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049816160 | description | DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the invention will now be described with reference to the drawing. The FIGURE is a view showing an embodiment of the spent fuel treatment method of this invention, in which (1) represents a dissolving tank, (2) a solvent extraction process, (3) a plutonium nitrate solution and uranyl nitrate solution, (4) a freeze-vacuum drying apparatus, (5) a nitrate, (6) a condensate, (7) a denitrification process, (8) a roasting reduction process, (9) a product, (10) a spent solvent, (11) a freeze-vacuum drying apparatus, (12) TBP, DBP, etc., (13) n-dodecan, (14) a vacuum distillation apparatus, (15) DBP, etc., (16) TBP, (17) a preparation process, (18) an incinerator, (19) liquid waste, (20) a freeze-vacuum drying apparatus, (21) residue, (22) water and nitric acid, (23) storage or solid waste treatment system, (24) a preparation process, (25) a utilization process, and (26) an emission process. In the drawing, nuclear fuel scrap which contains impurities generated at a fuel manufacturing plant or the like is supplied to (1) the dissolving tank along with a nitric acid solution, heated there and dissolved. Then uranium and plutonium solutions are sent to the solvent extraction process (2) after preparation. Solvents consisting of TBP, n-dodecan, etc., and the nitric acid solution are employed to effect separation into plutonium nitrate and uranyl nitrate solutions (3), spent solvent (10) and liquid waste (19). The plutonium nitrate and uranyl nitrate solutions (3) are separated into nitrates (5) and condensate (6) by the freeze-vacuum drying process (4). The condensate (6) is fed to the freeze-vacuum drying apparatus (4). Meanwhile, the nitrates (5) are sent to the denitrification process (7). After microwave heating, for example, for conversion to oxide, powder is prepared as needed by the roasting reduction process (8) employing a roasting reduction furnace or the like. The result is the product (9). Spent solvent (10) is separated into TBP, DBP, etc. at (12) and into n-dodecan (13) by freeze-vacuum drying apparatus (11). TBP, OBP (12) are separated into DBP, etc. (15) and TBP (16) by the vacuum distillation apparatus (14). DBP, etc. (15) is sent to the incinerator (18). Meanwhile, TBP (16) and n-dodecan (13) are blended in the preparation process (17) and the result is sent to the solvent extraction process (2) after preparation by the further addition of TBP, n-dodecan and so on as necessary. Liquid waste (19) is sent to the freeze-vacuum drying apparatus (20) and separated into residue (21) consisting of plutonium, uranium and americium impurities and the like, and into water and nitric acid (22). For recovery, residue (nitrates) (21) is sent to storage at process (23) or to a solid waste treating system. At the preparation process (24), water and nitric acid (22) are prepared by either concentration or dilution by means of adding water or nitric acid as necessary. The result is used at the process (25) and is also sent to, e.g., the dissolving tank (1), the solvent extraction tank (2) or another process, such as an off-gas scrubbing process, not shown. If there is a surplus, this can be released at the process (26). In the embodiment described above, the freeze-vacuum dry apparatus is employed at three points, namely (4), (11) and (20). However, if the system is operated with storage tanks provided, a single freeze-vacuum drying apparatus would of course be quite satisfactory. In accordance with the present invention, TBP, DBP and the like and n-dodecan can be separated by using a freeze-vacuum drying method in a solvent cleansing process, TBP and DBP can be separated by using a vacuum evaporation method in the solvent cleansing process, and the use of sodium can be eliminated. As a result, the amount of liquid radioactive waste is reduced, it is possible to abbreviate treatment, the amount of sludge produced is reduced and neutralization and filtration are unnecessary. By treating the liquid radioactive waste using a freeze-vacuum drying process having a high decontamination efficiency, most of the radioactive substance can be recovered as residue, the recovered solution can be reutilized, liquid waste can be reduced and liquid waste treatment simplified. Furthermore, plutonium and uranium solutions are recovered as nitrates by the freeze-vacuum drying method, and these solutions are rendered into oxides by thermal decomposition, thereby obtaining a powdered oxide product. As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims. |
059986891 | summary | BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The invention relates to a method for recycling metal parts contaminated by radioactive elements, in particular by .alpha.-emitters, in which a melt and a slag are formed from the metal parts and the slag is then separated from the melt. When dismantling as well as when operating nuclear facilities, large amounts of contaminated metal, in particular iron-containing scrap, are produced and have to be recycled or disposed of. It is customary to subject the scrap to a melt decontamination. The radioactive elements which the scrap contains are, for example, .alpha.-emitters, such as uranium, thorium, transuranium elements and/or alkaline-earth metals. A method which includes a melt decontamination is known from German Published, Non-Prosecuted Patent Application DE 33 18 377 Al. In that method, the contaminated metal is melted, and a slagging agent is added to the melt which is formed. A slag which floats on the melt is then formed. Radioactive elements (e.g. uranium, neptunium and plutonium), which caused the contamination of the metal parts, accumulate in the slag. Thus, while the concentration of those elements in the slag increases, their concentration in the melt decreases. That is to be attributed to the fact that the solubility of the elements in question is greater in the slag than in the melt. Decontamination of the melt is thus achieved. The slag is later drawn off from the surface of the melt. Its volume is small as compared to the total volume of the melt and the slag. It is only that small volume of slag which has to be taken to final storage. Consequently, only a relatively small final storage capacity is required. The metal melt, which has a volume that is significantly greater than the volume of the slag, is decontaminated and can therefore be reused. For example, shaped metal parts may be produced from decontaminated metal. Although the elements which caused the contamination of the metal parts are substantially removed from the metal melt by using the known melt decontamination, a further increase in the degree of decontamination is nevertheless desirable. Published French Patent Application 2 479 540 discloses a method for the disposal of radioactive waste in which component parts of the waste are oxidized. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a method for recycling radioactively contaminated metal parts, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods of this general type and which removes elements that caused the radioactive contamination of the metal parts from the metal melt to an even greater extent than heretofore. It is therefore intended for a greater amount (volume or mass) of the radioactive elements to be removed from the melt and introduced into the slag. With the foregoing and other objects in view there is provided, in accordance with the invention, a method for recycling metal parts contaminated by radioactive elements, in particular .alpha.-emitters, which comprises oxidizing radioactive elements by holding contaminated metal parts in an oxygen-containing atmosphere for a period at a temperature below a melting temperature of the metal parts; forming a melt and a slag from the metal parts after oxidizing the radioactive elements; and then separating the slag from the melt. The temperature at which the oxidation occurs must be as high as possible, i.e. only slightly below the melting temperature, in order to ensure that on one hand, no melt is yet formed during the oxidation and on the other hand, oxidation which is as complete as possible is made possible. The oxidation process namely proceeds better at higher temperatures. This process is advantageously simplified due to the fact that all of the contaminated metal parts, and not just the radioactive elements exclusively, are subjected to an oxidation process. Since the radioactive elements adhere to the surfaces of the metal parts, complete oxidation of those elements is ensured. The fact that a relatively small part of the metal of which the metal parts themselves is formed is also oxidized in the process, does not impede the subsequent melt decontamination. The oxides of the elements then preferentially pass into the slag during the melting process. The method according to the invention particularly brings the advantage of ensuring that the radioactive elements which are to be dissolved in the slag are present as oxides, having a solubility which, surprisingly, is greater in the slag than the solubility of the unoxidized elements. As a result, a significantly increased proportion of the elements which caused the contamination is advantageously removed from the metal melt. As a result, the metal melt and thus also shaped metal parts produced therefrom, for example, are substantially decontaminated. The shaped metal parts can then be used without limitations. In accordance with another mode of the invention, the oxygen-containing atmosphere is dry or moist air. It is then advantageously unnecessary to provide any special oxygen-containing atmosphere. In accordance with a concomitant mode of the invention, the period for which the metal parts are held in the oxygen-containing atmosphere at a temperature below the melting temperature of the metal parts, for the purpose of oxidation, is 10 minutes to 60 minutes long. Sufficiently complete oxidation of the radioactive elements is ensured in a period of this length. The metal parts to be decontaminated may, for example, contain iron and/or copper. Other steps which are considered as characteristic for the invention are set forth in the appended claims. |
description | The present invention relates to nuclear reactors and more specifically to heat pipe cooled molten salt reactors. Climate change is perceived to be one of the biggest threats to the world's economy in our time. Therefore, interest in developing clean energy technologies that can reduce reliance on fossil fuels has heightened. To that end, there is renewed interest in thoroughly reinvestigating and improving nuclear power, particularly as a power generator in markets that have heretofore been underserved by these technologies. Very small (less than 10 MW-thermal) nuclear generation devices have been developed, but have met with problems. For example, Patent Application US2016/0027536 from Los Alamos National Labs describes a heat pipe portable reactor concept that has a solid stainless steel monolith core. Heat from the solid core is transported to the secondary side of the reactor via passive heat pipes. A heat pipe reactor has the advantage of not requiring an active pump. Although the design is very simple, there are some challenges. The solid monolith's fuel and heat pipe configuration result in a low-density nuclear fuel packing which may cause some neutron leakage and absorption of neutrons by the monolith, resulting in a lower neutron flux, necessitating a large amount of initial fuel loading to become critical. Although heat pipes have high heat removal capability, they take up space in the core. Therefore, for a heat pipe reactor, any space not occupied by a heat pipe, should be occupied by fuel in order to attain highest fuel density. However, due to the solid state core and heat pipes, the interface between core and heat pipes may be problematic. While at steady state, the reactor would be expected to work without issues, but during transient events, the heat pipe and monolith may expand and contract at different rates and contribute to significant stresses in junctions and in the monolith itself opening the possibility of a heat pipe or monolith failure. Furthermore, the core requires a relatively high enrichment, as much as 20%, to enable criticality due to the small size and neutron attenuation by the monolith. In addition, a large amount of fuel is needed to make the core critical and extend the life of the core. This leads to low fuel burn-up and poor fuel utilization. Another type of reactor investigated from time to time for more than fifty years, is the molten salt fast reactor, wherein the fuel is dissolved in molten salt and actively pumped to the primary heat exchanger. While traditional molten salt reactors have many advantages, such as simplicity and safety, there are inherent disadvantages and limitations. For example, molten salt fuel transfers heat poorly compared with sodium in a liquid metal fast breeder reactor and the high melting point (˜560° C.) of suitable fuel salts necessitates preheating. The high melting point of the fuel salt limits the Δt across a heat exchanger. Consequently, the mass flow rate must be increased. In addition, the presence of fission products in the fuel salt necessitates a high standard of plant reliability and leak tightness, reducing the utility of molten salt reactors for widespread or remote deployment. See, Eric H. Ottewitte, “Cursory First Look at the Molten Chloride Fast Reactor as an Alternative to the Conventional BATR Concept,” April 1992, (http://egeneration.org/wp-content/Repository/Feasibility_and_concept_study/MCFR BATR. pdf). The disadvantages with the prior reactors are addressed by the nuclear reactor described herein. In various embodiments, a reactor is provided for operative connection to a power conversion system or process heat system. In certain embodiments, the reactor may comprise a containment vessel, a reactor core housed within the containment vessel, a neutron reflector spaced from the containment vessel and positioned between the core and the containment vessel, a liquid fuel comprised of a nuclear fission material dissolved in a molten salt enclosed within the core, a plurality of heat transfer pipes, each pipe having a first and a second end, wherein the first end is positioned within the reactor core for absorbing heat from the fuel, a heat exchanger external to the containment vessel for receiving the second end of each heat transfer pipe for transferring heat from the core to the heat exchanger, and at least one reactor shut down system. In various aspects, there may be two shut down systems. In various embodiments of the reactor, there may be at least three shutdown mechanisms: 1) one or more melting plugs which allow molten salt fuel to drain into a chamber, changing the critical mass and volume of the core, thus stopping the fission reaction; 2) a neutron absorber material, that may be in the form of neutron absorbing spheres, such as boron carbide spheres, that can fill a central cavity within the core by gravity triggered by deactivating a release member manually or automatically through a sensor signal, such as temperature sensor; and 3) rotating control members, in the form of drums or hollow rings or pipes, that include a neutron absorbing material portion and a neutron reflecting material portion. In certain embodiments, the reactor may comprise very small (less than 10 MW-thermal) nuclear generation devices which, with the disclosed reactor design, provides a more reliable, sustainable, flexible, secure, resilient and/or affordable power generator than has heretofore been available. 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 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. The reactor described herein comprises of an inherently load following, very small modular reactor that has a high degree of inherent safety, self-regulation, and security, with a passive heat transport system, passive decay heat removal and at least one shut-down system. The system is expected to have a long life, to require no planned maintenance, and is inherently proliferation resistant, which makes it ideal for use as an autonomous nuclear power generation system for remote or decentralized power needs. In various embodiments, the reactor described herein is a molten halide salt reactor with a uranium halide (fluoride or chloride) or a uranium oxyhalide fuel dissolved in the molten salt, such as UO2Cl2 or UO4 dissolved in any one or more of KCl, MgCl2 and NaCl, with heat pipes to remove heat from the core to a secondary side of the reactor. Unlike previous molten salt reactor concepts where the fuel was pumped or moved to a primary heat exchanger, the liquid fuel in the reactor described herein is kept within the core container and heat pipes are used to carry only the heat from the liquid core to a secondary side of the reactor. In various embodiments, the reactor is a molten salt fast reactor, wherein, as stated, the liquid fuel is kept within its container and is not pumped around unlike traditional molten salt reactors. This eliminates problems arising from the complexities associated with pumping around liquid fuel. The various embodiments of the reactor described herein use heat pipes to transport only the heat from the core to a secondary fluid, for example, in a heat exchanger. In various embodiments, the reactor may be a small, less than 20 thermal Megawatt (MWth), molten salt fast reactor. The fuel is dissolved in an alkali metal or alkaline-earth-metal halide salt such as a mixture of sodium chloride, potassium chloride, magnesium chloride, calcium chloride, and the like. Fluorides can also be used instead of chlorides. The fuel can be uranium chloride or uranium fluoride, uranium oxide, or uranium oxyhalides, with possible mixtures of thorium halides and transuranics. In certain embodiments, the reactor is a thermal or epithermal reactor wherein moderators, such as graphite, are introduced in the core. Referring to the Figures, an embodiment of a heat pipe molten salt fast reactor 10 (HP-MSFR) is shown. The reactor 10 is operatively coupled on a secondary side to a heat exchanger that is used to heat air or gas to drive a gas turbine or reciprocating machine or to generate steam for use in a steam turbine, either of which are used to drive an electric generator (not shown), similar to those of conventional water cooled reactors and prior forms of molten salt reactors. These features are well known to those skilled in the nuclear power generation industry and will not be described herein. In various aspects, reactor 10 includes a containment vessel 12 having side walls 76, for example, in the form of a cylinder, a floor 78, and a ceiling 80. In various aspects, vessel 12 may be housed in a barrier housing 60 to provide a second containment function against leakage of fission material and to provide a secure barrier against external threats to the reactor core. The barrier housing 60 may be made of suitably thick and preferably reinforced concrete or a suitable structural metal that satisfies both desired functions. Barrier housing 60 may be placed for example, directly or indirectly in contact with the ground. The exterior of vessel 12 side walls 76 includes a plurality of cooling fins 16 that radiate outwardly from the vessel walls 76. A portion of the interior of sidewall 76 near ceiling 80 may optionally be lined with a fission gas adsorber 22. Reactor containment vessel 12 may be made of a structural material to satisfy the containment function of the vessel. The material also may have good conducting properties. Exemplary materials include stainless steel and other good conducting structural alloys. Cooling fins 16 are preferably made of a good conducting material, such as stainless steel. In certain embodiments, the reactor is a fast reactor having a thick neutron reflector 18 spaced from the interior of vessel 12 walls 76. The neutron reflector 18 may be annular in cross-section and have walls 88, a floor 58, and a ceiling 82. The neutron reflector 18 may be made of materials such as alumina oxide (Al2O3), beryllium oxide (BeO), or beryllium carbide (Be2C). The space between the walls 88 and ceiling 82 of the neutron reflector 18 and the walls 76 and ceiling 80 of the vessel 12 (whether or not lined with the absorber 22) defines a cavity 20. The core gas plenum or cavity 20 has dual function, both during normal operation and during shutdown. During normal operation the cavity 20 holds volatile fission product gases, which radiates significant thermal energy. The fission gases, such as helium, xenon, krypton, and radon, are generated by the nuclear fission reaction within the core 14. The volatile fission gases are poisonous for reactors, so to increase the useful life of the reactor, it is best to remove the gases. Exterior to the cavity 20 and vessel 12, are the cooling fins 16, which conduct heat away from vessel 12 to air and constantly cools the plenum. The plenum 20 may also include adsorber 22, which may be made from or include any suitable material that will adsorb fission gases, such as activated charcoal, zeolites or molecular sieves, to trap and retain fission gases to reduce vessel pressure. In the embodiments wherein the molecular sieves are cooled, cooling can be carried out passively via convective fins or by active air cooling. The fins can either be made out of conductive fins, such as stainless steel, copper, tungsten or heat pipe plates. The vessel 12 of reactor 10 may also include an optional gas extraction port 28 from cavity 20 which can be used to periodically remove the volatile fission gases in a batch or continuous mode. If gas is released continuously, the fission gases can be sent through delay beds or directly sent out through the stacks. Within the area defined by the neutron reflector 18 surfaces 88/58/82, lies the reactor core 14, designated by a dashed line in FIG. 1. The core 14 includes the molten salt fuel mixture 34 and heat pipes 32. Heat pipes 32 are preferably arranged evenly throughout the core and within the neutron reflector walls 88. The heat pipes 32 run parallel to the central axis 100 of reactor 10 and, in various embodiments, may be positioned along imaginary lines radiating from axis 100 in a spoke like configuration, as illustrated in FIG. 3. Heat pipes 32 extend upwardly through neutron reflector 18 ceiling 82, plenum space 20 and vessel 12 ceiling 80 to carry the heat generated by fuel 34 in core 14 from the ends 92 of heat pipes 32 into a primary heat exchanger 70 positioned above vessel 12. The ends 94 of heat pipes 32 form the heat exchanger by transferring the heat from the core to a secondary fluid that passes by ends 94. A secondary fluid inlet port 64 and a secondary fluid outlet port 66 create a pathway through heat exchanger 70 for a secondary fluid to carry the heat to, for example, a turbine (not shown) or comparable equipment, to convert the heat energy into electricity by well-known means, or to transfer the heat to another location. The secondary fluid may be any fluid that can absorb and transfer heat. Exemplary fluids include air and water or another fluid. A cold fluid, such as liquid water, cold air, or supercritical CO2, enters the heat exchanger inlet port 64, flows past the upper portions 94 of heat pipes 32 where the heat in the heat pipes 32 flows from the higher potential to the lower potential secondary fluid, elevating the temperature of the secondary fluid, and exits the heat exchanger 70 through outlet port 66 as, for example, hot air or steam or a heated version of the chosen fluid, for transfer to the secondary side of the reactor 10. In various embodiments, the heat pipes 32 are similar to those known in the art and may be constructed in the same way. The interior of heat pipes 32 may include, for example, a thin layer of a spongy material such as sintered stainless steel, wire mesh, or may be hollow tubes filled with a liquid to wick away the heat generated by the fission reaction of the fuel 34 in the core 14. The liquid may be sodium or potassium, or an alkali metal, such as lithium. In a fast reactor, the liquid in the heat pipes is selected from sodium and potassium and combinations thereof. However, unlike a solid core reactor with heat pipes, the heat pipes 32 used in various embodiments of the reactor 10 described herein are always in “wetted” thermal contact with the liquid fuel. In various embodiments, the evaporator end 92 of each heat pipe 32 is “dipped” into the reactor coolant system, while the condenser end 94 is integrated in a primary heat exchanger 70. The heat pipes 32 may be alkali metal heat pipes, where one end 92 of the heat pipe 32 is submerged in the molten salt 34 of reactor 10. The submerged end 92 of the heat pipes 32 and the interior of the reactor vessel 12 and any other structures that are in contact with molten salt can be made from corrosion resistant materials such as nickel or molybdenum steel alloys, ceramics, such as alumina or coated by a corrosion/erosion resistant material, such as high nickel steel, other metals or ceramics, such as alumina. In various aspects, heat pipe 32 bundles can also be located above the reactor core 14 to induce natural convective flow in the molten salt. If one or more heat pipes 32 fail, the others can pick up the heat without considering its relative position to other heat pipes 32, thus significantly improving the safety of the reactor. The reactor 10 preferably utilizes the molten salt fuel vessel geometry to place the fuel (in the molten salt) in an optimum shape such as a sphere that will reduce or minimize the neutron losses. This is particular important in small reactors where it is challenging to reach criticality due to the high neutron leakage. Although vessel 12 and core 14 are shown in the Figures as cylindrical in shape, other shapes may be used. However, the closer the shape is to being round, at least in the portions vulnerable to leakage, the more efficient the reactor is in preventing leakage. As stated, placing the fuel in a spherical shape is believed to be optimal. Appropriate changes to the structural elements (e.g., the containment vessel 12 and neutron reflector 18) of the reactor 10 from a cylinder as shown to spherical shapes may be made. In various embodiments, the reactor 10 includes a reactor shut-down system. In various embodiments, the reactor 10 includes at least one, and preferably multiple reactor shut-down systems, that together act as redundant fail-safe systems in the event another shut-down system is for whatever reason inoperative or inadequate under a given scenario. The shut-down systems may be active or passive, and in various embodiments, at least one passive shut-down system may be employed. In various embodiments, at least one active shut-down system may be employed. Active mechanisms may be automatically triggered or manually triggered or have both options available. In one embodiment of a shut-down system, one or more melt plugs may be used to change the geometry of the reactor core in the event of a failure to transfer heat from the core as designed. In certain aspects, an opening 48 may be cut in the neutron reflector floor 58 to expose a drain hole 62 in the vessel floor 78. The drain 62 leads to a drain chamber 36 under vessel 12 floor 78. Secondary heat dissipation elements or conductive fins 38 extend from chamber 36 into the barrier housing 60. The drain hole 62 is plugged during normal operation with a first melt plug 26. The melt plug 26 and drain 62 with chamber 36 act both to shut down the reactor and as a decay heat removal mechanism, transferring heat to the ground underneath the core. This feature shuts down the reactor and also forms a conductive bridge between the molten salt and the barrier material in housing 60 and/or ground, thus conducting decay heat to the ground. The fins 38 can be made out of either heat pipe plates or conductive materials, such as stainless steel, aluminum, copper, or tungsten. A second melt plug 24 may be positioned to plug under normal operating conditions, a passage 74 in the neutron reflector wall 88 leading to cavity 20. In the event that the heat transfer pipes 32 do not function as designed or there is another cause for an increase in the temperature of the reactor core above a predetermined level deemed to be safe, thereby indicating failure conditions, one or both of the melt plugs 24/26 melt in response to the predetermined temperature rise, draining the molten salt fuel through drain 62 into the drain chamber 36 where the heat dissipation elements 38 dissipate the heat to cool the molten fuel, and/or through passage 74 into plenum 20, where cooling fins 16 on the exterior of vessel walls 76 passively cool the molten fuel by air or coolant circulation. The melt plugs 24/26 may be made of any suitable material that melts only at the predetermined temperature deemed to be at the juncture between safe and unsafe operating conditions. For example, plugs 24/26 may be made of an aluminum alloy or stainless steel, or any alloy that will melt at, but not below, the predetermined critical temperature. For example, if the critical temperature at which conditions may be deemed to be unsafe is above 700° C. then an aluminum alloy that melts at, for example, 650 or 660° C. to sufficiently less than 700° C. for effective safety, would be used. By opening drain 62 and/or passage 74, the core geometry, which is important to the optimum functioning of a reactor, is altered, thus shutting down the reactor when coolant temperature rises beyond normal operating range. Another embodiment of a hybrid active and passive safety shutdown system is shown in inactivated and activated states in FIGS. 1 and 2, respectively. This embodiment of the reactor shutdown system can be activated by both passive activation or active activation features. Upon activation, a neutron absorbing material, such as a boron carbide material, may be inserted into the open cavity 40 of a hollow tube 72 in the center or outside of the core 14 volume that is not filled with the molten salt. Until activation, the absorbing material is held away from the core 14. Referring to FIG. 1, a tube 72 defining an absorbing cavity 40 runs through the center of core 14 coaxial to the center axis 100 of reactor 10. The tube 72 and cavity 40 extend through the neutron reflector ceiling 82, vessel ceiling 80 and into the primary heat exchanger 70. A holding gate 46 and ramped or sloped member 52 are inside the tube 72. The member 52 is suspended in absorbing cavity 40 by an actuation rod 54, which in turn extends downwardly into cavity 40 from a control rod drive or release mechanism member 56, positioned above and outside of the heat exchanger 70. In various embodiments, the tube 72 may further include a guide tube 42 positioned within cavity 40 beneath ramped member 52. Referring to FIG. 1, illustrative of the absorbing material shut-down system in the deactivated state, the neutron absorber material, for example, neutron absorber spheres 44, are held in the upper portion of cavity 40 by holding gate 46. In this position, the absorber spheres 44 are positioned outside of the core 14. Suitable dual active-passive activation mechanisms may be used. FIG. 2 shows the absorbing material shut-down system in the activated state wherein the holding gate 46 has opened, ramped member 52 and rod 54 have been lowered so that ramped member 52 rests on top of guide tube 42. When activated, control rod release member 56 releases rod 54 so that it lowers ramped member 52, thereby opening holding gate 46 and releasing absorber spheres 44 into the lower portion of cavity 40 within core 14 between the walls of tube 72 and guide tube 42. Guide tube 42 directs the spheres 44 towards the wall of tube 72 to be in close proximity to core 14. The absorbing material absorbs the neutrons from fuel mixture 34 which shuts down the fission reaction. The holding gate 46 is kept closed by the release member 56, which may be for example, an electromagnetic device that is structured to release the ramped member 52. Upon activation, the electromagnetic upwardly pulling force can be deactivated by breaking the electricity supply to the electromagnetic device. The electricity supply can be broken by manual or automatic activation through a signal, for example, when a pre-determined set point temperature below, for example, the melting point of the melt plugs 24/26, is exceeded. Upon release, the rod 54 falls into the open activated position by gravity to allow the release of the absorber spheres 44. Referring to FIG. 3, a plurality of rotatable member 30 positioned within neutron reflector walls 88 provide another embodiment of a shut-down system. Rotatable members 30 may be in the form of drums, rings or pipes that extend the length of the neutron reflector material. A portion or segment 68 of each rotatable member 30, shown in FIG. 3 as arcuate segments 68, includes an absorber material made, for example, of boron carbide, B4C. Rotatable members 30 are operatively connected to rotating drive mechanism 90 through actuation rods 96 to actuate rotation of rotatable members 30. The rotatable members 30 can be used to control the fission reaction. If the rotatable members 30 are rotated so that the absorber material on segments 68 face core 14, neutrons from the fuel 34 in core 14 will be absorbed into the boron carbide absorbing material, causing the reactor to reach subcritical neutron levels, and the reaction will stop. Referring to FIGS. 1 and 2, another embodiment of a shut-down system can be provided by adding a heating element 98 proximate to one or both of drain 62 and passage 74. The heating element may be positioned in the drain 62 or passage 74 or sufficiently close to heat the melting plug 24 and/or 26. The heating element 98 can be actuated by supplying an electric current through the heating element 98 that melts the melting plug 24/26 upon actuation. Electric current may be supplied by a heating element controller electrically connected to the heating element 98 for manual activation of the heating element 98. By opening drain 62 and/or passage 74, the fuel will flow out and the core geometry, which is important to the optimum functioning of a reactor, will be altered, thus shutting down the reactor. When the reactor 10 is initially loaded with the molten salt fuel mixture 34, the segments 68 of absorber material face reactor core 14. When the reactor 10 is activated or turned on, the rotatable members 30 are gradually rotated to move the segments 68 with the absorbing material away from the core 14. When the reactor reaches the desired power level, the rotation of rotatable members 30 is stopped. As the fuel is used up, rotatable members 30 in the position shown in FIG. 3 reflect more neutrons back towards core 14. When there is a build-up of fission products, the fission reaction stops and the reactor shuts down. In various embodiments, a gamma shield may be positioned outside of the reactor 10 as an extra precautionary measure and protective shield for people in the vicinity of the reactor 10. Alternatively, or in addition, the reactor 10 may have a double walled containment vessel 12. Molten salt is corrosive in nature. In a traditional molten salt reactor, all reactor vessels, fuel rods, pipes, primary heat exchangers, valves, pump impeller and other components had to be lined with corrosion/erosion-proof material. In the reactor described herein, only the heat pipes 32 and the internal wall 76 of the vessel 12 have to be made of or lined with corrosion resistant material. If the neutron reflector 18 is made of alumina, it can also act as an inner wall to the vessel 12, as shown in FIG. 1. Liquid fuel dissolved in molten salts has a very strong temperature reactivity feedback coefficient so the core 14 is self-regulating. This eliminates the need for active reactor controls. However, as shown in FIG. 3, the rotatable members 30, with absorber 68 in a crescent or arcuate form, may be used to control reactivity and temperature set point during normal operation. The control rotatable members 30 are mainly used as an active shutdown system. In addition to the rotatable members 30, an additional shutdown systems may be provided. In fast reactor embodiments, a neutron reflector 18 is positioned around the fuel mixture 34. Additional heat pipes 32 can be embedded in the neutron reflector wall 88 for preheating the secondary fluid before removing heat from the core 14. Volatile fission gases will evolve out of the liquid fuel mixture 34 and start collecting in the plenum 20 above the core 14. The plenum 20 is extended to a region outside the neutron reflector 18 to accommodate more volatile fission gases. A gas adsorber 22 may be used to reduce pressure of the vessel 12. Due to high activity of the fission gases, the adsorber needs to be constantly cooled. The gas adsorber 22 is attached to the inner layer of the reactor vessel 12. On the exterior of the reactor vessel there are finned channels 16. Natural convection of air through these channels 16 in the space between the vessel 12 and barrier housing 60 can constantly cool the reactor 10 down during normal operation. During off normal scenarios, if by any chance, the reactor core 14 becomes over-heated the passive shutdown mechanisms or the fuel negative feedback kick in. There may, for example, be any one of, or a combination of, the following three shutdown mechanisms: 1) one or more melting plugs 24/26 which allow molten salt fuel to drain into one or both of a chamber 36 and the cavity 20 surrounding the core 14, changing the critical mass and volume of the core 14, thus stopping the fission reaction; 2) a neutron absorber material, that may be in the form of neutron absorbing spheres 44, such as boron carbide spheres, that can fill a central cavity 40 within the core 14 by gravity triggered either by activating the release member 56 manually or automatically through a sensor signal; and 3) rotatable members that include a neutron absorbing material portion and a neutron reflecting material portion. The reactor 10 in various embodiments may include two independent decay heat removal systems. In one, a first melting plug 26 melts to open the core 14 to a drain chamber 36 to conduct much of the heat via conducting fins 38 or heat pipes 32 into the concrete or structural material of barrier housing 60 and eventually to the ground, once it is filled with molten salt fuel. A second melting plug 24 allows the bottom of the outer fission gas cavity 20 to be filled with molten salt and fuel thus allowing the decay heat removal by air utilizing the fins 16 outside the reactor vessel 12. The reactor 10 is designed to accommodate the liquid state of the already molten fuel so there is no risk of a fuel meltdown as is present where solid fuel can potentially melt down at high temperatures in certain severe accident scenarios. The reactor vessel 12 is preferably made with neutron reflector material such as alumina, which has a very high melting point of 2072° C., and has very little chance of failure due to temperature. Before the vessel 12 can be damaged by overheating, the melting plugs 24/26 will allow safe reactor shutdown. The reactor 10 described herein provides layers of inherent and passive safety by elegantly integrating multiple functions, without complicating the system. Nuclear micro-reactors can generate reliable, safe, emissions-free energy for heat and electricity production in decentralized locations. Various embodiments of the reactor described herein are expected to satisfy the desire for reliability, resiliency, efficiency, sustainability and security. The nuclear micro-reactor described herein can achieve all or any combination of these goals in most market applications. Decentralized generation of emissions free energy may supplement or even surpass centralized power generation capacity, thereby increasing distribution and availability of clean, reliable power at a significant cost savings. The reactor 10 is inherently safe. The reactor 10 has several key advantages. Compared to a solid monolith reactor, a heat pipe molten salt fast reactor (HP-MSFR) has a much more profound and prompt negative temperature feedback coefficient due to expansion and Doppler Effect. Under any normal or off-normal transient conditions, the reactor 10 is designed to self-regulate to ensure reactor safety. Since it is a fast reactor and the fuel is in liquid form, a change in reactor geometry can shut down the reactor. If the heat pipes 32 become ineffective in carrying away the heat generated from the core 14, the temperature in the core 14 rises. However, in certain embodiments, before the temperature can rise above a critical temperature determined to be unsafe, a very reliable passive shutdown system is triggered by the use of one or both melting plugs 24/26. The liquid fuel can drain to into cavity 20 or to chamber 36 which can also remove decay heat to the ground very effectively. Another advantage of the reactor 10 is its reduced fuel cost. Since the fuel is molten, there is no need for a fuel fabrication plant, which can be a significant upfront cost savings. Molten salt reactors, due to their high power density, can have a small core without the need of very high enrichment. This can significantly reduce licensing challenges and infrastructure needs. Molten salt reactors can burn any fuel ranging from uranium, thorium or transuranic from used fuel. The reactor 10 can potentially play a vital role in the deployment of a closed fuel cycle and reduce long lived waste. Due to the high power density of the core, one can design and build a very small HP-MSFR for small, decentralized power generation for both heat and electricity generation. Due to its overall simplicity and need of very little fuel, various embodiments of the reactor described herein are expected to be economically competitive, not only in price, but in the potential for off-grid decentralized power generation applications. The molten fuel neutronic behavior characteristic and inherent heat pipe behavior enhance the inherent control capability required for autonomous operation. 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. |
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062883003 | claims | 1. A process of heat treating of organic materials in the presence of air or oxygen, comprising; mixing the organic materials with a hydrated metal oxide; and heating the mixture. mixing the contaminated materials with hydrated ferric oxide; heating the mixture at a temperature to cause thermal decomposition of the organic materials; and pressing the heated mixture and gradually removing a large part of the water present in the mixture while under pressure for a period of time to produce a solid composition. mixing the contaminated materials with a metal oxide; heating the mixture to cause thermal decomposition of the organic materials; adding hydrated ferric oxide to the heated mixture; and pressing the heated mixture and gradually removing a large part of the water present in the mixture for a period of time to produce a solid composition. mixing the contaminated materials with hydrated ferric oxide, comprising at least about 20% Fe.sub.2 O.sub.3 by dry weight of the total weight of the mixture; and pressing the mixture at room temperature and gradually removing a large part of the water present in the mixture while under pressure for a period of time to produce a solid composition. precipitating hydrated ferric oxide in the solution to incorporate at least a fraction of the contaminated materials, wherein the hydrated ferric oxide comprises at least about 20% Fe.sub.2 O.sub.3, by dry weight of the total weight of the precipitate; and pressing the precipitate at room temperature and gradually removing a large part of the water while under pressure for a period of time to produce a solid composition containing the contaminated materials. mixing the contaminated materials with a metal oxide; heating the mixture; thermally decomposing the mixture; and immobilizing the heated mixture. mixing the contaminated materials with a hydrated metal oxide; heating the mixture; thermally decomposing the mixture; and containing the heated mixture in a high integrity container. 2. The process of claim 1, comprising mixing the hydrated metal oxide with organic materials chosen from the group consisting of organic solid hazardous wastes, organic solid radioactive wastes and organic solid municipal wastes. 3. The process of claim 1, comprising mixing the hydrated metal oxide with organic materials chosen from the group consisting of polymers, plastics, ion exchange resins and polymeric sorbents. 4. The process of claim 1, comprising mixing the organic materials with a hydrated ferric oxide. 5. The process of claim 4, further comprising immobilizing the heated mixture in a matrix of ferric oxide. 6. The process of claim 5, comprising pressing the heated mixture while under pressure for a period of time to immobilize the organic solids in a solid composition. 7. The process of claim 5, comprising pressing the heated mixture at room temperature. 8. The process of claim 5, comprising pressing the heated mixture at a pressure less than about 30,000 psi. 9. The process of claim 4, comprising mixing the organic materials with ferrihydrite. 10. The process of claim 1, comprising mixing the organic materials with hydrated aluminum oxide. 11. The process of claim 1, further comprising forming the hydrated metal oxide by precipitating the hydrated metal oxide from a solution comprising a metal salt, by mixing the solution with a base, prior to the mixing step. 12. The process of claim 1, further comprising immobilizing the heat treated mixture in a material chosen from the group consisting of cement, concrete, a polymer, bitumen and glass. 13. The process of claim 1, wherein the heating step is part of a process chosen from the group consisting of incineration and thermal decomposition. 14. The process of claim 1, comprising heating the mixture at a temperature of at least about 300.degree. C. 15. The process of claim 1, comprising heating the mixture up to a temperature of about 500.degree. C. 16. The process of claim 1, comprising mixing the hydrated metal oxide and the organic materials prior to the start of thermal decomposition of the organic materials. 17. The process of claim 1, comprising heating the organic materials to a temperature causing thermal decomposition of the organic material. 18. A process for immobilizing contaminated materials including organic materials, comprising: 19. The process of claim 18, comprising pressing the heated mixture at room temperature. 20. The process of claim 18, comprising pressing the heated mixture at a pressure less than about 30,000 psi. 21. The process of claim 20, further comprising adding at least one material chosen from the group consisting of a metal oxide, metallic iron powder, a ceramic binder, a silica, a silicate, an aluminosilicate, a phosphate, phosphoric acid, titania and a titanate, prior to pressing. 22. The process of claim 20, further comprising adding magnesium oxide and ammonium dihydrogen phosphate, prior to pressing. 23. The process of claim 22, comprising adding magnesium oxide such that the weight ratio of magnesium oxide to hydrated ferric oxide is between about 0.1 to about 4, and adding ammonium dihydrogen phosphate such that the weight ratio of ammonium dihydrogen phosphate to hydrated ferric oxide is between about 0.3 to about 3. 24. The process of claim 18, further comprising adding additional hydrated ferric oxide to the heated mixture prior to pressing, such that the mixture comprises at least about 20% Fe.sub.2 O.sub.3 by dry weight of the total weight of the mixture. 25. A process for immobilizing contaminated materials including organic materials, comprising: 26. The process of claim 25, comprising mixing the contaminated materials with a hydrated metal oxide. 27. The process of claim 25, comprising mixing the contaminated materials with hydrated ferric oxide. 28. The process of claim 25, comprising mixing the contaminated materials with hydrated aluminum oxide. 29. The process of claim 25, comprising pressing the heated mixture at room temperature. 30. The process of claim 25, further comprising adding at least one material chosen from the group consisting of a metal oxide, aluminum oxide, cupric oxide, zinc oxide, metallic iron powder, a ceramic binder, a silica, a silicate, an aluminosilicate, a phosphate, phosphoric acid, titania and a titanate, prior to pressing. 31. The process of claim 25, further comprising adding magnesium oxide and ammonium dihydrogen phosphate, prior to pressing and pressing at less than about 15,000 psi. 32. The process of claim 25, wherein the hydrated ferric oxide comprises at least about 20% Fe.sub.2 O.sub.3 by dry weight of the total weight of the mixture. 33. A process for immobilizing contaminated materials, comprising: 34. The process of claim 33, comprising pressing the mixture at a pressure less than about 30,000 psi. 35. The process of claim 33, comprising adding to the mixture at least one material chosen from the group consisting of a metal oxide, metallic iron powder, a ceramic binder, a silica, a silicate, an aluminosilicate, a phosphate, phosphoric acid, titania and a titanate, prior to pressing. 36. The process of claim 34 comprising adding to the mixture magnesium oxide and ammonium dihydrogen phosphate and pressing at less than about 15,000 psi. 37. A process for immobilizing contaminated materials contained in an aqueous solution, comprising: 38. The process of claim 37, further comprising adding to the resulting precipitate at least one material selected from the group consisting of a metal oxide, metallic iron powder, a ceramic binder, alumina, a silicate, an aluminosilicate, a phosphate, phosphoric acid, titania and a titanate before pressing. 39. The process of claim 37, wherein the pressing step is conducted at a pressure of less than about 30,000 psi. 40. A process for immobilizing contaminated materials, comprising: 41. The process of claim 40, comprising immobilizing the heated mixture in a material chosen from the group consisting of cement, concrete, ferric oxide, a polymer, bitumen and glass. 42. The process of claim 41, packaging the material in a storage container. 43. The process of claim 40, comprising heat treating the mixture at a temperature of at least about 300.degree. C. 44. The process of claim 40, comprising heating the mixture up to a temperature of about 500.degree. C. 45. A process for processing contaminated materials including organic materials, comprising: 46. The process of claim 45, wherein the heating step is chosen from the group consisting of incineration and thermal decomposition. |
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abstract | A device (100; 200; 300; 400) comprising a sealed cavity (210) containing n volumes of fluids (80, 87; 220, 230; 320, 332; 420, 422, 430, 432) is described, where n is an integer and n≧2. Each volume of fluid is substantially immiscible with every contiguous volume of fluid. The cavity is defined by an interior surface divided into n continuous areas (60, 170; 260, 270; 360, 362, 370; 460, 462, 470, 472), each continuous area corresponding to and being in contact with a respective one of the volumes of fluid. The wettability of each area is such that each volume of fluid preferentially adheres to the corresponding continuous area rather than any one of the continuous areas adjacent to the corresponding area. |
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abstract | An inspecting apparatus for reducing a time loss associated with a work for changing a detector is characterized by comprising a plurality of detectors 11, 12for receiving an electron beam emitted from a sample W to capture image data representative of the sample W, and a switching mechanism M for causing the electron beam to be incident on one of the plurality of detectors 11, 12, where the plurality of detectors 11, 12 are disposed in the same chamber MC. The plurality of detectors 11, 12 can be an arbitrary combination of a detector comprising an electron sensor for converting an electron beam into an electric signal with a detector comprising an optical sensor for converting an electron beam into light and converting the light into an electric signal. The switching mechanism M may be a mechanical moving mechanism or an electron beam deflector. |
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description | This application claims the benefit of U.S. provisional application Ser. No. 60/988,153 filed Nov. 15, 2007, which is incorporated herein by reference. The present application finds particular application in medical imaging and treatment systems, particularly involving cone-beam computed tomography (CT) and/or image-guided radiation therapy. However, it will be appreciated that the described technique may also find application in other imaging systems, other medical scenarios, or other medical techniques. Many CT systems employ a compensator (bow-tie filter) to reduce X-ray scatter and patient dose during single or multi-slice CT scans. However, the compensators do not always fit to the size and shape of the imaged object and are not adjustable. In conventional CT, the bowtie-shaped element is positioned between the x-ray source and the examined subject. In radiation therapy imaging, a wedge-shaped element is similarly positioned. In cone-beam CT, in which the detector is offset to enlarge the field-of-view, no compensating element is used. In diagnostic CT, use of a compensator is standard due to its benefits on image quality. Similar benefits were described in a recent publication for cone-beam CT (S. A. Graham et al., Med. Phys. 34, 2691, 2007). Fitting the compensator to the size and shape of the imaged object has proven difficult to achieve in practice. The compensator is commonly a machined aluminum block. The thickness of the block can be selected such that the line integral of attenuation through the block and a water phantom (e.g., a circular or elliptical cylinder) is constant. When the size or shape of the patient differs from that of the phantom, the compensation is less than ideal. The present application provides new and improved compensator systems and methods that improve image quality, which have the advantages of adjustable X-ray filtering, and which overcome the above-referenced problems and others. In accordance with one aspect, an imaging system includes a rotating gantry with an examination region in which a volume of interest (VOI) is positioned, and a transmission X-ray source, mounted for rotation with the gantry, the X-ray beam emitted across the examination region to an X-ray detector. The system further includes a movable wedge-shaped attenuation filter, positioned between the X-ray source and the examination region, for the wedge being movable relative to the X-ray beam to adjust an attenuation of the X-ray beams. In accordance with another aspect, a method of generating a 3D image of a subject includes evaluating a VOI in an examination region of an X-ray imaging device to determine size, shape, and density information about a portion of the VOI in the examination region, and positioning an adjustable wedge-shaped attenuation filter at a position in a cone-shaped X-ray beam, the wedge-shaped attenuation filter being movable in front of the X-ray source. Once the wedge-shaped attenuation filter has been positioned, CT data acquisition and gantry rotation are initiated. In accordance with yet another aspect, an apparatus for generating a 3D patient image includes means for generating a cone-shaped X-ray beam which traverses half of a VOI, such that the beam traverses the other half of the VOI when rotated 180°, and means for detecting the X-ray beam. The apparatus further includes means for adjustably attenuating of the X-ray beam, and means for monitoring size, shape, and density of the VOI presented to the X-ray beam during a 360° revolution of the X-ray generating means and means around the VOI. The apparatus further includes means for selectively adjusting X-ray attenuation by the attenuation means as the X-ray generation means and the detecting means rotate around the VOI. One advantage is that the compensation or filtering is adjustable. Another advantage resides in X-ray dose optimization. Another advantage is that the filtering can be dynamically adjusted during a scan. Another advantage resides in increased field of view relative to detector size. Still further advantages of the subject innovation will be appreciated by those of ordinary skill in the art upon reading and understanding the following detailed description. With reference to FIG. 1, an imaging system 10 includes an imaging device 12, such as a computed tomography (CT) imager or the like, which includes a rotatable gantry 14 defining a bore 15 extending therethrough, into which a subject support 16 is inserted for imaging a volume of interest (VOI) 18 of the subject. An X-ray source 20a is mounted to the gantry 14. The X-ray source emits a cone-beam of X-rays across an examination region in which the VOI is positioned. The cone-beam is received by an X-ray detector 22a, such as a flat panel X-ray detector. The detector is offset or misaligned relative to projected center of radiation. Specifically, the cone beam and the detector are offset such that half of the examination region is examined when the x-ray source and detector are in a first position 20a, 22a, and the other half is examined when the X-ray source and detector are in a second position 20b, 22b. Alternatively, two X-ray sources 20a, 20b, and two detectors 22a, 22b, can be provided. Systems with three or more source-detector pairs are also contemplated. A wedge-shaped, or half-bowtie-shaped, attenuation filter 24a, 24b, is used in conjunction with the cone-beam X-ray source(s) 20a, 20b and offset detector(s) 22a, 22b. Moreover, the position of the wedge is adjustable in accordance with patient geometry. By shifting the position of the wedge, the thickness of the wedge through which each ray of x-rays passes is adjusted. The position of the wedge can be selected in various ways. For example, after a scout image is generated, a wedge position is selected which holds the line integral of the x-ray rays through the patient substantially constant. Alternatively, the position can be estimated from the protocol, such as a head position versus torso position. Further, the wedge can be moved dynamically during a scan to adjust for different projections through an elliptical body portion or VOI. The gantry rotates 360° to generate multiple views of the VOI to ensure that a complete X-ray data set is available for reconstruction into a viewable image. The wedge(s) 24a, 24b, can be formed of Aluminum (Al), Molybdenum (Mo), Teflon, or some other suitable material. The system 10 further includes a CT acquisition module 40 that receives detected X-ray data from the imaging device 12, and a CT data store 42 that stores and/or buffers raw X-ray data. An overlap analyzer 44 resolves redundant data if the cone beams overlap slightly to ensure that a complete data set is present for the VOI. For example, redundant rays can be weighted and combined. A reconstruction processor 46 reconstructs an image of the VOI, for instance using a filtered back-projection reconstruction technique for viewing by an operator on a workstation 48. In one embodiment, the contour of the subject perimeter and a density map are determined from a pilot scan. A wedge position calculator 50 monitors the VOI to determine the shape, size, position, and density of the VOI. A compensator adjustment controller 52 receives information from the wedge position calculator and adjusts the wedge(s) 24a, 24b accordingly to compensate for the size, shape, and density of the imaged portion of the subject. For example, the X-ray detector has a range of X-ray intensities over which it is designed to operate, and the wedge(s) can be adjusted to manipulate attenuation or gain so that X-rays are received at the detector at an intensity level that is within the detector's range. The controller 52 can adjust the wedge(s) so that a line integral of X-ray attenuation through the wedge and irradiated portion of the subject is relatively constant and results in a radiation intensity at a selected point or in a selected region in the detector's operating range (e.g., a mid-point). In a dynamically adjusted embodiment, as the gantry rotates during CT acquisition, the path length and density through the VOI will change as seen from the X-ray source(s). These path length and density changes are determined by the wedge position calculator 50, and the controller 52 adjusts the wedge(s) accordingly. Wedge positions can be calibrated at a fixed number of wedge positions, and intermediate wedge positions can be interpolated there from. For example, the reconstruction processor can send pilot scan information to the calculator and the controller, which operate to reposition the wedge according to the path length and density changes. In one embodiment, the system 10 is employed in conjunction with a C-arm X-ray system (not shown) (e.g., X-ray source, image intensifier, and video read-out system) with offset detector, and/or for radiation therapy devices with an on-board imager. For use on a C-arm system, the wedge may be shifted outside the beam if not needed for certain parts of an examination. In another embodiment, patient-specific optimized beam shaping is achieved by positioning the movable wedge (or half bow-tie), and adjusting the lateral position of the wedge according to the patient's attenuation profile. “Lateral,” as used herein, describes movement across the X-ray cone beam, as indicated by the arrow in FIG. 2. The wedge position may either be fixed during the entire scan (e.g., an optimized position may either be pre-specified by the scan protocol or be determined using image processing algorithms according to the attenuation distribution in a pilot scan before the imaging scan), or the wedge may be moved relative to the source during the scan. Alternatively, in hybrid cone-beam CT (CBCT) systems with positron emission tomography (PET) or single photon emission computed tomography (SPECT), an optimal wedge position may be derived from the PET or SPECT image. For instance, nuclear detector heads (not shown), such as PET or SPECT detector heads, can be movably coupled to the gantry 14. PET or SPECT data captured by the detector heads can be reconstructed into corresponding image data, which is then used to identify an optimal wedge position for CT data acquisition. In SPECT imaging, a projection image representation is defined by the radiation data received at each coordinate on the detector head. In SPECT imaging, a collimator defines the rays along which radiation is received. In PET imaging, the detector head outputs are monitored for coincident radiation events on two heads. From the position and orientation of the heads and the location on the faces at which the coincident radiation is received, a ray between the coincident event detection points is calculated. This ray defines a line along which the radiation event occurred. In both PET and SPECT, the radiation data from a multiplicity of angular orientations of the heads is stored to data memory, and then reconstructed by a reconstruction processor into a transverse volumetric image representation of the region of interest, which is stored in a volume image memory. For proper gain correction, air scans, water scans, and bone equivalent scans may be acquired for different reproducible positions of the wedge to calibrate a plurality of wedge positions, and image gain corrections for intermediate positions of the wedge may be interpolated. The lateral position of the wedge is a parameter for its beam shaping function and can be adjusted before or during a CBCT scan according to the detected attenuation distribution of the patient (e.g., the larger the patient or patient portion, the thinner the wedge portion that is positioned in the X-ray beam). In this manner, a “one wedge fits all patients” scenario can be realized. In another embodiment, a single cone-beam X-ray source is positioned on the gantry opposite an offset flat panel X-ray detector, with a wedge positioned between the X-ray source and an examination region in which the VOI is positioned. The beam emitted by the X-ray source can pass through approximately half or more of the VOI, and a complete X-ray data set can be collected as the gantry rotates through a 360° revolution. The wedge can be adjusted as the gantry rotates to maintain relatively constant attenuation line integrals for X-ray paths through the VOI to keep the image gain substantially constant, which turn facilitates reconstruction of an image with good contrast. FIG. 2 is an illustration of the X-ray source and offset detector portion of the system 10, illustrated concurrently at 180° opposite positions (a) and (b). The two X-ray source positions 20a, 20b generate cone-shaped X-ray beams in opposite orientations to generate a field of view (FOV) that encompasses the VOI 18 (e.g., a human thorax in this example). Each X-ray cone is received at a flat-panel detector 22a, 22b, and is generated using a filter and/or collimator (not shown) to create a desired cone shape. To change the lateral position of the wedge 24a, 24b with respect to the source and the detector in a reproducible way, various mechanical means may be employed. In one embodiment, the means for its automated translation of the wedges is integrated into the collimator of the particular X-ray system employed. Additionally, different means may be employed to determine an optimal lateral position of the wedge for a given shape and size of the VOI. In one embodiment, the position of the wedge is fixed during the CBCT acquisition, and is determined before the scan either depending on the chosen scan protocol (e.g., different pre-determined positions for scans of the head, the torso, the periphery, and/or pediatric protocols, etc.), or depending on information extracted from a scout projection or low-dose scan. In the latter case, the relevant parameters, in particular the spatial dimensions (width and depth) of the object and the position of its projected boundary, are extracted from the attenuation profile using image processing techniques, and the wedge is optimally placed according to these parameters. In other embodiments, the position of the wedge is automatically adjusted during the scan, depending on the information extracted in real-time from the acquired projections. Additionally or alternatively, in a hybrid CBCT system with SPECT or PET, the optimal location of the wedge may be derived from the PET or SPECT image. For proper gain correction and correction of the beam hardening and scattering effects of the wedge, calibrations may be made beforehand for different positions of the wedge. In a scenario where continuous shifting of the wedge is desired, appropriate calibration images may be obtained by means of interpolation between the calibration images acquired at a number of fixed positions. FIG. 3 illustrates another configuration of the CBCT scanning system 10, illustrated concurrently at 180° opposite positions (a) and (b), in which the wedge is shifted to adjust the width/thickness of the wedge material through which the beam travels in accordance with a path length through the VOI at different rotational positions. For instance, an intermediate-thickness portion of the VOI is positioned in the field of view of the X-ray source position 20a, and the wedge 24a is positioned with a central portion in the beam to attenuate the cone beam in accordance with the relative thickness of the VOI. A thinner portion of the VOI is positioned in the field of view of the X-ray source at 20b, and the wedge 24b is positioned with a thicker portion in the X-ray beam, to increase attenuation of the beam emitted by the second X-ray source so that line integrals for the X-rays through the thinner portion of the VOI are substantially equal to line integrals for X-rays through the thicker portion of the VOI. Analogously, when the X-ray source at 20c projects the X-ray beam through the longest paths through the VOI, the wedge 24c is shifted such that its thinnest portion is in the beam. In this manner, a complete set of accurate X-ray data can be collected with relatively constant intensity over a single gantry revolution, and the VOI is subjected to less radiation than would otherwise be present (e.g., in a multi-revolution CT scan). FIG. 4 illustrates one embodiment of the wedge 24, in which the wedge has a typical wedge shape, including a substantially rectangular base 60 and top and bottom sides that taper to an edge 62 opposite the base. As the wedge position is adjusted in front of an X-ray source, the thickness of the wedge material changes, thereby altering beam hardness or energy content of the X-ray cone beam. It will be appreciated that the wedge 24 is not limited to having the illustrated geometry, and that any suitable geometry may be used in conjunction with the systems and/or methods described herein. FIG. 5 illustrates another embodiment of the wedge 24 in which the base 60 is triangular, and two top surfaces taper to edges, 62 and 64. This configuration permits cone-beam adjustment in two planes as the position of the wedge 24 is adjusted in front of an X-ray source. FIG. 6 illustrates another embodiment of the wedge 24 in which the base 60 has a bow-tie-like shape and the sides of the wedge taper to the edge 62. In this embodiment, the top surface 66 has a crease running approximately down the center of the surface between the base and the edge 62. This causes the wedge to be thinner in the center than at the sides along a longitudinally axis from the base 60 to the edge 62, which permits further manipulation of the X-ray beam as wedge position is manipulated. FIG. 7 illustrates an embodiment of the wedge 24 in which the top and bottom surfaces are non-planar. FIG. 8 illustrates an embodiment in which two wedges 25a, 25b are positioned with flat bottom or other surfaces contiguous to each other. The two wedges can be moved independently for greater freedom in the adjustment of the attenuation properties. In another embodiment, a mechanically adjustable wedge has an adjustable slope that can be manipulated to adjust attenuation of the X-ray beam. The innovation has been described with reference to several embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the innovation be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. |
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abstract | A system for exchanging and storing collimators for medical imaging devices includes a first frame having a first receptacle and a second frame having a first docking member. A collimator can be attached to and removed from the first receptacle. The first docking member can be positioned adjacent to the first receptacle such that the first docking member can contact the collimator to remove the collimator from the first receptacle. The collimator is coupled to the first docking member while the collimator is removed from the first receptacle. |
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abstract | A system according to an exemplary aspect of the present disclosure includes, among other things, a generator-detector configured to be attached to a pipe. The generator-detector is configured to measure the concentration of mercury in the pipe in a non-destructive manner. A method is also disclosed. |
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048150142 | description | DESCRIPTION OF PREFERRED EMBODIMENTS The invention is directed to improvements in the method and apparatus for monitoring the execution of process operating procedures disclosed in copending commonly owned U.S. patent application Ser. No. 825,427 filed on Feb. 3, 1986 and entitled "On-line Interactive Monitoring of the Execution of Process Operating Procedures". As in that application, the invention will be described as applied to a pressurized water reactor (PWR) nuclear power plant, but it should be realized that it has application to a variety of complex process facilities such as, for example, chemical plants, refineries and the like. In fact, it is adaptable to various facilities where a human operator is required to follow preconceived, although not necessarily rigid, step-by-step procedures, and is most useful in those facilities in which the sequence of steps depends at least in part upon current conditions in the complex process facility. It may be used in monitoring the execution of a variety of types of procedures, however, for purposes of illustration the invention will be described as applied to monitoring the execution of emergency procedures for a PWR, and specifically, the Emergency Operating Procedures developed by the Westinghouse Owners Group. FIG. 1 illustrates a PWR power plant adapted for implementing the above application of the invention. The plant 1 is instrumented with numerous sensors 3 which monitor various plant parameters such as temperatures, pressures, flows, radiation levels, tank levels, equipment status and the like. The signals, S, generated by the sensors are initially processed by instrumentation 5 which provides conventional signal processing such as surge protection, buffering, isolation, filtering, span control, conversion to engineering units, et cetera. Instrumentation 5 also generates logical signals representative of plant conditions of interest by comparing certain of the initially processed signals with set point values. The processed sensor signals and logical signals generated by the instrumentation 5 are supplied over the line 7 to the control board displays in the plant control room for visual presentation to the user 11, namely, the control room personnel. The control board displays 9, which are conventional, utilize the processed sensor signals to generate visual representations of the measured values of the associated parameters and the logical signals and to produce visual indications of the status of the plant conditions of interest. The operator then uses this information in analyzing the state of the plant and its various subsystems and in making decisions such as, if, when, and what adjustments should be made to the plant controls 13. In making these decisions, the operator presently has access to paper procedures 15 which provide step-by-step guidance through a logical sequence of analysis and action. The initially processed sensor signals and the logical signals generated by the instrumentation 5 are also fed over a line 17 to plant computer 19 which performs surveillance and logging functions. This computer 19 also calculates certain parameters such as for instance the departure from nucleate boiling ratio (DNBR) and degrees of core exit coolant subcooling from the measured parameters. Some of these calculated parameters are transmitted to the control board displays 9 over lead 21 for presentation to the operator. All of what has been described to this point is currently found in a typical PWR plant. The present invention provides improvements to the system described in U.S. patent application Ser. No. 825,427 filed Feb. 3, 1986, which in turn replaces the paper procedures 15 in providing guidance for the operator 11 in monitoring plant performance and manipulating the plant controls 13. It is an on-line tracking system which allows the operating staff to access and follow procedures in an easy, reliable and logical manner. The system performs irs functions through the execution of several concurrent but independent tasks including a read task, a main task, a parallel process task, and a display task. The read task gathers the plant parameter information and processes it for use by the main task. The main task controls the flow of the procedures and of the individual steps of the current procedure, and performs the logging functions. The parallel process task tracks system or safety conditions that may affect plant operations. The display task generates the visual displays for interfacing with the operator. The system includes a digital computer 23 which receives sensor and logical signals from the instrumentation 5 and calculated parameter values from the plant computer 19. The computer 23 may consist of a single main frame unit with an operating system which supports real time multi-task operations, or several microprocessors linked by a data highway, with each microprocessor handling a single task. Alternatively, the software may be resident in the plant computer 19 when that unit has a dual processing capability and the capacity to perform the routines required. The computer 23 interfaces to a display generator 25 (such as a Raster Technology Model 1/80 unit) to provide the color graphics output of the procedures program on a visual display device such as a high-resolution color monitor 27. The user, or operator, 11 observes the visual display generated on the color monitor 27 and provides input to the system through a touch screen device 29 on the monitor, or an equivalent device such as a keyboard. The architecture of the software for the system is illustrated within the broken line of FIG. 1 representing the computer 23. Central to the software package is a library 31 of subroutines or overlays, each of which embodies the logic, and as required, recommended action steps, for one procedure. This library of step-by-step procedures is stored in computer memory or in a high capacity, on-line device such as a computer disc. Operation of the system is initiated by a trigger 33 in response either to a user input from the touch screen 29, or to an indication of a particular set of plant conditions, such as a reactor trip, received from the control board displays 9 or the plant data processor 41. The trigger 33 activates a procedure selector 35 containing the necessary logic to select the appropriate procedure from the library 31 and to establish that procedure as the active procedure. A step selector 37 sequentially selects the steps of the active procedure one at a time for a current step processor 39. Inputs to the current step processor 39 are obtained from two sources. A plant data processor 41 collects plant data, including calculated parameters from the instrumentation 5 and plant computer 19, and transforms numerical (and analog) information into logical form for use in the procedure logic. For instance, a temperature signal is transformed into a logical signal which indicates whether the current value of the temperature is above or below a given set point value. The human user provides input to the current step processor through the touch screen or equivalent device 29 and a response processor 43. User inputs usually take the form of interactive responses to specific prompts generated by the current step processor 39. In the present invention the current step processor 39 repetitively performs the step logic. In so doing, the current step processor 39 repetitively checks the response processor 43 for an operator response to the instructions generated by the current step processor, as indicated by the dash dot line in FIG. 1. After the user response is received, the response processor communicates with the procedure selector 35 and step selector 37 to implement selection of a next step, and if necessary, a new procedure, dependent upon the user's response. A parallel condition processor 45 operates independently of the current step processor 3 to provide parallel, concurrent monitoring of overall plant and individual system conditions in areas not directly considered by the procedure in effect. If the parallel condition processor detects an unexpected condition, such as might arise as a result of operations or casualties unrelated to the currently active procedure, a priority evaluator 47 applies a preset logic to determine whether the unexpected condition is sufficiently severe to recommend to the user suspension of the current procedure in the current step processor 39 and initiation of a different course of action involving another procedure, to treat those conditions. In the present invention, text embedded in the logic of the current step processor 39 or the parallel condition processor 45, whichever is given priority by the priority evaluator 47, is passed to the display generator 25. The display generator accepts the specified lines of text and constructs a display image on the color monitor 27 as will be seen in more detail below. This display provides the user with information regarding plant and system status as seen both by the active procedure and by the parallel condition processor 45, with recommended action as required, and with appropriate instruction for the operator to generate user responses. The user observes the textual images produced by the display generator 25 on the color monitor 27 and communicates his desired responses by the way of the touch screen or equivalent 29, which transmits them to the response processor 43. The user's response is stored in a buffer 30, which may be incorporated in the touch screen device 29 as shown, so that the current step processor 39 can continue to repetitively execute the step logic without waiting for the user's response. User responses to action recommended by the current step processor typically result in indexing the step selector 37 to a next step in the active procedure. Responses to action recommended by the parallel condition processor 45 can result in a return to the current step, or a signal to the procedure selector 35 to transfer to another procedure. The last piece of software in the software package is a condition logger 53 which accepts current step information and parallel condition information from the current step processor 39 and parallel condition processor 45, respectively, in addition to operator responses received from the response processor 43, and generates a complete record of the displays presented to the operator, and the operator's responses. A permanent chronological record of the displays and responses is obtained by coupling a permanent copy device, such as a printer 55, or a magnetic tape recorder or the like, to the logger 53. An additional monitor 51 provides an on-line review capability which allows the operator to scan the recorder listing while simultaneously executing procedure steps. The additional monitor 51 is provided with a touch screen 52 or other device by which a user can initiate and control the on-line review. FIG. 2 illustrates the format of a typical display generated by the display generator 25 for presentation on the color monitor 27. Working from the top to the bottom of the display, the current clock time is displayed in the upper left hand corner at 59. Information generated by the parallel condition processor is displayed in a box 61 just below the time. The system allows the user to understand overall system or safety status by displaying a summary of those items which are considered crucial in a window 63 at the left end of the box 61. This enables the users to judge at any time whether these items are satisfied or not. As shown in FIG. 2, status tree information can be displayed in this block when pertinent to the current plant condition. The details of a suitable display are discussed in the related application. It is sufficient to say that each of the six safety functions in the exemplary display are represented by a horizontal bar graph. The greater the width of the particular bar graph, the more serious the situation. The system also allows the user to understand the status of the other conditions which require monitoring during the procedure execution. Typically, Notes and Cautions appear throughout a set of procedures. These relate to additional items which the user must remember to monitor, such as tank levels and component states. The system alleviates this burden on the operator's memory by automatically keeping track of these items. If a Note or Caution requires attention, the system informs the user of this fact along with a statement of action necessary, if any, to solve the problem, in the center window 65 of the box 61. Should a condition arise in either the safety or system status displayed in block 63 or the continuously monitored conditions displayed in block 65 which requires user attention, a flashing indication appears in a special alert indicator window 67. Information related to the procedure being executed is presented in the large block 69 in the center of the display. The title of the active procedure is displayed at the top of the block at 71. Information related to the current step being executed is displayed between the dashed lines 73. In order to allow the user to quickly understand the status of the current step, a high level statement of the step is presented first as at 75. In the example, the function of the procedure step 11 is to verify the reactor coolant system feed path. In this case, the condition is not verified, hence the high level statement reads "RCS FEED PATH - NOT VERIFIED". The system also informs the user of the status of the parameters or components which led to the high-level statement immediately below that statement at 77. In the example, these components are charging/safety injection pumps A and B, both of which are not running. Using the paper procedures, the operator would have to check the status of these pumps on the control board displays personally in order to then come to the conclusion that the pumps were not running. Alignment of the safety injection valves is also required to establish the RCS feed path, but in the example, valve alignment is verified. The system also informs the user of the actions, if any, which are recommended in response to the current procedure step, through a verbal statement 79 below the component status. In the example, the recommended action is "MANUALLY START PUMPS". To provide the operator with more perspective, the system also displays at 81 above the current step, the high-level results statement of a preselected number, in the preferred embodiment two, of the previously executed steps of a current procedure. Likewise, the high-level textual statement of the preselected number, again preferably two, of the next steps in the procedure are displayed immediately below the current step at 83. User touch screen prompts or instructions for generating a response are presented at the bottom of the display below the block 69. Each step presents the operator with the choice of either accepting the status information and recommended action, if any, or in rejecting it. In a case such as the example shown in FIG. 2 where action is recommended, the operator accepts the recommendation by performing the action and then pressing the touch screen button 85 labeled "ACTION COMPLETED". If the operator chooses to reject the recommendation, the touch screen button 87 labeled "ACTION OVERIDDEN" is touched. The displays make extensive use of color coding to enhance the operator interface. Different colors are used to distinguish between parameter or component stares, required actions, and high-level statements of procedure steps. Since the figures are monochromatic as presented, the various portions of the display are succeeded by one of the following symbols, which do not appear on the display, but are used here to indicate the following color designations: (b) blue PA1 (g) green PA1 (w) white PA1 (y) yellow The prompts are presented in the same color as the recommended action so that the operator knows whether he is responding to a current step or a continuously monitored condition. Thus in the example, the prompts are yellow indicating that they are to be used for indicating responses to the recommended action in the current step which is also in yellow. Action required in connection with a continuously monitored condition would be displayed in blue as would be the prompts so that the operator knows what function the response is associated with. In order to further enhance the presentation and to highlight important conditions, reverse video is used. This is represented in FIG. 2 by the asterisk after the high-level statement and the component status statements. Color coding is also used for the status of the critical safety functions with (g) green representing the normal condition, (y) yellow representing an off-normal condition, (o) orange representing a potentially dangerous condition, and (r) red representing an existing hazardous condition. FIG. 2 represents a typical initial display for a current step which requires operator action. Thus FIG. 2 indicates that the charging/safety injection pumps are nor operating and prompts or instructs the operator to start them. The earlier version of this system locked up at this point waiting for the operator response. When the response was received, a numerical code indicating the conditions existing at the time the step was entered and the operator response was logged, and the system advanced to the next step. Among the limitations with this approach is the lack of feedback to the operator and to the computer since the step logic is executed only once. The only way for the operator to verify that an action was complete was for him to use the step backup feature to execute the step again. Further, when the pumps had been started, the display was not updated to reflect the new status of the pumps. Without this feedback the operator may have been presented with false information. Finally the system never attempted to verify that the action was in fact complete. Instead it relied entirely on the operator's response. In accordance with the present invention the step logic, which includes the determination of the component or condition status and the generation of the text for the display, is continuously, repetitively executed. The operator's response is stored separately in a buffer 30 or temporary storage space. If the operator has responded, the response is saved in the buffer 30, otherwise, the buffer is empty. The response processor 43 can check the buffer 30 at any time, but it need not wait for the response. Thus, while the system is waiting for the operator to generate a response to the prompt, the computer is free to perform two functions: (1) it continues to re-execute the step logic and to update the display appropriately, thus providing feedback to the operator. (2) it checks the response buffer to see if an acceptable character has been entered. Once an acceptable response is registered, the system follows one of three courses of action. If the response is to override the suggested action, the system moves on to the next appropriate step. If the response is to indicate a completed action, the system uses the plant data to determine whether the action is complete. If the action is complete, the system moves to the next appropriate step. If it is not complete, the screen updating and buffer checking resume until the next acceptable response is entered. Thus, for the example of FIG. 2, when the operator performs the recommended action and turns on the pumps, a subsequent repetition of the step logic will detect from the sensors that the pumps are turned on, and the display will be updated as shown in FIG. 3. At this point the operator touches the "ACTION COMPLETED" button on the touch screen, and the system will advance to the next appropriate step. Had the operator touched the "ACTION COMPLETED" button under the circumstances shown in FIG. 2, the system would not have advanced since the process condition under consideration was not verified by the sensors. If due to some malfunction, one of the pumps would not start, the operator could advance to the next step by touching the "ACTION OVERRIDE" button. At least under the circumstances, he knows what the situation is, whereas with the earlier version of the system he would not have been made aware of such a malfunction by the system. In some instances, a recommended action may require a prolonged period of time for completion. For instance, the main isolation valves may require several minutes to fully close. In order that the remaining steps of the procedure are not unduly delayed, an "ACTION INITIATED" prompt is generated for such a step. When the operator generates an "ACTION INITIATED" response, indicating that the action has been initiated, the system advances to a next step without waiting for verification of operator action. FIG. 4 illustrates a display generated for the next step in the current procedure in which the condition monitored by the current step has been verified at the time the step is entered. Thus, the prompts generated for the operator are labeled "CONTINUE" and "OVERRIDE". If the operator selects "CONTINUE", the system proceeds to the next applicable step. If, however, the operator selects "OVERRIDE", the recommended action which would have been presented had the condition monitored by the current step not been verified is added to the display and the prompts are replaced by "ACTION COMPLETED" and "ACTION OVERRIDDEN". Such action would be taken for instance by the operator when other information available to him such as from the control board displays indicate that the condition is not verified. This could occur for example in the case of a faulty sensor. Thus the system allows the operator to remain in control at all times. It should be noted that Step 12 shown in FIG. 4 is somewhat different from that in FIGS. 2 and 3 in that the monitored parameter, SI RESET is at the level of the high level statement and there are no supporting components or parameters. This does not affect execution of the step, however. Upon receipt of an acceptable response from the operator, the response processor 43 activates the conditions logger 53 to record the contents of the current display exactly as the operator sees it, except that the operator response, such as, "OPERATOR COMPLETED ACTION" as shown in the partial display of FIG. 5, or "OPERATOR OVERRODE ACTION" et cetera, is recorded rather than the prompts. Hence, a permanent record is made of the time, the representation of the status tree status, any caution text which appears in the caution text window, the procedure title, the current step information, including the high-level statement and all plant status information as displayed for the user on the display, high-level results statements of the preceding two steps and high-level textual statements of the following two steps, and the operator's response to the system prompts. It is clear then, that the record generated by the invention is far more complete and understandable than the cryptic summary of the condition of the plant and/or the operator's responses for each step recorded by the earlier version of the system which required the use of both the paper version of the procedure and the listing of the program to interpret. Recording is performed immediately following every acceptable operator response. Hence, the information stored by the recorder is that confirmed or rejected (override option) by the operator. Since an operator response is required to all prompts, recorded entries are not limited simply to step execution; they include all interaction with the parallel process functions as well. The conditions recording is accomplish by calling a subroutine at the end of every acceptable operator response. In the procedures subroutines, all the text which is displayed for the operator is stored in text buffers. When the conditions recording subroutine is called, it recreates the operator display using the information stored in the text buffers. A listing is built, line by line, by spacially locating the text strings and the text buffers so as to simulate the display as seen by the operator. As each line is built, it is written to an appropriate storage device 55 such as disc drive, a magnetic tape, or line printer. The improved recording feature is particularly valuable for post-situation review. Given some sort of accident from which procedures are used to recover, the review feature allows an expert to conveniently inspect and verify every action taken by the operator. This feature is also valuable when training new operators as the instructor can constructively comment on a new operator's recovery by reviewing the recorder listing. Another feature of the present invention is the ability to view the procedure as a whole. The earlier version of the system presented to the user the high-level results statements of a small number, preferably two, of the previous steps and high-level textual statements of the steps to be executed in the future, assuming normal order of execution. The present system has this capability as well, as seen from FIGS. 2 through 4. It also enables the operator, on request, to scan the high-level textual statements of all of the steps of the current procedure. This can help the operators anticipate steps to be executed in the future. It can also help them understand the intent of a given procedure. This feature can be further used to re-execute a prior step. Scanning the high level textual statements of all of the steps of the current procedure is accomplished by accessing the library of steps stored in a file. The scanning option is presented to the operator at the completion of each step. As shown by the partial display in FIG. 6 the operator is presented with prompts which provide a choice between proceeding to the next step in the sequence or selecting another step by scanning forward or backward. If the "FORWARD/BACKWARD" button is touched, the operator is then presented with four options as shown in FIG. 7. The operator can scan forward or backward by touching the "FORWARD" or "BACKWARD" buttons respectively. The high level statement of the step selected appears in yellow above the touch buttons. In the example of FIG. 7, the operator as indexed back to step 11. The operator can return to the point in the procedure prior to requesting a scan of the steps, by touching the "RETURN TO MAIN MENU" touch button. Since step 11 is a prior step, the operator also has the option of reexecuting the step by touching the "EXECUTE" touch burton. This option is only available with a prior step so that the operator cannot easily jump over steps which have not yet been executed. Upon touching the "EXECUTE" button the operator is presented with the display shown in FIG. 8 wherein step 11 is shown as the current step. While this display is very similar to that of FIG. 3, there is a noticeable difference. Namely, it can be seen that the two high-level statements of previous steps are for steps 11 and 12 which indicate to the operator where he was in the procedure when backward scanning was requested. The two subsequent steps presented are the steps 12 and 13 which follow the step which is being re-executed. Since the condition addressed by step 11 is verified, the operator is presented with the "CONTINUE" and "OVERRIDE" prompts, as would be presented for any verified step. Upon selecting a response, the operator is presented with the option of continuing in sequence to the next step or returning to the main menu as shown in FIG. 9. If "RETURN TO MAIN MENU" is selected, the prompts of FIG. 6 would be presented again offering the choice to advance to the next step after the step from which the scan was initiated, step 13 in the example, or of scanning forward or backward again. In addition to the permanent chronological record, additional records are maintained for on-line review of plant conditions throughout an event and corresponding operator actions. The on-line review feature is called by an on-line review signal generated at the additional monitor 51 by a touch screen 52 or other input device. This feature permits the operator to review the previously executed steps on the additional monitor 51 simultaneously with the execution of the current step using the color monitor 27 so that operation of the system is not interrupted. The additional monitor 51 can be replaced by, or supplemented with, a printer, if a permanent copy of a past step or series of steps is desired. The on-line review feature is particularly useful during shift changes in the plant. The incoming operator need only use the review feature to learn the recent course of events and prepare for the upcoming shift. If the operator should forget something learned during the initial review, the listing can be reviewed again. This relieves both the exiting and incoming operators from having to commit a large amount of information to memory. The on-line review feature is accomplished by the digital computer 23 utilizing three additional files which may be referred to as files A, B, and C. Let file C be the summary file which contains a complete listing of all the conditions and operator actions. This file contains the same information as the conditions recording which is used strictly for archival purposes, as described above. Files A and B are temporary files used to gather the information generated in a manner similar to that used in the conditions recording subroutine. These files alternatively receive information from the subroutine and dump information to file C, the file which the operator reviews. Consider the situation in which file A is currently open and being written to by the subroutine, and files B and C are closed. When the operator desires to use the on-line review feature, such as by entering a request through a touch screen on the additional monitor 51, file B is opened and becomes the receiving file for new information. File A is closed and appended at the end of file C. File C is then opened for review by the operator using a file review subroutine. When the operator has completed his review, file C is closed. At the next on-line review, the situation proceeds as described above with the roles of files A and B reversed. With this approach, file C always contains a complete listing of the current event conditions, which is available for operator examination. In summary, the present invention provides a more powerful computer based system for assisting an operator in executing procedures for a complex process facility. In particular, the use of buffering allows for continuous plant data monitoring and continuous re-execution of procedure logic to provide feedback to the operator and verify operator action. The use of text strings in a conditions recording subroutine generates a more complete and understandable record of plant conditions during an event in the form of a copy of the screen as the operator sees it. The provision of an override feature allows the operator to maintain control of the procedure and assists in identifying and making a record of faulty sensors. The use of multiple files in a review subroutine provide the capability for on-line review of the plant conditions record. Finally, the use of a high level textual statement file and a scanning subroutine provide the ability to view the high level textual statements of the procedure steps. 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. |
