of a bottomed container according to the first embodiment. FIGS. 2(a) through 2(e), 2(a′) through 2(e′) and 2(e″) are explanatory diagrams showing the steps of manufacturing the bottomed container 1 shown in FIGS. 1(a) and 1(b). FIGS. 3(a) through 3(c) are perspective views showing one example of a metal billet 200 to be used in the first embodiment. As shown in FIGS. 1(a), a bottom section 1b of the bottomed container 1 according to the first embodiment is formed integrally with a body section la, and as is clear from FIG. 1(b), a sectional form of the bottomed container 1 of the first embodiment is circular. In this manufacturing method, the bottomed container 1 is manufactured by a punch according to hot dilation forming. Firstly there will be explained below the metal billet to be used in the manufacturing steps. The metal billet 200 is manufactured by cutting or free forging a foundry molding lump or a metal lump of molten metal before the hot dilation forming step, explained next. At the forging step, it is desirable that at least forward side of the metal billet in a pressing direction is formed into an angular section. As a result, an effect of the hot dilation forming can be utilized more effectively. Here, in the first embodiment, the metal lump is freely forging so that the integral metal billet 200 without a joint is manufactured, but a metal billet which can be used in the present invention is not limited to this. As shown in FIGS. 3(a) through 3(c), a sectional form of the metal billet 200 used in the present embodiment is circular on a backward side in the pressing direction (hereinafter, pressing backward side) 200a, and is square on a forward side in the pressing direction (hereinafter, pressing forward side) 200b. An outer diameter of the pressing backward side 200a is larger than a diagonal length of the pressing forward side 200b, and the outer diameter of the pressing backward side 200a is substantially equal with an inner diameter of a container 300, and the diagonal length of the pressing forward side 200b is shorter than the inner diameter of the body section 300 of the container for forming (FIG. 3(b)). Here, the inner diameter of the body section 300 of the container for forming is represented by a broken line. Further in this example, a section 970 of the metal billet 200 vertical to an axial direction on the pressing forward side 200b includes a projection image 920 of a section of a boring punch 410 vertical to the axial direction (FIGS. 3(b) and 3(e)). Here, the sectional form, dimension, outer form and the like of the metal billet 200 are not limited to this example. Another example of the billet which can be applied to the present invention will be explained later. Next, there will be explained below the hot dilation forming steps. Prior to the hot dilation forming, the metal billet 200 is heated to a temperature where it is easily hot-dilated by a heating furnace (not shown). Since the heating temperature is determined by a material and the like of the metal billet 200, it is not determined ultimately. Here, as for a carbon steel material which is used for a body of the cask for containing and transporting and temporarily storing used nuclear fuel, it is desirable that the heating temperature is set to 1100° C. to 1300° C. When the heating temperature exceeds this range, crystal grains become coarse and oxidation and decarbonization occur on the surface, and the material is embrittled and is easily cracked. In the case of the carbon steel, as a percentage of the carbon is higher, the heating temperature becomes lower with in the range. The metal billet 200, which is heated to the temperature at which it is easily hot-dilated by an electric furnace or the like, is upset into a body section 300 of the container for forming as shown in FIG. 2(a). The metal billet 200 which is upset into the body section 300 of the container for forming is upset by a large punch 400 having an outer diameter substantially equal with or larger than the outer diameter of the metal billet 200, and an extended section 201 extended to the container 300 is provided on the pressing backward side 200a of the metal billet 200 (see FIG. 2(b)). The extended section 201 is provided on an end portion of the metal billet 200 so that a constraint force in the axial direction of the metal billet 200 can be heightened at the hot dilation forming step by means of the punch. Since the upsetting phenomenon of the metal billet 200 is suppressed by this function and metal flow towards the pressing direction and the opposite direction can be reduced, a rise in the press pressure can be suppressed. At the same time, the form of the end surface of the bottomed container after the forming can be satisfactory. Here, even if the extended section 201 is not provided on the metal billet 200, the upsetting phenomenon can be suppressed by a friction force which is generated between an inner wall of the body section 300 of the container for forming and a side surface of the pressing backward side 200a of the metal billet 200. However, in order to lower the press pressure and obtain more organized end surface form, it is desirable that the extended section 201 is provided. After the extended section 201 is formed on the end portion of the metal billet 200, the sequence goes to the hot dilation forming step by means of a boring punch 410. In order to bore a hole at the center of the metal billet 200, firstly the punch 410 is placed on the center of the end surface of the metal billet 200 by a locating guide 310 attached to the body section 300 of the container for forming (see FIG. 2(c)). Next, the boring punch 410 is pushed by a pressing machine (not shown) so that the metal billet 200 is hot-dilated. When the boring punch 410 is pushed into the metal billet 200 by the pressing machine, the extended section 201 of the metal billet 200 is engaged with the upper end portion of the body section 300 of the container for forming, and the upsetting phenomenon of the metal billet 200 is suppressed. Moreover, the metal of the thick portion on the pressing backward side 200a of the metal billet 200 is deformed so as to fill the container 300 (see FIG. 2(d)). As a result, since the metal of the pressing backward side 200a is pushed against the inner wall of the body section 300 of the container for forming, the upsetting phenomenon of the metal billet 200 can be suppressed also by this function. As a result, the rise in the press pressure is suppressed, and the end surface form of the bottomed container after the forming can be satisfactory. When the boring punch 410 is pushed into the metal billet 200, the metal which exists just below the boring punch 410 becomes a hemispheric metal lump and moves to the pressing forward side 200b together with the boring punch 410. Due to this phenomenon, a cross section of the pressing backward side 200a of the metal billet 200 vertical to the axial direction should be larger than that of the pressing forward side 200b. Further, when the boring punch 410 is pushed into the metal billet 200, the metal on the pressing forward side 200b of the metal billet 200 is deformed so as to be spread towards the inner wall of the container 300 due to excellent property of plastic deformation owned by the steel heated to high temperature and a phenomenon that the metal lump is supplied from the pressing backward side 200a. Namely, it is the hot dilation forming. When the boring punch 410 is pushed to a predetermined depth previously set, the hot dilation forming is ended there (see FIG. 2(e)). There will be explained below types of the metal billet 200 applicable to the manufacturing method of the present invention. As shown in FIG. 3(a), at least the metal billet which is formed so that a sectional form vertical to the axial direction on the pressing forward side is polygonal can be applied to the manufacturing method of the present invention. When such a metal billet is used, since the metal billet is deformed so as to be spread towards the inner wall of the container for forming in the hot dilation forming, the press pressure can be suppressed in comparison with backward extrusion or the like. Here, the section of the metal billet 200 vertical to the axial direction shown in FIG. 3(a) is tetragonal, but the section is not limited to this. Further, the metal billet applicable to the manufacturing method of the present invention may have the following relationship. Namely, the section vertical to the axial direction on the pressing backward side has a relationship that it contains a projection image of a section vertical to the axial direction on the pressing forward side. Moreover, a projection image of the section vertical to the axial direction on the pressing backward side has a relationship that it is contained in the inner side of the section of the container for forming vertical to the axial direction. Here, the form of the inner side of the section of the container for forming vertical to the axial direction may be identical to the projection image of the section vertical to the axial direction on the pressing backward side. It is desirable that the metal billet which satisfies these relationships is used when a large bottomed container with large thickness is formed. The projection image will be explained below with reference to FIGS. 3(c) and 3(d). Here, the projection image is represented by a broken line, and the section is represented by a continuous line. FIG. 3(c) shows a state that a section 950 vertical to the axial direction on the pressing backward side of the metal billet contains a projection image 900 of the section vertical to the axial direction on the pressing forward side. FIG. 3(d) shows a state that a projection image 910 of the section vertical to the axial direction on the pressing backward side is contained in an inner side 960 of the section of the container for forming vertical to the axial direction. Moreover, FIG. 3(e) shows a state of the boring punch having a relationship that a projection image 920 of the section vertical to the axial direction is contained in a section 970 vertical to the axial direction on the pressing forward side of the metal billet. Here, the projection image 920 of the section vertical to the axial direction in FIG. 3(e) is a projection image of the boring punch. “Containing” in the present invention means that a whole portion surrounded by the broken line representing the projection image exists on the inside of a portion surrounded by the continuous line representing the section. In the case where even a part of the portion surrounded by the broken line exist on the outside of the portion surrounded by the continuous line, this is not included in the concept of “containing”. Moreover, in the case where the form of the section is identical to the projection image, this is not included in the concept of “containing”. It is desirable that the projection image of the boring punch is contained in the section vertical to the axial direction on the pressing forward side of the metal billet (see FIG. 3(e)). However, the projection image of the boring punch vertical to the axial direction may be identical to the section of the metal billet vertical to the axial direction on the pressing forward side. Moreover, the section of the boring punch vertical to the axial direction may contain the projection image of the section of the metal billet vertical to the axial direction on the pressing forward side. However, in the case of a relationship that the section of the boring punch vertical to the axial direction contains the projection image of the section vertical to the axial direction on the pressing forward side of the metal billet, when the cross section of the boring punch becomes large, a thickness of the body section of the container to be formed becomes thin. For this reason, the body section of the container easily ruptures during the hot dilation forming. Therefore, in the case of the relationship that the section of the boring punch vertical to the axial direction contains the projection image of the section of the metal billet vertical to the axial direction on the pressing forward side, it is necessary to suppress the cross section of the boring punch within a range that the body section of the container does not rapture. FIGS. 4(a) through 4(e) are explanatory diagrams showing examples of another metal billet according to the first embodiment. As is clear from FIGS. 4(a) through 4(d), these examples satisfy a condition that the section 950 vertical to the axial direction on the pressing backward side 200a of the metal billet 200 contains the projection image 900 of the section vertical to the axial direction of the pressing forward side 200b (FIG. 3(c)). In the case where the section of the pressing forward side 200b is tetragonal as shown in FIGS. 4(a) and 4(b), the metal billet 200 is deformed by mainly a force which directs towards an outer side in the radial direction of the body section 300 of the container, namely, a force which bends the side surface of the metal billet having a plane to the inner wall side of the body section 300 of the container for forming at the time of the hot dilation forming. FIG. 5 is a conceptual diagram showing a state of the deformation. A broken line in the diagram shows a process that the metal billet 200 is expanded towards the inner wall of the body section 300 of the container for forming. In the case where section vertical to the axial direction is tetragonal, the boring punch is pushed into so that a force F, which directs from the center of the metal billet 200 to the outside in the radial direction of the body section 300 of the container for forming, acts. Since the force F bends the side surface of the metal billet 200 towards the inner wall of the container for forming, the metal billet 200 is dilated to the inner wall of the container for forming by this bending function. Particularly this bending function acts effectively when the metal billet having a plane on the side surface is set into the container for forming having a circular inside of the section. Therefore, when the metal billet is dilated by the bending function, the hot dilation forming requires less press pressure, and defects such as a crack generated at the time of the forming can be suppressed. Even if the metal billet whose section vertical to the axial direction has a tetragonal shape is hot-dilated by using the boring punch whose section vertical to the axial direction has a tetragonal shape, the metal billet is dilated by the bending function. Therefore, also in this case, the hot dilation forming requires the less press pressure, and the defects such as a crack generated at the time of the forming can be suppressed. In the case where the section vertical to the axial direction on the pressing forward side 200b is circular as shown in FIG. 4(c), the above-mentioned effect is slightly reduced, but the metal billet 200 has spaces between the pressing forward side 200b and the inner wall of the body section 300 of the container. Therefore, since the metal on the pressing forward side is dilated in the spaces at the time of the hot dilation forming, the dilating deformation of the metal billet is not constrained by the body section 300 of the container for forming, and the press pressure can be suppressed small. Moreover, in this case, when the metal billet is hot-dilated by using the boring punch whose section vertical to the axial direction has a tetragonal shape, the bending function acts. For this reason, the press pressure is less than the case where the metal billet is hot-dilated by using the boring punch having a circular section vertical to the axial direction. As shown in FIGS. 4(a), 4(c) and 4(d), in the case where the section of the pressing backward side 200a vertical to the axial direction is circular and its diameter is substantially equal with the diameter of the body section 300 of the container, the extended section 201 formed on the body section 300 of the container for forming (see FIG. 3(b)) can be formed uniformly. Therefore, at the time of hot dilation forming, since the upsetting phenomenon of the metal billet 200 can be suppressed effectively by the extended section 201, the press pressure can be suppressed low, and the form of the end surface of the container after the forming can be satisfactory. As shown in FIG. 4(b), in the case where the section of the pressing backward side 200a is tetragonal and its diagonal length is substantially equal with the diameter of the body section 300 of the container for forming, the extended section 201 is formed partially on the upper end of the body section 300 of the container. For this reason, in comparison with the case where the extended section 201 is not provided, the upsetting phenomenon of the metal billet 200 can be suppressed more effectively, but in comparison with the case where the extended section 201 is formed along the whole periphery of the upper end of the body section of the container for forming, this effect is slightly reduced. Further, the metal billet, which is formed so that the form of the section vertical to the axial direction of the pressing forward side 200b or the pressing backward side 200a of the metal billet 200 is pentagonal (or more) or triangular, can also be applied to the manufacturing method according to the first embodiment. Moreover, as shown in FIGS. 4(d), when the side surface of the pressing forward side 200b has at least one or more planes, since this portion is hot-dilated by the bending, the press pressure can be suppressed low. In addition, when a number of planes on the side surfaces of the metal billet 200 is not less than three, namely, the section of the metal billet 200 vertical to the axial direction is triangular or more, the press pressure can be suppressed lower. However, when a number of planes increases, namely, a number of angles of the polygonal section vertical to the axial direction increases, since the form of the polygonal section closes to a circle, the effect which suppresses the press pressure low is reduced. Therefore, it is desirable that a number of the planes of the side surface on the pressing forward side 200b is selected within the range that the press pressure can be suppressed low. Moreover, even in the case where the form of the section of the metal billet 200 vertical to the axial direction is of substantially drum as shown in FIG. 4(e), it is included in the metal billet of the present invention as long as the planes are provided on the side surface of the metal billet 200. FIGS. 6(a) through 6(c) are perspective views showing another metal billet applicable to the first embodiment. The metal billet 200 is provided with a stepped section on the pressing forward side 200b so that it becomes thinner gradationally towards the pressing direction. As a result, at the time of the hot dilation forming, the timing that the metal of the pressing forward side 200b fills the vicinity of the bottom of the body section 300 of the container for forming can be slow. Therefore, particularly at the final state of the hot dilation forming, a rise in the press pressure can be suppressed. Moreover, since slight flow to the pressing direction is formed by this stepped section, a forging degree of a molded form can be heightened, and shortage of forging materials can be prevented. Further, since a change of the section is gradate, the manufacturing becomes easier than the case where a taper, mentioned later, is provided. Here, a number of the stepped sections is not limited to the above example, but the number can be suitably increased or decreased according to a pressing condition and the like. In addition, as shown in FIG. 6(b), even if the pressing forward side 200b is provided with a taper which becomes thinner towards the pressing direction, the function and effect which are similar to the case where the stepped section is provided on the pressing forward side 200b can be obtained. Further, as shown in FIG. 6(c), when the metal billet 200 is manufactured, the extended section 201 which is engaged with the upper end portion of the container 300 for forming may be previously provided on the metal pressing backward side 200a. As a result, the step of forming the extended section 201, which is extended to the body section 300 of the container, on the pressing backward side 200a of the metal billet 200 of is not required before the hot dilation forming, and the container manufacturing steps can be simplified. FIGS. 7(a) through 7(c) are explanatory diagrams showing another metal billet applicable to the first embodiment. As shown in the diagrams, the metal billet 200 is characterized in that the section vertical to the axial direction is constant along the pressing direction and the extended section 201 which is engaged with the upper end portion of the container 300 for forming is provided on its one end. Even in the case where such a metal billet 200 is used, since the upsetting phenomenon of the metal billet can be suppressed by the extended section 201 at the time of hot dilation forming, a rise in the press pressure can be suppressed. Here, the sectional form of the metal billet 200 is not limited to a tetragonal form, and it may be polygonal, and the side surface of the metal billet 200 may provided with at least one plane. Moreover, the extended section 201 may be previously provided on the metal billet 200, or the metal billet 200 is set into the container 300 for forming and the extended section 201 may be provided. When the metal billet 200 is dilated to be deformed so as to be spread towards the inner wall of the container 300 (see FIGS. 2(d) and 2(e)), a friction force is generated between the metal of the metal billet 200 and the inner wall of the container 300. This friction force is caused by transfer of the metal of the pressing forward side 200b to the opposite direction to the pressing direction along the inner wall of the body section 300 of the container. Here, at the middle process of the hot dilation forming in the manufacturing method according to the first embodiment, this friction force is hardly generated. However, at the final stage of the hot dilation forming, since the metal fills the lower section of the container for forming, this friction force is generated. Due to this friction force, the body section 300 of the container for forming tries to move to the opposite direction to the pushing direction of the punch 410. When the body section 300 of the container for forming and the bottom section 301 of the container for forming are fixed, the metal of the pressing forward side moves against the friction force, and at the final stage of the hot dilation forming, excessive load is required. In order to solve this problem, in the first embodiment, the body section 300 of the container for forming and the bottom section 301 of the container for forming can move relatively. With such a structure, when the body section 300 of the container for forming tries to move to the opposite direction to the pushing direction of the punch 410 due to the friction force, the body section 300 of the container for forming also moves to the opposite direction to the pushing direction of the punch 410 together with the metal billet 200 to be formed (FIG. 2(e)). Namely, since the body section 300 of the container for forming and the metal billet 200 to be formed seldom move relatively, an increase in the load can be suppressed at the final stage of the hot dilation forming. In the present embodiment, the body section 300 of the container for forming and the bottom section 301 of the container can move relatively, and also the body section 300 of the container for forming is divided so as to capable of moving along the whole metal billet 200. As a result, even in the case where the bottomed container which is long in the axial direction is formed, an increase in the load can be suppressed at the final stage of the hot dilation forming. FIGS. 8(a) and 8(b) are sectional views showing divided sections of the body section 300 of the container for forming to be used in the first embodiment. The divided sections of the body section 300 of the container for forming may be constituted so as to be overlapped with each other as shown in FIG. 8(a) and the body sections 300a and 300b of the container for forming may move relatively at the time of hot dilation forming. Moreover, as shown in FIG. 8(b), a concave section is formed on the one body section 300a of the container for forming, and a convex section is provided on the other body section 300b of the container for forming so as to be combined. As a result, the body sections 300a and 300b of the container for forming may move relatively at the time of hot dilation forming. In the present embodiment, the body section 300 of the container for forming is divided into two, but a number of divisions can be changed suitably according to the height of the metal billet 200. Moreover, the body section 300 of the container for forming is only divided, or the body section 300 of the container is not divided and the body section 300 and the bottom section 301 of the container for forming can only move relatively. As a result, an increase in the load can be suppressed at the time of hot dilation forming. When the punch 410 is pushed into a set predetermined depth, the hot dilation forming is ended there (FIG. 2(e)). As shown in FIG. 2(e″), a cylindrical spacer 302 is placed instead of the bottom section 301 of the container for forming and the bottom section of the metal billet 200 is punched so that a drum can be formed by this manufacturing method. When the thick container whose length in the axial direction reaches a several meters is manufactured by the manufacturing method, since the press pressure can be reduced to a several part of the conventional press pressure, the container can be manufactured by the conventional facilities. Moreover, since the bottomed container in which the bottom section and the body section are integral with each other can be manufactured by one-time working, the manufacturing requires less steps, and this method is suitable also for mass production. The metal billet 200 which was subject to the hot dilation forming is cooled to normal temperature by natural cooling, forced cooling or control cooling. In order that the sectional form is adjusted or the external shape or the internal shape of the formed container is finished to a predetermined dimension, the container may be subject to the cutting or the like. Next, a result of the concrete example that an integrally cylindrical bottomed container is manufactured by the above-mentioned method will be shown. As a comparative example, a result of manufacturing the integrally cylindrical bottomed container by applying the Erhardt (elrhardt or Ehrhard) boring method which has been conventionally used is also shown. In both the concrete example and the comparative example, the cylindrical container was formed by using a cylindrical container having an inner diameter of 943 mm. As a material of the cylindrical container, carbon steel in which a percentage of carbon (C) is 0.1% was used, but stainless steel maybe used. Firstly after the metal billet of carbon steel was heated to 1250° C., it was forged into a metal billet in which its section in the axial direction has a T-shaped different diameter section by the free forging method. The pressing forward side of this metal billet has a quadrate section whose diagonal length is 875 mm smaller than an inner diameter of the container for forming, and a length of the axial direction is 1896 mm. The pressing backward side has a circular section having an outer diameter of 928 mm substantially equal with the inner diameter of the container for forming, and a length of the axial direction is 574 mm. After the metal billet was again heated to 1250° C., it is set into the container for forming, and the center of the workpiece is hot-dilated by the punch so that an elongate cup-shaped cylindrical container having length of 2420 mm and thickness of 165 mm was formed. Meanwhile, in the comparative example, the carbon steel which is the same as the concrete example was used as the material of the cylindrical container, and after the metal billet was heated to 1250° C., it was forged into an angular metal billet having uniform sectional form along the whole length of the axial direction was manufactured by the free forging method. The sectional form of this metal billet is quadrate, and a length of its diagonal is 928 mm which is substantially equal with the inner diameter of the container for forming. Moreover, a length of the axial direction is 2470 mm. After the metal billet was again heated to 1250° C., it was hot-dilated so that the cylindrical container whose size is the same as the concrete example was manufactured. Table 1 shows evaluated results of the forming load and the end surface form in both the pressing methods. As is clear from the comparison, according to the manufacturing method of the present invention, the press forming load is smaller and the product yield is higher than the conventional manufacturing method. Moreover, since defective portions of the end surface form which are seen in the conventional manufacturing method seldom exist, the product can be finished by simple working after the hot dilation forming. TABLE 1EVALUATION RESULTS OF BOTH METHODSPress formingMethodloadSectional formYieldMethod of the2400 tonSatisfactory70%Present InventionMethod of the3980 tonDefective portions60%Comparative Exampleof form are generated There will be explained below an example of cutting an outer side and an inner side of the bottomed container formed to be used as the radioactive substance container such as a cask or a canister. FIG. 9 is a schematic diagram showing an apparatus for cutting the outer side of the hot-dilated bottomed container. The bottomed container 1 is placed on a rotation supporting platform 154 having a roller and can be rotated freely in a circumferential direction. A fixing table 141 is provided on the outer side of the bottomed container 1, and a movable table 142 which slides on the fixing table in the axial direction of the bottomed container 1 is provided. A cutting tool 160 is attached to the movable table 142, and the cutting tool 160 cuts the outer circumference of the bottomed container 1. A roller 161 attached to the rotation supporting platform 154 is connected to a motor 162. The rotation of the motor 161 is transmitted to the bottomed container 1 via the roller 161 so as to rotate the bottomed container 1. When the motor 161 rotates and the bottomed container 1 starts to rotate to a direction of an arrow in the diagram, a servo motor 157 and a ball screw 158 which are provided to the end portion of the fixing table 141 moves the movable table 142 to the axial direction of the bottomed container 1, and the cutting tool 160 attached to the movable table 142 cuts the outer circumference of the bottomed container 1. Moreover, when the outer circumference is cut by a face mill or the like so that a plane is provided, not only the bottomed container having a circular section vertical to the axial direction but also the polygonal bottomed container can be formed. There will be explained below an example that the inner side of the formed bottomed container is cut. FIG. 10 is a schematic diagram showing an apparatus for working the inner side of the bottomed container. A machining apparatus 140 is composed of a fixing table 141 which pierces through a body 101 and is placed and fixed inside the bottomed container 1, a movable table 142 which slides on the fixing table 141 in the axial direction, a saddle 143 which is located and fixed onto the movable table 142, a spindle unit 146 which is composed of spindles 144 and a driving motor 145 provided on the saddle 143, and a face mill 147 provided on a spindle axis. Moreover, a reactive force receiver 148 which is formed with a contact portion according to the internal shape of the bottomed container 1 is provided on the spindle unit 146. The reactive force receiver 148 is detachable and slides along a dovetail groove (not shown) in a direction of an arrow in the diagram. Moreover, the reactive force receiver 148 has a clamp device 149 for the spindle unit 146, and is fixed to a predetermined position. Further, a plurality of clamp devices 150 are attached into a lower groove of the fixing table 141. The clamp devices 150 is composed of a hydraulic cylinder 151, a wedge-shaped moving block 152 which is provided on an axis of the hydraulic cylinder 151, and a fixing block 153 whose tilt surface comes in contact with the moving block 152. The shaded portions in the diagram are attached to the groove inner surface of the fixing table 141. When the axis of the hydraulic cylinder 151 is driven, the moving block 152 comes in contact with the fixing block 153, and the moving block 152 moves slightly downward due to the wedge effect (represented by dotted line). As a result, since the lower surface of the moving block 152 is pushed against the inner surface of a cavity 102, the fixing table 141 can be fixed inside the bottomed container 1. In addition, the bottomed container 1 is placed on a rotation supporting platform 154 composed of a roller, and is freely rotated in a radial direction. Moreover, a spacer 155 is attached between the spindle unit 146 and the saddle 143 so that a height of the tool 147 on the fixing table 141 can be adjusted. A thickness of the spacer 155 is the same as a dimension of one side of an angular pipe composing the basket. A servo motor 156 provided on the movable table 142 is driven so that the saddle 143 moves in the radial direction of the body 101. The movement of the movable table 142 is controlled by a servo motor 157 and a ball screw 158 provided on the end portion of the fixing table 141. Here, as the working proceeds, the shape of the inside of the bottomed container 1 changes, and thus it is necessary that the reactive force receiver 148 and the moving block 152 of the clamp devices 150 are replaced by ones having suitable shape. FIGS. 11(a) through 11(d) are explanatory diagrams showing one example of the method of working the inner side of the bottomed container 1. Firstly the fixing table 141 is fixed to a predetermined position inside the bottomed container 1 by the clamp devices 150 and the reactive force receiver 148. As shown in FIG. 11(a), the spindle unit 146 is moved along the fixing table (not shown) at a predetermined cutting speed, and the inner side of the bottomed container 1 is cut by the tool 147. When the cutting in this position is finished, the clamp devices 150 are removed so that the fixing table 141 is released. As shown in FIG. 11(b), the body 101 is rotated through an angle of 90° on the rotation supporting platform 154, and the fixing table 141 is fixed by the clamp devices 150. Similarly the cutting is carried out by the tool 147. Thereafter, the similar steps are repeated twice. Next, the spindle unit 146 is rotated through an angle of 180°, and as shown in FIG. 11(c), the inside of the cavity 102 is cut successively. Also in this case, similarly to the above-mentioned case, while the body 101 is being rotated through an angle of 90°, the working is repeated. Thereafter, as shown in FIG. 11(d), the spacer 155 is attached to the spindle unit 146 so that the height of the spindle unit is heightened. The tool 147 is fed in the axial direction in this position, and the inside of the bottomed container 1 is cut. This is repeated while the body 101 is being rotated through an angle of 90° so that a shape which is required for inserting an angular pipe (not shown) for containing used nuclear fuel is inserted is approximately finished. Here, the working can be carried out not only by a special machine but also by general horizontal boring machine and vertical boring machine. The above explanation referred to the example that the bottomed container 1 is laid and the outer side and the inner side are cut. However, the bottomed container 1 stands upright and the outer side and the inner side of the container may be cut by a working machine, explained below. Concretely, this working machine has a rotation table where a bottomed container to be worked is placed and the container is rotated, a crane for placing the bottomed container on the rotation table and moving it from the rotation table after the working, a movable table placed on a base, a saddle which is placed on the movable table and can move in a movable direction and a right angle direction of the movable table, a column which is placed on the saddle and supports an arm moving up and down, and the arm which is mounted to the column to be movable up and down in a vertical direction and has an attachment with a tool on its forward end and moves up and down with respect to a workpiece so as to work the workpiece. The attachment to be attached to the forward end of the arm is replaced so that this working machine can cope with various workings such as milling and boring. When the bottomed container is started to be worked, the bottomed container as a workpiece is placed on the rotation table by the crane, it is centered and is fixed onto the rotation table. When the outside of the bottomed container is cut into a cylindrical shape, a cutting tool is mounted to the attachment attached to the arm, and while the rotation table is being rotated at a predetermined number of revolution, the bottomed container is cut by this cutting tool. Here, since the column to which the arm is attached is placed on the saddle, the saddle and the movable table are moved so that the arm can be moved. Therefore, the arm is moved to an arbitrary position, and an arbitrary cutting amount can be set. Moreover, since the arm can be moved up and down, the arm is moved along the axial direction of the bottomed container so that the whole side surface of the bottomed container can be cut. Since these movements require accuracy, it is desirable that the rotating movement of the servo motor or the stepping motor is converted into a linear motion by a ball screw or the likes and the movable table or the like is moved. When the outer circumference of the bottomed container having circular sectional shape in a radial direction is cut and the container is desired to be worked into a polygonal shape such as an octagonal prism, an attachment for a face mill is attached to the arm, and the side surface of the bottomed container is cut by face milling cutting. When the arm is moved up and down and one surface of the bottomed container in the whole axial direction is cut, the rotation table is rotated through an angle of 45°, and a next side surface is worked. When this work is repeated eight times, the bottomed container whose external shape of the section in the radial section is octagonal can be manufactured. In such a manner, the bottomed container having an arbitrary polygonal section can be manufactured. When the bottomed container is used as a cask for containing used nuclear fuel, it is desirable that the internal shape of the bottomed container matches with at least one part of the outer periphery of a basket for containing used nuclear fuel aggregate. This is because the basket can be easily inserted and fixed into the container. Such a sectional shape is shown in FIG. 15(d), for example. In order to form the inner side of the bottomed container into such a shape, the attachment to be mounted to the arm is replaced by one for end mill, for example, and an angular portion inside the section is worked into a stepped form. Firstly, the saddle and the movable table are moved, the arm is moved to above the bottomed container placed on the rotation table. Next, the arm is lowered and the attachment with the end mill is inserted into the bottomed container so that the end mill is located. Thereafter, the inner side of the bottomed container is cut with a predetermined cutting amount. The cutting is given to the bottomed container some times and its inner side is cut, and when a predetermined shape is obtained, the cutting at the first stage is finished. When the necessary stepped form is obtained at one angular portion, the rotation table is rotated through an angle of 90°, and a next angular portion is worked. When this work is repeated four times, the bottomed container for the cask having the sectional shape in the radial direction as shown in FIG. 15(d) can be manufactured. Here, when the inner side of the bottomed container is cut, since there is not space for cutting chips and cutting oil, the cutting cannot be carried out during the working. For this reason, it is desirable that the cutting chips and the like are removed from the inner side of the on-working bottomed container by discharging means such as a vacuum pump. When the bottomed container is cut with it is in the upright state, the process for the cutting chips and the like is required unlike the case of the laid state, but an influence of deformation due to gravity can be reduced. When this working machine is inverted to be constituted, since the bottomed container is worked with its opening faces downward, when the inner side of the bottomed container is cut, the cutting chips and the like can be easily discharged. There will be explained below an example that the bottomed container manufactured by the method of the present invention is applied to the cask as the used nuclear fuel container. FIGS. 12(a) and 12(b) show the cask according to the embodiment of the invention: FIG. 12(a) is an axially sectional view; and FIG. 12(b) is a radially sectional view. The cask 100 is composed of a bottomed container 1 having a basket 3 in its inner side, a neutron shielding material 2 such as resin or silicone rubber provided on the outer side of the bottomed container 1, and an outer drum 4 to be the outer surface of the cask 100. The bottomed container 1 is formed with the inner side and the outer side by the above-mentioned cutting. A primary cover 5 and a secondary cover 6 are provided on the upper section of the bottomed container 1, and resin 7 for shielding neutron is sealed into the secondary cover 6. Moreover, the bottomed container 1 has a cylindrical shape with a bottom formed by punching draw working, and is made of carbon steel or stainless steel having γ rays shielding function. The neutron shielding material 2 is a polymeric material containing a lot of hydrogen and has a neutron shielding function. Moreover, a shielding body 9 into which a neutron shielding material 8 such as resin or silicone rubber is sealed is mounted to the lower section of the bottomed container 1. The basket 3 is constituted so that cells for containing used nuclear fuel aggregate (no shown) are arranged in a lattice form, and is composed of a composite martial of boron and aluminum. In addition, in order to secure sealing performance for a pressure resistant container, a metal gasket is provided between the primary cover 5, the secondary cover 6 and the bottomed container 1. A plurality of internal fins 10 made of copper for heat conduction are welded between the bottomed container land the outer drum 4, and the neutron shielding material 2 is poured into spaces formed by the internal fins 10 in a flow state and is heated to be solidified. The primary cover 5 and the secondary cover 6 are made of carbon steel or stainless steel having γ rays shielding function. Since the cask 100 uses the bottomed container 1, in comparison with the conventional case where a bottom plate is welded, the manufacturing steps can be reduced. Moreover, since a bottom plate was conventionally welded to the bottomed container, the sealing property of the welded portion depends upon the quality of the welding, but a problem of the sealing property on this welded portion in the bottomed container 1 is extremely low. Here, in order to realize the cask 100 of the present invention, shape and material of the basket 3, a charging state of the neutron shielding material 2, and a number and positions of the internal fins 10 in the bottomed container 1 are not limited to the example shown in FIGS. 12(a) and 12(b). The method of manufacturing the bottomed container according to the first embodiment is suitable for manufacturing a so-called thick container having a thickness with respect to the diameter of the cylinder. Further, the manufacturing method of the present invention is suitable particularly for the case where a container, in which a ratio of the axial length to the inner diameter is 1:1 or more, is manufactured in the thick containers. When the ratio exceeds the above-mentioned numerical value, as the forming proceeds in general hot working, the press pressure increases, but in the manufacturing method according to the embodiment the press pressure does not greatly increase at the beginning and the end of the working. Concretely, the manufacturing method according to the embodiment is particularly suitable for the case where a cask or the like as a large bottomed container, in which the thickness is thick with respect to the diameter and the axial length reaches a several meters, is manufactured. In order to manufacture such a large and thick bottomed container with its bottom integral, a several ten-thousand ton scale pressing machine was conventionally required. However, when such a so-called thick and large bottomed container is manufactured by the manufacturing method of the present invention, the press pressure of only about ten-thousand ton is required. For this reason, even if a pressing machine of several ten-thousand ton is not used, the bottomed container can be formed by an existing large pressing machine. Moreover, since the formed container has excellent sectional form and defects do not occur on the surface and the inner side, adjustment after the forming is seldom required. Here, the manufacturing method of the present invention is not limited to such a thick and large bottomed container, and a canister as a radioactive substance container with comparatively thin thickness can be manufactured by this method. In addition, with the manufacturing method of the present invention, a cylinder for a large pressing machine, a container for chemical plant, a reactor container for petroleum refining plant, an ammonia synthetic cell, a heat exchange container, a pressure container such as a boiler, a casing for a large rotational equipment for containing a hydroelectric water turbine, a body of submarine and ship can be manufactured. Moreover, the material which can be used in the method of the present invention is not limited to carbon steel, and the material includes iron materials such as stainless steel, low alloy steel and the like, nonferrous metal such as nickel alloy, aluminum metal, copper metal and magnesium metal. FIG. 13 is a perspective view showing the bottomed container according to a second embodiment of the present invention. The bottomed container 1 shown in FIG. 13 is characterized in that its external shape and internal shape are octagonal. Moreover, at least one of the external shape and the internal shape of the container may be octagonal. Since a basket for supporting a fuel bar aggregate is housed in the bottomed container of the cask as a radioactive substance container, it is preferable that the internal shape of the bottomed container is formed into a shape which matches with the basket particularly in a cask. Therefore, the internal shape of the cask is desirably octagonal instead of circular. Moreover, in the case where the internal shape of the cask is octagonal, since it is advantageous to a dimension and a weight that the thickness of the cask body is uniform as much as possible, it is desirable that the external shape of the cask body is also octagonal. This bottomed container can cope with such requirements. FIGS. 14(a) and 14(b) are sectional view showing the container body and a punch for manufacturing the bottomed container according to the second embodiment. A sectional shape of the inside of a body section 300 of the container for forming is substantially octagonal, and the external shape of the boring punch 410 is also substantially octagonal. When the body section 300 of the container for forming and the boring punch 410 are used in the hot dilation forming described in the first embodiment, an increase in the load at the time of the hot dilation forming is suppressed, and defects on a surface of a workpiece is suppressed after the forming, and the bottomed container having excellent end surface shape and a polygonal section can be manufactured. FIGS. 15(a) through 15(d) are sectional views vertical to the axial direction showing examples of the bottomed container capable of being formed by the manufacturing method according to the second embodiment. The section of the body section of the container for forming and the external shape of the boring punch are changed suitably so that the bottomed container having such a sectional shape can be formed. It is desirable that particularly the boring punch 410 for forming the internal shape of the cask is changed suitable according to the shape of the basket for containing used nuclear fuel. When the boring punch whose external shape is the internal sectional shape shown in FIG. 15(d) is used, for example, the internal shape can be formed according to the shape of the basket. FIGS. 16(a) through 16(d) are axially sectional views showing examples of the bottomed container capable of being formed by the manufacturing method according to the second embodiment. The internal shape of the bottom section of the container 300 for forming and the forward end shape of the boring punch 410 match with the shape of the bottom of the bottomed container so that these containers can be formed. The container shown in FIG. 16(d) maybe bored on the bottom section after the forming, or may be formed by the boring punch provided with a protrusion at its forward end. The container formed in such a manner can be used as a container, in which its bottom section formed integrally with the body section requires not a plane but a curved surface. For example, this container can be applied to a casing and the like for a large rotating equipment for containing a hydroelectric water turbine. FIG. 17 is a axially sectional view showing the bottomed container according to a third embodiment. The bottomed container 1 is characterized in that a body and a bottom are formed integrally and a spot facing section is also formed on the bottom of the container. The bottom provided with the spot facing section was conventionally mounted to a thick cylinder by welding, but in the manufacturing method, besides the step of providing the spot facing section on the bottom, the welding step and the heat treating step after the welding are required. For this reason, there arises a problem that the manufacturing requires troublesome steps. According to the method of manufacturing a bottomed container of the present invention, since the bottom provided with the spot facing section can be formed integrally with the body by one step, there is an advantage that the manufacturing becomes very easy. FIGS. 18(a) through 18(e), 18(b′), 18(c′) and 18(e′) are explanatory diagrams showing a method of providing the spot facing section 800 on the bottomed container. As shown in FIG. 18(a), before the punch 410 is pushed into so that the hot dilation forming is carried out, a cylinder 302 which is a cylindrical member is previously provided on the bottom section 301 of the container for forming. Here, the spot facing section 800 can be formed only by the cylinder 302, but in order to easily taken out the cylinder 302 after the metal billet 200 is formed, an annular metal plate 303 may be previously placed on the upper section of the cylinder 302 (see FIG. 18(a)). The annular metal plate 303 is constituted so that its radial width is slightly larger than a radial width of the cylinder 302. As a result, after the metal billet 200 is formed into the bottomed container, the cylinder 302 can be taken out easily. In this case, the annular metal plate 303 is fitted into the bottom section after the bottomed container is formed. When the spot facing section 800 is formed only by the cylinder 303, it is desirable that the diameter of the cylinder on a side coming in contact with the metal billet 200 is smaller than the diameter of the cylinder on a side coming in contact with the bottom section 301 of the container 300 for forming. Namely, it is desired that a taper is provided on the cylinder 302 so that the diameter of the cylinder 302 becomes larger towards the pressing direction. As a result, the cylinder 302 is removed from the metal billet 200 which was subject to the hot dilation forming. The metal billet 200 is set into the body section 300 of the container and is placed on the cylinder 302 and the annular metal plate 303 (FIG. 18(b)), and the punch 410 is pushed into the billet 200 so that the metal billet 200 is hot dilated into the shape of the bottomed container (FIG. 18(c)). As shown in FIG. 18(d), an annular groove is formed on the bottom section of the metal billet 200 formed in such a manner by the cylinder 302 and the annular metal plate 303. In this state, only a simple annular groove is provided, and thus a column section as a pillar section existing in the annular groove is cut by cutting means (not shown) such as gas burner in order to form the spot facing section 800. As a result, the bottomed container having the spot facing section 800 on the bottom section can be manufactured (FIG. 18(e)). The spot facing section 800 may be finished by cutting or the like as the need arises. In addition, as shown in FIG. 18(b′), a column 304 is used instead of the cylinder 301 so that the spot facing section 800 can be formed on the bottom section of the bottom container. Also in this case, the metal plate 305 is placed on the column 304 so that the spot facing section 800 can be formed (FIG. 18(c′)). In the case where the column 304 is used, unlike the case of using the cylinder 302, the bottomed container is formed and simultaneously the spot facing section 800 can be formed on its bottom section (FIG. 18(e′)). For this reason, the step of cutting the column section shown in FIG. 18(d) is not required, but the press pressure which is larger than the case of using the cylinder 302 is required. Therefore, the performance of the pressing machine, a size of the bottomed container to be produced and the like are taken into consideration, and it is desirable that a determination is made as to which is used the cylinder 302 or the column 304. Here, in the case where the spot facing section 800 is formed only by the column 304, it is desirable that a taper which becomes larger towards the pressing forward side is provided. Namely, it is desirable that the axial section of the column 304 is trapeziform. As a result, the column 302 is easily removed from the metal billet 200 which was subject to the hot dilation forming. The shapes of the sections of the cylinder 302 and the column 304 vertical to the axial direction are changed so that the spot facing section 800 having a desired sectional shape can be formed. For example, the shape of the section vertical to the axial direction is polygonal so that the spot facing section 800 whose internal surface shape is polygonal can be formed. As a result, since the spot facing section can be formed according to the external shape of the bottomed container, the thickness of the spot facing section in the radial direction can be kept constant. FIGS. 19(a) through 19(c) are axially sectional views showing examples of the bottomed container capable of being formed by the manufacturing method of the present invention. The bottomed container 1 shown in FIGS. 19(a) and 19(b) can be formed by selecting suitably a diameter of the cylinder or the column in the above explanation. FIG. 19(c) shows the example that two-staged spot facing section 800 is provided on the bottom section. In order to provide the two-staged spot facing section 800, for example, a forming tool, in which two columns having different diameters are stacked and its axially section has a convex shape, is set on the bottom of the metal billet 200. Moreover, the two-staged spot facing section 800 can be formed also by setting two cylinders having different diameters and heights on the bottom of the metal billet 200. Moreover, a step section may be provided on the external surface of the cylinder in an axial direction so as to be used. FIGS. 20(a) and 20(b) are axially sectional views showing the bottomed container according to a fourth embodiment of the present invention. The bottomed container 1 is characterized in that in the manufacturing method described in the first embodiment the extended section 201 formed before the hot dilation forming (see FIG. 2(b)) is directly utilized as the flange of the container. Since the cask as the used nuclear fuel container is constituted so that helium gas of a several atm. is sealed between the primary cover and the secondary cover, a great pressure is applied to the mounting portion of the secondary cover. Moreover, since the secondary cover occasionally receives impact of falling, the flange section as the mounting portion of the secondary cover requires the firm structure. Since the container of the present invention is formed so that the flange and the body are integral and a diameter of the flange is larger than a diameter of the body, measure such that bolts are arranged in two lines is easily taken. Therefore, the secondary cover can be fixed firmly. Since in the conventional cask the flange section is manufactured separately and is welded to the cask body, troublesome steps are required. According to the manufacturing method of the present invention, since the thick bottomed container having excellent end surface shape can be manufactured, the extended section 201 formed on the end surface of the container (see FIG. 2(b)) is seldom worked and can be utilized as the flange. Therefore, the welding step and the post-welding hot processing step can be omitted, and thus the manufacturing steps can be simplified. Here, in FIGS. 20(a) and 20(b), the extended section 201 formed before the hot dilation forming (see FIG. 2(b)) is directly used as the flange of the container, but the extended section 201 is removed by cutting or the like and the inner side of the opening of the container is worked so that a flange without extended section ever used can be formed. Also in this case, since the flange section and the body section are formed integrally, strength and sealing property can be secured sufficiently. There will be explained below another manufacturing method of the bottomed container 1. FIGS. 21 through 26 are explanatory diagrams showing the manufacturing steps of the bottomed container 1 of the cask 100 shown in FIG. 12. The bottomed container 1 is formed by combining the upsetting step and the punching draw step. Firstly, ring-shaped first die 21, second die 22 and third die 23 are stacked on a slide table 20 of the pressing machine (not shown), and first pressurizing platform 24 second pressurizing platform 25 and third pressurizing platform 26 are placed in the first through third dies 23 so that a mold is structured (FIG. 21(a)). A boring punch 27 is positioned on an upper surface of a metal billet W. The boring punch 27 is pressurized by a stem 28 attached to a punch of the pressing machine (FIG. 21(b)). The metal billet W is set in the first die 21. The metal billet W is made of low carbon steel or stainless steel formed by vacuum forging, and its upper surface is circular and its lower surface has a circular shape smaller than the upper surface so that the metal billet W is conically trapeziform (angle of tilt surface is not shown). At the time of pressurizing, the metal billet W is heated within a range of 1000° C. to 1200° C. The heating is carried out in an electric oven and the metal billet W in a red state is placed on the slide table 20. The cylinder 30 and the annular metal plate 303 (see FIG. 18(a)) are provided between the first die 21 and the metal billet W so that a spot facing section may be formed on the bottom. Next, when the metal billet W is placed, the boring punch 27 is pressurized so as to be upset (FIG. 21(c)). Since the inner end portion of the first die 21 is an opened, material flows between the boring punch 27 and an opening portion 21a of the first die so that the metal billet W is deformed into a dished shape. A hang tool 29 is used so as to hang the stem 28 together with the first die 21 (FIG. 21(d)), and the slide table 20 is moved to be conveyed out so that the first pressurizing platform 24 is removed (FIG. 21(e)). When the first die 21 including the metal billet W is hung, in this state a spacer 30 is placed on the second die 22 (FIG. 22(a)). Thereafter, the slide table 20 is moved and the mold is conveyed in (FIG. 22(b)), and the boring punch 27 is pushed down so that the metal billet W is subject to the punching draw working by the first die 21 (FIG. 22(c)). As a result, when the metal billet W passes through the first die 21, the dished section at the top is drawn so that the metal billet W has a cup shape and is positioned in the second die 22. Next, the stem 28 and the hang tool 29 are allowed to recede upward and the slide table 20 is moved so that the metal billet W and the mold are conveyed out and the spacer 30 is removed (FIG. 22(d)). In this state, the boring punch 27 remains at the bottom of the metal billet W having cup shape (FIG. 23(a)). Next, the stem 28 in a state that the spacer 30 is removed is lowered and is pressurized by the boring punch (FIG. 23(b)). As a result, the metal billet W is further upset, and the material flows from between an opening section 22a of the second die 22 and the boring punch 27 so that the metal billet W is deformed. The hang tool 29 is used so as to hang the second die 22 together with the metal billet W (FIG. 23(c)). In this state the slide table 20 is moved so that the mold is conveyed out, and the second pressurizing platform 25 is removed (FIG. 23(d)). Next, the spacer 31 is placed on the third die 23 (FIG. 24(a)). The slide table 20 is moved so that the mold is conveyed in (FIG. 24(b)), and the boring punch 27 is pushed down so that the punching draw working is carried out by the second die 22 (FIG. 24(c)). As a result, when the metal billet W passes through the second die 22, its bellied portion is drawn and the metal billet W has a cup shape. Next, the stem 28 and the hang tool 29 are allowed to recede upward, and the slide table 20 is moved so that the metal billet W and the mold are conveyed out and the spacer 31 is removed (FIG. 24(d)). In this state the boring punch 27 remains at the bottom of the metal billet W having a cup shape. Next, the slide table 20 is moved and the metal billet W is positioned below the stem 28 (FIG. 25(a)), and the stem 28 is lowered so that the metal billet W is pressurized by the boring punch 27 (FIG. 25(b)). As a result, the metal billet W is further upset, and the material flows from between an opening section 23a of the third die 23 and the boring punch 27 so that the metal billet W is deformed. Thereafter, the hang tool 29 is used so as to hang the third die 23 together with the metal bit W (FIG. 25(c)). In this state, the slide table 20 is moved, and the third pressurizing platform 26 is removed (FIG. 25(d)). Next, the spacer 32 is placed on the slide table 20 (FIG. 26(a)). The slide table 20 is moved and the spacer 32 is conveyed in (FIG. 26(b)), and the boring punch 27 is pushed down so that the punching draw working is carried out by the third die 23 (FIG. 26(c)). As a result, when the metal billet W passes through the third die 23, its bellied portion is drawn. Next, the step 28 and the hang tool 29 are allowed to recede upward, and the slide table 20 is moved so that the metal billet W is conveyed out and the spacer 32 is removed (FIG. 26(d)). In this state the boring punch 27 remains at the bottom of the metal billet W having a cup shape, but it is directly used as the bottom of the bottomed container 1. The boring punch 27 is removed and the metal billet W can be used. Moreover, when the slide table 20 is moved and the spacer 32 is conveyed in (FIG. 26(b)), a cylindrical spacer 302 is provided in the spacer 32 (see FIG. 2(e″))), and the bottom of section the metal billet W is punched by the boring punch 27. The cylindrical container can be formed by this method. When the above-mentioned forming is finished, predetermined heat treatment is given to the bottomed container, and its inner surface is mechanically worked. The bottomed container 1 formed in such a manner has a percentage that its section is reduced from the metal billet W becomes about 40%. Moreover, in comparison with the case where the bottomed container is formed by the normal backward extrusion forming, in the case of the backward extrusion forming, the bottom section of the bottomed container becomes thick, and this causes an increase in the weight of the cask. Moreover, the pressing machine requires a great pressure, and a bottomed container cannot be occasionally manufactured depending upon its scale. On the contrary, according to the manufacturing method of this embodiment, since the bottomed container is formed by combining the upsetting and the punching draw as mentioned above, only low pressure is required at the time of the upsetting or the draw. For this reason, a large-sized pressing machine which has been conventionally used is used so as to enable the forming. FIGS. 27(a) through 27(d) are explanatory diagrams showing another manufacturing method of the bottomed container. As mentioned above, in the above-mentioned manufacturing method, the cylindrical boring punch 27, the pressurizing platforms 24 through 26, and the dies 21 through 23 having annular ring-shaped inner side were used, but the boring punch 27, etc. are not limited to these shapes. For example as shown in FIG. 27(b), in the case where the outer shape of the bottomed container 1 is octagonal, the internal shapes of a first die 21b through a third die may be octagonal. In this case, a pressurizing platform 24b is also octagonal. In addition, in the case where the internal shape of the bottomed container 1 is octagonal, as shown in FIG. 27(c), a boring punch 27c may be a octagonal prism. The metal billet W in this case has a octagonal cone trapezoid shape (detail of a tapered angle is not shown). Further, in the case where the internal shape of the bottomed container 1 has a step, as shown in FIG. 27(d), a boring punch 27d may be a prism having steps. Here, as not shown, in the case where the internal and external shapes of the bottomed container are octagonal, the die 21b shown in FIG. 27(b) and the boring punch 27c shown in FIG. 27(c) may be used. Moreover, also in the case where these shapes are not octagonal, according to the manufacturing method of the present invention, the bottomed container can be formed similarly to the above-mentioned method by changing the shapes of the boring punch, die and the like. Here, the boring punches 27c and 27d shown in FIG. 27 can be applied also to the manufacturing method according to the first embodiment. FIGS. 28(a) through 28(f) are explanatory diagrams showing an embodiment of different manufacturing method. This manufacturing method is realized in that a stem 51 is set on a slide table 52 of a pressing machine and a boring punch 53 is attached to a top portion of the stem 51, and a pressurizing platform 54 is mounted to a punch 55 of the pressing machine. Namely, as shown in FIG. 28(a), the metal billet W is placed on the boring punch 53, and the punch 55 of the pressing machine is lowered so that the metal billet W is upset by the first die 56. Next, after the lower end of the metal billet W is deformed into a dished shape, the punch 55 is allowed to recede so that the metal billet W is conveyed out (not shown). A plurality of spacers 57 are placed on the upper portion of the first die 56 so as to be conveyed in below the punch 55. Moreover, the pressurizing platform 54 is removed from the punch 55. When the punch 55 is lowered and pressurizes the metal billet W in this state, as shown in FIG. 28(b), the metal billet W is drawn by the first die 56. The first die 56 directly recedes downward. Next, similarly the metal billet W is conveyed out so that the spacer 57 is removed and the pressurizing platform 54 is mounted, and the metal billet W is conveyed in below the punch 55 again (not shown). Moreover, a second die 58 is mounted to the punch 55 side. In this state the punch 55 is lowered so as to upset and pressurize the metal billet W (FIG. 28(c)). The metal billet W is once conveyed out, and a plurality of spacers 59 are placed on the second die 58 and simultaneously the pressurizing platform 54 is removed, and the metal billet W is again conveyed in below the punch 55. In this state when the punch 55 is lowered and the pressurizes the metal billet W, as shown in FIG. 28(d), the bellied portion of the bottomed container 1 is drawn. The second dice 58 directly recedes downward. Thereafter, similarly the metal billet W is conveyed out and the spacer 59 is removed, and the metal billet W is again conveyed in below the punch 55 (not shown). Moreover, a third die 60 and the pressurizing platform 54 are attached to the punch 55 side. In this state when the punch 55 is lowered so as to pressurize the metal billet W, as shown in FIG. 28(e), the metal billet W is further deformed. The metal billet W is once conveyed out and a plurality of spacers 61 are placed on the third die 60, and simultaneously the pressurizing platform 54 is removed so that the metal billet W is again conveyed in. When the punch 55 is lowered so as to pressurize the metal billet W in this state, as shown in FIG. 28(f), the bellied portion of the bottomed container 1 is drawn. The third die 60 recedes downward. Here, the material which can be used in the method according to this invention includes ferrous materials such as carbon steel, stainless steel and low-alloy steel, and also nonferrous metal such as nickel alloy, aluminum alloy, copper alloy and magnesium alloy. There will be explained below concrete forming conditions. When a metal billet made of low carbon steel is heated to 1000° C. and a distortion speed is 0.1 to 1 s, the deformation resistance becomes 1.5 to 3 kgf/mm2. For example, if a length of 1 cm is punched within 1 minute, in the case where the outer diameter is reduced from 2500 mm to 2200 mm by a die of 30°, when the inner diameter is 1420 mm, the distortion becomes:1n((25002−14202)/(22002−14202)))=0.4 and the metal billet is worked for time (2500−2200/2/tan 30°)/(1000/60)=15.6 sec. Therefore, the distortion speed becomes 0.025−1 s. Next, when the distortion resistance is 3 kgf/mm2 and the friction coefficient is 0.3, the punching force becomes:3×π/4×(22002−14202)×1n((25002−14202)/(22002−14202))×(1+0.1×0.3/tan 30°)+4π/(6·3·√3)=5460640 kgf However, since the temperature is initially high, the punching force is reduced to half, namely, becomes 2730 tonf. Moreover, the final thickness which prevents the bottom from dropping off at the time of punching is:54640/(3/√3)/π/1420=7.7 mm Therefore, the thickness of more than this value is required. When a length of a product is 4885 mm, since a length of a material is:4885×(22002−14202)/22002=2850 nm,an upsetting amount for one time is reduced to ⅓, namely, 950 mm and a height up to the mold constraining portion is set to the final bottom thickness of 700 mm, a necessary upsetting force becomes:3×π/4×14202×(1+0.3×1420/700/2)=61967With the pressure of this value, the bottomed container can be formed by pressing of 8000 ton which has usage accomplishment. On the contrary, since the extruding force in the case of the backward extrusion forming becomes:3×π/4×22002×1n((22002/(22002−14202))×(1+2×0.5/tan 45°))+4π/(4·3·√3)=19186103 kgfthe pressing force of 20000 ton is required. As mentioned above, in the radioactive substance container and the container of the present invention, the bottomed container in which the bottom and the body are formed integrally is used so that the conventional welding of a bottom plate is not required, and the heating treatment thereafter can be omitted. As a result, troublesome manufacturing steps can be reduced greatly. Moreover, since the bottomed container is formed by the hot dilation, only a press pressure, which is lower than that at the time of hot backward extrusion forming, for example, is required. In addition, as for the radioactive substance container and the container of the present invention, the bottomed container was formed by using the metal billet having a polygonal section vertical to the axial direction and the container for forming having a circular internal shape of the section vertical to the axial direction. For this reason, since the conventional welding of a bottom plate is not required, troublesome steps required for the manufacturing can be reduced. Further in the dilation forming, the bottomed container can be formed with a lower pressure than conventional one due to a function which bends one side of the polygon of the metal billet. In addition, as for the radioactive substance container and the container of the present invention, the bottomed container was formed by the metal billet having a polygonal section vertical to the axial direction and the container for forming having polygonal internal shape of the section vertical to the axial direction. For this reason, since the conventional welding of a bottom plate is not required, troublesome steps required for the manufacturing can be reduced. Further in the dilation forming, the bottomed container can be formed with a lower pressure than conventional one due to the function which bends one side of the polygon of the metal billet. Further, bottomed containers having external shapes according to various applications can be formed easily. In addition, as for the radioactive substance container of the present invention, the bottomed container, which is enough long in the axial direction to contain the basket of used nuclear fuel aggregate used as fuel of nuclear reactor and has large inner diameter, namely, is thick, was formed by the hot dilation forming in the container for forming so that the bottom and the body were integral. For this reason, since the conventional welding of a bottom plate is not required and the post-welding heat treatment can be omitted, troublesome steps required for the manufacturing can be reduced. Particularly in the bottomed container whose thickness is thick and dimension in the axial direction is a several meters and inner diameter reaches 2 to 2.5 meters, the effect which can omit the steps is extremely great. In addition, since the radioactive substance container of the present invention has a dimension such that the section of the boring punch is approximate to the section of the basket for used nuclear fuel aggregate, an operation for cutting the inside of the container becomes easy after the hot dilation forming, and the manufacturing does not require the troublesome steps. In addition, as for the radioactive substance container of the present invention, in the case where radioactive substance is stored in the bottomed container in which the bottom and the body are formed integrally by the hot dilation forming in the container for forming, a dosage equivalent factor of the γ rays is not more than 200 μSv/h. In order to satisfy the requirement that a dosage equivalent factor of the γ rays is not more than 200 μSv/h on the surface of an external wall on the substantially center of the side surface of the container, it is necessary to manufacture the container whose thickness reaches several dozens cm using stainless steel, carbon steel or the like. Since the body and the bottom of such a thick container were formed integrally, the conventional welding of a bottom plate is not required, and since the post-welding heat treatment can be omitted, the manufacturing does not require troublesome steps. Particularly in such a thick bottomed container, the effect which can omit the steps is extremely great. In addition, as for the radioactive substance container of the present invention, in the above-mentioned radioactive substance container and the container, the outer diameter of the bottomed container was not less than 1000 mm to not more than 3000 mm, and the thickness was not less than 150 mm to not more than 300 mm. Since the thick container was formed so that the bottom and the body were integral, the conventional welding of a bottom plate is not required, and since the post-welding heat treatment is omitted, the manufacturing does not require troublesome steps. Particularly in the thick bottomed container whose axial dimension is large, the effect which can omit the above-mentioned steps is extremely great. In addition, as for the radioactive substance container and the container of the present invention, the metal billet, whose section vertical to the axial direction on the pressing forward side was at least formed into polygonal shape, was set into the container for forming, and the boring punch is pushed into the metal billet so that the metal billet is hot-dilated. For this reason, the conventional welding of a bottom plate is not required, and the post-welding heat treatment can be omitted. In addition, since a number of defects caused on the end portion and the surface of the bottomed container is small, only less troublesome steps of correcting these defects are required after the forming, and thus the manufacturing does not require troublesome steps. In addition, in the radioactive substance container of the present invention, the bottom and the body were formed integrally by hot press working. Moreover, in the radioactive substance container of the present invention, the metal billet was heated and was upset and drawn so that the bottom and the body were formed integrally. For this reason, since the welding step and the heat treating step after that can be omitted, the manufacturing does not require troublesome steps. In addition, the radioactive substance container of the present invention, the spot facing section was provided integrally with the bottom section of the bottomed container. Since in the bottomed container the spot facing section is also formed integrally at the time of the hot dilation forming, the step of providing the spot facing section can be omitted, and thus the manufacturing does not require troublesome steps. In addition, as for the radioactive substance container of the present invention, since the flange was provided integrally with the body of the bottomed container, the welding step and the post-welding heat treating step can be omitted, and troublesome steps for the manufacturing can be omitted. Moreover, the sealing performance and the strength of the container itself can be secured. In addition, as for the radioactive substance container and the container of the present invention, at least one of the section outside the body and the section inside the body of the bottomed container was polygonal. For this reason, when the bottomed container is dilated to be formed, the internal section of the container can be formed into a shape according to a basket. As a result, the step of cutting the inside of the container which has been conventionally required can be omitted, and thus the manufacturing does not require troublesome steps. In addition, the metal billet for the hot dilation forming according to the present invention was formed so that at least the section vertical to the axial direction on the pressing forward side was polygonal. For this reason, the performance for bending a side of the polygon and the function for restraining the upsetting of the metal billet accrue. Because of these functions, the thick container, in which a ratio of the length to the diameter in the axial direction is not less than 1, can be formed with a lower press pressure than conventional one. Moreover, the defects caused on the end portion and the surface of the container after the forming can be suppressed. In addition, the metal billet for the hot dilation forming of the present invention was provided with at least one plane on at least one of the side surface on the pressing forward side and the side surface of the pressing backward side. At the time of the hot dilation forming, since the metal billet is dilated to be formed by the function for bending this plane, a force required for the hot dilation forming is weaker than the case where the side surface is a curbed surface. Therefore, the thick container which is long in the axial direction can be formed with a lower press pressure than conventional one. Moreover, in comparison with the case where the side surface is a curved surface, defects on the inner side such as a crack can be reduced. In addition, the metal billet for the hot dilation forming of the present invention was further provided with a taper which becomes thinner toward the pressing direction on the pressing forward side of the metal billet in the above-mentioned metal billet for the hot dilation forming. Moreover, in the metal billet for the hot dilation forming of the present invention, at least one or more step sections were provided so that the pressing forward side of the metal billet becomes thinner gradationally towards the pressing direction. As for the metal billet, timing at which the metal fills the vicinity of the bottom of the container for forming can be delayed at the final stage of the hot dilation forming. For this reason, the upsetting is suppressed at the final stage of the hot dilation forming, and thus the press pressure can be reduced at the time of the hot dilation forming. In addition, since the metal billet for the hot dilation forming of the present invention is provided with the extended section at the end portion on the pressing backward side, the metal billet is engaged with the end portion of the container by the extended section at the time of the hot dilation forming. With this function, constraint of the container on the metal billet becomes stronger so that the upsetting on the pressing forward side can be suppressed. Moreover, since the side surface is provided with at least one plane, the function for bending this plane and the function for suppressing the upsetting of the metal billet accrue. Therefore, due to their interaction, the press pressure can be suppressed to be small. Moreover, since the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing, the step of manufacturing the container can be simplified. In addition, since the metal billet for the hot dilation forming of the present invention is provided with the extended section on the pressing backward side, this extended section engages the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least the section vertical to the axial direction on the pressing forward side was formed into a polygonal shape, the function for bending each side of the polygonal section and the function for suppressing the upsetting of the metal billet accrue. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, since he metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing, the step of forming the extended section on the pressing backward side is not required, and thus the steps of manufacturing the container can be simplified. In addition, since the metal billet for the hot dilation forming of the present invention was provided with the extended section on the pressing backward side, this extended section engages the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least the section vertical to the axial direction on the pressing forward side was formed into a polygonal shape, the bending function and the function for suppressing flow of the metal accrue. Further, the pressing forward side was set to be thinner gradationally towards the pressing direction, the upsetting phenomenon is suppressed at the final stage of the hot dilation forming, and an increase in the press pressure can be suppressed. Therefore, the press pressure can be suppressed to be small due to their interaction. Moreover, since the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing, the step of forming the extended section on the pressing backward side is not required. In addition, since the metal billet for the hot dilation forming of the present invention is provided with the extended section on the pressing backward side, the extended section latches the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least one plane is provided on at least one of the side surfaces of the metal billet, the bending function and the function for suppressing the flow of the metal accrue further, since the pressing forward side becomes thinner gradationally, the upsetting phenomenon is suppressed at the final stage of the hot dilation forming, and an increase in the press pressure can be suppressed. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, since the metal billet is previously provided with the extended section on the pressing backward end at the time of the manufacturing, the step of forming the extended section on the pressing backward side is not required. In addition, in the above-mentioned radioactive substance container and the container, the bottomed container of the present invention has an outer diameter of not less than 200 mm to not more than 4000 mm and a thickness of not less than 20 mm to not more than 400 mm. Since such a thick container is formed so that the bottom and the body are integral, the conventional welding of a bottom plate is not required, and the post-welding heat treatment can be can be omitted so that the manufacturing does not require troublesome steps. Particularly in the thick bottomed container having a large axial dimension, the effect which can omit the above-mentioned steps is extremely great. In addition, since the container manufacturing apparatus of the present invention is provided with the container for forming, whose body and the bottom can relatively move with respect to the axial direction of the body of the container, and the boring punch which is attached to the pressing machine and pressurizes the metal billet set in the container for forming. For this reason, since the body of the container and the metal billet hardly relatively move at the time of hot dilation forming, increase in the press pressure can be suppressed at the time of the hot dilation forming. In addition, in the method of manufacturing a container of the present invention the body section of the container for forming is divided in the axial direction. For this reason, even in the case where the metal billet which is long in the axial direction is formed, the deformation of the metal billet in the axial direction at the time of hot dilation forming can be absorbed by the whole container. Therefore, an increase in the press pressure can be suppressed. In addition, according to the manufacturing method of the radioactive substance container of the present invention, the container in which the bottom and body are formed integrally is finished by cutting its external side, and its internal side is cut into a stepped shape so that a portion for containing a basket for used nuclear fuel aggregate is provided, or the internal section is cut to be finished so that the radioactive substance container is manufactured. For this reason, the internal side of the bottomed container can be cut and finished easily. In addition, according to the manufacturing method of the radioactive substance container of the present invention, the bottomed container is formed by hot dilation forming so that the bottom and the body are integral, and the external side of the bottomed container is cut to be finished, and the internal section is cut into a stepped shape so that a portion for storing a basket for used fuel aggregate is provided, or the internal side is cut to be finished so that the radioactive substance container is manufactured. For this reason, the inner side of the bottomed container can be cut and finished easily. In addition, according to the method of manufacturing a bottomed container of the present invention, the boring punch is pushed into the metal billet, and the plane of the metal billet is bent by the force directing to the inner wall of the container so that the metal billet is dilated to a gap existing between the container for forming and the metal billet. In this method of manufacturing a container, the metal billet is dilated to the inner wall side of the container for forming by function for bending the plane of the metal billet on the side surface. Moreover, since the metal billet is dilated to the space existing between the metal billet and the inner wall of the container for forming, the upsetting phenomenon of the metal billet can be suppressed. With these functions, in this method of manufacturing a container, only lower press pressure than the conventional one is required, and defects caused on the end portion and the surface of the container after the forming can be suppressed. In addition, according to the method of manufacturing a bottomed container of the present invention, the metal billet provided with the extended section engaging with the end portion of the opening of the container for forming is used for the end portion of the pressing backward side. For this reason, the extended section engages the metal billet with the end portion of the container at the time of hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting of the pressing forward side can be suppressed. Moreover, since at least one plane is provided on the side surface, the bending function and the function for suppressing the phenomenon that the metal flows to the opposite side to the pressing direction accrue. Therefore, the press pressure can be suppressed small by their interaction. Further, the defects which are caused on the end portion and the surface of the container after the forming can be also suppressed. In addition, in the method of manufacturing a bottomed container of the present invention, the metal billet, in which at least the section vertical to the axial direction on the pressing forward side is formed into polygonal, is hot-dilated. For this reason, on the pressing forward side, since the metal billet is dilated in the space existing between the pressing forward side and the body of the container, the upsetting phenomenon of the metal billet can be suppressed. Therefore, in this method of manufacturing a container, only lower press pressure than conventional one is required, and the defects which are caused on the end portion and the surface of the container after the forming can be suppressed. In addition, in the method of manufacturing a bottomed container of the present invention, the metal billet, which is provided with at least one plane on at least one of the side surface of the pressing forward side and the side surface of the pressing backward side, is hot-dilated. For this reason, only a weak force suffices for the hot dilation forming in comparison with the case where the side surface is a curved surface. Therefore, a lower press pressure is required in comparison with the conventional method of manufacturing a container, and internal defects such as a crack can be also reduced. In addition, in the hot pressing method of the thick metal cylinder or the cylindrical container of the present invention, a metal billet without joint of different diameter sections, in which its pressing forward side is composed of a member having a section with an outer diameter smaller than the inner diameter of the container or an outer diameter equal with the diagonal length, or a member having a section with an outer diameter of the diagonal length equal with the inner diameter of the container, and its pressing backward side is composed of a member having a section with an outer diameter or a diagonal length equal with the inner diameter of the container, is heated to a pressing temperature and is set into the container for press forming, and thereafter while the center of the workpiece as the metal billet without joint is being bored by the punch, the billet is pressed. For this season, the metal billet without joint reduces a press forming load and improves yield of the product. Further, a press formed product having excellent end surface shape can be obtained. In addition, in the method of manufacturing a drum or a container of the present invention, the metal billet, in which its pressing forward side has a section with an outer diameter smaller than the inner diameter of the container and its backward side has a section with an outer diameter substantially equal with the inner diameter of the container, is hot-dilated. For this reason, the thick container can be formed with lower press pressure than conventional one, and a number of defects caused on the end portion or the surface of the container is small. As a result, less steps are suffice for correcting the defects after the forming. Moreover, since both the pressing forward side and backward side have angular section, the metal billet can be worked comparatively easily in comparison with a billet having a circular sectional. Therefore, the manufacturing does not require troublesome steps. In addition, in the method of manufacturing a drum or a container of the present invention, the metal billet, in which the pressing forward side has a section with a diagonal length smaller than the inner diameter of the container and the backward side has a section with a diagonal length substantially equal with the inner diameter of the container, is hot-dilated. For this reason, the thick container can be formed with lower press pressure than conventional one, and a number of defects caused on the end portion and the container surface is small. As a result, less troublesome steps suffice for correcting the defects after the forming. Moreover, since both the pressing forward side and backward side have angular sections, the metal billet can be worked comparatively easily in comparison with a metal billet having a round section. In addition, in the method of manufacturing a drum or a container of the present invention, the metal billet, in which the pressing forward side has a section with an outer diameter smaller ethan the inner diameter of the container and the backward side has a section with an outer diameter substantially equal with the inner diameter of the container, is hot-dilated. For this reason, the press pressure can be lower than conventional one, and a number of defects caused on the end portion and the container surface is small. As a result, only less troublesome steps suffice for correcting these defects after the forming. In addition, the method of manufacturing a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before hot dilation forming. Moreover, in this container forming method, the metal billet is dilated by the function for bending the plane of the metal billet towards the inner wall of the container for forming. Since the extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, the press pressure can be reduced by the function for bending the plane of the metal billet towards the inner wall of the container for forming. With their interaction, this method of manufacturing a container can form the thick container with lower press pressure than the backward extrusion method or the like. In addition, in the method of manufacturing a container of the present invention, the metal billet, which is previously provided with the extended section engaging with the opening end portion of the container for forming on the end portion of the pressing backward side, is hot-dilated. For this reason, since the step of extending the pressing backward side of the metal billet to above the body of the container is not required before the hot dilation forming, time required for the hot dilation becomes short. As a result, since the forming can be finished until the temperature of the metal billet drops, the shape of the end portion becomes satisfactory. Moreover, since the extending step can be also omitted, the manufacturing does not require troublesome steps. In addition, the method of manufacturing a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since the extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which at least the section vertical to the axial direction on the pressing forward side is formed into a polygonal shape, is dilated to be formed, the function for bending each side of the polygon towards the inner wall of the container for forming acts. With their interaction, the thick container can be formed with lower press pressure than the backward extrusion method or the like. In addition, the method of manufacturing a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since the extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, which is provided at least one plane on at least one side, is dilated to be formed, the function for bending the plane of the metal billet towards the inner wall of the container for forming acts. With the interaction, this method of manufacturing a container can form the thick container with lower press pressure than the backward extrusion method or the like. In addition, the thick metal-made cylinder or drum container hot pressing method includes the step of extending the pressing backward side of the metal billet to the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet wit the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, the pressing forward side is a member having a section with an outer diameter smaller than the inner diameter of the container or an outer diameter of the diagonal length, or a member having a section with an outer diameter of the diagonal length equal with the inner diameter of the container. For this reason, as the metal is supplied from the pressing backward side and due to the function of the satisfactory plastic working of steel heated to high temperature, the metal billet is pushed to be spread sideways and simultaneously is worked, and is formed to fill the space of the container. As a result, the metal billet without joint is manufactured into a press product having a predetermined shape. With these interaction, the thick container can be formed with lower press pressure than the backward extrusion method or the like. In addition, the method of manufacturing a drum or a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet wit the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which the pressing forward side is a tetragonal section with its diagonal length is smaller than the inner diameter of the container, is used, the metal billet is dilated by the function for bending each side of the tetragonal section. Moreover, the pressing backward side of the metal billet suppresses the upsetting on the pressing forward side. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. In addition, the method of manufacturing a drum or a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of hot dilation forming, the constraint of the container on the metal billet becomes stronger, and upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which the pressing forward side is a tetragonal section with a sectional length smaller than the internal diameter of the container, is used, the metal billet is dilated to be formed by the function for bending each side of the tetragonal section. Moreover, the pressing backward side of the metal billet suppresses upsetting on the pressing forward side. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. Further, the metal billet to be used in this method is worked comparatively easier than the metal billet having one round section. In addition, the method of manufacturing a drum or a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of hot dilation forming, the constraint of the container on the metal billet becomes stronger, and upsetting on the pressing forward side can be suppressed. Moreover, since the pressing backward side of the metal billet has a diameter substantially equal with the inner diameter of the container for forming, upsetting on the pressing forward side can be suppressed. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. Further, the metal billet to be used in this method can be worked comparatively easier than the metal billet having different sectional shapes. Moreover, in the method of manufacturing a container of the present invention, the metal billet is formed by the forging step, and this method includes the step of forming at least the pressing forward side of the metal billet so as to provide an angular section. Moreover, the container manufacturing method of the present invention includes the step of providing a taper which becomes narrower towards the pressing direction on the pressing forward side of the metal billet. Further, in the method of manufacturing a container of the present invention, the forging step includes the step of providing at least one stepped section so that the pressing forward side of the metal billet becomes narrow gradationally towards the pressing direction. In this method of manufacturing a container, at the final stage of the hot dilation forming, timing at which the metal fills the vicinity of the bottom of the container for forming can be delayed. For this reason, the upsetting phenomenon of the metal billet can be suppressed, and the press pressure at the final stage of the hot dilation forming can be low. In addition, in the method of manufacturing a container of the present invention, the bottomed container is formed by a drum-shaped member provided at the bottom of the metal billet, and simultaneously a spot facing section is formed on the bottom of the bottomed container. Since the spot facing section has been conventional provided by cutting, the working requires troublesome steps. However, since a pillar-shaped section which remains on the bottom of the container is only removed after the dilation forming, the working does not require less troublesome steps than the conventional method. In addition, in the method of manufacturing a container of the present invention, the bottomed container is formed by the pillar-shaped member provided at the bottom of the metal billet, and simultaneously the spot facing section is formed on the bottomed container. For this reason, since the metal billet can be dilated and the spot facing section can be formed simultaneously, the working requires less troublesome steps than the conventional method. Moreover, since the step of removing the pillar-shaped member can be omitted, troublesome steps are not required for forming the spot facing section in comparison with the method where the spot facing section is formed. In addition, in the method of manufacturing a container of the present invention, the aforementioned method of manufacturing a container can relatively move the body of the container for forming with respect to the bottom of the container for forming. For this reason, since the body of the container and the metal billet to be formed hardly move relatively at the time of the hot dilation forming, an increase in the press pressure can be suppressed. In addition, in the method of manufacturing a container of the present invention, the aforementioned method of manufacturing a container divides the body of the container for forming in the axial direction. For this reason, even in the case where the metal billet which is long in the axial direction is formed, an increase in the press pressure can be suppressed. In addition, the radioactive substance container manufacturing method of the present invention includes the upsetting step of setting the pressurizing platform in a ring-shaped die provided with an opening on its inner end portion and putting the metal billet into the mold composed of the die and the pressurizing platform so as to pressurize the metal billet by means of the boring punch, and the metal billet drawing step of setting a spacer to the lower portion of the mold and pushing the metal billet by means of the boring punch. For this reason, the bottomed container is easily formed. In addition, this radioactive substance container manufacturing method includes the upsetting preparation step of stacking a plurality of ring-shaped dies provided with an opening on its inner end portion and stacking a plurality of pressurizing platforms in the dies and putting the metal billet into the mold composed of the die and the pressurizing platform, the upsetting step of pressurizing the metal billet from above the mold using the boring punch operated by the pressing machine, and the receding step of allowing the whole metal billet including the boring punch and the upper die to recede, the drawing preparation step of removing the used pressurizing platform and setting the drum-shaped spacer onto the next die and placing the whole metal billet including the receded die onto the spacer, the drawing step of pushing the metal billet using the boring punch and drawing the metal billet by means of the die, and the repeating step of repeating the above-mentioned steps using the next pressurizing platform and die and a spacer with a length according to deformation of the metal billet. For this reason, the pressurizing force can be suppressed small, and thus the manufacturing becomes easy. As mentioned above, the radioactive substance container, the radioactive substance container manufacturing apparatus and manufacturing method of the present invention are practical for a thick container such as a cask for containing, transporting and storing used nuclear fuel aggregate and substances contaminated with radioactive rays, and are suitable for providing a container such that manufacturing does not require troublesome steps and suppressing defects generated on an end portion of a cylinder and a surface of the container.
	abstract	A hydrogen/oxygen reaction is initiated in a catalytic recombination or ignition device. The device has one or more catalyst bodies with a predetermined catalytic surface. Only a small portion of the entire available catalytic surface, preferably less than 5% of the surface, is permanently maintained at a temperature level above ambient temperature. The temperature is raised by introducing energy with a heater. The heated surface portion acts as an initial igniter.
	summary
048715085	claims	1. A method for the operation of a light water boiling reactor with a core comprising a plurality of vertical fuel assemblies having an at least substantially square cross-section, each fuel assembly consisting of a bundle of fuel rods surrounded by a fuel channel, and a plurality of control rods, each control rod comprising four vertical blades arranged in a cruciform and provided with a neutron absorber material, the fuel assemblies being arranged in a symmetrical lattice with each fuel assembly included in two rows of fuel assemblies which are perpendicular to each other and the control rods being arranged with each one of their blades between two fuel assemblies located in the same row, so that each control rod together with four fuel assemblies arranged around the blades of the control rod form one unit, a control rod unit having an at least substantially square cross-section, the control rod units being arranged in a symmetrical lattice, with each control rod unit included in two rows of control rod units perpendicular to each other, said method comprising the steps of (a) operating said light water boiling reactor for an operating period until refuelling is needed, and then (b) exchanging in some control rod units control rods used during the operating period for new control rods having a reactivity worth which in a cold shut-down reactor is at least 6% higher than the original reactivity worth of the exchanged control rods, while maintaining in other control rod units control rods used during the operating period, the number of control rods with a higher reactivity worth after the exchange amounting to 40-60% of the total number of control rods of the reactor core. 2. Method according to claim 1, wherein in a cold shutdown reactor the reactivity worth of the new control rods with the higher reactivity worth is 10-20% higher than the original reactivity worth of the control rods which are present in the reactor at the time of exchange and have been used during the operating period. 3. Method according to claim 1, wherein the exchange of control rods for new control rods with a higher reactivity worth takes place in control rod units in a central zone of the reactor core, which is located inside an edge zone extending around the reactor core and comprising those control rod units which are located furthest out in the reactor core in each row of control rod units. 4. Method according to claim 3, in a number of control rod units distributed over the central zone of the reactor core there are used control rods which have been used in the reactor during the operating period and that at least the control rods in those control rods units which are located adjacent to each such control rod unit with a used control rod and in the same rows, perpendicular to each other, as said control rod unit with a used control rod, are exchanged for new control rods having a higher reactivity worth. 5. Method according to claim 3 or 4, wherein in regions within the central zone of the reactor core comprising 3.times.3 control rod units, control rods used during the operating period are exchanged for new control rods with a higher reactivity worth in that control rod unit which is located in the centre and in those four control rod units which are located in the same rows, perpendicular to each other, as said control rod unit located in the centre, whereas control rod which have been used during the operating period are used in the remaining four control rod units 6. Method according to claim 3 or 4, wherein when exchanging control rods new control rods with a higher reactivity worth are arranged in three control rod units at most, positioned adjacent to each other, in the same row of control rod units. 7. Method according to claim 3 or 4, wherein of the control rods used during the operating period, 50-50% of the total number of control rods in the central part of the reactor core are exchanged for new control rods with a higher reactivity worth.
	claims	1. A radiology garment storage and cleaning system, for storing cleaning, and tracking radiology garments, each garment including a radiology apron and a radiology collar, for use by a plurality of users, each user having an ID card with a unique identifier, comprising:a storage apparatus having a front end, a rear end, a first side extending from the front end to the rear end, a second side extending from the front end to the rear end, a top surface, and a bottom surface, the top surface and bottom surface each being connected to the front end, the rear end, the first side, and the second side,wherein the storage apparatus is divided into a plurality of compartments for accepting a radiation apron, the plurality of compartments extending from the first side to the second side, wherein each compartment spans from the front end to the rear end and extends partially from the first side to the second side,wherein each compartment is equipped with a cleaning system, a door disposed on the front end, a lock integrated with the door, and at least one status light, andwherein each compartment has a hanging mechanism and a liquid-impermeable bottom portion extending from the bottom surface;an ID card reader, disposed on the storage apparatus,wherein the ID card reader is capable of reading the ID card and determining its unique identifier; anda monitoring system, having a memory unit, and a processor,wherein the monitoring system is in electronic communication with the ID card reader, the lock of each compartment, each of the cleaning mechanisms, and the status light of each compartment, andwherein the monitoring system is configured to monitor when the radiation apron is placed in one of the plurality of compartments, monitor the last time each of the cleaning mechanisms were used, and operate each of the cleaning systems wherein the cleaning mechanism further comprises two scrubbing mechanisms, each scrubbing mechanism having: a first pulley, rotatably attached to the compartment and proximate to the top surface;a second pulley, rotatably attached to the compartment and within the volume of cleaning solution; a cable rotatably attached to the first and second pulley; and at least one brush fixed to cable, the cable selectively moving the brush downwardly to the liquid-impermeable bottom portion and moving the brush vertically alongside the garment to engage and clean the garment on a hanger. 2. The system of claim 1, wherein the hanging mechanism in each of the compartments includes a hanger, the hanger having:a neck with a top and bottom end;a hook fixed to the top end of the neck;a triangular part fixed to the bottom end of the neck for directly supporting one of the radiology aprons;a clamp mechanism for supporting one of the radiology collars; anda stem extending downwardly from the triangular part,the stem having a top portion fixed to the triangular part anda bottom portion attached to the clamp mechanism. 3. The system of claim 2, the clamp mechanism comprising a horizontal component that is fixed to the bottom portion, and at least two clamps rotatably attached to the horizontal component. 4. The system of claim 2, wherein the triangular part is sized to accommodate a radiation apron and the clamp mechanism is sized to accommodate a radiation collar. 5. The system of claim 1, wherein a volume of cleaning solution is located within the liquid-impermeable bottom portion. 6. The system of claim 1, wherein the cleaning system is configured to move the brush downwardly to the liquid-impermeable bottom portion and move the brush vertically alongside the garment to engage and clean the garment on the hanger only when the lock is engaged.
	abstract	A method of correcting a magnification of a mask pattern formed on a mask substrate. The method includes applying forces to four pressurizing points of an outer periphery of an approximately ring-shaped frame, which supports the mask substrate and has a rectangular window, on substantially extended lines of two diagonal lines of the rectangular window, and adjusting at least an angle, to the extended lines, of a vector of the forces applied to each of the pressurizing points.
	summary
	claims	1. A method for exposing an object to X-rays comprising the steps of:providing an X-ray machine including an X-ray tube equipped for emitting X-rays with an energy lower than or equal to 70 keV and a phototimer coupled to said X-ray tube for switching said tube on and off in accordance with an X-ray dose in the range from 0.75 up to 0.85 mR reaching said phototimer,placing an object between said X-ray tube and said phototimer,placing a cassette with a binderless storage phosphor panel or screen between said object and said phototimer andactivating said X-ray tube for exposing said object, said cassette and said phototimer until said phototimer switches said X-ray tube off, wherein said binderless storage phosphor panel comprises on a support (2) having a layer of amorphous carbon (23) with a thickness between 500 μm and 2000 μm, and a vacuum deposited phosphor layer (1) having a needle shaped CsBr:Eu phosphor, wherein amounts of Eu are in the range of from 100 up to 400 p.p.m. versus CsBr. 2. Method according to claim 1,wherein amounts of Eu are in the range of from 100 up to 200 p.p.m. versus CsBr. 3. Method according to claim 1,wherein amounts of Eu are in the range from 150 to 130 p.p.m. versus CsBr. 4. Method according to claim 1,wherein said support further includes a reflective auxiliary aluminum layer (22) with a thickness between 0.2 μm and 200 μm. 5. Method according to claim 2,wherein said support further includes a reflective auxiliary aluminum layer (22) with a thickness between 0.2 μm and 200 μm. 6. Method according to claim 3,wherein said support further includes a reflective auxiliary aluminum layer (22) with a thickness between 0.2 μm and 200 μm. 7. Method according to claim 1,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 8. Method according to claim 2,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 9. Method according to claim 3,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 10. Method according to claim 4,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 11. Method according to claim 5,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 12. Method according to claim 6,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 13. Method according to claim 7,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 14. Method according to claim 8,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 15. Method according to claim 9,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 16. Method according to claim 10,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 17. Method according to claim 11,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 18. Method according to claim 12,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene C and parylene HT. 19. Method according to claim 1,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 20. Method according to claim 2,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 21. Method according to claim 3,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 22. Method according to claim 4,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 23. Method according to claim 5,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 24. Method according to claim 6,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 25. Method according to claim 7,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 26. Method according to claim 8,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 27. Method according to claim 9,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 28. Method according to claim 10,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 29. Method according to claim 11,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 30. Method according to claim 12,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 31. Method according to claim 13,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 32. Method according to claim 14,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 33. Method according to claim 15,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 34. Method according to claim 16,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 35. Method according to claim 17,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 36. Method according to claim 18,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 37. Method according to claim 1,wherein said method is a mammographic application method.
043303676	description	DESCRIPTION OF THE PREFERRED EMBODIMENT In the art of reactor control the objectives to be achieved are the maximization of plant capacity and availability without violating the specified acceptable fuel design limits as a result of normal operation and anticipated operational occurrences. These limits are defined to provide a high degree of assurance that the fuel cladding integrity is maintained. Each of the design limits can be formulated in a mathematical description which gives an indication of the proximity to the violation of the design limit. These indices are mathematically dependent upon the various reactor parameters. One such index is the Departure from Nucleate Boiling Ratio (DNBR) which is indicative of the probability of occurrence of Departure from Nucleate Boiling (DNB). For each index a critical value of index may be determined which indicates when the probability is acceptably low that a reactor design limit is violated. In the case of DNBR the critical value has been determined to be 1.3. The DNBR index is functionally dependent upon the reactor parameters of coolant mass flow rate, coolant pressure, coolant inlet temperature, reactor power and reactor power distribution. The reactor must be operated in such a way that the critical values of each index are not violated in either of two cases: (1) steady state operation and (2) in the event of the occurrence of an anticipated operational occurrence which affects the values of the parameters on which the index is dependent. For the purposes of this description of the preferred embodiment, discussion will focus on the index called DNBR and the dependece of DNBR on the reactor parameter of reactor coolant mass flow rate. This choice has been made because the one most rapid anticipated operational occurrence that can occur in the operation of a pressurized water reactor is the simultaneous loss of power to all reactor coolant pumps. Other anticipated operational occurrences can occur that affect either the coolant mass flow rate or the other parameters but these other incidents result in a less rapid change in the DNBR. Therefore, the approach to the critical value of DNBR is less rapid and more time is available for initiation of protective measures. If a reactor protection system is devised to adequately handle the most rapid anticipated operational occurrence, then, by definition, its dynamic performance is adequate to accommodate the slower anticipated operational occurrences. For a better understanding of the dependence of DNBR on coolant flow rate, refer to FIGS. 2 and 3. FIG. 2 is a plot of the decay in the reactor coolant mass flow rate on the occurrence of an incident which interrupts electrical power to all of the reactor coolant pumps. Examination of the plot of FIG. 2 shows that the incident occurs at time t.sub.0 and the mass flow rate shows an exponential decay from the steady state condition existing before the occurrence of the incident. FIG. 3 shows the rapid fall in DNBR for two situations. The first situation is one in which the steady state operation before the accident was such that the initial value of DNBR was slightly above the critical value of 1.3. In this case the DNBR falls almost immediately below the critical value, indicating that the probability of damage to the fuel cladding is higher than desired. In the second situation, the operation of the reactor prior to the incident resulted in a steady state DNBR of approximately 1.8. On the occurrence of the indicent, the value of DNBR drops rapidly toward the critical value of 1.3. Unlike the first situation, in which the critical value was almost immediately violated, the second situation will bring about a violation of the critical value of DNBR only after a certain time period. It can be seen from these two described situations that operation of the reactor with an initial margin to the critical value of DNBR is preferable since a period of time is available for the sensing and calculating of the occurrence of the accident and the initiation and completion of the corrective measures (reactor scram). FIG. 4 shows the dependance of DNBR in the two situations described above including the effect of the actions of a protection system which detects the fall of coolant flow rate and which initiates a reactor scram. It can be seen in the second situation which starts from a DNBR value of 1.8, that due to the time delay between the time of occurrence of the incident and the time at which the DNBR violates the critical value, the protection apparatus has sufficient time to terminate the decrease in DNBR before the critical value of DNBR is violated. Such is not the case for the first situation and the critical value of DNBR is violated regardless of the initiation of protective action. A further modification to these concepts is necessary since the time available for preventing limit violation depends upon the power configuration within the reactor core. The reactor is controlled by inserting neutron absorbing control rods into the core from the top. If the power distribution in the core is such that a power peak is near the top of the core, control rod insertion effects are felt earlier than if the power was peaked in the bottom of the core. This phenomenon is reflected in FIG. 5. The lower curve illustrates the behavior of DNBR for the case in which the power is peaked toward the top of the core. The upper curve shows the behavior of DNBR where the power is peaked toward the bottom of the core. The minimum DNBR in both cases is 1.3; however, the curve for the case where power is peaked toward the bottom of the core starts from a slightly higher initial DNBR due to the fact that a greater time is required to halt the DNBR decrease. Therefore, the reactor should be operated either with a larger initial DNBR margin when the power is peaked in the bottom of the core, or the reactor protection system should initiate control action at an earlier time than in the case where the power is peaked in the top of the core. The instant invention provides means and methods for (1) maintaining a margin sufficient to avoid the violation of the critical value of the index on the occurrence of the most rapid anticipated operational occurrence and (2) predicting the imminent violation of the critical value of the index in sufficient time to allow the initiation and completion of successful control measures. FIG. 1 shows a typical pressurized reactor steam generating system with the inclusion of the margin maintaining system 60 and the protecting and predicting system 58. The reactor 10 consists of a core 12 and control rods 14 (only one of which is shown) which are movable into the core for reactor control. The core is constructed of a multitude of fuel pins 20 (only a few of which are shown) which define coolant channels 22 through which the coolant is circulated. The reactor coolant system 25 has a number of coolant loops only one of which is shown and which includes a hot leg 28 which delivers the heated reactor coolant to a steam generator 26. Heat from the heated reactor coolant is transferred to the secondary coolant in the steam generator 26 to form steam which is contained in a secondary coolant system 40. The steam is delivered to a turbine 42 which converts the thermal energy of the steam into mechanical rotation for subsequent conversion into electrical energy in a generator. The secondary coolant, after passing through the turbine, is delivered to a condenser 44 and recirculated by feed water pumps 46 back to the steam generator where it again picks up heat energy from the reactor coolant. After passing through the steam generator 26, the reactor coolant is circulated back to the reactor by reactor coolant pumps 32 and through cold leg 34. A pressurizing system (not shown) is provided to maintain the pressure of the primary coolant within certain acceptable limits. After being delivered to the reactor pressure vessel through the cold leg 34, the coolant is forced to circulate downwardly around the outside of the core 12 and upwardly through the interior of the core, through coolant channels 22, where the reactor coolant cools the core and its fuel pins 20. Proper control of the nuclear reactor system requires the sensing of all those parameters necessary for a computation of the various design limit indices. Ex-core detectors 16 are provided to monitor the neutron flux originating in the reactor core. Such ex-core detectors are convention, commercially available pieces of equipment such as manufactured by Reuter Stokes, Inc. or Westinghouse Electric Corp., Electronic Tube Division and the particular construction does not form a part of the invention. Also provided are strings of in-core detectors 18 for monitoring the local power of individual sectors of the reactor core. Such in-core detectors are conventional, commercially available pieces of equipment such as manufactured by Reuter Stokes Canada Ltd. and the particular construction does not form a part of the present invention. Information from the in-core detectors is necessary for the calculation of azimuthal tilt magnitude and is also used to calculate the axial power distribution. Resistance temperature detectors 36 and 38 are provided on the hot leg 28 and the cold leg 34 respectively to generate signals indicative of the temperature of the coolant as it enters the core and as it leaves the core. These temperatures are subsequently used in a calculation of .DELTA.T and average temperature for calculation purposes in determining core thermal power. The temperature of the cold leg T.sub.c is also used in a calculation of DNBR. Shaft speed detector 50 is positioned on the rotating shaft of the primary coolant pump 32 for the purposes of determining the speed of rotation (W) of the coolant pump shaft which can be used to calculate the coolant mass flow rate. Additional information is obtained by pressure sensors 51 and 53 which enable the calculation of the pressure head across the cooland pump. This information enables a more direct calculation of coolant mass flow rate. Also provided is a pressure sensor 48 to give an indication of the reactor coolant system pressure. Turbine 42 has a pressure sensor 56 which give an indication of the turbine first stage pressure for the purposes of calculating a total plant power. The method for calculating total plant power from a plant load signal (turbine first stage pressure) is fully and completely disclosed in U.S. Pat. No. 3,423,285 issued on Jan. 21, 1969 to C. F. Curry, et al., the disclosure of which is incorporated herein by reference. In addition to the turbine first stage pressure sensor 56, the secondary coolant system is equipped with mainstream pressure sensor 43, feedwater temperature detector 45 and feedwater flow detector 47. The reactor 10 and its control rods 14 are provided with a rod position detection system 54. This system is composed of two or more reed switch assemblies positioned adjacent to and outside of each control rod housing 52 that extends external to the reactor pressure vessel. The reed switches 54 are activated through the control rod housing which is made of magnetically permeable material, by a permanent magnet affixed to a control rod extension shaft. By this means the position of every control rod can be redundantly determined and logged for the purposes of determining various factors to be used in the calculation of DNBR and local power density. More specifically, CEA group position signals and CEA deviation signals are used to calculate pin and channel planar radial peaking factors (see infra.). All of the various signals described above which describe various reactor parameters are delivered to either the calculation means 58 called the core protection calculator, the calculation means called COLSS 60, or both. The parametric signals generated for and utilized by the core protection calculator and COLSS are quite similar although minor differences exist. These differences in detail are differences which would be obvious to one skilled in the art of reactor control. Therefore, for the sake of simplification and clarity, FIG. 1 shows the same signals delivered to both the Core Protection Calculator 58 and COLSS 60. However, it should be understood that in actual practice the signals delivered to these two systems are derived from separate isolated sources. It should also be recognized that in order to meet protection criteria, the core protection calculator must be redundant with redundant input signals. The calculation means 58 and 60 may either be hard-wired analogue systems or special purpose digital computers. The core protection calculator calculates and projects a value of DNBR over a certain time period T. Similar calculations and predictions can also be made for other design limits according to this invention. The core protection calculator then compares the calculated and projected value of DNBR to a fixed setpoint indicative of the violation of the specified acceptable fuel design limit on DNBR. When the calculated and projected value of DNBR is equal to or falls below the fixed setpoint, a signal is generated by the core protection calculator 58 and is sent to the control rod control means 62 for the purposes of scramming the control rods 14 into the core thereby terminating the chain reaction within the reactor core. Some of the variously generated signals representing the reactor parameters described above are also delivered to a calculation means 60 called the Core Operating Limit Supervisory System (COLSS). It is the function of COLSS 60 to make a very accurate calculation of a DNBR operating limit which contains sufficient margin to allow the core protection calculator to sense, calculate, predict and shut the reactor down in a timely fashion that avoids the violation of any fuel design limits. The operating limit thus generated may be utilized in either of two fashions in order to control the operation of the reactor. The first is merely to register the limit on a visual indicator 170 which would allow the reactor operator to compare the actual reactor operating condition to the COLSS limit. With this knowledge available to the operator, he will be able to operate the reactor in such a way that a sufficient margin is continuously maintained while at the same time maximizing the capability and availability of the reactor. The second method would be to automatically restrict the plant power to be within the COLSS limit thereby insuring that the necessary margin is maintained. Core Protection Calculator The low DNBR and high local power density trips initiated by a plurality of Core Protection Calculators of which only one is shown (see FIGS. 6 and 6a) function to assure that specified acceptable fuel design limits are not exceeded during anticipated operational occurrences (which are defined as those conditions of normal operation which are expected to occur one or more times during the life of a nuclear power plant). In particular, these occurrences include single electrical component or control system failures, that can result in transients which could lead to a violation of specified acceptable fuel design limits if protective action were not initiated. The protection system is designed so that reactor core protective action will not be initiated during normal operation of the reactor. Four measurement channels are provided for each parameter monitored by the system. The four measurement channels are independent and isolated from each other as are the core protection calculators. These four channels provide trip signals to six independent logic matrices, arranged to effect a two-out-of-four coincidence logic or a two-out-of-three coincidence logic. This redundancy enables one of the four measurement and calculation channels to be taken out of service for maintenance or testing while still providing the necessary protective function for the operating reactor. In this case, the protection system logic is changed to a two-out-of-three coincidence logic for the actuation of plant protective action. It should be understood that this discussion and the functional block diagram of FIG. 6 describe only one out of the four parallel and independent protective channels. For the purposes of illustrating the invention and for the purpose of teaching a preferred mode of operation, attention will be focused on the two fuel design limits called: 1. A 1.3 Departure from Nucleate Boiling Ratio (DNBR) in the limiting coolant channel in the core, and 2. The peak local power density in the limiting fuel pin in the core. The low DNBR trip maintains the integrity of the DNBR fuel design limit and the high local power density trip maintains the integrity of the fuel design limit on peak local power density. While discussion will be focused on these two indices, it should be apparent that the invention is not so limited and applies to any design limits which may be represented by a mathematical index. The subject trips monitor all the Nuclear Steam Supply System parameters that affect these fuel design limits. The pertinent parameters are monitored directly where possible, or indirectly by monitoring Nuclear Steam Supply System variables that can be related to the particular parameters of interest through well defined mathematical relationships. The number and location of sensors are chosen to assure that the reactor core is adequately monitored for all allowable operating configurations. The parameters of primary importance for the calculation of DNBR and local power density are the following: 1. Reactor Core Power; PA1 2. Reactor Core Power Distribution; PA1 3. Reactor Coolant Flow Rate; PA1 4. Reactor Coolant System Pressure; and PA1 5. Reactor Coolant Inlet Temperature. PA1 .phi.=core power; PA1 .phi.m=coolant mass flow rate; PA1 P =reactor coolant system pressure; PA1 T.sub.c =cold leg temperature; PA1 F.sub.z (z)=axial power distribution; PA1 F.sub.r =integral radial peaking factor; and PA1 T.sub.r =azimuthal tilt magnitude. PA1 .phi.=core power in percent of full power; PA1 T.sub.c =coolant inlet temperature; PA1 P=coolant pressure; PA1 m=coolant mass flow rate; PA1 F.sub.r =integral radial peaking factor; PA1 F.sub.z (z)=axial power distribution; and PA1 T.sub.r =azimuthal tilt magnitude. PA1 a. core average power; PA1 b. normalized core average axial power distribution; and PA1 c. normalized radial power distribution. PA1 .phi. is an n element column vector whose typical element is .phi..sub.i representing the flux at the detector at node i. PA1 .phi. is an n element column vector whose typical element is .phi..sub.j representing the flux at the core periphery at node j. PA1 S is an n.times.n square matrix whose typical element is S.sub.ij representing the shape annealing factor for the flux at node i at the detector due to the flux at the periphery at node j. PA1 .phi. is an n element column vector whose typical element is .phi..sub.k representing the core average axial power distribution at node k. PA1 F is an n.times.n diagonal matrix whose typical element is F.sub.kk representing the CEA shadowing factor associated with a CEA insertion at node k. PA1 .phi. is an n element column vector whose typical element is .phi..sub.j representing the flux at the core periphery at node j. PA1 a. Location and insertion of full and part-length CEA groups; PA1 b. The relative position of individual CEAs within the same CEA group; and PA1 c. Azimuthal Flux tilt magnitude. PA1 D.sub.rj --the maximum deviation of an individual CEA in subgroup j from the subgroup position. PA1 D.sub.si --the maximum deviation of the subgroup in group i from the group position. PA1 F.sub.r.sup.p (z)--the pin planar radial peaking factor at node z. PA1 F.sub..alpha.n --the precalculated pin planar radial peaking factor for the CEA configuration represented by the subscript .alpha.n. PA1 G.sub.i --the position of group i. PA1 F.sub.r.sup.CHAN (z)--the channel planar radical peaking factor at node z. PA1 P.sub.os --out-of-sequence penalty factor. PA1 P.sub.r+ --CEA deviation penalty factor for positive deviation. PA1 P.sub.r- --CEA deviation penalty factor for negative deviation. PA1 P.sub.s+ --subgroup deviation penalty factor for positive deviation. PA1 P.sub.s- --subgroup deviation penalty factor for negative deviation. PA1 S.sub.ij --position of subgroup j, which belongs to group i. PA1 T.sub.j --position of the target CEA for group j. PA1 .DELTA..sub.os --limit on allowed group deviation for out-of-sequence condition. PA1 .DELTA..sub.r --limit on allowed CEA deviation. PA1 .DELTA..sub.s --limit on allowed subgroup deviation. PA1 D is an n element column vector representing the external detector segment responses whose typical element is D.sub.i PA1 S is an n.times.n element matrix representing the shape annealing factors whose typical element is S.sub.ij PA1 P is an n element column vector representing the peripheral detector segment responses whose typical element is P.sub.i. PA1 X.sub.i =Fraction of the overall segment length to which CEA shadowing factor F.sub.i applies PA1 F.sub.i =Planar CEA shadowing factor for a given CEA insertion PA1 N=Number of different shadowing regions over a detector segment. PA1 .phi.is an n element column vector representing a quantity proportional to nuclear power whose typical element is .phi..sub.i PA1 R is an n.times.n diagonal matrix of average CEA shadowing factors whose typical element is R.sub.ii PA1 P is an n element column vector representing the peripheral detector response whose typical element is P.sub.i. PA1 .phi..sub.i =Corrected response of segment i PA1 T.sub.cmin : is the minimum of the two cold leg temperature signals PA1 C.sub.T : is a proportionality constant relating the percent change in indicated neutron flux power for a one degree temperature change PA1 T.sub.co : is a base cold leg temperature which is set at the limit of the usable sensor range. PA1 .phi..sub.cal =Calibrated neutron flux power for the channel PA1 K.sub.cal =Calibration factor for detector response to power PA1 .phi..sub.N =Total corrected detector channel response for the channel. PA1 B.sup.static.sub..DELTA.T : core power expressed in percent of rated power PA1 .DELTA.T: defined as T.sub.h .times.T.sub.c PA1 T.sub.h : defined as ##EQU5## T.sub.c : defined as ##EQU6## T.sub.h : hot leg temperature T.sub.c : cold leg temperature PA1 f.sub.i (m): are fitted coefficients that are a function of coolant mass flow rate and are of the general form: f.sub.i (m)=a.sub.i m+b.sub.i m.sup.2 +c.sub.i with a.sub.i, b.sub.i, c.sub.i being constants. PA1 with W.sub.1 and W.sub.2 being weighting coefficients. PA1 f.sub.4 (m): is a flow dependent coefficient as described above. ##EQU8## is a "filtered" derivative of T.sub.x which is Laplace transform notation is EQU (.tau.s)/(.tau..sub.1s +1) PA1 .tau..sub.1 is an equivalent coolant transport lag time constant and PA1 .tau.is a derivative gain which accounts for coolant transport delays and sensor time constants. PA1 .phi..sub.cal : calibrated neutron flux power PA1 K.sub.cal : calibration factor PA1 .phi..sub.N : total corrected detector response PA1 K.sub.cal (t-.DELTA.S): is the calibration factor that was calculated at the previous sampling time PA1 .DELTA.S: is the time internal between K.sub.cal updates PA1 .tau..sub.k : is an error weighting factor which is less than unity PA1 B: .DELTA.T power PA1 V.sub.i =volumetric flow rate of pump i PA1 RPM=pump shaft rotational speed PA1 .DELTA.P=pump head (obtained for periodic calibration) PA1 N=number of coolant pumps running PA1 K.sub.bypass =flow bypass correction factor PA1 m: is the mass flow rate PA1 C.sub.0 : is the density correction at base inlet temperature (T.sub.co) and pressure (P.sub.o) PA1 C.sub.1 and C.sub.2 : are coefficients that reflect the density change from the base conditions PA1 T.sub.c : is the maximum of the two cold leg temperature inputs PA1 P: is the measured pressurizer pressure PA1 a. a power trip set point is calculated as described above; PA1 b. the indicated core average power is compared to the set point; PA1 c. a trip will occur if the core average power remains equal to the set point for a fixed time interval. PA1 B.sub.sp.sup.st --local power density static set point PA1 B.sub.sp --local power density trip set point PA1 C.sub.s --a constant based on the allowed limiting value of the local power density PA1 F.sub.r.sup.p (z)--the pin planar radial peaking factor at node z PA1 F.sub.z (z)--the normalized core average axial power distribution, expressed as the ratio of the power at axial node z to the average power PA1 T.sub.r --the azimuthal tilt magnitude PA1 .DELTA.S--time between static set point samples PA1 .tau..sub.1 --protective system equivalent delay time PA1 .tau..sub.2 --fuel, gap and clad effective time constant PA1 Go=channel mass velocity at nominal reactor condition; PA1 A.sub.i, B.sub.i =analytically determined form coefficients for ith parameter; PA1 .DELTA..psi..sub.i =change of ith parameter from its nominal value. PA1 a. Input parameters are varied individually in many calculations performed off-line with detailed thermal hydraulic design codes. PA1 b. The same input is used in the on-line version of the model with the exception that mass velocity is iterated upon such that the DNBR obtained from the off-line codes is matched. PA1 c. Knowing the input variations from one case to the next and the required mass velocity for prediction of the minimum DNBR, the coefficients of the mass velocity correlation can be determined via multiple regression calculations. However, the minimum DNBRs produced here must equal or exceed those which result from the on-line calculations. PA1 a. calculated normalized core average axial power distribution, PA1 b. calculated axially dependent one-pin and coolant channel planar radial peaking factors, PA1 c. calculated reactor coolant mass flow rate, PA1 d. calibrated neutron flux power, PA1 e. maximum of the two input cold leg temperatures, and PA1 f. the monitored reactor coolant system pressure. PA1 m: is the calculated coolant mass flow rate in percent of 4 pump design flow PA1 (P.sub.box /P.sub.core): is the fuel assembly peaking factor defined by EQU (P.sub.box /P.sub.core)=.alpha..sub.1 F.sub.r.sup.int PA1 F.sub.r.sup.int : is the integrated planar radial peaking factor defined by ##EQU15## F.sub.z (z): is the normalized core average axial power distribution F.sub.r.sup.p (z): is the axially dependent planar radial peaking factor PA1 P.sub.pri : is the pressurizer pressure PA1 .phi..sub.cal : is the calibrated neutron flux power PA1 T.sub.cmax : is the maximum of the two cold leg temperature inputs C.sub.i 's and .alpha..sub.1 : are constants PA1 H.sub.z=0 : coolant enthalpy at core inlet PA1 .DELTA.H.sub.i : coolant enthalpy rise at node i PA1 F.sub.r.sbsb.i.sup.chan : channel planar radial peaking factor at node i, defined by EQU F.sub.r.sbsb.i.sup.chan =constant .times.F.sub.r.sbsb.i.sup.p PA1 T.sub.r : azimuthal tilt magnitude PA1 Qlocal.sub.i (t): is the heat flux at node i PA1 T.sub.cmax (t): is the inlet temperature PA1 Qlocal.sub.i.sup.periodic : is the value of the heat flux at node i used in the periodic DNBR calculation PA1 T.sub.cmax.sup.periodic : is the value of the inlet temperature used in the periodic DNBR calculation PA1 m(t): is the current calculated value of the reactor coolant mass flow rate PA1 m.sup.periodic : is the mass flow rate used in the periodic DNBR calculation PA1 P.sub.pri (t): is the current sampled value of the primary coolant pressure PA1 P.sub.pri.sup.periodic : is the value of the primary pressure used in the periodic DNBR calculation Several methods of calculation are possible and it is not intended that this invention be limited to any particular mathematical expression of the parameters used to calculate true DNBR or local power density or any other index. The discussion following is only intended to distinctly set out and describe one mode of practicing this invention. In the spirit of the foregoing statement, it should be recognized that there are various methods for determining the controlling parameters in the "hottest" pin or the "hottest" channel of the reactor core. One extreme method is to make calculations for each and every pin and channel in the core. A comparison of the calculations for each and every pin and channel would determine which pin and which channel are limiting for the operation of the reactor. A simplification of this extreme method is to take representative samples throughout the core and make conservative approximations that assure that the selected sampling method does not overlook the "hottest" or limiting pin or channel. Either of these methods is tedious and expensive to implement. An alternative method, the one to be described in this discussion, consists of making one calculation to synthesize a pseudo "hottest" or limiting pin or channel which, by making conservative assumptions, ensures that no actual pin or channel can exceed the conditions calculated for the pseudo limiting pin or channel. A brief mathematical description of this method of synthesizing a pseudo pin or channel follows with a detailed description of the terms and methods used in obtaining the various parameters included in the "Appendix to the Description of the Preferred Embodiment." For a better understanding of the following brief description reference may be made to FIG. 11. In order to calculate DNBR and the KW/ft. local power density limits, the reactor core power distribution must be known. In actuality, the method for finding the absolute power distribution for one pin or channel requires the subdivision of the reactor core into its constituent parts of individual fuel pins and coolant channels. Therefore, the function to be calculated is "the absolute power distribution for the limiting pseudo pin or channel" which is a function varying with axial position along the channels (see 108 in FIG. 11). In the interest of simplification, the following discussion will refer only to the channel and not make reference to the associated fuel pin. Mathematically, this term, the channel absolute power distribution 108, can be factored into three independent terms, two of which are constants, and one of which is a function dependent on axial position or the z coordinate. The function term is defined to be "the normalized channel axial power distribution" 110. This function can be thought of as a curve indicative of power which begins at the bottom of the core (z=0) and ends at the top of the core (z= 1). Every value of this function, the normalized channel axial power distribution 110, or every part of the curve generated by this function is called a "channel axial peaking factor." The one point on the curve generated by the function which represents the point of maximum power generation is called "the channel axial peaking factor" 111. The constant terms factored out of the "channel absolute power distribution" are selected to normalize the axial power distribution curve 110 or, restated, the constant terms are chosen so that the integration of the "channel axial power distribution" 110 from the bottom of the core (z=0) to the top of the core (z=1) gives a value of unity. The two constants which have been factored out of the "channel absolute power distribution" to give the "normalized channel axial power distribution" are called the "integral radial peaking factor" 112 and the "total power generated in the average channel" 114. The "integral radial peaking factor" 112 is defined to be the ratio of the power generated in the referenced channel to the power generated in the average core channel. In order to synthesize the limiting pseudo channel, a method is developed for generating a "pseudo absolute power distribution for a pseudo channel" 106. In order to mathematically do this, the core is thought of as being a right cylindrical solid which has been divided into slices along the z axis with each slice having a thickness of dz (see FIG. 11a and 11b). This mathematical manipulation allows each slice to be treated as a two-dimensional model; the core, therefore, comprises a simple summation of all of the two-dimensional slices. For each slice a ratio of the power of the "hottest" pin in that slice to the average power of all of the pins in that slice is generated. (See FIG. 11a.) This ratio is called the "planar radial peaking factor." The "planar radial peaking factor" is analogous to the "channel axial peaking factor" discussed above for a single channel with the exception that there exists one "planar radial peaking factor" for each slice. If all of the planar radial peaking factors for all of the slices of thickness dz are plotted on a curve, thereby melding the "hottest" pins in each slice into a single pseudo pin (see FIG. 11b), the result is a step curve which varies with axial position 102. The step function 102 generated by the "planar radial peaking factors" may be multiplied by the "core average axial power distribution" 100 which is a z dependent function and the "average over all slices of average pin power per slice" 104, which is a constant for one set of reactor operating conditions. This former term, the core average axial power distribution, is a cross-plot of the linear power density in each axial slice versus the axial position of the slice, with appropriate normalization such that integration of the curve from z=9 to z=1 gives a value of unity. This latter term, the "average over all slices of average pin power per slice" 104 is a number proportional to the total power generated in the core. The function which results from the multiplication of the function generated by the "planar radial peaking factors" 102, the "core average axial power distribution" 100 and the "average over all slices of average pin power per slice" 104 is the "pseudo absolute power distribution for the pseudo limiting pin or channel" 106. In this way a pseudo pin can be generated representing the worst possible power distribution or the power distribution for the limiting pin, since the hottest pins in each slice have been stacked into one pseudo pin extending through all of the slices (see FIG. 11b). Once this "pseudo absolute power distribution for the pseudo limiting pin" 106 has been generated, it may then be factored into a normalized axial power distribution 110 and an integral radial peaking factor 112 for the pseudo limiting pin just as in the case for an actual pin. The purpose of making these calculations is to obtain a normalized axial power distribution F.sub.z (z) and an integral radial peaking factor F.sub.r which are two of the input signals necessary for the calculation of the DNBR index and the KW/ft. local power density index according to functional equations (1) and (2) which appear in the "Background of the Invention." The above concepts are discussed in greater detail in the "Appendix to the Description of the Preferred Embodiment". Referring now to FIG. 6, the method used by the core protection calculator will be described. A calculation of DNBR is made in functional element 76 according to the equation: EQU DNBR=f(.phi., m, P, T.sub.c, F.sub.z (z), F.sub.r, T.sub.r) where An explanation of .phi., F.sub.z (z), F.sub.r and T.sub.r is to be found in detail in the "Appendix to the Description of the Preferred Embodiment." The signals are monitored by the DNBR calculator 76 and a snapshot is taken of these signals approximately every two seconds. The snapshot values are used in a calculation of DNBR which takes approximately 2 seconds. The two to four second old indication of DNBR which results is not sufficiently responsive to the actual condition of the reactor core to allow adequate core protection; therefore, means are provided for continuously updating this basic DNBR calculation. For the purposes of this discussion and claims, the word "continuously" should be taken to mean "of a periodicity which is substantially higher than the frequency of the periodic calculation." The values of each of the parameters used in the most recently completed periodic snapshot calculation of DNBR are compared at 78 with the continuously monitored parameters. This comparison results in an update change in parameter for each of the parameters used in the DNBR calculation of element 76. Element 78 then multiplies each change of parameter by a value which conservatively converts each change of parameter into a change of DNBR. An example of the type of multiplication which may be made is the multiplication of change in temperature by a value which is equivalent to the partial derivative of the DNBR equation with respect to temperature. The partial derivative may either be taken to be a constant value or it may be taken to be a function which is dependent upon all other parameters except T, the choice depending upon which multiplication gives a more satisfactory result. After element 78 generates a change in DNBR for the change in each parameter, all of the chages in DNBR are summed to obtain a net change in DNBR. This net change in DNBR and the snapshot calculation of DNBR are added in element 80, the result being a continuously updated value of DNBR which is dynamically responsive to changes in the reactor core. The above described calculation standing alone is not sufficient for the function of protecting the reactor core. The reason for this is that it is not sufficient merely to know what the DNBR is but it is necessary to be able to predict what the DNBR will be far enough in advance to allow the initiation and completion of corrective action that will avoid limit violation. Therefore, a projection of DNBR must be made. The projection is accomplished by elements 82, 84 and 86 of the Core Protection Calculator. These elements sense the dynamic response of one of the reactor parameters and take the slope or the rate of change of the parameter with respect to time. This slope or rate of change is then projected over a period of time T. In this manner, a projection of change of DNBR may be calculated on the basis of the instantaneous rate of change of one of the reactor parameters. Projections of the rate of change of each parameter are continuously made, thereby providing a continuous projection of change of DNBR. Now, referring to FIG. 6 element 82 takes the derivative of the parameter, for example pressure, with respect to time. Element 84 then multiplies the derivative of the signal with respect to time by a value which is proportional to the partial derivative of DNBR with respect to the parameter being projected. The product of these two terms is a term indicative of the change in DNBR with respect to time due to the change in the parameter in question. This change in DNBR with respect to time is then multiplied by a time period T.sub.p to obtain a projected change in DNBR. The time period over which the change in DNBR is projected is a period which is calculated to allow the sensing, calculating and predicting of a limit violation and to allow the initiation and completion of corrective action before the violation of a design limit all of which represent system inertia or system reaction time. The time period T is illustrated in FIG. 2. It should be recognized from FIG. 5 and from the discussion above that the multiplicative time period T may have to be modified depending upon the power distribution within the core. The projected change in DNBR which has been derived by elements 82, 84 and 86 is then limited to negative values in element 88 before it is summed in element 90 with the calculated value of DNBR of element 80. A calculation similar to the one which has just been described may be made for any of the reactor parameters on which DNBR depends. In actual operation, it is likely that the dynamic response of DNBR to changes in coolant temperature, axial power distribution, integral radial power distribution, and azimuthal tilt magnitude will not be fast enough to require the projection techniques just described. However, it is likely that the parameters of core power, reactor coolant system pressure and coolant mass flow rate will be projected as above since DNBR is very responsive to changes in these parameters. It is indeed likely that DNBR will be so responsive to changes in coolant mass flow rate that a technique for desensitizing the DNBR response should be provided in order to avoid unnecessary reactor trips. In order to do this, the core protection calculator may take advantage of the presence of a system similar to COLSS which insures that an appropriate margin to the fuel design limit will be maintained. It is a characteristic of the COLSS system that it need not be as dynamically responsive to changes in reactor parameters as the core protection calculator since COLSS assures that adequate margin is maintained to fuel design limits only during normal operation. The core protection calculator, on the other hand, must be dynamically responsive to changes in reactor parameters in order to adequately protect the reactor core during anticipated operational occurrences. The differences between these two systems mandate an extremely fast, although approximate, core protection calculation of DNBR and predicted DNBR, and a slower but extremely accurate COLSS DNBR calculation. Therefore, it is a possibility that the approximations made to allow a rapid and dynamic calculation of DNBR in the core protection calculator may result in a value of DNBR that is less accurate than the values of DNBR calculated by COLSS. Advantage may be taken of these calculational differences to provide a method to desensitize the responses of the core protection calculator value of DNBR to changes in the coolant mass flow rate. A brief description of this desensitization process follows. The DNBR value calculated by COLSS which includes an opening margin is used as the static base value of DNBR to which continuous instantaneous modifications are made rather than that value of DNBR which is periodically calculated by the core protection calculator. This static base value of DNBR called DNBR.sub.limit is then updated to make it responsive to changes in coolant mass flow rate to obtain an updated value of DNBR. This updated value of DNBR is then compared to the updated values of DNBR obtained from the above-described periodic calculation and the higher of the two values is selected. To this higher value is added a projected change in DNBR which is obtained in a projection procedure similar to that described above for a projection of primary pressure. Reactor coolant pump speed signals (W.sub.i) are continuously delivered to element 120 along with the cold leg temperature and the reactor coolant system pressure where a calculation is made converting these signals into coolant mass flow rate signals (m.sub.i). FIG. 6 shows four sets of signals from the usual configuration of four reactor coolant pumps. Element 122 sums the four mass flow rate signals (m.sub.i) to produce a total coolant mass flow rate signal (m.sub.t). In a series of steps similar to the pressure signal projection sequence described above, the derivative of m.sub.t is taken at 124 with respect to time to get the rate of change of m.sub.t. This value is then multiplied by the partial derivative of DNBR with respect to the mass flow rate at 126 to obtain a signal indicative of the rate of change of DNBR due to the rate of change of the coolant mass flow rate. Next, a projection of the change of DNBR over a period of time T is made in element 128 by multiplying the signal indicative of the rate of change of DNBR with respect to time by a period of time T. Just as in the example described above for the projection of DNBR due to changes in pressure, the value of T may be dependent on the axial power distribution that exists in the reactor core. Thus, element 134 generates a time period T.sub.fz as a function of axial power distribution. There are numerous acceptable ways that element 134 can handle this function generation. A suggested preferred mode is illustrated in FIGS. 7 and 8. FIG. 7 is a plot of axial power distribution F.sub.z (z) from the bottom to the top of the core. The core is divided into two equal parts; the lower half of the core and the upper half of the core. The axial power distribution is integrated in element 134 over each half to get two numbers symbolized by L and U meaning lower and upper halves. FIG. 7 represents a reactor configuration in which the axial power distribution is weighed more heavily toward the bottom of the core. In such a situation the area under the axial power distribution curve toward the bottom of the core, L, would be larger than that at the top of the core U. By using these two, values L and U, an Axial Shape Index can be generated which gives a single number indicative of the axial power distribution. One possible axial shape index can be generated by the following equation: EQU ASI=(L-U/L+U) An examination of this equation shows that the Axial Shape Index will be negative when the power distribution is peaked toward the top of the core, and positive when the power distribution is peaked toward the bottom of the core. FIG. 8 shows one way in which this fact may be utilized to generate a time period T which is a function of the axial power distribution. The shape of the curve plotted with Axial Shape Index versus T is chosen by the reactor designers to make best use of the fact that it takes less time to terminate the reactor's nuclear chain reaction when the power is peaked in the top of the core. Thus, the greater the power peak in the top of the core, the larger is the negative value of the Axial Shape Index. The curve of FIG. 8 shows that T decreases with larger and larger negative values of Axial Shape Index. Once the projection of change in DNBR is obtained, it is limited to negative valves in element 130 (for the sake of being conservative) and is then added to an updated base value of DNBR in element 132. The appropriate updated base value of DNBR is the larger of either the value computed by the periodic sampling method described above and which is computed in elements 76, 78 and 80, or the value computed by using a fixed based value of DNBR which includes an operating margin. For the sake of simplifying the discussion, this base value of DNBR, which includes a margin, will be called DNBR.sub.limit and may be found on FIG. 6 as an input to element 98. The DNBR.sub.limit value is a constant that may be determined once the physical characteristics of the reactor protection equipment are determined. The functioning of the COLSS margin system will be described, infra, following the completion of the discussion of the core protection calculator. The inclusion of the DNBR.sub.limit value as a base value for the desensitization of the response of DNBR to the coolant mass flow rate is justified by the fact that when the reactor is operated on the basis of the COLSS margin, as it should be when operated on a steady-state basis, then reactor conditions should always be such that this margin exists. The DNBR.sub.limit must be dynamically updated for recent changes in DNBR. The following is a description of the preferred method for accomplishing this update. The values of m.sub.t (total coolant mass flow rate) are continuously fed to element 92 where the following calculation is made. The mass flow rate is continuously integrated over a period of time which is long compared to typical transients which effect the parametric value of mass flow rate. The value obtained from the integration is continuously divided by the same time period over which the integration was made, thereby generating an average value of coolant mass flow rate over the stated period of time (m). This flow average value (m) is then compared in element 94 to the instantaneous value of coolant mass flow rate (m.sub.t) to obtain a recent deviation of mass flow rate (.DELTA.m) from the long term average value of mass flow rate. If the deviation is negative, i.e., m less than m.sub.t, the value of the mass flow rate integral (m) is automatically increased such that m=m.sub.t. In element .DELTA.m is multiplied by the partial derivative of DNBR with respect to the coolant mass flow rate (m). The product of this multiplication is a value indicative of any decrease in DNBR (.DELTA.DNBR) due to any recent decrease of coolant mass flow rate from the long term average value of mass flow rate. The derived .DELTA.DNBR is then used as the value which updates the DNBR.sub.limit in element 98 to give an updated value of DNBR.sub.limit. A signal representing DNBR.sub.limit is then sent to element 118 where it is compared to the updated DNBR value which has concurrently been calculated in elements 76, 78 and 80. Element 118 selects the higher value of the two updated DNBR signals to be used as the value to which the projected change in DNBR over time T is to be subtracted in element 132 to generate a predicted value of DNBR T seconds into the future due to a projected change in coolant mass flow rate. At this point in this description, two values of predicted DNBR have been described as having been generated in elements 90 and 132. Element 136 selects the lower value of all of the predicted values of DNBR and element 138 compares that selected low value to the critical value of DNBR (1.3). If the predicted value is equal to or less than the critical value of DNBR, then a signal is sent to element 140 which generates a signal for tripping the reactor. The tripping action de-energizes the electromagnetic circuits on a magnetic jack control rod drive mechanism and the control rods are allowed to fall into the reactor core. A second embodiment of the core protection calculator 58 appears in FIG. 6A. In this embodiment, the same signals are generated as inputs with the exception that one or more of them are modified in element 75. Element 75 generates signals which are dynamically commensurate with the parameters that the fuel design limit is most closely related to; these dynamically compensated signals are then projected over time T. The time projection technique described in the first embodiment as occurring in elements 82, 84 and 86 is thus handled by element 75. The net result of this modification is that elements 82, 84 and 86 have been embodied in new element 75 and the dynamic compensation and time projection is made before the DNBR calculation and update are made in elements 76 and 78. Thus, the DNBR calculation and update are calculations which work with at least one parameter which has been dynamically compensated and projected into the future, resulting in a calculation of projected DNBR. Core Operating Limit Supervisory System (COLSS) The function of the Core Operating Limit Supervisory System is to insure that the nuclear reactor is operated with sufficient margin to critical core design limits so that the Reactor Protection System has time to terminate an incident (anticipated operational occurrence) before the violation of a fuel design limit. The method and apparatus described herein can be adapted to calculate a limiting value of a reactor parameter which encompasses such a sufficient margin. This method is applicable to any design limit; however, for the sake of simplification, the following discussion will be focused primarily on fuel element clad integrity and overheating as indicated by the occurrence of departure from nucleate boiling and the appropriate index, DNBR. As described above, DNBR is a function of a number of either measured or calculated reactor parameters and may be expressed in the functional notation: EQU DNBR=f(.phi., T.sub.c, P, m, 0, F.sub.r, F.sub.z (z), T.sub.r) where: A detailed description of how core power, axial power distribution, integral radial peaking factors and azimuthal tilt magnitude are generated appears in the "Appendix to the Description of the Preferred Embodiment." The critical value of DNBR is 1.3 (or 1 depending on the definition used). One possibility for operating the reactor with a sufficiently large margin (see FIG. 4) is to make sure that the reactor is operated at a DNBR value (say 1.8) which is sufficiently distant from the critical value of 1.3 to allow enough time to sense the occurrence of an accident, predict the effect of the occurrence of the accident and take appropriate protective action, such as scramming the reactor. Using this approach, the value of one of the parameters on which DNBR, if functionally dependent, (for example, core power) may be calculated from the above equation by using the false input of 1.8 for DNBR (see FIG. 10A). The result of the on-line calculation is a false value of the parameter which is then used as an operating limit. This calculated false parameter (power) or limit is then displayed to the operator informing him that if he were to allow the actual reactor power to exceed the computed false power or power limit, then the DNBR margin would be less than that required for the avoidance of design limit violation on the occurrence of an anticipated operational occurrence. At this point the operator has at least two choices. He can cause control rods to be inserted into the reactor, thereby decreasing actual reactor power until the actual power no longer exceeds the power limit, or, he can cause variations in one of the other parameters on which DNBR is dependent, such as inlet temperature T.sub.c, coolant pressure P, or coolant mass flow rate m, in such a way as to raise the computed power limit (i.e., increase the existing DNBR margin) so that the actual power no longer exceeds the computed power limit. In the above described procedure and calculation, the variables in the DNBR equation were treated in three distinct ways: (1) one variable (DNBR) was modified (from 1.3 to 1.8); (2) one variable (power) was calculated as the unknown, the solution for which gave a limit, and (3) the remaining variables were treated as known values whose true values were used in the calculation of the limit on the basis of the false or modified input. It is mathematically possible, and perhaps desirable to switch the variables and their roles. Thus, an operating limit may be calculated for the temperature variable rather than for power. In this case, a temperature limit is obtained by utilizing the actual reactor power in the calculation and then the temperature limit is compared to the actual temperature by the operator just as in the case described above for the power limit and the actual power. Another way in which a power limit may be obtained is to generate as input signals the true values for al the variables (except power) including a DNBR signal of 1.3. The calculation results in an actual power which may then be modified to obtain an adjusted power or power limit which encompasses an operating margin. (See FIG. 10B). Another way in which a modification can be made to incorporate an operating margin is the modification of one of the other variables, such as coolant mass flow rate, rather than the DNBR index or the power. In fact, in practical application, it turns out that the more desirable method is to falsify the coolant mass flow rate signal rather than the DNBR value, and to iterate on reactor power as the calculated operating limit. Therefore, as the preferred mode of practicing this invention, FIG. 10 illustrates the method described above with the coolant mass flow rate being modified to build in a margin which is then used in calculating a core power operating limit. Referring to FIG. 10, the inputs of feedwater temperature (T.sub.s), mainstream pressure (P.sub.s), feedwater flow (m.sub.2), turbine first stage pressure (P.sub.t), core inlet temperature (T.sub.c), reactor coolant pump speed (W), reactor coolant pump head (.DELTA.P), reactor coolant pressure (P.sub.pri), CEA positions, and in-core flux (.phi.) are delivered to the core operating limit supervisory system (COLSS) 60. From these signals, core calorimetric power 142, core plant power 144, coolant volumetric flow rate (m) 146, fuel pin and coolant channel planar radials (F.sub.r) 148, azimuthal tilt magnitude (T.sub.r) 150, and normalized axial power distribution F.sub.z (z) 152 are computed. See U.S. Pat. No. 3,752,735 entitled "Instrumentation for Nuclear Reactor" as an illustration of one prior art technique for the derivation of core calorimetric power. See U.S. Pat. Nos. 3,423,285 and 3,356,577, which patents are illustrative of the prior art techniques for computing core (plant) power from turbine first stage pressure and for automatically calibrating the core (plant) power to core (calorimetric) power respectively. The signals indicative of fuel pin and coolant channel planar radials, azimuthal tilt magnitude and normalized axial power distribution are computed as described in the "Appendix to the Description of the Preferred Embodiment." One of the totality of generated signals is modified by a factor which is calculated to build a margin into the operating limit to be computed. For purposes of this description, the variable chosen to be modified is the coolant flow rate, m, as shown in element 156. The modification made in element 156 is of the general form EQU m.sub.adj =m[f(reactor condition] where m is the true value of the coolant flow rate and f(reactor condition) is the modification increment which falsifies the flow rate to generate an adjusted flow rate m.sub.adj. One possible modification function, f (reactor condition) is [1+(.DELTA.m/m)]. The value of .DELTA.m is a value which is a function of reactor reaction time or system inertia including delays in sensing, calculating, predicting and actuating the reactor's protection mechanisms and which also varies with the reactor core axial power distribution similar to the way in which T varied in the discussion of the projected technique supra. If the core power is peaked toward the top of the core, effective control of the core's chain reaction may be accomplished in a shorter time than if the core power were peaked in the bottom of the core. Thus, with the peak toward the top of the core, a smaller margin is adequate to ensure that timely control action may be taken than for the case in which the power is peaked toward the bottom of the core. The functional dependence of .DELTA.m on axial power distribution may be developed by using the Axial Shape Index and a technique which is similar to that described for the T dependence on axial power distribution discussed above in the description of the core protection calculator. Using the signals generated from the various input parameters, including the adjusted coolant flow rate signal, an operating limit (.phi.=powder) is calculated in element 162 according to the equation: EQU DNBR=f(.phi., m.sub.adj, T.sub.c, P, F.sub.z (z), F.sub.r, T.sub.r). A similar core power limit based on local power density or any other design limit may also be calculated in element 158 and additional elements (not shown) in accordance with the equation EQU KW/ft.=f(.phi., F.sub.z (z), F.sub.r) or an appropriate aternative equation. A similar signal modification (not shown) could be made for this calculation. The signals from elements 142 and 144 representing core (calorimetric) power and core (plant) power respectively are delivered to element 154 where the core (plant) power is automatically calibrated to the core (calorimetric) power. The resultant signal generated by element 154 is a signal representative of the actual power of the reactor. This signal is compared to the lowest power limit available. FIG. 10 shows three available power limits, one generated in element 162 by the DNBR equation, one generated in element 158 by the KW/ft. equation and one determined by the licensed power limit as generated in the element 166. These three (or more) power limit signals are compared and the lowest selected in element 164 for comparison with the actual power as generated in element 154. If the comparison indicates that the actual power is in violation of the power limit, then an alarm means 170 is activated alerting the operator that remedial action is required. Core Protection Calculator and the Core Operating Limit Supervisory System The third invention herein disclosed consists of the combination of the Core Protection Calculator 58 and the Core Operating Limit Supervisory System (COLSS) 60. See FIG. 1, FIG. 6 and FIG. 9. The combination of these two systems creates a symbiotic relationship designed to protect the nuclear reactor 10 from design limit violations, both in steady-state operation and during the transients caused by anticipated operational occurrences. The combination of the Core Protecting Calculator 58 and the COLSS 60 systems takes advantage of each of their design characteristics. The Core Protection Calculator 58 is a protection device which must respond rapidly to system transients so that safety and system design limits are not violated. This system must not only be rapidly responsive to reactor system transients, but the protection system must also be redundant so that a single failure cannot prevent the required protective action from occurring. In order to meet these requirements, the core protection calculator has been designed to consist of four mini-computers, each mini-computer comprising an independent and redundant channel. Increasing cost for increasing computer complexity and the need for rapid response requires the core protection calculator 58 mini-computers to be limited in their degree of calculational accuracy. The COLSS system 60, on the other hand, need not include either the characteristics of system redundancy or rapid response. Therefore, COLSS calculations may be made in a high powered plant computer at a relatively slow rate of calculation with the highest degree of accuracy which can possibly be achieved. As a result, the operating limit values calculated by COLSS are much more accurate than the calculations made by the core protection calculator. By recognizing these differences, utilization of the two systems may be made in a way that takes advantage of the strong points of each. Therefore, reactor steady-state operation is based on the slow, although very accurate, operating limit calculated by COLSS. As has been described above, this limit is a value which provides sufficient margin to the design limits to allow the core protection calculator to respond to an incident and terminate the reactor core chain reaction before the design limits are violated. Due to the inaccuracies of the core protection calculator and the need to provide allowances for them, the core protection calculator will normally indicate that the reactor is running closer to a specified limit than would be indicated by the COLSS. When this is the case, the reactor tripping system will be conservative and will trip the reactor before a trip is actually required. This unavoidable behavior is acceptable since it represents increased conservatism and does not raise the potential for design limit violation. For most efficient operation, the core protection calculator must be made sufficiently accurate to avoid reactor trips on normal parameter fluctuations which are expected to occur during steady-state operation of the reactor system at the COLSS operating limit. In the above described manner, COLSS and the core protection calculator operate hand-in-hand to assure the efficient and safe operation of the nuclear reactor system. The Core Protection Calculator 58 projects ahead in time a prediction of DNBR and trips the reactor when the predicted value is seen to violate the design limit. COLSS calculates a parametric limit value which incorporates a margin, and on the basis of which the reactor should be operated. When both of these systems are joined in combination, the reactor can be operated in a safe and efficient manner. While a preferred embodiment has been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation. The following "Appendix to the Description of the Preferred Embodiment" is a description of the generation of some of the input signals used in the above described calculation. In reading the appendix, it may be helpful to refer to FIG. 9 and to FIG. 11. APPENDIX TO THE DESCRIPTION OF THE PREFERRED EMBODIMENT Methods Used to Determine Core Power Distributions The axial heat flux distributions in the channels 22 and the local power density distributions in the fuel pins 20 are dependent upon the core power distribution. The core power distribution can be thought of as being composed of three distinct components: The method used to define the "hot" pin and "hot" channel power distribution consists of measuring core average power, synthesizing the normalized core average axial power distribution from out-of-core neutron flux signals and synthesizing the radial peaking factor from CEA 14 position measurements. Normalized Core Average Axial Power Distribution A normalized axial flux distribution is synthesized from the response of the ex-core detectors 16 and corrected for shape annealing and rod shadowing in order to determine a core average normalized axial power distribution. The ex-core detectors 16 are sensitive to the leakage neutron flux from the reactor core 12. The neutron flux seen by each of the three axially positioned detectors 16 is dependent upon the axial flux distribution near the core periphery, the radial flux distribution near the core periphery, and the diffusion and capture of neutrons leaking from the core periphery to the point of incidence with the ex-core detectors 16. Shape annealing, or the relative effect of contributions of the core peripheral axial flux distribution to a given detector, is due to the placement of the detector 16 away from the core periphery 12. The detectors "see" a distorted peripheral flux distribution due to the scattering and diffusion of neutrons between the core periphery and the detector locations. The shape annealing factors are dependent only upon the geometric location of the detectors 16 and do not depend on the axial flux distribution. Rod shadowing is the effect of rod insertion on the peripheral axial flux distribution relative to the core average power distribution. Rod shadowing factors are dependent upon the rods 14 inserted in the core 12. Since the ex-core detectors 16 primarily measure the flux distribution of the peripheral fuel elements 24 and since the detectors 16 are located a fixed distance from these elements 24, correction factors in the form of rod shadowing and shape annealing and required to obtain the core average axial power distribution in the core 12 from the flux distribution at the detectors. Calculation of Normalized Core Average Axial Power Distribution The method by which the normalized axial flux distribution at the ex-core detectors is used to yield the normalized core average axial power distribution is described in the following. Let the normalized axial flux distribution at the detectors be designated at .phi..sub.i where i designates the axial node at which the flux is defined. Let S.sub.ij be the contribution of a unit flux at axial node j at the core periphery to the flux at node i at the detector. Let .phi..sub.j be the flux at the core periphery in axial node j. Thus, the flux at the detector is given by ##EQU1## Representing the fluxes at the detector and periphery as n element column vectors implies that EQU .phi.=S.phi. where the flux shape at the periphery is then given by EQU .phi.=S.sup.-1 .phi. Having determined the peripheral axial flux distribution, it is now necessary to determine the core average axial power distribution .phi..sub.k by correcting for control element assembly (CEA) insertion. CEA position indications are used to generate axial CEA shadowing factors. A planar CEA shadowing factor for a particular planar CEA configuration is defined as the ratio of the peripheral flux at an axial node having the planar CEA configuration to the peripheral flux at an unrodded node, given that the power in the node is the same in both cases. When the entire node is not uniformly rodded, the planar CEA shadowing factor for that node is given by ##EQU2## where .gamma..sub.k is the fraction of the node which is rodded with the planar CEA configuration associated with planar CEA shadowing factor F.sub.k. These factors are conveniently arranged into an n.times.n diagonal matrix F. The resultant correction process is mathematically defined as follows: EQU .phi.=F.sup.-1 .phi. where: Net Algorithm The required correction algorithm for converting the ex-core axial flux distribution as synthesized using the ex-core detector responses to the normalized core average axial power distribution is expressed by the following matrix equation: EQU .phi.=F.sup.-1 S.sup.-1 .phi. Normalized Radial Power Distribution The radial power distribution that exists at any time for a specified fuel loading pattern, fuel enrichment, and fuel burnup is dependent primarily upon the following: The radial power distribution will vary along the core 12 height as a function of the CEA 14 configuration that exists in a given axial segment. The peak value of the normalized radial power distribution in a given axial segment is defined as the one-pin planar radial peaking factor for that segment. The product of the one-pin planar radial peaking factor, core average power and normalized core average axial power distribution in an axial segment defines the power produced in the "hottest" fuel pin 20 in that axial segment. The product also defines a quantity that is directly proportional to the heat flux in the "hottest" channel 22 in the axial segment. The combination of planar radial peaking factors in axial segments having known CEA 14 configurations, synthesized normalized core average axial power distribution and the measured core average power conservatively defines the axial heat flux distribution and local power density distribution in the "hot" channel 22 and fuel pin 20, respectively, without consideration for azimuthal tilt. The presence of an azimuthal tilt will have the effect of increasing an "untilted" planar radial peaking factor by an amount that is directly proportional to the magnitude of the tilt. The core protection calculator will monitor the position of every CEA group and receive information from CEA Calculators on the relative position of CEAs 14 in the same group. This information is used to define the CEA 14 configuration along the entire core 12 height. Knowing the total CEA configuration, precalculated planar radial peaking factors are chosen that apply to each axial segment. The radial peaking factors are then modified to reflect an azimuthal tilt magnitude that is at its worst case operating limit. This latter adjustment is made because the azimuthal tilt magnitude is not a directly monitored parameter. The result is a set of axially varying planar radial peaking factors that can be used in defining the "hot" pin and "hot" channel conditions. Calculation of Planar Radial Peaking Factors Planar radial peaking factors are used in conjunction with the core average axial power shape to determine the three dimensional peaking factor and the heat flux axial profile. Since the radial peaking depends upon the number and location of control CEAs, planar radial peaking factors depend upon CEA bank insertion and hence upon axial position in the core. The planar radial peaking factors are determined by the Core Protection Calculators (CPCs) for each of 25 axial nodes. Determination of the pin planar radial peaking factors at any given axial node by the CPC is done by a table look-up routine using a precalculated table of values for the planar radial associated with those CEA groups which have been inserted in normal sequence into the axial node of interest. Penalty factors are then applied to account for the increased radial peaking which would result from conditions other than normal. These include CEA groups being inserted out of sequence, excessive misalignment of subgroups from the group average position, and excessive misalignment of individual CEAs from its average subgroup position. In the discussion which follows, the term subgroup is defined as being any one of the sets of four CEA combinations which are operated and positioned as a unit. A group is then any combination of one or more subgroups which are operated and positioned as a unit. Regulating CEA groups are considered to be inserted in a prescribed sequence, with group withdrawal in the reverse order. Any insertion of regulating groups in other than this prescribed sequence is considered to be an out-of-sequence condition. In additon to the normal sequential insertion of regulating groups, either or both of the part length rod (PLR) groups or selected subgroups used for reactor power cutback can be inserted at the same time. F.sub..alpha.n is the pin planar radial peaking factor associated with the configuration of CEA groups identified by the .alpha.n subscript. Here n represents the number of regulating groups, inserted according to their specific sequence, and .alpha. represents the combination of PLR and/or the selected reactor power cutback subgroups inserted in addition to the regulating groups. As an example, if A represents PLR group 1, B represents PLR group 2, and C represents a reactor power cutback subgroup then the group configurations and their associated planar radials can be designated as in the following example: ______________________________________ Subscript Pin planar radial Designation, Peaking factor, CEA Configuration .alpha.n F.sub..alpha.n ______________________________________ First 3 regulating groups 3 F.sub.3 First 2 regulating groups + PLR group 1 A2 F.sub.A2 First 4 regulating groups + cutback group C4 F.sub.C4 PLR group 2 only, no regulating groups BO F.sub.BO First regulating group + both PLR groups + cutback group. ABCl F.sub.ABCl ______________________________________ The information available for calculating the planar radials includes the position of a target CEA in each of the 22 subgroups, and CEA deviation information identifying the CEA with maximum deviation from the average position of the CEAs making up the subgroup and the magnitude and direction of the deviation. The sequence of steps by which the planar radial peaking factors are determined is as follows: In the first step, target CEA positions and CEA deviations are used as inputs in a calculation where subgroup position is determined. The subgroup position is taken to be the target CEA position, unless the target CEA is the one identified as deviating from the group average. In that case the indicated position is adjusted by the amount of the deviation to give the subgroup position. Subgroup position is used as input to step two, where the average position of those subgroups making up a group is taken to get the group position. Group position and subgroup position is used as input to step three, where subgroup deviation from group position is obtained. The group position from step two is also input to step four where pin planar radial peaking factors for normal CEA sequencing are determined. For each of the 25 axial nodes in the core, the group position signals are used to determine which CEA groups have been inserted in their normal sequence into that node. The appropriate pin planar radial peaking factor, F.sub..alpha.n, associated with that CEA configuration is then obtained from a precalculated table of peaking factors. In the fifth step, the group position information is evaluated at each axial node to determine whether any CEA groups are inserted into a node other than those of which follow the normal sequence of insertion. If an out-of-sequence condition exceeds the allowable limits, an out-of-sequence penalty factor, P.sub.os, is applied to the planar radial, F.sub..alpha.n, at the nodes affected. In steps six and seven additional penalty factors, P.sub.s and P.sub.r, are applied to account for subgroup or CEA deviations in excess of the allowable limits. Group position is evaluated to identify those nodes at which the group insertion terminates. If subgroup or CEA deviation exceeds the allowable limits, then the appropriate penalty factor is applied at the nodes affected. As might be expected, the penalty factors have different values depending on the direction of the deviation. The product F.sub..alpha.n P.sub.os P.sub.s P.sub.r emerging from step seven is the nodewise pin planar radial peaking factor, F.sub.r.sup.p (z). The pin planar radial peaking factor is used in the high local power density set point and DNBR calculation. The channel planar radial peaking factor (F.sub.r.sup.CHAN (z) is used on the DNBR calculation and is related to the pin planar radial by a proportionality constant. The algorithm is summarized below. Algorithm For Planar Radial Peaking Factor Calculation Definitions: Algorithm: Subgroup positions are found from EQU S.sub.ij =T.sub.j for each subgroup j. If the target CEA is the rod which deviates from the average subgroup position, then for that j EQU S.sub.ij =T.sub.j -D.sub.rj The group position for group i is found from EQU G.sub.i =.sub.j contained in i{S.sub.ij } The subgroup deviation for group i is then EQU D.sub.si =max{S.sub.ij -G.sub.i } Having obtained the necessary position information, each axial node is evaluated to find: EQU F.sub.r.sup.p (z)=F.sub..alpha.n P.sub.os P.sub.s P.sub.r node z EQU F.sub.R.sup.CHAN (Z)=Constant F.sub.R.sup.P (Z) where: ##STR1## and F.sub..alpha.n is obtained from the precalculated table of pin planar radial peaking factors for normal sequence group configurations. Detection and Calculation of Azimuthal Flux Tilt Magnitude Indications of any azimuthal flux tilting are obtained by comparing the signals from symmetrically located detectors. These signals are fitted to the functional form: EQU .phi.(r,.theta.)=.phi..sub.o (r, .theta.) [1.times.sg(r) cos (.theta.-.theta..sub.o)] where the fundamental flux pattern is designated as .phi..sub.o, the amplitude of the tilt by s, and the orientation of the tilt by .phi..sub.o. The separable functional form in the second term of the equation has been suggested by examination of tilted flux shapes from diffusion theory representations of mild xenon oscillations. The functional g(r), which is given approximately by J.sub.1 (.alpha..sub.1, r)/J.sub.o (.alpha..sub.o, r), can be used to relate the tilt signals from different sets of detectors if no asymmetrically placed rods are inserted. Where J.sub.1 and J.sub.o are Bessel functions of the first kind of the zero and first orders. Signals from symmetrically placed in core rhodium detector strings 18 are analyzed to obtain s and .phi..sub.o. One symmetrical set such as this gives five values of s and .phi..sub.o since there are five axially located detectors in each string. Since there are five detectors per string, this gives 20 values of the pair (s, .phi..sub.o). The average value of s and .phi..sub.o are then found as well as the respective standard deviations. A study of the standard deviations gives some insight into whether or not the average values of s and .phi..sub.o indicate a true flux tilt. Core Average Power The core average power is determined by two distinct methods. The first method uses the response of the out-of-core detectors 16. The detectors responses are indicative of the neutron flux level in the core 12; when the ex-core detectors 16 are calibrated to the plant calorimetric, any change in detector response is indicative of a change in neutron flux level which in turn is indicative of a change in the power being produced in the fuel. The second method provides a measure of core power by relating the temperature rise through the core 12 to the total thermal power. The temperature rise through the core is obtained from the temperature detectors (RTD's) in the hot and cold leg coolant piping. In order to calculate the power level from the neutron flux level signals from the ex-core detectors, corrections for shape annealing, rod shadowing and inlet temperature are necessary. The shape annealing and rod shadowing corrections have been discussed above. The correction for inlet temperature change is necessary because the detector responses are dependent upon the diffusion and absorption of neutrons that occur between the core periphery and the detector locations. Reactor coolant from the steam generators 26 passes between the core 12 and detectors 16 before entering the core 12. As a result, the total neutron leakage out of the reactor vessel and thus the ex-core detector response is affected by the reactor coolant, which has temperature dependent neutronic properties. This effect is defined as temperature shadowing and has the characteristic of causing a decrease in detector response for a decrease in inlet temperature. The temperature shadowing effect is independent of the power distribution at the peripheral core assemblies. The corrected detector responses for each detector segment are summed to yield a quantity that is proportional to neutron power level. A constant of proportionality is then applied to convert the detector response to a calibrated thermal power level. The ex-core detectors 16 respond very rapidly to changes in the neutron flux and, after the above corrections are made, provide a dynamically accurate signal that can be used to sense core power changes. The core power measurement obtained from the temperature rise across the core 12, on the other hand, is a statically accurate indicator of core power. This latter core power measurement, after calibration to the plant calorimetric, provides a very accurate steady-state indication of core power. The thermal energy added to the coolant as it passes through the core 12 serves as an indicator of core power. This relationship depends upon the fuel, cladding and coolant channel configuration, their heat transfer characteristics and the primary coolant .DELTA.T rise. The coolant hot leg 28 temperature is measured by sensor 36 and the coolant cold leg 34 temperature is measured by sensor 38. The difference between these two measurements is .DELTA.T. The heat transfer properties of the fuel, clad and coolant are dependent upon the core temperatures and volumetric flow rate of the coolant. Using standard thermodynamic models and methods the core power can be determined by measuring the primary coolant hot 28 and cold 34 leg temperatures. These measurements are used in a mathematical expression which accurately models the core power taking into account the variation of the heat transfer properties that are temperature dependent. The core volumetric flow rate is obtained from the reactor coolant pump 32 speed measurements. The .DELTA.T power measurement described above is also made more accurate for slow variations in core thermal energy by dynamically compensating the basic steady-state expression with a measure of the rate of heat addition to the stored thermal energy in the primary coolant. This is accomplished by differentiating the measured hot and cold leg temperatures, multiplying by constants and adding the products to the steady-state expression. Since the .DELTA.T power measurement provides a very accurate steady-state indication of core power, it is used as a calibration standard against which the core power from the ex-core detectors is calibrated during steady-state operation. See U.S. Pat. No. 3,356,577 issued to Ygal Fishman on Dec. 5, 1967. The calibration technique provides an updated nuclear power signal proportional to the time averged integral error between the calculated nuclear power and .DELTA.T power. The updating interval is long compared to reactor dynamic characteristics to ensure that the dynamic accuracy of the ex-core detectors is maintained during transients. In addition to providing a .DELTA.T power calculation that is accurate for slow transients, the dynamic compensation described above will provide an accurate .DELTA.T power signal during incidents that involve rapid single CEA deviations (especially a dropped CEA). In the event that a CEA drops, the power distribution will be distorted. This distortion can have the effect that the nuclear power signals in any one or more of the four core protection calculator channels will not be indicative of the core average power. The indicated nuclear power in any channel may be higher or lower than the core average power depending upon how the dropped CEA distorts the power distribution. The .DELTA.T power, however, will not be significantly affected. Therefore, the nuclear flux power will be used as the core average power indication unless a large CEA deviation is detected, at which time the CPCs will automatically use the .DELTA.T power as the core average power indication for the low DNBR and high local power density trips. Calculation of Corrected Neutron Flux Power In order to calculate the power level as indicated by the neutron flux level measured by the ex-core detectors, corrections for shape annealing and rod shadowing are necessary for each segment of each of the four detector channels. These corrected responses for each detector segment are summed to yield a quantity proportional to the neutron flux power level for each detector channel. A constant of proportionality is then applied to this quantity to convert the detector channel response to a previously calibrated thermal power level. The shape annealing correction factor is applied to the uncorrected response of each segment of the four ex-core detector strings. Let D.sub.i be the uncorrected response of segment i of a detector string. Let Pi be the detector response of segment i had the detector been placed at the core periphery. In order to determine the external detector segment response, the peripheral segment response must be corrected for the effects of shape annealing. Let S.sub.ij be the shape annealing correction factor which allows for contributions to an external detector segment due to the distribution of point sources along the core periphery. This relationship can conveniently expressed in matrix notation as EQU D=S P where In order to determine the peripheral detector response, the following calculation is required; EQU P=S.sup.-1 D Having determined the detector response had the detectors been at the core periphery, it is now necessary to determine the detector response in the absence of flux perturbations due to CEA insertion in the region of the detector. Average CEA shadowing factors R are determined for each detector segment dependent on CEA insertion in the region of detector segment. This can mathematically be expressed as: ##EQU3## where R.sub.j =Average CEA shadowing correction factor for detector segment j The X.sub.i are determined by CEA position indications. The average CEA shadowing correction factor for each segment is applied to the peripheral detector response to yield a detector signal .phi..sub.i for segment i proportional to nuclear power. EQU .phi.=R.sup.-1 P where The individual segments i of each string are then summed. ##EQU4## where .phi.=Total corrected detector channel response proportional to reactor power The total summed response .phi.is corrected for temperature changes by applying a factor that is proportional to the change in cold leg temperature. Let T.sub.f be the temperature correction factor then EQU .phi..sub.N (t)=.phi.(t).multidot.T.sub.f (t) where EQU T.sub.f (t)=1+C.sub.T (T.sub.co -T.sub.cmin (t)) This number is proportional to the nuclear flux power as measured by that detector channel. The constant of proportionality is determined during power range testing by calibrating the detector response to a known steady state thermal power level. The final calculation required for determining the calibrated neutron flux power is to correct the total average corrected detector response by the calibration factor K.sub.cal. EQU .phi..sub.cal =K.sub.cal .phi..sub.N where: The calibration factor is obtained on-line by making use of the calculated .DELTA.T power. The algorithms employed to perform this calibration are described infra. Calculation of Core Thermal Power (.DELTA.T Power) The calculated core thermal power is obtained using a correlation based on measured coolant conditions. The information available as input to this calculation consists of hot and cold leg temperature signals, the reactor coolant system mass flow rate and an indication of the number of reactor coolant pumps running. The .DELTA.T power is calculated in two portions, a static calculation and a dynamic calculation. Static Calculation: The two hot and cold leg temperature inputs are averaged and using in the following equation: EQU B.sub..DELTA.T.sup.static =f.sub.i (m).DELTA.T+f.sub.2 (m).DELTA.T.multidot.T.sub.c +f.sub.3 (m).DELTA.T.sup.2 where The static .DELTA.T power equation conservatively calculated the variation in-core power with temperature changes, mass flow rate and coolant specific heat (which is temperature dependent). Dynamic Calculation The hot and cold leg temperature signals used in the static calculation are obtained from Resistance Temperature Detectors (RTDs) in the primary coolant piping. As a result of their location the hot leg temperature signal will "lag" the core outlet temperature due to the transport time involved through the coolant piping; similarly the cold leg temperature signal will "lead" the core inlet temperature. Dynamic compensation is provided to accommodate this effect as follows: ##EQU7## where: T.sub.x : is a weighted average of the hot and cold leg temperature signals, defined as EQU T.sub.x =W.sub.1 T.sub.h +W.sub.2 T.sub.c where Dynamic Compensation assures that the .DELTA.T power agrees with the heat flux transmitted out of the fuel pin. The dynamic portion of the .DELTA.T power is implemented using a Z transform of the above equation as ##EQU9## where: .DELTA.S: is the time between updates of B.sub..DELTA.T.sup.dyn The total .DELTA.T power is then EQU B=B.sub..DELTA.T.sup.static +B.sub..DELTA.T.sup.dyn The calculated .DELTA.T power is used to calibrate the nuclear flux power, as described hereinafter. Calculations for Calibration of Neutron Flux Power to .DELTA.T Power The calculated .DELTA.T Power, described supra, is used to calibrate the corrected neutron flux power. The calibration algorithm is specified to provide an accurate calibration during steady-state operation but retain the rapid neutron flux power response during anticipated operational occurrences. The flux power calibration is achieved by means of a proportionality constant multiplier (K.sub.cal) on the corrected neutron flux power. The calibrated neutron flux is defined by EQU .phi..sub.cal =K.sub.cal .times..phi..sub.N where The calibration factor is defined by the following equation: EQU K.sub.cal (t)=K.sub.cal (t-.DELTA.S)+.tau..sub.k [.phi..sub.cal (t)-B(t)].DELTA.S with EQU .phi..sub.cal (t)=K.sub.cal (t-.DELTA.S).multidot..phi..sub.N (t) where The error weighting factor is chosen to result in an effective neutron flux power calibration time constant of approximately 4 minutes. This means that if there were a step change in .DELTA.T power it would take about 16 minutes for the difference between neutron flux power and .DELTA.T power to be reduced to 2 percent of its initial value. The above technique is used for calibration during conditions when there is no excessive CEA deviation. In the event that excessive CEA deviation occurs the neutron flux power indication can rapidly decalibrate. The flux power calibration algorithm consists of logic which will change the mode operation of calibrated neutron flux power as the power input to the DNBR and local power density set point calculation. This mode will occur for those CEA configurations which are not accommodated by the CEA shadowing correction. In this mode, the .DELTA.T power is the primary power input. The .DELTA.T power conservatively predicts the core average power for these conditions. Method Used to Determine Reactor Coolant System Mass Flow Rate The primary coolant mass flow rate is used in the low DNBR trip. The mass flow rate is obtained using the pump speed inputs from the reactor coolant pumps 32, the primary coolant pressure, and the core inlet temperature. The volumetric flow rate through each reactor coolant pump is dependent upon the rotational speed of the pump and the pump head. This relationship is typically shown in pump characteristic curves. Flow changes resulting from changes in the loop flow resistances occur slowly (i.e., core crud buildup, increase in steam generator resistance, etc.). Calibration of the pump speed signal, relating pump rotational speed to volumetric flow, will be performed periodically using pump .DELTA.P instrumentation which is not part of this invention. Flow reductions associated with pump speed reductions are more rapid than those produced from loop flow resistance changes. The pump rotational speed signal is converted to a pump volumetric flow using mathematical relationships based on pump characteristics and periodic loop flow calibrations. The algorithm used is conservative relative to the actual pump performance. The volumetric flow rates calculated for each pump 32 are summed to give a vessel flow. The vessel flow is corrected for core bypass and density and the result is the core mass flow rate. Calculation of Reactor Coolant System Mass Flow Rate The reactor coolant system mass flow rate is obtained from the rotational speed of each reactor coolant pump 32. This is done by making use of the pump characteristic curves, summing the four pump volumetric flow rates, correcting for internal and external vessel flow leakage and correcting for density variations. Proximity probes are used to measure the shaft rotational speed. The volumetric flow rate for each pump is defined by the general relationship shown below: EQU V.sub.i =f.sub.i (RPM, .DELTA.P(RPM,N),N) where: The total vessel volumetric flow rate is the sum of the pump flows. The summed flows are adjusted for core bypass as follows: ##EQU10## where: V.sub.core =volumetric flow rate through the core The core volumetric flow rate is corrected for density to obtain the mass flow rate as follows: EQU m=V.sub.core (C.sub.0 +C.sub.1 (T.sub.c -T.sub.co)+C.sub.2 (P-P.sub.o)) where: Method Used to Determine Local Power Density Trip Set Point The local power density distribution in the fuel is dependent upon the core power distribution. The objective of establishing a trip on high local power density is to prevent the centerline fuel temperature in the "hottest" fuel pellet in the core from exceeding the melting point. The centerline fuel temperature is dependent upon the pellet geometry, pellet composition, the amount of energy deposited in the fuel, the local power density, the gap and cladding configuration and their heat transfer characteristics. The core power distribution can be related to the local power density deposited in the fuel by a proportionality constant. The power distribution in the "hot" pin can be obtained by the methods described supra. In steady-state, the deposited local power density can be related to the centerline fuel temperature by a proportionality constant when the temperature profile across the fuel diameter is known. Therefore, the centerline fuel temperature can be directly related to the power distribution in the "hot" fuel pin. During transient conditions, the deposited local power density can be related to the centerline fuel temperature through standard heat conduction models which predict the spatial variation in the fuel temperature profile as a function of the heat transfer time constant of the fuel. Therefore, changes in centerline fuel temperature can be directly related to changes in the power distribution in the "hot" pin. The local power density trip set point is defined as that value of core average power which corresponds to a power density in a fuel pellet which would result in raising the steady-state centerline fuel temperature to the melting point for a given three-dimensional peaking factor. The three-dimensional peaking factor is defined as the maximum product of the normalized core average axial power distribution and axially varying planar radial peaking factor adjusted for allowed azimuthal tilt magnitude. This definition of the set point assures that the local power density in the "hottest" fuel pellet is accommodated and that the dynamics of the fuel temperature variation are correctly defined. The basic operation of the high local power density trip is as follows: If the core average power becomes greater than the set point a trip will occur in a time interval that is a function of the amount by which the core average power exceeds the set point and the transient characteristics of the set point. The time interval that occurs before a trip signal is generated is obtained by delaying the setpoint transient response to account for the fuel temperature time constant and accelerating the set point transient response to account for protective system time delays, CEDM de-energization time and the time required to effectively terminate the occurrence. Calculation of Local Power Density Set Point The local power density set point is that value of the core power which could correspond to the limiting local power density for a given three-dimensional peaking factor and azimuthal tilt magnitude. The information available for calculating the set point consists of the planar radial peaking factors, (F.sub.r.sup.p (z)), the normalized core average axial power shape, (F.sub.z (z)), and the azimuthal tilt magnitude (T.sub.r). The maximum value of the product F.sub.z (z)F.sub.r.sup.p (z) is the 3-D peaking factor, which is defined as the ratio of the peak local power density to the core average power density. Thus, if C.sub.S represents the limiting local power density divided by the full power average local power density, the ratio of C.sub.s to F.sub.z (z)F.sub.r.sup.p expressed as a percentage gives the core power at which this limit will be reached. Reducing this by the amount of azimuthal tilt results in the static local power density set point, B.sub.sp.sup.st, which is the value of core average power that would result in centerline fuel melt in steady-state. This set point is passed through the digital equivalent of a lead-lag filter of the form: EQU (.tau..sub.1 s+1)/(.tau..sub.2 s+1). The output of the filtered set point is the local power density trip set point, B.sub.sp. The algorithm is summarized below. Algorithms for Local Power Density Set Point Calculation Definitions: Algorithm: The local power density static set point is found from: ##EQU11## where max F.sub.z (z)F.sub.r.sup.p (z) is the maximum value of the product evaluated at each node z. The trip set point is found using a z-transform of the lead-lag filter as follows: ##EQU12## High Local Power Density Trip Signal Generation The local power density trip set point that is calculated as described above is compared to the measured core average power. The core average power measurement is either the corrected neutron flux power or the calculated .DELTA.T power depending upon the conditions discussed above. Based upon this comparison the following actions are initiated: EQU a. if: M.sub.2 <B.sub.sp -.phi..sub.cal .ltoreq.M1 a contact opening output is sent to the Reactor Protective System 2/4 CEA withdrawal prohibit logic. EQU b. if: 0.0<B.sub.sp -.phi..sub.cal .ltoreq.M2 a contact opening output provides a channel pre-trip annunciation and a contact closure output provides a signal to the 2/4 Power Reduction Control Signal logic. EQU c. if: B.sub.sp -.phi..sub.cal .ltoreq.0.0 a contact opening output provides a channel trip signal to the 2/4 RPS trip logic. The values of M1 and M2 are margins to trip expressed in terms of percent of core power, with M1>M2. Method Used to Determine DNBR The Core Protection Calculators require a correlation for calculation of on-line minimum DNBR. Input to the calculation will include the instant reactor conditions of mass flow, integrated nuclear planar radial peaking factor, the ratio of maximum average fuel assembly power to core average assembly power, coolant inlet temperature, a measure of the axial power distribution, and reactor coolant system pressure. A simplified closed channel hydraulic model is used as the algorithm for the on-line computation of DNBR. This on-line computation may most easily be done in a special purpose digital computer. Since the closed hot channel calculation does not take into consideration turbulent interchange of coolant between the hot channel and the neighboring channels, an adjustment to the algorithm's input is required. This adjustment is made to the mass velocity in the channel such that when all other conditions are the same, the closed channel minimum DNBR equals the minimum DNBR obtained by considering the interchange of coolant between channels. The adjusted mass velocity is found by evaluating an analytically derived Equivalent Mass Velocity Correlation. The Equivalent Mass Velocity Correlation is an expression of the form: ##EQU13## where Ge=algorithm required mass velocity; The coefficients of the polynominal fit are analytically determined by the following scheme. The Equivalent Mass Velocity Correlation is dependent upon all the input parameters mentioned above. The integrated planar radial peaking factor is defined as the integrated value of the product of the axial dependent planar radial peaking factors and the normalized core average axial power distribution. The ratio of the maximum assembly power to the core average assembly power is related to the integrated planar radial peaking factor by a proportionality constant. The DNB trip is basically composed of two distinct levels of calculation. The first level can be termed as the periodic static or snapshot calculation and the second level as the update calculation. In the periodic calculation, the most recent values of the monitored variables or calculated parameters that affect the DNBR are used to determine the DNBR. This calculation employs the Equivalent Mass Velocity Correlation discussed above and a simplified version of the W-3 correlation. The update calculation will be used to update the DNBR between period calculations. The relationships involved consist of polynominal functions that have been obtained from extensive analysis using a standard DNBR analysis method. Calculations for Minimum DNBR The minimum DNBR in the "hot" channel is calculated using the following calculated parameters and monitored NSSS variables: Periodic DNBR Calculation The periodic calculation uses the inputs listed above, in the Equivalent Mass Velocity Correlation and simplified version of the W-3 correlation. The equivalent mass velocity is defined by ##EQU14## where: g*: is the channel mass velocity at a datum (for instance nominal reactor conditions) Knowing the equivalent mass velocity and the axial distribution at 25 axial nodes, the coolant enthalpy rise can be calculated up the channel from ##EQU16## where: H.sub.i : average coolant enthalpy at node i With the coolant enthalpy known at each node, the critical heat flux is calculated EQU Qcrit.sub.i =f.sub.1 (.phi..sub.cal, Ge, H.sub.i, T.sub.cmax, P.sub.pri, F.sub.r.sbsb.i.sup.chan, F.sub.z.sbsb.i) The complicated expressions for Qcrit.sub.i involving several empirically derived constants utilizes the standard W-3 correlation developed by Tong and may be found in: L. S. Tong, "Prediction of Departure from Nucleate Boiling for an Axially Non-uniform Heat Flux Distribution," Journal of Nuclear Energy, 21:241-248, 1967. The local heat flux at each node defined by EQU Qlocal.sub.i =f.sub.2 (.phi..sub.cal, T.sub.r, F.sub.r.sbsb.i.sup.p, F.sub.z.sbsb.i) is divided into the critical heat flux to give the DNBR at each node, EQU DNBR.sub.i.sup.periodic =Qcrit.sub.i /Qlocal.sub.i The minimum DNBR resulting from the periodic portion of the calculation is then EQU DNBR.sup.periodic =min[DNBR.sub.i.sup.periodic ] DNBR Update Calculation The periodic calculation described above will be performed approximately every 2 seconds. In this time interval the update calculation is used to update the statically calculated DNBR. In this context, when "continuously" is used in the description and in the claims it should be taken to mean: "of a substantially higher periodicity than the frequency of the periodic calculation." The local heat flux defined as EQU Qlocal.sub.i =constant.multidot..phi..sub.cal .multidot.F.sub.z.sbsb.i .multidot.F.sub.r.sbsb.i.sup.p .multidot.(1+T.sub.r) is calculated on a nodal basis. The update calculation is performed approximately every 20 millisec. Therefore, the periodic calculation will never have an input that is more than 20 millisec delayed. The periodic DNBR is updated within the interval between periodic calculations by comparing the inputs to the values of the inputs used in the most recently completed periodic calculation. The differences in the input values are used to calculate the change in the periodic DNBR at each node by a partial derivative approach as shown below ##EQU17## where: .DELTA.DNBR.sub.i (t): is the change in DNBR at node i ##EQU18## are functions that relate a change in a particular parameter to an equivalent change in DNBR. These functions will be conservatively chosen constants or polynomial expressions that depend upon the measured values of other pertinent parameters. The updated DNBR at each node is then EQU DNBR.sub.i (t)=DNBR.sub.i.sup.periodic +.DELTA.DNBR.sub.i (t) The minimum DNBR is then EQU DNBR(t)=min[DNBR.sub.i (t)] It will be understood that the embodiment shown and described herein is merely illustrative and that changes may be made without departing from the scope of the invention as claimed.
	claims	1. A method for electroplating a nonmetallic grating comprising:providing a nonmetallic grating having an aspect-ratio of from 50:1 to 60:1;performing an atomic layer deposition (ALD) reaction to form a seed layer directly on the nonmetallic grating, wherein the seed layer uniformly and conformally coats the nonmetallic grating such that a thickness of the seed layer is substantially the same along an entire surface of the nonmetallic grating, and wherein performing the ALD reaction comprises:(i) placing the nonmetallic grating in an ALD reactor, the ALD reactor having (a) a back-purge mechanism to facilitate delivery of an ALD precursor vapor across the non-metallic grating, and (b) a pressure of about 1.5 Ton; and(ii) alternating pulses of an ALD precursor and a precursor vapor to grow the seed layer, wherein the pulses are alternated to produce pressure pulses of 30 mTorr of the ALD precursor and 75 mTorr of the precursor vapor and result in a seed layer having a uniform thickness of from 22 nm to 27 nm; andelectroplating a metallic layer directly on the seed layer, wherein the metallic layer uniformly and conformally coats the seed layer such that a thickness of the metallic layer is substantially the same along an entire surface of the seed layer, wherein electroplating the metallic layer on the seed layer comprises pulsing an electroplating bath solution comprising thallium across the nonmetallic grating at a pulse frequency of from 50 Hz to 100 Hz, a pulse profile of ¼ duty and a pulse current of 2 mA/cm2, and the metallic layer comprises a thickness of from 0.1 microns to 0.6 microns. 2. The method of claim 1 wherein the aspect-ratio is 60:1. 3. The method of claim 1 wherein the nonmetallic grating comprises elongated elements having a depth of at least 26 microns and a pitch between each of the adjacent elongated elements is 2 microns or less. 4. The method of claim 1 wherein the nonmetallic grating comprises silicon. 5. The method of claim 1 wherein the seed layer comprises palladium. 6. The method of claim 1 wherein the seed layer comprises platinum. 7. The method of claim 1 wherein electroplating comprises exposing the nonmetallic grating having the seed layer thereon to an electrical current forward pulse of from 340 to 365 microseconds. 8. The method of claim 1 wherein electroplating comprises submerging the nonmetallic grating having the seed layer thereon within an electroplating reaction tank, the electroplating reaction tank having outer sidewalls that form chamfered corners, wherein the chamfered corners are dimensioned to generate a laminar flow profile of an electroplating bath solution within the tank, and wherein the electroplating bath solution comprises a thallium concentration of from 40 ppm to 60 ppm. 9. The method of claim 1 wherein the metallic layer comprises gold. 10. A method for electroplating a high aspect-ratio silicon grating comprising:providing a silicon substrate having gratings with an aspect-ratio of 60:1;coating the gratings with a conducting seed layer using atomic layer deposition (ALD),wherein the conducting seed layer comprises a same thickness along an entire surface of the gratings, wherein performing ALD comprises:(i) placing the silicon grating in an ALD reactor, the ALD reactor having (a) a back-purge mechanism to facilitate delivery of an ALD precursor vapor across the silicon grating, and (b) a pressure of about 1.5 Torr; and(ii) alternating pulses of an ALD precursor and a precursor vapor to grow the conducting seed layer, wherein the pulses are alternated to produce pressure pulses of 30 mTorr of the ALD precursor and 75 mTorr of the precursor vapor and result in the conducting seed layer having a uniform thickness of 22 nm to 27 nm; andelectroplating a gold layer on the conducting seed layer such that the gold layer comprises a same thickness along an entire surface of the conducting seed layer, wherein electroplating the gold layer comprises:(iii) placing the silicon grating having the conducting seed layer thereon into an electroplating chamber, the electroplating chamber comprising outer sidewalls that form chamfered corners dimensioned to produce a laminar flow profile of an electroplating bath across the silicon grating, and wherein the electroplating bath comprises a thallium concentration of from 40 ppm to 60 ppm; and(iv) forming the gold layer on the conducting seed layer by pulsing the electroplating bath across the silicon grating at a pulse frequency of from 50 Hz to 100 Hz, a pulse profile of ¼ duty and a pulse current of 2 mA/cm2, and wherein the thickness of the gold layer is from 0.6 microns to 1.5 microns. 11. The method of claim 10 wherein the gratings have a depth of at least 50 microns and a pitch of less than 5 microns. 12. The method of claim 10 wherein performing ALD comprises exposing the silicon grating to from 300 to 375 cycles of ALD, wherein the cycles comprise alternating pulse and purge cycles of the ALD precursor and the precursor vapor, wherein each ALD precursor comprises 1 second pulse and 20 second purge and each precursor vapor cycle comprises 2 second pulse and 15 second purge. 13. The method of claim 10 wherein the chamfered corners form an angle with respect to the outer sidewalls of the electroplating chamber of greater than 90 degrees. 14. The method of claim 10 further comprising:prior to coating the gratings with a conducting seed layer, using ALD to apply an adhesive layer comprising aluminum oxide to the gratings. 15. A method for electroplating a nonmetallic grating comprising:providing a nonmetallic grating having an aspect-ratio of 50:1 to 60:1; performing an atomic layer deposition (ALD) reaction to form a seed layer directly on the nonmetallic grating, wherein the seed layer uniformly and conformally coats the nonmetallic grating such that a thickness of the seed layer is substantially the same along an entire surface of the nonmetallic grating, and wherein performing the ALD reaction comprises:(i) placing the nonmetallic grating in an ALD reactor, the ALD reactor having (a) a back-purge mechanism to facilitate delivery of an ALD precursor vapor across the non-metallic grating, and (b) a pressure of about 1.5 Ton; and(ii) alternating pulses of an ALD precursor and a precursor vapor to grow the seed layer, wherein the pulses are alternated to produce pressure pulses of 30 mTorr of the ALD precursor and 75 mTorr of the precursor vapor and result in a seed layer having a uniform thickness of 22 nm to 27 nm; andelectroplating a metallic layer directly on the seed layer, wherein the metallic layer uniformly and conformally coats the seed layer such that a thickness of the metallic layer is substantially the same along an entire surface of the seed layer, and wherein electroplating the metallic layer comprises:(iii) placing the nonmetallic grating having the seed layer thereon into an electroplating chamber, the electroplating chamber comprising outer sidewalls that form chamfered corners dimensioned to produce a laminar flow profile of an electroplating bath across the nonmetallic grating, and wherein the electroplating bath comprises a thallium concentration of from 40 ppm to 60 ppm; and(iv) forming the metallic layer on the seed layer by pulsing the electroplating bath across the nonmetallic grating at a pulse frequency of from 50 Hz to 100 Hz, a pulse profile of 1 A duty and a pulse current of 2 mA/cm2, and wherein the thickness of the metallic layer is from 0.6 microns to 1.5 microns. 16. The method of claim 15 wherein the nonmetallic grating comprises elongated elements having a depth of at least 26 microns and a pitch between each of the adjacent elongated elements is 2 microns or less. 17. The method of claim 15 wherein the nonmetallic grating comprises silicon, the seed layer comprises palladium or platinum, and the metallic layer comprises gold. 18. The method of claim 15 further comprising:prior to coating the gratings with a conducting seed layer, using ALD to apply an adhesive layer comprising aluminum oxide to the gratings.
	summary
056028873	abstract	A tool for pushing a suspended shroud repair tie rod assembly radially inward in the downcomer annulus of a boiling water reactor. The tie rod pusher tool includes a pole adaptor for coupling to the end of a service pole, a pole adaptor extension having one end connected to the pole adaptor, a mounting channel connected to the other end of the pole adaptor extension, a hydraulic spreader mounted on the mounting channel, an adaptor bracket having a proximal end connected to the pivoting member of the hydraulic spreader, a rocker plate pivotably mounted on the distal end of the adaptor bracket, and a saddle mounted on the rocker plate. Using a service pole, the tool is lowered into a position whereat the saddle contacts the tie rod assembly when the hydraulic spreader is actuated. In the open position, the saddle bears against the tie rod assembly with sufficient force to displace the assembly radially inward until the clevis hook clears the clevis pin. Then the tie rod assembly is lowered until the tip of the clevis hook clears the bottom of the clevis pin. During descent of the tie rod assembly, it slides against the saddle of the pusher tool. The saddle is made of ultra-high-molecular weight polyethylene to prevent scratching of the tie rod assembly. When the pressurized fluid to the spreader is cut off, the bottom end of the suspended tie rod assembly drifts radially outward, causing the clevis pin to engage the clevis hook.
	description	This application claims the benefit of priority to U.S. Provisional Application No. 60/819,057, filed Jul. 7, 2006, and entitled, “Electron Beam Apparatus To Collect Side-View and/or Plane-View Image With In-Lens Sectional Detector”, all of which is incorporated herein by reference. The present invention is related generally to scanning electron microscopes and more particularly to a system and method to collect the side-view and plane-view SEM image. A low-landing energy, high resolution SEM (scanning electron microscope) with the capability of capturing a side-view and plane-view image is a very important metrology tool to inspect and review defects in a semiconductor wafer. This SEM accelerates the new wafer processing technology ramp and improves the yield during mass production. For the conventional SEM with capability of collecting side-view SEM image of specimen, one or more side-detectors are placed very close to the specimen surface. The objective magnetic lens usually has a conical shape to make space for the side detectors. The space between the specimen surface and the lens pole-piece has no or very weak axial magnetic field and electrostatic field so that the secondary electrons emanating from the specimen with a polar angle can be collected by the side-detector. In order to improve the collection efficiency, a positive voltage with respect to the specimen will be applied to the side-detector to attract the secondary electron signal. This conventional SEM layout has a poor aberration property, and it is difficult to achieve high resolution, especially for low landing energy SEM imaging. It is known that the combination of immersion magnetic lens and retarding electrostatic lens has very low aberration coefficients and can achieve high resolution for the low landing energy. Due to strong axial magnetic field and extraction electric field between the specimen surface and lens pole-piece of this compound lens, the layout of the side-detector near the specimen surface to collect the side-view SEM image cannot work anymore. The presented invention will solve the conflict between high-resolution achieving and side-view imaging for low landing energy SEM. An object of the present invention is to provide an apparatus and method to collect the secondary electrons emanating from specimen surface without influencing the primary electron beam thereafter form side-view and/or plane-view image of a high resolution and low landing energy SEM. This and other objects are achieved in an electron detector structure and aperture arrangement around the primary beam optical axis to capture the secondary and backscattered electrons emanating from specimen surface with different azimuth and polar angles. In one embodiment, an apparatus for generating side-view and plane-view image from a specimen is disclosed. The apparatus includes a charged particle beam generator arranged to generate and control a charged particle beam substantially towards a portion of the specimen and a detector arranged to detect charged particles emanating from the specimen to allow generation of an image of interested portion of the specimen. In another embodiment, a charge particle detector for obtaining an image of a portion of specimen surface is disclosed. An in-lens sectional detector composed of at least two segments with an aperture is arranged to capture secondary electrons and backscattered electrons emanating from specimen surface with different azimuth and polar angles. For further embodiment, an ExB filter is positioned to guide the secondary electrons and backscattered electrons emanating from specimen surface substantially toward the off-axis sectional detector. In yet another embodiment, a detector for generating quality side-view image is disclosed. An aperture on the detector with 3 millimeters diameter is calculated for quality side-view image and image aberration. Reference will now be made in detail to specific embodiments of the invention. Examples of these embodiments are illustrated in accompanying drawings. While the invention will be described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to these embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a through understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. The present invention may be implemented within any suitable measurement device that detects charged particles towards a sample and then detects emitted particles from the sample. FIG. 1 is a diagrammatic representation of an electron beam apparatus 100 (SEM) in accordance with one embodiment of the present invention. The SEM system 100 includes an electron beam generator (101 through 112) that generates and directs an electron beam 102 substantially toward an area of interest on a specimen 113. The SEM system 100 includes an electron beam gun tip 101 for providing the electron beam to an anode 102 to create an electron field. Gun lenses 104 and 105 retain the electric field. A blanking plate 106 retains the electron beam shape. The SEM system 100 also includes an in-lens sectional detector 107 arranged to detect charged particles 111 (secondary electrons SE and/or backscattered electrons BSE) emanating from the specimen surface 113. The SEM system 100 includes deflectors 108 and 110 to deflect the electric field. The SEM system also includes a bottom seal 112 for holding the assembly. The SEM system 100 includes an objective lens 109 which provides a magnetic immersion function and an electrostatic retarding function. The SEM system 100 also includes an image generator (not shown) for forming an image from the emanated particles. The electron beam generator and sectional detector are further described below, along with other features of the SEM system 100. The landing location of these charged particles when they arrive at the detector plane is determined by their initial energy and escaping angle emanating from the specimen surface. FIG. 2 is a diagrammatic representation of electron trajectory simulation of the emanating SE from specimen surface 113 with initial trajectory condition of azimuth angle 0 degree 202 and 135 degree 203. The corresponding landing position image on the detector plane is illustrated on FIG. 3. 302 is the landing area for SE from specimen surface 113 with 0 degree azimuth angle and 303 is the landing area for SE from specimen surface 113 with 135 degree azimuth angle. FIG. 4 is a diagrammatic representation of the trajectory of SE emanating from specimen surface 113 to in-lens sectional detector 107 without any other electronic and magnetic field affection except the objective lens field. FIGS. 6 through FIG. 11 illustrate different sectional detectors samples for SEM image processing. FIG. 6 is a diagrammatic representation of a sample sectional detector with an aperture in the center. FIG. 7 is a diagrammatic representation of a sample in which a 4 segments detector forms a hole at optical axis. FIG. 8 is a diagrammatic representation of a sample in which an 8 segments detector forms a hole at optical axis. FIG. 9 is a diagrammatic representation of a sample in which one of the detector segments has a hole, which is located at optical axis. FIG. 10 is a diagrammatic representation of a sample in which some detector segments form a hole, which is located at optical axis center. FIG. 11 is a diagrammatic representation of a sample detector that does not locate at the beam optical axis. An ExB filter is utilized to guide the secondary electrons onto the off-axis sectional detector. The sectional detector is divided into at least two sections with an aperture in the center 600, 700 and 800. The size of the center aperture 601 is less than 3 mm to let the primary charged particle 102 to pass. The aperture 601 can also be located at section of the sectional detector 900 and between the boundaries of the sections of detector 1000. If the detector is located off-axis of the optical system, the aperture hole can also be removed, shown as 114 in FIG. 11, then the SE emanating from specimen surface 113 is guided to the off-axis sectional detector 114 by an ExB filter 108 as FIG. 5 illustrates. Each section of the detector collects only the secondary charged particles with particular range of the polar and azimuth angle with respect to the specimen surface 113. The SEM image generated by a particular secondary charge particle is the side-view image, which corresponds to the side-view SEM image collected by a conventional side-detector. The signal from all sections of the sectional detector can be processed to achieve a plane-view SEM image of the scanned specimen area. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.
	summary
	summary
	abstract	Electron-optical systems are disclosed that are especially useful in mapping-projection electron microscopy and related uses. The systems achieve high magnification with excellent control of aberrations, and low magnification at wide optical fields with excellent control of aberrations.
	summary
040000370	summary	BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear steam turbine power plant controls, and in particular, to a power plant control system which permits operation beyond the normally defined end of the fuel cycle of the reactor element. 2. Description of the Prior Art In a nuclear reactor power plant, heat energy generated within the nuclear reactor element is utilized to provide a source of pressurized dry and saturated or low superheat steam. The steam so produced drives an associated turbine system linked mechanically to an electrical generator which produces electrical energy for an associated electrical load. In general, there are two main classifications of nuclear power plants, the Boiling Water Reactor or BWR System, and the Pressurized Water Reactor, or PWR System. The Boiling Water Reactor, or BWR, conducts a water coolant through a single closed loop comprising the reactor element and the turbine system. As the water coolant passes through the reactor core, heat taken from the reactor core raises the temperature of the water coolant above the vaporization point of the coolant, thereby producing steam. The steam is then directly passed through the turbine system and the heat energy within the steam is converted into useful mechanical energy. The steam passes into a condenser element and is returned to a liquid state. The condensate, now in the form of low temperature water coolant, is reintroduced as a coolant into the reactor element, thus completing the single closed loop. However, since the water coolant and the steam produced therefrom are exposed to the highly radioactive reactor core and have undergone extensive bombardment of high energy neutron particles therein, the turbine system through which the radioactively contaminated steam has passed experiences an increase in radioactivity levels. Also, extreme care must be taken to prevent leakage of the contaminated steam or water out of the single, closed BWR loop. On the other hand, power plants employing a Pressurized Water Reactor use a double closed loop arrangement for the production of electrical energy by use of the nuclear reactor element. The first, or primary, side connects the reactor element in a closed loop with a steam generator, or heat exchanger element. The water coolant is maintained under a high pressure, approximately 2200 p.s.i. and takes heat from the reactor core as the pressurized water coolant passes therethrough. Since the water coolant is under pressure, the heat taken from the reactor does not raise the temperature of the water coolant above the vaporization temperature for water at the appropriate pressure. Instead, the heated, but not boiling, water enters the steam generator element. In the steam generator, the heat carried by the pressurized water coolant is transferred to secondary water disposed within the steam generator. The transfer of heat from the pressurized water to the secondary water sufficiently cools the pressurized water coolant to permit its reintroduction into the reactor core, thus completing the primary side loop. Within the steam generator, heat taken from the pressurized water coolant and transmitted to the secondary water therein raises the temperature of the secondary water above its boiling point, thus producing steam. However, in the case of the PWR, the steam produced within the steam generator element is not radioactively contaminated since the pressurized water coolant is physically segregated from the secondary water in the steam generator element. From the steam generator, the steam produced by the heating of the secondary water is conveyed through the second enclosed loop, called the secondary or steam side. The secondary or steam side comprises a high pressure turbine element, a low pressure turbine element, a condenser element, and a series of feedwater heater elements. Each turbine element has an inlet orifice of the predetermined size with the inlet orifice of the high pressure turbine being physically smaller than the inlet orifice of the low pressure turbine element. Steam passes through the turbine elements and converts the heat energy carried therein to mechanical energy. The expanded steam is returned to the liquid state in the condenser. The condensate is then reintroduced into the steam generator after passing through a series of feedwater heater elements to complete the second closed loop. It is evident then, that the pressure of the steam flowing in the secondary loop is directly dependent upon the amount of heat transferred from the pressurized water coolant to the secondary water in the steam generator. If the temperature of the water coolant is high, a greater amount of heat energy will be transferred to the secondary water than would occur if the primary water coolant were at a lower temperature. The heat energy transferred to the secondary water thus raises the temperature of a greater volume of secondary water above the applicable vaporization temperature of the secondary water, thus producing more steam in the secondary loop. Steam flow through the secondary or steam side loop is dependent upon the physical sizing of the high pressure turbine inlet orifice. The heat content of the primary water coolant is obviously dependent upon the heat output in the reactor core. Heat output in the reactor core is in turn dependent upon the reactivity levels within the fuel elements which comprise the core. Fuel element reactivity is dependent upon the age of the individual fuel elements. Early in the fuel element life, reactivity levels are usually above some predetermined reference point and controlled by the manipulation of a plurality of neutron absorbing control rods. If, for example, the reactivity level within the reactor element is desired to be raised, the control rods are withdrawn a predetermined distance from within the core, thus increasing the neutron level within the core, thereby increasing the reactivity level within the core. With the reactivity level within the core increased, heat is generated within the reactor and therefore the heat content of the pressurized water coolant increases. A higher coolant temperature increases the amount of heat transferred from the pressurized water coolant to the secondary water in the steam generator. The increased amount of heat transferred increases the pressure of the steam produced in the steam generator for use in the secondary or steam side. The ability to change steam pressure to change the overall electrical output of the power plant to meet the demand of the system is termed the load follow capability of the system. As is well known to those skilled in the art, the load follow capability of the system is inhibited during a xenon transient condition. The useful life, or fuel cycle, of the fuel elements utilized by the reactor is measured in terms of certain predetermined parameters. One such parameter is the amount of reactivity in excess of a certain predetermined reference level. This level of excess reactivity is above the reference level at the start of the fuel cycle, but the level of excess reactivity decreases toward the reference value as the fuel element is utilized. During the life of the fuel element, the decrease in excess reactivity is offset by the withdrawal of the neutron absorbing control rods, which permits the reactor to operate at its constant rated power levels. The point in time denominated as the end of the fuel cycle is normally defined as that time when, with the control rods fully withdrawn and the overall system at full thermal load, the excess reactivity of the reactor core is zero. When these parameters are met, the end of the fuel cycle occurs, despite the fact that some residuum reactivity remains in the fuel elements. Of course, other parameters may be used to define at what point in time the end of the fuel cycle occurs. When the reactor element operation nears the end of the fuel cycle, that is near the end of the useful life of the fuel element, even though there is still the residuum reactivity in the core, it has been observed that even total withdrawal of the control rods fails to sufficiently increase reactivity levels within the reactor to provide thermal output demanded by the electrical load. When reactivity decreases, the heat produced, or thermal output, within the core decreases. Decrease in thermal output of the reactor core due to decreased reactivity causes a concomitant lowering of water coolant temperatures. This in turn, decreases the amount of heat transfer in the steam generator. Thus, the thermal output of the steam generator is lowered, lowering the electrical output of the power plant as a whole. It is well known to those skilled in the art that the density of the pressurized water coolant is dependent upon temperature of the coolant. As the temperature of the coolant decreases, the coolant becomes more dense. At the end of the fuel cycle, fissions of atoms within the fuel element still produce the residuum of reactivity noted above, but, the level of reactivity is not sufficient to maintain the rated thermal output of the reactor. The fissions produce both high energy and lower energy "thermal" neutrons. The thermal neutrons, although only a certain percentage of the neutrons released by the fission process, are important since they are required to enable the fission chain reaction to continue. Since the number of fissions, and therefore, the number of neutrons produced, is decreased at the end of the fuel cycle, and since only a predetermined percentage of neutrons produced are thermal neutrons, the reactivity, and hence the thermal output of the reactor, decreases. However, since the pressurized water coolant becomes denser with a lower temperature, it is possible to increase the excess reactivity above the reference value by lowering coolant temperature. With coolant temperature lower, and the coolant more dense, more of the high energy neutrons produced by those fissions which provide the residuum reactivity will be slowed sufficiently to enable them to produce a fission. Thus, although the total number of neutrons released is not increased, the cooler, and therefore, denser, pressurized water coolant lowers the energy of enough high energy neutrons to provide enough thermal neutrons to sustain a fission chain reaction. In this way, the reactivity of the core is increased over the reference reactivity value, thus enabling the core to maintain its rated thermal output. The increase in excess reactivity thus offsets the loss of reactivity attendant upon the end of the fuel cycle. However, decreases in coolant temperature have a deleterious effect upon pressure of the steam in the turbine side. In order to increase excess reactivity, without diminishing steam pressure, the prior art has to either insert a new fuel element or enrich the old fuel element. However, a further alternative which can maintain the reactivity at an excess level above the reference and the thermal output at its rated level, and yet still maintain the electrical output of the entire system within a predetermined close range of values to the rated electrical output is needed. SUMMARY OF THE INVENTION This invention relates to a central system which extends the life of a fuel element of a pressurized water reactor beyond the end of a predetermined fuel cycle and to a method of improving the operation of a nuclear steam power plant having a pressurized water reactor therein. The control system extends the life of the fuel elements beyond the end of the fuel cycle without a diminution in the thermal output of the reactor and with a diminution in the electrical output of the system within only a small predetermined range of values. The control system utilizes temperature sensing means to indicate that the reactor element is unable to maintain or raise the temperature of a primary coolant to produce steam pressure sufficient to meet a load demand upon the overall system. A predetermined signal from the temperature sensing means initiates flow interruption means which provide an appropriate steam system response. The response will be to maintain the plant output as high as possible with the load of steam pressure conditions by utilization of one or more flow interruption modifications within the steam system. This permits the plant to maintain electrical output within a close range of the rated output over an extended fuel cycle. The method for improving the operation of the nuclear steam power plant having the pressurized water reactor element therein comprises the steps of increasing reactivity levels within the reactor core to maintain the reactivity level at its constant rated value, and modifying the turbine system associated with the pressurized water reactor element to compensate for the increase in reactivity levels within the reactor core. In addition to extended fuel cycle application, utilization of the control system or method taught by this invention provides an improved load follow capability for the reactor element when operating during a xenon transient condition. Although the control system and method to improve reactor operation can be utilized at any time during operation of the system, they are most advantageously employed if initiated during a xenon transient condition or at the end of the fuel cycle.
	abstract	A control system, computer, method and computer program that imposes a monitoring load to the extent necessary to carry out pattern analysis on the operation of a system and failure analysis and does not apply any excessive monitoring load. The invention includes an interface for receiving an operation performance metric value of each of a plurality of first monitored items from a monitored computer, and a control section for, based on the operation performance metric value of each first monitored item, determining a second monitored item whose data should be obtained and instructing the monitored computer to obtain an operation performance metric value of the second monitored item which is associated with each first monitored item.
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	claims	1. A method of forming tungsten tetraboride, the method comprising the steps of:combining tungsten and boron in a molar ratio of from about 1:6 to about 1:12, respectively, andfiring the combined tungsten and boron in a hexagonal boron nitride crucible at a temperature of from about 1600 C to about 2000 C, to form tungsten tetraboride wherein the tungsten and the boron are combined with carbon in the crucible. 2. The method of claim 1, wherein the molar ratio is one of about 1:9 or 1:6. 3. The method of claim 1, wherein the temperature is about 1800 C. 4. The method of claim 1, wherein the firing is accomplished at about one atmosphere. 5. The method of claim 1, wherein the firing is accomplished in an argon environment. 6. The method of claim 1, wherein the tungsten is provided as tungsten oxide. 7. The method of claim 1, wherein the boron is provided as boric acid. 8. The method of claim 1, wherein the tungsten is provided as tungsten metal. 9. The method of claim 1, wherein the boron is provided as boron metal. 10. The method of claim 1, wherein the boron is provided as 10B enriched boron. 11. The method of claim 1, further comprising milling the tungsten tetraboride to a powder. 12. The method of claim 1, further comprising milling the tungsten tetraboride to a powder and compressing the powder into a desired shape. 13. The method of claim 1, further comprising milling the tungsten tetraboride to a powder, compressing the powder into a desired shape, and sintering the desired shape. 14. A method of forming tungsten tetraboride into a desired shape, the method comprising the steps of:combining tungsten and boron in a molar ratio of from about 1:6 to about 1:12, respectively,firing the combined tungsten and boron in a hexagonal boron nitride crucible at a temperature of from about 1600 C to about 2000 C, to form tungsten tetraboride,milling the tungsten tetraboride to a powder,compressing the powder into a desired shape, andsintering the desired shape wherein the tungsten and the boron are combined with carbon in the crucible. 15. The method of claim 14, wherein the boron is provided as 10B enriched boron. 16. The method of claim 14, wherein the firing is accomplished in one of an argon environment or a vacuum environment, and the sintering is accomplished using spark plasma sintering in one of an argon environment or a vacuum environment. 17. A method of forming tungsten tetraboride into a fission reactor shield, the method comprising the steps of:combining tungsten and 10B enriched boron in a molar ratio of from about 1:6 to about 1:12, respectively,firing the combined tungsten and 10B boron in a hexagonal boron nitride crucible at a temperature of from about 1600 C to about 2000 C, to form tungsten tetraboride,milling the tungsten tetraboride to a powder,compressing the powder into a desired shape of the fission reactor shield, andsintering the fission reactor shield wherein the tungsten and the boron are combined with carbon in the crucible. 18. The method of claim 17, wherein the boron is provided as 10B enriched boric acid. 19. The method of claim 17, wherein the firing is accomplished in one of an argon environment or a vacuum environment, and the sintering is accomplished using spark plasma sintering in one of an argon environment or a vacuum environment.
047770120	summary	The invention concerns a gas cooled high temperature reactor charge with spherical fuel elements with a reflector surrounding the pile of fuel elements on all sides with a hot gas collector compartment adjoining the bottom part (bottom reflector) of said reflector consisting of graphite columns, the hot gas collector compartment being bounded in the downward direction by the bottom layers of graphite blocks of the high temperature reactor with a charging means and a removal means for the spherical fuel elements wherein the removal means comprises at least one ceramic pebble removal tube passing through the bottom reflector and the bottom layers and a metal pebble removal tube ajoining the ceramic pebble removal tube underneath the bottom layers. High temperature reactors having a core formed by a pile of spherical fuel elements that are continuously added to the core and removed after their burnup from the core are part of the state-of-the-art. Nuclear reactors of this type have very high gas temperatures in their lower parts, wherefore, only high heat resistant materials may be used in the bottom reflector, the bottom layers and the part of the pebble removal installation located within the core structure. These materials are thus made of a ceramic material, such as graphite, but graphite cannot be exposed to high tensile and bending stresses. The forces of the reactor core are thus transmitted to a thermal shield made of a metal, preferably a cast metal, which is surrounding the entire reflector. In the removal installation for the spherical fuel elements only the part of the removal tubes located within the space enclosed by the thermal shield is made of a ceramic material. The continuing pebble removal tubes connecting the ceramic removal tubes with a scrap separator are made of metal for the sake of higher strength. It has now been found that even the metal pebble removal tubes are exposed to high temperatures in the areas adjoining the ceramic removal tubes. This is the result of the fact that the fuel elements standing in front of the ceramic removal tubes and located in them are not completely burned up and are thus still generating heat. The temperature in the above-described area may thus be reduced only by eliminating or limiting the flow of neutrons in the ceramic pebble removal tubes. It is known in the prior art (West German Offenlegungsshrift No. 23 47 817) to provide neutron absorbing materials in a nuclear reactor with spherical fuel elements and a single passage of the fuel elements through the reactor core in the roof reflector and/or the upper part of the side reflector in order to reduce the flow of fast neutrons in the areas exposed to doses of radiation of the reflector, thereby preventing damage to the graphite of the reflector. There is also known a process for the operation of a high temperature reactor of the above-described structural type (West German Offenlegungsschrift No. 22 41 873) whereby the output distribution in the reactor core may be affected so that the axial output density declines only slightly in the downward direction. Such a mode of operation permits a higher load on the fuel elements which is utilized either for an improvement in the economy of the nuclear reactor or to increase its safety. A series of measures have been proposed to obtain this result, among others the addition of combustible poisons to the loading charge, whereby the generation of power output is transferred to the lower reaches of the reactor core. The combustible poisons may also be provided in the roof, bottom and/or side reflectors. It is further known (West German Offenlegungsschrift No. 23 65 531) to improve the shutdown efficiency of absorber rods directly insertable in a nuclear reactor with spherical fuel elements and directly insertable absorber rods by a special measure without increasing the number of absorber rods or their depth of insertion. This measure consists of doping the graphite of the bottom reflector and the lower part of the side reflector with neutron absorbing substances. This thus represents a quasi-homogeneous poisoning of the lower range of the reflector. It has the disadvantage that the neutron absorbing substances are burning up in a very short period of time and their effectiveness is, therefore, inadequate for the life time of a nuclear reactor estimated at approximately 30 to 40 years. Based on this state-of-the-art, it is the object of the invention to design a gas cooled high temperature reactor of the above-described type so that the temperature is sufficiently lowered from the core bottom to the metal pebble removal tubes so that the thermal exposure of the last-mentioned removal tubes remains within permissible limits. This object is attained according to the invention by that each ceramic pebble removal tube within the area of the bottom reflector and the hot gas collector compartment is surrounded directly by a ring of graphite columns, wherein a plurality of recesses, a number of boron bodies is arranged. The boron bodies reduce the neutron flux in the area affected to the extent that there is no more output by the fuel elements. A further number of boron bodies may be provided within the area of the bottom layers of the high temperature reactor to reinforce the absorption of neutrons. The boron bodies are arranged preferably directly outside each ceramic pebble removal tube. As a solution extremely favorable from an economic standpoint, the boron bodies are designed in the graphite columns in the form of rods and inserted into vertical bores adapted to their shape and size. Advantageously, the length of the boron rods is determined so that the graphite columns are almost entirely penetrated by the boron rods. Preferably, the boron rods are arranged in at least two rows around each ceramic pebble removal tube and the boron rods belonging to different rows are staggered so that they always face a gap. The boron bodies located in the area of the bottom layers may advantageously consist of solid plates arranged so that each ceramic pebble removal tube in this area is completely or almost completely surrounded by them. The form of the boron bodies chosen and the mode of their location effect a gradual decline of the temperature in the direction of the metal pebble removal tubes, so that no or only slight thermal stresses are generated in the structural parts of the high temperature reactor. Furthermore, the boron rods and boron plates are readily built into the corresponding graphite structural parts (graphite columns or bottom layers). The realization of the invention thus does not require a substantial effort.
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	abstract	The invention comprises a method and apparatus for determining a radiation beam treatment path to a tumor, comprising the steps of: (1) delivering charged particles from an accelerator, along a first beam transport path, through an output nozzle, and along a treatment path to the tumor relative to a calibrated reference beam path from the output nozzle toward a patient position and (2) prior to the step of delivering, a main controller verifying an unobstructed linear path of the treatment path using a set of fiducial indicators positioned at least: on a first element physically affixed and co-movable with the output nozzle and on a moveable object in the treatment room. Optionally, voxels of the treatment beam path and potentially obstructing objects are defined in the treatment room using an axis system relative to the calibrated reference beam path and a reference beam point.
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	description	The invention concerns generally the technology of irradiating a sample with X-rays for producing fluorescence. Especially the invention concerns the task of selectively irradiating a desired small target area in the sample. X-ray fluorescence (XRF) is an analysis method in which incident X-rays irradiate a sample, causing sample atoms to emit fluorescent radiation at characteristic wavelengths. By analysing the intensity spectrum of the received fluorescent radiation it is possible to deduce the material composition of the sample. The X-ray source used in an XRF analyzer device is typically an X-ray tube. Due to the structure of an X-ray tube, it usually emits X-rays into a half-spherical (2*pi steradians) spatial angle from an area of the anode referred to as the focal spot. If the sample is small and/or if only a small part of the sample is to be investigated, a need arises for collimating the incident X-rays so that they only fall upon the desired target area. Conventional X-ray collimators were made by stacking metal plates parallel to each other, leaving narrow slits between them for only radiation propagating in the desired direction to pass through. The stack of parallel metal plates must have a certain length, for example in the order of 5 cm, in the propagation direction of the X-rays in order to achieve the desired angular selectivity. This makes them ill suited for portable XRF analyzer units, where space is scarce. It is also difficult to use the metal plate stack principle for reconfigurable applications, where the angular selectivity, irradiation spot size or some similar parameter should be changed quickly and easily. An objective of the present invention is to present a collimator arrangement for collimating incident X-rays onto a desired target area in a sample. Another objective of the invention is to present an X-ray fluorescence analyzer device utilizing such a collimator arrangement. A further objective of the invention is to minimize the space required for collimating incident X-rays in an X-ray analyzer device. A yet further objective of the invention is to present an arrangement for changing the parameters of a collimating arrangement in an X-ray fluorescence analyzer quickly and easily. The objectives of the invention are achieved by using a thin plate with microscopic pores therethrough to realize angular selectivity, and a layer opaque to X-rays with an opening to only let through incident radiation to the desired target area. A collimator arrangement according to the invention comprises: a collimator plate with a plurality of pores that penetrate through at least an essential part of the thickness of the collimator plate and that have a diameter smaller than 100 micrometers, and an annular plate comprising material essentially opaque to X-rays, said annular plate defining an area transparent to X-rays;wherein said collimator plate and said annular plate are adapted to be placed in said X-ray fluorescence analyzer device between an X-ray source and a sample, and wherein said plurality of pores in said collimator plate are adapted to let through at least a part of X-rays radiated by said X-ray source, and wherein edges of said area transparent to X-rays in said annular plate are adapted to spatially limit in a transverse direction a beam of X-rays radiated by said X-ray source. The invention concerns also an X-ray fluorescence analyzer device, which comprises: an X-ray source, a sample window to be placed adjacent to a sample, and between the X-ray source and the sample window a collimator plate with a plurality of pores that penetrate through at least an essential part of the thickness of the collimator plate and that have a diameter smaller than 100 micrometers, and an annular plate comprising material essentially opaque to X-rays, said annular plate defining an area transparent to X-rays;wherein said plurality of pores in said collimator plate are adapted to let through at least a part of X-rays radiated by said X-ray source towards said sample window, and wherein edges of said area transparent to X-rays in said annular plate are adapted to spatially limit in a transverse direction a beam of X-rays radiated by said X-ray source towards said sample window. A capillary tube has a strong collimating effect on X-rays, because X-rays only reflect at very shallow incident angles, and thus only rays the propagating directions of which are within a small range around the longitudinal axis of the capillary tube will pass through. Capillary tubes in macroscopic scale share some of the clumsiness and other drawbacks of metal plate stacks as collimators. Additionally using a long macroscopic capillary tube as a collimator means that fluctuations in focal spot location and spatial intensity become clearly visible in the characteristics of the collimated incident X-ray beam. It is possible to produce a large number of microscopic pores through a plate having a thickness in the order from less than a millimetre to a few millimetres. A piece of such plate placed in front of an X-ray source will effectively implement accurate angular selectivity in a relatively small space. A plate with microscopic pores having the required aspect ratio can be made through etching from a semiconductor wafer. Another possibility is to fuse together a large number of glass fibres with a suitably selected core, cut a plate from the fused bunch of fibres and etch away the fibre cores. An annular plate made of a material that is opaque to X-rays will only let those rays pass that go through the opening in the plate. Together with a collimator plate with microscopic pores such an annular plate will produce a collimator that only allows a strictly selected, highly collimated portion of incident radiation to irradiate the sample at a desired location. The annular plate can be built as an integral part of the plate with microscopic pores, or the two plates may be structurally different parts. At least one of them may be located in a movable frame, with which it is possible to move the collimator plate and/or the annular plate between a storage position and an operating position in front of the X-ray source. The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. FIG. 1 illustrates schematically certain parts of an X-ray fluorescence analyzer device. An X-ray tube 101, which here is schematically shown as being of the side window type but which could also be of the end window type, acts as the source of incident X-rays. The incident X-rays are directed through a plate 102 that comprises a large number of microscopic pores therethrough. The microscopic pores in the plate 102 have a collimating effect, so that only X-rays the propagation direction of which is within a relatively narrow range around the longitudinal direction of the microscopic pores pass through the plate 102. A holder 103 keeps the plate 102 in place. Those incident X-rays that pass through the plate 102 meet next an annular plate 104 made of a material that is generally opaque to X-rays. The opening in the annular plate 104 lets through those X-rays that hit said opening. There is a holder 105 to keep the annular plate 104 in place. A sample 106 is shown, from which fluorescent and/or scattered X-rays go to a detection arrangement (not shown in FIG. 1). At least two principal approaches are possible for producing the plate 102 with the microscopic pores. We will first consider the possibility of using a glass capillary plate. Glass capillary plates have conventionally been used as an image intensifiers, i.e. analog amplifying components in detecting charged particles or electro-magnetic radiation. The conventional designation of a glass capillary plate used in such applications is “microchannel plate”. It consists of a glass plate with a periodic array of microscopic holes therethrough. The thickness of the glass plate is usually slightly less or slightly more than one millimeter, and a typical diameter of the holes is in the order of about ten micrometers. Thus each hole constitutes a channel through the glass plate, with an aspect ratio of typically about 100, although large deviations from these exemplary values are possible. For use as an image intensifier, the walls of the channels have been treated so that they enable the easy emission of photoelectrons and an avalanche-like multiplication of emitted electrons under the influence of an electric field between electrode metallizations on the top and bottom surfaces of the plate. According to an aspect of the present invention, it is possible to use a glass capillary plate as such as an X-ray collimator. However, the transmission efficiency at acceptable incoming angles becomes much better, if the channel walls of a glass capillary plate are treated to act like X-ray mirrors, so that they reflect incoming X-rays instead of causing photoelectric emission. Thus each channel in the glass capillary plate acts as a miniature waveguide that exhibits high transmissivity of X-rays at a relatively narrow range of acceptable input angles around the nominal channel direction. A suitable treatment is the plating of the channel walls with a layer of a metal such as iridium, ruthenium, platinum, osmium or nickel, having a thickness of a few nanometers. An exemplary method for applying such a treatment is ALD (Atomic Layer Deposition). FIG. 2 illustrates the principle of plating the channels of a glass capillary plate so that it becomes a better collimator for X-rays. On the left is a schematic cross section of a small portion of an ordinary glass capillary plate, of which a large part of the middle section has been omitted in order to add graphical clarity. In reality the channel length would be dozens of times larger than the channel width. The body 201 of the glass capillary plate consists of lead oxide glass or other material that is suitable for the manufacturing process. A conventional microchannel plate also comprises a top electrode layer 202 and a bottom electrode layer 203 usually made of chromium and/or nickel alloys. On the right in FIG. 2 is a schematic cross section of a small portion of a glass capillary plate for use as an effective X-ray collimator. A very thin conformal coating 210 has been added. The thickness of the coating 210 is preferably in the order of nanometers, like 5 nanometers. In the drawing it has been vastly exaggerated: taken that the channel width is in the order of several micrometers, drawn to scale the coating 210 would be hardly visible in the drawing. Whether or not the coating 210 also covers the top and bottom surfaces of the glass capillary plate is immaterial to the present invention. It is much more important that the coating 210 covers the walls of the channels and has as smooth a surface as possible. The smoothness requirement is one reason for not making the coating 210 thicker than a few nanometers, since the thicker the layer, the more easily its surface becomes uneven. Another reason for the small thickness of the coating 210 on the walls of the channels is that unnecessarily decreasing the channel cross-section will just reduce the transmission ratio of X-rays. A number of important criteria are set to the material used for the coating 210. The material should have a high atomic ordinal number in order to reflect X-rays as effectively as possible. The material should be well suited for application as very thin conformal layers, using atomic layer deposition (ALD) or other suitable coating method. Additionally it is advantageous if the material of the coating 210 does not have characteristic X-ray fluorescence peaks that could be easily confused with those of analysed materials in the target. The most suitable material for the coating 210 is believed to be iridium. Other suitable materials include but are not limited to ruthenium, osmium and nickel, of which at least the last-mentioned is more suitable for application through wet chemistry than ALD. Platinum and gold are known to be applicable as X-ray mirror materials, but they may have other disadvantages that may make them a suboptimal choice for the material of the coating 210. FIG. 3 illustrates an X-ray 301 that enters a channel 302 in a glass capillary plate at an incident angle a that is greater than zero (here incident angle is defined as the angle between the axial direction of the channel and the propagation direction of the X-ray). If the incident angle was zero, the X-ray 301 would pass straight through the channel, assuming that it did enter the channel in the first place and did not hit some of the solid parts of the front surface of the glass capillary plate, in which case it would become absorbed. Let us first assume that the channel walls have not been plated. In that case there is some relatively small limiting value for the angle a, so that at incident angles larger than the limiting value the X-ray could not pass directly through the channel but would hit the wall instead. An unplated glass wall is not a very good reflector for X-rays, but would be very likely to cause absorption. Depicting transmittivity as a function of incident angle would give something like the qualitative curve 401 in FIG. 4. Let us now assume that the channel walls have been plated in accordance with an aspect of the present invention. The plating allows the obliquely entering X-rays to reflect once or several times on their way through the channel, which in terms of transmittivity as a function of incident angle gives the qualitative curve 402 of FIG. 4. At a zero incident angle the transmittivity curve 402 is slightly lower than the curve 401, because the plating causes a small decrease in channel diameter and correspondingly increases the possibility of a directly coming X-ray to miss the channel and hit the front surface of the glass capillary plate instead. However, taken that the thickness of the plating is easily less than one thousandth of the channel diameter, this decrease in zero-angle transmittivity is almost infinitesimally small, and only exaggerated in FIG. 4 for clarity. On the other hand, due to the highly increased reflectivity of the channel walls, transmittivity experiences a large increase at larger values of the incident angle. The overall transmittivity of a collimator is proportional to an integral of the area that is left under the transmittivity curve in FIG. 4. The increased overall transmittivity due to better transmittivity at larger incident angles is shown with a simple hatch, while the small decrease in small-incident-angle transmittivity is shown with a cross hatch. The applicability of a glass capillary plate with plated channel walls as a collimator comes from the fact that a collimator can well have a certain allowance function of finite width around the nominal propagating direction that should pass directly through the collimator, as long as the maximum deviation a1 from the zero incident angle, at which radiation will still pass, is not so large that it would cause serious degradation in the required degree of collimating. How wide the allowance function can be, i.e. how much a propagating direction is allowed to differ from the nominal propagating direction and still be accepted to pass the collimator, depends on the application for which the collimator is used. According to the invention, it is easy to design and manufacture collimators with differently dimensioned allowance functions by simply selecting the aspect ratio of the glass capillary plate, i.e. by selecting a suitable plate thickness (typically between 0.4 and 3 millimeters) and channel width (typically between 5 and 15 micrometers). Also the material selected for the plating of the channel walls, and the resulting degree of reflectivity of the channel walls, is a parameter to be considered when the maximum allowable value of a1 is decided. It is expected that the increase in the allowance function width will in any cases be less than two degrees compared to the allowance function of a correspondingly dimensioned glass capillary plate with unplated channels. X-ray reflection at grazing incidence is known to be non-dispersive. This means that the collimator according to the embodiment of the invention described above does not significantly change the spectrum characteristics of the incident radiation of an X-ray fluorescence analyzer device. FIG. 5 illustrates schematically certain method steps that aim at manufacturing an X-ray collimator according to an embodiment of the invention. The process may contain some previous steps of unspecified nature, illustrated as 501. In step 502 at least the body of a glass capillary plate is produced. In a typical manufacturing process of glass capillary plates, a rod of etchable core glass is used as a support for a hollow billet of lead oxide cladding glass. A composite fibre is pulled from the combination. A number of these first draw fibres are stacked into an array, which is drawn again to produce a so-called multifiber. Several multifibres are stacked and fused together under vacuum, which results in a thick rod-like product known as a boule. The boule is sliced and polished to the required thickness and outline of the desired glass capillary plates. The solid cores, which at this stage still perforate the plate, are etched away, thus producing the characteristic array of microscopic holes through the plate. A complete manufacturing process of conventional microchannel plates involves firing the plates in a hydrogen oven to produce a semiconducting surface layer with the de-sired resistance and secondary electron yield, as well as producing the top and bottom electrode layers. For the purposes of the present invention these are un-necessary steps and can be left out. However, they do not cause much change either to the operation of the glass capillary plate as an X-ray collimator, so concerning the present invention it is immaterial, whether step 502 of the manufacturing process includes the hydrogen firing and electrode producing sub-steps or not. Step 503 involves plating the channel walls with the thin coating reflective of X-rays, for example in an ALD process. Other method steps may follow after that as is illustrated as 504. A microchannel plate meant for use as a particle detector or image intensifier has often a so-called nonzero bias angle, which means that the channels are not perpendicular to the planar surfaces of the plate. The bias angle is selected in step 502 mentioned above, by tilting the blade that is used to cut slices from the boule (or by tilting the boule with respect to the blade). If a glass capillary plate with plated channel walls is to be used as a collimator according to an embodiment of the invention, it should either have a zero bias angle, or the glass capillary plate should be placed at a non-perpendicular angle with respect to the desired propagation direction of X-rays, so that the channel direction coincides with the desired propagation direction of X-rays. The second principal approach for producing the plate 102 with the microscopic pores in FIG. 1 is to use a semiconductor wafer or a corresponding piece of etchable solid material as the starting point. From prior art there is known a general principle of using an anisotropically etched silicon semiconductor plate as the collimator for the scattered X-rays or fluorescent X-rays somewhere along the radiation path from the sample to a detector. A prior art publication U.S. Pat. No. 6,477,226 suggests anisotropically etching deep pores of diameter between 0.1 and 100 micrometers to a semiconductor plate. The publication suggests either leaving a thin layer of the semiconductor material intact at the bottom of the pores, or lining the inside of each pore with a so-called stabilizing layer made of silicon nitride, silicon carbide, boron nitride, boron hydride, boron carbide and/or carbon, after which the remaining thin semiconductor layer could be removed, which leaves the previously closed end of each pore visible but covered with the stabilizing layer material. The materials that said publication suggests for the stabilizing layer have the drawback that they do not function as X-ray mirrors. The performance of a collimator plate made of a semiconductor material and having a plurality of micropores anisotropically etched therethrough could be remarkably improved by using an additional coating of iridium, ruthenium, osmium, platinum, nickel, or gold either instead of the so-called stabilizing layer of said prior art publication or on top of it. We should also note that silicon is relatively transparent to X-rays, which means that the silicon-based collimator plates known from prior art would hardly be suitable for collimating incident radiation. The intensity of the radiation produced by the X-ray source is simply so high that a significant portion of the rays propagating in other directions than those that a collimator should let through would also penetrate through the solid silicon parts that constitute the structural matrix of the plate. However, germanium is a substrate that lends itself easily to anisotropic etching, and that is relatively easily available in wafer thicknesses suitable for collimator plates. Germanium is also very seldom among those elements that should be analysed from a sample, so it is unlikely that excited spectral lines or scattered radiation from germanium would interfere with the measurement. In the following we will use the designation “collimator plate” to mean either a glass capillary plate based collimator plate or a semiconductor (preferably germanium) based collimator plate with a plurality of (preferably iridium-coated) microscopic pores therethrough. FIG. 6 is a cross section that shows how an assembly of collimator plates with differently directed channels therethrough can be used as a combined collimator and focusing lens for X-rays. We assume that radiation comes from top to down. The upper row of collimator plate sections has a central plate 603 with zero bias angle. On each side of it there are plates 602 and 604, with small bias angles of equal absolute magnitude but opposite sign. The utmost plates 601 and 605 constitute a similar pair, having bias angles of equal absolute magnitude but opposite sign, said magnitude being slightly larger than that of plates 602 and 604. In the lower row the same principal arrangement is repeated, with the central plate 613 having zero bias angle, the intermediate plates 612 and 614 constituting an equal-magnitude and opposite-sign pair and the utmost plates 611 and 615 likewise. The increasing steps in the absolute value of the bias angle are larger in the lower row, with plates 611 and 615 having the largest absolute bias angle magnitude in the whole assembly. X-rays that pass through the collimator plate assembly of FIG. 6 from top to down will experience certain convergence that directs them at least approximately towards a point or area of convergence located further down. Similar lens-like effects are achieved if the manufacturing process of the collimator plate allows the bias angle to be gradually changed across the plate like in the plate 701 of FIG. 7, or if a collimator plate with initially parallel channels is afterwards made to exhibit some curvature like plate 801 in FIG. 8. Collimator plates and collimator plate arrangements that include a focusing characteristic, like the ones shown in FIGS. 6, 7, and 8, can be said to be special cases of the concept “collimator”, because they still act as pure collimators at least for those X-rays that pass through the central region (or more generally: the region where the channel direction is the same as the incident direction of those X-rays that should pass), and because they only allow X-rays with incident propagating directions within the (relatively narrow) allowance function to pass. Thus also the embodiments shown in FIGS. 6, 7, and 8 fall within the scope of the claims directed to a collimator. It should be noted that for reasons of graphical clarity, the differences in bias angle and the curvature of the collimator plate in FIG. 8 have been exaggerated. Two consecutive collimator plates can be used to implement a variable-attenuation X-ray attenuator that also has a collimating effect. This requires there to be a possibility of causing relative movement between the two collimator plates, so that in one relative position the channel direction in both collimator plates is essentially the same and in some other relative position the channel direction in the collimator plates is not the same. In practice this principle can be implemented for example in the way shown in FIG. 9. A first collimator plate 901 and a second collimator plate 902 have an equal, non-zero bias angle. Both collimator plates are assumed to be round, but are shown in FIG. 9 schematically as cut through to illustrate the concept of channel direction. The planar direction of both collimator plates is the same. At least one of them, here the second collimator plate 902, can be rotated around an axis 903 that is perpendicular to said planar direction of the collimator plates. In one rotational position, shown in the upper part of FIG. 9, the channel direction in the second collimator plate 902 differs so much from the channel direction in the first collimator plate 901 that at least a majority of X-rays that pass through the first collimator plate 901 cannot pass through the second collimator plate 902, due to the difference in channel direction between the collimator plates. In another rotational position, shown in the lower part of FIG. 9, the channel direction in the second collimator plate 902 is essentially the same as the channel direction in the first collimator plate 901 that at least a majority of X-rays that pass through the first collimator plate 901 also pass through the second collimator plate 902. Since the transmittivity of both plates as a function of incident angle has a finite width around the nominal channel direction, between the position shown in the upper and lower parts of FIG. 9 there are a number of intermediate positions in which the relative amount of X-rays coming through varies between these extremities. A collimator plate alone does not limit the cross section of the incident radiation beam. According to an aspect of the invention, an annular plate or layer can be used together with the collimator plate to achieve the desired incident radiation beam with a limited width. Various ways exist for combining the use of a collimator plate and an annular plate or layer. FIG. 10 is a cross section of a plate 1000, a middle section 1001 of which comprises a number of microscopic pores through the plate, with their walls preferably coated with X-ray-reflecting material. The edge sections 1002 of the plate 1000 are without said microscopic pores, so that the body of the plate 1000 also constitutes the annular plate or layer referred to above. Since the middle section 1001 is not actually an opening in the same sense as in the other annular plates discussed in this description, we may generalize the definition of an annular plate so that it defines an area transparent to X-rays, which in true annular plates is an opening and in combined embodiments like that of FIG. 10 is the section that comprises the microscopic pores. FIG. 11 illustrates another alternative embodiment, in which a collimator plate 1100 has a (relatively wide) middle section 1101 with pores and only a narrow edge section 1102 without pores, for making it easier to attach the collimator plate 1100 to a holder (not shown). It would not be necessary to have any edge section at all. On one surface of the collimator plate 1100 there is an annular layer 1103 of a material essentially opaque to X-rays. At its center the annular layer 1103 defines an opening 1104. The dimensions of the opening 1104 define, how wide is the collimated X-ray beam that will come through the collimator plate 1100. The annular layer 1103 may be formed as an integral layer of the collimator plate 1100, for example by applying a suitable annular metallization pattern of sufficient thickness to one side of the collimator plate. Alternatively the annular layer 1103 may originally be a mechanically different component that is just stacked together with the collimator plate and possibly attached thereto by glueing, soldering or other means. FIG. 12 illustrates yet another alternative embodiment, in which the collimator plate 1200 and an annular plate 1210 are separate mechanical entities and can be separately attached to corresponding holders (not shown). A section 1201 of the collimator plate 1200 is equipped with microscopic pores, with their walls preferably coated with X-ray-reflecting material. The collimator plate 1200 may have a solid edge section 1202 for making attachment to holder easier, but this is not necessary. The body of the annular plate 1210 constitutes an annular barrier 1213 essentially opaque to X-rays, and defines an opening 1214 for the X-rays to come through. Together the collimator plate 1200 and the annular plate 1210 are configured to both collimate and spatially limit an incident X-ray beam. Since not all X-ray fluorescence measurements require a well collimated and spatially limited incident X-ray beam, it is in many cases advantageous to equip an X-ray fluorescence analyzer device with a mechanism that allows easily selecting into use the incident-beam-shaping means that are needed. FIG. 13 illustrates schematically certain parts of an X-ray fluorescence analyzer device, in which an X-ray tube 101 of the end window type acts as the source of incident X-rays. Within the space between the X-ray source and a sample window 1301 of the analyzer device there are located a first movable holder 1302 and a second movable holder 1303. The first movable holder 1302 comprises at least one location in which resides a collimator plate 1304. The form of the first movable holder 1302 shown here (a circular plate rotatable around its central normal axis, and with circular openings for attaching collimator plates and/or other equipment) is naturally only exemplary. From the technology of X-ray fluorescence analyzers it is known to use movable holders of various kinds for selectively placing e.g. exchangeable filters to the path of the incident X-rays. The other circular openings of the first movable holder 1302 may comprise such filters. The second movable holder 1303 is configured to offer into use various annular openings so that the transverse dimensions of the incident X-ray beam can be limited to the desired size. This can be accomplished either so that there is one or more attachment locations in the second movable holders into which the user may removably attach separate annular plates according to need, or like in FIG. 13 in which the second movable holder 1303 itself constitutes the annular plate by having a number of differently dimensioned openings therethrough. If the X-ray fluorescence analyzer only comprises a single movable holder, it is most advantageous to use a combined structure of a collimator plate and an annular layer like that shown in FIG. 11 and to attach it to a suitable location in the movable holder. Alternatively, especially in the absence of a second movable holder, a changeable annular plate can be constructed from a number of partially overlapping, radially movable parts in the same way as the controllable aperture is implemented in a systems camera. The invention allows incident X-rays to be directed to a relatively small area in the sample. The irradiated sample area has essentially the same form and dimensions as the opening in the annular plate. A natural exception occurs if the incident X-ray beam comes at an oblique angle to the sample, in which case the irradiated sample area is the area the projection of which, looked from the direction of the incident X-rays, has the form and dimensions of the opening in the annular plate. The invention does not limit the selection of the form and dimensions of the opening in the annular plate. In exemplary cases the invention could be used to only irradiate a spot-like sample area with transverse dimensions in the order of a millimeter or a couple of millimeters with incident X-rays. In order to help a human user to perceive, what part of the sample will be subjected to incident X-rays, it is advantageous if the X-ray fluorescence analyzer device includes an optical aiming aid. FIG. 14 illustrates schematically certain considerations related to optical aiming aids. The X-ray fluorescence analyzer device of FIG. 14 comprises a fiber optic sight generally designated as 1401, with an objective 1402 aimed to look at the sample 106 and an ocular or display 1403 for conveying a visual image to a human user. Using the fixed structures of the X-ray fluorescence analyzer device to aim and attach the objective 1402 accurately enough may as such be enough for ensuring that the user gets a good perception of which area of the target will be irradiated with incident X-rays. In addition the X-ray fluorescence analyzer device of FIG. 14 comprises a light source 1404 and a mirror 1405, which may be either a dichroic mirror that reflects visible light but lets X-rays through, or a movable mirror that can be moved to the location seen in FIG. 14 for the duration of optical aiming. Together the light source 1404 and the mirror 1405 produce a beam of visible light, which comes from the same direction and through the same collimating and spatial limiting means as the beam of incident X-rays. Thus the sample area illuminated with visible light from the light source 1404 will be exactly the same as the sample area irradiated with incident X-rays from the X-ray tube 101. The user sees, which part of the sample is illuminated, through the fiber optic sight 1401. Illumination of the appropriate sample area with visible light can be accomplished also with other kinds of arrangements, for example with a fiber optic illumination system either integrated with the fiber optic sight 1401 or built separately.
	summary
	abstract	A method of manufacturing nuclear fuel elements may include: forming a base portion of the fuel element by depositing a powdered matrix material including a mixture of a graphite material and a fibrous material; depositing particles on the base portion in a predetermined pattern to form a first particle layer, by controlling the position of each particle in the first particle layer; depositing the matrix material on the first particle layer to form a first matrix layer; depositing particles on the first matrix layer in a predetermined pattern to form a second particle layer by controlling positions of each particle in the second particle layer; depositing the matrix material on the second particle layer to form a second matrix layer; and forming a cap portion of the fuel pebble by depositing the matrix material. The particles in the first particle layer and the second particle layer include nuclear fuel particles.
040574679	abstract	An integrated pressurized-water reactor and steam generator is disclosed, particularly for ship propulsion and smaller stationary installations, in which circulating pumps of primary coolant loops are arranged in the upper part of a vertical pressure vessel close to the vessel's removable head. An annular steam generator is arranged centrally within the vessel, and above the outer periphery of a reactor core in the vessel's lower portion, and is suspended via a separate hollow ring supported by the vessel's upper portion and in which the impellers of the circulating pumps are arranged. The primary cooling water flows upwardly through the reactor core and the heat exchanger to the pumps located above, and from there, through a separate ring passage outside of the heat exchanger housing and core downward to again flow upwardly through the reactor core. Secondary steam and primary feed water nozzles can be disassembled sideways, so that although normally connecting with internal parts, all internals can be removed upwardly from the pressure vessel so that the pressure vessel can be checked everywhere. In the event of a failure of the circulating pumps, the afterheat given off by the reactor core after being shut down, is transmitted to the heat exchanger through natural thermal circulation of the water in the vessel.
039742695	summary	RELATED APPLICATION This application relates to improvements in the methods of detection disclosed in copending and commonly assigned patent application Ser. No. 385,863 filed on Aug. 6, 1973, the disclosure of which is hereby incorporated by reference. FIELD OF INVENTION This invention relates generally to methods for screening large number of persons for current or past gonorrhea infection. Gonorrhea is one of the most commonly reported bacterial diseases in man and its persistence as a major health problem has intensified the search for new and better methods of detection. The present mass screening method is a bacteriological method which requires two to seven days for completion. Moreover, it requires that a specimen of the gonorrhea caused discharge arrive at the testing laboratory with the fragile gonococcus organism still viable, a natural time limit of as little as two days. In the above identified patent application, a novel serological method for detecting antibodies in sera is described. The method is generally based on the discovery and isolation of a heat labile antigen produced by Neisseria gonorrhoeae (N.g.) organisms. This antigen does not react with cross reacting antibodies which may also be present in the sera, with the result that the number of false positive reactions which have reduced the value of previously employed serological procedures is substantially reduced. In accordance with the several detection methods described and claimed in the application, the antigens are caused to react with the antibody and the presence of the resulting complex is determined. In the principal methods described, the complex is caused to react with a labelled anti-human immunoglobulin G. The immunoglobulin is labelled with a fluorescent compound, a radioactive element, or an enzyme. The label is then detected by suitable procedures such as radioactive counting. The optimum sources of the antigen are growth cultures of Neisseria gonorrhoeae ATCC 21823 (B-585), 21824 (B-370) and 21825 (B-1094), although a number of N.g. microorganisms will also serve as antigen sources. Order: Eubacteriales Family: Neissericeae Genus: Neisseria Species: Gonorrhoeae Morphology: Gram negative spherical or bean shaped diplococci with adjacent sides flattened usually 0.6 .times. 1.0 .mu. and more uniform in size. Biochemical and Cultural: Aerobic, optimal growth requires 4 - 10% CO.sub.2 and incubation at 36.degree.C. The cultures grow slowly on chocolate agar producing small barely visible colonies after 24 hours (0.1 mm in diameter) with typical morphology seen on 48-72 hours cultures. The colonies are small 1.0 mm in diameter, gray white, transparent, smooth, with round entire edge, glistening surface and butyrous consistency. B-1094 produced slightly larger colonies and grows more rapidly. Oxidase +, catalase +; ferments glucose but not maltose, lactose or sucrose. Antigenicity: All three isolates share common antigens which is heat labile "L". Virulence: All three strains were originally isolated from patients with symptomatic gonorrhea. A novel radioimmunoassay technique has now been discovered which makes possible the detection of the antigen-antibody conjugate in a facile manner.
053032730	abstract	An apparatus for assembling a nuclear fuel assembly is disclosed. The apparatus includes a deflecting device which is disposed adjacent to the grid and deflects grid springs away from dimples opposing thereto. The deflecting device includes a tubular member, a rod member and a drive mechanism. The tubular member has a plurality of circumferentially divided sleeve pieces. The rod member is releasably inserted in the tubular member for sliding movement therealong. The drive mechanism is drivingly connected to the rod member and moves the rod member in the tubular member in a longitudinal direction thereof to bring the rod member into urging engagement with the sleeve pieces of the tubular member, whereby the sleeve pieces are deflected to be urged against the spring to deflect the same.
051715198	abstract	A nuclear reactor having a chemical decontamination system is provided in which every piece of decontamination equipment which processes radioactive materials is located within a decontamination building. The decontamination system is designed to provide for adequate protection for personnel safety and also incorporates a modular design for equipment transportation and storage.
048083704	description	Referring now to the figures in detail and first, particularly, to FIG. 1 thereof, there is seen a concrete structure serving as a radiation shield 1, inside of which a steel pressure vessel 2 is disposed. The pressure vessel is equipped with a removable cover 3 at the upper end thereof Disposed inside the pressure vessel 2 is a metallic core barrel 4, which has a circular projection 5 that rests on a circular flange 6 formed on the inside of the pressure vessel 2. The core barrel 4 contains a lining of built-in ceramic parts or internals 9 formed of carbon blocks and/or graphite which surround a space for receiving a fission zone 10 formed of a multiplicity of spherical fuel assemblies Among other things, canals 11 extend through the built-in parts 9. Absorber rods can be moved in the canals 11 by means of conventional drives 12 which are disposed at the ceiling of the core barrel 4, for controlling the fission zone The built-in parts 9 include other canals 13, through which cooling gas coming from non-illustrated heat sink is conducted into an upper plenum and is sent from the plenum through the fission zone 10 from the top down The heated cooling gas converging in a lower plenum 15 is fed to a non-illustrated heat sink through a hot gas line 16 disposed in a stub or connecting piece 17 of the pressure vessel 2. As seen in FIG. 2, the hot gas line 16 includes an outer tight metallic pipe 22 The flow of the hot gas itself is guided by an inner pipe 23 which is concentric with the pipe 22 and is formed of carbon fiber-reinforced carbon The hot gas line 16 is inserted into a wider stub or connecting piece 24 which is fastened to the core barrel 4 and is fastened to the line 16 by a number of screws 25 distributed over the periphery, as seen in FIG. 3. The breakdown of the hot gas temperature which is as high as 950.degree. C., to a level of about 300.degree. C. which is peraissible for the pressure-carrying pipe 22, is accomplished by heat insulation which can be partially formed of a filamentary ceramic material 26 and partially (at particularl highly mechanically stressed points) of a highly porous solid ceramic 27. A bellows compensator 28 is disposed in the interior of the wider stub 24 and is tightly connected thereto The bellows compensator may be optionally formed of two parts that are joined together by a rigid intermediate part. The bellow:s compensator is connected by means of screws 29 to a sleeve 30, which is also formed of carbon fiberreinforced carbon The sleeve 30 is in turn fastened by means of further screws 31 formed of the same material, to the builtin graphite or carbon block parts of the core barrel 4. The temperature breakdown at the built-in parts 9 from the hot gas temperature to the temperature permissible for the bellows compensator 28, takes place by way of the sleeve 30 A displacement which may occur during operation due to temperature changes between the core barrel 4 and the hot gas line 16 (even in the vertical direction) is compensated by the bellows compensator 28, while at the same time tightness is maintained, so that hot gas leaks from the line 16 which could endanger the core barrel 4 do not take place The outside of the bellows compensator 28 is then cooled by part of the stream of the cooled gas which returns from the heat sink and flows in a space 32 between the hot gas line 16 and a cold gas line 33 which surrounds the line 16 coaxially and is connected to the stub 17. To this end, holes 34 are provided in the wider stub 24 The wider stub 24 is detachably fastened to the core barrel 4 by means of additional screws 35 distributed over the periphery. It can become necessary to detach the stub 24 if the hot gas line 16 is to be replaced because of damage. Due to the necesary close fit between the line 16 and the wider stub 24, it appears more advantageous to replace both parts together If the screws 35 cannot be loosened(such as because of mechanical bending or due to a weld in the thread) they can be cut off between the wall of the core barrel 4 and the flange of the wider stub 24, by means of non-illustrated cutting tools During the reassembly, new tapped holes are cut in the core barrel 4, offset at an angle thereto. During the disassembly of the gas line 16, insulating plugs 36 as well as the screws 25 covered by the plugs can be removed from the inside of the flow guiding inner pipe 23 by chip-producing or milling machinery. This can be done more easily with remotely controlled tools Undesirable friction welding of the pressure pipe 22 to the wider stub 24 in the hot helium atmosphere, is prevented by a suitable coating of these parts, such as with chromium carbide The advantage of the illustrated construction is that during installation the wider flange 24, the bellows compensator 28 and the sleeve 30 can be tested for leaks in a completely assembled condition, before the hot gas line 16 is placed in the wider stub 24. The foregoing is a description corresponding in substance to German Application No. P 35 18 609.7, filed May 23, 1985, the International priority of which is being claimed for the instant application, and which is hereby made part of this application. Any material discrepancies between the foregoing specification and the aforementioned corresponding German application are to be resolved in favor of the latter.
