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06178219& | abstract | A holder for fuel elements in a reactor, in particular in a boiling water reactor, is provided, as well as a method for repairing such a holder. A core grid is demounted from a core shroud and replaced by a new forged core grid for the purpose of repairing the holder. An adapter ring is to be inserted between the new core grid and the core shroud. There is provision for connecting the adapter ring to the core grid without a welding operation, for example by shrink fitting. |
045444994 | claims | 1. A process for separating and immobilizing radioactive anions from a liquid containing same comprising contacting said liquid with a porous silica glass or gel comprising at least 82 mol percent silica having interconnected pores and non-radioactive cationic polyvalent metals bonded to silicon of the glass or gel through divalent oxygen linkages on the internal surfaces of said pores and non-radioactive anions ionically bonded to said cationic polyvalent metals and displaceable by said radioactive anions, to provide a distribution of radioactive anions internally bonded within the pores to silicon of the glass or gel through a cationic polyvalent metal and a divalent oxygen linkage as represented by the formula: ##STR15## Wherein M.sup.+ are said cationic polyvalent metals. 2. Process as claimed in claim 1 wherein a porous silica glass is used and said non-radioactive cationic polyvalent metal is selected from the group consisting of --Zr.sup.3+, --Pb.sup.+, --Th.sup.3+ and --Ti.sup.3+. 3. Process as claimed in claim 1 wherein the porous silica glass is represented by the formula: ##STR16## in which M is a polyvalent metal and the two unfilled valences of M are bonded ionically to additional OH.sup.- anions, are bonded through divalent oxygen linkage to another silicon of the glass and/or are bonded through divalent oxygen linkage to other M atoms. 4. Process as claimed in claim 1 wherein porous silica glass containing integral boron is used and some of said polyvalent metals are bonded through oxy linkages to boron of said porous glass. 5. The process as claimed in claim 1 wherein said porous silica glass or gel contains at least 1 ppb radioactive material encapsulated and immobilized in said porous silica glass or gel. 6. The process as claimed in claim 1 wherein said porous silica glass or gel contains at least 10 ppb radioactive material encapsulated and immobilized in said porous silica glass or gel. 7. The process as claimed in claim 1 wherein said radioactive anions are chromium, technetium and/or molybdenum anions. 8. The process as claimed in claim 1 wherein said porous silica glass or gel is characterized by a radiation activity of at least one microcurie per cubic centimeter. 9. The process as claimed in claim 1 wherein said radioactive anions are derived from radioactive nuclear waste. 10. The process as claimed in claim 1 wherein said radioactive anions are iodine. |
claims | 1. A blade device for use in forming a hollow cone-like radiation, the blade device comprising:a pair of first blade members opposed to each other symmetrically at a fixed inclination angle in a first direction perpendicular to an axis of the cone;a pair of second blade members opposed to each other symmetrically at a variable inclination angle in a second direction perpendicular to the axis of the cone, the second direction perpendicular to the first direction;a pair of lever members each fixed respectively at a first end thereof to a respective face of each the pair of second blade members on the side opposite to a mutually confronting side; anda lever actuating unit configured to pivot the pair of lever members about respective support shafts, wherein the lever actuating unit comprises:a pair of shaft members supported axially movably and unrotatably, the pair of shaft members having outer peripheries formed with thread grooves respectively and being engaged at front end portions thereof with a second end of each of the pair of lever members respectively;a pair of wheels supported axially unmovably and rotatably, the pair of wheels having inner peripheries formed with thread grooves respectively and being threadedly engaged with the pair of shafts respectively;a common entraining member engaged with the pair of wheels; anda driving wheel configured to drive the entraining member. 2. A blade device according to claim 1, wherein the lever actuating unit further comprises a pair of springs configured to urge the second end of each of the pair of lever members toward the pair of shaft members respectively. 3. A blade device according to claim 1, wherein the lever actuating unit further comprises a tension imparting device configured to impart tension to the entraining member. 4. A blade device according to claim 3, wherein the tension imparting device comprises:an idler wheel engaged with the entraining member; anda spring configured to urge an axis of the idler wheel in a direction to expand a loop of the entraining member. 5. A blade device according to claim 4, wherein the pair of wheels, the driving wheel, and the idler wheel each comprises a toothed wheel, and the entraining member is a toothed belt. 6. A blade device for use in forming a hollow cone-like radiation, the blade device comprising:a pair of first blade members opposed to each other symmetrically at a fixed inclination angle in a first direction peapendicular to an axis of the cone;a pair of second blade members opposed to each other symmetrically at a variable inclination angle in a second direction perpendicular to the axis of the cone, the second direction perpendicular to the first direction;a pair of lever members each fixed respectively at a first end thereof to a respective face of each the pair of second blade members on the side opposite to a mutually confronting side; anda lever actuating unit configured to pivot the pair of lever members about respective support shafts, wherein the lever actuating unit comprises:a ring configured to rotate coaxially with the axis of the cone; anda pair of cam members formed on an end face of the ring so as to engage a second end of each of the pair of lever members. 7. A blade device according to claim 6, wherein the lever actuating unit further comprises a pair of springs configured to urge the second end of each of the pair of lever members toward the cam members respectively. 8. A blade device according to claim 7, wherein the pair of springs comprises a pair of leaf springs. 9. An X-ray imaging apparatus comprising:an X-ray tube;a collimator configured to adjust an irradiation field;an X-ray receiver; anda blade device configured to form a hollow cone-like radiation for excluding off-focal irradiation, the blade device comprising:a pair of first blade members opposed to each other symmetrically at a fixed inclination angle in a first direction perpendicular to an axis of the cone;a pair of second blade members opposed to each other symmetrically at a variable inclination angle in a second direction perpendicular to the axis of the cone, the second direction perpendicular to the first direction;a pair of lever members each fixed respectively at a first end thereof to a respective face of each the pair of second blade members on the side opposite to a mutually confronting side; anda lever actuating unit configured to pivot the pair of lever members about respective support shafts, wherein the lever actuating unit comprises:a pair of shaft members supported axially movably and unrotatably, the pair of shaft members having outer peripheries formed with thread grooves respectively and being engaged at front end portions thereof with, opposite end portions of the pair of lever members respectivelya pair of wheels supported axially unmovably and rotatably, the pair of wheels having inner peripheries formed with thread grooves respectively and being threadedly engaged with the pair of shafts respectively;a common entraining member engaged with the pair of wheels; anda driving wheel configured to drive the entraing member. 10. An X-ray imaging apparatus according to claim 9, wherein the lever actuating unit further comprises a pair of springs configured to urge a second end of each of the pair of lever members toward the pair of shaft members respectively. 11. An X-ray imaging apparatus according to claim 9, wherein the lever actuating unit further comprises a tension imparting device configured to impart tension to the entraining member. 12. A blade device according to claim 11, wherein the tension imparting device comprises:an idler wheel engaged with the entraining member; anda spring configured to urge an axis of the idler wheel in a direction to expand a loop of the entraining member. 13. An X-ray imaging apparatus according to claim 12, wherein the pair of wheels, the driving wheel, and the idler wheel each comprises a toothed wheel, and the entraining member is a toothed belt. 14. An X-ray imaging apparatus comprising:an X-ray tube;a collimator configured to adjust an irradiation field;an X-ray receiver; anda blade device configured to form a hollow cone-like radiation for excluding off-focal irradiation, the blade device comprising:a pair of first blade members opposed to each other symmetrically at a fixed inclination angle in a first direction perpendicular to an axis of the cone;a pair of second blade members opposed to each other symmetrically at a variable inclination angle in a second direction perpendicular to the axis of the cone, the second direction perpendicular to the first direction;a pair of lever members each fixed respectively at a first end thereof to a respective face of each the pair of second blade members on the side opposite to a mutually confronting side; anda lever actuating unit configured to pivot the pair of lever members about respective support shafts, wherein the lever actuating unit comprises:as ring configured to rotate coaxially with the axis of the cone; anda pair of cam members formed on an end face of the ring so as to engage a second end of each of the pair of lever members. 15. An X-ray imaging apparatus according to claim 14, wherein the lever actuating unit further comprises a pair of springs configured to urge the second end of each of the pair of lever members toward the cam members respectively. 16. An X-ray imaging apparatus according to claim 15, wherein the pair of springs comprises a pair of leaf springs. 17. A method of assembling a blade device for use in forming a hollow cone-like radiation, the method comprising:positioning a pair of first blade members opposed to each other symmetrically at a fixed inclination angle in a first direction perpendicular to an axis of the cone;positioning a pair of second blade members opposed to each other symmetrically at a variable inclination angle in a second direction perpendicular to the axis of the cone, the second direction perpendicular to the first direction;fixedly coupling each of a pair of lever members at a respective first end thereof to a respective face of each of the pair of second blade members on the side opposite to a mutually confronting side;coupling a lever actuating unit to the pair of lever members, the lever actuating unit configured to pivot the pair of lever members about respective support shafts;coupling a pair of shaft members to the lever members, the pair of shaft members supported axially movably and unrotatably and having outer peripheries formed with thread grooves respectively and being engaged at front end portions thereof with a second end of each of the pair of lever members respectively;coupling a pair of wheels to the pair of shafts, the pair of wheels supported axially unmovably and rotatably and having inner peripheries formed with thread grooves respectively and being threadedly engaged with the pair of shafts respectively;coupling a common entraining member to the pair of wheels; andcoupling a driving heel to the entraining member. |
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051805456 | description | DETAILED DESCRIPTION FIG. 1 shows the lower part of a nuclear fuel assembly with a lower end nozzle 1. This end nozzle comprises an adaptor plate 2, through which water-passage holes pass, and supporting feet 3, which come to rest on the lower core plate 4 of the reactor when the fuel assembly is in operation. As can be seen in FIG. 2, the lower end nozzle 1 is closed by means of a frame 5 joining the feet 3. The fuel assembly has guide tubes, such as the guide tube 8, having a lower part attached to the adaptor plate 2 of the end nozzle 1 and an upper part attached to the upper end nozzle of the assembly (not shown). The framework of the assembly comprises, in addition to the end nozzles and guide tubes 8, spacer grids, such as 9, which are intended to ensure that the rods of the assembly are held secure transversely and axially and in which the guide tubes 8 are engaged and attached. The assembly can also have guide tubes, such as 8', which are attached only in the upper end nozzle and in the spacer grids 9 and which are not connected to the lower end nozzle. In contrast to the guide tubes 8 ensuring both the guidance of the absorbent rods of the control clusters and the mechanical stability of the framework of the assembly, the guide tubes 8' serve only for guiding absorbent rods of the control cluster. An aperture 18 passes through the adaptor plate 2' in line with each of the tubes 8', the lower end of which is freely engaged in the upper part of the plate 2'. According to the invention, a filter plate 6 pierced with holes is fastened against the adaptor plate 2 of the lower end nozzle 1, substantially over its entire surface. FIG. 2 shows the holes passing through the filter plate 6 in a zone 7, but in actual fact these holes pass through the plate over virtually its entire surface. To avoid having to dismount the filter plate 6, it is necessary to ensure that the zones of connection of the guide tubes 8 to the lower end nozzle remain accessible. Moreover, installed in the central part of the adaptor plate of the lower end nozzle in some types of fuel assembly is a device making it possible to protect the instrumentation