abstract | A hohlraum for an inertial confinement fusion power plant is disclosed. The hohlraum includes a generally cylindrical exterior surface, and an interior rugby ball-shaped surface. Windows over laser entrance holes at each end of the hohlraum enclose inert gas. Infrared reflectors on opposite sides of the central point reflect fusion chamber heat away from the capsule. P2 shields disposed on the infrared reflectors help assure an enhanced and more uniform x-ray bath for the fusion fuel capsule. |
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048266469 | summary | BACKGROUND OF THE INVENTION The invention pertains to a method and apparatus for controlling charged particles, and more particularly to a method and apparatus for confining ionized gases or plasmas. Confinement of dense ionized gases is a necessary step in several processes which are currently the object of intense research. These processes include nuclear fusion. Research on the confinement and heating of ionized gases and of their electron and ionic charged particle gaseous components has concentrated principally on the methods of inertial confinement and magnetic confinement. An example of non-magnetic and non-electric inertial confinement is the so-called "laser-fusion" process in which large pressures are to be induced over the surface of small spheres of material which it is desired to heat and confine by the vaporization and "blowoff" of surface material by laser light energy heating thereof. Such high pressures cause the compression and resulting compressional heating of the material within the small spheres. The principal difficulty of achieving useful fusion-reaction-producing conditions by such hydrodynamic-inertial confinement means is that of the lack of stability of compression, to the very high densities required, of small amounts of material in spherical or other convergent geometries. In such inertial-compression schemes it is necessary to create the forces and drive the compression sufficiently rapidly that radiation and electron conduction energy loss processes from the compressing plasma/material do not radiate and/or carry away the material internal energy before the desired particle energy (i.e., temperature) has been achieved. Typically, time scales of fractions of microseconds are required for such systems. Attainable compression temperatures are limited by deviations from perfect symmetry of compression which lead to non-spherical compressed geometries, mixing of material, and other effects. Because of these requirements and physical difficulties such inertial-mechanical means for confining and heating plasma material have been shown to require large power input to the devices and machinery used for studies of such processes. This approach is of little interest in connection with and has no fundamental relevance to the present invention, and will not be further considered. In magnetic confinement methods, strong magnetic fields are used in various geometries in attempts to confine or hold plasmas in well-defined spatial regions for periods of time long enough to allow their heating to desired levels of plasma ion particle energy or temperature. An example of a device which employs a magnetic confinement method is the tokamak. The tokamak has a toroidal magnetic confinement geometry in which plasma ions and electrons are to be held within a toroidal volume by the magnetic field lines which circle through that volume. Another example of such a device is a mirror machine. A mirror machine uses a "mirror" geometry in which opposing (facing) magnet coils act to provide a high-field surface around a central volume at lower field strength. Use of similarly-directed currents in such end coils causes "end-plugging" of a solenoidal field configuration, while oppositely-directed currents give rise to a bi-conic mirror geometry around a region in which the central magnetic field strength can drop to zero. This latter arrangement is described in more detail below. Charged particles (e.g., plasma ions and/or electrons) gyrate around (normal to) the magnetic field lines in all such systems, in orbits whose gyro radii are set by their individual particle charge, mass, and energy of motion transverse to the magnetic fields. As just mentioned, one of the magnetic field geometries which have been studied for plasma confinement is the simple bi-conic mirror cusp geometry. Such a system is depicted in FIGS. 1A-1C. In this system two opposing coils 100 and 110 carrying oppositely-directed currents face ("mirror") each other and produce a bipolar and equatorial field "cusp" geometry. The advantage of this system is that it is (i.e., has been shown to be) inherently stable to macroscopic "collective" losses of plasma across the field. This is in contrast to the toroidal tokamak and to the solenoidal mirror geometries which are not inherently stable to such losses. Losses in the bi-conic mirror sytem are principally out through the polar end cusps one of which is labelled 130, and the belt or equatorial "ring" cusp 135 of the magnetic field geometry. Particles 140 making up plasma 120 approach point cusp 130 from a range of angles. Most are deflected due to their having a nonaxial velocity component. Some particles, however, approach the cusp along paths which permits them to escape out through the cusp. The total loss rate through these cusp regions is determined by the "loss cone angle" and by the solid angle (in velocity space) subtended by the leakage cusp system. The loss cone angle is set by the field strength in the current-carrying coils which make the field, and the solid angle is simply the result of the geometry chosen. In the bi-conic mirror system the losses are predominantly through the equatorial cusp 135 because of its great extent entirely surrounding the plasma region. In experimental and theoretical work aimed at using such "mirror" field systems to confine plasmas, a remedy for this defect was attempted by twisting half the field through 90.degree. (so-called "baseball" geometry), so that the cusp leakage ring is split into orthogonal half-hemispheres. This has the effect of removing equatorial plane coherence for particles able to scatter out through the equatorial field ring loss angle. However, this twisting of the field half-space does not change the basic topology of the bi-conic system and, although still macroscopically stable, the plasma will continue to be lost predominantly through the (now-bifurcated) equatorial ring cusp in the confining field. Studies have also been made of the solenoidal mirror geometry, in which end point cusps act as "reflectors" of particles at the ends of the quasi-solenoidal field system. As previously noted, however, the solenoidal field region itself and its connection regions to the end cusps are inherently macro-unstable with respect to plasma/field interchange displacements. Thus, while the ring cusp losses of the bi-conic cusp mirror geometry have been removed, they have been replaced by new losses due to instability of the basic central field configuration. In experimental and theoretical work on such systems a partial remedy for this defect was obtained by adding current-carrying conductors between the mirror coils, parallel to the coil system axis. These parallel conductors (sometimes called "Ioffe bars", after their Soviet inventor) provide fields which yield longitudinal surface cusp geometries which are inherently stable, but which embody particle losses through the longitudinal line cusps thus formed between conductors. In addition, locally unstable regions still exist between the central longitudinal fields and the end mirror point cusps, which lead to enhanced losses. The virtue of the simple bi-conic mirror cusp field system is that it is inherently macroscopically stable and is thus subject only to microscopic plasma loss phenomena (e.g., collisional guiding center transport). Its defect is the very large conical sector equatorial ring cusp loss region. Polar end point cusp losses can be made small in most systems of interest by use of large mirror field ratios (particle velocity-space loss cone angle is given by sin.sup.2 .theta.=[Bo/Bm], where Bo is field strength at plasma center and Bm is field strength on the mirror cusp axis). In addition, the point cusps can be used easily for injection of plasma ions and/or electrons into the central region of such a magnetic confinement geometry. A mirror system which contained only point cusps would provide an ideal stable field/confinement geometry. Some efforts have been expended investigating other geometries for magnetic plasma confinement. One such effort is that of R. Keller and I. R. Jones, "Confinement d'un Plasma par un Systeme Polyedrique a' Courant Alternatif", Z. Naturforschg. Vol. 21 n, pp. 1085-1089 (1966). Octahedral and truncated cube sutems were explored. These are labeled 200 and 230 in FIG. 2. Keller and Jones noted that the octadedron has two symmetry axes A and B. The truncated cube system, on the other hand, has three axes of symmetry C, D, and E. Keller and Jones experimentally explored neutral plasma heating and confinement by driving alternate interlaced coil sets at a (high) frequency of 4.7 MHz. It should be noted that the two interlaced fields used in this work were of opposite type, one being solenoidal and the other opposing bi-conical; thus alternation between field sets/states caused an inversion of field direction with each half cycle. It is not clear that alternation between such states is most effective for plasma confinement or heating. However, losses are greatly reduced from those of a conventional bi-conic equatorial ring cusp system of comparable size. Stable confinement was obtained at the experimental conditions, with modest heating observed, and the presence of a "spherical wave" was noted in the plasma. Another extreme of magnetic confinement geometry is disclosed in U.S. Pat. No. 4,233,537 to Limpaecher. Therein it is proposed to confine neutral plasmas within a cylindrically symmetric volume whose surface contains an array of 1250 alternating magnetic poles, with axial electron injection to establish a negative plasma potential. In this system, plasma is to be confined by the surface magnetic multipole fields and constrained by the cylindrical interior negative electric potential field. In all of the arrangements considered there were always macroscopically unstable regions somewhere on the surface (e.g., at the end regions of the cylindrical volume), and the electrostatic field was never suggested as the primary or sole confinement field for plasma ions. In both magnetic and inertial-confinement approaches the plasma heating is made to occur by statistically random collisional processes, either while under growing compression conditions or (with externally-driven energetic particles) while "trapped" for a sufficient length of time in a "confining" magnetic field geometry. It would appear to be desirable, however, to provide a more direct and non-statistical non-random process for energy addition to gain energy directly by "falling" through the negative electric potential which provides their confinement. In the tokamak (and all other magnetic confinement systems) configuration, charged particles are lost from the system (to its walls) by transport of plasma ions and electrons across the magnetic fields by microscopic inter-particle collisions (which abruptly shift the particle gyration radius "guiding center"), and by other processes in which plasma particles, ions, and electrons act collectively to yield macroscopic transport losses of "groups" of particles across the supposed "confining" magnetic fields. Microscopic inter-particle collisions are both inevitable and necessary in plasmas in which it is desired to achieve inter-particle nuclear reactions. Thus in a plasma confinement system of interest for the attainment of nuclear fusion reactions, it is inherently necessary that plasma particles be lost from the field geometry by collisional "jumping" of the gyro centers (above). Without collisions there can be no fusion (or other nuclear) reactions; thus the attainment of conditions for fusion reactions ensures that the magnetic field can not confine the plasma, but can only constrain its (inherent) loss rate. In short, magnetic fields can confine (without losses) only plasmas and charged particle systems in which no collisions occur between particles. Since collisions will occur if two or more particles are in a given magnetic field system, it is evident that magnetic fields can not completely confine plasmas at densities of utility for nuclear fusion (or other nuclear) reaction production; they can only inhibit their unavoidable loss rate. In such systems the particle losses therefrom constitute an energy loss which must be made up by continuous injection of power to the system, in order to keep it operating at the desired conditions of plasma density and/or temperature. Research work to date in nuclear fusion has shown that considerable losses are inherent in the use of magnetic fields for plasma confinement. In general it has been found that conceptual magnetic confinement systems for the production of useful fusion power generation must be very large when based on low-power-consumption magnet coils of super-conducting material. Alternatively it has been found that small tokamak systems with small power input to the plasma region can be based on magnet coils of normal-conducting materals but will require very large power input to drive these coils. Thus, all conventional magnetic approaches to the generation of fusion power are practically unable to take advantage of the natural large energy gain (G=ratio of energy output to energy input per fusion reaction) inherent in the fusion process. This "natural" gain can be as large as G=2000 for the fusion of deuterium (D or H.sub.2) and tritium (T or H.sub.3), the two heavy isotopes of hydrogen (p or H.sub.1). Furthermore, a magnetic field can not produce a force on a charged particle unless that particle is in motion. If it is at rest with respect to the system--and therefore not attempting to leave the system--it will not feel any force in a magnetic system. It will experience a force in such a system only if it is trying to escape from (or otherwise moving in) the system. When moving, the force exerted on such a charged particle by a magnetic field is not oppositely directed to its direction of motion, it is at right angles to its direction of motion. The magnetic force on such a moving particle is thus not a "restoring" force, it is a "deflecting" force. Because of this the field is relatively ineffective in holding neutral plasmas of equal numbers of charged particles together, as in tokamaks or mirror field geometries used in fusion research. This results in large power requirements for the machinery needed/used for confinement and plasma heating in such devices constructed according to these concepts, and the energy gain (G) potentially achievable is found to be limited by practical engineering considerations to the order of G.congruent.2 to 6. Electrostatic systems have also been explored for the confinement of plasmas. The simplest such system is that with a spherical geometry, in which a negative potential is maintained at the center of a spherical shell by an electrode (cathode) mounted at the center. Positive ions introduced into such a geometry will be forced toward (and will "fall" to) the center until their mutual Coulombic repulsive forces exactly balance the inward-directed forces on them by the applied radial fields. In this "fall" the ions will acquire particle energies equal to the electric field potential drop. In principle, such systems offer very efficient means of reaching particle energies of interest for fusion reactions (efficiency of energy addition and thus to particle "heating" by this means is nearly 100%, which enables the achievement of very high gain G values). Unfortunately the ion densities which can be achieved by this means, within the limits of externally-supplied electric fields which are practically attainable, are too small to be of interest for fusion plasma reaction production at a useful level. The absolute density of ions can be increased by the addition of electrons to such a system, to yield a (net) neutral plasma whose ion and electron densities are grossly equal. However, it can be shown (Earnshaw's theorem) that a (neutral) plasma can not be confined by an electrostatic field of this type. This is because the plasma ions and electrons will be subject to oppositely-directed forces in the static field and will separate, thus producing a local field gradient (due to their charge separation) which exactly cancels the applied field. In this condition the plasma can move across the field as fast as electrons are lost from its outer boundary. The speed of motion of electrons escaping from such a system is limited to that of ion motion, as the two oppositely-charged particles are tied together by their dielectric field. In a system with a static centrally-negative spherical electric field configuration, as described above, there is no force field to inhibit electron loss from the outer boundary or periphery of such a neutral plasma. Previous workers have recognized the value of electrostatic forces for plasma/ion confinement. The earliest work was reported by William C. Elmore, James L. Tuck, and Kenneth M. Watson, "On the Inertial-Electrostatic Confinement of a Plasma", Phys. Fluids, Vol. 2, No. 3, pp. 239-246 (May-June 1959). Elmore et al proposed to overcome the difficulty of the Earnshaw's theorem limit (mentioned above) through the generation of the desired spherical radial field by the injection of energetic electrons in a radially-inward direction. This is depicted in FIG. 3. In this pioneering work electrons were to be emitted from the inner surface of a spherical shell 300 through a (screen) grid 310 at high positive potential (100 kev). Electrons so injected would pass through the grid and converge radially to a central region 330 where their electrostatic potential at the sphere center was equal to the grid injection energy. This large negative electrostatic potential, maintained by continuous electron injection (to make up losses) could then be used to trap positive ions in the system. Ions "dropped" into such a potential well would acquire energy at the "bottom" of the well (i.e., at the sphere center) equal to the negative potential established by the electron injection energy. This scheme obviously depends on the conversion of kinetic energy of injected electrons to negative electric potential fields and is thus an inertial-electrostatic method of plasma confinement. No means were provided to inhibit electron loss at the sphere surface. Somewhat later it was shown that such a negative potential well system is unstable to various perturbations if the confined ion density exceeds a certain level. H. P. Furth, "Prevalent Instability of Nonthermal Plasma", Phys. Fluids, Vol. 6, No. 1, pp. 48-53 (January 1963). This level was shown to be so low that the system was not of practical interest. Furth agreed with Elmore et al that the system would be unstable, and further showed that such self-confined inertial electron/ion systems using electrostatic confinement were inherently unstable. That is, systems in which confining non-equilibrium electrostatic fields are to be produced by inertial-electrostatic conversion of one charged component would be unstable above some critical density of the other component. For confinement by electron injection the ion density limit is too small to be of interest. Another system for electrostatic confinement of plasmas is set forth in U.S. Pat. Nos. 3,258,402 (June 28, 1966) and 3,386,883 (June 4, 1968) to P. T. Farnsworth. Following the approach further research has been conducted into the feasibility of electrostatic confinement of ions. See, e.g, Robert L. Hirsch, "Inertial-Electrostatic Confinement of Ionized Fusion Gases", Jour. Appl. Phys, Vol. 38, No. 11 (October 1967). Hirsch also utilized conversion of inertial energy for the production of central electrostatic fields. His work followed along the lines developed by Farnsworth (above), and utilized spherical grid structures and geometries as outlined in his U.S. Pat. Nos. 3,530,036 and 3,530,497 (both Sept. 22, 1970). Hirsch used injected ions (of D and T) rather than electrons. The several-thousand-fold mass difference (ions heavier than electrons) allowed the attainment of much more stable field/ion structures than predicted for electron injection, and the devices tested by Hirsch achieved fusion reaction rates in excess of 1.0E10 reactions/second on a continuous basis. However, the geometry which was used was not completely spherical; the ions were injected by six opposing ion guns mounted in opposite cubic-faced array. Later analysis suggests that this geometry as well as other conditions of the experiment caused intersecting beam phenomena and ion/gas collisions to dominate over other phenomenologies important to electrostatic confinement, as these were envisioned by Farnsworth and Hirsch in their earlier work. D. C. Baxter and G. W. Stuart, "The Effect of Charge Exchange and Ionization in Electrostatic Confinement Devices", Jour. Appl. Phys., Vol. 53, No. 7, pp. 4597-4601 (July 1982). In particular, it appears that current amplification by multiple transits across the potential cavity did occur, with consequent beam buildup, in part due to reflection of ions by the grid structures opposing their own injector structures, as indicated by the sensitivity of neutron output to injection beam alignment. Here (as in the work of Elmore et al) no mechanism was invoked to provide any other confinement of electrons at the surface or periphery of the approximately-spherical system geometry. The use of electron injection to produce negative plasma potentials for enhanced confinement in magnetic systems was examined in Soviet work on magnetic mirror systems. Work of the Soviet group at Kharkov, as reported in the Annals of the New York Academy of Sciences, Vol. 251, the proceedings of a conference held Mar. 5-7, 1974 on Electrostatic and Electromagnetic Confinement of Plasmas and the Phenomenology of Relativistic Electron Beams, (L. C. Marshall and H. L. Sahlin, ed., 1975). See, for example, Levrent'ev "Electrostatic and electromagnetic High-Temperature Plasma Traps" and also Dolan "Electric-Magnetic Confinement". These systems used physical ring electrodes in the ring cusp region of bi-conic cusp systems to inhibit plasma ion losses, and employed axial electron injection in cylindrical geometry to enhance ion magnetic confinement by producing negative potentials in the plasma region of this and of solenoidal Ioffe-bar-type mirror systems. Similar work by Blondin and Dolan invoked fixed cusp-region anode and cathode structures to aid magnetic cusp/miror plasma ion confinement by the imposition of electrostatic fields in both the polar and equatorial loss cones. D. C. Blondin and T. J. Dolan, " Equilbrium Plasma Conditions in Electrostatically Plugged Cusps and Mirrors", J. Appl. Phys., Vol. 47, No. 7, pp. 2903-2906 (July 1976). R. L. Hirsch had earlier studied this method to aid confinement in solenoidal mirror magnetic confinement systems. U.S. Pat. No. 3,655,508 (Apr. 11, 1972). Still other work utilized ion injection to establish positive potential fields in bi-conic or mirror cusp geometries, or in twisted bi-conic mirrors used as "plugs" at the ends of linear solenoids (See, e.g., F. L. Hinton and M. N. Rosenbluth, "Stabilization of Axisymmetric Mirror Plasmas by Energetic Ion Injection", Nucl. Fus., Vol. 22, No. 12, pp. 1547-1557 (1982), and P. J. Catto and J. B. Taylor, "Electrostatic Enhancement of Mirror Confinement", Nucl. Fus., Vol. 24, No. 2, pp 229-233 (1984).) All of these approaches used fixed electrodes and/or ion or electron injection to establish electric potentials to aid magnetic plasma confinement systems, not for the direct electrostatic confinement of ions. In summary, previous work in inertial, magnetic, and electrostatic confinement aimed at the confinement of charged particles (ions), for the purpose of creating conditions useful for the generation of nuclear fusion reactions between them, has shown that: (1) Magnetic fields do not provide restoring forces to charged particles in motion, or to confine plasma particles; they provide deflecting forces, at righ angles to the direction of motion of the particles. Electrostatic, electrodynamic, and other electric fields can provide direct restoring forces for the confinement of charged particles. (2) Even the most favorable magnetic confinement geometries lose charged particles by gyro guiding center shifting due to microscopic collisions between particles. Such collisions are essential for the creation of nuclear reactions. (3) Collisions between particles of like sign have the most effect on ion losses. Such collisional losses are governed by the gyro radii of ion/ion collisions in conventional magnetic confinement schemes. Electron gyro radii are very much less than those of ions of comparable energy. (4) Electron and ion motions in magnetic fields are of opposite sign. This results in the electric polarization of the plasma, with the establishment of an ambipolar dielectric field. Plasma losses are then set by the rate of ion/ion transport collisions across the field. (5) Inertial-electrostatic potential wells estblished and maintained by charged particle injection alone and held solely within electric field structures are stable only for confinement at particle densities below a certain critical value. This is found to be too low for the production of nuclear fusion reaction rates useful for power generation. SUMMARY OF THE INVENTION Considering all of these facts and features it appears that useful confinement of plasmas can be achieved by an improvement on all prior concepts, by new and unique uses and combinations of magnetic and electric fields, and by use of inertial forces. The current invention accomplishes this by: (a) Using a substantially spherical magnetic field geometry which is macroscopically and magnetohydrodynamically (MHD) stable for confinement of charged particles to confine a plasma which is non-neutral with excess density of electrons. This requires use of a magnetic field geometry which is everywhere convex towards its confined ion/electron/plasma system. (b) Using a magnetic field geometry with minimum losses; e.g., a "mirror" type system without line or ring cusps--its cusps are all point cusps. The geometries of interest utilize special polyhedral configurations for magnetic field generating means. Oscillation of single polyhedral or multiply-faceted interlaced polyhedral surface fields may be useful to provide good magnetic surface "reflection" of confined electrons, by causing the time-averaged fields to appear "quasi-spherical" over the electron gyration time at the local field strength. (c) Using the excess electrons thus confined to produce a central electrostatic field which is stable (by virtue of stable electron confinement by the magnetic field) and in which the desired negative potential can be maintained at any density of "trapped" ions (less than the total density of electrons). (d) Injecting electrons at high energies (e.g., 10 kev to more than 1 Mev, depending on the ions chosen for confinement) into stable magnetic configuration, to establish negative potential wells of depths sufficient to confine ions at energies at which nuclear fusion reactions will occur. (e) Adding ions to the system by injection, to attain ion densities needed for useful nuclear fusion reaction rates, and to provide high pressure to support the central plasma core, by dynamic conversion of the energy of injected ions falling into the confining potential well (e.g., as by two-stream instability coupling of momentum from the in-falling ions to the core field structure). It is the object of this invention to overcome the defects and deficiencies of previous concepts for electrostatic confinement of ions by utilizing the confinement ability of (MHD stable configurations) of magnetic fields for the confinement of electrons, so that stable electric fields produced by their confined distribution in turn may be used to confine the ions. It is further the object of this invention to utilize these means for ion confinement to achieve densities of such confined ions at values large enough to allow nuclear fusion reactions to occur at useful rates. It is further the object of this invention to provide a confinement means for ions which can confine a variety of ions of interest for nuclear fusion, at particle energies up to the range of 400 kev to 2 Mev, as well as at smaller ion particle energies. The concepts of the current invention provide for a device able to ensure stable entrapment of positive ions which are injected into negative electric field configurations capable of confining these positive particles. These electric potential configurations are formed by spatially-stable distributions of electrons from electron injection into stable magnetic field configurations, to yield net excess electron (over ion) density, and thus net negative internal potential distributions. These stable quasi-spherical magnetic field configurations are all point-cusp mirror fields, placed with alternating sign (or sense) on the surface planes of any of the regular polyhedra when truncated (except for the octahydron untruncated; the octahedron is just a truncated tetrahedron); or on any other arrangement of polygonal faces on or extending from these surface planes, or forming any other ordered polyhedron. A feature of importance to optimal functioning of the current invention is that the arrangement of polygonal faces must be such that all intersection points (between faces) are surrounded by an even number of faces. Another feature of importance is the possible oscillation of the surface magnetic fields at frequencies high enough so that good surface magnetic "reflection" is provided to the confined electrons. Ions may be injected with any energy from (nearly) zero up to (or greater than) that of the energy of injected electrons. Electrons may be injected with energies of 10 kev up to several Mev, but must be injected with sufficient energy to establish central negative electric potential wells of greater depth (strength) than the energy level at which it is desired to promote or contain ion-collisional interactions among the ions trapped in the negative potential well. Increasing electron injection energy (voltage) may lead to negative potential well depths great enough to initiate nuclear fusion reactions between injected/confined plasma ions of the light elements and their isotopes (e.g., H, D, T, Li, B, Be, etc.). Increasing injection energy of the ions may likewise lead to such wells, through the mechanism of "virtual electrodes" orginally discussed by Hirsch and Farnsworth. Alternatively, some form of both effects may be used in the current invention. In devices operated at conditions capable of creating nuclear fusion reactions, the strength of the (surface) magnetic fields and (internal) electric potential fields is such that: (a) fusion products will escape entirely from the field regions; (b) unreacted ions will be trapped in the well by the electric fields, and; (c) electrons will be trapped internally by the magnetic fields. This is a result of the fact that the radii of gyration of charged particles in the magnetic fields of the device are much larger for fusion products than the dimensions of the device, are comparable for trapped ions, and are much smaller for electrons. This feature is unique among all other concepts for the confinement and generation of nuclear fusion reactions. One consequence of this feature is that fusion reaction energy carried by fusion product ions is not deposited locally in the entrapped plasma by collisions therewith but, rather, is carried outside and away from the source region in which the reactions themselves are caused to occur. Thus, fusion energy so created does not contribute to the "heating" of plasma ion, and the device of the current invention is not an "ignition" machine, nor does its functioning depend upon "heating" a mass of plasma ions to a sufficiently large temperature to ensure significant fusion reactions, as is the case for all other magnetic and/or inertial confinement concepts for the attainment of fusion. Electron and/or ion injection may be steady-state (cw) or pulsed at frequencies from a few Hz to several hundred MHz. This frequency may be made equal to the frequency of oscillation of current flow in the magnetic field coils, if these are driven in an oscillatory mode. Such pulsing of the injection beam which is responsible for establishing the potential well which confines the ions so that they will react among themselves, will naturally cause the inter-ionic reactions to oscillate with the frequency of oscillation of the well induced by the injection pulsation. In such conditions the release of energy from ion nuclear collisional interactions will oscillate as well, and so will the output of (charged) collisional reaction products. When operated in such a pulsed or oscillatory mode, nuclear-reaction-generated energy may be coupled into oscillations of the confined plasma, itself, to yield amplification of the pulsations and thus to yield high frequency radiative power output, or to yield oscillation of surface potentials on the container walls surrounding the plasma region, and thus to surface (spherical) wave generation. By this means the device can become a self-amplifying, self-powered generator of microwave or other radio-frequency energy. If operated with electron injection currents, ion densities, and magnetic fields which allow large power gain to occur, such a device can be used as a powerful source of radio-frequency energy, for radar, communications, power-beaming, energy beam weapons, etc., with no external source of main power. At the onset of fusion reactions in the device, the fusion product ions will escape from the system, leaving behind their electrons, to yield a still-more-negative confining well. This could lead to a "runaway" effect in which continuing reactions yielding fusion products create an ever-deepening well, which in turn increases the fusion rate, which deepens the well more, etc. This process will be stopped by burnout of the ion fuel in the well, by arcing or other "shorting" or destabilizing effects, or by reaching stable burn conditions in balance with the ion and electron injection rate, and the rate of escape of electrons and of ions from the surface of the confinement region (i.e., from the magnetic field). The nature of this self-initiated creation of deeper potential wells will depend upon the species of ions used in the device, and undergoing fusion reactions. If all the ions involved carry only single nuclear charges (i.e., if all have only one proton in the nucleus), then the onset of fusion reactions can lead to a well-deepening effect, as described above, only for a limiting transient period. This transient will be damped out by the continuous injection of new electrons and ions, to reach a new stable density distribution. However, if the ions involved carry more than one proton in their nucleus and are injected only partially stripped, with a single charge, then the onset of fusion will trigger an exponential well-deepening process stabilizing at a new, deeper well depth as a result of each fusion reaction leaving more electrons behind in the well than were originally injected. Particle injection (ion or electron) may be along the magnetic cusp field axes, or offset but parallel to them, or in an annular sheath around these axes. If annular, the particle sheath may be injected with or without rotation; if offset-axial or on-axis the particle beam may be injected with or without rotational or nutational motions. If rotation/nutation is used in the injection process, this will have the effect of preventing the particles from "falling" directly radially inward into the potential well, towards zero radius. Rather, such particles will be constrained to converge to a minimum radius greater than zero on account of the angular momentum they possess by virtue of the rotational momentum with which they were injected. The beneficial effect of such angular momentum on stabilization of the confining potential well has been shown by experiment. J. T. Verdeyen, et al, "Recent Developments in Electrostatic Confinement--Experimental", Ann. of the N.Y. Acad. of Sci., Vol. 251, May 8, 1975. In these experiments it was also shown tht the introduction of angular momentum results in formation of a "virtual" central electrode of small radius, and in the reduction of well depth and maintenance of parabolic shape of the well. Ions also may be injected in parallel with electrons (annular and axial beams) or opposing the electrons (opposite magnetic cusp faces on the polyhedral configuration (used), or in opposing each other, or in parallel with ions, or in any other fashion recognized to be appropriate by one of ordinary skill in the art. Fusion reaction products will generally escape regions containing the plasma, electrostatic fields, and electrons, and be deposited in and on structures around but outside of these regions. Since these products are all positively charged and carry high energy (several Mev each) their energy may be converted directly to electrical energy in external circuits by causing the external structures (on which the fusion products impinge) to operate at high positive electric potentials (voltages). With such an arrangement the positively-charged fusion products must escape "up hill" against the applied positive potentials, and can drive electrical energy into any circuit to which the external structures are connected and which closes back to the plasma/electron/well region. The pressure of particles in the well will be supported by the external magnetic field which confines the electrons, through the inertial effects of conversion of the kinetic energy of in-falling ions to electrostatic dynamic pressure on ions confined in (but moving outward from) the central core of the potential well reacting volume. This well itself is produced by the conversion of the inertial energy of injected electrons to the energy of the well depth at the electrostatic potentals established by the stably-confined electron density. This electrostatic well gives energy to the in-falling ions, which in turn couple their thus-acquired energy of motion (and momentum) into confinement pressure on the core. The kinetic energy of injected electrons thus, through the medium of the electrostatic well, is transformed into the kinetic energy of in-falling ions. The ratio of momenta of ions and electrons of the same energy is given by the square root of the ratio of their masses, thus this transformation has the effect of producing an ionic "gas" whose (dynamic) pressure is very much larger than the equivalent (dynamic) pressure of the injected electrons. In addition, the convergence of the quasi-spherical geometry of the polyhedral configurations of interest increases the local dynamic pressure by the square of the inverse ratio of radii from the outer (electron injection) radius to the inner (ion dynamic pressure confinement) radius. As an example of these effects, if the ions are those of deuterium (D=H.sub.2) and the radius of the inner core is 0.1 that of the inner "surface" of the confinement field region, then the ratio of ion-generated pressure on the core to electron pressure on the (external) confining field will be roughly 6100:1. The physics phenomena invoked in this invention thus have the effect of creating an electrical "gas" inside the magnetic field region whose (dynamic) pressure at large radii is very much less than its pressure at small radii within the volume which it occupies. For this reason, it is possible to contain and confine a high density of reactive ions in a small radius within a larger radius at which a relatively weak magnetic field is placed; it is not necessary for the magnetic field to provide the confining pressure over a large radius to hold the high density plasma together at the pressure at which it operates within the smaller radius core. For example, ion densities of 1.0E15 to 1.0E17 per cm.sup.3 may be sustained in D at 100 kev with surface magnetic fields of only 1 to 10 kG (kilogauss). The coupling of in-falling momemtum of the ions to ions from the core will be accomplished by multi-stream instability interactions, through quasi-spherical waves. In this the particles streaming inward and outward through each other will interact collectively, above some ion current density, through the generation of electrostatic waves which transfer momentum (and energy) from one stream to the other and operate over very short interaction distances (wavelengths). There will always exist a radius sufficiently close to the center at which the convergent density becomes large enough to initiate electrostatic wave instability interaction, hence this phenomena will always be available to provide dynamic pressure coupling to the core region. Note that scattering collisions in this geometry do not directly increase particle losses, because they occur near the center and their effect of changing the direction of motion still leaves all motion predominantly radial in vector direction. It is important to note that requirements on ion beam injection power are minimal in an electron-injection-driven device, since most of the ion energy is acquired by "falling" down into the negative electric potential well set up by the electron injection. This potential well depth is established by the net excess electron density which is set up over the ion density contained therein. This is limited by the loss rate of electrons from the surface region of the polyhedral magnetic field configuration. Higher fields give smaller loss rates; larger injection currents allow larger losses. Thus, the balance between losses and input will set the level of excess electron density. Numerical calculations show that only modest electron injection powers are required with modest fields (as above) to yield very deep electrostatic wells for ion confinement (e.g., over 200 kev). The power output is set by the rate of reaction within the central region, integrated over the volume of this region. The reaction rate is determined by the square of the ion density (and the product of reaction cross-section and particle speed), thus is limited by the ion current density in the central region. In-falling ions will converge as the inverse square of the radius, thus the reaction rate will tend to vary as the inverse fourth power of the radius. This very rapid dependence ensures that nearly all of the fusion energy generated in such a device will be generated in and around the center of the (structurally-empty) cavity confined by the external magnetic field, at the largest possible distance from the walls of the system. It will be somewhat like a little "star" burning in the center of electrostatic well cavity "void". Numerical calculations show that the ion current densities required for total fusion power output at "useful" levels is much larger than those required for power balance makeup against electron losses, or for pressure support of the reaction core. To achieve this state requires a current multiplication or "gain" (G.sub.j) achieved by the recirculation of ion (and electron) currents across the machine volume many times, until a sufficient ion density is achieved. The required current recirculation factor is found to vary roughly as the inverse fifth power of the major radius (R) of the device, so that Gj.congruent.(1/R.sup.5). Large devices thus will require less current "gain" than small devices, and it is clear that there must exist a size sufficiently large that the "gain" may be unity (G.sub.j =1), and that no current multiplication is required for operation of the machine at a breakeven power balance. Numerical calculations show that this size is approximately R.congruent.10-20 m, and that G.sub.j .congruent.1.0E6 will allow R.congruent.20-30 cm (G.sub.j values greater than 1.0E6 have been obtained in a variety of electron and ion magnetron tubes of other types). Another feature of the device is the existence of a "black hole effect" (BHE) in respect to fusion burn reactions. This results from the fact that there must exist a radius at which the fusion reaction collision rate is sufficiently large that the total fusion reactions occuring per unit volume over the time of flight of an ion from this radius to the center of the machine would equal or exceed the total density of ions at this radius. For such a condition it is clear that all ions entering (i.e., falling into) this radius region will undergo fusion reactions. |
claims | 1. A method for the quantitative determination of fissile material content in small size particles present in samples wherein: the samples are sandwiched in organic sheets and then submitted to a defined thermal neutrons fluence whereupon fission products of the fissile material in the sample create fission tracks in the sheets, the tracks are enhanced by a chemical etching process, the size of selected particles having created such tracks is determined by means of a microscope, and thereafter these visible tracks of said selected particles are compared to pre-established standard tracks obtained by the same process from particles of different stepped down size and enrichment ratio. 2. A method according to claim 1 , wherein many distinct samples are collected on a common carrier tape and are submitted simultaneously to the neutron flux. claim 1 3. A method according to claim 1 or 2 , wherein the fission tracks are interpreted by an automated image processing treatment using reference data derived from said pre-established standard tracks. claim 1 2 4. A method according to claim 1 or 2 , wherein samples are sandwiched between a first organic sheet which is covered on the side of the ,sample with a metal foil, and a second organic sheet without such foil, in order to permit both the analysis of alpha tracks and that of fission products. claim 1 2 |
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claims | 1. A radiation generator comprising:an insulator;a ion source carried within the insulator and configured to directly generate ions and indirectly generate undesirable particles;a plurality of extractor electrodes, a first extractor electrode of the plurality of extractor electrodes carried within the insulator downstream of the ion source and having a first potential, and a second extractor electrode of the plurality of extractor electrodes carried within the insulator downstream of the ion source and having a second potential, wherein the first extractor electrode terminates farther downstream from the ion source than the second extractor electrode, and wherein the first potential is closer to ground than the second potential;an intermediate electrode carried within the insulator downstream of the extractor electrodes and being shaped to capture at least some of the undesirable particles;a suppressor electrode carried within the insulator downstream of the intermediate electrode and having a third potential opposite in sign to the first potential and the second potential;the intermediate electrode being at an intermediate potential between the first and third potential; anda target carried within the insulator downstream of the suppressor electrode;the extractor electrodes and the suppressor electrode having a voltage therebetween such that an electric field generated in the insulator accelerates the ions generated by the ion source toward the target. 2. The radiation generator of claim 1, wherein the intermediate potential is at ground potential. 3. The radiation generator of claim 1, wherein the first extractor electrode is curved inwardly toward a longitudinal axis of the insulator and a portion of the first extractor electrode that is curved has a substantially uniform thickness. 4. The radiation generator of claim 1, wherein the suppressor electrode is curved inwardly toward a longitudinal axis of the insulator. 5. The radiation generator of claim 1, wherein the first extractor electrode is shaped to capture the undesirable particles indirectly generated by the ion source, wherein the first extractor electrode is curved inwardly toward a longitudinal axis of the insulator, whereby an inner diameter of the first extractor electrode is greater at a longitudinal location nearer to the ion source than at a longitudinal location farther from the ion source. 6. The radiation generator of claim 1, wherein the intermediate electrode is shaped to attenuate x-rays undesirably generated in the radiation generator. 7. The radiation generator of claim 1, wherein the intermediate electrode is T-shaped. 8. The radiation generator of claim 1, wherein the intermediate electrode comprises a base extending along the longitudinal axis of the insulator, and a projection extending outwardly from the base. 9. The radiation generator of claim 8, wherein the projection has a concave triangular shape. 10. The radiation generator of claim 1, wherein the intermediate electrode comprises a material having a Z of less than or equal to 13. 11. The radiation generator of claim 1, comprising a sealed housing carrying the insulator, and ionizable gas molecules within the sealed housing; andwherein the ion source comprises:a cathode configured to emit electrons;a cathode grid downstream of the cathode;an extractor grid downstream of the cathode grid;the cathode and the cathode grid having a first voltage therebetween such that the electrons emitted by the cathode are accelerated toward the grid and downstream;the cathode grid and the extractor grid having a second voltage therebetween less than the first voltage such that the electrons are decelerated as they approach the extractor grid, at least some of the electrons striking the ionizable gas molecules to create the ions. 12. A well logging instrument comprising:a sonde housing;a radiation generator carried by the sonde housing;a solid insulator carried by the sonde housing between an inner surface of the sonde housing and an outer surface of the radiation generator; andan insulating gas in the sonde housing;the radiation generator comprisinga sealed generator tube,a charged particle source carried within the sealed generator tube and configured to emit charged particles,an extractor electrode carried within the sealed generator tube downstream of the charged particle source at a first potential,an intermediate electrode carried within the sealed generator tube downstream of the extractor electrode wherein the intermediate electrode comprises a base extending along the longitudinal axis of the sealed generator tube, and a projection extending outwardly from the base, wherein the projection comprises a first portion extending from a central point in the base toward the charged particle source and a second portion extending from the central point in the base away from the charged particle source, wherein the first and second portions are substantially symmetrical to each other,a suppressor electrode carried within the sealed generator tube downstream of the intermediate electrode at a second potential opposite in sign to the first potential, anda target within the sealed generator tube downstream of the suppressor electrode,the intermediate electrode being at an intermediate potential between the first and second potential,the difference in the first and second potentials being such that an electric field generated in the sealed generator tube accelerates the charged particles emitted by the charged particle source toward the target;wherein the intermediate electrode curves in a generally complementary trajectory to the extractor electrode and in a generally complementary trajectory to the suppressor electrode, thereby allowing an acceleration gap between the ion source and the target to be shorter than otherwise, and thereby reducing a number of charge exchange reactions that might otherwise occur. 13. The well logging instrument of claim 12, wherein the intermediate potential is a ground potential. 14. The well logging instrument of claim 12, wherein the extractor electrode is curved inwardly toward a longitudinal axis of the sealed generator tube in a direction away from the charged particle source. 15. The well logging instrument of claim 12, wherein the suppressor electrode is curved inwardly toward a longitudinal axis of the sealed generator tube. 16. The well logging instrument of claim 12, wherein the intermediate electrode is T-shaped. 17. The well logging instrument of claim 12, wherein the projection has a concave triangular shape. 18. A method of generating radiation comprising:generating ions and indirectly generating undesirable particles, using an ion source within an insulator, the undesirable particles generated on a trajectory toward the insulator;accelerating the ions toward a target within the insulator using an extractor electrode downstream of the ion source at a first potential and a suppressor electrode downstream of the extractor electrode at a second potential opposite in sign to the first potential; andshielding the insulator from the undesirable particles that would otherwise strike the insulator, using an intermediate electrode downstream of the extractor electrode and upstream of the suppressor electrode at an intermediate potential between the first and second potential and using the extractor electrode, wherein the extractor electrode shields the insulator by curving inwardly toward a longitudinal axis of the insulator away from the ion source, and using the suppressor electrode, wherein the suppressor electrode shields the insulator by curving inwardly toward a longitudinal axis of the insulator toward the ion source, wherein the intermediate electrode curves in a generally complementary trajectory to the extractor electrode and in a generally complementary trajectory to the suppressor electrode, thereby allowing an acceleration gap between the ion source and the target to be shorter than otherwise, and thereby reducing a number of charge exchange reactions that might otherwise occur. 19. The method of claim 18, comprising reducing an electric field that would otherwise be at a surface of the suppressor electrode by shaping the suppressor electrode to be curved inwardly toward a longitudinal axis of the insulator. 20. The method of claim 18, comprising shielding the insulator from the undesirable particles that would otherwise strike the insulator by shaping the extractor electrode to capture the undesirable particles. 21. The method of claim 18, wherein the intermediate electrode comprises a base extending along the longitudinal axis of the housing, and a projection extending outwardly from the base. 22. The method of claim 18, wherein generating the ions comprises:emitting electrons using a cathode; andaccelerating the electrons away from the cathode using a grid downstream of the cathode so that some of the electrons accelerated away from the cathode strike ionizable gas molecules to create the ions. |
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claims | 1. A clamp for sealing a slip joint formed by a juncture of a diffuser and inlet mixer in a nuclear reactor, the clamp comprising:a plurality of moveable clamp arms shaped to seat on a top end of the diffuser and against an outer surface of the inlet mixer;a joint moveably connecting at least two of the clamp arms to permit movement of the two clamp arms around the inlet mixer; anda lateral drive in one of the clamp arms, wherein the lateral drive extends in a transverse direction against the inlet mixer to bias the inlet mixer against an opposite one of the clamp arms, and wherein the lateral drive is positioned in the clamp arm to directly contact and bias only the inlet mixer in the slip joint. 2. The clamp of claim 1, wherein the plurality of moveable clamp arms are two annular halves that join to form a continuous annulus around an entire upper and inner perimeter of the diffuser. 3. The clamp of claim 2, wherein the joint is a pin allowing rotation of the two annular halves with respect to each other about a single axis with no other relative movement. 4. The clamp of claim 1, further comprising:a bolt between the two clamp arms configured to prevent the movement of the two clamp arms around the inlet mixer. 5. The clamp of claim 1, further comprising:an axial mount on one of the clamp arms, wherein the axial mount is configured to secure to only an exterior of the diffuser so as to prevent relative movement between the one clamp arm and the diffuser. 6. The clamp of claim 5, wherein the axial mount is a guide ear clamp shaped to clamp against an axial bottom end of a guide ear of the diffuser. 7. The clamp of claim 6, further comprising:a plurality of the axial mounts sufficient to clamp to each guide ear of the diffuser. 8. The clamp of claim 1, wherein the lateral drive includes a leaf spring captured in the one clamp arm. 9. The clamp of claim 8, wherein the leaf spring has a length in the clamp arm so as to extend at least 12 percent of a perimeter of the inlet mixer, and wherein the leaf spring has a thickness and spring constant to provide 750 pounds-force when compressed by an inch in the transverse direction. 10. The clamp of claim 8, wherein the lateral drive further includes a bolt connected to a wedge seating against the leaf spring in the one clamp arm, and wherein axial movement of the bolt causes transverse compression of the leaf spring via the wedge. 11. A clamp for sealing a slip joint formed by a juncture of a diffuser and inlet mixer in a nuclear reactor, the clamp comprising:at least two opposite clamp arms shaped to fit inside of the diffuser and against the inlet mixer in the slip joint and fill the slip joint completely; anda spring in one of the clamp arms configured to bias the inlet mixer in a transverse direction against at least one of the clamp arms without transversely loading the diffuser, and wherein the clamp in its entirety seats around only a single slip joint. 12. The clamp of claim 11, wherein the two opposite clamp arms are semi-circular and join to create a continuous annulus that fills the slip joint, the clamp further comprising:a bolt rigidly connecting the two opposite clamp arms, wherein the bolt may be loosened to allow movement of the clamp arms relative to each other. 13. The clamp of claim 12, further comprising:a driving bolt on a top of the clamp, wherein the driving bolt biases the spring in the transverse direction when rotated. 14. The clamp of claim 11, wherein the leaf spring is positioned in the clamp arm to directly contact and bias only the inlet mixer in the slip joint. 15. A clamp for sealing a slip joint formed by a juncture of a diffuser and inlet mixer in a nuclear reactor, the clamp comprising:a plurality of moveable clamp arms shaped to seat on a top end of the diffuser and against an outer surface of the inlet mixer;a joint moveably connecting at least two of the clamp arms to permit movement of the two clamp arms around the inlet mixer; anda lateral drive in one of the clamp arms, wherein the lateral drive extends in a transverse direction against the inlet mixer to bias the inlet mixer against an opposite one of the clamp arms, and wherein the clamp in its entirety seats around only a single slip joint. |
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055901620 | summary | FIELD OF THE INVENTION This invention generally relates to stand-alone means for generating low-power direct current or voltage. BACKGROUND OF THE INVENTION FIG. 1 shows an apparatus, disclosed in U.S. patent application Ser. No. 08/384,997, for electrically suppressing the electrochemical potential (ECP) near a BWR component which is susceptible to intergranular stress corrosion cracking (IGSCC). The apparatus is a self-contained means of locally protecting critical portions of metals, such as welds, by suppressing ECP in the immediate vicinity of that portion of the metal requiring protection in operating BWR plants. The apparatus shown in FIG. 1 is based on the concept of supplying electrons directly and locally to the surface of a sensitized metallic structural member 2, as in the case of the heat affected zone 6 of a weld 4, thereby inhibiting IGSCC. The electrical system depicted in FIG. 1 is capable of supplying sufficient electrons to the metal surface to inhibit the corrosion reaction due to local ECF exceeding the threshold value at which IGSCC can occur. In the circuit of FIG. 1, the center electrical conductor of a small mineral-insulated steel sheathed cable 16 is attached to the metal surface to be protected against IGSCC and connected to an electrical control circuit 10 that operates off the low-voltage DC power supply 20. The control circuit 10 and DC power supply 20 are enclosed in a housing 8 made of material able to withstand thermal and radiological conditions inside a boiling water reactor, but outside the reactor core. The passive conductor of a twisted-shielded pair of cable conductors is connected to a reference electrode 18 located in the oxidizing coolant near the metal surface and to a terminal of the control circuit. The current collected at the metal surface is controlled by the applied voltage on the load resistor R via an electrical conductor connected to the surface of the metal to be protected and to another terminal of the control circuit. This current I is converted to a voltage drop across R, which is input to a differential amplifier 12 of gain G. The differential amplifier output is the effective voltage "error signal" which is integrated by the operational amplifier 14 with time constant .tau.=R.sub.1 C. The small stand-off resistor R.sub.2 depletes excess charge build-up on the feedback capacitor C to eliminate any possibility of integrator malfunction. The collected current is dissipated in the load resistor R. Electron depletion of the metal and IGSCC are defeated since electrons are forced to flow into the metal to compensate for those that would be lost by oxidation of the metal. The apparatus shown in FIG. 1 has a power supply 20 which requires no external power source, but rather is energized by electrons (also referred to herein as .beta.-particles) produced during nuclear decay. The source current I.sub.s (see FIG. 1) arises from the collection of nuclear decay electrons and produces a voltage across the source resistor R.sub.s which is a slowly decreasing function of time (because of the emitter decay). The Zener diode 24 and load resistor R.sub.L stabilize and limit the output voltage B.sub.+, since the voltage drop across the diode is essentially the same for all reverse currents I.sub.Z flowing through it in the breakdown region of the device. The voltage B.sub.+ is regulated and stabilized, since large changes in diode current produce small changes in diode voltage. The resulting voltage across the load resistor R.sub.L, due to the load current I.sub.L, is insensitive to the .beta.-emitter decay and can be used to power the active components in the control circuit. In accordance with the foregoing teaching, the source of electrons was the decay of a radioactive isotope, depicted in FIG. 1 as a current source 22. For ease of handling and fabrication, the proposed isotope was a .beta.-emitter (nuclear electrons) without decay .gamma.-radiation. The following .beta.-emitting isotopes were identified as being suitable candidates: H.sup.3, C.sup.14, Si.sup.32, Sr.sup.90 and Ru.sup.106. The Ru.sup.106 isotope was preferred because of its 368-day half-life and 39.4 keV .beta.-ray. All these isotopes have a single decay mode with no prompt .gamma. emissions. However, further study of the table of isotopes has revealed that Ru.sup.106 decays to Rh.sup.106, which is both a .beta.-emitter and a .gamma.-emitter in its decay to stable Pd.sup.106. A preferred .beta.-emitter is one with adequate half-life for practical application and with no .gamma.-emission in its decay chain. The avoidance of .gamma.-emission serves to simplify handling and fabrication, and to reduce leakage currents. SUMMARY OF THE INVENTION The present invention is a stand-alone low-voltage direct current power supply, for use as a battery, which is energized by the decay of a radioactive isotope. During this decay, either .alpha.- or .beta.-particles are emitted. The .beta.-emitting radioactive decay is activated by exposing the battery to a substantial neutron flux, e.g., inside the neutron flux of a nuclear reactor core. After the .beta.-emitting material has been activated, the battery can be installed inside the reactor, as needed, at a location which is not exposed to a substantial neutron flux, i.e., out of core. The group of .beta.-emitting radioactive isotopes which satisfy the criteria of adequate half-life and no .gamma.-emission include the following additional isotopic candidates Sn.sup.121, Cs.sup.134, Sm.sup.151, Ra.sup.228 and Tl.sup.204. In accordance with a preferred embodiment of the invention, the .beta.-emitting radioisotope is thallium, which decays directly to the ground state of Pb.sup.204 by 763-keV .beta.-decay with no .gamma.-emission. The resulting .beta. particles are collected to form a current which can be used for various purposes inside a reactor. |
claims | 1. A system for storing high level radioactive waste comprising:an overpack body having an outer surface and a cavity for storing high level radioactive waste, the overpack body comprising an air inlet vent for introducing cool air into a bottom portion of the cavity;a plurality of plates disposed within a portion of the air inlet vent in a spaced apart manner, each of the plates extending from a first end proximate the outer surface of the overpack body to a second end, adjacent ones of the plates spaced apart a first distance at the first ends of the plates and a second distance at the second ends of the plates, the first distance being greater than the second distance; andan overpack lid comprising an air outlet vent enclosing a top end of the cavity. 2. The system of claim 1 wherein a line connecting the first ends of the plates forms a first reference circle having a first diameter and a line connecting the second ends of the plates forms a second reference circle having a second diameter, the first diameter being greater than the second diameter. 3. The system of claim 1 wherein the distance between the adjacent ones of the plurality of plates continuously decreases from the first ends of the plates to the second ends of the plates. 4. The system of claim 1 wherein the air inlet vent comprises an air inlet plenum in the spaces between the plurality of plates and an air inlet passageway that extends from the air inlet plenum to an opening in a floor of the cavity. 5. The system of claim 4 wherein the air inlet plenum extends substantially horizontally from an opening in the outer surface of the overpack body to a terminal end and wherein the air inlet passageway extends upwardly from the air inlet plenum to the opening in the floor at an oblique angle relative to a vertical axis of the overpack body. 6. The system of claim 5 wherein the air inlet passageway circumferentially surrounds the vertical axis of the overpack body. 7. The system of claim 1 wherein the air inlet vent comprises an air inlet plenum that extends substantially horizontally from an opening in the outer surface of the overpack body and an air inlet passageway that extends obliquely from the air inlet plenum in a direction away from a vertical axis of the overpack body. 8. The system of claim 7 wherein the overpack body extends along a first vertical axis, and wherein a second vertical axis that is parallel to the first vertical axis intersects both the air inlet plenum and the air inlet passageway. 9. The system of claim 1 wherein each of the plurality of plates is formed of steel. 10. A system for storing high level radioactive waste comprising:an overpack body extending along a vertical axis and having an outer surface and a cavity for storing high level radioactive waste, the cavity having an open top end and a floor;an air inlet vent for introducing cool air into the cavity, the air inlet vent configured so that aerodynamic performance of the air inlet vent is substantially independent of an angular direction of a horizontal component of an air-stream applied to the outer surface of the overpack body; andan overpack lid positioned atop the overpack body to enclose the open top end of the cavity, an air outlet vent in the overpack lid for removing warmed air from the cavity, the air outlet vent configured so that aerodynamic performance of the air outlet vent is substantially independent of an angular direction of a horizontal component of an air-stream applied to the outer surface of the overpack body. 11. The system of claim 10 further comprising a plurality of plates disposed within a portion of the air inlet vent, each of the plates extending along a reference line that is tangent to a first reference circle having a center point coincident with the vertical axis of the overpack body. 12. The system of claim 10 further comprising a plurality of plates disposed within a portion of the air inlet vent in a spaced apart manner, each of the plates extending from a first end proximate the outer surface of the overpack body to a second end, adjacent ones of the plates spaced apart a first distance at the first ends of the plates and a second distance at the second ends of the plates, the first distance being greater than the second distance. 13. The system of claim 12 wherein a line connecting the first ends of the plates forms a first reference circle having a first diameter and a line connecting the second ends of the plates forms a second reference circle having a second diameter, the first diameter being greater than the second diameter. 14. The system of claim 12 wherein the distance between the adjacent ones of the plurality of plates continuously decreases from the first ends of the plates to the second ends of the plates. 15. The system of claim 12 wherein the air inlet vent comprises an air inlet plenum in the spaces between the plurality of plates and an air inlet passageway that extends from the air inlet plenum to an opening in the floor of the cavity. 16. The system of claim 14 wherein the air inlet plenum extends substantially horizontally from an opening in the outer surface of the overpack body to a terminal end and wherein the air inlet passageway extends upwardly from the air inlet plenum at an oblique angle away from the vertical axis of the overpack body. |
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063046326 | claims | 1. A method for improving the quality of a radiographic image of an object obtained by a radiographic X-ray unit containing an anti-diffusion grid, arranged between the object and a receiver of radiographic images, comprising the steps of displacing the grid in rectilinear translation in its plane, at the time that the image is taken, between a starting position and an ending position and according to a predetermined temporal law of displacement, wherein the law of displacement is a continuous curve exhibiting point symmetry with respect to the point whose temporal coordinate is equal to one-half of the duration of imaging, and whose spatial derivative of the temporal variable exhibits two portions symmetrical with respect to an axis of symmetry passing through the center of the area of displacement of the grid, and displacing the grid according to the law of displacement at a high rate of displacement in the vicinity of the starting position and of the ending position. 2. The method according to claim 1, wherein the high rate of displacement is between about 3 times and about 10 times the value of the ratio between the area of displacement and the duration imaging. 3. The method according to claim 1, wherein the two portions of the spatial derivative of the temporal variable are linear. 4. The method according to claim 2, wherein the two portions of the spatial derivative of the temporal variable are linear. 5. The method according to claim 1, wherein the continuous curve is formed of two portions symmetrical with respect to the point whose temporal coordinate is equal to one-half of the duration of photography, each of these portions representing a profile of evolution of the variable position, a function of the square root of the variable time. 6. The method according to claim 2, wherein the continuous curve is formed of two portions symmetrical with respect to the point whose temporal coordinate is equal to one-half of the duration of photography, each of these portions representing a profile of evolution of the variable position, a function of the square root of the variable time. 7. The method according to claim 3, wherein the continuous curve is formed of two portions symmetrical with respect to the point whose temporal coordinate is equal to one-half of the duration of photography, each of these portions representing a profile of evolution of the variable position, a function of the square root of the variable time. 8. A radiographic X-ray unit for radiographic images of an object comprising: a source X-ray radiation to produce a beam of radiation; a receiver of the beam of radiation; an anti-scatter grid disposed between the source and the receiver; and means for displacing the grid in rectilinear translation in its plane, at the time the image is taken between a starting position and an ending position, according to a predetermined temporal law of displacement, the law of displacement being a continuous curve exhibiting point symmetry with respect to the point whose temporal coordinate is equal to one-half of the duration of imaging, and whose spatial derivative of the temporal variable exhibits two portions symmetrical with respect to an axis of symmetry passing through the center of the area of displacement of the grid, and the grid is displaced at a high rate of displacement in the vicinity of the starting position and of the ending position. |
abstract | An imaging optical unit for EUV projection lithography serves to image an object field into an image field. Mirrors guide imaging light from the object field to the image field. An aperture stop is tilted by at least 1° in relation to a normal plane which is perpendicular to an optical axis. The aperture stop has a circular stop contour. In mutually perpendicular planes, a deviation of a numerical aperture NAx measured in one plane from a numerical aperture NAy measured in the other plane is less than 0.003, averaged over the field points of the image field. What emerges is an imaging optical unit, in which homogenization of an image-side numerical aperture is ensured so that an unchanging high structure resolution in the image plane is made possible, independently of an orientation of a plane of incidence of the imaging light in the image field. |
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047330872 | abstract | A tilting mechanism tilts the wafer holding portions for wafers in any direction with respect to the direction of ion beams emitted from an ion source. The tilting mechanism is comprised of a linking mechanism for transmitting the rotation of the receiving plates to the wafer holding portions, tilted links for tilting the wafer holding portions, and a shaft being coupled to the tilted links and moving in the rotary disc. The wafer holding portions provide spherical portions, and the spherical portions are inserted in the receiving plates. A pin of the preceiving plates is fitted into the groove of the wafer holding portion. The distances between the ion source and the respective wafers are maintained equal. The ion beam irradiates uniformly the respective wafers and treats uniformly the respective wafers. |
abstract | A planar pattern (11), having a plurality of apertures of the same size (Wx×Wy), is determined by a two-dimensional layout determination tool (10), and a three-dimensional structure, having a depth d and an undercut amount Uc for making the phase of the transmitted light be shifted by 180 degrees with every even-numbered aperture, is determined by a three-dimensional structure determination tool (20). Simulation of transmitted light is executed for a structural body having the planar pattern (11) and the three-dimensional structure (21) by a three-dimensional simulator (30) to determine the light intensity deviation D of transmitted light for an odd-numbered aperture without a trench and an even-numbered aperture with a trench. At a two-dimensional simulator (40), simulations using a two-dimensional model prepared based on this deviation D are performed to determine a correction amount δ for making the deviation D zero and obtain a new planar pattern (12). The work load spent on designing a trench-type, Levenson-type phase shift mask can be lightened and the working time for the designing process can be shortened. |
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summary | ||
047629967 | abstract | This die-shaped coarse-approach positioning device is particularly suited for the sample holder of a scanning tunneling microscope or the like. It comprises two blocks (21, 23), one (21) stationary, the other tiltable with respect to the stationary one. The blocks (21, 23) are connected on one side by a spring-like sheet metal (20). The tiltable block (23) can be moved, and with it the sample (33), by turning a screw (28) which presses down onto an elastic pad (30). The tilting of the tiltable block (23) occurs against the effect of the sheet metal (20) until the block (23) touches down onto a wedge (39) insertable in the gap (24) between the blocks (21, 23), and consisting of an elastic material, however, having a spring constant much higher than the effective spring constant of the sheet metal (20). In the application shown, the sample can be lowered onto a tunnel tip (34) attached to a fine-approach positioning device (35). |
abstract | A control rod drive system and an inspection method of the control rod drive system capable of performing inspection safely and effectively. A control rod drive system controlling operation by supplying a hydraulic pressure to control rod drive mechanisms. A plurality of first hydraulic control units supply the hydraulic pressure to corresponding control rod drive mechanisms and a second hydraulic control unit supplies the hydraulic pressure to corresponding control rod drive mechanisms of the first hydraulic control unit as an inspection target instead of the first hydraulic control unit. |
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description | The present invention pertains to an ion implantation device; in particular, it pertains to a system which corrects the temperature of a wafer in an ion implantation device. Ion implantation is implemented in various steps in the semiconductor manufacturing process; for example, it is implemented during formation of the diffusion region of the source/drain of a MOS transistor and in the formation of a polysilicon gate electrode. For example, Japanese Kokai Patent Application No. Hei 9[1997]-213258 discloses a large current ion implantation device which can increase the beam current without degrading the device characteristics. Furthermore, Japanese Kokai Patent Application No. 2009-87603 discloses an ion implantation device which is capable of controlling the amount of ion implantation very precisely even when the divergence angle or the beam gradient of the ion beam changes. Methods for processing wafers with an ion implantation device can be broadly classified as a batch method or a single-substrate method. FIG. 1(a) shows an overview of a batch-type ion implantation device; FIG. 1(b) shows an overview of a single-substrate type ion implantation device. As shown in FIG. 1(a), a batch-type ion implantation device has a disk 1 on which are formed pedestals that respectively retain multiple wafers W which have been transported thereto. Disk 1 is rotated at a high speed of, for example, 1200 rpm and disk 1 is scanned mechanically in the vertical direction V, according to the amount of an ion beam B with which it is to be irradiated, from a more or less perpendicular or at an oblique angle with respect to disk 1, thus performing ion implantation of the wafers W. This method is primarily employed with high-current implantation devices. In particular, with a high-current process the wafers W are exposed to extremely high temperature, so that the wafers W must be cooled or the resist pattern formed on the wafers will be deformed by the heat, causing degradation of or variation in the device characteristics. Therefore, disk 1 is connected to a heat exchanger 3 via pipes 2A, 2B; water that is heated by disk 1 passes through pipe 2A and is cooled by heat exchanger 3, and cooled water is supplied to disk 1 through pipe 2B to cool wafers W. In addition, a chiller capable of temperature adjustment can be used in place of a heat exchanger. Furthermore, as shown in FIG. 1(b), a single-substrate ion implantation device has a platen 4 that supports a wafer W. With respect to wafer W retained on platen 4, ion implantation of said wafer W is performed by a scanning beam B that scans in the horizontal direction H and by mechanically controlling the scan in the vertical direction V of scanning beam B and platen 4. This method primarily is employed with medium-current implantation devices. As with the aforementioned disk, platen 4 is connected to a heat exchanger 3 via pipes 2A, 2B, and the wafer retained on platen 4 is cooled by cooling water or a gas. For both the batch-type and the single-substrate type methods shown in FIG. 1(a) and (b), the temperature (the heat exchange efficiency) of a wafer W is a critical factor in the functioning of the implantation device, but the ion implantation device is not provided with a system to continuously monitor the wafer temperature. With both the batch processing method and the single-substrate processing method the pedestal and the wafer W are tightly adhered, or the platen and the wafer W are tightly adhered, and a basic premise is that heat exchange takes place normally between them. However, cooling systems these become completely ineffective when the pedestal or platen deteriorates or when contaminants or particles adhere to the pedestal or platen, causing wafer contact defects and resulting in an insufficient heat exchange, and in some cases the occurrence of a large number of product defects may not be noticed for a long time. The present invention solves the aforementioned problem of the prior art, and its objective is to provide an ion implantation device with which the wafer temperature can be continuously monitored and corrected. The ion implantation device according to the present invention is a device that implants ions in a substrate and that has: an irradiation means that radiates ions; a retention means that retains at least one substrate; a detection means that detects, in a noncontact state, temperature information pertaining to the temperature of a substrate retained by the aforementioned retention means; a supply means that supplies a cooling medium to the aforementioned retention means to enable heat exchange for the substrate retained by the aforementioned retention means; and a control means that calculates the surface temperature of the substrate retained by the aforementioned retention means based on the temperature information detected by the aforementioned detection means and determines whether the calculated substrate surface temperature is within a permissible temperature range. Preferably, the aforementioned control means halts the radiation of ions by the aforementioned irradiation means when it is determined that the aforementioned calculated substrate surface temperature is outside of the aforementioned permissible temperature range. Preferably, the aforementioned permissible temperature range is a temperature range permitted with the ion implantation process, and the aforementioned control means records information related to the permissible temperature range in a memory. Preferably, the aforementioned control means controls the aforementioned supply means based on the aforementioned calculated substrate surface temperature. Preferably, the aforementioned control means controls at least one of: the temperature of the cooling medium or the amount of cooling medium supplied by the aforementioned supply means. Preferably, the aforementioned control means records in a memory a correlation between the temperature information for a dummy silicon substrate and the surface temperature thereof, and the aforementioned control means calculates the surface temperature of a process silicon substrate based on the temperature information for the process silicon substrate and the aforementioned relational expression. Preferably, the aforementioned detection means includes an infrared sensor and the aforementioned temperature information is a voltage generated by the aforementioned infrared sensor according to the heat radiated from the substrate. Preferably, the surface temperature of the dummy silicon substrate included in the aforementioned correlation is a value observed on a thermo label attached to the surface of a dummy silicon substrate. Preferably, the aforementioned retention means includes a rotatable disk that retains multiple substrates and is used for batch processing, and the aforementioned disk exchanges heat by means of a cooling medium supplied by the aforementioned supply means. Preferably, the aforementioned retention means includes a retention member that retains one substrate and is used for single-substrate processing, and the aforementioned retention member exchanges heat by means of a cooling medium supplied by the aforementioned supply means. The method of the present invention for correcting the temperature of a substrate in an ion implantation device includes steps wherein: a dummy silicon substrate on which is attached a thermo label for the purpose of monitoring the substrate surface temperature is retained by a retention member; the temperature information for the dummy silicon substrate is detected when ion implantation is performed on the dummy silicon substrate, and the substrate surface temperature is monitored based on the aforementioned thermo label; the relationship between the temperature information for the aforementioned dummy silicon substrate and the observed substrate surface temperature is recorded; next, the temperature information for an actual process silicon substrate is detected when ion implantation will be performed or is being performed on the process silicon substrate; the surface temperature of the process silicon substrate is calculated based on the detected temperature information and the aforementioned recorded relationship; and it is determined whether the calculated surface temperature is within a permissible temperature range. Preferably, the temperature correction method further includes a step wherein implantation of ions in the process silicon substrate is halted when the aforementioned calculated surface temperature is outside of the aforementioned permissible temperature range. Preferably, the temperature correction method further includes a step wherein an alert is provided when the aforementioned calculated surface temperature is outside of the aforementioned permissible temperature range. Preferably, the temperature correction method controls the temperature of the aforementioned retention member based on the aforementioned calculated surface temperature and corrects the surface temperature of the process silicon substrate such that the surface temperature of the process silicon substrate is within the permissible temperature range. Preferably, the temperature correction method further includes a step wherein the calculated surface temperature of the process silicon substrate is displayed on a display in real time. In the FIG. 10 represents an ion implantation device, 112 represents a disk, 114 represents a rotary shaft, 116 represents a positioning mark, 118 represents a cover, 118A represents a through-hole, 119 represents a chamber, 122 represents an infrared sensor, P represents a measurement region, V represents a vertical scan, W represents a wafer By means of the present invention an ion implantation device can be provided that monitors and corrects the temperature of the substrate in real time. Therefore, variation in the manufacture of semiconductor devices can be reduced and the yield rate can be improved. In the following an embodiment of the present invention will be explained in detail with reference to the figures. However, the shape and the scale of the components recorded in the figures have been emphasized to facilitate an understanding of the invention, and it should be noted that they do not necessarily match those of an actual product. FIG. 2 is a schematic block diagram showing the configuration of an ion implantation device according to an embodiment of the present invention. As shown in FIG. 2, an ion implantation device 10 according to the present embodiment is configured to include: an ion irradiation unit 100 that irradiates a semiconductor wafer with ions; a wafer retention unit 110 that retains a semiconductor wafer; a temperature information detection unit 120 that detects temperature information for a semiconductor wafer retained by the wafer retention unit; a cooling medium supply unit 130 that supplies to wafer retention unit 110 a cooling medium, such as cooling water or a cooling gas, that enables heat exchange for the semiconductor wafer retained by wafer retention unit 110; a display unit 140 that displays various information; an input unit 150 that receives input from an operator; a data communication unit 160 that transmits and receives data using an external electronic device, external server, etc., in a wired or wireless manner; a control unit 170 that controls each unit; and a memory 180 that stores programs for execution by control unit 170, as well as other information. As is publicly known, ion irradiation unit 100 includes an ion source that generates ions to be implanted, an acceleration unit that accelerates the ions generated by the ion source, a deflection plate that deflects the accelerated ions, etc. Wafer retention unit 110 includes a disk for batch processing on which are formed pedestals that respectively retain multiple wafers, or a platen for single-wafer use. The following explanation will discuss batch processing. As shown in FIG. 3(a), a disk 112 is circular in shape and comprises electroconductive material, and can retain multiple semiconductor wafers W arranged circumferentially. Each pedestal is provided with a mechanism for clamping the edge of a semiconductor wafer W. The rotary shaft 114 of disk 112 is rotated by a motor not shown in the figure, and disk 112 can be moved in the vertical direction V. In addition, a positioning mark 116 is formed on the surface of disk 112, and positioning mark 116 is detected by a position detection sensor not shown in the figure. Thus, control unit 170 can identify the rotational angle of the wafer and the pedestal positions. Preferably, temperature information detection unit 120 measures the temperature of the wafer W retained on disk 112 in a noncontact manner. In the present embodiment an infrared sensor (thermopile) is used to measure heat radiated from the wafer. Generally, it is difficult to measure the temperature of a silicon wafer with a radiation thermometer, the primary reason being that many of the radiation wavelengths of the temperature measurement region pass through silicon, Consequently, the wafer is in a semi-translucent state with respect to the radiation measurement device, which ends up measuring extraneous energy other than that of the object to be measured. Furthermore, factors other than transmissivity which affect the measurement of the wafer temperature include emissivity, the visual field of measurement, responsiveness, resolution, etc. If a specific factor evaluation is preformed however, since many factors are stable, it can be said to be a suitable measurement environment. With regard to the transmissivity of silicon, the temperature of the back side is stable, and if the total energy can be determined then it is possible to determine the wafer temperature. For these reasons, the present embodiment utilizes a thermopile module used with a typical radiation measurement device. As shown in FIG. 3(b), a U-shaped cover 118 is mounted such that it faces the edge portion of the rotating disk 112. Cover 118 forms a chamber 119 in which disk 112 can be arranged. A small, cylindrical through-hole 118A is formed in a portion of cover 118 and a thermopile 122 is arranged in said though-hole 118A. The environment which includes disk 112 within chamber 119 and cover 118 is preferably a vacuum. As shown in FIG. 3(a), thermopile 122 measures a measurement region P of each wafer W on the circumference, but because through-hole 118A has a small radius and the distance between thermopile 122 and the wafers W is small, the field of view of thermopile 122 is small, so that the effect due to the measurement field of view can be limited. When disk 112 rotates, thermopile 122 measures the heat energy radiated by the wafers W placed on disk 112 and produces an electromotive force corresponding to that heat energy. Thermopile 122 is arranged within cover 118 such that the surfaces of the wafers W are parallel to the surface of thermopile 122. Furthermore, with the present embodiment a thermo label is used to obtain the relationship between the thermopile's output voltage and the substrate surface temperature of a wafer W. A thermo label exhibits a change in color when it reaches a specific temperature, and the change in color that occurs when it reaches that specific temperature is irreversible. With a thermo label the temperature can be viewed or checked in step widths of from 1 to 5 degrees. The thermo label cannot be attached to the actual product (wafer W); instead, as will be explained later, it is attached to a dummy wafer and the dummy wafer is used to determine the relationship between the thermopile's output voltage and the substrate surface temperature. Cooling medium supply unit 130 supplies cooling water or cooling gas to disk 112; for example, cooling medium supply unit 130 supplies the cooling medium to disk 112 via a pipe and collects the heat-exchanged cooling medium from disk 112 via a pipe. Cooling medium supply unit 130 can include a temperature sensor that detects the temperature of the cooling medium, a supply valve for adjusting the amount of cooling medium supplied, and a heat exchanger that adjusts the temperature of the cooling medium. Cooling medium supply unit 130 can adjust the temperature of the cooling medium and the amount of cooling medium supplied in response to commands from control unit 170. Display unit 140 includes a display and, for example, displays the substrate surface temperature of a semiconductor wafer in real time and displays information such as the process conditions for execution of ion implantation, the permissible substrate temperature for execution of ion implantation, and the characteristics of the semiconductor device being manufactured by ion implantation. Input unit 150 receives various information from an operator and transmits this to control unit 170. The operator can, for example, specify the beginning and end of ion implantation, and cans search for and select process conditions stored in memory 180. Data communication unit 160 can exchange data with other semiconductor manufacturing equipment and computer devices, and can exchange data with a server which manages the manufacturing process. For example, the ion implantation device can obtain process conditions from the server, with said process conditions including information such as the type of ions to be implanted, the ion beam power, and the permissible range of substrate surface temperature for ion implantation. Control unit 170 can control ion irradiation unit 100 in response to the received process conditions. Control unit 170 includes a device such as a central processing unit, microcomputer or arithmetic processor, and controls the various units. Memory 180 stores information required for execution of ion implantation, programs executed by control unit 170, etc. This information is prepared in advance or is received from the outside via data communication unit 160. Memory 180 stores, for example, the process conditions (including ion implantation conditions and the permissible wafer temperature during ion implantation) on a device-by-device basis, including information such as the formula for calculating the substrate surface temperature based on the temperature information detected by temperature information detection unit 120. Next, the operation of the ion implantation device according to the present embodiment will be explained with reference to the flow charts in FIGS. 4A and 4B. First, a dummy wafer is placed on a pedestal of disk 112 (step S101). The number of dummy wafers used does not necessarily have to be more than one. In addition, a thermo label which changes color when a specific temperature is attained is attached to the surface of the mounted dummy wafer. When multiple dummy wafers are mounted a thermo label can be attached to each dummy wafer. Next, control unit 170 rotates disk 112 at a fixed rotational velocity and the dummy wafer is subjected to ion implantation under actual process conditions (step S102). Control unit 170 reads the process conditions from memory 180 and controls the irradiation for the purpose of ion implantation by controlling the type of ion for ion irradiation unit 100, the ion beam power, etc. Next, temperature information detection unit 120 detects the dummy wafer temperature information—that is, the voltage generated in response to the radiant heat—in two stages and transmits the information to control unit 170 (step S103). First, temperature information detection unit 120 detects the mean temperature information for all dummy wafers on disk 112 being rotated at high speed in chamber 119 while ion implantation is being performed on the dummy wafers. In other words, the temperature within the batch is detected by thermopile 122. Next, ion implantation is stopped or paused, the rotational velocity of disk 112 is reduced, and when it becomes possible to detect individual dummy wafer temperature information, the temperature information for each dummy wafer is detected. Thus the two sets of information—the mean temperature information within the batch and the individual temperature information for the dummy wafers—are respectively stored in memory 180. On the other hand, the operator observes the substrate surface temperature from the thermo label attached to the surface of the dummy wafer. This observation is performed when the disk is rotating at high speed—in other words, during detection of mean temperature information for the batch—and when the disk's rotational velocity is reduced and temperature information is detected for each dummy wafer in the batch. Sampling of the information using dummy wafers is performed under multiple process conditions, and the information for each situation, such as the ion beam power, thermopile output voltage and the substrate surface temperature, are extracted. FIG. 5 is a table showing the relationship between the beam power (W), the thermopile output voltage (V), and the thermo label's substrate surface temperature (° C.) for sampling under multiple process conditions. FIG. 6 is a graph showing the relationship between the beam power, thermopile output voltage, and substrate surface temperature from the table of FIG. 5; the left vertical axis is the thermopile output voltage (V), the right vertical axis is the thermo label substrate surface temperature, and the horizontal axis is the beam power. The line shape obtained shows how the thermopile output voltage and the substrate surface temperature increase as the beam power increases. The correlation shown in FIG. 6 is created based on the mean temperature information within the batch and the individual temperature information for the dummy wafers. The substrate surface temperature observed by means of the thermo label is input by the operator at input unit 150, and control unit 170 can determine the correlation of the line shape shown in FIG. 6 based on the sample information by means of a prepared program, storing this in memory 180 (step S105). Next, the dummy wafers are removed from the disk and multiple process wafers are mounted (step S106). Control unit 170 rotates disk 112 at a fixed rotational velocity and controls ion irradiation unit 100 to perform ion implantation for the process wafers under actual process conditions (step S107). Just as with the dummy wafers, temperature information detection unit 120 detects the temperature information for the process wafers in two stages, outputting the information to control unit 170. First, temperature information detection unit 120 detects the mean temperature information for all process wafers on disk 112 being rotated at high speed in chamber 119 while ion implantation is performed. In other words, the temperature within the batch is detected by thermopile 122. Next, ion implantation is stopped or paused, the rotational velocity of disk 112 is reduced, and when it becomes possible to detect the individual process wafer temperature information, the temperature information for each process wafer is detected. Control unit 170 calculates substrate surface temperature information for the process wafers from the 2 output voltages for the process wafers—that is, the mean thermopile output voltage and the individual thermopile output voltage—and from the correlation obtained when the dummy wafers were sampled (step S108). FIG. 7 is a graph showing the relationship between the thermopile output voltage for process wafers and the calculated substrate surface temperature. A resist pattern and the like are formed on the surface of an actual process wafer, so that emissivity differs from that of a dummy wafer; in other words, with identical process conditions the thermopile output voltage detected with a process wafer differs from the thermopile output voltage detected with a dummy wafer. Therefore, a correlation is calculated wherein the corresponding substrate surface temperature has been sampled for the detected thermopile output voltage of the process wafer. Control unit 170 determines whether the mean temperature for each process wafer—that is, the temperature within the batch—is within the permissible temperature range for the corresponding process and the semiconductor devices manufactured with said process (step S109). Ion implantation continues (step S110) when the substrate surface temperature is within the permissible range. However, if the substrate surface temperature is outside of the permissible temperature range control unit 170 stops the operation of ion irradiation unit 100, pauses ion implantation (step S111), and displays an alert on display unit 140 indicating that a wafer temperature error has occurred (step S112). This alert is either a ‘Warning’ from which ion implantation is restarted or an ‘Alarm’ which cancels ion implantation. The difference between a ‘Warning’ and an ‘Alarm’, for example, can be set with a given temperature (60° C.) serving as the boundary, with anything at or above that being an ‘Alarm’ and anything below that being a ‘Warning’. In addition, the alert can be audible. When the alert is a ‘Warning’ the operator restarts ion implantation (S114) and the process returns to step S109. On the other hand, when the alert is an ‘Alarm’, ion implantation is stopped (S116). In addition, when ion implantation continues (S110), a determination is made regarding whether the specified ion implantation has been completed (S115); if it has not been completed the process returns to step S109, and if it has been completed, ion implantation is stopped (S116). When the ion implantation process is complete (step S116) the rotation of disk 112 is slowed enough to allow individual wafers to be identified, and the surface temperature is measured separately for each wafer while disk 112 is rotating (step S117). Then a determination is made regarding whether the surface temperature for each process wafer is within the permissible temperature range (step S118). If the surface temperature is within the permissible temperature range a new process wafer is mounted on disk 112 and the process repeats from step S106. On the other hand, if the surface temperature is not within the permissible temperature range an alert indicating that a wafer temperature error has occurred is displayed on display unit 140 (step S119), the device is stopped, and monitoring is complete. Preferably, control unit 170 detects positioning mark 116 on disk 112 with a position detection sensor during the aforementioned step S118 to identify which process wafer has a temperature error. This can be identified based on the time at which positioning mark 116 is detected and the angle between positioning mark 116 and each pedestal. The wafer temperature can thus be monitored on a per-wafer basis even for a batch. Furthermore, it is preferable that display unit 140 display the substrate surface temperature on a per-wafer basis in real time, with the operator confirming the wafer temperature. On the other hand, for a batch, the mean temperature within the batch can be monitored and corrected. By means of the present embodiment, it is possible when ion implantation is performed to correct the wafer temperature to a permissible temperature, so that variation in the characteristics of elements and circuits formed on the wafer can be limited and devices can be manufactured within the design margin. In particular, with a high-current ion implantation process it is well known that the wafer temperature has a significant effect on the amplification characteristic of the transistor (the HFE parameter). This is thought to be caused by an insufficient exchange of heat, which causes the wafer temperature to rise and increases crystal defects, thus hindering diffusion. In other words, the wafer temperature during processing is a critical factor not only with respect to product defects, but also with respect to variability in product quality. FIG. 8 is a diagram showing the relationship between wafer temperature and the HFE characteristic; the vertical axis is the amplification characteristic and the horizontal axis is the wafer temperature at the time of ion implantation. In this example, when the wafer temperature exceeds 60° C. a large variation in the HFE parameter begins to occur. Conversely, if the wafer temperature is too low (for example, 45° C.) the yield rate decreases. As with the current embodiment, correcting the wafer temperature within a permissible temperature range during ion implantation makes it possible to limit variability in the amplification characteristic of the transistor. Next, a second embodiment of the present invention will be explained with reference to the flow chart in FIG. 9. In the aforementioned embodiment, ion implantation is automatically stopped when the substrate surface temperature of the process wafer is outside of a permissible temperature range; however, in the second embodiment the substrate surface temperature is controlled such that it is within a permissible temperature range. First, control unit 170 determines whether the substrate surface temperature is outside of a permissible temperature range (step S201) and controls cooling medium supply unit 130 to increase the supply of cooling medium by a fixed amount (step S202). Cooling of disk 112 thus is promoted and the process wafer heat exchange efficiency is improved (step S203). Control unit 170 monitors the substrate surface temperature of process wafers based on temperature information from temperature information detection unit 120, and determines whether said temperature is within a permissible temperature range (step S204). If it is outside of the permissible temperature range, the flow rate of the cooling medium is further increased in the aforementioned step S202, and the heat exchange efficiency can be further improved. By means of the second embodiment the substrate surface temperature of the process wafer is corrected within a permissible range, enabling ion implantation to be performed continuously. A decrease in throughput can thus be prevented. In the case of the second embodiment, it is preferable that the permissible temperature be set such that it includes more of a margin than that of the first embodiment. Next, a third embodiment of the present invention will be explained with reference to the flow chart in FIG. 10. The second embodiment illustrated an example in which the flow rate of the cooling medium was adjusted, but here the temperature of the cooling medium is controlled. Control unit 170 determines whether the substrate surface temperature is outside of the permissible temperature range (step S301) and controls cooling medium supply unit 130, lowering the temperature of the cooling medium. Preferably the temperature of the cooling medium is detected with cooling medium supply unit 130 (step S302); next, the temperature of the cooling medium is lowered by a fixed amount (step S303) by means of a heat exchanger with which cooling medium supply unit 130 is provided. Cooling of the disk is thus promoted, and the heat exchange efficiency of the process wafer is improved (step S304). Control unit 170 monitors the substrate surface temperature of process wafers based on temperature information from temperature information detection unit 120 and determines whether the substrate surface temperature is within the permissible temperature range (step S305). If it is outside of the permissible temperature range, the temperature of the cooling medium is lowered by means of the aforementioned step S303, further improving heat exchange efficiency. In addition, control can be performed by combining the third embodiment and the second embodiment. The aforementioned embodiments involved a batch process ion implantation device, but the present invention can also be applied to a single-wafer ion implantation device. For the batch type a thermopile was used as a noncontact temperature detection means, but if the platen which retains the wafer in the single-wafer type does not rotate, the temperature detection means can be mounted on the platen itself. Furthermore, in the aforementioned embodiments, the examples for the batch type illustrated control involving detection of mean temperature information for dummy wafers and process wafers within a batch, but for the single-wafer type a thermopile (sensor) can track temperature information even during ion implantation, so it is not necessary to average the temperature within chamber 119. Accordingly, for the single wafer type it is possible to monitor the substrate surface temperature for the process wafer during ion implantation and to pause or stop ion implantation, or to control the cooling temperature of the process wafer in response to the detected temperature. Furthermore, the wafer retention method for the batch type or single-wafer type can use an electrostatic chuck or another adhesion method in addition to a mechanical clamp. Furthermore, the thermopile used to measure the radiant heat of the wafer was arranged in a vacuum environment, but instead of being in a vacuum the measurement can be performed from the outside through a special glass. The aforementioned embodiments illustrated an example whereby a dummy wafer is used to sample the relationship between the output voltage of a thermopile and the substrate surface temperature but when the relationship there-between (for example, a correlation such as that shown in FIG. 6) is already known, sampling of a dummy wafer is not always required if said correlation is stored in memory 180. For both the batch type and the single-wafer type it is possible to determine the temperature during processing by creating monitoring timing. For example, the ion implantation process includes cycles of ion implantation start, ion implantation hold, ion implantation start, and ion implantation end, and it is possible to determine the wafer temperature within these cycles. In the above a preferred embodiment of the present invention was described, but the present invention is not limited to a specific embodiment. Various modifications and changes are possible without departing from the scope of the invention recorded in the claims. |
