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patent_number stringlengths 0 9	section stringclasses 4 values	raw_text stringlengths 0 954k
	abstract	A seal apparatus for a jet pump slip joint in a boiling water nuclear reactor pressure vessel, in an exemplary embodiment, includes a split seal ring and a segmented diaphragm spring engaging the split seal ring at an inner circumference of the diaphragm spring. The diaphragm spring includes a plurality of latch assemblies spaced circumferentially around an outer circumference. A plurality of slots, spaced circumferentially around the inner circumference, extend from the inner circumference to the support portion. Each latch assembly includes a latch bolt extending through and threadendly engaging a corresponding latch bolt opening in the diaphragm spring. Each latch bolt includes a head and a plurality of ratchet teeth spaced around the periphery of the latch bolt head. A locking spring is positioned to engage the ratchet teeth of the latch bolt head. The latch assembly further includes a latch arm coupled to the latch bolt. The latch arm includes a slot sized to receive a diffuser guide ear.
	abstract	Disclosed embodiments include nuclear fission reactor cores, nuclear fission reactors, methods of operating a nuclear fission reactor, and methods of managing excess reactivity in a nuclear fission reactor.
	claims	1. An extreme ultraviolet (EUV) radiation source, comprising:a target droplet generator configured to generate target droplets;a first laser source configured to generate pre-pulses that heat the target droplets to produce target plumes;a second laser source configured to generate main pulses that heat the target plumes to produce plasma emitting EUV radiation;a controller configured to adjust at least one parameter of the first and second laser sources comprising a delay between one of the pre-pulses and a corresponding one of the main pulses, and positions of the pre-pulses in a Y direction that is different from an X direction along which the target droplets travel;an energy detector configured to monitor an energy of the EUV radiation and record the parameters of the first and second laser sources with which the energy of the EUV radiation is maximized;a first laser beam generator and a second laser beam generator configured to generate first and second laser beams, respectively, that are directed onto a travel path of the target plumes, wherein the first and second laser beams are substantially parallel; anda laser beam monitor configured to receive the first and second laser beams reflected by the target plumes to determine a real-time velocity of the target plumes in their traveling path. 2. The EUV radiation source as claimed in claim 1, wherein one of the main pulses heats the target plumes produced by the corresponding one of the pre-pulses. 3. The EUV radiation source as claimed in claim 1, wherein a range for tuning the delay is 100 ns (nanoseconds), and an amount in each adjustment for tuning the delay is 10 ns. 4. The EUV radiation source as claimed in claim 1, wherein the Y direction is perpendicular to the X direction. 5. The EUV radiation source as claimed in claim 1, wherein a range for tuning each of the positions is 6 μm, and an amount in each adjustment for tuning each of the positions is 1 μm. 6. The EUV radiation source as claimed in claim 1, wherein the energy detector records the energy of the EUV radiation at an initial time period of each EUV burst. 7. The EUV radiation source as claimed in claim 1, wherein the controller is configured to adjust the delay between the one of the pre-pulses and the corresponding one of the main pulses according to the real-time velocity of the target plumes in its traveling path. 8. The EUV radiation source as claimed in claim 1, wherein the pre-pulses heat the target droplets along a Z direction and the Y direction is substantially perpendicular to both the X direction and the Z direction. 9. The EUV radiation source as claimed in claim 6, wherein the initial time period is the earliest 5% of the entire time period of the each EUV burst. 10. An extreme ultraviolet (EUV) lithography system, comprising:a radiation source, wherein the radiation source comprises:a target droplet generator configured to generate target droplets;a first laser source configured to generate pre-pulses that heat the target droplets to produce target plumes;a second laser source configured to generate main pulses that heat the target plumes to produce plasma emitting EUV radiation;a controller configured to adjust at least one parameter of the first and second laser sources comprising a delay between one of the pre-pulses and a corresponding one of the main pulses, and positions of the pre-pulses in a Y direction that is different from an X direction along which the target droplets travel;an energy detector configured to monitor an energy of the EUV radiation and record the parameters of the first and second laser sources with which the energy of the EUV radiation is maximized;a collector configured to collect and reflect the EUV radiation;a mask stage configured to secure an EUV mask;a wafer stage configured to secure a semiconductor wafer;one or more optical modules configured to direct the EUV radiation from the radiation source to image an integrated circuit (IC) pattern defined on the EUV mask onto the semiconductor wafer;a first laser beam generator and a second laser beam generator configured to generate first and second laser beams, respectively, that are directed onto a travel path of the target plumes, wherein the first and second laser beams are substantially parallel; anda laser beam monitor configured to receive the first and second laser beams reflected by the target plumes to determine a real-time velocity of the target plumes in their traveling path. 11. The EUV lithography system as claimed in claim 10, wherein one of the main pulses heats the target plumes produced by the corresponding one of the pre-pulses. 12. The EUV lithography system as claimed in claim 10, wherein a range for tuning the delay is 100 ns (nanoseconds), and an amount in each adjustment for tuning the delay is 10 ns. 13. The EUV lithography system as claimed in claim 10, wherein the Y direction is perpendicular to the X direction. 14. The EUV lithography system as claimed in claim 10, wherein a range for tuning each of the positions is 6 μm, and an amount in each adjustment for tuning each of the positions is 1 μm. 15. The EUV lithography system as claimed in claim 10, wherein the energy detector monitors the energy of the EUV radiation at an initial time period of each EUV burst. 16. A method for extreme ultraviolet (EUV) lithography, the method comprising:generating a target droplet;producing a target plume by heating the target droplet with a pre-pulse generated by a first laser source;producing EUV-radiating plasma by heating the target plume with a main pulse generated by a second laser source;adjusting at least one parameter of the first and second laser sources comprising a delay between the pre-pulse and the main pulse, and a position of the pre-pulse in a Y direction that is different from an X direction along which the target droplet travel;monitoring an energy of the EUV radiation and recording the parameters of the first and second laser sources with which the energy of the EUV radiation is maximized;generating first and second laser beams that are directed onto a travel path of the target plume, wherein the first and second laser beams are substantially parallel; andreceiving the first and second laser beams reflected by the target plume to determine a real-time velocity of the target plume in its traveling path. 17. The method as claimed in claim 16, wherein a range for tuning the delay is 100 ns (nanoseconds), and an amount in each adjustment for tuning the delay is 10 ns. 18. The method as claimed in claim 16, wherein a range for tuning the position is 6 μm, and an amount in each adjustment for tuning the position is 1 μm. 19. The method as claimed in claim 16, wherein a mean value of the energy of the EUV radiation of each EUV burst is monitored and recorded. 20. The method as claimed in claim 16, wherein the energy of the EUV radiation at an initial time period of each EUV burst is monitored and recorded, and the initial time period is the earliest 5% of the entire time period of the each EUV burst.
048896632	description	EXAMPLE 1 The starting oxide, obtained by a dry process in a furnace of the kind described in U.S. Pat. No. 4,397,824 is a UO.sub.2 powder. The powder obtained is separated into two portions. One of the portions, which represents 19.1% by weight of the total, is oxidized by passing in boats through a furnace at 330.degree. C. through which air flows. The stay time in the furnace is 3 hours. The remaining UO.sub.2 powder and the U.sub.3 O.sub.8 powder thus obtained are calibrated by passing through a rotary sifter, of turbo-calibrator type. The powders obtained are mixed in a 5000 liter "NAUTAMIX" mixer. The mesh size of the sifter may range from 200 to 350 microns. The homogenized mixture is granulated by precompression, then crushed into grains of about 800 microns, using a conventional technique. The granulates are then compressed into pellets of a diameter of 10 mm and a height of 17 mm. A comparison between the green pellets thus obtained, without addition of binder, and pellets manufactured from UO.sub.2 coming directly from the conversion furnace and also sifted, was carried out. Tests have shown that the green pellets manufactured from the intimate UO.sub.2 --U.sub.3 O.sub.8 mixture have a much higher crushing strength. The crushing test consisted in measuring the force required to break a given pellet by compressing it between two parallel surfaces (Brazilian test). For UO.sub.2 powder pellets, about 20 daN was obtained with O/U=2.04. For pellets made from a UO.sub.2 --U.sub.3 O.sub.8 mixture, it required 67 daN for O/U=2.15. EXAMPLE 2 Tests were carried out to determine the influence of the temperature for oxidizing the powder and the unfavorable effect of high temperature oxidation. For that, mixtures were prepared by sifting together the same powders as those described in Example 1, but with U.sub.3 O.sub.8 powders obtained by oxidation at different temperatures. The results obtained are given in the following table 1. They show that the advantages of the method of the invention decrease if the oxidation temperature increases. TABLE I ______________________________________ % powder O/U of strength of oxidized oxidation t.degree. mixture pellets (daN) ______________________________________ 24.6 300.degree. C. 2.22 65 24.6 550.degree. C. 2.22 45 24.6 900.degree. C. 2.22 23 UO.sub.2 powder 2.05 18 ______________________________________ EXAMPLE 3 12,000 kg of UO.sub.2 powder obtained by the "dry" process described in the document FR-A-2 060242 was sifted using a depression sifter having a mesh size of 104 microns. An amount of 25% of the powder to be sifted was oxidized at 350.degree. C. for 4 hours, in air, then mixed with the non-oxidized powder in a plough share mixer having a disagglomeration turbine, for 60 minutes. The UO.sub.2 --U.sub.3 O.sub.8 mixture obtained had an O/U ratio of about 2.22. The powder mixture was granulated by pre-compacting, then compacting using a conventional procedure. The granulates were compressed into pellets 10 mm in diameter and 15.2 mm in height, having a green density of 6.1 g/cm.sup.3, in a rotary press. The pellets were then sintered at 700.degree. C. for 3 hours in a hydrogen atmosphere. The fragility of the green pellets was determined by abrasion of the pellets in a squirrel cage and by measuring the loss in weight after rotating for more than 600 revolutions. This method of measurement characterizes the fragility of the edges, determinant during handling of the green pellets. The fragility test carried out on the green pellets prepared from the UO.sub.2 --U.sub.3 O.sub.8 mixture revealed a loss of material of 6.5%, whereas pellets manufactured from UO.sub.2 suffered a loss of 35%. In addition, the sinterability of the mixture was improved since a density of 97.39% of the theoretical density was obtained instead of 97.19% for the UO.sub.2 powder. EXAMPLE 4 700 kg of powder formed of: 80% by weight of UO.sub.2 powder obtained by a dry process, and PA1 20% of powder obtained by oxidation at 330.degree. C. for 4 hours of UO.sub.2 powder obtained by a dry process, were mixed for 60 minutes in a twin screw orbital mixer having a disagglomeration turbine. These powders had been previously sifted using a depression sifter with a mesh of 150 microns. The mixture was granulated by precompacting and crushing. The granulates were compressed into pellets with different rates of compression, in a die of 9.90 mm in diameter so as to produce green pieces 16 mm in height. The pellets were then sintered in a continuous furnace, being kept at the sintering temperature of 1760.degree. C. for 3.5 hours. The same compacting and sintering tests were carried out with the UO.sub.2 powder. The results of the comparative tests are given in the following table 2. TABLE 2 ______________________________________ With the UO.sub.2 powder Compacting Density Fragility of Density pressure (T) (g/cm3) the edges (%) (% TD) ______________________________________ 2.5 5.77 25.4 97.42 3.5 6.02 19.1 97.63 4.5 6.29 20.0 97.81 5.5 6.44 15.5 97.94 ______________________________________ With the UO.sub.2 --U.sub.3 O.sub.8 powder Compacting Density Fragility of Density pressure (T) (g/cm3) the edges (%) (% TD) ______________________________________ 2.5 5.60 11.05 97.31 3.5 5.90 10.0 97.49 4.5 6.11 8.3 97.75 5.5 6.29 6.1 97.90 ______________________________________ It can be seen that the green pellets made from the UO.sub.2 --U.sub.3 O.sub.8 mixture are appreciably stronger. In addition, the sinterability is slightly improved. Depending on the intrinsic characteristics of the oxides to be formed into pellets and of the pellet forming installation, optimum strength for green pellets can be obtained by adjusting the U.sub.3 O.sub.8 content and the compacting pressure. Similar tests were made on pellets including, in addition to uranium oxide, an absorbant material oxide (gadolinium). These tests showed that the same favorable results are obtained from the dual point of view of reduction of green fragility and the absence of degradation of the sintered product.
	claims	1. An electrical penetration assembly for a nuclear reactor vessel that can be mounted in an aperture of a nuclear reactor vessel, said electrical penetration assembly comprising:a penetration body comprising:a first end configured to be positioned inside the vessel;a second end configured to be positioned outside the vessel;a sealed electrical connector forming a first seal for the electrical penetration assembly, said sealed electrical connector hermetically sealing the penetration body at the first end;a feed-through carrier flange at the second end of the penetration body comprising a plurality of unitary electrical feed-throughs, each unitary electrical feed-through allowing a single electrical conductor to pass therethrough thereby ensuring the continuity of the electrical connections, each unitary electrical feed-through having a narrowed portion at an outer end and being individually insulated by an individual insulator forming a second seal for the electrical penetration assembly, said unitary electrical feed-throughs hermetically sealing the penetration body at the second end, each of the electrical conductors having a shoulder provided thereon that is larger in size than the narrowed portion, and said individual insulator being provided between the narrowed portion and the shoulder;an anti-ejection device formed by the engagement between the narrowed portion provided at each unitary electrical feed-through and the shoulder that is provided on each of the electrical conductors of said unitary feed-throughs. 2. The electrical penetration assembly for a nuclear reactor vessel according to claim 1, further comprising means for making the electrical penetration assembly integral, in a leak-tight manner, with the outside of said reactor vessel. 3. The electrical penetration assembly for a nuclear reactor vessel according to claim 1, wherein said individual insulators are insulators made of ceramic or prestressed vitroceramic. 4. The electrical penetration assembly for a nuclear reactor vessel according to claim 3, wherein said sealed electrical connector is an electrical connector having an insulator made of ceramic or an insulator made of prestressed vitroceramic. 5. The electrical penetration assembly for a nuclear reactor vessel according to claim 4, wherein the material of the insulator of the sealed electrical connector is different from the material forming the individual insulators. 6. The electrical penetration assembly for a nuclear reactor vessel according to claim 1, further comprising means for detecting a leak-tightness failure of the unitary feed-throughs. 7. The electrical penetration assembly for a nuclear reactor vessel according to claim 6, wherein the electrical penetration body is pressurised by an inert gas and wherein the means for detecting a leak-tightness failure are formed by a detection device detecting an increase in pressure downstream of the unitary feed-throughs. 8. The electrical penetration assembly for a nuclear reactor vessel according to claim 1, wherein the penetration body comprises, at the first end thereof, at least one rigid part at the aperture and one flexible part adjacent to the sealed electrical connector that is capable of deforming at least along one direction. 9. The electrical penetration assembly for a nuclear reactor vessel according to claim 1, wherein the narrowed portion of each of the unitary electrical feed-throughs and the shoulder are further configured for limiting a leakage rate of liquid upon engagement, in the event of failure of the sealed electrical connector combined with the failure of at least one individual insulator. 10. The electrical penetration assembly for a nuclear reactor vessel according to claim 1, wherein the electrical penetration assembly further comprises a plurality of unitary ducts provided inside the penetration body between the first end and the second end thereof, each unitary duct being suitable for allowing a single electrical conductor to pass therethrough for going through said penetration body. 11. The electrical penetration assembly for a nuclear reactor vessel according to claim 1, wherein the second end of the penetration body further comprises a leakage and connection detection cell connected to an outer end of the feed-through carrier flange and a sealed connector provided at the second end of the penetration body. 12. A nuclear reactor vessel comprising at least one electrical feed-through according to claim 1. 13. The a nuclear reactor vessel according to claim 12, further wherein the feed-through carrier flange is part of an end plate arranged at the second end of the penetration body, and wherein the end plate engages the nuclear reactor vessel via fastening elements and at least one ring joint. 14. A nuclear reactor comprising a vessel according to claim 12. 15. The nuclear reactor according to claim 14, wherein said nuclear reactor is an integrated reactor or a small modular reactor.
	summary
051805430	description	DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 3, a first aspect of the present invention is to provide a flow path 52 from the reactor coolant system hot leg 54 to the flood up level in the containment (but above the top of the core elevation). This flow path provides for a circulatory flow of water from the containment through the core as indicated by the broken lines and arrows which limits the concentration of boron from in the reactor vessel in the long term cooling mode. The flow path 52 is attached at the bottom of the hot leg 54 so that steam can vent through the upper portion of the pipe. The added dilution flow path is below the flood up water elevation but high enough to achieve natural circulation driven by the lighter (heated) water in and above the core. With the hot leg flow path 52, containment water flows through the core to keep the boron concentration low. Natural circulation of water is shown by the broken lines and arrows and is driven by the differences in water densities. The flow path 52 (one of two shown) works in conjunction with the flow path 56 (one of two shown) which includes a sump screen 13 for communicating water from within the containment into the reactor vessel. A portion of the second flow path is also used to introduce borated water from the passive safety injection system from the various sources, such as the core makeup tanks and the accumulators, as compared with the previous invention shown in FIG. 3 where the normal flow is simply to introduce containment water into the reactor vessel 10 through the second flow path. However, the water is boiled off the core, boron accumulates in the reactor vessel, thereby creating a dangerous situation in which the ability of the reactor core to effect heat transfer is diminished. The flow path 52 thus induces a natural circulatory flow of water from the water flooding the containment through the reactor core based on the differences in water density produced by the heating of water by the reactor core, thereby limiting the concentration on boron in the reactor vessel. Another problem associated with the use of borated water is that in a pressurized water reactor you need to increase the concentration of boron in the reactor when you go from hot to cold conditions while maintaining the core in a subcritical condition. According to the present invention, the core makeup tank elevation relative to the pressurizer elevation and core makeup tank boric acid concentration can be established to ensure that sufficient borated water will be drained by gravity from the core makeup tanks to achieve the required cold shut down boric acid concentration. As shown in FIG. 5, the core makeup tank and pressurizer elevation are established, along with the core makeup tank boric acid concentration, such that when the reactor coolant system water is cooled and shrinks in volume, sufficient core makeup tank boric acid solution will drain into the reactor coolant system to raise the overall concentration to that required to keep the reactor sub-critical. FIG. 4 represents the normal operation with the reactor coolant system water volume at around 6,000 cubic feet. The water in the reactor vessel contains less than 100 parts per million boric acid, while the core makeup tank may contain greater than or equal to 2,000 ppm boric acid. The core makeup tank is located substantially below the pressurizer normal water level. Balance lines 58 and 60 are also illustrated schematically, while flow path 56 connects the core makeup tank to the reactor vessel 10. After cool down, and referring to FIG. 5, the reactor core system water volume shrinks allowing the core makeup tank 40 to drain into the reactor vessel 10 thereby increasing the reactor coolant system boric acid concentration. Thus, high concentration boric acid in the core makeup tanks drain into the reactor coolant system to make up for water shrinkage, and thereby achieves a concentration in the reactor vessel of greater than or equal to about 500 PPM boric acid. An alternative to the partial draining of the core makeup tanks compensating for water shrinking discussed above with respect to FIGS. 4 and 5 is to create a natural circulation flowpath to mix the high concentration boric acid solution in the core makeup tank with the low concentration boric acid solution in the reactor coolant system. Referring to FIGS. 6 and 7, an alternative embodiment is illustrated in which the normal mode of operation would be the same as what is illustrated in FIG. 4. However, in this embodiment, a flowpath 62 is provided between the core makeup tank 40 (which is full of cold borated water) and the pressure balance line 60 which connects the top of the core makeup tank with the cold leg 24. The boration mode illustrated in FIG. 6 can be accomplished prior to the cool down since it is not necessary for the water level to shrink. The circulation flow is illustrated in FIG. 7, which is an enlargement of the broken circle portion of FIG. 6. Basically, hot water rises from the cold leg 24 into the pressure balance line 60 and then through the vent 62 into the core makeup tank 40. The hot water forces the cold core makeup water containing boric acid into the reactor coolant system. An equilibrium boric acid concentration of about 1,000 PPM is achieved as a result of the natural circulation induced by the vent 62. FIG. 8 is another illustration of the embodiment described with reference to FIGS. 6 and 7. The vent 62 has a pipe segment which extends into the core makeup tank 40 and terminates at an elevation below the depressurization system actuator core makeup tank level. The valves 46 are preferably located at or below the depressurization system actuation core makeup tank level. When the reactor coolant pumps are running, the pressurizer is about 60 PSIG less than the cold leg. Thus, check valves will be closed and the pressure difference between the core makeup tank inlet/outlet will be small. Borated water in the core makeup tanks will be mixed simply when the operator opens the core makeup tank inlet/outlet. As the reactor coolant system water shrinks, the pressurizer level will fall and the reactor coolant pumps will trip. Thus, the core makeup tank level and pressurizer level will equilibriate. When the reactor coolant pumps are not running, the natural circulation flow path will allow approximately 100 to 200 gallons per minute from the cold leg to the cold core makeup tank. Another aspect of the present invention incorporates use of two accumulators (one shown in FIG. 1) (tanks partially filled with water with a pressurized covergas such as nitrogen) which provide additional water flow to the reactor in the event of a large pipe break, when the required water addition rate is highest. These tanks deliver water to the reactor when the reactor coolant pressure falls below the nitrogen covergas pressure. This feature permits the high design pressure CMT's and associated piping to be reduced in size since the highest required flow for core cooling after a large pipe break can be initially provided by the accumulators, followed by a lower flow from the CMT's. The check valves provided in the core makeup tank discharge line prevent water from the accumulator from going into the core makeup tank which may be partially drained with accumulator injection. Also since the accumulators can be designed for a lower pressure (corresponding to the covergas pressure of about 100 psig), equipment cost can be reduced. Another aspect of the present invention is illustrated in FIG. 9 in which the core makeup tanks 40 and 41 are provided with fill, drain and sample capability, schematically illustrated by fill and sample lines 40a, 41a, and drain lines 40b and 41b. Similar fill, drain and sample capabilities are provided for the accumulators 42 and 43, as well as the in-containment refueling water storage tank 36. The fill lines 40a, 41a, 42a, 43a and 36a are used to inject water into the respective tanks with an appropriate concentration of boric acid in order to adjust the concentration in the tanks to a desired level. Thus, in a sampling mode, borated water is removed and sampled from the drain and sample lines 40b, 41b, 42b, 43b and 36b to determine the concentration of boric acid. If the concentration is determined to be low, higher concentration boric acid can be injected into the respective tanks through the fill lines while simultaneously removing an equal volume from the drain lines until the desired concentration level is achieved. A refilling supply tank (not shown) can be temporarily or permanently connected to the fill lines. The various tanks containing borated water are isolated normally from the reactor coolant system by means of isolation valves 64 which prevent the contents of the tanks from entering the reactor coolant system. The drain and sample lines 40b, 41b, 42b and 43b include isolation valves 66 to ensure that accidental drain does not occur. The two parallel normally closed valves 50 which are in the pressure balance line from the reactor coolant system cold legs to the top of each core makeup tank are actuated to their open position simultaneously with the parallel core makeup tank discharge isolation valves, on receipt of a core makeup tank actuation signal. Thus, the balance line can be used to permit a large amount of steam to float to the top of the core makeup tank which results in a high flow rate of water to the reactor coolant system from the core makeup tank. It will be recognized by those of skill in the art that numerous modifications and additions may be made to the various structures and the systems disclosed herein and thus it is intended by the appended claims to encompass all such modifications which fall within the true spirit and scope of the invention.
047675918	abstract	A probe for determining the energy and flux of particles in a plasma comprises a carbon film adapted to be exposed to the plasma, the film havinmg an electrical resistance which is related to the number of particles impacting the film, contacts for passing an electrical current through the film, and contacts for determining the electrical resistance of the film. An improved method for determining the energy or flux of particles in a plasma is also disclosed.
	summary
	description	This invention relates generally to printed circuit cards used in a control system and more particularly, to printed circuit cards used within a nuclear reactor protection system. The generation of electrical power in a nuclear power plant is a complex process. Numerous parameters (such as, without limitation, pressure, temperature, flow, radiation level, valve position, pump status, etc.) must be constantly monitored and measured. These measurements are used by plant operators to regulate the process (e.g., actuate valves, pumps, control rod drive mechanisms, etc.), monitor the process (e.g., monitor tank levels, flows, temperatures, etc.), and provide protection to the equipment used within the process (e.g., prevent low coolant levels, overheating, over pressurization, etc.; trigger a reactor “trip”, a unit runback; etc.). In the case of a nuclear power plant, the protection function, in particular, is very demanding. Thus, nuclear power plants employ a nuclear reactor protection system. To increase reliability of the protection system, redundant sets of critical sensors are provided to measure the numerous process parameters. For instance, four redundant sets of sensors are typically employed to measure a critical parameter, such as reactor core temperature. The sensors may be divided into a number of data channels which are in communication with the nuclear power plant's protection system. If one sensor fails, the three remaining sensors are available to measure the reactor core temperature. To prevent an unintended interruption of normal operations, the signals produced by the redundant sensors are correlated before initiating an emergency or safety response. In the current example, for instance, an indication by at least two out of the four sensors may be required as a prerequisite to actuating an emergency or safety response to lower the reactor core temperature. Many protection systems employed today are Solid State Protection Systems (“SSPS”) which employ discrete digital electronics, mechanical switches, and electromechanical relays, among others. The components of the SSPS are typically arranged in redundant logic “trains” which insures, for example, that the reactor does not inadvertently trip due to a component failure in a single logic train. Instead, the second logic train maintains the proper control. More specifically, if one logic train is off-line, malfunctioning, etc., the other train is able to provide the necessary protection. Examples of protection systems can be found in commonly assigned U.S. Pat. Nos. 6,062,412 issued to Stucker et al., 4,804,515 issued to Crew et al., and 4,399,095 issued to Morris. Each logic train of the SSPS typically includes a number of printed circuit cards which are used, for example and without limitation, to check the correlation between redundant sensors, check for under-voltage conditions, check for over-voltage conditions, etc. Each train may include, for example and without limitation, thirty-five (35) Universal Logic cards, one (1) Undervoltage Driver card (UV Card), and four (4) Safeguard Driver Cards. Most existing printed circuit cards, however, employ dated technology. Existing printed circuit cards, for example, operate from a 15 volt power supply and thus consume large amounts of power. Additionally, most existing printed circuit cards do not have adequate indicators (such as LED's) incorporated therein to convey the current status of the card, the inputs/outputs, etc. As a result, troubleshooting of the SPSS is difficult. Generally, the existing printed circuit cards must be periodically tested by taking a single train out of service (i.e., taking it off-line). Once off-line, a series of test pulses are applied to the inputs of the off-line train to test the printed circuit cards in that train; the second train remains on-line and provides protection to the nuclear power plant. After testing is completed, the first train is placed back on-line. Next the second train is taken off-line and a series of test pulses are applied to the inputs of the second train to test its printed circuit cards; the first train remains on-line and provides protection to the nuclear power plant. After testing is completed, the second train in placed back on-line. Under a typical test protocol, the trains are alternatingly tested every three to six months. For example, train-1 is tested in January, train-2 is tested in April, train-1 is tested in July, and train-2 is tested in October. As is evident, each train only undergoes testing once every six months. Thus, a malfunction that occurs shortly after a train is tested may not be discovered until the next scheduled test for that train (e.g., six months later; or even worse, only discovered after the train has caused an inadvertent reactor trip). Newer protection systems may be available; however, a wholesale change out of the protection system is cost prohibitive and complicated. Consequently, there is a need in the art for an improved printed circuit cards for an existing SSPS. More particularly, there is a need in the art for improved printed circuit cards that provide high reliability, low power consumption, which may be incorporated as a direct replacement to existing cards, and which provides continuous self-testing, among others. One aspect of the present invention relates to a nuclear reactor plant protection system printed circuit card comprising a first logic device having a number of basic logic circuits, and a second logic device operatively connected with the first logic device for testing the number of basic logic circuits without taking the printed circuit card out of service. Another aspect of the present invention relates to a nuclear reactor plant protection system printed circuit card comprising a first logic device having a number of basic logic circuits structured to produce a first output signal in response to a test signal, a second logic device having a number of basic logic circuits structured to produce a second output signal in response to the test signal, and a comparator for comparing the first output signal and the second output signal; wherein the test signal has a pulse duration that is less than a latching period associated with the printed circuit card. Another aspect of the present invention relates to a method for testing a printed circuit card without taking the printed circuit card out of service, the printed circuit card having a first logic device including a number of basic logic circuits structured to produce a first output signal and a second logic device including a number of basic logic circuits structured to produce a second output signal, the basic logic circuits of the second logic device being substantially the same as the basic logic circuits of the first logic device. The method comprises receiving a test signal having a pulse duration at an input of the first logic device; wherein the basic logic circuits of the first logic device include a component having a latching period, the latching period being greater than the pulse duration, receiving the test signal at an input of the second logic device, and comparing the first output to the second output. Another aspect of the present invention relates to a nuclear reactor control system comprising a plurality of sensors structured to measure numerous process parameters of a nuclear reactor and to produce a sensor output signal in response thereto, a nuclear reactor protection system having a printed circuit card electrically connected to at least some of the plurality of sensors and structured to receive the sensor output signal from each of the at least some of the plurality of sensors. The printed circuit card comprises a first logic device having a number of basic logic circuits, and a second logic device operatively connected with the first logic device for testing the number of basic logic circuits without taking the printed circuit card out of service. Therefore, it should now be apparent that the invention substantially achieves all the above aspects and advantages. Additional aspects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. Moreover, the aspects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. The expression “a number of” and variations thereof, as employed herein, shall refer broadly to any quantity, including a quantity of one. FIG. 1 is a block diagram of a printed circuit card 10 according to one embodiment of the present invention. The printed circuit card 10 is structured to be used, for example and without limitation, in a nuclear reactor protection system (e.g., an SSPS). More specifically, the printed circuit card 10 is structured to have the same basic functions, the same dimensions, the same interfaces, etc. as the existing printed circuit cards that it is intended to replace. For instance, the printed circuit card is structured to have the same basic functions, the same dimensions, the same interfaces, etc. as an existing safeguards driver card, an existing universal logic card, an existing undervoltage driver card, etc. Referring to FIG. 1, the printed circuit card 10 includes a main complex programmable logic device (main CPLD) 11, a test complex programmable logic device (test CPLD) 17, a test pulse generator and counter 15, tri-state buffers 16, result comparator 18 and power supply 25, among others. A card edge connector 22 is provided which allows the printed circuit card 10 to interface with the existing nuclear reactor protection system. For instance, a printed circuit card 10 structured to replace a universal driver card in one train of an SSPS will include a card edge connector 22 that is substantially the same as the universal driver card being replaced. The printed circuit card 10 includes status indicators 20 and additional test connectors for electrically connecting the printed circuit card to, for example, an external test apparatus. The main CPLD 11 includes a number of inputs (I1 . . . IN), each having a signal attenuator 12 associated therewith and a number of basic logic circuits (not shown in FIG. 1). The inputs (I1 . . . IN) are structured to receive signals, for example, from the numerous sensors which measure the process parameters associated with the power generation process. The number of basic logic circuits are structured to complete certain functions (e.g., 2/4 logic operations, ⅔ logic operations, multiplexer operations, AND operations, OR operations, driver operations, etc.) for producing an output signal which is provided to an associated buffer/driver 13. The test CPLD 17 is operatively connected with the main CPLD 11. The test CPLD 17 permits the number of basic logic circuits of the main CPLD 11 to be tested while the main CPLD 11 remains in service (i.e., without taking the main CPLD 11, or the train of which it is a part, off-line). In the current embodiment, for example, the test CPLD 17 includes a number of basic logic circuits (not shown) which are substantially the same as the number of basic logic circuits of the main CPLD 11. As a result, the logic circuits of the test CPLD 17 are structured to implement the same function as the main CPLD 11. The logic circuits of the test CPLD 17 are structured to produce a second output signal. During testing, the test pulse generator and counter 15 cycles through a number of test steps and generates a number of test pulses. For each test step, various signals of a logic test pattern are applied to each input (I1 . . . IN) of the main CPLD 11 through tri-state buffers 16. The various signals of the logic test pattern are capacitively coupled to each input (I1 . . . IN) via the associated attenuator 12 and override the normal input signal for an approximately 2 μS (i.e., 2×10−6 seconds) test period. The same signals of the logic test pattern are applied to the inputs of test CPLD 17. The output signal produced by the main CPLD 11 in response to the logic test pattern is compared by comparator 18 to the output signal produced by the test CPLD 17 in response to the logic test pattern. If the output of the main CPLD 111 fails to match the output of the test CPLD 17, an indication is provided (for example, with status indicators 20) that one set of basic logic circuits (i.e., either the basic logic circuits of main CPLD 11 or the basic logic circuits of test CPLD 17) is in error. As discussed above, the test pulses have a duration of approximately 2 μS (i.e., 2×10−6 seconds). Because relays on the printed circuit card 10 (not shown) require approximately 12 mS (i.e., 12×10−3 seconds) to latch, the use of the 2 μS test pulse (which has a duration over 1000 times shorter than the latch time) insures that the test pulses do not propagate from the printed circuit card 10 to another printed circuit card within the train. Additionally, a filter (not shown) may be added to the output of the printed circuit card 10 to further insure that the test pulses do not propagate through the train. For example, in the current embodiment, the output buffer/driver 13 may be tested as part of the sequence of tests discussed above. When buffer/driver 13 is used as a driver, the buffer/driver 13 is designed to filter out the two 2 μS test pulses. More specifically, an approximately 256 μS test pulse (which is over 100 times longer than the 2 μS test pulse) is applied to turn off the output transistor of the buffer/driver 13 (or the driver output FET of the buffer/driver 13) one at a time. The output voltage of the buffer/driver 13 is measured to ensure that there is no output voltage present. If there is an output voltage present and the buffer/driver 13 is not turned on by the main CPLD 11, one of the output devices of the buffer/driver 13 is shorted and an error is generated. Also, if the buffer/driver 13 is activated and there is no output voltage an error is generated. As discussed above, the main CPLD 11 can be tested while the printed circuit card 10 remains in service. In the current embodiment, the main CPLD 11 is continuously tested when in service. More specifically, the main CPLD 11 is tested at least once a second without forcing the main CPLD 11 off-line. The printed circuit card 10 may also include a “dead man” circuit 19. The dead man circuit 19 is operatively connected with the main CPLD 11 and the test CPLD 17. The dead man circuit 19 is structured to permit the main CPLD 11 to verify that the test CPLD 17 is operating correctly. More specifically, the dead man circuit is structured to receive a test signal from the main CPLD 11, generate a response indicative of the status of the test CPLD 17, and transmit the response back to the main CPLD 11. Responsive to the response signal generated by the dead man circuit, the main CPLD 11 provides an indication (for example, at status indicators 20) if the test CPLD 17 is not functioning properly. In the current embodiment, status indicators 20 include a number of LEDs which display the status of the card itself, the inputs, the outputs, etc. For example, the printed circuit card 10 includes a “heartbeat” LED which flashes at a constant rate to indicate that the printed circuit card 10 is powered-up and operating correctly. As a result of the status indicators 20, troubleshooting procedures are simplified. To reduce power consumption, most of the components of the printed circuit card 10 are operated at 3.3 volts. Because the printed circuit board 10 is a one-to-one replacement for an existing printed circuit card which operates at 15 volts, power supply 25 is structured to convert the 15 volt power supplied to the existing printed circuit cards to 3.3 volts for use by the printed circuit card 10. FIG. 2 is a block diagram of the power supply 25 used by the printed circuit board 10 of FIG. 1. The power supply 25 includes a filter 26, redundant DC-DC converters 27a, 27b, and a regulator 28. In the current embodiment, the existing 15 volt power signal is conditioned/filtered by filter 26. The output of the filter 26 is electrically connected to a 15 volt output pin, to the input of DC-to-DC converter 27a, to the input of DC-to-DC converter 27b, and to the input of regulator 28. The DC-to-DC converter 27a and the DC-to-DC converter 27b each convert the 15 volt power signal to 3.3 volts for use by the printed circuit board 10. The DC-to-DC converter 27a and the DC-to-DC converter 27b provide a redundant 3.3 volt supply. Regulator 28 provides a 5 volt power supply to the printed circuit card 10. Dual 48 volt supplies (48V1 and 48V2) are connected to power supply 25 and a single 48 volt output is provided for use by the printed circuit card 10. FIG. 3 is a block diagram of a printed circuit card 10′ structured to function as a Universal Logic Board according to one embodiment. FIG. 4 illustrates the main CPLD 11′ of the Universal Logic Board printed circuit card 10′ of FIG. 3 according to one embodiment. Referring to FIG. 4, the Universal Logic Board printed circuit card 10′ includes a main CPLD 11′ having basic logic circuits for implementing five logic blocks. The basic logic functions are implemented in main CPLD 11′, with supporting discrete components (not all of which are shown) on the printed circuit card 10′. The main CPLD 11′ implements 4-input logic block 50, a 3-input logic block 51, a 3-input logic block 52, a 2-input logic block 53, and a 6 output multiplexer block 54. The 4-input logic block 50 receives inputs I1-I4; the 3-input logic block 51 receives inputs I5-I7, the 3-input logic block 52 receives inputs I8-I10, and the 2-input logic block 53 receives inputs SP I1-I2. Each input (i.e., I1-I10 and SP I1-SP I2) has an attenuator 12 associated therewith. In the current embodiment, the Universal Logic Board printed circuit card 10′ is active logic low. The 4-input logic block provides a low output when any 2 of its 4 inputs are low, each of the 3-input logic blocks provide a low output when any 2 of their 3 inputs are low, and the 2-input logic block provides a high output when any 1 of its 2 inputs are low. Signals from the 4, 3, and 2 input logic blocks (i.e., 50, 51, 52, 53) are routed to outputs M1-M6 by multiplexer 54. Each output of multiplexer 54 has a buffer 13′ associated therewith. The outputs M1-M6 may be displayed, for example, by the plant protection system. Referring to FIG. 3, it should be noted that the test CPLD 17′ includes substantially the same basic logic circuits for implementing the five basic logic functions that are implemented in main CPLD 11′. During testing, the same signals of the logic test pattern are applied to the inputs of test CPLD 17′. The output signal produced by the main CPLD 11′ in response to the logic test pattern is compared by comparator 18 to the output signal produced by the test CPLD 17′ in response to the logic test pattern. If the output of the main CPLD 11′ fails to match the output of the test CPLD 17′, an indication is provided (for example, with status indicators 20) that one set of basic logic circuits (i.e., either the basic logic circuits of main CPLD 11′ or the basic logic circuits of test CPLD 17′) is in error. It should further be noted that all inputs and outputs of the Universal Logic Board printed circuit card 10′ are structured to be of the same logic level and impedance as the existing Universal Logic Board printed circuit card being replaced. It should be apparent that additional or alternative logic functions may be implemented on the Universal Logic Board printed circuit card 10′ while remaining within the scope of the present invention. FIG. 5 is a block diagram of an Undervoltage Driver Board printed circuit card 10″ according to one embodiment. FIG. 6 illustrates the main CPLD 11″ of the Undervoltage Driver Board printed circuit card 10″ of FIG. 5 according to one embodiment. The basic logic functions are implemented in main CPLD 11″, with supporting discrete components (not all of which are shown) on the printed circuit card 10″. Referring to FIG. 6, the Undervoltage Driver Board printed circuit card 10″ includes a main CPLD 11″ having basic logic circuits for implementing an “AND” logic function and a current limit retry logic function. More specifically, a number of signals are divided into five input groups G1-G5; each of which are provided at the inputs of AND gates 54a and to the inputs of AND gate 54b. The output of AND gate 54a is provided to a current/voltage monitor 55a via drivers 13″. Likewise, the output of AND gate 54b is provided to a current/voltage monitor 55b via drivers 13″. An output of the current/voltage monitor 55a and an output of the current/voltage monitor 55b are compared by comparator 18 which produces an error signal when the actual output being monitored differs from the test injected expected output. If the output becomes low impedance (e.g. short circuited), the driver circuit will experience an over-current condition that can damage electronic components on the driver card. During the over current condition, the current monitor turns off the output driver to prevent damage to the driver circuit. The current monitor tests for an over-current condition and retries the output circuit on a periodic basis. Once the over-current condition is eliminated, the driver circuit returns to normal operation. Referring to FIG. 5, it should be noted that the test CPLD 17″ includes substantially the same basic logic circuits for implementing the “AND” logic function and the current limit retry logic function which are implemented in main CPLD 11″. During testing, the same signals of the logic test pattern are applied to the inputs of test CPLD 17″. The output signal produced by the main CPLD 11″ in response to the logic test pattern is compared by comparator 18 to the output signal produced by the test CPLD 17″ in response to the logic test pattern. If the output of the main CPLD 11″ fails to match the output of the test CPLD 17″, an indication is provided (for example, with status indicators 20) that one set of basic logic circuits (i.e., either the basic logic circuits of main CPLD 11″ or the basic logic circuits of test CPLD 17″) is in error. It should further be noted that all inputs and outputs of the Undervoltage Driver Board printed circuit card 10″ are structured to be of the same logic level and impedance as the existing Undervoltage Driver Board printed circuit card being replaced. It should be apparent that additional or alternative logic functions may be implemented on the Undervoltage Driver Board printed circuit card 10″ while remaining within the scope of the present invention. FIG. 7 is a block diagram of a Safeguards Driver Board printed circuit card 10′″ according to one embodiment. FIG. 8 illustrates the main CPLD 11′″ of the Safeguards Driver Board printed circuit card 10′″ of FIG. 7 according to one embodiment. Referring to FIG. 8, the Safeguards Driver Board printed circuit card 10′″ includes a main CPLD 11′″ having basic logic circuits for implementing eight driver circuits with latch reset (e.g., driver-1 thru driver-8). The basic logic functions are implemented in main CPLD 11′″, with supporting discrete components (such as driver current/voltage monitors 61, but not all of which are shown) on the printed circuit card 10′″. Although each driver (i.e., driver-1 thru driver-8) includes the same components, for simplicity, exemplary logic is illustrated only for driver-8. Driver-8 includes a filter 56, two NAND gates 57-58, an AND gate 59 and a transistor 60. The reset memory function is performed by the two NAND gates 57-58, which provide the latching function on reset by taking the output of NAND gate 58 and feeding it back to the input of NAND gate 57 through filter 56. Filter 56 prevents noise glitches from resetting the driver output by setting the latch and acts like a typical RC filter but is implemented in the main CPLD 11′″ by using an up/down counter that emulates the charging and discharging of a capacitor. The output of NAND gate 58 is also provided to and input of AND gate 59. The output of AND gate 59 drives the gate of output driver transistor 60. The output of each driver circuit (e.g., driver-1 thru driver-8) is fed to an associated driver 13′″. An output of each driver 13′″ is provided, via its associated driver current monitor 61, to the comparator 18. The test CPLD 17′″ includes substantially the same basic logic circuits for implementing the driver logic function as main CPLD 11′″. The output of the test CPLD 17′″ is also provided to the comparator 18. During testing, the same signals of the logic test pattern are applied to the inputs of test CPLD 17′″. The output signal produced by the main CPLD 11′″ in response to the logic test pattern is compared by comparator 18 to the output signal produced by the test CPLD 17′″ in response to the logic test pattern. If the output of the main CPLD 11′″ fails to match the output of the test CPLD 17′″, an indication is provided (for example, with status indicators 20) that one set of basic logic circuits (i.e., either the basic logic circuits of main CPLD 11′″ or the basic logic circuits of test CPLD 17′″) is in error. It should further be noted that all inputs and outputs of the Safeguards Driver Board printed circuit card 10′″ are structured to be of the same logic level and impedance as the existing Safeguards Driver Board printed circuit card being replaced. It should be apparent that additional or alternative logic functions may be implemented on the Safeguards Driver Board printed circuit card 10′″ while remaining within the scope of the present invention. While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.
	claims	1. An apparatus comprising:an x-ray source assembly configured to revolve about a central axis;an x-ray detector assembly configured to revolve about the central axis to capture and store a plurality of radiographic images of a portion of a patient positioned at or near the central axis;sensors to detect a position of the patient at or near the central axis;a network connected microphone, monitor, video camera and speaker each selectively activatable to allow audio/visual communication over the network between the patient and remote personnel; andan x-ray shielded enclosure attached to the x-ray source assembly and the x-ray detector assembly, the x-ray shielded enclosure configured to entirely enclose the source assembly, the detector assembly and the patient,wherein the apparatus is configured to be transportable as a unit. 2. The apparatus of claim 1, wherein the source assembly and the detector assembly are configured to revolve along a radial path or a helical path relative to the patient positioned at or near the central axis. 3. The apparatus of claim 1, wherein the x-ray shielded enclosure comprises a cylindrical shape having an interior diameter between about 28 inches and about 38 inches and a height of about 9 feet. 4. The apparatus of claim 1, wherein the enclosure comprises an electronically controlled lock configured to unlock only in response to receiving a digital access code. 5. The apparatus of claim 4, wherein the enclosure comprises a door to allow entry into, and exit out of, the enclosure, the door configured to be locked and unlocked by the electronically controlled lock. 6. The apparatus of claim 4, wherein the enclosure further comprises transducer circuitry to convert magnetic signals, reflected laser signals, iris scanned signals, biomarker signals, audio signals, or optical signals into the digital access code. 7. The apparatus of claim 4, wherein the enclosure further comprises a reader to identify an encoded digital access code on an ID card or token. 8. The apparatus of claim 1, further comprising a support within the enclosure to support the patient at an angle of between about 5 degrees and about 15 degrees away from a vertical position. 9. The apparatus of claim 1, wherein the sensors comprise a laser source and a laser detector configured to detect a position of the patient at or near the central axis. 10. The apparatus of claim 1, wherein the speaker is configured to output live or electronically recorded audio instructions. 11. The apparatus of claim 1, wherein the x-ray detector assembly is configured to revolve about the central axis simultaneously with the x-ray source assembly. 12. The apparatus of claim 1, further comprising a height adjustable overhead apparatus configured to retain the patient's arms in an extended position while capturing radiographic images of the patient. 13. The apparatus of claim 12, wherein the height adjustable overhead apparatus comprises handles configured to be grasped by the patient. 14. The apparatus of claim 1, wherein the enclosure comprises a window fabricated from a radiopaque material transparent to visible light. 15. The apparatus of claim 1, further comprising an x-ray assembly controller, wherein the x-ray source comprises an array of x-ray sources configured to be independently energized, and wherein the controller is configured to selectively energize two or more of the x-ray sources in a predetermined sequence and a predetermined timing. 16. The apparatus of claim 1, further comprising a platform for the patient to stand on, the platform having an area of between about 600 square inches and about 1200 square inches. 17. The apparatus of claim 1, further comprising an interior volume to allow movement of the patient therein of between about 17 cubic feet and about 71 cubic feet. 18. An apparatus comprising:a stationary x-ray shielded cylindrical enclosure having a central axis;an x-ray source assembly positioned on one side of the central axis and attached to the cylindrical enclosure;an x-ray detector assembly positioned on a second side of the central axis opposite the x-ray source assembly and attached to the cylindrical enclosure, the x-ray detector assembly configured to capture and store a plurality of radiographic images of a patient standing at or near the central axis; anda circular platform within the cylindrical enclosure, the platform configured to support the patient standing thereon and to rotate the standing patient between the source assembly and the detector assembly, wherein the source assembly and the detector assembly are configured to capture and store a plurality of radiographic images of the patient; andthe stationary x-ray shielded cylindrical enclosure configured to completely enclose the source assembly, the detector assembly, the platform and the standing patient, wherein the apparatus is configured to be transportable as a unit. 19. The apparatus of claim 18, wherein the cylindrical enclosure comprises an interior diameter between about 28 inches and 38 inches and a height of about 9 feet. 20. An apparatus comprising:an x-ray detector, the x-ray detector fixed in a stationary position;an x-ray source, the x-ray source configured to rotate while translating between two terminal positions about a patient in order to emit x-rays toward the patient and the x-ray detector at predetermined times while translating between the two terminal positions;sensors to detect a position of the patient;a network connected microphone, monitor, video camera and speaker each selectively activatable to allow audio/visual communication over the network between the patient and remote personnel to assist in proper positioning of the patient; anda cylindrical x-ray shielded enclosure comprising an interior diameter between about 28 inches and 38 inches and a height of about 9 feet, the x-ray shielded enclosure attached to the x-ray source and the x-ray detector, the x-ray shielded enclosure configured to entirely enclose the x-ray source, the x-ray detector, the sensors and the patient,wherein the apparatus is configured to be transportable as a unitary integrated whole, and wherein the x-ray detector is configured to capture radiographic images of the patient standing between the x-ray source and the x-ray detector.
047327308	summary	BACKGROUND OF THE INVENTION 1. Field of Invention The present invention relates broadly to nuclear reactors wherein steam is generated by heat from nuclear fuel rods arranged in fuel assemblies. Periodically, these fuel rods must be replaced. In doing so the used fuel rods are removed from spent fuel assemblies. The present invention is particularly directed to a device to be used during derodding of spent fuel assemblies if a fuel rod becomes stuck in a partially withdrawn position. 2. Description of the Related Art During the operation of a nuclear reactor, the fuel assemblies, which include cylindrical fuel rods, periodically must be renewed. This is accomplished by replacing the rods in spent fuel assemblies. Initially, the spent fuel assemblies must be derodded and on occasion a fuel rod may become stuck in a partially withdrawn position. Continued handling of the fuel assembly requires severing of the stuck fuel rod. Severing of the fuel rod using standard hydraulic shears, bolt cutters, or pipe cutters results in the gross release of radioactive debris, which is, of course, highly undesirable. SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to prevent the gross release of radioactive debris upon the severing of a spent fuel rod. It is a further object of the present invention to accomplish the substantial closure of the severed ends of a spent fuel rod. Another object of the present invention is to provide for the closure of the severed ends of a spent fuel rod through the use of caps or capping sleeves. Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. In accordance with the present invention, a stuck fuel rod capping sleeve is provided in order to permit severing of a fuel rod without the gross release of radioactive material. The stuck fuel rod capping sleeve is a bi-metallic cylindrical device made up of an inner sleeve and an outer sleeve. The inner sleeve is made of a low work hardening highly ductile material (e.g., Inconel 600). The outer sleeve is made of a moderately ductile material (e.g., 304 stainless steel). Severing of the fuel rod is performed by using a bolt cutter device. Upon initial pressure being applied by the bolt cutter device, the bi-metallic sleeve provides strength to permit fracturing of the ceramic fuel without severing of the fuel rod cladding. Continued application of pressure by the bolt cutter device results in separation of the fractured surfaces and a closure of the rod ends. During this stage of cutting, ductile flow of the inner sleeve results in a barrier that restricts the release of radioactive fuel material. The final application of pressure by the bolt cutter device fractures the outer sleeve and results in a separation of the upper and lower portions of the fuel rod. Although the capping sleeve does not hermetically seal the fuel rod, it does prevent the gross release of radioactive fuel material. The stuck fuel rod capping sleeve of the present invention has been tested on unirradiated LWBR fuel rod tubing and simulated (aluminum oxide) fuel pellets. Cutting forces of approximately 20,000 lbs. were needed to sever a 0.30 inch diameter fuel rod. In the tests, end gaps as small as approximately 0.0005 inch were achieved, thus demonstrating substantial closure of the severed ends of the fuel rod. Tests wherein a single Inconel capping sleeve was utilized have also been performed. Although comparable end closures were obtained, in many cases severing of the sleeve was not obtained. In a second embodiment of the stuck fuel rod capping sleeve, each end of the inner sleeve is tapered and split in the region beyond the outer sleeve. Lower and upper locking sleeves are threaded onto threaded portions at each end of the outer sleeve prior to placement of the stuck fuel rod capping sleeve over the stuck fuel rod. Following severing of the stuck fuel rod capping sleeve and the fuel rod, the locking sleeves are advanced to seal the shank of the fuel rod to the inner sleeve and caps are threaded onto the locking sleeves to seal the severed end of the fuel rod. In a third embodiment of the stuck fuel rod capping sleeve, a single Inconel capping sleeve with a recess is used to seal the rod. The recess is packed with an underwater epoxy prior to placing the sleeve on the rod. The epoxy reduces the amount of fission product release by providing a seal between the rod and sleeve. The epoxy also locks the capping sleeve on the fuel rod.
039768899	claims	1. In an X-ray diagnostic apparatus for the preparation of dental X-ray exposures including means for fixedly setting values for the X-ray tube voltage and current, the improvement comprising: means for fixedly setting the exposure time; a plurality of filters for the X-radiation each coordinated with a particular exposure object in conformance with an exposure program in which the fixed values for said X-ray tube voltage, X-ray tube current and exposure time cannot be varied by a user of said diagnostic apparatus; and support means for said filters, said support means being adapted to selectively position a preselected one of said filters at any one time in the path of said X-radiation in conformance with a predetermined selected exposure object. 2. An apparatus as claimed in claim 1, said apparatus including an X-ray outlet aperture facilitating through passage of a central X-ray beam, said filter support means comprising a disc located proximate to said X-ray outlet aperture and being rotatable about an axis eccentric relative to said central X-ray beam, said filters being circumferentially spaced about said disc so as to have the centerpoints thereof define a circle concentric to the rotational axis of said disc, the radius of said circle corresponding to the distance between said central X-ray beam and said rotational axis. 3. An apparatus as claimed in claim 2, comprising a housing encompassing said filters and said filter support means, said disc having a rim portion thereof projecting outwardly of said housing adapted to facilitate manual rotational movement of said filters. 4. An apparatus as claimed in claim 3, comprising indicia indicative of the exposure object associated with a respective of said filters being provided on said portion of the disc projecting outwardly of said housing. 5. An apparatus as claimed in claim 1, said apparatus comprising a single-enclosure collectively housing the components of said X-ray diagnostic apparatus adapted for dental X-ray exposures. 6. An apparatus as claimed in claim 5, comprising primary switching circuit means for actuation of said apparatus, said single-enclosure including a generator, said generator and said circuit being constructed as a unitary entity.
	claims	1. A probe for a near-field microscope comprising: a probe body is inserted into an evanescent field generated in a sample surface so that scattered light is detected by scattering the evanescent field using a probe tip of the probe body, one part or all of the probe body which interacts with the evanescent field being coated with metal particles which do not mutually adhere, each of the metal particles having a particle diameter of 10 nm or larger and a radius of curvature of 50 nm or smaller. 2. A method of manufacturing a probe for a near-field microscope, comprising: a first vapor deposition process of vapor-depositing gold-palladium as a substrate metal film on a probe body; a second vapor deposition process of vapor-depositing a metal selected from one of silver, a silver alloy, gold and a gold alloy onto the substrate metal film; and adjusting a vapor deposition time and a vapor deposition speed in the second vapor deposition process so that particles of the metal vapor-deposited in the second vapor deposition process are coated on one part or all a surface of the probe body, the metal particles comprising particles which do not mutually adhere, each of the metal particles having a particle diameter of 10 nm or larger and a radius of curvature of 50 nm or smaller. 3. A method of manufacturing a probe for a near-field microscope, comprising: proving a mixed solution by adding an aqueous solution containing an aldehyde compound as a reducing agent to an aqueous solution containing silver nitrate to thereby reduce the silver nitrate to silver; growing silver particles on a surface of a probe body by immersing the probe body into the mixed solution; and adjusting a concentration of the silver nitrate aqueous solution and a time for immersing the probe body into the mixed solution so that silver particles which do not mutually adhere are formed on one part or all of the surface of the probe body, each of the silver particles having a particle diameter of 10 nm or larger and a radius of curvature of 50 nm or smaller. 4. A scanning probe microscope having a probe according to claim 1.
	claims	1. An x-ray device, comprising: a vacuum enclosure having disposed therein an electron-producing cathode and an anode positioned to receive the electrons produced by the cathode, the electrons impacting the anode such that a bean of x-rays is emitted through an x-ray transmissive window in a wall of the enclosure; an outer housing defining an interior portion within which is disposed the vacuum enclosure, the outer housing defining a port; and an adjustable collimator comprising: a base member at least partially disposed within the port of the outer housing; a collimating plate mounted on an end of the base member and having a primary x-ray passage area that is substantially adjacent to the x-ray transmissive window so as to permit at least a portion of the emitted x-rays to pass, and wherein at least a portion of the collimating plate is composed of an x-ray attenuating material; a fluid tight seal disposed between the primary x-ray passage area and the interior of the outer housing; and a block that is moveable in a substantially linear direction, the block having at least one blocking member that is at least partially composed of an x-ray attenuating material, the blocking member being movable in the linear direction between at least a retracted and a extended position, the blocking member partially blocking the aperture defined in the collimating plate when the blocking member is in the extended position such that a secondary x-ray passage area is defined through which the emitted x-rays can pass, the second passage area having smaller dimensions than the primary passage area. 2. An x-ray device as defined in claim 1 , wherein the blocking member comprises at least two blocking portions, wherein each blocking portion blocks at least a portion of the primary x-ray passage area when the blocking member is in the extended position. claim 1 3. An x-ray device as defined in claim 1 further comprising a rigid arm that is selectively moveable between a first and a second position, the arm operably attached to the block via a lever, wherein movement of the arm from the first to the second position actuates the lever and causes the block to move the at least one blocking member from the retracted position to the extended position. claim 1 4. An x-ray device as defined in claim 3 , wherein the lever comprises first and second legs disposed substantially at a right angle to one another, and wherein the arm actuates the lever by applying a force to the first leg. claim 3 5. An x-ray device as defined in claim 4 , wherein the second leg of the lever includes an elongated hole, and wherein the block is connected to the lever by a pin that passes through the elongated hole and into the block. claim 4 6. An x-ray device as defined in claim 1 , wherein the collimating plate and the at least one blocking member comprise a material selected from the group consisting of lead, tungsten, and bismuth. claim 1 7. An x-ray device as defined in claim 1 , wherein the adjustable collimator further comprises a mounting plate that connects the adjustable collimator to the outer housing of the x-ray device. claim 1 8. An x-ray device as defined in claim 7 , wherein the mounting plate is integrally formed with the base member. claim 7 9. An x-ray device as defined in claim 1 , further comprising a second blocking member composed of an x-ray attenuating material, the second blocking member being movable in a linear direction independent of the at least one blocking member to partially block the aperture defined in the collimating plate. claim 1 10. An x-ray beam collimator assembly comprising: a base member having an outer periphery that is adapted to be received within a port of an x-ray generating device; a collimator plate attached to an end of the base member, the collimator plate being substantially comprised of an x-ray attenuating material and defining a primary x-ray passing region; a blocking member that is attached to the base member in a manner such that the blocking member is selectively movable in a substantially linear direction between a retracted position and an extended position, wherein in the extended position at least a portion of the blocking member obstructs the primary x-ray passing region so as to define a second radiation passing region; and at least one guide rod, the guide rod being operable to effect movement of the blocking member between the retracted position and the extended position. 11. An x-ray beam collimator assembly as defined in claim 10 , further comprising a retractable arm operably connected to the blocking member via a lever, the lever being connected to the blocking member such that movement of the retractable arm actuates the lever and moves the blocking member between the retracted position and the extended position. claim 10 12. An x-ray beam collimator assembly as defined in claim 11 , wherein the retractable arm is at least partially disposed within a hole defined in the base member, and wherein the retractable arm is retained within the hole via a C-clip that is attached to the retractable arm. claim 11 13. An x-ray beam collimator assembly as defined in claim 11 , wherein the lever comprises first and second legs disposed in an xe2x80x9cLxe2x80x9d-shaped configuration, and wherein the arm actuates the lever by applying a force to the first leg. claim 11 14. An x-ray beam collimator assembly as defined in claim 13 , wherein the second leg of the lever is pivotably connected to the blocking member via a pivot pin. claim 13 15. An x-ray beam collimator assembly as defined in claim 14 , wherein the second leg includes an elongated hole, and wherein the pin that pivotably connects the lever to the blocking member is slidably disposed within the elongated hole. claim 14 16. An x-ray beam collimator assembly as defined in claim 10 , wherein the blocking member comprises at least two extended portions, the extended portions obstructing portions of the primary x-ray passing region when the blocking member is in the extended position. claim 10 17. An x-ray beam collimator assembly as defined in claim 10 , further comprising a resilient member that is disposed in relation to the blocking member such that it provides a force to move the blocking member into the retracted position. claim 10 18. An x-ray beam collimator assembly as defined in claim 10 , wherein the collimator plate and the blocking member comprise an x-ray attenuating material selected from the group consisting of lead, tungsten, and bismuth. claim 10 19. An adjustable collimating device for use in collimating an x-ray signal emitted from the surface of an anode within an x-ray tube, the collimating device comprising: a base member; a stationary collimator plate attached to the base member in a manner such that the plate is substantially adjacent to the anode, the collimator plate defining at least one slot through which at least a portion of the x-ray beam passes, the collimator plate comprising an x-ray attenuating material; a blocking member; means for selectively moving the blocking member between a retracted position and an extended position, wherein when in the extended position the blocking member at least partially obstructs a portion of the slot and thereby prevents a portion of the x-ray signal from passing through the slot; and a resilient member that is disposed in relation to the blocking member such that it provides a force to move the blocking member into the retracted position or the extended postion. 20. An adjustable collimating device as defined in claim 19 , wherein the means for selectively moving comprises: claim 19 a retractable arm operably connected to the blocking member via a lever, the lever pivotably connected to the blocking member such that selective movement of the retractable arm actuates the lever and moves the blocking member from the retracted position to the extended position in a substantially linear direction. 21. An adjustable collimator as defined in claim 20 , wherein the lever comprises first and second legs disposed in an xe2x80x9cLxe2x80x9d-shaped configuration, and wherein the arm actuates the lever by applying a force to the first leg. claim 20 22. An adjustable collimating device as defined in claim 21 , wherein the resilient member comprises: claim 21 a spring that is disposed in relation to the blocking member so as to provide a force that moves the blocking member from the extended position to the retracted position. 23. An adjustable collimator as defined in claim 19 , wherein the collimator plate and the blocking member are substantially comprised of lead. claim 19 24. An x-ray beam collimator assembly comprising: a base member having an outer periphery that is adapted to be received within a port of an x-ray generating device, the base member including a cavity; a collimator plate attached to an end of the base member, the collimator plate being substantially comprised o f an x-ray attenuating material and defining a primary x-ray passing region; and a blocking member at least partially disposed in the base member cavity, the blocking member comprising: a block having at least one extending tab, the at least one tab engaging a surface within the base member cavity to enable the blocking member to slide in a linear direction between a retracted and an extended position; and at least two extended portions comprising an x-ray attenuating material, the extended portions being attached to the block and obstructing portions of the primary x-ray passing region to define a secondary radiation passing region when the blocking member is in the extended position. 25. An x-ray beam collimator assembly as defined in claim 24 , further comprising a retractable arm operably connected to the block via a lever, the lever being connected to the block such that movement of the retractable arm actuates the lever and moves the blocking member between the retracted position and the extended position. claim 24 26. An x-ray beam collimator assembly as defined in claim 25 , further comprising a resilient member that is disposed in relation to the blocking member such that it provides a force to move the blocking member into the retracted position. claim 25 27. An x-ray beam collimator assembly as defined in claim 26 , wherein the resilient member comprises a spring, and wherein the spring is connected to lever. claim 26 28. An x-ray beam collimator assembly as defined in claim 27 , further comprising at least one guide rod, the guide rod being operable to effect movement of the blocking member between the retracted position and the extended position. claim 27 29. An x-ray beam collimator assembly as defined in claim 28 , wherein the block includes two tabs oppositely disposed on the block. claim 28 30. An x-ray beam collimator assembly comprising: a base member having an outer periphery that is adapted to be received within a port of an x-ray generating device; a collimator plate attached to an end of the base member, the collimator plate being substantially comprised of an x-ray attenuating material and defining a primary x-ray passing region; first and second pivot arms pivotally attached at one end of each arm to the base member, the first and second pivot arms being interconnected in a scissor-like fashion so as to be selectively movable between a retracted position and an extended position; and a first blocking member attached to the first pivot arm, and a second blocking member attached to the second pivot arm, the first and second blocking members obstructing the primary x-ray passing region so as to define a secondary x-ray passing region when the first and second pivot arms are in the extended position. 31. An x-ray beam collimator assembly as defined in claim 30 , further comprising a retractable arm operably connected to at least one of the pivot arms such that movement of the retractable arm pivots the first and second pivot arms between the retracted position and the extended position. claim 30 32. An x-ray beam collimator assembly as defined in claim 31 , further comprising a resilient member that is disposed in relation to the pivot arms such that it provides a force to move the pivots arms into the retracted position. claim 31 33. An x-ray beam collimator assembly as defined in claim 32 , wherein the resilient member comprises a spring that operably attaches to at least one of the pivot arms. claim 32 34. An x-ray beam collimator assembly as defined in claim 33 , wherein the first pivot arm includes an extended pin that interconnects with an elongated slot defined in the second pivot arm. claim 33 35. An x-ray beam collimator assembly as defined in claim 34 , wherein the first and second pivot arms and the first and second blocking members are disposed in a cavity defined in the base member. claim 34 36. An x-ray beam collimator assembly as defined in claim 35 , wherein the first blocking member is attached to an end of the first pivot arm that is opposite the end that is pivotally attached to the base member, and wherein the second blocking member is attached to an end of the second pivot arm that is opposite the end that is pivotally attached to the base member. claim 35
	abstract	In apparatus for radiation analysis, for example, X-ray spectrometers, the aperture angle of the analysing radiation beam 45 is often desired to vary during the measuring process. The aperture angle of the radiation beam is determined, for example, by the length of the collimating elements 46, 60 in the collimator. In accordance with the invention, this is achieved by displacing or rotating the collimator through the radiation beam 45, so that the collimating element length L exposed to the radiation beam can be varied in consequence. A collimator comprising rectangular plates 46 (Soller collimator) can be rotated around a shaft 50 perpendicular to the plates, or a collimator comprising X-ray fibres 60 can be arranged with varying fibre lengths and displace them through the radiation beam transversely to the longitudinal direction of the fibres.
	summary
	summary
056278659	description	DETAILED DESCRIPTION OF THE INVENTION The trend in reactor fuel design has been to increase the number of fuel rods in an assembly to more finely divide the uranium fuel so that the heat generated in the fuel may be more readily transferred to the coolant. This allows progressively more heat to be generated in an assembly but must be within limits on fuel rod power to assure reliable performance of the fuel rod. Increased power generated by a fuel assembly can (1) improve nuclear fuel utilization through greater freedom to optimize the core power distribution, (2) improve plant capacity factor by permitting more rapid power changes, or (3) increase the overall power density of the core and the plant power output. In BWRs, fuel rod array designs have progressed from 7.times.7 through 8.times.8 and 9.times.9 to the 10.times.10. In PWRs, the early fuel rod array designs were 14.times.14 and 15.times.15 and later designs are 16.times.16 and 17.times.17. To prevent degradation of the heat transfer efficiency, a minimum separation between fuel rods is required. To be assured of the required separation, an allowance must be made for both fabrication tolerances as well as irradiation induced bow of the fuel rods. A square lattice BWR 10.times.10 fuel assembly has 100 fuel rod positions available and the rod-to-rod pitch will be approximately 0.51 inches. The required rod-to-rod spacing will determine the maximum allowable fuel rod diameter without degradation of heat transfer efficiency during the fuel assembly operation. This places a limit on the volume of fuel that can be incorporated in a given fuel assembly length. However, in accordance with the present invention, if the fuel rods are configured in a 10.times.12 triangular lattice array, then 120 fuel rod locations are made available with a larger rod diameter than in the 11.times.11 square lattice array, thus increasing the volume of fuel in a given fuel assembly length. In a square lattice array, the 120 potential fuel rod locations would require an 11.times.11 lattice. The 11.times.11 rod pitch would be about 0.464 inch and the maximum rod diameter would be reduced by the rod-to-rod spacing requirement so that the volume of fuel that could be incorporated in a given length of fuel assembly would be reduced to less than for the 10.times.12 array. Referring to FIG. 1, a boiling water reactor fuel assembly 100 has fuel rods 11 positioned within the area formed by outer channel 15. According to the prior art, a 10.times.10 array of fuel rods would be positioned within the area formed by the outer channel. Fuel rods 11 are arranged with their centers located at the vertices of equilateral triangles rather than the corners of squares. Six adjacent triangles having a common vertex form a hexagon. Fuel rods 11 are arranged in 12 rows with 10 rods in each row so as to fit within the confines of outer channel 15. The arrangement shown in FIG. 1 is a 10.times.12 hexagonal BWR fuel assembly and contains 120 fuel rod locations which is within one rod position available in a square lattice array of 11.times.11. The hexagonal arrangement shown in FIG. 1 has the advantage that a larger fuel rod diameter can be used while maintaining the same rod surface to rod surface spacing and rod surface to channel wall spacing of a square lattice array of about the same number of fuel rods, because of the more efficient utilization of cross sectional area within the outer channel. In a square lattice array, an 11.times.11 arrangement would be necessary to provide 120 fuel rod locations provided by the 10.times.12 hexagonal arrangement. Consequently, the 11.times.11 rod pitch would be reduced (about 0.464 inches) and a smaller fuel rod diameter (less than 10/11) would be necessary to maintain the required rod surface to rod surface spacing for adequate cooling. The smaller diameter fuel rods would provide a lower amount of uranium fuel per rod. Referring to the earlier example, the 11.times.11 lattice could have a rod diameter of 0.35 inches, for a rod-surface-to-rod surface spacing of 0.114 inches. The 10.times.12 triangular lattice would have a rod pitch of 0.485 inches and could have a rod diameter of about 0.371 inches, providing an increase in relative fuel cross sectional area of approximately ##EQU2## Thus, the triangular lattice arrangement of FIG. 1 enables the use of larger fuel rod diameters that could be used in an 11.times.11 array. Because of the larger fuel rod diameter(s), more water rods and part length fuel rods can be used while maintaining the fuel assembly uranium content. Thus, increased diameter of fuel rods, permissible in a triangular lattice relative to a square lattice, allows a greater fraction of the fuel rod positions to be used for water channels, water rods, part length fuel rods, and/or part length fuel rods having part length water rods. Referring to FIG. 2, fuel rods 11, water rods 17 and part length fuel rods 13 are positioned in fuel assembly 10 so that every fuel rod 11 and part length fuel rod 13 is: (a) directly adjacent to either a water rod 17 or the outer channel 15; or (b) has a direct line of sight to a water rod or the outer channel. This configuration results in a greater probability that fast neutrons born by fission in the fuel rods will escape resonance capture in another fuel rod and instead be thermalized in the water rods or in the water outside of the fuel channel. The higher probability of neutron thermalization produces a larger thermal neutron source throughout the fuel assembly with resultant increased fuel utilization. This arrangement could not be achieved in a square lattice fuel assembly having a similar number of possible rod locations without severely reducing the uranium content of the fuel assembly. Satisfactory uranium loading can be maintained in the triangular lattice because of the larger number of fuel rod locations and the inherently larger possible uranium loading. In most cases, some fuel rod positions will be occupied by water rods or water channels to add moderation and obtain better uranium utilization. The 10.times.12 triangular lattice array shown in FIG. 1 makes about 6 more fuel rod positions available for water rods or channels while maintaining an equal volume for uranium fuel as a 10.times.10 square lattice array. BWR fuel assemblies are typically under-moderated at their centers. Accordingly, supplemental water moderation is most effective when placed near the center of the assembly cross-section. Referring to FIG. 3, a center water channel composed of (a) a tube 18 (having a diameter up to two rod pitches and a fuel rod diameter) and (b) six smaller tubes 19 (each having the diameter of a fuel rod) is substituted for thirteen fuel rod positions near the assembly center. Center water channel assembly comprising tubes 18 and 19 fits into the triangular array so as not to disrupt the uniformity of coolant area distribution. Although an equilateral triangular lattice has the advantage of the most dense and uniform packing of fuel rods, a very limited number of equilateral triangular lattices fit into an approximately square boundary. One of these the 10.times.12 hexagonal BWR Fuel Assembly is shown in FIGS. 1-3 and is discussed above. In accordance with another embodiment of the present invention, other triangular lattice arrays could be made by slightly modifying the triangular lattice from equilateral to fit into a square boundary with approximately uniform distribution of coolant area to each fuel rod. One of these non-equilateral triangular arrays, a 9.times.11 array is shown in FIG. 13a. In this embodiment of the invention, a triangular fuel rod lattice for a square fuel assembly is obtained by placing the centers of the fuel rods at the vertices of isosceles triangles. Referring to FIG. 13a, BWR fuel assembly 111 having a 9.times.11 fuel rod arrangement within outer channel 15 is shown with the centers of fuel rods 11 located at the vertices of isosceles triangles with a height h to base b ratio (h/b) of about 0.85. The angles at the base, of the isosceles triangle are approximately 59.53 degrees and the angle opposite the base, the apex, is approximately 60.94 degrees as shown in FIG. 13b. If the required rod surface to rod surface spacing is 0.114 inch as in the square lattice array is to be maintained, then the maximum fuel rod diameter in the 9.times.11 is limited to about 0.415 inch. This is determined by the sides of the triangle opposite the base angles because they are, in this arrangement, shorter than the base of the triangle. The dimension b for a typical BWR fuel assembly which has an inside channel dimension of about 5.278 inches and a rod to channel wall spacing of 0.145 inch is 0.537 inch. The sides of the triangles opposite the base angles will be 0.986.times.0.537 inch =0.529 inch. This dimension of 0.529 inch less the rod-to-rod space of 0.114 inch leaves 0.415 inch for the rod diameter. This is larger than the rod diameter of a 10.times.10 square lattice array which was 0.396 inch and provides ##EQU3## In addition, orthogonal symmetry which is symmetry across each centerline perpendicular to the assembly faces so that the assembly can be divided into identical quarters can be obtained for lattices with an odd number of fuel rod rows by removing one fuel rod from every other row. Another non-equilateral triangular array, a 8:9.times.11 array is shown in FIG. 4a. In this embodiment of the invention, a triangular fuel rod lattice for a square fuel assembly is also obtained by placing the centers of the fuel rods at the vertices of isosceles triangles. The designation 8:9.times.11 identifies that this array has a number of fuel rods that alternates from row to row from 8 to 9 to 8 etc. Thus, the array in FIG. 4a begins with 8 fuel rods in the bottom row, and alternates in the next row to 9 fuel rods, and again alternates back to 8 fuel rods for all 11 rows. The ratio of the height to the base of the isosceles triangle of the triangular lattice is selected to create a high density triangular arrangement of fuel rods. In a specific application, a fuel rod diameter less than the maximum allowable based on minimum rod-to-rod spacing requirements may be selected to reduce flow resistance or optimize the water-to-fuel ratio for reactivity characteristics. In a preferred embodiment, the isosceles triangles should be as near equilateral as possible to maximize the packing density of the fuel rods with a nearly uniform rod-to-rod spacing. Referring to FIG. 4a, BWR fuel assembly 102 having a 8:9.times.11 fuel rod arrangement is shown with the centers of fuel rods 11 located at the vertices of isosceles triangles with a height h to base b ratio (h/b) of 8 to 10, or 0.8. The angles at the base of the isosceles triangle are approximately 58 degrees, and the apex angle is approximately 64 degrees as shown in FIG. 4b. This arrangement is symmetrical across any centerline, so that all four corners present the same geometry to the channel walls, and the assembly may be divided into identical quarters for convenience in reactivity and power distribution calculations. Referring to FIG. 5, this symmetry across any centerline facilitates the substitution of a centrally located hexagonal water channel 22 for the seven centrally located fuel rods providing water moderation of neutrons in the center of the fuel assembly in order to flatten the thermal neutron flux. Further, the absence of fuel rods in the corners of fuel assembly 103 shown in FIG. 5 will reduce corner fuel rod power peaking and will permit larger radii for the outer channel corners making the channel easier to fabricate with thick corners. It is a further advantage that the absence of fuel rods in the corner of assembly 103 shown in FIG. 5 will permit reduction of the rod surface to channel wall surface spacing to that provided for rod surface to rod surface spacing thereby allowing an increase in the rod pitch and consequently the rod diameter. In an alternative embodiment shown in FIG. 6, a BWR fuel assembly 104 having a 9:8.times.11 fuel rod arrangement (i.e. eleven rows with the number of fuel rods per row alternating from 9 to 8 to 9 etc.) with the centers of fuel rods 11 at the vertices of isosceles triangles similar to FIGS. 4a and 5, but further includes fuel rods positioned in the four corners of the assembly. Fuel assembly 104 further differs from fuel assembly 103 shown in FIG. 4a in that the fuel rod loading is increased by one fuel rod to increase the amount of uranium in the fuel assembly when fuel loading takes priority over corner rod power peaking. In order to alter the moderation of the assemblies shown in FIGS. 4-6, water rods, water channels and/or part length fuel rods can be positioned in place of the fuel rod(s) in either a symmetrical or asymmetrical pattern. In each of the assemblies shown in FIGS. 4-6 and 13, outer channel 15 is shown having straight walls. Referring to FIG. 7, the indentation of every other row of fuel rods 11 allows two of the outer channel walls 16 of the fuel assembly 105 to be corrugated on two sides giving it greater rigidity to resist seismic forces, and improves the uniformity of distribution of the coolant flow area. As stated above, nuclear fuel rods are spaced apart from one another to provide adequate flow area for coolant to remove heat from each of the fuel rods. In addition, it is highly desirable to have a method of verifying that the minimum required space between fuel rods has been provided. In a square lattice array, rod-to-rod spacing is readily verified by passing a calibrated shim through the line-of-sight between the rows and columns of fuel rods. A difficulty with a triangular lattice is that the line-of-sight between a column of fuel rods is blocked by the rods in the next row. This difficulty is overcome by recognizing, in accordance with the present invention, that a line-of-sight may be maintained parallel to the line of fuel rod centers by judicious choice of height to base ratios of the isosceles triangular lattice or pitches and rod diameters of interest. The resulting three line-of-sight directions can then be used to verify the adequacy of rod-to-rod spacing. A BWR core is composed of repetitions of control rod modules each of which comprises a control rod blade surrounded by four fuel assemblies. The preferred loading of triangular lattice BWR assemblies (for example four fuel assemblies 111 in accordance with the present invention) will put the assembly faces with uniform rod to channel spaces adjacent to assembly faces that have alternating rod to channel spaces as shown in FIG. 14 so that the water gap area between channels will be substantially uniform. This arrangement will also present uniform corner patterns without rods at the assembly corners located at the juncture of the control rod blades 115 as shown in FIG. 14. Not only will this arrangement provide a substantially uniform distribution of water moderator around the assemblies, but the core will be stiffened against seismic forces in each direction by corrugating the channel faces of fuel assemblies 112 as shown for example in FIG. 15. As contrasted to BWR fuel assemblies, fuel assemblies for pressurized water reactors (PWRs) have larger cross-sectional areas and have more fuel rods. Typical present day PWR fuel assemblies include 15.times.15 and 17.times.17 fuel rod arrays distributed on a square lattice. In accordance with another embodiment of the present invention, an isosceles triangular fuel rod lattice with a height h to base b ratio h/b of about 0.875, with base angles only slightly greater than 60 degrees (i.e. approximately 60.255) would be used to distribute fuel rods 12 in PWR fuel assembly 106 to form a 15.times.17 array as shown in FIGS. 8a and 8b. In another embodiment, an isosceles triangular fuel rod lattice with a height h to base b ratio h/b of about 0.824, with base angles only slightly less than 60 degrees (i.e. approximately 58.75) would be used to distribute fuel rods 12 in PWR fuel assembly 107 to form a 15.times.18 array as shown in FIGS. 11a and 11b. Fuel assemblies 106 and 107 each has the same overall cross-sectional area of the fuel assemblies of the prior art having a 15.times.15 array distributed on a square lattice. Slight modification of the PWR control rod cluster assemblies might be necessary since control rod guide tubes typically take fuel rod positions within the lattice. Thus, control rod guide tubes will be selectively positioned within the fuel rod arrays shown in FIGS. 8a and 8b as well as those discussed below in accordance with reactivity control requirements. Referring to FIGS. 9a and 9b, a 17.times.19 PWR fuel assembly array 109 of triangular lattice fuel rods 12 with a height h to base b ratio h/b of about 0.889 and base angles of approximately 60.65 degrees can be used as an alternative to the prior art fuel assemblies having a 17.times.17 square lattice array. FIG. 9a shows a typical position of guide tubes 20 in fuel assembly 109, but can be located in alternative positions within the array. Instrumentation tube 21 is typically located in the very center of the array. A 17.times.20 PWR fuel assembly array 110 of triangular lattice fuel rods 12 with a height h to base b ratio h/b of about 0.842 and base angles approximately 59.3.degree. shown in FIGS. 12a and 12b can be similarly used as an alternative to the prior art PWR fuel assemblies having a 17.times.17 square lattice array. Fuel assemblies 109 and 110 each has the same overall cross-sectional area of the fuel assemblies of the prior art having a 17.times.17 array distributed on a square lattice. Since PWR fuel assemblies do not include an outer channel as in BWR fuel assemblies, each fuel assembly (e.g. FIGS. 8 or 9) can interfit with one another at their boundaries as shown for example in FIG. 10. Four fuel assemblies 106 interface together (shown as dashed lines) to form a regular distribution of fuel rods without any discontinuities. While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
	description	This application claims the benefits of the Taiwan Patent Application Serial Number 100123732, filed on Jul. 5, 2011, the subject matter of which is incorporated herein by reference. 1. Field of the Invention The present invention relates to specimen box with one or more through holes for use with an electron microscope, especially to a specimen box with one or more photoelectric elements for use with an electron microscope. 2. Description of Related Art As known in the prior art, a vacuum environment inside an electron microscope is critical for high resolution and accuracy during the observation of specimens under the electron microscope. Considering the vacuum environment, the conventional electron microscope is usually used to observe the structures of solid substances or specimens, such as dehydrated bio-tissueor dehydrated virus. Hence, the conventional electron microscope has a limitation on the selection of specimens and is invalid for the dynamic observation of specimens. Even the stimuli-induced responses of specimens are unobservable under the conventional electron microscope. Due to the above-mentioned limitations, the application of the electron microscope is restricted. In order to improve the above-mentioned drawbacks, a specimen box for an electron microscope suitable for the observation of specimens (such as chemical particles, biochemical molecules, or bio-tissues) in a gas or liquid state was proposed. After the specimen is injected into this specimen box, a sealant or a polymer sealant is used for sealing the specimen box. However, the vacuum degree of the electron microscope may be degraded due to the easy evaporation of liquid from the specimen into the vacuum environment through the sealant or the polymer sealant. Accordingly, the resolution and the observation efficiency of the electron microscope would be greatly affected by the above-mentioned factors. Another specimen box for an electron microscope was also suggested to solve the above issues. The specimen box further includes a gas chamber in addition to the specimen chamber. Accordingly, the gas diffusion or the liquid evaporation from the specimens in the specimen chamber can be inhibited by the pressure equilibrium between the specimen chamber and the gas chamber. However, the inert gas filled in the gas chamber for the pressure equilibrium may affect the observation resolution. In addition, the structure of this specimen box is complicated so the cost is increased. In all of the current and above specimen boxes, none could be opened again (=reopen) after the specimen boxes are sealed. Due to finite oxygen contained in the closed space, the long-term dynamic changes and the light- or current-induced responses of the specimens cannot be observed under the electron microscope and thus the observation of living tissues or cells is limited. According to above, providing a specimen box, which has high sealing and reopening features and allows the application of ambient stimuli (such as light, or current), is advantageous to the long-term dynamic observation of specimens in a gas or liquid state and the variation of the observation condition under an electron microscope. The object of the present invention is to provide a specimen box for an electron microscope, in which the specimen box comprises through holes and plugs. Hence, the specimen box can be reopened to inject gas or liquid again so as to prolong the in-situ observation time effectively. Another object of the present invention is to provide a specimen box with photoelectric elements. Hence, when the specimen is stimulated by light or current by the photoelectric elements, the observation of dynamic changes and response of the specimen could be obtained. To achieve the object, the current specimen box for an electron microscope comprises: a first substrate, a second substrate, a metal adhesion layer, and one or more photoelectric elements. The first substrate has a first surface, a second surface, a first concave, and one or more first through holes, wherein the first concave is disposed on the second surface, a first thin film corresponding to the first concave is disposed on the first surface, and the first through hole is disposed around the first concave and penetrates through the first substrate. The second substrate has a third surface, a fourth surface, and a second concave, wherein the second concave is disposed on the fourth surface, and a second thin film corresponding to the second concave is disposed on the third surface. The metal adhesion layer is disposed between the first substrate and the second substrate. In addition, a space, which is formed by the first substrate, the second substrate, and the metal adhesion layer, could contain the gas or liquid specimens in the specimen box. The photoelectric element comprises one or more ends, wherein the photoelectric element is disposed between the first substrate and the second substrate, and the end is disposed in the space. The specimen used for the specimen box, such as chemical atoms, molecules, complexes, mixtures, bio-tissues, cells, enzymes, nerve cells, photoreceptor cells, or stem cells is not particularly limited as long as it could be observed by the electron microscope. The photoelectric element can transmit light or transfer current to the end of the photoelectric element. Therefore, the dynamic change and response of the specimen could be observed by the stimuli of light or current. According to the specimen box of the present invention, the first through hole penetrates through the first thin film and the first substrate to connect the space in the specimen box with the outside space. Therefore, the specimen box could be opened by the first through hole, and the specimens, gas, or liquid could be inserted or injected into the specimen box through the first through hole. In the present invention, the first concave and the second concave are formed by photolithography process accompanied by wet etching process, dry etching process, or deep reactive-ion etching process. The shape of the first concave and the second concave could be regular shape or irregular shape. Preferably, the shape of the first concave and the second concave is independently a cylinder, a cone, a cube, or a cuboid. In the present invention, one or more third concave are disposed on the second surface of the first substrate, and one or more fourth concave are disposed on the fourth surface of the second substrate. One or more photoelectric elements are disposed on the third concave and the fourth concave. The photoelectric element could transmit light or transfer current to the end. The end could release light or current in the space. Therefore, the light or current could stimulate the specimen in the space. The third concave and the fourth concave are formed by photolithography process accompanied by wet etching process, dry etching process, or deep reactive-ion etching process, particularly, are formed by photolithography process accompanied by wet etching process. The disposition of the third concave and the fourth concave are not especially limited. The preferable position of the third concave and the fourth concave is at the diagonal position of the first substrate and the second substrate. The photoelectric element is independently a photoelectric conversion element, an optical element, or an electric element. Preferably, the photoelectric element is independently an optical fiber, or an electrode. The optical fiber is a gradient fiber, a multi-mode mutant fiber, a single mode fiber, a multi-mode fiber, a photonic crystal fiber, or etc. In the present invention, the photoelectric element could be disposed, sealed, and fixed on the third concave and the fourth concave by O-ring, sealant, polymer sealant, solder, or etc. The metal adhesion layer of the specimen box could be disposed between the second surface and the fourth surface, the second surface and the second thin film, or the first thin film and the second thin film, in order to form a space with different volume and shape. Hence, according to the different specimen volume and different observed resolution, the volume and the shape of the space could be adjusted by the disposition of the metal adhesion layer. The volume of the space is 0.01 mm3 to 100 mm3. Preferably, the volume of the space is 0.05 mm3 to 50 mm3. Most preferably, the volume of the space is 0.1 mm3 to 10 mm3. The height of the space is between 10 μm to 1000 μm. Preferably, the height of the space is between 20 μm to 700 μm. Most preferably, the height of the space is between 30 μm to 550 μm. The material of the metal adhesion layer preferably comprises a metal material to form a solder, in which the metal material is selected from a group consisting of Ti, Cr, Sn, In, Bi, Cu, Ag, Ni, Zn, Au, and Ti—W alloy. Preferably, the metal material is Sn, Ni, Zn, Au, In, or a combination thereof. Most preferably, the metal material is Sn, Au, or a combination thereof. In addition, the metal adhesion layer could further comprise an adhesion layer, a metallurgy layer, and a solder layer, in which the material of the adhesion layer is Ti, Ti—W alloy, or Cr; and the material of the metallurgy layer is Ni, Cu, or Au. The metal material used in the present invention has excellent features of waterproofing, high sealing, and biocompatibility. However, the material of the metal adhesion layer has to be heated to a high temperature so as to allow the upper substrate and the lower substrate to adhere together. The high temperature thus may destroy the specimen in the specimen box. Hence, the preferably method to solve this problem is that the metal adhesion layer adheres the first substrate and the second substrate together at 70° C., then the specimen is inserted or injected into the specimen box. Therefore, the specimen would not be destroyed by the high temperature. In the present invention, the second substrate could further comprise one or more second through holes, in which the second through hole is disposed around the second concave and penetrates through the second substrate. Therefore, the space of the specimen box could connect with the outside space, and the specimen, gas, and liquid could be inserted or injected into the space through the second through hole. The hole size of the mentioned first through hole is 10 μm to 1000 μm. Preferably, the hole size of the first through hole is 50 μm to 700 μm. Most preferably, the hole size of the first through hole is 100 μm to 500 μm. In addition, the hole size of the mentioned second through hole is 10 μm to 1000 μm. Preferably, the hole size of the second through hole is 50 μm to 700 μm. Most preferably, the hole size of the second through hole is 100 μm to 500 μm. The hole size of the first through hole and the second through hole could be adjusted according to the different requirements for observation. The method for forming the first through hole and the second through hole is preferably a deep reactive-ion etching process or laser drilling process. In fact, the first through hole and the second through hole are passages to inject a gas specimen or a liquid specimen. Additionally, the first through hole and the second through hole also could be used to inject gas or liquid such as oxygen, nitrogen, buffer, or medium, as is required by the specimen. Therefore, the observation time of the specimen could be prolonged. For example, if oxygen and medium are injected into the space via the through hole, the lifetime of the cell specimen in the specimen box could not only be prolonged, but also the in-situ observation time is prolonged. The dynamic changes of the cell specimen could therefore also be observed. In the present invention, the specimen box for an electron microscope of the present invention could further comprise one or more plugs assembled into the first through holes and the second through holes. The material of the plug is not especially limited, which could be metal, memory metal, polymer, plastic, ceramic, acrylic, or a combination thereof. Preferable, the material of the plug is memory metal, polymer, plastic, ceramic, or a combination thereof. Most preferably, the material of the plug is memory metal. Then, the material of memory metal could select from a group consisting of Ni—Ti alloy, copper-base alloy, Cu—Zn alloy, Cu—Al—Mn alloy, Cu—Al—Ni alloy, Cu—Al—Be alloy, Cu—Al—Be—Zr alloy, and Cu—Al—Ni—Be alloy. Preferably, the material of memory metal is Ti—Ni alloy, Cu—Zn alloy, Cu—Al—Ni alloy, or a combination thereof. Most preferably, the material of memory metal is Ni—Ti alloy. Because memory metal has a property of thermal expansion and contraction, the plugs of the present invention preferably are used for sealing the through hole, and a tight sealing of the specimen box in the present invention could be accomplished. In the specimen box for an electron microscope of the present invention, the material of the first thin film and the second thin film is independently silicon dioxide (SiO2), silicon nitride (Si3N4), or a combination thereof. The function of the first thin film and the second thin film is increasing the selectivity in the etching process and enhancing the hardness of the surface of the first substrate and the second substrate. In addition, the thickness of the first thin film and the second thin film is independently 1 nm to 100 nm. Preferably, the thickness of the first thin film and the second thin film is independently 5 nm to 80 nm. In the present invention, the specimen box further comprises a first protective layer on the surface of the first thin film, in which the first protective layer is disposed on the surface of the first thin film. Similarly, the specimen box also comprises a second protective layer on the surface of the second thin film, in which the second protective layer is disposed on the surface of the second thin film. Preferably, the material of the first protective layer and the second protective layer is silicon nitride (Si3N4), in which silicon nitride (Si3N4) is hard enough to protect the first thin film and the second thin film, and could prevent cracking of the first thin film and the second thin film. Furthermore, the first protective layer and the second protective layer could increase the selectivity in the etching process. In the specimen box of the present invention, the first substrate and the second substrate is independently silicon substrate, glass substrate, or polymer substrate. Preferably, the first substrate and the second substrate is silicon substrate. In addition, the thickness of the first substrate and the second substrate is independently about 10 μm to 1000 μm. Preferably, the thickness of the first substrate and the second substrate is independently about 100 μm to 250 μm. According to above, before the specimen is injected in the specimen box, the photoelectric elements are disposed, fixed, and sealed on the third concave and the fourth concave by O-ring, sealant, polymer sealant, or solder, preferably by solder. Then, the metal adhesion layer adheres the first substrate and the second substrate together at high temperature. Besides, the specimen box comprises the first through hole and the second through hole, which are passages to insert or inject the specimen into the space of the specimen box. When the specimen is inserted or injected into the specimen box, the plugs, especially the memory metal plugs, would be assembled into the first through holes and the second through holes to tightly seal the specimen box by the feature of thermal expansion and contraction of the plugs. Therefore, the specimen was totally sealed in the specimen box so as to be observed through the electron microscope. When the specimen box is removed from the electron microscope, the plugs could be removed to reopen the specimen box temporarily. Therefore, a gas or liquid, as may be required by the specimen, could additionally be injected into the specimen box to prolong the lifetime of the specimen. In addition, the manufacturing method of the specimen box in the present invention is less complicated than the prior art, and the materials of the specimen box of the present invention are also easy to obtain. According to the above improvements of the specimen box in the present invention, the kinds of specimens which could be observed by an electron microscope are increased. After the specimen is stimulated by the photoelectric elements, the in-situ observation of the specimen could be obtained by using the specimen box of the present invention. Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. As showing in FIG. 1, FIG. 2, and FIG. 3, FIG. 1 a three-dimensional view showing the specimen box for electron microscope of the example 1; FIG. 2 is a three-dimensional view showing the photoelectric element of the specimen box for an electron microscope; and FIG. 3 is a three-dimensional view, which is shown along the A-A′ section line in FIG. 1, showing the specimen box for an electron microscope of the example 1. According to FIG. 1, FIG. 2, and FIG. 3, the specimen box of the present invention comprises: a first substrate 11, a second substrate 12, a metal adhesion layer 13, and four photoelectric elements 16. In the present example, the first substrate 11 and the second substrate 12 are (001) silicon substrate. The thickness of the first substrate 11 is 250 μm, and the thickness of the second substrate 12 is also 250 μm. The first substrate 11 has a first surface 111, a second surface 112, a first concave 113, and two first through holes 114, in which the first concave 113 is disposed on the second surface 112, and a first thin film 111 corresponding to the first concave 113 is disposed on the first surface 111. In addition, the first through holes 114 is disposed around the first concave 113 and penetrates through the first substrate 11. The second substrate 12 has a third surface 121, a fourth surface 122, and a second concave 123, in which the second concave 123 is disposed on the fourth surface 122, and a second thin film 125 corresponding to the second concave 123 is disposed on the third surface 121. Additionally, the metal adhesion layer 13 is disposed between the second surface 112 of the first substrate 11 and the fourth surface 122 of the second substrate 12, in which a space (not shown) was formed by the second surface 112, the fourth surface 122, and the metal adhesion layer 13. Gas or liquid specimens could be contained in the space (not shown). In the present example, the specimen is not especially limited as long as the specimens could be observed by an electron microscope. According to FIG. 1, FIG. 2, and FIG. 3, four fourth concaves 127 are formed on the diagonal position of the second surface 122 of the second substrate, in which the fourth concaves 127 are formed along the direction of <110> by photolithography process accompanied by the wet etching process. The etching solution of the wet etching process is NaOH solution. The shape of the fourth concave is V-shape. According to FIG. 2, the photoelectric element 16 is disposed on each of the fourth concave 127, and the end 161 of the photoelectric element 16 connects with the space (not shown). Therefore, the specimen in the space (not shown) could be stimulated by light or electricity. The photolithography process and the wet etching process are used for forming the first concave 113 on the second surface 112 and the second concave 123 on the fourth surface 122. The shape of the first concave 113 and the second concave 123 is cone. In addition, the deep reactive-ion etching process is used for forming the first through holes 114, in which the first through holes 114 penetrate through the first thin film 115 and the first substrate 11. The hole size of the first through holes 114 is 250 μm. The function of the first through holes 114 is as passages to insert or inject the specimen into the space (not shown). Besides, the first through holes 114 could also inject gas (such as oxygen, or nitrogen) or liquid (such as buffer, acidic solution, or basic solution) to further observe the dynamic changes of the specimen. The material of the first thin film 115 and the second thin film 125 in the present example is SiO2, in which the function is enhancing the hardness of the first substrate 11 and the second substrate 12 to avoid cracking of the substrates and increase the selectivity in the etching process. In the present example, the photoelectric element 16 is formed and fixed on the fourth concave 127 by solder. The end 161 of the photoelectric element 16 is connected with the space (not shown), the other end of the photoelectric element 16 is extended out from the specimen box, which connects with the source of light or electricity. In the present example, the photoelectric element 16 is an optical fiber, an electrode, or the combination of optical fiber and electricity. The metal adhesion layer 13 in the present example comprises an adhesion layer, a metallurgy layer, and a solder layer, in which the material of the adhesion layer is Ti—W alloy, and the material of the metallurgy layer is Cu. In the present example, the metal adhesion layer 13 adheres the first substrate 11 and the second substrate 12 to form the space 14 by the method of automatic alignment packaging method at 150° C. After the first substrate 11 and the second substrate 12 are adhered together, the specimen would be inserted or injected into the space (not shown) of the specimen box. An electron beam from the electron microscope would penetrate through the first concave 113 to the space (not shown) and penetrate through the second concave 123. The volume of the space (not shown) is 4 mm3, and the height of the space 14 is 550 μm. In order to enhance the hardness and etching selectivity of the substrate, a first protective layer 116 is disposed on the surface of the first thin film 115, and a second protective layer 126 is disposed on the surface of the second thin film 125. Furthermore, the material of the first protective layer 116 and the second protective layer 126 is silicon nitride (Si3N4). Finally, the specimen box has two plugs 15, which could seal the first through holes 114, to totally seal the specimen box. In addition, the plugs 15 also could be removed from the first through holes 114, therefore, the specimen box could be reopened according to the requirement of the in-situ observation. The material of the plugs 15 is Ti—Ni alloy. Because Ti—Ni alloy has the property of thermal expansion and contraction, the volume of the plugs 15 is smaller below freezing point than at room temperature. Therefore, when the plugs 15 below freezing point are assembled in the first through holes 114 of the specimen box with the room temperature, the volume of the plugs 15 would gradually expand according to the gradually warming plugs 15. Afterwards, the specimen box would be sealed completely as long as the first through holes 114 are sealed by the plugs 15. In the present example, a method of using a specimen box for observing a living specimen is shown. First, the cell specimen is inserted or injected in the space 14 through the first through holes 114. The plugs 15 below freezing point are assembled in the first through holes 114. After the temperature of the plugs 15 are warmed to room temperature, the specimen box would be sealed. Then, the specimen box is placed in the electron microscope to observe the cell specimen. In the process of the observation, the specimen could be stimulated by light, current, or the combination thereof, to complete the in-situ observation of the dynamic changes and response of the specimen. According to the requirements of the observation, one could further inject oxygen or medium by removing and then replacing the plugs 15 to complete the in-situ observation of the cell specimen. FIG. 4 is a perspective view showing the specimen box for an electron microscope of the example 2. According to FIG. 4, the specimen box of the present example is roughly the same as example 1. The only difference is the disposition of the metal adhesion layer 13. In the present example, the metal adhesion layer 13 is disposed between the second surface 112 and the second protective layer 126. Four of the third concaves 117 are formed on the second surface 112 of the first substrate 11 by a dry etching process. The photoelectric element 16, which is disposed on the third concave 117, is an optical fiber, an electrode, or the combination thereof. The volume of the space (not shown) in the present example is 2 mm3. The height of the space (not shown) is 550 μm. In the present example, the volume of the space (not shown) is smaller than example 1, so the resolution is higher than example 1. Therefore, different volumes of the space (not shown) of the specimen box could be chosen according to different requirements of the observation, such as the volume of the specimen, and the required resolution. FIG. 5 is a perspective view showing the specimen box for an electron microscope of the example 3. According to the FIG. 5, the specimen box of present example is roughly the same as example 1. The only difference in the present example is the first thin film 115, the first protective layer 116, the second thin film 125, and the second protective layer 126 are only disposed on the first concave 113 and the second concave 123 to enhance the structure of the first concave 113 and the second concave 123. Therefore, the first thin film 115 and the second thin film 125 would not be cracked so as to avoid the specimen escaping from the space (not shown). A fourth concave 127 is formed on the second surface 122 of the second substrate 12 by a wet etching process. The photoelectric element 16 is disposed on the fourth concave 127, in which the photoelectric element 16 is an optical fiber, an electrode, or a combination thereof. In addition, the first through holes 114 and the second through holes 124 are formed by a deep reactive-ion etching process. Therefore, the first through holes 114 are penetrated through the first surface 111, and the second through holes 124 are penetrated through the third surface 121. The hole size of the first through holes 114 is 250 μm, and the hole size of the second through holes 124 is also 250 μm. Finally, the material of the plugs 15 in the present example is Ni—Ti alloy, and the plugs 15 could also seal the first through holes 114 and the second through holes 124 to seal the specimen box completely. Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.