glove finger from the hydraulic stresses. Apertures can therefore be provided in the filter plate in all the zones where this proves necessary. There is, for example, a circular zone of a diameter of approximately 20 mm in the center of the plate, in respect of an assembly having a device for protecting the instrumentation glove finger. If apertures of a particular dimension are made in the filter plate laid against the adaptor plate, it is necessary to modify the arrangement and dimension of the water passages in the corresponding zone of the adaptor plate. The conventional water passages can be replaced by holes of smaller diameter, ensuring the retention of the debris, instead of the filter plate, according to a known technique. At all events, the holes passing through the filter plate have dimensions assiduously determined according to the maximum size of the debris which can be allowed to pass through the fuel assembly together with the coolant flow. The active part of the filter plate having the water-passage holes occupies virtually the entire surface of the filter plate, with the exception of the zones of connection between the guide tubes and the end nozzle and the central zone of the assemblies which has a system for protecting the instrumentation glove finger. The connection between the filter plate and the adaptor plate can be made permanently or removably. Thus, it is possible to make this connection by welding (spot welding or bead welding), soldering or riveting. If appropriate, a removable connection can be used between the guide tube and the lower end nozzle in order to ensure the fastening of the filter plate. When the coolant flow comes under the lower end nozzle of the fuel assembly as it passes through the lower core plate 4 in the region of through-passages, particles of a dimension above a particular limit are retained by the filter plate under the lower end nozzle of the assembly. This debris is therefore not liable to be introduced into the assembly and to be jammed between the fuel rods and the cells of the lowermost spacer grid. The apertures passing through the filter plate can be of any form and be made by drilling or cutting processes of various types. The zones of the plate separating the cutouts can be narrow, since the filter plate is adhesively bonded to the adaptor plate of the end nozzle and therefore does not undergo any appreciable mechanical stress under the effect of the passage of the coolant flow at high speed through the lower end nozzle of the assembly. The passage apertures through the filter plate can likewise be made by stamping, but in this case the stamped parts must coincide perfectly with the water passages of the adaptor plate, since they have parts pushed back outside the plane of the filter plate and forming deflectors for the circulating cooling water. FIG. 3 illustrates a lower end nozzle 10 of a fuel assembly, into which a filter plate 11 is introduced, utilizing the elasticity of this plate to introduce it between the feet 12 of the assembly. The plate is first bent so as to pass between the inner ends of the feet 12, is then introduced into the lower part of the end nozzle 10 and is finally restored to a plane arrangement 11' underneath the adaptor plate 2'. The filter plate 11 can be fastened by any means, such as welding, soldering, riveting or snapping or even by elastic wedging between the feet 12, in contact with the bottom face of the adaptor plate of the end nozzle 10. FIG. 3A (or FIG. 4) shows the method for holding the plate 11 (or 16), in each of the corners of an end nozzle, in recesses 20 made in the feet 12 (or 15) of the end nozzle. The plate 11 (or 16) has cutouts which are introduced into the grooves 20. Such cutouts are similar to the cutouts 21 of the filter plate shown in FIG. 6. FIG. 4 shows a lower end nozzle 13 of a fuel assembly, having a frame 14 surrounding an adaptor plate and supporting feet 15 in each of the corners of the frame 14. A filter plate 16 in two parts 16a, 16b connected along a median line 17 of the adaptor plate is introduced into the end nozzle and fastened against the adaptor plate, for example by welding, soldering or snapping. FIG. 4A shows a detail of the positioning of the two-part filter plate 16 shown in FIG. 4. The edge of the plate 16 is introduced into a groove 24 delimited by a shoulder 25 which protects the plate against being torn out during the handling of the assemblies. FIG. 5 is a bottom view of a part, adjacent to a corner, of the lower end nozzle of a fuel assembly having guide tubes, only part of which forms the framework of the assembly. The reference 27 denotes the location of such a guide tube (similar to the tube 8 of FIGS. 1 and 3). This guide tube has, at its end, a plug (not shown) with an internally threaded hole. A hollow screw 28 is screwed into the plug. The screw 28 has a collar 29 which is deformed as a result of expansion in recesses of the adaptor plate of the end nozzle. This ensures that the screw 28 for fastening the guide tube is locked against rotation. The screw 28, accessible via an aperture in the filter plate 30, contributes to holding and fastening the plate 30 under the adaptor plate of the end nozzle. The locations 31 situated vertically in line with the guide tubes similar to the tubes 8' of FIGS. 1 and 3 not connected to the lower end nozzle of the assembly, serve for the fastening of the filter plate 30, as can be seen in FIG. 7. At each of the locations 31, the adaptor plate 2" is pierced with an aperture 33 in the lower part of which is engaged and locked by deformation a rivet 32 ensuring that the filter plate 30 is held against the bottom face 34 of the adaptor plate 2". The lower end of the guide tubes 8' comprising a plug is freely engaged in the upper part of the aperture 33. The rivet 32 is fixed in an extension having a reduced diameter of the upper part of the aperture in which the plug of the guide tube is received. FIG. 6 illustrates a filter plate 30 which can be fastened in the way just described under the adaptor plate of the lower end nozzle of an assembly which has a set of guide tubes connected to the lower end nozzle and ensuring the rigidity of the framework of the assembly, and a set of guide tubes not connected to the lower end nozzle and serving solely for guiding absorbent rods. The filter plate 30 has passing through it a central aperture 35 for the passage of the instrumentation tube of the assembly, sixteen clover-shaped apertures 36 for the passage of the guide tubes connected to the lower end nozzle, and eight circular apertures 37 allowing the passage of the rivets 32 for attaching the filter plate 30 to the adaptor plate of the end nozzle. Furthermore, the plate 30 has passing through it small, square apertures 40 which are disposed in square array, in positions corresponding to the positions of the water-passage apertures 41 passing through the adaptor plate (FIG. 5). The square apertures 40 are of such a dimension that the filter plate 30 can retain the particles liable to be jammed between the fuel rods and the cells of the first grid of the assembly. This arrangement of the apertures in square array can be especially advantageous for some arrangements of the apertures of the adaptor plate, for example when these apertures are disposed in square array. The plate 30 is fastened against the adaptor plate both by means of the rivets and guide tubes passing through the apertures 36 and 37 and by means of the feet of the assembly which have recesses, in which cutouts 21 of the filter plate 30 are engaged. FIGS. 8, 8A and 9 show part of a filter plate, the active zone of which is produced in the form of a grating comprising parallel lamellae 45, the spacing of which corresponds to the minimum size of the particles to be retained in the region of the filter plate 46. The lamellae 45 are placed next to one another within circular apertures which are made in the plate 46 and the dimension of which corresponds to the dimension of the water-passage holes of the lower core plate. These lamellae 45 can be welded at their ends to the edges of the aperture of the filter plate. To limit the head loss of the coolant flow during the passage through the filter plate, the lamellae 45 are profiled, as can be seen in FIG. 8A. This profiled form adapted to the flow of fluid in the direction perpendicular to the filter plate allows an appreciable reduction in head loss. The lamellae 45 of the active parts of the filter plate 46 are fastened at their ends only and are all parallel to one another. They are liable to vibrate under the effect of the hydraulic stresses of the coolant flow passing through the active zones of the filter plate 46. To prevent these vibrations, it is possible to place stiffeners 47 in directions perpendicular to those of the lamellae 45, as can be seen in FIG. 9. The stiffeners 47 can be welded or otherwise fastened to the lamellae and to the edges of the aperture passing through the filter plate 46. At all events, the filter plate according to the invention ensures effective retention of those particles contained in the coolant flow of the reactor whose size exceeds a predetermined limit. This filter plate introduces only a moderate head loss into the circuit of the coolant flow, inasmuch as the apertures of the filter plate are in the extension of the passage apertures of the lower core plate. If these apertures are made by the stamping of a plate, the deflector elements of the plate are pointed in the direction of flow, thus making it possible to reduce the head loss. According to one alternative embodiment, the parallel lamellae equip the entire cross-section of the filter plate 46. In the production of lower end nozzles of fuel assemblies according to the invention, to produce the filter plate it is possible to use any material resistant to the coolant flowing in the reactor and to the mechanical stresses exerted as a result of the passage of the fluid. Preferably, this adaptor plate can be produced from a nickel-based alloy with structural hardening, or from a martensitic steel. At all events, during operation the filter plate is bonded against the adaptor plate and covers virtually the entire surface of this plate. The advantage of the lower end nozzle according to the invention is that it makes it possible to ensure effective filtration of the coolant flow of the reactor, in such a way that particles transported by this fluid whose size exceeds a particular limit are retained under the lower end nozzle. The particle retainer used, consisting of a filter plate bonded under the adaptor plate of the lower end nozzle, does not increase the head loss of the coolant flow passing through the lower end nozzle to any appreciable extent, and the filter plate is held perfectly against the adaptor plate, with the result that it is capable of withstanding the forces generated by the circulating coolant fluid, without being of large thickness, even when it has solid parts of only small thickness separating the filter holes passing through it. The general overall size of the lower end nozzle is not increased, and this end nozzle can remain dismountable, inasmuch as there are in the filter plate apertures for access to the end for the fastening of the guide tubes to the adaptor plate. The invention is not limited to the embodiments described. Thus, it is possible to fastening the filter plate under the adaptor plate than those described. For example, the mounting of the edge of the filter plate in a groove delimited by a shoulder, described in respect of a filter plate in two parts, can be used in a general way for a filter plate produced in any form. As regards an assembly having two sets of guide tubes, one of which ensures the rigidity of the framework and the other of which serves only for guiding the absorbent rods, the filter plate may be fastened by means other than rivets engaged in apertures of the adaptor plate vertically in line with the tubes serving only for guidance, although this method of fastening is especially suitable. |
summary | ||