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050733052 | abstract | A method of evacuating a container to a vacuum for use in treating radioactive wastes by placing the waste into the container, evacuating, sealing off and thereafter compressing the container, the method being characterized by placing the waste into the container, forming over the waste a filter layer of particulate material fulfilling one of the following requirements: and sealing off and compressing the container.. (1) A layer having a thickness of at least 5 mm and formed of a particulate material not smaller than 40 .mu.m to less than 105 .mu.m in mean particle size; PA0 (2) A layer having a thickness of D in mm and formed of a particulate material not smaller than 105 .mu.m to not greater than 210 .mu.m in mean particle size d in .mu.m, the thickness D and the mean particle size d having the relationship represented by: EQU D.gtoreq.(20/105).times.d-15;. and thereafter aspirating a gas through the filter layer from thereabove to evacuate the container and sealing off and compressing the container. |
abstract | A coating of niobium oxide, zirconium titanate, or nickel titanate is formed on at least a part of a surface of a jet pump member constituting a jet pump serving as a reactor internal structure of a boiling water reactor. Further, a solution containing, e.g., a niobium compound is applied to at least a part of the surface of the jet pump member constituting the jet pump, and the jet pump member coated with the solution is heat-treated to form a coating of, e.g., niobium oxide. With this configuration, the jet pump member constituting the jet pump of the boiling water reactor is provided such that deposition of crud can be sufficiently suppressed on the jet pump member. |
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063317133 | description | DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, an ion implanter apparatus 1 is shown. The apparatus comprises an ion source assembly 10 in schematic form only, whose structure will be described in more detail in connection with FIGS. 2-5 below. The ion source assembly includes an ion source 20 which is fed with a supply of atoms or molecules from a gas bottle 12, for example. The ion source has an extraction assembly shown generally at 14 from which an ion beam 16 is produced. The ion beam 16 is directed through an ion mass selector 17 including a magnetic analyser 18. Ions of the chosen mass to charge ratio follow a curved path through the magnetic analyser 18 and pass through an exit slit 15 before impinging upon a target substrate 19 mounted upon a substrate holder 19a. As will be appreciated by the skilled reader, the above elements are all housed in a vacuum housing although this is not shown for clarity. Referring next to FIG. 2a, 2b, 3a and 3b, a schematic plan view of the ion source assembly 10 embodying the present invention is shown in various views. In FIGS. 2a and 3a, the ion source assembly 10 is shown in a first, closed position. FIGS. 2b and 3b show the assembly 10 in a second, open position. The ion source assembly comprises an ion source 20 which may be of any suitable type such as a Freeman or Bernas source, for example. In the example shown in the Figures, the ion source 20 has a base portion 25, and a generally elongate portion upon that base. The end of the generally elongate portion contains an arc chamber 30. As will be familiar to those skilled in the art, the arc chamber 30 has an aperture therein to allow ions generated within the ion source to exit. The ion source assembly also includes an extraction electrode 40, which is mounted immediately adjacent the face plate 35 to allow ions formed within the ion source 20 to be extracted in the form of an ion beam. In order to support the extraction electrode 40 next to the face plate 35, an extraction electrode support member 50 is employed. As seen in FIG. 2a, the extraction electrode support member 50 is U-shaped in section, with the base thereof holding the extraction electrode 40. The ion source 20 is usually at a common potential of a few kV or more. In order to accelerate positive ions away from the arc chamber 30, the extraction electrode 40 needs to be at a net negative potential relative to the potential of the ion source 20. Therefore, both the ion source 20 and the extraction electrode 40 (via the extraction electrode support member 50, which is electrically conducting) are connected to separate voltage supplies (not shown). Furthermore, the extraction electrode support ember 50 is electrically insulated from the base of the ion source 20 by a first high voltage bushing 60, formed from a suitable insulating material. The first high voltage bushing 60 acts not only to separate the extraction electrode support member 50 from the base portion 25 of the ion source 20, but also to support the extraction electrode support member 50 mechanically relative to the ion source base portion 25. The aperture of the arc chamber 30, and the extraction electrode 40, extend into an evacuatable chamber 70. This chamber 70 contains a suppressor electrode 80 at a net negative potential with respect to the extraction electrode 40. Downstream of the suppressor electrode 80 is a fourth, ground electrode 90. The suppressor and ground electrodes 80,90 together form an extraction assembly 100 (shown in perspective view in FIG. 3b). The purpose of the various electrodes in the tetrode structure does not form part of the present invention and will not be described in further detail. The chamber 70 and the ground electrode 90 are typically at a common ground potential relative to the ion source 20 and extraction electrode 40. Therefore, it is again necessary to insulate the chamber 70 from the extraction electrode support member 50, and this is accomplished with a second high voltage bushing 110. As with the first high voltage bushing 60, the second high voltage bushing 110 not only electrically insulates the extraction electrode support member 50 from the chamber 70, but also provides mechanical support for the extraction electrode support member 50. The end face of the second high voltage bushing 110, proximal to the chamber 70, has a bushing flange 120. The ion source 20, extraction electrode 40, extraction electrode support member 50, first high voltage bushing 60, second high voltage bushing 110 and bushing flange 120 together constitute an ion source sub assembly 130, as indicated in FIGS. 2a and 2b. The ion source sub assembly 130 is mounted against an end face 140 of the chamber 70 but movable relative thereto, as will be described in further detail referring in particular to FIGS. 2b and 3b. In use, the bushing flange 120 of the ion source sub assembly 130 abuts against the end face 140 of the chamber 70. The ion source assembly 10 must be evacuated in use and an O-ring seal (not shown) is therefore employed to allow the bushing flange 120 to form a vacuum-tight seal with the end face 140 of the chamber 70. A hinge 150 is attached between an outside wall of the chamber 70, and the bushing flange 120. Previously, in order to access the inside of the chamber 70, or the extraction electrode 40, the ion source 20 first had to be lifted away from the assembly 10 by detaching it from the first high voltage bushing 60 and the extraction electrode support member 50. Even then, to access the inside of the chamber 70, the extraction electrode support member 50 also had to be removed. Using the hinge 150, the ion source sub assembly 130 can be rotated away from the chamber 70 by pivoting about the hinge 150. This is shown in FIGS. 2b and 3b. Not only does the hinge 150 allow ready access to the inside of the chamber 70, but it also supports the weight of the ion source sub assembly 1when in the second position shown in FIGS. 2b and 3b, that is, when the bushing flange 120 does not abut the end face 140 of the chamber 70. The inner walls of the chamber 70 may be lined with a liner 160 which is preferably formed from aluminum sheet. Aluminum is relatively cheap and a liner formed from it may therefore be disposable. Moreover, aluminum is inert to the process. The use of a liner is advantageous because, over time, the walls of the chamber accrue a layer of material formed from the ion beam. As the layer builds up, it deleteriously affects the vacuum pumping rate and introduces the risk of species cross contamination in the wafer to be implanted. By lining the walls of the chamber and then removing the liner and disposing of it on a regular basis, the problems associated with material build up on the chamber wall are alleviated. It will be appreciated that the hinge described herein provides the ready access to the chamber 70 desirable to allow a liner to be used beneficially. FIG. 4 shows a close up perspective view of the hinge 150 of FIGS. 2a, 2b, 3a and 3b, in situ. FIG. 5 shows an exploded view of the hinge 150. As may be seen, the hinge comprises a first hinge part 200 attached in use to the wall of the chamber 70 via screws or bolts (not shown) extending through apertures 205 in the first hinge part. A second hinge part 210 is in use attached in a similar manner to the bushing flange 120. The first and second hinge parts 200, 210 are linked to each other via two hinge linking members 220,230. Each of the two hinge parts 200,210 and hinge linking members 220,230 which together constitute the hinge 150 are connected together via pins or dowels 240 which slide into cooperating needle bearings 260 inserted into holes 265 formed axially in the upper and lower portions of the second hinge part 210 and in the upper and lower portions of both of the hinge linking members 220,230. To assemble the hinge 150, needle bearings are first inserted into the axial holes 265 in the second hinge part 210 and in the hinge linking members 220,230. Next, a plurality of thrust bearing assemblies 250, each comprising a needle thrust race and a thrust washer, are aligned with thrust bearing apertures 275 in the lower portions of each of the first hinge part 200 and the first and second hinge linking members 220,230. The second hinge linking member 230 is aligned with the first hinge part 200 and dowels 240 are inserted through the first hinge part 200 to pivotally connect that first hinge part 200 to the second hinge linking member 230. Next, the first hinge linking member 220 is pivotally attached to the second hinge linking member 230 by inserting dowels 240 through the second hinge linking member 230 into the first hinge linking member 220. Finally, the second hinge part 210 is pivotally connected to the first hinge linking member 220 again by insertion of dowels through the first hinge linking member 220 into the second hinge part 210. It will be appreciated that the ion source sub assembly 130 has significant mass, and the first and second hinge parts 200,210 as well as the first and second hinge linking members 220,230 are therefore preferably formed of a relatively high tensile material such as aluminum. The hinge 150 is also relatively elongate to provide additional strength. Whilst the invention has been described in connection with a fixed axis hinge to connect the source sub assembly 130 with the chamber 70, it will be understood that a number of variations are possible. For example, rather than pivoting about a fixed axis, the hinge may instead move through an arc as the source sub assembly 130 moves relative to the chamber 70. In that case, the sub assembly will move axially away from the chamber as well as rotating relative to it. In other words, upon opening the source sub assembly, all of the bushing flange 120 moves away from the chamber, not just those parts away from the hinge itself. This arrangement may be advantageous if the ion source 20 and extraction electrode 40 extend a long way into the chamber; with a fixed point pivot, the ends of these elements might `catch` upon the wall of the chamber when the source sub assembly 130 is pivoted about the hinge. Likewise, rather than a single hinge, two or more sliders could be employed to connect the source sub assembly to the chamber. This would allow linear sliding of the former relative to the latter. Whilst this arrangement is not preferred as access to the inside of the chamber is more difficult, it does at least allow support for the bulky source sub assembly during disassembly of the ion source assembly. Finally, it is to be appreciated that the hinge need not constrain the source sub assembly to move relative to the chamber in a horizontal plane. However, given the weight of the source sub assembly, if the hinge is to allow movement in a vertical plane as well, then a mechanical energy storage device such as a spring or gas strut is desirably mounted between the source sub assembly and chamber. Thus, the decrease in potential energy when the source sub assembly moves downwards relative to the chamber can be stored in the spring or gas strut. Then, when the source sub assembly is to be moved back upwards against gravity, the energy stored in the spring or gas strut can be utilised to assist the person moving it. Whilst a tetrode structure ion source assembly has been described in the above embodiment, it will of course be understood that the traditional triode structure is also more easily dismantled for cleaning and servicing when a hinge is employed. |
claims | 1. Apparatus for particle contamination control on an EUV reflective surface, comprising: one or more electron sources adapted to shower particles produced by an EUV source in the area around and on the reflective surface with electrons sufficient to impart a negative charge to particles therein; and one or more electrostatic elements adjacent the reflective surface having a positive charge sufficient to attract the particles off of and from around the reflective surface. 2. The apparatus of claim 1 , wherein the reflective surface is adapted to be electrically biased sufficient to repel the particles. claim 1 3. The apparatus of claim 1 , wherein the one or more electron sources are positioned above the reflective surface. claim 1 4. The apparatus of claim 1 , wherein the one or more electron sources are positioned to the side of the reflective surface. claim 1 5. The apparatus of claim 1 , wherein the one or more electrostatic elements are positioned above the reflective surface. claim 1 6. The apparatus of claim 1 , wherein the one or more electrostatic elements are positioned to the side of the reflective surface. claim 1 7. The apparatus of claim 1 , wherein the reflective surface is adapted to be electrically biased sufficient to repel the particles, the reflective surface comprising an EUV mirror having a quartz substrate upon which are alternating layers of silicon and molybdenum. claim 1 8. The apparatus of claim 1 , the reflective surface comprising an EUV reflective mask having a quartz substrate upon which are multilayers of silicon and molybdenum, and an absorber pattern upon the multilayers. claim 1 9. The apparatus of claim 1 , the reflective surface comprising an EUV mirror having a quartz substrate upon which are alternating layers of silicon and molybdenum. claim 1 10. The apparatus of claim 1 , wherein the reflective surface is adapted to be electrically biased sufficient to repel the particles, the reflective surface comprising an EUV reflective mask having a quartz substrate upon which are multilayers of silicon and molybdenum, and an absorber pattern upon the multilayers. claim 1 11. An EUV lithography system with contamination control apparatus for one or more EUV reflective surfaces, comprising: an EUV lithography system having an EUV source that produces particles; and a contamination control apparatus adjacent at least one of the EUV reflective surfaces, the contamination control apparatus comprising: one or more electron sources adapted to shower the particles in the area around and on the reflective surface with electrons sufficient to impart a negative charge to particles therein; and one or more electrostatic elements adjacent the reflective surface having a positive charge sufficient to attract the particles off of and from around the reflective surface. 12. The apparatus of claim 11 , wherein the at least one of the reflective surfaces is adapted to be electrically biased sufficient to repel the particles. claim 11 13. The apparatus of claim 11 , wherein the at least one of the reflective surfaces comprise an EUV mirror having a quartz substrate upon which are alternating layers of silicon and molybdenum. claim 11 14. The apparatus of claim 11 , wherein the at least one of the reflective surfaces comprises an EUV reflective mask having a quartz substrate upon which are multilayers of silicon and molybdenum, and an absorber pattern upon the multilayers. claim 11 15. A method for controlling contamination on reflective surfaces, comprising: using a charging source to electrically charge particles produced by an EUV source on and in the area around a reflective surface of an EUV apparatus; and attracting the charged particles to electrostatic elements adjacent the reflective surface having an opposite charge sufficient to attract the particles off of and from the area around the reflective surface. 16. The method of claim 15 , wherein providing an electric charge to particles on and in the area around a reflective surface comprises: claim 15 showering the reflective surface and surrounding area with electrons from an electron source negatively charging the particles. 17. The method of claim 16 , wherein attracting the negatively charged particles to electrostatic elements comprises: claim 16 electrically biasing electrostatic elements with a positive charge to attract the negatively charged particles. 18. The method of claim 16 , wherein showering the reflective surface and surrounding area with electrons from an electron source comprises showering the reflective surface and surrounding area with electrons from an electron source prior to attracting the charged particles to electrostatic elements. claim 16 19. The method of claim 16 , wherein attracting the charged particles to electrostatic elements comprises attracting the charged particles to electrostatic elements subsequent to showering the reflective surface and surrounding area with electrons from an electron source. claim 16 20. The method of claim 15 , wherein attracting the particles to electrostatic elements further comprises electrically biasing the reflective surface with the same electrical charge as the charged particles to repel the charged particles away from the reflective surface. claim 15 |
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abstract | System and Method is described that controls the release of contaminated water by rapidly freezing the ground water, including salt water, which permeates the area underneath the a contamination source, so that the resulting ice lens mitigates the extent to which radioactive water is released into the environment. An aperture in the containment area allows the dispersal and dilution of the contaminates by allowing in ground water from outside, and/or removing water from the containment area. The variable aperture may be a physical valve or preferably an opening in the ice shield which size may be controlled by freezing or thawing portions of the ice shield. |
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042971675 | abstract | Nuclear reactor installation having a concrete cell disposed beneath the earth of a hill for enclosing activity-carrying components includes at least one additional concrete cell disposed in the earth separated from the first-mentioned concrete cell, the additional concrete cell having at most one-fortieth the volume of the first-mentioned concrete cell and being at least predominantly of shell-like construction, and including equipment of use for the nuclear reactor installation received in the additional concrete cell. |
053533224 | claims | 1. A lens system for a projection lithography camera having a source of x-ray radiation, a wafer, and a mask to be imaged on the wafer, the lens system having a convex mirror M.sub.2 arranged optically between concave mirrors M.sub.1 and M.sub.3 to form an imaging system, and the lens system comprising: a. the mask is reflective and is arranged on the same side of the mirrors as the wafer; b. a chief ray angle of the radiation at the mask is up to 10.degree.; and c. a chief ray of the radiation incident on the mask is inclined away from the optical axis of the lens system in a direction from the source toward the mask. a. one of the concave mirrors M.sub.3 is arranged optically between the mask and the convex mirror M.sub.2, and the other concave mirror M.sub.1 is arranged optically between the convex mirror M.sub.2 and the wafer; b. the mask is reflective and is arranged on the same side of the convex mirror as the wafer; c. a chief ray of the radiation incident on the mask is inclined away from the optical axis of the lens system in a direction from the source toward the mask; d. a chief ray angle of the radiation at the mask is up to 10.degree.; and e. chief rays of the radiation diverge in a direction from the source toward the mask. 2. The lens system of claim 1 wherein chief rays of the radiation incident on opposite edges of the mask diverge in a direction from the source toward the mask. 3. The lens system of claim 1 including a real aperture stop arranged downstream of the M.sub.2 mirror in a region where chief rays of the radiation cross the optical axis of the lens system. 4. The lens system of claim 1 wherein components of the lens system are arranged so that the wafer is movable relative to an image field of the lens system so that a succession of mask images can be formed in juxtaposed registry with each other on the wafer. 5. The lens system of claim 1 wherein the chief ray angle of the radiation at the mask is from 3.degree.-10.degree.. 6. A lens system for an x-ray projection lithography camera having a source of x-ray radiation directed from a mask through an imaging system to a wafer so that the mask is imaged on the wafer, the imaging system being formed of a pair of concave mirrors optically on opposite sides of a convex mirror, the lens system comprising: 7. The lens system of claim 6 wherein a numerical aperture of the lens system is less than 0.15 and a real aperture stop is arranged in a region downstream of the convex mirror where chief rays of the radiation cross the optical axis of the lens system. 8. The lens system of claim 6 wherein an image field at the wafer does not include the optical axis and allows juxtaposition of a succession of images of the mask on the wafer. 9. The lens system of claim 6 including a real aperture stop arranged downstream of the M.sub.2 mirror in a region where chief rays of the radiation cross the optical axis of the lens system. 10. The lens system of claim 6 wherein the chief ray angle of the radiation at the mask is from 3.degree.-10.degree.. |
description | This application is a National Phase filing under 35 U.S.C. §371 of PCT/FR2006/001430 filed Jun. 22, 2006, which claims priority to Patent Application No. 0506512, filed in France on Jun. 27, 2005. The entire contents of each of the above-applications are incorporated herein by reference. The invention relates in general to heat exchangers, in particular for a high temperature or a very high temperature nuclear reactor (HTR or VHTR). More precisely, the invention relates to a heat exchanger assembly for exchanging heat between a first fluid and a second fluid, the assembly comprising: an outer enclosure presenting a central axis and provided with at least one inlet and outlet for the first fluid and with at least one inlet and outlet for the second fluid; a central manifold extending along the central axis and communicating with one of the inlet and the outlet for the first fluid; an annular manifold disposed around the central manifold and communicating with the other one of the inlet and the outlet for the first fluid; a plurality of heat exchangers distributed around the central axis and radially interposed between the central manifold and the annular manifold; a plurality of axial inlet manifolds communicating with the inlet for the second fluid, and a plurality of axial outlet manifolds communicating with the outlet for the second fluid, the axial inlet and outlet manifolds being circumferentially interposed between the heat exchangers; and each heat exchanger comprises a plurality of channels for flow of the first fluid between the central and annular manifolds, and a plurality of channels for flow of the second fluid from at least one inlet manifold towards at least one outlet manifold. Assemblies of this type are known from patent document JP-2004/144422 which describes a heat exchanger assembly provided with a respective secondary fluid inlet for each axial inlet manifold. In such an assembly, each inlet is generally connected to the corresponding axial inlet manifold by a welded pipe. In operation, the connection between the pipe and the manifold is subjected to high levels of thermomechanical stress. It therefore presents a risk of premature rupture. In this context, the invention seeks to propose a heat exchanger assembly in which the risk of such rupture is greatly reduced, both in normal operation, and in an accidental situation. To this end, the invention provides an assembly of the above-specified type, characterized in that it comprises an inlet chamber provided at a first axial end of the heat exchangers and putting the inlet(s) for the second fluid into communication with at least a plurality of the axial inlet manifolds. The assembly may also present one or more of the following characteristics considered individually or in any technically feasible combination: the inlet chamber is annular in shape and surrounds the central manifold; it includes an outlet chamber provided at a second axial end of the heat exchangers opposite from the first axial end and putting the outlet(s) for the second fluid into communication with at least a plurality of axial outlet manifolds; it includes an inspection channel extending the central manifold axially from the second end, and isolated therefrom by a removable hatch, the outlet chamber being annular in shape and surrounding the inspection channel; at least the heat exchangers, the inlet and outlet chambers, and the axial inlet and outlet manifolds are united in a mechanical subassembly that can be extracted as a single piece from the enclosure; the enclosure has a vertical central axis, the enclosure comprising a vessel within which the subassembly is disposed and presenting towards the top an opening for extracting said subassembly, and a removable closure head for closing the opening of the vessel in leaktight manner; the vessel comprises a cylindrical shell coaxial with the central axis and having the inlet and outlet for the second fluid formed therein, the inlet and outlet chambers being connected in leaktight manner to the inlet and outlet for the second fluid by removable sleeves that can be retracted into the chambers; the sleeves are suitable for being dismounted from inside the chambers; the enclosure has a plurality of inlets for the second fluid and a plurality of outlets for the second fluid, these inlets and outlets being brought together in a single circumferential half of the shell; the subassembly comprises a cylindrical outer envelope coaxial about the central axis, defining the outlet chamber and the annular manifold radially outwards; the assembly includes bottom inlet and outlet manifolds that are coaxial and in communication respectively with the inlet and outlet for the first fluid, and that are disposed beneath the subassembly, the bottom of the subassembly being defined by a frustoconical envelope converging from the cylindrical envelope, said frustoconical envelope surrounding the central manifold and co-operating therewith to define the annular manifold, the bottom manifolds being terminated upwards by flanges suitable for receiving the bottom free ends of the central manifold and of the frustoconical envelope in leaktight manner merely by mutual engagement; the central manifold presents an inspection hole that is closed by a removable hatch and that communicates with the inlet chamber, and the inspection channel presents an opening communicating with the outlet chamber; the enclosure presents a bottom end wall, and the assembly includes a circulation member fastened to the bottom end wall and suitable for sucking in the first fluid coming from the annular channel or from the central channel and of delivering it to the outlet for the first fluid; the axial inlet and outlet manifolds, the central manifold, and the annular manifold, all have through sections that are sufficient to enable an operator to act directly on the heat exchangers; the inlet and outlet for the first fluid are coaxial; the heat exchangers are disposed regularly spaced apart in a circle around the central axis, each axial manifold being defined both inwards and outwards by respective inner and outer circumferential sheets welded to the two heat exchangers between which said manifolds extend; the annular manifold is defined inwardly by the heat exchangers and by the outer sheets; the central manifold is defined by the heat exchangers and by the inner sheets; each heat exchanger comprises a plurality of heat exchange modules that are stacked axially; the modules present, perpendicularly to the central axis, a section that is rectangular, and present corners that are machined over the full axial height of the heat exchanger, the heat exchanger further including forged and/or machined metal bars disposed in the machined corners and onto which the modules are welded; and each bar presents a flange projecting circumferentially relative to the modules and towards the neighboring axial manifold, having the inner or outer sheet defining said axial manifold welded thereto. In a second aspect, the invention provides the use of an assembly presenting the above-described characteristics: with a first fluid mainly comprising helium and a second fluid mainly comprising helium and/or nitrogen; with a first fluid mainly comprising helium and a second fluid mainly comprising water, the second fluid being vaporized in the heat exchanger assembly; with first and second fluids mainly comprising water, the second fluid being vaporized in the heat exchanger assembly; and with one of the first and second fluids coming from a nuclear reactor. The assembly 1 shown in FIGS. 1 and 2 is for use in a high temperature or very high temperature nuclear reactor (HTR/VHTR) for exchanging heat between a first fluid and a second fluid. The first fluid is the primary fluid of the nuclear reactor, and it flows therethrough in a closed loop. It passes through the core of the nuclear reactor (not shown), then through the assembly 1, and finally returns to the inlet of the core. The primary fluid becomes heated in the reactor core, leaving it for example at a temperature of about 850° C. Inside the assembly 1, it yields a fraction of its heat to the secondary fluid, and it leaves the assembly 1 at a temperature of about 450° C., for example. The primary fluid is typically substantially pure gaseous helium. The second fluid is the secondary fluid of the nuclear reactor and it flows therethrough in a closed loop. It passes through the assembly 1 and then passes through a gas turbine driving an electricity generator and returns to the inlet of the assembly 1. The secondary fluid enters into the assembly 1 at a temperature of about 405° C., for example, and it leaves it at a temperature of about 805° C., for example. The secondary fluid is a gas comprising mainly helium and nitrogen. The assembly 1 comprises: an outer enclosure 2 presenting a central axis 1 that is substantially vertical, provided with an inlet 4 and an outlet 6 for primary fluid, and four inlets 8 and four outlets 10 for the secondary fluid; eight heat exchangers 12 disposed inside the enclosure 2, within which heat is exchanged between the primary and secondary fluids; primary fluid flow manifolds 14 and 16 inside the enclosure 2; secondary fluid flow manifolds 18 and 20 inside the enclosure 2; an inlet chamber 22 distributing the secondary fluid amongst the manifolds 18, and an outlet chamber 24 collecting the secondary fluid at the outlets from the manifolds 20; bottom internal equipments 26 channeling the primary fluid between firstly the manifolds 14 and 16 and secondly the primary fluid inlet and outlet 4 and 6; and a primary fluid circulator 28 secured to the enclosure 2. The enclosure 2 comprises a vessel 30 within which the heat exchangers 12 and the manifolds 14, 16, 18, and 20 are disposed, the vessel presenting towards the top an opening 32 and a removable closure head 34 for closing the opening 32 of the vessel 30 in leaktight manner. The vessel 30 comprises a cylindrical top shell 36, coaxial with the axis X, a cylindrical bottom shell 38 coaxial with the axis X that is disposed beneath the top shell 36 and that is of slightly smaller diameter than the shell 36, a frustoconical shell 40 interposed between the shells 36 and 38, and a rounded bottom 42 closing the bottom of the shell 38. The top free edge of the shell 36 surrounds the opening 32 and forms a flange 44. The closure head 34 is upwardly domed, and presents a free edge forming a flange 46 complementary to the flange 44 of the vessel 30. In a plane containing the axis X, the closure head 34 presents a top wall of section that constitutes substantially a portion of an ellipse. As can be seen in FIG. 11, the closure head 34 can be secured rigidly on the vessel 30 with the help of eighty tierods 50 engaged in holes 52 formed in the flange 46 and screwed into tapped orifices 54 formed in the flange 44. The flange 46 carries a highly leaktight metal gasket 55, e.g. of the type sold under the trade name “Helicoflex”, providing sealing between the closure head 34 and the vessel 30 when they are fastened together. The secondary fluid inlets 8 are provided in the bottom of the shell 36 on a common circumference thereof. All four of them are disposed on one-half of the shell 36, as shown in FIG. 4. These inlets are circular, and they present axes disposed at 42° from one another. The secondary fluid outlets 10 are formed in the top of the top shell 36, and they lie on a common circumference of said shell (FIG. 3). They are situated in the same half of the shell 36 as are the inlets 8. Like the inlets, these outlets 10 are circular and their axes are spaced apart at 42°. The bottom shell 38 has a single tapping point through which the primary fluid inlet 4 and outlet 6 are provided. The inlet 4 and the outlet 6 are coaxial, as shown in FIG. 2, with the outlet 6 surrounding the inlet 4. The rounded bottom 42 bulges downwards, and presents a round central opening centered on the axis X and in which the circulation 28 is secured. As can be seen in FIG. 5, the eight heat exchangers 12 are disposed in a circle around the axis X, and they are regularly distributed thereabout. The heat exchangers 12 are heat exchangers of the plate type. Each heat exchanger 12 comprises a vertical stack of eight mutually-identical modules 56. As shown in FIG. 7, each module 56 is in the form of a rectangular parallelepiped. Each module 56 comprises both an outer envelope 58 having inlet and outlet slots 60 and 62 for the primary fluid and inlet and outlet slots 64 and 66 for the secondary fluid machined therein, and also a plurality of plates 67 disposed inside the envelope 58 in an axial stack. The slots 60 and 62 are disposed in two opposite faces of the envelope 58, facing respectively towards the inside and the outside of the assembly 1. The slots 64 and 66 are formed in two substantially radial and opposite faces of the envelope 58 (FIGS. 6A to 6C). The stacked plates 67 define between them a plurality of primary fluid flow channels extending radially from the slot 60 to the slot 62. The plates 67 also define between one another a plurality of secondary fluid flow channels extending substantially circumferentially from the slot 64 to the slot 66. It should be observed that the slot 64 is offset radially outwards from the slot 66, such that the secondary fluid follows a Z-shaped path through the module 56, as shown in FIG. 6B. The primary and secondary fluid flow channels are superposed in alternation within the module 56, so as to improve the efficiency of heat exchange between the fluids. The radial flow channels for the primary fluid do not open out along the two radial faces of the module 56, such that the secondary fluid cannot penetrate into said channels via the slots 64 and 66. Similarly, the substantially circumferential flow channels for the secondary fluid do not open out along the inside and outside faces of the module 56, such that the primary fluid cannot penetrate into these channels through the slots 60 and 62. As shown in FIG. 7, the rectangular modules 56 present machined corners along the full axial height of the heat exchanger 12. The heat exchanger 12 also has forged and machined metal bars 68 disposed in the machined corners of the modules 56. These bars 68 extend over the full axial height of the heat exchanger 12. The modules 56 are welded to one another via their respective envelopes 58, and they are also welded to the metal bars 68. Each bar 68 has both a main portion 70 of rectangular section perpendicularly to the axis X that is placed in a machined portion of a module 56, and a flange 72 projecting circumferentially relative to the module 56. The main portion 70 is welded to the corresponding module 56 along two axial weld lines 74 and 76, visible in FIGS. 7, 8A, and 8B. The line 74 extends along radial faces of the modules 56, and the line 76 extends along inside faces or along outside faces of the modules 56, as appropriate. It should be observed that the empty axial channels 78 are machined in the modules 56 and in the bars 68 behind the weld lines 74 and 76, and along the entire length thereof. The presence of these empty channels 78 enables the quality of the welds 74 and 76 to be verified by ultrasound. It should be observed that the flanges 72 are connected to the radial faces of the modules 56 with a predetermined radius of curvature R that is determined in such a manner as to reduce stresses in the bars 68. The modules 56 are also welded to one another along weld lines 79. These weld lines 79 follow the edges defining the inner and outer radial faces of the modules 56 at the tops and bottoms thereof. The assembly 1 has four axial inlet manifolds 18 communicating with the secondary fluid inlet 8 via the inlet chamber 22, and four axial outlet channels 20 communicating with the secondary fluid outlet 10 via the outlet chamber 24. The manifolds 18 and 20 are circumferentially interposed between the heat exchangers 12, as shown in FIG. 5. The axial inlet and outlet manifolds 18 and 20 are distributed in alternation around the central axis X, such that on going around the central axis X there are to be found in succession: a heat exchanger 12; an axial inlet manifold 18; a heat exchanger 12; an axial outlet manifold 20; a heat exchanger 12; an axial inlet manifold 18; etc. . . . Each axial manifold 18 and 20 presents a section perpendicular to the axis X that is in the form of a sector of a ring, being defined towards the inside and towards the outside by respective circumferential sheets 80 and 82, and towards its sides by the radial faces of the heat exchangers 12 between which said manifold extends. The inner and outer sheets 80 and 82 of a given axial manifold 18 or 20 are welded edge to edge on the flanges 72 of the bars 68 of the two heat exchangers 12 adjacent to the manifold. The shapes of the flanges 72 are determined so that these flanges lie in continuity with the inner or outer sheets 80 or 82 (FIGS. 8A and 8B). The modules 56 are oriented in such a manner that the inlet window 64 opens out into an axial inlet channel 18, and the outlet window 66 opens out into an axial outlet channel 20. The assembly 1 also includes a central manifold 14 extending along the axis X and communicating with the primary fluid inlet 4, and an annular channel 16 communicating with the primary fluid outlet 6. The central manifold 14 extends radially inside the heat exchangers 12 and is defined by the bottom faces of the modules 56 and by the inner sheets 80. It presents a section perpendicular to the axis X that is substantially circular. The windows 60 open out into the central manifold 14. The annular manifold 16 extends around the heat exchangers 12, radially outside them. It is defined inwardly by the outer sheets 82 and the outer faces of the modules 56. The windows 62 open out into the annular manifold 16. The inlet and outlet chambers 22 and 24 for the secondary fluid are disposed respectively under the heat exchangers 12 and over the heat exchangers 12 (FIGS. 1 and 2). The central manifold 14 extends axially downwards in the form of an intermediate cylindrical segment 84 disposed under the heat exchangers 12. Similarly, the annular manifold 16 extends axially downwards in the form of an intermediate annular segment 86 surrounding the intermediate cylindrical segment 84. The inlet chamber 22 is annular in shape and is situated axially level with the secondary fluid inlet 8. It surrounds the intermediate cylindrical segment 84 and extends radially inside the intermediate annular segment 86. The inlet chamber 22 is defined radially outwards by a cylindrical wall 85. Furthermore, the assembly 1 includes an inspection channel 88 extending the manifold 14 axially upwards beyond the heat exchangers 12. This inspection channel 88 is isolated from the central manifold 14 by a removable hatch 90. It is also closed upwards by another removable inspection hatch 92. The outlet chamber 24 is also annular in shape and it surrounds the inspection channel 88. The axial inlet channels 18 are downwardly open and communicate with the inlet chamber 22. They are upwardly closed and isolated from the outlet chamber 24. Conversely, the axial outlet channels 20 are downwardly closed and isolated from the inlet chamber 22 and they are upwardly open and communicate with the outlet chamber 24. The annular manifold 16 is upwardly closed and does not communicate with the outlet chamber 24. According to another important aspect of the invention, the heat exchangers 12, the inlet and outlet chambers 22 and 24, and the manifolds 14, 16, 18, and 20 are united in a mechanical subassembly 94 that can be extracted as a single piece from the enclosure 2. This subassembly is shown in FIG. 9. The subassembly 94 is generally cylindrical in shape about the axis X. The subassembly 94 is defined upwards by a plane circular plate 96, radially outwards by a cylindrical envelope 98, and downwards by a frustoconical envelope 100 extending the cylindrical envelope 98 downwards and converging therefrom. The top plate 96 defines the top of the outlet chamber 24 (FIGS. 1 and 2). The inspection channel 88 is extended upwards and projects above the plate 96 forming a mushroom-shaped part 102 for griping the subassembly 94. The hatch 92 is situated level with the top plate 96. The subassembly 94 also comprises an engagement ring 104 surrounding the top plate 96 (FIG. 9) and projecting radially outwards relative to the envelope 98. On its underside, this ring 96 forms a bearing surface 106. On a radially inner side, the flange 94 has a complementary bearing surface 108 against which the bearing surface 106 rests when the subassembly 94 is placed inside the vessel 30. The subassembly 94 also has four stiffeners 108 extending radially from the mushroom-shaped part 102 towards the ring 104. The outer envelope 98 defines radially outwards the outlet chamber 24 and the annular manifold 16, and in particular the intermediate segment 86 of said manifold. It is pierced by four circular holes 110 in an upper portion and by four circular holes 112 in a lower portion, disposed respectively in register with the secondary fluid outlet 10 and the secondary fluid inlet 8 when the subassembly 94 is placed in the enclosure 2. The subassembly 94 also has an annular horizontal floor 114 (FIGS. 1 and 2) defining the bottom of the inlet chamber 22 and extending between the respective segments 84 and 86 respectively of the central and annular manifolds 14 and 16. Furthermore, the central manifold 14 extends under the segment 84 in the form of a bottom cylindrical segment 116 of axis X and terminates downwards by a free edge 118 (FIG. 2). The frustoconical envelope 100 surrounds the bottom segment 116 and is downwardly terminated by a cylindrical rim 120 of axis X. The annular segment 86 of the annular manifold 16 opens out downwards between the bottom segment 116 and the frustoconical envelope 100. It can be seen in FIG. 1 that the subassembly 94 includes a stiffener shell 122 that is disposed around the bottom segment 116 and that is perforated to allow the primary fluid to flow therethrough. This bottom shell 122 is welded at the top to the floor 114 and at the bottom to the frustoconical shell 100. Radial stiffeners 124 are welded simultaneously to the floor 114, to the frustoconical shell 100, and to the bottom shell 122, and they increase the stiffness of the subassembly 94 in its bottom portion. An outer cylindrical shell 126 (FIG. 12) is welded under the frustoconical envelope 100. It extends close to the frustoconical shell 40 of the vessel 30. This outer shell is reinforced by six radial stiffeners 128 welded both to the frustoconical envelope 100 and to the outer shell 126. Between them, these stiffeners 128 carry three keys 130, shown in FIG. 12, co-operating with axial grooves 132 formed in the shell 40 of the vessel 30. The keys 130 and the grooves 132 are disposed at 120° from one another about the axis X and enable the subassembly 94 to be indexed in rotation about the axis X. The outlet chamber 24 is connected in leaktight manner to the secondary fluid outlet 10 via outer and inner sleeves 140 and 142, that can be seen in FIG. 1A. The outer sleeve 140 is screwed onto an annular part 144 welded in the outlet 10. It is tubular in shape and extends from the outlet 10 towards the inside, so as to be engaged in the hole 110 of the outer envelope 98. The fastener screws 146 are accessible from inside the outlet chamber 24. The hole 110 is surrounded by an edge 148 projecting towards the inside of the outlet chamber 24 from the envelope 98. The inner sleeve 142 is tubular in shape and is interposed between the outer sleeve 140 and the projecting edge 148. It is fastened by screws 150 to the free end of the projecting edge 148. Highly leaktight metal gaskets of known type, as sold under the trade name “Helicoflex”, are interposed firstly between the outer sleeve 140 and the ring-shaped part 144, and secondly between the inner sleeve 142 and the projecting edge 148. Furthermore, a tubular bellows 154 interconnects the sleeves 140 and 142 in leaktight manner. The sleeves 140 and 142 are free to slide relative to each other in a radial direction relative to the axis X, with sealing being maintained by the bellows 144. Blocks of lagging 156 isolate the bellows 154 and the screws 146 from the secondary fluid flowing from the outlet chamber 24 towards the outlet 10. The inlet chamber 22 is connected in leaktight manner to the inlets 8 by outer and inner sleeves 158 and 160 similar to the outer and inner sleeves 140 and 142 described above (FIG. 10B). Nevertheless, it should be observed that in this example the projecting edge 148 extends from the outer envelope 98 beyond the cylindrical wall 85 to the inside of the inlet chamber 22. The cylindrical wall 85 is welded to the projecting edge 148. The projecting edge 148 thus serves to provide a leaktight passage from the inlet chamber 22 through the annular intermediate segment 86 of the manifold 16, to the outer envelope 98. Furthermore, it should be observed that the outer and inner sleeves 158 and 160 and the bellows 154 are not lagged, given the moderate temperature of the secondary gas at its inlet to the assembly 1. The inspection channel 88 has a large opening (163) that gives access to the systems for disconnecting the outlet chamber 24. The intermediate segment 84 of the manifold 14 has an inspection hole 164 communicating with the inlet chamber 22 (FIG. 2). This inspection hole 164 is closed in leaktight manner by a removable hatch. An inspection hole (not shown) provided with a removable hatch gives access to the annular channel 16 from one of the axial outlet channels 20. The bottom inner equipments 26 comprise bottom inlet and outlet manifolds 170 and 172 coaxial about the axis X and communicating respectively with the primary fluid inlet 4 and outlet 6 (FIG. 2). The bottom outlet manifold 172 surrounds the bottom inlet manifold 170. The bottom inlet manifold 172 is connected to the inlet 4 by radial pipework 174 passing through the bottom outlet manifold 172. The manifold 172 is welded in leaktight manner around the pipework 174. The bottom inlet and outlet manifolds 170 and 172 are both terminated upwards by flanges 176 suitable for receiving in leaktight manner the free edge 118 of the central manifold 14 and the edge 120 of the frustoconical envelope 100 merely by mutual engagement. Towards the inside, the flanges 176 present frustoconical bearing surfaces that serve to guide the free edge 118 and the rim 120. Furthermore, the edge and the rim carry outer metal gaskets providing leaktight contact with the inside faces of the flanges 176. The bottom outlet manifold 172 is closed downwards by a bottom wall 178 extending perpendicularly to the axis X. The bottom inlet manifold 170 comprises a cylindrical shell 180 about the axis X and extending as far as the bottom wall 178, and its own bottom wall 182 perpendicular to the axis X and closing the shell 180 at an intermediate level between the pipework 174 and the bottom wall 178. The bottom wall 178 is pierced by a central opening 184 receiving the suction side of the circulator 28. The shell 180 also presents through openings 186 under the bottom wall 182, thus creating a path allowing the primary fluid to pass from the bottom outlet manifold 172 through the openings 186 into the volume that extends between the bottom walls 178 and 182, and then to the suction side of the circulator 28. Furthermore, the bottom internal equipments 26 include another frustoconical shell 188 that converges upwards, with its large base welded to the bottom shell 38 of the vessel 30 and with its small base welded around the bottom outlet manifold 172. The frustoconical shell 188 has through openings 190. These openings put the volume situated beneath the bottom inlet and outlet manifolds 170 and 172 into communication with the volume situated around said bottom manifolds. The primary fluid outlet 6 opens out directly into the volume situated around the bottom manifolds 170 and 172. The circulator 28 delivers the primary fluid through the radial openings in the rounded bottom wall 42, with the primary fluid being suitable for flowing upwards from there through the openings 190 and on via the outlet 6. Finally, the vessel 30 includes three support blocks 194 integrated with and welded to the bottom shell 38. The blocks 194 are disposed at 120° to one another around the axis X. As shown in FIG. 13, the assembly 1 rests via the blocks 194 on concrete foundations 196 projecting from the walls of the cell 197 in which the assembly 1 is disposed. Buttresses 198 interposed between the walls of the cell and the top shell 36 of the vessel 30 serves to stabilize the assembly 1 in the vertical position. The hottest portions of the assembly 1 are lagged, e.g. by blocks comprising Al2O3 fibers or carbon fibers. These portions operate at temperatures that are close to or greater than 800° C. in nominal operation. They comprise the pipework 176, the bottom inlet manifold 170, the central manifold 14, including its intermediate and bottom segments 84 and 116, the axial outlet manifolds 20, the outlet chamber 24, and the sleeves 140 and 142 connecting the outlet chamber 24 to the secondary fluid outlets 10. The enclosure 2 presents a total height of about 27 meters (m), and a diameter of about 7 m. The cylindrical envelope 98 presents a diameter of about 6300 millimeters (mm). Each heat exchanger 12 presents an axial height of about 4800 mm, a radial depth of about 1300 mm, and a circumferential width of about 560 mm. Each module 56 presents a height of about 600 mm. The diameter of the central manifold 14 is about 2800 mm. It is determined in such a manner that the inner sheets 80 defining the axial manifolds 18 and 20 present flexibility and respective developed lengths in the circumferential direction that are sufficient to accommodate the deformation that the heat exchangers 12 impose in a plane perpendicular to the axis X. The radial depth of the annular manifold 16 is about 500 mm. It is determined in such a manner as to make it possible for an operator to pass inside the annular manifold 16 so as to carry out inspections and/or repairs on the outside faces of the heat exchangers 12. The secondary fluid inlets 8 present through diameters of at least 850 mm, and the secondary fluid outlets 10 present through diameters of at least 1 m. The assembly 1 is dimensioned, for example, for a primary fluid pressure of about 50 bars, a primary fluid flow rate of about 200 kilograms per second (kg/s), a secondary fluid flow rate of about 600 kg/s, and a pressure difference in normal operation between the primary and second fluids of about 5 bars. There follows a description of the flow paths of the primary and secondary fluids through the assembly 1 (see FIG. 1). The primary fluid enters into the assembly 1 via the inlet 4, passes into the pipework 174, into the bottom inlet manifold 170, and then into the central manifold 14. It is delivered from the central manifold 14 to the various heat exchangers 12 distributed around the central manifold, it passes radially through the heat exchangers to the annular manifold 16 while yielding a fraction of its heat to the secondary fluid. The primary fluid then flows downwards along the annular manifold 16, along its bottom portion 86, passes through the openings in the perforated shell 122, and then flows around the bottom segment 116 of the central manifold 14, and then between the bottom manifold 170 and the bottom manifold 172. Thereafter the primary fluid passes through the openings 186 in the shell 180, is sucked into the circulator 28 and is delivered radially into the bottom of the vessel 30. Thereafter it passes through the openings 190 in the frustoconical shell 188 and leaves the assembly 1 via the outlet 6 formed around the inlet 4. The secondary fluid enters into the assembly 1 via the inlets 8, flows through the sleeves 158 and 160 to the inlet chamber 22, and is then distributed from the inlet chamber 22 into the various axial inlet manifolds 18. The secondary fluid passes through the heat exchangers 12 circumferentially and is collected in the axial outlet manifolds 20. It travels along the manifolds 20 axially to the outlet chamber 24 and is delivered from the chamber 24 to the various outlets 10. The procedures for maintaining the assembly 1 are described below. In the event of a minor action to be carried out on the heat exchangers 12, e.g. plugging a flow channel for the primary fluid or the secondary fluid, an operator acts directly on the heat exchangers 12 while they remain in place inside the enclosure 2. For this purpose, the closure head 34 is initially removed from the outer enclosure 2. Thereafter, the operator opens the hatch 92 and moves into the inspection channel 88. If the repair is to be made on a face of a heat exchanger 12 that faces towards an axial outlet channel 20, the operator passes through the opening 163 (FIG. 2) and penetrates into the outlet chamber 24, then going down inside the appropriate axial outlet manifold from the chamber 24. If the repair is to be made on an outside face of a heat exchanger 12, the operator penetrates into the annular manifold 16 from the chamber 24 via the axial outlet channel 20 presenting an inspection hole, and carries out the repair from the manifold 16. If the action is to be performed on an inside face of a heat exchanger 12, the operator opens the hatch 90 and goes from the inspection channel 88 to the central manifold 14. The repair is carried out from the central manifold 14. If the action is to be performed on a side of a heat exchanger 12 facing towards an axial inlet manifold 18, the operator moves down along the central manifold 14 to the intermediate segment 84, opens the hatch 164, penetrates into the inlet chamber 22, and moves up inside the appropriate axial inlet manifold 18 from the chamber 22. If a major repair is to be performed on the heat exchangers 12, e.g. replacing a module 56, then it is necessary initially to remove the subassembly 94 from the vessel 30. For this purpose, a maintenance cell 200 (FIG. 13) is provided above the cell 197 in which the assembly 1 is located. These two cells communicate via an opening 202 that is closed by an isolating hatch 203 extending above the assembly 1. Initially, a sealing ring 204 is placed around the top portion of the assembly 1. Gaskets provide sealing firstly between the ring 204 and the flange 44 of the vessel 30, and secondly between the ring 204 and the peripheral edge of the hatch 202. A vinyl sock 206 is placed above the sealing ring 204 and is suspended from the lifting beam of the bridge crane 201 in the cell 200. The closure head 34 is removed initially from the enclosure 2 using the crane 201. Thereafter the enclosure 2 is isolated from the maintenance cell 200 by putting the hatch 203 into place while removing the closure head 34. After the vinyl sock 206 has been put into place and the hatch 203 has been opened, operators penetrate into the outlet chamber 24 through the hatch 92 and the opening 163. They then remove the blocks of lagging 156 that protect the sleeves 140 and 142, and then undo the screws 146 and 150 using appropriate tools. Once the sleeves 140 and 142 have been released, the operators pull the sleeves into the inside of the outlet chamber 24 (using special tooling). They proceed in this manner for all four secondary fluid outlets 10. Thereafter, the operators penetrate into the inlet chamber 22 via the hatches 90 and 164. They release the sleeves 158 and 160 connecting the secondary fluid inlets 8 to the inlet chamber 22 and they use special tooling to pull the sleeves into the inside of the chamber. They then leave the assembly 1. The beam of the crane 201 is then coupled to the mushroom 102 of the subassembly 94. The subassembly is then lifted by raising the beam of the crane 201, thereby extracting the subassembly from the vessel 30, and it is lifted through the hatch 202 into the cell 200. It is then located inside the vinyl sock 206, being isolated from the enclosure 1 by reclosing the hatch 202. The crane then moves inside the maintenance cell 200 so as to put the subassembly 94 down onto an appropriate reception stool. Major maintenance operations are then performed in the cell 200. The subassembly 94 is put back into place inside the vessel 30 by a procedure that is exactly the reverse of the procedure described above. The subassembly 94 needs to be guided in turning about the axis X while being put back into place so as to cause the indexing keys 130 to engage in the appropriate grooves 132. Once the bearing surface 106 of the flange 104 bears on the complementary bearing surface 108 of the vessel 30, the beam of the crane 201 is uncoupled from the grip mushroom 102. The maintenance cell 200 may be common to a plurality of assemblies 1, all serving the same nuclear reactor, or indeed serving a plurality of different nuclear reactors. The above-described assembly presents numerous advantages. The axial manifolds 18 and 20 open out into the inlet and outlet chambers 22 and 24 and they are not directly connected mechanically to the secondary fluid inlets and outlets 8 and 10. This configuration is favorable in terms of differential expansion between the inlets and outlets 8 and 10 connected to the vessel and the chambers 22 and 24 belonging to the heat exchanger subassembly 94, thereby considerably restricting thermomechanical stresses on these connections. The disposition of the heat exchangers 12 and of the axial outlet and inlet manifolds 18 and 20 enables the manifolds 18 and 20 to be given respective large through sections. The axial speed of flow of the secondary fluid along these manifolds lies for example in the range 10 meters per second (m/s) to 20 m/s. In other heat exchanger designs, these speeds can be as great as 60 m/s. Slower speeds are favorable for maintaining hydraulic equilibrium between the secondary fluid inlets and outlets 64 and 66 in each manifold 56 during normal operation. These smaller speeds also enable the secondary fluid to be distributed uniformly amongst the various modules 56 stacked along a given axial manifold 18, and from a thermo-hydraulic point of view, they are favorable during transient operation. The overall efficiency of the heat exchangers 12 is improved. The thermomechanical behavior of the manifolds is also particularly favorable. The axial manifolds 18 and 20 are defined by inner and outer circumferential sheets 80 and 82 that are flexible, deforming easily under the effect of the stresses imposed by the heat exchangers 12. The heat exchangers 12 are blocks that are very rigid compared with the sheets 80 and 82, which means that deformation is imposed on the sheets. The sheets 80 and 82 constitute thin shells of large radius of curvature, thereby giving them a large amount of flexibility. The inlet and outlet chambers 22 and 24 are of large size and they do not have internal partitions. As a result, the inlet chamber allows the secondary fluid to be distributed uniformly amongst the various axial inlet manifolds 18. Furthermore, because of their large through sections, these chambers offer little resistance to the flow of secondary fluid. They also provide easy access to the inlets 8 and outlets 10, and thus enable the sleeves 140, 142, 158, and 160 to be disconnected easily and quickly from the inlets 8 and outlets 10. Finally, because the chambers do not have any internal partitioning, it is possible to place all of the inlets 8 and outlets 10 on the same side of the enclosure 2. It is thus possible to place the assembly 1 close to one of the walls of the cell 97, since the inlet and outlet pipework for the secondary fluid is all located away from that wall. The subassembly 94 containing all of the heat exchangers and the main primary and secondary fluid flow manifolds can be withdrawn as a single piece from the outer enclosure 2. This operation is performed in a manner that is particularly simple and convenient, using the crane in the maintenance cell situated above the heat exchanger assembly 1, after removing the closure head 34 and withdrawing the sleeves 40 and 42 into the inlet and outlet chambers 22 and 24. The sleeves 40 and 42 are retracted quickly and easily using special tools, such that the doses of radiation to which the operators are exposed are small. Once the sleeves 140 and 142 have been retracted, the subassembly 94 is extracted from and reinserted into the enclosure 2, merely by mutual disengagement and engagement. The bottom manifolds 170 and 172 present flanges 176 of shape adapted to guide the bottom portion of the subassembly 94 while it is being put back into place. The central manifold 14 and the annular manifold 16 are connected in leaktight manner with the bottom manifolds 170 and 172, merely by mutual engagement in a vertical direction. Major maintenance operations are performed on the heat exchangers 12 in convenient manner in a special maintenance cell that is fitted with suitable equipment. Furthermore, small repairs can be carried out on the heat exchangers 12 in situ, i.e. without withdrawing the subassembly 94 from the enclosure 2. The central manifold, the annular manifold, and the axial inlet and outlet manifolds present sections that are of sufficiently large size to enable an operator to enter them and work inside them. The heat exchangers 12 are accessible on all four faces for repair. The modules 56 constituting each heat exchanger 12 are welded to one another along edges that define, upwards and downwards, the inner, outer, and radial faces of these modules. Corner welds are eliminated by the presence of the bars 68 disposed in the machined corners of the modules 56. The inner and outer circumferential sheets 80 and 82 are welded to the flanges 72 of the bars 68. This welding is situated at a distance from the modules 56 and can be inspected in practical manner using X-rays. The critical zone C in which thermomechanical stresses are at a maximum (see FIGS. 9A and 9B) is situated at the junction between a flange 72 and the main portion 70 of a bar 68, so this zone extends in the material of the bar 68 and not in the weld. Finally, the flanges 72 are connected to the radial faces of the modules 56 via radii of curvature (R) that are optimized as a function of the thermomechanical stresses in the critical zones C. These various constructional dispositions enable the heat exchangers 12 to be made to be particularly good at withstanding thermomechanical stresses. The heat exchanger assembly described above may present numerous variants. Thus, for example, the heat exchangers 12 need not be plate type heat exchangers, but they could be heat exchangers of the type having tubes and shells. The circulator 28 need not be disposed at the bottom of the vessel 30, but could be secured to the closure head 34. It is then necessary to modify the path followed by the primary fluid leaving the heat exchangers 12. The annular manifold 16 is extended upwards towards the circulator 28 and is partitioned so as to define an up portion, channeling the primary flow to the circulator 28, and a down portion, channeling the primary flow from the circulator 28 to the outlet 6. This makes removing the subassembly 94 more complex, since it is necessary to begin by removing the circulator 28 before removing the closure head 34 from the enclosure 2. The heat exchanger assembly may have a number of heat exchangers 12 that is greater than or less than eight. The secondary fluid inlets 8 could be disposed at the top of the top shell 36, with the secondary fluid outlets 10 then being disposed beneath the exchangers 12. The primary fluid can flow from the inlet 4 towards the heat exchangers 12 in the annular manifold 16 and return from the heat exchangers to the outlet 6 via the central manifold. The primary fluid could flow from the inlet chamber 22 through the axial channels 18 and 20 to the outlet chamber 24, with the secondary fluid then flowing through the central manifold 14 and the annular manifold 16. The primary fluid need not be substantially pure helium, but could be a mixture of helium and nitrogen. The primary fluid could also mainly comprise water. The secondary fluid may be substantially pure helium or a mixture of helium and nitrogen (e.g. 20% helium and 80% nitrogen or 40% helium and 60% nitrogen). The secondary fluid may also be constituted mainly by water, and may be vaporized within the heat exchanger assembly. Under such circumstances, the heat exchanger acts as a steam generator. It should be observed that the heat exchanger assembly 1 described above presents several original aspects suitable for being protected independently of one another. Thus, it is possible to make provision for the assembly 1 to have a mechanical subassembly that can be extracted in a single piece such as the subassembly 94, even though the axial manifolds 18 and 20 are connected to the inlets 8 and outlets 10 via connecting pipework and not via chambers such as 22 and 24. Under such circumstances, the terminal portions of the connecting pipework should be suitable for being disconnected manually from the inlets and outlets 8 and 10, e.g. from the empty space between the closure head 34 and the heat exchangers 12 and from the empty space lying between the frustoconical envelope 100 and the heat exchangers 12. These terminal portions are retracted into the inside of the connection pipework, or they are completely separated therefrom and extracted manually from the enclosure 2 by the operators. Similarly, it is possible to make provision for the assembly 1 to have heat exchangers 12 provided with bars 68 of the kind described above while the axial manifolds 18 and 20 are not connected to the inlets 8 and outlets 10 by chambers 22 and 24 and/or it is possible for the assembly 1 not to include a subassembly 94 that can be removed. |
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description | 1. Field The present disclosure relates to devices for harvesting water from the channel walls of a boiling water reactor (BWR). 2. Description of Related Art Developments in fuel bundle designs have improved the critical power ratio (CPR) performance of interior rods. As a consequence, the periphery rods have become limiting in design. To improve the critical power ratio (CPR) performance of periphery rods, features have been machined in the channel walls to act as flow trippers. However, such machined features may be difficult to produce, inspect, and require specialized equipment. As a result, machining flow tripping features in the channel walls can be costly. A flow tripping device according to a non-limiting embodiment of the present invention may include a peripheral band surrounding a central space; a plurality of flow tabs extending from an upper portion of the peripheral band toward the central space; and a plurality of finger structures extending from a lower portion of the peripheral band. The peripheral band may have a symmetrical shape based on a plan view. The peripheral band may include a plurality of corners based on a plan view, and each corner of the peripheral band may be provided with at least one of the plurality of flow tabs. Similarly, the peripheral band may include a plurality of corners based on a plan view, and each corner of the peripheral band may be provided with at least one of the plurality of finger structures. Each of the plurality of flow tabs may taper as it extends from the peripheral band. Each of the plurality of flow tabs may also curve inward toward the central space. The plurality of flow tabs may be spaced apart from each other by a space that corresponds to a shape of an inverted flow tab. Each of the plurality of finger structures may have a segment that is parallel to but on a different plane than an outer wall of the peripheral band. Each of the plurality of finger structures may extend upward and back toward the peripheral band. Alternatively, each of the plurality of finger structures may extend downward and away from the peripheral band. The plurality of finger structures may be separated from each other by a plurality of slits. A total quantity of the plurality of finger structures may exceed a total quantity of the plurality of flow tabs. For instance, a ratio of the total quantity of the plurality of finger structures to the total quantity of the plurality of flow tabs may be about 3:1. The flow tripping device may additionally include a plurality of dividers in the central space bounded by the peripheral band. The plurality of dividers may be connected to opposing inner walls of the peripheral band. A ratio of a total quantity of the plurality of dividers to a total quantity of the plurality of flow tabs may be about 1:6. The plurality of dividers may include a first group of parallel dividers that intersect a second group of parallel dividers. The first group of parallel dividers may be orthogonal to the second group of parallel dividers. The flow tripping device may further include a plurality of convex portions on the peripheral band. The plurality of convex portions may protrude away from the central space bounded by the peripheral band. A fuel bundle according to a non-limiting embodiment of the present invention may include a fuel channel; a plurality of fuel rods within the fuel channel; at least one water rod arranged in parallel with the plurality of fuel rods; and a flow tripping device secured to the at least one water rod, the flow tripping device including a peripheral band surrounding the plurality of fuel rods, a plurality of flow tabs extending from an upper portion of the peripheral band toward the plurality of fuel rods, and a plurality of finger structures extending from a lower portion of the peripheral band and configured to apply a spring force to inner walls of the fuel channel. It should be understood that when an element or layer is referred to as being “on,” “connected to,” “coupled to,” or “covering” another element or layer, it may be directly on, connected to, coupled to, or covering the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout the specification. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It should be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of example embodiments. Spatially relative terms (e.g., “beneath,” “below,” “lower,” “above,” “upper,” and the like) may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It should be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” may encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. The terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes,” “including,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Example embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and inter mediate structures) of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, including those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. FIG. 1A is a plan view of a flow tripping device according to a non-limiting embodiment of the present invention. Referring to FIG. 1A, the flow tripping device 100 includes a peripheral band 102 surrounding a central space. The peripheral band 102 may have a symmetrical shape based on a plan view. As shown in the drawings, the peripheral band 102 may be square-shaped. However, it should be understood that the peripheral band 102 may have other dimensions depending on the shape of the channel that is intended to accommodate it. A plurality of flow tabs 104 and finger structures 106 extend from the peripheral band 102. A total quantity of the plurality of finger structures 106 exceeds a total quantity of the plurality of flow tabs 104. As shown in the drawings, the flow tripping device 100 may be provided with nine flow tabs 104 and twenty-six finger structures 106′ on each side of the peripheral band 102 (for a total of thirty-six flow tabs 104 and one hundred and four finger structures 106). Thus, a ratio of the total quantity of the plurality of finger structures 106 to the total quantity of the plurality of flow tabs 104 may be about 3:1, although example embodiments are not limited thereto. Although not shown, each corner of the peripheral band 102 may be provided with at least one of the plurality of flow tabs 104. Similarly, although not shown, each corner of the peripheral band 102 may be provided with at least one of the plurality of finger structures 106. The flow tripping device 100 also includes a plurality of dividers 108 in the central space bounded by the peripheral band 102. The plurality of dividers 108 are connected to opposing inner walls of the peripheral band 102. The plurality of dividers 108 includes a first group of parallel dividers intersecting a second group of parallel dividers. The first group of parallel dividers may be orthogonal to the second group of parallel dividers. As illustrated in the drawings, the first group may include three dividers 108, and the second group may also include three dividers 108 (for a total of six dividers 108). Thus, a ratio of a total quantity of the plurality of dividers 108 to a total quantity of the plurality of flow tabs 104 may be about 1:6, although example embodiments are not limited thereto. FIG. 1B is a side view of a flow tripping device according to a non-limiting embodiment of the present invention. Referring to FIG. 1B, the plurality of flow tabs 104 extend from an upper portion of the peripheral band 102 toward the central space. The plurality of flow tabs 104 taper as they extend from the peripheral band 102. Although the plurality of flow tabs 104 are illustrated as being flat-tipped, it should be understood that the tips may be more (or less) pointed. The plurality of flow tabs 104 also curve inward toward the central space. In particular, the base of each flow tab 104 may be linear, while the tip may curve inwards. The plurality of flow tabs 104 may be spaced apart from each other by a space that corresponds to a shape of an inverted flow tab, although it should be understood that the spacing may be varied. Furthermore, the plurality of flow tabs 104 may be integrally formed as part of the peripheral band 102. Alternatively, the plurality of flow tabs 104 may be individually secured (e.g., welded) to the perimeter band 102. The plurality of finger structures 106 extend from a lower portion of the peripheral band 102. In a non-limiting embodiment, each of the plurality of finger structures 106 extends upward and back toward the peripheral band 102. Although the distal ends of the finger structures 106 are illustrated as contacting the peripheral band 102, the present invention is not limited thereto. Each of the plurality of finger structures 106 may have a segment that is parallel to but on a different plane than an outer wall of the peripheral band 102. Furthermore, the plurality of finger structures 106 may be separated from each other by a plurality of slits. It is beneficial for the plurality of slits to be as small as possible to minimize the bypass flow through it when the flow tripping device 100 is put to use in a fuel bundle during the operation of a boiling water reactor (BWR). For instance, the width of each slit may be no more than the thickness of the sheet material used to form the finger structures 106. Providing the flow tripping device 100 with a plurality of finger structures 106 is advantageous, because it ensures that the flow tripping device 100 will remain in continuous contact with the inner walls of the fuel channel throughout the life of the fuel bundle. For instance, the walls of the fuel channel are subjected to pressure from the internal flow which may cause the fuel channel to bulge/distort over time. Additionally, the fuel channel may experience irradiation growth which will contribute to the bulge/distortion. As a result, having a plurality of individual finger structures 106 configured to apply a spring force to the fuel channel allows the flow tripping device 100 to accommodate to any in-service changes that the fuel channel may undergo during the life of the fuel bundle. The number of finger structures 106 may vary from that shown in the drawings. Having a greater number of finger structures 106 will increase the ability of the flow tripping device 100 to adjust to any in-service changes/distortions that the fuel channel may undergo. However, a greater number of finger structures 106 will mean a greater number of slits which will consequently increase the bypass flow. Conversely, having a smaller number of finger structures 106 (and, thus, less slits) will decrease the bypass flow but will also decrease the ability of the flow tripping device 100 to adjust to the in-service changes/distortions to the fuel channel. That being said, it may be beneficial for the number of finger structures 106 to be within the range of ±10-15% of that shown in the drawings. For instance, each side of the peripheral band 102 of the flow tripping device 100 may be provided with about twenty-two to thirty finger structures 106. The flow tripping device 100 also includes a plurality of convex portions 110 on the peripheral band 102. The plurality of convex portions 110 may be elliptical in shape and vertically oriented, although the present invention is not limited thereto. The plurality of convex portions 110 also protrude away from the central space bounded by the peripheral band 102. The extent of the protrusion of the convex portions 110 is less than that of the finger structures 106 based on a side view of the flow tripping device 100. The plurality of convex portions 110 are located in a middle region between the flow tabs 104 at the upper portion of the peripheral band 102 and the finger structures 106 at the lower portion of the peripheral band 102. FIG. 1C is a perspective view of a flow tripping device according to a non-limiting embodiment of the present invention. Referring to FIG. 1C, the height of the dividers 108 may be comparable to that of the peripheral band 102 while not exceeding the height of the flow tabs 104. Although not explicitly labeled, the two central diagonal compartments formed by the dividers 108 may be provided with springs (two each for a total of four springs) to accommodate a water rod in each of the two central diagonal compartments when the flow tripping device 100 is mounted within a fuel channel. Each of the water rods (which may be more or less than the two mentioned above) may have a diameter that is about twice that of each fuel rod. That being said, while the two central diagonal compartments formed by the dividers 108 may each accommodate a water rod, each of the other compartments may accommodate a plurality of fuel rods. Also, as shown in the drawings, four convex portions 110 are provided on each side of the peripheral band 102 (for a total of sixteen convex portions 110). However, it should be understood that the quantity and spacing of the convex portions 110 may be varied. The flow tripping device according to non-limiting embodiments of the present invention is intended to be incorporated within a fuel bundle. Such a fuel bundle may include a fuel channel, a plurality of fuel rods within the fuel channel, at least one water rod arranged in parallel with the plurality of fuel rods, and a flow tripping device secured to the at least one water rod. An upper tie plate and a lower tie plate may be disposed at opposing ends of the fuel bundle. A plurality of spacers may be placed at predetermined intervals in the fuel bundle to space the fuel rods in an array. A flow tripping device may be arranged between adjacent spacers in the upper region of the fuel bundle. Unlike the spacers, the flow tripping device is designed to not interface with the fuel rods. Stated more clearly, the flow tripping device is configured so as to not contact the fuel rods when installed. The flow tripping device may include a peripheral band surrounding the plurality of fuel rods, a plurality of flow tabs extending from an upper portion of the peripheral band toward the plurality of fuel rods, and a plurality of finger structures extending from a lower portion of the peripheral band and configured to apply a spring force to inner walls of the fuel channel. FIG. 1D is a side view illustrating a flow tripping device with the main film flow, bypass flow, and diverted flow according to a non-limiting embodiment of the present invention. Referring to FIG. 1D, the flow tripping device is designed to be attached to one or more water rods (not shown) in a fuel channel and positioned between spacers at the higher elevations of the fuel bundle. Additionally, the structure of the flow tripping device is designed to be at a minimum so as to minimize the pressure drop within the fuel channel. During the operation of a boiling water reactor (BWR), a water film flows upward on the inner walls of the fuel channel 112. When the water film reaches the flow tripping device 100, a small portion of the water film will pass through the slits between the finger structures 106 as a bypass flow (broken arrows), while a majority of the water film will be diverted into the flow tripping device 100 and redirected by the flow tabs 104 toward the periphery fuel rods (not shown). As a result, the critical power ratio (CPR) performance of the periphery rods may be increased, thereby also increasing overall performance. Consequently, the increased power translates to lower fuel cycle costs. In view of the above, it should be understood that a method of increasing critical power ratio (CPR) performance may involve installing a flow tripping device according to the present invention in a fuel, channel of a boiling water reactor (BWR). One or more of the flow tripping devices may be installed in the upper region of the fuel bundle. For instance, a flow tripping device may be installed between the first (uppermost) spacer and second spacer. Another flow tripping device may be installed between the second spacer and third spacer. Furthermore, yet another flow tripping device may be installed between the third spacer and fourth spacer. FIG. 2A is a side view of a flow tripping device according to another non-limiting embodiment of the present invention. FIG. 2B is a perspective view of a flow tripping device according to another non-limiting embodiment of the present invention. FIG. 2C is a side view illustrating a flow tripping device with the main film flow, bypass flow, and diverted flow according to another non-limiting embodiment of the present invention. The flow tripping device 100′ of FIGS. 2A-2C may be as described above in connection with the flow tripping device 100 of FIGS. 1A-1D with the exception of the finger structures. Accordingly, common features already discussed above will not be repeated below for purposes of brevity. Referring to FIGS. 2A-2C, each of the plurality of finger structures 206 extends downward and away from the peripheral band 102. For instance, each of the plurality of finger structures 206 may initially extend downward at an angle away from the lower portion of the peripheral band 102 before extending downward vertically so as to be parallel to an outer wall of the peripheral band 102. Although not shown, each corner of the peripheral band 102 may be provided with at least one of the plurality of finger structures 206. Similarly, although not shown, each corner of the peripheral band 102 may be provided with at least one of the plurality of flow tabs 104. While a number of example embodiments have been disclosed herein, it should be understood that other variations may be possible. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. |
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abstract | Provided are a safety protection system facility 43 outputting a normal actuating signal S1 in a case where the safety protection system facility 43 controls actuation of a unit provided in a nuclear facility to a safe side based on an abnormality detecting signal output at the time of occurrence of an abnormality in the nuclear facility, and where this causes the unit to be actuated normally, and a CCF countermeasure facility 44 outputting a CCF-case actuating signal S2 actuating the unit to a safe side in a case where the CCF countermeasure facility 44 determines from output results of the abnormality detecting signal and the normal actuating signal S1 that the unit is not actuated normally at the time of occurrence of the abnormality in the nuclear facility. |
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summary | ||
abstract | A seal arrangement for providing a seal between a nuclear reactor in-core instrument housing and an instrument contained within the housing includes a lower seal assembly surrounding an outer portion of the in-core instrument housing, an upper seal assembly surrounding an outer portion of the in-core instrument, a seal housing enclosing the lower and upper seal assemblies, and lower and upper compression assemblies positioned on respective ends of the seal housing. The compression assemblies each include a drive nut and a compression collar. The compression collars engage and apply an axial load on the seal assemblies to maintain a reliable seal between the seal housing and the outer portion of the in-core instrument housing, and between the seal housing and the outer portion of the in-core instrument. |