	description	In a pressurized water reactor (PWR) or other type of nuclear reactor, movable control rods are used to control the nuclear reaction. The control rods include a neutron absorbing material, and are arranged to be inserted into the reactor core. In general, the further the control rods are inserted into the core, the more neutrons are absorbed and the more the nuclear reaction rate is slowed. Precise control of the amount of insertion, and accurate measurement of same, is useful in order to precisely control the reactivity. The control rods drive mechanism (CRDM) provides this control. In an emergency, the control rods can be fully inserted in order to quickly quench the nuclear reaction. In such a “scram”, it is useful to have an alternative fast mechanism for inserting the control rods. Additionally or alternatively, it is known to have dedicated control rods that are either fully inserted (thus turning the nuclear reaction “off”) or fully withdrawn (thus making the reactor operational). In such systems, the “on/off” rods are sometimes referred to as “shutdown rods” while the continuously adjustable control rods are sometimes referred to as “gray rods”. Given these considerations, it is known to construct a CRDM employing a lead screw that is engaged by a separable roller-nut assembly. During normal operation, the roller-nut assembly is clamped onto the lead screw by an affirmative magnetic force acting against biasing springs. By turning the roller nut the lead screw, and hence the attached control rods, are moved in precisely controllable fashion toward or away from the reactor core. In a scram, the electrical current is cut thus cutting the magnetic force, the biasing springs open the separable roller nut, and the gray rod including the lead screw scrams. An example of such a configuration is disclosed, for example, in Domingo Ricardo Giorsetti, “Analysis of the Technological Differences Between Stationary & Maritime Nuclear Power Plants”, M.S.N.E. Thesis, Massachusetts Institute of Technology (MIT) Department of Nuclear Engineering (1977) which is incorporated herein by reference in its entirety. For an integral pressurized water reactor (integral PWR), it is known to mount the CRDM externally and to couple with the control rods inside the pressure vessel by suitable feedthroughs. To reduce the extent of feedthroughs, it has also been proposed to integrate the CRDM within the pressure vessel. See, for example, Ishizaka et al., “Development of a Built-In Type Control Rod Drive Mechanism (CRDM) For Advanced Marine Reactor X (MRX)”, Proceedings of the International Conference on Design and Safety of Advanced Nuclear Power Reactors (ANP '92), Oct. 25-29, 1992 (Tokyo Japan) published by the Atomic Energy Society of Japan in October 1992, which is incorporated herein by reference in its entirety. Existing CRDM designs have certain disadvantages. These disadvantages are enhanced when an internal CRDM design is chosen in which the complex electro-mechanical CDRM is internal to the high pressure and high temperature environment within the pressure vessel. Placement of the CRDM internally within the pressure vessel also imposes difficult structural challenges. The separable roller-nut creates a complex linkage with the lead screw that can adversely impact gray rod insertion precision during normal operation. Reattachment of the roller-nut to the lead screw can be complex, and it may not be immediately apparent when contact is reestablished, thus introducing a positional offset after recovery from the scram event. Scramming the lead screw also has the potential to cause irrecoverable damage to the threading or structural integrity of the lead screw. Still further, wear over time can be a problem for the complex separable roller-nut. Another consideration is reliability. Because rod scramming is a safety-critical feature, it must operate reliably, even in a loss of coolant accident (LOCA) or other failure mode that may include interruption of electrical power, large pressure changes, or so forth. The control rod position detector is also typically a complex device. In some systems, an external position detector is employed, which requires feedthroughs across the pressure vessel wall. For the internal CRDM of the MRX reactor, a complex position detector was designed in which a transducer generates a torsional strain pulse that passes through a magnetoresistive waveguide, and magnetic field interactions are measured to adduce the rod position. In general, an internal position detector operating on an electrical resistance basis is prone to error due to temperature-induced changes in material resistivity. In one aspect of the disclosure, a control rod mechanism for use in a nuclear reactor comprises: at least one control rod configured for insertion in a reactor core to absorb neutrons; a hollow lead screw; a motor operatively coupled with the hollow lead screw to drive the hollow lead screw linearly toward or away from the reactor core; a connecting rod connected with the aforementioned at least one control rod and disposed partially inside the hollow lead screw; a latch assembly having latches that when closed operatively connect the connecting rod and the lead screw so that when the latches are closed the connecting rod and the aforementioned at least one control rod move together with the lead screw when the lead screw is driven by the motor; and a release mechanism configured to cause the latches of the latch assembly to open responsive to a scram condition to detach the connecting rod from the lead screw such that the connecting rod and the aforementioned at least one control rod scram but the lead screw remains operatively coupled with the motor and does not scram. In another aspect of the disclosure, a control rod drive mechanism (CRDM) comprises: a lead screw; a motor threadedly coupled with the lead screw to linearly drive the lead screw in an insertion direction or an opposite withdrawal direction; a latch assembly secured with the lead screw and configured to (i) latch to a connecting rod and to (ii) unlatch from the connecting rod, the connecting rod being free to move in the insertion direction when unlatched; and a release mechanism configured to selectively unlatch the latch assembly from the connecting rod. In another aspect of the disclosure, a control rod drive mechanism (CRDM) comprises: a plurality of CRDM units each comprising a lead screw and a motor configured to drive the lead screw; and a support mounting the plurality of CRDM units in a nuclear reactor vessel with the motors of adjacent CRDM units arranged at different heights respective to a reactor core of the nuclear reactor vessel. Each CRDM unit is connected with one or more control rods such that the motor driving the lead screw moves the connected one or more control rods toward, away from, or within the reactor core. In another aspect of the disclosure, a control rod drive mechanism (CRDM) comprises: a lead screw; a drive assembly configured to linearly drive the lead screw in an insertion or opposite withdrawal direction, the drive assembly including a motor and at least one non-separable ball nut coupling with the lead screw; and a latch assembly connected with the lead screw and having (i) a latched state in which the latch assembly is latched to a connecting rod and (ii) an unlatch state in which the latch assembly is not latched to the connecting rod. In another aspect of the disclosure, a control rod mechanism for use in a nuclear reactor comprises: at least one control rod; a connecting rod connected with the aforementioned at least one control rod at a lower end of the connecting rod; and a control rod drive mechanism (CRDM) including a latch assembly having (i) a latched state in which the latch assembly is latched to an upper end of the connecting rod and (ii) an unlatched state in which the latch assembly is not latched to the upper end of the connecting rod, and a linear drive mechanism configured to drive the latch assembly linearly toward or away from a nuclear reactor core. In another aspect of the disclosure, in a control rod mechanism as set forth in the immediately preceding paragraph the CRDM is configured to allow the connecting rod to be removed by placing the latch assembly in the unlatched state and drawing the connecting rod away from the nuclear reactor core through the CRDM. With reference to FIG. 1, an illustrative nuclear reactor vessel of the pressurized water reactor (PWR) type is diagrammatically depicted. An illustrated primary vessel 10 contains a reactor core 12, internal helical steam generators 14, and internal control rods 20. The illustrative reactor vessel includes four major components, namely: 1) a lower vessel 22, 2) upper internals 24, 3) an upper vessel 26 and 4) an upper vessel head 28. A mid-flange 29 is disposed between the lower and upper vessel sections 22, 26. Other vessel configurations are also contemplated. Note that FIG. 1 is diagrammatic and does not include details such as pressure vessel penetrations for flow of secondary coolant into and out of the steam generators, electrical penetrations for electrical components, and so forth. The lower vessel 22 of the illustrative reactor vessel 10 of FIG. 1 contains the reactor core 12, which can have substantially any suitable configuration. One suitable configuration includes a stainless steel core former structure that contains the fuel assemblies and is replaceable in order to refuel the reactor, and which is supported by the lower vessel. The illustrative upper vessel 26 houses the steam generators 14 for this illustrative PWR which has an internal steam generator design (sometimes referred to as an integral PWR design). In FIG. 1, the steam generator 14 is diagrammatically shown. A cylindrical inner shell or upper flow shroud 30 separates a central riser region 32 from an annular downcomer region 34 in which the helical steam generators 14 are located. The illustrative steam generator 14 is a helical coil design, although other designs are contemplated. Primary reactor coolant flows across the outside of tubes of the steam generator 14 and secondary coolant flows inside the tubes of the steam generator 14. In a typical circulation pattern the primary coolant is heated by the reactor core 12 and rises through the central riser region 32 to exit the top of the shroud 30 whereupon the primary coolant flows back down via the downcomer region 34 and across the steam generators 14. Such primary coolant flow may be driven by natural convection, by internal or external primary coolant pumps (not illustrated), or by a combination of pump-assisted natural convection. Although an integral PWR design is illustrated, it is also contemplated for the reactor vessel to have an external steam generator (not illustrated), in which case pressure vessel penetrations allow for transfer of primary coolant to and from the external steam generator. The illustrative upper vessel head 28 is a separate component. It is also contemplated for the vessel head to be integral with the upper vessel 26, in which case the steam generator 14 and upper shroud 30 are optionally supported by lugs on the inside of the vessel head. The illustrative embodiment is an integral PWR in that it includes the internal steam generators 14, which in general may have various geometric configurations such as helical, vertical, slanted, or so forth. For the purpose of redundancy, it is generally advantageous to have more than one steam generator, whose pipes or tubes are typically interleaved within the downcomer region 34 to facilitate thermal uniformity; however, it is contemplated to include only a single steam generator. Although the illustrative steam generators 14 are shown disposed or wrapped proximate to the shroud 30, in general the steam generators may fill a substantial volume of the downcomer region 34, and in some embodiments the steam generators may substantially fill the annular volume between the outer surface of the shroud 30 and the inside surface of the pressure vessel 10. It is also contemplated for the internal steam generators or portions thereof to be disposed in whole or in part in the riser region 32, above the shroud 30, or elsewhere within the pressure vessel 10. On the other hand, in some embodiments the PWR may not be an integral PWR, that is, in some embodiments the illustrated internal steam generators may be omitted in favor of one or more external steam generators. Still further, the illustrative PWR is an example, and in other embodiments a boiling water reactor (BWR) or other reactor design may be employed, with either internal or external steam generators. With reference to FIG. 2, the upper internals section 24 in greater detail. In the illustrative design the upper internals section 24 provides support for control rod drives or drive mechanisms 40, 42 and control rod guide frames 44 and is also the structure through which control rod drive power and control instrumentation signals pass. This allows the upper vessel 26 and integral steam generator 14 to be removed independently of the control rod drives and associated structure. However, a more integrated design is also contemplated, such as using a common section for both the CRDM support and the integral steam generator support. With particular reference to the illustrative embodiment of FIG. 2, the upper internals structure 24 includes an upper internals basket 46, a CRDM support structure 48, control rod guide frames 44, and the control rod drive mechanisms 40, 42 themselves. The upper internals basket 46 is suitably a welded structure that includes the mid-flange 29 and the support structure for the control rod guide frames 44. In one suitable embodiment, the control rod guide frames 44 are separate 304L stainless steel welded structures that are bolted in place, the mid-flange 29 is a SA508 Gr 4N Cl 2 carbon steel forging, and the balance of the structure is 304L stainless steel. The CRDM support structure 48 includes support lattices for the control rod drives 40, 42 and guide structure for the in-core instruments. All of these are suitably 304L stainless steel. The CRDM support structure 48 is bolted to the upper internals basket 46. These are merely illustrative materials and construction, and other configurations and/or reactor-compatible materials are also contemplated. The illustrative example of FIG. 2 employs two types of control rod drives 40, 42: a hydraulic control rod drive type 42 that operates the shutdown rods which are either fully withdrawn or fully inserted into the core; and an electrical control rod drive type 40 that operates the gray rods which are inserted various amounts throughout the life of the core to control the nuclear reaction rate during normal reactor operation. The gray rods are also configured to scram, that is, to be rapidly inserted into the reactor core 12, during certain emergency conditions. In other embodiments, it is contemplated to omit the shutdown rods entirely in which case the gray rods also provide shutdown operation. With continuing reference to FIG. 2 and with further reference to FIGS. 3-5, aspects of the shutdown rods are illustrated. The shutdown rods are suitably arranged in clusters mounted on spiders or the like that are all operated in single bank and are all moved by a single shutdown rod drive 42. FIGS. 3-5 show only the single shutdown rod drive 42, but not the spiders and individual shutdown rods. This configuration is cognizant of the fact that the shutdown rods are used in a binary “on/off” manner, and are either all wholly inserted into the reactor core 12 in order to shut down the reaction, or are all wholly withdrawn from the reactor core 12 in order to allow normal reactor operation. With particular reference to FIG. 3, the shutdown rod drive 42 includes a cylinder housing 50, a cylinder cap 52, a cylinder base plate 54, and a connecting rod 56 providing connection to the shutdown rod lattice (not shown). The illustrative shutdown rod drive 42 of FIGS. 3-5 is a hydraulically actuated drive using reactor coolant inventory clean-up return fluid from high pressure injection pumps at approximately 500° F. (260° C.) and 1600 psi to hold the shutdown rod bank out of the reactor core 12. With particular reference to FIG. 4, a sectional view of the piston region with the rod in the withdrawn position is shown. In an enlarged portion of FIG. 4 a vent port 60 of the cylinder cap 52 is shown, together with a lift piston 62, piston rings 64 (which in some embodiments are metallic), a scram buffer 66, and a buffer cocking spring 68. The withdrawn position shown in FIG. 4 corresponds to the shutdown control drive cylinder 42 being pressurized. With particular reference to FIG. 5, a sectional view of the piston region with the rod in the inserted position is shown. An enlarged portion of FIG. 5 shows the lift piston 62, the piston rings 64, the scram buffer or scram buffer piston 66, a rod guide bushing 70, and rod sealing rings 72 (which in some embodiments are metallic). The cylinder base plate 54 is seen in the enlarged portion to include a pressure port or inlet port 74. The inserted position shown in FIG. 5 corresponds to the shutdown control drive cylinder 42 being unpressurized. In some embodiments, the coolant is allowed to bleed past the piston and shaft seals 64, 72 and becomes part of the inventory returned to the reactor vessel 10. The shutdown rod drive cylinder 42 is mounted above the reactor core 12. A hydraulic line (not shown) to actuate the cylinder 42 is routed through the flange 29 and instrument lines are routed through pressure tight conduit to common connectors that are also optionally used for the gray rod drives 40. The extension rods that connect the control rod spiders to the shutdown rod lattice are optionally designed so that they will slide through the lattice so that a single stuck cluster will not prevent the other sets of control rods from dropping. Additionally, the extension rods are designed to be disengaged from the control rod spider so that the shutdown rods remain in the core when the upper internals 24 are removed. Disengagement and reengagement is done using remote tooling at during refueling operations. During normal reactor operation, the shutdown rods are suspended completely out of the reactor core (that is, in the withdrawn position) by pressurization of the shutdown rod hydraulic cylinder 42. For example, in one suitable embodiment coolant inventory clean-up return fluid from the high pressure injection pumps is supplied at 500° F. (260° C.) and 1600 psi to the underside of the lift cylinder piston 62, via the inlet port 74 of the cylinder base 54. In this example, the fluid present in the cylinder 50 above the piston 62 is supplied from the reactor vessel 10 through the cylinder cap vent port 60, and is therefore at the reactor vessel conditions of 600° F. (315° C.) and 1500 psi, resulting in a net 100 psi pressure differential across the piston 62. Piston sizing is selected such that the developed pressure differential is sufficient to support the specified load of the shutdown rods and supporting spiders and other associated components and lift the shutdown rod bank through the cylinder stroke to the top stop of the piston 62. In the event of a vessel-pressurized scram, the shutdown rod bank is abruptly released by ceasing the supply of pressurized coolant to the bottom side of the lift piston 62 and venting the supply line to atmospheric pressure. In the aforementioned example the vessel pressure at the top surface of the lift piston 62 is expected to create an initial 1500 psig pressure differential across the lift piston, which acts along with the influence of gravity to propel the translating assembly (including the lift piston 62, scram buffer piston 66, cocking spring 68, connecting rod 56, and shutdown rod lattice (not shown) downward toward the full insertion position illustrated in FIG. 5. During the descent of the translating assembly, the force of the buffer cocking spring 68 holds the buffer piston 66 out of the bore of the lift piston 62, preserving a fluid-filled buffer cavity between the two pistons 62, 66. When the bottom surface of the buffer piston 66 impacts the fixed base plate 54 of the cylinder assembly, the continued travel of the lift piston 62 expels the trapped fluid through controlled flow restrictions, thereby dissipating the kinetic energy of the translating assembly. Additionally, kinetic energy is dissipated through elastic deformation of the translating assembly components, especially the long, relatively slender, connecting rod 56. Other kinetic energy dissipation mechanisms are also contemplated. When the fluid is expelled from the cavity, the lift piston 62 impacts the buffer piston 66, bringing the translating assembly to rest. With continuing reference to FIGS. 1 and 2 and with further reference to FIGS. 6-14, an illustrative embodiment of the gray rods and associated drive mechanisms 40 is described. As seen in FIG. 6, in the illustrative embodiment there are two different gray rod configurations (Type 1 and Type 2). The gray rods 80 are arranged as gray rod clusters, which in turn are yoked together in groups of two or four and supported by connecting rods 82 as shown in FIG. 6. The configuration Type 1 also includes a counterweight 84 in place of one connecting rod/cluster unit. More particularly, a yoke 86 connects two connecting rods 82 and the counterweight 84 to form a configuration of Type 1. A yoke 88 connects three connecting rods 84 to form a configuration of Type 2. The gray rod drives 40 are mounted above the reactor core 12. FIG. 7 shows a plan view of the locations of the gray rod drives 40 and of the shutdown rods lift cylinder 50, respective to the CRDM support structure 48. The shutdown rods lift cylinder 50 is centrally located. Four outboard gray rod drives 40, each moving two rod configurations of Type 1 including yokes 86, move simultaneously. Two inboard drives 40, each moving four rod configurations of Type 2 including yokes 88, move simultaneously. These different sets of drives 40 optionally move together or independently. Power and signal connections are suitably routed through a pressure tight conduit or in-core instrumentation guide 90 to connectors on the mid-flange 29 (not shown in FIG. 7). As with the shutdown rods, the extension rods that connect the control rod spiders to the rod lattice are optionally designed so that they will slide through the lattice so that a single stuck cluster will not prevent the other sets of control rods from dropping. Additionally, the extension rods are optionally designed to be disengaged from the control rod spider so that the gray rods can remain in the core when the upper internals are removed or be removed while the upper internals are on their support stand. Two suitable design styles for the gray rod control mechanism include the “magnetic jack” type and the “power screw” type. Of these, the power screw type is expected to provide more precise position control for the gray rod clusters, and accordingly the illustrated embodiment employs the power screw type control mechanism. With reference to FIG. 8, in one illustrated embodiment the gray rod control mechanism 40 employs a ball nut lifting rod configuration. FIG. 8 shows both the fully inserted state (left-side drawing) and fully withdrawn state (right-side drawing). The drawings of FIG. 8 show the yoke 88 of the Type 2 configuration; for the Type 1 arrangement the yoke 88 is replaced by the yoke 86. In the embodiment shown in FIG. 8, a bottom stop/buffer assembly 100 is mounted on a reactor support 101, optionally with additional lateral support provided for the electromagnet coil assembly. Lower and upper support tubes 102, 104, which mount to the top of the bottom stop 100, provide the guidance for the lead screw/torque taker assembly. A ball nut/motor assembly 106 mounts on top of the upper support tube 104 and an electromagnet coil assembly 108 mounts to the top of the motor. Within the electromagnet coil assembly 108 resides a lifting rod-to-lead screw latching assembly 110 that (when latched) supports a lifting/connection rod assembly 112 (seen extended in the inserted state, i.e. left-side drawing). A position indicator assembly is mounted to the support tubes 102, 104 between the ball nut/motor assembly 106 and the bottom stop assembly 100. In some embodiments, the position indicator is a string potentiometer suitably mounted below the latching assembly 110, although other mounting locations are contemplated. The illustrated string potentiometer includes a tensioned spool 120 mounted on the support tube 102 and a “string” or cable or the like 122 having an end attached to the lifting/connection rod assembly 112 such that the string or cable 122 is drawn off the spool 120 against the tension as the lifting/connection rod assembly 112 (and, hence the attached gray rod clusters) move toward the reactor core 12 (not shown in FIG. 8). When the motion is reversed, the tension in the tensioned spool 120 causes the string or cable 122 to roll back onto the spool 120. A rotational sensor 124 measures the rotation of the tensioned spool 120 using an encoder that counts passage of fiducial markers or another rotational metric. The mounting of the string potentiometer can be otherwise than that illustrated, so long as the tensioned spool 120 is mounted at a location that does not move with the gray rods and the string or cable 122 is secured to move with the gray rods. It is also contemplated to integrate the rotational sensor 124 with the tensioned spool 120. The string potentiometer provides an electrical output signal consistent with the location of the connecting rod or other component 112 that moves with the gray control rod, thus providing positional information for the gray control rods within the reactor core 12. The electrical position indication signal is conveyed out of the reactor vessel 10 through an electrical feedthrough (not shown), which can be made small and/or integrated with other electrical feedthroughs. The position indicator device is configured and calibrated for operation at reactor vessel temperature and radiation level. With continuing reference to FIG. 8 and with further reference to FIGS. 9-14, in the illustrated embodiment the translating assembly of the gray rod CRDM 40 includes three elements: a lead screw/torque taker assembly; a lifting rod/connecting rod assembly; and a latching system that operatively connects the lifting rod with the lead screw. FIG. 9 shows the lead screw/torque taker assembly in perspective (left side) and sectional (right side) views. A motor assembly includes a stator housing 130 housing a stator 132 and a rotor 134. An upper stator end plate 136 and a radial bearing 138 with adjustable spacer 140 complete an upper portion of the motor assembly, while a lower housing 142 and a thrust bearing 144 complete a lower portion of the motor assembly. A lower ball-nut assembly 150 disposed within the lower housing 142 is threaded to the rotor 134, and an upper ball nut assembly 152 is also threaded to the rotor 134. Both ball-nut assemblies 150, 152 are coupled in threaded fashion with a lead screw 160 (shown in part in FIG. 9). FIG. 9 further shows portions of the lifting rod 112 and the upper support tube 104. With reference to FIG. 10, the latching system is illustrated, including the lifting rod-to-lead screw latching assembly 110 and a portion of the electromagnet coil assembly 108. Also shown in FIG. 10 are an end 111 of the lifting rod 112 and a proximate end of the lead screw 160 terminating at or in the latching assembly 110. Latches 170 directly connect the top end 111 of the lifting rod 112 to the lead screw 160 for normal operation, and disconnect the lifting rod 112 during scram (see FIG. 11). The bottom of the lifting rod 112 is threaded to the top of the connecting rod 82 (optionally by the intermediary yoke 86 or intermediary yoke 88) thereby creating a continuous lifting rod/connecting rod assembly. The bottom of the connecting rod 82 couples directly to the control rod spiders thereby attaching the control rods to the mechanism. Optionally, a magnet 113 is disposed proximate to the top 111 of the lifting rod 112 to provide a magnetic signal for a magnetically-based position indicator (see FIG. 21). FIG. 10 also shows a portion of the motor including portions of the motor housing 130, stator 132, and rotor 134, which is shown in full in FIG. 9. The latches 170 are housed in a latch housing 172 that includes a spring guide for a latch spring 174. Additional components of the illustrated latching system embodiment include an electromagnet housing 176 housing electromagnets 177 forming an electromagnet coil stack, and permanent magnets 178 on the latches 170. The lead screw 160 is threaded into a latching system base 179 of the latch housing 172. The latches 170 are arranged to pivot about pivot locations 180 to provide a failsafe scram due to downward rod load. In this embodiment, the lead screw 160 is continuously supported by a ball nut motor assembly (best seen in FIG. 9) which allows for very fine control of lead screw position and consequently very fine control of the position of the control rod assembly. In the illustrated embodiment, the motor (e.g., stator 132, rotor 134) is a synchronous motor in which the rotor 134 is a permanent magnet. This design has advantages such as compactness and simplicity; however, other motor configurations are also contemplated. The lead screw 160 does not scram. Instead, during a scram the top end of the lifting rod 112 of the lifting rod/connection rod assembly is disconnected from the lead screw 160 by the magnetically activated latching system (see FIG. 11). When power is cut to the electromagnets 177 the failsafe latching system releases the lifting/connection rod assembly (and thus the control rod assembly) from the lead screw 160 thereby initiating a scram. A bottom stop and buffering system (not illustrated, but suitably similar to the bottom stop and buffering system of the illustrative shutdown rods described herein with reference to FIGS. 4 and 5) is incorporated into the base/buffer assembly to dissipate the kinetic energy at the end of the scram stroke and to set the rod bottom elevation. A torque taker (not shown) is attached to the lead screw 160 to react the motor torque thereby providing translation of the lead screw/control rod assembly. The normal state, that is, the state prior to scram, is shown in FIGS. 9 and 10. FIG. 9 illustrates the ball nut motor assembly and FIG. 10 shows the latching system engaged for normal operation. As seen in FIG. 10, the permanent magnets 178 on the latches 170 are magnetically attracted toward the powered electromagnets 177 thus pivoting the latches 170 about the pivot locations 180 and engaging the latches 170 with a mating region of the lifting rod 112. Thus, the latches 170 are secured with the lifting rod 112 in the normal state shown in FIG. 10. Further, the latching system base 179 is threaded to or otherwise secured with the lead screw 160. Accordingly, in the normal state of FIG. 10 the lifting rod 112 is secured with the lead screw 160 via the latching system, and so as the ball nut motor assembly shown in FIG. 9 translates the lead screw 160 the lifting rod 112 is translated with the lead screw 160. Scram is described with reference to FIG. 11, which shows the lifting rod 112, and consequently the control rod assembly, during a scram. To initiate scram the power to the electromagnets 177 is cut, that is, turned off. This removes the attractive force on the permanent magnets 178 on the latches 170, and the latch spring 174 extends to pivot the latches 170 about the pivot locations 180 and away from the mating region of the lifting rod 112. This disengages the latches 170 from the lifting rod 112, and the lifting/connection rod assembly (and thus the control rod assembly) falls toward the reactor 12. The lead screw 160 is seen in FIG. 11 still at the previous withdrawal height (that is, the lead screw 160 is not scrammed), but power to the electromagnet coils 177 has been cut so that the magnetic field from the coils is removed. As further shown in FIG. 11, the pivoting of the latches 170 about the pivot locations 180 is stopped by impingement at a location 181 with the spring guide of the latch housing 172. With continuing reference to FIG. 11 and further reference to FIGS. 12 and 13, to re-engage the mechanism after a scram, the lead screw 160 is driven to the fully inserted position via the ball nut motor (see again FIG. 9). A lead screw on-bottom sensor is used to confirm lead screw full insertion. With particular reference to FIG. 12, as the lead screw 160 nears the fully inserted position an angled camming surface 182 on the top 111 of the lifting rod 112, which is scrammed to the bottom, will cam the latches 170 to their near full out position. With particular reference to FIG. 13, when power is restored to the electromagnets 177, the latches 170 will fully re-engage with the mating region of the lifting rod 112 so that the lifting/connection rod assembly is once again connected to the lead screw 160. Normal operation can then resume as per FIG. 10. To reiterate, FIG. 12 shows the lead screw 160 being driven back down to the fully inserted position in preparation for re-engagement of the lifting rod 112. Power to the electromagnet coils 177 is still cut and the latches 160 are still disengaged. The angled camming surfaces 182 on the top 111 of the lifting rod 112 are camming the latches 170 back into partial engagement with the top 111 of the lifting rod 112. FIG. 13 shows the lead screw 160 still on bottom but with the power restored to the electromagnet coils 177. The restored magnet field has now re-engaged the latches 170 with the mating region of the lifting rod 112. FIG. 9 diagrammatically shows a suitable embodiment of the ball nut/motor assembly 106, including lower and upper ball nut assemblies 150, 152. In general, substantially any type of motor can be used, suitably configured for operation in the pressure vessel environment. With reference to FIGS. 14 and 15, an illustrative embodiment is shown which employs a brushless DC electronically controlled (BLDC) motor 184 with lower ball nut assembly 185. The assembly 184, 185 is an illustrative embodiment of the ball nut/motor assembly 106. With particular reference to FIG. 14, the illustrative BLDC motor 184 includes a wound stator core assembly 186 disposed between a stator outer shell 187 and a stator inner shell 188 and secured by a stator upper housing 189 and stator lower housing 190. A permanent magnet rotor 191 includes permanent magnets 192. The BLDC motor 184 produces torque from interaction of magnetic flux of the rotor magnets 192 and the current carrying stator conductors of the stator core assembly 186. The lower ball nut assembly 185 is analogous to the lower ball-nut assembly 150 of FIG. 9; however, in the illustrative assembly of FIG. 14 there is no upper ball-nut assembly corresponding to the upper ball nut assembly 152 of FIG. 9. The assembly of FIG. 14 also includes a radial bearing 193, a thrust bearing 194 secured by a thrust bearing lock nut 195, and a motor top cap 196. An insulated and environmentally robust electrical connection to the motor is provided by a lead wire gland 197. For example, some suitable insulated lead wire glands are available from Conax® Technologies (Buffalo, N.Y., USA). With particular reference to FIG. 15, the BLDC motor 184 and lower ball-nut assembly 185 are illustrated in the context of the control rod drive mechanism (CRDM) of FIGS. 10-13. The illustrative CRDM of FIG. 15 also includes the previously described electromagnet coil stack assembly 177, lifting rod-to-lead screw latching assembly 110, lead screw 160, and lifting rod 112. The ball-nut assembly 185 engages the lead screw 160 so that, as the motor 184 rotates the permanent magnet rotor 191 and the secured ball-nut assembly 185, the lead screw 160 is driven linearly. With returning reference to FIGS. 1 and 2, an advantage of the disclosed reactor design is that the middle section includes the internals support flange or “mid-flange” 29. This section can be made relatively thin, and provides support for the control rod drive mechanism and guides for the in-core instrumentation. This section provides electrical and hydraulic inputs for the control rod drive mechanisms (CRDMs). A reactor coolant drain penetration (not illustrated) is optionally also incorporated in this section. This drain line, if incorporated, is optionally isolated by an internal valve whenever the reactor is pressurized to limit or eliminate its potential as a loss of coolant accident (LOCA) site. The illustrated upper internals 24 including the CRDM do not include illustrated thermal insulation. However, it is contemplated to insulate these components using an insulation system capable of withstanding a design temperature of at least about 650° F. (343° C.). By using the insulation system, external cooling water will not be required although may optionally also be used. For example, cooling water can be supplied to the electrical devices to reduce the severity of the heat duty imposed by the operating environment. The insulation system facilitates locating the electrical CRDM within the pressure vessel, which reduces the overall height of the reactor vessel 10, significantly reduces the number of penetrations into the reactor vessel 10, and enables a complete reactor module to be shipped as a single unit. Another advantage is reduction of the overall height of the containment structure (not shown). Although the use of insulation is believed to be advantageous, other contemplated solutions include the use of water cooling and/or selecting materials capable of withstanding the high operating temperature without insulation. The illustrative reactor embodiment is an integral pressurized water reactor (PWR) configuration. However, one or more of the disclosed techniques, apparatuses, or so forth are also expected to be suitably used in other types of nuclear reactor vessels, such as boiling water reactors (BWRs) that can advantageously incorporate internal CRDM assemblies, efficient control rod position sensors, and so forth. The CDRM configuration of FIGS. 2-15 provides two separate scram mechanisms: a hydraulic scram provided by the shutdown rods described with reference to FIGS. 3-5; and a magnetic latch scram mechanism described with reference to FIGS. 6-15 with the magnetic latch system described with particular reference to FIGS. 10-15. This advantageously provides redundant hydraulic and magnetic scram mechanisms thus reducing likelihood of a complete scram failure in the event of a loss of coolant accident (LOCA) or other safety-related event. With reference to FIGS. 16-20, in another control rod system embodiment is described, which provides electromagnetic gray rod functionality and a hydraulic latch system providing scram functionality. Like the control rod system of FIGS. 6-15, the control rod system of FIGS. 16-20 allows for failsafe scram of the control rod cluster without scramming the lead screw. With particular reference to FIG. 16, the motor/ball nut assembly of FIG. 15 is employed, such that a lead screw 200 is permanently engaged to the ball-nut assembly 185 which provides for axial translation of the lead screw 200 by driving the motor 184. A control rod cluster (not shown in FIG. 16) is connected to the lead screw 200 via a connecting rod or connecting rod assembly 204 and a latch assembly 206. The lead screw 200 is substantially hollow, and the connecting rod assembly 204 fits coaxially inside the inner diameter of the lead screw 200 and is free to translate vertically within the lead screw 200. The latch assembly 206, with two spring loaded latches, is permanently attached to the top of the lead screw 200. When the latches are engaged with the connecting rod 204 they couple the connecting rod 204 to the lead screw 200 and when the latches are disengaged they release the connecting rod 204 from the lead screw. In the illustrated embodiment, latch engagements and disengagements are achieved by using a four-bar linkage cam system including two cam bars 208 and at least two (and, in the illustrated embodiment four) cam bar links 209 per cam bar 208. The additional cam bar links add support for the cam bar. When the cam bars 208 move upward the cam bar links 209 of the four-bar linkage also cams the cam bars 208 inward so as to cause the latches to rotate into engagement with the connecting rod 204. In the illustrated embodiment, a hydraulic lift assembly 210 is used to raise the cam bar assemblies 208. In an alternative embodiment (not illustrated), an electric solenoid lift system is used to raise the cam bar assemblies. When a lift force is applied to the cam system, the upward and inwardly-cammed motion of the cam bars 208 rotates the latches into engagement thereby coupling the connecting rod 204 to the lead screw 200. This causes the control rod cluster to follow lead screw motion. When the lift force is removed, the cam bars 208 swing down and are cammed outward by the cam bar links 209 of the four-bar linkage allowing the latches to rotate out of engagement with the connecting rod 204. This de-couples the connecting rod 204 from the lead screw 200 which causes the control rod cluster to scram. During a scram, the lead screw 200 remains at its current hold position. After the scram event, the lead screw 200 is driven to the bottom of its stroke via the electric motor 202. When the lift force is reapplied to the cam system via the hydraulic lift assembly 210, the latches are re-engaged and the connecting rod is re-coupled to the lead screw 200, and normal operation can resume. Still further, other latch drive modalities are contemplated, such as a pneumatic latch drive in which pneumatic pressure replaces hydraulic pressure in the illustrated lift assembly 210. With continuing reference to FIG. 16, the lead screw 200 is arbitrarily depicted in a partially withdrawn position for illustration purposes. It can be seen in FIG. 16 that the latching assembly 206 is attached to the top of the lead screw 200. In FIG. 16 the latches are engaged the connecting rod 204, which is coupled to the lead screw, is also at the same partially withdrawn position as the lead screw 200. The ball nut 185 and motor 184 are at the bottom of the control rod drive mechanism (CDRM), the latch cam bars 208 extend for the full length of mechanism stroke, and the hydraulic lift system 210 is located at the top of the mechanism. In some embodiments, the CRDM of FIGS. 16-20 is an integral CDRM in which the entire mechanism, including the electric motor 184 and ball nut 185, the latching system 206, and a position indicator (not shown in FIG. 16), are located within the reactor pressure vessel 10 at full operating temperature and pressure conditions. With reference to FIGS. 17 and 18, the lower end of the control rod drive mechanism (CRDM) including the latching assembly 206 is illustrated in additional detail. The latching assembly 206 includes a latch housing 212 and latches 214. The latch housing 212 provides a frame or support for pivot positions 216 (e.g., pivot pins or rods) about which the latches 214 can pivot. In FIG. 16, the connecting rod 204 is withdrawn, that is, latches 214 of the latching assembly 206 are pivoted inwardly into engagement with mating region at an upper end 215 of the connecting rod 204. In the illustrative embodiment, the top of the connecting rod 204 includes the optional magnet 113 to provide a magnetic signal for a magnetically-based position indicator (see FIG. 21). FIG. 17 shows the connecting rod 204 scrammed, that is, latches 214 are pivoted outwardly so as to be disengaged from the mating region at the upper end 215 of the connecting rod 204 so that the connecting rod 204 is mechanically decoupled from the lead screw 200 and is free to move within the inner diameter of the lead screw 200. Thus decoupled as shown in FIG. 17, the connecting rod 204 (and hence the control rod bundle or bundles secured to the connecting rod 204) fall toward the reactor core 12 under the influence of gravity. In both FIGS. 16 and 17, the lead screw 200 is again shown slightly withdrawn to an arbitrarily position—as seen in FIG. 17 the lead screw 200 does not scram. Referring particularly to FIG. 17, the two cam bars 208 are shown raised at their maximum inward (that is, engaged) position. The inward movement of the cam bars 208 caused by the cam bar links 209 rotates or pivots the latches 214 inward into full engagement with the mating region at the upper end 215 of the connecting rod 204. When moved inward to full engagement, the cam bars 208 are supported along their full length by a cam bar housing cover 222 which provides a positive stop for the inward movement of the cam bars 208. The cam bar housing cover 222 is slotted down its center for the full mechanism stroke length to allow latch fingers 224 or other outward extensions of the latches 214 to pass through the cam bar housing cover 222 and contact the cam bars 208 at any withdrawal position of the lead screw 200. In the illustrative embodiment, there are two latches 214 and two cam bars 208, one per latch. However, other numbers of latches and cam bars are contemplated—as another example, there can be four latches and four corresponding cam bars spaced at 90° intervals around the central axis of the lead screw 200/connecting rod 204. The illustrated two cam bars 208 drive a corresponding two latches 214 in a two-fold rotationally symmetric arrangement respective to the central axis of the lead screw 200/connecting rod 204. Again, more generally, it is contemplated for the number of cam bars/latches to be greater than two, with the number of cam bars/latches being selected and arranged to provide balanced latching support for the connecting rod 204. The lower portions of FIGS. 17 and 18 also show an upper portion of the motor 184, whose details are described with reference to FIG. 14 herein. Again, the illustrative motor 184 is merely an illustrative example, and various types of motors can be employed, such as the illustrative brushless DC electronically controlled (BLDC) motor 184 with a wound stator core and a permanent magnet rotor which produces torque from interaction of magnetic flux and the current carrying stator conductors, or a variable reluctance stepper motor (VRS) having a wound stator core and a laminated steel rotor which produces torque from the variation in reluctance as a function of rotor position, or a hybrid stepper motor (HBS) which is a combination of the BLDC and VRS types and utilizes permanent magnets in the rotor and a reluctance component to produce torque, or so forth. In some embodiments it is contemplated to omit the separate the ball nut assemblies and instead or additionally provide engagement with the lead screw directly via the rotor by forming thread engagements on an inner diameter surface of the rotor. Additionally, a torque taker (not shown) is provided to prevent rotation of the lead screw 200 responsive to operation of the motor 184. In some suitable embodiments, the cam bar housing cover 222 includes guide features (not shown) that engage the latch housing 212 to prevent the latch housing 212 from rotating and thus serve as a torque taker to prevent rotation of the lead screw 200 responsive to operation of the ball nut motor 202. In this arrangement, the lead screw 200 is suitably secured together with a bottom portion 226 of the latch housing 212 so that preventing rotation of the latch housing 212 also prevents rotation of the lead screw 200. Again with particular reference to FIG. 17, the cam bars 208, when rotated inward, provide a positive full stroke track to guide the engaged latches 214 via camming of the latch fingers 224 against the cam bars 220 as the translating assembly (including the lead screw 200, latch housing 212 and latches 214, and latched connecting rod 204) is withdrawn (i.e., moved upward) or inserted (i.e., moved downward). The hydraulic lifting of the cam bars 208 instigates a four-bar linkage action via cam bar links 209 that connect the cam bars 208 with a cam bar support housing 232. Each cam bar link 209 is pivotally pinned to the cam bar support housing 232 via a pivot location 234 and to the cam bar 208 by a pivot location 235. In this way, two cam bar links 209 together with the portion of the cam bar support housing 232 between the pivot locations 234 of the cam bar links and the portion of the cam bar 208 between the pivot locations 235 of the cam bar links together define a four-bar linkage. Optionally, more than two cam bar links 209 per cam bar 208 can be provided—in the illustrative example four cam bar links 209 per cam bar 208 are provided (see FIG. 16). Hydraulic lifting of the cam bars 208 causes the cam bar links 209 to pivot upward about the pivot locations 234 and thus force the lifting cam bars 208 inward via the pivot locations 235. When the cam bars 208 are moved to their full inward position, the cam bar links 209 are closest to, but below, horizontal, e.g. at a minimum angle of 20° from the horizontal in some contemplated embodiments, which reduces the likelihood that the four-bar linkage may jam in a horizontal null position. With particular reference now to FIG. 18, the cam bars 208 are shown lowered at their maximum outward position. Again said briefly, hydraulic lowering of the cam bars 208 (or, gravitational, spring-biased, and/or other lowering of the cam bars 208 responsive to removal of the hydraulic lifting force) causes the cam bar links 209 to pivot downward about the pivot locations 234 and thus force the lifting cam bars 208 outward by a four-bar linkage action. The outward movement of the cam bars 208 allows the latches 214 to freely rotate or pivot outward about the pivot locations 216 and disengage from the connecting rod 204 to initiate scram of the connecting rod 204 and hence of the control rods connected with the connecting rod 204. The scram action is failsafe in that the weight of the connecting rod 204, with the assist of latch springs 240, disengages the latches 214. More particularly, the latch springs 240 are compressively held between the latch housing 212 and the upper portions of the latches 214 (above the pivot positions 216) so that they bias the upper portions of the latches 214 inward and consequently bias outward the lower portions of the latches 214 (below the pivot positions 216, i.e. including the latch fingers 224). When moved outward to full disengagement, the cam bars 208 are supported along their full length by the cam bar support housing 232 which provides a positive stop for their outward movement. When the cam bars 208 are moved to their full outward position, the cam bar links 209 are closest to, but not exactly, vertical, for example at a minimum angle of 20° from the vertical in some embodiments, which reduces the likelihood that the four-bar linkage may jam in a vertical null position. With reference to FIGS. 19 and 20, the upper end of the control rod drive mechanism (CRDM) including the hydraulic lift system 210 is illustrated in additional detail. The hydraulic lift system 210 includes a hydraulic cylinder 250 and hydraulic piston 252. Cam bar hangers 254 are coupled with the top of the piston 252, and connection links 256 extend downward from the cam bar hangers 254 to the upper portions of the cam bars 208. During normal operation (FIG. 19) the hydraulic cylinder 250 is pressurized so as to raise the piston 252 and so raise the cam bars 208 via the linkages 254, 256. This causes the latches 214 to engage with the upper end 215 of the connecting rod 204, as described herein with reference to FIG. 17. During scram (FIG. 20), the hydraulic cylinder 250 is depressurized so that the piston 252, linkages, 254, 256, and cam bars 208 fall under the force of gravity. This causes the latches 214 to disengage from the connecting rod 204, as described herein with reference to FIG. 18. In the hydraulic lift system illustrated in FIGS. 19 and 20, the scram is made failsafe by inclusion of a scram assist spring 260 that spring-biases the piston 252 downward by compression of the scram assist spring 260 between the piston 252 and a hydraulic assembly cap 262. In FIGS. 19 and 20, the lead screw 200 is shown fully withdrawn for illustration purposes, so that the latch system is also visible in the view of FIGS. 19 and 20. However, the operation of the hydraulic lift system 210 as described with reference to FIGS. 19 and 20 is applicable for any position of the lead screw 200. With particular reference to FIG. 19, as was described previously with reference to FIG. 17, in the normal (latched) state the cam bars 208 are raised and, due to action of the cam bar links 209, are at their maximum inward position. The inward movement of the cam bars 208 rotates or pivots the latches 214 into full engagement with the top end 215 of the connecting rod 204. Also, when moved inward to full engagement the cam bars 208 are supported along their full length by the cam bar housing cover 222 which provides a positive stop for their inward movement. With continuing reference to FIG. 19, in the engaged condition the hydraulic piston 252 is in the fully raised position due to pressurization of the hydraulic cylinder 250. As the piston is raised the cam bar hanger 254 is lifted by the piston 252 and pulls upward on the pair of connection links 256 which in turn lift the cam bars 208. The piston 252 also lifts against the downward force produced by the scram assist spring 260. In some contemplated embodiments, the hydraulic piston lift assembly operates at a differential pressure of only 5.5 psi, although design for higher differential pressure operation is also contemplated. With particular reference to FIG. 20, as was described previously with reference to FIG. 18, in the scrammed (unlatched) state the cam bars 208 are lowered and, due to the four-bar linkage action of the cam bar links 209, are at their maximum outward position. The outward movement of the cam bars 208 allows the latches 214 to freely pivot or rotate and disengage from the connecting rod 204. In illustrative FIG. 20, the connecting rod 204 has scrammed out of view to the fully inserted position, and hence the connecting rod 204 is not shown in FIG. 20. When moved outward to full disengagement, the cam bars 208 are supported along their full length by the cam bar support housing 232 which provides a positive stop for their outward movement. With continuing reference to FIG. 20, in order to scram the pressure in the hydraulic cylinder 250 at the bottom side of the piston 252 is evacuated to allow the piston 252 to lower. In a suitable approach, the depressurization is accomplished by a double-acting valve (not shown) that simultaneously cuts the supply pressure to the piston 252 while evacuating the piston cavity to the reactor environment. If the valve fails, it fails in an open state to the dump side for scram reliability. A large flow area valve is optionally employed to provide fast evacuation of the (typically small-volume) piston cavity. Once the pressure is dumped, the combined weight of the cam bars 208, the linkages 254, 256, and the piston 252 gravitationally drive lowering of the cam bars 208 and resultant disengagement of the latches 214. Optionally, as in the illustrated embodiment the scram assist spring 260 is provided in or with the hydraulic lift assembly to assist in lowering the piston 252 and cam bars 208. The scram action is preferably also failsafe in that the connecting rod weight, with the assist of the latch springs, disengages the latches. Camming action by the cam bar links 209 also pushes the cam bars 208 outward toward disengagement. Reengagement of the latch assembly 206 with the connecting rod 204 after a scram can be performed similarly to the reengagement process described with particular reference to FIGS. 12 and 13 for the embodiment of FIGS. 6-15. The electric motor 184 is driven to move the latching assembly 206 and lead screw 200 (which, again, are secured together) downward toward the top 215 of the scrammed connecting rod 204. The hydraulic cylinder 250 remains depressurized and the latches 214 remain in the disengaged position due to bias of the latch springs 240, as shown in FIG. 18. Thus, the latches 214 can be driven downward by the motor 184 to align with the mating region at the upper end 215 of the connecting rod 204. In the illustrated embodiment, the magnet 113 disposed at or near the top 215 of the connecting rod 204 is magnetically sensed by a position indicator (not shown) in the latching assembly 206 in order to detect when the latches 214 are aligned with the mating region at the upper end 215 of the connecting rod 204. Once the latches 214 are aligned with the mating region at the upper end 215 of the connecting rod 204, the hydraulic cylinder 250 is re-pressurized to lift the hydraulic piston 252 and thus raise the cam bars 208 and reengage the latches 214. Thereafter, the electric motor 184 can be operated to drive the lead screw 200 and re-latched connecting rod 204 upward to a desired operational position. In an alternative embodiment, the hydraulic lift system 210 described with illustrative reference to FIGS. 19 and 20 can be replaced by an electric solenoid lift assembly, for example suitably located at the top of the control rod drive mechanism (CRDM) in place of the illustrative hydraulic lift assembly 210. Such an electric solenoid lift assembly can be suitably connected with the illustrative four-bar linkage latch cam system, and the latch assembly 206 functions as described herein. In this alternative embodiment, instead of applying pressure to the hydraulic piston 252 to provide the lifting force for engaging the cam bar assemblies, the lifting force is provided by applying electrical power to the solenoid. When electric power is cut the lifting force is immediately lost, the cam bars disengage the latches and the control rod cluster scrams as described herein. With reference back to FIG. 17 and with further reference to FIG. 21, a section S indicated in FIG. 17 is shown in FIG. 21. The section S passes through a coupling between each cam bar 208 and one of its coupling cam bar links 209, and through the finger 224 of each latch 214, and through the position sensor magnet 113. The sectional view S shown in FIG. 21 includes the cam bar support housing 232 and an supporting cam housing assembly 232a, and the latch housing 212, and the top 215 of the connecting rod 204 with sectioning through the position sensor magnet 113. The sectional view S further includes sectioning through the two cam bars 208 and their latch fingers 224, and shows cam links 209 and their pivot locations 234 connecting with the cam bar support housing 232, with sectioning through their pivot locations 235 connecting with the latch housing 212. As seen in FIG. 21, the pivot locations 234, 235 are suitably embodied by pins. The sectional view S of FIG. 21 also shows an illustrative magnetic position indicator assembly 270 that senses the magnet 113 in the top end 215 of the connecting rod 204 based on magnetic coupling between the indicator assembly 270 and the magnet 113. As already mentioned, the connecting rod 204 is connected at its lower end with a control rod bundle. Optionally, this connection is via one or more intermediate linkages, for example the illustrative yokes 86, 88 shown in FIG. 6. With reference to FIGS. 22 and 23, the nuclear reactor typically includes an array or other plurality of control rod clusters operated by corresponding control rod drive mechanisms supported by a suitable support frame 274 (for example, as shown in greater detail in FIG. 2). In some embodiments, the electric motor 184 is the bulkiest component of the CDRM. In the illustrative array shown in FIGS. 22 and 23, the bulky motors 184 are accommodated in a compact array by vertically staggering the positions of the motors 184 so that the motors 184 of any two adjacent CRDM are not at the same vertical level or height. This enables a more compact array as compared with conventional arrangements in which all the motors are at the same vertical level or height. The CRDM embodiments described with reference to FIGS. 6-21 advantageously provide both “grey rod” incremental control capability and also provide an efficient scram capability and hence can perform the task normally allocated to dedicated shutdown rods (e.g., as described herein with reference to FIGS. 3-5). Accordingly, it is contemplated to omit dedicated shutdown rods and instead rely wholly on control rods of embodiments such as those of FIGS. 6-21, for example arranged as shown in FIGS. 22 and 23. In a variant embodiment, to provide further redundancy in a LOCA or other emergency event, it is contemplated to employ a configuration including: (i) no dedicated shutdown rods; (ii) a first set of control rods with hydraulic lift as described herein with reference to FIGS. 16-21 so that in an emergency the rods perform the shutdown function via a hydraulic mechanism; and (iii) a second set of control rods with configured to perform the shutdown function via an electromagnetic mechanism. The latter set (iii) can be embodied, for example, by control rods conforming with the embodiment described herein with reference to FIGS. 6-15, or alternatively by control rods conforming with the embodiment described herein with reference to FIGS. 16-21 but with the hydraulic lift system 210 replaced by a solenoidal lift system. Such an arrangement advantageously uses (or can use) all available control rods for reactivity control while also providing a two-fold redundant (hydraulic and electromagnetic) safety system. With reference back to the CRDM embodiments of FIGS. 6-20, an advantage of employing a latch to decouple the connecting rod 204 from the lead screw 200 (and, hence, to decouple the connecting rod 204 from the CRDM) is that the CRDM can be configured for removal of the connecting rod 204 through the CRDM without first removing the CRDM. To provide this capability, the CRDM is constructed with a hollow central region providing a pass-through by which the connecting rod 204, once unlatched from the lead screw 200, may pass. A cylindrical opening 280 (see FIGS. 18 and 20) through the latch assembly is made large enough for the connecting rod 204 to pass through when the latches 214 are open. In the embodiment of FIGS. 6-15, such an opening can be provided by replacing the centrally positioned latch spring 174 with a side-positioned biasing mechanism similar to the latch springs 240 of the embodiment of FIGS. 16-21. For the embodiment of FIGS. 16-21, a cylindrical opening 282 is also provided through the hydraulic lift system 210 (see FIGS. 19 and 20). Both openings 280, 282 are made large enough for the connecting rod 204 to pass through when the latches 214 are open. Regarding the latter opening 282, the scram assist spring 260 is suitably an annular spring providing for the opening 282, and the piston 252 is also suitably made hollow with the requisite inner diameter. In the case of an alternative lifting mechanism, such as a solenoidal lift, the solenoid is suitably hollow. With reference to FIGS. 24 and 25, for the connecting rod 204 to be removable through the CRDM it should be detachably connected with the spider or other mechanical control rod structure in such a way that (i) it can be detached from the spider from above the CRDM and (ii) so that the outer diameter of the connecting rod 204 at the detachable connection is not so large so as to prevent withdrawal of the lower end of the connecting rod 204 through the openings 280, 282. FIGS. 24 and 25 show one suitable detachable connection, in which a low-profile “J-groove” couples the connecting rod 204 with the control rod bundle. In this illustrative detachable connection, one or more inverted “J” shaped grooves 300 are formed in the lower end of the connecting rod 204. Since these grooves are recessed into the connecting rod 204, the J-grooves 300 do not increase the outer diameter of the connecting rod 204 at the lower end. A biasing spring 302 is terminated at the end proximate to the connecting rod 204 by a spring guide/capture element 304, and the elements 302, 304 are disposed inside a generally cylindrical rod cluster threaded cap 306 that secures to the top of a rod cluster 310. The cluster cap 306 includes mating tabs 312 that are sized and positioned to slide into the inverted J-shaped grooves 300 of the connecting rod 204. To establish the coupling, the long vertical portions of the inverted J-shaped grooves 300 are aligned with the mating tabs 312 of the cluster cap 306, and the connecting rod 204 is then pushed downward against the compressive force of the spring 302 such that the tabs 312 enter the long vertical portions of the grooves 300. When the connecting rod 204 is pushed down far enough for the tabs 312 to reach the horizontal portions of the inverted J-shaped grooves 300, the connecting rod is rotated by a rotation 314 (which is clockwise in FIGS. 24 and 25) until the tabs 312 align with the short vertical portions of the inverted J-shaped grooves 300. At that point, removal of the downward force allows the upward spring force generated by the spring 302 to push the connecting rod 204 upward in order to lock the tabs 312 into the short vertical portions of the inverted J-shaped grooves 300. The process is reversed to decouple the connecting rod 204 from the rod cluster 310. After removal, the spring 302 and guide/capture element 304 are retained at the rod cluster 310 by the cluster cap 306. Thus, the coupling/decoupling of the connecting rod 204 with the rod cluster 310 advantageously can be performed with the latches 214 disengaged, so that the connecting rod 204 can be installed or removed with the CDRM in place. This is made possible because the lead screw 200 and the connecting rod 204 are not directly connected together, but rather are coupled by the latch assembly 206. When the latches 214 are disengaged, the connecting rod 204 can move freely inside the substantially hollow lead screw 200, and if the hydraulic piston 252 (or hollow solenoid, in the case of an electromagnetic lifting mechanism) is also made substantially hollow and the hydraulic cylinder 250 is annular with a sufficiently large inner diameter, then the connecting rod 204 can also pass through the hydraulic lift assembly 210. Thus, installation of the connecting rod 204 amounts to inserting the connecting rod 204 into the opening of the CDRM and pushing it down until it presses against the spring 302 (see FIGS. 24 and 25) and then rotating the connecting rod 204 as per the illustrated rotation 314 and releasing the connecting rod 204 so that the force of the spring 302 completes locking of the coupling. To remove the connecting rod 204, the process is reversed. The preferred embodiments have been illustrated and described. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
055770835	claims	1. An electrical system for locally reducing electrochemical potential in the vicinity of a metal surface immersed in water, comprising a control circuit powered by a battery power supply, said control circuit and said battery power supply being enclosed in a housing, said control circuit having first and second terminals which penetrate said housing, a first electrical conductor for electrically coupling said first terminal to the metal surface, a reference electrode for placement outside said housing in water near the metal surface, a second electrical conductor for electrically coupling said reference electrode to said second terminal, and a sheathed cable which encases said first and second electrical conductors, wherein said control circuit comprises means for supplying electrons to the metal surface via said first electrical conductor whenever the electrochemical potential between the metal surface and the water is above a threshold level, above which the metal is susceptible to stress corrosion cracking. 2. The system as defined in claim 1, wherein said electron supplying means comprise a differential amplifier having a first input electrically connected to a first junction, a second input electrically connected to a first resistance, and an output for outputting a voltage signal proportional to the difference between the voltage signals at said first and second inputs, wherein said first terminal is electrically connected to said first junction, said second terminal is electrically connected to a second junction, and said first resistance is electrically connected to said second junction. 3. The system as defined in claim 2, wherein said electron supplying means further comprises a second resistance electrically connected to said output of said differential amplifier, a third resistance electrically connected to said first junction, and an operational amplifier having a first input electrically connected to said second resistance, a second input electrically connected to said second junction, and an output connected to said third resistance. 4. The system as defined in claim 1, wherein said first electrical conductor is connected to a heat-affected zone in a welded component. 5. An electrical system for locally reducing electrochemical potential in the vicinity of a portion of metal to be protected, comprising a control circuit powered by a battery power supply and having first and second terminals, a first electrical conductor for electrically coupling said first terminal to the surface of the metal to be protected, a reference electrode for placement in electron-accepting fluid near the metal surface, and a second electrical conductor for electrically coupling said reference electrode to said second terminal, wherein said control circuit comprises means for supplying electrons to the metal surface via said first electrical conductor under conditions when the metal is susceptible to stress corrosion cracking, wherein said battery power supply comprises a current source containing .beta.-emitting material. 6. The system as defined in claim 5, wherein said .beta.-emitting material is the Ru-106 isotope. 7. The system as defined in claim 1, wherein said battery power supply comprises a current source, a first resistance and a Zener diode connected in series to form a closed circuit, and a second resistance connected in parallel across said Zener diode. 8. The system as defined in claim 5, wherein said current source comprises a flat disk of .beta.-emitting material housed inside a metallic collector and electrically isolated from said metallic collector by ceramic insulating means. 9. An electrical system comprising a control circuit powered by a battery power supply and having first and second terminals, said control circuit and said battery power supply being contained inside a housing which is penetrated by said first and second terminals, first and second electrical conductors electrically connected to said first and second terminals respectively, a sheathed cable encasing said first and second electrical conductor, and an electrode electrically connected to said second electrical conductor, wherein said control circuit comprises: a first junction electrically connected to said first terminal; a second junction electrically connected to said second terminal; a first resistance electrically connected to said first junction; a second resistance electrically connected to said second junction; a differential amplifier having a first input electrically connected to said first junction, a second input electrically connected to said second resistance, and an output for outputting a voltage signal proportional to the difference between the voltage signals at said first and second inputs; a third resistance electrically connected to said output of said differential amplifier; an operational amplifier having a first input electrically connected to said third resistance, a second input electrically connected to said second junction, and an output electrically connected to said first resistance. a first amplifier having a first input electrically coupled to said first terminal, a second input electrically coupled to said second terminal by way of a first resistor, and an output for outputting a voltage signal proportional to the difference between the voltage signals at said first and second inputs; and a second amplifier having a first input electrically coupled to said output of said first amplifier by way of a second resistor, a second input electrically coupled to said second terminal, and an output connected to said first terminal by way of a third resistor, wherein said battery power supply comprises a current source containing .beta.-emitting material. immersing a reference electrode in the water coolant in the vicinity of said metal surface; connecting an electrical conductor to said metal surface; detecting a difference in potential between the metal surface and the water coolant surrounding said reference electrode; and supplying a current of electrons to said metal surface via said electrical conductor whenever the difference in potential between the metal surface and the water coolant surrounding said reference electrode is above a threshold level corresponding to a predetermined electrochemical potential above which the metal is susceptible to stress corrosion cracking. placing a reference electrode in the electron-accepting fluid in the vicinity of said portion of metal; connecting an electrical connector to said portion of metal; and supplying a current of electrons from a source of electrons to the metal surface via said electrical conductor, said current having a magnitude which is a function of the difference between the potential of said reference electrode and the potential of said portion of metal, wherein said source of electrons comprises .beta.-emitting material. 10. The system as defined in claim 9, wherein said control circuit comprises a third junction located between said second input of said differential amplifier and said second resistance, a fourth junction located between said first input of said operational amplifier and said third resistance, and a capacitor having one terminal connected to said third junction and a second terminal connected to said fourth junction. 11. An electrical system comprising a control circuit powered by a battery power supply and having first and second terminals, said control circuit and said battery power supply being contained inside a housing, wherein said control circuit comprises: 12. The system as defined in claim 11, wherein said .beta.-emitting material is the Ru-106 isotope. 13. The system as defined in claim 9, wherein said battery power supply comprises a current source, a fourth resistance and a Zener diode connected in series to form a closed circuit, and a fifth resistance connected in parallel across said Zener diode. 14. The system as defined in claim 11, wherein said current source comprises a flat disk of .beta.-emitting material housed inside a metallic collector and electrically isolated from said metallic collector by ceramic insulating means. 15. A method for locally reducing electrochemical potential in the vicinity of a metal surface immersed in the water coolant of a light water reactor high-temperature, comprising the steps of: 16. The method as defined in claim 15, wherein the current of electrons is sufficient to compensate for the electrons lost by the metal to said water coolant. 17. The method as defined in claim 15, wherein said metal is a heat-affected zone in a welded component. 18. A method for locally reducing electrochemical potential in the vicinity of a portion of metal in contact with electron-accepting fluid, comprising the steps of: 19. The method as defined in claim 18, wherein said .beta.-emitting material is the Ru-106 isotope.
	claims	1. A method of inspecting an operation of sealed closure by welding an end opening of a filling channel axially traversing an upper plug for closing a cladding of a fuel rod for a nuclear reactor, the cladding of the rod configured to contain a plurality of pellets of nuclear fuel stacked in an axial direction of the cladding and two closure plugs, at least one of the plugs and an upper plug traversed by the channel for filling the cladding of the rod with an inert pressurized gas and the sealed closure by welding of the filling channel of the upper plug after filling the cladding with the inert pressurized gas, in a filling apparatus, by melting an end central part of the plug adjacent to the opening of the filling channel comprising: prior to the sealed closure of the filling channel, acquiring images of an end surface of the plug on which the filling channel emerges by the substantially circular inlet opening to obtain a digitized image, wherein the rod is in a position for filling and for sealed welding of the upper plug in the filling apparatus and wherein the inlet opening is configured to be substantially circular; determining a position of a center of the circular inlet opening of the filling channel with respect to a reference position and a diameter of the inlet opening of the filling channel through analyzing the digitized image; deducing whether it is possible to weld the filling channel; and acquiring images of the end of the plug after welding and determining a presence and position of a spot weld for sealed closure of the filling channel, where the sealed closure of the filling channel is carried out by welding. 2. The method according to claim 1 , wherein the reference position consists of a center of a reticule comprising a horizontal axis and a vertical axis, the position of the center of the reticule corresponding to a position for adjusting a welding arrangement for carrying out the sealed closure by welding of the filling channel of the plug, the method further comprising: claim 1 seeking the edges of the inlet opening of the filling channel on the end surface of the plug along a first axis of the reticule; deducing a first position of the center of the circular inlet opening of the filling channel and a first value of the diameter of the inlet opening; seeking the edges of the opening along a second axis perpendicular to the first axis of the reticule passing through the center deduced position; deducing a second position of the center of the circular opening of the filling channel and a second value of the diameter of the circular inlet opening of the filling channel; seeking the edges of the circular opening along a third axis perpendicular to the second axis passing through the second position of the center; deducing a third position of the center of the circular opening and a third value of the diameter of the circular opening, the third position of the circular opening considered as an actual center of the opening; comparing the second value of the diameter and the third value of the diameter to deduce a consistency of the second and the third parameter values considered as parameters of the circular opening; determining a distance between the third position of the center of the opening and the center of the reticule; and comparing the distance calculated between the centers of the circular opening and of the reticule together with the calculated value of the diameter of the circular inlet opening of the filling channel to threshold values to determine a compliance of the inlet opening of the filling channel and of a filling channel position and a possibility of carrying out sealed closure by welding. 3. The method according to claim 2 , wherein the edges of the inlet opening of the filling channel are sought along each of the axes from a graph providing gray levels along at least one of the mean gray levels along N rows and N columns of the digitized image parallel to the axis along which a search is carried out and which are located on each side of the axis. claim 2 4. The method according to claim 3 , wherein the edges of the circular inlet opening of the filling channel are determined by using a threshold value of the gray levels constituting a mean between the gray levels of the image of the filling channel and the gray levels of the surface of the upper plug around the circular inlet opening of the filling channel. claim 3 5. The method according to claim 1 , wherein a reflection of the spot weld, having a central part in the shape of a crater reflecting light directed axially is sought on the digitized image of the end surface of the plug after carrying out the sealed closure of the filling channel of the upper plug, and determining a position of the center and the size of the reflection. claim 1 6. The method according to claim 5 , wherein the position of the center of the reflection is determined with respect to a center of a reticule defined by two horizontal and vertical axes, respectively, on the digitized image, corresponding to a welding position and a diameter of the reflection, and in that the distance from the center of the reflection to the center of the reticule and a calculated diameter of the reflection are compared to threshold values to define whether the upper plug of the fuel rod is compliant after welding. claim 5 7. The method according to claim 6 , further comprising: claim 6 seeking the edges of the reflection along a first axis of the reticule; deducing a first position of the center and a first value of the diameter of the reflection; seeking the edges of the reflection along a second axis perpendicular to the first axis passing through the first center of the reflection; deducing a second position of the center and a second value of the diameter of the reflection; comparing the first and second diameters in order to verify a consistency of the values obtained; determining a distance between the second position of the center of the reflection and the center of the reticule; and comparing a distance between the center of the reflection and the center of the reticule and the calculated diameter of the reflection to threshold values to determine a compliance of carrying out the sealed closure by welding the filling channel of the upper plug of the rod. 8. The method according to claim 7 , wherein the edges of the reflection are sought along each of the first and second axes by determining a graph providing grey levels along a mean row parallel to the first and second axes respectively, corresponding to a mean of the gray levels along at least one of several rows and columns of the digitized image which are parallel to one of the first and second axes respectively and placed on each side of one of the first and second axes. claim 7
	description	The present invention relates to retention systems for nuclear fuel rods, and more particularly to magnetic retention systems. In conventional nuclear reactor systems, the fuel rods are held in position axially and laterally with mechanical components such as springs, braces, end plugs, and other devices positioned along the length and at each end of a fuel rod. Such traditional means of retention and alignment sacrifice system pressure without providing a corresponding thermal efficiency benefit. The flow of fluid coolant around the fuel rods past the mechanical retention and alignment components reduces the coolant pressure causing a pressure drop in the coolant flow. Further, the retention and alignment components may cause wear of the fuel rods due to contact between the structural retention features and the fuel, which may lead to fuel rod damage. Elimination of these retention and alignment contact features would eliminate or reduce the coolant pressure drop. Avoiding loss of pressure would increase fuel efficiency. The problems associated with physical contact-based retention and alignment features are addressed by the system for retention and alignment of nuclear fuel rods described herein wherein retention is achieved by magnetizing certain contacts between adjacent components. Magnetization may be achieved by using precision magnets keyed to the polarity of confronting precision magnets. An improved retention and alignment system for nuclear fuel rods may, in various aspects, include an upper plate and a lower nozzle, at least one nuclear fuel rod having an upper end and a lower end and extending axially between the upper and lower nozzles, a first precision magnet incorporated onto the lower end of the at least one fuel rod, and, a second precision magnet incorporated onto the lower nozzle in a position confronting the at least one first precision magnet. The first precision magnet has at least one of a magnetic north or south polarity and the second precision magnet has at least one of a magnetic south or north polarity opposite the polarity of the confronting first precision magnet to effect magnetic attraction between the confronting first and second precision magnets. In various aspects, there is a first precision magnet incorporated onto the lower ends of the at least one fuel rod and a second precision magnet incorporated onto the lower nozzle to axially retain the fuel rod between the upper and lower nozzles by magnetic attraction. In various aspects of the system, each of the at least one first and second precision magnets has at least one, and in certain aspects, two or more, paired sections. Each paired section has a polarity opposite the other section in the pair. The paired sections may be configured in a locked configuration wherein confronting precision magnet sections attract each other to an unlocked configuration wherein confronting precision magnet sections repel each other. In various aspects, the polarity of each member of the pair may be selectively switchable, for example by rotation, to the opposite polarity to selectively switch one of the first or second precision magnets from the locked configuration to the unlocked configuration. In various aspects of the system, the paired sections of at least one of the first and second precision magnets may be rotatable for rotating the paired sections of one of the first and second magnets into the locked or the unlocked configuration. The improved retention and alignment system may address problems of maintaining fuel rod alignment during seismic events. The system may include at least one grid substantially parallel to and positioned between the upper and lower nozzles. The at least one grid defines a perimeter and has within the perimeter, a first set of grid strap extending laterally and longitudinally across the grid to define at least one, and in various aspects, multiple cells. Each cell has an interior and an exterior, wherein one of the at least one fuel rods passes axially through the interior of one cell. The grid strap walls of the grid may include at least one third precision magnet incorporated onto the interior of the cell. At least one fourth precision magnet may be incorporated onto a side of the fuel rod, fuel rod cladding, or a sleeve over the fuel rod in a position confronting the at least one third precision magnet. The third precision magnet has at least one of a magnetic north or south polarity and the fourth precision magnet has at least one of a magnetic north or south polarity the same as the polarity of the confronting third precision magnet to effect magnetic repulsion between the confronting third and fourth precision magnets for maintaining a gap between the fuel rod and the grid strap onto which the confronting third precision magnet is incorporated. The system may have in certain aspects, a plurality of cells and a plurality of fuel rods, wherein each cell is sized to receive one of the plurality of fuel rods extending axially therethrough. In various aspects, each enclosure through which a fuel rod passes has at least two third precision magnets incorporated onto different grid strap walls of the enclosure and the fuel rod (or its cladding or sleeve) has at least two fourth precision magnets. Each fourth precision magnet is positioned on the fuel rod to confront a different one of the at least two third precision magnets incorporated onto the grid strap walls. As used herein, the singular form of “a”, “an”, and “the” include the plural references unless the context clearly dictates otherwise. Thus, the articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element. Directional phrases used herein, such as, for example and without limitation, top, bottom, left, right, lower, upper, front, back, and variations thereof, shall relate to the orientation of the elements shown in the accompanying drawing and are not limiting upon the claims unless otherwise expressly stated. In the present application, including the claims, other than where otherwise indicated, all numbers expressing quantities, values or characteristics are to be understood as being modified in all instances by the term “about.” Thus, numbers may be read as if preceded by the word “about” even though the term “about” may not expressly appear with the number. Accordingly, unless indicated to the contrary, any numerical parameters set forth in the following description may vary depending on the desired properties one seeks to obtain in the compositions and methods according to the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter described in the present description should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Further, any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include any and all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. An exemplary nuclear reactor fuel rod and coolant system 10 is shown in part in FIG. 1. The system 10 includes a control rod assembly 12 positioned at the upper end of the system 10, a top nozzle 14 with leaf springs 16, a top nozzle plate 28, a bottom nozzle 22, bottom nozzle plate 30, and several grids 68, 88, and 18 positioned between the top and bottom nozzle plates 28, 30 to support rows of fuel rods 20 extending between the top and bottom nozzles plates 28, 30. The plurality of grids 68, 88, and 18 are substantially parallel to, but separated from each other, and supported by support rods 24, sometimes referred to as guide thimbles, positioned inside the grid perimeter and inside several rows of fuel rods 20. Cut-away sections of FIG. 1 show a pattern of holes 26 through each of a top grid 68, mid-grid 88, and bottom grid 18 through which the fuel rods 20 pass from the bottom nozzle plate 30 to the top nozzle plate 28. Holes 26 are sized to allow coolant fluid flow around the fuel rods 20. Additional openings, for example, venturi openings, may be formed in the top and bottom nozzles plates 28, 30. The system 10 is enclosed in a reactor housing (not shown). As stated above, in conventional nuclear reactor systems, the fuel rods 20 are held in position laterally with springs 48 and/or dimples 58 on the inner sides of the grids 68, 88, and 18. In a typical conventional system, each cell has two dimples and two springs. Both the dimples and the springs arch into the cell from the grid strap wall forming raised portions 56 for contact with the fuel rod 20. The more flexible spring 48 forces the fuel rod against the raised portion 56 of the dimple 58 to laterally secure the rod within the cell. The flow of fluid coolant around the fuel rods 20 past the springs 48 and other retention features to retain the fuel rods 20 and supports 24 in position reduces the coolant pressure causing a pressure drop in the coolant flow. In the retention system described herein, fuel rods 20 are held in position using keyed patterns of confronting precision magnets. The methods of axial and lateral fuel rod retention described herein provides opportunities to eliminate components, reduce aforementioned pressure drop, and improve grid to rod fretting (e.g., reduce or eliminate wear of the fuel rods due to contact with structural retention features of the grid). By incorporating a means of dampening or preventing adjacent fuel assembly impact with features of grids 68, 88, and 18, the improved retention and alignment system will reduce seismic forces on the fuel assemblies and reduce, and preferably, eliminate the risk of associated loss-of-coolant accidents. The improved retention and alignment system will also prevent fuel assembly lift off at the axial retention features, and allow for easier removal of components for reconstitution, repair or replacement. Precision magnets are fundamentally different than conventional magnets. Most off the shelf conventional magnets have a simple configuration: a north pole on one side and a south pole on the other. Software-driven magnetizers, such as those sold under the mark, POLYMAGNETS®, by Correlated Magnetics Research LLC of California, USA, have been commercially developed that enable manufacture of customizable patterns of magnetism designed in software and programmed into a magnet. See, for example, U.S. Pat. Nos. 8,179,219; 9,219,403; 9,245,677 and 9,404,776, incorporated in relevant part herein by reference. These precision magnets can be up to five times stronger than conventional magnets because their magnetic energy may be concentrated near the surface, as shown in FIGS. 2A and 2B. 3-D printed precision magnet technology is an emerging field that prints individual (digital/pixelated) magnetic poles into customizable orientations and 3-D geometries. This ability to print small field magnetic circuits allows for increased magnetic forces over a smaller distance due to the reduced energy loss of the field. Conventional magnets 206, 208 as shown in FIG. 2A do not necessarily align when attaching to each other. The magnetic field lines 210 of conventional magnets 206, 208 show that much of the magnetic energy is lost, directed away from the magnets 206, 208. Precision magnets, as shown in FIG. 2B, concentrate the magnetic field. The magnetic field lines 210 in the precision magnets 202, 204, form a smaller, tighter field so that the magnetic force remains with the magnets. Precision magnets, such as those sold under the mark, POLYMAGNETS®, may be designed to align with a wide variety of alignment functions. Latch precision magnets, for example, are designed to repel until the magnet pair pass through a defined transition point. After the transition point they are designed to reverse polarity and attract. Spring precision magnets are designed to attract until they pass through a defined transition point, past which they will repel. These precision magnets will come to rest at an equilibrium distance. At equilibrium, the opposing precision magnets maintain a predetermined distance from each other so that the components into which the precision magnets are placed can be held apart, spaced from each other at or about the predetermined distance. Referring to FIGS. 3 and 4, variations in precision magnet configurations are shown for illustration purposes. In FIG. 3, two opposing discs 200 are shown having a plurality of precision magnets on each disc keyed to the precision magnets on the confronting disc. Precision magnets having, for example, a north magnetic pole 202 are shown with a plus sign and precision magnets having a south magnetic pole 204 are shown with a negative sign. When discs A and B are moved toward each other, they can be aligned so that the north (+) poles on disc A directly align with the south (−) poles on disc B, and the south (−) poles on disc A align with the north (+) poles on disc B, forcing the discs A and B to attract and join together. Because of the tight magnetic field, as shown in FIG. 2B, the force of attraction between discs A and B is very strong. If it is necessary or desirable to have the two discs 200 repel each other, the precision magnets 202, 204 on the discs 200 may be aligned so that the north (+) and south (−) poles on disc A align with the north (+) and south (−) poles, respectively, on disc B. The like magnetic poles on opposing discs will repel each other, forcing the discs A and B apart. In FIG. 3, an image of two discs 200 placed side-by-side show the magnetic fields of the discs through a sheath 300. The disc on the right side of the image has precision magnets having (+) poles 202 on the outer ring and precision magnets having (−) poles 204 in a center ring. The disc 200 on the left side of the image has precision magnets 202, 204 with alternating north (+) and south (−) poles, respectively. FIGS. 3 and 4 illustrate the possible geometries and patterns that may be used in configuring precision magnet retention assemblies. Keyed confronting precision magnets for use in the environment of a nuclear fuel and coolant system 10 may be made of any suitable materials that are believed to retain their magnetic properties under reactor conditions. Research has shown that certain materials, such as Sm2Co17, have temperature and irradiation resistance with regard to degradation of magnetic properties. The ability to axially secure and maintain the alignment of fuel rods 20 by means of a non-lateral contact method, such as by use of precision magnets, may make it possible to eliminate the bottom grid 88, and would significantly reduce pressure drop penalties in current fuel designs. The retention geometries may be magnetically keyed to allow for easy fuel rod 20 reconstitution. Referring to FIG. 5 [A], part (A), a modification of the conventional end plug 32 design of a fuel rod 20 is shown. The end plug 32 includes a boss section 38 that is welded to one or both of the ends of a fuel rod 20 (not shown in this view). An end surface 34, for example, on the bottom end plug 32 may, in various aspects, have first precision magnets 36 incorporated on the surface 34. The first magnets may be a single magnet of a single polarity. As shown, in certain aspects, the first magnet may comprise paired sections of one or both positive and negative polarities, such as the alternating positive (+) 202 and negative (−) 204 pattern of poles shown on surface 34. FIG. 5[B], part (B) shows the bottom nozzle plate 30 including a plurality of holes 42 for passage of reactor coolant, such as water, around the fuel rods 20 and feet 44 for supporting the bottom nozzle plate 30. Bottom nozzle plate 30 further includes a second precision magnet 40 incorporated therein for each fuel rod 20 for alignment with the first precision magnet 36 on end surface 34 of end plug 32 of fuel rod 20. The second precision magnet 40 may be a single magnet of a single polarity opposite the polarity of the first single magnet 36, or may comprise paired sections of one or both positive and negative polarities, as shown in FIG. 5[B] part (B), able to be positioned or programmed such that the polarity of the paired sections on surface 34 are opposite the polarity of the paired sections on nozzle plate 30. The second precision magnet 40 includes alternating positive (+) 202 and negative (−) 204 poles that, when aligned in an orientation opposite that of the poles on the first precision magnet 36 on the end surface 34, exhibit strong magnetic attraction, locking the fuel rod 20 in position on bottom nozzle plate 30 when the two are brought into contact with each other, as shown in FIG. 6. In various aspects, a similar end surface with a precision magnet 36 may be incorporated on the upper end of the fuel rod 20 for magnetic attachment to a mating precision magnet 40 incorporated into the top nozzle plate 28. With precision magnets on the lower end of the fuel rods 20 in a confronting position relative to the bottom nozzle plate 30, the fuel rods 20 may be locked into axial alignment within the reactor system 10. When it is necessary to move a fuel rod 20, for example, to reconstitute, replace or repair it, one of the first or second precision magnets 36, 40 on the end of the fuel rod 20 is turned to position the positive (+) and negative (−) poles 202, 204 of one precision magnet 36 or 40 into alignment with the like poles of the opposing precision magnet 40 or 36 so that the bottom end surfaces of the fuel rod (or fuel rod end plug) and associated nozzle plate 30 repel each other, moving into an unlocked position, as shown in FIG. 7. In certain aspects, each of the first and second precision magnets 36, 40 may be formed from a plurality of paired sections, wherein each section of a pair may have the same polarity as the other section of the pair or each section of a pair may have the opposite polarity of the other section of the pair. The polarity of each section may be selectively switchable to the opposite polarity to selectively switch one of the first or second precision magnets 36, 40 from the locked configuration wherein at least a majority of the confronting precision magnet sections attract each other to an unlocked position wherein at least a the majority of the confronting precision magnet sections repel each other. In this embodiment, the strength of the attractive or repelling force may be controlled by polarities of confronting sections of the precision magnets. In another aspect, as shown in FIG. 8, a second precision magnet 40 may be placed by suitable means, for example, by 3-D printing, in each of a plurality of spaced recessed portions 46 in a bottom nozzle plate 30′, also having holes 42 in the nozzle plate 30′ for coolant flow about each fuel rod 20. The nozzle plate 30′ may, in certain aspects, have an egg crate-like structure comprised of the plurality of recessed portions 46 and coolant flow holes 42. Each such recessed portion 46 is configured to seat the end plug 32 of one of a plurality of fuel rods 20. The plurality of recessed portions 46 may include a floor section onto which a second precision magnet 40 is incorporated and openings around the floor that lead to venturi type openings directly below the recess 46 for coolant flow. The flow holes 42 may also form venturi type openings. In use, the first precision magnet 36 on the end surface 34 of each end plug 32 is positioned to align with precision magnet 40 on the floor of the recessed portion 46 to either attract or repel each other for locking or unlocking, respectively, the fuel rod 20 to the nozzle plate 30′. The precision magnets 36, 40 may, as described above, have paired sections of alternating patterns of positive (+) 202 and negative (−) 204 poles on each of the precision magnets which may be rotated into an attracting or a repelling alignment, or each may have a single positive (+) 202 or negative (−) 204 pole on one and a single negative (−) 204 or positive (+) 202 pole on the other, opposite the polarity of the confronting precision magnet, to attract each other to axially lock the fuel rod 20 into the recessed portion 46. Unlocking may occur by reversing the polarity of one of the two confronting precision magnets, for example, by rotating the fuel rod. A conventional grid includes laterally positioned grid straps 50 that crisscross within the grid perimeter to define cells 60 through which the fuel rods 20 pass. The grid straps 50 serve to align the fuel rods 20 laterally and prevent adjacent fuel rods 20 from contacting each other. The grid straps 50 may include springs 48. The embodiment of exemplary springs 48 is shown in FIGS. 9-10. Each cell 60 may include one or two springs 48, on different sides of the grid strap sections that define the cell 60. In various embodiments, each cell may include two springs 48 or two dimples 58. The springs 48 or dimples 58 extend or arch from the grid strap wall into the cell 60 forming a raised plateau 56 which bows toward the fuel rod 20 when the rod is positioned within cell 60 such that the elevated plateau 56 of spring 48 is pressed or wedged laterally against the adjacent fuel rod 20. The springs 48 may be arranged so that at least two plateaus 56 extend into each cell 60 to laterally secure the rod 20 within the cell 60. In certain aspects, when there are adjacent fuel assemblies 10, the grid design may include a first set of grid straps 50 on the perimeter of one fuel assembly 10 and a second set of grid straps 52 on the perimeter of the adjacent fuel assembly 10 on each grid 68, 88, and 18. The second set of grid straps 52 are positioned adjacent the first set of grid straps 50 to define a space between adjacent grid strap walls 50, 52. Adjacent grid strap walls are positioned in planes substantially parallel and spaced from each other. In certain aspects, shown in FIGS. 9 and 10, precision magnets 361 and 401, keyed to precision magnets 362 and 402, may be incorporated into the exterior surfaces of adjacent grid straps 50, 52 of the first and second sets for lateral impact dampening to maintain a distance between the adjacent fuel assemblies 10. A novel grid design for accident tolerant fuel configurations may add one or both sets of precision magnets 361, 362 and 401, 402, incorporating them into the outer surfaces of the grid straps 50, 52 during the manufacturing process. Manufacture of the grid straps 50, 52 may, for example, be by any suitable known 3-D printing method or any other process for forming a molded three dimensional product or surface. Precision magnet patterns may be printed, for example, into the adjacent areas of grid straps 50, 52 to create resistance at a pre-determined distance or gap 54 between the outer sides of adjacent grid straps 50, 52. The gaps 54 reduce seismic and loss of coolant accident forces without the need for external features on the grid straps that may cause damage to the fuel rods 20 held within the cells 60 in the event of unplanned movement of any significant force. When each aligned pole 202 or 204 of the opposing precision magnets 361, 362 and 401, 402, respectively, is of the same polarity, the grid straps 50, 52 will repel each other and resist impact. By controlling the strength of the magnetic field generated by the precision magnets 361, 362, 401 and/or 402, the distance 54 between the exteriors of adjacent grid straps 50, 52 can be controlled and maintained under adverse conditions. Referring to FIGS. 11-12, an alternative fuel rod lateral positioning configuration is shown that incorporates precision magnets 72 into the inside of the cells 60 and confronting precision magnets 70 on the fuel rods 20. Precision magnet patterns may be incorporated into the interior side of grid strap walls 50 in place of or in addition to dimples 58, which align fuel rods 20 within the grid cells 60. A thin sleeve 62 can be attached to the fuel rod 20. The sleeve 62 may in various aspects, be printed with the opposite magnetic pole from the pole incorporated into grid strap 50 at some or all grid 68, 18, or 88 elevations within the system 10. As shown, the repelling force of the confronting like-pole precision magnets (i.e., each of the confronting precision magnets having positive (+) poles 202 or each of the confronting precision magnets having negative (−) poles 204) maintains a desired gap 74 between the fuel rod 20 or a sleeve covering the rods 20. This arrangement will provide a significant grid-to-rod fretting margin because there would be much less (if any) rod 20, 24 contact support required. Referring to FIG. 11, cells 60 defined by grid straps 50 are shown in an alternative arrangement from the grid strap arrangement described above and shown in FIG. 10. Each cell 60 also includes mixing vanes 78 for controlling coolant flow around rods 20. Coolant flow runs parallel to the rods 20. In certain aspects, as shown in FIGS. 13-14, dimples 58 may be eliminated as a retention means from cells 60 so that precision magnets 72 on the interior of grid straps 50 and precision magnets 70 on rod 20 or a rod sleeve 62 provide the sole means of maintaining the separation, gap 74, between the rods 20 and grid straps 50 within each cell 60. A significant cost savings can be gained by combining and eliminating components. Components can be more easily controlled and result in a cost savings by removing tightly tolerance features such as springs and dimples in sheet metal components. There will be less pressure drop in coolant flow resulting in increased fuel efficiency and a higher burn-up. Movements between fuel assemblies and fuel rods that may occur in accident conditions can be better controlled, preventing damage due to sudden impacts between adjacent components. The use of precision magnets for maintaining lateral rod position control provides contact free retention. This provides more clearance between the fuel rods and other components, improving wear because debris will not be trapped against the fuel rods. The improved retention features described herein create opportunities to simplify structural components and, importantly, create safer fuel designs for use in higher seismic locations. The present invention has been described in accordance with several examples, which are intended to be illustrative in all aspects rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. All patents, patent applications, publications, or other disclosure material mentioned herein, are hereby incorporated by reference in their entirety as if each individual reference was expressly incorporated by reference respectively. All references, and any material, or portion thereof, that are said to be incorporated by reference herein are incorporated herein only to the extent that the incorporated material does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as set forth herein supersedes any conflicting material incorporated herein by reference and the disclosure expressly set forth in the present application controls. The present invention has been described with reference to various exemplary and illustrative embodiments. The embodiments described herein are understood as providing illustrative features of varying detail of various embodiments of the disclosed invention; and therefore, unless otherwise specified, it is to be understood that, to the extent possible, one or more features, elements, components, constituents, ingredients, structures, modules, and/or aspects of the disclosed embodiments may be combined, separated, interchanged, and/or rearranged with or relative to one or more other features, elements, components, constituents, ingredients, structures, modules, and/or aspects of the disclosed embodiments without departing from the scope of the disclosed invention. Accordingly, it will be recognized by persons having ordinary skill in the art that various substitutions, modifications or combinations of any of the exemplary embodiments may be made without departing from the scope of the invention. In addition, persons skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the various embodiments of the invention described herein upon review of this specification. Thus, the invention is not limited by the description of the various embodiments, but rather by the claims.
	summary
	abstract	A segmented lattice rack to store fuels coming from nuclear reactors, whose walls are made of plates joined orthogonally forming a mesh defining multiple cell cavities which are longitudinally coupled forming a sandwich comprising a central part coinciding with the active part of the stored element, being from a material obtained from neutronic poisons, preferably boron treated steel; whilst the end areas coinciding with the non-active part of the stored radioactive element are of normal stainless steel, joined to each other and with the adjacent components by means of welding or pretensioning.
	summary
	summary
	description	The invention relates generally to collimating treatment beams for radiotherapy. More particularly, the invention relates to a flexible aperture assembly that enables collimating a treatment beam in close proximity to a patient to limit radiation dispersion in a treatment area. When beams of high energy x-rays or electrons are used for radiotherapy, it is important to direct the beams to a tumor within the patient, while restricting the beams from striking healthy tissue outside the tumor region. Tumors commonly have irregular shapes and it is necessary to shape the beam cross-section to the corresponding irregular shape. It is common for the treatment plan to prescribe the beam to be directed at the tumor from a number of different angles, where the beam profile is unique for each corresponding angle. Currently, radiotherapy accelerators producing therapeutic electron beams utilize “applicators”, also known as “cones”, are attached to the therapy machine to provide a final collimation aperture along the beam path before the tumor is exposed. The applicator defines the final beam cross-section profile and it is desirable to place the applicator as close to the patient as possible to limit exposure to healthy tissue. Because the tumor has a unique shape for each prescribed exposure angle, a unique collimating aperture is required for each corresponding angle. It is common to have multiple apertures fabricated for treating a single tumor, where alloys with low melting temperatures are typically cast into the required irregular shapes. The cast apertures can be interchanged with the radiotherapy device to provide a beam that conforms to the shape of the area to be irradiated. These unique apertures are expensive and time consuming to fabricate. In an attempt to alleviate the need to fabricate a unique aperture for each exposure, multi-leaf collimators (MLC) have been implemented as a way to shape the beam cross-section. These devices include a set of flat, thin leaves made from a high-density material, such as tungsten, where each leaf in moved transversely in and out of the radiation field to selectively attenuate portions of the beam to create a unique beam cross-section. The shape of the beam can be altered dynamically during the therapy session using motorized controls connected to each leaf. By dynamically attenuating select portions of the beam, intensity-modulated radiotherapy (IMRT) has been made possible, where by moving the leaves during beam exposure, the beam can be delivered in a manner such that the spatial fluence of the irradiation is not constant over the irradiated area. IMRT can also be accomplished by making multiple irradiations, each with a different field shape, the sum of which creates a field of non-uniform intensity. The leaves must be thick enough to highly attenuate the beam. For example, when using x-ray beams, at least a 6 cm thickness of tungsten is required. X-ray MLC's are typically mounted as far from the patient as practicable to ensure maximum clearance between the radiotherapy machine and the patient. In accordance with some accelerators, the MLC has been used to replace the standard field-shaping jaws of the accelerator. The shape of the portion of the leaf that defines the edge of the field is designed for minimum penumbra to create the sharpest edge of the beam as possible between the irradiated and protected areas. In electron radiotherapy, fabricated electron applicators are typically used, where the applicators are customized for each patient to define each unique final beam aperture. This process is very time consuming and expensive. The custom aperture must be installed by hand for each treatment field. If two or more fields are used for a therapy session, the aperture must be changed for each field. Further, the aperture must be redesigned to accommodate changes in the tumor size during the course of treatment. The beneficial practice of IMRT delivery cannot be used with these fixed apertures. It is desirable to be able to use multi-leaf collimators for electron irradiation as well as for x-ray irradiation. Currently, MLC's that are designed for x-rays are not suitable to this end. To produce a desired penumbra, a collimator for electron beams must be close to the patient surface, typically within 5 cm. Conversely, the usual location of an x-ray MLC is far from the patient, which makes creating desirable beam characteristics unfeasible. It is possible to move the patient closer, but the penumbra achievable still cannot match that which is attained with an electron applicator. Attempts to create the final aperture of an electron applicator using a form of MLC have been reported. In these efforts, the final aperture of the applicator has been constructed of a bank of leaves that can be moved to a variable position relative to the beam, similar to an x-ray MLC. The leaves do not have to be as thick as those for x-ray MLC's, where it requires only approximately 1 cm of brass to stop 20 MeV electrons compared to the 6 cm of more of tungsten required for an effective x-ray MLC. FIG. 1 shows a perspective view of a prior art applicator and multi-leaf collimator assembly 100 for use with electron beams 101, where shown is an applicator 102 and a multi-leaf collimator 104 having two opposing sets of movable leaves 106 configured to move parallel with one another in a collimator housing 108 disposed as a treatment aperture 110. As shown, the leaves 108 are positioned manually however motorized leaves are also known in the art. FIG. 2 is a planar view of a treatment configuration 200, where a patient 202 is positioned close to the prior art multi-leaf collimator assembly 100 attached to an accelerator 204. Here, the large extensions of the MLC 104 to each side of the treatment field prevent the applicator 102 from being positioned close to irregular surfaces on the patient 202, such as near the head and neck. This results in the final aperture 110 being further away from the patient 202 than desired, and prevents optimization of the penumbra. Accordingly, there is a need to minimize the lateral extention of the electron applicator near the patient and minimize clearance issues to overcome the current shortcomings in the art. According to the current invention, a flexible multi-leaf collimator is provided that includes a plurality of flexible assemblies, at least one guide supporting the assemblies, and a plurality of assembly drivers. The driver engages the assembly and moves it along the guide. The assembly has an extended state and a retracted state relative to the guide, such that when in the extended state, the assembly is held in an aperture plane and when in the retracted state, the assembly conforms along the guide. In one aspect of the invention, the assembly is a flexible assembly that includes at least two collimator segments, where the segment has a first side and a second side. The first side of one the segment interfaces the second side of an adjacent segment. The flexible assembly further includes a flexible conveyor, where the conveyor can be a flexible top strap attached to the top of each the segment and is disposed along the guide. The first segment in the series is an assembly collimation end and a last segment in the series is an assembly actuation end. In another aspect of the invention, the conveyor can further include a flexible opposing strap, where the assembly collimation end is connected to a first end of the opposing strap and the assembly actuation end is connected to a second end of the opposing strap. According to a further aspect of the invention, the conveyor can be a plurality of pivotable linkages to provide a pivotable connection between the segments. Here, a first end of the segment top surface is pivotably connected to a second end of an adjacent segment top surface. In another aspect of the invention, the conveyor can include an interlock strap having interlock nodes disposed to engage a node socket on a bottom surface of the segment. When the assembly is in the extended state the node is engaged in the socket and when the assembly is in the retracted state the node is disengaged from the socket. In yet another aspect of the invention, the segment has a first side and a second side, where the first side has a first engagement feature and the second side has a second engagement feature such that the first engagement feature engages the second engagement feature. In another aspect of the invention, the assembly can be a flexible assembly made from a graduated-length stack of at least two flexible straps disposed along the guide. According to another aspect of the invention, the guide is a curved guide having an upper guide surface and a lower guide surface, where the upper surface has a smaller radius of curvature than a radius of curvature of the lower surface. In this aspect, the guide further has a guide collimation end and a guide actuation end, where the guide collimation end is disposed about perpendicular to a radiation beam path and the guide actuation end is disposed about parallel to the beam path. In a further aspect of the invention, the driver can be a computer-controlled actuator, where the actuator engages an actuation end of the assembly and moves the assembly along the guide. Some key advantages are the leaves of the flexible MLC being curved away from the patient surface to allow better access to the patient. The invention resolves the problem of having an array of rigid leaves occupying a plane near the patient surface, and can replace the standard final aperture in an electron applicator. A further advantage is the ability of the flexible leaves to be positioned dynamically and remotely, enabling faster patient treatments and the use of IMRT techniques with electron radiotherapy. Although the following detailed description contains many specifics for the purposes of illustration, anyone of ordinary skill in the art will readily appreciate that many variations and alterations to the following exemplary details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. The present invention provides leaves with an MLC for electron radiotherapy where the leaves are not a single rigid component, but are configured in a manner that curves away from the patient to provide greater clearance. According to the current invention, a flexible multi-leaf collimator is provided that includes a plurality of flexible assemblies, at least one guide supporting the assembly, and a plurality of assembly drivers. The driver engages the assembly and moves the assembly along the guide. The assembly has an extended state and a retracted state relative to the guide, such that when in the extended state the assembly is held in the aperture plane and when in the retracted state the assembly conforms along the guide. FIG. 3 shows a perspective view of an applicator and flexible multi-leaf collimator assembly 300 for use with an electron beam 101. As shown, an applicator 102 has an opposing pair of flexible multi-leaf collimators 302 attached to the end of the applicator 102. The flexible leaves 304 are driven by drivers 306, such as computer controlled positioning motors, to move within a guide 308 and extend horizontally out of the guide 308 to form an aperture 110. Accordingly, the aperture 110 can be dynamic or static during treatment. The flexible leaves 304 are able to bend away from the patient according to the profile of the guide 308. In the drawing, one guide 308 is shown as a partial-cutaway to illustrate the flexible leaves 304. FIG. 4 shows a planar view of a flexible MLC treatment configuration 400 with a patient 202 positioned close to the applicator and flexible MLC-applicator assembly 300 attached to an accelerator 202. As shown, the flexible MLC 302 curves away from the patient 202 to provide greater clearance to the curvatures of the patient 202. FIGS. 5a-5b show perspective views of the flexible multi-leaf collimator 302 according to one embodiment of the invention. Shown is a flexible MLC 302 having multiple flexible leaves 304. The flexible leaves 304 include multiple collimator segments 500 attached to a flexible conveyor 502. The segments 500 are thick enough to provide the desired attenuation of the electron beam 101. The conveyor 502 and attached segments 500 move along a guide 308 when actuated by the driver 306. The guides 308 are shown as curved and illustrated in partial perspective cutaway views to show the aspects of the flexible leaves 304 and how they translate within the guides 308. The drivers 306 can be operated using computer commands (not shown). FIG. 6 shows one embodiment of the flexible leaf 304 of collimator elements 500 in a guide 308 according to the current invention. The flexible leaves 304 include at least two collimator segments 500, where the segment 500 has a first segment side 600 and a second segment side 602. Here, the first side 600 of one the segment 500 interfaces the second side 602 of an adjacent segment 500. The conveyor 502 of FIG. 5 is shown in FIG. 6 as a flexible top strap 604 attached to a topside 606 of each the segment 500, where the top strap 600 is disposed along the guide 308. In this embodiment, a first segment 606 in the series is the assembly collimation end 606 and a last segment 608 in the series is the assembly actuation end 608, where the collimation end 606 provides the desired penumbra, and the actuation end 608 is incorporated to the driver 306 (not shown). The adjacent segments 500 form a rigid assembly 610 as they extend from the guide 308. FIG. 7 shows another embodiment of the flexible leaf 304 of collimator elements 500 oriented upside down, relative to the embodiment of FIG. 6, in an inverted guide 700. In this embodiment, the conveyor 502 of FIG. 5 includes the aspects of the flexible top strap 604 embodiment of FIG. 6, but inverted here, in addition to a flexible opposing strap 702, where the assembly collimation end 606 is connected to a opposing strap first end 704 and the assembly actuation end 608 is connected to a the opposing strap second end 706. The segments 500 between the collimation end 606 and actuation end 608 are not attached to the opposing strap 702. In this embodiment, the combination of straps (604, 702) serves to hold the segments 500 rigidly in place as they extend from the guide 308 to define the radiation field. FIG. 8 shows another embodiment of the invention, where the collimator elements 500 of the flexible assembly 304 of have interlock features between the elements 500 allowing them to join with each other when in an extended state. As shown, the elements 500 have a first engagement feature 800 on the first side and a second engagement feature 802 on the second side, where the first engagement feature 800 of one the segment 500 interfaces the second engagement feature 802 of an adjacent segment 500. In this example, the flexible top strap 604 is shown, however other conveyors may be used to provide similar results. In this embodiment, the engagement of the features (800, 802) on the adjacent segments 500 forms a rigid assembly 610 as they extend from the guide 308. FIG. 9 shows a further embodiment of the flexible leaves 304 of collimator elements 500 having pivotable interconnects 900 between the elements 500. Here, the conveyor 502 of FIG. 5, is shown as a plurality of pivotable linkages 900 that provide a pivotable connection between the segments 500, where a first end 902 of a segment top surface 606 is pivotably connected to a second end 904 of an adjacent segment top surface 606. FIG. 10 shows one embodiment of the flexible leaves 304 of collimator elements 500 having a pivotable-linkage top conveyor 1000 and a lower interlock conveyor 1002 with nodes 1004 for fitting to element sockets 1006. As shown, the pivotable-linkage top conveyor 1000 has of pivotable linkages 1008 to provide a pivotable connection between the segments 500, where a first end 902 of a segment top surface 606 is pivotably connected to a second end 904 of an adjacent segment top surface 606. The interlock conveyor 1002 has interlock nodes 1004 disposed to engage the node socket 1006 on a segment bottom surface 1010. When the assembly 304 is in the extended state the node 1004 is engaged in the socket 1006 and when the assembly is in the retracted state the node 1004 is disengaged from the socket 1006. The engagement of the interlock node 1004 to the node socket 1006 is facilitated by an engagement mechanism 1010 to hold the elements 500 as a rigid assembly as they extend from the guide 308, and disengagement of the node 1004 to the socket 1006 is facilitated by a disengagement mechanism 1012. FIG. 11 shows one embodiment of a flexible leaves 304 having a stack of graduated-length flexible elements 1100 fastened together at each end 606 and 608 in a guide 308 according to the current invention. Here, the guide 308 is curved having an upper guide surface 1102 and a lower guide surface 1104, where the upper surface 1102 has a smaller radius of curvature than a radius of curvature of the lower surface 1104. In this aspect, the guide 308 further has a guide collimation end 1106 and a guide actuation end 1108, where the guide collimation end 1106 is disposed about perpendicular to a radiation beam path (not shown) and the guide actuation end 1108 is disposed about parallel to the beam path (not shown). The combined thickness of the elements 1100 in the direction parallel to the radiation beam 101 (not shown) is sufficient to provide the desired attenuation of the beam 101 (not shown). It should be evident from the above descriptions that many combinations of conveyors 502, guides 308, drivers 306 and elements 500 are possible without detracting from the spirit of the invention. The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. All such variations are considered to be within the scope and spirit of the present invention as defined by the following claims and their legal equivalents.