claims | 1. An X-ray distribution adjusting filter apparatus for adjusting a distribution of penetration intensity of X-rays emitted from an X-ray source and expanding in a predetermined shape outward from center axis of the X-rays, said X-ray distribution adjusting filter apparatus comprising:a curved face having a predetermined curvature along said center axis;an X-ray absorbing portion formed of an X-ray absorbing material, wherein said distribution of the penetration intensity of X-rays is adjusted by varying a shape of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus;a fixed section including a base portion uniform in thickness along said center axis, and inclined portions linked to or formed integrally with the base portion, symmetrically formed about said center axis and on both sides of said center axis, and each having an inclined face with a predetermined inclination relative to a flat face of said base portion;first and second movable sections formed on both sides of said center axis, each configured to pass said center axis and to be tiltable on a plane orthogonal to said center axis, pivoting on a center point, which is the position where one-side ends of said inclined faces of said fixed section are coupled, and having a flat face positioned on the side opposite to said inclined faces of said fixed section and a curved face opposite to the flat face; andfirst and second deformable sections having opposite ends each opposite to said coupling position of each of said inclined faces of said fixed section, and expansible means disposed between the ends of said flat faces of said first and second movable sections, opposite to the opposite ends, and expanding or contracting according to the pivoting of said first and second movable sections, in which cavities defined by said inclined faces of said fixed section, said flat faces of said movable sections and said expansible means are filled with fluid to keep the insides of said cavities in a filled state, wherein said fixed section and said movable sections are formed of an X-ray absorbing material to constitute said X-ray absorbing portion. 2. The X-ray distribution adjusting filter apparatus according to claim 1, wherein said inclined faces of said fixed section and said flat faces of said movable sections are caused to approach or move away from each other by the tilting of said first and second movable sections pivoting on said center point to vary the quantities of said fluid in the cavities of said movable sections, and to vary the sectional shape of said X-ray absorbing portion of the X-ray distribution adjustment filter apparatus. 3. An X-ray distribution adjusting filter apparatus for adjusting a distribution of penetration intensity of X-rays emitted from an X-ray source and expanding in a predetermined shape outward from center axis of the X-rays, said X-ray distribution adjusting filter apparatus comprising:a curved face having a predetermined curvature along said center axis;an X-ray absorbing portion formed of an X-ray absorbing material, wherein said distribution of the penetration intensity of X-rays is adjusted by varying a shape of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus,a basic X-ray distribution adjusting filter portion symmetrically shaped about said center axis and having a curved inner wall; anda removable X-ray distribution adjusting filter portion symmetrically shaped about said center axis and having a first curved outer wall whose shape is identical with the shape of said curved inner wall of said basic X-ray distribution adjusting filter portion and a first curved inner wall on a face opposite to the first curved outer wall, capable of being inserted to or discharged from an inside of said basic X-ray distribution adjusting filter portion, with said first curved outer wall being run along said curved inner wall of said basic X-ray distribution adjusting filter portion, wherein:said basic X-ray distribution adjusting filter portion and said removable X-ray distribution adjusting filter portion are formed of a material that can absorb X-rays, andthe insertion or removal of said removable X-ray distribution adjusting filter portion into or from said basic X-ray distribution adjusting filter portion causes the sectional shape of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus to vary. 4. The X-ray distribution adjusting filter apparatus according to claim 3, further including:a second removable X-ray distribution adjusting filter portion symmetrically shaped about said center axis and having a second curved outer wall whose shape is identical with the shape of said first curved inner wall of said removable X-ray distribution adjusting filter portion and a second curved inner wall on a face opposite to the second curved outer wall, capable of being inserted to or discharged from an inside of said removable X-ray distribution adjusting filter portion, with said second curved outer wall being run along said first curved inner wall of said removable X-ray distribution adjusting filter portion, wherein:said second removable X-ray distribution adjusting filter portion is formed of a material that can absorb X-rays, andthe insertion or removal of said removable X-ray distribution adjusting filter portion and said second removable X-ray distribution adjusting filter portion causes the sectional shape of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus. 5. The X-ray distribution adjusting filter apparatus according to claim 4, wherein:within said basic X-ray distribution adjusting filter portion, an inner guide member is arranged which is formed of a member which absorbs less of said X-rays and whose curved outer wall is identical in shape with said curved inner wall of said removable X-ray distribution adjusting filter portion or said curved inner wall of said second removable X-ray distribution adjusting filter portion, andsaid removable X-ray distribution adjusting filter portion and/or said second removable X-ray distribution adjusting filter portion are inserted into or removed from a space between said basic X-ray distribution adjusting filter portion and said inner guide member with said inner guide member as guiding means. 6. An X-ray CT apparatus comprising:an X-ray source;X-ray detecting section; andan X-ray distribution adjusting filter apparatus configured to adjust the distribution of the penetration intensity of X-rays emitted from said X-ray source and configured to disperse in a predetermined shape from the center axis of the X-rays linking the focal position of said X-ray source and the center of said X-ray detecting section on a plane orthogonal to said center axis, wherein said X-ray distribution adjusting filter apparatus comprises:a curved face along said center axis;an X-ray absorbing portion formed of an X-ray absorbing material, in which the distribution of the penetration intensity of said X-rays can be adjusted by varying the sectional shape of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus;a fixed section including a base portion uniform in thickness along said center axis, and inclined portions linked to or formed integrally with the base portion, symmetrically formed about said center axis and on both sides of said center axis, and each having an inclined face with a predetermined inclination relative to a flat face of said base portion;first and second movable sections formed on both sides of said center axis, each configured to pass said center axis and to be tiltable on a plane orthogonal to said center axis, pivoting on a center point, which is the position where one-side ends of said inclined faces of said fixed section are coupled, and having a flat face positioned on the side opposite to said inclined faces of said fixed section and a curved face opposite to the flat face; andfirst and second deformable sections having opposite ends each opposite to said coupling position of each of said inclined faces of said fixed section, and an expansible section disposed between the ends of said flat faces of said first and second movable sections, opposite to the opposite ends, and expanding or contracting according to the pivoting of said first and second movable sections, in which cavities defined by said inclined faces of said fixed section, said flat faces of said movable sections and said expansible section are filled with fluid to keep the insides of said cavities in a filled state, wherein:said fixed section and said movable sections are formed of an X-ray absorbing material to constitute said X-ray absorbing portion. 7. The X-ray CT apparatus according to claim 6, wherein said inclined faces of said fixed section and said flat faces of said movable sections are caused to approach or move away from each other by the tilting of said first and second movable sections pivoting on said center point to vary the quantities of said fluid in the cavities of said movable sections, and to vary the sectional shape of said X-ray absorbing portion of the X-ray distribution adjustment filter apparatus. 8. The X-ray CT apparatus according to claim 7, comprising:a fluid accommodating section for pressing said fluid to said cavity so as to fill said cavities with said fluid without obstructing the rotation of said movable sections and in response to the rotation of said movable sections; anda movable device for rotating said movable sections. 9. The X-ray CT apparatus according to claim 8, wherein:said movable sections are continuously tilted via said movable device to continuously vary the sectional shape of said X-ray absorbing portion of the X-ray distribution adjustment filter apparatus. 10. The X-ray CT apparatus according to claim 8, wherein:said movable sections can be continuously tilted via said movable device to continuously vary the sectional shape of said X-ray absorbing portion of the X-ray distribution adjustment filter apparatus. 11. The X-ray CT apparatus according to claim 7, further including an X-ray distribution adjusting filter apparatus control section, in which the shape and characteristics of said X-ray absorbing portion of said X-ray distribution adjusting filter apparatus are found on each individual occasion according to the tilted position of said movable sections, or has memory section in which are stored said shape and characteristics figured out in advance, and said found results or the results stored in said memory section are referenced to tilt said movable sections according to the desired shape and characteristics of the X-ray absorbing portion of said X-ray distribution adjusting filter apparatus. 12. An X-ray CT apparatus comprising:an X-ray source;X-ray detecting section; andan X-ray distribution adjusting filter apparatus configured to adjust a distribution of a penetration intensity of X-rays emitted from said X-ray source and configured to disperse in a predetermined shape from a center axis of the X-rays linking a focal position of said X-ray source and a center of said X-ray detecting section on a plane orthogonal to said center axis, said X-ray distribution adjusting filter apparatus comprising:a curved face along said center axis;an X-ray absorbing portion formed of an X-ray absorbing material, in which the distribution of the penetration intensity of said X-rays can be adjusted by varying the sectional shape of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus;a basic X-ray distribution adjusting filter portion symmetrically shaped about said center axis and having a curved inner wall; anda removable X-ray distribution adjusting filter portion symmetrically shaped about said center axis and having a first curved outer wall whose shape is identical with the shape of said curved inner wall of said basic X-ray distribution adjusting filter portion and a first curved inner wall on a face opposite to the first curved outer wall, capable of being inserted to or discharged from an inside of said basic X-ray distribution adjusting filter portion, with said first curved outer wall being run along said curved inner wall of said basic X-ray distribution adjusting filter portion, wherein:said basic X-ray distribution adjusting filter portion and said removable X-ray distribution adjusting filter portion are formed of a material that can absorb X-rays, andthe insertion or removal of said removable X-ray distribution adjusting filter portion into or from said basic X-ray distribution adjusting filter portion causes the sectional shape of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus to vary. 13. The X-ray CT apparatus according to claim 12, in which said X-ray distribution adjusting filter apparatus further includes:a second removable X-ray distribution adjusting filter portion symmetrically shaped about said center axis and having a second curved outer wall whose shape is identical with the shape of said first curved inner wall of said removable X-ray distribution adjusting filter portion and a second curved inner wall on a face opposite to the second curved outer wall, capable of being inserted to or discharged from an inside of said removable X-ray distribution adjusting filter portion, with said second curved outer wall being run along said first curved inner wall of said removable X-ray distribution adjusting filter portion, wherein:said second removable X-ray distribution adjusting filter portion is formed of a material that can absorb X-rays, andthe insertion or removal of said removable X-ray distribution adjusting filter portion and said second removable X-ray distribution adjusting filter portion causes the sectional shape of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus. 14. The X-ray CT apparatus according to claim 13, wherein:said X-ray distribution adjustment filter apparatus includes, within said basic X-ray distribution adjusting filter portion, an inner guide member arranged, which is formed of a member which absorbs less of said X-rays and whose curved outer wall is identical in shape with said curved inner wall of said removable X-ray distribution adjusting filter portion or said curved inner wall of said second removable X-ray distribution adjusting filter portion, andsaid removable X-ray distribution adjusting filter portion and/or said second removable X-ray distribution adjusting filter portion are inserted into or removed from a space between said basic X-ray distribution adjusting filter portion and said inner guide member with said inner guide member as guiding means. 15. The X-ray CT apparatus according to claim 12, further including:an X-ray distribution adjusting filter apparatus control section which has memory section in which the shape and characteristics of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus when using said one or a plurality of removable X-ray distribution adjusting filter portions in combination are found in advance and stored, and which inserts or removes said one or a plurality of removable X-ray distribution adjusting filter portions according to the desired shape and characteristics of said X-ray absorbing portion with reference to the results stored in said memory section. 