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description | The invention relates to the process of fabricating semiconductor chips. More specifically, the invention relates to a lithographic method. Integrated circuit technology improvements are mostly driven by the decrease of the feature size of the semiconductor chips. As the feature size of the circuit decreases, circuit designers have to deal with the limitations of the lithography process used to manufacture the integrated circuits. The lithography process starts first by coating the surface of the semiconductor wafer with a material called resist. A source of radiation is then shone through the mask in the case of a transparent mask. For a reflective mask the radiation is reflected by the mask. The transparent mask is made of a substrate transparent to the radiation and coated with a patterned opaque layer defining clear and opaque regions to the radiation. Transparent masks are mostly used in optical lithography with typical wavelengths of 436 nm, 405 nm, 365 nm, 248 nm, 193 nm, and 157 nm. The reflective masks are made using a substrate reflective to the radiation and coated with a patterned non-reflective layer defining reflective and non-reflective regions to the radiation. Alternatively, a reflective mask could be made of a non-reflective substrate coated with a reflective layer. Reflective masks are mostly used for shorter radiation wavelength on the order of 13 nm usually referred to as EUV or Extreme Ultra Violet. During the exposure to the radiation source, an image of the mask is formed using an optical system on top of the resist layer. Various optical systems can be used to produce an image of the mask. In the techniques called contact printing and proximity printing, the mask is placed in contact or in close proximity to the resist. Light is shone through the backside of the mask thereby exposing the entire resist-coated wafer through the openings of the mask. Since the mask image and the wafer image are of the same dimension, this technique was partially abandoned for volume manufacturing because of the tight requirements on the mask image and the difficulty to obtain good contact over the entire wafer area. The main technique used today in volume production relies on the projection of the image of the mask onto the wafer. Typically the wafer image is reduced by a factor of 4 (usually named mask image magnification factor or wafer image demagnification factor) as compared to the mask image, thus relaxing the mask fabrication requirements. The field on the wafer corresponding to the image of the mask is exposed multiple times to cover the entire wafer. The entire field can be exposed in one shot, in this case the equipment is named a stepper. Alternatively, the field can be scanned by moving the mask and the wafer relative to the projection lens. In this case the equipment is named a scanner. Scanners offer the advantage to mitigate some field non-uniformities observed in steppers but the scanning mechanism adds residual noise that partially degrades the aerial image. Moreover scanners show differences of the aerial image for features perpendicular to the scan direction versus features parallel to the scan direction. As the quality of the projection lenses improves, the advantage of scanners over steppers becomes less apparent. The resist layer is exposed by the radiation passing through the mask in case of transparent mask or reflected by the mask in the case of a reflective mask. The resist is then developed in a developer bath and depending on the polarity of the resist (positive or negative), the exposed regions or the unexposed regions of the resist are removed. The end result is a semiconductor wafer with a resist layer having a desired pattern. This resist pattern can then be used by subsequent processing steps of the underlying regions of the wafer. As the feature size decreases, distortion in the pattern transfer process becomes more severe. The design shapes must be modified in order to print the desired images on the wafer. The modifications account for the limitation in the lithography process. One such modification is referred to as Optical Proximity Correction (OPC) in the case of optical lithography. In the case of OPC, modifications of the design image account for optical limitations as well as mask fabrication limitations and resist limitations. Modifications of the design image can also account for the subsequent process steps like dry etching or implantation. It can also account for flare in the optical system as well as pattern density variations. Another application of proximity effect correction is the compensation of the effects of aberrations of the optical system used to print the image of the mask onto the wafers. In this case, a mask with aberration correction would be dedicated to a given lithography tool as the aberrations are tool-specific. FIG. 1 illustrates the modification of the mask data to correct for proximity effects. The processing of the mask data starts with a target layout 101 representing the desired dimensions of the image on the wafer. The printed image 102 of the target layout 101 differs from the desired image due to proximity effect. For reference, the target image 101 is shown with the printed image 102. The edges of the features are then moved (103) so that the corresponding printed image on the wafer 104 is correct (as close to the target as possible). In FIG. 1, all the areas of the layout have been corrected but different degrees of proximity effect correction aggressiveness can be applied to different regions depending on the criticality of the region in the integrated circuit. The corrections to layout 101 can be applied using a rule-based approach or a model-based approach. For a rule-based approach (Rule-based OPC), the displacement of the segments would be set by a list of rules depending, for example, on the feature size and its environment. For a model-based approach (Model-based OPC), the printed image on the wafer would be simulated using a model of the pattern transfer process. The correction would be set such that the simulated image matches the desired wafer image. A combination of rule-based OPC and model-based OPC sometimes referred to as hybrid OPC can also be used. In the case of model-based OPC, the original layout 201 as shown in FIG. 2 is dissected in smaller segments 203 shown in modified layout 202. Each segment is associated an evaluation point 204. The printed errors of the evaluation points are compensated by moving the corresponding segment in a direction perpendicular to the segment as shown in the final layout 205. The segments are corrected using multiple iterations in order to account for corrections of neighboring segments. The image quality can be improved by adding printing or non-printing assist features along the edges of the main features. These assist features modify the diffraction spectrum of the pattern in a way that improves the printing of the main feature. The practical implementation of assist features is enhanced with the use of proximity effect correction as described above to correct for any optical printing artifact as well as resist and etch artifacts. The image quality can also be improved by using phase-shifting masks. In this case, at least two different regions are created on the masks corresponding to different phase and transmission of the light either going through these regions (for transparent mask) or reflected by these regions (for reflective mask). The phase difference between the two regions is chosen to be substantially equal to 180 degrees. The destructive interference between adjacent regions of opposite phase creates a very sharp contrast at the boundary between the regions, thus leading to the printing of small features on the wafer. Two main classes of phase-shifting masks are in use today. For the first class, the amount of light transmitted for transparent masks (or reflected for reflective masks) by one region is only a portion of the light transmitted (or reflected) by the other region, typically 5% to 15%. These masks are referred to as attenuated phase-shifting masks or half-tone phase-shifting masks. For the second class, the light transmitted (for transparent masks) or reflected (for reflective masks) by one region is substantially equal to the light transmitted (for transparent masks) or reflected (for reflective masks) by the other region. The second class of masks includes the following types of phase-shifting masks: alternating aperture phase-shifting masks, chromeless phase-shifting masks, and rim phase-shifting masks. The practical implementation of these techniques is improved with the use of proximity effect correction as described above to correct for any optical printing artifact as well as resist and etch artifacts. All the techniques can be combined with the use of assist features. The image quality can also be improved by using off-axis illumination. To achieve off-axis illumination, the illuminator of the stepper or scanner is shaped in a way that only the light at certain angles with respect to the optical axis is used to create the image thereby favoring certain spatial frequencies of the mask pattern. The off-axis setting can be adjusted for a given feature size and type or for a collection of feature sizes and types. Off-axis illumination can be used in combination with binary masks, attenuated phase-shifting masks, chromeless phase-shifting masks, or rim phase-shifting masks. Off-axis illumination will also be improved by the use of proximity effect correction as described in a previous paragraph. Off-axis illumination can also be combined with the use of assist-features. For the various techniques described above, i.e. proximity effect correction, phase-shifting masks, off-axis illumination, a very accurate model of the overall patterning process is required. This model strongly depends on the optical set up of the exposure tool used to expose the wafers. In particular, the wavelength of the light source, the numerical aperture of the projection lens and the illumination setting (partial coherence, off-axis illumination) are critical. The limitation of current optical systems is driven by the following equation:R=k1λ/NA R=resolution λ=wavelength of the illumination source NA=numerical aperture of the exposure system The maximum resolution (or smallest line in a pattern made of equal lines and spaces) achieved for standard optical system is achieved for k1=0.25. But for values of k1 below 0.5, severe distortion of the pattern can be observed on the wafer thus requiring the correction of the mask in order to print the desired image on the wafer. In spite of all the techniques described above, the theoretical limit of k1=0.25 cannot be exceeded for current exposure tools. One approach that was recently proposed to extend the life of optical lithography is to create the image in a liquid having a high refractive index medium thereby reducing the wavelength of the light by the reflective index. The limitation of optical systems with immersion will be driven by the following equation:R=k1λ/(nNA) R=resolution λ=wavelength in vacuum of the illumination source NA=numerical aperture of the exposure system n=refractive index of the immersion mediumIn this case, the k1=0.25 limitation still holds but much smaller feature sizes can be printed. For example, for 193 nm wavelength, water was proposed as the immersion liquid. Water has a refractive of 1.47 at 193 nm, thereby lowering the smallest printable feature by a factor of 1.47. For 157 nm wavelength, a polymer referred to as PFPE (Perfluoropolyether) was proposed with a refractive index of 1.38. The main challenge of immersion lithography is the manufacturing of steppers and scanners capable of handling a film of liquid between the lens and the wafer without creating bubble or non-uniformities. The addition of an immersion film could result in vibration coupling between the stage and the lens. Moreover the resist used must be compatible with the immersion liquid. Another challenge is the limited availability of transparent liquids with a high refractive index at 193 nm or 157 nm. Another option would be to use solid immersion lithography as described in U.S. Pat. No. 5,121,256. In this case a solid immersion lens is added to an existing optical system and placed closely adjacent to the sample. The optical system images the mask onto the wafer and the immersion lens is shaped with a spherical surface and placed at a distance such that the beams enter the solid lens with no refraction. For today's exposure systems the distance between the lens and the wafer is on the order of a few millimeters while the field size image is on the order of 20 to 40 millimeters thus rendering the set up described in U.S. Pat. No. 5,121,256 not viable. Indeed, to image such a field size, the solid immersion lens would be too thick compared to the distance between the projection lens and the wafer. Moreover the set up proposed is limited by aberrations when a large field is imaged. Building two separate lenses, one objective lens and one solid immersion lens also puts extreme requirements on the alignment of the lenses, the vibration of the lenses, and the overall correction of the aberrations across the field. Projection lenses used today for optical lithography are made of a large number of lens elements, typically on the order of 30 lens elements, as described in U.S. Pat. No. 6,522,484. The large number of lens elements is required in order to lower the aberration level across the field of the image. The shape of each lens elements is optimized by taking into account all the other lens elements. Each element is accurately positioned inside the lens assembly and kept in an environment where the pressure, temperature and atmosphere are controlled. These tight requirements make the use of two separate lenses as described in U.S. Pat. No. 5,121,256 impractical. The present invention provides a lithography system based on a projection lens in which a final lens element has a surface adapted to be placed in contact or in close proximity with the sample being exposed, and a stage that supports the sample in contact or in close proximity with the surface of the final lens element. The projection lens typically comprises a plurality of lens elements including a first lens element adapted to face a mask, and the final lens element. The final lens element comprises a solid material having a high index of refraction, or an index of refraction greater than one. For example, in systems projecting an image using radiation having a wavelength of about 193 nm, or having a wavelength of about 157 nm, the final lens element may comprise one of silicon dioxide, calcium fluoride, aluminum oxide, yttrium fluoride, lanthanum fluoride, strontium fluoride. In embodiments of the invention, the plurality of lens elements of the projection lens demagnifies an object on the mask by a factor greater than 4 at an image plane on or near the sample. In some embodiments, the image plane is on or near the surface of the final lens element. In such cases, evanescent waves from the image plane can be used to transfer the image into the radiation sensitive layer on the sample. In yet other embodiments, the index of refraction of the final lens element matches the index of refraction of the radiation sensitive layer on the sample. The projection lens in typical embodiments comprises a plurality of lenses which are encased, to provide controlled atmosphere and temperature in the spaces among the lenses. The final lens element has a high index of refraction, and is adapted to be placed in contact with the radiation sensitive layer on the sample. In some embodiments, the final lens element is a slab of high index of refraction material, which is adapted to be removed from the projection lens assembly for ease of replacement, and cleaning. Further embodiments of the invention comprise a lithography system that includes a projection lens which has one side adapted to be placed in contact or in close proximity with the sample, and an other side adapted to be placed in contact or in close proximity with the mask. The present invention also provides a method for manufacturing integrated circuits. The method includes providing a wafer having a layer adapted to be developed in response to radiation. Also, a layout object to be projected on the layer is provided. According to the method, the layer on wafer which is sensitive to radiation is placed in contact or close proximity with a high index of refraction lens element on the projection lens. The object is imaged on the radiation sensitive layer by the projection lens. According to embodiments of the method, the step of imaging the object includes imaging the object at an image plane, so that evanescent waves emanating from the lens element transfer the image to the layer on the sample, at least in part. In other embodiments, the image of the object is formed at an image plane near a top surface of the radiation sensitive layer. Alternatively, the image plane may be placed anywhere within the radiation sensitive layer, according to the pattern being transferred, the characteristics of the material of the layer, and other factors. In yet other embodiments of the invention, the method includes preventing adhesion of the lens element to the layer. Also, embodiments the invention include placing a mask including the object to be imaged, in contact or close proximity with another high index of refraction lens element of the projection lens. Embodiment of the invention includes laying out the layout pattern on the mask to be imaged on the radiation-sensitive layer. Laying out includes applying proximity correction using a lithography model comprising, for an incident material different than air characterized by its refractive index and absorption coefficient, calculating fields in the resist, accounting for the incident material refractive index and absorption coefficient, performed using thin film optics or by solving Maxwell equations. Embodiment of the invention includes laying out the layout pattern on the mask to be imaged on the radiation-sensitive layer. Laying out includes applying proximity correction using a lithography model comprising, for an incident material different than air characterized by its refractive index and absorption coefficient, calculating fields in the resist, accounting for the incident material refractive index and absorption coefficient, performed using thin film optics or by solving Maxwell equations, and accounting for a gap between the incident material and the resist using thin film modeling or by solving Maxwell equations. Embodiment of the invention includes laying out the layout pattern on the mask to be imaged on the radiation-sensitive layer. The layout pattern comprises an alternating aperture phase-shifting mask layout. Laying out includes applying proximity correction using a lithography model comprising, for an incident material different than air characterized by its refractive index and absorption coefficient, calculating fields in the resist, accounting for the incident material refractive index and absorption coefficient, performed using thin film optics or by solving Maxwell equations. Embodiment of the invention includes applying an off-axis setting for the projection lens, the off-axis setting obtained using a lithography model comprising, for an incident material different than air characterized by its refractive index and absorption coefficient, calculating fields in the resist, accounting for the incident material refractive index and absorption coefficient, performed using thin film optics or by solving Maxwell equations. Embodiment of the invention includes laying out the layout pattern on the mask to be imaged on the radiation-sensitive layer. The layout pattern comprises an assist feature having a size and a distance from a corresponding main feature, and laying out includes determining said size and distance using a lithography model comprising, for an incident material different than air characterized by its refractive index and absorption coefficient, calculating fields in the resist, accounting for the incident material refractive index and absorption coefficient, performed using thin film optics or by solving Maxwell equations. Embodiment of the invention includes laying out the layout pattern on the mask to be imaged on the radiation-sensitive layer. The layout pattern comprises an attenuated phase-shifting mask having sizing parameters, and laying out includes determining said sizing parameters using a lithography model comprising, for an incident material different than air characterized by its refractive index and absorption coefficient, calculating fields in the resist, accounting for the incident material refractive index and absorption coefficient, performed using thin film optics or by solving Maxwell equations. Embodiment of the invention includes laying out the layout pattern on the mask to be imaged on the radiation-sensitive layer. Laying out includes applying proximity correction using a lithography model comprising, for an incident material different than air characterized by its refractive index and absorption coefficient, dividing the refractive indices and absorption coefficients of all the materials in the wafer stack by the refractive index of the incident material. A technique described in FIG. 3a was developed to address the issues encountered with immersion lithography. A wafer 302 is placed on the stage 301. The projection lens 303 projects an image of the mask 304 on top of the wafer 302. The bottom surface of the lens is placed either in close proximity or in contact with the radiation sensitive layer (photoresist) on the wafer 302. Close proximity means that the distance between the bottom of the lens and the wafer is small compared to the wavelength of the exposure, typically smaller than the wavelength divided by 5. The set up described in FIG. 3a will avoid the issue of having a liquid between the lens and the wafer. Moreover, there is no need to insert a solid “immersion” lens between the projection lens and the wafer as the image of the mask 304 is created in the vicinity of the bottom surface (surface facing the wafer) of the last lens element of the projection lens 303. This set up allows the reduction of potential aberrations of the images created by the lens. A possible lens implementation is shown in FIG. 3b. The object plane, i.e. the mask is represented by O and the image plane, i.e. the wafer position is represented by IM. The image of the object is formed at the image plane in the vicinity of the surface of the last lens element 335 (the surface in proximity or in contact with the wafer). The lens elements are placed in an environment control casing not shown on the drawing. The temperature, pressure, and atmosphere are precisely controlled in the environment of the casing. The material of the last lens element needs to be chosen with the highest possible refractive index in order to achieve the best resolution. At the same time the material must be compatible with all other lens requirements including for example the fact that the material must be transparent. Potential candidates are listed below with their approximate values of the refractive indices in parenthesis and compared to liquid immersion lithography. WavelengthLiquid ImmersionLens Materials193 nmWater (1.47)Al2O3 (1.88)CaF2 (1.5)SiO2 (1.56)YF3 (1.55)SrF2 (1.55)LaF3 (1.63)157 nmPFPE (1.38)CaF2 (1.56)SiO2 (1.69)YF3 (1.64)SrF2 (1.62)LaF3 (1.72) For both 193 nm and 157 nm wavelengths, the refractive indices for the lens materials are higher than the best liquid immersion materials available thus enabling this technique to resolve smaller resist feature sizes than liquid immersion lithography. Some of the materials listed as potential candidates for the lens material, i.e. CaF2 and SiO2, are already used today in the manufacturing of lenses for 193 nm and 157 nm lithography. The lens manufacturing process could use the same material as for conventional lenses, only the overall design of the lens needs to be changed so that the image of the mask is created in the vicinity of the boundary of the last lens element. The exact location of the image of the mask will need to be chosen carefully such that the image is created at the best location with respect to the resist. Typically the image should be formed inside the resist. The exact location will depend on the resist thickness, on the resist refractive index, and on the resist processing characteristics. For some resist the image should be formed closer to the top of the resist, for other resists the image should be closer to the bottom of the resist. The difference in best image position can be attributed to differences in chemistry and to different processing conditions like for example, pre-bake time and temperature, post-exposure bake time and temperature, developer normality, development time and temperature. Alternatively, as current projection lenses are made of multiple lens elements, only the last element could be made of a high refractive index material in order to achieve the best possible resolution. For example a lens for 193 nm lithography could use CaF2 and SiO2 for all the lens elements except for the last element in contact or proximity with the resist that could be made of Al2O3. For a 157 nm lens, all elements could be made of CaF2 except for last element close to the resist that could be made of LaF3. FIG. 4 shows a detailed view of the lower surface of the projection lens 403 in contact with the resist 402 coated on the wafer 401. Marker 404 indicates light beams propagating through the projection lens 403 and creating an image inside the resist 402. Preferably the image of the mask is created inside the resist to obtain the best resist image quality after resist processing. FIG. 4 assumes that the refractive index of the lens material is lower than the refractive index of the resist and the beams at the lens/resist interface are refracted following Snell's law:n1·sin(i1)=n2·sin(i2) where n1 is the refractive index of the lens material, n2 the refractive index of the resist, i1 the angle between the beam inside the lens material and the optical axis, i2 the angle between the beam inside the resist and the optical axis. The refraction of the beams entering the resist will create an aberration of the image inside the resist equivalent to spherical aberration. The aberration can be reduced if n1 and n2 are substantially the same as shown in FIG. 5. In this case the previous formula shows that i1 and i2 are substantially equal. The beams propagating through the lens enter the resist with minimal direction change and the aberration effect is therefore reduced. The lens material could be chosen to achieve a good refractive index match with the resist. Vice-versa, the resist composition could be modified to match the refractive index of the lens material. The composition of the resist could also be modified so that the refractive index of the resist is larger than the refractive index of the last lens element in order to ensure that no resolution loss occurs at the interface between the resist and the lens. If the projection lens is not in perfect contact with the resist and a gap is left in between the lens and the resist, the propagation of the beams with high angles of incidence (i1) through the gap could be altered if the refractive index of the gap ng is lower than the refractive index of the lens material as shown in FIG. 6. Total reflection would take place for sin(i1)>ng/n1. The beam 605 going through the lens 604 is reflected into the beam 606 because of the gap 603 before it reaches the resist 602 coated on the wafer 601. In this case evanescent waves present at the lens/gap interface could be used to image the resist. As their amplitude decays exponentially, the gap thickness will have to be kept small compared to the wavelength of the light. To alleviate the effects of exponentially decaying evanescent fields, a negative refractive index slab can be placed in between the lens and the wafer as shown in FIG. 7. A wafer 702 is placed on the stage 701. The projection lens 703 combined with a negative refractive index slab 705 creates an image of the mask 704 on top of the wafer 702. The propagating fields are imaged by the slab which acts like a lens. The evanescent fields are amplified throughout the slab and can be used on the resist side to create the final image inside the resist. FIG. 8 shows a detailed view of the lower portion of the projection lens 806, the negative refractive index slab 804, the resist 802 and the wafer 801. A gap 805 is left between the lens 806 and the negative refractive index slab 804 and a gap 803 is left between the negative refractive index slab 804 and the resist 802. The dotted line 807 shows fields propagating from the lens 806, through the gaps 805 and 803 and through the negative refractive index material 804 to create an image into the resist 802. For beams with large incidence angle, gap 805 results in total reflection inside the lens 806 as described in FIG. 6. Evanescent fields inside gap 805 decay exponentially but are amplified through the negative refractive index slab 804. These evanescent fields decay again exponentially in the gap 803. They can create an image in the resist if the widths of the gaps 805 and 803 are small compared to wavelength of the light. Preferably the width of the gaps 805 and 803 is chosen equal to half the width of the negative refractive index slab 804 and the refractive index of the slab 804 is chosen equal to −1. The negative index slabs can be obtained by manufacturing three dimensional photonic crystals. For all the cases described above the mask is placed in air or in a purged environment for 157 nm lithography as air attenuates the 157 nm radiation. Under these conditions, the diffraction pattern emanating from the mask and captured by the projection lens could be limiting the resolution of the image created on the wafer. A magnification of the mask image is required if the mask is imaged in air. The condition imposed on the magnification of the mask is as follows:m>nw/nm where m is the magnification of the image on the mask compared to the image on the wafer, nm is the refractive index of the material in contact or in close proximity with the mask and nw is the refractive index of the material in contact or in close proximity with the wafer. In general, the resolution of the image created on the wafer is limited by the minimum of the refractive index of the last lens element, the refractive index of the resist, and the refractive index of any material that could be present in-between the last lens element and the resist. These limitations apply to any type of optical system currently in use and even apply to future imaging systems using liquid immersion lithography. Using this technique, very small features will be printed on the wafer using a relatively large wavelength as measured in the air. The originating mask features will be small as well and given by the following equation:MFS=mk1λ/(nNA) where MFS is the mask feature size. For example, if NA=0.85, k1=0.3, n=1.5, m=4, then MFS=0.67λ. The mask feature size is actually smaller than the wavelength. A magnification of 4 is the standard for today's scanners and steppers. While the mask feature size is small compared to the wavelength, the thickness of the mask absorber is relatively large compared to the wavelength, typically the chrome absorber thickness is on the order of 70 nm for advanced masks. The high aspect ratio of the mask features creates large scattering at the edges of the mask feature that will degrade the mask transmission and ultimately the quality of the aerial image printed on the wafer. Moreover the problem becomes more acute for alternating phase-shifting masks as the quartz substrate is etched to create a 180 degree phase-shift by a depth given by the following equation:d=λ/2(ns−1) where ns is the refractive index of the mask substrate. If the alternating aperture mask is created using an additive process, a shifter layer is coated and patterned on top of the chrome features. The thickness of the shifter layer is given by the following equation:d=λ/2(nsh−1) where nsh is the refractive index of the shifter layer. In the case of alternating aperture phase-shifting mask the aspect ratio is larger and the effect of scattering is more severe. For resolving smaller feature size compared to the exposure wavelength, a larger magnification should be used for example 5, 6 or 7. Integer number for the mask magnification is not required but it is preferred as it simplifies the scaling of the wafer data to create the mask. In particular, it makes it easier to put the mask data on a grid compatible with the mask write tools. The limitation brought up by mask scattering effects at the edge of high aspect ratio features and the idea of increasing the mask magnification accordingly applies to any types of optical system currently in use and even applies to future imaging systems using liquid immersion lithography. Another way to solve this problem would be to immerse the mask side of the imaging systems in high index liquid, or place the mask surface in contact or close proximity with a high index solid lens element, as well as the target wafer. This can be accomplished for the optical systems described in the present invention but also for any type of optical system currently in use and including future imaging systems using liquid immersion lithography. FIG. 9 shows a drawing representing such a system as an extension of the system described in FIG. 3a. A wafer 902 is placed on the stage 901. The projection lens 903 projects an image of the mask 904 on top of the wafer 902. The top of the lens is placed either in close proximity or in contact with the mask 904. The bottom of the lens is placed either in close proximity or in contact with the wafer 902. This type of set up would limit the constraints on the mask magnification described above. This type of set-up precludes the use of a pellicle on the mask. This type of set up can also be used in combination with negative refractive index slab as described in FIG. 7. In all the cases described in FIGS. 3a, 4, 5, 6, 7, 8, and 9 the distance between the lens and the wafer is set by some mechanical constraints. For example in contact mode as described in FIG. 4, no gap is left between the lens and the resist. The contact mode can be precisely adjusted by monitoring the pressure exerted by the wafer stage on the lens. For FIG. 6, the gap left between the lens and the resist is controlled very accurately since the evanescent waves are decaying exponentially. Any non-uniformity of the gap or any small change of the gap would lead to large differences in the image formed into the resist. One method to monitor the gap distance is to use an interferometer, where the waves reflected by the surface of the resist and the waves reflected by the surface of the lens (lens air interface) are interfered. As the distance between the lens and the resist is fixed because of the nature of the imaging technique used, the position of the image can be changed by adjusting the position of the mask with respect to the lens. On the other hand as the focus position inside the resist is set by the optical system when the wafer is in contact with the lens, a better control of the image position within the resist should be obtained compared to conventional exposure tool. For conventional exposure tools like steppers or scanners, the image position within the resist, i.e. the focus setting, is adjusted by moving up or down the wafer stage with respect to the lens which may lead to positioning inaccuracies. In addition the effect of vibrations or vibration coupling (liquid immersion) should be minimized in contact mode as described in the present invention. One of the main differences between this technique and current lithography techniques is that the lens is in close proximity or even in contact with the wafer which increases the risk of contamination of the lens and of the wafer. In close proximity the risk of contamination is lower but the control of the distance between the wafer and the lens is critical. On the other hand, contact printing offers the ultimate resolution but the largest risk of contamination. In order to prevent the contamination of the lens or of the wafer, a thin layer preventing the adhesion of the resist to the lens can be coated either on top of the lens or on top of the resist. This layer can be made for example of a fluoro-polymer similar to the one used for mask pellicle. This type of material presents the advantage of being transparent to short wavelengths and to reduce surface tension. The refractive index of this film should be high enough not to limit the resolution of the printed images by total reflection as described in FIG. 6. The resist chemistry can also be changed to lower surface tension. Resist can be made of fluoro-polymers combined with photo-acid generators. Upon irradiation, the photo-acid generator generates an acid that can de-protect chemical functions on the fluoro-polymer that are soluble in the resist developer. The resist would then be a positive resist. Alternatively soluble functions could be removed (negative resist) or photo-base generators could be used. In order to improve the contact between the lens and the wafer or to better control the gap between the lens and the wafer, the wafer surface should be as flat as possible. Techniques such as chemical mechanical polishing can be used to improve the wafer surface flatness. Dummy fill patterns placed in sparse areas of the layout can be combined with chemical mechanical polishing to further improve the flatness. The contact between the lens and the wafer can also be improved by using resists specially formulated to create a planar coating of the topography of the wafer. The resist can also be formulated to conform to the surface of the lens when the lens and the wafer are put in contact. To avoid any printing degradation due to the lens cleanliness, the surface of the lens that was in contact with the wafer can be inspected and cleaned after a certain number of exposures. Preferably, both the inspection and the cleaning systems should be placed either on the wafer stage or in the near proximity of the stage to minimize throughput loss. Many different inspection techniques can be used. A laser system can be used to scan the surface of the lens at a grazing incidence. Any change in the amount of light reflected by the surface indicates the presence of a defect. A pinhole combined with a detector could be used to monitor the uniformity of the aerial image of the lens. A phase measurement interferometer could be used to monitor the uniformity of the wave-fronts across the field. If defects have been found during the inspection step, the lens surface should be cleaned. The cleaning process can combine wet and dry cleaning methods similar to the techniques used for cleaning wafers or photo-mask in order to remove resist residues or particles. The portion of the lens that was in contact with the wafer could be immersed in liquid chemical capable of dissolving the resist residues, and then dried. The wet cleaning step could be performed in combination with acoustic waves (typically 750 to 1000 kHz) in order to dislodge any particle left on the lens surface. The portion of the lens that was in contact with the wafer could be placed in a plasma environment. The plasma could contain oxygen or nitrogen as the byproducts of the reaction of polymer with oxygen or nitrogen are volatile compounds like for example H2O, CO, CO2 in the case of oxygen. Another technique involves the use of UV radiation combined with an atmosphere of ozone. The UV radiation could be generated by an external source or could be generated by the exposure tool source itself. To address the issue where the lens surface cannot be cleaned, the lens could be built in such as way that the last element of the lens can be removed and replaced. For example the last lens element in contact with the wafer could be a disposable sheet of the high refractive index material. As the image on the mask is magnified compared to the expected wafer image, multiple exposures will be required to expose an entire wafer. FIG. 10a describes the required flow to expose and process an entire wafer. The wafer is aligned globally to the mask through an alignment step. The resist is first coated on the wafer, and then the wafer is loaded onto the stage. The wafer is aligned to the mask using global alignment keys on the wafer. The stage is moved to the location of the first field to be exposed. The stage is then moved up in contact or in proximity with the wafer. The resist is then exposed to the desired exposure dose. The stage is then moved down to release the wafer from the lens. If more fields need to be exposed the stage moves to the location of the next field to expose and the same procedure is repeated. When all the fields are exposed, the wafer is unloaded from the stage and the resist is processed. In FIG. 10b, the alignment is performed for each field individually. The flow is similar to the flow described on the right side except that the alignment is performed just before the exposure of each field with alignment keys placed on each field. If the lens is placed in contact with the wafer, the exposure for a given field is performed as a whole in which the entire field is imaged by the imaging system onto the wafer (stepper mode). If the wafer is placed in proximity of the lens, the entire field can be imaged as a whole (stepper mode) or the field can be scanned (scanner mode). The lens elements are typically circular. To avoid neighboring fields overlapping, the final lens element can be shaped into the geometry of the field, for example the final lens element may be rectangular. The typical field sizes for current stepper and scanner is a rectangle (or square) on the order of 20 to 40 mm in each direction. This reduced field size (typically less than 2000 mm2) compared with the previous non-magnified implementations (which needs to cover the entire wafer as a whole), in combination with printing with a magnified mask image (such that mask critical dimension tolerance is relaxed), makes contact and proximity printing described in the present invention more viable than the previous non-magnified implementations. The reduced field size also makes the resist less prone to sticking to the lens. As far as alignment is concerned, the global or local alignment systems can re-use existing alignment systems where the position of alignment keys on the mask is compared to the position of alignment keys on the wafer using an optical beam either going through the projection lens or going through an optical system adjacent to the projection lens. The exposure systems described in FIG. 3a, FIG. 7, and FIG. 9 could also be used to produce an image on the wafer without the need for a mask. In this case, the mask would be replaced by a radiation source. An image of the source would be created on the wafer in the form of a small exposed area. The image of the source could be scanned on the wafer using a deflection mechanism like an acousto-optics modulator or a rotating mirror placed between the source and the projection lens. The source could be turned on and off to create the desired pattern on the wafer. The image of the source on the wafer could have a range of possible shapes in order to re-create the desired wafer image. Despite the increased resolution provided by imaging into a high refractive index material, the distortion in the pattern transfer will require proximity effect correction as the feature size decreases. The design shapes must be modified in order to print the desired images on the wafer. To achieve an accurate correction of the layout, new models are required to account for the imaging into a high refractive index and to account for the evanescent waves coupling when the lens and wafer are in close proximity. Modeling can be approximated using a conventional lithography simulator. Denoting the refractive index of the last lens element material by n_s, modeling can be performed using a conventional simulator by specifying a wavelength given by the wavelength in vacuum divided by n_s. Refractive indexes and extinction coefficients (n,k) of all materials in the wafer stack should also be divided by n_s. Other parameters should remain unchanged. The simple approximation described above is adequate for first-order analysis. A more accurate modeling is necessary when the user is interested in secondary effects such as aberrations, focus errors, and the impact of mask topography on imaging. In these situations, the simulator should model the thin-film stack using n_s as the refractive index of the incident material. Spacing of the diffraction orders would change from 1/p to 1/(n_s*p) (where p is the period of the printed feature) to reflect the capturing of more diffracted harmonics. For proximity or contact printing in which evanescent waves play a non-negligible role, the model should also include extensions of thin-film optics. Alternatively, for the calculation of the propagation through the gap between the lens and the resist and the field inside the resist, Maxwell's equations can be solved using for example the FDTD (Finite Difference Time Domain) method. With these changes, modeling of secondary effects such as aberrations and mask topography becomes accurate. Using this methodology, an accurate model of the image process can be built for both contact and proximity modes. The model built using this methodology can be used in many applications. It can be used to accurately correct the original layout to compensate for proximity effects or to verify the printing of a given layout or the correction of a layout after the compensations have been performed. This model can also be used to calculate the image of a given layout, determine its dose and focus latitude, or evaluate the printability of defects. Other uses of the model are the calculations of the optimum shifter width for alternating aperture phase-shifting mask, the optimum design shape sizing for attenuating phase-shifting mask, the optimum assist feature size and distance from the main feature, and the optimum illuminator setting for off-axis illumination. FIG. 11 illustrates a computer system that can be used for one or more of the following tasks: correcting proximity effects on data layouts, verifying the correction, simulating the image of the layout, simulating the optimum shifter width for alternating aperture phase-shifting masks, simulating the optimum sizing for attenuating phase-shifting masks, simulating the optimum illuminator setting for off-axis illumination. This computer system represents a wide variety of computer systems and computer architectures suitable for this application. A processor 101 is connected to receive data indicating user signals from user input circuitry 1102 and to provide data defining images to display 1103. Processor 1101 is also connected for accessing mask layout data 1104, which define a mask layout under construction and a layout for a layer of material to be exposed using the mask. Processor 1101 is also connected for receiving instruction data from instruction input circuitry 1105, which can provide instructions received from connections to memory 1106, storage medium access device 1107, or network 1108. FIG. 12 illustrates the manufacturing process of an IC (Integrated Circuit). At step 1201, the layout file of the integrated circuit is first read using a computer system described in FIG. 11. At step 1202, the layout is corrected for proximity effect and the data is converted to mask data format. The correction of the data is only performed if required to meet the lithography specifications. The data resulting from step 1202 is used to create a mask at step 1203, and the mask is finally used in the fabrication process of an IC at step 1204. At least one lithographic step used to manufacture the IC in step 1204 uses one of the techniques described above from either, FIG. 3a, FIG. 7 or FIG. 9. In summary, the present invention as described in FIG. 3a, FIG. 7, and FIG. 9 requires substantial modifications of many aspects of the semiconductor manufacturing process. FIG. 13 summarizes some of the ramifications of this technique that were described in more details in the previous paragraphs. The use of the invention directly impacts the following areas: Semiconductor-MEMS-integrated optics manufacturing, Mask manufacturing, EDA (Electronic Design Automation), Resist Manufacturing, and Lithography Equipment Manufacturing. The data structures and code described in this description can be stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tapes, CD (compact discs) and DVD (digital video disks), and computer instruction signals embodied in a transmission medium. For example, the transmission medium may include a communication network, such as the Internet. The invention can be applied to any binary masks, rim phase-shifting masks, chromeless phase-shifting masks, attenuated phase-shifting masks, alternating aperture phase-shifting masks used in single or multiple exposure methodologies. While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims. |
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description | 1. Field of the Invention The present invention relates to a scanning apparatus which performs scan on an object with a charged particle beam, a drawing apparatus which includes the scanning apparatus and performs drawing on a substrate, and a method of manufacturing an article using the drawing apparatus. 2. Description of the Related Art A charged particle beam irradiation apparatus blanks a charged particle beam, emitted by a charged-particle source, in accordance with drawing data while making a deflector scan the charged particle beam to irradiate a predetermined position on a sample with a predetermined amount of charged particle beam. A charged particle beam drawing apparatus blanks a charged particle beam, emitted by a charged-particle source, in accordance with drawing data while making a deflector scan the charged particle beam to irradiate a predetermined position on a substrate with a predetermined amount of charged particle beam, thereby drawing a circuit pattern on the substrate. The blanking is an operation of switching between ON and OFF of the irradiation of the substrate with a charged particle beam. Controlling the timing of this switching operation makes it possible to control the time taken to irradiate a unit region with a charged particle beam. Also, the deflector can change the deflection amount of a charged particle beam by controlling a voltage applied across the electrodes. The drawing data is circuit pattern bitmap data generated from CAD data of circuit design. The charged particle beam drawing apparatus performs drawing in accordance with the drawing data, and therefore requires no circuit pattern mask used in the conventional exposure apparatus. Hence, various developments are in progress in order to reduce the running cost intended for a miniaturization process of increasing the mask cost, and limited production of a wide variety of products which require a large number of masks. The current mainstream charged particle beam drawing apparatus is an electron-beam exposure apparatus which uses an electron beam. An electron-beam exposure apparatus will be taken as an example hereinafter. The electron-beam exposure apparatus performs drawing while scanning an electron beam, and therefore has a low throughput. Hence, to improve the throughput, a method of drawing by simultaneously using a large number of electron beams has been proposed. In this case, it is possible to set a deflector for each electron beam to control its deflection amount. However, deflector electrodes, electrode driving circuits, applied voltage command circuits, and wiring lines which connect them to each other are required in numbers equal to the number of electron beams, entailing a high cost. Also, the requirement of a large number of circuits increases the probability that a failure will occur, thus increasing the maintenance load. For this reason, a method of guiding electron beams between the electrodes of one deflector to collectively deflect them is used. When electron beams are collectively deflected using one deflector, it is desired to deflect all electron beams in the same amount in the same direction. However, the deflection amount is different for each electron beam if the electric field produced between the deflector electrodes is not uniform. In such a case, because the applied voltage cannot be adjusted individually for each electron beam, International Publication No. 2010/134018 discloses a method of expanding/contracting drawing data in accordance with the deflection amount of each electron beam to draw a pattern at a desired position on a substrate. Also, because an error occurs in the relationship between the applied voltage and the deflection amount of each electron beam due, for example, to nonuniformity of the electric field between the deflector electrodes, Japanese Patent No. 4074240 proposes a method of correcting the applied voltage in accordance with the deflection amount using a minimum amount of data to obtain a desired deflection amount. A more uniform electric field can be produced when the electrodes used in the deflector are larger and are more sufficiently spaced apart from the region through which each electron beam passes. However, the use of such electrodes increases the size and cost of the deflector. Under the circumstances, the electric field between the deflector electrodes is not uniform and considerably varies not only in strength but also in direction especially near the ends of the electrodes. Therefore, electron beams which pass near the ends of the electrodes generate errors not only in the amount of deflection but also in the deflection direction with respect to desired values. In International Publication No. 2010/134018, drawing data is expanded/contracted for each electron beam to draw a predetermined pattern even if an error occurs in the deflection amount, but an error of the deflection direction is not corrected. Also, International Publication No. 2010/134018 describes neither an obtaining method nor a holding method for correction data indicating the amount of expansion/contraction of drawing data. Also, as the number of electron beams increases to improve the throughput, it becomes more difficult to correct errors for each electron beam. More specifically, since an enormous amount of correction data is set for each electron beam, the cost for holding data using, for example, a memory increases. Also, the measurement time for obtaining correction data, and the update time of the correction data become considerable. In Japanese Patent No. 4074240, in one electron beam, correction data is set for each region having a size that changes depending on the deflection amount, thereby reducing the required amount of correction data. However, it is necessary to provide correction data corresponding to a plurality of regions for each electron beam. Accordingly, with an increase in number of electron beams, the amount of correction data, the measurement time for obtaining correction data, and the update time of the correction data become considerable as well. The present invention provides, for example, a technique advantageous in compensating for a deflection error, of each of a plurality of charged particle beams, generated by a scanning deflector which collectively deflects the plurality of charged particle beams and performs scan on an object with the deflected plurality of charged particle beams. The present invention in its one aspect provides a scanning apparatus which performs scan on an object with a charged particle beam, the apparatus comprising: a blanking deflector configured to individually blank a plurality of charged particle beams based on control data; a scanning deflector configured to collectively deflect the plurality of charged particle beams to perform the scan; and a controller, wherein the controller is configured to hold first data used to obtain error in a scanning amount and a scanning direction of the scanning deflector relative to a reference scanning amount and a reference scanning direction with respect to each of the plurality of charged particle beams, and to generate the control data based on the first data so that the scan is performed for a target region on the object. Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. Embodiments of the present invention will be described below with reference to the accompanying drawings. In the following embodiments, an electron-beam exposure apparatus will be described as an irradiation apparatus which irradiates an object with a plurality of charged particle beams based on irradiation data indicating the target position on the object to be irradiated with each charged particle beam, and the target dose of this charged particle beam. However, the present invention is also applicable to charged particle beam irradiation apparatuses other than a charged particle beam drawing apparatus, such as an electron microscope and an ion beam injection apparatus. The present invention is moreover applicable to an irradiation apparatus which uses an ion beam other than an electron beam as a charged particle beam. FIG. 1 shows the configuration of an electron-beam exposure apparatus in the first embodiment. An electron optical system 10 is a multibeam electron optical system which generates an electron beam to be guided onto a substrate (wafer) 8. An electron beam emitted by an electron gun 11 is split into a plurality of beams by apertures 12. The plurality of split electron beams are individually blanked by a blanking deflector (blanker) 13 based on control data. The plurality of electron beams having passed through the blanker 13 are collectively deflected and scanned by two scanning deflectors (an X-deflector 21 and a Y-deflector 22) to irradiate a predetermined position on the wafer 8 set on a substrate stage (wafer stage) 9. A drawing data generation unit 1 generates drawing data drawn by the blanker 13, based on design data created by circuit design CAD. The generated drawing data is bitmap data indicating the pattern information of a semiconductor circuit. The drawing data generation unit 1 may directly generate drawing data from CAD design data, or accumulate generated drawing data in, for example, a memory and read it out. The drawing data generated by the drawing data generation unit 1 undergoes various types of correction by a drawing data correction unit 2 in accordance with the states of the electron-beam exposure apparatus and wafer 8, such that a desired position on the wafer 8 is irradiated with an electron beam having a desired intensity, thereby generating drawing data to be actually used for drawing. The various types of correction can include correction for compensating for a position error of each electron beam, correction for compensating for the intensity of this electron beam, and correction for overlaying a drawing pattern on a circuit pattern formed on the wafer 8. A blanker controller 131 drives the blanker 13 in accordance with drawing data to control the passage time of each electron beam. An X-deflector controller 211 and a Y-deflector controller 221 drive the X-deflector 21 and the Y-deflector 22, respectively, to collectively deflect the electron beams in the X- and Y-directions. This makes it possible to draw a predetermined pattern at a predetermined position on the surface of the wafer 8. The position of the wafer stage 9 with reference to the electron optical system 10 is calculated by a stage position measurement unit 61 based on the value measured by a position sensor 6. The wafer stage 9 can move in the X- and Y-directions, and position the wafer 8 at the electron beam irradiation position in response to a command from a wafer stage controller 91 based on the calculated stage position. A synchronization controller 5 controls the blanker controller 131, X-deflector controller 211, Y-deflector controller 221, and wafer stage controller 91 in accordance with a predetermined drawing procedure. Upon this operation, the blanker 13, X-deflector 21, Y-deflector 22, and wafer stage 9 operate in synchronism with each other, thereby drawing a circuit pattern at a predetermined position on the wafer 8. Each of the controllers 5, 131, 211, 221, and 91, the drawing data generation unit 1, the drawing data correction unit 2, a correction data output unit 3, an electron beam position measurement unit 71, and the stage position measurement unit 61 constitute a controller 100. FIG. 2 illustrates a detailed example of the procedure of drawing a semiconductor chip. Assume that the X-direction is the main scanning direction, and the Y-direction is the sub scanning direction. Assume also that m electron beams are juxtaposed in the X-direction, and n electron beams are juxtaposed in the Y-direction. First, the X-deflector 21, Y-deflector 22, and wafer stage 9 are controlled so that an upper left drawing grid 501 in a drawing area 500 of each electron beam is irradiated with this electron beam. The grid size is set to, for example, a half of the line width, that is, 11 nm for a line width of 22 nm, and 8 nm for a line width of 16 nm. In this case, the blanker 13 is driven to irradiate each drawing grid with an electron beam for a predetermined period of time determined for each drawing grid based on drawing data, thereby drawing a predetermined pattern. Each electron beam sequentially moves in the rightward, main scanning (X−) direction (a solid arrow) by the X-deflector 21, and a predetermined pattern is sequentially drawn on the grids with this movement. When drawing on one row is complete, the X-deflector 21 returns to the left end (a broken arrow) and starts drawing on the next row. At this time, the wafer stage 9 moves at a constant velocity in the upward, sub scanning (Y−) direction. The Y-deflector 22 adjusts the deflection amount with the movement of the wafer stage 9, and returns to the initial position to start drawing on the next row upon completion of drawing on one row. Therefore, the Y-deflector 22 can deflect a grid width corresponding to one row. By repeating this operation, a predetermined pattern can be drawn in the drawing area 500 of each electron beam. By parallelly drawing a predetermined pattern in adjacent drawing areas 500 with respective electron beams, this pattern can be drawn in a wide region with a high throughput. For example, when drawing is done using 100×100=10,000 electron beams, the throughput can be improved to ten thousand times. Although FIG. 2 illustrates an example in which the sequence of drawing in the main scanning direction has the same direction on each row, drawing can also be done so that the direction of the sequence of drawing reverses for every other row. FIGS. 3A to 3C illustrate examples of the arrangements of drawing areas 500. Each electron beam is guided to the same position in each drawing area 500. Referring to FIG. 3A, drawing areas 500 are arranged in a grid pattern, similarly to that shown in FIG. 2. Referring to FIG. 3B, drawing areas 500 are arranged in a staggered pattern in the Y-direction. This makes it possible to set wide spacings in the X-direction between electron beams on the same column, so it becomes easy to arrange, for example, wiring lines and cooling pipes and, in turn, to fabricate the apertures 12 and blanker 13. Referring to FIG. 3C, the drawing areas 500 shown in FIG. 3B are further divided into groups to ensure wiring line and cooling pipe spaces and to ensure a given strength upon equipment with beams. The wafer stage 9 is further moved in the X-direction to draw a predetermined pattern in the divided spacings. FIG. 4 is a schematic view of the arrangement of electron beams in the X-deflector 21 when viewed from the optical axis direction of each electron beam. Note that coordinates (X, Y) are deflector coordinates representing the arrangement of electron beams in the deflector, and have, as their origin, the center of an electron beam passage region in the deflector. A region through which electron beams pass is sandwiched between parallel plate electrodes 212 and 213 from two sides along the X-direction. For the sake of simplicity, only a small number of electron beams are shown in FIG. 4, but a large number of electron beams pass through the electron beam passage region in correspondence with the arrangement of drawing areas shown in one of FIGS. 3A to 3C in practice. Electron beams may be deflected using one deflector, or may be grouped to set deflectors in one-to-one correspondence with groups. In the case of the arrangement of drawing areas shown in, for example, FIG. 3C, deflectors may be set in one-to-one correspondence with groups. An electric field is produced between the electrodes by applying a voltage across the electrodes. Each electron beam is deflected in accordance with the direction and strength of the electric field. For example, each electron beam is deflected in the rightward direction, as described by a deflection vector indicated by an arrow in FIG. 4, by applying a negative voltage to the electrode 212, and a positive voltage to the electrode 213. At this time, to obtain a correct drawing result, all electron beams are expected to be deflected and scanned in the same amount in the same direction. However, the electric field between the electrodes is not uniform especially at the ends of the electrodes, so the strength of the electric field varies and the direction of the electric field is not exactly the X-direction. This is because the electric field spreads outwards from the parallel plate electrodes 212 and 213. The uniformity of the electric field also degrades as the electrodes are not parallel to each other due to manufacturing errors. Note that the same applies to the Y-deflector 22 upon replacement of the X-axis with the Y-axis. That is, the deflection amount (scanning amount) and deflection direction (scanning direction) of at least one of a plurality of electron beams by each of the X-deflector 21 and Y-deflector 22 have errors with respect to a reference deflection amount (reference scanning amount) and a reference deflection direction (reference scanning direction), respectively. The reference deflection amount and the reference deflection direction are the deflection amount and the deflection direction, respectively, at, for example, the center of the electron beam passage region in the deflector. FIGS. 5A and 5B show the influence that the differences in direction and strength of the electric field from a predetermined direction and strength have on the deflection error and the drawing result. Note that coordinates (x, y) are local coordinates for each electron beam, and represent the position of a beam grid 503 to actually undergo drawing relative to a data grid (target region) 502 to undergo drawing. The coordinates (x, y) have, as their origin, a data grid position to undergo drawing while the deflector is kept stopped, so the midpoint of the drawing area 500 is normally defined as a local coordinate origin to uniformly deflect each electron beam to the two sides of the origin upon reverse of the direction of the electric field. Alternatively, a grid to undergo drawing first in the drawing area may be defined as an origin to apply a voltage across the deflector electrodes so as to deflect it in one direction. The following description assumes that each electron beam is used to correctly draw a predetermined pattern at a desired origin grid position while the deflector is kept stopped. FIG. 5A shows the state in which a predetermined electric field is obtained. By applying a predetermined voltage corresponding to the number k of the data grid 502 across the electrodes, each electron beam can be deflected as:(x,y)=(a·v·k,0) (1)where v is the unit voltage applied to the deflector, so the actually applied voltage is v·k in accordance with the parameters of each grid, and a is the deflection sensitivity to v. Since the electric field runs exactly in the X-direction, the electron beam is not deflected in the Y-direction. FIG. 5B shows the case wherein the electric field has a direction different from a predetermined direction. The deflection amount at this time is given by:(x,y)=(α·v·k,β·v·k) (2)where (α, β) is a two-dimensional deflection vector. When the strength of the electric field varies, α≠a, so a predetermined deflection amount cannot be obtained in the X-direction. Also, when the direction of the electric field varies, β≠0, so other components are generated in the Y-direction. When this happens, the result of actually deflecting each electron beam exhibits the beam grid 503 indicated by a broken line, which does not coincide with the data grid 502 indicated by a solid line. The deflection vector (α, β) is data (first data) indicating errors in deflection amount and deflection direction of each electron beam with respect to a reference deflection amount and a reference deflection direction, respectively. As the number of electron beams increases, the deflection width of each electron beam becomes sufficiently smaller than the distance between the electrodes. When, for example, 100×100 electron beams are used, the deflection width becomes 100/1 or less of the distance between the electrodes. Hence, the electric field can be regarded as uniform within the range of the deflection width of each electron beam. This means that α and β are constant in each individual electron beam. However, the deflector coordinates are different for each electron beam, so α and β have values different for each electron beam. If a deflector is set individually for each electron beam, the beam grid 503 of this electron beam can be matched with the data grid 502 by individually adjusting the voltage applied to the deflector. However, as shown in FIG. 4, when one deflector collectively deflects a large number of electron beams, the voltage applied to it cannot be adjusted individually for each electron beam. This means that all beam grids 503 of electron beams cannot be matched with data grids 502. Hence, the drawing data is corrected to correctly perform drawing even when the beam grid 503 is different from the data grid 502. FIGS. 6A to 6C illustrate examples of how to correct drawing data. FIG. 6A shows drawing data indicating the data grid 502 as a target region on the substrate to be irradiated with an electron beam, and its target dose. Referring to FIG. 6A, drawing is done by irradiating a 3×3 region with an electron beam. Assume that the beam grid 503 of an electron beam then becomes a beam grid as indicated by a broken line of FIG. 6B due to errors in deflection amount and deflection direction. Since each electron beam is used to draw a predetermined pattern along the beam grid 503, it is necessary to assign drawing data of the data grid 502 to originally undergo drawing to the beam grid 503. To meet this requirement, the area overlapping the data grid to undergo drawing is evaluated for each beam grid 503. The electron beam intensity for the beam grid 503 is determined in accordance with the drawing area in the beam grid 503, and the intensity of an electron beam used to irradiate this area. That is, drawing data is corrected so as to irradiate a target region with an electron beam in a target dose corresponding to the drawing data, using an electron beam to be guided to the target region, and an electron beam to be guided to a position adjacent to the target region. In an example shown in FIGS. 6A to 6C, as shown in FIG. 6C, a 2×2 central beam grid 503 completely falls within the drawing range, and therefore fully undergoes drawing. A 4×4 beam grid 503 around the 2×2 central beam grid 503 includes both the drawing range and non-drawing range, so the electron beam intensity is determined in accordance with the area of the drawing range. For example, the electron beam intensities of 12 grids other than four central grids among 4×4 grids are 70% for two grids, 50% for six grids, 20% for two grids, and 10% for two grids. The electron beam intensity is adjusted by controlling the ON time of each electron beam for each grid using the blanker. In this way, drawing data for the beam grid 503 is generated from the drawing data of the data grid 502 to perform drawing using the drawing data. To generate drawing data for the beam grid 503, it is necessary to obtain a deflection vector (α, β) for each electron beam. This is done by detecting the position of each electron beam using an electron beam sensor (detector) 7, and measuring the deflection amount. A Faraday cup or a CCD sensor, for example, is used as the electron beam sensor 7. The electron beam sensor 7 moves the wafer stage 9 so as to irradiate it with an electron beam to be measured, thereby measuring the electron beam intensity. The electron beam position measurement unit 71 calculates the position of an electron beam with reference to the electron optical system 10 based on the value measured by the electron beam sensor 7, and the stage position information at the time of measurement. α and β can be obtained from the difference between the reference position of each electron beam while the deflector is kept stopped (while no voltage is applied across the deflector electrodes) and the position of this electron beam while the deflector is driven. Obtained α and β are held in the correction data output unit 3, and used to correct the beam grid 503 by the drawing data correction unit 2. Note that the correction data output unit 3 holds various correction parameters other than α and β. A calibration process of the irradiation position and deflection amount of each electron beam by the electron beam position measurement unit 71 will be described with reference to FIG. 7. In step S1, the electron beam position measurement unit 71 performs position measurement using the electron beam sensor 7 for all electron beams while the deflectors 21 and 22 are kept stopped (first deflection state). With this operation, the difference between the data grid 502 and the beam grid 503 can be measured at the local coordinate origin of each electron beam. In step S2, the electron beam position measurement unit 71 calculates an electron beam position correction amount (offset amount) based on the measurement result obtained in step S1. The electron beam position correction amount calculated in step S2 constitutes second data indicating an error in irradiation position of each charged particle beam while this charged particle beam is not deflected by the deflector. The electron beam position measurement unit 71 obtains an offset amount which errors the drawing data in the X- and Y-directions so as to match the origins of the data grid 502 and beam grid 503. The obtained offset amount is stored in the correction data output unit 3 and used for correction. In step S3, the electron beam position measurement unit 71 uses to measure the electron beam irradiation position of a reference electron beam using the electron beam sensor 7 while a predetermined voltage is applied across the deflector electrodes. The reference electron beam is normally an electron beam around the origin of the deflector coordinates (X, Y), that is, around the center of the deflector. At this time, a plurality of electron beams around the center of the deflector may be measured to determine their average as a measurement result. A voltage nearly corresponding to the deflection width of the drawing area 500 can also be used. In step S4, the electron beam position measurement unit 71 determines a deflection voltage which matches the data grid 502 and the beam grid 503 for the reference electron beam, based on the measurement result obtained in step S3. Upon this operation, a deflection sensitivity a in equation (1) is obtained to determine a unit applied voltage v. The obtained unit applied voltage v is sent to the X-deflector controller 211 to determine a voltage applied to the X-deflector 21. In step S5, the electron beam position measurement unit 71 measures the electron beam irradiation position using the electron beam sensor 7 while a voltage (reference deflection voltage) corresponding to the deflection width of the drawing area 500 determined from the unit applied voltage v obtained in step S4 is applied across the deflector electrodes (second deflection state). Although it is desired to measure all electron beams, they may be thinned at a predetermined interval to measure some of them, because adjacent electron beams generate measurement results with little difference. Also, a plurality of adjacent electron beams may be measured to determine their average as a measurement result. In step S6, the electron beam position measurement unit 71 obtains the deflection amount of each electron beam when a voltage is applied to the deflector, based on the measurement results obtained in steps S1 and S5. Upon this operation, α and β in equation (2) are determined. Obtained α and β are stored in the correction data output unit 3 and used for correction. A correction process of the position and amount of deflection of each electron beam will be described next with reference to FIG. 8. In step S11, the drawing data correction unit 2 reads out α and β stored in the correction data output unit 3 to calculate the deflection correction amount of beam grid k to undergo drawing for the coordinate values (x, y). In step S12, the drawing data correction unit 2 reads out the offset amount of each electron beam stored in the correction data output unit 3 to calculate an electron beam position correction amount for beam grid k to undergo drawing. In step S13, a drawn circuit pattern mark position is measured for each wafer, independently of the above-mentioned calibration process. Based on this information, the drawing data correction unit 2 obtains the amount of error, amount of rotation, and magnification of drawing data to be used for drawing upon precise overlay of the drawn circuit pattern on the wafer to calculate the coordinate position of the data grid 502 in accordance with these parameters. In step S14, the drawing data correction unit 2 sums the coordinate values upon individual correction, which are calculated in steps S11, S12, and S13, thereby obtaining the coordinate value (x, y) of beam grid k to undergo drawing. In step S15, the drawing data correction unit 2 reads out pixel data corresponding to the calculated beam grid coordinate values from the drawing data generated by the drawing data generation unit 1 to obtain drawing data indicating the electron beam intensity, that is, the electron beam ON time using the above-mentioned method based on the area ratio. In step S16, the drawing data correction unit 2 reflects correction for the intensity of each electron beam on the drawing data as needed. The drawing data correction unit 2 supplies the obtained drawing data to the blanker controller 131 to make the synchronization controller 5 drive the blanker 13 in synchronism with the set of deflectors 20 and the wafer stage 9. The above-mentioned correction operation is repeatedly executed for each beam grid. A large number of process circuits may be set to parallelly process a plurality of grids and a plurality of electron beams. As described above, the deflection amount of each electron beam is measured to correct drawing data, thereby deflecting a large number of electron beams using only one deflector. This makes it possible to accurately perform drawing even if errors are generated in the deflection amount and deflection direction of each electron beam based on the electric field distribution in the deflector. It takes much time to individually measure the amounts of deflection of a large number of electron beams in step S5 described in the first embodiment. On the other hand, a deflection vector (α, β) can be obtained by simulation. More specifically, an electric field generated by a deflector is simulated to obtain the state (strength and direction) of the electric field at each electron beam position. Further, the behavior of each electron beam as this electron beam passes through the obtained electric field is simulated to obtain a deflection vector (α, β). The deflection vector of each electron beam obtained by simulation can be stored in the correction data output unit 3 and used for correction. A deflection vector need only be calculated in advance during, for example, design. Hence, steps S5 and S6 can be omitted from the calibration process in the first embodiment shown in FIG. 7. A correction process of drawing data at this time is the same as in that of the first embodiment shown in FIG. 8. In the second embodiment, a process of measuring a deflection vector can be omitted from the calibration process, thus significantly shortening the calibration time. The deflection vector of each electron beam shown in FIG. 4 has a given correlation with the deflector coordinates. Especially when electrodes with a shape symmetrical about a coordinate system, such as parallel plate electrodes, are used, a deflection vector can be expressed as a relatively simple approximation for the coordinate values. Since the distribution of a deflection amount α in the X-direction in equation (2) is axisymmetrical about the X- and Y-axes, its approximation can be expressed as an even-order polynomial such as a quadratic or quartic polynomial. This approximation can be expressed as, for example, a quadratic polynomial: α = f ( X , Y ) = 1 + f 10 X + f 20 X ⋀ 2 + f 01 Y + f 02 Y ⋀ 2 + f 11 X Y ( 3 ) where the constant of the first term is 1 assuming that a unit applied voltage v is calibrated in steps S3 and S4. Since the distribution of a deflection amount β in the Y-direction is point-symmetrical about the X- and Y-axes, its approximation can be expressed as an odd-order polynomial such as a linear or cubic polynomial. This approximation can be expressed as, for example, a cubic polynomial: β = g ( X , Y ) = g 00 + g 10 X + g 20 X ⋀ 2 + g 30 X ⋀ 3 + g 01 Y + g 02 Y ⋀ 2 + g 03 Y ⋀ 3 + g 11 X Y + g 12 X Y ⋀ 2 + g 21 X ⋀ 2 Y ( 4 ) With this expression, as long as the coefficients f10 and f11 in equation (3) and the coefficients g00 to g21 in equation (4) are obtained, a deflection vector (α, β) can be obtained based on equation (4) from the deflector coordinates (X, Y) of each electron beam. The types and coefficient values of equations (3) and (4) can be obtained by simulating the electric field of the deflector, as in the second embodiment. At this time, the order of the polynomial is determined so that the error between a deflection vector (α, β) obtained by an approximate polynomial and a directly obtained deflection vector (α, β) falls below a tolerance. Although the tolerance of the error varies depending on the accuracy and process condition of each unit of the drawing apparatus, the error of the beam grid determined based on the calculated deflection vector (α, β) desirably falls below about 1/10 of the grid size. The types and coefficient values of equations (3) and (4) can also be obtained by measuring the deflection amount of each electron beam, as in the first embodiment. The coefficients of an approximate polynomial can be calculated using, for example, the least-squares method by substituting the deflection amount of each electron beam into the approximate polynomial. As for the types of equations (3) and (4), a low-order polynomial as mentioned above is desirably used so as to obtain a deflection vector by a small amount of calculation, a polynomial need not always be used. A trigonometric or exponential function can also be used. A calibration process of the position and amount of deflection of each electron beam in this embodiment will be described with reference to FIG. 9. Unlike the process shown in the second embodiment, instead of calibration of the deflection amount, drawing can be done using approximate polynomial coefficients obtained from the measurement result or by simulation in advance. The case wherein calibration is performed with a change in state of the apparatus will be described herein. Steps S1 to S4 are the same as in the first embodiment. In step S7, a voltage corresponding to the deflection width of a drawing area 500 determined from the unit applied voltage v obtained in step S4 is applied to the deflector electrodes for a reference electron beam. An electron beam position measurement unit 71 measures the electron beam position at that time using an electron beam sensor 7. Unlike step S5, in step S7, only reference electron beams in a number that allows the coefficients in equations (3) and (4) to be specified need only be measured. To improve the measurement accuracy, electron beams near the electrode ends considerably spaced apart from the vicinity of an origin serving as a reference are measured as reference electron beams. However, a plurality of adjacent electron beams may be measured to determine their average as a measurement result. In step S8, the electron beam position measurement unit 71 obtains the deflection amount of each reference electron beam when a voltage is applied to the deflector, based on the measurement results obtained in steps S1 and S7. Upon this operation, α and β in equation (2) are determined. Obtained α and β are substituted into equations (3) and (4) to obtain the coefficients of the polynomials f and g using the least-squares method. The obtained coefficients are stored in a correction data output unit 3 and used for correction. Also, depending on the variation factors, it is often unnecessary to individually obtain each coefficient. α = f ( X , Y ) = 1 + ( f 10 X + f 20 X ⋀ 2 + f 01 Y + f 02 Y ⋀ 2 + f 11 X Y ) F ( 3 ′ ) β = g ( X , Y ) = ( g 00 + g 10 X + g 20 X ⋀ 2 + g 30 X ⋀ 3 + g 01 Y + g 02 Y ⋀ 2 + g 03 Y ⋀ 3 + g 11 X Y + g 12 X Y ⋀ 2 + g 21 X ⋀ 2 Y ) G ( 4 ′ ) For example, as long as the coefficients f10, f11, and g00 to g21 remain the same when α and β are transformed into those in equations (3′) and (4′), the coefficients F and G as representatives of the entire equations need only be calibrated. In this case, the number of electron beams which measure the amounts of deflection can further be reduced. A correction process of the position and amount of deflection of each electron beam will be described next with reference to FIG. 10. Except for the addition of step S10, this process is the same as in FIG. 8 of the first embodiment. In step S10, a drawing data correction unit 2 reads out the coefficients of the polynomials f and g stored in the correction data output unit 3, and substitutes them into the polynomials f and g, together with the deflector coordinate values (X, Y) of each electron beam, thereby calculating α and β. This calculation can be done simply by a product-sum operation, so the amount of calculation increases only a little. In step S11, the drawing data correction unit 2 calculates the coordinate values (x, y) of beam grid k to undergo drawing, using α and β calculated in step S10. In the third embodiment, it is only necessary to measure the amounts of deflection of only a small number of reference electron beams, thus significantly shortening the calibration time. Also, although α and β must be stored in the correction data output unit 3 for all electron beams in the first embodiment, only the coefficients F and G of the same polynomials f and g need only be stored for all electron beams in the third embodiment, thus greatly reducing the storage capacity. A correction process of the position and amount of deflection of each electron beam in the fourth embodiment will be described with reference to FIG. 11. The fourth embodiment is different from the third embodiment in that the correction process is divided into an offline process for each calibration operation and an online process for each chip. In the offline process, steps S10, S11, and S12 are performed, correction of only the position and amount of deflection of each electron beam is applied to drawing data, and intermediate drawing data is generated in step S17. Calibration is done when, for example, the drawing apparatus is started up, drawing of a plurality of wafers is complete, or a predetermined time has elapsed from the previous calibration operation. Note that the correction parameters are updated, so the drawing pattern is corrected with this operation. On the other hand, the overlay correction amount is obtained in step S13 by measurement for each wafer, and the correction parameters are different for each chip having a pattern to be drawn on the wafer. Hence, steps S13, S18, and S16 are performed every time one chip undergoes drawing. In step S18, the overlay correction amount generated in step S13 is applied to the intermediate drawing data S17 generated in the offline process. In this embodiment, since the amount of process executed online can be reduced, the scale of the process circuit can also be reduced. The offline process can be executed in parallel with the calibration process, and therefore does not directly influence the throughput of the apparatus. Hence, the scale of the process circuit can be reduced using a software process to reduce the apparatus cost although the process time prolongs. [Method of Manufacturing Article] A method of manufacturing an article according to an embodiment of the present invention is suitable for manufacturing an article including a semiconductor device or an original (it can also be called, for example, a reticle or a mask). This manufacturing method can include a step of drawing a pattern on a substrate, coated with a photosensitive agent, using the above-mentioned charged particle beam drawing apparatus, and a step of developing the substrate having the pattern drawn on it. In manufacturing a device, this manufacturing method can also include subsequent known steps (for example, oxidation, film formation, vapor deposition, doping, planarization, etching, resist removal, dicing, bonding, and packaging). While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. This application claims the benefit of Japanese Patent Application No. 2011-258217 filed Nov. 25, 2011, which is hereby incorporated by reference herein in its entirety. |
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claims | 1. A particle beam radiotherapy system comprising:a gantry configured to apply particle beams to a subject;an ultrasonic diagnostic apparatus configured to scan the subject with ultrasonic waves via an ultrasonic probe, and acquire an ultrasonic image concerning a radiotherapy target region of the subject;processing circuitry configured tospecify a first planned point of a Bragg peak in the ultrasonic image, which anatomically coincides approximately with a second planned point of the Bragg peak determined using a radiotherapy planning apparatus, the first planned point and the second planned point being positioned so as to be included in the radiotherapy target region, andestimate a sighting point of the Bragg peak of a particle beam applied by the gantry based on information concerning a body surface position of the subject and an actual range of the particle beam applied by the gantry; anda display configured to display the ultrasonic image so as to indicate the first planned point and the sighting point. 2. The particle beam radiotherapy system of claim 1, wherein the information concerning the body surface position is a body surface region concerning the subject in the ultrasonic image. 3. The particle beam radiotherapy system of claim 2, wherein the processing circuitry decides placement of the ultrasonic probe for acquiring the ultrasonic image in particle beam radiotherapy, based on an irradiation direction of the particle beam applied from the gantry. 4. The particle beam radiotherapy system of claim 3, wherein the processing circuitry decides a vicinity of an incident point of the particle beam from the gantry into a body surface of the subject as the placement of the ultrasonic probe, based on the irradiation direction. 5. The particle beam radiotherapy system of claim 3, wherein the processing circuitry decides the placement of the ultrasonic probe so that both of the radiotherapy target region and an incident point of the particle beam from the gantry into the body surface of the subject are included in an ultrasonic scanning region. 6. The particle beam radiotherapy system of claim 3, wherein the processing circuitry decides the placement of the ultrasonic probe so that both of the radiotherapy target region and an incident point of the particle beam into an organ including the radiotherapy target region are included in the ultrasonic scanning region. 7. The particle beam radiotherapy system of claim 2, further comprising control circuitry configured to switch between irradiation and a stoppage of the irradiation of the particle beam by the gantry based on a comparison between an actual range of the particle beam applied by the gantry and an ideal range from the body surface region to the first planned point. 8. The particle beam radiotherapy system of claim 7, whereinthe processing circuitry registers a medical image targeted for the radiotherapy target region acquired in radiotherapy planning with the ultrasonic image,specifies a third planned point of the Bragg peak in the registered medical image, which anatomically coincides approximately with the second planned point, anddecides a distance from a body surface region in the registered medical image to the third planned point as the ideal range. 9. The particle beam radiotherapy system of claim 1, further comprising control circuitry,wherein the processing circuitry identifies a radiotherapy target region included in the ultrasonic image by image processing, andwherein the control circuitry switches between irradiation and a stoppage of the irradiation of the particle beam by the gantry based on the radiotherapy target region or a position of the first planned point and the actual range. 10. The particle beam radiotherapy system of claim 1, further comprising a position sensor mounted on the ultrasonic probe,wherein the processing circuitry detects three-dimensional coordinates of the ultrasonic probe in a real-space coordinate system based on an output signal from the position sensor,specifies coordinates of a radiotherapy target region in an image coordinate system which defines the ultrasonic image, andestimates coordinates of the sighting point in the real-space coordinate system or the image coordinate system based on the actual range by using the coordinates of the ultrasonic probe in the real-space coordinate system and the coordinates of the radiotherapy target region in the image coordinate system. 11. The particle beam radiotherapy system of claim 10, wherein the processing circuitry detects coordinates of an incident point of the particle beam from the gantry into the body surface of the subject in the real-space coordinate system. 12. The particle beam radiotherapy system of claim 11, wherein the detected coordinates are used as information concerning the body surface position. 13. The particle beam radiotherapy system of claim 2, further comprising control circuitry configured to modulate energy of the particle beam applied from the gantry in accordance with a difference between the actual range of the particle beam applied from the gantry and an ideal range from the body surface region to the first planned point so as to reduce the difference. 14. The particle beam radiotherapy system of claim 13, further comprising storage circuitry configured to store in each of a plurality of differential values an association with a modulation value of the energy of the particle beam for reducing the differential value to be approximately zero,wherein if the difference is within a permissible amount, the control circuitry decides a modulation value corresponding to the difference based on the association, and modulates the energy of the particle beam applied from the gantry in accordance with the decided modulation value. 15. The particle beam radiotherapy system of claim 13, further comprising:a display configured to display an inquiry of whether to modulate the energy of the particle beam; andinput circuitry configured to input a response to the inquiry,wherein the control circuitry modulates the energy of the particle beam applied from the gantry if a response of adopting is input via the input circuitry. 16. A particle beam radiotherapy system, comprising:a gantry configured to apply particle beams to a subject;an ultrasonic diagnostic apparatus configured to scan the subject with ultrasonic waves via an ultrasonic probe, and acquire an ultrasonic image concerning a radiotherapy target region of the subject;processing circuitry configured tospecify a first planned point of a Bragg peak in the ultrasonic image, which anatomically coincides approximately with a second planned point of the Bragg peak determined using a radiotherapy planning apparatus, the first planned point and the second planned point being positioned so as to be included in the radiotherapy target region, andestimate a sighting point of the Bragg peak of a particle beam applied by the gantry based on a predetermined anatomical region inside a body of the subject specified by the ultrasonic image and an actual range of the particle beam applied by the gantry; anddisplay configured to display the ultrasonic image so as to indicate the first planned point and the sighting point. 17. A particle beam radiotherapy system, comprising:a gantry configured to apply particle beams to a subject;an ultrasonic diagnostic apparatus configured to scan the subject with ultrasonic waves via an ultrasonic probe, and acquire an ultrasonic image concerning a radiotherapy target region of the subject;processing circuitry configured tospecify a first planned point of a Bragg peak in the ultrasonic image, which anatomically coincides approximately with a second planned point of the Bragg peak determined using a radiotherapy planning apparatus, the first planned point and the second planned point being positioned so as to be included in the radiotherapy target region, andestimate a sighting point of the Bragg peak of a particle beam applied by the gantry based on information concerning a body surface position of the subject and an actual range of the particle beam applied by the gantry; andcontrol circuitry configured to switch between irradiation and a stoppage of the irradiation of the particle beam by the gantry based on a comparison between an actual range of the particle beam applied by the gantry and an ideal range from a body surface region to the first planned point. |
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052271293 | abstract | A corrosion resistant metallic coating (60) of zirconium nitride is applied to the cladding tube (40) of a nuclear fuel rod (20). The zirconium nitride is reactively deposited on a zirconium-alloy cladding tube by a cathodic arc plasma deposition process. The zirconium nitride coating provides superior wear test results and enhances the corrosion resistance of the cladding tube. |
abstract | A method is provided for coating the substrate of a component, such as a zirconium alloy cladding tube, for use in a water cooled nuclear reactor under normal operating conditions and under high temperature oxidation conditions. The method includes heating a pressurized carrier gas to a temperature between 200° C. and 1200° C., adding chromium or chromium-based alloy particles having an average diameter of 20 microns or less to the heated carrier gas, and spraying the carrier gas and particles onto the substrate at a velocity, preferably from 800 to 4000 ft./sec. (about 243.84 to 1219.20 meters/sec.), to form a chromium and/or chromium-based alloy coating on the substrate to a desired thickness. |
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061480622 | description | DETAILED DESCRIPTION OF THE INVENTION In the figures, the same elements are identified by the same reference numbers. Reference is more particularly made to FIG. 3, in which a first embodiment of an X-ray beam-shaping filter 10 of variable area has been depicted. This filter, like the filter of the prior art in FIG. 2, comprises a main frame 11 provided with sliding rails 13, 14 and two compensating plates 17, 18 made of X-ray-absorbent material. Since the general structure is similar to that of the conventional filter in FIG. 2, in particular with regard to the arrangement of the plates 17, 18 on the rails 13, 14 by means of carriages (not depicted), reference should be made to the previous description of this figure for greater details. The shaping filter 10 of one embodiment of the invention differs from that of the prior art by the arrangement of the compensating plates 17, 18. Given that the compensating plates 17, 18 are identical, the following description relating to one of the plates applies in extends to the other compensating plate. The compensating plate 17 comprises a first plate element 17a having the general shape of a biconvex meniscus comprising a first end joined conventionally to a carriage which can move translationally on the rail 13 and a second end, opposite the first, provided with a pivot pin 20. A second plate element 17b, having the general shape of a scythe blade, has a first end mounted so as to pivot on the pivot pin 20 and an enlarged second end, opposite the first, having a curved elongate slot 22. A stud 23 projects from the first plate element 17a in order to fit into the elongate curved slot 22 at the enlarged second end of the second plate element 17b. A stress spring 21, placed around the pivot pin 20, is joined by one of its ends, respectively, to the first and second plate elements 17a, 17b. Finally, a stop 24 is provided on the main frame 11 in order to keep the second plate element 17b in its initial position, as will be seen later. In FIG. 3, the plate 17 has been depicted in its retracted position in which the first and second plate elements 17a, 17b are in their initial position in which these plate elements overlap almost entirely. In contrast, the plate 18 has been depicted in its final active position and the plate elements have been depicted in their maximum deployed position in which the first and second plate elements 17a and 18b now overlap only over a minimum area 25 along the outer edge of the first plate element 18a and along the inner edge of the second plate element 18b. Those parts of the plate elements 18a and 18b which correspond to this overlap area 25 are bevelled so that the overlap area 25 has a thickness identical to the rest of the plate 18, as may be seen in FIG. 6. The plate elements 18a, 18b have an identical thickness in their non-bevelled parts. Although the second plate elements 17b, 18b have been depicted as pivoting above the first plate elements 17a, 18a, it is also possible to place them in the same manner below the first plate elements 17a, 18a. The operation of the filter 10 is described with reference to FIGS. 3 to 6. Initially, the plates are in the retracted position, depicted in FIG. 3 in respect of the plate 17, in which the first plate element 17a is pushed back by sliding at one end of the rail 13 as far as a position in which, by rotation under the effect of the bearing force of the stop 24 against the spring 21, the stud 23 bears on one end of the slot 22 and the plate elements are in their initial position and overlap almost entirely. When the user wishes to shape the field of view of a region 1 of a patient under examination which includes a wide area 2 of low absorption density, so as to compensate for the lowest absorption of the X-ray beam 3 by the low-density area 2 as depicted in FIG. 5, he moves the first plate element, for example 18a, by sliding it along the rail 14 in order to end up over the low-density area. Under the action of the spring 21, the second plate element 18b pivots about the pin 20 and is deployed, the sliding of the stud 23 in the slot 22 during pivoting of the second plate element causing this element to pivot uniformly. At this stage, the plate is in an active position. Referring more particularly to FIG. 4, the plate 17 is brought by the user into an intermediate active first position in which the overlap area 25 of the plate elements 17a and 17b is large, but as may be seen in FIG. 4 this relatively large overlap area 25, because of the shape and size of the plate elements, lies entirely over the frame 11 and, consequently, only part of the plate element 17a lies in the field of view 19 of the X-ray beam and is active in order to absorb part of this X-ray beam. Since the thickness of this plate element 17a is constant, the compensation produced is uniform. The other plate 18, in the case of FIG. 4, is in an intermediate active position close to the final active position and, as may be seen in FIG. 4, the plate element 18b was deployed under the effect of the spring 21 and the overlap area of the plate elements 18a, 18b is almost the minimum, but it lies slightly within the field of view 19. However, because of the fact that this small overlap area 25 corresponds to appropriate bevelled parts of the plate elements 18a, 18b, the thickness in this overlap area 25 is almost equal to the thickness of the rest of the plate element 18a lying in the field of view 19. Uniform compensation over the entire desired part of the field of view 19 is therefore obtained. Referring again to FIG. 3, the plate 18 has been depicted in its final active position in which the spring 21 has pivoted the plate element 18b until the end of the slot 22 butts against the stud 23. In this position, the area of the field of view 19 covered by the plate is the maximum area. The overlap area 25 of the plate elements 18a, 18b is the minimum and, as previously, because of the bevelling of the corresponding parts of the plate elements, has a thickness equal to the remaining parts of the plate elements. Thus, uniform compensation over the entire area of the field of view covered by the plate 18 is obtained. Depicted in FIGS. 7 and 8 is a filter according to another embodiment of the invention which differs from the filter described above by the fact that the plate elements 17a, 17b and 18a, 18b can be moved in relative translation, one with respect to the other, and by the means allowing this relative translation of the plate elements. The second plate element 18a, 18b is a curved plate of almost constant width provided at both its ends with parallel straight slots 31, 32. Since the slots 30, 31 allow the second plate element 18a, 18b to move translationally along two studs 32, 33 fixed to the first plate element 17a, 17b. Return springs 34, 35 are fixed both to the support plate and to the second plate element 17b, 18a. FIG. 7 depicts the plate 17 in its initial retracted position in which the overlap of the plate elements is the maximum. In this position, the plate rests on the stops 36. When the user moves the first plate element 18a, by sliding it along the rail, in order to bring it into a first intermediate active position, as depicted in FIG. 7, the second plate element 18b remains stationary. The overlap area 25 remains large but it lies entirely over the main frame 11 and only part of the first plate element lies in the field of view 19. Because of the constant thickness of the plate element, the X-ray beam is therefore uniformly attenuated. When the user brings the first plate element into the position depicted in respect of the plate 18 in FIG. 8, the second plate element 18b has still not been moved, but the studs 32, 33 butt against the front end of the slots 30, 31. The overlap area 25 of the plate elements is the minimum area and, because of the fact that the plate elements have suitable bevelled parts in this minimum overlap area 25, the thickness remains constant. When the user slides the first plate element into the final active position of the plate 17 in FIG. 8, the second plate element 17b is driven by the first plate element 17a under the action of the studs 32, 33 on the front ends of the slots 30, 31 and against the return force of the springs 34, 35. Of course, conventional locking means are provided in order to keep the plates in the positions chosen by the user. In the position of the plate 17 depicted in FIG. 8, the area of the field of view covered by the plate is the maximum. Although the minimum overlap area 25 lies within the field of view, the thickness of material through which the X-ray beam passes remains constant, for the reasons given above, and uniform attenuation is obtained. Although the embodiments of the invention have been described with filters having two rails and two plates, it is possible to produce filters having a single rail and a single plate, or more than two rails and two plates, for example four rails and four plates diametrically opposed in pairs. Various modifications in structure and/or function and/or steps may be made by one skilled in the art to the disclosed embodiments without departing from the scope and extent of the invention. |