	abstract	There are provided a light water reactor core which has the same levels in cost efficiency and degree of safety as those of an existing BWR under operation now, that is, which is oriented to plutonium multi-recycle having a breeding ratio near 1.0 or slightly larger and having a negative void coefficient with minimizing modification of the reactor core structure of the existing BWR under operation now, and to fuel assemblies used for the boiling water reactor. The light water reactor core having an effective water-to-fuel volume ratio of 0.1 to 0.6 is formed by combining closed compact lattice fuel assemblies each composed of fuel rods having fuel which is enriched by adding plutonium or plutonium and an actinide to a uranium containing at least one of a depleted uranium, natural uranium, a degraded uranium and a low enriched uranium; high void fraction coolant of 45% to 70%; and large-diameter control rods to be inserted into the fuel assemblies, the large-diameter control rod comprising at least one absorption rod having a transverse cross-sectional area larger than a cross-sectional area of a unit lattice cell of the fuel rod.
	summary
	summary
	description	The present invention relates to a quality degradation point estimating system and a quality degradation point estimating method, which estimate a failure point and a quality drop point in a network, and a test flow determining method. In accompaniment with complexity of an information system that uses a network, it has become more and more difficult to specify a failure point when a communication failure or a communication quality degradation has occurred. For the sake of a quick recovery from a quality degradation (QoS degradation), a technique that can estimate a quality degradation point (point of QoS degradation) at a high precision is desired. Conventionally, in order to estimate a quality degradation point in the network, for example, the quality of a flow (user flow) in the network is measured. Instead, a test communication (hereinafter, to be referred to as “a test flow) is performed in the network, and the quality of the test flow is measured. Here, the flow is a flow of packets between terminals in a certain application. The quality degradation point can be estimated in accordance with the quality of a flow passing through various routes. In order to improve the estimation precision of the quality degradation point, the selection of a route for the test flow is important. In “Estimating points of QoS degradation in the network from the aggregation of per-flow quality information” (The Institute of Electronics, Information and Communication Engineers, TM Research Group, Vol. 104, No. 707, Pp. 31-36, May 3, 2005) by Masayoshi Kobayashi, Yohei Hasegawa, and Tsutomu Murase, a method of estimating a quality degradation point is disclosed. According to this method, the quality degradation point is estimated by using both of the quality of a user flow and the quality of a test flow in a network. Specifically, a group of test flows is determined such that the test flows pass through links included in a set of links through which the user flow passes. Here, the test flow set, namely, test flow routes are determined such that the links through which the respective test flows pass are different from each other. Measuring the quality degradation in the different test flow route allows the quality of each link to be determined to be degraded. FIG. 1 is one example of a flow/link correspondence table that indicates a relation a plurality of test flows used in an estimating method and links to which the test flows are applied. A set of links to which the user flows are applied includes links L0 to L3. The links to which respective test flows are applied are different from each other. Each test flow is configured to be applied to only a certain link and not to be applied to the other links. Since the plurality of test flows are used, the link whose quality is degraded is estimated. However, this method requires to search the test flow which passes through only a targeted link. The restriction to search the test flow is severe, and a probability at which the test flow can be discovered is low. When the test flow that passes through only one link cannot be generated, the link in which the quality degradation occurs cannot be detected. Moreover, the search for the routing is required at each node or terminal that serves as the end point of each link. Thus, the search cost becomes expensive. As other techniques related to control of a communication route, the followings are known. Japanese Laid Open Patent Application (JP-P2002-64493A) describes a control method of a communication route in a plurality of networks. The plurality of networks are connected to each other and managed by a network managing system. Each network has a network apparatus. According to this control method, a communication conductivity of the route from a network apparatus in a first network to a network apparatus in a second network is held. Japanese Laid Open Patent Application (JP-P2002-271392A) describes a voice quality managing method for each call in an IP network. A telephone communication quality for each call is monitored from a remote end. A quality degradation is detected in substantially real time. Since the measurement is performed without any installation of an external measuring apparatus, a cost is suppressed. Also, delay information in end-to-end is collected. Japanese Laid Open Patent Application (JP-P2003-258903A) discloses a communication path monitoring system. The communication path monitoring system monitors a communication path established between data processors, in the communication network composed of a plurality of data processors and propagation paths. In particular, the communication monitoring system contains an attribute value obtaining unit for obtaining an attribute value of a communication path as a monitor target. The attribute obtaining unit has first to third units. The first unit obtains control information to establish the communication path that serves as the information transferred between the data processors. The second unit extracts the setting information with regard to the obtainment of the attribute value, from the obtained control information. The third unit uses extracted setting information and obtains the attribute value from the information that passes on the established communication path. An object of the present invention is to provide a technique that can search a link including a quality degradation point in a network at a high probability. Another object of the present invention is to provide a technique that can efficiently set a test flow set to estimate a quality degradation point in the network. Still another object of the present invention is to provide a technique that can reduce a cost to search a route at a node or terminal in a network, when a quality degradation point in the network is estimated. In a first exemplary embodiment of the present invention, a quality degradation point estimating method for estimating a quality degradation point in a directed link set through which a communication flow passed is provided. The quality degradation point estimating method has: (A) determining a test flow set for estimating a quality degradation point; and (B) estimating the quality degradation point in the directed link set by sending the test flow set to the network. The (A) step includes a step of setting the flow, which passes through a partial set as a part of the directed link set, as the test flow and adding the set test flow to the test flow set. The test flow is sent from the test terminal on the network to a predetermined node in the partial set. A response is obtained at the predetermined node, and the response is sent from the predetermined node to a predetermined terminal. The (A) step includes: (a) setting continuous directed links included in the directed link set as the partial set and then setting an interval of the continuous directed links as a directed link interval; (b) setting the flow, which passes through at least a part of the directed link interval, as the test flow; and (c) adding the set test flow to the test flow set. The test flow is sent from the test terminal to the predetermined node in the directed link interval. The response is obtained at the predetermined node, and the response is sent from the predetermined node to the predetermined terminal. The determined test flow set may include a first test flow. The first test flow is sent from the test terminal to a termination point in the directed link interval, the response is obtained at the termination point, and the response is sent from the termination point to the predetermined terminal. The determined test flow set may include a second test flow. The second test flow is sent from the test terminal to a start point in the directed link interval, the response is obtained at the start point, and the response is sent from the start point to the predetermined terminal. The determined test flow set may include a plurality of third test flows. Here, the number of the hops until the intersection point at which the flows from the test terminal overlap on the directed link interval is assumed to be H1, and the number of the hops until the termination point of the directed link interval from the test terminal is assumed to be H2. At this time, each TTL (Time To Live) value in the plurality of third test flows is set to H1 or more and H2 or less, and each of destinations is set at the termination point of the directed link interval. Each of the plurality of third test flows is sent to the plurality of nodes in the directed link interval from the test terminal, the response is obtained at each of the plurality of nodes, and the response is sent from each node to the predetermined terminal. In that case, the (b) step may include: (b1) setting the start point of the directed link interval at the predetermined node; (b2) setting the flow sent to the predetermined node as one of the plurality of third test flows; and (b3) repeatedly executing the (b2) step while changing the predetermined nodes from the start point of the directed link interval to the termination point, one hop at a time. The (A) step further includes (d1) determining the route from the terminal, in which the flow can be generated in the directed link interval, to the termination point of the directed link interval; and (d2) setting the terminal corresponding to the route having the longest overlapping with the directed link interval, in the determined routes, as the test terminal. The (A) step may further include (e1) setting the link set included in the directed link interval as an indefinite link set, if the test terminal is not found out; and (e2) removing the indefinite link set from the directed link set and updating the directed link set. The response obtained in the predetermined node may be an ECHO response resulting from ICMP (Internet Control Message Protocol). Also, the response may be the response resulting from a packet survival time excess. The predetermined terminal receiving the response may be the test terminal. The distance between the predetermined node sending the response and the predetermined terminal may be shorter than the distance between the predetermined node and the test terminal. In a second exemplary embodiment of the present invention, the quality degradation point estimating method for estimating the quality degradation point in the directed link set through which the communication flow passed is provided. The quality degradation point estimating method has: (A) determining the test flow set to estimate the quality degradation point; and (B) sending the test flow set to the network and consequently estimating the quality degradation point in the directed link set. The (A) step includes: (f) setting one link, which is included in the partial set serving as a part of the directed link set, as a targeted link; (g) removing the one link from the partial set and consequently updating the partial set and then setting the link included in the updated partial set as a non-targeted link; (h) setting the flow, which passes through the targeted link and does not pass through the non-targeted link, as the test flow; (i) adding the set test flow to the test flow set; and (j) repeating the (f) to (i) steps until the partial set becomes an empty set. At the (f) step, continuous directed links included in the directed link set may be set as the partial set. In a third exemplary embodiment of the present invention, a quality degradation point estimating system for monitoring the quality degradation point in the directed link set through which the communication flow passed is provided. The quality degradation point estimating system contains: a plurality of terminals that are connected to a network and can be communicated through a router to each other; and a monitoring server that is connected to the network and monitors the quality of the communication between the plurality of terminals. The monitoring server sets the flow, which passes through the partial set serving as a part of the directed link set, as the test flow to estimate the quality degradation point. The test flow is sent from the test terminal among the plurality of terminals to the predetermined node in the partial set. Then, the response is obtained at the predetermined node, and the response is sent from the predetermined node to the predetermined terminal among the plurality of terminals. The predetermined terminal reports the quality of the test flow to the monitoring server. The monitoring server estimates the quality degradation point in accordance with the quality of the test flow. The monitoring server may set the continuous directed links included in the directed link set, as the partial set. In this case, the monitoring server sets the interval of the continuous directed links as the directed link interval, and sets the flow, which passes through at least a part of the directed link interval, as the test flow. According to the present invention, a link including the quality degradation point in the network can be searched at a high probability. Also, the test flow set to estimate the quality degradation point in the network can be efficiently set. Moreover, when the quality degradation point is estimated in the network, a cost to search the routing at the node or terminal in the network can be reduced. Hereinafter, a quality degradation point estimating system, a quality degradation point estimating method and a test flow determining method according to exemplary embodiments of the present invention will be described with reference to the attached drawings. A network quality measuring system in a packet switching network is exemplified as the quality degradation point estimating system in the exemplary embodiments. 1-1. Configuration FIG. 2 schematically shows the configuration of the network quality measuring system (quality degradation point estimating system) according to the first exemplary embodiment. The network quality measuring system contains a plurality of routers 100 (100-0 to 100-N; N is a natural number), a plurality of terminals 200 (200-0 to 200-5) as computer apparatuses; and a monitoring server 400. The plurality of terminals 200 and the monitoring server 400 are connected to each other through a network. The plurality of routers 100 are arranged on the network and connected to each other. In short, the plurality of terminals 200, and these terminals 200 and the monitoring server 400 are connected to each other through the routers 100, and bidirectional communication can be carried out between them. It should be noted that FIG. 2 shows six terminals 200-0 to 200-5. However, the number of terminals is not limited to 6. A physical connection between the routers 100 or the physical connection between an end router 100 and the terminal 200 is referred to as a “link”. In particular, when a direction is also considered, each connection is referred to as a “directed link”. The directed link is defined with an address of a start point and an address of a final point. FIG. 1 shows a plurality of directed links L0 to L(N+1). A route from the terminal 200-0 to the terminal 200-1 passes through the plurality of routers 100-0 to 100-N, and this is constituted by the plurality of links (link set) L0 to L(N+1). A flow of packets between the terminals 200 in a certain application is referred to as a “flow”. In particular, when the direction is also considered, a flow of packets is referred to as a “directed flow”. For example, in FIG. 1, a communication from the terminal 200-0 to the terminal 200-1 is carried out through a directed flow 300. A communication from the terminal 200-2 to the terminal 200-3 is carried out through a directed flow 310. A communication from the terminal 200-4 to the terminal 200-5 is carried out through a directed flow 320. With regard to these flows, a flow/link correspondence table can be defined to indicate a relation of each flow and the link through which the flow passes. FIG. 3A shows one example of the flow/link correspondence table. FIG. 3A shows the relation between the directed flows 300 to 320 on the network and the links (L0 to L(N+1)) through which the flows pass. With regard to each flow, the link through which the flow passes is represented by “1”, and the link through which the flow does not pass is represented by “0”. Also, the flow/link correspondence table shows the quality on the routes of the respective flows. For example, the quality of the flow 300 is degraded, and the good quality is obtained with regard to the flows 310 and 320. The degradation in the quality is indicated by, for example, a flag. When a certain communication flow is generated between the terminals 200, the terminal 200 on a receiving side measures the quality of its communication flow. Then, the terminal 200 on the receiving side sends a “quality data” as its quality measurement result to the monitoring server 400. In FIG. 2, the quality data are transferred from the terminals 200-1, 200-3 and 200-5 to the receiving side among the terminals 200-0 to 200-5, which carry out the communications, to the monitoring server 400, respectively. When the quality degradation in the communication flow is detected and determined, it is necessary to estimate the link in which the degradation is generated. According to this exemplary embodiment, in order to estimate the quality degradation point, a “test flow” is sent to the network. In order to improve the estimation precision of the quality degradation point, the selection of the route to which the test flow is sent. It is the monitoring server 400 that determines a route of the test flow. In this exemplary embodiment, the monitoring server 400 has a function of determining the route of the test flow in accordance with the quality data received from the terminal 200. Also, the monitoring server 400 has a function of instructing the generation of the determined test flow to the terminal 200. Moreover, the monitoring server 400 has a function of estimating a quality degradation point in accordance with the quality data of the test flow. FIG. 4 schematically shows the configuration of the monitoring server 400 according to this exemplary embodiment. The monitoring server 400 contains an input output control section 401, a flow quality collecting section 402, a route data collecting section 403, a quality degradation point estimating section 404, a flow set determining section 405, a flow/link correspondence table managing section 406 and a flow generation instructing section 407. The input output control section 401 controls the input/output of packets from/to the network. The flow quality collecting section 402 collects the quality data of the flows that are reported from the terminals 200 through the network. The route data collecting section 403 collects a route data (a routing table) from the router 100 on the network. The quality degradation point estimating section 404 integrates the quality data collected from the terminals 200 and estimates a quality degradation point in the network. The flow set determining section 405 sets a set of the test flows (hereinafter, to be referred to as a “test flow set Z”) required to specify the quality degradation point. The flow/link correspondence table managing section 406 manages a flow/link correspondence table (refer to FIG. 3A). The flow generation instructing section 407 instructs the terminal 200 to generate the test flow determined by the flow set determining section 405. As mentioned above, the flow set determining section 405 sets the test flow set Z. Here, the link set of a test target has a high possibility that the continuous directed links are included. Hereinafter, the continuous directed links are referred to as “concatenated directed links”. Also, there is a case that an interval of the concatenated directed links is referred to as a “directed link interval P”. A quality degradation point has a high possibility that it is included in the concatenated directed links. Thus, the flow set determining section 405 according to this exemplary embodiment especially pays attention to this concatenated directed links. As shown in FIG. 4, the flow set determining section 405 contains a route overlap investing section 4051, a route searching section 4052, a concatenated directed link searching section 4053, a concatenated directed link table 4054 and a flow set table 4055. The concatenated directed link table 4054 is a table for indicating the concatenated directed links and is stored in a storage unit. Also, the flow set table 4055 is a table for indicating the test flow set Z used to estimate the quality degradation point and is stored in the storage unit. The concatenated directed link searching section 4053 searches the concatenated directed links (the directed link interval P) from the link set as the test target and prepares and updates the concatenated directed link table 4054 indicating the concatenated directed links. The route searching section 4052 refers to the concatenated directed link table 4054 and searches a route for a test flow to estimate the quality degradation point. Also, the route searching section 4052 prepares and updates the flow set table 4055 to indicate the test flow set Z. The route overlap investing section 4051 checks the overlap between the route of the test flow and the directed link interval P. It should be noted that the respective sections are attained through cooperation of an operational process and a software program executed by an operational processor. 1-2. Detailed Process A process of the network quality measuring system according to this exemplary embodiment will be described below in detail with reference to FIG. 4. It should be noted that in the following description, there is a case that a quality degradation point, namely, a link in which the quality degradation has been caused is referred to as a “degradation link”. (Generation for Flow/Link Correspondence Table) The input output control section 401 in the monitoring server 400 receives a quality data with regard to each flow from the terminal 200 on the receiving side and transfers the received quality data to the flow quality collecting section 402. The flow quality collecting section 402 inquires of the route data collecting section 403 about a link through which each flow passes. The route data collecting section 403 collects a routing table (route data) from each router 100 through the input output control section 401. In response to the inquiry from the flow quality collecting section 402, the route data collecting section 403 reports a relation of each flow and a link through which each flow passed, to the flow quality collecting section 402. Here, the link is a directed link that is defined based on the address of a start point and the address of an end point. The flow quality collecting section 402 adds the quality data (a good state, a degraded state etc) received from the input output control section 401, to the data received from the route data collecting section 403, and generates the flow/link correspondence table as shown in FIG. 3A. In the example shown in FIG. 3A, the route of the flow 300 includes the links (L0 to L(N+1)), and the quality of the flow 300 is degraded. The flow quality collecting section 402 transfers the generated flow/link correspondence table to the flow/link correspondence table managing section 406. The flow/link correspondence table managing section 406 stores the flow/link correspondence table in the storage unit such as a memory and a hard disc. Also, the flow/link correspondence table managing section 406 reports that the flow/link correspondence table has been stored and updated, to the quality degradation point estimating section 404. The quality degradation point estimating section 404 refers to the updated flow/link correspondence table and issues a setting instruction of test flows to specify a degradation link, to the flow set determining section 405. The flow set determining section 405 determines a test flow set Z for specifying the degradation link in response to the setting instruction of the test flows. The determination of the test flow set Z by the quality degradation point estimating section 404 and the flow set determining section 405 will be described below in detail. (Determination of Test Flow Set Z) FIG. 5A and FIG. 5B are a flowchart showing the determining method of the test flow set Z according to this exemplary embodiment. The quality degradation point estimating section 404 refers to the flow/link correspondence table and sets the link set through which the flow whose quality is degraded passes, to a “test target link Set” (Step S2). Also, the quality degradation point estimating section 404 extracts the link set through which the flow whose quality is good passes, as a “tested link set” (Step S4). For example, in case of the flow/link correspondence table shown in FIG. 3A, the links (L0 to L(N+1)) are set as the test target link set. Also, the links L1 and L(N+1) through which the flows 310 and 320 whose qualities are good pass are extracted as the tested link set. Next, the quality degradation point estimating section 404 removes the tested link set from the flow/link correspondence table. Specifically, the quality degradation point estimating section 404 removes the tested link set from the test target link set and consequently determines a “test link set” (Step S6). For example, in case of the example as shown in FIG. 3A, the quality degradation point estimating section 404 removes the tested link set (Links L1, L(N+1)) from the test target link set (links (L0 to L(N+1)) and sets the test link set (links L1, L2 to LN). The test link set is reported to the flow/link correspondence table managing section 406. The flow/link correspondence table managing section 406 updates the flow/link correspondence table as shown in FIG. 3A to that shown in FIG. 3B. If the test link set does not exist (Step S8: No), the quality degradation point does not exist. Thus, the process is finished. If the test link set exists, (Step S8: Yes), the quality degradation point estimating section 404 issues an instruction to set the test flows to the flow set determining section 405. In response to the setting instruction of the test flows, the concatenated directed link searching section 4053 in the flow set determining section 405 sets a “directed link interval P” (Step S10). Specifically, the concatenated directed link searching section 4053 refers to the flow/link correspondence table and sets the interval of the concatenated directed links in the test link set, as the directed link interval P. If the concatenated directed links do not exist in the test link set, the interval of a single link is set as the directed link interval P. In case of the example shown in FIG. 3B, the test link set (links L0, L2 to LN) includes the concatenated directed links (links L2 to LN), and the interval (L2 to LN) is set as the directed link interval P. The concatenated directed link searching section 4053 records the set directed link interval P (the concatenated directed links) on the concatenated directed link table 4054 and reports the setting of the directed link interval P to the route searching section 4052. In response to the report (update) of the directed link interval P, the route searching section 4052 determines a “test flow route” through which the test flow passes and a “test Terminal T” at which the test flow can be generated (Steps S12 to S18). Specifically, in response to the report of the directed link interval P, the route searching section 4052 refers to the directed link interval P (L2 to LN) recorded on the concatenated directed link table 4054. Then, the route searching section 4052 inquires of each of the terminals 200 on the network about whether or not the new test flow can be generated in the directed link interval P (Step S12). If there is not the terminal 200 responding to the inquiry (Step S16; No), the route searching section 4052 determines a link set included in the directed link interval P as am “indefinite link set X” and adds to the tested link set (Step S20). After that, the process proceeds to the step S6. On the other hand, if there is the terminal 200 responding to the inquiry (Step S16; Yes), the route overlap investing section 4051 and the route searching section 4052 determine a route between each terminal 200 at which the new flow can be generated and the end point of the directed link interval P. Then, the route overlap investing section 4051 and the route searching section 4052 select the terminal 200 corresponding to a route having the longest overlap with the directed link interval P as a “test Terminal T” (Step S18). For example, in this example, it is supposed that the terminal 200-3 and the terminal 200-4 can generate the test flow and respond to the inquiry. In this case, at the step S18, the route searching section 4052 reports that the test flow can be generated from the terminals 200-3 and 200-4, to the route overlap investing section 4051. The route overlap investing section 4051 determines the route from each of the terminals 200-3 and 200-4 to the router 100-N serving as the end point of the directed link interval P (L2 to LN). FIG. 6 shows the route corresponding to each of the terminals. The route overlap investing section 4051 checks the overlap between each of the routes shown in FIG. 6 and the directed link interval P and selects the terminal 200-3 corresponding to the route having the longest overlap as the test terminal T. Then, the route overlap investing section 4051 reports the selected terminal 200-3 to the route searching section 4052. When the test terminal T is determined at the step S18, the route searching section 4052 can determine the test flow generated at the test terminal T (Step S22). For example, the test flow is sent from the test terminal T to the end point of the directed link interval P. Then, the response destined to the test terminal T is obtained at the end point, and the response is sent to the test terminal T and received by the test terminal T. FIG. 7 shows one example of the set test flow. A test flow 500 is sent from the terminal 200-3 (the test terminal T) to the router 100-N (the end point of the directed link interval P). At the router 100-N, the response destined to the terminal 200-3 is obtained, and the response is sent to the terminal 200-3 and received by the terminal 200-3. As shown in FIG. 7, the test flow 500 shows a series of the flows. The route searching section 4052 adds this test flow 500 to the test flow set Z indicated on the flow set table 4055. It should be noted that in order to obtain the response, it is possible to use a protocol such as ICMP (Internet Control Message Protocol) ECHO. Also, it is possible to use a response resulting from packet survival time excess. The use of the service in which the response is obtained at the router serving as the end point inverts the direction of the test flow. As a modification example, the terminal 200 serving as the end point of the test flow can be set to a terminal different from the test terminal T. There is a possibility that measurement noise becomes greater, as the distance from the response point to the end point becomes longer. Thus, the terminal 200 serving as the end point may be set such that the distance becomes shorter. For example, the test flow is set such that the response destined to the terminal 200-4, instead of the terminal 200-3, is sent from the router 100-N. For this purpose, the designation address of the packet sent from the terminal 200-3 may be set to the router 100-N, and a transmission source address may be set to the terminal 200-4. Or, there is a possibility that the test flow passes through the larger number of different directed links, as the distance from the response point to the end point becomes longer. Thus, the terminal 200 serving as the end point may be set such that the distance becomes longer. In that case, it is possible to reduce the entire number of the test flows. Moreover, an IP source route option may be used. Also, the following test flow is generated. A plurality of test flows are sent to each of the nodes existing in the interval in which the directed link interval P overlaps with the route from the test terminal T to the end point of the directed link interval P. The plurality of test flows are sent from the test terminal T to each of the nodes, the response is obtained at each of the nodes, and the respective responses are sent to the test terminal T. Here, the number of the hops until the intersect point at which the flow from the test terminal T overlaps on the directed link interval P is assumed to be H1. Also, the number of the hops until the end point of the directed link interval P from the test terminal T is assumed to be H2. The route searching section 4052 determines the numbers H1 and H2 and sets each of TTL (Time To Live) values of the plurality of test flows to H1 or more and H2 or less (H1·TTL·H2). It should be noted that “Designations from Test Terminal T” with regard to all the test flows are set as the end point of the directed link interval P. The route searching section 4052 adds each of the plurality of determined test flows to the test flow set Z indicated on the flow set table 4055. FIG. 8 shows an example of the set test flow. The route searching section 4052 determines the hop number H1 (=1) until the intersect point at which the flow from the terminal 200-3 overlaps on the directed link interval P and the hop number H2 (=4) until the end point of the directed link interval P. Then, the route searching section 4052 determines the plurality of test flows in which the designation from the test terminal 200-3 is set as an end point L100-N, and the TTL value is set to satisfy H1·TTL·H2. For example, the nodes are changed from the start point to the end point in the directed link interval P one hop by one hop, and the test flow is set for each node. The plurality of test flows are sent from the test terminal 200-3 to the respective nodes, the responses are obtained at the respective nodes, and the respective responses are sent to the test terminal 200-3. In FIG. 8, a test flow 510 that passes through the router 100-3, and a test flow 520 that passes through the router 100-2 are set. The plurality of test flows 510 and 520 are added to the test flow set Z indicated on the flow set table 4055. In this way, the route searching section 4052 sets the test flow set Z and generates and updates the flow set table 4055 (Step S22). Next, the route searching section 4052 reports the newly set test flow set Z and the link set corresponding to each test flow to the flow/link correspondence table managing section 406. The flow/link correspondence table managing section 406 records the data with regard to the reported test flow set Z onto the flow/link correspondence table. Thus, the flow/link correspondence table is updated. In the foregoing example, the correspondence relation between the flows 500 to 520 and the respective links is recorded on the flow/link correspondence table (refer to FIG. 10A). If an interval in which a test route through which the set test flow set Z passes, namely, the route between the test terminal T and the end point in the directed link interval P and the directed link interval P overlap is identical to the directed link interval P (Step S24; Yes), the route searching section 4052 adds the link set included in the directed link interval P to a “tested link set” (Step S26). Thus, the tested link set is updated, and the new tested link set is set. In case of the foregoing example, the interval (L2 to LN) in which the route between the test terminal 200-3 and the end point (L100-N) and the directed link interval P overlap is identical to the directed link interval P (L2 to LN) (Step S24; Yes). Therefore, the route searching section 4052 adds the link set (L2 to LN) included in the directed link interval P to the tested link set (Step S26). Next, the process proceeds to the step S6. At the step S6, the quality degradation point estimating section 404 removes the new tested link set from the flow/link correspondence table. Specifically, the quality degradation point estimating section 404 removes the new tested link set (L1, L(N+1), L2 to LN) from the test target link set (links L0 to L(N+1)) and sets the new test link set (link L0)]. Only the link L0 exists in the test link set updated at the step S6. Accordingly, the link L0 is set as a “new directed link interval P” (Step S8; Yes, Step S10). Also, at the steps S12 to S16, it is supposed that the responses can be obtained from the terminal 200-3 and the terminal 200-4. At the step S18, the route overlap investing section 4051 determines the route from each of the terminal 200-3 and the terminal 200-4 to the router 100-0 serving as the end point in the directed link interval P (L0). In this case, there is no overlap between the determined route and the directed link interval P (L0). Therefore, the route overlap investing section 4051 next checks a route from the start point (the terminal 200-0) of the directed link interval P to each of the terminals 200-3 and 200-4. Then, the route overlap investing section 4051 checks the overlap between each of the routes and the directed link interval P and selects the terminal 200 corresponding to the route having the longest overlap as the test terminal T. In the foregoing example, since the overlap lengths are equal, the route overlap investing section 4051 selects one of the terminal 200-3 and the terminal 200-4 as the test terminal T. For example, the terminal 200-3 is selected as the test terminal T. When the test terminal T is determined, the route searching section 4052 can determine the test flow to be generated by the test terminal T (Step S22). FIG. 9 shows one example of the set test flow. A test flow 700 is sent from the terminal 200-3 (test terminal T) to the terminal 200-0 (the start point of the directed link interval P). At the terminal 200-0, the response destined to the terminal 200-3 is obtained, and its response is sent to the terminal 200-3 and received by the terminal 200-3. The route searching section 4052 adds this test flow 700 to the test flow set Z indicated on the flow set table 4055. Next, the route searching section 4052 reports the newly set test flow 700 and the link L0 corresponding to it to the flow/link correspondence table managing section 406. The flow/link correspondence table managing section 406 updates the flow/link correspondence table. FIG. 10A shows the updated flow/link correspondence table. The test route through which the test flow passes is only the link L0, and this is identical to the directed link interval P (L0) (Step S24; Yes). Thus, the link L0 in the directed link interval P is added to the tested link set (Step S26). The process again proceeds to the step S6. At the step S6, the link L0 is further removed from the test target link set, and the test link set becomes an empty set. Since the test link set is the empty set (Step S8; No), the flow set determining section 405 ends the searching process for the test flow set Z. As mentioned above, the test flow set Z, which includes the test flows 500 to 520 and the test flow 700, is recorded on the flow set table 4055. Also, as shown in FIG. 10A, the correspondence relation between the test flows 500 to 520 and 700 and the links through which they pass is recorded onto the flow/link correspondence table. At the step S24, if the test route and the directed link interval P are not identical to each other (Step S24; No), the concatenated directed link searching section 4053 sets the directed link interval P, which does not overlap with the test route, as a new directed link interval P (Step S32). Then, the concatenated directed link searching section 4053 updates the concatenated directed link table 4054 and reports the setting of the directed link interval P to the route searching section 4052. In response to the report of the setting (updating) of the directed link interval P, the route searching section 4052 refers to the directed link interval P recorded on the concatenated directed link table 4054. Then, the route searching section 4052 inquires of each of the terminals 200 on the network about whether or not the new test flow can be generated in the directed link interval P (Step S34). If there is not the terminal 200 that responds to the inquiry (Step S36; No), the process proceeds to the step S20. On the other hand, if there is the terminal 200 that responds to the inquiry (Step S36; Yes), the route overlap investing section 4051 and the route searching section 4052 determine the route from the start point of the directed link interval P to each terminal 200 that can generate a new flow. Then, the route overlap investing section 4051 and the route searching section 4052 select the terminal 200 corresponding to the route, which has the longest overlap with the directed link interval P, as the test terminal T. Moreover, the concatenated directed link searching section 4053 sets the overlap route as the new directed link interval P and sets the non-overlap route as a “directed link interval Q”. The concatenated directed link searching section 4053 updates the concatenated directed link table 4054 (Step S40). When the test terminal T is determined, the route searching section 4052 can determine the test flow generated by the test terminal (Step S42). The test flow is sent from the test terminal T to the start point of the directed link interval P. At the start point, the response destined to the test terminal T is obtained, and the response is sent to the test terminal T and received by the test terminal T. The route searching section 4052 adds the test flow to the test flow set Z indicated on the flow set table 4055. Moreover, the route searching section 4052 sets the test flow sent to a node, which is moved by one hop from the start point of the directed link interval P to the end point. The test flow is sent from the test terminal T to the node, and the response is obtained at the node. The response is sent from the node to the test terminal T and received by the test terminal T. The route searching section 4052 adds the test flow to the test flow set Z (Step S42). If the node is not the end point of the directed link interval P (Step S44; No), the step S42 is again executed (further moved by one hop). When the node serves as the end point of the directed link interval P (Step S44; Yes), the process proceeds to the step S46. After the test flow set Z is set, if the directed link interval Q exists (Step S46; No), the process proceeds to the step S32. In that case, the concatenated directed link searching section 4053 sets the directed link interval Q as a new directed link interval P. If the directed link interval Q does not exist (Step S46; Yes), the process proceeds to the step S26. In accordance with the foregoing procedures, the flow set determining section 405 according to this exemplary embodiment can determine the test flow set Z to specify a link that causes the quality degradation. At this time, even if the link set (L0, L2 to LN) causing the quality degradation in the flow 300 is not continuous, the test flow set Z can be determined such that the sets of the flows which passes through the respective links included in the link set are different from each other. Thus, the respective links can be independently determined. (Generation of Test Flow) When the searching process for the test flow set Z has been ended, the flow set determining section 405 sends a report to the flow generation instructing section 407. In response to the report, the flow generation instructing section 407 refers to the test flow set Z of the flow set table 4055 and issues an instruction to the test terminal T to generate each of the test flows. The terminal 200 (test terminal T) receiving the flow generation instruction generates test flows in response to the instruction. Then, each test terminal T reports the quality data with regard to each test flow to the monitoring server 400. In the foregoing example, the terminal 200-3 generates the test flows 500 to 520 and 700 in response to the instruction from the flow generation instructing section 407. Then, the terminal 200-3 reports the respective quality data of the test flows 500 to 520 and 700 to the monitoring server 400. (Specification of Degradation Link) The flow quality collecting section 402 updates the flow/link correspondence table in accordance with the received quality data. FIG. 10B shows the updated flow/link correspondence table. The quality degradation point estimating section 404 refers to this flow/link correspondence table and estimates a link whose quality is degraded. At first, the link through which the flow having the good quality passes is removed from the candidates of the degradation links. As a result, the link L3 and the link L(N) remain as the candidates of the degradation links. If the link L(N) is assumed to be the degradation link, the reason why the quality of the flow 510 is degraded cannot be explained. On the other hand, if the link L3 is assumed to be the degradation link, the quality data with regard to all of the flows can be explained without any contradiction. Thus, the quality degradation point estimating section 404 determines the link L3 to be the degradation link. 1-3. Effect As described above, the monitoring server 400 according to this exemplary embodiment can set the test flow set Z (test link set) for specifying the degradation link in accordance with the link set through which the flow 300 having the degraded quality passes. Since the respective test flows included in the test flow set Z are generated, the degradation link causing the quality degradation can be specified. At this time, even if the link set (L0, L2 to LN) causing the quality degradation in the flow 300 is not continuous, the test flow set Z can be determined such that the classes of the flows passing through the respective links included in the link set are different from each other. Thus, the respective links can be independently determined. Also, when the test for specifying the link causing the quality degradation is carried out, the link set as a test target has a high possibility that the continuous directed links are included. Using the quality degradation point estimating system according to this exemplary embodiment can efficiently determine the test flow set. According to the determining method of the test flow set Z according to this exemplary embodiment, after the search of the test flow that passes through the directed link interval P for the longest time, it is possible to set the test flow that passes through the directed link interval P for the time shorter than that test flow. For example, the test flows can be successively set by reducing the TTL value or tracing the directed link intervals P in turn. As a result, the number of times of the search for the routing table is reduced. That is, the route having the longest overlap with the concatenated directed links is firstly searched. Then, using the searched route can generate the test flow without any search for the short route. Thus, it is possible to reduce the number of the searches for the test flow set Z to specify the quality degradation point. In addition, this method has a merit that the indefinite link set X can be determined. As a comparison example, according to the conventional technique, the test flows that pass through only one link and do not pass through the links except it are sequentially searched. When the number of the terminals that can generate the test flows is K, the usable flow is determined based on the combinations (K×(K−1)) of the terminals. Also, when the number of the links included in the link set is N, the combinations that pass through only one link and do not pass through the links except it are (N−1). Since the test flow is searched for the (N−1) combinations, the K×(K−1)×(N−1) searches are required. On the other hand, according to this exemplary embodiment, a partial set (test link set) in the test target link set is considered. While this partial set is changed, the test flow is determined, which can reduce the number of times of the search. For example, the search for N times is executed in order to detect the concatenated directed link interval P. After that, the search is executed K times in order to detect the routes from the respective terminals 200 to the end point in the directed link interval P. Also, the search for K times is performed in order to detect the routes from the start point of the directed link interval P to the respective terminals 200. Since those searches are executed independently, the entirely (N+2K) searches are adequate. Thus, the number of the searches is reduced. Moreover, the service in which the response is obtained in the router serving as the end point of the link is used, thereby inverting the direction of the test flow. The property that the flow does not pass through a certain link is jointly used, which greatly reduces the cost necessary for the searching process for the test flow set Z. The quality degradation point estimating system and the quality degradation point estimating method according to the second exemplary embodiment of the present invention will be described below. In the second exemplary embodiment, the same reference numerals and symbols are allocated to the same components as in the first exemplary embodiment, and the duplex explanations are properly omitted. 2-1. Configuration FIG. 11 schematically shows the configuration of the network quality measuring system (the quality degradation point estimating system) according to the second exemplary embodiment of the present invention. The network quality measuring system according to this exemplary embodiment contains a monitoring server 400′ instead of the monitoring server 400 in the first exemplary embodiment. The monitoring server 400′ does not have the route overlap investing section 4051, the concatenated directed link searching section 4053 and the concatenated directed link table 4054 in the first exemplary embodiment. Also, the network quality measuring system according to this exemplary embodiment further contains a terminal 200-6 and a terminal 200-7. It should be noted that this exemplary embodiment is assumed that the flow 300, the flow 310 and the flow 320, which are identical to the first exemplary embodiment, are generated. 2-2. Detailed Process (Generation of Flow/Link Correspondence Table) Similarly to the first exemplary embodiment, the route searching section 4052 in the monitoring server 400′ adds the quality data (the good state, the degraded state, etc.) received from the input output control section 401, to the data received from the route data collecting section 403, and generates the flow/link correspondence table as shown in FIG. 3A. The flow quality collecting section 402 transfers the generated flow/link correspondence table to the flow/link correspondence table managing section 406. The flow/link correspondence table managing section 406 stores the flow/link correspondence table in the storage unit such as the memory and the hard disc. Also, the flow/link correspondence table managing section 406 updates the flow/link correspondence table stored in the storage unit. Then, the flow/link correspondence table managing section 406 reports that the flow/link correspondence table is stored and updated, to the quality degradation point estimating section 404. The quality degradation point estimating section 404 refers to the updated flow/link correspondence table and issues the setting instruction of the test flow to specify the degradation link, to the flow set determining section 405. The flow set determining section 405 determines the test flow set Z to specify the degradation link in response to the setting instruction of the test flow. (Determination of Test Flow Set Z) FIG. 12 is a flowchart showing the determining method of the test flow set Z according to this exemplary embodiment. The quality degradation point estimating section 404 refers to the flow/link correspondence table and sets the link set, through which the flow having the degraded quality passes, to a “test target Link set” (Step S52). Also, the quality degradation point estimating section 404 extracts the link set through which the flow having the good quality passes as the “tested link set” (Step S54). For example, in case of the flow/link correspondence table shown in FIG. 3A, the links L0 to L(N+1) are set as the test target link set. Also, the links L1 and L(N+1) through which the flows 310 and 320 having the good qualities are extracted as the tested link set. Next, the quality degradation point estimating section 404 removes the tested link set from the flow/link correspondence table. Specifically, the quality degradation point estimating section 404 removes the tested link set from the test target link set and consequently determines the test link set (Step S56). For example, in case of the example shown in FIG. 3A, the quality degradation point estimating section 404 removes the tested link set (the links L1, L(N+1)) from the test target link set (the links L0 to L(N+1)) and sets the test link set (the links L0, L2 to LN). The test link set is reported to the flow/link correspondence table managing section 406. The flow/link correspondence table managing section 406 updates the flow/link correspondence table shown in FIG. 3A to that shown in FIG. 3B. If the test link set does not exist (Step S58; No), the quality degradation point does not exist. Thus, the process is ended. If the test link set exists (Step S58; Yes), the quality degradation point estimating section 404 issues the instruction for setting the test flow to the flow set determining section 405. In response to the setting instruction of the test flow, the route searching section 4052 in the flow set determining section 405 extracts one link from the test link set and sets the extracted link as the “targeted link Li” (Step S60). When it is extracted, the targeted link Li is removed from the test link set and added to the tested link set. At this time, the link included in the test link set is referred to as a “non-targeted link”. Next, the route searching section 4052 searches a combination of the terminals 200 (the sending side terminal and the receiving side terminal) that can send a test flow which passes through the targeted link Li and does not pass through the non-targeted link. The combination of the terminals 200 from which the response is obtained serves as the test terminals. The route searching section 4052 sets a flow between the test terminals as the test flow, and adds the test flow to the test flow set Z indicated on the flow set table 4055 (Step S62). After that, the process proceeds to the step S56. Then, until the test link set becomes the empty set, the steps S56 to S62 are repeated. In accordance with the foregoing procedures, the flow set determining section 405 can determine the test flow set Z to specify the degradation link at the high probability. For example, with reference to FIG. 13A, the route searching section 4052 extracts the link L0 as the targeted link Li from the test link set (L0, L2 to N). When it is extracted, the targeted link L0 is removed from the test link set and added to the tested link set. Next, the route searching section 4052 searches a test flow 1 which passes through the targeted link L0 and does not pass through the non-targeted links (L2 to LN). Here, the combination of a transmission terminal and a reception terminal is searched, and a flow 800 in which the terminal 200-0 serves as the transmission terminal and the terminal 200-2 serves as the reception terminal is detected (refer to FIG. 11). Then, as shown in FIG. 13B, the flow 800 is recorded as the test flow 1 on the flow set table 4055. Next, the route searching section 4052 extracts a link L2 as the targeted link Li from the test link set (L2 to LN). When it is extracted, the targeted link L2 is removed from the test link set and added to the tested link set. Next, the route searching section 4052 searches a test flow 2 which passes through the targeted link L2 and does not pass through the non-targeted links (L3 to LN). Here, the already-targeted link L0 is set at “Don't Care (May Pass or May Not Pass) (indicated by * in FIG. 13A). The combination of the transmission terminal and the reception terminal is searched, and a flow 810 in which the terminal 200-0 or terminal 200-3 serves as the transmission terminal, and the terminal 200-6 serves as the reception terminal is detected (refer to FIG. 11). As shown in FIG. 13B, the flow 810 (the transmission terminal 200-3 and the reception terminal 200-6) is recorded as the test flow 2 on the flow set table 4055. Next, the route searching section 4052 extracts the link L3 as the targeted link Li from the test link set (L3 to LN). When it is extracted, the targeted link L3 is removed from the test link set and added to the tested link set. Next, the route searching section 4052 searches a test flow 3 which passes through the targeted link L3 and does not pass through the non-targeted link (LN). Here, the already-targeted links L0 and L2 are set to “Don't Care”. The combination of the transmission terminal and the reception terminal is searched, and a flow 820 in which the terminal 200-0 or terminal 200-3 is serves as the transmission terminal, and the terminal 200-7 serves as the reception terminal is detected (refer to FIG. 11). As shown in FIG. 13B, the flow 810 (the transmission terminal 200-3 and the reception terminal 200-7) is recorded as the test flow 3 on the flow set table 4055. Finally, the route searching section 4052 extracts the link LN from the test link set (LN). When it is extracted, the targeted link LN is removed from the test link set and added to the already-targeted link set. Next, the route searching section 4052 searches a test flow 4 which passes through the targeted link LN and does not pass through the non-targeted link. Here, the already-targeted links L0, L2 and L3 are set to “Don't Care”. As shown in FIG. 11 and FIG. 13B, a flow 830 (the transmission terminal 200-3 and the reception terminal 200-4) is recorded as the test flow 4 on the flow set table 4055. (Generation of Test Flow, Specification of Degradation Link) When the searching process for the test flow set Z has been ended, the flow set determining section 405 reports to the flow generation instructing section 407. In response to the report, the flow generation instructing section 407 refers to the test flow set Z of the flow set table 4055 and issues an instruction to the respective test terminals T to generate the respective test flows. Hereinafter, the specifying process for the degradation link is similar to the first exemplary embodiment, and its explanation is omitted. 2-3. Effect As explained above, according to the second exemplary embodiment, among the degraded link sets (the test targeted link sets), the respective links in the test link set are targeted in turn. Then, the already-targeted link, which passes through the targeted link and does not pass through the non-targeted link, is “Don't Care”, and the flow is searched. Differently from the conventional technique, a flow which passes through only the targeted link and does not pass through the other links is not always targeted. Thus, the search range becomes wide. Therefore, the test flow set Z in which the quality degradation point can be specified can be determined at the higher probability.
	summary
	description	The present invention relates to a handheld radiation detector and, more particularly, to a radiation detector which has a replaceable radiation detection probe. A handheld medical radiation detector is disclosed in U.S. Pat. No. 6,236,880 B1. This radiation detector has a probe and a probe tip detachably mounted to the distal end of the probe. An object of the present invention is to provide a radiation detector which detects radiation from a place to be measured with high accuracy. In an aspect, the present invention relates to a radiation detector comprising a main body, and a radiation detection probe detachably attached to the main body. The radiation detection probe has a detection unit including a radiation detection element, and a first terminal electrically connected to the radiation detection element. The main body has a connector to which the proximal end of the radiation detection probe is detachably mounted. The connector includes a second terminal which is detachably connected to the first terminal when the radiation detection probe is mounted to the connector. A collimator for collimating radiation is provided in the distal end portion of the radiation detection probe. When the distal end of the radiation detection probe is directed toward a place to be measured, the radiation from the place is collimated by the collimator and then enters the radiation detection element. The radiation detection element detects the radiation to generate a detection signal corresponding to its dose. The first terminal receives the detection signal, which is in turn sent to the second terminal. The main body may have a circuit which is electrically connected to the second terminal and which processes the detection signal to determine the radiation dose. Furthermore, the main body may have a device for displaying the resulting radiation dose. Since the collimator limits the angle of incidence of radiation, the incidence of radiation from places other than the place to be measured onto the radiation detection element is prevented or suppressed. Therefore, an enhanced directivity for the detection of radiation is obtained, and therefore it is possible to detect the radiation from the place to be measured with high accuracy. The detection unit may have an input face which transmits the radiation. The radiation detection element may be arranged so as to receive the radiation which has passed through the input face. The collimator may be an opening which faces the input face. In this case, it is not necessary to install the collimator as a single component. Thus, the number of components is reduced, thereby simplifying the structure of the radiation detector. The radiation detection probe and the detection unit may have an elongated shape which extends along the common axis. In this case, the opening as the collimator may extend along the common axis. The radiation detection probe may further have a cap-shaped shield member which is mounted to the detection unit so as to cover the radiation detection element. The shield member is made of a material which blocks the radiation. The shield member may have a front wall facing the radiation detection element and a cylindrical side wall which extends from the edge of the front wall. The collimator may be a through-hole provided in the front wall. The radiation detection probe may further include a cap-shaped probe cover which covers the shield member and the detection unit and is detachably mounted to the connector; and a seal ring sandwiched between the probe cover and the connector to seal the main body and the radiation detection probe when the probe cover is mounted to the connector. To replace the radiation detection element, the probe cover is removed form the connector of the main body, and accordingly the first terminal on the radiation detection probe is separated from the second terminal on the connector. Mounting a new detection unit and a probe cover in the reverse procedures allows the radiation detection element to be replaced on a detection unit basis. Preferably, the radiation detection element is adapted to be separated from the detection unit. In this case, the radiation detection element can be replaced independently. Since the main body and the radiation detection probe are sealed by the seal ring, the radiation detector can be sterilized using a sterilizing gas such as EOG or washed in water. That is, the radiation detector has improved resistance to sterilization and anti-contamination. The shield member may be disposed in the probe cover to allow a hollow portion of the shield member and a hollow portion of the probe cover to communicate with each other. The detection unit is fitted into these hollow portions which communicate with each other. The shield member may be either detachably provided or fixed in the probe cover. The probe cover may have a cap-shaped first component detachably mounted to the connector, a cap-shaped second component detachably attached to the first component to accommodate and fix the shield member, and a seal ring sandwiched between the outer surface of the first component and the inner surface of the second to seal the probe cover when the second component is attached to the first component. The second component may be attached at positions variable along the axis of the probe cover. In this case, the distance between the collimator and the radiation detection element can be adjusted depending on the position of the second component. Consequently, the sensitivity of the radiation detector can be readily adjusted. The probe cover may have an input plate facing the front wall of the shield member, and a cylindrical side wall extending from an edge of the input plate to surround the side surfaces of the shield member and the detection unit. The input plate closes an end of the opening which is the collimat. The input plate is made of a material which transmits the radiation and blocks an electromagnetic wave having an energy of 1 keV or less. Preferably, the interface between the input plate and the side wall is sealed. The detection unit may have a casing for accommodating the radiation detection element. An opening is provided on the distal end of the casing so as to extend from an end face of the casing toward the radiation detection element. This opening may have substantially the same cross-section as that of the above-mentioned opening which is the collimator and communicate with the collimator. The radiation detection probe may further include a cap-shaped probe cover which covers the detection unit and is detachably mounted to the connector, and a seal ring sandwiched between the probe cover and the connector to seal the main body and the radiation detection probe when the probe cover is mounted to the connector. The probe cover may be made of a material which blocks the radiation. The collimator may be an opening provided on the distal end of the probe cover so as to extend toward the radiation detection element. An input plate for closing an end of the collimator may be provided on the distal end surface of the probe cover. The input plate may be made of a material which transmits the radiation and blocks an electromagnetic wave having an energy of 1 keV or less. The connector may further include a support bar protruding from the distal end of the main body and being thinner than the radiation detection probe. The support bar may have a proximal end connected to the distal end of the main body and a distal end connected to the radiation detection probe. The connector may further include a slide member slidably attached to the support bar. The collimator may move along with the slide member. In this case, the distance between the collimator and the radiation detection element varies when the slide member slides relative to the support bar. One of the first and second terminals may be a pin, and the other may be a socket into which the pin is fitted. The pin may include a plurality of pins having different fitting lengths and different polarities. The socket may include a plurality of sockets having fitting lengths and polarities corresponding to these pins. When replacing the detection unit, fitting the pins and sockets having the corresponding fitting lengths to each other reliably prevents the pins and sockets having different polarities from being fitted to each other accidentally. In another aspect, the present invention relates to a radiation detector which includes a main body, and a radiation detection probe detachably attached to the main body. The radiation detection probe has a radiation detection element, and a first terminal electrically connected to the radiation detection element, a cylindrical element cover surrounding the radiation detection element, and a cylindrical casing for accommodating the element cover. The main body has a connector to which the proximal end of the radiation detection probe is detachably mounted. The connector includes a second terminal which is detachably connected to the first terminal when the radiation detection probe is mounted to the connector. The element cover is made of a material which blocks radiation. The radiation detection element is disposed behind the distal end of the element cover. The portion of the element cover placed in front of the radiation detection element not only prevents the sideward radiation incidence on the radiation detection element but also limits the frontward radiation incidence. As a result, the radiation incidence from places other than the place to be measured is prevented or suppressed. Accordingly, it is possible to obtain an improved directivity in the detection of radiation, and detect the radiation from the place to be measured with high accuracy. The radiation detector may further comprise a fastener detachably mounted to the main body to fasten the radiation detection probe to the connector. The radiation detector may further comprise a seal ring sandwiched between the fastener and the connector to seal the main body when the fastener is mounted to the connector. An input plate facing the radiation detection element may be provided on the distal end surface of the casing. The input plate may be made of a material which transmits the radiation and blocks an electromagnetic wave having an energy of 1 keV or less. The present invention will be fully understood when taken with the following detailed descriptions and the accompanying drawings. The accompanying drawings are only illustrative, and therefore it is to be understood that the accompanying drawings are not considered to limit the present invention. A further applicable scope of the invention will become apparent from the following detailed descriptions. However, the detailed descriptions and specific examples describe the preferred embodiments of the invention but are only illustrative thereof. It will thus become apparent to those skilled in the art from the detailed descriptions that various modifications and variations can be made without departing from the scope of the invention. Now, the present invention will be described below in more detail with reference to the accompanying drawings in accordance with the embodiments. In the drawings, identical elements are indicated by identical symbols and overlapping description will be omitted. First Embodiment FIG. 1 is a perspective view showing a radiation detector according to a first embodiment. FIG. 2 is a longitudinal sectional view showing the radiation detector shown in FIG. 1. FIG. 3 is an exploded sectional view showing the radiation detector shown in FIG. 2. FIG. 4 is an enlarged exploded sectional view showing the radiation detection probe shown in FIG. 3. FIG. 5 is an enlarged sectional view showing the assembled radiation detection probe. A radiation detector 100 is a handheld, cordless, surgical probe. As shown in FIG. 1, the radiation detector 100 has a main body 1, and a radiation detection probe 2 provided at the distal end of the main body 1 so as to protrude from the main body 1. The radiation detector 100 is manipulated by grasping the main body 1. For example, the radiation detector 100 is used for detecting a metastatic breast cancer nidus using a radiative medicine. The radiation detection probe 2 is detachably mounted to a support member 1A of the main body 1. The support member 1A is pivotably supported at the distal end of the main body 1. The orientation of the probe 2 can be adjusted by rotating the support member 1A. Behind the support member 1A on the surface of the main body 1, provided are a liquid crystal display panel 1B and a switch 1C. As shown in FIG. 2, the main body 1 is hollow. Although not shown, a signal processing circuit, a drive circuit, an electronic sound generator, a power supply circuit, a battery and the like are provided inside the main body 1. The signal processing circuit processes a detection signal sent from the radiation detection probe 2 to generate a data signal indicative of the radiation dose. The data signal is sent to the drive circuit. The drive circuit displays the radiation dose indicated by the data signal on the display panel 1B as well as drives the electronic sound generator to produce an electronic sound corresponding to the radiation dose. As shown in FIG. 3 and FIG. 4, the radiation detection probe 2 has a detection unit 3, a probe cover 4, a seal ring 5, and a side shield 6. As shown in FIG. 5, the seal ring 5 and the side shield 6 are disposed so as to surround the outer circumference surface of the detection unit 3. The probe cover 4 covers the detection unit 3, the seal ring 5, and the side shield 6. The radiation detection probe 2 and the detection unit 3 have elongated shapes which have a common axis. As shown in FIG. 6, the detection unit 3 has an approximately cylindrical casing 3A. A radiation detection element 7 is disposed inside the distal end of the casing 3A. The radiation detection element 7 has a front face 7A serving as a detecting face to receive radiation and a rear face 7B placed opposite the front face 7A. A coupler 8 for the radiation detection element 7 is provided in the proximal end of the casing 3A. Behind the radiation detection element 7 inside the casing 3A, a partition 3D having a through-hole 3C is formed to define an accommodating portion 3B of the radiation detection element 7. A circular support plate 3E for holding the coupler 8 is fixed on the proximal end of the casing 3A. For example, the casing 3A is made of a resin material such as polyoxymethylene or an electrically conductive metal material. The casing 3A may be composed either of a material which transmits the radiation to be detected or a material which blocks the radiation to be detected. An electrically insulating adhesive 3F such as a silicone resin is filled in the accommodating portion 3B to fix the radiation detection element 7 with its front face 7A facing the distal end of the casing 3A. As the coupler 8, a socket 8A having a longer fitting length and a socket 8B having a shorter fitting length are fixed to the support plate 3E, with the sockets passing through the support plate 3E. The socket 8A having the longer fitting length is connected to the front face 7A of the radiation detection element 7 via a lead wire 9A inserted through the through-hole 3C. The socket 8B having the shorter fitting length is connected to the rear face 7B of the radiation detection element 7 via a lead wire 9B inserted through the through-hole 3C. The detection unit 3 has an input face 3G placed opposite the front face 7A of the detection element 7. The radiation passes through the input face 3G to enter the front face 7A of the detection element 7. The radiation detection element 7 is a semiconductor element which generates a voltage pulse having a pulse height corresponding to the energy of the radiation photon. The detection element 7 may be replaced by a combination of a scintillator which emits light when illuminated with radiation and a photoelectric converter. The scintillator is made of rare-earth oxides such as CdWO4. For example, the photoelectric converter has a structure in which a TFT (Thin Film Transistor) is overlaid on a photodiode. As shown in FIG. 4 and FIG. 5, the probe cover 4 is formed in the shape of a cylindrical cap so as to cover the detection unit 3 and the side shield 6. In this embodiment, the probe cover 4 is made of a material which transmits radiation. An example of this material includes a metal material such as stainless steel or aluminum or an electrically conductive resin material. A front wall 4A disposed on the distal end of the probe cover 4 is reduced in thickness to readily transmit the radiation. An internal thread 4B used to mount the probe 2 onto the main body 1 and an annular shoulder portion 4C for accommodating the seal ring 5 are formed on the inner surface of the proximal end portion of the probe cover 4. The annular shoulder portion 4C is disposed adjacent to the distal end of the internal thread 4B. As shown in FIG. 7 and FIG. 8, on the outer circumferential surface at the proximal end of the probe cover 4, formed is a knurl 4D for screwing operations. The side shield 6 is a component for enhancing the directivity in the detection of radiation. The side shield 6 is made of a material which is capable of blocking the radiation, e.g., lead (Pb) or tungsten (W). This material may be coated with rubber. As shown in FIG. 4 and FIG. 5, the side shield 6 is an approximately cylindrical member which fits into the hollow portion of the probe cover 4. The side shield 6 covers the distal end portion of the detection unit 3. The hollow portion of the side shield 6 engages with the detection unit 3. The front wall disposed on the distal end of the side shield 6 is provided with a radiation-introducing window 6A with a small diameter which window faces the radiation detection element 7 in the detection unit 3. The window 6A is a cylindrical opening which extends coaxially with the side shield 6. The radiation passes through the window 6A to enter the radiation detection element 7. Since the side surface of the radiation detection element 7 is covered with the side shield 6, the radiation from the side of the radiation detection element 7 is prevented from entering the radiation detection element 7. As a result, only the radiation from the direction in which the radiation detection probe 2 is directed is detected, whereby the directivity in the detection of radiation is improved. Furthermore, the side shield 6, which has the window 6A, also serves as a collimator for the radiation. The window 6A is formed coaxial with the side shield 6, thereby allowing only such radiation as traveling approximately in parallel to the axis of the window 6A and the side shield 6 to enter the radiation detection element 7. This is the collimating operation of the window 6A. Such an operation of the collimator will further improve the directivity in the detection of radiation. As shown in FIG. 2 and FIG. 3, the radiation detection probe 2 is detachably mounted to the distal end of the main body 1. The support member 1A disposed at the distal end of the main body 1 has a connector 10, to which the radiation detection probe 2 is detachably mounted. The connector 10 is a cylindrical body which protrudes from the middle of the support member 1A. The connector 10 extends coaxially with the main body 1. The opening of the connector 10 fits detachably over the detection unit 3. As shown in FIG. 4 and FIG. 5, the outer circumferential surface of the connector 10 is formed with an external thread 10A which screws into the internal thread 4B of the probe cover 4. This makes it possible to screw the probe cover 4 over the connector 10. The seal ring 5 is supportably sandwiched between a distal end surface 10B of the connector 10 and the annular shoulder portion 4C of the probe cover 4. In the connector 10, a terminal pin 11A having a longer fitting length and a terminal pin 11B having a shorter fitting length are disposed in parallel to the connector 10. The terminal pins 11A and 11B are electrically connected to a signal processing circuit (not shown) inside the main body 1 via lead wires 12A and 12B. As shown in FIG. 4 and FIG. 7, to mount the radiation detection probe 2 to the distal end of the main body 1, the detection unit 3 is first inserted into the connector 10 of the main body 1 to insert the terminal pins 11A and 11B into the sockets 8A and 8B. The terminal pin 11A having the longer fitting length and the socket 8A having the longer fitting length are connected together, while the terminal pin 11B having the shorter fitting length and the socket 8B having the shorter fitting length are connected together. This makes it possible to reliably prevent a terminal pin and a socket which have different polarities from being accidentally connected together. Thereafter, the seal ring 5 is mounted onto the outer circumference surface of the detection unit 3 so as to abut the distal end surface 10B of the connector 10, while the side shield 6 is fitted into the distal end portion of the probe cover 4. Subsequently, the detection unit 3 is inserted into the probe cover 4 so that the internal thread 4B at the proximal end of the probe cover 4 is engaged with the external thread 10A of the connector 10. This simple procedures allows the cap-shaped probe cover 4 to cover the side shield 6 and the detection unit 3, as shown in FIG. 5, and to be mounted onto the connector 10. The seal ring 5 is sandwiched between the distal end surface 10B of the connector 10 and the annular shoulder portion 4C of the probe cover 4, thereby allowing the main body 1 and the probe 2 to be sealed. For example, the radiation detector 100 having the radiation detection probe 2 mounted thereto is used for detecting a metastatic breast cancer nidus using a radiative medicine. In this case, the radiation detection probe 2, which directly contacts the patient skin, may be sterilized using a sterilizing gas such as ethylene oxide gas (hereinafter referred to as “EOG”) or washed in water. Since the radiation detection probe 2 is sealed by the seal ring 5, no problem will be raised in the sterilization or washing. On the other hand, the radiation detector 100 described in U.S. Pat. No. 6,236,880 B1 does not have such a structure that seals a probe chip containing a radiation detection unit therein. Accordingly, when the probe distal end is sterilized using a gas such as EOG, the gas may intrude into the probe chip to have an adverse effect on the radiation detection element or its signal transmitting system. It is also difficult to wash a contaminated probe chip in water. The operation of the radiation detector 100 will now be described below. During use of the radiation detector 100, the distal end of the radiation detection probe 2 is directed to a portion to be measured of a patient. The radiation coming from the portion to be measured passes through the front wall 4A of the probe cover 4 and the radiation-introducing window 6A of the side shield 6 to enter the radiation detection element 7. The side shield 6 and the window 6A block the radiation from portions other than the portion to be measured. This makes it possible to detect the dose of the radiation from the portion to be measured with high accuracy. The detection element 7 generates a detection signal corresponding to the radiation dose. This detection signal is sent to the signal processing circuit (not shown) in the main body 1 via the lead wires 9A and 9B, the sockets 8A and 8B, the terminal pins 11A and 11B, and the lead wires 12A and 12B. As a result, the data signal indicative of the radiation dose is generated, and the radiation dose is displayed on the liquid crystal display panel 1B. Also, the electronic sound is generated corresponding to the radiation dose. As shown in FIG. 4 and FIG. 8, to replace the radiation detection element 7, procedures reverse to the above-mentioned procedures are followed to remove the probe cover 4 by rotating it in the direction reverse to that in which it is screwed onto the connector 10. Thereafter, the terminal pins 11A and 11B are withdrawn from the sockets 8A and 8B of the detection unit 3 to remove the detection unit 3. Then, the terminal pins 11A and 11B of a new detection unit 3 are inserted into the sockets 8A and 8B of the detection unit 3 to mount the detection unit 3. Thereafter, the above-mentioned procedure is followed to mount the probe cover 4 to the connector 10 along with the seal ring 5. This simple procedure makes it possible to replace the radiation detection element 7 on detection unit 3 basis. Since the side shield 6 can be separated from the probe cover 4 (see FIG. 4), the probe cover 4 can be removed from the connector 10, thereby allowing the side shield 6 to be easily replaced. Thus, a plurality of side shields with a radiation-introducing window 6A having a different length and diameter can be replaced for use, thereby facilitating the adjustment of the sensitivity of the radiation detection element 7. Second Embodiment FIG. 9 to FIG. 11 show the main portion of a radiation detector according to a second embodiment. The second embodiment is different from the first embodiment in the fixing structure of the radiation detection element to be built in the detection unit and the fixing structure for fixing the side shield to the probe cover. Except for these structures, the second embodiment is configured in the same manner as the first embodiment. As shown in FIG. 9 and FIG. 10, the radiation detector according to this embodiment has a structure in which the radiation detection probe 2 according to the first embodiment is replaced by a radiation detection probe 22. The radiation detection probe 22 has a detection unit 23, a probe cover 24, the seal ring 5, and the side shield 6. As shown in FIG. 11, the detection unit 23 includes the radiation detection element 7 at the distal end portion of an approximately cylindrical casing 23A. The casing 23A is made of the same material as that of the casing 3A according to the first embodiment. The casing 23A may also be made of a material which transmits the radiation to be detected or of a material which blocks the radiation to be detected. The hollow portion of the casing 23A includes an increased diameter portion 23B having a larger diameter and a reduced diameter portion 23D having a smaller diameter. The increased diameter portion 23B and the reduced diameter portion 23D form a continuous portion via an annular shoulder portion 23C. The reduced diameter portion 23D is located at the distal end of the casing 23A, and is an opening which extends from the distal end surface of the detection unit 23 toward the radiation detection element 7. The casing 23A includes therein a cylindrical element cover 23E and a fixture member 23F. In this embodiment, the element cover 23E is made of resin, and surrounds and contacts the radiation detection element 7. The element cover 23E may be made of metal, and in this case, the radiation detection element 7 is disposed so as not to contact the detection unit 23. The element cover 23E is fitted into the increased diameter portion 23B with an end thereof being in contact with the annular shoulder portion 23C. The fixture member 23F is fitted into the increased diameter portion 23B so as to abut the other end of the element cover 23E, thereby latching the element cover 23E. Inside the hollow portion 23e of the element cover 23E, the radiation detection element 7 is fixed with its front face (detecting face) 7A being oriented toward the distal end of the element cover 23E. Also, the hollow portion 23e is filled with an electrically insulating adhesive 3F, such as a silicone resin. The lead wires 9A and 9B connected to the front face 7A and rear face 7B of the radiation detection element 7 are connected to the sockets 8A and 8B via through-holes 23G and 23H which are formed in the fixture member 23F. The probe cover 24 has a cylindrical cap-shaped seal structure for covering the detection unit 23. As shown in FIG. 9, the probe cover 24 has a cylindrical body 24A, and an input plate 24B fitted into the opening at the distal end of the cylindrical body 24A. The input plate 24B is fixed to the cylindrical body 24A using an adhesive or the like while the cylindrical body 24A being sealed. The cylindrical body 24A may be made of either a material which transmits the radiation to be detected or a material which blocks the radiation to be detected. The input plate 24B is made of a material which blocks visible and infrared light but transmits the radiation to be detected, e.g., aluminum or amorphous carbon. This is because incidence of electromagnetic waves other than the radiation to be detected upon the radiation detection element 7 generates noise signals. Preferably, the input plate 24B is made of a material which blocks electromagnetic waves having an energy of 1 keV or less but transmits the radiation to be detected. The hollow portion of the cylindrical body 24A includes an increased diameter portion 24C having a larger diameter and a reduced diameter portion 24D having a smaller diameter. The side shield 6 is inserted from the distal end of the cylindrical body 24A into the increased diameter portion 24C and fixed therein, and also the input plate 24B is fixed in the increased diameter portion 24C to seal it. The reduced diameter portion 24D has such a diameter that allows the detection unit 23 to be fitted therein. An annular shoulder portion 24E is provided between the reduced diameter portion 24D and the increased diameter portion 24C so as to abut the proximal end face of the side shield 6. As shown in FIG. 10, the radiation-introducing window 6A of the side shield 6 which is fitted into the increased diameter portion 24C communicates with the reduced diameter portion 23D of the casing 23 to face the radiation detection element 7. The window 6A and the reduced diameter portion 23D have substantially the same cross-section. Like the first embodiment, before using the radiation detector according to the second embodiment, the radiation detection probe 22 may be sterilized using a sterilizing gas such as EOG or washed in water. During the operation of the radiation detector, the high directivity of the radiation detection probe 22 makes it possible to detect the dose of the radiation from the portion to be measured with high accuracy. Furthermore, the radiation detection element 7 of the radiation detection probe 22 can be replaced on a detection unit 23 basis as necessary. In this case, the installation of the side shield 6 is not necessary because the side shield 6 is integrally fixed to the probe cover 24. Third Embodiment The third embodiment is different from the second embodiment in the structure of the probe cover, and the other structures are configured in the same manner as in the second embodiment. That is, the radiation detector according to this embodiment is configured such that the probe cover 24 (see FIG. 9) according to the second embodiment is replaced by a probe cover 34 shown in FIG. 12. In the probe cover 34, the portion corresponding to the cylindrical body 24A shown in FIG. 9 is divided into a proximal end cover 34A and a distal end cover 34B, which can be fastened to each other. These covers 34A and 34B are formed cylindrically to have a common axis. The covers 34A and 34B may be made of either a material which transmits the radiation to be detected or a material which blocks the radiation to be detected. The proximal end cover 34A has the internal thread 4B to be fastened to the connector 10 of the main body 1. The distal end cover 34B integrally includes the side shield 6. When the proximal end cover 34A is fastened to the distal end cover 34B, the outer surfaces of the covers 34A and 34B are flush with each other. A fitting portion 34C is provided at the distal end of the proximal end cover 34A. The fitting portion 34C has an outer diameter smaller than the distal end cover 34B, and protrudes into the hollow portion of the distal end cover 34B to be slidably fitted in the distal end cover 34B. On the inner circumference surface of the distal end cover 34B, a mount groove 34E for a seal ring 34D is annually formed. For example, the seal ring 34D is an O-ring, and seals between the inner surface of the distal end cover 34B and the outer surface of the fitting portion 34C. An internal thread 34F having a diameter larger than that of the outer surface of the fitting portion 34C is formed on the inner surface of the proximal end of the distal end cover 34B. An external thread 34G to engage with the internal thread 34F is formed at the middle of the proximal end cover 34A. It is possible to adjust the position of the distal end cover 34B relative to the proximal end cover 34A along the axis of the probe cover 34 depending on the amount of screwing the internal thread 34F onto the external thread 34G. The distal end cover 34B has the side shield 6 integrally built therein. This allows the side shield 6 to be readily placed closer to or farther away from the radiation detection element 7 in the detection unit 23, thereby facilitating the adjustment of the sensitivity of the radiation detection element 7. Fourth Embodiment The fourth embodiment is different from the second embodiment in the structure of the probe cover, and the other structures are configured in the same manner as in the second embodiment. That is, the radiation detector according to this embodiment is configured such that the probe cover 24 (see FIG. 9) according to the second embodiment is replaced by a probe cover 44 shown in FIG. 13. The probe cover 44 has the input plate 24B secured at the distal end of the probe cover 44 and a cylindrical body 44A extending from the edge of the input plate 24B. The cylindrical body 44A may be made of either a material which transmits the radiation to be detected or a material which blocks the radiation to be detected. The interface between the input plate 24B and the cylindrical body 44A is sealed. The side shield 6 can be detachably fitted into the hollow portion of the cylindrical body 44A. Unlike the above-mentioned embodiments, the side shield 6 is inserted from the proximal end of the probe cover 44 in this embodiment. An annular projection 44C is formed on the inner surface of the distal end of the cylindrical body 44A behind the input plate 24B. The annular projection 44C latches the distal end surface of the side shield 6. In this embodiment, the side shield 6 is detachably housed in the cylindrical body 44A. Accordingly, by removing the probe cover 44 from the connector 10, the side shield 6 can be easily replaced. Thus, a plurality of side shields having a radiation-introducing window 6A with a different length and diameter can be replaced for use, thereby facilitating the adjustment of the sensitivity of the radiation detection element. Fifth Embodiment The fifth embodiment is different from the second embodiment in the structure of the probe cover, and the other structures are configured in the same manner as in the second embodiment. That is, the radiation detector according to this embodiment is configured such that the probe cover 24 (see FIG. 9) according to the second embodiment is replaced by a probe cover 54 shown in FIG. 14. The probe cover 54 has the input plate 24B secured at the distal end of the probe cover 54 and a cylindrical body 54A extending from the edge of the input plate 24B. The interface between the input plate 24B and the cylindrical body 54A is sealed. The cylindrical body 54A is made of a material, e.g., lead (Pb) or tungsten (W), which can block the radiation, and serves as a side shield and a collimator. The radiation-introducing window 6A facing the input plate 24B is formed at the distal end of the cylindrical body 54A. The window 6A serves to collimate the radiation. Furthermore, a unit accommodating portion 54B which communicates with the window 6A is formed in the cylindrical body 54A. The distal end portion of the detection unit 23 can be detachably fitted into the unit accommodating portion 54B. Since the cylindrical body 54A serves as a side shield and a collimator, the side shield 6 as a single component can be eliminated. This allows the number of components to be reduced, thereby simplifying the structure of the radiation detector. Also, the probe cover 54 can be removed from the connector 10, thereby allowing the detection unit 23 to be readily replaced. Furthermore, since the probe cover 54 can block the radiation by itself, it is possible to improve the capability of blocking sideward radiation without increasing the diameter of the radiation detection probe 22, or reduce the diameter of the radiation detection probe 22 while approximately keeping the same capability of blocking radiation. Sixth Embodiment The sixth embodiment is different from the second embodiment in the radiation detection probe and its mounting structure, and the other structures are configured in the same manner as in the second embodiment. A radiation detection probe 32 according to this embodiment has a detection unit 33 shown in FIG. 15 instead of the detection unit 23 (see FIG. 11) in the second embodiment. Furthermore, the radiation detection probe 32 has no probe cover. The detection unit 33 has an element cover 33A instead of the element cover 23E shown in FIG. 11. The element cover 33A has the same shape as that of the element cover 23E. However, unlike the element cover 23E, the element cover 33A is made of a material, e.g., lead (Pb) or tungsten (W), which can block the radiation to be detected. Accordingly, like the side shield 6, the element cover 33A serves as a shield member which prevents sideward radiation from impinging upon the radiation detection element 7. Accordingly, the detection unit 33 does not have the side shield 6 built therein. The element cover 33A surrounds the radiation detection element 7 without contacting the radiation detection element 7. Furthermore, the element cover 33A also serves as a collimator which collimates the radiation. The radiation detection element 7 is not disposed to be flush with the distal end of the element cover 33A but behind the distal end of the element cover 33A. In other words, the radiation detection element 7 is backwardly positioned by a certain distance from the distal end of the element cover 33A toward the proximal end of the element cover 33A. The portion of the element cover 33A placed at the front of the radiation detection element 7 not only prevents the sideward radiation from entering the radiation detection element 7 but also limits the frontward radiation incidence. As a result, only the radiation that travels substantially in parallel to the axis of the element cover 33A enters the radiation detection element 7. The radiation is collimated in this manner, thereby providing an enhanced directivity in the detection of radiation. An input plate 33B is fitted into the distal end of the casing 23A so as to close an end of the reduced diameter portion 23D. The interface between the input plate 33B and the casing 23A is sealed. The input plate 33B functions in the same manner as the input plate 24B shown in FIG. 9. The input plate 33B is made of a material, e.g., aluminum or amorphous carbon, which blocks visible and infrared light but transmits the radiation. This is because incidence of electromagnetic waves other than the radiation to be detected upon the radiation detection element 7 generates noise signals. Preferably, the input plate 33B is made of a material which blocks electromagnetic waves having an energy of 1 keV or less. Other structures are the same as those of the detection unit 23, and thus detailed description will be omitted. Since the detection unit 33 is sealed by the input plate 33B, the probe cover 24 shown in FIG. 9 is not required. That is, the radiation detection probe 32 according to this embodiment is configured of the detection unit 33 and the sockets 8A and 8B. In this embodiment, a coupling nut 84 replacing the probe cover 34, as a fastener for the detection unit 33, is mounted onto the proximal end portion of the element cover 33A. The detection unit 33 is fitted into the connector 10 and an internal thread 84A of the coupling nut 84 is screwed onto the external thread 10A of the connector 10, thereby attaching the detection unit 33 to the connector 10. At this time, the seal ring 5 is sandwiched between the distal end surface 10B of the connector 10 and a top wall 84B of the coupling nut 84 facing the distal end surface 10B. Consequently, the seal ring 5 is closely attached to the outer circumferential surface of the casing 23A, whereby the connector 10 and the main body 1 are sealed. The input plate 33B seals the detection unit 33, while the seal ring 5 seals the main body 1. This allows the radiation detection probe according to this embodiment to be sterilized using a sterilizing gas such as EOG or washed in water. Also, it is possible to replace the radiation detection element 7 on a detection unit 33 basis by removing the coupling nut 64 from the connector 10. Seventh Embodiment The seventh embodiment is different from the first and second embodiments in the radiation detection probe and its mounting structure, and the other structures are configured in the same manner as in those embodiments. As shown in FIG. 16, a radiation detection probe 62 has the detection unit 3, a probe cover 64, and a side shield 66. The detection unit 3 is housed in the side shield 66 which is in turn housed in the probe cover 64. The probe cover 64 is formed in the shape of a cylindrical cap so as to cover the entirety of the side shield 66. For example, the probe cover 64 is made of a metal material such as stainless steel or aluminum or of an electrically conductive resin material. The probe cover 64 may be made of either a material which transmits the radiation to be detected or a material which blocks the radiation to be detected. At the center of a front wall 64A of the probe cover 4, there is formed an opening 64B which facilitates the transmission of the radiation. On the inner surface at the proximal end portion of the probe cover 64, there are formed an internal thread 64C used to mount the probe 2 to the main body 1 and an annular shoulder portion 64D for receiving the seal ring 5. The annular shoulder portion 64D is disposed adjacent to the distal end of the internal thread 64C. The side shield 66 has a structure similar to that of the side shield 6 according to the first embodiment. However, the side shield 66 is longer than the side shield 6. The side shield 66 is made of a material, e.g., lead (Pb) or tungsten (W), which can block the radiation. The side shield 66 fits into the hollow portion of the probe cover 64. The hollow portion of the side shield 66 fits onto the detection unit 3. A radiation-introducing window 66A is perforated in the front wall of the side shield 66. The radiation enters the radiation detection element 7 through the window 66A. The side shield 66 prevents the sideward radiation from entering the radiation detection element 7. Also, the radiation-introducing window 66A serves as a collimator for the radiation. The radiation detection probe 62 is detachably mounted to the distal end of the main body 1. The support member 1A disposed at the distal end of the main body 1 has a connector 70 to which the radiation detection probe 62 is detachably mounted. The connector 70 has an elongated support bar 70A which protrudes from the middle of the support member 1A. The support bar 70A is a cylindrical body extending coaxially with the main body 1 and has an outer diameter smaller than the outer diameter of the radiation detection probe 62. A connector main body 70B is attached to the middle portion of the support bar 70A. The support bar 70A passes through the main body 70B, and the main body 70B is able to slide relative to the support bar 70A. The front half portion of the main body 70B has an external thread 70C which screws into the internal thread 64C of the probe cover 64. The seal ring 5 is mounted to the distal end of the main body 70B adjacent to the external thread 70C. A cylindrical socket 70D which fits onto the detection unit 3 is provided on the distal end of the support bar 70A. The socket 70D has an outer diameter larger than the outer diameter of the support bar 70A. The detection unit 3 can be detachably fitted into the socket 70D. A groove into which a seal ring 90 is fitted is circumferentially formed on the side surface of the socket 70D. The terminal pins 11A and 11B are disposed in the opening of the socket 70D. These terminal pins are connected to a signal processing circuit (not shown) in the main body 1 via lead wires which extend inside the support bar 70A. In this embodiment, a threaded hole 91 is provided in the side surface of the detection unit 3. Also, a through-hole 92 is provided in the side surface of the socket 70D. Inserting the detection unit 3 into the opening of the socket 70D will allow the threaded hole 91 and the through-hole 92 to be aligned. A screw 93 can be screwed into the threaded hole 91 and the through-hole 92. As shown in FIG. 16, since the connector 70 has the external thread 70C, the proximal end portion of the probe cover 64 can be fastened to the connector 70. After fastening the proximal end portion, the seal ring 5 is sandwiched and held between the distal end surface 10B of the connector 10 and the annular shoulder portion 64C of the probe cover 64. To mount the radiation detection probe 62 to the distal end of the main body 1, the detection unit 3 is first inserted into the connector 70D, thereby inserting the terminal pins 11A and 11B into the sockets 8A and 8B. Thereafter, the screw 93 is screwed into the threaded hole 91 and the through-hole 92 to tightly fix the detection unit 3 to the socket 70D. The seal ring 90 is also mounted to the socket 70D. Then, the seal ring 5 is mounted to the connector 70 and the side shield 66 is fitted into the distal end portion of the probe cover 64. Subsequently, the probe cover 64 is attached to the connector 70 so that the detection unit 3 is inserted into the side shield 66, and the internal thread 64C of the probe cover 64 is engaged with the external thread 70C of the connector 70. This causes the seal ring 5 to be sandwiched between the distal end surface of the connector 70 and the annular shoulder portion 64C of the probe cover 64. The seal ring 90 is also sandwiched between the outer surface of the socket 70D and the inner surface of the side shield 66. This simple procedure allows the cap-shaped probe cover 64 to be mounted to the connector 70 so as to cover the side shield 66 and the detection unit 3. When the radiation detection probe 62 is mounted to the connector 70, the probe cover 64 and the side shield 66 are able to slide along with the connector main body 70B relative to the support bar 70A. On the other hand, the detection unit 3 mounted to the socket 70D cannot move relative to the support bar 70A. Accordingly, sliding the radiation detection probe 62 will cause the side shield 66 to move relative to the detection unit 3 along the axis of the probe 62. However, during this movement, the radiation detection element 7 always stays within the side shield 66. Since the detection unit 3 is attached to the socket 70D by the screw, the detection unit 3 will never fall off from the socket 70D while the side shield 66 is sliding. The seal ring 90 sandwiched between the side shield 66 and the socket 70D not only seals the radiation detection element 7 but also serves as an anti-skidding means while the side shield 66 is sliding. This embodiment also has an advantage that the orientation of the radiation detection probe 62 can be readily determined because the probe 62 is connected to the main body 1 via the elongated support bar 70A. According to the first to the sixth embodiments, the radiation detection probe larger in diameter than the support bar 70A is directly connected to the main body 1. Hence, when the user grasps the main body to direct the probe toward a place to be measured, the proximal end portion of the probe can obstruct the line of sight, thereby obscuring the distal end of the probe. In contrast to this, according to this embodiment, the support bar 70A which is thinner than the radiation detection probe 62 is provided between the probe 62 and the main body 1, thereby allowing the distal end of the radiation detection probe 62 to be readily viewed. Accordingly, the user can readily determine the orientation of the probe 62, thereby smoothly proceeding with the detection of the radiation. Furthermore, sliding the main body 70B along the support bar 70A makes it possible to readily position the radiation-introducing window 66A, which is a collimator, closer to or farther away from the radiation detection element 7. Consequently, it is possible to easily adjust the sensitivity of the radiation detection element 7. Eighth Embodiment The eighth embodiment is different from the first embodiment in the fixing structure of a socket for a terminal pin, and the other structures are configured in the same manner as in the first embodiment. As shown in FIG. 17 and FIG. 18, sockets 78A and 78B to be employed in this embodiment are fixed to the proximal end of a casing 73A using a socket fixture member 15 and a socket cover 16 instead of the support plate 3E shown in FIG. 6. The casing 73A is different from the casing 3A only in the structure of the proximal end, and the other structures are configured in the same manner as those of the casing 3A. The casing 73A has an opening 73B at its proximal end for receiving the distal end of the socket cover 16. The casing 73A may be composed either of a material which transmits the radiation to be detected or a material which blocks the radiation to be detected. The socket fixture member 15 is an approximately cylindrical column and made of an electrically highly insulating material. In one end face of the socket fixture member 15, socket accommodating holes 15A and 15B are perforated into which the socket 78A and 78B are fitted. At the center of the end face, a threaded hole 15E used to fasten the socket cover 16 is formed. In the opposite end face of the socket fixture member 15, through-holes 15C and 15D are perforated which have the diameters smaller than those of the socket accommodating holes 15A and 15B. When sockets 68A and 68B are inserted into the socket accommodating holes 15A and 15B, connection pins 78C and 78D that protrude from the ends of the sockets 8A and 8B will penetrate the through-holes 15C and 15D. The socket cover 16 is formed in the shape of a cap so as to cover the socket fixture member 15. In an upper wall 16D of the socket cover 16, through-holes 16A and 16B corresponding to the sockets 8A and 8B are provided. When the sockets 8A and 8B are fitted into the socket accommodating holes 15A and 15B, the sockets 8A and 8B will face the through-holes 16A and 16B. At the center of the upper wall 16D, there is also formed a through-hole 16C for a set screw 17. The set screw 17 is screwed into the threaded hole 15E of the socket fixture member 15 through the hole 16C. As shown in FIG. 17, the socket cover 16 receives the socket fixture member 15, and is fixed to the socket fixture member 15 using the set screw 17. The socket cover 16 has a spigot 16E at its distal end. The spigot 16E is fitted into the opening 73B of the casing 73A and adhered to the casing 73A. In the foregoing, the present invention has been described in detail in accordance with the embodiments. However, the present invention is not limited to the above-mentioned embodiments. Various modifications can be made to the present invention without deviating from the scope of the invention. In the first to seventh embodiments, the detection unit includes the sockets 8A and 8B for terminal pins having different fitting lengths, however, instead of this, the detection unit may include sockets having an equal fitting length. The main body 1 includes the terminal pins 11A and 11B having different fitting lengths, however, instead of this, the main body 1 may include terminal pins having an equal fitting length. Furthermore, the detection unit may be provided with a terminal pin, while the connector of the main body may be provided with a socket for the terminal pin. The distal end portion of the radiation detection probe 2 shown in FIG. 5 and the radiation detection probe 22 shown in FIG. 10 is not limited to a planar shape, and may be a rounded shape such as a spherical shape. In the radiation detector 100 shown in FIG. 1, the radiation detection probe 2 is mounted while being inclined with respect to the axis of the main body 1. However, the radiation detection probe 2 may be mounted to protrude along the axis of the main body 1. Furthermore, the ratio between the diameter and the length of the radiation detection probe 2 is not limited to those in the examples shown in the figures, and may be modified as appropriate. The radiation detector according to the above-mentioned embodiments is a medical surgical probe; however, the use of the radiation detector of the present invention is not limited thereto, and the radiation detector of the present invention may be employed in a wide range of other applications. The radiation detector in accordance with the present invention collimates radiation from a place to be measured and introduce the radiation to the radiation detection element. This makes it possible to detect the dose of the radiation from the place to be measured with high accuracy. Furthermore, since the detection unit having the radiation detection element is detachably attached to the connector of the main body, it is possible to replace the radiation detection element with simple operation.
	abstract	Devices and methods are provided for generating laser-accelerated high energy polyenergetic positive ion beams that are spatially separated and modulated based on energy level. The spatially separated and modulated high energy polyenergetic positive ion beams are used for radiation therapy. In addition, methods are provided for treating patients in radiation treatment centers using therapeutically suitable high energy polyenergetic positive ion beams that are provided by spatially separating and modulating positive ion beams. The production of radioisotopes using spatially separated and modulated laser-accelerated high energy polyenergetic positive ion beams is also provided.
	abstract	In one characterization, the present invention relates to a radiation-shielding assembly for holding a container having a radioactive material disposed therein. The assembly may, at least in one regard, be referred to as an elution shield and/or a dispensing shield. The assembly includes a body at least partially defining a cavity. There is at least one opening through the body into the cavity. The assembly may include a cap that at least generally hinders escape of radiation from the assembly through the opening. The cap may be releasably attached to the body in one orientation and may establish non-attached engagement with the body in another orientation. The assembly may include an adjustable spacer system for adapting the assembly for use with containers having different heights.
	summary
052727416	claims	1. A nuclear fuel assembly comprising: a plurality of fuel rods; a polygonal channel box surrounding said fuel rods; a plurality of round cell type spacers disposed in several stages in a longitudinal direction within said channel box, each of said spacers having a plurality of cylindrical cells for holding respective ones of said plurality of fuel rods therein and for keeping said fuel rods spaced from each other; and a plurality of vanes each extending along an outer cylindrical surface of each cylindrical cell in the longitudinal direction with an inclination to the longitudinal direction and disposed only in a corner region at and around a corner, within said channel box, farthest from a corner facing a control rod disposed adjacent to said channel box for imparting swirling motion to fluid flowing along said fuel rods in said region. a plurality of fuel rods; a channel box surrounding said fuel rods; and a plurality of spacers disposed in a plurality of stages in a longitudinal direction of said fuel rods within said channel box for keeping said fuel rods spaced from each other; wherein at least one of said spacers have a plurality of vane-formed cylindrical cells each having at least one vane on an outer cylindrical surface thereof extending along the outer cylindrical surface in the longitudinal direction for generating swirling fluid flows and disposed at a farther region from a control rod and smooth surface cylindrical cells each without vanes on an outer cylindrical surface thereof and disposed in a closer region to the control rod than the farther region. 2. The nuclear fuel assembly according to claim 1, wherein said vanes generate such swirling flows as direct liquid drops to said fuel rods, in spaces defined by opposite adjacent fuel rods, in spaces defined by said channel box and said fuel rods facing said channel box, in said corner region. 3. The nuclear fuel assembly according to claim 2, wherein and at least one of said round cell type spacers disposed in an upper region of said fuel rods is provided with said vanes formed in said outer cylindrical surfaces of said cylindrical cells in said corner region. 4. The nuclear fuel assembly according to claim 3, wherein said vanes are provided on said round cell spacers of a second upper stage and a third upper stage of said several stages, a first upper stage and other stages of said several stages having cylindrical cells without said vanes. 5. The nuclear fuel assembly according to claim 2, wherein said corner region is defined by adjacent two sides of said round type cell spacer within said channel box and said vanes are provided on said cylindrical cells for fuel rods in first and second rows each of said two sides. 6. The nuclear fuel assembly according to claim 5, wherein said vanes each are provided so as to project substantially normally to said outer cylindrical surface of each cell in said corner region and with an inclination to the axial direction. 7. The nuclear fuel assembly according to claim 2, wherein said cylindrical cells are surrounded and tied by a side band, and said vanes are provided on said side band. 8. The nuclear fuel assembly according to claim 7, wherein said side band with said vanes is provided on each of second and third upper stages of said round cell spacer of said several stages, a first upper stage and other stages of said several stages having a side band without said vanes. 9. The nuclear fuel assembly according to claim 2, wherein said vanes are provided on inner sides of said channel box defining said corner region so as to incline against the longitudinal direction. 10. The nuclear fuel assembly comprising: 11. The nuclear fuel assembly according to claim 10, wherein said channel box is polygonal in cross-section, and said farther region is a corner and an adjacent region to the corner, said corner being defined by two sides of said channel box and farthest from said control rod adjacent to said channel box. 12. The nuclear fuel assembly according to claim 11, wherein said second and third upper stages of said plurality of stages of said spacers have said vane-formed cylindrical cells each having a vane on said outer surface thereof for generating swirling fluid flows in said corner region and smooth surface cylindrical cells having no vane thereon in said closer region other than said corner region, a first upper stage and other states of said plurality of stages of said spaces having smooth surface cylindrical cells. 13. The nuclear fuel assembly according to claim 11, wherein said vanes each are formed on the outer surface of said cell to project therefrom, extend axially and be inclined against a plane on which an axis of said cylindrical cell lie. 14. The nuclear fuel assembly according to claim 11, wherein the number of said vanes is at most 15. 15. The nuclear fuel assembly according to claim 11, wherein said cylindrical walls have rectangular cut out portions extending axially at cylindrical walls thereof, are inverted alternatively and are joined to adjacent cylindrical cells so that the cut out portions receive a part of said cylindrical wall and the assembled two cylindrical cells are partially overlapped axially and laterally. 16. The nuclear fuel assembly according to claim 1, wherein said control rod is disposed at least at a center position among a grouping of four adjacent ones of the nuclear fuel assembly. 17. The nuclear fuel assembly according to claim 10, wherein said control rod is disposed at least at a center position among a grouping of four adjacent ones of the nuclear fuel assembly.