16. An X-ray CT apparatus including:an X-ray source;X-ray detecting section;an X-ray distribution adjusting filter apparatus having, in order to adjust the distribution of the penetration intensity of X-rays emitted from said X-ray source and dispersing in a predetermined shape from the center axis of the X-rays linking the focal position of said X-ray source and the center of said X-ray detecting section on a plane orthogonal to said center axis, a curved face along said center axis, and including an X-ray absorbing portion formed of an X-ray absorbing material; andan X-ray distribution adjusting filter apparatus control section for adjusting the distribution of the penetration intensity of X-rays penetrating said X-ray absorbing portion by varying the position of said X-ray absorbing portion of said X-ray distribution adjusting filter apparatus relative to the focal position of said X-ray source, said X-ray distribution adjusting filter apparatus comprising:a fixed section including a base portion uniform in thickness along said center axis, and inclined portions linked to or formed integrally with the base portion, symmetrically formed about said center axis and on both sides of said center axis, and each having an inclined face with a predetermined inclination relative to a flat face of said base portion;first and second movable sections formed on both sides of said center axis, each configured to pass said center axis and to be tiltable on a plane orthogonal to said center axis, pivoting on a center point, which is the position where one-side ends of said inclined faces of said fixed section are coupled, and having a flat face positioned on the side opposite to said inclined faces of said fixed section and a curved face opposite to the flat face; andfirst and second deformable sections having opposite ends each opposite to said coupling position of each of said inclined faces of said fixed section, and expansible section disposed between the ends of said flat faces of said first and second movable sections, opposite to the opposite ends, and expanding or contracting according to the pivoting of said first and second movable sections, in which cavities defined by said inclined faces of said fixed section, said flat faces of said movable sections and said expansible section are filled with fluid to keep the insides of said cavities in a filled state, wherein:said fixed section and said movable sections are formed of an X-ray absorbing material to constitute said X-ray absorbing portion. 17. The X-ray CT apparatus according to claim 16,wherein said inclined faces of said fixed section and said flat faces of said movable sections are caused to approach or move away from each other by the tilting of said first and second movable sections pivoting on said center point to vary the quantities of said fluid in the cavities of said movable sections, and to vary the sectional shape of said X-ray absorbing portion of the X-ray distribution adjustment filter apparatus. 18. An X-ray CT apparatus comprising:an X-ray source;X-ray detecting section;an X-ray distribution adjusting filter apparatus configured to adjust a distribution of a penetration intensity of X-rays emitted from said X-ray source and configured to disperse in a predetermined shape from a center axis of the X-rays linking a focal position of said X-ray source and a center of said X-ray detecting section on a plane orthogonal to said center axis, said X-ray distribution adjusting filter apparatus comprising:a curved face along said center axis;an X-ray absorbing portion formed of an X-ray absorbing material;a basic X-ray distribution adjusting filter portion symmetrically shaped about said center axis and having a curved inner wall; anda removable X-ray distribution adjusting filter portion symmetrically shaped about said center axis and having a first curved outer wall whose shape is identical with the shape of said curved inner wall of said basic X-ray distribution adjusting filter portion and a first curved inner wall on a face opposite to the first curved outer wall, capable of being inserted to or discharged from an inside of said basic X-ray distribution adjusting filter portion, with said first curved outer wall being run along said curved inner wall of said basic X-ray distribution adjusting filter portion, wherein:said basic X-ray distribution adjusting filter portion and said removable X-ray distribution adjusting filter portion are formed of a material that can absorb X-rays, andthe insertion or removal of said removable X-ray distribution adjusting filter portion into or from said basic X-ray distribution adjusting filter portion causes the sectional shape of said X-ray absorbing portion of the X-ray distribution adjusting filter apparatus to vary; andan X-ray distribution adjusting filter apparatus control section for adjusting the distribution of the penetration intensity of X-rays penetrating said X-ray absorbing portion by varying the position of said X-ray absorbing portion of said X-ray distribution adjusting filter apparatus relative to the focal position of said X-ray source. |
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052895092 | abstract | A comb-line antenna structure (80) includes a multiplicity of parallel current straps (86) through which an appropriate rf electrical current passes in order to launch a desired magnetosonic wave (42) into an adjacent plasma mass (39). The current straps are mounted within a conductive, shallow, open box (88, 90) that faces the plasma mass. The current straps are inductively coupled, thereby requiring only a single rf input port (82) at one end of the comb-line structure. The rf input port provides a substantially constant impedance to an rf power source. A single rf output port (84) at the other end of the comb-line structure allows for the recirculation of the rf power. A multiplicity of U-shaped wickets (92) loop over and enclose each current strap. Such wickets function as a Faraday shield to shield the plasma and adjacent current straps from electrostatic fields. The comb-line antenna structure finds primarily applicability in providing plasma heating and/or current drive in a tokamak, or equivalent plasma-forming structures. |
description | Using the drawings, the preferred embodiments of the present invention will now be explained. FIG. 1 shows an ICI assembly 16 (compressed flexible hose 10 is removed for clarity) that is used to measure conditions such as the power level and temperature of a nuclear reactor while the reactor is in operation. The assembly 16 includes a bullet nose 17 with a rounded tip. The hard metallic bullet nose 17 is relatively narrowed compared with the rearward components of the assembly 16. The shape and size of the bullet nose 17 aids in guiding the assembly 16 through the tortuous bend guide paths found in many reactors. As shown in FIG. 5, a central tensile member 20 is at one end connected to the bullet nose 17, and extends into a central bore of the bullet nose 17. At the opposite end of the central tensile member 20 is a seal plug 18, to which the central tensile member 20 is connected.. A plurality of detectors 21 is disposed around the periphery of the central tensile member 20. The detectors 21 are shown in FIG. 5 according to the present invention. Each of the detectors 21 includes a signal wire and an emitter. The signal wire and emitter can be insulated using an insulation material such as Al2O3, and housed within a sheath made of a metallic material. If the detectors 21 are subjected to the liquid and the high-pressure environment inside the nuclear reactor, the detector sheath may become pitted and cracked. The sheath is especially vulnerable while it is being directed through the interior of the nuclear reactor to detect conditions within the reactor. If water penetrates a detector wall, that detector will fail. A flexible outer hose 10 is provided to protect the detectors 21 and other nuclear applications within the ICI assembly from the harsh environment of the nuclear reactor during reactor operation. The hose 10 is pressure tight and, has properties that ensure that the interior of the hose is dry. Specifically, the starting sheath material that makes up the hose is a flexible bellows type hose 10 shown in, FIGS. 2 and 2a, where FIG. 2a is a close-up view. The hose 10 is made of a watertight material, and is provided with corrugations or ribs that impart flexibility to the hose 10. Each rib has a peak 11 with sides 12, on either side of the peak 1l, that each lead to a valley 13. The ends 14, 15 of the hose 10 will normally be attached to the ICI assembly 16 using a welding process, or other process that will provide a hermetic seal. Before the hose 10 is subjected to a collapse process, the sides 12 of the ribs are spaced apart from each other, and FIG. 1 represents the hose in its pre-collapsed form prior to being attached to the ICI assembly. FIGS. 3 and 3a show the hose 10 after it has been collapsed and is ready to be attached to the ICI assembly 16. The hose. 10 is collapsed using a hydro-collapse process that begins by placing the hose 10 in a hydro test fixture (not shown). A rod (not shown) is fitted to the inside of the hose. The rod is uniform in diameter, and extends along at least the entire length of the hose 10 to ensure that the hose 10 will collapse over its full length in a uniform manner during the hydro-collapse process. The rod is then sealed to one end of the hydro test fixture. Hydro-pressure is then applied to the hose 10 in the hydro test fixture, and the hose 10 collapses to the degree shown in FIG. 2, while the rod traverses outside the hydro test fixture. In its collapsed state, the hose 10 no longer has spaced apart ribs. The sides 12 of the ribs are no longer at an inclining angle relative to one another, and instead are in contact with one another. The peaks also are disposed substantially adjacent to one another and together form an outer diameter of the hose 10. The portions of the inner surface of the hose 10 that formed the valleys 13 also are disposed substantially adjacent to one another and together form the inner diameter of the hose 10. In a preferred embodiment of the invention, a commercially available flexible bellows hose 10 of the type shown in FIGS. 2 and 2a initially had a maximum outer diameter of 0.3.90 inches, measured from the peaks 11 of the ribs. The minimum inner diameter, measured from the inner surface of the hose 10 at the innermost portion of the valleys, was measured at 0.250 inches. The hose 10 was subjected to a series of applications of heat and pressure, where the pressure application was preformed using the above-described hydro test fixture process. After repeating the heat and pressure applications two times, the hose 10 was measured again. The hose 10 was measured to be less than half its starting length. The maximum outer diameter, again measured from the peaks of the ribs, was 0.418 inches. The minimum innermost diameter, measured from the portions of the inner surface of the hose 10 that formed the valleys, was 0.235 inches. Following the hydro-collapsing process, the hose 10 was hermetically sealed to the ICI assembly by, for example, a welding process. FIG. 4 shows the ICI assembly with the hose 10 of the present invention attached thereto. One end 14 of the hose 10 is attached to the bullet nose 17, while the opposite end 15 of the hose 10 is attached to the surface 19 of the seal plug 18 that is connected to the central tensile member 20. Accordingly, an airtight and watertight protection is provided to the detectors 21 by the hose 10 that surrounds the detectors 21 and the central tensile member 20. FIG. 5 shows a cross section of the front portion of the ICI assembly, showing the relationship between the hose 10, the central tensile member 20, the detectors 21, and the bullet nose 17. The preceding description has been presented only to illustrate and describe the invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. The preferred embodiment was chosen and described in order to best explain the principles of the invention and its practical application. The preceding description is intended to enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims. |
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abstract | An extreme ultraviolet light generation apparatus includes: A. a chamber in which extreme ultraviolet light is generated by a target substance being irradiated with a laser beam to generate plasma from the target substance; B. a vessel as a tubular member forming the chamber; C. a reference member supporting the vessel; D. a collector mirror configured to condense the extreme ultraviolet light in the chamber, the collector mirror being attached to the reference member in a replaceable manner and covered by the vessel to be housed in the chamber; and E. a vessel movement mechanism provided to the reference member and configured to move the vessel between a first position at which the vessel covers the collector mirror and a second position at which the vessel is retracted from the first position to expose the collector mirror. |
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description | The present invention relates to a sensor for sensing a metallic object through another metallic object. In particular, but not exclusively, the present invention relates to a sensor for measuring the relative position of a control rod within a nuclear reactor from within a metallic probe tube housing the sensor. Means for measuring or detecting the position of a control rod within a nuclear reactor are limited by the fact that the measurement needs to be made within the primary water for the nuclear reactor. A conventional method for determining the relative location of a control in a nuclear reactor is to use a metallic probe tube which extends into the primary water region, and which houses a coil of wire forming an inductive element that forms part of an electrical circuit. The probe tube is positioned such that a metallic leadscrew attached to the control rod moves telescopically over the probe tube as the control rod is moved in and out of the nuclear reactor to regulate the fission reaction therein. As the leadscrew moves over the probe tube the voltage across the inductor changes because of magnetic coupling effects. This change in voltage is directly proportional to the position of the leadscrew and thus the control rod. A problem with using this method is that it is typically not very accurate. In particular, it has a low span to offset ratio and a low signal span. This is problematic because the measurement instrumentation is typically limited to relatively low signal voltages, and it is thus desirable to maximise the signal span to offset ratio so that the relative position of the leadscrew (and therefore the control rod) can be known with high accuracy. A further problem with the prior art techniques is that the flux density of the field that is generated around the inductive element is difficult to predict before manufacture. It is common practice, therefore, to manufacture a multitude of inductive elements, the one with the best magnetic field in terms of the spread of the flux ultimately being selected for use. Indeed, each element may need to be calibrated in situ, so that variations in the local operating environment can be accounted for in the calibration. This