050248057 | description | DETAILED DESCRIPTION The present method for decontaminating metal surfaces having an oxide coating containing radioactive substances, such as the primary system surfaces of a pressurized water nuclear reactor, uses an aqueous solution of weak chelants and iron (II) or ferrous iron. The weak chelant maintains the dissolved metals in solution and prevents precipitation, while the ferrous iron improves the dissolution rate and minimizes base metal corrosion. The radioactive metals that are to be removed in a pressurized water reactor primary system include ferric iron (Fe.sup.III), nickel, chromiun, cobalt and manganese, which are metals forming the primary system components. The process uses an aqueous decontamination solution containing a weak chelant, capable of forming multiligand complexes with the metals of the oxide coating, in an amount of between 0.1 to 2.0 percent by weight based on the weight of the solution. The weak chelants are complexing agents generally having an equilibrium constant for metal ions, such as ferric ions, of between about 10.sup.12 to 10.sup.19. Examples of such chelants are nitrilotriacetic acid (NTA), hydroxyethylenediamine tetraacetic acid (HEDTA), citric acid, and iminodiacetic acid (IDA), with NTA being preferred because of its high iron capacity, multiligand ability, and relatively low complexation constant. Preferably, the concentration of the chelant is about 0.2 percent based on the weight of the aqueous solution. The use of less than about 0.1 percent chelant will not keep the ions in solution and chelate ions removed from the surface, while more than about 2.0 percent is inefficient and unnecessary. In addition to the weak chelant, the aqueous solution contains an organic ferrous salt in an amount to provide a ferrous iron (Fe.sup.II) concentration of between about 50 to 500 parts per million (ppm) based on the weight of solution. If less than about 50 ppm ferrous iron is present, the decontamination will not be effected, while more than about 500 ppm is inefficient and wasteful. Preferably about 100 ppm of ferrous iron of such an organic ferrous salt is used. These salts are ferrous salts of polyfunctional organic acids that are compatible with the materials of the primary system during operation of the pressurized water nuclear reactor. Organic acids are required to form the ferrous salts because inorganic acids can leave residual ions that can cause corrosion problems in the reactor during subsequent operations, whereas organic acids decompose to produce water and carbon dioxide. Such ferrous salts include ferrous acetate, ferrous oxalate, and ferrous gluconate. While the latter two ferrous salts are relatively insoluble in water, the same will dissolve in dilute chelant solutions. The ferrous iron (Fe.sup.II), with NTA, provides for reduction dissolution of the metal oxide with rapid kinetics (equations 1 and 2): ##EQU1## Multiple ligand complexes can then form. Corrosion of the base metal is inhibited by reactions such as equation 3, as compared to equation 4 for ferric ion corrosion: ##EQU2## The presence of a relatively large concentration of ferrous iron (Fe.sup.II) shifts the equilibrium and also inhibits ferric iron (Fe.sup.III) corrosion by equation 4. Additional ferrous iron is provided during decontamination. During the decontamination, the metal oxide film dissolves, and iron is present generally as ferric iron (Fe.sup.III). This can be reduced in a sidestream, electrolytic reactor using porous electrodes, as described in U.S. Pat. No. 4,537,666, assigned to the assignee of the present invention and incorporated by reference herein, i.e.: ##STR1## The electrolytic approach is effective for concentrated solutions (say 1 wt %), and will provide for a gradual buildup of ferrous iron (Fe.sup.II). However, entire loop decontamination will use dilute solutions, and will require a consistent ferrous iron (Fe.sup.II) presence throughout the application for corrosion and kinetic purposes. After passing the decontamination solution over the metal surface to remove radioactive substances therefor, the solution is regenerated and returned for further contact with those surfaces. Regeneration may be effected by treating a portion or sidestream thereof, either by use of cation exchange resins or electrolytically. The use of cation exchange resins to remove contaminants and recover reagents for reuse in decontamination methods is known. Solution regeneration by cation exchange is somewhat complicated, here, however, as ferrous iron (Fe.sup.II) complexes are more readily removed than ferric iron (Fe.sup.III) complexes. It is thus advisable to valve in an ion exchange column after the method has been running for a period of time, e.g. two hours. Electrolytic regeneration is the preferred regeneration method since it preferentially reduces the ferric iron (Fe.sup.III), albeit at a reduced efficiency in the dilute solution. Such electrolytic regeneration, as described in U.S. Pat. No. 4,537,666, passes the decontamination solution through a permeable electrode formed by a stainless steel wire or copper mesh in order to plate out the ions. When the electrode becomes spent, it is replaced. Or, as described in U.S. Pat. No. 4,792,385, assigned to the assignee of the present invention, the contents of which are incorporated herein, the permeable electrode may be comprised of a bed of carbon, or graphite particles, or an electrically conductive plastic material such as polyacetylene. Regardless of the method of regeneration used, however, slipsream regeneration of a large pressurized water reactor will have a long time constant, such as approximately 6 hours, and thus, will be incomplete. The time for decontamination of a pressurized water invention system using a present process would be expected to be in a range of about 6 to 24 hours. The temperature of the decontamination solution does not need adjustment and will typically be at a temperature of 70.degree. C. to 150.degree. C. during the decontamination method. The present process thus provides a chemical method for decontaminating pressurized water nuclear reactor systems utilizing a ferrous salt in the decontamination solution with the benefits described herein. |
description | The present application is a continuation of U.S. patent application Ser. No. 12/024,071, filed Jan. 31, 2008, now U.S. Pat. No. 8,345,813, which in turn claims the benefit of U.S. Provisional Patent Application Ser. No. 60/887,505, filed Jan. 31, 2007, the entireties of which are incorporated herein by reference. The present invention relates generally to apparatus, systems and methods for transporting high level waste “HLW”), such as spent nuclear fuel rods, and specifically to low profile translation of high level waste containment casks. In the operation of nuclear reactors, the nuclear energy source is in the form of hollow zircaloy tubes filled with enriched uranium, typically referred to as fuel assemblies. When the energy in the fuel assembly has been depleted to a certain level, the assembly is removed from the nuclear reactor. At this time, fuel assemblies emit both considerable heat and extremely dangerous neutron and gamma photons (i.e., neutron and gamma radiation). Thus, great caution must be taken when the fuel assemblies are handled, transported, packaged and stored. To protect the environment from radiation exposure, spent nuclear fuel is both transported and stored in large cylindrical containers called casks. A transfer cask is used to transport spent nuclear fuel between locations while a storage cask is used to store spent nuclear fuel for a determined period of time. Casks are typically designed to shield the environment from the dangerous radiation in two ways. First, shielding of gamma radiation requires large amounts of mass. Gamma rays are best absorbed by materials with a high atomic number and a high density, such as concrete, lead, and steel. The greater the density and thickness of the blocking material, the better the absorption/shielding of the gamma radiation. Second, shielding of neutron radiation requires a large mass of hydrogen-rich material. One such material is water, which can be further combined with boron for a more efficient absorption of neutron radiation. The transfer cask must perform the vital function of providing adequate radiation shielding for both neutron and gamma radiation emitted by the enclosed spent nuclear fuel. The transfer cask must also be designed to provide adequate heat transfer. Guided by the shielding principles discussed above, transfer casks are made of lead or a gamma absorbing material and contain a neutron absorbing material as well. As stated previously, greater radiation shielding is provided by increased thickness and density of the gamma and neutron absorbing materials. The weight of a fully loaded transfer cask is typically in the range of 100-125 tons. Similarly, storage casks are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because storage casks are not handled as much as transfer casks, the primary focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of spent nuclear fuel. Size and weight are at best secondary considerations. As a result of maximizing the thickness of radiation absorbing materials, the weight and size of storage casks often cause problems associated with lifting and handling. Typically, storage casks weigh approximately 150 tons and have a height greater than 15 ft. A common problem is that storage casks cannot be lifted by nuclear power plant cranes because their weight exceed the rated capacity of the crane. A common problem arises when the fully loaded transfer cask must be transported to the storage cask for the canister transfer procedure. Generally, the storage cask is located in a truck bay, or other location outside of the staging area. To get to the transfer cask, the storage cask may have to pass through a door of a nuclear planes truck bay. The doors are typically 17-24 feet tall. The transfer casks are typically about 16 feet and 3 inches tall. The need to move casks into and out of enclosed facilities limits the size and shape of machines that can be used to move the casks. For example, a low ceiling in such a facility makes it impractical to use a boom or overhead crane to lift and transport casks. Similarly, a doorway not much larger than the cask itself limits the extent to which a lifting device can extend beyond the sides, top or bottom of the cask. Thus, a need exists for a low profile transporter that can withstand the weight of the storage cask. It is therefore an object of the present invention to provide an apparatus for translating casks having a low profile. It is another object of the present invention to provide an apparatus for translating casks that can withstand high moment forces. A further object of the present invention is to provide an apparatus for translating casks that can be raised and lowered. A still further object of the present invention is to provide an apparatus for translating casks that can fit through an overhead, door. Another object of the present invention is to provide an apparatus that supports a cask during spent nuclear fuel transfer procedures. A still further object of the present invention is to provide an apparatus that can translate a cask close to the surface of the ground while avoiding interference with irregularities on the floor surface. A yet further aspect of the present invention is to provide an apparatus for translating casks that has a variable height. These and other objects are met by the present invention which in one aspect can be an apparatus for translating a nuclear waste storage cask comprising: a body for supporting a cask; and at least two rollers adapted to move between a retracted position and an extended position, wherein when the rollers are in the retracted position, the rollers do not contact a ground surface. In another aspect the invention can be an apparatus for translating a cask comprising: a body comprising a top surface, an open top end and a cavity for receiving a cask, at least one support member for supporting a cask close to a ground surface; and rollers for translating the apparatus. In a yet further aspect the invention can be A system for translating spent nuclear fuel comprising: an apparatus for supporting and translating a cask comprising: a body having an open top end and a cavity for receiving a cask; and at least two rollers adapted to move between a retracted position and an extended position; a cask positioned in the cavity, wherein the cask is supported close to a ground surface. In another aspect the invention can be a method of supporting and translating a storage cask comprising the steps of: (a) providing an apparatus comprising: to body for supporting a cask; and at least two rollers adapted to move between a retracted position and an extended position (b) placing a cask onto the body of the apparatus; loading spent nuclear fuel into the cask; (c) moving the rollers of the apparatus into the extended position; and (d) translating the apparatus. In yet another aspect, the invention can be a system for translating high level radioactive waste across a ground surface, the system comprising: a cask loaded with high level radioactive waste; an apparatus comprising: a body having a cavity having an open top end; and a plurality of rollers coupled to the body to be adjustable between: (1) an extended position in which the plurality of rollers contact the ground surface and support the body above the ground surface; and (2) a retracted position in which the plurality of rollers do not contact the ground surface and the body contacts the ground surface; and the cask positioned in the cavity, the cask supported above the ground surface by the apparatus when the plurality of rollers are in the extended position. In still another embodiment, the invention can be a system for translating high level radioactive waste across a ground surface, the system comprising: a cask loaded with high level radioactive waste; an apparatus comprising: a body having a cavity having an open top end; a plurality of rollers coupled to the body; and at least one contact member coupled to the body and extending into the cavity; a lower portion of the cask positioned in the cavity so that the cask rests atop the at least one contact member and is above the ground surface, an upper portion of the cask protruding from the open top end of the cavity. In a further embodiment, the invention can be a system for translating high level radioactive waste across a ground surface, the system comprising: a cask loaded with high level radioactive waste; an apparatus comprising: a body having a cavity having an open top end; and a plurality of rollers coupled to the body; the cask supported by the apparatus above the ground surface so that a lower portion of the cask is positioned within the cavity and an upper portion of the cask protrudes from the open top end of the cavity. FIG. 1 is a front perspective view of a low profile transporter (“LPT”) 100. The LPT 100 translates casks that are used in the handling and storing of spent nuclear fuel, such as storage casks used in the long-term dry storage of spent nuclear fuel. The LPT 100 is not limited to storage casks, however and other types of casks and/or structures can be translated in the LPT 100. The LPT 100 supports and translates loads in excess of 200 tons and high overturning moments without deforming. As will be discussed below, the LPT 100 carries the cask close to the ground while avoiding interference with the irregularities in the ground surface. The LPT 100 can either ride in tracks 90 on the ground surface 6 (shown in FIG. 9) or move along the ground surface 6 itself. Preferably, as will be discussed in greater detail below, the LPT 100 supports and translates a cask so that the top surface of the cask is less than 24 feet from the ground. Additionally, the LPT 100 has a width W1 that allows it to fit through standard over head door. The LPT 100 is designed so that its width W1 is smaller than its length. Referring to FIGS. 1 and 2, the LPT 100 generally comprises a body 10 and roller assemblies 60. The body 10 has an open top end 12 and a cavity 23 for receiving a cask. The cavity 23 has a horizontal cross-sectional profile that is circular. The diameter of the cavity 23 is slightly larger than the diameter of the cask 200 (shown in FIG. 7) to be loaded therein. Preferably, there is a small clearance between the cask. 290 (shown in FIG. 7) and the body 10 equal to about ½ inch to 1 inch. The invention is not so limited however, and the size and shape of the cavity 23 will vary depending upon the size and shape of the cask to be positioned therein so long as the small clearance is maintained between the cask and the body 10. The roller assemblies 60 include rollers 61 that allow for the translation of the LPT 100. The roller assemblies are positioned at opposite ends of the LPT 100 so that the width W1 of the LPT 100 is minimized. When the LPT 100 is in motion, the rollers 61 are at the lead and trail ends of the motion. This makes it easier for the LPT 100 to fit through narrow passageways like overhead doors. Additionally, as will be discussed in further detail below with reference to FIGS. 4 and 5, the roller assemblies 60 are designed so that the rollers 61 can be extended and retracted in the vertical direction between a fully extended position and a fully retracted position. When the rollers 61 are in the extended position, the rollers 61 contact the ground and support the full weight of the ZPT 100 and its load. When the rollers 61 are in the retracted position, they are moved in the vertical direction so as to be raised from the ground so that the rollers no longer support the weight of the ZPT 100 or its load. Preferably, when the rollers 61 are in the fully extended position, the height of the LPT 100 is less than 37 inches. The body 10 comprises a ring 20, an upper plate 40 and a lower plate 50. The ring 20 has an outer surface 21, an inner surface 22, a top surface 24 and a bottom surface 25. The ring 20 is preferably made of steel between 2 and 4 inches in thickness. The invention is not so limited, however, and other materials, including but not limited to other metals, may be used. The thickness of the ring 20 will vary depending upon the material used to form the ring 20. The inner surface 22 of the ring 20 forms the cavity 23 for receiving a cask. The ring 20 further comprises a plurality of notches 26 at the bottom surface 25. The notches 26 are rectangular shaped cutouts that will be discussed in more detail below. The upper plate 40 and the lower plate 50 are connected to the outer surface 21 of the ring 20 and provide structural robustness to the LPT 100. The upper plate 40 is connected to the outer surface 21 of the ring 20 at or near the top surface 24. The upper plate 40 extends laterally from the outer surface 21 of the ring 20. The upper plate 40 comprises a top surface 41 and a bottom surface 42. The ring 20 is fitted into an opening (not visible) in the upper plate 40 and then the tipper plate 40 and the ring 20 are welded together along their connection area. Other attachment means between the ring 20 and the upper plate 40 may be used, such as bolts, fasteners and/or fastening techniques, if desired. Alternatively the ring 20 and the upper plate 40 could be a unitary structure. The upper plate 40 is preferably made of steel and/or other metals. Where the upper plate 40 is made of steel, it is preferably between 1 and 2 inches thick. The lower plate 50 is connected to the ring 20 at or near the bottom surface 25 of the ring 20. The lower plate 50 comprises a top surface 51 and a bottom surface 52. The ring 20 is fitted into an opening (not visible) in the lower plate 50 and the ring 20 and the lower plate 50 are then welded together along, their contact area. Other attachment methods may be used however including mechanical means like bolts, fasteners and the like. The lower plate 50 is preferably made of steel having a thickness of between 1 and 2 inches. The invention is not so limited, however, and other materials of various thicknesses may be used. The body 10 of the LPT 100 further comprises cover plates 16. The cover plates 16 are positioned spaced from the outer surface 21 of the ring 20 and extend from the bottom surface 42 of the upper plate 40 to the top surface 51 of the lower plate 50. The cover plates 16 are preferably made of steel or another metal. The invention is not so limited, however, and other materials may be used. The cover plates are preferably welded to the upper and lower plates 40, 50. Other attachment means may be used however, including, mechanical means such as brackets, bolts, fasteners and the like. The upper plate 40 and the lower plate 50 protrude at the front and rear end of the LPT 100 so as to form the top and bottom, respectively of two housings 30 for the roller assemblies 60. The housings 30 for the roller assemblies 60 are positioned equidistant from each other, or 180 degrees apart. Each housing 30 comprises two outer plates 31 and two inner plate 32 that surround the roller assemblies 60. The outer plates and the inner plates 31, 32 extend laterally from the cover plate 16 to the lateral edge of 53 of the lower plate 50. The outer and inner plates 31, 32 additionally extend from the bottom surface 42 of the upper plate 40 to the top surface 51 of the lower plate 50. Preferably, the outer plates 31 of the housing 30 are welded to the cover plate 16 and to the upper and lower plates 40, 50. Other attachment means may be used however, including mechanical means such as fasteners, bolts, brackets and the like. The inner and outer plates 31, 32 are preferably 2 to 4 inches thick and made of steel. Other materials of different thicknesses may be used however, including other metals, so long as the robustness of the LPT 100 is maintained. The housing 30 further comprises a plurality of reinforcement plates 33 for structural stability. There are three reinforcement plates 33 positioned between the inner plates 32 and extending from the top plate 40 to the bottom plate 50. Preferably, the reinforcement plates 33 are welded to the top plate and the bottom plate 50, but other attachment means may be used. The reinforcement plates 33 extend laterally from the cover plate 16 to the lateral edge 53 of the lower plate 50 and are preferably welded to the cover plate 16. The LPT 100 further comprises a plurality of support members 70. In the illustrated embodiment there are four support members 70. The invention is not so limited, however, and more or less support members 70 may be used so long as they can support the weight of a fully loaded storage cask. In operation, the support members 70 contact a cask to be supported and translated in the LPT 100. Each one of the support members 70 comprises a contact plate 72 and three support plates 74. As will be discussed in further detail with reference to FIG. 3, the support plates 74 are L-shaped plates having a portion which extends into the cavity 23 near the bottom of the cavity 23. The support plates 74 extend through the notches 26 in the ring 20. The notches 26 are slightly larger than the support plates 71 in order provide a passageway for the support plates 74 of the support members 70 to extend through the ring 20 into the cavity 23. Preferably the support plates 71 are welded to the ring 20 so that there is no movement between the support members 70 and the ring 20. The contact plate 72 is connected with the portion of support plate 74 that extends into the cavity 23. The contact plate 72 also contacts and supports a shoulder of the cask. The shoulder of the cask could either be an opening or cutout in an outer surface of the cask or a ledge, ridge, flange or other protrusion from the outer surface of the cask. The contact plate 72 is a rectangular plate made of two sections, a top section 75 and a bottom section 76. The top section 75 is in surface contact with the cask and is therefore made of a softer material so as to not damage the cask. The bottom section 76 is made of steel of another metal. The top section and bottom section 75, 76 of the contact plate 72 are connected to each other using either mechanical means, welding or gluing. The contact plate 72 is positioned near the bottom of the cavity 23 and are sufficiently robust to support the weight of a cask loaded into cavity 23. Additionally, the contact plate 72 could be a ring rather than rectangular plates, additionally, the contact plate 52 could be a bar extending the entire diameter of the body 20, so long as a surface that engages the cask is created. Referring now to FIG. 3, the LPT 100 is shown with a cover plate 16 removed on one side. The support plates 74 of the support members 70 are attached to the outer surface 22 of the ring 20. The support plates 74 extend from the bottom surface 42 of the upper plate 40 and through slots in the bottom plate 50 so that a bottom portion of the support plates 74 extends through the bottom plate 50. The support plate 74 additionally extends laterally from the body 20 to the cover plate 16 (shown in FIG. 2). The support plate 74 is preferably welded to the body 20, but other attachment means may be used. Each support member 70 comprises three support plates 74, but the invention is not so limited, and more or less support plates 74 may be used so long as the support member 70 is able to withstand the weight of a fully loaded storage cask without deforming. The LPT 100 further comprises a plurality of reinforcement plates 45 are attached to the outer surface 21 of the ring 20 to provide structural integrity. The reinforcement plates 45 are arranged in series around the outer surface 23 of the ring 20. FIG. 3 shows three reinforcement plates 45. The invention is not so limited however and there could be less or more reinforcement plates 45 per group, arranged closer or farther apart along the ring 20. The reinforcement plates 45 are generally rectangular in shape and preferably made of steel and/or other metals. The invention is not so limited however, and the shape and material of the reinforcement plates 45 can vary. The reinforcement plates 45 extend between the top and bottom plates 40, 50 and are preferably attached to the top and bottom plates 40, 50 by welding. Other attachment means may be used, such as bolts, fasteners and/or fastening techniques, if desired. Referring now to FIG. 4, a close-up view of section IV-IV of FIG. 3 showing the roller assembly 60 is illustrated so that its various components and their interaction with one another is more clearly visible. The roller assembly 60 comprises rollers 61, a horizontal beam 62, a jack 63, a locking ring 64, low-friction plates 65 and base plate 66. The jack 63 is designed to raise and lower the rollers 61 between the fully extended position (illustrated) and the fully retracted position (not shown). When the rollers 61 are in the fully retracted position, they do not contact a ground surface. As shown in FIG. 2, the LPT 100 comprises tour roller assemblies each having, a jack 63. The four jacks 63 are hydraulic jacks having hydraulic hoses (not illustrated) extending from a power skid unit not illustrated) comprises controls for operating, the jacks 63. The jacks 63 are designed to work in conjunction with each other so that no jack 63 will lift or lower the roller 61 independent of the other jacks. This avoids the tipping of the LPT 100. Additionally, other means of controlling the jacks 63 may be used including via motors that powered remotely of the LPT 100. The locking ring 64 is designed to keep the jack 63 from accidentally releasing and thereby dropping the load in the LPT 100. The locking ring 64 is a metal ring, that is threaded on its inner surface. When the jack 63 is powered so that the rollers 61 are in the extended position, the locking ring is locked into place so that loss of hydraulic power to the jack 63 will not cause the load to be dropped. The rollers 61 are bolted to the base plate 66 which comprises an indentation 68 (shown in FIG. 5) for the cylinder of the jack 63. The indentation 68 keeps the jack 63 from moving in the lateral direction. The base of the jack 63 is connected to the horizontal beam 62. The horizontal beam 62 is a steel I-beam that spans between the inner plate 31 and the outer plate 32. The ends of the horizontal beam 63 extend through openings in the inner and outer plates 31, 32. A cover is put on the end of the horizontal beam 63 that protrudes from the outer plate 32. The cover is bolted to the outer surface of the outer plate 31. The low-friction plates 65 are rectangular shaped pads made of nylon or another low-friction material having the capability to withstand high compression loads. The low-friction plates 65 are positioned between a vertical beam 69 and the outer and inner plates 31, 32 of the housing 30. When the jack 63 moves the rollers vertically, the low-friction plates 65 move along the outer and inner plates 31, 32. The low friction plates 65 provide support for the roller assembly 60 so that the rollers 61 do not supinate or pronate, meaning the roller assembly 60 does not bend inwards or outwards (horizontally) at any point from the moment forces caused by the weight of the load in the LPT 100. Rather the roller assembly 60 moves only vertically. The roller assembly 60 further comprises a shims 67. The shim 67 is preferably a rectangular thin metal plate, such as steel or aluminum. The shim 67 can be constructed of other materials and in other shapes if desired. The shim 67, provide a simple way to change the overall height of the LPT 100 because the height of the shim 67 can be easily varied. The shims 67 are positioned between the base plate 66 and the rollers 61. The rollers 61 are best illustrated in FIG. 5. Referring to FIG. 5, a roller 61 is illustrated removed from the LPT device 100. Although a particular roller design is illustrated, the rollers 61 can be any kind of linear motion device including devices commercially available. The rollers 61 comprise wheels 92, inner plate 93 and outer plate 94. The inner and outer plates 93, 94 keep the wheels 92 vertically in line so that they do not supinate or pronate. The inner and outer plates 93, 94 additionally acts as a connection to a rail of a track (shown in FIG. 8) in the ground surface that may be used with the LPT 100. Referring now to FIG. 6, a cask 200 is shown positioned above the LPT 100. A cutout 210 can be seen in the bottom surface 212 of the cask 200. As stated previously, the cask 200 could have a ridge or lug extending outwardly to engage with support members 70. The cutout 210 is aligned with the contact plate 72 so that the cask 200 can be lowered through the open top end of the body 20, into the cavity 23 until the contact plate 72 slides through the cutout 210 and engages the cask 200, as shown in FIG. 7. Referring now to FIG. 7, the cask 200 is shown positioned in the LPT 100. The contact plates 72 are positioned close to the bottom plate 50 of the LPT 100 so that when the cask 200 rests on the support members 70, the cask 200 can sit as close to the ground floor 6 as possible without the bottom surface 212 engaging the irregularities in the floor surface 6 (shown in FIG. 8). Preferably the distance between the bottom surface 212 of the cask 200 and the surface of the ground floor 6 is between 0.1 and 6 inches, more preferably between 0.1 and 3 inches, and most preferably between 0.1 and 1 inch. Referring now to FIG. 8, the LPT 100 is designed to glide along rails 90 of a ground floor 6. More specifically rollers 60 are designed to fit within rails 90 so that the LPT 100 can be laterally moved (i.e. translated) along the floor 6. The bottom surface of container 212 does not touch the floor 6. The LPT 100 is designed so that the loaded container 200 can be transported underneath a doorway 14 (shown in FIG. 9). Referring to FIGS. 6 through 8, a method of supporting and translating as storage cask 200 will now be discussed. The LPT 100 is positioned on the ground surface 6. The cask 200 is raised using a transporter 220 or any other crane like apparatus so that the bottom surface 212 of the cask 200 clears the tops surface of the upper plate 40 of the LPT 100. The cutouts 210 in the bottom surface 212 of the cask 200 are aligned with the contact plates 72 of the support members 70. the cask is lowered through the open top 12 end of the body 10 and into the cavity 23 of the LPT 100. The cask 200 is lowered until the cutouts 210 of the cask 200 are resting on the contact plate 72 of the support members 79. The rollers 61 are vertically moved into the extended position so that the rollers contact the ground surface 6. The LPT 100 is translated along the rails 90 (or on a ground surface having no rails). The rollers 61 are vertically moved so as to be raised from the ground surface. The support members 74 contact the ground surface 8 and the rollers 61 no longer support any weight of the LPT 100 or cask 200. Spent nuclear fuel is then loaded into the cask 200 by means known in the art. Such means of transferring spent nuclear fuel into storage casks including by resting a transfer cask (not shown) on top of the cask 200 so that the spent nuclear fuel can be transferred from the transfer cask into the storage cask. In such a transfer method, the full weight of the transfer cask and the cask 200 are supported by the LPT 100. The transfer cask is then removed from the cask 200. The rollers 61 of the LPT 100 are vertically moved into the extended position so that the rollers contact the ground surface 8. The LPT 100 and the fully loaded cask 200 are then translated. The LPT 100 and the cask 200 may be translated underneath overhead door 14. The LPT 100 can engage with the cask transporter 220 so that the cask 200 can be raised out of the cavity 23 until the bottom surface 210 of the cask 200 clears the top surface of the upper plate 40. The LPT 100 can be translated from underneath the cask 200 and the cask 200 lowered to the ground surface 6. The foregoing description of the preferred embodiment of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. |
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054815797 | summary | TECHNICAL FIELD This invention relates generally to boiling water nuclear reactors and, more specifically, to a latching and lifting mechanism facilitating removal of fuel rods from fuel bundle assemblies within such reactors. BACKGROUND It is well known that fuel rods in boiling water reactor (BWR) fuel bundles are supported on a lower tie plate and extend upwardly to an upper tie plate. Between the upper and lower tie plates, the fuel rods are supported laterally by a plurality of spacers. In some BWR, bundle designs, the upper and lower tie plates are directly connected to each other by several special fuel rods, called tie rods. These tie rods have lower end plugs which are threadably secured in the lower tie plate. The tie rods also have threaded upper end plugs which extend through the upper tie plate. Conventional nuts are used on these upper end plugs to secure the tie rods to the upper tie plate. Special lock washers are used to prevent rotation of the nuts. Specifically, the washers are formed with tabs which are bent upward to lock the nuts in position and thus prevent rotation. The upper tie plate typically includes a handle extending upwardly from its base. To lift the fuel bundle, a grapple engages this upwardly extending handle. The weight of the fuel rods is transmitted to the lower tie plate, through the tie rods to the upper tie plate, and then to the handle. Occasionally, a fuel bundle must be disassembled partway through its service life or at the end of its use. At such time, the lock washer tabs are bent to free the nuts, and the nuts and washers are then removed. Thereafter, the upper tie plate can be removed followed by extraction of one or more fuel rods as necessary. The invention here relates to a different constructions for transferring the fuel bundle weight to the upper tie plate, and for latching and unlatching the upper tie plate from the fuel bundle to facilitate removal of fuel rods from the bundle. DISCLOSURE OF THE INVENTION In accordance with this invention, highly reliable latching and lifting mechanisms are provided for fuel bundles in boiling water reactors. More specifically, the invention here relates to BWR fuel bundles which incorporate a pair of large water rods, each occupying a space which would otherwise be occupied by four fuel rods. The water rods typically have lower end plugs inserted into the lower tie plate and upper end plugs inserted into the upper tie plate. These end plugs are free to slide vertically within holes in the tie plates. The present invention utilizes these water rods to lift the fuel bundle. To this end, the lower water rod end plugs are rigidly attached to the lower tie plate by suitable means (e g, by threaded attachment), but the specific manner of attachment to the lower tie plate forms no part of this invention per se. A latching mechanism (three embodiments are disclosed) is utilized to attach the upper end plugs to the upper water rod tie plate Thus, when the fuel bundle is lifted, the load is transferred from the fuel rods to the lower tie plate, through the water rods to the latching mechanism and then to the upper tie plate and finally to the lifting handle. In an exemplary embodiment, the upper tie plate is formed with an enlarged double boss for receiving the end plugs for the pair of water rods. This double boss is formed integrally with the tie plate and is connected to the other fuel rod bosses in the upper tie plate by a series of relatively narrow webs similar to those webs which interconnect the various fuel rod bosses. This enlarged double boss is formed with a centering hole located between the pair of bosses and adapted to receive a center post utilized to center and secure a latching bar on the double boss. The double boss area is also formed with a forwardly extending projection midway between the bosses projecting normal to a line between the boss centers. The projection has a vertical hole on a center axis which is parallel to and located forwardly of the centering hole. At the same time, the upper ends of the water rods are provided with end plugs having upper ends formed with axially extending cut-outs which, in the assembled orientation, face each other. A latching bar in accordance with this first exemplary embodiment is formed with a vertically extending central through bore adapted to receive the centering pin, enabling the latching bar to be properly oriented on the double boss of the upper tie plate. The latching bar is also formed with lower ends laterally projecting in opposite directions and, as explained below, rotatable into and out of the cut-outs provided on the end plugs of the water rods. This latching bar is also formed with a forwardly extending projection containing a through hole, the center line or axis of which may be aligned with the projection hole in the enlarged double boss. The lower surface of the latching bar is provided with a centrally located, elongated rib extending from front to back in a direction perpendicular to the oppositely and outwardly extending lower end portions of the latching bar. With the latching bar in place on the double boss, and after the upper tie plate is lowered into position such that the end plugs (including the axial cut-outs) of the water rods project through the double boss of the upper fie plate, the latching bar may be rotated such that the oppositely extending lower end portions move into the cut-out areas of the end plugs. Springs are located on the end plugs so that when the lower tie plate is lowered into position as described above, the springs will exert a biasing force in the opposite direction. Accordingly, when the latching bar is rotated into position, dove, award pressure on the lower-tie plate is released, causing the tie plate and latching bar to be resiliently biased upwardly against shoulders formed in the end plugs. As a result, the upper tie plate, water rods and lower tie plate are now rigidly connected. A locking pin is then used to secure the latching bar against rotation relative to the tie plate, thereby preventing accidental rotation of the latching bar out from its locking position. At the same time, the locking pin is designed to prevent accidental withdrawal of the pin itself from the latching bar through the utilization of a compression ring described in greater detail below. In an alternative embodiment of the invention, the latching bar and locking pin are modified so that the locking pin is spring loaded in a downward direction, again preventing accidental withdrawal of the pin. In a third exemplary embodiment of the invention, the locking pin is provided with a pair of spring fingers which, in combination with a pair of adjacent stops, lock the latching bar against rotation and also prevent accidental withdrawal of the pin. In a fourth exemplary embodiment, the water rod end plugs are modified to include 360.degree. grooves, and the latch bar is of a "double horseshoe-type" where oppositely facing recesses on the latch bar are pivoted into locking engagement with the reduced diameter grooves in the end plugs The advantage of this approach is that the 360.degree. circumferential grooves on the end plugs makes the angular orientation of the water rods unimportant, at least insofar as the reliability of the locking mechanism per se is concerned. By making the radial position unimportant, the tolerances in the water rod will not need to be as tight, thus enhancing the manufacturability of the water rods. The anti-rotation feature of the water rods in this embodiment may be achieved by keying in the lower or upper tie plate with shaped holes. Another advantage of the above described "double horseshoe type" latch bar is that additional surface engagement of the locking surfaces can be achieved. In a variation of this fourth embodiment, the oppositely facing recesses in the latch bar are generally square, and the 360.degree. grooves or cut-outs in the water rod end plugs are also squared at the base of the respective grooves. This arrangement not only provides even more surface locking engagement, but also provides locked radial positioning of the water rods. This, then, simplifies the tie plate manufacturing in that no shaped holes for keyed positioning of the water rod end plugs is required. Additional objects and advantages of the invention will become apparent from the detailed description which follows. |
048448613 | abstract | A fuel assembly for nuclear reactors comprising a skeleton having two end pieces connected together by elongate elements and several grids spaced apart along the guide tubes and forming cells for holding a bundle of fuel elements in position at the nodes of a regular lattice. The grids comprise median grids adapted for resisting lateral shocks and having fins for creating turbulences in the flow of the coolant along the assembly, bottom grids without fins and ensuring the cross bracing of the elements and top grids with fins and ensuring cross bracing of the elements, the top and bottom grids imposing on the coolant a pressure loss lower than the median grids. |
H00009202 | summary | BACKGROUND OF THE INVENTION The present invention relates to a method for removing radioactive cesium from a vapor stream. In particular, the present invention relates to a method for removing radioactive cesium from high temperature steam. It is important to provide a method for the removal of cesium in the event of a release of radioactivity in a reactor environment. Cesium includes three radioactive isotopes of which Cs-139 has a half-life of only 9.3 minutes. However, two other isotopes of cesium have an intermediate half-life. Cs-133 has a half-life of about 12 years, and Cs-137 has a half-life of about 30 years. Accordingly, a release of radioactive cesium will contaminate equipment and the plant facility for a long time. Sufficient radioactive cesium may be released by the failure of fuel rod cladding in nuclear power reactors, such as boiling water reactors and pressurized water reactors, to create hazardous conditions which limit access to the reactor facility by the plant personnel. In some instances, the radioactive cesium may be transferred and transported by steam or some other vapor medium to various remote locations in the plant facility and thereby increase the area of concern for personnel safety. Inorganic zeolites were successfully used to completely remove the radioactive cesium from the primary water at the TMI-2 plant subsequent to the well-known accident which had contaminated that water at Three-Mile Island. The removal, however, was from water at ambient temperatures, and not from high temperature steam. Furthermore, the zeolites break down physically in steam so as to release loose powders capable of plugging filters and other equipment downstream of the zeolite. Organic materials such as ion exchange resins are also capable of removing cesium ions, but they are damaged by radiation, thereby making them also unacceptable for removing radioactive cesium from a hot vapor such as 750.degree. F. steam. Cesium and its compounds are volatile at temperatures above 700.degree. F., and are not easily removed in a demisting device or an aerosol filter. It appears in the hot steam as CsI and CsOH species. Accordingly, it is an object of the present invention to remove the various isotopes of cesium from a hot vapor stream either by chemical combination or by physical combination, or by both. It is another object of the present invention to remove the various isotopes of cesium from a hot vapor, such as 750.degree. F. steam, in the various forms in which the cesium could exist. It is yet another object of the present invention to remove the various forms of radioactive cesium from a high temperature vapor stream, such as steam, in a bed of particulate matter which will retain the radioactive cesium. It is a further object of the present invention to achieve the removal of radioactive cesium from a high temperature vapor stream in a bed of particulate matter which has a physical form that will produce an acceptably low pressure drop when placed inside of a closed vapor system, such as a steam circulating system. It is a still further object of the present invention to provide a bed of particulate matter for the removal of cesium from high temperature steam which has physical integrity in the dynamic high temperature steam system, so that the bed will not break up to release powdery substances which could plug filters and other equipment within the steam piping system. It is an additional further object of the present invention to provide a particulate matter which will remove radioactive cesium from high temperature steam, whereby the particulate matter will withstand break-down, both physical and chemical, under the intense gamma radiation emitted by the radioactive isotopes of cesium and other fission product isotopes present in the circulating steam system. These and other objects of the invention, as well as the advantages thereof, will become clear from the disclosure which follows. SUMMARY OF THE INVENTION The foregoing objects of the present invention are achieved by a method for removing radioactive cesium from a hot vapor, such as steam, by a technique wherein the cesium chemically reacts with a filtering material which retains the cesium without causing degradation of the filtering material. The method is carried out at temperatures in the range of from about 700.degree. F. to about 1,000.degree. F. and even higher, but it preferably is utilized at a temperature of at least about 800.degree. F. The method uses a silica glass which is preferably in the form of spheres as the filter material. The preferred material is a borosilicate glass (Pyrex). The degree of removal of the radioactive cesium from the hot steam or other vapor approaches 90 to 100%. Accordingly, the present invention comprehends a method for the removal of radioactive cesium from a vapor stream which includes the steps of passing an input vapor stream containing radioactive cesium into a bed of silicate glass particles at a temperature of at least about 700.degree. F. At least a portion of the cesium is chemically incorporated in silicate glass particles, and an output vapor stream is withdrawn from the bed of silicate glass particles which contains a reduced content of radioactive cesium. As pointed out, it is particularly preferred that the silicate glass particles comprise a borosilicate glass and that they be in the form of spheres. In addition, it is preferred that the bed of silicate glass particles be maintained at a temperature of at least about 700.degree. F., and it is particularly preferred that the temperature be at least about 800.degree. F. |
summary | ||
043137920 | description | DETAILED DESCRIPTION A portion of the gamma thermometer string of the Rolstad et al application, shown at 1 in FIG. 1, is shown partly inserted into a fuel assembly tube or thimble 2, in a non-fueled location. Coolant flows through the fuel assembly tubes or thimbles at high rates of speed, and thus through the annular space 3 as well. The gamma thermometer string includes sheath 4, which is swaged down into good thermal contact with the enlarged gamma sensor portions 5 and which bridges the reduced gamma sensor portions 6 (only one of which is shown). A plurality of thermocouple junctions, not shown, are used to sense the temperatures of the different gamma sensor portions, such as the constant diameter portions 5 and 6, and the resistance gap portions the resulting electric potentials are led off, by way of thermocouple leads 7, to instruments, not shown, located outside of the reactor. These thermocouple leads 7 are formed of conductors, electrically insulated in ceramic compacted powder, immune to the reactor environment, and covered by a metallic sheath. Thus, the construction of the ceramic insulated, metal sheathed thermocouple leads 7 is somewhat analagous to that of an electric heating element of an electric range, such as that sold under the trade name Calrod. This construction of thermocouple leads is in quite common use in nuclear systems. The leads 7 for different thermocouples are supported and spaced inside the uniform bore of the gamma sensor portions 5 and 6 by means of a splined central support rod 8. In accordance with the instant invention, the central rod 8 is provided with a bore 9. The bore 9 is used as an instrument tube, and one particular kind of instrument that can advantageously be inserted is the traveling gamma thermometer of the invention. The central rod 8, when modified by the provision of the bore 9, will be designated as an instrument tube establishing a path of movement therefor. At this point it is important to note the size of the parts here discussed in order to understand this particular example of the invention. The fuel assembly tube 2 typically has an internal diameter 0.441 inches (11.2 millimeters). The central rod 8 typically has a diameter of 0.13 inches (3.3 millimeters). The bore 9 typically has a diameter of 0.079 inches (2 millimeters). Thus, the size of bore 9 is very small. It is so small that it will accommodate the 5/64 inch diameter lead of an ordinary pencil only with a push-fit. In other less miniature applications, discussed below, the traveling gamma thermometer itself is larger, having a diameter of 3/8 inch (10.5 millimeters). The bore extending through the core of the nuclear reactor, through which the gamma thermometer travels, can assume several forms, as follows: A. The bore can be in the form of an added coaxial longitudinal passageway, formed through the central rod of the gamma thermometer of the above-identified Rolstad application. A typical use of such a gamma thermometer is with the pressurized nuclear reactors manufactured by Combustion Engineering, but such a gamma thermometer is of general utility for use with other atomic reactors. PA1 B. The bore can be that of an instrument thimble, such as those which extend through the core of pressurized atomic reactors manufactured by Westinghouse, and normally used for scanning by a traveling fission chamber. PA1 C. The bore may be that of a dry guide tube extending through the core of a boiling water atomic reactor, such as those manufactured by General Electric, and normally used for scanning with a self-powered neutron detector. PA1 D. The bore may be that of a wet guide tube, such as those used in the pressurized water atomic reactor manufactured by Combustion Engineering. Such a wet guide tube is filled with circulating reactor coolant and is normally loaded with fuel pins, but when left unfueled, is adapted to be scanned by the traveling gamma thermometer. A gamma thermometer of such size and construction as to permit it to be traversed through the bore 9 of the Rolstad et al gamma thermometer string of FIG. 1 is seen in FIG. 2. The head or heat generator 11 of the gamma thermometer is surrounded by shield 12, which has the primary function of acting as a heat shield and also acts as a protective fairleader during the scanning operation through bore 19 of an instrument tube, corresponding to bore 9 of FIG. 1. The head 11 contains two thermocouple junctions 15 and 17, formed as welds between the wires 22, 23 and 24. Wires 22 and 23 are made of a suitable material, such as "Chromel", while wire 24 and short link 20 are made of another suitable and compatible material such as "Alumel". The differing thermoelectric properties of these two materials, as is well known in the art, are used to generate voltages at each junction, which are a measure of the temperature at the respective junctions. The wires 22, 23 and 24 are encased in a sheath 25 which is attached to base 14 of the head 11. The left end of this sheath is available, outside of the reactor core, for connection of measuring instruments to wires 22, 23 and 24. In order to prevent short circuiting of the wires 22 to 24 to each other or to the head 11 or sheath 25, they are insulated by a compacted powder 26. The outside of sheath 25 is tightly wrapped with a helix of wire. The wrap 27 stiffens the sheath, which could easily buckle during handling without the wrap. It also permits the propulsion of the gamma thermometer through the bore which is to receive the miniature traveling gamma thermometer, such as bore 9 of FIG. 1, or 19 of FIG. 2, by means of a propulsion gear, not shown, which engages successive turns of the wrap 27 in the manner of a rack and gear. Other propulsion systems may drive with friction wheels directly on the outside of a sheath member 25. The propulsion gear is located outside of the reactor in the area susceptible of service. The sheath 25 is long enough so that a considerable amount will still be stored on a storage drum, adjacent the propulsion gear, when the traveling gamma thermometer is extended as far into the reactor core as possible. The bitter end of the sheath, on the storage drum, is connected to electrical instrumentation. The head end of the sheath and the gamma thermometer become highly radioactive during the scan of the reactor core. When the scan is completed, the sheath is withdrawn from the core by means of the propulsion gear and momentarily stored on the storage drum, whereupon it is immediately propelled forward again, through an array of wye switches, into a different instrument tube for further measurements. The radioactivity induced in the thermometer and sheath does not interfere with measurements. During periods of non-measurement the probe and sheath are stored in a parking position where the induced radioactivity presents no problem. During operation of the gamma thermometer, the ambient gamma radiation will heat up the various parts to an extent which depends upon the material involved. In order to simplify the explanation, it will be assumed that all parts are equally heated by the gamma flux and that the temperature of the bore 19 is uniform. The sheath 25 will thereupon assume a uniform temperature, loosing heat radially outwardly. The head 11 will be hotter at its right or tip end than at its left end, adjacent base 14, because the base 14 will act as the heat sink for almost all the heat generated in the right tip end and because there is a thermal resistance between the heat sink at base 14 and the heat source at the right end of head 11. Thermocouple junction 17 will therefore be hotter than thermocouple junction 15, to an extent dependent on the ambient gamma flux. The electrical circuit which can be traced through wire 22, junction 17, short link 20, junction 15, and wire 23, constitutes a differential thermocouple between junctions 17 and 15. This differential thermocouple generates a voltage which is a measure of the temperature difference from the right end of the head 11 to the left or sink end 14. This voltage, proportional to temperature difference, is led to a meter, not shown, but outside the reactor core, by way of wires 22 and 23. The temperature between the hot thermocouple junction 17 and the cold thermocouple junction 15 is closely related to the intensity of the ambient gamma flux, and is largely independent of the temperature of the base 14 except as thermal conductivity of the material varies slightly with temperature. The base 14 should be at a temperature within about 50.degree. C. of the bore 19 for high precision measurement. In order to avoid the risk of an improper reading, wire 24 also leads to the thermocouple junction 15, which acts as a hot junction for lead pair 23 and 24. Wire 24 leads to the outside of the reactor, whereat, in a manner not shown, but well understood by those skilled in the art, it is connected to a cold junction which in turn leads to one terminal of a meter, the second terminal of which is connected to wire 23. Thus, wire 23 is a common lead for two separate thermocouple circuits, one circuit of which measures the temperature difference along the head 11 and one circuit of which, with an external cold junction, measures the instrument sink temperature 14. The reading of the sink temperature at junction 15 can be used to correct the calculation of ambient gamma flux obtained from the difference thermocouple comprising junctions 15 and 17. No correction is usually necessary if the sink temperature 14 is within about 50.degree. C. of the temperature of bore 19. The use may prefer to withdraw and reinsert the unit to make better thermal contact, if high sink temperature indicates that thermal contact with bore 19 is poor. The outside of head 11 and the inside of shield 12 are preferably polished, so that there is little transfer of heat therebetween by way of radiation. Furthermore, the space 21 between the head 11 and shield 12 is (a) made narrow and (b) is either evacuated or filled with a gas of low thermal conductivity, such as argon or krypton, so as to eliminate or reduce transfer of heat between head 11 and shield 12 by convection. The result of this is that the right end of head 11 looses heat principally by longitudinal heat conduction to base 14. The resulting thermal gradient along head 11 can be accurately calculated since the dimensions of head 11, machined on a lathe, are accurately known, and the head 11 is massive compared to the wire 22, short link 20 and compacted powder 26 within the head, which wire, link and powder have dimensions and characteristics which are not as well defined. Thus, the embodiment of FIG. 2 has the advantageous characteristic that it has an accurately calculatable calibration, which can be checked against an experimental calibration. These two calibrations, which in agreement, can be used as a standard to calibrate other gamma thermometers. Another embodiment of gamma thermometer is shown in FIG. 3, at 28, inserted into bore 29. Here the sheath 30 is extended to form a head end 30A, which is hermetically sealed off with an electron-beam weld seal. The thermocouple junctions and wires inside the gamma thermometer 28 are similar to those described for FIG. 2. Junctions 15A and 17A correspond in construction and function to junctions 15 and 17, respectively, of FIG. 2. A novel feature is the series of spring elements 32 which are fastened about the periphery of the sheath. The spring elements are shown fastened to the sheath by rivets 33. It will be understood that in the nuclear environment, rivets are not used, and welding would be taken for granted. However, welding does not show up well in a drawing, and rivets are substituted for purposes of exposition. The spring elements 32 have two functions. They act as thermal bridges, so as to keep the temperature of the sheath adjacent rivets 33 close to that of the bore 29. They also act as centering means for the gamma thermometer within the bore 29. During operation, it is important that the right head end 30 of the gamma thermometer 28 does not touch the bore 29, as any contact would interfere with the thermal gradient between head end 30 and junction 15A. The centering action of spring elements 32 ensures that improper contact does not occur. In view of the presence of the thermal bridge 32, one function of the thermoelectric junction 15A (with its remote cold junction, not shown) is that of monitoring to warn if there is something wrong about the sheath temperature rather than that of measuring, as in the FIG. 2 embodiment, for purposes of correcting the indicated temperature difference. In use, the spring means 32 can be lubricated with "Neolube", a proprietary lubricant made of graphite in an organic suspension with alcohol for ease of distribution. Such a lubricant not only makes it easier to push the gamma thermometer through the instrument tube, but increases the thermal conductivity between the bore 29 and the gamma thermometer. When no thermal bridge is used with a traveling gamma thermometer, it has been found that the use of Neolube lubricant will frequently provide sufficient thermal contact between the gamma thermometer and the bore of the instrument tube to permit accurate readings to be made. However, in each such instance the quality of the thermal contact must be qualified by observation of the absolute temperature of the gamma thermometer. In case of the Rolstad type of gamma thermometer, when used inside a dry tube, the desire for such thermal bridge devices is more frequent, for two reasons. The accuracy is more affected by sink temperature variations in the biaxial heat flow system of Rolstad gamma thermometers than in the uniaxial heat flow devices of the instant application. In Rolstad the problem is often eliminated by water cooling which is not usual for traveling gamma thermometers. The embodiment of FIG. 4 uses a different type of thermal bridge and centering element and has a geometry which combines the simple swaged sheath construction of the embodiment of FIG. 3 with the excellent characteristics of the shielded head construction of the embodiment of FIG. 2. Gamma thermometer 41 is inserted in bore 39. The sheath 43, containing thermocouples 15B and 17B has been swaged down to a greatly reduced cross section at the head 44. Such swaging down, when properly done, by laminar flow, changes the scale inversely on the longitudinal and transverse axis without altering the thermocouple properties. The thin and fragile head 44 is covered by a protective shield and fairleader 45. The thermal bridge and centering means, which acts as a heat sink, is in the form of a highly conductive collar 46 which has only a small clearance with the bore 39. The small clearance ensures that collar 46 will touch the bore 39, to establish thermal contact, and it will also prevent the axis of the gamma thermometer 41 from being greatly cocked with respect to that of the bore, so that the shield 45 will not touch the bore 39. The space 47 between the swaged down portion or head 44 and the shield 45 can be evacuated or filled with argon or krypton. In an actual embodiment, when the gamma thermometer was inserted into an atomic reactor core operating at full power, the right end of the tip 44 was about 40 degrees hotter than its left end, adjacent collar 46. It is estimated that, for this condition, of all the heat energy liberated within the materials of the right end of head 44 (principally the material of the sheath) about 1% was lost to the shield by radiation, while about 15% was lost to the shield by convection through an argon gas filling. The remainder flowed by longitudinal conduction through the length of head 44 to the heat sink at collar 46. The embodiment of FIG. 5 is similar to that of FIG. 3 but has considerably higher sensitivity. The gamma thermometer 51 is inserted in bore 49. The sheath 53 contains a compacted insulating powder 55 which supports and insulates the thermocouple junctions 15C and 17C. At the fore end 54 of the sheath 53, the powder has been removed to leave the space 57, which can be evacuated or filled with a gas of poor conductivity. In this embodiment it is the wires 58 and 59 which form the sensors of the gamma radiation, by internally heating up in the presence of the gamma flux, and which also provide thermal conduction along a longitudinal path to a heat sink. The head end 54 of the sheath 53 acts as a heat shield for wires 58 and 59. In this embodiment, therefore, the differential thermocouple has two functions--first, to capture gamma radiation and convert it into heat and, second, to indicate resulting differential heat. This contrasts with the other embodiments, discussed above, wherein the differential thermocouple is used, essentially, only for the latter function. It is apparent from what has been described above that the herein described traveling gamma thermometer is of general utility. Besides being of use to calibrate an array of gamma thermometers, it can be used to calibrate an array of other types of nuclear activity measuring instruments, such as self-powered neutron detectors. Furthermore, it can be used by itself, either as a stationary and permanently installed measuring instrument, or as a continuously available traveling measuring instrument. |
summary | ||
048329029 | summary | CROSS-REFERENCE TO RELATED APPLICATION U.S. Pat. No. 4,511,531 granted Apr. 16, 1985 to Kenneth J. Swidwa, Leonard P. Hornak and Edward F. Kowalski for Transfer of Nuclear Reactor Component Assemblies and assigned to Westinghouse Electric Corporation (herein Swidwa). Swidwa is incorporated herein by reference. BACKGROUND OF THE INVENTION This invention relates to the art of nuclear reactor power plants. It has particular relationship to the refueling of the reactors of such plants. In carrying out the refueling, the reactor to be refueled is at the base of a pit in a containment filled with water to a depth of 20 or 30 feet. During refueling, component assemblies of the reactor or from refueling racks are engaged by grippers or grapples of a mast assembly, raised, transported from their position of origin and lowered into the position where they are to be deposited. The component assemblies are highly radioactive and are engaged, raised, transported and lowered under a substantial depth of water. To carry out this operation, there is provided apparatus including a bridge moveable along a track on the containment. A trolley is moveable on a track on the bridge. The trolley carries a mast assembly having a rotatable supporting mast from which the component-assembly engaging-raising-and-lowering means is suspended. This means is sometimes herein referred to as "component-assembly handling mechanism" or "assembly-handling mechanism" or simply "mechanism". With the bridge and trolley at selectably different positions along their tracks, the mast assembly is suspended with the assembly-handling mechanisms at selectably different positions of the area of the pit or of the reactor within the pit. It is indispensable to successful refueling that the mechanism on the mast assembly be positioned during each operation to engage and raise the exact assembly which is selected for transport. Since the reactor is under water, the positioning of the mechanisms must be carried out with the necessary precision with at best a heavily clouded view of the nuclear core. It is also necessary that the depth of the mechanisms in the water be known. The tracks on the bridge and containment define a coordinate system whose coordinates serve to determine the positions of the mechanisms on the mast assembly. The tracks in Swidwa, and predominantly in prior art apparatus, are linear and at right angles to each other defining a Cartesian coordinate system. With the tracks of other configurations, other coordinate systems may define the positions of these mechanisms. For example, if the bridge moves in a circular track on the containment and the trolley moves on a radial track, the coordinate system would be a polar coordinate system. The coordinates of the positions of the bridge and trolley along the tracks are magnitudes which indicate the positions of the engaging-raising-and-lowering means over the pit. In addition, there are provided indications of the vertical positions of these mechanisms. The mechanisms are raised or lowered by a hoist which is typically a winch. In accordance with the teachings of the prior art the positions of the mast assembly along the tracks are derived from selsyn-type indicators. The readings of the indicators for different positions of the mechanisms of the mast assembly are compared with marks along the containment walls. The marks are coordinated with the indications of the selsyns by a calibration process. Impairment of the calibration was experienced and when this occurred, recalibration was a difficult and time consuming experience. The height of the mechanisms on the mast assembly was determined from marks on a tape along the assembly. Recalibration in this case was also difficult and time consuming. In accordance with the teachings of the prior art, index marks were also provided on the trolley and bridge to locate the component assemblies to be moved in the reactor core. The index marks served to position the mast assembly over the theoretical location of a component assembly. If the assembly is out of position, difficult visual determinations were required to make the necessary adjustments. This operation was time consuming and required the utmost caution to preclude damage to the component assemblies. It is the object of this invention to overcome the difficulties and drawbacks of the prior art. An object of this invention is to provide refueling apparatus for a reactor having facilities, whose calibration shall be maintained throughout a refueling operation, for reliably and precisely identifying the position of the mast assembly with reference to the reactor core. A further object of this invention is to provide refueling apparatus including position-visual facilities for aligning the mast assembly with the component assembly to be transported for identifying and recording the location of component assemblies to be transferred and, if desirable, to provide a permanent record of the refueling and core-mapping operation. SUMMARY OF THE INVENTION In apparatus in accordance with this invention, the trolley is provided with a pulse generator which produces pulses as the trolley moves along the track on the bridge. The pulses are entered as counts in a computer. Each pulse enters in the computer a positive count for each increment of movement of the trolley in a forward direction away from a reference point on the track and a negative count for each increment of the movement of the trolley in a backward direction towards the reference point. There is thus a one-to-one relationship between the position of the trolley on the track and the count of each train of pulses measuring the increments from the reference point to a position. Likewise the bridge is provided with a pulse generator which produces pulses resulting in a positive count in the computer for each increment of the forward movement of the bridge on the track on the containment walls from a reference point and a negative count for each increment of backward movement. The drive for the hoist also has a pulse generator which produces pulses resulting in a positive count in the computer for each increment of forward, typically downward, movement of the component-assembly-handling mechanism from a reference point and a negative count for each increment of backward, typically upward, movement of this assembly-handling mechanism from the reference point. For each set of positions of the trolley, bridge and handling mechanism, the computer contains digital entries which identify these positions. The apparatus may be calibrated so that each set of counts defines the position of the trolley, bridge and component-assembly handling mechanism relative to a known predetermined component assembly in the core. To test the integrity of the calibration and reset the coordinate system if the calibration is impaired, a limit switch is actuated by each, the trolley, bridge and handling mechanism at a predetermined position along its path. The switch may be carried by the vehicle and may be actuated by a cam along its path or it may be provided at the position along the path and actuated by a cam on the vehicle. The actuation by the trolley and bridge takes place at a convenient position along their tracks. In the case of the handling assembly, the limit switch is typically actuated at the maximum height of this assembly. The actuation of each limit switch is entered in the computer together with the count at the position where each limit switch is actuated. The memory of the computer carries intelligence of the counts that should be entered for the actuation of the limit switches if the calibration is maintained. The memory is programmed to recalibrate the coordinate system if there is a disparity between a count entered in the memory and a count entered on actuation of a limit switch. |
041683943 | description | DETAILED DESCRIPTION OF THE DRAWING Referring to FIG. 1, there is shown one embodiment of electrical penetration assembly 11 employing my novel features, as would be typically installed in a penetration nozzle 15, piercing a concrete wall 16 enclosing a nuclear reactor (not shown) which would be located to the right of FIG. 1. The nozzle 15 is preferably made of an alloy steel and has an annular steel flange 17 fixed to the end thereof. The nozzle 15 is normally cylindrical and the flange 17 has its exposed surface perpendicular to the nozzle axis. Suitably bolted by bolts 19 to flange 17 is a steel header plate 18 having internal bores or passageways 21 suitably placed in a manner that will be made more apparent hereinafter, and having at least one transverse bore 20. Between the flange 17 and plate 18 is disposed an annular gasket 22 having novel features, as will be explained hereinafter. Into each transverse bore is inserted an electrical penetration assembly 11. The passageways 21 are made to communicate with each transverse bore 20 and to a pressure gauge 25 in a manner such as is taught in U.S. Pat. No. 3,828,118. One will note that the transverse bore 20 consists of two cylindrical outer bore sections 20a and 20b connected by a conical section 20c so that the assembly 11 can be removed from the outside of the reactor containment vessel represented by wall 16. The embodiment of the electrical penetration assembly 11 shown in FIG. 1 has a central conductor 31 preferably made of copper surrounded by ceramic insulator 32 preferably shaped as shown. The insulator 32 has the necessary annular ribs 33 to provide a high resistance to electrical leakage patterns. Between the ribs 33 the insulator has two cylindrical portions or sections 20a' and 20b' disposed on each side of a tapered portion or section 20c' to match the respective cylindrical sections 20a and 20b and the tapered section 20c on the transverse bore in the header plate 18. Within the tapered section 20c' of the insulator 32, a gas pervious means, in the form of a porous ceramic section 35, is placed between non-porous ceramic sections 36 and 37. The porous section 35 is, for example, made of equal amounts of alumina and silica with 100 micron-sized pores and is disc shaped. The sections 36 and 37 are, for example, made of alumina which is formed dense and rigid and with a glazed surface. Section 35 is bonded to sections 36 and 37 with a suitable glaze and the bonding is preferably done in the green state before firing of the ceramic to achieve uniform bonding strength. The ends of the conductor 31 are provided with suitable terminals such as terminal 38 which is called a NEMA type lug terminal. Since the bonding between the copper conductor 31 and the insulator 32 is inherently poor, I have provided a flexible metal apertured cap 41 at each end of the insulator 32, the cap 41 is shaped as shown to provide a lateral or axial movement between the conductor and insulator. The cap 41 is made of, for example, Monel, which is brazed to the copper conductor 31 and brazed to an annular metallized surface 42 on the insulator 32, which surface is made in a well known manner. To prevent relatively large axial movements, a copper ring 43 is brazed on the conductor 31 adjacent to each end of the insulator 32. These rings 43 are located on the copper when the copper is at ambient temperature and then bonded thereto. The electrical penetration assembly 11 is installed as shown, with a pair of suitable O-ring seals 45 and 46 disposed in suitable grooves in the header 18 and a retaining ring 47 bolted to the outside of the header and holding the assembly 11 in place. As in the prior art, the passageway 21 is filled with nitrogen. The nitrogen communicates with a pair of opposing circumferential grooves 51 in gasket 22 through a passageway 52. The gasket 22 has aperture 53 between grooves 51. Therefore, if any leaks develop, the nitrogen would pass through the porous ceramic 35 and through the space between the gasket 22 and header 18 or flange 17 and the pressure read on the gauge 25 would drop. Referring to FIG. 2, another embodiment of my electrical penetration assembly is shown, wherein like numbered items refer to the same functional thing. This embodiment also has the electrical conductor 31, the ceramic insulator 32 with ribs 33, cylindrical sections 20a' and 20b' and tapered section 20c', flexible end caps 41, metallized surfaces 42 and rings 43. Since no porous section 35 is provided, as this insulator 32 is made of a dense alumina, I have provided a T-shaped leakage path or ducts 61 and 62 with path 61 parallel to the conductor 31 and opening at opposite ends of the insulator 32 within the end caps and with path 62 communicating with path 61 and opening at the tapered section 20c'. Now, when this embodiment is installed, such as the embodiment shown in FIG. 1, path 62 communicates with the passageway 21 to perform a like function. Referring to FIG. 3 there is shown another embodiment wherein like numbered items refer to the same functional thing. This embodiment also has the electrical conductor 31, insulator 32 with ribs 33, cylindrical sections 20a', 20b' and tapered section 20c', flexible end caps 41, metallized surfaces 42 and rings 43. This embodiment also eliminates the porous section 35, but includes a zigzag duct 63 which terminates against the conductor 31, as shown, and opens at the tapered section 20c'. Also, when this embodiment is installed, such as in the embodiment of FIG. 1, path 63 communicates with path passageway 21 to perform a like function. Referring to FIG. 4, there is shown still another embodiment which has utility for low voltage application. This embodiment has a plurality of conductors 71, preferably seven conductors, arranged with six conductors evenly disposed around a central conductor. The conductors 71 are covered by a suitable insulator shaped similar to the insulator of the embodiment in FIG. 1, but made simpler. The insulator has two outer cylindrical sections 72 and 73 of different diameters connected by a conical section 74. At the conical section is placed a wafer-shaped porous insulator 75 while on each side there is disposed a dense insulation material as will be further described. This electrical penetration assembly can also be installed in a system shown in FIG. 1 so that the porous insulator 75 communicates with passageway 21. Since the bonding between conductor 71 and the insulator should prevent leakage at extreme conditions and after long life, the preferred process for making these electrical penetration assemblies, as shown in FIG. 4, will now be explained. The coefficient of thermal expansion for any conductor material such as copper is greater than most thermosetting dielectric materials. Further, shrinkage of the thermosetting dielectric materials after molding tends to move in the opposite direction from the conductors causing some leak paths between the conductors and the dielectric materials. The process described herein consists of applying a flexible coating between the conductors and the dielectric materials to produce a bonding in-between insuring a hermetically sealed thermosetting dielectric material on the conductors. The preferred process includes the following steps: 1. Each copper conductor is cleaned in an alkaline solution, rinsed and descaled in an ammonium persulfate solution (90 gms/liter), rinsed with de-ionized water and dried. 2. Each cleaned conductor is immersed in gamma-glycidoxypropyltrimethoxysilane and drained. Treated conductors are air dried for approximately 30 minutes. 3. A coating of solventless silicone resin is applied on each conductor, consisting of (a) a solventless liquid organosilicone resin containing 30 to 65 mol percent C.sub.6 H.sub.5 SiO.sub.3/2 units, 15 to 30 mol percent CH.sub.3 (CH.sub.2 .dbd.CH)SiO units, 20 to 40 mol percent (CH.sub.3).sub.2 SiO units, and 0 to 5 mol percent (CH.sub.3).sub.3 SiO.sub.1/2 units; (b) an organopolysiloxane fluid having at least two .tbd.SiH groups per molecule, the organopolysiloxane being of the formula ##STR1## in which n is an integer having a value of 2 or more, m is an integer having a value of 1 or more, n and m having a total value sufficient to result in a fluid having a viscosity of from 20 to 2000 cs. at 25.degree. ., the diphenylsiloxy units comprising from about 30 to 40 mol percent of said organopolysiloxane, which is present in the mixture in an amount sufficient to provide from 0.75 to 1.5 mol of .tbd.SiH per mol of vinyl substituent in (a); and (c) a platinum catalyst such as chloroplatinic acid containing approximately 10 p.p.m. by weight of platinum. 4. Each conductor is loaded into a mold retention insert, such as porous insulator 75, and preheated in an oven at 200.degree. C. The applied coating on the conductors is cured at 200.degree. C. for one hour. A complete cure of the coating is not necessary so that a maximum bonding may be obtained on the dielectric materials to be applied next. 5. The dielectric material consists of solventless silicone resin as described in (3) used in conjunction with filler materials of approximately 60 to 75 percent by weight of alumina powder or barium titanate or silica powder treated with gamma-glycidoxypropyltrimethoxysilane and between 1 to 3 percent of coarse-grain magnesium oxide to enhance the thermal conductivity. An example of the composition of the dielectric materials is as follows: (a) and (b): 34.5 gm PA1 (c): 3.5 gm PA1 Alumina Powder: 60 gm PA1 50-mesh Magnesium Oxide: 2 gm PA1 Dow Corning R-4-3157 Base: 34.5 gm PA1 Curing agent R-4-3157: 3.5 gm PA1 Tabular alumina: 60 gm PA1 350-mesh magnesium oxide: 2 gm PA1 Pyromellitic dianhydride: 8.5 gm PA1 N-glycidelphthalimide: 35 gm PA1 Tabular alumina: 60 gm 6. The above composition is cast into the mold and cured at 200.degree. C. for 16 hours. The electric feed-through module thus produced is hermetically sealed on each conductor. The dielectric material can withstand gamma irradiation to 2.times.10.sup.10 rads, thermal aging to 40 years design life, superheated steam at 600.degree. F. under pressure of 1200 psig without losing its hermeticity. Making the hermetically sealed electric penetration assembly feed-through modules, as shown in FIG. 4, by the standard vacuum cast method, ordinarily takes about 16 hours at 200.degree. C. This is not very suitable for high volume production. By the pressure gelation method on the other hand, it takes only about 30 minutes. Further, electric feed-through modules produced by the pressure gelation process are void free, more densely packed and better sealed with conductors because of the pressure exerted on the material during gel. As a result, considerable improvement can be achieved in the mechanical tensile strength, volume resistivity, insulation resistance and corona extinction level for high voltage applications. Referring to FIG. 5, the preferred apparatus and method of operation for making modules shown in FIG. 4 is now described: 1. After a coating of solventless silicone resin is applied on each conductor as described in (3) of the above mentioned process, each conductor is loaded into the mold 101 for pressure gelation molding. 2. The automatic pressure gelation system consists of a hydraulic press 102 to close the molds, a mold heating system 103 with thermostat control, a resin supply tank 104 with stirrer 105. The solventless silicone resin and the filler materials are first thoroughly mixed under vacuum drawn through tube 106. Then nitrogen pressure is applied through tube 106 so as to deliver the resin mixture into the mold 101. The mold is pre-heated to 200.degree. C. and material in the mold is held under a constant nitrogen pressure of approximately 15 psig during gel. 3. After about 30 minutes, the hermetically sealed electric feed-through module is cured and can be taken out of the mold. The module may be placed in an air-circulating oven preheated to a lower temperature to allow slower cooling rate to prevent any thermal shock during sudden cooling which may produce cracking of the molded parts. Examples of typical resin mixtures used to make insulators 72 and 73 of FIG. 4 are: 1. Solventless silicone resin Process: Pressure gelation molding at 200.degree. C. & 15 psig during gel for 30 minutes, followed by post-curing at 150.degree. C. for 4 hours. 2. PMDA Process: Compression molding at 400.degree. F. and 1,500 psig. Having described the preferred embodiments of my invention, one skilled in the art could devise other embodiments without departing from the spirit of my invention. Therefore, my invention is not to be considered as limited to the embodiments described but includes all embodiments which fall within the scope and breadth of my appended claims. |
claims | 1. A system comprising:an article of personal protection equipment (PPE) that can be worn by a user, and at least one component coupled to the article of PPE;a smart tag coupled to the at least one component or the article of PPE;a computing device comprising one or more computer processors and a memory that further comprises instructions that when executed by the one or more computer processors cause the one or more computer processors to:receive concentration data about a working environment using one or more sensors;determine a duration of usage of the component based on tracked data received from the smart tag; anddetermine a condition of the component based at least in part on the duration of usage of the component, the concentration data, and a service life of the component. 2. The system of claim 1, wherein the instructions that cause the one or more computer processors to determine the condition of the component further comprise instructions that when executed by the one or more computer processors cause the one or more computer processors to determine the condition of the component based at least in part on additional data corresponding to the user from the smart tag that indicates at least one of identity, medical, fit test, job description, seniority, training, or qualification information of the user. 3. The system of claim 1, wherein the tracked data indicates at least one of: an entry time when the user entered the working environment, an exit time when the user exited the working environment, an entry location where the user entered the working environment, an exit location where the user exited the working environment, a type of personal protection equipment or type of component, historical information relating to the article of PPE or component, an identifier of the user, a location where the article of PPE was used, a condition under which the article of PPE was used, maintenance performed on the article of PPE, a requirement for using the article of PPE, or a description of the working environment. 4. The system of claim 1 further comprising at least one smart tag scanner configured to read the smart tag, wherein to determine a duration of usage of the component, the at least one smart tag scanner is communicatively coupled to the computing device and configured to receive the tracked data from the smart tag and send the tracked data to the computing device. 5. The system of claim 1, wherein the instructions that cause the one or more computer processors to determine the condition of the component further comprise instructions that determine whether the duration of usage of the component exceeds the service life of the component. 6. The system of claim 1, wherein the memory further comprises instructions that when executed by the one or more computer processors cause the one or more computer processors to output a notification based on the condition of the component. 7. The system of claim 1, wherein the memory further comprises instructions that when executed by the one or more computer processors cause the one or more computer processors to perform, based on the determination of the condition of the component, one or more operations. 8. A method comprising:providing at least one component coupled to an article of personal protection equipment (PPE) that can be worn by a user;providing a smart tag coupled to at least the component or the article of PPE;receiving concentration data about a working environment using one or more sensors;determining a duration of usage of the component based on tracked data received from the smart tag; anddetermining, by a computing device, a condition of the component based at least in part on the duration of usage of the component, the concentration data, and a service life of the component. 9. The method of claim 8, wherein determining the condition of the component further comprises determining the condition of the component based at least in part on additional data corresponding to the user from the smart tag that indicates at least one of identity, medical, fit test, job description, seniority, training, or qualification information of the user. 10. The method of claim 8, wherein the tracked data indicates at least one of:an entry time when the user entered the working environment, an exit time when the user exited the working environment, an entry location where the user entered the working environment, an exit location where the user exited the working environment, a type of personal protection equipment or type of component, historical information relating to the article of PPE or component, an identifier of the user, a location where the article of PPE was used, a condition under which the article of PPE was used, maintenance performed on the article of PPE, a requirement for using the article of PPE, or a description of the working environment. 11. The method of claim 8, wherein at least one smart tag scanner is configured to read the smart tag, wherein determining a duration of usage of the component further comprises:receiving, by the computing device and from the at least one smart tag scanner communicatively coupled to the computing device, the tracked data from the smart tag. 12. The method of claim 8, wherein determining the condition of the component further comprises determining whether the duration of usage of the component exceeds the service life of the component. 13. The method of claim 8, further comprising outputting a notification based on the condition of the component. 14. A computing device comprising: one or more computer processors; and a memory comprising instructions that when executed by the one or more computer processors cause the one or more computer processors to: receive concentration data about a working environment from one or more sensors, wherein a smart tag is coupled to at least one component coupled to an article of personal protection equipment (PPE) that can be worn by a user or the article of PPE; determine a duration of usage of the component based on tracked data received from the smart tag; and determine a condition of the component based at least in part on the duration of usage of the component, the concentration data, and a service life of the component. 15. The computing device of claim 14, wherein the memory comprises instructions that when executed by the one or more computer processors cause the one or more computer processors to determine the condition of the component based at least in part on additional data corresponding to the user from the smart tag that indicates at least one of identity, medical, fit test, job description, seniority, training, or qualification information of the user. 16. The computing device of claim 14, wherein the tracked data indicates at least one of: an entry time when the user entered the working environment, an exit time when the user exited the working environment, an entry location where the user entered the working environment, an exit location where the user exited the working environment, a type of personal protection equipment or type of component, historical information relating to the article of PPE or component, an identifier of the user, a location where the article of PPE was used, a condition under which the article of PPE was used, maintenance performed on the article of PPE, a requirement for using the article of PPE, or a description of the working environment. 17. The computing device of claim 14, wherein the memory comprises instructions that when executed by the one or more computer processors cause the one or more computer processors to receive, from at least one smart tag scanner communicatively coupled to the computing device, the tracked data from the smart tag. 18. The computing device of claim 14, wherein the memory comprises instructions that when executed by the one or more computer processors cause the one or more computer processors to determine whether the duration of usage of the component exceeds the service life of the component. 19. The computing device of claim 14, wherein the memory comprises instructions that when executed by the one or more computer processors cause the one or more computer processors to output a notification based on the condition of the component. 20. The computing device of claim 14, wherein the memory comprises instructions that when executed by the one or more computer processors cause the one or more computer processors to perform, based on the determination of the condition of the component, one or more operations. |
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claims | 1. An ultrasonic reactor water level measuring device comprising:an upper tube that extends from a gas phase portion in a reactor;a lower tube that extends from a liquid phase portion in the reactor;measurement tubes that are connected to each other and arranged at multiple stages between the upper tube and the lower tube; andunits for generating and receiving ultrasonic waves and measuring levels of water within the measurement tubes, the units being arranged at bottom portions of the measurement tubes,wherein a water level within the reactor is calculated from the sum of the measured water levels, the sum excluding the height of an overlapped part of the measurement tubes. 2. The ultrasonic reactor water level measuring device according to claim 1, further comprisingseparation valves that are arranged in the middle of the upper and lower tubes,wherein when pressure on the side of the measurement tubes is equal to or lower than a set value, the separation valves are automatically closed. 3. The ultrasonic reactor water level measuring device according to claim 1,wherein ultrasonic probes are directly in contact with the water and integrated with flanges so that transmittances of the ultrasonic waves in the water are high, compared with a conventional method using an acoustic couplant. 4. The ultrasonic reactor water level measuring device according to claim 1, further comprisingreflective plates that are arranged in the middle of the measurement tubes,wherein the temperature and pressure of the water are corrected on the basis of ratios of periods of time for waves reflected from surfaces of the water or upper end portions of the measurement tubes to propagate from the surfaces of the water or the upper end portions of the measurement tubes to the generating/receiving units to periods of time for waves reflected from the reflective plates to propagate from the reflective plates to the generating/receiving units. 5. The ultrasonic reactor water level measuring device according to claim 4,wherein the reflective plates have reflection intensities that can be adjusted by adjusting diameters of holes of central portions of the measurement tubes or the areas of constituent members of the measurement tubes. 6. The ultrasonic reactor water level measuring device according to claim 1, further comprisinga temperature sensor that is arranged in at least one of the measurement tubes,wherein a sound speed is determined using a temperature instruction value for the temperature of the water within the measurement tube, and the temperature of the water is corrected on the basis of the sound speed and a time period in which the ultrasonic wave that propagates to the liquid surface or the upper end portion of the measurement tube reciprocates. 7. The ultrasonic reactor water level measuring device according to claim 1,wherein connection tubes and the upper and lower tubes that connect the pressure vessel to the measurement tubes are inclined so that gas-phase-side end portions of the connection tubes are higher than the other end portions of the connection tubes, gas-phase-side end portions of the upper and lower tubes are higher than the other end portions of the upper and lower tubes, and air bubbles are not stored in the connection tubes and the upper and lower tubes. |
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041490878 | abstract | The proposed drum for storing fuel assemblies of a nuclear reactor comprises a holder rotatable around its axis and provided with tubular sockets arranged in concentric rows along the circumference of the holder so that the axis of at least one socket of each row intersects the trajectory described by the grip of the recharging mechanism in the course of its movement. The proposed drum design makes it possible to facilitate and speed up the process of recharging fuel assemblies. |