	description	This application claims the benefit of U.S. Provisional Application No. 61/625,188 filed Apr. 17, 2012. U.S. Provisional Application No. 61/625,188 filed Apr. 17, 2012 is hereby incorporated by reference in its entirety. The following relates to the nuclear reactor arts and related arts. In nuclear reactor designs of the pressurized water reactor (PWR) type, a nuclear reactor core is immersed in primary coolant water at or near the bottom of a pressure vessel. In a typical design, the primary coolant is maintained in a subcooled liquid phase in a cylindrical pressure vessel that is mounted generally upright (that is, with its cylinder axis oriented vertically). A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. Primary coolant flows upward through the reactor core where it is heated and rises through the central riser, discharges from the top of the central riser, and reverses direction to flow downward back toward the reactor core through a downcomer annulus defined between the pressure vessel and the central riser. The nuclear core is built up from multiple fuel assemblies each comprising a bundle of fuel rods containing fissile material (usually 235U). In a typical PWR design, upper internals located above the reactor core include control rod assemblies with neutron-absorbing control rods that are inserted into/raised out of the reactor core by control rod drive mechanisms (CRDMs). Conventionally, the CRDMs employ motors mounted on tubular pressure boundary extensions extending above the pressure vessel. In this design, the complex motor stator can be outside the pressure boundary and magnetically coupled with the motor rotor disposed inside the tubular pressure boundary extension. The pressure vessel of the PWR is conventionally connected with an external pressurizer and external steam generators via large-diameter piping. More recent small modular reactor (SMR) designs have been driven by a desire to make the PWR more compact and to have fewer large diameter vessel penetrations. Toward this end, in so-called “integral” PWR design, the steam generator is located inside the pressure vessel, typically in the downcomer annulus. This replaces the external primary coolant loop carrying radioactive primary coolant by a secondary coolant/steam loop carrying nonradioactive secondary coolant. The use of an internal pressurizer comprising a steam bubble at the top of the pressure vessel and suitable baffling is contemplated to eliminate the large-diameter penetration for the external pressurizer. Still further, fully internal CRDM motors are contemplated, which eliminates the tubular pressure boundary extensions above the reactor vessel. Some illustrative PWR designs incorporating these advances are described in, e.g.: Thome et al., “Integral Helical-Coil Pressurized Water Nuclear Reactor”, U.S. Pub. No. 2010/0316181 A1 published Dec. 16, 2010 which is incorporated by reference in its entirety; Malloy et al., U.S. Pub. No. 2012/0076254 A1 published Mar. 29, 2012 which is incorporated by reference in its entirety; Stambaugh et al., U.S. Pub. No. 2010/0316177 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety; and DeSantis, U.S. Pub. No. 2011/0222640 A1 published Sep. 15, 2011 which is incorporated herein by reference in its entirety. Such SMR designs introduce numerous challenges not faced in more conventional PWR designs. One such challenge is reactor refueling. SMR components are tightly packed into a compact pressure vessel, with the nuclear reactor core located at or near the bottom of the pressure vessel in order to minimize the possibility of the primary coolant water level falling to a point that exposes the reactor core during a loss of coolant accident (LOCA). This means that all major components are located above the reactor core, and must be removed in order to access the reactor core for refueling. For example, in PWR designs disclosed in Stambaugh et al., U.S. Pub. No. 2010/0316177 A1, an upper internals basket welded to a mid-flange of the pressure vessel supports the internal CRDMs and the control rod guide frames, and power and signal connections for the CRDMs are routed to connectors on the mid-flange. These components must be removed in order to access the reactor core. Disclosed herein are improvements that provide various benefits that will become apparent to the skilled artisan upon reading the following. In accordance with one aspect, a nuclear reactor includes at least: a pressure vessel including an upper vessel section and a lower vessel section connected by a mid-flange and containing primary coolant; a nuclear reactor core disposed in the lower vessel section and immersed in the primary coolant; and upper internals suspended from the mid-flange of the pressure vessel. The upper internals include at least internal control rod drive mechanisms (CRDMs) with CRDM motors immersed in the primary coolant and control rod guide frames disposed between the CRDM motors and the nuclear reactor core. In a disclosed method, the nuclear reactor is depressurized. The upper vessel section is disconnected and removed from the remainder of the pressure vessel while leaving the mid-flange in place on the lower vessel section with the upper internals remaining suspended from the mid-flange. The mid-flange is removed from the lower vessel section with the upper internals remaining suspended from the mid-flange. Fuel of the nuclear reactor core is replaced, the mid-flange is placed back onto the lower vessel section with the upper internals remaining suspended from the mid flange, and the upper vessel section is placed back onto the remainder of the pressure vessel and is re-connected with the remainder of the pressure vessel. In accordance with another aspect, a nuclear reactor includes at least: a pressure vessel including an upper vessel section and a lower vessel section connected by a flange assembly and containing primary coolant wherein the flange assembly includes at least a flange on the upper vessel section and a flange on the lower vessel section; a nuclear reactor core disposed in the lower vessel section and immersed in the primary coolant; a support element supported by the lower vessel section; and upper internals disposed in the pressure vessel and suspended from the support element. The upper internals include at least internal control rod drive mechanisms (CRDMs) with CRDM motors immersed in the primary coolant and control rod guide frames disposed between the CRDM motors and the nuclear reactor core. In a disclosed method, the nuclear reactor is depressurized, the upper vessel section is disconnected from the lower vessel section and lifted away from the lower vessel section with the support element remaining supported by the lower vessel section and the upper internals remaining suspended from the support element, the support element is lifted away from the lower vessel section with the upper internals remaining suspended from the support element so as to lift both the support element and the suspended upper internals away from the lower vessel section, fuel of the nuclear reactor core disposed in the lower vessel section is replaced, the support element is lowered back onto the lower vessel section with the upper internals remaining suspended from the support element, the upper vessel section is reconnected to the lower vessel section, and the nuclear reactor is re-pressurized. In some embodiments the support element comprises a flange of the flange assembly connecting the upper and lower vessel sections. In accordance with another aspect, a nuclear reactor includes at least: a pressure vessel including an upper vessel section and a lower vessel section connected by a flange assembly and containing primary coolant wherein the flange assembly includes at least a flange on the upper vessel section and a flange on the lower vessel section; a nuclear reactor core disposed in the lower vessel section and immersed in the primary coolant; a support element supported by the lower vessel section; and upper internals disposed in the pressure vessel and suspended from the support element. The upper internals include at least internal control rod drive mechanisms (CRDMs) with CRDM motors immersed in the primary coolant and control rod guide frames disposed between the CRDM motors and the nuclear reactor core. The nuclear reactor is configured to be prepared for refueling by operations including disconnecting the upper vessel section from the lower vessel section and lifting the upper vessel section away from the lower vessel section with the support element remaining supported by the lower vessel section and the upper internals remaining suspended from the support element, and lifting the support element away from the lower vessel section with the upper internals remaining suspended from the support element so as to provide access from above to the nuclear reactor core disposed in the lower vessel section. In some embodiments the support element comprises a mid-flange of the flange assembly that is disposed between the flange on the upper vessel section and the flange on the lower vessel section. Optionally, the mid-flange includes vessel penetrations connected by cables with the upper internals that remain connected by cables with the upper internals during the lifting of the mid-flange away from the lower vessel section. With reference to FIG. 1, a small modular reactor (SMR) 1 of the of the integral pressurized water reactor (PWR) variety is shown in partial cutaway to reveal selected internal components. The illustrative PWR 1 includes a nuclear reactor core 2 disposed in a pressure vessel comprising a lower vessel portion 3 and an upper vessel portion 4. The lower and upper vessel portions 3, 4 are connected by a mid-flange 5. Specifically, a lower flange 5L at the open top of the lower vessel portion 3 connects with the bottom of the mid-flange 5, and an upper flange 5U at the open bottom of the upper vessel portion 4 connects with a top of the mid-flange 5. The reactor core 2 is disposed inside and at or near the bottom of the lower vessel portion 3, and comprises a fissile material (e.g., 235U) immersed in primary coolant water. A cylindrical central riser 6 is disposed coaxially inside the cylindrical pressure vessel and a downcomer annulus 7 is defined between the central riser 6 and the pressure vessel. The illustrative PWR 1 includes internal control rod drive mechanisms (internal CRDMs) 8 with internal motors 8m immersed in primary coolant that control insertion of control rods to control reactivity. Guide frames 9 guide the translating control rod assembly (e.g., each including a set of control rods comprising neutron absorbing material yoked together by a spider and connected via a connecting rod with the CRDM). The illustrative PWR 1 employs one or more internal steam generators 10 located inside the pressure vessel and secured to the upper vessel portion 4, but embodiments with the steam generators located outside the pressure vessel (i.e., a PWR with external steam generators) are also contemplated. The illustrative steam generator 10 is of the once-through straight-tube type with internal economizer, and is fed by a feedwater inlet 11 and deliver steam to a steam outlet 12. See Malloy et al., U.S. Pub. No. 2012/0076254 A1 published Mar. 29, 2012 which is incorporated by reference in its entirety. The illustrative PWR 1 includes an integral pressurizer 14 at the top of the upper vessel section 4 which defines an integral pressurizer volume 15; however an external pressurizer connected with the pressure vessel via suitable piping is also contemplated. The primary coolant in the illustrative PWR 1 is circulated by reactor coolant pumps (RCPs). In the illustrative embodiment of FIG. 1, each RCP comprises an external RCP motor 16 driving an impeller located in a RCP casing 17 disposed inside the pressure vessel. The illustrative PWR 1 also includes an optional support skirt 18. With reference to FIGS. 2-30, an illustrative refueling sequence is disclosed for a reactor of the type shown in FIG. 1. The refueling sequence utilizes the flange arrangement in which (1) the upper flange 5U is integral with the upper vessel 4; the lower flange 5L is integral with the lower vessel 3; and the mid-flange 5 is disposed between the upper and lower flanges 5U, 5L and supports the upper internals (including the CRDMs 8 and guide frames 9) in suspended fashion. In brief, the upper vessel 4 is removed with the integral steam generator 10 secured inside the upper vessel 4; then the mid-flange 5 is removed along with the upper internals 8, 9 suspended from the mid-flange 5 so that the reactor core is then accessible from above for refueling. This approach avoids the need to disassemble internal components. FIG. 2 illustrates a flowchart of a first portion 100 of the refueling sequence, including: starting the reactor shutdown sequence 110, allowing the reactor to cool down 120; depressurizing the reactor 130; draining the secondary side of the upper vessel 140; draining the primary side of the upper vessel to the lower vessel assembly level 150; removing the insulation panels from the appropriate areas of the reactor (i.e. the main closure area, pressurizer heater area, and pressure relief area) 160; and breaking the hydraulic connections (i.e. feedwater lines, main stream lines, and pressure relief lines) 170. FIG. 3 illustrates a flowchart of a second portion 200 of the sequence for removing spent fuel from the reactor, starting with disconnecting the upper vessel external electrical connections (i.e. pressurizer heaters and instrumentation) 210. If the electrical and piping for the lower vessel assembly are routed down the side of the upper vessel and also ride with the upper vessel, then in a further operation 215 all of the connections through the mid-flange are broken. An upper vessel crane is connected 220. The reactor vessel nuts at the main closure are de-tensioned in an operation 230. The reactor vessel studs are backed out of the lower vessel flange and the reactor vessel studs are backed out of the lower vessel flange and the reactor vessel nuts and reactor vessel studs are parked in the transport position on the upper flange in an operation 240. Optionally, the reactor vessel nuts and the reactor vessel studs are completely removed and placed into a separate transport rack, as indicated in FIG. 3 by an operation 245. Finally, the upper vessel is moved to the upper vessel maintenance stand for inspection in an operation 250. FIG. 4 illustrates a flowchart of a third portion 300 of the refueling sequence, including: removing the riser cone from the upper internals 310; disconnecting the external connections around the mid-flange from the lower vessel assembly 320; pulling the incore instruments into the upper internals and wrapping the incore instruments around the mid-flange 330; positioning the upper internals platform on the mid-flange 340; inserting a tool to disconnect control rods from the control rod drive mechanism (CRDM) 350; and hooking the upper internals crane up to the upper internals 360. FIG. 5 illustrates a flowchart of a fourth portion 400 of the refueling sequence. The portion 400 includes: verifying that all connecting rods are in a parked position and that all external connections are disconnected from the mid-flange 410; moving the upper internals and refueling bridge to the upper internals maintenance stand 420; and in sequence with the upper internals being pulled from the lower vessel, slowly filling the refueling canal 430. At the end of this portion 400, the fuel exchange can begin in operation 440. With reference to FIGS. 6-19, the state of an illustrative SMR of the integral PWR variety is shown at various stages of the process flowcharted in FIGS. 2-5. The SMR of FIGS. 6-19 is similar to that of FIG. 1. A difference between the illustrative SMR of FIG. 1 and that of FIGS. 6-19 is that in the SMR of FIG. 1 the reactor coolant pumps (RCPs) are mounted on the upper vessel 4 proximate to the integral pressurizer 14 with external RCP motors 16 driving impellers located in RCP casings 17 disposed inside the pressure vessel; whereas, in the SMR of FIGS. 6-19 the RCPs are wholly internal RCPs 167 that are part of the upper internals suspended from the mid-flange 5 (see, e.g. FIGS. 14-17). It is to be understood that the disclosed refueling process is also applicable to a reactor design such as that of FIG. 1 in which the RCPs have external motors (e.g., wholly dry motor/stator assemblies or dry stators coupled with wet stators via tubular pressure boundary extensions). FIG. 6 diagrammatically shows the SMR in an operational state 500. The operational reactor 500 includes components not shown in FIG. 1, including metal reflective insulation 505, upper vessel hydraulic connections 510, pressurized heater electrical connections 515, and upper internals electrical connections 520. (Additional components ancillary to the operating reactor 500, such a surrounding containment structure, reactor support including a floodwell, piping running to/from a turbine, various emergency core cooling and other safety-related components, and so forth are not illustrated). The operational state 500 corresponds to the state of the reactor just before reactor shutdown 110, and illustrates the externally observed state of the reactor during the operations 110, 120, 130, 140, 150 of FIG. 2. FIG. 7 diagrammatically shows the SMR in a state 600 with the insulation 505 partially removed, corresponding to operation 160 of FIG. 2. FIG. 8 diagrammatically shows the SMR in a state 700 with the insulation removed from the pressurizer, mid-flange, and lower vessel regions, and with upper vessel connections 510 detached. This state 700 corresponds to operation 170 of FIG. 2. FIG. 9 diagrammatically shows the SMR in a state 800 with pressurized heater electrical connections 815 and instrumentation connections 816 partially removed. This state 800 corresponds to operation 210 of FIG. 3. FIG. 10 diagrammatically shows the SMR in a state 900 corresponding to the alternative embodiment in which electrical and piping for the lower vessel assembly are routed down the side of the upper vessel and also ride with the upper vessel. FIG. 10 corresponds to further operation 215 of FIG. 3 in which the connections 920 through the mid-flange to the upper internals are broken. FIG. 11 diagrammatically shows the SMR in a state 1000, which is ready for hook-up with the upper vessel crane, i.e. operation 220 of FIG. 3. As seen in FIG. 11, the upper vessel includes upper vessel crane attachment points 1030 for attachment of the lifting crane (not shown). FIG. 12 diagrammatically shows the SMR in a state 1100 including reactor vessel nuts 1135 at the mid-flange 5 in tensioned position. The state 1100 corresponds to just before commencement of the nut detensioning operation 230 of FIG. 3. FIG. 13 diagrammatically shows the SMR in a state 1200 corresponding to immediately subsequent to the operation 240 of FIG. 3, with the reactor vessel studs 1240 backed out of the lower vessel flange 5L and parked in their transport positions in the upper vessel flange 5U. As indicated in alternative operation 245 of FIG. 3, the studs 1240 are alternatively completely removed and placed into a separate transport rack (variant not illustrated). An advantage of the approach of FIG. 13 in which the bolts remain at least partially inserted into the upper vessel flange 5U is that the bolts are not likely to be lost. (It is also contemplated to leave the fasteners parked at least partially inserted in the lower vessel flange rather than being parked at least partially inserted in the upper vessel flange 5L as shown in FIG. 13). FIG. 14 diagrammatically shows the SMR in a state 1300 in which the upper vessel portion 1342 is lifted off the lower vessel portion 1344 via the crane lifting at the crane attachment points 1030 (labeled in FIG. 11; note that the drawings do not show the crane). The state 1300 of FIG. 14 corresponds to the operation 250 of FIG. 3. The upper vessel portion 1342 of the SMR of the illustrated refueling process is analogous to the upper vessel portion 4 of the SMR of FIG. 1, but does not include the RCPs 16; rather, the SMR of the illustrative refueling process includes wholly internal RCPs 167 which are part of the upper internals disposed in the lower vessel portion 1344 (corresponding to the lower vessel portion 3 of the SMR of FIG. 1 but sized to accommodate the RCPs 167). As seen in FIG. 14, the tops of the internal RCPs 167 are visible after lifting off the upper vessel portion 1342. Also visible is a riser cone 1445 that that provides the transition from the central riser 6 in the upper vessel portion (see FIG. 1) into the lower vessel portion. Instead of the illustrative riser cone, this transition could alternatively be cylindrical or otherwise shaped depending on the reactor design. It is to be appreciated that the internal steam generator (if present, e.g. steam generator 10 of the SMR of FIG. 1) is secured inside the upper vessel portion and is lifted off with the upper vessel portion 1342. Advantageously, this enables removal of the steam generator as a unit without disconnecting it from its mountings inside the pressure vessel. Similarly, in a reactor such as that of FIG. 1 in which the RCPs each comprise an external RCP motor 16 driving an impeller located in a RCP casing 17 disposed inside the pressure vessel, the RCPs 16, 17 are secured with the upper vessel portion and are optionally removed as a unit together with the upper vessel portion. (Alternatively, the RCPs 16, 17 or portions thereof, e.g. the motors 16, are removed prior to lifting off the upper vessel section). As further seen in FIG. 14, the mid-flange 5 remains in place atop the lower flange 5L of the lower vessel portion 1344. The weight of the mid-flange 5 together with the weight of the suspended upper internals ensures that this component is held firmly in place during liftoff of the upper vessel portion. The next operations perform removal of the mid-flange 5 and the upper internals that are suspended from the mid-flange 5. FIG. 15 diagrammatically shows the SMR in a state 1400. In FIG. 15 and subsequently, the upper vessel portion 1342 is not shown, it having been moved off to the upper vessel maintenance stand (not shown) in the upper vessel liftoff operation 250 of FIG. 3. The state shown in FIG. 15 corresponds to operation 310 of FIG. 4, that is, to removal of the riser cone 1445 from the upper internals. FIG. 16 diagrammatically shows the SMR in a state 1500 corresponding to the operation 320 of FIG. 4, in which external connections 1550 around the mid-flange are disconnected. FIG. 17 diagrammatically shows the SMR in a state 1600 corresponding to the operation 330 of FIG. 4 in which the incore instruments (not shown) are pulled through vessel penetrations 1655 of the mid-flange 5 and wrapped around the mid-flange 5 or otherwise secured with the mid-flange 5. FIG. 18 diagrammatically shows the SMR in a state 1700 corresponding to the operations 340, 350, 360 of FIG. 4. An upper internals refueling platform 1750 is delivered (if not already on-site) and positioned on the mid-flange 5 as per operation 340. A suitable tool 1752 is employed in operation 350 to disconnect the control rods from the control rod drive mechanisms (CRDMs). It is to be appreciated that as part of the reactor shutdown sequence 110 (FIG. 2), the control rods were fully inserted into the reactor core in order to extinguish the nuclear chain reaction. Operation 350 serves to disconnect the CRDMs 8 (see FIG. 1), and a crane (not shown) is hooked up to the upper internals in operation 360. This hookup suitably employs the mid-flange 5 as the anchor point for hooking up the crane (not shown). FIG. 19 diagrammatically shows the SMR in a state 1800 corresponding to operation 420 of FIG. 5, i.e. to the lifting out of the mid-flange 5 with the upper internals suspended therefrom. In the SMR of the illustrated refueling process, the suspended upper internals include the CRDMs 8 (including the internal CRDM motors 8m), the guide frames 9, and the internal RCPs 167 (only the bottoms of few of which are visible in FIG. 19). The upper internals 8, 9, 167 are removed as a unit, again advantageously avoiding breakdown of these intricate assemblies. In the alternative SMR of FIG. 1, the suspended upper internals include the CRDMs 8 (and motors 8m) and the guide frames 9, but do not include the RCPs. Rather, as seen in FIG. 1, in that embodiment the RCPs (each comprising a motor 16 and impeller in RCP casing 17) are mounted proximate to the pressurizer 14. Thus in this embodiment the upper internals 8, 9 are removed as a unit. FIG. 19 shows the mid-flange 5 with the upper internals 8, 9, 167 remaining suspended from the mid-flange 5, and the refueling bridge or platform 1750 mounted on the mid-flange 5, with this complete assembly lifted vertically out of the lower vessel section 1344. The assembly is suitably then moved laterally (possibly with vertical adjustments as needed) to an upper internals maintenance stand (not shown). With the upper internals removed, the lower vessel 1344 remains with a large opening encircled by the lower flange 5L through which refueling can be performed. Advantageously, the removal of the upper internals suspended from the mid-flange 5 is efficient, as the CRDMs and guide frames do not need to be individually dismounted and removed. The mid-flange 5 provides a suitable support. Additionally, in embodiments in which the electrical (and, if needed, hydraulic) lines penetrate the pressure vessel through the mid-flange 5, the cables external to the pressure vessel are removed (see FIGS. 10 and 16 and operations 215, 320 of FIGS. 3 and 4 respectively), but the electrical (and hydraulic) cabling internal to the pressure vessel and running from the mid-flange 5 to the CRDM motors 8m can remain in place, further reducing the number of operations needed to remove the upper internals. With reference to FIGS. 20-22, the refueling and restart operations are flowcharted. FIG. 20 illustrates a first portion 1900 of this process. Operation 1910 is the spent fuel removal operation. A refueling crane is brought in and removes the spent fuel, which is transported through a refueling canal to a fuel transport system and then transported to the spent fuel pool (ancillary components not shown). The spent fuel is preferably removed in operation 1910 before new fuel is brought in, and then in an operation 1920 new fuel assemblies are transported through the refueling canal, removed from the fuel transport system, and placed by the crane into the lower vessel 1344 to assemble the fresh nuclear reactor core inside the lower vessel 1344. In an operation 1930, mid-flange standoffs are installed onto the lower vessel flange. New o-rings are placed onto the surface of the lower vessel in an operation 1940. The upper internals and refueling platform are placed onto standoffs in sequence with the refueling canal being drained in an operation 1950. Existing o-rings are removed, the o-ring sealing surfaces are inspected, and the new O-rings are installed, in an operation 1960. Alternatively, the o-rings are installed on the lower vessel flange but not the mid-flange (alternative operation 1965 of FIG. 20). The upper internals are lifted and removed from standoffs in an operation 1970. The upper internals are lowered onto the lower vessel and control rods are reconnected to the control rod drive mechanism (CRDM) with a connection tool in an operation 1980. FIG. 21 illustrates the continuation 2000 of the process. The refueling platform and existing o-rings are removed, the sealing surfaces are inspected, and new o-rings installed in an operation 2010. In some embodiments, redundant o-rings are installed on the upper vessel flange in an operation 2015. The incore instruments are pushed back into the core and incore feedthrough seals are replaced in an operation 2020. The electrical connections into the mid-flange are reconnected in an operation 2030. If appropriate for the reactor design, connections into the mid-flange are also completed after the upper vessel is in place in an operation 2035. The riser cone is replaced on the upper internals in an operation 2040. The upper vessel crane is attached to the upper vessel and the upper vessel is transported and positioned on top of the lower vessel in an operation 2050. The hydraulic tensioning nut and reactor vessel studs are moved from their parked position and loosely threaded into the lower vessel flange and prepared for tensioning in an operation 2060. If the studs were removed completely (as per optional operation 245 of FIG. 3), then the reactor vessel studs and hydraulic tensioning nuts are brought back in a transport rack in alternative operation 2065. The studs may be threaded into the lower vessel flange and prepared for tensioning. FIG. 22 illustrates the continuation 2100 of the process. The hydraulic tensioning nuts are tensioned to their specification requirements in an operation 2110. The crane is disconnected from the upper vessel in an operation 2120. Electrical connections (i.e. instrumentation) are reconnected to the upper vessel in an operation 2130, or alternatively are reconnected into the mid-flange (alternative operation 2135). The hydraulics (i.e. steam lines and pressurizer relief) are reconnected to the upper vessel in an operation 2140. The reactor is leak tested in an operation 2150. The removable insulation is reinstalled on the reactor vessel in an operation 2160. The reactor startup sequence is initiated in an operation 2170. With reference to FIGS. 23-30, further states of the SMR of the illustrative refueling are shown during the refueling/restart process of FIGS. 20-22. FIG. 23 diagrammatically shows the SMR in a state 2200 corresponding to the operations 1910, 1920 of FIG. 20. FIG. 23 shows an illustrative fuel assembly 2262 being unloaded (for operation 1910) or loaded (for operation 1920). FIG. 24 diagrammatically shows the SMR in a state 2300 corresponding to the operation 1930 of FIG. 20 in which standoffs 2364 are installed onto the lower vessel flange 5L of the lower vessel 1344. FIG. 25 diagrammatically shows the SMR in a state 2400 corresponding to the operation 1940 of FIG. 20 in which a new o-ring 2466 is placed onto the surface of the lower vessel flange 5L of the lower vessel 1344. FIG. 26 diagrammatically shows the SMR in a state 2500 corresponding to the operation 1950 of FIG. 20 in which the mid-flange 5 (with the upper internals suspended from the mid-flange 5) is placed onto the standoffs 2364. FIG. 27 diagrammatically shows the SMR in a state 2600 corresponding to the operation 1960 of FIG. 20. The old o-rings 2610 are cut and removed, and new o-rings 2620 are installed. FIG. 28 diagrammatically shows the SMR in a state 2700 corresponding to the operation 1970 of FIG. 20 in which the mid-flange 5 (with the upper internals suspended therefrom) is lifted and the standoffs 2764 removed. FIG. 29 diagrammatically shows the SMR in a state 2800 corresponding to the operation 1980 of FIG. 20 in which the mid-flange 5 (with the upper internals suspended therefrom) is placed onto the lower vessel flange 5L of the lower vessel 1344, and the tool 1752 (cf. FIG. 18) is used to reconnect the CRDMs with the connecting rods. FIG. 30 diagrammatically shows the SMR in a state 2900 corresponding to the operation 2010 of FIG. 21 in which the refueling platform 1750 (see FIG. 29) is removed, existing o-rings are removed and new o-rings 2910 are installed on the upper surface of the mid-flange 5. The remaining SMR re-assembly operations of FIGS. 21 and 22 are not explicitly illustrated, but comprise a reversal of corresponding disassembly operations of FIGS. 2-4. For example, operation 2040 of FIG. 21 corresponds to FIG. 15; operation 2050 of FIG. 21 corresponds to FIG. 14; and so forth. The use of the mid-flange 5 advantageously provides a support element for the upper internals that optionally integrates the electrical (and, if needed, hydraulic) vessel penetrations. This enables the upper internals, including the electrical cabling from the vessel penetrations to the CRDM motors 8m (or other elements) to be kept in place as the mid-flange 5 and suspended upper internals are lifted out (e.g., as shown in FIG. 19). However, in alternative embodiments (not shown) it is contemplated to employ a support element other than the illustrated mid-flange 5 for supporting the upper internals in suspended fashion. For example, the support element can be a forged annular ring with outer diameter is equal to or smaller than the inner diameter of the lower vessel section so as to fit inside the lower vessel and sit on discrete ledges (or a continuous annular ledge) on the inside surface of the lower vessel section. Such a ledge or ledges can be formed integrally as part of the forged lower vessel section, or can be welded to the lower vessel section. The upper internals are then lifted out by anchoring the crane to the annular ring or other support element and lifting it out of the lower vessel section. In these alternative embodiments, the support element is not part of the pressure boundary and cannot include the vessel penetrations for accessing the upper internals, and so an additional operation of disconnecting cabling inside the pressure vessel is needed before the support element and suspended upper internals can be lifted out. The illustrative embodiments have been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the illustrative embodiments be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
	description	The current invention relates to control rod absorbers for nuclear power plants. More specifically, the current invention relates to a control rod absorber and associated stack support which may be used in a nuclear power plant for an extended time period and which minimally degrades from thermal creep. Pressurized water reactors (PWRs) use fuel assemblies which contain a nuclear fuel, such as enriched uranium dioxide, to produce a nuclear chain reaction. The nuclear chain reaction is moderated in several ways, including maintaining specific fuel enrichment levels, maintaining a specific geometry of the fissionable nuclear fuel assemblies, and placing absorber material through the nuclear reactor core. The purpose of control rod absorbers is to slow or capture neutrons as the neutrons traverse the nuclear reactor. The absorber material allows sections of the core which potentially have greater amounts of nuclear activity to be reduced to more moderate levels. Because the control rod absorbers perform this task, the overall performance of the nuclear reactor is more consistent and the reactor is able to run with a greater degree of operational safety. Over time, conventional control absorber rods start to degrade. This degradation leads to the sections of the core potentially becoming more radiologically active than other sections of the core. The degradation then leads to localized “hot spots” in the reactor core and may require nuclear power plant operators to limit overall reactor operation to maintain safe operating margins, thereby negatively impacting the economic operation of the facility. When the rods become severely degraded, the reactor cannot be safely operated, necessitating rod replacement. As these absorber assemblies which contain the rods are located in the reactor core itself, the absorber assembly replacement occurs during a reactor outage. The replacement of these absorber rods and assemblies must be performed very carefully as the materials themselves are highly radioactive after their residence in the reactor core. The removal of the assemblies from the core is performed remotely by use of a crane and the removal is therefore a difficult and expensive operation. Ultimate disposal of the highly radioactive components removed is also very expensive as such highly radioactive waste must be cooled with cooling water for a long period of time. The components may then be stored in a “dry” condition in a specially prepared cask with a gaseous cooled interior. All of these disposal costs increases the overall cost of operation of the facility. Control rod assemblies may be provided in several various arrangements. “Black” clusters may be used in a reactor, wherein such “black” clusters are highly absorbent to neutrons traversing the nuclear reactor core. Other clusters, commonly known as “gray” clusters, may also be positioned around the reactor. The “gray” clusters are less absorbent than the “black” clusters and are therefore used in core positions that do not need as much radiation attenuation as other sections of the core. “Black” clusters are generally constituted of rods containing materials which are highly absorbent to neutrons, such as silver-indium-cadmium alloys. Absorber rods and assemblies degrade through a variety of degradation mechanisms. Absorber rods made of silver-indium-cadmium alloys are subject to both creep and swelling under irradiation. Boron carbide alloy based absorber rods undergo a large amount of swelling under irradiation and as a result are not heavily used in portions of rods inserted into high activity core areas, such as the bottom of absorber rods. For this reason, silver-indium-cadmium alloys are primarily used in the bottom parts of control rods. For silver-indium-cadmium alloys, the chief degradation mechanism limiting operability is diametral expansion. Diametral expansion of rodlets is a result of irradiation-induced expansion, thermal creep and thermal expansion. The overall geometric shape of the absorber assembly causes degradation to occur most frequently at the lower tip of a rod because of a high fluence exposure and high stresses due to the force exerted from an internal absorber stack of the rod. This is a need to produce a rod with absorber material which will limit diametral expansion. This is also a need to produce an absorber rod which will have an increased service life in the nuclear core compared to conventional absorber rods and that will not have to be replaced as often as other absorber rod units. There is also a need to produce an absorber rod assembly which will limit the amount of nuclear waste generated for a nuclear reactor. It is therefore an objective of the present invention to produce an absorber rod which will limit diametral expansion of rodlets in an absorber assembly. It is also an objective of the present invention to produce an absorber rod which will be operable longer in the nuclear core and that will not have to be replaced as often as other absorber assembly units. It is also an objective of the present invention to produce an absorber rod which will limit the amount of nuclear waste generated for a nuclear reactor. The objectives of the present invention are achieved as shown and described. The invention provides an absorber rod for a nuclear reactor comprising a rod cladding defining in internal volume, the rod cladding having an upper end and a lower end, an upper end fitting positioned at the upper end of the rod cladding, a rod internal arrangement configured in the internal volume of the rod cladding, a lower end cap positioned at the lower end of the rod cladding, the lower end cap having an upper surface and a lower surface, a stack support with a stack support top end and a stack support bottom end, the stack support bottom end placed in contact with the upper surface of the lower end cap, the stack support top end configured to support the rod internal arrangement, and an annulus of material configured around the stack support and contacting the upper surface of the lower end cap. The annulus of material may be made of a silver-indium-cadmium alloy and the cladding may be configured in a tube shape. The diameter of the absorber rod may be approximately 2.16 centimeters. The stack support may be placed inside the annulus of material. The stack support rod may be configured to carry an entire weight of the rod internal arrangement and transport the entire weight to the lower end cap. The stack support top end may also be configured to have a larger outer perimeter than an outer perimeter of a body of the stack support. The present invention also provides a stack support for a nuclear reactor absorber rod. The stack support comprises a stack support top end and a stack support bottom end, the stack support bottom end placed in contact with an upper surface of a lower end cap, the stack support top end configured to support a rod internal arrangement. Referring to FIG. 1, a control absorber rod 10 is illustrated in cross-sectional view. The control absorber rod may be part of a larger control rod assembly placed inside a nuclear reactor core. The control absorber rod 10 has an outer absorber cladding 16 which defines an interior volume. The cladding 16 is illustrated in a tubular arrangement, however other arrangements are possible and the illustrated arrangement in FIG. 1 is but one possibility. The cladding 16 is made of INCONEL® (high strength austenitic nickel-chromium-iron alloys) in the current illustrated embodiment, however other non-corrosive materials may be used such that corrosion products are not introduced into the reactor coolant stream if a corrosive environment is encountered. The thickness of the cladding 16 may be either constant or varied along the body of the cladding 16 to provide more or less protection to the internal components placed within the volume. In the illustrated embodiment, the thickness of the cladding is constant. The cladding 16 has an absorber rod upper end 14 and an absorber rod lower end cap 18. As illustrated, the absorber rod lower end cap 18 is configured such that the cladding 16 smoothly interfaces with the absorber rod lower end cap 18 at a connection 38. The connection 38 locks the respective side wall of the cladding together with the absorber rod lower end cap 18 such that the two pieces are not separable. The absorber rod lower end cap 18 is configured to limit hydraulic drag on the rod, thereby reducing overall pressure drop for the absorber rod 10. To this end, the absorber rod lower end cap 18 may have a rounded face wherein the fluid passing from a lower elevation to an upper elevation smoothly envelops the absorber rod 10. Other absorber rod lower end caps 18 may be configured with a cone shaped end or may be flat faced as non-limiting examples. The overall length of the absorber rod lower end cap 18 may also be configured such that the absorber rod may be lengthened to a desired amount, further limiting drag, if necessary. The overall width of an absorber rod 10 can be any size according to the needs of the reactor in which the absorber rod/assembly is placed. In the illustrated embodiment of the present invention, the diameter of the absorber rod 10 is approximately 2.41 centimeters. The control absorber rod 10 is also provided with an upper end filling 60, the upper end filling 60 sealing the upper end of the rod cladding 14. The absorber rod cladding 16 defines an interior volume into which materials may be placed for either structural support or moderation of the nuclear reaction. To moderate the nuclear reaction, a rod internal arrangement 36 is provided to either capture or slow down neutrons as they traverse the nuclear reactor. The rod internal arrangement 36, as illustrated, is configured to fit within a top portion of the absorber rod cladding 16. The rod internal arrangement 36 is configured to be generally tubular in nature, however other configurations are possible. The rod internal arrangement 36, has an internal arrangement annulus 40 thereby providing a space between the rod internal arrangement 36 and the absorber rod cladding 16. The amount of space between the absorber rod cladding 16 and the rod internal arrangement 36 may be zero in the case of actual abutment between a part of the rod internal arrangement and the absorber rod cladding 16. The amount of space between the absorber rod cladding 16 and the rod internal arrangement 36 is be established such that if the rod internal arrangement 36 undergoes seismic acceleration in vertical, horizontal, or a combination of directions, contact between the rod internal arrangement 36 and the absorber rod cladding 16 is prevented or minimized as desired. The rod internal arrangement 36 is configured out of neutron absorbing material. The absorber rod internal arrangement 36 is supported by a stack support 28. The purpose of the stack support 28 is to transfer the weight and/or forces on the rod internal arrangement 36 to the absorber rod lower end cap 18. The stack support 28 may simply support the absorber rod internal arrangement 36 or it may be connected to the absorber rod internal arrangement 36 such that in the event of a seismic event, such as when an upwards or lateral force is encountered, the rod internal arrangement 36 is maintained in a relatively consistent orientation and not allowed to impact the absorber rod cladding 16. The stack support 28 may be configured with an upper surface 34 and a stack support lower end 30. The stack support upper surface 34 provides the resting surface for the rod internal arrangement 36. The stack support upper surface 34, as illustrated, is configured in a circular configuration to fully support the rod internal arrangement 36. The thickness 42 of the stack support upper surface 34 is chosen such that the cantilever parts 44 of the stack support 28 do not deflect an appreciable amount, while providing the support capabilities needed during operational and accident loading conditions. The stack support 28 also is configured with a stem 46 which connects the stack support upper surface 34 with the stack support lower end 30. The stack support stem 46 is configured to allow the force transfer from the stack support upper surface 34 to the stack support lower end 30. The stack support stem 46 is designed such that bending of the stem 46 does not affect the operability of the absorber rod 10. The stack support lower end 30 is also designed such that the forces which are exerted upon the absorber rod lower end cap 18 do not punch through the absorber rod lower end cap 18 during any loading conditions. Although illustrated as a simple post end for the stack support lower end 30, the lower end 30 can be a flared unit, or may be countersunk into the absorber rod lower end cap 18 to provide a positive connection. The stack support 28 may be manufactured from high strength steel to provide sufficient support capacity for the rod internal arrangement 36. The materials may be stainless steel or other non-corrosive material to limit the effects of corrosion inside the absorber rod 10 and to limit galvanic corrosion from occurring. A material annulus 22 may be positioned inside the absorber rod cladding 16 to allow for attenuation of the nuclear chain reaction at the lower end of the absorber rod 10. The material annulus 22 is positioned around the stem 46 and stack support lower end 30 to provide the absorber rod 10 with sufficient neutron absorber rod capacity. The material annulus 22 may be positioned in the interior volume of the absorber rod cladding 16 such that the material annulus bottom end 26 contacts the absorber rod lower end cap 18. The material annulus 22 is positioned underneath the cantilever portions 44 of the stack support 28 such that there is a gap 48 between the material annulus top end 24 and the bottom face of the stack support upper end 50. The size of the gap 48 between the bottom face 50 of the stack support upper end and the material annulus top end 24 may be minimized such that little to no gap 48 is present. The material annulus bottom end 26 may be countersunk into the absorber rod lower end cap 18. A locking arrangement may be established between the material annulus bottom end 26 and the absorber rod lower end cap 18 to eliminate the possibility for movement of the material annulus 22 inside the absorber rod cladding 16. The material annulus 22 may be additionally configured such that an annular space 20 is created around the material annulus 22. The annular space 20 may be varied such that a lesser or greater amount of space is provided between the interior wall 52 of the absorber rod cladding 16 and the exterior surface 54 of the material annulus 22. The present invention solves problems associated with life-limiting considerations for control rods as a result of diametral expansion of the silver-indium-cadmium absorber. The present invention reduces the effect of thermal creep at the lower tip of the absorber stack which in turn lengthens the operable life-time of control rods. The equation for strain rate due to thermal creep is as follows:ε′=Kdnσmexp(−Q/RT)where ε′ is the axial creep strain rate measured in s−1. The grain size of the material is provided in the variable (d), stress in σ and temperature T. K, n, m and Q are material specific parameters. R is the universal gas constant. In the case of silver-indium-cadmium material, the parameter m has a value of approximately 1, therefore the creep strain rate is nearly linearly proportional to the stress of the material. By reducing the stress level of the material, the creep strain rate is also reduced. The present invention provides a rod configuration which removes the forces exerted from plenum springs and internal stack weights from the lower portion of absorber material positioned inside an absorber rod. The forces exerted from the internals are transmitted through the stack support 28 to the absorber rod lower end cap 18. As a result, the load path bypasses the silver-indium-cadmium material annulus reducing the stress level of this component. In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a restrictive sense.
	summary
054147425	description	SUMMARY OF THE INVENTION Disclosed herein is a leak-detection system and method for detecting a leaking container. The system includes an enclosure defining a cavity therein surrounding the container, which container may be a nuclear fuel rod having fission products leaking through a breach in the exterior surface of the fuel rod. The fission products are capable of adhering to the exterior surface of the fuel rod as the fission products leak through the breach in the fuel rod. A radiation detector is in communication with the cavity for detecting the fission products leaking through the breach. Moreover, a gas injector is in communication with the cavity for injecting a multiplicity of carrier gas bubbles into the cavity to remove any fission products adhering to the exterior surface of the fuel rod and to carry the fission products removed thereby to the radiation detector. In this manner, the detector detects the leaking fission products even though they may tend to adhere to the exterior surface of the fuel rod. In addition, an elevator is connected to the fuel assembly for elevating the fuel assembly in the cavity, so that the external pressure acting against the exterior surface of the fuel rods is reduced in order to relieve the internal pressure in the fuel rods. As the fuel assembly is elevated, any fission product gases and/or solids tending to "hide-out" in the fuel rods expand and migrate through the breach. As the fission products migrate through the breach, they are carried by the carrier by the carrier gas bubbles to the radiation detector where they are detected by the radiation detector. In this manner, otherwise undetectable leaking fuel rods become detectable. The invention in its broad form is a leak-detection system for detecting a leaking container having a surface thereon and a material leaking therefrom, the material capable of adhering to the surface of the container as the material leaks from the container, comprising enclosure means surrounding the container for enclosing the container; detector means associated with said enclosure means for detecting the leaking material; and fluid injection means associated with said enclosure means for injecting a fluid into said enclosure means to remove the material adhering to the surface and to carry the material removed thereby to said detector means, so that said detector means detects the material leaking from the container. The invention in its broad form is also a leak-detection method for detecting a leaking container having a material leaking from an exterior surface thereof, the material capable of adhering to the exterior surface, comprising the steps of enclosing the container by surrounding the container with an enclosure; detecting the material leaking from the container by operating a detector associated with the enclosure; and removing the material adhering to the exterior surface and carrying the material to the detector by injecting a fluid into the enclosure, so that the detector detects the material leaking from the container. An object of the present invention is to provide a leak-detection system and method for detecting a leaking container, which leaking container may be a leaking nuclear fuel rod having a radioactive fission product material leaking therefrom. Another object of the present invention is to provide a leak-detection method that is less time consuming than prior art leak-detection techniques. Yet another object of the present invention is to provide a leak-detection system and method that detects a leaking fuel rod even though the fission product material leaking therefrom may adhere to the exterior surface of the fuel rod and even though the fission product material may tend to "hide-out" in the fuel rod. A feature of the present invention is the provision of a radiation detector for detecting the fission product material leaking from an internally pressurized fuel rod. Another feature of the present invention is the provision of a gas injector for injecting a gas into a liquid medium contained in an enclosure and surrounding the fuel assembly which includes the leaking fuel rod so as to form a multiplicity of gas bubbles in the liquid in order to remove the fission product material adhering to the exterior surface of the fuel rod and to carry the fission product material removed thereby to the detector for detecting the leaking fuel rod. Yet another feature of the present invention is the provision of pressure relief means connected to the fuel rod for relieving the internal pressure in the fuel rod, so that the fission product material leaks from the fuel rod and into the liquid medium as the internal pressure is relieved in order to prevent the fission product material from "hiding-out" in the fuel rod. An advantage of the present invention is that time required to detect leaking fuel rods is reduced. Another advantage of the present invention is that leaking fuel rods otherwise undetectable are now detectable. These and other objects, features and advantages of the present invention will become evident to those having ordinary skill in the art upon a reading of the following detailed description taken in conjunction with the accompanying drawings wherein there is shown illustrative embodiments of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there is shown a typical nuclear reactor, generally referred to as 10, for producing heat by the controlled fission of fissile nuclear fuel material. Reactor 10 includes a reactor pressure vessel 20 disposed in a reactor cavity 15 which is defined by a pressure vessel containment structure 17 surrounding pressure vessel 20. Containment structure 17 has an upper surface 18 thereon having a rail 19 attached thereto spanning reactor cavity 15, for reasons disclosed hereinbelow. Pressure vessel 20 is open at its top end and has a plurality of inlet nozzles 30 and outlet nozzles 40 attached to the upper portion thereof (only one of each nozzle is shown). A hemispherical closure head 50 is mounted atop pressure vessel 20 and is sealingly attached to the open top end of pressure vessel 20, such that closure head 50 sealingly caps pressure vessel 20. Capping pressure vessel 20 in this manner allows for suitable pressurization of a liquid moderator coolant (not shown) within pressure vessel 20. Moreover, disposed in pressure vessel 20 are a plurality of nuclear fuel assemblies 60, each fuel assembly 60 comprising a plurality of elongate nuclear fuel rods 70 having the fissile nuclear fuel material sealingly contained therein in the form of a plurality of coaxially stacked fuel pellets 80 (see FIG. 6). As shown in FIG. 1, each fuel rod 70 also has an exterior surface 75 and is prepressurized to a predetermined internal pressure of approximately 100 to 250 psia. In addition, pressure vessel 20 is submerged in a liquid medium 85 that serves as a biological shield for service personnel who may be located in the vicinity of pressure vessel 20. During operation of nuclear reactor 10, the liquid moderator coolant enters inlet nozzle 30, circulates through pressure vessel 20 and then exits pressure vessel 20 through exit nozzle 40, whereupon it is piped to a heat exchange device (not shown) for generating steam. The steam is then piped from the heat exchange device to a turbine-generator set (not shown) for producing electricity in a manner well understood in the art. Moreover, as the nuclear material forming fuel pellets 80 fissions, gaseous and/or solid radioactive fission products 90 (e.g., Xenon-135 and Kr-85) are produced within the fuel rod 70 (see FIG. 6). Such radioactive fission products 90 are normally sealingly contained within fuel rod 70 because fuel pellets 80 producing the fission products 90 are themselves sealingly contained within fuel rods 70. However, such fuel rods 70 may nonetheless occasionally leak and thus may radioactively contaminate reactor system components in fluid communication with the liquid moderator coolant. It is therefore desirable to detect such leaking fuel rods 70 in order to replace the leaking fuel rods 70, so that radioactive contamination of reactor system components is avoided. Therefore, referring to FIG. 2, 3, 4, 5, 6, 7 and 8, there is shown the subject matter of the present invention, which is a leak-detection system, generally referred to as 100, for detecting a leaking container, which leaking container may be the leaking nuclear fuel rod 70 having the radioactive fission product material 90 leaking through a breach 110 in exterior surface 75 thereof. The fission product material 90 is capable of adhering to exterior surface 75 as it leaks through breach 110. Leak-detection system 100 comprises enclosure means, such as an elongate generally cylindrical enclosure 120 defining a cavity 130 axially therethrough, the cavity surrounding a selected one of the fuel assemblies 60 for enclosing fuel assembly 60 therein . As more fully disclosed hereinbelow, enclosure 120 is caused to penetrate liquid medium 85, so that surface 87 of liquid medium 85 is at a higher elevation than the top of fuel assembly 60. Also, for reasons disclosed in detail hereinbelow, enclosure 120 is preferably stationary during the leak-detecting process. Enclosure 120 has an open lower end portion 140 for allowing liquid medium 85 to enter and substantially fill cavity 130 of enclosure 120 and also has a substantially closed or capped upper end portion 135. However, in the preferred embodiment, liquid medium 85 does not completely fill cavity 130. That is, liquid medium 85 fills cavity 130 so as to define a liquid-free volume 150 in upper end portion 135 of enclosure 120 (i.e., the top portion of cavity 130). Thus, it will be understood from the description hereinabove, that exterior surface 75 of each fuel rod 70 is covered by liquid medium 85 and that the liquid medium 85 defines a hydrostatic pressure gradient acting against exterior surface 75 of fuel rod 70, the hydrostatic pressure gradient increasing as a function of depth of liquid medium 85. Still referring to FIG. 2, 3, 4, 5, 6, 7 and 8, leak-detection system 100 further comprises detector means, such as a radiation detector assembly, generally referred to as 160, connected to enclosure 120 for detecting and measuring the leaking fission product material 90. Detector assembly 160 includes a suction pump 170 in communication, such as by a first conduit 180, with liquid-free volume 150 for suctioning fission product 90 from liquid-free volume 150, as described more fully hereinbelow. Detector assembly 160 also includes radiation-sensitive sensor 190 housed in a sensor chamber 195 that is in communication with suction pump 170, such as by the previously mentioned first conduit 180. Sensor 190 senses the radiation emitted by the radioactive fission product material 90 that is suctioned by suction pump 170. Sensor 190 is adapted to generate a sensor output signal in response to the radiation sensed thereby. An analyzer, generally referred to as 210, is electrically connected to sensor 190, such as by wiring 220, for receiving and then analyzing the sensor output signal. Analyzer 210 is adapted to generate an analyzer output signal associated with the analysis provided thereby. In the preferred embodiment of the invention, analyzer 210 may include a high voltage power supply 230 electrically connected, such as by wiring 220, to sensor 190 for activating sensor 190. Sensor 190 is also electrically connected to an analog ratemeter 240 that displays the average counts per unit time (e.g., per second) of radiation detected by sensor 190. By use of ratemeter 240 the quantity or intensity of radiation from fission product material 90 in sensor chamber 195 is detected as a function of time. A suitable amplifier 250 electrically interconnects sensor 190 and ratemeter 250. The purpose of amplifier 250 is to generate a sensor output signal useable by ratemeter 250. Electrically connected to analyzer 210 is a controller 260 for controlling the operation of analyzer 210. Controller 260 is capable of operating leak-detection system 100 in either a "manual" or "automatic" mode as described more fully hereinbelow. In addition, a display 270 is electrically connected to analyzer 210, such as by wiring 280, for receiving the analyzer output signal and for displaying the analyzer output signal received thereby. In this manner, the analysis provided by analyzer 210 is visually displayed to the operator of leak-detection system 100. As best seen in FIGS. 5 and 8, leak-detection system 100 also comprises recirculation means, generally referred to as 287, and connected to enclosure 120 and to fluid injection means, for selectively discharging the gas to atmosphere or recirculating the gas to injection means 287, as disclosed in detail hereinbelow. In this regard, recirculation means 285 includes first conduit 180 which interconnects liquid-free volume 150 and suction pump 170 with sensor chamber 195 such that suction pump 170 suctions fission products 90 from liquid-free volume 150, through first conduit 180 and into sensor chamber 195. A second conduit 290 extends from sensor chamber 195 to the atmosphere for venting fission products 90 to the atmosphere. Disposed in second conduit 290 is a first valve 295 which is capable of opening and closing. A third conduit 300 has an end thereof connected, such as at location 305, to second conduit 290 and has the other end thereof connected, such as at location 308, to a fourth conduit 310, the purpose of which is described in more detail hereinbelow. Disposed in third conduit 300 so as to be interposed between locations 305/308 is a second valve 320 which is capable of opening and closing. Fourth conduit 310 is connected, as at location 325, to a fifth conduit 330. Fifth conduit 330 interconnects, as at location 335, a pressurized gas reservoir 340 with first conduit 180. In addition, disposed in fourth conduit 310, so as to be interposed between locations 308/325, is a third valve 345 which is capable of opening and closing. Moreover, disposed in fifth conduit 330, so as to be interposed between locations 325/335, is a fourth valve 350 which is capable of opening and closing. Furthermore, surrounding sensor chamber 195 may be a lead shielding wall 360 for shielding sensor 190 from external background radiation. Shielding sensor 190 enhances the radiation sensitivity of radiation-sensitive sensor 190. Radiation detector assembly 160 may be what is commonly referred to in the art as a "flow-through" beta-gama scintillation detector of the type available from Bicron Industries, Incorporated located in Newbury, Ohio. Referring to FIGS. 5 and 7, leak-detection system 100 also comprises the previously mentioned fluid injection means 285, such as a gas injection manifold 370, connected to open lower end portion 140 of enclosure 120 for injecting a pressurized carrier fluid (e.g., air) into cavity 130 which is defined by enclosure 120. Gas injection manifold 370 may comprise a split ring member 380 surrounding open lower end portion 140. Injecting gas into cavity 130 assists in removing any fission product material 90 that may be adhering to exterior surface 75 of fuel rod 70 and also assists in carrying the fission material 90 removed thereby to liquid-free volume 150. In this regard, gas injection manifold 370 includes a plurality of injector nozzles 390 in fluid communication-with liquid medium 85 for injecting the gas into liquid medium 85. As the gas is thus injected into liquid medium 85, a multiplicity of voids or bubbles 400 are formed in liquid medium 85, which bubbles 400 upwardly travel or rise in liquid medium 85 to encounter fission product material 90 adhering to exterior surface 75 of fuel rod 70. As bubbles 400 encounter fission product material 90, they will dislodge, disassociate, scrub or remove fission product material 90 from exterior surface 75 and upwardly carry fission product material 90 to liquid-free volume 150, so that fission product material 90 can be suctioned from liquid-free volume 150. Turning now to FIGS. 3 and 5, leak-detection system 100 also comprises alignment means, such as a plurality of spaced-apart roller assemblies 410, for aligning fuel assembly 60 coaxially within cavity 130, as described more fully hereinbelow. Roller assemblies 410 are spaced along the length of enclosure 120 and are respectively radially distributed around enclosure 120 for performing their alignment function. In this regard, each roller assembly 410 comprises a roller 420 penetrating enclosure 120 through an aperture 425 cut through enclosure 120. Fuel assembly 60 is preferably coaxially aligned within cavity 130 for proper alignment and insertion of the fuel assembly 60 back into the reactor core during routine refueling. A sealing cup 430 attached to the exterior of enclosure 120 sealingly surrounds each aperture 425 so that all the fission product material 90 is carried to liquid-free volume 150 to be suctioned therefrom. In this manner, fission product material 90 will not undesirably leak through aperture 425 and into reactor cavity 15. Referring to FIGS. 2, 3, and 4, system 100 further comprises pressure relief means connected to fuel assembly 60 for reducing or relieving .the internal pressure in fuel rod 70, so that the fission product material 90 leaks from fuel rod 70 as the internal pressure is relieved. This feature of the invention is important in order to prevent fission product material 90 from "hiding-out" in fuel rod 70. In this regard, the pressure relief means comprises an elongate and axially movable elevator 440 capable of removably gripping fuel assembly 60 and then elevating fuel assembly 60 in cavity 130. Fuel assembly 60 is elevated so that the hydrostatic external pressure gradient acting against exterior surface 75 of the breached fuel rod 70 is reduced to relieve the internal pressure in the breached fuel rod 70. As the internal pressure is relieved, the radioactive fission product material 90 expands and migrates through breach 110. In this manner, fission product material 90 is prevented from "hiding-out" in the breached fuel rod 70. More specifically, elevator 440 has a capped top end portion 450 and includes a gripper 455 adapted to removably grip fuel assembly 60. Attached to top end portion 450 of elevator 440 is an adaptor 460 connected to a cable 470 that in turn engages a pulley 480. Pulley 480 is connected to a winch (not shown) for rotating pulley 480. When the winch is operated, pulley 480 rotates in a first direction for raising elevator 440, which in turn will elevate fuel assembly 60 in cavity 430. Similarly, the winch is capable of rotating pulley 480 in an opposite second direction for lowering elevator 440, which in turn will lower the elevation of fuel assembly 60 in cavity 430. Moreover, the previously mentioned rollers 420 are capable of slidably abutting and guiding elevator 440 for aligning fuel assembly 60 along the centerline of cavity 130. Referring to FIGS. 2 and 3, a manipulator bridge or support frame, generally referred to as 490, is connected to axially movable elevator 440 and to stationary enclosure 120 for supporting elevator 440 and enclosure 120. Moreover, detector assembly 160 and pulley 480 may be mounted on support frame 490, as shown, so that detector assembly 160 and pulley 480 are suitably supported thereby. In addition, support frame 490 includes a plurality of wheels 500 adapted to slidably engage rail 19, so that support frame 490 is movable on rail 19 in order to align support frame 490 with a preselected one of fuel assemblies 60 to be leak-tested. OPERATION Leak-detection system 100 is capable of detecting leaks of fission product material 90 from any one of fuel assemblies 60 during routine refueling operations of the reactor core without the need for transporting fuel assembly 60 to a remote test chamber. In this regard, pressure vessel 20 is removed from service and closure head 50 is disconnected from atop pressure vessel 20. The upper internal structure of pressure vessel 20 is then removed in order to expose fuel assemblies 60. Next, support frame 490 is moved on rail 19 by rotatably engaging wheels 500 with rail 19, so that support frame 490 is aligned with a preselected one of the fuel assemblies 60 to be leaktested. At this point, stationary enclosure 120 will be caused to penetrate the liquid medium 85 of reactor cavity 15. AS enclosure 120 penetrates reactor cavity 15, the liquid medium 85 therein will flow through open lower end portion 140 of enclosure 120 to substantially fill cavity 130 to a level that defines fluid-free volume 50. The winch (not shown) is then operated to rotate pulley 480 so that elevator 440 is lowered, via cable 470, to a position that is coaxially aligned with the preselected one of the fuel assemblies 60. Next, gripper 455 is caused to securely grip the preselected fuel assembly 60 and raise it into cavity 15. Leak-detection system 100 is then used to perform the leak-detection in accordance with the following modes of operation which may be performed sequentially: (a) "vacuum test" mode, (b) "flow test" mode, (c) "recirculation" mode, and (d) "purge" mode. Each of these modes of operation will be described in turn hereinbelow. First, with respect to the vacuum test mode, first valve 295 is opened and then second valve 320, third valve 345 and fourth valve 350 are closed as suction pump 170 is operated. As suction pump 170 operates, the air within liquid-free volume 150 will be suctioned through first conduit 180 and into sensor chamber 195 to be detected by sensor 190. After passing into sensor chamber 195, the air will next flow into second conduit 290 and through open valve 295 on its way to the atmosphere. It will be understood from the description hereinabove that bubbles 400 are not generated to transport fission products 90 to liquid-free volume 150; rather, the purpose for operating in the vacuum test mode is to confirm that suction pump 170 will draw the necessary sample. Secondly, with respect to the flow test mode, first valve 295 remains open as third valve 345 is opened. Second valve 320 and first valve 350 are closed as suction pump 170 is operated. The pressurized gas reservoir 340 is caused to supply the pressurized gas (e.g., air) into fifth conduit 330 and thence through fourth conduit 310 and its associated third valve 345. The gas then flows to gas injection manifold 370 for generating bubbles 400. As the bubbles 400 transport fission products 90 into liquid-free volume 150, suction pump 170 will suction fission products 90 through first conduit 180, into sensor chamber 195, through second conduit 290 and its associated first valve 295. The fission product material 90 will then flow to the atmosphere. Thirdly, with respect to the recirculation or closed-loop mode, fourth valve 350 remains closed as first valve 295 and third valve 345 are closed and as second valve 320 is opened. Suction pump 170 continues operating. It will be understood from the description hereinabove that bubbles 400 are again generated to transport fission products 90 to liquid-free volume 150. More specifically, the bubbles are now provided by the output of vacuum pump 170. The vacuum pump 170 discharges the gas to gas injection manifold 370 to generate bubbles 400. As bubbles 400 transport fission products 90 into liquid-free volume 150, suction pump 170 will suction fission products 90 through first conduit 180, into sensor chamber 195, into second conduit 290, into third conduit 300, through second valve 320 and then into fourth conduit 310 on its way back to gas injection manifold 370. It will be appreciated from the description hereinabove that in the recirculation mode no fission products 90 are released to the atmosphere; rather, the fission products 90 are recirculated through enclosure 120. An important advantage of operating leak-detection system 100 in the recirculation or closed-loop mode is that it is possible to detect relatively small amounts of fission product material 90 released by relatively small-sized breaches 110 in fuel rod 70 because the recirculation mode allows the small releases of fission product-material 90 to accumulate within the closed-loop for detection. Next, with respect to the purge mode, third valve 345 remains closed as second valve 320 is closed and as first valve 295 and fourth valve 350are opened. Suction pump 170 continues operating. The pressurized gas flows from gas reservoir 340, through fifth conduit 330 and then through open fourth valve 350. The gas then flows through suction pump 170 and into sensor chamber 195 via first conduit 180. The gas next flows from sensor chamber 195 through second conduit 290 and through open first valve 295 on its way to the atmosphere. The purpose of the purge mode is to purge or remove residual fission products 90 from enclosure 120, so that test results for the next fuel assembly to be leak-tested are not corrupted, influenced or contaminated by such residual fission products that would otherwise remain in leak-detection system 100. It will be appreciated from the description hereinabove that the purging mode is considered complete when sensor 190 detect relatively no fission products 90. It will be understood from the description hereinabove that as the gas flows through fourth conduit 310 to injection manifold 370, it will exit injection nozzles 390 which are in fluid communication with liquid medium 85 in cavity 130. The gas injected into cavity 130 upwardly travels or rises therein and forms the multiplicity of voids or bubbles 400. The bubbles 400 encounter any fission product material 90 that may be passing through breach 110 and adhering to exterior surface 75 of fuel rod 70. As bubbles 400 encounter such fission product material 90, they will dislodge, disassociate, scrub or remove the fission product material 90 from exterior surface 75 and carry the fission product material removed thereby to liquid-free volume 150. The bubbles 400 will dissipate as they enter liquid-free volume 150 and will expel the fission product material 90 and gas mixture into liquid-free volume 150, so that fission product material 90 is substantially suspended in liquid-free volume 150. As fission product material 90 enters liquid-free volume 150, it is suctioned through first conduit 180 and into sensor chamber 195 by means of suction pump 170. Once inside sensor chamber 195, sensor 190 will detect the presence of fission product material 90 therein. In this regard, high voltage supply 230 is caused to supply power to sensor 190 for activating sensor 190, so that sensor 190 detects the fission product material 90 in sensor chamber 195. As sensor 190 operates, it generates a sensor output signal. The output signal of sensor 190 is passed to analyzer 210 for analysis. Amplifier 250 which belongs to analyzer 210, electrically interconnects sensor 190 and analog ratemeter 240. Ratemeter 240 displays the average counts per unit time (e.g., per second) of radiation detected by sensor 190. In this manner, the quantity or intensity of radiation is detected as a function of time. Controller 260, which is electrically connected analyzer 210, controls the operation of analyzer 210. In addition, display 270, which is electrically connected to analyzer 210, receives the analyzer output signal and displays the analyzer output signal received thereby. As previously described, some of the fission product material 90 may tend to "hide-out" within fuel rod 70 rather than readily migrate through breach 110. Fission product "hide-out" is undesirable because such "hide-out" of fission product material 90 within fuel rod 70 may cause a breached fuel rod 70 to be otherwise undetectable or at least imprecisely detected and analyzed. Therefore, it is desirable to coax such fission products from fuel rod 70 prior to beginning the flow test mode of operation. In order to coax the fission product material 90 through breach 110 of fuel rod 70, the winch (not shown) rotates pulley 480 for elevating elevator 440 a predetermined amount (e.g., approximately 24 feet). As elevator 440 is elevated by the predetermined amount, the hydrostatic pressure will acting against the exterior surface 75 of each breached fuel rod 70 is preferably reduced by about 20 pounds per square inch. This reduction in external hydro-static pressure relieves the internal fuel rod pressure and will cause the fission product gases and/or solids "hiding-out" within fuel rod 70 to expand and migrate through breach 110 because the internal pressure of fuel rod 70 after elevation thereof is less than before elevation of fuel rod 70. However, as the fission product material 90 is coaxed through breach 110, it may tend to adhere to exterior surface 75 of fuel rod 70. The fission product material 90 tending to adhere to exterior surface 75 of fuel rod 70 will be removed therefrom by action of bubbles 400, as previously described, so that fission product material 90 is carried to liquid-free volume 150 to be suctioned therefrom in order to be precisely detected by sensor 190. Although the invention is fully illustrated and described herein, it is not intended that the invention as illustrated and described be limited to the details shown, because various modifications may be obtained with respect to the invention without departing from the spirit of the invention or the scope of equivalents thereof. For example, leak-detection system 100 need not be limited to detecting leaks of fission product material from nuclear fuel rods; rather, leak-detection system 100 including a detector suitable for the material being detected is usable for detecting leaks from any similar container having material leaking therefrom. Therefore, what is provided is a leak-detection system and method for detecting a leaking container, which leaking container may be a leaking nuclear fuel rod having a radioactive fission product material leaking therefrom.
042591538	description	In the text of the present disclosure, the term "can of the control and safety system" is to be understood as a housing, wherein a rod of the control and safety system is installed. When in the housing, the dimensions of the rod are equal to those of a fuel assembly. The device of this invention can be used to remove both empty cans and cans containing rods of the control and safety system; it can also be used to remove fuel assemblies. The proposed device 1 for the removal of fuel assemblies and cans of the control and safety system from the core 2 of a nuclear reactor, shown in FIG. 1, is installed in a cover 3 of the nuclear reactor, over the core 2 containing fuel assemblies 4 secured in sockets of a collector 5. The device 1 includes a hollow bar 6, wherein there is arranged a grip 7 intended to grip the fuel assembly 4 by its head to withdraw said fuel assembly 4 from the reactor core 2. The grip 7 is coupled to the hollow bar 6 by means of rollers 8 arranged in a housing 9 of the grip 7 at an angle of 120.degree. relative to each other and interacting with the internal surface of the hollow bar 6. As a result, the grip 7 is centered in relation to the axis of the hollow bar 6 and is movable along said axis of the hollow bar 6 over a distance which is not less than the length of the fuel assemblies 4. The internal diameter of the hollow bar 6 enables the bar 6 to envelop the fuel assembly 4 so that there is a clearance between the bar 6 and the fuel assembly 4 being removed; as a result, the fuel assembly 4 can freely move inside the hollow bar 6 over the entire length of said fuel assembly 4. On the core-facing end of the hollow bar 6 there is mounted an auxiliary grip 10. A magnified view of the auxiliary grip 10 is presented in FIG. 2. The auxiliary grip 10 is intended to grip the lower ends of the fuel assemblies 4 being removed (FIG. 1). The grip 10 (FIGS. 1 and 2) comprises six jaws 11 movable mounted on axles 12 which are rigidly secured in the hollow bar 6. Fitted over the hollow bar 6 is a sleeve member 13 which envelops the auxiliary grip 10 and is adapted for axial movement in relation to the hollow bar 6. The bar 6 is sufficiently long for the auxiliary grip 10 to reach the end of the fuel assembly 4 being removed, while the main grip 7 is on the head of said fuel assembly 4, which is at the opposite end of the fuel assembly 4. Opposite each of the jaws 11, there is an opening 14 (FIG. 2) in the sleeve member 13. Each opening 14 has bevelled sides 15 and 16 to interact with external surfaces 17 and 18, respectively, of the jaws 11, bevelled so as to correspond to the profile of the bevelled surfaces 15 and 16. In FIG. 2, the auxiliary grip 10 is opened; the bevelled external surfaces 17 of the jaws 11 interact in this case with the bevelled sides 15 of the openings 14. The hollow bar 6 (FIG. 1) is coupled to the sleeve member 13 by means of a nut 19 which makes it possible for the hollow bar 6 to axially move with respect to the sleeve member 13 over a distance necessary to close (open) the jaws 11 of the auxiliary grip 10. At the end of the sleeve member 13, on the side of the auxiliary grip 10, there is provided an external conical surface 20 (FIG. 2) intended for centering the main grip 7 relative to the axis of the fuel assembly 4 being removed, as the latter is gripped by its head. On the sleeve member 13 there is mounted a support 21 (FIG. 1) which is slidable along the axis of the sleeve member 13 and is secured on the cover 3 of the nuclear reactor. Mounted on the support 21 are hydraulic cylinders 22 which provide enough traction to extract the assembly 4 from the collector 5, as the auxiliary grip 10 grips the lower end of the fuel assembly 4. To withdraw the fuel assembly 4, rods 23 of the hydraulic cylinders 22 interact with stops 24 rigidly mounted on the sleeve member 13. In the portions of the hollow bar 6 and the sleeve member 13, extending above the cover 3 of the reactor, and in the support 21 there are installed sealings 25, 26, 27 and 28 to prevent the penetration of air into the nuclear reactor, as well as a discharge of contaminated gas from the nuclear reactor into the atmosphere. FIG. 3 shows the device of FIG. 1, the only difference being that the fuel assemblies, adjacent to the fuel assembly 4 being withdrawn, have been removed, and the hollow bar 6 with the auxiliary grip 10 and the sleeve member 13 is lowered to the stop into the collector 5; the auxiliary grip 10 is released. FIG. 4 shows the area A of FIG. 3; the bevelled external surfaces of the jaws 11 of the auxiliary grip 10 interact with the bevelled sides 16 of the openings 14 provided in the sleeve member 13. FIG. 5 shows an alternative embodiment of the proposed device for the removal of fuel assemblies and cans of the control and safety system from the core of a nuclear reactor. The device of FIG. 5 is used to remove jammed adjacent fuel assemblies. FIG. 5 only shows part of this device, located just above the core 2 of the nuclear reactor. According to this second embodiment, the hollow bar 6 and the sleeve member 13 are provided with a longitudinally extending slot 30 to receive the fuel assembly 4 which is adjacent to the one being removed. As a fuel assembly is received in the slot 30, there is a clearance between said fuel assembly and the walls of the slot 30. The slot 30 is somewhat longer than the fuel assembly 4; in all other respects, the device of FIG. 5 is similar to that of FIG. 1. FIG. 6 is a sectional view taken on line VI--VI of FIG. 5 and shows the arrangement of the slot 30 with respect to the jammed fuel assembly 4; all the adjacent fuel assemblies have been removed. The width of the slot 30 is determined by the size of the fuel assembly 4. The hollow bar 6 and the sleeve member 13 may be provided with several slots 30 arranged at an angle to one another. The number, arrangement, shape and dimensions of the slots 30 depend on the arrangement, shape and dimensions of the fuel assemblies 4 to be withdrawn from the reactor core 2. The device provided with the slots 30 can also be used to extract single jammed fuel assemblies 4. The proposed device 1 (FIG. 1) for the removal of fuel assemblies and cans of the control and safety system from the core 2 of a nuclear reactor operates as follows. In order to remove unjammed fuel assemblies 4 from the reactor core 2, the device 1 is oriented in the known manner with respect to the coordinates of the fuel assembly 4 to be removed. After this, the sleeve member 13, the hollow bar 6 and the opened auxiliary grip 10 are lowered with the aid of a drive 29 until they abut against the upper end faces of the fuel assemblies 4 adjacent to the one that has to be removed. The sleeve member slides under gravity with respect to the support 21; at the end of its travel it interacts with its external conical surface 20 with the heads of the fuel assemblies 4 adjacent to the one that is to be extracted and thus accurately centers the main grip 7 relative to the axis of the fuel assembly 4 to be removed. It also forces the adjacent fuel assemblies 4 aside from the one that is to be removed; the width of the clearance corresponds to the overall design of the fuel assemblies 4. The main grip 7 is then lowered with the aid of the same drive 29 onto the head of the fuel assembly 4 to be removed, grips the head and draws it. As this takes place, the force directed into the hollow bar 6 is limited so as not to destroy the fuel assembly 4 being removed. The drive 29 then raises the sleeve member 13, the hollow bar 6, the opened auxiliary grip 10 and the fuel assembly 4 being removed, gripped by the main grip 7, somewhat above the heads of the fuel assemblies 4 in the reactor core 2, and the device 1 is oriented with respect to the coordinates of the socket, wherein the fuel assembly 4 being removed is to be inserted. The removed fuel assembly 4 is installed in its socket in a similar manner, although the sequence of operations in this case is reversed. As fuel assemblies are being removed from the core of a nuclear reactor, there may be a situation when a fuel assembly is jammed in the collector so that it cannot be taken out by using the traction force developed by the main grip 7. In such a case, all the fuel assemblies 4 that are adjacent to the jammed one are first removed from the core 2 (FIGS. 1 and 3) of the nuclear reactor, which is done as described above. After this, the device 1 is oriented in the known manner with respect to the coordinates of the jammed fuel assembly 1. The sleeve member 13, the hollow bar 6 and the opened auxiliary grip 10 move down by gravity and with the aid of the drive 29. As this takes place, the sleeve member freely slides in the support 21, whereas the fuel assembly 4 to be removed enters the hollow bar 6. As the end face of the sleeve member 13 abuts against the collector 5, the auxiliary grip 10 grips the fuel assembly 4 by its lower end, which is done by rotating the nut 19. The rotation of the nut 19 causes downward movement of the hollow bar 6 with respect to the sleeve member 13. As this takes place, the bevelled surfaces 18 of the jaws 11 (FIG. 4) of the auxiliary grip 10 slide along the bevelled sides 16 of the openings 14 provided in the sleeve member 13. Upon reaching the end face of the collector 5, the grip 10 closes and firmly grips the lower end of the fuel assembly 4 (FIG. 3). The main grip 7 is then lowered onto the head of the fuel assembly 4 being removed and grips it by the head. In order to provide the necessary traction force to withdraw the fuel assembly 4 from its socket in the collector 5, working fluid is directed under pressure into the hydraulic cylinders 22. As a result, the rods 23 of the hydraulic cylinders 22 interact with the stop 24 rigidly mounted on the sleeve member 13 and produce axial pressure on the sleeve member 13, which is necessary to withdraw the fuel assembly 4 from its socket in the collector 5. As the tail of the fuel assembly 4 leaves the collector 5, the drive 29 raises the sleeve member 13, the hollow bar 6 and the fuel assembly 4, gripped by the closed auxiliary grip 10 and the main grip 7, somewhat above the heads of the fuel assemblies 4 in the reactor core 2, and the device 1 is oriented with respect to the socket, into which the removed fuel assembly 4 is to be inserted. By rotating the nut 19, the auxiliary grip 10 is released, and the fuel assembly 4 is installed by the main grip 7 into its socket in the manner described above. The fuel assembly can also be placed in its socket by the auxiliary grip 10; in this case the main grip 7 first disengages from the head of the fuel assembly 4. The adjacent jammed fuel assemblies 4 (FIGS. 5 and 6) are removed from the reactor core 2 as the single jammed fuel assembly 4, although in this case prior to lowering the sleeve member 13 with the auxiliary grip 10 and the hollow bar 6, the slot 30 is oriented with due regard for the position of the adjacent jammed fuel assemblies 4. All the other operations involved in the removal of jammed fuel assemblies 4 are carried out as described above. The device 1 (FIGS. 1 and 3) is capable of the removal of empty cans of the control and safety system, as well as cans containing rods of the control and safety system. In the case of empty cans, all the operations are carried out by the auxiliary grip 10.
	summary
	summary
	claims	1. A process for reducing radioactive contamination of a surface of a component which is used in a nuclear reactor and is in contact with radioactively contaminated water, comprising: producing a hydrophobic film on the surface of the component by wetting the surface with an aqueous solution containing a film-forming amphiphilic substance that comprises a primary, secondary, tertiary or quaternary amino group as a polar group, wherein the hydrophobic film prevents at least some radioactive particles from adhering to the surface of the component. 2. The process as claimed in claim 1, characterized in that the hydrophobic film is produced on an interior surface of the component, the component comprising a water-conducting circuit of the nuclear reactor. 3. The process as claimed in claim 2, characterized in that the hydrophobic film is produced after a part-circuit or full-circuit decontamination of the water-conducting circuit. 4. The process as claimed in claim 2, further comprising replacing the component with a new component, and forming a new hydrophobic film on the new component. 5. The process as claimed in claim 2, further comprising introducing a chemical compound or element into the aqueous solution that forms at least one layer on the surface before production of the hydrophobic film. 6. The process as claimed in claim 5, characterized in that the chemical compound or element is a noble metal. 7. The process as claimed in claim 5, characterized in that the chemical compound or element is a salt of chromic acid. 8. The process as claimed in claim 1, characterized in that the hydrophobic film is produced at a point in time outside load operation. 9. The process as claimed in claim 8, characterized in that the hydrophobic film is produced during a start-up phase of the nuclear reactor. 10. The process as claimed in claim 1, wherein the component is a tool used for inspecting the nuclear reactor. 11. The process as claimed in claim 10, characterized in that the tool is dipped into an aqueous solution of the amphiphilic substance. 12. The process as claimed in claim 10, characterized in that the surface of the tool is sprayed with an aqueous solution of the amphiphilic substance. 13. The process as claimed in claim 1, characterized in that an application of the amphiphilic substance to the surface is stopped when the surface has been covered with the hydrophobic film. 14. The process as claimed in claim 1, characterized by use of an amphiphilic substance which has a polar end formed by at least one polar group and a nonpolar end formed by at least one hydrocarbon radical. 15. The process as claimed in claim 1, characterized in that the contaminated water contains radioactive colloids and a film is produced by means of an amphiphilic substance which has a polar group having a charge having a same sign as the charge on the radioactive colloids. 16. The process as claimed in claim 1, characterized in that an amphiphilic substance which contains an aliphatic group as nonpolar radical is used. 17. The process as claimed in claim 1, characterized by a nonpolar radical having from 8 to 22 carbon atoms. 18. The process as claimed in claim 1, wherein producing the hydrophobic film on the surface of the component is performed before the surface comes into contact with radioactively contaminated water. 19. The process as claimed claim 14, characterized in that the aqueous solution is influenced in such a way that a charge which is opposite to the charge of the polar group of the amphiphilic substance arises on the surface or that a surface charge opposite to that of the polar group of the amphiphilic substance is increased. 20. The process as claimed in claim 19, characterized in that an influence on the aqueous solution is exerted via a pH of the aqueous solution. 21. A process for reducing radioactive contamination of a surface of a component which is used in a nuclear reactor and is in contact with radioactively contaminated water, comprising: producing a hydrophobic film on the surface of the component, before the surface comes into contact with radioactively contaminated water, by wetting the surface with an aqueous solution containing a film-forming amphiphilic substance, wherein the hydrophobic film prevents at least some radioactive particles from adhering to the surface of the component.
058870458	description	Other characteristics will become more clear from the description of particular embodiments. The following compositions found of interest: ______________________________________ COMPOSITIONS I II III ______________________________________ Tin 1.3 1.3 1.3 Iron 0.6 0.6 0.6 Vanadium 0.3 0.25 0 Chromium 0 0 0.25 Oxygen 0.12 0.14 0.14 Carbon 140 140 140 Silicon 90 90 90 ______________________________________ the other components being zirconium and impurities. The starting alloy was in the form of an ingot. It was formed into a bar by forging or rolling and, after heating to the .beta. phase, was water quenched at a controlled rate to bring it into the .alpha. region, for example at a cooling rate in the range 5.degree. C. per second to 30.degree. C. per second until the temperature was less than about 800.degree. C. After quenching, annealing was effected at a temperature of less than 800.degree. C. to prevent transformation of the .alpha. phase into the .beta. phase. Extrusion was carried out after machining a tubular billet and heating to between 600.degree. C. and 700.degree. C. The drawn blank, after undergoing any required annealing at a temperature of less than 800.degree. C., then underwent the required number of successive cold rolling steps to bring it to the required thickness, with intermediate annealing steps carried out in argon, each for one to three hours, to produce a suitable .SIGMA.A. In practice, four or five rolling steps were generally carried out to produce solid cladding tubes of conventional diameter and thickness. Finally, a final annealing step was carried out in an inert atmosphere, at about 485.degree. C. for one to three hours if a stress-relieved structure was required, or at about 580.degree. C. for about two hours if a recrystallized structure was required. The tests were carried out on samples to compare the alloys of the invention containing different tin contents with Zircaloy-4 type alloys. Generalized Corrosion Tests were carried out on recrystallized samples in an autoclave, in water and steam. The results are shown in Table I below. TABLE I ______________________________________ Weight gain .DELTA.P (mg/dm.sup.2) Water Steam ALLOY 350.degree. C. - 210 days 400.degree. C. - 30 days ______________________________________ 1 Zr 0.6 Fe; 0.3 V 29.2 26.4-38.5 2 Zr 0.6 Fe; 0.3 V; 0.5 Sn 31 27.5 3 Zr 0.6 Fe; 0.3 V; 1.0 Sn 32.2 30.4 4 Zr 0.6 Fe; 0.3 V; 1.5 Sn 32 30.9 (invention) 5 Zircaloy 4 43.9-47.2 32 ______________________________________ The results obtained, in particular for alloy 4 which was in accordance with the invention, show that an increase in the tin content from 0 to 1.5% had no effect on generalized corrosion resistance in water and steam. Corrosion in a Lithium Medium and Creep Resistance The influence of tin content on the corrosion resistance of Zircaloy 4 type alloys in a medium containing lithium hydroxide was studied in water containing 70 ppm of lithium at 360.degree. C. The results are shown in Table II. TABLE 2 ______________________________________ % Sn in Weight gain .DELTA.P (mg/dm.sup.2) alloy 50 days 100 days 150 days ______________________________________ 1.5 48 78 112 1.3 51 85 148 0.5 35 72 740 ______________________________________ The highly favorable influence of a high tin content (between 1.2% and 1.5%) on the corrosion resistance in a lithium hydroxide medium was observed in alloys in accordance with the invention. A high tin content was also shown to be favorable to the creep resistance of this alloy. Measurements of diametral creep .epsilon..sub.D at 400.degree. C. over 240 hours at a pressure of 130 MPa gave the following values for a stress-relieved alloy: ______________________________________ Sn content (%) .epsilon..sub.D (%) ______________________________________ 1.5 (invention) 1.5 1.3 (invention) 2 0.5 4.2 ______________________________________ The results obtained show a quasi-linear relationship between the Sn content and the creep characteristics.
	description	Referring now to the drawings, particularly to FIG. 1, there is illustrated a spacer for use in a fuel bundle or assembly in a nuclear reactor, the spacer being generally designated 10. The spacer includes a plurality of cylindrical ferrules 12 having stops 13 projecting from interior wall surfaces thereof and springs 16 for bearing against fuel rods 18 extending through the spacer in each of the ferrules 12. The springs 16 impart a lateral force to the fuel rods to maintain the fuel rods in bearing engagement against the stops 13. The spacer 10 also has a surrounding band 20 laterally encompassing the ferrules. As illustrated, a pair of water rods 22 pass through enlarged openings in the spacer 10. A clip 24 is secured to the ferrules, for example, by welding, and has a vertical extent corresponding to the vertical extent of the ferrules. One of the water rods 22 carries upper and lower tabs 26 which engage upper and lower margins, respectively, of the clip 24 to prevent relative axial displacement of the spacer and one water rod. As previously stated, in unchanneled bundles, the twist of the water rods and spacers may be sufficient to misalign the tabs 26 with the clip, enabling the spacer and water rod for relative axial displacement. Referring to FIGS. 2A-2C, there is illustrated in a preferred form of the present invention, a spacer/water rod connecting structure or retention assembly, generally designated 30, for use in a spacer of the type illustrated in FIG. 1 but having a different clip and water rod retention tab arrangement, as will now be explained. As best illustrated in FIG. 2C, the clip 32 is generally U-shaped with laterally outwardly directed flanges 34 to facilitate welding to adjoining ferrules 12 (FIG. 1). The base of the U-shaped clip 32 defines a slot or opening 36 bounded by axially opposite structural portions 38 and 40, as well as lateral or side structural portions 42 and 44 (FIG. 2B). Thus, the portions 38, 40, 42 and 44 perimetrically enclose the slot 36. Along the side portion 42 of clip 32, there is provided a clip tab 45 which projects toward the opposite side portion 44 of slot 36. The metal material from which the clip 32 is made enables the clip tab 45 to be resiliently deflected. A water rod tab 46 projects generally radially outwardly from the water rod and is sized for reception in the slot 36. To secure the spacer and water rod relative to one another, the water rod is disposed through the opening in the spacer with the tab 46 outside of the opening or slot 36 as illustrated in FIG. 2B. By rotating the spacer and water rod relative to one another, for example, by rotating the water rod in a counterclockwise direction as illustrated in FIG. 2B, the water rod tab 46 engages the tab 45 of the clip 32, resiliently deflecting tab 45 such that the water rod tab 46 obtains the position illustrated in FIG. 3B within slot 36. The clip tab 45, once the water rod tab 46 passes by out of engagement with tab 45 and into slot 36, resiliently deforms back to its initial position, generally within the slot 36. As a consequence, the side portion 44 of clip 32 and tab 45 lie on opposite sides of the water rod tab 46, preventing relative rotation between the spacer and water rod. Even with a twist of an unchanneled fuel bundle assembly, the water rod tab 46 remains captured in the slot between the side portion 44 and clip tab 45. To disassemble the water rod and spacer, a tool, not shown, may be used to deflect the spring in the same direction as in the initial assembly whereby the tab 46 can be rotated out of the slot past the tab 45. Referring now to the embodiment of the present invention illustrated in FIGS. 4A-4B and 5A-5C, wherein like reference numerals as in the prior embodiment apply to like parts followed by suffix xe2x80x9ca,xe2x80x9d the water rod 22a includes a tab 46a. The clip 32a includes a slot 36a which is perimetrically bounded by axially opposite end portions 38a and 40a, as well as bounded on its sides by side portions 50 and 52. The clip 32a is secured to the spacer similarly as in the prior embodiment. In this form, however, the tab 45 projecting from one side portion of the clip is omitted. To secure the spacer and one water rod against axial displacement relative to one another, the tab 46a is rotated into the slot 36a by relative rotation of the spacer and water rod. The tab 46a is thus prevented from rotational movement by one side portion 50 of the slot 36a. To prevent rotational movement of the tab in a direction displacing the tab from the slot, a second water rod 23 is disposed through an opening in the spacer adjacent the first water rod. The second water rod prevents rotation of the tab out of the slot 36a. The water rod does not rely on a square lower end plug and tie plate hole to prevent water rod rotation as in certain current designs. Thus, the present spacer/water rod retention assembly not only prevents relative rotation of the water rod and spacer to an extent permitting axial displacement of the water rod and spacer, particularly when installed in unchanneled fuel bundle assemblies, but also permits the less costly fabrication of a round water rod end plug and round openings in the lower tie plate of the fuel assembly. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
	claims	1. A CT system comprising:a rotatable gantry having an opening for receiving an object to be scanned;an x-ray tube having a focal spot from which x-rays emit;a detector comprising an imaging area of pixels and a calibration area of pixels;a pre-patient collimator positioned between the x-ray tube and the detector having first and second apertures that pass x-rays respectively to at least a portion of the imaging area of pixels, and to the calibration area of pixels; anda computer programmed to:determine a focal spot location in a Z-direction using energy derived from x-rays that fall upon the calibration area of pixels; andissue commands to adjust a Z-position of the focal spot based on the determined position of the focal spot from the x-rays that fall upon the calibration area of pixels, by adjusting a grid voltage within a cathode of the x-ray tube;wherein the pre-patient collimator includes a plurality of apertures that are not contiguous with one another, each of which includes both imaging apertures and calibration apertures that are contiguous with one another, wherein the calibration apertures in each of the plurality of apertures have different widths from one another along the Z-direction and at least one of the calibration apertures is narrower than its corresponding imaging aperture, resulting in a different amount of pixels used for reference channels. 2. The CT system of claim 1, wherein the imaging area of pixels and the calibration area of pixels are contiguous with one another. 3. The CT system of claim 1, wherein the imaging area of pixels and the calibration area of pixels have different widths in the Z-direction of the CT scanning system. 4. The CT system of claim 1, wherein the calibration area of pixels includes:a first area of pixels for providing reference signals; anda second area of pixels for tracking focal spot motion such that the computer determines a location of the focal spot. 5. The CT system of claim 4, wherein:the first area of pixels is one or more rows of central pixels as defined along the Z-direction; andthe second area of pixels are outermost rows of pixels on either side of the first area of pixels as defined along the Z-direction. 6. The CT system of claim 5, wherein the computer is programmed to determine the Z-position of the location of the focal spot using the energy derived from the x-rays that fall upon the second area of pixels. 7. The CT system of claim 1, wherein the computer is programmed to issue commands to a motor to adjust a position of the pre-patient collimator in the Z-direction, based on the determined position of the focal spot. 8. The CT system of claim 1, wherein the imaging apertures have widths in the Z-direction that are different from one another such that the x-rays that pass therethrough fall upon different sized portions of the imaging area of pixels. 9. A method of CT imaging, comprising:passing x-rays through an opening in a pre-patient collimator, through an object, and to at least a portion of a detector, the detector including an imaging area of pixels and a calibration area of pixels;determining a focal spot location using energy derived from x-rays that fall upon the calibration area of pixels; andadjusting the focal spot location in a Z-direction by adjusting a grid voltage on a cathode, based on the determination of the focal spot location using the energy derived from x-rays that fall upon the calibration area of pixels;wherein the pre-patient collimator includes a plurality of more than two apertures that are not contiguous with one another, each of which includes both imaging apertures and calibration apertures that are contiguous with one another, wherein the calibration apertures in each of the plurality of apertures have different widths from one another along the Z-direction and at least one of the calibration apertures is narrower than its corresponding imaging aperture, resulting in a different amount of pixels used for reference channels. 10. The method of claim 9, wherein passing the x-rays through the opening includes passing the x-rays to the imaging area of pixels and the calibration area of pixels that are contiguous with one another. 11. The method of claim 9, wherein the imaging area of pixels and the calibration area of pixels have different widths in the Z-direction of a CT scanning system. 12. The method of claim 9, wherein the calibration area of pixels includes:a first area of pixels for providing reference signals; anda second area of pixels for tracking focal spot motion such that a computer determines the focal spot location. 13. The method of claim 12, wherein:the first area of pixels is one or more rows of central pixels as defined along the Z-direction; andthe second area of pixels are outermost rows of pixels on either side of the first area of pixels as defined along the Z-direction. 14. The method of claim 13, further comprising determining a Z-position of the focal spot location using the energy derived from the x-rays that fall upon the second area of pixels. 15. The method of claim 9, further comprising adjusting a position of the pre-patient collimator based on the determined focal spot location. 16. The method of claim 9, wherein the imaging apertures have widths in the Z-direction that are different from one another such that the x-rays that pass therethrough fall upon different sized portions of the imaging area of pixels.
	summary
	description	This application is a U.S. national phase under the provisions of 35 U.S.C. §371 of International Patent Application No. PCT/EP12/51435 filed Jan. 30, 2012, which in turn claims priority of French Patent Application No. 11 50871 filed Feb. 3, 2011. The disclosures of such international patent application and French priority patent application are hereby incorporated herein by reference in their respective entireties, for all purposes. The present invention relates to a process for separating at least one platinoid element from an acidic aqueous solution comprising, besides this platinoid element, one or more other chemical elements. Hereinabove and hereafter, it is pointed out that “platinoid element” is taken to mean an element that may be chosen from platinum, palladium, rhodium, ruthenium, iridium, osmium. The invention is likely to find application in the field of the treatment and the recycling of irradiated nuclear fuels where it has a quite particular interest for recovering in a selective manner a platinoid element from aqueous solutions of high activity such as, for example, raffinates from the treatment of irradiated nuclear fuels. The processes that make it possible to extract and to purify uranium and plutonium present in the dissolution liquors of irradiated nuclear fuels generate effluents, which are given the name of raffinates. Raffinates are aqueous solutions with high nitric acidity, typically from 2 to 5 M, which contain two minor actinides, namely americium and curium, lanthanides such as lanthanum, cerium, praseodymium, neodymium, samarium and europium, fission products other than lanthanides such as molybdenum, zirconium, rubidium, ruthenium, rhodium, palladium and yttrium, as well as corrosion products such as iron and chromium. The management thereof presently consists in concentrating them as much as possible then conditioning them in vitreous matrices with a view to storage before ultimate disposal. Nevertheless, the presence of platinoid elements (such as palladium, rhodium and ruthenium) generates, at the level of vitrification, among others, the following difficulties: they exhibit limited solubility in the containment glasses; they tend to form particles that precipitate in the vitrification crucibles and, as a result, considerably perturb the functioning of the vitrification processes. It is to overcome these difficulties and also with a view to recycling the platinoid elements (with regard, in particular, to the predictable rarefaction of the natural resources of these elements) that certain authors have put in place processes for recovering these elements from a solution containing them in addition also to other radioactive elements. Different separation techniques have been implemented, among which may be cited: electrochemical reduction techniques in nitric acid medium, so as to deposit on an electrode the platinoid elements, which it is wished to separate, as described in US 2003/0099322, with, nevertheless, as drawback, a relatively complex implementation; liquid-liquid extraction techniques involving the use of extraction agents, such as tricaprylmethylammonium nitrate (as described in U.S. Pat. No. 4,162,231) or dialkyl sulphide (as described in U.S. Pat. No. 5,503,812), these techniques nevertheless have the drawback of generating an important amount of secondary effluents; precipitation techniques by chemical reduction of platinoid elements in nitric acid medium involving the use of a chemical reducing agent, such as saccharose as in U.S. Pat. No. 4,290,967, this embodiment bringing about problems of denitration reaction, which reaction has a highly exothermic character which could be the source of self-ignition of the nitric acid medium. With a view to overcoming the drawbacks inherent in the techniques implemented in the prior art, the authors of the present invention have proposed developing a novel method for recovering one or more platinoid elements contained in an acidic aqueous solution, for example, an aqueous nitric solution, comprising other chemical elements, such as radioactive elements. They have thus discovered, in a surprising manner, that by using certain organic alcohols, it is possible to extract selectively the platinoid elements contained in an acidic aqueous solution, by a simple and cheap implementation, without this generating large amounts of secondary effluents and without this resulting in the drawbacks linked to important denitration reactions, when the acidic aqueous solution is an aqueous nitric solution. The invention thus relates to a method for recovering at least one platinoid element contained in an acidic aqueous solution comprising chemical elements other than said platinoid element, said process comprising the following steps: a step of bringing said acidic aqueous solution into contact with a reducing amount of a reducing agent which is a non-sulphurous and non-glucidic alcoholic compound chosen from cyclic, optionally aromatic, alcohols and aliphatic polyols, by means of which said platinoid element is reduced to its 0 oxidation state; a step of separating said thus reduced platinoid element from said acidic aqueous solution. Before going into more detail in the description of the invention, the following definitions will be defined. Hereinabove and hereafter, platinoid element is taken to mean a metal element existing in an oxidation state different to 0, which metal element is chosen from platinum, palladium, rhodium, ruthenium, iridium, osmium. Thanks to the choice of specific alcohols used in specific amounts (namely, amounts enabling reduction to its 0 oxidation state of the platinoid element(s) present in the acidic aqueous solution), it is possible to obtain a selective reduction of the platinoid element(s) compared to other chemical elements present in the acidic aqueous solution, without there being pollution from said solution by sulphur (due to the fact that the alcohols are non-sulphurous), which would be totally unacceptable for the vitrification of the solution thereby obtained after extraction of said platinoid elements. As mentioned above, the non-sulphurous and non-glucidic alcoholics that can be used for the process of the invention may be cyclic, optionally aromatic, alcohols i.e. in other words: cyclic, optionally aromatic, hydrocarbon compounds comprising at least one ring directly bearing at least one hydroxyl group; or cyclic, optionally aromatic, hydrocarbon compounds, the ring of which bears at least one, linear or branched, saturated or unsaturated hydrocarbon group, which hydrocarbon group bears at least one hydroxyl group. Concerning cyclic hydrocarbon compounds, comprising at least one ring directly bearing at least one hydroxyl group, alicyclic and monocyclic compounds comprising from 4 to 10 carbon atoms, bearing at least one hydroxyl group may be cited. By way of example of compounds complying with this definition, cyclohexanol may be cited. Concerning cyclic, optionally aromatic, hydrocarbon compounds, the ring of which bears at least one, linear or branched, saturated or unsaturated hydrocarbon group bearing at least one hydroxyl group, monocyclic aromatic compounds may be cited, the ring of which bears at least one, linear or branched, hydrocarbon group bearing at least one hydroxyl group and the ring of which also optionally bears one or more groups other than the aforementioned hydrocarbon group, such as alkoxy, —OH groups. More precisely, it may be phenylic compounds, the phenyl group of which bears at least one, linear or branched, saturated or unsaturated hydrocarbon group, being able to comprise from 4 to 10 carbon atoms, which hydrocarbon group bears at least one hydroxyl group, which phenyl group may also bear one or more groups other than the aforementioned hydrocarbon group. According to a first variant, specific compounds complying with the definition given above may be phenylic compounds, the phenyl group of which bears a linear or branched, saturated hydrocarbon group comprising from 1 to 4 carbon atoms, which hydrocarbon group bears a hydroxyl group. More precisely, specific compounds complying with this definition may be phenylic compounds, the phenyl group of which bears a —CH2—OH group and optionally at least one group chosen from alkoxy or —OH groups. When the compound consists of a phenyl group, only bearing a —CH2OH group, it corresponds to the benzylic alcohol of following formula: Other specific compounds complying with this definition may be phenylic compounds, the phenyl group of which bears a —CH(OH)—CH3 group and optionally at least one group chosen from alkoxy or —OH groups. When the compound consists of a phenyl group, only bearing a —CH(OH)—CH3 group, it corresponds to the alcohol of following formula: commonly designated under the terminology of 1-phenyl-1-ethanol. When the compound consists of a phenyl group, bearer of a —CH2OH group and at least one group other than a —CH2OH group, it may correspond to a benzylic alcohol derivative of following formula: in which R1 is an alkoxy group or a hydroxyl group. R1 may be situated in ortho, meta or para position with respect to the —CH2—OH group. Advantageously, R1 is situated in para position with respect to the —CH2—OH group, in which case the compound meets the following formula: R1 being as defined above. Specific compounds meeting the definition given above may be 4-methoxybenzylic alcohol or 4-hydroxybenzylic alcohol, which correspond respectively to the following formulas: According to a second variant, specific compounds meeting the definition given above (namely, compounds belonging to the family of phenylic compounds, the phenyl group of which bears at least one, linear or branched, saturated or unsaturated hydrocarbon group, being able to comprise from 1 to 4 carbon atoms, which group bears at least one hydroxyl group) may be phenylic compounds, the phenyl group of which bears at least one, linear or branched, unsaturated hydrocarbon group being able to comprise from 2 to 4 carbon atoms, which hydrocarbon group bears at least one hydroxyl group. More precisely, specific compounds complying with this definition may be phenylic compounds, the phenyl group of which bears a linear or branched, monounsaturated hydrocarbon group, being able to comprise from 2 to 4 carbon atoms, such as a —CH═CH—CH2—OH group, a specific example of such compounds being the cinnamyl alcohol of following formula: As mentioned above, the non-sulphurous and non-glucidic alcohols that may be used for the process of the invention may be aliphatic polyols, namely linear or branched hydrocarbon compounds, comprising at least two hydroxyl groups. These compounds may comprise from 2 to 4 carbon atoms. Advantageously, these compounds may be ethylene glycol or glycerine of following respective formulas: In particular, the platinoid element according to the invention may be palladium. The reducing amount of reducing agent needed to reduce the platinoid element(s) to their 0 oxidation state may be chosen, by those skilled in the art, by simple experimental tests (the reduction to the oxidation state being able to be materialised visually by a precipitation of the platinoid element(s)). According to the invention, the acidic aqueous solution may be a nitric solution (i.e. in other words an aqueous solution of nitric acid). In this instance, the use of specific alcohols as defined above contributes to avoiding, in addition, highly exothermic denitration reactions of the nitric solution, which are conventionally caused by the presence in a nitric solution of reducing glucides (such as saccharose). According to the invention, the nitric solution intended to be treated according to the process of the invention may be a raffinate (or aqueous solution) from processes of treating irradiated nuclear fuels, which process conventionally comprises: a step of dissolution of a spent fuel in a highly concentrated aqueous solution of nitric acid, by means of which an aqueous nitric solution is obtained comprising uranium, plutonium, fission products (such as lanthanides, yttrium elements, one or more platinoid elements), minor actinides (such as americium and curium), corrosion products; a step of co-extraction of uranium and plutonium from said aqueous solution by means of an extracting organic phase, at the end of which there remains an organic phase comprising uranium and plutonium and an aqueous phase corresponding to the aforementioned raffinate comprising, apart from the aforementioned platinoid element(s) (such as palladium, ruthenium, rhodium), two minor actinides, namely americium and curium, lanthanides such as lanthanum, cerium, praseodymium, neodymium, samarium and europium, fission products other than lanthanides and platinoids such as molybdenum, zirconium, rubidium and yttrium, as well as corrosion products such as iron and chromium. The aqueous solution of nitric acid may be an aqueous solution with strong nitric acidity, typically from 2 to 5 M. Once the step of adding a reducing agent has been implemented, the process of the invention comprises a step of separating said thus reduced platinoid element from said acidic aqueous solution, this step being able to be carried out, for example, by a simple filtration, a decantation operation or a centrifugation operation. The solution thereby obtained rid of all or part of the platinoid elements may then be used with a view to being vitrified by a conventional vitrification process. The invention will now be described with respect to the following examples given by way of illustration and non-limiting. The aim of the present example is the study of palladium/cerium separation in nitric solution implementing the use of benzylic alcohol. In this example, palladium chloride ((NH4)2PdCl4) and cerium nitrate (Ce(NO3)3) are used, palladium being a platinoid element and cerium being one of the fission products present in the raffinates and considered as representative of all the lanthanides. For each of the tests of this example, 10 mL of 1 mol/L nitric acid solution are used, to which is added 71 mg of palladium chloride and 87 mg of cerium nitrate i.e. 0.2 mmol of palladium and 0.2 mmol of cerium. After addition of the desired amount of benzylic alcohol, the solutions are placed in Parr digestion bombs and heated to a desired temperature for 16 hours. Different series of tests are carried out with variable amounts of benzylic alcohol at a given temperature: a series of tests at 150° C. with respective benzylic alcohol amounts of 0; 0.35; 0.4; 0.5 and 0.7 g; a series of tests at 180° C. with respective benzylic alcohol amounts of 0; 0.35; 0.4; 0.5 and 0.7 g. For each of these series, the measurement was carried out (by ICP-AES) respectively of the amount of palladium and of the amount of cerium after 16 hours of reaction, the results of these measurements being reported respectively in the following figures: FIG. 1 illustrating the evolution of the amount of palladium C1 (in g/L) as a function of the added amount of benzylic alcohol B1 (in g) after 16 hours of reaction; FIG. 2 illustrating the evolution of the amount of cerium C2 (in g/L) as a function of the added amount of benzylic alcohol B2 (in g) after 16 hours of reaction. As may be noted in these figures, a temperature of 180° C. makes it possible to precipitate all the palladium for amounts of benzylic alcohol from 0.3 g and also a non-negligible amount of cerium, whereas a temperature of 150° C. makes it possible to precipitate all the palladium from benzylic alcohol from 0.3 g while enabling a precipitation of cerium in lesser amounts than at 180° C. It has also been noted that the palladium precipitates in the form of particles of a hundred or so nanometers diameter within an organic matrix constituted of an aromatic polymer from the polycondensation of the benzylic alcohol and the oxidation products thereof. The formation of this matrix largely facilitates the separation between the solution containing cerium and palladium having precipitated. Furthermore, no substantial extra pressure is observed in the reactor at the end of reaction, which attests to the absence of denitration or, at the least, to a very limited denitration phenomenon, which proves the safety aspect of the process of the invention. The aim of the present example is the study of palladium/cerium separation in nitric solution implementing the use of different alcohols: benzylic alcohol (part b of FIG. 3), 4-methoxybenzylic alcohol (part c of FIG. 3), glycerine (part d of FIG. 3), 1-phenyl-1-ethanol (part e of FIG. 3), ethylene glycol (part f of FIG. 3), cinnamyl alcohol (part g of FIG. 3), 4-hydroxybenzylic alcohol (part h of FIG. 3) and cyclohexanol (part i of FIG. 3). A test was carried out without alcohol (part a of FIG. 3). In this example, palladium chloride ((NH4)2PdCl4) and cerium nitrate (Ce(NO3)3) are used, palladium being a platinoid element and cerium being one of the fission products present in the raffinates and considered as representative of all the lanthanides. For each of the tests of this example, 10 mL of a 1 mol/L nitric acid solution is used, to which is added 71 mg of palladium chloride and 87 mg of cerium nitrate i.e. 0.2 mmol of palladium and 0.2 mmol of cerium. After addition of the desired amount of alcohol (here, 4.8 mmol), the solutions are placed in Parr digestion bombs and heated to a temperature of 150° C. for 16 hours. FIG. 3 illustrates the concentration of metal ions (palladium and cerium) remaining in solution (in g/L) after heating to 150° C. for 16 hours for the aforementioned different alcohols tested. As may be seen from this figure, all of the alcohols tested have a good selectivity vis-à-vis palladium. In order to get as close as possible to a system encountered in the treatment of aqueous effluents from the reprocessing of spent fuels, selective precipitation tests of palladium were carried out on a model solution of nitric acid (1.5 M) corresponding to the composition described in the table below and a total nitrates load of 3.5 mol/L. OxideConcentrationconcentrationMetalof metalOxide used(g/L)elementelement (g/L)Na2O18.739Na13.90Al2O37.378Al3.90ZnO0.209Zn0.17ZrO29.479Zr7.02TeO21.272Te1.02Cs2O5.757Cs5.43SrO1.703Sr1.44BaO3.632Ba3.25SnO20.128Sn0.10Cr2O30.222Cr0.15Fe2O31.698Fe1.19MnO21.997Mn1.26La2O36.444La5.49Nd2O314.878Nd12.75Ce2O35.195Ce4.44Pr2O32.821Pr2.41MoO39.413Mo6.27P2O50.925P0.40——Pd2.98Total91.8970.61 To carry out the tests, 10 mL of the solution were used. After addition of 500 mg of benzylic alcohol, the solution was placed in a Parr digestion bomb and heated to 150° C. for 16 hours. This manipulation was repeated 4 times to offset any experimental errors. The amounts of palladium and cerium present in the solution at the end of the test were measured by ICP-AES (in other words by atomic emission spectroscopy). The average results calculated on the basis of 4 tests are as follows: Palladium 0.05 g/L Cerium 4.09 g/L As may be seen from these results, the palladium has been almost totally eliminated, whereas the cerium remains very largely in solution.
	summary
	abstract	An apparatus for cleaning an irradiated nuclear fuel assembly includes a housing adapted to engage a nuclear fuel assembly. A set of ultrasonic transducers is positioned on the housing to supply radially emanating omnidirectional ultrasonic energy to remove deposits from the nuclear fuel assembly.
	abstract	An detector-assembly for measuring flux in a nuclear reactor core includes self-powered in-core detector arrangements each for measuring flux at a different one of a plurality of axial locations in the core, and an assembly connector configured to be connected to a power plant connector. The assembly connector includes a plurality flux signal terminals each connected to one of self-powered in-core detector arrangements. At least one of the self-powered in-core detector arrangements comprises a set of at least two self-powered in-core detectors for measuring flux at a same one of the axial locations in the nuclear reactor core. Each of the at least two self-powered in-core detectors includes a sheath, a detector material section inside the sheath, an insulator between the sheath and the detector material, and a flux signal output line. The flux signal output lines of the at least two self-powered in-core detectors are joined together.
044850697	claims	1. A moisture separator reheater comprising: a generally horizontally disposed cylindrical shell; a head on each end of said shell forming end closures for said moisture separator reheater; a plurality of long U-shaped tubes disposed in a generally parallel array to form a generally round tube bundle with shell side inlet and outlet portions of said tube bundle being generally flat; a tubesheet having a plurality of holes which receive the ends of the tubes, said ends of said tubes being in a sealed relationship with said holes; a plurality of tube supports spaced along the length of said tube bundle; a pair of arcuate plates disposed on opposite sides of said tube bundle and fastened to said tube supports forming a wrapper subtending only the round portions of said tube bundle; a pair of cylindrically shaped elongated plates disposed longitudinally in the central portion of said shell, said cylindrically shaped plates having generally horizontal upper and lower margins which generally extend the length of the shell; a plurality of upper elongated plates which extend from the upper margins of said cylindrically shaped plates to the upper portions of the shell; a plurality of lower elongated plates which extend downwardly from the lower margins of said cylindrically shaped plates toward the lower portion of said shell; said upper, lower and cylindricaly shaped plates and said shell being cooperatively associated in a sealed relationship to form within the shell a central chamber flanked by two side chambers; openings in said lower plates placing the side chambers in fluid communication with the central chamber; moisture separating means disposed adjacent the openings in said lower plates; a sliding sealed juncture disposed between adjacent arcuate and cylindrically shaped plates, said sliding sealed juncture generally extending the length of said plates to provide a seal and sliding engagement between said arcuate and cylindrically shaped plates to allow easy installation and removal of said tube bundle within said central chamber and to allow for differential thermal expansion between said arcuate and said cylindrical shaped plates; a fluid inlet in fluid communication with each side chamber in said shell; and a fluid outlet in fluid communication with the upper portion of said central chamber in said shell to form a moisture separator reheater. 2. A moisture separator reheater as set forth in claim 1, wherein the sliding sealed juncture is formed by elongated flat bars generally horizontally disposed so the bars are slidably engaged in such a manner to prevent upward movement of said tube bundle due to the pressure differential caused by fluid flowing over the outer surface of the tubes. 3. A moisture separator reheater as set forth in claim 1 and further comprising a head disposed in a sealed relationship with the tubesheet, means for dividing said head into at least two chambers, a heating fluid supply nozzle in fluid communication with one of said head chambers, a manifold forming a vent chamber in fluid communication with a plurality of tubes, said manifold being so disposed that the fluid flowing through the tubes makes four passes through said central chamber. 4. A moisture separator reheater as set forth in claim 3 and further comprising a duct disposed in said tubesheet in fluid communication with said manifold for draining fluid from said manifold.
	claims	1. A cylindrical gamma ray generator, comprising:a cylindrical RF-driven plasma ion source for producing a single proton hydrogen ion containing plasma;a single proton hydrogen gas source in fluid communication with said plasma ion source whereby single proton ionized hydrogen gas can be introduced into said ion source;a cylindrically-shaped radial ion extractor system, the system disposed coaxially about the ion source for extracting proton ions radially from the ion source; whereinthe proton ions extracted from the source by the cylindrical extractor system are directed with an energy of approximately 165 keV at a cylindrical target comprising boron carbide (B4C) disposed coaxially outside and spaced from the proton plasma ion source, wherein the target material undergoes proton/gamma (p,γ) reaction when irradiated by the proton ions to produce gamma rays having an energy level above 6 MeV. 2. The generator of claim 1, further comprising an RF antenna disposed within the ion source. 3. The generator of claim 1, further comprising water cooling within the RF antenna. 4. The generator of claim 1, wherein the cylindrically-shaped radial ion extractor system comprises a plurality of axially extending slots. 5. The generator of claim 1, further comprising a vacuum chamber disposed to contain the target. 6. A generator for detecting special nuclear materials, comprising:a cylindrical gamma generator of claim 1;a shield surrounding parts of the generator to prevent gammas from escaping in undesired directions;an associated neutron detector for detecting neutrons produced by gamma reaction with the special nuclear materials. 7. A higher power gamma generator, comprising, a plurality of individual gamma ray generators of claim 1, stacked one on top of another. 8. The generator of claim 7, wherein a first and a second adjoining generators share high voltage sources. 9. The generator of claim 8, wherein the second and third adjoining generators share a pumping chamber.
052020827	abstract	A replacement nozzle and heater sleeve for a nuclear reactor coolant system pressurizer. The heater, damaged heater sleeve exterior of the pressurizer and a portion of the heater sleeve inside the heater sleeve bore are removed. The heater sleeve bore is partially tapped to provide threads at its lower end. A sleeve having a first seal ring positioned at its upper end is threaded into the heater sleeve bore such that the first seal ring presses against the original heater sleeve and provides a seal. A second seal ring on a flange on the sleeve is pressed into sealing engagement with the exterior of the pressurizer.
	description	This application claims benefit under 35 USC 119(e) from U.S. provisional patent application 60/992,691, filed on Dec. 5, 2007. The contents of all of the above documents are incorporated by reference as if fully set forth herein. The present invention, in some embodiments thereof, relates to radiation detection systems used in the vicinity of pulsed radiation beams and other radiation sources; and, more particularly, but not exclusively, to x-ray and gamma-ray imaging and tracking systems used to monitor patients while they are treated by pulsed radiation therapy beams. Radiation therapy is often used to treat cancer and other abnormal growths. Such therapy can use implanted radioactive sources (brachytherapy), or external radiation sources, generally beams, including x-ray beams and electron beams produced by linacs, as well as proton beams and heavy ion beams. Such beams are also used for radiosurgery, for example for ablating cardiac tissue to prevent atrial fibrillation. Because radiation beams can harm healthy tissue, radiation beam therapy and radiosurgery are carefully planned, with beams aimed precisely at a target such as a tumor, often with several doses of radiation given from different angles, to make sure that the target receives enough radiation, while minimizing the exposure of healthy tissue to radiation. U.S. Pat. No. 6,683,318 to Haberer et al describes a heavy ion beam therapy system, in which positron emission tomography (PET) is used to locate radioactive nuclei that decay by positron emission, produced in the target tissue by the heavy ion beam. The PET results can verify that the heavy ion beam was aimed properly. In order to locate these positron-emitting nuclei before they have moved away from the target, PET is performed during the treatment session. The most convenient time for doing this is said to be in the time slots between beam spills, when the PET signal is less obscured by background noise than in the periods with the beam on. As defined in other publications by the inventors and their research group at Darmstadt, “beam spills” refers to periods of one to five seconds during which the beam is on, separated by time slots of similar length during which the beam is off. This use of “beam spill” is found, for example, in Parodi et al, “The Time Dependence of the γ-Ray Intensity Seen by an In-Beam PET Monitor,” downloaded from www.fzd.de/FWK/jb02/PDF/page77.pdf, on Nov. 23, 2008; Peters et al, “Spill Structure Measurements at the Heidelberg Ion Therapy Centre,” Proceedings of EPAC08, Genoa, Italy, paper TUPP127, pages 1824-1826, downloaded from epaper.kek.jp/e08/papers/tupp127.pdf, on Nov. 23, 2008; Crespo et al, “First In-Beam PET Imaging With LSO/APD Array Detectors,” IEEE Trans. Nucl. Sci. 15, 2654-2661 (2004); and Pshenichnov et al, “PET monitoring of cancer therapy with 3He and 12C beams: a study with the GEANT4 toolkit,” submitted to Phys. Med. Biol., downloaded from arxiv.org/PS_cache/arxiv/pdf/0708/0708.1691v1.pdf, on Nov. 23, 2008. U.S. Pat. Nos. 7,438,685 and 6,804,548 describe using ultrasound to monitor the position of a target organ or tumor in real time during beam therapy. US 2005/0197564 to Dempsey describes using real time MRI during beam therapy. U.S. Pat. No. 7,349,522 describes software for simulating dynamic radiation therapy, for example gated to respiration, using fluoroscope images fused to previously acquired reference images. But they do not suggest the use of such fluoroscope images during actual beam therapy, instead using index markers or other known methods of gating to respiration. U.S. Pat. No. 7,302,033 describes real time “image guided radiation treatment,” using a linac for treatment, and a stereo x-ray system for real time imaging, arranged so neither one blocks the other. “Real time” is defined to mean anytime during a treatment delivery phase, with the linac turned on or off. Specifically, they describe making an x-ray image before turning on the linac, delivering a dose of radiation with the linac, then making another image with the linac turned off, delivering another dose of radiation, etc. US 2008/0130825 to Fu et al describes using image guided radiation therapy, including x-ray imaging, while the beam is turned on or off. Image segmentation is used in real time to better identify the target, for example a tumor. US 2005/0080332 to Shiu describes using “near simultaneous” CT image guided radiotherapy. U.S. Pat. No. 7,295,648 describes using linac x-rays for imaging “by suitable variation of the output energy”. U.S. Pat. No. 5,233,990 describes using a lower energy therapeutic x-ray beam, from an x-ray tube, for imaging in real time, to verify the position of the patient. U.S. Pat. No. 6,839,404 describes using linac x-rays for imaging before delivering a dose of x-rays for therapy, and using the detector to monitor the dose during therapy. U.S. Pat. No. 6,618,467 describes using linac x-rays to produce CT images in real time. Because the therapy x-rays do not make up a complete set of angles for CT, they supplement them with low level x-rays at other angles, obtained from leakage through the shutters of the linac, or from sources other than the linac, collected either before or during treatment. They also describe using only the low level x-rays to produce the CT images. U.S. Pat. No. 7,263,164 describes using an x-ray imaging system in real time, during treatment by a linac beam. Scattering from the linac beam into the detector is estimated, using a phantom, and subtracted from the image. U.S. Pat. No. 7,171,257 describes doing x-ray imaging just before radiosurgery, finding the change in position of the beam target, for example cardiac tissue to be ablated, as a function of cardiac phase and breathing phase, then using that information, with the imaging system turned off, to keep the beam aimed correctly during the radiosurgery, monitoring the breathing and cardiac cycles in real time. U.S. Pat. No. 6,865,411 states that it is a disadvantage that imaging and radiation beam therapy cannot be done at the same time. U.S. Pat. No. 6,662,036 and U.S. Pat. No. 6,405,072 describe using index markers to track movement of the patient in real time, during beam therapy. “Answer to Question #4511 Submitted to ‘Ask the Experts’” on the Health Physics Society website, downloaded on Nov. 27, 2008 from hps.org/publicinformation/ate/q4511.html, states that most medical linacs are pulsed with repetition rates of 100 to 400 pulses per second, and pulse lengths of 1 to 10 microseconds, resulting in a very low duty cycle, less than 1% or less than 0.1%, and peak intensities of radiation much higher than the average intensity. Radiation detectors that have long dead times, such as Geiger-Muller and proportional counters, tend to become saturated at such high peak intensities, and are not suitable for safety monitoring of radiation levels outside rooms where linacs are used. Similar points are made by R. McCall and N. Ipe, “The Response of Survey Meters to Pulsed Radiation Fields,” SLAC-PUB-4488 (1987), downloaded from www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-4488.pdf, on Nov. 27, 2008. Radiation Detection and Measurement by Glenn Knoll, 3rd edition (2000), ISBN 0-471-07338-5, describes instruments for detecting x-ray and gamma-ray photons. An aspect of some embodiments of the invention concerns a system for treating a body with a pulsed beam of ionizing radiation, for example a radiation therapy beam, while using a detector system to obtain data of the body being treated, from another ionizing radiation source such as an x-ray or gamma-ray imaging or tracking system, in real time during operation of the beam. The detection system reduces or avoids interference from the beam, by not recording data and/or by having reduced sensitivity during the pulses of the beam, while using the intervals between pulses to obtain the data. There is thus provided, in accordance with an exemplary embodiment of the invention, a detector system adapted for monitoring a radiation treatment system comprising a pulsed beam radiation source for treating a body with a given beam intensity and beam configuration, with pulse times and intervals between pulses less than 100 milliseconds, using at least one monitoring radiation source located inside or outside the body, the detector system comprising; a) a detector designed to detect radiation from the monitoring source, and subject to interference radiation from the beam source; and b) control circuitry that creates a data record of radiation received by the detector, to provide information about the body;wherein, when the detector detects radiation in real time during operation of the beam, the data record selectively excludes data for radiation received by the detector during the pulses, as opposed to data for radiation received by the detector between pulses. Optionally, the detector sensitivity is controllable by the control circuitry, and the data record selectively excludes data for radiation received during the pulses because the detector system is configured to make the sensitivity of the detector lower during the pulses than between the pulses. Optionally, the detector has a bias voltage, and the control circuitry makes the sensitivity of the detector lower by changing the bias voltage. In an embodiment of the invention, the data record selectively excludes data for radiation received during the pulses because the control circuitry is configured not to add data for radiation detected during the pulses to the data record. Optionally, the detector system uses a triggering element that signals the timing of the pulses to the control circuitry. Optionally, the triggering element is comprised in the beam source, or in a timing element that controls the timing of the beam pulses. Alternatively, the triggering element is comprised in a sensor which senses when the beam source produces a pulse. Optionally, the sensor is the detector. Optionally, the data record selectively excludes data for radiation received during the pulses because the control circuitry is configured to remove said data from the data record data. In an embodiment of the invention, the data record selectively excludes data for radiation received during the pulses because the detector is configured to saturate at a level of radiation received during the pulses at the given beam intensity and beam configuration, but not to saturate at a level of radiation received from the monitoring source between pulses. Optionally, the detector is a scintillation detector with decay time shorter than the intervals between pulses. Optionally, the time-averaged relative contribution of the interference to the data record is less, by at least a factor of 5, than the time-averaged relative contribution of the interference to the radiation received by the detector, when the beam is operating at the given beam intensity and beam configuration. Optionally, the interference contributes to the data record less than 20% as much as the radiation from the monitoring source, averaged over any time interval that includes many pulses, when the beam is operating at the given beam intensity and beam configuration. In an embodiment of the invention, the radiation treatment system that the detector system is adapted for monitoring is a radiation therapy system using a beam intensity of at least 1 centiGray per second, and the detector system can locate a beam therapy target inside the patient to within 2 mm in an acquisition time of less than 2 seconds, using an internal monitoring source of less than 1 milliCurie or an external x-ray monitoring source of less than 20 centiGray per acquisition time. Optionally, the data record provides information on one or more of motion of the patient's body, position of the patient's body, motion and position of one or more parts of the patient's body, all relative to the beam in real time when the beam is on, and a dose of radiation received from the beam by one or more parts of the patient's body. Optionally, the pulsed beam source in the radiation treatment system that the detector system is adapted to monitor comprises a linac beam source. Optionally, the linac beam source comprises an x-ray beam source. Alternatively, the pulsed beam source in the radiation treatment system that the detector system is adapted to monitor comprises an ion beam source. In an embodiment of the invention, the detector system comprises an x-ray imaging system, using a monitoring source comprising an x-ray source. Optionally, the detector system comprises a CT system. Alternatively or additionally, the detector system comprises a radioactive tracking system, using a monitoring source comprising a radioactive source inside the body being treated. Alternatively or additionally, the detector system comprises a gamma imaging system, using a monitoring source comprising radioactive material inside the body being treated. Optionally, the control circuitry is adapted to determine a difference between a position of a treatment target in the body and planned position with respect to the beam, and to adjust a position of the beam and/or reduce the power of the beam in response to the difference. Optionally, the control circuitry is fast enough, or the detector has a fast enough decay time, or both, so that the data record can selectively exclude data for radiation received by the detector during a time period shorter than 100 milliseconds, as opposed to data for radiation received by the detector outside the time period. Optionally, the time period is shorter than 10 milliseconds. There is further provided, in accordance with an exemplary embodiment of the invention, a method of monitoring a body in real time while the body is being treated by a pulsed beam of treatment radiation, with pulse lengths and interval between pulses both shorter than 100 milliseconds, the method comprising: a) passing monitoring radiation from a source other than the beam through at least part of the body; b) receiving the monitoring radiation together with any interfering radiation from the beam, and detecting and recording at least some of the radiation in a data record; c) using the data record to monitor the body in real time during the treatment;wherein detecting and recording the radiation selectively excludes from the data record data for radiation received during the beam pulses. Optionally, using the data record to monitor comprises reconstructing images. Optionally, the monitoring radiation comes from a source inside the body, and using the data record to monitor comprises tracking a location of the source. Optionally, the body is a patient's body, and the treatment by the beam comprises radiation therapy on the patient. Optionally, recording the radiation comprises selectively failing to record, or selectively removing from the data record, data for radiation detected during the beam pulses. Optionally, the method also includes lowering a detection sensitivity to radiation during the beam pulses, and raising the detection sensitivity during intervals between pulses. Optionally, the method also includes receiving triggering signals indicating the beginning and end of each pulse, wherein lowering and raising the detection sensitivity is done in response to the triggering signals. Optionally, detecting the radiation comprises detecting with a saturation level lower than a level of radiation received during the beam pulses, but higher than a level of radiation received during intervals between the beam pulses. Optionally, using the data record to monitor in real time comprises: a) determining a difference between a position of a beam therapy target in the patient, and a planned position with respect to the beam; and b) adjusting a position of the beam, and/or reducing the power of the beam, in response to the difference. Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting. The present invention, in some embodiments thereof, relates to radiation detection systems used in the vicinity of pulsed radiation beams and other radiation sources, and, more particularly, but not exclusively, to x-ray and gamma-ray imaging and tracking systems used to monitor patients while they are treated by pulsed radiation therapy beams. An aspect of some embodiments of the invention concerns a radiation treatment system that uses a pulsed beam of radiation to treat a body, with pulse lengths and intervals between pulses shorter than 100 milliseconds, together with a detector system that uses another source of radiation to monitor the body during the treatment. In an exemplary embodiment of the invention, the detector system reduces or avoids interference from the beam by having reduced sensitivity during the pulses, or not recording data for radiation detected during the pulses, and recording data for radiation detected in the intervals between pulses. Optionally, the sensitivity of the detector is decreased by lowering a bias voltage used by the detector, triggered, for example, by a sensor that responds to the beam pulses, or triggered by the increased intensity of radiation that the detector measures just as a pulse is beginning. Alternatively or additionally, the detector has a lower sensitivity during the beam pulses because the detector saturates at a level well below the intensity of radiation it receives during the beam pulses, but optionally at a higher level than the highest intensity of radiation expected from the other source of radiation. The detector may be paralyzed when it saturates, not responding at all, or non-paralyzed, responding at a maximum rate, or something in between these two extremes. Additionally or alternatively, any radiation detected during the beam pulses is removed, for example by software, from a data record of the detector, with the beam pulses optionally identified by the higher levels of radiation detected by the detector then. Optionally, the beam of radiation is a radiation therapy beam, for example a linac-generated x-ray beam or electron beam, or an ion beam, used to treat a patient, and the detector system monitors the patient in real time during the therapy, for example to determine the position of the patient's body, or of parts of the body, and to detect any change in their position, and optionally adjust the aim of the beam in response. Additionally or alternatively, the detector system monitors the patient in real time, in order to accurately determine the dose of radiation delivered to a desired target in the patient's body, for example a tumor, and/or the dose of radiation delivered to healthy tissue, and optionally adjust the beam intensity and/or the beam path. Optionally, the detector system is an x-ray or gamma-ray imaging or tracking system, and the other source of radiation is an x-ray source such as an x-ray tube, or a radioactive marker or other radioactive material in the patient's body. Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details set forth in the following description. The invention is capable of other embodiments or of being practiced or carried out in various ways. Referring now to the drawings, FIG. 1 illustrates a radiation treatment system 100, with detector systems for monitoring using separate monitoring radiation sources, according to an exemplary embodiment of the invention. A patient 102, lying on a bed or table 104, is being treated by a radiation therapy beam 106 generated by a pulsed beam source 108, for example a linac producing hard x-rays for treating cancer. Alternatively, the pulsed beam source is a linac producing an electron beam, or a proton beam source, or a heavy ion beam source, or any other kind of pulsed beam source used for radiation therapy. The pulses, which typically each last for 1 to 10 microseconds, generally occur at regular intervals shorter than 100 milliseconds, typically at intervals between 2.5 and 10 milliseconds, when the beam is on, and typically the beam is on for 1 to 20 seconds at a time. In some linacs, certain pulses may be missing, in some pattern, to adjust the dose rate, so the pulses will not all occur at fixed intervals, but in a more complicated pattern. As used herein, the beam is said to be “on” or “operating” or “treating the patient” when it is producing pulses at one of its normal rates and patterns, including during the intervals between pulses, and is said to be “off” or “pausing” when it is not generating pulses at all. The beam source operates the beam at a given beam intensity and beam configuration, including beam diameter, spread, energy distribution and composition, appropriate for radiation therapy, and characteristics of the monitoring detector systems may be adjusted depending on the beam intensity and beam configuration. Optionally, beam source 108 is mounted on an arm or gantry 110, with a control 112 for adjusting the direction and/or position of the beam. Optionally, there is a detector system using another radiation source 114, for example an x-ray tube, producing radiation 116, which is detected by detector 118, for imaging patient 102 in real time, for monitoring during radiation treatment. In a prior art radiation treatment system with a linac beam and a radiation detector operating at the same time, photons, electrons, and emitted neutrons from the beam would enter the detector directly, or after scattering from the patient or other objects in the room, and cause substantial interference in data generated by the detector. But for reasons that will be explained below, the data generated by detector 118 suffers from relatively little interference from beam 106. Additionally or alternatively, there is a detector system using an internal radioactive source 120 in the patient, for example an implanted compact marker, or extended radioactive material, injected or ingested to mark the location of a tumor or other target being treated by beam 106, which emits radiation 122, for example gamma rays, which are detected by a detector 124. In some embodiments of the invention, radioactive source 120 is generated inside the patient's body by the beam, for example it is a positron source generated by spallation of a heavy ion beam. The data generated by detector 124 also has relatively little interference from beam 106, for reasons that will be explained. Detector 124 is, for example, a gamma camera, or a radioactive tracker, used to image or track the radioactive marker or material in the patient's body, in real time, for monitoring during the beam treatment. A controller 126, for example a computer, optionally reconstructs images of patient 102 and/or source 120, and/or tracks the position of source 120, using data generated by detectors 118 and/or 124, and detects any changes in position of the patient or of internal tissues or organs being targeted by the beam, in real time. An image can be reconstructed, for example, from measurements of the absorption of x-rays on different chords going through the body, forming a 2-D x-ray image. Alternatively, instead of reconstructing an image, a compact radioactive source in the body, generally a gamma-ray source, can be tracked, for example by using three differential detectors to determine the position of its center of mass in each of three dimensions, and then following changes in the position, as described for example in WO 2006/016368 and WO 2007/017846, both assigned to Navotek Medical Ltd. A 2-D image can also be reconstructed by a gamma camera, scanning a detector over the body to measure a spatial distribution of a gamma-ray emitting radioactive material in the body, but x-ray imaging and gamma-ray tracking have the potential advantage that they are much faster, and may be better suited for finding changes in the position of a beam therapy target in real time. This information is used by controller 126, either automatically or in conjunction with a human operator, to make any needed changes in the location or direction of the beam, using control 112 and arm 110, as well as possibly turning off beam source 108 in real time to avoid damaging healthy tissue, if beam 106 is no longer aimed properly, and leaving beam source 108 off until the aim of the beam and/or the position of the patient can be adjusted. As used herein, each detector used for monitoring the patient using a given radiation source, for example detector 118 using source 114, or detector 124 using source 120, including any control circuitry, is referred to as a detector system. The control circuitry can be common to more than one detector system, as in the case of controller 126 in FIG. 1, or can be separate for one or more detector systems, packaged with the detector for example. As used herein, the control circuitry of a detector system includes any elements which create or modify data records; use data records for imaging or tracking; use results of the imaging or tracking for control, for example of the beam source; actively control the sensitivity or other characteristics of the detector; receive signals for actively controlling the detector, for example signals about the beginning and end of beam pulses. All of these functions of the control circuitry need not be present in a given detector system, and they may be performed by the same or different elements, packaged together or separately. A given detector system may use more than one radiation source, and a given source may be used by more than one detector system. The sources used by the detector systems, such as external source 114 and internal source 120, are referred to as monitoring sources, and their radiation referred to as monitoring radiation, to distinguish them from the beam source, and from the interfering radiation coming directly and indirectly from the beam. To reduce or avoid interference from beam 106 when it is operating, controller 126 uses a data record with data from detectors 118 and/or 124, for imaging and tracking. The data record selectively excludes data for the radiation received during the pulses of beam 106, as opposed to data for radiation received between the pulses. Since interference from beam 106 occurs primarily during the pulses, from beam radiation hitting the detectors directly or after scattering, this selective exclusion decreases the effect of interference from beam 106 on the imaging or tracking. As used herein, “selectively excludes” does not necessarily mean that data is excluded for all radiation received during the beam pulses, but data is excluded for a greater proportion of such radiation, than for radiation received during the intervals between pulses. Optionally, data is excluded for most or all radiation received during the beam pulses. As used herein, excluding data can include not adding data to the data record because the radiation was not detected, not recording data to the data record even though the radiation was detected, and removing data from the data record after it was recorded. As used herein, “data record” can include any data from the detector used for imaging or tracking, even if the data is only kept in a computer memory and used continuously in real time for imaging or tracking, and never saved. As used herein, “interfering radiation” or “interference radiation” includes radiation directly entering a detector from the beam source, as well as radiation from the beam source which is scattered before entering the detector, or is absorbed and re-emitted in much less time than the time between pulses. It does not include radiation from activated nuclei or excited atoms which is emitted in a time that is not short compared to the time between pulses. System 100 has a potential advantage over prior art systems in which a radiation beam is turned on to provide a dose of radiation to the patient, then paused for a few seconds or longer while the patient is imaged to see if he or she has moved, then turned on again, and the procedure repeated several times. In such a prior art system, the pauses in treatment to image the patient make the treatment session last longer, making the session more difficult for the patient, and resulting in lower throughput for the expensive beam therapy system. The longer treatment sessions also make it more likely that the patient will move. In addition, because the imaging is not done in real time, it is not possible to stop or adjust the beam if the patient moves in the middle of treatment, and one cannot be certain that the patient's position as measured after the treatment is exactly the same as the patient's position while the treatment was going on. In system 100, imaging may be done in real time during treatment, so there are no pauses in the treatment session, it is possible to stop or make adjustments in the beam as soon as the patient moves, and it is possible to verify that the beam was aimed properly during the treatment. In some embodiments of the invention, decreasing the contribution to the data record of radiation received during the beam pulses is accomplished by actively making the detectors less sensitive during the pulses. For example, a sufficiently fast sensor 128 in the path of beam 106 senses the timing of the pulses, and communicates this information to controller 126, which decreases the sensitivity of the detectors, for example by lowering their bias voltage. Additionally or alternatively, information about the timing of the pulses may come directly from beam source 108. In some embodiments of the invention, detectors 118 and/or 124 are not necessarily less sensitive to radiation during the pulses than in the intervals between the pulses, but the detectors, or controller 126, do not record data on any radiation received during the pulses, or controller 126 removes this data from a data record after it has been written. In some embodiments of the invention, the detectors are less sensitive to radiation during the pulses because they have a saturation level that is much lower than the radiation level received during the pulses, primarily from the beam, but optionally the saturation level is higher than the radiation level received during the intervals between pulses, primarily from the other radiation sources 114 and/or 120. In those embodiments, controller 126 and detectors 118 and 124 do not need to use information about the timing of the pulses. There are a variety of types of radiation detectors that are optionally used for detectors 118 and 124. These include rare earth screens, photostimulable phosphors, Geiger counters, proportional counters, scintillators, direct semiconductor detectors, and a combination of scintillators with semiconductor detectors (indirect detectors). These devices generally have a sensitivity that is controllable, for example by controlling a bias voltage, and/or a saturation level that is controllable, for example by controlling an integration time and a maximum recharging current. In principle even photographic plates or x-ray film could be used, with, for example, a shield with an aperture and a shutter than can be closed during the pulses, or a rotating shield with one or more openings that are timed to be closed during the pulses. But shielding that is effective against the hard x-rays typically used for radiation therapy, above 1 MeV, is generally quite massive, and devices whose sensitivity or saturation level can be controlled electronically have the potential advantage that it is not necessary to apply the high inertial forces that would be used to open and close a massive, high speed mechanical shutter, or to use the high rotational energy associated with a massive, rapidly rotating shield. Detector 118 and/or 124 may be divided into a plurality of sections each corresponding to a pixel of an image, optionally with a collimator so that each section is sensitive primarily to radiation from a relatively narrow range of directions, and/or radiation that has passed through the body along a narrow range of paths. The detector may produce a single 1-D or 2-D image from radiation reaching the detector when the detector and source are at fixed position. Examples of such systems include x-ray imaging systems using an external x-ray tube, and gamma cameras, such as an Anger camera, using a distributed internal radioactive source. Alternatively, the detector may be used to detect radiation with the detector at a series of different positions or orientation, and/or with the source, particularly an external source, at a series of different positions and orientations, to produce a 2-D or 3-D tomographic image. Systems producing tomographic images include CT (computerized tomography) systems using an external x-ray source, and SPECT (single photon emitted computer tomography) and PET (positron emission tomography) systems, using an internal radioactive source. Alternatively, the detector may be used in a tracking system, to locate and track a compact radioactive source, or a plurality of compact sources, inside the body. Examples of such tracking systems, using gamma ray sources, are described, for example, in WO 2006/016368 and in WO 2007/017846, both assigned to Navotek Medical, Ltd. Whether the detector is used for imaging or tracking or both, if the radiation treatment system is used for beam therapy of patients, as system 100 is, then the detector system optionally provides information that is useful for real-time monitoring of patients during beam therapy. For example, the detector system can find the position of a therapy target inside the patient's body, to within a precision, such as 2 mm or 1 mm, that is adequate for providing effective and safe aiming of the beam, in an acquisition time, such as 1 second or 2 seconds, that is short enough so that if an error in aiming is found, the beam can be adjusted or turned off without any significant harm done. This can be done using a medically safe monitoring radiation source, either a safe internal radioactive source, for example no more than 1 milliCurie, or no more than 100 microCuries, or a safe external x-ray source, for example no more than 20 centiGray per acquisition time, or no more than 2 centiGray, or no more than 0.2 centiGray. And it can be done even in the presence of an x-ray therapy beam as strong as 1 centiGray per second, or 3 centiGrays per second, or 10 centiGrays per second. Certain characteristics of the detectors may be advantageous: 1) Short integration time. It is potential advantageous if the integration time of the detector is short compared to the length of a pulse, so that the detector sensitivity during a pulse can be lower than the detector sensitivity during the intervals between pulses. Alternatively, if the integration time is longer than or comparable to a pulse length but much shorter than the interval between pulses, then the detector sensitivity can be lower for several integration times around the pulse, still leaving most of the much longer interval between pulses during which the detector can have high sensitivity. For scintillation based detectors, the integration time depends on the decay time of the scintillation pulses, which may be defined as the time required for the scintillation pulse to fall to 1/e of its maximum value. 2) Low afterglow. Some detectors produce an afterglow for a period of time, for example a few milliseconds, after radiation is received by the detector. It is potentially advantageous if the afterglow is sufficiently low, or sufficiently short-lived, or both, so that the afterglow from radiation received during a pulse does not substantially interfere with the detection of radiation between pulses. Halide scintillation crystals, particularly thallium doped sodium iodide and thallium doped cesium iodide tend to exhibit long afterglow, as high as a few percent after 3 milliseconds. Cadmium tungstate (CdWO4) crystals, bismuth germinate (BGO, or Bi4Ge3O12), and zinc selenide (ZnSe) doped with oxygen or tellurium, are examples of low afterglow scintillation materials. 3) High radiation hardness. Because the detector is exposed to a relatively high level of radiation from the beam, it is potentially advantageous to use a detector with relatively high radiation hardness, for example a detector whose functioning will not be substantially affected after exposure to 10,000 gray. Examples of radiation hard scintillation materials include cadmium tungstate, gadolinium silicate (Gd2SiO5), and undoped cesium iodide. Alternatively, if the detector material is low enough in cost, it can be replaced when it is damaged by radiation. 4) Low neutron activation. X-rays from a linac, particularly x-ray of energy 10 MeV or greater, can release neutrons from nuclei from material exposed to the beam, and these neutrons can activate materials in the vicinity of the beam, including detectors 118 and 124. The resulting radioactive isotopes can interfere with the detectors, producing spurious data. Since most radioactive isotopes that are of concern have half-lives much longer than the typical time intervals between pulses, this source of interference cannot be avoided by making the detector less sensitive during pulse times. It is optionally minimized by using detector materials with low neutron activation levels, and/or by using a linac beam with energy below 10 MeV, so that few neutrons will be released. It is potentially advantageous to avoid the use of gold and silver, which have high neutron activation levels, and to use molybdenum and tungsten, which have low neutron activation levels. FIG. 2 schematically shows a plot 200 of the beam intensity 202 as a function of time 204, showing pulses 206 and intervals 208 between pulses. Other quantities as a function of time, with the same time axis 204, are shown below the plot of beam intensity, for an embodiment of the invention in which the sensitivity of detector 118 and/or 124 is lowered during each beam pulse by decreasing the bias voltage of the detector. Optionally, the duty cycle of beam source 108, the ratio of the interval of pulse 206 to the interval between pulses 208, is small, for example less than 20%, less than 10%, less than 5%, less than 2%, less than 1%, less than 0.5%, less than 0.2%, or less than 0.1%. For example, in a typical linac generating x-rays for radiation therapy, the pulse length may be 1 to 10 microseconds, or 0.1 to 1 microsecond, or 10 to 100 microseconds, or smaller or larger values, and the intervals between pulses may be 2.5 to 10 milliseconds, or 0.5 to 2.5 milliseconds, or 10 to 50 milliseconds, or smaller or larger values. In general, both the pulse time and the interval between pulses may be less than 100 milliseconds. The pulses in a linac are due to the fact that the microwaves responsible for accelerating electrons in the linac are generally produced by pulsed power from a discharging capacitor, which then takes a much longer time to charge up again, using a lower level of power from the power grid, than the time over which it discharges. In addition to this pulse structure, linac beams also have structure on a shorter timescale, on the order of a nanosecond, due to bunching of electrons in phase with the microwaves. In the second plot from the top, the detector bias voltage 210 is shown schematically as a function of time 204. Many types of radiation detectors require outside power to detect radiation, which is supplied as a bias voltage, and these detectors cannot detect radiation at all if the bias voltage is too low. As used herein, “bias voltage” means any voltage applied to a detector, which affects the sensitivity of the detector. Examples include the voltage applied to the photomultiplier tube in a scintillation detector, the voltage supplied between the anode and cathode of a proportional counter, and the voltage applied to a semiconductor diode detector. During each pulse 206, the bias voltage is lowered to a level 214, so that the detector has decreased sensitivity to radiation. Optionally, level 214 is low enough so that the detector substantially does not detect radiation at all. During intervals 208 between pulses, the bias voltage is at a higher level 212, so that the detector is sensitive to radiation from the source, either an external source such as source 114 or an internal source such as source 120, that the detector is designed to be used with. The third plot from the top schematically shows the amplitude 216 of trigger signals that trigger controller 126 to change the bias voltage. Trigger signals 218 at the beginning of each pulse trigger the controller to decrease the bias voltage of the detector, and trigger signals 220 at the end of each pulse trigger the controller to increase the bias voltage of the detector back to a higher value. In some embodiments of the invention, there is only one trigger signal per pulse, for example only at the beginning or only at the end of the pulse, and controller 126 calculates the beginning and/or end of the pulse from the single trigger signal, using a known pulse length and a known timing of the signal relative to the pulse. In some embodiments of the invention, the trigger signals are generated by a signal from sensor 128 which intercepts the beam, or intercepts scattered radiation from the beam, and measures its intensity as a function of time. For example, the trigger signals are optionally generated by taking a time derivative of the sensed beam intensity as a function of time, with oppositely biased diodes optionally used to separate “pulse start” trigger signals 218 from “pulse end” trigger signals 220, or the “pulse start” and “pulse end” signals may be distinguished by their polarity. Other analog and digital methods of generating trigger signals from sensor 128 will be apparent to one of skill in the art. Additionally or alternatively, information about the timing of the beginning and/or end of each beam pulse is obtained from beam source 108, or from a timing element that sends synchronized triggering signals both to the detector system and to beam source 108, to control the timing of the beam pulses. Additionally or alternatively, timing information about the beam pulses comes from one of the detectors. However, using a beam sensor, or using the beam source, or using a timing element, has the potential advantage that it may more accurately reflect the timing of the beam pulses than the detectors, which may be positioned and/or shielded to reduce their exposure to the beam radiation. The fourth plot from the top in FIG. 2 schematically shows an intensity 222 of radiation received by detector 118 or 124, as a function of time. During each pulse 206, the direct and indirect interference radiation from beam source 108 dominates the signal from source 114 or source 120 that the detector is supposed to be measuring, raising it to a level 224 much greater than the lower level 226 during the intervals between pulses, which is due mostly to source 114 or 120. Even integrating over time, the contribution of the interference radiation may dominate the contribution from source 114 or source 120, or at least may be comparable to it. If the detector were equally sensitive to all radiation it receives, then the interference might substantially degrade images that are produced from data the detector generates, or substantially degrade the accuracy of tracking based on data that the detector generates. The bottom plot in FIG. 2 schematically shows the actual response 228 of the detector to received radiation, as a function of time 204. In the intervals between pulses, when the detector has a high bias voltage 212, the detector response is at a level 230, due to the radiation from source 114 or 120 that the detector is designed to detect. At the beginning of each pulse, when the received radiation jumps up, the response may momentarily jump up to a still higher level 232, before the bias voltage has time to fall to level 214, because the rate at which the bias voltage can fall may be limited by the response time of the circuit, or there may be a short time delay before controller 126 receives a trigger signal indicating that the pulse has begun. This might be true, for example, if controller 126 relies on the detector itself to provide the trigger signal. Once the bias level falls sufficiently low that the sensitivity of the detector is substantially reduced, however, the response of the detector will fall to a level 234 which is optionally even lower than level 230, or at least is much lower, relative to level 230, than level 224 is relative to level 226. As a result, the integrated contribution of interference radiation from the beam to detector response 228 is less, sometimes much less, than the integrated contribution of radiation from source 114 or 120 to detector response 228. For example, the integrated contribution of interference radiation is at least 2 times less, or at least 5 times less, or at least 10 times less, or at least 20 times less, or at least 50 times less, than the integrated contribution from source 114 or 120. Images that are produced, or tracking that is done, using detector response 228 will then not be so degraded by interference radiation, and optionally are substantially not degraded at all. It should be noted that the detector response 228 shown in FIG. 2 is a measure of radiation detected per unit time, integrated over a long enough time so that many photons (in the case of x-ray or gamma-ray sources) would be detected in an integration time, and there would be relatively small statistical fluctuations in the number of photons or the total energy of photons detected in an integration time. In some embodiments of the invention, the detector has a long enough integration time so that, at the level of radiation it is exposed to from source 114 or 120, it generates output data directly that look similar to detector response 228 in FIG. 2. This mode of operation of a detector is called “current mode,” because the detector produces an output current that is proportional to the photon power received. In other embodiments of the invention, a shorter integration time is used, shorter than an average time between photons detected, and the direct output of the detector as a function of time looks like a series of individual narrow peaks, each corresponding to the detection of one photon, with the height of each peak proportional to the energy of the photon. This mode of operation of a detector is called “pulse mode.” The detector may count the narrow peaks to produce a digital output signal proportional to the number of photons received per unit time. In this case, detector response 228 as plotted in FIG. 2 may be understood as a plot of an average number of photons per second, or an average energy of photons detected per second, found by integrating such a series of many individual peaks over time. These remarks apply as well to the plots of detector response as a function of time in FIGS. 3, 4, and 5. In some embodiments of the invention, instead of or in addition to lowering the bias voltage of the detector during a beam pulse, controller 126 may decrease the voltage to digital counting circuitry of a detector operating in pulse mode, during the beam pulses, so that no photons are counted during the beam pulses, even though the detector is detecting photons. This option may be used as well in FIG. 6. In some embodiments of the invention, rather than lowering the bias voltage, or decreasing the voltage to digital counting circuitry, only during the beam pulses, controller 126 lowers the bias voltage, or the voltage to the counting circuitry, for longer periods of time each including the time of a beam pulse. These longer periods of time may extend to several times as long as the decay time of the detector response, following the beam pulse, even if the decay time is comparable to or longer than the time of each beam pulse. In such a case, keeping the detector insensitive to radiation for at least a few times the decay time prevents the detector from producing a delayed high level response to the radiation received directly or indirectly from the beam, after the beam pulse. This option may be used as well for FIG. 6. However, optionally the detector is insensitive to radiation for a time much shorter than the interval between pulses. This allows the detector to detect radiation most of the time, and to detect most of the radiation it receives from the monitoring source. FIG. 3 schematically shows a plot 300 of the detector response 302 as a function of time 204, for the same arrangement of beam pulses as a function of time, and received radiation as a function of time, as is shown in FIG. 2. The detector in FIG. 3 does not have a changing bias voltage, but has a relatively low saturation level, lower than the high level of received radiation during the pulses, but higher than the level of received radiation between pulses. Photon detectors generally operate by discharging a charge, typically proportional to the photon energy, whenever they detect a photon, which reduces the bias voltage until the charge can be replaced. If the rate at which the charge can be replaced is limited, for example by an impedance in series with the source of the bias voltage, then the response of the detector will saturate at a current equal to the maximum current with which the detector can be recharged. In FIG. 3, the response is at a level 304, below saturation, in the intervals 208 between beam pulses, and is at a somewhat higher level 306, at the saturation level, during beam pulses 206, when the amount of radiation received by the detector is much greater. The integrated detector response during the beam pulses, however, is small compared to the integrated detector response between beam pulses, because the beam has a small duty cycle of pulses. As a result, interference from the beam contributes only a small amount, for example less than 20%, 10%, 5%, 2% or 1%, to the integrated detector response, and to the data record based on it, which is used for imaging or tracking. The detector response shown in FIG. 3 is the response for a non-paralyzing detector. Such a detector will respond at the saturation level when it receives radiation above the saturation level, because radiation received during the “dead time,” when the detector has not yet recovered its bias voltage, do not prolong the dead time. FIG. 4 shows a plot 400 of the response 402 of a paralyzing detector as a function of time 204, for the same beam intensity, and received radiation, as a function of time, as shown in FIG. 2. A paralyzing detector does not respond at all if it receives radiation above the saturation level, since radiation received during the dead time can restart the dead time, prolonging recovery. In FIG. 4, the detector response 402 is at a level 404, below the saturation level, during the intervals 208 between pulses, but during the pulses 206 the detector response is zero. Due to any finite rise and fall times of the beam pulses, as well as the finite decay time of the detector response, the detector may rise somewhat at the beginning of each pulse, before falling to zero, and may take some time to return to level 404 after the beam pulse, as shown in FIG. 4. However, if the saturation level is not too far above level 404, then the small rise at the beginning of each pulse will not contribute very much to the integrated detector response. And if the decay time and pulse length are short compared to the interval between pulses, then the detector will not miss more than a small fraction of the radiation in the interval between pulses, due to the finite rise time in the detector response after the pulse. It should be noted that “paralyzing” and “non-paralyzing” detectors are abstract models of detector behavior, and in practice most types of detectors fall somewhere in between these two extremes. FIG. 5 schematically shows a plot 500 of a data record of a detector response 502 as a function of time 204, for the same beam pulses 206, and intervals 208 between pulses, as shown in FIG. 2, and for the same level 222 of radiation received by the detector as shown in FIG. 2. In the case shown in FIG. 5, the detector may fully respond to of the radiation it receives. However, the radiation received during beam pulses is not written to the data record of the detector response, or is removed from the data record. The data record shows a level 504 during intervals 208 between pulses, but a level of zero during pulses 206. This may be done, as described above, by having controller 126 reduce the voltage to counting circuitry of the detector, during pulses, so any photons detected during beam pulses are not counted. Alternatively, controller 126 may use software which sets the data record to zero for pulse times 206, after a data record is written which includes information about the time dependence of the intensity of radiation detected in each section of the detector. The times 206 of the beam pulses may be identified by using information from beam source 108, or sensor 128, or the times of the beam pulses may be identified because they have high levels of radiation detected by the detector. FIG. 6 schematically shows plots 600 of beam intensity 602 and other quantities as functions of time 204, including a bias voltage 604 for detector 118 detecting radiation from external source 114, a bias voltage 606 for detector 124 detecting radiation from source 120 inside the patient's body, and an intensity 608 of radiation from source 114. As in FIGS. 2-5, the beam intensity has pulse times 610 separated by intervals 612 between pulses. Also as in FIG. 2, the bias voltages of both detector 118 and detector 124 are lowered during the beam pulse times 610, optionally using any of the methods for doing this described above for FIG. 2. But in FIG. 6, unlike in FIG. 2, there is a time division between a period 614 when detector 118 operates, and a period 616 when detector 124 operates, in order to avoid interference from radiation source 114 on detector 124. Radiation source 114 produces radiation during period 614, but does not produce radiation, or produces radiation at a low level, during period 616. The time-averaged level of radiation produced by source 114 may be set by a trade-off between quality of images produced by detector 118, and possible dangers of radiation exposure to the patient. If so, then the same optimal average level of radiation from source 114 may be achieved to using a higher level of radiation with a lower duty cycle. It may be advantageous to use a relatively low duty cycle for radiation source 114 and detector 118, well below 50%, and to use a relatively high duty cycle for detector 124, not too far below 100%, because using a lower duty cycle will reduce the quality of imaging or tracking by detector 124, while using a lower duty cycle will not adversely affect the quality of imaging by detector 118. However any duty cycles may be used. Although source 120, inside the body, is not turned off during period 614 when detector 118 is on, any interference from source 120 can be reduced by using a lower duty cycle and correspondingly higher intensity for source 114. In any case, source 120, being implanted inside the body and producing radiation isotropically, is likely to produce very little interference in detector 118, compared to the radiation received from source 114, which is directed at detector 118, and which can safely be of higher intensity than radiation source 120 because it is only used for a limited time. Periods 614 and 616 need not be longer than intervals 612 between beam pulses, as shown in FIG. 6, but could be shorter than intervals 612, and each interval 612 could, for example, be divided into an interval 614 and an interval 616. Periods 614 and 616 may also be much longer than intervals 612, for example between 10 milliseconds and 100 milliseconds, or longer than 100 milliseconds. However, it may be advantageous not to make these periods so long that the patient, or a part of the patient's body, could move significantly, for example a distance comparable to the diameter of the beam, within one of these periods, because then the detector that is not operating will not be able to detect the motion. It should be noted that, for any of the embodiments in which controller 126 actively controls the detector, by changing its bias voltage, or affecting its ability to record data, or in which controller 126 fails to record data or removes data based on the timing of the detection of the radiation, the corresponding control circuitry and/or timing circuitry in controller 126 is optionally fast enough to respond in much less than the interval between pulses, and optionally in less than a pulse length. If the detector system, in these embodiments, is adapted to monitor a given radiation treatment system, this means that it has control and/or timing electronics with the appropriate speed, depending on the pulse length and interval between pulses of that radiation treatment system. But this need not be true in embodiments where saturation of the detector is used to reduce detection of radiation during beam pulses, although in those embodiments it is still advantageous for the detector to have a short enough decay time. For example, the detector system is capable of selectively excluding data for radiation received over a time period as short as 100 milliseconds, or 10 milliseconds, or 1 millisecond, or 100 microseconds; or 10 microseconds, as opposed to radiation received outside this time period. In some embodiments of the invention, this is accomplished by actively controlling the detector to be less sensitive or not to record data during that time period, or by timing the radiation received and removing data corresponding to that time period. In these embodiments, the control circuitry, including the timing circuitry, is fast enough to accomplish this. In other embodiments of the invention, this is accomplished by having a detector that saturates at a low enough level. In all these embodiments, the detector optionally has a decay time shorter than the time period during which radiation is excluded. Optionally, in any of the radiation treatment systems described above, the decreased contribution to the detector data record of radiation during the beam pulses is such that this contribution of the direct and indirect radiation from the beam pulses is at least 2 times less, or at least 5 times less, or at least 10 times less, or at least 20 times less, or at least 50 times less, than the contribution that radiation from the beam pulses make to the total radiation received by the detector, for at least one detector used in an imaging or tracking system. Optionally, the contribution to the detector data record of radiation during the beam pulses is sufficiently low that interference from beam radiation is at least a factor of 2 less, or at least a factor of 5 less, or at least a factor of 10 less, or at least a factor of 20 less, or at least a factor of 50 less, than the contribution of radiation from the radiation source, external or internal, that is designed to be used with that detector for imaging or tracking. Optionally, any remaining interference from beam radiation is sufficiently small that it substantially does not affect the quality of images produced, for example the SNR, or the accuracy of tracking, for a given acquisition time. In some implementations, the invention can be used with existing beam therapy systems, without the need to modify the beam source. In some embodiments of the invention, imaging with an external radiation source 114, and imaging or tracking with an internal radiation source 120, are used on a patient, even without using a beam for radiation treatment. In these embodiments, periods 614 when source 114 and corresponding detector 118 are operating, are alternated with periods 616 when source 114 and detector 118 are not operating, while detector 124 for source 120 is operating. This reduces or eliminates interference of source 114 on detector 124, and, for a given average power of source 114, reduces interference of source 120 on detector 118. For the reasons given above, it may be advantageous to make periods 614 much shorter than periods 616. Although the embodiments of the invention shown in FIGS. 1-6 all involve patients receiving medical treatment with the radiation beam, similar methods are optionally used in industrial processes where an inanimate body is being treated by a radiation beam, and is being monitored by an imaging or tracking system using another source of radiation, and a radiation detector, to avoid or reduce interference of the beam on the detector. It is expected that during the life of a patent maturing from this application many relevant radiation beams and detectors will be developed and the scope of the terms radiation beam and radiation detector is intended to include all such new technologies a priori. As used herein the term “about” refers to ±10%. The terms “comprises”, “comprising”, “includes”, “including”, “having” and their conjugates mean “including but not limited to”. This term encompasses the terms “consisting of” and “consisting essentially of”. The phrase “consisting essentially of” means that the composition or method may include additional ingredients and/or steps, but only if the additional ingredients and/or steps do not materially alter the basic and novel characteristics of the claimed composition or method. As used herein, the singular form “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a compound” or “at least one compound” may include a plurality of compounds, including mixtures thereof. Throughout this application, various embodiments of this invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range. Whenever a numerical range is indicated herein, it is meant to include any cited numeral (fractional or integral) within the indicated range. The phrases “ranging/ranges between” a first indicate number and a second indicate number and “ranging/ranges from” a first indicate number “to” a second indicate number are used herein interchangeably and are meant to include the first and second indicated numbers and all the fractional and integral numerals therebetween. As used herein, the term “treating” includes abrogating, substantially inhibiting, slowing or reversing the progression of a condition, substantially ameliorating clinical or aesthetical symptoms of a condition or substantially preventing the appearance of clinical or aesthetical symptoms of a condition. It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. To the extent that section headings are used, they should not be construed as necessarily limiting.
	abstract	A material is exposed to a neutron flux by distributing it in a neutron-diffusing medium surrounding a neutron source. The diffusing medium is transparent to neutrons and so arranged that neutron scattering substantially enhances the neutron flux to which the material is exposed. Such enhanced neutron exposure may be used to produce useful radio-isotopes, in particular for medical applications, from the transmutation of readily-available isotopes included in the exposed material. It may also be used to efficiently transmute long-lived radioactive wastes, such as those recovered from spent nuclear fuel. The use of heavy elements, such as lead and/or bismuth, as the diffusing medium is particularly of interest, since it results in a slowly decreasing scan through the neutron energy spectrum, thereby permitting very efficient resonant neutron capture in the exposed material.
	claims	1. An apparatus for determining residual energy of positively charged particles after passing through a patient, comprising:a multi-layer detector, comprising;a first layer comprising a first scintillation material, said first scintillation material, responsive to passage of the positively charged particles, emitting first secondary photons over a first wavelength range;a second layer comprising a second scintillation material, said second scintillation material, responsive to passage of the positively charged particles, emitting second secondary photons over a second wavelength range, the first scintillation material differing from the second scintillation material;a third layer comprising a third scintillation material, said third scintillation material, responsive to passage of the positively charged particles, emitting third secondary photons over a third wavelength range, said third scintillation material differing from both said first scintillation material and said second scintillation material; andorientation of said first layer, said second layer, and said third layer within thirty degrees of orthogonal to a path of the positively charge particles, said second layer positioned between said first layer and said third layer, a front surface of said second layer within ten centimeters of said first layer, a back surface of said second layer within ten centimeters of said third layer. 2. The apparatus of claim 1, said multi-layer detector further comprising:a first sub-stack of scintillation materials comprising said first layer, said second layer, and said third layer; anda second sub-stack comprising a manufactured copy of said first sub-stack. 3. The apparatus of claim 2, said multi-layer detector further comprising:at least ten layers of scintillation materials, said at least ten layers of scintillation materials comprising:said first sub-stack; andsaid second sub-stack. 4. The apparatus of claim 1, further comprising:an imaging system configured to use output from said multi-layer detector to generate an image of a tumor of the patient. 5. A multi-layer detector, comprising:a first layer comprising a first scintillation material, said first scintillation material, responsive to passage of the positively charged particles, emitting first secondary photons over a first wavelength range; anda second layer comprising a second scintillation material, said second scintillation material, responsive to passage of the positively charged particles, emitting second secondary photons over a second wavelength range, the first scintillation material differing from the second scintillation material,wherein said first scintillation material further comprises a first responsivity to the positively charged particles at least twice as responsive, in terms of number of emitted photons per unit energy released from the positively charged particles, as said second scintillation material. 6. The apparatus of claim 5, further comprising:an accelerator configured to generated the positively charged particles;a beam transport system configured to transport the positively charged particles from said accelerator, over a patient positioning system, and into said multi-layer detector. 7. The apparatus of claim 5, further comprising:an imaging system configured to use output from said multi-layer detector to generate an image of the patient. 8. A method for determining residual energy of positively charged particles after passing through a patient, comprising the steps of:passing the positively charged particles into a multi-layer detector element;detecting first secondary photons, resultant from passage of the positively charged particles, over a first wavelength range from a first layer of said multi-layer detector, said first layer comprising a first scintillation material; anddetecting second secondary photons, resultant from passage of the positively charged particles, over a second wavelength range from a second layer of said multi-layer detector element, the first wavelength range differing from the second wavelength range; andgenerating a set of response signals, each of at least six individual members of said set of response signals relating to a corresponding layer of a set of at least six layers of said multi-layer detection element. 9. The method of claim 8, further comprising the step of:detecting third secondary photons, resultant from passage of the positively charged particles, over a third wavelength range from a third layer of said multi-layer detector element, a third mean wavelength of the third wavelength range differing from both a first mean wavelength of the first wavelength range and a second mean wavelength of the second wavelength range by at least ten nanometers. 10. The method of claim 8, further comprising the step of:generating said set of response signals using two or three distinct scintillation material types.
	abstract	Device and method for checking fuel rods with IFBA, their zirconium diboride coating. The device includes a variable magnetic field generator and a magnetic field pickup device, arranged in the vicinity of the rod, as well as a control system for comparing both fields in order to measure the electric conductivity of the rod. The method includes the steps of: arranging the rod to be measured between the generator and the pickup device; generation of a variable magnetic field in the generator; picking-up of the magnetic field; comparison between the generated magnetic field and the picked-up one in order to quantify the electric conductivity of the rod; if the electric conductivity differs from a reference value, consider the rod for checking or recycling.
	description	The present application claims the benefit of U.S. Provisional Application Ser. No. 62/080,560, filed Nov. 17, 2014, which is incorporated by reference herein in its entirety, including any figures, tables, and drawings. Computed Tomography (CT) is a mainstay in diagnostic imaging. It plays key roles in disease detection and characterization, and also in patient follow-up during and after treatment. Biological soft tissue consists mainly of light elements, and its density is nearly uniform with little variation. Conventional attenuation-based X-ray imaging cannot provide sufficient contrast for biological soft tissue. X-ray phase imaging is sensitive to structural variation of soft tissue and offers superior contrast resolution for characterization of cancerous and other diseased tissues. The cross-section of an X-ray phase shift image is a thousand times greater than that of X-ray attenuation in soft tissue over the diagnostic energy range. This implies that phase imaging can achieve a much higher signal-to-noise ratio and substantially lower radiation dose than attenuation-based X-ray imaging. Grating interferometry is a state of the art X-ray imaging approach, which can simultaneously acquire information of X-ray phase-contrast, dark-field, and linear attenuation. Conventional grating interferometers often use flat gratings, with serious limitations in the field of view and the flux of photons. The subject invention provides novel and advantageous systems and methods for X-ray phase-contrast imaging (PCI), including the use of one or more period-varying or quasi-periodic gratings. An X-ray PCI system can include a phase grating that is period-varying or quasi-periodic and can be positioned between an object being imaged and a detector. A second grating, such as an absorption grating or an analyzer grating can also be present and disposed between the phase grating and the detector. One or both of the gratings (phase grating and absorption or analyzer grating) can be curved, such as a cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere) grating. Alternatively, one or both of the gratings (phase grating and absorption or analyzer grating) can be flat. The detector array can be curved or flat. For example, the detector can be a flat detector array. The subject invention also provides second-order approximation models for X-ray phase retrieval, for example using paraxial Fresnel-Kirchhoff diffraction theory. An iterative method can be used to reconstruct a phase-contrast image and/or a dark-field image. The models can be iteratively solved using the algebraic reconstruction technique (ART). State of the art compressive sensing techniques can be incorporated to achieve high quality image reconstruction. In an embodiment, an X-ray PCI system can include an X-ray source, a detector, a first grating disposed between the energy source and the detector, and a second grating disposed between the first grating and the detector, wherein the first grating is a quasi-periodical phase grating such that the period of the grating varies across the grating. In another embodiment, a method of performing X-ray PCI can include providing an X-ray PCI system as described herein, positioning an object to be imaged between the X-ray source and the first grating, and imaging the object using the system. In another embodiment, a method of reconstructing or retrieving an X-ray phase image can include approximating the image with a second-order approximation model using paraxial Fresnel-Kirchhoff diffraction theory. The subject invention provides novel and advantageous systems and methods for X-ray phase-contrast imaging (PCI), including the use of one or more period-varying or quasi-periodic gratings. An X-ray PCI system can include a phase grating that is period-varying or quasi-periodic and can be positioned between an object being imaged and a detector. A second grating, such as an absorption grating or an analyzer grating can also be present and disposed between the phase grating and the detector. One or both of the gratings (phase grating and absorption or analyzer grating) can be curved, such as a cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere) grating. Alternatively, one or both of the gratings (phase grating and absorption or analyzer grating) can be flat. The detector array can be curved or flat. For example, the detector can be a flat detector array. In an embodiment, a system for imaging (e.g., X-ray imaging such as X-ray PCI) can include an energy source (e.g., an X-ray source), a detector, a first grating that is a period-varying or quasi-periodical phase grating positioned between the energy source and the detector, and a second grating positioned between the first grating and the detector. The second grating can be an absorption grating or an analyzer grating. Each of the first and second gratings can be curved, such as a cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere) grating or flat. The object to be imaged would be positioned between the energy source and the first grating. The system can optionally include a source grating positioned near the energy source, wherein the object to be imaged would be positioned between the source grating and the first grating. The source grating can be curved, such as a cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere) grating or flat. In an embodiment, the detector is a flat detector, such as a flat detector array. The first grating can be curved, such as a cylindrical or spherical grating. The second grating can be flat to match the curvature (or lack thereof) of the detector. The optional source grating, if present, is curved, such as a cylindrical or spherical grating. The period-varying or quasi-periodical grating (e.g., period-varying or quasi-periodical phase grating) can have a predetermined or set period at the lateral point where the energy source is centered on the grating. The period can then change as the position on the grating is moved outwardly away along the surface of grating. In an embodiment, the period can change equally in both directions, such that the period at a given angle from the center point is the same as the negative of that given angle from the center point (e.g., as shown in FIGS. 3A and 11C). Embodiments of the subject invention also provide spherical and/or cylindrical grating interferometers, and Talbot self-imaging with gratings of a slow-varying-period. Image quality can be improved and field of view (FOV) can be enlarged. Imaging systems of the subject invention allow perpendicular incidence of X-rays on one or more of the gratings, giving higher photon flux and visibility, and a larger FOV. Imaging can be performed on larger sized objects than with a conventional interferometer with flat gratings. Imaging systems and methods of the subject invention have great utility for many preclinical and clinical applications. The subject invention also provides second-order approximation models for X-ray phase retrieval, for example using paraxial Fresnel-Kirchhoff diffraction theory. An iterative method can be used to reconstruct a phase-contrast image and/or a dark-field image. The models can be iteratively solved using the algebraic reconstruction technique (ART). State of the art compressive sensing techniques can be incorporated to achieve high quality image reconstruction. In an embodiment, an iterative method can be used for phase reconstruction (e.g., based on Equation (42) below), and the method can include Equation (46) below. Equation (46) can be discretized to a system of linear equations with respect to variables pk(r). Then, the ART can be employed for phase image reconstruction. The second-order phase approximation models for X-ray phase retrieval, and iterative algorithms, can be used for both in-line phase-contrast and dark-field imaging. These models accurately establish quantitative correspondence between phase shifts and image intensities, outperforming the related art linear phase approximation model used for X-ray in-line phase contrast imaging, because the linear model assumes slow phase variation and weak linear attenuation of an object. The second-order approximation models disclosed herein can accurately establish a quantitative correspondence between phases and recorded intensity images, outperforming the related art linear phase approximation models used in the conventional methods of X-ray in-line PCI. X-ray PCI is fundamentally different from conventional X-ray imaging and exhibits superior contrast resolution in various basic and applied areas. PCI relies on phase-shifts of X-ray waves, instead of their amplitude changes. In principle, the X-ray PCI is much more sensitive than attenuation-based imaging in the cases of weak-absorption materials. PCI can be used in many areas, including clinical applications. Three main types of X-ray phase retrieval for PCI are the propagation-based method, the analyzer-based method, and the grating-based method. Among these, the grating-based method has particularly good cost-effectiveness and established performance. Grating-based PCI has been used extensively in, for example, studying breast tissues and joint structures, and even with prototype machines. The grating-based PCI method has been implemented with synchrotron radiation and has been extended to work under laboratory conditions by adding a third grating in certain situations. Grating-based PCI is based on the Talbot self-imaging principle, and it utilizes a near-field diffraction effect. That is, when coherent light passes through a periodic grating, the wave pattern right after the grating repeats itself at multiple distances. Based on the Talbot effect, phase shifts of incident X-ray waves induced by an object in the wave field can be extracted by comparing the grating self-imaging patterns with and without the object. The phase-stepping method is often used for data collection in such a grating interferometer. Grating-based PCI can also be used for small field-of-view (FOV) imaging. However, it can sometimes be difficult to increase the FOV with conventional systems, due to several issues related to not only grating fabrication but also physical modeling. Theoretically speaking, the beam divergence cannot be neglected in the case of a large beam angle for which the paraxial approximation is no longer valid. The subject invention addresses this problem by utilizing the Talbot self-imaging method with one or more quasi-periodic gratings. This is effective for many applications, including performing user-defined self-imaging and being part of an optimal system design for grating-based PCI with a large FOV. X-ray imaging machines are widely used in pre-clinical and clinical examinations. Such machines use X-ray tubes and X-ray detectors, integrated with electrical and mechanical parts. According to the detector type, they can be classified into flat- and curved-detector systems. FIG. 1A shows a schematic of a curved-detector-based X-ray imaging system, and FIG. 1B shows a schematic of a flat-detector-based X-ray imaging system. Much of the derivation presented herein focuses on fan-beam imaging geometry (FIG. 1A) and not the cone-beam counterpart (FIG. 1B), but the fan-beam principles can be extended to the cone-beam case. The imaging plane can be considered where X-rays stay, and are characterized by an angle λ with respect to a central line on the imaging plane. In the curved-detector system, as shown in FIG. 1A, the distance between an X-ray source and each detector cell is a constant, D=D0. In the flat-detector system, as shown in FIG. 1B, the distance is described as a parabolic functionDi=√{square root over (D02+(iΔx)2)}, (1)where i=0, ±1, ±2, . . . , indexing the positions of detector cells, and Δx is the cell pitch. Approximated by the Taylor series, Equation (1) can be written as D i ≈ D 0 + 1 2 ⁢ ⁢ D 0 ⁢ ( i ⁢ ⁢ Δ x ) 2 . ( 2 ) The Talbot self-imaging phenomenon is understood in the art. Let a periodic grating be infinite long along the x-axis: t ⁡ ( x ) = ∑ i = - ∞ + ∞ ⁢ ⁢ g ⁡ ( x - ip ) = g ⁡ ( x ) ⊗ ∑ i = - ∞ + ∞ ⁢ δ ⁡ ( x - ip ) ( 3 ) where i=0, ±1, ±2, . . . , is a convolution operator, p is the grating period, and g(x) gives the complex amplitude at individual grating apertures. If the case of a fully coherent parallel beam with a wavelength λ is considered, the wave field, wt(x), right after the grating, is described by t(x) (Equation (3)). Then, the Fourier transform of wt(x) is W ⁡ ( k x ) = FT ⁢ { w t ⁡ ( x ) } = FT ⁢ { t ⁡ ( x ) } ∝ ∑ j = - ∞ + ∞ ⁢ G ⁡ ( k x ) · δ ⁡ ( k x - jK 0 ) = ∑ j = - ∞ + ∞ ⁢ G ⁡ ( jK 0 ) ( 4 ) in which FT denotes the Fourier transform, j=0, ±1, ±2, . . . , G(kx)=FT{g(x)}, and K 0 = 2 ⁢ ⁢ π p . With the X-ray propagation after the grating, the diffraction phenomenon is expressed as by the Fresnel approximationwi,d(x)=wt(x)Fd(x) (5)in which d specifies the propagation distance, and F(x) is the Fresnel operator defined by F d ⁡ ( x ) = exp ⁡ ( i ⁢ 2 ⁢ ⁢ π λ ⁢ d ) i ⁢ ⁢ λ ⁢ ⁢ d ⁢ exp ⁡ ( i ⁢ ⁢ π λ ⁢ ⁢ d ⁢ x 2 ) ( 6 ) The Fourier transform of Equation (5) can be found to be W d ⁡ ( k x ) = FT ⁢ { w t , d ⁡ ( x ) } ∝ exp ⁡ ( i ⁢ λ ⁢ ⁢ d 4 ⁢ ⁢ π ⁢ k x 2 ) ⁢ ∑ j = - ∞ + ∞ ⁢ G ⁡ ( jK 0 ) ≈ ∑ j = - ∞ + ∞ ⁢ exp ⁡ ( i ⁢ λ ⁢ ⁢ d 4 ⁢ ⁢ π ⁢ ( jK 0 ) 2 ) ⁢ G ⁡ ( jK 0 ) = ∑ j = - ∞ + ∞ ⁢ exp ⁡ ( i ⁢ λ ⁢ ⁢ d ⁢ ⁢ π p 2 ⁢ j 2 ) ⁢ G ⁡ ( jK 0 ) ( 7 ) According to Equation (7), when d = d T = 2 ⁢ ⁢ n ⁢ p 2 λ ,where n=1, 2, 3 . . . , a similar wave front is formed, and dT is called the Talbot distance. Further, the same intensity pattern can be repeated at the distance d = m ⁢ p 2 8 ⁢ ⁢ λ ,where m is an odd integer. In the case of fan-beam illumination, as shown in FIG. 1B, the wave front at the grating position is w, and its wave phase is distributed as a quadratic function w ⁡ ( x ) = exp ⁡ ( i ⁢ ⁢ α ⁢ 2 ⁢ ⁢ π λ ⁢ x 2 ) ,in which α defines the curvature of the quadratic phase profile, determined by the distance between the X-ray source and the grating position D, α = 1 2 ⁢ ⁢ D ,as derived from Equations (1) and (2). Thus, the wave complex amplitude wt(x) immediately after the phase grating is w t fan ⁡ ( x ) = w ⁡ ( x ) · t ⁡ ( x ) = w ⁡ ( x ) ⁢ ∑ i = - ∞ + ∞ ⁢ g ⁡ ( x - ip ) ≈ ∑ i = - ∞ + ∞ ⁢ w ⁡ ( ip ) ⁢ g ⁡ ( x - ip ) ( 8 ) According to Equation (5), the Fourier transform of the wave front after a short propagation distance d is w d fan ⁡ ( k x ) = ⁢ FT ⁢ { w t , d fan ⁡ ( x ) } ∝ ⁢ ∑ j = - ∞ + ∞ ⁢ exp ⁡ ( i ⁢ λ ⁢ ⁢ d ⁢ ⁢ π p 2 ⁢ j 2 ) ⁢ exp ⁡ ( i ⁢ λ ⁢ ⁢ D ⁢ ⁢ π p 2 ⁢ j 2 ) ⁢ G ⁡ ( jK 0 ) = ⁢ ∑ j = - ∞ + ∞ ⁢ exp ⁡ ( i ⁢ λ ⁡ ( d + D ) ⁢ π p 2 ⁢ j 2 ) ⁢ G ⁡ ( jK 0 ) ( 9 ) With the magnification effect of fan-beam geometry, the fractional Talbot distance is d = m ⁢ p 2 8 ⁢ ⁢ λ ⁢ D + d D ( 10 ) which is the counter part of the Talbot lengths in the parallel-beam case, with the magnification factor (D+d)/D. FIG. 2A shows a representation of flat-grating-based fan-beam Talbot self-imaging. FIG. 2B shows a representation of curved-grating-based fan-beam Talbot self-imaging. Talbot self-imaging is equivalent to parallel-beam imaging with an appropriate phase initialization. The magnification factor (M) due to fan-beam geometry should be considered. Comparing Equations (7) and (9), it is clear that the behavior of fan-beam Talbot self-imaging is similar to that of the parallel-beam case, except that there is a quadratically-distributed initial phase at the grating position in the fan-beam case. Hence, as shown in FIG. 2A, flat-grating-based fan-beam Talbot imaging is equivalent to the parallel-beam situation by setting the initial phase distribution and taking the fan-beam magnification. In addition, with curved-grating-based fan-beam Talbot imaging, the distance between the X-ray source and each grating aperture is constant, and only the system magnification is needed, as shown in FIG. 2B. The Talbot effect is based on periodic gratings, which repeats the same intensity pattern at different distances after the grating. Embodiments of the subject invention can utilize Talbot self-imaging with quasi-periodic period gratings. This can result in more freedom in designing the intensity profile for phase-contrast imaging. A quasi-periodic grating can be defined as t ′ ⁡ ( x ) = ∑ i = - ∞ + ∞ ⁢ g p ′ ⁡ ( i ) ⁡ ( x - X ⁡ ( i ) ) ( 11 ) where X(i) denotes the grating aperture position along the x-axis, indexed by i, and p′(i) is the width of the corresponding grating aperture. A grating with a slow-varying period can be approximated by a composition of periodic gratings, written as t ′ ⁡ ( x ) ≈ ⁢ 1 2 ⁢ ⁢ N ⁢ ∑ i = - ∞ + ∞ ⁢ ⁢ ∑ i L = i - N i + N ⁢ ⁢ g p ′ ⁡ ( i ) ⁡ ( x - X ⁡ ( i ) ) = ⁢ 1 2 ⁢ ⁢ N ⁢ ∑ i = - ∞ + ∞ ⁢ ( g p ′ ⁡ ( i ) ⁡ ( x ) ⊗ ∑ p L = - N + N ⁢ ⁢ δ ⁡ ( x - X ⁡ ( i ) - i L ⁢ p ′ ⁡ ( i ) ) ) = ⁢ 1 2 ⁢ ⁢ N ⁢ ∑ i = - ∞ + ∞ ⁢ ( g p ′ ⁡ ( i ) ⁡ ( x ) ⊗ ( t trunc ⁡ ( i , 2 ⁢ ⁢ N ) ⁢ ∑ i L = - ∞ + ∞ ⁢ δ ⁡ ( x - X ⁡ ( i ) - i L ⁢ p ′ ⁡ ( i ) ) ) ) ( 12 ) where iL defines a local grating aperture location corresponding to the index i, and ttrunc(i,2N) is a grating envelope associated with the finite grating length 2N, centered at the ith-indexed position. If each individual grating aperture is short enough relative to the grating truncation length 2N, Equation (12) can be further simplified to t ′ ⁡ ( x ) ≈ 1 2 ⁢ ⁢ N ⁢ ∑ i = - ∞ + ∞ ⁢ ( g p ′ ⁡ ( i ) ⁡ ( x ) ⊗ ∑ i L = - ∞ + ∞ ⁢ δ ⁢ ( x - X ⁡ ( i ) - i L ⁢ p ′ ⁡ ( i ) ) ) ( 13 ) The accuracy of the approximation in Equation (13) can be measured as  N ⁢ ⁢ p ′ ⁡ ( i ) - ( X ⁡ ( i ) - X ⁡ ( i - N ) )  p ′ ⁡ ( i ) ( 14 ) that quantifies the maximum local linear representation error. The accuracy of this is discussed in Examples 1 and 2 based on a real imaging scenario, where the system size was much larger than the grating aperture width. Comparing Equations (3) and (13), it is found that the period-varying grating is a set of super-positioned periodic gratings, and has the same Talbot self-imaging property as that of periodic gratings. Thus, the fractional Talbot length of the period-varying grating should resemble that of the conventional periodic gratings, but the Talbot length is changed along the x-axis and determined by corresponding grating aperture size p′(i) expressed as d = m ⁢ p ′ ⁡ ( i ) 8 ⁢ ⁢ λ ( 15 ) in the parallel-beam case and d = m ⁢ D + d D ⁢ p ′ ⁡ ( i ) 8 ⁢ ⁢ λ ( 16 ) in the fan-beam case. With Equation (15) or Equation (16), it is feasible to design the self-imaging patterns for various benefits. The length between the grating and its self-imaging pattern is determined by the grating aperture size p′(i). In a grating-based PCI system, a phase grating G1 and an absorption grating G2 can be placed between an object and a detector, as shown in FIGS. 4A-4D. Curved gratings can be used as well, as shown in FIG. 1B, to match the X-ray beam divergence. Similarly, in a curved-detector system (FIG. 1A), flat and curved grating sets can be installed for phase-contrast imaging. Thus, four kinds of grating-based PCI systems include: flat grating, flat detector; curved grating, flat detector; flat grating, curved detector; and curved grating, curved detector. FIG. 4A shows a schematic view of a flat grating, flat detector system. FIG. 4B shows a schematic view of a curved grating, flat detector system. The red arrow highlights the non-uniform gaps between the absorption grating G2 and the detector. FIG. 4C shows a schematic view of a flat grating, curved detector system. The red arrow highlights the non-uniform gaps between the absorption grating G2 and the detector. FIG. 4D shows a schematic view of a curved grating, curved detector system. If the detector and the absorption grating G2 have different curvatures as indicated by the red arrows in FIGS. 4B and 4C, there are non-uniform gaps between the grating G2 and the detector. The non-uniform gap distribution can introduce non-uniform image artifacts due to the blurring effect from the X-ray spot. Therefore, the absorption grating G2 should be placed as closely as possible to the detector plane, for example as in FIGS. 4A and 4D. The designs in FIGS. 4A and 4D can also have issues due to imperfect fabrication of the gratings and/or difficulty in matching the gratings to the beam divergence. This can lead to the phase grating G1 and absorption grating G2 not being exactly paired, as shown in FIGS. 5A and 5B. FIG. 5A shows a schematic view of a grating-based phase-contrast system with a curved G1 and a flat G2 (and flat detector), and FIG. 5B shows a schematic view of a grating-based phase-contrast system with a flat G1 and a curved G2 (and curved detector). FIG. 5C shows a plot of the visibility versus angle (in degrees) for the setup of FIG. 5A, and FIG. 5D shows a plot of the visibility versus angle (in degrees) for the setup of FIG. 5B. FIG. 6A shows a plot of visibility versus angle (in degrees) for the unpaired G1-G2 system of FIG. 5A and a period-unmatched imaging system. The red line (upper line) is for the period-unmatched system, and the blue line (lower line) is for the system of FIG. 5A). FIG. 6B shows a plot of visibility versus angle (in degrees) for a period-matched imaging system. In a grating-based PCI system, a periodic phase grating can be adopted to produce the same interference patterns at a Talbot length. However, if the phase grating and absorption grating are not paired (as in FIGS. 5A and 5B), the absorption grating may not be located exactly on the interference patterns. The visibility of the stepping curve can then be degraded. FIGS. 5C and 5D show results with respect to the angle in the fan beam. In many embodiments of the subject invention, a phase grating can be a period-varying grating, and can be paired with, e.g., a flat absorption grating G2 (FIG. 5A) or a curved absorption grating G2 (FIG. 5B). The phase gratings shown in FIGS. 5A and 5B can be designed using Equation (16). The results are shown in FIG. 6A, compared with the unpaired G1-G2 system of FIG. 5A. To match the period-varying G1, the grating G2 can be accordingly modified by p G ⁢ ⁢ 2 ′ ⁡ ( i ) = p G ⁢ ⁢ 1 ′ ⁡ ( i ) 2 ⁢ D + d D . ( 17 ) FIG. 6B shows that the period-matched G1 and G2 is an optimized grating-based PCI system with a nearly-uniform visibility distribution within the fan beam range. In the related art, high-energy and large FOV PCI imaging is a major technical challenge. In the case of a large FOV, as the fan beam angle increases, the X-ray beam divergence becomes significant enough to degrade the imaging performance. A curved grating structure can thus be used in embodiments of the subject invention. Flat-panel detectors are widely used in practice due to low cost and high spatial resolution. This requires a flat absorption grating G2 to match the detector shape, avoiding the non-uniform gap between the detector and G2. This can lead to a curved phase gating G1 and flat absorption grating G2 being unpaired, as shown in FIG. 5A. Similar issues exist for a curved-detector imaging system, as shown in FIG. 5B. To solve this problem, embodiments of the subject invention advantageously use one or more period-varying gratings (e.g., a Talbot self-imaging method with a period-varying grating). By modifying the individual grating aperture width, the Talbot self-imaging lengths can be determined to match the spatial non-uniformity between G1 and G2, allowing the G2 to be placed exactly (or nearly exactly) on the self-imaging pattern of G1. The performance of a grating design of the subject invention can be quantified, for example, by the maximum local linear representation error defined by Equation (14). To make the error less than 5%, in an imaging system used in practice (e.g., a 4.8 μm G1 period and 1 m source-to-G1 distance), the value of N in Equation (14) should be about 1×104, which is large enough to satisfy the approximation in Equation (13). The grating length truncation will not influence the beam decomposition and reformation (see also, e.g., FIGS. 2A and 2B). Period-varying gratings of the subject invention generate uniform visibility distribution within the fan beam range, leading to excellent imaging performance. Thus, Talbot self-imaging methods with period-varying grating(s) can facilitate system design and improve imaging performance. The phase grating G1 can be curved to match the divergence of an X-ray fan-beam, and the absorption grating G2 can be adapted to match a flat detector (e.g., FIGS. 4D and 5A). This can lead to large FOV clinically-oriented imaging systems. X-ray PCI based on grating interferometry has exhibited superior contrast in biological soft tissue imaging. The high sensitivity relies on the high-aspect ratio structures of the planar gratings, which prohibit the large field of view applications with a diverging X-ray source. Curved gratings allow a high X-ray flux for a wider angular range, but the interference fringes are only visible within ˜10° range due to the geometrical mismatch with the commonly-used flat array detectors. Embodiments of the subject invention use a curved quasi-periodic grating for large FOV imaging, even with a flat detector array. This can lead to, for example, an interference fringe pattern with less than a 10% decrease in visibility over 25°. X-ray PCI can detect small variations of the refractive index, providing superior contrast for low-elements compared with attenuation contrast, and thus is advantageous in imaging biological soft tissues, such as vessels, lesions, and micro-calcifications. FIGS. 10A and 10B show setups of grating-based PCI systems. FIG. 10A shows two flat gratings G1 and G2, and FIG. 10B shows a curved phase grating G1 and a flat analyzer grating G2. A flat panel detector can be placed immediately after G2. In the case of using an X-ray tube with low spatial coherence, another grating G0 (omitted in FIGS. 10A and 10B; see also FIG. 9A) can be used between the source and the object. During the imaging process, phase stepping of the analyzer grating G2 can be induced, and the information of X-ray attenuation, phase contrast, and small angle scattering can be acquired simultaneously. The PCI sensitivity relies on the frequency of the interference fringes. Flat gratings with high aspect ratio structures are ideal for parallel beams. However, in most non-synchrotron setups, the divergent radiation emerges from the focus of the X-ray tube; therefore, the FOV in the X-ray interferometer is limited to only a few centimeters. The use of a curved phase grating G1 allows perpendicular incidence of X-rays on the gratings, permitting a high X-ray flux with a wider acceptance angle than that associated with a flat grating. In such a setup, a curved phase grating G1 can generate a self-image on a curved analyzer grating G2, posing a geometrical mismatch with a flat panel detector. This mismatch is position-dependent and grows nonlinearly as the angle (from straight ahead from the source onto the grating) increases. As a result, the mismatch can cause a reduction of the visibility of the stepping curve at larger angles and limit the FOV of the curved grating setups. The stepping visibility is defined as the ratio between the amplitude and the mean of the phase stepping curve. FIG. 9A shows a schematic of an X-ray imaging system with spherical gratings. The system can include an X-ray source, a spherical phase grating G1, and a spherical analyzer grating G2. The object to be imaged can be positioned between the source and the phase grating G1. An optional spherical source grating G0 is also shown in FIG. 9A. FIG. 9B shows a spherical grating for 1D grating. FIG. 9C shows a spherical grating for 2D grating. The use of spherical gratings (e.g., with a common X-ray tube as a point source) can allow perpendicular incidence of X-rays on the gratings, thereby resulting in higher flux and higher visibility, and a larger field of view than a conventional interferometer with flat gratings. The source grating G0, if present, can be placed close to the source to form an aperture mask with transmission elements and can generate an array of periodically repeating sources. The phase grating G1 can act as a phase mask for the phase modulations onto the incoming wave field. Through the Talbot effect, the phase modulation can be transformed into an intensity modulation in the plane of the analyzer grating G2 to form a periodic fringe pattern. The third (analyzer) grating G2 can have the same period and orientation as the fringes created by G1 to extract X-ray phase variation induced by the object being imaged. In many embodiments, a curved quasi-periodic phase grating G1′ can be used in a PCI system or method. The grating can have a self-image that occurs on a flat plane, thereby advantageously allowing the use of a flat analyzer (or absorption grating) G2 to match the detector (i.e., flat detector) to eliminate the geometric mismatch for a larger FOW in PCI with a flat panel detector, as shown in FIG. 10B. The self-imaging effect of an infinite periodic planar structure can be formulated by geometrical ray tracing and scalar diffraction theory under paraxial approximation. An angular decomposition approach can be used to derive the Talbot distance for one dimensional (1D) curved gratings in polar coordinates. The analysis presented herein is not limited to binary transmission gratings, but also applicable to phase gratings, which are more common as gratings G1 in X-ray PCI setups. In diffraction theory, the vector field can be treated as a scalar field under two conditions: 1) the feature size of the object along the polarization direction of the field is much larger than the wavelength; 2) all components of vector field obey the same scalar wave equation. In general, the second condition is not satisfied in cylindrical coordinates. Scalar diffraction theory is applicable, though, for the grating analysis for the X-ray PCI imaging. Assuming r0>>p>>λ, the electric field components in free space diffracted by the grating G1 can be expressed as a sum of its Fourier series: E z = ∑ m ⁢ ∈ Z ⁢ ⁢ a m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 18 ) E θ = i ⁢ ⁢ η 0 k ⁢ ∑ m ⁢ ∈ Z ⁢ b m ⁢ ∂ ∂ r ⁢ H m ( 2 ) ⁡ ( kr ) ⁢ e i ⁢ ⁢ m ⁢ ⁢ θ , ( 19 ) E r = η 0 ⁢ m kr ⁢ ∑ m ⁢ ∈ Z ⁢ ⁢ b m ⁢ H m ( 2 ) ⁡ ( kr ) ⁢ e i ⁢ ⁢ m ⁢ ⁢ θ , ( 20 ) where η0=1/(ε0c), is the impedance of the free space, k=2π/λ, is the wavenumber, r>r0, Hm(2)(kr) is Hankel function of the second kind representing the outgoing wave, and m is an integer. For a curved grating with period p, m is given by 2nπr0/p, where n is an integer. In the case of an X-ray PCI setup, r0>>p>>λ. Thus, the asymptotic expansions of modulus and the phase of Hm(2)(kr) can be used: ⁢  H m ( 2 ) ⁡ ( kr )  = 2 π ⁢ ⁢ k ⁢ ⁢ r + 4 ⁢ ⁢ m 2 - 1 2 · ( 2 ⁢ ⁢ kr ) 2 ⁢ + … , ( 21 ) arg ⁢ ⁢ H m ( 2 ) ⁡ ( kr ) = ⁢ - kr + ( 1 2 ⁢ m + 1 4 ) ⁢ π - 4 ⁢ ⁢ m 2 - 1 8 ⁢ ⁢ kr + ( 4 ⁢ ⁢ m 2 - 1 ) ⁢ ( 4 ⁢ ⁢ m 2 - 9 ) 6 · ( 4 ⁢ ⁢ kr ) 3 ⁢ + … ⁢ , ( 22 ) These asymptotic expansions are valid for kr much greater than m, i.e., m/kr<<1. Substituting Equation (21) and Equation (22) into Equation (18), (19), and (20) yields the approximated expressions of the outwardly propagating electric field. Under the condition r0>>p>>λ, further approximation can be made as H′m(2)/k≈−i Hm(2). The three components of the electric field reduce to E z = ∑ m ⁢ ∈ Z ⁢ ⁢ a m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 23 ) E θ = η 0 ⁢ ∑ m ⁢ ∈ Z ⁢ b m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 24 ) E r = m kr ⁢ η 0 ∑ m ⁢ ∈ Z ⁢ ⁢ b m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 25 ) where r>r0, Hm(2)(kr) are approximated by Equation (21), and Equation (22) shows that Er decays faster along the direction of r than the other two components. Because m/kr<<1, Er<<E0. The component Er can be ignored. The other two components of the electric fields have the same form. The components of the magnetic field have similar results. Therefore, under the condition r0<<p<<λ, all the field components have the same form, and the polarization of the electric field is perpendicular to the r direction. Because the two conditions for using scalar diffraction theory are both satisfied, one scalar field u(θ, r0) can be used to represent any field component in cylindrical coordinates, u ⁡ ( θ , r ) = ∑ m ⁢ ∈ Z ⁢ c m · 1 kr ⁢ e - kr + m 2 ⁢ 4 ⁢ ⁢ m 2 - 1 8 ⁢ ⁢ kr · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 26 ) where cm is the complex Fourier coefficient. Referring to FIG. 10B, a curved grating G1, whose center is located at the origin of the coordinate system, can have a radius of r0. The point source can also be placed at the origin. The angular span of the grating can be assumed to be small in the vertical direction, so the irradiance is uniform along the z-axis. In the case of an X-ray PCI setup, the radius of grating G1 (˜1 m) is much greater than the period of G1 (˜1 μm), and the period of G1 is much greater than the wavelength (˜0.1 nm), i.e., r0>>p>>λ. Under these conditions, a scalar field, u(θ, r) representing the field after the grating G1 in cylindrical coordinates can be expressed by the sum of the angular Fourier series. u ⁢ ( θ , r ) = ∑ m = - ∞ ∞ ⁢ ⁢ c m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 27 ) where r>r0, m is an integer representing the angular frequency, k=2π/λ, is the wave number, Hm(2)(kr) is the Hankel function of second kind representing the outgoing wave, and cm is the complex coefficient. For kr>>m, the modulus and the phase of Hm(2)(kr) can be expressed asymptotically as:  H m ( 2 ) ⁡ ( kr )  = 2 π ⁢ ⁢ k ⁢ ⁢ r + O ⁡ ( 1 ( kr ) 2 ⁢ ) , ( 28 ) arg ⁢ ⁢ H m ( 2 ) ⁡ ( kr ) = ⁢ - kr ⁡ ( 1 2 ⁢ m + 1 4 ) ⁢ π - 4 ⁢ ⁢ m 2 - 1 8 ⁢ ⁢ kr + O ⁡ ( 1 ( kr ) 3 ⁢ ) . ( 29 ) The scalar field at the boundary, r=r0, can be considered, and the periodic scalar field can be also be expressed as the sum of its angular Fourier harmonics, u ⁡ ( θ , r 0 ) = ∑ m ⁢ ∈ Z ⁢ A m · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 30 ) where Am denotes the angular Fourier coefficients. Matching the angular Fourier components of Equation (27) with Equation (30) and plugging in the asymptotic expansions in Equation (28) and Equation (29), u ⁡ ( θ , r ) = r 0 r ⁢ e i ⁡ ( kr 0 - kr + 1 8 ⁢ ⁢ kr ⁢ 1 8 ⁢ ⁢ kr 0 ) · ∑ m = - ∞ ∞ ⁢ { e i ⁡ ( m 2 2 ⁢ ⁢ k ⁢ ⁢ ( 1 r 0 ⁢ ⁢ 1 r ⁢ ) ) } · A m · e i ⁢ ⁢ m ⁢ ⁢ θ . ( 31 ) Equation (31) shows that the amplitude of the field decays as 1/√r. The common phase term outside of the summation operation can be neglected, because only the phase term, exp(im2(1/r0−1/r)/2k), affects the intensity of the field. In analogy to the infinite periodic planar grating, the case where the circumference of the circle is a multiple of the grating period can be considered, and this ratio, 2πr0/p, can be considered as the fundamental angular frequency. Under this condition, the angular Fourier frequency components contain the harmonics of the fundamental angular frequency only, i.e., m=2πnr0/p, where n is an integer. When the field at radius rD, satisfies the equation 1 r D ⁢ = 1 r 0 ⁢ - l · 2 ⁢ ⁢ p 2 λ ⁢ ⁢ r 0 2 ⁢ , l = 1 , 2 , 3 ⁢ ⁢ … ⁢ , ( 32 ) the phase term, exp(im2(1/r0−1/r)/2k), is equal to a multiple of 2π for all the angular frequency m, which means that the field g(θ, rD) and g(θ, r0) have the same intensity distribution along the θ direction. Equation (32) indicates three facts of the Talbot effect of a curved periodic grating. First, the self-image is a magnified curved periodic grating sharing the same origin with the original grating G1. Second, the Talbot distance, rD−r0, has a non-linear relation with p2/λ, which is different from the case of planar periodic gratings. Third, there are only a limited number of the self-images, since l is bounded. Thus, a curved periodic grating can generate a magnified self-imaging grating on a curved plane. In a particular setup, the analyzer grating G2 is designed so that its period and curvature matches the self-image of G1. However, array detectors with matching curvature can be difficult or impractical to fabricate. Using a planar analyzer (absorption) grating G2 and flat detector, as depicted in FIG. 10B, will inevitably result in a loss of the contrast of the stepping curves at large angles if the phase grating has a constant period. In many embodiments of the subject invention, one or more quasi-periodic gratings can be used to improve the contrast of the stepping curve on a flat detector and provide one more degree of freedom in the design. Specifically, the periodicity of the curved grating can be perturbed with a slow-varying term to create a flat self-imaging plane. As shown in Equation (32), the Talbot distance depends on the grating period p. By changing the period along the direction of θ, a quasi-periodic grating can be designed with a flat self-image plane. The quasi-periodic grating with slow-varying period can be expressed as: g ⁡ ( θ , r 0 ) = ∑ i ∈ Z ⁢ ⁢ rect ⁡ ( 2 ⁢ ⁢ r 0 p i ⁢ θ ) * δ ⁡ ( θ - θ i ) , ( 33 ) where rect(•) denotes the rectangular function, * denotes the convolution operation, and δ(•) is the Dirac-delta function. The term pi denotes the period of the grating at angle θi, indexed by i. In a strictly periodic grating, pi is a constant, and θi=ipi/r0. In a quasi-periodic grating, the angle θi holds the relation with pi, θ i = 1 r 0 ⁢ ( p 0 + p i 2 + ∑ j = 1  i - 1  ⁢ ⁢ p j ) , i = 1 , 2 , 3 ⁢ ⁢ … ⁢ , ( 34 ) where p0 denotes the central period of the quasi-periodic grating G1′ at θ=0. The quasiperiodic grating can be treated as a locally periodic grating for self-imaging, when 1) the amplitude of high harmonic diffraction order is small, pi>>λ, which is satisfied in X-ray PCI setups, and 2) the period of the quasi-periodic grating varies slowly. To design a flat self-image plane, the period of the quasi-periodic grating, pi, satisfies the following equation: 2 ⁢ ⁢ p i 2 λ ⁢ ⁢ r 0 2 + 1 r 0 = ( 2 ⁢ ⁢ p 0 2 λ ⁢ ⁢ r 0 2 + 1 r 0 ) · cos ⁡ ( θ i ) , ( 35 ) Using Equation (34) and Equation (35), the period of the quasi-periodic grating G1′ can be calculated at each angle iteratively. In many embodiments, a curved quasi-periodic grating can be used for a large FOV X-ray PCI, even with a flat array detector. In comparison with using a periodic phase grating G1, a quasi-periodic grating can generate a Talbot self-imaging effect on a flat plane, and can thus increase the visibility over a larger range of view angle. Examples 3-5 demonstrate that the decrease of the fringe visibility is less than 10% within the view angle of 25°. X-ray phase imaging is sensitive to structural variation of soft tissue, and offers excellent contrast resolution for characterization of cancerous tissues. Also, the cross-section of an X-ray phase shift is a thousand times greater than that of X-ray attenuation in soft tissue over the diagnostic energy range, allowing a much higher signal-to-noise ratio at a substantially lower radiation dose than attenuation-based X-ray imaging. In an embodiment of the subject invention, a second-order approximation model can be used to reconstruct an X-ray image. The model can be with respect to phase shift based on the paraxial Fresnel-Kirchhoff diffraction theory. In-line dark-field imaging can also be performed based on the second-order model. The model can accurately establish a quantitative correspondence between phases and recorded intensity images, outperforming related art linear phase approximation models that are used in conventional methods of X-ray in-line phase-contrast imaging. In a further embodiment, the models of the subject invention can be iteratively solved using the algebraic reconstruction technique (ART). The state of the art compressive sensing techniques can be incorporated to achieve high quality image reconstruction. Numerical simulation studies presented in Examples 6 and 7 demonstrate the feasibility of the models that are more accurate and stable, and more robust against noise, than related art methods. Biological soft tissues consist mainly of light elements with low atomic numbers, and the elemental composition is nearly uniform with little density variation. Hence, the X-ray attenuation contrast cannot yield sufficient sensitivity and specificity in many biomedical scenarios. On the other hand, the cross-section of X-ray phase shift is three orders of magnitude larger than that of X-ray attenuation in soft tissues over the diagnostic energy range. Hence, X-ray PCI is also advantageous to reveal subtle features of soft tissues and offer superior contrast for analyses of various normal and diseased conditions at a reduced radiation dose. When a spatial coherent X-ray wave passes through an object, the X-ray wavefront is deformed by variation of the refractive index in an object. After coherent X-rays propagate a sufficient distance in a free space, a phase contrast is formed. In-line X-ray PCI can reconstruct phase shifts from image intensity measurements. This scheme offers unique merits in terms of simplicity and implementation. A relationship exists between the Fourier transform of a Fresnel diffraction pattern and the autocorrelation of a transmittance function, and a contrast transfer function (CTF) method can be used to reconstruct phase shifts based on an assumption of weak absorption and slow phase change. Paraxial Fresnel-Kirchhoff diffraction theory can be applied to establish a transport of intensity equation (TIE) to describe a relationship between intensity data and phase shifts. With intensity measurements at several distances along its propagation direction, the phase information can be quantitatively retrieved. An auxiliary function can transform TIE into a classical two-dimensional Poisson equation, which can be solved using a Fourier transform method, or a Green function method with suitable boundary conditions. However, TIE relates to intensity derivatives along the optic axis, which cannot be directly measured. These derivatives must be approximated using differences between intensity images on two adjacent planes, which is not in accordance with a relatively large separation between the planes to meet the phase-contrast requirement in the presence of noise. A mixed-phase retrieval method can directly combine CTF and TIE, and iteratively reconstruct phase shifts assuming a slowly varying absorption distribution. This algorithm alleviates the weak absorption assumption in CTF and the short propagation distance in TIE. To minimize the effect of measurement noise and enhance the accuracy of the conventional linear approximation model for X-ray in-line phase-contrast imaging, multiple intensity measurements at different distances from an object can be performed in the Fresnel diffraction region for reconstruction of a phase shift image. X-ray small-angle scattering, or dark-field, imaging is used to acquire photons slightly deflected from the primary beam through an object. Small-angle scattering signals reflect structural texture on length scales between 1 nm to several hundred nanometers. This imaging mode can reveal subtle texture of tissues. For example, the growth of tumors causes remarkable differences of small-angle scattering patterns from that of healthy tissues. It is clinically important that the structural variation in tumors modifies the refractive index. The propagation of X-rays in a medium is characterized by the complex index of refraction. The cross-section of X-ray phase shift is one thousand times larger than that of linear attenuation in the 20-100 keV range. This means that phase-contrast imaging has much higher sensitivity for light elements than attenuation-contrast imaging. The contrast-to-noise ratio of differential phase contrast CT images is superior to the attenuation-contrast CT counterpart. Therefore, PCI can be used to observe unique critical structures of soft biological tissues. Moreover, the refractive index of a tissue is inversely proportional to the square of the X-ray energy while the absorption coefficient decreases as the fourth power of the X-ray energy. Hence, X-ray PCI is suitable to operate at higher energies (>30 keV) for lower radiation dose than attenuation imaging. Higher energy X-ray imaging is important for studies on large animals or patients. Approximation models of the subject invention can be second-order approximation models for X-ray phase retrieval using paraxial Fresnel-Kirchhoff diffraction theory. An iterative method can be used to reconstruct a phase-contrast image (e.g., and in-line image) and a dark-field image. To describe the X-ray and matter interaction at a specific wavelength of λ, an object can be characterized by a complex-valued refractive index distribution n=1−δ+iβ, where δ is for refraction, and β is for attenuation. When a spatial coherent X-ray wave passes through an object, the X-ray wave front is deformed by variation of the refractive index in the object. δ (10−6−10−8) is about 1,000 times larger than β (10−9−10−11) in biological soft tissues over the diagnostic radiation energy range. This implies that X-ray phase contrast imaging can achieve a higher signal-to-noise ratio at a lower radiation dose than attenuation contrast imaging. The wave-object interaction process can be described with a transmittance functionT(r)=A(r)exp[iΦ(r)], (36)where r denotes transverse coordinates (x, y) in a transverse plane along the propagation direction z, A(r) is the amplitude modulus of the wave, and Φ(r) is the phase shift of the wave. The amplitude modulus and phase shift is related to the X-ray refractive index distribution A ⁡ ( r ) = exp ⁡ [ - 2 ⁢ ⁢ π λ ⁢ ∫ β ⁡ ( r , z ) ⁢ dz ] , ⁢ Φ ⁡ ( r ) = - 2 ⁢ ⁢ π λ ⁢ ∫ δ ⁡ ( r , z ) ⁢ dz ( 37 ) Based on Fresnel-Kirchhoff diffraction theory, the wave field on the transverse plane in a free space is described by the Fresnel diffraction formula ( 38 ) where [T(r)] is the Fresnel diffraction pattern of the transmittance function at a distance of z from the object, and k=2π/λ is a wavenumber. The Fourier transform of the Fresnel diffraction pattern at z can be formulated in terms of the autocorrelation of the transmittance function ∫ ∫ ⁡ [ T ⁡ ( r ) ] ⁢ ❘ 2 ⁢ exp ⁡ ( - i2 ⁢ ⁢ r · w ) ⁢ dr = ∫ ∫ { A ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) ⁢ A ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ⁢ exp ⁡ [ i ⁢ ⁢ Φ ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - i ⁢ ⁢ Φ ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ] } ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr ( 39 ) Hard X-ray radiation has a very short wavelength λ, and Φ(r) has only moderate variations for biological tissues. Thus, a second-order approximation to the exponential function of the phase in Equation (39) would be sufficiently accurate for practical applications ∫ ∫  ⁢ ⁡ [ T ⁡ ( r ) ]  2 ⁢ exp ⁡ ( - i2π ⁢ ⁢ r · w ) ⁢ dr = ∫ ∫ A ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) ⁢ A ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ⁡ [ 1 + i ⁢ ⁢ Φ ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - i ⁢ ⁢ Φ ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) - 1 2 ⁢ ( Φ ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - Φ ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ) 2 ] ⁢ exp [ - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr ( 40 ) Applying the autocorrelation formula to the parts involving A(r) and A(r)Φ(r) on the right-hand side of Equation (40) respectively, ∫ ∫ [  ⁢ ⁡ [ T ⁡ ( r ) ]  2 -  ⁢ ⁡ [ A ⁡ ( r ) ]  2 -  ⁡ [ A ⁡ ( r ) ⁢ Φ ⁡ ( r ) ]  2 ] ⁢ exp ⁡ ( - i2π ⁢ ⁢ r · w ) ⁢ dr = ∫ ∫ A ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) ⁢ A ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ⁡ [ i ⁢ ⁢ Φ ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - i ⁢ ⁢ Φ ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) - 1 2 ⁢ Φ 2 ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - 1 2 ⁢ Φ 2 ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ] ⁢ exp [ - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr ( 41 ) Changing integral variables on the right-hand side of Equation (41), ∫ ∫ [  ⁡ [ T ⁡ ( r ) ]  2 -  ⁡ [ A ⁡ ( r ) ]  2 -  ⁡ [ A ⁡ ( r ) ⁢ Φ ⁡ ( r ) ]  2 ] ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr = i ⁢ ∫ ∫ [ exp ⁡ ( - i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r + λ ⁢ ⁢ zw ) - exp ⁡ ( i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r - λ ⁢ ⁢ zw ) ] ⁢ A ⁡ ( r ) ⁢ Φ ⁡ ( r ) ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr - 1 2 ⁢ ∫ ∫ [ exp ⁡ ( - i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r + λ ⁢ ⁢ zw ) + exp ⁡ ( i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r - λ ⁢ ⁢ zw ) ] ⁢ A ⁡ ( r ) ⁢ Φ 2 ⁡ ( r ) ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr ( 42 ) During in-line phase-contrast imaging, phase contrast is manifested through a free-space propagation of a coherent X-ray beam, which transforms phase deformation on the object plane into intensity variation on an image plane. Equation (40) describes a relationship between measured intensity images and phase shifts. This model of the subject invention is plainly superior to the linear phase approximation model widely used for conventional X-ray in-line phase imaging, which assumes slow phase variation and weak linear attenuation of the object. Dark-field imaging is also feasible along this direction. A dark-field image is formed through scattering of X-rays, which is sensitive to microstructures in the object. Boundaries and interfaces in biological tissues commonly produce strong dark-field signals. Hence, X-ray in-line dark-field imaging can be important to reveal detailed critical features of tissues. When a coherent X-ray beam goes through an object, X-ray photon scattering can change spatial coherence and transmitted intensity of X-rays, inducing a variation in the transmittance functionT(r)−(Q(r)+\|ΔQ(r))exp[i(Φ(r)+ΔΦ(r))], (43)where ΔΦ(r) and ΔQ(r) are the phase and the amplitude variations on the object plane induced by scattered X-rays, respectively. Although the formulation itself in Equation (43) covers both small and large angle scattered photons, the main contributions should be from small angle scattering because the field of view for in-line imaging is typically small to satisfy the paraxial approximation assumption. The scattering effect would change the original transport path in the object, leading to an additional phase variation ΔΦ. The amplitude modulus A(r)=Q(r)+ΔQ(r) on the object plane and the intensity I on an image plane can be measured respectively. Hence, from Equation (38), I =  exp ⁡ ( i ⁢ ⁢ kz ) i ⁢ ⁢ λ ⁢ ⁢ z ⁢ ∫ ∫ ( ( Q ⁡ ( r ) + Δ ⁢ ⁢ Q ⁡ ( r ) ) ⁢ exp ⁡ [ i ⁢ ⁢ ( Φ ⁡ ( r ) + Δ ⁢ ⁢ Φ ⁡ ( r ) ) ] ) ⁢ exp ⁡ ( i ⁢ ⁢ k 2 ⁢ ⁢ z ⁢  r ′ - r  2 ) ⁢ dr ′  , ( 44 ) With a relative small variation ΔΦ(r), the first-order approximation of exp(iΔΦ(r)) can be made to reduce Equation (44) to I =  exp ⁡ ( i ⁢ ⁢ kz ) i ⁢ ⁢ λ ⁢ ⁢ z ⁢ ∫ ∫ A ⁡ ( r ) ⁢ exp ⁡ [ i ⁢ ⁢ Φ ⁡ ( r ) ] ⁢ ( 1 + i ⁢ ⁢ D ⁡ ( r ) ) ⁢ exp ⁡ ( i ⁢ ⁢ k 2 ⁢ ⁢ z ⁢  r ′ - r  2 ) ⁢ dr ′  ( 45 ) where D ⁡ ( r ) = Δ ⁢ ⁢ Q ⁡ ( r ) ⁢ Δ ⁢ ⁢ Φ ⁡ ( r ) A ⁡ ( r ) represents a dark-field image to be reconstructed. The phase image Φ(r) can be recovered using the method described herein. Hence, the dark-field image D(r) can be reconstructed from intensity measurements, based on Equation (45). In an embodiment, an iterative method can be used for phase reconstruction (e.g., based on Equation 42). The method can include Equations (46) below. i ⁢ ∫ ∫ [ exp ⁡ ( - i ⁢ ⁢ πλ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r + λ ⁢ ⁢ zw ) - exp ⁡ ( iπ ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r - λ ⁢ ⁢ zw ) ] ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ p k + 1 ⁡ ( r ) ⁢ dr = ∫ ∫ [  ⁡ [ T ⁡ ( r ) ]  2 -  ⁡ [ A ⁡ ( r ) ]  2 -  ⁡ [ p k ⁡ ( r ) ]  2 ] ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ π ⁢ ⁢ r · w ) ⁢ dr + 1 2 ⁢ ∫ ⁢ ∫ [ exp ⁡ ( - iπ ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r + λ ⁢ ⁢ zw ) A ⁡ ( r ) + exp ⁡ ( i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ⁢ ·  w  2 ) ⁢ A ⁡ ( r - λ ⁢ ⁢ zw ) A ⁡ ( r ) ] ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ p k 2 ⁡ ( r ) ⁢ dr ⁢ ⁢ ⁢ p 0 ⁡ ( r ) = 0 , p k ⁡ ( r ) = A ⁡ ( r ) ⁢ Φ k ⁡ ( r ) , k = 0 , 1 , … ⁢ , ( 46 ) where \|[T(r)]\|2 is the measurement on the Fresnel diffraction pattern at a distance z, \|[A(r)]\|2 is the measurement on the object plane, [A(r)]\|2 gives the modulus of the function pk(r). Equation (46) can be discretized to a system of linear equations with respect to variables pk(r). Then, the algebraic reconstruction technique (ART) can be employed for phase image reconstruction. ART is a widely-used iterative method for its robustness and efficiency. In the iterative scheme, the absorption term is accurate without any approximation. Hence, no limitation is imposed on the attenuation property of an object. The exponential function of phase variation has been approximated to the second order, which should be very accurate for hard X-ray imaging. Therefore, the second-order phase models of the subject invention can be very informative and useful for biomedical imaging. The second-order model can be computationally complex, and can have a high computational cost under certain circumstances. With the development of high-performance computing techniques, especially Graphics Processing Units (GPUs), any computational issues should not be a problem. X-ray in-line imaging can have higher spatial resolution than X-ray grating-based imaging. The field of view for in-line imaging ought to be small to satisfy the paraxial approximation requirement, and yet practical applications often focus on a small region of interest (ROI) in an object. Therefore, interior tomographic imaging methods would be useful to deliver unique and stable ROI reconstructions from truncated projection data. The second-order phase approximation models for X-ray phase retrieval, and iterative algorithms, can be used for both in-line phase-contrast and dark-field imaging. These models accurately establish quantitative correspondence between phase shifts and image intensities, outperforming the related art linear phase approximation model used for X-ray in-line phase contrast imaging, because the linear model assumes slow phase variation and weak linear attenuation of an object. The iterative method can incorporate the state of the art compressive sensing techniques to achieve high quality image reconstruction. In an embodiment, a system can include a machine-readable medium (e.g., a computer-readable medium) having machine-executable instructions (e.g., computer-executable instructions) for performing a method of reconstructing an image (e.g., an X-ray image), the method comprising using a second-order approximation model as described herein. The machine-readable medium (e.g., computer-readable medium) can be non-transitory (e.g., a non-transitory computer-readable medium). In an embodiment, a method of performing imaging (e.g., X-ray PCI) can include providing an imaging system (e.g., X-ray PCI system) as described herein, positioning an object to be imaged in the appropriate location within the system (e.g., between the energy source and the period-varying grating), and imaging the object using the system. In a further embodiment, the method can include using a second-order approximation model as described herein to reconstruct the image (e.g., X-ray image). The methods and processes described herein can be embodied as code and/or data. The software code and data described herein can be stored on one or more computer-readable media, which may include any device or medium that can store code and/or data for use by a computer system. When a computer system reads and executes the code and/or data stored on a computer-readable medium, the computer system performs the methods and processes embodied as data structures and code stored within the computer-readable storage medium. It should be appreciated by those skilled in the art that computer-readable media include removable and non-removable structures/devices that can be used for storage of information, such as computer-readable instructions, data structures, program modules, and other data used by a computing system/environment. A computer-readable medium includes, but is not limited to, volatile memory such as random access memories (RAM, DRAM, SRAM); and non-volatile memory such as flash memory, various read-only-memories (ROM, PROM, EPROM, EEPROM), magnetic and ferromagnetic/ferroelectric memories (MRAM, FeRAM), and magnetic and optical storage devices (hard drives, magnetic tape, CDs, DVDs); network devices; or other media now known or later developed that is capable of storing computer-readable information/data. Computer-readable media should not be construed or interpreted to include any propagating signals. A computer-readable medium of the subject invention can be, for example, a compact disc (CD), digital video disc (DVD), flash memory device, volatile memory, or a hard disk drive (HDD), such as an external HDD or the HDD of a computing device, though embodiments are not limited thereto. A computing device can be, for example, a laptop computer, desktop computer, server, cell phone, or tablet, though embodiments are not limited thereto. The subject invention includes, but is not limited to, the following exemplified embodiments. An imaging system, comprising: an energy source; a detector; a first grating disposed between the energy source and the detector; and a second grating disposed between the first grating and the detector, wherein the first grating is a quasi-periodical grating (such that the period of the grating varies across the grating (e.g., in a lateral direction across the grating)). The system according to embodiment 1, wherein the first grating is a quasi-periodical phase grating. The system according to any of embodiments 1-2, wherein the second grating is an analyzer grating. The system according to any of embodiments 1-3, wherein the detector is flat. The system according to any of embodiments 1-4, wherein the detector is a flat detector array. The system according to any of embodiments 1-3, wherein the detector is curved (such as cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere)). The system according to any of embodiments 1-6, wherein the second grating is flat. The system according to any of embodiments 1-6, wherein the second grating is curved (such as cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere)). The system according to any of embodiments 1-8, wherein the first grating is flat. The system according to any of embodiments 1-8, wherein the first grating is curved (such as cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere)). The system according to any of embodiments 1-10, further comprising a source grating disposed between the energy source and the first grating. The system according to any of embodiments 1-11, wherein the source grating is flat. The system according to any of embodiments 1-11, wherein the source grating is curved (such as cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere)). The system according to any of embodiments 1-13, wherein the imaging system is an X-ray phase-contrast imaging (PCI) system, and wherein the energy source is an X-ray source. The system according to any of embodiments 11-14, wherein the system is configured such that an object to be imaged is positioned between the source grating and the first grating while it is being imaged. The system according to any of embodiments 1-15, wherein the system is configured such that an object to be imaged is positioned between the energy source and the first grating while it is being imaged. The system according to any of embodiments 1-16, wherein the first grating has a slow-varying period. The system according to any of embodiments 1-17, wherein the period of the first grating varies the same way in both lateral directions moving away (e.g., as the angle is varied positively or negatively by the same amount) from a point on the first grating where radiation from the energy source is centered during imaging (see, e.g., FIGS. 3A and 11C). The system according to any of embodiments 1-18, wherein the second grating matches the curvature (or lack thereof) of the detector. A method of reconstructing or retrieving an image, the method comprising: using a second-order approximation model using paraxial Fresnel-Kirchhoff diffraction theory as described herein. The method according to embodiment 20, further comprising performing an iterative method to reconstruct a phase-contrast image and/or a dark-field image. The method according to any of embodiments 20-21, further comprising iteratively solving the model using the algebraic reconstruction technique (ART). The method according to any of embodiments 20-22, further comprising incorporating state of the art compressive sensing techniques to achieve high quality image reconstruction. The method according to any of embodiments 21-23, wherein the iterative method is based on Equation (42) herein The method according to any of embodiments 21-24, wherein the iterative method includes Equation (46) herein. The method according to embodiment 25, further comprising discretizing Equation (46) to a system of linear equations with respect to variables pk(r). The method according to embodiment 26, further comprising employing the ART for phase image reconstruction. The method according to any of embodiments 20-27, wherein the image is an X-ray phase image. A machine-readable medium (e.g., a computer-readable medium) having machine-executable instructions (e.g., computer-executable instructions) for performing the method according to any of embodiments 20-28. The machine-readable medium according to embodiment 29, wherein the machine-readable medium is non-transitory (e.g., a non-transitory computer-readable medium). A system comprising the machine-readable medium according to any of embodiments 29-30. A method of performing imaging, comprising: providing the imaging system according to any of embodiments 1-19; positioning an object to be imaged in the appropriate location within the system (e.g., between the energy source and the first grating); and imaging the object using the system. The method according to embodiment 32, further comprising performing the method according to any of embodiments 20-28 to reconstruct and/or retrieve an X-ray image (e.g., an in-line and/or dark-field image). The method according to embodiment 33, wherein the method according to any of embodiments 20-28 is performed using the machine-readable medium according to any of embodiments 29-30 or the system according to embodiment 31. A greater understanding of the present invention and of its many advantages may be had from the following examples, given by way of illustration. The following examples are illustrative of some of the methods, applications, embodiments and variants of the present invention. They are, of course, not to be considered as limiting the invention. Numerous changes and modifications can be made with respect to the invention. A numerical analysis was performed to show the modified interference pattern along the x-axis parallel to the grating plane for both a constant-period grating and a period-varying phase grating according to an embodiment of the subject invention. In this analysis, a pure phase grating with a λ phase shift was illuminated by a fully coherent parallel-beam of 28 keV. FIG. 3A shows a plot of the period (in μm) along the x-axis (in μm) for the period-varying phase grating. FIG. 3B shows a plot of the period (in μm) along the x-axis (in μm) for the constant-period grating. FIG. 3C shows the interference pattern at different propagation distances from the grating for the period-varying phase grating. FIG. 3D shows the interference pattern at different propagation distances from the grating for the constant-period grating. FIG. 3E shows an enlarged view of the two boxes highlighted in FIG. 3D. The red arrows in FIG. 3E show the difference caused by the grating aperture widths (plotted in FIG. 3A). To analyze the Talbot self-imaging performances with different combinations of gratings and detectors, as well as with periodic and quasi-periodic gratings designs, numerical experiments were performed. The energy of incident X-rays was set to 28 keV. The distance between the X-ray source and the phase grating G1 grating was 1 m, the phase grating G1 and absorption grating G2 were separated by the first-order fractional Talbot distance, calculated according to Equation (10). The period of the phase grating G1 (pG1 was 4.8 μm, while the period of the absorption grating G2 (pG2) was determined according to imaging geometry. In the data collection step, the classic phase stepping method was employed, and the visibility of the stepping curve was used to quantify the imaging performance. The detector cell pitch was 50 μm. First, for a grating-based phase-contrast imaging system, if the detector and the absorption grating have different curvatures as indicated by the red arrows in FIGS. 4B and 4C, there would be non-uniform gaps to the detector. The non-uniform gap distribution would introduce non-uniform image artifacts due to the blurring effect from the X-ray spot. Therefore, a better or optimal design would place the absorption grating G2 with as closely aligned curvature or shape as possible to that of the detector plane, leading to the grating-detector system designs illustrated in FIGS. 4A and 4D. Due to the imperfect fabrication of the gratings and/or difficulty in matching the gratings to the beam divergence, the phase grating G1 and absorption grating G2 may not be able to be exactly paired, as shown in FIG. 5, where curved G1 and flat G2, or flat G1 and curved G2 are assumed for optimized PCI system designs, as shown in FIG. 6A. In a conventional grating-based PCI system, a periodic phase grating is adopted to produce the same interference patterns at a Talbot length. However, in FIGS. 5A and 5B the phase grating and absorption grating are not paired, which means that the absorption grating cannot be located exactly on the interference patterns. FIG. 5A shows a schematic view of a grating-based phase-contrast system with a curved G1 and a flat G2 (and flat detector). FIG. 5B shows a schematic view of a grating-based phase-contrast system with a flat G1 and a curved G2 (and curved detector). FIG. 5C shows a plot of the visibility versus angle (in degrees) for the setup of FIG. 5A. FIG. 5D shows a plot of the visibility versus angle (in degrees) for the setup of FIG. 5B. FIG. 6A shows a plot of visibility versus angle (in degrees) for the unpaired G1-G2 system of FIG. 5A and a period-unmatched imaging system. The red line (upper line) is for the period-unmatched system, and the blue line (lower line) is for the system of FIG. 5A). FIG. 6B shows a plot of visibility versus angle (in degrees) for the period-matched imaging system. To demonstrate the performance of the quasi-periodic grating, numerical simulations with Matlab (Mathwork) were conducted for both curved periodic phase grating G1 and quasiperiodic phase grating G1′ based on the general setup shown in FIG. 10B. In the simulation, the energy of the X-ray beam was 28 keV. The period of the phase grating G1 was set to 4.8 μm, with a radius, r0=1.0 m. The center of planar analyzer (absorption) grating G2 was at the first fractional Talbot image of the center of G1. 1/rD−1/r0=p2/8λr02. G1′ had the central period set to be the same as G1. The calculated period of G1′ as a function of θ is shown in FIG. 11C, and the period of G2′ is calculated accordingly. The period of the quasi-periodic grating is larger than 4.8 μm, and the same fabrication process for periodic grating can be adapted (see, e.g., David et al., “Fabrication of diffraction gratings for hard X-ray phase contrast imaging,” Microelectron. Eng. 84(5-8), 1172-1177 (2007), which is incorporated by reference herein in its entirety). The pixel size of the detector was set to 24 μm, five times the period of G1. FIG. 11A shows a plot of visibility versus angle (in degrees) for the simulations in the X-ray regime. The red line (uppermost line) is for G1′ (quasi-periodic phase grating), and the blue line (lower most line sloping downward) is for G1. FIG. 11B shows intensity profiles at the first fractional Talbot distance of both G1 and G1′ at 0° and 16°. The upper-left is for G1 at 0°; the upper-right is for G1 at 16°; the lower-left is for G1′ at 0°; and the lower-right is for G1′ at 16°. FIG. 11C shows a plot of the period (in μm) versus the angle (in degrees) for the quasi-periodic phase grating G1′. The visibility of the stepping curves at first fractional Talbot distance G1 and G1′ are shown in FIG. 11A. Referring to FIGS. 11A-11C, a decrease of 58% in visibility for G1 at 16° was observed, while no obvious decrease was observed in the visibility for G1′. The source in the X-ray regime is not truly an absolute point source. The intensity of a fringe pattern can be expressed as a convolution of the intensity profile of the source and the fringe pattern of an ideal point source. This convolution will decrease the visibility of the fringes pattern. This is always an issue in the X-ray regime for both quasi-periodic and periodic gratings. If necessary, a G0 grating can be used for needed spatial coherence. To experimentally verify the analysis of Example 3, experiments were designed in the visible optical regime. Numerical simulations were also conducted. Different from the X-ray simulation, G1 was set as an absorption grating. G2 was separated from G1 by one Talbot distance. The wavelength of the source, λ, was 0.67 μm to match the optical experiment setup. The period of G1, p0, was set to be 91 μm with a radius, r0=75 mm. The quasi-periodic grating G1′ had the central period set to be the same as that of G1. Given that G1′ had the same radius as G1, Equation (34) and Equation (35) were used to calculate its period p ranging from −30° to 30°. The calculated period of G1′ as a function of θ is shown in FIG. 12C. The diagram of the experimental setup is shown in FIG. 13A. The point source came from a laser with fiber coupled output (Suprlum, SLD-261, λ=670 nm). The driven current was set at 133 mA (Suprlum, pilot-2). Two lenses (f1=100 mm, f2=62.9 mm) were used to relay a relayed focal point of 41.0°. The radius of the acrylic tube was 75 mm. The periodic grating and the quasi-periodic grating with central period of 91 μm were made on a mylar film with a thickness of 180 μm (CAD/Art Services, Inc.), which could be bent easily. Two gratings were taped on the outer surface of the acrylic tube. The detection microscope system included a microscope objective (Nikon, 10X/0.3), a tube lens (f=200 mm), and a CMOS camera (JAI, GO5000M), all of which were installed on a motorized translation stage (Thorlab, NRT-100). A stepper motor controller (Thorlab, apt) drove the stage, so that the detection system was able to scan the field in the x direction. The scan range of the experiment was 50 mm, with 5 mm step size, covering an angular range from −13° to 13°. The microscope objective could adjust its focus in the y direction. The plane with the highest fringes contrast near the calculated Talbot distance was set as the observation plane. FIG. 12A shows the Talbot carpet of the periodic grating G1; the vertical axis is Z (in mm), and the horizontal axis is X (in mm). The period of the grating was 91 μm. The blown-up portions are for 0° and 12.6°. FIG. 12B shows the Talbot carpet of the quasi-periodic grating G1′; the vertical axis is Z (in mm), and the horizontal axis is X (in mm). The period of the grating at the X=0 line was 91 μm. The blown-up portions are for 0° and 12.6°. FIG. 12C shows a plot of the period (in μm) versus the angle (in degrees) for the quasi-periodic phase grating G1′. The period was calculated from Equation (35). These results are from the simulations in the visible optical regime. The wavelength of the source was 0.67 μm; the radius of the grating was 75 mm; and the white line in both FIGS. 12A and 12B, indicating the self-imaging plane, was located at Y=111.9 mm. The Talbot carpets are shown in Cartesian coordinates. It can be observed that the self-imaging surface of grating G1 is on a curved plane. FIG. 12B shows that the self-imaging surface of the quasi-periodic grating G1′ is almost flat. The ideal Talbot image is located at rD=111.9 mm. If a flat array detector were placed at this plane, indicated by the white solid lines in FIGS. 12A and 12B, the interference fringes would be distorted at 12.6° for periodic grating G1, while the fringes maintain the original fringe profiles for the quasiperiodic grating G1′. FIG. 13A shows a schematic view of the experimental setup. S is the point source λ=0.67 μm. A periodic grating G1 or quasi-periodic grading G1′ is fixed on a cylindrical surface. P denotes the observation plane; Obj denotes the microscope objectives; L denotes the tube lens; and D denotes the CMOS detector. FIG. 13B shows images of interference fringes at 0° on the observation plane. FIG. 13C shows images of interference fringes at 7.4° on the observation plane. FIG. 13D shows images of interference fringes at 12.24° on the observation plane. In these optical experiments, the fields were scanned along the x-axis. The images acquired by the CMOS camera at 0°, 7.4°, and 12.2° angles from G1 and G1′ are shown in FIGS. 13B-13D, respectively. The results also show that the fringes from the periodic grating G1 had a severe distortion at larger angles, while those from the quasi-periodic grating G1′ did not change much or at all. In the X-ray PCI setups, the pixel size of the detector was ˜5 times larger than the period of G1, and the analyzer grating G2 should resolve the lateral shifts of the fringes. In this experiment, the effective pixel size of the CMOS camera after the microscope magnification (˜0.5 μm) was less than the period of fringes on the observation plane. In order to compare the experimental results, the images were processed to add in a ‘virtual’ analyzer grating G2. The readout from a virtual pixel consists of five periods of the fringes. Because for quasi-periodic grating the periods have slight variance with respect to the angle, the period of G2′ is calculated accordingly at the observation plane. The virtual grating G2 and G2′ scan 40 phase steps for one period of the fringes. FIG. 14 shows the experimental visibility of the periodic grating G1 and quasi-periodic grating G1′. The visibility was calculated from averaging the visibility from 100 sub-regions with 3 mm in width and 2 μm in height on the CMOS camera. FIG. 14 shows a plot of average visibility versus angle (in degrees) for the periodic grating G1 and the quasi-periodic grating G1′ on the observation plane. The red line (upper line) is for G1′, and the blue line (lower line) is for G1. It can be observed that the quasi-periodic grating G1′ maintains a uniform (or nearly uniform) visibility on the observation plane. From −13° to 13°, the visibility variance of G1′ is less than 10%, while the visibility for G1 dropped by over 10% after only 5° from center. The results in Example 4 show an overall decrease in the visibility. Three reasons could possibly have caused this difference: the speckle noise; low contrast of the binary mask; and the optical aberration from the mask film. First, a coherent source was used in Example 4, which introduced laser speckles to the measurement. In addition, the artifacts on the grating surface can contribute to the blur of the interference fringes. From the measured images of the fringes, the average size of the speckles was ˜13 μm, and the standard deviation of the intensity was 30%. These artifacts, coupling the light to the non-harmonic frequency components of the fringes, can reduce the experimental visibility. Second, the opaque part of the grating was not strictly dark in Example 4. The opaque region had a measured intensity of ˜10% of the maximum intensity, which essentially acted as a bias to the field. This low contrast of the grating mask can also decrease the visibility. Third, due to the mask printing process, the thickness of the transparent region was not completely uniform in Example 4. A profilemeter measured the average difference of the thickness to be ˜0.13 μm. This non-uniform thickness of the mask can introduce a lensing effect. Evidence for this speculation is that the field distribution along the Talbot distance was asymmetric. This aberration effect can also affect the visibility. The three aforementioned effects were incorporated into the simulation in this example. Specifically, to include the speckle noise, the images were divided into small regions with a size of 13 μm, and random Gaussian noise with a standard deviation of 30% of the local intensity was added to each region. The transmittance bias and the lensing effect were also included in the simulation. FIG. 15A shows a plot of visibility versus the radius direction of the periodic grating (in mm) for the simulated visibility profile. The origin was set at the observation plane. FIG. 15B shows experimental images of interference fringes at −2 mm (top image), 0 mm (middle image), and 2 mm (bottom image) from the observation plane. FIG. 15C shows profiles of interference fringes. The red curves denote simulated profiles, and the blue curves denote the grayscale value profiles of the blue (near the center) lines marked in FIG. 15B. The profiles are for −2 mm (top profile), 0 mm (middle profile), and 2 mm (bottom profile). The simulated visibility is shown in FIG. 15A. The red point marks the observation plane. The visibility curve, as predicted, was asymmetric from the observation plane. The visibility at the red point was 0.7, which was close to the peak visibility in the experiment. The intensity pattern was measured at −2 mm, 0 mm, and 2 mm away from the observation plane. The experimental images are shown in FIG. 15B. The simulated and experimental intensity profiles of the interference fringes are shown in FIG. 15C. The ratio between the grating period and the wavelength, p/λ, is on the order of 104 in the X-ray regime, and p/λ is on the order of 102 for the experiment in the visible regime, and both of these are sufficiently large for the approximation in Equations (21) and Equation (22) to be valid. Numerical tests were performed to demonstrate the feasibility of second-order approximation models for X-ray phase retrieval described herein (see, e.g., Equations (36)-(45)). The phase contrast image shown in FIG. 7A was used as the phase distribution after the wave-object interaction. FIG. 7A shows the true phase image. FIG. 7B shows a reconstructed version of the image of FIG. 7A, reconstructed using a method of the subject invention. FIG. 7C shows a reconstructed version of the image of FIG. 7A, reconstructed using a related art TIE-based reconstruction method. The image of 7A is from https://www.maxlab.lu.se/sv/MEDMAX. The phase image had 498×500 pixels with a pixel size of 3 μm. The radiation wavelength was set to λ=0.25 Å, corresponding to hard X-rays. It was assumed that the intensity distribution on the object plane was uniform to mimic a homogeneous attenuation scenario. The wave amplitudes on the transverse plane through free space propagation can be simulated using the Fresnel transform Equation (38). The intensity distribution on the image plane at a propagation distance of 1000 mm was calculated, with Poisson noise added to the intensity image. Equation (43) was discretized into a system of linear equations, and the ART algorithm was employed to reconstruct the phase image. The procedure took 4 iterations to produce the phase contrast image as shown in FIG. 7B. For the comparison, the phase reconstruction was performed using the TIE-based reconstruction method. The same image plane and the same Poisson noise were used, and the differences between the intensity images on the object and image planes were employed to approximate the intensity derivatives along the propagation direction. The Fourier transform method was implemented for phase reconstruction based on TIE. The reconstructed image was plainly inferior, as shown in FIG. 7C. The experiment of Example 6 was repeated, using the image of FIG. 8A as the phase contrast image. FIG. 8A shows the true phase image. FIG. 8B shows a reconstructed version of the image of FIG. 8A, reconstructed using a method of the subject invention. FIG. 8C shows a reconstructed version of the image of FIG. 8A, reconstructed using a related art TIE-based reconstruction method. The image of 8A is from Tapfer et al. (“Three-dimensional imaging of whole mouse models: comparing nondestructive X-ray phase-contrast micro-CT with cryotome-based planar epi-illumination imaging,” Journal of Microscopy 253(1), 24-30 (2014)). The phase image had 256×238 pixels with a pixel size of 3 μm. The radiation wavelength was set to λ=0.25 Å. The intensity distribution on the object plane was uniform again. The intensity distribution on the image plane at a propagation distance of 1000 mm was simulated based on Equation (38), with Poisson noise similarly added. The ART-based reconstruction procedure took 5 iterations to generate the phase contrast image shown in FIG. 8B. In the same setting, the phase reconstruction was performed using the TIE-based reconstruction method, as shown in FIG. 8C. These results suggest that the phase imaging method of the subject invention is plainly superior to the TIE-based method. It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application. All patents, patent applications, provisional applications, and publications referred to or cited herein (including those in the “References” section) are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification. T. Davis, D. Gao, T. 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	abstract	A method for mitigating stress corrosion cracking in high temperature water includes introducing catalytic nanoparticles and dielectric nanoparticles to the high temperature water in an amount effective to reduce a electrochemical corrosion potential of the high temperature water.
	summary