is undesirable. Some prior art methods of measurement use the transformer principle rather than the simple inductor principle. The transformer principle also involves a metallic probe tube and a metallic leadscrew, but the probe tube houses a series of transformer windings alternating between electromagnetically coupled primary and secondary windings along a core. When in operation, a magnetic field is generated between the primary and secondary windings. As the leadscrew moves over the probe tube the magnetic field between the windings is affected such that the voltage generated across the secondary windings changes proportionately to the position of the leadscrew over the probe tube. An example of a transformer effect sensor is U.S. Pat. No. 5,563,922, which shows the use of a transformer effect to sense the moving metallic item through a metallic enclosure. However, in the arrangement shown in U.S. Pat. No. 5,563,922, the output signal typically suffers from a low span to offset ratio. As mentioned above is undesirable because it reduces the sensitivity of the sensor and therefore the accuracy to which the relative position of the leadscrew (and therefore the control rod) can be known. In particular, in arrangements similar to that of U.S. Pat. No. 5,563,922, the signal span is relatively small. And, typically, a large residual magnetic field exists between the primary and secondary windings when the leadscrew is “covered” (i.e. the leadscrew is arranged to cover the probe tube). This typically results in a large voltage offset on the output signal of the sensor, which is undesirable. In particular, when an output signal is amplified the voltage offset of the signal is also amplified, which causes difficulty for subsequent signalling processing of the output signal; indeed, it can make it difficult to detect the relevant part of the signal, because it is swamped by the amplified offset level (and any associated noise on the offset level). The present invention seeks to provide a way to remove the undesirable offset, thus improving the sensor significantly with respect to the known prior art sensors by providing a sensor with an improved signal span to offset ratio, thereby providing a sensor with higher resolution. In other words, the present invention seeks to provide a sensor and/or method which provides a signal indicating the relative location of a metallic object with a higher degree of accuracy than the prior art. A first aspect provides a sensor assembly for indicating the relative location of a metallic object, the sensor assembly including: a primary electromagnetic coil arranged to generate a time varying magnetic field; and a secondary electromagnetic coil arranged to detect the time varying magnetic field as affected, directly or indirectly, by the object and to output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the object; wherein at least one of the primary and secondary electromagnetic coils is wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. Accordingly, the signal span to offset ratio of the output of the sensor has a higher resolution than prior art sensors. The primary and secondary coils may be arranged coaxially. A plurality of primary electromagnetic coils may be provided. A plurality of secondary electromagnetic coils may be provided. The plurality of primary and secondary coils may be arranged in a mutually alternating sequence of primary and secondary coils. The or each primary coil may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The or each secondary coil may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary and secondary coils may each be wound about the same core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary coils may be mutually arranged in electrical series; and/or wherein the secondary coils may be separately mutually arranged in electrical series. The primary and/or secondary coils may be formed of an alloy comprising 86% copper, 12% Manganese and 2% Nickel, e.g. Manganin® wire. The or each core body may be formed of a material having the same conductivity and/or magnetic permeability as the object. The or each core body may be formed of the same material as the object. The metallic object may be attached to a (movable) nuclear reactor control rod. A second aspect provides a method of optimising the output of a sensor as described herein, the method including the steps of: supplying the primary coil(s) with an alternating current to result in the generated time varying magnetic field; locating the object in a first position and recording the signal output by the secondary electromagnetic coil(s) for a range of respective frequencies of the supplied alternating current; locating the object in a second position and recording the signal output by the secondary electromagnetic coil(s) for the range of respective frequencies of the supplied alternating current; calculating, for each of the respective frequencies, a value for the span to offset ratio of the measured signals on the basis of the respective signals measured for the object in the first and second positions; and determining the frequency of the supplied alternating current which provides the maximum span to offset ratio on the basis of the calculations. When the object is in the first location, the output from the secondary coil(s) may be a maximum. When the object is in the second location, the output from the secondary coil(s) may be a minimum. The calculation step may include, for each respective frequency: calculating the difference between the amplitudes of the signals measured for the object in the first and second positions; and dividing the difference by the amplitude of the signal measured for the object in the second position. The sensor assembly may include a metallic body, within which the primary and secondary coils and core body/bodies are located, and outside of which the metallic object is located. Thus the sensor assembly is configured to be capable of indicating the relative location of the metallic object even though the coils are separated from the metallic object by the metallic body (within which the coils are located). A third aspect provides sensor assembly for indicating the location of a leadscrew relative to a probe tube, the leadscrew forming part of a nuclear control rod and the probe tube being moveably connected to the leadscrew, the sensor assembly including: a primary electromagnetic coil arranged to generate a time varying magnetic field; and a secondary electromagnetic coil arranged to detect the time varying magnetic field as affected, directly or indirectly, by the leadscrew moving relative to the probe tube and to output, on the basis of the detected time varying magnetic field, a signal indicative of the location of the leadscrew relative to the probe tube; wherein the primary electromagnetic coil and the secondary electromagnetic coil comprises copper and nickel. The primary electromagnetic coil and the secondary electromagnetic coil may be formed from a copper-manganese-nickel alloy. The copper-manganese-nickel alloy may comprise by weight equal to or between 77 and 89% Copper, 10 and 18% Manganese, 1 and 5% Nickel. The copper-manganese-nickel alloy may comprise by weight 86% Copper, 12% Manganese and 2% Nickel. Both the primary electromagnetic coil and the secondary electromagnetic coil may comprise copper and nickel. The sensor assembly may include a temperature indicator to indicate the temperature of the sensor assembly. The sensor assembly may comprise a processor configured to receive the voltage from the primary electromagnetic coil, the voltage from the secondary electromagnetic coil and an output from the temperature indicator and output a calibrated output that compensates for the temperature of the sensor assembly. The sensor assembly may comprise a tertiary coil. The tertiary coil may comprise at least 95% by weight copper, for example at least 98% by weight copper, or at least 99% by weight copper. The tertiary coil may be positioned to surround the primary electromagnetic coil. The sensor assembly of the third aspect may have one or more of the optional features of the sensor assembly of the first aspect. A fourth aspect provides a method of indicating the relative location of a leadscrew relative to a probe tube, the leadscrew forming part of a nuclear control rod and the probe tube being moveably connected to the leadscrew, the sensor being of the type according to the first or the third aspect, the method including the steps of: supplying the primary electromagnetic coil with an alternating current to result in the generated time varying magnetic field; recording a voltage from the primary coil; recording the signal output by the secondary electromagnetic coil; recording a temperature indicator indicative of the temperature of the sensor; modifying the voltage from the secondary electromagnetic coil based upon the temperature indicator to produce a calibrated secondary voltage; and calculating a position of the leadscrew based on the calibrated secondary voltage and the voltage recorded from the primary coil. A fifth aspect provides a method of optimising the output of a sensor for indicating the relative location of a metallic object, the sensor being of the type having a primary electromagnetic coil arranged to generate a time varying magnetic field; and a secondary electromagnetic coil arranged to detect the time varying magnetic field as affected, directly or indirectly, by the object and to output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the object, the method including the steps of: supplying the primary coil with an alternating current to result in the generated time varying magnetic field; locating the object in a first position and recording the signal output by the secondary electromagnetic coil for a range of respective frequencies of the supplied alternating current locating the object in a second position and recording the signal output by the secondary electromagnetic coil for the range of respective frequencies of the supplied alternating current; calculating, for each of the respective frequencies, a value for the span to offset ratio of the measured signals on the basis of the respective signals measured for the object in the first and second positions; and determining the frequency of the supplied alternating current which provides the maximum span to offset ratio on the basis of the calculations. The sensor may be a sensor assembly according to the first or the third aspects. When the object is in the first location, the output from the secondary coil may be a maximum; and/or when the object is in the second location, the output from the secondary coil may be a minimum. The calculation step may include, for each respective frequency: calculating the difference between the amplitudes of the signals measured for the object in the first and second positions; and dividing the difference by the amplitude of the signal measured for the object in the second position. The sensor may be positioned within a metallic tube and the metallic object may be arranged to move relative to the tube between a position of minimum overlap and a position of maximum overlap of the tube and the object. The first position may be a position where there is minimum overlap between the tube and the object. The second position may be a position where there is maximum overlap between the tube and the object. At least one of the primary and secondary electromagnetic coils may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary and secondary coils may be arranged coaxially. The sensor may comprise a plurality of primary electromagnetic coils. The sensor may comprise plurality of secondary electromagnetic coils. The plurality of primary and secondary coils may be arranged in a mutually alternating sequence of primary and secondary coils. The or each primary coil may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The or each secondary coil may be wound about a core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary and secondary coils may be each wound about the same core body formed of a material having the same conductivity and/or magnetic permeability as the object. The primary coils may be mutually arranged in electrical series; and/or wherein the secondary coils may be separately mutually arranged in electrical series. The or each core body may be formed of a material having the same conductivity and/or magnetic permeability as the object. The or each core body may be formed of the same material as the object. The metallic object may be attached to a nuclear reactor control rod. FIG. 1 shows schematic cross sections of a sensor 10. The schematic only shows half of the full arrangement; the full arrangement being mirrored about the dashed line A-A to shown in FIG. 1. The sensor 10 comprises a plurality of primary coils 12, coaxially arranged with a plurality of secondary coils 14. One or more primary coils 12 may be provided. One or more secondary coils 14 may be provided. Where a plurality of primary coils 12 are provided, the respective primary coils may be connected in electrical series. Where a plurality of secondary coils 12 are provided, the respective secondary coils may be connected in electrical series. The primary and secondary coils are arranged in a mutually alternating (physical) series or sequence, such that the sequence of coils along the long axis of the series alternates between individual primary and secondary coils. The primary and secondary coils are not in electrical connection. In other words, between each pair of adjacent primary coils 12 a secondary coil 14 may be provided; and/or between each pair of adjacent secondary coils 14 a primary coil 12 may be provided. In the embodiment shown, each coil 12, 14 is wound around a single core body 16. However, the coils may be each be wound around a respective core body 16. Or plural sets of two or more of the coils may be wound around respective core bodies. The coils 12, 14 may be wound around a supporting body, which is itself mounted on to the core body 16. However, the coils 12, 14 may be wound directly on to the core body 16. In either case the coils 12, 14 