claims | 1. An electrochemical decontamination plant for radioactive waste metal, comprising:a pipeline equipped with shutoff valves,a treatment module for radioactive waste metal comprising an electrochemical decontamination module for radioactive waste metal;a ventilation channel;a ventilation module;decontamination solution supply and discharge pipeline having shutoff valves and at least one pump;a decontamination solution receipt module; anddecontamination solution preparation module;the electrochemical decontamination module connected via the ventilation channel with the ventilation module, and the electrochemical decontamination module connected with the decontamination solution receipt module via the decontamination solution supply and discharge pipeline equipped with the shutoff valves;the decontamination solution preparation module connected via the decontamination solution supply and discharge pipeline and the at least one pump to the electrochemical decontamination module for radioactive waste metal and to the decontamination solution receipt module;the decontamination solution receipt module being equipped with decontamination solution treatment and pH correction devices;the electrochemical decontamination module for radioactive waste metal, the decontamination solution receipt module and the decontamination solution preparation module are equipped with pH measurement components. 2. The electrochemical decontamination of plant for radioactive waste metal according to claim 1, wherein the electrochemical decontamination module for radioactive waste metal comprises: a cylindrical tank with a conical bottom; a basket for a treated component of radioactive waste metal installed inside the tank; a high-pressure water supply pipeline equipped with shutoff valves; a direct current source; a negative terminal of the direct current source connected to a cathode of the electrochemical decontamination module for radioactive waste metal, and a positive terminal of the direct current source connected to (i) an anode of the electrochemical decontamination module for radioactive waste metal and (ii) the basket for the treated component of radioactive waste; a mixing device connected to the tank; a sediment discharge unit located in a bottom of the tank; an ionizing radiation dose rate monitor connected to the tank; a temperature monitor connected to the tank; and a decontamination solution monitor connected to the tank;the decontamination solution supply and discharge pipeline being connected to the tank. 3. The electrochemical decontamination plant for radioactive waste metal according to claim 1, wherein the decontamination solution receipt module comprises:an alkaline agents supply pipeline equipped with shutoff valves;a decontamination solution processing container connected to the decontamination solution supply and discharge pipeline, the decontamination solution supply and discharge pipeline equipped with an ion indicating filter, and the decontamination solution processing container connected to the alkaline agents supply pipeline equipped with shutoff valves;an ionizing radiation monitor connected to the decontamination solution processing container;a mixing device connected to the decontamination solution processing container;sediment discharge device located in a bottom of and connected to the decontamination solution treatment container; andand a decontamination solution level monitor located inside and connected to the decontamination solution processing container. 4. The electrochemical decontamination plant for radioactive waste metal according to claim 1, wherein the decontamination solution preparation module comprises:wateracid agents supply pipelines equipped with a shutoff valve;a decontamination solution preparation container connected to the water, the acid agents supply pipelines equipped with the shutoff valve, and the decontamination solution supply and discharge pipeline equipped with shut-off valves;a mixing device connected to the decontamination solution preparation container; anda decontamination solution level monitor installed inside the decontamination solution preparation container. 5. The electrochemical decontamination plant for radioactive waste metal according to claim 1, wherein the ventilation module comprises the ventilation channel, a moisture separator, and a hydrogen burner installed therein. 6. The electrochemical decontamination plant for radioactive waste metal according to claim 1, wherein the treatment module for radioactive waste metal comprises a radioactive waste metal degreasing unit, the radioactive waste metal degreasing unit comprising:pipelines equipped with shutoff valves and supplying degreasing solutions, high pressure water, steam and air;a radioactive waste metal degreasing container connected to the pipelines equipped with shutoff valves and supplying degreasing solutions, high pressure water, steam and air;a pH meter connected to the radioactive waste metal degreasing container; anda sediment discharge device connected to the radioactive waste metal degreasing container. 7. The electrochemical decontamination of plant for radioactive waste metal according to claim 1, wherein the treatment module for radioactive waste metal comprises a non-metallic coating etching unit for radioactive waste metal, the non-metallic coating etching unit for radioactive waste metal comprising:pipelines equipped with shutoff valves and supplying acid agents, high pressure water and air;a container for non-metallic etching of radioactive waste metal, the container for non-metallic coating etching of radioactive waste metal connected to the pipelines equipped wth shutoff valves and supplying acid agents, high pressure water and air;a pH meter connected to the container for non-metallic coating etching of radioactive waste metal; anda sediment discharge device connected to the container for non-metallic coating etching of radioactive waste metal. 8. The electrochemical decontamination of plant for radioactive waste metal according to claim 1, wherein the treatment module for radioactive waste comprises a flush unit, the flush unit connected with a high pressure water supply pipeline to the electrochemical decontamination module for radioactive waste metal, a radioactive waste metal degreasing unit, and a non-metallic coating etching unit for radioactive waste metal; the high pressure water supply pipeline equipped with shutoff valves. 9. The electrochemical decontamination plant for radioactive waste metal according to claim 1, wherein the decontamination solution preparation module, the decontamination solution receipt module, a radioactive waste metal degreasing unit, and a non-metallic coating etching unit for radioactive waste metal are connected to the ventilation module via the ventilation channel. 10. The electrochemical decontamination of plant for radioactive waste metal according to claim 1, wherein the decontamination solution supply and discharge pipeline is designed to transfer decontamination solution:(i) from the decontamination solution preparation module to the electrochemical decontamination module for radioactive waste metal and the decontamination solution receipt module,(ii) from the electrochemical decontamination module for radioactive waste metal to the decontamination solution receipt module, and(iii) from the decontamination solution receipt module to the electrochemical decontamination module for radioactive waste metal. 11. The electrochemical decontamination plant for radioactive waste metal according to claim 6, wherein the treatment module for radioactive waste comprises a flush unit, the flush unit connected with a high pressure water supply pipeline to the electrochemical decontamination module for radioactive waste metal, the radioactive waste metal degreasing unit and a non-metallic coating etching unit for radioactive waste metal; the high pressure water supply pipeline equipped with shutoff valves. 12. The electrochemical decontamination plant for radioactive waste metal according to claim 7, wherein the treatment module for radioactive waste comprises a flush unit, the flush unit connected with a high pressure water supply pipeline to the electrochemical decontamination module for radioactive waste metal, a radioactive waste metal degreasing unit and the non-metallic coating etching unit for radioactive waste metal; the high pressure water supply pipeline equipped with shutoff valves. 13. The electrochemical decontamination plant for radioactive waste metal according to claim 6, wherein the decontamination solution preparation module, the decontamination solution receipt module, the radioactive waste metal degreasing unit, and a non-metallic coating etching unit for radioactive waste metal are connected to the ventilation module via the ventilation channel. 14. The electrochemical decontamination plant for radioactive waste metal according to claim 7, wherein the decontamination solution preparation module, the decontamination solution receipt module, a radioactive waste metal degreasing unit, and the non-metallic coating etching unit for radioactive waste metal are connected to the ventilation module via the ventilation channel. |
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abstract | A workpiece or semiconductor wafer is tilted as a ribbon beam is swept up and/or down the workpiece. In so doing, the implant angle or the angle of the ion beam relative to the workpiece remains substantially constant across the wafer. This allows devices to be formed substantially consistently on the wafer. Resolving plates move with the beam as the beam is scanned up and/or down. This allows desired ions to impinge on the wafer, but blocks undesirable contaminants. |
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description | This application relates generally to systems and methods for obtaining and displaying an X-ray image. In particular, this application relates to systems and methods for using an X-ray collimator to generate an X-ray image in which one or more corners of an X-ray detector that is used to capture the image are not displayed as part of the image. In this manner, a relatively large view of the image can be displayed and rotated on a square or rectangular display device without changing the image's shape or size as the image is rotated. A typical X-ray imaging system comprises an X-ray source and an X-ray detector. The X-rays that are emitted from the X-ray source can impinge on the X-ray detector and provide an X-ray image of the object (or objects) that are placed between the X-ray source and the X-ray detector. In one type of X-ray imaging system, a fluoroscopic imaging system, the X-ray detector is often an image intensifier or, more recently, a flat panel digital detector. In many medical imaging applications, a collimator is placed between the X-ray source and the X-ray detector to limit the size and shape of the field of the X-ray beam. The collimator can shape or limit the X-ray beam to an area of a patient's body that requires imaging, preventing unnecessary X-ray exposure to areas surrounding the body part that is being imaged and protecting the patient from needless X-ray exposure. And because the collimator can limit the X-rays impinging on the X-ray detector near the body part being imaged, the collimator helps improve image contrast and quality. For example, the collimator can reduce excess X-rays from impinging on a flat panel digital detector, reducing or preventing image blooming or bleeding (which tend to occur when the detector is overloaded with X-rays). Thus, some conventional collimators can minimize X-ray exposure and maximize the efficiency of the X-ray dosage to obtain an optimum amount of data for diagnosis. This application relates to systems and methods for obtaining and displaying a collimated X-ray image. The methods can include providing an X-ray device having an X-ray source, a square or rectangular X-ray detector, and a collimator. The collimator can be sized and shaped to collimate an X-ray beam from the X-ray source that exposes a receptor region on the detector. The collimator can allow the X-ray image received by the X-ray detector to have any suitable shape that allows a relatively large view of the image to be displayed and rotated on the display device without changing the shape or size of the image as it rotated. In some instances, the collimator provides the image with superellipse shapes or cornerless shapes having four substantially straight edges with a 90 degree corner missing between at least two edges that run substantially perpendicular to each other (e.g., a squircle, a rounded square, rounded rectangle, a chamfered square, chamfered rectangle, etc.). The Figures illustrate specific aspects of the systems and methods for displaying collimated X-ray images. Together with the following description, the Figures demonstrate and explain the principles of the structures, methods, and principles described herein. In the drawings, the thickness and size of components may be exaggerated or otherwise modified for clarity. The same reference numerals in different drawings represent the same element, and thus their descriptions will not be repeated. Furthermore, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the described devices. Moreover, for clarity, the Figures may show simplified or partial views, and the dimensions of elements in the Figures may be exaggerated or otherwise not in proportion. The following description supplies specific details in order to provide a thorough understanding. Nevertheless, the skilled artisan would understand that the described systems and methods for obtaining and displaying collimated X-ray images can be implemented and used without employing these specific details. Indeed, the described systems and methods can be placed into practice by modifying the illustrated devices and methods and can be used in conjunction with any other apparatus and techniques conventionally used in the industry. For example, while the description below focuses on systems and methods for displaying collimated X-ray images that were created using a fluoroscopic X-ray device that obtains X-ray images in near real time, the described systems and methods (or portions thereof) can be used with any other suitable device or technique. For instance, the described systems and methods (or portions thereof) may be used with X-ray devices that produce traditional, plain X-ray images; with X-ray treatment procedures used in radiation therapy; in procedures for collimating gamma radiation; in nuclear medicine; and/or for a combination of different imaging and/or treatment techniques. As the terms on, attached to, connected to, or coupled to are used herein, one object (e.g., a material, an element, a structure, etc.) can be on, attached to, connected to, or coupled to another object, regardless of whether the one object is directly on, attached, connected, or coupled to the other object or whether there are one or more intervening objects between the one object and the other object. Also, directions (e.g., on top of, below, above, top, bottom, side, up, down, under, over, upper, lower, horizontal, vertical, etc.), if provided, are relative and provided solely by way of example and for ease of illustration and discussion and not by way of limitation. Where reference is made to a list of elements (e.g., elements a, b, c), such reference is intended to include any one of the listed elements by itself, any combination of less than all of the listed elements, and/or a combination of all of the listed elements. Furthermore, as used herein, the terms a, an, and one may each be interchangeable with the terms at least one and one or more. Additionally, the terms X-ray image, image, collimated image, and collimated X-ray image may refer to an X-ray image that is produced from a portion of an X-ray detector that is exposed to an X-ray beam that has been collimated with a collimator. As used herein, in some embodiments the term square may refer to a shape with four sides of equal length that also has four 90 degree corners. The term circle, in some embodiments, may refer to a closed plane curve having all points at a given distance from a common center point. The term squircle, in some embodiments, may refer to a Boolean intersection of a concentric circle and square, where the final shape has an area less than either the circle or the square. The term squircle, in other embodiments, may refer to a Boolean intersection of a square and a concentric circle whose diameter is greater than the length of the side of the square, but less than the diagonal of the square. The term mathematical squircle, in some embodiments, may refer to a specific type of superellipse with a shape between those of a concentric square and circle and may be expressed as a quadric planar curve or as a quadric Cartesian equation. A mathematical squircle, as opposed to the squircle shapes immediately above, maintains the tangent continuity between the circular corners with the flatter edges of a superellipse. The terms rounded square and rounded rectangle, in some embodiments, may respectively refer to a square or a rectangle with fillets breaking the corners (e.g., circular corners that are tangent to the edges of the square or rectangle). Additionally, in some embodiments the terms chamfered square and chamfered rectangle may respectively refer to a square and rectangle having any number of chamfers breaking their corners. As mentioned above, this application describes systems and methods for displaying collimated X-ray images. In some embodiments, the described systems and methods use a collimator to prevent an X-ray beam from impinging on one or more corners of an X-ray detector. The collimator can provide the image with any suitable shape that allows one or more corners of the X-ray detector that is used to obtain the image not to be displayed in the image. In some instances, the X-ray image has a perimeter with a (i) a superellipse shape and (ii) a cornerless shape with at least two substantially straight edges that run substantially perpendicular to each other, wherein such edges do not physically intersect with each other at a 90-degree corner. Some embodiments of a collimated image 10 are shown in FIG. 1. By having any of the described shapes, the collimated image can be shown on a relatively large portion of a display area of a display device (e.g., a square or rectangular monitor, screen, projector, TV, etc.), and the entire image can be viewed as it is rotated about its center, without requiring the image to be reshaped or resized. Thus, the described collimated image can maintain its size and geometry during rotation on the display device, while maximizing its on-screen, image size and the amount of the receptor area of the X-ray detector that is used to take the image. FIG. 2 shows some embodiments of a method 150 for displaying the described collimated X-ray images. Although this method can be modified in any suitable manner (including by rearranging, adding to, removing, modifying, substituting, and otherwise modifying various portions of the method), FIG. 2 shows those embodiments in which the method begins at 155 by providing an X-ray system 15. The X-ray system 15 can comprise any suitable X-ray device that is capable of capturing the described X-ray images 10. For example, the X-ray system can comprise a mobile X-ray device (e.g., an X-ray device comprising a C-arm, a mini C-arm, an O-arm, a non-circular arm, etc.), and a fixed X-ray device. By way of illustration, FIG. 3 shows an X-ray imaging system 15 comprises that a C-arm X-ray device 18. The X-ray system 15 can also comprise any component that allows it to take the collimated X-ray images 10. In some embodiments, FIG. 3 shows the X-ray imaging system 15 comprises an X-ray source 20, an X-ray detector 25, and a collimator 30. Any suitable X-ray source can be used, including a standard X-ray source, a rotating anode X-ray source, a stationary or fixed anode X-ray source, a solid state X-ray emission source, or a fluoroscopic X-ray source 35 (as shown in FIG. 3). Any suitable X-ray detector can be used, such as an image intensifier or a flat panel digital detector 40 (as shown in FIG. 3). Indeed, in some embodiments, the X-ray detector comprises a square or a rectangular flat panel detector. FIG. 3 shows some embodiments in which the collimator 30 comprises an X-ray attenuating material 45 that defines an aperture 50. The collimator 30 can comprise any suitable X-ray attenuating material 45 that allows it to collimate an X-ray beam. Some examples of suitable X-ray attenuating materials include tungsten, lead, gold, copper, tungsten-impregnated substrates (e.g., glass or a polymer impregnated with tungsten), coated substrates (e.g., glass or a polymer coated with tungsten, lead, gold, etc.), steel, aluminum, bronze, brass, rare earth metals, or combinations thereof. In some embodiments, however, the collimator comprises tungsten. The collimator 30 collimates an X-ray beam (not shown) so that a resultant image 10 comprises any suitable shape that does not include one or more corners of the X-ray detector 40 that is used to obtain the image. In some embodiments, however, the collimator provides the image with a shape corresponding to a shape of the aperture, wherein the image shape is a superellipse shape or a cornerless shape. A cornerless shape comprises a shape missing one or more 90 degree corners (i.e., two edges that run substantially perpendicular to each other without containing a 90 degree corner between those edges). The cornerless shape may contain corners with a degree less than 90 degrees. Some examples of such shapes include a rounded square, a rounded rectangle, a chamfered square, a chamfered rectangular, a rectangle with curved borders, a truncated circle, an octagon, a hexagon, or any other suitable shape. Where the aperture 50 has the shape of a superellipse, it can have any suitable characteristic that allows the shape of the aperture to be classified as a superellipse (as described above) and that allows the collimator 30 to prevent the X-ray beam from impinging on the corners of the X-ray detector 40. By way of example, the aperture can be a shape that is generated by a formula selected from: (i) (x−a)4+(y−b)4=r4, (ii) |x−a|n+|y−b|n=|r|n, and (iii) ( x - a ) r a n + ( y - b ) r b n = 1 ,wherein a, b is the center point; r is the minor; n is equal to 4; and ra and rb are the semi-major and semi-minor axes, respectively. FIG. 4 shows some embodiments in which the collimator 30 defines an aperture that has a shape of a superellipse. Furthermore, FIG. 4 shows the aperture is sized so that a portion of the collimator 30 overlaps (and thereby collimates) the corners 55 of a corresponding square X-ray detector 25 (e.g., a flat panel detector 40), wherein the perimeter 60 of a receptor area 62 of the detector 25 is illustrated by a dotted line. FIG. 5 shows one example of a collimated image 10 that has been taken with the collimator 30 of FIG. 4. In particular, FIG. 5 shows that image 10 has a shape of a superellipse, in which a first 51 and second 52 images edge, a second 52 and third 53 image edge, a third 53 and fourth 54 edge, and a fourth 54 and first 51 image edge, respectively, do not physically intersect at a 90 degree corner. Instead, FIG. 5 shows the image's corners 56 are trimmed (or collimated) so the image's first edge 51 and third edge 53 are each separated from the image's second edge 52 and fourth edge 54 by a non-linear (i.e., substantially curved) image border 65. Where the aperture 50 comprises the cornerless shape, the aperture can have any suitable characteristic that allows it to function as intended. In one example, FIG. 6 shows the aperture 50 comprises a first 91 and third 93 aperture edge, which run substantially parallel to each other, and which run substantially perpendicular to both a second 92 and a fourth 94 aperture edge. In another example, FIG. 6 shows that instead of physically intersecting at a 90 degree corner, one or more corresponding aperture edges that run perpendicular to each other (e.g., the second 92 and third 93 aperture edges) can be attached to each other with a border 63 that allows the collimator 30 to shield a 90 degree corner 55 of a corresponding detector 40. Additionally, while this disclosure focuses on using an aperture in which all of the apertures edges are substantially equal in length (e.g., an aperture having the appearance of a trimmed square), the skilled artisan will recognize that the aperture could be modified so that any two edges running parallel to each other may be longer or shorter than the other edges of the aperture (e.g., the aperture could have the appearance of a rectangle with trimmed corners). Where the aperture 50 comprises one of the described cornerless shapes, the aperture can be missing any suitable number of corners (e.g., one or more corners of the aperture can be filled in with an X-ray attenuating material), including 1, 2, 3, 4, or more. Indeed, FIG. 6 shows configurations where a portion of the collimator 30 shields two corners 55 (located diagonally from each other) of the X-ray detector's receptor area 62. Accordingly, FIG. 7 shows that an image 10 captured with the configuration of FIG. 6 contains two corners 56 that lack a 90 degree corner between two perpendicular edges of the image (e.g., between the image's second 52 and third 53 edges and between the image's first 51 and fourth 54 edges). Where the aperture 50 is missing one or more corners (e.g., contains an X-ray attenuating material that prevents the X-ray beam from impinging on one or more corners of a corresponding X-ray detector 40), the collimator can collimate the X-ray beam so that the resultant image 10 has any suitably shaped border 63 between adjacent aperture edges that run perpendicular to each other. Some examples of suitable borders include a border with the shape of an arc of a circle, a chamfered border, a rounded border, a convex border, a concave border, a zigzagged border, a curved border, an irregular border, etc. In this regard, FIG. 8 shows that in some embodiments in which all four borders 63 of the aperture 50 (and therefore borders of the image 63) comprise an arc-shaped border 66, the aperture 50 defines a squircle. FIG. 10 shows some configurations in which the each of the aperture's four borders 63 comprises a rounded border 67, the aperture 50 can comprise rounded square (or rectangle where applicable). Additionally, FIG. 12 shows some embodiments in which each of the aperture's four borders 63 comprises a chamfered border 68, the aperture 50 comprises a chamfered square (or rectangle where applicable). Images with shapes corresponding to the collimators 30 of FIGS. 8, 10, and 12 are respectively shown in FIGS. 9, 11, and 13. Additionally, where a border 63 (as described above) separates two substantially perpendicular edges (e.g., 91 and 92, 92 and 93, 93, and 94, and/or 91 and 94) of the aperture 50, the borders can be any suitable shape that allows the collimator to function as described herein. By way of example, FIGS. 14-16 illustrate additional embodiments in which the image 10 has the shape of a squircle, wherein the image 10 in FIG. 14 is predominantly square shaped, the image 10 in FIG. 16 is predominantly circular in shaped, and the image 10 in FIG. 15 has a shape between those shown in FIGS. 14 and 16. Where the collimator 30 shields a portion of the X-ray detector 25 (e.g. one or more of the detector's corners 55), the aperture can leave any suitable amount of the receptor area 62 exposed to X-rays from the X-ray source 20. This configuration allows an image 10 taken with the collimator to be rotated on a display device without being resized or reshaped. In some cases, the collimator allows less than an amount selected from about 100%, about 98.5%, about 94%, about 90%, about 87%, or about 80% of the detector's receptor area to be exposed to X-rays from the X-ray source. In other cases, the collimator allows more than an amount selected from about 78.5%, about 79%, about 80%, about 82%, about 84%, and about 85% of the detector's receptor area to be exposed to X-rays from the X-ray source. In yet other cases, the aperture can allow any suitable combination or sub-range of these amounts of the detector's receptor area to be exposed to X-rays. For example, FIG. 17 shows some embodiments in which the collimator (not shown) allows (from left to right) about 98.2%, about 93.7%, and about 86.1% of the detector's receptor area 62 to be exposed to X-rays (exposed area 105) and in which about 1.8%, about 6.4%, and about 17.9% of the receptor area 62, respectively is shielded by the collimator (unexposed area 110). In other words, FIG. 17 shows the trade-off between an on-screen image size and the detector utilization. The more square the image is, the smaller it has to appear on the screen in order to be rotatable. In some embodiments of the squircles described herein, the geometry could range between a full square (100% of the detector utilized) and a full circle (78.5% utilization). Returning to the method 150 in FIG. 2, after an image 10 has been taken of an object (as shown at 160), the method continues at box 165, where the collimated X-ray image is shown on a display device (e.g., a screen, monitor, tablet/handheld device, etc.). The image can take up any suitable amount of the display device's display area that allows the entire image to be viewed as it is rotated at least 45 degrees about its center, without being resized or reshaped. The height H (e.g., the distance between the first 51 and third 53 or second 52 and fourth 54 edges) of the image 10 can be any height that allows the entire image to be rotated on the display device 115 without the image being resized or reshaped. In some embodiments, the height H is greater than an amount selected from about 71.6%, about 75%, about 80%, or about 82.5% of the narrower of the width and length of the display area. In other embodiments, the height H of the image is less than an amount selected from about 100%, about 98%, about 95%, and about 90% of the height of the display area. In yet other embodiments, the image's height H falls between any suitable combination or sub-range of these amounts. For example, FIG. 17 shows that where the display device 115 comprises an HD display device (e.g., a device having a pixel resolution of 1080 pixels by 1920 pixels), the image 10 can have a height of about 841 pixels (where about 98.2% of the detector's receptor area is exposed), about 921 pixels (where about 93.7% of the detector's receptor area 62 is exposed), or about 1012 pixels (where about 78.5% of the detector's receptor area is exposed to X-rays). Because some embodiments of the aperture 50 can have any shape between a full circle (e.g., in which about 78.5% of a square detector are is utilized) and a true square (e.g., in which about 100% of the square detector is utilized), the widest portion D (e.g., a diagonal measurement) of the collimated image can be any length that allows the entire image to be rotated on the display device 115 without the image being resized or reshaped. In some embodiments, the widest portion D of the image is less than an amount selected from about 100%, about 99%, and about 96% of the width or length of the display's display area, whichever is narrower. In other embodiments, the widest portion D of the image is greater than an amount selected from about 85%, about 90%, and about 95% of the width or length of the display area, whichever is narrower. In still other embodiments, the widest portion D of the image can be between any suitable combination or sub-range of these amounts. In some instances, the shape of the aperture 50 helps provide a desired balance between the on-screen image size of the image 10 and detector utilization. By way of illustration, FIG. 17 shows that, in some cases, the more square the image 10 is, the smaller it has to be on the display device 115 in order to be entirely seen as it is rotated. In contrast, where the aperture has borders 63 that are arcs of a true circle, thereby providing a squircle image, the entire squircle can be rotated on the display device without clipping any part of the image and without rescaling the shape as long as the true circle could be fully displayed on the device. Returning to FIG. 2, the method 150 continues at box 165 where the collimated image is optionally shown on a display device 115. At box 170, the method 150 optionally includes a process of rotating the image clockwise and/or counterclockwise. FIGS. 18 through 20 show successive views of the image 10 being rotated counter-clockwise on a display device 115. As the image 10 is rotated, the entire image can be viewed on the display device 115, without any resizing or reshaping of the image. Thus, in some embodiments, the exposed area of the live image (and not the processed image) can be substantially equal to the displayed area. Where the collimated X-ray images 10 are shown, rotated, or otherwise manipulated on a display device 115, the display device can be used with any suitable computing environment. FIG. 21 describes some embodiments of one exemplary computing environment. These embodiments can include one or more processing units in a variety of customizable enterprise configurations, including in a networked or combination configuration. These embodiments can include one or more computer readable media, wherein each medium may be configured to include or includes thereon data or computer executable instructions for manipulating data. The computer executable instructions can include data structures, objects, programs, routines, or other program modules that may be accessed by one or more processors, such as one associated with a general-purpose modular processing unit capable of performing various different functions or one associated with a special-purpose modular processing unit capable of performing a limited number of functions. Computer executable instructions cause the one or more processors of the enterprise to perform a particular function or group of functions and are examples of program code means for implementing steps for methods of processing. Furthermore, a particular sequence of the executable instructions provides an example of corresponding acts that may be used to implement such steps. Examples of computer readable media (including non-transitory computer readable media) include random-access memory (“RAM”), read-only memory (“ROM”), programmable read-only memory (“PROM”), erasable programmable read-only memory (“EPROM”), electrically erasable programmable read-only memory (“EEPROM”), compact disk read-only memory (“CD-ROM”), any solid state storage device (e.g., flash memory, smart media, etc.), or any other device or component capable of providing data or executable instructions that may be accessed by a processing unit. With reference to FIG. 21, a representative enterprise includes modular processing unit 200, which may be used as a general-purpose or special-purpose processing unit. For example, modular processing unit 200 may be employed alone or with one or more similar modular processing units as a personal computer, a notebook computer, a personal digital assistant (“PDA”) or other hand-held device, a workstation, a minicomputer, a mainframe, a supercomputer, a multi-processor system, a network computer, a processor-based consumer device, a cellular phone, a smart appliance or device, a control system, or the like. Using multiple processing units in the same enterprise provides increased processing capabilities. For example, each processing unit of an enterprise can be dedicated to a particular task or can jointly participate in distributed processing. In FIG. 21, the modular processing unit 200 includes one or more buses and/or interconnects 205, which may be configured to connect various components thereof and enables data to be exchanged between two or more components. The bus(es)/interconnect(s) 205 may include one of a variety of bus structures, including a memory bus, a peripheral bus, or a local bus that uses any of a variety of bus architectures. Typical components connected by the bus(es)/interconnect(s) 205 include one or more processors 210 and one or more memories 215. Other components may be selectively connected to the bus(es)/interconnect(s) 205 through the use of logic, one or more systems, one or more subsystems and/or one or more I/O interfaces, hereafter referred to as data manipulating system(s) 220. Moreover, other components may be externally connected to the bus(es)/interconnect(s) 205 through the use of logic, one or more systems, one or more subsystems and/or one or more I/O interfaces, and/or may function as logic, one or more systems, one or more subsystems, and/or one or more I/O interfaces, such as one or more modular processing unit(s) 245 and/or proprietary device(s) 255. Examples of I/O interfaces include one or more mass storage device interfaces, one or more input interfaces, one or more output interfaces, and the like. Accordingly, embodiments of the described systems and methods embrace the ability to use one or more I/O interfaces and/or the ability to change the usability of a product based on the logic or other data manipulating system employed. The logic may be tied to an interface, part of a system, subsystem and/or be used to perform a specific task. Accordingly, the logic or other data manipulating system may allow, for example, for IEEE1394 (firewire), wherein the logic or other data manipulating system is an I/O interface. Alternatively or additionally, logic or another data manipulating system may be used that allows a modular processing unit to be tied into another external system or subsystem. For example, an external system or subsystem that may or may not include a special I/O connection. Alternatively or additionally, logic or another data manipulating system may be used wherein no external I/O is associated with the logic. Embodiments of the described systems and methods also embrace the use of specialty logic, such as for ECUs for vehicles, hydraulic control systems, etc. and/or logic that informs a processor how to control a specific piece of hardware. Moreover, those skilled in the art will appreciate that embodiments of the described systems and methods embrace a plethora of different systems and/or configurations that utilize logic, systems, subsystems and/or I/O interfaces. As provided above, embodiments of the described systems and methods embrace the ability to use one or more I/O interfaces and/or the ability to change the usability of a product based on the logic or other data manipulating system employed. For example, where a modular processing unit is part of a personal computing system that includes one or more I/O interfaces and logic designed for use as a desktop computer, the logic or other data manipulating system can be changed to include flash memory or logic to perform audio encoding for a music station that wants to take analog audio via two standard RCAs and broadcast them to an IP address. Accordingly, the modular processing unit may be part of a system that is used as an appliance rather than a computer system due to a modification made to the data manipulating system(s) (e.g., logic, system, subsystem, I/O interface(s), etc.) on the back plane of the modular processing unit. Thus, a modification of the data manipulating system(s) on the back plane can change the application of the modular processing unit. Accordingly, embodiments of the described systems and methods embrace very adaptable modular processing units. As provided above, processing unit 200 includes one or more processors 210, such as a central processor (or CPU) and optionally one or more other processors designed to perform a particular function or task. It is typically the processor 210 that executes the instructions provided on computer readable media, such as on the memory(ies) 215, a magnetic hard disk, a removable magnetic disk, a magnetic cassette, an optical disk, or from a communication connection, which may also be viewed as a computer readable medium. The memory(ies) 215 includes one or more computer readable media that may be configured to include or includes thereon data or instructions for manipulating data, and may be accessed by the processor(s) 210 through the bus(es)/interconnect(s) 205. The memory(ies) 215 may include, for example, ROM(s) 225, used to permanently store information, and/or RAM(s) 226, used to temporarily store information. The ROM(s) 225 may include a basic input/output system (“BIOS”) having one or more routines that are used to establish communication, such as during start-up of the modular processing unit 200. During operation, the RAM(s) 226 may include one or more program modules, such as one or more operating systems, application programs, and/or program data. As illustrated, at least some embodiments of the described systems and methods embrace a non-peripheral encasement, which provides a more robust processing unit that enables use of the unit in a variety of different applications. In FIG. 21, one or more mass storage device interfaces (illustrated as data manipulating system(s) 220) may be used to connect one or more mass storage devices 230 to the bus(es)/interconnect(s) 205. The mass storage devices 230 are peripheral to the modular processing unit 200 and allow the modular processing unit 200 to retain large amounts of data. Examples of mass storage devices include hard disk drives, magnetic disk drives, tape drives and optical disk drives. A mass storage device 230 may read from and/or write to a magnetic hard disk, a removable magnetic disk, a magnetic cassette, an optical disk, or another computer readable medium. The mass storage devices 230 and their corresponding computer readable media provide nonvolatile storage of data and/or executable instructions that may include one or more program modules, such as an operating system, one or more application programs, other program modules, or program data. Such executable instructions are examples of program code means for implementing steps for methods disclosed herein. The data manipulating system(s) 220 may be employed to enable data and/or instructions to be exchanged with the modular processing unit 200 through one or more corresponding peripheral I/O devices 235. Examples of the peripheral I/O devices 235 include input devices such as a keyboard and/or alternate input devices, such as a mouse, trackball, light pen, stylus, or other pointing device, a microphone, a joystick, a game pad, a satellite dish, a scanner, a camcorder, a digital camera, a sensor, and the like, and/or output devices such as a display device 115 (e.g., a monitor or display screen), a speaker, a printer, a control system, and the like. Similarly, examples of the data manipulating system(s) 220 coupled with specialized logic that may be used to connect the peripheral I/O devices 235 to the bus(es)/interconnect(s) 205 include a serial port, a parallel port, a game port, a universal serial bus (“USB”), a firewire (IEEE 1394), a wireless receiver, a video adapter, an audio adapter, a parallel port, a wireless transmitter, any parallel or serialized I/O peripherals or another interface. The data manipulating system(s) 220 enable an exchange of information across one or more network interfaces 240. Examples of the network interfaces 240 include a connection that enables information to be exchanged between processing units, a network adapter for connection to a local area network (“LAN”) or a modem, a wireless link, or another adapter for connection to a wide area network (“WAN”), such as the Internet. The network interface 240 may be incorporated with or peripheral to modular processing unit 200, and may be associated with a LAN, a wireless network, a WAN and/or any 260 connection (see FIG. 22) between processing units. The data manipulating system(s) 220 enables the modular processing unit 200 to exchange information with one or more other local or remote modular processing units 245 or computer devices. A connection between modular processing unit 200 and modular processing unit 245 may include hardwired and/or wireless links. Accordingly, embodiments of the described systems and methods embrace direct bus-to-bus connections. This enables the creation of a large bus system. It also eliminates hacking as currently known due to direct bus-to-bus connections of an enterprise. Furthermore, the data manipulating system(s) 220 enable the modular processing unit 200 to exchange information with one or more proprietary I/O connections 250 and/or one or more proprietary devices 255. Program modules or portions thereof that are accessible to the processing unit may be stored in a remote memory storage device. Furthermore, in a networked system or combined configuration, the modular processing unit 200 may participate in a distributed computing environment where functions or tasks are performed by a plurality of processing units. Alternatively, each processing unit of a combined configuration/enterprise may be dedicated to a particular task. Thus, for example, one processing unit of an enterprise may be dedicated to video data, thereby replacing a traditional video card, and provides increased processing capabilities for performing such tasks over traditional techniques. While those skilled in the art will appreciate that the described systems and methods may be practiced in networked computing environments with many types of computer system configurations, FIG. 22 represents an embodiment of a portion of the described systems in a networked environment that includes clients (265, 270, 275, 280, etc.) connected to a server 285 via a network 260. While FIG. 22 illustrates an embodiment that includes four clients connected to the network, alternative embodiments include one client connected to a network or many clients connected to a network. Moreover, embodiments in accordance with the described systems and methods also include a multitude of clients throughout the world connected to a network, where the network is a wide area network, such as the Internet. Accordingly, in some embodiments, the described systems and methods can allow a collimated image 10 to be taken in a first location and a user (e.g., a radiologist, technician, physician, etc.) to view, rotate, and otherwise manipulate the image from a second location. As previously mentioned, the described systems and methods can be modified in any suitable manner. In one example, where computer software is used to display the described collimated images 10 on a display device, the software can be used to clean up the images in any suitable manner. For instance, the software can be used to remove shadows, fuzzy lines, or to otherwise sharpen the image's edges. The described systems and methods for displaying collimated X-ray images 10 have several useful features. First, unlike some conventional methods that use a collimator to shield a relatively large amount of the detector's receptor area, some embodiments of the described systems and methods shield a relatively small amount of the detector's receptor area (as discussed above). Thus, some conventional methods are limited to using a collimator having a circular aperture with a circumference that falls completely within all of the perimeters of a four-sided flat panel detector. As a result, a relatively large amount of the receptor area in such conventional methods is not used. Second, while some conventional methods shrink an X-ray image as the image is rotated, some embodiments of the described systems and methods allow the image to be relatively large with respect to the display's display area and to be rotated while maintaining a substantially constant size and shape. And third, unlike some conventional methods for displaying an X-ray image that only show a small square image that can be rotated without being resized or reshaped, some embodiments of the described methods allow the described images 10 to use a relatively large amount of the display's display area without needing any resizing or reshaping. Thus, users of the described systems can see better detail on the collimated images than may be obtained through some other conventional methods. In addition to any previously indicated modification, numerous other variations and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of this description, and appended claims are intended to cover such modifications and arrangements. Thus, while the information has been described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred aspects, it will be apparent to those of ordinary skill in the art that numerous modifications, including, but not limited to, form, function, manner of operation, and use may be made without departing from the principles and concepts set forth herein. Also, as used herein, the examples and embodiments, in all respects, are meant to be illustrative only and should not be construed to be limiting in any manner. |
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046648712 | summary | BACKGROUND OF THE INVENTION Field Of The Invention The invention relates to a nuclear power installation with a high-temperature pebble bed reactor, whose core consists of a pile of spherical fuel elements which is surrounded on all sides by a reflector. A cooling gas flows through the core from the bottom to top. The nuclear power plant has a heat utilization system which is arranged above the high-temperature pebble bed reactor and located, along with said reactor, in a multi-component, cylindrical, steel pressure vessel. The heat utilization system also has cooling gas flowing through it from bottom to top. The nuclear power plant also has at least two circulating blowers adjusted to the direction of flow of the heat utilization system, and two shut-down arrangements comprising absorber elements of different types. A high-temperature reactor with spherical fuel elements (AVR installation) is known in which the heat utilization system consists of a steam generator arranged above the reactor. The blowers for circulating the cooling gas, flowing from bottom to top through the reactor core, are located under the reactor core in this nuclear reactor. In case the blowers fail, the residual heat is transferred by natural convection to both the heat generator and the constructions surrounding the reactor core. The latter comprises, in addition to a graphite reflector jacket, a carbon brick jacket. In order to safely contain the fission products released, the nuclear reactor and the steam generator are surrounded by a double steel pressure vessel. For the shut-down of the nuclear reactor, absorber rods are provided, which may be inserted from the top into graphite columns located in the reactor core. There are no additional shut-down arrangements. The state of the art further includes a nuclear reactor installation with a high-temperature pebble bed reactor and a plurality of steam generators housed, together with the reactor, in a cylindrical steel pressure vessel (German Offenlegungsschriften Nos. 32 12 264 and 32 12 266). Cooling gas flows from bottom to top through the reactor core. The heated cooling gas is conducted to the steam generators from above through a central hot gas line in the vessel. The circulating blowers are mounted horizontally on the outside of the jacket of the steel pressure vesel, requiring much space. The control and shut-down arrangements, which comprise a plurality of absorber rods that may be inserted from below into the bores of the side reflector and drives associated with them, also require much space. The side reflector may be provided with graphite projections, projecting into the pile of fuel elements. These projections would have bores into which additional absorber rods may be inserted. Small absorber spheres are provided as a diverse shut-down installation, for introduction into the fuel element pile. Another nuclear reactor installation with a high-temperature pebble bed reactor also comprises several steam generators installed above the small reactor, in which the steam generators and the small reactor are both contained in a cylindrical steel pressure vessel. The cooling gas flows from bottom to top through both the reactor core and the steam generators. The circulating blowers are located entirely inside the steel pressure vessel, above the steam generators. This presumes a relatively large height of the steel pressure vessel. The small reactor is equipped with two diverse shut-down devices, which are again completely arranged in the steel pressure vessel. The effective elements in both shut-down installations consist of absorber spheres which may be inserted into vertical channels in the side reflector. SUMMARY OF THE INVENTION In view of the aforementioned state of the art, an object of the present invention is to provide a nuclear power plant of the abovedescribed type with a compact structure in which the heights of the steel pressure vessel and of the core structure are reduced, the core diameter is increased with respect to known reactor installations. In addition, it is an object of the present invention to provide a small, high-temperature reactor with a high power density, which is highly economical. According to the invention, this object is attained by a nuclear power plant of the aforedescribed type in which an upper part of a cylindrical steel pressure vessel, which contains a heat utilization system, is retracted and equipped with a cover upon which circulating blowers are placed. The heat utilization system comprises, in a known manner, a single steam generator. The steam generator comprises at least two sub-systems independent of one another, with their own distributors and collectors, and with their own inlet and outlet lines. A first means for shutting down the reactor comprises, in an actually known manner, a plurality of absorber rods insertable into bores of the side reflector from above and comprises rod drives arranged outside the steel pressure vessel in the area of its retracted upper part. A second means for shutting down the reactor comprises small absorber spheres, and several storage containers and annular conduits for said spheres. The storage containers are also arranged outside the steel pressure vessel in the area of its retracted upper part. Annular conduits for the introduction of the small absorber spheres are, however, disposed inside the steel pressure vessel and connected with channels provided, in a known manner, in projections of the side reflector protruding into the core. As the nuclear power installation according to the invention has only one steam generator, the steel pressure vessel requires a smaller diameter only above the high-temperature pebble bed reactor, and there is sufficient space available for housing the rod drives and the storage containers of the first and second means for shutting down the reactor. These structural components are further readily accessible as they are arranged outside the steel pressure vessel. The subdivision of the steam generator assures the safe operation of the reactor even in case of the failure of one of the sub-systems. By broadening the core structure (fuel element pile and reflector) and arranging the circulating blowers outside and on top of the steel pressure vessel, the height of the steel pressure vessel may be considerably economized. |
052176819 | summary | BACKGROUND OF THE INVENTION This invention relates generally to pressure vessel enclosures and in particular to pressure vessel enclosures in which the compression in the enclosure walls can be continuously controlled and adjusted. This invention also relates to pressure safety enclosures that can be disassembled and assembled for inspection of the primary pressure vessel and its welds with a minimum of time and effort. Increasingly, in the operation of nuclear reactors, environmental degradation due to corrosion, mechanical or radiation effects on reactor vessel steels and weld materials have become a major concern. In order to extend its useful life, operating pressure or volume, a concept of a prestressed safety enclosure has been developed a nuclear reactor system. For safety reasons, pressure vessels, particularly nuclear reactor pressure vessels, must be designed so that all pressure or load carrying welds can be inspected periodically. This inspection can be performed on the pressure vessel either prior to service or later while the system is temporarily out of service, such as, during a reactor refueling outage. Most of the primary pressure vessels of the prior art comprised a single large vessel with one or more openings to gain access to the interior of the vessel. A flanged dome cover, usually fastened by studs to one end of the pressure vessel, provided the primary access to the interior of the vessel. To inspect the interior of the pressure vessel the studs in the peripheral flange surrounding the dome cover had to be removed in order for the dome to be removed. The main purpose of the pressure safety enclosure (PSE) of the present invention is to contain the fragments of the primary pressure vessel (PPV), the hot radioactive coolant fluid, the fragments of the nuclear core and any fission products in the unlikely event of an explosion, leak or other failure of the primary pressure vessel (PPV). SUMMARY OF THE INVENTION Since the primary safety enclosure (PSE) is designed to function during an accident, the PSE must always be prepared for an accident. Therefore, it must be permanently stressed so that it is in a state of three-dimensional (3-D) compression. This is necessary so that the "gapping," that is, opening or separation of the cast-iron blocks, will never occur even during the worst accident scenario, when the PSE becomes pressurized and hot due to a (postulated) primary pressure vessel explosion. The pressure vessel safety enclosure of the present invention is also adapted to enclose a primary pressure vessel, such as, a nuclear reactor pressure vessel, and contain it during operation. The pressure vessel safety enclosure of the present invention comprises a first pressure vessel containment assembly enclosing the primary pressure vessel. This first pressure vessel enclosure comprises a set of cast-iron blocks defining a vault or enclosure under pressure. The cast-iron block core vault must be so thick that it will not buckle when subjected to the external pressure load provided by the surrounding filler, jackets and tendons. The pressure vessel safety enclosure of the present invention further comprises a pair of first upper and first lower pressure vessel jackets that are adapted to enclose and be spaced apart from, respectively, the upper and lower halves of the pressure vessel safety enclosure (PSE). The space between the upper and lower pressure vessel jackets is filled with a low melting point, high boiling point metal. The rims of the upper and lower pressure vessel jackets are adapted to slidably engage, in a sealed relationship, the outer surface of the pressure vessel safety enclosure (PSE). The upper and lower jackets are supported, respectively, by upper and lower bearing plates or seats attached to respective upper and lower frusto-conical skirts. The upper and lower skirts are, respectively, attached to upper and lower ring girders. The upper and lower ring girders are connected to each other by a number of equally spaced, high-strength, post-tensioned tendon cables contained in insulated sleeves for corrosion protection. The ends of each of the tendon cables are connected to the respective upper and lower ring girders by a base anchor at the lower end and a permanent hydraulic or screw jack at the upper end. The hydraulic or screw jacks provide the force necessary to move the upper and lower jackets toward or away from each other to increase or decrease the compression in the pressure vessel safety enclosure. It is, therefore, an object of the present invention to provide a pressure vessel safety enclosure. It is another object of the present invention to provide a pressure vessel safety enclosure in which the compression in the enclosure can be continuously monitored, controlled and adjusted. It is a further object of the present invention to provide a pressure vessel safety enclosure in which moving jackets are used to control the compression in the vessel enclosure walls. It is still another object of the present invention to provide a pressure vessel safety enclosure that is easily dismantled without the use of stud and flange connections. It is another object of the present invention to provide a pressure vessel safety enclosure in which the compression in the walls of the enclosure is controlled and adjusted by the tension members holding the upper and lower pressure vessel jackets together. These and other objects of the present invention will become manifest upon study of the following specification taken together with the drawings. |
052672794 | claims | 1. A method for repairing an elongated metal hollow member welded to a wall of a pressure vessel of a nuclear reactor and extending into a coolant within said pressure vessel, the method comprising the steps of: removing said coolant from said hollow member; cutting and removing a defective portion of said hollow member; replacing the removed defective portion of said hollow member with a new hollow element; welding the new hollow element with a remaining part of said hollow member so as to form a hollow member consisting of the remaining part of the hollow member and the new hollow element; smoothing an inner peripheral surface of a wall portion of said new hollow member including a weld formed by said welding; locating a stainless steel sleeve on said inner peripheral surface of said wall portion in a coaxial relationship; fitting said sleeve onto said inner peripheral surface of said wall portion along an entire length of said sleeve; and heating said sleeve throughout so as to produce a molten metal portion including 4% or more by wt of .delta. ferrite penetrating into both said wall portion and said sleeve. 2. A method according to claim 1, wherein the step of heating is conducted by an electric arc welding machine. 3. A method according to claim 2, wherein the step of heating is conducted by heat sink welding. 4. A method according to claim 1, wherein the step of heating is carried out while cooling said wall portion. 5. A method according to claim 4, wherein said cooling is carried out by coolant within said pressure vessel. 6. A method according to claim 2, wherein the step of heating is conducted using a non-filler tungsten inert gas welding machine. 7. A method according to claim 1, wherein the step of fitting includes radially outwardly expending said sleeve. 8. A repaired elongated metal hollow member welded to a wall of a pressure vessel of a nuclear reactor and extending into a coolant within said pressure vessel, said metal hollow member including an original portion of the metal hollow member and a repairing hollow member portion, a stainless steel sleeve fitted to inner peripheral surfaces of said original portion of said metal hollow member and said repairing hollow member portion, a molten metal portion including 4% or more by wt of .delta. ferrite formed at a portion of the inner peripheral surfaces of the original portion of said metal hollow member and said repairing hollow member portion so as to form a new hollow member by a weld, and wherein, at said weld, said molten metal portion penetrates into both a well portion of said new hollow member and said sleeve. |
description | The instant application is a national phase of PCT International Application No. PCT/RU2014/000282 filed Apr. 18, 2014, and claims priority to Russian Patent Application Serial No. 2013139258, filed Aug. 26, 2013, the entire specifications of both of which are expressly incorporated herein by reference. A mass transfer apparatus relates to a field of energy mechanical engineering and can be used in power installations involving a liquid-metal heat carrier containing lead. The closest analogue to the claimed technical solution is a mass transfer apparatus according to RF Patent No. 2246561, C23F 11/00, 20.02.2005], comprising a housing and, provided therein, a flow reaction chamber filled with a solid-phase granulated oxidation agent, an electric heater positioned in the reaction chamber, a perforated grille for removal of enriched liquid-metal heat carrier, located above the reaction chamber, used for removal of oxygen-enriched liquid-metal heat carrier, a perforated grille for supplying liquid-metal heat carrier to the reaction chamber. Return line is made in the form of a ring channel. The housing is arranged inside a cylindrical shell ring having openings for heat carrier passage, and forms a ring channel in conjunction with it, the lower end of the cylindrical shell ring is blinded off, and its upper end is partially covered, in plain view, with a ring-shaped deflector screen. The disadvantages of the device known in the art device lie in the limited time of operation determined by reserves of the solid-state oxidation agent. Increasing the operational life by increasing the volume and load of the reaction chamber will lead to an increase in electrical energy consumption, since along with increasing the volume of the reaction chamber, the dimensions and power of the heater need to be increased, too. In addition, there are problems related to maintenance of the mass transfer apparatus, since, during extraction of the apparatus for reloading of the reaction chamber, the liquid-metal heat carrier, the filling apparatus and the cylindrical shell ring are extracted at the same time. The object of the invention is to create a mass transfer apparatus, ensuring a substantial increase of operational life during a single-shot loading of a solid-state granulated oxidation agent without increasing the electrical energy consumption when operating in a liquid-metal heat carrier enrichment mode. Another object of the invention is to create a mass transfer apparatus, during the extraction of which, a minimal amount of liquid-metal heat carrier is extracted. In order to achieve the set objects, a mass transfer device is provided as described below. The advantageous effect consists in increasing the operational life and the service life of a mass transfer apparatus, decreasing the electrical energy consumption, ensuring a possibility of its arrangement in limited space conditions, ensuring an automatic supply of a fresh oxidation agent, and ensuring the removal of a liquid-metal heat carrier from the apparatus during its extraction. The aforementioned advantageous effects are influenced by the following essential features of the mass transfer apparatus. The mass transfer apparatus comprises a housing and, provided therein, a flow reaction chamber filled with a oxidation agent, provided with an adjustable heating system, and systems for inlet and outlet of oxidizable material, wherein the housing of the apparatus is equipped with a repository for reserves of the oxidation agent. In addition, in the mass transfer apparatus, in the capacity of the adjustable heating system, an electric heater, particularly a rod-type electric heater, is used, and in the electric heater, in the capacity of the heating element, a high-resistance wire made of nichrome or fechral is used. In addition, a repository for reserves of the oxidation agent consists of a bottom and a side wall formed by the lower part of the housing, and in the upper part of the side wall of the repository for reserves of the oxidation agent, adjacent to the reaction chamber, there are openings made. Moreover, in the lower part of the side wall of the repository for reserves of the oxidation agent, there are openings made. In addition, the repository for reserves of the oxidation agent is located below the reaction chamber and below the lower end of the electric heater. Moreover, in the initial state, the volume of the repository for reserves of the oxidation agent is filled with the oxidation agent. In addition, the flow reaction chamber is formed by the middle part of the housing, defined from below by the upper part of the repository for reserves of the oxidation agent, and from above—by the restrictive grille, and in the restrictive grille, there are openings made. In addition, the system for inlet of the oxidizable material is formed by the upper part of the side wall of the repository of reserves of the oxidation agent. In addition, the system for outlet of the oxidizable material is formed by the restrictive grille of the reaction chamber and the openings in the wall of the housing of the mass transfer apparatus, and is located above the reaction chamber. In addition, the oxidation agent is made as a solid-phase one and consisting of separate particles. Moreover, in the capacity of the solid-phase oxidation agent, a granulated lead oxide is used. In addition, all of the openings, except for the openings in the wall of the housing of the mass transfer apparatus, forming the system for outlet of the oxidizable material, are made in the form of a series of slits having a width lesser than the size of the particles of the solid-phase oxidation agent. In addition, the mass transfer apparatus is arranged horizontally in the oxidation agent reservoir. Equipping the mass transfer apparatus with the repository for reserves of the solid-phase oxidation agent ensures the increase in the apparatus service life, since, as the particles loaded in the reaction chamber outflow, the feeding of the reaction chamber with the particles of the oxidation agents is ensured. However, the consumption of the electrical energy does not increase, since the volume of the reaction chamber and the dimensions of the heater have not changed. Presence of the openings in the lower part of the repository for reserves of the oxidation agent ensures the removal of the oxidizable material (liquid-metal heat carrier) from the apparatus during its extraction. The lead oxide has a density lower than the density of pure lead, and the granules of the lead oxide enter the reaction chamber under the action of the buoyancy force, which ensures automatic supply of fresh oxidation agent until the granules of the lead oxide outflow completely. The recovered lead is carried away by the flow of liquid-metal heat carrier. In the FIGURE, the following conventional symbols are adopted: 1—housing; 2—bottom; 3—cover; 4—perforated grille; 5—electric heater; 6—solid-phase granulated oxidation agent; 7—outlet openings; 8—inlet openings; 9—openings (drainage); 10—volume with the heat carrier; 11—heat carrier, 13—flow reaction chamber, 14—bottom part of the repository for reserves of the oxidation agent (cup) 15—repository for reserves of the solid-phase granulated oxidation agent, 16—pocket for the housing of the mass transfer apparatus. The mass transfer apparatus includes a reservoir formed by the housing 1, defined by the bottom 2 and a ring-shaped cover 3. In the reservoir, a flow reaction chamber 13 located inside the reservoir below the level of the liquid-metal heat carrier and defined from above by the perforated grille 4, is arranged. The restrictive grille 4 is intended for restraining the solid-phase granulated oxidation agent 6 from floating up under the action of the buoyancy force. Through the restrictive grille 4 and the openings 7 in the wall of the housing 1, located in the upper part of the wall of the housing 1 above the restrictive grille 4, the oxygen-enriched liquid-metal heat carrier leaves the mass transfer apparatus and mixes with the heat carrier of the main circuit of the installation. The solid-phase oxidation agent 6, housed below the grille 4, when interacting with the liquid-metal heat carrier, is dissolved enriching the heat carrier with oxygen. The heater 5, located in the reaction chamber 13 and passing through the perforated grille 4, is intended to heat the heat carrier in the reaction chamber 13. The inlet openings 8 are located in the wall of the housing 1 at the level of the lower end of the electric heater 5, so that during operation of the mass transfer apparatus, the liquid-metal heat carrier moves substantially through the layer of the solid-phase oxidation agent, located in the reaction chamber 13 in the gap between the housing 1 and the electric heater 5. Below the reaction chamber, the housing 1 is made in the form of a cup 14 having a bottom 2, in which the repository 15 for reserves of the solid-phase granulated oxidation agent 6 is located. The drainage openings 9 located in the lower part of the reservoir, are intended for draining the liquid-metal heat carrier during extraction of the mass transfer apparatus from the installation. The outlet openings 7, the inlet openings 8, the drainage openings 9 and the perforation holes in the grille 4 are made, preferably, in the form of narrow slits having a size lesser than the granules of the solid-phase oxidation agent. When in operating position, the mass transfer apparatus is immersed into the lead-containing heat carrier, so that the outlet openings 7 are located below the level of the liquid-metal heat carrier. The mass transfer apparatus is arranged in the reservoir of the installation, wherethrough the liquid-metal heat carrier flows. If the height of the layer of the liquid-metal heat carrier is insufficient for immersing the housing of the mass transfer apparatus thereinto, the reservoir is equipped with the pocket 16, into which the housing 1 of the mass transfer apparatus is embedded. The flow of the liquid-metal heat carrier through the pocket 16 is ensured as a result of a convective flow of the liquid-metal heat carrier through the reaction chamber during operation of the electric heater 5. The mass transfer apparatus operates as follows. Upon switching of the electric heater 5, due to the natural convection, an outflow of the liquid-metal heat carrier through the granulated solid-phase oxidation agent 6, located in the flow reaction chamber 13 in the gap between the housing 1 and the electric heater 5, is created. The liquid-metal heat carrier 11 from the ambient volume enters the mass transfer apparatus through the inlet openings 8 and moves bottom-upwards through the granulated solid-phase oxidation agent 6 located in the reaction chamber 13. The granules of the solid-phase oxidation agent, when interacting with the heat carrier, are dissolved therein enriching the liquid-metal heat carrier with oxygen. The oxygen-enriched liquid-metal heat carrier leaves the mass transfer apparatus through the outlet openings 7, and mixes with the liquid-metal heat carrier of the main circuit of the installation. The value of throughput, i.e. the amount of oxygen inflowing from the mass transfer apparatus per unit of time, is adjusted by altering the power level of the electric heater. During operation of the mass transfer apparatus, there is practically no outflow of the liquid-metal heat carrier through the reserves of the solid-phase oxidation agent, located in the repository 15 positioned in the cup 14 in the lower part of the housing 1 between the bottom 2 and the reaction chamber. In the process of operation, first the layer of the granulated solid-phase oxidation agent, located in the reaction chamber 13 in the gap between the housing 1 of the mass transfer apparatus and the electric heater 5, wherethrough the outflow of the heat carrier is ensured, begins to run out. Moreover, this layer is under elevated temperature, which facilitates the dissolution of the solid-phase oxidation agent. Since the density of the solid-phase oxidation agent (lead oxide) is lower than the density of the liquid-metal heat carrier, as the above-said layer runs out, the reserves of the solid-phase oxidation agent, located in the repository 15, when floating up, fill the freed up space in the reaction chamber 13 between the housing of the mass transfer apparatus and the electric heater. Specific exemplary embodiment of the mass transfer apparatus. Design characteristics of the mass transfer apparatus and the materials used: housing 1: inner diameter—64 mm, height—1500 mm, size of the inlet and drainage openings—2 mm, size of the outlet openings—10 mm, material—stainless steel 12H18N10T; perforated grille 4: size of perforation holes—2 mm, material—stainless steel 12H18N10T; electric heater 5: type—electric rod heater having a power capacity of 7 kW, height of the heating part—820 mm, heater housing dia. 25 mm, heating element—nichrome wire (H20N80) dia. 1.6 mm; solid-phase oxidation agent 6: pebble fill consisting of granules dia. 8-9 mm, material—lead oxide (PbO) of a “Ch” grade, TU 6-09-5382-88. Lead-containing liquid-metal heat carrier: Pb—Bi alloy, temperature—340° C. Oxygen throughput (at an inlet temperature of 340° C.): ˜1 g[O]/h. |
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claims | 1. A seal arrangement for a nuclear reactor in-core instrument housing, comprising: a first seal assembly surrounding an outer portion of an in-core instrument housing; a second seal assembly surrounding an outer-portion of an in-core instrument inserted within said in-core instrument housing; a seal housing having first and second ends, said seal housing enclosing said first and second seal assemblies, said seal housing having external threads engaged with corresponding internal threads of the in-core instrument housing to resist slipping when system pressure is applied; and first and second compression assemblies positioned on and threadedly engaging said first and second ends of said seal housing, respectively. 2. The seal arrangement as set forth in claim 1 , wherein said first and second compression assemblies compress said first and second seal assemblies to form respective seals between said first end of said seal housing and said outer portion of the in-core instrument housing, and between said second end of said seal housing and said outer portion of the in-core instrument. claim 1 3. The seal arrangement as set forth in claim 1 , wherein said first seal assembly comprises a first pair of graphite seal rings. claim 1 4. The seal arrangement as set forth in claim 3 , wherein said second seal assembly comprises a second pair of graphite seal rings. claim 3 5. The seal arrangement as set forth in claim 1 , wherein said first compression assembly comprises a first threaded drive nut threadably engaged to said first end of said seal housing, and a first compression collar positioned between a flange of said first drive nut and said first seal assembly. claim 1 6. The seal arrangement as set forth in claim 5 , wherein said second compression assembly comprises a second threaded drive nut threadably engaged to said second end of said seal housing, and a second compression collar positioned between a flange of said second drive nut and said second seal assembly. claim 5 7. The seal arrangement as set forth in claim 6 , further comprising a first spacer ring positioned between said first compression collar and said first seal assembly, and a second spacer ring positioned between said second compression collar and said second seal assembly. claim 6 8. The seal arrangement as set forth in claim 1 , wherein said seal housing is fabricated of a stainless steel alloy that resists galling and seizing of threads. claim 1 9. The seal arrangement as set forth in claim 1 , further comprising a retainer nut having external threads threadably engaged with corresponding internal threads of the in-core instrument housing to resist slipping when system pressure is applied, said retainer nut being separate from said seal housing. claim 1 10. The seal arrangement as set forth in claim 1 , wherein said first and second compression assemblies comprise first and second drive nuts and first and second compression collars, respectively, said compression collars each having a first portion engageable by one of said drive nuts and a second portion that protrudes axially from said compression assemblies for engagement by an installation tool. claim 1 11. The seal arrangement as set forth in claim 1 , wherein said first compression assembly comprises a first threaded drive nut threadably engaged to said first end of said seal housing, and a first compression collar positioned between a flange of said first drive nut and said first seal assembly, said first compression collar having an anti-rotation key received in a first mating keyway of said seal housing. claim 1 12. The seal arrangement as set forth in claim 11 , wherein said second compression assembly comprises a second threaded drive nut threadably engaged to said second end of said seal housing, and a second compression collar positioned between a flange of said second drive nut and said second seal assembly, said second compression collar having an anti-rotation key received in a second mating keyway of said seal,housing. claim 11 13. The seal arrangement as set forth in claim 12 , wherein said first drive nut has a larger threaded diameter than said second drive nut. claim 12 14. In combination, a nuclear reactor in-core instrument housing, an in-core instrument inserted within said instrument housing, and a seal arrangement for providing a seal between said instrument and said instrument housing, the seal arrangement comprising: a first seal assembly surrounding an outer portion of said in-core instrument housing; a second seal assembly surrounding an outer portion of said in-core instrument, a seal housing having first and second ends, said seal housing enclosing said first and second seal assemblies, said seal housing having external threads engaged with corresponding internal threads of the in-core instrument housing; and first and second compression assemblies positioned on and threadedly engaged with said first and second ends of said seal housing, respectively, said first compression assembly engaging said first seal assembly to maintain a seal between said first end of said seal housing and said outer portion of said in-core instrument housing, and said second compression assembly engaging said second seal assembly to maintain a seal between said second end of said seal housing and said outer portion of said in-core instrument. 15. The combination as set forth in claim 14 , wherein said first and second compression assemblies each comprise a threaded drive nut threadably engaged to said seal housing, and a compression collar positioned between a flange of said drive nut and a respective one of said seal assemblies. claim 14 16. The combination as set forth in claim 15 , wherein said compression collars each have a first portion engageable by one of said drive nuts and a second portion that protrudes axially from said compression assemblies for engagement by an installation tool. claim 15 |
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052746836 | description | DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The drawing illustrates a portion of a pressurizer vessel wall 10 having a nozzle bore 12 therethrough. The interior of the pressurizer is provided with a corrosion resistant cladding 14 such as stainless steel. In this drawing, existing nozzle remnant 16 remains in its original installed position at weld 18 approximately at the junction of cladding 14 and the interior of pressure vessel wall 10. The method of the invention is carried out as follows. The original nozzle is cut by any suitable means such as machining approximately at the junction of cladding 14 and the interior of pressure vessel wall 10. The portion of the original nozzle that extends beyond the exterior of pressure vessel wall 10 is removed. A weld pad 22 is deposited on the exterior of pressure vessel wall 10 around nozzle bore 12. The remaining portion of the original nozzle is removed by any suitable means such as machining. The surface of nozzle bore 12 is then prepared to receive a thermal spray coating. Thermal spray coating 20 is then applied to nozzle bore 12. Thermal spray coating 20 is comprised of a material such as stainless steel, nickel-chromium, or monel that is corrosion resistant and acts as a liquid barrier to prevent corrosion of the carbon steel of pressure vessel wall 10. Replacement nozzle 24 is inserted into nozzle bore 12 and welded in position to weld pad 22 using partial penetration weld 26 all the way around replacement nozzle 24 and weld pad 22. Weld 26 provides the necessary seal at the exterior of pressure vessel wall 10. Replacement nozzle 24 is formed from a corrosion resistant material as the original nozzle was. Therefore, no corrosion will occur to pressure vessel wall 10 or replacement nozzle 24 as a result of any liquid seepage between replacement nozzle 24 and thermal spray coating 20. Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense. |
description | This patent application claims priority to Provisional Patent Application No. 60/949,554, filed on Jul. 13, 2007. 1. Field of the Invention The present invention relates to a method of controlling a nuclear reactor during a transient period. The method includes actuating the steam dump (bypass) system in response to a temperature error signal and a power mismatch signal. 2. Description of the Prior Art In commercial PWRs that are utilized to generate electrical power, reactor coolant water (or primary water) recirculates between a reactor pressure vessel and one of a plurality of in-parallel steam generators in a closed loop known as a reactor coolant system (or a primary system). In a steam generator, the heat in the recirculating primary water flowing through the primary side (i.e., the tube side) passes through the walls of the tubes and is absorbed by relatively cool secondary water flowing on the secondary side (or shell side). The transferred heat generates steam on the secondary side at a temperature of about 500° F. or more and at a pressure of about 800 psi or more. The steam flows out of the steam generators to turbines that generate the electrical power. The exhaust steam from the turbines is condensed and recirculated to the steam generators as feedwater. An increase in reactor power can increase the rate of heat transfer to the reactor coolant water which can increase the rate of heat transfer to the secondary water causing more steam supplied to the turbine for transformation into electrical power. Conversely, if less electrical power is required, the power requirement of the turbine diminishes. The steam flow to the turbine is reduced and the turbine utilizes less of the steam being transferred to the secondary water. Since less steam is being drawn from the secondary side when the steam flow is reduced, both the temperature and pressure of the steam generator secondary side can increase. The effect of this increase in secondary water temperature is reflected in the reactor coolant water since less heat can be transferred from the primary water to the secondary water in the steam generator. As a result, both the temperature and pressure of the reactor coolant water can increase. A decrease in turbine power over a period of time is referred to in the art as a load rejection. If the load rejection is such that the reactor regulating systems, like the rod control system and the steam dump system, are unable to compensate rapidly enough for the change in power and, the temperature and pressure of the primary water increases uncontrollably, protective systems come into operation to trip the reactor and/or to open steam safety valves to avoid an overpressurization in the primary and secondary systems. The steam dump valves operate in conjunction with the turbine and the reactor to enable the prevention of excessive pressures in the primary and secondary systems, thereby allowing the reactor to stay operational in a partial or even a complete load rejection transient. The steam dump valves operate to remove excess steam from the system. The steam dump valves can be actuated when the reactor coolant average temperature (Tavg) exceeds an established setpoint or reference temperature. A load rejection can be initiated by the operator or by an automatic signal. A 50% load rejection is a design basis requirement for commercial PWRs. In this situation, the turbine power is reduced from 100% power to 50% power; and the nuclear power, i.e., the power generated by the reactor pressure vessel, initially remains at 100%. Since the nuclear power is greater than the turbine power, the reactor coolant average temperature and pressure will increase. The rod control system will insert the rods to reduce the nuclear power; however, it will take some time to reduce the nuclear power. Thus, opening of the steam dump valves can quickly dissipate the additional nuclear power thereby slowing or precluding the increases in reactor coolant water temperature and pressure. There are some nuclear plants that have implemented variable temperature operation at 100% power which means that the plants are operating at a reactor coolant average temperature (Tavg) that is lower than the typical value. Operation at a lower Tavg can reduce the steam dump capacity and thus, can limit the capability of a plant to sustain load rejection transients. When plants are operating at a lower Tavg, the nominal steam pressure is lower. This lower steam pressure can reduce the steam dump capacity at early stages of the transient. Currently, this may be addressed by revising the deadband and proportional band of the steam dump controller. One disadvantage to this solution is that it introduces primary and secondary side parameter fluctuations. Thus, there is room for improvement in the art to provide a method of controlling a nuclear reactor during a transient, the nuclear reactor being operated at a lower reactor coolant average temperature, while maintaining the capacity of the steam dump system to provide rapid and early relief to avoid a reactor trip in order to improve plant operability. As one aspect of the present invention, a method is provided for controlling a nuclear reactor during a transient. The method includes generating a first temperature error signal based on the amount by which reactor coolant average temperature exceeds a reference temperature; generating a second temperature error signal based on a power error signal provided when turbine power is reduced and the power of the turbine changes relative to the power of the nuclear reactor at a rate that exceeds a preselected rate; summing the temperature error signals to generate a valve control signal; and actuating the steam dump system in response to the valve control signal. As another aspect of the present invention, a steam dump control system is provided for controlling the response of a nuclear reactor to a transient. The steam dump control system includes at least one steam dump valve having a positioner operable to open the valve. Further included, is a coolant sensor system for monitoring an average temperature of a coolant of the nuclear reactor and providing a temperature error signal when the average temperature of the coolant exceeds a reference temperature; a nuclear power plant power sensing system for monitoring the power of the reactor and the power of a turbine which is driven by the power of the reactor and providing a power error signal when the turbine power is reduced and the power of the turbine changes relative to the power of the reactor at a rate that exceeds a preselected rate; and a control means having an input from the coolant sensor system and the nuclear power plant power sensing system for combining the temperature error signal and the power error signal to produce a valve control signal to control said valve positioner. In still another aspect of the present invention, a method is provided for dissipating steam from the secondary side of a steam generator in a nuclear reactor. The method includes monitoring the nuclear reactor power and turbine power; monitoring the reactor coolant average temperature and a reference temperature; generating a power error signal when the turbine power is reduced and the power of the turbine changes relative to the power of the reactor at a rate that exceeds a preselected rate; generating an error based on the reactor coolant average temperature and the reference temperature; summing the errors to generate a valve control signal to open at least one steam dump valve to dissipate the steam. It is an object of the present invention to provide a method for controlling a nuclear reactor during a transient by introducing a temperature error signal proportional to a power mismatch (between the nuclear reactor power and the turbine power) in addition to a temperature error signal based on Tavg (and Tref) to actuate the steam dump system in response to the combined temperature error signals. In commercialized PWRs for nuclear power generation, it is known for the steam dump valves in the steam dump system to open due to the actual reactor coolant average temperature (Tavg) exceeding an established reference temperature setpoint (Tref). The amount by which Tavg exceeds Tref is referred to as a temperature error. However, when a nuclear plant is operating at a lower Tavg, the Tref may not be exceeded at the initial onset or early stages of a transient such as a load rejection transient. Thus, a signal to open the steam dump valves in order to dissipate the additional nuclear power will not be generated and the valves will not open during the these initial or early stages of the transient. As a result of the steam dump valves not opening and the additional nuclear power not being dissipated, continued operability of the nuclear plant could be jeopardized. In the present invention, the steam dump system is actuated and the steam dump valves opened based on a combination of a temperature error based on a rate of power mismatch and a temperature error based on Tavg. As used herein and the claims, the term “rate” when used to describe a power mismatch or turbine power relative to nuclear reactor power, refers to a change in turbine power (e.g., when the turbine power is reduced) over a specified or preselected time period. For example, when the turbine is operating at 50% power and the reactor pressure vessel is generating 100% power, the nuclear power exceeds the turbine power 50%, e.g., the power mismatch is 50%. Depending on the time period during which the reduction in turbine power occurs, a temperature error based on the power mismatch may be generated. The steam dump valves will be armed (e.g., ready to operate) when the turbine power is reduced based on the change in turbine power relative to the nuclear reactor power at a rate that exceeds a specified or preselected rate. For example, at some nuclear plants, the steam dump valves are armed when the turbine power step decreases at least 10% over a specified time period. This feature is referred to as interlock and precludes the steam dump valves from opening unnecessarily. This interlock criterion is plant specific and thus, can vary from one nuclear plant to another. Further, in a hypothetical transient where the turbine is operating at 100% power and the reactor pressure vessel is generating 50% power, the nuclear power does not exceed the turbine power and therefore, a temperature error based on power mismatch will not be generated. The nuclear reactor power is measured using ex-core detectors located outside the reactor pressure vessel. “Nuclear reactor power” is also referred to herein as “nuclear power” or “reactor power”. The nuclear reactor power is expressed as a percentage value. The turbine power is determined by measuring the steam pressure in the impulse chamber of the turbine. The turbine impulse pressure value can be correlated with or correspond to a percent power value. For example, a turbine impulse pressure of 800 psi corresponds to a turbine power of 100% and, a turbine impulse pressure of 400 psi corresponds to a turbine power of 50%. As used herein and the claims, the terms “steam dump system” and “steam dump control system” can also be referred to as “steam bypass system” and “steam bypass control system”, respectively. Further, the use of the term “steam dump valves” refers to the steam dump valves typically located in the steam dump valve system and/or steam dump control system of a PWR, and can include condenser steam dump valves or atmospheric steam dump valves. A temperature error based solely on the difference between Tavg and Tref is conventionally used to generate an error signal to open the steam dump valves (as previously discussed herein). A nuclear plant can include a coolant sensor system for monitoring an average temperature of a coolant of the nuclear reactor and providing a temperature error signal when the average temperature of the coolant exceeds a reference temperature. In the present invention, the combination, e.g., summation, of the temperature errors based on both power mismatch and Tavg is used to generate a signal, such as a valve control signal, to actuate the steam dump system to open steam dump valves rapidly and early to dissipate steam in response to a transient such as load rejection. Each steam dump valve can include a positioner operable to open the valve in response to a valve control signal. Further, a nuclear plant can include a power sensing system for monitoring the power of the reactor and the power of a turbine which is driven by the power of the reactor and providing a power error signal when the turbine power is reduced and the power of the turbine changes relative to the power of the reactor at a rate that exceeds a preselected rate. The nuclear plant can also include a control means having an input from the coolant sensor system and the nuclear power plant power sensing system for combining the temperature error signal and the power error signal to produce a valve control signal to control said valve positioner. Referring to the drawings in detail and in particular to FIG. 1, there is shown a steam generator 1 in a commercial pressurized water reactor (PWR) with a control system that may be employed in a preferred practice of the present invention when the PWR is generating power. The steam generator 1 has thousands of small diameter tubes in a tube bundle represented by tube 2, which may be U tubes extending above a tube sheet 6 as shown or straight tubes extending between two tube sheets. The primary water from the reactor pressure vessel (not shown) flows into the primary side of the steam generator 1 through an inlet nozzle 4 in a lower hemispherical head, through the tubes 2 in the tube sheet, out of the steam generator 1 through an outlet nozzle 8 and back to the reactor pressure vessel. On the secondary side of the steam generator 1, steam is generated and flows out through steam line 12 and main steam valve 14 to turbines (not shown) for generating electrical power. The low pressure steam exhausted from the turbines is condensed and then pumped back to the steam generator 1 by a main feedwater pump 15 through a feedwater water line 16. In the practice of the present invention, selected process variables around the secondary side of the steam generator 1 are monitored. Such sensors may be electrical resistance level indicators, venturi meters, ultrasonic flow meters and the like. Sensors (not shown) may be employed to monitor process variables such as the turbine impulse pressure. Transducers (not shown) may be employed to send process signals based upon the sensed process variables to a control system 17. Referring to FIG. 2, the steam dump system 21 is comprised of four banks of valves. The steam line 12 which exits from steam generator 1 can be delivered as input to the turbine 31. Alternatively, the valves can bypass the steam line 12 from the turbine 31 to the condenser 33. These valves have a total capacity of typically forty percent (40%) of the full load turbine steam flow at full load steam pressure. The valves receive flow from the steam line 12 downstream of the main steam stop valves 27. The steam dump valves, such as the condenser steam dump valves 29 and atmospheric steam dump valves (not shown), have two modes of operation, i.e., (i) load rejection and (ii) reactor trip. In response to a reactor trip signal, all steam dump valves may fully open essentially immediately to dissipate steam. The reactor is brought to no-load conditions. The steam dump valves open based on the difference between Tavg and T-no-load. Based on the magnitude of this temperature error the steam dump valves may trip open or modulate open. This mode of operation is conventional in the art. In response to a load rejection transient, the position to which the valves are opened (i.e., partially or fully) and how rapidly (immediately or slower) the valves are opened can depend on the magnitude of the temperature error signal generated. Conventionally, the temperature error is based on Tavg only. In the present invention, the total temperature error is based on Tavg and on power mismatch. A load rejection controller is provided having a dead-band and a proportional band. A dead-band magnitude can be, for example, 2 to 5 degrees. If the temperature error is less than the magnitude of deadband, the steam dump valves will not open. The proportional error magnitude varies from plant to plant and depends on the nominal Tavg at 100% power and the no-load Tavg. For example, at some nuclear plants, the proportional temperature error can be 16 degrees and may control four banks of steam dumps. If the deadband in this example is 2 degrees, and if the temperature error exceeds 18 degrees, all (four banks) of the steam dump valves will fully open essentially immediately. The temperature at which the steam dump valves fully open essentially immediately can be referred to as a trip open setpoint. However, if the total temperature error is only 6 degrees, the first bank of valves may trip open from 0% to 100% over a period of three seconds. If, for instance, the total temperature error is only 4 degrees, the first bank of valves may open to only 50% of full open over a time period of ten seconds. Further, if for instance, the total temperature error was 8 degrees, the first bank of valves may fully open within 3 seconds and the second bank will open to a position of 50% full open over a period of 10 seconds. The trip open setpoints specified herein are provided herein for illustrative purposes only. The trip open setpoints are determined on a plant specific basis and thus, can vary from one nuclear plant to another. Further, the number of banks of steam dump valves is plant specific and therefore, can also vary amongst various nuclear plants. In the load rejection mode of operation, a modulate signal is sent to the valve positioner and the dump valve position depends upon the magnitude of the modulate signal. The dump valves are typically modulated one bank at a time. For example, when a nuclear plant has four banks of valves, the second bank does not begin to modulate open until the first bank has received a signal to modulate full open. The sequence for modulating the valves closed is the reverse of the opening sequence. For example, the fourth bank to open is the first bank to close, and the third bank starts to close after the fourth bank has received a signal to close. The first bank to modulate open is also the first bank to be tripped open. The second bank to modulate open is the second bank to trip open. The valves in the first bank can be designated as the cooldown dump valves. In the present invention, the temperature error used to open the steam dump valves can be increased due to the temperature error being generated based on power mismatch in addition to the conventional temperature error based on Tavg. Thus, as a result of the total temperature error, the steam dump valves can open earlier and more fully during a load rejection transient to provide increased steam relief capacity early in the transient as compared with the conventional Tavg temperature error generated. The steam dump control system can allow a nuclear plant to accept a sudden 50 percent loss of load without incurring reactor trip. Conventionally, in response to such a loss of load, the nuclear power has been reduced by inserting the rods and using the steam dump valves to remove excess energy. Based on the value of Tavg and Tref, the steam dump valves remove stored energy and residual heat following a load rejection and along with the rod control system bring the plant to an equilibrium condition without actuation of the steam generator safety valves or reactor trip. Various interlocks minimize any possibility of an inadvertent actuation of the steam dump valves. Referring to FIG. 3, there is provided a schematic flow diagram showing the determination and utilization of a temperature error based on power mismatch over time and a temperature error based on Tavg, to open the steam dump valves in response to initiation of a transient such as load rejection. A turbine impulse pressure measurement 41 and a nuclear power measurement 43 are used to determine if a power mismatch 35, e.g., loss of load, has occurred. As previously indicated, the turbine impulse pressure 41 is derived from the steam pressure measured in the impulse chamber of the turbine, and nuclear power 43 is measured using ex-core detectors located outside the reactor pressure vessel. The turbine impulse pressure 41 corresponds to a turbine power value. The turbine power is subtracted from the nuclear power 43. If there is a negative (−) result such that the turbine power exceeds the nuclear power 43, no temperature error based on power mismatch is generated. However, if there is a positive (+) result such that the nuclear power 43 exceeds the turbine power, and depending on the length of time over which the turbine power 41 is reduced (e.g., the rate at which the turbine power 41 changes relative to the nuclear power 43 compared to a preselected rate), a temperature error 38 based on power mismatch 35 may be generated. A percent turbine reduction per time is representative of a specific temperature error. The temperature error corresponding to the power/time value is determined on a plant specific basis and therefore, varies from one plant to another. The correlation between power/time and temperature error is determine based on the configuration of a certain plant thus, often times involves analysis and modeling of the nuclear plant. For example, as an illustration only, for a 50% load rejection whereby the turbine power decreases by 50% over a time period of fifteen seconds, the power error (e.g., gain) could be adjusted to yield a temperature error 38 of 16 degrees. Also in FIG. 3, the hot leg temperature (THL) 32 and the cold leg temperature (TCL) 34 are measured and input to calculate Tavg 48. Turbine impulse pressure 41 is used to determine a Tref value 44. The Tavg 48 and the Tref 44 are combined in the summator 46 wherein Tref 44 is subtracted from Tavg 48. If the temperature difference is positive (+) such that Tavg 48 exceeds Tref 44, a temperature error 50 is generated. If the temperature difference is negative (−) such that Tref 44 exceeds Tavg 48, a temperature error is not generated. The temperature error 50 is representative of the amount by which Tavg 48 exceeds Tref 44. The temperature error 50 (based on Tavg and Tref) and the power mismatch temperature error 38 (based on the nuclear and turbine power) are added in summator 40. The resultant temperature error is used to generate a signal, such as a valve control signal, which enables actuation of the steam dump system and opening of the steam dump valves early and rapidly so that the steam build-up in the secondary side can be dissipated. The use of the power mismatch temperature error in addition to the Tavg temperature error, allows the steam dump valves to open at a lower Tavg than if only the Tavg temperature error was used (i.e., without the power mismatch temperature error). As shown in FIG. 4, Tavg will increase during a load rejection. For example, the load rejection can include a reduction in turbine power from 100% to 50%, and while the nuclear power remains initially at 100%. Since the reactor power exceeds the turbine power, the Tavg will increase. The rod control system will insert the rods to reduce the nuclear power based on the amount by which Tavg exceeds Tref. However, since it will take a period of time for the rods to mitigate the loss of load and reduce the nuclear power, the extra power may be dissipated by opening the steam dump valves. The opening of the steam dump valves minimizes the increase of Tavg. As shown in FIG. 4, Tavg increases to a maximum of 578.1° F. with the prior art steam dump system operation such that the steam dump valves open based only on a temperature error between Tavg and Tref. Further, in FIG. 4, it is shown that Tavg increases to a maximum of 574.8° F. with the steam dump system operation in accordance with the present invention such that the steam dump valves open based on a power mismatch temperature error in addition to the Tavg temperature error. The lower Tavg temperature increase provides operating margin and will not imitate a trip function that is based on Tavg. As shown in FIG. 5, the reactor coolant system pressure will increase during a load rejection. The load rejection can cause the turbine to reduce power such as from 100% to 50%, while the nuclear power remains initially at 100%. Since the reactor power exceeds the turbine power, the pressure of the reactor coolant system will increase. As previously indicated, the rod control system will insert the rods to reduce the nuclear power but it will take a period of time for the rods to mitigate the loss of load and reduce the nuclear power, thus, the extra power may be dissipated quickly by opening the steam dump valves. The opening of the steam dump valves minimizes the reactor pressure increase. In FIG. 5, the reactor coolant pressure increases to a maximum of 2367 psia with the prior art steam dump system operation such that the steam dump valves open based only on a temperature error between Tavg and Tref. Further, in FIG. 5 it is shown that the reactor coolant pressure increases to a maximum of 2311 psia with the steam dump system operation in accordance with the present invention such that the steam dump valves open based on a power mismatch temperature error in addition to the Tavg temperature error. The lower pressure increase provides operational margin since the opening of the steam dump valves precludes the need for the pressure relief/steam safety valves to open. In the present invention, the summation of the power mismatch error and temperature error is used to open the steam dump valves. Typically, the opening (and closing) of the valves is modulated through the valve positioners. The steam dump valves are not opened unless the condenser is available, i.e., unless a vacuum exists and circulating water is available. The air supplied to the steam dump valves is blocked on high condenser pressure or loss of all circulating water. 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 claims appended and any and all equivalents thereof. |