	summary
	description	Priority is claimed to French Application Serial No. FR 06 07424 filed Aug. 21, 2006 through International Patent Application Serial No. PCT/FR2007/001032, filed Jun. 21, 2007. The invention relates in general to transport containers for nuclear fuel assemblies. To be more precise, the invention relates, according to a first aspect, to a transport container for nuclear fuel assemblies of elongate shape in a longitudinal direction, of the type comprising a support having at least a first longitudinal bearing surface delimiting a longitudinal housing for receiving a nuclear fuel assembly, and a door having a second longitudinal bearing surface, the door being movable between a position holding the nuclear fuel assembly between the two longitudinal surfaces and a release position in which the assembly is free with respect to the support. The document WO-99/41754 describes such a container. In this container, the second longitudinal surface rests on a nuclear fuel assembly located in the housing by means of bearing runners mounted movably on the door. These runners are distributed longitudinally so that they each rest on a grid of the skeleton of the nuclear fuel assembly. Since each type of assembly has a specific cross-section and specific grid positions, it is necessary to use a specific door for each type of assembly, which is complicated and expensive. In this context, an object of the invention is to provide a container which is suitable for transporting several types of assembly and which is more readily adaptable to each of them. To that end, the invention relates to a transport container of the above-mentioned type, characterized in that it comprises means for adjusting the spacing between the first and second surfaces in the holding position of the door. The container may also have one or more of the following features, considered individually or in accordance with any technically possible combination: the first surface comprises a first pair of longitudinal faces arranged in the shape of a V, and the second surface comprises a second pair of longitudinal faces which are arranged in the shape of a V and which are parallel with and opposite the faces of the first pair when the door is in the holding position; the first and second pairs of faces in a V shape converge towards first and second vertices, respectively, the adjusting means comprising means for adjusting the position of the door with respect to the support by translation of the door in a transverse adjusting direction passing via the first and second vertices when the door is in the holding position; the support comprises parallel longitudinal surfaces for guiding the translation of the door in the direction of adjustment; the container comprises means for displacing the door with respect to the support between its holding and release positions by translation in the direction of adjustment and then rotation about at least one longitudinal shaft; the faces of the first pair form between them an angle substantially equal to that which the faces of the second pair form between them, this angle being from 60° to 135°; the second longitudinal bearing surface is free from movable runners for resting on a nuclear fuel assembly; and the second longitudinal bearing surface is suitable for resting directly on a nuclear fuel assembly. According to a second aspect, the invention relates to the use of a container as defined above for transporting a nuclear fuel assembly. According to one variant, the container is used with the same support and the same door to transport nuclear fuel assemblies of at least two different types. FIGS. 1 and 2 show a container 1 for transporting fresh fuel assemblies for a pressurized water nuclear reactor. The transport container 1, which is intended to transport two fuel assemblies in the horizontal position, comprises an external casing 2 formed by a lower half-shell 2a and an upper half-shell 2b mounted one on top of the other in accordance with a junction plane. Each of the half-shells 2a and 2b is produced from sheet-steel and comprises respective reinforcing bows 3a, 3b distributed along the length of the half-shell. Sectional members 4 and 4′ forming support feet for the container are also secured to the lower portion of the lower half-shell 2a. In addition, adjustable bearing members 5 and 5′ which comprise screw jacks and which are fixedly joined to a longitudinal end portion of the container enable the inclination of the container resting on a support surface to be adjusted, about the longitudinal axis of the container and about a transverse axis, respectively. The two half-shells 2a and 2b are mounted one on top of the other by way of peripheral end-plates constituting an upper flat bearing portion of the lower half-shell 2a and a flat lower bearing portion of the upper half-shell 2b of the container. In the closed position of the container shown in FIGS. 1 and 2, the end-plates of the two half-shells 2a and 2b are mounted and secured one on top of the other by screws and nuts and form an assembly flange 6. FIGS. 3A and 3B show a portion of the container in the open state, that is to say, with the upper half-shell of the casing of the container separated from the lower half-shell and removed. In FIGS. 3A and 3B it is possible to see the internal structure of the container which is indicated in a general manner by the reference 7 and which comprises, in particular, a cradle 8 which rests on supports 9 formed by damper studs, in the lower half-shell 2a of the external casing 2 of the container. A second portion of the internal structure of the container is formed by an assembly 10 for receiving and supporting two fuel assemblies placed side by side in a horizontal position. The assembly 10, which rests on the cradle 8, delimits two completely closed housings for two fuel assemblies, as will be explained hereinafter. The cradle 8 comprises two longitudinal members 8a, 8b formed by angle beams which are secured to the support studs 9 and which are maintained in parallel arrangements with a spacing corresponding to the width of the receiving assembly 10 by cross-members 8c. At one of its ends, the cradle comprises an assembly for stiffening and for pivot mounting, comprising two plates 11a and 11b which are parallel with each other, and two cross-members 12 formed by hollow sectional members secured below the longitudinal members of the cradle and to the plates 11a and 11b. The pivot mounting of the assembly 10 on the lower half-shell of the container, about a horizontal axis of transverse direction, is ensured by means of the stiffening and pivot-mounting assembly comprising the plates 11a and 11b. In addition, as will be explained hereinafter, a retaining plate 11c for the fuel assemblies is also mounted between the plates 11a and 11b. As shown in FIG. 3B, in order to limit the effect of impact on the fuel assemblies, for example the effect of the container 1 falling, a buffer 43 is interposed between the longitudinal end of the internal structure 7 and the internal end wall of the external casing 2, of circular shape. The buffer 43, in the form of a disc, the cross-section of which is identical to the internal cross-section of the container casing, is formed by a disc of balsa wood surrounded by a casing of stainless sheet-steel. An identical buffer is located at the second longitudinal end of the container, between the second longitudinal end of the internal structure and the second end of the external casing. As can be seen in FIG. 4, the assembly 10 comprises a parallelepipedal support 13 in which the housings 15A and 15B for receiving a nuclear fuel assembly are formed, and two doors 17A and 17B capable of closing the housings 15A and 15B. The support 13 is elongate longitudinally and has a rectangular cross-section which is constant over the entire longitudinal length of the support 13. The two housings 15A and 15B extend longitudinally, parallel with each other, and open out in an upper face 19 of the support 13. The housings 15A and 15B are identical. Only one of them will be described below. Likewise, the doors 17A and 17B are identical, and only one of them will be described below. The base of the housing 15B is delimited by a first V-shaped bearing surface 21, comprising a first pair of longitudinal faces 23 forming an angle of 90° between them. The first pair of faces 23, viewed in cross-section, converges towards a vertex 25, corresponding to the deepest point of the housing 15B and where the faces 23 join. The two faces 23 continue towards the top of FIG. 4, that is to say, towards the upper face 19, by way of two lower guide surfaces 27, which are parallel with each other and perpendicular to the face 19, then by way of two upper guide surfaces 29, which are also parallel with each other and perpendicular to the face 19. The surfaces 27 have between them a transverse spacing less than that of the surfaces 29, with the result that shoulders 31 are formed between the surfaces 27 and 29. The door 17B extends over the entire longitudinal length of the housing 15B. It is movable between a position, shown in FIG. 4, of holding the nuclear fuel assembly in the housing 15B, and a release position in which the assembly is free with respect to the support 13 and which is shown in FIG. 5. These positions will be described in detail hereinafter. The door 17B comprises an upper portion 33 and a lower portion 35 of reduced width compared with the portion 33, the width corresponding to the transverse direction when the door is in the holding position. The upper portion 33 therefore comprises two lateral edges 36 projecting one on each side of the portion 35. The respective widths of the portions 33 and 35 correspond to the transverse spacing between the upper guide surfaces 29 and the lower guide surfaces 27, respectively, and are constant along the entire housing 15B. The upper portion 33 is delimited on the side remote from the portion 35 by a substantially flat upper surface 37. The lower portion 35 is delimited on the side remote from the portion 33 by a second longitudinal bearing surface 39 having, in a transverse plane, the shape of a W. The second bearing surface 39 comprises at the centre a second pair of longitudinal faces 41 arranged in the shape of a V and forming an angle of 90° between them. The faces 41 converge towards a second vertex 43 where they join. The second bearing face 39 also comprises two lateral faces 45 extending the faces 41 away from the vertex 43. The faces 45 are substantially perpendicular to the faces 41. The faces 23 of the first pair are wider than the faces 41 of the second pair, viewed in a transverse plane. For each door 17A, 17B, the assembly 10 also comprises means for displacing the door with respect to the support 13 between its holding and release positions, these means also enabling the spacing between the first and second surfaces 23 and 39 to be adjusted when the door occupies its holding position. Only the means for displacing the door 17B will be described here, those of the door 17A being identical. The displacement means comprise, for example, two screws 47 mounted to rotate freely on the support 13, a plurality of nuts 49 which are movable along the screws 47 and which are each provided with two shaft ends 51 (FIG. 6), the door 17B being mounted to be movable in rotation about the shaft ends 51 and being connected, in terms of translation along the screws 47, to the nuts 49. As can be seen in FIG. 4, the screws 47 extend in a vertical direction in FIG. 4, perpendicularly to the upper face 19. They are engaged by their free ends in bearings 53 formed in the shoulder 31 of the support 13. The screws 47 are fixed in terms of translation vertically in the bearings 53 and are free to rotate in these bearings. The bearings 53 are located in the shoulder 31 furthest away from the housing 15A. The screws 47 are distributed longitudinally along the door 17B. The vertical length of the screws 47 is such that their heads 55 are located outside the support 13, projecting above the upper face 19. As can be seen in FIGS. 4 to 6, the door 17B comprises, in the region of each screw 47, a recess 57 formed in the edge 36 of the upper portion 33. The recesses 57 are formed through the entire vertical thickness of the edge 36, the screws 47 passing through the recesses 57. The nuts 49 are located in the recesses 57. The door 17B also comprises blind holes 59 formed longitudinally in the thickness of the edge 36 and opening out in each recess 57. As shown in FIG. 6, the shaft ends 51 are fixedly joined to the nuts 49 and extend longitudinally from the nuts 49. They are engaged in the blind holes 59 and can rotate freely in these holes. The assembly 10 also comprises for each housing 15A, 15B a plurality of threaded orifices 61 formed in the shoulder 31 remote from the screws 47, and a plurality of screws 63 for securing the door 17A, 17B in the holding position, which screws 63 can be screwed into the orifices 61. The number of screws 63 may be, for example, from ten to fifteen. Only the means for securing the door 17B will be described here. As shown in FIG. 4, each of the securing screws 63 comprises a threaded end portion 65, a head 67 remote from the portion 65, and a smooth portion 69 interposed between the head 67 and the threaded portion 65. The door 17B comprises a plurality of smooth holes 71 (FIG. 5) formed in the edge 36 located on the same side as the housing 15A in the holding position. The screws 63 are engaged in the smooth holes 71, the smooth portion 69 being located in the smooth hole 71, the head 67 bearing against the upper face 37 of the door 17B, and the threaded portion 65 being screwed into the threaded orifice 61 of the support. The orifices 61 and 71 and the securing screws 63 are distributed regularly along the housing 15B. In addition, each of the doors 17A, 17B comprises two handles 73 projecting towards the top relative to the face 37. These handles 73 are located in the vicinity of the longitudinal ends of the doors. When the door 17A, 17B is in the holding position, its upper portion 33 is engaged between the upper guide surfaces 29 and the lower portion 35 is engaged between the lower guide surfaces 27. The second bearing surface 39 faces the base of the housing 15A, 15B, and the faces 41 of the second pair are parallel with and opposite the faces 23 of the first pair. The first and second vertices 23 and 43 are then aligned vertically in FIG. 4, that is to say, in a direction perpendicular to the face 19, and the upper surface 37 is parallel with the face 19. The release position of the door 17B is illustrated in FIG. 5. In this position, the door 17B is mounted to the maximum extent along the screws 47 and is swung towards the outside of the housing 15B about the shafts 51. The nuts 49 are in abutment with the heads 55 of the screws 47. The upper surface 37 of the door 17B extends substantially horizontally, at the level of the upper face 19, the second bearing surface 39 facing the top of FIG. 5 and away from the housing 15B. The release position of the door 17A is symmetrical with the release position of the door 17B relative to a centre longitudinal plane of the housings 15A and 15B. The operation of the container described above will now be explained. In order to load nuclear fuel assemblies into the container, the two half-shells 2a and 2b are first of all detached from each other by unscrewing the screws of the flange 6, and the upper half-shell 2b is removed. The assembly 10 is then detached from the cradle 8 and the assembly 10 is swung into a substantially vertical position about the transverse axis located at one of the ends of the cradle. The doors 17A and 17B are then placed in the release position in order to give access to the housings 15A and 15B. A nuclear fuel assembly can then be placed in each of the housings 15A and 15B, by a fuel assembly lifting tool, such as the winch of a travelling crane, by displacing the assembly horizontally, in accordance with the arrow F1 in FIG. 5. The fuel assemblies come to rest, by way of their lower ends, on the fuel assembly support plate 11C secured between the two plates 11A and 11B of the assembly 10. In the case of fuel assemblies having a square cross-section, an assembly is placed in each housing 15A, 15B in such a manner that two adjacent lateral sides of this assembly rest on the faces 23 of the first bearing surface 21, as illustrated on the left in FIG. 5. The edge separating the two adjacent sides of the fuel assembly is located along the vertex 25. Once the assemblies are in place in the housings 15A, 15B, the doors 17A, 17B are closed. For that purpose, each door 17A, 17B is caused to pivot about the shafts 51, through approximately 180°, the door then occupying an intermediate position illustrated on the left in FIG. 5. In this intermediate position, the lower portion 35 of the door is engaged in the housing, the faces 41 of the second bearing surface 39 being separated from the fuel assembly by a free space. The screws 47 are then caused to turn in the bearings 53 by means of suitable tools in order to cause the nuts 49 to descend along the screws 47, the shaft ends 51 driving the door towards the assembly located in the housing. When the faces 41 of the second bearing surface 39 come into contact with the nuclear fuel assembly, the translational movement of the cover 17A, 17B is interrupted. It will be noted that the second bearing surface 39 comes into direct contact with the nuclear fuel assembly. In particular, the door 17B, just like the door 17A, is free from bearing runners, such as those provided in the prior art at right-angles to each of the grids of a nuclear fuel assembly to be transported. It will be appreciated that the translational movement is effected in a direction symbolized by the arrow F1 in FIG. 5 and passing via the vertices 25 and 43 of the two bearing surfaces 21 and 39. This second part of the movement of the door 17A, 17B enables the spacing between the first and second bearing surfaces 21 and 39 to be adjusted in the holding position of the door, in accordance with the size of the fuel. For, as shown in FIG. 4, for fuel having a square cross-section of large size, the translational movement of the door 17A, 17B will be interrupted earlier. The cross-section of such fuel is symbolized by the line marked CG on the right in FIG. 4. In that case, the faces 41 of the second bearing surface 39 bear against the two adjacent sides of the fuel that face the top of FIG. 4, but cover only a portion of those sides. A band 74 of those sides remains free, between the first and second surfaces 21 and 39. For fuel having a cross-section of intermediate size, symbolized by the dot-dash line CM in FIG. 4, the translational movement of the door 17A, 17B is stopped further away than in the case of fuel having a cross-section CG. The free band 74 is reduced. Finally, for fuel having a cross-section of small size, symbolized by the dot-dash line CP in FIG. 4, the descent movement of the door 17A, 17B is stopped even further away than for the cross-sections of size CG and CM, the door coming into contact with the faces 23 by way of the lateral faces 45 of the second bearing face. The sides of the assembly that face the top of FIG. 4 are completely covered by the faces 41 of the second bearing surface 39. There is no longer a free band 74. In its translational movement towards the vertex 25 of the housing 15A, 15B, the upper portion 33 of the door is guided by the upper guide surfaces 29, and the lower portion 35 of the door is guided by the lower guide surfaces 27. Once the door 17A, 17B is in its holding position, the screws 63 are screwed into the threaded orifices 61. The serpentine form, the surface state and the manufacturing tolerances of the guide surfaces 27 and 29 and of the shoulder 31, on the one hand, and of the doors 17A, 17B, on the other hand, are such that the housings 15A, 15B are well sealed and that the nuclear materials are confined in the housings 15A, 15B in the event of a serious accident which would have caused cladding bursts in the assemblies. The assembly 10 is subsequently swung as far as the horizontal position and then comes to rest on the cradle 8 where it is secured by bolts. After placing the upper half-shell back on the lower half-shell of the casing 2 and securing the two half-shells by screws and nuts, it is possible to handle and transport the container, for example, by lifting the container by means of the lifting feet 75 and 75′ secured to the upper half-shell of the external casing, as is shown in FIG. 1. The procedure for unloading the nuclear fuel assemblies is the reverse of the procedure for loading these assemblies into the container. It will not be explained in detail here. The transport container described above can be used for fresh or irradiated nuclear fuel assemblies, regardless of the nuclear fuel UO2,PuO2 . . . . It can also be used to transport equipment having a space requirement similar to that of a nuclear fuel assembly, for example rod boxes, quiver-like supports, or skeletons of nuclear fuel assemblies. The container described above has multiple advantages. It is possible in the same container, with the same internal structure, to transport nuclear fuel assemblies of different sizes. This result is achieved owing to the fact that it is possible to adjust the spacing between the first and second bearing surfaces 21 and 39 by displacing the doors 17A, 17B along the screws 47. The adjustment described above is also effected using simple and economical means: the screws 47 mounted in the bearings 53, and the nuts 49 provided with shafts 51 engaged in the blind holes 59 of the door. It is possible in the same container, with the same internal structure, to transport nuclear fuel assemblies having different grid positions. This result is achieved owing to the fact that the bearing surfaces 21 and 39 are smooth and the doors 17A, 17B do not have movable runners which are to rest on the grids of a transported nuclear fuel assembly. This feature is also advantageous with regard to the cleaning and decontamination of the receiving housings 15A and 15B and the doors 17A and 17B of the container, the surfaces 21 and 39 of which are smooth and may be free from retention zones. This feature is also advantageous with respect to the increase in mass associated with the absence of runners in that it enables more assemblies to be transported for the same external space requirement. The operation of the container is particularly simple owing to the fact that it comprises only a small number of screws 47 enabling the position of the door to be adjusted, and a small number of securing screws 63. The container described above may have multiple variants. Thus, the means for displacing the doors 17A and 17B on the support 13 may have structures other than that described above. By way of example, they may be in the form of connecting rods arranged to form an arm of the pantograph type. Such arms are known from the prior art and therefore will not be described in detail here. They make it possible to obtain a movement of the door, first of all of rotation and then of translation, to pass from its release position to its holding position, like the screw and nut displacement means described above. More generally, these displacement means do not necessarily ensure a movement of translation and then of rotation. It is thus possible to provide that each door will pass from its holding position to its release position by a simple translational movement along the screws 47 perpendicularly to the upper face 19 of the support 13, without a 180° rotation as in the embodiment described above. In that case, the release position corresponds substantially to the position of the door illustrated on the left in FIG. 5. The introduction of the nuclear fuel assemblies into the housings is then effected by a longitudinal movement, by means of a crane. The withdrawal of the assemblies from the housings is effected in the same manner. In a variant, it is also possible to provide that the door is dismountable, in which case the screws 47 can be replaced by securing screws of the same type as the screws 63. In order to load and unload the assemblies into and from the housings, all of the securing screws are then unscrewed and the doors 17A, 17B are subsequently removed completely, for example by means of a travelling crane. Protective means may be located around the nuclear fuel assemblies, inside the support 13 and/or the doors 17A, 17B. These protective means may be of different types. They may be of the mechanical type in order to stiffen the internal equipment of the container and to protect the fuel assemblies in the event of the container falling or in the event of impact. These protective means may also be of the neutron type and may absorb the neutrons emitted by the nuclear fuel assemblies. The protective means may also be of the thermal type in order to prevent the heat generated by the fuel assembly from being conducted through the support or the door. The protective means may also be of the biological type and may absorb the ionizing radiation emitted by the nuclear fuel assemblies, for example gamma radiation. It is even possible that these protective means may be sufficient to transport a nuclear fuel assembly without an external casing 2 being necessary. The container described above is suitable for transporting nuclear fuel assemblies for a BWR reactor (boiling water reactor) or a PWR reactor (pressurized water reactor). These assemblies may be of the type 17×17, 10×10, 18×18, or of any other type. It will be recalled that these numbers characterize the square network in accordance with which the fuel rods are arranged. Thus, a 17×17 assembly has a network of seventeen rows of seventeen rods or accessories. The container can also be adapted to transport nuclear fuel assemblies, the cross-section of which is not square but, for example, rectangular or hexagonal. In such cases, the adjustment of the position of the door relative to the support 13 is not necessarily effected by a vertical translation as described above and represented by the vertical arrow F1 in FIG. 5. Thus, in case of rectangular assemblies, this translation will be effected in a direction passing via the vertices 25 and 43 of the longitudinal bearing surfaces 21 and 39. In the case of a hexagonal nuclear fuel assembly, it is provided, for example, that the faces 23 of the first bearing surface 21 form between them an angle of approximately 60°. Likewise, it is provided, for example, that the faces 41 of the second bearing surface 39 form between them an angle of 60°. The assembly is arranged in the housing in such a manner that a first side of the hexagon is in contact with one of the faces 23, and a second side of the hexagon is in contact with the other face 23. A third side of the hexagon, connecting the first and second sides, extends from one face 23 to the other, opposite the vertex 25. This third side is not placed against the bearing surface 21. In the same manner, two other sides of the hexagon rest against the faces 41 of the second bearing surface 39, one side of the hexagon extending between these two faces 41, opposite the vertex 43. In the same manner, the internal structure of the container can be adapted to transport nuclear fuel assemblies having an octagonal or triangular cross-section or any other polygonal cross-section. In a variant, it is possible to provide that the two pairs of faces 23 and 41 of the first and second bearing surfaces 21 and 39 form a continuous square, of variable size depending on the cross-section of the assembly to be transported, as illustrated in FIG. 7A. In that case, the means for adjusting the spacing between the first and second surfaces 21 and 39 may comprise means for displacing in a co-ordinated manner the four faces 23 and 41 along each other, in order to vary the size of the square. The faces 23 and 41 remain perpendicular to each other in the course of this movement. According to another variant, illustrated in FIG. 7B, each of the first and second bearing surfaces 21 and 39 comprises a large face 23, 41 and a small face 23′, 41′ which is narrower than in the large face in a transverse plane. Each of the first and second surfaces 21, 39 also comprises an undercut 80 bordering the small face and delimited in part by a guide surface 82. The guide surfaces 82 extend substantially parallel with the diagonal passing via the vertices 25 and 43, that is to say, vertically in FIG. 7B. As shown in FIG. 7B, the two large faces 23, 41 are parallel and opposite, and the two small faces 23′, 41′ are parallel and opposite. The two faces of the same bearing surface are typically perpendicular to each other. As can be seen on the right in FIG. 7B, for a nuclear fuel assembly having a large cross-section, the two opposite sides of this assembly resting on the large faces 23 and 41 are completely covered by those faces. On the other hand, the two opposite sides resting on the small faces 23′ and 41′ are only partially covered by those faces. It can be seen on the left in FIG. 7B that, for nuclear fuel assemblies having a small cross-section, the four sides of these assemblies are completely covered by the faces 23, 23′, 41, 41′, the free edges of the large faces 23 and 41 engaging in the undercuts 80 of the opposite surface. The surfaces 82 enable the relative displacement of the first and second bearing surfaces 21 and 39 to be guided. Furthermore, they form baffles enabling sealing to be improved. Finally, the bearing surfaces 21 and 39 are delimited by two identical members which are fitted together head to tail, which enables the production costs to be reduced. Preferably, the faces of the first surface 21 form between them an angle substantially equal to the angle which the faces of the second surface 39 form between them. This angle is from 60° to 135°, depending on the geometry of the nuclear fuel assemblies to be transported. More generally, the container 1 according to the invention can receive a number of nuclear fuel assemblies other than two. Thus, it may be configured to receive a single nuclear fuel assembly, or in some variants a much higher number, for example six or eight. The container 1 may also comprise, in addition to the first and second longitudinal bearing surfaces, a third longitudinal bearing surface. Each of these longitudinal bearing surfaces may comprise, as in the examples described above, two longitudinal faces, but the number of faces may also be different, for example a single longitudinal face may be envisaged, and three longitudinal faces may be envisaged. Thus, for nuclear fuel assemblies having a hexagonal cross-section, it is possible to provide three longitudinal bearing surfaces each comprising a single bearing face, these faces being inclined relative to each other by 120° when they rest against an assembly. Still for the same type of assembly, it is possible in another variant to provide a first surface comprising three faces inclined relative to each other by 120° and intended to rest on consecutive faces of a nuclear fuel assembly. The second surface may then comprise a single bearing face. When the same surface comprises several longitudinal faces, the latter do not necessarily intersect at a vertex as described above. Likewise, in the examples described above, the longitudinal bearing surfaces rest directly on the nuclear fuel assemblies, without using movable holding runners. However, it is also possible to use such runners or other means to ensure contact between the longitudinal bearing surfaces and a transported nuclear fuel assembly. The invention described above can be readily implemented simply by modifying existing packages.
	description	This application is a continuation of, and claims priority under 35 U.S.C. §120 to, U.S. patent application Ser. No. 12/703,417, filed Feb. 10, 2010, which, in turn, claims priority under 35 U.S.C. §120 to international application PCT/EP2007/007326, filed Aug. 20, 2007. The contents of U.S. application Ser. No. 12/703,417 and international application PCT/EP2007/007326 are hereby incorporated by reference in their entirety. This disclosure relates to optical systems having mirror elements with reflective coatings for use with short wavelength radiation in photolithography equipment. Optical systems may be employed as projection objectives in projection exposure systems used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks (or reticles) onto an object having a photosensitive coating at ultrahigh resolution. In order to allow creating even finer structures, various approaches to improving the resolving power of projection objectives are being pursued. It is well known that resolving power may be improved by increasing the image-side numerical aperture (NA) of the projection objective. Another approach is employing shorter-wavelength electromagnetic radiation. Deep ultraviolet (DUV) lithography at 193 nm, for example, typically involves a projection system with a numerical aperture of 0.75 or higher to achieve 0.2 μm or smaller features. At this NA, the depth of focus (DOF) is some tenths of a micrometer. In addition, fabrication and assembly tolerances make it difficult to build optical systems with such as large NA. As is known in the art, short wavelength ultraviolet radiation (less than about 193 nm) is not generally compatible with many refractive lens materials due to the intrinsic bulk absorption. To reduce the radiation absorption within an optical system, reflective elements may be used in place of refractive optical elements. State of the art DUV systems often use catadioptric optical systems which include refractive lenses and reflective elements (mirrors). Systems that operate at moderate numerical apertures and improve resolving power largely by employing short-wavelength electromagnetic radiation from the extreme-ultraviolet (EUV) spectral region have been developed. In the case of EUV-photolithography employing operating wavelengths of 13.5 nm, resolutions of the order of 0.1 μm at typical depths of focus of the order of 1 μm may theoretically be obtained for numerical apertures of NA=0.1. It is well known that radiation from the extreme-ultraviolet spectral region cannot usually be focused using refractive optical elements, since radiation at the short wavelengths involved is usually absorbed by the known optical materials that are transparent at longer wavelengths. Pure mirror system (catoptric optical systems) that have several concavely and/or convexly curved mirrors that have reflective coatings are thus employed in EUV-photolithography. The reflective coatings employed are typically multilayer coatings having, for example, alternating lay-ers (films) of molybdenum and silicon. In the manufacture of semiconductor components and other finely structured components, a pattern from a mask to be imaged on a substrate is usually formed by lines and other structural units representing a specific layer of the component to be produced. The structures to be produced for semiconductor components typically include tiny metallic tracks and silicon tracks as well as other structural elements, which may be characterized by critical dimensions (CD) which, in the case of EUV-photolithography may be in the order of 100 nm or below. Where the pattern of a mask has structural features with given critical dimension on different parts of the mask, it is desired to reproduce the relative dimensions as precisely as possible in the structured substrate. However, various influences involved in the lithography process may result in undesirable variations of the critical dimensions (CD variations) in the structured substrate, which may affect the performance of the structured components negatively. Therefore, it is generally desired to improve lithography equipment and processes to minimize CD variations, especially lateral variations across the exposed field. In many applications, linear features of the pattern run in different directions. It has been observed that under certain conditions a contrast obtained in a lithographic process depends on the structural orientation, thereby leading to what is commonly denoted as horizontal-vertical differences (H-V differences), which may affect the performance of the structured components negatively. Therefore, it may be desired to improve lithographic equipment and processes to minimize H-V differences. Photolithographic equipment, or steppers, employ two different methods for projecting a mask onto a substrate, namely, the “step-and-repeat” method and the “step-and-scan” method. In the case of the “step-and-repeat” method, large areas of the substrate are exposed in turn, using the entire pattern present on the reticle. The associated projection optics thus have an image field that is large enough to allow imaging the entire mask onto the substrate. The substrate is translated after each exposure and the exposure procedure repeated. In the case of the step-and-scan method that is preferred here, the pattern on the mask is scanned onto the substrate through a movable slit, where the mask and slit are synchronously translated in parallel directions at rates whose ratio equals the projection objectives magnification. In some embodiments, the disclosure provides an EUV projection optical system operable at high numerical aperture capable of imaging patterns with a low level of variation of critical dimensions. In certain embodiments, the disclosure provides an EUV projection optical system operable at high numerical aperture capable of imaging patterns with little or low orientation dependence of contrast. In some embodiments, the disclosure provides an EUV projection optical system operable at high numerical aperture having a low level of image-side telecentricity error. In some embodiments, the disclosure provides an optical system that includes: a plurality of elements arranged to image radiation at a wavelength λ from an object field in an object surface to an image field in an image surface; the elements including mirror elements having a reflective surface formed by a reflective coating positioned at a path of radiation; at least one of the mirror elements having a rotationally asymmetrical reflective surface deviating from a best-fit rotationally symmetric reflective surface by about λ or more at one or more locations; the elements including an apodization correction element effective to correct a spatial intensity distribution in an exit pupil of the optical system relative to the optical system without the apodization correcting element. An optical system according to preceding paragraph includes at least one mirror element having a rotationally asymmetrically reflective surface deviating from the best-fit rotationally symmetric surface by about λ or more at one or more locations. Reflective elements having reflective surfaces according to this condition will be denoted “freeform surfaces” in this application. Utilizing at least one freeform surface in the optical system provides additional free parameters to optimize the optical system with regard to overall transmission, uniformity of field illumination, and other quality parameters. Further, freeform surfaces may be shaped and positioned such that only relatively small local angles of incidence, Θ, and/or relatively small ranges of angles of incidence, ΔΘ, of rays and/or relatively small average angles of incidence, Θavg on each reflective surface for a selected reflection of the optical system are obtained, thereby reducing problems typically associated with higher incidence angles. Unlike spherical or aspherical mirrors, freefrom mirror surfaces do not have an axis of rotational symmetry. Generally, a freeform surface deviates from a best fit rotationally symmetric surface (e.g., a spherical or aspherical surface). A freeform surface can, for example, have a maximum deviation from a best fit sphere of about λ or more. Definitions and descriptions of freeform surfaces and their use in optical systems for EUV lithography and other applications may be taken from applicant's US-patent application US 2007/0058269 A1. The disclosure of that patent application is incorporated herein by reference. While utilizing one or more freeform surfaces may be advantageous with respect to overall transmission, for example, it has been found that significant field dependent apodization effects may be caused by freeform surfaces due to the lack of rotational symmetry. In the context of this application, the term “apodization” is intended to characterize effects resulting from the fact that different rays originating from one and the same field point in the object surface may be characterized by different values of the overall transmission which characterizes the loss of radiant energy of the ray upon propagation between the object surface and the image surface. As different rays emanating from an object field point in different directions are typically incident at different positions and/or at different angles of incidence on various mirrors within the optical system, and each of the mirrors typically has different reflectivities for rays incident on different locations of the mirror and/or at different angles of incidence, a significant variation of transmission for each of the rays may occur. As seen from a selected image field point (field point in the image field) the apodization may be characterized by a given spatial intensity distribution in the exit pupil of the optical system. For example, in the absence of apodization, the spatial intensity distribution in the exit pupil may be uniform for selected field point. However, in general, there is an uneven distribution of intensity in the exit pupil for each of the field points. Further, in general, the spatial intensity distribution in the exit pupil varies for each of the image filed points such that each of the image field points “sees” a different spatial intensity distribution in the exit pupil. This effect is denoted as “field depend-ent apodization” in this application. It has been found that field dependent apodization may contribute to undesirable CD variations. According to the above aspect of the disclosure an apodization correction element is provided which may be specifically designed to decrease the field dependence of the spatial intensity distribution in the exit pupil when compared to the same optical system without the apodization correcting element. As a result, an improved optical performance of the optical system, e.g. with regard to generation of CD variations and other effects originating from field dependent apodization, may be obtained. The apodization correcting element may be designed to be effective to increase symmetry of the spatial intensity distribution in the exit pupil of the optical system as compared to the same optical system without the apodization correcting element. In some embodiments, the apodization correcting element may be designed to be effective to improve rotational symmetry of the intensity distribution in the exit pupil as compared to the same optical system without the apodization correcting element. In general, a variation of structural contrast with structure orientation may be avoided where the intensity distribution in the exit pupil is rotationally symmetric to the center of the exit pupil. On the other hand, significant deviations of the intensity distribution from rotational symmetry may cause or contribute to a significant dependence of structural contrast from structure orientation, usually denoted as horizontal-vertical differences (H-V difference). While spatial positions at or close to a center of the exit pupil correspond to low aperture rays, those rays corresponding to the largest aperture angles in the image space correspond to locations at or close to the outer edge of the exit pupil. Those rays are critical with respect to the resolution obtained at given image-side numerical aperture. It may be useful to define the rotational symmetry of the intensity distribution in the exit pupil, or a deviation therefrom, specifically with regard to rays originating from an edge region at or close to the outer edge of the pupil surface. In some embodiments, the spatial intensity distribution in the exit pupil is characterized by an apodization parameter APO representing a normalized azimuthal variation of the intensity in an edge region of the exit pupil according to:APO=(IMAX−IMIN)/(IMAX+IMIN),wherein IMAX is the maximum value of the intensity in the edge region of the exit pupil and IMIN is the minimum value of the intensity in the edge region of the exit pupil, which is typically found at another azimuthal (circumferential) position than the maximum intensity value. It is evident that this apodization parameter equals zero for a completely rotationally symmetric intensity distribution in the outer edge region of the exit pupil and becomes larger the larger the deviation from rotational symmetry becomes. In exemplary embodiments, the apodization correcting element is effective to decrease the normalized azimuthal variation of the intensity of the edge region of the exit pupil when compared to an optical system without the apodization correcting element. For example, the apodization parameter APO defined above may be decreased by at least 1% or more, such as at least 5% or more. In some embodiment the apodization correcting element may be designed to be effective to shift an intensity centroid of the intensity distribution in the exit pupil towards a center of the exit pupil when compared to the same optical system without the apodization correcting element. Where the centroid of the intensity distribution in the exit pupil is situated significantly outside the center of the exit pupil a telecentricity error may occur. A telecentricity error, in turn, may cause a shift of the image position with defocus, which is generally not desirable. Problems associated with image-side telecentricity error may be avoided or minimized where the centroid of the intensity distribution in the exit pupil is at or close to the center of the exit pupil. In some embodiments, the apodization correcting element may be designed to be effective to increase mirror symmetry of the intensity distribution in the exit pupil relative to a meridional plane when compared to the same optical system without the apodization correcting element. In some embodiments, the apodization correcting element may be designed to be effective to reduce the field-dependent apodization when compared to the same optical system without the apodization correcting element. In general, where a pupil apodization is essentially the same for all field points (field-constant pupil apodization) corresponding aberrations, such as telecentricity error and orientation-dependent contrast, will be essentially the same for all field points. In those cases, a compensation of those aberrations may be compensated for by modifications in the optical system. For example, where a telecentricity error is essentially the same for all field points, such error may be compensated for by tilting the illumination radiation incident on the object field. On the other hand, where the apodization varies significantly across the field, a significant amount of CD variations may be effected, which may be difficult to compensate for. It has been found that in some embodiments an improvement with respect to mirror symmetry and/or with respect to the position of the intensity centroid may be obtained concurrently with improving the rotational symmetry of the intensity distribution in the exit pupil. The apodization correcting element may be a filter element provided in addition to mirror elements and having a spatially varying distribution of transmission across the utilized area of the filter element. The filter element may include at least one layer of an absorbing material having significant absorption at the operating wavelength λ. The layer may have a geometrical thickness varying across a utilized area such that a spatial variation of transmission and/or reflectance is obtained. In some embodiments, the apodization correcting element is a mirror element having a reflective surface formed by a reflective coating designed as a non-rotationally symmetric, graded coating including a multilayer stack of different materials, at least one of the layers having a geometrical layer thickness which varies according to a first grading function in a first direction of the coating and according to a second grading function, different from the first grading function, in a second direction perpendicular to the first direction. A non-rotationally symmetric, graded reflective coating providing a rotationally asymmetrical reflectance across the reflective surface will also be denoted as “freeform coating” in this application. In some embodiments, the apodization correcting element is a mirror element having a reflective surface formed by a reflective coating including a multilayer stack of different materials, the layers including a cap layer on a radiation entry side facing away from a mirror substrate, wherein the cap layer has a geometrical layer thickness which varies according to a rotationally asymmetrical grading function across the reflective surface. In general, multilayer systems such as those used as mirrors in the extreme ultraviolet wavelength range may suffer contamination or oxidation during storage in air and in long-time operation. It is known to provide such multilayer systems with protective layers on the radiation entry side thereof to improve lifetime and constancy of reflectivity of those multilayer systems. The term “cap layer” as used here may refer to such protective layer or layers. A cap layer may be made from ruthenium, aluminium oxide, silicon carbide, molybdenum carbide, carbon, titanium nitride or titanium dioxide, for example. Alternatively, a mixture, an alloy or a compound of ruthenium, aluminium oxide, titanium nitride or titanium dioxide, and a further substance may be used to form the cap layer. Examples of multilayer systems with protective cap layers and production methods thereof are disclosed in U.S. Pat. No. 6,656,575 B2, which is incorporated herein by reference. For example, the geometrical layer thickness of the cap layer may vary according to a first grading function in a first direction of the coating and according to the second grading function, different from the first grading function, in a second direction perpendicular to the first direction. In some embodiments, the geometrical thickness of the cap layer increases from an origin in a center region of the cap layer towards the edge of the mirror slightly in the first direction and an amount of increase between the origin and the edge region is significantly larger in the second direction. Considering the intensity filtering effect of the material of the cap layer a strong absorbing effect may be obtained near the outer edge in the second direction and significantly less absorption may be effected the outer edge in the first direction. In some embodiments, the layer thickness of the cap layer is essentially uniform in central zone around the origin at least up to radial coordinates corresponding to an outer edge of a region corresponding to a first sub-aperture corresponding to a central field point and the layer thickness of the cap layer increases outside the central region slightly in the first direction and stronger in the second direction. If the spatial distribution of the layer thickness of the cap layer is designed generally in accordance with this teaching the apodization correcting element has little or no apodization changing effect on rays originating from the center of the object field, whereas the apodization may be changed in the targeted manner for rays originating from regions close to or at the edge of the object field, whereby a field dependence of apodization may be decreased. In some embodiments, the reflective coating includes a plurality of intermediate layers arranged between the cap layer and the mirror substrate, wherein each of the plurality of intermediate layers has a uniform layer thickness. The intermediate layers may include a Molybdenum-Silicon bilayer stack, for example. Where only the geometrical thickness of the cap layer is varied, substantially no additional interference effects occur which might be introduced if the geometrical thicknesses of other layers of the multilayer stack would be varied spatially. Therefore, a correction of phase to account for the variation of geometrical thickness of the cap layer may be relatively simple. In some embodiments, the material of the cap layer has significant absorbance for the radiation at wavelength λ and a corresponding refractive index which is at or close to 1 for the used wavelength. Under these conditions the cap layer has little or almost no influence on the phase of the reflected radiation, whereby correaction of imaging errors is facilitated. In some embodiments, the material of the cap layer has an absorbance for the radiation at wavelength λ which is greater than a specific absorbance of each of the materials of the intermediate layers. The absorbance of the material of the cap layer may be greater than the absorbance of Silicon and/or greater than the absorbance of Molybdenum. An absorbance difference may be 10% or more (or 20% or more, or 30% or more, or 50% or more) for example. In some embodiments, the apodization correcting element has a reflective surface formed by a reflective coating including a multilayer stack of layers of different materials, the layers forming a stack of bilayers, wherein a bilayer includes a (relatively thick) layer of a first material, e.g. Si, having a first refractive index and a (relatively thin) second layer of a second material, e.g. Mo, having a second refractive index which is lower than the first refractive index, wherein a thickness ratio between a geometrical thickness of the first layer and the second layer of at least one bilayer varies according to first grading function in a first direction of the coating and according to a second grading function, different from the first grading function, in a second direction perpendicular to the first direction. In some embodiments, the apodization correcting element includes at least one filter layer disposed on the cap layer on a radiation entry side thereof, wherein the filter layer is made from a filter layer material absorbing for radiation at wavelength λ and having a geometrical thickness which varies spatially. The layer system below the filter layer and including the cap layer may be formed in a conventional process and the filter layer may be added in a separate manufacturing step based on data evaluated for the optical design without filter layer. Process reliability may thereby be maintained. The filter layer material may have a greater specific absorbance at wavelength λ than the material of the cap layer. In this case, a smaller variation of geometrical thickness of the filter layer may be sufficient to obtain the same degree of variation of reflectance of the apodization correcting element. Alternatively, the filter layer material may have a smaller specific absorbance at wavelength λ than the material of the cap layer. In this case, the absolute layer thickness of the filter layer may be relatively thick and a desired variation of the overall absorbing effect across the filter layer may be achieved with high accuracy through appropriate variation of geometrical layer thickness of the filter layer. Where a relatively thick filter layer is used, it is desirable to use a material having a refractive index at or close to 1 to avoid negative effects of the filter layer on the phase of the reflected radiation. Alternatively, the filter layer material may have essentially the same specific absorbance at wavelength λ than the material of the cap layer. The filter layer may be made from the cap layer material, thereby allowing to form a cap layer with spatially varying geometrical layer thickness. The filter layer may be made from ruthenium, aluminium oxide, silicon carbide, molybdenum carbide, carbon, titanium nitride or titanium dioxide or the filter layer material may be a mixture, an alloy or a compound of ruthenium, aluminium oxide, titanium nitride or titanium dioxide, and a further substance. An apodization correcting element may be positioned at various positions along the radiation beam path between object surface and image surface. In some embodiments, the apodization correcting element is positioned optically remote from a pupil surface of the optical system. Where the apodization element is placed at a position sufficiently remote from a pupil surface it is generally possible to influence different field points differently from each other such that a field-dependent variation of apodization may be corrected or reduced. The apodization correcting element may be positioned at a position where condition P(M) <1 is fulfilled, whereinP(M)=D(SA)/(D(SA)+D(CR)),where D(SA) is a diameter of a sub-aperture of a ray bundle originating from a field point in the object surface on a respective surface M; and D(CR) is a maximum distance of chief rays of an effective object field imaged by the optical system measured in a reference plane of the optical system on the surface M. The reference plane may be a symmetry plane of the optical system. In systems having a meridional plane, the reference plane may be the meridional plane. As the diameter of a sub-aperture approaches zero at a field surface, the parameter P(M)=0 in a field surface. In contrast, the maximum distance of chief rays, D(CR), approaches zero in pupil surface. Therefore, the condition P(M)=1 is fulfilled for a position exactly in the pupil surface. In some embodiments, the condition P(M)<0.99 holds for a position optically remote from a pupil surface. In some embodiments, the apodization correcting element is positioned in an intermediate region optically between a pupil surface and a field surface of the optical system. In those embodiments, the apodization correcting element is placed neither exactly at a pupil surface nor exactly at a field surface, such as an intermediate image, for example, but at a position sufficiently remote from both a pupil surface and a field surface if it is desired to influence the field dependent variation of apodization. An apodization correcting element placed exactly at a pupil surface cannot influence different field points differently from each other. On the other hand, if an apodization correcting element would be placed at or very close to a field surface, the spatial distribution of intensity within the pupil surface cannot be influenced significantly. In some embodiments, the apodization correcting element is positioned at a position where condition 0.99>P(M)>0.95 is fulfilled. Under this condition, the apodization correcting element is neither very close to a pupil surface nor very close to a field surface. Therefore, the sub-apertures of ray bundles originating from different field points do not overlap completely at the position of the apodization correcting element, thereby enabling the apodization correcting element to influence apodization differently for different field points. Further, the apodization correcting element is sufficiently remote from a field surface such that a change in reflectivity at a certain location on the apodization correcting element will have different influence on different locations of the exit pupil, whereby it is possible to vary the spatial intensity distribution in the exit pupil. In general, an appropriate position of the apodization correcting element may be selected depending on whether or not a field dependence of pupil apodization is significant or not or whether or not a certain degree of field dependence of pupil apodization may be acceptable. In some embodiments, an apodization correcting element is positioned optically near to a field surface, such as the object surface, or the image surface, or an optional intermediate image surface, for example where 0<P(M)≦0.93. Where an apodization correcting element is positioned at or optically close to a field surface, it is possible to correct variations of critical dimensions (CD variations) across the field and/or to improve field uniformity, i.e. to obtain a more uniform intensity distribution in the image field. For example, where there is little or substantially no field dependence of pupil apodization, the apodization correcting element may be positioned at or optically close to a pupil surface of the optical system, for example where 0.98<P(M)≦1. Where an apodization correcting element is positioned at or very close to the pupil surface, it is possible to correct field-constant contributions to telecentricity error and/or structure orientation-dependent contrast variations (H-V differences). The previous and other properties can be seen not only in the claims but also in the description and the drawings, wherein individual characteristics may be used either alone or in sub-combinations as an exemplary embodiment of the disclosure and in other areas and may individually represent advantageous and patentable embodiments. Specific aspect of exemplary embodiments of the disclosure are now described in more detail using as examples catoptric projection objectives which can be used in microlithography tools, e.g. in a projection exposure apparatus for manufacturing semiconductor devices. Referring to FIG. 1, a microlithography tool 100 generally includes a light source 110, an illumination system 120, a projection objective 101, and a stage 130. A Cartesian coordinate system is shown for reference. Light source 110 produces radiation at a wavelength λ and directs a beam 112 of the radiation to illumination system 120. Illumination system 120 interacts with (e.g., expands and homogenizes) the radiation and directs a beam 122 of the radiation to a reticle 140 positioned at an object plane 103. Projection objective 101 images radiation 142 reflected from reticle 140 onto a surface of a substrate 150 positioned at an image plane 102. The radiation on the image-side of projection objective 101 is depicted as rays 152. As shown in FIG. 1, the rays are illustrative only and not intended to be accurately depict the path of the radiation with respect to reticle 140, for example. Substrate 150 is supported by stage 130, which moves substrate 150 relative to projection objective 101 so that projection objective 101 images reticle 140 to different portions of substrate 150. Projection objective 101 includes a reference axis 105. In embodiments where projection objective is symmetric with respect to a meridional section, reference axis 105 is perpendicular to object plane 103 and lies inside the meridional section. Light source 110 is selected to provide radiation at a desired operational wavelength, λ, of tool 100. Typically, for projection objectives designed for operation in lithography tools, wavelength λ is in the ultraviolet portion, the deep ultraviolet portion or the extreme ultraviolet portion of the electromagnetic spectrum. For example, λ can be about 200 nm or less. λ can be more than about 2 nm. In the exemplary embodiment, light source 110 is an EUV light source providing radiation at an operational wavelength about λ=13.5 nm. Illumination system 120 includes optical components arranged to form a collimated radiation beam with a homogeneous intensity profile. Illumination system 120 typically also includes beam steering optics to direct beam 122 to reticle 140. In some embodiments, illumination system 120 also includes components to pro-vide a desired polarization profile for the radiation beam. Image plane 103 is separated from object plane 102 by a distance L, which is also referred to as the lengthwise dimension, or track length, of projection objective 101. In general, this distance depends on the specific design of projection objective 101 and the wavelength of operation of tool 100. In some embodiments, such as in tools designed for EUV lithography, L is in a range from about 1 m to about 3 m. As shown in FIG. 2, rays 152 define a cone of light paths that form the reticle image at image plane 102. The angle of the cone of rays is related to the image-side numerical aperture (NA) of projection objective 101. Image-side NA can be expressed as NA=no sin θmax, where no refers to the refractive index of the medium adjacent the surface of substrate 150 (e.g., air, nitrogen, water or other immersion liquid, or evacuated environment), and θmax is the half-angle of the maximum cone of image forming rays from projection objective 101. In general, projection objective 101 can have an image side NA of about 0.1 or more, e.g., about 0.15 or more, about 0.2 or more, about 0.25 or more, about 0.3 or more, about 0.35 or more. In general, problems associated with pupil apodization may be more difficult to compensate the larger the image-side numerical aperture becomes. The number of mirrors in projection objective 101 may vary. Typically, the number of mirrors is related to various performance trade-offs associated with the optical performance characteristics of the objective, such as the desired throughput (e.g., the intensity of radiation from the object that forms the image at image plane 102), the desired image-side NA and related image resolution, and the desired maximum pupil obscuration (only in systems with pupil obscuration). Embodiments for EUV applications typically have at least three or at least four mirrors. Exactly six mirrors may be desirable in some cases. Typically no more than six or no more than seven or no more than eight mirrors are used. In embodiments where it is desirable that all the mirrors of the objective are positioned between the object plane and the image plane, objective 101 will typically have an even number of mirrors. In certain embodiments, an odd number of mirrors can be used where all the mirrors of the projection objective are positioned between the object plane and image plane. For example, where one or more mirrors are tilted at relatively large angles, a projection objective can include an odd number of mirrors where all the mirrors are positioned between the object plane and image plane. At least one of the mirrors in projection objective 101 has a freeform surface. Unlike spherical or aspherical mirrors, freeform mirror surfaces do not have an axis of rotational symmetry. Generally, a freeform surface deviates from a best fit rotationally symmetric surface (e.g., a spherical or aspherical surface). Best fit surfaces are calculated by first calculating the surface area of the mirror surface and then determining a best fit to that surface of a spherical or aspherical surface using a least squares fitting algorithm. The best fit spherical or aspherical surface can be titled or decentered with respect to a reference axis, where decenter and tilt are used as additional fitting parameters. A freeform surface can, for example, have a maximum deviation from a best fit sphere of about λ or more. A more general description of freeform surfaces and characterizing features thereof may be taken from applicant's US-application published as US 2007/0058269 A1, which is incorporated herein by reference. In certain embodiments, freeform mirror surfaces can be described mathematically by the equation: Z = cr 2 1 + 1 - ( 1 + k ) ⁢ c 2 ⁢ r 2 + ∑ j = 2 α ⁢ ⁢ C j ⁢ X m ⁢ Y n ⁢ ⁢ where ( 1 ) j = ( m + n ) 2 + m + 3 ⁢ ⁢ n 2 + 1 ( 2 ) and Z is the sag of the surface parallel to a Z-axis (which may or may not be parallel to the z-axis in projection objective 101), c is a constant corresponding to the vertex curvature, k is a conic constant, and Cj is the coefficient of the monomial Xm. Parameter α is an integer indicating the order of the terms considered in the summation. A value α=66, for example, corresponds to a sum including the 10th order. Typically, the values of c, k, and Cj are determined based on the desired optical properties of the mirror with respect to projection objective 101. Further, the order of the monomial, m+n, can vary as desired. Generally, a higher order monomial can provide a projection objective design with a higher level of aberration correction, however, higher order monomials are typically more computationally expensive to determine. In some embodiments, m+n is 10 or more (e.g., 15 or more, 20 or more). The parameters for the freeform mirror equation can be determined using commercially-available optical design software. In some embodiments, m+n is less than 10 (e.g., 9 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4 or less, 3 or less). A reference projection objective that includes six mirrors is shown in FIG. 3. Specifically, projection objective 300 includes six freeform mirrors 310, 320, 330, 340, 350, and 360. Data for projection objective 300 is presented in Table 3 and 3A. Table 3 presents optical data, while Table 3A presents freeform constants for each of the mirror surfaces. For the purposes of Table 3 and Table 3A, the mirror designations correlate as follows: mirror 1 (M1) corresponds to mirror 310; mirror 2 (M2) corresponds to mirror 320; mirror 3 (M3) corresponds to mirror 330; mirror 4 (M4) corresponds to mirror 340; mirror 5 (M5) corresponds to mirror 350; and mirror 6 (M6) corresponds to mirror 360. “Thickness” in Table 3 and subsequent tables refers to the distance between adjacent elements in the radiation path. The monomial coefficients, Cj, for the freeform mirrors, along with the amount the mirror is decentered and rotated from an initial projection objective design, are provided in Table 3A. R, the radius, is the inverse of the vertex curvature, c. Decenter is given in mm and rotation is given in degrees. Units for the monomial coefficients are mm−j+1. Nradius is a dimensionless scaling factor (see, for example, the manual for CODE V® by Optical Research Associates). TABLE 3, 3ASurfaceRadiusThicknessModeObjectINFINITY780.487Mirror 1−651.370−538.725REFLMirror 2−458.973954.889REFLMirror 3−1713.884−788.833REFLMirror 41822.4471037.281REFLMirror 5309.686−306.280REFLMirror 6406.556351.325REFLImageINFINITY0.000CoefficientM1M2M3M4M5M6K−5.903672E−012.073876E+00−1.833015E+003.271462E+001.890760E+002.288136E−01Y0.000000E+000.000000E+000.000000E+000.000000E+000.000000E+000.000000E+00X22.472443E−046.412975E−047.597407E−052.796077E−05−5.194312E−041.242933E−05Y21.860007E−046.109309E−05−2.585312E−05−7.188311E−05−3.216223E−045.312795E−05X2Y−3.583286E−08−3.071465E−061.092569E−071.012955E−073.020360E−06−2.535119E−08Y3−8.844243E−09−3.552973E−06−7.253427E−08−3.594092E−081.841453E−06−5.500854E−09X41.244536E−118.137457E−10−3.305282E−11−9.180029E−11−3.120770E−10−1.505816E−10X2Y24.205964E−114.214802E−09−2.413146E−11−7.897645E−111.723114E−08−7.741421E−11Y47.880677E−11−2.267546E−08−1.619900E−10−1.828672E−10−1.496597E−09−6.454720E−11X4Y−3.805662E−14−6.346439E−12−1.583112E−142.866668E−145.008931E−11−1.742178E−14X2Y3−2.029523E−13−2.541614E−10−1.674478E−132.799446E−132.596617E−104.271686E−15Y51.220704E−131.665324E−10−7.087076E−141.191804E−133.460656E−111.015356E−13X6−3.312394E−172.434475E−145.577328E−17−1.391180E−166.717835E−14−9.513669E−16X4Y24.241990E−17−5.674715E−13−1.846910E−16−4.213298E−163.645922E−13−1.757315E−15X2Y47.289250E−161.445006E−12−4.864362E−164.229043E−161.948906E−12−1.108117E−15Y6−1.932461E−15−5.889233E−132.208996E−193.372721E−16−2.972937E−13−5.629275E−16X6Y1.312317E−19−5.559829E−164.394337E−19−2.093226E−182.264426E−157.984310E−20X4Y3−7.425365E−193.498226E−15−3.102058E−19−2.302217E−186.866200E−153.285060E−19X2Y5−3.017211E−18−9.750526E−15−5.272924E−191.016950E−181.910446E−148.639175E−19Y77.041988E−18−1.193805E−15−1.040456E−201.216081E−19−3.171040E−151.122386E−18X8−1.167281E−22−2.037245E−19−2.161469E−21−3.119267E−211.712789E−18−5.553581E−21X6Y2−2.908994E−21−8.287593E−188.189808E−22−1.751502E−201.493607E−17−1.603818E−20X4Y44.197769E−22−5.375652E−17−1.274499E−22−1.018042E−204.780170E−17−1.716956E−20X2Y64.927775E−210.000000E+00−2.073924E−229.299910E−221.247675E−16−1.068401E−20Y8−9.565501E−210.000000E+00−5.017162E−23−8.283404E−22−3.764158E−18−4.376413E−21X8Y7.278748E−250.000000E+00−1.118538E−23−2.775571E−233.460801E−209.971498E−25X6Y31.052158E−230.000000E+005.116150E−25−5.328529E−231.294494E−193.651595E−24X4Y50.000000E+000.000000E+001.457517E−25−1.988964E−232.977259E−191.009834E−23X2Y70.000000E+000.000000E+003.156995E−266.916310E−254.647584E−192.016596E−23Y90.000000E+000.000000E+00−4.348593E−27−1.027479E−247.340789E−201.110871E−23X100.000000E+000.000000E+00−3.950695E−27−1.384689E−260.000000E+00−5.360850E−26X8Y20.000000E+000.000000E+00−1.366279E−26−5.441218E−260.000000E+00−2.218167E−25X6Y40.000000E+000.000000E+000.000000E+00−5.626818E−260.000000E+00−3.417889E−25X4Y60.000000E+000.000000E+000.000000E+00−1.607444E−260.000000E+00−2.595410E−25X2Y80.000000E+000.000000E+000.000000E+008.146110E−290.000000E+00−1.130081E−25Y100.000000E+000.000000E+000.000000E+00−8.501369E−280.000000E+00−3.760797E−26Nradius1.00E+001.00E+001.00E+001.00E+001.00E+001.00E+00Y-decenter−142.443−93.91442.888−6.282−4.405−11.420X-rotation−9.038−14.6021.511−3.471−7.366−2.186 In FIG. 3, projection objective 300 is shown in meridional section. The meridional plane is a symmetry plane for projection objective 300. Symmetry about the meridional plane is as the mirrors are decentered only with respect to the y-axis and tilted about the x-axis. Further, the coefficients for the freeform mirrors having an odd degree in the x-coordinate (e.g., x, x3, x5, etc.) are zero. Projection objective 300 is configured for operation with 13.5 nm radiation and has an image-side NA of 0.35 and a track length L of 1,490 mm. The optical path length of imaged radiation is 4,758 mm. Accordingly, the ratio of optical path length to track length is approximately 3.19. Projection objective has a de-magnification of 4×, a maximum distortion of less than 2 nm, a wavefront error WRMS of 0.030λ, and a field curvature of 30 nm. Additional characteristics of projection objective 300 are presented in the discussion of projection objective 101 that follows. The first mirror in the radiation path from object plane 103, concave mirror 310, has positive optical power. Mirrors 330, 340, and 360 are also concave P mirrors. Convex mirrors 320 and 350 have (N) negative optical power. The sequence of mirrors in the radiation path in projection objective 300 is thus PNPPNP. For the mirrors in projection objective 300, the maximum deviation of the freeform surfaces from a best fit sphere is significantly more than one micrometer for each mirror. Projection objective 300 images radiation from object plane 103 to an intermediate image at a location 305 near mirror 360. Embodiments that have one or more intermediate images, also include two or more pupil planes. In some embodiments, at least one of these pupil planes is physically accessible for the purposes of placing an aperture stop substantially at that pupil plane. An aperture stop is used to define the size of the projection objective's aperture. Each mirror in projection objective 100 can also be characterized by parameters defining the position of the mirror along the path of radiation in terms of proximity or distance from a field surface or a pupil surface, respectively. Reference is made to schematic FIG. 4 showing three mirrors M1, M2, M3 of a projection objective. Consider a field point FP1 in the object surface OS. A ray bundle RB1 (cone of radiation) having an opening angle proportional to the object-side numerical aperture originates at object field point FP1. As the optical distance from the object surface (corresponding to a field surface) increases, the diameter of such ray bundle increases. Where the ray bundle is incident on an optical surface, the ray bundle may be characterized by a “sub-aperture” of the ray bundle, where the sub-aperture is the area on the optical surface illuminated by the cone of light projected onto the x-y plane. Whereas sub-apertures of different field points FP1 and FP2 laterally offset in the object surface are laterally separated in regions close to the field surface, the sub-apertures of different field points overlap completely in a pupil surface. In a field surface, the diameter D(SA) of a sub-aperture is zero, whereas in a pupil surface the diameters of sub-apertures corresponding to different field points are substantially equal, and the sub-apertures overlap completely. Now, consider a meridional section of an effective object field OF in the object surface as shown in FIG. 4. The effective object field includes the plurality of field points actually used for the imaging process. In scanning systems, for example, the effective object field may be rectangular or arcuate with a high aspect ratio between width (in x-direction) and height (measured in the scanning direction, i.e. the y.direction). The diameter of the effective object field in the meridional plane corresponds to a maximum distance of chief rays, D(CR) in the object surface. The chief rays CR1 and CR2 corresponding to field points, FP1 and FP2 are drawn in dashed lines in FIG. 4 (In an optical system essentially telecentric on the object side, the chief rays are nominally orthogonal to the object plane.) As the chief rays propagate through the optical system, the distance D(CR) between the chief rays eventually decreases between a field surface and a subsequent pupil surface. The optical position of a pupil surface PS may be defined as the position where the chief rays CR1 and CR2 intersect. Therefore, the distance between the chief rays, D(CR), approaches zero close to a pupil surface and the condition D(CR)=0 is fulfilled at a pupil surface. Based on these considerations, a parameterP(M):=D(SA)/(D(SA)+D(CR))may be defined to characterize the optical proximity or distance of an optical surface M from a field surface or a pupil surface, respectively. Specifically, if the optical surface is positioned exactly in a field surface, D(SA) is =0 such that P(M)=0. On the other hand, if the optical surface M is exactly in a pupil surface, D(CR)=0 such that P(M)=1. In Table 3B the parameters D(SA), D(CR) and P(M) are given for each of the mirrors in projection objective 300. TABLE 3BMirror #D(SA) [mm]D(CR) [mm]P(M)1133.86614.1690.904257.4370.6120.9893252.14419.4430.9284185.00118.2220.910569.6981.2380.9836236.5671.9850.992 In the exemplary embodiment of FIG. 3 mirror 350 (M5, geometrically closest to the image surface, P(M)=0.983) is optically close to a pupil surface. Also, mirror 320 (M2) geometrically closest to the object surface (with P(M)=0.989) and mirror 360 (M6) (with P(M)=0.992) are optically close to a pupil surface In contrast, mirrors 310 (M1), 330 (M3) and 340 (M4) (all with P(M)<0.93) are optically closer to a field surface. Where it is desired to correct a certain amount of field variation of pupil apodization the apodization correction element may not be placed exactly in a pupil position (where P(M)=1), but at a distance therefrom where P(M)<1 such that the sub-apertures of ray bundles originating from different field points do not overlap completely at the position of the apodization correcting element. Since none of the mirrors 1 through 6 is placed exactly in a pupil surface, an apodization correcting element may be formed on each one of the mirrors 1 through 6. Where desired, two or more of the mirrors may be designed to cause, in combination, a desired apodization correcting effect. In general, the percentage of radiation at λ reflected by a mirror varies as a function of the angle of incidence of the radiation on the mirror surface. Because imaged radiation propagates through a catoptric projection objective along a number of different paths, the angle of incidence of the radiation on each mirror can vary. This effect is illustrated with reference to FIG. 5, which shows a portion of a mirror 500, in meridional section, that includes a concave reflective surface 501. Imaged radiation is incident on surface 501 along a number of different paths, including the paths shown by rays 510, 520, and 530. Rays 510, 520, and 530 are incident on portions of surface 501 where the surface normal is different. The direction of surface normal at these portions is shown by lines 511, 521, and 531, corresponding to rays 510, 520, and 530, respectively. Rays 510, 520, and 530 are incident on surface 501 at angles θ510, θ520, and θ530, respectively. In general, angles θ510, θ520, and θ530 may vary. For each mirror in projection objective 101, the incident angles of imaged radiation can be characterized in a variety of ways. One characterization is the maximum angle of incidence, θmax, of rays running in a meridional section of projection objective 100 on a mirror. Another characterization is the minimum angle of incidence, θmin, of rays running in a meridional section of projection objective 100 on each mirror. Each mirror in projection objective 100 can also be characterized by the maximum difference of angles of incidence of rays in the meridional section of the projection objective, where the maximum difference Δθ corresponds to the difference between Δθmax and Δθmin. Each mirror in projection objective 100 can also be characterized by the angle of incidence of a chief ray corresponding to a central field point of the projection objective on a respective mirror. This incidence angle will be denoted as chief ray angle of incidence, ΔθCR. Table 3C summarizes the values mentioned above for all mirrors of projection objective 300. TABLE 3CMirrorCR [°]max [°][°]M13.694.271.23M210.0910.853.23M36.487.563.70M410.0014.2210.16M513.7324.0923.41M67.148.773.89 The exemplary embodiment includes a number of reflective freeform surfaces having no rotational symmetry. It has been found that a significant variation of apodization across the image field exists. The apodization characteristics are demonstrated in FIGS. 6 and 7. FIGS. 6 and 7 present schematic plots of the spatial intensity distribution in the circular exit pupil of the projection objective for two different image field points FP1, FP2. FIG. 6 plots the distribution for field point FP1 lying at the center of the rectangular image field IF on the y axis, which is the symmetry axis of the projection objective, and FIG. 7 plots the distribution for a field point FP2 at the edge of the rectangular image field. The numbers associated with different contour lines represent the level of intensity at the respective pupil locations as a fraction of the intensity at the entrance of the projection objective (at the object surface). The pupil distribution corresponding to the center field point FP1 exhibits a relatively weak apodization with a maximum value 0.092 in the lower part of the exit pupil and a decreasing intensity towards the upper edge to a minimum value of about 0.082. The intensity distribution is essentially mirror symmetric to the meridional plane which forms the symmetry plane of the projection objective. On the other hand, the amount of apodization of the pupil corresponding to the edge field point FP2 is significantly larger, ranging from about 0.088 close to the center of the pupil towards 0.060 at the left edge of the pupil. Further, the intensity distribution is not symmetrical. A comparison of FIGS. 6 and 7 shows a relatively large variation of pupil apodization across the filed in the cross-scan direction (x direction) oriented perpendicular to the scanning direction (y direction) of the rectangular field. A symmetrical distribution of intensity at the pupil surface would be desirable from an imaging point of view. For example, contrast differences depending on the structure orientation (H-V differences) may be avoided where the intensity distribution in the exit pupil is essentially rotationally symmetric. The level of symmetry of the intensity distribution in the exit pupil may be de-scribed in a variety of ways. For example, the spatial intensity distribution may be described in terms of Zernike polynomials {Zn(r, φ)} which form a complete orthonormal function system in a unit circle which may be described in terms of polar coordinates (r, φ). The Zernike polynomials may be subdivided into rotationally symmetric polynomials and non-rotationally symmetric polynomials. A deviation from a complete rotationally symmetry in the circular exit pupil may therefore be described in terms of the root-mean-square (rms) value of the non-rotationally Zernike polynomials, which are equal to zero in a perfectly rotationally symmetric distribution and which should be a small as possible where rotational symmetry is desired. In this respect, embodiments having an apodization correcting element effective to increase rotational symmetry may be discerned by the fact that the rms value of the non-rotational symmetric Zernike polynomials is decreased when the apodization correcting element is introduced into the optical system. Another way to describe and quantify the rotational symmetry of the exit pupil (or a deviation therefrom) is to consider the local intensity distribution at the outer edge of the exit pupil (corresponding to the largest aperture rays) in the azimuthal (circumferential) direction. Pupil coordinates at or close to the outer edge of the exit pupil correspond to rays incident on the image surface with the largest aperture angles. Those rays typically define the resolution limit of the optical system at the image-side numerical aperture used. If the local intensity of the exit pupil is about the same for all positions at or close to the outer edge of the exit pupil those rays would contribute with comparable intensity to image formation. On the other hand, significant variations of critical dimensions may occur where the intensity at the outer edge of the exit pupil varies significantly in the circumferential (azimuthal) direction. An apodization parameter APO may be defined characterizing the normalized azimuthal variation of intensity in an edge region of the exit pupil in the azimuthal direction according toAPO=(IMAX−IMIN)/(IMAX+IMIN). In this equation, IMAX is a maximum intensity in the outer edge region of the exit pupil and IMIN is the minimum value of the intensity in this outer edge region such that a non-zero value of apodization parameter APO indicates a deviation from perfect rotational symmetry for the critical rays originating from the outer edge region of the exit pupil. The spatial intensity distribution corresponding to the edge field point FP2 shown in FIG. 7 may be describe by a minimum value IMIN=0.47 at the leftmost edge (in the x direction) and a maximum value IMAX=0.90 at the lower right edge of the exit pupil such that APO=0.314 (note that the contour lines in FIG. 7 and other corresponding figures indicate the intensity distribution semi-quantitative, whereas the analysis presented above has been performed on the actual values calculated for the optical system). It will be demonstrated below that an improvement of rotational symmetry particularly at the outer edge of the exit pupil may be obtained by providing an appropriate apodization correcting element. An analysis of contributions of each of the reflecting mirrors to pupil apodization shows that a relatively large contribution to field dependent apodization originates from mirror 350 (M5), geometrically closest to the image surface, optically close to a pupil surface. It can be seen from table 3C above that the chief ray angle of incidence, θCR, as well as the maximum angle of incidence, θmax in the meridional section and the maximum difference, Δθ, of rays in the meridional section have the relatively highest value for mirror 350 (mirror M5). Mirror 350 (M5) is situated relatively close to a pupil surface (P(M)=0.983). Mirror M5 is subject to relatively large variations of the angles of incidence. This is qualitatively shown in FIG. 8, which shows a schematic diagram for the spatial distribution of average angles of incidence, θavg, on mirror M5 in FIG. 3. In the figure, a generally elliptic shape of the utilized mirror surface is shown together with contour lines connecting locations having the same average angles of incidence as indicated for each contour line. It is evident that the distribution of average angles of incidence is symmetric to the meridional plane MP. The lowest value for the average angle of incidence is obtained at the lower edge (θavg<4°), whereas relatively large values are obtained at the upper edge (θavg>20°). The average angle of incidence varies substantially in the first direction (y-direction, lying in the meridional plane MP) between the lower edge and the upper edge of the mirror by more than 10° or more than 15°, for example. On the other hand, there is relatively little variation of the average angle of incidence in the second direction perpendicular to the first direction, i.e. perpendicular to the meridional plane. For example, in the center region of the mirror between upper edge and lower edge, the absolute value of the average angle of incidence is between about 12° and about 16° and does not vary by more than 4° or more than 3°, for example. Therefore, in a first approximation, the average angle of incidence varies strongly according to a roughly linear function in the first direction (y-direction, in the meridional plane MP), whereas the average angle of incidence is essentially constant in the second direction perpendicular thereto. As will explained in more detail below, specifically designed graded coatings on mirrors with characteristic variations of average angles of incidence can be applied to compensate some of the negative effects of incidence angle variation on reflectivity of mirrors such that a mirror may have only little variation of reflectivity despite relatively large variations of average angles of incidence across the mirror surface. Specifically, as will be explained below, mirror M5 is provided with a non-rotational symmetric coating designed as a one-dimensionally graded coating including a multilayer stack of layer of different materials, where the lay-ers have a geometrical layer thickness which varies according to a first grading function in the first direction of the coating (in the meridional plane) and which is substantially constant in the second direction perpendicular to the first direction. Coatings of this type are referred to as “linearly tilted coating” in this application. With regard to structure and advantages of linearly tilted coatings reference is made to U.S. provisional patent application No. 60/872,503 filed on Dec. 4, 2006, the disclosure of which is incorporated herein by reference. Each of the mirrors is coated with a reflective coating including a multilayer stack of layers of different materials. Such multilayer stacks can include about 20 or more, such as about 30 or more, about 40 or more or about 50 or more layers. In the exemplary embodiment, multiple alternating layers of molybdenum and silicon are used to form reflective coatings effective for EUV radiation wave-lengths in a range from about 10 nm to about 15 nm, specifically between about 13 nm and 14 nm. The reflective coatings were optimized for EUV lithography systems operating at 13.5 nm with NA=035. Optimization was performed using a coating stack (multilayer stack) as shown in Table 3D. TABLE 3DMaterialThickness [nm]FunctionASLayer167.60Anti stress layer (no optical(n = 0.99946, k = 0.0)function, does not influencereflectivity, because it is tofar below the top surface)Si3.66Bilayer structure for HighMoSi0.8reflectance multi layer,Mo1.64repeated 46 times.MoSi0.8Si3.73Last layer, interface to vacuum.MoSi0.8Cap Layer, RutheniumMo1.44Ru1.5 Table 3D shows the order of layers of the coating stack from the bottom surface (close to the substrate) to the top surface (in contact with vacuum). Si designates Silicon, while Mo designates Molybdenum. MoSi stands for an interlayer between Molybdenum and Silicon, which in a real coating stack is a result of inter-diffusion between the two layers. The interdiffusion layer was introduced to obtain a more physically relevant model. No interface roughness was considered in the calculation. As evident from the table, the multilayer stack includes a bilayer structure of relatively thick Silicon layers and relatively thin Molybdenum layers, which are repeated forty-six times in the multilayer stack. An anti stress layer (ASL) is positioned between the bilayer structure and the substrate. The anti stress layer has no optical function as it is positioned remote from the interface to vacuum. It does not influence reflectivity but improves mechanical stability of the reflective coating. A cap layer made of ruthenium is used on each of the mirrors. The cap layer is the layer on a radiation entry side of the reflective coating facing away from a mirror substrate. The cap layer is adjacent to the environment, which may be a vacuum in an assembled system and which may be air or another gas in during manufacturing and storage of the mirror. As evident from FIG. 9A the cap layer material ruthenium has an extinction coefficient k greater than 0.013 in a wave-length range between about 13 nm and 14 nm, where the extinction coefficient is about 0.015 or more in the region of a wavelength band pass from about 13.4 nm to 13.6 nm. On the other hand, molybdenum and silicon forming the bilayer structure below the cap layer have significantly smaller specific absorbance, characterized by an extinction coefficient k<0.008. As seen from 9B, the refractive index of ruthenium is generally between about 0.9 and 0.88 in the same wavelength range, which is significantly smaller than the refractive index of molybdenum (between about 0.925 and 0.92) and silicon (between about 1.01 and 0.99) in the same wavelength range. Optimization was performed by maximizing the overall transmission T of the optical system as retrieved by ray tracing a representative sample of all field points and averaging over all those rays. This approach is similar to averaging over the spatial reflectance distribution of each mirror. The transmission spectrum was optimized as an integrated value over a band pass from 13.36 nm to 13.64 nm. One or more mirrors may have reflective coatings having a uniform layer thick-ness. At least one of the mirrors may have a graded coating characterized by non-zero gradient of a layer thickness of the reflective coating in at least one direction of the mirror surface. The layer thickness profiles of graded coatings may be described as a variation of geometrical (physical) layer thickness relative to a local position (x,y) on a mirror surface. At each location, the layer thickness may be measured along the surface normal, i.e. perpendicular to a tangent to the mirror surface at the respective location. The real (geometrical) layer thickness d(x,y) may then be described as a product of a nominal thickness, d0, and a modification factor fac(x, y), which depends on the location. In some embodiments, the local geometrical film thickness d(x, y) of the layers of the multilayer stack deviates by λ/100 or less (or λ/1000 or less) from a grading function according tod(x,y)=d0●max(0,fac(x,y)) (3), wherefac(x,y)=c0+c1y●y+c2●r(x,y)2 (4),wherein r(x,y)=√{square root over (x2+y2)}, where y is a coordinate in the first direction, x is a coordinate in the second direction, d0 is a nominal thickness measured in a z direction normal to the reflective surface perpendicular to the x and y direction in a local coordinate system of the reflective surface. Introducing the max ( ) function prevents the function d(x, y) from attaining values smaller then zero. In this polynomial expression for fac(x, y) the grading profile may be understood as a superposition of a constant value (c0), and a “tilting” of the coating in y-direction (corresponding to a linear variation of layer thickness in the direction in the meridional plane) and a rotational symmetric parabolic term. Therefore, in a grading according to a linear grading function (tilted coatings), the term c1y deviates from 0 and the parabolic constant c2=0. In a parabolic coating, c1y=0 and c2≠0. Mixed graded coatings with c1y≠0 and c2≠0 are possible. In mirrors designed for reflecting EUV radiation a reflective coating is typically formed by a stack of so-called bilayers, where a bilayer includes a relatively thick layer of a first material (such as silicon) having a first refractive index and a relatively thin second layer of a second material (such as molybdenum) having a second refractive index which is higher than the first refractive index. In general, a thickness ratio (γ factor) between the thicknesses of the first layer and second layer should be maintained essentially constant in such bilayer although the absolute thickness of the bilayer may vary across the mirror surface in a graded coating. In cases where such bilayers are used, the above condition also applies for the geometrical thickness d(x,y) of the bilayer. Absolute values for parameters c0, c1y and c2 may vary depending on the design of the optical system. Specifically, those parameters will vary depending on the numerical aperture NA of the projection objective which also determines the angles of incidence and related properties of the rays passing through the optical system. In some embodiments, the condition 0.90≦c0≦1.2 or the condition 0.95≦c0≦1.05 applies, such as 0.98≦c0≦1.02. In some embodiments the amount of the parameter c1y may be 0.1 or less, for example 0.01 or less. Sometimes, the condition 0.001≦c1y≦0.002 applies. In some embodiments, the absolute value of parameter c2 is 10−5 or less, such as 10−6 or less. The absolute value of parameter c2 may be 10−8 or more, such as 10−7 or more. These values apply for a nominal thickness d0=6.9 nm and allow to calculate the real physical thickness of the graded coating according to the respective grading function. If a different nominal thickness d0 were used, the same physical thickness of the graded coating would be obtained with a different set of values for the parameters i, c1y and c2. Therefore, also conversions based on different values for d0 shall be covered by these exemplary parameter values. The modulation factor fac(x, y) is defined in a local coordinate system of the mirror. The origin of the local coordinate system may or may not coincide with a reference axis of the optical system, i.e. it may be centred or decentred with respect to that reference axis. The reference axis may coincide with an optical axis of the optical system. However, systems having no optical axis may also be utilized. Table 3E presents, for each of the mirrors, the parameters c0, c1y and c2 of equation (4) from which the geometrical layer thicknesses, d(x, y,) of the single layers of the multilayer stack are calculated. Furthermore, the maximum reflectance Rmax [%] within the band path from 13.36 to 13.64 nm, obtained by averaging over the whole mirror, is given for each of the mirrors. An average transmission TAVG=7.33% and a maximum transmission TMAX=8.86 are obtained. TABLE 3ENA = 0.35Mirrorc0c1yc2Rmax [%]M11.0070068.1M21.0250066.6M31.0070 3.0e−667.9M40.99603.044e−567.3M51.1230.15e−2063.0M61.0140067.6 In view of the fact that coefficient c1y represents a linear term indicating an increase or decrease of layer thickness in the y direction (first direction, in the meridional plane) and parameter c2 represents a parabolic term, it can be seen that mirrors M1, M2, and M6 each have a reflective coating with uniform thickness of all layers. In contrast, mirrors M3, M4 and M5 each have a graded reflective coating having non-uniform layer thickness in the meridional direction. Specifically, mirrors M3 and M4 each have a rotationally symmetric graded coating with a parabolic shape. Mirror M5 optically close to a pupil surface, has a one-dimensionally graded coating where the layer thickness increases linearly along the first direction (in the meridional section) according to coefficient c1y, whereas the layer thickness does not vary along the second direction perpendicular thereto (perpendicular to the drawing plane in FIG. 3). In FIG. 10 a schematic drawing of a local mirror coordinate system LMCS with axes x′, y′ and z′ on a mirror M is shown. The origin of LMCS is decentered by DEC relative to the reference axis RA in the y-direction and has a distance D from the origin of the coordinate system CS of the optical system along the reference axis. FIG. 11A shows a schematic drawing of a graded reflective coating COAT on a substrate SUB, where the geometrical layer thicknesses of the individual layers of the multilayer stack vary rotationally symmetric around the z′ axis according to a parabolic function. Such parabolic coating may, for example, be applied to mirror M3 and mirror M4. FIG. 11B shows a schematic drawing of a one-dimensionally graded coating COAT according to a linear grading function. An exemplary embodiment of such linearly tilted coating is applied to pupil mirror M5. It has been found that tilted graded coatings, such as formed on mirror M5, which may be advantageous in terms of improving overall transmissions, for example, tend to break a rotational symmetry of optical systems and, therefore, may contribute substantially to generating field-dependent apodization. Based on this analysis a modified projection objective 100 was designed, which includes an apodization correcting element effective to decrease the field dependence of apodization. A method of designing such apodization correcting element may include the following steps: (Step 1) Calculating, for a plurality of field points distributed across the image field, a pupil apodization represented by a spatial intensity distribution at the exit pupil of the optical system; (Step 2) Calculating sub-apertures on each of the mirror elements for a number of field points including a number of field points close to or at the edge of the image field and close to or at the center of the image field. (Step 3) Selecting, from the plurality of mirrors, a mirror where the sub-apertures of problematic field points close to or at the edge of the field are relatively positioned to each other such that the surfaces to be reduced in intensity of the pupil do not overlap with sub-apertures of other field points Where those “freely accessible” sub-apertures are present, the intensity distribution in the exit pupil can be modified by modifying the relative reflectivities of the mirror element in the areas of the sub-apertures. (Step 4) Modifying reflectance of the mirror surface in the regions of the critical field sub-apertures such that a symmetry of the spatial intensity distribution in the exit pupil is increased. For many applications a completely homogenous intensity distribution in the exit pupil may be desirable, which could be considered as a highly symmetric intensity distribution. In may practical cases it may be sufficient to improve symmetry such that the intensity distribution and the exit pupil is more rotationally symmetric as in the case without an apodization correcting element. A first and at least one second apodization correcting element may be used in some cases. For ex-ample, a first apodization correcting element may be optimized to minimize the field variation of pupil apodization, and a second apodization correcting element adapted to the first apodization correcting element may be used to effect further corrections. For example, an apodization correcting element placed very close to or at a pupil surface may be used to correct contributions to pupil apodization which are essentially constant across the entire field. Therefore, a combination of at least two mutually adapted apodization correcting elements may be used in embodiments. It can be seen from step 3 that the mirror element to be used as an apodization correcting element to influence the field dependence of apodization may not be positioned exactly in the pupil since the sub-apertures of all field points overlap essentially in the pupil surface. In many cases the reflective mirror to be modified may also not be positioned exactly in or very close to a field surface since in that case all rays of a ray bundle would be incident in one common point of incidence such that a change of reflectance in the location of the point of incidence would effect all locations in the exit pupil in the same way, thereby making it impossible to modify the relative intensity levels at different locations in the pupil. The analysis revealed that mirror 320 (mirror M2) may be used to be modified such that the mirror forms or includes an apodization correcting element. As seen from table 3C P(M)=0.989 for mirror 320, indicating that the mirror is close to, but at a sufficient optical distance from a pupil surface. The field dependent apodization described in connection with FIGS. 6 and 7 may be described more generally such that a significant asymmetry of the pupil intensity distribution is present for edge field points at or close to the left and right shorter edges of the image field, whereas apodization is relatively small at the center of the image field and generally along a central region on both sides of the symmetry plane. The mirror element to be modified as an apodization correcting element may be selected such that the critical edge field points (showing significant asymmetry of pupil apodization) and relatively uncritical next neighbours thereof (closer to the symmetry plane) correspond to regions freely accessible at the outer edge of the mirror (i.e. regions with little or no overlap of corresponding sub-apertures). FIG. 12 shows schematically a “footprint” on second mirror M2, where a relatively narrow edge region ER close to the right edge of the mirror (marked dark in the FIG. 12) has been identified as corresponding to field points critical with respect to pupil apodization. As used here, the term “footprint” denotes the region on a mirror which is actually used for reflecting the radiation beam. In general, the physical shape and size of the mirror may essentially correspond to the respective footprint such that all rays incident on a mirror are actually reflected from the mirror. The shape of the footprint may be described as a rectangular shape with rounded edges, where a diameter Dy of the footprint in the y-direction (scanning direction) is significantly smaller than the diameter Dx in the cross-scan direction (x-direction). In this exemplary embodiment, the aspect ratio Dy/Dx of the footprint is about 0.55. Considering that the cross section of the radiation beam is rectangular with a high aspect ratio exactly at the object surface or image surface, and essentially circular in the pupil surface, the footprint shape indicates that mirror M2 is optically between the closest field surface and the pupil surface. The local reflectance R of second mirror M2 in a narrow edge region ER close to the right edge (indicated dark in FIG. 12) is now reduced to R=0% (no reflection) for demonstration purposes to illustrate qualitatively how a modification of local reflectance may be used to influence the spatial intensity distribution in the exit pupil of the projection objective. FIGS. 13 and 14 show respective spatial intensity distribution in the exit pupil for the center field point FP1 (FIG. 13) and the edge field point FP2 (FIG. 14). It is evident from a comparison between FIGS. 13 and 6 that the mirror symmetry of the intensity distribution to the meridional plane re-mains unchanged. However, the total amount of intensity at different locations of the pupil is slightly reduced since the reflection losses at the right edge of the second mirror M2 influences the corresponding sub-apertures in the upper and lower part thereof. In contrast, the intensity distribution of the pupil corresponding to edge field point FP2 shown in FIG. 14 is significantly changed when compared to the distribution of FIG. 7. Specifically, the shape of the non-reflection edge region ER is reproduced in a C-shaped edge region with no transmission T=0 intensity level at the right edge of the pupil. It can be seen from the effects explained the connection with FIGS. 12 to 14 that the variation of pupil apodization with locations in the image field may be effectively changed in a targeted manner if the reflectivity of a selected mirror (not too close to a pupil surface) is modified to influence the intensity distribution in the exit pupil of the projection objective. An exemplary embodiment of a projection objective including an apodization correcting element formed by a mirror element with a targeted spatial distribution of reflectivity will now be explained in connection with FIGS. 15 to 17. The basic optical design is as described for the reference system in FIG. 3, the only structural difference is the layout of mirror M2, which is optimized to form or include an apodization correcting element. FIG. 15 shows schematically positions of two selected sub-apertures SA-FP1 and SA-FP2 on the illuminated area of second mirror M2, where the area corresponds to the respective footprint F2. A sub-aperture on a mirror is an area on a mirror illuminated by a cone of light originating from a specific field point. Where an optical surface is placed exactly in a field surface, all corresponding sub-apertures are punctiform and all sub-apertures of spatially separated field points are spatially separated from each other. On the other hand, where an optical surface is positioned in a pupil surface, the corresponding sub-apertures, which may be generally circular, may completely overlap. Where a mirror is positioned at a distance from a pupil surface, sub-apertures of the different field points spaced apart from each other in a field surface do not completely overlap. In the exemplary embodiment illustrated in FIG. 15, a first sub-aperture SA-FP1 corresponding to a central field point lies essentially centered on the mirror, whereas a second sub-aperture SA-FP2 corresponding to an edge-field point at the edge of the field in the x-direction (compare FIG. 7) is laterally offset to the first sub-aperture in the x-direction. Whereas the first sub-aperture SA-FP1 (solid line) does not extend into the edge region ER at the outer edge of the footprint F2 in the x-direction, the second sub-aperture SA-FP2 (bold dashed line) extends into the edge region ER. The spatial separation of sub-apertures indicates that it is possible to manipulate the cones of radiation originating from the different field points separately and independently from each other, for example by changing the reflectivity of the mirror in the edge region ER, which may be accomplished by an appropriate intensity filter element. If an independent manipulation of the sub-apertures is desired, the filtering effect of the filter element should not extend into the intersection region covered by both footprints SA-FP1 and SA-FP2. Instead, the intensity manipulation by a filter element or the like should be located outside the first sub-aperture SA-SP1 corresponding to the central field point if a change of the intensity distribution corresponding to a central field point is not desired. An ex-ample of a filter region FR influencing the second sub-aperture SA-FP2 of the edge field point without influencing the first sub-aperture SA-FP1 of the central field point in the edge region ER is indicated with small dashed line in FIG. 15 (compare FIG. 12). As explained in connection with FIGS. 6 and 7, only a relatively small amount of uneven pupil apodization is present for a central field point in the reference system (FIG. 6), whereas a highly asymmetric pupil apodization is found for the edge field point FP2. As a general trend, the asymmetry of apodization has been found to increase from the central field point FP1 towards the edge field point FP2 along the x-direction. From this analysis it is concluded that an improvement towards an increased symmetry of the intensity distribution in the exit pupil may be obtained by filtering the intensity corresponding to a filtering function with little or no filtering action close to or at the center of the filer element and an filtering efficiency which may be essentially zero in the region where the footprints SA-FP1 and SA-FP2 overlap and which may increase significantly towards the outer edge in the x-direction where the edge region ER is located. The schematic drawings in FIGS. 16A to 16C are used to further describe an exemplary embodiment of an apodization correcting element designed as an intensity filter which influences only the second sub-aperture SA-FP2 of the edge filed point without influencing the first sub-aperture SA-FP1 of the central field point. To this end, FIG. 16A shows the generally oval shape of second mirror M2, FIG. 16B shows a section (y-cut) through the mirror in a z-y plane (meridional plane) between the origin O of the mirror and the outer edge in y-direction, and FIG. 16C shows a section in x-direction (x-cut) from the origin to the outer edge in the x-direction, where the edge region ER shown in FIG. 15 is located. As explained in connection with Table 3D, for example, the reflective coating on second mirror M2 includes a multilayer stack MLS having a bilayer stack BS formed by a multitude of bilayers, where each bilayer includes a relatively thick silicon layer and a relative thin molybdenum layer. The bilayer structure is formed on an anti stress layer interposed between the bilayer structure and the substrate (not shown in FIG. 16). A cap layer CL made of ruthenium is formed on the bilayer stack BS and forms the interface of the reflective coating towards the environment on the radiation entry side. The cap layer has a uniform thickness in the reference example of FIG. 3 discussed above. The cap layer forms a protective layer to protect the bilayer stack from contamination and the like. Further, the cap layer absorbs a specific amount of radiation energy depending on the geometrical thickness of the cap layer and the absorption coefficient k of the cap layer material. It is evident that a spatial variation of absorbing effect of the cap layer may be obtained if the geometrical thickness of the cap layer varies across the mirror surface. In general, the geometrical layer thickness of the cap layer varies according to a non-rotationally symmetric grading function. The y-cut in FIG. 16B shows schematically the thickness variation in the first direction (y-direction) lying in the meridional plane, which may be described with a first grading function. FIG. 16C shows the variation of thickness in the second direction (x-direction, cross-scan-direction), which may be described by a second grading function. It is evident that the first grading function differs from the second grading function. The geometrical thickness increases from the origin O towards the edge of the mirror slightly in the first direction, whereas the amount of increase between the center and the edge region is significantly larger in the second direction such that a stronger absorbing effect is obtained in the edge region ER on the x-axis than in the direction perpendicular thereto. Both the first and second grading functions are continuous functions indicating a continuous distribution (without thickness steps) of geometrical layer thickness in the different radial direction from the origin. The layer thickness is generally uniform or has only a slight variation in a central zone around the origin O at least up to radial coordinates corresponding to the outer edge of the region corresponding to the first sub-aperture SA-FP1 corresponding to the central field point such that all the rays originating from the central field point “see” approximately the same geometrical layer thickness, whereby no significant variation of filtering action is obtained for the rays originating from the central field point. The central region having essentially uniform cap layer thickness may extend to more than 20% or more than 50% or more than 70% of the maximum radial distance to the origin in the respective direction. Outside the central region the layer thickness of the cap layer increases slightly in the y-direction and increases sharply in the x-direction corresponding to the edge region ER shown in FIG. 15. The schematics of FIGS. 16B and 16C are not to scale. The cap layer CL has a spatial variation of geometrical thickness which is non-rotational symmetric to the origin O. It has been found useful to describe the geometrical layer thickness g(x,y) of the cap layer using a non-rotationally symmetric polynomial in x and y according to: g ⁡ ( x , y ) = c ⁢ ⁢ 1 ⁢ ⁢ y * y + b 1 ⁢ x 10 + b 2 ⁢ x 8 ⁡ ( y - y ⁢ ⁢ 0 ) 2 + b 3 ⁢ x 6 ⁡ ( y - y ⁢ ⁢ 0 ) 4 + b 4 ⁢ x 4 ⁡ ( y - y ⁢ ⁢ 0 ) 6 + b 5 ⁢ x 2 ⁡ ( y - y ⁢ ⁢ 0 ) 8 + b 6 ⁡ ( y - y ⁢ ⁢ 0 ) 10 ( 5 ) It can be seen that the polynomial does not have uneven powers in x-coordinates indicating that the function is mirror symmetric with respect to the meridional plane (corresponding to a y-z plane). In other words, the left half and the right half of the mirror with respect to the meridional plane MP are mirror symmetric to each other. Further, the polynomial does not have uneven powers in y-coordinate, which indicates that the desired intensity attenuation to be effected by the filtering action of the cap layer has a smooth variation towards the edge of the mirror with no inflection points. In the exemplary embodiment optimized to at least partly compensate the influence of the tilted coating on fifth mirror M5, the thickness profile of the cap layer of mirror M2 may be described with the following coefficients: c1y=7.33e-3 y0=33.46 b1=2.316e-17 b2=3.976e-16 b3=−1.61e-15 b4=9.896e-15 b5=−5.029e-15 b6=3.209e-15 In this formulation, the function g(x,y) describes the spatial variation of the layer thickness of the cap layer across the surface of the mirror, y0 describes a decentering of the layers relative to the origin O of the local coordinate system of the substrate and the coefficients c1y and b1-b6 correspond to the coefficients of the polynomial description of the lateral layer thickness variation, which may be used as free parameters to be optimized for a specific optical system. The absolute value of the layer thickness of the cap layer CL at the origin O may be described in this exemplary embodiment, by d0′=d0*1.025, where d0=6.9 nm is the nominal thickness used in the exemplary embodiment described above for calculating the real physical thickness of the layers of the multilayer stack MLS. The thickness values of the multilayer stack MLS covered by the cap layer CL is the same as in the reference example described in FIG. 3. The optical effect of the intensity filtering by the apodization correcting element formed by modified mirror M2 will now be explained in connection with FIGS. 17A, 17B and 17C. Similar to FIGS. 6 and 7 these figures demonstrate the apodization characteristics and present schematic plots of the spatial intensity distribution in the circular exit pupil of the projection objective. In this case, this intensity distribution is shown for three different image field points FP2, FP3 and FP4, each positioned at the outer edge of the rectangular image field in the x-direction. While FP2 lies in the middle of this edge on the x axis (FIG. 17A), FP3 lies at the upper corner at this edge (FIG. 17B) and FP4 lies at the lower corner at this edge (FIG. 17C). It is evident that the intensity distribution in the pupil corresponding to of each of these edge field points exhibits only a relatively weak apodization when compared to the apodization without correction shown in FIG. 7. In general, the intensity value varies between a minimum value of about 0.085±0.001 in a large central region and less than 0.050 at the outer edge of the pupil. Remarkably, the pupil apodization is significantly improved with respect to rotational symmetry when compared to the same projection objective without apodization correcting element. In order to demonstrate the improvement in rotational symmetry, the apodization parameter APO calculated for the edge field point FP2 in connection with FIG. 7 is now calculated for the system including an apodization correcting element (compare FIG. 17A). The apodization correction is effective such that there is virtually no change of the local intensity at the edge of the exit pupil at the left side where the smallest value for the intensity for all coordinates at the edge of the exit pupil occurs. The minimum intensity value IMIN=0.47 is not changed significantly. However, the maximum value IMAX of intensity in the edge region of the exit pupil has been reduced in the region where the maximum was found in the distribution of FIG. 7 to a value IMAX=0.83 in the lower right edge region of the exit pupil. The corresponding apodization parameter is APO=0.277, which is about 10% smaller than the apodization parameter for the reference system without apodization filter (APO=0.314). Therefore, the rotational symmetry particular at the outer edge of the exit pupil is significantly improved by the effect of the apodization correcting element. A substantially rotational symmetric intensity distribution in the exit pupil allows to image structure features with different orientations with about the same contrast independent of the structure orientation, whereby H-V differences may be reduced when compared to the optical system without the apodization correcting element. Further, the energy centroid of the intensity distribution in the pupil is shifted very close to the center of the pupil, thereby improving image-side telecentricity. Embodiments of optical systems consisting of mirrors only (catoptric systems) may be designed for various wavelength ranges, for example for DUV wave-lengths at about 193 nm or less (operating e.g. with an ArF light source). Some embodiments are designed for EUV wavelength 2 nm<λ<20 nm and/or 10 nm<λ<15 nm and/or 13 nm<λ<14 nm Embodiments capable of use in a micro-lithographic projection exposure system are typically designed to provide high resolution such as resolutions less than 1 μm or less than 0.5 μm or less than 100 nm, for example. The above description of the embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present disclosure and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. It is sought, therefore, to cover all changes and modifications as fall within the spirit and scope of the disclosure, as defined by the appended claims, and equivalents thereof.
047160055	description	DESCRIPTION OF THE PREFERRED EMBODIMENT The invention will be described as applied to a pressure vessel in a nuclear reactor, however, it will be realized by those skilled in the art that it has wider application to a great many installations where it is desired to form a pressure tight seal between two planar sealing surfaces. Thus, by way of examples which are not meant to be limiting, the invention could also be utilized to provide a pressure tight seal for pump housings, fluid conduits and other types of pressure vessels. As illustrated in FIG. 1, the nuclear reactor pressure vessel 1 in connection with which the invention will be described, includes an upright cylindrical body 3 having a radially, outwardly extending flange 5 at its upper end which defines an axially facing planar sealing surface 7. A removable, hemispherical head 9 terminates at its lower end in a mating, radially extending flange 11 which also defines an axially facing sealing surface 13. The head 9 is secured to the pressure vessel body 3 by a number of bolts 15 which extend through bores 17 and 19 in flanges 5 and 11 respectively. Under normal operating conditions, reactor coolant is introduced into the pressure vessel 1 at a pressure between 2000 and 3000 psi. In accordance with the invention, a seal assembly 21 provides a pressure tight seal between the confronting sealing surfaces 7 and 13 on the flanges 5 and 11. As can best be seen from FIGS. 2 and 3, the seal assembly 21 includes a spacer unit 23 and a toroidal, crushable seal member 25. The spacer unit 23 includes a flat, ring shaped member 27 with a resilient inner portion which is formed by a circular coil spring 29 seated in a radially extending V-shaped recess 31 in the inner surface formed by the bore 33 of the ring shaped member. Alternatively, an elastomeric solid or hollow O-ring can be used in place of the circular coil spring 29. The toroidal, crushable seal member 25 is a circular metallic tube which is received in the bore 33 in the ring shaped member 27 and rests on a shoulder 35. The seal member 25 is locked in place within the ring-shaped member 27 by the circular coil spring 29 so that the seal assembly 21 can be maneuvered as a unit. This greatly facilitates alignment of the seal and is an important feature of the invention. The ring-shaped member 27 is provided with a number of axial bores 37 which register with the flange bores 17 and 19 to positively locate the seal assembly 21 between the sealing surfaces 7 and 13. The thickness t.sub.1 of the seal member 25 is greater than the thickness t.sub.2 of the ring-shaped member 27, so that as the bolts 15 are tightened to draw the sealing surfaces 7 and 13 toward each other the seal member 25 is contacted first and crushed betwen the sealing surfaces. As this occurs, the seal member 25 flattens out against the sealing surfaces 7 and 13 as shown in FIG. 4 to form an in depth seal. The spacer unit 23 maintains the proper position of the seal member 25 while it is being crushed yet the resiliency of the circular coil spring 29 permits it to expand radially as it is deformed. Preferably, the ring-shaped member 27 is made of a rigid material such as steel so that its thickness, t.sub.2, provides a positive limit for the crushing of the seal member 25. The tubular seal member 25 is provided with radial bores 39 along its inner surface to admit pressurized reactor coolant into the interior of the seal member to improve its sealing capability. The alignment of the toroidal seal member 25 relative to the planar sealing surfaces 7 and 13 without the necessity of cutting grooves in, or providing other special arrangements on the planar sealing surfaces, is another important feature of the invention. By way of example, a typical seal unit made in accordance with the teachings of the invention for use in sealing the head on a nuclear reactor pressure vessel includes an annular 160.822 inch O.D. seal member 25, made of 0.5 inch O.D. tubular stainless steel material with a 0.050 inch wall thickness and 0.06 to 0.07 inch apertures 39 and coated with a 0.001 to 0.002 layer of silver to fill in machining imperfections. The spacer unit 23 includes a flat 0.473 inch thick ring member 27 having a 184 inch O.D. and a 160.54 inch I.D., and a solid annular 161.35 inch O.D. resilient member 29 made of 0.25 inch elastomeric material. While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
	summary
	claims	1. An underground nuclear reactor, comprising:a containment member including:(a) a bottom wall having a first end, a second end, a first side, a second side, an upper side and a lower side;(b) an upstanding first end wall having a lower end, an upper end, an inner side, an outer side, a first end and a second end;(c) said first end wall extending upwardly from said first end of said bottom wall;(d) an upstanding second end wall having a lower end, an upper end, an inner side, an outer side, a first end and a second end;(e) said second end wall extending upwardly from said second end of said bottom wall;(f) said second end wall of said containment member having a door opening formed therein;(g) a door movably positioned in said door opening in said second end wall of said containment member with said door being movable from a normally closed position to an open position when an explosion or blast occurs in the nuclear reactor;(h) an upstanding first side wall having a lower end, an upper end, an inner side, an outer side, a first end and a second end;(i) said first side wall extending upwardly from said first side of said bottom wall;(j) an upstanding second side wall having a lower end, an upper end, an inner side, an outer side, a first end and a second end;(k) said second side wall extending upwardly from said second side of said bottom wall;(l) an upper wall having a first end, a second end, a first side, a second side, a lower side and an upper side;(m) said upper wall extending between said upper ends of said first end wall, said second end wall, said first side wall and said second side wall so that said containment member defines an interior compartment therebetween; and(n) said upper wall of said containment member being located below ground level whereby said containment member is completely buried in the ground;a nuclear reactor positioned in said interior compartment of said containment member;an elongated and hollow blast tunnel comprising:(a) a bottom wall with a first end, a second end, a first side, a second side, an upper side and a lower side;(b) an upstanding first side wall extending upwardly from said first side of said bottom wall with said first side wall having an upper end, a lower end, a first end, a second end, an inner side and an outer side;(c) an upstanding second side wall extending upwardly from said second side of said bottom wall with said second side wall having an upper end, a lower end, a first end, a second end, an inner side and an outer side;(d) a first end wall, having an upper end and a lower end, at said first end of said bottom wall and which has a door opening formed therein which communicates with said door opening in said second end wall of said containment member when said door is in said open position;(e) a second end wall, having an upper end and a lower end, at said second end of said bottom wall which extends between said second ends of first and second side walls;(f) an upper wall positioned at said upper end of said first end wall of said blast tunnel, said second end wall of said blast tunnel, said first side wall of said blast tunnel, and said second side wall of said blast tunnel; and(g) said walls of said blast tunnel defining a blast chamber configured to receive debris from said nuclear reactor in the event that said nuclear reactor explodes thereby creating a blast force extending therefrom. 2. The underground nuclear reactor of claim 1 wherein said upper wall of said blast tunnel has a roof opening formed therein and wherein a roof portion is positioned on said blast tunnel and which is positioned in said roof opening to normally close said roof opening but which is selectively movable to an open position. 3. The underground nuclear reactor of claim 1 wherein a plurality of spaced-apart first deflectors are secured to said inner side of said first side wall of said blast tunnel so as to be partially in the path of debris passing through said blast tunnel from said first end of said blast tunnel towards said second end of said blast tunnel and wherein a plurality of spaced-apart second deflectors are secured to said inner side of said second side wall of said blast tunnel so as to be partially in the path of debris passing through said blast tunnel from said first end of said blast tunnel towards said second end of said blast tunnel. 4. The underground nuclear reactor of claim 3 wherein said first deflectors are vertically disposed and horizontally spaced-apart and wherein said second deflectors are vertically disposed and horizontally spaced-apart. 5. The underground nuclear reactor of claim 4 wherein each of said first and second deflectors has an angular shape. 6. The underground nuclear reactor of claim 3 wherein said first and second deflectors are comprised of a concrete material. 7. The underground nuclear reactor of claim 3 wherein said first and second deflectors are selectively removably secured to said first and second side walls of said blast tunnel respectively. 8. The underground nuclear reactor of claim 3 wherein each of said first and second deflectors have upper and lower ends with said lower ends thereof being positioned on said upper side of said bottom wall of said blast tunnel. 9. The underground nuclear reactor of claim 3 wherein each of said first and second deflectors have a leading face which is disposed at an angle with respect to the longitudinal axis of said blast tunnel. 10. The underground nuclear reactor of claim 1 wherein said containment member is comprised of a concrete material. 11. The underground nuclear reactor of claim 1 wherein said blast tunnel is comprised of a concrete material. 12. The underground nuclear reactor of claim 1 wherein said door openings are large enough to permit said nuclear reactor to pass therethrough for repair or replacement. 13. The underground nuclear reactor of claim 2 wherein said roof opening is large enough to permit said nuclear reactor to pass therethrough for repair or replacement. 14. The underground nuclear reactor of claim 1 wherein said door is selectively movable from said closed position to said open position. 15. An in-ground nuclear power reactor, comprising:a containment member having a wall, a bottom wall, a first end wall, a second end wall, a first side wall, a second side wall, an upper wall and an interior compartment;a nuclear reactor positioned in said interior compartment of said containment member;said second end wall of said containment member having a door opening formed therein;a hollow blast tunnel having a first end wall, a second end wall, a first side wall having inner and outer sides, a second side wall having inner and outer sides, a bottom wall, an upper wall and a blast mitigation chamber;said first end wall of said blast tunnel having a door opening formed which communicates with said door opening in said second end wall of said containment member;a door movably positioned in said door openings; andsaid door being normally closed but being movable to an open position upon a predetermined blast force being exerted thereon should the nuclear reactor explode whereby debris from said exploded nuclear reactor will pass through said door openings into said blast mitigation chamber. 16. The in-ground nuclear power reactor of claim 15 wherein a plurality of spaced-apart first deflectors are mounted on said inner side of said first side wall of said blast tunnel and wherein a plurality of spaced-apart second deflectors are mounted on said inner side of said second side wall of said blast tunnel. 17. The in-ground nuclear power reactor of claim 16 wherein said second deflectors are off-set with respect to said first deflectors. 18. The in-ground nuclear power reactor of claim 16 wherein said first and second deflectors are selectively removably secured to said inner sides of said first and second side walls of said blast tunnel. 19. The in-ground nuclear power reactor of claim 15 wherein said upper wall of said blast tunnel has an opening formed therein which is selectively closed by a roof portion. 20. The in-ground nuclear power reactor of claim 15 wherein said door openings are large enough to permit said nuclear reactor to pass therethrough. 21. The in-ground nuclear power reactor of claim 19 wherein said opening in said upper wall of said blast tunnel is large enough to permit said nuclear reactor to pass therethrough.
	abstract	A vibration-type cantilever holder holds a cantilever opposed to a sample. The holder supports a main body part of the cantilever at only its base end so that a probe at the free end of the cantilever can contact the sample. The holder has a cantilever-attaching stand on which the main body part is placed and fastened such that the cantilever is tilted at a predetermined angle with respect to the sample. A first vibration source is fastened to the cantilever-attaching stand and vibrates with a phase and an amplitude depending on a predetermined waveform signal, and the first vibration source is fastened at a first location to a holder main body. A second vibration source is fastened at a second location, which is spaced from the first location, to the holder main body and generates vibrations to offset vibrations traveling from the first vibration source to the cantilever-attaching stand and holder main body. The holder allows the cantilever to vibrate according to the vibrational characteristics of only the cantilever by counteracting additional vibrations.
	abstract	Improved radiation devices and their associated fabrication and applications are described herein. The microirradiators generally include a non-radioactive conducting electrode, an insulating sheath, a radioactive source, and, optionally, a contact electrode. The microirradiators generally produce low absolute radiation levels with high radiation flux densities.
	description	The instant invention generally relates to nuclear medicine, and systems for obtaining nuclear medical images of interest. In particular, the instant invention relates to a novel detector configuration for preclinical single photon imaging including single photon emission computed tomography (SPECT) or planar imaging Nuclear imaging is a unique specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the subject, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed. In conventional nuclear imaging arrangements, collimators are used in a wide variety of equipment in which it is desired to permit only beams of radiation emanating along a particular path to pass a selected point or plane. Collimators are frequently used in nuclear imagers to ensure that only radiation beams passing along a direct path from the known radiation source strike the detector thereby minimizing detection of beams of scattered or secondary radiation. Particularly in nuclear imagers used for preclinical analysis or for non-destructive evaluation procedures, it is important that only radiation emanating from a known source and passing along a direct path from that source be detected and processed by the imaging equipment. If the detector is struck by undesired radiation such as that passing along non-direct paths to the detector, performance of the imaging system can be degraded. Two principal types of collimators have been used in nuclear imaging. The predominant type of collimation is the parallel-hole collimator. This type of collimator contains hundreds of parallel holes drilled or etched into a very dense material such as lead. The parallel-hole collimator accepts only photons traveling perpendicular to the scintillator surface, and produces a planar image of the same size as the source object. In general, the resolution of the parallel-hole collimator increases as the holes are made smaller in diameter and longer in length. The conventional pinhole collimator typically is cone-shaped and has a single small hole drilled in the center of the collimator material. The pinhole collimator generates a magnified image of an object in accordance with its acceptance angle, and is primarily used in studying small organs such as the thyroid or localized objects such as a joint. The pinhole collimator must be placed at a very small distance from the object being imaged in order to achieve acceptable image quality. Pinhole collimators offer the benefit of high magnification of a single object, but lose resolution and sensitivity as the field of view (FOV) gets wider and the object is farther away from the pinhole. U.S. Pat. No. 7,166,846, assigned to the same assignee herein and incorporated herein by reference in its entirety, discloses a multi-pinhole collimator nuclear medical imaging detector that divides a target object space into many non-overlapping areas and projects a minified image of each area onto a segmented detector, where each segment functions as an independent detector or imaging cell. Other known types of collimators include the slant-hole collimator, converging and diverging collimators, and the fan beam collimator. The slant-hole collimator is a variation of the parallel-hole collimator but with all holes slanted at a specific angle. This type of collimator is positioned close to the body and produces an oblique view for better visualization of an organ whose line of sight may be partially blocked by other parts of the body. The converging collimator has holes that are not parallel but instead are focused toward the organ, with the focal point being located in the center of the field of view. The image appears larger at the face of the scintillator using a converging collimator. A diverging collimator results by reversing the direction of the converging collimator. The diverging collimator is typically used to enlarge the FOV, such as would be necessary with a portable camera having a small scintillator. The fan beam collimator is typically used with a rectangular camera head to image smaller organs. The holes are parallel when viewed from one direction and converge when viewed from another direction. The fan beam collimator allows the maximum surface of the crystal to be used to capture imaging data. In most applications, the choice of collimation represents a trade-off between the size of the FOV and the sensitivity and spatial resolution required to properly visualize the target object or organ. Collimators are positioned to substantially absorb the undesired radiation before it reaches the detector. The collimator includes (or is manufactured from) a relatively high atomic number material and the collimator is positioned so that undesired radiation strikes the body of the collimator and is absorbed before being able to strike the detector. In a typical detector system the collimator includes barriers associated with the detector and located in the direction of the radiation source. An example exists in radiation imaging systems used for medical diagnosis which use a small point source of radiation to expose the patient under examination. The radiation passes through the patient and strikes a radiation detector that is oppositely positioned. Conventional single photon imaging systems with parallel-hole collimation use large area (on the order of 2000 cm2) monolithic scintillation detectors, and typically have an intrinsic spatial resolution of approximately 3.5 mm FWHM (Full Width Half Maximum). Such detectors are made either of sodium iodide crystals doped with thallium (NaI(Tl)), or cesium iodide (CsI). Scintillations within the NaI crystal caused by absorption of a gamma photon within the crystal, result in the emission of a number of light photons from the crystal. The scintillations are detected by an array of photomultiplier tubes (PMTs) in close optical coupling to the crystal surface. The intrinsic spatial resolution is primarily determined by the size of the PMTs. The design of the parallel-hole collimator (i.e., the length and diameter of the collimator holes) fixes the system resolution, and represents a trade-off between sensitivity (i.e., the number of detected gamma rays) and spatial resolution (i.e., sharpness of the image) of the imaged target object. The system spatial resolution is a quadrature sum of the geometric resolution of the collimator and the intrinsic resolution of the camera. In most clinical imaging studies, the predominant spatial resolution achieved is determined by the geometric resolution of the collimator, and thus there has not been a strong incentive to increase the intrinsic spatial resolution of the gamma camera. Conventional commercial gamma cameras are designed to minimize cost by using the largest possible size PMTs, and thus achieve an intrinsic spatial resolution of about 3.5 mm FWHM. However, recent detector technology has enabled the design of small gamma cameras with intrinsic spatial resolution of less than 1 mm FWHM. Thus, there exists a need in the art for improvements in collimator technology to take advantage of such increased intrinsic spatial resolution in the development of new commercial gamma cameras. In the instance of SPECT scanning, a subject (patient) is infused with a radioactive substance that emits radioactive or gamma rays. Conventionally, a gamma camera includes a transducer to receive the gamma rays and record an image therefrom. In order for the image to be a true representation of the subject being investigated, a collimator having collimating apertures is positioned between the transducer and the subject to screen out all of the radioactive rays except those directed along a straight line through the collimating apertures between a particular part of the subject and a corresponding particular part of the transducer. Traditionally, the collimator is made of a radiation opaque material such as lead, and collimating apertures have been formed therein by various means such as drilling holes therethrough. In conventional SPECT system designs, lead collimator gamma cameras have been supported on gantries that rotate the camera head through an angular range of one hundred eighty or three hundred sixty degrees around the patient. One drawback associated with this requirement however, is that such gantry systems are relatively expensive subsystems of the diagnostic tool because they must be capable of providing rapid rotation of the large and heavy camera heads through very precise orbits about the patient. Further, rotating gantries require a large degree of space for the actual unit as well as for full operational ability. This is especially problematic with preclinical SPECT wherein lab space limitations are more prevalent. As a result, one object of the present invention is to accommodate the use of space and money-saving simplified gantries, without sacrificing image quality. In general, the invention features a preclinical nuclear imaging detector system, comprising a gantry and one or more detector assemblies. The gantry is optionally fixed and configured for securing the one or more detector assemblies substantially about an axis to thereby describe a portion of a detector perimeter. The one or more detector assemblies each comprise a scintillator configured to interact with radiation emanating from a target object being imaged and at least one pinhole collimator, having one or more pinhole apertures formed therein. The pinhole collimator is disposed between the target object and the scintillator, wherein a distance between the pinhole aperture and the scintillator is selected as a function of the number of pinhole apertures provided in the collimator, such that the one or more pinhole apertures collectively project a unitary minified radiation image of the target object onto the scintillator. Further, one or more photosensors are optically coupled to the scintillator to receive interaction events from the scintillator. In one embodiment of the instant invention, the imaging detector system has a knife-edge pinhole collimator aperture. In another embodiment, the imaging detector system has a keel-edge pinhole collimator aperture. In another embodiment, the imaging detector system has a perimeter that is a portion of an ellipse. In other embodiments, the imaging detector system has a perimeter that is a portion of a polygon. In still other embodiments, the imaging detector system has a perimeter that is a portion of a rectangle. In still another embodiment, the detector assemblies form a substantially contiguous array disposed about a perimeter. In yet another embodiment, at least one or more detector assemblies have a pinhole plate, the adjustable pinhole plate is positionable relative to the scintillator and along an axis that is perpendicular to a plane of the scintillator. In yet still another embodiment, the pinhole collimator has a plurality of walls in slidable engagement with one another to allow the pinhole collimator to be positioned relative to the scintillator. In a further embodiment, the collimator has a plurality of walls disposed and slidable with respect to one another. In a yet further embodiment, the walls include a friction reducing element therebetween. Generally, the instant invention features a method of performing a preclinical scan of a test subject employing a preclinical nuclear imaging detector system. The preclinical nuclear imaging detector system therefore comprises a gantry and one or more detector assemblies. The gantry is optionally fixed and configured for securing the one or more detector assemblies substantially about an axis to thereby describe a portion of a detector perimeter. The one or more detector assemblies each comprise a scintillator configured to interact with radiation emanating from a target object being imaged and at least one pinhole collimator, having one or more pinhole apertures formed therein. The pinhole collimator is disposed between the target object and the scintillator, wherein a distance between the pinhole aperture and the scintillator is selected as a function of the number of pinhole apertures provided in the collimator, such that the one or more pinhole apertures collectively project a unitary minified radiation image of the target object onto the scintillator. Further, one or more photosensors are optically coupled to the scintillator to receive interaction events from the scintillator. The method includes positioning a test subject with respect to the one or more detector assemblies, positioning the pinhole aperture with respect to the scintillator, then obtaining image data. In another embodiment, a method of performing a preclinical scan of a test subject employs a preclinical nuclear imaging detector system including using a pinhole aperture that is positionable to effect a desired spatial resolution. In yet another embodiment, the method of performing a preclinical scan of a test subject employs a preclinical nuclear imaging detector system having a pinhole aperture that is positionable to effect a desired spatial sensitivity. In still another embodiment, a method of performing a preclinical scan of a test subject employs a preclinical nuclear imaging detector system according to the instant invention including using a plurality of detectors configured to describe a portion of a perimeter. In yet still another embodiment, a method of performing a preclinical scan of a test subject employs a preclinical nuclear imaging detector system including using slidable collimators to adjust one of distance d and/or focal length f. In other embodiments, a method of performing a preclinical scan of a test subject employs a preclinical nuclear imaging detector system including adjusting the distance d between the pinhole aperture and the subject between 100 mm and 300 mm. In yet other embodiments, a method of performing a preclinical scan of a test subject employing a preclinical nuclear imaging detector system includes adjusting distance d between the pinhole aperture and the subject between 125 mm and 225 mm. In still other embodiments, the method of performing a preclinical scan of a test subject employing a preclinical nuclear imaging detector system includes adjusting the distance d between the pinhole aperture and the subject to 125 mm. In yet still other embodiments, a method of performing a preclinical scan of a test subject employing a preclinical nuclear imaging detector system includes adjusting the distance d between the pinhole aperture and the subject to 225 mm. Examples of the main features of this invention have thus been outlined rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will also form the subject of the claims appended hereto. Referring now to the figures, FIGS. 1 and 6 disclose example embodiments of preclinical nuclear imaging detector systems according to the instant invention. The detectors of FIG. 1 are generally disposed in a rectangular or square configuration, whereas the detectors of FIG. 6 are generally disposed in an arcuate configuration. Generally, a preclinical nuclear imaging detector system 10 comprises one or more detector assemblies 12 secured to a gantry and/or support structure 11. Detector assemblies 12 are radially disposed about an axis 14 to thereby describe a perimeter 16 or a portion thereof and are optionally fixed relative to gantry and/or support structure 11. In the embodiment illustrated in FIG. 1, preclinical nuclear imaging detector system 10 further includes an object table 18 which is further shown supporting object 20, e.g., a phantom or test object such as a laboratory animal. Referring now to FIGS. 2 to 5, detector assembly 12 is shown as comprising a scintillator 22, at least one photosensor 26 optically coupled to said scintillator 22, and at least one collimator 24, which is preferably a pinhole collimator, and more preferably, a multi-pinhole collimator. For example, collimator 24 can be of a type disclosed and described in U.S. patent application Ser. No. 10/881,674 filed 30 Jun. 2004 and assigned to the same assignee herein, which application is incorporated herein by reference in its entirety. As shown in FIG. 2, collimator 24 comprises a multi-pinhole collimator spaced apart from scintillator 22 by a focal length f such that the one or more pinhole apertures 28 collectively project a unitary minified radiation image of the subject 20 onto the scintillator 22. Also, at least one photosensor 26 is optically coupled to the scintillator 22 to receive interaction events from the scintillator 22. In an aspect of the invention, collimator 24 can include a plurality of pinhole apertures 28 having aperture acceptance angles α, as shown in FIGS. 3 and 4. The acceptance angle can be selected such that each pinhole aperture 28 projects a non-overlapping area of a field of view of test object 20 being imaged onto scintillator 22 of the preclinical nuclear imaging detector system 10 so as to collectively project a unitary minified radiation image of test object 20 onto scintillator 22. In another aspect, alone or in combination with that described above, multi-pinhole collimator 24 can include one or more septa 30, e.g., made of suitable dense material, such as lead or tungsten polymer, etc., positioned between pinhole plate 34 and scintillator 22 and between at least two pinhole apertures 28. For example, FIG. 2 illustrates a preclinical nuclear imaging detector system 10 including septa 30 wherein multiple conical projections 32 do not overlap each other at their point of intersection with scintillator 22. Overlap can be further prevented and/or minimized by adjusting the pinhole acceptance angle α and focal length f without septa 30, as shown in FIGS. 3 and 4, or by adjusting the acceptance angle α and focal length f in addition to providing the septa 30. Pinhole apertures 28 can include a number of types thereof, including but not limited to, knife 38 or keel 40 as disclosed in FIGS. 7 and 8, respectively. Detector assembly 12 is configured such that the focal length f, between collimator plate 24 and scintillator 22 can be adjusted, as can distance d between object 20 and collimator plate 24. More specifically, FIGS. 3 and 4 illustrate detector assembly 12 wherein pinhole plate 34 is disposed at first and second positions, respectfully, relative to scintillator for purposes of adjusting focal length. As illustrated in FIGS. 3-4 and 6, in exemplary embodiments, distance d between the pinhole plate 34 and object 20 can be between 1 mm to 1 cm, preferably between 100 mm to 300 mm, and more preferably, between 125 mm and 225 mm. FIGS. 9A and 9B illustrate images of a phantom wherein the distance between pinhole plate 34 and object 20 is 225 mm and 125 mm, respectively. Adjustment of focal length f and/or distance d can be accomplished by providing septa 30 and/or collimator sidewalls 36 that are slidable relative to one another, as along tracks or channels, etc. As illustrated in FIG. 5 the septa 30 and/or side walls 36 can include friction reducing element 38, such as wheels, for easing movement therebetween. Referring now to FIG. 6, a preclinical nuclear imaging detector system according to the instant invention can comprise a plurality of detector assemblies optionally positionally fixed relative to gantry and/or support structure 11 and about axis 14 to describe a perimeter 16. In the embodiment illustrated in FIG. 6, perimeter 16 describes a portion of an ellipse, which ellipse can describe a circle and/or arc. Alternatively, perimeter 16 can describe a portion of a polygon for example, a rectangle, which can further describe a square. While an imaging device according to the instant invention can be used for obtaining images of a subject, it is preferably configured for use to conduct preclinical assessments on, for example, test objects such as laboratory animals 20. In one embodiment, when the preclinical nuclear imaging detector system according to the instant invention is configured for laboratory animal analyses, the footprint of the system is minimal when compared to known diagnostic SPECT devices used for clinical diagnoses, for example, with human medical patients. Consequently, a test object 20, such as a laboratory mouse or rat, can be positioned with respect to one or more detector assemblies 12, one or pinhole apertures 28 positioned relative to scintillator 22 to optimize one of spatial resolution and spatial sensitivity and an image of the subject 20 is obtained. It is understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is define by the scope of the appended claims. Other aspects, advantages and modifications are within the scope of the following claims.