	description	This invention was made with government support under grants R01 EB007168 and R21 EB007236 awarded by the National Institutes of Health. The Government has certain rights to this invention. The subject technology relates generally to radiological devices and methods and, in some embodiments, more particularly to devices and methods for computed tomography (CT). CT systems have included single-slice and multiple-slice detectors. CT systems with multiple-slice detectors are, in particular, able scan large volumes of interest. Some large volumes are imagined by helical CT scanning. In helical scanning, the subject moves axially relative to a radiation beam such that the beam traverses a helical path through the subject. Such scanning can be leveraged to quickly scan whole or large portions of organs. As aspect of at least one embodiment disclosed herein includes the realization that high radiation dose of conventional perfusion CT imaging and the lack of standardized perfusion CT imaging protocols limit the clinical potential of CT. Some embodiments disclosed herein significantly reduce the x-ray dose of CT perfusion scans without sacrificing clinical accuracy. Some embodiments that significantly reduce the x-ray dose of CT perfusion scans without sacrificing clinical accuracy, thereby expand use of perfusion CT as a diagnostic tool. Some embodiments of devices and methods for perfusion CT imaging can used for diagnosis, treatment of diseases, or both. In various embodiments, a radiation dose delivered to a subject can be reduced by application of any one of a transverse dynamic collimator, a grated collimator, an adaptive sampling algorithm, or an adaptive exposure algorithm, or a combination of some or all thereof. Some embodiments can be used for perfusion imaging of the kidneys, pancreas, liver, and heart. Primarily due to concerns about the magnitude of radiation dose delivered, perfusion CT imaging has not been used routinely in various fields, including stroke assessment, oncology, and cardiac and kidney function. In some embodiments, reduction of radiation dose delivered to a subject can permit application of perfusion CT to those applications wherein dose is a limiting factor, e.g. cardiac perfusion. Although some embodiments are discussed herein with respect to perfusion CT imaging, some embodiments can be used with other imaging. Similarly, although some embodiments may provide particular benefits for perfusion CT imaging, various embodiments can provide advantages with other imaging. The subject technology is illustrated, for example, according to various aspects described below. Various examples of aspects of the subject technology are described as numbered clauses (1, 2, 3, etc.) for convenience. These are provided as examples and do not limit the subject technology. It is noted that any of the dependent clauses may be combined in any combination, and placed into a respective independent clause, e.g., clause 1 or clause 55. The other clauses can be presented in a similar manner. 1. A collimator for a computed x-ray tomography imaging device, comprising a first grating and a second grating positioned on opposing sides of a primary radiation delivery window, each of the first and second gratings comprising a plurality of attenuating members with a plurality of secondary radiation delivery windows extending between adjacent attenuating members of the first grating and the second grating, respectively. 2. The collimator of clause 1, wherein a width of each secondary windows is less than a width of the primary window. 3. The collimator of clause 1, wherein a total area of each of the plurality of secondary windows is less than a total area of the primary window. 4. The collimator of clause 1, wherein a width of each secondary window is proportional to a distance between the secondary window and the primary window. 5. The collimator of clause 4, wherein the width of each secondary window is linearly proportional to the distance between the secondary window and the primary window. 6. The collimator of clause 4, wherein the width of each secondary window is positively proportional to the distance between the secondary window and the primary window. 7. The collimator of clause 1, wherein a width of each attenuating member is proportional to a distance between the attenuating member and the primary window. 8. The collimator of clause 7, wherein the width of each attenuating member is linearly proportional to the distance between the attenuating member and the primary window. 9. The collimator of clause 7, wherein the width of each attenuating member is positively proportional to the distance between the attenuating member and the delivery window. 10. The collimator of clause 1, wherein the secondary windows comprise open passages extending through the grating. 11. The collimator of clause 1, wherein the secondary windows comprise panes of substantially radio-transmissive material. 12. The collimator of clause 1, wherein the attenuating members are oriented generally parallel to sides of the primary window. 13. The collimator of clause 1, wherein the secondary windows are oriented generally parallel to sides of the primary window. 14. The collimator of clause 1, wherein the first grating is movable relative to the second grating. 15. The collimator of clause 14, wherein the first and second gratings are independently movable. 16. The collimator of clause 1, wherein the attenuating members of the first grating are integrally formed with each other, and the attenuating members of the second grating are integrally formed with each other. 17. The collimator of clause 16, wherein the attenuating members of the first grating are integrally formed with the attenuating members of the second grating. 18. A method of directing radiation during computed tomography (CT) imaging, comprising: emitting x-ray radiation from a radiation source toward an object; passing a first portion of the radiation through a primary window toward a target region in the object; passing a second portion of the radiation through at least one secondary window, on each of opposing sides of the primary window, to corresponding regions in the object outside the target region; attenuating, between the primary and secondary windows, at least a third portion of the radiation; and generating CT image data based on the first and second portions of the radiation. 19. The method of clause 18, further comprising rotating the radiation source and the primary and secondary windows about an axis, and wherein radiation is passed through the primary window and the secondary windows while either (i) centers of the primary window and at least two of the secondary windows lie in a plane non-parallel to the axis or (ii) a plane intersecting the centers and the axis is non-parallel to the axis. 20. The method of clause 18, further comprising rotating the radiation source and the primary and secondary windows about an axis, and translating the secondary windows relative to the primary window in a direction non-parallel to the axis. 21. The method of clause 20, wherein the secondary windows are translated relative to the primary window during rotation of the radiation source. 22. The method of clause 21, wherein a first of the secondary windows is translated independently of a second of the secondary windows on an opposing side of the primary window from the first of the secondary windows. 23. The method of clause 18, wherein the attenuating comprises blocking passage of at least the third portion of the radiation between the primary and secondary windows. 24. A computed tomography device, comprising: a gantry configured to rotate about an axis and comprising an opening configured to accommodate an object; a radiation source mounted to the gantry; a collimator positioned between the radiation source and the gantry opening, the collimator comprising first and second leaves respectively bounding first and second opposing sides of a radiation delivery window, the first leaf and the second leaf being movable to adjust at least one of a size or a location of the radiation delivery window relative to the radiation source in a direction non-parallel to the axis. 25. The computed tomography device of clause 24, wherein the first leaf is moveable independently of the second leaf. 26. The computed tomography device of clause 24, further comprising third and fourth leaves respectively bounding third and fourth opposing sides of the window and aligned with the window along the axis. 27. The computed tomography device of clause 26, wherein each of the first and second sides is substantially orthogonal to each of the third and fourth sides. 28. The computed tomography device of clause 24, wherein the collimator is mounted within about 27 cm of the radiation source. 29. The computed tomography device of clause 24, wherein the collimator is mounted within about 12 cm of the radiation source. 30. A computed tomography device, comprising: a gantry configured to rotate about an axis and comprising an opening configured to accommodate an object; a radiation source mounted to the gantry; a collimator positioned between the radiation source and the gantry opening, the collimator comprising a first leaf and a second leaf respectively bounding first and second opposing sides of a radiation delivery window, the first leaf and the second leaf being independently movable relative to the radiation source in a direction non-parallel to the axis. 31. The device of clause 30, wherein the first leaf and the second leaf are independently movable relative to the radiation source in a direction tangential to a circle (i) centered on the axis and (i) defining a plane that is not parallel to the axis. 32. The device of clause 31, wherein the first leaf is independently movable relative to the second leaf. 33. A method of radiologic imaging, comprising: rotating, about an axis, a gantry carrying a radiation source; emitting radiation from the radiation source toward an object between a pair of leaves; during rotation of the gantry, moving the pair of leaves relative to the radiation source in a direction nonparallel to the axis. 34. The method of clause 33, further comprising: performing a preliminary scan of a object; demarcating a region of interest in the object, based on the preliminary scan, that is at least one of (i) non-concentric with the axis or (ii) non-circular; and controlling movement of the pair of leaves during rotation of the gantry to adjust at least one of a location, relative to the radiation source, or a dimension, of a radiation delivery window such that substantially only the region of interest is exposed to radiation through the radiation delivery window. 35. The method of clause 34, further comprising directing radiation through a plurality of secondary windows, on opposing sides of the radiation delivery window, to regions in the object outside the region of interest; and substantially blocking the passage of radiation toward the object in regions between the radiation delivery window and the secondary windows. 36. The method of clause 33, further comprising repositioning the object such that a region of interest is located closer to the axis. 37. A method of contrast-enhanced computed tomography (CT) imaging, comprising: (a) repeatedly scanning a target region at a frequency during a session, the scanning comprising performing a CT scan by emitting x-ray radiation toward the target region, the frequency initially being a first rate; (b) monitoring, during the session, an indicator of attenuation of radiation by a contrast-enhanced first structure within the target region; (c) after detecting an increase of the attenuation, increasing the frequency to a second rate; and (d) after detecting a decrease in the attenuation after (c), decreasing the frequency to a third rate. 38. The method of clause 37, further comprising generating a representation of a relationship between time and radiation attenuation by a second structure within the target region. 39. The method of clause 38, wherein the radiation attenuation by the second structure with respect to time represents an indicator of vascular perfusion of the second structure. 40. The method of clause 37, further comprising monitoring of a rate of change of the attenuation. 41. The method of clause 40, wherein the frequency is increased to the second rate in response to detection of a decrease in a rate at which the attenuation is increasing. 42. The method of clause 41, further comprising beginning monitoring of the rate of change after detecting an increase of the attenuation to or beyond a threshold. 43. The method of clause 42, wherein the threshold is about 35 HU. 44. The method of clause 42, wherein the threshold comprises a degree of increase in the attenuation compared to a value indicated by an initial scan. 45. The method of clause 40, further comprising decreasing the frequency below the third rate in response to detection of a decrease in a rate at which the attenuation is decreasing. 46. The method of clause 45, wherein decreasing the frequency below the third rate comprises reducing the frequency with each successive scan. 47. The method of clause 46, wherein the frequency is approximately halved with each successive scan. 48. The method of clause 37, wherein the frequency is reduced to the third rate upon a first detection of a decrease in attenuation after (c). 49. The method of clause 37, wherein the first rate is one scan approximately every two seconds. 50. The method of clause 37, wherein the second rate is one scan approximately every second. 51. The method of clause 37, wherein the third rate is one scan approximately every two seconds. 52. The method of clause 37, wherein the structure comprises at least one of a heart chamber, an aorta, or another blood vessel. 53. The method of clause 37, further comprising terminating the scanning after a predetermined period of time and performing a final scan at the end of the predetermined period. 54. The method of clause 37, further comprising terminating the scanning after a predetermined period of time, and, if a remaining time between a latest scan and an end of the predetermined period is less than an interval between the latest scan and an immediately preceding scan, performing (i) a penultimate scan at approximately half of the remaining time after the latest scan and (ii) a final scan at the end of the predetermined period. 55. A computer-implemented system for controlling contrast-enhanced computed tomography imaging, comprising: an attenuation monitoring module configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region; a scanning-frequency control module configured to (i) increase a frequency of scanning from a first rate to a second rate after detection of an increase of the attenuation, and (ii) decrease the frequency to a third rate after detecting a decrease in attenuation after increasing the frequency to the second rate. 56. The computer-implemented system of clause 55, wherein the monitoring module is further configured to monitor a rate of change of the attenuation. 57. The computer-implemented system of clause 56, wherein the scanning-frequency control module is further configured to increase the frequency to the second rate in response to detection of a decrease in a rate at which the attenuation is increasing. 58. The computer-implemented system of clause 56, wherein the monitoring module is further configured to begin monitoring of the rate of change after detection of compliance of the attenuation with a threshold. 59. The computer-implemented system of clause 58, wherein the threshold is 35 HU. 60. The computer-implemented system of clause 58, wherein the threshold is a degree of increase in the attenuation compared to a value indicated by an initial scan. 61. The computer-implemented system of clause 55, further comprising a processing module configured to generate a representation of a relationship between time and radiation attenuation by a second structure within the target region. 62. The computer-implemented system of clause 61, wherein the radiation attenuation by the second structure with respect to time represents an indicator of vascular perfusion of the second structure. 63. The computer-implemented system of clause 55, wherein the scanning-frequency control module is further configured to decrease the frequency below the third rate in response to detection of a decrease in a rate at which the attenuation is decreasing. 64. The computer-implemented system of clause 63, wherein the scanning-frequency control module is further configured to decrease the frequency further below the third rate with each successive scan. 65. The computer-implemented system of clause 64, wherein the scanning-frequency control module is further configured to divide the frequency by approximately two with each successive scan. 66. The computer-implemented system of clause 55, wherein the scanning-frequency control module is further configured to reduce the frequency to the third rate upon a first detection of a decrease in attenuation after an increase to the second rate. 67. The computer-implemented system of clause 55, further comprising a termination module configured to terminate the scanning after a predetermined period of time and direct performance of a final scan at the end of the predetermined period. 68. The computer-implemented system of clause 55, further comprising a termination module configured to terminate the scanning after a predetermined period of time, and, if a remaining time between a latest scan and an end of the predetermined period is less than an interval between the latest scan and an immediately preceding scan, direct performance of (i) a penultimate scan at a half of the remaining time after the latest scan and (ii) a final scan at the end of the predetermined period. 69. A computed tomography imaging system, comprising: a gantry comprising an opening configured to accommodate an object; a radiation source mounted to the gantry; a radiation detector mounted to the gantry opposite the radiation source relative to the opening; an attenuation monitoring module configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region; a scanning-frequency control module configured to (i) increase a frequency of scanning from a first rate to a second rate after detection of an increase of the attenuation, and (ii) decrease the frequency to a third rate after detecting a decrease in attenuation after increasing the frequency to the second rate. 70. A method of computed tomography imaging, comprising: repeatedly emitting x-ray radiation into a target region at a frequency during a session; monitoring, during the session, an indicator of attenuation of radiation by a contrast-enhanced first structure within the target region; varying the frequency based on the attenuation. 71. The method of clause 70, wherein x-ray radiation is emitted at a minimum frequency when the attenuation is below a low threshold and at a maximum frequency when then attenuation is above a high threshold. 72. A method of contrast-enhanced computed tomography (CT) imaging, comprising: (a) repeatedly scanning a target region during a session, the scanning comprising performing a CT scan by emitting x-ray radiation at an applied power toward the target region, the applied power being a first power for a first scan; (b) monitoring an indicator of attenuation of radiation by a contrast-enhanced first structure within the target region; and (c) selecting the applied power for each of a plurality of scans, after a first scan, based on the attenuation indicated from a preceding scan in the session. 73. The method of clause 72, wherein the first power is a maximum power applied during the session. 74. The method of clause 72, wherein the applied power is determined by selection of an applied current. 75. The method of clause 74, wherein the first applied current is about 200 ma. 76. The method of clause 72, further comprising applying substantially the first applied power to individual scans until detection of an increase of the attenuation to or beyond a threshold attenuation magnitude. 77. The method of clause 76, wherein the threshold attenuation magnitude is about 35 HU. 78. The method of clause 76, wherein the threshold attenuation magnitude is a predetermined proportion of the attenuation determined from an initial scan. 79. The method of clause 76, wherein the threshold attenuation magnitude is a predetermined number of Hounsfield Units greater than the attenuation determined from an initial scan. 80. The method of clause 72, wherein the applied power is selected by multiplying a maximum current by an exponential function based on the attenuation determined from the preceding scan. 81. The method of clause 80, wherein the exponential function yields a value that is (i) greater than a minimum allowable current divided by a maximum allowable current, and (ii) less than 1. 82. The method of clause 80, wherein the exponential function is a function F determined byF=eC(TH-ΔHU)/TH wherein TH is a threshold attenuation magnitude and ΔHU is equal to a difference in magnitude, in Hounsfield Units, between the attenuation determined from a preceding scan and a baseline attenuation. 83. The method of clause 82, wherein the preceding scan is a scan immediately prior to a scan performed according the applied power as determined by the function F. 84. The method of clause 82, wherein the baseline attenuation is a magnitude of the attenuation indicated based on the initial scan. 85. The method of clause 82, wherein C is selected such that, when the function is applied, an applied current for a next scan is about a tenth of the maximum allowable current when the attenuation of the preceding scan is about ten times above the threshold attenuation magnitude. 86. The method of clause 82, wherein C is about 0.25. 87. The method of clause 82, further comprising selecting an applied power corresponding to a minimum allowable current for each scan for which the function F indicates, based on the attenuation indicated by a preceding scan, a current less than the minimum allowable current. 88. A computer-implemented system for controlling contrast-enhanced computed tomography imaging, comprising: an attenuation monitoring module configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region; a power control module configured to select an applied power for each of a plurality of scans based on the attenuation detected from a preceding scan. 89. The computer-implemented system of clause 88, wherein the power control module is further configured to direct application of a maximum power applied during the session in a first scan. 90. The computer-implemented system of clause 88, wherein the power control module is further configured to apply substantially the same amount of power to individual scans until detection of an increase of the attenuation to or beyond a threshold attenuation magnitude. 91. The computer-implemented system of clause 88, wherein the power control module is further configured to select the applied power by multiplying a maximum current by an exponential function. 92. A computed tomography imaging system, comprising: a gantry comprising an opening configured to accommodate an object; a radiation source mounted to the gantry; a radiation detector mounted to the gantry opposite the radiation source relative to the opening; an attenuation monitoring module configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region; a power control module configured to select an applied power for each of a plurality of scans based on the attenuation detected from a preceding scan. 93. A method of computed tomography imaging, comprising: repeatedly emitting x-ray radiation into a target region, each emission having an input power; monitoring an attenuation of radiation through a structure within the target region; varying the input power based on the attenuation. 94. The method of clause 93, further comprising applying a minimum input power when the attenuation is above a high threshold and applying a maximum input power when then attenuation is below a low threshold. Additional features and advantages of the subject technology will be set forth in the description below, and in part will be apparent from the description, or may be learned by practice of the subject technology. The advantages of the subject technology will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the subject technology as claimed. In the following detailed description, numerous specific details are set forth to provide a full understanding of the subject technology. It will be apparent, however, to one ordinarily skilled in the art that the subject technology may be practiced without some of these specific details. In other instances, well-known structures and techniques have not been shown in detail so as not to obscure the subject technology. A phrase such as “an aspect” does not imply that such aspect is essential to the subject technology or that such aspect applies to all configurations of the subject technology. A disclosure relating to an aspect may apply to all configurations, or one or more configurations. An aspect may provide one or more examples of the disclosure. A phrase such as “an aspect” may refer to one or more aspects and vice versa. A phrase such as “an embodiment” does not imply that such embodiment is essential to the subject technology or that such embodiment applies to all configurations of the subject technology. A disclosure relating to an embodiment may apply to all embodiments, or one or more embodiments. An embodiment may provide one or more examples of the disclosure. A phrase such “an embodiment” may refer to one or more embodiments and vice versa. A phrase such as “a configuration” does not imply that such configuration is essential to the subject technology or that such configuration applies to all configurations of the subject technology. A disclosure relating to a configuration may apply to all configurations, or one or more configurations. A configuration may provide one or more examples of the disclosure. A phrase such as “a configuration” may refer to one or more configurations and vice versa. Although many features, aspects and embodiments are described herein or shown in the accompanying drawings in the context of CT, the disclosed technology can also be applied in other imaging systems and methods, other medical scenarios, or other image data acquisition or processing techniques. Primarily due to concerns about the magnitude of radiation dose delivered, perfusion CT imaging has not been used routinely in various fields, including stroke assessment, oncology, and cardiac and kidney function. In some embodiments, reduction of radiation dose delivered to a subject can permit application of perfusion CT to those applications wherein dose is a limiting factor, e.g. cardiac perfusion. In some embodiments, perfusion CT can be applied to stroke assessment, oncology, and assessment of cardiac and kidney function. In various embodiments, a radiation dose delivered to a subject can be reduced by application of a transverse dynamic collimator, a grated collimator, an adaptive sampling algorithm, an adaptive exposure algorithm, or a combination thereof. In some embodiments, the radiation dose of perfusion CT can be significantly reduced without impacting diagnostic accuracy During a helical CT scan, an X-ray source generates a cone (or wedge) beam of radiation that moves relative to the patient. Portions of the cone beam of radiation may not pass through the volume to be reconstructed. While this extra radiation may have little adverse effect on the clinical use of the reconstructed image, it can subject the patient to more radiation than is necessary. Accordingly, various embodiments described herein relate to replacing a conventional collimator with a dynamically transversely adjustable collimator. The collimator can be actuated by an electromechanical servo system. The imaging system can comprise a control (e.g., an electronic control) that is responsive to sensor(s) for sensing the axial and rotational position of the X-ray source relative to a volume of interest. As the X-ray source rotates about an axis, the collimator is adjusted (i) to adjust the width, location, or both of the radiation beam so that radiation is primarily allowed to pass through the volume of interest and (ii) to block some or all of the rays of radiation that will not intersect the volume of interest. FIG. 1 illustrates an exemplifying CT imaging system 100 including a CT scanner 102 with a gantry portion 104, a radiation source unit 112, a detector 124, and a couch or support 126. The gantry portion 104 can comprise a gantry opening 106 and can rotate about an examination region 108. The rotating gantry portion 104 can support the radiation source unit 112 and the detector 124. The radiation source unit 112 can be an x-ray source, such as an X-ray tube, for example. The radiation source can emit a radiation beam. The radiation beam can be a cone beam, wedge beam, or other desirable beam shape. The beam can be collimated to have a generally conical geometry in some embodiments. The detector 124 is sensitive to radiation (e.g., x-ray) emitted by the radiation source unit 112. In some embodiments, the detector 124 can be a detector array comprising multiple radiation detectors. The detector 124 can be disposed opposite the x-ray source unit 112 on rotating gantry portion 104. In some embodiments, the detector 124 includes a multi-slice detector having a plurality of detector elements extending in the axial and transverse directions. Each detector element can detects radiation emitted by the radiation source unit 112 that traverses the examination region 108 and can generate corresponding output signals or projection data indicative of the detected radiation. Other detector configurations, such as those wherein stationary detectors surround the examination region, can also be used. The motion of the radiation source and emission of radiation thereby are coordinated to scan a volume of interest (VOI) 122 such as anatomy, or a portion of anatomy, disposed within the examination region 108. The volume of interest can be enhanced with a contrast agent in some embodiments, such as described below, for example. In some embodiments, coordinated motion and radiation emission can be used for fly-by scanning, for example. In some embodiments, the radiation source and detector move in coordination with a contrast agent through the subject such that the VOI is scanned in coordination with the flow of the agent as it is traced through the VOI. In another embodiment, the axial advancement is coordinated with a motion of the subject to capture a desired motion state. The support 126 can support a subject, such as a human patient for example, in which the VOI is defined within the examination region 108. As illustrated in FIG. 1, a drive mechanism 116 can move the radiation source longitudinally along a z-axis 120 on tracks 128 while the support 126 is stationary. In some embodiments, however, the support 126 can be translated axially along the z-axis 120 while the gantry 104 rotates in a fixed location along the z-axis. An operator of the system can define the VOI to encompass the whole subject or a portion thereof for scanning. In one embodiment, the CT scanner performs a helical scan of the VOI by rotating around the axis 120 during relative movement of the gantry and the support parallel to the axis. The system 100 can further comprise various computer hardware and software modules. As illustrated in FIG. 1, for example, the system can comprise data memory 130, a processor 132, a volume image memory 134, a user interface 136, and one or more controllers, such as a CT controller 128 and a collimator controller 140. In some embodiments, a single hardware or software module can control multiple parts of the system 110, such as the radiation source and one or more collimators, for example. The projection data generated by the detector 124 can be stored to a data memory 130 and reconstructed by a processor 132 to generate a volumetric image representation therefrom. The reconstructed image data can stored in a volume image memory 134 and displayed to a user via a user interface 136. Although FIG. 1 separately illustrates the data memory 130 and the volume image memory 134, both can be stored within common data storage hardware. The image data can be processed to generate one or more images of the scanned region or volume of interest or a subset thereof. The user interface 136 facilitates user interaction with the scanner 102 and can comprise various input and output devices. Software applications and modules can receive inputs from the user interface 136 to configure and/or control operation of the scanner 102, and other elements of the system 100. For instance, the user can interact with the user interface 136 to select scan protocols, and initiate, pause, and terminate scanning. The user interface 136 can display images, facilitate manipulation of the data and images and measurement of various characteristics of the data and images, etc. An optional physiological monitor (not shown) can monitor cardiac, respiratory, or other motion of the VOI. For example, the monitor can include an electrocardiogram (ECG) or other device that monitors the electrical activity of the heart. This information can be used to trigger one or more scans or to synchronize scanning with the heart electrical activity to reduce or eliminate adverse affects of heart motion on imaging. An optional injector (not shown) or the like can be used to introduce agents, such as contrast for example, into the subject. Introduction of the agent can be used to trigger one or more scans. The CT controller 138 can control rotational and axial movement of the radiation source unit 112 and the detector 124 relative to the support 126. The CT scanner and CT controller can be coupled to a collimator controller 140 that controls a collimator 142 positioned between the radiation source and the examination region 108. Although FIG. 1 illustrates the CT controller and the collimator controller as separate units, the CT scanner and one or more collimators can be controlled by the same hardware and software modules in some embodiments. The collimator controller 140 can control movement, and opening and closing, of a radiation delivery window of the collimator 142. In some embodiments, the collimator controller can independently control movement of individual leaves of the collimator. The collimator controller can be a software module configured to move leaves of a collimator to allow passage of radiation toward a region of interest while blocking radiation to portions of a subject outside the region of interest. In some embodiments, the collimator controller 140 can cause the collimator to function as a shutter to block radiation between scans and to open, close, and translate as the rotatable gantry 104 (and accordingly the source unit 112 and detector 124 coupled thereto) move around the VOI 122 during a scan. In some embodiments, the collimator controller 140 can include one or more electro-mechanical servo motors. In some embodiments, the collimator controller 140 can include an electronic controller. Repeated large area circular scans and helical scans can be used to perform perfusion CT. By opening, closing, and/or translating the collimator 142, radiation can be delivered primarily only along paths that intersect the VOI, thereby reducing the X-ray dose. In the case of helical scans, a dynamic axial collimator can be used to limit the x-ray exposure, axially, at either end or both ends of the helical scan. For example, in some embodiments, an axial collimator can be gradually opened at the leading end of the VOI and closed at the trailing end of the VOI. A dynamic transverse collimator positioned in a plane transverse to a gantry rotation axis and in front of the x-ray source can limit the x-ray exposure to a region of interest (ROI) 144 within a field of view (FOV) 146, as illustrated in FIG. 3. FIG. 3 is a schematic illustration of an imaging system showing two positions 152, 154, respectively at 0 and 90 degrees relative to a subject, of the radiation source unit 112, detector 124, and a dynamic transverse collimator 142 for an off-center ROI 144 surrounding the heart. FIG. 4 illustrates the outline of a VOI corresponding to the heart. Transverse and axial collimators can together limit the x-ray exposure to primarily only the VOI 148 for the heart illustrated in FIG. 4, for example. As illustrated, for example, in FIG. 4, the VOI 148 can include multiple ROIs 144. In some embodiments, a collimator can comprise a first leaf 170 and a second leaf 172 respectively bounding first and second opposing sides of a radiation delivery window 174, as illustrated in FIG. 5, for example. The first leaf and the second leaf can be movable to adjust at least one of a size or a location of the radiation delivery window relative to the radiation source in a direction non-parallel to the axis. The first leaf and the second leaf can be independently movable relative to the radiation source in a direction non-parallel to the axis. The first leaf 170 and the second leaf 172 can be moveable independently of each other. Each of the first and second sides can be substantially orthogonal to each of the third and fourth sides opposing sides of the radiation delivery window 174. In some embodiments, the first leaf and the second leaf can be independently movable relative to the radiation source in a direction tangential to a circle (i) centered on the axis and (i) defining a plane that is not parallel to the axis. In some embodiments, the leafs can be movable along guide rails 180, as illustrated in FIG. 7. In some embodiments, the collimator can comprise a third leaf 176 and fourth leaf 178 respectively bounding the third and fourth opposing sides of the window. The third leaf, the fourth leaf, and the window can be arranged generally along a line that is parallel to the axis of gantry rotation. The window can be interposed between the third and fourth leaves such that radiation is transmitted between the third and fourth leaves in a direction generally perpendicular to the axis of rotation. The third leaf and the fourth leaf can be independently movable relative to the radiation source with a direction of motion being generally parallel to the axis. The third leaf 176 and the fourth leaf 178 can be moveable independently of each other. In some embodiments, the third and fourth leaves can be movable independently of the first and second leaves. The transverse and axial collimators can be driven by the same motor or different motors. Similarly, the transverse and axial collimators can be controlled by the same hardware or software modules. In some embodiments, transverse and axial collimators can be integrated into a single unit. The VOI can be defined, for example, by a previously-acquired very low dose scan of the same region or two orthogonal localizer scans could be used. An operator can specify an axial extent of the VOI and the size, shape, and location of each ROI along the axial direction. In some embodiments, an outline of the entire VOI can be drawn from two orthogonal views, e.g. sagittal and coronal views, with the images zoomed according to the largest ROI in the sequence. The truncated region of each reconstructed image can be displayed with a dark background. The axial collimator leaves can be opened and closed based on the axial extent of the VOI. If the VOI is modeled using elliptical cross-sections, the transverse collimator leaves can move smoothly as they closely follow the outline of the VOI. In the case of a large cone-beam, the beam already encompasses a large portion of the VOI, therefore there can be less narrowing of the VOI profile at the ends of the scan. As in the case of the axial collimator, the position of the dynamic transverse collimator leaves can be based on the couch position. However, in the case of the dynamic transverse collimator, as the couch moves in the axial direction, the rotation angle can be used to determine where the current ROI is situated with respect to the source. Given both the couch position and rotation angle, the leaves can continuously follow the outline of the overall VOI. For example, in the case of the cardiac scan shown in FIG. 4, the collimator leaves can follow the ROIs located along the cardiac volume based on the couch location of the ROI as well as the rotation angle of the x-ray source. The ROI along the cardiac volume can have both a non-circular (e.g., elliptical) shape and a location away from a scan center 150. In some embodiments, for a sequence of axial or circular scans, one ROI can be determined for each scan in the sequence. For each scan, the rotation angle can be used alone to determine the motion of the collimator leaves, adjusting for an off-center and/or non-circular ROI. In some embodiments, the size of the radiation delivery window 174 between movable leaves of the transverse collimator can be adjusted prior to a scan and held in a fixed arrangement during the scan. For example, a low dose can be performed to identify a VOI, then a subsequent scan can be performed with an transverse collimator aperture dimension selected for imaging of the VOI while the transverse collimator leaves remain stationary during the scan. In some embodiments, the moveable leaves can comprise secondary windows and attenuating members, as discussed further below. In some embodiments, the collimator can comprise a primary radiation delivery window 174 of a fixed size. In some embodiments, between scans, a collimator having a window of a first fixed size can be removed from the CT scanner 102 and can be replaced with another collimator having a window of a second fixed size, different than the first fixed size. The collimators with primary radiation delivery windows of a fixed size can comprise secondary windows and attenuating members, discussed further below, in some embodiments. As illustrated in FIG. 8, attenuation information from the tissue surrounding the collimated ROI is desirable due to the extent of the convolution involved in image reconstruction. Attenuation information for the material outside the collimated VOI can be acquired to facilitate accurate reconstruction. This can be accomplished in various ways, including one or more of the following: (1) use the previously-acquired very low dose scan mentioned above to measure the attenuation outside the VOI; (2) use heavily-attenuated rays through the outer portions of the collimator, providing an estimate of the attenuation outside the VOI so long as the collimator attenuation is previously calibrated; (3) use a projection-completion estimate, extrapolating the truncated projections using a curve-fit based on a simple model; or (4) use limited, but known, attenuation information for isolated sub-regions in the surrounding tissue. In some embodiments, attenuation values for the region outside the ROI can be determined from radiation passing through windows in a grating that are separated from a primary window configured to allow passage of rays of radiation that would travel through the ROI. FIG. 9 shows an example of a display output, such as can be displayed on a monitor of a scanner, of a CT scan of an anthropomorphic thorax phantom using a grated collimator. The image of the display output is shown without application of any correction. In some embodiments, correction can be applied to the data used to form the image. In some embodiments, correction can be applied to image data before displaying an image. The VOI in the image of FIG. 9 includes a phantom cardiac region near the center of the thorax phantom. In some embodiments, the collimator 142 can comprise a first grating 170 and a second grating 172 positioned on opposing sides of a primary radiation delivery window 174. Each of the first and second gratings can comprise a plurality of attenuating members 182 with a plurality of secondary radiation delivery windows 184 extending between adjacent attenuating members of the first grating and the second grating, respectively. A width of each secondary window can be less than a width of the primary window, as illustrated in FIG. 6, for example. Although FIG. 6 illustrates attenuating members of the first grating and the second grating as being integral with each other, the first and second gratings can be part of separate first and second leaves 170, 172, as illustrated in FIG. 7, for example. A total area of each of the secondary windows can be less than a total area of the primary window. In some embodiments, the width of each secondary window can be proportional to a distance between the secondary window and the primary window, as illustrated in FIG. 7, for example. For example, the width of each secondary window can be linearly, exponentially, or geometrically proportional to the distance between the secondary window and the primary window. The width of each secondary window can be positively proportional to the distance between the secondary window and the primary window such that the width of the windows increase with their distance from the primary window. The secondary windows can be oriented generally parallel to sides of the primary window. The secondary windows can comprise open passages extending through the grating. In some embodiments, the secondary windows can comprise panes of substantially radio-transmissive or low-attenuating material. When panes of low-attenuating material are employed, the panes can attenuate the radiation to a lesser extent than the attenuating members 182. A width of each attenuating member, and thus the spacing between secondary windows, can be proportional to a distance between the attenuating member and the primary window, as illustrated in FIG. 7, for example. The width of each attenuating member can be linearly, exponentially, or geometrically proportional to the distance between the attenuating member and the primary window. The width of each attenuating member can be positively proportional to the distance between the attenuating member and the delivery window. The attenuating members can be oriented generally parallel to sides of the primary window. In some embodiments, the attenuating members can block passage of x-ray radiation therethrough. In some embodiments, the attenuating members can be made of materials that substantially prevent transmission of x-rays. The attenuating members can be made of lead, tungsten, or other materials or combinations thereof. In some embodiments, the size, number, position, spacing, or a combination thereof of the secondary windows and attenuating members can provide approximately a minimum or a near-minimum amount of radiation transmission for detector operation. The collimator controller, which can be a hardware or software module, can be configured to control operation of transverse dynamic collimators to significantly reduce the radiation dose delivered by CT scans. In some embodiments, the collimator controller can control both axial and transverse collimators. Further details regarding axial collimators and their control are provided in U.S. Patent Application Publication No. 2010/0246752 to Heuscher et al., entitled Dynamic Collimation in Cone Beam Computed Tomography to Reduce Patient Exposure, the entirety of which is hereby incorporated herein by reference. In some embodiments, a single collimator can be configured for both axial and transverse collimation. A axial collimator and a transverse collimator can be integrated as a single unit in some embodiments. The transverse dynamic collimator, whether alone or in combination with the axial collimator, can be used for both helical and axial scans. In some embodiments, the leaves of the axial and transverse collimators can be moved such that only or substantially only that radiation, e.g., x-rays, that intersects a predefined VOI is exposed to the patient. In some embodiments, control of the transverse collimator involves the velocity of each leaf. The velocity of each leaf can be determined for a region of interest (ROI) at a given distance from scan center. In some embodiments, the ROI in a particular slice can be circular or noncircular. The velocity and acceleration of leaf movement depend on how far off-center a given cross-sectional region of interest (ROI) is located from a scan center within a field of view (FOV). Referring to FIG. 10, the following equations can define the transverse collimator leaf position Q(t) and approximated velocity Q′ given the rotation speed p, collimator distance C, distance S from an ideal radiation point source to the scan isocenter, ROI radius R1, and offset R0 at angle θ0. Q ⁡ ( t ) = C · tan ⁡ ( arcsin ⁡ ( R 1 L ⁡ ( t ) ) + arcsin ⁡ ( R 0 L ⁡ ( t ) · sin ⁡ ( θ ⁡ ( t ) - θ 0 ) ) ) ( 1 ) L ⁡ ( t ) = ( S · sin ⁡ ( θ ⁡ ( t ) - θ 0 ) ) 2 + ( S · cos ⁡ ( θ ⁡ ( t ) - θ 0 ) - R 0 ) 2 ( 2 ) ∂ Q ⁡ ( t ) ∂ t ≈ Q ′ ⁡ ( t ) = C · π · R 0 ρ · S · ( cos ⁡ ( θ ⁡ ( t ) - θ 0 ) - R 1 S · sin ⁡ ( θ ⁡ ( t ) - θ 0 ) - R 0 S ) ( 1 - R 0 S · cos ⁡ ( θ ⁡ ( t ) - θ 0 ) ) 2 ( 3 ) L(t) can represent the distance from the ideal radiation point source to the center 156 of the ROI and can be used to obtain the angle between the line 158 of length L and the central ray of the projection (line 160 of length S). This angle can be added to the remaining angle between the line 158 and the line 162 tangent to the ROI. The distance Q(t) of the collimator leaf from the central ray can be obtained by multiplying the tangent of the resulting angle by a total distance C between the collimator 142 and the point source. The derivative of the expression for Q(t) can be calculated to calculate the velocity Q′. Assuming a constant rotation speed ρ, the expression 2πt/ρ can substituted for θ(t). Due to its complexity, the derivative of this equation, δQ(t)/δt, can be calculated numerically. In some embodiments, approximations can be made to obtain a closed form expression for obtaining the derivative of the equation δQ(t)/δt. By assuming R0 and R1 are relatively small compared to the point source to isocenter distance S, the higher order terms in the Taylor series expansions of both Q(t) and δQ(t)/δt can be eliminated and a good approximation Q′(t) can be obtained as shown in equation (3). The leaves of the axial collimator can be opened at the beginning of a scan, e.g., a helical scan, and closed at the end such that only that radiation, e.g., x-rays, that intersect the VOI are exposed to the subject, as illustrated in FIGS. 11 and 12 for the right end of the scanned volume, for example. As the beam, e.g., a cone-beam, approaches the cylindrical VOI from the right, the leaves can be closed, shifted to the far left. As the left edge of the beam touches the far edge of the cylinder, the right leaf can begin opening by moving to the right until the center of the beam reaches the edge of the cylinder. The right leaf then can immediately accelerate to a higher velocity to be able to follow the near edge of the cylinder, until the entire beam is exposed. The collimator can remain open as the scan proceeds until the left edge of the beam hits the near edge of the left side of the cylinder, at which point the left collimator leaf can begin closing in reverse order from the previous sequence for the right leaf. The following equations can define the axial collimator position Q(t) and velocity δQ/δt given the helical pitch P, helical position H(t), rotation speed ρ, detector width w, collimator distance C, point source to isocenter distance S, and cylindrical VOI radius R: H ⁡ ( t ) > 0 ⁢ : ⁢ ⁢ Q ⁡ ( t ) = C · w 2 · S · ( 1 + 2 · P · t ρ · ( 1 + R S ) ) ⁢ ⁢ ∂ Q ⁡ ( t ) ∂ t = C · w · P ρ · ( S + R ) ( 4 ) H ⁡ ( t ) < 0 ⁢ : ⁢ ⁢ Q ⁡ ( t ) = C · w 2 · S · ( 1 + 2 · P · t ρ · ( 1 - R S ) ) ⁢ ⁢ ∂ Q ⁡ ( t ) ∂ t = C · w · P ρ · ( S - R ) ( 5 ) From right to left, as the beam, e.g., cone-beam, approaches the cylindrical VOI, the collimator can attenuate all rays outside the far edge of the cylinder (H(t)>0). Once the center of the beam passes the end of the cylinder (H(t)<0), the collimator can attenuate all rays inside the near edge of the cylinder. Consequently, the denominator of the equations correspondingly changes from (S+R) to (S−R) between equations (4) and (5). FIGS. 13 and 14 are plots of the velocities (in cm/s) for a transverse collimator leaf where θ0=0, S=70, and ρ=0.3 s/rev. In FIG. 13, C=27, R0=18, and R1=6. In FIG. 14, C=12, R0=6, R1=6. As illustrated in FIG. 13, for a ROI of 12 cm within a 48 cm diameter FOV and an offset R0 of 18 cm, the transverse collimator leaf can achieve a maximum velocity of 198 cm/s. On the other hand, as illustrated in FIG. 14, if the collimator is located 12 cm from the idealized radiation point source and if the subject is positioned such that the ROI is 12 cm closer to the scan center, the collimator leaf reaches a maximum velocity of 24 cm/s for an offset of 6 cm. Therefore, some embodiments include moving the transverse dynamic collimator closer to the radiation point source, moving the subject closer to the scan center, or both. FIG. 15 is a plot of the velocities (in cm/s) for a transverse collimator leaf where θ0=0, S=70, ρ=0.3 s/rev, C=12, R0=18, and R1=6. Thus, if a velocity of 100 cm/s or less can be tolerated and the collimator is located within 12 cm of the idealized radiation point source, an off-center ROI can be tracked by the collimator and, thus, a subject can remain in the same position within the FOV between a preliminary full-field scan and subsequent scanning targeting to the VOI. FIG. 6 shows that a maximum velocity of 87 cm/s is reached for a 12 cm ROI located 18 cm off-center according to the equations provided herein. FIGS. 13-15 also compare the closed form approximation Q′ (dashed line) with the actual velocity δQ/64 (solid line) computed numerically from Q(t). The close match between the two curves confirms the validity of the approximation for the range of geometric values simulated. FIG. 16 is a plot of the velocities (in cm/s) for leaves of an axial collimator where C=27 cm, S=70 cm, a detector width w is 8 cm, a pitch factor P is 1.4, rotation speed p is 0.3 sec/rev, and a VOI has a radius R of 25 cm. FIG. 16 shows a maximum axial collimator leaf velocity of 22.9 cm/s. That maximum velocity occurs for H(t)<0, when the center of the cone-beam passes the edge of the cylinder closest to the x-ray source. The velocity profiles of the transverse collimator leaves and the axial collimator leaves can be compared. The maximum velocity incurred by the axial collimator leaves during a typical helical scan with an 8 cm detector, 0.3 second rotation time (i.e., revolution duration), with a pitch factor of 1.4 is 22.9 cm/sec. On the other hand, the maximum velocity of a transverse collimator positioned 27 cm from the source is 198 cm/sec for a 12 cm cardiac scan located 18 cm from the scan center. Thus, the velocities of the transverse collimator can be greater than the helical collimator. For a transverse collimator mounted 27 cm from the x-ray source, e.g., just beyond the opening of the gantry, the maximum velocity incurred can be more than 8 times that of the axial collimator. However, if the collimator is located closer to the source and if the VOI is positioned, or possibly repositioned, 12 cm or less off-center, the velocities of the transverse and axial collimators can be comparable. In some embodiments, a motor and control system for at least the transverse collimator can provide leaf velocities up to 90 cm/s, patient repositioning can be avoided in more instances. In some embodiments, a motor and control system for at least the transverse collimator can provide leaf velocities greater than 90 cm/s. Although movement of leaves in a transverse dynamic collimator have been discussed in connection with placement at 12 cm and 27 cm from the radiation source, a transverse dynamic collimator can be positioned at other distances from the radiation source. For example, the transverse dynamic collimator can be positioned within about 27 cm, about 20 cm, about 15 cm, or about 10 cm of the radiation source in some embodiments. In some embodiments, the collimator can be integrated into the radiation source unit 112. By integrating a transverse dynamic collimator into the radiation source unit 112, the distance from the radiation source (measured from the idealized point source location) can be shorter and, thereby, reduce the dynamic requirements (velocity and acceleration) of the transversely moving leaves. However, the distance between the idealized point source and the transverse dynamic collimator is preferably sufficiently long that the size of the radiation penumbra is acceptable. For example, in embodiments that comprise a grated collimator, the distance between the idealized point source and the transverse dynamic collimator is preferably sufficiently long to avoid crosstalk between adjacent secondary windows, e.g., slots, in the grated collimator. Modeling of heart and kidney scans, with and without transverse dynamic collimation, indicate significant skin dose reduction in both cases. Elliptical models were analyzed to determine the skin exposure for heart and kidney scans. A dose of radiation, e.g., X-rays, exposed to the subject, e.g., a patient, can be significantly reduced using a dynamic transverse collimator such as that illustrated in FIG. 5, for example. The use of other dynamic transverse collimators, such as the grated dynamic transverse collimator illustrated in FIG. 7, can also significantly reduce radiation dose. FIG. 17 shows three cross-sections from the middle of a five cross-section sequence. A target ROI corresponding to a heart is represented by an ellipse in each cross-section of FIG. 17. Dynamic transverse collimation can follows the ellipses outlining the heart and representing the target ROI. The cross-sections of FIG. 17 also include body outlines, approximated with ellipses, from which the skin exposure values were calculated. The five sections of the heart were equally spaced. In the un-collimated case, all sections are fully exposed, while in the collimated case, the exposure is limited to the target VOI, with the first and last sections fully collimated down to a 0 cm diameter circle. Reference lines are shown in FIG. 17 that intersect the scan center, the body ellipse, and target ROI. The average un-collimated skin exposure relative to the average exposure at the center of the target ROIs is compared to the average collimated skin exposure over the five slices again relative to the average exposure at the center of the target ROI. For the exposure analysis of the heart, reconstructions of an XCAT phantom were used with an X-ray source radius S of 57 cm. A compensator in front of the X-ray source can provide a more uniform signal to the detector and can reduce the overall dose to the patient. Therefore a compensator was modeled that compensates for a disc with a radius rc of 24 cm and an attenuation coefficient equal to water (0.183/cm at 80 KeV, approximately the average energy for a typical CT system performing 120 KeV scans). As an initial approximation, the water attenuation value of 0.183/cm was assumed for all path lengths throughout the body. The exposure for each of 100 points equally spaced in angle β on the surface of the body ellipse was calculated for 1000 angular views, θ, of the source covering 360 degrees. Rays emanating from the source pass either just through the compensator (FIG. 4), or both through the compensator and the body (FIG. 5). These exposure values were averaged over the 1000 views. The exposure at the center of the target ROI (FIG. 6), was averaged over 1000 views. This provided a reference for the skin exposure values. Thus, all overall skin exposure values were measured as a ratio with respect to the exposure at the center of the target ROI. For the scans utilizing a dynamic collimator, the fan angles for which the collimated x-rays intersect tangentially to the target ROI were calculated for each x-ray source position and used to exclude all exposure from the x-ray source that fell outside the corresponding angular range. Again, all skin exposure values were averaged over 1000 views. In the case of the heart, 5 equally-spaced sections of the heart were used to define the cardiac VOI, with the first and last sections fully collimated. The relative skin dose for the center section was compared with and without dynamic collimation. The final skin exposure values were averaged over the five sections, both with and without dynamic collimation. The boundaries of the ellipses were defined by specifying 6 user-defined points around the periphery of both the body outline and target ROI. A least-squares solution to the parameters of each ellipse, (a, b, r0, t0, tr)=(major axis, minor axis, polar radius of the origin of the ellipse, polar angle of the origin, and angular orientation of the ellipse), was then obtained given the (R,T) polar coordinates of these 6 points along with the following constraints: X 2 a + Y 2 b = 1 ⁢ ⁢ and ⁢ \| tr - π 2 \| < π 2 where:X=R·cos(T−tr)−r0·cos(t0−tr)Y=R·sin(T−tr)−r0·sin(t0−tr)(R,T) correspond to the polar coordinates of the 6 points and the following initial values are provided:(a,b,r0,t0,tr)=(TOL,TOL,com(R,T),mean(T),TOL)where:com(R,t)=√{square root over (mean(R·cos(T))2+mean(R·sin(T))2)}{square root over (mean(R·cos(T))2+mean(R·sin(T))2)}TOL=tolerance value=0.00001. Given both the body and target ellipses along with the compensator attenuation as a function of the fan angle, the skin exposures was calculated. For all ray angles up to the tangent to the body ellipse, the exposure was the attenuated exposure through the compensator (FIG. 18). For all other angles for which the rays pass through the body to the selected point on the ellipse, the x-ray exposure is further attenuated by the path length through the body. Given the source radius, source angle, and point on the body ellipse, the path length p relative to the distance p0 (FIG. 19) can be calculated as a solution to a quadratic equation resulting from the condition that the entrance point of the ray also satisfies the equation for the body ellipse. A point on the skin was treated like any other point within the body ellipse. In the case of a point on the boundary of the ellipse directly exposed by the x-ray source, the path lengths through the body converge to zero at all angles up to those angles tangent to ellipse. FIG. 18 illustrates geometry for a ray at angle α directly exposing the skin, emanating from the x-ray source located at angle θ. The ray illustrated in FIG. 18 is only attenuated by the compensator as a function of angle α. FIG. 19 illustrates another geometry for a ray at angle α indirectly exposing the skin, emanating from the x-ray source located at angle θ. The ray illustrated in FIG. 19 is further attenuated by the body path length (1−p) p0. The resulting skin exposure is then calculated as an average value over 360 degrees of the angular position (θ) of the x-ray source: Exposure ⁡ ( β ) = .001 · ∑ i = 1 1000 ⁢ ⁢ [ ( intensity ( α ⁡ ( β , θ i ) ] ) ) · att ⁡ ( β , θ i ) where:β=the angle of the point on the body ellipseθ=the source angleα=fan angle of the ray intersecting the point on the body ellipse intensity ⁡ ( α ) = ⅇ 2 · .189 · ( rc 2 - ( S · sin ⁢ ⁢ α ) 2 - rc ) att ⁡ ( β , θ ) = ⅇ - .183 · ( 1 - p ⁡ ( β , θ ) ) ⁢ p ⁢ ⁢ 0 ⁢ ( y , s ) p is the path length to the entrance point on the body ellipse relative to p0; andp0 is the path length to the selected point on the body ellipse. The average relative skin exposure is then the average of the exposure for all points around the body ellipse divided by the exposure at the center of the target ROI. For the average collimated exposure, the same exposure equation is used, but with the intensity set to zero for all ray angles whose fan angle exceeds that of the range of angles spanned by the two rays that intersect the tangents to the target ellipse, i.e.:intensity(α(β,θ))=0 if α>AC(θ) or α<ACC(θ)AC is the clockwise angular position of the collimator leaf for source angle θ; andACC is the counter-clockwise angular position of the collimator leaf Finally, the exposure at the center of the target ROI is calculated as a reference for the skin exposure values. In this case, p0 corresponds to the path length to the point at the center of the target ellipse and p is calculated as the relative path length to the point on the body ellipse (FIG. 20). As this is the center of the target ellipse, the same value applies whether or not dynamic collimation is used. FIG. 20 illustrates geometry for a ray at angle α exposing the center of the target ROI. The ray of FIG. 20 is further attenuated by the body path length (1−p) p0. For the kidneys, ellipses were used to outline the target ROI and body of the patient. A single multi-slice scan was used to acquire the kidney perfusion images with the central image shown in FIG. 21. The average relative (un-collimated) skin dose was compared to the average relative (collimated) skin dose. A second kidney study was used to not only corroborate the results of the first study, but to demonstrate the additional dose savings that would be achieved for a whole-organ kidney study. Five equally-spaced sections (FIGS. 22A-E) were selected spanning the entire kidney with the first and last sections fully collimated when utilizing dynamic collimation. The central section was used to compare with the central section of the previous study and the overall whole-organ un-collimated (relative) kidney dose was compared with the overall dynamically collimated (relative) kidney dose. The reduction in skin exposure that can be achieved using transverse dynamic collimation of heart and kidney scans was calculated using the above-described elliptical models. The results, collimated and un-collimated exposure values relative to the average exposure calculated at the center of the target ROI, are summarized in Table 1 below: TABLE 1Un-collimatedCollimatedExposureExposureExposureReductionHeart2.611.22.1:1Whole Organ2.605.7043.7:1Kidney Study I2.5451.581.61:1 Kidney Study II2.11.02.1:1Whole Organ II2.1.5843.6:1 These results indicate a significant skin dose reduction for both heart and kidney scans. A 3.7:1 reduction in skin exposure was calculated for the whole-heart scan. A 1.6:1 to 2.1:1 reduction in skin exposure was calculated for a kidney scan and a 3.6:1 reduction for a whole organ kidney scan (both kidneys are included in the target ROI). The reduced skin exposure calculated for heart and kidney scans demonstrates the significant benefit of transverse dynamic collimation, especially to whole organ studies Skin exposure values are 2.1 to 2.6 times higher than the exposure at the center of the target ROI, even with an x-ray compensator. This demonstrates how important it is to keep skin exposure values as low as possible. Dynamic collimation to target the VOI can greatly reduce patient dose (up to 4:1 reduction in skin exposure for whole organ cardiac and kidney scans). This reduction in dose may enable coronary CT angiography to be used on a much more routine basis. Likewise, significantly reducing the dose for CT perfusion scans and whole organ kidney scans will greatly benefit the clinical use of such scans. Dynamic collimation can greatly reduce dose for other clinical applications as well. In some embodiments, radiation dose delivered to a patient can be reduced by varying a scanning frequency or interval between scans during a series of scans. FIG. 23 illustrates a method of contrast-enhanced computed tomography (CT) imaging. The following description of FIG. 23 refers to FIG. 24, which schematically illustrates a curve 2410 representing a magnitude of radiation attenuation (vertical axis) by a contrast-enhanced structure over time (horizontal axis), and a threshold 2412. In some embodiments, the threshold 2412 is 35 HU. In some embodiments, the threshold can be other predetermined attention densities, such as 50, 75, or 100 HU, for example. In some embodiments, the threshold comprises a degree of increase, e.g., a percentage, in the attenuation compared to a value indicated by an initial scan. At step 2310, scanning can begin at a first rate while monitoring for an increase of attenuation above the threshold 2412. At step 2312, after detection of the increase of attenuation above the threshold, the rate of change of attenuation can be determined between scans. At step 2314, when the rate of change of attenuation decreases (corresponding to an inflection point 2414 on the ascending part of the curve 2410), the rate of scanning can be increased. At step 2316, when the rate of change of attenuation becomes negative (corresponding to a peak 2416 of the curve 2410), the rate of scanning can be decreased. In some embodiments, the frequency is reduced at step 2316, in response to detection of a decrease in attenuation. For example, the scan interval can be lengthen for a next scan upon detecting a first decrease in attenuation after the increase above the threshold 2412. At step 2318, when the rate of change of attenuation decreases (corresponding to an inflection point 2418 on the descending part of the curve 2410), the rate of scanning can be further decreased. The rate can be further decreased in some embodiments by increasing a scan interval with each successive scan. In some embodiments, the scan interval can be approximately doubled with each successive scan. Scanning can be terminated at step 2320 in response to expiration of a predetermined period of time, completion of a predetermined number of scans, increase of a scan interval beyond a predetermined interval, or other trigger. In some embodiments, if a remaining time between a latest scan and an end of the predetermined period is less than an interval between the latest scan and an immediately preceding scan, a penultimate scan can be performed at approximately half of the remaining time after the latest scan and a final scan can be performed at the end of the predetermined period. FIG. 25 is an exemplifying plot of magnitudes of radiation attenuation by various contrast-enhanced structures over time. The vertical axis corresponds to attenuation density in Hounsfield Units. The horizontal axis corresponds to time in seconds. In FIG. 25, the large circles represent scans initiated by a sampling frequency adjustment according to an embodiment. Small circles correspond to scans obtained at each of 70 seconds. Squares in FIG. 25 represent attention through the LAD territory as detected by scans each second. FIG. 25 shows that application of a sampling frequency or interval adjustment can result in 18 scans compared to 70 scans if a scan is performed each second. Triangles in FIG. 25 represent attention through remote territory as detected by scans each second. The methods described herein for controlling contrast-enhanced computed tomography imaging can be implemented by computer system. For example, such a system can comprise an attenuation monitoring and a scanning-frequency control module. The monitoring module can be configured to begin monitoring of the rate of change after detection of compliance of the attenuation with the threshold 2412. In some embodiments, the system can further comprise a processing module configured to generate a representation of a relationship between time and radiation attenuation by a second structure within the target region. In some embodiments, the system can further comprise a termination module configured to terminate the scanning The attenuation monitoring module can be configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region. The monitoring module can be configured to monitor a rate of change of the attenuation. The scanning-frequency control module configured to (i) increase a frequency of scanning from a first rate to a second rate after detection of an increase of the attenuation, and (ii) decrease the frequency to a third rate after detecting a decrease in attenuation after increasing the frequency to the second rate. The scanning-frequency control module can be configured to increase the frequency to the second rate in response to detection of a decrease in a rate at which the attenuation is increasing. The scanning-frequency control module can be configured to decrease the frequency below the third rate in response to detection of a decrease in a rate at which the attenuation is decreasing. The scanning-frequency control module can be further configured to decrease the frequency further below the third rate with each successive scan. The scanning-frequency control module can be configured to divide the frequency by approximately two with each successive scan. The scanning-frequency control module can be configured to reduce the frequency to the third rate upon a first detection of a decrease in attenuation after an increase to the second rate. The termination module can be configured after a predetermined period of time and direct performance of a final scan at the end of the predetermined period. The termination module can be configured to terminate the scanning after a predetermined period of time, and, if a remaining time between a latest scan and an end of the predetermined period is less than an interval between the latest scan and an immediately preceding scan, direct performance of (i) a penultimate scan at a half of the remaining time after the latest scan and (ii) a final scan at the end of the predetermined period. In some embodiments, radiation dose delivered to a patient can be reduced by varying an applied power during a series of scans. FIG. 26 illustrates a method of contrast-enhanced computed tomography (CT) imaging. At step 2610, scanning can be at a first applied power while monitoring for an increase of attenuation above the threshold 2412. At step 2612, the applied power for each of a plurality of scans can be selected. The applied power can be varied among scans of a series during a session. The applied power can be determined, at least in part, by selection of an applied current. In some embodiments, the applied power can be varied by changing an applied voltage or a resistance. In some embodiments, the power for a first scan can be a maximum power applied during the session. In some embodiments, a current applied to a first scan can be about 200 ma. The applied power can be selected in some embodiments by multiplying a maximum current by an exponential function based on the attenuation determined from the preceding scan. In some embodiments, the exponential function can yield a value that is (i) greater than a minimum allowable current divided by a maximum allowable current, and (ii) less than 1. In some embodiments, the exponential function can be function F determined byF=eC(TH-ΔHU)/TH wherein TH is a threshold attenuation magnitude and ΔHU is equal to a difference in magnitude, in Hounsfield Units, between the attenuation determined from a preceding scan and a baseline attenuation. The baseline attenuation can be a magnitude of the attenuation indicated based on the initial scan. In some embodiments, C is selected such that, when the function is applied, an applied current for a next scan is about a tenth of the maximum allowable current when the attenuation of the preceding scan is about ten times above the threshold attenuation magnitude. In some embodiments, C can be about 0.25. FIG. 27 is an exemplifying plot, similar to FIG. 25, of magnitudes of radiation attenuation by various contrast-enhanced structures over time, and indicates a current magnitude for a plurality of scans. The vertical axis corresponds to attenuation density in Hounsfield Units. The horizontal axis corresponds to time in seconds. In FIG. 27, each large circle represents a scan and an applied current is identified for each large circle. As shown by FIG. 27, an applied power corresponding to a minimum allowable current can be selected for each scan for which a determining function, such as the function F for example, indicates, based on the attenuation indicated by a preceding scan, a current less than the minimum allowable current. The methods described herein for controlling contrast-enhanced computed tomography imaging can be implemented by computer system. For example, such a system can comprise an attenuation monitoring and a power control module. In some embodiments, the system can comprise a power control module in addition or alternative to a scanning-frequency control module. The power control module can be configured to select an applied power for each of a plurality of scans based on the attenuation detected from a preceding scan. The power control module can be configured to direct application of a maximum power applied during the session in a first scan. The power control module can be configured to apply substantially the same amount of power to individual scans until detection of an increase of the attenuation to or beyond a threshold attenuation magnitude. The power control module can be configured to select the applied power by multiplying a maximum current by an exponential function, such as described above. The statistics for perfusion curves can be improved with a number of slices used for a VOI. Thus, in some embodiments, a VOI can utilize more than one slice. Motion may occur between the reference scan and perfusion series. Registration can be performed for the tissues containing the VOIs. Motion correction can be performed between all images acquired in the series. In some embodiments, motion correction can significantly improve the quality and accuracy of the perfusion curves. For example, motion can occur because of breathing as indicated by the oscillations repeating approximately every three seconds in the LAD territory data of FIGS. 25 and 27. In some embodiments, motion control can permit further dose reduction while retaining statistical accuracy of the perfusion curves and achieving comparable diagnostic results. A kidney perfusion study was performed applying an embodiment of scan frequency control. Kidney perfusion data was acquired using a first protocol. In the first protocol, a low dose reference scan was performed. From the reference scan, an axial location and extent of the scan was identified for a perfusion series. The target ROIs were identified. A 60-second scan series was performed, with each scan being acquired with a tube current of 200 ma. One scan was performed each second for 60 seconds. Each scan was directed to a 8 cm circular ROI. FIG. 21 illustrates a slice obtained from a first person using the first protocol. From the slice shown in FIG. 21, various perfusion parameters were determined, including mean arterial transit time (MTTa), renal plasma flow (RPF), and glomerular filtration rate (GFR). These parameters are shown in Table 2A, below, as “fully-sampled data.” Sub-sampled data was obtained by applying a second (emulated) protocol to select a subset of data from the fully-sampled data. According to the second protocol, a low dose reference scan would be performed. From the reference scan, the axial location and extent of the scan would be identified as well as the arterial VOI used to define the motion of a transverse dynamic collimator. The target ROIs would be identified. The sub-sampled data was selected as though a 60-second scan series had been performed with the scanning frequency being adjusted during the series as data indicative of the arterial input function (AIF) was acquired. A sampling algorithm was applied in the second (emulated) protocol. The sampling algorithm adaptively varied the sampling along the arterial input function (AIF) which corresponds, to blood flow in the aorta. FIGS. 28 and 29 illustrate the adaptive sampling based on the AIF. The second protocol was applied to select the sub-sampled data from the fully-sampled data as though following steps had been preformed in capturing the data. A first scan took place at time t=0. Another scan was performed every two seconds after the first scan until the arterial curve rose above a predefined threshold TH, e.g. 35 HU. After the curve rose above the threshold, the slope of the curve was tracked using a finite difference. When the magnitude of the slope was found to have decreased, the interval between scans was reduced such that one scan was performed every second. When the peak of the arterial curve was detected by the value of the slope becoming negative, the scanning frequency was made one scan every 2 seconds. After the inflection point of descent of the curve was detected by the magnitude of the slope again decreasing, the scan interval was doubled after each subsequent scan. Very sparse sampling can be performed over the slowly descending exponential portion of the curve. At the end of a predetermined scan period, one final scan was performed to complete the sampling of the arterial curve. FIG. 29 illustrates the ROI 144 for the second (emulated) protocol. FIG. 28 includes curves 190, 192, 194 corresponding respectively to attenuation by cortex 186, aorta 188, and kidney tissue, shown in FIG. 29. FIG. 28 indicates 16 subsamples chosen out of 60. Each subsample is represented by a vertical line intersecting an “x.” FIG. 29 corresponds to a subsample 29-29 in FIG. 28. Current magnitudes are indicated in FIG. 28 for each subsample. The minimum current was 40 ma, occurring at the peak 2816 of the AIF curve, and the maximum current was 200 ma, used at the beginning of the scan. From the data obtained using the second (emulated) protocol, various perfusion parameters were determined, including mean arterial transit time (MTTa), renal plasma flow (RPF), and glomerular filtration rate (GFR). These parameters are shown in Table 2A, below, as “sub-sampled data.” The structures within the ROIs that were used to generate the curves used to calculate these parameters included the cortex of the kidney and aorta. The parameters derived from the original fully-sampled data are compared to those derived from the sparsely-sampled (frequency adjusted) data in Table 2A. The kidney perfusion study compared MTTa/RPF/GFR values between original fully-sampled values and values obtained using sub-sampled data. Table 2A summarizes the results of the comparison. Maximum deviation in MTTa\|RPF\|GFR values is 5%. TABLE 2AMTTa/RPF/GFRFully-sampled data8.5\|144\|34Sub-sampled data8.3\|141\|36 The slice obtained from the first person shown in FIG. 21 (Case I) and a slice obtained from a second person shown in FIG. 22C (Case II) were used to estimate the dosage reduction that would be obtained though use of sparse sampling, modulated tube current, and dynamic collimation to the ROI (an elliptical region surrounding both kidneys and aorta). Those estimates were compared to the radiation exposure without sparse sampling, modulated tube current, and dynamic collimation to the ROI. The overall exposure reductions were also calculated and compared. The dosage reduction from sub-sampling was determined based on application of the second (emulated) protocol, described above. The dosage reduction from dynamic collimation was calculated using the calculation techniques described above under the heading “Dynamic Transverse Collimation Dose Reduction.” The average exposure due to modulated tube current was used to determine the dosage reduction from modulation of tube current. For both Case I and Case II, a scan-exposure algorithm was applied to calculate the exposure reduction that would have been attributable to current modulation had the algorithm been applied during data acquisition. The scan-exposure algorithm would have adaptively varied the input current and, therefore, the emitted radiation during scanning FIGS. 25 and 27 illustrate adaptive scan-exposure control based on the AIF. Given a maximum and a minimum current (ma) allowed, exposure was calculated as though the following scan-exposure algorithm and procedure had been used to acquire the data. Scans were performed at the maximum current, e.g. 200 ma, until the arterial curve was detected to have risen above the threshold. Thereafter, the applied current was reduced, using the predetermined contrast detection threshold TH, by multiplying the maximum current by the following factor F:F=eC(TH−ΔHU)/TH where:C=0.25(minimum allowable ma)/(maximum ma)<F<1ΔHU=the difference of the Hounsfield Unit value of the curve and the baseline value corresponding to the first HU value of the curve. C was chosen such that if the arterial curve rose 10 times above the threshold TH (e.g., 35 HU), the current would be reduced by 0.1. For example, if the attenuation is 275 HU and the baseline is 100 HU (ΔHU=175), then the current would be decreased from a maximum of 200 ma to 40 ma. The applied x-ray current was determined for the remaining scans using this formula above for the rest of the series. Table 2B summarizes the exposure reductions calculated from evaluation of Case I and Case II. The average overall reduction for Case I and Case II is 10.5:1. TABLE 2BCase ICase IISub-sampling Exposure Reduction3.8:13.8:1Current Modulation Exposure1.5:11.5:1ReductionDynamic Collimation Reduction1.6:12.1:1Overall Reduction9.1:1 12:1 Based on the results of this comparison, the second protocol can provide diagnostic results comparable to the first protocol. For kidney perfusion scans, the radiation dose using the second protocol can be expected to be one-tenth of the radiation dose to the patient using the first protocol. FIG. 30 is a conceptual block diagram illustrating an example of a system, in accordance with various aspects of the subject technology. A system 3001 may be, for example, a client device or a server. The system 3001 may include a processing system 3002. The processing system 3002 is capable of communication with a receiver 3006 and a transmitter 3009 through a bus 3004 or other structures or devices. It should be understood that communication means other than busses can be utilized with the disclosed configurations. The processing system 3002 can generate audio, video, multimedia, and/or other types of data to be provided to the transmitter 3009 for communication. In addition, audio, video, multimedia, and/or other types of data can be received at the receiver 3006, and processed by the processing system 3002. The processing system 3002 may include a processor for executing instructions and may further include a machine-readable medium 3019, such as a volatile or non-volatile memory, for storing data and/or instructions for software programs. The instructions, which may be stored in a machine-readable medium 3010 and/or 3019, may be executed by the processing system 3002 to control and manage access to the various networks, as well as provide other communication and processing functions. The instructions may also include instructions executed by the processing system 3002 for various user interface devices, such as a display 3012 and a keypad 3014. The processing system 3002 may include an input port 3022 and an output port 3024. Each of the input port 3022 and the output port 3024 may include one or more ports. The input port 3022 and the output port 3024 may be the same port (e.g., a bi-directional port) or may be different ports. The processing system 3002 may be implemented using software, hardware, or a combination of both. By way of example, the processing system 3002 may be implemented with one or more processors. A processor may be a general-purpose microprocessor, a microcontroller, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Programmable Logic Device (PLD), a controller, a state machine, gated logic, discrete hardware components, or any other suitable device that can perform calculations or other manipulations of information. A machine-readable medium can be one or more machine-readable media. Software shall be construed broadly to mean instructions, data, or any combination thereof, whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. Instructions may include code (e.g., in source code format, binary code format, executable code format, or any other suitable format of code). Machine-readable media (e.g., 3019) may include storage integrated into a processing system, such as might be the case with an ASIC. Machine-readable media (e.g., 3010) may also include storage external to a processing system, such as a Random Access Memory (RAM), a flash memory, a Read Only Memory (ROM), a Programmable Read-Only Memory (PROM), an Erasable PROM (EPROM), registers, a hard disk, a removable disk, a CD-ROM, a DVD, or any other suitable storage device. Those skilled in the art will recognize how best to implement the described functionality for the processing system 3002. According to one aspect of the disclosure, a machine-readable medium is a computer-readable medium encoded or stored with instructions and is a computing element, which defines structural and functional interrelationships between the instructions and the rest of the system, which permit the instructions' functionality to be realized. In one aspect, a machine-readable medium is a non-transitory machine-readable medium, a machine-readable storage medium, or a non-transitory machine-readable storage medium. In one aspect, a computer-readable medium is a non-transitory computer-readable medium, a computer-readable storage medium, or a non-transitory computer-readable storage medium. Instructions may be executable, for example, by a client device or server or by a processing system of a client device or server. Instructions can be, for example, a computer program including code. An interface 3016 may be any type of interface and may reside between any of the components shown in FIG. 30. An interface 3016 may also be, for example, an interface to the outside world (e.g., an Internet network interface). A transceiver block 3007 may represent one or more transceivers, and each transceiver may include a receiver 3006 and a transmitter 3009. A functionality implemented in a processing system 3002 may be implemented in a portion of a receiver 3006, a portion of a transmitter 3009, a portion of a machine-readable medium 3010, a portion of a display 3012, a portion of a keypad 3014, or a portion of an interface 3016, and vice versa. FIG. 31 illustrates a simplified diagram of a system 3100, in accordance with various embodiments of the subject technology. The system 3100 may include one ore more remote client devices 3102 (e.g., client devices 3102a, 3102b, 3102c, and 3102d) in communication with a server computing device 3106 (server) via a network 3104. In some embodiments, the server 3106 is configured to run applications that may be accessed and controlled at the client devices 3102. For example, a user at a client device 3102 may use a web browser to access and control an application running on the server 3106 over the network 3104. In some embodiments, the server 3106 is configured to allow remote sessions (e.g., remote desktop sessions) wherein users can access applications and files on the server 3106 by logging onto the server 3106 from a client device 3102. Such a connection may be established using any of several well-known techniques such as the Remote Desktop Protocol (RDP) on a Windows-based server. By way of illustration and not limitation, in one aspect of the disclosure, stated from a perspective of a server side (treating a server as a local device and treating a client device as a remote device), a server application is executed (or runs) at a server 3106. While a remote client device 3102 may receive and display a view of the server application on a display local to the remote client device 3102, the remote client device 3102 does not execute (or run) the server application at the remote client device 3102. Stated in another way from a perspective of the client side (treating a server as remote device and treating a client device as a local device), a remote application is executed (or runs) at a remote server 3106. By way of illustration and not limitation, a client device 3102 can represent a computer, a mobile phone, a laptop computer, a thin client device, a personal digital assistant (PDA), a portable computing device, or a suitable device with a processor. In one example, a client device 3102 is a smartphone (e.g., iPhone, Android phone, Blackberry, etc.). In certain configurations, a client device 3102 can represent an audio player, a game console, a camera, a camcorder, an audio device, a video device, a multimedia device, or a device capable of supporting a connection to a remote server. In one example, a client device 3102 can be mobile. In another example, a client device 3102 can be stationary. According to one aspect of the disclosure, a client device 3102 may be a device having at least a processor and memory, where the total amount of memory of the client device 3102 could be less than the total amount of memory in a server 3106. In one example, a client device 3102 does not have a hard disk. In one aspect, a client device 3102 has a display smaller than a display supported by a server 3106. In one aspect, a client device may include one or more client devices. In some embodiments, a server 3106 may represent a computer, a laptop computer, a computing device, a virtual machine (e.g., VMware® Virtual Machine), a desktop session (e.g., Microsoft Terminal Server), a published application (e.g., Microsoft Terminal Server) or a suitable device with a processor. In some embodiments, a server 3106 can be stationary. In some embodiments, a server 3106 can be mobile. In certain configurations, a server 3106 may be any device that can represent a client device. In some embodiments, a server 3106 may include one or more servers. In one example, a first device is remote to a second device when the first device is not directly connected to the second device. In one example, a first remote device may be connected to a second device over a communication network such as a Local Area Network (LAN), a Wide Area Network (WAN), and/or other network. When a client device 3102 and a server 3106 are remote with respect to each other, a client device 3102 may connect to a server 3106 over a network 3104, for example, via a modem connection, a LAN connection including the Ethernet or a broadband WAN connection including DSL, Cable, T1, T3, Fiber Optics, Wi-Fi, or a mobile network connection including GSM, GPRS, 3G, WiMax or other network connection. A network 3104 can be a LAN network, a WAN network, a wireless network, the Internet, an intranet or other network. A network 3104 may include one or more routers for routing data between client devices and/or servers. A remote device (e.g., client device, server) on a network may be addressed by a corresponding network address, such as, but not limited to, an Internet protocol (IP) address, an Internet name, a Windows Internet name service (WINS) name, a domain name or other system name. These illustrate some examples as to how one device may be remote to another device. But the subject technology is not limited to these examples. According to certain embodiments of the subject technology, the terms “server” and “remote server” are generally used synonymously in relation to a client device, and the word “remote” may indicate that a server is in communication with other device(s), for example, over a network connection(s). According to certain embodiments of the subject technology, the terms “client device” and “remote client device” are generally used synonymously in relation to a server, and the word “remote” may indicate that a client device is in communication with a server(s), for example, over a network connection(s). In some embodiments, a “client device” may be sometimes referred to as a client or vice versa. Similarly, a “server” may be sometimes referred to as a server device or vice versa. In some embodiments, the terms “local” and “remote” are relative terms, and a client device may be referred to as a local client device or a remote client device, depending on whether a client device is described from a client side or from a server side, respectively. Similarly, a server may be referred to as a local server or a remote server, depending on whether a server is described from a server side or from a client side, respectively. Furthermore, an application running on a server may be referred to as a local application, if described from a server side, and may be referred to as a remote application, if described from a client side. In some embodiments, devices placed on a client side (e.g., devices connected directly to a client device(s) or to one another using wires or wirelessly) may be referred to as local devices with respect to a client device and remote devices with respect to a server. Similarly, devices placed on a server side (e.g., devices connected directly to a server(s) or to one another using wires or wirelessly) may be referred to as local devices with respect to a server and remote devices with respect to a client device. As used herein, the word “module” refers to logic embodied in hardware or firmware, or to a collection of software instructions, possibly having entry and exit points, written in a programming language, such as, for example C++. A software module may be compiled and linked into an executable program, installed in a dynamic link library, or may be written in an interpretive language such as BASIC. It will be appreciated that software modules may be callable from other modules or from themselves, and/or may be invoked in response to detected events or interrupts. Software instructions may be embedded in firmware, such as an EPROM or EEPROM. It will be further appreciated that hardware modules may be comprised of connected logic units, such as gates and flip-flops, and/or may be comprised of programmable units, such as programmable gate arrays or processors. The modules described herein are preferably implemented as software modules, but may be represented in hardware or firmware. It is contemplated that the modules may be integrated into a fewer number of modules. One module may also be separated into multiple modules. The described modules may be implemented as hardware, software, firmware or any combination thereof. Additionally, the described modules may reside at different locations connected through a wired or wireless network, or the Internet. In general, it will be appreciated that the processors can include, by way of example, computers, program logic, or other substrate configurations representing data and instructions, which operate as described herein. In other embodiments, the processors can include controller circuitry, processor circuitry, processors, general purpose single-chip or multi-chip microprocessors, digital signal processors, embedded microprocessors, microcontrollers and the like. Furthermore, it will be appreciated that in one embodiment, the program logic may advantageously be implemented as one or more components. The components may advantageously be configured to execute on one or more processors. The components include, but are not limited to, software or hardware components, modules such as software modules, object-oriented software components, class components and task components, processes methods, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. The foregoing description is provided to enable a person skilled in the art to practice the various configurations described herein. While the subject technology has been particularly described with reference to the various figures and configurations, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the subject technology. There may be many other ways to implement the subject technology. Various functions and elements described herein may be partitioned differently from those shown without departing from the scope of the subject technology. Various modifications to these configurations will be readily apparent to those skilled in the art, and generic principles defined herein may be applied to other configurations. Thus, many changes and modifications may be made to the subject technology, by one having ordinary skill in the art, without departing from the scope of the subject technology. It is understood that the specific order or hierarchy of steps in the processes disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged. Some of the steps may be performed simultaneously. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented. As used herein, the phrase “at least one of” preceding a series of items, with the term “and” or “or” to separate any of the items, modifies the list as a whole, rather than each member of the list (i.e., each item). The phrase “at least one of” does not require selection of at least one of each item listed; rather, the phrase allows a meaning that includes at least one of any one of the items, and/or at least one of any combination of the items, and/or at least one of each of the items. By way of example, the phrases “at least one of A, B, and C” or “at least one of A, B, or C” each refer to only A, only B, or only C; any combination of A, B, and C; and/or at least one of each of A, B, and C. Terms such as “top,” “bottom,” “front,” “rear” and the like as used in this disclosure should be understood as referring to an arbitrary frame of reference, rather than to the ordinary gravitational frame of reference. Thus, a top surface, a bottom surface, a front surface, and a rear surface may extend upwardly, downwardly, diagonally, or horizontally in a gravitational frame of reference. Furthermore, to the extent that the term “include,” “have,” or the like is used in the description or the claims, such term is intended to be inclusive in a manner similar to the term “comprise” as “comprise” is interpreted when employed as a transitional word in a claim. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. A reference to an element in the singular is not intended to mean “one and only one” unless specifically stated, but rather “one or more.” Pronouns in the masculine (e.g., his) include the feminine and neuter gender (e.g., her and its) and vice versa. The term “some” refers to one or more. Underlined and/or italicized headings and subheadings are used for convenience only, do not limit the subject technology, and are not referred to in connection with the interpretation of the description of the subject technology. All structural and functional equivalents to the elements of the various configurations described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and intended to be encompassed by the subject technology. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the above description. While certain aspects and embodiments of the invention have been described, these have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms without departing from the spirit thereof. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.
	abstract	A beam line system includes a hollow tube and a plurality of protruding structures. The hollow tube has an inlet and an outlet. An ion beam emitted by the ion implanter is introduced into the hollow tube through the inlet and exited from the hollow tube through the outlet. The protruding structures are formed on an inner wall of the hollow tube. Each of the protruding structures has a reflective surface for reflecting a portion of the ion beam.
041586042	summary	This invention relates to power generating plant and has especial application to boiler systems for nuclear power stations. In order to cope with peak demands for electricity there is an increasing requirement for a power station having auxiliary generating capacity over and above the normal maximum continuous rating of the station. Such auxiliary capacity can be provided by gas turbine or diesel engine powered plant, but it would be convenient to associate at least some auxiliary generating plant with the heat source already present in the station. Further, where only large steam turbines are employed in a power station a degree of inflexibility in accommodating rapid changes in electricity demand exists because such turbines suffer considerable thermal stresses if subjected to rapid load changes. A further disadvantage lies in the fact that steam turbines are inefficient at low load so that it is undesirable to run a large steam turbine at low load in order to meet a considerably reduced demand for electricity. Finally, with some nuclear reactors, and in particular with the so-called High Temperature Reactor, it is desirable that the predominant flow of reactor coolant through the reactor core be downward. To simplify the reactor coolant circuit it is therefore convenient to pass the reactor coolant upwardly through associated boilers and so, in order to achieve counterflow within the boiler, water and steam must flow downwardly through the boiler. Owing to waterside instability such water downflow boilers could be difficult to operate at low flow rates which may be occasioned either by low load running of a large steam turbine to which they are connected, or by emergency use of such boilers as a heat sink for the reactor core. According to the invention there is provided power generating plant comprising a heat source, at least one main boiler arranged to be heated by heat from the heat source, and at least one main steam turbine arranged to be driven by steam generated in the said main boiler, wherein there is further provided at least one further boiler, of lower capacity than the said main boiler and also arranged to be heated by heat from the heat source, and at least one further steam turbine of lower capacity than the main steam turbine and arranged to be driven by steam generated in the said further boiler. Where the boiler system is used to supply steam to turbines which are each connected to a generator, the said further or auxiliary boiler(s) may be operated to enable the said further or auxiliary turbine(s) and generator(s) to provide auxiliary generating capacity over and above the normal maximum continuous rating of the main turbine and generator. Further, whilst the main boiler(s) are being operated to enable the main turbine to run under substantially constant load, the auxiliary boiler(s) may be operated to enable the auxiliary turbine(s), being smaller and thus more tolerant of rapid load changes, to run under varying load in order to accommodate changes in demand for electricity. The common heat source may advantageously be the core of a nuclear reactor, the boilers being disposed in a heat exchange circuit including the core such that reactor coolant fluid can flow through the core to abstract heat therefrom, and then through the boilers in which the heat is used to generate steam. Conveniently each boiler is disposed in a vertical channel or `pod` defined within the wall thickness of the reactor pressure vessel. To avoid operating the main boiler(s) at low flowrate when supporting a very considerably reduced load, it is possible to shut the main boiler(s) and turbine down and carry the load solely on the auxiliary boiler(s) and turbine(s). Thus the problem, mentioned above, of instability in a main boiler operating at low flowrate can be avoided since, in supporting the same load, the auxiliary boiler(s), being of smaller size, will operate at a considerably higher flowrate. Further, because of the smaller size of the auxiliary boiler(s), it is comparatively easy to arrange the heat exchange circuit such that reactor coolant flows downwardly through the auxiliary boiler(s), so enabling water flow therein to be upward. In order to maintain essential services in a power station during an emergency involving loss of normal power supplies, it is usual to provide emergency generating facilities. Such facilities are usually completely idle during normal running of the station. However, where a boiler system in accordance with the invention is associated with a nuclear heat source it becomes possible to dispense with at least part of the emergency generating facilities since, after emergency or even normal shut-down of the reactor core and the main boiler(s), the auxiliary boiler(s) can be operated to enable the auxiliary turbine(s) and generator(s) to run under at least part load. The auxiliary boiler(s) can continue operating in this manner for some considerable time simply by abstracting residual heat from the reactor core. This operating time can be maximised by designing the auxiliary boilers to produce steam at an appreciably lower temperature and pressure than the main boilers. To enable such continued operation of the auxiliary boiler(s) to occur a water feed and condensing system separate from that for the main boiler(s) is provided. It will be appreciated that a further advantage accrues from the use in such circumstances of a boiler system in accordance with the invention, in that, after shut-down of the reactor core, the core is effectively cooled by the auxiliary boiler(s) which act(s) as a core heat sink. The auxiliary boiler(s) can also be used for power generation during reactor and main boiler start-up.
056205368	claims	1. A method of manufacturing a composite cladding tube of a nuclear fuel element which is resistant to pellet-clad interaction, said composite cladding tube comprising an inner portion formed from a first component selected from the group consisting of zirconium and a zirconium alloy and an outer portion formed from a second component selected from the group consisting of Zircaloy 2, Zircaloy 4 and Zr 2.5 Nb, said method comprising the steps of: (a) providing an ingot of said first component, (b) forging rolling, extruding, and heat treating said ingot of said first component to form an inner billet, (c) positioning said inner billet from step (b) within an outer machined billet formed from an ingot of said second component, (d) extruding said joined billets from step (c) to form a joined tube blank, and (e) machining said tube blank from step (d) to provide said composite cladding tube, (f) said steps (b)-(e) being conducted at a temperature below that which causes incipient beta-phase transformation within said first component. 2. A method according to claim 1, including between steps (a) and (b) a step (a.sup.1) of preheating said ingot of said first component in an alpha-phase range temperature. 3. A method according to claim 2, wherein in step (b) said ingot of said first component is subjected to forging in two steps. 4. A method according to claim 2, wherein said first component consists essentially of zirconium-tin alloy comprising zirconium with 0.1-1% tin, less than 600 ppm iron and less than 600 ppm oxygen. 5. A method according to claim 4, wherein step (a.sup.1) is conducted at a temperature of 700.degree.-860.degree. C. 6. A method according to claim 4, wherein in step (b) said first component is heat treated at a temperature of 600.degree.-860.degree. C. 7. A method according to claim 6, wherein said temperature is 650.degree.-750.degree. C. 8. A method according to claim 2, wherein said first component consists essentially of zirconium. 9. A method according to claim 8, wherein step (a.sup.1) is conducted at a temperature of 700.degree.-800.degree. C. 10. A method according to claim 8, wherein in step (b) said first component is heat treated at a temperature of 600.degree.-800.degree. C. 11. A method according to claim 10, wherein said temperature is 650.degree.-750.degree. C. 12. A method according to claim 1, wherein step (d) is conducted at a temperature below 710.degree. C. 13. A method according to claim 1, wherein in step (b) said first component is rolled and extruded at a temperature below 710.degree. C. 14. A method according to claim 1, wherein step (e) includes cold rolling said tube blank, intermediate heating of said tube blank at a temperature of 525.degree.-700.degree. C., and a final heating of said tube blank at a temperature of 400.degree.-700.degree. C. 15. A method according to claim 1, including after step (d) a step (d.sup.1) of heat treating said tube blank at a temperature of 600.degree.-800.degree. C. 16. A method according to claim 15 including after step (d.sup.1) a step of beta-quenching said outer machined billet.
	claims	1. A method of modulating ions comprising:(a) generating an area beam of ions directed along an axis and having a longitudinal and latitudinal extent in cross-section and divided into latitudinally adjacent beamlets of ions;(b) occluding the area beam with a set of latitudinally arrayed ion-blocking shutters each having a latitudinal width substantially equal to a latitudinal width of a beamlet, the ion-blocking shutters controllably extended to different longitudinal distances at the cross-section according to a desired intensity of a beamlet of ions to block only a controllable portion of the longitudinal extent of the beamlet and to pass without blocking a remaining portion of the beamlet so that the intensity of the beamlet is substantially nonuniform in a longitudinal direction, some of the different longitudinal distances at the cross-section being less than a length of the longitudinal extent of the area beam; wherein an average intensity of the beamlet is defined by a portion of the beamlet occluded by a given shutter; and(c) refocussing the occluded area beam with a lens system to collapsing the area beam and beamlets in the longitudinal direction to form a fan beam directable toward a patient for radiation therapy to provide a substantially uniform longitudinal intensity in each collapsed beamlet. 2. The method of claim 1 further including the step of occluding the fan beam with a set of latitudinally adjacent ion attenuating wedges controllably extended to different longitudinal distances according to a desired energy of a beamlet of ions defined by a portion of the area beam occludable by a given wedge. 3. The method of claim 1 wherein each wedge is matched to a corresponding wedge providing mirror movement to the wedge to present a uniform thickness of material within the fan beam. 4. The method of claim 1 wherein step of the collapsing the area beam passes the area beam through a pair of quadrupole lenses aligned in rotation with each other about the axis. 5. The method of claim 1 wherein the step of generating the area beam passes a pencil beam through a scattering foil to form the area beam. 6. The method of claim 1 wherein the area beam is a proton beam. 7. The method of claim 1 including the step of rotating the fan beam with respect to the patient about a longitudinal axis while controllably varying the longitudinal distances of the shutters. 8. A modulator for ions comprising:(a) a beam generator generating an area beam of ions directed along an axis and having a longitudinal and latitudinal extent in cross-section composed of latitudinally adjacent beamlets of ions;(b) an intensity modulator occluding the area beam with a set of latitudinally arrayed ion-blocking shutters each having a latitudinal width substantially equal to a latitudinal width of a beamlet, the ion-blocking shutters controllably extended to different longitudinal distances at the cross-section according to a desired intensity of a beamlet of ions to block only a controllable portion of the longitudinal extent of the beamlet and to pass without blocking a remaining portion of the beamlet so that the intensity of the beamlet is substantially nonuniform in a longitudinal direction, some of the different longitudinal distances at the cross-section being less than a length of the longitudinal extent of the area beam; wherein an average intensity of the beamlet is defined by a portion of the beamlet occluded by a given shutter; and(c) a lens system receiving the area beam from the intensity modulator, the lens system collapsing the area beam and beamlets in the longitudinal direction to form a fan beam directable toward a patient for radiation therapy to provide a substantially uniform longitudinal intensity in each collapsed beamlet. 9. The modulator of claim 8 further including an energy modulator having a set of latitudinally adjacent ion attenuating wedges controllably extended to different longitudinal distances according to a desired energy of a beamlet of ions defined by a portion of the area beam occludable by a given wedge. 10. The modulator of claim 8 wherein each wedge is matched to a corresponding wedge providing minor movement to the wedge to present a uniform thickness of material within the fan beam. 11. The modulator of claim 8 wherein the lens system is a pair of quadrupole lenses aligned in rotation about the axis. 12. The modulator of claim 8 wherein the beam generator is a scattering foil receiving a pencil beam to spread it into an area beam. 13. The modulator of claim 8 wherein the beam generator generates a proton beam. 14. The modulator of claim 8 including a gantry holding the modulator for rotation of the fan beam with respect to the patient about a longitudinal axis while controllably varying the longitudinal distances of the shutters.
	summary
062401580	claims	1. An X-ray projection exposure apparatus, comprising: an X-ray source that generates X-rays; a mask stage configured to hold a mask having a mask pattern; an X-ray illumination optical system that directs the X-rays generated by the X-ray source towards the mask to project the mask pattern; a substrate stage configured to hold a substrate; an X-ray projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern onto the substrate, the X-ray projection focusing optical system including a plurality of reflective mirrors that reflect the X-rays; and a position detection optical system that optically detects a mark on the mask and a mark on the substrate, at least a portion of the position detection optical system being disposed between the plurality of reflective mirrors. an X-ray source that generates X-rays; an illumination optical system that directs the X-rays generated by the X-ray source towards a mask having a mask pattern; a substrate stage configured to hold a substrate; a projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern onto the substrate, the projection focusing optical system including a plurality of reflective mirrors that reflect the X-rays; and a position detection device that optically detects a position of the substrate in a direction substantially parallel to an optical axis of the projection focusing optical system, at least a portion of the position detection device being disposed between the plurality of reflective mirrors. An X-ray source that generates X-rays; an illumination optical system that directs the X-rays generated by the X-ray source towards a mask having a mask pattern; a substrate stage configured to hold a substrate; a projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern on the substrate, the projection focusing optical system including a plurality of reflective mirrors that reflect the X-rays; and a position detection mechanism that optically detects a position of the substrate in a direction substantially parallel to an optical axis of the projection focusing optical system, wherein a reflective mirror that is closest to the substrate among the plurality of reflective mirrors has a space which allows the passage of light by which the position detection mechanism detects the position of the substrate. an X-ray source that generates X-rays; an illumination optical system that directs the X-rays generated by the X-ray source towards a mask having a mask pattern; a substrate stage configured to hold a substrate; a projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern onto the substrate, the projection focusing optical system including a plurality of reflective mirrors that reflect the X-rays, the projection focusing optical system further including a holder that holds a reflective mirror that is closest to the substrate; and a position detection mechanism that optically detects a position of the substrate in a direction substantially parallel to an optical axis of the projection focusing optical system, wherein the holder has a space in its surface facing the substrate, the space allowing the passage of light by which the position detection mechanism detects the position of the substrate. an X-ray source that generates X-rays; a mask stage configured to hold a mask having a mask pattern; an X-ray illumination optical system that directs the X-rays generated by the X-ray source towards the mask; a substrate stage configured to hold a substrate; an X-ray projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern onto the substrate, the X-ray projection focusing optical system including a plurality of reflective mirrors that reflect X-rays, a reflective mirror closest to the substrate stage being adjacent the substrate stage; and a position detection optical system that optically detects a position of the substrate, wherein the X-ray projection focusing optical system is configured to accommodate at least a portion of the position detection optical system disposed between the plurality of reflective mirrors. wherein the reflective mirror closest to the substrate stage is configured to provide a passage for the optical path of the detection light. 2. The X-ray projection exposure apparatus according to claim 1, wherein a portion of the position detection optical system is disposed between a reflective mirror closet to the substrate and a reflective mirror second closest to the substrate among the plurality of reflective mirrors in the X-ray projection focusing optical system. 3. The X-ray projection exposure apparatus according to claim 1, wherein the position detection optical system includes an illumination optical system and a detection optical system, the illumination optical system illuminating the mark on the mask with light, the light reflected from the mark on the mask being guided towards the mark on the substrate via at least one of the reflective mirrors in the X-ray projection focusing optical system, the detection optical system detecting the light from the mark on the substrate. 4. The X-ray projection exposure apparatus according to claim 1, wherein the position detection optical system includes an illumination optical system and a detection optical system, the illumination optical system illuminating the mark on the substrate with light, the light reflected from the mark on the substrate being guided towards the mark on the mask via at least one of the reflective mirrors in the X-ray projection focusing optical system, the detection optical system detecting the light from the mark on the mask. 5. The X-ray projection exposure apparatus according to claim 1, wherein the position detection optical system includes a motion mechanism. 6. The X-ray projection exposure apparatus according to claim 1, wherein the numerical aperture of the position detection optical system is equal to or less than one half of the numerical aperture of the X-ray projection focusing optical system. 7. The X-ray projection exposure apparatus according to claim 1, wherein the portion of the position detection optical system that is disposed between the plurality of reflective mirrors includes a half-mirror. 8. The X-ray projection exposure apparatus according to claim 1, wherein the position detection optical system includes a temperature adjustment mechanism. 9. An X-ray projection apparatus, comprising: 10. The X-ray projection exposure apparatus according to claim 9, wherein at least a portion of the position detection device is disposed between a reflective mirror closest to the substrate and a reflective mirror second closest to the substrate among the plurality of reflective mirrors. 11. The X-ray projection exposure apparatus according to claim 9, wherein a through-hole is formed in a reflective mirror closest to the substrate to provide a passage for light by which the position detection device detects the position of the substrate. 12. The X-ray projection exposure apparatus according to claim 9, wherein the position detection device includes a motion mechanism. 13. The X-ray projection exposure apparatus according to claim 9, wherein the position detection device includes a temperature adjustment mechanism. 14. An X-ray projection exposure apparatus comprising: 15. The X-ray projection exposure apparatus according to claim 14, wherein the space which allows the passage of the light of the position detection mechanism includes a tapered part formed in the reflective mirror that is closest to the substrate. 16. The X-ray projection exposure apparatus according to claim 14, wherein the space which allows the passage of the light of the position detection mechanism includes at least one of a groove and a through-hole formed in the reflective mirror that is closest to the substrate. 17. An X-ray projection exposure apparatus, comprising: 18. The X-ray projection exposure apparatus according to claim 17, wherein the space, which allows the passage of the light of the position detection mechanism, includes a tapered part formed in the holder. 19. The X-ray projection exposure apparatus according to claim 17, wherein the space, which allows the passage of the light of the position detection mechanism, includes at least one of a groove and a through-hole formed in the holder. 20. An X-ray projection exposure apparatus, comprising: 21. The X-ray projection exposure apparatus according to claim 20, wherein the position detection optical system detects the position of the substrate in a direction substantially parallel to an optical axis of the X-ray projection focusing optical system. 22. The X-ray projection exposure apparatus according to claim 20, wherein the position detection optical system detects the position of the substrate relative to a position of the mask. 23. The X-ray projection exposure apparatus according to claim 20, wherein the position detection optical system detects the position of the substrate in a direction substantially perpendicular to an optical axis of the X-ray projection focusing optical system. 24. The X-ray projection exposure apparatus according to claim 20, wherein the portion of the position detection optical system includes a retractable optical element to be inserted between two of the plurality of reflective mirrors upon detecting the position of the substrate. 25. The X-ray projection exposure apparatus according to claim 24, wherein when inserted, the retractable optical element is positioned between the reflective mirror closest to the substrate stage and the reflective mirror second closest to the substrate stage. 26. The X-ray projection exposure apparatus according to claim 24, wherein when the retractable optical element is inserted, the position detection optical system utilizes at least one of the reflective mirrors in the X-ray projection focusing optical system to detect the position of the substrate. 27. The X-ray projection exposure apparatus according to claim 20, wherein the portion of the position detection optical system to be accommodated by the X-ray projection focusing optical system includes an optical path for a detection light by which the position detection optical system detects the position of the substrate, and 28. The X-ray projection exposure apparatus according to claim 27, wherein the reflective mirror closest to the substrate stage has a tapered portion providing the passage for the optical path of the detection light. 29. The X-ray projection exposure apparatus according to claim 27, wherein the reflective mirror closest to the substrate stage has a through-hole providing the passage for the optical path of the detection light.
044407157	abstract	A nuclear reactor is supplied with feed water through a feed water pump system. A primary steam flow produced from the reactor is controlled by regulating a recirculated flow of feed water. The feed water pump system comprises two main pumps each of 55%-capacity and two auxiliary pumps each of 27.5%-capacity. Normally, the two main pumps are operated. Upon occurrence of abnormal condition of at least one main pump, the auxiliary pumps are started to supply feed water. At that time, the recirculated flow is controlled for a predetermined time to a reduced rate which is smaller as compared with that of the primary steam flow decreased rapidly due to the shutdown of the main pump. Subsequently, the recirculated flow is so controlled that the primary steam flow rate is slightly smaller as compared with the feed water flow which is determined by the available capacity of the pumps.
052308588	description	DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a fuel bundle B is illustrated both partially broken away and cut off in length so that representative portions of the fuel bundle can be seen and understood. Fuel bundle B includes a lower tie plate L forming the upper portion of nose piece N which commonly communicates across a core separation plate (not shown) for receiving water from the lower portion of a reactor (also not shown). Upper tie plate U captures the upstanding end of a group of fuel rods R that extend the full length of fuel bundle B. It will be noted that some fuel rods are partial length, terminating at less than the full distance to upper tie plate U. Centrally of fuel bundle B there is water rod W. (In some designs more than one may be used.) Water rod W has upper compartment 14 with upwardly exposed open end 15. It is into this open end 15 that liquid moderator (water) falls during normal operation of the fuel bundle to fill the upper compartment 14. Lower compartment 17 is supplied with lower entrance apertures 18 and upper exit aperture 20. Water from the bottom of fuel bundle B flows through water rod W at lower compartment 17 and exits at exit apertures 20. In FIG. 1, such exit is shown between the first and second spacers S1, S2. Referring to FIG. 2, a schematic of the invention is illustrated with the schematic being compressed in the vertical direction. Fuel bundle B includes lower tie plate L, upper tie plate U with water rod W extending therebetween. Water rod W has upper compartment 14 with upward opening 15. Water rod W further includes lower compartment 17 with entrance apertures 18 and discharge apertures 20. All seven spacers S1-S7 are illustrated with discharge of the discharges apertures occurring in accordance with the preferred embodiment of this invention between spacers S2-S3. There is a practical limitation on the length of upper compartment 14. If this compartment becomes too long, it may become the site of unstable flow or "chugging." In this case upward flowing steam at the tube exit will be too great to permit water flow down into the tube. As the completely voided condition is approached, the steam flow will decrease and liquid flow will begin again. This cycle of volume and refilling will continue endlessly. This flouroscillation would not be desired and would defeat the nuclear characteristics of the water rod W. Therefore, we preferred to limit the length of upper compartment 14 to about 1/4 to 1/3 the total length of the fuel bundle depending on the counter current flow characteristics of the particular water rod design. Referring to FIG. 3, mention has been made that the upper two phase region is a portion of the fuel bundle B where the thickness of the liquid film coating the center of the fuel bundle B narrows as the upwardly flowing moderator passes along the fuel rods R between the spacers S3, S2, and S1. Liquid and vapor moderator passes upwardly in the direction of arrow 40. Thickness of the vapor film overlying the central fuel rods R is schematically shown lines T1, T2, and T3. Some remarks may be made relative to the graphic illustration. First, and relative to line T1, enriched nuclear fuel typically ends near the first spacer. As the line Tl suggests, thickness of the water film over the central rods is generally not a problem in these locations. Secondly, and with respect to lines T2 and T3, thickness of liquid film over the central fuel rods can be a problem between spacers S1 and S2 or S2 and S3. At flow rates up to in the order of 75%, maximum thinning of the film will typically occur just upstream of spacer S1. As flow increases above 75% flow rate, maximum thinning of the film will typically occur just upstream of spacer S2. Since spacer location may be vertically variable, optimal placement of discharge apertures 20 will be a matter of design.
041586026	summary	BACKGROUND OF THE INVENTION 1. Field of the Invention This invention generally relates to control rod assemblies for nuclear reactors and, more particularly, to the operative connections between control rods and control rod drive mechanisms. 2. Description of the Prior Art One of the most serious accidents that can occur to a nuclear power plant is a loss of the flow of coolant followed by the failure of the control system to accomplish a rapid shutdown of the reactor. A loss of coolant flow can occur from either the rupture of piping or the stoppage of one or more of the coolant circulating pumps. This type of accident is especially serious because the heat generated in the reactor cannot be carried off. If the reactor continues to generate heat, then tremendous pressures are built up in the coolant system. In addition, this heat generation, if it is not terminated by a scram, could melt down the majority of the core of the reactor. In the reactors using liquid sodium for primary coolant, there is a special problem caused by a partial or total loss of sodium flow if reactor scram does not follow promptly. In the present design of liquid metal fast breeder reactors there is a gain in reactivity called a positive sodium void coefficient that occurs when sodium flow is interrupted. The sodium temperature may increase to its boiling point, whereupon "voids" of sodium vapor are formed, resulting in increased reactivity, power, more boiling, and the possibility of serious consequences. This gain in reactivity occurs because although the neutron absorption effect of sodium is small, it is not zero. Any loss of sodium from the core causes a shift in the neutron absorption spectrum and increases the number of neutrons. This shift, in turn, increases the probability of neutron capture by the fissionable atoms in the fuel. Many organizations, government agencies, and corporations have studied the problem of minimizing the probability of the loss of low accident. A considerable design effort has been expended over a priod of many years in order to provide maximum assurance to both the public and the various reactor licensing agencies that this accident can be avoided. Heretofore, the most reliable and simplest system that has been proposed contemplates hydraulically supporting a plurality of tantalum absorber balls in a column above the reactor core by the flow of primary coolant. In the event coolant flow is reduced, these balls which have a large neutron absorption coefficient fall into the high flux region of the core and quickly shut down the reactor. Although the use of hydraulically supported, absorber balls provides a self-actuated and reliable reactor shut-down system, there are many inherent limitations with this system. For example, the absorber balls are hydraulically supported by the flow of primary coolant in a position substantially above the high flux region of the core. In this position the balls cannot be maneuvered to regulate the amount of neutron flux during normal operations. Secondly, the absorber balls are only controlled by the flow of primary coolant through the column. Therefore, the balls add reactivity to the system whenever the main coolant pumps are started and primary coolant flow is commenced. Further, the hydraulically supported absorber balls only shut down the reactor when the flow of primary coolant decreases. The balls cannot be commanded to scram a reactor by either the reactor operator or one of the other reactor safety systems. Finally, considerable testing will be necessary to demonstrate the exact response of the tantalum balls under actual reactor conditions of flow and wear. SUMMARY OF THE INVENTION It is an object of the present invention to provide a novel method and apparatus that overcomes the limitations and disadvantages of the prior art. A further object of the present invention is to develop a reactor shut-down system that can provide maximum assurance to both the public and the various reactor licensing agencies that the system will in fact operate during any emergency situation. Another object of the present invention is to provide a system for scramming a reactor that is directly initiated by a loss of coolant flow and a self-actuated system that will operate without electrical circuits, sensors, or rod drive mechanisms. An additional object of the present invention is to develop a self-actuated, loss of flow scramming system that can also be commanded through a conventional rod drive mechanism to initiate a scram. Still another object of the present invention is to provide a self-actuated, loss of flow scramming system that can be used to control the level of neutron flux during normal power operation, especially normal reactor start-ups and shut-downs. A further object of the present invention is to provide a system that will reliably scram a reactor during a severe earthquake. The foregoing and other objects are achieved by a control rod assembly that includes a separator plate having an orifice through which primary coolant flow and across which a differential pressure may be developed. Around the orifice is located a sealing surface that can be engaged by a control rod. The control rod can be independently moved with respect to the core of the reactor and can also be brought up and into engagement with the separator plate. The differential pressure across the separator plate and the control rod seal retains the sealing surfaces of the control rod and the separator plate together. In operation, normal flow through the reactor develops sufficient differential pressure across the separator plate and the control rod seal to retain the control rod against the separator plate and above the high neutron flux region of the core. When the flow of primary coolant is reduced or the differential pressure across the separator plate decreases sufficiently, the control rod falls by gravity and the reactor is scrammed. The reactor can also be scrammed on command by the downward motion of the rod drive shaft which causes the sealing surfaces to separate. During a severe earthquake the reactor will be scrammed because a lateral motion of the control rod with respect to the separator plate brought about by different inertia of the separator plate and the control rod will cause the sealing surfaces to separate. Additional objects and features of the invention will appear from the following description in which the preferred embodiment has been set forth in conjunction with the accompanying drawings.
039649687	claims	1. A fuel assembly for a nuclear reactor, comprising: a housing; fuel elements accommodated within said housing; a lateral surface of each of said fuel elements; spacer members, each of which is formed as a bunch of wires helically arranged on said lateral surface of at least some of said fuel elements; planes of contact, wherein said spacer members adjoin adjacent said fuel elements relative to those said fuel elements on which said spacer members are disposed; each of which said wires of said bunch of wires adjoins at least two adjacent wires along the entire length thereof; all said wires being rigidly interconnected between said planes of contact.
	description	This application claims the benefit of U.S. Provisional Application Ser. No. 61/161,964 filed 20 Mar. 2009, entitled “Sample Stage Apparatus for Reducing Energy of Electrons in an Annular-Acceptance Analyzer System,” which is incorporated herein by reference in its entirety. The disclosure herein relates generally to analyzer systems and methods, e.g., annular-acceptance analyzer systems. More particularly, the disclosure herein pertains to sample holder apparatus for such systems and methods (e.g., that may be mounted on an analyzer instrument stage). It has long been recognized that the energy resolution of a cylindrical mirror analyzer (CMA), or other analyzers with annular acceptance (whether full 360-degree azimuthal acceptance or partial, that is, less than 360 degree azimuthal acceptance), is determined by the geometry of the analyzer. In such an instrument, (ΔE)/E is equal to a constant determined by the geometry of the instrument (where E is the kinetic energy of electrons passing through the analyzer in electron·volt (eV) and ΔE is the energy width (i.e., the analyzer “slit width”) or the range of electron energies passed by the analyzer around and at E). Furthermore, it has been recognized that if the energy of electrons entering an annular-acceptance analyzer (such as a CMA) is reduced, the effective resolution of the instrument is increased. U.S. Pat. No. 3,699,331, entitled “Double Pass Coaxial Cylinder Analyzer with Retarding Spherical Grids,” discloses a device and method for reducing the energy of electrons entering a CMA using a retarding grid assembly constructed of two concentric spherical sections. One disadvantage of this approach is the loss in transmission due to two effects. First, the physical transparency (typically 60-90% each) of the grids limits the number of electrons transmitted. Second, when an electric field terminates on a grid of finite mesh, the equipotential surfaces are rippled close to the grid. Each grid opening acts as a lens which causes the transmitted electrons to be scattered from their original trajectory. The effect of grid scattering is to reduce transmission. A second disadvantage of this approach is the aberrations introduced by the non-sphericity of the fine grids (which are easily distorted during manufacture and assembly) and the non-concentricity of the two grid sections. A third disadvantage of this approach is the possibility of grid contamination and consequent need for grid replacement. A final disadvantage of this approach is the necessity of electrically floating the CMA, electron detector, and associated electronics. Japan Pat. Appl. No. JP2006-302689A, entitled “Auger Electron Spectral Analysis Device and Auger Electron Spectral Analysis Method,” discloses a method that applies positive bias voltage to a sample electrode holder and alleges that, with this, kinetic energy of Auger electrons is reduced and high-resolution spectra are obtained. There exists a need to reduce the energy of electrons entering a CMA without incurring the disadvantages caused by the use of grids for pre-retardation. In one or more embodiments described herein, the energy of electrons entering a CMA (or other analyzer with annular acceptance) is reduced without the use of grids. Further, in one or more embodiments, this is accomplished without modification to the analyzer or other critical components of the spectrometer. Rather, one or more embodiments described herein provide a sample holder apparatus (e.g., mountable on an instrument stage or provided as part of an instrument stage) to reduce the energy of electrons entering an annular-acceptance analyzer (e.g., a CMA) without the use of grids. The sample holder apparatus may include a grounded sample aperture member and an electrically isolated sample support member to which is applied a positive bias potential. The combination of the grounded sample aperture member along with the positive bias potential applied to the sample support member and electrically connected sample produces an electrical retarding field that reduces the energy of the electrons before they enter the analyzer. At least in one or more embodiments, due to the shape of the field, which is substantially planar near the sample surface and substantially spherical farther from the sample surface, the trajectories of said electrons are bent outward from the optical axis such that they fill the entrance to the analyzer and enter with the desired range of input angles. One or more embodiments of a sample holder apparatus for reducing the energy of charged particles entering an annular-acceptance analyzer (e.g., a cylindrical mirror analyzer (CMA)) described herein includes an electrically isolated sample support member having a sample receiving surface configured to receive a sample and electrically connect the sample to the sample support member. The sample support member is configured for application of a retarding bias potential (e.g., a positive retarding bias). Further, the sample holder apparatus includes a grounded sample aperture member defining an aperture (e.g., a circular aperture), wherein the grounded sample aperture member is positioned relative to the sample support member but electrically isolated therefrom such that the aperture is proximate the sample receiving surface to expose at least a portion of a surface of a sample received thereon to be analyzed. The grounded sample aperture member along with the sample support member are configured to produce an electrical retarding field about the aperture when a retarding bias potential is applied thereto that reduces the energy of emitted particles from a sample before they enter the annular-acceptance analyzer. In one or more embodiments, the grounded sample aperture member along with the sample support member are configured to produce an electrical retarding field about a circular aperture when a retarding bias potential is applied thereto that reduces the energy of emitted particles from a sample before they enter the annular-acceptance analyzer and further that modifies the trajectories of such emitted particles such that they enter the annular-acceptance analyzer in a predetermined range of input elevation angles. One or more embodiments of a method for reducing the energy of charged particles entering an annular-acceptance analyzer (e.g., a cylindrical mirror analyzer (CMA)) as described herein may include providing an electrically isolated sample support member having a sample receiving surface configured to receive a sample and electrically connect the sample to the sample support member (e.g., the sample support member is configured for application of a retarding bias potential). Further, the method may include positioning a grounded sample aperture member defining an aperture relative to the sample support member but electrically isolated therefrom such that the aperture (e.g., a circular aperture) is proximate the sample receiving surface to expose at least a portion of a surface of a sample received thereon to be analyzed (e.g., the surface of the sample may be above, flush or below the plane in which the aperture lies). Still further, the method may include applying a retarding bias potential (e.g., a positive bias potential) to the sample support member to produce an electrical retarding field about the aperture (e.g., a retarding field about the aperture that includes a planar portion proximate the surface of the sample to be analyzed and a more spherical portion farther from the surface of the sample to be analyzed) that reduces the energy of emitted particles from a sample before they enter the annular-acceptance analyzer. In one or more embodiments of the method, applying a retarding bias potential to the sample support member may include applying a retarding bias potential to the sample support member to produce an electrical retarding field about the aperture that reduces the energy of emitted particles from a sample before they enter the annular-acceptance analyzer and further that modifies the trajectories of such emitted particles such that they enter the annular-acceptance analyzer in a predetermined range of input, elevation angles. One or more embodiments of an analyzer system for use in analyzing a sample are also provided herein. The system may include an analyzer apparatus defining a full or a partial annular-acceptance input opening to receive emitted particles from the sample being analyzed. Further, the system includes an electrically isolated sample support member having a sample receiving surface configured to receive a sample and electrically connect the sample to the sample support member (e.g., wherein the sample support member is configured for application of a retarding bias potential). Still further, the system may include a grounded sample aperture member defining an aperture. At least a portion of the grounded sample aperture member is positioned between the annular-acceptance input opening of the analyzer apparatus and the sample support member such that the aperture is proximate the sample receiving surface to expose at least a portion of a surface of a sample received thereon to be analyzed (e.g., the sample support member is electrically isolated from the grounded sample aperture member). Yet further, the grounded sample aperture member along with the sample support member are configured to produce an electrical retarding field about the aperture when a retarding bias potential is applied thereto that reduces the energy of emitted particles from a sample before they enter the annular-acceptance input opening. The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings. The figures are rendered primarily for clarity and, as a result, are not necessarily drawn to scale. In the following detailed description of illustrative embodiments, reference is made to the accompanying figures of the drawing which form a part hereof, and in which are shown, by way of illustration, specific embodiments which may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the disclosure. In the following description, the polarities of the applied potentials are chosen for the analysis of negatively-charged particles, and in the embodiments of FIGS. 1-14 the charged particles are assumed to be electrons. It will, of course, be appreciated that positively-charged particles may be analyzed by reversing the polarities of the applied potentials. The present description is with regard to a sample holder apparatus for reducing the energy of electrons entering an annular-acceptance analyzer, such as a CMA. For example, the analyzer may be part of a system used to analyze the chemical state of a solid surface, such as an Auger electron spectroscopy (AES) system. Instrumentation for use in spectroscopy of charged particles makes use of electrons or ions which are emitted from a substance after being bombarded or irradiated with electrons or ions from a source such as an electron gun, e.g., such as in Auger electron spectroscopy. For example, in one embodiment of this technique, a target sample material is placed in a vacuum, and upon being bombarded with electrons from some source, such as an electron gun, the sample gives off a variety of emissions. Among these are X-rays, secondary electrons, and reflected primary electrons from the source. The emissions include Auger electrons (a particular class of secondary electrons) having a distribution of electron energies and a range of particle trajectories. In the art of Auger electron spectroscopy, as taught for example in U.S. Pat. No. 4,205,226 (Gerlach), instruments making use of cylindrical mirror analyzers (“CMA”) are known which analyze the energy and the energy spectrum of Auger electrons emitted by the sample material. Such instruments operate by introducing the diverging electrons into a radial electric field produced between a pair of coaxially mounted electrode cylinders held at different electric potentials. Auger electrons injected from the sample into the radial electric field between the cylindrical electrodes are deflected by the field back toward the common axis of the electrodes. Electrons of a predetermined energy are thereby brought to a focus. By positioning a collector apparatus at this focus, electrons of a predetermined energy are selected and detected. By sweeping the voltage impressed across the cylindrical electrodes through a range of values, and detecting as a function of these applied potentials such electrons as are collected, the energy spectrum of the injected electrons may be plotted and determined. One or more embodiments of the sample holder apparatus described herein is beneficial to enhance Auger electron spectroscopy energy resolution, or improve the energy resolution of the Auger electron spectrum measured by a cylindrical mirror analyzer (CMA) or any other analyzer with annular acceptance (whether full 360-degree azimuthal acceptance or partial, that is, less than 360 degree azimuthal acceptance). For simplicity herein, when the term CMA is used herein it shall refer to any of such types of analyzers, including those with annular acceptance (whether full 360-degree azimuthal acceptance or partial, that is, less than 360 degree azimuthal acceptance). In other words, even though CMA is a type of annular-acceptance analyzer, the terms CMA and annular-acceptance analyzer are used interchangeably herein. Further, as used herein annular-acceptance or annular-acceptance used with terms such as input, entrance, openings, etc. refers to any annular region or portion of an annular region defined by an analyzer for acceptance of particles to be analyzed (see, for example, the gap 42 defined in CMA 30 of system 10 shown in FIG. 1). The standard equation, ΔE/E=c, expresses the energy resolution of the analyzer as a ratio involving E, the kinetic energy of electrons passing through the analyzer in electron·volt (eV) and ΔE, the energy width (i.e., the analyzer “slit width”) or the range of electron energies passed by the analyzer around and at E; c is a constant determined by the design and operational characteristics of the analyzer and, for a CMA, c is typically about 0.5% by design. From the energy resolution expression above, it follows that there are two ways to increase the energy resolution. The first is to decrease the constant c of the analyzer. Reduction of the constant c by redesign of the analyzer is to be avoided and, in fact, may not be practical for optical reasons. However, second, if the natural kinetic energy, E0, of electrons of a given spectral feature (peak) can be reduced by a multiplicative factor, f, to kinetic energy Ef=E0·f before they enter the analyzer, then the corresponding analyzer energy window, ΔE0, through which those electrons would pass, is reduced by the same factor to ΔEf=ΔE0·f and sinceΔEf/Ef=c thenΔEf/E0=c·f=cf where, cf is the effective analyzer energy resolution for a spectral feature at natural kinetic energy, E0. Effective analyzer energy resolution means that the Auger spectral feature naturally appearing at energy, E0, will appear in the measured spectrum with the energy resolution that would be achieved had the measurement been made on an analyzer with designed energy resolution, cf (i.e., the designed energy resolution depending on the design of the instrument). The reduction in the natural kinetic energy, E0, can be made by applying a positive, DC electrical bias, VB, to the sample, as mentioned above and f is then given by the ratio of the natural and reduced kinetic energiesf=Ef/E0 whereEf=E0−eVB and, VB is the bias voltage, e is the charge on the electron and, in all cases, eVB<E0. Meaningful application (e.g., application that results in enhanced analyzer energy resolution) of a positive, DC electrical bias, VB, to the sample can be accomplished using a sample holder apparatus as generally and more specifically described herein. The energy resolution of the chemical element's measured Auger spectrum is improved by some amount that is dependent on this effect, the natural linewidth of the Auger peak when it is not distorted by any analyzer, and possibly other effects yet to be determined. Note that the energy scale on the measured spectrum must then be shifted back by +eVB for display purposes, therefore, the bias voltage, VB, must be known accurately, say to less than 0.1 eV. In other words, a system that uses an applied positive DC electrical bias, VB, to the sample, may include software that is configured to receive an input corresponding to the bias voltage, VB, and then manipulate the data representative of the measured spectrum based on the bias voltage such that the energy scale on the measured spectrum is shifted back by +eVB and the measured spectrum is properly displayed, or otherwise communicated to a user (e.g., printed, graphically or visually provided, etc.). For example, in one or more embodiments, the proper spectrum display, or other communicated spectrum, can be achieved by data processing software employing the following or similar steps: 1. Read the stored spectral values from the data-file; 2. Read the corresponding stored energy values from the data-file; 3. Read the stored bias voltage value from the data-file; 4. Produce the offset energy values by adding the bias voltage, VB to the energy values; 5. Optionally, display the spectral values and the offset energy values on a display, such as an (x,y) graph. In one or more embodiments, the analysis method includes applying a positive electrical bias to the sample analyzed, thereby, creating a retarding electric field between the sample surface and the energy analyzer input optics and, therefore, reducing the kinetic energy, E, of the Auger electrons for all spectral peaks. The improvement is gained by shifting the naturally occurring spectrum to lower energy where the ΔE of the analyzer is smaller. The analyzer need not be modified in any way and may operate always at its native energy resolution, c. As such, an existing instrument may be upgraded by addition of a sample holder apparatus such as described herein. In addition, a further benefit regarding sensitivity of the analyzer to sample topography is realized by biasing the sample. It is generally known that the positions of the Auger spectral peaks measured by a CMA are sensitive to the distance of the analyzed sample surface from the CMA input (i.e., the sample height or “working distance”) and that the sample surface must be accurately positioned at this working distance to obtain an accurate energy scale; hence accurate chemical information, in the measured Auger spectrum. The working distance is characteristic of the analyzer and is established by the design and tuning, or operating configuration, of the analyzer. The equation ΔE=k·Δz·E expresses the shift, ΔE, in the observed peak position (i.e., the peak energy), E, from the expected position in the Auger energy spectrum, as a function of Δz, the deviation in distance of the sample surface from the optimum working distance and of the kinetic energy, E, of the Auger electrons. “k” is a constant that is intrinsic to the analyzer. This shows that a sample surface, or a particular portion of a sample surface, that is not at the optimal working distance will result in there being a variance, ΔE, in the Auger peak position, hence, an error in the observed Auger peak energy. However, since the effect of sample biasing is to reduce the natural kinetic energy, E0, of the Auger electrons for a given Auger spectral peak by some factor, f, then it can be seen from the expression above that the sensitivity of the Auger peak positions to deviations from the optimal working distance is reduced since, if the final, biased kinetic energy of the electrons as they pass through the analyzer is reduced to some fraction of E0, Ef=E0·f, where f is less than 1.0, then ΔEf=k·Δz·Ef where, ΔEf=ΔE·f, the reduced shift in the peak position due to sample surface variance from optimal, or sample topographic differences. One or more embodiments of the apparatus, systems or methods provided herein may provide one or more of the following potential benefits: Increase the capability of Auger spectrometers, that use a cylindrical mirror analyzer (CMA), to obtain chemically-specific information about solid surfaces or to separate overlapping spectral features. An example of an existing product is the PHI 700 Scanning Auger Nanoprobe. However, the present disclosure may apply to all annular-acceptance electron energy analyzer types in the field of solid surface analysis. There does not need to be any modification to the analyzer or other critical optical components of the spectrometer or incident particle sources to achieve improvement in the energy resolution by this method. Other approaches to the problem include adding retarding fields inside the analyzer and this has deleterious effects, including reduction of the analyzer transmission. Energy resolution is continuously adjustable over the usable range. Secondary electron (SE) image is now from backscattered electrons which provide a different contrast in the image and can give grain structure information; may be able to measure grain orientation after a calibration step. Shifting Auger peak into 10-20 eV range may allow real-time elemental image with imaging secondary electron detector. FAT (fixed analyzer transmission) mode for CMA by scanning the sample potential instead of the analyzer; analyzer pass energy held constant at low kinetic energy. Better energy resolution (and signal rates) than available from retarding grids. No modification of analyzer, electron source or gun optics, or ultra high vacuum (UHV) chamber. Upgrades of existing equipment can be done on-site—return to factory is not required. Direct measurement: not a software deconvolution technique (although the method may enable a software deconvolution technique by allowing a direct determination of the analyzer broadening function). Measurement of electrically insulating samples may be enhanced by the biasing method since the Ar ion flux used for charge neutralization of these samples would have to be at higher incident energy, therefore, more ion current would be available for neutralization; low ion current is a problem with the ion neutralization at lower incident energies used at 0 V bias. The effect of the bias, when the sample is coated with a conductive layer, like normal airborne carbonaceous contamination, may be to drain static charge from the sample surface, thereby, aiding in “charge neutralization”. Sample access by auxiliary beams is maintained, for example, the ion gun can still sputter-clean the sample or provide low energy ions for sample neutralization. Full sample movement is maintained, including tilt, rotate and x and y position adjustment. The sensitivity of the Auger peak positions to sample height is reduced, as compared with the native energy resolution peak positions, by the application of the bias voltage. The method extends the region of the Auger spectrum that is accessible to analysis by an existing Auger instrument. Typical commercial instruments restrict—by design, to reduce cost and complexity while maintaining a practical analytical capability—the measurable range of Auger electrons to about 0-3200 eV; however, there are many Auger transitions (peaks) that are at higher kinetic energies than 3200 eV and are, therefore, not detectable by current commercial Auger instrument offerings. For example, the higher energy transitions of the element Pt were measured by a special apparatus in the range 5000-12000 eV (see L. H. Toburen and R. G. Albridge, Nuclear Physics A, 90(3) 529 (1967); such energies being well outside of the capabilities of current typical commercial instruments. Since these higher energy Auger electrons, that fall outside the designed energy range of the analyzer, can be reduced in energy by the apparatus and methods described herein and, therefore, brought into the range of the analyzer, the analytical usefulness of, for example, the typical commercial analyzer instrument is increased. This additional analytical energy range (i.e., kinetic energies higher than 3200 eV) enabled by the sample holder apparatus as described herein, allows the sample to be interrogated to greater depths below the sample surface since the escape of Auger electrons through the surface region, from greater depths is a strong, direct function of their initial kinetic energy. A description of one or more apparatus that may be modified with the sample holder apparatus described herein is provided in, for example, Japan Pat. Appl. No. JP2006-302689A, entitled “Auger Electron Spectral Analysis Device and Auger Electron Spectral Analysis Method,” and U.S. Pat. No. 5,032,724, entitled “Multichannel Charged-Particle Analyzer,” issued 16 Jul. 1991; all of which are incorporated herein in their entirety by reference thereto. However, the present disclosure is not limited to application of the sample holder apparatus to only such apparatus as described in such documents as will be appreciated by one skilled in the art but is applicable to any annular-acceptance systems and/or analyzers. For example, FIG. 1 is an illustrative simplified longitudinal section of analyzer system 10 including a cylindrical mirror analyzer (CMA) 30 (a type of annular-acceptance analyzer) and incorporating a sample holder apparatus 100. For example, the system 10 is an analyzer system as described in U.S. Pat. No. 5,032,724 modified using a sample holder apparatus 100. As described in U.S. Pat. No. 5,032,724, the electron energy analyzing system 10 generally includes the components of conventional analyzing systems such as the components in a Model 25-120A Auger electron spectroscopy analyzer available from The Perkin-Elmer Corporation, the CMA Models 25-130, 25-140 and 25-150 available from Physical Electronic, Inc. (MN), the Model CMA 100 available from Omicron NanoTechnology, the Models ESA 100, ESA 150, DESA 100 and DESA 150 available from Staib Instruments, the Model CMA-D 40B1 available from C4 Scientific Instruments, and the Model CMA 2000 available from LK Technologies, except as modified by the sample holder apparatus generally shown by reference numeral 100. This FIG. 1 is a simplified schematic diagram and various details may be found in references such as the aforementioned U.S. Pat. No. 4,205,226. The analyzing system 10 is enclosed in a multi-component housing 14 which is maintained at vacuum by an ion pump (e.g., conventional components not being shown). For clarity other standard parts such as viewing ports, internal valving, and the like are also not shown. An electron gun 18 located on the axis 16 of the system may be any desired type such as may be based on electrostatic or magnetic optics, or thermionic or field emission of electrons. A target of sample material to be surface analyzed is placed at an outer end 22 of the system using the sample holder apparatus 100. The system is generally cylindrical, and all of the components located between the electron gun and the target have an axial passage 24 extending therethrough so that a primary electron or other energy beam 25 can be directed from the gun 18 onto the target (e.g., directed from the gun 18 through an aperture of the sample holder apparatus 100 onto a surface of the target sample, directed from the gun 18 onto a surface of a target sample protruding through an aperture of the sample holder apparatus 100, etc.). Electron optical components for the beam, shown generally at 26 may also be disposed coaxially as required, such as a variable aperture objective, steering plates, an objective lens, a deflector and a stigmator, some of which may be magnetic. The beam from the gun causes charged particles 27 to be emitted from the target. The particles may be positive ions, negative ions or electrons, for example Auger electrons. In other cases the axial beam 25 to the target may be an ion stream, X-rays or any other suitable energy source, and the emission may include reflected electrons. The system 10 may have two energy analyzers operating in tandem with respect to emitted elections 27. The first, or primary analyzer may be a conventional cylindrical mirror analyzer 30 (CMA). This analyzer 30 may include an outer conductive cylinder 32 (e.g., a cylindrical electrode) and an inner conductive cylinder 34 (e.g., a cylindrical electrode). The outer cylinder may have a negative voltage applied thereto relative to a reference voltage on the inner cylinder; the reference generally being ground potential. Voltages are provided by a DC source 36 via leads 38 in a through-fitting 40 in the housing 14. Electrons 27 diverging from the target within a certain range of conical angles 29 (the angle of the diverging electrons being relative to a plane at the sample surface orthogonal to axis 16) pass through a first annular gap 42 and a grounded screen 43 and enter the space 44 between the cylinders. Because of the field associated with the negative voltage on the outer cylinder 32, the electrons will follow a path 28 that curves back toward the axis 16 of the system. The exact path or trajectory for each electron is dependent on the kinetic energy of the electron relative to the field. Those electrons having an energy within a small range of energies will exit the space 44 through another grounded screen 45 and an annular passage 47 in the inner cylinder 34, to be detected by a detection system 50 located within the inner cylinder 34 opposite the end 22. In one or more embodiments of analyzer systems, such as disclosed in the aforementioned U.S. Pat. No. 4,205,226, a small annular opening is provided for the electron detector. By changing the voltage on the outer cylinder the energy band selected by the detector is changed correspondingly. Thus scanning of the voltage combined with electron detection of electrons transmitting the gap provides a spectrum which may be correlated with characteristics of the target, such as material and topography. Processor 71 is suitably connected to the components of the system for control thereof, such as by leads 69, and, for example, to receive signals for conversion to spectrographic information of energy distribution (e.g., for use in creating a display of information or other presentation of information, for running routines for correction of data as described herein, for control of the application of bias potential to the sample support member and measurement thereof, etc.). In one or more embodiments, the sample holder apparatus 100 includes a grounded sample aperture member and an electrically isolated sample support member to which is applied a retarding bias potential (e.g., together shown as reference numeral 110). The sample support member includes a sample receiving surface upon which a sample (e.g., a sample to be analyzed) may be mounted such that the sample is electrically coupled or connected to the sample support member (e.g., the sample being held at the same potential as the sample support member). The bias potential is applied to the sample support member, and thus the sample, by a source 112 (e.g., via leads 113 in a through-fitting in the housing 14, or in any other manner). Generally, the grounded sample aperture member is electrically isolated from the sample support member (e.g., such as with use of electrical insulative spacers). As used herein, the term “grounded” when used in conjunction with a sample aperture member (e.g., aperture plate, etc.) refers to the sample aperture member generally being held at ground potential of the analyzer (i.e., at the reference voltage of the analyzer, which for a CMA is generally the voltage on the inner conductive cylinder of the analyzer, such as inner conductive cylinder 34 as shown in FIG. 1). In other words, a grounded sample aperture member refers to a sample aperture member that is grounded and has no potential applied thereto, or a sample aperture member that may have a small voltage (positive or negative) applied thereto. Such a small voltage applied thereto is generally a voltage that would not result in significant alterations to the planar-spherical field and resulting trajectories. Further, generally, such a small voltage is less than about 10 percent of the voltage applied to the sample support member. A voltage applied to the sample aperture member that is higher than about 10 percent of the voltage applied to the sample support member would result in significant alterations to the planar-spherical field and resulting trajectories, such that, for example, electrons would not enter the CMA with the proper range of input elevation angles. In one embodiment, the grounded sample aperture member surrounds a substantial portion of the sample support member (or otherwise includes one or more surfaces located between the sample support member and the analyzer components that receive the emitted electrons) and includes an aperture (e.g., a circular aperture having an axis therethrough, an aperture orthogonal to an axis at its center, etc.) in proximity to the sample (e.g., above the sample, flush with the sample, the sample extending through the aperture with a surface above the aperture, etc.). In other words, at least in one embodiment, the aperture (e.g., an aperture generally parallel to the sample receiving surface, at least in one embodiment) provides access to the surface of the sample to be analyzed and reduces the biased area directly under the analyzer such that an electrical retarding field (e.g., an axially symmetric electrical retarding field) is created in a reduced area about the aperture and over the sample surface to be analyzed. The aperture may be circular in order to create an axially symmetric retarding field. In one embodiment, the grounded sample aperture member surrounds the sample support member to shield all the retarding field except that portion in proximity to the circular aperture. The sample holder apparatus 100 may be configured to provide full sample movement, including, for example, tilt, rotation, and x, y, and z adjustment. For example, such sample movement may be provided using any known stage movement apparatus such as with the use of leadscrew or rack and pinion actuation, manually driven linear or rotary feedthroughs, electric motors internal or external to the vacuum, or piezoelectric motors. For example, similar movement of a conventional stage apparatus is provided in the PHI Model 15-680, available from Physical Electronics, Inc. (MN). Further, in one or more embodiments, the sample support member or the sample support member and the sample aperture member (together 110) may be mounted on an instrument stage, such as stage 115 generally shown in FIG. 1. As such, stage movement apparatus as described above may be used to provide full sample movement of the components. Further, for example, the sample support member may be mounted separately and independently (e.g., on an instrument stage) from the sample aperture member (e.g., the sample aperture member may be mounted separately to allow movement of the sample aperture member relative to the sample receiving surface of the sample support member, such as being movable into a retracted position using one or more components like those used for accomplishing sample stage movement). Still further, the sample support member may be mounted (e.g., as part of the instrument stage of an analyzer) such that it is movable apart from the independently mounted sample aperture member. FIG. 2A shows a schematic, isometric cutaway view of a portion of one generalized embodiment of a sample holder apparatus 210 that may be used to reduce energy of electrons in an annular-acceptance analyzer system, such as the analyzer system 10 shown in FIG. 1. The exemplary sample holder apparatus 210 includes a grounded sample aperture member 214 and an electrically isolated sample support member 212 to which is applied a retarding bias potential (e.g., source not shown). The sample support member 212 includes a sample receiving surface 222 (e.g., a planar surface) terminating a cylindrical body member 221 lying along axis 218 of the sample holder apparatus 210 and upon which a sample 220 (e.g., a sample to be analyzed) may be mounted such that the sample 220 is electrically coupled or connected to the sample support member 212 (e.g., the sample 220 being held at the same potential as the sample support member 212). The cylindrical body member 221 including a cylindrical outer surface 227 at a distance from axis 218. For example, the axis 218 may coincide with the axis 16 of an analyzer system, such as system 10 shown in FIG. 1. The grounded sample aperture member 214 is electrically isolated from the sample support member 212 (e.g., such as with use of electrical insulative spacers, not shown, or in any other suitable insulative manner). In one embodiment, as shown in FIG. 2A, the grounded sample aperture member 214 includes a cylindrical wall 241 including an inner surface 247 located at a distance from the axis 218 and surrounding the cylindrical body member 221 of sample support member 212 (e.g., the sample aperture member 214 may be physically fixed to the sample support member 212 such that they do not move independently of each other). Further, the grounded sample aperture member 214 includes an end portion 243 (e.g., a generally planar end having a inner surface 253 adjacent but isolated from the sample receiving surface 222 and an outer surface 255 facing the analyzer components of an analyzer system, such as system 10 in FIG. 1) terminating the cylindrical wall 241. In other words, the grounded sample aperture member 214 includes one or more surfaces or material portions located between the sample support member 212 and the analyzer components that receive the emitted electrons, such as the gap between the electrodes of analyzer 30 as shown in FIG. 1. The grounded sample aperture member 214 of FIG. 2A further includes a circular aperture 216 (e.g., having a thickness defined by the thickness or distance between the outer surface 255 and inner surface 253) defined in the end portion 243 by one or more surfaces 217 (e.g., along axis 218, or in other words, the aperture 216 is orthogonal to the axis 218 at its center) in proximity to the sample 220 when positioned for analysis (e.g., above the sample 220 as shown in FIG. 2A, but may be flush with the sample 220, the sample 220 may extend through the aperture 216 with a surface 232 above the aperture 216, etc.). In at least in one embodiment, a circular aperture 216 (e.g., an aperture generally parallel to the surface of the sample receiving surface 222, at least in one embodiment) provides access to the sample surface 232 to be analyzed (e.g., reducing the biased area directly under an analyzer 30 such as shown in FIG. 1 with which the sample holder apparatus 210 can be used such that an axially symmetric electrical retarding field is created in a reduced area about the aperture and over the sample surface 232 to be analyzed). The aperture 216 in the embodiment of FIG. 2A is circular to create an axially symmetric retarding field. The grounded sample aperture member 214 surrounds the sample support member 212 to shield all the retarding field except that portion in proximity to the circular aperture 216. The combination of grounded sample aperture member 214 along with the bias potential applied to the sample support member 212 and electrically connected sample 220 produces an electrical retarding field that reduces the energy of the electrons before they enter the analyzer (e.g., an electrical retarding field over the aperture 216, and thus, the sample surface 232 being analyzed). For example, in one or more embodiments, the electrical retarding field is created at least about the aperture 216 (e.g., a plurality of equipotential surfaces can be calculated that extend at least from edges 251 defining the aperture 216 and outward from the sample 220 towards the analyzer, such as analyzer 30 as shown in FIG. 1 with which the sample holder apparatus 210 may be used). Further, at least in one embodiment, the electrical retarding field is confined to the region in proximity to the aperture 216 by the grounded sample aperture member 214 (e.g., confined by the one or more grounded surfaces such as the grounded cylindrical wall 241 and grounded end portion 243 that includes the aperture 216 formed therein). At least in one or more embodiments, due to the shape of the field, which is substantially planar near the sample surface and non-planar (e.g., substantially spherical farther from the sample surface, or, for example, more non-planar the further the distance from the sample surface), the trajectories of the electrons are bent outward from the optical axis such that they fill the entrance to an analyzer and enter with the desired range of input angles (e.g., the electrons diverging from the target sample within a certain range of conical angles pass through a first annular gap 42 and a grounded screen 43 and enter the space 44 between the cylindrical electrodes 32 and 34 shown in FIG. 1). FIG. 2B shows a schematic, isometric cutaway view of a portion of one alternate generalized embodiment of a sample holder apparatus 610 that may be used to reduce energy of electrons in an annular-acceptance analyzer system, such as the analyzer system 10 shown in FIG. 1. The exemplary sample holder apparatus 610 is similar to the sample holder 210 shown in FIG. 2A except that the exemplary sample holder apparatus 610 includes a grounded sample aperture plate 614 that is mounted separately from electrically isolated sample support member 612 to which is applied a retarding bias potential (e.g., source not shown). With such separate mounting of the components, the aperture defined by the grounded sample aperture plate 614 and the sample receiving surface 622 can be moved relative to the other (e.g., the sample receiving surface 622 can be moved under the aperture 616). The sample support member 612 (e.g., which may be mounted on an instrument stage or provided as part of an instrument stage) includes a sample receiving surface 622 (e.g., a planar surface) terminating a cylindrical body member 621 lying along axis 618 of the sample holder apparatus 610 and upon which a sample 620 (e.g., a sample to be analyzed) may be mounted such that the sample 620 is electrically coupled or connected to the sample support member 612 (e.g., the sample 620 being held at the same potential as the sample support member 612). The cylindrical body member 621 includes a cylindrical outer surface 627 at a distance from axis 618. For example, the axis 618 may coincide with the axis 16 of an analyzer system, such as system 10 shown in FIG. 1. The grounded sample aperture plate 614 is electrically isolated from the sample support member 612 and movement is possible between the receiving surface 622 of the sample support member 612 and the grounded sample aperture plate 614 (e.g., the sample support member 612 may be moved relative to a fixed aperture, the aperture may be retracted from a position between the sample receiving surface 622 and the analyzer components, etc.). For example, in one embodiment, the grounded sample aperture plate 614 (e.g., a generally planar structure having an inner surface 653 adjacent but isolated from the sample receiving surface 622 and an outer surface 655 facing the analyzer components of an analyzer system, such as system 10 in FIG. 1) may be coupled to structure (e.g., components like those used to provide full instrument stage movement) configured to allow the aperture plate 614 to be moved relative to the receiving surface 622 (and thus movement of the aperture 616 relative to the sample 620 when mounted thereon). The grounded sample aperture member 614 of FIG. 2B further includes a circular aperture 616 (e.g., having a thickness defined by the thickness or distance between the outer surface 655 and inner surface 653) defined by one or more surfaces 617 (e.g., along axis 618, or in other words, the aperture 616 is orthogonal to the axis 618 at its center in proximity to the sample 620 when positioned for analysis (e.g., above the sample 620 as shown in FIG. 2B, but may be flush with the sample 620, the sample 620 may extend through the aperture 616 with a surface 632 above the aperture 616, etc.). In at least one embodiment, a circular aperture 616 (e.g., an aperture generally parallel to the surface of the sample receiving surface 622) provides access to the sample surface 632 to be analyzed (e.g., reducing the biased area directly under an analyzer 30 such as shown in FIG. 1 with which the sample holder apparatus 610 can be used such that an axially symmetric electrical retarding field is created in a reduced area about the aperture and over the sample surface 632 to be analyzed). Still further, in one embodiment, with the grounded sample aperture plate 614 being mounted separately from the sample support member 612 (e.g., not physically attached to one another), the sample support member 612 having the sample mounted thereon may be moved under the aperture 616, thereby, allowing the analyst to select and examine different areas on the sample 620 without having to extract and remount the sample 620 (e.g., allowing analysis of a region of the sample that is larger than the aperture without removing the sample). In other words, full sample movement may be possible under the aperture 616. Further, such separate mounting may allow the aperture plate 614 to be removed or retracted from the region of the sample 620 to allow for other sample analysis/processing that may have been restricted by the presence of the aperture plate 614. In other words, the sample aperture plate 614 is not physically fixed to the sample support member 612 and they are allowed to move independently of each other. FIG. 2C shows a schematic, isometric cutaway view of a portion of another alternate generalized embodiment of a sample holder apparatus 810 that may be used to reduce energy of electrons in an annular-acceptance analyzer system, such as the analyzer system 10 shown in FIG. 1. The exemplary sample holder apparatus 810 is similar to the sample holder 210 shown in FIG. 2A except that the exemplary sample holder apparatus 810 further includes a positioning plate structure 880 and a grounded sample aperture member 814 that includes a recessed aperture region 890. For example, FIG. 2C shows a sample holder apparatus 810 that includes a grounded sample aperture member 814 and an electrically isolated sample support member 812 to which is applied a retarding bias potential (e.g., source not shown). The sample support member 812 includes a sample receiving surface 822 (e.g., a planar surface) terminating a cylindrical body member 821 lying along axis 818 of the sample holder apparatus 810 and upon which a sample 820 (e.g., a sample to be analyzed) may be mounted such that the sample 820 is electrically coupled or connected to the sample support member 812 (e.g., the sample 820 being held at the same potential as the sample support member 812). Opposite the sample receiving surface 822, an elongated mating element 896 is provided for engagement with another mating element (not shown) to place the receiving surface 822 in a particular position in the analyzer (e.g., the elongated mating element 896 may mate with an instrument stage mating element). The sample support member 812 further includes a positioning plate structure 880. The positioning plate structure 880 includes a cylindrical element 887 at least partially about the cylindrical outer surface 827 of the cylindrical body member 821 (e.g., in contact with the cylindrical body member 821 at a distance from axis 818). The positioning plate structure 880 further includes a retaining plate 886 located between the grounded sample aperture member 814 and the cylindrical body member 821 of the electrically isolated sample support member 812. The retaining plate 886 includes an aperture 889 aligned with aperture 816 of the grounded sample aperture member 814 to allow access to the sample 820. Further, the retaining plate 886 is coupled to the cylindrical element 887 of the positioning plate structure 880 by one or more fasteners 888. The retaining plate 886 of the positioning plate structure 880 may be used to hold the sample 820 in place. The exemplary retaining plate 886 may include a raised center and the opening 889 defined therein for exposing the sample to the analysis instrument in the area of aperture 816 provided in the grounded sample aperture member 814. Although one exemplary positioning structure (e.g., positioning plate structure 880) is shown in FIG. 2C, it will be recognized that any number of techniques may be used to hold the sample 820 in place for analysis. For example, samples may be held down via their front side as shown in FIG. 2C, or alternatively, via their back side, or side edges. For example, fasteners including threaded screws, pins, retaining rings, hold-down plates, spring clips or clamps may be used to hold down the samples. Further, for example, and clearly not limited thereto, such samples may be held down using conductive adhesives such as glues, epoxies, or silver paint, and/or conductive sticky tapes (e.g., double sided conductive tape). Still further, for example, the sample may be set in a recessed hole and held down by gravity. The grounded sample aperture member 814 is electrically isolated from the sample support member 812 (e.g., such as with use of electrical insulative spacers, not shown, or in any other suitable insulative manner). In one embodiment, as shown in FIG. 2C, the grounded sample aperture member 814 includes a cylindrical wall member 841 including an inner surface 847 located at a distance from the axis 818 and surrounding the cylindrical element 887 of sample support member 812 (e.g., the sample aperture member 814 may be physically fixed to the sample support member 812 such that they do not move independently of each other). Further, the grounded sample aperture member 814 includes an end aperture plate 843 (e.g., a generally planar end having an inner surface 853 adjacent but isolated from the sample receiving surface 822 and an outer surface 855 facing the analyzer components of an analyzer system, such as system 10 in FIG. 1) terminating and coupled to the cylindrical wall 841 by one or more fasteners 849. In other words, the grounded sample aperture member 814 includes one or more surfaces or material portions located between the sample support member 812 and the analyzer components that receive the emitted electrons, such as the gap between the electrodes of analyzer 30 as shown in FIG. 1. The grounded end aperture plate 843 of FIG. 2C further includes a circular aperture 816 (e.g., having a thickness defined by the thickness or distance between the outer surface 855 and inner surface 853) defined in the end aperture plate 843 by one or more surfaces 817 (e.g., along axis 818, or in other words, the aperture 816 is orthogonal to the axis 818 at its center) in proximity to the sample 820 when in one position for analysis (e.g., above the sample 820 as shown in FIG. 2C). In at least one embodiment, a circular aperture 816 (e.g., an aperture generally parallel to the surface of the sample receiving surface 822) provides access to the sample surface 832 to be analyzed (e.g., reducing the biased area directly under an analyzer 30 such as shown in FIG. 1 with which the sample holder apparatus 810 can be used such that an axially symmetric electrical retarding field is created in a reduced area about the aperture and over the sample surface 832 to be analyzed). The aperture 816 in the embodiment of FIG. 2C is circular to create an axially symmetric retarding field and is located in a recessed aperture region 890 of the grounded end aperture plate 843. Such recessed aperture region 890 is configured to allow use of the positioning plate structure 880 and the fasteners 888 thereof. One will recognize that one or more various configurations of sample holder apparatus (e.g., the size and shape) may be used to provide the retarding field as described herein. The disclosure is not limited to only the generalized sample holder apparatus shown herein, but includes other apparatus that can be provided with use of various combinations of the structural elements shown in the various embodiments herein. However, some configurations may be more beneficial than others. Various other embodiments of sample holder apparatus are further described herein with reference to FIGS. 3-14. Generally, such apparatus provide an electrical retarding field over the sample surface being analyzed to reduce the energy of the electrons entering the analyzer. Sample surfaces to be analyzed are typically nearly planar. Applying a retarding potential to such a sample (e.g., without a grounded aperture being used such as described herein) generally creates a retarding field that would be substantially planar (e.g., above the planar sample surface). Use of this planar retarding field would undesirably bend the emitted electron trajectories significantly outward away from the central axis (e.g., axis 16 of the analyzer 30 along which the annular-acceptance region is located or aligned such as shown in FIG. 1) such that the emitted electrons would most likely not fill the entrance of the analyzer and would most likely not enter the analyzer with the proper range of input angles. In one or more embodiments, as described herein, by adding or using a grounded circular aperture in proximity to the sample surface (e.g., such as where the sample is protruding through and above the aperture, where the sample surface is flush with the aperture, or where the sample surface is below the aperture), a substantially spherical field can be created farther away from the sample surface. In other words, at least in one or more embodiments, the retarding field is then substantially planar close to the sample surface and substantially spherical farther away from the sample surface. For example, this combination planar-spherical field bends the trajectories of the emitted electrons outward away from the central axis but by a much smaller amount than does the substantially planar field when used by itself. In one or more embodiments, the relative amounts of planar and spherical fields are controlled. For example, the relative amounts of planar and spherical fields may be controlled by the design of the diameter and thickness of the aperture and its position relative to the sample (e.g., that is, such as the gap distance for the case of a sample located below the aperture, or in other words the gap distance between the surface 232 of sample 220 and an inner surface 253 of the end portion 243 facing the sample 220). With the proper proportion of planar to spherical field, this allows for filling the CMA annular input and adjusting the input angles to the CMA. Advantageously, these input angles can be chosen to be the conventional range of input angles, namely 36 to 48 degrees, so that the CMA input is substantially filled and no modification to the CMA is necessary. Because the planar-spherical field bends the electrons away from the central axis, it can be readily understood that it is particularly suitable to annular-acceptance analyzers where the beam is received off-axis. However, it could also be employed with an on-axis analyzer such as the common SCA (spherical capacitor analyzer), but the transmission would be reduced. Referring to FIG. 3, the right side of the figure depicts a coaxial CMA and electron gun 301, such as illustrated in FIG. 1. The left side depicts a sample holder apparatus 302 including a sample aperture member 322, sample support member 323, and sample 324. The sample aperture member 322 is electrically grounded. The sample support member 323 is electrically isolated from the sample aperture member 322 and is electrically connected to a positive bias potential (not shown). The sample 324 is mounted directly to the sample support member 323 (e.g., or a post of support member 323), and the two elements 323 and 324 are in electrical contact. The sample surface 325 (e.g., the surface closest to the CMA and which is to be analyzed) protrudes above an aperture 330 (e.g., a circular aperture) defined in the sample aperture member 322. Referring to FIG. 4, an enlarged view of a portion of FIG. 3 is depicted. Element 341 is a magnetic objective lens pole-piece of the coaxial electron gun and CMA 301. The sample aperture member 322 is electrically grounded. The sample support member 323 has +900 V applied. The sample 324 is mounted to the sample support member 323 and as such also has +900 V applied. A computer program SIMION 3D Version 7.0 was used to calculate nine equipotential surfaces 365 that range from +90 V to +810 V in 90 V increments. The shape of these surfaces changes from nearly planar close to the sample 324 to nearly spherical farther away from the sample 324. For this embodiment, where the sample surface 325 protrudes above the circular aperture 330 in the sample aperture member 322, the sample (and/or post of support member 323) has a small diameter to create enough of a spherical field so that there is the desired proportion of planar to spherical field. Furthermore, for a given sample (and/or post) diameter, the height of the sample above the aperture 330 and the diameter and thickness of the aperture 330 are chosen to produce a planar-spherical field shape that bends the electron trajectories advantageously to fill the annular entrance or gap 342 of the CMA 301 and enter with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Referring to FIG. 5, operation of the embodiment of FIG. 3 is depicted. The sample aperture member 322 is electrically grounded. The sample support member 323 has +900 V applied. The sample 324 is mounted to the sample support member 323 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate 21 electron trajectories 375 that originate from the center of the sample 324 (e.g., on axis of the aperture 330). The trajectories 375 were launched with 1000 eV kinetic energy and with 24 to 34 degrees elevation angle. The electrons undergo deflection near the sample 324 as they penetrate the planar-spherical field and are slowed from 1000 eV to 100 eV kinetic energy. Because of the shape of the planar-spherical equipotential surfaces 365 depicted in FIG. 4, the electrons are deflected advantageously to fill the CMA annular entrance or gap 342 and enter the CMA 301 with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Because the electrons undergo a factor of 10 reduction in kinetic energy, the effective CMA resolution is increased by a factor of ten, from a native resolution of about 0.5% to an effective resolution of about 0.05%. No reduction in transmission occurs due to grid transparency or grid scattering. No modification to the analyzer or other critical components of the spectrometer is required (e.g., other than to use the sample holder apparatus described herein). Referring to FIG. 6, the right side of the figure depicts a coaxial CMA and electron gun 401. The left side depicts a sample holder apparatus 402 including a sample aperture member 422, sample support member 423, and sample 424. The sample aperture member 422 is electrically grounded. The sample support member 423 is electrically isolated from the sample aperture member 422 and is electrically connected to a positive bias potential (not shown). The sample 424 is mounted directly to the sample support member 423, and the two elements 423 424 are in electrical contact. The sample surface 425 is flush with the top of the circular aperture 430 in the sample aperture member 422. Referring to FIG. 7, an enlarged view of a portion of FIG. 6 is depicted. Element 441 is the magnetic objective lens pole-piece of the coaxial electron gun and CMA 401. The sample aperture member 422 is electrically grounded. The sample support member 423 has +900 V applied. The sample 424 is mounted to the sample support member 423 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate nine equipotential surfaces 465 that range from +90 V to +810 V in 90 V increments. The shape of these surfaces changes from nearly planar close to the sample 424 to nearly spherical far away from the sample 424. For this embodiment, where the sample surface 425 is flush with the top of the aperture 430 in the sample aperture member 422, the sample 424 can have a larger diameter than was possible in the embodiment of FIGS. 3-5, and still generate enough of a spherical field so that there is the proper proportion of planar to spherical field. Furthermore, for a given sample diameter, the diameter and thickness of the circular aperture 430 are chosen to produce a planar-spherical field shape that bends the electron trajectories advantageously to fill the annular entrance or gap 442 of the CMA and enter with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Referring to FIG. 8, operation of the embodiment of FIG. 6 is depicted. The sample aperture member 422 is electrically grounded. The sample support member 423 has +900 V applied. The sample 424 is mounted to the sample support member 423 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate 22 electron trajectories 475 that originate from the center of the sample 424. The trajectories were launched with 1000 eV kinetic energy and with 23.5 to 34 degrees elevation angle. The electrons undergo deflection near the sample 424 as they penetrate the planar-spherical field and are slowed from 1000 eV to 100 eV kinetic energy. Because of the shape of the planar-spherical equipotential surfaces 465 depicted in FIG. 7, the electrons are deflected advantageously to enter the CMA 401 with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Because the electrons undergo a factor of 10 reduction in kinetic energy, the effective CMA resolution is increased by a factor of ten, from a native resolution of about 0.5% to an effective resolution of about 0.05%. No reduction in transmission occurs due to grid transparency or grid scattering. No modification to the analyzer or other critical components of the spectrometer is required (e.g., other than to use the sample holder apparatus described herein). Referring to FIG. 9, the right side of the figure depicts a coaxial CMA and electron gun 501. The left side depicts a sample holder apparatus 502 including a sample aperture member 522, sample support member 523, and sample 524. The sample aperture member 522 is electrically grounded. The sample support member 523 is electrically isolated from the sample aperture member 522 and is electrically connected to a positive bias potential (not shown). The sample 524 is mounted directly to the sample support member 523, and the two elements 523 and 524 are in electrical contact. The sample surface 525 is below the circular aperture 530 in the sample aperture member 522. Referring to FIG. 10, an enlarged view of a portion of FIG. 9 is depicted. Element 541 is the magnetic objective lens pole-piece of the coaxial electron gun and CMA 501. The sample aperture member 522 is electrically grounded. The sample support member 523 has +900 V applied. The sample 524 is mounted to the sample support member 523 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate nine equipotential surfaces 565 that range from +90 V to +810 V in 90 V increments. The shape of these surfaces changes from nearly planar close to the sample 524 to nearly spherical far away from the sample 524. For this embodiment, where the sample surface 525 is below the circular aperture 530 in the sample aperture member 522, the sample can have an even larger diameter than was possible in the embodiments shown in FIGS. 3-8 and still generate enough of a spherical field so that there is the proper proportion of planar to spherical field. Furthermore, the height of the aperture 530 above the sample 524 and the diameter and thickness of the circular aperture 530 are chosen to produce a planar-spherical field shape that bends the electron trajectories advantageously to fill the annular entrance of the CMA and enter the CMA 501 with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Referring to FIG. 11, operation of the embodiment of FIG. 9 is depicted. The sample aperture member 522 is electrically grounded. The sample support member 523 has +900 V applied. The sample 524 is mounted to the sample support member 523 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate 25 electron trajectories 575 that originate from the center of the sample 524. The trajectories were launched with 1000 eV kinetic energy and with 24 to 36 degrees elevation angle. The electrons undergo deflection near the sample as they penetrate the planar-spherical field and are slowed from 1000 eV to 100 eV kinetic energy. Because of the shape of the planar-spherical equipotential surfaces 565 depicted in FIG. 10, the electrons are deflected advantageously to fill the CMA annular entrance or gap 542 and enter the CMA 501 with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Because the electrons undergo a factor of 10 reduction in kinetic energy, the effective CMA resolution is increased by a factor of ten, from a native resolution of about 0.5% to an effective resolution of about 0.05%. No reduction in transmission occurs due to grid transparency or grid scattering. No modification to the analyzer or other critical components of the spectrometer is required (e.g., other than to use the sample holder apparatus described herein). Referring to FIG. 12, the right side of the figure depicts a coaxial CMA and electron gun 501 in the same manner as in FIG. 9. The left side depicts a sample holder apparatus 502 like that shown in FIG. 9 including a sample support member 523 and sample 524. However, unlike FIG. 9, FIG. 12 includes a sample aperture member 722 that is not physically connected to the sample support member 523, but rather is separated therefrom (e.g., such as like shown in FIG. 2B). The sample aperture member 722 is electrically grounded. The sample support member 523 is electrically isolated from the sample aperture member 722 and is electrically connected to a positive bias potential (not shown). The sample 524 is mounted directly to the sample support member 523, and the two elements 523 and 524 are in electrical contact. The sample surface 525 is below the circular aperture 730 defined in the sample aperture member 722. The sample aperture member 722 may be mounted off of the instrument stage or off of a separate port (not shown) in the chamber (not shown) of the analysis instrument. As such, sample aperture member 722 may be centered under the analyzer and the sample would be positioned under the aperture 730. In such a manner, the sample 524 would be movable under the aperture 730 so that any area of the sample 524 can be selected for analysis. As shown in FIG. 13, the sample aperture member 722 may be retracted from its position of FIG. 12 to allow for other sample analysis/processing that may have been restricted by the presence of the aperture member 722. In other words, the sample aperture member 722 is moved out of its position between the coaxial CMA/electron gun 501 and the sample 524. Referring to FIG. 14, an enlarged view of a portion of FIG. 12 is depicted. Element 541 is the magnetic objective lens pole-piece of the coaxial electron gun and CMA 501. The sample aperture member 722 is electrically grounded. The sample support member 523 has +900 V applied. The sample 524 is mounted to the sample support member 523 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate nine equipotential surfaces 565 that range from +90 V to +810 V in 90 V increments. The shape of these surfaces changes from nearly planar close to the sample 524 to nearly spherical far away from the sample 524. While certain embodiments have been illustrated and described, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made without deviating from the concepts set forth above. Illustrative embodiments of this invention are discussed and reference has been made to possible variations within the scope of this disclosure. These and other variations, combinations, and modifications in the disclosure will be apparent to those skilled in the art without departing from the scope of the invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein. Accordingly, the invention is to be limited only by the claims provided below and equivalents thereof.
	description	This application is a divisional of U.S. patent application Ser. No. 12/843,037, filed Jul. 25, 2010, the contents of which is incorporated by reference in its entirety. As shown in FIG. 1, a conventional fuel assembly 10 of a nuclear reactor, such as a Boiling Water Reactor (BWR), may include an outer channel 12 surrounding an upper tie plate 14 and a lower tie plate 16. A plurality of full length fuel rods 18 and/or part length fuel rods 19 may be arranged in a matrix within the fuel assembly 10 and pass through a plurality of spacers (also known as spacer grids) 15 axially spaced one from the other and maintaining the rods 18, 19 in the given matrix thereof. The fuel rods 18 and 19 are generally continuous from their base to terminal, which, in the case of the full length fuel rod 18, is from the lower tie plate 16 to the upper tie plate 14. Outer channel 12 encloses the fuel rods 18/19 within the assembly 10 and maintains water or other coolant flow within assembly 10 about fuel rods 18/19 and in contact with the fuel rods 18/19 to facilitate heat transfer from the fuel to the coolant. Outer channel 12 is traditionally uniform in mechanical design and material for each other assembly 10 provided to a particular core, to aid in assembly design standardization and manufacturing simplicity. Outer channel 12 may be fabricated conventionally of a material compatible with the operating nuclear reactor environment, such as a Zircaloy-2. As shown in FIG. 2, a conventional reactor core, such as a BWR core, may include a plurality of cells 40 in the reactor core. Each cell may include four fuel assemblies 10 having adjacent fuel channels 12. Other fuel assemblies 10 may be placed in the reactor core outside of cells 40 and not adjacent to control blades. The fuel assemblies 10 in FIG. 2 are shown in section to illustrate control blades 45, which are conventionally cruciform-shaped and movably-positioned between the adjacent surfaces of the fuel channels 12 in a cell 40 for purposes of controlling the reaction rate of the reactor core. Conventionally, there is one control blade 45 per cell 40. As a result, each fuel channel 12 has two sides adjacent to the control blade 45 and two sides with no adjacent control blade. The control blade 45 is formed of materials that are capable of absorbing neutrons without undergoing fission itself, for example, boron, hafnium, silver, indium, cadmium, or other elements having a sufficiently high capture cross section for neutrons. Thus, when the control blade 45 is moved between the adjacent surfaces of the fuel channels 12, the control blade 45 absorbs neutrons which would otherwise contribute to the fission reaction in the core. On the other hand, when the control blade 45 is moved out of the way, more neutrons will be allowed to contribute to the fission reaction in the core. Conventionally, only a fraction of all control blades 45 within a core will be exercised to control the fission reaction within the core during an operating cycle. As such, only a corresponding fraction of fuel assemblies will be directly adjacent to an extended control blade, or “subject to control,” during an operating cycle. After a period of time, a fuel channel 12 may become distorted as a result of differential irradiation growth, differential hydrogen absorption, and/or irradiation creep. Differential irradiation growth is caused by fluence gradients and results in fluence-gradient bow. Differential hydrogen absorption is a function of differential corrosion resulting from shadow corrosion on the channel sides adjacent to the control blades 45 and the percent of hydrogen liberated from the corrosion process that is absorbed into the fuel channel 12; this results in shadow corrosion-induced bow. Irradiation creep is caused by a pressure drop across the channel faces, which results in permanent distortion called creep bulge. As a result, the distortion (bow and bulge) of the fuel channel 12 may interfere with the movement of the control blade 45. Channel/control blade interference may cause uncertainty in control blade location, increased loads on reactor structural components, and decreased scram velocities. Conventionally, if channel/control blade interference has become severe, the control blade is declared inoperable and remains fully inserted. Example embodiments are directed to fuel assemblies usable in nuclear reactors and methods of optimizing and fabricating the same. Example embodiment fuel assemblies include an outer channel having a physical configuration determined based on a position of the fuel assembly within a core of the nuclear reactor, such as the position of the fuel assembly with respect to a control blade in the nuclear reactor that will be used to control core reactivity. When example embodiment fuel assemblies are to be directly adjacent to an inserted control blade, the outer channel may be thickened, reinforced, and/or fabricated of a material more resistant to deformation than Zircaloy-2, such as Zircaloy-4, NSF, and VB, so as to reduce or prevent distortion of the channel against the control blade and interfering with operation of the same. When example embodiment fuel assemblies are not in a controlled location, the outer channel may be thinned so as to increase water volume and reactivity in the assembly. As such, a reactor core including example embodiment fuel assemblies will include fuel assemblies having unique outer channels, in thickness, material, etc., unlike conventional power reactor cores. Example methods of configuring fuel assemblies include determining operational characteristics of the fuel assembly, such as the likelihood that the fuel assembly is controlled via control blade insertion in the nuclear reactor in a current or future fuel cycle, and physically selecting or modifying the outer channel of the fuel assembly based thereon. For example, if the fuel assembly is in a controlled location during the fuel cycle, the outer channel may be fabricated of a material more resistant to deformation than Zircaloy-2, such as Zircaloy-4, NSF, or VB, and/or thickened. Or, for example, if the fuel assembly is not in a controlled location, the outer channel may be approximately 20 mils (thousandths of an inch) or more thinner than outer channels of conventional fuel assemblies. Example methods are usable with or may further include configuring outer channel characteristics in order to meet desired neutronic properties of the fuel assembly. Hereinafter, example embodiments will be described in detail with reference to the attached drawings. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. The example embodiments may be embodied in many alternate forms and should not be construed as limited to only example embodiments set forth herein. It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that when an element is referred to as being “connected,” “coupled,” “mated,” “attached,” or “fixed” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the language explicitly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, “channel,” “outer channel,” and the like are defined in accordance with the conventional fuel assembly structures shown and described in FIG. 1 as element 12, subject to the modifications discussed hereafter. As used herein, “distortion” or “channel distortion” includes both channel bow and channel bulge in nuclear fuel assemblies that may cause interference with control blade operation. It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures or described in the specification. For example, two figures or steps shown in succession may in fact be executed in parallel and concurrently or may sometimes be executed in the reverse order or repetitively, depending upon the functionality/acts involved. The inventors of the present application have identified several potential fuel channel characteristics and/or modifications to reduce or prevent fuel channel distortion. The inventors of the present application have further identified the effect these characteristics, in combination with other fuel assembly parameters, have on whole core performance. Example embodiments and methods discussed below uniquely address these previously-unidentified effects to achieve several advantages, including improved core performance, increased energy generation, reduced control blade error, materials conservation, and/or other advantages discussed below or not, in commercial nuclear power plants, while departing from total fuel channel uniformity purposefully used in conventional commercial nuclear power plants. Example embodiment fuel assemblies include fuel channels with optimized physical properties. Example embodiment fuel assemblies may include one or more channel characteristics to decrease fuel channel distortion. For example, the fuel channel may be thickened in its shortest dimension or reinforced with additional material. The thicker or reinforced fuel channel has greater resistance to distortion from differential irradiation growth, differential hydrogen absorption, and/or irradiation creep experienced in operating nuclear reactor environments. The percent reduction in deformation is approximately proportional the percentage increase in channel thickness. Or, for example, materials may be used in the channel that are resistant to distortion. For example, Zircaloy-4, a known zirconium alloy excluding nickel, may replace Zircaloy-2, which contains nickel. The reduced nickel content in Zircaloy-4 reduces differential hydrogen absorption and resultant channel bow. Other materials more resistant to deformation than Zircaloy-2 may additionally be used in whole or in part in addition to Zircaloy-4. For example, additional materials more resistant to deformation than Zircaloy-2 are described in co-pending application Ser. No. 12/153,415 “Multi-layer Fuel Channel and Method of Fabricating the Same,” incorporated herein by reference in its entirety. That document discloses alloys hereinafter called “NSF” having about 0.6-1.4% niobium (Nb), about 0.2-0.5% iron (Fe), and about 0.5-1.0% tin (Sn), with the balance being essentially zirconium (Zr) and alloys hereinafter called “VB” having about 0.4-0.6% tin (Sn), about 0.4-0.6% Fe, and about 0.8-1.2% chromium (Cr), with the balance being essentially zirconium (Zr). Other configurations for decreasing fuel channel distortion are usable with example embodiment fuel assemblies. Example embodiment fuel assemblies may use multiple mechanisms in combination to further reduce fuel channel distortion. Configurations and fuel channel characteristics in example embodiment fuel channels may be selected in accordance with example methods, discussed in the following section. Example embodiment fuel assemblies may further include channel characteristics that improve fuel neutronic characteristics, decrease material usage and costs, and/or improve other fuel assembly parameters. Such characteristics may include, for example, a thinner channel that permits greater water volume and neutron moderation within example embodiment fuel assemblies. The thinner channel may consume less material in fabrication and improve fuel assembly reactivity, heat transfer characteristics, etc. Example embodiment fuel assemblies having thicker, reinforced, and/or thinner channels, different alloys, or other channel modification may be used instead of conventional fuel assemblies having standardized channels throughout an entire core. Example embodiment fuel assemblies may thus significantly improve performance of a core including example embodiment fuel assemblies and/or reduce fuel resource consumption. For example, thinning the channels of 75% of the fresh conventional fuel assemblies for a particular fuel cycle by approximately 20 mils (20 thousandths of an inch) in the thinnest dimension may result in a reduction in volume of approximately 16,500 in3 zirconium alloy used. In the same example, assuming 8 channels would not need to be replaced because they include channel mechanisms to decrease fuel channel deformation, an additional ˜2,000 in3 zirconium alloy volume may be saved. In the same example, assuming 8 channels are not needed to be fabricated because 8 fewer fuel assemblies are required in a fuel cycle with fuel savings from channel characteristics that improve fuel neutronic characteristics of example embodiment fuel assemblies, an additional ˜2,000 in3 zirconium alloy volume may be conserved. Thus, example embodiment fuel assemblies, having different channel characteristics selected and implemented in accordance with example methods discussed below, may result in significant materials savings and improved core performance. Example Methods As discussed above, increasing channel thickness decreases water volume and overall reactivity of an assembly having a thicker channel. Lower reactivity results in less optimal fuel usage and less power production in a nuclear core of a nuclear power reactor. Increasing channel thickness further increases costs of fuel assemblies having thicker channels. Increasing channel thickness also reduces the risk and/or magnitude of channel distortion and interference with control blade function. Decreasing channel thickness has a generally opposite effect of increasing water volume and overall reactivity of an assembly having a thinner channel, while also increasing distortion likelihood. Zircaloy-4 has similar fluence bow and creep bulge characteristics compared to Zircaloy-2. Zircaloy-4, however, resists channel bow caused by differential hydrogen absorption. NSF and VB are additionally resistant to other forms of bow and bulge causing channel deformation. Example methods uniquely leverage the above advantages and disadvantages of fuel channel modification to reduce or prevent channel distortion while minimizing negative effects on fuel economy, control blade function, and other core performance metrics. As shown in FIG. 3, example methods include an operation S100 of determining fuel assembly characteristics, including whether a fuel assembly is placed or will be located in a cell such that the fuel assembly will be directly adjacent to a control blade that will be operated in a current and/or future fuel cycle to control the fission reaction in the core. A fuel assembly positioned directly adjacent to a control blade that is likely to be exercised to control the fission reaction is herein defined as a “controlled fuel assembly” or in a “controlled location,” because it is most subject to control blade negative reactivity and most likely to affect control blade performance. The determination of whether a fuel assembly is subject to control may be based on one or more fuel assembly operational characteristics that determines placement/position of the fuel assembly within the reactor core over one or more fuel cycles, in addition to overall plant characteristics such as core size, thermal power rating, etc. For example, an operational characteristic may be reactivity of the fuel assembly. Reactivity determines the degree to which the fuel can contribute to the fission chain reaction during power operations. Reactivity is directly controllable with control blade insertion, due to the blades' neutron-absorbing properties. As such, fuel with higher reactivity may be placed in controlled locations to enhance core-wide overall control of the neutron chain reaction. Similarly, fuel with lower reactivity may be less likely to be subject to control. Although location with regard to utilized cruciform control blades is described in connection with example embodiments and methods, it is understood that other sources of negative reactivity may additionally be accounted for in example methods and embodiments. For example, proximity to burnable poisons or proximity to a control rod present in some plant designs may be accounted for by determining operational characteristics of the fuel assembly that determine the likelihood that the fuel assembly will be placed in that proximity. Controlled locations may also be determined in S100 by known core modeling and mapping methods and software. For example, a program may receive input of several fuel assembly operational characteristics for several fuel assemblies and determine an optimum core configuration with corresponding fuel assembly positions. Because example methods and embodiments may themselves affect fuel assembly operational characteristics as discussed below, such known core modeling and mapping methods may be alternatively and repetitively executed before and following fuel assembly modification in example methods to ensure optimized core performance. Following the determination in S100, one or more fuel assembly channels are configured based on the position determination. The configuring generally increases assembly reactivity, decreases distortion potential, and/or reduces material consumption in the configured assembly/assemblies. If it is determined from S100 that the assembly will be placed in a cell adjacent to an employed control blade, i.e., subject to control, then a first configuration S210 is pursued. S210 configures the assembly channel to reduce or eliminate channel distortion during power operations. For example, in S210, channel thickness may be increased by several hundredths of an inch or more to ensure decreased channel distortion. The degree of thickening may further be based on decreased reactivity or other operational characteristics desired of the assembly during operation in the nuclear reactor core. Or, channel thickness may be increased or the channel may be reinforced on only a side or wall directly adjacent to the control blade that will be operated, while remaining fuel channel sides may be unmodified or modified in accordance with S220. Additionally, or in the alternative, in S210, the channel may be fabricated out of a material more resistant to distortion than Zircaloy-2, including shadow-corrosion-bow-resistant Zircaloy-4, or fluence-gradient-bow and/or-creep-bulge-resistant NSF or VB. In this way, only assemblies determined to be at a position benefiting from a thicker or reinforced channel or a channel including Zircaloy-4, NSF, and/or VB, such as a controlled assembly likely to be placed in a cell adjacent to an employed control blade, are configured with channel features that decrease or eliminate distortion while leveraging other characteristics such as reactivity or fabrication expense. Further, because assemblies in a controlled core position typically possess higher excess reactivity, a thicker or reinforced channel that may decrease reactivity is not a significant disadvantage for the overall core reactivity; indeed, such reactivity-decreasing configuration may aid in balancing core power production and/or simplifying control blade operations. If it is determined from S100 that the assembly will be placed in an uncontrolled core position, such as an edge position in the core or adjacent to a control blade that will not be utilized, then a second configuration S220 is pursued. S220 configures the assembly channel to increase fuel assembly neutronic characteristics for the assembly in the operating core and decrease manufacturing burden in fabricating the assembly, without regard to distortion risk. For example, in S220, channel thickness may be decreased by several hundredths of an inch or more to increase water or moderator volume in the assembly, thereby increasing reactivity and fuel usage in the assembly. Reducing channel thickness in S220 further decreases an amount of expensive zirconium alloy or other channel material required to fabricate the assembly. In S220, assembly channel thickness may be reduced by a margin that takes into account the increased reactivity; the channel may be thinned such that the assembly has a determined or desired reactivity or other operational property when in use in the nuclear reactor core. In this way, a core may contain fuel assemblies with several different, unique channel thicknesses and other characteristics as determined in S210 and S220. Assemblies may be configured in S210 and S220 in several different manners and timeframes. For example, the configuring in S210 and S220 may be selecting a pre-existing assembly or ordering an assembly having the configuration determined in S210 and S220, by a power plant operator, for insertion or re-insertion during an upcoming fuel cycle in the nuclear reactor core. Alternatively, the configuring in S210 and S220 may be a physical fabricating or modifying of the fuel assembly to match the configuration determined in S210 and S220 by a fuel assembly manufacturer or refitter, for example. Example methods including S100 and S210/S220 may address fuel assembly location and configuration for use in an immediately approaching fuel cycle, a future fuel cycle, and/or multiple fuel cycles. For example, S100 may determine that a fuel assembly will be in a controlled position adjacent to an employed control blade in a first fuel cycle, and the same or later analysis may determine that the fuel assembly will be relocated to a position away from a control blade in a second layer fuel cycle. The assembly may be configured under S210 for the first cycle, and then reconfigured under S220 for the second cycle. Such reconfiguring may include re-channeling the fuel assembly by removing and replacing the channel used in the first fuel cycle with a channel having the configuration determined in S220 for use in the second fuel cycle. Similarly, a reverse determination may result in the reverse configuration. Or, for example, S100 may determine, based on multi-cycle operating parameters, that a particular fuel assembly will not be placed in a controlled location in its lifetime. Configuration of the assembly may then proceed under S220, without further modification of the assembly during its lifetime in the reactor. FIG. 4 is an illustration of an example reactor core 400 containing example embodiment fuel assemblies 100 and 200 modified in accordance with example methods. As shown in FIG. 4, four example embodiment assemblies 100 are in controlled locations about a control blade 45a that is anticipated to be used to control the fission chain reaction in the core. According to example methods, assemblies 100 about blade 45a have channels 120 configured in accordance with S210. For example, channels 120 may be thickened, reinforced, and/or fabricated of a material more resistant to deformation than Zircaloy-2. Or, for example, only select sides or walls 120b directly adjacent to control blade 45a may be configured in accordance with S210, including being thickened, reinforced, and/or fabricated of a material more resistant to deformation than Zircaloy-2. Other walls 120a may be unmodified or thinned and/or fabricated of a material equally or less resistant to deformation than Zircaloy-2, in accordance with S220. Example assemblies 200, adjacent to blade 45b that is not to be operated during the fuel cycle or adjacent to no control blade, may be configured in accordance with S220. For example, channels 121 in assemblies 200 may be thinned and/or fabricated of a material equally or less resistant to deformation than Zircaloy-2. Example methods including S100 and S210/S220 may be executed for each assembly to be placed within a core. Alternatively, example methods may be executed only with respect to particular assemblies in order to optimize core operating characteristics. For example, if example fuel assembly channel configuring methods are used in conjunction with other known core configuration methods, the calculated or desired fuel assembly locations and characteristics may require no fuel assembly channel configuring or reconfiguring as in S210 or S220. Example methods may be used as an integral part of core design or as a separate step performed alternatively and/or iteratively with other known methods of core design. For example, a known core design program may output a core map using fuel assembly characteristics with fuel having uniform channel properties. Example methods including S100 and S210/S220 may then be performed on some or all fuel assemblies involved in the map, changing their operational characteristics. The core design program may then be re-executed with the modified fuel assembly characteristics, and this alternating core configuring between example and known methods may continue until no further optimization is possible or desired. Or, example methods may be used as an integral part of otherwise known core design methods, treating reactivity, bow likelihood, and other fuel assembly parameters affected by channel configuring in S210 and S220 as additional variables in the core design process. Example embodiments and methods thus being described, it will be appreciated by one skilled in the art that example embodiments may be varied through routine experimentation and without further inventive activity. Variations are not to be regarded as departure from the spirit and scope of the example embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
047044136	abstract	According to the invention, a resin composition having an electromagnetic wave shielding effect is provided. The resin composition comprises 35 to 90 wt % of an ABS resin or a mixture of the ABS resin and an AS resin, 1 to 25 wt % of a plasticizer and 5 to 40 wt % of carbon fibers. The carbon fibers are uniformly dispersed in the resin ingredient without being broken or cut at the step of mixing and dispersing the resin components, thus exhibiting improved electromagnetic wave shielding effect.
062158538	claims	1. A method for calibrating a collimator, the method comprising: acquiring a digital image representative of collimator blades aligned relative to a region of interest; automatically determining a position of at least one collimator blade from the digital image; and automatically adjusting a position of at least one collimator blade relative to the region of interest. a communication interface for carrying at least one collimator blade position sensor signal, at least one collimator blade actuator signal, and an image detector signal; and a central processor, coupled to the communication interface, for automatically determining a position of a collimator blade based on an image detector signal, and for automatically generating a blade activator signal for adjusting a position of a collimator blade toward a desired position relative to the region of interest. an X-ray detector; at least one collimator blade located relative to an region of interest; a detector for detecting a position of the collimator blade; a blade position controller for moving the collimator blade through at least one of a rotation and a translation; and a calibrator for determining the position of collimator blades and automatically instructing the blade position controller to move the collimator blade. 2. A method as recited in claim 1, wherein the step of acquiring the digital image comprises acquiring a digital image with a solid state X-ray detector. 3. A method as recited in claim 1, wherein the step of automatically determining position comprises segmenting at least a portion of the digital image into at least one band along an axis, and determining an edge of the collimator blades based on an edge response function. 4. A method as recited in claim 1, wherein the step of automatically determining position comprises fitting a collimator blade edge in the digital image with a linear model. 5. A method as recited in claim 3, wherein the step of automatically determining position further comprises fitting a collimator blade edge in the digital image with a linear model. 6. A method as recited in claim 4, wherein the step of automatically determining position comprises determining a distance from a center of the digital image to a collimator blade edge. 7. A method as recited in claim 1, wherein the step of adjusting a position of at least one collimator blade comprises at least one of translating and rotating at least one collimator blade relative to a reference point. 8. A method as recited in claim 1, wherein the step of adjusting a position of at least one collimator blade comprises adjusting at least one of a width between collimator blades and a centering of the collimator blades in the digital image. 9. A method as recited in claim 1, further comprising the steps of rotating a collimator assembly so that the collimator blades appear in the region of interest, and reversing the rotation after calibration is completed. 10. A method according to claim 1, further comprising the step of iterating the acquiring, determining, and adjusting steps until the collimator blades expose the region of interest within a predetermined tolerance. 11. A collimator calibration subsystem comprising: 12. A collimator calibration subsystem according to claim 11, further comprising at least one sensor and at least one actuator for at least one collimator blade. 13. A collimator calibration subsystem according to claim 11, wherein the communication interface connects to a digital X-ray detector image detector. 14. A collimator calibration subsystem according to claim 11, wherein the processor segments at least a portion of the digital image into at least one band along an axis, averages pixels in each band perpendicular to the axis, and determines edges of the collimator blades using an edge response function. 15. A collimator calibration subsystem according to claim 11, wherein the processor fits a linear model to determine blade edges in the digital image with a linear model. 16. A collimator calibration subsystem according to claim 14, wherein the processor fits a linear model to determine collimator blade edges in the digital image with a linear model. 17. A collimator calibration subsystem according to claim 11, wherein the activator signal induces at least one of translation and rotation of at least one collimator blade using an actuator. 18. A collimator calibration subsystem according to claim 11, wherein the activator signal adjusts at least one of a width between collimator blades and a centering of the collimator blades in the digital image. 19. A collimator calibration subsystem according to claim 11, further comprising a memory coupled to the processor, and wherein the processor stores calibration position information for at least one collimator blade in the memory. 20. A collimator calibration subsystem according to claim 11, wherein the processor iterates the determining and generating steps until the collimator blades expose the region of interest to within a predetermined tolerance. 21. A collimator calibration subsystem according to claim 11, wherein the activator signal induces movement of the collimator blade by a predetermined increment, after which the processor determines a new position of the collimator blade. 22. A collimator calibration subsystem according to claim 11, wherein the processor determines current and destination calibrated positions of the collimator blade, and asserts the activator signal to move the collimator blade from the current position to the destination calibration position. 23. A collimator calibration system comprising: 24. A collimator calibration system of claim 23 wherein the calibrator directs the blade position controller to move the collimator blade by a predetermined increment, after which the calibrator determines a new position of the collimator blade. 25. A collimator calibration system of claim 23, wherein the calibrator calculates current and destination calibrated positions of the collimator blade, and instructs the blade position controller to move the collimator blade from a current position to the destination calibration position. 26. A collimator calibration subsystem according to claim 23, further comprising at least one sensor and at least one actuator for at least one collimator blade. 27. A collimator calibration subsystem according to claim 23, further comprising a communication interface coupled to the X-ray detector, the blade position controller, and the calibrator. 28. A collimator calibration subsystem according to claim 23, wherein the calibrator segments at least at portion of the digital image into at least one band along an axis, averages pixels in each band perpendicular to the axis, and determines edges of the collimator blades using an edge response function. 29. A collimator calibration subsystem according to claim 23, wherein the calibrator fits a linear model to determine blade edges in the digital image with a linear model. 30. A collimator calibration subsystem according to claim 28, wherein the calibrator fits a linear model to determine collimator blade edges in the digital image with a linear model. 31. A collimator calibration subsystem according to claim 23, wherein the blade position controller generates an activator signal that induces at least one of translation and rotation of at least one collimator blade using an actuator. 32. A collimator calibration subsystem according to claim 23, wherein the blade position controller adjusts at least one of a width between collimator blades and a centering of the collimator blades in the digital image. 33. A collimator calibration subsystem according to claim 28, wherein the axis lies in a width direction of the digital image. 34. A collimator calibration subsystem according to claim 23, wherein the calibrator iterates the determining and instructing until the collimator blades expose the region of interest to within a predetermined tolerance.
051270307	claims	1. A device useful in producing a tomographic image of a selected slice of an object to be examined comprising: a source of penetrating radiation, sweep means for forming energy from said source into a pencil beam and for repeatedly sweeping said pencil beam over a line in space to define a sweep plane, first means for supporting an object to be examined so that said pencil beam intersects said object along a path passing through said object, collimating means with a plane of symmetry forming an angle to said sweep plane for filtering radiation scattered by said object, said collimating means having: a source of penetrating radiation, sweep means for forming energy from said source into a pencil beam and for repeatedly sweeping said pencil beam over a line in space to define a sweep plane, first means for supporting an object to be examined so that said pencil beam intersects said object, second means for preferentially detecting radiation scattered, at any instant, by a selected volume element in said slice, said second means subtending a large solid angle relative to said selected volume element, said second means including: wherein said collimator has an axis of symmetry which does not coincide with said sweep plane. said radiation detector means and said collimator, on the one hand, and said second radiation detector means and said second collimator, on the other hand, being spaced from each other with said sweep plane and said object lying therebetween. 2. A device as recited in claim 1 wherein said collimating means has a face opposite said object, said channels of said collimating means have equal dimensions in said face of the collimating means. 3. A device as recited in claim 2 wherein said slice has a thickness determined by a dimension of said pencil beam. 4. A device as recited in claim 1 wherein said sweep plane intersects said plane of symmetry of said collimating means at about a right angle. 5. A device as recited in claim 1 wherein said sweep plane intersects said plane of symmetry of said collimating means at an angle other than 90.degree.. 6. A device as recited in claim 1 wherein a solid angle subtended by said collimating means at an elementary volume of said object illuminated by said pencil beam and along said bounded line is about .pi./2 steradians. 7. A device as recited in claim 1 wherein a solid angle subtended by said collimating means at an elementary volume of said object illuminated by said pencil beam and along said bounded line is in a range of about 0.05.pi. to 2.pi. steradians. 8. A device as recited in claim 1 which further includes means for providing relative motion between said object and said source and further includes imaging means responsive to a signal produced by said radiation detector means for producing an image of said selected slice. 9. A device as recited in claim 1 wherein said collimating means includes first and second collimators, each collimator having a field of view which intersects said sweep plane and said object in a bounded line and a plurality of channels each substantially planar in form to collectively define said field of view, said first and second collimators spaced from each other with said sweep plane lying therebetween and wherein said radiation detector means includes a first radiation detector associated with said first collimator and a second radiation detector associated with said second collimator, each radiation detector producing, at any instant a single valued signal representing detected radiation and summing means responsive to an output of each of said radiation detectors for producing a signal representing, at any instant in time, a sum of said output signals. 10. A device as recited in claim 9 wherein a solid angle subtended by said collimating means at an elementary volume of said object illuminated by said pencil beam and along said bounded line is at least about 0.05.pi. steradians. 11. A device as recited in claim 10 which further includes means for producing relative motion between said object and said source and further includes imaging means responsive to a signal produced by said radiation detector means for producing an image of said selected slice. 12. A device useful in producing a tomographic image of a selected slice of an object to be examined comprising: 13. A device as recited in claim 12 wherein said channels of said collimator have equal dimensions in that face of the collimator facing said object. 14. A device as recited in claim 13 wherein said slice has a thickness determined by a dimension of said pencil beam. 15. A device as recited in claim 12 wherein said sweep plane intersects said plane of symmetry of said collimator at about a right angle. 16. A device as recited in claim 12 wherein said sweep plane intersects said plane of symmetry of said collimator at an angle other than 90.degree.. 17. A device as recited in claim 12 wherein said solid angle is about .pi./2 steradians. 18. A device as recited in claim 12 wherein said solid angle is in a range of about 0.05.pi. to 2.pi. steradians. 19. A device as recited in claim 12 which further includes means for producing relative motion between said object and said source and further includes imaging means responsive to a signal produced by said radiation detector means for producing an image of said selected slice. 20. The device as recited in claim 12 wherein said second means includes second radiation detector means developing at any instant in time a single signal reflecting radiation impinging on said second radiation detector means, and a second collimator located between said object and said second radiation detector means, said second collimator including a plurality of radiation transmitting channels collectively establishing a field of view which intersects said sweep plane in a line which is a locus of said selected volume elements so that said collimator passes radiation scattered by different elementary volume elements of said object lying along said line as said sweep of said pencil beam illuminates the different volume elements lying along said line and wherein said second collimator has an axis of symmetry which does not coincide with said sweep plane, 21. The device recited in claim 20 wherein a solid angle subtended by said first and second collimator at an elementary volume of said object illuminated by said pencil beam and along said bounded line is at least about 0.05.pi. steradians. 22. The device recited in claim 21 which further includes means for producing relative motion between said object and said source, summing means for summing outputs of said radiation detector means and said second radiation detector means and imaging means responsive to an output of said summing means for producing an image of said selected slice.
043426212	description	DESCRIPTION OF THE PREFERRED EMBODIMENT While the invention is susceptible of various modifications and alternative constructions, there is shown in the drawings and there will hereinafter be described in detail a description of the preferred embodiment of the invention. It is to be understood, however, that the specific description and drawings are not intended to limit the invention to the specific form disclosed. On the contrary, it is intended that the scope of this patent include all modifications and alternative constructions thereof falling within the spirit and scope of the invention as expressed in the appended claims. In a preferred embodiment of the present invention, as depicted in FIG. 1, a nuclear reactor 10 having a vessel 14 and an internally mounted reactor core 12 is suspended within a reactor chamber 24 formed within the base 21 of the containment building 20. The reactor vessel 14 is supported by a cantilevered support member 22 upon which rests the reactor coolant piping 18 as it leaves the reactor vessel 14. Reactor coolant piping 18, in the case of a pressurized water reactor, fluidically connects the interior of the reactor vessel 14 with a steam generator 16 which generates steam for its ultimate delivery to a steam turbine. The general function of the present invention is to protect the concrete walls of chamber 24 from the damaging effects of both the extreme heat and radiation generated by a nuclear core 12 which has melted and dropped through the bottom of the reactor vessel 14 as shown generally at 38. Of primary concern are both the cavity floor 28 and the vertical walls 26. Damage to these walls is to be avoided if at all possible on the occurance of such an accident in order that the radioactive materials associated with a molten core 38 are prevented from escaping either to the exterior of chamber 24 or the exterior of the containment building 20. Therefore, the present invention provides a system of heat pipes 40 and 50 which respectively protect the chamber floor 28 and the chamber wall 26 by collecting a portion of the heat generated by the core in chamber 24 and transporting the heat to a location exterior to the containment building 20. It has been found that heat pipes are well suited for this function in that they can be designed to transfer large quantities of heat with very little thermal resistance, can remain passive for large periods of time without maintainance and can automatically begin their heat transfering function without need of human or mechanical intervention. Furthermore, a heat pipe is ideal for accomplishing these functions inasmuch as each heat pipe constitutes a hermetically sealed unit which is independent from all the other heat pipes of the system. The hermetically sealed feature of the heat pipe permits the heat pipe to penetrate from the interior of chamber 24 to the exterior of the containment building 20 without running the risk of pumping radioactive material to the exterior of the containment envelope in the event that one end of the heat pipes is breached. Heat pipes 40 and 50 are arranged with their evaporator sections 42 and 52 adjacent to the reactor chamber's floor 28 and wall 26 respectively so as to shield the concrete base 21 from the damaging heat and radiation of the molten core 38 and so as to contain the core. Each of these heat pipes has its condenser section 46 and 56 respectively located in a water reservoir 60 external to the reactor containment building 20. The condenser sections and the evaporator sections of the heat pipes are fluidically connected by adiabatic sections 44 and 54 respectively which penetrate through the reactor containment building 20 through base 21. Each of the adiabatic sections 44 and 54 are surrounded by thermal insulation 64 in order that the concrete of the base 21 and the containment building 20 not be exposed to excessive heat which could possibly cause concrete dehydration and subsequent failure. As may be seen in FIG. 1, water reservoir 60 is vented by a vent pipe 62 directly to the atmosphere exterior to the containment building 20. As may also be seen from FIG. 1, condenser sections 46 and 56 of heat pipes 40 and 50 respectively are positioned at elevations higher than the elevations of their respective evaporator sections so that the working fluid does not have to work against a gravitational head in its return to the evaporator section. An angle of slant no less than 30.degree. is preferred in order that the maximum capability of the heat pipe be achieved. As can be seen in FIG. 1, evaporator ends 42 of heat pipes 40 are disposed vertically below reactor vessel 14. A slightly conical upwardly facing shallow basin 30 is also disposed below evaporator sections 42. Basin 30 is provided to prevent direct contact between molten core 38 and base 21. Basin 30 desirably consists of a refractory metal having a high melting point such as tungsten, tantalum carbide, zirconium carbide, niobium carbide, hafnium carbide or graphite. However, it is predicted that the temperature of molten core 38 would exceed the melting point of refractory basin 30 so that refractory basin 30 must be either directly cooled by a cooling system or shielded from the elevated temperatures of the core. The present invention chooses the latter arrangement. Accordingly, heat pipes 40 are arranged in a star-like pattern radiating outwardly from a position under the core 12 in a manner which best shields refractory basin 30 from the temperature of the core. The preferred arrangement of the invention is illustrated in FIG. 2 in which heat pipes 40 are arranged in an outwardly radiating star-like array. Each heat pipe 40 includes thermal conducting fins 48 attached to its evaporator end 42. As can be seen, fins 48 are shaped to butt one against another to almost completely cover the upwardly facing surface of basin 30. Adjacent fins 48 however, are separated by a slight gap in order to accomodate the thermal expansion expected when heated by a molten core 38. Evaporator sections 42 of heat pipes 40 are vertically supported by but not anchored to underlying basin 30. In this manner, heat pipes 40 are permitted to operate at a temperature in excess of the temperature of basin 30 without incurring the significant problems of differential thermal expansion. Thermal expansion of the heat pipes 40 are further accommodated by the provision of anchoring heat pipes 40 to base 21 of the containment building 20 at only one point: the point at which the heat pipes enter the concrete foundation 21. At this position, the heat pipe is hermetically sealed to the base 21 by seal 34 so that the passage through which the adiabatic section 44 of the heat pipe passes is hermetically isolated from the interior of chamber 24. Seals 34 must be of such a nature as to be able to withstand radial expansion of heat pipe 40. As can be seen, with this arrangement, heat pipes 40 are permitted axial growth in both the inward and outward direction from the attachment point at seal 34 and the hermetic containment envelope is maintained. While the above described arrangement effectively shields basin 30 from excessive temperatures, it is still expected that the basin 30 will be exposed to an extreme elevated temperature. Accordingly, basin 30 will also experience thermal growth. In anticipation of the thermal growth expected in basin 30, the basin is attached to base 21 only in one centrally located point 32. This arrangement permits basin 30 to undergo unrestricted radial expansion so that warping effects are minimized. As can be seen in FIG. 1, a layer of thermal insulation 36 such as a layer of alumina bricks may be placed under basin 30 in order to protect the underlying concrete of base 21 from thermally caused dehydration. The above described arrangement exposes the evaporator sections 42 of heat pipes 40 directly to the molten core 38. It should be recognized, however, that basaltic blocks as taught in U.S. Pat. No. 3,702,802 may also be placed above the heat pipes in order to reduce the thermal shock placed on the heat pipes 40 as well as to reduce the heat release per unit volume of core material by diluting the molten core material with material from the basalt blocks. An additional measure which may be taken to avoid the excessive concentration of heat of the molten core is the formation of basin 30 and evaporator sections 42 of heat pipes 40 in a nearly horizontal manner. The molten core material would then be expected to spread out in a relatively thin layer. Other measures may be taken such as those taught in U.S. Pat. No. 4,036,688 in order to prevent the molten core from forming a critical geometry. In a manner similar to that described above for heat pipes 40, heat pipes 50 are provided with thermally conducting fins 58 which spread out and shield the inner surface 26 of chamber 24 at portions which are not directly shielded by the heat pipe evaporator section 52 itself. While not shown in the drawing of FIG. 1, it may also be desirable to line the inner surface of chamber 24 with a thermal insulator such as refractory bricks in order to prevent the dehydration of the concrete base 21. Heat pipes 50 are preferably arranged in a star-like pattern which radiates outwardly from the reactor. As can be seen from FIG. 1, an inwardly projecting "knee" of heat pipe 50 is provided in order to intercept and absorb the upwardly directed radiations emminating from molten core 38. In this manner, the upper portions of the cavity 24 are protected against the damaging heat and radiations emmited by the core. Heat pipes 50 are also anchored at that point at which each heat pipe enters the concrete wall of the chamber at seal 34 so that heat pipes 50 may undergo unrestricted thermal expansion in both the inward and outward directions. Turning now to an examination of FIG. 3, a typical cross-section of the evaporator sections of heat pipes 40 is shown. Also shown are a portion of the base 21, the thermal insulation 36, the refractory metal basin 30 and the fins 48. Wicking material 66 resides on the interior of the heat pipes 40. As is well understood, the materials from which heat pipes 40 are constructed depend upon a number of factors including the amount of heat which must be transported, the maximum temperatures expected, and the compatability of the materials used in the pipe which included the pipe itself, the working fluid, and the wicking material. It has been calculated that for a 3,800 megawatt thermal core, 136 six inch sodium filled heat pipes with evaporator lengths of eight feet and condenser lengths of twelve feet would be adequate to effectively remove the heat generated by molten core 38 so that basin 30 is protected from melting and core 38 is prevented from boiling. Other possible candidates for the working fluid of the heat pipes 40 and 50 includes potassium, cesium, mercury, and one of the eutectic alloys such as NaK. If the selected working fluid were to be liquid sodium, suitable heat pipe materials might include one of the alloys having trade names Nickel 200, Monel 400, Inconel 600, or Inconel 800. In addition, long heat pipes containing a plurality of working fluids are possible. Such heat pipes, when called upon to operate, would automatically separate themselves into zones determined by the latent heat of evaporation of the various working fluids as well as the temperature of the evaporation of the various working fluids.
055286410	abstract	A fuel assembly is provided with a coolant ascending path for making coolant rise and a water rod having a coolant descending path for conducting the coolant.. A ratio of a flow area in a coolant inlet port of the smallest in coolant ascending path 13 on the downstream side than large diameter tube portion 3E to a flow area of the largest in the axial direction of coolant ascending path 13 in large diameter tube portion 3E is set to be 0.2-20%.. In the normal operation, the declination degree from the liquid level in the coolant ascending path, corresponding to the coolant flow rate of the liquid level formed in the coolant ascending path can be controlled. Further, at the time of the excess the change speed of the liquid level can also be controlled.
	summary
	description	The present application claims priority from Japanese Patent application serial no. 2009-011479, filed on Jan. 22, 2009, the content of which is hereby incorporated by reference into this application. 1. Technical Field The present invention relates to jet pump and reactor and, in particular, to a jet pump and a reactor suitable for applying to a boiling water reactor. 2. Background Art A conventional boiling water reactor (BWR) has a jet pump in a reactor pressure vessel (hereinafter referred to as an RPV) to which a recirculation pipe is connected. The jet pump has a nozzle, a bell mouth, a throat, and a diffuser. Cooling water in a downcomer, where the jet pump is disposed, formed in the RPV is pressurized by operation of a recirculation pump, pumped through the recirculation pipe as a driving flow, and ejected from the nozzle into the throat. The nozzle increases the speed of the driving flow. The cooling water around the nozzle in the downcomer is sucked into the bell mouth as a suction flow due to the working of the ejected driving flow, passes the throat, and flows into the diffuser. The cooling water discharged from the diffuser is supplied to a core through a lower plenum in the RPV (see, for example, U.S. Pat. No. 3,625,820, Japanese Patent Laid-open No. Sho 59 (1984)-188100, Japanese Patent Laid-open No. Hei 7 (1995)-119700, and Japanese Patent Laid-open No. 2007-285165). Jet pumps disclosed in Japanese Patent Laid-open No. Sho 59 (1984)-188100, Japanese Patent Laid-open No. Hei 7 (1995)-119700, and Japanese Patent Laid-open No. 2007-285165 each have a plurality of nozzles. When the total area of each ejection opening formed in the plurality of nozzles remains constant, an increase in the number of nozzles increases the contact area between driving flows and suction flows, and thus mixing of the driving flows and the suction flows is promoted. Consequently, a mixing loss is decreased, increasing efficiency of the jet pump. A jet pump installed in a reactor has a nozzle connected to a raiser pipe that is installed in the RPV. In this jet pump, an elbow pipe, the nozzle, a bell mouth and a throat are unified into one body, which structure allows the elbow to the throat to be removed for inspection and maintenance. A connection portion between the throat and the diffuser has a joint structure in which a lower end portion of the throat is inserted into an upper end portion of the diffuser. This joint structure is a slip joint. The slip joint, where the throat and the diffuser are connected, has a structure which allows the upper end portion of the diffuser and the lower end portion of the throat to slide up and down, so that no stress is generated due to the difference between the thermal expansions of the raiser pipe and the diffuser. For this reason, a gap is formed between an inner surface of the diffuser's upper end portion and an outer surface of the throat's lower end portion. Part of the cooling water that flows into the diffuser from the throat leaks out to the downcomer through the gap. This leakage flow prevents a foreign object from being caught in the gap or deposited on the surfaces. However, when the flow rate of the leakage flow exceeds a limit, the jet pump may start to vibrate. Thus, in order to suppress the vibration of the jet pump, the leakage flow from the gap in the slip joint should be limited below the limit. Although it is not a jet pump, Japanese Examined Utility Model Application Publication No. Sho 52 (1977)-5301 discloses a fluid sealing joint used for pipes for introducing high-temperature and high-pressure gas (or steam). In this fluid sealing joint, a tubular inlet-side joint portion is inserted into a tubular outlet-side joint portion; and an end portion of the inlet joint portion has a narrowing portion whose flow passage cross-sectional area decreases and an expanding portion whose cross-sectional area increases toward the end. A communication hole is formed in the place where the narrowing portion and the expanding portion are connected, the flow passage cross-sectional area of which the place is the smallest in the inlet-side joint portion. This communication hole communicates with an annular space portion formed between the inlet-side joint portion and the outlet-side joint portion. Static pressure inside is reduced at the seam between the narrowing portion and the expanding portion so that a fluid in the annular space portion is sucked inside the narrowing portion through the communication hole. This effectively prevents a fluid from leaking out of the fluid sealing joint through a gap between the inlet-side joint portion and the outlet-side joint portion. Japanese Patent Laid-open No. Sho 59 (1984)-159489 discloses a jet pump for suppressing vibration. In this jet pump, a lower end portion of a throat, which is inserted into an upper end portion of a diffuser, has a flow passage cross-sectional area that diminishes toward the end. Other than that, for the purpose of reducing the amount of cooling water leaking from a slip joint of a jet pump, a way of forming a labyrinth seal on an outer surface of a lower end portion of a throat in the slip joint is known (see, for example, Japanese Examined Patent Application Publication No. Sho 59 (1984)-48360). A jet pump illustrated in FIG. 3 of Japanese Patent Laid-open No. 2001-90700 has a venturi tube and a nozzle that ejects a driving flow into the venturi tube a driving flow. This nozzle has an inner cylinder and an outer cylinder that surrounds the inner cylinder. A driving flow passage formed between the inner cylinder and the outer cylinder is an annular passage whose cross-sectional area gradually decreases towards the discharge side of the driving flow. The driving flow supplied to the driving flow passage is ejected from one end (a discharge opening) of the driving flow passage into the venturi tube. Cleaning water around the nozzle is sucked into the venturi tube due to the driving flow ejected from the nozzle. To be more specific, this cleaning water flows into the venturi tube through each of a first cooling water suction passage formed between the nozzle and the venturi tube and a second cooling water suction passage formed inside the inner cylinder. From the nozzle, the annular driving flow is ejected. Cross sections of the annular driving flow are similar to continuous rings. Japanese Patent Laid-open No. 2008-82752 discloses a jet pump applicable to a BWR. This jet pump has a ring header for supplying a driving flow surrounding a suction flow suction passage formed in the center of the jet pump; and a nozzle portion installed to a lower end of the ring header, surrounding the suction flow suction passage, having a plurality of ejection openings in an annular arrangement, where the ejection openings eject a driving flow fed to the ring header. Patent Literature 1: U.S. Pat. No. 3,625,820 Patent Literature 2: Japanese Patent Laid-open No. Sho 59 (1984)-188100 Patent Literature 3: Japanese Patent Laid-open No. Hei 7 (1995)-119700 Patent Literature 4: Japanese Patent Laid-open No. 2007-285165 Patent Literature 5: Japanese Examined Utility Model Application Publication No. Sho 52 (1977)-5301 Patent Literature 6: Japanese Patent Laid-open No. Sho 59 (1984)-159489 Patent Literature 7: Japanese Examined Patent Application Publication No. Sho 59 (1984)-48360 Patent Literature 8: Japanese Patent Laid-open No. 2001-90700 Patent Literature 9: Japanese Patent Laid-open No. 2008-82752 For the soundness of a jet pump, excessive vibration of the jet pump is undesirable. Each slip joint disclosed in Japanese Examined Utility Model Application Publication No. Sho 52 (1977)-5301 and Japanese Patent Laid-open No. Sho 59 (1984)-159489 can suppress a leakage flow from a slip joint and reduce vibration caused by the leakage flow. However, each slip joint has a flow passage cross-sectional area that changes, or decreases, where the structure somewhat increases a pressure loss in the slip joint. For this reason, when these slip joints are applied to a jet pump, the efficiency of the jet pump is reduced for the increased amount of pressure loss. In Japanese Examined Patent Application Publication No. Sho 59 (1984)-48360, a labyrinth seal is provided to a slip joint. When a labyrinth seal is fabricated on the outer surface of a throat, its fabrication range is limited to the thickness of the throat and the length of insertion. For this reason, when the fabrication range is insufficient, a desired effect in leakage flow reduction may not be achieved. An object of the present invention is to provide a jet pump and a reactor, which can suppress the vibration of the jet pump and improve the efficiency of the jet pump. The present invention for achieving the above object is characterized in that a nozzle apparatus has a nozzle base member, and a plurality of nozzles installed to the nozzle base member and forming a plurality of narrowing portions in which a fluid passage cross-sectional area of a driving fluid passage formed in the nozzle is reduced; and in a lower end portion of a throat inserted into a diffuser, a cross-sectional area of a fluid passage formed in the throat diminishes toward a downstream end of the throat. Since, in the lower end portion of the throat inserted into the diffuser, the cross-sectional area of the fluid passage formed in the throat diminishes toward the downstream end of the throat, the amount of a fluid leaking from a space between the throat and the diffuser can be reduced and the vibration of the jet pump can be suppressed. Since the nozzle apparatus has the nozzle base member and the plurality of nozzles installed to the nozzle base member and forming the plurality of narrowing portions, in which the fluid passage cross-sectional area of the driving flow passage is reduced, inside itself, the efficiency of the jet pump can be increased after compensating for a loss in jet pump efficiency caused by the diminishment of the fluid passage cross-sectional area in the throat. The above object can also be achieved by a jet pump comprising a nozzle apparatus having a header portion disposing a first pipe member forming a suction fluid passage for introducing a suction fluid, inside the head portion, and including an annular passage, which surrounds the first pipe member, for introducing a driving fluid, and a nozzle portion installed to the header portion, surrounding the first pipe member, and forming an ejection outlet, which is communicated with the annular passage formed in the header portion, for ejecting the driving fluid; and a second pipe member, one end of which is connected to the nozzle apparatus, forming a driving fluid passage for introducing the driving fluid to annular passage in the header portion, wherein the first pipe member is disposed inside the driving fluid passage formed in the second pipe member through the one end of the second pipe member, and an opening for the suction flow passage is formed on an outer surface of the second pipe member and opened toward outside of the second pipe member; the driving flow passage is formed in a way that the driving fluid flowing toward the one end of the second pipe member hits the first pipe member diagonally in the axial direction of the first pipe member; and, in the lower end portion of a throat inserted into a diffuser, a cross-sectional area of a fluid passage formed in the throat diminishes toward a downstream end of the throat. Since, in the lower end portion of the throat inserted into the diffuser, the cross-sectional area of the fluid passage formed in the throat diminishes toward the downstream end of the throat, the amount of a fluid leaking from a space between the throat and the diffuser can be reduced and the vibration of the jet pump can be suppressed. Since the driving fluid passage formed inside the second pipe member is formed so that the driving fluid flowing toward the one end of the second pipe member hits the first pipe member diagonally to the axial direction of the first pipe member, pressure loss inside the driving fluid passage is decreased. Since the speed of the driving fluid ejected from the annular ejection outlet of the nozzle portion becomes faster, the flow rate of the suction fluid sucked inside the jet pump body is increased. From above, efficiency of the jet pump is improved. Part of this increase in the jet pump efficiency can compensate for a decrease in the jet pump efficiency caused by the diminishment of the flow passage cross-sectional area in the throat. The above object can also be achieved by a jet pump comprising a nozzle apparatus having a first tubular member; a second tubular member disposed in the first tubular member, apart from the first tubular member; a fluid passage forming-member disposed in the first tubular member, and installed to an upper end portion of the second tubular member; a plurality of passage members fixing both ends to the first and the second tubular members and disposed in the circumferential direction of the nozzle apparatus; and an annular ejection outlet is formed between a lower portion of the first tubular member and a lower portion of the second tubular member; Wherein a suction passage formed in each of the passage members, for introducing a suction fluid from the outside to the inside, communicates with an inner region formed in the second tubular member, an annular driving fluid passage for introducing the driving fluid, across which each of the passage members is disposed, is formed between the first tubular member, and the second tubular member and the flow passage forming member, and communicated with the annular ejection outlet, the ejection outlet-side portion of the driving fluid passage slopes inward toward a lower end of the nozzle apparatus, and, in a lower end portion of a throat inserted into a diffuser, a cross-sectional area of a fluid passage formed in the throat diminishes toward a downstream end of the throat. Since, in the lower end portion of the throat inserted into the diffuser, the cross-sectional area of the fluid passage formed in the throat diminishes toward the downstream end of the throat, the amount of fluid leaking from a space between the throat and the diffuser can be reduced and the vibration of the jet pump can be suppressed. Since the ejection outlet-side portion of the driving fluid passage slopes inward toward the lower end of the nozzle apparatus, degree of negative pressure in the inner region is increased, increasing the flow rate of the suction fluid flowing into the inner region through the suction passage. Furthermore, since the ejection outlet-side portion of the driving fluid passage slopes inward toward the lower end of the nozzle apparatus, the width of a gap between the lower end of the outer circumference portion of the nozzle apparatus and the upper end of a jet pump body is increased. This increases the flow rate of a suction fluid flowing into the jet pump body through the gap. From these increases in the flow rates, the efficiency of the jet pump is further increased. Part of this increase in the jet pump efficiency can compensate for a decrease in jet pump efficiency caused by the diminishment of the fluid passage cross-sectional area in the throat. According to the present invention, the vibration of a jet pump can be suppressed and the efficiency of the jet pump can be improved. Various embodiments of the present invention are described below. A jet pump installed to a boiling water reactor, according to an embodiment of the present invention is described below with reference to FIGS. 1, 2, and 3. Before explaining a structure of the jet pump of the present embodiment, an overall structure of a boiling water reactor to which this jet pump is applied is described below with reference to FIGS. 1 and 4. The boiling water reactor (BWR) has a reactor pressure vessel (reactor vessel) 1 and a core shroud 3 installed in the reactor pressure vessel. Hereinafter, the reactor pressure vessel is referred to as an RPV. A core 2 loaded with a plurality of fuel assemblies (not shown) is disposed in the core shroud 3. A steam separator 4 and a steam dryer 5 are disposed above the core 2 in the RPV 1. A plurality of jet pumps 11 is disposed in an annular downcomer 6 formed between the RPV 1 and the core shroud 3. A recirculation system provided to the RPV 1 includes a recirculation pipe 7 and a recirculation pump 8 installed to the recirculation pipe 7. One end of the recirculation pipe 7 communicates with the downcomer 6. Another end of the recirculation pipe 7 is connected to a lower end of a raiser pipe 9 disposed in the downcomer 6. An upper end of the raiser pipe 9 is connected to a branching pipe 60. An elbow pipe (a curved pipe) 10 attached to the branching pipe 60 is connected to a nozzle apparatus 12 of the jet pump 11. A main steam pipe 39 and a feed water pipe 28 are connected to the RPV 1. The nozzle apparatus 12 is fixed to a bell mouth 21 using a plurality of supporting plates 33, and makes up one body with the bell mouth 21. Cooling water (suction fluid, coolant), which is suction flow existing in an upper portion of the RPV 1, is mixed with feed water supplied from the feed water pipe 28 to the RPV 1, and descends in the downcomer 6. This cooling water is sucked into the recirculation pipe 7 by operation of the recirculation pump 8, and pressurized by the recirculation pump 8. This pressurized cooling water is called a driving or motive fluid flow (a driving or motive fluid) 30 for descriptive purposes. The driving flow 30 flows through the recirculation pipe 7, the raiser pipe 9, the branching pipe 60, and the elbow pipe 10, and reaches the nozzle apparatus 12 of the jet pump 11 to be ejected from the nozzle apparatus 12. The cooling water 32, which is a suction flow around the nozzle apparatus 12 (see FIG. 3), is sucked into a throat 22 from the bell mouth 21 due to the working of a jet flow 31 of the driving flow 30 (see FIG. 3). The cooling water 32 descends with the driving flow 30 in the throat 22, and discharged from a lower end of a diffuser 25. The cooling water discharged from the diffuser 25 (including the suction flow 32 and the driving flow 30) is called cooling water 34 for descriptive purposes. The cooling water 34 passes through a lower plenum 29 and is supplied to the core 2. The cooling water 34 is heated while passing the core 2 and becomes a two-phase flow including water and steam. The steam separator 4 separates the gas-liquid two-phase flow into steam and water. The steam dryer 5 removes further moisture from the separated steam, and the steam from which the moisture is removed is exhausted to the main steam pipe 39. This steam is introduced to a steam turbine (not shown) and turns the steam turbine. A power generator (not shown) coupled to the stream turbine rotates to generate power. The steam exhausted from the steam turbine becomes water through condensation in a condenser (not shown). This condensed water is supplied into the RPV 1 as feed water through the feed water pipe 28. The water separated by the separator 4 and the dryer 5 descends and reaches the downcomer 6 as cooling water. The jet pump 11 of the present embodiment, which has the nozzle apparatus 12, the bell mouth 21, the throat 22, and the diffuser 25 as its main components, can supply more cooling water 34 to the core with less driving flow 30 by sucking the cooling water around the nozzle apparatus 12 in the downcomer 6. When the kinetic energy of the driving flow 30 given by the recirculation pump 8 effectively acts on the cooling water 32, more cooling water 32 is sucked into the jet pump 11 and the flow rate of the cooling water 34 is increased more. The jet pump 11 reduces static pressure in the throat 22 by ejecting the driving flow 30 (the jet flow 31) at high speed from the nozzle apparatus 12 into the throat 22. This makes the throat 22 suck in the cooling water 32, and allows the necessary core flow rate to be obtained with a small amount of power. The diffuser 25 has a flow passage cross-sectional area which gradually increases toward the downstream direction within a degree that prevents detachment of cooling water flow. This diffuser 25 changes the kinetic energy of the cooling water into pressure. In the diffuser 25, the pressure of the suction flow 32 is raised higher than the pressure at the position where the suction flow is sucked into the bell mouth 21. A flow passage cross-sectional area of the bell mouth 21 increases toward the upstream direction. The bell mouth 21, the throat 22, and the diffuser 25 are disposed in this order from the upper position to the lower position. A jet pump body comprises the bell mouth 21, the throat 22, and the diffuser 25. The nozzle apparatus 12 is disposed above the bell mouth 21. A structure of a slip joint 26 in the jet pump 11 of the present embodiment is described with reference to FIG. 2. This slip joint 26 has, in a lower end portion (a downstream end portion) of the throat 22, a flow passage reduction portion 23 whose flow passage cross-sectional area gradually diminishes toward a lower end of the throat 22. An inner diameter D6 of a downstream end (the lower end) of the flow passage reduction portion 23 is smaller than an inner diameter D5 of an upstream end (an upper end) of the flow passage reduction portion 23. Part of this flow passage reduction portion 23 is inserted into an upper end portion (an upstream end portion) of the diffuser 25. The flow passage reduction portion 23 has a thick-wall portion 24 on the outer surface. Formation of this thick-wall portion 24 reduces the width of a gap 27 in the radial direction of the throat 22, which gap is formed between the flow passage reduction portion 23 and the diffuser 25. A detailed structure of the nozzle apparatus 12 in the jet pump 11 is described below with reference to FIGS. 3, 5, and 6. The nozzle apparatus 12 has a nozzle base (a nozzle base member) 13 and six nozzles 14. The nozzle base 13 of the nozzle apparatus 12 is fixed to the bell mouth 21 by using the supporting plates 33 to make up one body, and connected to the elbow pipe 10. The nozzle apparatus 12 is disposed above the bell mouth 21. The nozzle base 13 has a protrusion 36 protruding downward, in the center of the nozzle apparatus. The six nozzles 14 are fixed to the nozzle base 13 in an annular arrangement, disposed around the protrusion 36. These nozzles 14 extend toward the bell mouth 21 from the nozzle base 13. A detailed structure of the six nozzles 14 provided to the nozzle apparatus 12 is described with reference to FIG. 6. In the nozzle 14, when the inner diameters, that is, the passage diameters of a jet passage 35 formed inside the nozzle 14, are sequentially defined as D1, D2, and D3 from the upstream end to the downstream end of the nozzle 14, these inner diameters have a relationship which is D1>D2>D3. In the nozzle apparatus 12, the nozzle 14 has a nozzle straight-tube portion 15, a nozzle narrowing portion 16, a nozzle straight-tube portion 17, a nozzle narrowing portion 18, and a nozzle lower end portion 19. The nozzle straight-tube portion 15 positioned in an uppermost position has a uniform inner diameter of D1. In the nozzle narrowing portion 16, which is the first stage of narrowing, connected to a downstream end of the nozzle straight-tube portion 15, a flow passage cross-sectional area in the narrowing portion 16 decreases toward a lower end of the nozzle 14, an inner diameter at an upper end is D1, the inner diameter at a lower end is D2, and the length is L1. The nozzle straight-tube portion 17 connected to the downstream end of the nozzle narrowing portion 16 has a uniform inner diameter of D2. In the nozzle narrowing portion 18, which is the second stage of narrowing, connected to a downstream end of the nozzle straight-tube portion 17, a flow passage cross-sectional area in the narrowing portion 18 decreases toward the lower end of the nozzle 14, an inner diameter at an upper end is D2, the inner diameter at a lower end is D3, and the length is L2. The nozzle lower end portion 19 located in a lowest position of the nozzle 14, connected to the lower end of the nozzle narrowing portion 18 has an inner diameter of D3 and forms an ejection outlet 20 in the end portion. Unlike the nozzle in Japanese Patent Laid-open No. Sho 59 (1984)-188100, in which a nozzle narrowing portion is formed only in one place in its end portion, the nozzle 14 narrows the jet passage 35 in two places in the nozzle narrowing portions 16 and 18. A narrowing angle θ1 of the nozzle narrowing portion 16 and a narrowing angle θ2 of the nozzle narrowing portion 18 can be calculated by the following Equation (1) and Equation (2) respectively.θ1=tan−1((D1−D2)/2/L1) (1)θ2=tan−1((D2−D3)/2/L2) (2) The narrowing angle θ2 of the nozzle narrowing portion 18 near the ejection outlet 20 is larger than the narrowing angle θ1 of the nozzle narrowing portion 16 (θ2>θ1). The nozzle straight-tube portion 15 having a larger flow passage cross-sectional area is disposed upstream from the nozzle narrowing portion 16, and the nozzle straight-tube portion 17 having a smaller flow passage cross-sectional area is disposed downstream from the nozzle narrowing portion 16 respectively. The nozzle lower end portion 19, which is a straight tube having an inner diameter of D3 and the ejection outlet 20 in its end, is preferably disposed at an outlet portion of the nozzle 14, that is, the lower end portion of the nozzle 14. However, in order to improve the flow speed of the jet flow 31 ejected from the ejection outlet 20, a nozzle narrowing portion having a flow passage cross-sectional area which gradually decreases toward the downstream end may be used in place of the nozzle lower end portion 19 being the straight-tube. When the nozzle narrowing portion having a flow passage cross-sectional area which gradually decreases toward the downstream end is used as the nozzle lower end portion 19, it is preferable to reduce the narrowing angle θ of the nozzle narrowing portion 18 of this nozzle to approximately less than 2 degrees in order to keep the spreading of the jet flow 31 from the ejection outlet 20 of the nozzle lower end portion 19, within a desirable range. The driving flow 30 discharged from the recirculation pump 8 during the operation of the boiling water reactor is introduced through the raiser pipe 9 and the elbow pipe 10 and supplied into the nozzle base 13 of the nozzle apparatus 12. This driving flow 30 is introduced to the jet passage 35 of each nozzle 14. A flow passage cross-sectional area of the jet passage 35 varies according to the inner diameters of the nozzle straight-tube portion 15, the nozzle narrowing portion 16, the nozzle straight-tube portion 17, the nozzle narrowing portion 18, and the nozzle lower end portion 19 disposed from the upper position to the lower position. The driving flow 30 flowing into the jet passage 35 flows through the nozzle straight-tube portion 15, the nozzle narrowing portion 16, the nozzle straight-tube portion 17, and the nozzle narrowing portion 18, and reaches the nozzle lower end portion 19. The driving flow 30 descends in the jet passage 35 gradually gains speed in the nozzle narrowing portion 16, and gains speed even faster in the nozzle narrowing portion 18 than in the nozzle narrowing portion 16. The accelerated driving flow 30 is ejected from the ejection outlet 20 into the throat 22. In the nozzle narrowing portion 18, a velocity component toward the central axis of the nozzle 14 is given to the driving flow 30. However, since a fluid has a characteristic to flow along a wall surface, the jet flow 31 ejected from the ejection outlet 20 formed at the lower end of the nozzle lower end portion 19 has a diameter of D3. Since the larger the narrowing angle θ2 of the nozzle narrowing portion 18, the more the momentum flows toward the central axis of the nozzle, the spreading of the jet flow 31 ejected from the ejection outlet 20 can be suppressed. As a consequence, and the diameter D4 of the jet flow 31, which is a distance L3 away from the ejection outlet 20 in the downstream direction, can be small within a desirable range. The diameter D4 of the jet flow 31 is a width of the jet flow 31. The smaller the diameter D4 of the jet flow 31, the faster the speed of this jet flow. When the jet flow 31 is ejected from the nozzle 14 into the throat 22 while the spreading of the jet flow 31 is suppressed and its speed maintained, the static pressure inside the throat 22 is reduced, making more suction flow 32 around the nozzle apparatus 12 in the downcomer 6 to be sucked into the bell mouth 21. Assume that no nozzle lower end portion 19 is disposed downstream from the nozzle narrowing portion 18. In this case, a diameter of the jet flow 31 keeps decreasing even after being ejected because of the momentum of the driving flow 30 toward the central axis of the nozzle 14, given in the nozzle narrowing portion 18. That is, since no straight-tube portion of the nozzle lower end portion 19 is provided, the jet flow 31 ejected from the ejection outlet 20 formed in the lower end of the nozzle 14 is affected by the nozzle narrowing portion 18. This makes the diameter D4 of the jet flow 31 at the distance L3 away from the ejection outlet 20 in the downstream direction, smaller than the inner diameter D3 of the ejection outlet 20. Thus, the jet speed is raised and the acceleration loss is increased, reducing the flow rate of the driving flow 30. For this reason, the nozzle lower end portion 19 being the straight-tube portion is installed in the downstream side of the nozzle narrowing portion 18 to keep the diameter of the jet flow 31 ejected from the ejection outlet 20 to be no smaller than the inner diameter D3 of the nozzle lower end portion 19 being the straight-tube portion. The installation of the nozzle lower end portion 19 prevents the reduction in the flow rate of the driving flow 30 caused by the increase in the acceleration loss. In addition, the nozzle narrowing portions are provided to the nozzle 14 in two or more locations to reduce the 25, pressure loss in the nozzle 14 as well as to widen the flow passage for the suction flow 32, formed between the nozzles 14. Next, the following case is considered where the inner diameter of the ejection outlet 20 is fixed to D3, the nozzle narrowing portion 16 is made straight, each inner diameter of the nozzle straight-tube portion 15 and the nozzle narrowing portion 16, which is now a straight tube, is set to D2, and a nozzle narrowing portion formed in the nozzle 14 is only in one place in the nozzle narrowing portion 18. When the length L2 of the nozzle narrowing portion 18 is unchanged, the flow passage cross-sectional areas of the nozzle straight-tube portion 15 and the nozzle narrowing portion 16, which is now straight, become smaller, increasing the flow speed of the driving flow 30 flowing inside. Consequently, a loss in friction is increased and the flow rate of the driving flow 30 is reduced. When the length L2 of the nozzle narrowing portion 18 is extended to enlarge the flow passage cross-sectional area of the nozzle narrowing portion 18 in the upstream side, the outer diameter of the nozzle 14 becomes larger and a flow passage cross-sectional area of the suction flow 32 formed among the plurality of nozzles 14 becomes smaller, reducing the suction amount of the suction flow 32 into the bell mouth 21. Therefore, in the present embodiment that two or more nozzle narrowing portions are provided to the nozzle 14, a flow passage cross-sectional area of the jet passage 35 in the nozzle 14 becomes smaller toward the ejection outlet 20, and the flow speed of the driving flow 30 flowing in the jet passage 35 is increased. Because of this, the area where the loss in friction is increased in the jet passage 35 can be reduced. In addition, since the outer diameter of the nozzle 14 can be made smaller below the nozzle narrowing portion 16, a space 37 (see FIG. 5) formed among the nozzles 14 can be larger, and the flow rate of the suction flow 32 sucked into a region 38 (see FIG. 3) inside the six nozzles 14 can be increased. As a result, the flow rate of the suction flow 32 sucked into the throat 21 is increased. As described above, the driving flow 30 flowing into the jet passage 35 is accelerated in the jet passage 35 by the nozzle narrowing portions 16 and 18, and ejected from the ejection outlet 20 into the throat 22 as the jet flow 31. In the present embodiment, the spreading of the jet flow 31 is kept small so that the speed of the jet flow 31 reached inside the throat 22 is higher, reducing the static pressure inside the throat 22. As a result, more suction flow 32 can be sucked into the throat 22. The present embodiment provides the nozzle 14 having two nozzle narrowing portions 16 and 18 so that the flow rate of the suction flow 32 sucked into the throat 22 can be increased, by the above-described working of the nozzle 14, more than the conventional jet pump disclosed in Japanese Patent Laid-open No. Sho 59 (1984)-188100 which provides five nozzles, each having one stage of a narrowing portion and a straight-tube portion. For this reason, the flow rate of the cooling water 34 discharged from the jet pump 11 is increased, and the efficiency of the jet pump 11 in a high-M ratio range can be improved more than that of the conventional jet pump. An example of a change in the differential pressure between the inside of the jet pump and the downcomer 6 in the axial direction of the jet pump from the inlet of the throat to the outlet of the diffuser is shown in FIG. 7. In FIG. 7, the broken line shows a characteristic of a conventional jet pump having five nozzles, which has been used in a boiling water reactor of a million kW class. As shown here, the high-speed ejection of a driving flow from the nozzle causes the static pressure in the throat to be lower than the static pressure in the downcomer 6, making the differential pressure between the inside and the outside of the throat inlet portion negative. The differential pressure between the inside of the jet pump and the downcomer 6 becomes positive at a position of a slip joint, and a magnitude of this positive pressure increases toward the diffuser outlet. In the conventional jet pump, in the lower portion of the throat, the static pressure in the throat is recovered by gradually increasing the flow passage cross-sectional area toward the downstream end of the throat. When the static pressure in the jet pump at the position of the slip joint is larger than the static pressure in the downcomer 6 at the same location, cooling water starts to leak from the inside of the jet pump to the downcomer 6 through a gap in the slip joint. When the amount of this leakage flow is excessive, the jet pump may vibrate undesirably. In the slip joint 26 of the jet pump 11 of the present embodiment, as described above, the flow passage reduction portion 23 formed in the downstream end portion of the throat 22 is inserted into the upstream end portion of the diffuser 25 so that the flow speed of the cooling water flowing into the diffuser 25 from the flow passage reduction portion 23 is increased, reducing the static pressure in the diffuser 25 in the vicinity of the downstream end of the flow passage reduction portion 23. This reduces the difference between the static pressure in the jet pump 11, that is, the static pressure in the diffuser 25, and the static pressure in the downcomer 6 at the installation position of the slip joint 26. By using the method that can reduce the difference between these static pressures, the amount of the cooling water leaking to the downcomer 6 through the gap 27 in the slip joint 26 can be reduced more surely than by using the method such as in Japanese Examined Patent Application Publication Sho 59 (1984)-48360 which provides a labyrinth seal whose effect in reducing the leakage flow is limited by an available range of fabrication. Consequently, in the present embodiment, the vibration of the jet pump 11 can be controlled. The solid line in FIG. 7 shows a change in the differential pressure between the inside of the jet pump 11 and the downcomer 6, when the jet pump 11 in the present embodiment is used, in which jet pump 11, the inner diameter of the downstream end of the throat 22 is made, by forming the flow passage reduction portion 23, 6% smaller than the inner diameter of the downstream end of the throat in the conventional jet pump having no flow passage reduction portion 23. In the present embodiment, the static pressure starts to decrease at the starting point of the flow passage reduction portion 23, and the differential pressure between the inside of the jet pump 11 and the downcomer 6 at the position of the slip joint 26 drops to about a half of that in the conventional example shown in the broken line. After that, the velocity energy of the cooling water is changed to pressure as the flow passage cross-sectional area in the diffuser 25 is increased, recovering the pressure in the diffuser 25. The drop in the differential pressure between the inside of the jet pump 11 and the downcomer 6 at the position of the slip joint 26 reduces the vibration of the jet pump 11 as described above. However, the present embodiment increases a pressure loss more than the conventional jet pump because of the formation of the flow passage reduction portion 23. As a result, in the present embodiment shown in the solid line, the pressure at the outlet of the diffuser 25 is lower than that in the conventional example shown in the broken line (see FIG. 7). This reduces the flow rate of the cooling water 34 supplied to the core 2 from the jet pump. In other words, the formation of the flow passage reduction portion 23 reduces the efficiency of the jet pump. The jet pump 11 of the present embodiment, as described above, tries to improve the efficiency of the jet pump by installing the nozzle apparatus 12 having six nozzles 14 with two stages of nozzle narrowing portions. In the jet pump 11, the reduction in the jet pump efficiency due to the formation of the flow passage reduction portion 23 can be compensated by part of the improvement in the jet pump efficiency achieved by using the nozzle apparatus 12. Thus, the jet pump 11 can prevent the vibration of the jet pump and at the same time, can improve the efficiency of the jet pump more than the conventional jet pump. The improvement in the efficiency of the jet pump of the present embodiment is explained in detail with reference to FIG. 8. In FIG. 8, the broken line shows the efficiency of the conventional jet pump (the conventional jet pump having the characteristic shown by the broken line in FIG. 7) having a nozzle apparatus with five nozzles. In this conventional jet pump, a flow passage cross-sectional area of the downstream end of the throat is set to a conventional ratio of 100%, and each nozzle has one stage of narrowing portion as in the jet pump disclosed in Japanese Patent Laid-open No. Sho 59 (1984)-188100. The alternate long and short dash line in FIG. 8 shows the efficiency of a jet pump of a comparative example, in which the throat in the conventional jet pump having the characteristic shown in the broken line is replaced with a throat having the same flow passage reduction portion 23 as in the present embodiment, in the lower end portion. In the jet pump of the comparative example, a flow passage cross-sectional area of the downstream end of the flow passage reduction portion 23 is 90% of a flow passage cross-sectional area of the corresponding position in the conventional jet pump having the characteristic shown in the broken line. For this jet pump, since the pressure loss is increased by forming the flow passage reduction portion 23 in the throat, the efficiency of the jet pump is lower than that shown in the broken line. The efficiency of the conventional jet pump having the flow passage reduction portion in the throat is reduced by approximately 0.7%. In FIG. 8, the solid line shows the jet pump efficiency of the jet pump 11 of the present embodiment. In the jet pump 11, a flow passage cross-sectional area of the downstream end of the flow passage reduction portion 23 in the throat 22 is also 90%. In the jet pump 11, the reduction in the jet pump efficiency caused by the formation of the flow passage reduction portion 23 in the throat 22 is covered by the increase in the jet pump efficiency achieved by using the nozzle apparatus 12. As a result, the jet pump efficiency is improved more than the jet pump efficiency of the jet pump of the conventional example shown in the broken line. In the present embodiment, the efficiency of the jet pump is improved by approximately 3% more at the peak compared to that of the conventional jet pump without the flow passage reduction portion in the throat. In the jet pump 11 of the present embodiment, the number of the nozzles 14 is increased to six. By using two stages of the nozzle narrowing portions 16 and 18, the spreading of the jet flow 31 ejected from the ejection outlet 20 can be kept small, suppressing the reduction in the speed of the jet flow 31 that has reached the inlet of the throat 22 as well as the decrease in the suction area for the suction flow 32 in the throat 22. This allows more suction flow 32 to be sucked into the throat 22 at the same ejecting speed of the jet flow 31. In addition, in the present embodiment, the total flow passage cross-sectional area of the ejection outlets 20 of the six nozzles 14 is made the same as that of the conventional five nozzles, while making the total length of wetted perimeter of the six nozzles 14 approximately 9% more than that of the conventional five nozzles. This increases the contact area between the suction flow 32 and the jet flow 31 of the driving flow 30 ejected from the ejection outlet 20, making both fluids to be mixed faster, which reduces a loss during the mixing. The jet pump 11 of the present embodiment can improve the jet pump efficiency compared to the conventional jet pump disclosed in Japanese Patent Laid-open No. Sho 59 (1984)-188100 which provides five nozzles, each having one stage of a narrowing portion and a straight-tube portion. In the present embodiment, since the narrowing angle θ2 of the nozzle narrowing portion 18 is made larger than the narrowing angle θ1 of the nozzle narrowing portion 16, the spread of the jet flow 31 is suppressed and which prevents the reduction in the speed of the driving flow 30 at the inlet of the throat 22 is also suppressed. At the same time, since the nozzle lower end portion 19 forming the ejection outlet 20 is provided, it can be prevented to accelerate excessively the driving flow 30 by the narrowing portion and to increase the pressure loss in the nozzle 14. Since the speed of the driving flow 30 in the throat 22 is not much slower than the speed at the ejection outlet 20, the static pressure in the throat 22 is reduced and the suction amount of the suction flow 32 into the throat 22 is increased. Consequently, the M ratio and the efficiency of the jet pump can be improved. In a boiling water reactor, a rotational speed of the recirculation pump 8 is controlled to adjust a flow rate of cooling water supplied to the core 2 (a core flow rate). The improvement in the M ratio and the jet pump efficiency allows the core flow rate to be increased using less power from the recirculation pump. Thus, power consumption for driving the recirculation pump 8 can be reduced. In addition, when a power upgrade of a reactor in the U.S. is to be implemented, the core flow rate can be further increased without increasing the capacity of the recirculation pump 8 by employing, for the existing reactor, the jet pump 11 of the present embodiment which can increase the M ratio and the jet pump efficiency. For this reason, the power upgrade of the boiling water reactor can be easily achieved. A jet pump according to embodiment 2, which is another embodiment of the present invention, is described below. The jet pump is also applied to a boiling water reactor. A jet pump 11A of the present embodiment has a structure in which the nozzle apparatus 12 in the jet pump 11 of the embodiment 1 is replaced with a nozzle apparatus 12A. Other components of the jet pump 11A are the same as the jet pump 11. The nozzle apparatus 12A is explained below with reference to FIGS. 9 and 10. In the jet pump 11A, minimizing the loss in pressure and making the most of the suction power induced by a driving flow are both important to increase the M ratio and the N ratio and to raise the efficiency of the jet pump. The jet pump 11A of the present embodiment has an inner cooling water suction passage 50 in and through the nozzle apparatus 12A in the axial direction. The inner cooling water suction passage 50 has, in its upper end, an opening 51 which communicates with the downcomer 6. Furthermore, in the jet pump 11A, the inner cooling water suction passage 50 extends upward inside the elbow pipe 10, and the opening 51 is formed on the outer surface of the elbow pipe 10 at a position lower than a top point TP of the elbow pipe 10. The nozzle apparatus 12A, as shown in FIG. 9, has a nozzle portion 40 and a nozzle header portion 46. The nozzle header portion 46 has an outer cylinder member 47 and an inner cylinder member 48 disposed inside the outer cylinder member 47. An annular header portion 49 is formed between the outer cylinder member 47 and the inner cylinder member 48, both of which are disposed in a concentric manner. The nozzle portion 40 is disposed below the nozzle header portion 46, and fixed to a lower end portion of the nozzle header portion 46. The nozzle portion 40 has an outer cylinder member 41, an inner cylinder member 42, an outer funnel member 43, and an inner funnel member 44. The outer cylinder member 41 surrounds the inner cylinder member 42, and the outer cylinder member 41 and the inner cylinder member 42 are concentrically disposed. The outer funnel member 43 surrounds the inner funnel member 44, and the outer funnel member 43 and the inner funnel member 44 are concentrically disposed. The outer funnel member 43 and the inner funnel member 44 each have a cross-sectional area that decreases downward. The outer funnel member 43 is fixed to an upper end of the outer cylinder member 41, and the inner funnel member 44 is fixed to an upper end of the inner cylinder member 42. The outer funnel member 43 is attached to a lower end of the outer cylinder member 47. The inner funnel member 44 is attached to a lower end of the inner cylinder member 48. An annular ejection outlet 20A is formed between the outer cylinder member 41 and the inner cylinder member 42. An outlet end 53 of the elbow pipe 10 is fixed to the nozzle header portion 46, that is, an upper end of the outer cylinder member 47. An inlet end 52 of the elbow pipe 10 is placed on an upper end of the branching pipe 60. The elbow pipe 10 and the branching pipe 60 are detachably coupled with a fixture. The center of the outlet end 53 of the elbow pipe 10 matches an axis of the nozzle header portion 46, that is, the outer cylinder member 47. The nozzle portion 40, the nozzle header portion 46, and the elbow pipe 10 are joined into one body by welding. The inner cylinder member 48 is inserted into the elbow pipe 10 through the outlet end 53, and extends upward. The opening 51 located in the upper end portion of the inner cylinder member 48 is formed on the outer surface of the elbow pipe 10 and communicates with the downcomer 6. The upper end of the inner cylinder member 48 is welded to the elbow pipe 10. A joint (a fixed position) 57 which is the highest position in a joint portion (a fixed portion) between the inner cylinder member 48 and the elbow pipe 10 is disposed lower than the top point TP which is the highest position on the outer surface of the elbow pipe 10. A flow-adjusting plate (a flow-adjusting member) 54 having the same curvature as the elbow pipe 10 is installed inside the elbow pipe 10, and disposed from the inlet end 52 of the elbow pipe 10 to the inner cylinder member 48 along the axis of the elbow pipe 10. The flow-adjusting plate 54 is disposed upstream from the inner cylinder member 48. An upper passage 55 and a lower passage 56 that are separated into the top and the bottom, are formed in the elbow pipe 10 by the installation of the flow-adjusting plate 54. Since the joint 57 is located lower than the top point TP, the upper flow passage 55 and the lower passage 56 in the elbow pipe 10 extending toward the outlet end 53 are formed diagonally to the axis of the inner cylinder member 48. In other words, the upper passage 55 and the lower passage 56 are formed so that the driving flows in the flow passages flowing toward the outlet end 53, hitting the inner cylinder member 48 diagonally in relation to the axial direction of the inner cylinder member 48. The inner cooling water suction passage 50 communicating with the downcomer 6 through the opening 51 is formed inside the joined inner cylinder member 48, inner funnel member 44, and inner cylinder member 42. The joined inner cylinder member 48, inner funnel member 44, and inner cylinder member 42 are a first pipe member. The inner cooling water suction passage 50 has a flow passage cross-sectional area which gradually decreases downward in the inner funnel member 44, and its lower end opens toward the bell mouth 21. An annular passage 45 formed between the outer funnel member 43 and the inner funnel member 44, communicating with the annular header portion (an annular passage) 49 and the annular ejection outlet 20A, has a flow passage cross-sectional area which gradually decreases downward. The driving flow pressurized by the recirculation pump 8 during the operation of the boiling water reactor reaches the raiser pipe 9 and is introduced into the annular header portion 49 through the elbow pipe 10. The flow-adjusting plate 54 disposed in the elbow pipe 10 reduces the pressure loss in the elbow pipe 10. Part of the driving flow flowing in each of the upper passage 55 and the lower passage 56 in the elbow pipe 10 toward the outlet end 53 hits the outer surface of the inner cylinder member 48 diagonally in relation to the axial direction of the first pipe member (the inner cylinder member 48 in particular). The driving flow introduced into the annular header portion 49 flows through the annular passage 45 and is ejected at high speed into the bell mouth 21 from the annular ejection outlet 20A. The cross-sectional area of the jet flow of the driving flow ejected from the annular ejection outlet 20A is annular. The high-speed supplying of the jet flow of the driving flow into the throat 22 reduces the static pressure in the throat 22, making the cooling water around the nozzle apparatus 12A in the downcomer 6 to be sucked into the bell mouth 21. There are two patterns for the cooling water being the suction flow around the nozzle apparatus 12A to be sucked into the bell mouth 21 due to the reduction in the static pressure in the throat 22. The first pattern is that the cooling water above the elbow pipe 10 flows into the inner cooling water suction passage 50 from the opening 51, and reaches the bell mouth 21 through the inner cooling water suction passage 50. In this pattern, the cooling water sucked through the inner cooling water suction passage 50 flows into the inside of the annular jet flow ejected from the annular ejection outlet 20A. The second pattern is that the cooling water in the downcomer 6 passes through an outer cooling water suction passage 58 formed between the nozzle portion 40 and the bell mouth 21, and reaches the bell mouth 21 at the outside of the annular jet flow. The driving flow ejected from the annular ejection outlet 20A and the cooling water (the suction flow) sucked into the bell mouth 21 by the working of the driving flow are mixed in the throat 22 by exchanging their momentum, and introduced to the diffuser 25 located below the throat 22. The cooling water 34 discharged from the diffuser 25 is introduced to the core 2 through the lower plenum 29. In the present embodiment, since the joint portion 57 is positioned lower than the top point TP, the upper passage 55 and the lower passage 565 in the elbow pipe 10 are formed toward the outlet end 53, diagonally to the inner cylinder member 48 forming the inner cooling water suction passage 50 in the axial direction of the inner cylinder member 48. For this reason, the pressure loss in the elbow pipe 10 where the inner cylinder member 48 exists is reduced, and the flow speed of the cooling water ejected from the annular ejection outlet 20A is increased. The reduction amount of the static pressure in the throat 22 is increased, increasing the flow rate of the cooling water sucked into the bell mouth 21 through the inner cooling water suction passage 50 and the outer cooling water suction passage 58. This increase in the flow rate of the cooling water improves the efficiency of the jet pump 11A. This improvement in the efficiency of the jet pump 11A is explained. FIG. 11 shows a relationship between the M ratio and the jet pump efficiency of a jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat, and that of a jet pump of a comparative example. In FIG. 11, the solid line shows a characteristic of the jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat, and the broken line shows a characteristic of the comparative jet pump. The jet pump of the comparative example uses the nozzle apparatus shown in FIG. 3 of Japanese Patent Laid-open No. 2001-90700 as a nozzle for the jet pump in a SWR, disclosed in U.S. Pat. No. 3,625,820. While in the comparative example, a pressurized driving flow hits an inner cylinder of the nozzle apparatus at a right angle, in the jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat, a driving flow flowing through a cooling water passage in the elbow pipe 10 hits the inner cylinder member 48 diagonally as described above. Because of such a difference in the driving flows, the pressure loss of the jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat is less than that of the comparative example. Consequently, in the jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat, the efficiency of the jet pump is increased by more than that of the comparative example for the amount of the reduced pressure loss in the nozzle. Since the jet pump 11A of the present embodiment has the flow passage reduction portion 23 in the lower end portion of the throat 22 in the same manner as the jet pump 11 of the embodiment 1, in the jet pump 11A, the flow passage reduction portion 23 causes the efficiency of the jet pump to decrease. However, this reduction in the efficiency can be compensated for by part of the increase in the efficiency achieved by the nozzle apparatus 12A. From the contribution of the remaining increase in the efficiency achieved by the nozzle apparatus 12A, the jet pump 11A, thus, can improve the efficiency of the jet pump more than that of the comparative example. In the present embodiment, a leakage flow from the gap 27 in the slip joint 26 can be reduced because the flow passage reduction portion 23 is formed in the lower end portion of the throat 22. For this reason, the vibration of the jet pump 11A can be suppressed. In the present embodiment, since the flow-adjusting plate 54 is installed in the elbow pipe 10, the pressure loss in the elbow pipe 10 can be further reduced. The reduction in the pressure loss further increases the efficiency of the jet pump 11A. Since the flow-adjusting plate 54 is disposed upstream from the inner cylinder member 48, separation of the flow and uneven distribution of speed in the elbow pipe 10 are improved, and the pressure loss in the elbow pipe 10 is reduced. Since the cooling water passages (the upper passage 55 and the lower passage 56) formed in the elbow pipe 10 are diagonal to the inner cylinder member 48 as described above, the driving flow flowing in the cooling water passages hits the outer surface of the inner cylinder member 48 diagonally to the axial direction of the inner cylinder member 48. This causes the stress generated at the contact portion between the inner cylinder member 48 and the elbow pipe 10 to be small. Thus, when the nozzle apparatus 12A is applied to a current BWR, it is not necessary to reinforce the joint portion by making the member particularly thick, or to modify the raiser pipe 9 and the fixture. In the present embodiment, Since the inner cooling water suction passage 50 is formed in the nozzle apparatus 12A, the effect of the pressure reduction in the area inside the ejected annular jet flow can be effectively used. This generates the flow of the cooling water reaching into the bell mouth 21 through the inner cooling water suction passage 50. Thus, since cooling water can flow into the bell mouth 21 through each of the inner cooling water suction passage 50 and the outer cooling water suction passage 58, the flow rate of the cooling water flowing into the bell mouth 21 is increased. The inner cooling water suction passage 50 is disposed in the axial direction of the RPV 1, and the opening 51 opens upward, so that the flow power of the cooling water descending in the downcomer 6, supplied to the inner cooling water suction passage 50, can be effectively utilized to increase the suction power of the jet pump 11A. This increases the rate of the cooling water sucked into the throat 22. In addition, since the nozzle portion 40 has the outer funnel member 43 whose outer diameter decreases downward, the nozzle apparatus 12A has a structure that allows the cooling water descending in the downcomer 6 to be easily sucked into the bell mouth 21 through the outer cooling water suction passage 58. This also increases the flow rate of the cooling water flowing into the bell mouth 21, increasing the efficiency of the jet pump 11A. In the boiling water reactor installed with the jet pump 11A, the core flow rate can be further increased without increasing the capacity of the recirculation pump 8 in the same manner as in the embodiment 1. For this reason, a power upgrade of the boiling water reactor can be easily achieved. Furthermore, in the present embodiment, the inverted U-shaped elbow pipe 10 is connected to the nozzle apparatus 12A so that a single raiser pipe 9 disposed in the downcomer 6 can be connected to two jet pumps 11A adjacent to the raiser pipe 9, with the elbow pipes 10 each connected to the nozzle apparatus 12A of each of the two jet pumps 11A. For this reason, a space between the jet pumps 11A can be made equal to the corresponding space in the existing boiling water reactor. A jet pump according to embodiment 3, which is another embodiment of the present invention, is described below. A jet pump 11B of the present embodiment has a structure in which the nozzle apparatus 12 in the jet pump 11 in the embodiment 1 is replaced with a nozzle apparatus 12B. Other components of the jet pump 11B are the same as the jet pump 11. The nozzle apparatus 12B is explained below with reference to FIG. 12. The nozzle apparatus 12B, as shown in FIG. 12, has a nozzle portion 61, a suction passage portion 65, and a nozzle holder 78. The suction passage portion 65 is disposed above the nozzle portion 61, and installed on the upper end of the nozzle portion 61. The nozzle holder 78 is disposed above the suction passage portion 65 and installed on an upper end of the suction passage portion 65. The suction passage portion 65 has a cylinder member (a third tubular member) 66, a flow passage forming member 67, and a passage member 72. The flow passage forming member 67 is disposed inside the cylinder member 66 in the center of the cylinder member 66. Six passage members 72 are disposed radially around the central axis of the cylinder member 66, 60 degrees apart from each other in the circumferential direction (see FIG. 13). The outer end portion of the passage member 72 is welded to the cylinder member 66, and the inner end portion of the passage member 72 is welded to the flow passage forming member 67. Each passage member 72 slopes downward and inward (toward the flow passage forming member 67), and has an oval cross-sectional area (see FIG. 15). An opening 74 is formed in the outer end portion of the passage member 72. An annular driving flow passage 76 is formed between the cylinder member 66 and the flow passage forming member 67. Each passage member 72 is placed across this driving flow passage 76. A suction passage 73 communicating with the downcomer 6 through the opening 74 is formed in each passage member 72. The inner surfaces of each passage member 72 at the inlet and the outlet of each suction passage 73 are curved surfaces. The total flow passage cross-sectional area of all the suction passages 73 is larger than the cross-sectional area of a decompression chamber (an inner region) 77 at the lower end of the nozzle portion 61. Each passage member 72 is provided with a streamline member 75 (see FIG. 15) having a cross-sectional area that decreases toward the upper course to reduce the pressure loss in the driving flow passage 76. The flow passage forming member 67 has a circular cross section at any point in the axial direction, and includes an upper region 68, a center region 69, and a lower region 70, each having a different cross-sectional area in the axial direction. The upper region 68 is cylindrical, and the center region 69 connected to a lower end of the upper region 68 is a truncated cone. The lower region 70 connected to a lower end of the center region 69 is an inverted cone. The center region 69 has a cross-sectional area that increases downward. This reduces a cross-sectional area of the driving flow passage 76 downward between the cylinder member 66 and the outer surface of the center region 69. The lower region 70 has a cross-sectional area that decreases downward, and has a curved surface 71 whose outer surface comes together in the axial direction. The nozzle portion 61 has an outer cylinder member (a first tubular member) 62 and an inner cylinder member (a second tubular member) 63 disposed in the outer cylinder member 62. The outer cylinder member 62 is welded to a lower end of the cylinder member 66, and an upper end of the inner cylinder member 63 is welded to the flow passage forming member 67. The outer cylinder member 62 has an outer diameter that is smaller in the lower end than in the upper end, and slopes inward. The inner cylinder member 63 has an outer diameter that becomes the largest in a center portion and smaller in the upper and a lower ends. An inner end portion of the passage member 72 is welded to the upper portion rather than the center portion of the inner cylinder member 63. Therefore, the inner cylinder member 63 exists between the adjacent passage members 72 in the circumferential direction of the inner cylinder member 63. An annular jet passage 64 is formed between the outer cylinder member 62 and the inner cylinder member 63. The annular jet passage 64 slopes inward, and has a flow passage cross-sectional area that becomes smaller downward. The jet passage 64 communicates with the driving flow passage 76. The jet passage 64 is also a part of the driving flow passage. An annular ejection outlet 20B is formed at the end of the jet passage 64. The decompression chamber 77 is formed in the inner cylinder member 63, and the suction passage 73 communicates with the decompression chamber 77. The curved surface 71 of the lower region 70 of the flow passage forming member 67 faces the decompression chamber 77. The inner cylinder member 63 separates the driving flow passage 76 and the decompression chamber 77. The nozzle holder 78 has a cylinder member 81, a reinforcing streamline plate 79, and a cone member 80. The cylinder member 81 is fixed to the upper end of the cylinder member 66 of the suction passage portion 65. The cone member 80 has a cross-sectional area that decreases upward, and disposed in the center of the cylinder member 81. Six reinforcing streamline plates (see FIG. 14) 79 are radially disposed around the central axis of the cylinder member 81, 60 degrees apart from each other in the circumferential direction, and disposed in the positions above the passage members 72 (see FIG. 13). The both ends of each reinforcing streamline plate 79 are fixed to the cylinder member 81 and the cone member 80. A lower end portion of the cone member 80 is fitted to the upper end portion of the flow passage forming member 67. An upper end of the cylinder member 81 is connected to the elbow pipe 10. It can be said that when the nozzle portion 61 and the suction passage portion 65 are unified, the outer cylinder member 62 and the cylinder member 66 make up the first tubular member and the inner cylinder member 63 is the second tubular member. Between these first and second tubular members, the driving flow passage including the jet passage 64 is formed. The driving flow 30 pressurized by the recirculation pump 8 during the operation of the boiling water reactor flows into the cylinder member 81 through the elbow pipe 10, and further reach the jet passage 64 through the driving flow passage 76. This driving flow 30 is ejected as a jet flow 31A into the bell mouth 21 from the ejection outlet 20B located at the end of the jet passage 64. The working of the jet flow 31A makes the suction flow 32, which is part of the cooling water around the nozzle apparatus 12B in the downcomer 6, to flow into the bell mouth 21 through the cooling water suction passage 58. This suction flow 32 is introduced into the throat 22 through the space between the bell mouth 21 and the jet flow 31A. Since the jet passage 64 is sloped, the jet flow 31A is ejected diagonally toward the central axis of the throat 22 from the ejection outlet 20B. Consequently, the working of the jet flow 31A makes the pressure in the decompression chamber 77 negative, so that the suction flow 32A, which is part of the cooling water descending in the downcomer 6, flows into the suction passage 73 to reach the decompression chamber 77. This suction flow 32A further flows into a decompression region 82 formed inside the jet flow 31A in the bell mouth 21. The suction flows 32 and 32A and the driving flow 30 flowing into the bell mouth 21 are mixed in the throat 22 and discharged from the diffuser 25 (see FIGS. 1 and 4). These flows, that is, the cooling water 34, discharged from the diffuser 25 is supplied to the core 2. The jet pump 11B in the present embodiment as described above has the following unique structures (a) to (c). (a) The jet passage 64 in the nozzle portion 61 slopes inward. (b) The suction passage 73 slopes inward. (c) A cross section of the passage member 72 forming the suction passage 73 is oval. Various effects obtained by the unique structures (a) to (c) are explained in detail. First of all, various effects obtained by the unique structure (a) are described. The jet passage 64 in the nozzle portion 61 slopes inward. That is, the jet passage 64 slopes inward and downward toward the central axis of the throat 22. As a consequence, the jet flow 31A ejected from the ejection outlet 20B is ejected downward toward the central axis of the throat 22. Such jet flow 31A reduces the volume of the inverted cone-shaped decompression region 82 formed inside the jet flow 31A below the flow passage forming member 67. The reduction in the volume of the decompression region 82 relatively increases the degree of the pressure reduction, increasing the degree of negative pressure in the decompression chamber 77. As a result, a flow rate Qb2 of the suction flow 32A sucked into the bell mouth 21 through the suction passage 73 is increased. In addition, in the present embodiment, since the jet passage 64 in the nozzle portion 61 slopes inward, a distance L4 between the bell mouth 21 and the end of the outer cylinder member 62 of the nozzle portion 61 can be larger. As a result, a distance L5 between the inner surface of the throat 22 and the jet flow 31A is increased, increasing a flow rate Qb1 of the suction flow 32 flowing into the space between the bell mouth 21 and the jet flow 31A through the cooling water suction passage 58. An increase in the flow rate Qb1 of the suction flow 32 and the flow rate Qb2 of the suction flow 32A increases the flow rate of the cooling water 34 discharged from the diffuser 25. That is, the efficiency of the jet pump 11B is further improved. Various effects obtained by the unique structure (b) are described. Since the suction passage 73 slopes inward, the cooling water descending in the downcomer 6 can flow into the suction passage 73 by only slightly changing its flow direction. This makes the suction flow 32A to be easily sucked into the suction passage 73. In addition, since the suction passage 73 slopes inward, the downward flow force (a flow speed of approximately 2 m/s) of the cooling water in the downcomer 6 can be effectively used, allowing the suction flow 32A to be easily sucked into the suction passage 73. These workings further increase the flow rate Qb2 of the suction flow 32A, further increasing the flow rate of the cooling water 34 as well. Various effects obtained by the unique structure (c) are described. Since a cross section of the passage member 72 forming the suction passage 73 is oval, the cross-sectional area of the suction passage 73 can be enlarged. Consequently, the pressure loss in the suction passage 73 can be reduced and the flow rate Qb2 of the suction flow 32A can be increased. In particular, since the passage members 72 are disposed in such a way that their major axes follow the axial direction of the nozzle apparatus 12B and their minor axes follow the circumferential direction of the nozzle apparatus 12B, the pressure loss in the suction flow passage 76 can be reduced and the cross-sectional area of the suction passage 73 can be enlarged. In addition, such an arrangement with respect to the major and minor axes allows the number of the passage members 72 disposed in the circumferential direction of the nozzle apparatus 12B to be increased. Consequently, the total flow passage cross-sectional area of all the suction passages 73 can be enlarged. This greatly contributes to the increase in the flow rate Qb2 of the suction flow 32A. Besides the unique structures (a) to (c), the nozzle apparatus 12B has some other structures that allow the yielding of new effects. These effects are described. In order to reduce the pressure loss in a flow passage for the driving flow 30, the nozzle apparatus 12B adapts some ideas. A structure for reducing the pressure loss, other than the structure in which the cross section of the passage member 72 is oval, is explained. Each passage member 72 forms, in the upstream side, a streamline member 75 having a cross section that decreases toward the upper course (see FIG. 15). The formation of this streamline member 75 reduces turbulence in the driving flow 30 flowing in the driving flow passage 76, reducing the pressure loss in the driving flow passage 76. The reinforcing streamline plate 79 also has a streamline shape whose cross section decreases toward the upper course (see FIG. 14). This structure reduces the pressure loss in the driving flow passage 76. Furthermore, since each reinforcing streamline plate 79 is disposed to the same position above the passage member 72 located downstream in the circumferential direction of the nozzle apparatus 12B, the pressure loss in the driving flow passage 76 is reduced. Since the flow passage cross-sectional area of the jet passage 64 gradually decreases from the upper course to the ejection outlet 20B, the pressure loss in the jet passage 64 is also reduced. The cone member 80 having a cross-sectional area that increases from the upper course to the lower course, is disposed on the upper end of the flow passage forming member 67, so that the driving flow 30 flowing in the elbow pipe 10 can be smoothly introduced to the annular driving flow passage 76. This reduces the pressure loss in the flow passage for the driving flow 30 in the nozzle apparatus 12B. Furthermore, in the present embodiment, the pressure loss in the nozzle apparatus 12B can be further reduced because the nozzle apparatus 12B forms no flow passage such as the one in the nozzle apparatus shown in FIG. 1 of Japanese Patent Laid-open No. 2008-82752, in which the flow passage turns the driving flow at a right angle. The nozzle apparatus 12B employs some ideas for reducing the pressure loss in the flow passage for the suction flow 32A. This reduction in the pressure loss is obtained by forming curved surfaces on the inlet and the outlet of the passage member 72 as described above. Since the total flow passage cross-sectional area of all the suction passages 73 is larger than the cross-sectional area of the decompression chamber 77 at the lower end of the nozzle portion 61, the pressure loss in the flow passage for the suction flow 32A formed in the nozzle apparatus 12B is reduced. Since the cross section of the passage member 72 is oval and this passage member 72 is disposed in such a way that it slopes downward toward the axial direction of the nozzle apparatus 12B, the opening area of the inlet of the suction passage 73 can be enlarged. This also decreases the pressure loss in the suction passage 73. Since the surface of the lower region 70 of the flow passage forming member 67, facing the decompression chamber 77, is the curved surface 71, the driving flow 32A discharged from the suction passage 73 can smoothly change the direction downward along the curved surface 71 in the decompression chamber 77. By forming the curved surface 71 functioning in this way, the pressure loss in the flow passage for the suction flow 32A, formed in the nozzle apparatus 12B, can be reduced as well. The lower region 70 of the flow passage forming member 67 protrudes below an upper end of the outlet side of the passage member 72. Adapting such a shape allows the negative pressure in the decompression chamber 77, which is increased by the unique structure of (a), to effectively act on the suction passage 73, and allows the flow rate Qb2 of the suction flow 32A flowing into the suction passage 72 to be increased. In other words, the lower region 70 prevents the formation of a decompression dead water region in the decompression chamber 77 by the suction flow 32A discharged from the suction passage 73. The lower region 70 is disposed in the area where the decompression dead water region is to be formed in the decompression chamber 77 when no lower region 70 is provided. For this reason, cavitation induced in the decompression dead water region is prevented from occurring, and the flow rate Qb2 of the suction flow 32A can be increased. In the present embodiment, the ejection outlet 20B is annular, making the jet flow 31A ejected from the ejection outlet 20B also annular. Thus, since a vortex generated by the jet flow 31A is evenly distributed in the circumferential direction, a random vortex formation that causes flow-induced vibration is prevented and consequently, the vibration of structures in the boiling water reactor can be prevented. Since the nozzle apparatus 12B has an annular flow passage for the driving flow 30, the ejection outlet 20B, and the suction passages 73 crossing the flow passage for the driving flow 30, for introducing the suction flow 32A, the nozzle apparatus 12B can be made compact. Therefore, by replacing a nozzle in a conventional jet pump to the nozzle apparatus 12B, the jet pump can be quickly and easily converted into the jet pump 11B having a higher nozzle efficiency. A characteristic of the jet pump having the nozzle apparatus 12B with no flow passage reduction portion in the throat is compared with the characteristics of the conventional jet pumps in FIG. 16. In this comparison, the conventional jet pumps are the jet pumps having five nozzles as shown in FIG. 2 of Japanese Patent Laid-open No. Hei 7 (1995)-119700 and the jet pumps having the nozzle apparatus provided with a ring header and a cooling water suction passage in the axis as shown in FIG. 1 of Japanese Patent Laid-open No. 2008-82752. In each jet pump in Japanese Patent Laid-open No. Hei 7 (1995)-119700 and Japanese Patent Laid-open No. 2008-82752, each ejection outlet is disposed parallel to the axis of the jet pump, facing downward. FIG. 16 shows a change in the jet pump efficiency to the M ratio for the jet pump having the nozzle apparatus 12B with no flow passage reduction portion in the throat and for the above-described jet pumps in the conventional examples. In the jet pump having the nozzle apparatus 12B with no flow passage reduction portion in the throat, as described above, the efficiency increases more than the conventional examples due to the reduction in the pressure loss in the nozzle apparatus 12B, and the increase in the flow rates Qb1 and Qb2 of the suction flows 32 and 32A. When the M ratio is increased for the power upgrade of the reactors, the efficiency of the jet pump having the nozzle apparatus 12B with no flow passage reduction portion in the throat is higher than the others as shown in FIG. 16. Since the jet pump 11B of the present embodiment has the flow passage reduction portion 23 in the lower end portion of the throat 22 in the same manner as the jet pump 11 of the embodiment 1, by the influence of this flow passage reduction portion 23, the efficiency of the jet pump is reduced. However, this reduction in the efficiency can be compensated for by part of the increase in the efficiency achieved by the nozzle apparatus 12B. From the contribution of the remaining increase in the efficiency achieved by the nozzle apparatus 12B, the jet pump 11B, thus, can improve the efficiency of the jet pump more than those of the conventional examples. The jet pump 11B of the present embodiment has the flow passage reduction portion 23 in the lower end portion of the throat 22 so that vibration can be suppressed. The present embodiment can increase the efficiency of the jet pump as well as the flow rate of the cooling water 34 supplied to the core 2. The boiling water reactor having the jet pump 11B of the present embodiment, including the nozzle apparatus 12B, can easily handle a power upgrade which requires a large increase in the core flow rate. By using the nozzle apparatus 12B, a nozzle in a jet pump in an existing boiling water reactor can be quickly replaced. In addition, the vibration of the jet pump can be kept low. A jet pump according to embodiment 4, which is another embodiment of the present invention, is described below. The jet pump of the present embodiment is a jet pump in which a leakage flow from the gap 27 in the slip joint 26 to the downcomer 6 is completely eliminated from the jet pump 11 of the embodiment 1. To completely eliminate the leakage flow from the gap 27, the differential pressure between the inside of the slip joint 26 and the downcomer 6 should be zero. When the water head of a jet pump is H (Pa), the density of the fluid is ρ (kg/m3), and its speed is v (m/s), a static pressure Pi (Pa) of the slip joint 26 is represented in Equation (3) based on the static pressure in the downcomer 6.Pi=H−0.5 ρv2 (3) When Pi=0, the differential pressure between the inside of the slip joint 26 and the downcomer 6 becomes zero. The speed v is represented in Equation (4) using a jet pump flow rate Q (m3/s) and the inner diameter D6 of the outlet of the throat 22.v=Q/(πD62/4) (4) From Equation (3) and Equation (4), a value of the inner diameter D6 that makes Pi=0 is as in Equation (5).D6=(8ρQ2/πH)0.25 (5) Therefore, when the inner diameter D6 is within a range of (8ρQ2/πH)0.25≦D6<D5, the differential pressure between the inside of the slip joint 26 and the downcomer 6 can be reduced. A change in the static pressure in the axial direction of the jet pump when D6=(8ρQ2/πH)0.25 in the present embodiment is shown as the alternate long and short dash line in FIG. 7. The differential pressure between the inside of the slip joint 26 and the downcomer 6 becomes zero at the position of the slip joint 26, eliminating the leakage flow from the gap 27 in the slip joint 26. In the present embodiment, each effect achieved in the embodiment 1 can be obtained. The vibration of the jet pump can be reduced more than in the embodiment 1. A jet pump according to embodiment 5, which is another embodiment of the present invention, is described below. A jet pump 11C of the present embodiment has a structure in which the throat 22 in the jet pump 11 of the embodiment 1 is replaced with a throat 22 having a labyrinth seal 85 on the outer surface of a thick-wall portion 24A of the flow passage reduction portion 23 shown in FIG. 17. Other components of the jet pump 11C are the same as the jet pump 11 of the embodiment 1. Since the jet pump 11C is provided with the labyrinth seal 85 on the outer surface of the thick-wall portion 24A of the flow passage reduction portion 23, the resistance of the flow passage of the gap 27 is increased and thus a leakage flow from the inside of the slip joint 26 to the downcomer 6 can be reduced even more than the jet pumps in the embodiments 1 to 3. Consequently, the vibration of the jet pump 11C can be reduced. The jet pump 11C has the nozzle apparatus 12 so that each effect achieved by the jet pump 11 can be obtained. The throat 22 provided with the labyrinth seal 85 on the outer surface of the thick-wall portion 24A of the flow passage reduction portion 23 may be applied to the jet pumps 11A and 11B. The present invention can be applied to a boiling water reactor. 1: reactor pressure vessel, 2: core, 3: core shroud, 6: downcomer, 7: recirculation pipe, 8: recirculation pump, 10: elbow pipe, 11, 11A, 11B, 11C: jet pump, 12, 12A, 12B: nozzle apparatus, 13: nozzle base, 14: nozzle, 15, 17: nozzle straight-tube portion, 16, 18: nozzle narrowing portion, 19: nozzle lower end portion, 20, 20B: ejection outlet, 20A: annular ejection outlet, 21: bell mouth, 22: throat, 23: flow passage reduction portion, 25: diffuser, 26: slip joint, 30: driving flow (driving fluid), 31, 31A: jet flow, 32, 32A: suction flow (suction fluid), 40, 61: nozzle portion, 41: outer cylinder member, 42: inner cylinder member, 43: outer funnel member, 44: inner funnel member, 45: annular passage, 46: nozzle header portion, 47, 62: outer cylinder member, 48, 63: inner cylinder member, 49: annular header portion, 50: inner cooling water suction passage, 54: flow-adjusting plate, 57: joint portion, 64: jet passage, 65: suction passage portion, 66, 81: cylinder member, 67: flow passage forming member, 70: lower region, 71: curved surface, 72: passage member, 73: suction passage, 74: opening, 77: decompression chamber, 78: nozzle holder, 79: reinforcing streamline plate, 80: cone member.
	summary
	summary
053645685	abstract	Anions of the formula (I): EQU [DA.sub.5 M.sub.30-x O.sub.110-x (M'L).sub.x ].sup.m- (I). in which D is Na.sup.+, Ca.sup.2+; A is P, As, Sb, Si, Ge, or combinations thereof M is W.sup.5+, W.sup.6.alpha., or mixtures thereof; M' is a metallic element from groups 2 to 15 of the periodic table; other than W; L is O.sup.2-, OH.sup.-, H.sub.2 O; x is 0-10; and m is 10-20; selectively react with cations Z.sup.n+ to afford anions of the formula (II): EQU [ZA.sub.5 M.sub.30-x O.sub.110-x (M'L).sub.x ].sup.(m+1-n)- (II). wherein n is 3 or 4; Z=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, or Bi, when n=3, and Z=Ce, U, Np, Pu, or Am, when n=4. This reaction may be used for the selective encapsultion of lanthanide or actinide cations, and salts containing anions of formula (II) may be vitrified to form glasses or reduced to form tungsten "bronze" materials suitable for the long-germ storage of radioactive lanthanides or actinides.
039742695	abstract	Gonorrhea antibodies in serum are detected by determination of radioactivity of conjugate formed between antibodies and antigens labelled with radioactive isotope.