may be referred to as bobbins. In one particular use, the sensor 10 is mounted inside a probe tube 18 which extends or projects into a region containing the primary water surrounding a nuclear reactor. In this example, for safety reasons the probe tube must be metallic. Within the aforementioned region the nuclear reactor control rods (not shown) are movable, to be inserted into or withdrawn from the nuclear reactor itself. Typically, each control rod is attached to a leadscrew 20, such that movement of the nuclear rod causes movement of its respective leadscrew. It is the accurate detection of the movement, or more accurately the relocation, of the leadscrew that the present disclosure aims to provide. As the control rod is moved, the leadscrew 20 moves along the probe tube 18. At one extreme, the leadscrew may not cover any part of the probe tube, as shown in FIG. 1A. This may occur for example when the control rod is fully inserted into the nuclear reactor core. At another extreme, the leadscrew may fully cover the probe tube, as shown in FIG. 1B, for example when the control rod is fully withdrawn from the nuclear reactor core. Therefore, to assist in the understanding of the present example, FIG. 1A shows the leadscrew in the “uncovered” position, whereby the leadscrew 20 is withdrawn from the probe tube 18; whereas FIG. 1B shows the leadscrew in the “covered” position, whereby the leadscrew 20 is arranged proximate to the sensor, e.g. to cover the probe tube 18. In FIG. 1A the nuclear reactor control rod (not shown) to which the leadscrew 20 is attached may be at a maximum insertion in the nuclear reactor core for example. Whereas, in FIG. 1B the nuclear reactor control rod (not shown) to which the leadscrew 20 is attached may be at a maximum extent of withdrawal from the nuclear reactor core, for example. In order to control the reaction within the nuclear reactor core in a reliable and safe manner it is important to know the relative location of the leadscrew 20, and therefore of the control rod, with a high degree of accuracy. During operation of a sensor, the primary coils 12 of the sensor 10 are supplied with an alternating (AC) current so as to result in a time varying magnetic field being produced by the primary coils 12. The time varying magnetic field interacts with the local environment, including the probe tube 18, the core body 16 and the leadscrew 20. The time varying magnetic field, as affected by the local environment, induces in the secondary coils 14 a corresponding AC current, and the secondary coils therefore output a corresponding signal indicative of the time varying magnetic field which induced the AC current in the secondary coils. Changes in the local environment, such as relocation, or repositioning, of the leadscrew 20 will alter the time varying magnetic field, and therefore will consequently alter the current induced in the secondary coils 14. Thus the corresponding output signal will be changed. This change in the signal output of the secondary coils is detectable, and can be used to establish the relative location of the leadscrew 20, and thus of the control rods. As discussed above, similar prior art sensor arrangements (e.g. U.S. Pat. No. 5,563,922) suffer from disadvantages that mean the accuracy of the determination of the relative location of the leadscrew can be improved significantly. The present inventor has realised that an important factor when considering how to improve the accuracy of said determination is the (signal) span to offset ratio. The signal span is the measurable signal span from the minimum signal to the maximum signal, and the offset is the minimum achievable signal. It is often difficult, if not impossible, to achieve a zero offset in a measured signal. Noise and residual signal inducing effects (e.g. residual magnetic fields in the context of the present discussion) mean that a non-zero signal offset is almost inevitable in any measurement system. Systems such as that shown in U.S. Pat. No. 5,563,922 often suffer from relatively small signal spans and undesirably large signal offsets, meaning that the overall accuracy of the measurement system can suffer. The present inventor currently considers that the (static) local environment around the sensor 10 is responsible for disadvantages discussed above. For example, as shown in FIG. 2, the time varying magnetic field 22 generated by the primary coils 12 results in a secondary electromagnetic field 24 in the metallic probe tube 18 (due to the produced eddy current 26) which can adversely affect the signal output by the secondary coils 14 by reducing the signal span to offset ratio for example. Other aspects of the local environment can also affect the signal span and signal offset. For example, the core body 16 about which the respective coils are wound. The present inventor has realised that one way to significantly improve the (signal) span to offset ratio to achieve excellent accuracy in determining the relative location of the leadscrew 20, is to ensure that the core body 16 is formed of a material having the same permeability and/or conductivity as the material from which the leadscrew 20 is formed. Indeed, in particularly preferred embodiments, the core body 16 is formed of the same material as the leadscrew 20. In such embodiments, ideally, the core body would be formed of the same production batch of material as the leadscrew 20, although this is not strictly necessary for the sensor to work. FIG. 3 shows a plot demonstrating the advantageous effect on the SoR (signal span to offset ratio) of the output signal of the secondary coils 12 when the material (or the permeability and/or conductivity characteristics) of the core body 16 is matched to the material from which the leadscrew is formed. Line 28 indicates the output signal VS against leadscrew position P for the sensor of the present embodiment. Line 30 is provided for comparison purposes and indicates an output signal against leadscrew position for a sensor where the material of the core 16 is different to the material of the leadscrew 20 (including having a different conductivity and a different magnetic permeability). To produce FIG. 3, an arbitrary frequency of 400 Hz for the AC current supply to the primary coils 12 was chosen. To calculate the SoR at the arbitrary frequency of 400 Hz, the output signal from the secondary coils 14 was measured for the uncovered leadscrew arrangement (i.e. where the leadscrew is distal to the sensor as in FIG. 1A) and separately for the covered leadscrew (i.e. the leadscrew at least partially ensheathing the sensor 10 and probe tube 16 as shown in FIG. 1B). Typically this provides values representative of the maximum output signal and the minimum output signal respectively. The difference between the measured values was then calculated to obtain the signal span. The result was then divided by the measured signal corresponding to the covered leadscrew (i.e. at least partially ensheathing the sensor) which typically corresponds with the offset of the measured signal. The result of the division operation gives the span to offset ratio (SoR) for the output signal at the chosen 400 Hz. For a typical prior art arrangement without core matching (without matching the material characteristics of the core body 16 to that of the leadscrew 20), the SoR at 400 HZ was determined to be around 0.8 only. However, for a sensor arrangement according to the present embodiment, which adopts the principle of matching the permeability and/or conductivity characteristics of the core body material to that of the leadscrew material (for example, by matching the material of the core body 16 to that of the leadscrew 20), the SoR at 400 HZ was determined to be 2.26. Thus, the described sensor arrangement can provide a very significant improvement in the SoR of the output signal from the secondary coils 14. This is particularly advantageous where the output signal of the secondary coils may be fed to a measurement system via a data acquisition card having a maximum input voltage. For example, such data acquisition cards may have a maximum input voltage of 5V. Therefore, improving the SoR within the available 5V range means that the resolution of the acquired signal is improved, and thus the subsequent processing can produce a more accurate result for the determination of the relative location of the leadscrew 20. To demonstrate that matching the conductivity and/or magnetic permeability of the core body material to that of the leadscrew 20 is particularly advantageous in achieving an optimum SoR for the output signal of the secondary coils 14, the present inventor has conducted extensive finite element analysis, a resulting plot of the SoR for various metals against the frequency of the AC current supply to the primary coils 12 is shown in FIG. 4. In FIG. 4, line 30 is the plot of HAS4104; line 32 is the plot for grey cast iron; line 34 is the plot for ingot iron; line 36 is the plot for powdered iron; line 38 is the plot for supermendur (a cobalt-iron alloy); line 40 is the plot for pure iron; line 42 is the plot for Sinimax (a nickel-iron alloy); line 44 is the plot for Mumetal® (a nickel-iron alloy); line 46 is the plot for Inconel 625 (a nickel-chromium alloy); and line 48 is the plot for stainless steel. The finite element analysis has shown that the particular characteristics of the material of the core body 16 which contribute to the significant improvement of SoR are the conductivity of the core body material and the magnetic permeability of the core body material. In particular, the finite element analysis has shown that the improvement in the SoR of the output of the sensor 10 to be most significant when the magnetic permeability and/or the conductivity values of the core body material is/are matched closely to the magnetic permeability and/or conductivity values of the material from which the object to be detected is formed—here, the object to be detected typically being a leadscrew 20 formed of a particular metal. To demonstrate this effect, FIG. 4 shows the primary coils 12 AC current frequency dependency of the SoR of the output signal of the secondary coils 14 for various different materials of core body 16 for a leadscrew formed of a material referred to as HAS 4104, or DGS MS HAS 4104, which is a stainless having a high magnetic permeability. This material was chosen for the leadscrew material in this study because it is the typical material from which the leadscrews in nuclear reactors are formed. As can be seen from FIG. 4, the highest SoR is achieved for a core material of HAS4104, i.e. a material matching the material of the leadscrew which is also formed of HAS4104. So, where leadscrews are typically formed of HAS4104, embodiments for use in nuclear reactors employing such leadscrews may also have a core body 16 formed of HAS 4104. The SoR is also dependent on frequency. Not only will the electrical circuitry typically demonstrate a resonance peak, but the materials in the local environment will demonstrate different responses depending on the frequency of the time varying magnetic field generated by the primary coils. For example, a peak at around 7.5 KHz is observed in FIG. 4 when the core body 16 is formed of HAS4104 material. The SoR at this frequency is calculated to be around 11. This result for the SoR is calculated as follows, taking the suitable voltage values from FIG. 5 (in FIG. 5, line 50 is the line plotted for a core body of HAS4104, and line 52 is for a core body of stainless steel 316): Core body of HAS4104: (2.35V−0.19V)/0.19V=˜11 As shown in FIG. 5, if an alternative material is used for the core body 16, which does not have conductivity and/or permeability characteristics which match with the HAS4104 of the leadscrew, the SoR is shown to be only around 2.8. This result for the SoR is calculated as follows, taking the suitable voltage values from FIG. 5: Core body of stainless steel 316: (0.27V−0.07)/0.07=˜2.8 Therefore, the present disclosure surprisingly offers an improvement in the SoR of almost four times. Interestingly, this is achieved with an alternative material which is not a wildly different material to HAS4104, but which is another stainless steel: stainless steel 316. The present inventor has therefore demonstrated that a careful selection of the material for the core body 16 can have a surprisingly large advantageous effect on the SoR of the output signal of the secondary coils 14. As can be seen from FIG. 4, the optimum SoR is provided at a particular frequency, and so the present disclosure also proposes a method for determining the frequency at which the optimum SoR exists for a particular system. The object to be detected, for example the leadscrew 20, is arranged distally from the sensor 10; for example at its furthest distance from the sensor 10. In the case of the leadscrew, the control rod may be fully inserted into the nuclear reactor, for example. With the leadscrew 20 in this position, the primary coils are provided with AC current at a range of (two or more) discrete frequencies f, and the output signal VS from the secondary coils measured and recorded for each respective frequency. The result of such an exercise is shown in FIG. 6 for example. The object to be detected, for example the leadscrew 20, is also arranged at proximally to the sensor 10; for example at its nearest position to the sensor 10. In the case of the leadscrew 20, the control rod may be at its maximum withdrawal from the nuclear reactor for example. With the leadscrew in this position, the primary coils 12 are provided with AC current at the same range of the same (two or more) discrete frequencies f, and the output signal VS from the secondary coils 14 measured and recorded for each respective frequency. The result of this exercise is shown in FIG. 7 for example, where the effect of the object (the leadscrew) on the signal output by the secondary coils 14 can clearly be seen by comparison of FIG. 7 with FIG. 6. Then the SoR at each frequency is determined in accordance with the calculation discussed above in relation to FIG. 5, to determine the frequency f at which the SoR is a maximum. In other words, for each