051857758	claims	1. A radiological apparatus, used in radiological examination of the lower limbs, the apparatus comprising: an X-ray source which emits a beam of X-rays towards a patient; a table on which the patient may be laid out; and an X-ray receiver disposed on the opposite side of the table to the side on which the source is placed, the table and/or the source-receiver pair being capable of displacement relative to each other so that the X-ray beam is capable at least of scanning the lower limbs of the patient in the longitudinal direction, wherein said apparatus further includes a variable X-ray attenuation filter which is disposed between the X-ray source and the receiver, said filter providing attenuation on each X-ray path in the beam such that the total attenuation to which the X-rays are subjected on any of the paths to the receiver is substantially the same for all of the paths in the beam, thereby homogenizing the exposure of the image-forming receiver, and means for changing the scale factor K=(a+b)/a between the source-patient distance (a+b) and the source-to-filter distance a in such a manner as to adapt said filter to the size of the patient. an X-ray source which emits a beam of X-rays towards a patient, a table on which the patient may be laid out; and an X-ray receiver disposed on the opposite side of the table to the side on which the source is placed, the table and/or the source-receiver pair being capable of displacement relative to each other so that the X-ray beam is capable at least of scanning the lower limbs of the patient in the longitudinal direction, wherein said apparatus further includes a variable X-ray attenuation filter device which is disposed between the X-ray source and the receiver, said filter device providing attenuation on each X-ray path in the beam such that the total attenuation to which the X-rays are subjected on any of the paths to the receiver is substantially the same for all of the paths in the beam, thereby homogenizing the exposure of the image-forming receiver, and wherein said filter device includes a plurality of interchangeable filters are provided, each filter being adapted to a type of patient morphology. an X-ray source which emits a beam of X-rays towards a patient; a table on which the patient may be laid out; and an X-ray receiver disposed on the opposite side of the table to the side on which the source is placed, the table and/or the source-receiver pair being capable of displacement relative to each other so that the X-ray beam is capable at least of scanning the lower limbs of the patient in the longitudinal direction, wherein said apparatus further includes a variable X-ray attenuation filter which is disposed between the X-ray source and the receiver, said filter providing attenuation on each X-ray path in the beam such that the total attenuation to which the X-rays are subjected on any of the paths to the receiver is substantially the same for all of the paths in the beam, thereby homogenizing the exposure of the image-forming receiver, and whereby the varying attenuation of said filter is obtained by varying the composition of the material constituting said filter. 2. Radiological apparatus according to claim 1, wherein said filter is associated with means for displacing it perpendicularly relative to an X-ray beam to ensure that said beam intersects corresponding portions of said filter and of the patient's body. 3. Radiological apparatus according to claim 1 wherein said filter is disposed in the proximity of the source and wherein means for displacing said filter are fixed to said source. 4. A radiological apparatus, used in radiological examination of the lower limbs, the apparatus comprising: 5. Radiological apparatus according to claim 4, wherein said filter device is associated with means for displacing it perpendicularly relative to an X-ray beam to ensure that said beam intersects corresponding portions of said filters and of the patient's body. 6. Radiological apparatus according to claim 4 wherein said varying attenuation of the filter device is obtained by varying thickness of the material constituting said filters. 7. Radiological apparatus according to claim 4 wherein said filter device is disposed in the proximity of the source and wherein means for displacing said filter device are fixed to said source. 8. A radiological apparatus, used in radiological examination of the lower limbs, the apparatus comprising: 9. Radiological apparatus according to claim 8, wherein said filter is associated with means for displacing it perpendicularly relative to an X-ray beam to ensure that said beam intersects corresponding portions of said filter and of the patient's body. 10. Radiological apparatus according to claim 8 wherein said filter is disposed in the proximity of the source and wherein means for displacing said filter are fixed to said source.
	summary
045267430	summary	BACKGROUND OF THE INVENTION This invention relates to containment vessels for nuclear reactors, and more particularly to a containment vessel for a nuclear reactor which is adapted to suppress a sudden increase in the pressure applied to the pressure suppressing chamber by air bubbles formed in the coolant within the pressure suppressing chamber at the initial stages of occurrence of an accident involving the escape of coolant from the nuclear reactor. Generally, a containment vessel for a boiling-water reactor is known which comprises a dry well for mounting therein a pressure vessel for the reactor, a pressure suppressing chamber having a pool of coolant therein, and a vent pipe device maintaining the dry well in communication with the coolant within the pressure suppressing chamber whereby steam of high temperature and high pressure generated by an accident involving the escape of the coolant from the reactor can be released from the dry well into the coolant within the pressure suppressing chamber. The vent pipe device comprises a plurality of vent pipe members inserted in the coolant within the pressure suppressing chamber and each having at least one exhaust port opening in the coolant. The vent pipe members are constructed and arranged such that, even if the pressure in the dry well becomes lower than the pressure in a space formed above the liquid level of the coolant within the pressure suppressing chamber, the exhaust ports of the vent pipe members are immersed in the coolant within the pressure suppressing chamber at all times. This type of containment vessel for a nuclear reactor is disclosed, for example, in the Japanese Patent Gazette of Laid-Open Patent Application No. 43091/74 of Apr. 23, 1974, issued for an application filed by Hitachi Ltd., to which the present invention has been assigned. In the event that an accident involving the escape of coolant from the pressure vessel of a reactor occurs in the aforementioned nuclear reactor containment vessel due to a failures of the piping system or some other trouble the space within the dry well will be filled with steam of high temperature and high pressure. Such steam will be caused to pass through the vent pipe members and to be released through the exhaust ports into the coolant or cooling water within the pressure suppressing chamber so that the steam may be condensed. This permits a rise in the pressure in the dry well to be avoided. However, in the initial stages of occurrence of an accident involving the escape of coolant from the nuclear reactor, non-condensable gas or air existing in the space in the dry well will be first released under high pressure into the cooling water in the pressure suppressing chamber through the vent pipe members. Therefore, a high pressure will be suddenly applied, though transiently, to the pressure suppressing chamber by bubble formation in the cooling water. This phenomenon occurs in about 0.3 to 0.7 second after the occurrence of the accident causing the escape of coolant from the nuclear reactor. There is the danger of the pressure suppressing chamber being damaged if a high pressure is suddenly applied thereto, even if the phenomenon is transient, thereby reducing the safety of the nuclear reactor containment vessel. SUMMARY OF THE INVENTION This invention has as its object the provision of a containment vessel for a nuclear reactor of an acceptable safety level which obviates the aforementioned disadvantage of the prior art by inhibiting a sudden increase in the pressure applied transiently to the pressure suppressing chamber in initial stages of the occurrence of an accident involving the escape of coolant from the reactor. The outstanding characteristic of the invention is that the containment vessel for a nuclear reactor comprises a vent pipe device including vent pipe members divided into a plurality of groups in such a manner that the vent pipe members of different groups differ from one another in the length of submerged portions of the vent pipe members interposed between the liquid level of the coolant within the pressure suppressing chamber and the exhaust ports of the vent pipe members. The length h.sub.n of the submerged portion of a vent pipe member between the liquid level of the coolant and the exhaust port of the vent pipe member, which is in the nth order by starting from the pipe of the shortest submerged length h.sub.1, is preferably determined by the formula: EQU h.sub.n =h.sub.1 +(n-1).DELTA.h where .DELTA.h=h.sub.1 when the vent pipe members are divided into 2 groups, and .DELTA.h=(h.sub.m -h.sub.1)/m-1) but .DELTA.h.ltoreq.h.sub.1 when the vent pipe members are divided into m more than 2 groups. Each vent pipe member may be formed with one exhaust port at its lower end or with a pair of exhaust ports disposed in its side wall near the lower end thereof in diametrically opposed positions. When this invention is applied to a nuclear reactor containment vessel of the over-under type (Mark-II type), communicating means is preferably provided to permit an internal space formed in the pedestal to communicate only with a space formed above the coolant in the pressure suppressing chamber. Additionally, the vent pipe members are preferably arranged such that the vent pipe members of a plurality of groups having submerged portions of different lengths interposed between the liquid level of the coolant in the pressure suppressing chamber and the lower ends of the vent pipe members are disposed symmetrically with respect to the center line of the pedestal.
	claims	1. A space vehicle comprising:a capsule; anda radiation shield device comprising:at least one first coil comprised of a superconductive material;first and second conduits extending about the capsule, wherein the at least one first coil is disposed within the first conduit so as to also extend within the first conduit about the area to be shielded from radiation, and wherein the first conduit is disposed within the second conduit;first and second cryogen liquids disposed within the first and second conduits, respectively, wherein the second cryogen liquid is disposed within the second conduit exterior of the first conduit, and wherein the first cryogen liquid has a lower boiling point than the second cryogen liquid;at least one second coil comprised of a superconductive material;third and fourth conduits extending about the capsule and also extending about the first and second conduits, wherein the at least one second coil is disposed within the third conduit so as to extend within the third conduit about the capsule, and wherein the third conduit is disposed within the fourth conduit; andfirst and third cryogen liquids disposed within the third conduit and the fourth conduit, respectively, wherein the third cryogen liquid is different than the first and second cryogen liquids. 2. The space vehicle according to claim 1 wherein the first cryogen liquid comprises liquid helium and the second cryogen liquid comprises one of liquid oxygen, liquid nitrogen or liquid hydrogen. 3. The space vehicle according to claim 1 wherein the second cryogen liquid comprises liquid hydrogen and the third cryogen liquid comprises liquid oxygen, and wherein the space vehicle further comprises a fuel cell configured to receive boil off of the second and third cryogen liquids. 4. The space vehicle according to claim 1 wherein the at least one first coil is spaced apart from the at least one second coil. 5. The space vehicle according to claim 1 wherein the radiation shield device further comprises thermal insulation surrounding the second conduit. 6. The space vehicle according to claim 5 wherein the radiation shield device further comprises a first intermediate conduit extending about the capsule, wherein the first conduit is disposed within the first intermediate conduit, and wherein the first intermediate conduit is disposed within the thermal insulation and the second conduit. 7. The space vehicle according to claim 1 wherein the first conduit is larger than the first coil. 8. The space vehicle according to claim 1 wherein the third conduit is larger than the second coil. 9. The space vehicle according to claim 1 wherein at least one first coil is comprised of a niobium titanium (NbTi) copper matrix multifilament superconducting wire winding. 10. The space vehicle according to claim 1 wherein at least one second coil is comprised of a niobium titanium (NbTi) copper matrix multifilament superconducting wire winding. 11. The space vehicle according to claim 1 further comprising a thermal control system in thermal communication with the superconductive material of the at least one first and second coils. 12. The space vehicle according to claim 1 wherein the at least one first conduit has a length in an axial direction that is greater than a radial width of the at least one first conduit. 13. The space vehicle according to claim 12 wherein the at least one second conduit has a length in the axial direction that is greater than the length of the at least one first conduit, and wherein the at least one second conduit has a radial width that is greater than the radial width of the at least one first conduit. 14. The space vehicle according to claim 1 wherein the at least one first and second conduits are formed of aluminum. 15. The space vehicle according to claim 1 further comprising a power source in communication with the at least one first and second coils. 16. The space vehicle according to claim 1 further comprising a source of the first, second and third cryogen liquids. 17. The space vehicle according to claim 16 wherein the source of the first, second and third cryogen liquids comprises a plurality of tanks configured to store the first, second and third cryogen liquids. 18. The space vehicle according to claim 17 wherein the plurality of tanks are nested. 19. The space vehicle according to claim 1 further comprising a controller for controlling flow of the first, second and third cryogen liquids through the first, second, third and fourth conduits. 20. The space vehicle according to claim 19 wherein the controller is configured to determine that particle radiation is approaching and to issue instructions regarding circulation of the first, second and third cryogen liquids.
054811171	claims	1. A shipping container for at least one nuclear fuel assembly including a top nozzle, a plurality of fuel rods held in an array by a plurality of grids spaced longitudinally along the fuel rods, and a bottom nozzle; said shipping container comprising: support means for supporting the top nozzle, the plurality of grids, and the bottom nozzle; said support means having a first surface for abutting the array and a second surface which is about perpendicular to the first surface of said support means; housing means for housing said support means and said at least one nuclear fuel assembly; top nozzle holding means secured to said support means for holding the top nozzle of said at least one nuclear fuel assembly; a plurality of grid support means for supporting the array, each of said plurality of grid support means for supporting a corresponding one of the plurality of grids on the second surface of said support means; a plurality of clamping means for clamping the array, each of said plurality of clamping means for clamping a corresponding one of said plurality of grids to a corresponding one of said plurality of grid support means; a plurality of guide plate means for guiding said at least one nuclear fuel assembly between adjacent ones of said plurality of grid support means; and bottom nozzle holding means secured to said support means for holding the bottom nozzle of said at least one nuclear fuel assembly. 2. The shipping container as recited in claim 1 wherein the array is a hexagonal array having six sides, the first surface of said support means abutting a first side of the array; wherein each of said guide plate means has two surfaces for guiding a second side and a third side of the hexagonal array; wherein each of said grid support means includes first support means for supporting the second side of the array, second support means for supporting the third side of the array, base plate means for fixedly supporting the first support means and the second support means thereto, bearing pad means for slidably supporting the base plate means, and limiting means for limiting a sliding motion of the base plate means on the bearing pad means; the bearing pad means being fixedly mounted to the second surface of said support means. 3. The shipping container as recited in claim 2 wherein the first support means is a first wedge and the second support means is a second wedge, and wherein the two wedges have about a 120 degree angle therebetween and a cork surface for supporting said at least one nuclear fuel assembly. 4. The shipping container as recited in claim 3 wherein the bearing pad means has a teflon surface for slidably supporting the base plate means. 5. The shipping container as recited in claim 3 wherein said at least one nuclear fuel assembly has a longitudinal axis which is parallel to the first surface of said support means, wherein the base plate means slides in a direction which is perpendicular to the longitudinal axis and the first surface of said support means, wherein the limiting means limits the sliding motion in the direction which is perpendicular to the longitudinal axis and the first surface of said support means, and wherein the limiting means prevents the sliding motion in the direction which is parallel to the longitudinal axis and the first surface of said support means. 6. The shipping container as recited in claim 2 wherein each of said guide plate means has two surfaces for guiding the second side and the third side of the hexagonal array, and wherein each of the two surfaces of said guide plate means have about a 120 degree angle therebetween, a guide side for guiding said at least one nuclear fuel assembly, and an absorbing side having a coating of gadolinium oxide.
	abstract	The invention relates to a component support (29) for use in a radioisotope generator, the component support comprising a latching member (5) movable between an engaging position and an open position characterised by further including a bracing member (13) mechanically associated with the latching member and adapted to prevent movement of the latching member to the open position.
	description	This application claims the benefit of priority from UK Patent Application No. GB 1720014.8, filed on 1 Dec. 2017, which is hereby incorporated herein in its entirety by reference. The present invention relates to safety systems for nuclear power generation systems, and methods of using such systems. More specifically, it relates to safety systems for use in pressurised water reactors (PWRs). Worldwide energy demands are constantly increasing, and nuclear energy is increasingly considered an attractive energy source for meeting this growing requirement, with current estimates that nuclear power provides between around 10 and 15% of the world's total energy production. Maintaining a reliable safety system that is capable of providing an effective response in various fault situations is paramount, particularly in view of the hazards posed by uncontrolled emission of radiation into the environment. In pressurised water reactors, safety systems are typically provided to deliver protection against a number of faults, including rapid cooldown events, in which the reactor coolant rapidly cools down and contracts, and loss of coolant accidents (LOCA), in which a significant volume of coolant is lost from the system. Such fault events can lead to the reactor pressuriser emptying due to reduction in coolant volume in the system. This volume reduction results either directly from loss of coolant from the system in a LOCA, or due to thermal contraction of the coolant in the case of a rapid cooldown event. Typically, the volume reduction will be greater in a LOCA than in the case of a rapid cooldown event. When the pressuriser empties, steam from the pressuriser may enter the reactor circuit (primary circuit) and the system pressure can rapidly fall to a saturation condition. As the system pressure falls, the reactor coolant may boil off, increasing the risk of uncovering the reactor core as coolant level is suppressed in the reactor pressure vessel. Uncovering of the reactor core is highly undesirable as it reduces the ability of the system to remove excess decay heat from the core, and leads to a risk of potential fuel melt. A commonly-employed method of resolving extreme system faults is implementation of an emergency core cooling system in which the reactor pressure vessel is rapidly depressurised through opening of emergency blowdown lines, and a large quantity of water is injected into the reactor pressure vessel by gravity drain, typically from a tank located inside or outside of the containment. However, for certain faults, such as rapid cooldown events and some LOCAs, their initiation frequency means that it is preferred to have two independent safety measures, rather than resorting to initiation of emergency core cooling systems which can be highly disruptive to plant operating conditions. Accordingly, it is desired to find methods of coolant injection into the reactor pressure vessel without the need to initiate emergency core cooling systems which require depressurisation of the system, i.e. methods of coolant injection which are able to operate under conditions where the nuclear power generation system remains at a relatively high pressure (e.g. above 5.0 MPa). Safety systems employed in nuclear power generation systems can generally be categorised into either active or passive safety systems. Active safety systems are typically defined as those for which external power input or forces are required, and typically have associated control and instrumentation systems. One example is the use of high-head pumps for emergency injection of coolant into the reactor pressure vessel in the case of a rapid cooldown event or LOCA. Whilst such systems can be effective, they are typically dependent on external power input, and often require complex control and instrumentation systems. Passive safety systems are classified by the International Atomic Energy Agency into four different categories (A-D) depending on the exact nature of the passive safety system. Category A includes physical barriers against the release of radioactive material, such as nuclear fuel cladding or the nuclear power generation system containment structure. Category B includes emergency cooling systems based on air or water natural circulation in heat exchangers to which decay heat is transferred. Category C includes accumulators, which are dependent on valve actuation (i.e. moving mechanical parts), but do not require intelligent signal inputs or external power input. Category D includes SCRAM or reactor trip systems in which control rods drop under gravity upon release. Known injection systems which are effective at higher pressures are typically active systems, for example the high head pumps discussed above. Use of active safety systems may introduce additional risk into the system, as the pumps may be more likely to fail in some types of fault event of the nuclear power generation system. Accordingly, it is desired to find a passive safety system which is nevertheless capable of injecting coolant into the reactor pressure vessel at elevated pressures. In a first aspect, there is provided a coolant injection system for a nuclear power generation system including a reactor pressure vessel having a reactor core, and a pressuriser in fluid communication with the reactor pressure vessel, the injection system comprising a make-up tank having a tank inlet and a tank outlet, wherein the injection system is configured to switch between an operating condition and a fault response condition when the coolant level in the pressuriser drops below a threshold level, and wherein: in the operating condition, the tank outlet is isolated from the reactor pressure vessel such that coolant is retained in the make-up tank, and the tank inlet is in fluid communication with the reactor pressure vessel and the pressuriser; and in the fault response condition, the tank inlet is isolated from the reactor pressure vessel, and the tank outlet is in fluid communication with the reactor pressure vessel such that coolant from the make-up tank can flow into the reactor pressure vessel to provide cooling of the reactor core. Operating condition is used here to refer to a ‘normal’ condition of the injection system. Typically, the injection system will run in the operating condition unless and until a fault event (in which coolant level in the pressuriser drops below a threshold level) in the nuclear power generation system is detected. When such a fault is detected, the injection system is configured to switch to the fault response condition in which water is injected into the reactor pressure vessel. In this way, activation of the fault response condition may reduce the risk of uncovering of the reactor core as a result of a fault event in the nuclear power generation system. Provision of such an injection system can enable a compact and efficient core cooling system for response to a number of fault events of the nuclear power generation system. By isolation of the make-up tank inlet from the reactor pressure vessel (RPV) in the fault response condition, backflow of hot coolant from the RPV into the make-up tank is prevented. Accordingly, the make-up tank can be significantly reduced in size in comparison to known systems, because the tank does not need to have the large capacity that has been necessary in previous system to hold the required mass of (relatively low density) hot coolant from the RPV. Rather, because the make-up tank is an independent cold-temperature system, the capacity can be smaller for the same mass of injected coolant. Accordingly, manufacture of the tank can be less expensive, and the overall space required for the system is reduced. The system may be configured such that in the fault condition, the tank inlet is in fluid communication with the pressuriser. Accordingly, the coolant injection may be driven by residual pressure in the pressuriser. By driving the coolant into the RPV using pressure from the pressuriser, the injection system may be able to operate under conditions where the nuclear power generation system remains at a relatively high pressure (e.g. above 5.0 MPa). The injection system may be classified as a passive system. Accordingly, in one embodiment, the injection system does not employ pumps for the injection of coolant into the reactor pressure vessel. As such, the system may offer improved safety in response to some fault events of the nuclear power generation system due to a lower risk of failure as compared to active systems, whilst not requiring total system depressurisation. In a second aspect, there is provided a method of operation of a coolant injection system for a nuclear power generation system including a reactor pressure vessel having a reactor core, a pressuriser in fluid communication with the reactor pressure vessel, and a coolant injection system comprising a make-up tank having a tank inlet and a tank outlet, the method including the steps of: detecting a coolant level in the pressuriser; in an operating condition in which the coolant level in the pressuriser is above a threshold level, isolating the tank outlet from the reactor pressure vessel such that coolant is retained in the make-up tank, and the tank inlet is in fluid communication with the reactor pressure vessel and the pressuriser; when the coolant level in the pressuriser drops below the threshold level, switching the injection system from the operating condition to a fault response condition, in which the tank inlet is isolated from the reactor pressure vessel, and the tank outlet is in fluid communication with the reactor pressure vessel; and allowing coolant from the make-up tank to flow into the reactor pressure vessel to provide cooling of the reactor core. In a third aspect, there is provided a nuclear power generation system comprising the coolant injection system of the first aspect. Optional features will now be set out. These are applicable singly or in any combination with any aspect. The tank inlet may be located in a top wall of the make-up tank. In the operating condition, the tank inlet provides a fluid connection between the make-up water tank and the pressuriser. The tank inlet may be arranged such that during normal operation of the system, coolant does not flow into the make-up tank through the tank inlet. Correspondingly, the tank outlet may be located in a bottom wall of the make-up tank. When the injection system is in the fault response condition, coolant contained in the make-up tank will flow out of the tank via the tank outlet. Fluid connection between the pressuriser and the RPV may be provided via a surge line connecting the pressuriser and the RPV in a known manner. Fluid connection between the make-up tank inlet and the pressuriser may be provided via a pressure balance line. By providing a pressure balance line, the make-up tank can be maintained at system pressure during normal operation of the nuclear power generation system. The pressure balance line may be formed by rising pipework connected to the surge line. Alternatively the pressure balance line may connect directly to the pressuriser. Fluid connection between the make-up tank outlet and the RPV may be provided via an outlet line connected to the make-up tank outlet. The outlet line may connect the make-up tank to the surge line, or may alternatively connect directly to the RPV, or to a hot leg of the primary circuit. There may be an auxiliary circuit branching from the primary circuit at first and second connection points. The pressure balance line, make-up tank and outlet lines may form all or part of such an auxiliary circuit. The first and second connection points may both be on the surge line. Where the pressure balance and outlet lines form part of an auxiliary circuit having both first and second connection points on the surge line, the connection of the pressure balance line to the surge line will typically be closer to the pressuriser than the connection of the outlet line to the surge line. The coolant injection system may comprise a valve arrangement operable in response to the coolant level in the pressuriser to switch the injection system between the operating condition and the fault response condition. Such valve arrangement may comprise one or more valves. For example, the valve arrangement may comprise a first valve located on the outlet line (hereafter referred to as an ‘outlet valve’), and a second valve located on the surge line (hereafter referred to as a ‘surge line valve’. Other valve arrangements are also contemplated. For example, the valve arrangement may comprise a single multi-way (e.g. three-way) valve, rather than separate first and second valves. The valve arrangement may be actuated in any suitable manner. For example, the valve arrangement may be operable in response to e.g. a control signal initiated by a detector in response to coolant level in the pressuriser falling below a threshold level. Actuation of the valve arrangement may be automatic in response to detection of a specified fault event in the nuclear power generation system. Where the valve arrangement comprises outlet and surge line valves as described above, actuating the valve arrangement may include the steps of opening the outlet valve, and closing the surge line valve. The outlet and surge line valves may be actuated simultaneously. Alternatively, the outlet and surge line valves may be actuated at different times. For example, the surge line valve may be closed before the outlet valve is opened, to ensure that there is no backflow of coolant liquid into the make-up tank upon opening of the outlet valve. As is known from typical pressurised water reactors, the pressuriser may contain one or more heaters to assist in control of pressure in the pressuriser. Accordingly, the threshold coolant level may be selected to be a level at which one or more of the heaters is/are at least partially uncovered. Alternatively, a margin may be provided such that the threshold level is selected to be above that at which the heaters would be uncovered. The precise level which is selected as the threshold level is not specifically limited. Selecting the threshold level to be higher will result in more rapid actuation of the injection system when a fault event occurs, but may also result in a greater risk of the injection system actuating unnecessarily due to normal temperature fluctuation of the system. Accordingly, the threshold level should be selected as appropriate by balancing the desired response time against the risk of unnecessary actuation. The make-up tank may be positioned vertically above the reactor vessel. It may be positioned either directly above the reactor vessel, or may instead be laterally offset from the reactor vessel. The precise arrangement of the make-up tank in relation to other components of the nuclear power generation system will generally depend in part on other constraints such as available space within the containment. Because the flow of coolant may be driven by residual pressure in the pressuriser, the exact positioning (vertical and lateral) of the makeup tank in relation to the RPV is not particularly limited. The volume of coolant injected from the make-up tank in the fault response condition may vary depending on the coolant volume reduction in the primary circuit. Accordingly, the size of the make-up tank may be selected such that the level of coolant in the make-up tank remains above a threshold level when the coolant volume reduction is due to a rapid cooldown fault event (which results in a maximum known volume reduction based on thermal contraction of the total volume of coolant in the circuit), and only drops below this threshold level in the case of a LOCA. In this way, the level of coolant in the make-up tank can itself be used as a trigger for additional safety systems which require initiation in response to more serious fault events. For example, there may be a detector located in the make-up tank and arranged to detect the level of coolant in the tank such that when the level of water in the make-up water tank drops below the selected threshold level, emergency blow down (EBD), or some other specified safety system or procedure initiates. This threshold level may be a level at which the make-up tank is completely empty, or may be at some selected level higher than this. When selecting the threshold level of the make-up tank, similar considerations apply as discussed above in relation to selection of the threshold level in the pressuriser. The injection system may be arranged to allow for total isolation of the pressuriser from the make-up tank and reactor pressure vessel under selected conditions. Typically, in a fault response condition of the injection system, the pressuriser will remain in fluid communication with the reactor pressure vessel via the make-up tank to assist in coolant injection under residual pressure from the pressuriser. However, where a fault event in the nuclear power generation system is caused by a leak from the pressuriser, maintaining such fluid connection may not be desirable. Accordingly, there may be provided an isolation valve operable to isolate the make-up tank inlet from the pressuriser in selected conditions, such as when a pressuriser leak is detected. The isolation valve may be provided on the pressure balance line connecting the make-up tank and the pressuriser. Where present, the isolation valve will typically remain open during normal operation of the nuclear power generation system, and be actuated to close when it is detected that the fault event in the nuclear power generation system is a pressuriser leak. In other words, the isolation valve may prevent fluid communication between the tank inlet and the pressuriser in the fault response condition. The make-up tank may further comprise a gas inlet line arranged to supply gas to the make-up tank. In this way, where the injection system is arranged to provide total isolation of the pressuriser from the make-up tank and the reactor pressure vessel, pressure may still be maintained in the primary circuit of the nuclear power generation system by injection of gas into the make-up tank whilst the injection system is in a fault response condition (i.e. the tank outlet is in fluid communication with the RPV), and when the pressuriser is isolated. The make-up tank may have a drain line. The make-up tank may be arranged to supply any suitable coolant fluid, however the coolant will typically comprise water, as a cost-effective coolant. The make-up tank may be arranged to supply boronated water. In this way, injection of water into the RPV can also provide a method of emergency boron injection directly into the RPV when the nuclear power generation system undergoes a fault event. Direct injection of boron can help to absorb excess neutrons in the RPV and according decrease the risk of nuclear runaway in the reactor core. FIG. 1 shows a schematic process flow arrangement of a primary circuit (also referred to as the reactor coolant circuit) of a nuclear power generation system (not shown) incorporating a passive injection system 100. The nuclear power generation system is a pressurised water reactor (PWR) including in the primary circuit a reactor pressure vessel 1 containing a reactor core for generation of heat by radioactive decay. The reactor core heats a coolant fluid (typically water) contained in the primary circuit. The reactor pressure vessel is connected to first and second steam generators 3a, 3b, via respective hot legs 5a, 5b and cold legs 7a, 7b, and there are respective primary pumps 9a, 9b arranged to provide active flow of coolant fluid from the reactor pressure vessel 1, through the hot legs 5a, 5b, to the first and second steam generators 3a, 3b, where heat is transferred to respective secondary circuits (not shown) in a well-known manner, before being returned to the reactor pressure vessel via cold legs 7a, 7b respectively. The reactor pressure vessel 1 is connected to a pressuriser 11 via surge line 13, and in normal operation acts to maintain the primary circuit at a suitable pressure. The pressuriser contains one or more heaters 15, and one or more detectors (not shown) for detecting the water level and pressure in the pressuriser. When the pressure in the circuit drops below a threshold level, the heaters can be activated to heat water held in the pressuriser. This causes water in the pressuriser to boil, creating steam and thus increasing pressure in the system to maintain the system at the required pressure. The passive injection system 100 comprises a make-up water tank 17 having a tank inlet 19 and tank outlet 21. Here, the tank inlet and tank outlet are conveniently located in top and bottom walls of the tank respectively. In normal operation, the make-up water tank contains a supply of boronated water. Here, the passive injection system forms part of an auxiliary circuit branching from the surge line 13 at first and second connection points. The auxiliary circuit includes rising pipe-work which forms a pressure balance line 23 connecting the tank inlet to the pressuriser. Accordingly, in normal operation, the make-up water tank will be maintained at system pressure due to this fluid connection with the pressuriser. The auxiliary circuit further includes an outlet line 25 connecting the tank outlet to the surge line at a point downstream of the connection of the surge line to the pressure balance line 23 relative to the pressuriser. This outlet line forms an injection conduit for injection of water into the reactor pressure vessel from the make-up water tank, in a manner that will be described later. There is a valve arrangement operable to switch the injection system between an operating condition, in which the tank outlet is isolated from the reactor pressure vessel, such that water is retained in the make-up water tank 17, and the tank inlet 19 is in fluid communication with the reactor pressure vessel 1 and the pressuriser 11 and a fault response condition, in which the tank inlet 19 is in fluid communication with the pressuriser 11 and isolated from the reactor pressure vessel 1, and the tank outlet 21 is in fluid communication with the reactor pressure vessel 1. In the fault condition, water is driven from the make-up tank 17 into the RPV 1 by residual pressure from the pressuriser 11. Here, the valve arrangement is provided as two separate valves, a first valve 27 disposed between the make-up water tank 17 and the reactor pressure vessel 1 on the outlet line 25 (the ‘outlet line valve’), and the second valve 29 disposed between the reactor pressure vessel and the pressuriser on the surge line (the ‘surge line valve’). In normal operation of the nuclear power generation system, when the injection system is in an operating condition, the surge line valve 29 is open, to allow equalisation of pressure between the pressuriser 11 and the reactor pressure vessel 1 in a manner described previously, and the outlet valve 27 is shut, to retain water in the make-up water tank 17. However, when the water level in the pressuriser 11 falls below a threshold level due to e.g. a fault event in the nuclear power generation system which results in a decrease in coolant volume, the valve arrangement is actuated to open the outlet valve 27 and close the surge line valve 29. In this way, the surge line 13 is isolated, and water held in the make-up tank 17 is able to flow into the reactor pressure vessel 1, driven by the residual pressure in the pressuriser 11, whilst backflow of water from the RPV into the make-up tank is prevented. This provides cooling of the reactor core by increasing coolant mass and providing additional boron directly to the RPV. The make-up water tank 17 is sized such that during a cooldown/contraction fault event, the tank does not empty due to the total volume of coolant remaining in the system. However, in a LOCA, where total volume losses are typically greater than for cooldown events, the tank may empty. A detector (not shown) in the make-up tank is arranged to detect the level of water in the tank, such that when the level of water in the make-up water tank drops below a threshold level, EBD initiates. The system includes an isolation valve 31 located on the pressure balance line. In normal operation, this valve is open, but it can be controlled to close when e.g. it is detected that there is a leak from the pressuriser such that isolation of the pressuriser from the make-up water tank is advantageous. When such situation is detected (e.g. by monitoring of pressuriser level with the surge line isolated), the isolation valve is closed. As loss of coolant from the pressuriser will also result in isolation of the surge line due to the passive injection system switching to the fault response condition as the water level drops below a threshold level, the pressuriser will accordingly be isolated from both the make-up water tank and the RPV. If necessary, gas can then be supplied through gas line 33 into the make-up water tank to maintain pressure control of the nuclear power generation system by provision of a pressurised gas bubble in the make-up water tank. A drain line 35 is located in the bottom of the make-up tank. It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.
	abstract	The present invention provides a drawing apparatus including a plurality of drawing units each of which is configured to perform drawing on a substrate with a charged particle beam, a plurality of first processors configured to be selectively connectable to each of the plurality of drawing units, an information processor configured to determine, from the plurality of first processors, a first processor to be connected to a first drawing unit among the plurality of drawing units, based on drawing data, and a connection unit configured to connect the determined first processor to the first drawing unit.
	summary
051868687	description	DETAILED DESCRIPTION OF THE INVENTION General Description and Definitions One feature of the present invention is its ability to achieve high levels of introduction of deuterium or tritium radiolabel into target molecules. This is achieved without harsh conditions which can give rise to disruption of or nonspecific addition to the target molecules. A labeling agent having a "high tritium or deuterium level" as used herein refers to a labeling agent in which at least 20% of the reducing hydrogen atoms are tritium or deuterium. By "reducing hydrogen atom" is meant a hydrogen atom that may ultimately be transferred to the target compound at the site of reduction. "High tritium or deuterium level" is also used to refer to the reduced target compound in which at least 20% of the reducing hydrogen atoms incorporated into the molecule via reduction are tritium or deuterium atoms. In preferred embodiments, at least about 50% of the reducing hydrogen atoms present in the labeling reducing agent and subsequently incorporated into the target molecule are tritium or deuterium atoms. In specially preferred embodiments at least about 80% of the reducing hydrogen atoms present in the labeling reagent and incorporated into the target molecule are tritium or deuterium atoms. Pure tritium has a specific activity of 28.72 Curies per milliatom (Ci/mmatom). Therefore, a highly tritiated compound, in which at least 20% of the reducing hydrogen is tritium, will have a specific activity of at least 5.74 Ci/mmole of reducible hydrogen atoms. In the preferred (50%) embodiment of this invention, the compounds will have a specific activity of at least 14.36 Ci/mmole of reducible hydrogen atoms. An organic compound having a "reducible site" as used herein refers to a compound which has a group or functionality within its covalent structure which is capable of being reduced by the labeling reagents of this invention. The "reducible site" will receive at least one and often two or more hydrogen, deuterium or tritium atoms when acted upon by the processes and reagents of this invention. Examples of reducible sites include aldehydes which are reduced to primary alcohols, ketones which are reduced to secondary alcohols, acid chlorides which are reduced to primary alcohols, lactones which are reduced to glycols, epoxides, esters, carboxylic acids, carboxylic acid salts and tertiary amides which are all reduced to primary alcohols, nitriles and nitros which are reduced to primary amines, olefins and alkyl halides which are reduced to alkanes, aryl halides which are reduced to aryls, p-toluene sulphonates which are reduced to alkanes and .alpha.,.beta.-enones which are reduced to ketones. Table 1, which follows, summarizes these reactions and correlates them with the reactivity of a variety of hydride reagents. TABLE 1 __________________________________________________________________________ Reactivity of Hydride Reagents, in THF Functional Gp. Product #H* LiEt.sub.3 BH Li(s-Bu).sub.3 BH LiAlH.sub.4 BH.sub.3 NaBH.sub.4 __________________________________________________________________________ Aldehyde 1.degree. OH 1 + + + + + Ketone 2.degree. OH 1 + + + + + Acid Chloride 1.degree. OH 2 + + + - + Lactone Glycol 2 + + + + - Epoxide 1.degree. OH 1 + + + + Ester 1.degree. OH 2 + + + .+-. - Carboxylic 1.degree. OH 2 - - + + - acid Carboxylic 1.degree. OH 2 - - + - - acid salt tert-Amide 1.degree. OH 1 + + + + - Nitrile 1.degree. Amine 2 + + + + - Nitro NH.sub.2 (2) + + + - - Olefin Alkane 2 - - - .times. - Alkyl Halide Alkane 1 + + + - + Aryl Halide CH 1 - - + - - -pToluene Alkane 1 + + - - - sulphonates .alpha.,.beta. Enone Ketone 1 + + - - - __________________________________________________________________________ + = reactive .+-. = slightly reactive - = very slow or unreactive .times. = organoborane formed aTHF,diglyme or ethanol solution In chemical formulae, tritium will be identified as T, deuterium as D and H.sup.* refers to a mixture of deuterium or tritium with hydrogen containing greater than >20% (molar) tritium or deuterium. The term "alkali metal", as used herein refers to lithium, sodium, or potassium. Thus, an alkali metal tritide refers to LiT, NaT or KT. An "alkali metal alkyl", as used herein refers to an organometallic compound of lithium, sodium, or potassium with a simple alkyl, such as a methyl, ethyl, n or i propyl or n, s, or t butyl. "Lower alkyl" as used herein refers to a straight chain, branched chain, cyclic or bicyclic saturated hydrocarbon containing from one to about six carbon atoms. A "lower alkylene" as used herein refers to a generally straight chain bridging saturated hydrocarbon containing from about two to about four carbon atoms. "Labeling reagent" as used herein refers to a compound capable of transferring a tritium or deuterium atom to a target compound in a reduction reaction. The Labeling Reagents It is in the area of high deuterium and tritium content H.sup.* that the present invention offers labeling reagents and methods vastly superior to materials and methods available heretofore. For that reason it is especially preferred that H.sup.* is at least 80% or as close to 100% deuterium or tritium as possible. The labeling reagents provided by this invention are highly active reducing agents which can achieve high levels of site-specific reduction in target molecules with minimal disruption of the target molecule's structure. The reagents fall into three general classes. The first are the deuterium and tritium analogs of alkali metal trialkyl borohydrides. These materials have the formula MR.sup.1 R.sup.2 R.sup.3 BH.sup.. In this formula M is an alkali metal (as defined), preferably lithium. R.sup.1, R.sup.2 and R.sup.3 are independently selected from lower alkyls. Preferably R.sup.1, R.sup.2 and R.sup.3 are identical. In especially preferred embodiments R.sup.1, R.sup.2 and R.sup.3 are each ethyl, each sec-butyl or each 1,2-dimethyl propyl. These first two materials are analogous to the "superhydride" and "selectride" reducing agents known in the art. The second general class of reagents are deuterium and tritium analogs of alkali metal aluminum hydrides and have the formula MAlh.sup.8.sub.4 with M and H.sup. being as defined above. The third general class of reducing agents are deuterium and tritium analogs of borane. They have the formula BH.sup..sub.3 where H.sup. is as previously defined. Preparation and Use of the Reagents The reagents listed above can, in theory, be prepared, isolated and stored prior to use and this practice is not excluded from the invention as claimed herein. However, best results are achieved when the reagents are prepared relatively immediately prior to use. This description of preparation and use will focus on this in situ preparation method but those of skill in the art will be able to readily adapt this to isolate the reagents if desired. A general preparation scheme for the reagents is shown in Reaction Sequence 1. ##STR1## Step 1 of this preparation process is optional. It is used to form sodium and potassium materials. If lithium materials are employed, step 1 is not used. Step 1 begins with an alkyl lithium material, R.sup.4 -Li. In this formula R.sup.4 is an alkyl, especially a lower alkyl, and more especially n-butyl. This reaction is carried out in solution. Typical solvents are saturated hydrocarbons, for example pentane, n-hexane, cyclcohexane, heptane, and saturated hydrocarbon fractions. The reaction is carried out in the presence of a tertiary amine, especially a tertiary alkyl amine, such as a tertiary lower alkyl amine. A preferred group of tertiary amines are the alkylene diamine ditertiary amines such as tetraalkylalkylenediamines. These can include materials such as tetramethylethylenediamine (TMEDA), tetraethylethylenediamine (TEEDA), tetramethylpropylene 1,3-diamine and the like. This reaction can be carried out at -10 to 50.degree. C. and typically is carried out at 10 to 25.degree. C. Typical reaction times range from 5 to 120 minutes with reaction time being inversely related to reaction temperature. 30 to 45 minutes at 15.degree.-20.degree. C. are conditions which have worked very effectively. Essentially equimolar amounts of amine, lithium, alkyl and sodium or potassium alkoxide give good results. The reaction mixture may be stirred. In step 2 the R.sup.4 -M solution is contacted with H.sup..sub.2 in the presence of tertiary amine. The solvent and tertiary amine may already be present from step 1. This reaction is carried out with about 1 mole of tertiary amine present for each mole of R.sup.4 -M. Generally an excess of H.sup..sub.2 is employed. This can be achieved by conducting the reaction under a substantial pressure, e.g. 0.25 to several (10) atmospheres, of H.sup..sub.2. Most preferred pressures are in the range of 0.5 to 2 atmospheres of H.sup..sub.2, primarily because of a convenient rate of reaction, and simplicity of laboratory equipment and handling under these conditions. This reaction goes quite rapidly and has a tendency to be substantially exothermic. Accordingly, vigorous stirring is desirable as well as cooling to hold the temperature in the desired range. The sodium and potassium reactions are somewhat more violent and are usually held in the -50.degree. C. to -10.degree. C. temperature range during initial H.sup..sub.2 addition. They can be allowed to warm to room temperature during later stages. The lithium reaction can be held at room temperature (15.degree.-25.degree. C.) to about 50.degree. C. without hazard. This reaction is complete within about 60 minutes in most cases. In this reaction it is helpful if all solvents and reagents are quite dry. These MH.sup. materials are pyrophoric in air. Accordingly they should be kept covered with an inert atmosphere if isolated. In the third step of Reaction Sequence 1 the metal hydride produced in step 2 is converted to an active reducing agent. In reaction 3A the metal hydride is reacted with a trialkyl borane. This can be carried out directly in the reaction mixture of step 2 following uptake of H.sup..sub.2. Essentially an equimolar amount of the trialkyl borane is added based on the moles of MH.sup. present. This reaction can be carried out at room temperature (15.degree.-25.degree. C.) and appears to be complete within a few seconds. Higher or lower temperatures could be used if desired. This reaction mixture as so formed can be used directly in the reduction and labeling of a reducible target compound. In reaction 3B, it is generally desirable to first isolate the MH.sup.* intermediate, such as by removing solvent in vacuo and capping with nitrogen gas. To this solid material can then be added the stoichiometrically required amount of aluminum trihalide, for example aluminum tribromide in a suitable solvent such as an aprotic polar solvent for example THF, DMF or DMSO. In a typical process, AlBr.sub.3 is obtained in solution in dibromomethane. The solvent is removed and the solid AlBr.sub.3 is then dissolved in THF with cooling to -50.degree. C. to 0.degree. C. When the solution of aluminum salt is mixed with the MH.sup.* to give the metal aluminum hydride the reaction is very rapid and is carried out at low to moderate temperatures, typically in the range of -20.degree. to 35.degree. C. and especially at about room temperature. This material as formed can be used directly in the reductive labeling. In the case of reaction 3C, again it is desirable to first isolate the MH.sup.* material. The reaction of the MH.sup.* with BF.sub.3, which BF.sub.3 is generally present as an etherate (Et.sub.2 O.multidot.BF.sub.3), is typically begun at low temperature, such as -50.degree. C. to 0.degree. C. This is to allow a gradual controlled evolution of the ether, which is released as the BF.sub.3 decomposes and reacts. The mixture is allowed to warm to room temperature or even beyond, such as to 50.degree. C. to complete the reaction and yield a solution of the desired borane analog. This material can be used directly in the reductive labeling. Use of the Reagents As detailed in the Examples which follow, the reagents prepared by reactions 3A, 3B and 3C are all highly reactive materials. They can be used with a wide variety of target compounds having a reducible site so as to introduce deuterium or tritium labels thereinto. In a typical reaction the products of reactions 3A, 3B or 3C are used directly in approximately the stoichiometric amount required to add the labeling deuterium or tritium atoms. This reaction can be carried out generally at room temperature, for example from about 10.degree. C. to about 35.degree. C. and is complete in from about 10 to about 30 minutes. Following reaction, the labeling reaction mixture is quenched such as by addition of methanol or the like, and the product is then worked up to recover the labeled reducible target. This invention will be further described by the following examples. These are provided to illustrate various preferred embodiments of the invention but are not to be construed as limitations on the scope of the invention. The scope of the invention is defined by the claims which follow. EXAMPLES Tritium gas was purchased from Oak Ridge National Laboratory, and contained 97.9% T.sub.2, with the largest contaminant being DT (1.76%). Ethyl-p-nitrobenzoate was purchased from ICN, and all other starting materials and reagents were purchased from Aldrich Chemical Co. All chemicals were used without further purification. n-Butyllithium (1.6M in hexane) was purchased from Aldrich, titrated with 2,5-dimethoxybenzylalcohol and determined to be 1.2M. Tetrahydrofuran was freshly distilled from LiAlH.sub.4 and stored over molecular sieves. Tritiated samples were counted with a Packard 2002 liquid scintillation counter, using Opti-Fluor scintillant from Packard. Thin layer chromatography was carried out on Whatman K6F analytical silica gel plates (250 .mu.m), using dichloromethane/hexane (2:1) as developing solvent. Analytical high pressure liquid chromatography was performed by using a Waters C-18 radial pak column, and Waters model 510 pumps. The HPLC mobile phase for analysis of most of the products was 60% methanol/water, pumped at 2 ml/min. Mass peaks were observed by UV detection at 275 nm on a Hewlett Packard 1040A diode array spectrophotometer. Radioactivity measurements were made with a Ramona-5-LS HPLC flow detector, using a lithium glass scintillant cell with an efficiency of ca. 0.05%. .sup.1 H and .sup.3 H NMR spectra were recorded in CD.sub.3 OD, on an IBM AF-300 NMR spectrometer (.sup.3 H at 320 MHz, .sup.1 H at 300 MHz), using a .sup.3 H/.sup.1 H 5 mm dual probe. Samples were made to a volume of 200 .mu.L in teflon tubes (Wilmad, #6005), which were then placed inside 5 mm glass NMR tubes having a screw-cap (Wilmad, 507-TR-8"). A quality .sup.3 H band stop-.sup.1 H band pass filter (Cir-Q-Tel Inc., FBT/20-300/3-6/50-3A/3A) was placed in the proton decoupling line of the instrument, and the observe channel had an in-line .sup.1 H band stop-.sup.3 H band pass filter. Tritium and proton spectra were acquired over approximately 12 ppm, using a 5 s total recycle time and excitation pulses of 3.6 .mu.s (3H, 65.degree.) and 2 .mu.s (.sup.1 H, 25.degree.). All spectra were acquired at 297.degree. K. with the sample spinning. Referencing of tritium chemical shifts was achieved by generation of a ghost .sup.3 H TMS signal from internal TMS in the .sup.1 H NMR spectrum. Tritiations were performed on several different chemical scales, but were generally 1 mmol scale using 10% T.sub.2 in H.sub.2, and 0.1-0.2 mmol scale using 100% T.sub.2. Reactions were carried out at room temperature, unless otherwise noted, in 5-15 mL side-arm flasks equipped with a septum. The reaction pressure was monitored with a Wallace & Tiernan gauge, and the reaction vessel was connected to a high vacuum line during the entire reaction. The results of these examples are summarized in Table 2 which follows Example 9. EXAMPLE 1 Reaction of 2-Naphthaldehyde with Supertritide 10% A 10% mixture of tritium/hydrogen was admitted to a final pressure of 650 mm Hg (91.5 kPa) to a flask containing n-butyllithium solution (n-BuLi, 630 .mu.L, 1.2M in hexane, 0.75 mmol). Injection of tetramethylethylenediamine (TMEDA, 150 .mu.L, 1.0 mmol) to the vigorously stirred solution gave a creamy white precipitate (LiT and LiH) after 15 minutes. It was observed that hydrogen gas uptake was more rapid when the reaction pressure was maintained above 360 mm Hg (51 kPa), so additional tritium/hydrogen gas was admitted to the flask after 15 minutes. After 30 minutes, uptake of tritium/hydrogen had ceased and triethylborane (1M, 1 mL, 1 mmol) in tetrahydrofuran (THF) solution was injected to generate the LiEt.sub.3 BT (supertritide) reagent. At this point there was a pressure increase, and the creamy suspension clarified within a few seconds. A THF solution of 2-naphthaldehyde (160 mg, 1 mL, 1 mmol) was slowly added with stirring and allowed to react with the supertritide for 30 minutes at room temperature. At the end of the reaction, methanol (2.times.1 mL) was added to quench the supertritide reagent and solvent and gas were removed in vacuo. Methanolic HCl (1 mL of a solution of 1.5 mL 1N HCl and 1 mL MeOH) was added to destroy any borane complexes and the pressure rose considerably, indicating the evolution of ethane. The solvent and gases were evacuated, and the reaction products were then removed from the vacuum line, dissolved in ethyl acetate (1 mL), and the borate salts were separated by washing with KOH (2.times.1 ml, 0.1N). EtOAc was removed by overnight lyophilization, and the residual solids were dissolved CD.sub.3 OD for liquid scintillation counting, TLC, HPLC, and NMR analyses. All analyses (TCL, HPLC AND NMR) of this product suggested that generation of the supertritide reagent had been successful and conversion of the aldehyde to product alcohol was quantitative, though with a moderate chemical yield (24%). It was determined that there were several yield limiting factors in this initial reaction including (a) the true molarity of the n-BuLi, (b) the dryness and purity of the TMEDA, and (c) the dryness of the THF solvent. These were addressed in Example 2. EXAMPLE 2 Reaction of Methyl 2-Naphthoate with Supertritide 20% The supertritide reagent was generated by stirring n-BuLi (415 .mu.L, 1.2M, 0.5 mmol) and TMEDA (85 .mu.L) in the presence of one atmosphere of tritium gas (20% T), and subsequent addition of Et.sub.3 B (0.5 mL, 1M, 0.5 mmol) in THF after approximately 30 minutes, when gas uptake had ceased. Methyl 2-naphthoate (94.5 mg, 0.5 mL, 0.51 mmol) was added as a THF solution. The reaction was stopped after two hours by the addition of methanol (1 mL), with subsequent methanolic HCl treatment and workup of the reaction products carried out in a similar fashion as that described above. The product was analyzed by tritium NMR. The spectrum so obtained is given in FIG. 1. Inset (A) shows an expansion (4.85-4.60 ppm) of the proton decoupled tritium spectrum, with the smaller singlet due to molecules containing two tritium atoms. Inset (B) shows the proton coupled tritium spectrum, with a doublet for the R-CHTOH species (J.sub.TH =13.83 Hz) and singlet arising from R-CT.sub.2 OH molecules. The ratios of singly and doubly tritiated products calculated from this spectrum was as expected if there were no isotope effect. (H.sub.2 /HT/T.sub.2 =64/32/4). That is, the percentage of R-CT.sub.2 OH and R-CHTOH species should have been 4% and 32% respectively with .sup.3 H NMR signals having intensities of 8:32 as observed (21:100) within the accuracy of the original T/H gas mixture. EXAMPLE 3 Reaction of Methyl 2-Naphthoate with Sucertritide 100% Tritium gas (100%) was admitted to a final pressure of 650 mm Hg (91.5 kPa) to a flask containing 3 .mu.L (0.1 mmol) of n-BuLi solution (1.2M in hexane). Injection of TMEDA (20 .mu.L, 0.1 mmol) to the stirred solution gave a creamy white precipitate (LiT). Since the scale of this reaction was much smaller than the above examples, it was not necessary to add additional tritium gas to keep the reaction pressure above 360 mm Hg (51 kPa). After 30 minutes uptake of tritium had ceased and triethylborane (0.1 mL, 1M in THF, 0.1 mmol) was injected, generating the LiEt.sub.3 BT reagent, giving a slight pressure increase, and causing clarification of the creamy solution. A THF solution of methyl 2-naphthoate (100 .mu.L, 9.7 mg, 0.05 mmol) was slowly added, and allowed to react with the supertritide at room temperature, with stirring, for 30 minutes. At the end of the reaction, methanol (3.times.0.5 mL) was added to quench the supertritide reagent, and both solvent and gas were removed in vacuo. Methanolic HCl was added, and removed in vacuo. The reaction products were then removed from the vacuum line, and worked up as for the larger scale, lower activity samples. This product was analyzed by tritium NMR. The spectrum so obtained is given in FIG. 2. Inset (A) shows an expansion (4.85-4.60 ppm) of the proton decoupled tritium spectrum, with the smaller singlet in this case due to molecules containing only one tritium atom. Inset (B) shows an expansion from 5.1-4.4 ppm, with larger vertical scale. Note the .sup.13 C satellite peaks (asterisks) with J.sub.CT =149.89 Hz, which help to give scale to the abundance of the singly labeled species. This product had a specific activity 95% of theoretical (as shown by HPLC and counting). NMR studies (FIG. 2.) supported this analysis, showing a trace of R-CH.sub.2 OH and R-CHTOH species in the proton NMR spectrum (the tritiated isotopomer being slightly isotope shifted upfield), and the R-CHTOH peak having 3.4% the intensity of the RCT.sub.2 OH peak in the spectrum of FIG. 2. EXAMPLE 4 Reaction of Ethyl p-Nitrobenzoate with Supertritide 10% The supertritide reagent was generated at 0.2 mmol scale, using 166 .mu.L n-BuLi, 34 .mu.l TMEDA and 200 .mu.L 1M ET.sub.3 B. This reagent was then syringed into a THF solution of ethyl p-nitrobenzoate (inverse addition, 20.6 mg, 0.1 mmol, 300 mL) in an adjacent flask in order to inhibit amine formation. This particular reaction was maintained at -15.degree. C. to control the reaction rate over the one hour reaction time. Workup of the reaction products was carried out in the same fashion as described above. The results of Examples 1-4 demonstrate the potential of the supertritide reagent for aldehyde and ester reductions. They also show that specific activity is retained with chemoselective reduction. The ester group of ethyl p-nitrobenzoate was successfully reduced while the nitro functionality was preserved. EXAMPLE 5 Reaction of Methyl 2-Naphthoate with Lithium Aluminum Tritide (LAT) 10% The LiT/LiH reagent was generated at 1 mmol scale (830 .mu.L n-BuLi, 170 .mu.L TMEDA). After one hour of stirring both the excess gas and solvent were removed in vacuo,. and nitrogen gas was admitted to a pressure of kPa. In a separate reaction flask, solvent was removed from a solution of aluminum tribromide in dibromomethane (1M AlBr.sub.3, 250 .mu.L, 0.25 mmol), and the solid was dissolved in 1 mL THF at -20.degree. C. to form a brownish solution. To the remaining solid LiT/LiH, 900 .mu.L of the AlBr.sub.3 solution was added slowly, yielding a tan-colored solution of LAT. A THF solution of methyl injected, and allowed to react with the LAT at room temperature, with stirring for one hour. At the end of the reaction, methanol (1 mL) was added to quench the LAT reagent, and gas evolution indicated the presence of residual hydride. The solution turned straw yellow, and the solvent volume was then reduced under vacuum. Methanolic HCl (1 mL 10% conc. HCl in MeOH) was injected, and again the solvent was removed in vacuo. The product was removed from the vacuum line and subjected to EtOAc extraction and KOH workup as described for earlier samples. Radio HPLC and UV HPLC data were obtained on this product and are given in FIG. 3. Trace (A) is the UV trace obtained by monitoring at 275nm. Trace (B) is the radioactive trace from the in-line solid scintillant detector. The traces are slightly offset in time since the sample passes through the UV cell before the radioactivity detector. These results show that the reaction is quantitative and clean. EXAMPLE 6 Reaction of Methyl 2-Naphthoate with Lithium Aluminum Tritide (LAT) 100% The LiT reagent was generated as for Example 3, the excess gas and solvent were removed in vacuo. and nitrogen gas was added to 100 kPa pressure. In a separate reaction flask, solvent was removed from a solution of aluminum tribromide in dibromomethane (1M AlBr.sub.3, 250 .mu.L, 0.25 mmol), and the solid was dissolved in 1 mL THF at -20.degree. C. To the solid LiT, 60 .mu.L of the AlBr.sub.3 solution was added with an additional 200 .mu.L THF, and the solution clarified. A THF solution of methyl 2-naphthoate (9.1 mg, 0.05 mmol, 100 .mu.L) was slowly added, and allowed to react with the LAT at room temperature, with stirring for one hour. At the end of the reaction, methanol (1 mL) was added to quench the LAT reagent, and the sample underwent methanolic HCl (1 mL) and EtOAc workup as above. Examples 5 and 6 show that LiAlT.sub.4 as reducing agent gave excellent conversion and yielded products of theoretical specific activity. Care should be exercised in the choice of stoichiometry for the generation of LiAlT.sub.4 so as to ensure that neither LiT nor AlT.sub.3 are the active agents. EXAMPLE 7 Reaction of Ethyl p-Nitrocinnamate with Selectride 10% This reaction was similar to the procedure of Fortunato, J. M., et al., supra. The LiT/LiH reagent was generated by stirring n-BuLi (830 .mu.L) and TMEDA (170 .mu.L) in the presence of one atmosphere of 10% T/H for one hour, and the flask was evaporated to dryness. Nitrogen gas was admitted to 100 kPa, and the LiH /LiT was reacted with tri-sec-butylborane (1M, 1 mL, 1.0 mmol) in THF, to give a turbid solution of Li(sec-Bu).sub.3 BT. The reaction flask was cooled to -78.degree. C., and 2.5 mL of a solution of ethyl p-nitrocinnamate (221 mg, 1 mmol) and t-butyl alcohol (266 mg) was slowly added to give a green solution. After 20 minutes the reaction temperature had risen to -70.degree. C., and the reaction was quenched with methanol to give an orange product. The solvent and excess gases were removed by evacuation. Oxidative workup (to destroy the borane complex) was achieved by the addition of NaOH (1 mL, 2N), H.sub.2 O.sub.2 (1 mL, 30%) and hexane (5 mL) with stirring overnight at room temperature. This yielded a precipitate, which was extracted by the addition of EtOAc. The organic layer was lyophilized, dissolved in MeOH, relyophilized and dissolved in CH.sub.2 Cl.sub.2. The product was then chromatographed on a 25 mL silica column, and developed with CH.sub.2 Cl.sub.2. Five fractions (each 10 mL) were collected and counted, and fractions 1 and 2 were found to contain the product with the most radioactivity. These were combined, lyophilized, and dissolved in CD.sub.3 OD for HPLC and NMR analyses. HPLC was conducted using a mobile phase of 80% CH.sub.3 OH in H.sub.2 O at 2 mL/min. This shows the utility of L-Selectride in tritium labeling experiments, making use of the outstanding selectivity of this reagent. In a 1,4 addition, the double bond of ethyl p-nitrocinnamate was reduced without affecting the nitro functional group. Some of the ester functionality appeared to have been hydrolyzed (by NMR analysis). Much less than 5% of the incorporated tritium was in the alpha-CH.sub.2 position, giving effectively quantitative tritiation in the CH.sub.2 position beta to the ester group. The specific activity of this sample was not calculated from the HPLC analyses, since an authentic standard was not freely available. EXAMPLE 8 Reaction of Naphthoic Acid and Methyl Myristate with Borane 10% The LiT/LiH reagent was generated as for Example 7 and solvent and gas were removed by evaporation. Nitrogen was admitted to 100 kPa and Et.sub.2 O.multidot.BF.sub.3 (164 .mu.L, 1.3 mmol) was added at -20.degree. C. The white solid dissolved rapidly as the solution was warmed to room temperature. After 30 minutes, borane (BH.sub.2 T) was removed by vacuum distillation to an adjoining flask, which contained a THF solution (300 .mu.L) of methyl myristate (12.5 mg, 0.05 mmol) and naphthoic acid (21.7 mg, 0.125 mmol). The reaction was allowed to proceed at room temperature, with stirring for 45 minutes. At the end of the reaction, H.sub.2 O (0.2 mL) was added to quench the borane reagent, the pressure rose significantly, and both solvent and gas were removed in vacuo. Methanol (2.times.0.6 mL) was injected and evaporated, followed by dissolution in ethyl acetate and extraction with NaOH (1N) as described in earlier examples. HPLC analysis of the products was achieved by monitoring at 215 nm and using a methanol/water mobile phase as follows: 60% CH.sub.3 OH for 10 minutes, gradient to 100% CH.sub.3 OH for a further five minutes. EXAMPLE 9 Reaction of Naphthoic Acid with Borane 100% The LiT reagent was generated as above at 0.2 mmol scale and solvent and gas were subsequently removed by evaporation. Nitrogen was admitted to 90 kPa and THF (200 .mu.L) was injected. Et.sub.2 O.multidot.BF.sub.3 (33 .mu.L, 0.26 mmol) was added at room temperature, and there was an immediate pressure rise as the solution turned to a turbid white mixture. A THF solution of 2-naphthoic acid (17 mg, 0.1 mmo, 200 .mu.l) was slowly injected with cooling. The reaction was allowed to proceed at room temperature, with stirring for 20 minutes. At the end of the reaction, methanol (1 ml) was added to quench the borane reagent, the pressure rose significantly, and both solvent and gas were removed in vacuo. Methanolic HCl (1 mL, 10%) was injected and evaporated, followed by dissolution in ethyl acetate and extraction with KOH (1N) as described in earlier examples. Example 8 shows that borane is a selective reagent, able to reduce acids in the presence of esters. Both the HPLC and NMR analyses were in agreement that some 10% of the product radioactivity was due to reduced ester. Tritium NMR study suggested that the R-CT.sub.2 OH to R-CHTOH ratio in the product was very close to the theoretical, while the specific activity of the product appeared to be considerably higher than theoretical (9.6 vs 5.76 Ci/mmol). Example 9 shows that borane reagents perform similarly when at almost theoretical maximum specific activity. TABLE 2 __________________________________________________________________________ Reactions of Tritide Reagents S.A. C;/ mmole Reagent Precursor and (Theor. Yield R.sub.x (% T/H) Product Value) % Comments on HPLC NMR Data __________________________________________________________________________ LiEt.sub.3 BT 2-Naphthaldehyde 2.42 24 Aldehyde 3% of UV. .delta.=4.7020, J.sub.HT =13.81Hz (10%) 2-(Hydroxymethyl)naphthalene (2.88) No other peaks in .sup.3 H. LiEt.sub.3 BT Methyl 2-Naphthoate 13.4 68 Acid 11% of UV. .delta..sub.1 =4.7145, .delta..sub.2 =4.6833, J.sub.HT =13.83Hz (20%) 2-(Hydroxymethyl)naphthalene (11.52) No other peaks in .sup.3 H. 1.degree. isotope effect=9.99Hz % CHT=100, % CT.sub.2 =21.00 LiEt.sub.3 BT Methyl 2-Naphthoate 54.2 27 Ester 3.7% of UV. .delta..sub.1 =4.7445, .delta..sub.2 =4.7133 (100%) 2-(Hydroxymethyl)naphthalene (57.6) No other peaks in .sup.3 H. 1.degree. isotope effect=9.98Hz % CHT=3.37, % CT.sub.2 =100 LiEt.sub.3 BT Ethyl p-Nitrobenzoate 7.1 26 Small peak at 7 min RT, .delta..sub.1 =4.7088, .delta..sub.2 =4.6762, J.sub.HT =15.38Hz (10%) p-Nitrophenyl benzyl (5.76) .sup.3 H and UV. UV similar 1.degree. isotope effect=10.44Hz alcohol to benzyl alcohol. % CHT=100, % CT.sub.2 =9.51 LiAlT.sub.4 Methyl 2-Naphthoate 8.5 66 Some small peaks .delta..sub.1 =4.7346, .delta..sub.2 =4.7033, J.sub.HT =13.82Hz (10%) 2-(Hydroxymethyl)naphthalene (5.76) in UV. 1.degree. isotope effect=10.02Hz No other peaks in .sup.3 H. % CHT=100, % CT.sub.2 =9.49 LiAlT.sub.4 Methyl 2-Naphthoate 60.5 38 Ester 9.4% of UV. .delta..sub.1 =4.7425, .delta..sub.2 =4.7113 (100%) 2-(Hydroxymethyl)naphthalene (57.6) No other peaks in .sup.3 H. 1.degree. isotope effect=10.01Hz % CHT=3.48, % CT.sub.2 =100 Li(s-Bu).sub.3 BT Ethyl p-Nitrocinnamate -- <10 3 other small peaks in .delta..sub.1 =3.0444, J.sub.HT =12.6Hz (10%) Ethyl p-Nitrophenyl (2.88) the UV. propanoate BT.sub.3 2-Naphthoic Acid/ 8.9 100 Methyl Myristate had .delta..sub.1 =4.7453, .delta..sub.2 =4.7140, J.sub. HT =13.82Hz (10%) Methyl Myristate (5.76) 9% of the UV, and 8.8% 1.degree. isotope effect=10.01Hz 2-(Hydroxymethyl)naphthalene of the .sup.3 H intensity. % CHT=100, % CT.sub.2 =11.96 BT.sub.3 2-Naphthoic Acid 55.0 2.5 Product had 13.3% of .delta..sub.1 =4.7500, .delta..sub.2 =4.7186, J.sub.HT =13.82Hz (100%) 2-(Hydroxymethyl)naphthalene (57.6) the UV, and 41% 1.degree. isotope effect=10.05Hz of the .sup.3 H intensity. % CHT=2.46, % CT.sub.2 __________________________________________________________________________ =100
046684670	description	DETAILED DESCRIPTION OF THE INVENTION The safety installation constructed in accordance with the present invention and illustrated by the single appended figure incorporates a reservoir 1 for storing a cooling liquid 2 such as water, a pump 3, a conduit 4 which connects the reservoir 1 to the pump 3 and a conduit 5 which leaves the pump 3 and terminates in the water reactor circuit (not shown) of a power station. The pump 3 makes it possible, in particular, to aspirate the cooling liquid 2 from the reservoir 1 and to discharge the said liquid, or part of it, via the conduits 4 and 5, into the circuit of the reactor. As is the case with the pump 3, the reservoir 1 is located outside the sealed containment 6 which encloses the reactor circuit. A spraying manifold 7 is located in the upper part of the containment 6. It consists of one or more distributing tubes 16 equipped with regularly spaced spray nozzles 17. The said manifold is designed to project part of the cooling liquid 2 of reservoir 1 into the interior of the containment over at least a large part of the volume occupied by the latter, the said manifold being supplied with cooling liquid by the pump 3, a pipe-line 8 connecting conduit 5 to the manifold, downstream of the said pump 3. Not only does this double injection of water make it possible efficiently to cool the circuit of the reactor and thus prevent the latter from melting, but it also serves to reduce the pressure in the containment by reducing the temperature in the latter when a break occurs in the reactor's circuit. In order to pass from the injection phase to the recirculation phase of the cooling liquid, the installation is provided with an ejector 9, the said ejector making it possible to recover the cooling liquid falling into the sump 10 of the containment 6, the cooling liquid being mixed with water from the reactor in the event of a break or rupture in the circuit of the latter, and conduct this liquid mixture by non gravitational flow towards the pump 3 after the liquid 2 has passed into the circuit of the reactor. To this end, a pipe-line 11 connects the ejector 9 to the conduit 4 upstream of pump 3, and a pipe-line 12 connects the conduit 5, downstream of pump 3, to the ejector 9. A valve 13, which is incorporated in the pipe-line 12, and two valves 14 and 15 which are incorporated in the pipe-line 11, are arranged to permit or block the flow of cooling liquid and of the water coming from the circuit of the reactor. In order to reactivate the ejector 9 and/or in order to raise the pressure of the liquid circulating in the pipe-lines 11, 12 associated with the ejector, to separate gases from the liquid the installation is provided with a hermetically sealed tank 18 whose axis is vertical. This tank 18, which is inserted in the pipe-line 11 between the valves 14 and 15, has, let into its upper part, an opening 19 to which is connected the part of the pipe-line 11 coming from the ejector 9, the lower part of the tank also being provided with an opening 20 to which is connected the part of the pipe-line 11 connected to the conduit 4. A vent pipe 21 leading to the containment 6 is set up to allow the gas or contaminated vapors, which accumulate in the top part of the tank when the ejector's pipe-lines are filled with cooling liquid and water, to escape to the interior of the containment. This tank 18 could, of course, be replaced by a normal pipe-line, that is, a pipe-line 11 could be provided without the tank 18. In this case, restarting of the ejector is effected via the principal reservoir 1. Means 22, inserted in the conduit 5 downstream of the pump 3, are provided to contain a cooling fluid and to permit an exchange of heat between this cooling fluid and the cooling liquid and reactor water to be cooled, after the liquid, and, if such be the case, the reactor water, have passed through the reactor circuit. The installation is provided with a non-return valve 23 incorporated in the conduit 4, the valve allowing cooling liquid and water to pass towards the pump 3 and prevents them from passing in the reverse direction, that is, it prevents them from discharging into the reservoir 1. In order, in particular, to verify that the pump 3 is functioning or to refill the reservoir 1 when the latter is empty, there is provided a conduit 24 which connects the conduit 5, downstream of the pump 2, to the reservoir 1, and also a valve 25 which is inserted in the said conduit 24 and which is normally closed when the installation is functioning. In addition to the above-mentioned advantages, the safety cooling installation of the invention has the following advantages when compared with conventional installations: (1) The ejector has no problem in pumping water loaded with debris such as pieces of concrete, heat-insulating rock wool, etc., with no risk of destruction. (2) The ejector can function at a very low inlet pressure and withstands cavitation which could either result in a partial blockage of the inlet or in too low a water level in the sump. (3) The ejector can easily be designed to withstand an earthquake. (4) The ejector may be installed at the location most appropriate as far as operation and protection against possible missiles are concerned. (5) In the event of the aspiration being blocked, unblocking may be effected by water pressure, by reversing the direction of flow. (6) Since the size of the safeguarding pumps are conditioed by the injection phase, the said pumps having a large margin during the recirculation phase, and a fraction of their flow can be used for driving the injectors. Some installations are not equipped with a spray circuit within the containment and, on the other hand, where such a circuit exists, it is preferable to stop the spraying as soon as possible by closing the valve 26 in such a way as to limit the dispersion of contaminated water over the equipment and the structures. Consequently, in the most probable case of a small rupture in the primary circuit, the recirculation flow-rate is very small when the spraying function is absent, which greatly reduces the rate at which the heat stored in the sump water is removed. In addition, if so desired, the installation of the invention makes it possible to maintain a high rate of recirculation, thus ensuring rapid cooling of the sump water. This has a direct effect on the temperature of the containment and, consequently, on the pressure reigning within it. The way in which the safety cooling installation of the invention functions may be easily understood by referring to the appended figure. Subsequent to a break which leads to a depressurization of the reactor circuit and, as a result, to an increase of pressure in the containment 6, the one or more pumps 3 are put into service automatically and inject water under adequate pressure into the circuit of the reactor and, if such be the case, into the interior of the containment via the conduits 4, 5 and 8, the water being drawn from the storage reservoir 1. When the water level in the reservoir 1 reaches a predetermined lower limit, the ejector circuit is put manually or automatically into operation by progressively opening the valves 13 and 14. The ejector 9 takes back the sump water, that is, the injection water coming from the storage reservoir 1 and the functional water coming from the so-called reactor circuit. After a certain time, the ejector circuit is completely filled with water, the air containing the contaminating radioactive products having been expelled to the interior of the containment via the evacuation vent-pipe 21 belonging to the tank 18. The ejector 9 also has the effect of raising the pressure of the water circulating in the conduits, the pressure of the water leaving the tank 18 via the section of tube 11 being appreciably higher than that reigning in the conduit 4 on the reservoir side. The valve 15 is then opened, the pump 3 then being supplied with water coming from the sump 10 and the ejector 9. This water, which is radioactive, is not discharged into the reservoir 1 because the non-return valve 23 blocks the flow of water in this direction. The heat exchanger 22, which is cooled by a different water circuit, makes it possible rapidly to reduce the temperature of the water put back into circulation. The water is next re-injected into the reactor circuit via the conduit 5 and eventually to the interior of the containment via the conduit 8. Part of the sump water likewise passes through the pipe-line 12 and is conveyed towards the ejector 9. For better understanding the circuit associated with the ejector 9 is shown by the slightly thicker lines. The return conduit 24 to the reservoir 1 is normally closed by the valve 25 and is only used to verify that the one or more pumps 3 are functioning, for filling the reservoir 1 or, if need be, for other purposes. In addition, in the event of a momentary stoppage of the circuit as, for example, during an interruption of the electric power supply, the ejector may be re-energized either by closing the valves 13, 14 and 15 and causing the injection to restart from the reservoir 1 and recommencing the above-defined sequence, or by using the reserve of water contained in the priming tank 18 insofar as the latter is filled. It will be noted that the water coming from the so-called reactor circuit, which generally contains a certain amount of boric acid, and the cooling liquid coming from the storage reservoir, which likewise normally consists of water, and more particularly borated water, flow in the installation in a closed circuit. This prevents any possible contamination from occurring outside the containment and the above-mentioned installation. It should be understood that the present invention is in no way limited to the embodiments described above, and modifications may be made without departing from the scope of the present patent.
054992834	summary	BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray computed tomography (CT), and more particularly to a so called helical scan imaging in which a body to be examined is moved along its body axis during the scanning operation in such an X-ray CT. 2. Description of the Background Art Recently, there has been a proposition of an X-ray CT apparatus capable of carrying out a so called helical scan imaging. As shown in FIG. 1, in the helical scan imaging, a body to be examined P located on a bed plate 100 is moved along a direction of its body axis while an X-ray tube 101 and a detector 102 is rotated around the body to be examined P, such that the X-ray tube 101 moves along a helical trajectory 103 shown In FIG. 2 relative to the body to be examined P. In reconstructing the image from the data collected by such a helical scan, tomographic image data are obtained from data collected during one rotation around the body to be examined P, such as those collected between points a and b shown in FIG. 2. Such a helical scan imaging has an advantage that the three-dimensional information on the body to be examined P can be obtained in a relatively short period of time. Now, in such a helical scan imaging, the slice plane obtained from data collected between the points a and b does not appear like a normal slice plane shown in FIG. 3A which can be obtained by ordinary scans, but appears as shown in FIG. 8B in which 0.degree. plane and 360.degree. plane do not coincide with each other. Hence, when these data are directly used in reconstructing the image, the strong artefacts appear on the reconstructed image. For this reason, the reduction of the artefacts is achieved by deriving the data of the same single slice plane from the collected data by using the interpolation as follows. For example, as shown in FIG. 4, the data at a point C of a desired rotational phase on a desired slice plane can be obtained by using the interpolation of the data d.sub.A of the point A in the same rotational phase as that of the point C and on a part of the trajectory 103 neighboring the point C, and the data d.sub.B of the point B in the same rotational phase as that of the point C and on another part of the trajectory 103 neighboring the point C. Therefore, in a case of using a linear Interpolation, the data d.sub.C at the point C can be obtained by the following expression: ##EQU1## where l is a distance between the points A and C, and m is a distance between the points B and C, as shown in FIG. 4. Now, as shown in FIG. 5, in reconstructing the image from the data collected by the helical scan, in order to obtain the necessary data for reconstructing the image at a slice center position E, the data must be collected at least at a main data region D which covers a half rotation (180.degree.) ahead and a half rotation (180.degree.) behind the slice center position E in a case of a full scan. In addition, in the case whose data for reconstruction are to be derived from the collected data by using the interpolation, the data must also be collected at supplementary data regions F and G which cover a half rotation (180.degree.) ahead and a half rotation (180.degree.) behind the main data region D. Namely, in order to obtain the data at a point C' on the slice center position E by the interpolation, the data at a point A' in the main data region D as well as the data at a point B' in the supplementary data region F become necessary. Also, in order to obtain the data at a point C" on the slice center position E by the interpolation, the data at a point A" in the main data region D as well as the data at a point B" in the supplementary data region G become necessary. Therefore, the operator must position the body to be examined P and the bed plate 100 and set up the scanning region such that the scan and the data collection can be carried out for the main data region D and possibly also for the supplementary data regions F and G if necessary, according to the desired imaging regions on the body to be examined P. Moreover, it is further preferable for the operator to position the body to be examined P and the bed plate 100 and set up the scanning region such that the scan also covers the regions for the initial acceleration and the final deceleration of the motion of the bed plate 100 along the body axis of the body to be examined P at which the data collection is unnecessary, so as to obtain the accurate data collected only while the bed plate 100 is moving at a constant speed. However, in a conventional X-ray CT apparatus capable of carrying out the helical scan imaging, the operator must carry out the initial set up operation including the positioning of the body to be examined P and the bed plate 100 and setting up of the scanning region described above, on his own discretion. Hence, these positioning and setting up operations are cumbersome and not very accurate. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method and an apparatus for helical scan imaging in an X-ray CT, in which the initial set up operation can be achieved easily and accurately, without relying heavily on the discretion of the operator. According to one aspect of the present invention an X-ray CT apparatus is provided for carrying out a helical scan imaging, comprising: Input means for entering a desired imaging region; a bed plate for carrying a body to be examined along a direction of the body axis of the body to be examined, which is linearly movable along the direction of the body axis of the body to be examined; an X-ray tube for irradiating X-rays on the body to be examined on the bed plate; a detector for detecting the X-rays irradiated by the X-ray tube and penetrated through the body to be examined, where the X-ray tube and the detector are Integrally rotatable around the body to be examined at a predetermined constant angular speed; data collection means for collecting data concerning the X-rays detected by the detector according to the desired imaging region entered by the input means; image reconstruction means for reconstructing tomographic images according to the data collected by the data collection means; and bed plate control means for controlling a linear motion of the bed plate according to the desired imaging region entered by the input means such that the bed plate is linearly moved through a distance covered by a scanning region appropriate for the data collection means to collect the data required by the image reconstruction means to reconstruct the tomographic images for the desired imaging region. According to another aspect of the present invention a method of a helical scan imaging in an X-ray CT is provided, comprising the steps of: placing a body to be examined on a bed plate which is linearly movable along a direction of a body axis of the body to be examined; entering a desired imaging region; integrally rotating an X-ray tube for irradiating X-rays on the body to be examined on the bed plate and a detector for detecting the X-rays irradiated by the X-ray tube and penetrated through the body to be examined, around the body to be examined at a predetermined constant annular speed; collecting data concerning the X-rays detected by the detector according to the desired imaging region entered at the entering step; reconstructing tomographic images according to the data collected at the collecting step; and automatically controlling a linear motion of the bed plate according to the desired imaging region entered at the entering step such that the bed plate is linearly moved through a distance covered by a scanning region appropriate for the collecting step to collect the data required by the reconstructing step to reconstruct the tomographic images for the desired imaging region. Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.
039708551	claims	1. A positron-emitting probe for use in testing samples of metals for defects by positron annihilation techniques comprising: a substrate made from the same material as the test sample; positron-emitting material supported by one surface of said substrate; and a cover for said material, said cover being sealed to said substrate to keep said material in place, said cover being of such thinness and density as to provide a window through which positron passage is uninhibited. a grounded support having a central hollow therein, the substrate-cover structure being located in said hollow and sealed hermetically at its edges to said support; an electrical insulator located within said hollow and being hermetically sealed at its edges to said support, the insulator being spaced from said substrate and forming a hermetically sealed space therebetween, said space being evacuated; and an electrode for connection to a source of electrical potential, said electrode extending into said hermetically sealed space so that an electric field is produced between the electrode and the substrate upon application of an electrical potential difference between said electrode and said substrate. a support having a central hollow therein, the substrate-cover structure being located in said hollow and sealed hermetically at its edges to said support; and a magnet, one pole of which extends into said hollow and is spaced from said substrate, said magnet being hermetically sealed to said support wherein a space is formed within said hollow bounded by said magnet, said support and said substrate, said space being evacuated. a sheet of scintillator material said sheet being placed in contact with said cover and extending beyond the substrate-cover structure; and at least one photon detector located on the contacting surface of said scintillator sheet beyond said substrate-cover structure. a substrate made of the same material as that to be tested but in the unfatigued condition, said substrate being of such thickness as to absorb all those positrons that enter it; positron-emitting material supported by one surface of said substrate; and a cover for said material, said cover being sealed to said substrate to keep said material in place, said cover being of such thinness and density as to provide a window through which positron passage is uninhibited, said probe to be used with a set of reference fatigue samples of the same material as that to be tested, each having a different but known amount of fatigue damage. an electrically conductive thin substrate; positron-emitting material supported by one side of said substrate; a thin window covering said positron-emitting material, a grounded support having a central hollow therein, the substrate-cover structure being located in said hollow and sealed hermetically at its edges to said support; an electrical insulator located within said hollow and being hermetically sealed at its edges to said support, the insulator being spaced from said substrate and forming a hermetically sealed space therebetween, said space being evacuated; and an electrode for connection to a source of electrical potential, said electrode extending into said hermetically sealed space so that an electric field is produced between the electrode and the substrate upon application of an electrical potential difference between said electrode and said substrate; said probe to be used with a set of reference fatigue samples of the same material as that being tested, each having a different but known amount of fatigue damage. a nonmagnetic substrate; positron-emitting material supported by one side of said substrate; a nonmagnetic thin window covering said positron-emitting material, said window being of such thinness and density as to not significantly impede the passage of positrons; a support having a central hollow therein, the substrate-cover structure being located in said hollow and sealed hermetically at its edges to said support; and a magnet, one pole of which extends into said hollow and is spaced from said substrate, said magnet being hermetically sealed to said support wherein a space is formed within said hollow bounded by said magnet, said support and said substrate, said space being evacuated, said probe to be used with a set of reference fatigue samples of the same material as that being tested, each having a different but known amount of fatigue damage. a gamma-ray-attenuating substrate; positron-emitting material supported by one side of said substrate; a thin sheet of scintillator material covering said positron-emitting material, said sheet extending beyond the substrate structure; and at least one photon detector located on the surface of said scintillator sheet, said probe to be used with a set of reference fatigue samples of the same material as that being tested, each having a different but known amount of fatigue damage. 2. A probe as in claim 1, wherein said cover and substrate are electrically conductive, said probe further including: 3. A probe as in claim 1, further including: 4. A probe as in claim 1, further including: 5. A probe as in claim 4, wherein said substrate is sufficiently thick to prevent passage of positrons through the substrate. 6. A positron-emitting probe designated as the "unfatigued substrate positron probe" for use in testing metal samples or objects for fatigue by positron lifetime techniques comprising: 7. A positron-emitting probe designated as the "electric-field positron probe" for use in testing metal samples or objects for fatigue by positron lifetime techniques comprising: 8. A positron-emitting probe designated as the "magnetic-field positron probe" for use in testing metal samples or objects for fatigue by positron lifetime techniques comprising: 9. A positron-emitting probe designated as the "thin-scintillator positron probe" for use in testing metal samples or objects for fatigue by positron lifetime techniques comprising:
	claims	1. A lithography tool for patterning resist coated substrates comprising:a charged particle source, configured to produce a charged particle beam;a first lens positioned below said charged particle source, said first lens being configured to form said charged particle beam into a substantially laminar charged particle beam;a stage positioned below said first lens, for carrying said resist coated substrates;a second lens positioned between said first lens and said stage, said second lens being configured to focus said substantially laminar charged particle beam onto the surface of said resist coated substrate; andan alignment deflector/stigmator positioned between said first lens and said second lens, said alignment deflector/stigmator comprising eight electrodes in an octupole configuration. 2. A lithography tool as in claim 1, wherein said eight electrodes are electrically connected to an alignment deflector control configured to supply voltages to each of said eight electrodes in said alignment deflector/stigmator to simultaneously generate a rotatable quadrupole electric field transverse to said charged particle beam for stigmating said charged particle beam and a rotatable dipole electric field transverse to said charged particle beam for deflecting said charged particle beam. 3. A lithography tool as in claim 1, further comprising a beam blanker positioned between said alignment deflector/stigmator and said second lens, for blanking said substantially laminar charged particle beam. 4. A lithography tool for patterning resist coated substrates comprising:a charged particle source, configured to produce a charged particle beam;a first lens positioned below said charged particle source, said first lens being configured to form said charged particle beam into a substantially laminar charged particle beam;a stage positioned below said first lens, for carrying said resist coated substrates;a second lens positioned between said first lens and said stage, said second lens being configured to focus said substantially laminar charged particle beam onto the surface of said resist coated substrate;an alignment deflector/stigmator positioned between said first lens and said second lens, said alignment deflector/stigmator comprising eight electrodes in an octupole configuration;a patterned beam-defining aperture positioned between said alignment deflector/stigmator and said second lens, said patterned beam-defining aperture being configured to block a large portion of charged particles in said substantially laminar charged particle beam that cannot be focused by said second lens into a predetermined beam profile at the surface of said resist coated substrate; andwherein said electron source, first lens, alignment deflector/stigmator, patterned beam-defining aperture, and second lens are configured (1) to form a non-circular shaped beam at the surface of said resist coated substrate and (2) to map charged particle current passing through multiple separated areas of said beam-defining aperture onto a single area at said resist coated substrate. 5. A lithography tool as in claim 4, wherein said eight electrodes are electrically connected to an alignment deflector control configured to supply voltages to each of said eight electrodes in said alignment deflector/stigmator to generate a rotatable dipole electric field transverse to said charged particle beam for deflecting said charged particle beam. 6. A lithography tool as in claim 4, wherein said eight electrodes are electrically connected to an alignment deflector control configured to supply voltages to each of said eight electrodes in said alignment deflector/stigmator to generate a rotatable quadrupole electric field transverse to said charged particle beam for stigmating said charged particle beam. 7. A lithography tool as in claim 4, wherein said eight electrodes are electrically connected to an alignment deflector control configured to supply voltages to each of said eight electrodes in said alignment deflector/stigmator to simultaneously generate a rotatable quadrupole electric field transverse to said charged particle beam for stigmating said charged particle beam and a rotatable dipole electric field transverse to said charged particle beam for deflecting said charged particle beam. 8. A lithography tool as in claim 4, further comprising a beam blanker positioned between said alignment deflector/stigmator and said patterned beam-defining aperture, for blanking said substantially laminar charged particle beam. 9. A lithography tool for patterning resist coated substrates comprising:a charged particle source, configured to produce a charged particle beam;a first lens positioned below said charged particle source, said first lens being configured to form said charged particle beam into a substantially laminar charged particle beam;a stage positioned below said first lens, for carrying said resist coated substrates;a second lens positioned between said first lens and said stage, said second lens being configured to focus said substantially laminar charged particle beam onto the surface of said resist coated substrate; anda beam deflector positioned between said first lens and said second lens, wherein said beam deflector is a double deflector, configured to allow telecentric scanning of said beam;wherein said second lens is configured to allow the effective axis of said second lens to move paraxially with said beam during said telecentric scanning. 10. A high throughput charged particle direct write lithography system comprising:a charged particle optical assembly configured to (1) produce a multiplicity, N, of high current density charged particle non-circular shaped-beams focused on a specimen plane and (2) vector scan said charged particle shaped-beams across said specimen plane;wherein each of said multiplicity of high current density charged particle shaped-beams has a current density, Ia, and an area A which satisfy the equations:Ia≧2000 Amperes per square centimeter;100≧N≧36;A=p2; and120>p>30 nanometers. 11. A charged particle direct write lithography system as in claim 10, wherein said charged particle optical assembly comprises a multiplicity of electron beam columns each with a patterned beam defining aperture.