046577270

abstract

The present system uses measurement inputs which reveal fission product barrier functional characteristics for emergency event classification in nuclear generating stations. The process determines the functional status of barriers to fission product release under transient or emergency conditions. The first part of the present process specifies which key indicators within a nuclear plant provide data indicative of fission product barrier status, and develops a logic table or diagram relating specific key symptoms to barrier status such that a computer identifies the functional status of each fission product barrier. In the second part of the present process nuclear power plant operators utilize the computer to determine emergency event classification. The computer utilizes the present process, using the installed nuclear generating station instrumentation which indicates flow rates in fluid and air handling systems, pressures in pipes and vessels, temperatures in pipes and vessels, radiation levels and other indications of equipment status to determine whether the fission product barriers are functional at a given point in time according to the criteria established during the first part of the process. The implentation is an orderly process in which nuclear power plant operator uses the process as programmed on the computer to determine and correctly characterize the condition or status of each fission product barrier at any time. The process results in direct indication of the status of fission product barriers and of the categorization of the emergency event according to established classes of emergencies. If the computer is non-functional, the nuclear plant operator uses the process as presented in implementing procedures to determine the status of each fission product barrier and to determine the proper event clasification. The operator relies upon installed plant instrumentation to determine the functional status of the barriers according to the process.

051724030

description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, denoted at 50 is an X-ray source accommodating chamber for accommodating an X-ray source therein. Barrel 5 is coupled to the X-ray source accommodating chamber and, to this barrel 5, a stage accommodating chamber 19 is coupled. The barrel 5 is equipped with a beryllium blocking window 6 through which the X-rays produced by the X-ray source are introduced into the stage accommodating chamber 19 along the z-axis direction. The stage accommodating chamber 19 accommodates therein a mask 13, a mask chuck 14 for holding the mask 13, a semiconductor wafer 15, a wafer chuck 16 for holding the wafer 15, a wafer stage 18 which is movable along the x-axis, y-axis and z-axis directions as well as in rotational directions about these axes, respectively, and an optical system 17 for the alignment of the mask and the wafer with respect to each of the x-axis, y-axis and z-axis directions. More particularly, the optical system 17 is operable to detect any positional deviation between alignment marks provided on the mask and the wafer. The wafer stage 18 can be moved stepwise in each of the x-axis and y-axis directions, for printing a circuit pattern formed on the mask 13 upon different shot areas on the wafer 15. By exposing the wafer 15 with the x-rays through the mask 13, the pattern of the mask 13 can be printed on a particular shot area of the wafer 15. Thus, the above-described structure constitutes what can be called an "X-ray stepper". To the barrel 5, a high vacuum pump 51 such as a turbo molecular pump, for example, is coupled, for vacuum-evacuation thereof. To the stage accommodating chamber 19, a low vacuum pump 11 such as an oil rotation pump, for example, is coupled by way of a discharging port 12 and a variable valve 10. The discharging port 12 is coupled to the same side of the stage accommodating chamber 19, to which the barrel 5 is coupled. However, the discharging port may be coupled to the wall of the stage accommodating chamber 19 on the opposite side remote from the barrel 5, as in the FIG. 6 example. The variable valve 10 is adapted to change the opening thereof automatically in response to a signal from a controller 9. The stage accommodating chamber 19 is equipped with an opening 31 for passage of X-rays, on the side of which opening a pressure detecting port 7 is provided to detect the pressure in an X-ray projection path 30 between the beryllium blocking window 6 and the mask 13. On the basis of the pressure detected by the pressure sensor 8, the controller 9 controls the opening of the variable valve 10. By this, the pressure in the stage accommodating chamber 19, more particularly, the pressure in the X-ray projection path (passageway) 30, can be controlled and maintained constant. The flow of helium gas from a tank 1 is adjusted by a manual adjusting valve 2 to a predetermined flow rate and, through a helium supply port 3 disposed just after (wafer 15 side) the beryllium blocking window 6 of the barrel 5, the helium gas is supplied into the stage accommodating chamber 19 through the X-ray projection path 30. The mask chuck 14 is provided with a helium discharging port 4 for discharging the supplied helium gas in the X-ray projection path 30 into the stage accommodating chamber 19. The helium discharging port 4 is arranged to avoid vibration or flexure of the mask 13 due to the gas flow of helium. FIG. 2 is a sectional view of the helium supply port 3, taken on line A--A in FIG. 1. FIG. 3 is a sectional view of the helium discharging port 4, taken on line B--B in FIG. 1. As seen in these drawings, in the neighborhood of the portion of the barrel 5 to which the helium supply port 3 is coupled, an inner cylinder 40 is provided in a concentric relationship with the barrel 5. The inner cylinder 40 has twelve (12) bores 41 formed equidistantly along the circumference of the cylinder 40. The helium introduced into the barrel 5 from the helium supply port 3, is then introduced into the X-ray projection path 30 through these bores 41 of the inner cylinder 40. Also, as best seen in FIG. 3, the mask chuck 14 has eight (8) helium discharging ports 4 which are formed equidistantly around the X-ray projection path 30, as passageways extending radially outwardly from the center of the X-ray projection path 30. The helium introduced into the X-ray projection path 30, is then introduced into the stage accommodating chamber 19 through these discharging ports 4. By introducing and discharging the helium gas radially inwardly and outwardly in the manner described above, it is possible to obtain a uniform or homogeneous helium ambience. For the exposure process in the X-ray exposure apparatus of the structure described above, first the inside atmosphere in the stage accommodating chamber 19 is replaced by a helium gas of a predetermined pressure. Then, in accordance with the quantity of impure gas introduction (leakage) into the stage accommodating chamber 19 (for example, the introduction of air through seal means), a constant amount of helium necessary for retaining the purity of helium in the stage accommodating chamber 19 is supplied continuously from the helium supply port 3. Additionally, the opening of the variable valve 10 coupled to the vacuum pump 10 is controlled continuously on the basis of the output of the pressure sensor 8. By this, the pressure in the X-ray projection path 30 can be controlled and maintained constant. Referring to FIG. 4 showing a second embodiment of the present invention, helium is supplied through a helium supply port 3 which is provided just before (light source side) the mask 13 held on the mask chuck 14. Additionally, a communication port 20 is formed to provide communication between the stage accommodating chamber 19 and the barrel 5, more particularly, a part of the barrel 5 just after (mask 13 side) of the blocking window 6. The helium gas from the tank 1 flowing through the manual valve 2 flows through the X-ray projection path 30 in a direction from the mask 13 to the blocking window 6 and, thereafter, through the communication port 20 as the helium gas introduced into the stage accommodating chamber 19. As in the first embodiment, the pressure detecting port 7 is provided on the side of the opening 31 of the stage accommodating chamber 19, for passage of X-rays, and it is operable to detect the pressure in the X-ray projection path 30. The remaining structure and operation of this embodiment are substantially the same as those of the first embodiment. Referring to FIG. 5 showing a third embodiment of the present invention, in this embodiment, the first embodiment is modified such that a thin film 21 is added between the mask 13 and the helium discharging port 4 to intercept them. Also, a port 22 is added to provide communication between the stage accommodating chamber 19 and the space which is defined between the thin film 21 and the mask 13. With this arrangement, it is possible to effectively suppress or minimize the vibration or flexure of the mask 13 due to the flow of helium, substantially without decreasing the amount of exposure of the wafer 16 with the X-rays. In FIG. 5, the helium from the tank 1 flowing through the valve 2, is supplied into the stage accommodating chamber 19 from the helium supply port 3 provided just after the blocking window 6 of the barrel 5. More particularly, the helium gas introduced from the helium supply port 3 into the X-ray projection path 30 flows to the discharging ports 4 which are provided just before (blocking window 6 side) the thin film 21, into the stage accommodating chamber. Into the space between the thin film 21 and the mask 13, helium is introduced from the stage accommodating chamber 19 side, through the port 22. The thin film 21 is disposed adjacent the helium discharging ports 4 which are formed in the mask chuck 14 for introduction of the helium gas supplied to the X-ray projection path 30, into the stage accommodating chamber 19. More particularly, it is disposed at one side of the ports 4 facing the mask 13. The communication port 22 provided between the thin film 21 and the mask 13 functions also to prevent the possible flexure or vibration of the thin film 21, due to the gas flow of helium, from adversely affecting the mask 13. Since substantially no differential pressure is produced on the opposite sides of the thin film 21, the thickness thereof may be very small on an order of a few microns. Examples of the material thereof are: an organic material such as polypropylene, polyethylene, polyamide, polycarbonate, vinyl chloride, fluorine plastic or the like; or an inorganic material such as Si.sub.3 N.sub.4, SiC, Be, SiO.sub.2 or the like. As in the first embodiment, the pressure detecting port 7 is provided on the side of the opening 31 of the stage accommodating chamber 19, for passage of X-rays and it is operable to detect the pressure in the X-ray projection path 30. The remaining structure and operation are similar to those of the first embodiment. Also, in the third embodiment, like the second embodiment, the supply port 3 may be provided on the mask chuck side, at a position between the blocking window 6 and the thin film 21, and the communication port 20 (see FIG. 4) may be provided on the barrel 5 side. Whether the structure without a thin film 21 as in the first and second embodiments or the structure with a thin film as in the third embodiment should be selected, may be determined on the basis of various conditions such as, for example, the mechanical structure of the components (such as the barrel 5, the optical system 17, the mask 14, etc.) disposed around the X-ray projection path 30, the flow rate of helium gas as supplied from the helium supply port 3, the material and thickness of the thin film 21, while taking into account the quantity of X-ray attenuation, the effect of vibration or flexure of the mask, and the like. In the preceding three embodiments, the supply and discharge of helium are executed through the ports 3 and 4 provided in the barrel 5 and the mask chuck 14, and the pressure detection is executed through the port 7 provided in the stage accommodating chamber 19. However, the present invention is not limited to such a form. By way of example, the supply port 3 may be provided in the stage accommodating chamber 19 so as to supply helium into the X-ray projection path 30, or the pressure detecting port 7 may be provided in the barrel 5 or the mask chuck 14 so as to detect the pressure in the X-ray projection path 30. Further, the position for the provision of the thin film 21 is not limited to the mask chuck 14. According to the present invention, as described hereinbefore, it is possible to control the helium ambience in the X-ray projection path mainly, to which the control of purity, pressure and the like of the helium gas should actually be executed. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

claims

1. An isotope production target rod for a light water nuclear power reactor, said isotope production target rod comprising:at least one rod central body including an outer shell that defines an internal cavity, the at least one rod central body configured to be inserted into the light water nuclear power reactor;a plurality of irradiation targets within the internal cavity, the irradiation targets not being nuclear fuel; anda low nuclear cross-section separating medium separating each of the irradiation targets from each other and positioning each of the irradiation targets in a spatial arrangement, the low nuclear cross-section separating medium having a neutron absorption cross-section less than about 1 barn for neutrons produced through nuclear fission in the light water nuclear power reactor and incident on the separating medium; and whereinthe irradiation targets and the low nuclear cross-section separating medium are not disposed in a container within the rod central body; andthe separating medium comprises at least one irradiation target receptacle within the internal cavity of each respective rod central body, each irradiation target receptacle including a plurality of target reservoirs in a pattern within at least a portion of an outer surface of each irradiation target receptacle. 2. The isotope production target rod of claim 1, wherein each target reservoir is sized to retain a single irradiation target. 3. The isotope production target rod of claim 1, wherein each target reservoir is sized to retain the respective target in a particular orientation. 4. The isotope production target rod of claim 2, wherein each target reservoir is sized to retain the respective target in a random orientation. 5. The isotope production target rod of claim 1, wherein each irradiation target receptacle is fabricated of at least one of zirconium and aluminum. 6. The isotope production target rod of claim 1, wherein each irradiation target receptacle comprises a solid body having the target reservoirs in the pattern along the at least a portion of an outer surface of the solid body. 7. The isotope production target rod of claim 6, wherein the target reservoirs are equally spaced around the body outer surface between opposing axial end portions of the body outer surface such that each target reservoir is a substantially equal distance from each adjacent target reservoir. 8. The isotope production target rod of claim 6, wherein the target reservoirs are equally spaced about at least one of opposing axial end portions of the body outer surface such that each target reservoir is a substantially equal distance for each adjacent target reservoir. 9. The isotope production target rod of claim 6, wherein each irradiation target receptacle comprises an axially threaded bore in at least one axial end portion. 10. The isotope production target rod of claim 1, wherein each irradiation target receptacle comprises a tubular body having the target reservoirs in the pattern equally spaced around an outer surface. 11. The isotope production target rod of claim 10, wherein the tubular body includes a longitudinal center cavity therethrough, the axial bore having threads in at least one axial end portion. 12. The isotope production target rod of claim 1, wherein each irradiation target receptacle comprises:a perforated tubular sleeve having a plurality of holes therein in the pattern; anda solid core fitted within the sleeve. 13. An isotope production target rod for a light water nuclear power reactor, comprising:at least one rod body including an outer shell that defines an internal cavity, the at least one rod body configured to be inserted into the light water nuclear power reactor;a separating structure disposed within the cavity that defines separate target spaces;a plurality of irradiation targets, the plurality of irradiation targets not being nuclear fuel, each of the plurality of irradiation targets being disposed in a different one of the target spaces such that the plurality of irradiation targets are separated from each other by the separating structure, and the irradiation targets not being disposed in a separate container within the rod body. 14. An isotope production target rod for a light water nuclear power reactor, comprising:at least one rod body including an outer shell that defines an internal cavity, the at least one rod body configured to be inserted into the light water nuclear power reactor;a cylinder disposed in the cavity of at least one of the rod bodies, the cylinder having a plurality of reservoirs formed in an outer lateral surface thereof;a plurality of irradiation targets, each irradiation target disposed in a different one of the plurality of reservoirs such that the plurality of irradiation targets are separated from one another, and each irradiation target not being nuclear fuel, wherein at least one of the plurality of reservoirs has an elliptical cross-section. 15. The isotope production target rod of claim 14, further comprising:a target container within the cavity of the at least one of the rod bodies, and the cylinder being disposed in the target container. 16. A fuel bundle for a light water nuclear power reactor, comprising:a plurality of fuel rods;at least one isotope production rod as recited in claim 1;a plurality of spacer grids maintaining the plurality of fuel rods and the isotope production rod in a spaced relationship in the light water nuclear power reactor; andan outer channel housing the plurality of fuel rods, the isotope production rod and the plurality of spacer grids. 17. The fuel bundle of claim 16, further comprising:an upper tie plate;a lower tie plate; and whereinthe plurality of fuel rods and the isotope production rod are disposed between the upper and lower tie plates.

description

The invention relates to the manufacture of a glovebox-type chamber, as schematically represented in FIG. 1, used in particular for radioactive substance handling. Such a box 1 comprises a structure or frame 2 carrying transparent panels 3 provided with openings receiving glove ports 4 in which gloves are mounted. The invention relates to the localisation of the positions of the glove ports 4 to make the openings at the proper locations in the panels of such a glove box. In practice, the position of the glove ports on a panel conditions the accessibility to the inner space of the chamber. One solution consists in approximately assessing the positions of these openings before making them, hoping that they will enable the operator to access the entire inner volume of the chamber. Since the shapes of the glovebox change from box to box, this solution can result in placing the glove ports into positions not enabling the manipulator to access the entire inner volume when his/her arms are engaged in the glove ports. Consequently, it is necessary to manufacture again the panel with better defined locations, which is disadvantageous considering the high cost of such panels, which are made of lead-loaded Plexiglas and the supply delay of which is generally long. Another solution consists in making a prototyping to attach to the structure itself free of panels the glove ports at the locations foreseen, and checking that these locations enable the entire inner space to be accessed. The prototype can then be modified to correct the locations up to enable the operator to access the entire inner space with his/her arms engaged in the glove ports. In practice, the prototyping is made with angle beams or other profiles, such that it is boring and complex to implement. This complexity is further increased when the goal is to place several glove ports on the prototyping. The invention relates to a device for identifying a position of at least one glove port intended to equip a panel carried by a glovebox structure delimiting a closed chamber, this device comprising a base, means for attaching this base to the glovebox structure, at least one template comprising an opening able to receive a glove port, each template being carried by the base while being movable with respect to this base, and means for locking each template in position with respect to the base. The invention thus makes it possible to very significantly reduce the time required to determine the positions of the glove ports. When the panel in question is withdrawn, the device is attached to the structure, at the location of this panel: the operator can pass his/her hands through the glove ports in order to determine whether the positions of these glove ports are adapted or not. If necessary, the operator unlocks the elements to replace one or more glove ports as he/she wants before locking them into their new position in order to make a new test. By trial and error, the operator thus succeeds in identifying the optimum configuration. The invention also relates to a device thus defined, comprising a main post carrying a base, this main post being provided at the ends thereof with means for attaching to two cross members of the glovebox structure intended to receive the panel. The invention also relates to a device thus defined, wherein the base is slidably mounted on this main post, and comprising means for locking the base with respect to the main post. The invention also relates to a device thus defined, wherein each template is carried by the base while being movable with respect to this base perpendicularly to the main post, and comprising means for locking each template with respect to the base. The invention also relates to a device thus defined, comprising a system of graduation marks along the main post and on each means for attaching the template to a base, to locate the position of each template by visual reading. The idea underlying the invention is a device carrying one or more glove ports the positions of which are adjustable, which device is intended to be mounted on a glovebox structure free of panel. An operator can thus make tests enabling him/her to determine simply the positions of the glove ports which are best adapted to the task that has to be made in the box, and best adapted to access the entire inner space of the chamber. The device which is schematically represented in FIG. 2, wherein it is marked as 6, includes a main post 7 in the form of a slider forming profile equipped at one end thereof with a fixed latching tab 8 and at the other end thereof with an adjustable latching tab 9. The fixed tab 8 is rigidly integral with the end of the post 7 whereas the adjustable tab 9 is able to slide along this post. In use, the main post 7 extends vertically by having its fixed tab 8 latched to an upper horizontal cross member 12 of the glovebox structure 11, and its movable tab latched to a lower horizontal cross member 13 of the structure of this box 11. These cross members 12 and 13 correspond to the upper and lower edges of an opening of the structure intended to receive a panel when the box is completed. This main post 7 carries several identical assemblies 14, 16, 17 each including two opening templates each able to receive a glove port and possibly a glove. As can be seen in FIG. 2, the assembly 14 includes a slider forming cross member 18 which projects on either side from the post 7 by extending perpendicularly to the same, that is horizontally in position of use. This slider forming cross member 18 is carried by a lockable fastener 19 which is slidably mounted on the slider forming post 7 and which is lockable in position on this post 7, to adjust vertically the position of the assembly 14 at any desired height. The slider forming cross member 18 is itself transversally movable in the fastener 19 and lockable in position in the same, which enables the position of this cross member 18 to be transversally adjusted by offsetting more or less to left or right with respect to the post 7. The cross member 18 thus includes a first portion 21 projecting from a side of the post 7 and a second portion 22 projecting from the other side of the post 7. The first portion 21 carries a first template 23 and the second portion carries a second template 24. Each template 23, 24 is slidably mounted on the slider forming cross member 18 and it can be locked at a given position along this cross member. As can be seen in FIG. 2, the template 23 includes a plate 27 comprising an upper region equipped with means for nesting and locking in the slider forming cross member 18, and a lower region including an opening 28 on which a glove port is mounted. The template 24 is identical to the template 23. The template 23 and the template 24 are thus adjustable in position on the cross member 18 and lockable in position along this cross member. The fastener 19 includes a base having a part which engages in the post 7 and which is equipped with a locking screw enabling it to be immobilised along the post 7 by tightening this screw, and another part in which the cross member 18 is engaged with another locking screw to lock this cross member in position. Analogously, the top part of each template is equipped with a base engaging in or about the cross member 18 to slide along the same and it is provided with a tightening screw enabling the template to be locked in position along this cross member 18. Thus, both templates 23 and 24 are located at a same height which is adjustable by virtue of the movable fastener 19, and the side positions of both these templates are adjustable by moving sideways the cross member 18 with respect to the base of the lockable fastener 19 which carries it, and/or by moving sideways each template with respect to this cross member and by tightening the different locking screws to fix the transverse position of these templates. In the configuration of FIG. 2, the assembly 14 is located at the top part of the post 7, and the other two assemblies, namely the assemblies 16 and 17 which are identical to the assembly 14 are located below the assembly 14 along the post 7. In use, the operator starts with placing the post 7 by suspending it to the upper cross member 12 by means of the fixed fastener 8, and by attaching its lower region to the lower cross member 13 by means of the movable fastener 9. Once the device is thus placed, the operator adjusts the height of each assembly 14, 16 and 17 along the post 7, and he/she tightens the corresponding locking screws. The operator can then adjusts sideways the positions of each of both templates of each assembly before tightening the corresponding locking screws. When this prepositioning has been made, the operator which is marked as 26 on FIG. 3 can make tests. At this time, he/she passes his/her arms in the different glove ports to check that he/she can access the different inner regions of the box, and he/she can make feeling minimum disturbance the task that has to be made in this box. Optionally, the operator readjusts the positions of each glove port of the device until optimum position settings are achieved. When the operator considers that the glove ports carried by the templates are immobilized at the optimum positions, he/she can note down these positions after he/she has measured them. These measurements can be manually made with a tape measure, or simply by reading the graduation marks 29 intended to that end on the main post 7 and on the cross members enabling the position of each glove port to be identified. Another possibility can consist in using a laser measuring system for identifying the positions of each glove port. The position data thus collected are then transferred on the panel that has to be placed in the box opening delimited by the cross members 12 and 13, before passing to the operation of cutting these openings in the panel. The invention thus enables the position of each glove port to be accurately determined by making real tests for the ergonomics of handlings to be made. By the way, it is possible to equip each glove port with a glove to make the tests still more realistic. The invention further enables the positions identified to be accurately measured. The invention which has been set forth within the context of radioactive substance handling is also applicable to other fields such as for example chemistry, biology and bacteriology. It can also be applied to the hospital environment for the case of incubators, and of works on viruses. The invention is further applicable to the field of quick-assembly movable chambers on theatres of civil or military operations.

055639259

claims

1. A method for adjusting the radiation output delivered to an object from a radiation source, comprising the following steps: generating a radiation beam having a variable radiation output and a substantially lossless beam path from a radiation source to the object; delimiting the beam path by moving at least one beam-shielding device; defining an irradiated field of the object; varying the radiation output of the beam as a predetermined function of the position of the beam-shielding device, a wedge factor of the radiation output thereby varying according to a predetermined profile, in which the wedge factor is defined as the ratio between a reference radiation output along a reference axis of the beam with a predetermined physical wedge in the beam path and an actual radiation output of the beam in a substantially lossless beam path; and varying the radiation output such that the wedge factor is constant regardless of the size of the irradiated field. a radiation source generating a radiation beam having a variable radiation output; an irradiated field of the object; beam-shielding means for delimiting the output beam to at least one predetermined irradiation field of the object; a dose controller for varying a degree of shielding of the beam and the radiation output; and processing means for generating and applying to the dose controller set dose signals, comprising nominal dose signals and wedge correction factors, and for thereby varying the radiation output such that a wedge factor is constant regardless of the degree of shielding, where the output factor is defined as the ratio between a reference radiation output along a reference axis of the beam with a predetermined physical wedge in the beam path and an actual radiation output of the beam in a substantially lossless beam path. 2. A method as in claim 1, in which the wedge factor is equal to unity. 3. A system for adjusting the radiation output delivered to an object from a radiation source, comprising:

description

This application is a National Stage Application of PCT/US2018/025216, filed Mar. 29, 2018, which claims the benefit of priority to U.S. Provisional patent application Ser. No. 62/478,419, filed Mar. 29, 2017, the entire disclosures of which are incorporated by reference in their entireties. To the extent appropriate, a claim of priority is made to each of the above disclosed applications. Nuclear fission reactors include breed-and-burn fast reactors (also referred to as traveling wave reactors, or TWRs). TWR means a reactor that would be designed to operate indefinitely using natural uranium, depleted uranium, spent light water reactor fuel, or thorium as a reload fuel after start up, and in which waves that breed and then burn would travel relative to the fuel. Thus, in some aspects, the TWR is a once-through fast reactor that runs on subcritical reload fuel which is bred up to a useful state and burned in situ. In a TWR, a wave of breeding and fissioning (a “breed-burn wave”) is originated in the core of the reactor and moves relative to the fuel. In cases where the fuel is stationary, the breed and burn wave expands outward from the ignition point. In some cases, the fuel is moved so that the breed and burn wave stays stationary relative to the core (e.g., a standing wave) but moves relative to the fuel; a standing wave is considered a type of TWR. Movement of fuel assemblies is referred to as “fuel shuffling” and is used to maintain the standing wave and for adjustment of reactor characteristics (heat, flux, power, fuel burn up, etc.). Nuclear fissioning occurs in the core in which the fuel assemblies are shuffled is housed in a reactor vessel. The fuel assemblies include fissile nuclear fuel assemblies and fertile nuclear fuel assemblies. Reactivity control is primarily accomplished by control rod assemblies also located in the core for adjustment of reactor characteristics. Fission energy developed by the standing wave creates thermal energy which is transferred in series through one or more heat transport loops to steam generators to produce electricity, and low temperature heat is rejected through a set of water-cooled vacuum condensers. The separation of coolant systems into both primary and intermediate coolant loops helps maintain the integrity of the core and the primary coolant loops. In the TWR, both the primary coolant and intermediate coolant loop utilizes liquid sodium. In one aspect, the technology relates to a method of replacing a cesium trap, the method includes: freezing a first cesium trap at least partially containing cesium therein, wherein the first cesium trap is located within a shielded cell; decoupling the first cesium trap from the shielded cell; removing the first cesium trap from the shielded cell; inserting a second cesium trap into the shielded cell; and attaching the second cesium trap to the shielded cell. In an example, the first cesium trap includes at least one of: remotely decoupling at least one lateral support anchor extending between the first cesium trap and the shielded cell; remotely decoupling the first cesium trap from a sodium processing circuit; and remotely disconnecting electrical power and instrument control attachments extending between the first cesium trap and the shielded cell. In another example, remotely decoupling the at least one lateral support anchor includes removing a connection member that couples a cell anchor extending from the shielded cell to a trap anchor extending from the first cesium trap. In yet another example, the connection member is at least one of a pin and a bolt. In still another example, remotely decoupling the first cesium trap from a sodium processing circuit includes: crimping at least one sodium line extending from the first cesium trap; and cutting the at least one sodium line adjacent the crimped portion such that a first portion of the at least one sodium line extends from a top portion of the first cesium trap and a second portion of the at least one sodium line remains part of the sodium processing circuit. In another example of the above aspect, remotely disconnecting electrical power and instrument control attachments includes unplugging at least one of an electrical power attachment and an instrument control attachment from a corresponding receiver disposed on a top portion of the first cesium trap. In an example, the first cesium trap includes: releasably coupling the first cesium trap to a lifting tool; and lifting, via the lifting tool, the first cesium trap out of the shielded cell such that a base of the first cesium trap slidably disengages with at least one locating pin extending from a bottom of the shielded cell. In another example, the method includes decoupling a cooling line inlet extending from the first cesium trap from a fixed cooling line extending from the shielded cell via the lifting operation. In yet another example, releasably coupling the first cesium trap to a lifting tool includes rotating and lifting at least one hook of the lifting tool into a corresponding lifting eye disposed on the first cesium trap. In still another example, inserting the second cesium trap includes: releasably coupling the second cesium trap to a lifting tool, wherein the lifting tool includes at least one hook and the second cesium trap includes at least one corresponding lifting eye; placing, via the lifting tool, the second cesium trap into the shielded cell; and simultaneously aligning a base of the second cesium trap with at least one locating pin extending from a bottom of the shielded cell. In another example of the above aspect, the method includes coupling, via a slidable engagement, a cooling line inlet extending from the second cesium trap to a fixed cooling line extending from the shielded cell. In an example, attaching the second cesium trap includes at least one of: coupling a cell anchor extending from the shielded cell to a trap anchor extending from the second cesium trap forming at least one lateral support anchor extending between the first cesium trap and the shielded cell; welding at least one first sodium line extending from the second cesium trap to at least one second sodium line extending from a sodium processing circuit; and connecting electrical power and instrument control attachments via plugging the attachments into a corresponding receiver disposed on a top portion of the second cesium trap. In another example, the first cesium trap contains a predetermined amount of cesium and the second cesium trap contains no amount of cesium. In yet another example, the shielded cell is an individualized shielded cell. In another aspect, the technology relates to a cesium trap having: a body having a top portion, the body enclosing a filter and an active material configured to remove cesium from a sodium stream; a cooling jacket disposed substantially around the body; and at least one first sodium line extending from the top portion, wherein the at least one first sodium line is configured to couple in flow communication with a corresponding at least one second sodium line of a sodium processing circuit. In an example, the cesium trap further includes at least one trap anchor extending from the cooling jacket, wherein the trap anchor is releasably couplable to at least one corresponding cell anchor extending from the shielded cell so as to form at least one lateral support anchor. In another example, the cesium trap includes at least one lifting eye disposed on the top portion, the at least one lifting eye configured to receive a lifting tool for placing the cesium trap into and removing the cesium trap from a shielded cell. In still another example, the cesium trap an inlet cooling line extending from a bottom portion of the cooling jacket, wherein the inlet cooling line has a free end that is releasably couplable in flow communication to a corresponding fixed cooling line within the shielded cell. In still another example, the cesium trap includes at least one receiver disposed on the top portion, wherein electrical power and instrument control attachments are configured to releasably plug into the at least one receiver. In another example, the cesium trap includes a base coupled to a bottom portion of the body, wherein at least one opening is defined within the base such that the base is positionable within a bottom of the shielded cell via at least one corresponding locating pin. In another aspect, the technology relates to a cesium trap having: a body having a top portion, the body enclosing a filter and an active material configured to remove cesium from a sodium stream containing cesium and argon; and at least one first sodium line extending from the top portion, wherein the at least one first sodium line is configured to couple in flow communication with a corresponding at least one second sodium line of a sodium processing circuit; wherein the filter is located to prevent the active material from leaving the cesium trap while allowing the sodium and argon to pass and the filter has a mean pore size of from 40 to 160 μm. In an example, the lower limit of the mean port size of the filter is selected from 40, 50, 60, 70, 80, and 90 μm. In another example, the upper limit of the mean port size of the filter is selected from 100, 110, 120, 130, 140, 150, and 160 μm. In yet another example, the filter is a sintered metal filter. In still another example, the sintered metal filter is sintered from one or more of stainless steel, Hastelloy®, Monel®, Inconel®, nickel, HT-9, and titanium. FIG. 1 illustrates, in a block diagram form, some of the basic components of a travelling wave reactor (TWR) 100. In general, the TWR 100 includes a reactor core 102 containing a plurality of fuel assemblies (not shown). The core 102 is disposed within a pool 104 holding a volume of liquid sodium coolant 106. The pool 104 is referred to as a hot pool and has a sodium temperature higher than that of a surrounding cold pool 108 (due to the energy generated by the fuel assemblies in the reactor core 102), which also contains liquid sodium coolant 106. The hot pool 104 is separated from the cold pool 108 by a redan 110. A headspace 112 above the level of the sodium coolant 106 is filled with an inert cover gas, such as argon. The reactor vessel 114 surrounds the reactor core 102, hot pool 104, and cold pool 108, and is sealed with a reactor head 116. The reactor head 116 provides various access points into the interior of the reactor vessel 114. The size of the reactor core 102 is selected based on a number of factors, including the characteristics of the fuel, desired power generation, available reactor 100 space, and so on. Various embodiments of a TWR may be used in low power (around 300 MWe—around 500 MWe), medium power (around 500 MWe—around 1000 MWe), and high power (around 1000 MWe and above) applications, as required or desired. The performance of the reactor 100 may be improved by providing one or more reflectors, not shown, around the core 102 to reflect neutrons back into the core 102. Additionally, fertile and fissile nuclear assemblies are moved (or “shuffled”) within and about the core 102 to control the nuclear reaction occurring therein. The sodium coolant 106 is circulated within the vessel 114 via a primary sodium coolant pump 118. The primary coolant pump 118 draws sodium coolant 106 from the cold pool 108 and injects it into a plenum below the reactor core 102. The coolant 106 is forced upward through the core and is heated due to the reactions taking place within the reactor core 102. Heated coolant 106 enters an intermediate heat exchanger(s) 120 from the hot pool 104, and exits the intermediate heat exchanger 120 and re-enters the cold pool 108. This primary coolant loop 122 thus circulates sodium coolant 106 entirely within the reactor vessel 114. The intermediate heat exchanger 120 incorporates a segment of a closed liquid sodium loop that is physically separated from the primary sodium pools 104 and 108 at all times (i.e., intermediate and primary sodium are never co-mingled). The intermediate heat exchanger 120 transfers heat from the primary coolant loop 122 (fully contained within the vessel 114) to an intermediate coolant loop 124 (that is only partially located within the vessel 114). The intermediate heat exchanger 120 passes through the redan 110, thus bridging the hot pool 104 and the cold pool 108 (so as to allow flow of sodium 106 in the primary coolant loop 122 therebetween). In an embodiment, four intermediate heat exchangers 120 are distributed within the vessel 114. Alternatively, two or six intermediate heat exchangers 120 are distributed within the vessel 114. The intermediate coolant loop 124 circulates sodium coolant 126 that passes through pipes into and out of the vessel 114, via the reactor head 116. An intermediate sodium pump 128 located outside of the reactor vessel 114 circulates the sodium coolant 126, for example, through a power generation system 129. Heat is transferred from the sodium coolant 106 of the primary coolant loop 122 to the sodium coolant 126 of the intermediate coolant loop 124 in the intermediate heat exchanger 120. The sodium coolant 126 of the intermediate coolant loop 124 passes through a plurality of tubes 130 within the intermediate heat exchanger 120. These tubes 130 keep separate the sodium coolant 106 of the primary coolant loop 122 from the sodium coolant 126 of the intermediate coolant loop 124, while transferring heat energy therebetween. A direct heat exchanger 132 extends into the hot pool 104 and provides cooling to the sodium coolant 106 within the primary coolant loop 122, usually in case of emergency. The direct heat exchanger 132 is configured to allow sodium coolant 106 to enter and exit the heat exchanger 132 from the hot pool 104. The direct heat exchanger 132 has a similar construction to the intermediate heat exchanger 120, where tubes 134 keep separate the NaK (Sodium-Potassium) of the primary coolant loop 122 from the direct heat exchanger coolant (NaK) 136 of a direct reactor coolant loop 138, while transferring heat energy therebetween. Additionally, TWR reactor 100 includes sodium processing circuit 140 extending into the hot pool 104. The sodium processing circuit 140 facilitates channeling sodium (Na) coolant 106 therethrough for receipt, storage, purification, and/or sampling and analysis of one or more subsystems. For example, the sodium processing circuit 140 may include, but is not limited to, any number of on-site storage tanks, cold traps (e.g., devices used to remove impurities from sodium), cesium traps (e.g., devices for trapping cesium (Cs), which are discussed in further detail below), electromagnetic pumps, heat exchangers, pipelines, testing equipment, and/or control valves. Other ancillary reactor components (both within and outside of the reactor vessel 114) include, but are not limited to, pumps, check valves, shutoff valves, flanges, drain tanks, etc., that are not depicted but would be apparent to a person of skill in the art. Additional penetrations through the reactor head 116 (e.g., a port for the primary coolant pump 118, inert cover gas and inspection ports, sodium processing, and cover gas ports, etc.) are not depicted. A control system 142 is utilized to control and monitor the various components and systems which make up the reactor 100. Broadly speaking, this disclosure describes configurations that improve the performance of the reactor 100 described in FIG. 1. Specifically, embodiments, configurations, and arrangements of a remotely removable cesium trap assembly are shown and described in more detail below with reference to FIGS. 2-5. In general, during operation of the reactor, radioactive cesium will enter the primary sodium coolant. To reduce the adverse radiological impact of this radionuclide, cesium traps are installed as part of the sodium processing circuit. The cesium trap assembly described below enables a cesium trap loaded with cesium to be removed from the sodium processing circuit and its shielded cell, and be replaced with another new empty cesium trap to facilitate the continued absorption and filtering of radioactive cesium from the primary sodium coolant. For example, the cesium trap assembly includes lifting eyes, lateral support anchors, sodium lines, and power and control attachments all positioned at a top portion of the cesium trap to increase remote access and ease of removal. Additionally, remotely controlled tools and lifts enable the loaded cesium trap to be removed from its shielded cell for transport to another location without direct access to the radioactive cesium. Installation of the new cesium trap may be performed with direct access to the shielded cell once the radioactive cesium is removed, thus decreasing installation time. By enabling the cesium traps to be removable and replaceable, the loaded cesium traps are no longer stored in place at the plant and adjacent the reactor. The loaded cesium traps may be transported to an off-plant facility for storage and/or preparation for disposal. Thus, the number of shielded cells adjacent the reactor may be reduced, thus increasing plant space around the reactor. Moreover, through remote access to the loaded cesium trap and direct access to the new cesium trap, removal and replacement time may be decreased, enabling replacement of a cesium trap to be performed while the reactor continues to operate and in a short amount to time. Additionally, the highly radioactive cesium trap is replaced with reducing exposure to the personnel performing the replacement. As, such risk to plant personnel and plant operations are reduced and overall reactor efficiency and viability may be increased. FIG. 2 is a schematic view of an example cesium trap assembly 200 and FIG. 3 is a top view of the cesium trap assembly 200 taken along section line 3-3. Referring to both FIGS. 2 and 3, the cesium trap assembly 200 includes a removable cesium trap 202 disposed within an interior chamber 204 of an individual shielded cell 206 that is configured to contain radioactive material therein. The cell 206 may be coverable via a removable cover 208 depicted in dashed lines in FIG. 2. In the example, the cesium trap 202 includes a body 210 at least partially surrounded by a cooling jacket 212 as discussed in further detail below. The cesium trap 202 is coupled in flow communication with a sodium line 214 that forms at least a portion of the sodium processing circuit 140 (shown in FIG. 1), such that a stream of sodium 216 may be channeled through the body 210. Sodium stream 216 may contain, but is not limited to, sodium, cesium, and argon. For example, an inlet sodium line 218 extends from the body 210 at a top portion 220 of the cesium trap 202, and an outlet sodium line 222 also extends from the body 210 at the top portion 220. Additionally, the cesium trap 202 is coupled in flow communication with a fixed cooling line 224 such that a stream of coolant 226 may be channeled through the cooling jacket 212. An inlet cooling line 228 extends from the cooling jacket 212 proximate a bottom portion 230 of the cesium trap 202 and includes a free end 232. The free end 232 is configured to releasably couple in flow communication with the fixed cooling line 224. In one example, the free end 232 has an enlarged diameter such that the free end 232 may be slidably engaged S over and around the fixed cooling line 224. In alternate examples, the inlet cooling line 228 may be releasably coupled to the fixed cooling line 224 via any other connection that enables the cesium trap 202 to function as described herein. An outlet cooling line 234 also extends from the cooling jacket 212 such that the coolant stream 226 may be exhausted into the interior chamber 204. The cesium trap 202 is coupled to the cell 206 within the interior chamber 204. A base 236 is coupled to the bottom portion 230 and includes at least one opening 238 defined therein. The openings 238 correspond to and are configured to slidably engage with locating pins 240 extending from a floor 242 of the cell 206. The locating pins 240 include a bullet nose 244 or other tapered tip to facilitate locating of and engagement with the openings 238. Additionally, the locating pins 240 reduce excessive lower cesium trap 202 movement during seismic events. In the example, the base 236 is sized larger than the cesium trap 202 to increase stability within the cell 206. The cesium trap 202 is also coupled within the interior chamber 204 via support anchors 246 configured to support lateral and vertical loads, such as seismic loads. The lateral support anchor 246 includes a cell anchor 248 coupled to and extending from a wall 250 of cell and a trap anchor 252 coupled to and extending from cooling jacket 212. The cell anchor 248 is releasably coupled to and on top of the trap anchor 252 via a connection member 254. The connection member 254 is a captive male fastener located on the cell anchor 248 that has a bullet nose entrance to allow a remote rotational device to be used to unthread it from a corresponding female capture nut located on the trap anchor 252. In alternative examples, the connection member 254 may be a pin, a bolt, or any other connection member that enables the lateral support anchor 246 to function as described herein. The cesium trap 202 is communicatively coupled to a control system, for example, control system 142, via electrical power attachment 256 and instrument control attachment 258 extending from cell 206. Electrical power and instrument control attachments 256 and 258 plug into the cesium trap 202 via at least one receiver 260 disposed at the top portion 220. Additionally, the cesium trap 202 includes at least one lifting eye 262, for example two or four for redundancy, extending from the top portion 220. In one example, lifting eyes 262 are welded to body 210, while in other examples lifting eyes 262 are unitarily formed with body 210. The lifting eyes 262 may be configured to facilitate rotation of the cesium trap 202 once lifted. Positioned above the cell 206 is a moveable removal cask 264 that facilitates replacing the cesium trap 202 within cell 206. The removal cask 264 defines an interior chamber 266 that is configured to contain radioactive material therein. Additionally, a floor valve 268 is positioned above the cell 206 that may be closed after removing the cover 208. The removal cask 264 includes a remote lifting tool 270 that releasably couples to the cesium trap 202 and facilities removing the cesium trap 202 from the cell 206. For example, lifting tool 270 includes an arm 272 with a plate 274 connected thereto. The plate 274 includes at least one hook 276 extending therefrom. The hooks 276 may be substantially “J”-shaped and correspond to the lifting eyes 262 disposed on cesium trap 202. In alternative examples, lifting tool 270 has any other configuration that enables function as described herein. Within interior chamber 266, removal cask 264 also includes a remote sodium line tool 278 that facilitates crimping and cutting the sodium line 214 as described further below, a remote anchor tool 280 that facilitates decoupling the lateral support anchors as described further below, and a remote attachment tool 282 that facilitates unplugging the electrical power and instrument control attachments 256 and 258 as described further below. In the example, each tool 270, 278, 280, and 282 are operatively coupled to a control system, for example, control system 142, for remote operation. Additionally, one or more cameras (not shown) are provided to assist remote operation. In alternative examples, removal cask 264 may include a single tool that combines all of the functions of tools 270, 278, 280, and 282 such that removal cask 264 may function as described herein. In operation, sodium coolant 106 is circulated through reactor 100 as described above in reference to FIG. 1. As the sodium coolant 106 is channeled through the reactor core 102, fissionable fuel transfers heat to the sodium coolant 106, in addition, volatile fission products such as cesium enter the sodium coolant 106. A portion 216 of the sodium coolant 106 is channeled through sodium processing circuit 140, including the inlet and outlet sodium lines 218 and 222 and the cesium trap 202, to, in part, remove the volatile fission products contained therein and reduce any adverse radiological impact. As the sodium stream 216 is channeled through cesium trap 202, the cesium contained therein is absorbed and filtered out of the sodium stream 216 before being channeled back into the reactor 100. The operation of cesium trap 202 will be discussed in further detail below. In some known reactors, the cesium traps are fixed within the shielded cells, and as such, once the traps are loaded with the extracted cesium, the entire cesium trap and extracted radioactive cesium are stored in place. However, long term operation of such reactors may produce a number of cesium traps loaded with radioactive cesium, with a number of radioactive shielded cells located in close proximity to the reactor core. In contrast, the above described cesium trap assembly 200 facilitates removal and replacement of the cesium traps 202 that are loaded with radioactive cesium. As such, the cesium trap 202 and radioactive cesium therein may be transferred to a more suitable long-term storage location either on-site and/or at a remote facility. Additionally, some of the reactor area surrounding the reactor core may be used for other reactor systems and processes. Once the cesium trap 202 contains a predetermined amount of cesium, the sodium stream 216 is redirected away from the cesium trap 202 and the remaining sodium within the sodium lines 214 and the cesium trap 202 is allowed to freeze. For example, the sodium stream 216 freezes at 208° Fahrenheit, such that turning off electrical heaters of the circuit (not shown) and stopping flow of the sodium stream 216 facilitates freezing the cesium trap 202 over time. In alternative examples, cooling gas may be circulated through the cooling jacket 212 to increase freezing time. To remove the cesium trap 202 from the shielded cell 206, the floor valve 268 is installed over the cell 206 and the cover 208 removed from the cell 206 and moved to a temporary storage location on-site. The floor valve 268 is closable such that reactor personnel are shielded from the high radiation (gamma) fields from the cesium trap 202 once the cover 208 is removed. The removal cask 264 is mated to the floor valve 268 and the floor valve 268 opened to provide access into interior chamber 204 of cell 206 while maintaining containment thereof. For example, the floor valve 268, cover 208, and removal cask 264 may all be positionable via a crane (not shown) within the reactor 100 and remotely operable. In alternative examples, the floor valve 268, cover 208, and removal cask 264 are movable via any other system that enables the cesium trap 202 to be removed and replaced as described herein. When the removal cask 264 is mated to the floor valve 268, the cesium trap 202 may be remotely decoupled from the shielded cell 206. To decouple the cesium trap 202, the sodium lines 214, the lateral support anchors 246, and the power and control attachments 256 and 258 extending to the cesium trap 202 are removed. For example, sodium line tool 278 is remotely operated to extend into interior chamber 204 and to decouple the cesium trap 202 from the sodium processing circuit 140. The sodium line tool 278 crimps both the inlet and outlet sodium lines 218 and 222 extending from the cesium trap 202 and cuts the sodium lines 218 and 222 adjacent to a crimped portion 284. As such, each sodium line 218 and 222 now includes two portions, a first portion 286 that remains extending from cesium trap 202 and a second portion 288 that remains part of the sodium processing circuit 140. Anchor tool 280 is remotely operated to extend into interior chamber 204 and decouple the lateral support anchor 246 extending between the cesium trap 202 and the cell 206. The anchor tool 280 removes connection member 254, for example, a pin or a bolt, such that trap anchor 252 is decoupled from cell anchor 248. Attachment tool 282 is remotely operated to extend into interior chamber 204 and disconnect the electrical power and instrument control attachments 256 and 258 from the cesium trap 202. The attachment tool 282 unplugs attachments 256 and 258 from the associated receivers 260. By remotely decoupling the cesium trap 202, direct access to the radioactive cell 206 is reduced. Additionally, by locating each of the sodium line 214, lateral support anchor 246, and attachments 256 and 258, proximate the top portion 220 ease of remote access increases, thus reducing replacement time. To remove the cesium trap 202 from the shielded cell 206, the lifting tool 270 extends into interior chamber 204 and is releasably coupled to the cesium trap 202. For example, lifting tool 270 rotates and lifts hooks 276 into the corresponding lifting eyes 262 such that cesium trap 202 is movable. The lifting tool 270 vertically lifts the cesium trap 202 away from the cell 206 and into the removal cask 264 such that the base 236 slidably disengages with the locating pins 240. As the lifting tool 270 vertically lifts the cesium trap 202, the inlet cooling line 228 is also decoupled from the fixed cooling line 224 as the free end 232 automatically slidably disengages with the fixed cooling line 224. In alternative examples, the inlet cooling line 228 may be cut to decouple the cesium trap 202, similar to the procedure with the sodium lines 214. Additionally, the trap anchor 252 is positioned above the cell anchor 248 so that the cesium trap 202 may be vertically lifted. Once the cesium trap 202 is within the removal cask 264, the cesium trap 202 with cesium therein and the removal cask 264 may be moved to another location, either on-site or to a remote site, for further processing and/or long-term storage. Additionally, once the radioactive cesium is removed, the cell 206 may be directly accessed for installing a new cesium trap 202. With the interior chamber 204 of cell 206 empty, a new replacement cesium trap, such as cesium trap 202 that is similar to the cesium trap removed but without any cesium therein, may be inserted into the shielded cell 206 using typical personnel and rigging methods on the overhead crane. The replacement cesium trap 202 is releasably coupled to a lifting tool, for example, the lifting tool 270 via corresponding hooks 276 and lifting eyes 262 such that the replacement cesium trap 202 may be placed into the interior chamber 204 of the cell 206. In alternative examples, the replacement cesium trap 202 is coupled to a different lifting tool, such as the crane within the reactor 100. As the replacement cesium trap 202 is lowered into the cell 206, the base 236 is simultaneously aligned with the locating pins 240 to position the replacement cesium trap 202 along the floor 242. By aligning the locating pins 240, the free end 232 of the inlet cooling line 228 is automatically slidably engaged with the fixed cooling line 224. Once the replacement cesium trap 202 is placed within the cell 206, the replacement cesium trap 202 is attached to the shielded cell 206. This attachment may be performed with direct access to the cell because no radioactive cesium is present therein. To attach the replacement cesium trap 202, the sodium lines 214, the lateral support anchors 246, and the power and control attachments 256 and 258 are extended to the cesium trap 202. For example, the replacement cesium trap 202 is coupled to the sodium processing circuit 140. The first portion 286 of the sodium line 214 extending from the cesium trap 202 is aligned and welded to the second portion 288 of the sodium line extending from the sodium processing circuit, thus forming the inlet and outlet sodium lines 218 and 222. The cell anchor 248 is coupled to the trap anchor 252 via a connection member 254, such as a pin or a bolt, forming the lateral support anchor 246. The electrical power and instrument control attachments 256 and 258 are connected, via plugging, to corresponding receivers 260. Once the replacement cesium trap 202 is attached to the shielded cell 206, the sodium processing circuit 140, and other piping and electrical connections, the cover 208 is replaced, and the replacement cesium trap 202 may begin to receive the sodium stream 216 and facilitate filtering and extracting cesium out of the sodium stream 216 as discussed further below. FIG. 4 is a perspective view of an example cesium trap 300 that includes a body 302 at least partially surrounded by a cooling jacket 304, similar to the cesium trap described above. The body 302 includes an inlet line 306 and an outlet line 308 such that a stream of sodium 310 is channeled therethrough. Within body 302, the cesium trap 300 includes a column of active material 312. For example, the active material is Reticulated Vitreous Carbon (RVC) that adsorbs cesium and removes the cesium from the sodium stream 310. Body 302 also includes a filter 314 that filters RVC particles that may become entrained within the sodium stream 310. Additionally, the cooling jacket 304 includes an inlet line 316 and an outlet line 318 such that a stream of coolant 320 is channeled therethrough. For example, the coolant 320 is nitrogen that reduces the operating temperature of the cesium trap 300. In alternative examples, the inlet line 306 and/or the outlet line 308 may extend into the cesium trap 300 such that the level of sodium therein to be lowered in preparation for disposal. In further alternative examples, the cesium trap 300 may include a capped line (not shown) that facilitates draining the sodium therein in a purpose built hot cell preparation facility and inserting material to reduce the chemical reactivity of sodium, such as lead, in preparation for disposal. In an example of the cesium trap 300, the filter 314 that prevents RVC from exiting the trap has a mean pore size of from 40 to 160 μm. In particular, a filter 314 having a mean pore size from a lower limit of any of 40, 50, 60, 70, 80, or even 90 μm to an upper limit of any of 100, 110, 120, 130, 140, 150 or even 160 μm are contemplated. Through testing, the above ranges of mean pore size where determined to be more effective in maintaining adequate filtration and throughput of the anticipated sodium and entrained argon stream to be passed through the cesium trap 300. Mean pore sizes of 25 or less for the filter 314 have been determined to have reduced throughput when there is significant amounts of argon entrained in the sodium to be passed through the cesium trap 300. This limits the power output of the reactor 100. The filter 314 may be any appropriate filter media. In an example, the filter 314 is a sintered metal filter. The sintered metal filter may be made of one or more of stainless steel, Hastelloy®, Monel®, Inconel®, nickel, HT-9, and titanium to name but a few possible metals. Alternatively, other filter types and filter media may be used having the mean pore size as described above. FIG. 5 is a flowchart illustrating a method 400 of replacing a cesium trap. The method 400 includes a first cesium trap at least partially containing cesium therein being frozen 402. The first cesium trap is decoupled 404 from a shielded cell and then removed 406 from the shielded cell. A second cesium trap is inserted 408 into the shielded cell and then attached 410 to the shielded cell. Decoupling 404 the first cesium trap includes removing lateral support anchors, sodium lines, and power and electrical connections extending thereto. For example, the first cesium trap may be remotely decoupled 412 from a lateral support anchor. As such, a connection member may be removed 414 between a cell anchor and a trap anchor. The first cesium trap may be remotely decoupled 416 from a sodium processing circuit. As such, a sodium line is crimped 418 and cut 420. The first cesium trap may be also remotely disconnected 422 from electrical power and instrument control attachments. As such, the electrical power and instrument control attachments are unplugged 424 from a corresponding receiver. Removing 406 the first cesium trap includes releasably coupling 426 the first cesium trap to a lifting tool. For example, the lifting tool may include a hook that is rotated and lifted 428 into a corresponding lifting eye of the cesium trap. The first cesium trap is then lifted 430 by the lifting tool out of the shielded shell. The lifting operation may also simultaneously decouple 432 a cooling line inlet from a fixed cooling line. Inserting 408 the second cesium trap includes coupling 434 the second cesium trap to a lifting tool, for example, via a hook extending from the lifting tool and a corresponding lifting eye of the cesium trap. The second cesium trap is then placed 436 into the shielded cell by the lifting tool, and simultaneously a base of the second cesium trap is aligned 438 with at least one locating pin extending from the floor of the shielded cell. Attaching 410 the second cesium trap includes re-coupling lateral support anchors, sodium lines, and power and electrical connections thereto. For example, a cell anchor may be coupled 440 to a trap anchor forming a lateral support anchor within the cell. A first sodium line of the second cesium trap may be welded 442 to a second sodium line of the sodium processing circuit forming the sodium lines of the second cesium trap. Additionally, the electrical power and instrument control attachments may be connected 444 into a corresponding receiver.

abstract

Methods for producing Tc-99m radioisotope by proton irradiation of a fluid target matrix. A method of producing Tc-99m includes irradiating a fluid target matrix comprising Mo-100 with a proton beam to transform at least a portion of Mo-100 to Tc-99m. Optionally, the fluid target matrix further includes at least one of O-18, O-16, or N-14, which upon exposure to the proton beam concurrently transform at least a portion of O-18 to F-18, at least a portion of O-16 to N-13, at least a portion of the O-16 to O-15, or at least a portion of N-14 to C-11. The method further includes isolating Tc-99m and optionally at least one of F-18, N-13, O-15, or C-11 from the irradiated fluid target matrix. An additional source of Tc-99m is available from the decay of Mo-99 that is co-produced from the Mo-100 during irradiation with the proton beam.

description

A detailed description will be presented below for a nuclear fuel assembly designed to be employable either for a UO2/light water reactor or for a MOX nuclear fuel/light water reactor in accordance with three independent embodiments of this invention. Referring to FIG. 4, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 fuel or MOX nuclear fuel in accordance with the first embodiment of this invention, will be described below. A thermal reactor currently in operation in any country in the world is required to observe the laws and regulations regarding the length of operation cycle thereof and the burn-up thereof, effective in the specific country in which the specific reactor is operating. Thus, possibilities exist that the entire requirements of this invention may not be allowed. In the first embodiment, however, a serious attention is paid to realize the feature and advantages of this invention as much as possible independently from the current legal restriction. In other words, the MOX enrichment grade of the only one kind of MOX nuclear fuel rods is selected as high as 14 weight % and the ratio of the quantity of the MOX rods with respect to the total quantity of the nuclear fuel rods, is selected to be as low as 33%. Referring to FIG. 4 illustrating a horizontal cross-section of a nuclear fuel assembly in accordance with the first embodiment of this invention, the assembly having MOX nuclear fuel rods and U O2 rods and having a burn-up capacity of 70 GWd/ton (heavy metal mass ton), symbol 1 shows highly enriched MOX nuclear fuel rods each of which contains U235 in 0.225 weight % and the fissionable Pu-s or Pu239 and Pu241 in approximately 14 weight %. The quantity employed is 24. This is the only one kind of MOX nuclear fuel rods employed for this assembly. Symbols 2 and 3 show UO2 rods each of which kinds contains U235 in 4.9 weight % and 4.5 weight % respectively. The quantity employed is 24 and 4 respectively. Symbol G shows gadolinium rods each of which contains U235 in 3.5 weight % and gadolinium in 3.5 weight % respectively. The quantity employed is 20. Thus, the MOX nuclear fuel rods accounts for 33% of the total quantity of the nuclear fuel rods contained in the nuclear fuel assembly. Each rod is approximately 4 m in length and approximately 11 mm in the external diameter. The nuclear fuel assembly is a square of which the length of each side is approximately 15 cm. The production process of the foregoing nuclear fuel assembly in accordance with this invention is nearly identical to that which is presently available. Firstly, Purex process or some other dry or wet spent nuclear fuel reprocessing processes are employed to separate Pu-s out of a spent fuel. The quantity of fissionable Pu-s or Pu239 and Pu241 contained in the separated Pu-s which usually is determined by a mass analysis process is 60 through 70%. An oxidation process is conducted to produce Pu-oxides. Secondly, the powder of PuO2 and UO2 are mixed to make the resultant grade of enrichment to a desired value. Thirdly, MINAS process or SBR process is conducted to well commingle the powder of PuO2 and UO2. Molding and sintering process are conducted to convert the powder of PuO2 and UO2 to sintered pellets of PuO2 and UO2. The product pellets are charged in a zircaloy sheath and the both ends thereof are sealed to produce fuel rods. MOX fuel rods, UO2 rods and other parts are fabricated to finish nuclear fuel assemblies. As is illustrated in FIG. 4, only one kind of MOX fuel rods 1 of which the enrichment grade is as high as 14 weight % are arranged at the area at which the effects of the moderator are less. Since the rods represented by symbols 2 and 3 are UO2 rods and since the rods represented by symbol 1 alone are MOX fuel rods, the ratio of the quantity of MOX fuel rods with respect to the total quantity of the nuclear fuel rods is as high as 33%. As a result, the grade of enrichment of 14 weight % is remarkably higher than that of the prior art of 5 weight %. On the other hand, the ratio (33%) of the quantity of the MOX fuel rods with respect to the total quantity of the nuclear fuel rods is remarkably less in comparison with that of the prior art or 80%. The function of the gadolinium fuel rods is identical to that of the prior art. Namely, it is to restrict fission to occur at the beginning of a reactor operation period. In other words, the gadolinium fuel rods are effective to reduce the possibility of fission to occur at the beginning of the reactor operation period but thereafter they lose the function and transit themselves to a fissionable fuel. In conclusion, the nuclear fuel assembly in accordance with this embodiment is provided with only one kind of MOX nuclear fuel rods each of which has remarkably large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, and the quantity of the MOX nuclear fuel rods is remarkably small. As was described earlier, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. Referring to FIG. 5, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 fuel alone or MOX nuclear fuel in accordance with the second embodiment of this invention, will be described below. As was described earlier, a thermal reactor currently in operation in any country in the world is required to observe the laws and regulations regarding the length of operation cycle thereof and the burn-up thereof, effective in the specific country in which the specific thermal reactor is operating. In the second embodiment as well, however, a serious attention is paid to realize the feature and advantages of this invention as much as possible within the limitation to observe the legal restriction presently effective generally in the world. In other words, the MOX enrichment grade is selected to be 6 weight % and the ratio of the quantity of the MOX rods with respect to the total quantity of the nuclear fuel rods, is selected to be 25%. Referring to FIG. 5 illustrating a horizontal cross-section of a nuclear fuel assembly in accordance with the second embodiment of this invention, the assembly having MOX nuclear fuel rods and UO2 nuclear fuel rods and a burn-up capacity of 45 GWd/ton (heavy metal mass ton), symbol 1 shows highly enriched MOX nuclear fuel rods each of which kinds contains U235 in 0.225 weight % and the fissionable Pu-s or Pu239 and Pu241 in 6 weight %. The quantity employed is 16. This is the only one kind of MOX nuclear fuel rods employed for this assembly. Symbols 2, 3 and 4 show UO2 fuel rods each of which kinds contains U235 in 4.0 weight %, 3.5 weight % and 3 weight % respectively. The quantity employed is 28, 8 and 4 respectively. Symbol G shows gadolinium rods each of which contains U235 in 2 weight % and gadolinium in 2 weight % respectively. The quantity employed is 16. Thus, the MOX nuclear fuel rods accounts for 25% of the total quantity of the nuclear fuel rods employed in the nuclear fuel assembly. The production process of the foregoing nuclear fuel assembly in accordance with this embodiment is entirely identical to that for the first embodiment. As is illustrated in FIG. 5, only one kind of MOX fuel rods 1 of which kind the enrichment grade is 6 weight % are arranged at the area at which the effects of the moderator are less. Since the rods represented by symbols 2, 3 and 4 are UO2 fuel rods and since the rods represented by symbol 1 alone are MOX fuel rods, the ratio of the quantity of MOX fuel rods with respect to the total quantity of the nuclear fuel rods is 25%. As a result, the grade of enrichment of 6 weight % is higher than that of the prior art or 5%. On the other hand, the ratio (25%) of the quantity of the MOX fuel rods with respect to the total quantity of the nuclear fuel rods is remarkably less than that of the prior art or 80%. The function of the gadolinium fuel rods is identical to that of the prior art. Namely, it is to restrict fission to occur at the beginning of the reactor operation period. In conclusion, the nuclear fuel assembly in accordance with this embodiment is provided with only one kind of MOX nuclear fuel rods each of which only one kind has large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, and the quantity of the MOX nuclear fuel rods is small. As was described earlier, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. Referring to FIG. 6, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 fuel alone or MOX nuclear fuel in accordance with the third embodiment of this invention, will be described below. As was described earlier, a thermal reactor currently in operation in any country in the world is required to observe the laws and regulations regarding the length of operation cycle thereof and the burn-up thereof, effective in the specific country in which the specific thermal reactor is presently operating. In the third embodiment, the best efforts are used to realize the feature and advantage of this invention as much as possible within the limitation of the design of the presently operating reactor. Referring to FIG. 6 illustrating a horizontal cross-section of a nuclear fuel assembly in accordance with the third embodiment of this invention, the assembly having MOX fuel rods and UO2 fuel rods and having a burn-up capacity of 45 GWd/ton (heavy metal mass ton), symbol 1 shows highly enriched MOX nuclear fuel rods each of which contains U235 in 0.225 weight % and the fissionable Pu-s or Pu239 and Pu241 in 6 weight %. The quantity employed is 24. This is the only one kind of the MOX nuclear fuel rods employed for this assembly. Symbols 2, 3 and 4 show UO2 fuel rods each of which kinds contains U235 in 4 weight %, 3.5 weight % and 3 weight % respectively. The quantity employed is 20, 8 and 4 respectively. Symbol G shows gadolinium rods each of which contains U235 in 2 weight % and gadolinium in 2.2 weight % respectively. The quantity employed is 16. Thus, the quantity of the MOX nuclear fuel rods accounts for 33% of the total quantity of the nuclear fuel rods employed in the assembly. The production process of the foregoing nuclear fuel assembly in accordance with this embodiment is entirely identical to that for the first and second embodiments. As is illustrated in FIG. 6, only one kind of MOX fuel rods 1 of which only one kind the enrichment grade is 6 weight % are arranged at the area at which the effects of the moderator are less. Since the rods represented by symbols 2, 3 and 4 are UO2 fuel rods and since the rods represented by symbol 1 alone are the MOX fuel rods, the ratio of the quantity of the MOX fuel rods with respect to the total quantity of nuclear fuel rods is 33%. As a result, the grade of enrichment of 6 weight % is higher than that of the prior art or 5%. On the other hand, the ratio (33%) of the quantity of the MOX fuel rods with respect to the total quantity of the nuclear fuel rods is remarkably less than that of the prior art or 80%. The function of the gadolinium fuel rods is identical to that of the prior art. As is identical to the first and second embodiments, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. In conclusion, the nuclear fuel assembly in accordance with this embodiment is provided with only one kind of MOX nuclear fuel rods each of which only one kind has relatively large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, and the quantity of the MOX nuclear fuel rods is small. As was described earlier, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. The above description has clarified that this invention has successfully provided an improvement applicable to a nuclear fuel assembly employable either for a thermal neutron reactor employing UO2 as the nuclear fuel and light water as the moderator/coolant or for a thermal neutron reactor employing the MOX nuclear fuel as the nuclear fuel and light water as the moderator/coolant, wherein the production cost is much less and the value of the spent fuel thereof is much larger than that of the nuclear fuel assembly available in the prior art.

description

A series of cylindrical extruded uranium rods about 8 inches long and 1.360 inches in diameter, after cleaning by a nitric acid pickling treatment, washing and drying as pre-viously described, were subjected to the following canning procedure: Each rod was immediately dipped through a xc2xdinch depth of potassium-sodium lithium chloride flux of the composition 53% potassiwm chloride, 42% lithium chloride and 5% sodium chloride, by weight, into a molten bronze bath consisting of 53 parts of tin and 47 parts of copper, by weight, at a tem-perature of 720xc2x0 C. for 45 seconds. Upon removal from the bronze bath each rod, which was uniformly coated with a bronze coating, was immersed in a bath of molten tin at 640xc2x0 C. for 20 seconds. Immersion in the tin bath was effected by dropping the rod onto the high end of a sloping wire rack sunk in the tin bath, allowing the rod to roll to the lower end, and removing it by a pair of tongs from the molten metal where the surface had just been carefully scraped to remove all traces of flux. Upon removal from the tin bath the rod was centrifuged at 640 rpm, in a centrifuge supporting the rod at about 6 inches from the axis of rotation, for 5 seconds to remove excess tin and then immersed in a bath of 0.1% sodium-modified, degassed 13X aluminum-silicon alloy (Federal specification AN-QQ-A-366, Amendment 4 Al-13X) at 600xc2x0 C. for 6 seconds. While the rod was being coated in this manner a 2S aluminum can which had an inside diameter 15 mils greater than the rod and had been cleaned by washing the can first with trichlorethene, then with an aqueous 0.1% soap solution containing 0.1% sodium pyrophosphate at 80xc2x0 C., and finally with aqueous 20% o-phosphoric acid solution for 5 minutes at 20xc2x0 C. was heated in a chromium steel supporting sleeve to a temperature of 640xc2x0 C. and 80 grams of 13X aluminum-silicon alloy and an aluminum plug xc2xdinch thick, with a hole through its center, preheated to about 630xc2x0 C. and about 640xc2x0 C., respectivey, were added to the can just before the metallic uranium rod was withdrawn from the aluminum-silicon bath. The plug was used to space the rod from the end of the can and provide good heat conduct-ivity at this location. It was inserted on the molten 13X alloy in the can by means of a rod having a tapered end fitting the hole in the plug snugly and having a sleeve for forcing the plug off the end. The coated uranium rod was passed from the bath immediately into the can and a slightly tapered aluminum cap about 5/16 inch thick, which had been preheated to a temperature of about 600xc2x0 C., was inserted into the molten aluminum-silicon alloy filling the open end of the can. The complete assembly was then immediately quenched by immersion in water and the canned rod was removed from its supporting sleeve. In this manner a firm uniform bond between the uranium rod and the protective metallic can was obtained. The total elapsed time from immersion of the uranium rods in the bronze bath to their immersion in the quench tank was about 80-90 seconds. FIG. 1 is photomicrograph at 500 magnifications show-ing a section of the bond between one of the uranium rods of this series and its protective can. Another series of uranium rods of the same size as employed in EXAMPLE 1 was canned under similar conditions with the following changes. After the rods were centrifuged to remove excess tin, they were dipped into a molten 1.0% sodium-modified, degassed aluminum-silicon alloy containing 88% aluminum and 12% silicon (before modifying) at 600xc2x0xc2x15xc2x0 C. for 6 seconds. They were then inserted in aluminum cans, containing 70 grams of 0.1% sodium-modified, degassed aluminum-silicon alloy of the same composition, maintained at 590xc2x0 C. The cans were capped and quenched as in the preceding example. A 1.360xe2x80x3xc3x978xe2x80x3 metallic uranium rod having its ends machined down to accommodate ferrules was coated as described in EXAMPLE 1. Upon removal of the coated rod from the alum-inum-silicon bath it was immediately capped by aluminum caps pressed over the machine ends, redipped in the aluminum-silicon bath for b 2 seconds, then placed in the valley formed by a pair of smooth xe2x80x9cTransitexe2x80x9d (asbestos cement) rollers rotating at about 200 ft. per minute peripheral velocity. Sixty-five grams of the modified degassed 88/12 aluminum silicon alloy at a temperature of 640xc2x0 C. was poured into the trough formed between the uranium rod and one of the xe2x80x9cTransitexe2x80x9d rollers. When all of the aluminum-silicon had solidified, the rod was removed from the rollers and quenched by immersing it in water. It was then machined down to a 1.42 inch diameter to provide a smooth, even 30-mil coating. A small metallic uranium rod, prepared for coating by pickling in nitric acid solution as previously described, was immersed for 45 seconds in a speculum metal bath (67% copper and 33% tin) at 810xc2x0 C. During this period the bath cooled to 795xc2x0 C. The rod was withdrawn from the bath and immersed in a eutectic bronze bath (47% copper and 35% tin) for about 1 minute at 710xc2x0 C. It was withdrawn from this bath and centrifuged for 8 seconds at about 640 rpm. It was then quenched by immersion in water. The bronze coated rod was dried and than dipped for 30 seconds in an aluminum-silicon bath comprising 88 parts of aluminum and 12 parts silicon at a temperature of 593xc2x0 C. The coated rod was rolled until the coating solidified, then washed with water. A firm over coating was obtained. A cross-section of the coated rod, showing the bond between the rod and the coating is illustrated in the photomicrograph, FIG. 2 of the drawing, at 100 magni-fications. A metallic uranium rod prepared for coating as previously described was immersed for 40 seconds in a bronze bath con-sisting of 47 parts of copper and 53 parts of tin at 730xc2x0 C. During the period of immersion the bath cooled from 730xc2x0 C. to 720xc2x0 C. The coated rod was then immersed in a tin bath at 420xc2x0 C. for 10 seconds. The tin bath was covered with a 53% KCl, 42% LiCl, 5% NaCl flux. Upon withdrawal from the tin bath the rod was centrifuged for 5 seconds at about 640 rpm. It was then dipped quickly into an aluminum-silicon alloy bath containing 88% aluminum and 12% silicon and quickly withdrawn. The rod was dipped seven times in this bath, which was held at a temperature of 615xc2x0 C. After the seventh dip it was withdrawn and laid on xe2x80x9cTransitexe2x80x9d rollers and 35 grams of additional aluminum-silicon alloy of the same composition was poured into the trough between the rod and the roller. The rod was rotated for 10 seconds until the aluminum-silicon had solidified and was then quenched by immersion in water. FIG. 3 of the drawing is a photomicrograph of a cross-section of the coated rod at 100 magnification. A small uranium rod, after pickling in nitric acid, washing and drying, as previously described, was immersed in a speculum metal bath at 850xc2x0 C. for 1xc2xd minutes during which the bath cooled to 840xc2x0 C. The rod, upon withdrawal from this bath, was immersed in peritectic bronze at 700xc2x0 C. for xc2xd minute, centrifuged for 15 seconds at 640 rpm, then quenched in water. The rod was coated with a continuous uniform protective bronze coating. Rods coated in this manner are afforded a substantial degree of corrosion resistance by the bronze coating. A bronze-coated rod prepared as described in EXAMPLE 6 was dipped in 88/12 almuminum-silicon alloy at 595-600xc2x0 C. for 30 seconds. Upon withdrawal from the aluminum-silicon alloy bath it was rotated at about 200 ft. per minute on xe2x80x9cTransitexe2x80x9d rollers for 15 seconds then quenched in water. The rod was coated with an aluminum-silicon coating over bronze. An extruded uranium rod was pickled for one minute in 50% HNO3 at room temperature, washed, dried and then dipped through a top flux of 50% NaCl and 50% KCl into a bronze bath containing 65% copper, 33% tin, and 2% nickel. After immersion in this alloy for 60 seconds, the rod was removed, dipped in a 50% LiCl, 40% KCl, 10% NaCl bath at about 550xc2x0 C. for about xc2xd minute then cooled. The rod was completely coated with a continuous corrosion-resistant bronze coating. An extruded uranium rod was dipped through a flux contain-ing 66% CaCl2, 29% NaCl, and 5% KCl into a molten bronze con-sisting of 53% copper and 47% tin to which 5% of nickel (based on the weight of the bronze) had been added. The bath was at 840xc2x0 C. After three minutes in the bath the rod was withdrawn, cooled and cleaned anodically in 98% H2SO4 solution. A polished protective bronze coating was thus obtained on the metal. A metallic uranium rod 1.1 inches in diameter and 4 inches long was first dipped through a chloride flux contain-ing 53% potassium chloride, 42% lithium chloride, and 5% sodium chloride, into a 47% copper 53% tin bath at a tempera-ture between 740xc2x0 and 760xc2x0 C. for 20 seconds. Upon withdrawal from the bronze bath the rod was centrifuged for 5 seconds to remove excess metal, it was then immersed in a salt bath having the same composition as the flux on the bronze bath and main-tained at 605xc2x0 C. After 20 seconds in this salt bath the rod was withdrawn and immersed in an unmodified 88% aluminum 12% silicon bath at 635xc2x0 C. for 20 seconds. The rod was then placed on xe2x80x9cTransitexe2x80x9d rollers and rolled slowly while 22 grams of the aluminum-silicon alloy was poured into the trough be-tween one of the rollers and the rod. Thirty-two grams of 88/12 aluminum-siliccn alloy modified by the addition of 1% of zinc and 0.02% of sodium was then poured on and the roll-ing was continued until the coating had solidified. The rod was then quenched by immersion in water. When cool the coat-ing was machined to a uniform 30 mil thickness. The resulting coating formed a protection for tle metal, which was resistant to the corrosive action of hot aqueous hydrogen peroxide solution. It will be understood that we intend to include variations and modification of the invention and that the preceding examples are illustrations only and in no wise to be construed as limitations upon the invention, the scope of which is defineed in the appended claims, wherein

047117583

summary

BACKGROUND OF THE INVENTION The present invention is related to the long-term storage of spent fuel that has been removed from a nuclear reactor, and more particularly, to a spent fuel storage cask having a basket which supports the spent fuel and which dissipates heat generated by the spent fuel. The basket includes a plurality of grid assemblies which provide storage slots for the spent fuel and which conduct heat to the walls of the cask. FIG. 1 illustrates a typical fuel assembly 20 for supplying nuclear fuel to a reactor. Assembly 20 includes a bottom nozzle 22 and a top nozzle 24, between which are disposed elongated fuel rods 26. Each fuel rod 26 includes a cylindrical housing made of a zirconium alloy such as commercially available "Zircalloy-4", and is filled with pellets of fissionable fuel enriched with U-235. Within the assembly of fuel rods 26, tubular guides (not shown) are disposed between nozzles 22 and 24 to accommodate movably mounted control rods (not illustrated) and measuring instruments (not illustrated). The ends of these tubular guides are attached to nozzles 22 and 24 to form a skeletal support for fuel rods 26, which are not permanently attached to nozzles 22 and 24. Grid members 28 have apertures through which fuel rods 26 and the tubular guides extend to bundle these elements together. Commercially available fuel assemblies for pressurized water reactors include between 179 and 264 fuel rods, depending upon the particular design. A typical fuel assembly is about 4.1 meters long, about 19.7 cm wide, and has a mass of about 585 kg, but it will be understood that the precise dimensions vary from one fuel assembly design to another. After a service life of about 3 years in a pressurized water reactor, the U-235 enrichment of a fuel assembly 20 is depleted. Furthermore, a variety of fission products, having various half-lives, are present in rods 26. These fission products generate intense radioactivity and heat when assemblies 20 are removed from the reactor, and accordingly the assemblies 20 are moved to a pool containing boron salts dissolved in water (hereinafter "borated water") for short-term storage. Such a pool is designated by reference number 30 in FIG. 2. Pool 30 is typically 12.2 meters deep. A number of spent fuel racks 32 positioned at the bottom of pool 30 are provided with storage slots 34 to vertically accommodate fuel assemblies 20. A cask pad 36 is located at the bottom of pool 30. During the period when fuel assemblies 20 are stored in pool 30, the composition of the spent fuel in rods 26 changes. Isotopes with short half-lives decay, and consequently the proportion of fission products having relatively long half-lives increases. Accordingly, the level of radioactivity and heat generated by a fuel assembly 20 decreases relatively rapidly for a period and eventually reaches a state wherein the heat and radioactivity decrease very slowly. Even at this reduced level, however, rods 26 must be reliably isolated from the environment for the indefinite future. Dry storage casks provide one form of long-term storage for the spent fuel. After the heat generated by each fuel assembly 20 falls to a predetermined amount--such as 0.5 to 1.0 kilowatt per assembly, after perhaps 10 years of storage in pool 30--an opened cask is lowered to pad 36. By remote control the spent fuel is transferred to the cask, which is then sealed and drained of borated water. The cask can then be removed from pool 30 and transported to an above-ground storage area for long-term storage. The requirements which must be imposed on such a cask are rather severe. The cask must be immune from chemical attack during long-term storage. Furthermore, it must be sufficiently rugged mechanically to avoid even tiny ruptures or fractures during long-term storage and during transportation, when the cask might be subjected to rough treatment or accidents such as drops. Moreover, the cask must be able to transmit heat generated by the spent fuel to the environment while nevertheless shielding the environment from radiation generated by the spent fuel. The temperature of the rods 26 must be kept below a maximum temperature, such as 375.degree. C., to prevent deterioration of the zirconium alloy housing. Provisions must also be made to ensure that a chain reaction cannot be sustained within the cask; that is, that the effective criticality factor K.sub.eff remains less than one so that a self-sustaining reaction does not occur. These requirements impose stringent demands upon the cask, which must fulfill its storage function in an utterly reliable manner. SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a mechanically rugged storage cask which prevents fission products and radiation from escaping into the environment while dissipating heat generated by spent fuel. Another object of the present invention is to provide a storage cask having a basket with a plurality of grid assemblies for supporting spent fuel, either in the form of fuel assemblies or consolidated fuel, or both, and for conducting heat generated thereby to the walls of the cask. Another object of the present invention is to provide a storage cask having a basket with disk-shaped grid assemblies which are spaced apart from one another in a column and which expand, after the basket is inserted into a container to form the cask, so that they come into contact with corresponding rings on the interior walls of the container in order to transmit heat from the basket to the rings, the diameters of the grid assemblies in the column and their corresponding rings decreasing slightly from the top of the column to the bottom in order to facilitate insertion of the basket into the container. Another object of the present invention is to provide a basket having legs which support grid assemblies at spaced-apart positions, the grid assemblies being movably mounted on the legs by top and bottom rings which are affixed to the legs and which are spaced further apart than the thickness of the grid assemblies. Another object of the present invention is to provide heat-transmitting wedges for positioning cells within the basket while permitting differential expansion of the cells with respect to the basket, the cells in turn enclosing spent fuel assemblies. These and other objects can be attained by providing a container having a cavity defined by substantially cylindrical walls. A basket disposed in the cavity has a plurality of substantially disk-shaped grid assemblies which are mounted at different vertical positions above the container floor. Each grid assembly includes plates which are joined together to provide a matrix of apertures and metal elements which are affixed to the plates to provide a substantially circular periphery. The apertures provided by the matrixes of the grid assemblies are aligned to provide storage slots for accepting spent fuel. When the cask elements are fabricated at normal shop temperatures, the diameters of the grid assemblies are slightly less than the diameter of the inner cask wall, so that the basket can be inserted into the container. When spent fuel is loaded and stored, however, the temperature within the cask rises and the grid assemblies expand with respect to the container so that the peripheries of the grid assemblies come into heat-conducting contact with the container walls. In accordance with one aspect of the invention, the container walls include vertically spaced rings which project slightly into the cavity. Each ring corresponds to a grid assembly and is positioned adjacent the periphery of the corresponding grid assembly. At the time of fabrication the diameter of a ring is slightly (e.g., about 0.3 cm) greater than the diameter of the corresponding grid assembly, and in order to alleviate the risk that the basket might jam while it is being inserted into the container, the diameter of each grid assembly and the diameter and of its corresponding ring are slightly less than the diameters of the grid assembly and ring above it. The changing diameters not only afford a greater latitude for error during the initial stages of insertion, they also provide visual guides for correcting the alignment as the insertion process progresses. In accordance with another aspect of the invention, the basket includes legs to which the grid assemblies are mounted at spaced-apart positions. Four of the metal elements of each grid assembly are provided with holes through which the legs extend. The grid assemblies are confined between top and bottom rings affixed to the legs on either side of the holes. The distance between the rings is slightly greater than the thickness of the grid assemblies in order to permit differential expansion of the elements as the temperature within the cask rises. In accordance with another aspect of the invention, open-ended cells for enclosing fuel assemblies can be inserted into the storage slots of the basket. Each cell has four walls which have dimensions corresponding to those of the fuel assembly to be enclosed and which support "neutron poison." The cells are positioned in the storage slots by heat-conducting wedges which are welded to the plates of the grid assemblies and which are spaced apart slightly from two of the cell walls. This slight spacing allows differential expansion of the cells with respect to the basket but permits transfer of heat to the basket, particularly if the cask is flooded with helium. In accordance with yet another aspect of the invention, the rings on the container wall are wider than the thicknesses of the corresponding grid assemblies, and moreover beveled surfaces are provided on both the rings and the peripheries of the grid assemblies in order to facilitate insertion and/or removal of the basket.

description

Pursuant to 35 U.S.C. § 119(a), this application claims the benefit of the earlier filing date and the right of priority to Korean Patent Applications No. 10-2017-0059996, filed on May 15, 2017, the contents of which are incorporated by reference herein in their entirety. The present invention relates to a nuclear reactor vessel cooling method, and more particularly, a power generation using heat of a reactor vessel during a normal operation, an emergency power generation using heat of the reactor vessel during an accident, and cooling of the reactor vessel. Nuclear reactors are divided according to installation positions of major components (steam generator, pressurizer, pump, etc.) into loop type reactors (e.g., commercial reactors: domestic) in which such major components are installed outside a reactor vessel and integral reactors (e.g., SMART reactors: domestic) in which the major components are installed inside a reactor vessel. Nuclear power plants are also divided into active plants and passive plants depending on a way of implementing a safety system. An active plant is a plant using an active component such as a pump operated by electric power of an emergency diesel generator (EDG) or the like to drive a safety system, and a passive plant is a plant using a passive component operated by passive power such as gravity, gas pressure or the like to drive the safety system. A passive safety system in a passive plant may maintain the reactor in a safe manner only with a natural force built in the system without an operator action or an AC power source of safety class such as an emergency diesel generator for more than a period of time (72 hours) required by regulatory requirements in the event of an accident, After 72 hours, using an operator action and a non-safety systems might be allowed to maintain the function of the safety systems and an emergency DC power source(battery). Unlike a general thermal power plant where heat generation is stopped when fuel supply is stopped, a reactor in a nuclear power plant generates residual heat from a reactor core for a significant period of time by a fission product produced and accumulated during a normal operation even when a fission reaction is stopped in the reactor core. Accordingly, a variety of safety systems for removing the residual heat of the core during an accident are installed in the nuclear power plant. In case of an active nuclear power plant (Conventional Nuclear Power Plant of Korea), a plurality of emergency diesel generators are provided in preparation for a case of interruption of electric power supply from the inside or outside at the time of an accident, and most active nuclear power plants use a pump to circulate cooling water, and thus a large-capacity emergency AC power source (a diesel generator) is provided due to the high power requirements of those active components. An operator action allowance time in an active nuclear power plant is estimated about 30 minutes. In order to exclude active components such as a pump and the like that require a large amount of electricity, a passive force such as gas pressure or gravity is introduced in a passive nuclear reactor (U.S. Westinghouse AP1000, Korean SMART) that has been developed or is being developed to enhance the safety of the nuclear power plant, and thus a large amount of power is not required other than small components such as a valve, which is essentially required for the operation of a passive safety system. However, to enhance the safety in a passive nuclear power plant, an operator action allowance time is drastically extended from 30 minutes to 72 hours or longer, and an emergency active power source (diesel generator) is excluded, and an emergency DC power source (battery) is adopted. And thus the emergency DC power source should be maintained for more than 72 hours. Therefore, the emergency power source capacity required per unit time in a passive nuclear power plant is relatively small compared to an active nuclear power plant, but it is very large in terms of the battery capacity because the emergency power should be supplied for 72 hours or more. In the other hand, a residual heat removal system (auxiliary feed water system or passive residual heat removal system) is employed as a system for removing the heat of a reactor coolant system (the sensible heat of the reactor coolant system and the residual heat of the core) using a residual heat removal heat exchanger connected to a primary system or secondary system when an accident occurs in various nuclear power plants including an integral reactor. (AP1000: U.S. Westinghouse, commercial loop type nuclear power plant and SMART reactor: Korea) Furthermore, a safety injection system is employed as a system for directly injecting cooling water into the reactor coolant system in case of a loss-of-coolant accident to maintain a water level of the reactor core and removing the heat of the reactor coolant system (the sensible heat of the reactor coolant system and the residual heat of the core) using the injected cooling water. (AP1000: U.S. Westinghouse, commercial loop type and SMART reactor: Korea) In addition, a reactor containment cooling system or spray system is a system for condensing steam using cooling or spraying to suppress a pressure rise when a pressure inside the reactor containment rises due to an accident such as a loss-of-coolant accident or a steam-line-break accident. Additionally, there are a method of directly spraying cooling water into the reactor containment (commercial loop type reactor: Korea), a method of inducing steam discharged in the reactor containment into a suppression tank (commercial boiling water reactor), a method of using a heat exchanger installed inside or outside the reactor containment (reinforced concrete containment building) (APR+: Korea), a method of using a surface of the steel containment vessel as a heat exchanger (AP1000: U.S. Westinghouse), or the like. As such, the nuclear power plant is provided with various safety systems, each of which consists of a plurality of trains of two or more trains, such as the residual heat removal system and the safety injection system for protecting the reactor core by cooling the reactor coolant system (including the reactor vessel) during an accident. However, in recent years, there has been a growing demand for safety enhancement of nuclear power plants due to the impact of Fukushima nuclear power plant (boiling water reactor) accident and the like, and thus there is a rising demand for safety facilities against a severe accident such as an external reactor vessel cooling system even in a pressurized water reactor (PWR) with a very low risk of leakage of large amounts of radioactive materials due to employing a very large-internal-volume nuclear reactor containment. In detail, the nuclear power plant is provided with various safety facilities for relieving accidents upon an occurrence of an accident. In addition, each safety facility consists of redundant trains, and probability that all trains fail simultaneously is very low. However, as public demands for safety of nuclear power plants increase, safety facilities have been enhanced in preparation of severe accidents even with very low probability of occurrence. The external reactor vessel cooling system is a system provided to cool the outside of reactor vessel during core meltdown to prevent damage of the reactor vessel, assuming that a serious damage occurs in the core cooling function and a severe accident that the core is melted occurs since various safety facilities do not adequately perform functions due to multiple failure causes at the time of an accident. (AP1000 Westinghouse of USA) When the reactor vessel is damaged, a large amount of radioactive materials may be discharged into a reactor building, and internal pressure of the reactor building may rise due to an large amount of steam generated by corium(melted core)-water reaction and gas generated by the corium-concrete reaction. The reactor building serves as a final barrier to prevent the radioactive material from being discharged into an external environment during an accident. If the reactor building is damaged due to an increase in internal pressure, a large amount of radioactive materials may be released to the external environment. Therefore, the external reactor vessel cooling system performs a very important function of suppressing radioactive materials from being discharged into the reactor containment and the increase of the internal pressure during a severe accident to prevent radioactive materials from being discharged into an external environment. The external reactor vessel cooling system which is adopted in many countries is a system in which cooling water is filled in the reactor cavity located at a lower part of the reactor vessel and the cooling water is introduced into the cooling flow path in a space between the thermal insulation material and the reactor vessel and then steam is discharged to an upper part of the cooling flow path. In addition, a method of injecting a liquid metal at the time of an accident to mitigate the critical heat flux phenomenon, a method of pressurized cooling water to induce single phase heat transfer, a method of modifying a surface of the external reactor vessel to increase the heat transfer efficiency, a method of forming a forced flow, and the like, may be taken into consideration. In the related art external reactor vessel cooling system, since a thermal insulation material has to perform an appropriate thermal insulation function during a normal operation of the nuclear power plant, a flow path is sealed such that inlet and outlet flow paths formed in the thermal insulating material must be properly opened in a timely manner at the time of an accident. Also, a time delay occurs to fill the reactor cavity, and the heat removal ability may be reduced due to a critical heat flux phenomenon or the like while evaporating cooling water to form a steam layer on the external reactor vessel. In addition, there is also a research on an external reactor cooling method using a liquid metal, but the liquid metal method has difficulties in the maintenance of the liquid metal. In addition, an external reactor cooling method in a pressurization manner has difficulties in the application of a natural circulation flow, and a method of modifying a surface of a reactor vessel has difficulties in the fabrication and maintenance of the surface, and a forced flow method has a disadvantage in that it must be supplied with electric power. In addition, since the external reactor vessel cooling system is operated by an operator action at the time of an accident, various instruments and components for monitoring the accident are required for the operation, and probability that a system in a standby mode fails to operate at the time of an accident is higher than probability that a system being operated is stopped to operate at the time of an accident. Thus, the present invention proposes an external reactor vessel cooling and electric power generation system, in which a large-scale turbine power generation facility in the related art is maintained almost same design, and a small-scale power generation facility is additionally installed to receive heat discharged from the reactor vessel during a normal operation or during an accident of the nuclear power plant and thus produce electricity. One aspect of the present invention is to provide an external reactor vessel cooling and electric power generation system having improved system reliability in which safety class or seismic design is easily applicable, and the reactor vessel cooling is carried out while continuously operating during an accident as well as during a normal operation to produce emergency power. Another aspect of the present invention is to provide an external reactor vessel cooling and electric power generation system having improved safety by removing residual heat of a predetermined scale or more during an accident as well as a normal operation. Another aspect of the present invention is to provide a nuclear power plant with improved economic efficiency and safety due to downsizing and reliability enhancement of an emergency power system. An external reactor vessel cooling and electric power generation system according to the present invention may include a reactor vessel, an external reactor vessel cooling section formed to enclose at least part of the reactor vessel so as to cool heat discharged from the reactor vessel, a power production section including a small turbine and a small generator to generate electric energy using a fluid that receives heat from the external reactor vessel cooling section, a condensation heat exchange section to perform a heat exchange of the fluid discharged after operating the small turbine, and condense the fluid to generate condensed water, and a condensed water storage section to collect therein the condensed water generated in the condensation heat exchange section, wherein the fluid receiving the heat from the reactor vessel may be circulated. In an embodiment, the condensed water in the condensed water storage section may be circulated through the external reactor vessel cooling section, the power production section, and the condensation heat exchange section, and the fluid may be phase-changed into gas by the heat received from the reactor vessel. In an embodiment, the system may further include an evaporation section connected to the external reactor vessel cooling section, to cause a heat exchange between the fluid inside the external reactor vessel cooling section and the condensed water of the condensed water storage section. The system may further include a first circulation part defined between the external reactor vessel cooling section and the evaporation section such that a fluid flows therealong, and a second circulation part defined sequentially along the evaporation section, the power production section, the condensation heat exchange section, and the condensed water storage section, such that a fluid flows therealong. In an embodiment, the first circulation part may be circulated by a single-phase fluid. In an embodiment, the power generation system may be operated during a normal operation of a nuclear power plant and during an accident of the nuclear power plant to produce electric power. The electric power generated during the normal operation of the nuclear power plant may be charged in an internal/external electric power system and an emergency battery. Also, the electric power charged in the emergency battery may be supplied as an emergency power source during the accident of the nuclear power plant. In an embodiment, the electric power generated during the accident of the nuclear power plant may be supplied as an emergency power source of the nuclear power plant. In an embodiment, the emergency power source may be used as power for operating a safety system of the nuclear power plant during the accident of the nuclear power plant, opening and closing a valve for the operation of the safety system, monitoring the safety system, or operating the external reactor vessel cooling and electric power generation system. In an embodiment, seismic design of seismic categories I, II or III may be applied, and safety classes 1, 2 or 3 may be applied. In an embodiment, the external reactor vessel cooling section may be provided with a discharge pipe that connects the external reactor vessel cooling section and the power production section to each other such that the fluid of the external reactor vessel cooling section is applied to the power production section. In an embodiment, the discharge pipe may be provided with a first discharge portion through which at least part of the fluid excessively supplied to the power production section bypasses the small turbine and the small generator. In an embodiment, the discharge pipe may further be provided with a liquid-gas separator that is connected to the discharge pipe such that only gas of the fluid is transferred to the power production section. In an embodiment, the condensation heat exchange section may be provided with a motor or pump that supplies a cooling fluid to the condensation heat exchange section to exchange heat with the fluid. The cooling fluid may include air, pure water, seawater, or a mixture thereof. In an embodiment, the condensed water storage section may be disposed below the condensation heat exchange section to collect the condensed water generated in the condensation heat exchange section. In an embodiment, the condensed water storage section may be connected to the external reactor vessel cooling section through a pipe so that the condensed water is supplied to the external reactor vessel cooling section. In an embodiment, the condensation heat exchange section or the condensed water storage section may be provided with an vent portion through which non-condensable gas accumulated in the condensation heat exchange section or in the condensed water storage section. In an embodiment, the vent portion may remove the non-condensable gas by pressure drop of the Venturi using a fan or a steam flow rate. In an embodiment, a shape of the external reactor vessel cooling section may include a cylindrical shape, a hemispherical shape, and a double vessel shape, or a combination thereof. In an embodiment, a pipe may be connected to an in-containment refueling water storage tank (IRWST) such that refueling water is supplied to the external reactor vessel cooling section. In an embodiment, the external reactor vessel cooling section may be provided with a second discharge portion through which the refueling water supplied from the IRWST is discharged. In an embodiment, a coating member may further be provided to prevent corrosion of the reactor vessel, and a surface of the coating member may be chemically processed to increase a surface area. In an embodiment, the system may further include a heat transfer member to smoothly transfer heat discharged from the reactor vessel and a surface of the heat transfer member may be chemically processed to increase a surface area. In an embodiment, the system may further include a core catcher provided inside the external reactor vessel cooling section to receive and cool corium when the reactor vessel is damaged. A loop or integral type nuclear power plant according to the present invention may include a reactor vessel, an external reactor vessel cooling section formed to enclose at least part of the reactor vessel so as to cool heat discharged from the reactor vessel, a power production section including a small turbine and a small generator to generate electric energy using a fluid that receives heat from the external reactor vessel cooling section, a condensation heat exchange section 140 to perform a heat exchange of the fluid discharged after operating the small turbine and condense the fluid to generate condensed water, and a condensed water storage section to collect therein the condensed water generated in the condensation heat exchange section, wherein the fluid receiving the heat from the reactor vessel may be circulated. The reactor external wall cooling and electric power generation system according to the present invention is configured to enclose the reactor vessel with small-scale facilities so as to produce electric power using heat transferred while cooling the reactor vessel. A phase of fluid is changed from liquid to gas by the transferred heat, and the power production section is driven using the gas. The external reactor vessel cooling section, the power production section, and the condensation heat exchange section of the present invention may continuously operate even during an accident as well as during a normal operation, to cool the reactor vessel and produce emergency power, thereby improving system reliability. The system may easily employ a safety class or seismic design using small scale facilities, which may result in improving reliability of the nuclear power plant including cooling of an external wall of the reactor vessel. The external reactor vessel cooling and electric power generation system according to the present invention can be designed to remove residual heat of a predetermined scale or more, which is generated in the reactor, by the external reactor vessel cooling section, and can continuously operate not only during a normal operation but also during an accident. This may result in lowering probability of operation failure during the accident, thereby improving safety of the nuclear power plant. The nuclear power plant according to the present invention can have improved economic efficiency by way of reducing a size of an emergency power system using the external reactor vessel cooling and electric power generation system. Description will now be given in detail according to exemplary embodiments disclosed herein, with reference to the accompanying drawings. For the sake of brief description with reference to the drawings, the same or equivalent components may be provided with the same or similar reference numbers, and description thereof will not be repeated. In describing the present invention, moreover, the detailed description will be omitted when a specific description for publicly known technologies to which the invention pertains is judged to obscure the gist of the present invention. The accompanying drawings are used to help easily understand the technical idea of the present disclosure and it should be understood that the idea of the present disclosure is not limited by the accompanying drawings. The idea of the present disclosure should be construed to extend to any alterations, equivalents and substitutes besides the accompanying drawings. It will be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are generally only used to distinguish one element from another. A singular representation may include a plural representation unless it represents a definitely different meaning from the context. Terms such as “include” or “has” are used herein and should be understood that they are intended to indicate an existence of several components, functions or steps, disclosed in the specification, and it is also understood that greater or fewer components, functions, or steps may likewise be utilized. FIG. 1A is a conceptual view of an external reactor vessel cooling and electric power generation system 100 according to an embodiment of the present invention. In the embodiment of the present invention, an insulating material 111 may be provided to surround a part of a reactor vessel 110, and a core 114 may be provided inside the reactor vessel 110. The core 114 refers to nuclear fuel. Electric power is produced by heat which is generated while nuclear fission is performed in the core 114. When an accident occurs in a nuclear power plant, residual heat may be generated for a considerable period of time even when the core 114 is stopped because a control rod is inserted in the core 114. If it is assumed that various safety and non-safety systems do not work at the time of an accident of a nuclear power plant, cooling water inside the reactor vessel 110 may be lost to increase the temperature of the nuclear fuel, thereby causing a core meltdown phenomenon. On the other hand, during a normal operation of the nuclear power plant, steam may be produced in the steam generator 113 by receiving heat from the reactor coolant system. The steam generator 113 may be a pressurized light water reactor. Further, the steam produced by the steam generator 113 may be steam that is phase-changed by receiving water through a main feedwater line 11 connected to a feedwater system 10 and an isolation valve 12. The steam produced by the steam generator 113 may be passed through a main steam line 14 connected to an isolation valve 13 and supplied to a large turbine 15 and a large generator (not shown) to produce electric power while fluid energy of the steam is converted into electric energy through mechanical energy. In addition, a reactor coolant pump 112 may circulate a coolant filled in the reactor vessel 110. A pressurizer 115 provided inside the reactor vessel 110 may control pressure of the reactor coolant system. In addition, a passive residual heat removal system including an emergency cooling water storage section 20 and a heat exchanger 21 may be provided therein, so that heat can also be discharged to the emergency cooling water storage section 20 by natural circulation due to a two-phase flow and opening and closing of a valve 24 during an accident. Further, when steam is generated while emergency cooling water is evaporated by heat transferred to the emergency cooling water storage section 20, the steam may be discharged through a steam discharge unit 25 such that the transferred heat can be discharged to the atmosphere. The external reactor vessel cooling and electric power generation system 100 is in an operating state even during a normal operation. And then the external reactor vessel cooling system continues to operate during an accident. Heat is continuously transferred to the reactor vessel 110 due to residual heat generated in the core 114 until the temperature of the reactor vessel 110 is remarkably reduced to reach a safe state of the reactor vessel 110 during an accident. Accordingly, an operator action for the operation of the external reactor vessel cooling system, various measuring instruments and control systems, valve operation or pump start and opening and closing of a thermal insulation material which are needed in the related art may not be required, and thus the probability of operation failure of the external reactor vessel cooling and electric power generation system 100 is greatly reduced to improve the safety of the nuclear power plant. In addition, since emergency electric power can be safely produced by the external reactor vessel cooling and electric power generation system 100 until the temperature of the reactor vessel is reduced to reach a safe state during an accident, the capacity of an emergency DC battery(emergency electric power source) can be decreased to improve the economic efficiency of the nuclear power plant and improve the reliability of an emergency power system of the nuclear power plant by securing an emergency power supply source of a safety system, thereby improving the safety of the nuclear power plant. In detail, in case of a passive nuclear power plant, emergency power required during an accident is less than about 0.05% compared to the power generation capacity generated from the nuclear power plant during a normal operation. However, it is designed to use a battery for 72 hours or more, and thus a very large battery is required, having a disadvantage of increasing the cost. However, the external reactor vessel cooling and electric power generation system 100 may be configured to produce an appropriate level of emergency power using residual heat continuously generated from the core (an amount of residual heat generated is several % (initial shutdown) to 1/several % (after 72 hours subsequent to shutdown) compared to a normal amount of thermal power) even after the reactor is shut down upon an occurrence of an accident. In addition, when power is produced using the external reactor vessel cooling and electric power generation system 100, the power production amount is about several tens kWe to several MWe, and the capacity is less than 1/several % compared to the feedwater system 10 and the large turbine 15 for a normal operation of the nuclear power plant. This system 100 has almost no influence on the operation of the nuclear power plant, and therefore, even when this system 100 fails during a normal operation. That is to say, this system 100 has a capacity less than 1/several %, so it has little effect on a nuclear power plant operation. In addition, when power is produced using the external reactor vessel cooling and electric power generation system 100, it may be constructed in a small scale compared to the large-capacity feedwater system 10 and the large turbine 15 for producing normal power. Therefore, it is easy to apply seismic design and safety class, and cost increase is not so great due to small facilities even when the seismic design and safety class are applied. Even in the event of an accident, the external reactor vessel cooling and electric power generation system can continue to operate as a normal operation without any additional valve operation, and therefore, during an accident, operation failure of valves, pumps, and etc. for operating the related art external reactor vessel cooling and electric power generation system, and probability of operation failure or malfunction due to errors of measuring instruments and control signals may be significantly reduced. Further, when a severe accident occurs and the external reactor vessel cooling section 120 and a power(electric power) production section 130 may do not work during the occurrence of the severe accident, a flow path through an in-containment refueling water storage tank (hereinafter, referred to as IRWST) 180 and a second discharge portion 127 is already formed, and therefore, smooth supply and discharge of cooling water may be enabled by a simple operation such as opening/closing of valves according to an operator action, such that the cooling water can be supplied well for cooling the reactor vessel 110. Particularly, since an integral type reactor in which a connecting line for a measuring instrument or the like is not installed at a lower portion of the reactor vessel has a simple structure, it is facilitated to apply the external reactor vessel cooling and electric power generation system 100. In addition, the external reactor vessel cooling and electric power generation system 100 may be utilized as additional residual heat removal means that plays a role of removing residual heat of the reactor core 114 during an accident. Hereinafter, the external reactor vessel cooling and electric power generation system 100 according to the present invention will be described in detail. A reactor vessel 110, an external reactor vessel cooling section 120 and an IRWST 180 may be provided inside a reactor building boundary 1. An insulating material 111 may additionally be provided on an external wall of the reactor vessel 110. The external reactor vessel cooling section 120 may be formed to enclose at least part of the reactor vessel 110. In addition, the external reactor vessel cooling section 120 may be formed to receive heat discharged from the reactor vessel 110 and cool the external wall of the reactor vessel 110. On the other hand, a power production section 130, a condensation heat exchange section 140, and a condensed water storage section 150 are provided outside the reactor building boundary 1. The power production section 130 may supply power by being connected to motors 141, 151 and 157, an internal/external electric power system 171, a charger 172, an emergency power consuming device(emergency safety systems, main control room, and etc.) 174 and an emergency battery 173. However, some of those components illustrated as being installed outside the reactor building boundary 1 may alternatively be disposed inside the reactor building boundary 1 depending on arrangement characteristics of nuclear power plants. The reactor vessel 110 may be a pressure vessel that circulates a reactor coolant therein, is provided with the core 114 therein, and is designed to withstand high pressure. In addition, the external reactor vessel cooling section 120 may be formed to enclose the reactor vessel 110, and receive heat discharged from the reactor vessel 110 so as to cool the external wall of the reactor vessel 110 using a fluid which is subjected to phase change from liquid to gas. In detail, the external reactor vessel cooling section 120 may be formed in a cylindrical shape. However, the shape of the external reactor vessel cooling section 120 is not limited to the cylindrical shape, but may alternatively include a hemispherical shape, a dual-container shape, or a combined shape thereof. In addition, the external reactor vessel cooling section 120 may further include a coating member 121 for preventing corrosion thereof. In this embodiment, the coating member 121 may have a surface modified in various ways and may be processed into an uneven shape (cooling fins) to increase a heat transfer surface area. Further, the coating member 121 may further include a heat transfer member (not shown) whose surface is chemically processed to increase a surface area so as to improve heat transfer efficiency. That is, the surfaces of the coating member 121 and the heat transfer member may be chemically processed to increase the surface areas thereof, such that the heat transfer can be efficiently carried out. The external reactor vessel cooling section 120 may be provided with a discharge pipe 122. The discharge pipe 122 may be connected to the external reactor vessel cooling section 120 and the power production section 130, respectively, such that the fluid of the external reactor vessel cooling section 120 can be supplied to the power production section 130 therethrough. The discharge pipe 122 may be branched to the pipe 124 to be connected to the power production section 130 through a valve 123. The discharge pipe 122 may be provided with a first discharge portion 126 connected to a valve 125. The fluid which could be excessively supplied to the power production section 130 may be discharged through the first discharge portion 126. In detail, the first discharge portion 126 is a pipe through which a fluid (gas, steam) is discharged from the external reactor vessel cooling section 120 to the outside of the reactor building (not shown). Accordingly, a part of the fluid (gas, steam) can be discharged through the first discharge portion 126 when pressure of the system is increased or the fluid (liquid) is excessively supplied. The present invention has illustrated that the fluid is discharged to the outside of the reactor building (not shown) through the first discharge portion 126. However, the fluid to be discharged may alternatively be bypassed the power production section 130 and supplied and condensed in a condensation heat exchange section 140 to be reused according to characteristics of the nuclear power plants. Further, the external reactor vessel cooling section 120 may be connected to the IRWST 180 such that refueling water is supplied through the pipe 183. In detail, the IRWST 180 may be connected to a valve 181 and a check valve 182. In addition, the external reactor vessel cooling section 120 may be provided with a second discharge portion 127 connected to a valve 127′. The second discharge portion 127 may be formed to discharge the refueling water supplied from the IRWST 180 therethrough. In detail, the second discharge portion 127 is a pipe through which a fluid (gas/steam or liquid/high temperature water) is discharged from the external reactor vessel cooling section 120 to the reactor building (not shown). Accordingly, the second discharge portion 127 can allow the reactor vessel to be cooled even when the external reactor vessel cooling section 120 and the power production section 130 are unable to perform cooling and electric power generation due to their failure or the like, which is caused by multiple failures of safety system, and may worsen to a severe accident and the like. Meanwhile, the power production section 130 may be configured such that the fluid moving from the external reactor vessel cooling section 120 is injected therein. As the moved fluid actuates the small turbine 131, fluid energy of the fluid may be converted into mechanical energy (rotational force), and the mechanical energy may be converted into electric energy through the small generator 132 connected to the small turbine 131 with a shaft, thereby producing electric power. The small turbine 131 may generate electric power by receiving heat of a preset scale from the reactor vessel 110 in consideration of characteristics during a normal operation and during an accident. In this embodiment, the present invention may have a configuration that power is produced in a variable manner in consideration of a rate of change in a steam flow rate which is caused due to a variation of a heat generated in the core 114 supplied during an accident, and may adjust a load of the power production section 130. Also, the small turbine 131 of the power production section 130 may be a small-capacity turbine, which may make it easy to apply seismic design or safety class to be described below. The electric power that can be generated by the power production section 130 has a capacity of several tens of kWe to several MWe, which is less than 1% compared to the large-capacity feedwater system 10 and the large turbine 15 for producing normal power of the nuclear power plant, and even when the facility operates or fails during a normal operation of the nuclear power plant, there is little influence on the operation of the large capacity feedwater system 10 and the large turbine 15 for producing the normal nuclear power. That is, the large-capacity feedwater system 10 and the large turbine 15 for producing the normal power are one of the biggest large-scale facilities of the nuclear power plant, and applying the seismic design and safety class above the whole facilities is very uneconomical because it causes a huge cost increase. Meanwhile, in case of the external reactor vessel cooling and electric power generation system 100 in which the small turbine 131 and the small generator 132 are provided, the size of the system 100 is much smaller than that of the feedwater system 10 and the large turbine 15. Thus, it is easy to apply seismic design or safety class thereto, and the increased cost due to applying the seismic design or the safety class is not so great. The small turbine 131 and the small generator 132 can be continuously operated to supply emergency electric power even when it is difficult to supply electric power due to an occurrence of an earthquake in conventional plants since seismic design is applied to the external reactor vessel cooling and electric power generation system 100. Also, the small turbine 131 and the small generator 132 can be continuously operated to supply emergency power even when various accidents occur since safety class is applied to the external reactor vessel cooling and electric power generation system 100 to secure system reliability. Considering that electric power required in case of a passive nuclear power plant during an accident is several tens kWe although the emergency power has a difference according to characteristics of nuclear power plants, sufficient power can be supplied with only electric power produced by the small turbine 131 and the small generator 132. Besides, since the emergency DC battery capacity of the passive nuclear power plant is not greater than the emergency power required in an active nuclear power plant, the DC battery may be recharged by electric power produced by the small turbine 131 and the small generator 132. The external reactor vessel cooling and electric power generation system 100 may be configured to have seismic design of seismic category I, II or seismic category III which are specified by ASME (American Society of Mechanical Engineers). In detail, seismic category I is applied to structures, systems and components which are classified as safety items, and should be designed to maintain an inherent ‘safety function’ in case of a safe shutdown earthquake (SSE). Also, seismic category I is designed such that the safety function is maintained even under an operating basis earthquake (OBE) in synchronization with a normal operation load, and appropriate allowable stresses and changes are within limits. Though not requiring nuclear safety or continuous functions, seismic category II is applied to items whose structural damages or interaction may lower the safety functions of the structures, systems and components of the seismic category I or cause damage to an operator located within a main control room. In detail, the structures, systems and components belonging to the seismic category II are not required to have functional integrity for the SSE, but required only to have structural integrity. In addition, the structures, systems and components of the seismic category II should be designed and arranged so as not to impair safety-related operations of the items belonging to the seismic category I. Seismic category III is designed according to uniform building codes (UBCs) or general industrial standards depending on individual design functions. The external reactor vessel cooling and electric power generation system 100 may be configured to have a safety class of safety classes 1, 2 or 3 of the nuclear power plant specified by the American Society of Mechanical Engineers (ASME). In detail, the safety class of the nuclear power plant is typically divided into safety classes 1 to 3. Safety class 1 is a grade assigned to a RCS(reactor coolant system) pressure-boundary portion of a facility and its support that constitute a reactor coolant pressure boundary (a portion that may cause a loss of coolant beyond a normal make-up capacity of the reactor coolant in the event of a failure). Safety class 2 may be assigned to a pressure-boundary portion of the reactor containment building and its support, and assigned to a facility and its support that perform the following safety functions while not belonging to safety class 1. A function of preventing the release of fission products or containing or isolating radioactive materials in the containment building A function of removing heat or radioactive materials generated in the containment building in case of an emergency (e.g., containment building spray system), a function of increasing negative reactivity to make the reactor in a subcritical state in case of an emergency or suppressing an increase of positive reactivity (e.g., boric acid injection system) A function of supplying coolant directly to the core during an emergency to ensure core cooling (e.g., residual heat removal, emergency core cooling system) and a function of supplying or maintaining sufficient reactor coolant for cooling the reactor core during an emergency (e.g., refueling water storage tank) Safety class 3 is not included in safety classes 1 and 2, and may be assigned to a facility that performs one of the following safety functions: A function of controlling concentration of hydrogen in the reactor containment building within an allowable limit A function of removing radioactive materials from a space (e.g., main control room, nuclear fuel building) outside the reactor containment building with safety class 1, 2 or 3 facilities A function of increasing negative reactivity to make or maintain the reactor in a subcritical state (e.g., boric acid make-up) A function of supplying or maintaining sufficient reactor coolant for core cooling (e.g., Reactor coolant replenishment system) A function of maintaining a geometric structure inside the reactor to ensure core reactivity control or core cooling capability (e.g., core support structure) A function of supporting or protecting the load for safety class 1, 2 or 3 facilities (concrete steel structures not included in KEPIC-MN, ASME sec. III). A function of shielding for radiation for people outside the reactor control room or nuclear power plant A cooling function of spent fuel (e.g., spent fuel pool and cooling system) A function of ensuring safety functions performed by safety class 1, 2 or 3 facilities (e.g., a function of removing heat from safety class 1, 2 or 3 heat exchangers, a safety class 2 or 3 pump lubrication function, a fuel supplying function of emergency diesel generator) A function of supplying electric power or motive power to safety class 1, 2 or 3 facilities A function of providing informations or controlling informations for safety class 1, 2 or 3 facilities to perform the safety functions automatically or manually A function of supplying or receiving signals for safety class 1, 2 or 3 facilities to perform the safety functions Manual or automatic interlocking function for safety class 1, 2 or 3 facilities to perform the safety functions A function of providing appropriate environmental conditions for safety class 1, 2 or 3 facilities and an operator A function corresponding to safety class 2 to which standards for the design and manufacture of pressure vessels, KEPIC-MN, ASME Sec. III, are not applied The condensation heat exchange section 140 is configured to perform heat exchange with the fluid discharged from the power production section 130 including the small turbine 131 and the small generator 132 subsequent to producing electric energy, and shrink the fluid to generate condensed water. In detail, the condensation heat exchange section 140 includes a heat exchanger to condense the fluid so as to recover the fluid in the form of condensed water. The heat exchanger of the condensation heat exchange section 140 may be a shell-and-tube type heat exchanger or a plate type heat exchanger. However, the heat exchanger disclosed herein is not limited to this and may be any heat exchanger capable of condensing the fluid to generate condensed water. The condensing heat exchange section 140 is provided with a motor 141 or a pump (not shown), and the motor 141 or the pump may supply a cooling fluid to the condensation heat exchange section 140 so as to exchange heat with the fluid. The cooling fluid may be air, pure water, seawater or a mixture thereof. The motor 141 may provide rotational power to a fan 142 or to a pump. The fan 142 may be a cooling fan when an air-cooling type heat exchanger is applied, and the condensation heat exchange section 140 may be downsized using the cooling fan. In addition, a discharge fan (not shown) may selectively be provided in an vent portion 143. Accordingly, when non-condensable gas is accumulated in the condensation heat exchange section 140, the discharge fan may remove the non-condensable gas, thereby improving the heat exchange of the condensation heat exchange section 140 and lowering pressure thereof. In addition, there may be various methods of discharging gas, such as using pressure drop of the Venturi by a steam flow rate, and thus the present invention is not limited to a specific form. The motor 141 may be supplied with electric power produced by the power production section 130 itself through a connecting line 133. The fan 142 connected to the motor 141 may supply cooling air to the condensation heat exchange section 140 to efficiently perform the heat exchange in the condensation heat exchange section 140. In addition, the motor 141 may be provided to charge electric power produced by the power production section 130 to an emergency battery 173 and receive electric power from the emergency battery 173. A pipe 144 may be provided between the condensation heat exchange section 140 and the condensed water storage section 150 such that the fluid flows therealong. The condensed water generated in the condensation heat exchange section 140 is transferred to the condensed water storage section 150 along the pipe 144. In detail, the condensed water storage section 150 may be disposed below the condensation heat exchange section 140 to collect the condensed water generated in the condensation heat exchange section 140. In the embodiment of the present invention, the condensed water in the condensation heat exchange section 140 is transferred to the condensed water storage section 150 by gravity. However, depending on characteristics of nuclear power plants, a pump may be installed in the connected pipe between the condensation heat exchange section 140 and the condensed water storage section 150 so that the condensed water is forcibly transferred. In addition, a pipe 144′ for transferring steam or non-condensable gas, which may be accumulated in the condensation heat exchange section 140, to the condensed water storage section 150 may be additionally provided. The condensed water collected in the condensed water storage section 150 may circulate through the external reactor vessel cooling section 120, the power production section 130, and the condensation heat exchange section 140. Further, the condensed water storage section 150 may be connected to the external reactor vessel cooling section 120 through a pipe 160 such that the condensed water is supplied to the external reactor vessel cooling section 120. Specifically, the condensed water of the condensed water storage section 150 may be supplied into a pipe 160 connected to the external reactor vessel cooling section 120 by gravity through a valve 154 and a check valve 155 connected to the pipe 153. The condensed water of the condensed water storage section 150 may be passed through a valve 161 and a check valve 162 by a motor 157 and a small feedwater pump 158 connected to a pipe 156 to be supplied into the pipe 160 connected to the external reactor vessel cooling section 120. Similar to the condensation heat exchange section 140 described above, the condensed water storage section 150 may include a motor 151 and a fan 152, and the motor 151 is capable of providing rotation power to the fan 152. The fan 152 may discharge non-condensable gas to lower pressure of the condensed water storage section 150 when the non-condensable gas is accumulated in the condensed water storage section 150. However, various methods of discharging gas including the method of using the pressure drop of the Venturi by the steam flow rate may be applied. Therefore, the method of discharging the non-condensing gas is not limited to a specific form. In the present invention, the motor 151 and the fan 152 are provided above the condensed water storage section 150, but the motor 151 and the fan 152 may alternatively be provided in the condensation heat exchange section 140. The motor 151 described above may receive power produced by the small turbine 131 itself through the connecting line 134. In addition, the motor 151 may be provided to charge electric power produced by the power production section 130 to the emergency battery 173 and receive electric power from the emergency battery 173. The power system 170 may be configured to use the power produced during the normal operation of the nuclear power plant as power of the internal/external electric power system 171. In detail, the internal/external electric power system 171 may be a system for processing electricity supplied from an on-site large turbine generator, the power production section 130, an on-site diesel generator, and an external power grid. In addition, electric energy may be stored in the emergency battery 173 through the charger 172, which is a facility for storing alternating current (AC) electricity supplied from the on-site, the outside, or the power production section 130 or the like. The emergency battery 173 may be a battery provided in an on-site to supply emergency DC power used during an accident. Further, the electric energy stored in the emergency battery 173 may be supplied to the emergency power consuming device 174 and used as emergency power. The emergency power may be used as power for operating the nuclear power plant safety system, opening or closing a valve for the operation of the nuclear power plant safety system, or monitoring the nuclear safety system during an accident of the nuclear power plant. Moreover, the electric power produced by the power production section 130 during an accident of the nuclear power plant may also be supplied as the emergency power of the nuclear power plant. Moreover, when the heat exchange section 120 and the power production section 130 fail due to multiple failures, a flow path through the IRWST 180 and the first discharge portion 127 is already formed, and therefore, smooth supply and discharge of cooling water may be enabled by a simple operation such as opening/closing valves according to an operator action, such that the cooling water can be supplied effectively for cooling the reactor vessel 110. FIG. 1B is a conceptual view illustrating a normal operation of the external reactor vessel cooling and electric power generation system 100 according to the embodiment of the present invention. Referring to FIG. 1B, it is a conceptual view illustrating system arrangement and a flow of fluid during a normal operation of a nuclear power plant. During the normal operation of the nuclear power plant, main feedwater (water) is supplied from the feedwater system 10 to the steam generator 113, and heat received from the core 114 by the reactor coolant circulation is transferred to a secondary system through the steam generator 113 so as to increase a temperature of the main feedwater and produce steam. The steam of the main feedwater produced from the steam generator 113 is supplied to the large turbine 15 along the main steam line 14 to rotate the large turbine 150 and rotate the large generator (not shown) connected through a shaft so as to produce electric power. The electric power produced through the large generator may be supplied to an on-site or off-site electric power system. On the other hand, feedwater supplied from the small feedwater pump 158 to the external reactor vessel cooling section 120 through the pipe 160 receives heat while rising along the external wall of the reactor vessel 110, thereby generating steam. The steam is supplied to the power production section 130 including the small turbine 131 and the small generator 132 along the discharge pipe 122 disposed at an upper part of the external reactor vessel cooling section 120. Fluid energy of the steam is converted into mechanical energy while rotating the small turbine 131, and the mechanical energy is converted into electric energy by the small generator 132 connected to the small turbine 131 by the shaft, thereby producing electric power. Further, the electric power produced by the power production section 130 may be supplied as electric power of the internal/external electric power system 171 through the electric power system 170. In addition, electric energy may be stored in the emergency battery 173 through the charger 172, which is a facility for storing alternating current (AC) electricity supplied from the on-site, the outside, or the power production section 130 or the like. The emergency battery 173 may be a battery provided in the on-site to supply emergency DC power used during an accident. Further, the electric power may be supplied to the emergency power consuming device 174 and used as emergency power. FIG. 1C is a conceptual view illustrating a design basis accident (DBA) operation of an external reactor vessel cooling and electric power generation system according to an embodiment of the present invention. Referring to FIG. 1C, it is a conceptual view illustrating the operation of the external reactor vessel cooling and electric power generation system 100 including the small feedwater pump 158, the power production section 130 and the like, during a design basis accident (DBA) of a nuclear power plant. Specifically, when an accident occurs in a nuclear power plant due to various causes, safety systems, such as a passive residual heat removal system, a passive safety injection system and a passive containment cooling system, including the emergency cooling water storage section 20, which are installed in a plurality of trains, may be automatically operated in response to related signals. Further, steam generated by the operation of the safety system may be discharged from a steam discharge portion 25 of the emergency cooling water storage section 20. The operation of the safety system may remove residual heat generated in the reactor coolant system 111 and the core 114. In addition, safety injection water may be supplied to the reactor coolant system 111 to lower pressure and temperature of the reactor coolant system 111 and lower the temperature of the core 114. Also, a pressure increase inside the reactor containment (not shown) may be suppressed by the operation of the passive containment cooling system, so as to protect the reactor containment. On the other hand, while the isolation valves 12, 13 provided in the main feedwater line 11 and the main steam line 14 are closed, the operation of the large turbine 15 is stopped. However, even when the reactor core 114 is stopped, residual heat is generated in the core 114 for a considerable period of time, and a lot of sensible heat is present in the reactor coolant system 110 and the reactor vessel 110. As a result, the temperature of the reactor coolant system 111 does not decrease rapidly. Accordingly, even when an accident occurs, the external reactor vessel cooling section 120 and the power production section 130 may be operated in the same state as a normal operation. Therefore, the power production section 130 may cool the reactor vessel 110 while continuously producing electric power. As time elapses, the residual heat generated in the core 114 may decrease and the temperature of the reactor vessel 110 may decrease while the reactor vessel 110 is cooled by the safety system. In this case, the external reactor vessel cooling and electric power generation system 100 may be operated in substantially the same manner as the normal operation while reducing the amount of electric power generated by the power production section 130 due to the reduction in an amount of heat transferred. FIG. 1D is a conceptual view illustrating a design basis accident (DBA) operation of an external reactor vessel cooling and electric power generation system 100 according to an embodiment of the present invention. Referring to FIG. 1D, it is a conceptual view of a case where the operation of the small feedwater pump 158 is disabled following the design basis accident and operation of the external reactor vessel cooling and electric power generation system 100 through the path 153 is activated. As illustrated in the foregoing case of FIG. 1C, the safety systems, such as the passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to relative signals. Accordingly, the reactor coolant system may be cooled, the residual heat of the core 114 may be removed and safety injection water may be supplied to the reactor coolant system so as to lower pressure and temperature of the reactor coolant system and lower the temperature of the core 114. Also, a pressure increase inside the reactor containment (not shown) may be suppressed so as to protect the reactor containment. On the other hand, while the isolation valves 12, 13 provided in the main feedwater line 11 and the main steam line 14 are closed, the operation of the large turbine 15 is stopped. In detail, when feedwater supplied from the small feedwater pump is interrupted for various reasons, the pipe 153 connected to the condensed water storage section 150 may be opened in response to a related signal or by an operator action to supply feedwater from the condensed water storage section 150. At this time, the feedwater may be supplied by natural circulation by gravity. The gravity may be applied to the condensed water in the condensed water storage section 150 so that the condensed water can be supplied in the natural circulation manner. Accordingly, the operation state of the external reactor vessel cooling section 120 and the power production section 130 may be similar to that during a normal operation except for the feedwater pump 158. As time elapses, when a steam generation amount is reduced due to a gradual reduction of the residual heat of the core 114, the operation state may become similar to that during the normal operation while adjusting the power production amount of the power production section 130. FIG. 1E is a conceptual view illustrating a severe accident operation of an external reactor vessel cooling and electric power generation system 100 in accordance with an embodiment of the present invention. Referring to FIG. 1E, it is a conceptual view in which the operation of the external reactor vessel cooling and electric power generation system 100 is disabled due to multiple failures including the failures of the system 100, and severe accidents(core melting) occur. As illustrated in the foregoing case of FIG. 1C, the safety systems, such as a passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to relative signals. However, when it is assumed that various safety systems do not operate although it is rarely likely to occur, an accident in which the core is melted down due to a temperature rise of the core may occur. For example, in order to block the discharge of radioactive materials to the outside of the reactor containment during a severe accident such as a generation of corium of nuclear accidents, the operation of the external reactor vessel cooling section 120 and the power production section 130 may be stopped. Accordingly, the pipe 183 connected to the IRWST 180 may be opened in response to a related signal or by an operator action to supply feedwater from the IRWST 180 so as to cool the lower part of the reactor vessel 110, and the valve 127′ installed on the second discharge portion 127 may be opened to discharge generated steam. Depending on characteristics of nuclear power plants, the feedwater may be forcibly injected by installing a pump (not shown) in the pipe 183 connected to the IRWST 180 or may be injected using gravity. Furthermore, even when a severe accident such as damage to the reactor vessel or exposure of the reactor core 114 occurs of nuclear accidents, in addition to the generation of the corium 114′ in the reactor, the operation of the external reactor vessel cooling section 120 and the power production section 130 may be stopped to allow the injection of feedwater through the IRWST 180 and the opening of the valve 127′ connected to the first discharge portion 127 in terms of protection. Furthermore, according to another embodiment described below, the same or similar reference numerals are designated to the same or similar configurations to the foregoing example, and the description thereof will be substituted by the earlier description. FIG. 2A is a conceptual view of an external reactor vessel cooling and electric power generation system in accordance with another embodiment of the present invention. An evaporation section 290 connected to an external reactor vessel cooling section 220 may further be provided. The evaporation section 290 may be configured to perform heat exchange between a fluid inside the external reactor vessel cooling section 220 and condensed water of a condensed water storage section 250. In detail, a first circulation part may be formed from the external reactor vessel cooling section 220 to the evaporation section 290 such that a fluid flow therealong. On the other hand, a second circulation part may be formed sequentially along the evaporation section 290, a power production section 230, a condensation heat exchange section 240, and the condensed water storage section 250 such that the fluid flows therealong. That is, the first circulation part and the second circulation part may have a dual circulation loop. The evaporation section 290 may be formed to be a boundary between the first circulation part and the second circulation part. The first circulation part may be configured such that a single-phase fluid flows therealong. In detail, the single-phase fluid of the first circulation part may be compressed gas. In addition, a compressor 293 and a blower (not shown) may be provided to allow the circulation of the single-phase fluid of the first circulation part, and activate the external reactor vessel cooling section 220 to transfer heat to the evaporation section 290. Further, the external reactor vessel cooling section 220 may be formed in a hemispherical shape and may also cool an external wall of a reactor vessel 210 without a coating member or a heat transfer enhancement member. FIG. 2B is a conceptual view illustrating a normal operation of the external reactor vessel cooling and electric power generation system according to the another embodiment of the present invention. Referring to FIG. 2B, it is a conceptual view illustrating system arrangement and a flow of fluid during a normal operation of a nuclear power plant. During a normal operation of a nuclear power plant, main feedwater (water) is supplied from the feedwater system 10 to a steam generator 213, and heat received from a core 214 by a reactor coolant circulation is transferred to a secondary system through the steam generator 213 to increase a temperature of the main feedwater and produce steam. The steam produced from the steam generator 213 is supplied to the large turbine 15 along the main steam line 14 to rotate the large turbine 150 and rotate the large generator (not shown) connected through the shaft so as to produce electric power. The electric power produced through the large generator may be supplied to an on-site or off-site from the power system. The single-phase fluid inside the external reactor vessel cooling section 220 provided along the external wall of the reactor vessel 210 is moved to the evaporation section 290 by receiving heat from the external wall of the reactor vessel 210. The single-phase fluid moved to the evaporation section 290 may exchange heat with a fluid, which is to be supplied to the power production section 230 including a small turbine 231 and a small generator 232, and circulate the first circulation part formed between the external reactor vessel cooling section 220 and the evaporation section 290. Further, a compressor 293 and a blower (not shown) connected to a motor 296 may be configured to allow the circulation of the single-phase fluid of the first circulation part. On the other hand, feedwater supplied from a small feedwater pump 258 to the evaporation section 290 through a pipe 260′ is supplied to the power production section 230 including a small turbine 231 and a small generator 232 while circulating the second circulation part. Fluid energy of the steam may be converted into mechanical energy while circulating the small turbine 231 and the mechanical energy may be converted into electric energy while rotating the small generator 232 connected through the shaft, thereby producing electric power. Further, the electric power produced by the power production section 230 may be utilized by the power system 270. FIG. 2C is a conceptual view illustrating a design basis accident operation of the external reactor vessel cooling and electric power generation system 200 according to the another embodiment of the present invention. Referring to FIG. 2C, it is a conceptual view illustrating an operation of the external reactor vessel cooling and electric power generation system 200 in case where the small feedwater pump 258, the power production section 230, and the like can be operated during a design basis accident of a nuclear power plant. Specifically, when an accident occurs in a nuclear power plant due to various causes, the safety systems, such as the passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to related signals. Further, steam generated by the operation of the safety system may be discharged from the steam discharge portion 25 of the emergency cooling water storage section 20. The operation of the safety system may remove residual heat generated in the reactor coolant system and the core 214. In addition, safety injection water may be supplied to the reactor coolant system to lower pressure and temperature of the reactor coolant system and lower temperature of the core 214. Also, a pressure increase in the reactor containment (not shown) may be suppressed by the operation of the passive containment cooling system so as to protect the reactor containment. On the other hand, while the isolation valves 12, 13 provided in the main feedwater line 11 and the main steam line 14 are closed, the operation of the large turbine 15 is stopped. However, since the temperature of the reactor vessel 210 is similar in the early stage of the accident, the external reactor vessel cooling section 220 and the power production section 230 connected to the evaporation section 290, respectively, may be operated in the same state as a normal operation. As time elapses, when the temperature of the reactor vessel 210 decreases in response residual heat generated in the core 214 being reduced and the reactor vessel 210 being cooled by the safety system, the power production section 230 may be operated similar to the normal operation while adjusting a power production amount according to an amount of heat transferred thereto. FIG. 2D is a conceptual view illustrating a design basis accident (DBA) operation of the external reactor vessel cooling and electric power generation system 200 according to the another embodiment of the present invention. Referring to FIG. 2D, it is a conceptual view illustrating the operation of the external reactor vessel cooling and electric power generation system 200 when the operation of the small feedwater pump 258 is disabled during a design basis accident of a nuclear power plant. As illustrated in the foregoing case of FIG. 2C, the safety systems, such as the passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to relative signals. Accordingly, the reactor coolant system may be cooled, the residual heat of the core 214 may be removed and safety injection water may be supplied to the reactor coolant system so as to lower pressure and temperature of the reactor coolant system and lower the temperature of the core 214. Also, a pressure increase in the reactor containment (not shown) may be suppressed so as to protect the reactor containment. On the other hand, while the isolation valves 12, 13 provided in the main feedwater line 11 and the main steam line 14 are closed, the operation of the large turbine 15 is stopped. In detail, when feedwater supplied from the small water supply pump is interrupted for various reasons, the pipe 253 connected to the condensed water storage section 250 may be opened in response to a related signal or by an operator action to supply water from the condensed water storage section 250. At this time, the feedwater may be supplied by natural circulation by gravity. The gravity may be applied to the condensed water in the condensed water storage section 250 so that the condensed water can be supplied in the natural circulation manner. Accordingly, the reactor vessel external wall cooling unit 220 and the power production section 230 can be operated in an operating state similar to that during normal operation. However, when the residual heat of the core 214 gradually decreases and the heat transferred to the evaporator 290 decreases, the power production section 230 may be operated in a similar manner to the normal operation while adjusting the power production amount. FIG. 2E is a conceptual view illustrating a severe accident operation of the external reactor vessel cooling and electric power generation system 200 in accordance with the another embodiment of the present invention. Referring to FIG. 2E, it is a conceptual view illustrating the operation of the external reactor vessel cooling and electric power generation system 200 when the power generation system 200 is shut down during a severe accident of a nuclear power plant. As illustrated in the foregoing case of FIG. 2C, the safety systems, such as the passive residual heat removal system, the passive safety injection system, and the passive containment cooling system, including the emergency cooling water storage section 20, which are installed in the plurality of trains, may be automatically operated in response to relative signals. For example, when a severe accident such as a generation of corium 214′ occurs of nuclear accidents, the external reactor vessel cooling section 220 and the power production section 230, which are respectively connected to the evaporation section 290, may be interrupted. Accordingly, the pipe 283 connected to the IRWST 280 may be opened in response to a related signal or by an operator action to supply feedwater from the IRWST 280 so as to cool the lower part of the reactor vessel 210, and the valve 226 installed on the second discharge portion 227 may be opened to discharge generated steam. Depending on characteristics of nuclear power plants, the feedwater may be forcibly injected by installing a pump (not shown) in the pipe 283 connected to the IRWST 280 or may be injected using gravity. Furthermore, even in case where a severe accident such as damage to the reactor vessel or exposure of the reactor core 214 has occurred of nuclear accidents, in addition to the generation of the corium 214′ in the reactor, when the operation of the external reactor vessel cooling section 220 and the power production section 230 is stopped, the injection of feedwater through the IRWST 280 and the opening of the valve 226 connected to the second discharge portion 127 may be enabled in terms of protection. FIGS. 3A through 3E are conceptual views of an external reactor vessel cooling and electric power generation system in accordance with another embodiment of the present invention. Referring to FIG. 3A, an external reactor vessel cooling section 320a of an external reactor vessel cooling and electric power generation system 300a may have a hemispherical shape. In addition, the external reactor vessel cooling section 320a may further include a coating member 321a for preventing corrosion thereof. In this embodiment, the coating member 321a may have a surface modified in various ways and may be processed into an uneven shape (cooling fins) to increase a heat transfer surface area. Further, the coating member 321 may further include a heat transfer member (not shown) whose surface is chemically processed to increase a surface area so as to improve heat transfer efficiency. That is, the surfaces of the coating member 321 and the heat transfer member may be chemically processed to increase the surface areas thereof, such that the heat transfer can be efficiently carried out. Referring to FIG. 3B, an external reactor vessel cooling section 320b of an external reactor vessel cooling and electric power generation system 300b may have a mixed shape of a hemispherical shape and a cylindrical shape. Referring to FIG. 3C, an external reactor vessel cooling and electric power generation system 300c may further include a core catcher 328 provided inside an external reactor vessel cooling section 320c. The core catcher 328 may receive and cool corium when a reactor vessel 310 is damaged. Further, the external reactor vessel cooling section 320c may be connected to an IRWST 380′ through a pipe 383′ so that nuclear refueling water is supplied. In detail, the IRWST 380′ may be connected to a valve 381′ and a check valve 382′. In addition, the external reactor vessel cooling section 320c may be further provided with a coating member 321c for preventing corrosion thereof. Referring to FIG. 3D, an external reactor vessel cooling section 320d in an external reactor vessel cooling and electric power generation system 300d may have a shape of a dual vessel such that a cooling vessel (container) covers the entire reactor vessel. The external reactor vessel cooling and electric power generation system 300d may further include an evaporation section 390 connected to the external reactor vessel cooling section 320d, similar to the external reactor vessel cooling section 220 of FIG. 2A. The evaporation section 390 may be configured such that a fluid inside the external reactor vessel cooling section 320d exchanges heat with condensed water of a condensed water storage section 350. That is, the external reactor vessel cooling and electric power generation system 300d may be formed to have a dual circulation loop of a first circulation part and a second circulation part. Referring to FIG. 3E, an external reactor vessel cooling and electric power generation system 300e may further include a liquid-gas(water-steam) separator 329 connected to a discharge pipe 322. The liquid-gas separator 329 may be configured to transfer only gas in a fluid circulating in the external reactor vessel cooling section 320e to a power production section 330. Further, the system 300e may further include a cooling water recovery pipe 359 and a pump 339 by which a liquid separated from the liquid-gas separator 329 is transferred into a condensed water storage section 350. Although those various embodiments of the external reactor vessel cooling and electric power generation system of the present invention have been described above, the present invention may not be limited to the external reactor vessel cooling and electric power generation system and may include a loop or integral type nuclear power plant. In detail, the loop or integral type nuclear power plant according to the present invention may include a reactor vessel, and an external reactor vessel cooling section that is formed to enclose the reactor vessel to cool an external wall of the reactor vessel using a fluid, which is phase-changed from liquid to gas by receiving heat discharged from the reactor vessel. The nuclear power plant may also include a power production section provided with a small turbine that allows the fluid to move in the external reactor vessel cooling section and generates electric energy using the moved fluid. Further, the nuclear power plant may further include a condensation heat exchange section that causes a heat exchange of the fluid, which is discharged from the small turbine after the generation of the electric energy, and condenses the fluid to generate condensed water. The nuclear power plant may further include a condensed water storage section that collects the condensed water generated in the condensation heat exchange section and allows the condensed water of the condensed water storage section to circulate sequentially through the external reactor vessel cooling section, the power production section and the condensation heat exchange section. It is obvious to those skilled in the art that the present disclosure can be embodied in other specific forms without departing from the concept and essential characteristics thereof. The above detailed description should not be limitedly construed and should be considered illustrative in all aspects. The scope of the present invention should be determined by rational interpretation of the appended claims, and all changes within the scope of equivalents of the present invention are included in the scope of the present invention.