frequency, the minimum measured output signal is subtracted from the maximum measured signal to generate a difference value, and the difference value is divided by the minimum value to generate the SoR value. For the range of frequencies f measured, FIG. 8 shows a plot of the SoR. As can be seen, for the particular arrangement used in the demonstration, the SoR reaches a maximum value of around SoR=11.4 at around 6.75 kHz. Therefore, for the particular sensor and the local environment in which the sensor was located in this demonstration, the AC current should ideally be supplied to the primary coils 12 at around 6.75 KHz in order to maximize the SoR of the output signal of the secondary coils. Accordingly, the present embodiment provides a position sensor which provides an output signal indicative of the relative position of an object to be detected with a higher resolution than equivalent sensor arrangements in the prior art. This is achieved by winding the primary coil(s) and secondary coil(s) around one or more core bodies formed of a material having similar characteristics to the material of the object to be detected. In particular, it is preferred that the material of the one or more core bodies has a conductivity and/or magnetic permeability which matches the material of the object to be detected. In most preferred embodiments, the material of the one or more bodies is the same as the material of the object to be detected. In this way, a sensor arrangement according to the present embodiment provides a higher SoR and span output signal when detecting metallic objects through another metallic body. This provides major advantages in high accuracy and resolution measurement systems. The ability to provide the downstream instrumentation detection electronics with good resolution sensor signals enables errors to be reduced significantly, thereby allowing the overall system to be more accurate and to offer better resolution. In particular, a sensor according to the present embodiment, especially when used in conjunction with the SoR optimisation technique disclosed herein, offers a greatly improved means to measure linear displacement of a metallic device through another metallic device. In the sense that a sensor 10 according to the present embodiment generates a signal for interaction with the local environment and measures the effect on the signal in order to output a signal indicative of a change in the local environment, the sensor 10 may be considered to be a transducer, and may be referred to as such. As mentioned above, a sensor according to the present embodiment is particularly suited to use in a nuclear reactor, where the temperature of the local environment may fluctuate to a large extent. A large fluctuation in temperature will likely change the resistive properties of the primary and/or secondary coils, and therefore will likely change the SoR of the output signal of the secondary coils. Referring to FIG. 9, the output VS from the secondary coil 14 at varying positions of the leadscrew 20 is illustrated for a sensor 10 where the primary coil 12 and the secondary coil 14 are made from copper. The line 60 indicates the variation of the output VS from the secondary coil for varying positions P of the leadscrew at 200° C. and line 62 indicates the variation of the output from the secondary coil for varying positions of the leadscrew at 20° C. The temperatures given are measured at a position on the sensor having a maximum temperature, in this case this is in a region at the bottom of the probe tube 18. It can be seen from FIG. 9, that a change in maximum temperature from 200° C. to 20° C. significantly affects the output from the secondary coil. This means that the output from the secondary coil is undesirably dependent upon the temperature of the system. Furthermore, at lower temperatures the change in output for a given position change of the leadscrew decreases, which in turn impacts the sensitivity of the sensor. Referring to FIG. 10 a sensing arrangement that attempts to address this temperature dependence problem is illustrated. The arrangement of FIG. 10 is similar to the arrangement previously described, and similar features are given a similar reference numeral but with a pre-fix “1” to distinguish between embodiments. Only the differences between the embodiments will be described in detail. As illustrated in FIG. 10, similar to the previously described embodiment, the arrangement includes a sensor 110 positioned in a probe tube 118, and the probe tube 118 is moveable relative to a leadscrew 120. Referring now to FIG. 11, the sensor 110 is shown in more detail. Similar to the previously described sensor, the sensor 110 includes a series of primary coils 112 and secondary coils 114. In the presently described embodiment, the primary coils and the secondary coils are made from a copper-manganese-nickel alloy. In particular, the primary and secondary coils are made from Manganin®. Manganin® is a Copper-manganese-nickel alloy, and is generally provided in the ratio of 86:12:2 by weight. FIG. 12 illustrates the output VS from the secondary coil 114 at varying positions P of the leadscrew. Line 64 illustrates the output from the secondary coil at 200° C. and line 66 illustrates the output from the secondary coil at 20° C. As can be seen from FIG. 12, the use of manganin wire significantly reduces the temperature dependence of the output from the secondary coil both in terms of magnitude for a given position of the leadscrew as well as in terms of the change in magnitude for a step change in position of the leadscrew. However, it can be seen from FIG. 12 that there is still some variation in the output VS from the secondary coil 114. It is believed that this variation is due to probe tube and leadscrew thermal effects, primarily probe tube thermal effects. Referring again to FIG. 11, the sensor 110 is further optimised to include a tertiary coil 115. The tertiary coil is arranged substantially coaxially with the primary coil 112 and is positioned radially outside of the primary coil. However, in alternative embodiments the tertiary coil could surround the secondary coil or surround both the primary and secondary coils, or be positioned at any other suitable position on the core 116. The tertiary coil is made from copper or an alloy thereof. Referring now to FIG. 13, the sensor is connected to a processor 168. The voltage VP from the primary coil, the Voltage VS from the secondary coil and the Voltage VT from the tertiary coil are transmitted to the processor. The processor receives the voltage from the primary coil, secondary coil and tertiary coil and performs a compensation procedure. The algorithm for the compensation procedure can be established using techniques known in the art and will vary depending on the specific environment in which the sensor is used. Once the processor has performed the compensation procedure, the processor outputs a calibrated secondary coil output VSC. The compensation procedure can remove the variation in output from the secondary coil illustrated in FIG. 12, so that the calibrated secondary coil output VSC is independent of the temperature of the sensor. In alternative embodiments, the primary and secondary coils may be made from an alloy such as constantan (a copper-nickel alloy). However, the inventor has found Manganin® to provide an optimum SoR. In the present embodiment, the tertiary coil is provided with an AC current, but in alternative embodiments the tertiary coil may be provided with a DC current. In further alternative embodiments, the tertiary coil may be replaced with another type of temperature indicator. It will be appreciated by one skilled in the art that, where technical features have been described in association with one or more embodiments, this does not preclude the combination or replacement with features from other embodiments where this is appropriate. Furthermore, equivalent modifications and variations will be apparent to those skilled in the art from this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. |
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summary | ||
048572608 | claims | 1. Automated apparatus for welding a separate end plug to one open end of each of a succession of nuclear fuel cladding tubes and for inspecting each end plug weld, said apparatus comprising, in combination: A. a welding station including: B. a serial number reader station for reading a unique serial number imprinted on each end plug; C. a weld inspection station for inspecting each end plug weld and for generating weld inspection data indicative of the weld characteristics; D. data acquisition means linked with said serial number reader station and said weld inspection station and operating to correlate the weld inspection data with the associated end plug serial number for each end plug weld; E. an input queue for holding a quantity of tubes; and F. a tube transporter for periodically picking individual tubes from said input queue and conveying said tubes successively to said welding station, A. a welding station including B. a cooldown station for cooling each end plug weld in an inert gas atmosphere; C. a serial number reader station for reading a unique serial number inprinted on each end plug; D. a first weld inspection station for inspecting each end plug weld and for generating first weld inspection data indicative of the weld internal characteristics; E. a second weld inspection station for inspecting each end plug weld and generating second weld inspection data indicative of the weld external characteristics; F. a computer system linked with said serial number reader and said first and second weld inspection stations, said computer system operating to correlate said first and second weld inspection data with the associated end plug serial number for each end plug weld and processing said inspection data to determine whether each end plug weld meets established engineering standards pursuant to issuing appropriate accept/reject signals in correlation with the associated end plug serial number; G. an input queue for holding a plurality of tubes; H. a tube transporter for periodically picking individual tubes from said input queque and conveying the tubes in a direction transverse to their tube axis in indexing steps to index positions respectively axially aligned with said welding, serial number reader, and first and second weld inspection stations, said tube transporter including separate drive means positioned at said index positions for axially reciprocating the tubes into and out of said welding, serial number reader, and first and second weld inspection stations; and I. a sorter positioned at an output end of said tube transporter and operating in response to said accept/reject signals from said computer system to sort the tubes successively transported thereto into separate accept and reject lots. 2. The apparatus defined in claim 1, wherein said tube transporter conveys the tubes in a direction transverse to their tube axis in indexing steps to index positions respective axially aligned with said stations, said transporter further including separate drive means positioned at said index positions for axially reciprocating the tubes into and out of said stations. 3. The apparatus defined in claim 2, wherein said tube transporter further includes a plurality of parallel spaced, endless conveyor chains carrying grooved rollers at corresponding spaced intervals, each tube being supported on an aligned set of said rollers. 4. The apparatus defined in claim 3, wherein each said drive means includes a bidirectionally driven pinch roller coacting with one of said grooved rollers to axially reciprocate a tube positioned therebetween. 5. The apparatus defined in claim 2, wherein said welding station includes a weld box enclosing said welder and into which the one open end of each tube is successively positioned by said drive means, said end plug handling means includes a manipulator and a mating ram carrying an adapter at one end, said manipulator picking individual end plugs from said supply and placing same in said adapter, and motive means for driving said ram into said weld box to mate the end plug held by said adapter with the tube open end positioned therein. 6. The apparatus defined in claim 5, wherein said welding station further includes means for commonly rotating said tube and said ram during the welding of the mated end plug to the tube. 7. The apparatus defined in claim 6, wherein said welder is a tungsten electrode-inert gas welder. 8. The apparatus defined in claim 7, wherein said weld station further includes a TV camera for imaging the interior of said weld box to provide a visual aid in the proper positioning of said welder electrode. 9. The apparatus defined in claim 7, wherein said weld station further includes means selectively operable to preheat said end plug-holding adapter prior to a end plug welding operation. 10. The apparatus defined in claim 1, wherein said weld station further includes means for generating weld parameter data pertaining to each end plug weld for correlation with the serial number of the associated end plug by said data acquisition means. 11. The apparatus defined in claim 5, which further includes a cooldown station having an enclosure into which the end plug welded end of each tube is immediately introduced by said tube transporter upon withdrawal from said weld station by said drive means, said cooldown station including means within said enclosure for bathing the end plug welds with streams of an inert cooling gas as the tubes are indexed therethrough by said tube transporter. 12. The apparatus defined in claim 11, wherein cooldown station enclosure is at least two tube index positions in length. 13. The apparatus defined in claim 2, wherein said weld inspection station includes an ultrasonic transducer for inspecting each end plug weld. 14. The apparatus defined in claim 13, wherein said weld inspection station further includes a bubbler of ultrasonic energy couplant liquid in which each end plug weld is positioned. 