047073233	summary	BACKGROUND OF THE INVENTION 1. Field of the Invention Broadly, the present invention provides a closure apparatus for an inclined duct. In a more specific embodiment of the invention there is provided a closure for such a duct which is utilized for the transfer of fuel elements into and out of a nuclear reactor vessel. 2. Description of the Prior Art In a typical liquid metal cooled breeder reactor, two vessels are utilized. One vessel houses the reactor core and associated components. The other vessel is utilized for storage of fuel elements. Generally, each of the two vessels will be interconnected via inclined ducts to a fuel transfer cell which moves the fuel elements between the two vessels via the inclined ducts. The duct leading into the reactor-containing vessel is subject to extreme temperatures and radiation during normal operation of the reactor. Thus, the closure for that duct must provide for both thermal insulation and radiation absorption. To accomplish this result, the plug portion of the closure will generally have substantially the same configuration as the duct in cross-section and have sufficient length to achieve the required degree of thermal insulation and radiation absorption. The removal and insertion of such plugs requires a mechanism capable of tilting a sufficient amount to achieve axial alignment with the open end of the duct and must be sufficiently strong to support the plug while moving it axially in an inclined direction corresponding to that of the duct. Obviously, such a mechanism is necessarily bulky and complex. Accordingly, there is need for a closure for such a duct which could be raised and lowered in a vertical direction by a hoist to simplify its insertion and removal from the duct. SUMMARY OF THE INVENTION The present invention provides a simplified closure apparatus for an inclined duct having an open upper end and defining a downwardly extending passageway. The closure comprises a cap means for sealing engagement with the open upper end of the duct and further includes an array of vertically aligned plug members. Each of the plug members has a cross-sectional area substantially conforming to the cross-sectional area of the passageway at least adjacent and upper end thereof, and are freely moveable therein. Means are provided for interconnecting each of the vertically aligned plug members to provide for free movement of plug members only in the plane in which the duct is inclined. The uppermost plug of the array is connected to the cap means. A hoist is provided directly over the open upper end of the duct and connected to the cap means for moving the closure apparatus between an upper position at which the lowermost plug of the array is above the open end of the duct, and a lower position at which the cap means is in engagement with the open upper end of the duct. In accordance with a preferred embodiment of the invention, the cap means further includes a latch for securing the cap to the open upper end of the duct. In accordance with another embodiment of the invention, each of the plug members comprises a cylinder, each cylinder having a longitudinally extending axle at each end. Axles of adjacent cylinders are interconnected by a plurality of end plates. Advantageously, the cylinders are rotatable to facilitate their movement into and out of the duct. In accordance with a particularly preferred embodiment there also is provided means for guiding at least a lower portion of the array of vertically aligned plug members into the open upper end of the duct. Typically, the guide means will comprise a vertical member in fixed relation to the open end of the duct and the cap includes an alignment member in vertically slideable engagement with the vertical member. It is an object of the invention to provide a simplified closure apparatus for inclined duct. It is another object of the invention to provide a closure system suitable for an inclined fuel transfer duct leading into a nuclear reactor vessel. Still another object of the invention is to provide a closure apparatus with a latch which will automatically secure a cap to a duct upon contact. Another object of the invention is to provide a closure system for a fuel transfer duct which will minimize the release of radiation and thermal energy from the interior of the duct. These and other objects of the invention will be more apparent from the drawings and the following detailed description thereof.
	summary
042319765	description	EXAMPLE 1 Sintered pellets from a mixed uranium-plutonium-oxide particularly for light water reactors: As starting materials a uranium xerogel and a plutonium xerogel are used. The uranium xerogel is produced by a sol-gel process corresponding to the so-called "Julich H-Process"(see L5) and the plutonium xerogel is made according to the so-called "EIR Sol-Gel Process" which is described in: (L7) Bischoff K. et al EIR Report No. 236,1973 PA1 (L8) Wymer R.G. PA1 IAEA, Vienna 1968 PA1 "Laboratory and Engineering Studies of Sol-Gel Processes at ORNL". PA1 (L9) Finney B.C. et al. PA1 ORNL-4802, 1972 PA1 "Sol-Gel Process Engineering-Scale Demonstration of the Preparation of High-Density UO.sub.2 Microspheres". PA1 (L10) Lloyd M.H. et al. PA1 Nucl. Appln. 5, 1968 PA1 "A Sol-Gel Process for preparing dense forms of PuO.sub.2 ". "Sol-Gel Processes for Carbide Preparation" To disperse the feed solutions into droplets of 10 to 50 .mu.m diameter a two-jet nozzle is used as is described e.g. in: The uranium and plutonium xerogels are available in the form of microspheres with a diameter of 5 to 25 .mu.m. It is important that the size distribution of the microspheres in the uranium xerogel and the plutonium xerogel should be the same and that in each xerogel the mean diameter should lie between 10 and 15 .mu.m. 5 g of plutonium oxide xerogel and 162 g of uranium oxide xerogel were well mixed together. From the flowable microspheres mixture green pellets were pressed in a usual pelletisation apparatus in the desired size, e.g., with a diameter of 9 mm, with the density of the pressed green pellets being not less than 4.0 g cm.sup.-3. Thus, the production of green pellets takes place here without addition of any binding material. The green pellets are then calcined at 500.degree. C. in a reducing gas of 80% argon and 20% hydrogen and then are sintered in argon at 1400.degree.-1600.degree. C. to the final pellets consisting of mixed oxides with a typical density of 95--98% of the theoretical density (Reaction sintering). The sintered pellets are here produced according to the previously illustrated Route II. EXAMPLE 2 Sintered pellets from a mixed uranium-plutonium oxide, particularly for LW reactors. A plutonium xerogel and a uranium oxide powder are the starting materials. The plutonium xerogel is produced as described in Example 1. The uranium oxide powder must have certain flowability which in this case is ensured by a grain size distribution in the powder of between 50 and 50 .mu.m. 5 g of the plutonium oxide xerogel described in Example 1 and 162 g of uranium oxide powder are well mixed in a tumble mixer. The production of green pellets from the thus obtained mixture and the subsequent heat treatment take place in the same manner as in Example 1. EXAMPLE 3 Sintered pellets from a mixed oxide of uranium and plutonium with a ratio of plutonium to heavy metal of 0.3, particularly for FB-reactors: Firstly, a uranium-plutonium xerogel is made, preferably by a sol-gel process corresponding to a so-called "Gel Precipitation Process" (See L2). The feed solution, which has been previously prepared for the production of xerogel microspheres, has the same ratio of plutonium to heavy metal as that of the desired sintered pellet, thus here 0.3. It is to be emphasized that in this case the mean size of the xerogel microspheres is not critical since the homogeneity range for uranium and plutonium in no way depends here on the size of the feed microspheres. Expediently, microspheres with a diameter in the range of 20 to 100 .mu.m are produced. To obtain a suitable dispersing of the feed solution in practice advantageously a sextuple two-jet nozzle is selected. The production of green pellets and the subsequent heat treatment are undertaken in the same manner as in Example 1. This production of sintered pellets corresponds to Route I mentioned above. EXAMPLE 4 Sintered pellets from a mixed carbide of uranium and plutonium, particularly for FB reactors: A uranium-plutonium xerogel is used as feed material for the production of green pellets, the microspheres of which contain about 12 weight parts, related to the total weight, of finely divided carbon black. The uranium-plutonium (carbon xerogel is preferably produced according to the "EIR Sol-Gel Process" (see L7) already mentioned in Example 1. 98.1 g of the feed solution containing 0.112 mol/kg of plutonium, 0.637 mol/kg of uranium and carbon in the form of carbon black of 2.397 mol/kg are dispersed into droplets of a diameter of 40 to 200 .mu.m and converted into a xerogel. Since, as in Example 2, the mean size of the microspheres is not critical for the homogeneity, the dispersing of the feed solution can be undertaken with a conventional pressure nozzle. From the uranium-plutonium (carbon) xerogel, green pellets are pressed at a pressure of about 6 tonnes/cm.sup.2. The green pellets are calcined, subjected to a carbon reduction and sintered. The principal advantage in this preferred embodiment lies in that the carbon reduction and the sintering takes place in a single operation, thus only one single process step is required for this. In practice, the green pellets are calcined in flowing gas containing 80% argon and 20% hydrogen at 500.degree. C. and then follows a heat treatment in flowing argon at 1700.degree. to 1800.degree. C. Mixed carbide fuel pellets with a density of a constant 95% of the theoretical density are achieved. This production mode corresponds again to Route I. EXAMPLE 5 Sintered pellets from a mixed oxide of uranium and plutonium particularly for FB reactors: The green pellets are produced from uranium oxide microspheres and plutonium oxide microspheres wherein the microspheres of both types have a diameter of 5-25 .mu.m. The uranium oxide microspheres are produced by the so-called "ORNL Sol-Gel Process" which is described in: The plutonium oxide microspheres are produced by a sol-gel process which is described in: The two kinds of microspheres are mixed in a ratio of one weight part of plutonium moxide microspheres and three weight parts of uranium oxide microspheres and from the mixture of microspheres green pellets are pressed in a conventional pelletising press. The subsequent heat treatment of the green pellets is in the form of a reaction sintering in a flowing gas of 96% argon and 4% hydrogen at 1200.degree. to 1300.degree. C. Uranium-plutonium mixed oxide sintered pellets are obtained with a density of 95-98% of the theoretical density. This process corresponds to the previously illustrated general Route III.
059006382	summary	BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to an arrangement for protecting attending personnel from x-rays emitted by an x-ray source in an x-ray diagnostics installation. 2. Description of the Prior Art German OS 30 12 463 discloses a radiation protection arrangement for an x-ray aiming device comprised of lead-rubber straps pivotedly suspended at a carrier. The carrier has two carrying elements connected to one another in articulated fashion that are adjustable via button-like guide elements at guide rails secured to the x-ray aiming device. The supporting elements can describe an angle relative to one another. The lead-rubber straps are secured to the two carrying elements of the carrier overlapping one another in the fashion of roofing tiles. The carrying element is guided at a lateral guide rail of the x-ray aiming device and is angled and has comb-like projections at which the lead-rubber straps are seated in articulated fashion. Another lower carrying element is fabricated of an elastically resilient flat material to which the lead-rubber straps are riveted flat at a number of locations. The lead-rubber straps are pivotable around a rotational axis at the comb-like projections of the carrying element. A number of lead-rubber straps are arranged at each carrying element. German OS 196 19 297 discloses a radiation protection arrangement for an x-ray diagnostics installation that has a carrier at which a lead-rubber flap is pivotably seated. The carrier is adjustable at a guide rail. A carrier is provided for each lead-rubber flap and the carriers are connected chain-like to one another via a connecting element. The lead-rubber straps are seated at the respective carrier pivotable around a transverse axis, so that a good radiation protection is assured both given a horizontal as well as given a vertical alignment of the x-ray aiming device together with an allocated support platform for an examination subject. When the x-ray aiming device is pivoted from the horizontal into the vertical alignment, then the lead-rubber straps are moved in the guide rail via their carrier as a result of the force of gravity and thus proceed into a limit position. SUMMARY OF THE INVENTION It is an object of the present invention to provide a radiation protection arrangement for an x-ray diagnostics installation of the type initially described which does not allow the lead-rubber straps to be unintentionally moved in the guide rail. This above object is achieved in accordance with the principles of the present invention in a radiation protection arrangement for an x-ray diagnostics installation having a guide rail surrounding a region in which x-rays are emitted, with a number of lead-rubber straps each having a carrier movable seated in the guide rail, and a brake arrangement for each carrier which at least impedes, and preferably prevents, movement of the carriers along the guide rail when the component to which the guide rail is attached is adjusted in position. An advantage of the invention is that an movement of the carrier along the rail is intentionally initiated by the operator and does not occur independently, such as by the force of gravity. It is advantageous for the braking arrangement to be a as friction brake or a mechanism which engages at the carrier, preferably at an end of the carrier. The position of the radiation protection arrangement can thus be fixed to achieve an optimum radiation protection. A further object of the present invention is to provide a radiation protection arrangement for an x-ray diagnostics installation of the type initially described wherein the lead-rubber straps do not seize or become impeded when the x-ray aiming device is pivoted. The above object is achieved in accordance with the principles of the present invention in a radiation protection arrangement for an x-ray diagnostics installation having at least two lead-rubber straps which are pivotably seated at a carrier, each strap having a formed part in the region of the carrier which effects a diversion of the lead-rubber strap when the strap is caused to pivot out of a perpendicular position. An advantage of providing a formed part at the carrier is that the lead-rubber straps are diverted at this formed part when pivoted, without the lead-rubber straps seizing or becoming impeded. It is particularly advantageous when, for this purpose, the formed part is fashioned as a bead surrounded by a swiveling axis of the lead-rubber strap fashioned as a ring. To this end, the formed part can be placed into or placed on the lead-rubber strap. It is advantageous for the same purpose when the lead-rubber straps are joined to one another in a first direction and are aligned in a second direction, with the first and second directions describing a non-zero angle relative to one another. As a result, seizing and impeding of the lead-rubber straps when pivoting the x-ray aiming device is also prevented. Lead-rubber straps pivotally suspended at a carrier are provided in the radiation protection arrangement disclosed by German OS 30 12 463. These lead-rubber straps serve as radiation protection but impede access to the examination subject. It is therefore also an additional object of the invention to provide a radiation protection arrangement of the type initially described wherein this access is facilitated. This object is achieved in accordance with the principles of the present invention in a radiation protection arrangement for an x-ray diagnostic installation wherein at least one lead-rubber strap is movably arranged in a guide rail so as to prevent radiation from being emitted beyond a region surrounding an x-ray source, and wherein the radiation protection strap can be folded over, and fixed in a folded-over position, so as to allow access to the protected region. An advantage of the invention is that the at least one lead-rubber strap can be folded over and fixed in the folded-over position. Access to the examination subject is thereby produced and the access is maintained.
051204886	summary	FIELD OF THE INVENTION The present invention relates to a sealing sleeve of memory metal, especially suited to repair such leaks in pipes or the like which are difficult to reach, where the sealing sleeve must also be able to endure great temperature differences without being influenced such that the sealing effect is discontinued. The sealing sleeve is particularly suited for use in a nuclear reactor environment. BACKGROUND OF THE INVENTION In a nuclear reactor there are numerous pipes, pipe sockets and the like, in which cracks, especially in the vicinity of welds, may arise and where, during repair work, one end of the pipe may be released so as to form an open end for the fitting of a sealing sleeve. A plurality of various types of sealing sleeves are known--also such of memory metal (see, e.g., U.S. Pat. Nos. 4,773,680 and 4,149,911). Characteristic of these sleeves is that they are not suited for sealing of pipe joints, for example control rod drive (CRD) stub tubes in a nuclear reactor, in which the pipes on either side of the joint are eccentrically arranged. Nor are they adapted to withstand the dimensional changes, caused by the temperature, which occur in a nuclear reactor in which the temperature varies from about +40.degree. C. to about +280.degree. C. SUMMARY OF THE INVENTION The present invention relates to a sealing sleeve for, for example, a pipe socket of such a design that the above-mentioned drawbacks are eliminated. The sealing sleeve according to the invention is characterized in that it comprises a bellows-like mid-portion with annular ends, at least these ends or part of these ends being made of a memory metal with a suitable transition temperature and being given an internal diameter suitable for achieving sealing around the relevant pipe section on each side or the leak. That part of the ends which consists of memory metal is intended to thereafter, at a temperature below the transition temperature, be deformed into a diameter which permits the sleeve to be freely fitted-onto the pipe. In fitted position, the sleeve is intended to be subjected to heating to a temperature above the transition temperature in order to achieve the desired sealing when the memory metal in the sleeve strives to recover its original shape. According to one embodiment, the entire sleeve, including the bellows, is made of memory metal, the annular ends around the inner side being provided with an elevation or a number or grooves for obtaining more reliable sealing. In order to improve the sealing further around, for example, pipe sockets, the elevation or the grooves are replaced by sealing rings of stainless steel, the outer limiting surface of which is spherical and has an external diameter which fits inside the sleeve in its deformed state below the transition temperature. The rings have an inner diameter which permits them to be fitted freely onto the pipe socket. The outer spherical limiting surface of the rings facilitates the adoption by the device of any eccentricity between the pipe sections over which the sealing sleeve is fitted by permitting the sleeve-to slide over the spherical contact surfaces on the outside of the sealing rings. The sleeve finally reaches an oblique position which coincides with the connecting line between the spherical centers of the sealing rings. The aim is to produce a sealing sleeve of memory metal which may be stocked in its deformed state and which withstands all eccentricities up to a certain maximum value without any further enlarging deformation of the diameter of the sleeve being necessary.
	summary
	description	This application is a continuation in part of U.S. Utility patent application Ser. No. 14/519,645, Oct. 21, 2014, now pending, that claimed the benefit of U.S. Provisional Application Ser. No. 61/894,162, Oct. 22, 2013. This invention relates to gas separation. More particularly, this invention relates to an apparatus for the removal of radon from a gas. Radon is a gaseous element having the atomic number 86, i.e., an atom of radon has 86 protons in its nucleus and 86 electrons. Radon is a member of a group of elements known as the noble gases because they are relatively unreactive. Radon exists in the form of eighteen different isotopes. Isotopes are atoms of an element that contain different numbers of neutrons in their nuclei. Particular isotopes are commonly identified by their total number of protons and neutrons. For example, radon-222 is an isotope containing 86 protons and 136 neutrons. All isotopes of radon are radioactive. Radioactivity is a process in which atoms undergo spontaneous nuclear transformations or decay by emitting atomic particles and/or electromagnetic energy. The most common types of radioactive decay are alpha decay (which produces an alpha particle consisting of two protons and two neutrons) and beta decay (which produces an electron). Electromagnetic radiation in the form of a gamma ray is also emitted as part of each alpha and beta decay. Each gamma ray is emitted in a random direction and travels in a straight line until absorbed. The rate of radioactivity is measured by its half-life. A half-life is the time for one-half of the atoms to undergo radioactive transformation. Seventeen of the eighteen radon isotopes have half-lives of a minute or less. Radon-222 is the most stable of the radon isotopes and has a half-life of 3.8 days. Radon-222 decays through several intermediates (also known as decay progeny) into lead-210, an isotope of lead (atomic number 82) having a half life of 22 years. Lead-210 decays through intermediates into the stable, nonradioactive lead-206. Radon is constantly being formed by the radioactive decay of subterranean uranium (atomic number 92). Uranium is present primarily in the form of the uranium-238 isotope. Uranium-238 is radioactive with a half life of 4.5 billion years. Uranium-238 decays through several intermediates into radon-222. The slow decay of uranium-238 has been occurring since the earth was formed. The decay chain from uranium-238 via radon-222 to lead-206 produces multiple alpha, beta, and gamma emissions. The highest energy gamma ray emission in this decay chain is about 7.7 MeV (mega electron volts). Radon is also being formed by the radioactive decay of subterranean thorium (atomic number 90). Thorium is present primarily in the form of the thorium-232 isotope. Thorium-232 is radioactive with a half life of 14 billion years. Thorium-232 decays through several intermediates into radon-220. Radon-220 has a half-life of about one minute. Radon-220 decays through several intermediates into stable, nonradioactive lead-208. The decay chain from thorium-232 via radon-220 to lead-208 also produces multiple alpha, beta, and gamma emissions. For brevity, the term “radon” is used hereinafter to refer to all the isotopes of radon. As a result of the ongoing formation of radon from radioactive decay, radon gas is constantly seeping upward through rock and soil toward the surface of the earth. The radon poses no risk if it decays before reaching the surface because its decay progeny are solids that remain wherever formed. Similarly, the radon poses no risk if it reaches the atmosphere because its concentration is so small. However, radon can enter buildings and concentrate to dangerous levels in the air, particularly in basements and first floors of buildings without basements. Radon levels vary considerably at different sites, and over time at any given site. Many factors cause these variations. For example, low pressure atmospheric conditions which often occur during storms are believed to draw higher levels of radon from the ground. Radon is the leading environmental cause of cancer in the United States and the second-leading cause of lung cancer. The harmful effects are due primarily to alpha, beta, and gamma ray emissions inside the body resulting from radon breathed into the lungs. The health risks posed by radon have become more widely recognized in the past decade. The United States Environmental Protection Agency has recommended that homeowners take corrective action if the level of radon in their homes exceeds 4 picocuries per liter (about 0.04 decays per second per liter of air). There are two basic ways to lower radon levels in a building. The first is to suppress the flow of radon into a the building. The second is to remove the radon that is already there. One common technique for suppressing the flow of radon into a building is to seal cracks and other openings in the building foundation, often in conjunction with sub-slab decompression. However, it is difficult to identify and permanently seal every opening. Furthermore, normal settling of buildings creates new openings and reopens old ones. The flow of radon into buildings is also suppressed by placing a barrier film on or under the lowest floor. An example of such a method is disclosed in Shahar, U.S. Pat. No. 6,676,780, Jan. 13, 2004. The most common technique for removing existing radon from the air in a building is to increase ventilation. Simply opening doors and windows can lower radon levels. However, ventilation is difficult in basements with few, if any, windows or doors. Ventilation also results in the loss of conditioned air, discomfort, security problems, and increased costs of conditioning outside air. Another technique for removing existing radon from the air in a building is to operate a liquid-gas contacting apparatus, commonly known as a wet scrubber. A wet scrubber is an apparatus in a which a gas stream is brought into contact with a working (scrubbing) liquid by forcing the gas through the liquid, by spraying the gas with the liquid, or by some other contact method. As the gas and liquid make contact, one or more components of the gas are absorbed, captured by the clathrate mechanism, or otherwise transferred from the gas to the liquid. The terms “absorb” and “dissolve” are used herein to refer to any mechanism by which a component of a gas becomes a component of a liquid. The removal of radon from air with a wet scrubber having an oil as the working liquid is disclosed in Gross et al., U.S. Pat. No. 5,743,944, Apr. 28, 1998. Gross et al. disclose that suitable oils include vegetable oils, animal oils, and petroleum oils. While a wet scrubber removes radon from the air, the radon absorbed into the liquid remains a health risk. If the level of radon or its decay progeny reach a level to generate 2000 picocuries per gram (pCi/g), the liquid becomes a “low-level radioactive waste” as defined by the U.S. Environmental Protection Agency and requires special handling. Gross et al. address this problem by removing the working liquid from the wet scrubber, agitating and heating the liquid to release the radon from the liquid, and then venting the released radon to the atmosphere. The immediate removal of radon from the working liquid is an expensive step that adds greatly to the complexity and cost of the wet scrubber system. Accordingly, a demand exists for an improved wet scrubber apparatus for removing radon from a gas, such as the air in the interior of a building, that does not require the immediate removal of radon from the working fluid. The general object of this invention is to provide an improved apparatus for removing radon from a gas, such as the air in the interior of a building. A more particular object is to provide a wet scrubber apparatus for removing radon from a gas that do not require the radon to be removed from the working liquid. We have invented an improved radon removal apparatus. The apparatus comprises: (a) a liquid-gas contacting enclosure containing a working liquid in which radon is soluble; (b) a shielded reservoir containing the working liquid; and (c) a means for causing a stream of gas containing radon to pass through the liquid-gas contacting enclosure. The apparatus of this invention provide an effective, simple, and cost effective means for removing radon from a gas. The radon is captured in a working liquid. When the radon level in the working liquid reaches a certain level, the working liquid is either taken out of service and stored until the radon level is reduced or the working liquid is diluted with additional working liquid having no radon. In either case, there is no need to remove the captured radon from the working liquid. This invention is best understood by reference to the drawings. Referring to FIG. 1, a gas stream 20 containing radon is treated in a first embodiment of the apparatus 10 of this invention. The first embodiment is especially suited for removing radon from the ambient air in a residential building. The apparatus comprises a blower 30 and an integral liquid-gas contacting enclosure/shielded reservoir 40 containing a liquid 50 in which the radon is soluble. In the first embodiment, the liquid-contacting enclosure and the shielded reservoir are an integral unit. If desired, the liquid-contacting enclosure and the shielded reservoir are separate components. The treated gas 21, which is depleted in radon, is vented back into the building. The components of the apparatus are discussed in detail below. Also shown in FIG. 1 are two other liquid-gas contacting enclosures/shielded reservoirs—a replacement unit 40′ and a spent unit 40″. The replacement unit is ready for future use. The spent unit has been placed in storage until substantially all the absorbed radon decays to lead. As explained in detail below, the current unit will be taken out of service after a period of time and placed into storage along with the spent unit. The replacement unit will simultaneously be placed into service. The gas stream 20 contains radon and is at a temperature and pressure that does not adversely affect the working liquid. A preferred gas stream is ambient air in a building interior that contains radon. Ambient air typically contains about 78 percent nitrogen, about 20 percent oxygen, about one percent water vapor, and about one percent other gases. Other suitable gas streams include natural gas (comprising primarily methane) and various industrial gas streams. The blower 30 causes the radon-containing gas to enter the liquid-gas contacting enclosure. The blower has a rotating impeller that propels the gas in a direction that is perpendicular to the axis of rotation. The size and air flow capability of the blower is a matter of choice that depends on the desired capability of the apparatus. In the preferred embodiment, the blower is a small, electrically-powered unit having sufficient capacity to cause the gas to pass through the liquid-gas contacting enclosure. Other means for causing the gas stream to flow through the liquid-gas contacting enclosure are also suitable, including displacement pumps, fans (that propel the gas in a direction parallel to the axis of rotation of the vanes), and the like. After passing through the blower, the radon-containing gas enters a duct 31. The outlet of the duct contains a sleeve 32 or other suitable means for connection to the inlet of the liquid-gas contacting enclosure. The means for connection is preferably simple enough that a homeowner can make the connection (and disconnection) quickly by hand or with commonly available tools. The liquid-gas contacting enclosure 40 is the component of the apparatus where a gas stream is brought into contact with a working (scrubbing) liquid by causing the gas to flow through the liquid (either with or without packing such as Raschig rings, structured packing, and the like) through a vertical column or tank, by spraying the gas with the liquid, or by some other contact method. A wide variety of such wet scrubbers are known in the art. One of the simplest types of enclosures is used in the preferred embodiment. This enclosure is a sealed tank partially filled with a working liquid 50 and having an inlet 41 for the radon-containing gas and an outlet 42 for the radon-depleted gas. The inlet is preferably above the level of the liquid so that liquid cannot flow out the inlet. The inlet preferably contains a replaceable filter to remove large particulates. Filters similar to those used in forced air heating units are suitable. The outlet preferably contains a coalescing metal mesh filter to entrap any entrained working liquid in the exiting gas stream. The inlet feeds into a perforated pipe or plate 43 near the bottom of the enclosure. The radon-containing gas passes through the perforations and upward through the working liquid (as represented by the wavy lines in FIG. 1). Screens, filters, or the like are preferably placed over the perforations to reduce the size of the gas bubbles and thereby increase the surface area of contact between the gas and the liquid. The radon-depleted gas then passes through the head space above the liquid and exits the enclosure. The enclosure of the preferred embodiment has no moving parts. Because the enclosure and reservoir are integral, there is no need to pump the liquid back and forth between the enclosure and the reservoir. Furthermore, the integral enclosure/reservoir can be removed and replaced by simply undoing the connection to the blower duct. The working liquid 50 captures some of the radon as it makes contact with the radon-containing gas. The working liquid also captures particulates and other solids in the gas, including radon decay progeny. Suitable working liquids are those in which radon is soluble. Preferred working liquids include hydrocarbon oils of vegetable, animal, petroleum, or synthetic origin (e.g., silicone oil) as known in the art, including those described in Gross et al., U.S. Pat. No. 5,743,944, Apr. 28, 1998. The selection of the oil depends on radon solubility, cost, stability of the liquid, and other factors. The level of radon and its decay progeny in the working liquid is never allowed to reach the point at which the working liquid becomes a low-level radioactive waste. The shielded reservoir contains all or a substantial portion of the working liquid. As previously discussed, the shielded reservoir of the preferred embodiment is integral with the liquid-gas contacting enclosure. In other embodiments, the shielded reservoir takes the form of a tank from which the working liquid is pumped to, and returned from, the liquid-gas contacting enclosure. The size, liquid capacity, and structure of the shielded reservoir are matters of choice that depend on the desired gas flow rate, the anticipated radon level in the gas, the solubility of radon in the working liquid, the desired service life of the working liquid, and other factors. For a typical single family residence, a reservoir in the form of a tank made of a shielding material having a liquid capacity of about one to ten gallons is sufficient for use for at least a year without needing replacement. A reservoir of this capacity typically weighs about ten to one hundred pounds and can be moved and transported relatively easily. A second suitable form of reservoir is made of a lightweight material such as a thermoplastic, and has a hollow wall that is then filled with a suitable shielding material such as sand or water. A third suitable form of reservoir is an enclosure made of a non-shielding material, for example a thermoplastic such as polyvinyl chloride, that is placed inside a shielding enclosure. A variety of materials are suitable for the shielding component of the enclosure walls. For example, metals, concrete, brick, sand, water, and soil are commonly used to shield radiation. The choice of preferred material depends on weight, cost, and other factors. The amount of shielding is a matter of choice to a certain extent. If the goal is to prevent a single alpha particle, beta particle, or gamma ray from escaping the reservoir, the thickness of the shielding for a given material can be calculated as follows. The shielding would need to absorb a gamma ray having an energy of about 7.7 MeV because that is the highest energy gamma ray emitted during the decay of radon to nonradioactive lead. Shielding that can absorb a 7.7 MeV gamma ray can also absorb alpha and beta particles. The thickness of a given material needed for containment of a 7.7 MeV gamma ray is easily calculated from published empirical correlations between gamma ray energy and the attenuation coefficient (also known as the linear attenuation coefficient) of the material. The attenuation coefficient is a measure of how easily the material can be penetrated by gamma rays or other energy or matter. For example, the attenuation coefficient for lead for a gamma ray of 7.7 MeV is about 0.5 cm−1. Accordingly, lead having a thickness of about 2 cm (or one inch) is needed for complete containment of a perpendicularly-directed 7.7 MeV gamma ray. As a practical matter, a reservoir that completely contains a 7.7 MeV gamma ray is unnecessary for several reasons. First of all, simply locating the reservoir in the lowest level of a building, preferably along an outside wall or at an outside corner, ensures that most of the emitted gamma rays will be directed into the soil surrounding the building rather than into the living space. The relocation of interior radon from room air that is breathed by occupants into a shielded oil reservoir greatly reduces the total radiation exposure of the occupants and accordingly reduces the frequency of radon decay events in close proximity to lung tissues, which in turn is believed to be the etiology of radon-caused lung cancer. Secondly, any amount of shielding between the working liquid and the living space causes some reduction in gamma rays. Thirdly, it makes little sense to use an expensive and heavy enclosure that will completely contain all gamma rays when some radon will always remain in the living space. More particularly, some radon will always remain because radon is constantly entering the living space, because some of the radon-containing air will never be drawn into the liquid-gas contacting apparatus, and because not all the radon that passes through the liquid-gas contacting apparatus is absorbed into the working liquid. In short, any reduction in the radon in the living space is beneficial and the more reduction the better, but a complete elimination of radon is impossible. The operation of the preferred embodiment of the apparatus can now be considered. The integral liquid-gas contacting enclosure/shielded reservoir is filled to the desired level with the desired working fluid (vegetable oil, petroleum oil, etc.). If the enclosure shell is of the hollow type, the shell is filled with the desired shielding material (water, sand, etc.). The enclosure/reservoir is moved to the lowest level of the building either before or after filling. As discussed above, the enclosure/reservoir is preferably placed along an outside wall or at an outside corner to increase the likelihood that any escaping radiation will be directed into the soil surrounding the building rather than into the living space. If desired, the enclosure/reservoir is placed into a separate enclosure, such as a small space defined by walls of brick, sand bags, concrete block, or the like, or into a recessed pit in the foundation. The blower (or other gas mover) is then connected to a source of electrical power and the blower duct is connected to the inlet of the integral liquid-gas contacting enclosure/shielded reservoir. The blower is then turned on and allowed to run continuously. After a period of time, the reservoir with the spent working liquid is taken out of service. If desired, the apparatus contains a timer similar to those used with water softeners that either automatically shuts off the apparatus, displays a visual signal, or emits an aural signal to remind the responsible person that the reservoir needs to be replaced. As previously discussed, the reservoir is replaced before the working liquid accumulates enough radioactive materials (radon, lead-210, etc.) to become a low-level radioactive waste. After being taken out of service, a first cap 44 is placed on its inlet and a second cap 45 is placed on its outlet. If exposure to radiation escaping out the inlet and outlet during capping (or at any time) is a concern, it can be minimized in various ways. One way is to include baffles of a suitable material at the inlet and outlet that reduce the escape of radiation at all times. Another way is to provide a solenoid or other mechanism that automatically moves the caps over the inlet and outlet at the desired time. The capped reservoir is then removed from the building and transported to a storage facility. As soon as one reservoir is removed from service, a new reservoir with fresh working liquid is brought in and connected. The removed reservoir is stored until substantially all the absorbed radon decays to lead. As previously discussed, the half life of radon is about four days. Accordingly, about 50 percent of the radon decays in four days, about 75 percent decays in 8 days, about 88 percent decays in 12 days, about 94 percent decays in 16 days, and so on. After substantially all the radon has decayed, the liquid is removed from the reservoir and treated to remove the lead. The lead includes several isotopes, including radioactive lead-210, nonradioactive lead-208, and nonradioactive lead-206. The liquid can then be recycled or discarded. The reservoir is refilled with fresh liquid and is ready for use again. The apparatus and method of this invention have many advantages over those of the prior art. One advantage over most radon reduction methods is that there is no need for exterior ventilation. An advantage over most wet scrubber methods is that there is no need to agitate, heat, or otherwise treat the spent working liquid to remove absorbed radon. Instead, the replaceable reservoir containing the spent working liquid is simply taken out of service and placed in storage until the radon decays to lead to the desired degree. An advantage of the preferred embodiment is that it is especially suited for residential use because of its size, low power consumption, and relatively silent operation. Referring now to FIG. 2, the second embodiment 110 of the invention is shown treating the ambient air 120 in the basement of a residential building 125. The apparatus comprises a blower 130, an outflow duct 135, and an integral liquid-gas contacting enclosure/shielded reservoir 140. The enclosure/reservoir has an access door 145 and is partially filled with a liquid 150 in which the radon is soluble. After the air is treated, the radon depleted air returns to the basement through the return duct 155. The enclosure/reservoir is buried under the ground 160 outside and in relative close proximity to the house with the access door just above ground level. The enclosure/reservoir is generally less than 100 feet from the house and preferably less than 50 feet from the house. An especially convenient location for the enclosure/reservoir is directly below a compressor or heat pump of a cooling system where it is very unlikely that the reservoir will be accidentally damaged by digging, trenching, or the like. The earth around the enclosure/reservoir provides the necessary shielding so the wall material is determined primarily by structural considerations rather than radiation considerations. The operation of the second embodiment is similar to the first. However, instead of replacing the entire enclosure/reservoir, the access door is opened and the spent working liquid is removed and new working liquid added. The spent working liquid can be treated as desired. It can be stored in a shielded container until the radon decays to the desired level or it can be added to other liquid of the same type. The dilution of the spent working liquid may provide the necessary reduction in radon level. For example, assume the enclosure/reservoir has been in use for many years with a working liquid consisting of five gallons of motor oil. Assume also that the radioactivity level has reached 10,000 picocuries per gram (pCi/g), a level that is well above the 2,000 pCi/g threshold for “low-level radioactive waste” as defined by the U.S. Environmental Protection Agency. Removing the spent motor oil and adding it to thirty gallons of used motor oil results in thirty-five gallons of used motor oil having a radioactivity level of 1,428 pCi/g that is below the threshold for “low level radioactive waste” and that can be safely handled as desired. Referring now to FIG. 3, the third embodiment 210 of the invention is shown treating the ambient air 220 in the basement of a residential building 225. The apparatus comprises a blower 230, an outflow duct 235, a liquid-gas contacting enclosure 240, a return duct 245, and a separate shielded reservoir 270. The sizes of the enclosure and reservoir are not drawn to scale. The reservoir preferably has a significantly greater volume than the enclosure. The liquid-gas contacting enclosure preferably contains a perforated plate or pipe submerged in the working liquid as previously described. A pump 280 transfers the working liquid from the enclosure to the reservoir via line 281. The working liquid flows by gravity from the reservoir to the enclosure via overflow return line 282. Lines 281 and 282 are relatively small in diameter and are preferably routed through the exterior wall at the same place as the refrigerant lines for the cooling system. The shielded reservoir is buried under the ground 260 outside with the same considerations discussed above with respect to the enclosure/reservoir of the second embodiment. The treatment of spent working fluid is also similar to the treatment in the second embodiment. The use of a separate shielded reservoir allows some advantageous changes to the liquid-gas contacting enclosure. For example, when compared to the integral enclosure/reservoir in the first and second embodiments, the enclosure of the third embodiment has a substantially smaller volume of working liquid and has substantially less shielding. The operation of the third embodiment of the radon removal apparatus is similar to the operation of the second embodiment.
	claims	1. An apparatus to monitor and determine parameter settings for beam control in a scanning microscope comprising:a processor to process images of a sample each generated by said microscope with a varying setting of a corresponding microscope parameter pertaining to beam control and including:an evaluation module to evaluate information of each image associated with said microscope parameter to identify a region with differentiation between said varying parameter settings;a model module to approximate behavior of said information in said identified region for each image associated with said microscope parameter to determine a response in that region for each of those images, wherein said response serves as an image quality value indicating quality of said associated image; anda parameter module to determine a resulting parameter setting for said associated microscope parameter pertaining to beam control based on an optimization of said responses of said images to control said microscope beam. 2. The apparatus of claim 1, wherein said microscope parameter includes at least one of a focus parameter and a stigmation parameter. 3. The apparatus of claim 1, wherein said model module approximates said information behavior via linear regression techniques to determine said responses and said parameter module determines said resulting parameter setting via curve fitting techniques applied to said responses, wherein a lowest slope of a generated curve indicates said resulting parameter setting. 4. The apparatus of claim 1, wherein said processor further includes:a comparison module to compare an image quality value of at least one currently scanned microscope image with at least one image quality value of prior images and initiating determination of said resulting parameter setting in response to said at least one current image quality value varying from said at least one prior image quality value in excess of a threshold. 5. The apparatus of claim 4, wherein said processor further includes:an inspection module to ascertain at least one currently scanned image and determine said image quality value for each currently scanned image for said comparison in response to occurrence of a particular condition to monitor said microscope parameters. 6. The apparatus of claim 1, wherein said microscope includes a critical dimension scanning electron microscope. 7. The apparatus of claim 1, wherein said response includes a change of intensity in said identified region for an associated image. 8. A program product apparatus including a computer readable medium with computer program logic recorded thereon for monitoring and determining parameter settings for beam control in a scanning microscope comprising:an evaluation module to evaluate information of each image of a sample generated by said microscope with a varying setting of a corresponding microscope parameter pertaining to beam control to identify a region with differentiation between said varying parameter settings;a model module to approximate behavior of said information in said identified region for each image associated with said microscope parameter to determine a response in that region for each of those images, wherein said response serves as an image quality value indicating quality of said associated image; anda parameter module to determine a resulting parameter setting for said associated microscope parameter pertaining to beam control from an optimization of said responses of said images to control said microscope beam. 9. The apparatus of claim 8, wherein said microscope parameter includes at least one of a focus parameter and a stigmation parameter. 10. The apparatus of claim 8, wherein said model module approximates said information behavior via linear regression techniques to determine said responses and said parameter module determines said resulting parameter setting via curve fitting techniques applied to said responses, wherein a lowest slope of a generated curve indicates said resulting parameter setting. 11. The apparatus of claim 8, wherein said processor further includes:a comparison module to compare an image quality value of at least one currently scanned microscope image with at least one image quality value of prior images and initiating determination of said resulting parameter setting in response to said at least one current image quality value varying from said at least one prior image quality value in excess of a threshold. 12. The apparatus of claim 11, wherein said processor further includes:an inspection module to ascertain at least one currently scanned image and determine said image quality value for each currently scanned image for said comparison in response to occurrence of a particular condition to monitor said microscope parameters. 13. The apparatus of claim 8, wherein said microscope includes a critical dimension scanning electron microscope. 14. The apparatus of claim 8, wherein said response includes a change of intensity in said identified region for an associated image. 15. An apparatus to monitor and determine parameter settings for beam control in a scanning microscope comprising:processing means for processing images of a sample each generated by said microscope with a varying setting of a corresponding microscope parameter pertaining to beam control and including:evaluation means for evaluating information of each image associated with said microscope parameter to identify a region with differentiation between said varying parameter settings;model means for approximating behavior of said information in said identified region for each image associated with said microscope parameter to determine a response in that region for each of those images, wherein said response serves as an image quality value indicating quality of said associated image; andparameter means for determining a resulting parameter setting for said associated microscope parameter pertaining to beam control from an optimization of said responses of said images to control said microscope beam. 16. The apparatus of claim 15, wherein said microscope parameter includes at least one of a focus parameter and a stigmation parameter. 17. The apparatus of claim 15, wherein said model means approximates said information behavior via linear regression techniques to determine said responses and said parameter means determines said resulting parameter setting via curve fitting techniques applied to said responses, wherein a lowest slope of a generated curve indicates said resulting parameter setting. 18. The apparatus of claim 15, wherein said processing means further includes:comparison means for comparing an image quality value of at least one currently scanned microscope image with at least one image quality value of prior images and initiating determination of said resulting parameter setting in response to said at least one current image quality value varying from said at least one prior image quality value in excess of a threshold. 19. The apparatus of claim 18, wherein said processing means further includes:inspection means for ascertaining at least one currently scanned image and determining said image quality value for each currently scanned image for said comparison in response to occurrence of a particular condition to monitor said microscope parameters. 20. The apparatus of claim 15, wherein said microscope includes a critical dimension scanning electron microscope. 21. The apparatus of claim 15, wherein said response includes a change of intensity in said identified region for an associated image. 22. A scanning microscope system monitoring and determining parameter settings for beam control comprising:a beam unit for generating and controlling a beam to scan a sample in accordance with microscope parameters pertaining to beam control;an image unit to generate an image of said sample based on information collected from said scan;a processor to process images of said sample each generated with a varying setting of a corresponding microscope parameter pertaining to beam control and including:an evaluation module to evaluate information of each image associated with said microscope parameter to identify a region with differentiation between said varying parameter settings;a model module to approximate behavior of said information in said identified region for each image associated with said microscope parameter to determine a response in that region for each of those images, wherein said response serves as an image quality value indicating quality of said associated image; anda parameter module to determine a resulting parameter setting for said associated microscope parameter pertaining to beam control from an optimization of said responses of said images to control said microscope beam. 23. The system of claim 22, wherein said microscope parameter includes at least one of a focus parameter and a stigmation parameter. 24. The system of claim 22, wherein said model module approximates said information behavior via linear regression techniques to determine said responses and said parameter module determines said resulting parameter setting via curve fitting techniques applied to said responses, wherein a lowest slope of a generated curve indicates said resulting parameter setting. 25. The system of claim 22, wherein said processor further includes:a comparison module to compare an image quality value of at least one currently scanned microscope image with at least one image quality value of prior images and initiating determination of said resulting parameter setting in response to said at least one current image quality value varying from said at least one prior image quality value in excess of a threshold. 26. The system of claim 25, wherein said processor further includes:an inspection module to ascertain at least one currently scanned image and determine said image quality value for each currently scanned image for said comparison in response to occurrence of a particular condition to monitor said microscope parameters. 27. The system of claim 22, wherein said scanning microscope system includes a critical dimension scanning electron microscope. 28. The system of claim 22, wherein said response includes a change of intensity in said identified region for an associated image.
045004889	abstract	This invention teaches an encapsulated fuel unit for a nuclear reactor, such as for an enriched uranium fuel plate of thin cross section of the order of 1/64 or 1/8 of an inch and otherwise of rectangular shape 1-2 inches wide and 2-4 inches long. The case is formed from (a) two similar channel-shaped half sections extended lengthwise of the elongated plate and having side edges butted and welded together to define an open ended tube-like structure and from (b) porous end caps welded across the open ends of the tube-like structure. The half sections are preferably of stainless steel between 0.002 and 0.01 of an inch thick, and are beam welded together over and within machined and hardened tool steel chill blocks. The porous end caps preferably are of T-316-L stainless steel having pores of approximately 3-10 microns size.
	summary
	summary
059109718	description	DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates the only method that currently exists for the production of Mo-99 that is approved by the U.S. Food and Drug Administration. An enriched uranium target is irradiated by neutrons in a nuclear reactor producing Mo-99 and a large quantity of radioactive wastes. The Mo-99 is chemically extracted from the target. A large quantity of radioactive fission byproducts are also produced by the neutron bombardment of the target that subsequently must be disposed of. The Mo-99 production process flow of the present invention is shown in a diagram in FIG. 2. The molybdenum-99 is extracted from the uranyl sulfate nuclear fuel of a homogeneous solution nuclear reactor. The uranyl sulfate reactor is operated at powers from 20 kW up to 100 kW for a period of from several hours to a week. During this time the fission products, including molybdenum-99, accumulate in the operating reactor solution. After the operating period, the reactor is shut down and kept at a subcritical condition to reduce the total fission product activity of the nuclear fuel solution and to cool the reactor down. The cooling down period can vary from 15 minutes to several days. The solution is then pumped from the reactor, through a heat exchanger to further reduce the temperature to below 40.degree. C., through a sorption column, and back to the reactor via a closed-loop path. Molybdenum-99 is extracted from this solution by the sorbent with at least 90% efficiency. Less than 2% of the other fission fragments are extracted by the sorbent and less than 0.01% of the uranium are absorbed by the sorbent. The sorbent radioactivity due to the absorbed Mo-99 is about 50 Curies per kW of reactor power. The sorbent material is the subject of a co-pending application. It is a solid polymer sorbent composed of a composite ether of a maleic anhydride copolymer and .alpha.-benzoin-oxime. This sorbent is capable of absorbing more than 99% of the Mo-99 from the uranyl sulfate reactor solution. The solution containing uranium sulfate and all fission products not adhering to the sorbent material is returned to the reactor vessel. Thus, waste is contained and uranium is conserved. The operation can then be repeated after any chemical adjustments to the solution to compensate for removed material or consumed uranium. FIG. 3 details the operation of the uranyl sulfate solution reactor in the preferred embodiment. The right-cylinder reactor container 1 holds about 20 liters of the uranyl sulfate solution 2 and has a free volume 3 above the solution to receive radiolytic gas formed during operation of the reactor. During operation, the reactor is critical and is operated at 20 kW. With increased cooling, the reactor could be operated up to 100 kW. Heat is removed from the uranyl sulfate solution through a cooling coil 4 containing circulating distilled water. A first pump 5 moves the cooling water through the coils to a first heat exchanger 6. The secondary side of the heat exchanger 6 uses city water. During operation of the reactor, H.sub.2 and O.sub.2 radiolytic gas is formed in the solution. This gas bubbles to the surface of the solution and rises 7 to the catalytic (platinum) recombiner 8 where the hydrogen and oxygen are burned to form pure steam. The heat of burning is removed in a second heat exchanger and the steam condensed to water. The secondary side of the second heat exchanger 9 can again use city water. The first liter of water so formed is directed to a water container 12 by opening valve-1 11. The remaining water is returned to the reactor container 1. The extraction process to isolate Mo-99 is shown in FIG. 4. After the reactor is shutdown, the radioactivity is allowed to decay for a selected period of time up to a day. Then valve-3 20, valve-4 21, and valve-7 22 are opened. All other valves remain closed. A second pump 23 is activated, drawing up the reactor fluid 2 containing uranium and fission products including Mo-99. This fluid is pumped through a third heat exchanger 24 to reduce its temperature to less than 30.degree. C. It then passes through the sorbent 25 and finally through valve-7 22 back to the bottom of the reactor container. Note that the pump 23 draws the reactor fluid 2 from the top and returns it to the bottom. This provides a "layering" effect caused by the difference in density between the warmer reactor solution 2 and the cooler, denser pumped fluid. The cooler pumped fluid has been stripped of Mo-99 and is thereby kept separated from the "unstripped" solution 2 in the reactor. The flow rate of the pumped fluid is about 4 liters per hour (.about.1 ml/second) and the entire 20 liters of reactor solution 2 takes about five hours to pass through the sorbent 25. With adjustments to the sorbent 25 size and packing and with greater pressure from the pump 23, the flow rate could vary from 1 to 10 ml/second. After all of the fluid 2 has passed through the sorbent container 25, valve-3 20 is closed and valve-2 27 is opened. This permits the liter of pure water 12 to "wash" the sorbent of reactor fluid and also maintains the concentration of the reactor fluid 2. After the wash, valve-2 27, valve-3 20, valve-4 21, and valve-7 22 are closed and valve-6 28 and valve-5 29 are opened. From a storage container, the eluting solution 30 of 10 molar nitric acid passes through the sorbent and into a transfer container 31. About 80 ml of eluting fluid is used. The reactor can be operated from one to five days at a time. Typically, the reactor is run for five days, allowed to cool for one day, and the Mo-99 extracted on the seventh day. This weekly cycle can vary depending on the demand for the product and the length of time used for the extraction process. The operation of the reactor at 20 kW power for five days results in a solution 31 containing 420 Curies of Mo-99 following a one day cooling period and a one day extraction period. The efficiency of the Mo-99 extraction by the sorbent 25 is at least 90%. Other fission fragments in the extracted solution 31 are less than 2% and the solution contains less than 0.01% uranium. The preferred sorbent is a composite ether of a maleic anhydride copolymer and .alpha.-benzoin-oxime, the subject of a pending patent application. Well-known purification processes are subsequently used to purify the concentrated Mo-99 solution 31. The method and apparatus of the present invention produces Mo-99 by a waste free, economical, and simple technology. Mo-99 is directly produced in the uranyl sulfate solution (pH.about.1) of a homogeneous solution nuclear reactor. No uranium is wasted because it is used again in the nuclear reactor as nuclear fuel after Mo-99 sorption from the solution. Radioactivity is not released beyond the reactor region due to a high selectivity of the sorbent used. Nuclear fuel reprocessing is not required for subsequent extraction cycles and the expense of manufacturing targets is not incurred. The present invention is, of course, in no way restricted to the specific disclosure of the specifications and drawings, but also encompasses any modifications within the scope of the appended claims. The reactor could be run continuously, for example, as long as the cooling system keeps the reactor solution below boiling. The burn up of uranium is insignificant and additions would only be needed after hundreds of days of operation.
	summary
	abstract	The invention relates to brachytherapy devices, methods for making brachytherapy devices, and methods for using brachytherapy devices. For example, the invention provides brachytherapy devices such as staging-sterilization devices and insert devices that can be used in various brachytherapy procedures.
	claims	1. A Marinelli beaker correction container comprising:a body container which is formed to have a diameter corresponding to the inner diameter of a recessed lower surface of the Marinelli beaker,wherein in the container body, a first groove which is attached to a detector of a detecting system for nuclide analysis is formed in a lower portion of the container body, a second groove having a smaller diameter than the first groove is formed in a upper portion of the container body, and an intake and exhaust hole is formed through the first groove to the second groove. 2. The Marinelli beaker correction container according claim 1, wherein the intake and exhaust hole formed in the container body is provided with a length of 8 mm to 12 mm. 3. The Marinelli beaker correction container according to claim 1, wherein the second groove formed in the upper portion of the container body is formed with an inner diameter corresponding to the diameter of the lower surface of the sample measuring bottle having a capacity of 80 ml used in radionuclide analysis. 4. The Marinelli beaker correction container according to claim 1, wherein, when the container body is made of a polyethylene resin.
	description	The present invention relates to a method of optimising the output of a sensor used for sensing a metallic object through another metallic object. In particular, but not exclusively, the present invention relates to a method of optimising the output of a sensor used for measuring the relative position of a control rod within a nuclear reactor from within a metallic probe tube housing the sensor. Means for measuring or detecting the position of a control rod within a nuclear reactor are limited by the fact that the measurement needs to be made within the primary water for the nuclear reactor. A conventional method for determining the relative location of a control in a nuclear reactor is to use a metallic probe tube which extends into the primary water region, and which houses a coil of wire forming an inductive element that forms part of an electrical circuit. The probe tube is positioned such that a metallic leadscrew attached to the control rod moves telescopically over the probe tube as the control rod is moved in and out of the nuclear reactor to regulate the fission reaction therein. As the leadscrew moves over the probe tube the voltage across the inductor changes because of magnetic coupling effects. This change in voltage is directly proportional to the position of the leadscrew and thus the control rod. A problem with using this method is that it is typically not very accurate. In particular, it has a low span to offset ratio and a low signal span. This is problematic because the measurement instrumentation is typically limited to relatively low signal voltages, and it is thus desirable to maximise the signal span to offset ratio so that the relative position of the leadscrew (and therefore the control rod) can be known with high accuracy. A further problem with the prior art techniques is that the flux density of the field that is generated around the inductive element is difficult to predict before manufacture. It is common practice, therefore, to manufacture a multitude of inductive elements, the one with the best magnetic field in terms of the spread of the flux ultimately being selected for use. Indeed, each element may need to be calibrated in situ, so that variations in the local operating environment can be accounted for in the calibration. This is undesirable. Some prior art methods of measurement use the transformer principle rather than the simple inductor principle. The transformer principle also involves a metallic probe tube and a metallic leadscrew, but the probe tube houses a series of transformer windings alternating between electromagnetically coupled primary and secondary windings along a core. When in operation, a magnetic field is generated between the primary and secondary windings. As the leadscrew moves over the probe tube the magnetic field between the windings is affected such that the voltage generated across the secondary windings changes proportionately to the position of the leadscrew over the probe tube. An example of a transformer effect sensor is U.S. Pat. No. 5,563,922, which shows the use of a transformer effect to sense the moving metallic item through a metallic enclosure. However, in the arrangement shown in U.S. Pat. No. 5,563,922, the output signal typically suffers from a low span to offset ratio. As mentioned above is undesirable because it reduces the sensitivity of the sensor and therefore the accuracy to which the relative position of the leadscrew (and therefore the control rod) can be known. In particular, in arrangements similar to that of U.S. Pat. No. 5,563,922, the signal span is relatively small. And, typically, a large residual magnetic field exists between the primary and secondary windings when the leadscrew is “covered” (i.e. the leadscrew is arranged to cover the probe tube). This typically results in a large voltage offset on the output signal of the sensor, which is undesirable. In particular, when an output signal is amplified the voltage offset of the signal is also amplified, which causes difficulty for subsequent signalling processing of the output signal; indeed, it can make it difficult to detect the relevant part of the signal, because it is swamped by the amplified offset level (and any associated noise on the offset level). The present invention seeks to provide a way to remove the undesirable offset, thus improving the sensor significantly with respect to the known prior art sensors by providing a sensor with an improved signal span to offset ratio, thereby providing a sensor with higher resolution. In other words, the present invention seeks to provide a sensor and/or method which provides a signal indicating the relative location of a metallic object with a higher degree of accuracy than the prior art. A first aspect provides a sensor assembly for indicating the relative location of a metallic object, the sensor assembly including: a primary electromagnetic coil arranged to generate a time varying magnetic field; and a secondary electromagnetic coil arranged to detect the time varying magnetic field as affected, directly or indirectly, by the object and to output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the object; wherein at least one of the primary and secondary electromagnetic coils is wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. Accordingly, the signal span to offset ratio of the output of the sensor has a higher resolution than prior art sensors. The primary and secondary coils may be arranged coaxially. A plurality of primary electromagnetic coils may be provided. A plurality of secondary electromagnetic coils may be provided. The plurality of primary and secondary coils may be arranged in a mutually alternating sequence of primary and secondary coils. The or each primary coil may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The or each secondary coil may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary and secondary coils may each be wound about the same core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary coils may be mutually arranged in electrical series; and/or wherein the secondary coils may be separately mutually arranged in electrical series. The primary and/or secondary coils may be formed of an alloy comprising 86% copper, 12% Manganese and 2% Nickel, e.g. Manganin® wire. The or each core body may be formed of a material having the same conductivity and/or magnetic permeability as the object. The or each core body may be formed of the same material as the object. The metallic object may be attached to a (movable) nuclear reactor control rod. A second aspect provides a method of optimising the output of a sensor as described herein, the method including the steps of: supplying the primary coil(s) with an alternating current to result in the generated time varying magnetic field; locating the object in a first position and recording the signal output by the secondary electromagnetic coil(s) for a range of respective frequencies of the supplied alternating current; locating the object in a second position and recording the signal output by the secondary electromagnetic coil(s) for the range of respective frequencies of the supplied alternating current; calculating, for each of the respective frequencies, a value for the span to offset ratio of the measured signals on the basis of the respective signals measured for the object in the first and second positions; and determining the frequency of the supplied alternating current which provides the maximum span to offset ratio on the basis of the calculations. When the object is in the first location, the output from the secondary coil(s) may be a maximum. When the object is in the second location, the output from the secondary coil(s) may be a minimum. The calculation step may include, for each respective frequency: calculating the difference between the amplitudes of the signals measured for the object in the first and second positions; and dividing the difference by the amplitude of the signal measured for the object in the second position. The sensor assembly may include a metallic body, within which the primary and secondary coils and core body/bodies are located, and outside of which the metallic object is located. Thus the sensor assembly is configured to be capable of indicating the relative location of the metallic object even though the coils are separated from the metallic object by the metallic body (within which the coils are located). A third aspect provides sensor assembly for indicating the location of a leadscrew relative to a probe tube, the leadscrew forming part of a nuclear control rod and the probe tube being moveably connected to the leadscrew, the sensor assembly including: a primary electromagnetic coil arranged to generate a time varying magnetic field; and a secondary electromagnetic coil arranged to detect the time varying magnetic field as affected, directly or indirectly, by the leadscrew moving relative to the probe tube and to output, on the basis of the detected time varying magnetic field, a signal indicative of the location of the leadscrew relative to the probe tube; wherein the primary electromagnetic coil and/or the secondary electromagnetic coil comprises copper and nickel. The primary electromagnetic coil and the secondary electromagnetic coil may be formed from a copper-manganese-nickel alloy. The copper-manganese-nickel alloy may comprise by weight equal to or between 77 and 89% Copper, 10 and 18% Manganese, 1 and 5% Nickel. The copper-manganese-nickel alloy may comprise by weight 86% Copper, 12% Manganese and 2% Nickel. Both the primary electromagnetic coil and the secondary electromagnetic coil may comprise copper and nickel. The sensor assembly may include a temperature indicator to indicate the temperature of the sensor assembly. The sensor assembly may comprise a processor configured to receive the voltage from the primary electromagnetic coil, the voltage from the secondary electromagnetic coil and an output from the temperature indicator and output a calibrated output that compensates for the temperature of the sensor assembly. The sensor assembly may comprise a tertiary coil. The tertiary coil may comprise at least 95% by weight copper, for example at least 98% by weight copper, or at least 99% by weight copper. The tertiary coil may be positioned to surround the primary electromagnetic coil. The sensor assembly of the third aspect may have one or more of the optional features of the sensor assembly of the first aspect. A fourth aspect provides a method of indicating the relative location of a leadscrew relative to a probe tube, the leadscrew forming part of a nuclear control rod and the probe tube being moveably connected to the leadscrew, the sensor being of the type according to the first or the third aspect, the method including the steps of: supplying the primary electromagnetic coil with an alternating current to result in the generated time varying magnetic field; recording a voltage from the primary coil; recording the signal output by the secondary electromagnetic coil; recording a temperature indicator indicative of the temperature of the sensor; modifying the voltage from the secondary electromagnetic coil based upon the temperature indicator to produce a calibrated secondary voltage; and calculating a position of the leadscrew based on the calibrated secondary voltage and the voltage recorded from the primary coil. A fifth aspect provides a method of optimising the output of a sensor for indicating the relative location of a metallic object, the sensor being of the type having a primary electromagnetic coil arranged to generate a time varying magnetic field; and a secondary electromagnetic coil arranged to detect the time varying magnetic field as affected, directly or indirectly, by the object and to output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the object, the method including the steps of: supplying the primary coil with an alternating current to result in the generated time varying magnetic field; locating the object in a first position and recording the signal output by the secondary electromagnetic coil for a range of respective frequencies of the supplied alternating current locating the object in a second position and recording the signal output by the secondary electromagnetic coil for the range of respective frequencies of the supplied alternating current; calculating, for each of the respective frequencies, a value for the span to offset ratio of the measured signals on the basis of the respective signals measured for the object in the first and second positions; and determining the frequency of the supplied alternating current which provides the maximum span to offset ratio on the basis of the calculations. The sensor may be a sensor assembly according to the first or the third aspects. When the object is in the first location, the output from the secondary coil may be a maximum; and/or when the object is in the second location, the output from the secondary coil may be a minimum. The calculation step may include, for each respective frequency: calculating the difference between the amplitudes of the signals measured for the object in the first and second positions; and dividing the difference by the amplitude of the signal measured for the object in the second position. The sensor may be positioned within a metallic tube and the metallic object may be arranged to move relative to the tube between a position of minimum overlap and a position of maximum overlap of the tube and the object. The first position may be a position where there is minimum overlap between the tube and the object. The second position may be a position where there is maximum overlap between the tube and the object. At least one of the primary and secondary electromagnetic coils may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary and secondary coils may be arranged coaxially. The sensor may comprise a plurality of primary electromagnetic coils. The sensor may comprise plurality of secondary electromagnetic coils. The plurality of primary and secondary coils may be arranged in a mutually alternating sequence of primary and secondary coils. The or each primary coil may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The or each secondary coil may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary and secondary coils may be each wound about the same core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary coils may be mutually arranged in electrical series; and/or wherein the secondary coils may be separately mutually arranged in electrical series. The or each core body may be formed of a material having the same conductivity and/or magnetic permeability as the object. The or each core body may be formed of the same material as the object. The metallic object may be attached to a nuclear reactor control rod. FIG. 1 shows schematic cross sections of a sensor 10. The schematic only shows half of the full arrangement; the full arrangement being mirrored about the dashed line A-A to shown in FIG. 1. The sensor 10 comprises a plurality of primary coils 12, coaxially arranged with a plurality of secondary coils 14. One or more primary coils 12 may be provided. One or more secondary coils 14 may be provided. Where a plurality of primary coils 12 are provided, the respective primary coils may be connected in electrical series. Where a plurality of secondary coils 12 are provided, the respective secondary coils may be connected in electrical series. The primary and secondary coils are arranged in a mutually alternating (physical) series or sequence, such that the sequence of coils along the long axis of the series alternates between individual primary and secondary coils. The primary and secondary coils are not in electrical connection. In other words, between each pair of adjacent primary coils 12 a secondary coil 14 may be provided; and/or between each pair of adjacent secondary coils 14 a primary coil 12 may be provided. In the embodiment shown, each coil 12, 14 is wound around a single core body 16. However, the coils may be each be wound around a respective core body 16. Or plural sets of two or more of the coils may be wound around respective core bodies. The coils 12, 14 may be wound around a supporting body, which is itself mounted on to the core body 16. However, the coils 12, 14 may be wound directly on to the core body 16. In either case the coils 12, 14 may be referred to as bobbins. In one particular use, the sensor 10 is mounted inside a probe tube 18 which extends or projects into a region containing the primary water surrounding a nuclear reactor. In this example, for safety reasons the probe tube must be metallic. Within the aforementioned region the nuclear reactor control rods (not shown) are movable, to be inserted into or withdrawn from the nuclear reactor itself. Typically, each control rod is attached to a leadscrew 20, such that movement of the nuclear rod causes movement of its respective leadscrew. It is the accurate detection of the movement, or more accurately the relocation, of the leadscrew that the present disclosure aims to provide. As the control rod is moved, the leadscrew 20 moves along the probe tube 18. At one extreme, the leadscrew may not cover any part of the probe tube, as shown in FIG. 1A. This may occur for example when the control rod is fully inserted into the nuclear reactor core. At another extreme, the leadscrew may fully cover the probe tube, as shown in FIG. 1B, for example when the control rod is fully withdrawn from the nuclear reactor core. Therefore, to assist in the understanding of the present example, FIG. 1A shows the leadscrew in the “uncovered” position, whereby the leadscrew 20 is withdrawn from the probe tube 18; whereas FIG. 1B shows the leadscrew in the “covered” position, whereby the leadscrew 20 is arranged proximate to the sensor, e.g. to cover the probe tube 18. In FIG. 1A the nuclear reactor control rod (not shown) to which the leadscrew 20 is attached may be at a maximum insertion in the nuclear reactor core for example. Whereas, in FIG. 1B the nuclear reactor control rod (not shown) to which the leadscrew 20 is attached may be at a maximum extent of withdrawal from the nuclear reactor core, for example. In order to control the reaction within the nuclear reactor core in a reliable and safe manner it is important to know the relative location of the leadscrew 20, and therefore of the control rod, with a high degree of accuracy. During operation of a sensor, the primary coils 12 of the sensor 10 are supplied with an alternating (AC) current so as to result in a time varying magnetic field being produced by the primary coils 12. The time varying magnetic field interacts with the local environment, including the probe tube 18, the core body 16 and the leadscrew 20. The time varying magnetic field, as affected by the local environment, induces in the secondary coils 14 a corresponding AC current, and the secondary coils therefore output a corresponding signal indicative of the time varying magnetic field which induced the AC current in the secondary coils. Changes in the local environment, such as relocation, or repositioning, of the leadscrew 20 will alter the time varying magnetic field, and therefore will consequently alter the current induced in the secondary coils 14. Thus the corresponding output signal will be changed. This change in the signal output of the secondary coils is detectable, and can be used to establish the relative location of the leadscrew 20, and thus of the control rods. As discussed above, similar prior art sensor arrangements (e.g. U.S. Pat. No. 5,563,922) suffer from disadvantages that mean the accuracy of the determination of the relative location of the leadscrew can be improved significantly. The present inventor has realised that an important factor when considering how to improve the accuracy of said determination is the (signal) span to offset ratio. The signal span is the measurable signal span from the minimum signal to the maximum signal, and the offset is the minimum achievable signal. It is often difficult, if not impossible, to achieve a zero offset in a measured signal. Noise and residual signal inducing effects (e.g. residual magnetic fields in the context of the present discussion) mean that a non-zero signal offset is almost inevitable in any measurement system. Systems such as that shown in U.S. Pat. No. 5,563,922 often suffer from relatively small signal spans and undesirably large signal offsets, meaning that the overall accuracy of the measurement system can suffer. The present inventor currently considers that the (static) local environment around the sensor 10 is responsible for disadvantages discussed above. For example, as shown in FIG. 2, the time varying magnetic field 22 generated by the primary coils 12 results in a secondary electromagnetic field 24 in the metallic probe tube 18 (due to the produced eddy current 26) which can adversely affect the signal output by the secondary coils 14 by reducing the signal span to offset ratio for example. Other aspects of the local environment can also affect the signal span and signal offset. For example, the core body 16 about which the respective coils are wound. The present inventor has realised that one way to significantly improve the (signal) span to offset ratio to achieve excellent accuracy in determining the relative location of the leadscrew 20, is to ensure that the core body 16 is formed of a material having the same permeability and/or conductivity as the material from which the leadscrew 20 is formed. Indeed, in particularly preferred embodiments, the core body 16 is formed of the same material as the leadscrew 20. In such embodiments, ideally, the core body would be formed of the same production batch of material as the leadscrew 20, although this is not strictly necessary for the sensor arrangement to be worked. FIG. 3 shows a plot demonstrating the advantageous effect on the SoR (signal span to offset ratio) of the output signal of the secondary coils 12 when the material (or the permeability and/or conductivity characteristics) of the core body 16 is matched to the material from which the leadscrew is formed. Line 28 indicates the output signal VS against leadscrew position P for the sensor of the present embodiment. Line 30 is provided for comparison purposes and indicates an output signal against leadscrew position for a sensor where the material of the core 16 is different to the material of the leadscrew 20 (including having a different conductivity and a different magnetic permeability). To produce FIG. 3, an arbitrary frequency of 400 Hz for the AC current supply to the primary coils 12 was chosen. To calculate the SoR at the arbitrary frequency of 400 Hz, the output signal from the secondary coils 14 was measured for the uncovered leadscrew arrangement (i.e. where the leadscrew is distal to the sensor as in FIG. 1A) and separately for the covered leadscrew (i.e. the leadscrew at least partially ensheathing the sensor 10 and probe tube 16 as shown in FIG. 1B). Typically this provides values representative of the maximum output signal and the minimum output signal respectively. The difference between the measured values was then calculated to obtain the signal span. The result was then divided by the measured signal corresponding to the covered leadscrew (i.e. at least partially ensheathing the sensor) which typically corresponds with the offset of the measured signal. The result of the division operation gives the span to offset ratio (SoR) for the output signal at the chosen 400 Hz. For a typical prior art arrangement without core matching (without matching the material characteristics of the core body 16 to that of the leadscrew 20), the SoR at 400 HZ was determined to be around 0.8 only. However, for a sensor arrangement according to the present embodiment, which adopts the principle of matching the permeability and/or conductivity characteristics of the core body material to that of the leadscrew material (for example, by matching the material of the core body 16 to that of the leadscrew 20), the SoR at 400 HZ was determined to be 2.26. Thus, the described sensor arrangement can provide a very significant improvement in the SoR of the output signal from the secondary coils 14. This is particularly advantageous where the output signal of the secondary coils may be fed to a measurement system via a data acquisition card having a maximum input voltage. For example, such data acquisition cards may have a maximum input voltage of 5V. Therefore, improving the SoR within the available 5V range means that the resolution of the acquired signal is improved, and thus the subsequent processing can produce a more accurate result for the determination of the relative location of the leadscrew 20. To demonstrate that matching the conductivity and/or magnetic permeability of the core body material to that of the leadscrew 20 is particularly advantageous in achieving an optimum SoR for the output signal of the secondary coils 14, the present inventor has conducted extensive finite element analysis, a resulting plot of the SoR for various metals against the frequency of the AC current supply to the primary coils 12 is shown in FIG. 4. In FIG. 4, line 30 is the plot of HAS4104; line 32 is the plot for grey cast iron; line 34 is the plot for ingot iron; line 36 is the plot for powdered iron; line 38 is the plot for supermendur (a cobalt-iron alloy); line 40 is the plot for pure iron; line 42 is the plot for Sinimax (a nickel-iron alloy); line 44 is the plot for Mumetal® (a nickel-iron alloy); line 46 is the plot for Inconel 625 (a nickel-chromium alloy); and line 48 is the plot for stainless steel. The finite element analysis has shown that the particular characteristics of the material of the core body 16 which contribute to the significant improvement of SoR are the conductivity of the core body material and the magnetic permeability of the core body material. In particular, the finite element analysis has shown that the improvement in the SoR of the output of the sensor 10 to be most significant when the magnetic permeability and/or the conductivity values of the core body material is/are matched closely to the magnetic permeability and/or conductivity values of the material from which the object to be detected is formed—here, the object to be detected typically being a leadscrew 20 formed of a particular metal. To demonstrate this effect, FIG. 4 shows the primary coils 12 AC current frequency dependency of the SoR of the output signal of the secondary coils 14 for various different materials of core body 16 for a leadscrew formed of a material referred to as HAS 4104, or DGS MS HAS 4104, which is a stainless having a high magnetic permeability. This material was chosen for the leadscrew material in this study because it is the typical material from which the leadscrews in nuclear reactors are formed. As can be seen from FIG. 4, the highest SoR is achieved for a core material of HAS4104, i.e. a material matching the material of the leadscrew which is also formed of HAS4104. So, where leadscrews are typically formed of HAS4104, embodiments for use in nuclear reactors employing such leadscrews may also have a core body 16 formed of HAS 4104. The SoR is also dependent on frequency. Not only will the electrical circuitry typically demonstrate a resonance peak, but the materials in the local environment will demonstrate different responses depending on the frequency of the time varying magnetic field generated by the primary coils. For example, a peak at around 7.5 KHz is observed in FIG. 4 when the core body 16 is formed of HAS4104 material. The SoR at this frequency is calculated to be around 11. This result for the SoR is calculated as follows, taking the suitable voltage values from FIG. 5 (in FIG. 5, line 50 is the line plotted for a core body of HAS4104, and line 52 is for a core body of stainless steel 316): Core body of HAS4104: (2.35V−0.19V)/0.19V=˜11 As shown in FIG. 5, if an alternative material is used for the core body 16, which does not have conductivity and/or permeability characteristics which match with the HAS4104 of the leadscrew, the SoR is shown to be only around 2.8. This result for the SoR is calculated as follows, taking the suitable voltage values from FIG. 5: Core body of stainless steel 316: (0.27V−0.07)/0.07=˜2.8 Therefore, the described sensor arrangement surprisingly offers an improvement in the SoR of almost four times. Interestingly, this is achieved with an alternative material which is not a wildly different material to HAS4104, but which is another stainless steel: stainless steel 316. The present inventor has therefore demonstrated that a careful selection of the material for the core body 16 can have a surprisingly large advantageous effect on the SoR of the output signal of the secondary coils 14. As can be seen from FIG. 4, the optimum SoR is provided at a particular frequency, and so the present disclosure also proposes a method for determining the frequency at which the optimum SoR exists for a particular system. The object to be detected, for example the leadscrew 20, is arranged distally from the sensor 10; for example at its furthest distance from the sensor 10. In the case of the leadscrew, the control rod may be fully inserted into the nuclear reactor, for example. With the leadscrew 20 in this position, the primary coils are provided with AC current at a range of (two or more) discrete frequencies f, and the output signal VS from the secondary coils measured and recorded for each respective frequency. The result of such an exercise is shown in FIG. 6 for example. The object to be detected, for example the leadscrew 20, is also arranged at proximally to the sensor 10; for example at its nearest position to the sensor 10. In the case of the leadscrew 20, the control rod may be at its maximum withdrawal from the nuclear reactor for example. With the leadscrew in this position, the primary coils 12 are provided with AC current at the same range of the same (two or more) discrete frequencies f, and the output signal VS from the secondary coils 14 measured and recorded for each respective frequency. The result of this exercise is shown in FIG. 7 for example, where the effect of the object (the leadscrew) on the signal output by the secondary coils 14 can clearly be seen by comparison of FIG. 7 with FIG. 6. Then the SoR at each frequency is determined in accordance with the calculation discussed above in relation to FIG. 5, to determine the frequency f at which the SoR is a maximum. In other words, for each frequency, the minimum measured output signal is subtracted from the maximum measured signal to generate a difference value, and the difference value is divided by the minimum value to generate the SoR value. For the range of frequencies f measured, FIG. 8 shows a plot of the SoR. As can be seen, for the particular arrangement used in the demonstration, the SoR reaches a maximum value of around SoR=11.4 at around 6.75 kHz. Therefore, for the particular sensor and the local environment in which the sensor was located in this demonstration, the AC current should ideally be supplied to the primary coils 12 at around 6.75 KHz in order to maximize the SoR of the output signal of the secondary coils. Accordingly, the present embodiment provides a position sensor which provides an output signal indicative of the relative position of an object to be detected with a higher resolution than equivalent sensor arrangements in the prior art. This is achieved by winding the primary coil(s) and secondary coil(s) around one or more core bodies formed of a material having similar characteristics to the material of the object to be detected. In particular, it is preferred that the material of the one or more core bodies has a conductivity and/or magnetic permeability which matches the material of the object to be detected. In most preferred embodiments, the material of the one or more bodies is the same as the material of the object to be detected. In this way, a sensor arrangement according to the present embodiment provides a higher SoR and span output signal when detecting metallic objects through another metallic body. This provides major advantages in high accuracy and resolution measurement systems. The ability to provide the downstream instrumentation detection electronics with good resolution sensor signals enables errors to be reduced significantly, thereby allowing the overall system to be more accurate and to offer better resolution. In particular, a sensor according to the present embodiment, especially when used in conjunction with the SoR optimisation technique disclosed herein, offers a greatly improved means to measure linear displacement of a metallic device through another metallic device. In the sense that a sensor 10 according to the present embodiment generates a signal for interaction with the local environment and measures the effect on the signal in order to output a signal indicative of a change in the local environment, the sensor 10 may be considered to be a transducer, and may be referred to as such. As mentioned above, a sensor according to the present embodiment is particularly suited to use in a nuclear reactor, where the temperature of the local environment may fluctuate to a large extent. A large fluctuation in temperature will likely change the resistive properties of the primary and/or secondary coils, and therefore will likely change the SoR of the output signal of the secondary coils. Referring to FIG. 9, the output VS from the secondary coil 14 at varying positions of the leadscrew 20 is illustrated for a sensor 10 where the primary coil 12 and the secondary coil 14 are made from copper. The line 60 indicates the variation of the output VS from the secondary coil for varying positions P of the leadscrew at 200° C. and line 62 indicates the variation of the output from the secondary coil for varying positions of the leadscrew at 20° C. The temperatures given are measured at a position on the sensor having a maximum temperature, in this case this is in a region at the bottom of the probe tube 18. It can be seen from FIG. 9, that a change in maximum temperature from 200° C. to 20° C. significantly affects the output from the secondary coil. This means that the output from the secondary coil is undesirably dependent upon the temperature of the system. Furthermore, at lower temperatures the change in output for a given position change of the leadscrew decreases, which in turn impacts the sensitivity of the sensor. Referring to FIG. 10 a sensing arrangement that attempts to address this temperature dependence problem is illustrated. The arrangement of FIG. 10 is similar to the arrangement previously described, and similar features are given a similar reference numeral but with a pre-fix “1” to distinguish between embodiments. Only the differences between the embodiments will be described in detail. As illustrated in FIG. 10, similar to the previously described embodiment, the arrangement includes a sensor 110 positioned in a probe tube 118, and the probe tube 118 is moveable relative to a leadscrew 120. Referring now to FIG. 11, the sensor 110 is shown in more detail. Similar to the previously described sensor, the sensor 110 includes a series of primary coils 112 and secondary coils 114. In the presently described embodiment, the primary coils and the secondary coils are made from a copper-manganese-nickel alloy. In particular, the primary and secondary coils are made from Manganin®. Manganin® is a Copper-manganese-nickel alloy, and is generally provided in the ratio of 86:12:2 by weight. FIG. 12 illustrates the output VS from the secondary coil 114 at varying positions P of the leadscrew. Line 64 illustrates the output from the secondary coil at 200° C. and line 66 illustrates the output from the secondary coil at 20° C. As can be seen from FIG. 12, the use of manganin wire significantly reduces the temperature dependence of the output from the secondary coil both in terms of magnitude for a given position of the leadscrew as well as in terms of the change in magnitude for a step change in position of the leadscrew. However, it can be seen from FIG. 12 that there is still some variation in the output VS from the secondary coil 114. It is believed that this variation is due to probe tube and leadscrew thermal effects, primarily probe tube thermal effects. Referring again to FIG. 11, the sensor 110 is further optimised to include a tertiary coil 115. The tertiary coil is arranged substantially coaxially with the primary coil 112 and is positioned radially outside of the primary coil. However, in alternative embodiments the tertiary coil could surround the secondary coil or surround both the primary and secondary coils, or be positioned at any other suitable position on the core 116. The tertiary coil is made from copper or an alloy thereof. Referring now to FIG. 13, the sensor is connected to a processor 168. The voltage Vp from the primary coil, the Voltage VS from the secondary coil and the Voltage VT from the tertiary coil are transmitted to the processor. The processor receives the voltage from the primary coil, secondary coil and tertiary coil and performs a compensation procedure. The algorithm for the compensation procedure can be established using techniques known in the art and will vary depending on the specific environment in which the sensor is used. Once the processor has performed the compensation procedure, the processor outputs a calibrated secondary coil output VSC. The compensation procedure can remove the variation in output from the secondary coil illustrated in FIG. 12, so that the calibrated secondary coil output VSC is independent of the temperature of the sensor. In alternative embodiments, the primary and secondary coils may be made from an alloy such as constantan (a copper-nickel alloy). However, the inventor has found Manganin® to provide an optimum SoR. In the present embodiment, the tertiary coil is provided with an AC current, but in alternative embodiments the tertiary coil may be provided with a DC current. In further alternative embodiments, the tertiary coil may be replaced with another type of temperature indicator. It will be appreciated by one skilled in the art that, where technical features have been described in association with one or more embodiments, this does not preclude the combination or replacement with features from other embodiments where this is appropriate. Furthermore, equivalent modifications and variations will be apparent to those skilled in the art from this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting.
	description	Exemplary surface designs are illustrated in FIGS. 1 through 13 for constructing a variety of optical systems suitable for converting x-rays into parallel or converging radiation. As used herein the term parallel means substantially parallel, including degrees of parallelism satisfactory for performing the functions of systems described herein. According to the invention, polycrystalline material is formed to define a curved surface, a portion of which is positioned to reflect x-rays at or near the Bragg angle. To achieve necessary conditions for Bragg reflection many of the individual grains in the polycrystalline material exhibit a common crystal orientation. Conventionally, a fiber texture orientation in such a polycrystalline material is understood to mean that the crystallographic direction [uvw] in most of the grains is parallel or nearly parallel to the wire axis. Fiber orientation is a measure of the degree that all of the crystalline units are oriented with a certain crystal plane normal to a reference direction. This is referred to herein as normal plane textural fiber orientation, which is to be distinguished from curvature plane texture orientation, as defined below. It is now recognized that the preferred orientation of some polycrystalline films in fiber textures, with the primary x-ray reflector normal to the surface, creates the ability to make a polycrystalline lens system which both collimates or focuses an x-ray beam to a spot below the lens itself. Deposition of certain polycrystalline films in fiber textures with their primary x-ray reflector plane normal to a reference surface provides an ability to realize Bragg reflection along a curved surface. Information from the ICDD (International Centre for Diffraction Data) database indicates that Aluminum (Al) crystallizes in a face centered cubic structure in the Fm3m(225) space group. The cell is=4.0494 with a z of 4. The primary low order reflections are the (111), (200), (220) and (311). Additional crystallographic data is available from the PDF (powder diffraction file) card. Aluminum, when exposed to copper K-alpha radiation, has specific reflections according to the Bragg condition for reflection: xcex=2 d sin xcex8, where xcex=reflection wavelength d=interatomic plane spacing xcex8=glancing incidence angle This condition results in the following reflections and their associated relative intensities l(f): As can be seen in the table, Aluminum""s strongest reflection is in the less than 111 greater than direction. This orientation has then a 2-theta Angle of approximately 38.472 degrees. Aluminum is used here as an example, while this effect can also be seen in other materials which exhibit similar orientation properties normal to the sample surface. An inverse pole figure map was constructed for Aluminum deposited onto a titanium nitride surface by chemical vapor deposition. The map allowed color shading corresponding to the automatic tiling of the unit triangle of the inverse pole figure. For this Orientation Imaging Microscope scan of aluminum, the color red was assigned to the [001] crystal direction, the color blue was assigned to blue to [111] and the color green was assigned to [101]. A particular point was then shaded in the OIM scan according to the alignment of these three directions in the crystal to the [001] direction (normal to the surface of the wafer). For the Aluminum sample the entire inverse pole figure was a shade of blue, indicating a texture whereby the [111] crystal direction is aligned with the normal direction of the surface. The fiber texture of aluminum was shown to be almost entirely on axis. An intensity pole figure plot of the aluminum sample for the 100, 110 and 111 directions confirmed a strong fiber texture in the [111] crystal direction of approximately 2500 times random at the center of the strongest rotational reflection on the pole plot. With this application of polycrystalline materials on curved surfaces, the invention is understood in the context of curved plane texture orientation which is now defined to mean that the polycrystalline film is such that the individual members in the film have a plane that is oriented at a certain angle with respect to an adjacent portion of the curved substrate surface. Therefore the texture orientation is with respect to the adjacent surface and not necessarily the same as that of other members which comprise the polycrystalline film. Further, curved plane fiber texture orientation is understood to mean that the crystallographic direction [uvw] in most of the grains is parallel or nearly parallel to the wire axis. Given that aluminum deposits along its strongest x-ray reflector plane in a position normal to the substrate surface, a three dimensional lens structure may be designed to provide a focal point below the lens (as needed for projection lithography) by solving the Bragg equation for multiple paths of reflections along the three dimensional lens surface. Once this three dimensional solution is found in space, glass (a good thermal conductor with good expansion properties) can be machined to the exact angular specifications of the lens structures and then the aluminum surface deposited on top of the glass will act as the Bragg reflector for the incident x-rays. The benefit of glass as the substrate is that, as an amorphous material, all x-rays of sufficient energy to migrate through the aluminum layer will become scattered internally to the amorphous glass atomic structure. Furthermore due to the initial conditions of a divergent x-ray source (such as by using an x-ray tube as the source) that is not delimited, e.g., by a slit, a much greater portion of the overall x-ray intensity can be used with a design that incorporates one or multiple sealed tubes or rotating anode x-ray sources. According to the invention the design of the lens structure is a three dimensional solution to the Bragg equation for the polycrystalline reflector overlaying the glass. This could form a singular lens system or a dual lens system. An optical system 10 for imaging with x-rays emitted from a divergent source 12 upon an ideal focal point 14 is shown in FIG. 1. The system includes a lens surface 18 which may be formed of one continuous reflective surface, or of multiple surface elements, positioned to reflect radiation impinging various regions along the surface 18 at the Bragg angle. FIG. 2 illustrates, as one example of the lens component, a full barrel-shaped surface 20, in contrast to a spaced-apart two-component surface which would exhibit inherently less throughput. With the source 12 and focal point 14 symmetrically positioned about the surface 20, a Bragg region 22 of width W along the surface 20 provides reflection of incident x-rays 24 to the focal point 14. In addition, rays 26 incident upon portions of the surface 20 near but outside the Bragg region will result in reflection of radiation within a useful focal region 28 about the focal point 14. A spaced-apart two component lens surface 32 is illustrated in FIG. 3. As described for the full barren surface 20 of FIG. 2, the two component surface 32 includes a Bragg region 34 of width W from which x-rays emanating from the source 12 are reflected to the focal point 14. The surface 32 also includes surface portions near but outside the Bragg region which reflect x-rays to a focal region 28 near the focal point 14. See, for example, F. In each of the schematic illustrations of FIGS. 1, 2 and 3, the source 12 and focal point 14 are ideally along an axis symmetric with the curvature of the lens surface. FIG. 1 thus provides a cross sectional view along a symmetric plane, illustrating for either the full barren surface 20 or the two component lens surface 32, reflection of x-rays from the Bragg region to the focal point 14. With reference to FIGS. 4 and 5, a single reflecting surface 40, comprising a series of axially symmetric partial circles, provides a suitable means for focusing the radiation about a point along a surface plane 44 of a work piece 46 such as a substrate. For example, a semiconductor wafer may be positioned along the axis defined by the source 1 and focal point 14 so that a selected portion of the surface is irradiated by x-rays reflected from the surface 40. This arrangement is beneficial for a variety of analyses , e.g., x-ray photo electron spectroscopy (XPS), and elemental spectroscopy for chemical analysis (ESCA), as well as treatments such as butt welding, cutting, and various forms of surface treatment (e.g., alloying, cladding, scribing hardening, glazing, cutting, etc.) The work piece 46 may be manipulated about the focal region 28 to effect sweeping of the x-rays along a pattern, this facilitating the various operations. Cylindrical reflective surfaces may employ the described concepts to converge x-rays about a focal point or along a focal line. The dual lens system 50 of FIG. 6 comprises a pair of cylindrical reflector surfaces 52 as described by Cosslett, et al. xe2x80x9cXray Microscopy, published by the Syndics of the Cambridge University Press, (1960) at page 5, which receive x-rays from a divergent source 12 and collimates the radiation about a focal point 28, i.e., at the focal point 14 or in a limited region 28 about the focal point 28 as afore-discussed with respect to FIGS. 1, 2 and 3. This lens combination facilitates reduction of optical aberrations, e.g., astigmatism. More generally, use of a single cylindrical lens surface 52, as shown in FIG. 7, enables convergence of the x-rays from the divergent source 12 along a focal line 54. Such a line 54 may be used in a scanning application for functions such as contact printing (e.g., photolithography), radiography and numerous forms of biological analyses. See again Coslett et al., at page 3. In other embodiments and applications of the invention it is desirable to generate a parallel beam of x-rays, e.g., to improve resolution of images. FIG. 8 illustrates the surface 40 of FIG. 4 applied to generate parallel x-rays 60 from a source 12 positioned along an arc having one half the radius of curvature as the Rowland circle. That is, the Rowland circle, having a center at 62, is one half the radius of curvature of the surface 40 and has a point which is tangent about the Bragg region. Thus, the source is placed along a circle 64, having a center at 66. The circle 64 includes a point 68 tangent about the corresponding Bragg region of the surface 40, and has a radius of curvature one fourth that of the surface 40. The reflected x-rays 60 are substantially parallel to one another, and may be expected to deviate from perfect parallelism based on, for example, possible misalignments such as orientation and height of crystal grains along the surface 40. However, with substantially parallel x-rays the resulting beam may be scanned to perform functions such as lithography. With reference to FIGS. 9 and 10, another geometric surface 70, suitable for generating parallel x-rays, corresponds to symmetric rotation of an arc of constant radius of curvature about a vertex point 72. The resulting axis 76 of symmetry passes through the vertex point 72 and the point 12 from which diverging radiation may emanate. X-rays from the point 12 undergo Bragg reflection about various portions of the surface 70 to create a parallel beam 80. Such generation of parallel rays is illustrated in the three dimensional view of FIG. 9, while the two dimensional view of FIG. 10 illustrates, for clarity, the same arrangement along a symmetric plane of the lens surface 70. It is noted that a similar effect can be achieved with multiple lens segments which, when assembled together, may comprise a sufficient portion of the geometric surface 70 as to provide satisfactory throughput. The geometric surface 70 of FIGS. 9 and 10 provides more throughput of reflected x-rays than does the surface 40 of FIG. 8. Moreover, the surface 40 is useful for constructing a telescope. That is, parallel x-ray radiation, e.g., from a distant source, may impinge upon the surface 70, undergo Bragg diffraction and converge upon or about the point 12. Theoretically such convergence can produce an image along a focal plane passing through the point 12. The quality of a diffraction limited image will depend, in part, on the orientation and height of adjoining crystal grains along the surface 70. Generally, x-ray lenses constructed with polycrystalline surfaces suitable for Bragg reflection may be constructed according to the Johannson symmetrical arrangement or the Guinier assymetrical arrangement. See Peiser, et al. published by the London Institute of Physics (1955) at page 130. Such geometries enlarge the effective area of agreement between the Rowland circle and the mirror surface. Thus, throughput at and about the focal point may be substantially increased. See, for example the reflective lens surface 100 of FIG. 11 wherein a portion 102 of the reflective surface corresponds to the Johann geometry and an adjoining portion 104 includes a radius of curvature coincident with the Rowland circle 106. This arrangement provides an increased surface area over which reflected x rays will traverse the same path length between the source 112 and focal point 114. An additional requirement for maximizing the throughput of this geometry is that of maintaining reflection at the Bragg angle over the entire surface portion. That is, throughput of the lens is dependent upon establishing an orientation of the individual polycrystalline surfaces which is normal to the original Johann curvature. While the foregoing geometries are generally difficult or impossible to achieve with a monocrystalline structure, all of the designs illustrated or contemplated can be constructed with a polycrytalline Bragg reflecting surface as aforedescribed. This includes but is not limited to the many complex shapes that are known to have desirable imaging properties but which heretofore have not been manufacturable or which have been fabricated with limited throughput. See, for example, Coslett et al. at pages 113, 114. All of the foregoing may be fabricated according to the invention by replacing conventional monocrystalline structures with polycrystalline materials formed along substrate surfaces of desired shapes. Another feature of the polycrystalline systems is that they may be scaled to a broad range of dimensions without the limitations associated with conventional crystals. Generally, with reference to FIG. 12, such a polycrystalline lens 120 is fabricated by initially forming a substrate surface 122, e.g., glass, to provide a surface 124 having curvature consistent with the Johann geometry or other complex shapes associated with differing lens designs. A polycrystalline metal film stack 124 is formed along the surface 124. As noted herein, an exemplary material suitable for Bragg reflection is Al. Accordingly, an initial layer 128 of Ti (e.g., 37.5 nm +/xe2x88x923.5 nm) is deposited, followed by a deposition of TiN layer 130 (60 nm +/xe2x88x925 nm). The TiN layer 130 facilitates formation of fiber texture in the Al layer, which is deposited to a desired thickness (e.g., 450 nm or more). Alternately, amorphous metal, e.g., Al, may be formed on the layer 130 and annealed to achieve desired fiber texture. The deposition conditions are conventional. For example, the Ti may be deposited at 150 C, the TiN may be deposited at 250 C and the Al may be deposited at 300 C. To effect a Johannson geometry, such as described for the lens surface 100 of FIG. 11, a layer 134 the polycrystalline material is first deposited to a desired thickness and a portion of the exposed metal surface 136 is then modified to provide desired curvature. This can be accomplished with conventional lens grinding techniques under thermally controlled conditions to minimize heating. To assure minimal heat generation the grinding may be performed at low rpm and may incorporate cooling techniques. The result will be removal of surface material without allowing substantial crystalline changes to occur, e.g., without alteration of grain structures or changes in grain orientations relative to the Johann surface. It is also noteworthy that the desired thickness of the lens design may be so great that a single deposition of the metal may not retain consistent orientation. That is, as the metal deposits, the fiber texture may transition to a more random orientation. To avoid this potential effect the film may be a stack created by repeated sequential depositions with an intervening amorphous material interposed between. For example, after the initial layers of Ti, TiN and Al are deposited, a minimal layer 140 of silicon dioxide is deposited thereover, followed by repeated deposition of the stack comprising layers of Ti, TiN and Al. Deposition of a silicon dioxide layer 140 is repeated between subsequent metal stacks. An exemplary structure is shown in FIG. 13 wherein like numerals reference layers of like materials as set forth in FIG. 12. Other amporphous materials may be used as materials intervening between the metal stacks. With a wide variety of lens designs now available for Bragg diffraction about polycrystalline surfaces (including those described in FIGS. 1 through 11), a variety of x-ray systems may be assembled to provide useful functions. These systems applications span multiple fields of interest. Examples include mass storage, medical and non-medical use of parallel x-rays for shadow imaging of surfaces such as bones and density variations in solid media, radiation therapy, butt welding such as applicable to sheet metal fabrication, numerous analyses in the sciences of materials, molecular biology, crystallography and astronomy, lithography, x-ray lasers and laser targets, microscopy, formation of thin films, surface treatments such as formation of hardened materials or formation of thin oxide layers to inhibit corrosion of underlying material, or treatments that alter surface properties to improve mechanical properties. Other applications include application of heat treatments, alloying, surface cladding, machining, texturing, non-contact bending and plating. From the following examples methods of applying the principles set forth to these and other systems applications will be apparent. Generally, the design of each lens structure is a three dimensional solution to the Bragg equation for the polycrystalline reflective surface 124 overlaying the substrate surface 122. Accordingly, systems applications may be formed with a single lens or a multiple lens system. As one example, a multiple lens assembly is illustrated in the plan view of FIG. 14 and the elevation view of FIG. 15 in a photolithographic system 150 suitable for fabrication of small geometry semiconductor products. The lens combination is designed to transmit x-rays from a divergent source 152 through two Bragg reflections toward a theoretical focal point 154. X-rays emitted from the source 152 are reflected by a first pair of lenses 158 and directed to a secondary lens 160. The first lenses are proportioned to capture a large flux of the x-rays generated from the source 152. The secondary lens 160 converges the reflected x-rays toward the focal point 154. The secondary lens 160 has a conical-like shape. The sizes and shapes and positions of the lenses 158 and 160 are based on a theoretical solution of the Bragg equation which focuses the x-rays. Once the angles for multiple reflections are calculated, different lens shapes may be determined. As described above, the lenses are formed on a substrate material having good thermal and mechanical stability. As illustrated in FIG. 15 a mask 164 containing an image and a substrate 166 are placed between the lens 160 and the focal point 154 so that collimating radiation passes through the mask to project an image of reduced size on to the substrate. The shape and focusing ability of the dual lens design allows for the resolution to be well below the limits of current x-ray lithography techniques using 1xc3x97 masks and eliminates the need to produce 1xc3x97 masks. With provision of a high throughput of x-rays, relative to the total flux generated from the source, relatively small x-ray sources may perform functions such as those provided with other types of optical sources such as LED lasers. Further, the ability to focus an x-ray beam enables formation of a narrow beam width capable of high-density storage such as achievable with laser read-write technology applied to optical media such as CD ROMs. Use of x-rays to read and write data also enables three-dimensional storage of information since x-rays easily pass through most media. That is, by defining multiple focal planes in a storage medium, information can be stored in stacked layers. By way of example, x-ray optics could generate Write Once Optical Storage in a manner analogous to CD ROM technology. The storage medium may consist of an absorptive thin metal layer, e.g., tellurium (Te) formed between two protective layers of plastic or glass with an air gap to allow for the displacement of material during the write step. Another embodiment comprises multiple absorptive metallic layers separated by layers of SiO2 similar to a thin film stack on a semiconductor. Such a system for storing information, illustrated in FIG. 16, may include a circularly rotating xe2x80x9caxisxe2x80x9d 200, a horizontal translation component 202, a vertical translation component 214, a storage disk 204, an x-ray source 206, focusing optics 208 and a detector 209 for sensing intensity of radiation transmitted through the disk 204. The disk may be rotated and linearly translated in a conventional manner to progressively pass discrete data locations through the radiation transmitted from the focusing optics. For high-density storage the translation component may displace the disk 204 along three orthogonal axes. The disk 204 will then comprise sequentially alternating 25 films of metal and insulator, each metal layer providing a level for storage of different information. In this example a Te layer 210 is alternately formed with a silicon dioxide layer 212. The process for writing information at any level of metal can be effected by providing sufficient intensity at each storage location to cause localized physical transformation which affects the intensity of transmitted x-rays during a read operation. Preferably, for a multi-layer storage disk, the radiation used to write data comes from two different sources to avoid incidental deformation of the storage medium at a different level. In a disk which stores information at only one level, a single focused source may perform the write operation at a first, relatively high intensity while the read operation may be performed at a lower intensity generated by the same source. For example, the focusing lens may be shifted to vary the flux transmitted for each of the two operations. The x-ray source 206 may be a low-cost rotating anode x-ray source and the x-rays may be generated from molybdenum or copper. Conventional medical x-ray imaging, e.g., to examine a bone for fractures, is based on use of divergent radiation. Commonly, a plate of film is positioned under the tissue to be examined. The distance from the tissue to the plate must be uniform and minimal to avoid fuzziness of the image caused by divergence of the x-rays. When the bone or other tissue cannot be aligned with the film to avoid effects of divergence, satisfactory imaging cannot be had. For example, it may not be possible to acquire a satisfactory image of a knee or elbow joint from desired views when, due to injury, the joint cannot be adjusted to a straight position. In contrast, provision of parallel x-rays will overcome such artifact and assure a relatively sharp image when the joint is not positioned a uniform distance from the film plate. Of course, in the past it has been possible to reduce the amount of divergence from a traditional source by moving it far away from the limb, but this approach has the disadvantage of requiring long exposure times or relatively higher powers of radiation. Thus, any prior efforts to address this problem have been countered with both health and economic disadvantages. Further, the distances which the x-rays must travel in order to approximate parallel radiation must be substantially larger than typical room dimensions. FIG. 17 illustrates in simple schematic form an x-ray imaging system 230 including a source 232 of parallel x-rays (corresponding to the source and lens arrangement of FIGS. 8, 9, and 10) and a photographic film plate 234 sufficiently spaced apart from the source 232 to permit a patient to interpose the body portion 236 of interest for examination. Similar arrangements can be constructed for non-medical applications. Numerous medical applications of x-rays may be undertaken according to the invention. Radiation therapy, one of the oldest and most cost-effective cancer therapies requires that healthy tissue as well as cancerous tissue be subjected to high exposure levels. External beam radiation, perhaps the most widely used type of cancer radiation therapy, allows relatively large areas of the body to be treated and permits treatment of more than a localized area such as the main tumor and nearby lymph nodes. External beam radiation is usually given in periodic doses over several weeks. An improved system 250 for imparting x-ray cancer radiation treatments is schematically shown in FIG. 18 as comprising a divergent source 252 generating x-rays which are reflected from a lens structure 254 (such as the two lenses shown in FIGS. 8-10), projecting substantially parallel x-rays 256 upon a desired region 258 of a patient""s body, e.g., positioned on a table 260. The source 252 and lens structure 254 are positioned in a suitable enclosure 262 from which the parallel x-rays emanate toward the table. The source 252 and lens structure 254 will vary substantially in size, depending on the application. For example, in order to examine a large portion of a person""s body, the enclosure 262 may have to be of dimensions exceeding 4 m3. On the other hand, if examination is limited to small specimens, such as a finger or tooth, the enclosure size may be less than 1 m3. According to another embodiment of the invention, internal radiation therapy, or, brachytherapy, may be performed with the High Energy Internal Spot Beam Radiation Therapy System 280 of FIG. 19. Brachytherapy is based on interstitial radiation or intracavitary radiation. In the past, interstitial radiation has been effected by placement of the radiation source in the affected tissue in small pellets, wires, tubes, or containers. Intracavitary radiation treatment has been performed by placing a container of radioactive material in a cavity of the body. The container is placed a short distance from the affected area. One objective of brachytherapy, delivery of a high dose of radiation within a small volume of tissue, is improved with the system 280 because the x-rays are projected from each of several sources 282 and focused via full barrel-shaped reflecting lens surfaces 284 (such as described with reference to FIGS. 1 and 2) about an irradiation volume 286. The system 280 enables delivery of a high dose of radiation within the volume 286. During operation the volume 286 includes tissue of a patient 288 undergoing treatment. Exposure of surrounding tissue is limited to tolerable, i.e., less damaging, levels. Three sources 282 and three lens surfaces 284 are employed in the example system 280 to illustrate that a relatively high dose is created within the volume 286 while the intensity in regions outside the volume is proportionately lower than would be if all of the flux were generated from a single source. Specifically, the convergence angles based on reflection of each lens surface 284 limit the flux outside the volume 286 to low levels so as to not destroy cells, while sufficient flux is delivered within the volume 286 to perform radiation treatment. Still another medical application of the invention may be based on one or more sources 282 and lens surfaces 284 to provide high energy and highly focused radiation in order to perform surgical procedures. Such a system may be configured as schematically described in FIG. 19 with the lens surfaces 282 adapted to narrow the focal region to a desired volume. If multiple sources are deployed, automated adjustment and alignment of the system may be effected with detector elements coupled to a feed back system and alignment mechanism. Operations of cutting, welding and other forms of surface treatment (e.g., hardening, modifying mechanical properties, melting, alloying , cladding, texturing, and machining) for industrial applications may be performed with the system 300 of FIG. 20 comprising a source 302, a barrel-shaped reflecting lens surface 304 (as described in FIGS. 1 and 2) configured to converge x-rays about a focal point 306 to perform an operation on a work piece 308. Either the focal point or the work piece may be displaced to irradiate a desired area of the work piece on or within the work piece. Alternately, and with application to low energy operations, lens surfaces such as illustrated in FIG. 7 may be employed in lieu of the surface 304 to create a focal line to effect the surface treatment. The focal line or the work piece may be displaced to effect irradiation of a desired region on or within the work piece. In the past x-ray photoemission spectroscopy (XPS) has been performed with unfocused x-rays, this resulting in a large beam spot. The size of the beam spot, e.g., ranging from tens of microns to millimeters in diameter, limits the spatial resolution of the technique. For XPS applications as well as other contexts in which a beam width substantially less than 10 microns is desired, converging x-rays emanating from a lens surface toward a desired focal region are passed through an aperture positioned relatively close to the focal region. Such apertures may be fabricated with focussed ion beam techniques. The exemplary XPS system 320 of FIG. 21 illustrates a source 322 which generates x-rays for reflection at a lens surface 324 to transmit converging radiation through an aperture 326 and on to a sample 328. The system 320 is positioned in a low pressure chamber 330 to detect emission of electrons 332 from about a focal region 334 by a collector 336. Other potential systems applications for the concepts described herein include x-ray microscopy and x-ray laser mirrors. Generally it should be recognized that the source and lens combination of each system should be statically fixed to one another in order to satisfy requisite tolerances for realizing optimum Bragg diffraction along the reflective surface. The invention has been described with exemplary embodiments while the principles disclosed herein provide a basis for practicing the invention in a variety of ways. Other constructions, although not expressly described herein, do not depart from the scope of the invention which is only to be limited by the claims which follow:
	description	A representative embodiment of a xe2x80x9cscattering aperturexe2x80x9d according to the invention is shown in FIGS. 1(a)-1(b). Turning first to FIG. 1(b), the scattering aperture 4 comprises a plate 4A made of a CPB-scattering material. The plate 4A defines aperture segments (xe2x80x9cvoidsxe2x80x9d) 4B that, in FIG. 1(b), collectively define essentially a ring-shaped aperture. The plate 4A can be silicon that is several micrometers thick. The aperture segments 4B are produced by micro-machining a silicon wafer using conventional techniques as used in semiconductor manufacturing. Turning now to FIG. 1(a), a charged particle beam 1 is incident on the scattering aperture 4 situated at a crossover plane A. Particles of the beam 1 passing through the void segments 4B become the transmitted hollow beam 2. Particles of the beam 1 that impact the plate 4A are scattered as they pass though the plate 4A and become the xe2x80x9cscattered beamxe2x80x9d 2xe2x80x2. The transmitted hollow beam 2 passes readily through a blocking aperture 5 located downstream of the scattering aperture 4. Most of the scattered beam 2xe2x80x2 is blocked by the blocking aperture 5. Specifically, the scattered beam 2xe2x80x2 is absorbed by the blocking aperture 5. The blocking aperture 5 defines a central void 5a and is situated between the scattering aperture 4 and the reticle 13. The blocking aperture 5 desirably is formed of a material that can withstand high temperatures (at least several hundred degrees C) and is several mm or more thick. The central void 5a has a diameter sufficiently large so as not to impede passage of the transmitted hollow beam 2. As can be seen in FIG. 1(a), some of the scattered beam 2xe2x80x2 xe2x80x9cleaksxe2x80x9d through the void 5a in the blocking aperture 5. However, the number of charged particles of the scattered beam 2xe2x80x2 leaking through is relatively small. Also, because the leaking particles are scattered, they pose no significant problem to pattern transfer. Furthermore, the number of such scattered particles reaching the reticle 13 can be reduced to substantially zero by adding one or more additional particle-absorbing apertures between the annular scattering aperture 4 and the reticle 13. An exemplary embodiment of a CPB microlithography system (e.g., electron-beam system) comprising the scattering aperture 4 and blocking aperture 5, as described above, is shown in FIG. 2. Components in FIG. 2 that are the same as described above have the same respective reference numerals and are not described further. An electron beam 1 is produced by an electron-beam source 6 (e.g., electron gun). The beam 1 passes through illumination lenses 7, 8 between which is a field-limiting aperture 9. A first beam crossover 10 is situated just downstream of the source 6. Downstream of the second illumination lens 8 is a current-limiting aperture 11. After passing through the current-limiting aperture 11, the beam 1 passes through the scattering aperture 4, the blocking aperture 5, and a third illumination lens 12 to the reticle 13. (The components located between the source 6 and the reticle 13 collectively are termed the xe2x80x9cillumination-optical system.xe2x80x9d) Electrons passing through the reticle 13 pass through first and second projection lenses 14, 15, respectively, to the wafer 16. Between the first and second projection lenses 14, 15 is a contrast aperture 17. (The components located between the reticle 13 and the wafer 16 collectively are termed the xe2x80x9cprojection-optical system.xe2x80x9d) The electron beam 1 emitted from the source 6 passes through the illumination lenses 7, 8 to the scattering aperture 4. The field-limiting aperture 9, which restricts the optical field illuminated by the beam 1 and shapes the beam, is situated at a position that is conjugate with the electron-emission surface in the electron source 6, with respect to the lens system consisting of the illumination lenses 7, 8. An image of the first crossover 10 is formed at the scattering aperture 4 by the illumination lenses 7, 8, as shown in FIG. 1(a). In other words, the scattering aperture 4 is disposed at a crossover position A of the illumination-optical system. The current-limiting aperture 11 is disposed upstream of the scattering aperture 4. The current-limiting aperture 11 pre-clips the edges of the electron beam 1 that were spread at the crossover, thereby alleviating the thermal load on the scattering aperture 4. The hollow beam 2 transmitted through the scattering aperture 4 forms an image of the electron-emission surface of the source 6 on the reticle 13 by means of the third illumination lens 12. The third illumination lens 12 achieves uniform illumination of the irradiated region of the reticle 13. The size and profile of the irradiated region is determined by the field-limiting aperture 9, which is disposed at an axial position that is conjugate with the reticle 13, with respect to the lens system consisting of the illumination lenses 8 and 12. As discussed above, most of the scattered electron beam 2xe2x80x2 from the scattering aperture 4 is absorbed by the blocking aperture 5. An image of the irradiated region (e.g., subfield) on the reticle 13 is formed on the wafer 16 by means of the projection lenses 14, 15. The contrast aperture 17 blocks particles of the beam 2 that were scattered by the reticle 13 (which can be a membrane type or a scattering-stencil type). Since the beam 2 is hollow before it irradiates the reticle 13 in this embodiment, Coulomb effects are reduced, thereby reducing image defocusing and distortion on the wafer 36, without having to reduce beam current. Consequently, this embodiment advantageously increases image resolution and pattern-transfer accuracy without decreasing throughput. In actual practice, image focal-point shift is reduced to less than several micrometers and image distortion is maintained at less than several nanometers. As noted above, although not shown in FIG. 2, multiple apertures may be disposed above the reticle 13 (but downstream of the apertures 4, 5) to block passage of particles in the scattered beam 2xe2x80x2 that were not absorbed by the blocking aperture 5. Exemplary alternative embodiments 40, 41 of the scattering aperture 4 are shown in FIGS. 3(a)-3(b), respectively, which are similar to FIGS. 8 and 9, respectively, of the JP Hei 11-297610 reference cited above. In FIG. 3(a), two half-ring-shaped void segments 40B, defined by the plate 40A, collectively define an essentially annular void. In FIG. 3(b), multiple small round void segments 41B, arranged in a bolt circle, collectively define an essentially annular void. Other configurations of void segments are also possible so long as the resulting scattering aperture effectively scatters particles of the beam incident within a central circular area having a first radius and transmits particles of the beam incident within an area outside the central region and having a first radius greater than the radius of the central region but also having a second radius greater than the first radius. The beam-transmitting area is surrounded by a beam-scattering area. Alternatively to a plate with voids, the scattering aperture 4 can be configured as a ring-shaped (or analogous segmented-ring-shaped) opening 42 in a layer 43 of a CPB-scattering material on a thin, relatively CPB-transmissive membrane 45 (see FIGS. 3(c)-3(d)). The charged particle beam passes with little or no scattering through the opening 42 (and supporting CPB-transmissive membrane 45 (arrow 46), but is blocked (highly scattered, arrow 47) by the CPB-scattering material 43. The respective materials and thicknesses of the CPB-scattering material 43 and membrane 45 are similar to materials and thicknesses that would be used in a so-called xe2x80x9cscattering-membranexe2x80x9d reticle as known in the art. Although this alternative configuration exhibits, with respect to the beam passing through the opening 42, greater dispersion of beam energy than exhibited by passage of the beam through actual voids, this alternative configuration (FIGS. 3(c)-3(d)) allows the opening 42 to be a complete ring (donut-shaped profile) as shown. Under some conditions, the beam passing through a scattering aperture 4 with a full-ring opening 42 (rather than a segmented-ring void such as shown in FIG. 1(b), 3(a), or 3(b)) exhibits less aberration. Since particles of the hollow beam 2 that are scattered (by passage through the scattering aperture 4) are absorbed by the downstream blocking aperture 5, neither the scattering aperture 4 nor the blocking aperture 5 reach high temperatures during use. Therefore, the scattering aperture 4 can be formed from a material such as silicon that is micro-machined easily. Also, the blocking aperture 5 can be made with a relatively large thickness. As a result, the problems inherent to the apparatus disclosed in JP Hei 11-297610 are solved while exploiting the advantages of that apparatus. The thermal load on the scattering aperture 4 can be alleviated further by passing the beam through a current-absorbing aperture before the beam enters the scattering aperture 4. Semiconductor devices can be manufactured having high resolution and high transfer accuracy using an apparatus according to the invention. Whereas the invention has been described in connection with a representative and alternative embodiments, it will be apparent that the invention is not limited to those embodiments. On the contrary, the invention encompasses all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.