claims

1. An x-ray tube, comprising: (a) a vacuum enclosure having an electron source and a target anode disposed therein, said target anode having a target surface positioned to receive electrons emitted by said electron source; and (b) a window mounted in said vacuum enclosure proximate to at least said target anode so that at least some x-rays produced as a result of said electrons striking said target surface pass through said window, and said window having a plurality of extended surfaces so that a coolant contacting said plurality of extended surfaces absorbs at least some heat dissipated by at least said window. 2. The x-ray tube as recited in claim 1 , wherein said vacuum enclosure comprises a plurality of extended surfaces. claim 1 3. The x-ray tube as recited in claim 1 , further comprising a cooling plenum joined to said vacuum enclosure and having mounted therein a compensating window, said compensating window cooperating with said extended surfaces of said window to define at least one flow passage capable of admitting at least a portion of said coolant so that said coolant absorbs at least some heat dissipated by at least said window. claim 1 4. The x-ray tube as recited in claim 1 , further comprising a cooling plenum joined to said vacuum enclosure and having mounted therein a compensating window disposed substantially proximate to said window and having an x-ray absorption coefficient value substantially equal to an x-ray absorption coefficient value of said window so that x-rays emitted from said compensating window are of substantially uniform intensity. claim 1 5. The x-ray tube as recited in claim 1 , wherein said plurality of extended surfaces comprise a plurality of fins. claim 1 6. The x-ray tube as recited in claim 1 , wherein said plurality of extended surfaces are disposed in a plane substantially parallel to a computerized tomography slice produced by the x-ray tube. claim 1 7. The x-ray tube as recited in claim 1 , wherein said vacuum enclosure substantially comprises copper. claim 1 8. A cooling system for use in conjunction with an x-ray tube and an x-ray tube vacuum enclosure, comprising: (a) a reservoir containing coolant in which the vacuum enclosure of the x-ray tube is at least partially immersed; (b) an external cooling unit continuously circulating said coolant through said reservoir; and (b) a window mounted in said vacuum enclosure, said window having a plurality of extended surfaces in contact with said coolant so that said coolant absorbs at least some heat dissipated by at least said window. 9. The cooling system as recited in claim 8 , further comprising a cooling plenum joined to said vacuum enclosure and having mounted therein a compensating window, said compensating window cooperating with said extended surfaces of said window to define at least one fluid passageway capable of admitting at least a portion of said coolant so that said coolant absorbs at least some heat dissipated by at least said window. claim 8 10. The cooling system as recited in claim 9 , wherein at least a portion of coolant exiting said external cooling unit is diverted initially to said fluid passageway. claim 9 11. The cooling system as recited in claim 9 , further comprising a plurality of extended surfaces disposed on said compensating window, said plurality of extended surfaces disposed on said compensating window cooperating with said extended surfaces of said window to define said fluid passageway. claim 9 12. The cooling system as recited in claim 9 , further comprising a fluid passageway defined by said cooling plenum, said fluid passageway defined by said cooling plenum being in fluid communication with said fluid passageway cooperatively defined by said compensating window and said extended surfaces of said window. claim 9 13. The cooling system as recited in claim 9 , wherein said coolant comprises a dielectric oil. claim 9 14. A method for cooling an x-ray tube, comprising the steps of: (a) providing a plurality of extended surfaces in a window of the x-ray tube; (b) placing a coolant in contact with said plurality of extended surfaces so that said coolant absorbs at least some heat dissipated by said window; (c) removing at least some heat from said coolant; and (d) repeating steps (b) and (c). 15. The method according to claim 14 , wherein said step of placing said coolant in contact with said extended surfaces comprises passing a flow of said coolant into contact with said extended surfaces. claim 14 16. The method according to claim 15 , further comprising the step of defining a fluid passageway substantially proximate to said extended surfaces of said window, said fluid passageway directing said flow of said coolant into contact with said extended surfaces. claim 15 17. The method according to claim 16 , wherein said step of removing at least some heat from said coolant occurs after said flow of coolant is discharged from said fluid passageway. claim 16 18. The method according to claim 16 , further comprising the step of placing at least a portion of said coolant discharged from said flow passageway in contact with the x-ray tube so that said at least a portion of said coolant absorbs at least some heat dissipated by the x-ray tube. claim 16 19. A window suitable for use in an x-ray device that includes an x-ray tube in contact with a coolant, the x-ray tube comprising a vacuum enclosure having an electron source and target anode disposed therein, and the target anode having a target surface positioned to receive electrons from the electron source so as to produce x-rays, wherein the window is configured to be mounted in the vacuum enclosure proximate to the target surface so that at least some of the x-rays pass through the window, the window comprising: (a) a body; and (b) a plurality of extended surfaces attached to said body, at least one of which is arranged for contact with the coolant. 20. The window as recited in claim 19 , wherein said plurality of extended surfaces are integral with said body. claim 19 21. The window as recited in claim 19 , wherein said plurality of extended surfaces comprise a plurality of fins, a slot being interposed between succeeding fins. claim 19 22. The window as recited in claim 21 , wherein a sum comprising a width of two fins added to a width of two slots is less than a minimum resolving power of the x-ray device. claim 21 23. The window as recited in claim 21 , wherein a width of each of said plurality of slots is substantially equal to a width of each of said plurality of said fins. claim 21 24. The window as recited in claim 21 , wherein each of said plurality of fins and said plurality of slots has a width greater than a minimum resolving power of the x-ray device. claim 21 25. The window as recited in claim 19 , wherein said window comprises beryllium. claim 19 26. The window as recited in claim 19 , further comprising an attenuator disposed at an end of each extended surface so that x-rays emitted through said window are of substantially uniform intensity. claim 19 27. The window as recited in claim 26 , wherein said attenuators comprise copper and said window comprises beryllium. claim 26 28. The window as recited in claim 19 , further comprising an attenuator disposed in a bottom of each slot defined between adjacent extended surfaces so that x-rays emitted through said window are of substantially uniform intensity. claim 19 29. The window as recited in claim 19 , wherein said extended surfaces are disposed in a plane substantially parallel to a computerized tomography slice produced by the x-ray device. claim 19 30. The window as recited in claim 19 , further comprising a clear area through which a useful beam portion of x-rays produced by the x-ray device pass. claim 19 31. The x-ray tube as recited in claim 1 , wherein said window substantially comprises a material selected from the group consisting of: beryllium, titanium, nickel, carbon, silicon, and aluminum. claim 1 32. The x-ray tube as recited in claim 1 , wherein at least one of said plurality of extended surfaces includes first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 1 33. The x-ray device as recited in claim 1 , further comprising a cooling plenum joined to said vacuum enclosure and including a compensating window, said compensating window having a plurality of extended surfaces, at least one of which is at least partially received within a corresponding recess defined by said plurality of extended surfaces of said window. claim 1 34. The window as recited in claim 19 , wherein said window comprises at least first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 19 35. The window as recited in claim 34 , wherein said first portion comprises a bottom of a slot cooperatively defined by a pair of adjacent extended surfaces, and said second portion comprises a part of at least one of said pair of adjacent extended surfaces. claim 34 36. The window as recited in claim 19 , wherein said window substantially comprises a material selected from the group consisting of: beryllium, titanium, nickel, carbon, silicon, and aluminum. claim 19 37. The window as recited in claim 19 , wherein at least one of said plurality of extended surfaces includes first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 19 38. An x-ray device, comprising: (a) a reservoir containing a volume of coolant; (b) a vacuum enclosure at least partially immersed in said volume of coolant, said vacuum enclosure having an electron source and a target anode disposed therein, said target anode including a target surface positioned to receive electrons emitted by said electron source; and (c) a window mounted in said vacuum enclosure proximate to said target anode, said window including a plurality of extended surfaces, at least one of which is in contact with said coolant. 39. The x-ray device as recited in claim 38 , wherein said vacuum enclosure includes a plurality of extended surfaces. claim 38 40. The x-ray device as recited in claim 39 , wherein at least one of said plurality of extended surfaces of said vacuum enclosure is in contact with said coolant. claim 39 41. The x-ray device as recited in claim 38 , further comprising a cooling plenum joined to said vacuum enclosure and including a compensating window, said compensating window cooperating with said extended surfaces of said window to define at least one flow passage in fluid communication with said reservoir. claim 38 42. The x-ray device as recited in claim 38 , further comprising a cooling plenum joined to said vacuum enclosure and including a compensating window disposed proximate to said window, and said compensating window having an x-ray absorption coefficient value substantially equal to an x-ray absorption coefficient value of said window. claim 38 43. The x-ray device as recited in claim 38 , further comprising a cooling plenum joined to said vacuum enclosure and including a compensating window, said compensating window including a plurality of extended surfaces, at least one of which is at least partially received within a corresponding recess defined by said plurality of extended surfaces of said window. claim 38 44. The x-ray device as recited in claim 38 , wherein said window includes at least first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 38 45. The x-ray device as recited in claim 38 , wherein at least one of said plurality of extended surfaces includes first and second portions, said first portion having a different x-ray absorption coefficient than said second portion. claim 38 46. A cooling system suitable for use in conjunction with an x-ray device that includes an x-ray tube vacuum enclosure, the cooling system comprising: (a) a reservoir containing a volume of coolant so that when the vacuum enclosure is received in said reservoir, the vacuum enclosure is at least partially immersed in said coolant; (b) an external cooling unit in fluid communication with said reservoir; and (c) a window mounted in said vacuum enclosure, said window having a plurality of extended surfaces, at least one of which is in contact with said coolant when the vacuum enclosure is received in said reservoir. 47. The cooling system as recited in claim 46 , further comprising a cooling plenum joined to said vacuum enclosure and cooperating with said window to define at least one fluid passageway configured for fluid communication with said external cooling unit. claim 46 48. The cooling system as recited in claim 47 , wherein at least a portion of said cooling plenum is in contact with said coolant contained in said reservoir when the vacuum enclosure is received in said reservoir. claim 47 49. The cooling system as recited in claim 47 , wherein said cooling plenum includes a compensating window disposed proximate said window and cooperating with said window to define said at least one fluid passageway. claim 47 50. The cooling system as recited in claim 49 , wherein said compensating window includes a plurality of extended surfaces, at least one of which is in contact with said coolant. claim 49 51. The cooling system as recited in claim 50 , wherein at least one of said plurality of extended surfaces of said compensating window is at least partially received within a corresponding recess defined by said plurality of extended surfaces of said window. claim 50

abstract

One embodiment relates to a method of controllably reflecting electrons from an array of electron reflectors. An incident electron beam is formed from an electron source, and the incident beam is directed to the array of electron reflectors. A first plurality of the reflectors is configured to reflect electrons in a first reflective mode such that the reflected electrons exiting the reflector form a focused beam. A second plurality of the reflectors is configured to reflect electrons in a second reflective mode such that the reflected electrons exiting the reflector are defocused. Another embodiment relates to an apparatus of a dynamic pattern generator for reflection electron beam lithography. Other embodiments, aspects and features are also disclosed.

description

1. Field of the Invention The present invention relates generally to an emergency core cooling system (ECCS) for a pressurized light water reactor, which directly injects emergency core cooling water, which is supplied from a high-pressure safety injection pump or a safety injection tank, into the downcomer of a reactor vessel, and, more particularly, to a downcomer injection extension duct technology for interrupting an emergency core cooling water direct bypass discharge phenomenon in which emergency core cooling water is guided by a strong cross-flow of the downcomer in the event of a large break loss of coolant accident (LBLOCA), and is then discharged out of the reactor. 2. Description of the Related Art A pressurized light water reactor can encounter unexpected safety problems even though it has been designed in consideration of a sufficient safety margin. If sufficient emergency core cooling water is not supplied when a safety problem in which a large quantity of cooling water leaks occurs, a core can overheat, resulting in damage to the reactor. In order to cool the core when the cooling water leaks, the pressurized light water reactor is equipped with a high-pressure safety injection pump and a safety injection tank such that the emergency core cooling water is exhausted externally. The supply of the emergency core cooling water is divided into two types according to the position of the injection nozzle end. Among the two types, one is a cold leg injection type, in which the injection nozzle is located at a cold leg, and the other is a direct vessel injection type, in which the injection nozzle is located at a reactor vessel. The cold leg injection type means that the emergency core cooling water is supplied to a reactor system through an injection line, which is connected to a cold leg corresponding to a pipe supplying cold water from the circulating pump of a reactor coolant circulatory system into the reactor vessel, and has a drawback in that, when the emergency core cooling water is supplied to a broken cold leg, the emergency core cooling water completely leaks out of the broken cold leg, and thus the reactor core cooling effect cannot be expected. As such, the direct vessel injection type is currently configured to include a direct vessel injection (DVI) nozzle, which supplies the emergency core cooling water to the reactor vessel, and to directly supply the emergency core cooling water to a downcomer between the reactor vessel and a core support barrel. However, the direct vessel injection type has a problem in that there is an increase in the emergency core cooling water direct bypass phenomenon, in which, when the cold leg is broken, the emergency core cooling water is headed to the broken cold leg by strong cross-flow of the downcomer, and is discharged out of the reactor vessel. As illustrated in FIGS. 1A through 1D, a conventional technology for preventing the emergency core cooling water direct bypass phenomenon is designed such that an injection extension duct 110 or 110′ is installed on the outer surface of a core barrel 100 of the downcomer 140 of FIG. 1A or on a baffle region in a core barrel 100 of FIG. 1B, and such that the injection extension duct 110 or 110′ is connected with the DVI nozzle 120 across the downcomer 140 using a pipe 130. Further, the conventional technology is designed such that an injection extension duct 210 is installed on the outer surface of a core barrel 200 of the downcomer 240 of FIG. 1C, and such that the injection extension duct 210 is connected with the DVI nozzle 220 across the downcomer 240 using projection nozzles 230 and 230′. As disclosed in U.S. Pat. Nos. 5,377,242 (James D. Carlton, et al.) and 5,135,708 (James D. Carlton, et al.), illustrated in FIGS. 1A and 1B respectively, the conventional method of connecting the DVI nozzle 120 with the injection extension duct 110 or 110′ in the downcomer 140 using the pipe 130 entails difficulty in the installation of the connecting portion because the gap in the downcomer 140 is narrow. When the reactor vessel is assembled with the core barrel, interference between the projections occurs. Further, according to this prior art, when a large cold leg 150 is broken, the emergency core cooling water can be effectively injected up to a lower portion or a core inlet of the downcomer 140. However, when a DVI line itself is broken, an outlet of the injection extension duct 110 or 110′ functions as an inlet of a break flow due to a siphon effect, and the level of the cooling water in the reactor vessel is lowered by the length of the injection extension duct 110 or 110′, so that the core cladding temperature is abnormally increased. This leads to a problem of noncompliance with safety regulations. As illustrated in FIG. 1C, another conventional technology similar to the aforementioned technology is adapted to directly connect the DVI nozzle 220 and the injection extension duct 210 in the downcomer 240 using a pipe, to position the protruding nozzles 230 and 230′ so as to be opposite to each other, and form a slight gap (Korean Patent Application Publication No. 10-2000-0074521). However, this conventional technology also has a problem in that, when the reactor vessel is assembled with the core barrel 200, interference between the nozzles 230 and 230′ protruding to the downcomer 240 occurs, thus making an assembly difficult, and thus a hole, which is used for a periodical withdrawal checkup of a neutron monitoring capsule installed at a lower portion of the reactor vessel, overlaps with the protruding nozzles, so that work becomes impossible. Further, when the DVI pipe line is broken, the gap between the upper connection nozzles of the injection extension duct 210 is narrow, and thus an inlet-outlet reverse phenomenon, in which the lowest outlet of the injection extension duct 210, located at the lowest position of the injection extension duct 210, functions as an inlet, is caused, although the quantity of intake of the break flow is not much. As such, there is a problem in that the level of the cooling water in the reactor vessel is significantly lowered down to the lowest outlet of the injection extension duct 210, and then the lowest outlet of the injection extension duct 210 functions as an inlet. There is a simpler technology in which an outlet of the DVI nozzle is vertically positioned at a right angle using an elbow 320 (Korean Patent Application Publication No. 10-2003-0064634). However, since the space occupied by the elbow 320 is similar to the gap in the downcomer 330, the reactor vessel 300 cannot be assembled with the core barrel 310. As a result of performing an emergency core cooling water bypass test, it was found that this simple vertical injection has little thermal hydraulic effect, because the direct bypass rate of the emergency core cooling water is very high (NED Vol. 225, “Effect of the yaw injection angle on the ECC bypass in comparison with the horizontal injection,” T. S. Kwon et al., 2003). According to the aforementioned conventional technology, the DVI line for the emergency core cooling water is broken, and thus the lowest outlet of the injection extension duct functions as an inlet for the break flow. In this case, the level of the cooling water in the reactor vessel is gradually lowered to reach a position equal to or lower than the lowest outlet of the injection extension duct, which is located at the lowest position of the injection extension duct. When the level of the cooling water is lowered, the reactor core is exposed. This has a lethal result when the reactor core is cooled. As described above, the conventional common technical problem is mostly attributable to a connection structure in which the DVI nozzle and the injection extension duct are connected to each other in the downcomer. Thus, in order to improve the assemblability between the reactor vessel and the core barrel, avoid an interference between the structures within a checkup work area during the operation, interrupt the inlet-outlet reverse phenomenon of an injection extension duct when the DVI line is broken, and avoid interference between the withdrawal inlet of the neutron monitoring capsule and the injection extension duct or the protruding nozzles, the concept of an injection extension duct having a new structure is required. An emergency core cooling water direct vessel injection system, which directly injects emergency core cooling water into the downcomer of a reactor vessel in a pressurized light water reactor complies with the following design requirements. First, the emergency core cooling water direct vessel injection system should be able to supply a larger quantity of emergency core cooling water to a core inlet through a lower portion of the downcomer by interrupting a phenomenon in which the emergency core cooling water is bypassed and discharged by a high-speed steam cross flow in the downcomer occurring in the event of a large break loss of coolant accident (LBLOCA). Second, a phenomenon in which the level of the cooling water in the reactor vessel is significantly reduced should not occur because an emergency core cooling water outlet of the lowest position of the injection extension duct functions as an inlet for a break flow so as to be able to be applied when the pipe of a direct vessel injection system is broken. Third, due to the injection extension duct installed in the downcomer, the cross flow resistance should not be excessively increased, and the flow induced vibration should not be excessively increased. Fourth, a connector of the injection extension duct and the direct vessel injection nozzle should not cause an interference when the reactor vessel is assembled with the core barrel in the downcomer or an interference with the withdrawal hole of a neutron monitoring capsule. Thereby, a practical application is possible, and the design can be certified, and continuous checkup during an operation of the reactor is possible. Accordingly, the present invention has been made by keeping in mind the above problems occurring in the related art. The present invention is directed to provide an emergency core cooling system having an injection extension duct for a pressurized light water reactor, in which the injection extension duct can be used in the event of a large break loss of coolant accident (LBLOCA) or a breakage accident of a direct vessel injection system pipe by interrupting the phenomenon in which emergency core cooling water is bypassed and discharged by a high-speed steam cross flow in a downcomer when a cold leg is broken, and preventing an inlet-outlet reverse phenomenon of the injection extension duct, thereby not interfering with the reactor assembly work and the periodical checkup work of the neutron monitoring capsule and reducing the cross flow resistance. According to one aspect of the present invention, there is provided an emergency core cooling system having core barrel injection extension ducts, which directly injects emergency core cooling water, which is supplied from a high-pressure safety injection pump or a safety injection tank for a pressurized light water reactor, into a reactor vessel downcomer, and in which the core barrel injection extension ducts interrupt an emergency core cooling water direct bypass discharge phenomenon, in which emergency core cooling water is controlled by a strong cross-flow of the downcomer in the event of a large break loss of coolant accident (LBLOCA) and is thereby discharged out of the reactor. According to the present invention, both a reactor assembly interference problem and an inlet-outlet reverse phenomenon occurring when any pipe of a direct vessel injection system is broken, which occurs in the prior art, are prevented. Thus, a concept of an emergency core cooling water injection extension duct of a downcomer having a new structure that is capable of preventing the level of the cooling water from being excessively lowered in the reactor vessel is realized. In the present invention, without a mechanical connection between the direct vessel injection nozzle and the injection extension duct using a pipe or a protrusion nozzle, the emergency core cooling water inlet and outlet, which are opposite each other, are designed to open in the downcomer. As illustrated in FIG. 2, although the direct vessel injection nozzle is not mechanically connected with the injection extension duct by means of a pipe, an emergency core cooling water intake port, through which the emergency core cooling water is injected from the direct vessel injection nozzle, is formed in the injection extension duct, located on an axis of the direct vessel injection nozzle, and a jet of the emergency core cooling water flows into the injection extension duct in a hydraulic connection fashion. The speed of the jet formed when the emergency core cooling water, supplied from a high-pressure safety injection pump or a safety injection tank, is injected into the reactor vessel downcomer through the direct vessel injection nozzle is high, and a jet stream capable of flowing across the downcomer between the direct vessel injection nozzle and the injection extension duct is formed by the momentum of the injected emergency core cooling water although the direct vessel injection nozzle is mechanically connected with the injection extension duct using a separate connection pipe or a separate protrusion nozzle. Thus, the direct vessel injection nozzle is hydraulically connected with the injection extension duct. Further, the emergency core cooling water does not yet flow into the injection extension duct, and thus part of the emergency core cooling water flows down into the downcomer, which is then stored in the downcomer of the reactor vessel, and contributes to the cooling of the reactor core. As a result, in a normal operation state, in which the emergency core cooling water is not injected, the direct vessel injection nozzle and the injection extension duct are open to the downcomer, and are connected by the water jet only when the emergency core cooling water is injected. Due to the connecting structure in the downcomer, interference, occurring when the reactor vessel is assembled with the core barrel, can be avoided. Further, interference between the withdrawal hole of a neutron monitoring capsule, which is attached to the downcomer of the reactor vessel and is periodically withdrawn and checked, and the injection extension duct can be fundamentally prevented. In the emergency core cooling system having core barrel injection extension ducts as illustrated in FIG. 3, the emergency core cooling water does not form water jets even in the event of an accident in which the direct vessel injection system pipe itself is broken, and thus the hydraulic connection between the direct vessel injection nozzle and the injection extension duct is automatically disconnected. As a result, only the cooling water in the hydraulically disconnected downcomer is discharged out of the reactor vessel through the direct vessel injection nozzle. Thus, in the prior art, in which the direct vessel injection nozzle and the injection extension duct are mechanically connected by a pipe and a nozzle, the inlet of the cooling water discharged out of the reactor vessel is located at the lowest position of the injection extension duct. However, in the present invention, the outlet of the cooling water is limited to the direct vessel injection nozzle, and thus the height of the inlet is further increased by the length of the injection extension duct compared to the prior art. The configuration in which the height of the inlet is further increased has a structural advantage in that it greatly contributes to preventing the level of the cooling water from being excessively lowered in the reactor vessel. Further, in the emergency core cooling system having core barrel injection extension ducts as illustrated in FIG. 5, lateral sides of the conventional injection extension duct are inclined as in a well known quadrilateral cross-sectional injection extension duct, so that the cross flow resistance of the downcomer is reduced. When the cross flow resistance against the jet of the cold leg is reduced, the magnitude of a flow vibration disturbance element is reduced. Thus, the present invention is provided to an emergency core cooling system having injection extension ducts for a pressurized light water reactor, in which the injection extension ducts can be used in the event of both a LBLOCA and a direct vessel injection line break. According to the present invention, the emergency core cooling system having core barrel injection extension ducts prevents the phenomenon in which the emergency core cooling water is directly bypassed and discharged in the event of the LBLOCA and thus makes a larger quantity of emergency core cooling water contribute to the cooling of the core. Further, in the event of a direct vessel injection pipe line break, the emergency core cooling system having core barrel injection extension ducts has an effect of preventing the level of the cooling water from being lowered in the downcomer due to an inlet-outlet reverse phenomenon at the lowest position of the outlet of the injection extension duct. Further, because the present invention features inclined lateral faces of the injection extension duct, which has a quadrilateral cross section in the prior art, the cross flow resistance of the downcomer is reduced. As a result of removing the connection structure between the direct vessel injection nozzle installed in the reactor vessel and the injection extension duct installed on the downcomer-side core barrel, the injection extension duct technology of the emergency core cooling water direct vessel injection system, which prevents an interference between the reactor vessel and the core barrel and a withdrawal interference for the neutron monitoring capsule, is provided, so that the safety and the safety regulatory requirements of the reactor can be sufficiently met. Reference will now be made in greater detail to an exemplary embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and a description to refer to the same or like parts. The detailed descriptions of known functions and constructions that might needlessly obscure the subject matter of the present invention will be avoided herein. As illustrated in FIG. 4A, an emergency core cooling system having injection extension ducts 26 of a core barrel 12 adopts a system of directly injecting emergency core cooling water into the downcomer 16 of a reactor vessel 10. Here, a pressurized light water reactor generally comprises an outer reactor vessel 10 and a core barrel 12, which has a diameter smaller than that of the reactor vessel 10 and is installed at the center of the reactor vessel 10. Further, a core 14 into which nuclear fuel rods are charged is located in the core barrel 12. The downcomer 16, which has an annular gap space due to the diameter difference between the core barrel 12 and the reactor vessel 10, is formed between the core barrel 12 and the reactor vessel 10. The reactor vessel 10 includes a plurality of cold legs 20, which function as inlets into a reactor cooling water circulatory path in the event of a normal operation, and a plurality of hot legs 22, which function as outlets through which cooling water, heated while sequentially flowing through the cold legs 20, the downcomer 16, and the core 14, flows to a steam generator. In the embodiment of the present invention, as illustrated in FIGS. 4A and 4B, four cold legs 20 and two hot legs 22 are installed. As illustrated in FIGS. 4A and 6, the emergency core cooling system of the pressurized light water reactor according to the present invention includes a plurality of direct vessel injection (DVI) nozzles 24 attached to an upper portion of the reactor vessel 10 constituting a nuclear reactor, and a plurality of injection extension ducts 26 for emergency core cooling water, which are installed on the outer surface of the core barrel 12 so as to face the DVI nozzles 24 with the downcomer 16 in between. Each injection extension duct 26 includes an emergency core cooling water intake port 28 in an outer surface thereof, wherein the emergency core cooling water intake port 28 passes through the outer surface of the injection extension duct 26, adopting a point intersecting an axis of the DVI nozzle 24 as the central point thereof, and has a diameter about twice the inner diameter DDVI of the DVI nozzle. According to this configuration of the present invention, no connection structure is installed between the DVI nozzle 24 and the injection extension duct 26 (particularly, the emergency core cooling water intake port). In other words, the DVI nozzle 24 is completely mechanically separated from the injection extension duct 26. In this manner, with the emergency core cooling system having the core barrel 12 and the injection extension duct 26 according to the present invention, the DVI nozzle 24 is separated from the injection extension duct 26 from a thermal hydraulic aspect when the pressurized light water reactor is normally operated without supply of the emergency core cooling water from the DVI nozzle 24, and no connection structure exists in the downcomer 16 between the DVI nozzle 24 and the injection extension duct 26 in the structural aspect. Thus, no interference occurs when the reactor vessel 10 and the core barrel 12 are assembled and when the neutron monitoring capsule is withdrawn. However, if a cold leg 20 is completely broken, the water jet speed of the emergency core cooling water injected through the DVI nozzle 24 amounts to about 22 m/sec while the safety injection tank injects the emergency core cooling water, and about 1.6 m/sec while the high-pressure safety injection pump injects the emergency core cooling water. The horizontal inertial force of the emergency core cooling water having such a water jet speed is sufficient for the emergency core cooling water to flow from the DVI nozzle 24 into the emergency core cooling water intake port 28 of the injection extension duct 26 facing the DVI nozzle 24 across the downcomer 16. Thus, the water jet formed while only the emergency core cooling water is injected causes the DVI nozzle 24 to be connected with the injection extension duct 26 in a thermal hydraulic fashion. The emergency core cooling water injected into the injection extension duct 26 can flow down to the lower portion of the downcomer 16 by means of gravity and flow momentum of the emergency core cooling water. The emergency core cooling water, which flows to the lower portion of the downcomer 16 in the injection extension duct 26 in the event of a large break loss of coolant accident (LBLOCA), is protected from a high-speed cross flow of the downcomer 16 by a wall of the injection extension duct. Further, although not all of the emergency core cooling water has flown into the injection extension duct 26, i.e. although part of the emergency core cooling water flows down to the downcomer 16, the emergency core cooling water that has not flown into the injection extension duct 26 is collected in the downcomer 16 of the reactor vessel 10, and thus helps cool the reactor core. In the case in which the pipe connected to the DVI nozzle 24 is broken, the emergency core cooling water is not injected into the broken DVI nozzle 24. Thus, the water jet of the emergency core cooling water is not formed. However, since the DVI nozzle 24 is separated from the injection extension duct 26 in a thermal hydraulic fashion, only the cooling water of the downcomer 16 around the DVI nozzle 24 leads to the formation of a break flow. In other words, when the DVI nozzle 24 is still connected with the injection extension duct 26 in spite of an accident in which the DVI line itself is broken, the lowest outlet of the injection extension duct 26 functions instead as an inlet for the break flow due to a siphon effect, so that the level of the cooling water in the reactor vessel 10 is remarkably lowered by the length of the injection extension duct 26. Thus, the break flow leaks out at a position where the DVI nozzle 24 faces the injection extension duct 26, i.e. at a position where the level of the cooling water is raised by the length of the injection extension duct 26 without the phenomenon in which the lowest outlet of the injection extension duct 26 functions as an inlet. Accordingly, the level of the cooling water of the reactor vessel 10 in the present invention can be maintained in a manner such that the level of the cooling water of the downcomer is much higher than in the structure in which the DVI nozzle 24 is mechanically connected with the injection extension duct 26 using a pipe, as in the prior art. Thus, the quantity of the cooling water that flows from the downcomer to the core due to the water level difference between the downcomer and the core is increased, so that the reactor core is more effectively cooled. As illustrated in FIG. 4A, the emergency core cooling system of the present invention is designed on a plane in a manner such that the angle α between the DVI nozzle 24 and the cold leg 20 is smaller than that between the DVI nozzle 24 and the hot leg 22. As also illustrated in FIG. 4A, the emergency core cooling system of the present invention is designed on a plane in a manner such that an angle β between the DVI nozzle 24 and the hot leg 22 is smaller than that between the DVI nozzle 24 and the cold leg 20. In the present invention, the angle between an axial line, which connects the DVI nozzle 24 and the injection extension duct 26, and the cold leg 20, and the angle between an axial line, which connects the DVI nozzle 24 and the injection extension duct 26, and the hot leg 22 are preferably smaller than the angle α between the DVI nozzle 24 and the cold leg 20 and the angle β between the DVI nozzle 24 and the hot leg 22, respectively. The embodiment of the present invention will be described in conjunction with the case in which an angle between the cold leg 20 and the hot leg 22 is 60°. An area between the angle α of FIG. 4A and the angle β of FIG. 4B is the movement path of an excore monitor that moves outside the reactor vessel in a vertical direction (in upward and downward directions). Thus, in consideration of the width of the injection extension duct 26 and the distance between the injection extension duct 26 and the neighboring hot leg 22, the maximum angle α is preferably less than 15°, and the maximum angle β is preferably less than 35°. The configuration of the injection extension duct 26 of the present invention will be described in greater detail with reference to FIG. 5A. The lowest outlet of the injection extension duct 26 is open, and the highest cap of the injection extension duct 26 is closed, and it includes at least one air vent 30 such that gas can be discharged when the nuclear reactor is filled with water. The injection extension duct 26 includes the emergency core cooling water intake port 28, which has about twice the inner diameter of the DVI nozzle 24 facing the injection extension duct 26, and is located on the axial line of the DVI nozzle 24. As illustrated in FIG. 6, the diameter DDUCT of the emergency core cooling water intake port 28 is about twice the inner diameter DDVI of the DVI nozzle 24 in consideration of a deflection of the jet by means of gravity in the case in which the jet of the DVI nozzle 24 has a small spreading and flow rate. Thereby, the emergency core cooling water is more easily introduced into the injection extension duct 26. As seen from FIG. 5A, the injection extension duct 26 includes lateral faces 32 inclined in a transverse direction at an angle of about 45° on opposite sides thereof, so that it can reduce the resistance of the cross flow of the downcomer 16 compared to an existing injection extension duct, the lateral faces of which are formed at an angle of 90°. The injection extension duct 26, which protrudes from the core barrel 12 toward the reactor vessel 10, preferably has a radial distance (h) limited to a range from about 3/25 to about 7/25 of the width of the diametrical gap of the downcomer 16 of FIG. 6. This is because, in the case in which the radial distance (h) must be smaller than the minimum inner diameter RKEY of the upper alignment key portion 34 of the reactor vessel 10 and the inner diameter RHL of the hot leg 22 of FIG. 4A, no interference occurs when the reactor vessel 10 and the core barrel 12 are assembled or when the neutron monitoring capsule is withdrawn. Thus, as for the cross-sectional shape of the injection extension duct 26 of the present invention, the radius of the curvature of the outer surface of the injection extension duct 26 protruding to the downcomer 16 is equal to the sum of the radius R of the core barrel 12 and the radial distance h of the injection extension duct 26, and the opposite lateral faces of the injection extension duct 26 are similar to the non-parallel opposite sides of an isosceles trapezoid. As in FIG. 5B, the injection extension duct 26 of the present invention has a vertical cross-sectional area defined by multiplying the average of the dimensions of the two circumferential parallel faces in the circumferential direction of the core barrel 12 by the radial distance, and is preferably equal to or greater than the effective cross-sectional area of the DVI nozzle 24. Here, considering all of the aforementioned conditions, including the condition that the radial distance h of the injection extension duct 26 be limited to a range from about 3/25 to about 7/25 of the width of the diametrical gap of the downcomer 16, a central angle Φ, subtending the longer circumferential face of the injection extension duct 26 in the circumferential direction of the core barrel 12, appropriately ranges from a minimum of 20° to a maximum of 35°. The length of the injection extension duct 26 installed on the core barrel 12 starts from the emergency core cooling water intake port 28 facing the DVI nozzle 24 of the reactor vessel 10, and then extends to a lower portion of the downcomer 16 below the positions of the cold leg 20 and the hot leg 22 along the outer surface of the core barrel 12. Further, the lowest position (length B of FIG. 6) of the outlet of the injection extension duct 26 is within a range from the central axis of the cold leg 20 to a lower portion of the downcomer 16. Considering that the surrounding cooling water is swept out by a strong intake break flow formed in the downcomer 16 around the broken cold leg 20 in the event of a LBLOCA, the lowest position of the outlet of the injection extension duct 26 must exist at a lower portion of the downcomer 16, which is lower than the position of the cold leg 20, and preferably extend toward a lower portion of the downcomer 16 from the central axis of the cold leg 20 within a range from about two to four times the inner diameter DCL of the cold leg, thereby being effective in preventing a direct bypass of the emergency core cooling water. Although the exemplary embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

claims

1. A method of installing an inlet mixer clamp assembly on a transition piece of a Boiling Water Reactor (BWR) jet pump assembly, the method comprising:forming a clamp plate, the clamp plate being formed to fit between lifting eyelets of the inlet mixer and cover upper surfaces of the transition piece,attaching the clamp plate to the transition piece by mounting a pair of mounting blocks onto a bottom surface of the clamp plate, each mounting block forming an aperture that is configured to allow the mounting block to cradle a bridge of the transition piece,attaching a jacking bolt to the clamp plate by penetrating the jacking bolt through the clamp plate and having the jacking bolt contact a top surface of the inlet mixer, andapplying a force to the top surface of the inlet mixer by tightening the jacking bolt. 2. The method of claim 1, further comprising:forming two beam bolt holes in the clamp plate, the beam bolt holes being configured to allow a pair of beam bolts of the inlet mixer to protrude through the clamp plate when the clamp plate is attached to the transition piece. 3. The method of claim 1, further comprising:forming first and second opposing lobes on the clamp plate, the first and second lobes being configured to extend away from the clamp plate at locations directly above the bridges of the transition piece when the clamp assembly is attached to the transition piece. 4. The method of claim 3, wherein the forming of the clamp plate includes bisecting the clamp plate into approximately two halves with a centerline, the centerline running through the middle of the first and second lobes, wherein the attaching of the jacking bolt includes penetrating at least one jacking bolt through the clamp plate on either side of the centerline. 5. The method of claim 4, further comprising:forming third and fourth opposing lobes on the clamp plate, the third and fourth lobes being configured to extend away from the centerline of the clamp plate, wherein the at least one jacking bolt on either side of the centerline penetrates the third and fourth lobes, respectively. 6. The method of claim 3, further comprising:attaching each mounting block to the first and second lobes, respectively.

description

The present invention relates to a container having thick walls (“thick container”) such as a cask for housing, transporting and storing used nuclear fuel aggregate and substance contaminated by radioactivity, for example. Particularly this invention relates to a thick container in which a body section and a bottom section are formed integrally, or a thick drum which can be used as a cylinder of a large size pressing machine, or a canister for storing substance contaminated by radioactivity. The invention relates to a container or a drum which requires less troublesome steps for manufacturing and has an excellent end surface form, a manufacturing apparatus thereof and a manufacturing method thereof. As a cylinder or the like to be used in a cask or a large size pressing machine for containing and transferring and temporarily storing used nuclear fuel generated from a nuclear reactor, a container in which height, diameter and the like of its drum reach several meters is used. Containers having a wall that is several dozens centimeter in thickness have been suggested from a viewpoint of shielding from γ rays or high pressure resistance. A cask for containing and transporting and temporarily storing used nuclear fuel will be exemplified and there will be explained below a conventional container which has been used for these applications. FIG. 29 is a sectional view showing one example of the conventional cask. The cask 500 is composed of a container 501 which is formed with a body section 501a and a bottom section 501b made of stainless or carbon steel, a basket 502 for used containing nuclear fuel aggregate which is arranged in the container 501, and a neutron shielding body 503 provided on an outer periphery of the container 501. The neutron shielding body 503 is charged into a space between an outer drum 504 and the container 501, and a plurality of heat transfer fins (not shown) are provided between the container 501 and the outer drum 504. As the basket 502, a material to which boron having neutron absorbing ability is added is used. The bottom plate 501b made of stainless or carbon steel is welded with tungsten-inert gas (TIG welding) or welded with submerged-arc (SAW welding) to the container 501. A neutron shielding material 506 is sealed into the bottom plate 501b. Moreover, a primary cover 507 and a secondary cover 508 are attached to an upper section of the container 501 by bolts. A neutron shielding material 509 is sealed into the secondary cover 508. γ rays generated from used nuclear fuel aggregate are shielded by the body section 501, the bottom plate 501b, the primary cover 507 and the secondary cover 508. Moreover, neutron is shielded by the neutron shielding material 503 provided on the outer periphery of the container 501, the bottom plate 501b, the neutron shielding material 506 sealed into the secondary cover 508, and the secondary cover 508. A degradation heat of the used fuel aggregate is transmitted from the container 501 via the heat transfer fins to the outer drum 504 and is radiated to the outside therefrom. Next, how a container having a bottom (“bottomed container”) for the cask shown in FIG. 29 is manufactured will be explained below. FIGS. 30(a) through 30(e) are explanatory diagrams showing one example of the method of manufacturing the bottomed container of the cask shown in FIG. 29. As shown in FIG. 30(a), a metal billet 61 which is cogged into a predetermined dimension is upset onto an anvil with bore and is bored by a punch 63. As shown in FIGS. 30(b) and 30(c), a mandrel 65 is inserted through a hole 64 of the metal billet 61 and while it is being rotated, the hole 64 is widened by a hammer 66. As shown in FIG. 30(d), the mandrel 65 is replaced by a large-diameter mandrel 67 and hollow cogging is carried out by a hammer 68. As a result, the metal billet 61 is thinned so that a cylindrical body is formed (FIG. 30(e)). FIGS. 31(a), 31(a′), 31(b) and 31(b′) are explanatory diagrams showing the method of manufacturing the bottomed container according to an Erhardt boring method. This method is for pushing a punch 410 into a metal billet 200 put into a container so as to form the metal billet 200 into a cylindrical shape. This metal billet 200 has a rectangular section, and its diagonal length is equal with an inner diameter of a body section 300 of the container. Moreover, since the section of the metal billet 200 is rectangular, spaces 350 exist between the metal billet 200 and the container 200 (FIG. 31(a′)). When the metal billet 200 is upset into the body section 300 of the container and the punch 410 is pushed into a center axis of the metal billet 200, metal flow occurs due to metal swelling function of the punch 410. While this metal flow is filling the spaces 350 and a part of the metal flow is rising in the body section 300 of the container, and the metal billet 200 is formed into the cylindrical form (FIG. 31(b)). In addition, the bottomed container of the cask can be manufactured also by a backward extrusion pressing method (not shown). In the backward extrusion pressing method, after the metal billet having a circular section substantially equal with the inner diameter of the container is upset into the container, metal flow is generated between the punch and the container by a compressive force of the punch pressurizing along the center axis of the metal billet. While the metal is being raised to a backward direction, the metal billet is formed into a long cylindrical form. After the cylindrical body section 501a is formed by one of the above-mentioned methods, the bottom plate 501b is welded to its lower section. Further, in order to remove a thermal stress due to the welding, the container 501 is subject to heating treatment. However, in order to obtain the bottomed container of the conventional cask 500, since the bottom plate 501b is jointed to the cylindrical body section 501a by welding, the container should be subject to the heating treatment after the welding. For this reason, there arose a problem that the manufacturing requires troublesome steps. Moreover in the Erhardt boring method, as shown in FIG. 31, a drop of a temperature in a metal forward end portion rising in the spaces 350 causes scratches and wave-shaped defects. Further, as shown in the diagrams, since a faulty form portion (FIG. 31(b)) is inevitably generated in the cylindrical end section, this portion should be eliminated by a constant amount, and thus yield is greatly lowered. In addition, in the backward extrusion pressing method, the metal billet is formed while high friction is generated between the container and metal billet. For this reason, a lot of defects such as pockmarks and ribs are generated on an outer surface of the metal billet, and it takes a long time to remove these defects. Further, in both the Erhardt boring method and the backward extrusion pressing method, when a dimension and a thickness of the container to be formed become large, a pressure which is required for pressing becomes extremely large. Therefore, in these methods, it was difficult to manufacture a container having large dimension and thickness. Therefore, the present invention is devised in order to solve the above-mentioned problems, and its object is to provide a container which requires less troublesome manufacturing steps or a container in which defects generated on its cylindrical end portion and its surface can be suppressed. The radioactive substance container according to this invention comprises a thick bottomed container in which a bottom section and a body section are formed integrally by hot-dilating a metal billet in a container for forming. The radioactive substance container according to the next invention comprises a thick bottomed container in which when a metal billet is hot-dilated in a container for forming and its body section is worked, a boring uncompleted section remains on one end side of the body section so as to be a bottom section and the bottom section and the body section are formed integrally. As the above-mentioned radioactive substance container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required, and heating treatment of the welding can be omitted. Moreover, since the bottomed container is formed by the hot dilation, only a press pressure, which is lower than that at the time of hot backward extrusion forming, for example, is required. Here, at the time of the hot press working and the upsetting drawing, punching and drawing which are normally known can be combined, and they are not limited to detailed working methods described below. Moreover, the radioactive substance container according to the present invention can store not only used nuclear fuel but also substances and the like contaminated by radioactive rays. The bottomed container provided to the radioactive substance container includes a so-called thick container in which a thickness is thick with respect to its radius like a container to be used for a cask for transporting and storing used nuclear fuel. Here, the thick container is such that a ratio of a difference between an outer radius R0 and an inner radius Ri, namely, a thickness t=R0−Ri to an average radius R=(Ro−Ri)/2 is (t/R)> 1/10. When the section of the container is not circular, an equivalent diameter de=s/π may be used for calculating the outer radius R0, the inner radius Ri and the average radius. Here, s is a peripheral length of the section, and when a length of one side is a, s=4×a in the case of quadrate section. In addition, the bottomed container of the invention is suitable for a container in which a ratio (L/Di) of an axial length L to an inner diameter Di is not less than 1 like a cask for storing used nuclear fuel in which an axial length reaches a several meters. Moreover, the present invention may be applied to a canister as a radioactive substance container. When (L/Di) is less than 1, a certain effect is produced, but as (L/D1) becomes larger than 1, the effect according to this invention is produced more remarkably. The radioactive substance container according to the next invention is the radioactive substance container described above in which a section of the metal billet vertical to an axial direction is polygonal and a shape in a section of the container for forming vertical to an axial direction is circular. As this radioactive substance container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required. Moreover, as for the bottomed container is hot-dilated by setting the metal billet having a polygonal section vertical to the axial direction into the container for forming having a circular internal shape of the section vertical to the axial direction. At this forming step, since the container is dilated to be formed by a function which bends each side of the polygon, the bottomed container in which the bottom section and the body section are integral can be formed with a lower pressure than conventional one. The radioactive substance container according to the next invention is the radioactive substance container described above in which a section of the metal billet vertical to an axial direction is polygonal and a section of the container for forming vertical to an axial direction is polygonal. As this radioactive substance container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required. Moreover, the bottomed container is hot-dilated by setting the metal billet having a polygonal section vertical to the axial direction into the container for forming having a polygonal internal shape of the section vertical to the axial direction. At this forming step, since the container is dilated to be formed by a function which bends each side of the polygon, the bottomed container in which the bottom section and the body section are integral can be formed with a lower pressure than conventional one. Moreover, an internal shape of the container for forming is changed so that bottomed containers having external forms according to various radioactive substance containers can be formed easily. The radioactive substance container according to the next invention comprises a bottomed container for storing a basket for used nuclear fuel aggregate in which a bottom section and a body section is integral by hot dilation forming in a container for forming. Here, the basket is constituted by collecting angular pipes, for example, its section vertical to the axial direction has like an internal shape of a section of the bottomed container shown in FIG. 15(d). Moreover, its outer diameter reaches about 2 to 2.5 m. As this radioactive substance container, the thick bottomed container for containing used nuclear fuel aggregate, in which a dimension in the axial direction reaches a several meters and inner diameter reaches 2 to 2.5 meters, is used so that the conventional welding of a bottom plate is not required and the heat treatment after the welding can be omitted. Particularly in the bottomed container whose thickness is thick and dimension in the axial direction is a several meters and inner diameter reaches 2 to 2.5 meters, the effect which can omit the steps is extremely great. In this container having such a size, the bottomed container of the present invention can be manufactured about one month earlier than a conventional container in which a bottom plate is welded. This difference depends upon time required for the welding itself and time required for the post-welding heat treatment and cooling. Since the bottomed container according to this invention does not require theses processes, the manufacturing time can be greatly shortened. The radioactive substance container according to the next invention is the radioactive substance container described above in which a section of the boring punch has a dimension and a shape which approach to the section of the basket for used nuclear fuel aggregate. Since this radioactive substance container has a dimension such that the section of the boring punch is approximate to the section of the basket for used nuclear fuel aggregate, an operation for cutting the inside of the container becomes easy after the hot dilation forming, and the manufacturing does not require the troublesome steps. Here, “dimension such that the section of the boring punch is approximate to the section of the basket for used nuclear fuel aggregate” means a relationship that the dimension of the section of the boring punch is substantially equal with a difference between the dimension of the basket for used nuclear fuel aggregate and a cutting allowance of the inner side of the bottomed container to be formed. As the boring punch of the present invention, a boring punch 27c or 27d shown in FIGS. 27(c) and 27(d), for example, can be used. Moreover, the sectional shape of the basket is like the internal shape of the section of the bottomed container shown in FIGS. 15(c) and 15(d), for example, the sectional shape of the boring punch of the invention can be approximate to the these shapes. Further, it is considered that when the sectional shape of the basket for used nuclear fuel aggregate is the internal shape of the section of the bottomed container shown in FIG. 15(d), for example, the inner side of the bottomed container having the inner shape of the section shown in FIG. 15(c) is cut so as to be formed into the internal shape of the section shown in FIG. 15(d). In this case, the section of the boring punch where the internal shape of the section shown in FIG. 15(c) is formed has a dimension that it is approximate to the section of the basket. The radioactive substance container according to the next invention comprises a bottomed container, in which a dosage equivalent factor of γ rays on an outer wall surface of a substantially center portion of a side surface of the body is not more than 200 μSv/h in the case where radioactive substance is contained in a bottomed container in which its bottom section and body section are formed integrally by hot dilation forming in a container for forming. Since the bottomed container to be used as this radioactive substance container transports and stores used nuclear fuel, it requires the function for shielding the γ rays radiated from the used nuclear fuel. It is desirable that the dosage equivalent factor of the γ rays on the outer wall surface of the substantially center portion of the sides surface of the radioactive substance container is smaller, not more than 2000 μSv/h confirms to the standard of transportation and storing based on “The rules relating to transportation of nuclear fuel substance and the like outside factory and business establishment (dated on Dec. 28, 1978, Prime Minister's Office Statute No. 57) (Final Amendment dated Nov. 28, 1990, Prime Minister's Office Statute No. 56)”, “The notice for defining details relating the technical standards relating to transportation of nuclear fuel substance and the like outside factory and business establishment (dated Dec. 28, 1978 Science and Technology Agency Notice No. 11) (Final Amendment dated Nov. 28, 1990, Science and Technology Agency Notice No. 5)” and “The technical study relating to storing of used fuel in a dry type cask (July, 1992, Agency of Natural Resources and Energy)”. The bottomed container to be used as the radioactive substance container of the present invention takes the safety into consideration, and the bottomed container is formed into a thick container made of stainless steel or carbon steel where the thickness reaches several dozens cm so that the dosage equivalent factor can be lowered to about 1/10 of this value. In this radioactive substance container, since the body section and the bottom section of the thick container are formed integrally, the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Particularly in such a thick bottomed container, the effect which can omit the above-mentioned steps is extremely great. The radioactive substance container according to the next invention is the radioactive substance container described above in which an outer diameter of the bottomed container is not less than 1000 mm to not more than 3000 mm and its thickness is not less than 150 mm to not more than 300 mm. As this radioactive substance container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Particularly in such a thick bottomed container, the effect which can omit the above-mentioned steps is extremely great. The radioactive substance container comprises a bottomed container where a metal billet, in which at least a section vertical to an axial direction on a forward side with respect to the pressing direction (“pressing forward side”) is formed into a polygonal shape, is set into a container for forming and a boring punch is pushed into the metal billet and the metal billet is hot-dilated so that a bottom section and a body section are formed integrally. This radioactive substance container has the bottomed container where the body section and the bottom section are formed integrally. When such a bottomed container is used, the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Moreover, since this bottomed container can be formed with lower press pressure than the conventional one and the movement of the metal to the axial direction of the container is minimum, defects generated on an end portion and a container surface is less. For this reason, the adjustment of these defects requires less troublesome steps. The bottomed container of the radioactive substance container is suitable for a container in which a ratio (L/Di) of an axial length L to an inner diameter Di is not less than 1 like a cask for storing used nuclear fuel in which an axial length reaches a several meters. When such a thick bottomed container which is long with respect to the axial direction is tried to be formed by the conventional hot working method, the press pressure of several dozen-thousand ton is required, and a lot of defects occur on the end portion or the surface of the formed bottomed container. Therefore, such a thick container was conventionally manufactured by welding the bottom to a thick cylinder which was manufactured by the roll forging method or the like. On the contrary, the bottomed container according to this invention can be manufactured by one-time working as a thick bottomed container which is long with respect to the axial direction and can store used nuclear fuel aggregate. Moreover, since the press pressure is about ten-thousand tons, an existing large pressing machine can be used. Moreover, since defects do not occur on the end portion or the surface of the container, adjustment after the forming is seldom required. In addition, in the case where the container does not have such a large dimension, even if the thick container in which (t/R) exceeds 1/10 is formed by the hot press forming, high press pressure is not required. However, a lot of defects occur on the end portion or the surface of the formed bottomed container. For this reason, it is difficult to form such a container according to hot press forming, but this thick bottomed container in which (t/R) exceeds 1/10 can be formed by one-time working, and defects seldom occur on the end portion or the surface. The radioactive substance container according to the next invention comprises: a bottomed container where a bottom section and a body section are formed integrally by hot press pressure and γ rays generated from radioactive substance such as used fuel is shielded; a neutron shielding member which is provided around the bottomed container and shields neutron generated from the radioactive substance; and a cover for covering an opening of the bottomed container. The radioactive substance container according to the next invention comprises: a bottomed container which contains a radioactive substance such as used fuel into a body section with a bottom section and shields γ rays generated from the radioactive substance; a neutron shielding material which is arranged around the bottomed container and shields neutron generated from the radioactive substance, wherein a metal billet is heated and is upset and drawn so that the bottom section and the body section are formed integrally. In this radioactive substance container, the bottomed container, where the body section and the bottom section are integral, is formed. When such a bottomed container is used, the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. At the time of the hot press working and upsetting draw forming, the punching and drawing which are normally known can be combined, the forming is not limited to the detailed working method mentioned below. This radioactive substance container can contain not only used fuel but also substances contaminated by radioactive rays. The radioactive substance container according to the next invention is the radioactive substance container described above in which a spot facing section is further formed integrally with the bottom section at the time of forming the bottomed container. This radioactive substance container is a bottomed container in which the metal billet is hot-dilated and simultaneously the spot facing section is also provided on the bottomed section. In a cask, since the bottomed section has a neutron absorbing member, the spot facing section is provided on the bottomed section of the container. Since the spot facing section has been conventionally provided by the cutting work or by welding a bottom plate previously provided with a spot facing section to the body section, the manufacturing requires the troublesome steps. In the bottomed container, since the spot facing section is formed integrally at the time of the hot-dilation forming, the step of forming the spot facing section can be omitted. The radioactive substance container according to the next invention is the radioactive substance container described above in which a flange is further provided integrally with the body section of the bottomed container. In the conventional radioactive substance container, since the flange section is separately manufactured and is welded to the body section so as to be attached, the heat treatment after the welding is required, namely, the manufacturing requires the troublesome steps. Moreover, since the radioactive substance container itself requires sealing property and strength, the welded portion requires high solidity. According to the bottomed container of the present invention, since the flange and the body section are formed integrally, the welding and post-welding heat treatment steps are omitted, and simultaneously the sealing property and the strength of the container itself can be secured. The radioactive substance container according to the next invention is the radioactive substance container described above in which at least one of an external section and an internal section of the bottomed container vertical to the axial direction is polygonal. Since a basket is contained in the bottomed container to be used particularly for a cask as the radioactive substance container, it is preferable that the sectional shape of the inside of the bottomed container is formed according to the shape the basket. In the conventional method, after the section of the inside of the bottomed container is formed into a circular shape, it is formed into a shape according to the basket by the cutting or the like. When this radioactive substance container is hot-dilated, the section of the inside of the container can be formed into a shape according to the basket. For this reason, the cutting step which was conventionally required can be omitted. The polygonal shape of the inner section of the body section includes so-called polygonal shapes such as triangular and tetragonal shapes and also shapes shown in FIGS. 15(c) and 15(d). Thereafter, this is applied also to the following inventions. The hot dilation forming-use metal billet according to the next invention in which at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape. Since this hot dilation forming-use metal billet is formed so that at least the section vertical to the axial direction on the pressing forward side is formed into a polygonal shape, the metal billet is dilated towards the inner wall of the container by a function for bending each side of the polygon at the time of the hot dilation forming. Since the metal billet is dilated in a space between the pressing forward side and the body of the container at the time of the hot dilation forming, the phenomenon that the metal flows to the opposite direction to the pressing direction is suppressed. Due to these functions, with this hot dilation forming-use metal billet, a thick container where a ratio of the axial length to the diameter is not less than 1 can be formed with a press pressure which is a several part of the conventional press pressure. Moreover, defects which occurs on the end portion or the surface of the container after the forming can be suppressed. The hot dilation forming-use metal billet according to the next invention in which at least one plane is provided on at least one of a side surface on a pressing forward side and a side surface on a backward side with respect to the pressing direction (“pressing backward side”). In this hot dilation forming-use metal billet, since its side surface has at least one plane and the metal billet is hot-dilated by the function for bending the plane towards the inner wall of the container for forming, the press pressure required at the time of the hot dilation forming is lower than the case where the side surface is a curved surface. Therefore, the thick container which is long in the axial direction can be formed with the press pressure smaller than the conventional pressure. Moreover, internal defects such as cracks can be reduced in comparison with the case where the side surface is a bent surface. The hot dilation forming-use metal billet according to the next invention is the hot dilation forming-use metal billet described above in which a taper which becomes thinner towards the pressing direction is provided on the pressing forward side of the metal billet. The hot dilation forming-use metal billet according to the next invention is the hot dilation forming-use metal billet described above in which at least one or more stepped sections are provided so that the pressing forward side of the metal billet becomes thinner gradationally towards the pressing direction. In the above-mentioned hot dilation forming-use metal billet, since the timing at which the metal fills the vicinity of the bottom of the container for forming can be delayed at the final stage of the hot dilation forming, the upsetting of the metal billet can be suppressed at the final stage of the hot dilation forming. As a result, thus the press pressure can be reduced at the time of the hot dilation forming. The hot dilation forming-use metal billet according to the next invention is the one in which at least one plane is provided on a side surface and an extended section which engages with an end portion of an inlet of a container for forming is provided on an end portion on a pressing backward side. Since this hot dilation forming-use metal billet is provided with the extended section at the end portion on the pressing backward side, the metal billet is engaged with the end portion of the container by the extended section at the time of the hot dilation forming. With this function, constraint of the container on the metal billet becomes stronger so that the upsetting of the metal billet on the pressing forward side can be suppressed. Moreover, since the side surface is provided with at least one plane, the function for bending this plane and the function for suppressing the upsetting of the metal billet accrue. Therefore, due to their interaction, the press pressure can be suppressed to be small. Moreover, the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing. For this reason, since the step of forming the extended section which is extended to above the body section of the container on the pressing backward side is not required before the hot dilation forming step, the container manufacturing step can be simplified. Here, in this metal billet, the sectional shape is uniform along the axial direction. The hot dilation forming-use metal billet according to the next invention is the one in which at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape, and an extended section which engages with an end portion of an inlet of a container for forming is provided on a pressing backward side. Since this hot dilation forming-use metal billet is provided with the extended section on the pressing backward side, this extended section engages the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting of the metal billet on the pressing forward side can be suppressed. Moreover, since at least the section vertical to the axial direction on the pressing forward side was formed into a polygonal shape, the function for bending each side of the polygonal section and the function for suppressing the upsetting of the metal billet accrue. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing. For this reason, since the step of forming the extended section which is extended to above the body section of the container on the pressing backward side is not required before the hot dilation forming step, the steps of manufacturing the container can be simplified. The hot dilation forming-use metal billet according to the next invention is the one in which at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape, and at least one or more stepped sections are provided so that the pressing forward side becomes thinner gradationally towards a pressing direction, and an extended section which engages with an end portion of an inlet of a container for forming is provided on a pressing backward side. Since this hot dilation forming-use metal billet is provided with the extended section on the pressing backward side, the extended section latches the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least the section vertical to the axial direction on the pressing forward side was formed into a polygonal shape, the function for bending each side of the polygonal section and the function for suppressing the flow of the metal accrue. Further, since the pressing forward side becomes thinner gradationally towards the pressing direction, the timing at which the metal fills the bottom section of the container for forming can be delayed. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing. For this reason, since the step of forming the extended section which is extended to above the body section of the container on the pressing backward side is not required before the hot dilation forming, the steps of manufacturing the container can be simplified. Here, since the pressing forward side becomes thinner gradationally, the forming becomes comparatively easy. The hot dilation forming-use metal billet according to the next invention is the one in which at least one plane is provided on at least one of a side surface on a pressing forward side and a side surface on a pressing backward side, and at least one or more stepped sections are provided so that the pressing forward side becomes thinner gradationally towards the pressing direction, and an extended section which engages with an end portion of an inlet of a container for forming is provided on the pressing backward side. Since this hot dilation forming-use metal billet is provided with the extended section on the pressing backward side, the extended section latches the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least one of the side surfaces of the metal billet has at least one plane, the function for bending the plane and the function for suppressing the flow of the metal accrue. Further, since the pressing forward side becomes thinner gradationally towards the pressing direction, the timing at which the metal fills the bottom section of the container for forming can be delayed. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing. For this reason, since the step of forming the extended section which is extended to above the body section of the container on the pressing backward side is not required before the hot dilation forming, the steps of manufacturing the container can be simplified. Here, since the pressing forward side becomes thinner gradationally, the forming becomes comparatively easy. The container according to the next invention is the one in which a metal billet is hot-dilated in a container for forming, and a bottom section and body section are formed integrally and a thick bottomed container is obtained. The container according to the next invention is the one in which, when a metal billet is hot-dilated in a container for forming and a body section is worked, a boring uncompleted section is allowed to remain on one end side of the body section so as to be a bottom section, and an integrally thick bottomed container is obtained. As the container of the above-mentioned invention, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required, and heat treatment after that can be omitted. Moreover, since the bottomed container is formed by the hot dilation, only a press pressure, which is lower than that at the time of hot backward extrusion forming, for example, is required. Here, at the time of the hot press working and the upsetting drawing, punching and drawing which are normally known can be combined, and they are not limited to detailed working methods described below. In addition, like a cylinder or the like for a large pressing machine, the above-mentioned container includes a so-called thick container in which a thickness is thick with respect to its radius. Here, the thick container is such that a ratio of a difference between an outer radius R0 and an inner radius Ri, namely, a thickness t=R0−Ri to an average radius R=(Ro−Ri)/2 is (t/R)> 1/10. When the section of the container is not circular, an equivalent diameter de=s/π may be used for calculating the outer radius R0, the inner radius Ri and the average radius. Here, s is a peripheral length of the section, and when a length of one side is a, s=4×a in the case of quadrate section. The container according to this invention is suitable for a container in which a ratio (L/Di) of an axial length L to an inner diameter Di is not less than 1. Another container of the present invention includes a comparatively thin container like a boiler as a pressure container. Further, another container of the invention includes a container for chemical plant, a reactor container for petroleum refining plant, an ammonia synthetic cell, a heat exchange container, a pressure container such as a boiler, a casing for a large rotational equipment for containing a hydroelectric water turbine, a container to be used as a body of submarine and ship. The container according to the next invention is a container described above in which a section of the metal billet vertical to an axial direction is polygonal and an internal shape of a section of the container for forming vertical to the axial direction is circular. As this container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required. Moreover, since the metal billet, where a section vertical to the axial direction is polygonal, is set into the container for forming, where an internal shape of the section vertical to the axial direction is circular so that the bottomed container is hot-dilated. At this forming step, since the metal billet is dilated by the function for bending one side of the polygonal section, the bottomed container can be formed with a press pressure which is lower than the conventional pressure. The container according to the next invention is a container described above in which a section of the metal billet vertical to an axial direction is polygonal and an internal shape of a section of the container for forming vertical to the axial direction is polygonal. As this container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required. Moreover, since the metal billet, where a section vertical to the axial direction is polygonal, is set into the container for forming, where an internal shape of the section vertical to the axial direction is circular so that the bottomed container is hot-dilated. At this forming step, since the metal billet is dilated by the function for bending each side of the polygonal section, the bottomed container where the bottom section and the body section are integral can be formed with a press pressure which is lower than the conventional pressure. Moreover, the internal shape of the container for forming is changed so that bottomed containers having external shapes according to appellations can be obtained. The container according to the next invention is a container described above in which an outer diameter of the bottomed container is not less than 200 mm to not more than 4000 mm, and a thickness is not less than 20 mm to not more than 400 mm. As this container, a bottomed container in which the bottom section and the body section are formed integrally is used so that the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Particularly in the thick bottomed container, the effect that the above-mentioned steps can be omitted is great. The container according to the next invention is the one in which a metal billet, where at least a section vertical to an axial direction on a pressing forward side is polygonal, is set into a container for forming, and a boring punch is pushed into the metal billet and the metal billet is hot-dilated to be formed into a bottomed container where a bottom section and a body section are integral. This container is a thick container and includes both a drum without a bottom and a bottomed container where a bottom section and a body section are integral. Particularly in the case of the thick bottomed container, the conventional welding of a bottom plate is not required, and the heat treatment after the welding can be omitted. Moreover, since the pressure required for pressing is lower than the conventional pressure, even in the case of a container which is thick and whose axial dimension reaches a several meters particularly like a container to be used as a cylinder or the like for a large pressing machine, it can be manufactured by the conventional facilities. Moreover, since a number of defects which occur on the end portion of the container surface is small, only less steps of correcting these defects after the forming are required. The container according to the next invention is a container described above in which the bottomed container is constituted so that at least one of an external section and an internal section of the bottomed container vertical to the axial direction is polygonal. Since a basket is contained in the bottomed container to be used particularly for a cask as the radioactive substance container, it is preferable that the inner section of the bottomed container is formed into a shape according to the basket. In the conventional method, after the inner section of the bottomed container is formed into a circular shape, and it is formed into the shape according to the basket by the cutting or the like. Since the inner section of this container can be formed into the shape according to the basket when the bottomed container is dilated, the cutting step which was required conventionally can be omitted. The bottomed container manufacturing apparatus according to the next invention comprises: a container for forming having at least a container body section and a container bottom section in which the container body section and the container bottom section can move relatively with respect to an axial direction of the container body section; and a boring punch which is mounted to a pressing machine and pressurizes a metal billet for hot dilation forming set into the container for forming. This bottomed container manufacturing apparatus has the container in which the bottomed section and the body section can move relatively. For this reason, when the body section of the container is tried to moved to the opposite direction to the pressing direction at the time of the hot dilation forming, the body section of the container moves to the opposite direction to the pressing direction together with the metal billet. Namely, since the body section of the container and the metal billet to be formed seldom move relatively, an increase in the press pressure at the time of the hot dilation forming can be suppressed. The bottomed container manufacturing apparatus according to the next invention comprises: a container for forming having at least container body sections and container bottom sections divided in an axial direction in which the container body section and the container bottom section can move relatively with respect to an axial direction of the container body section; and a boring punch which is mounted to a pressing machine and pressurizes a metal billet for hot dilation forming set into the container for forming. In this bottomed container, since the body section of the container extends along the whole axial direction, even in the case where the metal billet which is long in the axial direction, deformation of the metal billet in the axial direction at the time of the hot dilation forming can be absorbed by the whole container. Therefore, even in the case of the container which is long in the axial direction, an increase in the press pressure can be suppressed. The radioactive substance method of manufacturing a container according to the next invention comprises: the step of rounding a drum-shaped bottomed container where a bottom section and a body section are formed integrally by hot dilation and setting a tool so as to cut an external side of the bottomed container; and the step of cutting an internal section of the bottomed container into a shape according to at least one portion of an outer peripheral shape of a basket for containing used nuclear fuel aggregate. In this radioactive substance method of manufacturing a container, the outer side of the bottomed container where the bottom section and the body section are formed integral is finished by cutting, and the inner side is cut into a stepped shape so that a portion for containing a basket for used nuclear fuel aggregate is provided, or the inner side is finished by cutting so that the radioactive substance container is manufactured. The radioactive substance method of manufacturing a container according to the next invention comprises: the step of hot-dilating a bottomed container so that its bottom section and body section are integral; and the step of cutting an internal section of the bottomed container into a shape according to at least one portion of an outer peripheral shape of a basket for containing used nuclear fuel aggregate. In this radioactive substance method of manufacturing a container, the bottomed container where the bottom section and the body section are integral is formed by the hot dilation forming, and the outer side of the bottomed container is finished by cutting, and the inner side is cut into a stepped shape so that a portion for containing a basket for used nuclear fuel aggregate is provided, or the inner side is finished by cutting so that the radioactive substance container is manufactured. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet having at least one plane on a side surface into a container for forming with a gap from an inner wall; and the step of pushing a boring punch into the metal billet and bending the plane towards the inner wall so as to hot-dilate the metal billet. In this method of manufacturing a container, the metal billet is extended towards the inner wall of the container for forming by the function for bending the plane on the side surface of the metal billet. Moreover, since the metal billet is extended in space between the metal billet and the inner wall of the container for forming, the upsetting phenomenon of the metal billet can be suppressed. With these function, the method of manufacturing a container requires the press pressure which is lower than the conventional pressure, and defects which occurs on the end portion or the surface of the container after the forming can be suppressed. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet, which has at least one plane on a side surface and an extended section engaging with an end portion of an inlet of a container for forming on an end portion of a pressing backward side, into the container for forming with a gap from an inner wall; and the step of pushing a boring punch into the metal billet and bending the plane towards the inner wall so as to hot-dilate the metal billet. In this method of manufacturing a container, since the metal billet provided with the extended section engaging with the end portion of the opening of the container for forming is used for the end portion of the pressing backward side, the extended section engages the metal billet with the end portion of the container at the time of hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting of the pressing forward side can be suppressed. Moreover, since at least one plane is provided on the side surface, the bending function and the function for suppressing the phenomenon that the metal flows to the opposite side to the pressing direction accrue. Therefore, the press pressure can be suppressed small by their interaction, and deterioration of the end surface shape can be also suppressed. The method of manufacturing a container according to the next invention is characterized in that a metal billet, where at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape, is set into a container for forming, and a boring punch is pushed into the metal billet and the metal billet is hot-dilated. In this method of manufacturing a container, on the pressing forward side, since the metal billet is extended in the space between the pressing forward side and the container body, the flow of the metal directing to the opposite side of the pressing direction can be suppressed. For this reason, the phenomenon of the metal billet can be suppressed. For this reason, the method of manufacturing a container requires the press pressure which is lower than the conventional one, and defects which occur on the end portion or the surface of the container after the forming can be suppressed. The method of manufacturing a container according to the next invention is characterized in that a metal billet having at least one plane on at least one of a side surface on a pressing forward side and a side surface on a pressing backward side is set into a container for forming, and a boring punch is pushed into the metal billet and the metal billet is hot-dilated. In this method of manufacturing a container, since the metal billet having at least one plane on the side surface is hot-dilated, a force required for the hot dilation forming is weaker than the case where the side surface is a curved surface. Therefore, the press pressure is lower than the conventional method of manufacturing a containers, and internal defects such as cracks can be reduced. The hot pressing method of manufacturing a thick metal-made drum or a cylindrical container according to the next invention is characterized in that a metal billet having different diameter sections without joint, where its pressing forward side is composed of a member having a section with an outer diameter smaller than an inner diameter of a container or an outer diameter of a diagonal length or a member having a section with an outer diameter of a diagonal length equal with the inner diameter of the container and its backward side is composed of a member having a section with an outer diameter or a diagonal length equal with the inner diameter of the container, is set into the container for press forming which was heated to a press working temperature, and while a center of a workpiece of the metal billet without joint is being bored by a punch, the metal billet is press-worked. In this pressing method, the metal on the thick portion on the pressing backward side fills the container and simultaneously is worked, the constraint force is heightened, and the upsetting phenomenon of the metal billet without joint is suppressed, and the shape of the end surface is made to be satisfactory. Moreover, since the metal is supplied from the pressing backward side to the pressing forward side and the pressing forward side is pushed to be spread sideways and is simultaneously worked according to the effect of satisfactory plastic working of the steel heated to high temperature, it fills the space of the container to be formed. For this reason, a pressed product having a predetermined shape is manufactured from the metal billet without joint. As a result, the metal billet without joint reduces a pressing forming load and improves yield of the product, and a pressed product having excellent end surface shape can be obtained. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section having an outer diameter with a diagonal length of not more than an inner diameter of the container and its backward side has a section having an outer diameter substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; and the step of boring a center of a workpiece of the metal billet by means of the boring punch and simultaneously press-working the metal billet. In this method, since the metal billet, where the pressing forward side has a quadrate section whose the diagonal length of the pressing forward side is smaller than the inner diameter of the container, is used, the press pressure can be small by the function for bending the side surface composing the plane on the pressing forward side. Moreover, since the pressing backward side of the metal billet suppresses the upsetting of the pressing forward side, defects on the end portion and the container surface can be suppressed, and the press pressure can be reduced. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section having an outer diameter with a diagonal length of smaller than an inner diameter of the container and its backward side has a section having a diagonal length substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; and the step of boring a center of a workpiece of the metal billet by means of the punch and simultaneously press-working the metal billet. In this method, since the metal billet, where the pressing forward side has a quadrate section whose the diagonal length of the pressing forward side is smaller than the inner diameter of the container, is used, the press pressure can be small by the function for bending the side surface composing the plane on the pressing forward side. Moreover, the metal billet to be used in this method can be worked comparatively easier than the case of a circular sectional shape because the pressing forward side and backward side have angular sectional shape. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section with an outer diameter smaller than an inner diameter of the container and its backward side has a section with an outer diameter substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; and the step of boring a center of a workpiece of the metal billet by means of the punch and simultaneously press-working the metal billet. In the above-mentioned method of manufacturing a container, since the pressing forward side has a circular section whose diagonal length is smaller than the inner diameter of the container, the metal billet is extended to the space between the metal billet and the inner wall of the container for forming. For this reason, the press pressure can be lower than the conventional pressure. Moreover, since the pressing backward side of the metal billet suppresses the upsetting of the pressing forward side, defects on the end portion and the container surface can be also suppressed, and the press pressure can be also reduced. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet having at least one plane on a side surface into a container for forming with a gap from an inner wall; the step of pushing the metal billet so as to extend a pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of pushing a boring punch into the metal billet and bending the plane towards the inner wall so as to hot-dilate the metal billet. This method of manufacturing a container includes the step of extending the pressing backward side of the metal billet to above the body section of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, the metal billet is extended by the function for bending the plane of the metal billet towards the inner wall of the container for forming. With their interaction, this method of manufacturing a container can form a thick container by the lower press pressure than the backward extrusion method or the like. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet, where at least one plane is provided on a side surface and an extended section engaging with an end portion of an inlet of a container for forming is provided on a pressing backward side, into a container for forming with a gap from an inner wall; and the step of pushing a boring punch into the metal billet and bending the plane towards the inner wall so as to hot-dilate the metal billet. Since the metal billet to be used in this method of manufacturing a container is previously provided with the extended section engaging with the end portion of the inlet of the container for forming on the end portion on the pressing forward side. For this reason, since the step of extending the pressing backward side of the metal billet to above the body section of the container before the hot dilation forming, time required for the hot dilation is shortened. As a result, since the forming can be ended until the temperature of the metal billet is lowered, the end portion shape becomes more satisfactory. Moreover, since the extending step can be also omitted, the manufacturing does not require troublesome steps. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet, where at least a section vertical to an axial direction on a pressing forward side is formed into a polygonal shape, into a container for forming; the step of pushing the metal billet so as to extend a pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of pushing a boring punch into the metal billet so as to hot-dilate the metal billet. This method of manufacturing a container includes the step includes the step of extending the pressing backward side of the metal billet to above the body section of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which at least the section vertical to the axial direction on the pressing forward side is formed into a polygonal shape, is dilated to be formed, the function for bending each side of the polygon towards the inner wall of the container for forming acts. With their interaction, the thick container can be formed with lower press pressure than the backward extrusion method or the like. The method of manufacturing a container according to the next invention comprises: the step of setting a metal billet, where at least one plane is provided on at least one of a side surface of a pressing forward side and a side surface of a pressing backward side, into a container for forming; the step of pushing the metal billet so as to extend a pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of pushing a boring punch into the metal billet so as to hot-dilate the metal billet. This method of manufacturing a container includes the step includes the step of extending the pressing backward side of the metal billet to above the body section of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, which is provided at least one plane on at least one side, is dilated to be formed, the function for bending the plane of the metal billet towards the inner wall of the container for forming acts. With the interaction, this method of manufacturing a container can form the thick container with lower press pressure than the backward extrusion method or the like. The method of hot pressing a thick metal-made cylinder or a cylindrical container according to the next invention is characterized in that a metal billet having different diameter sections without joint, where its pressing forward side is composed of a member having a section with an outer diameter smaller than an inner diameter of a container or an outer diameter of a diagonal length or a member having a section with an outer diameter of a diagonal length equal with the inner diameter of the container and its backward side is composed of a member having a section with an outer diameter or a diagonal length equal with the inner diameter of the container, is set into the container for press forming which was heated to a press working temperature, and the metal billet is pushed so that the pressing backward side of the metal billet is extended to an end portion of an inlet of the container for forming, and while a center of a work piece of the metal billet without joint is being bored by a punch, the metal billet is press-worked. This thick metal made cylinder or cylindrical container hot pressing method includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet wit the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal is supplied from the pressing backward side to the pressing forward side and the pressing forward side is pushed to be spread sideways and is simultaneously worked according to the effect of satisfactory plastic working of the steel heated to high temperature, it fills the space of the container to be formed. For this reason, a pressed product having a predetermined shape is manufactured from the metal billet without joint. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section having an outer diameter with a diagonal length of not more than an inner diameter of the container and its backward side has a section having an outer diameter substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; the step of pushing the metal billet so as to extend the pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of boring a center of a workpiece of the metal billet by means of the boring punch and simultaneously press-working the metal billet. This method of manufacturing a drum or a container includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet wit the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which the pressing forward side is a tetragonal section with its diagonal length is smaller than the inner diameter of the container, is used, the metal billet is dilated by the function for bending each side of the tetragonal section. Moreover, the pressing backward side of the metal billet suppresses the upsetting on the pressing forward side. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section having an outer diameter with a diagonal length of smaller than an inner diameter of the container and its backward side has a section having a diagonal length substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; the step of pushing the metal billet so as to extend the pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of boring a center of a workpiece of the metal billet by means of the punch and simultaneously press-working the metal billet. This method of manufacturing a drum or a container includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of hot dilation forming, the constraint of the container on the metal billet becomes stronger, and upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which the pressing forward side is a tetragonal section with a sectional length smaller than the internal diameter of the container, is used, the metal billet is dilated to be formed by the function for bending each side of the tetragonal section. Moreover, the pressing backward side of the metal billet suppresses upsetting on the pressing forward side. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. Further, since the metal billet to be used in this method has the pressing forward side and backward side whose sections are angular shape, the metal billet is worked comparatively easier than the metal billet having one round section. The method of manufacturing a drum or a container of setting a metal billet into a container for forming and hot-dilating the metal billet by means of a boring punch to be operated by a pressing machine, according to the next invention comprises: the step of setting the metal billet, where its pressing forward side has a section with an outer diameter smaller than an inner diameter of the container and its backward side has a section with an outer diameter substantially equal with the inner diameter of the container, into a container for press forming which was heated to a press working temperature; the step of pushing the metal billet so as to extend the pressing backward side of the metal billet to an end portion of an inlet of the container for forming; and the step of boring a center of a workpiece of the metal billet by means of the punch and simultaneously press-working the metal billet. This method of manufacturing a drum or a container includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of hot dilation forming, the constraint of the container on the metal billet becomes stronger, and upsetting on the pressing forward side can be suppressed. Moreover, since the pressing backward side of the metal billet has a diameter substantially equal with the inner diameter of the container for forming, upsetting on the pressing forward side can be suppressed. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. Further, since the metal billet to be used in this method has the pressing forward side and backward side whose sectional shapes are circular, the metal billet can be worked comparatively easier than the metal billet having different sectional shapes. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container comprises the step of forming the metal billet by means of a forging step and forming at least the pressing forward side of the metal billet into an angular section. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container is characterized in that the forging step includes the step of providing a taper which becomes thinner towards the pressing direction on the pressing forward side of the metal billet. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container is characterized in that the forging step includes the step of providing at least one stepped section so that the pressing forward side of the metal billet becomes thinner gradationally towards the pressing direction. In this method of manufacturing a container, at the final stage of the hot dilation forming, timing at which the metal fills the vicinity of the bottom of the container for forming can be delayed. With this function, since the upsetting phenomenon of the metal billet can be suppressed, an increase in the press pressure at the final stage of the hot dilation forming can be suppressed. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container comprises: the step of providing a drum-shaped member between the metal billet and the bottom of the container for forming and setting the metal billet into the container for forming; the step of pushing the boring punch into the metal billet and hot-dilating the metal billet so as to form the bottomed container where the bottom section and the body section are integral; the step of removing the drum-shaped member from the bottom section of the bottomed container after the forming; and the step of removing a pillar-shaped portion formed on the bottom section of the bottomed container by means of the drum-shaped member. In this manufacturing method, the bottomed container is formed by a drum-shaped member provided at the bottom of the metal billet, and simultaneously a spot facing section is formed on the bottom of the bottomed container. Since the spot facing section has been conventionally provided by cutting, the working requires troublesome steps. However, according to this method, since a pillar-shaped section which remains on the bottom of the container is only removed after the dilation forming, the working does not require less troublesome steps than the conventional method. Here, the drum-shaped member includes polygonal members whose section vertical to the axial direction is triangular, tetragonal or the like, a polygonal member whose angle of the polygon is rounded off and an elliptic member, so the shape is not limited to the drum. The method of manufacturing a container according to the next invention comprises: the step of providing a pillar-shaped member between the metal billet and the bottom of the container for forming and setting the metal billet into the container for forming; the step of pushing the boring punch into the metal billet and hot-dilating the metal billet so as to form the bottomed container where the bottom section and the body section are integral; and the step of removing the pillar-shaped member from the bottom section of the bottomed container after the forming. In this method, the bottomed container is formed by the pillar-shaped member provided at the bottom of the metal billet, and simultaneously the spot facing section is formed on the bottomed container. Since the spot facing section has been conventionally provided by the cutting, the working requires troublesome steps. According to this method, since the spot facing section can be formed simultaneously with the dilation forming, the working requires less troublesome steps than the conventional method. Moreover, since the step of removing the pillar-shaped member can be omitted, troublesome steps are not required for forming the spot facing section in comparison with the method where the spot facing section is formed. The method of manufacturing a container according to the next invention depending from the above-mentioned method of manufacturing a container is characterized in that the body section of the container for forming can move relatively with respect to the bottom section of the container for forming. In this method of manufacturing a container, the metal billet is set into the container where the body section and the bottom section can move relatively so as to be hot-dilated. For this reason, when the metal billet tries to move the body section of the container to the opposite direction to the pressing direction at the time of the hot dilation forming, the body section of the container moves together with the metal billet to the opposite direction to the pressing direction. Namely, since the body of the container and the metal billet to be formed hardly move relatively, an increase in the press pressure can be suppressed at the time of the hot dilation forming. The method of manufacturing a container according to the next invention is characterized in that the body section of the container for forming is divided in an axial direction. In this method of manufacturing a container, since the body section of the container extends along the whole axial direction, even in the case where the metal billet which is long in the axial direction is formed, deformation of the metal billet with respect to the axial direction can be absorbed by the whole container at the time of the hot dilation forming. Therefore, even in the case where the thick container which is long along the axial direction is formed, an increase in the press pressure can be suppressed. The method of manufacturing a container according to the next invention comprises: the upsetting step of placing a pressurizing platform into a ring-shaped die formed with an opening at its inner end portion and putting a metal billet into a mold composed of the die and the pressurizing platform so as to pressurize the metal billet by means of a boring punch; and the metal billet drawing step of supporting the die by means of a drum-shaped spacer and pushing the metal billet by means of the boring punch. At the upsetting step, the metal billet is pressurized by the boring punch so that a material flows to between the opening portion of the die and the boring punch, and the metal billet is deformed into a dish shape. At this time, since the boring punch is held by the metal billet, the boring punch and the die including the metal billet are once allowed to recede. At the drawing step, the spacer is arranged below or above the die so as to support the die, and the boring punch is pushed into so that the metal billet is drawn by the die. As a result, the metal billet is deformed into a cup shape. These two steps are repeated plural times as the need arises so that the metal billet is formed into a final shape. Here, since these steps are heat working, it is necessary to heat the metal billet before the forming. This invention includes the case where the above-mentioned two steps are carried out once so that the metal billet is formed into a final shape. Moreover, the metal billet may be pressurized from above (see FIGS. 21 through 27), or pressurized from below (see FIG. 28). In such a manner the upsetting and the drawing are combined to be used so that an excessive pressure which is required in the backward extrusion forming is not required. For this reason, since the bottomed container can be formed by a normal large-size pressing machine, it is easily manufactured. The method of manufacturing a container according to the next invention comprises: the upsetting preparation step of stacking a plurality of ring-shaped dies formed with an opening on its inner end portion and stacking a plurality of pressurizing platforms respectively in the dies and putting a metal billet into a mold composed of the die and the pressurizing platform; the upsetting step of pressurizing the metal billet from above the mold using a boring punch to be operated by a pressing machine; the receding step of allowing the boring punch and the whole metal billet including and the upper die to recede; the drawing preparation step of removing the used pressurizing platform and placing a drum-shaped spacer onto the next die and placing the receded whole metal billet including the die onto the spacer; the drawing step of pushing the metal billet by means of the boring punch and drawing the metal billet by means of the die; and the repeating step of repeating the above-mentioned steps on the next pressurizing platform and die using a spacer of a length according to deformation of the metal billet. At the upsetting step, the metal billet is pressurized by the boring punch so that a material flows between the opening portion and the boring punch, and the metal billet is deformed with the boring punch being held. At the drawing step, the space is provided below the die and the metal billet is pressurized so that the die is allowed to pass and the metal billet is subject to the draw working. As a result, the metal billet is deformed into a cup shape. Here, the spacer may have any shape as long as it has a drum shape into which the drawn metal billet can be put. The upsetting preparation step through the drawing step are ended, these are further repeated. At this time, as for the die and pressurizing platform, the second ones from the top of the stacked dies and platforms are used. Since the spacer should be longer as the metal billet deforms, they are prepared for each step. In this manufacturing method, since the upsetting and the drawing are combined to be used, the pressurizing force can be suppressed small in comparison with the backward extrusion forming. For this reason, the container can be formed by a normal large-size pressing machine. The present invention will be detailed below with reference to the diagrams. However, the present invention is not limited to the embodiments described below. Moreover, a manufacturing method of a container or a drum of the present invention is not limited to methods to be disclosed below. Further, components of the following embodiments include ones which can be assumed by person skilled in the art. FIGS. 1(a) and 1(b) are explanatory diagrams showing one example of a bottomed container according to the first embodiment. FIGS. 2(a) through 2(e), 2(a′) through 2(e′) and 2(e″) are explanatory diagrams showing the steps of manufacturing the bottomed container 1 shown in FIGS. 1(a) and 1(b). FIGS. 3(a) through 3(c) are perspective views showing one example of a metal billet 200 to be used in the first embodiment. As shown in FIGS. 1(a), a bottom section 1b of the bottomed container 1 according to the first embodiment is formed integrally with a body section la, and as is clear from FIG. 1(b), a sectional form of the bottomed container 1 of the first embodiment is circular. In this manufacturing method, the bottomed container 1 is manufactured by a punch according to hot dilation forming. Firstly there will be explained below the metal billet to be used in the manufacturing steps. The metal billet 200 is manufactured by cutting or free forging a foundry molding lump or a metal lump of molten metal before the hot dilation forming step, explained next. At the forging step, it is desirable that at least forward side of the metal billet in a pressing direction is formed into an angular section. As a result, an effect of the hot dilation forming can be utilized more effectively. Here, in the first embodiment, the metal lump is freely forging so that the integral metal billet 200 without a joint is manufactured, but a metal billet which can be used in the present invention is not limited to this. As shown in FIGS. 3(a) through 3(c), a sectional form of the metal billet 200 used in the present embodiment is circular on a backward side in the pressing direction (hereinafter, pressing backward side) 200a, and is square on a forward side in the pressing direction (hereinafter, pressing forward side) 200b. An outer diameter of the pressing backward side 200a is larger than a diagonal length of the pressing forward side 200b, and the outer diameter of the pressing backward side 200a is substantially equal with an inner diameter of a container 300, and the diagonal length of the pressing forward side 200b is shorter than the inner diameter of the body section 300 of the container for forming (FIG. 3(b)). Here, the inner diameter of the body section 300 of the container for forming is represented by a broken line. Further in this example, a section 970 of the metal billet 200 vertical to an axial direction on the pressing forward side 200b includes a projection image 920 of a section of a boring punch 410 vertical to the axial direction (FIGS. 3(b) and 3(e)). Here, the sectional form, dimension, outer form and the like of the metal billet 200 are not limited to this example. Another example of the billet which can be applied to the present invention will be explained later. Next, there will be explained below the hot dilation forming steps. Prior to the hot dilation forming, the metal billet 200 is heated to a temperature where it is easily hot-dilated by a heating furnace (not shown). Since the heating temperature is determined by a material and the like of the metal billet 200, it is not determined ultimately. Here, as for a carbon steel material which is used for a body of the cask for containing and transporting and temporarily storing used nuclear fuel, it is desirable that the heating temperature is set to 1100° C. to 1300° C. When the heating temperature exceeds this range, crystal grains become coarse and oxidation and decarbonization occur on the surface, and the material is embrittled and is easily cracked. In the case of the carbon steel, as a percentage of the carbon is higher, the heating temperature becomes lower with in the range. The metal billet 200, which is heated to the temperature at which it is easily hot-dilated by an electric furnace or the like, is upset into a body section 300 of the container for forming as shown in FIG. 2(a). The metal billet 200 which is upset into the body section 300 of the container for forming is upset by a large punch 400 having an outer diameter substantially equal with or larger than the outer diameter of the metal billet 200, and an extended section 201 extended to the container 300 is provided on the pressing backward side 200a of the metal billet 200 (see FIG. 2(b)). The extended section 201 is provided on an end portion of the metal billet 200 so that a constraint force in the axial direction of the metal billet 200 can be heightened at the hot dilation forming step by means of the punch. Since the upsetting phenomenon of the metal billet 200 is suppressed by this function and metal flow towards the pressing direction and the opposite direction can be reduced, a rise in the press pressure can be suppressed. At the same time, the form of the end surface of the bottomed container after the forming can be satisfactory. Here, even if the extended section 201 is not provided on the metal billet 200, the upsetting phenomenon can be suppressed by a friction force which is generated between an inner wall of the body section 300 of the container for forming and a side surface of the pressing backward side 200a of the metal billet 200. However, in order to lower the press pressure and obtain more organized end surface form, it is desirable that the extended section 201 is provided. After the extended section 201 is formed on the end portion of the metal billet 200, the sequence goes to the hot dilation forming step by means of a boring punch 410. In order to bore a hole at the center of the metal billet 200, firstly the punch 410 is placed on the center of the end surface of the metal billet 200 by a locating guide 310 attached to the body section 300 of the container for forming (see FIG. 2(c)). Next, the boring punch 410 is pushed by a pressing machine (not shown) so that the metal billet 200 is hot-dilated. When the boring punch 410 is pushed into the metal billet 200 by the pressing machine, the extended section 201 of the metal billet 200 is engaged with the upper end portion of the body section 300 of the container for forming, and the upsetting phenomenon of the metal billet 200 is suppressed. Moreover, the metal of the thick portion on the pressing backward side 200a of the metal billet 200 is deformed so as to fill the container 300 (see FIG. 2(d)). As a result, since the metal of the pressing backward side 200a is pushed against the inner wall of the body section 300 of the container for forming, the upsetting phenomenon of the metal billet 200 can be suppressed also by this function. As a result, the rise in the press pressure is suppressed, and the end surface form of the bottomed container after the forming can be satisfactory. When the boring punch 410 is pushed into the metal billet 200, the metal which exists just below the boring punch 410 becomes a hemispheric metal lump and moves to the pressing forward side 200b together with the boring punch 410. Due to this phenomenon, a cross section of the pressing backward side 200a of the metal billet 200 vertical to the axial direction should be larger than that of the pressing forward side 200b. Further, when the boring punch 410 is pushed into the metal billet 200, the metal on the pressing forward side 200b of the metal billet 200 is deformed so as to be spread towards the inner wall of the container 300 due to excellent property of plastic deformation owned by the steel heated to high temperature and a phenomenon that the metal lump is supplied from the pressing backward side 200a. Namely, it is the hot dilation forming. When the boring punch 410 is pushed to a predetermined depth previously set, the hot dilation forming is ended there (see FIG. 2(e)). There will be explained below types of the metal billet 200 applicable to the manufacturing method of the present invention. As shown in FIG. 3(a), at least the metal billet which is formed so that a sectional form vertical to the axial direction on the pressing forward side is polygonal can be applied to the manufacturing method of the present invention. When such a metal billet is used, since the metal billet is deformed so as to be spread towards the inner wall of the container for forming in the hot dilation forming, the press pressure can be suppressed in comparison with backward extrusion or the like. Here, the section of the metal billet 200 vertical to the axial direction shown in FIG. 3(a) is tetragonal, but the section is not limited to this. Further, the metal billet applicable to the manufacturing method of the present invention may have the following relationship. Namely, the section vertical to the axial direction on the pressing backward side has a relationship that it contains a projection image of a section vertical to the axial direction on the pressing forward side. Moreover, a projection image of the section vertical to the axial direction on the pressing backward side has a relationship that it is contained in the inner side of the section of the container for forming vertical to the axial direction. Here, the form of the inner side of the section of the container for forming vertical to the axial direction may be identical to the projection image of the section vertical to the axial direction on the pressing backward side. It is desirable that the metal billet which satisfies these relationships is used when a large bottomed container with large thickness is formed. The projection image will be explained below with reference to FIGS. 3(c) and 3(d). Here, the projection image is represented by a broken line, and the section is represented by a continuous line. FIG. 3(c) shows a state that a section 950 vertical to the axial direction on the pressing backward side of the metal billet contains a projection image 900 of the section vertical to the axial direction on the pressing forward side. FIG. 3(d) shows a state that a projection image 910 of the section vertical to the axial direction on the pressing backward side is contained in an inner side 960 of the section of the container for forming vertical to the axial direction. Moreover, FIG. 3(e) shows a state of the boring punch having a relationship that a projection image 920 of the section vertical to the axial direction is contained in a section 970 vertical to the axial direction on the pressing forward side of the metal billet. Here, the projection image 920 of the section vertical to the axial direction in FIG. 3(e) is a projection image of the boring punch. “Containing” in the present invention means that a whole portion surrounded by the broken line representing the projection image exists on the inside of a portion surrounded by the continuous line representing the section. In the case where even a part of the portion surrounded by the broken line exist on the outside of the portion surrounded by the continuous line, this is not included in the concept of “containing”. Moreover, in the case where the form of the section is identical to the projection image, this is not included in the concept of “containing”. It is desirable that the projection image of the boring punch is contained in the section vertical to the axial direction on the pressing forward side of the metal billet (see FIG. 3(e)). However, the projection image of the boring punch vertical to the axial direction may be identical to the section of the metal billet vertical to the axial direction on the pressing forward side. Moreover, the section of the boring punch vertical to the axial direction may contain the projection image of the section of the metal billet vertical to the axial direction on the pressing forward side. However, in the case of a relationship that the section of the boring punch vertical to the axial direction contains the projection image of the section vertical to the axial direction on the pressing forward side of the metal billet, when the cross section of the boring punch becomes large, a thickness of the body section of the container to be formed becomes thin. For this reason, the body section of the container easily ruptures during the hot dilation forming. Therefore, in the case of the relationship that the section of the boring punch vertical to the axial direction contains the projection image of the section of the metal billet vertical to the axial direction on the pressing forward side, it is necessary to suppress the cross section of the boring punch within a range that the body section of the container does not rapture. FIGS. 4(a) through 4(e) are explanatory diagrams showing examples of another metal billet according to the first embodiment. As is clear from FIGS. 4(a) through 4(d), these examples satisfy a condition that the section 950 vertical to the axial direction on the pressing backward side 200a of the metal billet 200 contains the projection image 900 of the section vertical to the axial direction of the pressing forward side 200b (FIG. 3(c)). In the case where the section of the pressing forward side 200b is tetragonal as shown in FIGS. 4(a) and 4(b), the metal billet 200 is deformed by mainly a force which directs towards an outer side in the radial direction of the body section 300 of the container, namely, a force which bends the side surface of the metal billet having a plane to the inner wall side of the body section 300 of the container for forming at the time of the hot dilation forming. FIG. 5 is a conceptual diagram showing a state of the deformation. A broken line in the diagram shows a process that the metal billet 200 is expanded towards the inner wall of the body section 300 of the container for forming. In the case where section vertical to the axial direction is tetragonal, the boring punch is pushed into so that a force F, which directs from the center of the metal billet 200 to the outside in the radial direction of the body section 300 of the container for forming, acts. Since the force F bends the side surface of the metal billet 200 towards the inner wall of the container for forming, the metal billet 200 is dilated to the inner wall of the container for forming by this bending function. Particularly this bending function acts effectively when the metal billet having a plane on the side surface is set into the container for forming having a circular inside of the section. Therefore, when the metal billet is dilated by the bending function, the hot dilation forming requires less press pressure, and defects such as a crack generated at the time of the forming can be suppressed. Even if the metal billet whose section vertical to the axial direction has a tetragonal shape is hot-dilated by using the boring punch whose section vertical to the axial direction has a tetragonal shape, the metal billet is dilated by the bending function. Therefore, also in this case, the hot dilation forming requires the less press pressure, and the defects such as a crack generated at the time of the forming can be suppressed. In the case where the section vertical to the axial direction on the pressing forward side 200b is circular as shown in FIG. 4(c), the above-mentioned effect is slightly reduced, but the metal billet 200 has spaces between the pressing forward side 200b and the inner wall of the body section 300 of the container. Therefore, since the metal on the pressing forward side is dilated in the spaces at the time of the hot dilation forming, the dilating deformation of the metal billet is not constrained by the body section 300 of the container for forming, and the press pressure can be suppressed small. Moreover, in this case, when the metal billet is hot-dilated by using the boring punch whose section vertical to the axial direction has a tetragonal shape, the bending function acts. For this reason, the press pressure is less than the case where the metal billet is hot-dilated by using the boring punch having a circular section vertical to the axial direction. As shown in FIGS. 4(a), 4(c) and 4(d), in the case where the section of the pressing backward side 200a vertical to the axial direction is circular and its diameter is substantially equal with the diameter of the body section 300 of the container, the extended section 201 formed on the body section 300 of the container for forming (see FIG. 3(b)) can be formed uniformly. Therefore, at the time of hot dilation forming, since the upsetting phenomenon of the metal billet 200 can be suppressed effectively by the extended section 201, the press pressure can be suppressed low, and the form of the end surface of the container after the forming can be satisfactory. As shown in FIG. 4(b), in the case where the section of the pressing backward side 200a is tetragonal and its diagonal length is substantially equal with the diameter of the body section 300 of the container for forming, the extended section 201 is formed partially on the upper end of the body section 300 of the container. For this reason, in comparison with the case where the extended section 201 is not provided, the upsetting phenomenon of the metal billet 200 can be suppressed more effectively, but in comparison with the case where the extended section 201 is formed along the whole periphery of the upper end of the body section of the container for forming, this effect is slightly reduced. Further, the metal billet, which is formed so that the form of the section vertical to the axial direction of the pressing forward side 200b or the pressing backward side 200a of the metal billet 200 is pentagonal (or more) or triangular, can also be applied to the manufacturing method according to the first embodiment. Moreover, as shown in FIGS. 4(d), when the side surface of the pressing forward side 200b has at least one or more planes, since this portion is hot-dilated by the bending, the press pressure can be suppressed low. In addition, when a number of planes on the side surfaces of the metal billet 200 is not less than three, namely, the section of the metal billet 200 vertical to the axial direction is triangular or more, the press pressure can be suppressed lower. However, when a number of planes increases, namely, a number of angles of the polygonal section vertical to the axial direction increases, since the form of the polygonal section closes to a circle, the effect which suppresses the press pressure low is reduced. Therefore, it is desirable that a number of the planes of the side surface on the pressing forward side 200b is selected within the range that the press pressure can be suppressed low. Moreover, even in the case where the form of the section of the metal billet 200 vertical to the axial direction is of substantially drum as shown in FIG. 4(e), it is included in the metal billet of the present invention as long as the planes are provided on the side surface of the metal billet 200. FIGS. 6(a) through 6(c) are perspective views showing another metal billet applicable to the first embodiment. The metal billet 200 is provided with a stepped section on the pressing forward side 200b so that it becomes thinner gradationally towards the pressing direction. As a result, at the time of the hot dilation forming, the timing that the metal of the pressing forward side 200b fills the vicinity of the bottom of the body section 300 of the container for forming can be slow. Therefore, particularly at the final state of the hot dilation forming, a rise in the press pressure can be suppressed. Moreover, since slight flow to the pressing direction is formed by this stepped section, a forging degree of a molded form can be heightened, and shortage of forging materials can be prevented. Further, since a change of the section is gradate, the manufacturing becomes easier than the case where a taper, mentioned later, is provided. Here, a number of the stepped sections is not limited to the above example, but the number can be suitably increased or decreased according to a pressing condition and the like. In addition, as shown in FIG. 6(b), even if the pressing forward side 200b is provided with a taper which becomes thinner towards the pressing direction, the function and effect which are similar to the case where the stepped section is provided on the pressing forward side 200b can be obtained. Further, as shown in FIG. 6(c), when the metal billet 200 is manufactured, the extended section 201 which is engaged with the upper end portion of the container 300 for forming may be previously provided on the metal pressing backward side 200a. As a result, the step of forming the extended section 201, which is extended to the body section 300 of the container, on the pressing backward side 200a of the metal billet 200 of is not required before the hot dilation forming, and the container manufacturing steps can be simplified. FIGS. 7(a) through 7(c) are explanatory diagrams showing another metal billet applicable to the first embodiment. As shown in the diagrams, the metal billet 200 is characterized in that the section vertical to the axial direction is constant along the pressing direction and the extended section 201 which is engaged with the upper end portion of the container 300 for forming is provided on its one end. Even in the case where such a metal billet 200 is used, since the upsetting phenomenon of the metal billet can be suppressed by the extended section 201 at the time of hot dilation forming, a rise in the press pressure can be suppressed. Here, the sectional form of the metal billet 200 is not limited to a tetragonal form, and it may be polygonal, and the side surface of the metal billet 200 may provided with at least one plane. Moreover, the extended section 201 may be previously provided on the metal billet 200, or the metal billet 200 is set into the container 300 for forming and the extended section 201 may be provided. When the metal billet 200 is dilated to be deformed so as to be spread towards the inner wall of the container 300 (see FIGS. 2(d) and 2(e)), a friction force is generated between the metal of the metal billet 200 and the inner wall of the container 300. This friction force is caused by transfer of the metal of the pressing forward side 200b to the opposite direction to the pressing direction along the inner wall of the body section 300 of the container. Here, at the middle process of the hot dilation forming in the manufacturing method according to the first embodiment, this friction force is hardly generated. However, at the final stage of the hot dilation forming, since the metal fills the lower section of the container for forming, this friction force is generated. Due to this friction force, the body section 300 of the container for forming tries to move to the opposite direction to the pushing direction of the punch 410. When the body section 300 of the container for forming and the bottom section 301 of the container for forming are fixed, the metal of the pressing forward side moves against the friction force, and at the final stage of the hot dilation forming, excessive load is required. In order to solve this problem, in the first embodiment, the body section 300 of the container for forming and the bottom section 301 of the container for forming can move relatively. With such a structure, when the body section 300 of the container for forming tries to move to the opposite direction to the pushing direction of the punch 410 due to the friction force, the body section 300 of the container for forming also moves to the opposite direction to the pushing direction of the punch 410 together with the metal billet 200 to be formed (FIG. 2(e)). Namely, since the body section 300 of the container for forming and the metal billet 200 to be formed seldom move relatively, an increase in the load can be suppressed at the final stage of the hot dilation forming. In the present embodiment, the body section 300 of the container for forming and the bottom section 301 of the container can move relatively, and also the body section 300 of the container for forming is divided so as to capable of moving along the whole metal billet 200. As a result, even in the case where the bottomed container which is long in the axial direction is formed, an increase in the load can be suppressed at the final stage of the hot dilation forming. FIGS. 8(a) and 8(b) are sectional views showing divided sections of the body section 300 of the container for forming to be used in the first embodiment. The divided sections of the body section 300 of the container for forming may be constituted so as to be overlapped with each other as shown in FIG. 8(a) and the body sections 300a and 300b of the container for forming may move relatively at the time of hot dilation forming. Moreover, as shown in FIG. 8(b), a concave section is formed on the one body section 300a of the container for forming, and a convex section is provided on the other body section 300b of the container for forming so as to be combined. As a result, the body sections 300a and 300b of the container for forming may move relatively at the time of hot dilation forming. In the present embodiment, the body section 300 of the container for forming is divided into two, but a number of divisions can be changed suitably according to the height of the metal billet 200. Moreover, the body section 300 of the container for forming is only divided, or the body section 300 of the container is not divided and the body section 300 and the bottom section 301 of the container for forming can only move relatively. As a result, an increase in the load can be suppressed at the time of hot dilation forming. When the punch 410 is pushed into a set predetermined depth, the hot dilation forming is ended there (FIG. 2(e)). As shown in FIG. 2(e″), a cylindrical spacer 302 is placed instead of the bottom section 301 of the container for forming and the bottom section of the metal billet 200 is punched so that a drum can be formed by this manufacturing method. When the thick container whose length in the axial direction reaches a several meters is manufactured by the manufacturing method, since the press pressure can be reduced to a several part of the conventional press pressure, the container can be manufactured by the conventional facilities. Moreover, since the bottomed container in which the bottom section and the body section are integral with each other can be manufactured by one-time working, the manufacturing requires less steps, and this method is suitable also for mass production. The metal billet 200 which was subject to the hot dilation forming is cooled to normal temperature by natural cooling, forced cooling or control cooling. In order that the sectional form is adjusted or the external shape or the internal shape of the formed container is finished to a predetermined dimension, the container may be subject to the cutting or the like. Next, a result of the concrete example that an integrally cylindrical bottomed container is manufactured by the above-mentioned method will be shown. As a comparative example, a result of manufacturing the integrally cylindrical bottomed container by applying the Erhardt (elrhardt or Ehrhard) boring method which has been conventionally used is also shown. In both the concrete example and the comparative example, the cylindrical container was formed by using a cylindrical container having an inner diameter of 943 mm. As a material of the cylindrical container, carbon steel in which a percentage of carbon (C) is 0.1% was used, but stainless steel maybe used. Firstly after the metal billet of carbon steel was heated to 1250° C., it was forged into a metal billet in which its section in the axial direction has a T-shaped different diameter section by the free forging method. The pressing forward side of this metal billet has a quadrate section whose diagonal length is 875 mm smaller than an inner diameter of the container for forming, and a length of the axial direction is 1896 mm. The pressing backward side has a circular section having an outer diameter of 928 mm substantially equal with the inner diameter of the container for forming, and a length of the axial direction is 574 mm. After the metal billet was again heated to 1250° C., it is set into the container for forming, and the center of the workpiece is hot-dilated by the punch so that an elongate cup-shaped cylindrical container having length of 2420 mm and thickness of 165 mm was formed. Meanwhile, in the comparative example, the carbon steel which is the same as the concrete example was used as the material of the cylindrical container, and after the metal billet was heated to 1250° C., it was forged into an angular metal billet having uniform sectional form along the whole length of the axial direction was manufactured by the free forging method. The sectional form of this metal billet is quadrate, and a length of its diagonal is 928 mm which is substantially equal with the inner diameter of the container for forming. Moreover, a length of the axial direction is 2470 mm. After the metal billet was again heated to 1250° C., it was hot-dilated so that the cylindrical container whose size is the same as the concrete example was manufactured. Table 1 shows evaluated results of the forming load and the end surface form in both the pressing methods. As is clear from the comparison, according to the manufacturing method of the present invention, the press forming load is smaller and the product yield is higher than the conventional manufacturing method. Moreover, since defective portions of the end surface form which are seen in the conventional manufacturing method seldom exist, the product can be finished by simple working after the hot dilation forming. TABLE 1EVALUATION RESULTS OF BOTH METHODSPress formingMethodloadSectional formYieldMethod of the2400 tonSatisfactory70%Present InventionMethod of the3980 tonDefective portions60%Comparative Exampleof form are generated There will be explained below an example of cutting an outer side and an inner side of the bottomed container formed to be used as the radioactive substance container such as a cask or a canister. FIG. 9 is a schematic diagram showing an apparatus for cutting the outer side of the hot-dilated bottomed container. The bottomed container 1 is placed on a rotation supporting platform 154 having a roller and can be rotated freely in a circumferential direction. A fixing table 141 is provided on the outer side of the bottomed container 1, and a movable table 142 which slides on the fixing table in the axial direction of the bottomed container 1 is provided. A cutting tool 160 is attached to the movable table 142, and the cutting tool 160 cuts the outer circumference of the bottomed container 1. A roller 161 attached to the rotation supporting platform 154 is connected to a motor 162. The rotation of the motor 161 is transmitted to the bottomed container 1 via the roller 161 so as to rotate the bottomed container 1. When the motor 161 rotates and the bottomed container 1 starts to rotate to a direction of an arrow in the diagram, a servo motor 157 and a ball screw 158 which are provided to the end portion of the fixing table 141 moves the movable table 142 to the axial direction of the bottomed container 1, and the cutting tool 160 attached to the movable table 142 cuts the outer circumference of the bottomed container 1. Moreover, when the outer circumference is cut by a face mill or the like so that a plane is provided, not only the bottomed container having a circular section vertical to the axial direction but also the polygonal bottomed container can be formed. There will be explained below an example that the inner side of the formed bottomed container is cut. FIG. 10 is a schematic diagram showing an apparatus for working the inner side of the bottomed container. A machining apparatus 140 is composed of a fixing table 141 which pierces through a body 101 and is placed and fixed inside the bottomed container 1, a movable table 142 which slides on the fixing table 141 in the axial direction, a saddle 143 which is located and fixed onto the movable table 142, a spindle unit 146 which is composed of spindles 144 and a driving motor 145 provided on the saddle 143, and a face mill 147 provided on a spindle axis. Moreover, a reactive force receiver 148 which is formed with a contact portion according to the internal shape of the bottomed container 1 is provided on the spindle unit 146. The reactive force receiver 148 is detachable and slides along a dovetail groove (not shown) in a direction of an arrow in the diagram. Moreover, the reactive force receiver 148 has a clamp device 149 for the spindle unit 146, and is fixed to a predetermined position. Further, a plurality of clamp devices 150 are attached into a lower groove of the fixing table 141. The clamp devices 150 is composed of a hydraulic cylinder 151, a wedge-shaped moving block 152 which is provided on an axis of the hydraulic cylinder 151, and a fixing block 153 whose tilt surface comes in contact with the moving block 152. The shaded portions in the diagram are attached to the groove inner surface of the fixing table 141. When the axis of the hydraulic cylinder 151 is driven, the moving block 152 comes in contact with the fixing block 153, and the moving block 152 moves slightly downward due to the wedge effect (represented by dotted line). As a result, since the lower surface of the moving block 152 is pushed against the inner surface of a cavity 102, the fixing table 141 can be fixed inside the bottomed container 1. In addition, the bottomed container 1 is placed on a rotation supporting platform 154 composed of a roller, and is freely rotated in a radial direction. Moreover, a spacer 155 is attached between the spindle unit 146 and the saddle 143 so that a height of the tool 147 on the fixing table 141 can be adjusted. A thickness of the spacer 155 is the same as a dimension of one side of an angular pipe composing the basket. A servo motor 156 provided on the movable table 142 is driven so that the saddle 143 moves in the radial direction of the body 101. The movement of the movable table 142 is controlled by a servo motor 157 and a ball screw 158 provided on the end portion of the fixing table 141. Here, as the working proceeds, the shape of the inside of the bottomed container 1 changes, and thus it is necessary that the reactive force receiver 148 and the moving block 152 of the clamp devices 150 are replaced by ones having suitable shape. FIGS. 11(a) through 11(d) are explanatory diagrams showing one example of the method of working the inner side of the bottomed container 1. Firstly the fixing table 141 is fixed to a predetermined position inside the bottomed container 1 by the clamp devices 150 and the reactive force receiver 148. As shown in FIG. 11(a), the spindle unit 146 is moved along the fixing table (not shown) at a predetermined cutting speed, and the inner side of the bottomed container 1 is cut by the tool 147. When the cutting in this position is finished, the clamp devices 150 are removed so that the fixing table 141 is released. As shown in FIG. 11(b), the body 101 is rotated through an angle of 90° on the rotation supporting platform 154, and the fixing table 141 is fixed by the clamp devices 150. Similarly the cutting is carried out by the tool 147. Thereafter, the similar steps are repeated twice. Next, the spindle unit 146 is rotated through an angle of 180°, and as shown in FIG. 11(c), the inside of the cavity 102 is cut successively. Also in this case, similarly to the above-mentioned case, while the body 101 is being rotated through an angle of 90°, the working is repeated. Thereafter, as shown in FIG. 11(d), the spacer 155 is attached to the spindle unit 146 so that the height of the spindle unit is heightened. The tool 147 is fed in the axial direction in this position, and the inside of the bottomed container 1 is cut. This is repeated while the body 101 is being rotated through an angle of 90° so that a shape which is required for inserting an angular pipe (not shown) for containing used nuclear fuel is inserted is approximately finished. Here, the working can be carried out not only by a special machine but also by general horizontal boring machine and vertical boring machine. The above explanation referred to the example that the bottomed container 1 is laid and the outer side and the inner side are cut. However, the bottomed container 1 stands upright and the outer side and the inner side of the container may be cut by a working machine, explained below. Concretely, this working machine has a rotation table where a bottomed container to be worked is placed and the container is rotated, a crane for placing the bottomed container on the rotation table and moving it from the rotation table after the working, a movable table placed on a base, a saddle which is placed on the movable table and can move in a movable direction and a right angle direction of the movable table, a column which is placed on the saddle and supports an arm moving up and down, and the arm which is mounted to the column to be movable up and down in a vertical direction and has an attachment with a tool on its forward end and moves up and down with respect to a workpiece so as to work the workpiece. The attachment to be attached to the forward end of the arm is replaced so that this working machine can cope with various workings such as milling and boring. When the bottomed container is started to be worked, the bottomed container as a workpiece is placed on the rotation table by the crane, it is centered and is fixed onto the rotation table. When the outside of the bottomed container is cut into a cylindrical shape, a cutting tool is mounted to the attachment attached to the arm, and while the rotation table is being rotated at a predetermined number of revolution, the bottomed container is cut by this cutting tool. Here, since the column to which the arm is attached is placed on the saddle, the saddle and the movable table are moved so that the arm can be moved. Therefore, the arm is moved to an arbitrary position, and an arbitrary cutting amount can be set. Moreover, since the arm can be moved up and down, the arm is moved along the axial direction of the bottomed container so that the whole side surface of the bottomed container can be cut. Since these movements require accuracy, it is desirable that the rotating movement of the servo motor or the stepping motor is converted into a linear motion by a ball screw or the likes and the movable table or the like is moved. When the outer circumference of the bottomed container having circular sectional shape in a radial direction is cut and the container is desired to be worked into a polygonal shape such as an octagonal prism, an attachment for a face mill is attached to the arm, and the side surface of the bottomed container is cut by face milling cutting. When the arm is moved up and down and one surface of the bottomed container in the whole axial direction is cut, the rotation table is rotated through an angle of 45°, and a next side surface is worked. When this work is repeated eight times, the bottomed container whose external shape of the section in the radial section is octagonal can be manufactured. In such a manner, the bottomed container having an arbitrary polygonal section can be manufactured. When the bottomed container is used as a cask for containing used nuclear fuel, it is desirable that the internal shape of the bottomed container matches with at least one part of the outer periphery of a basket for containing used nuclear fuel aggregate. This is because the basket can be easily inserted and fixed into the container. Such a sectional shape is shown in FIG. 15(d), for example. In order to form the inner side of the bottomed container into such a shape, the attachment to be mounted to the arm is replaced by one for end mill, for example, and an angular portion inside the section is worked into a stepped form. Firstly, the saddle and the movable table are moved, the arm is moved to above the bottomed container placed on the rotation table. Next, the arm is lowered and the attachment with the end mill is inserted into the bottomed container so that the end mill is located. Thereafter, the inner side of the bottomed container is cut with a predetermined cutting amount. The cutting is given to the bottomed container some times and its inner side is cut, and when a predetermined shape is obtained, the cutting at the first stage is finished. When the necessary stepped form is obtained at one angular portion, the rotation table is rotated through an angle of 90°, and a next angular portion is worked. When this work is repeated four times, the bottomed container for the cask having the sectional shape in the radial direction as shown in FIG. 15(d) can be manufactured. Here, when the inner side of the bottomed container is cut, since there is not space for cutting chips and cutting oil, the cutting cannot be carried out during the working. For this reason, it is desirable that the cutting chips and the like are removed from the inner side of the on-working bottomed container by discharging means such as a vacuum pump. When the bottomed container is cut with it is in the upright state, the process for the cutting chips and the like is required unlike the case of the laid state, but an influence of deformation due to gravity can be reduced. When this working machine is inverted to be constituted, since the bottomed container is worked with its opening faces downward, when the inner side of the bottomed container is cut, the cutting chips and the like can be easily discharged. There will be explained below an example that the bottomed container manufactured by the method of the present invention is applied to the cask as the used nuclear fuel container. FIGS. 12(a) and 12(b) show the cask according to the embodiment of the invention: FIG. 12(a) is an axially sectional view; and FIG. 12(b) is a radially sectional view. The cask 100 is composed of a bottomed container 1 having a basket 3 in its inner side, a neutron shielding material 2 such as resin or silicone rubber provided on the outer side of the bottomed container 1, and an outer drum 4 to be the outer surface of the cask 100. The bottomed container 1 is formed with the inner side and the outer side by the above-mentioned cutting. A primary cover 5 and a secondary cover 6 are provided on the upper section of the bottomed container 1, and resin 7 for shielding neutron is sealed into the secondary cover 6. Moreover, the bottomed container 1 has a cylindrical shape with a bottom formed by punching draw working, and is made of carbon steel or stainless steel having γ rays shielding function. The neutron shielding material 2 is a polymeric material containing a lot of hydrogen and has a neutron shielding function. Moreover, a shielding body 9 into which a neutron shielding material 8 such as resin or silicone rubber is sealed is mounted to the lower section of the bottomed container 1. The basket 3 is constituted so that cells for containing used nuclear fuel aggregate (no shown) are arranged in a lattice form, and is composed of a composite martial of boron and aluminum. In addition, in order to secure sealing performance for a pressure resistant container, a metal gasket is provided between the primary cover 5, the secondary cover 6 and the bottomed container 1. A plurality of internal fins 10 made of copper for heat conduction are welded between the bottomed container land the outer drum 4, and the neutron shielding material 2 is poured into spaces formed by the internal fins 10 in a flow state and is heated to be solidified. The primary cover 5 and the secondary cover 6 are made of carbon steel or stainless steel having γ rays shielding function. Since the cask 100 uses the bottomed container 1, in comparison with the conventional case where a bottom plate is welded, the manufacturing steps can be reduced. Moreover, since a bottom plate was conventionally welded to the bottomed container, the sealing property of the welded portion depends upon the quality of the welding, but a problem of the sealing property on this welded portion in the bottomed container 1 is extremely low. Here, in order to realize the cask 100 of the present invention, shape and material of the basket 3, a charging state of the neutron shielding material 2, and a number and positions of the internal fins 10 in the bottomed container 1 are not limited to the example shown in FIGS. 12(a) and 12(b). The method of manufacturing the bottomed container according to the first embodiment is suitable for manufacturing a so-called thick container having a thickness with respect to the diameter of the cylinder. Further, the manufacturing method of the present invention is suitable particularly for the case where a container, in which a ratio of the axial length to the inner diameter is 1:1 or more, is manufactured in the thick containers. When the ratio exceeds the above-mentioned numerical value, as the forming proceeds in general hot working, the press pressure increases, but in the manufacturing method according to the embodiment the press pressure does not greatly increase at the beginning and the end of the working. Concretely, the manufacturing method according to the embodiment is particularly suitable for the case where a cask or the like as a large bottomed container, in which the thickness is thick with respect to the diameter and the axial length reaches a several meters, is manufactured. In order to manufacture such a large and thick bottomed container with its bottom integral, a several ten-thousand ton scale pressing machine was conventionally required. However, when such a so-called thick and large bottomed container is manufactured by the manufacturing method of the present invention, the press pressure of only about ten-thousand ton is required. For this reason, even if a pressing machine of several ten-thousand ton is not used, the bottomed container can be formed by an existing large pressing machine. Moreover, since the formed container has excellent sectional form and defects do not occur on the surface and the inner side, adjustment after the forming is seldom required. Here, the manufacturing method of the present invention is not limited to such a thick and large bottomed container, and a canister as a radioactive substance container with comparatively thin thickness can be manufactured by this method. In addition, with the manufacturing method of the present invention, a cylinder for a large pressing machine, a container for chemical plant, a reactor container for petroleum refining plant, an ammonia synthetic cell, a heat exchange container, a pressure container such as a boiler, a casing for a large rotational equipment for containing a hydroelectric water turbine, a body of submarine and ship can be manufactured. Moreover, the material which can be used in the method of the present invention is not limited to carbon steel, and the material includes iron materials such as stainless steel, low alloy steel and the like, nonferrous metal such as nickel alloy, aluminum metal, copper metal and magnesium metal. FIG. 13 is a perspective view showing the bottomed container according to a second embodiment of the present invention. The bottomed container 1 shown in FIG. 13 is characterized in that its external shape and internal shape are octagonal. Moreover, at least one of the external shape and the internal shape of the container may be octagonal. Since a basket for supporting a fuel bar aggregate is housed in the bottomed container of the cask as a radioactive substance container, it is preferable that the internal shape of the bottomed container is formed into a shape which matches with the basket particularly in a cask. Therefore, the internal shape of the cask is desirably octagonal instead of circular. Moreover, in the case where the internal shape of the cask is octagonal, since it is advantageous to a dimension and a weight that the thickness of the cask body is uniform as much as possible, it is desirable that the external shape of the cask body is also octagonal. This bottomed container can cope with such requirements. FIGS. 14(a) and 14(b) are sectional view showing the container body and a punch for manufacturing the bottomed container according to the second embodiment. A sectional shape of the inside of a body section 300 of the container for forming is substantially octagonal, and the external shape of the boring punch 410 is also substantially octagonal. When the body section 300 of the container for forming and the boring punch 410 are used in the hot dilation forming described in the first embodiment, an increase in the load at the time of the hot dilation forming is suppressed, and defects on a surface of a workpiece is suppressed after the forming, and the bottomed container having excellent end surface shape and a polygonal section can be manufactured. FIGS. 15(a) through 15(d) are sectional views vertical to the axial direction showing examples of the bottomed container capable of being formed by the manufacturing method according to the second embodiment. The section of the body section of the container for forming and the external shape of the boring punch are changed suitably so that the bottomed container having such a sectional shape can be formed. It is desirable that particularly the boring punch 410 for forming the internal shape of the cask is changed suitable according to the shape of the basket for containing used nuclear fuel. When the boring punch whose external shape is the internal sectional shape shown in FIG. 15(d) is used, for example, the internal shape can be formed according to the shape of the basket. FIGS. 16(a) through 16(d) are axially sectional views showing examples of the bottomed container capable of being formed by the manufacturing method according to the second embodiment. The internal shape of the bottom section of the container 300 for forming and the forward end shape of the boring punch 410 match with the shape of the bottom of the bottomed container so that these containers can be formed. The container shown in FIG. 16(d) maybe bored on the bottom section after the forming, or may be formed by the boring punch provided with a protrusion at its forward end. The container formed in such a manner can be used as a container, in which its bottom section formed integrally with the body section requires not a plane but a curved surface. For example, this container can be applied to a casing and the like for a large rotating equipment for containing a hydroelectric water turbine. FIG. 17 is a axially sectional view showing the bottomed container according to a third embodiment. The bottomed container 1 is characterized in that a body and a bottom are formed integrally and a spot facing section is also formed on the bottom of the container. The bottom provided with the spot facing section was conventionally mounted to a thick cylinder by welding, but in the manufacturing method, besides the step of providing the spot facing section on the bottom, the welding step and the heat treating step after the welding are required. For this reason, there arises a problem that the manufacturing requires troublesome steps. According to the method of manufacturing a bottomed container of the present invention, since the bottom provided with the spot facing section can be formed integrally with the body by one step, there is an advantage that the manufacturing becomes very easy. FIGS. 18(a) through 18(e), 18(b′), 18(c′) and 18(e′) are explanatory diagrams showing a method of providing the spot facing section 800 on the bottomed container. As shown in FIG. 18(a), before the punch 410 is pushed into so that the hot dilation forming is carried out, a cylinder 302 which is a cylindrical member is previously provided on the bottom section 301 of the container for forming. Here, the spot facing section 800 can be formed only by the cylinder 302, but in order to easily taken out the cylinder 302 after the metal billet 200 is formed, an annular metal plate 303 may be previously placed on the upper section of the cylinder 302 (see FIG. 18(a)). The annular metal plate 303 is constituted so that its radial width is slightly larger than a radial width of the cylinder 302. As a result, after the metal billet 200 is formed into the bottomed container, the cylinder 302 can be taken out easily. In this case, the annular metal plate 303 is fitted into the bottom section after the bottomed container is formed. When the spot facing section 800 is formed only by the cylinder 303, it is desirable that the diameter of the cylinder on a side coming in contact with the metal billet 200 is smaller than the diameter of the cylinder on a side coming in contact with the bottom section 301 of the container 300 for forming. Namely, it is desired that a taper is provided on the cylinder 302 so that the diameter of the cylinder 302 becomes larger towards the pressing direction. As a result, the cylinder 302 is removed from the metal billet 200 which was subject to the hot dilation forming. The metal billet 200 is set into the body section 300 of the container and is placed on the cylinder 302 and the annular metal plate 303 (FIG. 18(b)), and the punch 410 is pushed into the billet 200 so that the metal billet 200 is hot dilated into the shape of the bottomed container (FIG. 18(c)). As shown in FIG. 18(d), an annular groove is formed on the bottom section of the metal billet 200 formed in such a manner by the cylinder 302 and the annular metal plate 303. In this state, only a simple annular groove is provided, and thus a column section as a pillar section existing in the annular groove is cut by cutting means (not shown) such as gas burner in order to form the spot facing section 800. As a result, the bottomed container having the spot facing section 800 on the bottom section can be manufactured (FIG. 18(e)). The spot facing section 800 may be finished by cutting or the like as the need arises. In addition, as shown in FIG. 18(b′), a column 304 is used instead of the cylinder 301 so that the spot facing section 800 can be formed on the bottom section of the bottom container. Also in this case, the metal plate 305 is placed on the column 304 so that the spot facing section 800 can be formed (FIG. 18(c′)). In the case where the column 304 is used, unlike the case of using the cylinder 302, the bottomed container is formed and simultaneously the spot facing section 800 can be formed on its bottom section (FIG. 18(e′)). For this reason, the step of cutting the column section shown in FIG. 18(d) is not required, but the press pressure which is larger than the case of using the cylinder 302 is required. Therefore, the performance of the pressing machine, a size of the bottomed container to be produced and the like are taken into consideration, and it is desirable that a determination is made as to which is used the cylinder 302 or the column 304. Here, in the case where the spot facing section 800 is formed only by the column 304, it is desirable that a taper which becomes larger towards the pressing forward side is provided. Namely, it is desirable that the axial section of the column 304 is trapeziform. As a result, the column 302 is easily removed from the metal billet 200 which was subject to the hot dilation forming. The shapes of the sections of the cylinder 302 and the column 304 vertical to the axial direction are changed so that the spot facing section 800 having a desired sectional shape can be formed. For example, the shape of the section vertical to the axial direction is polygonal so that the spot facing section 800 whose internal surface shape is polygonal can be formed. As a result, since the spot facing section can be formed according to the external shape of the bottomed container, the thickness of the spot facing section in the radial direction can be kept constant. FIGS. 19(a) through 19(c) are axially sectional views showing examples of the bottomed container capable of being formed by the manufacturing method of the present invention. The bottomed container 1 shown in FIGS. 19(a) and 19(b) can be formed by selecting suitably a diameter of the cylinder or the column in the above explanation. FIG. 19(c) shows the example that two-staged spot facing section 800 is provided on the bottom section. In order to provide the two-staged spot facing section 800, for example, a forming tool, in which two columns having different diameters are stacked and its axially section has a convex shape, is set on the bottom of the metal billet 200. Moreover, the two-staged spot facing section 800 can be formed also by setting two cylinders having different diameters and heights on the bottom of the metal billet 200. Moreover, a step section may be provided on the external surface of the cylinder in an axial direction so as to be used. FIGS. 20(a) and 20(b) are axially sectional views showing the bottomed container according to a fourth embodiment of the present invention. The bottomed container 1 is characterized in that in the manufacturing method described in the first embodiment the extended section 201 formed before the hot dilation forming (see FIG. 2(b)) is directly utilized as the flange of the container. Since the cask as the used nuclear fuel container is constituted so that helium gas of a several atm. is sealed between the primary cover and the secondary cover, a great pressure is applied to the mounting portion of the secondary cover. Moreover, since the secondary cover occasionally receives impact of falling, the flange section as the mounting portion of the secondary cover requires the firm structure. Since the container of the present invention is formed so that the flange and the body are integral and a diameter of the flange is larger than a diameter of the body, measure such that bolts are arranged in two lines is easily taken. Therefore, the secondary cover can be fixed firmly. Since in the conventional cask the flange section is manufactured separately and is welded to the cask body, troublesome steps are required. According to the manufacturing method of the present invention, since the thick bottomed container having excellent end surface shape can be manufactured, the extended section 201 formed on the end surface of the container (see FIG. 2(b)) is seldom worked and can be utilized as the flange. Therefore, the welding step and the post-welding hot processing step can be omitted, and thus the manufacturing steps can be simplified. Here, in FIGS. 20(a) and 20(b), the extended section 201 formed before the hot dilation forming (see FIG. 2(b)) is directly used as the flange of the container, but the extended section 201 is removed by cutting or the like and the inner side of the opening of the container is worked so that a flange without extended section ever used can be formed. Also in this case, since the flange section and the body section are formed integrally, strength and sealing property can be secured sufficiently. There will be explained below another manufacturing method of the bottomed container 1. FIGS. 21 through 26 are explanatory diagrams showing the manufacturing steps of the bottomed container 1 of the cask 100 shown in FIG. 12. The bottomed container 1 is formed by combining the upsetting step and the punching draw step. Firstly, ring-shaped first die 21, second die 22 and third die 23 are stacked on a slide table 20 of the pressing machine (not shown), and first pressurizing platform 24 second pressurizing platform 25 and third pressurizing platform 26 are placed in the first through third dies 23 so that a mold is structured (FIG. 21(a)). A boring punch 27 is positioned on an upper surface of a metal billet W. The boring punch 27 is pressurized by a stem 28 attached to a punch of the pressing machine (FIG. 21(b)). The metal billet W is set in the first die 21. The metal billet W is made of low carbon steel or stainless steel formed by vacuum forging, and its upper surface is circular and its lower surface has a circular shape smaller than the upper surface so that the metal billet W is conically trapeziform (angle of tilt surface is not shown). At the time of pressurizing, the metal billet W is heated within a range of 1000° C. to 1200° C. The heating is carried out in an electric oven and the metal billet W in a red state is placed on the slide table 20. The cylinder 30 and the annular metal plate 303 (see FIG. 18(a)) are provided between the first die 21 and the metal billet W so that a spot facing section may be formed on the bottom. Next, when the metal billet W is placed, the boring punch 27 is pressurized so as to be upset (FIG. 21(c)). Since the inner end portion of the first die 21 is an opened, material flows between the boring punch 27 and an opening portion 21a of the first die so that the metal billet W is deformed into a dished shape. A hang tool 29 is used so as to hang the stem 28 together with the first die 21 (FIG. 21(d)), and the slide table 20 is moved to be conveyed out so that the first pressurizing platform 24 is removed (FIG. 21(e)). When the first die 21 including the metal billet W is hung, in this state a spacer 30 is placed on the second die 22 (FIG. 22(a)). Thereafter, the slide table 20 is moved and the mold is conveyed in (FIG. 22(b)), and the boring punch 27 is pushed down so that the metal billet W is subject to the punching draw working by the first die 21 (FIG. 22(c)). As a result, when the metal billet W passes through the first die 21, the dished section at the top is drawn so that the metal billet W has a cup shape and is positioned in the second die 22. Next, the stem 28 and the hang tool 29 are allowed to recede upward and the slide table 20 is moved so that the metal billet W and the mold are conveyed out and the spacer 30 is removed (FIG. 22(d)). In this state, the boring punch 27 remains at the bottom of the metal billet W having cup shape (FIG. 23(a)). Next, the stem 28 in a state that the spacer 30 is removed is lowered and is pressurized by the boring punch (FIG. 23(b)). As a result, the metal billet W is further upset, and the material flows from between an opening section 22a of the second die 22 and the boring punch 27 so that the metal billet W is deformed. The hang tool 29 is used so as to hang the second die 22 together with the metal billet W (FIG. 23(c)). In this state the slide table 20 is moved so that the mold is conveyed out, and the second pressurizing platform 25 is removed (FIG. 23(d)). Next, the spacer 31 is placed on the third die 23 (FIG. 24(a)). The slide table 20 is moved so that the mold is conveyed in (FIG. 24(b)), and the boring punch 27 is pushed down so that the punching draw working is carried out by the second die 22 (FIG. 24(c)). As a result, when the metal billet W passes through the second die 22, its bellied portion is drawn and the metal billet W has a cup shape. Next, the stem 28 and the hang tool 29 are allowed to recede upward, and the slide table 20 is moved so that the metal billet W and the mold are conveyed out and the spacer 31 is removed (FIG. 24(d)). In this state the boring punch 27 remains at the bottom of the metal billet W having a cup shape. Next, the slide table 20 is moved and the metal billet W is positioned below the stem 28 (FIG. 25(a)), and the stem 28 is lowered so that the metal billet W is pressurized by the boring punch 27 (FIG. 25(b)). As a result, the metal billet W is further upset, and the material flows from between an opening section 23a of the third die 23 and the boring punch 27 so that the metal billet W is deformed. Thereafter, the hang tool 29 is used so as to hang the third die 23 together with the metal bit W (FIG. 25(c)). In this state, the slide table 20 is moved, and the third pressurizing platform 26 is removed (FIG. 25(d)). Next, the spacer 32 is placed on the slide table 20 (FIG. 26(a)). The slide table 20 is moved and the spacer 32 is conveyed in (FIG. 26(b)), and the boring punch 27 is pushed down so that the punching draw working is carried out by the third die 23 (FIG. 26(c)). As a result, when the metal billet W passes through the third die 23, its bellied portion is drawn. Next, the step 28 and the hang tool 29 are allowed to recede upward, and the slide table 20 is moved so that the metal billet W is conveyed out and the spacer 32 is removed (FIG. 26(d)). In this state the boring punch 27 remains at the bottom of the metal billet W having a cup shape, but it is directly used as the bottom of the bottomed container 1. The boring punch 27 is removed and the metal billet W can be used. Moreover, when the slide table 20 is moved and the spacer 32 is conveyed in (FIG. 26(b)), a cylindrical spacer 302 is provided in the spacer 32 (see FIG. 2(e″))), and the bottom of section the metal billet W is punched by the boring punch 27. The cylindrical container can be formed by this method. When the above-mentioned forming is finished, predetermined heat treatment is given to the bottomed container, and its inner surface is mechanically worked. The bottomed container 1 formed in such a manner has a percentage that its section is reduced from the metal billet W becomes about 40%. Moreover, in comparison with the case where the bottomed container is formed by the normal backward extrusion forming, in the case of the backward extrusion forming, the bottom section of the bottomed container becomes thick, and this causes an increase in the weight of the cask. Moreover, the pressing machine requires a great pressure, and a bottomed container cannot be occasionally manufactured depending upon its scale. On the contrary, according to the manufacturing method of this embodiment, since the bottomed container is formed by combining the upsetting and the punching draw as mentioned above, only low pressure is required at the time of the upsetting or the draw. For this reason, a large-sized pressing machine which has been conventionally used is used so as to enable the forming. FIGS. 27(a) through 27(d) are explanatory diagrams showing another manufacturing method of the bottomed container. As mentioned above, in the above-mentioned manufacturing method, the cylindrical boring punch 27, the pressurizing platforms 24 through 26, and the dies 21 through 23 having annular ring-shaped inner side were used, but the boring punch 27, etc. are not limited to these shapes. For example as shown in FIG. 27(b), in the case where the outer shape of the bottomed container 1 is octagonal, the internal shapes of a first die 21b through a third die may be octagonal. In this case, a pressurizing platform 24b is also octagonal. In addition, in the case where the internal shape of the bottomed container 1 is octagonal, as shown in FIG. 27(c), a boring punch 27c may be a octagonal prism. The metal billet W in this case has a octagonal cone trapezoid shape (detail of a tapered angle is not shown). Further, in the case where the internal shape of the bottomed container 1 has a step, as shown in FIG. 27(d), a boring punch 27d may be a prism having steps. Here, as not shown, in the case where the internal and external shapes of the bottomed container are octagonal, the die 21b shown in FIG. 27(b) and the boring punch 27c shown in FIG. 27(c) may be used. Moreover, also in the case where these shapes are not octagonal, according to the manufacturing method of the present invention, the bottomed container can be formed similarly to the above-mentioned method by changing the shapes of the boring punch, die and the like. Here, the boring punches 27c and 27d shown in FIG. 27 can be applied also to the manufacturing method according to the first embodiment. FIGS. 28(a) through 28(f) are explanatory diagrams showing an embodiment of different manufacturing method. This manufacturing method is realized in that a stem 51 is set on a slide table 52 of a pressing machine and a boring punch 53 is attached to a top portion of the stem 51, and a pressurizing platform 54 is mounted to a punch 55 of the pressing machine. Namely, as shown in FIG. 28(a), the metal billet W is placed on the boring punch 53, and the punch 55 of the pressing machine is lowered so that the metal billet W is upset by the first die 56. Next, after the lower end of the metal billet W is deformed into a dished shape, the punch 55 is allowed to recede so that the metal billet W is conveyed out (not shown). A plurality of spacers 57 are placed on the upper portion of the first die 56 so as to be conveyed in below the punch 55. Moreover, the pressurizing platform 54 is removed from the punch 55. When the punch 55 is lowered and pressurizes the metal billet W in this state, as shown in FIG. 28(b), the metal billet W is drawn by the first die 56. The first die 56 directly recedes downward. Next, similarly the metal billet W is conveyed out so that the spacer 57 is removed and the pressurizing platform 54 is mounted, and the metal billet W is conveyed in below the punch 55 again (not shown). Moreover, a second die 58 is mounted to the punch 55 side. In this state the punch 55 is lowered so as to upset and pressurize the metal billet W (FIG. 28(c)). The metal billet W is once conveyed out, and a plurality of spacers 59 are placed on the second die 58 and simultaneously the pressurizing platform 54 is removed, and the metal billet W is again conveyed in below the punch 55. In this state when the punch 55 is lowered and the pressurizes the metal billet W, as shown in FIG. 28(d), the bellied portion of the bottomed container 1 is drawn. The second dice 58 directly recedes downward. Thereafter, similarly the metal billet W is conveyed out and the spacer 59 is removed, and the metal billet W is again conveyed in below the punch 55 (not shown). Moreover, a third die 60 and the pressurizing platform 54 are attached to the punch 55 side. In this state when the punch 55 is lowered so as to pressurize the metal billet W, as shown in FIG. 28(e), the metal billet W is further deformed. The metal billet W is once conveyed out and a plurality of spacers 61 are placed on the third die 60, and simultaneously the pressurizing platform 54 is removed so that the metal billet W is again conveyed in. When the punch 55 is lowered so as to pressurize the metal billet W in this state, as shown in FIG. 28(f), the bellied portion of the bottomed container 1 is drawn. The third die 60 recedes downward. Here, the material which can be used in the method according to this invention includes ferrous materials such as carbon steel, stainless steel and low-alloy steel, and also nonferrous metal such as nickel alloy, aluminum alloy, copper alloy and magnesium alloy. There will be explained below concrete forming conditions. When a metal billet made of low carbon steel is heated to 1000° C. and a distortion speed is 0.1 to 1 s, the deformation resistance becomes 1.5 to 3 kgf/mm2. For example, if a length of 1 cm is punched within 1 minute, in the case where the outer diameter is reduced from 2500 mm to 2200 mm by a die of 30°, when the inner diameter is 1420 mm, the distortion becomes:1n((25002−14202)/(22002−14202)))=0.4 and the metal billet is worked for time (2500−2200/2/tan 30°)/(1000/60)=15.6 sec. Therefore, the distortion speed becomes 0.025−1 s. Next, when the distortion resistance is 3 kgf/mm2 and the friction coefficient is 0.3, the punching force becomes:3×π/4×(22002−14202)×1n((25002−14202)/(22002−14202))×(1+0.1×0.3/tan 30°)+4π/(6·3·√3)=5460640 kgf However, since the temperature is initially high, the punching force is reduced to half, namely, becomes 2730 tonf. Moreover, the final thickness which prevents the bottom from dropping off at the time of punching is:54640/(3/√3)/π/1420=7.7 mm Therefore, the thickness of more than this value is required. When a length of a product is 4885 mm, since a length of a material is:4885×(22002−14202)/22002=2850 nm,an upsetting amount for one time is reduced to ⅓, namely, 950 mm and a height up to the mold constraining portion is set to the final bottom thickness of 700 mm, a necessary upsetting force becomes:3×π/4×14202×(1+0.3×1420/700/2)=61967With the pressure of this value, the bottomed container can be formed by pressing of 8000 ton which has usage accomplishment. On the contrary, since the extruding force in the case of the backward extrusion forming becomes:3×π/4×22002×1n((22002/(22002−14202))×(1+2×0.5/tan 45°))+4π/(4·3·√3)=19186103 kgfthe pressing force of 20000 ton is required. As mentioned above, in the radioactive substance container and the container of the present invention, the bottomed container in which the bottom and the body are formed integrally is used so that the conventional welding of a bottom plate is not required, and the heating treatment thereafter can be omitted. As a result, troublesome manufacturing steps can be reduced greatly. Moreover, since the bottomed container is formed by the hot dilation, only a press pressure, which is lower than that at the time of hot backward extrusion forming, for example, is required. In addition, as for the radioactive substance container and the container of the present invention, the bottomed container was formed by using the metal billet having a polygonal section vertical to the axial direction and the container for forming having a circular internal shape of the section vertical to the axial direction. For this reason, since the conventional welding of a bottom plate is not required, troublesome steps required for the manufacturing can be reduced. Further in the dilation forming, the bottomed container can be formed with a lower pressure than conventional one due to a function which bends one side of the polygon of the metal billet. In addition, as for the radioactive substance container and the container of the present invention, the bottomed container was formed by the metal billet having a polygonal section vertical to the axial direction and the container for forming having polygonal internal shape of the section vertical to the axial direction. For this reason, since the conventional welding of a bottom plate is not required, troublesome steps required for the manufacturing can be reduced. Further in the dilation forming, the bottomed container can be formed with a lower pressure than conventional one due to the function which bends one side of the polygon of the metal billet. Further, bottomed containers having external shapes according to various applications can be formed easily. In addition, as for the radioactive substance container of the present invention, the bottomed container, which is enough long in the axial direction to contain the basket of used nuclear fuel aggregate used as fuel of nuclear reactor and has large inner diameter, namely, is thick, was formed by the hot dilation forming in the container for forming so that the bottom and the body were integral. For this reason, since the conventional welding of a bottom plate is not required and the post-welding heat treatment can be omitted, troublesome steps required for the manufacturing can be reduced. Particularly in the bottomed container whose thickness is thick and dimension in the axial direction is a several meters and inner diameter reaches 2 to 2.5 meters, the effect which can omit the steps is extremely great. In addition, since the radioactive substance container of the present invention has a dimension such that the section of the boring punch is approximate to the section of the basket for used nuclear fuel aggregate, an operation for cutting the inside of the container becomes easy after the hot dilation forming, and the manufacturing does not require the troublesome steps. In addition, as for the radioactive substance container of the present invention, in the case where radioactive substance is stored in the bottomed container in which the bottom and the body are formed integrally by the hot dilation forming in the container for forming, a dosage equivalent factor of the γ rays is not more than 200 μSv/h. In order to satisfy the requirement that a dosage equivalent factor of the γ rays is not more than 200 μSv/h on the surface of an external wall on the substantially center of the side surface of the container, it is necessary to manufacture the container whose thickness reaches several dozens cm using stainless steel, carbon steel or the like. Since the body and the bottom of such a thick container were formed integrally, the conventional welding of a bottom plate is not required, and since the post-welding heat treatment can be omitted, the manufacturing does not require troublesome steps. Particularly in such a thick bottomed container, the effect which can omit the steps is extremely great. In addition, as for the radioactive substance container of the present invention, in the above-mentioned radioactive substance container and the container, the outer diameter of the bottomed container was not less than 1000 mm to not more than 3000 mm, and the thickness was not less than 150 mm to not more than 300 mm. Since the thick container was formed so that the bottom and the body were integral, the conventional welding of a bottom plate is not required, and since the post-welding heat treatment is omitted, the manufacturing does not require troublesome steps. Particularly in the thick bottomed container whose axial dimension is large, the effect which can omit the above-mentioned steps is extremely great. In addition, as for the radioactive substance container and the container of the present invention, the metal billet, whose section vertical to the axial direction on the pressing forward side was at least formed into polygonal shape, was set into the container for forming, and the boring punch is pushed into the metal billet so that the metal billet is hot-dilated. For this reason, the conventional welding of a bottom plate is not required, and the post-welding heat treatment can be omitted. In addition, since a number of defects caused on the end portion and the surface of the bottomed container is small, only less troublesome steps of correcting these defects are required after the forming, and thus the manufacturing does not require troublesome steps. In addition, in the radioactive substance container of the present invention, the bottom and the body were formed integrally by hot press working. Moreover, in the radioactive substance container of the present invention, the metal billet was heated and was upset and drawn so that the bottom and the body were formed integrally. For this reason, since the welding step and the heat treating step after that can be omitted, the manufacturing does not require troublesome steps. In addition, the radioactive substance container of the present invention, the spot facing section was provided integrally with the bottom section of the bottomed container. Since in the bottomed container the spot facing section is also formed integrally at the time of the hot dilation forming, the step of providing the spot facing section can be omitted, and thus the manufacturing does not require troublesome steps. In addition, as for the radioactive substance container of the present invention, since the flange was provided integrally with the body of the bottomed container, the welding step and the post-welding heat treating step can be omitted, and troublesome steps for the manufacturing can be omitted. Moreover, the sealing performance and the strength of the container itself can be secured. In addition, as for the radioactive substance container and the container of the present invention, at least one of the section outside the body and the section inside the body of the bottomed container was polygonal. For this reason, when the bottomed container is dilated to be formed, the internal section of the container can be formed into a shape according to a basket. As a result, the step of cutting the inside of the container which has been conventionally required can be omitted, and thus the manufacturing does not require troublesome steps. In addition, the metal billet for the hot dilation forming according to the present invention was formed so that at least the section vertical to the axial direction on the pressing forward side was polygonal. For this reason, the performance for bending a side of the polygon and the function for restraining the upsetting of the metal billet accrue. Because of these functions, the thick container, in which a ratio of the length to the diameter in the axial direction is not less than 1, can be formed with a lower press pressure than conventional one. Moreover, the defects caused on the end portion and the surface of the container after the forming can be suppressed. In addition, the metal billet for the hot dilation forming of the present invention was provided with at least one plane on at least one of the side surface on the pressing forward side and the side surface of the pressing backward side. At the time of the hot dilation forming, since the metal billet is dilated to be formed by the function for bending this plane, a force required for the hot dilation forming is weaker than the case where the side surface is a curbed surface. Therefore, the thick container which is long in the axial direction can be formed with a lower press pressure than conventional one. Moreover, in comparison with the case where the side surface is a curved surface, defects on the inner side such as a crack can be reduced. In addition, the metal billet for the hot dilation forming of the present invention was further provided with a taper which becomes thinner toward the pressing direction on the pressing forward side of the metal billet in the above-mentioned metal billet for the hot dilation forming. Moreover, in the metal billet for the hot dilation forming of the present invention, at least one or more step sections were provided so that the pressing forward side of the metal billet becomes thinner gradationally towards the pressing direction. As for the metal billet, timing at which the metal fills the vicinity of the bottom of the container for forming can be delayed at the final stage of the hot dilation forming. For this reason, the upsetting is suppressed at the final stage of the hot dilation forming, and thus the press pressure can be reduced at the time of the hot dilation forming. In addition, since the metal billet for the hot dilation forming of the present invention is provided with the extended section at the end portion on the pressing backward side, the metal billet is engaged with the end portion of the container by the extended section at the time of the hot dilation forming. With this function, constraint of the container on the metal billet becomes stronger so that the upsetting on the pressing forward side can be suppressed. Moreover, since the side surface is provided with at least one plane, the function for bending this plane and the function for suppressing the upsetting of the metal billet accrue. Therefore, due to their interaction, the press pressure can be suppressed to be small. Moreover, since the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing, the step of manufacturing the container can be simplified. In addition, since the metal billet for the hot dilation forming of the present invention is provided with the extended section on the pressing backward side, this extended section engages the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least the section vertical to the axial direction on the pressing forward side was formed into a polygonal shape, the function for bending each side of the polygonal section and the function for suppressing the upsetting of the metal billet accrue. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, since he metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing, the step of forming the extended section on the pressing backward side is not required, and thus the steps of manufacturing the container can be simplified. In addition, since the metal billet for the hot dilation forming of the present invention was provided with the extended section on the pressing backward side, this extended section engages the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least the section vertical to the axial direction on the pressing forward side was formed into a polygonal shape, the bending function and the function for suppressing flow of the metal accrue. Further, the pressing forward side was set to be thinner gradationally towards the pressing direction, the upsetting phenomenon is suppressed at the final stage of the hot dilation forming, and an increase in the press pressure can be suppressed. Therefore, the press pressure can be suppressed to be small due to their interaction. Moreover, since the metal billet is previously provided with the extended section on the pressing backward side at the time of the manufacturing, the step of forming the extended section on the pressing backward side is not required. In addition, since the metal billet for the hot dilation forming of the present invention is provided with the extended section on the pressing backward side, the extended section latches the metal billet with the end portion of the container at the time of the hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since at least one plane is provided on at least one of the side surfaces of the metal billet, the bending function and the function for suppressing the flow of the metal accrue further, since the pressing forward side becomes thinner gradationally, the upsetting phenomenon is suppressed at the final stage of the hot dilation forming, and an increase in the press pressure can be suppressed. Therefore, the press pressure can be suppressed to be small by their interaction. Moreover, since the metal billet is previously provided with the extended section on the pressing backward end at the time of the manufacturing, the step of forming the extended section on the pressing backward side is not required. In addition, in the above-mentioned radioactive substance container and the container, the bottomed container of the present invention has an outer diameter of not less than 200 mm to not more than 4000 mm and a thickness of not less than 20 mm to not more than 400 mm. Since such a thick container is formed so that the bottom and the body are integral, the conventional welding of a bottom plate is not required, and the post-welding heat treatment can be can be omitted so that the manufacturing does not require troublesome steps. Particularly in the thick bottomed container having a large axial dimension, the effect which can omit the above-mentioned steps is extremely great. In addition, since the container manufacturing apparatus of the present invention is provided with the container for forming, whose body and the bottom can relatively move with respect to the axial direction of the body of the container, and the boring punch which is attached to the pressing machine and pressurizes the metal billet set in the container for forming. For this reason, since the body of the container and the metal billet hardly relatively move at the time of hot dilation forming, increase in the press pressure can be suppressed at the time of the hot dilation forming. In addition, in the method of manufacturing a container of the present invention the body section of the container for forming is divided in the axial direction. For this reason, even in the case where the metal billet which is long in the axial direction is formed, the deformation of the metal billet in the axial direction at the time of hot dilation forming can be absorbed by the whole container. Therefore, an increase in the press pressure can be suppressed. In addition, according to the manufacturing method of the radioactive substance container of the present invention, the container in which the bottom and body are formed integrally is finished by cutting its external side, and its internal side is cut into a stepped shape so that a portion for containing a basket for used nuclear fuel aggregate is provided, or the internal section is cut to be finished so that the radioactive substance container is manufactured. For this reason, the internal side of the bottomed container can be cut and finished easily. In addition, according to the manufacturing method of the radioactive substance container of the present invention, the bottomed container is formed by hot dilation forming so that the bottom and the body are integral, and the external side of the bottomed container is cut to be finished, and the internal section is cut into a stepped shape so that a portion for storing a basket for used fuel aggregate is provided, or the internal side is cut to be finished so that the radioactive substance container is manufactured. For this reason, the inner side of the bottomed container can be cut and finished easily. In addition, according to the method of manufacturing a bottomed container of the present invention, the boring punch is pushed into the metal billet, and the plane of the metal billet is bent by the force directing to the inner wall of the container so that the metal billet is dilated to a gap existing between the container for forming and the metal billet. In this method of manufacturing a container, the metal billet is dilated to the inner wall side of the container for forming by function for bending the plane of the metal billet on the side surface. Moreover, since the metal billet is dilated to the space existing between the metal billet and the inner wall of the container for forming, the upsetting phenomenon of the metal billet can be suppressed. With these functions, in this method of manufacturing a container, only lower press pressure than the conventional one is required, and defects caused on the end portion and the surface of the container after the forming can be suppressed. In addition, according to the method of manufacturing a bottomed container of the present invention, the metal billet provided with the extended section engaging with the end portion of the opening of the container for forming is used for the end portion of the pressing backward side. For this reason, the extended section engages the metal billet with the end portion of the container at the time of hot dilation forming. With this function, the constraint of the container on the metal billet becomes stronger, and the upsetting of the pressing forward side can be suppressed. Moreover, since at least one plane is provided on the side surface, the bending function and the function for suppressing the phenomenon that the metal flows to the opposite side to the pressing direction accrue. Therefore, the press pressure can be suppressed small by their interaction. Further, the defects which are caused on the end portion and the surface of the container after the forming can be also suppressed. In addition, in the method of manufacturing a bottomed container of the present invention, the metal billet, in which at least the section vertical to the axial direction on the pressing forward side is formed into polygonal, is hot-dilated. For this reason, on the pressing forward side, since the metal billet is dilated in the space existing between the pressing forward side and the body of the container, the upsetting phenomenon of the metal billet can be suppressed. Therefore, in this method of manufacturing a container, only lower press pressure than conventional one is required, and the defects which are caused on the end portion and the surface of the container after the forming can be suppressed. In addition, in the method of manufacturing a bottomed container of the present invention, the metal billet, which is provided with at least one plane on at least one of the side surface of the pressing forward side and the side surface of the pressing backward side, is hot-dilated. For this reason, only a weak force suffices for the hot dilation forming in comparison with the case where the side surface is a curved surface. Therefore, a lower press pressure is required in comparison with the conventional method of manufacturing a container, and internal defects such as a crack can be also reduced. In addition, in the hot pressing method of the thick metal cylinder or the cylindrical container of the present invention, a metal billet without joint of different diameter sections, in which its pressing forward side is composed of a member having a section with an outer diameter smaller than the inner diameter of the container or an outer diameter equal with the diagonal length, or a member having a section with an outer diameter of the diagonal length equal with the inner diameter of the container, and its pressing backward side is composed of a member having a section with an outer diameter or a diagonal length equal with the inner diameter of the container, is heated to a pressing temperature and is set into the container for press forming, and thereafter while the center of the workpiece as the metal billet without joint is being bored by the punch, the billet is pressed. For this season, the metal billet without joint reduces a press forming load and improves yield of the product. Further, a press formed product having excellent end surface shape can be obtained. In addition, in the method of manufacturing a drum or a container of the present invention, the metal billet, in which its pressing forward side has a section with an outer diameter smaller than the inner diameter of the container and its backward side has a section with an outer diameter substantially equal with the inner diameter of the container, is hot-dilated. For this reason, the thick container can be formed with lower press pressure than conventional one, and a number of defects caused on the end portion or the surface of the container is small. As a result, less steps are suffice for correcting the defects after the forming. Moreover, since both the pressing forward side and backward side have angular section, the metal billet can be worked comparatively easily in comparison with a billet having a circular sectional. Therefore, the manufacturing does not require troublesome steps. In addition, in the method of manufacturing a drum or a container of the present invention, the metal billet, in which the pressing forward side has a section with a diagonal length smaller than the inner diameter of the container and the backward side has a section with a diagonal length substantially equal with the inner diameter of the container, is hot-dilated. For this reason, the thick container can be formed with lower press pressure than conventional one, and a number of defects caused on the end portion and the container surface is small. As a result, less troublesome steps suffice for correcting the defects after the forming. Moreover, since both the pressing forward side and backward side have angular sections, the metal billet can be worked comparatively easily in comparison with a metal billet having a round section. In addition, in the method of manufacturing a drum or a container of the present invention, the metal billet, in which the pressing forward side has a section with an outer diameter smaller ethan the inner diameter of the container and the backward side has a section with an outer diameter substantially equal with the inner diameter of the container, is hot-dilated. For this reason, the press pressure can be lower than conventional one, and a number of defects caused on the end portion and the container surface is small. As a result, only less troublesome steps suffice for correcting these defects after the forming. In addition, the method of manufacturing a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before hot dilation forming. Moreover, in this container forming method, the metal billet is dilated by the function for bending the plane of the metal billet towards the inner wall of the container for forming. Since the extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, the press pressure can be reduced by the function for bending the plane of the metal billet towards the inner wall of the container for forming. With their interaction, this method of manufacturing a container can form the thick container with lower press pressure than the backward extrusion method or the like. In addition, in the method of manufacturing a container of the present invention, the metal billet, which is previously provided with the extended section engaging with the opening end portion of the container for forming on the end portion of the pressing backward side, is hot-dilated. For this reason, since the step of extending the pressing backward side of the metal billet to above the body of the container is not required before the hot dilation forming, time required for the hot dilation becomes short. As a result, since the forming can be finished until the temperature of the metal billet drops, the shape of the end portion becomes satisfactory. Moreover, since the extending step can be also omitted, the manufacturing does not require troublesome steps. In addition, the method of manufacturing a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since the extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which at least the section vertical to the axial direction on the pressing forward side is formed into a polygonal shape, is dilated to be formed, the function for bending each side of the polygon towards the inner wall of the container for forming acts. With their interaction, the thick container can be formed with lower press pressure than the backward extrusion method or the like. In addition, the method of manufacturing a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since the extended section has the function for engaging the metal billet with the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes strong, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, which is provided at least one plane on at least one side, is dilated to be formed, the function for bending the plane of the metal billet towards the inner wall of the container for forming acts. With the interaction, this method of manufacturing a container can form the thick container with lower press pressure than the backward extrusion method or the like. In addition, the thick metal-made cylinder or drum container hot pressing method includes the step of extending the pressing backward side of the metal billet to the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet wit the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, the pressing forward side is a member having a section with an outer diameter smaller than the inner diameter of the container or an outer diameter of the diagonal length, or a member having a section with an outer diameter of the diagonal length equal with the inner diameter of the container. For this reason, as the metal is supplied from the pressing backward side and due to the function of the satisfactory plastic working of steel heated to high temperature, the metal billet is pushed to be spread sideways and simultaneously is worked, and is formed to fill the space of the container. As a result, the metal billet without joint is manufactured into a press product having a predetermined shape. With these interaction, the thick container can be formed with lower press pressure than the backward extrusion method or the like. In addition, the method of manufacturing a drum or a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet wit the container end portion at the time of the hot dilation forming, the constraint of the container on the metal billet becomes stronger, and the upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which the pressing forward side is a tetragonal section with its diagonal length is smaller than the inner diameter of the container, is used, the metal billet is dilated by the function for bending each side of the tetragonal section. Moreover, the pressing backward side of the metal billet suppresses the upsetting on the pressing forward side. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. In addition, the method of manufacturing a drum or a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of hot dilation forming, the constraint of the container on the metal billet becomes stronger, and upsetting on the pressing forward side can be suppressed. Moreover, since the metal billet, in which the pressing forward side is a tetragonal section with a sectional length smaller than the internal diameter of the container, is used, the metal billet is dilated to be formed by the function for bending each side of the tetragonal section. Moreover, the pressing backward side of the metal billet suppresses upsetting on the pressing forward side. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. Further, the metal billet to be used in this method is worked comparatively easier than the metal billet having one round section. In addition, the method of manufacturing a drum or a container of the present invention includes the step of extending the pressing backward side of the metal billet to above the body of the container before the hot dilation forming. Since this extended section has the function for engaging the metal billet with the container end portion at the time of hot dilation forming, the constraint of the container on the metal billet becomes stronger, and upsetting on the pressing forward side can be suppressed. Moreover, since the pressing backward side of the metal billet has a diameter substantially equal with the inner diameter of the container for forming, upsetting on the pressing forward side can be suppressed. With their interaction, in this manufacturing method, the thick container can be formed with lower press pressure than the backward extrusion method or the like. Further, the metal billet to be used in this method can be worked comparatively easier than the metal billet having different sectional shapes. Moreover, in the method of manufacturing a container of the present invention, the metal billet is formed by the forging step, and this method includes the step of forming at least the pressing forward side of the metal billet so as to provide an angular section. Moreover, the container manufacturing method of the present invention includes the step of providing a taper which becomes narrower towards the pressing direction on the pressing forward side of the metal billet. Further, in the method of manufacturing a container of the present invention, the forging step includes the step of providing at least one stepped section so that the pressing forward side of the metal billet becomes narrow gradationally towards the pressing direction. In this method of manufacturing a container, at the final stage of the hot dilation forming, timing at which the metal fills the vicinity of the bottom of the container for forming can be delayed. For this reason, the upsetting phenomenon of the metal billet can be suppressed, and the press pressure at the final stage of the hot dilation forming can be low. In addition, in the method of manufacturing a container of the present invention, the bottomed container is formed by a drum-shaped member provided at the bottom of the metal billet, and simultaneously a spot facing section is formed on the bottom of the bottomed container. Since the spot facing section has been conventional provided by cutting, the working requires troublesome steps. However, since a pillar-shaped section which remains on the bottom of the container is only removed after the dilation forming, the working does not require less troublesome steps than the conventional method. In addition, in the method of manufacturing a container of the present invention, the bottomed container is formed by the pillar-shaped member provided at the bottom of the metal billet, and simultaneously the spot facing section is formed on the bottomed container. For this reason, since the metal billet can be dilated and the spot facing section can be formed simultaneously, the working requires less troublesome steps than the conventional method. Moreover, since the step of removing the pillar-shaped member can be omitted, troublesome steps are not required for forming the spot facing section in comparison with the method where the spot facing section is formed. In addition, in the method of manufacturing a container of the present invention, the aforementioned method of manufacturing a container can relatively move the body of the container for forming with respect to the bottom of the container for forming. For this reason, since the body of the container and the metal billet to be formed hardly move relatively at the time of the hot dilation forming, an increase in the press pressure can be suppressed. In addition, in the method of manufacturing a container of the present invention, the aforementioned method of manufacturing a container divides the body of the container for forming in the axial direction. For this reason, even in the case where the metal billet which is long in the axial direction is formed, an increase in the press pressure can be suppressed. In addition, the radioactive substance container manufacturing method of the present invention includes the upsetting step of setting the pressurizing platform in a ring-shaped die provided with an opening on its inner end portion and putting the metal billet into the mold composed of the die and the pressurizing platform so as to pressurize the metal billet by means of the boring punch, and the metal billet drawing step of setting a spacer to the lower portion of the mold and pushing the metal billet by means of the boring punch. For this reason, the bottomed container is easily formed. In addition, this radioactive substance container manufacturing method includes the upsetting preparation step of stacking a plurality of ring-shaped dies provided with an opening on its inner end portion and stacking a plurality of pressurizing platforms in the dies and putting the metal billet into the mold composed of the die and the pressurizing platform, the upsetting step of pressurizing the metal billet from above the mold using the boring punch operated by the pressing machine, and the receding step of allowing the whole metal billet including the boring punch and the upper die to recede, the drawing preparation step of removing the used pressurizing platform and setting the drum-shaped spacer onto the next die and placing the whole metal billet including the receded die onto the spacer, the drawing step of pushing the metal billet using the boring punch and drawing the metal billet by means of the die, and the repeating step of repeating the above-mentioned steps using the next pressurizing platform and die and a spacer with a length according to deformation of the metal billet. For this reason, the pressurizing force can be suppressed small, and thus the manufacturing becomes easy. As mentioned above, the radioactive substance container, the radioactive substance container manufacturing apparatus and manufacturing method of the present invention are practical for a thick container such as a cask for containing, transporting and storing used nuclear fuel aggregate and substances contaminated with radioactive rays, and are suitable for providing a container such that manufacturing does not require troublesome steps and suppressing defects generated on an end portion of a cylinder and a surface of the container.

abstract

A hydrogen/oxygen reaction is initiated in a catalytic recombination or ignition device. The device has one or more catalyst bodies with a predetermined catalytic surface. Only a small portion of the entire available catalytic surface, preferably less than 5% of the surface, is permanently maintained at a temperature level above ambient temperature. The temperature is raised by introducing energy with a heater. The heated surface portion acts as an initial igniter.

summary

048715085

claims

1. A method for the operation of a light water boiling reactor with a core comprising a plurality of vertical fuel assemblies having an at least substantially square cross-section, each fuel assembly consisting of a bundle of fuel rods surrounded by a fuel channel, and a plurality of control rods, each control rod comprising four vertical blades arranged in a cruciform and provided with a neutron absorber material, the fuel assemblies being arranged in a symmetrical lattice with each fuel assembly included in two rows of fuel assemblies which are perpendicular to each other and the control rods being arranged with each one of their blades between two fuel assemblies located in the same row, so that each control rod together with four fuel assemblies arranged around the blades of the control rod form one unit, a control rod unit having an at least substantially square cross-section, the control rod units being arranged in a symmetrical lattice, with each control rod unit included in two rows of control rod units perpendicular to each other, said method comprising the steps of (a) operating said light water boiling reactor for an operating period until refuelling is needed, and then (b) exchanging in some control rod units control rods used during the operating period for new control rods having a reactivity worth which in a cold shut-down reactor is at least 6% higher than the original reactivity worth of the exchanged control rods, while maintaining in other control rod units control rods used during the operating period, the number of control rods with a higher reactivity worth after the exchange amounting to 40-60% of the total number of control rods of the reactor core. 2. Method according to claim 1, wherein in a cold shutdown reactor the reactivity worth of the new control rods with the higher reactivity worth is 10-20% higher than the original reactivity worth of the control rods which are present in the reactor at the time of exchange and have been used during the operating period. 3. Method according to claim 1, wherein the exchange of control rods for new control rods with a higher reactivity worth takes place in control rod units in a central zone of the reactor core, which is located inside an edge zone extending around the reactor core and comprising those control rod units which are located furthest out in the reactor core in each row of control rod units. 4. Method according to claim 3, in a number of control rod units distributed over the central zone of the reactor core there are used control rods which have been used in the reactor during the operating period and that at least the control rods in those control rods units which are located adjacent to each such control rod unit with a used control rod and in the same rows, perpendicular to each other, as said control rod unit with a used control rod, are exchanged for new control rods having a higher reactivity worth. 5. Method according to claim 3 or 4, wherein in regions within the central zone of the reactor core comprising 3.times.3 control rod units, control rods used during the operating period are exchanged for new control rods with a higher reactivity worth in that control rod unit which is located in the centre and in those four control rod units which are located in the same rows, perpendicular to each other, as said control rod unit located in the centre, whereas control rod which have been used during the operating period are used in the remaining four control rod units 6. Method according to claim 3 or 4, wherein when exchanging control rods new control rods with a higher reactivity worth are arranged in three control rod units at most, positioned adjacent to each other, in the same row of control rod units. 7. Method according to claim 3 or 4, wherein of the control rods used during the operating period, 50-50% of the total number of control rods in the central part of the reactor core are exchanged for new control rods with a higher reactivity worth.

claims

1. A radiology garment storage and cleaning system, for storing cleaning, and tracking radiology garments, each garment including a radiology apron and a radiology collar, for use by a plurality of users, each user having an ID card with a unique identifier, comprising:a storage apparatus having a front end, a rear end, a first side extending from the front end to the rear end, a second side extending from the front end to the rear end, a top surface, and a bottom surface, the top surface and bottom surface each being connected to the front end, the rear end, the first side, and the second side,wherein the storage apparatus is divided into a plurality of compartments for accepting a radiation apron, the plurality of compartments extending from the first side to the second side, wherein each compartment spans from the front end to the rear end and extends partially from the first side to the second side,wherein each compartment is equipped with a cleaning system, a door disposed on the front end, a lock integrated with the door, and at least one status light, andwherein each compartment has a hanging mechanism and a liquid-impermeable bottom portion extending from the bottom surface;an ID card reader, disposed on the storage apparatus,wherein the ID card reader is capable of reading the ID card and determining its unique identifier; anda monitoring system, having a memory unit, and a processor,wherein the monitoring system is in electronic communication with the ID card reader, the lock of each compartment, each of the cleaning mechanisms, and the status light of each compartment, andwherein the monitoring system is configured to monitor when the radiation apron is placed in one of the plurality of compartments, monitor the last time each of the cleaning mechanisms were used, and operate each of the cleaning systems wherein the cleaning mechanism further comprises two scrubbing mechanisms, each scrubbing mechanism having: a first pulley, rotatably attached to the compartment and proximate to the top surface;a second pulley, rotatably attached to the compartment and within the volume of cleaning solution; a cable rotatably attached to the first and second pulley; and at least one brush fixed to cable, the cable selectively moving the brush downwardly to the liquid-impermeable bottom portion and moving the brush vertically alongside the garment to engage and clean the garment on a hanger. 2. The system of claim 1, wherein the hanging mechanism in each of the compartments includes a hanger, the hanger having:a neck with a top and bottom end;a hook fixed to the top end of the neck;a triangular part fixed to the bottom end of the neck for directly supporting one of the radiology aprons;a clamp mechanism for supporting one of the radiology collars; anda stem extending downwardly from the triangular part,the stem having a top portion fixed to the triangular part anda bottom portion attached to the clamp mechanism. 3. The system of claim 2, the clamp mechanism comprising a horizontal component that is fixed to the bottom portion, and at least two clamps rotatably attached to the horizontal component. 4. The system of claim 2, wherein the triangular part is sized to accommodate a radiation apron and the clamp mechanism is sized to accommodate a radiation collar. 5. The system of claim 1, wherein a volume of cleaning solution is located within the liquid-impermeable bottom portion. 6. The system of claim 1, wherein the cleaning system is configured to move the brush downwardly to the liquid-impermeable bottom portion and move the brush vertically alongside the garment to engage and clean the garment on the hanger only when the lock is engaged.

abstract

A method of correcting a magnification of a mask pattern formed on a mask substrate. The method includes applying forces to four pressurizing points of an outer periphery of an approximately ring-shaped frame, which supports the mask substrate and has a rectangular window, on substantially extended lines of two diagonal lines of the rectangular window, and adjusting at least an angle, to the extended lines, of a vector of the forces applied to each of the pressurizing points.

summary

claims

1. A method for exposing an object to X-rays comprising the steps of:providing an X-ray machine including an X-ray tube equipped for emitting X-rays with an energy lower than or equal to 70 keV and a phototimer coupled to said X-ray tube for switching said tube on and off in accordance with an X-ray dose in the range from 0.75 up to 0.85 mR reaching said phototimer,placing an object between said X-ray tube and said phototimer,placing a cassette with a binderless storage phosphor panel or screen between said object and said phototimer andactivating said X-ray tube for exposing said object, said cassette and said phototimer until said phototimer switches said X-ray tube off, wherein said binderless storage phosphor panel comprises on a support (2) having a layer of amorphous carbon (23) with a thickness between 500 μm and 2000 μm, and a vacuum deposited phosphor layer (1) having a needle shaped CsBr:Eu phosphor, wherein amounts of Eu are in the range of from 100 up to 400 p.p.m. versus CsBr. 2. Method according to claim 1,wherein amounts of Eu are in the range of from 100 up to 200 p.p.m. versus CsBr. 3. Method according to claim 1,wherein amounts of Eu are in the range from 150 to 130 p.p.m. versus CsBr. 4. Method according to claim 1,wherein said support further includes a reflective auxiliary aluminum layer (22) with a thickness between 0.2 μm and 200 μm. 5. Method according to claim 2,wherein said support further includes a reflective auxiliary aluminum layer (22) with a thickness between 0.2 μm and 200 μm. 6. Method according to claim 3,wherein said support further includes a reflective auxiliary aluminum layer (22) with a thickness between 0.2 μm and 200 μm. 7. Method according to claim 1,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 8. Method according to claim 2,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 9. Method according to claim 3,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 10. Method according to claim 4,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 11. Method according to claim 5,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 12. Method according to claim 6,wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 13. Method according to claim 7,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 14. Method according to claim 8,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 15. Method according to claim 9,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 16. Method according to claim 10,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 17. Method according to claim 11,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 18. Method according to claim 12,wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene C and parylene HT. 19. Method according to claim 1,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 20. Method according to claim 2,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 21. Method according to claim 3,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 22. Method according to claim 4,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 23. Method according to claim 5,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 24. Method according to claim 6,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 25. Method according to claim 7,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 26. Method according to claim 8,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 27. Method according to claim 9,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 28. Method according to claim 10,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 29. Method according to claim 11,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 30. Method according to claim 12,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 31. Method according to claim 13,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 32. Method according to claim 14,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 33. Method according to claim 15,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 34. Method according to claim 16,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 35. Method according to claim 17,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 36. Method according to claim 18,wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 37. Method according to claim 1,wherein said method is a mammographic application method.