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055531075 | claims | 1. A pressurized water nuclear reactor pressure vessel, comprising: an upper core support plate having passageways for the passage of coolant; an upper support plate above the upper core support plate, the upper support plate and the upper core support plate defining a plenum; and peripheral hollow support columns extending upwardly through the plenum above the passageways in the upper core support plate with each of the peripheral hollow support columns being aligned with a passageway for supporting the upper support plate above the upper core support plate and for guiding coolant from the passageways into the plenum, each peripheral support column having an unslotted axial lower portion for guiding the coolant flowing through the aligned passageway upwardly through the peripheral support column and having a peripherally slotted upper axial portion for guiding the coolant in the peripheral support column into the plenum. 2. The pressurized water nuclear reactor vessel of claim 1 wherein the vessel has a hot leg nozzle and the peripheral support columns are unslotted below the hot leg nozzle. 3. The pressurized water nuclear reactor vessel of claim 1 wherein the slots in each peripheral support column comprises at least 50% of the peripheral surface area of the upper portion of the support column. 4. The pressurized water nuclear reactor vessel of claim 3 wherein the slots in each peripheral support column comprises from 50% to 66% of the peripheral surface area of the upper portion of the support column. 5. The pressurized water nuclear reactor vessel of claim 1 wherein guide tubes extend from the upper core support plate to the upper support plate, the guide tubes having peripheral slots for guiding coolant into the plenum, and wherein the slotted portion of each peripheral support column is above the slots of the guide tubes. |
055531091 | summary | TECHNICAL FIELD The present invention relates to apparatus and methods for simulating a nuclear fuel rod bundle transient and particularly relates to such apparatus and methods for simulating the effect of the change in both integrated bundle power and the axial power or flux shape as a function of time during a transient, for example, in a boiling water reactor. BACKGROUND There are many different types of nuclear reactor transient events. For example, the load on a turbine driven by steam from a nuclear power plant may be removed from the turbine by any one of a number of events, causing a short in the electric power transmission lines. Should such transient event occur, typically the stop valves to the turbine are closed, shutting down the delivery of steam to the turbine. The nuclear reactor, however, is still producing full power. To control the reactor in view of the transient, the control rods are driven into the reactor and valves are opened to bypass steam to a condenser. Safety considerations, however, require preparation for failures in the system, including, for example, those where the condenser bypass valve cannot be closed and the reactor continues to produce full steam power. In that event, and in a boiling water reactor (BWR), a pressure spike occurs, the position of the boiling boundary between the single and two-phase regions within the reactor vessel shifts upwardly, the average void distribution of the fuel changes downwardly and the overall power output increases. This changes the axial power or flux shape of the fuel rod bundle. The axial power or flux shape is the power associated with each axial location in the fuel bundle. In a typical situation, and early in the life of the fuel, the power distribution is at a peak adjacent the bottom of the fuel bundle and the instantaneous power along the fuel bundle upwardly from the peak falls off. When a transient occurs, the boiling boundary moves upwardly along the fuel bundle and displaces the peak of the heat flux curve upwardly along the bundle. The value of the peak of the heat flux curve also changes in response to movement of the boiling boundary. Thus, as the transient progresses, the peak of the heat flux curve is not only displaced upwardly along the fuel bundle but also changes in value so that a greater outlet peak power distribution momentarily occurs. As the effect of control rod insertion and increased reactor voiding decrease neutron flux levels, the peak reactor surface heat flux will be in approximately two seconds. Present-day nuclear fuel bundle simulators employ fuel rod simulators in a closed vessel containing a coolant. Such fuel bundle simulators, however, are limited in their capability. For example, present testing matches total bundle fuel rod surface heat flux with anticipated nuclear fuel bundle surface heat flux and it is the result of these tests which are used to quantify transient computer codes for application to reactor transient analyses. More particularly, today all test facilities employ heating elements in which the only control over each heating element is the magnitude of the power input to the heating elements. While more or less power could be supplied to the heating elements, the simulated axial power or flux shape is fixed. Thus, while the power supply to the heating elements could be varied over time to obtain the correct total bundle power as a function of time, the change in the axial power or flux shape could not be simulated. DISCLOSURE OF THE INVENTION According to the present invention, it has been recognized that during a transient event in a nuclear reactor core not only does the fuel bundle total power change but also the axial power shape varies with time. To my knowledge, this effect has not been studied in an out of reactor test using fuel rod simulators. Obtaining this type of experimental data is highly desirable in order to be able to check the ability of computer codes which predict the transient performance of nuclear reactor fuel to evaluate this more realistic simulation of postulated reactor transient events. The power of the fuel bundle can be represented mathematically as a function of axial position and time, i.e.: EQU P=f(x,t); Equation (1) where x and t represent axial position upwardly along the heated length and time from the beginning of the transient event, respectively. Mathematically, a function f(x,t) can be approximated as used in Equation (1) by EQU f(x,t).apprxeq.f.sub.1 (x,t)+f.sub.2 (x,t); Equation (2). To accomplish the foregoing, first, each fuel rod simulator (FRS) will have similar performance characteristics, i.e., power versus axial position and time, as the other FRS's in the bundle. Each FRS will have two separate, independent heating elements with identical power versus length characteristics. Each of the two heating element groups will be connected to two separate independent power supplies and each group will also have its own axial power shape. In this way by varying each group of heating elements separately with time, the functions f.sub.1 (x,t) and f.sub.2 (x,t) can be generated during a simulated nuclear reactor transient event, thus providing time varying axial power shapes typical of reactor transient events. Various designs of the heating elements can be provided. For example, a double helix heating member may be provided comprised of an internal double helix formed of the two heating elements separated from an outer tubular metallic cladding by suitable electrically insulating material. The heating element can be fabricated from a uniform wall thickness tube using a numerically controlled machine tool. In this way, two continuous helices are generated with a width versus length variation using the same table which represents the desired power versus length relationship for each of the length terms in Equation (2). One end of each of the two helices is connected to a common ground and the other ends to the two independently varied power supplies. In another form of the invention, the two heating elements may comprise two coaxial heating members separated by electrical insulating material. The outer element may also serve as the cladding for the fuel rod simulator. These elements can be either solid like the direct heater presently in use, or of the helix (either single or double helix) type. If the elements are solid, the axial power profile can be realized by either using tapered wall tubes of one material or uniform wall thickness tubes of s more than one material, with different coefficients of electrical resistivity. The method of FRS removal from the test vessels is also important to their design. In single-ended heaters, the heating element simulative of the fuel rod exits the pressure vessel at only one end while the opposite end remains in the vessel. In double-ended heaters, the heating elements exit the pressure vessel at both the top and bottom ends and, accordingly, pass straight through the vessel. Either heater style (single or double-ended) may be of the helix or coaxial type, or a combination of both types. In a preferred embodiment according to the present invention, there is provided apparatus for simulating a nuclear fuel rod bundle transient comprising a vessel for containing a coolant, a pair of heating elements disposed in the vessel for disposition in the coolant, a power supply for supplying power over time to each of the heating elements and means for independently controlling and thereby varying the supply of power over time to each heating element whereby an approximation of the variation in power and axial flux shape in a nuclear fuel bundle as a function of time can be obtained. In a further preferred embodiment according to the present invention, there is provided apparatus for simulating a nuclear fuel rod bundle transient comprising a vessel for containing a coolant, a plurality of nuclear fuel rod simulating members disposed in the vessel for disposition in the coolant and forming a simulated nuclear fuel rod bundle, each member including a pair of heating elements, a power supply for supplying power over time to each of the heating elements and means for independently controlling and thereby varying the supply of power over time to each heating element of the plurality of nuclear fuel rod simulating members whereby an approximation of the variation in power and axial flux shape in a nuclear fuel bundle as a function of time can be obtained. In a still further preferred embodiment according to the present invention, there is provided a method for simulating a nuclear fuel rod bundle transient comprising the steps of disposing a pair of heating elements simulative of a nuclear fuel rod in a vessel containing a coolant, independently supplying power to the heating elements and controlling the supply of power to the heating elements independently to vary the power supplied over time to simulate the variation over time of the power output and axial flux shape in a nuclear fuel bundle. Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for simulating an out-of-pile nuclear reactor transient event to approximate the variation over time of the power and axial flux shape in a nuclear fuel bundle during a transient event. |
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description | This application claims the benefit of DE 10 2012 201 855.7, filed Feb. 8, 2012, which is hereby incorporated by reference. The present embodiments relate to a contour collimator or an adaptive filter and to an associated method for adjusting a contour in a ray path in x-ray radiation. A contour collimator is used in radiation therapy for the treatment of tumors. In radiation therapy, a tumor is irradiated with energy-rich radiation (e.g., with high-energy x-ray radiation of a linear accelerator). In such treatment, the contour collimator is brought into the ray path of the x-ray radiation. The contour collimator has an opening, through which radiation may pass. The contour of the opening is intended to correspond to the contour of the tumor. The contour thus forms an aperture for the passage of the x-ray radiation. This provides that the tumor, and not the adjoining healthy body tissue, is irradiated with the x-ray radiation. By embodying the contour collimator in a suitable manner, almost any given contour of a tumor may be mapped. Collimators widely used for radiation therapy are multi-leaf collimators, as described, for example, in patent DE 10 2006 039793 B3. The multi-leaf collimator has a number of leaves (e.g., 160 leaves) able to be moved by motors in relation to one another to form the opening. The leaves include a material absorbing the x-ray radiation. Two packages of leaves are disposed opposite one another so that the leaves may be moved with end face sides towards one another or away from one another. Each of the leaves is able to be displaced individually by an electric motor. Since there may be slight deviations in the positioning of the leaves between a required specification and the actual position of the leaves currently set, each leaf has a position measurement device, with which the position currently set may be determined. In examinations with the aid of x-rays, it often occurs that the patient or organs of the patient exhibit a greatly differing absorption behavior with respect to the applied x-ray radiation in the area under examination. For example, in images of the thorax, the attenuation in the area in front of the lungs is very large, as a result of the organs disposed there, while in the area of the lungs, the attenuation is small. Both to obtain an informative image and also to protect the patient, the applied dose may be adjusted as a function of the area so that more x-ray radiation than necessary is not supplied. This provides that a larger dose is to be applied in the areas with high attenuation than in the areas with low attenuation. In addition, there are applications in which only a part of the area under examination is to be imaged with high diagnostic quality (e.g., with little noise). The surrounding parts are of importance for orientation but not for the actual diagnosis. These surrounding areas may thus be mapped with a lower dose in order to reduce the overall applied dose. Filters are used to attenuate the x-ray radiation. Such a filter is known, for example, from DE 44 22 780 A1. This has a housing with a controllable electrode matrix, by which an electrical field that acts on a fluid connected to the electrode matrix, in which x-ray radiation-absorbing ions are present, is able to be generated. The x-ray radiation-absorbing ions are freely movable and move around according to the field applied. In this way, by forming an appropriate field, many or few irons may be correspondingly accumulated in the area of one or more electrodes in order to change the absorption behavior of the filter locally. The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, a further contour collimator and a further adaptive filter that may map a contour robustly and rapidly are provided. In a further example, an appropriate method for forming a contour is provided. An aperture forming the contour is generated with the aid of a magnetic fluid absorbing x-ray radiation or with a fluid impermeable to x-ray radiation (e.g., a ferrofluid). In a magnetic field, magnetic moments of the particles of the ferrofluid tend to travel in a direction and achieve macroscopic magnetization. Magnet elements generating magnetic fields are used to magnetize the fluid or parts of the fluid. Ferrofluids are magnetic fluids that react to magnetic fields without solidifying. The ferrofluids are attracted by magnetic fields. The ferrofluids includes magnetic particles a few nanometers in size that are suspended in a colloidal manner in a carrier fluid. The particles may be stabilized with a polymer surface coating. True ferrofluids are stable dispersions, which provides that the solid particles do not break off over time and do not themselves accumulate on one another in extremely strong magnetic fields or separate from the fluid as another phase. Ferrofluids are supermagnetic and have a very low hysteresis. A contour collimator or an adaptive filter for adjusting a contour of a ray path of x-ray radiation is provided. The apparatus includes a magnetic fluid impermeable to x-ray radiation and switchable magnet elements, by which an aperture forming the contour may be formed in the magnetic fluid by the magnetic fluid being attracted by the magnetic fields of the magnet elements. The contour forms the aperture (i.e., an opening in the contour collimator or the filter). An aperture may be a free opening or the diameter of the free opening, through which x-rays may be emitted or received. The embodiment offers the advantage of a robust collimator or filter, with which rapidly changing contours may be adjusted precisely In a further embodiment, the magnetic fluid may be a ferrofluid. In one development, the magnetic fluid may be arranged in the form of a layer with limited expansion. Furthermore, the apparatus may include at least one second layer, in which the magnet elements are arranged. The second layer may be arranged above or below the first layer. Alternatively, a second layer may be arranged above or below the first layer in each instance. In a further embodiment, an electric grid structure formed from conductor paths is embodied in the second layer. The magnet elements are arranged at the points of intersection of the conductor paths. In a development, the magnet elements may include coils, through which current passes. The contour collimator or the filter may include an electric control unit, with the aid of which the magnet elements may be switched on and off according to the contour to be formed. A number of first and second layers may also be stacked in order to form the contour collimator. In one embodiment, a method for adjusting a contour of a ray path of x-ray radiation using a contour collimator or an adaptive filter is provided. Magnetic fields form an aperture forming the contour in a magnetic fluid that is impermeable to x-ray radiation, by the magnetic fields attracting the magnetic fluid. In one embodiment, the magnetic fields may be formed by switchable magnet elements. The magnetic fields may be formed by electric currents. FIG. 1 shows a spatial representation of one embodiment of a contour collimator 1 having a number of stacked contour plates 3. An aperture 11 forming a contour 10 is embodied in the collimator plates 3. The aperture 11 allows x-ray radiation 12 to pass through to an object 13 (e.g., a tumor). Except for the aperture 11, the collimator plates 3 are impermeable to x-ray radiation 12. The layers absorbing x-ray radiation 13 are formed by a magnetic fluid 9. Where the magnetic fluid 9 is absent, the aperture 11 is formed. FIG. 2 shows a spatial representation of one embodiment of an adaptive filter 2 having three stacked filter plates 3. An aperture 11 forming the contour 11 is embodied in the filter plates 3. The aperture 11 allows x-ray radiation 12 to pass through. Except for the aperture 11, the filter plates 3 are impermeable to x-ray radiation 12. The layers absorbing x-ray radiation 12 are formed by a magnetic fluid 9. Where the magnetic fluid 9 is absent, the aperture 11 is formed. FIG. 3 shows a spatial view of one embodiment of a collimator plate and/or a filter plate 3. The plate 3 includes a first layer 4 that is formed by a magnetic fluid 9 that is impermeable to x-ray radiation. Magnetic fields may be generated by magnet elements (not shown in FIG. 3) arranged in second layers 5 using a second layer 5 including material transparent for x-ray radiation arranged thereabove and below. At the location of the aperture 11, the magnetic fluid 9 is “drawn in” (e.g., attracted) through the magnetic fields lying outside of the aperture, and x-ray radiation may pass therethrough unhindered. FIG. 4 shows one embodiment of the plate 3 from FIG. 3 in a sectional view. The two second layers 5 including the material that is transparent to x-ray radiation are visible. A plurality of magnet elements 6 (e.g., coils) is embodied in the second layers 5. The more magnet elements 6 there are available, the more precisely a contour 10 and/or the aperture 11 forming the same may be mapped. The first layer 4 with the magnetic fluid 9 that is not transparent for x-ray radiation is located between the two second layers 5 and is, for example, a ferrofluid. At the locations, at which the magnet elements 6 are active (e.g., generate a magnetic field H), the magnetic fluid 9 is attracted (e.g., removed from the area of the aperture 11 to be formed). As a result, the aperture 11 is produced. FIG. 5 shows a schematic representation of one embodiment of a grid structure 9 embodied in the second layer. The grid structure 8 is formed by conductor paths 7. Magnet elements 6 are disposed at points of intersection of the conductor paths 7 (e.g., two coils connecting conductor paths). The magnet elements 6 generate a magnetic field H at right angles to the second layer when current is flowing through the conductor paths. A control unit 14 is able to switch each magnet element 6 on and/or off at each point of intersection. The more points of intersection there are available, the more precisely the contour may be mapped. While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description. |
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description | This application is a National Phase of PCT/EP2011/060001, filed Jun. 16, 2011, entitled, “SOLID INTERFACE JOINT WITH OPEN POROSITY, FOR NUCLEAR CONTROL ROD”, which claims the benefit of French Patent Application No. 10 54781, filed Jun. 16, 2010, the contents of which are incorporated herein by reference in their entirety. This invention relates to the interface between the stack of pellets and the cladding surrounding them, in a nuclear control rod used in a nuclear reactor. Target applications for the invention include: gas-cooled fast reactors (GFR) said to be generation IV reactors that operate with a coolant in the form of a gas such as pressurised helium, and use nuclear fuel rods with cladding made of a ceramic matrix composite (CMC) material, and mixed uranium and plutonium carbide type fuel pellets, [9]; fast neutron reactors operating with a sodium coolant (SFR) [12]; pressurised water reactors (PWR) [3] or boiling water reactors (BWR). The invention relates to control rods for power reactors in which the pellets are made from a B4C neutron absorber material [8], [5]. Throughout this application, the term <<nuclear reactors>> has its normal meaning as understood at the present time, namely power plants for the generation of energy based on nuclear fission reactions using fuel elements in which fission reactions occur releasing thermal power, which is extracted from elements by heat exchange with a coolant fluid that cools them. Throughout this application, <<Nuclear control rod>> (or <<absorber>>) has its official meaning as defined for example in the Dictionnaire des Sciences et Techniques nucléaires (Nuclear Sciences & Techniques Dictionary), namely a rod containing a neutron absorber material and that has an influence on the reactivity of a nuclear reactor depending on its position in the core. There are different types of control rods depending on operating conditions and the performances of nuclear reactors. The main functions to be performed by a nuclear control rod are to: enable controlled absorption of neutrons by nuclear reactions, which imposes performance constraints (density of absorbing nuclei) and safety constraints (geometric stability necessary for control of the nuclear reactivity and cooling), guarantee controlled extraction of energy released by nuclear reactions, which imposes performance constraints (limitation of thermal barriers that could degrade transfers to the coolant) and safety constraints (integrity of the coolant channel, margin before melting of the absorber, limitation of temperature gradients that cause differential expansion that could lead to an excessive mechanical load on structures, etc.). Absorbers conventionally encountered in nuclear installations may be classified as a function of their geometry as follows: cylinders: rods in control rods, for example in FNR reactors or PWR reactors; plates: for control rods, for example in Boiling Water Reactors (BWR). The invention exclusively concerns nuclear control rods with cylindrical geometry and circular cross-section in which cylindrical B4C neutron absorber pellets with a circular cross-section are stacked in a sealed tubular cladding with a zone at one of its ends without any pellets called the expansion vessel, which accommodates elongation of the column of pellets under irradiation due to swelling phenomena induced by nuclear reactions. In this cylindrical configuration, there is an interface between the column of stacked pellets and the cladding. Up to now, this interface might be reduced during assembly to a contact surface only or it might correspond to a functional clearance that may then be composed of one or several materials in gas or liquid form or in layers, as explained below. The inventors have made a list of functions to be performed by this interface in an absorbing element. They are described below. Primary Functions: f1/ manage mechanical decoupling between absorber pellets and the cladding, so as to limit mechanical interaction between pellets and the cladding (this interaction is hereinafter referred to as PCMI), by enabling free expansion of the column of stacked pellets along a radial direction and an axial direction; f2/ enable transport of gas products derived from neutron absorption reactions (helium and tritium in the case of B4C pellets), released by the absorber as far as the expansion vessel located at the axial end of the control rod or at vents formed in the cladding of the control rod to release these gases into the primary system (from where they are then eliminated through special purpose purifying loops), f3/ manage thermal coupling between the absorber and the cladding: i. minimising thermal barriers, particularly along the radial direction, to prevent any excessive temperature rise of the absorber; ii. guaranteeing continuity of this function, particularly along the axial and azimuth directions, so as to minimise temperature heterogeneities that can cause differential expansion that could in particular induce large mechanical loads on the cladding. Functions Induced by the Environment: f4/ perform primary functions (f1 to f3) minimising the neutron impact at the interface, so as to preserve performances of the reactor core: i. by minimising the geometric dimensions; ii. by making use of materials that may have a large interaction cross-section with neutrons. f5/ perform primary functions (f1 to f3) guaranteeing chemical compatibility of the interface with its environment: i. guaranteeing chemical compatibility of the interface with the cladding (no increase in rates at high temperature, for example under accident condition); ii. guaranteeing chemical compatibility of the interface with the absorber (no <<low temperature>> eutectic that could for example reduce the absorber melting margin). Secondary Functions: f6/ limit transfer of constituents from the absorber (particularly carbon for B4C pellets) to the cladding, to prevent the risk of internal corrosion that could cause embrittlement that might occur as a result of this transfer; this is a function related to the primary function f1; f7/ optimise absorber/cladding centring so as to minimise temperature heterogeneities that cause hot points and increased mechanical loads at the cladding; this is a secondary function related to primary functions f1 and f3; f8/ minimise (without introducing) the risk of the movement of absorber splinters into the clearance, if any, between the absorber and the cladding, that could cause an integrity defect in the cladding by ovalling and/or punching of the cladding when this clearance is reduced under the effect of differential strains (thermal expansion and swelling); this is a function related to the primary function f1. In the case of a control rod, functions f1 and f8 are possibly indissociable: unlike a fuel element, a control rod may have large radial dimensions (this is usually the case for FNRs but not necessarily for PWRs) that make a large pellet/cladding clearance necessary, which thus increases the risk that a pellet splinter could get trapped in this clearance, such that management of pellet splinters is a serious problem when attempting to maintain the integrity of the cladding under mechanical loads. Auxiliary Functions: f9/ satisfy usual economic constraints: i. life: perform primary and secondary functions for an absorber operating time compatible with target economic performances; ii. capacity for procurement of materials and implementation of fabrication methods; iii. cost. f10/ exclude any significant prejudice to safety in an accident condition (for example, chemical reactivity of the interface with structural materials in the core during an advanced core degradation phase); f11/ minimise technical fabricability problems, particularly implementation of the absorber assembly process (absorber, interface and cladding); f12/ satisfy separation and recycling requirements on the output side of the nuclear reaction cycle, with minimum constraints. The interface between pellets and cladding in control rods with circular geometry and circular cross-section is in the form of a gas, typically helium or in the form of sodium for an SFR reactor, which has optimum properties (among possible gases) regarding thermal conductivity (function f3.i), chemical neutrality (function f5 and auxiliary functions (functions f9 to f12). Functions for mechanical decoupling between fuel pellets and cladding (function f1) and gas transport to the expansion vessel and/or any vents (function f2) are ideally performed by an interface in gas form, provided that a sufficient functional clearance is created during fabrication between pellets and cladding to prevent filling of the gap under irradiation due to differential strains of the absorber and the cladding [6]. However, a control rod with cylindrical geometry and a circular cross-section and an interface in gas form shows antagonism because it cannot perform firstly functions f1 and f2 and secondly functions f3.i and f4.i simultaneously, except within very strict performance limits. Beyond the dimensional constraints that adversely affect neutron performances (density of absorber material in the absorbing element), since the thermal conductivity of the gas interface is relatively mediocre, any increase in the functional clearance between pellets and the cladding during assembly will increase the thermal barrier that it forms, leading to increased temperatures of the absorber. Apart from the fact that the temperature increase takes place at the detriment of safety requirements (particularly a reduction in the absorber melting margin), it is accompanied by an increase in the three-dimensional expansion of the pellet that tends to reduce said gap under irradiation, thus reducing the efficiency of the increased thickness of the interface and consequently the increase in the life of the absorbing element. One solution to reduce this thermal prejudice has been disclosed in patent JP 11183674 (applied to a fuel element, but in exactly the same way as is done elsewhere on control rods) and in which experiments have been made in various experimental irradiation programs [10], [11]. This solution consists of making the interface no longer in gas form but rather in the form of a metal with a low melting point and that is liquid under operating conditions of the fuel element, generally sodium. The conductivity of the metal is higher than that of gas and can thus considerably reduce problems related to conductance of the interface, which then makes a negligible contribution to the thermal balance of the fuel element/absorber and potentially makes greater interface thicknesses possible. Another advantage of having an interface in liquid metal form is that it reduces circumferential thermal heterogeneity problems resulting from possible eccentricity of the fuel pellet/absorber relative to the cladding, due to its good thermal conductivity. The concentricity requirement (function f7) is not a priori guaranteed by an interface in gas or liquid metal form, due to the lack of rigidity of a liquid metal or a gas. Any eccentricity will also mean that the heat flux is heterogeneous around the circumference. The consequences of this thermal heterogeneity (hot point at the cladding and mechanical load induced by differential thermal strains) are thus attenuated when the interface is in the liquid metal form due to better heat transfers firstly between the liquid metal and the cladding and secondly between the liquid metal and the pellets. However, the interface in liquid metal form cannot be made without creating some problems. Firstly, compatibility with the environment (function f5, for example for chemical aspects), is found to be very restrictive. Thus in the case of sodium, that is naturally applicable for SFRs, there is clearly an incompatibility with a water coolant (PWR), and with a reactor operating at high temperature and consequently leading to an insufficient margin (or even non-existent margin, for example in the case of the GFR) against the risk of sodium boiling (sodium boils at a temperature of the order of 880° C.). For example, concerning thermal heterogeneities (function f3.ii), it is clear that any discontinuity in the interface induced by the presence of gas bubbles in the liquid metal (bubbles formed during fabrication or by fission gases released under irradiation), would mitigate the thermal benefits of this solution: this problem was observed during experimental irradiation during which it was seen that it could lead to a premature end of life of the fuel element/absorber due to early failure of the cladding [11]. Furthermore, concerning the limitation of fuel/absorber constituent transfers (function f6), experimental irradiations of carbide fuels in SFR type reactors with the purpose of comparing the behaviour of helium and sodium interfaces have shown that the liquid metal contributes to embrittlement of the cladding due to carburization of the cladding induced by an increased transfer of carbon originating from fuel through the sodium, although this problem does not appear to arise through helium [11] unless there is pellet/cladding contact due to eccentricity. With a fuel control rod using a B4C absorber, a steel-based cladding and a sodium joint, there is a similar problem of embrittlement of the cladding due to decarburization of absorber pellets, migration of free carbon through the sodium path and thermochemical attack of the internal face of the cladding [8]. Finally, concerning function f8, the lack of inherent stiffness of the joint enables movement of fuel/absorber splinters which, if they move into the interface, could lead to ovalling or punching of the cladding by compression of the splinter between pellets and cladding during irradiation. Such punching implies a premature loss of the cladding integrity/seal safety function while ovalling will degrade performances because it affects heat exchanges and mechanical interactions, if any, between nearby fuel elements/absorbers. In practice, operating experience with irradiation of fuel elements shows that an initial value of the radial functional clearance between pellets and cladding of less than about 4% of the radius of pellets can minimize the risk of cladding failure by punching, by limiting the probability of a pellet splinter moving into the interface [13]. This limit, made necessary by safety requirements, nevertheless has proved to be relatively prejudicial to the operating life of the fuel element/absorber, in that it substantially reduces the operating life without PCMI. In this context, long term use of a fuel/absorber in a nuclear reactor, necessary for its economic performances, will make functioning with PCMI inevitable during a variable time period before the end of life. Very significant pellet/cladding clearances must be provided during fabrication of a B4C absorber characterised by a particularly high swelling ratio, in order to delay the PCMI; for example, typically more than 10% of the radius of pellets in SFR reactors, therefore more than the 4% mentioned above, which is particularly critical because the diameter of a control rod, at least in fast spectrum reactors, is potentially larger than the diameter of a fuel element; for example for SUPERPHENIX [8], the diameter of absorber pellets is 17.4 mm in comparison with the order of 7 mm for fuel pellets. Thus, there is a particularly severe risk of a pellet splinter moving into the pellet/cladding clearance, which is why a sleeve system has been developed to contain these splinters [8]. Various solutions have been proposed to enable acceptable operation with PCMI regarding economic and safety performances. They are aimed at overcoming two residual difficulties that neither the interface in gas form nor the interface in liquid metal form can solve individually, namely: the need to reduce the mechanical load imposed on the cladding in a situation of contact with the absorber, minimising embrittlement of the cladding due to thermochemical aggressions. All proposed solutions consist of depositing one or several intermediate layers of materials, as all or part of the interface. Patent GB 1187929 discloses the use of an intermediate layer between fuel pellets and the cladding, based on metal uranium, for a fuel rod with metal cladding operating at a temperature of at least 700° C. in an FNR reactor. This patent describes: intimate contact between the intermediate layer and the cladding; another part of the interface performing a temperature function, typically made of sodium, between the intermediate layer and the cladding; an additional layer performing a chemical compatibility function, typically alumina, between the intermediate layer and the cladding; grooves forming vacuum zones between the fuel and the intermediate layer; the possibility that the porosity of the intermediate layer and/or the fuel pellet will be such that its (their) density will be equal to not more than 85% of its (their) theoretical density; uranium alloy, or uranium and molybdenum alloy as constituents of the intermediate layer. Similar solutions have been disclosed for fuel rods with zirconium-based cladding used in PWR reactors. Thus, U.S. Pat. No. 4,818,477 discloses how to make a liner based on consumable neutron poisons (boride enriched in 10B), coating fuel pellets with a thickness of between 10 μm and 100 μm, so as to attenuate the PCMI. U.S. Pat. No. 3,969,186 discloses how to make a metal liner deposited on the inner face of the cladding, so as to prevent the risk of perforation or failure of the cladding induced by stress corrosion cracking and/or pellets/cladding mechanical interaction. U.S. Pat. No. 4,783,311 discloses how to make a combination of liners on the inner face of the cladding (thickness from 4 μm to 50 μm) and on the surface of fuel pellets (thickness from 10 μm to 200 μm), the liner on the inner face of the cladding, from a material such as graphite, particularly performing a <<lubricant>> role. Patent JP 3068895A discloses how to make a ductile intermediate layer provided with grooves, to absorb stresses induced by a potential PCMI, the layer being plastically deformable thus avoiding propagation of cracks on the inner face of the cladding. There are also fuel particles with a spherical geometry used in HTR reactors, as described in international patent application WO2009079068. As described in this application, a multilayer structure is made with a fuel ball at the centre and a surrounding cladding, providing mechanical integrity and a seal for fuel ball fission gases, and between which a porous pyrocarbon layer performing a buffer function is deposited in order to create an expansion volume for fission gases and the fuel ball. Moreover, the problem raised by nuclear control rods with cylindrical geometry and circular cross-section already considered the movement of splinters of neutron absorber into the interface between pellets and cladding (function f8), as described in important operating experience on use of the B4C material in the SFR reactors [8]. The absorber pellet becomes fragmented under the effect of swelling induced by the production of helium by neutron absorption on 10B. It thus releases micro-fragments that fill in the interface between the pellets and the cladding and consequently accelerate PCMI, creating a mechanical load on the cladding that quickly leads to unacceptable damage. One solution that consists of placing absorber pellets in a thin metal sleeve has been used [8]: this solution confines pellet fragments (including in a sleeve failure condition) and thus prolongs the life of the control rod within given limits. U.S. Pat. No. 4,235,673 discloses the use of a sleeve, either in the form of a fabric of metal wires (embodiment in FIGS. 1 and 2) or in the form of metal ribbons (embodiment in FIGS. 3 and 4), wound helically about the column of fuel pellets, fixed to closing elements at the ends of the column of fuel pellets and the sleeve being inserted between the column of fuel pellets and the cladding. This technological sleeve solution according to this U.S. Pat. No. 4,235,673 is aimed exclusively at confining pellet fragments or splinters that might be created. Thus, the only function of the sleeve according to this U.S. Pat. No. 4,235,673 is to confine fuel pellet splinters, and the function to transfer heat between the pellets and cladding is necessarily done by an infill fluid such as sodium as explained for example in column 4, lines 23-30 in this document and the function accommodating three-dimensional swelling of pellets is done through the compulsory existence of a functional clearance between the sleeve and cladding sized for this purpose, as is very clearly expressed in the text in claim 1 of this document. In other words, U.S. Pat. No. 4,235,673 discloses a necessarily composite interface solution between the sleeve fixed to the ends of the pellet column and a sufficiently large thickness of heat transfer liquid between the cladding and the pellet column to define a functional clearance sufficiently large to accommodate the three-dimensional swelling of the pellets. Furthermore, the combined interface solution according to this U.S. Pat. No. 4,235,673 is complex to implement and introduces risks of non-reproducibility, due to the sleeve being fixed to closing elements at the ends of the fuel pellet stack, which therefore requires an additional step during fabrication of a fuel rod in a nuclear environment. According to U.S. Pat. No. 4,235,673, this technical solution is applicable to nuclear control rods as shown in column 3, line 36. Patent FR 2769621 discloses the use of a SiC sleeve reinforced with Sic fibres, inserted between a stack of typically B4C neutron absorber pellets, and the cladding. The solution according to this patent FR 2769621 cannot genuinely function: the material described for the sleeve is the equivalent of a ceramic matrix composite CMC. Studies done by the inventors show that such a composite cannot accommodate expansion or three-dimensional swelling of the stacked pellets in the long term. A CMC is intrinsically very stiff (Young's modulus of the order of 200 to 300 GPa) and its ductility is low (elongation at failure less than 1%) which quickly causes its destruction as soon as a mechanical interaction between pellets and cladding situation develops under the effect of three-dimensional swelling of the neutron absorber. Furthermore, the sleeve thicknesses mentioned in this patent FR 2769621 imply volume fractions of neutron absorber very much lower than allowable values. Reducing the volume fraction of absorber makes it necessary to increase the 10B content, which has the disadvantage of high cost. Patent JP 2004245677 discloses the use of a metal sleeve made from fibres, particularly a braid inserted between a stack of boron carbide B4C absorber pellets over its entire height. As for U.S. Pat. No. 4,235,673, this sleeve alone cannot perform all functions required for a pellet/cladding interface joint in a control rod: it acts essentially to confine fragments of absorber pellets (function f8), but it must also be associated with a filling fluid (liquid metal such as sodium mentioned in Patent JP 2004245677) in particular to satisfy the primary mechanical (function f1) and thermal (function f3) functions. Consequently, this solution hardly seems applicable to situations in which the proposed sleeve is immersed in sodium, which limits its use to SFR and appears to exclude its use for PWR or GFR for example, in so far as these reactors prohibit the use of sodium (problem of compatibility with the coolant in PWR and the boiling temperature in GFR). Finally, U.S. Pat. No. 4,172,262 discloses the use of a metal sleeve inserted between the stack of neutron absorber pellets and the cladding, the sleeve being inserted only on the lower part of the stack. The specific material proposed in this document, namely 347 type stainless steel, is not compatible with very high temperatures and therefore makes it unsuitable for GFR reactors and in accident scenarios in other reactors. Therefore, the general purpose of the invention is to propose an improved interface between pellets and cladding in a nuclear control rod with a cylindrical geometry and circular cross section that does not have the disadvantages of interfaces according to prior art as presented above. Another purpose of the invention is to propose a method for fabricating a nuclear control rod with an improved pellet/cladding interface that is not completely unrelated to the industrial facility set up to fabricate existing nuclear control rods with circular cross-section. To achieve this, the purpose of the invention is a nuclear control rod extending along a longitudinal direction comprising a plurality of pellets made of a neutron absorber material, stacked on each other in the form of a column and cladding surrounding the column of pellets, in which the cladding and the pellets have a circular cross-section transverse to the longitudinal direction, and in which an interface joint also with a circular cross-section transverse to the longitudinal direction (XX′), made of a solid material transparent to neutrons and with open pores is inserted between the cladding and the column of stacked pellets, at least over the height of the column. According to the invention, the interface joint is a structure, mechanically decoupled from the cladding and from the column of pellets, with a high thermal conductivity and open pores, adapted to deform by compression across its thickness so as to be compressed under the effect of the three-dimensional swelling of the pellets under irradiation, the initial thickness of the joint and its compression ratio being such that the mechanical load transmitted to the cladding by the pellets under irradiation remains less than a predetermined threshold value. A high thermal conductivity means a coefficient of thermal conductivity sufficiently high to achieve heat transfer between the column of B4C absorber pellets and the cladding so as to guarantee that the core temperature in the absorber pellets remains below their melting point. Therefore the invention concerns an interface joint between the stacked pellets and the cladding, the joint having a solid structure, high porosity preferably between 30 and 95% of the volume of the joint in the cold state and that is adapted to perform the following functions up to nominal operating temperatures in nuclear reactors: due to its compression, enable radial expansion of the stacked neutron absorber pellets under irradiation, without any excessive mechanical load on the cladding; due to deformations not causing loss of continuity of its structure, enable accommodation of differential axial strains between the stacked pellets and the cladding surrounding them, at a high temperature and under irradiation without an excessive load on the cladding; facilitate transfer of heat generated by nuclear reactions within the pellets, to the coolant circulating along the cladding, in a uniform manner; enable the transfer of gases released under irradiation (helium and tritium) to vents formed in the cladding and/or the expansion vessel located at the end of the cladding and in which there is no neutron absorber; protect the cladding against compatibility problems with the absorber in the pellets by retention of products released by the absorber in the pellets that could corrode the cladding. The interface joint according to the invention may be made in any nuclear control rod for use in reactors in which the coolant is either pressurised (as for GFR reactors) or is not pressurised. For pressurised coolants, care will be taken to assure that the cladding used is sufficiently resistant to creep deformation so that it will not come into contact with the fuel pellets during operation. Typically, cladding made of a ceramic matrix composite CMC is perfectly suitable. A solid interface joint is defined with open pores that durably enable three-dimensional expansion of the B4C absorber pellets without applying an excessive mechanical load on the cladding, for irradiation durations that do not impose more severe shutdown constraints for reloading than shutdown constraints for fuel elements. “Excessive” means any load, particularly in the circumferential direction, that could exceed limits imposed by usual design criteria for a nuclear control rod [14]. Note also the thermal constraints (performance and lack of discontinuities) neutron constraints (neutron absorption capacity and dimensions) and constraints on the transfer of fission gases released to the expansion vessel also have to be respected. One or more materials for the interface joint according to the invention could be used, that would contribute to making non-mechanical interactions between the absorber and the cladding material unimportant. Thus, the solid interface joint with open pores can trap some or all products released by the absorber that can react chemically with the cladding and degrade its mechanical performances (for example stress corrosion problem). The open pores of the joint and any functional clearances separating the interface joint from the pellets and/or the cladding may be filled with a gas, preferably helium and/or a liquid metal such as sodium. Due to is consistence (intrinsic stiffness up to the mechanical load threshold beyond which it starts to be compressed), the solid interface joint according to the invention guarantees centring of the pellets in the cladding and prevents any movement of B4C neutron absorber fragments. One way of creating a long-term delay in the mechanical interaction between pellets and the cladding would be to envisage a solid interface joint several hundred microns thick. In any case, care will be taken to assure that its thermal properties, possibly taking account of the thermal properties of the gas and/or the liquid metal in which it is immersed, guarantee control of the temperature of the B4C neutron absorber. Care will be taken to make sure that the solid interface joint has ad hoc mechanical properties. Thus, care will be taken to assure that it has sufficiently high strain capacities in compression, in other words radially along the direction of the control rod, and in shear (around the circumference and along the direction parallel to the axis of revolution of the fuel rod or the control rod), to accommodate differential strains of neutron absorber pellets and the cladding under irradiation, without inducing any excessive mechanical load on the cladding, or any axial and circumferential discontinuity of the joint. These mechanical properties must be guaranteed under irradiation for doses of up to the order of 100 dpa-Fe to 200 dpa-Fe (fluences from 2 to 4×1027 n/m2). Neutron absorber pellets are subject to three-dimensional swelling, such that their diameter and length increase. Since the cladding a priori swells much less than the absorber, the interface between pellets and the cladding reduces during irradiation. Furthermore, the stack of pellets extends much more than the cladding, causing longitudinal shear between them. Thus, care will be taken to assure that the interface joint can: due to its compression strain, compensate for reduction of the interface with a stiffness compatible with the mechanical strength of the cladding, which excludes the presence of any locally dense zones (defects resulting from the fabrication method, densification in irradiation, etc.), compensate for the longitudinal sliding deformation between the neutron absorber stack and the cladding by its elongation (effect of Poisson's ratio) resulting from its radial compression and/or by shear deformation (assuming surface sticking on the cladding and/or the absorber with transmission of an axial force compatible with the mechanical strength of the cladding); and/or by a viscous axial extrusion flow into the gap under the action of its radial compression. The interface joint according to the invention is made continuously over its entire height: in any case, the objective is to reach a compromise such that by compensating for the longitudinal sliding deformation described above, no axial discontinuity of the joint occurs. Finally, care will be taken to assure that joint deformation modes do not cause fragmentation of the joint in a way that could lead to fragments moving when the interface is partially reopened, typically during an unscheduled or scheduled reactor shutdown, which would induce a risk of later punching of the cladding, for example when the power/temperature rise. Care may also be taken to assure that the material(s) to be envisaged for the solid interface joint is (are) neutron absorber(s) as much as possible. The high open porosity of the structure as fabricated must facilitate transport of released gases to vents (if any) formed in the cladding and/or the expansion vessel located near the top of the absorber element, with an efficiency that does not degrade much under irradiation (compression of the structure leading to a reduction in the total porosity and the open pores ratio). The large exchange surface area provided by the structure must facilitate retention of products released by the absorber under irradiation (for example carbon in the case of the B4C) that might contribute to embrittlement of the cladding by corrosion. Due to the structural solid interface joint according to the invention, it can be thicker than is possible with interfaces usually encountered between the pellets and cladding, so as to extend the life of pellets made of a B4C neutron absorber material, resulting in an appreciable economic saving without affecting safety. The open pores of the interface joint according to the invention may have a volume equal to at least 30% of the total volume of the interface joint as produced in fabrication. Preferably, this volume is between 30% and 95% of the total volume of the interface joint as produced in fabrication and is more preferably between 50% and 85%. Obviously, the described porosity and geometric dimensions of the interface joint are those for the cold interface joint as produced in fabrication and before it is used in a nuclear reactor. The same is true for other elements of the control rod according to the invention. The open porosity targeted by the invention may be quantified by various known measurement techniques: for example density measurement for braids and fibres, or for example image analysis by X tomography or optical microscopy or optical macroscopy. Advantageously, the thickness of the interface joint in its section transverse to the (XX′) direction is more than at least 10% of the radius of the pellets. The interface joint may be composed of one or several fibrous structures such as braid(s) and/or felt(s) and/or web(s) and/or fabric(s) and/or knit(s). Its volume percentage of fibres is then advantageously between 15 and 50%, which corresponds approximately to a porosity of between 50 and 85%, in other words an optimum compromise between the required joint compressibility and high thermal conductivity accompanied by effective confinement of any absorber splinters that might be formed. According to one embodiment, the interface joint may be made from a braid comprising a plurality of carbon fibre layers and a plurality of layers comprising silicon carbide fibres superposed on carbon fibre layers. Alternately, the interface joint may be made from one or several honeycomb materials such as foam. The interface joint may be based on ceramic or metal. For a gas-cooled fast reactor (GFR), the basic material of the cladding could preferably be envisaged to be a refractory ceramic matrix composite (CMC) such as SiC—SiCf, possibly associated with a liner based on a refractory metal alloy. For a sodium-cooled fast reactor (SFR), it would be preferable to envisage the cladding made of a metallic material. Finally, the invention relates to a method for making a nuclear control rod comprising the following steps: a/ at least partially make a joint with a circular cross-section made of a material transparent to neutrons, in the form of a structure made of a material with good thermal conductivity with open pores, capable of deforming under compression across its thickness; b/ insert the at least partially produced joint into a cylindrical cladding with a circular cross-section that is open at least at one of its ends, made of material that may or may not be transparent to neutrons; c/ insert a plurality of pellets made of boron carbide B4C neutron absorber material over not more than the height of the joint, inside the joint inserted into the cylindrical cladding with circular cross-section, d/ completely close the cladding once the joint has been entirely produced. According to a first embodiment, step a/ is performed using the following sub-steps: superpose a plurality of braid layers comprising silicon carbide fibres on a plurality of layers of carbon fibre braids themselves on a mandrel; compress the multilayer braid in a cylindrical mould; add a soluble binder into the compressed braid; evaporate the solvent; step b/ is performed using the mandrel around which the braid is in contact, the mandrel then being removed; and later in step c/, a heat treatment is performed under a vacuum to eliminate the binder and thus bring the joint into contact with the plurality of stacked pellets and with the cladding. The braid layers may be of the two-dimensional type with a braiding angle of 45° relative to the axis of the mandrel. The carbon fibres may be of the Thornel® P-100 type, each containing 2000 filaments and cracked. The silicon carbide fibres are of the HI-NICALON0™ type S each containing 500 filaments. The soluble binder is advantageously a polyvinyl alcohol. According to a second embodiment, step a/ is performed using the following sub-steps: needlebonding of carbon fibre webs in the form of a tube on a mandrel; performance of a heat treatment (for example at 3200° C. under Argon); compression of the heat-treated tube in a cylindrical mould; addition of soluble binder into the compressed tube; evaporation of the solvent; step b/ is performed using the mandrel around which the tube is in contact, the mandrel subsequently being removed; and later in step c/, a heat treatment is performed under a vacuum to eliminate the binder and thus bring the joint into contact with the plurality of stacked pellets and with the cladding. The carbon fibres may then be of the Thornel® P-25 type. As in the first embodiment, the soluble binder is advantageously a polyvinyl alcohol. According to a third embodiment, step a/ is performed using the following sub-steps: production of a carbon foam tube composed of open honeycombs; chemical vapour deposition (CVD) of a W—Re alloy on the carbon foam tube. Note that the element shown is a nuclear control rod. This element is shown cold, in other words once the final control rod has been fabricated and before use in a nuclear reactor. The control rod according to the invention comprises the following from the outside to the inside: cladding 1 made of a metallic or CMC (ceramic matrix composite) material(s), possibly coated with a liner on its internal wall, a first assembly set 2 (optional), to the extent that it may possibly be eliminated during fabrication following the binder evaporation process described above), a solid joint 3 with open pores according to the invention; a second assembly set 4 (optional, to the extent that it can possibly be eliminated during fabrication following the binder evaporation process described above); a stack of pellets 5 of neutron absorbing boron carbide B4C material forming a column. The solid joint with open pores 3 according to the invention has a height greater than the height of the column of stacked pellets 5. The difference in height between the porous solid joint 3 and the column of stacked pellets is chosen to assure that this column remains axially facing the joint throughout the irradiation phase during operation of the nuclear reactor during which its length increases due to swelling under irradiation. Thus according to [8], the absorber in the SUPERPHENIX reactor targets functioning with 1022 captures per cm3 of absorber and per year, and the elongation rate due to swelling of B4C is of the order of 0.05% for 1020 captures per cm3 of absorber, giving an elongation of the order of 5% per year of irradiation. Several types of materials may be suitable for fabrication of the porous solid joint 3 according to the invention, and advantageously fibrous structures possibly with a matrix deposited in these structures, or honeycomb materials with open pores. Fibrous structures that may be suitable include braids, felts, webs, fabrics or knits, or a combination of them, comprising a volume percentage of fibres equal to at least 15%, or possibly at least 5% in the case of felts, before densification. The fibres may be made of ceramic compounds (carbon, carbides, nitrides or oxides) or metallic compounds (such as W, W—Re alloys, Mo—Si2, etc.). One way of making fibrous structures suitable for a porous joint 3 according to the invention may be to use conventional braiding, felt forming techniques or webbing, needlebonding, weaving or knitting [4]. It is possible to envisage increasing the thermal conductivity of the material or protecting the fibres by depositing chemical compounds that are also refractory (ceramic or metallic compounds) on the fibres. These depositions then represent a volume percentage such that the open porosity of the final material, fibrous structure reinforced by a deposit, is between 30% and 85%, or even up to 95% in the case of felts. These depositions on fibrous structures may be made using conventional chemical vapour deposition (CVD) techniques [1] or other techniques such as impregnation of ceramic polymer precursor, pyrolysis, etc. The joint 3 may be placed either by positioning it around the pellets 5 and then inserting the joint 3/pellets 5 assembly into the cladding 1, or by inserting it into the cladding 1, the pellets then being inserted later. Physical contact firstly between the cladding 1 and the joint 3 and secondly between the joint 3 and the pellets 5 may be formed during the temperature rise in the nuclear reactor by differential thermal expansion, since joint 3 expands more. Another way of achieving this physical contact is radial compression of the joint 3, and then the joint 3 can be released after placement of the cladding 1-joint 3-pellets 5 assembly, before the assembly is put into service in the nuclear reactor for which the control rod is to be used. Honeycomb materials or foams that might be suitable are open pore materials with between 30% and 85% of porosity, with cell diameters preferably less than 100 μm to prevent movement of “macro-fragments” of pellets, but sufficiently high for interconnection of the pores. The composition of these materials may be based on ceramic or metallic compounds. It would be possible to make honeycomb materials suitable for porous joints 3 according to the invention using conventional techniques for the injection of gas bubbles or compounds generating bubbles in the molten material or a precursor compound (organic resin for carbon), powder metallurgy with porogenic compounds or particles, deposition of a compound on a foam acting as a substrate [2],[7]. The basic foam can then be reinforced by deposition of a compound (among ceramic or metallic compounds) with a nature that may be identical to or different from the foam compound. This deposition may for example be obtained by chemical vapour deposition (CVD) [1]. Three examples of nuclear control rods according to the invention are given below, with the characteristics of the main control system (SCP) for the SUPERPHENIX reactor [8]: in all these examples, the control rod comprises a stack of cylindrical boron carbide neutron absorber pellets 5 with a diameter of 17.4 mm and cladding 1 surrounding the column of stacked pellets with an inside diameter of 19.8 mm, namely a radial pellet/cladding clearance of 1.2 mm (cold). For comparison with the joint solution that will be presented below, for an SCP control rod for the SUPERPHENIX reactor [8], the absorber pellets column is surrounded by a 200 μm thick liner confining pellet fragments formed under irradiation, and the residual pellet/cladding space is filled with liquid sodium to provide efficient heat transfer. The end of life of such a control rod is associated particularly with the occurrence of a mechanical interaction between pellets and the cladding situation, when the three-dimensional expansion of B4C pellets eventually fills in the free radial space that initially separated the column of pellets from the cladding, leading to a mechanical load that quickly makes the cladding unusable. The thickness of the liner (200 μm) should be naturally subtracted from the initial value of the pellet/cladding clearance (1.2 mm), therefore the allowable future expansion of the pellets is of the order of 1 mm for a pellet radius of 8.7 mm, which gives an allowable expansion ratio of the order of 11.5% before the mechanical interaction between pellets and the cladding is reached. These characteristics are usually sufficient to achieve neutron capture ratios of the order of 200*1020 per cm3 of absorber. With a porous solid joint according to the invention, and considering the end of life reached for complete disappearance of the joint porosity (by compression under three-dimensional expansion of B4C pellets), the gain on the neutron absorption ratio that could be envisaged from the design fabrication porosity for the joint according to the invention can be evaluated. For changing from a 200 μm thick liner to a 1.2 mm thick joint, the required value of the joint porosity is typically a value equal to a ratio of 1/1.2, namely of the order of 83% (joint with 17% of the theoretical density of the material of which it is composed), to achieve the capture ratio obtained with a sleeve type solution and also to benefit from the advantage of centring the pellets in the cladding. Note that the thermal effect induced by the joint is neglected (calculations show that this is a second order effect concerning the swelling ratio of the absorber). A first series of three layers of superposed braids is made with carbon fibres (trade name Thornel® P-100 each containing 2000 filaments and that are cracked to reduce the thread diameter) on a mandrel with the following characteristics: inside diameter: 17.5 mm outside diameter: 19.0 mm braiding type: 2D braiding angle: 45° A second series of three braid layers is made on the previous series of braid layers with silicon carbide fibres (trade name HI-NICALON™ type S, each containing 500 filaments), with the following characteristics: inside diameter: 19.0 mm, outside diameter: 21.2 mm braiding type: 2D braiding angle 45° The multi-layer braid 3 thus formed is compressed in a cylindrical mould with an inside diameter of 19.7 mm. An eliminable soluble binder, in this case a polyvinyl alcohol, is then added into the braid and the solvent is then evaporated. The braid 3 is then stripped and inserted into a metal cladding 1 with inside diameter of 19.8 mm. The central mandrel is then removed, and a column of 17.4 mm diameter boron carbide B4C neutron absorber pellets 5 is then inserted into the braid. The binder is eliminated by heat treatment of the assembly under a vacuum. The braid 3 then expands and comes into physical contact with the pellets 5 and the cladding 1. Therefore, the fabricated thickness of the braid 3 is equal to the total assembly clearance between the cladding 1 and the pellets 5, namely 1.2 mm. The cladding 1 may then be closed at its ends, for example by welding. Even if not shown, before the final closing step is performed, a helical compression spring is housed in the expansion chamber or vessel 6 with its lower end bearing in contact with the stack of pellets 5 and its other end bearing in contact with the upper plug. The main functions of this spring are to hold the stack of pellets 5 along the direction of the longitudinal axis XX′ and to absorb the elongation of the fuel column with time under the effect of longitudinal swelling of the pellets 5. The nuclear control rod thus made with a porous solid joint 3 according to the invention can then be used for application in a fast neutron nuclear reactor. Carbon fibre layers (trade name Thornel® P-25) are needlebonded in the form of a tube with inside diameter 17.5 mm and outside diameter 21.2 mm, on a graphite mandrel. A heat treatment is then applied on the assembly at 3200° C. under Argon. The tube thus formed is compressed in a cylindrical mould with an inside diameter of 19.7 mm. An eliminable soluble binder, in this case a polyvinyl alcohol, is then added into the structure and the solvent is then evaporated. The porous solid joint 3 thus obtained is then stripped and inserted into a cladding 1 with inside diameter of 19.8 mm. The central mandrel is then removed, and a column of 17.4 mm diameter boron carbide B4C neutron absorber pellets 5 is then inserted into the mixed joint 3/cladding 1 structure. The binder is then eliminated by heat treatment of the assembly under a vacuum. The joint 3 then expands and comes into contact with the stacked pellets 5 and the cladding 1. The cladding 1 may then be closed at its ends, for example by welding. Even if not shown, before the final closing step is performed, a helical compression spring is housed in the expansion chamber or vessel 6 with its lower end bearing in contact with the stack of pellets 5 and its other end bearing in contact with the upper plug. The main functions of this spring are to hold the stack of pellets 5 along the direction of the longitudinal axis XX′ and to absorb the elongation of the fuel column with time under the effect of longitudinal swelling of the pellets 5. The nuclear control rod thus made with a porous solid joint 3 according to the invention can then be used for application in a fast neutron nuclear reactor. A tube with an inside diameter of 17.4 mm and outside diameter of 19.8 mm made of carbon foam composed of 40 μm diameter open honeycombs is placed in a chemical vapour deposition (CVD) furnace. An approximately 7 μm thick deposition of W—Re 5% alloy obtained from the decomposition of a mix of tungsten and rhenium halide compounds is applied on the ligaments forming the foam. This foam tube is then inserted into the cladding 1 with inside diameter 19.8 mm, and the column of 17.4 mm diameter boron carbide B4C neutron absorber pellets 5 is in turn inserted into the foam tube. The cladding 1 may then be closed at its ends, for example by welding. Even if not shown, before the final closing step is performed, a helical compression spring is housed in the expansion chamber or vessel 6 with its lower end bearing in contact with the stack of pellets 5 and its other end bearing in contact with the upper plug. The main functions of this spring are to hold the stack of pellets 5 along the direction of the longitudinal axis XX′ and to absorb the elongation of the fuel column with time under the effect of longitudinal swelling of the pellets 5. The nuclear control rod thus made with a porous solid joint 3 according to the invention can then be used for application in a fast neutron nuclear reactor. Other improvements would be possible without going outside the scope of the invention. Thus, in all examples 1 to 3 mentioned above, the fabrication thickness of the porous solid joint 3, in other words the thickness after the cladding 1 has been closed and the control rod is ready for application, is equal to the total design assembly clearance between the cladding 1 and the column of pellets 5 made of B4C neutron absorber material. Obviously, clearances could be provided (see references 2, 4 in FIG. 1) that are maintained once the control rod is ready, provided that the fabrication methods and properties (particularly differential thermal expansion firstly of the cladding 1 and the porous solid joint 3, and secondly of the joint 3 and the pellets 5) make it possible. These clearances as shown in references 2, 4 in FIG. 1 are a priori filled with a gas or a liquid metal that then naturally occupies the open pores of the porous solid joint 3 according to the invention, and the open pores of the B4C neutron absorber pellets 5. But according to the invention and unlike solutions according to the state of the art, and more particularly the solution according to U.S. Pat. No. 4,235,673, assembly clearances are not essential and therefore are not functional clearances provided to accommodate the three-dimensional swelling of the pellets under irradiation. Furthermore, the mandrel used to form the porous solid joint as in the examples described may be made of different materials compatible with the materials used in the joint, such as graphite and quartz. Similarly, for the final step in the process before the cladding is closed, examples 1 to 3 describe placement of a helical compression spring. More generally, during this final step before the actual closing step of the cladding, it would be possible to use what is currently referred to as an “internals system” in the nuclear domain, in other words an assembly of components such as springs, packing, etc., the function of which is to position the column of pellets axially within the cladding. FIG. 2 shows the compression behaviour of interface joints according to the invention with high open porosity and based on braids or based on felt made of a SiC material. More precisely, as shown, these are tests in cycled compression, with each cycle alternating a load and an unload, which in FIG. 2 is illustrated by loading loops in the strain-stress plane. The abscissa indicates the values of the compression ratio (strain in %) of the joint across its thickness. The ordinate indicates values of mechanical loads (stress in MPa) transferred by the joint under the effect of its compression. Thus, the indicated stresses actually correspond to the radial mechanical load σr applied to the cladding of a nuclear control rod under the effect of the three-dimensional swelling of B4C neutron absorber pellets stacked on each other, the stresses being transmitted to the cladding directly by compression of the joint between the pellets and the cladding. This radial load introduces a controlling circumferential load σθ, the intensity of which corresponds to the intensity of the radial load to which a multiplication factor is applied, which is approximately equal to the ratio of the average radius rG of the cladding to its thickness eG, which is typically equal to 5 to 10: σθ≈(rG/eG)σr. FIG. 2 thus illustrates the fact that an interface joint according to the invention is adapted to function like a stress absorber: the transmitted load only becomes significant for a sufficiently high compression ratio beyond which the transmitted load increases progressively with the compression ratio, until it reaches the threshold value of the allowable limiting load (without any sudden changes). Thus, for a load σr considered to be significant starting from 1 MPa, the compression ratio is of the order of 40% and 70% respectively for the braid and felt type joints considered in FIG. 2. In a situation of operation under irradiation in a reactor, the cladding of a nuclear control rod cannot resist a mechanical load from B4C neutron absorbers unless it remains below a limit guaranteeing that there is no cladding failure. Thus, for example if the threshold value of the allowable circumferential load σθ is fixed at 100 MPa (which is a reasonable value considering usually allowed loads), namely a radial load θr of the order of 10 MPa (for a ratio rG/eG of the order of 10), FIG. 2 shows that braid and felt type joints considered will give a compression ratio of the order of 60% and 95% respectively, below which the mechanical load transmitted to the cladding remains acceptable. Note that the tests done according to FIG. 2 showed that the interface joint according to the invention based on braids and the joint based on felt maintained their integrity; thus, the braid/felt structure is preserved without any formation of fragments that could move into a reopened gap between pellets and the cladding in a control rod in a fast neutron reactor FNR. A control rod must be kept for as long as possible in a fast neutron reactor if economic performances are to be optimised. These performances are usually limited by various operating constraints so as to satisfy safety objectives. One of the most severe constraints is imposed by the need to guarantee mechanical integrity of the control rod cladding under all circumstances. This leads to the definition of an allowable limiting load on the cladding (stress and/or strain beyond which the integrity of the cladding can no longer be guaranteed). However under irradiation, the B4C neutron absorber pellets are affected by a continuous three-dimensional swelling that leads to a pellet/cladding mechanical interaction (PCMI) that could eventually lead to an unacceptable load on the cladding. Therefore, the operating life for a nuclear control rod with B4C nuclear absorbers is strongly dependent on the time for such an excessive interaction to occur. The interface joint according to the invention as defined above provides a satisfactory response because it enables long term expansion or three-dimensional swelling of the pellets. For a fixed three-dimensional swelling of the pellets, the durability depends on the initial thickness of the joint and the compression ratio that it can accommodate before its compression state causes the transmission of an unacceptable mechanical load to the cladding; the initial thickness of the joint to be installed reduces as the allowable compression ratio increases. FIG. 2 illustrates the fact that very high compression ratios are necessary to reach the compression limit of the proposed braid or felt type joints, which means that increased irradiation times can be reached if a reasonably thick joint is installed. The large joint thicknesses characteristic of control rods for a fast spectrum reactor, support the installation of high porosity joints that can easily reach and probably exceed the performances of the sleeve type solution used in SUPERPHENIX. Furthermore, shear tests were carried out by imposing forces on an approximately 1 cm thick fibrous structure according to the invention, corresponding to cyclic displacements of the order of 100 μm at temperatures of the order of 400° C. For these elongations of 1%, the fibrous structure remained perfectly intact. In the case of control rods for fast spectrum reactors, the large thicknesses of the joints also enable the use of joints according to the invention comprising several layers of superposed braids and/or felts. Concerning the axial shear to which the joint under irradiation is submitted, due to the elongation of the column of pellets (effect of swelling) that is more pronounced than the elongation of the cladding, this multi-layer structure reduces the mechanical load on the joint by enabling relative sliding of layers on each other, and consequently limits the risk that the joint would be damaged by shear. [1] S. Audisio, Dépôts chimiques à partir d'une phase gazeuse (Chemical depositions starting from a gaseous phase), Techniques de l'ingénieur, M1660, 1985. [2] J. Banhart, Manufacture, characterisation and application of cellular metals and metal foams, Progress in Materials Science, Vol. 46, pp. 559-632, 2001. [3] A. Berthet, B. Kaputsa, R. Traccucci, P. Combette, F. couvreur, D. Gouaillardou, J. C. Leroux, J. Royer & M. Trotabas, Pressurized Water Reactor Fuel Assembly, in The nuclear fuel of pressurized water reactors and fast reactors—Design and behaviour, (H. Bailly, D. Menessier and C. Prunier, Editors), Lavoisier Publishing, Paris, pp. 271-436, 1999. [4] L. Caramaro, Textiles à usages techniques (Fabrics for Engineering Applications), Techniques de l'ingénieur N2511, 2006. [5] D. Gosset and P. Herter, Matériaux absorbants neutroniques pour le pilotage des reacteurs (Neutron absorber materials for control of reactors), Techniques de l'ingénieur B3720, 2007. [6] Y. Guérin, In-reactor behaviour of fuel materials, in The nuclear fuel of pressurized water reactors and fast reactors—Design and behaviour, (H. Bailly, D. Menessier and C. Prunier, Editors), Lavoisier Publishing, Paris, pp. 77-158, 1999. [7] L. Kocon and T. Piquero, Les aérogels et les structures alveolaires: deux examples de mousses de carbone (Aerogels and honeycomb structures: two examples of carbon foam), L'Actualité Chimique, No. 295-296, pp. 119-123, 2006. [8] B. Kryger & J. M. Escleine, Absorber elements, in The nuclear fuel of pressurized water reactors and fast reactors—Design and behaviour, (H. Bailly, D. Menessier and C. Prunier, Editors), Lavoisier Publishing, Paris, pp. 531-565, 1999. [9] J. Y. Malo, N. Alpy, F. Bentivoglio, F. Bertrand, L. Cachon, G. Geffraye, D. Haubensack, A. Messié, F. Morin, Y. Péneliau, F. Pra, D. Plancq & P. Richard, Gas Cooled Fast Reactor 2400 MWTh, status on the conceptual design studies and preliminary safety analysis, Proceedings of the ICAPP'09 conference, (Tokyo, Japan, May 10-14, 2009). [10] R. B. Matthews and R. J. Herbst, Nuclear Technology, Vol. 63, pp. 9-22, 1983. [11] Hj. Matzke, Science of advanced LMFBR fuels, North Holland, Amsterdam, 1986. [12] P. Millet, J. L. Ratier, A. Ravenet and J. Truffert, Fast Reactor Fuel Subassembly, in The nuclear fuel of pressurized water reactors and fast reactors—Design and behaviour, (H. Bailly, D. Ménessier and C. Prunier, Editors), Lavoisier Publishing, Paris, pp. 437-529, 1999. [13] K. Tanaka, K. Maeda, K. Katsuyama, M. Inoue, T. Iwai and Y. Arai, Journal of Nuclear Materials, Vol. 327, pp. 77-87, 2004. [14] Design and construction rules for fuel assemblies of PWR nuclear power plants, AFCEN, 2005. |
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059206039 | abstract | A forged core plate fabricated from a single piece stainless steel forging is described. The plate has a substantially circular shape and penetrations are provided in the core plate to allow passage of control rod blades into the core and direct recirculation flow past the fuel assemblies. The penetrations are formed from machining "5 holes" per penetration. Particularly, each penetration includes a center through hole to allow passage of the control rod blade and control rod blade guide tube. The through hole has a smaller diameter at the plate top surface to provide a tighter fit to the outside diameter of the guide tube to maintain alignment and minimize flow leakage. The through hole is enlarged at the core plate bottom surface to allow clearance in order to improve the installation and removal of the guide tube. Each penetration also include four peripheral blind holes to direct the recirculation flow into the flow holes located on the sides of the guide tube. The blind holes do not extend completely through the plate. When the guide tube is installed in the core plate, the flow holes in the side of the guide tube is located approximately 1/3 of the way down from the plate top surface. |
description | This application is a National Phase Application of International Application No. PCT/EP2004/013095, filed Nov. 18, 2004, which claims the benefit under 35 U.S.C. 119 (a-e) of TV2004A000072 filed Jun. 24, 2004, which is herein incorporated by reference. The present invention relates to a garment for sporting activity, in particular for motorcyclists. It is usual for those who practice sporting activities, for example motorcycling, to make use of rigid protective elements in order to protect the parts of the body which are most at risk in the event of falls or violent impacts. In particular, a protective element is used for the back, said element consisting normally of a resistant shield which is applied underneath a protective garment which, for the sake of simplicity here and by way of example, is considered to be a riding suit. It is clear that despite the fact that reference is made to a motorcyclist's suit, the subject of the invention may also be used in any other category of garments, of the sporting type or not, where the particular characteristics features of the invention are required. Normally fixing means (buckles, laces, adjustable straps, etc.) are associated with the abovementioned shield in order to ensure that it may adhere firmly and be gripped tightly against the motorcyclist's body, in order to prevent the shield from moving and thus exposing the parts which are to be protected. This protective solution has, however, certain disadvantages. In fact, in the event of a fall, the suit may tear and uncover the motorcyclist's body, exposing him/her to wounds or abrasions. Moreover, owing to tearing of the suit, the shield and any other protective elements which are fixed to the suit may move, uncovering the back and the protected parts and exposing them to the risk of injuries. The main object of the present invention is to improve this state of the art, i.e. provide a garment, in particular a suit, which ensures improved protection for the rider. Another object of the invention is to increase the wearing comfort of the garment. These objects are achieved by the garment designed in accordance with the claims present at the end of this description. The Figures show a suit 10 which surrounds the whole of the rider's body except for the head, hands and feet Said suit 10 comprises two leg portions 12, a trunk portion 14 and two sleeves 16. Protection for the back against knocks resulting from falls or impacts is ensured by a known elongated plastic shield 40, the top end 42 of which has a widened form in order to protect the shoulder blades, while the bottom end or tail 44—shown in broken lines in FIGS. 1 and 2—protects the lumbar region with an appendage 45 and surrounds the rider's waist with a fixing belt 46. Said shield may be fixed to the fabric of the suit 10, for example by means of a zip (not shown) along its perimeter. In this way it is possible to remove the shield 40, which may be damaged after a fall, by simply opening the zip fastener and replacing the damaged shield 40 with a new one. In FIGS. 1 and 2 the parts shown as a shaded dotted area are made of a light and breathable mesh fabric (known as “mesh”). In particular, the trunk portion 15, from which two segments 18 covering the arm and two segments 20 covering the leg extend, are made of said material. The segments 18 which surround the arm completely extend with a strip 120 above the forearm as far as the rider's wrist, while the segments 20 surround the leg completely, except for an area on the front part of the knee. As can be seen from the Figures, in the lumbar zone of the suit 10 the “mesh” fabric is interrupted approximately along an upper unstitched edge 48 by a layer of puckered elastic material 58 fixed along its side edges 49 to the mesh fabric. Therefore the tail 44 of the shield 40 is able to be inserted underneath the puckered material 58—cf. FIG. 3—being fixed directly onto the rider's back. With this solution it is possible to insert the lumbar appendage 45 of the shield 40 inside the suit 10, causing it to adhere perfectly to the riders waist and ensuring optimum fixing by means of the belt 46. As an alternative it is also possible to line internally the puckered layer 58 so as to contain the shield 40, perforating the lining so that the belt 46 is able to pass through. Since what is essential is to form a slit in the suit 10 so that the tail end 44 of the shield 40 can be inserted underneath the fabric, a simple incision in the suit itself may also be sufficient. The arrangement of the shield 40 on top of the suit 10—which is a characteristic feature of the invention—allows more effective protection of the fabric of the suit 10. In this way tearing of the said suit 10 on the back is prevented in the event of a fall, with the result that the safety of the entire garment improves overall. Moreover the use of a light and breathable material such as the “mesh”, which is possible because it is protected by the shield 40, increases the comfort of the rider. In fact, the use of a breathable material for the abovementioned parts allows increased ventilation of the rider's body. In particular, the “mesh” material has produced excellent results in this connection. The fact of incorporating the shield 40 inside the suit 10 also ensures that the suit 10 adheres better to the body, improving its wearing comfort. It should be noted that another characteristic feature as regards comfort for the rider consists of the puckered material 58, owing to the fact that it manages to stretch elastically and be deformed in accordance with the rider's posture, especially when the rider bends forwards over the motorcycle. In the suit 10 the parts covering the shoulders, the forearms, the thighs and the front part of the knee are made of leather (or material with a high resistance both to abrasion and to knocks)—indicated by 50 in the figures—so as to offer adequate protection. Another problem solved by the present invention is as follows. Usually rigid protective elements (not shown) are worn on the elbows, being placed inside the suit using known fixing systems. As already mentioned, the protective elements following a knock or fall may move. The suit 10 according to the invention improves the state of the art. It has a strip of “mesh” material 120 which interrupts the leather fabric 50 around the forearms. An adjustable buckle 60 is also fixed onto the portions of said leather fabric. By varying the tension of the buckle 60 the adhesion of the leather fabric 50 in contact with the forearms is tightened or loosened until optimum fixing of the element protecting the elbow is obtained, further increasing the wearing comfort of the suit 10, since the rider is able to adapt the suit 10 to his own body build. It should be noted that, despite the fact that “mesh” material has been used for most of the surface of the suit 10, it is also possible to imagine using a stronger material, for example leather. In this case also, the advantages of better protection resulting from positioning of the shield 40 on top of the suit are retained. It is understood that minor variations to the idea proposed by the present invention are nevertheless included in the following claims. For example, parts of the suit other than those described may be made of breathable material. |