051768771	abstract	A nuclear fuel assembly for a nuclear reactor has a plurality of vertically extending fuel rods arranged side by side in a square array and containing fissile material. The array has two adjacent first sides which are next to a control rod region of the core and two adjacent second sides which are next to a non-control rod region of the core. When the rods of the array as seen in horizontal cross section, excluding the rods lying on the diagonal line joining the opposite corners of the array at which the control rod region meets the non-control rod region, is divided into four regions which are. a: the rods in the row and column of said array adjoining said first sides; PA0 b: the rods lying between said region a and said diagonal line; PA0 c: the rods lying between said region d and said diagonal line, PA0 d: the rods in the row and column of said array adjoining said second sides and; over at east part of the height containing fissile material, the average concentration of fissile material per fuel rod is higher in said region b than said region c and by at least 5% and, among all said regions a, b, c, d, is a minimum in said region a. This provides good control rod worth and low local power peaking.
	summary
040381323	summary	BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and nuclear fuel elements for use in nuclear reactors cooled by light or heavy water. The fuel elements for these reactors commonly comprise nuclear fuel rods supported in parallel spaced apart positions in suitably proportioned clusters or bundles so that within the nuclear reactor core, coolant passed over the fuel rods in a direction parallel to their surfaces becomes heated. Thereafter the heated coolant is employed in a heat utilisation plant for example a heat exchanger or a steam turbine. Various proposals have been made for providing an auxiliary means of cooling the nuclear fuel clusters by means of one or more spray cooling tubes which, connected to their own supply of coolant extend within the cluster parallel with the rods, and from holes in the side walls of these tubes, coolant water may be sprayed laterally into the cluster against the rods. The flow of main coolant may thus be supplemented by the auxiliary coolant or replaced by the auxiliary coolant in some circumstances. A difficulty in these arrangements is that only those fuel rods close to a spray cooling tube have a line of sight to it and can readily be reached by the auxiliary coolant. SUMMARY OF THE INVENTION According to the present invention a nuclear fuel element comprising a cluster of nuclear fuel rods supported axes parallel in spaced apart relationship by transverse grids so as to define interspaces for the axial flow of reactor coolant has at least one of the interspaces occupied in part by an axially extending auxiliary coolant conduit with lateral holes through which auxiliary coolant is sprayed into the cluster and deflector means extending from a transverse grid into a position in front of the holes for deflecting auxiliary coolant spray on to parts of the fuel rods inaccessible to auxiliary coolant. The deflecting means preferably include target plates secured to a grid and dependent therefrom into the trajectory of one or more jets of auxiliary coolant emitted from the lateral holes in the conduit. If, as is usual, the fuel rods are disposed axes vertical, in rows on a uniform pitch then the face of the target plate directed towards the auxiliary coolant conduit is bounded by two longitudinal edges spaced by a distance substantially equivalent to the pitch of the fuel rods. When pitched on concentric circles about a central auxiliary coolant conduit the target plate is preferably so disposed that its vertical centre line lies in plane passing through a mid pitch position. If the edges of the target plate are made sharp then there is a well defined deflection of auxiliary coolant and preferably the vertical longitudinal edges of the target plate make acute angles with the face of the plate which is directed towards the auxiliary coolant conduit.
	description	1. Field of the Invention The present invention relates to uniformity correction modules, lithographic apparatus comprising a uniformity correction module, a method of increasing the uniformity of an illumination beam, and a device manufacturing method using a uniformity correction module. 2. Background of the Related Art A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. The invention relates to uniformity correction modules (sometimes referred to as “unicoms”) that consist of a plurality of light-absorbing elements the positions of which are adjustable in the scanning direction in order to set the outer boundary of the illumination slit. Such a unicom (sometimes also called dynamic adjustable slit or “DYAS”) is described in U.S. Pat. No. 6,097,474. The elements may for example be embodied as transmissive quartz plates which are coated with a semi-transparent layer. A disadvantage of these plates is that refraction occurs at the air-quartz interface at the edges leading to ellipticity, telecentricity, local straylight or hotspot errors. Current uniformity correction modules function in air or vacuum i.e. in an ambient with a refractive index n=1. For the coming generation high numerical aperture (NA) systems (NA>0.5) no feasible solution exists to use a known concept that does not affect the pupil and telecentricity distribution negatively. The pupil distribution is the intensity distribution in the pupil plane of the projection system, and corresponds with the angular intensity distribution of the projection beam. The telecentricity of a radiation beam impinging on a point on a wafer is the average incident angle. Preferably the telecentricity is perpendicular to the wafer surface but in practice it shows a slight variation over the illuminated field/slit. In accordance with a first aspect of the invention there is provided a uniformity correction module for improving the uniformity of a radiation distribution in a rectangular illumination slit having two long sides and two short sides, including a plurality of movable blades arranged along each long side of the illumination slit, and a chamber containing a fluid wherein said movable blades are at least partly immersed in said fluid, and wherein the difference between the refractive index of each blade and the refractive index of said fluid is sufficiently small to prevent significant reflection and refraction at the surface of each blade. In a particular embodiment, the difference between the refractive index of each blade and the refractive index of said fluid may be 0.15 or less. In a second embodiment, the refractive index of said fluid and the refractive index of said blades may be substantially equal at the wavelength of said radiation. In alternate embodiments, the blades may be formed from quartz, fused silica, or calcium fluoride. The fluid may be water. In yet another embodiment, each blade may be provided with a light absorbing coating on its upstream or downstream surface. In another embodiment, the light absorbing coating may vary in a gradual way, so that the degree of absorption increases with increasing distance from the center of the illumination slit. The degree of absorption at the end of each blade nearest to the center of the illumination slit may be 0%. In variations on the above embodiments, each blade may be triangular when viewed in the direction of the optical axis. Likewise, each blade may have the shape of an elongate rectangle. Each blade may be arranged along an axis perpendicular to the longitudinal axis of the illumination slit. Each blade may be arranged along an axis which is slanted with respect to the direction perpendicular to the longitudinal axis of the illumination slit. The end of each blade nearest the center of the illumination slit may be substantially parallel with the longitudinal axis of the illumination slit. In another embodiment, the uniformity correction module of the first aspect of the present invention may be provided with a transparent top cover located upstream of said blades, and a transparent bottom cover located downstream of said blades. An absorptive coating may be applied to the top or bottom cover at locations between adjacent blades, so as to prevent light leakage between the blades. In a particular application, the liquid may be more absorptive than said blades. The uniformity correction module may be provided with a transparent top cover located upstream of said blades, and a transparent bottom cover located downstream of the blades, and wherein the distance between each blade and the top cover, and between each blade and the bottom cover, is about 0.05 mm to 0.1 mm. The blades may be transparent. The liquid may have an absorption coefficient of about 0.2/cm. Likewise, the blades may be slanted at an angle relative to the direction perpendicular to the longitudinal axis of the illumination slit, and wherein the end of each blade nearest the center of the illumination slit is substantially parallel with the longitudinal axis of the illumination slit. In yet another embodiment, the blades may be provided with complementary shaped protrusions and grooves along their sides, each protrusion fitting into a respective groove, so as to prevent radiation from passing through the uniformity correction module without also passing through the blades. Furthermore, the edges of adjacent blades on the same side of the illumination slit may have a V-shaped profile, and wherein a V-shaped projection along the edge of each blade fits into a V-shaped groove along the edge of an adjacent blade, so as to prevent radiation from passing through the uniformity correction module without also passing through the blades. In a particular embodiment, the thickness of each finger, from its upstream edge to its downstream edge, may be between about 1 mm and 2 mm. In another variation, the edges of each finger may be polished. The uniformity correction module may further comprise a circulation arrangement for introducing liquid into the chamber and removing liquid from the chamber during use. Each blade may be formed from an opaque member supported by a quartz substrate. In another embodiment, the opaque member may be a metallic foil. In another embodiment, the liquid may contain at least one additive which affects the level of absorption of light by the liquid. In accordance with a further aspect of the present invention there is provided a lithographic apparatus including an illumination system for providing a projection beam of radiation, a support structure for supporting a patterning device, the patterning device serving to impart the projection beam with a pattern in its cross-section, a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate, and a uniformity correction module including a plurality of movable blades arranged along each side of the projection beam, and a chamber containing a fluid, wherein the movable blades are at least partly immersed in the fluid, and wherein the difference between the refractive index of each blade and the refractive index of the fluid is sufficiently small to prevent significant reflection and refraction at the surface of each blade. In such an apparatus, the uniformity correction module may be located just above the patterning device. In accordance with a further aspect of the present invention, there is provided a method of increasing the uniformity of an illumination beam, the method including placing in the illumination beam a uniformity correction module including a plurality of movable blades arranged along each side of the illumination beam, and a chamber containing a fluid, wherein the movable blades are at least partly immersed in the fluid, and adjusting the positions of the blades of the uniformity correction module so as to increase the uniformity of the illumination beam. In accordance with a still further aspect of the present invention, there is provided a device manufacturing method including providing a substrate, providing a projection beam of radiation using an illumination system, using a patterning device to impart the projection beam with a pattern in its cross-section, projecting the patterned beam of radiation onto a target portion of the substrate, placing in the projection beam a uniformity correction module including a plurality of movable blades arranged along each side of the projection beam, and a chamber containing a fluid, wherein the movable blades are at least partly immersed in the fluid, and adjusting the positions of the blades of the uniformity correction module so as to increase the uniformity of the projection beam. Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers. The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams. The term “patterning device” used herein should be broadly interpreted as referring to devices that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. Patterning devices may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned. The support structure supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support can be using mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device”. The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system”. The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. FIG. 1 schematically depicts a lithographic apparatus according to a particular embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL for providing a projection beam PB of radiation (e.g. UV radiation); a first support structure (e.g. a mask table) MT for supporting a patterning device (e.g. a mask) MA and connected to first positioner PM for accurately positioning the patterning device with respect to item PL; a substrate table (e.g. a wafer table) WT for holding a substrate (e.g. a resist-coated wafer) W and connected to second positioner PW for accurately positioning the substrate with respect to item PL; and a projection system (e.g. a refractive projection lens) PL for imaging a pattern imparted to the projection beam PB by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W. As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above). The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system. The illuminator IL may comprise adjustable optical element or elements AM for adjusting the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam of radiation, referred to as the projection beam PB, having a desired uniformity and intensity distribution in its cross-section. The projection beam PB is incident on the mask MA, which is held on the mask table MT. Having traversed the mask MA, the projection beam PB passes through the lens PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in FIG. 1) can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the object tables MT and WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the positioners PM and PW. However, in the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. The depicted apparatus can be used in the following preferred modes: 1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the projection beam is projected onto a target portion C in one go (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure. 2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the projection beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT is determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. 3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the projection beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array of a type as referred to above. In FIG. 1 the radiation from the source SO travels “downwardly” or “downstream” towards the substrate/wafer W. In this specification references to “top” and “upper” and the like are to be understood as corresponding to “upstream” in the lithographic apparatus of FIG. 1, and references to “bottom” and “lower” and the like are to be considered as corresponding to “downstream”. Thus “top and bottom” and “upper and lower” are defined only by the direction of the radiation in the device, and have no connection with the direction of gravity in relation to the device. FIG. 2 shows a uniformity correction module 1, in accordance with an embodiment of the invention, in which an illumination slit 2 is provided with two inner rows of triangular blades 4 and 6, and two outer rows of triangular blades 8 and 10. In this specification “illumination slit” refers to the slit-shaped area of illumination produced by a physical slit. Although the blades are shown arranged in rows, it should be understood that each triangular blade can be moved independently of any other blade in the same row, or indeed in a different row. Any suitable mechanism can be used for moving the blades, and the mechanism is not shown in the drawings. The mechanism for manipulating the blades can allow all blades within a row to move together, or separately. Also, the movement of a blade on one side of the slit can be linked to the movement of the corresponding blade on the other side of the slit 2, so that for example when the blade moves inwardly, its partner on the opposite side of the slit moves inwardly by the same amount. However, there is no requirement for the movement of the blades to be linked in this way, and as noted above the movement of each blade can be completely independent of the movement of any other blade, thus allowing total flexibility when using the uniformity correction module to smooth an intensity distribution. The blades may be moved together for adjusting a long wave distribution, and may be moved separately if a high frequency distribution needs to be corrected. Each triangular blade is provided with a light-absorbing coating, eg chromium, on its upper surface 12. FIG. 2 is a view of the uniformity correction module 1 from the top, so the triangular surface (labelled 12) of each blade which is visible in FIG. 2 is considered as the upper surface of the blade. The degree of absorption varies across the triangle in a gradual (ie. “graded”) way, and this is indicated in FIG. 2 by the variation in shading on each triangular blade. In this embodiment, the degree of absorption varies from 0% at the inner tip of each triangular blade to a maximum of between 5 and 10% at the base of each blade. The graded coating can be fabricated using evaporation, where the coating is deposited on part of the blade using a mask, where the shadow causes the grading. The blades 2, 4, 6, 8 are themselves formed from a radiation transparent material, for example quartz, and are immersed in a transparent liquid, which may be water. A chamber 14 contains the liquid, and surrounds the illumination slit 2 and the blades. Liquid flows into the chamber 14 through an inlet port 16, and leaves the chamber 14 through an outlet port 18. The thickness of each blade, from the triangular top surface 12 to the triangular bottom surface, is between 0.5 and 3.5 mm. The distance from the tip 20 to the base 22 of each blade is between 2 and 8 mm. The width of each blade is between 2 and 15 mm. The smaller the blades, the more are needed to cover the length of the slit 2. Because the blades 2, 4, 6, 8 are formed from thin graded coated quartz plates mounted in water, only the graded coating affects the light beam. Due to the small refractive index difference between quartz and water, the quartz substrates (ie plates) do not need an anti-reflective (AR) coating because there is no reflection at the quartz-water interface, while the edges of the substrates have no influence on the beam. This is because the coating is preferably applied only to the top or bottom surface of each quartz plate. The preferred limit for the difference in refractive index between the blades and the liquid is a maximum of 0.15, which is slightly greater than the difference (of 0.126) between water (1.437) and quartz (1.563). Given the equation for reflectance R=((n1−n2)/n1+n2))^2 the difference between water and quartz gives a reflectance of less than 0.2%, while the difference of 0.15 results in a reflectance of 0.25%. Obviously using a smaller refractive index difference will decrease the reflection even further. The liquid can be the same as the one used for immersion imaging either in series or parallel flow, but may be a different circuit and even a different liquid depending on what is preferable in view of having substantially equal refractive indices. Immersion imaging is a recent technique which uses a liquid, such as water, between the lens and the wafer. Instead of, or in addition to, using an absorbing coating, one could use a thin absorbing foil (such as a metallic foil, or other opaque member) supported by a quartz substrate. One advantage of such an arrangement is that the shape of the absorbing structure can be easily made by cutting the foil. The uniformity correction module 1 is intended to be used in the same manner and for the same purpose as the dynamic adjustable slit described in U.S. Pat. No. 6,097,474 mentioned above. The illumination field is continually adjusted and controlled, but “continually” may mean one adjustment of the blades per hour, or even less frequently. In the lithographic apparatus of FIG. 1, the uniformity correction module 1 may be located just above the reticle, or at a different position in the illuminator which can be treated as an intermediate image. The uniformity of the radiation intensity distribution can be measured with a sensor at wafer level. Based on this measurement the uniformity correction module is adjusted so that most of the intensity variations are removed. If the exposure of a die starts, the wafer stage, the reticle stage and the internal REMA (reticle masking blades that determine the field size, being something completely different from the blades 2, 4, 6, 8 described above) start moving, while the laser starts exposing. The combination of these effects results in a non-uniform dose behaviour during the scan in the non-scanning direction. If this behaviour is known, one can correct for this by using the uniformity correction module. Obviously one can also correct slit position dependent dose variations caused by large changes in reticle reflections caused by changing from dark field to bright field areas. Dark field areas are covered with chromium and thus reflect light back into the illuminator. This back-reflected light might also be back-reflected again towards the reticle to cause small dose differences between light and dark areas. FIG. 3 shows a uniformity correction module 24 in accordance with a further embodiment. This time the blades are in the form of rectangular fingers which are arranged to be moved into and out of the slit 26. The fingers may be arranged perpendicular to the slit 26 as indicated by fingers 28 at the right hand side of FIG. 3, or the fingers may be slanted at a different angle to the slit 26 as indicated by the fingers 30 at the left hand side of FIG. 3. Although FIG. 3 shows both the fingers 28 and the fingers 30 in the same drawing, these represent alternative configurations. The other features of the uniformity correction module 24 of FIG. 3 are generally the same as those of FIG. 2. That is, the fingers 28, 30 are formed from quartz having a graded coating on the top surface thereof, and the fingers are immersed in a liquid, such as water. Again, the fingers may be manipulated individually, or together with other fingers. The fingers 28, 30 may be completely absorbing or may have a graded coating with the absorption increasing from center to the edge of the slit. The graded coating may be such that the level of absorption increases linearly from the inner end of the finger (ie the end closest to the center of the slit) for a certain distance, and then becomes constant. The absorption profile would then resemble one end of a trapezium. FIG. 4 shows the cross-section along the line A-B in FIG. 3. It shows the top and bottom covers 32 and 34 respectively of the illumination slit 26. These covers are transparent and may be formed from quartz. Water 36 is contained between the covers 32 and 34, and the fingers 28 (or 30) can be moved in and out of the slit 26 as required. Motors for moving the fingers may be provided in the water or outside, depending on the chosen construction. FIG. 5 is a cross-sectional view taken along the line C-D in FIG. 3. It shows 3 of the fingers 28 positioned above the bottom cover 34. FIG. 5 shows an optional absorbing coating 38 deposited in lines on the top surface 40 of the bottom cover 34. The coating 38 is positioned in the open spaces between each pair of adjacent fingers 28, thus preventing light leakage caused by scattering of light at the edges of the fingers 28. The coating 38 may have an absorption up to 100% (ie opaque). The coating could of course alternatively be placed on the top cover. The illumination slits 2 and 26 shown in FIGS. 2 and 3 actually have a trapezium-shaped intensity profile. The slit size is determined by the positions on the slope were the intensity is 50% of the maximum intensity. We now turn to the embodiment of FIGS. 6, 7 and 8, which also relates to a uniformity correction module using blades immersed in liquid, but in this case the liquid is more absorptive than the blades. It is known to use a set of “fingers” or blades with a predetermined optical transmission placed from the sides of an illumination slit in the optical path in order to correct uniformity. The device itself is located in a strongly defocused space which means that the correction does not only affect uniformity in the field but pupils as well. This concept may cause the following problems which will be explained below: fingers have edges which produce shadows and/or bright stripes in the field gaps between fingers may cause light leaks—bright stripes fingers have to be very thin and may be difficult to make.Infringing into the field from the sides causes significant ellipticity. If fingers are inserted deep enough in the field, the ellipticity does not suffer but losses of light become significant. FIG. 6 shows a uniformity correction module (unicom) 40 which comprises a plurality of fingers 42 which are movably mounted within a chamber 44 containing a liquid. The fingers 42 are immersed in the liquid. The chamber 44 has upper and lower transparent portions 46 and 48 which allow light to pass through the illumination slit. Motors 50 are provided for moving the fingers 42 individually or together with other fingers, as discussed above. In this embodiment, the fingers 42 have a thickness, from top to bottom, of 1.4 mm, and the spacing between the top each finger 42 and the upper wall of the chamber 44 is 0.05 to 0.18 mm. Likewise, the spacing between the bottom of each finger 42 and the lower wall of the chamber 44 is 0.05 to 0.18 mm. The width of the upper and lower transparent portions 46 and 48 (in the scan direction, which is also the direction of movement of the fingers 42) is 62 mm. The uniformity correction module (unicom) 40 uses a liquid which absorbs some of the light at the working wavelength, such as 193 nm. The liquid can be water with some absorbing additive, and the fingers 42 and the upper and lower transparent portions 46 and 48 can be made out of fused silica. Several goals are achieved in the proposed design: Attenuation occurs in the middle of the field, which has no effect on ellipticity. When no adjustment is required, the fingers can be placed all way to the middle of the field and attenuation is minimal. No edge effects, like shadows or bright stripes are created. Gaps between fingers still can cause problems which can be resolved by cutting the edges of the fingers at an angle, as will be explained below. This becomes possible because fingers are thicker than in the previous embodiments. The whole construction is immersed into absorptive water which has an additive with certain coefficient of absorption at 193 nm. The whole unicom package is about 7 mm thick which is comparable with the current 6.25 mm compensation plate plus some allowances for the fingers 42. A top view of the unicom 40 is shown in FIG. 7. FIG. 7 is a schematic diagram which shows that the ends of the fingers 42 are actually parallel with the longitudinal axis of the slit, so that opposite pairs of fingers 42 come together with no gap between them. The adjustment works in the following way. When both sets of the fingers 42 are inserted all the way and touch each other in the middle of the illumination slit there is low attenuation. As the fingers 42 move apart, absorption grows in the middle of the field, which is the most desirable form of attenuation. Reflection from the fingers 42, and the edges of the fingers, will be negligible. The difference between the refractive indices of water and fused silica give rise to 0.1% reflection and between water and CaF2 (fluorite), 0.01% reflection. As will be explained below with respect to FIG. 9, all edges of the fingers 42 are polished and shadows therefore do not exist. Bevels will be made and they will be shined also, and therefore scatter from edge chips will not exist. FIG. 9 shows how light can be reflected from the ends 51 of a pair of fingers 42 if no liquid is used to surround the fingers 42. The fingers 42 are thick (1 to 2 mm) and create significant shadows 52 at the reticle plane 54. Light that hits the edge 51 of a finger 42 from the inside of the finger 42 reflects 100% because of total internal reflection and this causes a part of the field (ie shadows 52 shown in the drawing as bold black lines) not to be illuminated. Although the light striking the edge of the fingers 42 from outside (illustrated by arrow 56) will complement the lost light, it can be shown that it does not do so exactly and it does not complement the lost light at all at the edges of the field. If the edge of each finger 42 is ground instead of polished, light does not reflect specularly from outside at all and this causes shadows everywhere. This is why prior art arrangements have been forced to use thin fingers (0.2 mm). Shadows caused by thin fingers are significantly reduced but still they present a serious problem and do not allow adjustment of the field uniformity to achieve good flatness of light intensity. Ripples from the shadows still remain. Another problem with thin fingers is that it is impossible to process the edge appropriately, and chips remain which create stray light (shown in FIG. 9 by arrows 58). From a manufacturing point of view, it is preferable to work with thick fingers but they have the set of problems described above. In the proposed configuration of FIGS. 6 to 8, the fingers 42 are thick (1 to 2 mm), and the edges are polished (shined). Because the fingers 42 are immersed in a liquid with a matching refractive index, there is no internal total reflection and no shadows. There is also no reflection from outside the edges. Stray light is not a problem also because there are no sharp edges and chips. Extra absorption between the fingers 42 on the same side of the illumination slit also represents a serious problem, as will be explained below. This problem is avoided by shaping the sides of the fingers 42 as shown in FIG. 8. The (long) sides of the fingers 42 are angled or tapered, or formed in a V-shape, as shown in FIG. 8. In the prior art (where fingers were surrounded by air, rather than a liquid) the fingers are partially absorptive and gaps (0.1 mm wide) between fingers on the same side of the illumination slit are transmissive. The contrast 100% (gap)−85% (finger) is enough to create noticeable ripples in the uniformity. In the case of Y dipole illumination, for example, almost all light will leak through this gap. The reason for this is that the illuminator can create an illumination pattern such that all points at the reticle (and also at the wafer) are illuminated by two narrow beams converging to this point. In the case of Y dipole illumination these two beams are positioned along the scan direction, ie along a line which is perpendicular to the longitudinal axis of the illumination slit, and if the fingers are also perpendicular to the longitudinal axis of the illumination slit then light leaking through the gaps between adjacent fingers is significant. Rotating the fingers at an angle to the scan direction reduces the problem significantly but it still exists. If fingers were not rotated, the prior art design would not have been usable at all. In the proposed configuration shown in FIGS. 6 to 8 we have similar problems. The finger area is not 100% transmissive because there is some absorptive liquid above and below the fingers 42, and this is unavoidable. The depth of absorptive liquid above (and below) the fingers is typically 0.05 mm to 0.1 mm. This depth can be made very small if a viscosity reducing component is added to the liquid. If the area of the fingers is 98.2% transmissive (absorption coefficient of liquid assumed to be 0.2/cm.), the narrow gaps (labelled G in FIG. 7) between the fingers would be 83% transmissive and we would have approximately the same finger/gap contrast as in the prior art but in reverse: ie more transmissive fingers and less transmissive gaps. This will create ripples as in the prior art. To solve this problem the fingers 42 are rotated so that they are not perpendicular to the illumination slit as shown in FIG. 7, as is done in the prior art, and because the fingers 42 are thick the sides of the fingers are made V-shaped as shown in FIG. 8 (something which is impossible to do for thin 0.2 mm fingers). The fingers 42 in FIG. 7 are shown in an exemplary arrangement in which they are creating a certain attenuation pattern. The fingers at the top of the figure are spaced further apart, so at the top there is more attenuation than at the bottom. This means that uniformity curve showed a bump at the top which is being compensated for using the device. FIG. 8 shows how the effect of the gaps G between the fingers 42 are significantly reduced by giving the sides of the fingers 42 complementary V-shaped profiles so that they fit together as shown in FIG. 8. The key is that angle of the V-groove must be shallow enough to ensure that rays with extreme angles, present in the illumination light (illustrated by rays A and B in the FIG. 8), will cross it instead of traveling along it. If rays travelled along the gap they would suffer too much aborption. Provided the rays cross the gap, the gap will represent minimum absorption and the intensity distribution will be smeared over a certain “smear area” along the edge of the fingers 42 as illustrated in FIG. 8. The embodiment of FIGS. 6 to 8 may further include a water circulation system, which allows a change to be made to the level of absorption of the water, and thus allows a change to the effective finger transmission. The liquid may contain one or more additives which affect the degree of absorption of light, and as the liquid is circulated the amount or type of additive may be changed in order to change the level of absorption. The liquid circulation circuit also allows for the recycling and “refreshment” of the used liquid. Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. All embodiments of the invention are also suitable for use in CCD applications. A CCD is a Charge Coupled Device. The application area of CCD production needs an extremely good uniformity of the intensity of the beam, and the uniformity value should be better (ie. lower) than 0.02%. Uniformity=(Imax−Imin)/(Imax+Imin)*100%. ‘Uniformity’ usually refers to the ‘scanning uniformity’ or ‘scanning dose uniformity’. In any embodiment of the invention, the blades may be either partly or fully immersed in the liquid which is being used. Although we have referred to liquids in the specification, a pressurised gas could also be used. The refractive index of a gas increases with increasing pressure, and the pressure could be increased so that the refractive index is similar to that of the blades. Therefore the invention can be used with fluids, which can be either liquids or pressurised gases. While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention.
	claims	1. A radiation to optical converter, comprising:a semiconductor substrate having first and second opposed surfaces, wherein the semiconductor substrate is formed of a direct band gap semiconductor;a first mirror formed on the first surface of the semiconductor substrate;a fixed transmission grating positioned adjacent to the first mirror formed on the first surface of the semiconductor substrate; anda probe beam source positioned to provide a probe beam that is incident on the second surface of the semiconductor substrate, passes through the semiconductor substrate, and is reflected back therethrough by the first mirror,wherein radiation from an external source passing through the fixed transmission grating is thereby modulated, the modulated radiation passing through the first mirror into the semiconductor substrate and producing a transient radiation induced grating therein, andwherein a portion of the probe beam passing through the semiconductor substrate is diffracted out of the probe beam by the transient grating. 2. The converter of claim 1, wherein the direct band gap semiconductor is a proton or neutron or ion damaged direct band gap semiconductor. 3. The converter of claim 1, wherein the direct band gap semiconductor is selected from GaAs, CdTe, Cd1-yZnyTe, ZnTe, InP InGaAsP, ZnSe, ZnO, TiO2, and GaP. 4. The converter of claim 1, wherein the semiconductor substrate has a thickness of about 5-200 microns. 5. The converter of claim 1, wherein the semiconductor substrate ic cooled to liquid nitrogen temperature. 6. The converter of claim 1, wherein the probe beam source is an optical or infrared source. 7. The converter of claim 6, wherein the probe beam source is an optical laser. 8. The converter of claim 1, wherein the probe beam source is tuned just below the band gap of the semiconductor substrate. 9. The converter of claim 1, further comprising a reference optical grating formed on the first surface of the semiconductor substrate and having the same spatial frequency and phase as the fixed grating. 10. The converter of claim 9, wherein the reference optical grating is an etched surface relief grating or ion implanted grating formed on the first surface of the semiconductor substrate, the first mirror being formed on the etched surface relief grating or ion implanted grating. 11. The converter of claim 9, wherein the reference optical grating is a metallic phase shifting grating formed on the first surface of the semiconductor substrate over the first mirror. 12. The converter of claim 1, further comprising a second mirror formed on the second surface of the semiconductor substrate, the second mirror being partly transmissive and partly reflective to the probe beam, thereby forming a Gires-Tournois etalon. 13. The converter of claim 12, wherein the first and second mirrors are dielectric mirrors. 14. The converter of claim 12, further comprising a reference optical grating formed on the first surface of the semiconductor substrate and having the same spatial frequency and phase as the fixed grating. 15. A radiation to optical converter apparatus, comprising:a radiation to optical converter of claim 1;external source imaging optics positioned to direct radiation from the external source onto the fixed transmission grating of the converter;diffracted beam imaging optics positioned to direct the diffracted probe beam to an imaging plane; andan image detector positioned at the imaging plane. 16. The apparatus of claim 15, wherein the external source is an x-ray source, and the external source imaging optics comprises an x-ray pinhole camera. 17. The apparatus of claim 15, wherein the external source is an optical source, and the external source imaging optics comprises one or more lenses. 18. The apparatus of claim 15, wherein the diffracted beam imaging optics comprises a beam splitter mirror positioned between the probe beam source and the converter to change the direction of the diffracted beam, and at least one lens positioned after the beam splitter mirror to focus an image produced by the diffracted beam onto the imaging plane. 19. The apparatus of claim 18, further comprising a beam block positioned at the Fourier transform plane to block the zero order portion of he diffracted beam. 20. The apparatus of claim 15, wherein the image recoding device comprises a CCD or a photographic plate. 21. The apparatus of claim 15 wherein the converter further comprises a reference optical grating formed on the first surface of the semiconductor substrate and having the same spatial frequency and phase as the fixed grating. 22. The apparatus of claim 15, wherein the converter further comprises a second minor formed on the second surface of the semiconductor substrate, the second mirror being partly transmissive and partly reflective to the probe beam, thereby forming Gires-Tournois etalon. 23. The apparatus of claim 22, wherein the converter further comprises a reference optical grating formed on the first surface of the semiconductor substrate and having the same spatial frequency and phase as the fixed grating. 24. The apparatus of claim 15, wherein the diffracted beam imaging optics further comprises a reference grating. 25. A radiation to optical converter, comprising:a semiconductor substrate having first and second opposed surfaces, wherein the semiconductor substrate is formed of an indirect band gap semiconductor;a first mirror formed on the first surface of the semiconductor substrate;a fixed transmission grating positioned adjacent to the first mirror formed on the first surface of the semiconductor substrate; anda probe beam source positioned to provide a probe beam that is incident on the second surface of the semiconductor substrate, passes through the semiconductor substrate, and is reflected back therethrough by the first mirror,wherein radiation from an external source passing through the fixed transmission grating is thereby modulated the modulate radiation passing through the first mirror into the semiconductor substrate and producing a transient radiation induced grating therein, andwherein a portion of the probe beam passing through the semiconductor substrate is diffracted out of the probe beam by the transient grating. 26. The converter of claim 25, wherein e indirect band gap semiconductor is selected from Si and Ge. 27. A method of converting radiation from an external source to optical radiation, comprising:directing radiation from the external source onto a fixed transmission grating to modulate the radiation;passing the modulated radiation into a semiconductor substrate to produce a transient induced grating therein, wherein the semiconductor substrate is formed of a direct band gap semiconductor;directing a probe beam into the semiconductor substrate so that it travels twice through the semiconductor substrate and interacts with the transient grating to diffract a portion of the probe beam, wherein the probe beam travels twice through the semiconductor substrate b being reflected off a first minor on the substrate; andimaging diffracted portions of the probe beam. 28. The method of claim 27, further comprising forming the semiconductor substrate of a proton or neutron or ion damaged direct band gap semiconductor. 29. The method of claim 27, further comprising tuning the probe beam to just below the band gap of the semiconductor substrate. 30. The method of claim 27, further comprising providing a reference optical grating on the semiconductor substrate, the reference optical grating having the same spatial frequency and phase as the fixed grating. 31. The method of claim 27, further comprising providing a second minor on the opposed surface of the substrate from the first mirror to form an etalon. 32. The method of claim 31, further comprising providing a reference optical grating on the semiconductor substrate, the reference optical grating having the same spatial frequency and phase as the fixed grating.
051695669	description	DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides novel cementitious contaminant barriers useful for containment of waste materials including highly toxic and radioactive waste materials. The cementitious contaminant barriers within the scope of the present invention are formed by positioning a hydraulic cement composition and one or more getters into a predetermined configuration and then hydrating the cement composition. The getters are capable of binding or absorbing undesirable liquids, ions, or gases which externally penetrate the barrier or which internally leak from contained waste material surrounded by the barrier. The contaminant barriers within the scope of the present invention provide a barrier which separates a contaminated environment from an uncontaminated environment. The present invention includes in situ barriers for isolating large volumes of waste materials, as well as smaller waste containers. Such containers may be divided into two general categories: (1) preformed containers having lids; and (2) containers which are prepared by compressing hydraulic cement and at least one getter around waste material and allowing at least a portion of the hydraulic cement to hydrate. Containers within each of the general categories above may include contaminant barriers prepared with (a) a mixture of powdered hydraulic cement and a single getter; (b) two or more getters mixed with a powdered hydraulic cement composition; (c) one or more getters packed into a homogeneous layer adjacent to a layer of packed hydraulic cement; and (d) two or more getter layers adjacent one or more hydraulic cement layers. Referring now to FIG. 1, one possible contaminant barrier within the scope of the present invention is illustrated in the form of a waste container. Waste container 10 includes a mixture 12 of at least one liquid, ion, or gas getter and powdered hydraulic cement. The mixture 12 is compressed around solid hazardous waste 14. The outer surface layer 16 of the cement mixture is subsequently hydrated. The average thickness of outer surface layer 16 may vary from as little as 0.001 inches to as much as 100 inches. In most cases, the thickness will range from about 0.25 inches to about 3 inches. Desired strength characteristics often dictate the thickness of the hydrated outer surface layer. In some cases, natural water vapor in the atmosphere may hydrate a thin outer surface layer prior to depositing the waste container in an underground storage site. More complete hydration would then occur over the years as ground water contacts the waste container. Although the waste container shown in FIG. 1 is generally spherical in shape, it will be appreciated that the waste containers within the scope of the present invention may be prepared in a variety of different shapes. For instance, triangular, rectangular, hexagonal, and many other geometric cross-sectional configurations may be used. These cross-sectional configurations enable waste containers to be packed together more efficiently than cylindrical waste containers for transportation and final storage of the waste containers. Waste containers within the scope of the present invention may also be prepared by compressing powdered hydraulic cement around the solid hazardous waste and thereafter applying a layer of cement paste over the compressed powdered hydraulic cement. Aggregates, such as fibers, may be added to the powdered hydraulic cement or to the cement paste to provide desired mechanical properties. It is also within the scope of the present invention to compress a getter layer containing one or more getters around waste material, compress a cement layer containing a powdered hydraulic cement composition around the getter layer, and then hydrate the cement composition. Any number of cement layers and getter layers can be included in such a containment system. FIG. 2 illustrates one possible waste container 20 within the scope of the present invention having a getter layer 22 surrounding a quantity of waste material 24. Outer wall 26 includes substantially hydrated cement. FIG. 3 illustrates another possible cementitious contaminant barrier configuration 30 in which multiple layers of liquid, ion, or gas getters 32A and 32B are mixed within a hydraulic cement composition layer 34. Contaminant barriers within the scope of the present invention may be prepared with a single barrier layer comprising a mixture of one or more liquid, ion, or gas getters and a hydraulic cement composition. The outer surface of this single layer would then be partially hydrated to provide mechanical strength. Aggregates may also be incorporated into one or more hydraulic cement layers of the contaminant barriers within the scope of the present invention to obtain desired structural or mechanical characteristics. In those embodiments in which the powdered hydraulic cement is compressed around the waste materials, the void space within the waste container is substantially reduced. The waste materials are essentially "precrushed" inside the container walls. In this pre-stabilized, high density, condition, the waste containers can be made so that the whole is much closer to equilibrium with the ground without the need for further compaction, grouting, or sealing. In the case where the waste containers are buried in underground vaults, the fewer number of void spaces within the waste containers enables the ground to reach equilibrium density faster when the underground storage room collapses. In addition, the problems with ground water seeping into void spaces are reduced. It is also within the scope of the present invention to form a hollow waste container and later add the waste materials to the container. FIG. 4 illustrates another possible hazardous waste container within the scope of the present invention. The waste container 40 shown in FIG. 4 has a removable lid 42. The contaminant barrier forming the container wall includes a getter layer 44 having at least one liquid, ion, or gas getter. A layer of hydrated cement 46 provides mechanical strength to the waste container. The lid 42 serves to seal the container after a quantity of hazardous waste is placed inside the container. The lid preferably includes a getter layer 44 to prevent passage of contaminants through the lid. It is anticipated that cement paste or mortar may be used to seal the lid 42 with the container 40. In this way, the lid and container are bonded together. The cement paste preferably includes the getters included within the container wall. 1. Liquid, Gas, and Ion Getters According to the present invention, the cementitious contaminant barriers include at least one getter. As defined above, getters are materials which adsorb, absorb, chemically react, ionically bond, trap, attract, or otherwise bind to selected liquid, gases, or ions. The getters may be mixed with a powdered hydraulic cement composition prior to forming the contaminant barrier. In other cases, one or more getter layers may be used to form the contaminant barrier. As applied to the present invention, getters, including zeolites, layered clays, and similar compounds, are included in the cementitious contaminant barrier to remove contaminants which might leach from waste material or otherwise penetrate the cementitious barrier. In those cases where many different types of contaminants may need to be isolated by the cementitious barrier, then more than one getter may be required to adequately contain the contaminants. A few examples of common getters which may be used in connection with the present invention are listed in Table 1. TABLE 1 ______________________________________ Liquid, Ion, or Gas Getter Liquid, Ion, or Gas ______________________________________ Unhydrated cement CO.sub.2, H.sub.2 O Zeolite A Tl, Ag, Sr, Zn, Cd, Hg, Pb, Ba Zeolite F NH.sub.4.sup.+, nitrogen containing organics Zeolite X Fe, Cr, U, Lanthanides, Actinides Zeolite AgX Iodides, CH.sub.3 I, HI Zeolite PbX Iodides, CH.sub.3 I, HI Zeolite Y Organics, Ni, Co, Cr, Fe, transuranics Chabazite (AW500) Tl, K, Ag, Rb, NH.sub.4.sup.+, Pb, Na, Ba, Sr Mordenite (Zeolon) Cs, Co, Cr, Fe, NO.sub.x Faujasite (Y) Rare earths and transuranium, I Clinoptilolite NH.sub.4.sup.+, Sr, Cs Silicalite Organics ZSM-5 Organics Na.sub.3 PO.sub.4 Di and trivalent ions Zeolite/Pd,Cu,Ag H.sub.2 FeTi(H.sub.x) H.sub.2 LaNi.sub.5 (H.sub.x) H.sub.2 LaNi.sub.4.7 Al.sub.0.3 (H.sub.x) H.sub.2 Ca(OH).sub.2 CO.sub.2 Kaolinite Cs, Rb, polar organics Smectite Hg, Pb, Tl, Lanthanides, Actinides Vermiculite Hydrotalcite SeO.sub.4.sup. 2-, MoO.sub.4.sup.2-, V.sub.10 O.sub.28.s up.6-, Mo.sub.7 O.sub.24.sup.6-, CO.sub.3.sup.2-, I.sup.- Montmorillonite Na.sub.x Al.sub.(2-x) Mg.sub.x Si.sub.4 O.sub.10 (OH).sub.2, Pb, Cs, amines, benzene, alcohols, ketones, glycols, large functionalized organics Mica (phlogopite) Cs, Sr ______________________________________ Zeolites are an important class of getters used within the scope of the present invention. Zeolites are aluminosilicate framework minerals having a general formula: EQU M.sub.x/n.sup.+n [Al.sub.x Si.sub.y O.sub.2x+2y ].sup.-x.zH.sub.2 O Where n is the cation valence. They are characterized by their open structures that permit exchange of ions or molecules. Both natural and synthetic zeolites find wide application as ion exchangers, adsorbents, and catalysts. The ion exchange and molecular sieve properties of zeolites make them important in a variety of filtering processes. One important use of zeolites is the removal of radioactive cesium and strontium from waters contaminated with these elements. Because of their differing structures, particular zeolites can be used as molecular sieves to capture molecules of different sizes and shapes. As applied to the present invention, zeolites may also be used as getters for ions, liquids, organics, and gasses such as H.sub.2 and CO.sub.2. Mixtures of different zeolites and clays may be used to prevent a wide variety of different ions and molecules from escaping the waste container. The following are a few common zeolites which may be used within the scope of the present invention: Zeolite A has the following typical oxide formula: Na.sub.2 O. Al.sub.2 O.sub.3.2SiO.sub.2.4.5H.sub.2 O. Zeolite A has a highly charged framework which will selectively adsorb thallium, silver, strontium, zinc, cadmium, mercury, lead, and barium over calcium. For example, .sup.90 Sr, a common nuclide in nuclear explosion fallout with a half life of 28 years is preferentially removed in the presence of calcium ions. This zeolite is not effective for trivalent ions. Zeolite X has the following typical oxide formula: Na.sub.2 O.Al.sub.2 O.sub.3.2.5SiO.sub.2.6H.sub.2 O. Zeolite X is preferred for higher valent ions such as iron, chromium, uranium, lanthanides, and actinides. The ion exchange of the actinides is irreversible if the temperature is above .about.85.degree. C. Neither Zeolite A nor Zeolite X is stable in acidic environments with protons as the cations. Zeolite Y has the following typical oxide formula: Na.sub.2 O.Al.sub.2 O.sub.3.4.8SiO.sub.2.8.9H.sub.2 O. Zeolite Y has more silicon in the framework (Si/Al.about.3). As a result, it is more stable in acidic conditions. Cation exchange tends to be faster but less complete than for zeolite X. The same cations can be exchanged. Aluminum can be removed from the framework to give increasingly acid stable structures, but with less ion exchange capacities. However, the higher silicon compositions selectively remove organics from water. Chabazite has the following typical oxide formula: CaO.Al.sub.2 O.sub.3.4SiO.sub.2.6.5H.sub.2 O. Chabazite is important because calcium is less selectively exchanged than for the above zeolites. For example, the following cation selectivity has been reported: EQU T1.sup.30 >K.sup.+ >Ag.sup.+ >Rb.sup.+ >NH.sub.4.sup.30 >Pb.sup.2+ >Na.sup.+ =Ba.sup.2+ >Sr.sup.2+ >Ca.sup.2+ >Li.sup.30. Chabazite can be made in the acid form. Mordenite has the following typical oxide formula: Na.sub.2 O.Al.sub.2 O.sub.3.10SiO.sub.2.6H.sub.2 O. Mordenite, also known as Zeolon, has been used to isolate cesium 137 and radioactive strontium (.sup.90 Sr). The hydrogen form selectively absorbs NO.sub.x. Mordenite can be made in the acid form. Clinoptilolite has the following typical oxide formula: (Na.sub.2,K.sub.2)O.Al.sub.2 O.sub.3.10SiO.sub.2.8H.sub.2 O. Clinoptilolite has a strong preference for ammonium ions and also prefers strontium over calcium. High silica content molecular sieves, such as silicalite and ZSM-5, may also be used in acidic environments. They do not have high exchange capacities for cations, but will adsorb organic molecules. Boron can be substituted for silicon to give a molecular sieve with a large neutron capture cross section. Arsenates, iodates, sulfides, sulfates, selenides, selenates, and fluorides are anions, and in general, will not be selectively adsorbed by zeolites unless they react with a cation, such as lead, which is already within the zeolite framework to form a substantially insoluble compound. These compounds can also be absorbed in hydrotalcite clays as well. Mixtures of different zeolites may be used to absorb a wider variety of hazardous waste ions and gases than using a single zeolite or layered clay material. Mixtures of zeolites and/or clays can improve the efficiency of "getting" or sieving out specific hazardous substances. The following combinations of zeolites are a few currently preferred zeolite mixtures. It should be noted that due to the large concentrations of calcium present when the zeolite mixtures are combined with hydraulic cement compositions, it is possible a "mass action" effect could in some cases overwhelm the selectivity of the noted zeolites for other ions. ______________________________________ Zeolite or Molecular Sieve Ion or Gas ______________________________________ Combination A 1. Mordenite (Zeolon) .sup.137 Cs 2. Chabazite (AW500) Sr selectivity over Ca 3. Faujasite (Y) Rare earths and trans- uranium, Hg 4. Linde 5A Tl, Pb Combination B 1. NaZSM5 (high silica) Organic residues and/or Silicate (optional) 2. Zeolite X, Organic radioactive Zeolite Y, or thermal decomposition Zeolite A with H.sub.2 "getter" 3. Clinoptilolite or NH.sub.3 /NH.sub.4.sup.+ decomposition from Zeolite F (Linde) nitrogen residues. 4. Linde 5A Tl, Pb Combination C 1. Zeolon (large pore mordenite) .sup.137 Cs 2. Zeolite X .sup.90 Sr .fwdarw. Anorthite with vitrification 3. Chabazite (AW500) .sup.90 Sr 4. Linde 5A Tl, Pb ______________________________________ Anorthite, referred to above, is known as an "early condensate." Early condensates are formed at high temperatures and subsequently suffer little loss by chemical change or decomposition. The terrestrial abundance of the early condensates is similar to their cosmic abundance. Examples include iron with 12% nickel, which condenses at 1500.degree. K.; diopside, CaMgSi.sub.2 O.sub.6, which condenses at 1450.degree. K.; and anorthite, CaAl.sub.2 Si.sub.2 O.sub.8, which condenses at 1350.degree. K. It is for this reason that Fe, O, Mg and Si make up more than 90% of the earth. The elements Ca, Al, Ni and S add up to another 6 to 7 percent. Zeolite X, one of the most open zeolites, can be easily exchanged with Ca or Sr and directly converted by vitrification to the very stable anorthite phase Ca(Sr)Al.sub.2 Si.sub.2 O.sub.8. Selective exchange for Sr followed by condensation to anorthite at elevated temperatures may be a useful way to deal with .sup.90 Sr decay which generates considerable heat. ______________________________________ Zeolite or Molecular Sieve Ion or Gas ______________________________________ Combination D 1. Zeolon (large pore mordenite) .sup.137 Cs 2. Zeolite AgY CH.sub.3 I, I.sub.2, .sup.129 I 3. Zeolite PbX HI 4. Linde 5A Tl, Pb ______________________________________ It will be appreciated that many other possible combinations of zeolites and clays may be used depending on the particular contaminants of interest and the desired barrier efficiency. Layered Clays are another important class of getters used within the scope of the present invention. A common structural feature in the layered clays is one or more hexagonal sheets of linked MO.sub.4 (M=Si, Al) tetrahedra and one or more sheets of linked M'O.sub.6 (M'=Mg, Ca, Al, Fe) octahedra. These sheets are co-condensed in a variety of ways to form the various layered clays. The layered clays can be used to absorb large organic molecules, gases, cations, and even anions under certain conditions. Large molecules can be absorbed in layered clays because of the large pore openings in the layered structure. Layered clays have been used to trap highly toxic organic compounds such as dioxins. The following are a few common layered clays which are typical of those used within the scope of the present invention: Kaolinite has the following typical oxide formula: Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4. Kaolinite is a two-layered sheet clay composed of a SiO.sub.4 tetrahedra sheet and an AlO.sub.6 octahedra sheet which is useful for the absorption of polar organic molecules. Smectite and Vermiculite has the following typical oxide formula: Al.sub.2 Si.sub.4 O.sub.10 (OH).sub.2. Smectite and Vermiculite have three layer sheets with one octahedra layer between two tetrahedral layers. Substitutions of Mg.sup.2+, Ca.sup.2+, and Fe.sup.3+ for aluminum can be used to change the charge of the framework and structural features. Hydrotalcite has the following typical oxide formula: (Mg.sub.4 Al.sub.2 (OH).sub.12)A.sup.n- (A.sup.n- =anion). Hydrotalcite is an unusual anion exchanger clay which can be modified to adsorb various negatively charged ions. The anions can be large metal oxide anionic aggregates, simple inorganic anions such as selenate (SeO.sub.4.sup.2-) or organic in nature. Some gas getters may be specifically included within the contaminant barriers of the present invention for the purpose of trapping certain gasses that might be generated by waste materials. Gaseous formation of hydrogen and carbon dioxide from organics and metals in radioactive and other hazardous wastes and waste containers is a serious problem. Of course, unhydrated cement will act as a CO.sub.2 getter, but some zeolites such as hydroxy cancrinite, and some nonzeolite compounds such as hydrotalcite clays may also be included in the hazardous waste container to function as CO.sub.2 getters. Zeolites impregnated with palladium may be used to adsorb and remove hydrogen gas. Palladium is one of the most effective hydrogen getters known in the art. Other compounds such as FeTi(H.sub.x) and LaNi.sub.5 (H.sub.x) are also good hydrogen getters at high pressure. FeTi(H.sub.1.2) is formed by trapping 0.1 grams H.sub.2 /ml which is greater than the density of liquid hydrogen (0.07 grams H.sub.2 /ml). LaNi.sub.5 (H.sub.6) is formed by trapping 0.09 grams H.sub.2 /ml. 2 Engineering the Cementitious Contaminant Barrier Before selecting specific getters to be included in a cementitious contaminant barrier, the waste material to be contained must be identified. The waste material is preferably assayed and characterized to determine the nature and quantity of contaminants per unit mass. For example, a low-level radioactive waste form may consist of 50 liters of soil containing only 1 gram of mercury or radioactive cesium that causes the entire mass to be classified as hazardous. Specific hazardous materials that have a particularly high toxicity and/or a propensity or probability of diffusion or leaching through the containment barrier are preferably further identified and characterized. The potential of generating and diffusing or leaching a maximum amount of each contaminant of interest per unit mass of waste is determined. A cementitious contaminant barrier is then engineered and fabricated with one or more getters disbursed therein (either randomly or in layers) having the capacity of trapping the maximum quantity of potential contaminants of interest. Thus, getters are selected for inclusion in cementitious contaminant barrier depending on the type and quantity of contaminants for which there is concern of diffusion or leaching through the barrier. Once the getter is selected, the amount of the getter necessary to trap specific contaminants must be calculated. It will be appreciated that those skilled in the art would be capable of calculating the amount getter required by taking into consideration the molecular or formula weight of the getter, the amount of potential contaminant, and the getter efficiency. The ultimate fabrication method and design based used will depend upon the economics of fabrication which include the manufacturing process costs and the getter costs. The following examples are offered to illustrate how to calculate the amount of getter to be included in a cementitious barrier in order to trap a given amount of contaminant material These examples are intended to be purely exemplary and should not be viewed as a limitation on any claimed embodiment. EXAMPLE 1 This example calculates the amount of hydrogen getter required to absorb hydrogen generated by five kilograms of hazardous waste material within 10 cubic feet of mass. It is assumed the hazardous waste material is 5% hydrogen by weight. The hydrogen gas (H.sub.2) is preferably converted to a stable hydride or hydroxide at low temperature. One possible mechanism for converting the H.sub.2 is the use of palladium and/or silver cations highly dispersed in a zeolitic framework. The palladium is preferably loaded into the zeolite by ion exchange as Pd(NH.sub.3).sub.4.sup.+2. All molecular sieves or zeolites containing ten or twelve rings (channel openings with ten or twelve oxygen atoms coordinated to ten or twelve main group element atoms, respectively) are suitable for this method of exchange. For smaller pore zeolites, ion exchange using aqueous solutions of halide salts of palladium or dry impregnation (incipient wetness) is used. Coexchange with transition metals such as cobalt or iron is used to enhance the dispersion of the palladium(0) phase. In this example, sodium zeolite Y, Na.sub.56 (AlO.sub.2).sub.56 (SiO.sub.2).sub.136, is prepared by conventional hydrothermal crystallization (see D. W. Breck and E. M. Flanigen, "Molecular Sieves," Soc. Chem. Ind., London 1968), p. 47 and H. Kacirek and H. Lechert, J. Phys. Chem. 1975, vol. 79, p. 1589) or purchased commercially (Linde LZ-Y52). After aqueous cation exchange with Pd(NH.sub.3).sub.4.sup.+2 (0.01M) the zeolite sample is washed, filtered, and subsequently dehydrated in a flow of oxygen (570.degree.-870.degree. K.) at a heating rate of 1.degree. K./min to form the active absorber. The prepared zeolite has the following chemical analysis: Pd.sub.13.7 Na.sub.28.5 [(AlO.sub.2).sub.56 (SiO.sub.2).sub.136 ], Formula Weight, 13589. With 5000 gm hazardous waste which is 5% hydrogen (250 gms), 250 moles of hydrogen atoms are potentially generated. One mole of the above molecular sieve contains 13.7 moles of Pd.sup.2+ which will potentially convert 27.4 moles of hydrogen atoms to hydrogen ions. Assuming an efficiency of 50%, then 250/13.7=18.3 moles or 248.7 kg of zeolite are needed to convert the hydrogen. This corresponds to 106.4 * 250=26.6 kg of palladium are needed. EXAMPLE 2 This example calculates the amount of hydrogen getter required to absorb hydrogen generated by five kilograms of hazardous waste material within 10 cubic feet of mass according to the procedure of Example 1, except that iron or cobalt is further substituted for the sodium in the sodium zeolite Y. This has been demonstrated to increase the room temperature reactivity of palladium with hydrogen (see Karin Moller and Thomas Bein, J. Phys. Chem. 1990, vol. 84, p. 845; K. Moller and T. Bein, "Studies in Surface Science and Catalysis, Zeolite: Facts, Figures, Future," P. A. Jacobs and R. A. van Santen, Eds., Elsevier, Amsterdam, Vol. 49, p. 985, 1989). The zeolite is first exchanged with iron, then dehydrated under oxygen at 623.degree. K. under oxygen, and then ion exchanged with Pd(NH.sub.3).sub.4.sup.+2 (0.01M). Alternatively, ion exchange procedure can be reversed. The resulting zeolite has the following chemical analysis: Na.sub.29 Fe.sub.3.8 Pd.sub.10 AlO.sub.2).sub.56 (SiO.sub.2).sub.136 (Formula weight 13418). With the same assumptions as in Example 1, 25 moles or 335.5 kg of zeolite containing the same amount of palladium (26.6 kg) is required to convert the hydrogen. EXAMPLE 3 This example calculates the amount of hydrogen getter required to absorb hydrogen generated by five kilograms of hazardous waste material within 10 cubic feet of mass according to the procedure of Example 1, except that silver and copper exchanged zeolites are used. The silver zeolitic phase undergoes reduction with hydrogen below 100.degree. C. (see H. K. Beyer and P. A. Jacobs in "Metal Microstructures in Zeolites", ed. P. A. Jacobs, et al., Elsevier, Amsterdam, p. 95, 1982.) The copper zeolite is reduced below 200 C, although the lower limit is not well established. This is readily done with small pore size zeolites, such as zeolite A, as well as zeolite X or Y. Ag.sub.12 [AlO.sub.2).sub.12 (SiO.sub.12)], zeolite A, or Ag.sub.36 Cu.sub.10 (AlO.sub.2).sub.56 (SiO.sub.2).sub.136, zeolite Y, compositions can be used. The latter has a formula weight of 15997. For conversion of 250 moles of hydrogen, 500/36=13.9 moles (222.2 kg) of zeolite are required at 50% efficiency. In this case, the copper may assist in the reduction, but has not been included as a backup factor. The amount of silver needed is 500 moles or 53.95 kg. EXAMPLE 4 This example calculates the amount of ion getter required to absorb mercury contained in five kilograms of hazardous waste material within 10 cubic feet of mass. It is assumed the hazardous waste material contains 1% Hg.sup.2+. In this example, sodium zeolite A is prepared by conventional hydrothermal crystallization or purchased commercially (Linde 5A). Sodium zeolite A has the following typical oxide formula: Na.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2.4.5H.sub.2 O, with a formula weight of 365. One mole of the above molecular sieve contains one mole of Na.sub.2 O which will potentially be replaced by one mole of Hg.sup.2+. Assuming an efficiency of 50%, then (0.25 moles Hg.sup.2+)/(0.5 moles Hg.sup.2+ converted per mole zeolite A)=0.5 moles or 182.5 grams of zeolite A are needed to convert the mercury. 3. Powdered Hydraulic Cements The family of cements known as hydraulic cements used in the present invention is characterized by the hydration products that form upon reaction with water. It is to be distinguished from other cements such as polymeric organic cements. The term powdered hydraulic cement, as used herein, includes clinker, crushed, ground, and milled clinker in various stages of pulverizing and in various particle sizes. The term powdered hydraulic cement also includes cement particles which may have water associated with the cement; however, the water content of the powdered hydraulic cement is preferably sufficiently low that the cement particles are not fluid. The water to cement ratio is typically less than about 0.25. Examples of typical hydraulic cements known in the art include the broad family of Portland cements (including ordinary Portland cement without gypsum), calcium aluminate cements (including calcium aluminate cements without set regulators, e.g., gypsum), plasters, silicate cements (including .beta. dicalcium silicates, tricalcium silicates, and mixtures thereof), gypsum cements, phosphate cements, magnesium oxychloride cements, as well as mixtures of hydraulic cements. Hydraulic cements generally have particle sizes ranging from 0.1 .mu.m to 100 .mu.m. The cement particles may be gap-graded and recombined to form bimodal, trimodal, or other polymodal systems to improve packing efficiency. For example, a trimodal system having a size ratio of 1:5:25 and a mass ratio of 21.6:9.2:69.2 (meaning that 21.6% of the particles, by weight, are of size 1 unit and 6.9% of the particles, by weight, are of size 5 units and 69.2% of the particles, by weight are of size 25 units) can theoretically result in 85% of the space filled with particles after packing. Another trimodal system having a size ratio of 1:7:49 and a mass ratio of 13.2:12.7:66.1 can result in 88% of the space filled with particles after packing. In yet another trimodal system having the same size ratio of 1:7:49 but a different mass ratio of 11:14:75 can result in 95% of the space filled with particles after packing. It will be appreciated that other particle size distributions may be utilized to obtain desired packing densities. A bimodal system having a size ratio of 0.2:1 and a mass ratio of 30:70 (meaning that 30% of the particles, by weight, are of size 0.2 units and 70% of the particles, by weight, are of size 1 unit) can theoretically result in 72% of the space filled with particles after packing. Another bimodal system having a size ratio of 0.15:1 and a mass ratio of 30:70 can result in 77% of the space filled with particles after packing. 4. Pressure Compaction Processes Many of the general principles regarding pressure compaction of powdered hydraulic cement as well as various techniques for hydrating packed hydraulic cement are discussed in copending patent application Serial No. 07/526,231, filed May 18, 1990, in the names of Hamlin M. Jennings and Simon K. Hodson and entitled "HYDRAULICALLY BONDED CEMENT COMPOSITIONS AND THEIR METHODS OF MANUFACTURE AND USE," which was incorporated reference above. The compressing of powdered hydraulic cement within the scope of the present invention is not to be confused with prior art processes which mold and shape cement pastes. As used herein, the term "cement paste" includes cement mixed with water such that the hydration reaction has commenced in the cement paste. Cement pastes are continuous, fluid mixtures having a measurable viscosity. Pressure compaction processes, such as dry pressing and isostatic pressing, may be used to compress powdered hydraulic cement and getters in the form of waste containers described above. Dry pressing consists of compacting powders between die faces in an enclosed cavity. Pressures can range from about 500 psi to greater than 100,000 psi in normal practice. In some cases, additives are mixed with the powdered hydraulic cement to make molding easier and to provide sufficient strength so that the article does not crumble upon removal from the press. Suitable additives preferably neither initiate hydration nor inhibit later hydration of the hydraulic cement. Grading the cement particles and getters, as discussed above, may also provide a certain fluidity to the cement powder and getters during compressing. Because cement particles are formed by crushing and grinding larger cement clinker pieces, the individual particles have rough edges. Rounding the edges of the cement particles enhances their ability to slide over each other, thereby improving the packing efficiency of the cement particles. Techniques for rounding cement particles known in the art may be used. Some of the air enclosed in the pores of the loose cement powder and getter has to be displaced during pressing. The finer the mix and the higher the pressing rate, the more difficult the escape of air. The air may then remain compressed in the mix. Upon rapid release of the pressure, the pressed piece can be damaged by cracks approximately perpendicular to the direction of pressing. This pressure lamination, even though almost imperceptible, may weaken the resulting product. This problem is usually solved by repeated application of pressure, by releasing the pressure more slowly, or by creating a vacuum before pressing. Isostatic pressing is another powder pressing technique in which pressure is exerted uniformly on all surfaces of the cement article being formed. The method is particularly suitable in forming of symmetric shapes, and is similarly employed in the shaping of large articles which could not be pressed by other methods. In practice, the powdered mix is encased in a pliable rubber or polymer mold. The mold is then preferably sealed, evacuated to a pressure between 0.1 atm and 0.01 atm, placed in a high-pressure vessel, and gradually pressed to the desired pressure. An essentially noncompressible fluid such as high-pressure oil or water is preferably used. Pressures may range from 1000 psi to 100,000 psi. The forming pressure is preferably gradually reduced before the part is removed from the mold. Vibrational compaction techniques, as described more fully in copending patent application Ser. No. 07/526,231, may be used to help pack the hydraulic cement composition and getter into molds and into in situ barrier configurations. In vibrational compaction processes, the powdered hydraulic cement particles and getter particles are typically compacted by low-amplitude vibrations. Inter-particle friction is overcome by application of vibrational energy, causing the particles to pack to a density consistent with the geometric and material characteristics of the system and with the conditions of vibration imposed. Packed densities as high as 100% of theoretical are possible using vibration packing processes. As used herein, the term "theoretical packing density" is defined as the highest conceivable packing density achievable with a given powder size distribution. Hence, the theoretical packing density is a function of the particle size distribution. Vibration packing processes may also be combined with pressure compaction processes to more rapidly obtain the desired packing densities or even higher packing densities. Typical vibration frequencies may range from 1 Hz 20,000 Hz, with frequencies from about 100 Hz to about 1000 Hz being preferred and frequencies from about 200 Hz to about 300 Hz being most preferred. Typical amplitudes may range from about one half the diameter of the largest cement particle to be packed to about 3 mm, with amplitudes in the range from about one half the diameter of the largest cement particle to about 1 mm. If the amplitude is too large, sufficient packing will not occur. Once the amplitude is determined, the frequency may be varied as necessary to control the speed and rate of packing. For particle sizes in the range from 0.1 .mu.m to 50 .mu.m, the vibration amplitude is preferably in the range from about 10 .mu.m to about 500 .mu.m. Although it is not necessary to have a specific particle size distribution in order to successfully use vibrational compaction processes, carefully grading the particle size distribution usually improves compaction. 5. Aggregates and Composite Materials It is within the scope of the present invention to include aggregates commonly used in the cement industry with the powdered hydraulic cement prior to hydration. Examples of such aggregates include sand, gravel, pumice, perlite, and vermiculite. One skilled in the art would know which aggregates to use to achieve desired characteristics in the final cementitious waste container. For many uses it is preferable to include a plurality of differently sized aggregates capable of filling interstices between the aggregates and the powdered hydraulic cement so that greater density can be achieved. In such cases, the differently sized aggregates have particle sizes in the range from about 0.01 .mu.m to about 2 cm. In addition to conventional aggregates used in the cement industry, a wide variety of other fillers, fibers, and strengtheners, including balls, filings, pellets, powders, and fibers such as graphite, silica, alumina, fiberglass, polymeric fibers, and such other fibers typically used to prepare composites, may be combined with the powdered hydraulic cement prior to hydration. It is also within the scope of the present invention to use frozen ice and dry ice as aggregates, which upon hydration of the hydraulic cement composition, leave voids dispersed throughout the waste container. The voids act as crack attenuators and improve ductility. The use of ice, dry ice, and other similar aggregates in cement compositions is discussed in greater detail in copending patent application Ser. No. 07/565,602 which is incorporated herein by specific reference. 6 Cement Hydration Techniques a. Cement Hydration in General The term hydration as used herein is intended to describe the chemical reactions that take place between the cement and water. The chemistry of hydration is extremely complex and can only be approximated by studying the hydration of pure cement compounds. For simplicity in describing cement hydration, it is often assumed that the hydration of each compound takes place independently of the others that are present in the cement mixture. In reality, cement hydration involves complex interrelated reactions of the each compound in the cement mixture. With respect to Portland cement, the principal cement components are about 55% tricalcium silicate (3CaO.SiO.sub.2, also referred to as C.sub.3 S), about 25% dicalcium silicate (2CaO.SiO.sub.2, also referred to as C.sub.2 S), about 10% tricalcium aluminate (3CaO.Al.sub.2 O.sub.3, also referred to as C.sub.3 A), and about 8% tetracalcium aluminoferrite (4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3, also referred to as C.sub.4 AF). In addition, some minor components are also present in Portland cement. The hydration reaction of the two silicates with water produces calcium silicate hydrates (C-S-H) and calcium hydroxide. The C-S-H make the largest contribution to the strength of the hydrated cement. Tricalcium aluminate also forms a hydrate, but it contributes little to the strength of the cement. Moreover, the hydration reaction of tricalcium aluminate is so rapid that it has to be controlled by gypsum. The presence of tricalcium aluminate is, however, advantageous in the preparation of Portland cement. Tetracalcium aluminoferrite is not particularly important except that it contributes to the characteristic gray color of Portland cement. If a white cement is desired, the presence of tetracalcium aluminoferrite has to be kept down to about 1 percent. On first contact with water, C and S dissolve from the surface of each C.sub.3 S grain, and the concentration of calcium and hydroxide ions rapidly increases. The pH rises to over 12 in a few minutes. The rate of this hydrolysis slows down quickly but continues throughout a dormant period. After several hours under normal conditions, the hydration products, CH and C-S-H, start to form rapidly, and the reaction again proceeds rapidly. Dicalcium silicate hydrates in a similar manner, but is much slower because it is a less reactive compound than C.sub.3 S. For additional information about the hydration reactions, reference is made to F. M. Lea, Chemistry of Cement and Concrete, 3rd edition, pp. 177-310 (1970). It has been observed that the better the contact between individual cement particles both before and during hydration, the better the hydration product and the better the strength of the bond between the particles. Hence, the positioning of cement particles in close proximity one to another before and during hydration plays an important role in the strength and quality of the final cementitious waste container. b. Hydration With Gaseous and Liquid Water It is within the scope of the present invention to hydrate the powdered hydraulic cement after the cement particles have been compressed into a hazardous waste container. Hydration is accomplished without mechanical mixing of the cement and water. Thus, diffusion of water (both gaseous and liquid) into the compressed hazardous waste container is an important hydration technique within the scope of the present invention. In most cases, hydration occurs immediately after the container is compressed. In other cases, initial hydration may occur from water vapor in the atmosphere, with a more complete hydration occurring from ground water exposure after the container is placed in underground storage. When hydration is achieved by contacting the cementitious waste container with gaseous water, the gas may be at atmospheric pressure; however, diffusion of the water into the article, and subsequent hydration, may be increased if the gaseous water is under pressure. The pressure may range from 0.001 torr to about 2000 torr, with pressures from about 0.1 torr to 1000 torr being preferred, and pressures from about 1 torr to about 50 torr being most preferred. Even though water vapor is introduced into the cement compact, it is possible that the water vapor may immediately condense into liquid water within the pores of the cement compact. If this happens, then gaseous water and liquid water may be functional equivalents. Atomized liquid water may, in some cases, be used in place of gaseous water vapor. As used herein, atomized water is characterized by very small water droplets, whereas gaseous water is characterized by individual water molecules. Gaseous water is currently preferred over atomized water under most conditions because it can permeate the pore structure of the compressed cementitious container better than atomized water. The temperature during hydration can affect the physical properties of the hydrated cement container. Therefore, it is important to be able to control and monitor the temperature during hydration. Cooling the cement container during hydration may be desirable to control the reaction rate. The gaseous water may also be combined with a carrier gas. The carrier gas may be reactive, such as carbon dioxide or carbon monoxide, or the carrier gas may be inert, such as argon, helium, or nitrogen. Reactive carrier gases are useful in controlling the morphology and chemical composition of the final cementitious container. Reactive carrier gases may be used to treat the hazardous waste container before, during, and after hydration. The partial pressure of the water vapor in the carrier gas may vary from about 0.001 torr to about 2000 torr, with 0.1 torr to about 1000 torr being preferred, and 1 torr to about 50 torr being most preferred. An autoclave may be conveniently used to control the gaseous environment during hydration. It is also possible to initially expose the cement container to water vapor for a period of time and then complete the hydration with liquid water. In addition, the cement container may be initially exposed to water vapor and then to carbon dioxide. Heating the gaseous water will increase the rate of hydration. Temperatures may range from about 25.degree. C. to about 200.degree. C. It should be noted that the temperature at which hydration occurs affects certain physical characteristics of the final cement container, especially if an additional silica source is added. For example, when hydration temperature is greater than 50.degree. C., the formation of a hydrogarnet crystalline phase is observed, and when the hydration temperature is greater than 85.degree. C. other crystalline phases are observed. These crystalline phases, which often weaken the cement structure, are not always desirable. However, in some cases, the pure crystalline phases may be desired. In order to form the pure crystalline phase, it is important to use pure starting materials and to accurately control the hydration temperature. It should be remembered that obtaining a contaminant barrier with high chemical and structural stability may be more important than obtaining mechanical strength when hydrating the powdered hydraulic cement. c. The Effect of Carbon Dioxide on Hydration The inventors have found that when carbon dioxide is introduced during the stages of hydration, significant structural benefits can be realized, such as high strength and reduced shrinkage on drying. These concepts are disclosed in copending patent application Ser. No. 07/418,027, filed Oct. 10, 1989, entitled PROCESS FOR PRODUCING IMPROVED BUILDING MATERIAL AND PRODUCT THEREOF, which is incorporated herein by specific reference. More specifically, as applied to the cementitious contaminant barrier within the scope of the present invention, it has been found that CO.sub.2 can be used to prepare contaminant barriers having improved water resistance, surface toughness, and dimensional stability. These results may be obtained by exposing the contaminant barrier to an enriched CO.sub.2 atmosphere while rapidly desiccating the cement container. For best results, the CO.sub.2 is preferably at a partial pressure greater than its partial pressure in normal air. d. Control of the Aqueous Solution Aqueous solutions may also be used to hydrate the cementitious contaminant barriers and containers within the scope of the present invention. As used herein, the term aqueous solution refers to a water solvent having one or more solutes or ions dissolved therein which modify the hydration of hydraulic cement in a manner different than deionized water. For instance, it is possible to simply immerse the unhydrated cement container in lime water to achieve adequate hydration. Lime water is an aqueous solution containing Ca.sup.2+ and OH.sup.- ions formed during the hydration reactions. Because of the presence of hydroxide ions, lime water typically has a pH in the range from about 9 to about 13. Other aqueous solutions, such as extracts from cement paste, silica gel, or synthetic solutions may be used to hydrate the contaminant barriers of the present invention. Other ions in addition to Ca.sup.2+ and OH.sup.-, such as carbonates, silica, sulfates, sodium, potassium, iron, and aluminum, may also be included in aqueous phase solutions. In addition, solutes such as sugars, polymers, water reducers, and superplasticizer may be used to prepare aqueous solutions within the scope of the present invention. A typical aqueous solution within the scope of the present invention may contain one or more of the following components within the following ranges: ______________________________________ Most Preferred Component Concentration (ppm) Concentration (ppm) ______________________________________ calcium 50-3000 400-1500 silicon 0-25 0.25-5 carbon 0-5000 5-250 iron 0.001-10 0.01-0.2 aluminum 0.001-10 0.01-0.2 sulfur 0-5000 200-2000 sodium 0-2000 400-1500 potassium 0-4000 800-2000 sugars sdr sdr polymers sdr sdr water reducers sdr sdr superplasticizer sdr sdr ______________________________________ Where the term "sdr" refers to the standard dosage rate used in the concrete industry, and where the term "ppm" means the number of component atoms or molecules containing the component compound per million molecules of water. Apparatus capable of monitoring the concentrations of ions in the aqueous solution include pH meters and spectrometers which analyze absorbed and emitted light. e. Addition of Hydrated Crystals It is also within the scope of the present invention to provide the water necessary for hydration from compounds which release water upon mild heating. For instance, many compounds which contain water in a crystalline form, such as gypsum (a hydrated calcium sulphate, CaSO.sub.4.2H.sub.2 O), ettringite (a calcium sulphoaluminate, 3CaO.Al.sub.2 O.sub.3.3CaSO.sub.4.31H.sub.2 O), zeolites and layered clays containing water, and various hydrated crystals such as Na.sub.2 CO.sub.3.10H.sub.2 O, release water when heated to temperatures in the range from about 60.degree. C. to about 120.degree. C. These water-containing compounds are preferably added to the powdered hydraulic cement prior to forming the cementitious contaminant barrier. Subjecting the contaminant barrier to mild heating, typically less than about 100.degree. C., causes water to be released. The water is then capable of partially hydrating the hydraulic cement. High green strengths are obtained using this technique. Cementitious contaminant barriers formed in this manner would also be excellent water getters. 7. Examples of Cementitious Barriers Various cementitious contaminant barriers and their method of manufacture within the scope of the present invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the use of the invention and should not be viewed as a limitation on any claimed embodiment. EXAMPLE 5 In this example, an engineered waste container having a contaminant barrier was prepared. A waste material was identified that had the potential of generating 0.1 moles of hydrogen gas per kilogram of waste by way of oxidation of iron. Twenty kilograms of the waste (having a unit density of 2.0 grams/cm.sup.3) was selected to be contained in a preformed containment system with a separate lid. The container was manufactured to have an interior capacity of 10 liters. The container was in the shape of a box having an interior dimension of approximately 22 centimeters per side. 7.3 kilograms of ordinary Portland cement was combined in a dry powder mixer with 300 grams of LaNi.sub.4.7 Al.sub.0.3. The LaNi.sub.4.7 Al.sub.0.3 has the capacity to absorb up to three moles of hydrogen gas under ambient conditions. The resultant dry powder mixture was placed in a mold. The mold had a latex exterior with an inside steel cubic mandrel having a cross section of 22.times.22 centimeters. The cement/LaNi.sub.4.7 Al.sub.0.3 mixture was placed within the latex mold. The mold was then sealed and placed in an isostatic press and pressurized to 30,000 psi for 30 seconds and released. After removing the "green" container from the mold, it was then immersed in water for 15 minutes, removed, and allowed to cure or hydrate for 24 hours. The containment wall was approximately 1 centimeter thick and had a rated flexural strength of greater than 10,000 psi. The waste was then placed inside, the lid was placed on top and sealed with cement paste as a bonding agent having a 0.30 water to cement ratio and including 5% LaNi.sub.4.7 Al.sub.0.3. EXAMPLE 6 In this example, an engineered waste container having a contaminant barrier is prepared. A waste material is identified that has the potential of generating 0.1 moles of hydrogen gas per kilogram of waste by way of oxidation of iron. One hundred (100) kilograms of the waste, having a unit density of 2.0 grams/cm.sup.3 are selected to be contained in a preformed containment system with a separate screw lid. The container is manufactured to have an interior capacity of 50 liters. The preformed container is designed to be a cylinder with an interior diameter of 21 centimeters and an interior height of 36 centimeters. It is further designed to have an outside diameter of 23 centimeters and an overall height of 38 centimeters. The container is also designed to have a nearly homogenized layer of LaNi.sub.4.7 Al.sub.0.3, a hydrogen gas getter, approximately 0.1 centimeter thick on the interior of the container. The container is formed using the following procedure: An appropriate mold and mandrel having a cross-section of 21 centimeters is selected. 1500 grams of LaNi.sub.4.7 Al.sub.0.3 having an average particle size of 5 microns are uniformly placed in the mold and isostatically pressed to 10,000 psi. The mold is then released, a larger mold housing is selected, and 21.2 kilograms of ordinary white Portland cement are uniformly placed in the mold. The mold and contents are then vibrated for one minute and evacuated. The mold is then sealed and isostatically pressed to 30,000 psi. The "green" container has a total wall thickness of approximately 1.1 centimeters with an interior diameter of 21 centimeters. The container is placed in a chamber which is subsequently evacuated and backfilled with an aqueous solution. The aqueous solution is extracted from cement paste prepared with ordinary Portland cement having a water to cement ratio of 1.0. The container is then allowed to cure for 24 hours. Once the container has cured, the waste material is placed in the container, the preformed screw lid is coated with a 0.3 water to cement ratio cement paste containing 5% LaNi.sub.4.7 Al.sub.0.3, and the lid is screwed into place. EXAMPLE 7 In this example, an engineered waste container having a contaminant barrier was prepared. A waste material was identified that had the potential of generating 0.2 moles of NH.sub.4.sup.+ and leaching a maximum of 0.1 moles of mercury per kilogram of waste. Ten kilograms of this waste were to be contained. The waste had an average density of 1.6 grams/cm.sup.3 after compaction to 20,000 psi. The container was designed to have 400 grams of zeolite A and 850 grams of zeolite F mixed with 4500 grams of ordinary Portland cement. A mold was selected having a cubic shape with an interior dimension of 18 centimeters and an exterior diameter of 20 centimeters. The 10 kilograms of waste were placed in the interior cavity of the mold and 5750 grams of uniformly blended powder were evenly distributed in the exterior cavity of the mold and pressurized to 20,000 psi. Ultrasonic measurement indicated that the resultant container had a uniform cementitious wall of approximately one centimeter. The "green" container wall was formed to an engineered voids content of 19%. The container was hydrated by initially spraying the entire container with an aqueous solution of water, CaOH, and SiO.sub.2. Subsequently, approximately one-third liter of aqueous solution was uniformly sprayed on the exterior of the container in order to create a hydrated bond of the packed wall to an average depth of 0.75 centimeters. Approximately 0.25 centimeters wall thickness was substantially unhydrated. EXAMPLE 8 In this example, an engineered waste container having a contaminant barrier is prepared. A waste material is identified and characterized having 0.2 moles of cesium and 0.05 moles of nickel per kilogram of waste. The cesium and nickel contaminants are identified as being particularly hazardous to the environment in which they were going to be placed. Because the waste has a moisture content of approximately 2%, there is also the possibility that the contaminants could leach out of the waste material. Twenty kilograms of waste are selected for containment having a unit density of 1.6 grams/cm.sup.3. A cementitious cylindrical container is designed having an approximate interior diameter of 20 centimeters and an interior height of approximately 10 centimeters such that the cylinder is capable of containing 12,500 cubic centimeters of hazardous waste material when sealed. The container is manufactured as follows: 1100 grams of Kaolinite and 730 grams of Zeolite Y are mixed together as dry powders, both having an average particle size less than 10 microns. A matrix mold is selected having a diameter of approximately 25 centimeters and a height of 15 centimeters. A cylindrical mandrel is placed inside the mold having a cross-sectional diameter of 20 centimeters. Approximately 2500 grams of ordinary Portland cement are placed in the mold. The mold is then sealed and pressurized in an isostatic press to 10,000 psi. The mold is released and the 1830 gram getter mixture is evenly placed in the cavity around the packed cement layer and the mold. The mold is sealed again and pressurized to 15,000 psi and released. Finally, an additional 2500 grams of cement are placed in the cavity between the getter layer and the exterior mold wall. The mold is sealed and pressurized to 30,000 psi and released. The mandrel is removed and the "green" container has an interior diameter of 20 centimeters and the wall has a cross section of approximately 0.4 centimeters of cement on the outside of the container, a sandwiched getter layer of approximately 0.5 centimeters, and a final layer of 0.4 centimeters of cement as the interior wall of the container. A lid is made using a similar process. The 20 kilograms of waste are placed inside the container. The lid is screwed into place and sealed with a cement paste containing kaolinite and zeolite Y. The entire container with waste is then immersed in water for 15 minutes. The container is removed and allowed to hydrate and form an integral barrier to the hazardous waste and particularly to the cesium and nickel contaminants. EXAMPLE 9 In this example, an engineered waste container having a contaminant barrier is prepared. 150 kilograms of hazardous waste having a moisture content of less than 5% and an iodide content of 0.1 moles per kilogram of waste are selected for containment. The waste has a density of 1.6 grams/cm.sup.3 when compacted at 25,000 psi. The waste occupies a volume of 94 liters in a pressed cylindrical shape having a diameter of 20 centimeters and a height of 75 centimeters. The waste is placed in an appropriately selected latex mold, and a dry powder mixture of 10 kilograms of cement and 9 kilograms of zeolite PbX is placed uniformly around the waste. The mold is sealed and pressurized to 20,000 psi. The mold is then released and opened. 20 kilograms of dry powdered cement is placed around the consolidated mass. The mold is then sealed and pressurized again to 25,000 psi and released. The final wall has a "green" density, as determined by ultrasound, of approximately 2.4 grams/cm.sup.3. Approximately 1.5 liters of water are then uniformly sprayed on the surface of the cylindrical mass. This amount of water is calculated to hydrate approximately 2/3 of a centimeter of the wall leaving approximately 3/4 of a centimeter of unhydrated cement and zeolite to capture any moisture or iodide that would potentially penetrate the container wall. EXAMPLE 10 In this example, an engineered waste container having a contaminant barrier is prepared. A waste material is identified and characterized having 0.1 moles of cesium, 0.1 moles of iron and 0.05 moles of cobalt per kilogram of waste. The waste has a unit density of approximately 2 grams/cm.sup.3. A preformed container having multiple layers of getter and cement is formed using a latex mold and an isostatic press. The inner layer is compressed around a cylindrical mandrel having a diameter of 20 centimeters and a height of 24 centimeters. The inner layer includes approximately 7200 grams of dry white Portland cement (free of gypsum) and is consolidated at 25,000 psi to an approximately thickness of 0.5 centimeters. A getter layer is then pressed onto the inner layer at a pressure of about 25,000 psi. The getter layer includes a pre-blended dry powder mixture of 6000 grams of clinoptilolite, 900 grams of LaNi.sub.4.7 Al.sub.0.3, and 3000 grams of dry gypsum-free cement. An exterior layer is made by uniformly distributing 7200 grams of gypsum-free, white Portland cement between the getter layer and the container mold. The mold is sealed and pressurized to a final pressure of 30,000 psi. The container is then immersed in aqueous solution saturated with lime for ten minutes, removed, and allowed to cure for six hours. The container has a final outside diameter of 23 centimeters and an interior diameter of 20 centimeters. A lid is prepared using a similar process. Approximately 60 kilograms of the waste are placed in the container. A 0.3 water to cement ratio paste is used as a bonding agent to chemically bond the lid to the container and seal the waste in the containment system. EXAMPLE 11 In this example, an engineered waste container having a contaminant barrier is prepared. A pre-processed, mixed hazardous waste form is identified and characterized as having 0.2 moles of radioactive cesium, 0.1 moles of benzene, and 0.5 moles of mixed alcohol per kilogram of waste. The pre-processed waste is placed in a spherical latex mold approximately 42 centimeters in diameter and isostatically compressed to 30,000 psi. Subsequently, a dry mixture of 15 kilograms of montmorillonite, 4.2 kilograms of phlogopite (mica), and 5 kilograms of ordinary Portland cement is uniformly placed around the compacted waste inside the spherical mold. The mold is sealed and pressed to 20,000 psi and released An additional 7 kilograms of bi-modal, gap-graded ordinary Portland cement without gypsum is uniformly placed around the previously compacted spherical mass inside the mold. The mold is sealed and pressed to 25,000 psi and released. The sphere is placed in sea water and allowed to cure underwater. EXAMPLE 12 In this example, a waste container having a contaminant barrier is prepared by encasing hazardous waste with powdered hydraulic cement containing a liquid, ion, or gas getter. Ordinary Portland cement and about 200 grams of zeolite A are mixed. The hazardous waste material is known to include approximately 50 grams of Hg.sup.2+. The hazardous waste and the cement/zeolite mixture are positioned within a pliable polymeric cylindrical mold such that from 1 to 2 inches of the cement/zeolite mixture surrounds the waste material. The cement/zeolite mixture also fills irregularities around the exterior surface of the hazardous waste materials. The waste container is then compressed at a pressure of 30,000 psi. The hazardous waste container is then hydrated by immersing the container in saturated lime water, maintained at a temperature between 22.degree. C. and 25.degree. C. at atmospheric pressure during hydration. Testing to determine leach rates of the cured hazardous waste container show that no measurable amounts of mercury escape the waste container. EXAMPLE 13 In this example, a waste container is prepared by encasing hazardous waste with a getter layer and a cement layer. The hazardous waste material is known to include approximately 50 grams of Hg.sup.2+. Ordinary Portland cement and about 200 grams of powdered zeolite A are used in this example. The hazardous waste and the zeolite are positioned within a pliable polymeric cylindrical mold such that a zeolite layer surrounds the waste material. If necessary, a binder may be used to hold the powdered zeolite together. Binders known to those skilled in the art, including hydraulic cement, may be used. The zeolite layer fills irregularities around the exterior surface of the hazardous waste materials. A layer of the ordinary Portland cement is then positioned around the zeolite layer in the mold. The cement and zeolite layers are isostatically compressed at a pressure of 35,000 psi. The outer surface of the Portland cement layer is then hydrated in immersion saturated lime water, maintained at a temperature between 22.degree. C. and 25.degree. C. at atmospheric pressure during hydration. Testing to determine leach rates of the cured hazardous waste container show that no measurable amounts of mercury escape the waste container. EXAMPLE 14 A waste container is prepared according to the procedure of Example 13, except that the hazardous waste is encased with a layer containing multiple liquid, ion, or gas getters rather than a single getter. The hazardous waste material is known to include a variety of radioactive and nonradioactive hazardous constituents. Mordenite, Chabazite, Faujasite, and Linde 5A are selected as suitable liquid, ion, or gas getters. The amount of each respective getter is calculated according to the general procedure outlined in Examples 1-4 above. EXAMPLE 15 A hazardous waste container is prepared according to the procedure of Example 14, except that the various getters are graded by size to improve packing efficiency. EXAMPLE 16 A hazardous waste container is prepared according to the procedure of Example 14, except that the hazardous waste is encased with multiple layers of liquid, ion, or gas getters rather than a single layer. The hazardous waste material is known to include a variety of radioactive and nonradioactive hazardous constituents including organic residues. Zeolite X, NaZSM5, Clinoptilolite, and Linde 5A are selected as suitable liquid, ion, or gas getters. The amount of each respective getter is calculated according to the general procedure outlined in Examples 1-4 above. The hazardous waste and getters are positioned within a pliable polymeric cylindrical mold such that layer containing zeolite X surrounds the waste material. If necessary, a binder may be used to hold the zeolite X together. Binders known to those skilled in the art, including hydraulic cement, may be used. A second layer containing NaZSM5 is positioned around the zeolite X layer, followed by a third layer containing Clinoptilolite and a fourth layer containing Linde 5A. Finally, a layer of the ordinary Portland cement is then positioned around the getter layers in the mold. The cement and getter layers are isostatically compressed, and the outer cement layer is hydrated. EXAMPLE 17 A waste container is prepared according to the procedure of Example 13, except that the hazardous waste is encased with multiple getter layers having mixtures of different getters. The waste material includes a variety of radioactive and nonradioactive hazardous constituents. Zeolon, zeolite X, chabazite, and Linde 5A are selected as suitable liquid, ion, or gas getters. The amount of each respective getter is calculated according to the general procedure outlined in Examples 1-4 above. The waste material and a first getter layer are positioned within a pliable polymeric cylindrical mold such that the first getter layer, containing a mixture of zeolon and zeolite X surrounds the waste material. If necessary, a binder may be used to hold the getter layer together. A second getter layer containing a mixture of chabazite and Linde 5A is positioned around the first getter layer. Finally, a layer of the ordinary Portland cement is then positioned around the getter layers in the mold. The cement and getter layers are isostatically compressed, and the outer cement layer is hydrated. EXAMPLE 18 In this example, a waste container is prepared by compressing a getter and a powdered hydraulic cement composition into a mold. The mold is capable of defining an internal cavity within the waste container. The container wall includes a layer containing the getter and a layer containing the cement composition. The hazardous waste container is designed to hold hazardous waste material which includes approximately 50 grams of Hg.sup.2+. Ordinary Portland cement and about 200 grams of powdered zeolite A are used in this example. The zeolite A is positioned within a pliable polymeric mold and compressed to form a zeolite layer. A layer of the ordinary Portland cement is then positioned around the exterior surface of the zeolite layer. The cement is isostatically compressed. A removable lid for the waste container is prepared by compressing separate zeolite and hydraulic cement layers as described above. The Portland cement layers of the container and lid are then hydrated by immersion in saturated lime water. EXAMPLE 19 A waste container is prepared according to the procedure of Example 18, except that a mixture of various liquid, ion, or gas getters is used rather than a single getter. The container wall includes a layer containing a mixture of various getters and a layer containing the cement composition. The waste container is designed to hold waste material having a variety of radioactive and nonradioactive hazardous constituents. Mordenite, Chabazite, Faujasite, and Linde 5A are selected as suitable liquid, ion, or gas getters. The amount of each respective getter is calculated according to the general procedure outlined in Examples 1-4 above. EXAMPLE 20 A waste container is prepared according to the procedure of Example 19, except that the various getters are graded by size to improve packing efficiency. EXAMPLE 21 A waste container is prepared according to the procedure of Example 18, except that the container wall includes multiple layers containing various getters and a layer containing the cement composition. The hazardous waste container is designed to hold waste material having a variety of radioactive and nonradioactive hazardous constituents. Mordenite, Chabazite, Faujasite, and Linde 5A are selected as suitable liquid, ion, or gas getters. The amount of each respective getter is calculated according to the general procedure outlined in Examples 1-4 above. EXAMPLE 22 In this example, a multi-layered waste container is prepared according to the procedure of Example 21, except that the outer layer of Portland Cement also contains a plurality of fibers wrapped around the compressed high alumina cement to improve the mechanical properties of the final hazardous waste container. EXAMPLE 23 In this example, a multi-layered waste container is prepared according to the procedure of Example 21, except that the outer layer of Portland Cement also contains electrical and thermal conducting aggregates dispersed therein to improve the mechanical properties of the final hazardous waste container. EXAMPLE 24 In this example, a cementitious contaminant barrier capable of preventing passage of radon gas is prepared in situ. The cementitious contaminant barrier is prepared at a site for a proposed building known to release unacceptably high levels of radon gas. Rather than pouring a concrete floor directly on compacted excavated ground, a 50:50 dry mixture by weight of mordenite and Portland cement is placed on the ground to a depth of about 1.5 inches. The mixture is compacted by vibration compaction to a finished depth of less than 1 inch. The Portland cement is partially hydrated by spraying the surface with water. Conventional concrete is thereafter poured over the mordenite/cement mixture to provide additional mechanical and structural strength. It will be appreciated by those skilled in the art that there are many different possible combinations of liquid, ion, or gas getters that were not specifically mentioned in the foregoing examples. A few possible combinations are listed in the above section entitled "Liquid, Gas, and Ion Getters." The use of multiple layers of liquid, ion, or gas getters may be advantageously used in the design of contaminant barriers as molecular sieves to prevent specific waste constituents from passing through one layer into another. For instance, knowing the relative selectivities of various liquid, ion, and gas getters, one layer of the barrier may be selected to trap a specific ion or class of ions with additional layers selected to trap other ions. 8. Summary From the foregoing, it will be appreciated that the present invention provides novel cementitious contaminant barriers which are constructed of strong materials that do not intrinsically corrode to produce gases. The present invention also provides novel contaminant barriers which include liquid, ion, and gas getters. In addition, the present invention provides contaminant barriers constructed of materials which are self-healing upon contact with aqueous solution. The present invention further provides contaminant barriers which do not require high temperature vitrification processes. Finally, it will be further appreciated that the present invention provides novel contaminant barriers which are inexpensive. The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
050013548	abstract	Pinhole-free, radiation absorbing, surgical quality latex gloves are formed from homogeneous suspension of natural rubber and high specific gravity metal or metal compund. The suspension is formed under conditions which prevent entrapment of gas in the suspension.
	claims	1. An X-ray imaging system that produces one or more fan-shaped beams, comprising:a target for emitting X rays upon being struck by electrons from an electron source, said target comprising at least one target focal spot; andone or more graded multilayer optic devices in communication with said target to transmit at least a portion of said X rays through total internal reflection to produce said one or more fan-shaped beams, said graded multilayer optic devices, comprising:a first graded multilayer section for redirecting and transmitting X rays through total internal reflection including,a high-index layer of material comprising a first complex refractive index n1 having a real part Re (n1) and an imaginary part β1;a low-index layer of material comprising a second complex refractive index n2 having a real part Re (n2) and an imaginary part β2; anda grading zone disposed between said high-index layer of material and said low-index layer of material, said grading zone having a grading layer comprising a third complex real refractive index n3 having a real part Re (n3) and an imaginary part β3 such that Re(n1)>Re(n3)>Re(n2). 2. The system of claim 1, wherein said target is enclosed within a housing having an X-ray transparent window, said one or more graded multilayer optic devices are mounted within the housing, mounted external to the housing, fabricated as the window, or integrated into the window. 3. The system of claim 1, wherein said one or more graded multilayer optic devices is mounted on said target for transmission targets or proximate to said target for reflection targets. 4. The system of claim 1, wherein said fan-shaped beam comprises a continuous fan-shaped beam or a discrete stack of parallel fan-shaped beams. 5. The system of claim 1, wherein said graded multilayer optic devices each have a collection angle of up to about 90 degrees. 6. The system of claim 1, wherein said one or more graded multilayer optic devices comprise a circular cross-sectional profile, and wherein the graded multilayer optic devices are arranged in one or two two-dimensional array to produce said one or more fan-shaped beams. 7. The system of claim 1, wherein each of the graded multilayer optic devices has a rectangular cross-sectional profile. 8. The system of claim 7, wherein said one or more graded multilayer optic devices comprises pairs of stacked graded multilayer optic devices, wherein one half of a pair is positioned to be a mirror image of the other half of the pair. 9. The system of claim 1, wherein said one or more graded multilayer optic devices are configured to compensate for movement of said at least one focal spot on said target. 10. The system of claim 1, wherein said X-ray imaging system is employed in one of a computed tomography (CT) system, an X-ray radiographic system, a tomosynthesis system, or an X-ray diffraction system. 11. The system of claim 1, wherein a ratio of a change in an imaginary part (β) of a complex refractive index to a change in a decrement (δ) from unity of a real part (1−δ) of a complex refractive index between adjacent multilayer materials is a minimum. 12. The system of claim 1, wherein the grading zone comprises a plurality of layers. 13. A multi-energy X-ray imaging system that produces one or more fan-shaped beams, comprising:an electron source;a target for forming X rays upon being struck by electrons from said electron source;a vacuum chamber housing the target;a window through which the X rays exit the vacuum chamber;at least one graded multilayer optic device configured to transmit a desired range of X-ray energies to produce the one or more fan-shaped beams, said at least one graded multilayer optic device comprises:a first optic portion for redirecting first optic X rays through total internal reflection; anda second optic portion for redirecting second optic X rays, said second optic X rays being at a different mean energy level than said first optic X rays. 14. The multi-energy X-ray imaging system of claim 13, wherein:said first optic portion is configured for producing a spectrum with a high mean energy; andsaid second optic portion is configured for a spectrum with a low mean energy, wherein the mean energy of said first optic energy spectrum is greater than or equal to the mean energy in the said second optic energy spectrum. 15. The multi-energy X-ray imaging system of claim 13, comprising a filtering mechanism for filtering out certain energies from a beam transmitted by said at least one optic device, wherein said filtering mechanism is at least one of a filtering mechanism external to said optic device and a filtering mechanism integral to said optic device. 16. A method for imaging an object, comprising:emitting electron beams from at least one electron beam emitter towards a target having at least one target focal spot;producing X rays from the target in response to being struck by the electron beams;forming the X rays into one or more fan-shaped beams, wherein the fan-shaped beams are produced via total internal reflection of the X rays through one or more graded multilayer optic devices positioned such that at least one of the one or more graded multilayer optic devices is in optical communication with the at least one target focal spot; andgenerating an image of the object by using the emitted X rays transmitted via the one or more graded multilayer optics. 17. The method of claim 16, further comprising filtering at least a portion of an energy spectrum. 18. The method of claim 16, further comprising two-dimensional (2D) reconstruction computations that are used to generate an image of the object. 19. The method of claim 16, further comprising performing computed tomography, tomosynthesis, X-ray diffraction or X-ray radiography of the object. 20. The method of claim 16, further comprising performing diffraction analysis and identifying at least one of the crystal structure, composition, and stress/strain of the object.