043303676

description

DESCRIPTION OF THE PREFERRED EMBODIMENT In the art of reactor control the objectives to be achieved are the maximization of plant capacity and availability without violating the specified acceptable fuel design limits as a result of normal operation and anticipated operational occurrences. These limits are defined to provide a high degree of assurance that the fuel cladding integrity is maintained. Each of the design limits can be formulated in a mathematical description which gives an indication of the proximity to the violation of the design limit. These indices are mathematically dependent upon the various reactor parameters. One such index is the Departure from Nucleate Boiling Ratio (DNBR) which is indicative of the probability of occurrence of Departure from Nucleate Boiling (DNB). For each index a critical value of index may be determined which indicates when the probability is acceptably low that a reactor design limit is violated. In the case of DNBR the critical value has been determined to be 1.3. The DNBR index is functionally dependent upon the reactor parameters of coolant mass flow rate, coolant pressure, coolant inlet temperature, reactor power and reactor power distribution. The reactor must be operated in such a way that the critical values of each index are not violated in either of two cases: (1) steady state operation and (2) in the event of the occurrence of an anticipated operational occurrence which affects the values of the parameters on which the index is dependent. For the purposes of this description of the preferred embodiment, discussion will focus on the index called DNBR and the dependece of DNBR on the reactor parameter of reactor coolant mass flow rate. This choice has been made because the one most rapid anticipated operational occurrence that can occur in the operation of a pressurized water reactor is the simultaneous loss of power to all reactor coolant pumps. Other anticipated operational occurrences can occur that affect either the coolant mass flow rate or the other parameters but these other incidents result in a less rapid change in the DNBR. Therefore, the approach to the critical value of DNBR is less rapid and more time is available for initiation of protective measures. If a reactor protection system is devised to adequately handle the most rapid anticipated operational occurrence, then, by definition, its dynamic performance is adequate to accommodate the slower anticipated operational occurrences. For a better understanding of the dependence of DNBR on coolant flow rate, refer to FIGS. 2 and 3. FIG. 2 is a plot of the decay in the reactor coolant mass flow rate on the occurrence of an incident which interrupts electrical power to all of the reactor coolant pumps. Examination of the plot of FIG. 2 shows that the incident occurs at time t.sub.0 and the mass flow rate shows an exponential decay from the steady state condition existing before the occurrence of the incident. FIG. 3 shows the rapid fall in DNBR for two situations. The first situation is one in which the steady state operation before the accident was such that the initial value of DNBR was slightly above the critical value of 1.3. In this case the DNBR falls almost immediately below the critical value, indicating that the probability of damage to the fuel cladding is higher than desired. In the second situation, the operation of the reactor prior to the incident resulted in a steady state DNBR of approximately 1.8. On the occurrence of the indicent, the value of DNBR drops rapidly toward the critical value of 1.3. Unlike the first situation, in which the critical value was almost immediately violated, the second situation will bring about a violation of the critical value of DNBR only after a certain time period. It can be seen from these two described situations that operation of the reactor with an initial margin to the critical value of DNBR is preferable since a period of time is available for the sensing and calculating of the occurrence of the accident and the initiation and completion of the corrective measures (reactor scram). FIG. 4 shows the dependance of DNBR in the two situations described above including the effect of the actions of a protection system which detects the fall of coolant flow rate and which initiates a reactor scram. It can be seen in the second situation which starts from a DNBR value of 1.8, that due to the time delay between the time of occurrence of the incident and the time at which the DNBR violates the critical value, the protection apparatus has sufficient time to terminate the decrease in DNBR before the critical value of DNBR is violated. Such is not the case for the first situation and the critical value of DNBR is violated regardless of the initiation of protective action. A further modification to these concepts is necessary since the time available for preventing limit violation depends upon the power configuration within the reactor core. The reactor is controlled by inserting neutron absorbing control rods into the core from the top. If the power distribution in the core is such that a power peak is near the top of the core, control rod insertion effects are felt earlier than if the power was peaked in the bottom of the core. This phenomenon is reflected in FIG. 5. The lower curve illustrates the behavior of DNBR for the case in which the power is peaked toward the top of the core. The upper curve shows the behavior of DNBR where the power is peaked toward the bottom of the core. The minimum DNBR in both cases is 1.3; however, the curve for the case where power is peaked toward the bottom of the core starts from a slightly higher initial DNBR due to the fact that a greater time is required to halt the DNBR decrease. Therefore, the reactor should be operated either with a larger initial DNBR margin when the power is peaked in the bottom of the core, or the reactor protection system should initiate control action at an earlier time than in the case where the power is peaked in the top of the core. The instant invention provides means and methods for (1) maintaining a margin sufficient to avoid the violation of the critical value of the index on the occurrence of the most rapid anticipated operational occurrence and (2) predicting the imminent violation of the critical value of the index in sufficient time to allow the initiation and completion of successful control measures. FIG. 1 shows a typical pressurized reactor steam generating system with the inclusion of the margin maintaining system 60 and the protecting and predicting system 58. The reactor 10 consists of a core 12 and control rods 14 (only one of which is shown) which are movable into the core for reactor control. The core is constructed of a multitude of fuel pins 20 (only a few of which are shown) which define coolant channels 22 through which the coolant is circulated. The reactor coolant system 25 has a number of coolant loops only one of which is shown and which includes a hot leg 28 which delivers the heated reactor coolant to a steam generator 26. Heat from the heated reactor coolant is transferred to the secondary coolant in the steam generator 26 to form steam which is contained in a secondary coolant system 40. The steam is delivered to a turbine 42 which converts the thermal energy of the steam into mechanical rotation for subsequent conversion into electrical energy in a generator. The secondary coolant, after passing through the turbine, is delivered to a condenser 44 and recirculated by feed water pumps 46 back to the steam generator where it again picks up heat energy from the reactor coolant. After passing through the steam generator 26, the reactor coolant is circulated back to the reactor by reactor coolant pumps 32 and through cold leg 34. A pressurizing system (not shown) is provided to maintain the pressure of the primary coolant within certain acceptable limits. After being delivered to the reactor pressure vessel through the cold leg 34, the coolant is forced to circulate downwardly around the outside of the core 12 and upwardly through the interior of the core, through coolant channels 22, where the reactor coolant cools the core and its fuel pins 20. Proper control of the nuclear reactor system requires the sensing of all those parameters necessary for a computation of the various design limit indices. Ex-core detectors 16 are provided to monitor the neutron flux originating in the reactor core. Such ex-core detectors are convention, commercially available pieces of equipment such as manufactured by Reuter Stokes, Inc. or Westinghouse Electric Corp., Electronic Tube Division and the particular construction does not form a part of the invention. Also provided are strings of in-core detectors 18 for monitoring the local power of individual sectors of the reactor core. Such in-core detectors are conventional, commercially available pieces of equipment such as manufactured by Reuter Stokes Canada Ltd. and the particular construction does not form a part of the present invention. Information from the in-core detectors is necessary for the calculation of azimuthal tilt magnitude and is also used to calculate the axial power distribution. Resistance temperature detectors 36 and 38 are provided on the hot leg 28 and the cold leg 34 respectively to generate signals indicative of the temperature of the coolant as it enters the core and as it leaves the core. These temperatures are subsequently used in a calculation of .DELTA.T and average temperature for calculation purposes in determining core thermal power. The temperature of the cold leg T.sub.c is also used in a calculation of DNBR. Shaft speed detector 50 is positioned on the rotating shaft of the primary coolant pump 32 for the purposes of determining the speed of rotation (W) of the coolant pump shaft which can be used to calculate the coolant mass flow rate. Additional information is obtained by pressure sensors 51 and 53 which enable the calculation of the pressure head across the cooland pump. This information enables a more direct calculation of coolant mass flow rate. Also provided is a pressure sensor 48 to give an indication of the reactor coolant system pressure. Turbine 42 has a pressure sensor 56 which give an indication of the turbine first stage pressure for the purposes of calculating a total plant power. The method for calculating total plant power from a plant load signal (turbine first stage pressure) is fully and completely disclosed in U.S. Pat. No. 3,423,285 issued on Jan. 21, 1969 to C. F. Curry, et al., the disclosure of which is incorporated herein by reference. In addition to the turbine first stage pressure sensor 56, the secondary coolant system is equipped with mainstream pressure sensor 43, feedwater temperature detector 45 and feedwater flow detector 47. The reactor 10 and its control rods 14 are provided with a rod position detection system 54. This system is composed of two or more reed switch assemblies positioned adjacent to and outside of each control rod housing 52 that extends external to the reactor pressure vessel. The reed switches 54 are activated through the control rod housing which is made of magnetically permeable material, by a permanent magnet affixed to a control rod extension shaft. By this means the position of every control rod can be redundantly determined and logged for the purposes of determining various factors to be used in the calculation of DNBR and local power density. More specifically, CEA group position signals and CEA deviation signals are used to calculate pin and channel planar radial peaking factors (see infra.). All of the various signals described above which describe various reactor parameters are delivered to either the calculation means 58 called the core protection calculator, the calculation means called COLSS 60, or both. The parametric signals generated for and utilized by the core protection calculator and COLSS are quite similar although minor differences exist. These differences in detail are differences which would be obvious to one skilled in the art of reactor control. Therefore, for the sake of simplification and clarity, FIG. 1 shows the same signals delivered to both the Core Protection Calculator 58 and COLSS 60. However, it should be understood that in actual practice the signals delivered to these two systems are derived from separate isolated sources. It should also be recognized that in order to meet protection criteria, the core protection calculator must be redundant with redundant input signals. The calculation means 58 and 60 may either be hard-wired analogue systems or special purpose digital computers. The core protection calculator calculates and projects a value of DNBR over a certain time period T. Similar calculations and predictions can also be made for other design limits according to this invention. The core protection calculator then compares the calculated and projected value of DNBR to a fixed setpoint indicative of the violation of the specified acceptable fuel design limit on DNBR. When the calculated and projected value of DNBR is equal to or falls below the fixed setpoint, a signal is generated by the core protection calculator 58 and is sent to the control rod control means 62 for the purposes of scramming the control rods 14 into the core thereby terminating the chain reaction within the reactor core. Some of the variously generated signals representing the reactor parameters described above are also delivered to a calculation means 60 called the Core Operating Limit Supervisory System (COLSS). It is the function of COLSS 60 to make a very accurate calculation of a DNBR operating limit which contains sufficient margin to allow the core protection calculator to sense, calculate, predict and shut the reactor down in a timely fashion that avoids the violation of any fuel design limits. The operating limit thus generated may be utilized in either of two fashions in order to control the operation of the reactor. The first is merely to register the limit on a visual indicator 170 which would allow the reactor operator to compare the actual reactor operating condition to the COLSS limit. With this knowledge available to the operator, he will be able to operate the reactor in such a way that a sufficient margin is continuously maintained while at the same time maximizing the capability and availability of the reactor. The second method would be to automatically restrict the plant power to be within the COLSS limit thereby insuring that the necessary margin is maintained. Core Protection Calculator The low DNBR and high local power density trips initiated by a plurality of Core Protection Calculators of which only one is shown (see FIGS. 6 and 6a) function to assure that specified acceptable fuel design limits are not exceeded during anticipated operational occurrences (which are defined as those conditions of normal operation which are expected to occur one or more times during the life of a nuclear power plant). In particular, these occurrences include single electrical component or control system failures, that can result in transients which could lead to a violation of specified acceptable fuel design limits if protective action were not initiated. The protection system is designed so that reactor core protective action will not be initiated during normal operation of the reactor. Four measurement channels are provided for each parameter monitored by the system. The four measurement channels are independent and isolated from each other as are the core protection calculators. These four channels provide trip signals to six independent logic matrices, arranged to effect a two-out-of-four coincidence logic or a two-out-of-three coincidence logic. This redundancy enables one of the four measurement and calculation channels to be taken out of service for maintenance or testing while still providing the necessary protective function for the operating reactor. In this case, the protection system logic is changed to a two-out-of-three coincidence logic for the actuation of plant protective action. It should be understood that this discussion and the functional block diagram of FIG. 6 describe only one out of the four parallel and independent protective channels. For the purposes of illustrating the invention and for the purpose of teaching a preferred mode of operation, attention will be focused on the two fuel design limits called: 1. A 1.3 Departure from Nucleate Boiling Ratio (DNBR) in the limiting coolant channel in the core, and 2. The peak local power density in the limiting fuel pin in the core. The low DNBR trip maintains the integrity of the DNBR fuel design limit and the high local power density trip maintains the integrity of the fuel design limit on peak local power density. While discussion will be focused on these two indices, it should be apparent that the invention is not so limited and applies to any design limits which may be represented by a mathematical index. The subject trips monitor all the Nuclear Steam Supply System parameters that affect these fuel design limits. The pertinent parameters are monitored directly where possible, or indirectly by monitoring Nuclear Steam Supply System variables that can be related to the particular parameters of interest through well defined mathematical relationships. The number and location of sensors are chosen to assure that the reactor core is adequately monitored for all allowable operating configurations. The parameters of primary importance for the calculation of DNBR and local power density are the following: 1. Reactor Core Power; PA1 2. Reactor Core Power Distribution; PA1 3. Reactor Coolant Flow Rate; PA1 4. Reactor Coolant System Pressure; and PA1 5. Reactor Coolant Inlet Temperature. PA1 .phi.=core power; PA1 .phi.m=coolant mass flow rate; PA1 P =reactor coolant system pressure; PA1 T.sub.c =cold leg temperature; PA1 F.sub.z (z)=axial power distribution; PA1 F.sub.r =integral radial peaking factor; and PA1 T.sub.r =azimuthal tilt magnitude. PA1 .phi.=core power in percent of full power; PA1 T.sub.c =coolant inlet temperature; PA1 P=coolant pressure; PA1 m=coolant mass flow rate; PA1 F.sub.r =integral radial peaking factor; PA1 F.sub.z (z)=axial power distribution; and PA1 T.sub.r =azimuthal tilt magnitude. PA1 a. core average power; PA1 b. normalized core average axial power distribution; and PA1 c. normalized radial power distribution. PA1 .phi. is an n element column vector whose typical element is .phi..sub.i representing the flux at the detector at node i. PA1 .phi. is an n element column vector whose typical element is .phi..sub.j representing the flux at the core periphery at node j. PA1 S is an n.times.n square matrix whose typical element is S.sub.ij representing the shape annealing factor for the flux at node i at the detector due to the flux at the periphery at node j. PA1 .phi. is an n element column vector whose typical element is .phi..sub.k representing the core average axial power distribution at node k. PA1 F is an n.times.n diagonal matrix whose typical element is F.sub.kk representing the CEA shadowing factor associated with a CEA insertion at node k. PA1 .phi. is an n element column vector whose typical element is .phi..sub.j representing the flux at the core periphery at node j. PA1 a. Location and insertion of full and part-length CEA groups; PA1 b. The relative position of individual CEAs within the same CEA group; and PA1 c. Azimuthal Flux tilt magnitude. PA1 D.sub.rj --the maximum deviation of an individual CEA in subgroup j from the subgroup position. PA1 D.sub.si --the maximum deviation of the subgroup in group i from the group position. PA1 F.sub.r.sup.p (z)--the pin planar radial peaking factor at node z. PA1 F.sub..alpha.n --the precalculated pin planar radial peaking factor for the CEA configuration represented by the subscript .alpha.n. PA1 G.sub.i --the position of group i. PA1 F.sub.r.sup.CHAN (z)--the channel planar radical peaking factor at node z. PA1 P.sub.os --out-of-sequence penalty factor. PA1 P.sub.r+ --CEA deviation penalty factor for positive deviation. PA1 P.sub.r- --CEA deviation penalty factor for negative deviation. PA1 P.sub.s+ --subgroup deviation penalty factor for positive deviation. PA1 P.sub.s- --subgroup deviation penalty factor for negative deviation. PA1 S.sub.ij --position of subgroup j, which belongs to group i. PA1 T.sub.j --position of the target CEA for group j. PA1 .DELTA..sub.os --limit on allowed group deviation for out-of-sequence condition. PA1 .DELTA..sub.r --limit on allowed CEA deviation. PA1 .DELTA..sub.s --limit on allowed subgroup deviation. PA1 D is an n element column vector representing the external detector segment responses whose typical element is D.sub.i PA1 S is an n.times.n element matrix representing the shape annealing factors whose typical element is S.sub.ij PA1 P is an n element column vector representing the peripheral detector segment responses whose typical element is P.sub.i. PA1 X.sub.i =Fraction of the overall segment length to which CEA shadowing factor F.sub.i applies PA1 F.sub.i =Planar CEA shadowing factor for a given CEA insertion PA1 N=Number of different shadowing regions over a detector segment. PA1 .phi.is an n element column vector representing a quantity proportional to nuclear power whose typical element is .phi..sub.i PA1 R is an n.times.n diagonal matrix of average CEA shadowing factors whose typical element is R.sub.ii PA1 P is an n element column vector representing the peripheral detector response whose typical element is P.sub.i. PA1 .phi..sub.i =Corrected response of segment i PA1 T.sub.cmin : is the minimum of the two cold leg temperature signals PA1 C.sub.T : is a proportionality constant relating the percent change in indicated neutron flux power for a one degree temperature change PA1 T.sub.co : is a base cold leg temperature which is set at the limit of the usable sensor range. PA1 .phi..sub.cal =Calibrated neutron flux power for the channel PA1 K.sub.cal =Calibration factor for detector response to power PA1 .phi..sub.N =Total corrected detector channel response for the channel. PA1 B.sup.static.sub..DELTA.T : core power expressed in percent of rated power PA1 .DELTA.T: defined as T.sub.h .times.T.sub.c PA1 T.sub.h : defined as ##EQU5## T.sub.c : defined as ##EQU6## T.sub.h : hot leg temperature T.sub.c : cold leg temperature PA1 f.sub.i (m): are fitted coefficients that are a function of coolant mass flow rate and are of the general form: f.sub.i (m)=a.sub.i m+b.sub.i m.sup.2 +c.sub.i with a.sub.i, b.sub.i, c.sub.i being constants. PA1 with W.sub.1 and W.sub.2 being weighting coefficients. PA1 f.sub.4 (m): is a flow dependent coefficient as described above. ##EQU8## is a "filtered" derivative of T.sub.x which is Laplace transform notation is EQU (.tau.s)/(.tau..sub.1s +1) PA1 .tau..sub.1 is an equivalent coolant transport lag time constant and PA1 .tau.is a derivative gain which accounts for coolant transport delays and sensor time constants. PA1 .phi..sub.cal : calibrated neutron flux power PA1 K.sub.cal : calibration factor PA1 .phi..sub.N : total corrected detector response PA1 K.sub.cal (t-.DELTA.S): is the calibration factor that was calculated at the previous sampling time PA1 .DELTA.S: is the time internal between K.sub.cal updates PA1 .tau..sub.k : is an error weighting factor which is less than unity PA1 B: .DELTA.T power PA1 V.sub.i =volumetric flow rate of pump i PA1 RPM=pump shaft rotational speed PA1 .DELTA.P=pump head (obtained for periodic calibration) PA1 N=number of coolant pumps running PA1 K.sub.bypass =flow bypass correction factor PA1 m: is the mass flow rate PA1 C.sub.0 : is the density correction at base inlet temperature (T.sub.co) and pressure (P.sub.o) PA1 C.sub.1 and C.sub.2 : are coefficients that reflect the density change from the base conditions PA1 T.sub.c : is the maximum of the two cold leg temperature inputs PA1 P: is the measured pressurizer pressure PA1 a. a power trip set point is calculated as described above; PA1 b. the indicated core average power is compared to the set point; PA1 c. a trip will occur if the core average power remains equal to the set point for a fixed time interval. PA1 B.sub.sp.sup.st --local power density static set point PA1 B.sub.sp --local power density trip set point PA1 C.sub.s --a constant based on the allowed limiting value of the local power density PA1 F.sub.r.sup.p (z)--the pin planar radial peaking factor at node z PA1 F.sub.z (z)--the normalized core average axial power distribution, expressed as the ratio of the power at axial node z to the average power PA1 T.sub.r --the azimuthal tilt magnitude PA1 .DELTA.S--time between static set point samples PA1 .tau..sub.1 --protective system equivalent delay time PA1 .tau..sub.2 --fuel, gap and clad effective time constant PA1 Go=channel mass velocity at nominal reactor condition; PA1 A.sub.i, B.sub.i =analytically determined form coefficients for ith parameter; PA1 .DELTA..psi..sub.i =change of ith parameter from its nominal value. PA1 a. Input parameters are varied individually in many calculations performed off-line with detailed thermal hydraulic design codes. PA1 b. The same input is used in the on-line version of the model with the exception that mass velocity is iterated upon such that the DNBR obtained from the off-line codes is matched. PA1 c. Knowing the input variations from one case to the next and the required mass velocity for prediction of the minimum DNBR, the coefficients of the mass velocity correlation can be determined via multiple regression calculations. However, the minimum DNBRs produced here must equal or exceed those which result from the on-line calculations. PA1 a. calculated normalized core average axial power distribution, PA1 b. calculated axially dependent one-pin and coolant channel planar radial peaking factors, PA1 c. calculated reactor coolant mass flow rate, PA1 d. calibrated neutron flux power, PA1 e. maximum of the two input cold leg temperatures, and PA1 f. the monitored reactor coolant system pressure. PA1 m: is the calculated coolant mass flow rate in percent of 4 pump design flow PA1 (P.sub.box /P.sub.core): is the fuel assembly peaking factor defined by EQU (P.sub.box /P.sub.core)=.alpha..sub.1 F.sub.r.sup.int PA1 F.sub.r.sup.int : is the integrated planar radial peaking factor defined by ##EQU15## F.sub.z (z): is the normalized core average axial power distribution F.sub.r.sup.p (z): is the axially dependent planar radial peaking factor PA1 P.sub.pri : is the pressurizer pressure PA1 .phi..sub.cal : is the calibrated neutron flux power PA1 T.sub.cmax : is the maximum of the two cold leg temperature inputs C.sub.i 's and .alpha..sub.1 : are constants PA1 H.sub.z=0 : coolant enthalpy at core inlet PA1 .DELTA.H.sub.i : coolant enthalpy rise at node i PA1 F.sub.r.sbsb.i.sup.chan : channel planar radial peaking factor at node i, defined by EQU F.sub.r.sbsb.i.sup.chan =constant .times.F.sub.r.sbsb.i.sup.p PA1 T.sub.r : azimuthal tilt magnitude PA1 Qlocal.sub.i (t): is the heat flux at node i PA1 T.sub.cmax (t): is the inlet temperature PA1 Qlocal.sub.i.sup.periodic : is the value of the heat flux at node i used in the periodic DNBR calculation PA1 T.sub.cmax.sup.periodic : is the value of the inlet temperature used in the periodic DNBR calculation PA1 m(t): is the current calculated value of the reactor coolant mass flow rate PA1 m.sup.periodic : is the mass flow rate used in the periodic DNBR calculation PA1 P.sub.pri (t): is the current sampled value of the primary coolant pressure PA1 P.sub.pri.sup.periodic : is the value of the primary pressure used in the periodic DNBR calculation Several methods of calculation are possible and it is not intended that this invention be limited to any particular mathematical expression of the parameters used to calculate true DNBR or local power density or any other index. The discussion following is only intended to distinctly set out and describe one mode of practicing this invention. In the spirit of the foregoing statement, it should be recognized that there are various methods for determining the controlling parameters in the "hottest" pin or the "hottest" channel of the reactor core. One extreme method is to make calculations for each and every pin and channel in the core. A comparison of the calculations for each and every pin and channel would determine which pin and which channel are limiting for the operation of the reactor. A simplification of this extreme method is to take representative samples throughout the core and make conservative approximations that assure that the selected sampling method does not overlook the "hottest" or limiting pin or channel. Either of these methods is tedious and expensive to implement. An alternative method, the one to be described in this discussion, consists of making one calculation to synthesize a pseudo "hottest" or limiting pin or channel which, by making conservative assumptions, ensures that no actual pin or channel can exceed the conditions calculated for the pseudo limiting pin or channel. A brief mathematical description of this method of synthesizing a pseudo pin or channel follows with a detailed description of the terms and methods used in obtaining the various parameters included in the "Appendix to the Description of the Preferred Embodiment." For a better understanding of the following brief description reference may be made to FIG. 11. In order to calculate DNBR and the KW/ft. local power density limits, the reactor core power distribution must be known. In actuality, the method for finding the absolute power distribution for one pin or channel requires the subdivision of the reactor core into its constituent parts of individual fuel pins and coolant channels. Therefore, the function to be calculated is "the absolute power distribution for the limiting pseudo pin or channel" which is a function varying with axial position along the channels (see 108 in FIG. 11). In the interest of simplification, the following discussion will refer only to the channel and not make reference to the associated fuel pin. Mathematically, this term, the channel absolute power distribution 108, can be factored into three independent terms, two of which are constants, and one of which is a function dependent on axial position or the z coordinate. The function term is defined to be "the normalized channel axial power distribution" 110. This function can be thought of as a curve indicative of power which begins at the bottom of the core (z=0) and ends at the top of the core (z= 1). Every value of this function, the normalized channel axial power distribution 110, or every part of the curve generated by this function is called a "channel axial peaking factor." The one point on the curve generated by the function which represents the point of maximum power generation is called "the channel axial peaking factor" 111. The constant terms factored out of the "channel absolute power distribution" are selected to normalize the axial power distribution curve 110 or, restated, the constant terms are chosen so that the integration of the "channel axial power distribution" 110 from the bottom of the core (z=0) to the top of the core (z=1) gives a value of unity. The two constants which have been factored out of the "channel absolute power distribution" to give the "normalized channel axial power distribution" are called the "integral radial peaking factor" 112 and the "total power generated in the average channel" 114. The "integral radial peaking factor" 112 is defined to be the ratio of the power generated in the referenced channel to the power generated in the average core channel. In order to synthesize the limiting pseudo channel, a method is developed for generating a "pseudo absolute power distribution for a pseudo channel" 106. In order to mathematically do this, the core is thought of as being a right cylindrical solid which has been divided into slices along the z axis with each slice having a thickness of dz (see FIG. 11a and 11b). This mathematical manipulation allows each slice to be treated as a two-dimensional model; the core, therefore, comprises a simple summation of all of the two-dimensional slices. For each slice a ratio of the power of the "hottest" pin in that slice to the average power of all of the pins in that slice is generated. (See FIG. 11a.) This ratio is called the "planar radial peaking factor." The "planar radial peaking factor" is analogous to the "channel axial peaking factor" discussed above for a single channel with the exception that there exists one "planar radial peaking factor" for each slice. If all of the planar radial peaking factors for all of the slices of thickness dz are plotted on a curve, thereby melding the "hottest" pins in each slice into a single pseudo pin (see FIG. 11b), the result is a step curve which varies with axial position 102. The step function 102 generated by the "planar radial peaking factors" may be multiplied by the "core average axial power distribution" 100 which is a z dependent function and the "average over all slices of average pin power per slice" 104, which is a constant for one set of reactor operating conditions. This former term, the core average axial power distribution, is a cross-plot of the linear power density in each axial slice versus the axial position of the slice, with appropriate normalization such that integration of the curve from z=9 to z=1 gives a value of unity. This latter term, the "average over all slices of average pin power per slice" 104 is a number proportional to the total power generated in the core. The function which results from the multiplication of the function generated by the "planar radial peaking factors" 102, the "core average axial power distribution" 100 and the "average over all slices of average pin power per slice" 104 is the "pseudo absolute power distribution for the pseudo limiting pin or channel" 106. In this way a pseudo pin can be generated representing the worst possible power distribution or the power distribution for the limiting pin, since the hottest pins in each slice have been stacked into one pseudo pin extending through all of the slices (see FIG. 11b). Once this "pseudo absolute power distribution for the pseudo limiting pin" 106 has been generated, it may then be factored into a normalized axial power distribution 110 and an integral radial peaking factor 112 for the pseudo limiting pin just as in the case for an actual pin. The purpose of making these calculations is to obtain a normalized axial power distribution F.sub.z (z) and an integral radial peaking factor F.sub.r which are two of the input signals necessary for the calculation of the DNBR index and the KW/ft. local power density index according to functional equations (1) and (2) which appear in the "Background of the Invention." The above concepts are discussed in greater detail in the "Appendix to the Description of the Preferred Embodiment". Referring now to FIG. 6, the method used by the core protection calculator will be described. A calculation of DNBR is made in functional element 76 according to the equation: EQU DNBR=f(.phi., m, P, T.sub.c, F.sub.z (z), F.sub.r, T.sub.r) where An explanation of .phi., F.sub.z (z), F.sub.r and T.sub.r is to be found in detail in the "Appendix to the Description of the Preferred Embodiment." The signals are monitored by the DNBR calculator 76 and a snapshot is taken of these signals approximately every two seconds. The snapshot values are used in a calculation of DNBR which takes approximately 2 seconds. The two to four second old indication of DNBR which results is not sufficiently responsive to the actual condition of the reactor core to allow adequate core protection; therefore, means are provided for continuously updating this basic DNBR calculation. For the purposes of this discussion and claims, the word "continuously" should be taken to mean "of a periodicity which is substantially higher than the frequency of the periodic calculation." The values of each of the parameters used in the most recently completed periodic snapshot calculation of DNBR are compared at 78 with the continuously monitored parameters. This comparison results in an update change in parameter for each of the parameters used in the DNBR calculation of element 76. Element 78 then multiplies each change of parameter by a value which conservatively converts each change of parameter into a change of DNBR. An example of the type of multiplication which may be made is the multiplication of change in temperature by a value which is equivalent to the partial derivative of the DNBR equation with respect to temperature. The partial derivative may either be taken to be a constant value or it may be taken to be a function which is dependent upon all other parameters except T, the choice depending upon which multiplication gives a more satisfactory result. After element 78 generates a change in DNBR for the change in each parameter, all of the chages in DNBR are summed to obtain a net change in DNBR. This net change in DNBR and the snapshot calculation of DNBR are added in element 80, the result being a continuously updated value of DNBR which is dynamically responsive to changes in the reactor core. The above described calculation standing alone is not sufficient for the function of protecting the reactor core. The reason for this is that it is not sufficient merely to know what the DNBR is but it is necessary to be able to predict what the DNBR will be far enough in advance to allow the initiation and completion of corrective action that will avoid limit violation. Therefore, a projection of DNBR must be made. The projection is accomplished by elements 82, 84 and 86 of the Core Protection Calculator. These elements sense the dynamic response of one of the reactor parameters and take the slope or the rate of change of the parameter with respect to time. This slope or rate of change is then projected over a period of time T. In this manner, a projection of change of DNBR may be calculated on the basis of the instantaneous rate of change of one of the reactor parameters. Projections of the rate of change of each parameter are continuously made, thereby providing a continuous projection of change of DNBR. Now, referring to FIG. 6 element 82 takes the derivative of the parameter, for example pressure, with respect to time. Element 84 then multiplies the derivative of the signal with respect to time by a value which is proportional to the partial derivative of DNBR with respect to the parameter being projected. The product of these two terms is a term indicative of the change in DNBR with respect to time due to the change in the parameter in question. This change in DNBR with respect to time is then multiplied by a time period T.sub.p to obtain a projected change in DNBR. The time period over which the change in DNBR is projected is a period which is calculated to allow the sensing, calculating and predicting of a limit violation and to allow the initiation and completion of corrective action before the violation of a design limit all of which represent system inertia or system reaction time. The time period T is illustrated in FIG. 2. It should be recognized from FIG. 5 and from the discussion above that the multiplicative time period T may have to be modified depending upon the power distribution within the core. The projected change in DNBR which has been derived by elements 82, 84 and 86 is then limited to negative values in element 88 before it is summed in element 90 with the calculated value of DNBR of element 80. A calculation similar to the one which has just been described may be made for any of the reactor parameters on which DNBR depends. In actual operation, it is likely that the dynamic response of DNBR to changes in coolant temperature, axial power distribution, integral radial power distribution, and azimuthal tilt magnitude will not be fast enough to require the projection techniques just described. However, it is likely that the parameters of core power, reactor coolant system pressure and coolant mass flow rate will be projected as above since DNBR is very responsive to changes in these parameters. It is indeed likely that DNBR will be so responsive to changes in coolant mass flow rate that a technique for desensitizing the DNBR response should be provided in order to avoid unnecessary reactor trips. In order to do this, the core protection calculator may take advantage of the presence of a system similar to COLSS which insures that an appropriate margin to the fuel design limit will be maintained. It is a characteristic of the COLSS system that it need not be as dynamically responsive to changes in reactor parameters as the core protection calculator since COLSS assures that adequate margin is maintained to fuel design limits only during normal operation. The core protection calculator, on the other hand, must be dynamically responsive to changes in reactor parameters in order to adequately protect the reactor core during anticipated operational occurrences. The differences between these two systems mandate an extremely fast, although approximate, core protection calculation of DNBR and predicted DNBR, and a slower but extremely accurate COLSS DNBR calculation. Therefore, it is a possibility that the approximations made to allow a rapid and dynamic calculation of DNBR in the core protection calculator may result in a value of DNBR that is less accurate than the values of DNBR calculated by COLSS. Advantage may be taken of these calculational differences to provide a method to desensitize the responses of the core protection calculator value of DNBR to changes in the coolant mass flow rate. A brief description of this desensitization process follows. The DNBR value calculated by COLSS which includes an opening margin is used as the static base value of DNBR to which continuous instantaneous modifications are made rather than that value of DNBR which is periodically calculated by the core protection calculator. This static base value of DNBR called DNBR.sub.limit is then updated to make it responsive to changes in coolant mass flow rate to obtain an updated value of DNBR. This updated value of DNBR is then compared to the updated values of DNBR obtained from the above-described periodic calculation and the higher of the two values is selected. To this higher value is added a projected change in DNBR which is obtained in a projection procedure similar to that described above for a projection of primary pressure. Reactor coolant pump speed signals (W.sub.i) are continuously delivered to element 120 along with the cold leg temperature and the reactor coolant system pressure where a calculation is made converting these signals into coolant mass flow rate signals (m.sub.i). FIG. 6 shows four sets of signals from the usual configuration of four reactor coolant pumps. Element 122 sums the four mass flow rate signals (m.sub.i) to produce a total coolant mass flow rate signal (m.sub.t). In a series of steps similar to the pressure signal projection sequence described above, the derivative of m.sub.t is taken at 124 with respect to time to get the rate of change of m.sub.t. This value is then multiplied by the partial derivative of DNBR with respect to the mass flow rate at 126 to obtain a signal indicative of the rate of change of DNBR due to the rate of change of the coolant mass flow rate. Next, a projection of the change of DNBR over a period of time T is made in element 128 by multiplying the signal indicative of the rate of change of DNBR with respect to time by a period of time T. Just as in the example described above for the projection of DNBR due to changes in pressure, the value of T may be dependent on the axial power distribution that exists in the reactor core. Thus, element 134 generates a time period T.sub.fz as a function of axial power distribution. There are numerous acceptable ways that element 134 can handle this function generation. A suggested preferred mode is illustrated in FIGS. 7 and 8. FIG. 7 is a plot of axial power distribution F.sub.z (z) from the bottom to the top of the core. The core is divided into two equal parts; the lower half of the core and the upper half of the core. The axial power distribution is integrated in element 134 over each half to get two numbers symbolized by L and U meaning lower and upper halves. FIG. 7 represents a reactor configuration in which the axial power distribution is weighed more heavily toward the bottom of the core. In such a situation the area under the axial power distribution curve toward the bottom of the core, L, would be larger than that at the top of the core U. By using these two, values L and U, an Axial Shape Index can be generated which gives a single number indicative of the axial power distribution. One possible axial shape index can be generated by the following equation: EQU ASI=(L-U/L+U) An examination of this equation shows that the Axial Shape Index will be negative when the power distribution is peaked toward the top of the core, and positive when the power distribution is peaked toward the bottom of the core. FIG. 8 shows one way in which this fact may be utilized to generate a time period T which is a function of the axial power distribution. The shape of the curve plotted with Axial Shape Index versus T is chosen by the reactor designers to make best use of the fact that it takes less time to terminate the reactor's nuclear chain reaction when the power is peaked in the top of the core. Thus, the greater the power peak in the top of the core, the larger is the negative value of the Axial Shape Index. The curve of FIG. 8 shows that T decreases with larger and larger negative values of Axial Shape Index. Once the projection of change in DNBR is obtained, it is limited to negative valves in element 130 (for the sake of being conservative) and is then added to an updated base value of DNBR in element 132. The appropriate updated base value of DNBR is the larger of either the value computed by the periodic sampling method described above and which is computed in elements 76, 78 and 80, or the value computed by using a fixed based value of DNBR which includes an operating margin. For the sake of simplifying the discussion, this base value of DNBR, which includes a margin, will be called DNBR.sub.limit and may be found on FIG. 6 as an input to element 98. The DNBR.sub.limit value is a constant that may be determined once the physical characteristics of the reactor protection equipment are determined. The functioning of the COLSS margin system will be described, infra, following the completion of the discussion of the core protection calculator. The inclusion of the DNBR.sub.limit value as a base value for the desensitization of the response of DNBR to the coolant mass flow rate is justified by the fact that when the reactor is operated on the basis of the COLSS margin, as it should be when operated on a steady-state basis, then reactor conditions should always be such that this margin exists. The DNBR.sub.limit must be dynamically updated for recent changes in DNBR. The following is a description of the preferred method for accomplishing this update. The values of m.sub.t (total coolant mass flow rate) are continuously fed to element 92 where the following calculation is made. The mass flow rate is continuously integrated over a period of time which is long compared to typical transients which effect the parametric value of mass flow rate. The value obtained from the integration is continuously divided by the same time period over which the integration was made, thereby generating an average value of coolant mass flow rate over the stated period of time (m). This flow average value (m) is then compared in element 94 to the instantaneous value of coolant mass flow rate (m.sub.t) to obtain a recent deviation of mass flow rate (.DELTA.m) from the long term average value of mass flow rate. If the deviation is negative, i.e., m less than m.sub.t, the value of the mass flow rate integral (m) is automatically increased such that m=m.sub.t. In element .DELTA.m is multiplied by the partial derivative of DNBR with respect to the coolant mass flow rate (m). The product of this multiplication is a value indicative of any decrease in DNBR (.DELTA.DNBR) due to any recent decrease of coolant mass flow rate from the long term average value of mass flow rate. The derived .DELTA.DNBR is then used as the value which updates the DNBR.sub.limit in element 98 to give an updated value of DNBR.sub.limit. A signal representing DNBR.sub.limit is then sent to element 118 where it is compared to the updated DNBR value which has concurrently been calculated in elements 76, 78 and 80. Element 118 selects the higher value of the two updated DNBR signals to be used as the value to which the projected change in DNBR over time T is to be subtracted in element 132 to generate a predicted value of DNBR T seconds into the future due to a projected change in coolant mass flow rate. At this point in this description, two values of predicted DNBR have been described as having been generated in elements 90 and 132. Element 136 selects the lower value of all of the predicted values of DNBR and element 138 compares that selected low value to the critical value of DNBR (1.3). If the predicted value is equal to or less than the critical value of DNBR, then a signal is sent to element 140 which generates a signal for tripping the reactor. The tripping action de-energizes the electromagnetic circuits on a magnetic jack control rod drive mechanism and the control rods are allowed to fall into the reactor core. A second embodiment of the core protection calculator 58 appears in FIG. 6A. In this embodiment, the same signals are generated as inputs with the exception that one or more of them are modified in element 75. Element 75 generates signals which are dynamically commensurate with the parameters that the fuel design limit is most closely related to; these dynamically compensated signals are then projected over time T. The time projection technique described in the first embodiment as occurring in elements 82, 84 and 86 is thus handled by element 75. The net result of this modification is that elements 82, 84 and 86 have been embodied in new element 75 and the dynamic compensation and time projection is made before the DNBR calculation and update are made in elements 76 and 78. Thus, the DNBR calculation and update are calculations which work with at least one parameter which has been dynamically compensated and projected into the future, resulting in a calculation of projected DNBR. Core Operating Limit Supervisory System (COLSS) The function of the Core Operating Limit Supervisory System is to insure that the nuclear reactor is operated with sufficient margin to critical core design limits so that the Reactor Protection System has time to terminate an incident (anticipated operational occurrence) before the violation of a fuel design limit. The method and apparatus described herein can be adapted to calculate a limiting value of a reactor parameter which encompasses such a sufficient margin. This method is applicable to any design limit; however, for the sake of simplification, the following discussion will be focused primarily on fuel element clad integrity and overheating as indicated by the occurrence of departure from nucleate boiling and the appropriate index, DNBR. As described above, DNBR is a function of a number of either measured or calculated reactor parameters and may be expressed in the functional notation: EQU DNBR=f(.phi., T.sub.c, P, m, 0, F.sub.r, F.sub.z (z), T.sub.r) where: A detailed description of how core power, axial power distribution, integral radial peaking factors and azimuthal tilt magnitude are generated appears in the "Appendix to the Description of the Preferred Embodiment." The critical value of DNBR is 1.3 (or 1 depending on the definition used). One possibility for operating the reactor with a sufficiently large margin (see FIG. 4) is to make sure that the reactor is operated at a DNBR value (say 1.8) which is sufficiently distant from the critical value of 1.3 to allow enough time to sense the occurrence of an accident, predict the effect of the occurrence of the accident and take appropriate protective action, such as scramming the reactor. Using this approach, the value of one of the parameters on which DNBR, if functionally dependent, (for example, core power) may be calculated from the above equation by using the false input of 1.8 for DNBR (see FIG. 10A). The result of the on-line calculation is a false value of the parameter which is then used as an operating limit. This calculated false parameter (power) or limit is then displayed to the operator informing him that if he were to allow the actual reactor power to exceed the computed false power or power limit, then the DNBR margin would be less than that required for the avoidance of design limit violation on the occurrence of an anticipated operational occurrence. At this point the operator has at least two choices. He can cause control rods to be inserted into the reactor, thereby decreasing actual reactor power until the actual power no longer exceeds the power limit, or, he can cause variations in one of the other parameters on which DNBR is dependent, such as inlet temperature T.sub.c, coolant pressure P, or coolant mass flow rate m, in such a way as to raise the computed power limit (i.e., increase the existing DNBR margin) so that the actual power no longer exceeds the computed power limit. In the above described procedure and calculation, the variables in the DNBR equation were treated in three distinct ways: (1) one variable (DNBR) was modified (from 1.3 to 1.8); (2) one variable (power) was calculated as the unknown, the solution for which gave a limit, and (3) the remaining variables were treated as known values whose true values were used in the calculation of the limit on the basis of the false or modified input. It is mathematically possible, and perhaps desirable to switch the variables and their roles. Thus, an operating limit may be calculated for the temperature variable rather than for power. In this case, a temperature limit is obtained by utilizing the actual reactor power in the calculation and then the temperature limit is compared to the actual temperature by the operator just as in the case described above for the power limit and the actual power. Another way in which a power limit may be obtained is to generate as input signals the true values for al the variables (except power) including a DNBR signal of 1.3. The calculation results in an actual power which may then be modified to obtain an adjusted power or power limit which encompasses an operating margin. (See FIG. 10B). Another way in which a modification can be made to incorporate an operating margin is the modification of one of the other variables, such as coolant mass flow rate, rather than the DNBR index or the power. In fact, in practical application, it turns out that the more desirable method is to falsify the coolant mass flow rate signal rather than the DNBR value, and to iterate on reactor power as the calculated operating limit. Therefore, as the preferred mode of practicing this invention, FIG. 10 illustrates the method described above with the coolant mass flow rate being modified to build in a margin which is then used in calculating a core power operating limit. Referring to FIG. 10, the inputs of feedwater temperature (T.sub.s), mainstream pressure (P.sub.s), feedwater flow (m.sub.2), turbine first stage pressure (P.sub.t), core inlet temperature (T.sub.c), reactor coolant pump speed (W), reactor coolant pump head (.DELTA.P), reactor coolant pressure (P.sub.pri), CEA positions, and in-core flux (.phi.) are delivered to the core operating limit supervisory system (COLSS) 60. From these signals, core calorimetric power 142, core plant power 144, coolant volumetric flow rate (m) 146, fuel pin and coolant channel planar radials (F.sub.r) 148, azimuthal tilt magnitude (T.sub.r) 150, and normalized axial power distribution F.sub.z (z) 152 are computed. See U.S. Pat. No. 3,752,735 entitled "Instrumentation for Nuclear Reactor" as an illustration of one prior art technique for the derivation of core calorimetric power. See U.S. Pat. Nos. 3,423,285 and 3,356,577, which patents are illustrative of the prior art techniques for computing core (plant) power from turbine first stage pressure and for automatically calibrating the core (plant) power to core (calorimetric) power respectively. The signals indicative of fuel pin and coolant channel planar radials, azimuthal tilt magnitude and normalized axial power distribution are computed as described in the "Appendix to the Description of the Preferred Embodiment." One of the totality of generated signals is modified by a factor which is calculated to build a margin into the operating limit to be computed. For purposes of this description, the variable chosen to be modified is the coolant flow rate, m, as shown in element 156. The modification made in element 156 is of the general form EQU m.sub.adj =m[f(reactor condition] where m is the true value of the coolant flow rate and f(reactor condition) is the modification increment which falsifies the flow rate to generate an adjusted flow rate m.sub.adj. One possible modification function, f (reactor condition) is [1+(.DELTA.m/m)]. The value of .DELTA.m is a value which is a function of reactor reaction time or system inertia including delays in sensing, calculating, predicting and actuating the reactor's protection mechanisms and which also varies with the reactor core axial power distribution similar to the way in which T varied in the discussion of the projected technique supra. If the core power is peaked toward the top of the core, effective control of the core's chain reaction may be accomplished in a shorter time than if the core power were peaked in the bottom of the core. Thus, with the peak toward the top of the core, a smaller margin is adequate to ensure that timely control action may be taken than for the case in which the power is peaked toward the bottom of the core. The functional dependence of .DELTA.m on axial power distribution may be developed by using the Axial Shape Index and a technique which is similar to that described for the T dependence on axial power distribution discussed above in the description of the core protection calculator. Using the signals generated from the various input parameters, including the adjusted coolant flow rate signal, an operating limit (.phi.=powder) is calculated in element 162 according to the equation: EQU DNBR=f(.phi., m.sub.adj, T.sub.c, P, F.sub.z (z), F.sub.r, T.sub.r). A similar core power limit based on local power density or any other design limit may also be calculated in element 158 and additional elements (not shown) in accordance with the equation EQU KW/ft.=f(.phi., F.sub.z (z), F.sub.r) or an appropriate aternative equation. A similar signal modification (not shown) could be made for this calculation. The signals from elements 142 and 144 representing core (calorimetric) power and core (plant) power respectively are delivered to element 154 where the core (plant) power is automatically calibrated to the core (calorimetric) power. The resultant signal generated by element 154 is a signal representative of the actual power of the reactor. This signal is compared to the lowest power limit available. FIG. 10 shows three available power limits, one generated in element 162 by the DNBR equation, one generated in element 158 by the KW/ft. equation and one determined by the licensed power limit as generated in the element 166. These three (or more) power limit signals are compared and the lowest selected in element 164 for comparison with the actual power as generated in element 154. If the comparison indicates that the actual power is in violation of the power limit, then an alarm means 170 is activated alerting the operator that remedial action is required. Core Protection Calculator and the Core Operating Limit Supervisory System The third invention herein disclosed consists of the combination of the Core Protection Calculator 58 and the Core Operating Limit Supervisory System (COLSS) 60. See FIG. 1, FIG. 6 and FIG. 9. The combination of these two systems creates a symbiotic relationship designed to protect the nuclear reactor 10 from design limit violations, both in steady-state operation and during the transients caused by anticipated operational occurrences. The combination of the Core Protecting Calculator 58 and the COLSS 60 systems takes advantage of each of their design characteristics. The Core Protection Calculator 58 is a protection device which must respond rapidly to system transients so that safety and system design limits are not violated. This system must not only be rapidly responsive to reactor system transients, but the protection system must also be redundant so that a single failure cannot prevent the required protective action from occurring. In order to meet these requirements, the core protection calculator has been designed to consist of four mini-computers, each mini-computer comprising an independent and redundant channel. Increasing cost for increasing computer complexity and the need for rapid response requires the core protection calculator 58 mini-computers to be limited in their degree of calculational accuracy. The COLSS system 60, on the other hand, need not include either the characteristics of system redundancy or rapid response. Therefore, COLSS calculations may be made in a high powered plant computer at a relatively slow rate of calculation with the highest degree of accuracy which can possibly be achieved. As a result, the operating limit values calculated by COLSS are much more accurate than the calculations made by the core protection calculator. By recognizing these differences, utilization of the two systems may be made in a way that takes advantage of the strong points of each. Therefore, reactor steady-state operation is based on the slow, although very accurate, operating limit calculated by COLSS. As has been described above, this limit is a value which provides sufficient margin to the design limits to allow the core protection calculator to respond to an incident and terminate the reactor core chain reaction before the design limits are violated. Due to the inaccuracies of the core protection calculator and the need to provide allowances for them, the core protection calculator will normally indicate that the reactor is running closer to a specified limit than would be indicated by the COLSS. When this is the case, the reactor tripping system will be conservative and will trip the reactor before a trip is actually required. This unavoidable behavior is acceptable since it represents increased conservatism and does not raise the potential for design limit violation. For most efficient operation, the core protection calculator must be made sufficiently accurate to avoid reactor trips on normal parameter fluctuations which are expected to occur during steady-state operation of the reactor system at the COLSS operating limit. In the above described manner, COLSS and the core protection calculator operate hand-in-hand to assure the efficient and safe operation of the nuclear reactor system. The Core Protection Calculator 58 projects ahead in time a prediction of DNBR and trips the reactor when the predicted value is seen to violate the design limit. COLSS calculates a parametric limit value which incorporates a margin, and on the basis of which the reactor should be operated. When both of these systems are joined in combination, the reactor can be operated in a safe and efficient manner. While a preferred embodiment has been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation. The following "Appendix to the Description of the Preferred Embodiment" is a description of the generation of some of the input signals used in the above described calculation. In reading the appendix, it may be helpful to refer to FIG. 9 and to FIG. 11. APPENDIX TO THE DESCRIPTION OF THE PREFERRED EMBODIMENT Methods Used to Determine Core Power Distributions The axial heat flux distributions in the channels 22 and the local power density distributions in the fuel pins 20 are dependent upon the core power distribution. The core power distribution can be thought of as being composed of three distinct components: The method used to define the "hot" pin and "hot" channel power distribution consists of measuring core average power, synthesizing the normalized core average axial power distribution from out-of-core neutron flux signals and synthesizing the radial peaking factor from CEA 14 position measurements. Normalized Core Average Axial Power Distribution A normalized axial flux distribution is synthesized from the response of the ex-core detectors 16 and corrected for shape annealing and rod shadowing in order to determine a core average normalized axial power distribution. The ex-core detectors 16 are sensitive to the leakage neutron flux from the reactor core 12. The neutron flux seen by each of the three axially positioned detectors 16 is dependent upon the axial flux distribution near the core periphery, the radial flux distribution near the core periphery, and the diffusion and capture of neutrons leaking from the core periphery to the point of incidence with the ex-core detectors 16. Shape annealing, or the relative effect of contributions of the core peripheral axial flux distribution to a given detector, is due to the placement of the detector 16 away from the core periphery 12. The detectors "see" a distorted peripheral flux distribution due to the scattering and diffusion of neutrons between the core periphery and the detector locations. The shape annealing factors are dependent only upon the geometric location of the detectors 16 and do not depend on the axial flux distribution. Rod shadowing is the effect of rod insertion on the peripheral axial flux distribution relative to the core average power distribution. Rod shadowing factors are dependent upon the rods 14 inserted in the core 12. Since the ex-core detectors 16 primarily measure the flux distribution of the peripheral fuel elements 24 and since the detectors 16 are located a fixed distance from these elements 24, correction factors in the form of rod shadowing and shape annealing and required to obtain the core average axial power distribution in the core 12 from the flux distribution at the detectors. Calculation of Normalized Core Average Axial Power Distribution The method by which the normalized axial flux distribution at the ex-core detectors is used to yield the normalized core average axial power distribution is described in the following. Let the normalized axial flux distribution at the detectors be designated at .phi..sub.i where i designates the axial node at which the flux is defined. Let S.sub.ij be the contribution of a unit flux at axial node j at the core periphery to the flux at node i at the detector. Let .phi..sub.j be the flux at the core periphery in axial node j. Thus, the flux at the detector is given by ##EQU1## Representing the fluxes at the detector and periphery as n element column vectors implies that EQU .phi.=S.phi. where the flux shape at the periphery is then given by EQU .phi.=S.sup.-1 .phi. Having determined the peripheral axial flux distribution, it is now necessary to determine the core average axial power distribution .phi..sub.k by correcting for control element assembly (CEA) insertion. CEA position indications are used to generate axial CEA shadowing factors. A planar CEA shadowing factor for a particular planar CEA configuration is defined as the ratio of the peripheral flux at an axial node having the planar CEA configuration to the peripheral flux at an unrodded node, given that the power in the node is the same in both cases. When the entire node is not uniformly rodded, the planar CEA shadowing factor for that node is given by ##EQU2## where .gamma..sub.k is the fraction of the node which is rodded with the planar CEA configuration associated with planar CEA shadowing factor F.sub.k. These factors are conveniently arranged into an n.times.n diagonal matrix F. The resultant correction process is mathematically defined as follows: EQU .phi.=F.sup.-1 .phi. where: Net Algorithm The required correction algorithm for converting the ex-core axial flux distribution as synthesized using the ex-core detector responses to the normalized core average axial power distribution is expressed by the following matrix equation: EQU .phi.=F.sup.-1 S.sup.-1 .phi. Normalized Radial Power Distribution The radial power distribution that exists at any time for a specified fuel loading pattern, fuel enrichment, and fuel burnup is dependent primarily upon the following: The radial power distribution will vary along the core 12 height as a function of the CEA 14 configuration that exists in a given axial segment. The peak value of the normalized radial power distribution in a given axial segment is defined as the one-pin planar radial peaking factor for that segment. The product of the one-pin planar radial peaking factor, core average power and normalized core average axial power distribution in an axial segment defines the power produced in the "hottest" fuel pin 20 in that axial segment. The product also defines a quantity that is directly proportional to the heat flux in the "hottest" channel 22 in the axial segment. The combination of planar radial peaking factors in axial segments having known CEA 14 configurations, synthesized normalized core average axial power distribution and the measured core average power conservatively defines the axial heat flux distribution and local power density distribution in the "hot" channel 22 and fuel pin 20, respectively, without consideration for azimuthal tilt. The presence of an azimuthal tilt will have the effect of increasing an "untilted" planar radial peaking factor by an amount that is directly proportional to the magnitude of the tilt. The core protection calculator will monitor the position of every CEA group and receive information from CEA Calculators on the relative position of CEAs 14 in the same group. This information is used to define the CEA 14 configuration along the entire core 12 height. Knowing the total CEA configuration, precalculated planar radial peaking factors are chosen that apply to each axial segment. The radial peaking factors are then modified to reflect an azimuthal tilt magnitude that is at its worst case operating limit. This latter adjustment is made because the azimuthal tilt magnitude is not a directly monitored parameter. The result is a set of axially varying planar radial peaking factors that can be used in defining the "hot" pin and "hot" channel conditions. Calculation of Planar Radial Peaking Factors Planar radial peaking factors are used in conjunction with the core average axial power shape to determine the three dimensional peaking factor and the heat flux axial profile. Since the radial peaking depends upon the number and location of control CEAs, planar radial peaking factors depend upon CEA bank insertion and hence upon axial position in the core. The planar radial peaking factors are determined by the Core Protection Calculators (CPCs) for each of 25 axial nodes. Determination of the pin planar radial peaking factors at any given axial node by the CPC is done by a table look-up routine using a precalculated table of values for the planar radial associated with those CEA groups which have been inserted in normal sequence into the axial node of interest. Penalty factors are then applied to account for the increased radial peaking which would result from conditions other than normal. These include CEA groups being inserted out of sequence, excessive misalignment of subgroups from the group average position, and excessive misalignment of individual CEAs from its average subgroup position. In the discussion which follows, the term subgroup is defined as being any one of the sets of four CEA combinations which are operated and positioned as a unit. A group is then any combination of one or more subgroups which are operated and positioned as a unit. Regulating CEA groups are considered to be inserted in a prescribed sequence, with group withdrawal in the reverse order. Any insertion of regulating groups in other than this prescribed sequence is considered to be an out-of-sequence condition. In additon to the normal sequential insertion of regulating groups, either or both of the part length rod (PLR) groups or selected subgroups used for reactor power cutback can be inserted at the same time. F.sub..alpha.n is the pin planar radial peaking factor associated with the configuration of CEA groups identified by the .alpha.n subscript. Here n represents the number of regulating groups, inserted according to their specific sequence, and .alpha. represents the combination of PLR and/or the selected reactor power cutback subgroups inserted in addition to the regulating groups. As an example, if A represents PLR group 1, B represents PLR group 2, and C represents a reactor power cutback subgroup then the group configurations and their associated planar radials can be designated as in the following example: ______________________________________ Subscript Pin planar radial Designation, Peaking factor, CEA Configuration .alpha.n F.sub..alpha.n ______________________________________ First 3 regulating groups 3 F.sub.3 First 2 regulating groups + PLR group 1 A2 F.sub.A2 First 4 regulating groups + cutback group C4 F.sub.C4 PLR group 2 only, no regulating groups BO F.sub.BO First regulating group + both PLR groups + cutback group. ABCl F.sub.ABCl ______________________________________ The information available for calculating the planar radials includes the position of a target CEA in each of the 22 subgroups, and CEA deviation information identifying the CEA with maximum deviation from the average position of the CEAs making up the subgroup and the magnitude and direction of the deviation. The sequence of steps by which the planar radial peaking factors are determined is as follows: In the first step, target CEA positions and CEA deviations are used as inputs in a calculation where subgroup position is determined. The subgroup position is taken to be the target CEA position, unless the target CEA is the one identified as deviating from the group average. In that case the indicated position is adjusted by the amount of the deviation to give the subgroup position. Subgroup position is used as input to step two, where the average position of those subgroups making up a group is taken to get the group position. Group position and subgroup position is used as input to step three, where subgroup deviation from group position is obtained. The group position from step two is also input to step four where pin planar radial peaking factors for normal CEA sequencing are determined. For each of the 25 axial nodes in the core, the group position signals are used to determine which CEA groups have been inserted in their normal sequence into that node. The appropriate pin planar radial peaking factor, F.sub..alpha.n, associated with that CEA configuration is then obtained from a precalculated table of peaking factors. In the fifth step, the group position information is evaluated at each axial node to determine whether any CEA groups are inserted into a node other than those of which follow the normal sequence of insertion. If an out-of-sequence condition exceeds the allowable limits, an out-of-sequence penalty factor, P.sub.os, is applied to the planar radial, F.sub..alpha.n, at the nodes affected. In steps six and seven additional penalty factors, P.sub.s and P.sub.r, are applied to account for subgroup or CEA deviations in excess of the allowable limits. Group position is evaluated to identify those nodes at which the group insertion terminates. If subgroup or CEA deviation exceeds the allowable limits, then the appropriate penalty factor is applied at the nodes affected. As might be expected, the penalty factors have different values depending on the direction of the deviation. The product F.sub..alpha.n P.sub.os P.sub.s P.sub.r emerging from step seven is the nodewise pin planar radial peaking factor, F.sub.r.sup.p (z). The pin planar radial peaking factor is used in the high local power density set point and DNBR calculation. The channel planar radial peaking factor (F.sub.r.sup.CHAN (z) is used on the DNBR calculation and is related to the pin planar radial by a proportionality constant. The algorithm is summarized below. Algorithm For Planar Radial Peaking Factor Calculation Definitions: Algorithm: Subgroup positions are found from EQU S.sub.ij =T.sub.j for each subgroup j. If the target CEA is the rod which deviates from the average subgroup position, then for that j EQU S.sub.ij =T.sub.j -D.sub.rj The group position for group i is found from EQU G.sub.i =.sub.j contained in i{S.sub.ij } The subgroup deviation for group i is then EQU D.sub.si =max{S.sub.ij -G.sub.i } Having obtained the necessary position information, each axial node is evaluated to find: EQU F.sub.r.sup.p (z)=F.sub..alpha.n P.sub.os P.sub.s P.sub.r node z EQU F.sub.R.sup.CHAN (Z)=Constant F.sub.R.sup.P (Z) where: ##STR1## and F.sub..alpha.n is obtained from the precalculated table of pin planar radial peaking factors for normal sequence group configurations. Detection and Calculation of Azimuthal Flux Tilt Magnitude Indications of any azimuthal flux tilting are obtained by comparing the signals from symmetrically located detectors. These signals are fitted to the functional form: EQU .phi.(r,.theta.)=.phi..sub.o (r, .theta.) [1.times.sg(r) cos (.theta.-.theta..sub.o)] where the fundamental flux pattern is designated as .phi..sub.o, the amplitude of the tilt by s, and the orientation of the tilt by .phi..sub.o. The separable functional form in the second term of the equation has been suggested by examination of tilted flux shapes from diffusion theory representations of mild xenon oscillations. The functional g(r), which is given approximately by J.sub.1 (.alpha..sub.1, r)/J.sub.o (.alpha..sub.o, r), can be used to relate the tilt signals from different sets of detectors if no asymmetrically placed rods are inserted. Where J.sub.1 and J.sub.o are Bessel functions of the first kind of the zero and first orders. Signals from symmetrically placed in core rhodium detector strings 18 are analyzed to obtain s and .phi..sub.o. One symmetrical set such as this gives five values of s and .phi..sub.o since there are five axially located detectors in each string. Since there are five detectors per string, this gives 20 values of the pair (s, .phi..sub.o). The average value of s and .phi..sub.o are then found as well as the respective standard deviations. A study of the standard deviations gives some insight into whether or not the average values of s and .phi..sub.o indicate a true flux tilt. Core Average Power The core average power is determined by two distinct methods. The first method uses the response of the out-of-core detectors 16. The detectors responses are indicative of the neutron flux level in the core 12; when the ex-core detectors 16 are calibrated to the plant calorimetric, any change in detector response is indicative of a change in neutron flux level which in turn is indicative of a change in the power being produced in the fuel. The second method provides a measure of core power by relating the temperature rise through the core 12 to the total thermal power. The temperature rise through the core is obtained from the temperature detectors (RTD's) in the hot and cold leg coolant piping. In order to calculate the power level from the neutron flux level signals from the ex-core detectors, corrections for shape annealing, rod shadowing and inlet temperature are necessary. The shape annealing and rod shadowing corrections have been discussed above. The correction for inlet temperature change is necessary because the detector responses are dependent upon the diffusion and absorption of neutrons that occur between the core periphery and the detector locations. Reactor coolant from the steam generators 26 passes between the core 12 and detectors 16 before entering the core 12. As a result, the total neutron leakage out of the reactor vessel and thus the ex-core detector response is affected by the reactor coolant, which has temperature dependent neutronic properties. This effect is defined as temperature shadowing and has the characteristic of causing a decrease in detector response for a decrease in inlet temperature. The temperature shadowing effect is independent of the power distribution at the peripheral core assemblies. The corrected detector responses for each detector segment are summed to yield a quantity that is proportional to neutron power level. A constant of proportionality is then applied to convert the detector response to a calibrated thermal power level. The ex-core detectors 16 respond very rapidly to changes in the neutron flux and, after the above corrections are made, provide a dynamically accurate signal that can be used to sense core power changes. The core power measurement obtained from the temperature rise across the core 12, on the other hand, is a statically accurate indicator of core power. This latter core power measurement, after calibration to the plant calorimetric, provides a very accurate steady-state indication of core power. The thermal energy added to the coolant as it passes through the core 12 serves as an indicator of core power. This relationship depends upon the fuel, cladding and coolant channel configuration, their heat transfer characteristics and the primary coolant .DELTA.T rise. The coolant hot leg 28 temperature is measured by sensor 36 and the coolant cold leg 34 temperature is measured by sensor 38. The difference between these two measurements is .DELTA.T. The heat transfer properties of the fuel, clad and coolant are dependent upon the core temperatures and volumetric flow rate of the coolant. Using standard thermodynamic models and methods the core power can be determined by measuring the primary coolant hot 28 and cold 34 leg temperatures. These measurements are used in a mathematical expression which accurately models the core power taking into account the variation of the heat transfer properties that are temperature dependent. The core volumetric flow rate is obtained from the reactor coolant pump 32 speed measurements. The .DELTA.T power measurement described above is also made more accurate for slow variations in core thermal energy by dynamically compensating the basic steady-state expression with a measure of the rate of heat addition to the stored thermal energy in the primary coolant. This is accomplished by differentiating the measured hot and cold leg temperatures, multiplying by constants and adding the products to the steady-state expression. Since the .DELTA.T power measurement provides a very accurate steady-state indication of core power, it is used as a calibration standard against which the core power from the ex-core detectors is calibrated during steady-state operation. See U.S. Pat. No. 3,356,577 issued to Ygal Fishman on Dec. 5, 1967. The calibration technique provides an updated nuclear power signal proportional to the time averged integral error between the calculated nuclear power and .DELTA.T power. The updating interval is long compared to reactor dynamic characteristics to ensure that the dynamic accuracy of the ex-core detectors is maintained during transients. In addition to providing a .DELTA.T power calculation that is accurate for slow transients, the dynamic compensation described above will provide an accurate .DELTA.T power signal during incidents that involve rapid single CEA deviations (especially a dropped CEA). In the event that a CEA drops, the power distribution will be distorted. This distortion can have the effect that the nuclear power signals in any one or more of the four core protection calculator channels will not be indicative of the core average power. The indicated nuclear power in any channel may be higher or lower than the core average power depending upon how the dropped CEA distorts the power distribution. The .DELTA.T power, however, will not be significantly affected. Therefore, the nuclear flux power will be used as the core average power indication unless a large CEA deviation is detected, at which time the CPCs will automatically use the .DELTA.T power as the core average power indication for the low DNBR and high local power density trips. Calculation of Corrected Neutron Flux Power In order to calculate the power level as indicated by the neutron flux level measured by the ex-core detectors, corrections for shape annealing and rod shadowing are necessary for each segment of each of the four detector channels. These corrected responses for each detector segment are summed to yield a quantity proportional to the neutron flux power level for each detector channel. A constant of proportionality is then applied to this quantity to convert the detector channel response to a previously calibrated thermal power level. The shape annealing correction factor is applied to the uncorrected response of each segment of the four ex-core detector strings. Let D.sub.i be the uncorrected response of segment i of a detector string. Let Pi be the detector response of segment i had the detector been placed at the core periphery. In order to determine the external detector segment response, the peripheral segment response must be corrected for the effects of shape annealing. Let S.sub.ij be the shape annealing correction factor which allows for contributions to an external detector segment due to the distribution of point sources along the core periphery. This relationship can conveniently expressed in matrix notation as EQU D=S P where In order to determine the peripheral detector response, the following calculation is required; EQU P=S.sup.-1 D Having determined the detector response had the detectors been at the core periphery, it is now necessary to determine the detector response in the absence of flux perturbations due to CEA insertion in the region of the detector. Average CEA shadowing factors R are determined for each detector segment dependent on CEA insertion in the region of detector segment. This can mathematically be expressed as: ##EQU3## where R.sub.j =Average CEA shadowing correction factor for detector segment j The X.sub.i are determined by CEA position indications. The average CEA shadowing correction factor for each segment is applied to the peripheral detector response to yield a detector signal .phi..sub.i for segment i proportional to nuclear power. EQU .phi.=R.sup.-1 P where The individual segments i of each string are then summed. ##EQU4## where .phi.=Total corrected detector channel response proportional to reactor power The total summed response .phi.is corrected for temperature changes by applying a factor that is proportional to the change in cold leg temperature. Let T.sub.f be the temperature correction factor then EQU .phi..sub.N (t)=.phi.(t).multidot.T.sub.f (t) where EQU T.sub.f (t)=1+C.sub.T (T.sub.co -T.sub.cmin (t)) This number is proportional to the nuclear flux power as measured by that detector channel. The constant of proportionality is determined during power range testing by calibrating the detector response to a known steady state thermal power level. The final calculation required for determining the calibrated neutron flux power is to correct the total average corrected detector response by the calibration factor K.sub.cal. EQU .phi..sub.cal =K.sub.cal .phi..sub.N where: The calibration factor is obtained on-line by making use of the calculated .DELTA.T power. The algorithms employed to perform this calibration are described infra. Calculation of Core Thermal Power (.DELTA.T Power) The calculated core thermal power is obtained using a correlation based on measured coolant conditions. The information available as input to this calculation consists of hot and cold leg temperature signals, the reactor coolant system mass flow rate and an indication of the number of reactor coolant pumps running. The .DELTA.T power is calculated in two portions, a static calculation and a dynamic calculation. Static Calculation: The two hot and cold leg temperature inputs are averaged and using in the following equation: EQU B.sub..DELTA.T.sup.static =f.sub.i (m).DELTA.T+f.sub.2 (m).DELTA.T.multidot.T.sub.c +f.sub.3 (m).DELTA.T.sup.2 where The static .DELTA.T power equation conservatively calculated the variation in-core power with temperature changes, mass flow rate and coolant specific heat (which is temperature dependent). Dynamic Calculation The hot and cold leg temperature signals used in the static calculation are obtained from Resistance Temperature Detectors (RTDs) in the primary coolant piping. As a result of their location the hot leg temperature signal will "lag" the core outlet temperature due to the transport time involved through the coolant piping; similarly the cold leg temperature signal will "lead" the core inlet temperature. Dynamic compensation is provided to accommodate this effect as follows: ##EQU7## where: T.sub.x : is a weighted average of the hot and cold leg temperature signals, defined as EQU T.sub.x =W.sub.1 T.sub.h +W.sub.2 T.sub.c where Dynamic Compensation assures that the .DELTA.T power agrees with the heat flux transmitted out of the fuel pin. The dynamic portion of the .DELTA.T power is implemented using a Z transform of the above equation as ##EQU9## where: .DELTA.S: is the time between updates of B.sub..DELTA.T.sup.dyn The total .DELTA.T power is then EQU B=B.sub..DELTA.T.sup.static +B.sub..DELTA.T.sup.dyn The calculated .DELTA.T power is used to calibrate the nuclear flux power, as described hereinafter. Calculations for Calibration of Neutron Flux Power to .DELTA.T Power The calculated .DELTA.T Power, described supra, is used to calibrate the corrected neutron flux power. The calibration algorithm is specified to provide an accurate calibration during steady-state operation but retain the rapid neutron flux power response during anticipated operational occurrences. The flux power calibration is achieved by means of a proportionality constant multiplier (K.sub.cal) on the corrected neutron flux power. The calibrated neutron flux is defined by EQU .phi..sub.cal =K.sub.cal .times..phi..sub.N where The calibration factor is defined by the following equation: EQU K.sub.cal (t)=K.sub.cal (t-.DELTA.S)+.tau..sub.k [.phi..sub.cal (t)-B(t)].DELTA.S with EQU .phi..sub.cal (t)=K.sub.cal (t-.DELTA.S).multidot..phi..sub.N (t) where The error weighting factor is chosen to result in an effective neutron flux power calibration time constant of approximately 4 minutes. This means that if there were a step change in .DELTA.T power it would take about 16 minutes for the difference between neutron flux power and .DELTA.T power to be reduced to 2 percent of its initial value. The above technique is used for calibration during conditions when there is no excessive CEA deviation. In the event that excessive CEA deviation occurs the neutron flux power indication can rapidly decalibrate. The flux power calibration algorithm consists of logic which will change the mode operation of calibrated neutron flux power as the power input to the DNBR and local power density set point calculation. This mode will occur for those CEA configurations which are not accommodated by the CEA shadowing correction. In this mode, the .DELTA.T power is the primary power input. The .DELTA.T power conservatively predicts the core average power for these conditions. Method Used to Determine Reactor Coolant System Mass Flow Rate The primary coolant mass flow rate is used in the low DNBR trip. The mass flow rate is obtained using the pump speed inputs from the reactor coolant pumps 32, the primary coolant pressure, and the core inlet temperature. The volumetric flow rate through each reactor coolant pump is dependent upon the rotational speed of the pump and the pump head. This relationship is typically shown in pump characteristic curves. Flow changes resulting from changes in the loop flow resistances occur slowly (i.e., core crud buildup, increase in steam generator resistance, etc.). Calibration of the pump speed signal, relating pump rotational speed to volumetric flow, will be performed periodically using pump .DELTA.P instrumentation which is not part of this invention. Flow reductions associated with pump speed reductions are more rapid than those produced from loop flow resistance changes. The pump rotational speed signal is converted to a pump volumetric flow using mathematical relationships based on pump characteristics and periodic loop flow calibrations. The algorithm used is conservative relative to the actual pump performance. The volumetric flow rates calculated for each pump 32 are summed to give a vessel flow. The vessel flow is corrected for core bypass and density and the result is the core mass flow rate. Calculation of Reactor Coolant System Mass Flow Rate The reactor coolant system mass flow rate is obtained from the rotational speed of each reactor coolant pump 32. This is done by making use of the pump characteristic curves, summing the four pump volumetric flow rates, correcting for internal and external vessel flow leakage and correcting for density variations. Proximity probes are used to measure the shaft rotational speed. The volumetric flow rate for each pump is defined by the general relationship shown below: EQU V.sub.i =f.sub.i (RPM, .DELTA.P(RPM,N),N) where: The total vessel volumetric flow rate is the sum of the pump flows. The summed flows are adjusted for core bypass as follows: ##EQU10## where: V.sub.core =volumetric flow rate through the core The core volumetric flow rate is corrected for density to obtain the mass flow rate as follows: EQU m=V.sub.core (C.sub.0 +C.sub.1 (T.sub.c -T.sub.co)+C.sub.2 (P-P.sub.o)) where: Method Used to Determine Local Power Density Trip Set Point The local power density distribution in the fuel is dependent upon the core power distribution. The objective of establishing a trip on high local power density is to prevent the centerline fuel temperature in the "hottest" fuel pellet in the core from exceeding the melting point. The centerline fuel temperature is dependent upon the pellet geometry, pellet composition, the amount of energy deposited in the fuel, the local power density, the gap and cladding configuration and their heat transfer characteristics. The core power distribution can be related to the local power density deposited in the fuel by a proportionality constant. The power distribution in the "hot" pin can be obtained by the methods described supra. In steady-state, the deposited local power density can be related to the centerline fuel temperature by a proportionality constant when the temperature profile across the fuel diameter is known. Therefore, the centerline fuel temperature can be directly related to the power distribution in the "hot" fuel pin. During transient conditions, the deposited local power density can be related to the centerline fuel temperature through standard heat conduction models which predict the spatial variation in the fuel temperature profile as a function of the heat transfer time constant of the fuel. Therefore, changes in centerline fuel temperature can be directly related to changes in the power distribution in the "hot" pin. The local power density trip set point is defined as that value of core average power which corresponds to a power density in a fuel pellet which would result in raising the steady-state centerline fuel temperature to the melting point for a given three-dimensional peaking factor. The three-dimensional peaking factor is defined as the maximum product of the normalized core average axial power distribution and axially varying planar radial peaking factor adjusted for allowed azimuthal tilt magnitude. This definition of the set point assures that the local power density in the "hottest" fuel pellet is accommodated and that the dynamics of the fuel temperature variation are correctly defined. The basic operation of the high local power density trip is as follows: If the core average power becomes greater than the set point a trip will occur in a time interval that is a function of the amount by which the core average power exceeds the set point and the transient characteristics of the set point. The time interval that occurs before a trip signal is generated is obtained by delaying the setpoint transient response to account for the fuel temperature time constant and accelerating the set point transient response to account for protective system time delays, CEDM de-energization time and the time required to effectively terminate the occurrence. Calculation of Local Power Density Set Point The local power density set point is that value of the core power which could correspond to the limiting local power density for a given three-dimensional peaking factor and azimuthal tilt magnitude. The information available for calculating the set point consists of the planar radial peaking factors, (F.sub.r.sup.p (z)), the normalized core average axial power shape, (F.sub.z (z)), and the azimuthal tilt magnitude (T.sub.r). The maximum value of the product F.sub.z (z)F.sub.r.sup.p (z) is the 3-D peaking factor, which is defined as the ratio of the peak local power density to the core average power density. Thus, if C.sub.S represents the limiting local power density divided by the full power average local power density, the ratio of C.sub.s to F.sub.z (z)F.sub.r.sup.p expressed as a percentage gives the core power at which this limit will be reached. Reducing this by the amount of azimuthal tilt results in the static local power density set point, B.sub.sp.sup.st, which is the value of core average power that would result in centerline fuel melt in steady-state. This set point is passed through the digital equivalent of a lead-lag filter of the form: EQU (.tau..sub.1 s+1)/(.tau..sub.2 s+1). The output of the filtered set point is the local power density trip set point, B.sub.sp. The algorithm is summarized below. Algorithms for Local Power Density Set Point Calculation Definitions: Algorithm: The local power density static set point is found from: ##EQU11## where max F.sub.z (z)F.sub.r.sup.p (z) is the maximum value of the product evaluated at each node z. The trip set point is found using a z-transform of the lead-lag filter as follows: ##EQU12## High Local Power Density Trip Signal Generation The local power density trip set point that is calculated as described above is compared to the measured core average power. The core average power measurement is either the corrected neutron flux power or the calculated .DELTA.T power depending upon the conditions discussed above. Based upon this comparison the following actions are initiated: EQU a. if: M.sub.2 <B.sub.sp -.phi..sub.cal .ltoreq.M1 a contact opening output is sent to the Reactor Protective System 2/4 CEA withdrawal prohibit logic. EQU b. if: 0.0<B.sub.sp -.phi..sub.cal .ltoreq.M2 a contact opening output provides a channel pre-trip annunciation and a contact closure output provides a signal to the 2/4 Power Reduction Control Signal logic. EQU c. if: B.sub.sp -.phi..sub.cal .ltoreq.0.0 a contact opening output provides a channel trip signal to the 2/4 RPS trip logic. The values of M1 and M2 are margins to trip expressed in terms of percent of core power, with M1>M2. Method Used to Determine DNBR The Core Protection Calculators require a correlation for calculation of on-line minimum DNBR. Input to the calculation will include the instant reactor conditions of mass flow, integrated nuclear planar radial peaking factor, the ratio of maximum average fuel assembly power to core average assembly power, coolant inlet temperature, a measure of the axial power distribution, and reactor coolant system pressure. A simplified closed channel hydraulic model is used as the algorithm for the on-line computation of DNBR. This on-line computation may most easily be done in a special purpose digital computer. Since the closed hot channel calculation does not take into consideration turbulent interchange of coolant between the hot channel and the neighboring channels, an adjustment to the algorithm's input is required. This adjustment is made to the mass velocity in the channel such that when all other conditions are the same, the closed channel minimum DNBR equals the minimum DNBR obtained by considering the interchange of coolant between channels. The adjusted mass velocity is found by evaluating an analytically derived Equivalent Mass Velocity Correlation. The Equivalent Mass Velocity Correlation is an expression of the form: ##EQU13## where Ge=algorithm required mass velocity; The coefficients of the polynominal fit are analytically determined by the following scheme. The Equivalent Mass Velocity Correlation is dependent upon all the input parameters mentioned above. The integrated planar radial peaking factor is defined as the integrated value of the product of the axial dependent planar radial peaking factors and the normalized core average axial power distribution. The ratio of the maximum assembly power to the core average assembly power is related to the integrated planar radial peaking factor by a proportionality constant. The DNB trip is basically composed of two distinct levels of calculation. The first level can be termed as the periodic static or snapshot calculation and the second level as the update calculation. In the periodic calculation, the most recent values of the monitored variables or calculated parameters that affect the DNBR are used to determine the DNBR. This calculation employs the Equivalent Mass Velocity Correlation discussed above and a simplified version of the W-3 correlation. The update calculation will be used to update the DNBR between period calculations. The relationships involved consist of polynominal functions that have been obtained from extensive analysis using a standard DNBR analysis method. Calculations for Minimum DNBR The minimum DNBR in the "hot" channel is calculated using the following calculated parameters and monitored NSSS variables: Periodic DNBR Calculation The periodic calculation uses the inputs listed above, in the Equivalent Mass Velocity Correlation and simplified version of the W-3 correlation. The equivalent mass velocity is defined by ##EQU14## where: g*: is the channel mass velocity at a datum (for instance nominal reactor conditions) Knowing the equivalent mass velocity and the axial distribution at 25 axial nodes, the coolant enthalpy rise can be calculated up the channel from ##EQU16## where: H.sub.i : average coolant enthalpy at node i With the coolant enthalpy known at each node, the critical heat flux is calculated EQU Qcrit.sub.i =f.sub.1 (.phi..sub.cal, Ge, H.sub.i, T.sub.cmax, P.sub.pri, F.sub.r.sbsb.i.sup.chan, F.sub.z.sbsb.i) The complicated expressions for Qcrit.sub.i involving several empirically derived constants utilizes the standard W-3 correlation developed by Tong and may be found in: L. S. Tong, "Prediction of Departure from Nucleate Boiling for an Axially Non-uniform Heat Flux Distribution," Journal of Nuclear Energy, 21:241-248, 1967. The local heat flux at each node defined by EQU Qlocal.sub.i =f.sub.2 (.phi..sub.cal, T.sub.r, F.sub.r.sbsb.i.sup.p, F.sub.z.sbsb.i) is divided into the critical heat flux to give the DNBR at each node, EQU DNBR.sub.i.sup.periodic =Qcrit.sub.i /Qlocal.sub.i The minimum DNBR resulting from the periodic portion of the calculation is then EQU DNBR.sup.periodic =min[DNBR.sub.i.sup.periodic ] DNBR Update Calculation The periodic calculation described above will be performed approximately every 2 seconds. In this time interval the update calculation is used to update the statically calculated DNBR. In this context, when "continuously" is used in the description and in the claims it should be taken to mean: "of a substantially higher periodicity than the frequency of the periodic calculation." The local heat flux defined as EQU Qlocal.sub.i =constant.multidot..phi..sub.cal .multidot.F.sub.z.sbsb.i .multidot.F.sub.r.sbsb.i.sup.p .multidot.(1+T.sub.r) is calculated on a nodal basis. The update calculation is performed approximately every 20 millisec. Therefore, the periodic calculation will never have an input that is more than 20 millisec delayed. The periodic DNBR is updated within the interval between periodic calculations by comparing the inputs to the values of the inputs used in the most recently completed periodic calculation. The differences in the input values are used to calculate the change in the periodic DNBR at each node by a partial derivative approach as shown below ##EQU17## where: .DELTA.DNBR.sub.i (t): is the change in DNBR at node i ##EQU18## are functions that relate a change in a particular parameter to an equivalent change in DNBR. These functions will be conservatively chosen constants or polynomial expressions that depend upon the measured values of other pertinent parameters. The updated DNBR at each node is then EQU DNBR.sub.i (t)=DNBR.sub.i.sup.periodic +.DELTA.DNBR.sub.i (t) The minimum DNBR is then EQU DNBR(t)=min[DNBR.sub.i (t)] It will be understood that the embodiment shown and described herein is merely illustrative and that changes may be made without departing from the scope of the invention as claimed.

claims

1. A method for electroplating a nonmetallic grating comprising:providing a nonmetallic grating having an aspect-ratio of from 50:1 to 60:1;performing an atomic layer deposition (ALD) reaction to form a seed layer directly on the nonmetallic grating, wherein the seed layer uniformly and conformally coats the nonmetallic grating such that a thickness of the seed layer is substantially the same along an entire surface of the nonmetallic grating, and wherein performing the ALD reaction comprises:(i) placing the nonmetallic grating in an ALD reactor, the ALD reactor having (a) a back-purge mechanism to facilitate delivery of an ALD precursor vapor across the non-metallic grating, and (b) a pressure of about 1.5 Ton; and(ii) alternating pulses of an ALD precursor and a precursor vapor to grow the seed layer, wherein the pulses are alternated to produce pressure pulses of 30 mTorr of the ALD precursor and 75 mTorr of the precursor vapor and result in a seed layer having a uniform thickness of from 22 nm to 27 nm; andelectroplating a metallic layer directly on the seed layer, wherein the metallic layer uniformly and conformally coats the seed layer such that a thickness of the metallic layer is substantially the same along an entire surface of the seed layer, wherein electroplating the metallic layer on the seed layer comprises pulsing an electroplating bath solution comprising thallium across the nonmetallic grating at a pulse frequency of from 50 Hz to 100 Hz, a pulse profile of ¼ duty and a pulse current of 2 mA/cm2, and the metallic layer comprises a thickness of from 0.1 microns to 0.6 microns. 2. The method of claim 1 wherein the aspect-ratio is 60:1. 3. The method of claim 1 wherein the nonmetallic grating comprises elongated elements having a depth of at least 26 microns and a pitch between each of the adjacent elongated elements is 2 microns or less. 4. The method of claim 1 wherein the nonmetallic grating comprises silicon. 5. The method of claim 1 wherein the seed layer comprises palladium. 6. The method of claim 1 wherein the seed layer comprises platinum. 7. The method of claim 1 wherein electroplating comprises exposing the nonmetallic grating having the seed layer thereon to an electrical current forward pulse of from 340 to 365 microseconds. 8. The method of claim 1 wherein electroplating comprises submerging the nonmetallic grating having the seed layer thereon within an electroplating reaction tank, the electroplating reaction tank having outer sidewalls that form chamfered corners, wherein the chamfered corners are dimensioned to generate a laminar flow profile of an electroplating bath solution within the tank, and wherein the electroplating bath solution comprises a thallium concentration of from 40 ppm to 60 ppm. 9. The method of claim 1 wherein the metallic layer comprises gold. 10. A method for electroplating a high aspect-ratio silicon grating comprising:providing a silicon substrate having gratings with an aspect-ratio of 60:1;coating the gratings with a conducting seed layer using atomic layer deposition (ALD),wherein the conducting seed layer comprises a same thickness along an entire surface of the gratings, wherein performing ALD comprises:(i) placing the silicon grating in an ALD reactor, the ALD reactor having (a) a back-purge mechanism to facilitate delivery of an ALD precursor vapor across the silicon grating, and (b) a pressure of about 1.5 Torr; and(ii) alternating pulses of an ALD precursor and a precursor vapor to grow the conducting seed layer, wherein the pulses are alternated to produce pressure pulses of 30 mTorr of the ALD precursor and 75 mTorr of the precursor vapor and result in the conducting seed layer having a uniform thickness of 22 nm to 27 nm; andelectroplating a gold layer on the conducting seed layer such that the gold layer comprises a same thickness along an entire surface of the conducting seed layer, wherein electroplating the gold layer comprises:(iii) placing the silicon grating having the conducting seed layer thereon into an electroplating chamber, the electroplating chamber comprising outer sidewalls that form chamfered corners dimensioned to produce a laminar flow profile of an electroplating bath across the silicon grating, and wherein the electroplating bath comprises a thallium concentration of from 40 ppm to 60 ppm; and(iv) forming the gold layer on the conducting seed layer by pulsing the electroplating bath across the silicon grating at a pulse frequency of from 50 Hz to 100 Hz, a pulse profile of ¼ duty and a pulse current of 2 mA/cm2, and wherein the thickness of the gold layer is from 0.6 microns to 1.5 microns. 11. The method of claim 10 wherein the gratings have a depth of at least 50 microns and a pitch of less than 5 microns. 12. The method of claim 10 wherein performing ALD comprises exposing the silicon grating to from 300 to 375 cycles of ALD, wherein the cycles comprise alternating pulse and purge cycles of the ALD precursor and the precursor vapor, wherein each ALD precursor comprises 1 second pulse and 20 second purge and each precursor vapor cycle comprises 2 second pulse and 15 second purge. 13. The method of claim 10 wherein the chamfered corners form an angle with respect to the outer sidewalls of the electroplating chamber of greater than 90 degrees. 14. The method of claim 10 further comprising:prior to coating the gratings with a conducting seed layer, using ALD to apply an adhesive layer comprising aluminum oxide to the gratings. 15. A method for electroplating a nonmetallic grating comprising:providing a nonmetallic grating having an aspect-ratio of 50:1 to 60:1; performing an atomic layer deposition (ALD) reaction to form a seed layer directly on the nonmetallic grating, wherein the seed layer uniformly and conformally coats the nonmetallic grating such that a thickness of the seed layer is substantially the same along an entire surface of the nonmetallic grating, and wherein performing the ALD reaction comprises:(i) placing the nonmetallic grating in an ALD reactor, the ALD reactor having (a) a back-purge mechanism to facilitate delivery of an ALD precursor vapor across the non-metallic grating, and (b) a pressure of about 1.5 Ton; and(ii) alternating pulses of an ALD precursor and a precursor vapor to grow the seed layer, wherein the pulses are alternated to produce pressure pulses of 30 mTorr of the ALD precursor and 75 mTorr of the precursor vapor and result in a seed layer having a uniform thickness of 22 nm to 27 nm; andelectroplating a metallic layer directly on the seed layer, wherein the metallic layer uniformly and conformally coats the seed layer such that a thickness of the metallic layer is substantially the same along an entire surface of the seed layer, and wherein electroplating the metallic layer comprises:(iii) placing the nonmetallic grating having the seed layer thereon into an electroplating chamber, the electroplating chamber comprising outer sidewalls that form chamfered corners dimensioned to produce a laminar flow profile of an electroplating bath across the nonmetallic grating, and wherein the electroplating bath comprises a thallium concentration of from 40 ppm to 60 ppm; and(iv) forming the metallic layer on the seed layer by pulsing the electroplating bath across the nonmetallic grating at a pulse frequency of from 50 Hz to 100 Hz, a pulse profile of 1 A duty and a pulse current of 2 mA/cm2, and wherein the thickness of the metallic layer is from 0.6 microns to 1.5 microns. 16. The method of claim 15 wherein the nonmetallic grating comprises elongated elements having a depth of at least 26 microns and a pitch between each of the adjacent elongated elements is 2 microns or less. 17. The method of claim 15 wherein the nonmetallic grating comprises silicon, the seed layer comprises palladium or platinum, and the metallic layer comprises gold. 18. The method of claim 15 further comprising:prior to coating the gratings with a conducting seed layer, using ALD to apply an adhesive layer comprising aluminum oxide to the gratings.

summary

056028873

abstract

A tool for pushing a suspended shroud repair tie rod assembly radially inward in the downcomer annulus of a boiling water reactor. The tie rod pusher tool includes a pole adaptor for coupling to the end of a service pole, a pole adaptor extension having one end connected to the pole adaptor, a mounting channel connected to the other end of the pole adaptor extension, a hydraulic spreader mounted on the mounting channel, an adaptor bracket having a proximal end connected to the pivoting member of the hydraulic spreader, a rocker plate pivotably mounted on the distal end of the adaptor bracket, and a saddle mounted on the rocker plate. Using a service pole, the tool is lowered into a position whereat the saddle contacts the tie rod assembly when the hydraulic spreader is actuated. In the open position, the saddle bears against the tie rod assembly with sufficient force to displace the assembly radially inward until the clevis hook clears the clevis pin. Then the tie rod assembly is lowered until the tip of the clevis hook clears the bottom of the clevis pin. During descent of the tie rod assembly, it slides against the saddle of the pusher tool. The saddle is made of ultra-high-molecular weight polyethylene to prevent scratching of the tie rod assembly. When the pressurized fluid to the spreader is cut off, the bottom end of the suspended tie rod assembly drifts radially outward, causing the clevis pin to engage the clevis hook.

description

This application claims the benefit of priority to U.S. Provisional Application No. 60/819,057, filed Jul. 7, 2006, and entitled, “Electron Beam Apparatus To Collect Side-View and/or Plane-View Image With In-Lens Sectional Detector”, all of which is incorporated herein by reference. The present invention is related generally to scanning electron microscopes and more particularly to a system and method to collect the side-view and plane-view SEM image. A low-landing energy, high resolution SEM (scanning electron microscope) with the capability of capturing a side-view and plane-view image is a very important metrology tool to inspect and review defects in a semiconductor wafer. This SEM accelerates the new wafer processing technology ramp and improves the yield during mass production. For the conventional SEM with capability of collecting side-view SEM image of specimen, one or more side-detectors are placed very close to the specimen surface. The objective magnetic lens usually has a conical shape to make space for the side detectors. The space between the specimen surface and the lens pole-piece has no or very weak axial magnetic field and electrostatic field so that the secondary electrons emanating from the specimen with a polar angle can be collected by the side-detector. In order to improve the collection efficiency, a positive voltage with respect to the specimen will be applied to the side-detector to attract the secondary electron signal. This conventional SEM layout has a poor aberration property, and it is difficult to achieve high resolution, especially for low landing energy SEM imaging. It is known that the combination of immersion magnetic lens and retarding electrostatic lens has very low aberration coefficients and can achieve high resolution for the low landing energy. Due to strong axial magnetic field and extraction electric field between the specimen surface and lens pole-piece of this compound lens, the layout of the side-detector near the specimen surface to collect the side-view SEM image cannot work anymore. The presented invention will solve the conflict between high-resolution achieving and side-view imaging for low landing energy SEM. An object of the present invention is to provide an apparatus and method to collect the secondary electrons emanating from specimen surface without influencing the primary electron beam thereafter form side-view and/or plane-view image of a high resolution and low landing energy SEM. This and other objects are achieved in an electron detector structure and aperture arrangement around the primary beam optical axis to capture the secondary and backscattered electrons emanating from specimen surface with different azimuth and polar angles. In one embodiment, an apparatus for generating side-view and plane-view image from a specimen is disclosed. The apparatus includes a charged particle beam generator arranged to generate and control a charged particle beam substantially towards a portion of the specimen and a detector arranged to detect charged particles emanating from the specimen to allow generation of an image of interested portion of the specimen. In another embodiment, a charge particle detector for obtaining an image of a portion of specimen surface is disclosed. An in-lens sectional detector composed of at least two segments with an aperture is arranged to capture secondary electrons and backscattered electrons emanating from specimen surface with different azimuth and polar angles. For further embodiment, an ExB filter is positioned to guide the secondary electrons and backscattered electrons emanating from specimen surface substantially toward the off-axis sectional detector. In yet another embodiment, a detector for generating quality side-view image is disclosed. An aperture on the detector with 3 millimeters diameter is calculated for quality side-view image and image aberration. Reference will now be made in detail to specific embodiments of the invention. Examples of these embodiments are illustrated in accompanying drawings. While the invention will be described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to these embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a through understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. The present invention may be implemented within any suitable measurement device that detects charged particles towards a sample and then detects emitted particles from the sample. FIG. 1 is a diagrammatic representation of an electron beam apparatus 100 (SEM) in accordance with one embodiment of the present invention. The SEM system 100 includes an electron beam generator (101 through 112) that generates and directs an electron beam 102 substantially toward an area of interest on a specimen 113. The SEM system 100 includes an electron beam gun tip 101 for providing the electron beam to an anode 102 to create an electron field. Gun lenses 104 and 105 retain the electric field. A blanking plate 106 retains the electron beam shape. The SEM system 100 also includes an in-lens sectional detector 107 arranged to detect charged particles 111 (secondary electrons SE and/or backscattered electrons BSE) emanating from the specimen surface 113. The SEM system 100 includes deflectors 108 and 110 to deflect the electric field. The SEM system also includes a bottom seal 112 for holding the assembly. The SEM system 100 includes an objective lens 109 which provides a magnetic immersion function and an electrostatic retarding function. The SEM system 100 also includes an image generator (not shown) for forming an image from the emanated particles. The electron beam generator and sectional detector are further described below, along with other features of the SEM system 100. The landing location of these charged particles when they arrive at the detector plane is determined by their initial energy and escaping angle emanating from the specimen surface. FIG. 2 is a diagrammatic representation of electron trajectory simulation of the emanating SE from specimen surface 113 with initial trajectory condition of azimuth angle 0 degree 202 and 135 degree 203. The corresponding landing position image on the detector plane is illustrated on FIG. 3. 302 is the landing area for SE from specimen surface 113 with 0 degree azimuth angle and 303 is the landing area for SE from specimen surface 113 with 135 degree azimuth angle. FIG. 4 is a diagrammatic representation of the trajectory of SE emanating from specimen surface 113 to in-lens sectional detector 107 without any other electronic and magnetic field affection except the objective lens field. FIGS. 6 through FIG. 11 illustrate different sectional detectors samples for SEM image processing. FIG. 6 is a diagrammatic representation of a sample sectional detector with an aperture in the center. FIG. 7 is a diagrammatic representation of a sample in which a 4 segments detector forms a hole at optical axis. FIG. 8 is a diagrammatic representation of a sample in which an 8 segments detector forms a hole at optical axis. FIG. 9 is a diagrammatic representation of a sample in which one of the detector segments has a hole, which is located at optical axis. FIG. 10 is a diagrammatic representation of a sample in which some detector segments form a hole, which is located at optical axis center. FIG. 11 is a diagrammatic representation of a sample detector that does not locate at the beam optical axis. An ExB filter is utilized to guide the secondary electrons onto the off-axis sectional detector. The sectional detector is divided into at least two sections with an aperture in the center 600, 700 and 800. The size of the center aperture 601 is less than 3 mm to let the primary charged particle 102 to pass. The aperture 601 can also be located at section of the sectional detector 900 and between the boundaries of the sections of detector 1000. If the detector is located off-axis of the optical system, the aperture hole can also be removed, shown as 114 in FIG. 11, then the SE emanating from specimen surface 113 is guided to the off-axis sectional detector 114 by an ExB filter 108 as FIG. 5 illustrates. Each section of the detector collects only the secondary charged particles with particular range of the polar and azimuth angle with respect to the specimen surface 113. The SEM image generated by a particular secondary charge particle is the side-view image, which corresponds to the side-view SEM image collected by a conventional side-detector. The signal from all sections of the sectional detector can be processed to achieve a plane-view SEM image of the scanned specimen area. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.

summary

abstract

Electron-optical systems are disclosed that are especially useful in mapping-projection electron microscopy and related uses. The systems achieve high magnification with excellent control of aberrations, and low magnification at wide optical fields with excellent control of aberrations.

summary

040000370

summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear steam turbine power plant controls, and in particular, to a power plant control system which permits operation beyond the normally defined end of the fuel cycle of the reactor element. 2. Description of the Prior Art In a nuclear reactor power plant, heat energy generated within the nuclear reactor element is utilized to provide a source of pressurized dry and saturated or low superheat steam. The steam so produced drives an associated turbine system linked mechanically to an electrical generator which produces electrical energy for an associated electrical load. In general, there are two main classifications of nuclear power plants, the Boiling Water Reactor or BWR System, and the Pressurized Water Reactor, or PWR System. The Boiling Water Reactor, or BWR, conducts a water coolant through a single closed loop comprising the reactor element and the turbine system. As the water coolant passes through the reactor core, heat taken from the reactor core raises the temperature of the water coolant above the vaporization point of the coolant, thereby producing steam. The steam is then directly passed through the turbine system and the heat energy within the steam is converted into useful mechanical energy. The steam passes into a condenser element and is returned to a liquid state. The condensate, now in the form of low temperature water coolant, is reintroduced as a coolant into the reactor element, thus completing the single closed loop. However, since the water coolant and the steam produced therefrom are exposed to the highly radioactive reactor core and have undergone extensive bombardment of high energy neutron particles therein, the turbine system through which the radioactively contaminated steam has passed experiences an increase in radioactivity levels. Also, extreme care must be taken to prevent leakage of the contaminated steam or water out of the single, closed BWR loop. On the other hand, power plants employing a Pressurized Water Reactor use a double closed loop arrangement for the production of electrical energy by use of the nuclear reactor element. The first, or primary, side connects the reactor element in a closed loop with a steam generator, or heat exchanger element. The water coolant is maintained under a high pressure, approximately 2200 p.s.i. and takes heat from the reactor core as the pressurized water coolant passes therethrough. Since the water coolant is under pressure, the heat taken from the reactor does not raise the temperature of the water coolant above the vaporization temperature for water at the appropriate pressure. Instead, the heated, but not boiling, water enters the steam generator element. In the steam generator, the heat carried by the pressurized water coolant is transferred to secondary water disposed within the steam generator. The transfer of heat from the pressurized water to the secondary water sufficiently cools the pressurized water coolant to permit its reintroduction into the reactor core, thus completing the primary side loop. Within the steam generator, heat taken from the pressurized water coolant and transmitted to the secondary water therein raises the temperature of the secondary water above its boiling point, thus producing steam. However, in the case of the PWR, the steam produced within the steam generator element is not radioactively contaminated since the pressurized water coolant is physically segregated from the secondary water in the steam generator element. From the steam generator, the steam produced by the heating of the secondary water is conveyed through the second enclosed loop, called the secondary or steam side. The secondary or steam side comprises a high pressure turbine element, a low pressure turbine element, a condenser element, and a series of feedwater heater elements. Each turbine element has an inlet orifice of the predetermined size with the inlet orifice of the high pressure turbine being physically smaller than the inlet orifice of the low pressure turbine element. Steam passes through the turbine elements and converts the heat energy carried therein to mechanical energy. The expanded steam is returned to the liquid state in the condenser. The condensate is then reintroduced into the steam generator after passing through a series of feedwater heater elements to complete the second closed loop. It is evident then, that the pressure of the steam flowing in the secondary loop is directly dependent upon the amount of heat transferred from the pressurized water coolant to the secondary water in the steam generator. If the temperature of the water coolant is high, a greater amount of heat energy will be transferred to the secondary water than would occur if the primary water coolant were at a lower temperature. The heat energy transferred to the secondary water thus raises the temperature of a greater volume of secondary water above the applicable vaporization temperature of the secondary water, thus producing more steam in the secondary loop. Steam flow through the secondary or steam side loop is dependent upon the physical sizing of the high pressure turbine inlet orifice. The heat content of the primary water coolant is obviously dependent upon the heat output in the reactor core. Heat output in the reactor core is in turn dependent upon the reactivity levels within the fuel elements which comprise the core. Fuel element reactivity is dependent upon the age of the individual fuel elements. Early in the fuel element life, reactivity levels are usually above some predetermined reference point and controlled by the manipulation of a plurality of neutron absorbing control rods. If, for example, the reactivity level within the reactor element is desired to be raised, the control rods are withdrawn a predetermined distance from within the core, thus increasing the neutron level within the core, thereby increasing the reactivity level within the core. With the reactivity level within the core increased, heat is generated within the reactor and therefore the heat content of the pressurized water coolant increases. A higher coolant temperature increases the amount of heat transferred from the pressurized water coolant to the secondary water in the steam generator. The increased amount of heat transferred increases the pressure of the steam produced in the steam generator for use in the secondary or steam side. The ability to change steam pressure to change the overall electrical output of the power plant to meet the demand of the system is termed the load follow capability of the system. As is well known to those skilled in the art, the load follow capability of the system is inhibited during a xenon transient condition. The useful life, or fuel cycle, of the fuel elements utilized by the reactor is measured in terms of certain predetermined parameters. One such parameter is the amount of reactivity in excess of a certain predetermined reference level. This level of excess reactivity is above the reference level at the start of the fuel cycle, but the level of excess reactivity decreases toward the reference value as the fuel element is utilized. During the life of the fuel element, the decrease in excess reactivity is offset by the withdrawal of the neutron absorbing control rods, which permits the reactor to operate at its constant rated power levels. The point in time denominated as the end of the fuel cycle is normally defined as that time when, with the control rods fully withdrawn and the overall system at full thermal load, the excess reactivity of the reactor core is zero. When these parameters are met, the end of the fuel cycle occurs, despite the fact that some residuum reactivity remains in the fuel elements. Of course, other parameters may be used to define at what point in time the end of the fuel cycle occurs. When the reactor element operation nears the end of the fuel cycle, that is near the end of the useful life of the fuel element, even though there is still the residuum reactivity in the core, it has been observed that even total withdrawal of the control rods fails to sufficiently increase reactivity levels within the reactor to provide thermal output demanded by the electrical load. When reactivity decreases, the heat produced, or thermal output, within the core decreases. Decrease in thermal output of the reactor core due to decreased reactivity causes a concomitant lowering of water coolant temperatures. This in turn, decreases the amount of heat transfer in the steam generator. Thus, the thermal output of the steam generator is lowered, lowering the electrical output of the power plant as a whole. It is well known to those skilled in the art that the density of the pressurized water coolant is dependent upon temperature of the coolant. As the temperature of the coolant decreases, the coolant becomes more dense. At the end of the fuel cycle, fissions of atoms within the fuel element still produce the residuum of reactivity noted above, but, the level of reactivity is not sufficient to maintain the rated thermal output of the reactor. The fissions produce both high energy and lower energy "thermal" neutrons. The thermal neutrons, although only a certain percentage of the neutrons released by the fission process, are important since they are required to enable the fission chain reaction to continue. Since the number of fissions, and therefore, the number of neutrons produced, is decreased at the end of the fuel cycle, and since only a predetermined percentage of neutrons produced are thermal neutrons, the reactivity, and hence the thermal output of the reactor, decreases. However, since the pressurized water coolant becomes denser with a lower temperature, it is possible to increase the excess reactivity above the reference value by lowering coolant temperature. With coolant temperature lower, and the coolant more dense, more of the high energy neutrons produced by those fissions which provide the residuum reactivity will be slowed sufficiently to enable them to produce a fission. Thus, although the total number of neutrons released is not increased, the cooler, and therefore, denser, pressurized water coolant lowers the energy of enough high energy neutrons to provide enough thermal neutrons to sustain a fission chain reaction. In this way, the reactivity of the core is increased over the reference reactivity value, thus enabling the core to maintain its rated thermal output. The increase in excess reactivity thus offsets the loss of reactivity attendant upon the end of the fuel cycle. However, decreases in coolant temperature have a deleterious effect upon pressure of the steam in the turbine side. In order to increase excess reactivity, without diminishing steam pressure, the prior art has to either insert a new fuel element or enrich the old fuel element. However, a further alternative which can maintain the reactivity at an excess level above the reference and the thermal output at its rated level, and yet still maintain the electrical output of the entire system within a predetermined close range of values to the rated electrical output is needed. SUMMARY OF THE INVENTION This invention relates to a central system which extends the life of a fuel element of a pressurized water reactor beyond the end of a predetermined fuel cycle and to a method of improving the operation of a nuclear steam power plant having a pressurized water reactor therein. The control system extends the life of the fuel elements beyond the end of the fuel cycle without a diminution in the thermal output of the reactor and with a diminution in the electrical output of the system within only a small predetermined range of values. The control system utilizes temperature sensing means to indicate that the reactor element is unable to maintain or raise the temperature of a primary coolant to produce steam pressure sufficient to meet a load demand upon the overall system. A predetermined signal from the temperature sensing means initiates flow interruption means which provide an appropriate steam system response. The response will be to maintain the plant output as high as possible with the load of steam pressure conditions by utilization of one or more flow interruption modifications within the steam system. This permits the plant to maintain electrical output within a close range of the rated output over an extended fuel cycle. The method for improving the operation of the nuclear steam power plant having the pressurized water reactor element therein comprises the steps of increasing reactivity levels within the reactor core to maintain the reactivity level at its constant rated value, and modifying the turbine system associated with the pressurized water reactor element to compensate for the increase in reactivity levels within the reactor core. In addition to extended fuel cycle application, utilization of the control system or method taught by this invention provides an improved load follow capability for the reactor element when operating during a xenon transient condition. Although the control system and method to improve reactor operation can be utilized at any time during operation of the system, they are most advantageously employed if initiated during a xenon transient condition or at the end of the fuel cycle.

abstract

A control system, computer, method and computer program that imposes a monitoring load to the extent necessary to carry out pattern analysis on the operation of a system and failure analysis and does not apply any excessive monitoring load. The invention includes an interface for receiving an operation performance metric value of each of a plurality of first monitored items from a monitored computer, and a control section for, based on the operation performance metric value of each first monitored item, determining a second monitored item whose data should be obtained and instructing the monitored computer to obtain an operation performance metric value of the second monitored item which is associated with each first monitored item.

summary

claims

1. A method of forming tungsten tetraboride, the method comprising the steps of:combining tungsten and boron in a molar ratio of from about 1:6 to about 1:12, respectively, andfiring the combined tungsten and boron in a hexagonal boron nitride crucible at a temperature of from about 1600 C to about 2000 C, to form tungsten tetraboride wherein the tungsten and the boron are combined with carbon in the crucible. 2. The method of claim 1, wherein the molar ratio is one of about 1:9 or 1:6. 3. The method of claim 1, wherein the temperature is about 1800 C. 4. The method of claim 1, wherein the firing is accomplished at about one atmosphere. 5. The method of claim 1, wherein the firing is accomplished in an argon environment. 6. The method of claim 1, wherein the tungsten is provided as tungsten oxide. 7. The method of claim 1, wherein the boron is provided as boric acid. 8. The method of claim 1, wherein the tungsten is provided as tungsten metal. 9. The method of claim 1, wherein the boron is provided as boron metal. 10. The method of claim 1, wherein the boron is provided as 10B enriched boron. 11. The method of claim 1, further comprising milling the tungsten tetraboride to a powder. 12. The method of claim 1, further comprising milling the tungsten tetraboride to a powder and compressing the powder into a desired shape. 13. The method of claim 1, further comprising milling the tungsten tetraboride to a powder, compressing the powder into a desired shape, and sintering the desired shape. 14. A method of forming tungsten tetraboride into a desired shape, the method comprising the steps of:combining tungsten and boron in a molar ratio of from about 1:6 to about 1:12, respectively,firing the combined tungsten and boron in a hexagonal boron nitride crucible at a temperature of from about 1600 C to about 2000 C, to form tungsten tetraboride,milling the tungsten tetraboride to a powder,compressing the powder into a desired shape, andsintering the desired shape wherein the tungsten and the boron are combined with carbon in the crucible. 15. The method of claim 14, wherein the boron is provided as 10B enriched boron. 16. The method of claim 14, wherein the firing is accomplished in one of an argon environment or a vacuum environment, and the sintering is accomplished using spark plasma sintering in one of an argon environment or a vacuum environment. 17. A method of forming tungsten tetraboride into a fission reactor shield, the method comprising the steps of:combining tungsten and 10B enriched boron in a molar ratio of from about 1:6 to about 1:12, respectively,firing the combined tungsten and 10B boron in a hexagonal boron nitride crucible at a temperature of from about 1600 C to about 2000 C, to form tungsten tetraboride,milling the tungsten tetraboride to a powder,compressing the powder into a desired shape of the fission reactor shield, andsintering the fission reactor shield wherein the tungsten and the boron are combined with carbon in the crucible. 18. The method of claim 17, wherein the boron is provided as 10B enriched boric acid. 19. The method of claim 17, wherein the firing is accomplished in one of an argon environment or a vacuum environment, and the sintering is accomplished using spark plasma sintering in one of an argon environment or a vacuum environment.

047770120

summary

The invention concerns a gas cooled high temperature reactor charge with spherical fuel elements with a reflector surrounding the pile of fuel elements on all sides with a hot gas collector compartment adjoining the bottom part (bottom reflector) of said reflector consisting of graphite columns, the hot gas collector compartment being bounded in the downward direction by the bottom layers of graphite blocks of the high temperature reactor with a charging means and a removal means for the spherical fuel elements wherein the removal means comprises at least one ceramic pebble removal tube passing through the bottom reflector and the bottom layers and a metal pebble removal tube ajoining the ceramic pebble removal tube underneath the bottom layers. High temperature reactors having a core formed by a pile of spherical fuel elements that are continuously added to the core and removed after their burnup from the core are part of the state-of-the-art. Nuclear reactors of this type have very high gas temperatures in their lower parts, wherefore, only high heat resistant materials may be used in the bottom reflector, the bottom layers and the part of the pebble removal installation located within the core structure. These materials are thus made of a ceramic material, such as graphite, but graphite cannot be exposed to high tensile and bending stresses. The forces of the reactor core are thus transmitted to a thermal shield made of a metal, preferably a cast metal, which is surrounding the entire reflector. In the removal installation for the spherical fuel elements only the part of the removal tubes located within the space enclosed by the thermal shield is made of a ceramic material. The continuing pebble removal tubes connecting the ceramic removal tubes with a scrap separator are made of metal for the sake of higher strength. It has now been found that even the metal pebble removal tubes are exposed to high temperatures in the areas adjoining the ceramic removal tubes. This is the result of the fact that the fuel elements standing in front of the ceramic removal tubes and located in them are not completely burned up and are thus still generating heat. The temperature in the above-described area may thus be reduced only by eliminating or limiting the flow of neutrons in the ceramic pebble removal tubes. It is known in the prior art (West German Offenlegungsshrift No. 23 47 817) to provide neutron absorbing materials in a nuclear reactor with spherical fuel elements and a single passage of the fuel elements through the reactor core in the roof reflector and/or the upper part of the side reflector in order to reduce the flow of fast neutrons in the areas exposed to doses of radiation of the reflector, thereby preventing damage to the graphite of the reflector. There is also known a process for the operation of a high temperature reactor of the above-described structural type (West German Offenlegungsschrift No. 22 41 873) whereby the output distribution in the reactor core may be affected so that the axial output density declines only slightly in the downward direction. Such a mode of operation permits a higher load on the fuel elements which is utilized either for an improvement in the economy of the nuclear reactor or to increase its safety. A series of measures have been proposed to obtain this result, among others the addition of combustible poisons to the loading charge, whereby the generation of power output is transferred to the lower reaches of the reactor core. The combustible poisons may also be provided in the roof, bottom and/or side reflectors. It is further known (West German Offenlegungsschrift No. 23 65 531) to improve the shutdown efficiency of absorber rods directly insertable in a nuclear reactor with spherical fuel elements and directly insertable absorber rods by a special measure without increasing the number of absorber rods or their depth of insertion. This measure consists of doping the graphite of the bottom reflector and the lower part of the side reflector with neutron absorbing substances. This thus represents a quasi-homogeneous poisoning of the lower range of the reflector. It has the disadvantage that the neutron absorbing substances are burning up in a very short period of time and their effectiveness is, therefore, inadequate for the life time of a nuclear reactor estimated at approximately 30 to 40 years. Based on this state-of-the-art, it is the object of the invention to design a gas cooled high temperature reactor of the above-described type so that the temperature is sufficiently lowered from the core bottom to the metal pebble removal tubes so that the thermal exposure of the last-mentioned removal tubes remains within permissible limits. This object is attained according to the invention by that each ceramic pebble removal tube within the area of the bottom reflector and the hot gas collector compartment is surrounded directly by a ring of graphite columns, wherein a plurality of recesses, a number of boron bodies is arranged. The boron bodies reduce the neutron flux in the area affected to the extent that there is no more output by the fuel elements. A further number of boron bodies may be provided within the area of the bottom layers of the high temperature reactor to reinforce the absorption of neutrons. The boron bodies are arranged preferably directly outside each ceramic pebble removal tube. As a solution extremely favorable from an economic standpoint, the boron bodies are designed in the graphite columns in the form of rods and inserted into vertical bores adapted to their shape and size. Advantageously, the length of the boron rods is determined so that the graphite columns are almost entirely penetrated by the boron rods. Preferably, the boron rods are arranged in at least two rows around each ceramic pebble removal tube and the boron rods belonging to different rows are staggered so that they always face a gap. The boron bodies located in the area of the bottom layers may advantageously consist of solid plates arranged so that each ceramic pebble removal tube in this area is completely or almost completely surrounded by them. The form of the boron bodies chosen and the mode of their location effect a gradual decline of the temperature in the direction of the metal pebble removal tubes, so that no or only slight thermal stresses are generated in the structural parts of the high temperature reactor. Furthermore, the boron rods and boron plates are readily built into the corresponding graphite structural parts (graphite columns or bottom layers). The realization of the invention thus does not require a substantial effort.

summary

abstract

The invention comprises a method and apparatus for determining a radiation beam treatment path to a tumor, comprising the steps of: (1) delivering charged particles from an accelerator, along a first beam transport path, through an output nozzle, and along a treatment path to the tumor relative to a calibrated reference beam path from the output nozzle toward a patient position and (2) prior to the step of delivering, a main controller verifying an unobstructed linear path of the treatment path using a set of fiducial indicators positioned at least: on a first element physically affixed and co-movable with the output nozzle and on a moveable object in the treatment room. Optionally, voxels of the treatment beam path and potentially obstructing objects are defined in the treatment room using an axis system relative to the calibrated reference beam path and a reference beam point.

summary

description

The invention concerns generally the technology of irradiating a sample with X-rays for producing fluorescence. Especially the invention concerns the task of selectively irradiating a desired small target area in the sample. X-ray fluorescence (XRF) is an analysis method in which incident X-rays irradiate a sample, causing sample atoms to emit fluorescent radiation at characteristic wavelengths. By analysing the intensity spectrum of the received fluorescent radiation it is possible to deduce the material composition of the sample. The X-ray source used in an XRF analyzer device is typically an X-ray tube. Due to the structure of an X-ray tube, it usually emits X-rays into a half-spherical (2*pi steradians) spatial angle from an area of the anode referred to as the focal spot. If the sample is small and/or if only a small part of the sample is to be investigated, a need arises for collimating the incident X-rays so that they only fall upon the desired target area. Conventional X-ray collimators were made by stacking metal plates parallel to each other, leaving narrow slits between them for only radiation propagating in the desired direction to pass through. The stack of parallel metal plates must have a certain length, for example in the order of 5 cm, in the propagation direction of the X-rays in order to achieve the desired angular selectivity. This makes them ill suited for portable XRF analyzer units, where space is scarce. It is also difficult to use the metal plate stack principle for reconfigurable applications, where the angular selectivity, irradiation spot size or some similar parameter should be changed quickly and easily. An objective of the present invention is to present a collimator arrangement for collimating incident X-rays onto a desired target area in a sample. Another objective of the invention is to present an X-ray fluorescence analyzer device utilizing such a collimator arrangement. A further objective of the invention is to minimize the space required for collimating incident X-rays in an X-ray analyzer device. A yet further objective of the invention is to present an arrangement for changing the parameters of a collimating arrangement in an X-ray fluorescence analyzer quickly and easily. The objectives of the invention are achieved by using a thin plate with microscopic pores therethrough to realize angular selectivity, and a layer opaque to X-rays with an opening to only let through incident radiation to the desired target area. A collimator arrangement according to the invention comprises: a collimator plate with a plurality of pores that penetrate through at least an essential part of the thickness of the collimator plate and that have a diameter smaller than 100 micrometers, and an annular plate comprising material essentially opaque to X-rays, said annular plate defining an area transparent to X-rays;wherein said collimator plate and said annular plate are adapted to be placed in said X-ray fluorescence analyzer device between an X-ray source and a sample, and wherein said plurality of pores in said collimator plate are adapted to let through at least a part of X-rays radiated by said X-ray source, and wherein edges of said area transparent to X-rays in said annular plate are adapted to spatially limit in a transverse direction a beam of X-rays radiated by said X-ray source. The invention concerns also an X-ray fluorescence analyzer device, which comprises: an X-ray source, a sample window to be placed adjacent to a sample, and between the X-ray source and the sample window a collimator plate with a plurality of pores that penetrate through at least an essential part of the thickness of the collimator plate and that have a diameter smaller than 100 micrometers, and an annular plate comprising material essentially opaque to X-rays, said annular plate defining an area transparent to X-rays;wherein said plurality of pores in said collimator plate are adapted to let through at least a part of X-rays radiated by said X-ray source towards said sample window, and wherein edges of said area transparent to X-rays in said annular plate are adapted to spatially limit in a transverse direction a beam of X-rays radiated by said X-ray source towards said sample window. A capillary tube has a strong collimating effect on X-rays, because X-rays only reflect at very shallow incident angles, and thus only rays the propagating directions of which are within a small range around the longitudinal axis of the capillary tube will pass through. Capillary tubes in macroscopic scale share some of the clumsiness and other drawbacks of metal plate stacks as collimators. Additionally using a long macroscopic capillary tube as a collimator means that fluctuations in focal spot location and spatial intensity become clearly visible in the characteristics of the collimated incident X-ray beam. It is possible to produce a large number of microscopic pores through a plate having a thickness in the order from less than a millimetre to a few millimetres. A piece of such plate placed in front of an X-ray source will effectively implement accurate angular selectivity in a relatively small space. A plate with microscopic pores having the required aspect ratio can be made through etching from a semiconductor wafer. Another possibility is to fuse together a large number of glass fibres with a suitably selected core, cut a plate from the fused bunch of fibres and etch away the fibre cores. An annular plate made of a material that is opaque to X-rays will only let those rays pass that go through the opening in the plate. Together with a collimator plate with microscopic pores such an annular plate will produce a collimator that only allows a strictly selected, highly collimated portion of incident radiation to irradiate the sample at a desired location. The annular plate can be built as an integral part of the plate with microscopic pores, or the two plates may be structurally different parts. At least one of them may be located in a movable frame, with which it is possible to move the collimator plate and/or the annular plate between a storage position and an operating position in front of the X-ray source. The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. FIG. 1 illustrates schematically certain parts of an X-ray fluorescence analyzer device. An X-ray tube 101, which here is schematically shown as being of the side window type but which could also be of the end window type, acts as the source of incident X-rays. The incident X-rays are directed through a plate 102 that comprises a large number of microscopic pores therethrough. The microscopic pores in the plate 102 have a collimating effect, so that only X-rays the propagation direction of which is within a relatively narrow range around the longitudinal direction of the microscopic pores pass through the plate 102. A holder 103 keeps the plate 102 in place. Those incident X-rays that pass through the plate 102 meet next an annular plate 104 made of a material that is generally opaque to X-rays. The opening in the annular plate 104 lets through those X-rays that hit said opening. There is a holder 105 to keep the annular plate 104 in place. A sample 106 is shown, from which fluorescent and/or scattered X-rays go to a detection arrangement (not shown in FIG. 1). At least two principal approaches are possible for producing the plate 102 with the microscopic pores. We will first consider the possibility of using a glass capillary plate. Glass capillary plates have conventionally been used as an image intensifiers, i.e. analog amplifying components in detecting charged particles or electro-magnetic radiation. The conventional designation of a glass capillary plate used in such applications is “microchannel plate”. It consists of a glass plate with a periodic array of microscopic holes therethrough. The thickness of the glass plate is usually slightly less or slightly more than one millimeter, and a typical diameter of the holes is in the order of about ten micrometers. Thus each hole constitutes a channel through the glass plate, with an aspect ratio of typically about 100, although large deviations from these exemplary values are possible. For use as an image intensifier, the walls of the channels have been treated so that they enable the easy emission of photoelectrons and an avalanche-like multiplication of emitted electrons under the influence of an electric field between electrode metallizations on the top and bottom surfaces of the plate. According to an aspect of the present invention, it is possible to use a glass capillary plate as such as an X-ray collimator. However, the transmission efficiency at acceptable incoming angles becomes much better, if the channel walls of a glass capillary plate are treated to act like X-ray mirrors, so that they reflect incoming X-rays instead of causing photoelectric emission. Thus each channel in the glass capillary plate acts as a miniature waveguide that exhibits high transmissivity of X-rays at a relatively narrow range of acceptable input angles around the nominal channel direction. A suitable treatment is the plating of the channel walls with a layer of a metal such as iridium, ruthenium, platinum, osmium or nickel, having a thickness of a few nanometers. An exemplary method for applying such a treatment is ALD (Atomic Layer Deposition). FIG. 2 illustrates the principle of plating the channels of a glass capillary plate so that it becomes a better collimator for X-rays. On the left is a schematic cross section of a small portion of an ordinary glass capillary plate, of which a large part of the middle section has been omitted in order to add graphical clarity. In reality the channel length would be dozens of times larger than the channel width. The body 201 of the glass capillary plate consists of lead oxide glass or other material that is suitable for the manufacturing process. A conventional microchannel plate also comprises a top electrode layer 202 and a bottom electrode layer 203 usually made of chromium and/or nickel alloys. On the right in FIG. 2 is a schematic cross section of a small portion of a glass capillary plate for use as an effective X-ray collimator. A very thin conformal coating 210 has been added. The thickness of the coating 210 is preferably in the order of nanometers, like 5 nanometers. In the drawing it has been vastly exaggerated: taken that the channel width is in the order of several micrometers, drawn to scale the coating 210 would be hardly visible in the drawing. Whether or not the coating 210 also covers the top and bottom surfaces of the glass capillary plate is immaterial to the present invention. It is much more important that the coating 210 covers the walls of the channels and has as smooth a surface as possible. The smoothness requirement is one reason for not making the coating 210 thicker than a few nanometers, since the thicker the layer, the more easily its surface becomes uneven. Another reason for the small thickness of the coating 210 on the walls of the channels is that unnecessarily decreasing the channel cross-section will just reduce the transmission ratio of X-rays. A number of important criteria are set to the material used for the coating 210. The material should have a high atomic ordinal number in order to reflect X-rays as effectively as possible. The material should be well suited for application as very thin conformal layers, using atomic layer deposition (ALD) or other suitable coating method. Additionally it is advantageous if the material of the coating 210 does not have characteristic X-ray fluorescence peaks that could be easily confused with those of analysed materials in the target. The most suitable material for the coating 210 is believed to be iridium. Other suitable materials include but are not limited to ruthenium, osmium and nickel, of which at least the last-mentioned is more suitable for application through wet chemistry than ALD. Platinum and gold are known to be applicable as X-ray mirror materials, but they may have other disadvantages that may make them a suboptimal choice for the material of the coating 210. FIG. 3 illustrates an X-ray 301 that enters a channel 302 in a glass capillary plate at an incident angle a that is greater than zero (here incident angle is defined as the angle between the axial direction of the channel and the propagation direction of the X-ray). If the incident angle was zero, the X-ray 301 would pass straight through the channel, assuming that it did enter the channel in the first place and did not hit some of the solid parts of the front surface of the glass capillary plate, in which case it would become absorbed. Let us first assume that the channel walls have not been plated. In that case there is some relatively small limiting value for the angle a, so that at incident angles larger than the limiting value the X-ray could not pass directly through the channel but would hit the wall instead. An unplated glass wall is not a very good reflector for X-rays, but would be very likely to cause absorption. Depicting transmittivity as a function of incident angle would give something like the qualitative curve 401 in FIG. 4. Let us now assume that the channel walls have been plated in accordance with an aspect of the present invention. The plating allows the obliquely entering X-rays to reflect once or several times on their way through the channel, which in terms of transmittivity as a function of incident angle gives the qualitative curve 402 of FIG. 4. At a zero incident angle the transmittivity curve 402 is slightly lower than the curve 401, because the plating causes a small decrease in channel diameter and correspondingly increases the possibility of a directly coming X-ray to miss the channel and hit the front surface of the glass capillary plate instead. However, taken that the thickness of the plating is easily less than one thousandth of the channel diameter, this decrease in zero-angle transmittivity is almost infinitesimally small, and only exaggerated in FIG. 4 for clarity. On the other hand, due to the highly increased reflectivity of the channel walls, transmittivity experiences a large increase at larger values of the incident angle. The overall transmittivity of a collimator is proportional to an integral of the area that is left under the transmittivity curve in FIG. 4. The increased overall transmittivity due to better transmittivity at larger incident angles is shown with a simple hatch, while the small decrease in small-incident-angle transmittivity is shown with a cross hatch. The applicability of a glass capillary plate with plated channel walls as a collimator comes from the fact that a collimator can well have a certain allowance function of finite width around the nominal propagating direction that should pass directly through the collimator, as long as the maximum deviation a1 from the zero incident angle, at which radiation will still pass, is not so large that it would cause serious degradation in the required degree of collimating. How wide the allowance function can be, i.e. how much a propagating direction is allowed to differ from the nominal propagating direction and still be accepted to pass the collimator, depends on the application for which the collimator is used. According to the invention, it is easy to design and manufacture collimators with differently dimensioned allowance functions by simply selecting the aspect ratio of the glass capillary plate, i.e. by selecting a suitable plate thickness (typically between 0.4 and 3 millimeters) and channel width (typically between 5 and 15 micrometers). Also the material selected for the plating of the channel walls, and the resulting degree of reflectivity of the channel walls, is a parameter to be considered when the maximum allowable value of a1 is decided. It is expected that the increase in the allowance function width will in any cases be less than two degrees compared to the allowance function of a correspondingly dimensioned glass capillary plate with unplated channels. X-ray reflection at grazing incidence is known to be non-dispersive. This means that the collimator according to the embodiment of the invention described above does not significantly change the spectrum characteristics of the incident radiation of an X-ray fluorescence analyzer device. FIG. 5 illustrates schematically certain method steps that aim at manufacturing an X-ray collimator according to an embodiment of the invention. The process may contain some previous steps of unspecified nature, illustrated as 501. In step 502 at least the body of a glass capillary plate is produced. In a typical manufacturing process of glass capillary plates, a rod of etchable core glass is used as a support for a hollow billet of lead oxide cladding glass. A composite fibre is pulled from the combination. A number of these first draw fibres are stacked into an array, which is drawn again to produce a so-called multifiber. Several multifibres are stacked and fused together under vacuum, which results in a thick rod-like product known as a boule. The boule is sliced and polished to the required thickness and outline of the desired glass capillary plates. The solid cores, which at this stage still perforate the plate, are etched away, thus producing the characteristic array of microscopic holes through the plate. A complete manufacturing process of conventional microchannel plates involves firing the plates in a hydrogen oven to produce a semiconducting surface layer with the de-sired resistance and secondary electron yield, as well as producing the top and bottom electrode layers. For the purposes of the present invention these are un-necessary steps and can be left out. However, they do not cause much change either to the operation of the glass capillary plate as an X-ray collimator, so concerning the present invention it is immaterial, whether step 502 of the manufacturing process includes the hydrogen firing and electrode producing sub-steps or not. Step 503 involves plating the channel walls with the thin coating reflective of X-rays, for example in an ALD process. Other method steps may follow after that as is illustrated as 504. A microchannel plate meant for use as a particle detector or image intensifier has often a so-called nonzero bias angle, which means that the channels are not perpendicular to the planar surfaces of the plate. The bias angle is selected in step 502 mentioned above, by tilting the blade that is used to cut slices from the boule (or by tilting the boule with respect to the blade). If a glass capillary plate with plated channel walls is to be used as a collimator according to an embodiment of the invention, it should either have a zero bias angle, or the glass capillary plate should be placed at a non-perpendicular angle with respect to the desired propagation direction of X-rays, so that the channel direction coincides with the desired propagation direction of X-rays. The second principal approach for producing the plate 102 with the microscopic pores in FIG. 1 is to use a semiconductor wafer or a corresponding piece of etchable solid material as the starting point. From prior art there is known a general principle of using an anisotropically etched silicon semiconductor plate as the collimator for the scattered X-rays or fluorescent X-rays somewhere along the radiation path from the sample to a detector. A prior art publication U.S. Pat. No. 6,477,226 suggests anisotropically etching deep pores of diameter between 0.1 and 100 micrometers to a semiconductor plate. The publication suggests either leaving a thin layer of the semiconductor material intact at the bottom of the pores, or lining the inside of each pore with a so-called stabilizing layer made of silicon nitride, silicon carbide, boron nitride, boron hydride, boron carbide and/or carbon, after which the remaining thin semiconductor layer could be removed, which leaves the previously closed end of each pore visible but covered with the stabilizing layer material. The materials that said publication suggests for the stabilizing layer have the drawback that they do not function as X-ray mirrors. The performance of a collimator plate made of a semiconductor material and having a plurality of micropores anisotropically etched therethrough could be remarkably improved by using an additional coating of iridium, ruthenium, osmium, platinum, nickel, or gold either instead of the so-called stabilizing layer of said prior art publication or on top of it. We should also note that silicon is relatively transparent to X-rays, which means that the silicon-based collimator plates known from prior art would hardly be suitable for collimating incident radiation. The intensity of the radiation produced by the X-ray source is simply so high that a significant portion of the rays propagating in other directions than those that a collimator should let through would also penetrate through the solid silicon parts that constitute the structural matrix of the plate. However, germanium is a substrate that lends itself easily to anisotropic etching, and that is relatively easily available in wafer thicknesses suitable for collimator plates. Germanium is also very seldom among those elements that should be analysed from a sample, so it is unlikely that excited spectral lines or scattered radiation from germanium would interfere with the measurement. In the following we will use the designation “collimator plate” to mean either a glass capillary plate based collimator plate or a semiconductor (preferably germanium) based collimator plate with a plurality of (preferably iridium-coated) microscopic pores therethrough. FIG. 6 is a cross section that shows how an assembly of collimator plates with differently directed channels therethrough can be used as a combined collimator and focusing lens for X-rays. We assume that radiation comes from top to down. The upper row of collimator plate sections has a central plate 603 with zero bias angle. On each side of it there are plates 602 and 604, with small bias angles of equal absolute magnitude but opposite sign. The utmost plates 601 and 605 constitute a similar pair, having bias angles of equal absolute magnitude but opposite sign, said magnitude being slightly larger than that of plates 602 and 604. In the lower row the same principal arrangement is repeated, with the central plate 613 having zero bias angle, the intermediate plates 612 and 614 constituting an equal-magnitude and opposite-sign pair and the utmost plates 611 and 615 likewise. The increasing steps in the absolute value of the bias angle are larger in the lower row, with plates 611 and 615 having the largest absolute bias angle magnitude in the whole assembly. X-rays that pass through the collimator plate assembly of FIG. 6 from top to down will experience certain convergence that directs them at least approximately towards a point or area of convergence located further down. Similar lens-like effects are achieved if the manufacturing process of the collimator plate allows the bias angle to be gradually changed across the plate like in the plate 701 of FIG. 7, or if a collimator plate with initially parallel channels is afterwards made to exhibit some curvature like plate 801 in FIG. 8. Collimator plates and collimator plate arrangements that include a focusing characteristic, like the ones shown in FIGS. 6, 7, and 8, can be said to be special cases of the concept “collimator”, because they still act as pure collimators at least for those X-rays that pass through the central region (or more generally: the region where the channel direction is the same as the incident direction of those X-rays that should pass), and because they only allow X-rays with incident propagating directions within the (relatively narrow) allowance function to pass. Thus also the embodiments shown in FIGS. 6, 7, and 8 fall within the scope of the claims directed to a collimator. It should be noted that for reasons of graphical clarity, the differences in bias angle and the curvature of the collimator plate in FIG. 8 have been exaggerated. Two consecutive collimator plates can be used to implement a variable-attenuation X-ray attenuator that also has a collimating effect. This requires there to be a possibility of causing relative movement between the two collimator plates, so that in one relative position the channel direction in both collimator plates is essentially the same and in some other relative position the channel direction in the collimator plates is not the same. In practice this principle can be implemented for example in the way shown in FIG. 9. A first collimator plate 901 and a second collimator plate 902 have an equal, non-zero bias angle. Both collimator plates are assumed to be round, but are shown in FIG. 9 schematically as cut through to illustrate the concept of channel direction. The planar direction of both collimator plates is the same. At least one of them, here the second collimator plate 902, can be rotated around an axis 903 that is perpendicular to said planar direction of the collimator plates. In one rotational position, shown in the upper part of FIG. 9, the channel direction in the second collimator plate 902 differs so much from the channel direction in the first collimator plate 901 that at least a majority of X-rays that pass through the first collimator plate 901 cannot pass through the second collimator plate 902, due to the difference in channel direction between the collimator plates. In another rotational position, shown in the lower part of FIG. 9, the channel direction in the second collimator plate 902 is essentially the same as the channel direction in the first collimator plate 901 that at least a majority of X-rays that pass through the first collimator plate 901 also pass through the second collimator plate 902. Since the transmittivity of both plates as a function of incident angle has a finite width around the nominal channel direction, between the position shown in the upper and lower parts of FIG. 9 there are a number of intermediate positions in which the relative amount of X-rays coming through varies between these extremities. A collimator plate alone does not limit the cross section of the incident radiation beam. According to an aspect of the invention, an annular plate or layer can be used together with the collimator plate to achieve the desired incident radiation beam with a limited width. Various ways exist for combining the use of a collimator plate and an annular plate or layer. FIG. 10 is a cross section of a plate 1000, a middle section 1001 of which comprises a number of microscopic pores through the plate, with their walls preferably coated with X-ray-reflecting material. The edge sections 1002 of the plate 1000 are without said microscopic pores, so that the body of the plate 1000 also constitutes the annular plate or layer referred to above. Since the middle section 1001 is not actually an opening in the same sense as in the other annular plates discussed in this description, we may generalize the definition of an annular plate so that it defines an area transparent to X-rays, which in true annular plates is an opening and in combined embodiments like that of FIG. 10 is the section that comprises the microscopic pores. FIG. 11 illustrates another alternative embodiment, in which a collimator plate 1100 has a (relatively wide) middle section 1101 with pores and only a narrow edge section 1102 without pores, for making it easier to attach the collimator plate 1100 to a holder (not shown). It would not be necessary to have any edge section at all. On one surface of the collimator plate 1100 there is an annular layer 1103 of a material essentially opaque to X-rays. At its center the annular layer 1103 defines an opening 1104. The dimensions of the opening 1104 define, how wide is the collimated X-ray beam that will come through the collimator plate 1100. The annular layer 1103 may be formed as an integral layer of the collimator plate 1100, for example by applying a suitable annular metallization pattern of sufficient thickness to one side of the collimator plate. Alternatively the annular layer 1103 may originally be a mechanically different component that is just stacked together with the collimator plate and possibly attached thereto by glueing, soldering or other means. FIG. 12 illustrates yet another alternative embodiment, in which the collimator plate 1200 and an annular plate 1210 are separate mechanical entities and can be separately attached to corresponding holders (not shown). A section 1201 of the collimator plate 1200 is equipped with microscopic pores, with their walls preferably coated with X-ray-reflecting material. The collimator plate 1200 may have a solid edge section 1202 for making attachment to holder easier, but this is not necessary. The body of the annular plate 1210 constitutes an annular barrier 1213 essentially opaque to X-rays, and defines an opening 1214 for the X-rays to come through. Together the collimator plate 1200 and the annular plate 1210 are configured to both collimate and spatially limit an incident X-ray beam. Since not all X-ray fluorescence measurements require a well collimated and spatially limited incident X-ray beam, it is in many cases advantageous to equip an X-ray fluorescence analyzer device with a mechanism that allows easily selecting into use the incident-beam-shaping means that are needed. FIG. 13 illustrates schematically certain parts of an X-ray fluorescence analyzer device, in which an X-ray tube 101 of the end window type acts as the source of incident X-rays. Within the space between the X-ray source and a sample window 1301 of the analyzer device there are located a first movable holder 1302 and a second movable holder 1303. The first movable holder 1302 comprises at least one location in which resides a collimator plate 1304. The form of the first movable holder 1302 shown here (a circular plate rotatable around its central normal axis, and with circular openings for attaching collimator plates and/or other equipment) is naturally only exemplary. From the technology of X-ray fluorescence analyzers it is known to use movable holders of various kinds for selectively placing e.g. exchangeable filters to the path of the incident X-rays. The other circular openings of the first movable holder 1302 may comprise such filters. The second movable holder 1303 is configured to offer into use various annular openings so that the transverse dimensions of the incident X-ray beam can be limited to the desired size. This can be accomplished either so that there is one or more attachment locations in the second movable holders into which the user may removably attach separate annular plates according to need, or like in FIG. 13 in which the second movable holder 1303 itself constitutes the annular plate by having a number of differently dimensioned openings therethrough. If the X-ray fluorescence analyzer only comprises a single movable holder, it is most advantageous to use a combined structure of a collimator plate and an annular layer like that shown in FIG. 11 and to attach it to a suitable location in the movable holder. Alternatively, especially in the absence of a second movable holder, a changeable annular plate can be constructed from a number of partially overlapping, radially movable parts in the same way as the controllable aperture is implemented in a systems camera. The invention allows incident X-rays to be directed to a relatively small area in the sample. The irradiated sample area has essentially the same form and dimensions as the opening in the annular plate. A natural exception occurs if the incident X-ray beam comes at an oblique angle to the sample, in which case the irradiated sample area is the area the projection of which, looked from the direction of the incident X-rays, has the form and dimensions of the opening in the annular plate. The invention does not limit the selection of the form and dimensions of the opening in the annular plate. In exemplary cases the invention could be used to only irradiate a spot-like sample area with transverse dimensions in the order of a millimeter or a couple of millimeters with incident X-rays. In order to help a human user to perceive, what part of the sample will be subjected to incident X-rays, it is advantageous if the X-ray fluorescence analyzer device includes an optical aiming aid. FIG. 14 illustrates schematically certain considerations related to optical aiming aids. The X-ray fluorescence analyzer device of FIG. 14 comprises a fiber optic sight generally designated as 1401, with an objective 1402 aimed to look at the sample 106 and an ocular or display 1403 for conveying a visual image to a human user. Using the fixed structures of the X-ray fluorescence analyzer device to aim and attach the objective 1402 accurately enough may as such be enough for ensuring that the user gets a good perception of which area of the target will be irradiated with incident X-rays. In addition the X-ray fluorescence analyzer device of FIG. 14 comprises a light source 1404 and a mirror 1405, which may be either a dichroic mirror that reflects visible light but lets X-rays through, or a movable mirror that can be moved to the location seen in FIG. 14 for the duration of optical aiming. Together the light source 1404 and the mirror 1405 produce a beam of visible light, which comes from the same direction and through the same collimating and spatial limiting means as the beam of incident X-rays. Thus the sample area illuminated with visible light from the light source 1404 will be exactly the same as the sample area irradiated with incident X-rays from the X-ray tube 101. The user sees, which part of the sample is illuminated, through the fiber optic sight 1401. Illumination of the appropriate sample area with visible light can be accomplished also with other kinds of arrangements, for example with a fiber optic illumination system either integrated with the fiber optic sight 1401 or built separately.

summary

abstract

A method of manufacturing nuclear fuel elements may include: forming a base portion of the fuel element by depositing a powdered matrix material including a mixture of a graphite material and a fibrous material; depositing particles on the base portion in a predetermined pattern to form a first particle layer, by controlling the position of each particle in the first particle layer; depositing the matrix material on the first particle layer to form a first matrix layer; depositing particles on the first matrix layer in a predetermined pattern to form a second particle layer by controlling positions of each particle in the second particle layer; depositing the matrix material on the second particle layer to form a second matrix layer; and forming a cap portion of the fuel pebble by depositing the matrix material. The particles in the first particle layer and the second particle layer include nuclear fuel particles.

040574679

abstract

An integrated pressurized-water reactor and steam generator is disclosed, particularly for ship propulsion and smaller stationary installations, in which circulating pumps of primary coolant loops are arranged in the upper part of a vertical pressure vessel close to the vessel's removable head. An annular steam generator is arranged centrally within the vessel, and above the outer periphery of a reactor core in the vessel's lower portion, and is suspended via a separate hollow ring supported by the vessel's upper portion and in which the impellers of the circulating pumps are arranged. The primary cooling water flows upwardly through the reactor core and the heat exchanger to the pumps located above, and from there, through a separate ring passage outside of the heat exchanger housing and core downward to again flow upwardly through the reactor core. Secondary steam and primary feed water nozzles can be disassembled sideways, so that although normally connecting with internal parts, all internals can be removed upwardly from the pressure vessel so that the pressure vessel can be checked everywhere. In the event of a failure of the circulating pumps, the afterheat given off by the reactor core after being shut down, is transmitted to the heat exchanger through natural thermal circulation of the water in the vessel.

039742695

summary

RELATED APPLICATION This application relates to improvements in the methods of detection disclosed in copending and commonly assigned patent application Ser. No. 385,863 filed on Aug. 6, 1973, the disclosure of which is hereby incorporated by reference. FIELD OF INVENTION This invention relates generally to methods for screening large number of persons for current or past gonorrhea infection. Gonorrhea is one of the most commonly reported bacterial diseases in man and its persistence as a major health problem has intensified the search for new and better methods of detection. The present mass screening method is a bacteriological method which requires two to seven days for completion. Moreover, it requires that a specimen of the gonorrhea caused discharge arrive at the testing laboratory with the fragile gonococcus organism still viable, a natural time limit of as little as two days. In the above identified patent application, a novel serological method for detecting antibodies in sera is described. The method is generally based on the discovery and isolation of a heat labile antigen produced by Neisseria gonorrhoeae (N.g.) organisms. This antigen does not react with cross reacting antibodies which may also be present in the sera, with the result that the number of false positive reactions which have reduced the value of previously employed serological procedures is substantially reduced. In accordance with the several detection methods described and claimed in the application, the antigens are caused to react with the antibody and the presence of the resulting complex is determined. In the principal methods described, the complex is caused to react with a labelled anti-human immunoglobulin G. The immunoglobulin is labelled with a fluorescent compound, a radioactive element, or an enzyme. The label is then detected by suitable procedures such as radioactive counting. The optimum sources of the antigen are growth cultures of Neisseria gonorrhoeae ATCC 21823 (B-585), 21824 (B-370) and 21825 (B-1094), although a number of N.g. microorganisms will also serve as antigen sources. Order: Eubacteriales Family: Neissericeae Genus: Neisseria Species: Gonorrhoeae Morphology: Gram negative spherical or bean shaped diplococci with adjacent sides flattened usually 0.6 .times. 1.0 .mu. and more uniform in size. Biochemical and Cultural: Aerobic, optimal growth requires 4 - 10% CO.sub.2 and incubation at 36.degree.C. The cultures grow slowly on chocolate agar producing small barely visible colonies after 24 hours (0.1 mm in diameter) with typical morphology seen on 48-72 hours cultures. The colonies are small 1.0 mm in diameter, gray white, transparent, smooth, with round entire edge, glistening surface and butyrous consistency. B-1094 produced slightly larger colonies and grows more rapidly. Oxidase +, catalase +; ferments glucose but not maltose, lactose or sucrose. Antigenicity: All three isolates share common antigens which is heat labile "L". Virulence: All three strains were originally isolated from patients with symptomatic gonorrhea. A novel radioimmunoassay technique has now been discovered which makes possible the detection of the antigen-antibody conjugate in a facile manner.

053032730

abstract

An apparatus for assembling a nuclear fuel assembly is disclosed. The apparatus includes a deflecting device which is disposed adjacent to the grid and deflects grid springs away from dimples opposing thereto. The deflecting device includes a tubular member, a rod member and a drive mechanism. The tubular member has a plurality of circumferentially divided sleeve pieces. The rod member is releasably inserted in the tubular member for sliding movement therealong. The drive mechanism is drivingly connected to the rod member and moves the rod member in the tubular member in a longitudinal direction thereof to bring the rod member into urging engagement with the sleeve pieces of the tubular member, whereby the sleeve pieces are deflected to be urged against the spring to deflect the same.

051715198

abstract

A nuclear reactor having a chemical decontamination system is provided in which every piece of decontamination equipment which processes radioactive materials is located within a decontamination building. The decontamination system is designed to provide for adequate protection for personnel safety and also incorporates a modular design for equipment transportation and storage.

048083704

description

Referring now to the figures in detail and first, particularly, to FIG. 1 thereof, there is seen a concrete structure serving as a radiation shield 1, inside of which a steel pressure vessel 2 is disposed. The pressure vessel is equipped with a removable cover 3 at the upper end thereof Disposed inside the pressure vessel 2 is a metallic core barrel 4, which has a circular projection 5 that rests on a circular flange 6 formed on the inside of the pressure vessel 2. The core barrel 4 contains a lining of built-in ceramic parts or internals 9 formed of carbon blocks and/or graphite which surround a space for receiving a fission zone 10 formed of a multiplicity of spherical fuel assemblies Among other things, canals 11 extend through the built-in parts 9. Absorber rods can be moved in the canals 11 by means of conventional drives 12 which are disposed at the ceiling of the core barrel 4, for controlling the fission zone The built-in parts 9 include other canals 13, through which cooling gas coming from non-illustrated heat sink is conducted into an upper plenum and is sent from the plenum through the fission zone 10 from the top down The heated cooling gas converging in a lower plenum 15 is fed to a non-illustrated heat sink through a hot gas line 16 disposed in a stub or connecting piece 17 of the pressure vessel 2. As seen in FIG. 2, the hot gas line 16 includes an outer tight metallic pipe 22 The flow of the hot gas itself is guided by an inner pipe 23 which is concentric with the pipe 22 and is formed of carbon fiber-reinforced carbon The hot gas line 16 is inserted into a wider stub or connecting piece 24 which is fastened to the core barrel 4 and is fastened to the line 16 by a number of screws 25 distributed over the periphery, as seen in FIG. 3. The breakdown of the hot gas temperature which is as high as 950.degree. C., to a level of about 300.degree. C. which is peraissible for the pressure-carrying pipe 22, is accomplished by heat insulation which can be partially formed of a filamentary ceramic material 26 and partially (at particularl highly mechanically stressed points) of a highly porous solid ceramic 27. A bellows compensator 28 is disposed in the interior of the wider stub 24 and is tightly connected thereto The bellows compensator may be optionally formed of two parts that are joined together by a rigid intermediate part. The bellow:s compensator is connected by means of screws 29 to a sleeve 30, which is also formed of carbon fiberreinforced carbon The sleeve 30 is in turn fastened by means of further screws 31 formed of the same material, to the builtin graphite or carbon block parts of the core barrel 4. The temperature breakdown at the built-in parts 9 from the hot gas temperature to the temperature permissible for the bellows compensator 28, takes place by way of the sleeve 30 A displacement which may occur during operation due to temperature changes between the core barrel 4 and the hot gas line 16 (even in the vertical direction) is compensated by the bellows compensator 28, while at the same time tightness is maintained, so that hot gas leaks from the line 16 which could endanger the core barrel 4 do not take place The outside of the bellows compensator 28 is then cooled by part of the stream of the cooled gas which returns from the heat sink and flows in a space 32 between the hot gas line 16 and a cold gas line 33 which surrounds the line 16 coaxially and is connected to the stub 17. To this end, holes 34 are provided in the wider stub 24 The wider stub 24 is detachably fastened to the core barrel 4 by means of additional screws 35 distributed over the periphery. It can become necessary to detach the stub 24 if the hot gas line 16 is to be replaced because of damage. Due to the necesary close fit between the line 16 and the wider stub 24, it appears more advantageous to replace both parts together If the screws 35 cannot be loosened(such as because of mechanical bending or due to a weld in the thread) they can be cut off between the wall of the core barrel 4 and the flange of the wider stub 24, by means of non-illustrated cutting tools During the reassembly, new tapped holes are cut in the core barrel 4, offset at an angle thereto. During the disassembly of the gas line 16, insulating plugs 36 as well as the screws 25 covered by the plugs can be removed from the inside of the flow guiding inner pipe 23 by chip-producing or milling machinery. This can be done more easily with remotely controlled tools Undesirable friction welding of the pressure pipe 22 to the wider stub 24 in the hot helium atmosphere, is prevented by a suitable coating of these parts, such as with chromium carbide The advantage of the illustrated construction is that during installation the wider flange 24, the bellows compensator 28 and the sleeve 30 can be tested for leaks in a completely assembled condition, before the hot gas line 16 is placed in the wider stub 24. The foregoing is a description corresponding in substance to German Application No. P 35 18 609.7, filed May 23, 1985, the International priority of which is being claimed for the instant application, and which is hereby made part of this application. Any material discrepancies between the foregoing specification and the aforementioned corresponding German application are to be resolved in favor of the latter.

description

This application claims the benefit of U.S. Provisional Application No. 61/625,188 filed Apr. 17, 2012. U.S. Provisional Application No. 61/625,188 filed Apr. 17, 2012 is hereby incorporated by reference in its entirety. The following relates to the nuclear reactor arts and related arts. In nuclear reactor designs of the pressurized water reactor (PWR) type, a nuclear reactor core is immersed in primary coolant water at or near the bottom of a pressure vessel. In a typical design, the primary coolant is maintained in a subcooled liquid phase in a cylindrical pressure vessel that is mounted generally upright (that is, with its cylinder axis oriented vertically). A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. Primary coolant flows upward through the reactor core where it is heated and rises through the central riser, discharges from the top of the central riser, and reverses direction to flow downward back toward the reactor core through a downcomer annulus defined between the pressure vessel and the central riser. The nuclear core is built up from multiple fuel assemblies each comprising a bundle of fuel rods containing fissile material (usually 235U). In a typical PWR design, upper internals located above the reactor core include control rod assemblies with neutron-absorbing control rods that are inserted into/raised out of the reactor core by control rod drive mechanisms (CRDMs). Conventionally, the CRDMs employ motors mounted on tubular pressure boundary extensions extending above the pressure vessel. In this design, the complex motor stator can be outside the pressure boundary and magnetically coupled with the motor rotor disposed inside the tubular pressure boundary extension. The pressure vessel of the PWR is conventionally connected with an external pressurizer and external steam generators via large-diameter piping. More recent small modular reactor (SMR) designs have been driven by a desire to make the PWR more compact and to have fewer large diameter vessel penetrations. Toward this end, in so-called “integral” PWR design, the steam generator is located inside the pressure vessel, typically in the downcomer annulus. This replaces the external primary coolant loop carrying radioactive primary coolant by a secondary coolant/steam loop carrying nonradioactive secondary coolant. The use of an internal pressurizer comprising a steam bubble at the top of the pressure vessel and suitable baffling is contemplated to eliminate the large-diameter penetration for the external pressurizer. Still further, fully internal CRDM motors are contemplated, which eliminates the tubular pressure boundary extensions above the reactor vessel. Some illustrative PWR designs incorporating these advances are described in, e.g.: Thome et al., “Integral Helical-Coil Pressurized Water Nuclear Reactor”, U.S. Pub. No. 2010/0316181 A1 published Dec. 16, 2010 which is incorporated by reference in its entirety; Malloy et al., U.S. Pub. No. 2012/0076254 A1 published Mar. 29, 2012 which is incorporated by reference in its entirety; Stambaugh et al., U.S. Pub. No. 2010/0316177 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety; and DeSantis, U.S. Pub. No. 2011/0222640 A1 published Sep. 15, 2011 which is incorporated herein by reference in its entirety. Such SMR designs introduce numerous challenges not faced in more conventional PWR designs. One such challenge is reactor refueling. SMR components are tightly packed into a compact pressure vessel, with the nuclear reactor core located at or near the bottom of the pressure vessel in order to minimize the possibility of the primary coolant water level falling to a point that exposes the reactor core during a loss of coolant accident (LOCA). This means that all major components are located above the reactor core, and must be removed in order to access the reactor core for refueling. For example, in PWR designs disclosed in Stambaugh et al., U.S. Pub. No. 2010/0316177 A1, an upper internals basket welded to a mid-flange of the pressure vessel supports the internal CRDMs and the control rod guide frames, and power and signal connections for the CRDMs are routed to connectors on the mid-flange. These components must be removed in order to access the reactor core. Disclosed herein are improvements that provide various benefits that will become apparent to the skilled artisan upon reading the following. In accordance with one aspect, a nuclear reactor includes at least: a pressure vessel including an upper vessel section and a lower vessel section connected by a mid-flange and containing primary coolant; a nuclear reactor core disposed in the lower vessel section and immersed in the primary coolant; and upper internals suspended from the mid-flange of the pressure vessel. The upper internals include at least internal control rod drive mechanisms (CRDMs) with CRDM motors immersed in the primary coolant and control rod guide frames disposed between the CRDM motors and the nuclear reactor core. In a disclosed method, the nuclear reactor is depressurized. The upper vessel section is disconnected and removed from the remainder of the pressure vessel while leaving the mid-flange in place on the lower vessel section with the upper internals remaining suspended from the mid-flange. The mid-flange is removed from the lower vessel section with the upper internals remaining suspended from the mid-flange. Fuel of the nuclear reactor core is replaced, the mid-flange is placed back onto the lower vessel section with the upper internals remaining suspended from the mid flange, and the upper vessel section is placed back onto the remainder of the pressure vessel and is re-connected with the remainder of the pressure vessel. In accordance with another aspect, a nuclear reactor includes at least: a pressure vessel including an upper vessel section and a lower vessel section connected by a flange assembly and containing primary coolant wherein the flange assembly includes at least a flange on the upper vessel section and a flange on the lower vessel section; a nuclear reactor core disposed in the lower vessel section and immersed in the primary coolant; a support element supported by the lower vessel section; and upper internals disposed in the pressure vessel and suspended from the support element. The upper internals include at least internal control rod drive mechanisms (CRDMs) with CRDM motors immersed in the primary coolant and control rod guide frames disposed between the CRDM motors and the nuclear reactor core. In a disclosed method, the nuclear reactor is depressurized, the upper vessel section is disconnected from the lower vessel section and lifted away from the lower vessel section with the support element remaining supported by the lower vessel section and the upper internals remaining suspended from the support element, the support element is lifted away from the lower vessel section with the upper internals remaining suspended from the support element so as to lift both the support element and the suspended upper internals away from the lower vessel section, fuel of the nuclear reactor core disposed in the lower vessel section is replaced, the support element is lowered back onto the lower vessel section with the upper internals remaining suspended from the support element, the upper vessel section is reconnected to the lower vessel section, and the nuclear reactor is re-pressurized. In some embodiments the support element comprises a flange of the flange assembly connecting the upper and lower vessel sections. In accordance with another aspect, a nuclear reactor includes at least: a pressure vessel including an upper vessel section and a lower vessel section connected by a flange assembly and containing primary coolant wherein the flange assembly includes at least a flange on the upper vessel section and a flange on the lower vessel section; a nuclear reactor core disposed in the lower vessel section and immersed in the primary coolant; a support element supported by the lower vessel section; and upper internals disposed in the pressure vessel and suspended from the support element. The upper internals include at least internal control rod drive mechanisms (CRDMs) with CRDM motors immersed in the primary coolant and control rod guide frames disposed between the CRDM motors and the nuclear reactor core. The nuclear reactor is configured to be prepared for refueling by operations including disconnecting the upper vessel section from the lower vessel section and lifting the upper vessel section away from the lower vessel section with the support element remaining supported by the lower vessel section and the upper internals remaining suspended from the support element, and lifting the support element away from the lower vessel section with the upper internals remaining suspended from the support element so as to provide access from above to the nuclear reactor core disposed in the lower vessel section. In some embodiments the support element comprises a mid-flange of the flange assembly that is disposed between the flange on the upper vessel section and the flange on the lower vessel section. Optionally, the mid-flange includes vessel penetrations connected by cables with the upper internals that remain connected by cables with the upper internals during the lifting of the mid-flange away from the lower vessel section. With reference to FIG. 1, a small modular reactor (SMR) 1 of the of the integral pressurized water reactor (PWR) variety is shown in partial cutaway to reveal selected internal components. The illustrative PWR 1 includes a nuclear reactor core 2 disposed in a pressure vessel comprising a lower vessel portion 3 and an upper vessel portion 4. The lower and upper vessel portions 3, 4 are connected by a mid-flange 5. Specifically, a lower flange 5L at the open top of the lower vessel portion 3 connects with the bottom of the mid-flange 5, and an upper flange 5U at the open bottom of the upper vessel portion 4 connects with a top of the mid-flange 5. The reactor core 2 is disposed inside and at or near the bottom of the lower vessel portion 3, and comprises a fissile material (e.g., 235U) immersed in primary coolant water. A cylindrical central riser 6 is disposed coaxially inside the cylindrical pressure vessel and a downcomer annulus 7 is defined between the central riser 6 and the pressure vessel. The illustrative PWR 1 includes internal control rod drive mechanisms (internal CRDMs) 8 with internal motors 8m immersed in primary coolant that control insertion of control rods to control reactivity. Guide frames 9 guide the translating control rod assembly (e.g., each including a set of control rods comprising neutron absorbing material yoked together by a spider and connected via a connecting rod with the CRDM). The illustrative PWR 1 employs one or more internal steam generators 10 located inside the pressure vessel and secured to the upper vessel portion 4, but embodiments with the steam generators located outside the pressure vessel (i.e., a PWR with external steam generators) are also contemplated. The illustrative steam generator 10 is of the once-through straight-tube type with internal economizer, and is fed by a feedwater inlet 11 and deliver steam to a steam outlet 12. See Malloy et al., U.S. Pub. No. 2012/0076254 A1 published Mar. 29, 2012 which is incorporated by reference in its entirety. The illustrative PWR 1 includes an integral pressurizer 14 at the top of the upper vessel section 4 which defines an integral pressurizer volume 15; however an external pressurizer connected with the pressure vessel via suitable piping is also contemplated. The primary coolant in the illustrative PWR 1 is circulated by reactor coolant pumps (RCPs). In the illustrative embodiment of FIG. 1, each RCP comprises an external RCP motor 16 driving an impeller located in a RCP casing 17 disposed inside the pressure vessel. The illustrative PWR 1 also includes an optional support skirt 18. With reference to FIGS. 2-30, an illustrative refueling sequence is disclosed for a reactor of the type shown in FIG. 1. The refueling sequence utilizes the flange arrangement in which (1) the upper flange 5U is integral with the upper vessel 4; the lower flange 5L is integral with the lower vessel 3; and the mid-flange 5 is disposed between the upper and lower flanges 5U, 5L and supports the upper internals (including the CRDMs 8 and guide frames 9) in suspended fashion. In brief, the upper vessel 4 is removed with the integral steam generator 10 secured inside the upper vessel 4; then the mid-flange 5 is removed along with the upper internals 8, 9 suspended from the mid-flange 5 so that the reactor core is then accessible from above for refueling. This approach avoids the need to disassemble internal components. FIG. 2 illustrates a flowchart of a first portion 100 of the refueling sequence, including: starting the reactor shutdown sequence 110, allowing the reactor to cool down 120; depressurizing the reactor 130; draining the secondary side of the upper vessel 140; draining the primary side of the upper vessel to the lower vessel assembly level 150; removing the insulation panels from the appropriate areas of the reactor (i.e. the main closure area, pressurizer heater area, and pressure relief area) 160; and breaking the hydraulic connections (i.e. feedwater lines, main stream lines, and pressure relief lines) 170. FIG. 3 illustrates a flowchart of a second portion 200 of the sequence for removing spent fuel from the reactor, starting with disconnecting the upper vessel external electrical connections (i.e. pressurizer heaters and instrumentation) 210. If the electrical and piping for the lower vessel assembly are routed down the side of the upper vessel and also ride with the upper vessel, then in a further operation 215 all of the connections through the mid-flange are broken. An upper vessel crane is connected 220. The reactor vessel nuts at the main closure are de-tensioned in an operation 230. The reactor vessel studs are backed out of the lower vessel flange and the reactor vessel studs are backed out of the lower vessel flange and the reactor vessel nuts and reactor vessel studs are parked in the transport position on the upper flange in an operation 240. Optionally, the reactor vessel nuts and the reactor vessel studs are completely removed and placed into a separate transport rack, as indicated in FIG. 3 by an operation 245. Finally, the upper vessel is moved to the upper vessel maintenance stand for inspection in an operation 250. FIG. 4 illustrates a flowchart of a third portion 300 of the refueling sequence, including: removing the riser cone from the upper internals 310; disconnecting the external connections around the mid-flange from the lower vessel assembly 320; pulling the incore instruments into the upper internals and wrapping the incore instruments around the mid-flange 330; positioning the upper internals platform on the mid-flange 340; inserting a tool to disconnect control rods from the control rod drive mechanism (CRDM) 350; and hooking the upper internals crane up to the upper internals 360. FIG. 5 illustrates a flowchart of a fourth portion 400 of the refueling sequence. The portion 400 includes: verifying that all connecting rods are in a parked position and that all external connections are disconnected from the mid-flange 410; moving the upper internals and refueling bridge to the upper internals maintenance stand 420; and in sequence with the upper internals being pulled from the lower vessel, slowly filling the refueling canal 430. At the end of this portion 400, the fuel exchange can begin in operation 440. With reference to FIGS. 6-19, the state of an illustrative SMR of the integral PWR variety is shown at various stages of the process flowcharted in FIGS. 2-5. The SMR of FIGS. 6-19 is similar to that of FIG. 1. A difference between the illustrative SMR of FIG. 1 and that of FIGS. 6-19 is that in the SMR of FIG. 1 the reactor coolant pumps (RCPs) are mounted on the upper vessel 4 proximate to the integral pressurizer 14 with external RCP motors 16 driving impellers located in RCP casings 17 disposed inside the pressure vessel; whereas, in the SMR of FIGS. 6-19 the RCPs are wholly internal RCPs 167 that are part of the upper internals suspended from the mid-flange 5 (see, e.g. FIGS. 14-17). It is to be understood that the disclosed refueling process is also applicable to a reactor design such as that of FIG. 1 in which the RCPs have external motors (e.g., wholly dry motor/stator assemblies or dry stators coupled with wet stators via tubular pressure boundary extensions). FIG. 6 diagrammatically shows the SMR in an operational state 500. The operational reactor 500 includes components not shown in FIG. 1, including metal reflective insulation 505, upper vessel hydraulic connections 510, pressurized heater electrical connections 515, and upper internals electrical connections 520. (Additional components ancillary to the operating reactor 500, such a surrounding containment structure, reactor support including a floodwell, piping running to/from a turbine, various emergency core cooling and other safety-related components, and so forth are not illustrated). The operational state 500 corresponds to the state of the reactor just before reactor shutdown 110, and illustrates the externally observed state of the reactor during the operations 110, 120, 130, 140, 150 of FIG. 2. FIG. 7 diagrammatically shows the SMR in a state 600 with the insulation 505 partially removed, corresponding to operation 160 of FIG. 2. FIG. 8 diagrammatically shows the SMR in a state 700 with the insulation removed from the pressurizer, mid-flange, and lower vessel regions, and with upper vessel connections 510 detached. This state 700 corresponds to operation 170 of FIG. 2. FIG. 9 diagrammatically shows the SMR in a state 800 with pressurized heater electrical connections 815 and instrumentation connections 816 partially removed. This state 800 corresponds to operation 210 of FIG. 3. FIG. 10 diagrammatically shows the SMR in a state 900 corresponding to the alternative embodiment in which electrical and piping for the lower vessel assembly are routed down the side of the upper vessel and also ride with the upper vessel. FIG. 10 corresponds to further operation 215 of FIG. 3 in which the connections 920 through the mid-flange to the upper internals are broken. FIG. 11 diagrammatically shows the SMR in a state 1000, which is ready for hook-up with the upper vessel crane, i.e. operation 220 of FIG. 3. As seen in FIG. 11, the upper vessel includes upper vessel crane attachment points 1030 for attachment of the lifting crane (not shown). FIG. 12 diagrammatically shows the SMR in a state 1100 including reactor vessel nuts 1135 at the mid-flange 5 in tensioned position. The state 1100 corresponds to just before commencement of the nut detensioning operation 230 of FIG. 3. FIG. 13 diagrammatically shows the SMR in a state 1200 corresponding to immediately subsequent to the operation 240 of FIG. 3, with the reactor vessel studs 1240 backed out of the lower vessel flange 5L and parked in their transport positions in the upper vessel flange 5U. As indicated in alternative operation 245 of FIG. 3, the studs 1240 are alternatively completely removed and placed into a separate transport rack (variant not illustrated). An advantage of the approach of FIG. 13 in which the bolts remain at least partially inserted into the upper vessel flange 5U is that the bolts are not likely to be lost. (It is also contemplated to leave the fasteners parked at least partially inserted in the lower vessel flange rather than being parked at least partially inserted in the upper vessel flange 5L as shown in FIG. 13). FIG. 14 diagrammatically shows the SMR in a state 1300 in which the upper vessel portion 1342 is lifted off the lower vessel portion 1344 via the crane lifting at the crane attachment points 1030 (labeled in FIG. 11; note that the drawings do not show the crane). The state 1300 of FIG. 14 corresponds to the operation 250 of FIG. 3. The upper vessel portion 1342 of the SMR of the illustrated refueling process is analogous to the upper vessel portion 4 of the SMR of FIG. 1, but does not include the RCPs 16; rather, the SMR of the illustrative refueling process includes wholly internal RCPs 167 which are part of the upper internals disposed in the lower vessel portion 1344 (corresponding to the lower vessel portion 3 of the SMR of FIG. 1 but sized to accommodate the RCPs 167). As seen in FIG. 14, the tops of the internal RCPs 167 are visible after lifting off the upper vessel portion 1342. Also visible is a riser cone 1445 that that provides the transition from the central riser 6 in the upper vessel portion (see FIG. 1) into the lower vessel portion. Instead of the illustrative riser cone, this transition could alternatively be cylindrical or otherwise shaped depending on the reactor design. It is to be appreciated that the internal steam generator (if present, e.g. steam generator 10 of the SMR of FIG. 1) is secured inside the upper vessel portion and is lifted off with the upper vessel portion 1342. Advantageously, this enables removal of the steam generator as a unit without disconnecting it from its mountings inside the pressure vessel. Similarly, in a reactor such as that of FIG. 1 in which the RCPs each comprise an external RCP motor 16 driving an impeller located in a RCP casing 17 disposed inside the pressure vessel, the RCPs 16, 17 are secured with the upper vessel portion and are optionally removed as a unit together with the upper vessel portion. (Alternatively, the RCPs 16, 17 or portions thereof, e.g. the motors 16, are removed prior to lifting off the upper vessel section). As further seen in FIG. 14, the mid-flange 5 remains in place atop the lower flange 5L of the lower vessel portion 1344. The weight of the mid-flange 5 together with the weight of the suspended upper internals ensures that this component is held firmly in place during liftoff of the upper vessel portion. The next operations perform removal of the mid-flange 5 and the upper internals that are suspended from the mid-flange 5. FIG. 15 diagrammatically shows the SMR in a state 1400. In FIG. 15 and subsequently, the upper vessel portion 1342 is not shown, it having been moved off to the upper vessel maintenance stand (not shown) in the upper vessel liftoff operation 250 of FIG. 3. The state shown in FIG. 15 corresponds to operation 310 of FIG. 4, that is, to removal of the riser cone 1445 from the upper internals. FIG. 16 diagrammatically shows the SMR in a state 1500 corresponding to the operation 320 of FIG. 4, in which external connections 1550 around the mid-flange are disconnected. FIG. 17 diagrammatically shows the SMR in a state 1600 corresponding to the operation 330 of FIG. 4 in which the incore instruments (not shown) are pulled through vessel penetrations 1655 of the mid-flange 5 and wrapped around the mid-flange 5 or otherwise secured with the mid-flange 5. FIG. 18 diagrammatically shows the SMR in a state 1700 corresponding to the operations 340, 350, 360 of FIG. 4. An upper internals refueling platform 1750 is delivered (if not already on-site) and positioned on the mid-flange 5 as per operation 340. A suitable tool 1752 is employed in operation 350 to disconnect the control rods from the control rod drive mechanisms (CRDMs). It is to be appreciated that as part of the reactor shutdown sequence 110 (FIG. 2), the control rods were fully inserted into the reactor core in order to extinguish the nuclear chain reaction. Operation 350 serves to disconnect the CRDMs 8 (see FIG. 1), and a crane (not shown) is hooked up to the upper internals in operation 360. This hookup suitably employs the mid-flange 5 as the anchor point for hooking up the crane (not shown). FIG. 19 diagrammatically shows the SMR in a state 1800 corresponding to operation 420 of FIG. 5, i.e. to the lifting out of the mid-flange 5 with the upper internals suspended therefrom. In the SMR of the illustrated refueling process, the suspended upper internals include the CRDMs 8 (including the internal CRDM motors 8m), the guide frames 9, and the internal RCPs 167 (only the bottoms of few of which are visible in FIG. 19). The upper internals 8, 9, 167 are removed as a unit, again advantageously avoiding breakdown of these intricate assemblies. In the alternative SMR of FIG. 1, the suspended upper internals include the CRDMs 8 (and motors 8m) and the guide frames 9, but do not include the RCPs. Rather, as seen in FIG. 1, in that embodiment the RCPs (each comprising a motor 16 and impeller in RCP casing 17) are mounted proximate to the pressurizer 14. Thus in this embodiment the upper internals 8, 9 are removed as a unit. FIG. 19 shows the mid-flange 5 with the upper internals 8, 9, 167 remaining suspended from the mid-flange 5, and the refueling bridge or platform 1750 mounted on the mid-flange 5, with this complete assembly lifted vertically out of the lower vessel section 1344. The assembly is suitably then moved laterally (possibly with vertical adjustments as needed) to an upper internals maintenance stand (not shown). With the upper internals removed, the lower vessel 1344 remains with a large opening encircled by the lower flange 5L through which refueling can be performed. Advantageously, the removal of the upper internals suspended from the mid-flange 5 is efficient, as the CRDMs and guide frames do not need to be individually dismounted and removed. The mid-flange 5 provides a suitable support. Additionally, in embodiments in which the electrical (and, if needed, hydraulic) lines penetrate the pressure vessel through the mid-flange 5, the cables external to the pressure vessel are removed (see FIGS. 10 and 16 and operations 215, 320 of FIGS. 3 and 4 respectively), but the electrical (and hydraulic) cabling internal to the pressure vessel and running from the mid-flange 5 to the CRDM motors 8m can remain in place, further reducing the number of operations needed to remove the upper internals. With reference to FIGS. 20-22, the refueling and restart operations are flowcharted. FIG. 20 illustrates a first portion 1900 of this process. Operation 1910 is the spent fuel removal operation. A refueling crane is brought in and removes the spent fuel, which is transported through a refueling canal to a fuel transport system and then transported to the spent fuel pool (ancillary components not shown). The spent fuel is preferably removed in operation 1910 before new fuel is brought in, and then in an operation 1920 new fuel assemblies are transported through the refueling canal, removed from the fuel transport system, and placed by the crane into the lower vessel 1344 to assemble the fresh nuclear reactor core inside the lower vessel 1344. In an operation 1930, mid-flange standoffs are installed onto the lower vessel flange. New o-rings are placed onto the surface of the lower vessel in an operation 1940. The upper internals and refueling platform are placed onto standoffs in sequence with the refueling canal being drained in an operation 1950. Existing o-rings are removed, the o-ring sealing surfaces are inspected, and the new O-rings are installed, in an operation 1960. Alternatively, the o-rings are installed on the lower vessel flange but not the mid-flange (alternative operation 1965 of FIG. 20). The upper internals are lifted and removed from standoffs in an operation 1970. The upper internals are lowered onto the lower vessel and control rods are reconnected to the control rod drive mechanism (CRDM) with a connection tool in an operation 1980. FIG. 21 illustrates the continuation 2000 of the process. The refueling platform and existing o-rings are removed, the sealing surfaces are inspected, and new o-rings installed in an operation 2010. In some embodiments, redundant o-rings are installed on the upper vessel flange in an operation 2015. The incore instruments are pushed back into the core and incore feedthrough seals are replaced in an operation 2020. The electrical connections into the mid-flange are reconnected in an operation 2030. If appropriate for the reactor design, connections into the mid-flange are also completed after the upper vessel is in place in an operation 2035. The riser cone is replaced on the upper internals in an operation 2040. The upper vessel crane is attached to the upper vessel and the upper vessel is transported and positioned on top of the lower vessel in an operation 2050. The hydraulic tensioning nut and reactor vessel studs are moved from their parked position and loosely threaded into the lower vessel flange and prepared for tensioning in an operation 2060. If the studs were removed completely (as per optional operation 245 of FIG. 3), then the reactor vessel studs and hydraulic tensioning nuts are brought back in a transport rack in alternative operation 2065. The studs may be threaded into the lower vessel flange and prepared for tensioning. FIG. 22 illustrates the continuation 2100 of the process. The hydraulic tensioning nuts are tensioned to their specification requirements in an operation 2110. The crane is disconnected from the upper vessel in an operation 2120. Electrical connections (i.e. instrumentation) are reconnected to the upper vessel in an operation 2130, or alternatively are reconnected into the mid-flange (alternative operation 2135). The hydraulics (i.e. steam lines and pressurizer relief) are reconnected to the upper vessel in an operation 2140. The reactor is leak tested in an operation 2150. The removable insulation is reinstalled on the reactor vessel in an operation 2160. The reactor startup sequence is initiated in an operation 2170. With reference to FIGS. 23-30, further states of the SMR of the illustrative refueling are shown during the refueling/restart process of FIGS. 20-22. FIG. 23 diagrammatically shows the SMR in a state 2200 corresponding to the operations 1910, 1920 of FIG. 20. FIG. 23 shows an illustrative fuel assembly 2262 being unloaded (for operation 1910) or loaded (for operation 1920). FIG. 24 diagrammatically shows the SMR in a state 2300 corresponding to the operation 1930 of FIG. 20 in which standoffs 2364 are installed onto the lower vessel flange 5L of the lower vessel 1344. FIG. 25 diagrammatically shows the SMR in a state 2400 corresponding to the operation 1940 of FIG. 20 in which a new o-ring 2466 is placed onto the surface of the lower vessel flange 5L of the lower vessel 1344. FIG. 26 diagrammatically shows the SMR in a state 2500 corresponding to the operation 1950 of FIG. 20 in which the mid-flange 5 (with the upper internals suspended from the mid-flange 5) is placed onto the standoffs 2364. FIG. 27 diagrammatically shows the SMR in a state 2600 corresponding to the operation 1960 of FIG. 20. The old o-rings 2610 are cut and removed, and new o-rings 2620 are installed. FIG. 28 diagrammatically shows the SMR in a state 2700 corresponding to the operation 1970 of FIG. 20 in which the mid-flange 5 (with the upper internals suspended therefrom) is lifted and the standoffs 2764 removed. FIG. 29 diagrammatically shows the SMR in a state 2800 corresponding to the operation 1980 of FIG. 20 in which the mid-flange 5 (with the upper internals suspended therefrom) is placed onto the lower vessel flange 5L of the lower vessel 1344, and the tool 1752 (cf. FIG. 18) is used to reconnect the CRDMs with the connecting rods. FIG. 30 diagrammatically shows the SMR in a state 2900 corresponding to the operation 2010 of FIG. 21 in which the refueling platform 1750 (see FIG. 29) is removed, existing o-rings are removed and new o-rings 2910 are installed on the upper surface of the mid-flange 5. The remaining SMR re-assembly operations of FIGS. 21 and 22 are not explicitly illustrated, but comprise a reversal of corresponding disassembly operations of FIGS. 2-4. For example, operation 2040 of FIG. 21 corresponds to FIG. 15; operation 2050 of FIG. 21 corresponds to FIG. 14; and so forth. The use of the mid-flange 5 advantageously provides a support element for the upper internals that optionally integrates the electrical (and, if needed, hydraulic) vessel penetrations. This enables the upper internals, including the electrical cabling from the vessel penetrations to the CRDM motors 8m (or other elements) to be kept in place as the mid-flange 5 and suspended upper internals are lifted out (e.g., as shown in FIG. 19). However, in alternative embodiments (not shown) it is contemplated to employ a support element other than the illustrated mid-flange 5 for supporting the upper internals in suspended fashion. For example, the support element can be a forged annular ring with outer diameter is equal to or smaller than the inner diameter of the lower vessel section so as to fit inside the lower vessel and sit on discrete ledges (or a continuous annular ledge) on the inside surface of the lower vessel section. Such a ledge or ledges can be formed integrally as part of the forged lower vessel section, or can be welded to the lower vessel section. The upper internals are then lifted out by anchoring the crane to the annular ring or other support element and lifting it out of the lower vessel section. In these alternative embodiments, the support element is not part of the pressure boundary and cannot include the vessel penetrations for accessing the upper internals, and so an additional operation of disconnecting cabling inside the pressure vessel is needed before the support element and suspended upper internals can be lifted out. The illustrative embodiments have been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the illustrative embodiments be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

description

The current invention relates to control rod absorbers for nuclear power plants. More specifically, the current invention relates to a control rod absorber and associated stack support which may be used in a nuclear power plant for an extended time period and which minimally degrades from thermal creep. Pressurized water reactors (PWRs) use fuel assemblies which contain a nuclear fuel, such as enriched uranium dioxide, to produce a nuclear chain reaction. The nuclear chain reaction is moderated in several ways, including maintaining specific fuel enrichment levels, maintaining a specific geometry of the fissionable nuclear fuel assemblies, and placing absorber material through the nuclear reactor core. The purpose of control rod absorbers is to slow or capture neutrons as the neutrons traverse the nuclear reactor. The absorber material allows sections of the core which potentially have greater amounts of nuclear activity to be reduced to more moderate levels. Because the control rod absorbers perform this task, the overall performance of the nuclear reactor is more consistent and the reactor is able to run with a greater degree of operational safety. Over time, conventional control absorber rods start to degrade. This degradation leads to the sections of the core potentially becoming more radiologically active than other sections of the core. The degradation then leads to localized “hot spots” in the reactor core and may require nuclear power plant operators to limit overall reactor operation to maintain safe operating margins, thereby negatively impacting the economic operation of the facility. When the rods become severely degraded, the reactor cannot be safely operated, necessitating rod replacement. As these absorber assemblies which contain the rods are located in the reactor core itself, the absorber assembly replacement occurs during a reactor outage. The replacement of these absorber rods and assemblies must be performed very carefully as the materials themselves are highly radioactive after their residence in the reactor core. The removal of the assemblies from the core is performed remotely by use of a crane and the removal is therefore a difficult and expensive operation. Ultimate disposal of the highly radioactive components removed is also very expensive as such highly radioactive waste must be cooled with cooling water for a long period of time. The components may then be stored in a “dry” condition in a specially prepared cask with a gaseous cooled interior. All of these disposal costs increases the overall cost of operation of the facility. Control rod assemblies may be provided in several various arrangements. “Black” clusters may be used in a reactor, wherein such “black” clusters are highly absorbent to neutrons traversing the nuclear reactor core. Other clusters, commonly known as “gray” clusters, may also be positioned around the reactor. The “gray” clusters are less absorbent than the “black” clusters and are therefore used in core positions that do not need as much radiation attenuation as other sections of the core. “Black” clusters are generally constituted of rods containing materials which are highly absorbent to neutrons, such as silver-indium-cadmium alloys. Absorber rods and assemblies degrade through a variety of degradation mechanisms. Absorber rods made of silver-indium-cadmium alloys are subject to both creep and swelling under irradiation. Boron carbide alloy based absorber rods undergo a large amount of swelling under irradiation and as a result are not heavily used in portions of rods inserted into high activity core areas, such as the bottom of absorber rods. For this reason, silver-indium-cadmium alloys are primarily used in the bottom parts of control rods. For silver-indium-cadmium alloys, the chief degradation mechanism limiting operability is diametral expansion. Diametral expansion of rodlets is a result of irradiation-induced expansion, thermal creep and thermal expansion. The overall geometric shape of the absorber assembly causes degradation to occur most frequently at the lower tip of a rod because of a high fluence exposure and high stresses due to the force exerted from an internal absorber stack of the rod. This is a need to produce a rod with absorber material which will limit diametral expansion. This is also a need to produce an absorber rod which will have an increased service life in the nuclear core compared to conventional absorber rods and that will not have to be replaced as often as other absorber rod units. There is also a need to produce an absorber rod assembly which will limit the amount of nuclear waste generated for a nuclear reactor. It is therefore an objective of the present invention to produce an absorber rod which will limit diametral expansion of rodlets in an absorber assembly. It is also an objective of the present invention to produce an absorber rod which will be operable longer in the nuclear core and that will not have to be replaced as often as other absorber assembly units. It is also an objective of the present invention to produce an absorber rod which will limit the amount of nuclear waste generated for a nuclear reactor. The objectives of the present invention are achieved as shown and described. The invention provides an absorber rod for a nuclear reactor comprising a rod cladding defining in internal volume, the rod cladding having an upper end and a lower end, an upper end fitting positioned at the upper end of the rod cladding, a rod internal arrangement configured in the internal volume of the rod cladding, a lower end cap positioned at the lower end of the rod cladding, the lower end cap having an upper surface and a lower surface, a stack support with a stack support top end and a stack support bottom end, the stack support bottom end placed in contact with the upper surface of the lower end cap, the stack support top end configured to support the rod internal arrangement, and an annulus of material configured around the stack support and contacting the upper surface of the lower end cap. The annulus of material may be made of a silver-indium-cadmium alloy and the cladding may be configured in a tube shape. The diameter of the absorber rod may be approximately 2.16 centimeters. The stack support may be placed inside the annulus of material. The stack support rod may be configured to carry an entire weight of the rod internal arrangement and transport the entire weight to the lower end cap. The stack support top end may also be configured to have a larger outer perimeter than an outer perimeter of a body of the stack support. The present invention also provides a stack support for a nuclear reactor absorber rod. The stack support comprises a stack support top end and a stack support bottom end, the stack support bottom end placed in contact with an upper surface of a lower end cap, the stack support top end configured to support a rod internal arrangement. Referring to FIG. 1, a control absorber rod 10 is illustrated in cross-sectional view. The control absorber rod may be part of a larger control rod assembly placed inside a nuclear reactor core. The control absorber rod 10 has an outer absorber cladding 16 which defines an interior volume. The cladding 16 is illustrated in a tubular arrangement, however other arrangements are possible and the illustrated arrangement in FIG. 1 is but one possibility. The cladding 16 is made of INCONEL® (high strength austenitic nickel-chromium-iron alloys) in the current illustrated embodiment, however other non-corrosive materials may be used such that corrosion products are not introduced into the reactor coolant stream if a corrosive environment is encountered. The thickness of the cladding 16 may be either constant or varied along the body of the cladding 16 to provide more or less protection to the internal components placed within the volume. In the illustrated embodiment, the thickness of the cladding is constant. The cladding 16 has an absorber rod upper end 14 and an absorber rod lower end cap 18. As illustrated, the absorber rod lower end cap 18 is configured such that the cladding 16 smoothly interfaces with the absorber rod lower end cap 18 at a connection 38. The connection 38 locks the respective side wall of the cladding together with the absorber rod lower end cap 18 such that the two pieces are not separable. The absorber rod lower end cap 18 is configured to limit hydraulic drag on the rod, thereby reducing overall pressure drop for the absorber rod 10. To this end, the absorber rod lower end cap 18 may have a rounded face wherein the fluid passing from a lower elevation to an upper elevation smoothly envelops the absorber rod 10. Other absorber rod lower end caps 18 may be configured with a cone shaped end or may be flat faced as non-limiting examples. The overall length of the absorber rod lower end cap 18 may also be configured such that the absorber rod may be lengthened to a desired amount, further limiting drag, if necessary. The overall width of an absorber rod 10 can be any size according to the needs of the reactor in which the absorber rod/assembly is placed. In the illustrated embodiment of the present invention, the diameter of the absorber rod 10 is approximately 2.41 centimeters. The control absorber rod 10 is also provided with an upper end filling 60, the upper end filling 60 sealing the upper end of the rod cladding 14. The absorber rod cladding 16 defines an interior volume into which materials may be placed for either structural support or moderation of the nuclear reaction. To moderate the nuclear reaction, a rod internal arrangement 36 is provided to either capture or slow down neutrons as they traverse the nuclear reactor. The rod internal arrangement 36, as illustrated, is configured to fit within a top portion of the absorber rod cladding 16. The rod internal arrangement 36 is configured to be generally tubular in nature, however other configurations are possible. The rod internal arrangement 36, has an internal arrangement annulus 40 thereby providing a space between the rod internal arrangement 36 and the absorber rod cladding 16. The amount of space between the absorber rod cladding 16 and the rod internal arrangement 36 may be zero in the case of actual abutment between a part of the rod internal arrangement and the absorber rod cladding 16. The amount of space between the absorber rod cladding 16 and the rod internal arrangement 36 is be established such that if the rod internal arrangement 36 undergoes seismic acceleration in vertical, horizontal, or a combination of directions, contact between the rod internal arrangement 36 and the absorber rod cladding 16 is prevented or minimized as desired. The rod internal arrangement 36 is configured out of neutron absorbing material. The absorber rod internal arrangement 36 is supported by a stack support 28. The purpose of the stack support 28 is to transfer the weight and/or forces on the rod internal arrangement 36 to the absorber rod lower end cap 18. The stack support 28 may simply support the absorber rod internal arrangement 36 or it may be connected to the absorber rod internal arrangement 36 such that in the event of a seismic event, such as when an upwards or lateral force is encountered, the rod internal arrangement 36 is maintained in a relatively consistent orientation and not allowed to impact the absorber rod cladding 16. The stack support 28 may be configured with an upper surface 34 and a stack support lower end 30. The stack support upper surface 34 provides the resting surface for the rod internal arrangement 36. The stack support upper surface 34, as illustrated, is configured in a circular configuration to fully support the rod internal arrangement 36. The thickness 42 of the stack support upper surface 34 is chosen such that the cantilever parts 44 of the stack support 28 do not deflect an appreciable amount, while providing the support capabilities needed during operational and accident loading conditions. The stack support 28 also is configured with a stem 46 which connects the stack support upper surface 34 with the stack support lower end 30. The stack support stem 46 is configured to allow the force transfer from the stack support upper surface 34 to the stack support lower end 30. The stack support stem 46 is designed such that bending of the stem 46 does not affect the operability of the absorber rod 10. The stack support lower end 30 is also designed such that the forces which are exerted upon the absorber rod lower end cap 18 do not punch through the absorber rod lower end cap 18 during any loading conditions. Although illustrated as a simple post end for the stack support lower end 30, the lower end 30 can be a flared unit, or may be countersunk into the absorber rod lower end cap 18 to provide a positive connection. The stack support 28 may be manufactured from high strength steel to provide sufficient support capacity for the rod internal arrangement 36. The materials may be stainless steel or other non-corrosive material to limit the effects of corrosion inside the absorber rod 10 and to limit galvanic corrosion from occurring. A material annulus 22 may be positioned inside the absorber rod cladding 16 to allow for attenuation of the nuclear chain reaction at the lower end of the absorber rod 10. The material annulus 22 is positioned around the stem 46 and stack support lower end 30 to provide the absorber rod 10 with sufficient neutron absorber rod capacity. The material annulus 22 may be positioned in the interior volume of the absorber rod cladding 16 such that the material annulus bottom end 26 contacts the absorber rod lower end cap 18. The material annulus 22 is positioned underneath the cantilever portions 44 of the stack support 28 such that there is a gap 48 between the material annulus top end 24 and the bottom face of the stack support upper end 50. The size of the gap 48 between the bottom face 50 of the stack support upper end and the material annulus top end 24 may be minimized such that little to no gap 48 is present. The material annulus bottom end 26 may be countersunk into the absorber rod lower end cap 18. A locking arrangement may be established between the material annulus bottom end 26 and the absorber rod lower end cap 18 to eliminate the possibility for movement of the material annulus 22 inside the absorber rod cladding 16. The material annulus 22 may be additionally configured such that an annular space 20 is created around the material annulus 22. The annular space 20 may be varied such that a lesser or greater amount of space is provided between the interior wall 52 of the absorber rod cladding 16 and the exterior surface 54 of the material annulus 22. The present invention solves problems associated with life-limiting considerations for control rods as a result of diametral expansion of the silver-indium-cadmium absorber. The present invention reduces the effect of thermal creep at the lower tip of the absorber stack which in turn lengthens the operable life-time of control rods. The equation for strain rate due to thermal creep is as follows:ε′=Kdnσmexp(−Q/RT)where ε′ is the axial creep strain rate measured in s−1. The grain size of the material is provided in the variable (d), stress in σ and temperature T. K, n, m and Q are material specific parameters. R is the universal gas constant. In the case of silver-indium-cadmium material, the parameter m has a value of approximately 1, therefore the creep strain rate is nearly linearly proportional to the stress of the material. By reducing the stress level of the material, the creep strain rate is also reduced. The present invention provides a rod configuration which removes the forces exerted from plenum springs and internal stack weights from the lower portion of absorber material positioned inside an absorber rod. The forces exerted from the internals are transmitted through the stack support 28 to the absorber rod lower end cap 18. As a result, the load path bypasses the silver-indium-cadmium material annulus reducing the stress level of this component. In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a restrictive sense.

summary

054147425

description

SUMMARY OF THE INVENTION Disclosed herein is a leak-detection system and method for detecting a leaking container. The system includes an enclosure defining a cavity therein surrounding the container, which container may be a nuclear fuel rod having fission products leaking through a breach in the exterior surface of the fuel rod. The fission products are capable of adhering to the exterior surface of the fuel rod as the fission products leak through the breach in the fuel rod. A radiation detector is in communication with the cavity for detecting the fission products leaking through the breach. Moreover, a gas injector is in communication with the cavity for injecting a multiplicity of carrier gas bubbles into the cavity to remove any fission products adhering to the exterior surface of the fuel rod and to carry the fission products removed thereby to the radiation detector. In this manner, the detector detects the leaking fission products even though they may tend to adhere to the exterior surface of the fuel rod. In addition, an elevator is connected to the fuel assembly for elevating the fuel assembly in the cavity, so that the external pressure acting against the exterior surface of the fuel rods is reduced in order to relieve the internal pressure in the fuel rods. As the fuel assembly is elevated, any fission product gases and/or solids tending to "hide-out" in the fuel rods expand and migrate through the breach. As the fission products migrate through the breach, they are carried by the carrier by the carrier gas bubbles to the radiation detector where they are detected by the radiation detector. In this manner, otherwise undetectable leaking fuel rods become detectable. The invention in its broad form is a leak-detection system for detecting a leaking container having a surface thereon and a material leaking therefrom, the material capable of adhering to the surface of the container as the material leaks from the container, comprising enclosure means surrounding the container for enclosing the container; detector means associated with said enclosure means for detecting the leaking material; and fluid injection means associated with said enclosure means for injecting a fluid into said enclosure means to remove the material adhering to the surface and to carry the material removed thereby to said detector means, so that said detector means detects the material leaking from the container. The invention in its broad form is also a leak-detection method for detecting a leaking container having a material leaking from an exterior surface thereof, the material capable of adhering to the exterior surface, comprising the steps of enclosing the container by surrounding the container with an enclosure; detecting the material leaking from the container by operating a detector associated with the enclosure; and removing the material adhering to the exterior surface and carrying the material to the detector by injecting a fluid into the enclosure, so that the detector detects the material leaking from the container. An object of the present invention is to provide a leak-detection system and method for detecting a leaking container, which leaking container may be a leaking nuclear fuel rod having a radioactive fission product material leaking therefrom. Another object of the present invention is to provide a leak-detection method that is less time consuming than prior art leak-detection techniques. Yet another object of the present invention is to provide a leak-detection system and method that detects a leaking fuel rod even though the fission product material leaking therefrom may adhere to the exterior surface of the fuel rod and even though the fission product material may tend to "hide-out" in the fuel rod. A feature of the present invention is the provision of a radiation detector for detecting the fission product material leaking from an internally pressurized fuel rod. Another feature of the present invention is the provision of a gas injector for injecting a gas into a liquid medium contained in an enclosure and surrounding the fuel assembly which includes the leaking fuel rod so as to form a multiplicity of gas bubbles in the liquid in order to remove the fission product material adhering to the exterior surface of the fuel rod and to carry the fission product material removed thereby to the detector for detecting the leaking fuel rod. Yet another feature of the present invention is the provision of pressure relief means connected to the fuel rod for relieving the internal pressure in the fuel rod, so that the fission product material leaks from the fuel rod and into the liquid medium as the internal pressure is relieved in order to prevent the fission product material from "hiding-out" in the fuel rod. An advantage of the present invention is that time required to detect leaking fuel rods is reduced. Another advantage of the present invention is that leaking fuel rods otherwise undetectable are now detectable. These and other objects, features and advantages of the present invention will become evident to those having ordinary skill in the art upon a reading of the following detailed description taken in conjunction with the accompanying drawings wherein there is shown illustrative embodiments of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there is shown a typical nuclear reactor, generally referred to as 10, for producing heat by the controlled fission of fissile nuclear fuel material. Reactor 10 includes a reactor pressure vessel 20 disposed in a reactor cavity 15 which is defined by a pressure vessel containment structure 17 surrounding pressure vessel 20. Containment structure 17 has an upper surface 18 thereon having a rail 19 attached thereto spanning reactor cavity 15, for reasons disclosed hereinbelow. Pressure vessel 20 is open at its top end and has a plurality of inlet nozzles 30 and outlet nozzles 40 attached to the upper portion thereof (only one of each nozzle is shown). A hemispherical closure head 50 is mounted atop pressure vessel 20 and is sealingly attached to the open top end of pressure vessel 20, such that closure head 50 sealingly caps pressure vessel 20. Capping pressure vessel 20 in this manner allows for suitable pressurization of a liquid moderator coolant (not shown) within pressure vessel 20. Moreover, disposed in pressure vessel 20 are a plurality of nuclear fuel assemblies 60, each fuel assembly 60 comprising a plurality of elongate nuclear fuel rods 70 having the fissile nuclear fuel material sealingly contained therein in the form of a plurality of coaxially stacked fuel pellets 80 (see FIG. 6). As shown in FIG. 1, each fuel rod 70 also has an exterior surface 75 and is prepressurized to a predetermined internal pressure of approximately 100 to 250 psia. In addition, pressure vessel 20 is submerged in a liquid medium 85 that serves as a biological shield for service personnel who may be located in the vicinity of pressure vessel 20. During operation of nuclear reactor 10, the liquid moderator coolant enters inlet nozzle 30, circulates through pressure vessel 20 and then exits pressure vessel 20 through exit nozzle 40, whereupon it is piped to a heat exchange device (not shown) for generating steam. The steam is then piped from the heat exchange device to a turbine-generator set (not shown) for producing electricity in a manner well understood in the art. Moreover, as the nuclear material forming fuel pellets 80 fissions, gaseous and/or solid radioactive fission products 90 (e.g., Xenon-135 and Kr-85) are produced within the fuel rod 70 (see FIG. 6). Such radioactive fission products 90 are normally sealingly contained within fuel rod 70 because fuel pellets 80 producing the fission products 90 are themselves sealingly contained within fuel rods 70. However, such fuel rods 70 may nonetheless occasionally leak and thus may radioactively contaminate reactor system components in fluid communication with the liquid moderator coolant. It is therefore desirable to detect such leaking fuel rods 70 in order to replace the leaking fuel rods 70, so that radioactive contamination of reactor system components is avoided. Therefore, referring to FIG. 2, 3, 4, 5, 6, 7 and 8, there is shown the subject matter of the present invention, which is a leak-detection system, generally referred to as 100, for detecting a leaking container, which leaking container may be the leaking nuclear fuel rod 70 having the radioactive fission product material 90 leaking through a breach 110 in exterior surface 75 thereof. The fission product material 90 is capable of adhering to exterior surface 75 as it leaks through breach 110. Leak-detection system 100 comprises enclosure means, such as an elongate generally cylindrical enclosure 120 defining a cavity 130 axially therethrough, the cavity surrounding a selected one of the fuel assemblies 60 for enclosing fuel assembly 60 therein . As more fully disclosed hereinbelow, enclosure 120 is caused to penetrate liquid medium 85, so that surface 87 of liquid medium 85 is at a higher elevation than the top of fuel assembly 60. Also, for reasons disclosed in detail hereinbelow, enclosure 120 is preferably stationary during the leak-detecting process. Enclosure 120 has an open lower end portion 140 for allowing liquid medium 85 to enter and substantially fill cavity 130 of enclosure 120 and also has a substantially closed or capped upper end portion 135. However, in the preferred embodiment, liquid medium 85 does not completely fill cavity 130. That is, liquid medium 85 fills cavity 130 so as to define a liquid-free volume 150 in upper end portion 135 of enclosure 120 (i.e., the top portion of cavity 130). Thus, it will be understood from the description hereinabove, that exterior surface 75 of each fuel rod 70 is covered by liquid medium 85 and that the liquid medium 85 defines a hydrostatic pressure gradient acting against exterior surface 75 of fuel rod 70, the hydrostatic pressure gradient increasing as a function of depth of liquid medium 85. Still referring to FIG. 2, 3, 4, 5, 6, 7 and 8, leak-detection system 100 further comprises detector means, such as a radiation detector assembly, generally referred to as 160, connected to enclosure 120 for detecting and measuring the leaking fission product material 90. Detector assembly 160 includes a suction pump 170 in communication, such as by a first conduit 180, with liquid-free volume 150 for suctioning fission product 90 from liquid-free volume 150, as described more fully hereinbelow. Detector assembly 160 also includes radiation-sensitive sensor 190 housed in a sensor chamber 195 that is in communication with suction pump 170, such as by the previously mentioned first conduit 180. Sensor 190 senses the radiation emitted by the radioactive fission product material 90 that is suctioned by suction pump 170. Sensor 190 is adapted to generate a sensor output signal in response to the radiation sensed thereby. An analyzer, generally referred to as 210, is electrically connected to sensor 190, such as by wiring 220, for receiving and then analyzing the sensor output signal. Analyzer 210 is adapted to generate an analyzer output signal associated with the analysis provided thereby. In the preferred embodiment of the invention, analyzer 210 may include a high voltage power supply 230 electrically connected, such as by wiring 220, to sensor 190 for activating sensor 190. Sensor 190 is also electrically connected to an analog ratemeter 240 that displays the average counts per unit time (e.g., per second) of radiation detected by sensor 190. By use of ratemeter 240 the quantity or intensity of radiation from fission product material 90 in sensor chamber 195 is detected as a function of time. A suitable amplifier 250 electrically interconnects sensor 190 and ratemeter 250. The purpose of amplifier 250 is to generate a sensor output signal useable by ratemeter 250. Electrically connected to analyzer 210 is a controller 260 for controlling the operation of analyzer 210. Controller 260 is capable of operating leak-detection system 100 in either a "manual" or "automatic" mode as described more fully hereinbelow. In addition, a display 270 is electrically connected to analyzer 210, such as by wiring 280, for receiving the analyzer output signal and for displaying the analyzer output signal received thereby. In this manner, the analysis provided by analyzer 210 is visually displayed to the operator of leak-detection system 100. As best seen in FIGS. 5 and 8, leak-detection system 100 also comprises recirculation means, generally referred to as 287, and connected to enclosure 120 and to fluid injection means, for selectively discharging the gas to atmosphere or recirculating the gas to injection means 287, as disclosed in detail hereinbelow. In this regard, recirculation means 285 includes first conduit 180 which interconnects liquid-free volume 150 and suction pump 170 with sensor chamber 195 such that suction pump 170 suctions fission products 90 from liquid-free volume 150, through first conduit 180 and into sensor chamber 195. A second conduit 290 extends from sensor chamber 195 to the atmosphere for venting fission products 90 to the atmosphere. Disposed in second conduit 290 is a first valve 295 which is capable of opening and closing. A third conduit 300 has an end thereof connected, such as at location 305, to second conduit 290 and has the other end thereof connected, such as at location 308, to a fourth conduit 310, the purpose of which is described in more detail hereinbelow. Disposed in third conduit 300 so as to be interposed between locations 305/308 is a second valve 320 which is capable of opening and closing. Fourth conduit 310 is connected, as at location 325, to a fifth conduit 330. Fifth conduit 330 interconnects, as at location 335, a pressurized gas reservoir 340 with first conduit 180. In addition, disposed in fourth conduit 310, so as to be interposed between locations 308/325, is a third valve 345 which is capable of opening and closing. Moreover, disposed in fifth conduit 330, so as to be interposed between locations 325/335, is a fourth valve 350 which is capable of opening and closing. Furthermore, surrounding sensor chamber 195 may be a lead shielding wall 360 for shielding sensor 190 from external background radiation. Shielding sensor 190 enhances the radiation sensitivity of radiation-sensitive sensor 190. Radiation detector assembly 160 may be what is commonly referred to in the art as a "flow-through" beta-gama scintillation detector of the type available from Bicron Industries, Incorporated located in Newbury, Ohio. Referring to FIGS. 5 and 7, leak-detection system 100 also comprises the previously mentioned fluid injection means 285, such as a gas injection manifold 370, connected to open lower end portion 140 of enclosure 120 for injecting a pressurized carrier fluid (e.g., air) into cavity 130 which is defined by enclosure 120. Gas injection manifold 370 may comprise a split ring member 380 surrounding open lower end portion 140. Injecting gas into cavity 130 assists in removing any fission product material 90 that may be adhering to exterior surface 75 of fuel rod 70 and also assists in carrying the fission material 90 removed thereby to liquid-free volume 150. In this regard, gas injection manifold 370 includes a plurality of injector nozzles 390 in fluid communication-with liquid medium 85 for injecting the gas into liquid medium 85. As the gas is thus injected into liquid medium 85, a multiplicity of voids or bubbles 400 are formed in liquid medium 85, which bubbles 400 upwardly travel or rise in liquid medium 85 to encounter fission product material 90 adhering to exterior surface 75 of fuel rod 70. As bubbles 400 encounter fission product material 90, they will dislodge, disassociate, scrub or remove fission product material 90 from exterior surface 75 and upwardly carry fission product material 90 to liquid-free volume 150, so that fission product material 90 can be suctioned from liquid-free volume 150. Turning now to FIGS. 3 and 5, leak-detection system 100 also comprises alignment means, such as a plurality of spaced-apart roller assemblies 410, for aligning fuel assembly 60 coaxially within cavity 130, as described more fully hereinbelow. Roller assemblies 410 are spaced along the length of enclosure 120 and are respectively radially distributed around enclosure 120 for performing their alignment function. In this regard, each roller assembly 410 comprises a roller 420 penetrating enclosure 120 through an aperture 425 cut through enclosure 120. Fuel assembly 60 is preferably coaxially aligned within cavity 130 for proper alignment and insertion of the fuel assembly 60 back into the reactor core during routine refueling. A sealing cup 430 attached to the exterior of enclosure 120 sealingly surrounds each aperture 425 so that all the fission product material 90 is carried to liquid-free volume 150 to be suctioned therefrom. In this manner, fission product material 90 will not undesirably leak through aperture 425 and into reactor cavity 15. Referring to FIGS. 2, 3, and 4, system 100 further comprises pressure relief means connected to fuel assembly 60 for reducing or relieving .the internal pressure in fuel rod 70, so that the fission product material 90 leaks from fuel rod 70 as the internal pressure is relieved. This feature of the invention is important in order to prevent fission product material 90 from "hiding-out" in fuel rod 70. In this regard, the pressure relief means comprises an elongate and axially movable elevator 440 capable of removably gripping fuel assembly 60 and then elevating fuel assembly 60 in cavity 130. Fuel assembly 60 is elevated so that the hydrostatic external pressure gradient acting against exterior surface 75 of the breached fuel rod 70 is reduced to relieve the internal pressure in the breached fuel rod 70. As the internal pressure is relieved, the radioactive fission product material 90 expands and migrates through breach 110. In this manner, fission product material 90 is prevented from "hiding-out" in the breached fuel rod 70. More specifically, elevator 440 has a capped top end portion 450 and includes a gripper 455 adapted to removably grip fuel assembly 60. Attached to top end portion 450 of elevator 440 is an adaptor 460 connected to a cable 470 that in turn engages a pulley 480. Pulley 480 is connected to a winch (not shown) for rotating pulley 480. When the winch is operated, pulley 480 rotates in a first direction for raising elevator 440, which in turn will elevate fuel assembly 60 in cavity 430. Similarly, the winch is capable of rotating pulley 480 in an opposite second direction for lowering elevator 440, which in turn will lower the elevation of fuel assembly 60 in cavity 430. Moreover, the previously mentioned rollers 420 are capable of slidably abutting and guiding elevator 440 for aligning fuel assembly 60 along the centerline of cavity 130. Referring to FIGS. 2 and 3, a manipulator bridge or support frame, generally referred to as 490, is connected to axially movable elevator 440 and to stationary enclosure 120 for supporting elevator 440 and enclosure 120. Moreover, detector assembly 160 and pulley 480 may be mounted on support frame 490, as shown, so that detector assembly 160 and pulley 480 are suitably supported thereby. In addition, support frame 490 includes a plurality of wheels 500 adapted to slidably engage rail 19, so that support frame 490 is movable on rail 19 in order to align support frame 490 with a preselected one of fuel assemblies 60 to be leak-tested. OPERATION Leak-detection system 100 is capable of detecting leaks of fission product material 90 from any one of fuel assemblies 60 during routine refueling operations of the reactor core without the need for transporting fuel assembly 60 to a remote test chamber. In this regard, pressure vessel 20 is removed from service and closure head 50 is disconnected from atop pressure vessel 20. The upper internal structure of pressure vessel 20 is then removed in order to expose fuel assemblies 60. Next, support frame 490 is moved on rail 19 by rotatably engaging wheels 500 with rail 19, so that support frame 490 is aligned with a preselected one of the fuel assemblies 60 to be leaktested. At this point, stationary enclosure 120 will be caused to penetrate the liquid medium 85 of reactor cavity 15. AS enclosure 120 penetrates reactor cavity 15, the liquid medium 85 therein will flow through open lower end portion 140 of enclosure 120 to substantially fill cavity 130 to a level that defines fluid-free volume 50. The winch (not shown) is then operated to rotate pulley 480 so that elevator 440 is lowered, via cable 470, to a position that is coaxially aligned with the preselected one of the fuel assemblies 60. Next, gripper 455 is caused to securely grip the preselected fuel assembly 60 and raise it into cavity 15. Leak-detection system 100 is then used to perform the leak-detection in accordance with the following modes of operation which may be performed sequentially: (a) "vacuum test" mode, (b) "flow test" mode, (c) "recirculation" mode, and (d) "purge" mode. Each of these modes of operation will be described in turn hereinbelow. First, with respect to the vacuum test mode, first valve 295 is opened and then second valve 320, third valve 345 and fourth valve 350 are closed as suction pump 170 is operated. As suction pump 170 operates, the air within liquid-free volume 150 will be suctioned through first conduit 180 and into sensor chamber 195 to be detected by sensor 190. After passing into sensor chamber 195, the air will next flow into second conduit 290 and through open valve 295 on its way to the atmosphere. It will be understood from the description hereinabove that bubbles 400 are not generated to transport fission products 90 to liquid-free volume 150; rather, the purpose for operating in the vacuum test mode is to confirm that suction pump 170 will draw the necessary sample. Secondly, with respect to the flow test mode, first valve 295 remains open as third valve 345 is opened. Second valve 320 and first valve 350 are closed as suction pump 170 is operated. The pressurized gas reservoir 340 is caused to supply the pressurized gas (e.g., air) into fifth conduit 330 and thence through fourth conduit 310 and its associated third valve 345. The gas then flows to gas injection manifold 370 for generating bubbles 400. As the bubbles 400 transport fission products 90 into liquid-free volume 150, suction pump 170 will suction fission products 90 through first conduit 180, into sensor chamber 195, through second conduit 290 and its associated first valve 295. The fission product material 90 will then flow to the atmosphere. Thirdly, with respect to the recirculation or closed-loop mode, fourth valve 350 remains closed as first valve 295 and third valve 345 are closed and as second valve 320 is opened. Suction pump 170 continues operating. It will be understood from the description hereinabove that bubbles 400 are again generated to transport fission products 90 to liquid-free volume 150. More specifically, the bubbles are now provided by the output of vacuum pump 170. The vacuum pump 170 discharges the gas to gas injection manifold 370 to generate bubbles 400. As bubbles 400 transport fission products 90 into liquid-free volume 150, suction pump 170 will suction fission products 90 through first conduit 180, into sensor chamber 195, into second conduit 290, into third conduit 300, through second valve 320 and then into fourth conduit 310 on its way back to gas injection manifold 370. It will be appreciated from the description hereinabove that in the recirculation mode no fission products 90 are released to the atmosphere; rather, the fission products 90 are recirculated through enclosure 120. An important advantage of operating leak-detection system 100 in the recirculation or closed-loop mode is that it is possible to detect relatively small amounts of fission product material 90 released by relatively small-sized breaches 110 in fuel rod 70 because the recirculation mode allows the small releases of fission product-material 90 to accumulate within the closed-loop for detection. Next, with respect to the purge mode, third valve 345 remains closed as second valve 320 is closed and as first valve 295 and fourth valve 350are opened. Suction pump 170 continues operating. The pressurized gas flows from gas reservoir 340, through fifth conduit 330 and then through open fourth valve 350. The gas then flows through suction pump 170 and into sensor chamber 195 via first conduit 180. The gas next flows from sensor chamber 195 through second conduit 290 and through open first valve 295 on its way to the atmosphere. The purpose of the purge mode is to purge or remove residual fission products 90 from enclosure 120, so that test results for the next fuel assembly to be leak-tested are not corrupted, influenced or contaminated by such residual fission products that would otherwise remain in leak-detection system 100. It will be appreciated from the description hereinabove that the purging mode is considered complete when sensor 190 detect relatively no fission products 90. It will be understood from the description hereinabove that as the gas flows through fourth conduit 310 to injection manifold 370, it will exit injection nozzles 390 which are in fluid communication with liquid medium 85 in cavity 130. The gas injected into cavity 130 upwardly travels or rises therein and forms the multiplicity of voids or bubbles 400. The bubbles 400 encounter any fission product material 90 that may be passing through breach 110 and adhering to exterior surface 75 of fuel rod 70. As bubbles 400 encounter such fission product material 90, they will dislodge, disassociate, scrub or remove the fission product material 90 from exterior surface 75 and carry the fission product material removed thereby to liquid-free volume 150. The bubbles 400 will dissipate as they enter liquid-free volume 150 and will expel the fission product material 90 and gas mixture into liquid-free volume 150, so that fission product material 90 is substantially suspended in liquid-free volume 150. As fission product material 90 enters liquid-free volume 150, it is suctioned through first conduit 180 and into sensor chamber 195 by means of suction pump 170. Once inside sensor chamber 195, sensor 190 will detect the presence of fission product material 90 therein. In this regard, high voltage supply 230 is caused to supply power to sensor 190 for activating sensor 190, so that sensor 190 detects the fission product material 90 in sensor chamber 195. As sensor 190 operates, it generates a sensor output signal. The output signal of sensor 190 is passed to analyzer 210 for analysis. Amplifier 250 which belongs to analyzer 210, electrically interconnects sensor 190 and analog ratemeter 240. Ratemeter 240 displays the average counts per unit time (e.g., per second) of radiation detected by sensor 190. In this manner, the quantity or intensity of radiation is detected as a function of time. Controller 260, which is electrically connected analyzer 210, controls the operation of analyzer 210. In addition, display 270, which is electrically connected to analyzer 210, receives the analyzer output signal and displays the analyzer output signal received thereby. As previously described, some of the fission product material 90 may tend to "hide-out" within fuel rod 70 rather than readily migrate through breach 110. Fission product "hide-out" is undesirable because such "hide-out" of fission product material 90 within fuel rod 70 may cause a breached fuel rod 70 to be otherwise undetectable or at least imprecisely detected and analyzed. Therefore, it is desirable to coax such fission products from fuel rod 70 prior to beginning the flow test mode of operation. In order to coax the fission product material 90 through breach 110 of fuel rod 70, the winch (not shown) rotates pulley 480 for elevating elevator 440 a predetermined amount (e.g., approximately 24 feet). As elevator 440 is elevated by the predetermined amount, the hydrostatic pressure will acting against the exterior surface 75 of each breached fuel rod 70 is preferably reduced by about 20 pounds per square inch. This reduction in external hydro-static pressure relieves the internal fuel rod pressure and will cause the fission product gases and/or solids "hiding-out" within fuel rod 70 to expand and migrate through breach 110 because the internal pressure of fuel rod 70 after elevation thereof is less than before elevation of fuel rod 70. However, as the fission product material 90 is coaxed through breach 110, it may tend to adhere to exterior surface 75 of fuel rod 70. The fission product material 90 tending to adhere to exterior surface 75 of fuel rod 70 will be removed therefrom by action of bubbles 400, as previously described, so that fission product material 90 is carried to liquid-free volume 150 to be suctioned therefrom in order to be precisely detected by sensor 190. Although the invention is fully illustrated and described herein, it is not intended that the invention as illustrated and described be limited to the details shown, because various modifications may be obtained with respect to the invention without departing from the spirit of the invention or the scope of equivalents thereof. For example, leak-detection system 100 need not be limited to detecting leaks of fission product material from nuclear fuel rods; rather, leak-detection system 100 including a detector suitable for the material being detected is usable for detecting leaks from any similar container having material leaking therefrom. Therefore, what is provided is a leak-detection system and method for detecting a leaking container, which leaking container may be a leaking nuclear fuel rod having a radioactive fission product material leaking therefrom.

042591538

description

In the text of the present disclosure, the term "can of the control and safety system" is to be understood as a housing, wherein a rod of the control and safety system is installed. When in the housing, the dimensions of the rod are equal to those of a fuel assembly. The device of this invention can be used to remove both empty cans and cans containing rods of the control and safety system; it can also be used to remove fuel assemblies. The proposed device 1 for the removal of fuel assemblies and cans of the control and safety system from the core 2 of a nuclear reactor, shown in FIG. 1, is installed in a cover 3 of the nuclear reactor, over the core 2 containing fuel assemblies 4 secured in sockets of a collector 5. The device 1 includes a hollow bar 6, wherein there is arranged a grip 7 intended to grip the fuel assembly 4 by its head to withdraw said fuel assembly 4 from the reactor core 2. The grip 7 is coupled to the hollow bar 6 by means of rollers 8 arranged in a housing 9 of the grip 7 at an angle of 120.degree. relative to each other and interacting with the internal surface of the hollow bar 6. As a result, the grip 7 is centered in relation to the axis of the hollow bar 6 and is movable along said axis of the hollow bar 6 over a distance which is not less than the length of the fuel assemblies 4. The internal diameter of the hollow bar 6 enables the bar 6 to envelop the fuel assembly 4 so that there is a clearance between the bar 6 and the fuel assembly 4 being removed; as a result, the fuel assembly 4 can freely move inside the hollow bar 6 over the entire length of said fuel assembly 4. On the core-facing end of the hollow bar 6 there is mounted an auxiliary grip 10. A magnified view of the auxiliary grip 10 is presented in FIG. 2. The auxiliary grip 10 is intended to grip the lower ends of the fuel assemblies 4 being removed (FIG. 1). The grip 10 (FIGS. 1 and 2) comprises six jaws 11 movable mounted on axles 12 which are rigidly secured in the hollow bar 6. Fitted over the hollow bar 6 is a sleeve member 13 which envelops the auxiliary grip 10 and is adapted for axial movement in relation to the hollow bar 6. The bar 6 is sufficiently long for the auxiliary grip 10 to reach the end of the fuel assembly 4 being removed, while the main grip 7 is on the head of said fuel assembly 4, which is at the opposite end of the fuel assembly 4. Opposite each of the jaws 11, there is an opening 14 (FIG. 2) in the sleeve member 13. Each opening 14 has bevelled sides 15 and 16 to interact with external surfaces 17 and 18, respectively, of the jaws 11, bevelled so as to correspond to the profile of the bevelled surfaces 15 and 16. In FIG. 2, the auxiliary grip 10 is opened; the bevelled external surfaces 17 of the jaws 11 interact in this case with the bevelled sides 15 of the openings 14. The hollow bar 6 (FIG. 1) is coupled to the sleeve member 13 by means of a nut 19 which makes it possible for the hollow bar 6 to axially move with respect to the sleeve member 13 over a distance necessary to close (open) the jaws 11 of the auxiliary grip 10. At the end of the sleeve member 13, on the side of the auxiliary grip 10, there is provided an external conical surface 20 (FIG. 2) intended for centering the main grip 7 relative to the axis of the fuel assembly 4 being removed, as the latter is gripped by its head. On the sleeve member 13 there is mounted a support 21 (FIG. 1) which is slidable along the axis of the sleeve member 13 and is secured on the cover 3 of the nuclear reactor. Mounted on the support 21 are hydraulic cylinders 22 which provide enough traction to extract the assembly 4 from the collector 5, as the auxiliary grip 10 grips the lower end of the fuel assembly 4. To withdraw the fuel assembly 4, rods 23 of the hydraulic cylinders 22 interact with stops 24 rigidly mounted on the sleeve member 13. In the portions of the hollow bar 6 and the sleeve member 13, extending above the cover 3 of the reactor, and in the support 21 there are installed sealings 25, 26, 27 and 28 to prevent the penetration of air into the nuclear reactor, as well as a discharge of contaminated gas from the nuclear reactor into the atmosphere. FIG. 3 shows the device of FIG. 1, the only difference being that the fuel assemblies, adjacent to the fuel assembly 4 being withdrawn, have been removed, and the hollow bar 6 with the auxiliary grip 10 and the sleeve member 13 is lowered to the stop into the collector 5; the auxiliary grip 10 is released. FIG. 4 shows the area A of FIG. 3; the bevelled external surfaces of the jaws 11 of the auxiliary grip 10 interact with the bevelled sides 16 of the openings 14 provided in the sleeve member 13. FIG. 5 shows an alternative embodiment of the proposed device for the removal of fuel assemblies and cans of the control and safety system from the core of a nuclear reactor. The device of FIG. 5 is used to remove jammed adjacent fuel assemblies. FIG. 5 only shows part of this device, located just above the core 2 of the nuclear reactor. According to this second embodiment, the hollow bar 6 and the sleeve member 13 are provided with a longitudinally extending slot 30 to receive the fuel assembly 4 which is adjacent to the one being removed. As a fuel assembly is received in the slot 30, there is a clearance between said fuel assembly and the walls of the slot 30. The slot 30 is somewhat longer than the fuel assembly 4; in all other respects, the device of FIG. 5 is similar to that of FIG. 1. FIG. 6 is a sectional view taken on line VI--VI of FIG. 5 and shows the arrangement of the slot 30 with respect to the jammed fuel assembly 4; all the adjacent fuel assemblies have been removed. The width of the slot 30 is determined by the size of the fuel assembly 4. The hollow bar 6 and the sleeve member 13 may be provided with several slots 30 arranged at an angle to one another. The number, arrangement, shape and dimensions of the slots 30 depend on the arrangement, shape and dimensions of the fuel assemblies 4 to be withdrawn from the reactor core 2. The device provided with the slots 30 can also be used to extract single jammed fuel assemblies 4. The proposed device 1 (FIG. 1) for the removal of fuel assemblies and cans of the control and safety system from the core 2 of a nuclear reactor operates as follows. In order to remove unjammed fuel assemblies 4 from the reactor core 2, the device 1 is oriented in the known manner with respect to the coordinates of the fuel assembly 4 to be removed. After this, the sleeve member 13, the hollow bar 6 and the opened auxiliary grip 10 are lowered with the aid of a drive 29 until they abut against the upper end faces of the fuel assemblies 4 adjacent to the one that has to be removed. The sleeve member slides under gravity with respect to the support 21; at the end of its travel it interacts with its external conical surface 20 with the heads of the fuel assemblies 4 adjacent to the one that is to be extracted and thus accurately centers the main grip 7 relative to the axis of the fuel assembly 4 to be removed. It also forces the adjacent fuel assemblies 4 aside from the one that is to be removed; the width of the clearance corresponds to the overall design of the fuel assemblies 4. The main grip 7 is then lowered with the aid of the same drive 29 onto the head of the fuel assembly 4 to be removed, grips the head and draws it. As this takes place, the force directed into the hollow bar 6 is limited so as not to destroy the fuel assembly 4 being removed. The drive 29 then raises the sleeve member 13, the hollow bar 6, the opened auxiliary grip 10 and the fuel assembly 4 being removed, gripped by the main grip 7, somewhat above the heads of the fuel assemblies 4 in the reactor core 2, and the device 1 is oriented with respect to the coordinates of the socket, wherein the fuel assembly 4 being removed is to be inserted. The removed fuel assembly 4 is installed in its socket in a similar manner, although the sequence of operations in this case is reversed. As fuel assemblies are being removed from the core of a nuclear reactor, there may be a situation when a fuel assembly is jammed in the collector so that it cannot be taken out by using the traction force developed by the main grip 7. In such a case, all the fuel assemblies 4 that are adjacent to the jammed one are first removed from the core 2 (FIGS. 1 and 3) of the nuclear reactor, which is done as described above. After this, the device 1 is oriented in the known manner with respect to the coordinates of the jammed fuel assembly 1. The sleeve member 13, the hollow bar 6 and the opened auxiliary grip 10 move down by gravity and with the aid of the drive 29. As this takes place, the sleeve member freely slides in the support 21, whereas the fuel assembly 4 to be removed enters the hollow bar 6. As the end face of the sleeve member 13 abuts against the collector 5, the auxiliary grip 10 grips the fuel assembly 4 by its lower end, which is done by rotating the nut 19. The rotation of the nut 19 causes downward movement of the hollow bar 6 with respect to the sleeve member 13. As this takes place, the bevelled surfaces 18 of the jaws 11 (FIG. 4) of the auxiliary grip 10 slide along the bevelled sides 16 of the openings 14 provided in the sleeve member 13. Upon reaching the end face of the collector 5, the grip 10 closes and firmly grips the lower end of the fuel assembly 4 (FIG. 3). The main grip 7 is then lowered onto the head of the fuel assembly 4 being removed and grips it by the head. In order to provide the necessary traction force to withdraw the fuel assembly 4 from its socket in the collector 5, working fluid is directed under pressure into the hydraulic cylinders 22. As a result, the rods 23 of the hydraulic cylinders 22 interact with the stop 24 rigidly mounted on the sleeve member 13 and produce axial pressure on the sleeve member 13, which is necessary to withdraw the fuel assembly 4 from its socket in the collector 5. As the tail of the fuel assembly 4 leaves the collector 5, the drive 29 raises the sleeve member 13, the hollow bar 6 and the fuel assembly 4, gripped by the closed auxiliary grip 10 and the main grip 7, somewhat above the heads of the fuel assemblies 4 in the reactor core 2, and the device 1 is oriented with respect to the socket, into which the removed fuel assembly 4 is to be inserted. By rotating the nut 19, the auxiliary grip 10 is released, and the fuel assembly 4 is installed by the main grip 7 into its socket in the manner described above. The fuel assembly can also be placed in its socket by the auxiliary grip 10; in this case the main grip 7 first disengages from the head of the fuel assembly 4. The adjacent jammed fuel assemblies 4 (FIGS. 5 and 6) are removed from the reactor core 2 as the single jammed fuel assembly 4, although in this case prior to lowering the sleeve member 13 with the auxiliary grip 10 and the hollow bar 6, the slot 30 is oriented with due regard for the position of the adjacent jammed fuel assemblies 4. All the other operations involved in the removal of jammed fuel assemblies 4 are carried out as described above. The device 1 (FIGS. 1 and 3) is capable of the removal of empty cans of the control and safety system, as well as cans containing rods of the control and safety system. In the case of empty cans, all the operations are carried out by the auxiliary grip 10.

summary

claims

1. A process for reducing radioactive contamination of a surface of a component which is used in a nuclear reactor and is in contact with radioactively contaminated water, comprising: producing a hydrophobic film on the surface of the component by wetting the surface with an aqueous solution containing a film-forming amphiphilic substance that comprises a primary, secondary, tertiary or quaternary amino group as a polar group, wherein the hydrophobic film prevents at least some radioactive particles from adhering to the surface of the component. 2. The process as claimed in claim 1, characterized in that the hydrophobic film is produced on an interior surface of the component, the component comprising a water-conducting circuit of the nuclear reactor. 3. The process as claimed in claim 2, characterized in that the hydrophobic film is produced after a part-circuit or full-circuit decontamination of the water-conducting circuit. 4. The process as claimed in claim 2, further comprising replacing the component with a new component, and forming a new hydrophobic film on the new component. 5. The process as claimed in claim 2, further comprising introducing a chemical compound or element into the aqueous solution that forms at least one layer on the surface before production of the hydrophobic film. 6. The process as claimed in claim 5, characterized in that the chemical compound or element is a noble metal. 7. The process as claimed in claim 5, characterized in that the chemical compound or element is a salt of chromic acid. 8. The process as claimed in claim 1, characterized in that the hydrophobic film is produced at a point in time outside load operation. 9. The process as claimed in claim 8, characterized in that the hydrophobic film is produced during a start-up phase of the nuclear reactor. 10. The process as claimed in claim 1, wherein the component is a tool used for inspecting the nuclear reactor. 11. The process as claimed in claim 10, characterized in that the tool is dipped into an aqueous solution of the amphiphilic substance. 12. The process as claimed in claim 10, characterized in that the surface of the tool is sprayed with an aqueous solution of the amphiphilic substance. 13. The process as claimed in claim 1, characterized in that an application of the amphiphilic substance to the surface is stopped when the surface has been covered with the hydrophobic film. 14. The process as claimed in claim 1, characterized by use of an amphiphilic substance which has a polar end formed by at least one polar group and a nonpolar end formed by at least one hydrocarbon radical. 15. The process as claimed in claim 1, characterized in that the contaminated water contains radioactive colloids and a film is produced by means of an amphiphilic substance which has a polar group having a charge having a same sign as the charge on the radioactive colloids. 16. The process as claimed in claim 1, characterized in that an amphiphilic substance which contains an aliphatic group as nonpolar radical is used. 17. The process as claimed in claim 1, characterized by a nonpolar radical having from 8 to 22 carbon atoms. 18. The process as claimed in claim 1, wherein producing the hydrophobic film on the surface of the component is performed before the surface comes into contact with radioactively contaminated water. 19. The process as claimed claim 14, characterized in that the aqueous solution is influenced in such a way that a charge which is opposite to the charge of the polar group of the amphiphilic substance arises on the surface or that a surface charge opposite to that of the polar group of the amphiphilic substance is increased. 20. The process as claimed in claim 19, characterized in that an influence on the aqueous solution is exerted via a pH of the aqueous solution. 21. A process for reducing radioactive contamination of a surface of a component which is used in a nuclear reactor and is in contact with radioactively contaminated water, comprising: producing a hydrophobic film on the surface of the component, before the surface comes into contact with radioactively contaminated water, by wetting the surface with an aqueous solution containing a film-forming amphiphilic substance, wherein the hydrophobic film prevents at least some radioactive particles from adhering to the surface of the component.