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047284897 | summary | FIELD OF THE INVENTION The present invention relates to nuclear reactors and more particularly to supporting spaced fuel elements in bundles or assemblies in the reactor by means of a welded fuel element support grid with integral flow directing vanes which direct fluid flow for increased heat transfer. BACKGROUND OF THE INVENTION Fuel assemblies for nuclear reactors are generally provided in the form of fuel element or rod arrays maintained by a structure which includes a plurality of welded spacer grids, a lower end fitting and an upper end fitting. Guide thimbles provide the structural integrity between the lower end fitting, the upper end fitting and the spacer grids intermediate the ends of the fuel assembly. The spacer grids define an array of fuel rods which, typically, may be rows and columns of 16 rods each. One such spacer and support grid is disclosed in U.S. Pat. No. 3,481,832. The typical fuel element support grid for supporting a spaced array of nuclear fuel elements or rods intermediate their ends includes a generally quadrangular or other polygonal perimeter. A plurality of fuel element compartments or cells within the perimeter are defined by first and second grid-forming members or strips welded to the perimeter and joined to each other at their lines of intersection. The grid-forming members of the fuel element support grid are slotted for part of their width along lines of intersection with the other grid-forming members of the array such that they may be assembled and interlocked at their lines of intersection in what is termed "egg-crate" fashion. This structure has been utilized because it provides a good strength-to-weight ratio without severely affecting the flow of cooling or moderating fluid through the grid of the nuclear reactor. The grid strips typically include projecting springs and arches for engaging and supporting the fuel elements within the compartments. Thus, at each fuel rod grid position in the fuel assembly, axial, lateral and rotational restraint is provided against fuel rod motion due to coolant flow, seismic disturbance or external impact. The spacer grids also act as lateral guides during insertion and withdrawal of the fuel assembly from the reactor. All of the elements of the fuel lattice, including the springs and the arches within the compartments, are arranged with respect to the fuel coolant flow in order to minimize pressure drop across the grid. In U.S. Pat. No. 3,764,470, a flow twister, mixing vane, or fluid flow directing vane was disclosed for redirecting the cooling fluid in the channels between the spaced parallel nuclear fuel elements. Those twisters were U-shaped metal sheets which straddled one grid member at an intersection with the free ends of the "U" folded on themselves to form two pairs of oppositely directed spirals and a pair of slots receiving the other grid member. The purpose of the twisters was to direct cooling fluid inwardly toward and spirally around the adjacent fuel rods. The desirability and theory of their use is described in the "Background of the Invention" of U.S. Pat. No. 3,764,470. The same background is applicable to the invention described herein. SUMMARY OF THE INVENTION Fluid flow directing vanes or "mixing vanes" provided according to the principals of the invention are integral to the strips and provide improved strength for the grid, improved damage resistance to the grid during fabrication and fuel reconstitution and improved hydraulic performance of the type previously provided by the separate "twisters" of U.S. Pat. No. 3,764,470. A major advantage of the fluid flow directing vanes being integral is that there is little chance of them becoming loose parts or debris within the flow stream circulating in the reactor in a manner which would damage the internals of the reactor. Moreover, the particular design of the integral fluid flow directing vanes of the instant invention provide the grid with increased strength over conventional grids with integral fluid flow directing vanes because the vanes themselves are "contained" and provide a strong means of attaching each of the orthogonal strips of an intersecting pair to each other. The advantages provided by the invention are accomplished in a spacer grid assembly of typical egg-crate assembly but with strips intersecting at at least three points for each of the intersections formed by two orthogonal strips. Individual strips of only two different types are required to produce the interior area of the grid but additional types to produce special fuel rod support features or special cells to accommodate guide thimbles or guide tubes can be made compatible with the two basic strips. The attachment welds at any given pair of intersecting strips in the region of the flow directing vanes can be, optionally, made at either one, two, or three locations in a manner that will be described hereinafter. A grid constructed according to the principles of the invention, with its novel integral flow directing vanes, provides superior performance over other designs of grids during fabrication, during seismic events and other off-normal conditions, during normal reactor operation, and during repair of the fuel assembly after a period of operation in the reactor (reconstitution). During fabrication, the strip shape is stamped, therefore eliminating any manual or other post assembly bending requirements to form and position the vanes. Accordingly, the fabrication is less expensive. Because of the particular shape of the integral flow directing vanes, they pass the fuel rod support springs and arches more readily during assembly than do bent mixing vanes of a conventional design. Moreover, because of the particular design of the grid and "contained" integral fluid flow directing vanes, there is easier access to the welds and less criticality in the least accessible or intermediate weld, if it is chosen to use one, in the area adjacent the integral attachment of the vanes to the strips. Since the vanes are integral and "contained" within the normal width of the strips without projecting beyond the strip edges, the flow directing vanes are less likely to be damaged during use and during fuel assembly fabrication than are the projecting types of integral vanes previously utilized. If desired, fluid flow directing vanes according to the invention can be provided on both the upstream and downstream side of the grid structure. The novel flow directing vanes' performance during seismic events or other off normal conditions provides resistance to lateral loading because they are "contained" and not projecting. The grid reinforcement is in part due to the fact that span lengths of the thin cross-sections are reduced at each corner by means of the shape of the fluid flow directing vanes. This is because each corner on one side of a fuel element cell is reinforced with a vane. Accordingly, a reduced strip cross-section will provide a resistance to lateral loading that is equivalent to that achieved with a conventional design. The benefits of the reduced cross-section can be utilized elsewhere. For example, the thinner cross-section will effect a lower pressure drop for a given strength or given resistance to damage from seismic events or it will permit the use of larger diameter fuel elements with no net effect on pressure drop. Alternately, a change could be made from a conventional grid design of a given material to the current invention with an inherently weaker, but more economical material, while maintaining the cross-section of the strips. The structural improvement afforded by the design would offset the inferior material properties. During normal operation, the fluid flow directing vanes provide a good mix from the open or corner areas of the fuel element cells to the tight areas. This mix affords better heat transfer and a better "thermal margin" for reasons discussed in U.S. Pat. No. 3,764,470. This is accomplished with an acceptable pressure drop because of the reduced crosssection of the strips for a given required grid strength. The "contained" integral flow directing vanes also reduce pressure drop from conventional grids by permitting smaller than conventional weld nugget sizes. Moreover, the weld geometry at the intersection of two orthogonal strips in a conventional grid structure provides greater flow restriction and undesirable turbulence than welds required at the vanes' intersections of the present invention. Also, without the abrupt flow control surface bends which integral flow control vanes have exhibited in the past, a lower pressure drop across the grid than would otherwise be the case is produced by the "contained" vanes of the invention. During fuel assembly reconstitution, individual fuel elements may be removed from and reinserted into the assembly. Individual mixing vanes which project beyond the strip edges, as in a conventional design, can become bent during the reinsertion process as the tip of the fuel element first approaches the grid. This bending can lead to blockage of further insertion, or to contact with the reinserted element or adjacent elements during subsequent operation. Such contact can initiate local wear and possibly breaching of the fuel element cladding tube. Also, if the bending of a conventional vane is severe enough, the vane could fracture and become debris within the circulating fluid of the nuclear reactor. Such debris is a common source of fuel element breaching in operating reactors. The "contained" vanes of the instant invention provide a geometry which is impervious to damage by fuel elements during reconstitution, thereby eliminating concerns of contact or debris during subsequent operation. |
abstract | This adjusting method can determine the precise deflection amount of mask deflectors corresponding to each aperture pattern by precisely measuring the position of the beam deflected by the mask deflectors in relation to that of each aperture pattern in an electron beam exposure apparatus, comprising an electron gun, a block mask, plural mask deflectors that deflect the electron beam so as to pass through one of the plural aperture patterns selectively, convergent devices that converge the electron beam onto a specimen, and deflectors that deflect the electron beam on the specimen. The electron beam exposure apparatus has the ability to expose the patterns corresponding to the selected aperture patterns, at one time, and in which the plural aperture patterns are square or rectangular and arranged in a matrix form, each aperture pattern has a square or rectangular maximum aperture area that limits the area in which each aperture is formed, the block mask has at least one adjusting aperture pattern equipped with independent apertures of the same shape arranged along the opposite sides of the maximum aperture area, and the mask deflectors are adjusted so that the intensity of the beam, which is radiated onto the specimen, at the portion of the independent apertures of the same shape arranged along the opposite sides of the adjusting aperture pattern, is uniform and maximum. |
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description | 1. Field of the Invention The invention is related to a novel structure for a Ga-68 radionuclide generator that uses two washing solutions to pass through two Ge-68 absorbents to wash out different chemical forms of Ga-68 nuclide. 2. Description of the Prior Art Because parent nuclide Ge-68 has long half-life, Ge-68/Ga-68 nuclide generator can be used for more than one year to steadily provide Ga-68 nuclide. In the past, Ge-68 is adsorbed on inorganic materials like silicon dioxide, aluminum oxide or tin oxide to produce Ga-68 nuclide generator. With these materials, there is inevitably inconvenience. For example, if a generator uses aluminum oxide as adsorption column, it will need EDTA (ethylediaminetetraacetic acid) solution to wash out daughter nuclide Ga-68. Since Ga-68-EDTA is a complex with very stable structure, it takes some complicated processes to convert Ga-68 into radiopharmaceuticals. Also because Ga-68 has short half-life, the conversion process will lose some materials. If a generator uses tin oxide as adsorption column, it will need 1N HCl solution or more concentrated to wash out daughter nuclide Ga-68. Thus, Ga-68 nuclide exists as chlorinated Ga and can be used after neutralization. However, the trace amount of inorganic salt can be dissolved and carried out by a strong acid(HC1 concentration >1N) and easily cause metal ion contamination. Its convenience lies in the further labeling for Ga-68 with ligand. The Ga-68 nuclide generators that are made from adsorption of Ge-68 in silicon dioxide, aluminum oxide or tin oxide have the following common features: (1) existing as a form of strong acid solution (HCl concentration>1N), (2) easily causing ion metal contamination, (3) if the chemical form of Ga-68-gallium citrate is needed, it is more time consuming and is not suitable for pharmaceutical production or clinical use because the exposure of operators to radiation is higher, (4) if Ga-68 nuclide exists as gallium chloride, it cannot be directly used for clinical purpose and needs further labeling for Ga-68 nuclide with ligand. The Ga-68 nuclide generator that is made from adsorption of Ge-68 in organic resins has the following features: (1) it needs diluted sodium citrate or sodium phosphate solution to wash Ga-68 nuclide, (2) it exists as Ga-68-gallium citrate, (3) it does not cause ion metal contamination, (4) if the chemical form of Ga-68-gallium chloride is needed, it is more time consuming and is not suitable for pharmaceutical production or clinical use because the exposure of operators to radiation is higher. Ga-68 nuclide has found increasing applications in PET radiopharmaceuticals. Single ion form of Ga-68 nuclide generator is not sufficient to meet the demand of diversified development of Ga-68 applications in radiopharmaceuticals. In review of existing literatures and patents, such as U.S. application Ser. No. 12/745,715, which from Fe (III) conducts purification with automatic apparatus to produce single chemical form of Ga-68 nuclide; or another U.S. publication 60/928,723, which uses washing solution and adsorption column for multiple purification operation to produce single chemical form of Ga-68 nuclide, it is found that there is no example like this invention that adopts a process to combine two different washing solutions and two different adsorption columns to obtain different chemical forms of Ga-68 nuclide, i.e. Ga-68-gallium citrate and Ga-68-gallium chloride. The objective of the invention is to propose a novel Ga-68 radionuclide generator structure, which uses two washing solutions to pass through two Ge-68 adsorbent to wash out different chemical forms of Ga-68 nuclide for Ge-68/Ga-68 nuclide generator. Since Ga-68 nuclide has found increasing applications in PET radiopharmaceuticals, single ion form of Ga-68 nuclide generator is not sufficient to meet the demand of diversified development of Ga-68 applications in radiopharmaceuticals. Besides, there is no similar example in existing literatures and patents to this invention that uses two washing solutions to pass two Ge-68 adsorbents to wash out different chemical forms of Ga-68 nuclide for novel Ge-68/Ga-68 nuclide generator. Ga-68 gallium citrate solution or Ga-68 gallium chloride solution can be chelated with chelating agent DotA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) as radioisotope solution. The invention, by using organic resins to absorb Ge-68 and using diluted sodium citrate or sodium phosphate to wash out Ga-68 nuclide, can minimize metal ion contamination. Although Ga-68 nuclide, when exists as gallium citrate, has narrower application range than Ga-68-gallium chloride, its advantage is less metal ion content, which total Ge-68 is approximately 0.0004%. In addition, Ga-68 nuclide is suitable for PET (Positron Emission Tomography) in radiopharmaceutical molecular labeling, such as application in tumor imaging and diagnostics via crosslink with ligand or peptide or protein and forming covalent bonding. Since Ga-68 is the daughter nuclide from the decay of Ge-68. Ge-68 has a half-life for as long as 271 days, maximum energy 511 KeV. It is produced via cyclotron irradiation to Ga-69. The nuclear reaction formula is 69Ga (p,2n)68Ge. Ga-68 has a half-life for 68.1 minutes, maximum energy β+=1.89 MeV. Since Ga-68 has a short half-life and decays in the form of positron, it is mainly used to prepare positron imaging agent for clinical radiopharmaceuticals and very suitable for hospitals or research institution without cyclotrons. The objective and feature for the invention lies in an operation process that combines two different washing solutions and two different absorbent columns to obtain different chemical forms Ga-68 nuclide, i.e. Ga-68 gallium citrate and Ga-68 gallium chloride. FIG. 1 is the block flow diagram for the operation process of the present invention Ga-68 generator, in which two washing solutions pass through two different Ge-68 absorbents to wash out different chemical forms of Ga-68 nuclide. FIG. 2 has detailed explanation. As shown in FIG. 2, the present invention structure includes: (1) first washing bottle 1, containing 0.1M sodium citrate stock; (2) second washing bottle 2, containing 0.1M hydrogen chloride stock; (3) organic resin absorbent column 3 (styrene-divinylbenzene copolymer column); (4) inorganic resin absorbent 4 (TiO2 column); (5) silica-gel cartridge 5; (6) sodium citrate stock 6. The two chemical forms of radioisotope solutions from the above method contain (7) radioisotope solution of Ga-68 gallium citrate solution 7 and (8) radioisotope solution of Ga-68 gallium chloride solution 8, which can be used for many different areas such as PET imaging and diagnostics. Ga-68 gallium citrate can be obtained as shown in FIG. 2. There are two methods. The first is to withdraw the sodium citrate solution from the first washing bottle 1 and allow it to pass through organic resin absorbent column 3, and then drain out radioisotope solution of Ga-68 gallium citrate solution 7; the second is to pass the radioisotope solution of Ga-68 gallium chloride solution 8 through sodium citrate solution 6 to mix, and thus obtain the radioisotope solution of Ga-68 gallium citrate 7′. Ga-68 gallium chloride can be obtained as in FIG. 2. There are two methods. The first is to withdraw the hydrochloric acid solution from the second washing bottle 2 and allow it to pass through inorganic resin absorbing column 4 to drain out the radioisotope solution of Ga-68 gallium chloride; the second is to pass the radioisotope solution of Ga-68 gallium citrate solution 7 to pass organic resin absorbing column 3 and then silica-gel cartridge 5, followed by washing with the hydrochloric acid solution in the second washing bottle 2 to obtain the radioisotope solution of Ga-68 gallium chloride 8′. Valve 51 and valve 52 in FIG. 2 are open or closed at the same time during operation. Valve 61 and valve 62 are also operatively open or closed at the same time during operation. |
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044141777 | abstract | An apparatus for detecting nominal phase conditions of coolant in a reactor vessel comprising one or more lengths of tubing each leading from a location being monitored to a closed outer end exterior of the vessel. Temperature is sensed at the open end of each length of tubing. Pressure within the tubing is also sensed. Both measurements are directed to an analyzer which compares the measured temperature to the known saturated temperature of the coolant at the measured pressure. In this manner, the nominal phase conditions of the coolant are constantly monitored. |
059995858 | abstract | A nuclear fuel based on UO.sub.2, ThO.sub.2 and/or PuO.sub.2 having improved retention properties for fission products. The fuel comprises a metal such as Cr or Mo able to trap oxygen in order to form an oxide having a free formation enthalpy equal to or below that of the superstoichiometric oxide or oxides (U, Th)O.sub.2+x and/or (U, Pu)O.sub.2+x (O<x.ltoreq.0.01). Thus, it is possible to trap oxygen atoms released during the fission of U, Th and/or Pu. This leads to an increase in the retention level of the fission products and a possibility of obtaining a high burn-up of nuclear fuel elements. |
claims | 1. A method comprising:providing a plurality of tristructural-isotropic fuel particles;mixing the plurality of tristructural-isotropic fuel particles with silicon carbide powder and at least two different rare earth oxide neutronic poisons to form a precursor mixture in which the silicon carbide powder separates at least one of the plurality of tristructural-isotropic fuel particles embedded in the silicon carbide powder from the other tristructural-isotropic fuel particles embedded in the silicon carbide powder; andcompacting the precursor mixture at a predetermined pressure and temperature to form a fuel element in which the silicon carbide powder becomes a silicon carbide matrix having a density substantially equal to the theoretical density of stoichiometric silicon carbide and having pockets of porosity of not more than 4%,wherein the pockets include the rare earth oxide neutronic poisons,wherein one of the rare earth oxide neutronic poisons is Eu2O3, andwherein the rare earth neutronic poisons are in an amount of up to 6 weight percent. 2. The method according to claim 1, wherein the rare earth oxide neutronic poisons include rare earth oxides having a large neutron capture cross-section and ability to suppress a sintering temperature of the silicon carbide powder below a critical damage temperature of the tristructural-isotropic fuel particles. 3. The method according to claim 1, wherein additional rare earth oxide neutronic poisons are selected from the group consisting of Gd2O3, Er2O3, and Dy2O3. 4. The method according to claim 1, further comprising: mixing additional sintering additives to the precursor mixture of the silicon carbide powder and the rare earth oxide neutronic poisons. 5. The method according to claim 4, wherein the additional sintering additives include alumina, yttria, or other rare earth oxides, or combinations thereof. 6. The method according to claim 1, wherein one or more of the rare earth oxide neutronic poisons are oxide sintering additives in the precursor mixture. 7. The method according to claim 1, wherein the precursor mixture consists essentially of the silicon carbide powder and the rare earth oxide neutronic poisons. 8. The method according to claim 1, wherein the precursor mixture includes the rare earth oxide neutronic poisons in an amount up to 10 weight percent of a total weight of the precursor mixture. 9. The method according to claim 1, wherein a combination of the rare earth oxide neutronic poisons and any additional sintering additives is in an amount up to 10 weight percent of a total weight of the precursor mixture. 10. The method according to claim 1, wherein the predetermined temperature is less than 1900° C. 11. A nuclear fuel comprising:a fuel element comprising a plurality of tristructural-isotropic fuel particles intermixed in a silicon carbide matrix,wherein the silicon carbide matrix separates a least one of the plurality of tristructural-isotropic fuel particles embedded in the silicon carbide matrix from the other tristructural-isotropic fuel particles embedded in the silicon carbide matrix,wherein the silicon carbide matrix has a density substantially equal to the theoretical density of stoichiometric silicon carbide and has pockets of porosity of not more than 4%,wherein the pockets include at least two different rare earth oxide neutronic poisons,wherein one of the rare earth rare earth oxide neutronic poisons is Eu2O3, andwherein the rare earth neutronic poisons are in an amount of up to 6 weight percent. 12. The nuclear fuel according to claim 11, wherein the pockets consist essentially of the rare earth oxide neutronic poisons. 13. The nuclear fuel according to claim 11, wherein the pockets consist essentially of the rare earth oxide neutronic poisons and sintering additives. 14. The nuclear fuel according to claim 11, wherein additional rare earth oxide neutronic poisons are selected from the group consisting of Gd2O3, Er2O3, and Dy2O3. 15. A nuclear fuel comprising:a fuel element comprising a plurality of tristructural-isotropic fuel particles intermixed in a silicon carbide matrix,wherein the silicon carbide matrix separates a least one of the plurality of tristructural-isotropic fuel particles embedded in the silicon carbide matrix from the other tristructural-isotropic fuel particles embedded in the silicon carbide matrix,wherein the silicon carbide matrix has a density substantially equal to the theoretical density of stoichiometric silicon carbide and has pockets of porosity of not more than 4%,wherein the pockets include rare earth oxide neutronic poisons, andwherein the rare earth oxide neutronic poisons include combinations of Gd2O3 and Er2O3 in a range of 1.57 to 2.07 total weight percent. 16. The nuclear fuel according to claim 15, wherein the pockets consist essentially of the rare earth oxide neutronic poisons. 17. The nuclear fuel according to claim 15, wherein the pockets consist essentially of the rare earth oxide neutronic poisons and sintering additives. 18. The nuclear fuel according to claim 15, wherein additional rare earth oxide neutronic poisons further include Dy2O3. |
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abstract | The method of improving nuclear reactor performance involves generating an operational solution for a nuclear reactor based on a constraint accounting for a problem with operation of the nuclear reactor. The generated operational solution can then be implemented at the nuclear reactor. |
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abstract | High-contrast, subtraction, x-ray images of an object are produced via scanned illumination by a laser-Compton x-ray source. The spectral-angle correlation of the laser-Compton scattering process and a specially designed aperture and/or detector are utilized to produce/record a narrow beam of x-rays whose spectral content consists of an on-axis region of high-energy x-rays surrounded by a region of slightly lower-energy x-rays. The end point energy of the laser-Compton source is set so that the high-energy x-ray region contains photons that are above the k-shell absorption edge (k-edge) of a specific contrast agent or specific material within the object to be imaged while the outer region consists of photons whose energy is below the k-edge of the same contrast agent or specific material. Scanning the illumination and of the object by this beam will simultaneously record and map the above edge and below k-edge absorption response of the object. |
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048045144 | summary | BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for neutron dosimetry and more particularly to a method and apparatus for neutron dosimetry in a nuclear power plant in applications where on-line measurement of neutron flux is necessary or desirable. 2. Background of the Invention The measurement of neutron exposure in selected regions within or outside of a nuclear reactor pressure vessel (RPV) is of interest for several reasons. Measurement of neutron exposure outside of the RPV can be used to obtain a determination of the plant operational power level. Measurement of neutron exposure outside of the RPV can also be used to determine the spatial power distribution within the RPV. Furthermore, measurement of neutron exposure within or outside of the RPV can provide dosimetry information with respect to fast neutron exposure, from which resulting vessel embrittlement may be inferred. The foregoing constitutes a broad range of applications, for which a variety of devices and techniques are currently employed. Detectors positioned outside of the reactor core (typically BF.sub.3 counters) are used to detect thermal neutrons for plant power/power distribution measurements. Passive activation samples are used for radiometric inference of vessel fluence from within vessel measurements. Passive samples containing both radiometric activation samples and solid state track recorders (SSTR) are used in outside of vessel measurements for inference of vessel fluence. Detectors positioned outside of the reactor pressure vessel are more accessible than those positioned within the RPV, but their removal can typically be effected only during a shutdown. While detectors so positioned are typically on-line devices, some are not. For example, passive counters must be transported to a laboratory in order to obtain data from them. Thus, experience has taught that exceptionally useful features in neutron dosimetry include the following: (1) for power level/power distribution purposes, sufficient sensitivity to enable meaningful inference of integrated neutron fluence over a period of minutes to hours; (2) for vessel dosimetry applications, a lifetime of years in a high radiation (neutron or gamma ray radiation) environment, with a relative insensitivity to gamma radiation; (3) also for vessel dosimetry applications, a capability of periodic on-line readout of integrated neutron dose (fluence), to provide data useful in forming decisions concerning the operation of the plant (e.g., heat up/cool down rates); and (4) capability of providing independent measurements of thermal and fast neutron flux. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a sensitive method and apparatus for neutron dosimetry on-line readout capability. It is a further object of the present invention to provide a neutron dosimeter having a long lifetime and capable of discriminating between thermal and fast neutron flux. Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. To achieve the foregoing and in accordance with the purposes of the invention, as embodied and broadly described herein, there is provided an apparatus for measuring intensity of neutron flux comprising means exposed to the neutron flux for generating fission fragments at a rate proportional to said intensity, means exposed to a known fraction of the fission fragments for generating light signals in response to the known fraction, means, optically coupled to the light signal generating means, for generating electrical signals in response to the light signals, and means, electrically coupled to the electrical signal generating means, for processing electrical signals to determine the intensity. In another aspect of the invention, multiple means for generating fission fragments having differing neutron energy thresholds for producing fission fragments are providing in order to obtain spectral data on neutron intensity. In yet another aspect of the invention, multiple means for generating light signals are positioned about the means for generating fission fragments, and the signals developed therein are checked for coincidence in order to discriminate events due to fission fragments from background events. Methods according to the invention are also disclosed and claimed. |
abstract | An apparatus is provided for one-dimensional magnetic scanning or switching of a charged particle beam. The apparatus can be extended to two dimensions at the cost of added complexity. |
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043494650 | summary | BACKGROUND OF THE INVENTION The present invention relates to a process for the treatment of combustible, solid radioactive wastes, especially wastes containing radionuclides emitting alpha radiation, in which the wastes are oxidized (that is, combusted in the wet state), at elevated temperatures, by a combination of concentrated (more than 16 moles/liter) sulfuric acid and concentrated nitric acid or NO.sub.x. Valuable radioactive materials may be recovered from the resultant solid residue. The treatment of combustible, solid radioactive wastes is based on the concept of converting such wastes into a noncombustible condition in a minimally hazardous way. A simultaneous, extensive reduction in volume is desirable to make the final, permanent disposal and/or storage of the thus-concentrated radioactive substances simpler and more economical. A normal combustion of, for example, cellulose-containing material, or rubber or synthetic resins in a furnace entails disadvantages in most cases in that part of the radioactive substances is entrained as suspended particles with the smoke, thus requiring special gas cleaning devices to free the evolving gases from the radioactive particles. Quite generally, a certain risk is incurred by the presence of suspended radioactive particles, inherent in practically any open combustion process. Additionally, with such a combustion method, the plutonium contained in the waste materials is converted to sparsely soluble plutonium oxides and plutonium mixed oxides. Attempts have been made to avoid these disadvantages of open combustion by conducting a chemical destruction of the aforementioned wastes. Using processes known as wet combustion, combustible wastes have been treated with strongly oxidizing acids, such as, for example, nitric acid, or the oxidative properties of concentrated sulfuric acid at elevated temperatures, for example in the proximity of the H.sub.2 SO.sub.4 boiling point, have been exploited. In this connection, oxidation catalysts have been used, such as selenium, as disclosed in German Pat. No. 1,295,724. The use of selenium is disadvantageous, however, since the element is toxic. In another prior art process, disclosed in U.S. Pat. No. 3,957,676, the solid wastes are allowed to react with concentrated sulfuric acid at a temperature in the range of from 503.degree. K. to 573.degree. K. (230.degree.-300.degree. C.), and simultaneously and/or thereafter, the waste materials subjected to the reaction are brought into contact with concentrated nitric acid or nitrogen oxides. These reactions cause oxidation of the wastes within the reaction liquid. The solid residue thus produced is separated from the liquid, and valuable radioactive materials are recovered from the residue. Thereafter, the residue, free of the valuable matter, is disposed of, and the H.sub.2 SO.sub.4 l and the HNO.sub.3 are recovered and recycled into the process. It is stated in U.S. Pat. No. 3,957,676 that in laboratory tests, volume reductions of up to a factor of 160 have been attained using this wet combustion process. A very small amount of acid is said to be consumed in the process, if the acid is processed after use, and reused. The oxidizing step in this process is conducted at or in the close proximity of the reflux temperature of sulfuric acid and should be within the temperature range of 503.degree. to 573.degree. K. For lower temperatures, the reaction rate is said to be lower, and although this offers a means to control the reaction, higher temperatures of about 543.degree. K. (270.degree. C.) are generally preferable for a complete reaction. The process can be conducted at or somewhat above atmospheric pressure, which is said to be an advantage in retaining the radioactive contaminants. Although a relatively high temperature (543.degree. K.) is utilized for the oxidizing step, the throughput of material to be combusted in this process per unit time is relatively small. Thus, approximately 81/2 hours are necessary for 100 g. of mixed waste material from the point of introduction of the waste material into the heated, concentrated sulfuric acid to complete oxidation of the waste material. It is a further disadvantage of this type of wet combustion process that large amounts of energy are required in the reaction of the waste with the concentrated sulfuric acid and in the oxidation with nitric acid. The conventional processes are further disadvantageous in that the reactions exert a relatively high stress on the materials of the reaction vessels, and there is an additional danger of lump formation and/or a partial conglomeration of waste pieces due to melting, factors which at least impede the normal course of the process. SUMMARY OF THE INVENTION It is an object of the present invention to provide a process for the treatment of combustible, solid radioactive wastes, especially wastes containing radionuclides emitting alpha radiation, which can be conducted continuously. It is also an object of the present invention to provide a process for the treatment of combustible, solid radioactive wastes which permits a complete oxidation of the waste materials. It is another object of the present invention to provide a process for the treatment of combustible, solid radioactive wastes which permits the attainment of a markedly higher throughput of waste materials than prior art processes, in a facility which is more compact than those of prior art processes. It is another object of the present invention to provide a process by which at least 20 kg per hour of solid, combustible radioactive waste may be treated. It is a further object of the present invention to provide a method for the treatment of combustible, solid radioactive wastes which is gentle to the materials of the apparatus used. Additional objects and advantages of the present invention will be set forth in part in the description which follows and in part will be obvious from the description or can be learned from practice of the invention. The objects and advantages are achieved by means of the processes, instrumentalities, and combinations particularly pointed out in the appended claims. To achieve these objects, and in view of its purpose, the present invention provides in a process for the treatment of solid, combustible radioactive wastes, wherein the wastes are contacted with sulfuric acid of a concentration of greater than 16 moles per liter and reacted with this sulfuric acid at an elevated temperature, and concentrated nitric acid or nitrogen oxides are added to the sulfuric acid, whereby oxidation of the wastes occurs below the surface of the sulfuric acid and gaseous by-products and a solid residue are formed, the improvement comprising subjecting the solid wastes, prior to their reaction with sulfuric acid, to mechanical processing which comprises (a) a preliminary comminution to waste pieces having a size less than or equal to 20 mm, and (b) a primary comminution by finely grinding the waste pieces produced in step (a) to a size of less than or equal to 1 mm at a temperature of less than about 123.degree. K. Preferably, the waste pieces from step (a) are made brittle in liquid nitrogen and then ground in a cold-grinding mill. It is also preferred to form a suspension from the ground material from step (b) and 90% strength sulfuric acid at less than 313.degree. K. By the fine comminution of the waste in step (b) and by forming a suspension with cold 90% sulfuric acid, the waste becomes pumpable and can be readily introduced into a reaction vessel with a rapid liquid circulation. If such a waste-H.sub.2 SO.sub.4 suspension is heated to the reaction temperature while being conveyed to the reaction vessel, the conversion rate is considerably higher than in the case of the prior art processes. In another preferred embodiment of the present invention, the reaction of the wastes with sulfuric acid is conducted at a vacuum in the range from 100 m bar to 500 m bar. Preferably, the reaction of the wastes with sulfuric acid is conducted at a temperature of at most 493.degree. K. It is to be understood that both the foregoing general description and the following detailed description are exemplary but are not restrictive of the invention. DETAILED DESCRIPTION OF THE INVENTION In the process of the present invention, solid, combustible, nuclear waste, especially that containing radionuclides emitting alpha radiation, is subjected to a two-step mechanical processing treatment, prior to a wet combustion treatment. Exemplary waste material is described in U.S. Pat. No. 3,957,676, incorporated in its entirety herein by reference, and includes both uranium and plutonium bearing wastes. It is contemplated that in the majority of cases, the waste material to be processed according to the present invention will be a mixed waste material comprising by weight, about 40% to 50% polyvinyl chloride, about 15% to 25% neoprene, about 10% to 20% cellulose, about 5% to 10% polyethylene, and about 5% to 10% polypropylene. Under actual operating conditions, it is expected that the waste material will also comprise noncombustibles such as metallic parts, glass, ceramic material, and the like. Since such components interfere with the controlled operation of the process, and can result in a reduction of the useful life of the blades used in the preliminary comminution stage of the mechanical treatment, it is absolutely necessary to inspect the waste as delivered, and possibly to pre-sort it. The noncombustible waste can then be diverted to waste-compaction and the combustible waste to the mechanical treatment of the present invention. The combustible waste which is directed to the two-step mechanical treatment is subjected in the first step to a preliminary comminution. In the preliminary comminution, the waste is reduced to pieces of a size less than or equal to about 20 mm. The preferred size range is from 5 to 20 mm. A slow running cutting mill or a shredder can be used as devices for carrying out the preliminary comminution. A nitrogen-containing waste gas is produced by the preliminary comminution and is purified by means of an exhaust gas line to such an extent that it corresponds in quality to the exhaust gas of the wet combustion process. The waste pieces are then subjected to the second step of the two-step mechanical treatment. This second step is a primary comminution of the waste pieces at a temperature of less than about 123.degree. K. (-150.degree. C.). The preferred temperature range is from 77.degree. K. (-196.degree. C.) up to 123.degree. K. In the primary comminution, the waste pieces are rendered brittle, for example, with the use of liquid nitrogen, and then ground in a cryogenic-grinding mill. In the primary comminution, a particle size of about 1 mm or less is achieved preferably a particle size in the range from 0.1 up to 1 mm. A screening means is generally a part of the grinding means. The exhaust gas from this cryogenic grinding is used to dilute the exhaust gas from the wet combustion. In a preferred embodiment of the present invention, a suspension then is formed from the extremely finely ground waste material produced by the primary comminution and concentrated sulfuric acid by introducing the waste material in metered amounts into concentrated sulfuric acid (greater than 16 M) by means such as a cell-wheel gate valve. By mixing with the waste, under agitation, fresh sulfuric acid of about 90% concentration or sulfuric acid recycled from the present process (and concentrated to about 90%) a homogeneous mixture is produced, in the form of a suspension, which is suitable for pumping and which can be readily introduced into a wet combustion reaction vessel having a rapid liquid circulation. In order to form this pumpable suspension, the mixing with the sulfuric acid takes place at a temperature maintained at less than about 313.degree. K. (40.degree. C.), preferably at a temperature in the range from room temperature to 313.degree. K. Advantageously, the mixture is now pumped and heated to close to the wet combustion reaction temperature directly into a transport line, so that the mixture is at the reaction temperature when it is delivered to the reaction vessel, where it is mixed with rapidly circulating reaction liquid. This preliminary heating insures a higher conversion rate than prior art processes in which the waste is added directly to hot H.sub.2 SO.sub.4. The HNO.sub.3 required for the oxidation is added to the circulating liquid. The HNO.sub.3 oxidizes carbonaceous material formed by the reaction of the wastes with the sulfuric oxide and is itself reduced principally to NO. In a preferred embodiment of the present invention, the decomposition of waste in the reactor takes place at a temperature from 453.degree. K. (180.degree. C.) to no more than about 493.degree. K. (220.degree. C.), and/or at a pressure of about 100 to 500 m bar. In a further preferred embodiment, the decomposition takes place at 493.degree. K. and a pressure of 300 m bar. The formation of a pumpable suspension as set forth above and the use of relatively low reaction and oxidation temperatures (of no more than about 493.degree. K.) ensure that disturbances during the course of the process caused by waste particles sticking together or by the conglomeration of molten waste particles can be avoided. A thermal syphon reactor well known as a thermal syphon evaporator of the ordinary state of the art, is preferably used for the reaction. For the radioactive criticality safe layout a modified jacketed annular gap reactor (in vertical position), with outside liquid circulation (thermal syphon) is used having a gap or layer thickness of about 5 cm, is preferably used for the reaction. The amount of heat required to maintain the reaction temperature is supplied by circulating through the jacket a thermal oil or concentrated H.sub.2 SO.sub.4. The reaction liquid, the gases generated, and steam leave the reactor at the top, and the vapors and gases are separated. The circulating (returning) reaction liquid then mixes first with freshly supplied waste suspension, and then with heated (353.degree. K.-393.degree. K.) concentrated (65-98 wt.%) niric acid, required for the oxidation. The nitric acid is introduced into the circulating liquid, which reenters the reaction vessel at the bottom. The nitric acid may be introduced into the circulating reaction liquid simultaneous with the start of the reaction or at a later time. In the reaction vessel, the combustible organic matter of the waste reacts with the sulfuric acid in reactions which may be greatly simplified by the following representations: EQU C.sub.m H.sub.n +(n)/(2)H.sub.2 SO.sub.4 .fwdarw.nH.sub.2 O+(n)/(2)SO.sub.2 +mC (1) EQU C+2H.sub.2 SO.sub.4 .fwdarw.2H.sub.2 O+2SO.sub.2 +CO.sub.2 (2) If nitric acid is present, reaction (2) tends to be suppressed in favor of the following reaction: EQU 3C+4HNO.sub.3 .fwdarw.4NO+2H.sub.2 O+3CO.sub.2 ( 3) Above the liquid in the reactor, a weak oxygen stream preferably is introduced. This stream oxidizes NO to NO.sub.2, which, in turn, oxidizes SO.sub.2 to SO.sub.3. It should be pointed out that as an alternative to adding HNO.sub.3, oxides of nitrogen, NO.sub.x, and especially NO.sub.2 may be added. HNO.sub.3, however, is preferred. As compared with dry-combustion process, an extensive breakdown of plutonium oxides is attained in the present wet-combustion process. The decomposition residues preferably are withdrawn as an approximately 5% (range: 2-20 wt.%) suspension in H.sub.2 SO.sub.4, cooled to less than 313.degree. K. (40.degree. C.), and separated by means of a pressure filter at a maximum pressure of 10 bar into a filter cake and H.sub.2 SO.sub.4 filtrate which may be recycled. The filter cake is then dried to remove sulfuric acid at about 743.degree. K. (470.degree. C.) at 200 m bar, and leached out with dilute The solution produced by this leaching contains Pu(SO.sub.4).sub.2, and is separated from the residue. The residue has become maximally free of plutonium and is passed on for waste compaction. The plutonium-containing solution can be rendered extensively free of sulfate by precipitation of sulfate with calcium and separation of the calcium sulfate precipitate, so that an extraction of plutonium with tributyl phosphate/kerosene is made possible. The purified CaSO.sub.4 also passes over to waste compaction. From the plutonium-containing solution, the uranium, likewise contained therein, and the plutonium are separated by extraction with a tributyl phosphate/kerosene mixture, and the re-extracted U/Pu solution is introduced, at a suitable point, into the extraction cycle of a reprocessing plant. After extraction of uranium and plutonium, the remaining aqueous waste solution is passed on for compaction. The exhaust gases from the reactor are, after cooling to about 423.degree. K. (150.degree. C.), freed of any entrained droplets by means of a hydrocyclone and by a wet electrostatic filter. During this step, oxygen is added as an oxidizing agent. Thereafter the exhaust gas is conducted countercurrently to the condensate and thus cooled to condensation temperature. Condensation takes place at about 341.degree. K. (68.degree. C.). The noncondensable gases are removed by suction with the use of a suitable vacuum-generating device, for example a water ring pump or a water jet pump, and transferred into a first absorption column. In the lower section of this absorption column, the major amount of the remaining nitrogen-containing gases is scrubbed out. Above this first absorption column, the exhaust gas is cooled to less than 283.degree. K. (10.degree. C.), and in a second absorption column, located thereabove, the exhaust gas is completely cleansed of nitrogen-containing gases by countercurrently conducted, dilute hydrogen peroxide solution having a temperature of less than 283.degree. K. (10.degree. C.). The absorber sump liquid and the condensate are degasified by heating to the boiling temperature and then introduced into an acid rectification stage, at a pressure of 100 to 300 m bar. In the acid rectification stage, the sulfuric acid is concentrated up to about 90% and then recycled into the process, and the vapors pass into the HNO.sub.3 rectification. In the HNO.sub.3 rectification, an approximately 68% strength HNO.sub.3 is obtained which is then recycled into the process. The head product of the HNO.sub.3 rectification is introduced in part as scrubbing liquor into the second exhaust gas absorption column and the remainder is discharged. Due to the use of the H.sub.2 O.sub.2 solution in the second absorption column, the nitrogen-containing gases are completely absorbed, as contrasted to the use of H.sub.2 O and air in the prior art processes wherein there has always been a considerable loss of NO.sub.x. The following example is given by way of illustration to further explain the principles of the invention. This example is merely illustrative and is not to be understood as limiting the scope and underlying principles of the invention in any way. All percentages referred to herein are by weight unless otherwise indicated. |
059237244 | abstract | In a medical X-ray diagnostic installation and a method for operating same, an X-ray source has a filter means optionally introducible into its beam path and emits X-rays which strike a detector having individual pixel elements and which generates image signals pixel-by-pixel dependent on the received radiation. The detector is followed by a signal processing chain which includes processing meansfor image generation and output, first and only image exposure of the examination subject initially ensues without the filter means introduced into the beam path, after which the image signals obtained from one or more picture element regions of the received image are processed pixel-by-pixel for generating a calculated image corresponding to the image of those regions which would be obtained with the filter means in the beam path, using signals which identify a position of the filter means in the beam path. The processed signals are combined with the unprocessed signals for the remainder of the original image to produce a calculated image, which is supplied as an output for display. The production of a second, actual exposure employing X-rays with the filter means in the beam path is thus avoided, thereby saving time and reducing the patient's overall X-ray dose. |
claims | 1. A particle beam therapy system that irradiates a charged particle beam, accelerated by an accelerator and scanned by a scanning electromagnet, onto an irradiation subject, the particle beam therapy system comprising:an irradiation management apparatus that controls the scanning electromagnet, based on target irradiation position coordinates of the charged particle beam; anda position monitor that measures measurement position coordinates of the charged particle beam, wherein the irradiation management apparatus has a command value creator that outputs a control input to the scanning electromagnet, for scanning the charged particle beam, based on the target irradiation position coordinates and correction data, said correction data having been created in a preliminary irradiation,wherein (i) said preliminary irradiation is performed prior to said irradiation of said subject and (ii) an excitation pattern of the scanning electromagnet of said preliminary irradiation is the same as an excitation pattern of the scanning electromagnet of a main irradiation plan for actual irradiation of the subject, andwherein the correction data is created on a basis of the target irradiation position coordinates and the measurement position coordinates measured by the position monitor in said preliminary irradiation. 2. The particle beam therapy system according to claim 1, wherein, the irradiation management apparatus includes (i) a correction data creator that creates the correction data in said preliminary irradiation and (ii) a scanning electromagnet command value creator that creates a basic control input from the measurement position coordinates, whereinthe command value creator outputs, as the control input, a corrected control input obtained by correcting the basic control input, created by the scanning electromagnet command value creator, with the correction data created by the correction data creator. 3. The particle beam therapy system according to claim 2, wherein the correction data is created based on a value obtained by dividing by a coefficient K the difference ΔBL between the value BL(me) of a BL product of the scanning electromagnet calculated from the measurement position coordinates measured in the preliminary irradiation and the value BL(ex) of a BL product of the scanning electromagnet calculated from the target irradiation position coordinates; andthe coefficient K is a gradient of a tangential line at a point, at which the value of the BL product is BL(ex), on the center line of the hysteresis loop configured with the BL product and the current of the scanning electromagnet. 4. The particle beam therapy system according to claim 3, wherein the irradiation management apparatus has a determination device that determines whether or not the position difference between the measurement position coordinates and the target irradiation position coordinates falls within a predetermined tolerance range; in the case where the position difference does not fall within the predetermined range, the irradiation management apparatus creates the correction data; and in the case where the position difference falls within the predetermined range, the irradiation management apparatus sets the control input to the scanning electromagnet to a value the same as a value in the preliminary irradiation. 5. The particle beam therapy system according to claim 2, wherein the irradiation management apparatus has a determination device that determines whether or not the position difference between the measurement position coordinates and the target irradiation position coordinates falls within a predetermined tolerance range; in the case where the position difference does not fall within the predetermined range, the irradiation management apparatus creates the correction data; and in the case where the position difference falls within the predetermined range, the irradiation management apparatus sets the control input to the scanning electromagnet to a value the same as a value in the preliminary irradiation. 6. The particle beam therapy system according to claim 1, wherein the correction data is created based on a value obtained by dividing by a coefficient K the difference ΔBL between the value BL(me) of a BL product of the scanning electromagnet calculated from the measurement position coordinates measured in the preliminary irradiation and the value BL(ex) of a BL product of the scanning electromagnet calculated from the target irradiation position coordinates; andthe coefficient K is a gradient of a tangential line at a point, at which the value of the BL product is BL(ex), on the center line of the hysteresis loop configured with the BL product and the current of the scanning electromagnet. 7. The particle beam therapy system according to claim 6, wherein the irradiation management apparatus has a determination device that determines whether or not the position difference between the measurement position coordinates and the target irradiation position coordinates falls within a predetermined tolerance range; in the case where the position difference does not fall within the predetermined range, the irradiation management apparatus creates the correction data; and in the case where the position difference falls within the predetermined range, the irradiation management apparatus sets the control input to the scanning electromagnet to a value the same as a value in the preliminary irradiation. 8. The particle beam therapy system according to claim 1, wherein the irradiation management apparatus has a determination device that determines whether or not the position difference between the measurement position coordinates and the target irradiation position coordinates falls within a predetermined tolerance range; in the case where the position difference does not fall within the predetermined range, the irradiation management apparatus creates the correction data; and in the case where the position difference falls within the predetermined range, the irradiation management apparatus sets the control input to the scanning electromagnet to a value the same as a value in the preliminary irradiation. 9. An irradiation method for irradiating a charged particle beam onto an irradiation subject, the method comprising:creating irradiation setting data, said setting data including at least target irradiation position coordinates and dosage amounts;outputting a control input to a scanning electromagnet performing a preliminary irradiation according to said setting data, wherein (i) said preliminary irradiation is performed prior to performing an actual irradiation on a subject and (ii) a scanning-electromagnet excitation pattern of the preliminary irradiation is the same as a scanning-electromagnet excitation pattern of actual irradiation;recording, in said preliminary irradiation, measurement data including at least measurement position coordinates;creating correction data based on the target irradiation position coordinates and the measurement position coordinates measured in said preliminary irradiation; andcorrecting the control input to the scanning electromagnet based on the correction data created in preliminary irradiation to create a corrected control input for actual irradiation on a subject. |
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048521418 | description | DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, and in particular to FIG. 1, there is shown a contemporary X-ray generator 10 having a shield 17 operative to shield a portion of the shielded radiation emitted from generator 10. This figure also illustrates the undesirable forms of radiation that are not eliminated with existing shielding devices, and which are sought to be substantially eliminated by the present invention. Such forms of radiation include (a) leakage, (b) primary, (c) scatter, and (d) transmitted. In general, the device illustrated at FIG. 2 includes an X-ray generator, a generally trapezoidal shaped shroud connected to the X-ray generator, a support fixture connecting the X-ray generator to the shroud and permitting relative movement therebetween, and a plurality of flaps secured to the shroud opposite the X-ray generator. It is to be understood that the teachings of the present invention may be used to construct a corresponding device or to modify contemporary X-ray devices for enhanced operation and convenience, as described below. The preferred embodiment of the present collimator and scatter shield is illustrated in FIG. 2, comprising an X-ray generator tank 13 and a shroud 12, which totally and detachably fits about and is secured to the X-ray generator tank 13. Generator tank 13 may be one of a variety of commercially available devices. As shown at FIGS. 2-5, the shroud 12, is carefully mounted under the cooling lines 33. Once engaged, the shroud 12, may be left on the generator 13, and stored therewith. The shroud 12, is preferably formed as a cylindrically shaped sheet of vinyl covered lead 0.32 cm. (1/8") thick. A cap 15 also preferably formed of vinyl covered lead is disposed at a bottom portion of the X-ray generator 13. The shroud 12, is installed by first attaching the shroud, to the generator tank 13, under the cooling lines 33. The electrical connector (not shown) is then removed from the back of the generator tank 13. Next, a rubber installation cap (not shown) is removed from the wire leads in back of the generator tank 13. The wire leads (not shown) are then disconnected from the lead screws (not shown). The lead disc 15, is then installed over the back of the generator tank 13. Finally, the electrical leads, connector and rubber installation cap are reconnected. The present apparatus 11, further comprises a tubehead shield 16, shown in more detail at FIG. 5, which covers the upper portion, i.e. first 10.16 cm (4"), of the generator tank 13. The tubehead shield 16, also covers the entire tubehead and port 34. The tubehead shield 16, is preferably made of aluminum lined, with lead, and is adapted to engage upper portion 25 of collimator cone 20, described in more detail below. The tubehead shield 16 is tightly affixed to the generator tank 13 by first removing two bolts (not shown) from the tubehead dome 38 and by replacing these bolts with two screws 36. The ends of the screws 36 are then secured to the tubehead by means of retainer nuts 41. The body and the ends of the screws project outwardly from the tubehead dome, and slide through two holes in the tubehead shield 16. Two winged nuts 39 are then fastened to the screws 36 to secure the tight connection between the tubehead 16, and the generator 13. With the tubehead shield 16 attached to the generator tank 13 the source of X-rays is generally shielded to limit the emitted X-rays from the generator 13. The collimator cone 20 serves to direct and shield X-rays eminating from the generator 13 towards the target. As described in more detail below the collimator cone 20 may be formed as a single piece, as shown at FIG. 3, or may be formed of a modular construction, as shown at FIG. 2, where in the upper cone support portion 25 and lower cone support 27 are joined by connecting panels that are detachably secured to upper and lower support portions 25 and 27. Where the cone is of a generally larger size modular construction is likely to be more suitable. Where the cone is smaller the integral construction is more likely to be suitable. The embodiment shown at FIGS. 2 and 5 illustrate modular construction, whereas the embodiment shown at FIG. 3 illustrates the unitary construction of the cone 20. As presently anticipated the unitary construction cone may be formed to be approximately 15.4 cm..times.15.4 cm. (6".times.6"). In one embodiment of the modular cone it is formed to be 35.56 cm. by 43.18 cm. (14".times.17"). It is to be understood, however, that various sizes of each type of cone construction may be implemented within the scope of the present invention. FIG. 5 illustrates the upper support portion 25 of cone 20 as it is connected to the tubehead 16. As shown therein portion 25 connects to the tubehead 16 via clamp 23 which, though shown as a clamp, may be substituted by any of a variety of different clamping means adapted to securely engage the portion 25 to the tubehead 16. It is to be understood that where the cone is of integral construction the means for connecting to the tubehead 16 may be identical as that shown in FIGS. 2 and 5, or may use equivalent means, as described above. In order to minimize the weight of the apparatus 11, the upper cone support portion 25 and lower cone support portion 27 may be formed of lightweight composite materials such as graphite, epoxy, or plastics. Four outer panels 26a, 26b, 26c and 26d are preferably formed of lead or lead covered materials and may be connected to upper support portion 25 and lower support portion 27 by means of fasteners 28, 29, or by any other conventional means. Holding fixture 21 extends between the generator tank 13 and the cone 20. The holding fixture 21 provides mechanical support for the apparatus 11 and may be formed to engage an adjustable tripod of conventional design. The tripod connection permits the apparatus 11 to be adjusted in orientation with respect to azimuth and elevation. The tripod further provides firm footing and support for the apparatus 11. The flaps 31 and 32 connect to the cone 20 and provide additional shielding to guard against radiation emitted from about the perimeter of cone 20. The flaps 31 and 32 are preferably formed of lead or lead coated material and may be disposed about and connected to cone 20 in any convenient fashion. Where cone 20 is formed as an integral unit the flaps 31 and 32 may be permanently secured to the cone 20. Where the cone is of a modular construction the flaps 31 and 32 may be independently secured to the outer surface of the panels forming the outer surface of cone 20. FIG. 3 illustrates the use of one embodiment of the present invention to expose a film to X-ray a portion of structure 33. As shown therein film 34 is held against the back of surface 33 and covered by means of backup lead shield 35. Thus, structure 33 may be X-rayed. The backbone panel 26, of a generally triangle shape is secured against the outer face of the upper cone support portion 25, by means of two panel bolts or panel fasteners 28, and to the base 22, of the holding fixture 21. Once the backbone panel 26a, is connected to the upper cone support portion 25, the holding fixture 21, is rotated to uppermost required elevation and azimuth. The lower support portion 27 is then secured against the inner side of the side wall panel 26a by the means of two conventional panel bolts 29. Another opposite panel 26b, which is substantially identical to the backbone panel 26a, (FIG. 4), is then secured against the outer face of the upper support portion 25, and the inner face of the lower support portion 27. It should be noted that the base 22, of the holding fixture 21, attaches to the backbone panel 26a. The remaining outer panels 26c and 26d are identicially dimensioned and generally shaped in a triangular or trapezoidal form. These panels 26c and 26d are attached to the outer faces of the upper support portion 25 and lower support portion 27. By means such as fastener 30 the outer panels 26c and 26d also partially overlap the outer faces of the backbone panel 26a and the opposite panel 26b. As shown in FIG. 4, the overlapping and interlocking of the four panels 26a, 26b, 26c, and 26d provides for a maximum containment of the radiation within the collimator cone 20. The backbone panel 26a and opposite panel 26b have identicial dimensions, and the other outer panel 26c and 26d have identicial dimensions larger than those of the backbone and opposite panels 26a and 26b. For this reason, lead flaps 31 and 32, are provided to interface between the collimator cone 20 and the object to be X-rayed, and to eliminate stray radiation. FIG. 2 further illustrates the interlocking and interposition of the flaps 31 and 32. The smaller flap 31, overlaps with and is stacked upon the larger flap 32, to provide additional precautionary shielding. While particular embodiment of the present invention have been disclosed, it is to be understood that various different modifications are possible and are contemplated in the true spirit and scope of the appended claims. There is no intention, therefore, of limitation to the exact abstract or disclosure herein presented. |
053389417 | abstract | Spherulitic cast iron container bodies for radiation shielding containers for irradiated fuel elements are provided with sealing coatings which prevent the body from acting as a galvanic element in a water basin during the filling of the container with the irradiated fuel elements. The coating of nickel, nickel based alloys or chromium/nickel austenitic alloys is applied by applying particles of a diameter less than the diameters of open pores of the body surfaces to these surfaces and then fusing the particles together and to the substrate with a laser beam, preferably in a back and forth motion. The pores are thus filled with the particle melt layer. |
claims | 1. A system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel, comprising an operating rod capable of extending into the nuclear reactor vessel, wherein the operating rod comprises:a suction pipe;a bendable rod section connected to an upper end of the suction pipe;an expandable rod section connected to an upper end of the bendable rod section;a suction opening disposed at a lower end of the suction pipe;an electric valve disposed at a connection of an upper end of the suction opening and the suction pipe;a filter mesh disposed in the suction pipe and above the electric valve;a suction pump disposed in the suction pipe and above the filter mesh;a touch switch disposed on the filter mesh, wherein the touch switch is in operative connection with the electric valve and wherein a foreign matter impact force to the filter mesh triggers the touch switch to close which causes the electric valve to close which prevents escape of foreign matter from the system; anda drainage pipe; wherein a water inlet of the suction pump is connected to the suction opening, a water outlet of the suction pump is connected to an outside space of the suction pipe though the drainage pipe, and the electric valve is controlled by the touch switch. 2. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 1, wherein the electric valve comprises a valve with mesh structure. 3. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 1, further comprising a control switch disposed on an upper end of the expandable rod section. 4. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 1, further comprising an operating handle disposed on the upper end of the expandable rod section. 5. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 1, wherein the drainage pipe comprises an outlet disposed upward, preventing the foreign matters flowing with water. 6. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 1, further comprising an alarm disposed on the upper end of the suction pipe, wherein the alarm is connected to and controlled by the touch switch, when the touch switch is triggered and then closed, the alarm will inform the operator to check whether there are foreign matters sucked. 7. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 2, further comprising an alarm disposed on the upper end of the suction pipe, wherein the alarm is connected to and controlled by the touch switch, when the touch switch is triggered and then closed, the alarm will inform the operator to check whether there are foreign matters sucked. 8. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 3, further comprising an alarm disposed on the upper end of the suction pipe, wherein the alarm is connected to and controlled by the touch switch, when the touch switch is triggered and then closed, the alarm will inform the operator to check whether there are foreign matters sucked. 9. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 4, further comprising an alarm disposed on the upper end of the suction pipe, wherein the alarm is connected to and controlled by the touch switch, when the touch switch is triggered and then closed, the alarm will inform the operator to check whether there are foreign matters sucked. 10. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 5, further comprising an alarm disposed on the upper end of the suction pipe, wherein the alarm is connected to and controlled by the touch switch, when the touch switch is triggered and then closed, the alarm will inform the operator to check whether there are foreign matters sucked. 11. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 6, wherein the expandable rod section is telescopic sleeve structure. 12. The system with function of bending and elongation for discharging foreign matters from nuclear reactor vessel according to claim 7, wherein the bendable rod section is coiled tube structure. |
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046702130 | abstract | An improved top nozzle has an enclosure with a lower adapter plate and an upstanding sidewall surrounding and attached to the periphery thereof and an upper hold-down plate spaced above the adapter plate for abutmnet with a lower side of an upper core plate by a plurality of coil springs disposed between the lower and upper plates. The lower adapted plate and upper hold-down plate have respective openings and passageways defined therethrough in a pattern which matches that of the upper ends of the guide thimbles of the fuel assembly to which the improved top nozzle is attached. The upper ends of the guide thimbles extend upwardly through the adapter plate which is stationarily mounted on the guide thimbles. Tubular sleeves disposed between the lower and upper plates are surrounded by respective coil springs. The lower ends of the sleeves are releasably threaded to the upper ends of the guide thimbles, while the upper ends of the sleeves extend into the passageways of the upper hold-down plate. Bosses are disposed above the hold-down plate and connected thereto such that respective bores through the bosses are aligned with passageways of the hold-down plate. The bosses extend into holes in the upper core plate for aligning the fuel assembly with the upper core plate and for receiving the upper ends of the guide thimbles as the fuel assembly vertically moves upwardly relative to the upper core plate. A plurality of lugs on the periphery of the upper hold-down plate are coupled to the sidewall of the enclosure for accommodating vertical movement of the fuel assembly relative to the upper core plate. |
description | This application is a continuation of application Ser. No. 11/205,086, filed on Aug. 17, 2005, now U.S. Pat. No. 7,189,982, which in turn claims the benefit of Japanese Patent Application No. 2004-263201, filed on Sep. 10, 2004, the disclosures of which Applications are incorporated by reference herein. The present invention relates to a focused ion beam (FIB) apparatus using a liquid metal ion source to perform, for example, cutting of a specimen and more particularly, to an FIB apparatus provided with an aperture having a less adverse influence upon the liquid metal ion source and a structure of the aperture used for the FIB apparatus. In the FIB apparatus, a liquid metal ion source is highly bright and reduced in source size and is therefore used in general as the ion source. In order to use the liquid metal ion source stably, a liquid metal of the ion source must be kept to be clean and for this end, formation of a gallium oxide film, surface contamination due to, for example, a sputter re-deposited film and intrusion of impurities must be suppressed to a minimum. Conventionally, with a view to attaining the object as above and protecting an aperture per se by taking advantage of the fluidity of liquid metal, a method of covering the aperture with a liquid metal has been proposed in, for example, Japanese Patent No. 3190395 (Patent Document 1) or JP-A-5-159730 (Patent Document 2). More particularly, a liquid metal used as an ion source material is held by coating it on the surface of an aperture or by permeating it into a sintered body so that contamination of a liquid metal ion source due to sputter re-deposition of a material of the aperture and damage of the aperture per se may be prevented. Especially, in an aperture disposed immediately beneath an ion source, a material constituting the aperture greatly affects the liquid metal ion source by its re-deposition on the source. Therefore, as described in JP-A-2001-160369 (Patent Document 3), an aperture having a dish-like vessel in which a liquid metal is carried or pooled is used to prevent instability attributable to re-deposition. In the technologies described in the Patent Documents 1 and 2, however, a liquid metal is in essence impregnated in a porous material and there is a possibility that during use, the porous material is sputtered and the ion source is contaminated, giving rise to a fear that the ion source becomes unstable. In the technology described in the Patent Document 3, the liquid metal is carried on the dish-like vessel and as compared to the techniques described in the Patent Documents 1 and 2, the amount of an ion beam impinging upon a portion other than the liquid metal can be decreased. But, in a hole of the dish-like vessel through which the ion beam passes, the ion beam is irradiated on a material other than the liquid metal and there is a possibility that the material sputtered from the hole portion will contaminate the liquid metal ion source. Further, since a portion of dish-like vessel at which the liquid metal is pooled is flat, the liquid metal coheres in an island pattern on the bottom of the dish-like vessel as the quantity of liquid metal decreases and there is also a possibility that the liquid metal is lost around the hole of disk-like vessel through which the ion beam passes. The present invention contemplates solving of the above problems and it is an object of this invention is to provide an aperture of a focused ion beam apparatus which can permit an ion source to operate stably for a long time and a focused ion beam apparatus using the aperture. According to the present invention, in a focused ion beam apparatus using a liquid metal ion source, an aperture for limiting the ion beam diameter comprises a vessel formed with a recess having, at its surface lowermost point, an aperture hole through which the ion beam passes and a liquid metal carried on the recess, which liquid metal is used for the liquid metal ion source. What is meant by the recess having an aperture hole at its surface lowermost point signifies that an aperture hole 2 is positioned at the lowest point of a recess 1 holding a liquid metal to cause the liquid metal to concentrate to the vicinity of the aperture hole by its weight, thereby making it possible to form a cover which does not expose till an edge portion of the hole. Exemplified as this type of recess is either a recess whose peripheral portion is tapered or a recess having its whole bottom slanted to enable the aperture hole to be positioned at the lowest point. According to this invention, a focused ion beam apparatus can be provided which can enable the ion source to operate stably. The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention. The inventor of the present invention has found the following problems encountered in the prior arts. (1) Stability is degraded under the influence of sputtering of a base material arising from the inner wall of an aperture hole not covered with liquid metal. (2) During use, the covering capability of liquid metal is deteriorated to cause an exposed portion at which the base material is sputtered and desired stability cannot be obtained. (3) An oxide film is formed on the surface of liquid metal on the aperture and fluidity necessary for the liquid metal to move in accordance with consumption of the liquid metal by sputtering under irradiation of ions is degraded. (4) The liquid metal is consumed by sputtering through use for a predetermined time to thereby expose the base material which in turn is sputtered to create contaminants. (5) Liquid metal sputtered from the aperture coated with the liquid metal is deposited and condensed on, for example, an overlying electrode so as to be turned into liquid droplets and there result dielectric breakdown and discharge which adversely affect stable operation of an optical system. To solve problematic point (1), taper-working 4 is applied to the downstream side of aperture hole 2 uncovered with liquid metal as shown in FIG. 1 to minimize the inner wall area of a beam-interactive aperture. Further, the aperture hole 2 is positioned at a lowermost point of the recess 1 holding the liquid metal to permit the liquid metal to be concentrated to the vicinity of the aperture hole by its weight, thereby making it possible to form a cover unexposed till the edge portion of the hole. To solve problematic point (2), wettability of the aperture surface for the liquid metal is improved. The wettability can be attained by forming fine irregularities 5 as shown in FIG. 2 through etching of the base material surface or work technique such as mechanical polishing and cleaning a resulting structure through, for example, high temperature heating. Alternatively, equivalent effect can be accomplished through dry treatment such as radical irradiation. To solve problematic point (3), the size of an area covered with the liquid metal is limited to such an extent that an oxide film formed on the overall surface of the covered area can be removed under irradiation of an ion beam. As shown in FIG. 3, by establishing not only a normal beam focus state 7 but also a maximally broadened focus state 8 on the aperture underlying a lens 6 and forming a liquid metal covered portion which is narrower than the state 8, the overall covered area can be irradiated and the oxide film on the surface can be cleaned out through sputtering. To practically solve problematic point (4), a mechanism is provided which refluxes sputtered liquid metal onto the aperture by utilizing fluidity of the liquid metal to thereby prolong time for the base material to expose and hence lifetime of the aperture. As shown in FIG. 4, a cover (shield plate) 10 made of a metal plate is disposed in close proximity to the upper surface of the aperture and a trough structure (guide) 12 for guidance to an aperture liquid metal pool is attached to the bottom of the cover 10. To solve problematic point (5), a shield plate is interposed between the aperture and an overlying electrode so that liquid metal can be prevented from being re-deposited on the overlying electrode. The aperture hole of the aperture according to the invention is not a circular hole having a uniform diameter like a mechanically worked hole but takes the form of such a tapered surface that a portion other than the circumference defining the aperture diameter, especially, an aperture conical hole portion on the downstream, by which ions pass, grows downstream diametrically. In other words, a structure is employed in which the area of an aperture hole inner wall surface liable to be sputtered by ions passing through the hole is minimized. This contrivance is made with the aim of making the ion source less contaminated by particles sputtered from the inner base material and returning to the upstream side to thereby permit the ion source to be used even for a long time without being decreased in its stability. Even when a base material having high wetting capability for a liquid metal is used, the aperture hole inner surface cannot be kept to be placed constantly in wet condition, that is, cannot be kept to be covered constantly with the liquid metal and the fact that generation of sputtered particles from the inner surface and re-deposition of them to the liquid metal ion source give rise to a fundamental cause of degraded stability of the liquid metal ion source is empirically proven through many trial manufactures and experiments, leading to the aforementioned contrivance. Without the tapered structure, the operational stability of the ion source is degraded expressly, with the result that the number of operations of flashing required during the use for a predetermined time increases to reduce the effective use time and the lifetime of the ion source. When the aperture is used by keeping the wetting capability of aperture surface lowered, a portion at which the wetting capability is low is exposed as the quantity of liquid metal decreases and the base material is sputtered at the exposed portion, so that the ion source is deposited with sputtered particles and is rendered unstable. In order to improve the wetting capability, it is effective to form fine irregularities of about 10 μm on the surface through wet etching, for instance. Adversely, irregularities formed through rough polishing result in a tendency toward exposure of raised or convex portions, giving rise to a reverse effect for the sake of maintenance of stability. In addition, when the surface is cleaned by heating it at high temperatures in vacuum to remove an oxide film from the surface, necessary wetting capability can be attained. A dry treatment such as radical irradiation is also effective. An ion beam optical system is used in vacuum but moisture and oxygen exist in the form of residual gases and an oxide film is formed on the liquid metal surface covering the aperture. Normally, in a range irradiated with a beam, the surface is sputtered and no oxide film remains but in the unirradiated periphery, an oxide film is formed which considerably reduces the fluidity of the liquid metal and the quantity of liquid metal usable for covering the aperture surface is substantially reduced. Ultimately, the surface oxide film is sputtered toward the upstream, causing re-deposition and degraded stability. Accordingly, in a method effectively adapted for long-term effective use of the aperture covered with liquid metal, the overall liquid surface or level is irradiated at intervals of suitable time to remove the oxide film. To this end, the radius of the portion covered with the liquid metal needs to be smaller than a maximum irradiation radius on the aperture. Accordingly, in the present invention, for the purpose of cleaning the overall surface through sputtering at intervals of suitable time, the radius of the covered portion is set to be smaller than the radius of maximum irradiation. Through this, even in a long-term use, the liquid metal surface on the aperture can be kept to be cleaned and a long lifetime can be attained. Even in the liquid metal, sputtering by an ion beam takes place as in the case of a normal solid, so that even in the aperture covered with liquid metal, there is such a life that the liquid metal withers and unstableness of ion source operation results. It has been known that though depending on accelerating energy at the time of irradiation, several atoms are normally sputtered on the aperture under irradiation of one ion and liquid metal is more consumed on the aperture than in the liquid metal source. To exchange the aperture or supplement the liquid metal in the ion beam optical system, the vacuum vessel must once be opened to the atmosphere and then again evacuated to vacuum. Therefore, it is of course preferable that once carried, an aperture can be used for as long a period as possible, that is, comparably to the liquid metal ion source. For this end, a method is conceivable according to which a great quantity of liquid metal is coated on the aperture or the liquid metal pool is made to be large but the method is found to be practically unsatisfactory because the arrangement of the liquid metal changes owing to vibration, shock and temperature change and such inconvenience as blocking of hole and dropping is caused to increase danger. Preferably, the quantity of liquid metal to be held is suppressed to a maximum of 100 mg or less and for the sake of long-term use, a structure may precedently be established which can catch sputtered liquid metal above the aperture and reflux again to the aperture by taking advantage of the fluidity of liquid metal. This can prolong the lifetime considerably. With a similar structure, the liquid metal can be prevented from being deposited on an unwanted portion, especially onto parts such as a lens applied with an intensive electric field and a high-voltage insulator whose reliability is deteriorated by surface contamination. In the following, the present invention will be described specifically by way of embodiments 1 and 2. Referring to FIG. 5, there is illustrated embodiment 1 of the invention in which an aperture according to teachings of the present invention is so used as to be disposed immediately beneath a gallium liquid metal ion source. As a base material, tungsten is used having a thickness of 1 mm, a diameter of 10 mm and an aperture hole diameter of 0.5 mm. A recess formed in the base has a diameter of 7 mm and a depth of 0.7 mm. The distance between an emitter and the aperture is about 7 mm and where an emission current of the gallium liquid metal ion source is 10 μA or more, the emission half-angle is about 22° and an ion beam can be irradiated on the whole of the recess. The taper is formed at 45° and the inner wall of aperture hole has a length of 200 μm or less in the thickness direction. The surface of the tungsten base is chemically etched in the atmosphere so as to be formed with fine irregularities. Thereafter, a resulting structure is heated at about 1000° C. in vacuum and after an oxide film has been removed, gallium is dropped onto the surface to form the aperture. If gallium is merely coated in the atmosphere, the wetting capability is bad and the gallium is turned into liquid droplets, so that a uniform liquid level cannot be formed. The quantity of carried gallium is 45 mg in the present embodiment. With the aperture constructed as above, emission of the ion source is examined to obtain experimental results as graphically illustrated in FIG. 6. Emission kept to be stable for a long time can be accomplished with the emission current changing at a rate of 0.5% per hour. By applying a flashing process at the time that the current value changes, the emission current can be restored to substantially the same value and stable ion emission can continue. The consumptive amount of the coated gallium per hour is known as being about (1.5E−2 mg)/h by measuring a change in weight of the aperture. Accordingly, the quantity of gallium mounted on the surface recess can assure a use of 2000 h or more. This usable time is substantially equal to the lifetime of the gallium liquid metal ion source put on the market. In embodiment 2 as shown in FIG. 7, the present invention is applied to a variable aperture. In the present embodiment, molybdenum is used as a base material of the aperture. A recess having a width of 1 mm, a length of 6 mm and a depth of 0.5 mm is formed above individual aperture holes of the variable aperture through mechanical working. The inner surface of the recess is worked through etching. A resulting structure is heated at high temperatures and gallium is mounted in the recess. In the optical system, the beam diameter is about 0.5 mm on the aperture during normal operation and the irradiation range is limited to the gallium liquid surface or level. Before being mounted to the apparatus, the aperture is conditioned as shown in FIG. 8. As will be seen from the figure, a shield plate 20 is mounted which is adapted for keeping sputtered gallium from being deposited on an aligner and the like on the upstream side and causing dielectric breakdown. With this construction, an aperture lifetime of more than 2000 hours can be attained. It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims. |
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description | This application claims priority to and the benefit of Korean Patent Application No. 10-2007-0140959, filed in the Korean Intellectual Property Office on Dec. 28, 2007, the entire contents of which are incorporated herein by reference. 1. Field of the Invention The present invention relates to a laser patterning apparatus. More particularly, the present invention relates to a laser patterning apparatus for handling a donor film and improving compression uniformity and transfer characteristics between the donor film and an acceptor substrate. 2. Description of the Related Art A laser induced thermal imaging (LITI) method is a transfer method using laser light, and includes a donor substrate and an acceptor substrate. Laser light is absorbed into a light to heat conversion (LTHC) layer of the donor substrate to be converted to heat energy, and a transfer layer of the donor substrate is transferred to the acceptor substrate by the heat energy. Because the transfer layer is transferred to the acceptor substrate, a desired image is formed in the acceptor substrate. In more detail, the LITI method is performed as follows. After the acceptor substrate is positioned on a stage, the donor substrate (or donor film) is laminated on the acceptor substrate. One surface of the donor film is covered with a transfer layer that is made of a material for transferring. When laminating, bubbles between the donor film and the acceptor substrate are removed using a roller. The laminated donor film is adsorbed and fixed by a vacuum groove of a chuck. By providing a shielding mask on the donor film and radiating laser light to the shielding mask, in a portion in the donor film that receives the laser light, light is converted to heat energy to transfer the transfer layer on the donor film to the acceptor substrate, and in a portion that does not receive laser light, the transfer layer is maintained on the donor film. After a transfer operation is completed, by removing the laminated donor film and shielding mask, the transfer layer that is transferred on the acceptor substrate forms an image of the same (or substantially the same) pattern as that of the shielding mask. The donor film should be aligned and compressed on the acceptor substrate, but because the donor film has flexibility, the donor film should not be handled when being maintained in a flat plate shape. Further, bubbles remaining between the donor film and the acceptor substrate are removed by being temporarily compressed to the acceptor substrate with the roller. Therefore, during a transfer process of radiating laser light, the donor film does not continuously receive a compression force acting on the acceptor substrate. As such, compression uniformity between the donor film and the acceptor substrate is lowered and transfer characteristics therebetween are deteriorated. The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. An aspect of an embodiment of the present invention is directed toward a laser patterning apparatus for handling a donor film, and improving compression uniformity and transfer characteristics between the donor film and an acceptor substrate. An exemplary embodiment of the present invention provides a laser patterning apparatus including: a stage that supports an acceptor substrate; a shielding mask that is placed on the acceptor substrate to form a pattern and attached to a donor film on one surface thereof; a laser gun that is disposed at an upper part of the stage to radiate laser light to a portion of the donor film through the pattern of the shielding mask; a pressing member that corresponds to a portion of the shielding mask; and an actuator that is connected to one side of the pressing member to press the pressing member. The shielding mask may be formed with a glass substrate. The shielding mask may include a transfer area corresponding to the donor film and a non-transfer area corresponding to an outer side of the donor film at an outer side of the transfer area, wherein the pressing member may be formed with a rim corresponding to at least a part of the non-transfer area. In one embodiment, with respect to a plane of the stage, an interval is formed between the end of an inner side of the pressing member and the end of an outer side of the transfer area. In one embodiment, with respect to a plane of the stage, the actuator corresponds to the center between the end of an outer side of the transfer area and the end of an outer side of the shielding mask. The actuator may be formed with a plurality of cylinders and be symmetrically disposed with respect to the center of the donor film. The pressing member may be formed with a quadrangular rim, and the actuator may be formed with four cylinders that are respectively disposed at four corners (or corner portions) of the quadrangular rim. The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which certain exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. The drawings and description are to be regarded as illustrative in nature and not restrictive. Also, in the context of the present application, when an element is referred to as being “on” another element, it can be directly on the another element or be indirectly on the another element with one or more intervening elements interposed therebetween. Like reference numerals designate like elements throughout the specification. FIG. 1 is a perspective schematic view of a laser patterning apparatus according to an exemplary embodiment of the present invention, and FIG. 2 is a top plan view of the laser patterning apparatus of FIG. 1. Referring to FIGS. 1 and 2, the laser patterning apparatus includes a stage 10, a shielding mask 20, a laser gun 30, a pressing member 40, and an actuator 50. The stage 10 is formed in a flat shape and provides a place for performing a laser induced thermal imaging process. The stage 10 primarily supports an acceptor substrate 61, and it also supports a donor film 62 and the shielding mask 20 (that are placed on the acceptor substrate 61) through the acceptor substrate 61. The acceptor substrate 61 may be made of thin film transistor (TFT) glass to form one substrate of a display device. The donor film 62 has flexibility and includes an LTHC layer 621 and a transfer layer 622 (see FIG. 4). The LTHC layer 621 generates heat energy though the radiation of laser light to transfer the transfer layer 622 that is made of a light emitting material to the acceptor substrate 61. The shielding mask 20 forms a pattern for transferring to the acceptor substrate 61 and selectively passes laser light to the donor film 62 through the pattern. The shielding mask 20 includes a shielding portion 21 for shielding laser light and a passing portion 22 for passing the laser light in order to form the pattern (see FIG. 4). The shielding mask 20 attaches the donor film 62 to one surface thereof and may be made of glass, for example, for passing laser light through to the passing portion 22. The donor film 62 has flexibility, and when the donor film 62 is placed on the acceptor substrate 61 to be absorbed by the acceptor substrate 61, the donor film 62, in terms of handling, should maintain a flat state corresponding to the acceptor substrate 61. As the donor film 62 is attached to the shielding mask 20, the donor film 62 can be integrally handled with the shielding mask 20. That is, as the donor film 62 maintains a flat state as in (or with) the shielding mask 20, the donor film 62 can be easily handled even though it has flexibility. The donor film 62 is attached to the shielding mask 20 through the LTHC layer 621 for forming one surface thereof, and the transfer layer 622 is provided in the other surface thereof positioned toward the acceptor substrate 61. The shielding mask 20 includes a transfer area A1 corresponding to (or overlapping) the donor film 62, and a non-transfer area A2 corresponding to (or overlapping) the outer side of the donor film 62 at the outer side of the transfer area A1 (see FIG. 3). The laser gun 30 is disposed at the upper part of the stage 10 to radiate laser light to the shielding mask 20. Further, the laser gun 30 is movably provided on an x-y plane of the stage 10 to radiate laser light to at least a partial area or an entire area of the transfer area A1 of the shielding mask 20. Referring again to FIGS. 1 and 2, the laser patterning apparatus according to the present exemplary embodiment includes an x-axis moving member 71 that is disposed in a pair at both sides in a y-axis direction of the stage 10 to move the laser gun 30 in an x-axis direction, and a y-axis moving member 72 that is provided in the y-axis direction with the x-axis moving member 71. The laser gun 30 is mounted to the y-axis moving member 72 to move in the x-axis direction by an operation of the x-axis moving member 71, and moves in the y-axis direction on the y-axis moving member 72. Therefore, the laser gun 30 can radiate laser light to the entire area of the stage 10. FIG. 3 is a schematic view illustrating an operation of compressing a shielding mask and a donor film to an acceptor substrate. Referring to FIG. 3, the pressing member 40 is formed to partially correspond to the shielding mask 20 in order to press the shielding mask 20 without interrupting laser light radiation of the laser gun 30. Because the pressing member 40 should not interrupt the laser gun 30 from radiating laser light while compressing the donor film 62 to the acceptor substrate 61 through the shielding mask 20, the pressing member 40 is formed with a rim to correspond to at least a part of the non-transfer area A2 of the shielding mask 20. When reflected on a plane of the stage 10, an interval C is formed between the end of an inner side of the pressing member 40 and the end of the outer side of the transfer area A1. Due to the interval C, the pressing member 40 does not interrupt the laser gun 30 from radiating laser light to the transfer area A1. The pressing member 40 is provided to ascend to and descend from the shielding mask 20, and may press the shielding mask 20 with its own weight when it is formed with a heavy body. The laser patterning apparatus according to the present exemplary embodiment may further include an actuator 50 for pressing by ascending to and descending from the pressing member 40. The actuator 50 is connected to one side of the pressing member 40 at the outside of a laser light radiation range of the laser gun 30 on an x-y plane. The actuator 50 may be suitably formed, and in the present exemplary embodiment, the actuator 50 is formed as a cylinder. According to an operation of the actuator 50, the pressing member 40 presses the shielding mask 20 and the donor film 62 in a state in which it is placed on the acceptor substrate 61 while ascending to and descending from the shielding mask 20. An extension operation of the actuator 50 compresses the donor film 62 to the acceptor substrate 61 through the pressing member 40 and the shielding mask 20. While the actuator 50 maintains an extension operating state, a compressing state of the donor film 62 and the acceptor substrate 61 is tightly (or substantially) maintained. Therefore, during a transfer process, compression uniformity between the donor film 62 and the acceptor substrate 61 is maintained, thereby improving transfer characteristics. When reflected on a plane of the stage 10, the actuators 50 that are formed with a plurality of cylinders are symmetrically disposed about (or with respect to) the center of the donor film 62 (see FIG. 2). For example, when the pressing member 40 is formed with a quadrangular rim, the actuator 50 that is formed with four cylinders is disposed at each of four corners (or corner portions) of the quadrangular rim. Therefore, a pressing force of the actuators 50 forms a uniform distribution load on the pressing member 40. The uniform distribution load of the pressing member 40 forms a uniform distribution load over the entire donor film 62 through the shielding mask 20. Because the shielding mask 20 is made of glass, the shielding mask 20 does not have substantial flexibility, as compared with the donor film 62. Further, the actuator 50 is disposed to correspond to (or on) the center L/2 of a distance L between the end of the outer side of the transfer area A1 and the end of the outer side of the shielding mask 20. Because the actuator 50 is disposed at the center of the non-transfer area A2, a concentrated load can be prevented from acting on the end part of the shielding mask 20. FIG. 4 is a cross-sectional schematic view illustrating a state before a transfer layer of a donor film is transferred, and FIG. 5 is a cross-sectional schematic view illustrating a state after a transfer layer of a donor film is transferred. Referring to FIGS. 4 and 5, in a state where the donor film 62 is attached to the shielding mask 20, the shielding mask 20 is placed on the acceptor substrate 61, and laser light is radiated to the shielding mask 20. Laser light is radiated to the LTHC layer 621 of the donor film 62 through the passing portion 22 of the shielding mask 20 to be converted to heat energy. The transfer layer 622 of the donor film 62 is transferred to the acceptor substrate 61 by the heat energy. After being transferred, when the shielding mask 20 is removed, a remaining portion 622b, excluding a portion 622a that is partially transferred from the transfer layer 622 of the donor film 62, is removed together with the LTHC layer 621 and the shielding mask 20. Therefore, the acceptor substrate 61 forms one substrate of a display device including the transferred portion 622a. As such, in view of the foregoing, during a series of transfer processes, the actuator 50 uniformly presses the pressing member 40 while maintaining an extended state. Therefore, compression uniformity between the donor film 62 and the acceptor substrate 61 is maintained and transfer characteristics are improved. While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof. |
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