051494957	summary	FIELD OF THE INVENTION The present invention relates to a water rod for a nuclear reactor and, in particular, to a tubular rod for enclosing substantially unvoided water in boiling water nuclear reactors. BACKGROUND OF THE INVENTION In a typical boiling-water reactor, fuel is provided in a number of fuel rods. The fuel itself is in the form of cylindrical pellets of enriched uranium. Enrichment is the proportion of fissionable U.sup.235 to the non-fissionable U.sup.238. These pellets are enclosed in a long cylindrical tube and sealed at both ends. The cylindrical tube with the enclosed fuel pellets is known as a "fuel rod." The rods are provided in the reactor in a number of discrete packages, which are called "fuel bundles." Each bundle includes a plurality of rods held between an upper tie plate and a lower tie plate. The tie plates contain seats or apertures for positioning and holding the ends of the fuel rods. Additionally, the tie plates include apertures for permitting a flow of water therethrough in the interstices between the fuel rods. Each fuel bundle is surrounded by a fuel channel. This channel, which is typically square in section, extends from the lower tie plate to the upper tie plate. The channel functions to confine water flow in between the tie plates and around the fuel rods. Typically seven spacers are substantially evenly spaced along the length of the fuel bundle inside the fuel channel. The spacers act to further position the fuel rods along their longitudinal extends. An upper handle portion typically attached to the upper tie plate and a lower nose piece protruding downward from the lower tie plate define the top and bottom of the fuel bundle. The handle and nose piece function to permit ready insertion and removal of the fuel bundles during so-called "reactor outages." Individual fuel rods in a bundle are disposed in a matrix, and are normally arrayed in rows and columns. Typically, some of the rows and columns in the matrix are occupied by tie rods. The tie rods are threaded fuel rods which engage the upper and lower tie plates to provide structural integrity to the fuel bundle. A typical fuel rod is approximately 160 inches in length. In a reactor, a plurality of fuel bundles are positioned in the reactor core. Fuel bundles are positioned between a lower core plate and an overlying top guide. The fuel bundles are supported within the core of the reactor at the elevation of the core plate, and are held in vertical spaced apart relationship at the top guide. Each fuel bundle in the reactor core is typically spaced apart from its neighboring fuel bundles. This spacing establishes a water filled volume in the reactor core known as the core bypass region. Water is maintained in this core bypass region by metering of a small amount of water through the fuel bundle nozzles. The nuclear reaction is controlled by a number of control rods or blades. These are typically in a cruciform shape so that each control blade is adjacent to four fuel bundles. The control rods are inserted into and out of the core bypass region. These control rods contain neutron absorbers such that insertion of the control rods will locally slow or stop the reaction from being critical. During operation of the reactor, water enters the fuel bundle through the lower tie plate. The water rises through the fuel bundles because of heating, and also, where used, from the action of one or more pumps in forcing circulation through the reactor. As the water rises through the fuel bundles and is increasingly heated, during normal operation, it eventually reaches its boiling point. Steam is formed from the boiling water, causing steam voids in the upper portion of the fuel bundle. The water in a boiling-water reactor performs two functions. First, the water carries away heat from the reactor so that it can be converted to useful energy by, for example, a turbine. Second, the water acts as a moderator, i.e., it slows down the "fast" neutrons. Neutrons in a nuclear reaction are present at a variety of energy levels. The neutrons are generally referred to as "fast" neutrons and "slow" (or "thermal") neutrons. Slowing down of the fast neutrons is desirable for at least two reasons. First, the slow neutrons are more reactive in the sense that they maintain the desired chain reaction involving the fission of U.sup.235 atoms. Second, slower neutrons are more easily captured by the control blades than the faster neutrons. Therefore, a moderator, in effect, increases the efficiency of the control blades. As noted, water is a moderator of fast neutrons. However, as water is heated, it becomes less dense and less effective as a moderator. When the water becomes steam, its effectiveness as a moderator decreases drastically, and can be, for some purposes, treated as a negligible moderator. In early fuel bundle designs, all lattice positions in the bundle were occupied by fuel rods. In these early designs, the only space for water in the interior of a fuel bundle was the space between the fuel rods and in the interstitial volume between discrete fuel bundles. Because the space between rods is typically filled with a mixture of water and steam, the moderating effectiveness of this space is less than space between bundles containing "solid" moderator. Accordingly, the most effective moderating water of the reactor was positioned between fuel bundles, i.e., in the core bypass region in the interstices between the bundles, exterior to the fuel channels. In such earlier configurations, the interior fuel rods in any fuel bundle were a large distance away from the large volumes of "solid" moderating water. Because of this distance, the most interior positions in the fuel bundle had large ratios of fast to slow neutrons, and were, therefore, less efficient in maintaining those nuclear reactions requiring "slow" or "thermal neutrons". Accordingly, interior rods were typically more enriched to compensate for this lack of efficiency. Such increase in rod enrichment, however, is rather expensive. Therefore, it was previously decided to provide for additional moderating water in interior positions of a fuel bundle. Initially, one or more fuel rods were replaced with a hollow rod (called a "water rod") of equal diameter for flowing water therethrough. The water rod communicated with the lower tie plate and extended through the upper tie plate. The water rod has its own confined water flow path and as a consequence is (like the bypass region) filled with water moderator. A water rod has nuclear and thermal advantages over simply leaving a spaced unoccupied by a fuel rod. By providing a hollow rod, the subcooled water inside is prevented from mixing with the other heated water in the bundle, and is somewhat insulated. The water in the rod, therefore, does not boil as does other water in the bundle. This scheme provided some advantages because of the additional moderator in the interior positions of a fuel bundle. Initially, the water rods were the same size as the fuel rods. Later, attempts were made to provide larger diameter water rods in the fuel bundles, these later water rods exceeding the size of the ordinary fuel rod. These attempts to provide larger water rods involved merely positioning a standard round cross-section pipe, or, in some cases, a square cross-section pipe, in interior positions of a fuel bundle so as to displace one or more fuel rods. No effective attempt was made, however, to depart from spaced-apart, standard round or square tubes, or to systematically analyze the effect of these shapes on reactor efficiency. Since fuel rods when viewed in horizontal section are arranged in rows and columns, it is common to refer to each fuel bundle as occupying a "lattice position". When the water rods were expanded in size, they intruded from one fuel rod position into those fuel rod positions occupied by adjacent fuel rods. As water rod design progressed, a configuration was provided in which a water rod had a circular cross-section with a diameter sufficiently large that it occupied more than one lattice position. In one such design, four lattice positions were sacrificed to accommodate a circular water rod. Water rods have also been developed which have a substantially square cross-sectional configuration, and occupy four or nine such lattice positions. Design Configuration In the development of new water rod design, it has been necessary to bring together certain design considerations. While these considerations have been generally known, I am unaware of their collective use which enables a design as herein disclosed. I therefore set forth the considerations, followed by the design. It will be understood that this assemblage constitutes together my invention. One aspect of water rod design, which has not previously been systematically addressed, is the displacement of fuel rods from their lattice positions. Provision of a water rod necessarily requires reduction of the number of fuel rods in a bundle, and thus results in a reduction of the amount of fuel in a bundle. In spite of the sacrifice of space for fuel, provision of the water rods has been found useful because of the greater overall efficiency obtained when moderating material is positioned interior to a fuel rod bundle. As noted above, because more fuel rods are positioned closer to the moderator, more fuel rods can be provided with lessened enrichment. This reduces fuel costs without sacrificing reactor power. Another factor related to reactor design is the impact on various safety factors. Understanding of this aspect will be promoted by a brief discussion of certain safety factors. Safety requirements provide several constraints on reactor design and operation. It must always be possible to shut down the reactor at any point during its operation. Because a boiling water reactor is most reactive when it is relatively cool, such as during start-up phases, the limiting factor of shut-down ability, is the cold-state reactivity margin. This must always be maintained at least 1% of reactivity. In a boiling water nuclear reactor, fast neutrons induce their own nuclear reactions. In many these fast neutron nuclear reactions, plutonium is produced. Unfortunately, plutonium is more reactive when the reactor is in the cold state. It is thus known that high fluxes of fast neurons can reduce the cold state reactivity margin. In addition to the cold shut-down margin, there is also a hot operating margin. It is desirable that the reactor be operated on a continuing basis near its fuel power. However, the normal continuing operation of a reactor requires that some control rods be positioned in the reactor, even at a full-power state, in order to shape the reaction, i.e., to reduce or eliminate hot spots in the reactor. Accordingly, the reactor must be designed so that the full power reactivity is less than the power that would result if all control rods were withdrawn. This difference in reactivity is known as the "hot excess margin." It is typically desirably about 1%. The reactor reactivity, then, can be viewed as constrained by a "window" of reactivity. It must maintain the cold shut-down margin, and it must also be capable of producing the hot excess margin. This window of operating constraints is referred to as the "hot-to-cold swing." Additional moderator in the bundle improves the above-described cold shut-down margin. This is at least partly because more moderating water produces a higher ratio of thermal neutrons. Thermal neutrons are not as efficient in producing plutonium. Therefore, more water generally results in less creation of plutonium. Plutonium is known to increase cold reactivity. Thus, more water, in general, will desirably resulting less cold reactivity. More water in the fuel bundle also improves the hot excess margin. This is because a larger amount of water increases reactivity by providing more thermal neutrons. Because the increase of water in the fuel bundles helps with both the cold shut-down margin and the hot excess margin, it provides a bigger hot-to-cold swing. In addition to safety considerations, another factor is the life span of a fuel load. Reactivity generally decreases as the particular fuel load ages. Thus, the cold shut-down margin must be within safety requirements when the fuel load is new and most reactive. This places an upper limit on the reactivity of a new fuel load. As the fuel load ages, reactivity drops to the point where refueling becomes necessary. Refueling is an enormously expensive proposition, and any extension of the amount of time between fuel loads is greatly beneficial. Thus, if the rate at which the reactivity drops, as a function of aging of the fuel load, can be lessened, it will take more time for reactivity to drop to the point where refueling becomes necessary. One way of decreasing the rate of this drop in reactivity is to add gadolinium oxide or other "burnable absorbers" to the fuel. These burnable absorbers capture thermal neutrons and inhibit the nuclear reaction; because of this property of inhibiting the nuclear reaction, they are sometimes called "poisons." Such poisons act to initially decrease reactivity of fuel in the discrete fuel rods. However, because these burnable absorbers are depleted or "burned up" as the reactor ages, it decreases reactivity of a new fuel load more than it decreases reactivity of an aging fuel load. In this way, the rate at which reactivity decreases with aging is reduced. However, these burnable absorbers have detrimental effects as well. During the aging stages of a fuel load, there is still some amount of residual burnable absorber, usually gadolinium, which reduces reactivity at a time when such a reduction is not desired. Therefore, it is generally preferable to reduce new-load reactivity without using (or using a reduced amount of) gadolinium. Another factor which is important in reactor design is the existence of non-nucleate boiling. Instabilities leading to non-nucleate boiling can include both thermal-hydraulic oscillations and coupled nuclear-thermal-hydraulic oscillations. These oscillations are manifested when the two phased pressure drop, particularly in the upper portions of the fuel bundle, becomes too high compared with the single phase pressure drop. The resulting fuel coolant flow has an oscillatory component superimposed on the normal steady state flow. The above-described hydraulic oscillation may be augmented by the dynamic nuclear-thermal feedback process. As steam voids are created, nuclear reactivity is reduced, since steam is a poor moderator, compared with liquid water. Thus, a negative feedback system can occur whereby the nuclear reaction creates heat which, in turn, creates steam voids. The steam voids then reduce the reactivity, because of poor moderation, leading to a reduction of the heat transferred out of the fuel and an increase in the water-to-steam ratio. The increase in water-to-steam ratio results in increased reactivity, thus beginning the cycle again. Under severe circumstances, this oscillatory behavior of the fuel surface hot flux and the coolant flow rate may result in a non-nucleate boiling process, resulting in a local increase in the fuel cladding temperature. Thus, hydrodynamic oscillations are undesirable. Some aspects of water rod design are elucidated by a brief discussion of the history of bundle designs. The designs of fuel bundles has shown a progression in the number of fuel rods in a bundle. Early bundles were formed with a 7.times.7 array of fuel rods, thus having 49 lattice positions. Fuel bundles having an 8.times.8 array of fuel rods were next produced. Most recently, fuel bundles having an array of 9.times.9 fuel rods have been produced. The physical size and cross-sectional area of the fuel bundles have not increased; rather, the progression has been to a larger number of smaller-diameter fuel rods in the bundle. The heat generated by smaller fuel rods is more quickly conveyed to the surrounding water. This increased rate of thermal transfer causes an increase in the tendency for nuclear-thermal augmented hydrodynamic oscillations. Water rods are useful in controlling such oscillations. More "solid" moderator is available to reduce the sensitivity of the nuclear fission rate to changes in the in-channel moderator density. Therefore, the tendency for a hydrodynamic oscillating is reduced. Although the provision and increase in size of water rods have lead to some desirable results, there are also undesirable effects of larger water rods. First, larger water rods displace more nuclear fuel so that the total heat-generating capacity of the reactor is affected. Second, larger water rods have a larger bundle pressure drop, i.e., difference in water pressure between the bottom tie plate and the top tie plate increases. This increase in pressure drop has been found to be associated in an increased tendency for hydraulic oscillations. Third, water is known to act not only to slow down neutrons, but also to absorb thermal neutrons. Thus, when too large an amount of water is provided, the water excessively absorbs thermal (as well as fast) neutrons and decreases reactivity of the reactor. Previous approaches to the problem of configuring a water rod for inclusion within a fuel bundle have generally been empirical in nature. No effective general procedure for analyzing or designing characteristics of water rods has been available. Accordingly, previous designs have largely been confined to conventional tube shapes, such as substantially circular or square cross-sectional tubes. Some of the problems associated with providing water rods, such as foregoing or sacrificing lattice positions for fuel, have been known. Because there was no general method of analysis, however, the relative benefits and problems of additional moderator were not systematically taken into account in the design. Further, practical considerations, such as the manufacturing feasibility of constructing various rod shapes and the methods for connecting the water rods and fuel rods to each other with the desired spacing, placed additional constrains on the types of water rods previously provided. Accordingly, as noted, previous water rods have typically included only spaced-apart circular or square tubular shapes. SUMMARY OF THE INVENTION The present invention includes the provision of a new design parameter for water rods which, in general terms, is a measure of how well the sacrificed fuel rod positions are utilized. This new parameter has been termed "water rod efficiency." The water rod efficiency includes consideration of three factors: 1) the cross-sectional area of the interior of the water rod; 2) the number of lattice positions which are sacrificed or displaced; and 3) the cross-sectional area of a single lattice position. The water rod efficiency is then calculated as the cross-sectional area of the water rod divided by the area of a single lattice position, relative to the number of sacrificed lattice positions. Water rod designs are provided which are efficient in terms of space utilization and, in particular, which have a water rod efficiency greater than about 0.6, preferably greater than about 0.7. The water rods occupy a number of lattice positions which have been found to be selectable to produce the desired amount of moderation, and yet to avoid too large a decrease in active flow area and number of fuel rods. By providing a water rod with increased efficiency, several advantages are produced. In general terms, these advantages relate to efficiency because they provide the benefits of water rods, but with a decreased need to sacrifice potential fuel rod positions. Efficient provision of a larger volume of moderator improves the cold margin by providing more moderator closer to fuel rods. The disclosed designs improve the hot margin by increasing hot reactivity, since larger amounts of moderator are present. Therefore, the hot-to-cold swing is improved. Accordingly, the amount of gadolinium oxide can be reduced to reduce gadolinium residual. Efficient provision of larger amounts of moderator also increases the water-to-steam ratio in the two-phase portion of the reactor. When this ratio is improved, the tendency for instabilities is reduced. Such a reduction in the instability tendency at least partially offsets the increase in pressure drop associated with larger water rods. By providing more fuel rod positions which are adjacent to a water rod, a lager number of fuel rods are in positions of high worth (i.e., close to moderator). Therefore, less enriched and less expensive fuel can be used without sacrificing reactivity. By providing a larger number of low-enrichment fuel rods in a bundle, a more even thermal distribution can be produced, reducing rod-to-rod and bundle-to-bundle peaking. A water rod with larger cross-sectional area concentration in one part of the fuel bundle has been found to be preferable to the same cross-sectional area provided by a plurality of spaced-apart, smaller rods. The efficient provision of larger amounts of moderator reduces the tendency for hydraulic and nuclear-thermal hydrodynamic instabilities. This, in turn, permits use of high-exposure fuel rods, such as a 9.times.9 array of potential fuel rod positions. This advantage is further beneficial because a larger number of lattice positions affords greater flexibility for placement of water rods. By providing a new design parameter for use in designing water rods, candidate water-rod shapes can be efficiently screened, and proposed designs can be selected on an objective, efficiency basis. Particular water rod cross-sectional shapes, which can be feasible and economically produced, are provided. The preferred configurations include a "FIG. 8" shape having two adjacent circular portions and a cross-sectional, "peanut" shape which has two substantially triangular rounded-cornered portions separated by a constricted portion. Other designs include a substantially "rectangular" cross-sectional design and a "cruciform" design having four concave portions separating four lobes. The particular designs are of feasible construction, and produce the desired efficiency and range of moderation. A method for analyzing water rod characteristics is provided, which includes the determination of a defined water rod efficiency and substantial occupation of a number of lattice positions in a predetermined range. A device and method for positioning and connecting the water rod with respect to the fuel rods and spacers is also provided.
	summary
	abstract	An apparatus for controlling movement of a first component integrated with a second component may include a first clamp configured to engage the first component, a second clamp configured to engage the second component, and a plurality of connectors configured to connect the first and second clamps. The connectors may allow movement of the first clamp relative to the second clamp in a first direction between the first and second clamps. The connectors may limit movement of the first clamp relative to the second clamp in a second direction perpendicular to the first direction.
	abstract	A MOX 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 fuels as the nuclear fuel and light water as the moderator/coolant is provided with only one kind of MOX nuclear fuel rods each of which has relatively large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, the quantity of the MOX nuclear fuel rods being relatively small.
041486854	abstract	Disclosed are a method and apparatus for regulating and shutting down a gas cooled nuclear reactor having a bed of spherical fuel elements, wherein regulation of the reactor is accomplished by a first set of absorber rods in the side reflector of the reactor, partial shutdown is achieved by a second set of absorber rods movable in the top reflector and in the space above the fuel elements, and total shutdown is accomplished by a third independent set of absorber rods which can be moved downwardly into the fuel elements.
046702121	claims	1. A temperature sensing instrument for direct mounting in a wall of a structure traversed by a primary coolant of a pressurized water reactor, comprising: a thermally conductive finger-shaped sheath closed at one end and extending through said wall so that a closed end of said sheath is disposed within said structure in direct contact with said coolant, said closed end being formed with a frustoconical blind bore tapering generally toward said closed end and having an internal surface; and a temperature sensor received in said sheath and having a frustoconical tip with an external surface tapered substantially to match said frustoconical bore and lodged therein in surface contact therewith at said surfaces, both of said surfaces being treated to limit the roughness of the surfaces in surface contact, said sensor comprising: 2. The instrument defined in claim 1, further comprising elastic means based between said temperature sensor and said sheath for pressing said sensor in the direction of taper of said tip against said sheath, thereby pressing said surfaces together. 3. The instrument defined in claim 2 wherein said sheath is formed externally of said structure with a thread, further comprising a sleeve engaging said thread and forming a seat for said spring, said sensor having a shoulder forming another seat for said spring. 4. The instrument defined in claim 3 wherein said tip and said blind bore are frustoconical with their respective generatrices including an angle of 3 degrees to a tolerance of about 1 minute with the respective axes thereof. 5. The instrument defined in claim 4 wherein said tip and said blind bore have mutually contacting surfaces, said surfaces having a roughness such that the arithmetic mean of the magnitude of said roughness is less than 0.2. 6. The instrument defined in claim 5 wherein said sheath is formed in two parts including a first cylindrical part open at both extremities, and a cap closing said first part and formed with said blind bore. 7. The instrument defined in claim 1 wherein said sheath is formed in two parts including a first cylindrical part open at both extremities, and a cap closing said first part and formed with said blind bore. 8. The instrument defined in claim 5 wherein the portion of said sheath provided with said blind bore is formed unitarily with the remainder of said sheath. 9. The instrument defined in claim 1 wherein the portion of said sheath provided with said blind bore is formed unitarily with the remainder of said sheath. 10. The instrument defined in claim 5 wherein said blind bore is formed on said sheath unitarily with a portion of said sheath formed with said cylindrical bore, said blind bore being closed at its small-diameter end by a plug received in said sensor.
	abstract	The present invention comprises systems and methods for determining stability margin of a combustor. One embodiment of the present invention includes the steps of providing a measuring device in communication with the combustor, wherein the measuring device generates signals indicative of combustor quantities; performing an autocorrelation calculation on the signals to determine the correlation time of the signals in the combustor; and determining the damping coefficient from the autocorrelation calculation, wherein the damping coefficient signifies a proximity of the combustor to instability. The damping coefficient may be estimated from the oscillatory envelope of the autocorrelation calculation data.
	description	This application is a divisional of U.S. application Ser. No. 13/523,277, filed on Jun. 14, 2012, the contents of which are hereby incorporated by reference herein in their entirety. The present general inventive concept relates to a diagnostic system to monitor rod position indication signals in a nuclear power plant, and more particularly relates to systems and methods of performing signal and data diagnostics on control rod position indication systems of a nuclear reactor, including non-intrusive error detection techniques for control and shutdown rod position in nuclear reactors. In a Pressurized Water Reactor (PWR), the power level of a nuclear reactor is typically controlled by inserting and retracting control rods, which for purposes of this application include the shutdown rods of the reactor core. The control rods are moved by a Control Rod Drive Mechanism (CRDM), which typically include electromechanical jacks that raise or lower the control rods in increments. Known CRDMs include a lift coil, a moveable gripper coil, and a stationary gripper coil that are controlled by a Rod Control System (RCS) and a ferromagnetic drive rod that is coupled to the control rod and moves within the pressure housing. The drive rod includes a number of circumferential grooves at intervals (“steps”) that define the range of movement for the control rod. For example, a typical drive rod contains approximately ⅝ inch intervals and 231 grooves, although this number may vary. The moveable gripper coil mechanically engages the grooves of the drive rod when energized and disengages from the drive rod when de-energized. Energizing the lift coil raises the moveable gripper coil (and the control rod if the moveable gripper coil is energized) by one step. Energizing the moveable gripper coil and de-energizing the lift coil moves the control rod down one step. Similarly, when energized, the stationary gripper coil engages the drive rod to maintain the position of the control rod and, when de-energized, disengages from the drive rod to allow the control rod to move. The RCS typically includes a logic cabinet and a power cabinet. The logic cabinet receives manual demand signals from an operator or automatic demand signals from Reactor Control and provides the command signals needed to operate the shutdown and control rods according to a predetermined schedule. The power cabinet provides the programmed DC current to the operating coils of the CRDM. Known PWR designs are challenged in that they do not have adequate direct indication of the actual position of each control rod. Instead, step counters associated with the control rods are typically maintained by the RCS and rod position indication (RPI) systems to monitor the positions of the control rods within the reactor. The associated step counter is incremented or decremented when movement of a control rod is demanded and successful movement is verified. Because the step counter only reports the expected position of the control rod, certain conditions can result in the step counter failing and deviating from the actual position of the control rod. In certain situations where the actual position of the control rod is known, the step counter can be manually adjusted to reflect the actual position. However, if the actual position of the control rod is not known, a plant shutdown may be required so that the step counters to be initialized to zero while the control rods are at core bottom. The RPI systems derive the axial positions of the control rods by direct measurement of drive rod positions. Currently both analog rod position indication (ARPI) systems and digital rod position indication (DRPI) systems are in use in PWRs. A conventional DRPI system includes two coil stacks for each control rod and the associated DRPI electronics for processing the signals from the coil stacks. Each coil stack is an independent channel of coils placed over the pressure housing. Each channel typically includes 21 coils, and the coils are interleaved and positioned at approximately 3.75 inch intervals (6 steps). The DRPI electronics for each coil stack of each control rod are located in a pair of redundant data cabinets (Data Cabinets A and B). Although intended to provide independent verification of the control rod position, conventional RPI systems are not accurate to fewer than 6 steps. For example, the overall accuracy of a DRPI system is considered to be about ±3.75 inches (6 steps) with both channels functioning and ±7.5 inches using a single channel (12 steps). In contrast to conventional DRPI systems, conventional ARPI systems determine the rod position based on the amplitude of the DC output voltage of an electrical coil stack linear variable differential transformer. The overall accuracy of a properly calibrated ARPI system is considered to be about ±7.5 inches (12 steps). Neither conventional ARPI systems nor conventional DRPI systems are capable of determining the actual positions of the control rods. In the event of a step counter failure, plant shutdown for re-initialization of the step counter is required as the approximate positions of the control rods reported by conventional RPI are of little or no value. For purposes of this application, the phrase “control rod” is used generically to refer to a unit for which separate axial position information is maintained, such as a group of control rods physically connected in a cluster assembly. The number of control rods can vary according to the plant design. For example, a typical four-loop PWR has 53 control rods. Each control rod requires its own sets of coils having one or more channels and the DRPI electronics associated with each channel. Thus, in a typical four-loop PWR, the entire DRPI system would include 53 coil stacks, each having two independent channels, and 106 DRPI electronics units. Further, in this application, the phrase “coil stack” is used generically to refer to the detector coils associated with each control rod and should be understood to include either or both channels of detector coils. Thus, a measurement across a coil stack contemplates the value across both channels combined and/or the value across a single channel. Over time, aging and obsolescence issues have led to an increase in problems with conventional DRPI systems including analog card failures and coil cable connection problems that, in some cases, may result in unplanned reactor trips. These problems, along with plans for plant life extension, have prompted the industry to actively seek viable options to monitor the health and accuracy of the DRPI systems and/or to replace failing systems in order to ensure reliable plant operations for decades to come. In addition to obsolescence concerns, the lack of diagnostic capabilities is a significant challenge. Since conventional RPI systems do not provide diagnostic information on their health other than the current rod position indication, diagnostics of the RPI system is limited to periods when the PWR is offline. The primary benefit of offline diagnostics is to catch obvious failures resulting from reassembly of the reactor. However, in between refueling outages, RPI failures can occur without warning, which leads to increased costs for the plant, especially if replacement parts cannot be obtained in a timely manner. Without active monitoring, plant engineers cannot identify problems developing in RPI systems and are unable to take preemptive actions, such as obtaining necessary replacement parts ahead of time and replacing failing components at the next scheduled outage. Instead, plants typically begin remedial actions after an actual failure occurs. Beyond the technical challenges of controlling conventional DRPI systems, regulatory issues exist. Many existing PWRs are approaching the end of qualified life for several components of the conventional DRPI systems causing a demand for replacement options. There has been a significant push in recent years for plants to replace aging analog systems with digital systems made from commercially-available off-the-shelf parts. Using readily-available commercial parts provide plants more options for replacement in the future. Example embodiments of the present general inventive concept provide improved systems and methods of monitoring digital rod position indication signals in nuclear power plants. Example embodiments can perform signal and data diagnostics on control rod position indication systems in a nuclear power reactor while the reactor is operating. Additional features and embodiments of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the present general inventive concept. Example embodiments of the present general inventive concept can be achieved by providing a diagnostic system to monitor digital rod position indication (DRPI) signals generated by detector coils of a DRPI system of a nuclear power plant, including a digital diagnostic unit connected between a DRPI display cabinet and a DRPI data cabinet of the DRPI system to monitor digital rod position signals of the DRPI data cabinet, wherein the digital diagnostic unit detects variations in the digital rod position signals to determine rod position errors of the DRPI system. The digital rod position signals can include rod address signals and rod position data signals, and wherein the rod position errors are determined based on signal level variation and/or signal timing variation of the rod address signals and the rod position data signals. The digital diagnostic unit can detect parity bit errors in the digital rod position signals between the DRPI display cabinet and the DRPI data cabinet. The digital diagnostic unit can store the measured voltages of the digital rod position signals when a rod position error or a parity bit error is detected. The digital diagnostic unit can monitor at least one of the signal level variation, the signal timing variation, and the parity bit error of the digital rod position signals to distinguish between errors of the DRPI display cabinet, the DRPI data cabinet, and/or the detector coils. The digital diagnostic unit can monitor variations in the digital rod position signals while the nuclear power plant is operating. The digital diagnostic unit can monitor variations in the digital rod position signals to isolate errors of a particular card, cable, or control rod. The diagnostic unit can monitor the Gray code rod drop signals of the digital rod position signals. Example embodiments of the present general inventive concept can also be achieved by providing a method of monitoring digital rod position indication (DRPI) signals of a DRPI system of a nuclear power plant, including acquiring digital rod position signals at a point between a DRPI display cabinet and a DRPI data cabinet of the DRPI system, and processing the digital rod position signals to identify variations in a signal level and a signal timing of the digital rod position signals to determine rod position errors of the DRPI system. The method can further include detecting parity bit errors in the digital rod position data signals between the DRPI display cabinet and the DRPI data cabinet, and using at least one of the signal level variation, the signal timing variation, and the parity bit error to distinguish between errors associated with the DRPI display cabinet and the DRPI data cabinet. The method can further include combining the DRPI system with a Coil Diagnostic System (CDS) to distinguish between problems with DRPI coils, the DRPI display cabinet, and the DRPI data cabinet. The following description is intended to describe various example embodiments of the present general inventive concept, but is in no way intended to limit its application, or uses. Various example embodiments are described below in order to explain the general inventive concept by referring to the figures. FIG. 1 is a block diagram of a conventional DRPI system in a pressurized water reactor (PWR), including an example DRPI diagnostic system according to an example embodiment of the present general inventive concept. The DRPI diagnostic system can continuously sense and display the positions of each of the control and shutdown rods during plant operation. This can be accomplished through the use of coil stacks which are mounted on the rod control housing above the reactor. The coils can be excited with an AC voltage and magnetically sense the presence of the control rod drive shaft in the center of the coil. As illustrated in FIG. 1, a typical DRPI system includes a DRPI coil stack 2 including a plurality of DRPI detector coils C1 to Cn to sense the rod position in containment. When the control rod shaft 1 enters the coil, it changes the coil impedance to the AC voltage provided to the coils, thus changing the AC current through the coils. The stepping of rod 1 generates an induced current in the detector coils C1 to Cn to produce DRPI coil signals. The analog electronics in the existing DRPI system detect the change in current and create a digital bit for each coil in the coil stack. Typically, each coil stack 2 is an independent channel of coils placed over a pressure housing 3 of the nuclear power reactor. These digital bits are transmitted to the control room to provide the rod position to a technician via the A and B data cabinets 4 and the display cabinet 5. DRPI data cabinets A and B (referred to as reference number 4 in FIG. 1) convert the rod position coil signals into digital information. The data cabinets A and B are generally redundant components located inside containment to monitor the coil currents and convert them into a digital position signal. The digital position information presented to the data cabinets A and B are converted to digital information and transmitted to the rod position display cabinet 5 in the control room. The display cabinet 5 addresses the data cabinets A and B, retrieves the digital rod position information, and displays the rod position on the display. The DRPI display cabinet 5 can operate under the control of a master controller 20 to display the rod position and diagnostic information, and/or other system information and controls, as desired. Referring to FIG. 1, the example DRPI diagnostic system 15 can include a Coil Diagnostic System (CDS) 7 and a Digital Diagnostic System (DDS) 22. The CDS 7 and DDS 22 can be formed as independent hardware subsystems, or could be integrated into a single unit. In one example embodiment, the CDS 7 is installed in containment at the DRPI A and B data cabinets to measure the 21 coil signals from each of the 21 DRPI A or DRPI B coils for every rod. The DDS can be connected in the control room at the display cabinet and can observe the digital rod address and digital rod position data signals between the DRPI A and B cabinets and the DRPI display cabinet. The CDS 7 and DDS 22 can be integrated with a master controller 20, although it is possible for the components to be formed as separate units, or as combinations of units, without departing from the scope of the present general inventive concept. The master controller 20 can include a human-machine interface (HMI) installed in the control room, to interface with the operator or technician. For example, the master controller 20 can include one or more displays and inputs/outputs to interact and display information to/from the operator or technician, as desired. The DRPI diagnostic system 15 can provide rod position, coil diagnostics, rod drop timing, digital diagnostics, or other information, to monitor operations of nuclear power plants. FIG. 2 illustrates an example embodiment of a Digital Diagnostic System (DDS) 22 used to retrofit existing conventional DRPI systems in nuclear power plants, according to an embodiment of the present general inventive concept. Referring to FIG. 2, the DDS 22 can be installed in the control room proximate the display cabinet 5, and can measure the digital rod address and digital rod position data signals from the DRPI coil signals, although other locations could be used to house the DDS 22 and/or display cabinets. The DDS 22 can be integrated with a master controller 20, including a human-machine interface. The master controller 20 can also be installed in the control room. As illustrated in FIG. 2, the DDS 22 can acquire rod position signals at a point between the output from the existing DRPI display cabinet 5 and the DRPI data cabinets 4A, 4B. This enables the DDS 22 to sample the DRPI signal voltages and convert them into digital signals. For example, the DDS 22 can acquire the rod position signals at the test points PT1-PTn (see FIG. 2) in the display cabinets of the conventional DRPI system. The test points PT1-PTn provide access to the rod position digital signals, which heretofore were not available during operation. The digital signals are then transmitted to the DDS 22, which may be located in the main control room. The DDS 22 detects changes in the level and/or timing of the digital rod position signals, including changes in the rod address and position data information, to determine rod position errors. Referring to FIG. 2, reference number 24 identifies a typical address and data communication sequence for a conventional DRPI system. Here, the rod address signals can be sent from the DRPI display cabinet 5 to the DDS 22. In this embodiment, the rod signals include 7 address signal lines (e.g., A0000110, et al.) and 6 data signal lines (e.g., D001111, et al.) for each of the data cabinets 4A, 4B, although various coding schemes and/or protocols could be chosen with sound engineering judgment. In this example, the detector/encoder card for the requested rod's address in the data cabinet can transmit the rod's position in a binary Gray code, wherein the Gray code is comprised of five data bits and one parity bit. In this case, the parity bit can be a 1 if the number of 1's in the address and data bits is odd. This position code can then be converted to a step number and displayed for that rod. In this way, the DDS 22 interprets the Gray code and displays diagnostic and status information. A system error can be identified as a one bit error which can be detected from a parity bit check. Other bit errors may include, for example, a valid rod position code that is mismatched between DRPI A rod position and DRPI B rod position, which should agree within 12 steps, in this example. The DDS 22 can be integrated with a Master Controller and human-machine interface 20 installed in the control room to form a system providing digital diagnostics for nuclear power plants. As mentioned above, the DDS 22 can be located in the control room at the display cabinet to measure the digital rod address and digital rod position data signals, although other locations could also be used. FIG. 3 is an example of a DDS 22 in communication with two independent channels of the data cabinets A and B, according to an example embodiment of the present general inventive concept. The illustrated Digital Diagnostic System (DDS 22) is in communication with the data cabinets A and B to sample the DRPI signal voltages and convert them into digital signals. The DDS 22 interprets the binary digital signals and displays diagnostic and status information. The components of the DDS 22 are selected to provide sufficient data transmission speeds to send the sampled data to the master controller and human-machine interface 20 in real time. In some embodiments, the DDS 22 can include a plurality of data acquisition modules to receive the respective DRPI coil signals from the A and B data cabinets. In FIG. 3 the modules are identified as acquisition Module 1 through acquisition module n. Although a variety of configurations for the DDS 22 could be chosen with sound engineering judgment, one suitable device for performing the functions of the data acquisition modules includes an analog-to-digital (A/D) module with a +/−60V input range capable of simultaneous, isolated, high-speed, differential analog acquisition for both the address bus and the data bus. The A/D modules can be each connected to an field-programmable gate array (FPGA) for acquiring various types of signals including the voltage signals used by the DDS 22. A high speed interface can be provided to allow an external computer to communicate with the FPGA, for example at data rates up to 50 MB/s, or higher. As illustrated in FIG. 3, the FPGA can be connected to an embedded controller, which can be, for example, a CompactRIO (cRIO) remote high speed interface system produced by National Instruments Corporation, which includes swappable I/O modules. The CompactRIO is capable of monitoring the rod address and rod position data. In some example embodiments, the DDS 22 is capable of driving the rod addresses by using a digital input/output (I/O) module that outputs a +/−15V TTL signal to the DRPI data cabinets, but a variety of other types and/or combinations of components could be chosen with sound engineering judgment to achieve the same or similar results. All such variations are intended to remain within the scope of the present general inventive concept. For example, one skilled in the art will recognize that the general specifications described above for the DDS 22 electronics are not intended to be limiting. A variety of other configurations could be used to acquire sufficient data containing information from which the positions of the control rods can be derived. As described herein, the DDS 22 can distinguish problems with the display cabinet in the control room and problems with the data cabinets in containment. It can also be used to monitor the Gray code signals directly. When used in conjunction with the CDS 7, the DDS 22 can identify a problem with a rod's position indication as a coil, data cabinet, or display cabinet problem. Additionally, the DDS is capable of isolating individual card, cable, and rod problems. In some embodiments, the DDS 22 can include an embedded system capable of measuring +/−15V signals that are used to transmit the rod address and rod position data. In these cases, the signals can be monitored using an A/D module with a +/−60V input range capable of simultaneous, isolated, high-speed, differential analog acquisition for both the address bus and the data bus. Instructions can be provided to both the FPGA and the real-time controller of the embedded system used to compute the position information. For example, in a typical address and data communication sequence, the rod address can be sent from the DRPI display. The detector/encoder card for the requested rod's address in the data cabinet in containment can transmit the rod's position in a binary Gray code. This position is then converted to a step number and displayed for that rod. The DDS can also act as a passive or active DRPI digital Gray code rod drop test system. Instructions provided to a real-time controller of the DDS can be used to collect voltage data and log anomalies with the Address or Data codes. The Master Controller 20 can handle the storage and display of rod diagnostic information. Instructions provided to the master controller 20, and the DRPI Diagnostic system 15 hardware, enable the system components to be used as a stand-alone system or for performing temporary diagnostic services. For example, the CDS 7 (FIG. 1) can be used as a stand-alone system, interfacing directly with instrumentation already installed in the plant. Likewise, the DDS 22 is capable of being used as an installed system or a temporary diagnostic system. If used as a stand-alone system, the CDS and DDS may incorporate a PC running a version of the Master Controller instructions in order to control the system and view the data and diagnostic information. Conventional DRPI systems can also perform a rod drop time test by monitoring the voltage across all the coils in the stack while the rod is dropped. The motion of the rod drive shaft through the coil stack induces a current in the coils which is proportional to the drive shaft velocity through the coil stack. Rod drop time testing is typically performed after each refueling outage. Example embodiments of the present general inventive concept provide a DDS module capable of monitoring the digital rod addresses and digital rod position at the DRPI display cabinet in the control room. When combined with the coil diagnostics system at the DRPI data cabinet in containment, the DDS 22 can identify card problems, cable and connector problems, and power supply problems. This will reduce the amount of reactor trips due to these problems, and minimize the off-line time due to DRPI reactor trips by identifying the DRPI problem during operation of the reactor. Example embodiment of the present general inventive concept can be achieved by providing a diagnostic system for a digital rod position indication (DRPI) system of a nuclear power plant designed to monitor in real time DRPI signals generated by a plurality of detector coils of the DRPI system while the nuclear power plant is operating, the diagnostic system including a digital diagnostic unit connected in parallel between a DRPI display cabinet and a redundant pair of DRPI A and DRPI B data cabinets, the digital diagnostic unit having inputs configured to receive DRPI signals communicated between the DRPI display cabinet and the DRPI A and B data cabinets, a coil diagnostic unit configured to receive voltage signals from each one of the detector coils, a plurality of data acquisition modules configured to receive digital rod position signals for each detector coil from the DRPI A and B data cabinets, at least one address input/output module configured to drive rod addresses of the digital rod position signals to the DRPI A and B data cabinets, and a gate array module configured to acquire the digital DRPI signals from the data acquisition and address input/output modules, the gate array module having an interface connected with a controller to monitor the digital rod position signals from the DRPI A and B data cabinets for each coil and identify mismatches between a DRPI A rod position of the DRPI A data cabinet and a DRPI B rod position of the DRPI B data cabinet for each coil while the nuclear power plant is operating. Example embodiments of the present general inventive concept can also be achieved by providing a method of controlling and monitoring digital rod position indication (DRPI) signals of control rods of a DRPI system of a nuclear power plant, including generating addresses signals for a subset of control rods, sequencing through the control rods at a faster rate than the display cabinet, acquiring digital rod position signals at a point between a DRPI display cabinet and a DRPI data cabinet of the DRPI system, and monitoring the Gray code rod drop signals of the digital rod position signals at a faster rate to obtain a more accurate time resolution, thus enabling improved rod diagnostics to detect any slow down or binding as the rods are dropped. Example embodiments of the present general inventive concept can also be achieved by providing a method of detecting errors in control and shutdown rod position during operation of nuclear reactors, including connecting inputs of a digital diagnostic unit in parallel between a digital rod position indication (DRPI) display cabinet and a redundant pair of DRPI A and DRPI B data cabinets of a DRPI system of a nuclear reactor, receiving DRPI signals that are generated by a plurality of detector coils of the DRPI system and communicated between the DRPI display cabinet and the DRPI A and B data cabinets while the nuclear reactor is operating via the inputs, receiving voltage signals from each one of the detector coils, receiving digital rod position signals for each detector coil from the DRPI A and B data cabinets, monitoring the digital rod position signals from the DRPI and B data cabinets for each coil, and identifying mismatches between a DRPI A rod position of the DRPI A data cabinet and a DRPI B rod position of the DRPI B data cabinet for each coil while the nuclear reactor is operating. Example embodiments of the present general inventive concept can also be achieved by providing a method of detecting errors in control and shutdown rod position during operation of the nuclear power plant using a diagnostic system for a digital rod position indication (DRPI) system of a nuclear power plant designed to monitor in real time DRPI signals generated by a plurality of detector coils of the DRPI system while the nuclear power plant is operating, the diagnostic system including a digital diagnostic unit having inputs configured to receive DRPI signals communicated between the DRPI display cabinet and the DRPI A and B data cabinets, a coil diagnostic unit configured to receive voltage signals from each one of the detector coils, a plurality of data acquisition modules configured to receive digital rod position signals for each detector coil from the DRPI A and B data cabinets, at least one address input/output module configured to drive rod addresses of the digital rod position signals to the DRPI A and B data cabinets, and a gate array module configured to acquire the digital DRPI signals from the data acquisition and address input/output modules, the gate array module having an interface connected with a controller to monitor the digital rod position signals from the DRPI A and B data cabinets for each coil. The method can include connecting the inputs of the digital diagnostic unit in parallel between a DRPI display cabinet and a redundant pair of DRPI A and DRPI B data cabinets of the DRPI system to receive the DRPI signals that are communicated between the DRPI display cabinet and the DRPI A and B data cabinets, receiving voltage signals from each one of the detector coils using the coil diagnostic unit, receiving digital rod position signals for each detector coil from the DRPI A and B data cabinets using the plurality of data acquisition modules, driving the rod addresses to the DRPI A and B data cabinets using the at least one address input/output module, acquiring the DRPI signals from the data acquisition and address input/output modules using the gate array module, monitoring the digital rod position signals from the DRPI A and B data cabinets for each coil, and identifying mismatches between a DRPI A rod position of the DRPI A data cabinet and a DRPI B rod position of the DRPI B data cabinet for each coil while the nuclear power plant is operating. It is noted that the simplified diagrams and drawings do not illustrate all the various connections and assemblies of the various components, however, those skilled in the art will understand how to implement such connections and assemblies, based on the illustrated components, figures, and descriptions provided herein, using sound engineering judgment. The present general inventive concept can also be embodied as computer-readable codes on a computer-readable medium. The computer-readable medium can include a computer-readable recording medium and a computer-readable transmission medium. The computer-readable recording medium is any data storage device that can store data as a program which can be thereafter read by a computer system. Examples of the computer-readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, DVDs, magnetic tapes, floppy disks, and optical data storage devices. The computer-readable recording medium can also be distributed over network coupled computer systems so that the computer-readable code is stored and executed in a distributed fashion. The computer-readable transmission medium can transmit carrier waves or signals (e.g., wired or wireless data transmission through the Internet). Also, functional programs, codes, and code segments to accomplish the present general inventive concept can be easily construed by programmers skilled in the art to which the present general inventive concept pertains. It is noted that numerous variations, modifications, and additional embodiments are possible, and accordingly, all such variations, modifications, and embodiments are to be regarded as being within the spirit and scope of the present general inventive concept. For example, regardless of the content of any portion of this application, unless clearly specified to the contrary, there is no requirement for the inclusion in any claim herein or of any application claiming priority hereto of any particular described or illustrated activity or element, any particular sequence of such activities, or any particular interrelationship of such elements. Moreover, any activity can be repeated, any activity can be performed by multiple entities, and/or any element can be duplicated. While the present general inventive concept has been illustrated by description of several example embodiments, it is not the intention of the applicant to restrict or in any way limit the scope of the inventive concept to such descriptions and illustrations. Instead, the descriptions, drawings, and claims herein are to be regarded as illustrative in nature, and not as restrictive, and additional embodiments will readily appear to those skilled in the art upon reading the above description and drawings as falling within the scope and spirit of the present general inventive concept, as defined in the appended claims.
	claims	1. Process for producing a composite material consisting of aggregates of a blend of ground UO2 and PuO2 powder dispersed in a UO2 matrix, comprising the following steps of:(a) dry co-grinding a UO2 powder and a PuO2 powder so as to obtain a homogenous primary blend,(b) consolidating the primary blend so as to obtain cohesive aggregates of the UO2—PuO2 blend, wherein the blend consolidation step in order to obtain cohesive aggregates comprises the following steps of: (b1) compacting the homogenous primary blend at a pressure in the range of 150 to 600 MPa in order to obtain a blank, (b2) crushing the blank obtained in order to obtain granules, and (b3) spheroidising the granules, or in lieu of steps (b1)–(b3) the blend consolidation step in order to obtain cohesive aggregates is carried out using heat treatment of the primary blend at a temperature of 1000 to 1400° C.,(c) sieving the aggregates between 20 and 350 um,(d) diluting the sieved aggregates in the UO2 matrix so as to obtain a powder blend,(e) pelleting the powder blend, and(f) sintering the pellets obtained in order to obtain the composite. 2. Process of claim 1, wherein the blend consolidation step in order to obtain cohesive aggregates comprises the following steps of:(a) compacting the homogenous primary blend at a pressure in the range of 150 to 600 MPa in order to obtain a blank,(b) crushing the blank obtained in order to obtain granules, and(c) spheroidising the granules. 3. Process of claim 2 wherein the compaction of the blend is carried out at a pressure of 300 MPa. 4. Process of claim 1, wherein the consolidation of the primary blend in order to obtain cohesive aggregates is carried out using heat treatment of the primary blend at a temperature of 1000 to 1400° C. 5. Process of claim 4, wherein the consolidation of the primary blend in order to cohesive aggregates is carried out using heat treatment of the primary blend at a temperature of 1000° C. 6. Process of claim 4, wherein consolidation heat treatment is carried out in a humidified or non-humidified atmosphere of 95 vol. % argon and 5 vol. % hydrogen. 7. Process of claim 1, wherein the size of the sieved cohesive aggregates ranges between 125 and 350 μm. 8. Process of claim 1, wherein the PuO2 powder is totally or partially replaced by a discarded manufactured powder comprised of mixed oxides (U,Pu)O2. 9. Process of claim 1, wherein the primary blend of UO2 and of PuO2 is comprised of 60 to 90 wt. % UO2 and 40 to 10 wt. % PuO2 relative to the total mass of the blend. 10. Process of claim 1, wherein the primary blend of UO2 and of PuO2 is comprised of 75 wt. % UO2 and 25 wt. % PuO2 relative to the total mass of the blend. 11. Process of claim 1, wherein the sieved cohesive aggregates are diluted in the UO2 matrix at a concentration of 20 to 35 vol. % relative to the total volume of the powder blend obtained. 12. Process of claim 1, wherein the sieved cohesive aggregates are diluted in the UO2 matrix at a concentration of 20 vol. % relative to the total volume of the powder blend. 13. Process of claim 1, wherein the pelletising step is carried out using a uniaxial hydraulic press. 14. Process of claim 1, wherein the sintering step is carried out at a temperature of approximately 1700° C. 15. Process of claim 1, wherein the sintering step is carried out in a furnace following a thermal cycle comprising the following successive steps of:(a) raising the temperature at 200° C./hour,(b) stabilizing at approximately 1700° C., and(c) cooling at approximately 400° C./hour down to 1000° C., then by following furnace inertia,wherein the sintering step is preferably carried out in a humidified or non-humidified atmosphere of 95 vol. % argon and 5 vol. % hydrogen. 16. Process of claim 5, wherein consolidation heat treatment is carried out in a humidified or non-humidified atmosphere of 95 vol. % argon and 5 vol. % hydrogen.
048760730	abstract	There is provided a generator for short-lived radionuclides. The generator comprises a support, an ion-exchange agent and a parent radionuclide in a steady-state equilibrium with a daughter nuclide, which daughter nuclide can be selectively eluted from said column. A suitable parent radionuclide is .sup.191 OS in equilibrium with .sup.191m Ir. There is also provided a specific Os(VI) complex which has certain advantages.
	summary
	description	This application is a continuation-in-part application of copending U.S. patent application Ser. No. 13/271,018 filed Oct. 11, 2011, and also a continuation-in-part of PCT Patent Application No. PCT/US2011/055480 filed Oct. 7, 2011, claiming the priority of Provisional Patent Application No. 61/391,536 filed Oct. 8, 2010. The present invention generally relates to grids used in radiation imaging including x-ray imaging. The fields of medical and industrial radiography use the technique of directing beams of electromagnetic radiation toward an object (or part of the human body), so that the radiation passes through the object, to obtain an image of the interior of the object, that is otherwise difficult to access or view directly without cutting through the body or other object. Usually, the electromagnetic radiations used for imaging purposes are x-rays, which tend to scatter as they travel through the object to be imaged. The scattered x-rays contribute to the degradation of the image of the object and more particularly to the degradation of the image contrast. The x-rays that travel through the object that are not scattered are referred to as primary transmissions and it is those transmissions that contribute the most useful information to the image. The various unscattered x-rays passing though the object are attenuated at differing levels by differing amounts and compositions of material within the object. The differences in x-ray attenuation along linear paths through the object produce an intensity pattern that comprises image information which is recorded by an image receptor. The image receptor may be a screen having a layer of x-ray sensitive material or x-ray sensitive electronic medium. The resulting image produced by the image receptor is based on the differences in the intensity of primary x-ray transmissions detected by the receptor. To improve the image quality, the primary x-ray transmissions and any scattered x-rays that would reach the image receptor after having traveled though the body, are first passed through a grid before they are allowed to impinge onto the image receptor. It is understood that, quantitatively speaking, the scattered x-rays degrade the image contrast by a factor approximately equal to (1-SF) where SF is the scatter fraction of the total radiation transmitted through the body. The scatter fraction SF is defined as: SF = S S + P where S and P are the intensities of the scattered and primary radiations incident on the image receptor, respectively. The present invention provides a device for, and method of manufacture of, a focused anti-scatter grid for improving the image contrast of x-ray images produced in medical, veterinary or industrial applications. In use, the grid is arranged to absorb as much of the scattered x-rays as possible, and to transmit as much of the primary x-rays as possible, thus reducing degradation of the image contrast. The performance of the grid in this respect is given by the Contrast Improvement Factor (CIF): (revised equation) CIF = C g C o = 1 - SF 1 - S × T s S × T s + P × T p where Cg and Co are the image contrasts with and without the grid, Ts and Tp are the transmissions of scatter and primary radiation by the grid, respectively. By design considerations, improvement in contrast can be accomplished by increasing Tp and by decreasing Ts. The design of the present invention is intended to reduce Ts and also increase Tp. In accordance with the present invention the grid comprises a plurality of channels that are substantially transparent to x-rays and higher energy level radiation, and a series of walls formed of a material that is capable of absorbing such high energy radiation, the walls being so placed and aligned as to define the channels in such a manner as to converge at the point location of the x-ray source. The walls thus aligned are designed to minimize absorption of radiation entering the grid that originates at the locus of the x-ray source while absorbing those x-rays scattered so that their directions are no longer along the paths of the radiation emitted from the x-ray source. The radiation absorbent walls are preferably supported by a frame, generally rectangular in outline. In one preferred embodiment, the frame, and the grid contained within the frame form a segment of a sphere, i.e. a portion of a spherical surface. The side of the grid facing the x-ray source and the object being imaged would have a radius of R, and the opposite surface a radius of R plus L, i.e., the height of the walls. To enhance the amount of T reaching the receptor, in one embodiment, the grid can be effectively lengthened by corrugating the walls. Another preferred embodiment of the focused grid of the present invention comprises an enclosed frame, comprising at least a pair of opposed side pieces, each supporting and positioning a ribbon of the material forming the grid walls. For example, each side piece is provided with a plurality of slots, or other openings, so spaced and disposed as to hold, preferably at each end, the material forming the walls defining the channels, in the proper alignment. The slots are so disposed relative to each other as to cause the wall materials held in the slots, to be in a configuration to focus any radiation impinging on one face of the grid to converge at a focus point, or line, beyond the opposing surface of the grid. Preferably, the grid is formed of a series of interconnected and mating modules, each module being substantially identical to the other modules. In one such embodiment, each module is essentially a ribbon, or plate, of the radiation absorbent heavy metal material, held in a frame so as to maintain their juxtaposition relative to each other and to the radiation source and the imaging device. In another such embodiment, the ribbon, or plate, of the radiation absorbent heavy metal material is secured to one side of a suitably shaped support formed of a radiation transparent material, also preferably held in a suitable frame, as above. In each preferred embodiment there is extending between, and defined by, the radiation absorbent walls, a substantially radiation transparent material, which most preferably, is only, or primarily, air, the material most transparent to x-rays. Alternatively, as a means of providing additional structural support and rigidity to the radiation absorbent walls, extending between and attached to at least one of the immediately adjacent pair of defining walls is a solid support material that is also substantially transparent to x-radiation, such as a hydrocarbon polymer or carboxylated hydrocarbon polymer; if the polymer is thick enough to completely fill the channel between the walls, the polymer is more preferably foamed to further increase radiation transmission. The grid design most preferably contains primarily air within the channels, so that transmission of primary radiation (Tp) through the grid is maximized, thus allowing the radiation dose to the patient to be lower, as compared to conventional aluminum,—or plastic or paper—supported grids. The thickness of the heavy metal, x-ray absorbent walls defining the channels and the depth of the channels (and thus the length, L, of the walls) can be varied to optimize primary transmission and reduce or eliminate transmission of the scattered radiation, for a given radiation energy. One preferred method of the present invention comprises the steps of forming a preferred grid frame by forming the frame sides, by casting or molding, of for example, aluminum or steel or a high strength polymer. In the method of forming one preferred embodiment of the frame, high precision machining of the light metal, such as aluminum or steel, or rigid polymer frame sides, produces a series of aligned slits on opposite sides of the frame. The planes containing the center lines of the pairs of opposed slits along the opposing frame sides, are so aligned and juxtaposed, as to converge at a line on the horizontal plane of the x-ray tube focus, as depicted in FIG. 1. The slits on opposite sides of the frame are precisely aligned so that slits on opposite sides are in the same planes orthogonal to the sides of the frame in which the slots are formed. The walls can be formed of thin ribbons of heavy metal foils held tightly in tension across the frame by the opposed slits. One embodiment is essentially a conventional linear grid where the metal foil ribbons define planes that extend from one edge of the frame to the other. In this embodiment the planes of all ribbons converge to a line through the x-ray focus. A second related embodiment is based upon the first embodiment, except that a second similar frame is positioned over the first but with the slits and ribbons orthogonal to those of the first layer. This design results in what is effectively a crossed linear grid, which further reduces scatter radiation striking the imaging surface and results in a further improved image. The grid ratio is the ratio of channel depth to spacing between walls and is typically between 5:1 and 16:1. Another embodiment of the present invention provides an improved, focused, antiscatter grid which comprises a plurality of substantially identical arc-shaped, mating modules, preferably comprising alternating layers of radiation transparent solid support material and radiation absorbent material. Preferably, each module is constructed from substantially radiation transparent solid material, such as a hydrocarbon polymer, and has at least one mating surface formed of a relatively thinner layer of radiation absorbent material. The modules are assembled to form a grid, preferably a plurality of focused channels, each bounded by radiation absorbent material. As in the ribbon embodiments, all of such channels are focused to the same point in space, intended to be located a certain distance from the assembled modules. The assembled modules are preferably mounted onto an appropriately shaped frame to form the focused grid of the present invention. In one preferred embodiment, the modules are corrugated, which corrugations are formed in a plane orthogonal to the direction of the focused radiation, and serve to extend the effective length of the grid, within a relatively compact frame allowing more primary radiation to pass to the receptor. In a third embodiment, the focused grid of the present invention comprises a plurality of substantially identical arc-shaped, mating modules, preferably comprising alternating layers of radiation transparent solid support material and radiation absorbent material. Preferably, each module is constructed in the shape of a segment of a sphere, and may also be transversely corrugated in a continuous wave or sawtooth form. The modules are assembled to form a grid having a plurality of substantially x-radiation transparent focused channels, each defined by the layer of radiation absorbent material; all of such channels are focused as before. The assembled modules are preferably mounted onto an appropriately shaped frame to form the focused grid of the present invention. A preferred embodiment of the present invention provides an air-interspaced, focused grid, where the scatter-radiation absorbing walls of the channels are formed as a single layer of heavy metal foil which is held tightly in a frame on opposite margins of the supporting frame. The ribbon walls are held by the frame so as to be aligned to converge to a line in the plane of the x-ray source that is parallel to the plane of the anti-scatter grid. In the preferred embodiment, the frame is substantially rigid and capable of holding the plurality of tapes or foil ribbons in correct alignment in slits that are so aligned and juxtaposed as to support the foil tape or ribbon. The focused grid of the present invention as described above can be used as an anti-scatter grid for x-ray imaging useful, for example, in the fields of medical and/or industrial radiography. Referring to FIG. 1, the x-rays are directed from the source (10) to pass through the object 12, to the image receptor 14, below. In passing through the object or patient, some of the x-rays are scattered 20, thus reducing the contrast in the recorded image. The grid of the present invention (16) is placed between the object or patient (12) and the image receptor (14). The grid is designed to absorb as much of the scatter radiation as possible while allowing the passage of the primary, direct imaging x-rays. The x-rays used for radiographic purposes usually include electromagnetic radiation having photon energies in the range of 10 keV to 1 MeV. For ease of explanation, the beam of radiation will henceforth be described as an x-ray beam in the range described. However, it should be understood that the claimed focused grid of the present invention may be operated and function as described using electromagnetic radiation having photon energies that fall outside of the range described above, with the boundaries or walls of the channels to be constructed from material that can absorb such scattered electromagnetic radiation as may be generated. Continuing with the description of FIG. 1, a uniform beam 18 of x-rays is directed toward a surface of the object 12 and travels through the object 12 emerging from an opposite surface of the object 12. Differences in x-ray attenuation along linear paths through the object 12 produce an x-ray intensity pattern that comprises image information recorded by the image receptor 14. The image receptor 14 can be a device such as an intensifying screen coupled with a photographic film or any layer of x-ray sensitive material or x-ray sensitive electronic medium, which through one or more steps converts the x-ray intensity pattern into a visible image or visible format. When x-rays 18 pass through the object 12, they are attenuated by a combination of scattering and absorption. X-rays which have passed through the object 14 and are “focused x-rays” (meaning they also pass through the grid 16, following a focused path as described herein) are referred to as ‘primary x-rays’; the primary x-rays contribute to the formation of the image. That is, unscattered focused beams—having passed directly though the object 12—will mostly pass through the channels of the focused grid 16 of the present invention. Radiation, including x-ray radiation, which do not follow a focused path leaving the object being imaged are referred to as scattered, and scattered radiation will intersect one of the metallic layers (or radiation absorbing layers) that define the channel boundaries, which are intended to absorb the scatter radiation to an extent depending on the composition and thickness of the boundary and the energy of the radiation. It must be noted that some focused x-rays, which pass in the plane of the foils will tend to be absorbed by those foils creating a shadow image of the foils in the resulting image. It is known to provide grid systems with a mechanism to move the grid during the x-ray exposure so that the image of the foils is reduced, if not eliminated, by the blurring resulting from the motion, without significantly reducing the resolution of the primary image. The radiation absorbent channel boundary can be designed to a desired or preferable state by changing the constituent elements, i.e., different atomic numbers of its elements, or the thickness or density of the absorbent layer, to better suit the absorption of x-rays of a specific range of photon energies. For example in an application using low energy x-rays such as in mammography, the absorbent layer may be only a few tens of microns thick and might include elements with atomic numbers as low as 29. Applications requiring more energetic x-rays, such as general medical radiography, may employ a thicker absorbent layer, which is preferably formed from heavy metal elements with atomic numbers above 65, such as Lead, Bismuth, Tungsten, or Tantalum. An x-ray transparent material (such as air, or a hydrocarbon polymer or other low molecular weight polymer which may also contain nitrogen or oxygen atoms) is a material through which an x-ray beam travels where the measurable intensity of the beam immediately prior to passing into the material is substantially equal to the measurable intensity of the beam immediately after exiting the material. Conversely, an x-ray absorbent material greatly reduces the amount of x-rays exiting such material compared to the strength of the x-rays that entered such material. X-rays passing through the object being imaged, and that are scattered, i.e., that do not follow a focused path through the channels of the focused grid intersect but impinge upon the x-ray absorbent wall boundaries of the focused channels, are thus absorbed by these walls. The ribbons of metal need not necessarily be pure metal but may be a powdered material mixed with binding agents (e.g., polymers) to bind a relatively high concentration of heavy metal in the form of a fine powder, or as a compound mainly containing elements with atomic numbers greater than 28 (preferably greater than 58), or as an alloy. Depending on the application for which the antiscatter grid is being used, the relatively high concentration of heavy metals may be in the range of 40% to 98% by weight. Because the channel boundaries are formed from foil under some degree of tension some desirable, highly radiation-absorbent metals, such as lead or bismuth or alloys thereof, will require the addition of fibers or coatings (e.g., Mylar) to provide adequate tensile strength to a ribbon of the metal. This may include the use of glass fiber reinforced lead foil, or a lead foil wrapped in a thin braided weave of high tensile strength glass, nylon, polyester or other fiber materials. Alternatively, a thin tape, formed, for example of Mylar or Kapton, may be adhered to the heavy metal ribbon. Mylar and Kapton are two commercially available polymer materials containing oxygen or nitrogen, respectively, in addition to carbon. As already described with respect to FIG. 1, the focused grid of the present invention has a plurality of focused channels that allow unscattered primary x-ray beams having passed through the object 12 to impinge upon the image receptor 14, and thus form a clear, focused image. Various portions of the ribbon embodiment of the focused grid of the present invention are depicted in different views, in FIGS. 2-10, to facilitate the description of the ribbon embodiment of the present invention. The drawings used in this application are not necessarily drawn to scale, but they are presented to clearly depict the various features and aspects of the present invention. Referring now to FIGS. 2 and 2A and 2B, the basic concept of the frame with a slitted tensioning assembly to support, grip and align the metal foils, is shown. In FIG. 2A, the slits 28 are formed between spacers 29 within the margins of two opposing sides 24 of the open frame, are in planes that are aligned along straight line paths 26 to the x-ray source 22, and extend fully through the side of each side of the frame 24. A thin heavy metal foil ribbon 30 (seen in cross-section within the slits) is stretched between a pair of the narrow slits 28 on opposite sides of the frame 24, so as to lie within the aligned plane extending between the slits. As shown by a comparison of FIGS. 2A and 2B, the ribbons on either side of a center line are aligned in oppositely facing angles relative to the center line. The angle of each ribbon to the perpendicular gradually is gradually reduced moving inwardly towards the center line, from each end. For example a grid focused to a distance of one meter may disperse the slits so that the angle changes at a rate of 0.0572 degrees for each slit along the bracket. Referring now to FIGS. 3A to 3C, the details of the foil ribbons are shown. The foils are made of suitable materials with adequate tensile strength and high atomic number such as tungsten, tantalum or alloys thereof, or lead coated on a high tensile strength substrate such as Mylar. The foils are fabricated into with the desired thickness, and cut to ribbons of height ‘d’ and sufficient length 30 to traverse the distance between the opposite sides of the open frame, including the opposing slits. A hole is formed through the ribbons 31 at either end. The purpose of the hole 31 shown at the ends of foil ribbons 30 in FIG. 3A, is to allow for the insertion of a tension rod 33 that prevents the ribbon from being pulled through the bracket slits 28 when under tension. As shown in FIGS. 2A, 2B, when the brackets are straight, the focusing can be accomplished by angling the ribbons relative to the side plane of the bracket, and splitting the bracket into two sections at opposite ends, as shown, so that the slits and the ribbons are angled in an opposed direction at each end of the bracket. Referring now to FIG. 4, the foil ribbon 30 with the hole at each of the ends 31 is inserted between spacers 29 constructed with a “T” shaped extension 34 that is held within a frame member with a “C” shaped cross-section 35. To tension the ribbons the inner frame member 34 is pulled outward toward the outer rigid frame 36 by tightening bolts 37. The assembly shown in FIG. 4 is duplicated at opposing sides of the frame to ensure that the foils are maintained in proper tension. As can be seen, the tension rod 33 passes through the holes at the ends of the foil ribbons 30 and through holes in spacers 29. Referring to the preferred embodiment of FIG. 5, the spacers that ensure gripping of the foils and alignment to the x-ray focus are formed by slicing a continuous aluminum extrusion 38 shown in detail in FIG. 5A. The slices are configured to ensure that the surface planes therein converge at the locus of the x-ray source as in FIG. 2. Slicing of the extrusion is done at high precision by the use of an advanced method such as wire EDM (electro-discharge machining) under computer control. Care must be taken to ensure that the width of the kerf corresponds to the thickness of the ribbon inserted between spacers. If this is not done it is necessary to adjust the cut angles so that the assembled spacers with foils are properly convergent on the x-ray source locus. Referring now to FIG. 6; to ensure that the foils are properly gripped between spacers, a tension rod 33 is inserted through all of the spacers 29 and foils 30. The tension rod is threaded on either end, such that tightening of nuts 39, causes the perforated ends of the ribbons 30 to be tightly gripped between spacers 29. In one embodiment the slits are present only on two sides of the grid frame (FIG. 7), creating narrow focused channels as in conventional linear grids. In this embodiment the foil ribbons 60 are tensioned between a fixed bracket 62 and an adjustable, movable bracket 56 on the opposite side of the frame. Referring back to FIG. 4 an important aspect of this design is that gripping assembly illustrated in FIG. 7A ensures that when under tension the foils are precisely aligned with the x-ray source focus. In this embodiment the scatter rejection capability would be similar to a conventionally fabricated linear grid with the same grid ratio, the same metal thickness and composition of channel walls, except that in accordance with this invention, the presence primarily of air in the channels between the metal ribbons ensures that the transmission of the primary radiation is substantially unimpeded and therefore superior. In a second embodiment, shown in FIG. 8, there are two sets of opposing fixed brackets 42 and adjustable brackets 56, one set positioned above the other in two layers, producing in effect a cross-hatched pattern. This embodiment would employ a single outer frame to which both sets of brackets are attached, so that the two sets of brackets are locked in orthogonal alignment with respect to the location of the x-ray focus. The advantage of this embodiment is that the scatter rejection would be considerably improved when compared to a conventional linear grid or that of the first embodiment, because scatter deflected into planes parallel to the grid ribbons in the first layer would be rejected by the transversely directed ribbons in the second layer. In both embodiments the resulting grid is preferably covered on both top and bottom surfaces with thin Mylar polyester sheets to prevent entry of foreign materials, such as dust, into the open channel spaces, that might cause image shadows. Mylar film is substantially transparent to x-rays. In another potentially less costly embodiment, the foil ribbons are made of lead or bismuth foil, possibly mixed in alloys also containing tin, antimony indium or cadmium. In at least some of these cases, the resulting foil will not have sufficient tensile strength to be held in tension on the frame 24, and will require reinforcement with, e.g., a thin layer of Mylar or Kapton tape, such as on one or both surfaces of the foil. The films of Mylar or Kapton used for such tapes are usually about 1 mil in thickness. Alternatively a single steel reinforcing ribbon of the same thickness can be used. The focused grid can achieves its focus in frames of different sizes for different applications ranging from a few cm of inner diameter to several meters in size to accommodate a variety of imaging applications in medicine, dentistry, veterinary medicine, security imaging and non-destructive testing. A preferred process for assembly of this reinforced foil is shown in FIG. 8, where two rolls of Mylar or Kapton tape 70 are adhered to the opposite major surfaces of the lead, or other heavy metal foil or alloy foil 72, then fed through compression/gauging rollers 74 then onto a take-up reel 76. A similar process may be used with tapes constructed of polymer loaded metal powders of suitable heavy metals. In the embodiment employing reinforced lead or alloy foils, it will not be possible to fold the ends of the ribbons creating a similar stop to fix the ribbon in position and to align it with the centerline of the bracket slit. Referring now to FIG. 9, the reinforced foil ribbons 80 are cut to precise lengths as required to traverse the frame but a steel, brass or other suitable metal clip is formed to a triangular shape in cross-section 82 and crimped over the ends of the reinforced ribbon. In this fashion, the crimped metal clip 82 forms a similar terminus on the reinforced foil ribbons so as to fix the position in the bracket slits and to align the foil to the slit margins. In an example of this embodiment, a grid is prepared to reduce the scatter radiation for image receptors up to 43 cm×43 cm in size, where the x-ray source 10 focus to the image receptor 14 is a distance of 100 cm. The grid ribbons are constructed of tungsten foils 10 mm high (“L”) and 100 microns in thickness, and cut to a length of 44 cm. A length of 4 mm at each end is folded to produce the triangular stop 34. Brackets 44 would be produced with slits 28 cut by wire electrical discharge machining (wire EDM) or laser cutting, to provide a slit width of 150 microns. The slits 28 would be spaced along the brackets with an angular alignment between center planes of 0.0573 degrees with respect to the x-ray focus and a depth of 10.5 mm. The brackets can be constructed of angle steel beams with L-shaped cross-sections with a thickness of 3 mm and web diameters of 11 mm. The grid frame is constructed of mild steel alloy with an inner open area of 45 cm×45 cm with a thickness of 3 mm and a depth of 15 mm. The heavy metal ribbons preferably should be arranged so as to be separated by a distance of about 1 mm. One embodiment of the tensioning mechanism for the adjustment of the movable frame bracket 56, as shown in FIG. 10, is provided preferably at both ends of the movable bracket 56. The tensioning mechanism is constructed using an m6 bolt 96, a coil spring 100 at each end of the adjustable ribbon bracket. The top and bottom surfaces of the grid frame would be covered with 25 micron Mylar sheets held in tension to prevent foreign material that might cause image artifacts from entering the space between ribbons. In an example of this embodiment the outer grid frame of the first example would be increased in depth to accommodate the second layer containing a second set of brackets and ribbons essentially identical to that in the first layer except that the slit separation angle would be increased to 0.0579 degrees with respect to the x-ray tube focus. In yet another embodiment of the present invention there is provided a device and method of manufacture of an improved, focused, antiscatter grid. The focused grid of the present invention comprises a plurality of substantially identical arc-shaped, corrugated mating modules, preferably comprising alternating layers of radiation transparent solid support material and radiation absorbent material. Preferably, each module is constructed from the radiation transparent support material coated with a relatively thinner layer of radiation absorbent material. The modules are assembled so that the support material and the absorbent material alternate to form a grid having a plurality of such focused channels, each bounded by radiation absorbent material; all of such channels are focused to the same point in space, intended to be located a certain distance from the assembled modules. The assembled modules are preferably mounted onto an appropriately shaped frame to form the focused arc-shaped grid of the present invention. The method of the present invention comprises the steps of forming, for example by injection molding, or by other thermoforming methods, a plurality of substantially identical mating modules made from radiation transparent material, and applying a layer of radiation absorbent material to each of the mating modules. The modules are then assembled to form focused channels and the assembly of focused channels is mounted onto a frame to form the focused grid of the present invention. In one preferred embodiment of the present invention, there is provided a focused grid comprising a plurality of mating modules; each module having at least one mating side surface covered with a layer of radiation absorbent material and each module being constructed from radiation transparent material. The modules are assembled to form a grid having a plurality of focused channels for the passage of x-rays, each such channel containing the radiation transparent material and bounded by the radiation absorbent material; each such channel is focused to the same point in space, said point located a certain distance from the assembled modules. The assembled modules are mounted onto an appropriately shaped frame to form the focused grid of the present invention. In one preferred embodiment, the shape of the focused grid is obtained from the projection of a square onto the surface of a sphere having a certain radius, R. The resulting structure is a spherical segment (i.e., a portion of a spherical surface) comprising a main inner surface having a radius R, a main outer surface, having a radius R′, a first end and a second end. The spherical segment further has outer side surfaces. The spherical segment is preferably formed from a plurality of substantially identical arc-shaped modules, constructed from radiation transparent material where each module has an outer radius, R′. Each module comprises opposing, mating side surfaces with mating structures, such as grooves (or corrugated surfaces), an arc-shaped top surface, an arc-shaped bottom surface, a first end and a second end. Further, preferably, for each arc-shaped module, at least one of the mating side surfaces (e.g., grooved or corrugated side surfaces) has a layer of a radiation absorbent material adhered thereto, such as a film or coating of a heavy metal. An assembly of the arc-shaped modules has adjacently positioned arc-shaped modules mated to each other via their respective mating side surfaces, thus forming channels between the absorbent metal coated side surfaces, permitting the guided passage of, e.g., x-ray radiation. The adjacently positioned modules may also be adhered to each other. Each such channel has a structure defined by the arrangement of radiation transparent material (i.e., the arc-shaped module) having one of its grooved side surfaces coated with a radiation absorbent layer (e.g., metal layer) and its opposing grooved side surface mated with or engaging a coated (with a radiation absorbent material such a metal layer) grooved side surface of an adjacently positioned arc-shaped module, so that the radiation transparent material is bounded by radiation absorbent material (i.e., the metallic side surfaces). Each of the channels has one or more focused axes where each focused axis is defined by a path (preferably a linear path) originating from a point in space a certain distance from the inner surface of the spherical segment extending to a point on the main inner surface of the spherical segment and through the radiation transparent material (e.g., an arc-shaped module) to a point on the main outer surface of the spherical segment without having intersected any of the surfaces coated with a layer of radiation absorbent material that define the channel boundaries; such a path is called a “focused path”, or “focused channel. The point in space that is located at the certain distance from the main inner surface of the spherical segment and from which the radiation emanates is herein defined as the “radiation source point”. Preferably, the radiation source point is located a distance equal to R from the main inner surface of the spherical segment. R can be any real number greater than zero. It will be readily understood that the radiation source point may be located a distance other than R from the main inner surface of the spherical segment. The focused grid of the present invention is therefore an assembly of a plurality of mating modules forming one or more focused channels where the assembly is mounted on a frame. That is, the assembly of the modules is mounted onto a suitably shaped frame having a structure that couples to or engages the first and second ends of the spherical segment and its outer side surfaces resulting in the focused grid of the present invention. The shape of the frame is also obtained from the projection of a square onto the surface of a sphere. The assembled top surfaces of the arc-shaped modules form the main inner surface of the spherical segment and the assembled bottom surfaces of the arc-shaped modules form the main outer surface of the spherical segment. The first and second ends of the assembled arc-shaped modules form the first and second ends respectively of the spherical segment. The outer side surfaces of the outer arc-shaped modules of the assembled arc-shaped modules form the side surfaces of the spherical segment. It should be noted that the outer side surfaces of the spherical segment need not be corrugated where the mating surfaces may be. The center of each arc-shaped module is the center of the spherical segment which is also the center of the sphere from which the spherical segment is created; said center is a point located in space a distance R away from any and all points on the main inner surface of the spherical segment; R is thus the radius of each of the arc-shaped modules and the spherical segment. The focused grid of the present invention as described above can be used as an anti scatter grid for x-ray imaging, useful, for example, in the fields of medical and/or industrial radiography. Referring to FIG. 11, the focused grid 100 of the present invention is viewed from one of its ends showing the substantially trapezoidal shapes of the ends of the arc-shaped modules. It should also be noted that each of the cross sections of the modules viewed from one of their ends is also substantially trapezoidal in shape; the grid frame is not included in FIG. 1 for ease of explanation and illustration. The grid 100 is shown positioned intermediate an object 200 to be imaged (e.g., portion of a human body) and an image receptor 300 as shown. A radiation source tube 400 is positioned so that a radiation beam 500 originates from a radiation source point 300 for the focused grid 100 as shown. The radiation may be x-rays comprising electromagnetic radiation having photon energies in the range of 10 keV to 1 MeV. For ease of explanation, the beam of radiation will henceforth be described as an x-ray beam in the range described. However, it should be understood that the claimed focused grid of the present invention may be operated and function as described using electromagnetic radiation having photon energies that fall outside of the range described above and the modules may be made from material that is transparent to such electromagnetic radiation with their boundaries made from material that can absorb such electromagnetic radiation. Continuing with the description of FIG. 1, a uniform beam 500 of x-rays is directed toward a surface of the object 200 and travels through the object 200 emerging from an opposite surface of the object 200. Differences in x-ray attenuation along linear paths through the object 200 produce an x-ray intensity pattern that comprises image information recorded by the image receptor 300. The image receptor 300 can be a device such as an intensifying screen coupled with a photographic film or any layer of x-ray sensitive material or x-ray sensitive electronic medium, which through one or more steps converts the x-ray intensity pattern into a visible image or visible format. When x-rays pass through the object 200 they are attenuated by a combination of scattering and absorption occurrences. X-rays which have passed through the object 200 and are focused x-rays (meaning they also pass through grid 100 following a focused path as described herein) are referred to as ‘primary x-rays’; the primary x-rays contribute to the formation of the image. That is, unscattered focused beams—having passed though the object 200—which then pass through the x-ray transparent materials of the focused grid of the present invention and do not impinge upon or intersect the radiation absorbent layers (defining the channel boundaries) lining the side surfaces of the radiation transparent modules contribute to the formation of the image. Radiation, including x-ray radiation, which do not follow a focused path and intersect one of the metallic layers (or radiation absorbing layers) that define the channel boundaries, are absorbed by such channel boundaries to an extent depending on the composition of the boundary and the energy of the radiation. The radiation absorbent layer can be designed to a desired or preferable state by altering its constituent elements based upon elements having higher or lower atomic numbers; or the thickness or density of the absorbent layer to better suit the absorption of x-rays of a specific range of photon energies. For example in an application using low energy x-rays, such as mammography, the absorbent layer may be only a few tens of microns thick and might include elements with atomic numbers as low as 40. Applications requiring more energetic x-rays, such as general medical radiography, may employ a thicker absorbent layer, consisting mainly of elements with atomic numbers above 65. An x-ray transparent material is a material through which an x-ray beam travels where the measurable intensity of the beam immediately prior to entering the material is substantially equal to the measurable intensity of the beam immediately after exiting the material. Conversely, an x-ray absorbent material does not allow any discernable (or only negligible) amounts of x-rays for the particular application to escape such material after the x-rays have entered such material. The x-ray transparent materials, forming the main body of the arc-shaped modules, are created through an injection molding method—discussed infra—where the molded material is a rigid polymer composed mainly of relatively low atomic number elements, (e.g., Hydrogen, Carbon, Oxygen, and Nitrogen) and having a physical density preferably less than 1.2 g/cm and is thus substantially transparent to x-rays. The x-ray transparency of these materials can be further enhanced by adding a foaming agent or micro bubbles to the polymer formulation during the molding process to further reduce the density of the final material. The unscattered and focused beams may or may not have been attenuated when passing through the object being examined. X-rays which were scattered during the passing through the object being examined do not follow a focused path through the channels of the focused grid, and thus intersect and impinge upon the x-ray absorbent boundaries of the focused channels and are thus absorbed by these layers. The absorbent layers are preferably formed of heavy metals such as Lead, Bismuth, Tungsten, or Tantalum. The layers of metal can also be made from low melting point alloys such as Low 117, Low 251 and Low 281, which are alloys of Bismuth with various combinations of Lead, Strontium, Cadmium and Indium. The layers of metal may not necessarily be pure metal, but may contain binding agents (e.g., polymers) to bind a relatively high concentration of heavy metal in the form of a fine powder or as a compound mainly containing elements with atomic numbers greater than 40. Depending on the application for which the antiscatter grid is being used, the relatively high concentration of heavy metals may be in the range of 40% to 98% by weight or volume of the absorbent layer. As already described with respect to FIG. 11, the focused grid of the present invention has one or more focused channels that allow unscattered primary x-ray beams having passed through the object 200 to impinge upon the image receptor 300. An end view of the focused grid 100 is depicted showing the cross sections of each arc-shaped modules 1001, . . . , 100M as being substantially trapezoidal in shape. As shown, there are M arc-shaped modules constituting the spherical segment, where M is an integer equal to 2 or greater. Various portions of the focused grid of the present invention are depicted in different fashions in FIGS. 12-18 to facilitate the description of one embodiment of the present invention. The drawings used in this application are not necessarily drawn to scale, but they are presented to clearly depict the various features and aspects of the present invention. A perspective view of the spherical segment of the focused grid of the present invention is shown in FIGS. 12 to 12C. FIGS. 12 A and B show individual arc-shaped modules 100 which can be assembled to form a spherical segment, as shown in FIG. 13, for example. Spherical segment 100A has a main inner surface and a main outer surface, generally designated by the numerals 106 and 108, respectively, formed by the mating arc-shaped modules 1001 to 100M. Each of the arc-shaped modules 1001 to 100M has respective first ends 1121 to 112M, respective second ends 1141 to 114M and respective side surfaces 1221, . . . , 122M and 1201, . . . , 120M. The spherical segment has outer side surfaces 1221 and 120M (partially shown). The spherical segment 100 comprises M substantially identical arc-shaped modules assembled so that adjacently positioned modules mate or engage each along their side surfaces. In this preferred example, the sides are corrugated side surfaces. The spherical segment of the present invention when cut across line C-C is shown in FIG. 1, where the cross section of each of the arc-shaped modules is substantially trapezoidal in shape. Referring now to FIG. 3, a partially exploded depiction of FIG. 2 is shown whereby each of the arc-shaped modules 1001, 1002, 1003 to 100M engages another adjacently positioned arc-shaped module along its corrugated side surface. The corrugations (which may be V-shaped grooves, as shown) for each of the modules are formed so that the assembly of arc-shaped modules (i.e., spherical segment 100 of FIG. 2) has aligned first and second ends as shown. As shown the peak of one corrugation fits into the groove of another. It should be noted that the dimensions of the corrugations relative to the dimensions of the arc-shaped modules are not necessarily drawn to scale. The arc-shaped modules have respective first ends 1121, 1122, 1123, to 112M, second ends 1141, 1142, 1143 to 114M, top surfaces 1061, 1062, 1063, to 106M, opposite side surfaces 1221, . . . , 122M and 1201, . . . , 120M and bottom surfaces 1081, . . . , 108M. Note the assembled top surfaces of the modules form main inner surface 106 and the assembled bottom surfaces form main outer surface 108. Referring now to FIG. 4, there is shown a perspective view of an arc-shaped module 1002 for the grid of the present invention. It will be readily understood that for the embodiment of the invention discussed with respect to FIGS. 2-4, the arc shape modules 1001 . . . , 100M are substantially identical to each other. Thus all of the geometrical, physical and/or chemical features attributed to or described with respect to arc-shaped module 1002 apply to the remaining arc-shaped modules as well (i.e., modules 1001, 1003 . . . , 100M). Arc-shaped module 1002 has a top surface 1062 and bottom surface 1082 (not shown) similar in shape to top surface 1062 but convex rather than concave. Most preferably, the top and bottom surfaces are concentric. Each arc-shaped module, e.g., 1002, has a first end 1122 having front surface 1122A, side surfaces 1122B and 1122C, top surface 1122E and bottom surface 1122D, a second end 1142 having front surface 1142A, opposing side surfaces 1142B and 1142C, top surface 1142E and bottom surface 1142D. Side surface 1222 is structured in a similar manner to side surface 1202 (not shown) having V-shaped grooves, or corrugations, as shown. The depth of the focused grid of the present invention corresponds to the depth of each of the identical arc-shaped modules, which is shown as “L” for module 1002. The angled side segments of the grooves, or corrugations, form the surface of each arc-shaped module; the groove segments have dimensions “s,” as shown, which are angled at 90° with respect to each other. Still referring to FIG. 4, the width “w” is measured from a peak of one groove to a valley of another oppositely positioned groove as shown. The measurement “w” can be said to be the width of each of the focused channels of the grid. Side surface 1222 of module 1002 may be coated with a heavy metal (as discussed above) able to absorb radiation such as x-rays. One side surface 1202 can be left uncoated. Thus, corresponding side surfaces (1221, 1223, . . . , 122M) of the other arc-shaped modules are similarly coated with a metallic layer and the corresponding opposite side surfaces (1201, 1203, . . . , 120M) of the other arc-shaped modules are similarly left uncoated. Referring temporarily to FIGS. 2 and 3, arc-shaped module 1002 is shown mating with or engaging a side surface of arc-shaped module 1001 and also engages a side surface of arc-shaped module 1003 as shown in FIGS. 2 and 3. Referring now to FIGS. 2, 3 and 4, while side surface 1202 of arc-shaped module 1002 is not coated, the side surface 1223 (not shown) of the adjacent module 1003 with which side surface 1202 of arc shape module 1002 mates is coated and thus a channel is formed with the coated metal layer on side surface 1222 (i.e., a first boundary of the channel), arc-shaped module 1002 (i.e., a radiation transparent material) and a metal layer coated onto the facing side of adjacent arc-shaped module 1003 (a second boundary of the channel). Each of the focused channels is thus formed with an arc-shaped module bounded by one of its side surfaces coated with a metal layer and a metal layer coating on the adjacently positioned arc-shaped module. Referring back to FIG. 4, when arc-shaped module 1002 is assembled with the remaining M−1 arc-shaped modules, resulting in the spherical segment 100 as shown in FIG. 2, and said spherical segment 100 is positioned a distance R (shown in FIG. 4 as 118) from a radiation source point 300, where R is the radius of the spherical segment, the channels of the spherical segment are focused to the radiation source point 300. As already explained and it will be readily understood, that the distance 118 need not be equal to R, but can be any suitable distance allowing the focused grid of the present invention to operate or otherwise function as described herein, i.e., to exclude scattered x-rays. An example of a focused axis is shown as path 116 originating from the radiation source point 300 and extending to top surface 1062 through arc-shaped module 1002 (made from x-ray transparent material) to outer surface 1082 where said path does not at all intersect or impinge upon the metallic layer boundaries, viz., side surface 1222 of arc-shaped module 1002 and the side surface 1223 (not shown) of adjacently positioned arc-shaped module 1003 that mates with side surface 1202 of arc-shaped module 1002. Thus, arc-shaped module 1002 forms a focused channel as defined herein where such channel has a channel depth of L and a channel width of w. The focused channel is bounded by x-ray absorbent layers 1222, bonded to the module 1002 and layer 1223 bounded to the module 1003. Referring now to FIG. 5, there is shown an exploded top view of a portion of FIG. 2, showing arc-shaped modules 1002 and 1003 positioned next to each other. Arc-shaped module 1003 has first end 1123, second end 1143, top surface 1063 and side surfaces 1223 and 1203. Arc-shaped module 1002 has first end 1122, second end 1142, top surface 1062 and side surfaces 1222 and 1202. A layer 1213 of metallic coating on side surface 1223 has a certain thickness t, for ease of illustration and description, the thickness of layer 1213 is drawn larger than if to scale relative to the width w (not shown in FIG. 5: see FIG. 4) of arc-shaped module 1003. Similarly, for arc-shaped module 1002 a layer 1212 of metallic coating on side surface 1222 has a certain thickness t; again for ease of illustration and description, the thickness of layer 1212 is not drawn to scale relative to the width w (not shown in FIG. 5; see FIG. 4) of arc-shaped module 1002. Still referring to FIG. 5, one channel comprises metallic layer 1212, arc-shaped module 1002 and metallic layer 1213. In general, for the specific embodiment being discussed, each of the channels of the grid of the present invention comprises an x-ray transparent material positioned between two metallic layers, one of which is a layer on one side of the x-ray transparent material and the other metallic layer is a layer on an adjacently positioned mating arc-shaped module. All of the corresponding arc-shaped modules of the spherical segment 100 have respective layers 1211, . . . , 121M of coated metal adhered to one of their side surfaces 1221, 1222, 1233, . . . , 123M. Each of the respective opposite side surfaces 1201, 1202, 1203, . . . , 120M-1 can be left uncoated, or may be coated if desired. Referring temporarily to FIG. 2, it is clearly shown that arc-shaped module 100M engages only one other module, viz., 100M-1, and thus to make an arc-shaped module function, as a focused channel, outer side 120M is also coated with a metal layer. Note that even though arc-shaped module 1001 also engages only one other module (i.e., module 1002), it functions as a focused channel because it is bounded by metal layers on side surfaces 1221 and 1222 (opposite side surface—not shown). Referring now to FIG. 6 there is shown frame 600 comprising sides 610, 612, 614 and 616 with respective inner slots 602, 604, 606 and 608. Only slot 602 is clearly shown in FIG. 6; oppositely positioned slot 606 is configured similarly to slot 602. Referring temporarily to FIGS. 6 and 2, oppositely positioned slots 602 and 606 are configured to receive outer surfaces 120M and 1221 respectively of spherical segment 100. The ends 114 and 112 respectively of each of the M arc-shaped modules engage the slots 604 and 608 respectively of frame 600. That is, slot 608 (not shown) is configured to frictionally receive respective ends 1121, . . . , 112M of the M arc-shaped modules. Similarly, and as shown in FIG. 7 for arc-shaped module 1001, slot 604 is configured to receive frictionally second ends 1141, . . . , 114M of the M arc-shaped modules of spherical segment 100. The shape of the frame is also obtained by projecting a square onto the surface of a sphere having radius R. For ease of explanation the spherical segment 100 as shown in FIGS. 2 and 3 and its portions shown in FIGS. 4 and 5 are oriented in the same manner so that the first ends 1121, . . . , 112M, face in the same direction. The focused grid of the present invention as shown in FIG. 1 (without frame 600) shows the front surfaces 1121A, . . . , 112MA of the M arc-shaped modules. FIG. 8 shows a perspective view of the grid of the present invention as described herein including frame 600. One example of the manufacture of one of the embodiments of the focused grid of the present invention comprises the following steps. Each of the arc-shaped modules is produced using an injection molding method using a form consisting of a pair of plates 700 and 800 with grooved surfaces facing each other as shown in FIGS. 9 and 10. The facing plates are curved to the radius of the arc-shaped modules and said facing plates have V-shaped grooves on their facing surfaces as shown. The grooves contain groove segments of length “s” and said segments are angled at 90° with respect to each other. The resulting arc-shaped module will have a radius, R, equal to that of the facing plates. For most medical radiography applications, the facing plates can be curved to have a radius of between 0.5 meters and 2.0 meters. The depth of the forms and thus of a resulting arc-shaped module (shown as L in FIG. 4) can be varied to obtain a desired performance. The plates are machined from aluminum or steel alloys commonly used to construct injection molds. The machining of the grooves employs computer controlled wire Electrical Discharge Machining (wire EDM) methods or other high resolution, high precision methods. The 90° grooves are machined into the surface of the curved plates so that lines along the grooves vertices and channels are convergent with the radiation source point when such point is located at distance, R, from the inner surface of the spherical segment 100. In a first step, the facing plates are aligned to each other with the grooves of one plate aligned with the peaks of another and a gap is left between the aligned plates creating a form as shown in FIGS. 9 and 10. The gap 900 left between the plates is shown in FIG. 10. Alternatively, the facing plates can be aligned such that the peaks on the surface of one plate align with the peaks of the surface of another plate forming diamond shaped gaps (not shown), instead of a corrugated shaped gap. In a second step, radiation transparent material (e.g., an x-ray transparent material such as a polymer material) is injected into the form, i.e., into the gap between the facing plates, and thus injection molds an arc shape mating module as shown in FIG. 4. As discussed previously the material injected into the form is x-ray transparent and such x-ray transparency can be enhanced by the addition of a foaming agent or micro bubbles to the material (e.g., a polymer formulation) during the molding process. Examples of suitable rigid polymers include ABS polymers, polyacetals, polyacrylates, polyamides (nylon), polycarbonates, polyethylenes, polypropylenes, polystyrenes, rigid vinyl polymers, as well as melamines, polyesters, epoxies, and blended polymers such as ABS/Polycarbonate, ABS/PVC, and PVC/Acrylic polymers and copolymers such as styrene/butadiene copolymers. In a third step a radiation absorbent material, such as a metal coating made from any one of various metals and/or alloys discussed herein, is applied to one mating side surface (e.g., a grooved surface) of the module and is caused to adhere to the side surface; the metal coating layer has a thickness, t. Any one of several methods for forming the layer is possible. For example, the layer can be formed through the use of electroplating the metal onto the module. Also casting or injection molding of a low melting point heavy metal alloy can be used to form the layer. Another method for forming the layer is the process of stamping, vacuum forming or pressure forming of a thin malleable metal layer onto a form or directly onto the radiation transparent material. Yet another layer forming method that can be used is a thermal spray process. One example of a thermal spray process is a plasma spray process wherein plasma gas is heated by an arc formed by two electrodes. As the plasma gas is heated by the arc, it expands and is accelerated through a shaped nozzle, creating relatively high velocities of the heated plasma. The metallic material (or a mixture of the metallic material and one or more polymers in powder form or molten form) is injected into the high speed plasma. The material is rapidly heated and accelerated to a relatively high velocity and impacts the surface being coated and rapidly cools forming the coating. In a fourth step a plurality (say M where M is an integer equal to 2 or greater) of the modules coated on one side with a metal layer are assembled. An adhesive can be used to adhere adjacently positioned arc-shaped mating modules to each other (see for example, FIG. 2) into an assembly and such assembly can be mounted onto a frame by sliding their ends onto receiving slots of the frame to frictionally attach to the frame and each such mounted module is caused to abut adjacently positioned modules. The assembly has outer modules with outer surfaces. The outer side surfaces of the outer modules of the assembly of modules (e.g., a spherical segment such as depicted in FIG. 2) can also frictionally fit into receiving slots of the frame. An adhesive may be used to attach the outer surfaces of the assembled modules (i.e., outer side surfaces of the outer modules—first module, module 1001 with outer side surface 1221 and the last module—module 100M with outer side surface 120M) to the frame 600. The outer side surfaces can also be attached to the frame with an adhesive and not necessarily be frictionally fit into a slot. Suitable adhesives depend upon the material forming the grid modules and the frame. Two-part and one-part epoxies are useful for a wide range of polymers, as are cyanoacrylates and polyurethane adhesives. The resulting grid, as shown in FIG. 18, shows the spherical segment 100 of FIG. 2 mounted onto the frame 600 of FIG. 16. Currently available grids are typically specified in terms of grid ratio, i.e., the ratio of channel depth to channel diameter or width. The same approach can be used for the focused grid of the present invention where the grid ratio is (L/W) (i.e., the ratio of channel depth, L, to channel width, W). A desirable set of dimensions for a grid—particularly a grid used generally for radiography purposes—is that the channel width, i.e., W, is approximately 1 mm. Thus, for a desired range of grid ratios of 8:1 to 16:1, the channel depth will fall in the range of 8-16 mm. An important performance characteristic of a grid is called the primary transmission P, which is defined by the following formula:P=s/(s+t)e−μ(E)L where t is the metal layer thickness as shown in FIG. 5, and it is assumed that W=s√2. The second term of the equation, viz., (e−μEL), is an expression that reflects the attenuation of the x-rays as they pass through the focused grid of the present invention, where L is the depth of the focus grid and μE is a linear attenuation coefficient for x-ray photons, of energy E, in the x-ray transparent material from which the arc-shaped modules are made. The primary transmission P represents the percentage of transmission that passes through the x-ray transparent material for a certain width, W, and depth, L, of the material and metal layer thickness, t. For a channel width, W, of 1.414 mm, the metal layer thickness would range from 0.0525 to 0.25 mm for primary transmissions, P, that range from 95% to 80% without the x-ray transparent material, respectively. For a channel made with a polymer material, the attenuation coefficient, μE, will vary with x-ray energy and with polymer density, which desirably should be less than 1.2 g/cm3. Considering the geometry and attenuation of the polymer material, the total primary transmission at 50 keV will range between 61% and 72% depending on metal thickness, t, and the density of the polymer material. Referring back to FIGS. 12-18, one particular grid designed and manufactured in accordance with the claimed focused grid of the present invention is a grid made from the arc-shaped modules of this invention, having a radius of R=985 mm, with L=13 mm, W=1.4 mm, t=0.1 mm and the grooves form 90° angles, and for the groove segments, s=1.0 mm; the arc-shaped modules are molded from ABS (Acrylonitrile Butidiene Styrene) polymer (specifically, a super high impact ABS plastic, such as TPI Porene® Grade ABS-SP-100-BK., having a μE=0.208 cm−1, at 50 keV, and plasma-coated with a layer of Bismuth metal. The final grid is installed in a frame having the dimensions 450 mm.×450 mm, which contains 430 individual modules adhesively connected to each other. It is understood that it is possible to avoid the use of adhesives, and merely mechanically press the modules together within the frame. Throughout this description supra, the values of variables, R, L, W, t, and s are real numbers greater than zero. It will be readily understood that the overall shape of the focused grid of the present invention may be obtained from the projection of an N-sided polygon onto a geometrical surface having at least one or more foci. In the case of a sphere, discussed above, there is one focus which is the center of the sphere. It should be noted that a rectangle, a triangle or any well known N-sided polygon (where N is an integer equal to 3 or greater) can be projected onto the surface of a sphere or any other three dimensional surface (e.g., surface of a spheroid or ellipsoid) to obtain the shape of the focused grid. It will therefore be readily understood that the shape of the claimed focused grid of the present invention is not limited to a spherical segment. The various aspects, characteristics and architecture of the device and method of the present invention have been described in terms of the embodiments described herein. It will be readily understood that the embodiments disclosed herein do not at all limit the scope of the present invention. One of ordinary skill in the art to which this invention belongs can, after having read the disclosure, may readily implement the device and method of the present invention using other implementations that are different from those disclosed herein but which are well within the scope of the claimed invention, as defined by the following claims.

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