058870458

description

Other characteristics will become more clear from the description of particular embodiments. The following compositions found of interest: ______________________________________ COMPOSITIONS I II III ______________________________________ Tin 1.3 1.3 1.3 Iron 0.6 0.6 0.6 Vanadium 0.3 0.25 0 Chromium 0 0 0.25 Oxygen 0.12 0.14 0.14 Carbon 140 140 140 Silicon 90 90 90 ______________________________________ the other components being zirconium and impurities. The starting alloy was in the form of an ingot. It was formed into a bar by forging or rolling and, after heating to the .beta. phase, was water quenched at a controlled rate to bring it into the .alpha. region, for example at a cooling rate in the range 5.degree. C. per second to 30.degree. C. per second until the temperature was less than about 800.degree. C. After quenching, annealing was effected at a temperature of less than 800.degree. C. to prevent transformation of the .alpha. phase into the .beta. phase. Extrusion was carried out after machining a tubular billet and heating to between 600.degree. C. and 700.degree. C. The drawn blank, after undergoing any required annealing at a temperature of less than 800.degree. C., then underwent the required number of successive cold rolling steps to bring it to the required thickness, with intermediate annealing steps carried out in argon, each for one to three hours, to produce a suitable .SIGMA.A. In practice, four or five rolling steps were generally carried out to produce solid cladding tubes of conventional diameter and thickness. Finally, a final annealing step was carried out in an inert atmosphere, at about 485.degree. C. for one to three hours if a stress-relieved structure was required, or at about 580.degree. C. for about two hours if a recrystallized structure was required. The tests were carried out on samples to compare the alloys of the invention containing different tin contents with Zircaloy-4 type alloys. Generalized Corrosion Tests were carried out on recrystallized samples in an autoclave, in water and steam. The results are shown in Table I below. TABLE I ______________________________________ Weight gain .DELTA.P (mg/dm.sup.2) Water Steam ALLOY 350.degree. C. - 210 days 400.degree. C. - 30 days ______________________________________ 1 Zr 0.6 Fe; 0.3 V 29.2 26.4-38.5 2 Zr 0.6 Fe; 0.3 V; 0.5 Sn 31 27.5 3 Zr 0.6 Fe; 0.3 V; 1.0 Sn 32.2 30.4 4 Zr 0.6 Fe; 0.3 V; 1.5 Sn 32 30.9 (invention) 5 Zircaloy 4 43.9-47.2 32 ______________________________________ The results obtained, in particular for alloy 4 which was in accordance with the invention, show that an increase in the tin content from 0 to 1.5% had no effect on generalized corrosion resistance in water and steam. Corrosion in a Lithium Medium and Creep Resistance The influence of tin content on the corrosion resistance of Zircaloy 4 type alloys in a medium containing lithium hydroxide was studied in water containing 70 ppm of lithium at 360.degree. C. The results are shown in Table II. TABLE 2 ______________________________________ % Sn in Weight gain .DELTA.P (mg/dm.sup.2) alloy 50 days 100 days 150 days ______________________________________ 1.5 48 78 112 1.3 51 85 148 0.5 35 72 740 ______________________________________ The highly favorable influence of a high tin content (between 1.2% and 1.5%) on the corrosion resistance in a lithium hydroxide medium was observed in alloys in accordance with the invention. A high tin content was also shown to be favorable to the creep resistance of this alloy. Measurements of diametral creep .epsilon..sub.D at 400.degree. C. over 240 hours at a pressure of 130 MPa gave the following values for a stress-relieved alloy: ______________________________________ Sn content (%) .epsilon..sub.D (%) ______________________________________ 1.5 (invention) 1.5 1.3 (invention) 2 0.5 4.2 ______________________________________ The results obtained show a quasi-linear relationship between the Sn content and the creep characteristics.

description

Referring now to the drawings, particularly to FIG. 1, there is illustrated a spacer for use in a fuel bundle or assembly in a nuclear reactor, the spacer being generally designated 10. The spacer includes a plurality of cylindrical ferrules 12 having stops 13 projecting from interior wall surfaces thereof and springs 16 for bearing against fuel rods 18 extending through the spacer in each of the ferrules 12. The springs 16 impart a lateral force to the fuel rods to maintain the fuel rods in bearing engagement against the stops 13. The spacer 10 also has a surrounding band 20 laterally encompassing the ferrules. As illustrated, a pair of water rods 22 pass through enlarged openings in the spacer 10. A clip 24 is secured to the ferrules, for example, by welding, and has a vertical extent corresponding to the vertical extent of the ferrules. One of the water rods 22 carries upper and lower tabs 26 which engage upper and lower margins, respectively, of the clip 24 to prevent relative axial displacement of the spacer and one water rod. As previously stated, in unchanneled bundles, the twist of the water rods and spacers may be sufficient to misalign the tabs 26 with the clip, enabling the spacer and water rod for relative axial displacement. Referring to FIGS. 2A-2C, there is illustrated in a preferred form of the present invention, a spacer/water rod connecting structure or retention assembly, generally designated 30, for use in a spacer of the type illustrated in FIG. 1 but having a different clip and water rod retention tab arrangement, as will now be explained. As best illustrated in FIG. 2C, the clip 32 is generally U-shaped with laterally outwardly directed flanges 34 to facilitate welding to adjoining ferrules 12 (FIG. 1). The base of the U-shaped clip 32 defines a slot or opening 36 bounded by axially opposite structural portions 38 and 40, as well as lateral or side structural portions 42 and 44 (FIG. 2B). Thus, the portions 38, 40, 42 and 44 perimetrically enclose the slot 36. Along the side portion 42 of clip 32, there is provided a clip tab 45 which projects toward the opposite side portion 44 of slot 36. The metal material from which the clip 32 is made enables the clip tab 45 to be resiliently deflected. A water rod tab 46 projects generally radially outwardly from the water rod and is sized for reception in the slot 36. To secure the spacer and water rod relative to one another, the water rod is disposed through the opening in the spacer with the tab 46 outside of the opening or slot 36 as illustrated in FIG. 2B. By rotating the spacer and water rod relative to one another, for example, by rotating the water rod in a counterclockwise direction as illustrated in FIG. 2B, the water rod tab 46 engages the tab 45 of the clip 32, resiliently deflecting tab 45 such that the water rod tab 46 obtains the position illustrated in FIG. 3B within slot 36. The clip tab 45, once the water rod tab 46 passes by out of engagement with tab 45 and into slot 36, resiliently deforms back to its initial position, generally within the slot 36. As a consequence, the side portion 44 of clip 32 and tab 45 lie on opposite sides of the water rod tab 46, preventing relative rotation between the spacer and water rod. Even with a twist of an unchanneled fuel bundle assembly, the water rod tab 46 remains captured in the slot between the side portion 44 and clip tab 45. To disassemble the water rod and spacer, a tool, not shown, may be used to deflect the spring in the same direction as in the initial assembly whereby the tab 46 can be rotated out of the slot past the tab 45. Referring now to the embodiment of the present invention illustrated in FIGS. 4A-4B and 5A-5C, wherein like reference numerals as in the prior embodiment apply to like parts followed by suffix xe2x80x9ca,xe2x80x9d the water rod 22a includes a tab 46a. The clip 32a includes a slot 36a which is perimetrically bounded by axially opposite end portions 38a and 40a, as well as bounded on its sides by side portions 50 and 52. The clip 32a is secured to the spacer similarly as in the prior embodiment. In this form, however, the tab 45 projecting from one side portion of the clip is omitted. To secure the spacer and one water rod against axial displacement relative to one another, the tab 46a is rotated into the slot 36a by relative rotation of the spacer and water rod. The tab 46a is thus prevented from rotational movement by one side portion 50 of the slot 36a. To prevent rotational movement of the tab in a direction displacing the tab from the slot, a second water rod 23 is disposed through an opening in the spacer adjacent the first water rod. The second water rod prevents rotation of the tab out of the slot 36a. The water rod does not rely on a square lower end plug and tie plate hole to prevent water rod rotation as in certain current designs. Thus, the present spacer/water rod retention assembly not only prevents relative rotation of the water rod and spacer to an extent permitting axial displacement of the water rod and spacer, particularly when installed in unchanneled fuel bundle assemblies, but also permits the less costly fabrication of a round water rod end plug and round openings in the lower tie plate of the fuel assembly. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

claims

1. A CT system comprising:a rotatable gantry having an opening for receiving an object to be scanned;an x-ray tube having a focal spot from which x-rays emit;a detector comprising an imaging area of pixels and a calibration area of pixels;a pre-patient collimator positioned between the x-ray tube and the detector having first and second apertures that pass x-rays respectively to at least a portion of the imaging area of pixels, and to the calibration area of pixels; anda computer programmed to:determine a focal spot location in a Z-direction using energy derived from x-rays that fall upon the calibration area of pixels; andissue commands to adjust a Z-position of the focal spot based on the determined position of the focal spot from the x-rays that fall upon the calibration area of pixels, by adjusting a grid voltage within a cathode of the x-ray tube;wherein the pre-patient collimator includes a plurality of apertures that are not contiguous with one another, each of which includes both imaging apertures and calibration apertures that are contiguous with one another, wherein the calibration apertures in each of the plurality of apertures have different widths from one another along the Z-direction and at least one of the calibration apertures is narrower than its corresponding imaging aperture, resulting in a different amount of pixels used for reference channels. 2. The CT system of claim 1, wherein the imaging area of pixels and the calibration area of pixels are contiguous with one another. 3. The CT system of claim 1, wherein the imaging area of pixels and the calibration area of pixels have different widths in the Z-direction of the CT scanning system. 4. The CT system of claim 1, wherein the calibration area of pixels includes:a first area of pixels for providing reference signals; anda second area of pixels for tracking focal spot motion such that the computer determines a location of the focal spot. 5. The CT system of claim 4, wherein:the first area of pixels is one or more rows of central pixels as defined along the Z-direction; andthe second area of pixels are outermost rows of pixels on either side of the first area of pixels as defined along the Z-direction. 6. The CT system of claim 5, wherein the computer is programmed to determine the Z-position of the location of the focal spot using the energy derived from the x-rays that fall upon the second area of pixels. 7. The CT system of claim 1, wherein the computer is programmed to issue commands to a motor to adjust a position of the pre-patient collimator in the Z-direction, based on the determined position of the focal spot. 8. The CT system of claim 1, wherein the imaging apertures have widths in the Z-direction that are different from one another such that the x-rays that pass therethrough fall upon different sized portions of the imaging area of pixels. 9. A method of CT imaging, comprising:passing x-rays through an opening in a pre-patient collimator, through an object, and to at least a portion of a detector, the detector including an imaging area of pixels and a calibration area of pixels;determining a focal spot location using energy derived from x-rays that fall upon the calibration area of pixels; andadjusting the focal spot location in a Z-direction by adjusting a grid voltage on a cathode, based on the determination of the focal spot location using the energy derived from x-rays that fall upon the calibration area of pixels;wherein the pre-patient collimator includes a plurality of more than two apertures that are not contiguous with one another, each of which includes both imaging apertures and calibration apertures that are contiguous with one another, wherein the calibration apertures in each of the plurality of apertures have different widths from one another along the Z-direction and at least one of the calibration apertures is narrower than its corresponding imaging aperture, resulting in a different amount of pixels used for reference channels. 10. The method of claim 9, wherein passing the x-rays through the opening includes passing the x-rays to the imaging area of pixels and the calibration area of pixels that are contiguous with one another. 11. The method of claim 9, wherein the imaging area of pixels and the calibration area of pixels have different widths in the Z-direction of a CT scanning system. 12. The method of claim 9, wherein the calibration area of pixels includes:a first area of pixels for providing reference signals; anda second area of pixels for tracking focal spot motion such that a computer determines the focal spot location. 13. The method of claim 12, wherein:the first area of pixels is one or more rows of central pixels as defined along the Z-direction; andthe second area of pixels are outermost rows of pixels on either side of the first area of pixels as defined along the Z-direction. 14. The method of claim 13, further comprising determining a Z-position of the focal spot location using the energy derived from the x-rays that fall upon the second area of pixels. 15. The method of claim 9, further comprising adjusting a position of the pre-patient collimator based on the determined focal spot location. 16. The method of claim 9, wherein the imaging apertures have widths in the Z-direction that are different from one another such that the x-rays that pass therethrough fall upon different sized portions of the imaging area of pixels.

summary

description

This application is a U.S. national phase under the provisions of 35 U.S.C. §371 of International Patent Application No. PCT/EP12/51435 filed Jan. 30, 2012, which in turn claims priority of French Patent Application No. 11 50871 filed Feb. 3, 2011. The disclosures of such international patent application and French priority patent application are hereby incorporated herein by reference in their respective entireties, for all purposes. The present invention relates to a process for separating at least one platinoid element from an acidic aqueous solution comprising, besides this platinoid element, one or more other chemical elements. Hereinabove and hereafter, it is pointed out that “platinoid element” is taken to mean an element that may be chosen from platinum, palladium, rhodium, ruthenium, iridium, osmium. The invention is likely to find application in the field of the treatment and the recycling of irradiated nuclear fuels where it has a quite particular interest for recovering in a selective manner a platinoid element from aqueous solutions of high activity such as, for example, raffinates from the treatment of irradiated nuclear fuels. The processes that make it possible to extract and to purify uranium and plutonium present in the dissolution liquors of irradiated nuclear fuels generate effluents, which are given the name of raffinates. Raffinates are aqueous solutions with high nitric acidity, typically from 2 to 5 M, which contain two minor actinides, namely americium and curium, lanthanides such as lanthanum, cerium, praseodymium, neodymium, samarium and europium, fission products other than lanthanides such as molybdenum, zirconium, rubidium, ruthenium, rhodium, palladium and yttrium, as well as corrosion products such as iron and chromium. The management thereof presently consists in concentrating them as much as possible then conditioning them in vitreous matrices with a view to storage before ultimate disposal. Nevertheless, the presence of platinoid elements (such as palladium, rhodium and ruthenium) generates, at the level of vitrification, among others, the following difficulties: they exhibit limited solubility in the containment glasses; they tend to form particles that precipitate in the vitrification crucibles and, as a result, considerably perturb the functioning of the vitrification processes. It is to overcome these difficulties and also with a view to recycling the platinoid elements (with regard, in particular, to the predictable rarefaction of the natural resources of these elements) that certain authors have put in place processes for recovering these elements from a solution containing them in addition also to other radioactive elements. Different separation techniques have been implemented, among which may be cited: electrochemical reduction techniques in nitric acid medium, so as to deposit on an electrode the platinoid elements, which it is wished to separate, as described in US 2003/0099322, with, nevertheless, as drawback, a relatively complex implementation; liquid-liquid extraction techniques involving the use of extraction agents, such as tricaprylmethylammonium nitrate (as described in U.S. Pat. No. 4,162,231) or dialkyl sulphide (as described in U.S. Pat. No. 5,503,812), these techniques nevertheless have the drawback of generating an important amount of secondary effluents; precipitation techniques by chemical reduction of platinoid elements in nitric acid medium involving the use of a chemical reducing agent, such as saccharose as in U.S. Pat. No. 4,290,967, this embodiment bringing about problems of denitration reaction, which reaction has a highly exothermic character which could be the source of self-ignition of the nitric acid medium. With a view to overcoming the drawbacks inherent in the techniques implemented in the prior art, the authors of the present invention have proposed developing a novel method for recovering one or more platinoid elements contained in an acidic aqueous solution, for example, an aqueous nitric solution, comprising other chemical elements, such as radioactive elements. They have thus discovered, in a surprising manner, that by using certain organic alcohols, it is possible to extract selectively the platinoid elements contained in an acidic aqueous solution, by a simple and cheap implementation, without this generating large amounts of secondary effluents and without this resulting in the drawbacks linked to important denitration reactions, when the acidic aqueous solution is an aqueous nitric solution. The invention thus relates to a method for recovering at least one platinoid element contained in an acidic aqueous solution comprising chemical elements other than said platinoid element, said process comprising the following steps: a step of bringing said acidic aqueous solution into contact with a reducing amount of a reducing agent which is a non-sulphurous and non-glucidic alcoholic compound chosen from cyclic, optionally aromatic, alcohols and aliphatic polyols, by means of which said platinoid element is reduced to its 0 oxidation state; a step of separating said thus reduced platinoid element from said acidic aqueous solution. Before going into more detail in the description of the invention, the following definitions will be defined. Hereinabove and hereafter, platinoid element is taken to mean a metal element existing in an oxidation state different to 0, which metal element is chosen from platinum, palladium, rhodium, ruthenium, iridium, osmium. Thanks to the choice of specific alcohols used in specific amounts (namely, amounts enabling reduction to its 0 oxidation state of the platinoid element(s) present in the acidic aqueous solution), it is possible to obtain a selective reduction of the platinoid element(s) compared to other chemical elements present in the acidic aqueous solution, without there being pollution from said solution by sulphur (due to the fact that the alcohols are non-sulphurous), which would be totally unacceptable for the vitrification of the solution thereby obtained after extraction of said platinoid elements. As mentioned above, the non-sulphurous and non-glucidic alcoholics that can be used for the process of the invention may be cyclic, optionally aromatic, alcohols i.e. in other words: cyclic, optionally aromatic, hydrocarbon compounds comprising at least one ring directly bearing at least one hydroxyl group; or cyclic, optionally aromatic, hydrocarbon compounds, the ring of which bears at least one, linear or branched, saturated or unsaturated hydrocarbon group, which hydrocarbon group bears at least one hydroxyl group. Concerning cyclic hydrocarbon compounds, comprising at least one ring directly bearing at least one hydroxyl group, alicyclic and monocyclic compounds comprising from 4 to 10 carbon atoms, bearing at least one hydroxyl group may be cited. By way of example of compounds complying with this definition, cyclohexanol may be cited. Concerning cyclic, optionally aromatic, hydrocarbon compounds, the ring of which bears at least one, linear or branched, saturated or unsaturated hydrocarbon group bearing at least one hydroxyl group, monocyclic aromatic compounds may be cited, the ring of which bears at least one, linear or branched, hydrocarbon group bearing at least one hydroxyl group and the ring of which also optionally bears one or more groups other than the aforementioned hydrocarbon group, such as alkoxy, —OH groups. More precisely, it may be phenylic compounds, the phenyl group of which bears at least one, linear or branched, saturated or unsaturated hydrocarbon group, being able to comprise from 4 to 10 carbon atoms, which hydrocarbon group bears at least one hydroxyl group, which phenyl group may also bear one or more groups other than the aforementioned hydrocarbon group. According to a first variant, specific compounds complying with the definition given above may be phenylic compounds, the phenyl group of which bears a linear or branched, saturated hydrocarbon group comprising from 1 to 4 carbon atoms, which hydrocarbon group bears a hydroxyl group. More precisely, specific compounds complying with this definition may be phenylic compounds, the phenyl group of which bears a —CH2—OH group and optionally at least one group chosen from alkoxy or —OH groups. When the compound consists of a phenyl group, only bearing a —CH2OH group, it corresponds to the benzylic alcohol of following formula: Other specific compounds complying with this definition may be phenylic compounds, the phenyl group of which bears a —CH(OH)—CH3 group and optionally at least one group chosen from alkoxy or —OH groups. When the compound consists of a phenyl group, only bearing a —CH(OH)—CH3 group, it corresponds to the alcohol of following formula: commonly designated under the terminology of 1-phenyl-1-ethanol. When the compound consists of a phenyl group, bearer of a —CH2OH group and at least one group other than a —CH2OH group, it may correspond to a benzylic alcohol derivative of following formula: in which R1 is an alkoxy group or a hydroxyl group. R1 may be situated in ortho, meta or para position with respect to the —CH2—OH group. Advantageously, R1 is situated in para position with respect to the —CH2—OH group, in which case the compound meets the following formula: R1 being as defined above. Specific compounds meeting the definition given above may be 4-methoxybenzylic alcohol or 4-hydroxybenzylic alcohol, which correspond respectively to the following formulas: According to a second variant, specific compounds meeting the definition given above (namely, compounds belonging to the family of phenylic compounds, the phenyl group of which bears at least one, linear or branched, saturated or unsaturated hydrocarbon group, being able to comprise from 1 to 4 carbon atoms, which group bears at least one hydroxyl group) may be phenylic compounds, the phenyl group of which bears at least one, linear or branched, unsaturated hydrocarbon group being able to comprise from 2 to 4 carbon atoms, which hydrocarbon group bears at least one hydroxyl group. More precisely, specific compounds complying with this definition may be phenylic compounds, the phenyl group of which bears a linear or branched, monounsaturated hydrocarbon group, being able to comprise from 2 to 4 carbon atoms, such as a —CH═CH—CH2—OH group, a specific example of such compounds being the cinnamyl alcohol of following formula: As mentioned above, the non-sulphurous and non-glucidic alcohols that may be used for the process of the invention may be aliphatic polyols, namely linear or branched hydrocarbon compounds, comprising at least two hydroxyl groups. These compounds may comprise from 2 to 4 carbon atoms. Advantageously, these compounds may be ethylene glycol or glycerine of following respective formulas: In particular, the platinoid element according to the invention may be palladium. The reducing amount of reducing agent needed to reduce the platinoid element(s) to their 0 oxidation state may be chosen, by those skilled in the art, by simple experimental tests (the reduction to the oxidation state being able to be materialised visually by a precipitation of the platinoid element(s)). According to the invention, the acidic aqueous solution may be a nitric solution (i.e. in other words an aqueous solution of nitric acid). In this instance, the use of specific alcohols as defined above contributes to avoiding, in addition, highly exothermic denitration reactions of the nitric solution, which are conventionally caused by the presence in a nitric solution of reducing glucides (such as saccharose). According to the invention, the nitric solution intended to be treated according to the process of the invention may be a raffinate (or aqueous solution) from processes of treating irradiated nuclear fuels, which process conventionally comprises: a step of dissolution of a spent fuel in a highly concentrated aqueous solution of nitric acid, by means of which an aqueous nitric solution is obtained comprising uranium, plutonium, fission products (such as lanthanides, yttrium elements, one or more platinoid elements), minor actinides (such as americium and curium), corrosion products; a step of co-extraction of uranium and plutonium from said aqueous solution by means of an extracting organic phase, at the end of which there remains an organic phase comprising uranium and plutonium and an aqueous phase corresponding to the aforementioned raffinate comprising, apart from the aforementioned platinoid element(s) (such as palladium, ruthenium, rhodium), two minor actinides, namely americium and curium, lanthanides such as lanthanum, cerium, praseodymium, neodymium, samarium and europium, fission products other than lanthanides and platinoids such as molybdenum, zirconium, rubidium and yttrium, as well as corrosion products such as iron and chromium. The aqueous solution of nitric acid may be an aqueous solution with strong nitric acidity, typically from 2 to 5 M. Once the step of adding a reducing agent has been implemented, the process of the invention comprises a step of separating said thus reduced platinoid element from said acidic aqueous solution, this step being able to be carried out, for example, by a simple filtration, a decantation operation or a centrifugation operation. The solution thereby obtained rid of all or part of the platinoid elements may then be used with a view to being vitrified by a conventional vitrification process. The invention will now be described with respect to the following examples given by way of illustration and non-limiting. The aim of the present example is the study of palladium/cerium separation in nitric solution implementing the use of benzylic alcohol. In this example, palladium chloride ((NH4)2PdCl4) and cerium nitrate (Ce(NO3)3) are used, palladium being a platinoid element and cerium being one of the fission products present in the raffinates and considered as representative of all the lanthanides. For each of the tests of this example, 10 mL of 1 mol/L nitric acid solution are used, to which is added 71 mg of palladium chloride and 87 mg of cerium nitrate i.e. 0.2 mmol of palladium and 0.2 mmol of cerium. After addition of the desired amount of benzylic alcohol, the solutions are placed in Parr digestion bombs and heated to a desired temperature for 16 hours. Different series of tests are carried out with variable amounts of benzylic alcohol at a given temperature: a series of tests at 150° C. with respective benzylic alcohol amounts of 0; 0.35; 0.4; 0.5 and 0.7 g; a series of tests at 180° C. with respective benzylic alcohol amounts of 0; 0.35; 0.4; 0.5 and 0.7 g. For each of these series, the measurement was carried out (by ICP-AES) respectively of the amount of palladium and of the amount of cerium after 16 hours of reaction, the results of these measurements being reported respectively in the following figures: FIG. 1 illustrating the evolution of the amount of palladium C1 (in g/L) as a function of the added amount of benzylic alcohol B1 (in g) after 16 hours of reaction; FIG. 2 illustrating the evolution of the amount of cerium C2 (in g/L) as a function of the added amount of benzylic alcohol B2 (in g) after 16 hours of reaction. As may be noted in these figures, a temperature of 180° C. makes it possible to precipitate all the palladium for amounts of benzylic alcohol from 0.3 g and also a non-negligible amount of cerium, whereas a temperature of 150° C. makes it possible to precipitate all the palladium from benzylic alcohol from 0.3 g while enabling a precipitation of cerium in lesser amounts than at 180° C. It has also been noted that the palladium precipitates in the form of particles of a hundred or so nanometers diameter within an organic matrix constituted of an aromatic polymer from the polycondensation of the benzylic alcohol and the oxidation products thereof. The formation of this matrix largely facilitates the separation between the solution containing cerium and palladium having precipitated. Furthermore, no substantial extra pressure is observed in the reactor at the end of reaction, which attests to the absence of denitration or, at the least, to a very limited denitration phenomenon, which proves the safety aspect of the process of the invention. The aim of the present example is the study of palladium/cerium separation in nitric solution implementing the use of different alcohols: benzylic alcohol (part b of FIG. 3), 4-methoxybenzylic alcohol (part c of FIG. 3), glycerine (part d of FIG. 3), 1-phenyl-1-ethanol (part e of FIG. 3), ethylene glycol (part f of FIG. 3), cinnamyl alcohol (part g of FIG. 3), 4-hydroxybenzylic alcohol (part h of FIG. 3) and cyclohexanol (part i of FIG. 3). A test was carried out without alcohol (part a of FIG. 3). In this example, palladium chloride ((NH4)2PdCl4) and cerium nitrate (Ce(NO3)3) are used, palladium being a platinoid element and cerium being one of the fission products present in the raffinates and considered as representative of all the lanthanides. For each of the tests of this example, 10 mL of a 1 mol/L nitric acid solution is used, to which is added 71 mg of palladium chloride and 87 mg of cerium nitrate i.e. 0.2 mmol of palladium and 0.2 mmol of cerium. After addition of the desired amount of alcohol (here, 4.8 mmol), the solutions are placed in Parr digestion bombs and heated to a temperature of 150° C. for 16 hours. FIG. 3 illustrates the concentration of metal ions (palladium and cerium) remaining in solution (in g/L) after heating to 150° C. for 16 hours for the aforementioned different alcohols tested. As may be seen from this figure, all of the alcohols tested have a good selectivity vis-à-vis palladium. In order to get as close as possible to a system encountered in the treatment of aqueous effluents from the reprocessing of spent fuels, selective precipitation tests of palladium were carried out on a model solution of nitric acid (1.5 M) corresponding to the composition described in the table below and a total nitrates load of 3.5 mol/L. OxideConcentrationconcentrationMetalof metalOxide used(g/L)elementelement (g/L)Na2O18.739Na13.90Al2O37.378Al3.90ZnO0.209Zn0.17ZrO29.479Zr7.02TeO21.272Te1.02Cs2O5.757Cs5.43SrO1.703Sr1.44BaO3.632Ba3.25SnO20.128Sn0.10Cr2O30.222Cr0.15Fe2O31.698Fe1.19MnO21.997Mn1.26La2O36.444La5.49Nd2O314.878Nd12.75Ce2O35.195Ce4.44Pr2O32.821Pr2.41MoO39.413Mo6.27P2O50.925P0.40——Pd2.98Total91.8970.61 To carry out the tests, 10 mL of the solution were used. After addition of 500 mg of benzylic alcohol, the solution was placed in a Parr digestion bomb and heated to 150° C. for 16 hours. This manipulation was repeated 4 times to offset any experimental errors. The amounts of palladium and cerium present in the solution at the end of the test were measured by ICP-AES (in other words by atomic emission spectroscopy). The average results calculated on the basis of 4 tests are as follows: Palladium 0.05 g/L Cerium 4.09 g/L As may be seen from these results, the palladium has been almost totally eliminated, whereas the cerium remains very largely in solution.

summary

abstract

An apparatus for cleaning an irradiated nuclear fuel assembly includes a housing adapted to engage a nuclear fuel assembly. A set of ultrasonic transducers is positioned on the housing to supply radially emanating omnidirectional ultrasonic energy to remove deposits from the nuclear fuel assembly.

abstract

An detector-assembly for measuring flux in a nuclear reactor core includes self-powered in-core detector arrangements each for measuring flux at a different one of a plurality of axial locations in the core, and an assembly connector configured to be connected to a power plant connector. The assembly connector includes a plurality flux signal terminals each connected to one of self-powered in-core detector arrangements. At least one of the self-powered in-core detector arrangements comprises a set of at least two self-powered in-core detectors for measuring flux at a same one of the axial locations in the nuclear reactor core. Each of the at least two self-powered in-core detectors includes a sheath, a detector material section inside the sheath, an insulator between the sheath and the detector material, and a flux signal output line. The flux signal output lines of the at least two self-powered in-core detectors are joined together.

044850697

claims

1. A moisture separator reheater comprising: a generally horizontally disposed cylindrical shell; a head on each end of said shell forming end closures for said moisture separator reheater; a plurality of long U-shaped tubes disposed in a generally parallel array to form a generally round tube bundle with shell side inlet and outlet portions of said tube bundle being generally flat; a tubesheet having a plurality of holes which receive the ends of the tubes, said ends of said tubes being in a sealed relationship with said holes; a plurality of tube supports spaced along the length of said tube bundle; a pair of arcuate plates disposed on opposite sides of said tube bundle and fastened to said tube supports forming a wrapper subtending only the round portions of said tube bundle; a pair of cylindrically shaped elongated plates disposed longitudinally in the central portion of said shell, said cylindrically shaped plates having generally horizontal upper and lower margins which generally extend the length of the shell; a plurality of upper elongated plates which extend from the upper margins of said cylindrically shaped plates to the upper portions of the shell; a plurality of lower elongated plates which extend downwardly from the lower margins of said cylindrically shaped plates toward the lower portion of said shell; said upper, lower and cylindricaly shaped plates and said shell being cooperatively associated in a sealed relationship to form within the shell a central chamber flanked by two side chambers; openings in said lower plates placing the side chambers in fluid communication with the central chamber; moisture separating means disposed adjacent the openings in said lower plates; a sliding sealed juncture disposed between adjacent arcuate and cylindrically shaped plates, said sliding sealed juncture generally extending the length of said plates to provide a seal and sliding engagement between said arcuate and cylindrically shaped plates to allow easy installation and removal of said tube bundle within said central chamber and to allow for differential thermal expansion between said arcuate and said cylindrical shaped plates; a fluid inlet in fluid communication with each side chamber in said shell; and a fluid outlet in fluid communication with the upper portion of said central chamber in said shell to form a moisture separator reheater. 2. A moisture separator reheater as set forth in claim 1, wherein the sliding sealed juncture is formed by elongated flat bars generally horizontally disposed so the bars are slidably engaged in such a manner to prevent upward movement of said tube bundle due to the pressure differential caused by fluid flowing over the outer surface of the tubes. 3. A moisture separator reheater as set forth in claim 1 and further comprising a head disposed in a sealed relationship with the tubesheet, means for dividing said head into at least two chambers, a heating fluid supply nozzle in fluid communication with one of said head chambers, a manifold forming a vent chamber in fluid communication with a plurality of tubes, said manifold being so disposed that the fluid flowing through the tubes makes four passes through said central chamber. 4. A moisture separator reheater as set forth in claim 3 and further comprising a duct disposed in said tubesheet in fluid communication with said manifold for draining fluid from said manifold.

claims

1. A cylindrical gamma ray generator, comprising:a cylindrical RF-driven plasma ion source for producing a single proton hydrogen ion containing plasma;a single proton hydrogen gas source in fluid communication with said plasma ion source whereby single proton ionized hydrogen gas can be introduced into said ion source;a cylindrically-shaped radial ion extractor system, the system disposed coaxially about the ion source for extracting proton ions radially from the ion source; whereinthe proton ions extracted from the source by the cylindrical extractor system are directed with an energy of approximately 165 keV at a cylindrical target comprising boron carbide (B4C) disposed coaxially outside and spaced from the proton plasma ion source, wherein the target material undergoes proton/gamma (p,γ) reaction when irradiated by the proton ions to produce gamma rays having an energy level above 6 MeV. 2. The generator of claim 1, further comprising an RF antenna disposed within the ion source. 3. The generator of claim 1, further comprising water cooling within the RF antenna. 4. The generator of claim 1, wherein the cylindrically-shaped radial ion extractor system comprises a plurality of axially extending slots. 5. The generator of claim 1, further comprising a vacuum chamber disposed to contain the target. 6. A generator for detecting special nuclear materials, comprising:a cylindrical gamma generator of claim 1;a shield surrounding parts of the generator to prevent gammas from escaping in undesired directions;an associated neutron detector for detecting neutrons produced by gamma reaction with the special nuclear materials. 7. A higher power gamma generator, comprising, a plurality of individual gamma ray generators of claim 1, stacked one on top of another. 8. The generator of claim 7, wherein a first and a second adjoining generators share high voltage sources. 9. The generator of claim 8, wherein the second and third adjoining generators share a pumping chamber.

052020827

abstract

A replacement nozzle and heater sleeve for a nuclear reactor coolant system pressurizer. The heater, damaged heater sleeve exterior of the pressurizer and a portion of the heater sleeve inside the heater sleeve bore are removed. The heater sleeve bore is partially tapped to provide threads at its lower end. A sleeve having a first seal ring positioned at its upper end is threaded into the heater sleeve bore such that the first seal ring presses against the original heater sleeve and provides a seal. A second seal ring on a flange on the sleeve is pressed into sealing engagement with the exterior of the pressurizer.

description

This application claims benefit under 35 USC 119(e) from U.S. provisional patent application 60/992,691, filed on Dec. 5, 2007. The contents of all of the above documents are incorporated by reference as if fully set forth herein. The present invention, in some embodiments thereof, relates to radiation detection systems used in the vicinity of pulsed radiation beams and other radiation sources; and, more particularly, but not exclusively, to x-ray and gamma-ray imaging and tracking systems used to monitor patients while they are treated by pulsed radiation therapy beams. Radiation therapy is often used to treat cancer and other abnormal growths. Such therapy can use implanted radioactive sources (brachytherapy), or external radiation sources, generally beams, including x-ray beams and electron beams produced by linacs, as well as proton beams and heavy ion beams. Such beams are also used for radiosurgery, for example for ablating cardiac tissue to prevent atrial fibrillation. Because radiation beams can harm healthy tissue, radiation beam therapy and radiosurgery are carefully planned, with beams aimed precisely at a target such as a tumor, often with several doses of radiation given from different angles, to make sure that the target receives enough radiation, while minimizing the exposure of healthy tissue to radiation. U.S. Pat. No. 6,683,318 to Haberer et al describes a heavy ion beam therapy system, in which positron emission tomography (PET) is used to locate radioactive nuclei that decay by positron emission, produced in the target tissue by the heavy ion beam. The PET results can verify that the heavy ion beam was aimed properly. In order to locate these positron-emitting nuclei before they have moved away from the target, PET is performed during the treatment session. The most convenient time for doing this is said to be in the time slots between beam spills, when the PET signal is less obscured by background noise than in the periods with the beam on. As defined in other publications by the inventors and their research group at Darmstadt, “beam spills” refers to periods of one to five seconds during which the beam is on, separated by time slots of similar length during which the beam is off. This use of “beam spill” is found, for example, in Parodi et al, “The Time Dependence of the γ-Ray Intensity Seen by an In-Beam PET Monitor,” downloaded from www.fzd.de/FWK/jb02/PDF/page77.pdf, on Nov. 23, 2008; Peters et al, “Spill Structure Measurements at the Heidelberg Ion Therapy Centre,” Proceedings of EPAC08, Genoa, Italy, paper TUPP127, pages 1824-1826, downloaded from epaper.kek.jp/e08/papers/tupp127.pdf, on Nov. 23, 2008; Crespo et al, “First In-Beam PET Imaging With LSO/APD Array Detectors,” IEEE Trans. Nucl. Sci. 15, 2654-2661 (2004); and Pshenichnov et al, “PET monitoring of cancer therapy with 3He and 12C beams: a study with the GEANT4 toolkit,” submitted to Phys. Med. Biol., downloaded from arxiv.org/PS_cache/arxiv/pdf/0708/0708.1691v1.pdf, on Nov. 23, 2008. U.S. Pat. Nos. 7,438,685 and 6,804,548 describe using ultrasound to monitor the position of a target organ or tumor in real time during beam therapy. US 2005/0197564 to Dempsey describes using real time MRI during beam therapy. U.S. Pat. No. 7,349,522 describes software for simulating dynamic radiation therapy, for example gated to respiration, using fluoroscope images fused to previously acquired reference images. But they do not suggest the use of such fluoroscope images during actual beam therapy, instead using index markers or other known methods of gating to respiration. U.S. Pat. No. 7,302,033 describes real time “image guided radiation treatment,” using a linac for treatment, and a stereo x-ray system for real time imaging, arranged so neither one blocks the other. “Real time” is defined to mean anytime during a treatment delivery phase, with the linac turned on or off. Specifically, they describe making an x-ray image before turning on the linac, delivering a dose of radiation with the linac, then making another image with the linac turned off, delivering another dose of radiation, etc. US 2008/0130825 to Fu et al describes using image guided radiation therapy, including x-ray imaging, while the beam is turned on or off. Image segmentation is used in real time to better identify the target, for example a tumor. US 2005/0080332 to Shiu describes using “near simultaneous” CT image guided radiotherapy. U.S. Pat. No. 7,295,648 describes using linac x-rays for imaging “by suitable variation of the output energy”. U.S. Pat. No. 5,233,990 describes using a lower energy therapeutic x-ray beam, from an x-ray tube, for imaging in real time, to verify the position of the patient. U.S. Pat. No. 6,839,404 describes using linac x-rays for imaging before delivering a dose of x-rays for therapy, and using the detector to monitor the dose during therapy. U.S. Pat. No. 6,618,467 describes using linac x-rays to produce CT images in real time. Because the therapy x-rays do not make up a complete set of angles for CT, they supplement them with low level x-rays at other angles, obtained from leakage through the shutters of the linac, or from sources other than the linac, collected either before or during treatment. They also describe using only the low level x-rays to produce the CT images. U.S. Pat. No. 7,263,164 describes using an x-ray imaging system in real time, during treatment by a linac beam. Scattering from the linac beam into the detector is estimated, using a phantom, and subtracted from the image. U.S. Pat. No. 7,171,257 describes doing x-ray imaging just before radiosurgery, finding the change in position of the beam target, for example cardiac tissue to be ablated, as a function of cardiac phase and breathing phase, then using that information, with the imaging system turned off, to keep the beam aimed correctly during the radiosurgery, monitoring the breathing and cardiac cycles in real time. U.S. Pat. No. 6,865,411 states that it is a disadvantage that imaging and radiation beam therapy cannot be done at the same time. U.S. Pat. No. 6,662,036 and U.S. Pat. No. 6,405,072 describe using index markers to track movement of the patient in real time, during beam therapy. “Answer to Question #4511 Submitted to ‘Ask the Experts’” on the Health Physics Society website, downloaded on Nov. 27, 2008 from hps.org/publicinformation/ate/q4511.html, states that most medical linacs are pulsed with repetition rates of 100 to 400 pulses per second, and pulse lengths of 1 to 10 microseconds, resulting in a very low duty cycle, less than 1% or less than 0.1%, and peak intensities of radiation much higher than the average intensity. Radiation detectors that have long dead times, such as Geiger-Muller and proportional counters, tend to become saturated at such high peak intensities, and are not suitable for safety monitoring of radiation levels outside rooms where linacs are used. Similar points are made by R. McCall and N. Ipe, “The Response of Survey Meters to Pulsed Radiation Fields,” SLAC-PUB-4488 (1987), downloaded from www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-4488.pdf, on Nov. 27, 2008. Radiation Detection and Measurement by Glenn Knoll, 3rd edition (2000), ISBN 0-471-07338-5, describes instruments for detecting x-ray and gamma-ray photons. An aspect of some embodiments of the invention concerns a system for treating a body with a pulsed beam of ionizing radiation, for example a radiation therapy beam, while using a detector system to obtain data of the body being treated, from another ionizing radiation source such as an x-ray or gamma-ray imaging or tracking system, in real time during operation of the beam. The detection system reduces or avoids interference from the beam, by not recording data and/or by having reduced sensitivity during the pulses of the beam, while using the intervals between pulses to obtain the data. There is thus provided, in accordance with an exemplary embodiment of the invention, a detector system adapted for monitoring a radiation treatment system comprising a pulsed beam radiation source for treating a body with a given beam intensity and beam configuration, with pulse times and intervals between pulses less than 100 milliseconds, using at least one monitoring radiation source located inside or outside the body, the detector system comprising; a) a detector designed to detect radiation from the monitoring source, and subject to interference radiation from the beam source; and b) control circuitry that creates a data record of radiation received by the detector, to provide information about the body;wherein, when the detector detects radiation in real time during operation of the beam, the data record selectively excludes data for radiation received by the detector during the pulses, as opposed to data for radiation received by the detector between pulses. Optionally, the detector sensitivity is controllable by the control circuitry, and the data record selectively excludes data for radiation received during the pulses because the detector system is configured to make the sensitivity of the detector lower during the pulses than between the pulses. Optionally, the detector has a bias voltage, and the control circuitry makes the sensitivity of the detector lower by changing the bias voltage. In an embodiment of the invention, the data record selectively excludes data for radiation received during the pulses because the control circuitry is configured not to add data for radiation detected during the pulses to the data record. Optionally, the detector system uses a triggering element that signals the timing of the pulses to the control circuitry. Optionally, the triggering element is comprised in the beam source, or in a timing element that controls the timing of the beam pulses. Alternatively, the triggering element is comprised in a sensor which senses when the beam source produces a pulse. Optionally, the sensor is the detector. Optionally, the data record selectively excludes data for radiation received during the pulses because the control circuitry is configured to remove said data from the data record data. In an embodiment of the invention, the data record selectively excludes data for radiation received during the pulses because the detector is configured to saturate at a level of radiation received during the pulses at the given beam intensity and beam configuration, but not to saturate at a level of radiation received from the monitoring source between pulses. Optionally, the detector is a scintillation detector with decay time shorter than the intervals between pulses. Optionally, the time-averaged relative contribution of the interference to the data record is less, by at least a factor of 5, than the time-averaged relative contribution of the interference to the radiation received by the detector, when the beam is operating at the given beam intensity and beam configuration. Optionally, the interference contributes to the data record less than 20% as much as the radiation from the monitoring source, averaged over any time interval that includes many pulses, when the beam is operating at the given beam intensity and beam configuration. In an embodiment of the invention, the radiation treatment system that the detector system is adapted for monitoring is a radiation therapy system using a beam intensity of at least 1 centiGray per second, and the detector system can locate a beam therapy target inside the patient to within 2 mm in an acquisition time of less than 2 seconds, using an internal monitoring source of less than 1 milliCurie or an external x-ray monitoring source of less than 20 centiGray per acquisition time. Optionally, the data record provides information on one or more of motion of the patient's body, position of the patient's body, motion and position of one or more parts of the patient's body, all relative to the beam in real time when the beam is on, and a dose of radiation received from the beam by one or more parts of the patient's body. Optionally, the pulsed beam source in the radiation treatment system that the detector system is adapted to monitor comprises a linac beam source. Optionally, the linac beam source comprises an x-ray beam source. Alternatively, the pulsed beam source in the radiation treatment system that the detector system is adapted to monitor comprises an ion beam source. In an embodiment of the invention, the detector system comprises an x-ray imaging system, using a monitoring source comprising an x-ray source. Optionally, the detector system comprises a CT system. Alternatively or additionally, the detector system comprises a radioactive tracking system, using a monitoring source comprising a radioactive source inside the body being treated. Alternatively or additionally, the detector system comprises a gamma imaging system, using a monitoring source comprising radioactive material inside the body being treated. Optionally, the control circuitry is adapted to determine a difference between a position of a treatment target in the body and planned position with respect to the beam, and to adjust a position of the beam and/or reduce the power of the beam in response to the difference. Optionally, the control circuitry is fast enough, or the detector has a fast enough decay time, or both, so that the data record can selectively exclude data for radiation received by the detector during a time period shorter than 100 milliseconds, as opposed to data for radiation received by the detector outside the time period. Optionally, the time period is shorter than 10 milliseconds. There is further provided, in accordance with an exemplary embodiment of the invention, a method of monitoring a body in real time while the body is being treated by a pulsed beam of treatment radiation, with pulse lengths and interval between pulses both shorter than 100 milliseconds, the method comprising: a) passing monitoring radiation from a source other than the beam through at least part of the body; b) receiving the monitoring radiation together with any interfering radiation from the beam, and detecting and recording at least some of the radiation in a data record; c) using the data record to monitor the body in real time during the treatment;wherein detecting and recording the radiation selectively excludes from the data record data for radiation received during the beam pulses. Optionally, using the data record to monitor comprises reconstructing images. Optionally, the monitoring radiation comes from a source inside the body, and using the data record to monitor comprises tracking a location of the source. Optionally, the body is a patient's body, and the treatment by the beam comprises radiation therapy on the patient. Optionally, recording the radiation comprises selectively failing to record, or selectively removing from the data record, data for radiation detected during the beam pulses. Optionally, the method also includes lowering a detection sensitivity to radiation during the beam pulses, and raising the detection sensitivity during intervals between pulses. Optionally, the method also includes receiving triggering signals indicating the beginning and end of each pulse, wherein lowering and raising the detection sensitivity is done in response to the triggering signals. Optionally, detecting the radiation comprises detecting with a saturation level lower than a level of radiation received during the beam pulses, but higher than a level of radiation received during intervals between the beam pulses. Optionally, using the data record to monitor in real time comprises: a) determining a difference between a position of a beam therapy target in the patient, and a planned position with respect to the beam; and b) adjusting a position of the beam, and/or reducing the power of the beam, in response to the difference. Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting. The present invention, in some embodiments thereof, relates to radiation detection systems used in the vicinity of pulsed radiation beams and other radiation sources, and, more particularly, but not exclusively, to x-ray and gamma-ray imaging and tracking systems used to monitor patients while they are treated by pulsed radiation therapy beams. An aspect of some embodiments of the invention concerns a radiation treatment system that uses a pulsed beam of radiation to treat a body, with pulse lengths and intervals between pulses shorter than 100 milliseconds, together with a detector system that uses another source of radiation to monitor the body during the treatment. In an exemplary embodiment of the invention, the detector system reduces or avoids interference from the beam by having reduced sensitivity during the pulses, or not recording data for radiation detected during the pulses, and recording data for radiation detected in the intervals between pulses. Optionally, the sensitivity of the detector is decreased by lowering a bias voltage used by the detector, triggered, for example, by a sensor that responds to the beam pulses, or triggered by the increased intensity of radiation that the detector measures just as a pulse is beginning. Alternatively or additionally, the detector has a lower sensitivity during the beam pulses because the detector saturates at a level well below the intensity of radiation it receives during the beam pulses, but optionally at a higher level than the highest intensity of radiation expected from the other source of radiation. The detector may be paralyzed when it saturates, not responding at all, or non-paralyzed, responding at a maximum rate, or something in between these two extremes. Additionally or alternatively, any radiation detected during the beam pulses is removed, for example by software, from a data record of the detector, with the beam pulses optionally identified by the higher levels of radiation detected by the detector then. Optionally, the beam of radiation is a radiation therapy beam, for example a linac-generated x-ray beam or electron beam, or an ion beam, used to treat a patient, and the detector system monitors the patient in real time during the therapy, for example to determine the position of the patient's body, or of parts of the body, and to detect any change in their position, and optionally adjust the aim of the beam in response. Additionally or alternatively, the detector system monitors the patient in real time, in order to accurately determine the dose of radiation delivered to a desired target in the patient's body, for example a tumor, and/or the dose of radiation delivered to healthy tissue, and optionally adjust the beam intensity and/or the beam path. Optionally, the detector system is an x-ray or gamma-ray imaging or tracking system, and the other source of radiation is an x-ray source such as an x-ray tube, or a radioactive marker or other radioactive material in the patient's body. Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details set forth in the following description. The invention is capable of other embodiments or of being practiced or carried out in various ways. Referring now to the drawings, FIG. 1 illustrates a radiation treatment system 100, with detector systems for monitoring using separate monitoring radiation sources, according to an exemplary embodiment of the invention. A patient 102, lying on a bed or table 104, is being treated by a radiation therapy beam 106 generated by a pulsed beam source 108, for example a linac producing hard x-rays for treating cancer. Alternatively, the pulsed beam source is a linac producing an electron beam, or a proton beam source, or a heavy ion beam source, or any other kind of pulsed beam source used for radiation therapy. The pulses, which typically each last for 1 to 10 microseconds, generally occur at regular intervals shorter than 100 milliseconds, typically at intervals between 2.5 and 10 milliseconds, when the beam is on, and typically the beam is on for 1 to 20 seconds at a time. In some linacs, certain pulses may be missing, in some pattern, to adjust the dose rate, so the pulses will not all occur at fixed intervals, but in a more complicated pattern. As used herein, the beam is said to be “on” or “operating” or “treating the patient” when it is producing pulses at one of its normal rates and patterns, including during the intervals between pulses, and is said to be “off” or “pausing” when it is not generating pulses at all. The beam source operates the beam at a given beam intensity and beam configuration, including beam diameter, spread, energy distribution and composition, appropriate for radiation therapy, and characteristics of the monitoring detector systems may be adjusted depending on the beam intensity and beam configuration. Optionally, beam source 108 is mounted on an arm or gantry 110, with a control 112 for adjusting the direction and/or position of the beam. Optionally, there is a detector system using another radiation source 114, for example an x-ray tube, producing radiation 116, which is detected by detector 118, for imaging patient 102 in real time, for monitoring during radiation treatment. In a prior art radiation treatment system with a linac beam and a radiation detector operating at the same time, photons, electrons, and emitted neutrons from the beam would enter the detector directly, or after scattering from the patient or other objects in the room, and cause substantial interference in data generated by the detector. But for reasons that will be explained below, the data generated by detector 118 suffers from relatively little interference from beam 106. Additionally or alternatively, there is a detector system using an internal radioactive source 120 in the patient, for example an implanted compact marker, or extended radioactive material, injected or ingested to mark the location of a tumor or other target being treated by beam 106, which emits radiation 122, for example gamma rays, which are detected by a detector 124. In some embodiments of the invention, radioactive source 120 is generated inside the patient's body by the beam, for example it is a positron source generated by spallation of a heavy ion beam. The data generated by detector 124 also has relatively little interference from beam 106, for reasons that will be explained. Detector 124 is, for example, a gamma camera, or a radioactive tracker, used to image or track the radioactive marker or material in the patient's body, in real time, for monitoring during the beam treatment. A controller 126, for example a computer, optionally reconstructs images of patient 102 and/or source 120, and/or tracks the position of source 120, using data generated by detectors 118 and/or 124, and detects any changes in position of the patient or of internal tissues or organs being targeted by the beam, in real time. An image can be reconstructed, for example, from measurements of the absorption of x-rays on different chords going through the body, forming a 2-D x-ray image. Alternatively, instead of reconstructing an image, a compact radioactive source in the body, generally a gamma-ray source, can be tracked, for example by using three differential detectors to determine the position of its center of mass in each of three dimensions, and then following changes in the position, as described for example in WO 2006/016368 and WO 2007/017846, both assigned to Navotek Medical Ltd. A 2-D image can also be reconstructed by a gamma camera, scanning a detector over the body to measure a spatial distribution of a gamma-ray emitting radioactive material in the body, but x-ray imaging and gamma-ray tracking have the potential advantage that they are much faster, and may be better suited for finding changes in the position of a beam therapy target in real time. This information is used by controller 126, either automatically or in conjunction with a human operator, to make any needed changes in the location or direction of the beam, using control 112 and arm 110, as well as possibly turning off beam source 108 in real time to avoid damaging healthy tissue, if beam 106 is no longer aimed properly, and leaving beam source 108 off until the aim of the beam and/or the position of the patient can be adjusted. As used herein, each detector used for monitoring the patient using a given radiation source, for example detector 118 using source 114, or detector 124 using source 120, including any control circuitry, is referred to as a detector system. The control circuitry can be common to more than one detector system, as in the case of controller 126 in FIG. 1, or can be separate for one or more detector systems, packaged with the detector for example. As used herein, the control circuitry of a detector system includes any elements which create or modify data records; use data records for imaging or tracking; use results of the imaging or tracking for control, for example of the beam source; actively control the sensitivity or other characteristics of the detector; receive signals for actively controlling the detector, for example signals about the beginning and end of beam pulses. All of these functions of the control circuitry need not be present in a given detector system, and they may be performed by the same or different elements, packaged together or separately. A given detector system may use more than one radiation source, and a given source may be used by more than one detector system. The sources used by the detector systems, such as external source 114 and internal source 120, are referred to as monitoring sources, and their radiation referred to as monitoring radiation, to distinguish them from the beam source, and from the interfering radiation coming directly and indirectly from the beam. To reduce or avoid interference from beam 106 when it is operating, controller 126 uses a data record with data from detectors 118 and/or 124, for imaging and tracking. The data record selectively excludes data for the radiation received during the pulses of beam 106, as opposed to data for radiation received between the pulses. Since interference from beam 106 occurs primarily during the pulses, from beam radiation hitting the detectors directly or after scattering, this selective exclusion decreases the effect of interference from beam 106 on the imaging or tracking. As used herein, “selectively excludes” does not necessarily mean that data is excluded for all radiation received during the beam pulses, but data is excluded for a greater proportion of such radiation, than for radiation received during the intervals between pulses. Optionally, data is excluded for most or all radiation received during the beam pulses. As used herein, excluding data can include not adding data to the data record because the radiation was not detected, not recording data to the data record even though the radiation was detected, and removing data from the data record after it was recorded. As used herein, “data record” can include any data from the detector used for imaging or tracking, even if the data is only kept in a computer memory and used continuously in real time for imaging or tracking, and never saved. As used herein, “interfering radiation” or “interference radiation” includes radiation directly entering a detector from the beam source, as well as radiation from the beam source which is scattered before entering the detector, or is absorbed and re-emitted in much less time than the time between pulses. It does not include radiation from activated nuclei or excited atoms which is emitted in a time that is not short compared to the time between pulses. System 100 has a potential advantage over prior art systems in which a radiation beam is turned on to provide a dose of radiation to the patient, then paused for a few seconds or longer while the patient is imaged to see if he or she has moved, then turned on again, and the procedure repeated several times. In such a prior art system, the pauses in treatment to image the patient make the treatment session last longer, making the session more difficult for the patient, and resulting in lower throughput for the expensive beam therapy system. The longer treatment sessions also make it more likely that the patient will move. In addition, because the imaging is not done in real time, it is not possible to stop or adjust the beam if the patient moves in the middle of treatment, and one cannot be certain that the patient's position as measured after the treatment is exactly the same as the patient's position while the treatment was going on. In system 100, imaging may be done in real time during treatment, so there are no pauses in the treatment session, it is possible to stop or make adjustments in the beam as soon as the patient moves, and it is possible to verify that the beam was aimed properly during the treatment. In some embodiments of the invention, decreasing the contribution to the data record of radiation received during the beam pulses is accomplished by actively making the detectors less sensitive during the pulses. For example, a sufficiently fast sensor 128 in the path of beam 106 senses the timing of the pulses, and communicates this information to controller 126, which decreases the sensitivity of the detectors, for example by lowering their bias voltage. Additionally or alternatively, information about the timing of the pulses may come directly from beam source 108. In some embodiments of the invention, detectors 118 and/or 124 are not necessarily less sensitive to radiation during the pulses than in the intervals between the pulses, but the detectors, or controller 126, do not record data on any radiation received during the pulses, or controller 126 removes this data from a data record after it has been written. In some embodiments of the invention, the detectors are less sensitive to radiation during the pulses because they have a saturation level that is much lower than the radiation level received during the pulses, primarily from the beam, but optionally the saturation level is higher than the radiation level received during the intervals between pulses, primarily from the other radiation sources 114 and/or 120. In those embodiments, controller 126 and detectors 118 and 124 do not need to use information about the timing of the pulses. There are a variety of types of radiation detectors that are optionally used for detectors 118 and 124. These include rare earth screens, photostimulable phosphors, Geiger counters, proportional counters, scintillators, direct semiconductor detectors, and a combination of scintillators with semiconductor detectors (indirect detectors). These devices generally have a sensitivity that is controllable, for example by controlling a bias voltage, and/or a saturation level that is controllable, for example by controlling an integration time and a maximum recharging current. In principle even photographic plates or x-ray film could be used, with, for example, a shield with an aperture and a shutter than can be closed during the pulses, or a rotating shield with one or more openings that are timed to be closed during the pulses. But shielding that is effective against the hard x-rays typically used for radiation therapy, above 1 MeV, is generally quite massive, and devices whose sensitivity or saturation level can be controlled electronically have the potential advantage that it is not necessary to apply the high inertial forces that would be used to open and close a massive, high speed mechanical shutter, or to use the high rotational energy associated with a massive, rapidly rotating shield. Detector 118 and/or 124 may be divided into a plurality of sections each corresponding to a pixel of an image, optionally with a collimator so that each section is sensitive primarily to radiation from a relatively narrow range of directions, and/or radiation that has passed through the body along a narrow range of paths. The detector may produce a single 1-D or 2-D image from radiation reaching the detector when the detector and source are at fixed position. Examples of such systems include x-ray imaging systems using an external x-ray tube, and gamma cameras, such as an Anger camera, using a distributed internal radioactive source. Alternatively, the detector may be used to detect radiation with the detector at a series of different positions or orientation, and/or with the source, particularly an external source, at a series of different positions and orientations, to produce a 2-D or 3-D tomographic image. Systems producing tomographic images include CT (computerized tomography) systems using an external x-ray source, and SPECT (single photon emitted computer tomography) and PET (positron emission tomography) systems, using an internal radioactive source. Alternatively, the detector may be used in a tracking system, to locate and track a compact radioactive source, or a plurality of compact sources, inside the body. Examples of such tracking systems, using gamma ray sources, are described, for example, in WO 2006/016368 and in WO 2007/017846, both assigned to Navotek Medical, Ltd. Whether the detector is used for imaging or tracking or both, if the radiation treatment system is used for beam therapy of patients, as system 100 is, then the detector system optionally provides information that is useful for real-time monitoring of patients during beam therapy. For example, the detector system can find the position of a therapy target inside the patient's body, to within a precision, such as 2 mm or 1 mm, that is adequate for providing effective and safe aiming of the beam, in an acquisition time, such as 1 second or 2 seconds, that is short enough so that if an error in aiming is found, the beam can be adjusted or turned off without any significant harm done. This can be done using a medically safe monitoring radiation source, either a safe internal radioactive source, for example no more than 1 milliCurie, or no more than 100 microCuries, or a safe external x-ray source, for example no more than 20 centiGray per acquisition time, or no more than 2 centiGray, or no more than 0.2 centiGray. And it can be done even in the presence of an x-ray therapy beam as strong as 1 centiGray per second, or 3 centiGrays per second, or 10 centiGrays per second. Certain characteristics of the detectors may be advantageous: 1) Short integration time. It is potential advantageous if the integration time of the detector is short compared to the length of a pulse, so that the detector sensitivity during a pulse can be lower than the detector sensitivity during the intervals between pulses. Alternatively, if the integration time is longer than or comparable to a pulse length but much shorter than the interval between pulses, then the detector sensitivity can be lower for several integration times around the pulse, still leaving most of the much longer interval between pulses during which the detector can have high sensitivity. For scintillation based detectors, the integration time depends on the decay time of the scintillation pulses, which may be defined as the time required for the scintillation pulse to fall to 1/e of its maximum value. 2) Low afterglow. Some detectors produce an afterglow for a period of time, for example a few milliseconds, after radiation is received by the detector. It is potentially advantageous if the afterglow is sufficiently low, or sufficiently short-lived, or both, so that the afterglow from radiation received during a pulse does not substantially interfere with the detection of radiation between pulses. Halide scintillation crystals, particularly thallium doped sodium iodide and thallium doped cesium iodide tend to exhibit long afterglow, as high as a few percent after 3 milliseconds. Cadmium tungstate (CdWO4) crystals, bismuth germinate (BGO, or Bi4Ge3O12), and zinc selenide (ZnSe) doped with oxygen or tellurium, are examples of low afterglow scintillation materials. 3) High radiation hardness. Because the detector is exposed to a relatively high level of radiation from the beam, it is potentially advantageous to use a detector with relatively high radiation hardness, for example a detector whose functioning will not be substantially affected after exposure to 10,000 gray. Examples of radiation hard scintillation materials include cadmium tungstate, gadolinium silicate (Gd2SiO5), and undoped cesium iodide. Alternatively, if the detector material is low enough in cost, it can be replaced when it is damaged by radiation. 4) Low neutron activation. X-rays from a linac, particularly x-ray of energy 10 MeV or greater, can release neutrons from nuclei from material exposed to the beam, and these neutrons can activate materials in the vicinity of the beam, including detectors 118 and 124. The resulting radioactive isotopes can interfere with the detectors, producing spurious data. Since most radioactive isotopes that are of concern have half-lives much longer than the typical time intervals between pulses, this source of interference cannot be avoided by making the detector less sensitive during pulse times. It is optionally minimized by using detector materials with low neutron activation levels, and/or by using a linac beam with energy below 10 MeV, so that few neutrons will be released. It is potentially advantageous to avoid the use of gold and silver, which have high neutron activation levels, and to use molybdenum and tungsten, which have low neutron activation levels. FIG. 2 schematically shows a plot 200 of the beam intensity 202 as a function of time 204, showing pulses 206 and intervals 208 between pulses. Other quantities as a function of time, with the same time axis 204, are shown below the plot of beam intensity, for an embodiment of the invention in which the sensitivity of detector 118 and/or 124 is lowered during each beam pulse by decreasing the bias voltage of the detector. Optionally, the duty cycle of beam source 108, the ratio of the interval of pulse 206 to the interval between pulses 208, is small, for example less than 20%, less than 10%, less than 5%, less than 2%, less than 1%, less than 0.5%, less than 0.2%, or less than 0.1%. For example, in a typical linac generating x-rays for radiation therapy, the pulse length may be 1 to 10 microseconds, or 0.1 to 1 microsecond, or 10 to 100 microseconds, or smaller or larger values, and the intervals between pulses may be 2.5 to 10 milliseconds, or 0.5 to 2.5 milliseconds, or 10 to 50 milliseconds, or smaller or larger values. In general, both the pulse time and the interval between pulses may be less than 100 milliseconds. The pulses in a linac are due to the fact that the microwaves responsible for accelerating electrons in the linac are generally produced by pulsed power from a discharging capacitor, which then takes a much longer time to charge up again, using a lower level of power from the power grid, than the time over which it discharges. In addition to this pulse structure, linac beams also have structure on a shorter timescale, on the order of a nanosecond, due to bunching of electrons in phase with the microwaves. In the second plot from the top, the detector bias voltage 210 is shown schematically as a function of time 204. Many types of radiation detectors require outside power to detect radiation, which is supplied as a bias voltage, and these detectors cannot detect radiation at all if the bias voltage is too low. As used herein, “bias voltage” means any voltage applied to a detector, which affects the sensitivity of the detector. Examples include the voltage applied to the photomultiplier tube in a scintillation detector, the voltage supplied between the anode and cathode of a proportional counter, and the voltage applied to a semiconductor diode detector. During each pulse 206, the bias voltage is lowered to a level 214, so that the detector has decreased sensitivity to radiation. Optionally, level 214 is low enough so that the detector substantially does not detect radiation at all. During intervals 208 between pulses, the bias voltage is at a higher level 212, so that the detector is sensitive to radiation from the source, either an external source such as source 114 or an internal source such as source 120, that the detector is designed to be used with. The third plot from the top schematically shows the amplitude 216 of trigger signals that trigger controller 126 to change the bias voltage. Trigger signals 218 at the beginning of each pulse trigger the controller to decrease the bias voltage of the detector, and trigger signals 220 at the end of each pulse trigger the controller to increase the bias voltage of the detector back to a higher value. In some embodiments of the invention, there is only one trigger signal per pulse, for example only at the beginning or only at the end of the pulse, and controller 126 calculates the beginning and/or end of the pulse from the single trigger signal, using a known pulse length and a known timing of the signal relative to the pulse. In some embodiments of the invention, the trigger signals are generated by a signal from sensor 128 which intercepts the beam, or intercepts scattered radiation from the beam, and measures its intensity as a function of time. For example, the trigger signals are optionally generated by taking a time derivative of the sensed beam intensity as a function of time, with oppositely biased diodes optionally used to separate “pulse start” trigger signals 218 from “pulse end” trigger signals 220, or the “pulse start” and “pulse end” signals may be distinguished by their polarity. Other analog and digital methods of generating trigger signals from sensor 128 will be apparent to one of skill in the art. Additionally or alternatively, information about the timing of the beginning and/or end of each beam pulse is obtained from beam source 108, or from a timing element that sends synchronized triggering signals both to the detector system and to beam source 108, to control the timing of the beam pulses. Additionally or alternatively, timing information about the beam pulses comes from one of the detectors. However, using a beam sensor, or using the beam source, or using a timing element, has the potential advantage that it may more accurately reflect the timing of the beam pulses than the detectors, which may be positioned and/or shielded to reduce their exposure to the beam radiation. The fourth plot from the top in FIG. 2 schematically shows an intensity 222 of radiation received by detector 118 or 124, as a function of time. During each pulse 206, the direct and indirect interference radiation from beam source 108 dominates the signal from source 114 or source 120 that the detector is supposed to be measuring, raising it to a level 224 much greater than the lower level 226 during the intervals between pulses, which is due mostly to source 114 or 120. Even integrating over time, the contribution of the interference radiation may dominate the contribution from source 114 or source 120, or at least may be comparable to it. If the detector were equally sensitive to all radiation it receives, then the interference might substantially degrade images that are produced from data the detector generates, or substantially degrade the accuracy of tracking based on data that the detector generates. The bottom plot in FIG. 2 schematically shows the actual response 228 of the detector to received radiation, as a function of time 204. In the intervals between pulses, when the detector has a high bias voltage 212, the detector response is at a level 230, due to the radiation from source 114 or 120 that the detector is designed to detect. At the beginning of each pulse, when the received radiation jumps up, the response may momentarily jump up to a still higher level 232, before the bias voltage has time to fall to level 214, because the rate at which the bias voltage can fall may be limited by the response time of the circuit, or there may be a short time delay before controller 126 receives a trigger signal indicating that the pulse has begun. This might be true, for example, if controller 126 relies on the detector itself to provide the trigger signal. Once the bias level falls sufficiently low that the sensitivity of the detector is substantially reduced, however, the response of the detector will fall to a level 234 which is optionally even lower than level 230, or at least is much lower, relative to level 230, than level 224 is relative to level 226. As a result, the integrated contribution of interference radiation from the beam to detector response 228 is less, sometimes much less, than the integrated contribution of radiation from source 114 or 120 to detector response 228. For example, the integrated contribution of interference radiation is at least 2 times less, or at least 5 times less, or at least 10 times less, or at least 20 times less, or at least 50 times less, than the integrated contribution from source 114 or 120. Images that are produced, or tracking that is done, using detector response 228 will then not be so degraded by interference radiation, and optionally are substantially not degraded at all. It should be noted that the detector response 228 shown in FIG. 2 is a measure of radiation detected per unit time, integrated over a long enough time so that many photons (in the case of x-ray or gamma-ray sources) would be detected in an integration time, and there would be relatively small statistical fluctuations in the number of photons or the total energy of photons detected in an integration time. In some embodiments of the invention, the detector has a long enough integration time so that, at the level of radiation it is exposed to from source 114 or 120, it generates output data directly that look similar to detector response 228 in FIG. 2. This mode of operation of a detector is called “current mode,” because the detector produces an output current that is proportional to the photon power received. In other embodiments of the invention, a shorter integration time is used, shorter than an average time between photons detected, and the direct output of the detector as a function of time looks like a series of individual narrow peaks, each corresponding to the detection of one photon, with the height of each peak proportional to the energy of the photon. This mode of operation of a detector is called “pulse mode.” The detector may count the narrow peaks to produce a digital output signal proportional to the number of photons received per unit time. In this case, detector response 228 as plotted in FIG. 2 may be understood as a plot of an average number of photons per second, or an average energy of photons detected per second, found by integrating such a series of many individual peaks over time. These remarks apply as well to the plots of detector response as a function of time in FIGS. 3, 4, and 5. In some embodiments of the invention, instead of or in addition to lowering the bias voltage of the detector during a beam pulse, controller 126 may decrease the voltage to digital counting circuitry of a detector operating in pulse mode, during the beam pulses, so that no photons are counted during the beam pulses, even though the detector is detecting photons. This option may be used as well in FIG. 6. In some embodiments of the invention, rather than lowering the bias voltage, or decreasing the voltage to digital counting circuitry, only during the beam pulses, controller 126 lowers the bias voltage, or the voltage to the counting circuitry, for longer periods of time each including the time of a beam pulse. These longer periods of time may extend to several times as long as the decay time of the detector response, following the beam pulse, even if the decay time is comparable to or longer than the time of each beam pulse. In such a case, keeping the detector insensitive to radiation for at least a few times the decay time prevents the detector from producing a delayed high level response to the radiation received directly or indirectly from the beam, after the beam pulse. This option may be used as well for FIG. 6. However, optionally the detector is insensitive to radiation for a time much shorter than the interval between pulses. This allows the detector to detect radiation most of the time, and to detect most of the radiation it receives from the monitoring source. FIG. 3 schematically shows a plot 300 of the detector response 302 as a function of time 204, for the same arrangement of beam pulses as a function of time, and received radiation as a function of time, as is shown in FIG. 2. The detector in FIG. 3 does not have a changing bias voltage, but has a relatively low saturation level, lower than the high level of received radiation during the pulses, but higher than the level of received radiation between pulses. Photon detectors generally operate by discharging a charge, typically proportional to the photon energy, whenever they detect a photon, which reduces the bias voltage until the charge can be replaced. If the rate at which the charge can be replaced is limited, for example by an impedance in series with the source of the bias voltage, then the response of the detector will saturate at a current equal to the maximum current with which the detector can be recharged. In FIG. 3, the response is at a level 304, below saturation, in the intervals 208 between beam pulses, and is at a somewhat higher level 306, at the saturation level, during beam pulses 206, when the amount of radiation received by the detector is much greater. The integrated detector response during the beam pulses, however, is small compared to the integrated detector response between beam pulses, because the beam has a small duty cycle of pulses. As a result, interference from the beam contributes only a small amount, for example less than 20%, 10%, 5%, 2% or 1%, to the integrated detector response, and to the data record based on it, which is used for imaging or tracking. The detector response shown in FIG. 3 is the response for a non-paralyzing detector. Such a detector will respond at the saturation level when it receives radiation above the saturation level, because radiation received during the “dead time,” when the detector has not yet recovered its bias voltage, do not prolong the dead time. FIG. 4 shows a plot 400 of the response 402 of a paralyzing detector as a function of time 204, for the same beam intensity, and received radiation, as a function of time, as shown in FIG. 2. A paralyzing detector does not respond at all if it receives radiation above the saturation level, since radiation received during the dead time can restart the dead time, prolonging recovery. In FIG. 4, the detector response 402 is at a level 404, below the saturation level, during the intervals 208 between pulses, but during the pulses 206 the detector response is zero. Due to any finite rise and fall times of the beam pulses, as well as the finite decay time of the detector response, the detector may rise somewhat at the beginning of each pulse, before falling to zero, and may take some time to return to level 404 after the beam pulse, as shown in FIG. 4. However, if the saturation level is not too far above level 404, then the small rise at the beginning of each pulse will not contribute very much to the integrated detector response. And if the decay time and pulse length are short compared to the interval between pulses, then the detector will not miss more than a small fraction of the radiation in the interval between pulses, due to the finite rise time in the detector response after the pulse. It should be noted that “paralyzing” and “non-paralyzing” detectors are abstract models of detector behavior, and in practice most types of detectors fall somewhere in between these two extremes. FIG. 5 schematically shows a plot 500 of a data record of a detector response 502 as a function of time 204, for the same beam pulses 206, and intervals 208 between pulses, as shown in FIG. 2, and for the same level 222 of radiation received by the detector as shown in FIG. 2. In the case shown in FIG. 5, the detector may fully respond to of the radiation it receives. However, the radiation received during beam pulses is not written to the data record of the detector response, or is removed from the data record. The data record shows a level 504 during intervals 208 between pulses, but a level of zero during pulses 206. This may be done, as described above, by having controller 126 reduce the voltage to counting circuitry of the detector, during pulses, so any photons detected during beam pulses are not counted. Alternatively, controller 126 may use software which sets the data record to zero for pulse times 206, after a data record is written which includes information about the time dependence of the intensity of radiation detected in each section of the detector. The times 206 of the beam pulses may be identified by using information from beam source 108, or sensor 128, or the times of the beam pulses may be identified because they have high levels of radiation detected by the detector. FIG. 6 schematically shows plots 600 of beam intensity 602 and other quantities as functions of time 204, including a bias voltage 604 for detector 118 detecting radiation from external source 114, a bias voltage 606 for detector 124 detecting radiation from source 120 inside the patient's body, and an intensity 608 of radiation from source 114. As in FIGS. 2-5, the beam intensity has pulse times 610 separated by intervals 612 between pulses. Also as in FIG. 2, the bias voltages of both detector 118 and detector 124 are lowered during the beam pulse times 610, optionally using any of the methods for doing this described above for FIG. 2. But in FIG. 6, unlike in FIG. 2, there is a time division between a period 614 when detector 118 operates, and a period 616 when detector 124 operates, in order to avoid interference from radiation source 114 on detector 124. Radiation source 114 produces radiation during period 614, but does not produce radiation, or produces radiation at a low level, during period 616. The time-averaged level of radiation produced by source 114 may be set by a trade-off between quality of images produced by detector 118, and possible dangers of radiation exposure to the patient. If so, then the same optimal average level of radiation from source 114 may be achieved to using a higher level of radiation with a lower duty cycle. It may be advantageous to use a relatively low duty cycle for radiation source 114 and detector 118, well below 50%, and to use a relatively high duty cycle for detector 124, not too far below 100%, because using a lower duty cycle will reduce the quality of imaging or tracking by detector 124, while using a lower duty cycle will not adversely affect the quality of imaging by detector 118. However any duty cycles may be used. Although source 120, inside the body, is not turned off during period 614 when detector 118 is on, any interference from source 120 can be reduced by using a lower duty cycle and correspondingly higher intensity for source 114. In any case, source 120, being implanted inside the body and producing radiation isotropically, is likely to produce very little interference in detector 118, compared to the radiation received from source 114, which is directed at detector 118, and which can safely be of higher intensity than radiation source 120 because it is only used for a limited time. Periods 614 and 616 need not be longer than intervals 612 between beam pulses, as shown in FIG. 6, but could be shorter than intervals 612, and each interval 612 could, for example, be divided into an interval 614 and an interval 616. Periods 614 and 616 may also be much longer than intervals 612, for example between 10 milliseconds and 100 milliseconds, or longer than 100 milliseconds. However, it may be advantageous not to make these periods so long that the patient, or a part of the patient's body, could move significantly, for example a distance comparable to the diameter of the beam, within one of these periods, because then the detector that is not operating will not be able to detect the motion. It should be noted that, for any of the embodiments in which controller 126 actively controls the detector, by changing its bias voltage, or affecting its ability to record data, or in which controller 126 fails to record data or removes data based on the timing of the detection of the radiation, the corresponding control circuitry and/or timing circuitry in controller 126 is optionally fast enough to respond in much less than the interval between pulses, and optionally in less than a pulse length. If the detector system, in these embodiments, is adapted to monitor a given radiation treatment system, this means that it has control and/or timing electronics with the appropriate speed, depending on the pulse length and interval between pulses of that radiation treatment system. But this need not be true in embodiments where saturation of the detector is used to reduce detection of radiation during beam pulses, although in those embodiments it is still advantageous for the detector to have a short enough decay time. For example, the detector system is capable of selectively excluding data for radiation received over a time period as short as 100 milliseconds, or 10 milliseconds, or 1 millisecond, or 100 microseconds; or 10 microseconds, as opposed to radiation received outside this time period. In some embodiments of the invention, this is accomplished by actively controlling the detector to be less sensitive or not to record data during that time period, or by timing the radiation received and removing data corresponding to that time period. In these embodiments, the control circuitry, including the timing circuitry, is fast enough to accomplish this. In other embodiments of the invention, this is accomplished by having a detector that saturates at a low enough level. In all these embodiments, the detector optionally has a decay time shorter than the time period during which radiation is excluded. Optionally, in any of the radiation treatment systems described above, the decreased contribution to the detector data record of radiation during the beam pulses is such that this contribution of the direct and indirect radiation from the beam pulses is at least 2 times less, or at least 5 times less, or at least 10 times less, or at least 20 times less, or at least 50 times less, than the contribution that radiation from the beam pulses make to the total radiation received by the detector, for at least one detector used in an imaging or tracking system. Optionally, the contribution to the detector data record of radiation during the beam pulses is sufficiently low that interference from beam radiation is at least a factor of 2 less, or at least a factor of 5 less, or at least a factor of 10 less, or at least a factor of 20 less, or at least a factor of 50 less, than the contribution of radiation from the radiation source, external or internal, that is designed to be used with that detector for imaging or tracking. Optionally, any remaining interference from beam radiation is sufficiently small that it substantially does not affect the quality of images produced, for example the SNR, or the accuracy of tracking, for a given acquisition time. In some implementations, the invention can be used with existing beam therapy systems, without the need to modify the beam source. In some embodiments of the invention, imaging with an external radiation source 114, and imaging or tracking with an internal radiation source 120, are used on a patient, even without using a beam for radiation treatment. In these embodiments, periods 614 when source 114 and corresponding detector 118 are operating, are alternated with periods 616 when source 114 and detector 118 are not operating, while detector 124 for source 120 is operating. This reduces or eliminates interference of source 114 on detector 124, and, for a given average power of source 114, reduces interference of source 120 on detector 118. For the reasons given above, it may be advantageous to make periods 614 much shorter than periods 616. Although the embodiments of the invention shown in FIGS. 1-6 all involve patients receiving medical treatment with the radiation beam, similar methods are optionally used in industrial processes where an inanimate body is being treated by a radiation beam, and is being monitored by an imaging or tracking system using another source of radiation, and a radiation detector, to avoid or reduce interference of the beam on the detector. It is expected that during the life of a patent maturing from this application many relevant radiation beams and detectors will be developed and the scope of the terms radiation beam and radiation detector is intended to include all such new technologies a priori. As used herein the term “about” refers to ±10%. The terms “comprises”, “comprising”, “includes”, “including”, “having” and their conjugates mean “including but not limited to”. This term encompasses the terms “consisting of” and “consisting essentially of”. The phrase “consisting essentially of” means that the composition or method may include additional ingredients and/or steps, but only if the additional ingredients and/or steps do not materially alter the basic and novel characteristics of the claimed composition or method. As used herein, the singular form “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a compound” or “at least one compound” may include a plurality of compounds, including mixtures thereof. Throughout this application, various embodiments of this invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range. Whenever a numerical range is indicated herein, it is meant to include any cited numeral (fractional or integral) within the indicated range. The phrases “ranging/ranges between” a first indicate number and a second indicate number and “ranging/ranges from” a first indicate number “to” a second indicate number are used herein interchangeably and are meant to include the first and second indicated numbers and all the fractional and integral numerals therebetween. As used herein, the term “treating” includes abrogating, substantially inhibiting, slowing or reversing the progression of a condition, substantially ameliorating clinical or aesthetical symptoms of a condition or substantially preventing the appearance of clinical or aesthetical symptoms of a condition. It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. To the extent that section headings are used, they should not be construed as necessarily limiting.

abstract

A material is exposed to a neutron flux by distributing it in a neutron-diffusing medium surrounding a neutron source. The diffusing medium is transparent to neutrons and so arranged that neutron scattering substantially enhances the neutron flux to which the material is exposed. Such enhanced neutron exposure may be used to produce useful radio-isotopes, in particular for medical applications, from the transmutation of readily-available isotopes included in the exposed material. It may also be used to efficiently transmute long-lived radioactive wastes, such as those recovered from spent nuclear fuel. The use of heavy elements, such as lead and/or bismuth, as the diffusing medium is particularly of interest, since it results in a slowly decreasing scan through the neutron energy spectrum, thereby permitting very efficient resonant neutron capture in the exposed material.

claims

1. An apparatus for determining residual energy of positively charged particles after passing through a patient, comprising:a multi-layer detector, comprising;a first layer comprising a first scintillation material, said first scintillation material, responsive to passage of the positively charged particles, emitting first secondary photons over a first wavelength range;a second layer comprising a second scintillation material, said second scintillation material, responsive to passage of the positively charged particles, emitting second secondary photons over a second wavelength range, the first scintillation material differing from the second scintillation material;a third layer comprising a third scintillation material, said third scintillation material, responsive to passage of the positively charged particles, emitting third secondary photons over a third wavelength range, said third scintillation material differing from both said first scintillation material and said second scintillation material; andorientation of said first layer, said second layer, and said third layer within thirty degrees of orthogonal to a path of the positively charge particles, said second layer positioned between said first layer and said third layer, a front surface of said second layer within ten centimeters of said first layer, a back surface of said second layer within ten centimeters of said third layer. 2. The apparatus of claim 1, said multi-layer detector further comprising:a first sub-stack of scintillation materials comprising said first layer, said second layer, and said third layer; anda second sub-stack comprising a manufactured copy of said first sub-stack. 3. The apparatus of claim 2, said multi-layer detector further comprising:at least ten layers of scintillation materials, said at least ten layers of scintillation materials comprising:said first sub-stack; andsaid second sub-stack. 4. The apparatus of claim 1, further comprising:an imaging system configured to use output from said multi-layer detector to generate an image of a tumor of the patient. 5. A multi-layer detector, comprising:a first layer comprising a first scintillation material, said first scintillation material, responsive to passage of the positively charged particles, emitting first secondary photons over a first wavelength range; anda second layer comprising a second scintillation material, said second scintillation material, responsive to passage of the positively charged particles, emitting second secondary photons over a second wavelength range, the first scintillation material differing from the second scintillation material,wherein said first scintillation material further comprises a first responsivity to the positively charged particles at least twice as responsive, in terms of number of emitted photons per unit energy released from the positively charged particles, as said second scintillation material. 6. The apparatus of claim 5, further comprising:an accelerator configured to generated the positively charged particles;a beam transport system configured to transport the positively charged particles from said accelerator, over a patient positioning system, and into said multi-layer detector. 7. The apparatus of claim 5, further comprising:an imaging system configured to use output from said multi-layer detector to generate an image of the patient. 8. A method for determining residual energy of positively charged particles after passing through a patient, comprising the steps of:passing the positively charged particles into a multi-layer detector element;detecting first secondary photons, resultant from passage of the positively charged particles, over a first wavelength range from a first layer of said multi-layer detector, said first layer comprising a first scintillation material; anddetecting second secondary photons, resultant from passage of the positively charged particles, over a second wavelength range from a second layer of said multi-layer detector element, the first wavelength range differing from the second wavelength range; andgenerating a set of response signals, each of at least six individual members of said set of response signals relating to a corresponding layer of a set of at least six layers of said multi-layer detection element. 9. The method of claim 8, further comprising the step of:detecting third secondary photons, resultant from passage of the positively charged particles, over a third wavelength range from a third layer of said multi-layer detector element, a third mean wavelength of the third wavelength range differing from both a first mean wavelength of the first wavelength range and a second mean wavelength of the second wavelength range by at least ten nanometers. 10. The method of claim 8, further comprising the step of:generating said set of response signals using two or three distinct scintillation material types.

abstract

Device and method for checking fuel rods with IFBA, their zirconium diboride coating. The device includes a variable magnetic field generator and a magnetic field pickup device, arranged in the vicinity of the rod, as well as a control system for comparing both fields in order to measure the electric conductivity of the rod. The method includes the steps of: arranging the rod to be measured between the generator and the pickup device; generation of a variable magnetic field in the generator; picking-up of the magnetic field; comparison between the generated magnetic field and the picked-up one in order to quantify the electric conductivity of the rod; if the electric conductivity differs from a reference value, consider the rod for checking or recycling.

description

The present application claims the benefit of U.S. Provisional Application Ser. No. 62/080,560, filed Nov. 17, 2014, which is incorporated by reference herein in its entirety, including any figures, tables, and drawings. Computed Tomography (CT) is a mainstay in diagnostic imaging. It plays key roles in disease detection and characterization, and also in patient follow-up during and after treatment. Biological soft tissue consists mainly of light elements, and its density is nearly uniform with little variation. Conventional attenuation-based X-ray imaging cannot provide sufficient contrast for biological soft tissue. X-ray phase imaging is sensitive to structural variation of soft tissue and offers superior contrast resolution for characterization of cancerous and other diseased tissues. The cross-section of an X-ray phase shift image is a thousand times greater than that of X-ray attenuation in soft tissue over the diagnostic energy range. This implies that phase imaging can achieve a much higher signal-to-noise ratio and substantially lower radiation dose than attenuation-based X-ray imaging. Grating interferometry is a state of the art X-ray imaging approach, which can simultaneously acquire information of X-ray phase-contrast, dark-field, and linear attenuation. Conventional grating interferometers often use flat gratings, with serious limitations in the field of view and the flux of photons. The subject invention provides novel and advantageous systems and methods for X-ray phase-contrast imaging (PCI), including the use of one or more period-varying or quasi-periodic gratings. An X-ray PCI system can include a phase grating that is period-varying or quasi-periodic and can be positioned between an object being imaged and a detector. A second grating, such as an absorption grating or an analyzer grating can also be present and disposed between the phase grating and the detector. One or both of the gratings (phase grating and absorption or analyzer grating) can be curved, such as a cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere) grating. Alternatively, one or both of the gratings (phase grating and absorption or analyzer grating) can be flat. The detector array can be curved or flat. For example, the detector can be a flat detector array. The subject invention also provides second-order approximation models for X-ray phase retrieval, for example using paraxial Fresnel-Kirchhoff diffraction theory. An iterative method can be used to reconstruct a phase-contrast image and/or a dark-field image. The models can be iteratively solved using the algebraic reconstruction technique (ART). State of the art compressive sensing techniques can be incorporated to achieve high quality image reconstruction. In an embodiment, an X-ray PCI system can include an X-ray source, a detector, a first grating disposed between the energy source and the detector, and a second grating disposed between the first grating and the detector, wherein the first grating is a quasi-periodical phase grating such that the period of the grating varies across the grating. In another embodiment, a method of performing X-ray PCI can include providing an X-ray PCI system as described herein, positioning an object to be imaged between the X-ray source and the first grating, and imaging the object using the system. In another embodiment, a method of reconstructing or retrieving an X-ray phase image can include approximating the image with a second-order approximation model using paraxial Fresnel-Kirchhoff diffraction theory. The subject invention provides novel and advantageous systems and methods for X-ray phase-contrast imaging (PCI), including the use of one or more period-varying or quasi-periodic gratings. An X-ray PCI system can include a phase grating that is period-varying or quasi-periodic and can be positioned between an object being imaged and a detector. A second grating, such as an absorption grating or an analyzer grating can also be present and disposed between the phase grating and the detector. One or both of the gratings (phase grating and absorption or analyzer grating) can be curved, such as a cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere) grating. Alternatively, one or both of the gratings (phase grating and absorption or analyzer grating) can be flat. The detector array can be curved or flat. For example, the detector can be a flat detector array. In an embodiment, a system for imaging (e.g., X-ray imaging such as X-ray PCI) can include an energy source (e.g., an X-ray source), a detector, a first grating that is a period-varying or quasi-periodical phase grating positioned between the energy source and the detector, and a second grating positioned between the first grating and the detector. The second grating can be an absorption grating or an analyzer grating. Each of the first and second gratings can be curved, such as a cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere) grating or flat. The object to be imaged would be positioned between the energy source and the first grating. The system can optionally include a source grating positioned near the energy source, wherein the object to be imaged would be positioned between the source grating and the first grating. The source grating can be curved, such as a cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere) grating or flat. In an embodiment, the detector is a flat detector, such as a flat detector array. The first grating can be curved, such as a cylindrical or spherical grating. The second grating can be flat to match the curvature (or lack thereof) of the detector. The optional source grating, if present, is curved, such as a cylindrical or spherical grating. The period-varying or quasi-periodical grating (e.g., period-varying or quasi-periodical phase grating) can have a predetermined or set period at the lateral point where the energy source is centered on the grating. The period can then change as the position on the grating is moved outwardly away along the surface of grating. In an embodiment, the period can change equally in both directions, such that the period at a given angle from the center point is the same as the negative of that given angle from the center point (e.g., as shown in FIGS. 3A and 11C). Embodiments of the subject invention also provide spherical and/or cylindrical grating interferometers, and Talbot self-imaging with gratings of a slow-varying-period. Image quality can be improved and field of view (FOV) can be enlarged. Imaging systems of the subject invention allow perpendicular incidence of X-rays on one or more of the gratings, giving higher photon flux and visibility, and a larger FOV. Imaging can be performed on larger sized objects than with a conventional interferometer with flat gratings. Imaging systems and methods of the subject invention have great utility for many preclinical and clinical applications. The subject invention also provides second-order approximation models for X-ray phase retrieval, for example using paraxial Fresnel-Kirchhoff diffraction theory. An iterative method can be used to reconstruct a phase-contrast image and/or a dark-field image. The models can be iteratively solved using the algebraic reconstruction technique (ART). State of the art compressive sensing techniques can be incorporated to achieve high quality image reconstruction. In an embodiment, an iterative method can be used for phase reconstruction (e.g., based on Equation (42) below), and the method can include Equation (46) below. Equation (46) can be discretized to a system of linear equations with respect to variables pk(r). Then, the ART can be employed for phase image reconstruction. The second-order phase approximation models for X-ray phase retrieval, and iterative algorithms, can be used for both in-line phase-contrast and dark-field imaging. These models accurately establish quantitative correspondence between phase shifts and image intensities, outperforming the related art linear phase approximation model used for X-ray in-line phase contrast imaging, because the linear model assumes slow phase variation and weak linear attenuation of an object. The second-order approximation models disclosed herein can accurately establish a quantitative correspondence between phases and recorded intensity images, outperforming the related art linear phase approximation models used in the conventional methods of X-ray in-line PCI. X-ray PCI is fundamentally different from conventional X-ray imaging and exhibits superior contrast resolution in various basic and applied areas. PCI relies on phase-shifts of X-ray waves, instead of their amplitude changes. In principle, the X-ray PCI is much more sensitive than attenuation-based imaging in the cases of weak-absorption materials. PCI can be used in many areas, including clinical applications. Three main types of X-ray phase retrieval for PCI are the propagation-based method, the analyzer-based method, and the grating-based method. Among these, the grating-based method has particularly good cost-effectiveness and established performance. Grating-based PCI has been used extensively in, for example, studying breast tissues and joint structures, and even with prototype machines. The grating-based PCI method has been implemented with synchrotron radiation and has been extended to work under laboratory conditions by adding a third grating in certain situations. Grating-based PCI is based on the Talbot self-imaging principle, and it utilizes a near-field diffraction effect. That is, when coherent light passes through a periodic grating, the wave pattern right after the grating repeats itself at multiple distances. Based on the Talbot effect, phase shifts of incident X-ray waves induced by an object in the wave field can be extracted by comparing the grating self-imaging patterns with and without the object. The phase-stepping method is often used for data collection in such a grating interferometer. Grating-based PCI can also be used for small field-of-view (FOV) imaging. However, it can sometimes be difficult to increase the FOV with conventional systems, due to several issues related to not only grating fabrication but also physical modeling. Theoretically speaking, the beam divergence cannot be neglected in the case of a large beam angle for which the paraxial approximation is no longer valid. The subject invention addresses this problem by utilizing the Talbot self-imaging method with one or more quasi-periodic gratings. This is effective for many applications, including performing user-defined self-imaging and being part of an optimal system design for grating-based PCI with a large FOV. X-ray imaging machines are widely used in pre-clinical and clinical examinations. Such machines use X-ray tubes and X-ray detectors, integrated with electrical and mechanical parts. According to the detector type, they can be classified into flat- and curved-detector systems. FIG. 1A shows a schematic of a curved-detector-based X-ray imaging system, and FIG. 1B shows a schematic of a flat-detector-based X-ray imaging system. Much of the derivation presented herein focuses on fan-beam imaging geometry (FIG. 1A) and not the cone-beam counterpart (FIG. 1B), but the fan-beam principles can be extended to the cone-beam case. The imaging plane can be considered where X-rays stay, and are characterized by an angle λ with respect to a central line on the imaging plane. In the curved-detector system, as shown in FIG. 1A, the distance between an X-ray source and each detector cell is a constant, D=D0. In the flat-detector system, as shown in FIG. 1B, the distance is described as a parabolic functionDi=√{square root over (D02+(iΔx)2)}, (1)where i=0, ±1, ±2, . . . , indexing the positions of detector cells, and Δx is the cell pitch. Approximated by the Taylor series, Equation (1) can be written as D i ≈ D 0 + 1 2 ⁢ ⁢ D 0 ⁢ ( i ⁢ ⁢ Δ x ) 2 . ( 2 ) The Talbot self-imaging phenomenon is understood in the art. Let a periodic grating be infinite long along the x-axis: t ⁡ ( x ) = ∑ i = - ∞ + ∞ ⁢ ⁢ g ⁡ ( x - ip ) = g ⁡ ( x ) ⊗ ∑ i = - ∞ + ∞ ⁢ δ ⁡ ( x - ip ) ( 3 ) where i=0, ±1, ±2, . . . , is a convolution operator, p is the grating period, and g(x) gives the complex amplitude at individual grating apertures. If the case of a fully coherent parallel beam with a wavelength λ is considered, the wave field, wt(x), right after the grating, is described by t(x) (Equation (3)). Then, the Fourier transform of wt(x) is W ⁡ ( k x ) = FT ⁢ { w t ⁡ ( x ) } = FT ⁢ { t ⁡ ( x ) } ∝ ∑ j = - ∞ + ∞ ⁢ G ⁡ ( k x ) · δ ⁡ ( k x - jK 0 ) = ∑ j = - ∞ + ∞ ⁢ G ⁡ ( jK 0 ) ( 4 ) in which FT denotes the Fourier transform, j=0, ±1, ±2, . . . , G(kx)=FT{g(x)}, and K 0 = 2 ⁢ ⁢ π p . With the X-ray propagation after the grating, the diffraction phenomenon is expressed as by the Fresnel approximationwi,d(x)=wt(x)Fd(x) (5)in which d specifies the propagation distance, and F(x) is the Fresnel operator defined by F d ⁡ ( x ) = exp ⁡ ( i ⁢ 2 ⁢ ⁢ π λ ⁢ d ) i ⁢ ⁢ λ ⁢ ⁢ d ⁢ exp ⁡ ( i ⁢ ⁢ π λ ⁢ ⁢ d ⁢ x 2 ) ( 6 ) The Fourier transform of Equation (5) can be found to be W d ⁡ ( k x ) = FT ⁢ { w t , d ⁡ ( x ) } ∝ exp ⁡ ( i ⁢ λ ⁢ ⁢ d 4 ⁢ ⁢ π ⁢ k x 2 ) ⁢ ∑ j = - ∞ + ∞ ⁢ G ⁡ ( jK 0 ) ≈ ∑ j = - ∞ + ∞ ⁢ exp ⁡ ( i ⁢ λ ⁢ ⁢ d 4 ⁢ ⁢ π ⁢ ( jK 0 ) 2 ) ⁢ G ⁡ ( jK 0 ) = ∑ j = - ∞ + ∞ ⁢ exp ⁡ ( i ⁢ λ ⁢ ⁢ d ⁢ ⁢ π p 2 ⁢ j 2 ) ⁢ G ⁡ ( jK 0 ) ( 7 ) According to Equation (7), when d = d T = 2 ⁢ ⁢ n ⁢ p 2 λ ,where n=1, 2, 3 . . . , a similar wave front is formed, and dT is called the Talbot distance. Further, the same intensity pattern can be repeated at the distance d = m ⁢ p 2 8 ⁢ ⁢ λ ,where m is an odd integer. In the case of fan-beam illumination, as shown in FIG. 1B, the wave front at the grating position is w, and its wave phase is distributed as a quadratic function w ⁡ ( x ) = exp ⁡ ( i ⁢ ⁢ α ⁢ 2 ⁢ ⁢ π λ ⁢ x 2 ) ,in which α defines the curvature of the quadratic phase profile, determined by the distance between the X-ray source and the grating position D, α = 1 2 ⁢ ⁢ D ,as derived from Equations (1) and (2). Thus, the wave complex amplitude wt(x) immediately after the phase grating is w t fan ⁡ ( x ) = w ⁡ ( x ) · t ⁡ ( x ) = w ⁡ ( x ) ⁢ ∑ i = - ∞ + ∞ ⁢ g ⁡ ( x - ip ) ≈ ∑ i = - ∞ + ∞ ⁢ w ⁡ ( ip ) ⁢ g ⁡ ( x - ip ) ( 8 ) According to Equation (5), the Fourier transform of the wave front after a short propagation distance d is w d fan ⁡ ( k x ) = ⁢ FT ⁢ { w t , d fan ⁡ ( x ) } ∝ ⁢ ∑ j = - ∞ + ∞ ⁢ exp ⁡ ( i ⁢ λ ⁢ ⁢ d ⁢ ⁢ π p 2 ⁢ j 2 ) ⁢ exp ⁡ ( i ⁢ λ ⁢ ⁢ D ⁢ ⁢ π p 2 ⁢ j 2 ) ⁢ G ⁡ ( jK 0 ) = ⁢ ∑ j = - ∞ + ∞ ⁢ exp ⁡ ( i ⁢ λ ⁡ ( d + D ) ⁢ π p 2 ⁢ j 2 ) ⁢ G ⁡ ( jK 0 ) ( 9 ) With the magnification effect of fan-beam geometry, the fractional Talbot distance is d = m ⁢ p 2 8 ⁢ ⁢ λ ⁢ D + d D ( 10 ) which is the counter part of the Talbot lengths in the parallel-beam case, with the magnification factor (D+d)/D. FIG. 2A shows a representation of flat-grating-based fan-beam Talbot self-imaging. FIG. 2B shows a representation of curved-grating-based fan-beam Talbot self-imaging. Talbot self-imaging is equivalent to parallel-beam imaging with an appropriate phase initialization. The magnification factor (M) due to fan-beam geometry should be considered. Comparing Equations (7) and (9), it is clear that the behavior of fan-beam Talbot self-imaging is similar to that of the parallel-beam case, except that there is a quadratically-distributed initial phase at the grating position in the fan-beam case. Hence, as shown in FIG. 2A, flat-grating-based fan-beam Talbot imaging is equivalent to the parallel-beam situation by setting the initial phase distribution and taking the fan-beam magnification. In addition, with curved-grating-based fan-beam Talbot imaging, the distance between the X-ray source and each grating aperture is constant, and only the system magnification is needed, as shown in FIG. 2B. The Talbot effect is based on periodic gratings, which repeats the same intensity pattern at different distances after the grating. Embodiments of the subject invention can utilize Talbot self-imaging with quasi-periodic period gratings. This can result in more freedom in designing the intensity profile for phase-contrast imaging. A quasi-periodic grating can be defined as t ′ ⁡ ( x ) = ∑ i = - ∞ + ∞ ⁢ g p ′ ⁡ ( i ) ⁡ ( x - X ⁡ ( i ) ) ( 11 ) where X(i) denotes the grating aperture position along the x-axis, indexed by i, and p′(i) is the width of the corresponding grating aperture. A grating with a slow-varying period can be approximated by a composition of periodic gratings, written as t ′ ⁡ ( x ) ≈ ⁢ 1 2 ⁢ ⁢ N ⁢ ∑ i = - ∞ + ∞ ⁢ ⁢ ∑ i L = i - N i + N ⁢ ⁢ g p ′ ⁡ ( i ) ⁡ ( x - X ⁡ ( i ) ) = ⁢ 1 2 ⁢ ⁢ N ⁢ ∑ i = - ∞ + ∞ ⁢ ( g p ′ ⁡ ( i ) ⁡ ( x ) ⊗ ∑ p L = - N + N ⁢ ⁢ δ ⁡ ( x - X ⁡ ( i ) - i L ⁢ p ′ ⁡ ( i ) ) ) = ⁢ 1 2 ⁢ ⁢ N ⁢ ∑ i = - ∞ + ∞ ⁢ ( g p ′ ⁡ ( i ) ⁡ ( x ) ⊗ ( t trunc ⁡ ( i , 2 ⁢ ⁢ N ) ⁢ ∑ i L = - ∞ + ∞ ⁢ δ ⁡ ( x - X ⁡ ( i ) - i L ⁢ p ′ ⁡ ( i ) ) ) ) ( 12 ) where iL defines a local grating aperture location corresponding to the index i, and ttrunc(i,2N) is a grating envelope associated with the finite grating length 2N, centered at the ith-indexed position. If each individual grating aperture is short enough relative to the grating truncation length 2N, Equation (12) can be further simplified to t ′ ⁡ ( x ) ≈ 1 2 ⁢ ⁢ N ⁢ ∑ i = - ∞ + ∞ ⁢ ( g p ′ ⁡ ( i ) ⁡ ( x ) ⊗ ∑ i L = - ∞ + ∞ ⁢ δ ⁢ ( x - X ⁡ ( i ) - i L ⁢ p ′ ⁡ ( i ) ) ) ( 13 ) The accuracy of the approximation in Equation (13) can be measured as  N ⁢ ⁢ p ′ ⁡ ( i ) - ( X ⁡ ( i ) - X ⁡ ( i - N ) )  p ′ ⁡ ( i ) ( 14 ) that quantifies the maximum local linear representation error. The accuracy of this is discussed in Examples 1 and 2 based on a real imaging scenario, where the system size was much larger than the grating aperture width. Comparing Equations (3) and (13), it is found that the period-varying grating is a set of super-positioned periodic gratings, and has the same Talbot self-imaging property as that of periodic gratings. Thus, the fractional Talbot length of the period-varying grating should resemble that of the conventional periodic gratings, but the Talbot length is changed along the x-axis and determined by corresponding grating aperture size p′(i) expressed as d = m ⁢ p ′ ⁡ ( i ) 8 ⁢ ⁢ λ ( 15 ) in the parallel-beam case and d = m ⁢ D + d D ⁢ p ′ ⁡ ( i ) 8 ⁢ ⁢ λ ( 16 ) in the fan-beam case. With Equation (15) or Equation (16), it is feasible to design the self-imaging patterns for various benefits. The length between the grating and its self-imaging pattern is determined by the grating aperture size p′(i). In a grating-based PCI system, a phase grating G1 and an absorption grating G2 can be placed between an object and a detector, as shown in FIGS. 4A-4D. Curved gratings can be used as well, as shown in FIG. 1B, to match the X-ray beam divergence. Similarly, in a curved-detector system (FIG. 1A), flat and curved grating sets can be installed for phase-contrast imaging. Thus, four kinds of grating-based PCI systems include: flat grating, flat detector; curved grating, flat detector; flat grating, curved detector; and curved grating, curved detector. FIG. 4A shows a schematic view of a flat grating, flat detector system. FIG. 4B shows a schematic view of a curved grating, flat detector system. The red arrow highlights the non-uniform gaps between the absorption grating G2 and the detector. FIG. 4C shows a schematic view of a flat grating, curved detector system. The red arrow highlights the non-uniform gaps between the absorption grating G2 and the detector. FIG. 4D shows a schematic view of a curved grating, curved detector system. If the detector and the absorption grating G2 have different curvatures as indicated by the red arrows in FIGS. 4B and 4C, there are non-uniform gaps between the grating G2 and the detector. The non-uniform gap distribution can introduce non-uniform image artifacts due to the blurring effect from the X-ray spot. Therefore, the absorption grating G2 should be placed as closely as possible to the detector plane, for example as in FIGS. 4A and 4D. The designs in FIGS. 4A and 4D can also have issues due to imperfect fabrication of the gratings and/or difficulty in matching the gratings to the beam divergence. This can lead to the phase grating G1 and absorption grating G2 not being exactly paired, as shown in FIGS. 5A and 5B. FIG. 5A shows a schematic view of a grating-based phase-contrast system with a curved G1 and a flat G2 (and flat detector), and FIG. 5B shows a schematic view of a grating-based phase-contrast system with a flat G1 and a curved G2 (and curved detector). FIG. 5C shows a plot of the visibility versus angle (in degrees) for the setup of FIG. 5A, and FIG. 5D shows a plot of the visibility versus angle (in degrees) for the setup of FIG. 5B. FIG. 6A shows a plot of visibility versus angle (in degrees) for the unpaired G1-G2 system of FIG. 5A and a period-unmatched imaging system. The red line (upper line) is for the period-unmatched system, and the blue line (lower line) is for the system of FIG. 5A). FIG. 6B shows a plot of visibility versus angle (in degrees) for a period-matched imaging system. In a grating-based PCI system, a periodic phase grating can be adopted to produce the same interference patterns at a Talbot length. However, if the phase grating and absorption grating are not paired (as in FIGS. 5A and 5B), the absorption grating may not be located exactly on the interference patterns. The visibility of the stepping curve can then be degraded. FIGS. 5C and 5D show results with respect to the angle in the fan beam. In many embodiments of the subject invention, a phase grating can be a period-varying grating, and can be paired with, e.g., a flat absorption grating G2 (FIG. 5A) or a curved absorption grating G2 (FIG. 5B). The phase gratings shown in FIGS. 5A and 5B can be designed using Equation (16). The results are shown in FIG. 6A, compared with the unpaired G1-G2 system of FIG. 5A. To match the period-varying G1, the grating G2 can be accordingly modified by p G ⁢ ⁢ 2 ′ ⁡ ( i ) = p G ⁢ ⁢ 1 ′ ⁡ ( i ) 2 ⁢ D + d D . ( 17 ) FIG. 6B shows that the period-matched G1 and G2 is an optimized grating-based PCI system with a nearly-uniform visibility distribution within the fan beam range. In the related art, high-energy and large FOV PCI imaging is a major technical challenge. In the case of a large FOV, as the fan beam angle increases, the X-ray beam divergence becomes significant enough to degrade the imaging performance. A curved grating structure can thus be used in embodiments of the subject invention. Flat-panel detectors are widely used in practice due to low cost and high spatial resolution. This requires a flat absorption grating G2 to match the detector shape, avoiding the non-uniform gap between the detector and G2. This can lead to a curved phase gating G1 and flat absorption grating G2 being unpaired, as shown in FIG. 5A. Similar issues exist for a curved-detector imaging system, as shown in FIG. 5B. To solve this problem, embodiments of the subject invention advantageously use one or more period-varying gratings (e.g., a Talbot self-imaging method with a period-varying grating). By modifying the individual grating aperture width, the Talbot self-imaging lengths can be determined to match the spatial non-uniformity between G1 and G2, allowing the G2 to be placed exactly (or nearly exactly) on the self-imaging pattern of G1. The performance of a grating design of the subject invention can be quantified, for example, by the maximum local linear representation error defined by Equation (14). To make the error less than 5%, in an imaging system used in practice (e.g., a 4.8 μm G1 period and 1 m source-to-G1 distance), the value of N in Equation (14) should be about 1×104, which is large enough to satisfy the approximation in Equation (13). The grating length truncation will not influence the beam decomposition and reformation (see also, e.g., FIGS. 2A and 2B). Period-varying gratings of the subject invention generate uniform visibility distribution within the fan beam range, leading to excellent imaging performance. Thus, Talbot self-imaging methods with period-varying grating(s) can facilitate system design and improve imaging performance. The phase grating G1 can be curved to match the divergence of an X-ray fan-beam, and the absorption grating G2 can be adapted to match a flat detector (e.g., FIGS. 4D and 5A). This can lead to large FOV clinically-oriented imaging systems. X-ray PCI based on grating interferometry has exhibited superior contrast in biological soft tissue imaging. The high sensitivity relies on the high-aspect ratio structures of the planar gratings, which prohibit the large field of view applications with a diverging X-ray source. Curved gratings allow a high X-ray flux for a wider angular range, but the interference fringes are only visible within ˜10° range due to the geometrical mismatch with the commonly-used flat array detectors. Embodiments of the subject invention use a curved quasi-periodic grating for large FOV imaging, even with a flat detector array. This can lead to, for example, an interference fringe pattern with less than a 10% decrease in visibility over 25°. X-ray PCI can detect small variations of the refractive index, providing superior contrast for low-elements compared with attenuation contrast, and thus is advantageous in imaging biological soft tissues, such as vessels, lesions, and micro-calcifications. FIGS. 10A and 10B show setups of grating-based PCI systems. FIG. 10A shows two flat gratings G1 and G2, and FIG. 10B shows a curved phase grating G1 and a flat analyzer grating G2. A flat panel detector can be placed immediately after G2. In the case of using an X-ray tube with low spatial coherence, another grating G0 (omitted in FIGS. 10A and 10B; see also FIG. 9A) can be used between the source and the object. During the imaging process, phase stepping of the analyzer grating G2 can be induced, and the information of X-ray attenuation, phase contrast, and small angle scattering can be acquired simultaneously. The PCI sensitivity relies on the frequency of the interference fringes. Flat gratings with high aspect ratio structures are ideal for parallel beams. However, in most non-synchrotron setups, the divergent radiation emerges from the focus of the X-ray tube; therefore, the FOV in the X-ray interferometer is limited to only a few centimeters. The use of a curved phase grating G1 allows perpendicular incidence of X-rays on the gratings, permitting a high X-ray flux with a wider acceptance angle than that associated with a flat grating. In such a setup, a curved phase grating G1 can generate a self-image on a curved analyzer grating G2, posing a geometrical mismatch with a flat panel detector. This mismatch is position-dependent and grows nonlinearly as the angle (from straight ahead from the source onto the grating) increases. As a result, the mismatch can cause a reduction of the visibility of the stepping curve at larger angles and limit the FOV of the curved grating setups. The stepping visibility is defined as the ratio between the amplitude and the mean of the phase stepping curve. FIG. 9A shows a schematic of an X-ray imaging system with spherical gratings. The system can include an X-ray source, a spherical phase grating G1, and a spherical analyzer grating G2. The object to be imaged can be positioned between the source and the phase grating G1. An optional spherical source grating G0 is also shown in FIG. 9A. FIG. 9B shows a spherical grating for 1D grating. FIG. 9C shows a spherical grating for 2D grating. The use of spherical gratings (e.g., with a common X-ray tube as a point source) can allow perpendicular incidence of X-rays on the gratings, thereby resulting in higher flux and higher visibility, and a larger field of view than a conventional interferometer with flat gratings. The source grating G0, if present, can be placed close to the source to form an aperture mask with transmission elements and can generate an array of periodically repeating sources. The phase grating G1 can act as a phase mask for the phase modulations onto the incoming wave field. Through the Talbot effect, the phase modulation can be transformed into an intensity modulation in the plane of the analyzer grating G2 to form a periodic fringe pattern. The third (analyzer) grating G2 can have the same period and orientation as the fringes created by G1 to extract X-ray phase variation induced by the object being imaged. In many embodiments, a curved quasi-periodic phase grating G1′ can be used in a PCI system or method. The grating can have a self-image that occurs on a flat plane, thereby advantageously allowing the use of a flat analyzer (or absorption grating) G2 to match the detector (i.e., flat detector) to eliminate the geometric mismatch for a larger FOW in PCI with a flat panel detector, as shown in FIG. 10B. The self-imaging effect of an infinite periodic planar structure can be formulated by geometrical ray tracing and scalar diffraction theory under paraxial approximation. An angular decomposition approach can be used to derive the Talbot distance for one dimensional (1D) curved gratings in polar coordinates. The analysis presented herein is not limited to binary transmission gratings, but also applicable to phase gratings, which are more common as gratings G1 in X-ray PCI setups. In diffraction theory, the vector field can be treated as a scalar field under two conditions: 1) the feature size of the object along the polarization direction of the field is much larger than the wavelength; 2) all components of vector field obey the same scalar wave equation. In general, the second condition is not satisfied in cylindrical coordinates. Scalar diffraction theory is applicable, though, for the grating analysis for the X-ray PCI imaging. Assuming r0>>p>>λ, the electric field components in free space diffracted by the grating G1 can be expressed as a sum of its Fourier series: E z = ∑ m ⁢ ∈ Z ⁢ ⁢ a m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 18 ) E θ = i ⁢ ⁢ η 0 k ⁢ ∑ m ⁢ ∈ Z ⁢ b m ⁢ ∂ ∂ r ⁢ H m ( 2 ) ⁡ ( kr ) ⁢ e i ⁢ ⁢ m ⁢ ⁢ θ , ( 19 ) E r = η 0 ⁢ m kr ⁢ ∑ m ⁢ ∈ Z ⁢ ⁢ b m ⁢ H m ( 2 ) ⁡ ( kr ) ⁢ e i ⁢ ⁢ m ⁢ ⁢ θ , ( 20 ) where η0=1/(ε0c), is the impedance of the free space, k=2π/λ, is the wavenumber, r>r0, Hm(2)(kr) is Hankel function of the second kind representing the outgoing wave, and m is an integer. For a curved grating with period p, m is given by 2nπr0/p, where n is an integer. In the case of an X-ray PCI setup, r0>>p>>λ. Thus, the asymptotic expansions of modulus and the phase of Hm(2)(kr) can be used: ⁢  H m ( 2 ) ⁡ ( kr )  = 2 π ⁢ ⁢ k ⁢ ⁢ r + 4 ⁢ ⁢ m 2 - 1 2 · ( 2 ⁢ ⁢ kr ) 2 ⁢ + … , ( 21 ) arg ⁢ ⁢ H m ( 2 ) ⁡ ( kr ) = ⁢ - kr + ( 1 2 ⁢ m + 1 4 ) ⁢ π - 4 ⁢ ⁢ m 2 - 1 8 ⁢ ⁢ kr + ( 4 ⁢ ⁢ m 2 - 1 ) ⁢ ( 4 ⁢ ⁢ m 2 - 9 ) 6 · ( 4 ⁢ ⁢ kr ) 3 ⁢ + … ⁢ , ( 22 ) These asymptotic expansions are valid for kr much greater than m, i.e., m/kr<<1. Substituting Equation (21) and Equation (22) into Equation (18), (19), and (20) yields the approximated expressions of the outwardly propagating electric field. Under the condition r0>>p>>λ, further approximation can be made as H′m(2)/k≈−i Hm(2). The three components of the electric field reduce to E z = ∑ m ⁢ ∈ Z ⁢ ⁢ a m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 23 ) E θ = η 0 ⁢ ∑ m ⁢ ∈ Z ⁢ b m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 24 ) E r = m kr ⁢ η 0 ∑ m ⁢ ∈ Z ⁢ ⁢ b m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 25 ) where r>r0, Hm(2)(kr) are approximated by Equation (21), and Equation (22) shows that Er decays faster along the direction of r than the other two components. Because m/kr<<1, Er<<E0. The component Er can be ignored. The other two components of the electric fields have the same form. The components of the magnetic field have similar results. Therefore, under the condition r0<<p<<λ, all the field components have the same form, and the polarization of the electric field is perpendicular to the r direction. Because the two conditions for using scalar diffraction theory are both satisfied, one scalar field u(θ, r0) can be used to represent any field component in cylindrical coordinates, u ⁡ ( θ , r ) = ∑ m ⁢ ∈ Z ⁢ c m · 1 kr ⁢ e - kr + m 2 ⁢ 4 ⁢ ⁢ m 2 - 1 8 ⁢ ⁢ kr · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 26 ) where cm is the complex Fourier coefficient. Referring to FIG. 10B, a curved grating G1, whose center is located at the origin of the coordinate system, can have a radius of r0. The point source can also be placed at the origin. The angular span of the grating can be assumed to be small in the vertical direction, so the irradiance is uniform along the z-axis. In the case of an X-ray PCI setup, the radius of grating G1 (˜1 m) is much greater than the period of G1 (˜1 μm), and the period of G1 is much greater than the wavelength (˜0.1 nm), i.e., r0>>p>>λ. Under these conditions, a scalar field, u(θ, r) representing the field after the grating G1 in cylindrical coordinates can be expressed by the sum of the angular Fourier series. u ⁢ ( θ , r ) = ∑ m = - ∞ ∞ ⁢ ⁢ c m ⁢ H m ( 2 ) ⁡ ( kr ) · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 27 ) where r>r0, m is an integer representing the angular frequency, k=2π/λ, is the wave number, Hm(2)(kr) is the Hankel function of second kind representing the outgoing wave, and cm is the complex coefficient. For kr>>m, the modulus and the phase of Hm(2)(kr) can be expressed asymptotically as:  H m ( 2 ) ⁡ ( kr )  = 2 π ⁢ ⁢ k ⁢ ⁢ r + O ⁡ ( 1 ( kr ) 2 ⁢ ) , ( 28 ) arg ⁢ ⁢ H m ( 2 ) ⁡ ( kr ) = ⁢ - kr ⁡ ( 1 2 ⁢ m + 1 4 ) ⁢ π - 4 ⁢ ⁢ m 2 - 1 8 ⁢ ⁢ kr + O ⁡ ( 1 ( kr ) 3 ⁢ ) . ( 29 ) The scalar field at the boundary, r=r0, can be considered, and the periodic scalar field can be also be expressed as the sum of its angular Fourier harmonics, u ⁡ ( θ , r 0 ) = ∑ m ⁢ ∈ Z ⁢ A m · e i ⁢ ⁢ m ⁢ ⁢ θ , ( 30 ) where Am denotes the angular Fourier coefficients. Matching the angular Fourier components of Equation (27) with Equation (30) and plugging in the asymptotic expansions in Equation (28) and Equation (29), u ⁡ ( θ , r ) = r 0 r ⁢ e i ⁡ ( kr 0 - kr + 1 8 ⁢ ⁢ kr ⁢ 1 8 ⁢ ⁢ kr 0 ) · ∑ m = - ∞ ∞ ⁢ { e i ⁡ ( m 2 2 ⁢ ⁢ k ⁢ ⁢ ( 1 r 0 ⁢ ⁢ 1 r ⁢ ) ) } · A m · e i ⁢ ⁢ m ⁢ ⁢ θ . ( 31 ) Equation (31) shows that the amplitude of the field decays as 1/√r. The common phase term outside of the summation operation can be neglected, because only the phase term, exp(im2(1/r0−1/r)/2k), affects the intensity of the field. In analogy to the infinite periodic planar grating, the case where the circumference of the circle is a multiple of the grating period can be considered, and this ratio, 2πr0/p, can be considered as the fundamental angular frequency. Under this condition, the angular Fourier frequency components contain the harmonics of the fundamental angular frequency only, i.e., m=2πnr0/p, where n is an integer. When the field at radius rD, satisfies the equation 1 r D ⁢ = 1 r 0 ⁢ - l · 2 ⁢ ⁢ p 2 λ ⁢ ⁢ r 0 2 ⁢ , l = 1 , 2 , 3 ⁢ ⁢ … ⁢ , ( 32 ) the phase term, exp(im2(1/r0−1/r)/2k), is equal to a multiple of 2π for all the angular frequency m, which means that the field g(θ, rD) and g(θ, r0) have the same intensity distribution along the θ direction. Equation (32) indicates three facts of the Talbot effect of a curved periodic grating. First, the self-image is a magnified curved periodic grating sharing the same origin with the original grating G1. Second, the Talbot distance, rD−r0, has a non-linear relation with p2/λ, which is different from the case of planar periodic gratings. Third, there are only a limited number of the self-images, since l is bounded. Thus, a curved periodic grating can generate a magnified self-imaging grating on a curved plane. In a particular setup, the analyzer grating G2 is designed so that its period and curvature matches the self-image of G1. However, array detectors with matching curvature can be difficult or impractical to fabricate. Using a planar analyzer (absorption) grating G2 and flat detector, as depicted in FIG. 10B, will inevitably result in a loss of the contrast of the stepping curves at large angles if the phase grating has a constant period. In many embodiments of the subject invention, one or more quasi-periodic gratings can be used to improve the contrast of the stepping curve on a flat detector and provide one more degree of freedom in the design. Specifically, the periodicity of the curved grating can be perturbed with a slow-varying term to create a flat self-imaging plane. As shown in Equation (32), the Talbot distance depends on the grating period p. By changing the period along the direction of θ, a quasi-periodic grating can be designed with a flat self-image plane. The quasi-periodic grating with slow-varying period can be expressed as: g ⁡ ( θ , r 0 ) = ∑ i ∈ Z ⁢ ⁢ rect ⁡ ( 2 ⁢ ⁢ r 0 p i ⁢ θ ) * δ ⁡ ( θ - θ i ) , ( 33 ) where rect(•) denotes the rectangular function, * denotes the convolution operation, and δ(•) is the Dirac-delta function. The term pi denotes the period of the grating at angle θi, indexed by i. In a strictly periodic grating, pi is a constant, and θi=ipi/r0. In a quasi-periodic grating, the angle θi holds the relation with pi, θ i = 1 r 0 ⁢ ( p 0 + p i 2 + ∑ j = 1  i - 1  ⁢ ⁢ p j ) , i = 1 , 2 , 3 ⁢ ⁢ … ⁢ , ( 34 ) where p0 denotes the central period of the quasi-periodic grating G1′ at θ=0. The quasiperiodic grating can be treated as a locally periodic grating for self-imaging, when 1) the amplitude of high harmonic diffraction order is small, pi>>λ, which is satisfied in X-ray PCI setups, and 2) the period of the quasi-periodic grating varies slowly. To design a flat self-image plane, the period of the quasi-periodic grating, pi, satisfies the following equation: 2 ⁢ ⁢ p i 2 λ ⁢ ⁢ r 0 2 + 1 r 0 = ( 2 ⁢ ⁢ p 0 2 λ ⁢ ⁢ r 0 2 + 1 r 0 ) · cos ⁡ ( θ i ) , ( 35 ) Using Equation (34) and Equation (35), the period of the quasi-periodic grating G1′ can be calculated at each angle iteratively. In many embodiments, a curved quasi-periodic grating can be used for a large FOV X-ray PCI, even with a flat array detector. In comparison with using a periodic phase grating G1, a quasi-periodic grating can generate a Talbot self-imaging effect on a flat plane, and can thus increase the visibility over a larger range of view angle. Examples 3-5 demonstrate that the decrease of the fringe visibility is less than 10% within the view angle of 25°. X-ray phase imaging is sensitive to structural variation of soft tissue, and offers excellent contrast resolution for characterization of cancerous tissues. Also, the cross-section of an X-ray phase shift is a thousand times greater than that of X-ray attenuation in soft tissue over the diagnostic energy range, allowing a much higher signal-to-noise ratio at a substantially lower radiation dose than attenuation-based X-ray imaging. In an embodiment of the subject invention, a second-order approximation model can be used to reconstruct an X-ray image. The model can be with respect to phase shift based on the paraxial Fresnel-Kirchhoff diffraction theory. In-line dark-field imaging can also be performed based on the second-order model. The model can accurately establish a quantitative correspondence between phases and recorded intensity images, outperforming related art linear phase approximation models that are used in conventional methods of X-ray in-line phase-contrast imaging. In a further embodiment, the models of the subject invention can be iteratively solved using the algebraic reconstruction technique (ART). The state of the art compressive sensing techniques can be incorporated to achieve high quality image reconstruction. Numerical simulation studies presented in Examples 6 and 7 demonstrate the feasibility of the models that are more accurate and stable, and more robust against noise, than related art methods. Biological soft tissues consist mainly of light elements with low atomic numbers, and the elemental composition is nearly uniform with little density variation. Hence, the X-ray attenuation contrast cannot yield sufficient sensitivity and specificity in many biomedical scenarios. On the other hand, the cross-section of X-ray phase shift is three orders of magnitude larger than that of X-ray attenuation in soft tissues over the diagnostic energy range. Hence, X-ray PCI is also advantageous to reveal subtle features of soft tissues and offer superior contrast for analyses of various normal and diseased conditions at a reduced radiation dose. When a spatial coherent X-ray wave passes through an object, the X-ray wavefront is deformed by variation of the refractive index in an object. After coherent X-rays propagate a sufficient distance in a free space, a phase contrast is formed. In-line X-ray PCI can reconstruct phase shifts from image intensity measurements. This scheme offers unique merits in terms of simplicity and implementation. A relationship exists between the Fourier transform of a Fresnel diffraction pattern and the autocorrelation of a transmittance function, and a contrast transfer function (CTF) method can be used to reconstruct phase shifts based on an assumption of weak absorption and slow phase change. Paraxial Fresnel-Kirchhoff diffraction theory can be applied to establish a transport of intensity equation (TIE) to describe a relationship between intensity data and phase shifts. With intensity measurements at several distances along its propagation direction, the phase information can be quantitatively retrieved. An auxiliary function can transform TIE into a classical two-dimensional Poisson equation, which can be solved using a Fourier transform method, or a Green function method with suitable boundary conditions. However, TIE relates to intensity derivatives along the optic axis, which cannot be directly measured. These derivatives must be approximated using differences between intensity images on two adjacent planes, which is not in accordance with a relatively large separation between the planes to meet the phase-contrast requirement in the presence of noise. A mixed-phase retrieval method can directly combine CTF and TIE, and iteratively reconstruct phase shifts assuming a slowly varying absorption distribution. This algorithm alleviates the weak absorption assumption in CTF and the short propagation distance in TIE. To minimize the effect of measurement noise and enhance the accuracy of the conventional linear approximation model for X-ray in-line phase-contrast imaging, multiple intensity measurements at different distances from an object can be performed in the Fresnel diffraction region for reconstruction of a phase shift image. X-ray small-angle scattering, or dark-field, imaging is used to acquire photons slightly deflected from the primary beam through an object. Small-angle scattering signals reflect structural texture on length scales between 1 nm to several hundred nanometers. This imaging mode can reveal subtle texture of tissues. For example, the growth of tumors causes remarkable differences of small-angle scattering patterns from that of healthy tissues. It is clinically important that the structural variation in tumors modifies the refractive index. The propagation of X-rays in a medium is characterized by the complex index of refraction. The cross-section of X-ray phase shift is one thousand times larger than that of linear attenuation in the 20-100 keV range. This means that phase-contrast imaging has much higher sensitivity for light elements than attenuation-contrast imaging. The contrast-to-noise ratio of differential phase contrast CT images is superior to the attenuation-contrast CT counterpart. Therefore, PCI can be used to observe unique critical structures of soft biological tissues. Moreover, the refractive index of a tissue is inversely proportional to the square of the X-ray energy while the absorption coefficient decreases as the fourth power of the X-ray energy. Hence, X-ray PCI is suitable to operate at higher energies (>30 keV) for lower radiation dose than attenuation imaging. Higher energy X-ray imaging is important for studies on large animals or patients. Approximation models of the subject invention can be second-order approximation models for X-ray phase retrieval using paraxial Fresnel-Kirchhoff diffraction theory. An iterative method can be used to reconstruct a phase-contrast image (e.g., and in-line image) and a dark-field image. To describe the X-ray and matter interaction at a specific wavelength of λ, an object can be characterized by a complex-valued refractive index distribution n=1−δ+iβ, where δ is for refraction, and β is for attenuation. When a spatial coherent X-ray wave passes through an object, the X-ray wave front is deformed by variation of the refractive index in the object. δ (10−6−10−8) is about 1,000 times larger than β (10−9−10−11) in biological soft tissues over the diagnostic radiation energy range. This implies that X-ray phase contrast imaging can achieve a higher signal-to-noise ratio at a lower radiation dose than attenuation contrast imaging. The wave-object interaction process can be described with a transmittance functionT(r)=A(r)exp[iΦ(r)], (36)where r denotes transverse coordinates (x, y) in a transverse plane along the propagation direction z, A(r) is the amplitude modulus of the wave, and Φ(r) is the phase shift of the wave. The amplitude modulus and phase shift is related to the X-ray refractive index distribution A ⁡ ( r ) = exp ⁡ [ - 2 ⁢ ⁢ π λ ⁢ ∫ β ⁡ ( r , z ) ⁢ dz ] , ⁢ Φ ⁡ ( r ) = - 2 ⁢ ⁢ π λ ⁢ ∫ δ ⁡ ( r , z ) ⁢ dz ( 37 ) Based on Fresnel-Kirchhoff diffraction theory, the wave field on the transverse plane in a free space is described by the Fresnel diffraction formula ( 38 ) where [T(r)] is the Fresnel diffraction pattern of the transmittance function at a distance of z from the object, and k=2π/λ is a wavenumber. The Fourier transform of the Fresnel diffraction pattern at z can be formulated in terms of the autocorrelation of the transmittance function ∫ ∫ ⁡ [ T ⁡ ( r ) ] ⁢ ❘ 2 ⁢ exp ⁡ ( - i2 ⁢ ⁢ r · w ) ⁢ dr = ∫ ∫ { A ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) ⁢ A ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ⁢ exp ⁡ [ i ⁢ ⁢ Φ ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - i ⁢ ⁢ Φ ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ] } ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr ( 39 ) Hard X-ray radiation has a very short wavelength λ, and Φ(r) has only moderate variations for biological tissues. Thus, a second-order approximation to the exponential function of the phase in Equation (39) would be sufficiently accurate for practical applications ∫ ∫  ⁢ ⁡ [ T ⁡ ( r ) ]  2 ⁢ exp ⁡ ( - i2π ⁢ ⁢ r · w ) ⁢ dr = ∫ ∫ A ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) ⁢ A ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ⁡ [ 1 + i ⁢ ⁢ Φ ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - i ⁢ ⁢ Φ ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) - 1 2 ⁢ ( Φ ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - Φ ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ) 2 ] ⁢ exp [ - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr ( 40 ) Applying the autocorrelation formula to the parts involving A(r) and A(r)Φ(r) on the right-hand side of Equation (40) respectively, ∫ ∫ [  ⁢ ⁡ [ T ⁡ ( r ) ]  2 -  ⁢ ⁡ [ A ⁡ ( r ) ]  2 -  ⁡ [ A ⁡ ( r ) ⁢ Φ ⁡ ( r ) ]  2 ] ⁢ exp ⁡ ( - i2π ⁢ ⁢ r · w ) ⁢ dr = ∫ ∫ A ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) ⁢ A ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ⁡ [ i ⁢ ⁢ Φ ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - i ⁢ ⁢ Φ ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) - 1 2 ⁢ Φ 2 ⁡ ( r - λ ⁢ ⁢ z 2 ⁢ w ) - 1 2 ⁢ Φ 2 ⁡ ( r + λ ⁢ ⁢ z 2 ⁢ w ) ] ⁢ exp [ - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr ( 41 ) Changing integral variables on the right-hand side of Equation (41), ∫ ∫ [  ⁡ [ T ⁡ ( r ) ]  2 -  ⁡ [ A ⁡ ( r ) ]  2 -  ⁡ [ A ⁡ ( r ) ⁢ Φ ⁡ ( r ) ]  2 ] ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr = i ⁢ ∫ ∫ [ exp ⁡ ( - i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r + λ ⁢ ⁢ zw ) - exp ⁡ ( i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r - λ ⁢ ⁢ zw ) ] ⁢ A ⁡ ( r ) ⁢ Φ ⁡ ( r ) ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr - 1 2 ⁢ ∫ ∫ [ exp ⁡ ( - i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r + λ ⁢ ⁢ zw ) + exp ⁡ ( i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r - λ ⁢ ⁢ zw ) ] ⁢ A ⁡ ( r ) ⁢ Φ 2 ⁡ ( r ) ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ dr ( 42 ) During in-line phase-contrast imaging, phase contrast is manifested through a free-space propagation of a coherent X-ray beam, which transforms phase deformation on the object plane into intensity variation on an image plane. Equation (40) describes a relationship between measured intensity images and phase shifts. This model of the subject invention is plainly superior to the linear phase approximation model widely used for conventional X-ray in-line phase imaging, which assumes slow phase variation and weak linear attenuation of the object. Dark-field imaging is also feasible along this direction. A dark-field image is formed through scattering of X-rays, which is sensitive to microstructures in the object. Boundaries and interfaces in biological tissues commonly produce strong dark-field signals. Hence, X-ray in-line dark-field imaging can be important to reveal detailed critical features of tissues. When a coherent X-ray beam goes through an object, X-ray photon scattering can change spatial coherence and transmitted intensity of X-rays, inducing a variation in the transmittance functionT(r)−(Q(r)+|ΔQ(r))exp[i(Φ(r)+ΔΦ(r))], (43)where ΔΦ(r) and ΔQ(r) are the phase and the amplitude variations on the object plane induced by scattered X-rays, respectively. Although the formulation itself in Equation (43) covers both small and large angle scattered photons, the main contributions should be from small angle scattering because the field of view for in-line imaging is typically small to satisfy the paraxial approximation assumption. The scattering effect would change the original transport path in the object, leading to an additional phase variation ΔΦ. The amplitude modulus A(r)=Q(r)+ΔQ(r) on the object plane and the intensity I on an image plane can be measured respectively. Hence, from Equation (38), I =  exp ⁡ ( i ⁢ ⁢ kz ) i ⁢ ⁢ λ ⁢ ⁢ z ⁢ ∫ ∫ ( ( Q ⁡ ( r ) + Δ ⁢ ⁢ Q ⁡ ( r ) ) ⁢ exp ⁡ [ i ⁢ ⁢ ( Φ ⁡ ( r ) + Δ ⁢ ⁢ Φ ⁡ ( r ) ) ] ) ⁢ exp ⁡ ( i ⁢ ⁢ k 2 ⁢ ⁢ z ⁢  r ′ - r  2 ) ⁢ dr ′  , ( 44 ) With a relative small variation ΔΦ(r), the first-order approximation of exp(iΔΦ(r)) can be made to reduce Equation (44) to I =  exp ⁡ ( i ⁢ ⁢ kz ) i ⁢ ⁢ λ ⁢ ⁢ z ⁢ ∫ ∫ A ⁡ ( r ) ⁢ exp ⁡ [ i ⁢ ⁢ Φ ⁡ ( r ) ] ⁢ ( 1 + i ⁢ ⁢ D ⁡ ( r ) ) ⁢ exp ⁡ ( i ⁢ ⁢ k 2 ⁢ ⁢ z ⁢  r ′ - r  2 ) ⁢ dr ′  ( 45 ) where D ⁡ ( r ) = Δ ⁢ ⁢ Q ⁡ ( r ) ⁢ Δ ⁢ ⁢ Φ ⁡ ( r ) A ⁡ ( r ) represents a dark-field image to be reconstructed. The phase image Φ(r) can be recovered using the method described herein. Hence, the dark-field image D(r) can be reconstructed from intensity measurements, based on Equation (45). In an embodiment, an iterative method can be used for phase reconstruction (e.g., based on Equation 42). The method can include Equations (46) below. i ⁢ ∫ ∫ [ exp ⁡ ( - i ⁢ ⁢ πλ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r + λ ⁢ ⁢ zw ) - exp ⁡ ( iπ ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r - λ ⁢ ⁢ zw ) ] ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ p k + 1 ⁡ ( r ) ⁢ dr = ∫ ∫ [  ⁡ [ T ⁡ ( r ) ]  2 -  ⁡ [ A ⁡ ( r ) ]  2 -  ⁡ [ p k ⁡ ( r ) ]  2 ] ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ π ⁢ ⁢ r · w ) ⁢ dr + 1 2 ⁢ ∫ ⁢ ∫ [ exp ⁡ ( - iπ ⁢ ⁢ λ ⁢ ⁢ z ·  w  2 ) ⁢ A ⁡ ( r + λ ⁢ ⁢ zw ) A ⁡ ( r ) + exp ⁡ ( i ⁢ ⁢ π ⁢ ⁢ λ ⁢ ⁢ z ⁢ ·  w  2 ) ⁢ A ⁡ ( r - λ ⁢ ⁢ zw ) A ⁡ ( r ) ] ⁢ exp ⁡ ( - i ⁢ ⁢ 2 ⁢ ⁢ π ⁢ ⁢ r · w ) ⁢ p k 2 ⁡ ( r ) ⁢ dr ⁢ ⁢ ⁢ p 0 ⁡ ( r ) = 0 , p k ⁡ ( r ) = A ⁡ ( r ) ⁢ Φ k ⁡ ( r ) , k = 0 , 1 , … ⁢ , ( 46 ) where |[T(r)]|2 is the measurement on the Fresnel diffraction pattern at a distance z, |[A(r)]|2 is the measurement on the object plane, [A(r)]|2 gives the modulus of the function pk(r). Equation (46) can be discretized to a system of linear equations with respect to variables pk(r). Then, the algebraic reconstruction technique (ART) can be employed for phase image reconstruction. ART is a widely-used iterative method for its robustness and efficiency. In the iterative scheme, the absorption term is accurate without any approximation. Hence, no limitation is imposed on the attenuation property of an object. The exponential function of phase variation has been approximated to the second order, which should be very accurate for hard X-ray imaging. Therefore, the second-order phase models of the subject invention can be very informative and useful for biomedical imaging. The second-order model can be computationally complex, and can have a high computational cost under certain circumstances. With the development of high-performance computing techniques, especially Graphics Processing Units (GPUs), any computational issues should not be a problem. X-ray in-line imaging can have higher spatial resolution than X-ray grating-based imaging. The field of view for in-line imaging ought to be small to satisfy the paraxial approximation requirement, and yet practical applications often focus on a small region of interest (ROI) in an object. Therefore, interior tomographic imaging methods would be useful to deliver unique and stable ROI reconstructions from truncated projection data. The second-order phase approximation models for X-ray phase retrieval, and iterative algorithms, can be used for both in-line phase-contrast and dark-field imaging. These models accurately establish quantitative correspondence between phase shifts and image intensities, outperforming the related art linear phase approximation model used for X-ray in-line phase contrast imaging, because the linear model assumes slow phase variation and weak linear attenuation of an object. The iterative method can incorporate the state of the art compressive sensing techniques to achieve high quality image reconstruction. In an embodiment, a system can include a machine-readable medium (e.g., a computer-readable medium) having machine-executable instructions (e.g., computer-executable instructions) for performing a method of reconstructing an image (e.g., an X-ray image), the method comprising using a second-order approximation model as described herein. The machine-readable medium (e.g., computer-readable medium) can be non-transitory (e.g., a non-transitory computer-readable medium). In an embodiment, a method of performing imaging (e.g., X-ray PCI) can include providing an imaging system (e.g., X-ray PCI system) as described herein, positioning an object to be imaged in the appropriate location within the system (e.g., between the energy source and the period-varying grating), and imaging the object using the system. In a further embodiment, the method can include using a second-order approximation model as described herein to reconstruct the image (e.g., X-ray image). The methods and processes described herein can be embodied as code and/or data. The software code and data described herein can be stored on one or more computer-readable media, which may include any device or medium that can store code and/or data for use by a computer system. When a computer system reads and executes the code and/or data stored on a computer-readable medium, the computer system performs the methods and processes embodied as data structures and code stored within the computer-readable storage medium. It should be appreciated by those skilled in the art that computer-readable media include removable and non-removable structures/devices that can be used for storage of information, such as computer-readable instructions, data structures, program modules, and other data used by a computing system/environment. A computer-readable medium includes, but is not limited to, volatile memory such as random access memories (RAM, DRAM, SRAM); and non-volatile memory such as flash memory, various read-only-memories (ROM, PROM, EPROM, EEPROM), magnetic and ferromagnetic/ferroelectric memories (MRAM, FeRAM), and magnetic and optical storage devices (hard drives, magnetic tape, CDs, DVDs); network devices; or other media now known or later developed that is capable of storing computer-readable information/data. Computer-readable media should not be construed or interpreted to include any propagating signals. A computer-readable medium of the subject invention can be, for example, a compact disc (CD), digital video disc (DVD), flash memory device, volatile memory, or a hard disk drive (HDD), such as an external HDD or the HDD of a computing device, though embodiments are not limited thereto. A computing device can be, for example, a laptop computer, desktop computer, server, cell phone, or tablet, though embodiments are not limited thereto. The subject invention includes, but is not limited to, the following exemplified embodiments. An imaging system, comprising: an energy source; a detector; a first grating disposed between the energy source and the detector; and a second grating disposed between the first grating and the detector, wherein the first grating is a quasi-periodical grating (such that the period of the grating varies across the grating (e.g., in a lateral direction across the grating)). The system according to embodiment 1, wherein the first grating is a quasi-periodical phase grating. The system according to any of embodiments 1-2, wherein the second grating is an analyzer grating. The system according to any of embodiments 1-3, wherein the detector is flat. The system according to any of embodiments 1-4, wherein the detector is a flat detector array. The system according to any of embodiments 1-3, wherein the detector is curved (such as cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere)). The system according to any of embodiments 1-6, wherein the second grating is flat. The system according to any of embodiments 1-6, wherein the second grating is curved (such as cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere)). The system according to any of embodiments 1-8, wherein the first grating is flat. The system according to any of embodiments 1-8, wherein the first grating is curved (such as cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere)). The system according to any of embodiments 1-10, further comprising a source grating disposed between the energy source and the first grating. The system according to any of embodiments 1-11, wherein the source grating is flat. The system according to any of embodiments 1-11, wherein the source grating is curved (such as cylindrical (e.g., curved like a portion of a cylinder) or spherical (e.g., curved like a portion of a sphere)). The system according to any of embodiments 1-13, wherein the imaging system is an X-ray phase-contrast imaging (PCI) system, and wherein the energy source is an X-ray source. The system according to any of embodiments 11-14, wherein the system is configured such that an object to be imaged is positioned between the source grating and the first grating while it is being imaged. The system according to any of embodiments 1-15, wherein the system is configured such that an object to be imaged is positioned between the energy source and the first grating while it is being imaged. The system according to any of embodiments 1-16, wherein the first grating has a slow-varying period. The system according to any of embodiments 1-17, wherein the period of the first grating varies the same way in both lateral directions moving away (e.g., as the angle is varied positively or negatively by the same amount) from a point on the first grating where radiation from the energy source is centered during imaging (see, e.g., FIGS. 3A and 11C). The system according to any of embodiments 1-18, wherein the second grating matches the curvature (or lack thereof) of the detector. A method of reconstructing or retrieving an image, the method comprising: using a second-order approximation model using paraxial Fresnel-Kirchhoff diffraction theory as described herein. The method according to embodiment 20, further comprising performing an iterative method to reconstruct a phase-contrast image and/or a dark-field image. The method according to any of embodiments 20-21, further comprising iteratively solving the model using the algebraic reconstruction technique (ART). The method according to any of embodiments 20-22, further comprising incorporating state of the art compressive sensing techniques to achieve high quality image reconstruction. The method according to any of embodiments 21-23, wherein the iterative method is based on Equation (42) herein The method according to any of embodiments 21-24, wherein the iterative method includes Equation (46) herein. The method according to embodiment 25, further comprising discretizing Equation (46) to a system of linear equations with respect to variables pk(r). The method according to embodiment 26, further comprising employing the ART for phase image reconstruction. The method according to any of embodiments 20-27, wherein the image is an X-ray phase image. A machine-readable medium (e.g., a computer-readable medium) having machine-executable instructions (e.g., computer-executable instructions) for performing the method according to any of embodiments 20-28. The machine-readable medium according to embodiment 29, wherein the machine-readable medium is non-transitory (e.g., a non-transitory computer-readable medium). A system comprising the machine-readable medium according to any of embodiments 29-30. A method of performing imaging, comprising: providing the imaging system according to any of embodiments 1-19; positioning an object to be imaged in the appropriate location within the system (e.g., between the energy source and the first grating); and imaging the object using the system. The method according to embodiment 32, further comprising performing the method according to any of embodiments 20-28 to reconstruct and/or retrieve an X-ray image (e.g., an in-line and/or dark-field image). The method according to embodiment 33, wherein the method according to any of embodiments 20-28 is performed using the machine-readable medium according to any of embodiments 29-30 or the system according to embodiment 31. A greater understanding of the present invention and of its many advantages may be had from the following examples, given by way of illustration. The following examples are illustrative of some of the methods, applications, embodiments and variants of the present invention. They are, of course, not to be considered as limiting the invention. Numerous changes and modifications can be made with respect to the invention. A numerical analysis was performed to show the modified interference pattern along the x-axis parallel to the grating plane for both a constant-period grating and a period-varying phase grating according to an embodiment of the subject invention. In this analysis, a pure phase grating with a λ phase shift was illuminated by a fully coherent parallel-beam of 28 keV. FIG. 3A shows a plot of the period (in μm) along the x-axis (in μm) for the period-varying phase grating. FIG. 3B shows a plot of the period (in μm) along the x-axis (in μm) for the constant-period grating. FIG. 3C shows the interference pattern at different propagation distances from the grating for the period-varying phase grating. FIG. 3D shows the interference pattern at different propagation distances from the grating for the constant-period grating. FIG. 3E shows an enlarged view of the two boxes highlighted in FIG. 3D. The red arrows in FIG. 3E show the difference caused by the grating aperture widths (plotted in FIG. 3A). To analyze the Talbot self-imaging performances with different combinations of gratings and detectors, as well as with periodic and quasi-periodic gratings designs, numerical experiments were performed. The energy of incident X-rays was set to 28 keV. The distance between the X-ray source and the phase grating G1 grating was 1 m, the phase grating G1 and absorption grating G2 were separated by the first-order fractional Talbot distance, calculated according to Equation (10). The period of the phase grating G1 (pG1 was 4.8 μm, while the period of the absorption grating G2 (pG2) was determined according to imaging geometry. In the data collection step, the classic phase stepping method was employed, and the visibility of the stepping curve was used to quantify the imaging performance. The detector cell pitch was 50 μm. First, for a grating-based phase-contrast imaging system, if the detector and the absorption grating have different curvatures as indicated by the red arrows in FIGS. 4B and 4C, there would be non-uniform gaps to the detector. The non-uniform gap distribution would introduce non-uniform image artifacts due to the blurring effect from the X-ray spot. Therefore, a better or optimal design would place the absorption grating G2 with as closely aligned curvature or shape as possible to that of the detector plane, leading to the grating-detector system designs illustrated in FIGS. 4A and 4D. Due to the imperfect fabrication of the gratings and/or difficulty in matching the gratings to the beam divergence, the phase grating G1 and absorption grating G2 may not be able to be exactly paired, as shown in FIG. 5, where curved G1 and flat G2, or flat G1 and curved G2 are assumed for optimized PCI system designs, as shown in FIG. 6A. In a conventional grating-based PCI system, a periodic phase grating is adopted to produce the same interference patterns at a Talbot length. However, in FIGS. 5A and 5B the phase grating and absorption grating are not paired, which means that the absorption grating cannot be located exactly on the interference patterns. FIG. 5A shows a schematic view of a grating-based phase-contrast system with a curved G1 and a flat G2 (and flat detector). FIG. 5B shows a schematic view of a grating-based phase-contrast system with a flat G1 and a curved G2 (and curved detector). FIG. 5C shows a plot of the visibility versus angle (in degrees) for the setup of FIG. 5A. FIG. 5D shows a plot of the visibility versus angle (in degrees) for the setup of FIG. 5B. FIG. 6A shows a plot of visibility versus angle (in degrees) for the unpaired G1-G2 system of FIG. 5A and a period-unmatched imaging system. The red line (upper line) is for the period-unmatched system, and the blue line (lower line) is for the system of FIG. 5A). FIG. 6B shows a plot of visibility versus angle (in degrees) for the period-matched imaging system. To demonstrate the performance of the quasi-periodic grating, numerical simulations with Matlab (Mathwork) were conducted for both curved periodic phase grating G1 and quasiperiodic phase grating G1′ based on the general setup shown in FIG. 10B. In the simulation, the energy of the X-ray beam was 28 keV. The period of the phase grating G1 was set to 4.8 μm, with a radius, r0=1.0 m. The center of planar analyzer (absorption) grating G2 was at the first fractional Talbot image of the center of G1. 1/rD−1/r0=p2/8λr02. G1′ had the central period set to be the same as G1. The calculated period of G1′ as a function of θ is shown in FIG. 11C, and the period of G2′ is calculated accordingly. The period of the quasi-periodic grating is larger than 4.8 μm, and the same fabrication process for periodic grating can be adapted (see, e.g., David et al., “Fabrication of diffraction gratings for hard X-ray phase contrast imaging,” Microelectron. Eng. 84(5-8), 1172-1177 (2007), which is incorporated by reference herein in its entirety). The pixel size of the detector was set to 24 μm, five times the period of G1. FIG. 11A shows a plot of visibility versus angle (in degrees) for the simulations in the X-ray regime. The red line (uppermost line) is for G1′ (quasi-periodic phase grating), and the blue line (lower most line sloping downward) is for G1. FIG. 11B shows intensity profiles at the first fractional Talbot distance of both G1 and G1′ at 0° and 16°. The upper-left is for G1 at 0°; the upper-right is for G1 at 16°; the lower-left is for G1′ at 0°; and the lower-right is for G1′ at 16°. FIG. 11C shows a plot of the period (in μm) versus the angle (in degrees) for the quasi-periodic phase grating G1′. The visibility of the stepping curves at first fractional Talbot distance G1 and G1′ are shown in FIG. 11A. Referring to FIGS. 11A-11C, a decrease of 58% in visibility for G1 at 16° was observed, while no obvious decrease was observed in the visibility for G1′. The source in the X-ray regime is not truly an absolute point source. The intensity of a fringe pattern can be expressed as a convolution of the intensity profile of the source and the fringe pattern of an ideal point source. This convolution will decrease the visibility of the fringes pattern. This is always an issue in the X-ray regime for both quasi-periodic and periodic gratings. If necessary, a G0 grating can be used for needed spatial coherence. To experimentally verify the analysis of Example 3, experiments were designed in the visible optical regime. Numerical simulations were also conducted. Different from the X-ray simulation, G1 was set as an absorption grating. G2 was separated from G1 by one Talbot distance. The wavelength of the source, λ, was 0.67 μm to match the optical experiment setup. The period of G1, p0, was set to be 91 μm with a radius, r0=75 mm. The quasi-periodic grating G1′ had the central period set to be the same as that of G1. Given that G1′ had the same radius as G1, Equation (34) and Equation (35) were used to calculate its period p ranging from −30° to 30°. The calculated period of G1′ as a function of θ is shown in FIG. 12C. The diagram of the experimental setup is shown in FIG. 13A. The point source came from a laser with fiber coupled output (Suprlum, SLD-261, λ=670 nm). The driven current was set at 133 mA (Suprlum, pilot-2). Two lenses (f1=100 mm, f2=62.9 mm) were used to relay a relayed focal point of 41.0°. The radius of the acrylic tube was 75 mm. The periodic grating and the quasi-periodic grating with central period of 91 μm were made on a mylar film with a thickness of 180 μm (CAD/Art Services, Inc.), which could be bent easily. Two gratings were taped on the outer surface of the acrylic tube. The detection microscope system included a microscope objective (Nikon, 10X/0.3), a tube lens (f=200 mm), and a CMOS camera (JAI, GO5000M), all of which were installed on a motorized translation stage (Thorlab, NRT-100). A stepper motor controller (Thorlab, apt) drove the stage, so that the detection system was able to scan the field in the x direction. The scan range of the experiment was 50 mm, with 5 mm step size, covering an angular range from −13° to 13°. The microscope objective could adjust its focus in the y direction. The plane with the highest fringes contrast near the calculated Talbot distance was set as the observation plane. FIG. 12A shows the Talbot carpet of the periodic grating G1; the vertical axis is Z (in mm), and the horizontal axis is X (in mm). The period of the grating was 91 μm. The blown-up portions are for 0° and 12.6°. FIG. 12B shows the Talbot carpet of the quasi-periodic grating G1′; the vertical axis is Z (in mm), and the horizontal axis is X (in mm). The period of the grating at the X=0 line was 91 μm. The blown-up portions are for 0° and 12.6°. FIG. 12C shows a plot of the period (in μm) versus the angle (in degrees) for the quasi-periodic phase grating G1′. The period was calculated from Equation (35). These results are from the simulations in the visible optical regime. The wavelength of the source was 0.67 μm; the radius of the grating was 75 mm; and the white line in both FIGS. 12A and 12B, indicating the self-imaging plane, was located at Y=111.9 mm. The Talbot carpets are shown in Cartesian coordinates. It can be observed that the self-imaging surface of grating G1 is on a curved plane. FIG. 12B shows that the self-imaging surface of the quasi-periodic grating G1′ is almost flat. The ideal Talbot image is located at rD=111.9 mm. If a flat array detector were placed at this plane, indicated by the white solid lines in FIGS. 12A and 12B, the interference fringes would be distorted at 12.6° for periodic grating G1, while the fringes maintain the original fringe profiles for the quasiperiodic grating G1′. FIG. 13A shows a schematic view of the experimental setup. S is the point source λ=0.67 μm. A periodic grating G1 or quasi-periodic grading G1′ is fixed on a cylindrical surface. P denotes the observation plane; Obj denotes the microscope objectives; L denotes the tube lens; and D denotes the CMOS detector. FIG. 13B shows images of interference fringes at 0° on the observation plane. FIG. 13C shows images of interference fringes at 7.4° on the observation plane. FIG. 13D shows images of interference fringes at 12.24° on the observation plane. In these optical experiments, the fields were scanned along the x-axis. The images acquired by the CMOS camera at 0°, 7.4°, and 12.2° angles from G1 and G1′ are shown in FIGS. 13B-13D, respectively. The results also show that the fringes from the periodic grating G1 had a severe distortion at larger angles, while those from the quasi-periodic grating G1′ did not change much or at all. In the X-ray PCI setups, the pixel size of the detector was ˜5 times larger than the period of G1, and the analyzer grating G2 should resolve the lateral shifts of the fringes. In this experiment, the effective pixel size of the CMOS camera after the microscope magnification (˜0.5 μm) was less than the period of fringes on the observation plane. In order to compare the experimental results, the images were processed to add in a ‘virtual’ analyzer grating G2. The readout from a virtual pixel consists of five periods of the fringes. Because for quasi-periodic grating the periods have slight variance with respect to the angle, the period of G2′ is calculated accordingly at the observation plane. The virtual grating G2 and G2′ scan 40 phase steps for one period of the fringes. FIG. 14 shows the experimental visibility of the periodic grating G1 and quasi-periodic grating G1′. The visibility was calculated from averaging the visibility from 100 sub-regions with 3 mm in width and 2 μm in height on the CMOS camera. FIG. 14 shows a plot of average visibility versus angle (in degrees) for the periodic grating G1 and the quasi-periodic grating G1′ on the observation plane. The red line (upper line) is for G1′, and the blue line (lower line) is for G1. It can be observed that the quasi-periodic grating G1′ maintains a uniform (or nearly uniform) visibility on the observation plane. From −13° to 13°, the visibility variance of G1′ is less than 10%, while the visibility for G1 dropped by over 10% after only 5° from center. The results in Example 4 show an overall decrease in the visibility. Three reasons could possibly have caused this difference: the speckle noise; low contrast of the binary mask; and the optical aberration from the mask film. First, a coherent source was used in Example 4, which introduced laser speckles to the measurement. In addition, the artifacts on the grating surface can contribute to the blur of the interference fringes. From the measured images of the fringes, the average size of the speckles was ˜13 μm, and the standard deviation of the intensity was 30%. These artifacts, coupling the light to the non-harmonic frequency components of the fringes, can reduce the experimental visibility. Second, the opaque part of the grating was not strictly dark in Example 4. The opaque region had a measured intensity of ˜10% of the maximum intensity, which essentially acted as a bias to the field. This low contrast of the grating mask can also decrease the visibility. Third, due to the mask printing process, the thickness of the transparent region was not completely uniform in Example 4. A profilemeter measured the average difference of the thickness to be ˜0.13 μm. This non-uniform thickness of the mask can introduce a lensing effect. Evidence for this speculation is that the field distribution along the Talbot distance was asymmetric. This aberration effect can also affect the visibility. The three aforementioned effects were incorporated into the simulation in this example. Specifically, to include the speckle noise, the images were divided into small regions with a size of 13 μm, and random Gaussian noise with a standard deviation of 30% of the local intensity was added to each region. The transmittance bias and the lensing effect were also included in the simulation. FIG. 15A shows a plot of visibility versus the radius direction of the periodic grating (in mm) for the simulated visibility profile. The origin was set at the observation plane. FIG. 15B shows experimental images of interference fringes at −2 mm (top image), 0 mm (middle image), and 2 mm (bottom image) from the observation plane. FIG. 15C shows profiles of interference fringes. The red curves denote simulated profiles, and the blue curves denote the grayscale value profiles of the blue (near the center) lines marked in FIG. 15B. The profiles are for −2 mm (top profile), 0 mm (middle profile), and 2 mm (bottom profile). The simulated visibility is shown in FIG. 15A. The red point marks the observation plane. The visibility curve, as predicted, was asymmetric from the observation plane. The visibility at the red point was 0.7, which was close to the peak visibility in the experiment. The intensity pattern was measured at −2 mm, 0 mm, and 2 mm away from the observation plane. The experimental images are shown in FIG. 15B. The simulated and experimental intensity profiles of the interference fringes are shown in FIG. 15C. The ratio between the grating period and the wavelength, p/λ, is on the order of 104 in the X-ray regime, and p/λ is on the order of 102 for the experiment in the visible regime, and both of these are sufficiently large for the approximation in Equations (21) and Equation (22) to be valid. Numerical tests were performed to demonstrate the feasibility of second-order approximation models for X-ray phase retrieval described herein (see, e.g., Equations (36)-(45)). The phase contrast image shown in FIG. 7A was used as the phase distribution after the wave-object interaction. FIG. 7A shows the true phase image. FIG. 7B shows a reconstructed version of the image of FIG. 7A, reconstructed using a method of the subject invention. FIG. 7C shows a reconstructed version of the image of FIG. 7A, reconstructed using a related art TIE-based reconstruction method. The image of 7A is from https://www.maxlab.lu.se/sv/MEDMAX. The phase image had 498×500 pixels with a pixel size of 3 μm. The radiation wavelength was set to λ=0.25 Å, corresponding to hard X-rays. It was assumed that the intensity distribution on the object plane was uniform to mimic a homogeneous attenuation scenario. The wave amplitudes on the transverse plane through free space propagation can be simulated using the Fresnel transform Equation (38). The intensity distribution on the image plane at a propagation distance of 1000 mm was calculated, with Poisson noise added to the intensity image. Equation (43) was discretized into a system of linear equations, and the ART algorithm was employed to reconstruct the phase image. The procedure took 4 iterations to produce the phase contrast image as shown in FIG. 7B. For the comparison, the phase reconstruction was performed using the TIE-based reconstruction method. The same image plane and the same Poisson noise were used, and the differences between the intensity images on the object and image planes were employed to approximate the intensity derivatives along the propagation direction. The Fourier transform method was implemented for phase reconstruction based on TIE. The reconstructed image was plainly inferior, as shown in FIG. 7C. The experiment of Example 6 was repeated, using the image of FIG. 8A as the phase contrast image. FIG. 8A shows the true phase image. FIG. 8B shows a reconstructed version of the image of FIG. 8A, reconstructed using a method of the subject invention. FIG. 8C shows a reconstructed version of the image of FIG. 8A, reconstructed using a related art TIE-based reconstruction method. The image of 8A is from Tapfer et al. (“Three-dimensional imaging of whole mouse models: comparing nondestructive X-ray phase-contrast micro-CT with cryotome-based planar epi-illumination imaging,” Journal of Microscopy 253(1), 24-30 (2014)). The phase image had 256×238 pixels with a pixel size of 3 μm. The radiation wavelength was set to λ=0.25 Å. The intensity distribution on the object plane was uniform again. The intensity distribution on the image plane at a propagation distance of 1000 mm was simulated based on Equation (38), with Poisson noise similarly added. 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abstract

A method for mitigating stress corrosion cracking in high temperature water includes introducing catalytic nanoparticles and dielectric nanoparticles to the high temperature water in an amount effective to reduce a electrochemical corrosion potential of the high temperature water.

summary

description

This invention was made with government support under grants R01 EB007168 and R21 EB007236 awarded by the National Institutes of Health. The Government has certain rights to this invention. The subject technology relates generally to radiological devices and methods and, in some embodiments, more particularly to devices and methods for computed tomography (CT). CT systems have included single-slice and multiple-slice detectors. CT systems with multiple-slice detectors are, in particular, able scan large volumes of interest. Some large volumes are imagined by helical CT scanning. In helical scanning, the subject moves axially relative to a radiation beam such that the beam traverses a helical path through the subject. Such scanning can be leveraged to quickly scan whole or large portions of organs. As aspect of at least one embodiment disclosed herein includes the realization that high radiation dose of conventional perfusion CT imaging and the lack of standardized perfusion CT imaging protocols limit the clinical potential of CT. Some embodiments disclosed herein significantly reduce the x-ray dose of CT perfusion scans without sacrificing clinical accuracy. Some embodiments that significantly reduce the x-ray dose of CT perfusion scans without sacrificing clinical accuracy, thereby expand use of perfusion CT as a diagnostic tool. Some embodiments of devices and methods for perfusion CT imaging can used for diagnosis, treatment of diseases, or both. In various embodiments, a radiation dose delivered to a subject can be reduced by application of any one of a transverse dynamic collimator, a grated collimator, an adaptive sampling algorithm, or an adaptive exposure algorithm, or a combination of some or all thereof. Some embodiments can be used for perfusion imaging of the kidneys, pancreas, liver, and heart. Primarily due to concerns about the magnitude of radiation dose delivered, perfusion CT imaging has not been used routinely in various fields, including stroke assessment, oncology, and cardiac and kidney function. In some embodiments, reduction of radiation dose delivered to a subject can permit application of perfusion CT to those applications wherein dose is a limiting factor, e.g. cardiac perfusion. Although some embodiments are discussed herein with respect to perfusion CT imaging, some embodiments can be used with other imaging. Similarly, although some embodiments may provide particular benefits for perfusion CT imaging, various embodiments can provide advantages with other imaging. The subject technology is illustrated, for example, according to various aspects described below. Various examples of aspects of the subject technology are described as numbered clauses (1, 2, 3, etc.) for convenience. These are provided as examples and do not limit the subject technology. It is noted that any of the dependent clauses may be combined in any combination, and placed into a respective independent clause, e.g., clause 1 or clause 55. The other clauses can be presented in a similar manner. 1. A collimator for a computed x-ray tomography imaging device, comprising a first grating and a second grating positioned on opposing sides of a primary radiation delivery window, each of the first and second gratings comprising a plurality of attenuating members with a plurality of secondary radiation delivery windows extending between adjacent attenuating members of the first grating and the second grating, respectively. 2. The collimator of clause 1, wherein a width of each secondary windows is less than a width of the primary window. 3. The collimator of clause 1, wherein a total area of each of the plurality of secondary windows is less than a total area of the primary window. 4. The collimator of clause 1, wherein a width of each secondary window is proportional to a distance between the secondary window and the primary window. 5. The collimator of clause 4, wherein the width of each secondary window is linearly proportional to the distance between the secondary window and the primary window. 6. The collimator of clause 4, wherein the width of each secondary window is positively proportional to the distance between the secondary window and the primary window. 7. The collimator of clause 1, wherein a width of each attenuating member is proportional to a distance between the attenuating member and the primary window. 8. The collimator of clause 7, wherein the width of each attenuating member is linearly proportional to the distance between the attenuating member and the primary window. 9. The collimator of clause 7, wherein the width of each attenuating member is positively proportional to the distance between the attenuating member and the delivery window. 10. The collimator of clause 1, wherein the secondary windows comprise open passages extending through the grating. 11. The collimator of clause 1, wherein the secondary windows comprise panes of substantially radio-transmissive material. 12. The collimator of clause 1, wherein the attenuating members are oriented generally parallel to sides of the primary window. 13. The collimator of clause 1, wherein the secondary windows are oriented generally parallel to sides of the primary window. 14. The collimator of clause 1, wherein the first grating is movable relative to the second grating. 15. The collimator of clause 14, wherein the first and second gratings are independently movable. 16. The collimator of clause 1, wherein the attenuating members of the first grating are integrally formed with each other, and the attenuating members of the second grating are integrally formed with each other. 17. The collimator of clause 16, wherein the attenuating members of the first grating are integrally formed with the attenuating members of the second grating. 18. A method of directing radiation during computed tomography (CT) imaging, comprising: emitting x-ray radiation from a radiation source toward an object; passing a first portion of the radiation through a primary window toward a target region in the object; passing a second portion of the radiation through at least one secondary window, on each of opposing sides of the primary window, to corresponding regions in the object outside the target region; attenuating, between the primary and secondary windows, at least a third portion of the radiation; and generating CT image data based on the first and second portions of the radiation. 19. The method of clause 18, further comprising rotating the radiation source and the primary and secondary windows about an axis, and wherein radiation is passed through the primary window and the secondary windows while either (i) centers of the primary window and at least two of the secondary windows lie in a plane non-parallel to the axis or (ii) a plane intersecting the centers and the axis is non-parallel to the axis. 20. The method of clause 18, further comprising rotating the radiation source and the primary and secondary windows about an axis, and translating the secondary windows relative to the primary window in a direction non-parallel to the axis. 21. The method of clause 20, wherein the secondary windows are translated relative to the primary window during rotation of the radiation source. 22. The method of clause 21, wherein a first of the secondary windows is translated independently of a second of the secondary windows on an opposing side of the primary window from the first of the secondary windows. 23. The method of clause 18, wherein the attenuating comprises blocking passage of at least the third portion of the radiation between the primary and secondary windows. 24. A computed tomography device, comprising: a gantry configured to rotate about an axis and comprising an opening configured to accommodate an object; a radiation source mounted to the gantry; a collimator positioned between the radiation source and the gantry opening, the collimator comprising first and second leaves respectively bounding first and second opposing sides of a radiation delivery window, the first leaf and the second leaf being movable to adjust at least one of a size or a location of the radiation delivery window relative to the radiation source in a direction non-parallel to the axis. 25. The computed tomography device of clause 24, wherein the first leaf is moveable independently of the second leaf. 26. The computed tomography device of clause 24, further comprising third and fourth leaves respectively bounding third and fourth opposing sides of the window and aligned with the window along the axis. 27. The computed tomography device of clause 26, wherein each of the first and second sides is substantially orthogonal to each of the third and fourth sides. 28. The computed tomography device of clause 24, wherein the collimator is mounted within about 27 cm of the radiation source. 29. The computed tomography device of clause 24, wherein the collimator is mounted within about 12 cm of the radiation source. 30. A computed tomography device, comprising: a gantry configured to rotate about an axis and comprising an opening configured to accommodate an object; a radiation source mounted to the gantry; a collimator positioned between the radiation source and the gantry opening, the collimator comprising a first leaf and a second leaf respectively bounding first and second opposing sides of a radiation delivery window, the first leaf and the second leaf being independently movable relative to the radiation source in a direction non-parallel to the axis. 31. The device of clause 30, wherein the first leaf and the second leaf are independently movable relative to the radiation source in a direction tangential to a circle (i) centered on the axis and (i) defining a plane that is not parallel to the axis. 32. The device of clause 31, wherein the first leaf is independently movable relative to the second leaf. 33. A method of radiologic imaging, comprising: rotating, about an axis, a gantry carrying a radiation source; emitting radiation from the radiation source toward an object between a pair of leaves; during rotation of the gantry, moving the pair of leaves relative to the radiation source in a direction nonparallel to the axis. 34. The method of clause 33, further comprising: performing a preliminary scan of a object; demarcating a region of interest in the object, based on the preliminary scan, that is at least one of (i) non-concentric with the axis or (ii) non-circular; and controlling movement of the pair of leaves during rotation of the gantry to adjust at least one of a location, relative to the radiation source, or a dimension, of a radiation delivery window such that substantially only the region of interest is exposed to radiation through the radiation delivery window. 35. The method of clause 34, further comprising directing radiation through a plurality of secondary windows, on opposing sides of the radiation delivery window, to regions in the object outside the region of interest; and substantially blocking the passage of radiation toward the object in regions between the radiation delivery window and the secondary windows. 36. The method of clause 33, further comprising repositioning the object such that a region of interest is located closer to the axis. 37. A method of contrast-enhanced computed tomography (CT) imaging, comprising: (a) repeatedly scanning a target region at a frequency during a session, the scanning comprising performing a CT scan by emitting x-ray radiation toward the target region, the frequency initially being a first rate; (b) monitoring, during the session, an indicator of attenuation of radiation by a contrast-enhanced first structure within the target region; (c) after detecting an increase of the attenuation, increasing the frequency to a second rate; and (d) after detecting a decrease in the attenuation after (c), decreasing the frequency to a third rate. 38. The method of clause 37, further comprising generating a representation of a relationship between time and radiation attenuation by a second structure within the target region. 39. The method of clause 38, wherein the radiation attenuation by the second structure with respect to time represents an indicator of vascular perfusion of the second structure. 40. The method of clause 37, further comprising monitoring of a rate of change of the attenuation. 41. The method of clause 40, wherein the frequency is increased to the second rate in response to detection of a decrease in a rate at which the attenuation is increasing. 42. The method of clause 41, further comprising beginning monitoring of the rate of change after detecting an increase of the attenuation to or beyond a threshold. 43. The method of clause 42, wherein the threshold is about 35 HU. 44. The method of clause 42, wherein the threshold comprises a degree of increase in the attenuation compared to a value indicated by an initial scan. 45. The method of clause 40, further comprising decreasing the frequency below the third rate in response to detection of a decrease in a rate at which the attenuation is decreasing. 46. The method of clause 45, wherein decreasing the frequency below the third rate comprises reducing the frequency with each successive scan. 47. The method of clause 46, wherein the frequency is approximately halved with each successive scan. 48. The method of clause 37, wherein the frequency is reduced to the third rate upon a first detection of a decrease in attenuation after (c). 49. The method of clause 37, wherein the first rate is one scan approximately every two seconds. 50. The method of clause 37, wherein the second rate is one scan approximately every second. 51. The method of clause 37, wherein the third rate is one scan approximately every two seconds. 52. The method of clause 37, wherein the structure comprises at least one of a heart chamber, an aorta, or another blood vessel. 53. The method of clause 37, further comprising terminating the scanning after a predetermined period of time and performing a final scan at the end of the predetermined period. 54. The method of clause 37, further comprising terminating the scanning after a predetermined period of time, and, if a remaining time between a latest scan and an end of the predetermined period is less than an interval between the latest scan and an immediately preceding scan, performing (i) a penultimate scan at approximately half of the remaining time after the latest scan and (ii) a final scan at the end of the predetermined period. 55. A computer-implemented system for controlling contrast-enhanced computed tomography imaging, comprising: an attenuation monitoring module configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region; a scanning-frequency control module configured to (i) increase a frequency of scanning from a first rate to a second rate after detection of an increase of the attenuation, and (ii) decrease the frequency to a third rate after detecting a decrease in attenuation after increasing the frequency to the second rate. 56. The computer-implemented system of clause 55, wherein the monitoring module is further configured to monitor a rate of change of the attenuation. 57. The computer-implemented system of clause 56, wherein the scanning-frequency control module is further configured to increase the frequency to the second rate in response to detection of a decrease in a rate at which the attenuation is increasing. 58. The computer-implemented system of clause 56, wherein the monitoring module is further configured to begin monitoring of the rate of change after detection of compliance of the attenuation with a threshold. 59. The computer-implemented system of clause 58, wherein the threshold is 35 HU. 60. The computer-implemented system of clause 58, wherein the threshold is a degree of increase in the attenuation compared to a value indicated by an initial scan. 61. The computer-implemented system of clause 55, further comprising a processing module configured to generate a representation of a relationship between time and radiation attenuation by a second structure within the target region. 62. The computer-implemented system of clause 61, wherein the radiation attenuation by the second structure with respect to time represents an indicator of vascular perfusion of the second structure. 63. The computer-implemented system of clause 55, wherein the scanning-frequency control module is further configured to decrease the frequency below the third rate in response to detection of a decrease in a rate at which the attenuation is decreasing. 64. The computer-implemented system of clause 63, wherein the scanning-frequency control module is further configured to decrease the frequency further below the third rate with each successive scan. 65. The computer-implemented system of clause 64, wherein the scanning-frequency control module is further configured to divide the frequency by approximately two with each successive scan. 66. The computer-implemented system of clause 55, wherein the scanning-frequency control module is further configured to reduce the frequency to the third rate upon a first detection of a decrease in attenuation after an increase to the second rate. 67. The computer-implemented system of clause 55, further comprising a termination module configured to terminate the scanning after a predetermined period of time and direct performance of a final scan at the end of the predetermined period. 68. The computer-implemented system of clause 55, further comprising a termination module configured to terminate the scanning after a predetermined period of time, and, if a remaining time between a latest scan and an end of the predetermined period is less than an interval between the latest scan and an immediately preceding scan, direct performance of (i) a penultimate scan at a half of the remaining time after the latest scan and (ii) a final scan at the end of the predetermined period. 69. A computed tomography imaging system, comprising: a gantry comprising an opening configured to accommodate an object; a radiation source mounted to the gantry; a radiation detector mounted to the gantry opposite the radiation source relative to the opening; an attenuation monitoring module configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region; a scanning-frequency control module configured to (i) increase a frequency of scanning from a first rate to a second rate after detection of an increase of the attenuation, and (ii) decrease the frequency to a third rate after detecting a decrease in attenuation after increasing the frequency to the second rate. 70. A method of computed tomography imaging, comprising: repeatedly emitting x-ray radiation into a target region at a frequency during a session; monitoring, during the session, an indicator of attenuation of radiation by a contrast-enhanced first structure within the target region; varying the frequency based on the attenuation. 71. The method of clause 70, wherein x-ray radiation is emitted at a minimum frequency when the attenuation is below a low threshold and at a maximum frequency when then attenuation is above a high threshold. 72. A method of contrast-enhanced computed tomography (CT) imaging, comprising: (a) repeatedly scanning a target region during a session, the scanning comprising performing a CT scan by emitting x-ray radiation at an applied power toward the target region, the applied power being a first power for a first scan; (b) monitoring an indicator of attenuation of radiation by a contrast-enhanced first structure within the target region; and (c) selecting the applied power for each of a plurality of scans, after a first scan, based on the attenuation indicated from a preceding scan in the session. 73. The method of clause 72, wherein the first power is a maximum power applied during the session. 74. The method of clause 72, wherein the applied power is determined by selection of an applied current. 75. The method of clause 74, wherein the first applied current is about 200 ma. 76. The method of clause 72, further comprising applying substantially the first applied power to individual scans until detection of an increase of the attenuation to or beyond a threshold attenuation magnitude. 77. The method of clause 76, wherein the threshold attenuation magnitude is about 35 HU. 78. The method of clause 76, wherein the threshold attenuation magnitude is a predetermined proportion of the attenuation determined from an initial scan. 79. The method of clause 76, wherein the threshold attenuation magnitude is a predetermined number of Hounsfield Units greater than the attenuation determined from an initial scan. 80. The method of clause 72, wherein the applied power is selected by multiplying a maximum current by an exponential function based on the attenuation determined from the preceding scan. 81. The method of clause 80, wherein the exponential function yields a value that is (i) greater than a minimum allowable current divided by a maximum allowable current, and (ii) less than 1. 82. The method of clause 80, wherein the exponential function is a function F determined byF=eC(TH-ΔHU)/TH wherein TH is a threshold attenuation magnitude and ΔHU is equal to a difference in magnitude, in Hounsfield Units, between the attenuation determined from a preceding scan and a baseline attenuation. 83. The method of clause 82, wherein the preceding scan is a scan immediately prior to a scan performed according the applied power as determined by the function F. 84. The method of clause 82, wherein the baseline attenuation is a magnitude of the attenuation indicated based on the initial scan. 85. The method of clause 82, wherein C is selected such that, when the function is applied, an applied current for a next scan is about a tenth of the maximum allowable current when the attenuation of the preceding scan is about ten times above the threshold attenuation magnitude. 86. The method of clause 82, wherein C is about 0.25. 87. The method of clause 82, further comprising selecting an applied power corresponding to a minimum allowable current for each scan for which the function F indicates, based on the attenuation indicated by a preceding scan, a current less than the minimum allowable current. 88. A computer-implemented system for controlling contrast-enhanced computed tomography imaging, comprising: an attenuation monitoring module configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region; a power control module configured to select an applied power for each of a plurality of scans based on the attenuation detected from a preceding scan. 89. The computer-implemented system of clause 88, wherein the power control module is further configured to direct application of a maximum power applied during the session in a first scan. 90. The computer-implemented system of clause 88, wherein the power control module is further configured to apply substantially the same amount of power to individual scans until detection of an increase of the attenuation to or beyond a threshold attenuation magnitude. 91. The computer-implemented system of clause 88, wherein the power control module is further configured to select the applied power by multiplying a maximum current by an exponential function. 92. A computed tomography imaging system, comprising: a gantry comprising an opening configured to accommodate an object; a radiation source mounted to the gantry; a radiation detector mounted to the gantry opposite the radiation source relative to the opening; an attenuation monitoring module configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region; a power control module configured to select an applied power for each of a plurality of scans based on the attenuation detected from a preceding scan. 93. A method of computed tomography imaging, comprising: repeatedly emitting x-ray radiation into a target region, each emission having an input power; monitoring an attenuation of radiation through a structure within the target region; varying the input power based on the attenuation. 94. The method of clause 93, further comprising applying a minimum input power when the attenuation is above a high threshold and applying a maximum input power when then attenuation is below a low threshold. Additional features and advantages of the subject technology will be set forth in the description below, and in part will be apparent from the description, or may be learned by practice of the subject technology. The advantages of the subject technology will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the subject technology as claimed. In the following detailed description, numerous specific details are set forth to provide a full understanding of the subject technology. It will be apparent, however, to one ordinarily skilled in the art that the subject technology may be practiced without some of these specific details. In other instances, well-known structures and techniques have not been shown in detail so as not to obscure the subject technology. A phrase such as “an aspect” does not imply that such aspect is essential to the subject technology or that such aspect applies to all configurations of the subject technology. A disclosure relating to an aspect may apply to all configurations, or one or more configurations. An aspect may provide one or more examples of the disclosure. A phrase such as “an aspect” may refer to one or more aspects and vice versa. A phrase such as “an embodiment” does not imply that such embodiment is essential to the subject technology or that such embodiment applies to all configurations of the subject technology. A disclosure relating to an embodiment may apply to all embodiments, or one or more embodiments. An embodiment may provide one or more examples of the disclosure. A phrase such “an embodiment” may refer to one or more embodiments and vice versa. A phrase such as “a configuration” does not imply that such configuration is essential to the subject technology or that such configuration applies to all configurations of the subject technology. A disclosure relating to a configuration may apply to all configurations, or one or more configurations. A configuration may provide one or more examples of the disclosure. A phrase such as “a configuration” may refer to one or more configurations and vice versa. Although many features, aspects and embodiments are described herein or shown in the accompanying drawings in the context of CT, the disclosed technology can also be applied in other imaging systems and methods, other medical scenarios, or other image data acquisition or processing techniques. Primarily due to concerns about the magnitude of radiation dose delivered, perfusion CT imaging has not been used routinely in various fields, including stroke assessment, oncology, and cardiac and kidney function. In some embodiments, reduction of radiation dose delivered to a subject can permit application of perfusion CT to those applications wherein dose is a limiting factor, e.g. cardiac perfusion. In some embodiments, perfusion CT can be applied to stroke assessment, oncology, and assessment of cardiac and kidney function. In various embodiments, a radiation dose delivered to a subject can be reduced by application of a transverse dynamic collimator, a grated collimator, an adaptive sampling algorithm, an adaptive exposure algorithm, or a combination thereof. In some embodiments, the radiation dose of perfusion CT can be significantly reduced without impacting diagnostic accuracy During a helical CT scan, an X-ray source generates a cone (or wedge) beam of radiation that moves relative to the patient. Portions of the cone beam of radiation may not pass through the volume to be reconstructed. While this extra radiation may have little adverse effect on the clinical use of the reconstructed image, it can subject the patient to more radiation than is necessary. Accordingly, various embodiments described herein relate to replacing a conventional collimator with a dynamically transversely adjustable collimator. The collimator can be actuated by an electromechanical servo system. The imaging system can comprise a control (e.g., an electronic control) that is responsive to sensor(s) for sensing the axial and rotational position of the X-ray source relative to a volume of interest. As the X-ray source rotates about an axis, the collimator is adjusted (i) to adjust the width, location, or both of the radiation beam so that radiation is primarily allowed to pass through the volume of interest and (ii) to block some or all of the rays of radiation that will not intersect the volume of interest. FIG. 1 illustrates an exemplifying CT imaging system 100 including a CT scanner 102 with a gantry portion 104, a radiation source unit 112, a detector 124, and a couch or support 126. The gantry portion 104 can comprise a gantry opening 106 and can rotate about an examination region 108. The rotating gantry portion 104 can support the radiation source unit 112 and the detector 124. The radiation source unit 112 can be an x-ray source, such as an X-ray tube, for example. The radiation source can emit a radiation beam. The radiation beam can be a cone beam, wedge beam, or other desirable beam shape. The beam can be collimated to have a generally conical geometry in some embodiments. The detector 124 is sensitive to radiation (e.g., x-ray) emitted by the radiation source unit 112. In some embodiments, the detector 124 can be a detector array comprising multiple radiation detectors. The detector 124 can be disposed opposite the x-ray source unit 112 on rotating gantry portion 104. In some embodiments, the detector 124 includes a multi-slice detector having a plurality of detector elements extending in the axial and transverse directions. Each detector element can detects radiation emitted by the radiation source unit 112 that traverses the examination region 108 and can generate corresponding output signals or projection data indicative of the detected radiation. Other detector configurations, such as those wherein stationary detectors surround the examination region, can also be used. The motion of the radiation source and emission of radiation thereby are coordinated to scan a volume of interest (VOI) 122 such as anatomy, or a portion of anatomy, disposed within the examination region 108. The volume of interest can be enhanced with a contrast agent in some embodiments, such as described below, for example. In some embodiments, coordinated motion and radiation emission can be used for fly-by scanning, for example. In some embodiments, the radiation source and detector move in coordination with a contrast agent through the subject such that the VOI is scanned in coordination with the flow of the agent as it is traced through the VOI. In another embodiment, the axial advancement is coordinated with a motion of the subject to capture a desired motion state. The support 126 can support a subject, such as a human patient for example, in which the VOI is defined within the examination region 108. As illustrated in FIG. 1, a drive mechanism 116 can move the radiation source longitudinally along a z-axis 120 on tracks 128 while the support 126 is stationary. In some embodiments, however, the support 126 can be translated axially along the z-axis 120 while the gantry 104 rotates in a fixed location along the z-axis. An operator of the system can define the VOI to encompass the whole subject or a portion thereof for scanning. In one embodiment, the CT scanner performs a helical scan of the VOI by rotating around the axis 120 during relative movement of the gantry and the support parallel to the axis. The system 100 can further comprise various computer hardware and software modules. As illustrated in FIG. 1, for example, the system can comprise data memory 130, a processor 132, a volume image memory 134, a user interface 136, and one or more controllers, such as a CT controller 128 and a collimator controller 140. In some embodiments, a single hardware or software module can control multiple parts of the system 110, such as the radiation source and one or more collimators, for example. The projection data generated by the detector 124 can be stored to a data memory 130 and reconstructed by a processor 132 to generate a volumetric image representation therefrom. The reconstructed image data can stored in a volume image memory 134 and displayed to a user via a user interface 136. Although FIG. 1 separately illustrates the data memory 130 and the volume image memory 134, both can be stored within common data storage hardware. The image data can be processed to generate one or more images of the scanned region or volume of interest or a subset thereof. The user interface 136 facilitates user interaction with the scanner 102 and can comprise various input and output devices. Software applications and modules can receive inputs from the user interface 136 to configure and/or control operation of the scanner 102, and other elements of the system 100. For instance, the user can interact with the user interface 136 to select scan protocols, and initiate, pause, and terminate scanning. The user interface 136 can display images, facilitate manipulation of the data and images and measurement of various characteristics of the data and images, etc. An optional physiological monitor (not shown) can monitor cardiac, respiratory, or other motion of the VOI. For example, the monitor can include an electrocardiogram (ECG) or other device that monitors the electrical activity of the heart. This information can be used to trigger one or more scans or to synchronize scanning with the heart electrical activity to reduce or eliminate adverse affects of heart motion on imaging. An optional injector (not shown) or the like can be used to introduce agents, such as contrast for example, into the subject. Introduction of the agent can be used to trigger one or more scans. The CT controller 138 can control rotational and axial movement of the radiation source unit 112 and the detector 124 relative to the support 126. The CT scanner and CT controller can be coupled to a collimator controller 140 that controls a collimator 142 positioned between the radiation source and the examination region 108. Although FIG. 1 illustrates the CT controller and the collimator controller as separate units, the CT scanner and one or more collimators can be controlled by the same hardware and software modules in some embodiments. The collimator controller 140 can control movement, and opening and closing, of a radiation delivery window of the collimator 142. In some embodiments, the collimator controller can independently control movement of individual leaves of the collimator. The collimator controller can be a software module configured to move leaves of a collimator to allow passage of radiation toward a region of interest while blocking radiation to portions of a subject outside the region of interest. In some embodiments, the collimator controller 140 can cause the collimator to function as a shutter to block radiation between scans and to open, close, and translate as the rotatable gantry 104 (and accordingly the source unit 112 and detector 124 coupled thereto) move around the VOI 122 during a scan. In some embodiments, the collimator controller 140 can include one or more electro-mechanical servo motors. In some embodiments, the collimator controller 140 can include an electronic controller. Repeated large area circular scans and helical scans can be used to perform perfusion CT. By opening, closing, and/or translating the collimator 142, radiation can be delivered primarily only along paths that intersect the VOI, thereby reducing the X-ray dose. In the case of helical scans, a dynamic axial collimator can be used to limit the x-ray exposure, axially, at either end or both ends of the helical scan. For example, in some embodiments, an axial collimator can be gradually opened at the leading end of the VOI and closed at the trailing end of the VOI. A dynamic transverse collimator positioned in a plane transverse to a gantry rotation axis and in front of the x-ray source can limit the x-ray exposure to a region of interest (ROI) 144 within a field of view (FOV) 146, as illustrated in FIG. 3. FIG. 3 is a schematic illustration of an imaging system showing two positions 152, 154, respectively at 0 and 90 degrees relative to a subject, of the radiation source unit 112, detector 124, and a dynamic transverse collimator 142 for an off-center ROI 144 surrounding the heart. FIG. 4 illustrates the outline of a VOI corresponding to the heart. Transverse and axial collimators can together limit the x-ray exposure to primarily only the VOI 148 for the heart illustrated in FIG. 4, for example. As illustrated, for example, in FIG. 4, the VOI 148 can include multiple ROIs 144. In some embodiments, a collimator can comprise a first leaf 170 and a second leaf 172 respectively bounding first and second opposing sides of a radiation delivery window 174, as illustrated in FIG. 5, for example. The first leaf and the second leaf can be movable to adjust at least one of a size or a location of the radiation delivery window relative to the radiation source in a direction non-parallel to the axis. The first leaf and the second leaf can be independently movable relative to the radiation source in a direction non-parallel to the axis. The first leaf 170 and the second leaf 172 can be moveable independently of each other. Each of the first and second sides can be substantially orthogonal to each of the third and fourth sides opposing sides of the radiation delivery window 174. In some embodiments, the first leaf and the second leaf can be independently movable relative to the radiation source in a direction tangential to a circle (i) centered on the axis and (i) defining a plane that is not parallel to the axis. In some embodiments, the leafs can be movable along guide rails 180, as illustrated in FIG. 7. In some embodiments, the collimator can comprise a third leaf 176 and fourth leaf 178 respectively bounding the third and fourth opposing sides of the window. The third leaf, the fourth leaf, and the window can be arranged generally along a line that is parallel to the axis of gantry rotation. The window can be interposed between the third and fourth leaves such that radiation is transmitted between the third and fourth leaves in a direction generally perpendicular to the axis of rotation. The third leaf and the fourth leaf can be independently movable relative to the radiation source with a direction of motion being generally parallel to the axis. The third leaf 176 and the fourth leaf 178 can be moveable independently of each other. In some embodiments, the third and fourth leaves can be movable independently of the first and second leaves. The transverse and axial collimators can be driven by the same motor or different motors. Similarly, the transverse and axial collimators can be controlled by the same hardware or software modules. In some embodiments, transverse and axial collimators can be integrated into a single unit. The VOI can be defined, for example, by a previously-acquired very low dose scan of the same region or two orthogonal localizer scans could be used. An operator can specify an axial extent of the VOI and the size, shape, and location of each ROI along the axial direction. In some embodiments, an outline of the entire VOI can be drawn from two orthogonal views, e.g. sagittal and coronal views, with the images zoomed according to the largest ROI in the sequence. The truncated region of each reconstructed image can be displayed with a dark background. The axial collimator leaves can be opened and closed based on the axial extent of the VOI. If the VOI is modeled using elliptical cross-sections, the transverse collimator leaves can move smoothly as they closely follow the outline of the VOI. In the case of a large cone-beam, the beam already encompasses a large portion of the VOI, therefore there can be less narrowing of the VOI profile at the ends of the scan. As in the case of the axial collimator, the position of the dynamic transverse collimator leaves can be based on the couch position. However, in the case of the dynamic transverse collimator, as the couch moves in the axial direction, the rotation angle can be used to determine where the current ROI is situated with respect to the source. Given both the couch position and rotation angle, the leaves can continuously follow the outline of the overall VOI. For example, in the case of the cardiac scan shown in FIG. 4, the collimator leaves can follow the ROIs located along the cardiac volume based on the couch location of the ROI as well as the rotation angle of the x-ray source. The ROI along the cardiac volume can have both a non-circular (e.g., elliptical) shape and a location away from a scan center 150. In some embodiments, for a sequence of axial or circular scans, one ROI can be determined for each scan in the sequence. For each scan, the rotation angle can be used alone to determine the motion of the collimator leaves, adjusting for an off-center and/or non-circular ROI. In some embodiments, the size of the radiation delivery window 174 between movable leaves of the transverse collimator can be adjusted prior to a scan and held in a fixed arrangement during the scan. For example, a low dose can be performed to identify a VOI, then a subsequent scan can be performed with an transverse collimator aperture dimension selected for imaging of the VOI while the transverse collimator leaves remain stationary during the scan. In some embodiments, the moveable leaves can comprise secondary windows and attenuating members, as discussed further below. In some embodiments, the collimator can comprise a primary radiation delivery window 174 of a fixed size. In some embodiments, between scans, a collimator having a window of a first fixed size can be removed from the CT scanner 102 and can be replaced with another collimator having a window of a second fixed size, different than the first fixed size. The collimators with primary radiation delivery windows of a fixed size can comprise secondary windows and attenuating members, discussed further below, in some embodiments. As illustrated in FIG. 8, attenuation information from the tissue surrounding the collimated ROI is desirable due to the extent of the convolution involved in image reconstruction. Attenuation information for the material outside the collimated VOI can be acquired to facilitate accurate reconstruction. This can be accomplished in various ways, including one or more of the following: (1) use the previously-acquired very low dose scan mentioned above to measure the attenuation outside the VOI; (2) use heavily-attenuated rays through the outer portions of the collimator, providing an estimate of the attenuation outside the VOI so long as the collimator attenuation is previously calibrated; (3) use a projection-completion estimate, extrapolating the truncated projections using a curve-fit based on a simple model; or (4) use limited, but known, attenuation information for isolated sub-regions in the surrounding tissue. In some embodiments, attenuation values for the region outside the ROI can be determined from radiation passing through windows in a grating that are separated from a primary window configured to allow passage of rays of radiation that would travel through the ROI. FIG. 9 shows an example of a display output, such as can be displayed on a monitor of a scanner, of a CT scan of an anthropomorphic thorax phantom using a grated collimator. The image of the display output is shown without application of any correction. In some embodiments, correction can be applied to the data used to form the image. In some embodiments, correction can be applied to image data before displaying an image. The VOI in the image of FIG. 9 includes a phantom cardiac region near the center of the thorax phantom. In some embodiments, the collimator 142 can comprise a first grating 170 and a second grating 172 positioned on opposing sides of a primary radiation delivery window 174. Each of the first and second gratings can comprise a plurality of attenuating members 182 with a plurality of secondary radiation delivery windows 184 extending between adjacent attenuating members of the first grating and the second grating, respectively. A width of each secondary window can be less than a width of the primary window, as illustrated in FIG. 6, for example. Although FIG. 6 illustrates attenuating members of the first grating and the second grating as being integral with each other, the first and second gratings can be part of separate first and second leaves 170, 172, as illustrated in FIG. 7, for example. A total area of each of the secondary windows can be less than a total area of the primary window. In some embodiments, the width of each secondary window can be proportional to a distance between the secondary window and the primary window, as illustrated in FIG. 7, for example. For example, the width of each secondary window can be linearly, exponentially, or geometrically proportional to the distance between the secondary window and the primary window. The width of each secondary window can be positively proportional to the distance between the secondary window and the primary window such that the width of the windows increase with their distance from the primary window. The secondary windows can be oriented generally parallel to sides of the primary window. The secondary windows can comprise open passages extending through the grating. In some embodiments, the secondary windows can comprise panes of substantially radio-transmissive or low-attenuating material. When panes of low-attenuating material are employed, the panes can attenuate the radiation to a lesser extent than the attenuating members 182. A width of each attenuating member, and thus the spacing between secondary windows, can be proportional to a distance between the attenuating member and the primary window, as illustrated in FIG. 7, for example. The width of each attenuating member can be linearly, exponentially, or geometrically proportional to the distance between the attenuating member and the primary window. The width of each attenuating member can be positively proportional to the distance between the attenuating member and the delivery window. The attenuating members can be oriented generally parallel to sides of the primary window. In some embodiments, the attenuating members can block passage of x-ray radiation therethrough. In some embodiments, the attenuating members can be made of materials that substantially prevent transmission of x-rays. The attenuating members can be made of lead, tungsten, or other materials or combinations thereof. In some embodiments, the size, number, position, spacing, or a combination thereof of the secondary windows and attenuating members can provide approximately a minimum or a near-minimum amount of radiation transmission for detector operation. The collimator controller, which can be a hardware or software module, can be configured to control operation of transverse dynamic collimators to significantly reduce the radiation dose delivered by CT scans. In some embodiments, the collimator controller can control both axial and transverse collimators. Further details regarding axial collimators and their control are provided in U.S. Patent Application Publication No. 2010/0246752 to Heuscher et al., entitled Dynamic Collimation in Cone Beam Computed Tomography to Reduce Patient Exposure, the entirety of which is hereby incorporated herein by reference. In some embodiments, a single collimator can be configured for both axial and transverse collimation. A axial collimator and a transverse collimator can be integrated as a single unit in some embodiments. The transverse dynamic collimator, whether alone or in combination with the axial collimator, can be used for both helical and axial scans. In some embodiments, the leaves of the axial and transverse collimators can be moved such that only or substantially only that radiation, e.g., x-rays, that intersects a predefined VOI is exposed to the patient. In some embodiments, control of the transverse collimator involves the velocity of each leaf. The velocity of each leaf can be determined for a region of interest (ROI) at a given distance from scan center. In some embodiments, the ROI in a particular slice can be circular or noncircular. The velocity and acceleration of leaf movement depend on how far off-center a given cross-sectional region of interest (ROI) is located from a scan center within a field of view (FOV). Referring to FIG. 10, the following equations can define the transverse collimator leaf position Q(t) and approximated velocity Q′ given the rotation speed p, collimator distance C, distance S from an ideal radiation point source to the scan isocenter, ROI radius R1, and offset R0 at angle θ0. Q ⁡ ( t ) = C · tan ⁡ ( arcsin ⁡ ( R 1 L ⁡ ( t ) ) + arcsin ⁡ ( R 0 L ⁡ ( t ) · sin ⁡ ( θ ⁡ ( t ) - θ 0 ) ) ) ( 1 ) L ⁡ ( t ) = ( S · sin ⁡ ( θ ⁡ ( t ) - θ 0 ) ) 2 + ( S · cos ⁡ ( θ ⁡ ( t ) - θ 0 ) - R 0 ) 2 ( 2 ) ∂ Q ⁡ ( t ) ∂ t ≈ Q ′ ⁡ ( t ) = C · π · R 0 ρ · S · ( cos ⁡ ( θ ⁡ ( t ) - θ 0 ) - R 1 S · sin ⁡ ( θ ⁡ ( t ) - θ 0 ) - R 0 S ) ( 1 - R 0 S · cos ⁡ ( θ ⁡ ( t ) - θ 0 ) ) 2 ( 3 ) L(t) can represent the distance from the ideal radiation point source to the center 156 of the ROI and can be used to obtain the angle between the line 158 of length L and the central ray of the projection (line 160 of length S). This angle can be added to the remaining angle between the line 158 and the line 162 tangent to the ROI. The distance Q(t) of the collimator leaf from the central ray can be obtained by multiplying the tangent of the resulting angle by a total distance C between the collimator 142 and the point source. The derivative of the expression for Q(t) can be calculated to calculate the velocity Q′. Assuming a constant rotation speed ρ, the expression 2πt/ρ can substituted for θ(t). Due to its complexity, the derivative of this equation, δQ(t)/δt, can be calculated numerically. In some embodiments, approximations can be made to obtain a closed form expression for obtaining the derivative of the equation δQ(t)/δt. By assuming R0 and R1 are relatively small compared to the point source to isocenter distance S, the higher order terms in the Taylor series expansions of both Q(t) and δQ(t)/δt can be eliminated and a good approximation Q′(t) can be obtained as shown in equation (3). The leaves of the axial collimator can be opened at the beginning of a scan, e.g., a helical scan, and closed at the end such that only that radiation, e.g., x-rays, that intersect the VOI are exposed to the subject, as illustrated in FIGS. 11 and 12 for the right end of the scanned volume, for example. As the beam, e.g., a cone-beam, approaches the cylindrical VOI from the right, the leaves can be closed, shifted to the far left. As the left edge of the beam touches the far edge of the cylinder, the right leaf can begin opening by moving to the right until the center of the beam reaches the edge of the cylinder. The right leaf then can immediately accelerate to a higher velocity to be able to follow the near edge of the cylinder, until the entire beam is exposed. The collimator can remain open as the scan proceeds until the left edge of the beam hits the near edge of the left side of the cylinder, at which point the left collimator leaf can begin closing in reverse order from the previous sequence for the right leaf. The following equations can define the axial collimator position Q(t) and velocity δQ/δt given the helical pitch P, helical position H(t), rotation speed ρ, detector width w, collimator distance C, point source to isocenter distance S, and cylindrical VOI radius R: H ⁡ ( t ) > 0 ⁢ : ⁢ ⁢ Q ⁡ ( t ) = C · w 2 · S · ( 1 + 2 · P · t ρ · ( 1 + R S ) ) ⁢ ⁢ ∂ Q ⁡ ( t ) ∂ t = C · w · P ρ · ( S + R ) ( 4 ) H ⁡ ( t ) < 0 ⁢ : ⁢ ⁢ Q ⁡ ( t ) = C · w 2 · S · ( 1 + 2 · P · t ρ · ( 1 - R S ) ) ⁢ ⁢ ∂ Q ⁡ ( t ) ∂ t = C · w · P ρ · ( S - R ) ( 5 ) From right to left, as the beam, e.g., cone-beam, approaches the cylindrical VOI, the collimator can attenuate all rays outside the far edge of the cylinder (H(t)>0). Once the center of the beam passes the end of the cylinder (H(t)<0), the collimator can attenuate all rays inside the near edge of the cylinder. Consequently, the denominator of the equations correspondingly changes from (S+R) to (S−R) between equations (4) and (5). FIGS. 13 and 14 are plots of the velocities (in cm/s) for a transverse collimator leaf where θ0=0, S=70, and ρ=0.3 s/rev. In FIG. 13, C=27, R0=18, and R1=6. In FIG. 14, C=12, R0=6, R1=6. As illustrated in FIG. 13, for a ROI of 12 cm within a 48 cm diameter FOV and an offset R0 of 18 cm, the transverse collimator leaf can achieve a maximum velocity of 198 cm/s. On the other hand, as illustrated in FIG. 14, if the collimator is located 12 cm from the idealized radiation point source and if the subject is positioned such that the ROI is 12 cm closer to the scan center, the collimator leaf reaches a maximum velocity of 24 cm/s for an offset of 6 cm. Therefore, some embodiments include moving the transverse dynamic collimator closer to the radiation point source, moving the subject closer to the scan center, or both. FIG. 15 is a plot of the velocities (in cm/s) for a transverse collimator leaf where θ0=0, S=70, ρ=0.3 s/rev, C=12, R0=18, and R1=6. Thus, if a velocity of 100 cm/s or less can be tolerated and the collimator is located within 12 cm of the idealized radiation point source, an off-center ROI can be tracked by the collimator and, thus, a subject can remain in the same position within the FOV between a preliminary full-field scan and subsequent scanning targeting to the VOI. FIG. 6 shows that a maximum velocity of 87 cm/s is reached for a 12 cm ROI located 18 cm off-center according to the equations provided herein. FIGS. 13-15 also compare the closed form approximation Q′ (dashed line) with the actual velocity δQ/64 (solid line) computed numerically from Q(t). The close match between the two curves confirms the validity of the approximation for the range of geometric values simulated. FIG. 16 is a plot of the velocities (in cm/s) for leaves of an axial collimator where C=27 cm, S=70 cm, a detector width w is 8 cm, a pitch factor P is 1.4, rotation speed p is 0.3 sec/rev, and a VOI has a radius R of 25 cm. FIG. 16 shows a maximum axial collimator leaf velocity of 22.9 cm/s. That maximum velocity occurs for H(t)<0, when the center of the cone-beam passes the edge of the cylinder closest to the x-ray source. The velocity profiles of the transverse collimator leaves and the axial collimator leaves can be compared. The maximum velocity incurred by the axial collimator leaves during a typical helical scan with an 8 cm detector, 0.3 second rotation time (i.e., revolution duration), with a pitch factor of 1.4 is 22.9 cm/sec. On the other hand, the maximum velocity of a transverse collimator positioned 27 cm from the source is 198 cm/sec for a 12 cm cardiac scan located 18 cm from the scan center. Thus, the velocities of the transverse collimator can be greater than the helical collimator. For a transverse collimator mounted 27 cm from the x-ray source, e.g., just beyond the opening of the gantry, the maximum velocity incurred can be more than 8 times that of the axial collimator. However, if the collimator is located closer to the source and if the VOI is positioned, or possibly repositioned, 12 cm or less off-center, the velocities of the transverse and axial collimators can be comparable. In some embodiments, a motor and control system for at least the transverse collimator can provide leaf velocities up to 90 cm/s, patient repositioning can be avoided in more instances. In some embodiments, a motor and control system for at least the transverse collimator can provide leaf velocities greater than 90 cm/s. Although movement of leaves in a transverse dynamic collimator have been discussed in connection with placement at 12 cm and 27 cm from the radiation source, a transverse dynamic collimator can be positioned at other distances from the radiation source. For example, the transverse dynamic collimator can be positioned within about 27 cm, about 20 cm, about 15 cm, or about 10 cm of the radiation source in some embodiments. In some embodiments, the collimator can be integrated into the radiation source unit 112. By integrating a transverse dynamic collimator into the radiation source unit 112, the distance from the radiation source (measured from the idealized point source location) can be shorter and, thereby, reduce the dynamic requirements (velocity and acceleration) of the transversely moving leaves. However, the distance between the idealized point source and the transverse dynamic collimator is preferably sufficiently long that the size of the radiation penumbra is acceptable. For example, in embodiments that comprise a grated collimator, the distance between the idealized point source and the transverse dynamic collimator is preferably sufficiently long to avoid crosstalk between adjacent secondary windows, e.g., slots, in the grated collimator. Modeling of heart and kidney scans, with and without transverse dynamic collimation, indicate significant skin dose reduction in both cases. Elliptical models were analyzed to determine the skin exposure for heart and kidney scans. A dose of radiation, e.g., X-rays, exposed to the subject, e.g., a patient, can be significantly reduced using a dynamic transverse collimator such as that illustrated in FIG. 5, for example. The use of other dynamic transverse collimators, such as the grated dynamic transverse collimator illustrated in FIG. 7, can also significantly reduce radiation dose. FIG. 17 shows three cross-sections from the middle of a five cross-section sequence. A target ROI corresponding to a heart is represented by an ellipse in each cross-section of FIG. 17. Dynamic transverse collimation can follows the ellipses outlining the heart and representing the target ROI. The cross-sections of FIG. 17 also include body outlines, approximated with ellipses, from which the skin exposure values were calculated. The five sections of the heart were equally spaced. In the un-collimated case, all sections are fully exposed, while in the collimated case, the exposure is limited to the target VOI, with the first and last sections fully collimated down to a 0 cm diameter circle. Reference lines are shown in FIG. 17 that intersect the scan center, the body ellipse, and target ROI. The average un-collimated skin exposure relative to the average exposure at the center of the target ROIs is compared to the average collimated skin exposure over the five slices again relative to the average exposure at the center of the target ROI. For the exposure analysis of the heart, reconstructions of an XCAT phantom were used with an X-ray source radius S of 57 cm. A compensator in front of the X-ray source can provide a more uniform signal to the detector and can reduce the overall dose to the patient. Therefore a compensator was modeled that compensates for a disc with a radius rc of 24 cm and an attenuation coefficient equal to water (0.183/cm at 80 KeV, approximately the average energy for a typical CT system performing 120 KeV scans). As an initial approximation, the water attenuation value of 0.183/cm was assumed for all path lengths throughout the body. The exposure for each of 100 points equally spaced in angle β on the surface of the body ellipse was calculated for 1000 angular views, θ, of the source covering 360 degrees. Rays emanating from the source pass either just through the compensator (FIG. 4), or both through the compensator and the body (FIG. 5). These exposure values were averaged over the 1000 views. The exposure at the center of the target ROI (FIG. 6), was averaged over 1000 views. This provided a reference for the skin exposure values. Thus, all overall skin exposure values were measured as a ratio with respect to the exposure at the center of the target ROI. For the scans utilizing a dynamic collimator, the fan angles for which the collimated x-rays intersect tangentially to the target ROI were calculated for each x-ray source position and used to exclude all exposure from the x-ray source that fell outside the corresponding angular range. Again, all skin exposure values were averaged over 1000 views. In the case of the heart, 5 equally-spaced sections of the heart were used to define the cardiac VOI, with the first and last sections fully collimated. The relative skin dose for the center section was compared with and without dynamic collimation. The final skin exposure values were averaged over the five sections, both with and without dynamic collimation. The boundaries of the ellipses were defined by specifying 6 user-defined points around the periphery of both the body outline and target ROI. A least-squares solution to the parameters of each ellipse, (a, b, r0, t0, tr)=(major axis, minor axis, polar radius of the origin of the ellipse, polar angle of the origin, and angular orientation of the ellipse), was then obtained given the (R,T) polar coordinates of these 6 points along with the following constraints: X 2 a + Y 2 b = 1 ⁢ ⁢ and ⁢ | tr - π 2 | < π 2 where:X=R·cos(T−tr)−r0·cos(t0−tr)Y=R·sin(T−tr)−r0·sin(t0−tr)(R,T) correspond to the polar coordinates of the 6 points and the following initial values are provided:(a,b,r0,t0,tr)=(TOL,TOL,com(R,T),mean(T),TOL)where:com(R,t)=√{square root over (mean(R·cos(T))2+mean(R·sin(T))2)}{square root over (mean(R·cos(T))2+mean(R·sin(T))2)}TOL=tolerance value=0.00001. Given both the body and target ellipses along with the compensator attenuation as a function of the fan angle, the skin exposures was calculated. For all ray angles up to the tangent to the body ellipse, the exposure was the attenuated exposure through the compensator (FIG. 18). For all other angles for which the rays pass through the body to the selected point on the ellipse, the x-ray exposure is further attenuated by the path length through the body. Given the source radius, source angle, and point on the body ellipse, the path length p relative to the distance p0 (FIG. 19) can be calculated as a solution to a quadratic equation resulting from the condition that the entrance point of the ray also satisfies the equation for the body ellipse. A point on the skin was treated like any other point within the body ellipse. In the case of a point on the boundary of the ellipse directly exposed by the x-ray source, the path lengths through the body converge to zero at all angles up to those angles tangent to ellipse. FIG. 18 illustrates geometry for a ray at angle α directly exposing the skin, emanating from the x-ray source located at angle θ. The ray illustrated in FIG. 18 is only attenuated by the compensator as a function of angle α. FIG. 19 illustrates another geometry for a ray at angle α indirectly exposing the skin, emanating from the x-ray source located at angle θ. The ray illustrated in FIG. 19 is further attenuated by the body path length (1−p) p0. The resulting skin exposure is then calculated as an average value over 360 degrees of the angular position (θ) of the x-ray source: Exposure ⁡ ( β ) = .001 · ∑ i = 1 1000 ⁢ ⁢ [ ( intensity ( α ⁡ ( β , θ i ) ] ) ) · att ⁡ ( β , θ i ) where:β=the angle of the point on the body ellipseθ=the source angleα=fan angle of the ray intersecting the point on the body ellipse intensity ⁡ ( α ) = ⅇ 2 · .189 · ( rc 2 - ( S · sin ⁢ ⁢ α ) 2 - rc ) att ⁡ ( β , θ ) = ⅇ - .183 · ( 1 - p ⁡ ( β , θ ) ) ⁢ p ⁢ ⁢ 0 ⁢ ( y , s ) p is the path length to the entrance point on the body ellipse relative to p0; andp0 is the path length to the selected point on the body ellipse. The average relative skin exposure is then the average of the exposure for all points around the body ellipse divided by the exposure at the center of the target ROI. For the average collimated exposure, the same exposure equation is used, but with the intensity set to zero for all ray angles whose fan angle exceeds that of the range of angles spanned by the two rays that intersect the tangents to the target ellipse, i.e.:intensity(α(β,θ))=0 if α>AC(θ) or α<ACC(θ)AC is the clockwise angular position of the collimator leaf for source angle θ; andACC is the counter-clockwise angular position of the collimator leaf Finally, the exposure at the center of the target ROI is calculated as a reference for the skin exposure values. In this case, p0 corresponds to the path length to the point at the center of the target ellipse and p is calculated as the relative path length to the point on the body ellipse (FIG. 20). As this is the center of the target ellipse, the same value applies whether or not dynamic collimation is used. FIG. 20 illustrates geometry for a ray at angle α exposing the center of the target ROI. The ray of FIG. 20 is further attenuated by the body path length (1−p) p0. For the kidneys, ellipses were used to outline the target ROI and body of the patient. A single multi-slice scan was used to acquire the kidney perfusion images with the central image shown in FIG. 21. The average relative (un-collimated) skin dose was compared to the average relative (collimated) skin dose. A second kidney study was used to not only corroborate the results of the first study, but to demonstrate the additional dose savings that would be achieved for a whole-organ kidney study. Five equally-spaced sections (FIGS. 22A-E) were selected spanning the entire kidney with the first and last sections fully collimated when utilizing dynamic collimation. The central section was used to compare with the central section of the previous study and the overall whole-organ un-collimated (relative) kidney dose was compared with the overall dynamically collimated (relative) kidney dose. The reduction in skin exposure that can be achieved using transverse dynamic collimation of heart and kidney scans was calculated using the above-described elliptical models. The results, collimated and un-collimated exposure values relative to the average exposure calculated at the center of the target ROI, are summarized in Table 1 below: TABLE 1Un-collimatedCollimatedExposureExposureExposureReductionHeart2.611.22.1:1Whole Organ2.605.7043.7:1Kidney Study I2.5451.581.61:1 Kidney Study II2.11.02.1:1Whole Organ II2.1.5843.6:1 These results indicate a significant skin dose reduction for both heart and kidney scans. A 3.7:1 reduction in skin exposure was calculated for the whole-heart scan. A 1.6:1 to 2.1:1 reduction in skin exposure was calculated for a kidney scan and a 3.6:1 reduction for a whole organ kidney scan (both kidneys are included in the target ROI). The reduced skin exposure calculated for heart and kidney scans demonstrates the significant benefit of transverse dynamic collimation, especially to whole organ studies Skin exposure values are 2.1 to 2.6 times higher than the exposure at the center of the target ROI, even with an x-ray compensator. This demonstrates how important it is to keep skin exposure values as low as possible. Dynamic collimation to target the VOI can greatly reduce patient dose (up to 4:1 reduction in skin exposure for whole organ cardiac and kidney scans). This reduction in dose may enable coronary CT angiography to be used on a much more routine basis. Likewise, significantly reducing the dose for CT perfusion scans and whole organ kidney scans will greatly benefit the clinical use of such scans. Dynamic collimation can greatly reduce dose for other clinical applications as well. In some embodiments, radiation dose delivered to a patient can be reduced by varying a scanning frequency or interval between scans during a series of scans. FIG. 23 illustrates a method of contrast-enhanced computed tomography (CT) imaging. The following description of FIG. 23 refers to FIG. 24, which schematically illustrates a curve 2410 representing a magnitude of radiation attenuation (vertical axis) by a contrast-enhanced structure over time (horizontal axis), and a threshold 2412. In some embodiments, the threshold 2412 is 35 HU. In some embodiments, the threshold can be other predetermined attention densities, such as 50, 75, or 100 HU, for example. In some embodiments, the threshold comprises a degree of increase, e.g., a percentage, in the attenuation compared to a value indicated by an initial scan. At step 2310, scanning can begin at a first rate while monitoring for an increase of attenuation above the threshold 2412. At step 2312, after detection of the increase of attenuation above the threshold, the rate of change of attenuation can be determined between scans. At step 2314, when the rate of change of attenuation decreases (corresponding to an inflection point 2414 on the ascending part of the curve 2410), the rate of scanning can be increased. At step 2316, when the rate of change of attenuation becomes negative (corresponding to a peak 2416 of the curve 2410), the rate of scanning can be decreased. In some embodiments, the frequency is reduced at step 2316, in response to detection of a decrease in attenuation. For example, the scan interval can be lengthen for a next scan upon detecting a first decrease in attenuation after the increase above the threshold 2412. At step 2318, when the rate of change of attenuation decreases (corresponding to an inflection point 2418 on the descending part of the curve 2410), the rate of scanning can be further decreased. The rate can be further decreased in some embodiments by increasing a scan interval with each successive scan. In some embodiments, the scan interval can be approximately doubled with each successive scan. Scanning can be terminated at step 2320 in response to expiration of a predetermined period of time, completion of a predetermined number of scans, increase of a scan interval beyond a predetermined interval, or other trigger. In some embodiments, if a remaining time between a latest scan and an end of the predetermined period is less than an interval between the latest scan and an immediately preceding scan, a penultimate scan can be performed at approximately half of the remaining time after the latest scan and a final scan can be performed at the end of the predetermined period. FIG. 25 is an exemplifying plot of magnitudes of radiation attenuation by various contrast-enhanced structures over time. The vertical axis corresponds to attenuation density in Hounsfield Units. The horizontal axis corresponds to time in seconds. In FIG. 25, the large circles represent scans initiated by a sampling frequency adjustment according to an embodiment. Small circles correspond to scans obtained at each of 70 seconds. Squares in FIG. 25 represent attention through the LAD territory as detected by scans each second. FIG. 25 shows that application of a sampling frequency or interval adjustment can result in 18 scans compared to 70 scans if a scan is performed each second. Triangles in FIG. 25 represent attention through remote territory as detected by scans each second. The methods described herein for controlling contrast-enhanced computed tomography imaging can be implemented by computer system. For example, such a system can comprise an attenuation monitoring and a scanning-frequency control module. The monitoring module can be configured to begin monitoring of the rate of change after detection of compliance of the attenuation with the threshold 2412. In some embodiments, the system can further comprise a processing module configured to generate a representation of a relationship between time and radiation attenuation by a second structure within the target region. In some embodiments, the system can further comprise a termination module configured to terminate the scanning The attenuation monitoring module can be configured to monitor, during an imaging session, an indicator of attenuation of radiation by a contrast-enhanced structure within a target region. The monitoring module can be configured to monitor a rate of change of the attenuation. The scanning-frequency control module configured to (i) increase a frequency of scanning from a first rate to a second rate after detection of an increase of the attenuation, and (ii) decrease the frequency to a third rate after detecting a decrease in attenuation after increasing the frequency to the second rate. The scanning-frequency control module can be configured to increase the frequency to the second rate in response to detection of a decrease in a rate at which the attenuation is increasing. The scanning-frequency control module can be configured to decrease the frequency below the third rate in response to detection of a decrease in a rate at which the attenuation is decreasing. The scanning-frequency control module can be further configured to decrease the frequency further below the third rate with each successive scan. The scanning-frequency control module can be configured to divide the frequency by approximately two with each successive scan. The scanning-frequency control module can be configured to reduce the frequency to the third rate upon a first detection of a decrease in attenuation after an increase to the second rate. The termination module can be configured after a predetermined period of time and direct performance of a final scan at the end of the predetermined period. The termination module can be configured to terminate the scanning after a predetermined period of time, and, if a remaining time between a latest scan and an end of the predetermined period is less than an interval between the latest scan and an immediately preceding scan, direct performance of (i) a penultimate scan at a half of the remaining time after the latest scan and (ii) a final scan at the end of the predetermined period. In some embodiments, radiation dose delivered to a patient can be reduced by varying an applied power during a series of scans. FIG. 26 illustrates a method of contrast-enhanced computed tomography (CT) imaging. At step 2610, scanning can be at a first applied power while monitoring for an increase of attenuation above the threshold 2412. At step 2612, the applied power for each of a plurality of scans can be selected. The applied power can be varied among scans of a series during a session. The applied power can be determined, at least in part, by selection of an applied current. In some embodiments, the applied power can be varied by changing an applied voltage or a resistance. In some embodiments, the power for a first scan can be a maximum power applied during the session. In some embodiments, a current applied to a first scan can be about 200 ma. The applied power can be selected in some embodiments by multiplying a maximum current by an exponential function based on the attenuation determined from the preceding scan. In some embodiments, the exponential function can yield a value that is (i) greater than a minimum allowable current divided by a maximum allowable current, and (ii) less than 1. In some embodiments, the exponential function can be function F determined byF=eC(TH-ΔHU)/TH wherein TH is a threshold attenuation magnitude and ΔHU is equal to a difference in magnitude, in Hounsfield Units, between the attenuation determined from a preceding scan and a baseline attenuation. The baseline attenuation can be a magnitude of the attenuation indicated based on the initial scan. In some embodiments, C is selected such that, when the function is applied, an applied current for a next scan is about a tenth of the maximum allowable current when the attenuation of the preceding scan is about ten times above the threshold attenuation magnitude. In some embodiments, C can be about 0.25. FIG. 27 is an exemplifying plot, similar to FIG. 25, of magnitudes of radiation attenuation by various contrast-enhanced structures over time, and indicates a current magnitude for a plurality of scans. The vertical axis corresponds to attenuation density in Hounsfield Units. The horizontal axis corresponds to time in seconds. In FIG. 27, each large circle represents a scan and an applied current is identified for each large circle. As shown by FIG. 27, an applied power corresponding to a minimum allowable current can be selected for each scan for which a determining function, such as the function F for example, indicates, based on the attenuation indicated by a preceding scan, a current less than the minimum allowable current. The methods described herein for controlling contrast-enhanced computed tomography imaging can be implemented by computer system. For example, such a system can comprise an attenuation monitoring and a power control module. In some embodiments, the system can comprise a power control module in addition or alternative to a scanning-frequency control module. The power control module can be configured to select an applied power for each of a plurality of scans based on the attenuation detected from a preceding scan. The power control module can be configured to direct application of a maximum power applied during the session in a first scan. The power control module can be configured to apply substantially the same amount of power to individual scans until detection of an increase of the attenuation to or beyond a threshold attenuation magnitude. The power control module can be configured to select the applied power by multiplying a maximum current by an exponential function, such as described above. The statistics for perfusion curves can be improved with a number of slices used for a VOI. Thus, in some embodiments, a VOI can utilize more than one slice. Motion may occur between the reference scan and perfusion series. Registration can be performed for the tissues containing the VOIs. Motion correction can be performed between all images acquired in the series. In some embodiments, motion correction can significantly improve the quality and accuracy of the perfusion curves. For example, motion can occur because of breathing as indicated by the oscillations repeating approximately every three seconds in the LAD territory data of FIGS. 25 and 27. In some embodiments, motion control can permit further dose reduction while retaining statistical accuracy of the perfusion curves and achieving comparable diagnostic results. A kidney perfusion study was performed applying an embodiment of scan frequency control. Kidney perfusion data was acquired using a first protocol. In the first protocol, a low dose reference scan was performed. From the reference scan, an axial location and extent of the scan was identified for a perfusion series. The target ROIs were identified. A 60-second scan series was performed, with each scan being acquired with a tube current of 200 ma. One scan was performed each second for 60 seconds. Each scan was directed to a 8 cm circular ROI. FIG. 21 illustrates a slice obtained from a first person using the first protocol. From the slice shown in FIG. 21, various perfusion parameters were determined, including mean arterial transit time (MTTa), renal plasma flow (RPF), and glomerular filtration rate (GFR). These parameters are shown in Table 2A, below, as “fully-sampled data.” Sub-sampled data was obtained by applying a second (emulated) protocol to select a subset of data from the fully-sampled data. According to the second protocol, a low dose reference scan would be performed. From the reference scan, the axial location and extent of the scan would be identified as well as the arterial VOI used to define the motion of a transverse dynamic collimator. The target ROIs would be identified. The sub-sampled data was selected as though a 60-second scan series had been performed with the scanning frequency being adjusted during the series as data indicative of the arterial input function (AIF) was acquired. A sampling algorithm was applied in the second (emulated) protocol. The sampling algorithm adaptively varied the sampling along the arterial input function (AIF) which corresponds, to blood flow in the aorta. FIGS. 28 and 29 illustrate the adaptive sampling based on the AIF. The second protocol was applied to select the sub-sampled data from the fully-sampled data as though following steps had been preformed in capturing the data. A first scan took place at time t=0. Another scan was performed every two seconds after the first scan until the arterial curve rose above a predefined threshold TH, e.g. 35 HU. After the curve rose above the threshold, the slope of the curve was tracked using a finite difference. When the magnitude of the slope was found to have decreased, the interval between scans was reduced such that one scan was performed every second. When the peak of the arterial curve was detected by the value of the slope becoming negative, the scanning frequency was made one scan every 2 seconds. After the inflection point of descent of the curve was detected by the magnitude of the slope again decreasing, the scan interval was doubled after each subsequent scan. Very sparse sampling can be performed over the slowly descending exponential portion of the curve. At the end of a predetermined scan period, one final scan was performed to complete the sampling of the arterial curve. FIG. 29 illustrates the ROI 144 for the second (emulated) protocol. FIG. 28 includes curves 190, 192, 194 corresponding respectively to attenuation by cortex 186, aorta 188, and kidney tissue, shown in FIG. 29. FIG. 28 indicates 16 subsamples chosen out of 60. Each subsample is represented by a vertical line intersecting an “x.” FIG. 29 corresponds to a subsample 29-29 in FIG. 28. Current magnitudes are indicated in FIG. 28 for each subsample. The minimum current was 40 ma, occurring at the peak 2816 of the AIF curve, and the maximum current was 200 ma, used at the beginning of the scan. From the data obtained using the second (emulated) protocol, various perfusion parameters were determined, including mean arterial transit time (MTTa), renal plasma flow (RPF), and glomerular filtration rate (GFR). These parameters are shown in Table 2A, below, as “sub-sampled data.” The structures within the ROIs that were used to generate the curves used to calculate these parameters included the cortex of the kidney and aorta. The parameters derived from the original fully-sampled data are compared to those derived from the sparsely-sampled (frequency adjusted) data in Table 2A. The kidney perfusion study compared MTTa/RPF/GFR values between original fully-sampled values and values obtained using sub-sampled data. Table 2A summarizes the results of the comparison. Maximum deviation in MTTa|RPF|GFR values is 5%. TABLE 2AMTTa/RPF/GFRFully-sampled data8.5|144|34Sub-sampled data8.3|141|36 The slice obtained from the first person shown in FIG. 21 (Case I) and a slice obtained from a second person shown in FIG. 22C (Case II) were used to estimate the dosage reduction that would be obtained though use of sparse sampling, modulated tube current, and dynamic collimation to the ROI (an elliptical region surrounding both kidneys and aorta). Those estimates were compared to the radiation exposure without sparse sampling, modulated tube current, and dynamic collimation to the ROI. The overall exposure reductions were also calculated and compared. The dosage reduction from sub-sampling was determined based on application of the second (emulated) protocol, described above. The dosage reduction from dynamic collimation was calculated using the calculation techniques described above under the heading “Dynamic Transverse Collimation Dose Reduction.” The average exposure due to modulated tube current was used to determine the dosage reduction from modulation of tube current. For both Case I and Case II, a scan-exposure algorithm was applied to calculate the exposure reduction that would have been attributable to current modulation had the algorithm been applied during data acquisition. The scan-exposure algorithm would have adaptively varied the input current and, therefore, the emitted radiation during scanning FIGS. 25 and 27 illustrate adaptive scan-exposure control based on the AIF. Given a maximum and a minimum current (ma) allowed, exposure was calculated as though the following scan-exposure algorithm and procedure had been used to acquire the data. Scans were performed at the maximum current, e.g. 200 ma, until the arterial curve was detected to have risen above the threshold. Thereafter, the applied current was reduced, using the predetermined contrast detection threshold TH, by multiplying the maximum current by the following factor F:F=eC(TH−ΔHU)/TH where:C=0.25(minimum allowable ma)/(maximum ma)<F<1ΔHU=the difference of the Hounsfield Unit value of the curve and the baseline value corresponding to the first HU value of the curve. C was chosen such that if the arterial curve rose 10 times above the threshold TH (e.g., 35 HU), the current would be reduced by 0.1. For example, if the attenuation is 275 HU and the baseline is 100 HU (ΔHU=175), then the current would be decreased from a maximum of 200 ma to 40 ma. The applied x-ray current was determined for the remaining scans using this formula above for the rest of the series. Table 2B summarizes the exposure reductions calculated from evaluation of Case I and Case II. The average overall reduction for Case I and Case II is 10.5:1. TABLE 2BCase ICase IISub-sampling Exposure Reduction3.8:13.8:1Current Modulation Exposure1.5:11.5:1ReductionDynamic Collimation Reduction1.6:12.1:1Overall Reduction9.1:1 12:1 Based on the results of this comparison, the second protocol can provide diagnostic results comparable to the first protocol. For kidney perfusion scans, the radiation dose using the second protocol can be expected to be one-tenth of the radiation dose to the patient using the first protocol. FIG. 30 is a conceptual block diagram illustrating an example of a system, in accordance with various aspects of the subject technology. A system 3001 may be, for example, a client device or a server. The system 3001 may include a processing system 3002. The processing system 3002 is capable of communication with a receiver 3006 and a transmitter 3009 through a bus 3004 or other structures or devices. It should be understood that communication means other than busses can be utilized with the disclosed configurations. The processing system 3002 can generate audio, video, multimedia, and/or other types of data to be provided to the transmitter 3009 for communication. In addition, audio, video, multimedia, and/or other types of data can be received at the receiver 3006, and processed by the processing system 3002. The processing system 3002 may include a processor for executing instructions and may further include a machine-readable medium 3019, such as a volatile or non-volatile memory, for storing data and/or instructions for software programs. The instructions, which may be stored in a machine-readable medium 3010 and/or 3019, may be executed by the processing system 3002 to control and manage access to the various networks, as well as provide other communication and processing functions. The instructions may also include instructions executed by the processing system 3002 for various user interface devices, such as a display 3012 and a keypad 3014. The processing system 3002 may include an input port 3022 and an output port 3024. Each of the input port 3022 and the output port 3024 may include one or more ports. The input port 3022 and the output port 3024 may be the same port (e.g., a bi-directional port) or may be different ports. The processing system 3002 may be implemented using software, hardware, or a combination of both. By way of example, the processing system 3002 may be implemented with one or more processors. A processor may be a general-purpose microprocessor, a microcontroller, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Programmable Logic Device (PLD), a controller, a state machine, gated logic, discrete hardware components, or any other suitable device that can perform calculations or other manipulations of information. A machine-readable medium can be one or more machine-readable media. Software shall be construed broadly to mean instructions, data, or any combination thereof, whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. Instructions may include code (e.g., in source code format, binary code format, executable code format, or any other suitable format of code). Machine-readable media (e.g., 3019) may include storage integrated into a processing system, such as might be the case with an ASIC. Machine-readable media (e.g., 3010) may also include storage external to a processing system, such as a Random Access Memory (RAM), a flash memory, a Read Only Memory (ROM), a Programmable Read-Only Memory (PROM), an Erasable PROM (EPROM), registers, a hard disk, a removable disk, a CD-ROM, a DVD, or any other suitable storage device. Those skilled in the art will recognize how best to implement the described functionality for the processing system 3002. According to one aspect of the disclosure, a machine-readable medium is a computer-readable medium encoded or stored with instructions and is a computing element, which defines structural and functional interrelationships between the instructions and the rest of the system, which permit the instructions' functionality to be realized. In one aspect, a machine-readable medium is a non-transitory machine-readable medium, a machine-readable storage medium, or a non-transitory machine-readable storage medium. In one aspect, a computer-readable medium is a non-transitory computer-readable medium, a computer-readable storage medium, or a non-transitory computer-readable storage medium. Instructions may be executable, for example, by a client device or server or by a processing system of a client device or server. Instructions can be, for example, a computer program including code. An interface 3016 may be any type of interface and may reside between any of the components shown in FIG. 30. An interface 3016 may also be, for example, an interface to the outside world (e.g., an Internet network interface). A transceiver block 3007 may represent one or more transceivers, and each transceiver may include a receiver 3006 and a transmitter 3009. A functionality implemented in a processing system 3002 may be implemented in a portion of a receiver 3006, a portion of a transmitter 3009, a portion of a machine-readable medium 3010, a portion of a display 3012, a portion of a keypad 3014, or a portion of an interface 3016, and vice versa. FIG. 31 illustrates a simplified diagram of a system 3100, in accordance with various embodiments of the subject technology. The system 3100 may include one ore more remote client devices 3102 (e.g., client devices 3102a, 3102b, 3102c, and 3102d) in communication with a server computing device 3106 (server) via a network 3104. In some embodiments, the server 3106 is configured to run applications that may be accessed and controlled at the client devices 3102. For example, a user at a client device 3102 may use a web browser to access and control an application running on the server 3106 over the network 3104. In some embodiments, the server 3106 is configured to allow remote sessions (e.g., remote desktop sessions) wherein users can access applications and files on the server 3106 by logging onto the server 3106 from a client device 3102. Such a connection may be established using any of several well-known techniques such as the Remote Desktop Protocol (RDP) on a Windows-based server. By way of illustration and not limitation, in one aspect of the disclosure, stated from a perspective of a server side (treating a server as a local device and treating a client device as a remote device), a server application is executed (or runs) at a server 3106. While a remote client device 3102 may receive and display a view of the server application on a display local to the remote client device 3102, the remote client device 3102 does not execute (or run) the server application at the remote client device 3102. Stated in another way from a perspective of the client side (treating a server as remote device and treating a client device as a local device), a remote application is executed (or runs) at a remote server 3106. By way of illustration and not limitation, a client device 3102 can represent a computer, a mobile phone, a laptop computer, a thin client device, a personal digital assistant (PDA), a portable computing device, or a suitable device with a processor. In one example, a client device 3102 is a smartphone (e.g., iPhone, Android phone, Blackberry, etc.). In certain configurations, a client device 3102 can represent an audio player, a game console, a camera, a camcorder, an audio device, a video device, a multimedia device, or a device capable of supporting a connection to a remote server. In one example, a client device 3102 can be mobile. In another example, a client device 3102 can be stationary. According to one aspect of the disclosure, a client device 3102 may be a device having at least a processor and memory, where the total amount of memory of the client device 3102 could be less than the total amount of memory in a server 3106. In one example, a client device 3102 does not have a hard disk. In one aspect, a client device 3102 has a display smaller than a display supported by a server 3106. In one aspect, a client device may include one or more client devices. In some embodiments, a server 3106 may represent a computer, a laptop computer, a computing device, a virtual machine (e.g., VMware® Virtual Machine), a desktop session (e.g., Microsoft Terminal Server), a published application (e.g., Microsoft Terminal Server) or a suitable device with a processor. In some embodiments, a server 3106 can be stationary. In some embodiments, a server 3106 can be mobile. In certain configurations, a server 3106 may be any device that can represent a client device. In some embodiments, a server 3106 may include one or more servers. In one example, a first device is remote to a second device when the first device is not directly connected to the second device. In one example, a first remote device may be connected to a second device over a communication network such as a Local Area Network (LAN), a Wide Area Network (WAN), and/or other network. When a client device 3102 and a server 3106 are remote with respect to each other, a client device 3102 may connect to a server 3106 over a network 3104, for example, via a modem connection, a LAN connection including the Ethernet or a broadband WAN connection including DSL, Cable, T1, T3, Fiber Optics, Wi-Fi, or a mobile network connection including GSM, GPRS, 3G, WiMax or other network connection. A network 3104 can be a LAN network, a WAN network, a wireless network, the Internet, an intranet or other network. A network 3104 may include one or more routers for routing data between client devices and/or servers. A remote device (e.g., client device, server) on a network may be addressed by a corresponding network address, such as, but not limited to, an Internet protocol (IP) address, an Internet name, a Windows Internet name service (WINS) name, a domain name or other system name. These illustrate some examples as to how one device may be remote to another device. But the subject technology is not limited to these examples. According to certain embodiments of the subject technology, the terms “server” and “remote server” are generally used synonymously in relation to a client device, and the word “remote” may indicate that a server is in communication with other device(s), for example, over a network connection(s). According to certain embodiments of the subject technology, the terms “client device” and “remote client device” are generally used synonymously in relation to a server, and the word “remote” may indicate that a client device is in communication with a server(s), for example, over a network connection(s). In some embodiments, a “client device” may be sometimes referred to as a client or vice versa. Similarly, a “server” may be sometimes referred to as a server device or vice versa. In some embodiments, the terms “local” and “remote” are relative terms, and a client device may be referred to as a local client device or a remote client device, depending on whether a client device is described from a client side or from a server side, respectively. Similarly, a server may be referred to as a local server or a remote server, depending on whether a server is described from a server side or from a client side, respectively. Furthermore, an application running on a server may be referred to as a local application, if described from a server side, and may be referred to as a remote application, if described from a client side. In some embodiments, devices placed on a client side (e.g., devices connected directly to a client device(s) or to one another using wires or wirelessly) may be referred to as local devices with respect to a client device and remote devices with respect to a server. Similarly, devices placed on a server side (e.g., devices connected directly to a server(s) or to one another using wires or wirelessly) may be referred to as local devices with respect to a server and remote devices with respect to a client device. As used herein, the word “module” refers to logic embodied in hardware or firmware, or to a collection of software instructions, possibly having entry and exit points, written in a programming language, such as, for example C++. A software module may be compiled and linked into an executable program, installed in a dynamic link library, or may be written in an interpretive language such as BASIC. It will be appreciated that software modules may be callable from other modules or from themselves, and/or may be invoked in response to detected events or interrupts. Software instructions may be embedded in firmware, such as an EPROM or EEPROM. It will be further appreciated that hardware modules may be comprised of connected logic units, such as gates and flip-flops, and/or may be comprised of programmable units, such as programmable gate arrays or processors. The modules described herein are preferably implemented as software modules, but may be represented in hardware or firmware. It is contemplated that the modules may be integrated into a fewer number of modules. One module may also be separated into multiple modules. The described modules may be implemented as hardware, software, firmware or any combination thereof. Additionally, the described modules may reside at different locations connected through a wired or wireless network, or the Internet. In general, it will be appreciated that the processors can include, by way of example, computers, program logic, or other substrate configurations representing data and instructions, which operate as described herein. In other embodiments, the processors can include controller circuitry, processor circuitry, processors, general purpose single-chip or multi-chip microprocessors, digital signal processors, embedded microprocessors, microcontrollers and the like. Furthermore, it will be appreciated that in one embodiment, the program logic may advantageously be implemented as one or more components. The components may advantageously be configured to execute on one or more processors. The components include, but are not limited to, software or hardware components, modules such as software modules, object-oriented software components, class components and task components, processes methods, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. The foregoing description is provided to enable a person skilled in the art to practice the various configurations described herein. While the subject technology has been particularly described with reference to the various figures and configurations, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the subject technology. There may be many other ways to implement the subject technology. Various functions and elements described herein may be partitioned differently from those shown without departing from the scope of the subject technology. Various modifications to these configurations will be readily apparent to those skilled in the art, and generic principles defined herein may be applied to other configurations. Thus, many changes and modifications may be made to the subject technology, by one having ordinary skill in the art, without departing from the scope of the subject technology. It is understood that the specific order or hierarchy of steps in the processes disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the processes may be rearranged. Some of the steps may be performed simultaneously. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented. As used herein, the phrase “at least one of” preceding a series of items, with the term “and” or “or” to separate any of the items, modifies the list as a whole, rather than each member of the list (i.e., each item). The phrase “at least one of” does not require selection of at least one of each item listed; rather, the phrase allows a meaning that includes at least one of any one of the items, and/or at least one of any combination of the items, and/or at least one of each of the items. By way of example, the phrases “at least one of A, B, and C” or “at least one of A, B, or C” each refer to only A, only B, or only C; any combination of A, B, and C; and/or at least one of each of A, B, and C. Terms such as “top,” “bottom,” “front,” “rear” and the like as used in this disclosure should be understood as referring to an arbitrary frame of reference, rather than to the ordinary gravitational frame of reference. Thus, a top surface, a bottom surface, a front surface, and a rear surface may extend upwardly, downwardly, diagonally, or horizontally in a gravitational frame of reference. Furthermore, to the extent that the term “include,” “have,” or the like is used in the description or the claims, such term is intended to be inclusive in a manner similar to the term “comprise” as “comprise” is interpreted when employed as a transitional word in a claim. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. A reference to an element in the singular is not intended to mean “one and only one” unless specifically stated, but rather “one or more.” Pronouns in the masculine (e.g., his) include the feminine and neuter gender (e.g., her and its) and vice versa. The term “some” refers to one or more. Underlined and/or italicized headings and subheadings are used for convenience only, do not limit the subject technology, and are not referred to in connection with the interpretation of the description of the subject technology. All structural and functional equivalents to the elements of the various configurations described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and intended to be encompassed by the subject technology. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the above description. While certain aspects and embodiments of the invention have been described, these have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms without departing from the spirit thereof. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.

abstract

A beam line system includes a hollow tube and a plurality of protruding structures. The hollow tube has an inlet and an outlet. An ion beam emitted by the ion implanter is introduced into the hollow tube through the inlet and exited from the hollow tube through the outlet. The protruding structures are formed on an inner wall of the hollow tube. Each of the protruding structures has a reflective surface for reflecting a portion of the ion beam.

041586042

summary

This invention relates to power generating plant and has especial application to boiler systems for nuclear power stations. In order to cope with peak demands for electricity there is an increasing requirement for a power station having auxiliary generating capacity over and above the normal maximum continuous rating of the station. Such auxiliary capacity can be provided by gas turbine or diesel engine powered plant, but it would be convenient to associate at least some auxiliary generating plant with the heat source already present in the station. Further, where only large steam turbines are employed in a power station a degree of inflexibility in accommodating rapid changes in electricity demand exists because such turbines suffer considerable thermal stresses if subjected to rapid load changes. A further disadvantage lies in the fact that steam turbines are inefficient at low load so that it is undesirable to run a large steam turbine at low load in order to meet a considerably reduced demand for electricity. Finally, with some nuclear reactors, and in particular with the so-called High Temperature Reactor, it is desirable that the predominant flow of reactor coolant through the reactor core be downward. To simplify the reactor coolant circuit it is therefore convenient to pass the reactor coolant upwardly through associated boilers and so, in order to achieve counterflow within the boiler, water and steam must flow downwardly through the boiler. Owing to waterside instability such water downflow boilers could be difficult to operate at low flow rates which may be occasioned either by low load running of a large steam turbine to which they are connected, or by emergency use of such boilers as a heat sink for the reactor core. According to the invention there is provided power generating plant comprising a heat source, at least one main boiler arranged to be heated by heat from the heat source, and at least one main steam turbine arranged to be driven by steam generated in the said main boiler, wherein there is further provided at least one further boiler, of lower capacity than the said main boiler and also arranged to be heated by heat from the heat source, and at least one further steam turbine of lower capacity than the main steam turbine and arranged to be driven by steam generated in the said further boiler. Where the boiler system is used to supply steam to turbines which are each connected to a generator, the said further or auxiliary boiler(s) may be operated to enable the said further or auxiliary turbine(s) and generator(s) to provide auxiliary generating capacity over and above the normal maximum continuous rating of the main turbine and generator. Further, whilst the main boiler(s) are being operated to enable the main turbine to run under substantially constant load, the auxiliary boiler(s) may be operated to enable the auxiliary turbine(s), being smaller and thus more tolerant of rapid load changes, to run under varying load in order to accommodate changes in demand for electricity. The common heat source may advantageously be the core of a nuclear reactor, the boilers being disposed in a heat exchange circuit including the core such that reactor coolant fluid can flow through the core to abstract heat therefrom, and then through the boilers in which the heat is used to generate steam. Conveniently each boiler is disposed in a vertical channel or `pod` defined within the wall thickness of the reactor pressure vessel. To avoid operating the main boiler(s) at low flowrate when supporting a very considerably reduced load, it is possible to shut the main boiler(s) and turbine down and carry the load solely on the auxiliary boiler(s) and turbine(s). Thus the problem, mentioned above, of instability in a main boiler operating at low flowrate can be avoided since, in supporting the same load, the auxiliary boiler(s), being of smaller size, will operate at a considerably higher flowrate. Further, because of the smaller size of the auxiliary boiler(s), it is comparatively easy to arrange the heat exchange circuit such that reactor coolant flows downwardly through the auxiliary boiler(s), so enabling water flow therein to be upward. In order to maintain essential services in a power station during an emergency involving loss of normal power supplies, it is usual to provide emergency generating facilities. Such facilities are usually completely idle during normal running of the station. However, where a boiler system in accordance with the invention is associated with a nuclear heat source it becomes possible to dispense with at least part of the emergency generating facilities since, after emergency or even normal shut-down of the reactor core and the main boiler(s), the auxiliary boiler(s) can be operated to enable the auxiliary turbine(s) and generator(s) to run under at least part load. The auxiliary boiler(s) can continue operating in this manner for some considerable time simply by abstracting residual heat from the reactor core. This operating time can be maximised by designing the auxiliary boilers to produce steam at an appreciably lower temperature and pressure than the main boilers. To enable such continued operation of the auxiliary boiler(s) to occur a water feed and condensing system separate from that for the main boiler(s) is provided. It will be appreciated that a further advantage accrues from the use in such circumstances of a boiler system in accordance with the invention, in that, after shut-down of the reactor core, the core is effectively cooled by the auxiliary boiler(s) which act(s) as a core heat sink. The auxiliary boiler(s) can also be used for power generation during reactor and main boiler start-up.

056205368

claims

1. A method of manufacturing a composite cladding tube of a nuclear fuel element which is resistant to pellet-clad interaction, said composite cladding tube comprising an inner portion formed from a first component selected from the group consisting of zirconium and a zirconium alloy and an outer portion formed from a second component selected from the group consisting of Zircaloy 2, Zircaloy 4 and Zr 2.5 Nb, said method comprising the steps of: (a) providing an ingot of said first component, (b) forging rolling, extruding, and heat treating said ingot of said first component to form an inner billet, (c) positioning said inner billet from step (b) within an outer machined billet formed from an ingot of said second component, (d) extruding said joined billets from step (c) to form a joined tube blank, and (e) machining said tube blank from step (d) to provide said composite cladding tube, (f) said steps (b)-(e) being conducted at a temperature below that which causes incipient beta-phase transformation within said first component. 2. A method according to claim 1, including between steps (a) and (b) a step (a.sup.1) of preheating said ingot of said first component in an alpha-phase range temperature. 3. A method according to claim 2, wherein in step (b) said ingot of said first component is subjected to forging in two steps. 4. A method according to claim 2, wherein said first component consists essentially of zirconium-tin alloy comprising zirconium with 0.1-1% tin, less than 600 ppm iron and less than 600 ppm oxygen. 5. A method according to claim 4, wherein step (a.sup.1) is conducted at a temperature of 700.degree.-860.degree. C. 6. A method according to claim 4, wherein in step (b) said first component is heat treated at a temperature of 600.degree.-860.degree. C. 7. A method according to claim 6, wherein said temperature is 650.degree.-750.degree. C. 8. A method according to claim 2, wherein said first component consists essentially of zirconium. 9. A method according to claim 8, wherein step (a.sup.1) is conducted at a temperature of 700.degree.-800.degree. C. 10. A method according to claim 8, wherein in step (b) said first component is heat treated at a temperature of 600.degree.-800.degree. C. 11. A method according to claim 10, wherein said temperature is 650.degree.-750.degree. C. 12. A method according to claim 1, wherein step (d) is conducted at a temperature below 710.degree. C. 13. A method according to claim 1, wherein in step (b) said first component is rolled and extruded at a temperature below 710.degree. C. 14. A method according to claim 1, wherein step (e) includes cold rolling said tube blank, intermediate heating of said tube blank at a temperature of 525.degree.-700.degree. C., and a final heating of said tube blank at a temperature of 400.degree.-700.degree. C. 15. A method according to claim 1, including after step (d) a step (d.sup.1) of heat treating said tube blank at a temperature of 600.degree.-800.degree. C. 16. A method according to claim 15 including after step (d.sup.1) a step of beta-quenching said outer machined billet.

claims

1. A method of modulating ions comprising:(a) generating an area beam of ions directed along an axis and having a longitudinal and latitudinal extent in cross-section and divided into latitudinally adjacent beamlets of ions;(b) occluding the area beam with a set of latitudinally arrayed ion-blocking shutters each having a latitudinal width substantially equal to a latitudinal width of a beamlet, the ion-blocking shutters controllably extended to different longitudinal distances at the cross-section according to a desired intensity of a beamlet of ions to block only a controllable portion of the longitudinal extent of the beamlet and to pass without blocking a remaining portion of the beamlet so that the intensity of the beamlet is substantially nonuniform in a longitudinal direction, some of the different longitudinal distances at the cross-section being less than a length of the longitudinal extent of the area beam; wherein an average intensity of the beamlet is defined by a portion of the beamlet occluded by a given shutter; and(c) refocussing the occluded area beam with a lens system to collapsing the area beam and beamlets in the longitudinal direction to form a fan beam directable toward a patient for radiation therapy to provide a substantially uniform longitudinal intensity in each collapsed beamlet. 2. The method of claim 1 further including the step of occluding the fan beam with a set of latitudinally adjacent ion attenuating wedges controllably extended to different longitudinal distances according to a desired energy of a beamlet of ions defined by a portion of the area beam occludable by a given wedge. 3. The method of claim 1 wherein each wedge is matched to a corresponding wedge providing mirror movement to the wedge to present a uniform thickness of material within the fan beam. 4. The method of claim 1 wherein step of the collapsing the area beam passes the area beam through a pair of quadrupole lenses aligned in rotation with each other about the axis. 5. The method of claim 1 wherein the step of generating the area beam passes a pencil beam through a scattering foil to form the area beam. 6. The method of claim 1 wherein the area beam is a proton beam. 7. The method of claim 1 including the step of rotating the fan beam with respect to the patient about a longitudinal axis while controllably varying the longitudinal distances of the shutters. 8. A modulator for ions comprising:(a) a beam generator generating an area beam of ions directed along an axis and having a longitudinal and latitudinal extent in cross-section composed of latitudinally adjacent beamlets of ions;(b) an intensity modulator occluding the area beam with a set of latitudinally arrayed ion-blocking shutters each having a latitudinal width substantially equal to a latitudinal width of a beamlet, the ion-blocking shutters controllably extended to different longitudinal distances at the cross-section according to a desired intensity of a beamlet of ions to block only a controllable portion of the longitudinal extent of the beamlet and to pass without blocking a remaining portion of the beamlet so that the intensity of the beamlet is substantially nonuniform in a longitudinal direction, some of the different longitudinal distances at the cross-section being less than a length of the longitudinal extent of the area beam; wherein an average intensity of the beamlet is defined by a portion of the beamlet occluded by a given shutter; and(c) a lens system receiving the area beam from the intensity modulator, the lens system collapsing the area beam and beamlets in the longitudinal direction to form a fan beam directable toward a patient for radiation therapy to provide a substantially uniform longitudinal intensity in each collapsed beamlet. 9. The modulator of claim 8 further including an energy modulator having a set of latitudinally adjacent ion attenuating wedges controllably extended to different longitudinal distances according to a desired energy of a beamlet of ions defined by a portion of the area beam occludable by a given wedge. 10. The modulator of claim 8 wherein each wedge is matched to a corresponding wedge providing minor movement to the wedge to present a uniform thickness of material within the fan beam. 11. The modulator of claim 8 wherein the lens system is a pair of quadrupole lenses aligned in rotation about the axis. 12. The modulator of claim 8 wherein the beam generator is a scattering foil receiving a pencil beam to spread it into an area beam. 13. The modulator of claim 8 wherein the beam generator generates a proton beam. 14. The modulator of claim 8 including a gantry holding the modulator for rotation of the fan beam with respect to the patient about a longitudinal axis while controllably varying the longitudinal distances of the shutters.

summary

062401580

claims

1. An X-ray projection exposure apparatus, comprising: an X-ray source that generates X-rays; a mask stage configured to hold a mask having a mask pattern; an X-ray illumination optical system that directs the X-rays generated by the X-ray source towards the mask to project the mask pattern; a substrate stage configured to hold a substrate; an X-ray projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern onto the substrate, the X-ray projection focusing optical system including a plurality of reflective mirrors that reflect the X-rays; and a position detection optical system that optically detects a mark on the mask and a mark on the substrate, at least a portion of the position detection optical system being disposed between the plurality of reflective mirrors. an X-ray source that generates X-rays; an illumination optical system that directs the X-rays generated by the X-ray source towards a mask having a mask pattern; a substrate stage configured to hold a substrate; a projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern onto the substrate, the projection focusing optical system including a plurality of reflective mirrors that reflect the X-rays; and a position detection device that optically detects a position of the substrate in a direction substantially parallel to an optical axis of the projection focusing optical system, at least a portion of the position detection device being disposed between the plurality of reflective mirrors. An X-ray source that generates X-rays; an illumination optical system that directs the X-rays generated by the X-ray source towards a mask having a mask pattern; a substrate stage configured to hold a substrate; a projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern on the substrate, the projection focusing optical system including a plurality of reflective mirrors that reflect the X-rays; and a position detection mechanism that optically detects a position of the substrate in a direction substantially parallel to an optical axis of the projection focusing optical system, wherein a reflective mirror that is closest to the substrate among the plurality of reflective mirrors has a space which allows the passage of light by which the position detection mechanism detects the position of the substrate. an X-ray source that generates X-rays; an illumination optical system that directs the X-rays generated by the X-ray source towards a mask having a mask pattern; a substrate stage configured to hold a substrate; a projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern onto the substrate, the projection focusing optical system including a plurality of reflective mirrors that reflect the X-rays, the projection focusing optical system further including a holder that holds a reflective mirror that is closest to the substrate; and a position detection mechanism that optically detects a position of the substrate in a direction substantially parallel to an optical axis of the projection focusing optical system, wherein the holder has a space in its surface facing the substrate, the space allowing the passage of light by which the position detection mechanism detects the position of the substrate. an X-ray source that generates X-rays; a mask stage configured to hold a mask having a mask pattern; an X-ray illumination optical system that directs the X-rays generated by the X-ray source towards the mask; a substrate stage configured to hold a substrate; an X-ray projection focusing optical system that receives the X-rays that have interacted with the mask and projects and focuses an image of the mask pattern onto the substrate, the X-ray projection focusing optical system including a plurality of reflective mirrors that reflect X-rays, a reflective mirror closest to the substrate stage being adjacent the substrate stage; and a position detection optical system that optically detects a position of the substrate, wherein the X-ray projection focusing optical system is configured to accommodate at least a portion of the position detection optical system disposed between the plurality of reflective mirrors. wherein the reflective mirror closest to the substrate stage is configured to provide a passage for the optical path of the detection light. 2. The X-ray projection exposure apparatus according to claim 1, wherein a portion of the position detection optical system is disposed between a reflective mirror closet to the substrate and a reflective mirror second closest to the substrate among the plurality of reflective mirrors in the X-ray projection focusing optical system. 3. The X-ray projection exposure apparatus according to claim 1, wherein the position detection optical system includes an illumination optical system and a detection optical system, the illumination optical system illuminating the mark on the mask with light, the light reflected from the mark on the mask being guided towards the mark on the substrate via at least one of the reflective mirrors in the X-ray projection focusing optical system, the detection optical system detecting the light from the mark on the substrate. 4. The X-ray projection exposure apparatus according to claim 1, wherein the position detection optical system includes an illumination optical system and a detection optical system, the illumination optical system illuminating the mark on the substrate with light, the light reflected from the mark on the substrate being guided towards the mark on the mask via at least one of the reflective mirrors in the X-ray projection focusing optical system, the detection optical system detecting the light from the mark on the mask. 5. The X-ray projection exposure apparatus according to claim 1, wherein the position detection optical system includes a motion mechanism. 6. The X-ray projection exposure apparatus according to claim 1, wherein the numerical aperture of the position detection optical system is equal to or less than one half of the numerical aperture of the X-ray projection focusing optical system. 7. The X-ray projection exposure apparatus according to claim 1, wherein the portion of the position detection optical system that is disposed between the plurality of reflective mirrors includes a half-mirror. 8. The X-ray projection exposure apparatus according to claim 1, wherein the position detection optical system includes a temperature adjustment mechanism. 9. An X-ray projection apparatus, comprising: 10. The X-ray projection exposure apparatus according to claim 9, wherein at least a portion of the position detection device is disposed between a reflective mirror closest to the substrate and a reflective mirror second closest to the substrate among the plurality of reflective mirrors. 11. The X-ray projection exposure apparatus according to claim 9, wherein a through-hole is formed in a reflective mirror closest to the substrate to provide a passage for light by which the position detection device detects the position of the substrate. 12. The X-ray projection exposure apparatus according to claim 9, wherein the position detection device includes a motion mechanism. 13. The X-ray projection exposure apparatus according to claim 9, wherein the position detection device includes a temperature adjustment mechanism. 14. An X-ray projection exposure apparatus comprising: 15. The X-ray projection exposure apparatus according to claim 14, wherein the space which allows the passage of the light of the position detection mechanism includes a tapered part formed in the reflective mirror that is closest to the substrate. 16. The X-ray projection exposure apparatus according to claim 14, wherein the space which allows the passage of the light of the position detection mechanism includes at least one of a groove and a through-hole formed in the reflective mirror that is closest to the substrate. 17. An X-ray projection exposure apparatus, comprising: 18. The X-ray projection exposure apparatus according to claim 17, wherein the space, which allows the passage of the light of the position detection mechanism, includes a tapered part formed in the holder. 19. The X-ray projection exposure apparatus according to claim 17, wherein the space, which allows the passage of the light of the position detection mechanism, includes at least one of a groove and a through-hole formed in the holder. 20. An X-ray projection exposure apparatus, comprising: 21. The X-ray projection exposure apparatus according to claim 20, wherein the position detection optical system detects the position of the substrate in a direction substantially parallel to an optical axis of the X-ray projection focusing optical system. 22. The X-ray projection exposure apparatus according to claim 20, wherein the position detection optical system detects the position of the substrate relative to a position of the mask. 23. The X-ray projection exposure apparatus according to claim 20, wherein the position detection optical system detects the position of the substrate in a direction substantially perpendicular to an optical axis of the X-ray projection focusing optical system. 24. The X-ray projection exposure apparatus according to claim 20, wherein the portion of the position detection optical system includes a retractable optical element to be inserted between two of the plurality of reflective mirrors upon detecting the position of the substrate. 25. The X-ray projection exposure apparatus according to claim 24, wherein when inserted, the retractable optical element is positioned between the reflective mirror closest to the substrate stage and the reflective mirror second closest to the substrate stage. 26. The X-ray projection exposure apparatus according to claim 24, wherein when the retractable optical element is inserted, the position detection optical system utilizes at least one of the reflective mirrors in the X-ray projection focusing optical system to detect the position of the substrate. 27. The X-ray projection exposure apparatus according to claim 20, wherein the portion of the position detection optical system to be accommodated by the X-ray projection focusing optical system includes an optical path for a detection light by which the position detection optical system detects the position of the substrate, and 28. The X-ray projection exposure apparatus according to claim 27, wherein the reflective mirror closest to the substrate stage has a tapered portion providing the passage for the optical path of the detection light. 29. The X-ray projection exposure apparatus according to claim 27, wherein the reflective mirror closest to the substrate stage has a through-hole providing the passage for the optical path of the detection light.

044407157

abstract

A nuclear reactor is supplied with feed water through a feed water pump system. A primary steam flow produced from the reactor is controlled by regulating a recirculated flow of feed water. The feed water pump system comprises two main pumps each of 55%-capacity and two auxiliary pumps each of 27.5%-capacity. Normally, the two main pumps are operated. Upon occurrence of abnormal condition of at least one main pump, the auxiliary pumps are started to supply feed water. At that time, the recirculated flow is controlled for a predetermined time to a reduced rate which is smaller as compared with that of the primary steam flow decreased rapidly due to the shutdown of the main pump. Subsequently, the recirculated flow is so controlled that the primary steam flow rate is slightly smaller as compared with the feed water flow which is determined by the available capacity of the pumps.

052308588

description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a fuel bundle B is illustrated both partially broken away and cut off in length so that representative portions of the fuel bundle can be seen and understood. Fuel bundle B includes a lower tie plate L forming the upper portion of nose piece N which commonly communicates across a core separation plate (not shown) for receiving water from the lower portion of a reactor (also not shown). Upper tie plate U captures the upstanding end of a group of fuel rods R that extend the full length of fuel bundle B. It will be noted that some fuel rods are partial length, terminating at less than the full distance to upper tie plate U. Centrally of fuel bundle B there is water rod W. (In some designs more than one may be used.) Water rod W has upper compartment 14 with upwardly exposed open end 15. It is into this open end 15 that liquid moderator (water) falls during normal operation of the fuel bundle to fill the upper compartment 14. Lower compartment 17 is supplied with lower entrance apertures 18 and upper exit aperture 20. Water from the bottom of fuel bundle B flows through water rod W at lower compartment 17 and exits at exit apertures 20. In FIG. 1, such exit is shown between the first and second spacers S1, S2. Referring to FIG. 2, a schematic of the invention is illustrated with the schematic being compressed in the vertical direction. Fuel bundle B includes lower tie plate L, upper tie plate U with water rod W extending therebetween. Water rod W has upper compartment 14 with upward opening 15. Water rod W further includes lower compartment 17 with entrance apertures 18 and discharge apertures 20. All seven spacers S1-S7 are illustrated with discharge of the discharges apertures occurring in accordance with the preferred embodiment of this invention between spacers S2-S3. There is a practical limitation on the length of upper compartment 14. If this compartment becomes too long, it may become the site of unstable flow or "chugging." In this case upward flowing steam at the tube exit will be too great to permit water flow down into the tube. As the completely voided condition is approached, the steam flow will decrease and liquid flow will begin again. This cycle of volume and refilling will continue endlessly. This flouroscillation would not be desired and would defeat the nuclear characteristics of the water rod W. Therefore, we preferred to limit the length of upper compartment 14 to about 1/4 to 1/3 the total length of the fuel bundle depending on the counter current flow characteristics of the particular water rod design. Referring to FIG. 3, mention has been made that the upper two phase region is a portion of the fuel bundle B where the thickness of the liquid film coating the center of the fuel bundle B narrows as the upwardly flowing moderator passes along the fuel rods R between the spacers S3, S2, and S1. Liquid and vapor moderator passes upwardly in the direction of arrow 40. Thickness of the vapor film overlying the central fuel rods R is schematically shown lines T1, T2, and T3. Some remarks may be made relative to the graphic illustration. First, and relative to line T1, enriched nuclear fuel typically ends near the first spacer. As the line Tl suggests, thickness of the water film over the central rods is generally not a problem in these locations. Secondly, and with respect to lines T2 and T3, thickness of liquid film over the central fuel rods can be a problem between spacers S1 and S2 or S2 and S3. At flow rates up to in the order of 75%, maximum thinning of the film will typically occur just upstream of spacer S1. As flow increases above 75% flow rate, maximum thinning of the film will typically occur just upstream of spacer S2. Since spacer location may be vertically variable, optimal placement of discharge apertures 20 will be a matter of design.

041586026

summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention generally relates to control rod assemblies for nuclear reactors and, more particularly, to the operative connections between control rods and control rod drive mechanisms. 2. Description of the Prior Art One of the most serious accidents that can occur to a nuclear power plant is a loss of the flow of coolant followed by the failure of the control system to accomplish a rapid shutdown of the reactor. A loss of coolant flow can occur from either the rupture of piping or the stoppage of one or more of the coolant circulating pumps. This type of accident is especially serious because the heat generated in the reactor cannot be carried off. If the reactor continues to generate heat, then tremendous pressures are built up in the coolant system. In addition, this heat generation, if it is not terminated by a scram, could melt down the majority of the core of the reactor. In the reactors using liquid sodium for primary coolant, there is a special problem caused by a partial or total loss of sodium flow if reactor scram does not follow promptly. In the present design of liquid metal fast breeder reactors there is a gain in reactivity called a positive sodium void coefficient that occurs when sodium flow is interrupted. The sodium temperature may increase to its boiling point, whereupon "voids" of sodium vapor are formed, resulting in increased reactivity, power, more boiling, and the possibility of serious consequences. This gain in reactivity occurs because although the neutron absorption effect of sodium is small, it is not zero. Any loss of sodium from the core causes a shift in the neutron absorption spectrum and increases the number of neutrons. This shift, in turn, increases the probability of neutron capture by the fissionable atoms in the fuel. Many organizations, government agencies, and corporations have studied the problem of minimizing the probability of the loss of low accident. A considerable design effort has been expended over a priod of many years in order to provide maximum assurance to both the public and the various reactor licensing agencies that this accident can be avoided. Heretofore, the most reliable and simplest system that has been proposed contemplates hydraulically supporting a plurality of tantalum absorber balls in a column above the reactor core by the flow of primary coolant. In the event coolant flow is reduced, these balls which have a large neutron absorption coefficient fall into the high flux region of the core and quickly shut down the reactor. Although the use of hydraulically supported, absorber balls provides a self-actuated and reliable reactor shut-down system, there are many inherent limitations with this system. For example, the absorber balls are hydraulically supported by the flow of primary coolant in a position substantially above the high flux region of the core. In this position the balls cannot be maneuvered to regulate the amount of neutron flux during normal operations. Secondly, the absorber balls are only controlled by the flow of primary coolant through the column. Therefore, the balls add reactivity to the system whenever the main coolant pumps are started and primary coolant flow is commenced. Further, the hydraulically supported absorber balls only shut down the reactor when the flow of primary coolant decreases. The balls cannot be commanded to scram a reactor by either the reactor operator or one of the other reactor safety systems. Finally, considerable testing will be necessary to demonstrate the exact response of the tantalum balls under actual reactor conditions of flow and wear. SUMMARY OF THE INVENTION It is an object of the present invention to provide a novel method and apparatus that overcomes the limitations and disadvantages of the prior art. A further object of the present invention is to develop a reactor shut-down system that can provide maximum assurance to both the public and the various reactor licensing agencies that the system will in fact operate during any emergency situation. Another object of the present invention is to provide a system for scramming a reactor that is directly initiated by a loss of coolant flow and a self-actuated system that will operate without electrical circuits, sensors, or rod drive mechanisms. An additional object of the present invention is to develop a self-actuated, loss of flow scramming system that can also be commanded through a conventional rod drive mechanism to initiate a scram. Still another object of the present invention is to provide a self-actuated, loss of flow scramming system that can be used to control the level of neutron flux during normal power operation, especially normal reactor start-ups and shut-downs. A further object of the present invention is to provide a system that will reliably scram a reactor during a severe earthquake. The foregoing and other objects are achieved by a control rod assembly that includes a separator plate having an orifice through which primary coolant flow and across which a differential pressure may be developed. Around the orifice is located a sealing surface that can be engaged by a control rod. The control rod can be independently moved with respect to the core of the reactor and can also be brought up and into engagement with the separator plate. The differential pressure across the separator plate and the control rod seal retains the sealing surfaces of the control rod and the separator plate together. In operation, normal flow through the reactor develops sufficient differential pressure across the separator plate and the control rod seal to retain the control rod against the separator plate and above the high neutron flux region of the core. When the flow of primary coolant is reduced or the differential pressure across the separator plate decreases sufficiently, the control rod falls by gravity and the reactor is scrammed. The reactor can also be scrammed on command by the downward motion of the rod drive shaft which causes the sealing surfaces to separate. During a severe earthquake the reactor will be scrammed because a lateral motion of the control rod with respect to the separator plate brought about by different inertia of the separator plate and the control rod will cause the sealing surfaces to separate. Additional objects and features of the invention will appear from the following description in which the preferred embodiment has been set forth in conjunction with the accompanying drawings.

039649687

claims

1. A fuel assembly for a nuclear reactor, comprising: a housing; fuel elements accommodated within said housing; a lateral surface of each of said fuel elements; spacer members, each of which is formed as a bunch of wires helically arranged on said lateral surface of at least some of said fuel elements; planes of contact, wherein said spacer members adjoin adjacent said fuel elements relative to those said fuel elements on which said spacer members are disposed; each of which said wires of said bunch of wires adjoins at least two adjacent wires along the entire length thereof; all said wires being rigidly interconnected between said planes of contact.

description

This application claims the benefit of U.S. Provisional Application Ser. No. 61/161,964 filed 20 Mar. 2009, entitled “Sample Stage Apparatus for Reducing Energy of Electrons in an Annular-Acceptance Analyzer System,” which is incorporated herein by reference in its entirety. The disclosure herein relates generally to analyzer systems and methods, e.g., annular-acceptance analyzer systems. More particularly, the disclosure herein pertains to sample holder apparatus for such systems and methods (e.g., that may be mounted on an analyzer instrument stage). It has long been recognized that the energy resolution of a cylindrical mirror analyzer (CMA), or other analyzers with annular acceptance (whether full 360-degree azimuthal acceptance or partial, that is, less than 360 degree azimuthal acceptance), is determined by the geometry of the analyzer. In such an instrument, (ΔE)/E is equal to a constant determined by the geometry of the instrument (where E is the kinetic energy of electrons passing through the analyzer in electron·volt (eV) and ΔE is the energy width (i.e., the analyzer “slit width”) or the range of electron energies passed by the analyzer around and at E). Furthermore, it has been recognized that if the energy of electrons entering an annular-acceptance analyzer (such as a CMA) is reduced, the effective resolution of the instrument is increased. U.S. Pat. No. 3,699,331, entitled “Double Pass Coaxial Cylinder Analyzer with Retarding Spherical Grids,” discloses a device and method for reducing the energy of electrons entering a CMA using a retarding grid assembly constructed of two concentric spherical sections. One disadvantage of this approach is the loss in transmission due to two effects. First, the physical transparency (typically 60-90% each) of the grids limits the number of electrons transmitted. Second, when an electric field terminates on a grid of finite mesh, the equipotential surfaces are rippled close to the grid. Each grid opening acts as a lens which causes the transmitted electrons to be scattered from their original trajectory. The effect of grid scattering is to reduce transmission. A second disadvantage of this approach is the aberrations introduced by the non-sphericity of the fine grids (which are easily distorted during manufacture and assembly) and the non-concentricity of the two grid sections. A third disadvantage of this approach is the possibility of grid contamination and consequent need for grid replacement. A final disadvantage of this approach is the necessity of electrically floating the CMA, electron detector, and associated electronics. Japan Pat. Appl. No. JP2006-302689A, entitled “Auger Electron Spectral Analysis Device and Auger Electron Spectral Analysis Method,” discloses a method that applies positive bias voltage to a sample electrode holder and alleges that, with this, kinetic energy of Auger electrons is reduced and high-resolution spectra are obtained. There exists a need to reduce the energy of electrons entering a CMA without incurring the disadvantages caused by the use of grids for pre-retardation. In one or more embodiments described herein, the energy of electrons entering a CMA (or other analyzer with annular acceptance) is reduced without the use of grids. Further, in one or more embodiments, this is accomplished without modification to the analyzer or other critical components of the spectrometer. Rather, one or more embodiments described herein provide a sample holder apparatus (e.g., mountable on an instrument stage or provided as part of an instrument stage) to reduce the energy of electrons entering an annular-acceptance analyzer (e.g., a CMA) without the use of grids. The sample holder apparatus may include a grounded sample aperture member and an electrically isolated sample support member to which is applied a positive bias potential. The combination of the grounded sample aperture member along with the positive bias potential applied to the sample support member and electrically connected sample produces an electrical retarding field that reduces the energy of the electrons before they enter the analyzer. At least in one or more embodiments, due to the shape of the field, which is substantially planar near the sample surface and substantially spherical farther from the sample surface, the trajectories of said electrons are bent outward from the optical axis such that they fill the entrance to the analyzer and enter with the desired range of input angles. One or more embodiments of a sample holder apparatus for reducing the energy of charged particles entering an annular-acceptance analyzer (e.g., a cylindrical mirror analyzer (CMA)) described herein includes an electrically isolated sample support member having a sample receiving surface configured to receive a sample and electrically connect the sample to the sample support member. The sample support member is configured for application of a retarding bias potential (e.g., a positive retarding bias). Further, the sample holder apparatus includes a grounded sample aperture member defining an aperture (e.g., a circular aperture), wherein the grounded sample aperture member is positioned relative to the sample support member but electrically isolated therefrom such that the aperture is proximate the sample receiving surface to expose at least a portion of a surface of a sample received thereon to be analyzed. The grounded sample aperture member along with the sample support member are configured to produce an electrical retarding field about the aperture when a retarding bias potential is applied thereto that reduces the energy of emitted particles from a sample before they enter the annular-acceptance analyzer. In one or more embodiments, the grounded sample aperture member along with the sample support member are configured to produce an electrical retarding field about a circular aperture when a retarding bias potential is applied thereto that reduces the energy of emitted particles from a sample before they enter the annular-acceptance analyzer and further that modifies the trajectories of such emitted particles such that they enter the annular-acceptance analyzer in a predetermined range of input elevation angles. One or more embodiments of a method for reducing the energy of charged particles entering an annular-acceptance analyzer (e.g., a cylindrical mirror analyzer (CMA)) as described herein may include providing an electrically isolated sample support member having a sample receiving surface configured to receive a sample and electrically connect the sample to the sample support member (e.g., the sample support member is configured for application of a retarding bias potential). Further, the method may include positioning a grounded sample aperture member defining an aperture relative to the sample support member but electrically isolated therefrom such that the aperture (e.g., a circular aperture) is proximate the sample receiving surface to expose at least a portion of a surface of a sample received thereon to be analyzed (e.g., the surface of the sample may be above, flush or below the plane in which the aperture lies). Still further, the method may include applying a retarding bias potential (e.g., a positive bias potential) to the sample support member to produce an electrical retarding field about the aperture (e.g., a retarding field about the aperture that includes a planar portion proximate the surface of the sample to be analyzed and a more spherical portion farther from the surface of the sample to be analyzed) that reduces the energy of emitted particles from a sample before they enter the annular-acceptance analyzer. In one or more embodiments of the method, applying a retarding bias potential to the sample support member may include applying a retarding bias potential to the sample support member to produce an electrical retarding field about the aperture that reduces the energy of emitted particles from a sample before they enter the annular-acceptance analyzer and further that modifies the trajectories of such emitted particles such that they enter the annular-acceptance analyzer in a predetermined range of input, elevation angles. One or more embodiments of an analyzer system for use in analyzing a sample are also provided herein. The system may include an analyzer apparatus defining a full or a partial annular-acceptance input opening to receive emitted particles from the sample being analyzed. Further, the system includes an electrically isolated sample support member having a sample receiving surface configured to receive a sample and electrically connect the sample to the sample support member (e.g., wherein the sample support member is configured for application of a retarding bias potential). Still further, the system may include a grounded sample aperture member defining an aperture. At least a portion of the grounded sample aperture member is positioned between the annular-acceptance input opening of the analyzer apparatus and the sample support member such that the aperture is proximate the sample receiving surface to expose at least a portion of a surface of a sample received thereon to be analyzed (e.g., the sample support member is electrically isolated from the grounded sample aperture member). Yet further, the grounded sample aperture member along with the sample support member are configured to produce an electrical retarding field about the aperture when a retarding bias potential is applied thereto that reduces the energy of emitted particles from a sample before they enter the annular-acceptance input opening. The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings. The figures are rendered primarily for clarity and, as a result, are not necessarily drawn to scale. In the following detailed description of illustrative embodiments, reference is made to the accompanying figures of the drawing which form a part hereof, and in which are shown, by way of illustration, specific embodiments which may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the disclosure. In the following description, the polarities of the applied potentials are chosen for the analysis of negatively-charged particles, and in the embodiments of FIGS. 1-14 the charged particles are assumed to be electrons. It will, of course, be appreciated that positively-charged particles may be analyzed by reversing the polarities of the applied potentials. The present description is with regard to a sample holder apparatus for reducing the energy of electrons entering an annular-acceptance analyzer, such as a CMA. For example, the analyzer may be part of a system used to analyze the chemical state of a solid surface, such as an Auger electron spectroscopy (AES) system. Instrumentation for use in spectroscopy of charged particles makes use of electrons or ions which are emitted from a substance after being bombarded or irradiated with electrons or ions from a source such as an electron gun, e.g., such as in Auger electron spectroscopy. For example, in one embodiment of this technique, a target sample material is placed in a vacuum, and upon being bombarded with electrons from some source, such as an electron gun, the sample gives off a variety of emissions. Among these are X-rays, secondary electrons, and reflected primary electrons from the source. The emissions include Auger electrons (a particular class of secondary electrons) having a distribution of electron energies and a range of particle trajectories. In the art of Auger electron spectroscopy, as taught for example in U.S. Pat. No. 4,205,226 (Gerlach), instruments making use of cylindrical mirror analyzers (“CMA”) are known which analyze the energy and the energy spectrum of Auger electrons emitted by the sample material. Such instruments operate by introducing the diverging electrons into a radial electric field produced between a pair of coaxially mounted electrode cylinders held at different electric potentials. Auger electrons injected from the sample into the radial electric field between the cylindrical electrodes are deflected by the field back toward the common axis of the electrodes. Electrons of a predetermined energy are thereby brought to a focus. By positioning a collector apparatus at this focus, electrons of a predetermined energy are selected and detected. By sweeping the voltage impressed across the cylindrical electrodes through a range of values, and detecting as a function of these applied potentials such electrons as are collected, the energy spectrum of the injected electrons may be plotted and determined. One or more embodiments of the sample holder apparatus described herein is beneficial to enhance Auger electron spectroscopy energy resolution, or improve the energy resolution of the Auger electron spectrum measured by a cylindrical mirror analyzer (CMA) or any other analyzer with annular acceptance (whether full 360-degree azimuthal acceptance or partial, that is, less than 360 degree azimuthal acceptance). For simplicity herein, when the term CMA is used herein it shall refer to any of such types of analyzers, including those with annular acceptance (whether full 360-degree azimuthal acceptance or partial, that is, less than 360 degree azimuthal acceptance). In other words, even though CMA is a type of annular-acceptance analyzer, the terms CMA and annular-acceptance analyzer are used interchangeably herein. Further, as used herein annular-acceptance or annular-acceptance used with terms such as input, entrance, openings, etc. refers to any annular region or portion of an annular region defined by an analyzer for acceptance of particles to be analyzed (see, for example, the gap 42 defined in CMA 30 of system 10 shown in FIG. 1). The standard equation, ΔE/E=c, expresses the energy resolution of the analyzer as a ratio involving E, the kinetic energy of electrons passing through the analyzer in electron·volt (eV) and ΔE, the energy width (i.e., the analyzer “slit width”) or the range of electron energies passed by the analyzer around and at E; c is a constant determined by the design and operational characteristics of the analyzer and, for a CMA, c is typically about 0.5% by design. From the energy resolution expression above, it follows that there are two ways to increase the energy resolution. The first is to decrease the constant c of the analyzer. Reduction of the constant c by redesign of the analyzer is to be avoided and, in fact, may not be practical for optical reasons. However, second, if the natural kinetic energy, E0, of electrons of a given spectral feature (peak) can be reduced by a multiplicative factor, f, to kinetic energy Ef=E0·f before they enter the analyzer, then the corresponding analyzer energy window, ΔE0, through which those electrons would pass, is reduced by the same factor to ΔEf=ΔE0·f and sinceΔEf/Ef=c thenΔEf/E0=c·f=cf where, cf is the effective analyzer energy resolution for a spectral feature at natural kinetic energy, E0. Effective analyzer energy resolution means that the Auger spectral feature naturally appearing at energy, E0, will appear in the measured spectrum with the energy resolution that would be achieved had the measurement been made on an analyzer with designed energy resolution, cf (i.e., the designed energy resolution depending on the design of the instrument). The reduction in the natural kinetic energy, E0, can be made by applying a positive, DC electrical bias, VB, to the sample, as mentioned above and f is then given by the ratio of the natural and reduced kinetic energiesf=Ef/E0 whereEf=E0−eVB and, VB is the bias voltage, e is the charge on the electron and, in all cases, eVB<E0. Meaningful application (e.g., application that results in enhanced analyzer energy resolution) of a positive, DC electrical bias, VB, to the sample can be accomplished using a sample holder apparatus as generally and more specifically described herein. The energy resolution of the chemical element's measured Auger spectrum is improved by some amount that is dependent on this effect, the natural linewidth of the Auger peak when it is not distorted by any analyzer, and possibly other effects yet to be determined. Note that the energy scale on the measured spectrum must then be shifted back by +eVB for display purposes, therefore, the bias voltage, VB, must be known accurately, say to less than 0.1 eV. In other words, a system that uses an applied positive DC electrical bias, VB, to the sample, may include software that is configured to receive an input corresponding to the bias voltage, VB, and then manipulate the data representative of the measured spectrum based on the bias voltage such that the energy scale on the measured spectrum is shifted back by +eVB and the measured spectrum is properly displayed, or otherwise communicated to a user (e.g., printed, graphically or visually provided, etc.). For example, in one or more embodiments, the proper spectrum display, or other communicated spectrum, can be achieved by data processing software employing the following or similar steps: 1. Read the stored spectral values from the data-file; 2. Read the corresponding stored energy values from the data-file; 3. Read the stored bias voltage value from the data-file; 4. Produce the offset energy values by adding the bias voltage, VB to the energy values; 5. Optionally, display the spectral values and the offset energy values on a display, such as an (x,y) graph. In one or more embodiments, the analysis method includes applying a positive electrical bias to the sample analyzed, thereby, creating a retarding electric field between the sample surface and the energy analyzer input optics and, therefore, reducing the kinetic energy, E, of the Auger electrons for all spectral peaks. The improvement is gained by shifting the naturally occurring spectrum to lower energy where the ΔE of the analyzer is smaller. The analyzer need not be modified in any way and may operate always at its native energy resolution, c. As such, an existing instrument may be upgraded by addition of a sample holder apparatus such as described herein. In addition, a further benefit regarding sensitivity of the analyzer to sample topography is realized by biasing the sample. It is generally known that the positions of the Auger spectral peaks measured by a CMA are sensitive to the distance of the analyzed sample surface from the CMA input (i.e., the sample height or “working distance”) and that the sample surface must be accurately positioned at this working distance to obtain an accurate energy scale; hence accurate chemical information, in the measured Auger spectrum. The working distance is characteristic of the analyzer and is established by the design and tuning, or operating configuration, of the analyzer. The equation ΔE=k·Δz·E expresses the shift, ΔE, in the observed peak position (i.e., the peak energy), E, from the expected position in the Auger energy spectrum, as a function of Δz, the deviation in distance of the sample surface from the optimum working distance and of the kinetic energy, E, of the Auger electrons. “k” is a constant that is intrinsic to the analyzer. This shows that a sample surface, or a particular portion of a sample surface, that is not at the optimal working distance will result in there being a variance, ΔE, in the Auger peak position, hence, an error in the observed Auger peak energy. However, since the effect of sample biasing is to reduce the natural kinetic energy, E0, of the Auger electrons for a given Auger spectral peak by some factor, f, then it can be seen from the expression above that the sensitivity of the Auger peak positions to deviations from the optimal working distance is reduced since, if the final, biased kinetic energy of the electrons as they pass through the analyzer is reduced to some fraction of E0, Ef=E0·f, where f is less than 1.0, then ΔEf=k·Δz·Ef where, ΔEf=ΔE·f, the reduced shift in the peak position due to sample surface variance from optimal, or sample topographic differences. One or more embodiments of the apparatus, systems or methods provided herein may provide one or more of the following potential benefits: Increase the capability of Auger spectrometers, that use a cylindrical mirror analyzer (CMA), to obtain chemically-specific information about solid surfaces or to separate overlapping spectral features. An example of an existing product is the PHI 700 Scanning Auger Nanoprobe. However, the present disclosure may apply to all annular-acceptance electron energy analyzer types in the field of solid surface analysis. There does not need to be any modification to the analyzer or other critical optical components of the spectrometer or incident particle sources to achieve improvement in the energy resolution by this method. Other approaches to the problem include adding retarding fields inside the analyzer and this has deleterious effects, including reduction of the analyzer transmission. Energy resolution is continuously adjustable over the usable range. Secondary electron (SE) image is now from backscattered electrons which provide a different contrast in the image and can give grain structure information; may be able to measure grain orientation after a calibration step. Shifting Auger peak into 10-20 eV range may allow real-time elemental image with imaging secondary electron detector. FAT (fixed analyzer transmission) mode for CMA by scanning the sample potential instead of the analyzer; analyzer pass energy held constant at low kinetic energy. Better energy resolution (and signal rates) than available from retarding grids. No modification of analyzer, electron source or gun optics, or ultra high vacuum (UHV) chamber. Upgrades of existing equipment can be done on-site—return to factory is not required. Direct measurement: not a software deconvolution technique (although the method may enable a software deconvolution technique by allowing a direct determination of the analyzer broadening function). Measurement of electrically insulating samples may be enhanced by the biasing method since the Ar ion flux used for charge neutralization of these samples would have to be at higher incident energy, therefore, more ion current would be available for neutralization; low ion current is a problem with the ion neutralization at lower incident energies used at 0 V bias. The effect of the bias, when the sample is coated with a conductive layer, like normal airborne carbonaceous contamination, may be to drain static charge from the sample surface, thereby, aiding in “charge neutralization”. Sample access by auxiliary beams is maintained, for example, the ion gun can still sputter-clean the sample or provide low energy ions for sample neutralization. Full sample movement is maintained, including tilt, rotate and x and y position adjustment. The sensitivity of the Auger peak positions to sample height is reduced, as compared with the native energy resolution peak positions, by the application of the bias voltage. The method extends the region of the Auger spectrum that is accessible to analysis by an existing Auger instrument. Typical commercial instruments restrict—by design, to reduce cost and complexity while maintaining a practical analytical capability—the measurable range of Auger electrons to about 0-3200 eV; however, there are many Auger transitions (peaks) that are at higher kinetic energies than 3200 eV and are, therefore, not detectable by current commercial Auger instrument offerings. For example, the higher energy transitions of the element Pt were measured by a special apparatus in the range 5000-12000 eV (see L. H. Toburen and R. G. Albridge, Nuclear Physics A, 90(3) 529 (1967); such energies being well outside of the capabilities of current typical commercial instruments. Since these higher energy Auger electrons, that fall outside the designed energy range of the analyzer, can be reduced in energy by the apparatus and methods described herein and, therefore, brought into the range of the analyzer, the analytical usefulness of, for example, the typical commercial analyzer instrument is increased. This additional analytical energy range (i.e., kinetic energies higher than 3200 eV) enabled by the sample holder apparatus as described herein, allows the sample to be interrogated to greater depths below the sample surface since the escape of Auger electrons through the surface region, from greater depths is a strong, direct function of their initial kinetic energy. A description of one or more apparatus that may be modified with the sample holder apparatus described herein is provided in, for example, Japan Pat. Appl. No. JP2006-302689A, entitled “Auger Electron Spectral Analysis Device and Auger Electron Spectral Analysis Method,” and U.S. Pat. No. 5,032,724, entitled “Multichannel Charged-Particle Analyzer,” issued 16 Jul. 1991; all of which are incorporated herein in their entirety by reference thereto. However, the present disclosure is not limited to application of the sample holder apparatus to only such apparatus as described in such documents as will be appreciated by one skilled in the art but is applicable to any annular-acceptance systems and/or analyzers. For example, FIG. 1 is an illustrative simplified longitudinal section of analyzer system 10 including a cylindrical mirror analyzer (CMA) 30 (a type of annular-acceptance analyzer) and incorporating a sample holder apparatus 100. For example, the system 10 is an analyzer system as described in U.S. Pat. No. 5,032,724 modified using a sample holder apparatus 100. As described in U.S. Pat. No. 5,032,724, the electron energy analyzing system 10 generally includes the components of conventional analyzing systems such as the components in a Model 25-120A Auger electron spectroscopy analyzer available from The Perkin-Elmer Corporation, the CMA Models 25-130, 25-140 and 25-150 available from Physical Electronic, Inc. (MN), the Model CMA 100 available from Omicron NanoTechnology, the Models ESA 100, ESA 150, DESA 100 and DESA 150 available from Staib Instruments, the Model CMA-D 40B1 available from C4 Scientific Instruments, and the Model CMA 2000 available from LK Technologies, except as modified by the sample holder apparatus generally shown by reference numeral 100. This FIG. 1 is a simplified schematic diagram and various details may be found in references such as the aforementioned U.S. Pat. No. 4,205,226. The analyzing system 10 is enclosed in a multi-component housing 14 which is maintained at vacuum by an ion pump (e.g., conventional components not being shown). For clarity other standard parts such as viewing ports, internal valving, and the like are also not shown. An electron gun 18 located on the axis 16 of the system may be any desired type such as may be based on electrostatic or magnetic optics, or thermionic or field emission of electrons. A target of sample material to be surface analyzed is placed at an outer end 22 of the system using the sample holder apparatus 100. The system is generally cylindrical, and all of the components located between the electron gun and the target have an axial passage 24 extending therethrough so that a primary electron or other energy beam 25 can be directed from the gun 18 onto the target (e.g., directed from the gun 18 through an aperture of the sample holder apparatus 100 onto a surface of the target sample, directed from the gun 18 onto a surface of a target sample protruding through an aperture of the sample holder apparatus 100, etc.). Electron optical components for the beam, shown generally at 26 may also be disposed coaxially as required, such as a variable aperture objective, steering plates, an objective lens, a deflector and a stigmator, some of which may be magnetic. The beam from the gun causes charged particles 27 to be emitted from the target. The particles may be positive ions, negative ions or electrons, for example Auger electrons. In other cases the axial beam 25 to the target may be an ion stream, X-rays or any other suitable energy source, and the emission may include reflected electrons. The system 10 may have two energy analyzers operating in tandem with respect to emitted elections 27. The first, or primary analyzer may be a conventional cylindrical mirror analyzer 30 (CMA). This analyzer 30 may include an outer conductive cylinder 32 (e.g., a cylindrical electrode) and an inner conductive cylinder 34 (e.g., a cylindrical electrode). The outer cylinder may have a negative voltage applied thereto relative to a reference voltage on the inner cylinder; the reference generally being ground potential. Voltages are provided by a DC source 36 via leads 38 in a through-fitting 40 in the housing 14. Electrons 27 diverging from the target within a certain range of conical angles 29 (the angle of the diverging electrons being relative to a plane at the sample surface orthogonal to axis 16) pass through a first annular gap 42 and a grounded screen 43 and enter the space 44 between the cylinders. Because of the field associated with the negative voltage on the outer cylinder 32, the electrons will follow a path 28 that curves back toward the axis 16 of the system. The exact path or trajectory for each electron is dependent on the kinetic energy of the electron relative to the field. Those electrons having an energy within a small range of energies will exit the space 44 through another grounded screen 45 and an annular passage 47 in the inner cylinder 34, to be detected by a detection system 50 located within the inner cylinder 34 opposite the end 22. In one or more embodiments of analyzer systems, such as disclosed in the aforementioned U.S. Pat. No. 4,205,226, a small annular opening is provided for the electron detector. By changing the voltage on the outer cylinder the energy band selected by the detector is changed correspondingly. Thus scanning of the voltage combined with electron detection of electrons transmitting the gap provides a spectrum which may be correlated with characteristics of the target, such as material and topography. Processor 71 is suitably connected to the components of the system for control thereof, such as by leads 69, and, for example, to receive signals for conversion to spectrographic information of energy distribution (e.g., for use in creating a display of information or other presentation of information, for running routines for correction of data as described herein, for control of the application of bias potential to the sample support member and measurement thereof, etc.). In one or more embodiments, the sample holder apparatus 100 includes a grounded sample aperture member and an electrically isolated sample support member to which is applied a retarding bias potential (e.g., together shown as reference numeral 110). The sample support member includes a sample receiving surface upon which a sample (e.g., a sample to be analyzed) may be mounted such that the sample is electrically coupled or connected to the sample support member (e.g., the sample being held at the same potential as the sample support member). The bias potential is applied to the sample support member, and thus the sample, by a source 112 (e.g., via leads 113 in a through-fitting in the housing 14, or in any other manner). Generally, the grounded sample aperture member is electrically isolated from the sample support member (e.g., such as with use of electrical insulative spacers). As used herein, the term “grounded” when used in conjunction with a sample aperture member (e.g., aperture plate, etc.) refers to the sample aperture member generally being held at ground potential of the analyzer (i.e., at the reference voltage of the analyzer, which for a CMA is generally the voltage on the inner conductive cylinder of the analyzer, such as inner conductive cylinder 34 as shown in FIG. 1). In other words, a grounded sample aperture member refers to a sample aperture member that is grounded and has no potential applied thereto, or a sample aperture member that may have a small voltage (positive or negative) applied thereto. Such a small voltage applied thereto is generally a voltage that would not result in significant alterations to the planar-spherical field and resulting trajectories. Further, generally, such a small voltage is less than about 10 percent of the voltage applied to the sample support member. A voltage applied to the sample aperture member that is higher than about 10 percent of the voltage applied to the sample support member would result in significant alterations to the planar-spherical field and resulting trajectories, such that, for example, electrons would not enter the CMA with the proper range of input elevation angles. In one embodiment, the grounded sample aperture member surrounds a substantial portion of the sample support member (or otherwise includes one or more surfaces located between the sample support member and the analyzer components that receive the emitted electrons) and includes an aperture (e.g., a circular aperture having an axis therethrough, an aperture orthogonal to an axis at its center, etc.) in proximity to the sample (e.g., above the sample, flush with the sample, the sample extending through the aperture with a surface above the aperture, etc.). In other words, at least in one embodiment, the aperture (e.g., an aperture generally parallel to the sample receiving surface, at least in one embodiment) provides access to the surface of the sample to be analyzed and reduces the biased area directly under the analyzer such that an electrical retarding field (e.g., an axially symmetric electrical retarding field) is created in a reduced area about the aperture and over the sample surface to be analyzed. The aperture may be circular in order to create an axially symmetric retarding field. In one embodiment, the grounded sample aperture member surrounds the sample support member to shield all the retarding field except that portion in proximity to the circular aperture. The sample holder apparatus 100 may be configured to provide full sample movement, including, for example, tilt, rotation, and x, y, and z adjustment. For example, such sample movement may be provided using any known stage movement apparatus such as with the use of leadscrew or rack and pinion actuation, manually driven linear or rotary feedthroughs, electric motors internal or external to the vacuum, or piezoelectric motors. For example, similar movement of a conventional stage apparatus is provided in the PHI Model 15-680, available from Physical Electronics, Inc. (MN). Further, in one or more embodiments, the sample support member or the sample support member and the sample aperture member (together 110) may be mounted on an instrument stage, such as stage 115 generally shown in FIG. 1. As such, stage movement apparatus as described above may be used to provide full sample movement of the components. Further, for example, the sample support member may be mounted separately and independently (e.g., on an instrument stage) from the sample aperture member (e.g., the sample aperture member may be mounted separately to allow movement of the sample aperture member relative to the sample receiving surface of the sample support member, such as being movable into a retracted position using one or more components like those used for accomplishing sample stage movement). Still further, the sample support member may be mounted (e.g., as part of the instrument stage of an analyzer) such that it is movable apart from the independently mounted sample aperture member. FIG. 2A shows a schematic, isometric cutaway view of a portion of one generalized embodiment of a sample holder apparatus 210 that may be used to reduce energy of electrons in an annular-acceptance analyzer system, such as the analyzer system 10 shown in FIG. 1. The exemplary sample holder apparatus 210 includes a grounded sample aperture member 214 and an electrically isolated sample support member 212 to which is applied a retarding bias potential (e.g., source not shown). The sample support member 212 includes a sample receiving surface 222 (e.g., a planar surface) terminating a cylindrical body member 221 lying along axis 218 of the sample holder apparatus 210 and upon which a sample 220 (e.g., a sample to be analyzed) may be mounted such that the sample 220 is electrically coupled or connected to the sample support member 212 (e.g., the sample 220 being held at the same potential as the sample support member 212). The cylindrical body member 221 including a cylindrical outer surface 227 at a distance from axis 218. For example, the axis 218 may coincide with the axis 16 of an analyzer system, such as system 10 shown in FIG. 1. The grounded sample aperture member 214 is electrically isolated from the sample support member 212 (e.g., such as with use of electrical insulative spacers, not shown, or in any other suitable insulative manner). In one embodiment, as shown in FIG. 2A, the grounded sample aperture member 214 includes a cylindrical wall 241 including an inner surface 247 located at a distance from the axis 218 and surrounding the cylindrical body member 221 of sample support member 212 (e.g., the sample aperture member 214 may be physically fixed to the sample support member 212 such that they do not move independently of each other). Further, the grounded sample aperture member 214 includes an end portion 243 (e.g., a generally planar end having a inner surface 253 adjacent but isolated from the sample receiving surface 222 and an outer surface 255 facing the analyzer components of an analyzer system, such as system 10 in FIG. 1) terminating the cylindrical wall 241. In other words, the grounded sample aperture member 214 includes one or more surfaces or material portions located between the sample support member 212 and the analyzer components that receive the emitted electrons, such as the gap between the electrodes of analyzer 30 as shown in FIG. 1. The grounded sample aperture member 214 of FIG. 2A further includes a circular aperture 216 (e.g., having a thickness defined by the thickness or distance between the outer surface 255 and inner surface 253) defined in the end portion 243 by one or more surfaces 217 (e.g., along axis 218, or in other words, the aperture 216 is orthogonal to the axis 218 at its center) in proximity to the sample 220 when positioned for analysis (e.g., above the sample 220 as shown in FIG. 2A, but may be flush with the sample 220, the sample 220 may extend through the aperture 216 with a surface 232 above the aperture 216, etc.). In at least in one embodiment, a circular aperture 216 (e.g., an aperture generally parallel to the surface of the sample receiving surface 222, at least in one embodiment) provides access to the sample surface 232 to be analyzed (e.g., reducing the biased area directly under an analyzer 30 such as shown in FIG. 1 with which the sample holder apparatus 210 can be used such that an axially symmetric electrical retarding field is created in a reduced area about the aperture and over the sample surface 232 to be analyzed). The aperture 216 in the embodiment of FIG. 2A is circular to create an axially symmetric retarding field. The grounded sample aperture member 214 surrounds the sample support member 212 to shield all the retarding field except that portion in proximity to the circular aperture 216. The combination of grounded sample aperture member 214 along with the bias potential applied to the sample support member 212 and electrically connected sample 220 produces an electrical retarding field that reduces the energy of the electrons before they enter the analyzer (e.g., an electrical retarding field over the aperture 216, and thus, the sample surface 232 being analyzed). For example, in one or more embodiments, the electrical retarding field is created at least about the aperture 216 (e.g., a plurality of equipotential surfaces can be calculated that extend at least from edges 251 defining the aperture 216 and outward from the sample 220 towards the analyzer, such as analyzer 30 as shown in FIG. 1 with which the sample holder apparatus 210 may be used). Further, at least in one embodiment, the electrical retarding field is confined to the region in proximity to the aperture 216 by the grounded sample aperture member 214 (e.g., confined by the one or more grounded surfaces such as the grounded cylindrical wall 241 and grounded end portion 243 that includes the aperture 216 formed therein). At least in one or more embodiments, due to the shape of the field, which is substantially planar near the sample surface and non-planar (e.g., substantially spherical farther from the sample surface, or, for example, more non-planar the further the distance from the sample surface), the trajectories of the electrons are bent outward from the optical axis such that they fill the entrance to an analyzer and enter with the desired range of input angles (e.g., the electrons diverging from the target sample within a certain range of conical angles pass through a first annular gap 42 and a grounded screen 43 and enter the space 44 between the cylindrical electrodes 32 and 34 shown in FIG. 1). FIG. 2B shows a schematic, isometric cutaway view of a portion of one alternate generalized embodiment of a sample holder apparatus 610 that may be used to reduce energy of electrons in an annular-acceptance analyzer system, such as the analyzer system 10 shown in FIG. 1. The exemplary sample holder apparatus 610 is similar to the sample holder 210 shown in FIG. 2A except that the exemplary sample holder apparatus 610 includes a grounded sample aperture plate 614 that is mounted separately from electrically isolated sample support member 612 to which is applied a retarding bias potential (e.g., source not shown). With such separate mounting of the components, the aperture defined by the grounded sample aperture plate 614 and the sample receiving surface 622 can be moved relative to the other (e.g., the sample receiving surface 622 can be moved under the aperture 616). The sample support member 612 (e.g., which may be mounted on an instrument stage or provided as part of an instrument stage) includes a sample receiving surface 622 (e.g., a planar surface) terminating a cylindrical body member 621 lying along axis 618 of the sample holder apparatus 610 and upon which a sample 620 (e.g., a sample to be analyzed) may be mounted such that the sample 620 is electrically coupled or connected to the sample support member 612 (e.g., the sample 620 being held at the same potential as the sample support member 612). The cylindrical body member 621 includes a cylindrical outer surface 627 at a distance from axis 618. For example, the axis 618 may coincide with the axis 16 of an analyzer system, such as system 10 shown in FIG. 1. The grounded sample aperture plate 614 is electrically isolated from the sample support member 612 and movement is possible between the receiving surface 622 of the sample support member 612 and the grounded sample aperture plate 614 (e.g., the sample support member 612 may be moved relative to a fixed aperture, the aperture may be retracted from a position between the sample receiving surface 622 and the analyzer components, etc.). For example, in one embodiment, the grounded sample aperture plate 614 (e.g., a generally planar structure having an inner surface 653 adjacent but isolated from the sample receiving surface 622 and an outer surface 655 facing the analyzer components of an analyzer system, such as system 10 in FIG. 1) may be coupled to structure (e.g., components like those used to provide full instrument stage movement) configured to allow the aperture plate 614 to be moved relative to the receiving surface 622 (and thus movement of the aperture 616 relative to the sample 620 when mounted thereon). The grounded sample aperture member 614 of FIG. 2B further includes a circular aperture 616 (e.g., having a thickness defined by the thickness or distance between the outer surface 655 and inner surface 653) defined by one or more surfaces 617 (e.g., along axis 618, or in other words, the aperture 616 is orthogonal to the axis 618 at its center in proximity to the sample 620 when positioned for analysis (e.g., above the sample 620 as shown in FIG. 2B, but may be flush with the sample 620, the sample 620 may extend through the aperture 616 with a surface 632 above the aperture 616, etc.). In at least one embodiment, a circular aperture 616 (e.g., an aperture generally parallel to the surface of the sample receiving surface 622) provides access to the sample surface 632 to be analyzed (e.g., reducing the biased area directly under an analyzer 30 such as shown in FIG. 1 with which the sample holder apparatus 610 can be used such that an axially symmetric electrical retarding field is created in a reduced area about the aperture and over the sample surface 632 to be analyzed). Still further, in one embodiment, with the grounded sample aperture plate 614 being mounted separately from the sample support member 612 (e.g., not physically attached to one another), the sample support member 612 having the sample mounted thereon may be moved under the aperture 616, thereby, allowing the analyst to select and examine different areas on the sample 620 without having to extract and remount the sample 620 (e.g., allowing analysis of a region of the sample that is larger than the aperture without removing the sample). In other words, full sample movement may be possible under the aperture 616. Further, such separate mounting may allow the aperture plate 614 to be removed or retracted from the region of the sample 620 to allow for other sample analysis/processing that may have been restricted by the presence of the aperture plate 614. In other words, the sample aperture plate 614 is not physically fixed to the sample support member 612 and they are allowed to move independently of each other. FIG. 2C shows a schematic, isometric cutaway view of a portion of another alternate generalized embodiment of a sample holder apparatus 810 that may be used to reduce energy of electrons in an annular-acceptance analyzer system, such as the analyzer system 10 shown in FIG. 1. The exemplary sample holder apparatus 810 is similar to the sample holder 210 shown in FIG. 2A except that the exemplary sample holder apparatus 810 further includes a positioning plate structure 880 and a grounded sample aperture member 814 that includes a recessed aperture region 890. For example, FIG. 2C shows a sample holder apparatus 810 that includes a grounded sample aperture member 814 and an electrically isolated sample support member 812 to which is applied a retarding bias potential (e.g., source not shown). The sample support member 812 includes a sample receiving surface 822 (e.g., a planar surface) terminating a cylindrical body member 821 lying along axis 818 of the sample holder apparatus 810 and upon which a sample 820 (e.g., a sample to be analyzed) may be mounted such that the sample 820 is electrically coupled or connected to the sample support member 812 (e.g., the sample 820 being held at the same potential as the sample support member 812). Opposite the sample receiving surface 822, an elongated mating element 896 is provided for engagement with another mating element (not shown) to place the receiving surface 822 in a particular position in the analyzer (e.g., the elongated mating element 896 may mate with an instrument stage mating element). The sample support member 812 further includes a positioning plate structure 880. The positioning plate structure 880 includes a cylindrical element 887 at least partially about the cylindrical outer surface 827 of the cylindrical body member 821 (e.g., in contact with the cylindrical body member 821 at a distance from axis 818). The positioning plate structure 880 further includes a retaining plate 886 located between the grounded sample aperture member 814 and the cylindrical body member 821 of the electrically isolated sample support member 812. The retaining plate 886 includes an aperture 889 aligned with aperture 816 of the grounded sample aperture member 814 to allow access to the sample 820. Further, the retaining plate 886 is coupled to the cylindrical element 887 of the positioning plate structure 880 by one or more fasteners 888. The retaining plate 886 of the positioning plate structure 880 may be used to hold the sample 820 in place. The exemplary retaining plate 886 may include a raised center and the opening 889 defined therein for exposing the sample to the analysis instrument in the area of aperture 816 provided in the grounded sample aperture member 814. Although one exemplary positioning structure (e.g., positioning plate structure 880) is shown in FIG. 2C, it will be recognized that any number of techniques may be used to hold the sample 820 in place for analysis. For example, samples may be held down via their front side as shown in FIG. 2C, or alternatively, via their back side, or side edges. For example, fasteners including threaded screws, pins, retaining rings, hold-down plates, spring clips or clamps may be used to hold down the samples. Further, for example, and clearly not limited thereto, such samples may be held down using conductive adhesives such as glues, epoxies, or silver paint, and/or conductive sticky tapes (e.g., double sided conductive tape). Still further, for example, the sample may be set in a recessed hole and held down by gravity. The grounded sample aperture member 814 is electrically isolated from the sample support member 812 (e.g., such as with use of electrical insulative spacers, not shown, or in any other suitable insulative manner). In one embodiment, as shown in FIG. 2C, the grounded sample aperture member 814 includes a cylindrical wall member 841 including an inner surface 847 located at a distance from the axis 818 and surrounding the cylindrical element 887 of sample support member 812 (e.g., the sample aperture member 814 may be physically fixed to the sample support member 812 such that they do not move independently of each other). Further, the grounded sample aperture member 814 includes an end aperture plate 843 (e.g., a generally planar end having an inner surface 853 adjacent but isolated from the sample receiving surface 822 and an outer surface 855 facing the analyzer components of an analyzer system, such as system 10 in FIG. 1) terminating and coupled to the cylindrical wall 841 by one or more fasteners 849. In other words, the grounded sample aperture member 814 includes one or more surfaces or material portions located between the sample support member 812 and the analyzer components that receive the emitted electrons, such as the gap between the electrodes of analyzer 30 as shown in FIG. 1. The grounded end aperture plate 843 of FIG. 2C further includes a circular aperture 816 (e.g., having a thickness defined by the thickness or distance between the outer surface 855 and inner surface 853) defined in the end aperture plate 843 by one or more surfaces 817 (e.g., along axis 818, or in other words, the aperture 816 is orthogonal to the axis 818 at its center) in proximity to the sample 820 when in one position for analysis (e.g., above the sample 820 as shown in FIG. 2C). In at least one embodiment, a circular aperture 816 (e.g., an aperture generally parallel to the surface of the sample receiving surface 822) provides access to the sample surface 832 to be analyzed (e.g., reducing the biased area directly under an analyzer 30 such as shown in FIG. 1 with which the sample holder apparatus 810 can be used such that an axially symmetric electrical retarding field is created in a reduced area about the aperture and over the sample surface 832 to be analyzed). The aperture 816 in the embodiment of FIG. 2C is circular to create an axially symmetric retarding field and is located in a recessed aperture region 890 of the grounded end aperture plate 843. Such recessed aperture region 890 is configured to allow use of the positioning plate structure 880 and the fasteners 888 thereof. One will recognize that one or more various configurations of sample holder apparatus (e.g., the size and shape) may be used to provide the retarding field as described herein. The disclosure is not limited to only the generalized sample holder apparatus shown herein, but includes other apparatus that can be provided with use of various combinations of the structural elements shown in the various embodiments herein. However, some configurations may be more beneficial than others. Various other embodiments of sample holder apparatus are further described herein with reference to FIGS. 3-14. Generally, such apparatus provide an electrical retarding field over the sample surface being analyzed to reduce the energy of the electrons entering the analyzer. Sample surfaces to be analyzed are typically nearly planar. Applying a retarding potential to such a sample (e.g., without a grounded aperture being used such as described herein) generally creates a retarding field that would be substantially planar (e.g., above the planar sample surface). Use of this planar retarding field would undesirably bend the emitted electron trajectories significantly outward away from the central axis (e.g., axis 16 of the analyzer 30 along which the annular-acceptance region is located or aligned such as shown in FIG. 1) such that the emitted electrons would most likely not fill the entrance of the analyzer and would most likely not enter the analyzer with the proper range of input angles. In one or more embodiments, as described herein, by adding or using a grounded circular aperture in proximity to the sample surface (e.g., such as where the sample is protruding through and above the aperture, where the sample surface is flush with the aperture, or where the sample surface is below the aperture), a substantially spherical field can be created farther away from the sample surface. In other words, at least in one or more embodiments, the retarding field is then substantially planar close to the sample surface and substantially spherical farther away from the sample surface. For example, this combination planar-spherical field bends the trajectories of the emitted electrons outward away from the central axis but by a much smaller amount than does the substantially planar field when used by itself. In one or more embodiments, the relative amounts of planar and spherical fields are controlled. For example, the relative amounts of planar and spherical fields may be controlled by the design of the diameter and thickness of the aperture and its position relative to the sample (e.g., that is, such as the gap distance for the case of a sample located below the aperture, or in other words the gap distance between the surface 232 of sample 220 and an inner surface 253 of the end portion 243 facing the sample 220). With the proper proportion of planar to spherical field, this allows for filling the CMA annular input and adjusting the input angles to the CMA. Advantageously, these input angles can be chosen to be the conventional range of input angles, namely 36 to 48 degrees, so that the CMA input is substantially filled and no modification to the CMA is necessary. Because the planar-spherical field bends the electrons away from the central axis, it can be readily understood that it is particularly suitable to annular-acceptance analyzers where the beam is received off-axis. However, it could also be employed with an on-axis analyzer such as the common SCA (spherical capacitor analyzer), but the transmission would be reduced. Referring to FIG. 3, the right side of the figure depicts a coaxial CMA and electron gun 301, such as illustrated in FIG. 1. The left side depicts a sample holder apparatus 302 including a sample aperture member 322, sample support member 323, and sample 324. The sample aperture member 322 is electrically grounded. The sample support member 323 is electrically isolated from the sample aperture member 322 and is electrically connected to a positive bias potential (not shown). The sample 324 is mounted directly to the sample support member 323 (e.g., or a post of support member 323), and the two elements 323 and 324 are in electrical contact. The sample surface 325 (e.g., the surface closest to the CMA and which is to be analyzed) protrudes above an aperture 330 (e.g., a circular aperture) defined in the sample aperture member 322. Referring to FIG. 4, an enlarged view of a portion of FIG. 3 is depicted. Element 341 is a magnetic objective lens pole-piece of the coaxial electron gun and CMA 301. The sample aperture member 322 is electrically grounded. The sample support member 323 has +900 V applied. The sample 324 is mounted to the sample support member 323 and as such also has +900 V applied. A computer program SIMION 3D Version 7.0 was used to calculate nine equipotential surfaces 365 that range from +90 V to +810 V in 90 V increments. The shape of these surfaces changes from nearly planar close to the sample 324 to nearly spherical farther away from the sample 324. For this embodiment, where the sample surface 325 protrudes above the circular aperture 330 in the sample aperture member 322, the sample (and/or post of support member 323) has a small diameter to create enough of a spherical field so that there is the desired proportion of planar to spherical field. Furthermore, for a given sample (and/or post) diameter, the height of the sample above the aperture 330 and the diameter and thickness of the aperture 330 are chosen to produce a planar-spherical field shape that bends the electron trajectories advantageously to fill the annular entrance or gap 342 of the CMA 301 and enter with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Referring to FIG. 5, operation of the embodiment of FIG. 3 is depicted. The sample aperture member 322 is electrically grounded. The sample support member 323 has +900 V applied. The sample 324 is mounted to the sample support member 323 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate 21 electron trajectories 375 that originate from the center of the sample 324 (e.g., on axis of the aperture 330). The trajectories 375 were launched with 1000 eV kinetic energy and with 24 to 34 degrees elevation angle. The electrons undergo deflection near the sample 324 as they penetrate the planar-spherical field and are slowed from 1000 eV to 100 eV kinetic energy. Because of the shape of the planar-spherical equipotential surfaces 365 depicted in FIG. 4, the electrons are deflected advantageously to fill the CMA annular entrance or gap 342 and enter the CMA 301 with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Because the electrons undergo a factor of 10 reduction in kinetic energy, the effective CMA resolution is increased by a factor of ten, from a native resolution of about 0.5% to an effective resolution of about 0.05%. No reduction in transmission occurs due to grid transparency or grid scattering. No modification to the analyzer or other critical components of the spectrometer is required (e.g., other than to use the sample holder apparatus described herein). Referring to FIG. 6, the right side of the figure depicts a coaxial CMA and electron gun 401. The left side depicts a sample holder apparatus 402 including a sample aperture member 422, sample support member 423, and sample 424. The sample aperture member 422 is electrically grounded. The sample support member 423 is electrically isolated from the sample aperture member 422 and is electrically connected to a positive bias potential (not shown). The sample 424 is mounted directly to the sample support member 423, and the two elements 423 424 are in electrical contact. The sample surface 425 is flush with the top of the circular aperture 430 in the sample aperture member 422. Referring to FIG. 7, an enlarged view of a portion of FIG. 6 is depicted. Element 441 is the magnetic objective lens pole-piece of the coaxial electron gun and CMA 401. The sample aperture member 422 is electrically grounded. The sample support member 423 has +900 V applied. The sample 424 is mounted to the sample support member 423 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate nine equipotential surfaces 465 that range from +90 V to +810 V in 90 V increments. The shape of these surfaces changes from nearly planar close to the sample 424 to nearly spherical far away from the sample 424. For this embodiment, where the sample surface 425 is flush with the top of the aperture 430 in the sample aperture member 422, the sample 424 can have a larger diameter than was possible in the embodiment of FIGS. 3-5, and still generate enough of a spherical field so that there is the proper proportion of planar to spherical field. Furthermore, for a given sample diameter, the diameter and thickness of the circular aperture 430 are chosen to produce a planar-spherical field shape that bends the electron trajectories advantageously to fill the annular entrance or gap 442 of the CMA and enter with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Referring to FIG. 8, operation of the embodiment of FIG. 6 is depicted. The sample aperture member 422 is electrically grounded. The sample support member 423 has +900 V applied. The sample 424 is mounted to the sample support member 423 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate 22 electron trajectories 475 that originate from the center of the sample 424. The trajectories were launched with 1000 eV kinetic energy and with 23.5 to 34 degrees elevation angle. The electrons undergo deflection near the sample 424 as they penetrate the planar-spherical field and are slowed from 1000 eV to 100 eV kinetic energy. Because of the shape of the planar-spherical equipotential surfaces 465 depicted in FIG. 7, the electrons are deflected advantageously to enter the CMA 401 with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Because the electrons undergo a factor of 10 reduction in kinetic energy, the effective CMA resolution is increased by a factor of ten, from a native resolution of about 0.5% to an effective resolution of about 0.05%. No reduction in transmission occurs due to grid transparency or grid scattering. No modification to the analyzer or other critical components of the spectrometer is required (e.g., other than to use the sample holder apparatus described herein). Referring to FIG. 9, the right side of the figure depicts a coaxial CMA and electron gun 501. The left side depicts a sample holder apparatus 502 including a sample aperture member 522, sample support member 523, and sample 524. The sample aperture member 522 is electrically grounded. The sample support member 523 is electrically isolated from the sample aperture member 522 and is electrically connected to a positive bias potential (not shown). The sample 524 is mounted directly to the sample support member 523, and the two elements 523 and 524 are in electrical contact. The sample surface 525 is below the circular aperture 530 in the sample aperture member 522. Referring to FIG. 10, an enlarged view of a portion of FIG. 9 is depicted. Element 541 is the magnetic objective lens pole-piece of the coaxial electron gun and CMA 501. The sample aperture member 522 is electrically grounded. The sample support member 523 has +900 V applied. The sample 524 is mounted to the sample support member 523 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate nine equipotential surfaces 565 that range from +90 V to +810 V in 90 V increments. The shape of these surfaces changes from nearly planar close to the sample 524 to nearly spherical far away from the sample 524. For this embodiment, where the sample surface 525 is below the circular aperture 530 in the sample aperture member 522, the sample can have an even larger diameter than was possible in the embodiments shown in FIGS. 3-8 and still generate enough of a spherical field so that there is the proper proportion of planar to spherical field. Furthermore, the height of the aperture 530 above the sample 524 and the diameter and thickness of the circular aperture 530 are chosen to produce a planar-spherical field shape that bends the electron trajectories advantageously to fill the annular entrance of the CMA and enter the CMA 501 with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Referring to FIG. 11, operation of the embodiment of FIG. 9 is depicted. The sample aperture member 522 is electrically grounded. The sample support member 523 has +900 V applied. The sample 524 is mounted to the sample support member 523 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate 25 electron trajectories 575 that originate from the center of the sample 524. The trajectories were launched with 1000 eV kinetic energy and with 24 to 36 degrees elevation angle. The electrons undergo deflection near the sample as they penetrate the planar-spherical field and are slowed from 1000 eV to 100 eV kinetic energy. Because of the shape of the planar-spherical equipotential surfaces 565 depicted in FIG. 10, the electrons are deflected advantageously to fill the CMA annular entrance or gap 542 and enter the CMA 501 with the conventional range of CMA elevation angles, namely from about 36 to 48 degrees. Because the electrons undergo a factor of 10 reduction in kinetic energy, the effective CMA resolution is increased by a factor of ten, from a native resolution of about 0.5% to an effective resolution of about 0.05%. No reduction in transmission occurs due to grid transparency or grid scattering. No modification to the analyzer or other critical components of the spectrometer is required (e.g., other than to use the sample holder apparatus described herein). Referring to FIG. 12, the right side of the figure depicts a coaxial CMA and electron gun 501 in the same manner as in FIG. 9. The left side depicts a sample holder apparatus 502 like that shown in FIG. 9 including a sample support member 523 and sample 524. However, unlike FIG. 9, FIG. 12 includes a sample aperture member 722 that is not physically connected to the sample support member 523, but rather is separated therefrom (e.g., such as like shown in FIG. 2B). The sample aperture member 722 is electrically grounded. The sample support member 523 is electrically isolated from the sample aperture member 722 and is electrically connected to a positive bias potential (not shown). The sample 524 is mounted directly to the sample support member 523, and the two elements 523 and 524 are in electrical contact. The sample surface 525 is below the circular aperture 730 defined in the sample aperture member 722. The sample aperture member 722 may be mounted off of the instrument stage or off of a separate port (not shown) in the chamber (not shown) of the analysis instrument. As such, sample aperture member 722 may be centered under the analyzer and the sample would be positioned under the aperture 730. In such a manner, the sample 524 would be movable under the aperture 730 so that any area of the sample 524 can be selected for analysis. As shown in FIG. 13, the sample aperture member 722 may be retracted from its position of FIG. 12 to allow for other sample analysis/processing that may have been restricted by the presence of the aperture member 722. In other words, the sample aperture member 722 is moved out of its position between the coaxial CMA/electron gun 501 and the sample 524. Referring to FIG. 14, an enlarged view of a portion of FIG. 12 is depicted. Element 541 is the magnetic objective lens pole-piece of the coaxial electron gun and CMA 501. The sample aperture member 722 is electrically grounded. The sample support member 523 has +900 V applied. The sample 524 is mounted to the sample support member 523 and as such also has +900 V applied. The computer program SIMION 3D Version 7.0 was used to calculate nine equipotential surfaces 565 that range from +90 V to +810 V in 90 V increments. The shape of these surfaces changes from nearly planar close to the sample 524 to nearly spherical far away from the sample 524. While certain embodiments have been illustrated and described, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made without deviating from the concepts set forth above. Illustrative embodiments of this invention are discussed and reference has been made to possible variations within the scope of this disclosure. These and other variations, combinations, and modifications in the disclosure will be apparent to those skilled in the art without departing from the scope of the invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein. Accordingly, the invention is to be limited only by the claims provided below and equivalents thereof.

description

This application is a divisional of U.S. patent application Ser. No. 12/843,037, filed Jul. 25, 2010, the contents of which is incorporated by reference in its entirety. As shown in FIG. 1, a conventional fuel assembly 10 of a nuclear reactor, such as a Boiling Water Reactor (BWR), may include an outer channel 12 surrounding an upper tie plate 14 and a lower tie plate 16. A plurality of full length fuel rods 18 and/or part length fuel rods 19 may be arranged in a matrix within the fuel assembly 10 and pass through a plurality of spacers (also known as spacer grids) 15 axially spaced one from the other and maintaining the rods 18, 19 in the given matrix thereof. The fuel rods 18 and 19 are generally continuous from their base to terminal, which, in the case of the full length fuel rod 18, is from the lower tie plate 16 to the upper tie plate 14. Outer channel 12 encloses the fuel rods 18/19 within the assembly 10 and maintains water or other coolant flow within assembly 10 about fuel rods 18/19 and in contact with the fuel rods 18/19 to facilitate heat transfer from the fuel to the coolant. Outer channel 12 is traditionally uniform in mechanical design and material for each other assembly 10 provided to a particular core, to aid in assembly design standardization and manufacturing simplicity. Outer channel 12 may be fabricated conventionally of a material compatible with the operating nuclear reactor environment, such as a Zircaloy-2. As shown in FIG. 2, a conventional reactor core, such as a BWR core, may include a plurality of cells 40 in the reactor core. Each cell may include four fuel assemblies 10 having adjacent fuel channels 12. Other fuel assemblies 10 may be placed in the reactor core outside of cells 40 and not adjacent to control blades. The fuel assemblies 10 in FIG. 2 are shown in section to illustrate control blades 45, which are conventionally cruciform-shaped and movably-positioned between the adjacent surfaces of the fuel channels 12 in a cell 40 for purposes of controlling the reaction rate of the reactor core. Conventionally, there is one control blade 45 per cell 40. As a result, each fuel channel 12 has two sides adjacent to the control blade 45 and two sides with no adjacent control blade. The control blade 45 is formed of materials that are capable of absorbing neutrons without undergoing fission itself, for example, boron, hafnium, silver, indium, cadmium, or other elements having a sufficiently high capture cross section for neutrons. Thus, when the control blade 45 is moved between the adjacent surfaces of the fuel channels 12, the control blade 45 absorbs neutrons which would otherwise contribute to the fission reaction in the core. On the other hand, when the control blade 45 is moved out of the way, more neutrons will be allowed to contribute to the fission reaction in the core. Conventionally, only a fraction of all control blades 45 within a core will be exercised to control the fission reaction within the core during an operating cycle. As such, only a corresponding fraction of fuel assemblies will be directly adjacent to an extended control blade, or “subject to control,” during an operating cycle. After a period of time, a fuel channel 12 may become distorted as a result of differential irradiation growth, differential hydrogen absorption, and/or irradiation creep. Differential irradiation growth is caused by fluence gradients and results in fluence-gradient bow. Differential hydrogen absorption is a function of differential corrosion resulting from shadow corrosion on the channel sides adjacent to the control blades 45 and the percent of hydrogen liberated from the corrosion process that is absorbed into the fuel channel 12; this results in shadow corrosion-induced bow. Irradiation creep is caused by a pressure drop across the channel faces, which results in permanent distortion called creep bulge. As a result, the distortion (bow and bulge) of the fuel channel 12 may interfere with the movement of the control blade 45. Channel/control blade interference may cause uncertainty in control blade location, increased loads on reactor structural components, and decreased scram velocities. Conventionally, if channel/control blade interference has become severe, the control blade is declared inoperable and remains fully inserted. Example embodiments are directed to fuel assemblies usable in nuclear reactors and methods of optimizing and fabricating the same. Example embodiment fuel assemblies include an outer channel having a physical configuration determined based on a position of the fuel assembly within a core of the nuclear reactor, such as the position of the fuel assembly with respect to a control blade in the nuclear reactor that will be used to control core reactivity. When example embodiment fuel assemblies are to be directly adjacent to an inserted control blade, the outer channel may be thickened, reinforced, and/or fabricated of a material more resistant to deformation than Zircaloy-2, such as Zircaloy-4, NSF, and VB, so as to reduce or prevent distortion of the channel against the control blade and interfering with operation of the same. When example embodiment fuel assemblies are not in a controlled location, the outer channel may be thinned so as to increase water volume and reactivity in the assembly. As such, a reactor core including example embodiment fuel assemblies will include fuel assemblies having unique outer channels, in thickness, material, etc., unlike conventional power reactor cores. Example methods of configuring fuel assemblies include determining operational characteristics of the fuel assembly, such as the likelihood that the fuel assembly is controlled via control blade insertion in the nuclear reactor in a current or future fuel cycle, and physically selecting or modifying the outer channel of the fuel assembly based thereon. For example, if the fuel assembly is in a controlled location during the fuel cycle, the outer channel may be fabricated of a material more resistant to deformation than Zircaloy-2, such as Zircaloy-4, NSF, or VB, and/or thickened. Or, for example, if the fuel assembly is not in a controlled location, the outer channel may be approximately 20 mils (thousandths of an inch) or more thinner than outer channels of conventional fuel assemblies. Example methods are usable with or may further include configuring outer channel characteristics in order to meet desired neutronic properties of the fuel assembly. Hereinafter, example embodiments will be described in detail with reference to the attached drawings. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. The example embodiments may be embodied in many alternate forms and should not be construed as limited to only example embodiments set forth herein. It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that when an element is referred to as being “connected,” “coupled,” “mated,” “attached,” or “fixed” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the language explicitly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, “channel,” “outer channel,” and the like are defined in accordance with the conventional fuel assembly structures shown and described in FIG. 1 as element 12, subject to the modifications discussed hereafter. As used herein, “distortion” or “channel distortion” includes both channel bow and channel bulge in nuclear fuel assemblies that may cause interference with control blade operation. It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures or described in the specification. For example, two figures or steps shown in succession may in fact be executed in parallel and concurrently or may sometimes be executed in the reverse order or repetitively, depending upon the functionality/acts involved. The inventors of the present application have identified several potential fuel channel characteristics and/or modifications to reduce or prevent fuel channel distortion. The inventors of the present application have further identified the effect these characteristics, in combination with other fuel assembly parameters, have on whole core performance. Example embodiments and methods discussed below uniquely address these previously-unidentified effects to achieve several advantages, including improved core performance, increased energy generation, reduced control blade error, materials conservation, and/or other advantages discussed below or not, in commercial nuclear power plants, while departing from total fuel channel uniformity purposefully used in conventional commercial nuclear power plants. Example embodiment fuel assemblies include fuel channels with optimized physical properties. Example embodiment fuel assemblies may include one or more channel characteristics to decrease fuel channel distortion. For example, the fuel channel may be thickened in its shortest dimension or reinforced with additional material. The thicker or reinforced fuel channel has greater resistance to distortion from differential irradiation growth, differential hydrogen absorption, and/or irradiation creep experienced in operating nuclear reactor environments. The percent reduction in deformation is approximately proportional the percentage increase in channel thickness. Or, for example, materials may be used in the channel that are resistant to distortion. For example, Zircaloy-4, a known zirconium alloy excluding nickel, may replace Zircaloy-2, which contains nickel. The reduced nickel content in Zircaloy-4 reduces differential hydrogen absorption and resultant channel bow. Other materials more resistant to deformation than Zircaloy-2 may additionally be used in whole or in part in addition to Zircaloy-4. For example, additional materials more resistant to deformation than Zircaloy-2 are described in co-pending application Ser. No. 12/153,415 “Multi-layer Fuel Channel and Method of Fabricating the Same,” incorporated herein by reference in its entirety. That document discloses alloys hereinafter called “NSF” having about 0.6-1.4% niobium (Nb), about 0.2-0.5% iron (Fe), and about 0.5-1.0% tin (Sn), with the balance being essentially zirconium (Zr) and alloys hereinafter called “VB” having about 0.4-0.6% tin (Sn), about 0.4-0.6% Fe, and about 0.8-1.2% chromium (Cr), with the balance being essentially zirconium (Zr). Other configurations for decreasing fuel channel distortion are usable with example embodiment fuel assemblies. Example embodiment fuel assemblies may use multiple mechanisms in combination to further reduce fuel channel distortion. Configurations and fuel channel characteristics in example embodiment fuel channels may be selected in accordance with example methods, discussed in the following section. Example embodiment fuel assemblies may further include channel characteristics that improve fuel neutronic characteristics, decrease material usage and costs, and/or improve other fuel assembly parameters. Such characteristics may include, for example, a thinner channel that permits greater water volume and neutron moderation within example embodiment fuel assemblies. The thinner channel may consume less material in fabrication and improve fuel assembly reactivity, heat transfer characteristics, etc. Example embodiment fuel assemblies having thicker, reinforced, and/or thinner channels, different alloys, or other channel modification may be used instead of conventional fuel assemblies having standardized channels throughout an entire core. Example embodiment fuel assemblies may thus significantly improve performance of a core including example embodiment fuel assemblies and/or reduce fuel resource consumption. For example, thinning the channels of 75% of the fresh conventional fuel assemblies for a particular fuel cycle by approximately 20 mils (20 thousandths of an inch) in the thinnest dimension may result in a reduction in volume of approximately 16,500 in3 zirconium alloy used. In the same example, assuming 8 channels would not need to be replaced because they include channel mechanisms to decrease fuel channel deformation, an additional ˜2,000 in3 zirconium alloy volume may be saved. In the same example, assuming 8 channels are not needed to be fabricated because 8 fewer fuel assemblies are required in a fuel cycle with fuel savings from channel characteristics that improve fuel neutronic characteristics of example embodiment fuel assemblies, an additional ˜2,000 in3 zirconium alloy volume may be conserved. Thus, example embodiment fuel assemblies, having different channel characteristics selected and implemented in accordance with example methods discussed below, may result in significant materials savings and improved core performance. Example Methods As discussed above, increasing channel thickness decreases water volume and overall reactivity of an assembly having a thicker channel. Lower reactivity results in less optimal fuel usage and less power production in a nuclear core of a nuclear power reactor. Increasing channel thickness further increases costs of fuel assemblies having thicker channels. Increasing channel thickness also reduces the risk and/or magnitude of channel distortion and interference with control blade function. Decreasing channel thickness has a generally opposite effect of increasing water volume and overall reactivity of an assembly having a thinner channel, while also increasing distortion likelihood. Zircaloy-4 has similar fluence bow and creep bulge characteristics compared to Zircaloy-2. Zircaloy-4, however, resists channel bow caused by differential hydrogen absorption. NSF and VB are additionally resistant to other forms of bow and bulge causing channel deformation. Example methods uniquely leverage the above advantages and disadvantages of fuel channel modification to reduce or prevent channel distortion while minimizing negative effects on fuel economy, control blade function, and other core performance metrics. As shown in FIG. 3, example methods include an operation S100 of determining fuel assembly characteristics, including whether a fuel assembly is placed or will be located in a cell such that the fuel assembly will be directly adjacent to a control blade that will be operated in a current and/or future fuel cycle to control the fission reaction in the core. A fuel assembly positioned directly adjacent to a control blade that is likely to be exercised to control the fission reaction is herein defined as a “controlled fuel assembly” or in a “controlled location,” because it is most subject to control blade negative reactivity and most likely to affect control blade performance. The determination of whether a fuel assembly is subject to control may be based on one or more fuel assembly operational characteristics that determines placement/position of the fuel assembly within the reactor core over one or more fuel cycles, in addition to overall plant characteristics such as core size, thermal power rating, etc. For example, an operational characteristic may be reactivity of the fuel assembly. Reactivity determines the degree to which the fuel can contribute to the fission chain reaction during power operations. Reactivity is directly controllable with control blade insertion, due to the blades' neutron-absorbing properties. As such, fuel with higher reactivity may be placed in controlled locations to enhance core-wide overall control of the neutron chain reaction. Similarly, fuel with lower reactivity may be less likely to be subject to control. Although location with regard to utilized cruciform control blades is described in connection with example embodiments and methods, it is understood that other sources of negative reactivity may additionally be accounted for in example methods and embodiments. For example, proximity to burnable poisons or proximity to a control rod present in some plant designs may be accounted for by determining operational characteristics of the fuel assembly that determine the likelihood that the fuel assembly will be placed in that proximity. Controlled locations may also be determined in S100 by known core modeling and mapping methods and software. For example, a program may receive input of several fuel assembly operational characteristics for several fuel assemblies and determine an optimum core configuration with corresponding fuel assembly positions. Because example methods and embodiments may themselves affect fuel assembly operational characteristics as discussed below, such known core modeling and mapping methods may be alternatively and repetitively executed before and following fuel assembly modification in example methods to ensure optimized core performance. Following the determination in S100, one or more fuel assembly channels are configured based on the position determination. The configuring generally increases assembly reactivity, decreases distortion potential, and/or reduces material consumption in the configured assembly/assemblies. If it is determined from S100 that the assembly will be placed in a cell adjacent to an employed control blade, i.e., subject to control, then a first configuration S210 is pursued. S210 configures the assembly channel to reduce or eliminate channel distortion during power operations. For example, in S210, channel thickness may be increased by several hundredths of an inch or more to ensure decreased channel distortion. The degree of thickening may further be based on decreased reactivity or other operational characteristics desired of the assembly during operation in the nuclear reactor core. Or, channel thickness may be increased or the channel may be reinforced on only a side or wall directly adjacent to the control blade that will be operated, while remaining fuel channel sides may be unmodified or modified in accordance with S220. Additionally, or in the alternative, in S210, the channel may be fabricated out of a material more resistant to distortion than Zircaloy-2, including shadow-corrosion-bow-resistant Zircaloy-4, or fluence-gradient-bow and/or-creep-bulge-resistant NSF or VB. In this way, only assemblies determined to be at a position benefiting from a thicker or reinforced channel or a channel including Zircaloy-4, NSF, and/or VB, such as a controlled assembly likely to be placed in a cell adjacent to an employed control blade, are configured with channel features that decrease or eliminate distortion while leveraging other characteristics such as reactivity or fabrication expense. Further, because assemblies in a controlled core position typically possess higher excess reactivity, a thicker or reinforced channel that may decrease reactivity is not a significant disadvantage for the overall core reactivity; indeed, such reactivity-decreasing configuration may aid in balancing core power production and/or simplifying control blade operations. If it is determined from S100 that the assembly will be placed in an uncontrolled core position, such as an edge position in the core or adjacent to a control blade that will not be utilized, then a second configuration S220 is pursued. S220 configures the assembly channel to increase fuel assembly neutronic characteristics for the assembly in the operating core and decrease manufacturing burden in fabricating the assembly, without regard to distortion risk. For example, in S220, channel thickness may be decreased by several hundredths of an inch or more to increase water or moderator volume in the assembly, thereby increasing reactivity and fuel usage in the assembly. Reducing channel thickness in S220 further decreases an amount of expensive zirconium alloy or other channel material required to fabricate the assembly. In S220, assembly channel thickness may be reduced by a margin that takes into account the increased reactivity; the channel may be thinned such that the assembly has a determined or desired reactivity or other operational property when in use in the nuclear reactor core. In this way, a core may contain fuel assemblies with several different, unique channel thicknesses and other characteristics as determined in S210 and S220. Assemblies may be configured in S210 and S220 in several different manners and timeframes. For example, the configuring in S210 and S220 may be selecting a pre-existing assembly or ordering an assembly having the configuration determined in S210 and S220, by a power plant operator, for insertion or re-insertion during an upcoming fuel cycle in the nuclear reactor core. Alternatively, the configuring in S210 and S220 may be a physical fabricating or modifying of the fuel assembly to match the configuration determined in S210 and S220 by a fuel assembly manufacturer or refitter, for example. Example methods including S100 and S210/S220 may address fuel assembly location and configuration for use in an immediately approaching fuel cycle, a future fuel cycle, and/or multiple fuel cycles. For example, S100 may determine that a fuel assembly will be in a controlled position adjacent to an employed control blade in a first fuel cycle, and the same or later analysis may determine that the fuel assembly will be relocated to a position away from a control blade in a second layer fuel cycle. The assembly may be configured under S210 for the first cycle, and then reconfigured under S220 for the second cycle. Such reconfiguring may include re-channeling the fuel assembly by removing and replacing the channel used in the first fuel cycle with a channel having the configuration determined in S220 for use in the second fuel cycle. Similarly, a reverse determination may result in the reverse configuration. Or, for example, S100 may determine, based on multi-cycle operating parameters, that a particular fuel assembly will not be placed in a controlled location in its lifetime. Configuration of the assembly may then proceed under S220, without further modification of the assembly during its lifetime in the reactor. FIG. 4 is an illustration of an example reactor core 400 containing example embodiment fuel assemblies 100 and 200 modified in accordance with example methods. As shown in FIG. 4, four example embodiment assemblies 100 are in controlled locations about a control blade 45a that is anticipated to be used to control the fission chain reaction in the core. According to example methods, assemblies 100 about blade 45a have channels 120 configured in accordance with S210. For example, channels 120 may be thickened, reinforced, and/or fabricated of a material more resistant to deformation than Zircaloy-2. Or, for example, only select sides or walls 120b directly adjacent to control blade 45a may be configured in accordance with S210, including being thickened, reinforced, and/or fabricated of a material more resistant to deformation than Zircaloy-2. Other walls 120a may be unmodified or thinned and/or fabricated of a material equally or less resistant to deformation than Zircaloy-2, in accordance with S220. Example assemblies 200, adjacent to blade 45b that is not to be operated during the fuel cycle or adjacent to no control blade, may be configured in accordance with S220. For example, channels 121 in assemblies 200 may be thinned and/or fabricated of a material equally or less resistant to deformation than Zircaloy-2. Example methods including S100 and S210/S220 may be executed for each assembly to be placed within a core. Alternatively, example methods may be executed only with respect to particular assemblies in order to optimize core operating characteristics. For example, if example fuel assembly channel configuring methods are used in conjunction with other known core configuration methods, the calculated or desired fuel assembly locations and characteristics may require no fuel assembly channel configuring or reconfiguring as in S210 or S220. Example methods may be used as an integral part of core design or as a separate step performed alternatively and/or iteratively with other known methods of core design. For example, a known core design program may output a core map using fuel assembly characteristics with fuel having uniform channel properties. Example methods including S100 and S210/S220 may then be performed on some or all fuel assemblies involved in the map, changing their operational characteristics. The core design program may then be re-executed with the modified fuel assembly characteristics, and this alternating core configuring between example and known methods may continue until no further optimization is possible or desired. Or, example methods may be used as an integral part of otherwise known core design methods, treating reactivity, bow likelihood, and other fuel assembly parameters affected by channel configuring in S210 and S220 as additional variables in the core design process. Example embodiments and methods thus being described, it will be appreciated by one skilled in the art that example embodiments may be varied through routine experimentation and without further inventive activity. Variations are not to be regarded as departure from the spirit and scope of the example embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

047044136

abstract

According to the invention, a resin composition having an electromagnetic wave shielding effect is provided. The resin composition comprises 35 to 90 wt % of an ABS resin or a mixture of the ABS resin and an AS resin, 1 to 25 wt % of a plasticizer and 5 to 40 wt % of carbon fibers. The carbon fibers are uniformly dispersed in the resin ingredient without being broken or cut at the step of mixing and dispersing the resin components, thus exhibiting improved electromagnetic wave shielding effect.

062158538

claims

1. A method for calibrating a collimator, the method comprising: acquiring a digital image representative of collimator blades aligned relative to a region of interest; automatically determining a position of at least one collimator blade from the digital image; and automatically adjusting a position of at least one collimator blade relative to the region of interest. a communication interface for carrying at least one collimator blade position sensor signal, at least one collimator blade actuator signal, and an image detector signal; and a central processor, coupled to the communication interface, for automatically determining a position of a collimator blade based on an image detector signal, and for automatically generating a blade activator signal for adjusting a position of a collimator blade toward a desired position relative to the region of interest. an X-ray detector; at least one collimator blade located relative to an region of interest; a detector for detecting a position of the collimator blade; a blade position controller for moving the collimator blade through at least one of a rotation and a translation; and a calibrator for determining the position of collimator blades and automatically instructing the blade position controller to move the collimator blade. 2. A method as recited in claim 1, wherein the step of acquiring the digital image comprises acquiring a digital image with a solid state X-ray detector. 3. A method as recited in claim 1, wherein the step of automatically determining position comprises segmenting at least a portion of the digital image into at least one band along an axis, and determining an edge of the collimator blades based on an edge response function. 4. A method as recited in claim 1, wherein the step of automatically determining position comprises fitting a collimator blade edge in the digital image with a linear model. 5. A method as recited in claim 3, wherein the step of automatically determining position further comprises fitting a collimator blade edge in the digital image with a linear model. 6. A method as recited in claim 4, wherein the step of automatically determining position comprises determining a distance from a center of the digital image to a collimator blade edge. 7. A method as recited in claim 1, wherein the step of adjusting a position of at least one collimator blade comprises at least one of translating and rotating at least one collimator blade relative to a reference point. 8. A method as recited in claim 1, wherein the step of adjusting a position of at least one collimator blade comprises adjusting at least one of a width between collimator blades and a centering of the collimator blades in the digital image. 9. A method as recited in claim 1, further comprising the steps of rotating a collimator assembly so that the collimator blades appear in the region of interest, and reversing the rotation after calibration is completed. 10. A method according to claim 1, further comprising the step of iterating the acquiring, determining, and adjusting steps until the collimator blades expose the region of interest within a predetermined tolerance. 11. A collimator calibration subsystem comprising: 12. A collimator calibration subsystem according to claim 11, further comprising at least one sensor and at least one actuator for at least one collimator blade. 13. A collimator calibration subsystem according to claim 11, wherein the communication interface connects to a digital X-ray detector image detector. 14. A collimator calibration subsystem according to claim 11, wherein the processor segments at least a portion of the digital image into at least one band along an axis, averages pixels in each band perpendicular to the axis, and determines edges of the collimator blades using an edge response function. 15. A collimator calibration subsystem according to claim 11, wherein the processor fits a linear model to determine blade edges in the digital image with a linear model. 16. A collimator calibration subsystem according to claim 14, wherein the processor fits a linear model to determine collimator blade edges in the digital image with a linear model. 17. A collimator calibration subsystem according to claim 11, wherein the activator signal induces at least one of translation and rotation of at least one collimator blade using an actuator. 18. A collimator calibration subsystem according to claim 11, wherein the activator signal adjusts at least one of a width between collimator blades and a centering of the collimator blades in the digital image. 19. A collimator calibration subsystem according to claim 11, further comprising a memory coupled to the processor, and wherein the processor stores calibration position information for at least one collimator blade in the memory. 20. A collimator calibration subsystem according to claim 11, wherein the processor iterates the determining and generating steps until the collimator blades expose the region of interest to within a predetermined tolerance. 21. A collimator calibration subsystem according to claim 11, wherein the activator signal induces movement of the collimator blade by a predetermined increment, after which the processor determines a new position of the collimator blade. 22. A collimator calibration subsystem according to claim 11, wherein the processor determines current and destination calibrated positions of the collimator blade, and asserts the activator signal to move the collimator blade from the current position to the destination calibration position. 23. A collimator calibration system comprising: 24. A collimator calibration system of claim 23 wherein the calibrator directs the blade position controller to move the collimator blade by a predetermined increment, after which the calibrator determines a new position of the collimator blade. 25. A collimator calibration system of claim 23, wherein the calibrator calculates current and destination calibrated positions of the collimator blade, and instructs the blade position controller to move the collimator blade from a current position to the destination calibration position. 26. A collimator calibration subsystem according to claim 23, further comprising at least one sensor and at least one actuator for at least one collimator blade. 27. A collimator calibration subsystem according to claim 23, further comprising a communication interface coupled to the X-ray detector, the blade position controller, and the calibrator. 28. A collimator calibration subsystem according to claim 23, wherein the calibrator segments at least at portion of the digital image into at least one band along an axis, averages pixels in each band perpendicular to the axis, and determines edges of the collimator blades using an edge response function. 29. A collimator calibration subsystem according to claim 23, wherein the calibrator fits a linear model to determine blade edges in the digital image with a linear model. 30. A collimator calibration subsystem according to claim 28, wherein the calibrator fits a linear model to determine collimator blade edges in the digital image with a linear model. 31. A collimator calibration subsystem according to claim 23, wherein the blade position controller generates an activator signal that induces at least one of translation and rotation of at least one collimator blade using an actuator. 32. A collimator calibration subsystem according to claim 23, wherein the blade position controller adjusts at least one of a width between collimator blades and a centering of the collimator blades in the digital image. 33. A collimator calibration subsystem according to claim 28, wherein the axis lies in a width direction of the digital image. 34. A collimator calibration subsystem according to claim 23, wherein the calibrator iterates the determining and instructing until the collimator blades expose the region of interest to within a predetermined tolerance.

051270307

claims

1. A device useful in producing a tomographic image of a selected slice of an object to be examined comprising: a source of penetrating radiation, sweep means for forming energy from said source into a pencil beam and for repeatedly sweeping said pencil beam over a line in space to define a sweep plane, first means for supporting an object to be examined so that said pencil beam intersects said object along a path passing through said object, collimating means with a plane of symmetry forming an angle to said sweep plane for filtering radiation scattered by said object, said collimating means having: a source of penetrating radiation, sweep means for forming energy from said source into a pencil beam and for repeatedly sweeping said pencil beam over a line in space to define a sweep plane, first means for supporting an object to be examined so that said pencil beam intersects said object, second means for preferentially detecting radiation scattered, at any instant, by a selected volume element in said slice, said second means subtending a large solid angle relative to said selected volume element, said second means including: wherein said collimator has an axis of symmetry which does not coincide with said sweep plane. said radiation detector means and said collimator, on the one hand, and said second radiation detector means and said second collimator, on the other hand, being spaced from each other with said sweep plane and said object lying therebetween. 2. A device as recited in claim 1 wherein said collimating means has a face opposite said object, said channels of said collimating means have equal dimensions in said face of the collimating means. 3. A device as recited in claim 2 wherein said slice has a thickness determined by a dimension of said pencil beam. 4. A device as recited in claim 1 wherein said sweep plane intersects said plane of symmetry of said collimating means at about a right angle. 5. A device as recited in claim 1 wherein said sweep plane intersects said plane of symmetry of said collimating means at an angle other than 90.degree.. 6. A device as recited in claim 1 wherein a solid angle subtended by said collimating means at an elementary volume of said object illuminated by said pencil beam and along said bounded line is about .pi./2 steradians. 7. A device as recited in claim 1 wherein a solid angle subtended by said collimating means at an elementary volume of said object illuminated by said pencil beam and along said bounded line is in a range of about 0.05.pi. to 2.pi. steradians. 8. A device as recited in claim 1 which further includes means for providing relative motion between said object and said source and further includes imaging means responsive to a signal produced by said radiation detector means for producing an image of said selected slice. 9. A device as recited in claim 1 wherein said collimating means includes first and second collimators, each collimator having a field of view which intersects said sweep plane and said object in a bounded line and a plurality of channels each substantially planar in form to collectively define said field of view, said first and second collimators spaced from each other with said sweep plane lying therebetween and wherein said radiation detector means includes a first radiation detector associated with said first collimator and a second radiation detector associated with said second collimator, each radiation detector producing, at any instant a single valued signal representing detected radiation and summing means responsive to an output of each of said radiation detectors for producing a signal representing, at any instant in time, a sum of said output signals. 10. A device as recited in claim 9 wherein a solid angle subtended by said collimating means at an elementary volume of said object illuminated by said pencil beam and along said bounded line is at least about 0.05.pi. steradians. 11. A device as recited in claim 10 which further includes means for producing relative motion between said object and said source and further includes imaging means responsive to a signal produced by said radiation detector means for producing an image of said selected slice. 12. A device useful in producing a tomographic image of a selected slice of an object to be examined comprising: 13. A device as recited in claim 12 wherein said channels of said collimator have equal dimensions in that face of the collimator facing said object. 14. A device as recited in claim 13 wherein said slice has a thickness determined by a dimension of said pencil beam. 15. A device as recited in claim 12 wherein said sweep plane intersects said plane of symmetry of said collimator at about a right angle. 16. A device as recited in claim 12 wherein said sweep plane intersects said plane of symmetry of said collimator at an angle other than 90.degree.. 17. A device as recited in claim 12 wherein said solid angle is about .pi./2 steradians. 18. A device as recited in claim 12 wherein said solid angle is in a range of about 0.05.pi. to 2.pi. steradians. 19. A device as recited in claim 12 which further includes means for producing relative motion between said object and said source and further includes imaging means responsive to a signal produced by said radiation detector means for producing an image of said selected slice. 20. The device as recited in claim 12 wherein said second means includes second radiation detector means developing at any instant in time a single signal reflecting radiation impinging on said second radiation detector means, and a second collimator located between said object and said second radiation detector means, said second collimator including a plurality of radiation transmitting channels collectively establishing a field of view which intersects said sweep plane in a line which is a locus of said selected volume elements so that said collimator passes radiation scattered by different elementary volume elements of said object lying along said line as said sweep of said pencil beam illuminates the different volume elements lying along said line and wherein said second collimator has an axis of symmetry which does not coincide with said sweep plane, 21. The device recited in claim 20 wherein a solid angle subtended by said first and second collimator at an elementary volume of said object illuminated by said pencil beam and along said bounded line is at least about 0.05.pi. steradians. 22. The device recited in claim 21 which further includes means for producing relative motion between said object and said source, summing means for summing outputs of said radiation detector means and said second radiation detector means and imaging means responsive to an output of said summing means for producing an image of said selected slice.

043426212

description

DESCRIPTION OF THE PREFERRED EMBODIMENT While the invention is susceptible of various modifications and alternative constructions, there is shown in the drawings and there will hereinafter be described in detail a description of the preferred embodiment of the invention. It is to be understood, however, that the specific description and drawings are not intended to limit the invention to the specific form disclosed. On the contrary, it is intended that the scope of this patent include all modifications and alternative constructions thereof falling within the spirit and scope of the invention as expressed in the appended claims. In a preferred embodiment of the present invention, as depicted in FIG. 1, a nuclear reactor 10 having a vessel 14 and an internally mounted reactor core 12 is suspended within a reactor chamber 24 formed within the base 21 of the containment building 20. The reactor vessel 14 is supported by a cantilevered support member 22 upon which rests the reactor coolant piping 18 as it leaves the reactor vessel 14. Reactor coolant piping 18, in the case of a pressurized water reactor, fluidically connects the interior of the reactor vessel 14 with a steam generator 16 which generates steam for its ultimate delivery to a steam turbine. The general function of the present invention is to protect the concrete walls of chamber 24 from the damaging effects of both the extreme heat and radiation generated by a nuclear core 12 which has melted and dropped through the bottom of the reactor vessel 14 as shown generally at 38. Of primary concern are both the cavity floor 28 and the vertical walls 26. Damage to these walls is to be avoided if at all possible on the occurance of such an accident in order that the radioactive materials associated with a molten core 38 are prevented from escaping either to the exterior of chamber 24 or the exterior of the containment building 20. Therefore, the present invention provides a system of heat pipes 40 and 50 which respectively protect the chamber floor 28 and the chamber wall 26 by collecting a portion of the heat generated by the core in chamber 24 and transporting the heat to a location exterior to the containment building 20. It has been found that heat pipes are well suited for this function in that they can be designed to transfer large quantities of heat with very little thermal resistance, can remain passive for large periods of time without maintainance and can automatically begin their heat transfering function without need of human or mechanical intervention. Furthermore, a heat pipe is ideal for accomplishing these functions inasmuch as each heat pipe constitutes a hermetically sealed unit which is independent from all the other heat pipes of the system. The hermetically sealed feature of the heat pipe permits the heat pipe to penetrate from the interior of chamber 24 to the exterior of the containment building 20 without running the risk of pumping radioactive material to the exterior of the containment envelope in the event that one end of the heat pipes is breached. Heat pipes 40 and 50 are arranged with their evaporator sections 42 and 52 adjacent to the reactor chamber's floor 28 and wall 26 respectively so as to shield the concrete base 21 from the damaging heat and radiation of the molten core 38 and so as to contain the core. Each of these heat pipes has its condenser section 46 and 56 respectively located in a water reservoir 60 external to the reactor containment building 20. The condenser sections and the evaporator sections of the heat pipes are fluidically connected by adiabatic sections 44 and 54 respectively which penetrate through the reactor containment building 20 through base 21. Each of the adiabatic sections 44 and 54 are surrounded by thermal insulation 64 in order that the concrete of the base 21 and the containment building 20 not be exposed to excessive heat which could possibly cause concrete dehydration and subsequent failure. As may be seen in FIG. 1, water reservoir 60 is vented by a vent pipe 62 directly to the atmosphere exterior to the containment building 20. As may also be seen from FIG. 1, condenser sections 46 and 56 of heat pipes 40 and 50 respectively are positioned at elevations higher than the elevations of their respective evaporator sections so that the working fluid does not have to work against a gravitational head in its return to the evaporator section. An angle of slant no less than 30.degree. is preferred in order that the maximum capability of the heat pipe be achieved. As can be seen in FIG. 1, evaporator ends 42 of heat pipes 40 are disposed vertically below reactor vessel 14. A slightly conical upwardly facing shallow basin 30 is also disposed below evaporator sections 42. Basin 30 is provided to prevent direct contact between molten core 38 and base 21. Basin 30 desirably consists of a refractory metal having a high melting point such as tungsten, tantalum carbide, zirconium carbide, niobium carbide, hafnium carbide or graphite. However, it is predicted that the temperature of molten core 38 would exceed the melting point of refractory basin 30 so that refractory basin 30 must be either directly cooled by a cooling system or shielded from the elevated temperatures of the core. The present invention chooses the latter arrangement. Accordingly, heat pipes 40 are arranged in a star-like pattern radiating outwardly from a position under the core 12 in a manner which best shields refractory basin 30 from the temperature of the core. The preferred arrangement of the invention is illustrated in FIG. 2 in which heat pipes 40 are arranged in an outwardly radiating star-like array. Each heat pipe 40 includes thermal conducting fins 48 attached to its evaporator end 42. As can be seen, fins 48 are shaped to butt one against another to almost completely cover the upwardly facing surface of basin 30. Adjacent fins 48 however, are separated by a slight gap in order to accomodate the thermal expansion expected when heated by a molten core 38. Evaporator sections 42 of heat pipes 40 are vertically supported by but not anchored to underlying basin 30. In this manner, heat pipes 40 are permitted to operate at a temperature in excess of the temperature of basin 30 without incurring the significant problems of differential thermal expansion. Thermal expansion of the heat pipes 40 are further accommodated by the provision of anchoring heat pipes 40 to base 21 of the containment building 20 at only one point: the point at which the heat pipes enter the concrete foundation 21. At this position, the heat pipe is hermetically sealed to the base 21 by seal 34 so that the passage through which the adiabatic section 44 of the heat pipe passes is hermetically isolated from the interior of chamber 24. Seals 34 must be of such a nature as to be able to withstand radial expansion of heat pipe 40. As can be seen, with this arrangement, heat pipes 40 are permitted axial growth in both the inward and outward direction from the attachment point at seal 34 and the hermetic containment envelope is maintained. While the above described arrangement effectively shields basin 30 from excessive temperatures, it is still expected that the basin 30 will be exposed to an extreme elevated temperature. Accordingly, basin 30 will also experience thermal growth. In anticipation of the thermal growth expected in basin 30, the basin is attached to base 21 only in one centrally located point 32. This arrangement permits basin 30 to undergo unrestricted radial expansion so that warping effects are minimized. As can be seen in FIG. 1, a layer of thermal insulation 36 such as a layer of alumina bricks may be placed under basin 30 in order to protect the underlying concrete of base 21 from thermally caused dehydration. The above described arrangement exposes the evaporator sections 42 of heat pipes 40 directly to the molten core 38. It should be recognized, however, that basaltic blocks as taught in U.S. Pat. No. 3,702,802 may also be placed above the heat pipes in order to reduce the thermal shock placed on the heat pipes 40 as well as to reduce the heat release per unit volume of core material by diluting the molten core material with material from the basalt blocks. An additional measure which may be taken to avoid the excessive concentration of heat of the molten core is the formation of basin 30 and evaporator sections 42 of heat pipes 40 in a nearly horizontal manner. The molten core material would then be expected to spread out in a relatively thin layer. Other measures may be taken such as those taught in U.S. Pat. No. 4,036,688 in order to prevent the molten core from forming a critical geometry. In a manner similar to that described above for heat pipes 40, heat pipes 50 are provided with thermally conducting fins 58 which spread out and shield the inner surface 26 of chamber 24 at portions which are not directly shielded by the heat pipe evaporator section 52 itself. While not shown in the drawing of FIG. 1, it may also be desirable to line the inner surface of chamber 24 with a thermal insulator such as refractory bricks in order to prevent the dehydration of the concrete base 21. Heat pipes 50 are preferably arranged in a star-like pattern which radiates outwardly from the reactor. As can be seen from FIG. 1, an inwardly projecting "knee" of heat pipe 50 is provided in order to intercept and absorb the upwardly directed radiations emminating from molten core 38. In this manner, the upper portions of the cavity 24 are protected against the damaging heat and radiations emmited by the core. Heat pipes 50 are also anchored at that point at which each heat pipe enters the concrete wall of the chamber at seal 34 so that heat pipes 50 may undergo unrestricted thermal expansion in both the inward and outward directions. Turning now to an examination of FIG. 3, a typical cross-section of the evaporator sections of heat pipes 40 is shown. Also shown are a portion of the base 21, the thermal insulation 36, the refractory metal basin 30 and the fins 48. Wicking material 66 resides on the interior of the heat pipes 40. As is well understood, the materials from which heat pipes 40 are constructed depend upon a number of factors including the amount of heat which must be transported, the maximum temperatures expected, and the compatability of the materials used in the pipe which included the pipe itself, the working fluid, and the wicking material. It has been calculated that for a 3,800 megawatt thermal core, 136 six inch sodium filled heat pipes with evaporator lengths of eight feet and condenser lengths of twelve feet would be adequate to effectively remove the heat generated by molten core 38 so that basin 30 is protected from melting and core 38 is prevented from boiling. Other possible candidates for the working fluid of the heat pipes 40 and 50 includes potassium, cesium, mercury, and one of the eutectic alloys such as NaK. If the selected working fluid were to be liquid sodium, suitable heat pipe materials might include one of the alloys having trade names Nickel 200, Monel 400, Inconel 600, or Inconel 800. In addition, long heat pipes containing a plurality of working fluids are possible. Such heat pipes, when called upon to operate, would automatically separate themselves into zones determined by the latent heat of evaporation of the various working fluids as well as the temperature of the evaporation of the various working fluids.

055286410

abstract

A fuel assembly is provided with a coolant ascending path for making coolant rise and a water rod having a coolant descending path for conducting the coolant.. A ratio of a flow area in a coolant inlet port of the smallest in coolant ascending path 13 on the downstream side than large diameter tube portion 3E to a flow area of the largest in the axial direction of coolant ascending path 13 in large diameter tube portion 3E is set to be 0.2-20%.. In the normal operation, the declination degree from the liquid level in the coolant ascending path, corresponding to the coolant flow rate of the liquid level formed in the coolant ascending path can be controlled. Further, at the time of the excess the change speed of the liquid level can also be controlled.

summary

description

The present application claims priority from Japanese Patent application serial no. 2009-011479, filed on Jan. 22, 2009, the content of which is hereby incorporated by reference into this application. 1. Technical Field The present invention relates to jet pump and reactor and, in particular, to a jet pump and a reactor suitable for applying to a boiling water reactor. 2. Background Art A conventional boiling water reactor (BWR) has a jet pump in a reactor pressure vessel (hereinafter referred to as an RPV) to which a recirculation pipe is connected. The jet pump has a nozzle, a bell mouth, a throat, and a diffuser. Cooling water in a downcomer, where the jet pump is disposed, formed in the RPV is pressurized by operation of a recirculation pump, pumped through the recirculation pipe as a driving flow, and ejected from the nozzle into the throat. The nozzle increases the speed of the driving flow. The cooling water around the nozzle in the downcomer is sucked into the bell mouth as a suction flow due to the working of the ejected driving flow, passes the throat, and flows into the diffuser. The cooling water discharged from the diffuser is supplied to a core through a lower plenum in the RPV (see, for example, U.S. Pat. No. 3,625,820, Japanese Patent Laid-open No. Sho 59 (1984)-188100, Japanese Patent Laid-open No. Hei 7 (1995)-119700, and Japanese Patent Laid-open No. 2007-285165). Jet pumps disclosed in Japanese Patent Laid-open No. Sho 59 (1984)-188100, Japanese Patent Laid-open No. Hei 7 (1995)-119700, and Japanese Patent Laid-open No. 2007-285165 each have a plurality of nozzles. When the total area of each ejection opening formed in the plurality of nozzles remains constant, an increase in the number of nozzles increases the contact area between driving flows and suction flows, and thus mixing of the driving flows and the suction flows is promoted. Consequently, a mixing loss is decreased, increasing efficiency of the jet pump. A jet pump installed in a reactor has a nozzle connected to a raiser pipe that is installed in the RPV. In this jet pump, an elbow pipe, the nozzle, a bell mouth and a throat are unified into one body, which structure allows the elbow to the throat to be removed for inspection and maintenance. A connection portion between the throat and the diffuser has a joint structure in which a lower end portion of the throat is inserted into an upper end portion of the diffuser. This joint structure is a slip joint. The slip joint, where the throat and the diffuser are connected, has a structure which allows the upper end portion of the diffuser and the lower end portion of the throat to slide up and down, so that no stress is generated due to the difference between the thermal expansions of the raiser pipe and the diffuser. For this reason, a gap is formed between an inner surface of the diffuser's upper end portion and an outer surface of the throat's lower end portion. Part of the cooling water that flows into the diffuser from the throat leaks out to the downcomer through the gap. This leakage flow prevents a foreign object from being caught in the gap or deposited on the surfaces. However, when the flow rate of the leakage flow exceeds a limit, the jet pump may start to vibrate. Thus, in order to suppress the vibration of the jet pump, the leakage flow from the gap in the slip joint should be limited below the limit. Although it is not a jet pump, Japanese Examined Utility Model Application Publication No. Sho 52 (1977)-5301 discloses a fluid sealing joint used for pipes for introducing high-temperature and high-pressure gas (or steam). In this fluid sealing joint, a tubular inlet-side joint portion is inserted into a tubular outlet-side joint portion; and an end portion of the inlet joint portion has a narrowing portion whose flow passage cross-sectional area decreases and an expanding portion whose cross-sectional area increases toward the end. A communication hole is formed in the place where the narrowing portion and the expanding portion are connected, the flow passage cross-sectional area of which the place is the smallest in the inlet-side joint portion. This communication hole communicates with an annular space portion formed between the inlet-side joint portion and the outlet-side joint portion. Static pressure inside is reduced at the seam between the narrowing portion and the expanding portion so that a fluid in the annular space portion is sucked inside the narrowing portion through the communication hole. This effectively prevents a fluid from leaking out of the fluid sealing joint through a gap between the inlet-side joint portion and the outlet-side joint portion. Japanese Patent Laid-open No. Sho 59 (1984)-159489 discloses a jet pump for suppressing vibration. In this jet pump, a lower end portion of a throat, which is inserted into an upper end portion of a diffuser, has a flow passage cross-sectional area that diminishes toward the end. Other than that, for the purpose of reducing the amount of cooling water leaking from a slip joint of a jet pump, a way of forming a labyrinth seal on an outer surface of a lower end portion of a throat in the slip joint is known (see, for example, Japanese Examined Patent Application Publication No. Sho 59 (1984)-48360). A jet pump illustrated in FIG. 3 of Japanese Patent Laid-open No. 2001-90700 has a venturi tube and a nozzle that ejects a driving flow into the venturi tube a driving flow. This nozzle has an inner cylinder and an outer cylinder that surrounds the inner cylinder. A driving flow passage formed between the inner cylinder and the outer cylinder is an annular passage whose cross-sectional area gradually decreases towards the discharge side of the driving flow. The driving flow supplied to the driving flow passage is ejected from one end (a discharge opening) of the driving flow passage into the venturi tube. Cleaning water around the nozzle is sucked into the venturi tube due to the driving flow ejected from the nozzle. To be more specific, this cleaning water flows into the venturi tube through each of a first cooling water suction passage formed between the nozzle and the venturi tube and a second cooling water suction passage formed inside the inner cylinder. From the nozzle, the annular driving flow is ejected. Cross sections of the annular driving flow are similar to continuous rings. Japanese Patent Laid-open No. 2008-82752 discloses a jet pump applicable to a BWR. This jet pump has a ring header for supplying a driving flow surrounding a suction flow suction passage formed in the center of the jet pump; and a nozzle portion installed to a lower end of the ring header, surrounding the suction flow suction passage, having a plurality of ejection openings in an annular arrangement, where the ejection openings eject a driving flow fed to the ring header. Patent Literature 1: U.S. Pat. No. 3,625,820 Patent Literature 2: Japanese Patent Laid-open No. Sho 59 (1984)-188100 Patent Literature 3: Japanese Patent Laid-open No. Hei 7 (1995)-119700 Patent Literature 4: Japanese Patent Laid-open No. 2007-285165 Patent Literature 5: Japanese Examined Utility Model Application Publication No. Sho 52 (1977)-5301 Patent Literature 6: Japanese Patent Laid-open No. Sho 59 (1984)-159489 Patent Literature 7: Japanese Examined Patent Application Publication No. Sho 59 (1984)-48360 Patent Literature 8: Japanese Patent Laid-open No. 2001-90700 Patent Literature 9: Japanese Patent Laid-open No. 2008-82752 For the soundness of a jet pump, excessive vibration of the jet pump is undesirable. Each slip joint disclosed in Japanese Examined Utility Model Application Publication No. Sho 52 (1977)-5301 and Japanese Patent Laid-open No. Sho 59 (1984)-159489 can suppress a leakage flow from a slip joint and reduce vibration caused by the leakage flow. However, each slip joint has a flow passage cross-sectional area that changes, or decreases, where the structure somewhat increases a pressure loss in the slip joint. For this reason, when these slip joints are applied to a jet pump, the efficiency of the jet pump is reduced for the increased amount of pressure loss. In Japanese Examined Patent Application Publication No. Sho 59 (1984)-48360, a labyrinth seal is provided to a slip joint. When a labyrinth seal is fabricated on the outer surface of a throat, its fabrication range is limited to the thickness of the throat and the length of insertion. For this reason, when the fabrication range is insufficient, a desired effect in leakage flow reduction may not be achieved. An object of the present invention is to provide a jet pump and a reactor, which can suppress the vibration of the jet pump and improve the efficiency of the jet pump. The present invention for achieving the above object is characterized in that a nozzle apparatus has a nozzle base member, and a plurality of nozzles installed to the nozzle base member and forming a plurality of narrowing portions in which a fluid passage cross-sectional area of a driving fluid passage formed in the nozzle is reduced; and in a lower end portion of a throat inserted into a diffuser, a cross-sectional area of a fluid passage formed in the throat diminishes toward a downstream end of the throat. Since, in the lower end portion of the throat inserted into the diffuser, the cross-sectional area of the fluid passage formed in the throat diminishes toward the downstream end of the throat, the amount of a fluid leaking from a space between the throat and the diffuser can be reduced and the vibration of the jet pump can be suppressed. Since the nozzle apparatus has the nozzle base member and the plurality of nozzles installed to the nozzle base member and forming the plurality of narrowing portions, in which the fluid passage cross-sectional area of the driving flow passage is reduced, inside itself, the efficiency of the jet pump can be increased after compensating for a loss in jet pump efficiency caused by the diminishment of the fluid passage cross-sectional area in the throat. The above object can also be achieved by a jet pump comprising a nozzle apparatus having a header portion disposing a first pipe member forming a suction fluid passage for introducing a suction fluid, inside the head portion, and including an annular passage, which surrounds the first pipe member, for introducing a driving fluid, and a nozzle portion installed to the header portion, surrounding the first pipe member, and forming an ejection outlet, which is communicated with the annular passage formed in the header portion, for ejecting the driving fluid; and a second pipe member, one end of which is connected to the nozzle apparatus, forming a driving fluid passage for introducing the driving fluid to annular passage in the header portion, wherein the first pipe member is disposed inside the driving fluid passage formed in the second pipe member through the one end of the second pipe member, and an opening for the suction flow passage is formed on an outer surface of the second pipe member and opened toward outside of the second pipe member; the driving flow passage is formed in a way that the driving fluid flowing toward the one end of the second pipe member hits the first pipe member diagonally in the axial direction of the first pipe member; and, in the lower end portion of a throat inserted into a diffuser, a cross-sectional area of a fluid passage formed in the throat diminishes toward a downstream end of the throat. Since, in the lower end portion of the throat inserted into the diffuser, the cross-sectional area of the fluid passage formed in the throat diminishes toward the downstream end of the throat, the amount of a fluid leaking from a space between the throat and the diffuser can be reduced and the vibration of the jet pump can be suppressed. Since the driving fluid passage formed inside the second pipe member is formed so that the driving fluid flowing toward the one end of the second pipe member hits the first pipe member diagonally to the axial direction of the first pipe member, pressure loss inside the driving fluid passage is decreased. Since the speed of the driving fluid ejected from the annular ejection outlet of the nozzle portion becomes faster, the flow rate of the suction fluid sucked inside the jet pump body is increased. From above, efficiency of the jet pump is improved. Part of this increase in the jet pump efficiency can compensate for a decrease in the jet pump efficiency caused by the diminishment of the flow passage cross-sectional area in the throat. The above object can also be achieved by a jet pump comprising a nozzle apparatus having a first tubular member; a second tubular member disposed in the first tubular member, apart from the first tubular member; a fluid passage forming-member disposed in the first tubular member, and installed to an upper end portion of the second tubular member; a plurality of passage members fixing both ends to the first and the second tubular members and disposed in the circumferential direction of the nozzle apparatus; and an annular ejection outlet is formed between a lower portion of the first tubular member and a lower portion of the second tubular member; Wherein a suction passage formed in each of the passage members, for introducing a suction fluid from the outside to the inside, communicates with an inner region formed in the second tubular member, an annular driving fluid passage for introducing the driving fluid, across which each of the passage members is disposed, is formed between the first tubular member, and the second tubular member and the flow passage forming member, and communicated with the annular ejection outlet, the ejection outlet-side portion of the driving fluid passage slopes inward toward a lower end of the nozzle apparatus, and, in a lower end portion of a throat inserted into a diffuser, a cross-sectional area of a fluid passage formed in the throat diminishes toward a downstream end of the throat. Since, in the lower end portion of the throat inserted into the diffuser, the cross-sectional area of the fluid passage formed in the throat diminishes toward the downstream end of the throat, the amount of fluid leaking from a space between the throat and the diffuser can be reduced and the vibration of the jet pump can be suppressed. Since the ejection outlet-side portion of the driving fluid passage slopes inward toward the lower end of the nozzle apparatus, degree of negative pressure in the inner region is increased, increasing the flow rate of the suction fluid flowing into the inner region through the suction passage. Furthermore, since the ejection outlet-side portion of the driving fluid passage slopes inward toward the lower end of the nozzle apparatus, the width of a gap between the lower end of the outer circumference portion of the nozzle apparatus and the upper end of a jet pump body is increased. This increases the flow rate of a suction fluid flowing into the jet pump body through the gap. From these increases in the flow rates, the efficiency of the jet pump is further increased. Part of this increase in the jet pump efficiency can compensate for a decrease in jet pump efficiency caused by the diminishment of the fluid passage cross-sectional area in the throat. According to the present invention, the vibration of a jet pump can be suppressed and the efficiency of the jet pump can be improved. Various embodiments of the present invention are described below. A jet pump installed to a boiling water reactor, according to an embodiment of the present invention is described below with reference to FIGS. 1, 2, and 3. Before explaining a structure of the jet pump of the present embodiment, an overall structure of a boiling water reactor to which this jet pump is applied is described below with reference to FIGS. 1 and 4. The boiling water reactor (BWR) has a reactor pressure vessel (reactor vessel) 1 and a core shroud 3 installed in the reactor pressure vessel. Hereinafter, the reactor pressure vessel is referred to as an RPV. A core 2 loaded with a plurality of fuel assemblies (not shown) is disposed in the core shroud 3. A steam separator 4 and a steam dryer 5 are disposed above the core 2 in the RPV 1. A plurality of jet pumps 11 is disposed in an annular downcomer 6 formed between the RPV 1 and the core shroud 3. A recirculation system provided to the RPV 1 includes a recirculation pipe 7 and a recirculation pump 8 installed to the recirculation pipe 7. One end of the recirculation pipe 7 communicates with the downcomer 6. Another end of the recirculation pipe 7 is connected to a lower end of a raiser pipe 9 disposed in the downcomer 6. An upper end of the raiser pipe 9 is connected to a branching pipe 60. An elbow pipe (a curved pipe) 10 attached to the branching pipe 60 is connected to a nozzle apparatus 12 of the jet pump 11. A main steam pipe 39 and a feed water pipe 28 are connected to the RPV 1. The nozzle apparatus 12 is fixed to a bell mouth 21 using a plurality of supporting plates 33, and makes up one body with the bell mouth 21. Cooling water (suction fluid, coolant), which is suction flow existing in an upper portion of the RPV 1, is mixed with feed water supplied from the feed water pipe 28 to the RPV 1, and descends in the downcomer 6. This cooling water is sucked into the recirculation pipe 7 by operation of the recirculation pump 8, and pressurized by the recirculation pump 8. This pressurized cooling water is called a driving or motive fluid flow (a driving or motive fluid) 30 for descriptive purposes. The driving flow 30 flows through the recirculation pipe 7, the raiser pipe 9, the branching pipe 60, and the elbow pipe 10, and reaches the nozzle apparatus 12 of the jet pump 11 to be ejected from the nozzle apparatus 12. The cooling water 32, which is a suction flow around the nozzle apparatus 12 (see FIG. 3), is sucked into a throat 22 from the bell mouth 21 due to the working of a jet flow 31 of the driving flow 30 (see FIG. 3). The cooling water 32 descends with the driving flow 30 in the throat 22, and discharged from a lower end of a diffuser 25. The cooling water discharged from the diffuser 25 (including the suction flow 32 and the driving flow 30) is called cooling water 34 for descriptive purposes. The cooling water 34 passes through a lower plenum 29 and is supplied to the core 2. The cooling water 34 is heated while passing the core 2 and becomes a two-phase flow including water and steam. The steam separator 4 separates the gas-liquid two-phase flow into steam and water. The steam dryer 5 removes further moisture from the separated steam, and the steam from which the moisture is removed is exhausted to the main steam pipe 39. This steam is introduced to a steam turbine (not shown) and turns the steam turbine. A power generator (not shown) coupled to the stream turbine rotates to generate power. The steam exhausted from the steam turbine becomes water through condensation in a condenser (not shown). This condensed water is supplied into the RPV 1 as feed water through the feed water pipe 28. The water separated by the separator 4 and the dryer 5 descends and reaches the downcomer 6 as cooling water. The jet pump 11 of the present embodiment, which has the nozzle apparatus 12, the bell mouth 21, the throat 22, and the diffuser 25 as its main components, can supply more cooling water 34 to the core with less driving flow 30 by sucking the cooling water around the nozzle apparatus 12 in the downcomer 6. When the kinetic energy of the driving flow 30 given by the recirculation pump 8 effectively acts on the cooling water 32, more cooling water 32 is sucked into the jet pump 11 and the flow rate of the cooling water 34 is increased more. The jet pump 11 reduces static pressure in the throat 22 by ejecting the driving flow 30 (the jet flow 31) at high speed from the nozzle apparatus 12 into the throat 22. This makes the throat 22 suck in the cooling water 32, and allows the necessary core flow rate to be obtained with a small amount of power. The diffuser 25 has a flow passage cross-sectional area which gradually increases toward the downstream direction within a degree that prevents detachment of cooling water flow. This diffuser 25 changes the kinetic energy of the cooling water into pressure. In the diffuser 25, the pressure of the suction flow 32 is raised higher than the pressure at the position where the suction flow is sucked into the bell mouth 21. A flow passage cross-sectional area of the bell mouth 21 increases toward the upstream direction. The bell mouth 21, the throat 22, and the diffuser 25 are disposed in this order from the upper position to the lower position. A jet pump body comprises the bell mouth 21, the throat 22, and the diffuser 25. The nozzle apparatus 12 is disposed above the bell mouth 21. A structure of a slip joint 26 in the jet pump 11 of the present embodiment is described with reference to FIG. 2. This slip joint 26 has, in a lower end portion (a downstream end portion) of the throat 22, a flow passage reduction portion 23 whose flow passage cross-sectional area gradually diminishes toward a lower end of the throat 22. An inner diameter D6 of a downstream end (the lower end) of the flow passage reduction portion 23 is smaller than an inner diameter D5 of an upstream end (an upper end) of the flow passage reduction portion 23. Part of this flow passage reduction portion 23 is inserted into an upper end portion (an upstream end portion) of the diffuser 25. The flow passage reduction portion 23 has a thick-wall portion 24 on the outer surface. Formation of this thick-wall portion 24 reduces the width of a gap 27 in the radial direction of the throat 22, which gap is formed between the flow passage reduction portion 23 and the diffuser 25. A detailed structure of the nozzle apparatus 12 in the jet pump 11 is described below with reference to FIGS. 3, 5, and 6. The nozzle apparatus 12 has a nozzle base (a nozzle base member) 13 and six nozzles 14. The nozzle base 13 of the nozzle apparatus 12 is fixed to the bell mouth 21 by using the supporting plates 33 to make up one body, and connected to the elbow pipe 10. The nozzle apparatus 12 is disposed above the bell mouth 21. The nozzle base 13 has a protrusion 36 protruding downward, in the center of the nozzle apparatus. The six nozzles 14 are fixed to the nozzle base 13 in an annular arrangement, disposed around the protrusion 36. These nozzles 14 extend toward the bell mouth 21 from the nozzle base 13. A detailed structure of the six nozzles 14 provided to the nozzle apparatus 12 is described with reference to FIG. 6. In the nozzle 14, when the inner diameters, that is, the passage diameters of a jet passage 35 formed inside the nozzle 14, are sequentially defined as D1, D2, and D3 from the upstream end to the downstream end of the nozzle 14, these inner diameters have a relationship which is D1>D2>D3. In the nozzle apparatus 12, the nozzle 14 has a nozzle straight-tube portion 15, a nozzle narrowing portion 16, a nozzle straight-tube portion 17, a nozzle narrowing portion 18, and a nozzle lower end portion 19. The nozzle straight-tube portion 15 positioned in an uppermost position has a uniform inner diameter of D1. In the nozzle narrowing portion 16, which is the first stage of narrowing, connected to a downstream end of the nozzle straight-tube portion 15, a flow passage cross-sectional area in the narrowing portion 16 decreases toward a lower end of the nozzle 14, an inner diameter at an upper end is D1, the inner diameter at a lower end is D2, and the length is L1. The nozzle straight-tube portion 17 connected to the downstream end of the nozzle narrowing portion 16 has a uniform inner diameter of D2. In the nozzle narrowing portion 18, which is the second stage of narrowing, connected to a downstream end of the nozzle straight-tube portion 17, a flow passage cross-sectional area in the narrowing portion 18 decreases toward the lower end of the nozzle 14, an inner diameter at an upper end is D2, the inner diameter at a lower end is D3, and the length is L2. The nozzle lower end portion 19 located in a lowest position of the nozzle 14, connected to the lower end of the nozzle narrowing portion 18 has an inner diameter of D3 and forms an ejection outlet 20 in the end portion. Unlike the nozzle in Japanese Patent Laid-open No. Sho 59 (1984)-188100, in which a nozzle narrowing portion is formed only in one place in its end portion, the nozzle 14 narrows the jet passage 35 in two places in the nozzle narrowing portions 16 and 18. A narrowing angle θ1 of the nozzle narrowing portion 16 and a narrowing angle θ2 of the nozzle narrowing portion 18 can be calculated by the following Equation (1) and Equation (2) respectively.θ1=tan−1((D1−D2)/2/L1) (1)θ2=tan−1((D2−D3)/2/L2) (2) The narrowing angle θ2 of the nozzle narrowing portion 18 near the ejection outlet 20 is larger than the narrowing angle θ1 of the nozzle narrowing portion 16 (θ2>θ1). The nozzle straight-tube portion 15 having a larger flow passage cross-sectional area is disposed upstream from the nozzle narrowing portion 16, and the nozzle straight-tube portion 17 having a smaller flow passage cross-sectional area is disposed downstream from the nozzle narrowing portion 16 respectively. The nozzle lower end portion 19, which is a straight tube having an inner diameter of D3 and the ejection outlet 20 in its end, is preferably disposed at an outlet portion of the nozzle 14, that is, the lower end portion of the nozzle 14. However, in order to improve the flow speed of the jet flow 31 ejected from the ejection outlet 20, a nozzle narrowing portion having a flow passage cross-sectional area which gradually decreases toward the downstream end may be used in place of the nozzle lower end portion 19 being the straight-tube. When the nozzle narrowing portion having a flow passage cross-sectional area which gradually decreases toward the downstream end is used as the nozzle lower end portion 19, it is preferable to reduce the narrowing angle θ of the nozzle narrowing portion 18 of this nozzle to approximately less than 2 degrees in order to keep the spreading of the jet flow 31 from the ejection outlet 20 of the nozzle lower end portion 19, within a desirable range. The driving flow 30 discharged from the recirculation pump 8 during the operation of the boiling water reactor is introduced through the raiser pipe 9 and the elbow pipe 10 and supplied into the nozzle base 13 of the nozzle apparatus 12. This driving flow 30 is introduced to the jet passage 35 of each nozzle 14. A flow passage cross-sectional area of the jet passage 35 varies according to the inner diameters of the nozzle straight-tube portion 15, the nozzle narrowing portion 16, the nozzle straight-tube portion 17, the nozzle narrowing portion 18, and the nozzle lower end portion 19 disposed from the upper position to the lower position. The driving flow 30 flowing into the jet passage 35 flows through the nozzle straight-tube portion 15, the nozzle narrowing portion 16, the nozzle straight-tube portion 17, and the nozzle narrowing portion 18, and reaches the nozzle lower end portion 19. The driving flow 30 descends in the jet passage 35 gradually gains speed in the nozzle narrowing portion 16, and gains speed even faster in the nozzle narrowing portion 18 than in the nozzle narrowing portion 16. The accelerated driving flow 30 is ejected from the ejection outlet 20 into the throat 22. In the nozzle narrowing portion 18, a velocity component toward the central axis of the nozzle 14 is given to the driving flow 30. However, since a fluid has a characteristic to flow along a wall surface, the jet flow 31 ejected from the ejection outlet 20 formed at the lower end of the nozzle lower end portion 19 has a diameter of D3. Since the larger the narrowing angle θ2 of the nozzle narrowing portion 18, the more the momentum flows toward the central axis of the nozzle, the spreading of the jet flow 31 ejected from the ejection outlet 20 can be suppressed. As a consequence, and the diameter D4 of the jet flow 31, which is a distance L3 away from the ejection outlet 20 in the downstream direction, can be small within a desirable range. The diameter D4 of the jet flow 31 is a width of the jet flow 31. The smaller the diameter D4 of the jet flow 31, the faster the speed of this jet flow. When the jet flow 31 is ejected from the nozzle 14 into the throat 22 while the spreading of the jet flow 31 is suppressed and its speed maintained, the static pressure inside the throat 22 is reduced, making more suction flow 32 around the nozzle apparatus 12 in the downcomer 6 to be sucked into the bell mouth 21. Assume that no nozzle lower end portion 19 is disposed downstream from the nozzle narrowing portion 18. In this case, a diameter of the jet flow 31 keeps decreasing even after being ejected because of the momentum of the driving flow 30 toward the central axis of the nozzle 14, given in the nozzle narrowing portion 18. That is, since no straight-tube portion of the nozzle lower end portion 19 is provided, the jet flow 31 ejected from the ejection outlet 20 formed in the lower end of the nozzle 14 is affected by the nozzle narrowing portion 18. This makes the diameter D4 of the jet flow 31 at the distance L3 away from the ejection outlet 20 in the downstream direction, smaller than the inner diameter D3 of the ejection outlet 20. Thus, the jet speed is raised and the acceleration loss is increased, reducing the flow rate of the driving flow 30. For this reason, the nozzle lower end portion 19 being the straight-tube portion is installed in the downstream side of the nozzle narrowing portion 18 to keep the diameter of the jet flow 31 ejected from the ejection outlet 20 to be no smaller than the inner diameter D3 of the nozzle lower end portion 19 being the straight-tube portion. The installation of the nozzle lower end portion 19 prevents the reduction in the flow rate of the driving flow 30 caused by the increase in the acceleration loss. In addition, the nozzle narrowing portions are provided to the nozzle 14 in two or more locations to reduce the 25, pressure loss in the nozzle 14 as well as to widen the flow passage for the suction flow 32, formed between the nozzles 14. Next, the following case is considered where the inner diameter of the ejection outlet 20 is fixed to D3, the nozzle narrowing portion 16 is made straight, each inner diameter of the nozzle straight-tube portion 15 and the nozzle narrowing portion 16, which is now a straight tube, is set to D2, and a nozzle narrowing portion formed in the nozzle 14 is only in one place in the nozzle narrowing portion 18. When the length L2 of the nozzle narrowing portion 18 is unchanged, the flow passage cross-sectional areas of the nozzle straight-tube portion 15 and the nozzle narrowing portion 16, which is now straight, become smaller, increasing the flow speed of the driving flow 30 flowing inside. Consequently, a loss in friction is increased and the flow rate of the driving flow 30 is reduced. When the length L2 of the nozzle narrowing portion 18 is extended to enlarge the flow passage cross-sectional area of the nozzle narrowing portion 18 in the upstream side, the outer diameter of the nozzle 14 becomes larger and a flow passage cross-sectional area of the suction flow 32 formed among the plurality of nozzles 14 becomes smaller, reducing the suction amount of the suction flow 32 into the bell mouth 21. Therefore, in the present embodiment that two or more nozzle narrowing portions are provided to the nozzle 14, a flow passage cross-sectional area of the jet passage 35 in the nozzle 14 becomes smaller toward the ejection outlet 20, and the flow speed of the driving flow 30 flowing in the jet passage 35 is increased. Because of this, the area where the loss in friction is increased in the jet passage 35 can be reduced. In addition, since the outer diameter of the nozzle 14 can be made smaller below the nozzle narrowing portion 16, a space 37 (see FIG. 5) formed among the nozzles 14 can be larger, and the flow rate of the suction flow 32 sucked into a region 38 (see FIG. 3) inside the six nozzles 14 can be increased. As a result, the flow rate of the suction flow 32 sucked into the throat 21 is increased. As described above, the driving flow 30 flowing into the jet passage 35 is accelerated in the jet passage 35 by the nozzle narrowing portions 16 and 18, and ejected from the ejection outlet 20 into the throat 22 as the jet flow 31. In the present embodiment, the spreading of the jet flow 31 is kept small so that the speed of the jet flow 31 reached inside the throat 22 is higher, reducing the static pressure inside the throat 22. As a result, more suction flow 32 can be sucked into the throat 22. The present embodiment provides the nozzle 14 having two nozzle narrowing portions 16 and 18 so that the flow rate of the suction flow 32 sucked into the throat 22 can be increased, by the above-described working of the nozzle 14, more than the conventional jet pump disclosed in Japanese Patent Laid-open No. Sho 59 (1984)-188100 which provides five nozzles, each having one stage of a narrowing portion and a straight-tube portion. For this reason, the flow rate of the cooling water 34 discharged from the jet pump 11 is increased, and the efficiency of the jet pump 11 in a high-M ratio range can be improved more than that of the conventional jet pump. An example of a change in the differential pressure between the inside of the jet pump and the downcomer 6 in the axial direction of the jet pump from the inlet of the throat to the outlet of the diffuser is shown in FIG. 7. In FIG. 7, the broken line shows a characteristic of a conventional jet pump having five nozzles, which has been used in a boiling water reactor of a million kW class. As shown here, the high-speed ejection of a driving flow from the nozzle causes the static pressure in the throat to be lower than the static pressure in the downcomer 6, making the differential pressure between the inside and the outside of the throat inlet portion negative. The differential pressure between the inside of the jet pump and the downcomer 6 becomes positive at a position of a slip joint, and a magnitude of this positive pressure increases toward the diffuser outlet. In the conventional jet pump, in the lower portion of the throat, the static pressure in the throat is recovered by gradually increasing the flow passage cross-sectional area toward the downstream end of the throat. When the static pressure in the jet pump at the position of the slip joint is larger than the static pressure in the downcomer 6 at the same location, cooling water starts to leak from the inside of the jet pump to the downcomer 6 through a gap in the slip joint. When the amount of this leakage flow is excessive, the jet pump may vibrate undesirably. In the slip joint 26 of the jet pump 11 of the present embodiment, as described above, the flow passage reduction portion 23 formed in the downstream end portion of the throat 22 is inserted into the upstream end portion of the diffuser 25 so that the flow speed of the cooling water flowing into the diffuser 25 from the flow passage reduction portion 23 is increased, reducing the static pressure in the diffuser 25 in the vicinity of the downstream end of the flow passage reduction portion 23. This reduces the difference between the static pressure in the jet pump 11, that is, the static pressure in the diffuser 25, and the static pressure in the downcomer 6 at the installation position of the slip joint 26. By using the method that can reduce the difference between these static pressures, the amount of the cooling water leaking to the downcomer 6 through the gap 27 in the slip joint 26 can be reduced more surely than by using the method such as in Japanese Examined Patent Application Publication Sho 59 (1984)-48360 which provides a labyrinth seal whose effect in reducing the leakage flow is limited by an available range of fabrication. Consequently, in the present embodiment, the vibration of the jet pump 11 can be controlled. The solid line in FIG. 7 shows a change in the differential pressure between the inside of the jet pump 11 and the downcomer 6, when the jet pump 11 in the present embodiment is used, in which jet pump 11, the inner diameter of the downstream end of the throat 22 is made, by forming the flow passage reduction portion 23, 6% smaller than the inner diameter of the downstream end of the throat in the conventional jet pump having no flow passage reduction portion 23. In the present embodiment, the static pressure starts to decrease at the starting point of the flow passage reduction portion 23, and the differential pressure between the inside of the jet pump 11 and the downcomer 6 at the position of the slip joint 26 drops to about a half of that in the conventional example shown in the broken line. After that, the velocity energy of the cooling water is changed to pressure as the flow passage cross-sectional area in the diffuser 25 is increased, recovering the pressure in the diffuser 25. The drop in the differential pressure between the inside of the jet pump 11 and the downcomer 6 at the position of the slip joint 26 reduces the vibration of the jet pump 11 as described above. However, the present embodiment increases a pressure loss more than the conventional jet pump because of the formation of the flow passage reduction portion 23. As a result, in the present embodiment shown in the solid line, the pressure at the outlet of the diffuser 25 is lower than that in the conventional example shown in the broken line (see FIG. 7). This reduces the flow rate of the cooling water 34 supplied to the core 2 from the jet pump. In other words, the formation of the flow passage reduction portion 23 reduces the efficiency of the jet pump. The jet pump 11 of the present embodiment, as described above, tries to improve the efficiency of the jet pump by installing the nozzle apparatus 12 having six nozzles 14 with two stages of nozzle narrowing portions. In the jet pump 11, the reduction in the jet pump efficiency due to the formation of the flow passage reduction portion 23 can be compensated by part of the improvement in the jet pump efficiency achieved by using the nozzle apparatus 12. Thus, the jet pump 11 can prevent the vibration of the jet pump and at the same time, can improve the efficiency of the jet pump more than the conventional jet pump. The improvement in the efficiency of the jet pump of the present embodiment is explained in detail with reference to FIG. 8. In FIG. 8, the broken line shows the efficiency of the conventional jet pump (the conventional jet pump having the characteristic shown by the broken line in FIG. 7) having a nozzle apparatus with five nozzles. In this conventional jet pump, a flow passage cross-sectional area of the downstream end of the throat is set to a conventional ratio of 100%, and each nozzle has one stage of narrowing portion as in the jet pump disclosed in Japanese Patent Laid-open No. Sho 59 (1984)-188100. The alternate long and short dash line in FIG. 8 shows the efficiency of a jet pump of a comparative example, in which the throat in the conventional jet pump having the characteristic shown in the broken line is replaced with a throat having the same flow passage reduction portion 23 as in the present embodiment, in the lower end portion. In the jet pump of the comparative example, a flow passage cross-sectional area of the downstream end of the flow passage reduction portion 23 is 90% of a flow passage cross-sectional area of the corresponding position in the conventional jet pump having the characteristic shown in the broken line. For this jet pump, since the pressure loss is increased by forming the flow passage reduction portion 23 in the throat, the efficiency of the jet pump is lower than that shown in the broken line. The efficiency of the conventional jet pump having the flow passage reduction portion in the throat is reduced by approximately 0.7%. In FIG. 8, the solid line shows the jet pump efficiency of the jet pump 11 of the present embodiment. In the jet pump 11, a flow passage cross-sectional area of the downstream end of the flow passage reduction portion 23 in the throat 22 is also 90%. In the jet pump 11, the reduction in the jet pump efficiency caused by the formation of the flow passage reduction portion 23 in the throat 22 is covered by the increase in the jet pump efficiency achieved by using the nozzle apparatus 12. As a result, the jet pump efficiency is improved more than the jet pump efficiency of the jet pump of the conventional example shown in the broken line. In the present embodiment, the efficiency of the jet pump is improved by approximately 3% more at the peak compared to that of the conventional jet pump without the flow passage reduction portion in the throat. In the jet pump 11 of the present embodiment, the number of the nozzles 14 is increased to six. By using two stages of the nozzle narrowing portions 16 and 18, the spreading of the jet flow 31 ejected from the ejection outlet 20 can be kept small, suppressing the reduction in the speed of the jet flow 31 that has reached the inlet of the throat 22 as well as the decrease in the suction area for the suction flow 32 in the throat 22. This allows more suction flow 32 to be sucked into the throat 22 at the same ejecting speed of the jet flow 31. In addition, in the present embodiment, the total flow passage cross-sectional area of the ejection outlets 20 of the six nozzles 14 is made the same as that of the conventional five nozzles, while making the total length of wetted perimeter of the six nozzles 14 approximately 9% more than that of the conventional five nozzles. This increases the contact area between the suction flow 32 and the jet flow 31 of the driving flow 30 ejected from the ejection outlet 20, making both fluids to be mixed faster, which reduces a loss during the mixing. The jet pump 11 of the present embodiment can improve the jet pump efficiency compared to the conventional jet pump disclosed in Japanese Patent Laid-open No. Sho 59 (1984)-188100 which provides five nozzles, each having one stage of a narrowing portion and a straight-tube portion. In the present embodiment, since the narrowing angle θ2 of the nozzle narrowing portion 18 is made larger than the narrowing angle θ1 of the nozzle narrowing portion 16, the spread of the jet flow 31 is suppressed and which prevents the reduction in the speed of the driving flow 30 at the inlet of the throat 22 is also suppressed. At the same time, since the nozzle lower end portion 19 forming the ejection outlet 20 is provided, it can be prevented to accelerate excessively the driving flow 30 by the narrowing portion and to increase the pressure loss in the nozzle 14. Since the speed of the driving flow 30 in the throat 22 is not much slower than the speed at the ejection outlet 20, the static pressure in the throat 22 is reduced and the suction amount of the suction flow 32 into the throat 22 is increased. Consequently, the M ratio and the efficiency of the jet pump can be improved. In a boiling water reactor, a rotational speed of the recirculation pump 8 is controlled to adjust a flow rate of cooling water supplied to the core 2 (a core flow rate). The improvement in the M ratio and the jet pump efficiency allows the core flow rate to be increased using less power from the recirculation pump. Thus, power consumption for driving the recirculation pump 8 can be reduced. In addition, when a power upgrade of a reactor in the U.S. is to be implemented, the core flow rate can be further increased without increasing the capacity of the recirculation pump 8 by employing, for the existing reactor, the jet pump 11 of the present embodiment which can increase the M ratio and the jet pump efficiency. For this reason, the power upgrade of the boiling water reactor can be easily achieved. A jet pump according to embodiment 2, which is another embodiment of the present invention, is described below. The jet pump is also applied to a boiling water reactor. A jet pump 11A of the present embodiment has a structure in which the nozzle apparatus 12 in the jet pump 11 of the embodiment 1 is replaced with a nozzle apparatus 12A. Other components of the jet pump 11A are the same as the jet pump 11. The nozzle apparatus 12A is explained below with reference to FIGS. 9 and 10. In the jet pump 11A, minimizing the loss in pressure and making the most of the suction power induced by a driving flow are both important to increase the M ratio and the N ratio and to raise the efficiency of the jet pump. The jet pump 11A of the present embodiment has an inner cooling water suction passage 50 in and through the nozzle apparatus 12A in the axial direction. The inner cooling water suction passage 50 has, in its upper end, an opening 51 which communicates with the downcomer 6. Furthermore, in the jet pump 11A, the inner cooling water suction passage 50 extends upward inside the elbow pipe 10, and the opening 51 is formed on the outer surface of the elbow pipe 10 at a position lower than a top point TP of the elbow pipe 10. The nozzle apparatus 12A, as shown in FIG. 9, has a nozzle portion 40 and a nozzle header portion 46. The nozzle header portion 46 has an outer cylinder member 47 and an inner cylinder member 48 disposed inside the outer cylinder member 47. An annular header portion 49 is formed between the outer cylinder member 47 and the inner cylinder member 48, both of which are disposed in a concentric manner. The nozzle portion 40 is disposed below the nozzle header portion 46, and fixed to a lower end portion of the nozzle header portion 46. The nozzle portion 40 has an outer cylinder member 41, an inner cylinder member 42, an outer funnel member 43, and an inner funnel member 44. The outer cylinder member 41 surrounds the inner cylinder member 42, and the outer cylinder member 41 and the inner cylinder member 42 are concentrically disposed. The outer funnel member 43 surrounds the inner funnel member 44, and the outer funnel member 43 and the inner funnel member 44 are concentrically disposed. The outer funnel member 43 and the inner funnel member 44 each have a cross-sectional area that decreases downward. The outer funnel member 43 is fixed to an upper end of the outer cylinder member 41, and the inner funnel member 44 is fixed to an upper end of the inner cylinder member 42. The outer funnel member 43 is attached to a lower end of the outer cylinder member 47. The inner funnel member 44 is attached to a lower end of the inner cylinder member 48. An annular ejection outlet 20A is formed between the outer cylinder member 41 and the inner cylinder member 42. An outlet end 53 of the elbow pipe 10 is fixed to the nozzle header portion 46, that is, an upper end of the outer cylinder member 47. An inlet end 52 of the elbow pipe 10 is placed on an upper end of the branching pipe 60. The elbow pipe 10 and the branching pipe 60 are detachably coupled with a fixture. The center of the outlet end 53 of the elbow pipe 10 matches an axis of the nozzle header portion 46, that is, the outer cylinder member 47. The nozzle portion 40, the nozzle header portion 46, and the elbow pipe 10 are joined into one body by welding. The inner cylinder member 48 is inserted into the elbow pipe 10 through the outlet end 53, and extends upward. The opening 51 located in the upper end portion of the inner cylinder member 48 is formed on the outer surface of the elbow pipe 10 and communicates with the downcomer 6. The upper end of the inner cylinder member 48 is welded to the elbow pipe 10. A joint (a fixed position) 57 which is the highest position in a joint portion (a fixed portion) between the inner cylinder member 48 and the elbow pipe 10 is disposed lower than the top point TP which is the highest position on the outer surface of the elbow pipe 10. A flow-adjusting plate (a flow-adjusting member) 54 having the same curvature as the elbow pipe 10 is installed inside the elbow pipe 10, and disposed from the inlet end 52 of the elbow pipe 10 to the inner cylinder member 48 along the axis of the elbow pipe 10. The flow-adjusting plate 54 is disposed upstream from the inner cylinder member 48. An upper passage 55 and a lower passage 56 that are separated into the top and the bottom, are formed in the elbow pipe 10 by the installation of the flow-adjusting plate 54. Since the joint 57 is located lower than the top point TP, the upper flow passage 55 and the lower passage 56 in the elbow pipe 10 extending toward the outlet end 53 are formed diagonally to the axis of the inner cylinder member 48. In other words, the upper passage 55 and the lower passage 56 are formed so that the driving flows in the flow passages flowing toward the outlet end 53, hitting the inner cylinder member 48 diagonally in relation to the axial direction of the inner cylinder member 48. The inner cooling water suction passage 50 communicating with the downcomer 6 through the opening 51 is formed inside the joined inner cylinder member 48, inner funnel member 44, and inner cylinder member 42. The joined inner cylinder member 48, inner funnel member 44, and inner cylinder member 42 are a first pipe member. The inner cooling water suction passage 50 has a flow passage cross-sectional area which gradually decreases downward in the inner funnel member 44, and its lower end opens toward the bell mouth 21. An annular passage 45 formed between the outer funnel member 43 and the inner funnel member 44, communicating with the annular header portion (an annular passage) 49 and the annular ejection outlet 20A, has a flow passage cross-sectional area which gradually decreases downward. The driving flow pressurized by the recirculation pump 8 during the operation of the boiling water reactor reaches the raiser pipe 9 and is introduced into the annular header portion 49 through the elbow pipe 10. The flow-adjusting plate 54 disposed in the elbow pipe 10 reduces the pressure loss in the elbow pipe 10. Part of the driving flow flowing in each of the upper passage 55 and the lower passage 56 in the elbow pipe 10 toward the outlet end 53 hits the outer surface of the inner cylinder member 48 diagonally in relation to the axial direction of the first pipe member (the inner cylinder member 48 in particular). The driving flow introduced into the annular header portion 49 flows through the annular passage 45 and is ejected at high speed into the bell mouth 21 from the annular ejection outlet 20A. The cross-sectional area of the jet flow of the driving flow ejected from the annular ejection outlet 20A is annular. The high-speed supplying of the jet flow of the driving flow into the throat 22 reduces the static pressure in the throat 22, making the cooling water around the nozzle apparatus 12A in the downcomer 6 to be sucked into the bell mouth 21. There are two patterns for the cooling water being the suction flow around the nozzle apparatus 12A to be sucked into the bell mouth 21 due to the reduction in the static pressure in the throat 22. The first pattern is that the cooling water above the elbow pipe 10 flows into the inner cooling water suction passage 50 from the opening 51, and reaches the bell mouth 21 through the inner cooling water suction passage 50. In this pattern, the cooling water sucked through the inner cooling water suction passage 50 flows into the inside of the annular jet flow ejected from the annular ejection outlet 20A. The second pattern is that the cooling water in the downcomer 6 passes through an outer cooling water suction passage 58 formed between the nozzle portion 40 and the bell mouth 21, and reaches the bell mouth 21 at the outside of the annular jet flow. The driving flow ejected from the annular ejection outlet 20A and the cooling water (the suction flow) sucked into the bell mouth 21 by the working of the driving flow are mixed in the throat 22 by exchanging their momentum, and introduced to the diffuser 25 located below the throat 22. The cooling water 34 discharged from the diffuser 25 is introduced to the core 2 through the lower plenum 29. In the present embodiment, since the joint portion 57 is positioned lower than the top point TP, the upper passage 55 and the lower passage 565 in the elbow pipe 10 are formed toward the outlet end 53, diagonally to the inner cylinder member 48 forming the inner cooling water suction passage 50 in the axial direction of the inner cylinder member 48. For this reason, the pressure loss in the elbow pipe 10 where the inner cylinder member 48 exists is reduced, and the flow speed of the cooling water ejected from the annular ejection outlet 20A is increased. The reduction amount of the static pressure in the throat 22 is increased, increasing the flow rate of the cooling water sucked into the bell mouth 21 through the inner cooling water suction passage 50 and the outer cooling water suction passage 58. This increase in the flow rate of the cooling water improves the efficiency of the jet pump 11A. This improvement in the efficiency of the jet pump 11A is explained. FIG. 11 shows a relationship between the M ratio and the jet pump efficiency of a jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat, and that of a jet pump of a comparative example. In FIG. 11, the solid line shows a characteristic of the jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat, and the broken line shows a characteristic of the comparative jet pump. The jet pump of the comparative example uses the nozzle apparatus shown in FIG. 3 of Japanese Patent Laid-open No. 2001-90700 as a nozzle for the jet pump in a SWR, disclosed in U.S. Pat. No. 3,625,820. While in the comparative example, a pressurized driving flow hits an inner cylinder of the nozzle apparatus at a right angle, in the jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat, a driving flow flowing through a cooling water passage in the elbow pipe 10 hits the inner cylinder member 48 diagonally as described above. Because of such a difference in the driving flows, the pressure loss of the jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat is less than that of the comparative example. Consequently, in the jet pump having the nozzle apparatus 12A with no flow passage reduction portion in the throat, the efficiency of the jet pump is increased by more than that of the comparative example for the amount of the reduced pressure loss in the nozzle. Since the jet pump 11A of the present embodiment has the flow passage reduction portion 23 in the lower end portion of the throat 22 in the same manner as the jet pump 11 of the embodiment 1, in the jet pump 11A, the flow passage reduction portion 23 causes the efficiency of the jet pump to decrease. However, this reduction in the efficiency can be compensated for by part of the increase in the efficiency achieved by the nozzle apparatus 12A. From the contribution of the remaining increase in the efficiency achieved by the nozzle apparatus 12A, the jet pump 11A, thus, can improve the efficiency of the jet pump more than that of the comparative example. In the present embodiment, a leakage flow from the gap 27 in the slip joint 26 can be reduced because the flow passage reduction portion 23 is formed in the lower end portion of the throat 22. For this reason, the vibration of the jet pump 11A can be suppressed. In the present embodiment, since the flow-adjusting plate 54 is installed in the elbow pipe 10, the pressure loss in the elbow pipe 10 can be further reduced. The reduction in the pressure loss further increases the efficiency of the jet pump 11A. Since the flow-adjusting plate 54 is disposed upstream from the inner cylinder member 48, separation of the flow and uneven distribution of speed in the elbow pipe 10 are improved, and the pressure loss in the elbow pipe 10 is reduced. Since the cooling water passages (the upper passage 55 and the lower passage 56) formed in the elbow pipe 10 are diagonal to the inner cylinder member 48 as described above, the driving flow flowing in the cooling water passages hits the outer surface of the inner cylinder member 48 diagonally to the axial direction of the inner cylinder member 48. This causes the stress generated at the contact portion between the inner cylinder member 48 and the elbow pipe 10 to be small. Thus, when the nozzle apparatus 12A is applied to a current BWR, it is not necessary to reinforce the joint portion by making the member particularly thick, or to modify the raiser pipe 9 and the fixture. In the present embodiment, Since the inner cooling water suction passage 50 is formed in the nozzle apparatus 12A, the effect of the pressure reduction in the area inside the ejected annular jet flow can be effectively used. This generates the flow of the cooling water reaching into the bell mouth 21 through the inner cooling water suction passage 50. Thus, since cooling water can flow into the bell mouth 21 through each of the inner cooling water suction passage 50 and the outer cooling water suction passage 58, the flow rate of the cooling water flowing into the bell mouth 21 is increased. The inner cooling water suction passage 50 is disposed in the axial direction of the RPV 1, and the opening 51 opens upward, so that the flow power of the cooling water descending in the downcomer 6, supplied to the inner cooling water suction passage 50, can be effectively utilized to increase the suction power of the jet pump 11A. This increases the rate of the cooling water sucked into the throat 22. In addition, since the nozzle portion 40 has the outer funnel member 43 whose outer diameter decreases downward, the nozzle apparatus 12A has a structure that allows the cooling water descending in the downcomer 6 to be easily sucked into the bell mouth 21 through the outer cooling water suction passage 58. This also increases the flow rate of the cooling water flowing into the bell mouth 21, increasing the efficiency of the jet pump 11A. In the boiling water reactor installed with the jet pump 11A, the core flow rate can be further increased without increasing the capacity of the recirculation pump 8 in the same manner as in the embodiment 1. For this reason, a power upgrade of the boiling water reactor can be easily achieved. Furthermore, in the present embodiment, the inverted U-shaped elbow pipe 10 is connected to the nozzle apparatus 12A so that a single raiser pipe 9 disposed in the downcomer 6 can be connected to two jet pumps 11A adjacent to the raiser pipe 9, with the elbow pipes 10 each connected to the nozzle apparatus 12A of each of the two jet pumps 11A. For this reason, a space between the jet pumps 11A can be made equal to the corresponding space in the existing boiling water reactor. A jet pump according to embodiment 3, which is another embodiment of the present invention, is described below. A jet pump 11B of the present embodiment has a structure in which the nozzle apparatus 12 in the jet pump 11 in the embodiment 1 is replaced with a nozzle apparatus 12B. Other components of the jet pump 11B are the same as the jet pump 11. The nozzle apparatus 12B is explained below with reference to FIG. 12. The nozzle apparatus 12B, as shown in FIG. 12, has a nozzle portion 61, a suction passage portion 65, and a nozzle holder 78. The suction passage portion 65 is disposed above the nozzle portion 61, and installed on the upper end of the nozzle portion 61. The nozzle holder 78 is disposed above the suction passage portion 65 and installed on an upper end of the suction passage portion 65. The suction passage portion 65 has a cylinder member (a third tubular member) 66, a flow passage forming member 67, and a passage member 72. The flow passage forming member 67 is disposed inside the cylinder member 66 in the center of the cylinder member 66. Six passage members 72 are disposed radially around the central axis of the cylinder member 66, 60 degrees apart from each other in the circumferential direction (see FIG. 13). The outer end portion of the passage member 72 is welded to the cylinder member 66, and the inner end portion of the passage member 72 is welded to the flow passage forming member 67. Each passage member 72 slopes downward and inward (toward the flow passage forming member 67), and has an oval cross-sectional area (see FIG. 15). An opening 74 is formed in the outer end portion of the passage member 72. An annular driving flow passage 76 is formed between the cylinder member 66 and the flow passage forming member 67. Each passage member 72 is placed across this driving flow passage 76. A suction passage 73 communicating with the downcomer 6 through the opening 74 is formed in each passage member 72. The inner surfaces of each passage member 72 at the inlet and the outlet of each suction passage 73 are curved surfaces. The total flow passage cross-sectional area of all the suction passages 73 is larger than the cross-sectional area of a decompression chamber (an inner region) 77 at the lower end of the nozzle portion 61. Each passage member 72 is provided with a streamline member 75 (see FIG. 15) having a cross-sectional area that decreases toward the upper course to reduce the pressure loss in the driving flow passage 76. The flow passage forming member 67 has a circular cross section at any point in the axial direction, and includes an upper region 68, a center region 69, and a lower region 70, each having a different cross-sectional area in the axial direction. The upper region 68 is cylindrical, and the center region 69 connected to a lower end of the upper region 68 is a truncated cone. The lower region 70 connected to a lower end of the center region 69 is an inverted cone. The center region 69 has a cross-sectional area that increases downward. This reduces a cross-sectional area of the driving flow passage 76 downward between the cylinder member 66 and the outer surface of the center region 69. The lower region 70 has a cross-sectional area that decreases downward, and has a curved surface 71 whose outer surface comes together in the axial direction. The nozzle portion 61 has an outer cylinder member (a first tubular member) 62 and an inner cylinder member (a second tubular member) 63 disposed in the outer cylinder member 62. The outer cylinder member 62 is welded to a lower end of the cylinder member 66, and an upper end of the inner cylinder member 63 is welded to the flow passage forming member 67. The outer cylinder member 62 has an outer diameter that is smaller in the lower end than in the upper end, and slopes inward. The inner cylinder member 63 has an outer diameter that becomes the largest in a center portion and smaller in the upper and a lower ends. An inner end portion of the passage member 72 is welded to the upper portion rather than the center portion of the inner cylinder member 63. Therefore, the inner cylinder member 63 exists between the adjacent passage members 72 in the circumferential direction of the inner cylinder member 63. An annular jet passage 64 is formed between the outer cylinder member 62 and the inner cylinder member 63. The annular jet passage 64 slopes inward, and has a flow passage cross-sectional area that becomes smaller downward. The jet passage 64 communicates with the driving flow passage 76. The jet passage 64 is also a part of the driving flow passage. An annular ejection outlet 20B is formed at the end of the jet passage 64. The decompression chamber 77 is formed in the inner cylinder member 63, and the suction passage 73 communicates with the decompression chamber 77. The curved surface 71 of the lower region 70 of the flow passage forming member 67 faces the decompression chamber 77. The inner cylinder member 63 separates the driving flow passage 76 and the decompression chamber 77. The nozzle holder 78 has a cylinder member 81, a reinforcing streamline plate 79, and a cone member 80. The cylinder member 81 is fixed to the upper end of the cylinder member 66 of the suction passage portion 65. The cone member 80 has a cross-sectional area that decreases upward, and disposed in the center of the cylinder member 81. Six reinforcing streamline plates (see FIG. 14) 79 are radially disposed around the central axis of the cylinder member 81, 60 degrees apart from each other in the circumferential direction, and disposed in the positions above the passage members 72 (see FIG. 13). The both ends of each reinforcing streamline plate 79 are fixed to the cylinder member 81 and the cone member 80. A lower end portion of the cone member 80 is fitted to the upper end portion of the flow passage forming member 67. An upper end of the cylinder member 81 is connected to the elbow pipe 10. It can be said that when the nozzle portion 61 and the suction passage portion 65 are unified, the outer cylinder member 62 and the cylinder member 66 make up the first tubular member and the inner cylinder member 63 is the second tubular member. Between these first and second tubular members, the driving flow passage including the jet passage 64 is formed. The driving flow 30 pressurized by the recirculation pump 8 during the operation of the boiling water reactor flows into the cylinder member 81 through the elbow pipe 10, and further reach the jet passage 64 through the driving flow passage 76. This driving flow 30 is ejected as a jet flow 31A into the bell mouth 21 from the ejection outlet 20B located at the end of the jet passage 64. The working of the jet flow 31A makes the suction flow 32, which is part of the cooling water around the nozzle apparatus 12B in the downcomer 6, to flow into the bell mouth 21 through the cooling water suction passage 58. This suction flow 32 is introduced into the throat 22 through the space between the bell mouth 21 and the jet flow 31A. Since the jet passage 64 is sloped, the jet flow 31A is ejected diagonally toward the central axis of the throat 22 from the ejection outlet 20B. Consequently, the working of the jet flow 31A makes the pressure in the decompression chamber 77 negative, so that the suction flow 32A, which is part of the cooling water descending in the downcomer 6, flows into the suction passage 73 to reach the decompression chamber 77. This suction flow 32A further flows into a decompression region 82 formed inside the jet flow 31A in the bell mouth 21. The suction flows 32 and 32A and the driving flow 30 flowing into the bell mouth 21 are mixed in the throat 22 and discharged from the diffuser 25 (see FIGS. 1 and 4). These flows, that is, the cooling water 34, discharged from the diffuser 25 is supplied to the core 2. The jet pump 11B in the present embodiment as described above has the following unique structures (a) to (c). (a) The jet passage 64 in the nozzle portion 61 slopes inward. (b) The suction passage 73 slopes inward. (c) A cross section of the passage member 72 forming the suction passage 73 is oval. Various effects obtained by the unique structures (a) to (c) are explained in detail. First of all, various effects obtained by the unique structure (a) are described. The jet passage 64 in the nozzle portion 61 slopes inward. That is, the jet passage 64 slopes inward and downward toward the central axis of the throat 22. As a consequence, the jet flow 31A ejected from the ejection outlet 20B is ejected downward toward the central axis of the throat 22. Such jet flow 31A reduces the volume of the inverted cone-shaped decompression region 82 formed inside the jet flow 31A below the flow passage forming member 67. The reduction in the volume of the decompression region 82 relatively increases the degree of the pressure reduction, increasing the degree of negative pressure in the decompression chamber 77. As a result, a flow rate Qb2 of the suction flow 32A sucked into the bell mouth 21 through the suction passage 73 is increased. In addition, in the present embodiment, since the jet passage 64 in the nozzle portion 61 slopes inward, a distance L4 between the bell mouth 21 and the end of the outer cylinder member 62 of the nozzle portion 61 can be larger. As a result, a distance L5 between the inner surface of the throat 22 and the jet flow 31A is increased, increasing a flow rate Qb1 of the suction flow 32 flowing into the space between the bell mouth 21 and the jet flow 31A through the cooling water suction passage 58. An increase in the flow rate Qb1 of the suction flow 32 and the flow rate Qb2 of the suction flow 32A increases the flow rate of the cooling water 34 discharged from the diffuser 25. That is, the efficiency of the jet pump 11B is further improved. Various effects obtained by the unique structure (b) are described. Since the suction passage 73 slopes inward, the cooling water descending in the downcomer 6 can flow into the suction passage 73 by only slightly changing its flow direction. This makes the suction flow 32A to be easily sucked into the suction passage 73. In addition, since the suction passage 73 slopes inward, the downward flow force (a flow speed of approximately 2 m/s) of the cooling water in the downcomer 6 can be effectively used, allowing the suction flow 32A to be easily sucked into the suction passage 73. These workings further increase the flow rate Qb2 of the suction flow 32A, further increasing the flow rate of the cooling water 34 as well. Various effects obtained by the unique structure (c) are described. Since a cross section of the passage member 72 forming the suction passage 73 is oval, the cross-sectional area of the suction passage 73 can be enlarged. Consequently, the pressure loss in the suction passage 73 can be reduced and the flow rate Qb2 of the suction flow 32A can be increased. In particular, since the passage members 72 are disposed in such a way that their major axes follow the axial direction of the nozzle apparatus 12B and their minor axes follow the circumferential direction of the nozzle apparatus 12B, the pressure loss in the suction flow passage 76 can be reduced and the cross-sectional area of the suction passage 73 can be enlarged. In addition, such an arrangement with respect to the major and minor axes allows the number of the passage members 72 disposed in the circumferential direction of the nozzle apparatus 12B to be increased. Consequently, the total flow passage cross-sectional area of all the suction passages 73 can be enlarged. This greatly contributes to the increase in the flow rate Qb2 of the suction flow 32A. Besides the unique structures (a) to (c), the nozzle apparatus 12B has some other structures that allow the yielding of new effects. These effects are described. In order to reduce the pressure loss in a flow passage for the driving flow 30, the nozzle apparatus 12B adapts some ideas. A structure for reducing the pressure loss, other than the structure in which the cross section of the passage member 72 is oval, is explained. Each passage member 72 forms, in the upstream side, a streamline member 75 having a cross section that decreases toward the upper course (see FIG. 15). The formation of this streamline member 75 reduces turbulence in the driving flow 30 flowing in the driving flow passage 76, reducing the pressure loss in the driving flow passage 76. The reinforcing streamline plate 79 also has a streamline shape whose cross section decreases toward the upper course (see FIG. 14). This structure reduces the pressure loss in the driving flow passage 76. Furthermore, since each reinforcing streamline plate 79 is disposed to the same position above the passage member 72 located downstream in the circumferential direction of the nozzle apparatus 12B, the pressure loss in the driving flow passage 76 is reduced. Since the flow passage cross-sectional area of the jet passage 64 gradually decreases from the upper course to the ejection outlet 20B, the pressure loss in the jet passage 64 is also reduced. The cone member 80 having a cross-sectional area that increases from the upper course to the lower course, is disposed on the upper end of the flow passage forming member 67, so that the driving flow 30 flowing in the elbow pipe 10 can be smoothly introduced to the annular driving flow passage 76. This reduces the pressure loss in the flow passage for the driving flow 30 in the nozzle apparatus 12B. Furthermore, in the present embodiment, the pressure loss in the nozzle apparatus 12B can be further reduced because the nozzle apparatus 12B forms no flow passage such as the one in the nozzle apparatus shown in FIG. 1 of Japanese Patent Laid-open No. 2008-82752, in which the flow passage turns the driving flow at a right angle. The nozzle apparatus 12B employs some ideas for reducing the pressure loss in the flow passage for the suction flow 32A. This reduction in the pressure loss is obtained by forming curved surfaces on the inlet and the outlet of the passage member 72 as described above. Since the total flow passage cross-sectional area of all the suction passages 73 is larger than the cross-sectional area of the decompression chamber 77 at the lower end of the nozzle portion 61, the pressure loss in the flow passage for the suction flow 32A formed in the nozzle apparatus 12B is reduced. Since the cross section of the passage member 72 is oval and this passage member 72 is disposed in such a way that it slopes downward toward the axial direction of the nozzle apparatus 12B, the opening area of the inlet of the suction passage 73 can be enlarged. This also decreases the pressure loss in the suction passage 73. Since the surface of the lower region 70 of the flow passage forming member 67, facing the decompression chamber 77, is the curved surface 71, the driving flow 32A discharged from the suction passage 73 can smoothly change the direction downward along the curved surface 71 in the decompression chamber 77. By forming the curved surface 71 functioning in this way, the pressure loss in the flow passage for the suction flow 32A, formed in the nozzle apparatus 12B, can be reduced as well. The lower region 70 of the flow passage forming member 67 protrudes below an upper end of the outlet side of the passage member 72. Adapting such a shape allows the negative pressure in the decompression chamber 77, which is increased by the unique structure of (a), to effectively act on the suction passage 73, and allows the flow rate Qb2 of the suction flow 32A flowing into the suction passage 72 to be increased. In other words, the lower region 70 prevents the formation of a decompression dead water region in the decompression chamber 77 by the suction flow 32A discharged from the suction passage 73. The lower region 70 is disposed in the area where the decompression dead water region is to be formed in the decompression chamber 77 when no lower region 70 is provided. For this reason, cavitation induced in the decompression dead water region is prevented from occurring, and the flow rate Qb2 of the suction flow 32A can be increased. In the present embodiment, the ejection outlet 20B is annular, making the jet flow 31A ejected from the ejection outlet 20B also annular. Thus, since a vortex generated by the jet flow 31A is evenly distributed in the circumferential direction, a random vortex formation that causes flow-induced vibration is prevented and consequently, the vibration of structures in the boiling water reactor can be prevented. Since the nozzle apparatus 12B has an annular flow passage for the driving flow 30, the ejection outlet 20B, and the suction passages 73 crossing the flow passage for the driving flow 30, for introducing the suction flow 32A, the nozzle apparatus 12B can be made compact. Therefore, by replacing a nozzle in a conventional jet pump to the nozzle apparatus 12B, the jet pump can be quickly and easily converted into the jet pump 11B having a higher nozzle efficiency. A characteristic of the jet pump having the nozzle apparatus 12B with no flow passage reduction portion in the throat is compared with the characteristics of the conventional jet pumps in FIG. 16. In this comparison, the conventional jet pumps are the jet pumps having five nozzles as shown in FIG. 2 of Japanese Patent Laid-open No. Hei 7 (1995)-119700 and the jet pumps having the nozzle apparatus provided with a ring header and a cooling water suction passage in the axis as shown in FIG. 1 of Japanese Patent Laid-open No. 2008-82752. In each jet pump in Japanese Patent Laid-open No. Hei 7 (1995)-119700 and Japanese Patent Laid-open No. 2008-82752, each ejection outlet is disposed parallel to the axis of the jet pump, facing downward. FIG. 16 shows a change in the jet pump efficiency to the M ratio for the jet pump having the nozzle apparatus 12B with no flow passage reduction portion in the throat and for the above-described jet pumps in the conventional examples. In the jet pump having the nozzle apparatus 12B with no flow passage reduction portion in the throat, as described above, the efficiency increases more than the conventional examples due to the reduction in the pressure loss in the nozzle apparatus 12B, and the increase in the flow rates Qb1 and Qb2 of the suction flows 32 and 32A. When the M ratio is increased for the power upgrade of the reactors, the efficiency of the jet pump having the nozzle apparatus 12B with no flow passage reduction portion in the throat is higher than the others as shown in FIG. 16. Since the jet pump 11B of the present embodiment has the flow passage reduction portion 23 in the lower end portion of the throat 22 in the same manner as the jet pump 11 of the embodiment 1, by the influence of this flow passage reduction portion 23, the efficiency of the jet pump is reduced. However, this reduction in the efficiency can be compensated for by part of the increase in the efficiency achieved by the nozzle apparatus 12B. From the contribution of the remaining increase in the efficiency achieved by the nozzle apparatus 12B, the jet pump 11B, thus, can improve the efficiency of the jet pump more than those of the conventional examples. The jet pump 11B of the present embodiment has the flow passage reduction portion 23 in the lower end portion of the throat 22 so that vibration can be suppressed. The present embodiment can increase the efficiency of the jet pump as well as the flow rate of the cooling water 34 supplied to the core 2. The boiling water reactor having the jet pump 11B of the present embodiment, including the nozzle apparatus 12B, can easily handle a power upgrade which requires a large increase in the core flow rate. By using the nozzle apparatus 12B, a nozzle in a jet pump in an existing boiling water reactor can be quickly replaced. In addition, the vibration of the jet pump can be kept low. A jet pump according to embodiment 4, which is another embodiment of the present invention, is described below. The jet pump of the present embodiment is a jet pump in which a leakage flow from the gap 27 in the slip joint 26 to the downcomer 6 is completely eliminated from the jet pump 11 of the embodiment 1. To completely eliminate the leakage flow from the gap 27, the differential pressure between the inside of the slip joint 26 and the downcomer 6 should be zero. When the water head of a jet pump is H (Pa), the density of the fluid is ρ (kg/m3), and its speed is v (m/s), a static pressure Pi (Pa) of the slip joint 26 is represented in Equation (3) based on the static pressure in the downcomer 6.Pi=H−0.5 ρv2 (3) When Pi=0, the differential pressure between the inside of the slip joint 26 and the downcomer 6 becomes zero. The speed v is represented in Equation (4) using a jet pump flow rate Q (m3/s) and the inner diameter D6 of the outlet of the throat 22.v=Q/(πD62/4) (4) From Equation (3) and Equation (4), a value of the inner diameter D6 that makes Pi=0 is as in Equation (5).D6=(8ρQ2/πH)0.25 (5) Therefore, when the inner diameter D6 is within a range of (8ρQ2/πH)0.25≦D6<D5, the differential pressure between the inside of the slip joint 26 and the downcomer 6 can be reduced. A change in the static pressure in the axial direction of the jet pump when D6=(8ρQ2/πH)0.25 in the present embodiment is shown as the alternate long and short dash line in FIG. 7. The differential pressure between the inside of the slip joint 26 and the downcomer 6 becomes zero at the position of the slip joint 26, eliminating the leakage flow from the gap 27 in the slip joint 26. In the present embodiment, each effect achieved in the embodiment 1 can be obtained. The vibration of the jet pump can be reduced more than in the embodiment 1. A jet pump according to embodiment 5, which is another embodiment of the present invention, is described below. A jet pump 11C of the present embodiment has a structure in which the throat 22 in the jet pump 11 of the embodiment 1 is replaced with a throat 22 having a labyrinth seal 85 on the outer surface of a thick-wall portion 24A of the flow passage reduction portion 23 shown in FIG. 17. Other components of the jet pump 11C are the same as the jet pump 11 of the embodiment 1. Since the jet pump 11C is provided with the labyrinth seal 85 on the outer surface of the thick-wall portion 24A of the flow passage reduction portion 23, the resistance of the flow passage of the gap 27 is increased and thus a leakage flow from the inside of the slip joint 26 to the downcomer 6 can be reduced even more than the jet pumps in the embodiments 1 to 3. Consequently, the vibration of the jet pump 11C can be reduced. The jet pump 11C has the nozzle apparatus 12 so that each effect achieved by the jet pump 11 can be obtained. The throat 22 provided with the labyrinth seal 85 on the outer surface of the thick-wall portion 24A of the flow passage reduction portion 23 may be applied to the jet pumps 11A and 11B. The present invention can be applied to a boiling water reactor. 1: reactor pressure vessel, 2: core, 3: core shroud, 6: downcomer, 7: recirculation pipe, 8: recirculation pump, 10: elbow pipe, 11, 11A, 11B, 11C: jet pump, 12, 12A, 12B: nozzle apparatus, 13: nozzle base, 14: nozzle, 15, 17: nozzle straight-tube portion, 16, 18: nozzle narrowing portion, 19: nozzle lower end portion, 20, 20B: ejection outlet, 20A: annular ejection outlet, 21: bell mouth, 22: throat, 23: flow passage reduction portion, 25: diffuser, 26: slip joint, 30: driving flow (driving fluid), 31, 31A: jet flow, 32, 32A: suction flow (suction fluid), 40, 61: nozzle portion, 41: outer cylinder member, 42: inner cylinder member, 43: outer funnel member, 44: inner funnel member, 45: annular passage, 46: nozzle header portion, 47, 62: outer cylinder member, 48, 63: inner cylinder member, 49: annular header portion, 50: inner cooling water suction passage, 54: flow-adjusting plate, 57: joint portion, 64: jet passage, 65: suction passage portion, 66, 81: cylinder member, 67: flow passage forming member, 70: lower region, 71: curved surface, 72: passage member, 73: suction passage, 74: opening, 77: decompression chamber, 78: nozzle holder, 79: reinforcing streamline plate, 80: cone member.

summary

053645685

abstract

Anions of the formula (I): EQU [DA.sub.5 M.sub.30-x O.sub.110-x (M'L).sub.x ].sup.m- (I). in which D is Na.sup.+, Ca.sup.2+; A is P, As, Sb, Si, Ge, or combinations thereof M is W.sup.5+, W.sup.6.alpha., or mixtures thereof; M' is a metallic element from groups 2 to 15 of the periodic table; other than W; L is O.sup.2-, OH.sup.-, H.sub.2 O; x is 0-10; and m is 10-20; selectively react with cations Z.sup.n+ to afford anions of the formula (II): EQU [ZA.sub.5 M.sub.30-x O.sub.110-x (M'L).sub.x ].sup.(m+1-n)- (II). wherein n is 3 or 4; Z=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, or Bi, when n=3, and Z=Ce, U, Np, Pu, or Am, when n=4. This reaction may be used for the selective encapsultion of lanthanide or actinide cations, and salts containing anions of formula (II) may be vitrified to form glasses or reduced to form tungsten "bronze" materials suitable for the long-germ storage of radioactive lanthanides or actinides.

039742695

abstract

Gonorrhea antibodies in serum are detected by determination of radioactivity of conjugate formed between antibodies and antigens labelled with radioactive isotope.