15. The apparatus defined in claim 14, wherein said weld inspection station further includes a live centering stop against which the end plug end of each tube is engaged when inserted into said weld inspection station by said tube transporter drive means, and means for rotating each tube to unltrasonically scan its end plug weld. 16. The apparatus defined in claim 15, wherein said weld inspection station further includes means for jointly incrementing said bubbler and transducer in a direction parallel to the tube axis to perform a spiral scan of the end plug weld. 17. The apparatus defined in claim 16, wherein said weld inspection station further includes a TV camera for imaging the end plug serial number to enable manual entry thereof into said data acquisition means in the event said serial number reader station fails to correctly read the serial number. 18. The apparatus defined in claim 2, which further includes a barrier detection station into which the other open end of tube is reciprocated by said tube transporter drive means, said barrier detection station including means for detecting the presence and sensing the thickness of any zirconium barrier on the interior surface of each tube, said barrier detection station linked with said data acquisition means to enter barrier data for correlation with the associated end plug serial number. 19. The apparatus defined in claim 18, wherein said barrier detection station includes an eddy current probe and means for articulating said probe into barrier sensing relation with each tube propelled into and out of said barrier detection station by said tube transporter drive means. 20. The apparatus defined in claim 2, which further includes a second end plug weld inspection station into which each tube is reciprocated by said tube transporter drive means, said second weld inspection station including means for gauging whether each end plug weld outer diameter exceeds a predetermined limit. 21. The apparatus defined in claim 20, wherein said gauging means includes a ring gauge, means for positioning said ring gauge in the tube entry path into said second weld inspection station, and a sensor responsive to the failure of a welded end plug to pass through said ring gauge for signalling said positioning means to remove said ring gauge from said tube entry path and thus permit full inspection of the tube into said second weld inspection station by said tube transporter drive means. 22. The apparatus defined in claim 21, wherein said second weld inspection station further includes means to check for any nonparallelism between the end plug and tube axes of each tube fully inserted into said second weld inspection station. 23. The apparatus defined in claim 2, which further includes means linked with said data acquisition means for processing said weld inspection data to determine whether each end plug weld meets established engineering standards and to issue appropriate accept/reject signals in correlation with the associated end plug serial number, and a sorter operating in response to said accept/reject signals for separating accepted tubes from rejected tubes. 24. The apparatus defined in claim 23, which further includes a tube conveyor for conveying accepted tubes successively away from said sorter, a visual inspection station capable of accepting a predetermined plurality of tubes in parallel, side-by-side relation, and an output queuing conveyor for successively conveying accepted tubes from said tube conveyor to fill said visual inspection station with said predetermined number of tubes. 25. The apparatus defined in claim 24, which further includes an off-load transfer mechanism for transferring those tubes passing visual inspection as a group from said visual inspection station to an accepted tube tray conveyor. 26. Automated apparatus for welding a separate end plug to one open end of each of a succession of nuclear fuel cladding tubes and for inspecting each end plug weld, said apparatus comprising, in combination: 27. The apparatus defined in claim 26, wherein said welding station includes a weld box enclosing said welder and into which the one open end of each tube is successively positioned by said tube transporter drive means, said end plug handling means including a manipulator and a mating ram carrying an adapter at one end, said manipulator picking individual end plugs from said supply and placing same in said adapter, and means for driving said ram into said weld box to mate the end plug held by said adapter with the tube open end position therein. 28. The apparatus defined in claim 27, wherein said welding station further includes means for commonly rotating said tube and said ram during the welding of the mated end plug to the tube. 29. The apparatus defined in claim 28, wherein said first weld inspection station includes an ultrasonic transducer for ultrasonically scanning each end plug weld. 30. The apparatus defined in claim 29, wherein said first weld inspection station includes a live centering stop against which the end plug of each tube is engaged when inserted into said first weld inspection station by said tube transporter drive means, means for rotating each inserted tube, and means for incrementing said transducer in a direction parallel to the tube axis to perform a spiral ultrasonic scan of each end plug weld. 31. The apparatus defined in claim 30, wherein said first weld inspection station further includes a TV camera for imaging the end plug serial number of an inserted tube to enable manual entry thereof into said computer system in the event said serial number reader station fails to correctly read the end plug serial number. 32. The apparatus defined in claim 26, which further includes a barrier liner inspection station into which the open end opposite the welded end plug end of each tube is reciprocated by said tube transporter drive means, said barrier liner inspection station including means for detecting the presence and thickness of any barrier liner applied to the tube interior surface. 33. The apparatus defined in claim 32, which further includes a tube conveyor for conveying accepted tubes successively away from said sorter, a visual inspection station capable of accepting a predetermined plurality of tubes in parallel, side-by-side relation, and an output queing conveyor for successively conveying accepted tubes from said tube conveyor to fill said visual inspection station with said predetermined number of tubes. 34. The apparatus defined in claim 33, which further includes an off-load transfer mechanism for transferring those tubes passing visual inspection as a group from said visual inspection station to an accepted tube tray conveyor. 35. The apparatus defined in claim 26, wherein said welding station further includes means for generating weld parameter data pertaining to each end plug weld, said computer system correlating said weld parameter data for each end plug weld with the serial number of the involved end plug. |
summary | ||
abstract | This disclosure describes various configurations and components for bimetallic and trimetallic claddings for use as a wall element separating nuclear material from an external environment. The cladding materials are suitable for use as cladding for nuclear fuel elements, particularly for fuel elements that will be exposed to sodium or other coolants or environments with a propensity to react with the nuclear fuel. |
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055442110 | description | DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described with reference to the accompanying drawings. A fuel assembly embodying the present invention has fuel rods arranged in a matrix-like pattern composed of 9 rows and 9 columns (referred to as "9.times.9 arrangement", hereinafter). This fuel assembly exhibits reduced average linear power distribution to provide required thermal margin. A mere increase in the number of fuel rods, however, poses various problems such as impairment of the thermal-hydraulic stability due to increase in the pressure loss in the reactor core. In order to overcome this problem, fuel rods having different axial lengths are used so as to suppress increase in the pressure loss. The present invention features a combined use of at least one large-diameter water rod which provides an increased cross-sectional area of water region and part length fuel rods having a comparatively low axial length. By virtue of this feature, it is possible to preserve safety margin of reactor performance such as thermal margin and shut down margin. In addition, power peaking such a local power peaking coefficient of the fuel assembly can be increased so as to further improve fuel economy, thanks to the reduction in the average linear power density. In order to achieve high fuel economy, the fuel assembly embodying the present invention offers: (A) efficient use of the power peaking, (B) reduction in the amount of residual burnable poison, and (C) improvement in water-to-fuel ratio. The efficient use of the power peaking means to increase the ratio of use of thermal neutrons by determining the distribution of fissile material, e.g. U-235, in relation to the neutron flux density in the fuel assembly such that the content of the fissile material is high in the region where the neutron flux density is high, and low in the region where the neutron flux density is low. As a consequence, the power peaking is increased and, at the same time, the reactivity of the reactor core increases. The reduction in the amount of residual burnable poison is to set the concentration of burnable poison, e.g., gadolinia, in the lower end region and in the upper end region of the reactor core where burning of the poison is slow due to high void fraction and high hardness of the neutron spectrum, thereby minimizing loss of reactivity attributable to presence of residual burnable poison. The improvement in the water-to-fuel ratio is intended to increase the ratio of water to the fuel so as to increase the ratio of use of the thermal neutrons thereby to enhance the reactivity in the reactor core. According to the invention, an enhanced exposure can be achieved with minimal increase in the enrichment, by adopting the above-described features (A) to (C). A description will now be given of outline of the fuel assembly of the invention, as well as the outline of the nuclear reactor of the invention, which adopts these improvement measures. I. Fuel Assembly of the Invention The fuel assembly of the present invention has the following basic features. In particular, the fuel assembly has an axial enrichment distribution an axial distribution of concentration of Gd as the burnable poison as shown in FIGS. 1A and 1B, respectively. (1) Large-diameter water rod As shown in FIG. 3, a fuel assembly embodying the present invention has a pair of water rods W which are arranged along a diagonal line of the cross-section of the fuel assembly. Each water rod has an outside diameter greater than the pitch of arrangement of the fuel rods. More specifically, the water rod W occupies an area which can accommodate 3-and-a-half fuel rods. Thus, these water rods W occupy an area corresponding to 7 fuel rods. The water rod W circulates non-boiling water. In the fuel assembly of the invention which incorporates two such water rods, the water-to-fuel ratio is increased as compared with the conventional fuel assemblies, so that the neutron spectrum is softened to suppress the aforesaid undesirable effects: (a) increase in the absolute value of the void coefficient, (b) increase in the core reactivity in cold state and (c) reduction in the ability of burnable poison to control the reaction. In addition, the reactivity of the fuel assembly can be increased because of the water-to-fuel ratio which is increased as compared with those in conventional fuel assemblies. (2) Part Length rods A mere increase in the number of fuel rods causes an increase in the length of the wet edge contacting the coolant and, hence, resistance due to friction, so that the pressure loss is increased particularly in the gas-liquid two-layer flow portion in the fuel assembly, particularly at upper part of the fuel assembly, resulting in problems such as reduction in the thermal hydraulic stability which leads to increase in the moderation ratio. In view of this problem, the fuel assembly of the present invention has two type of fuel rods having different lengths of fuel-charged zones: namely, first fuel rods each having a fuel-charged zone of a length equal to the axial overall length of fuel-charged zone of the fuel assembly (referred to as "effective fuel length", hereinafter), and second fuel rods each having a fuel-charged zone of a length smaller than the effective fuel length H. The first and second fuel rods therefore will be referred also to a "long" and "short" fuel rods. The long and part length fuel rods have different axial lengths. The fuel-charged zone will be referred to as "effective fuel zone", hereinafter. The use of the part length fuel rods realizes such an arrangement that the number of the fuel rods in the cross-section of the fuel assembly is smaller in upper part of the fuel assembly than in lower part of the same, thus contributing to reduction in the flow resistance in the gas-liquid two-phase flow region. The fuel assembly of the invention of this application has eight part length fuel rods. The axial length of the effective fuel zone in the part length fuel rod is 14/24 of the effective fuel length H. The effective fuel zone of the part length fuel rod extends from a position which is 1/24 of the effective fuel length H to a position which is at 15/24 of the effective fuel length H as measured from the bottom of the effective fuel zone of the fuel assembly. In general, a BWR exhibits such characteristic that the axial distribution of the power is rendered even to some extent during power generation in which voids exist but a peak of axial power generation appears in cold state in which no void exists. Therefore, the reduction in the number of the fuel rods in the upper part of the fuel assembly, which increases the water-to-fuel ratio in the upper part of the fuel assembly where the neutron importance is high in cold state of the reactor, serves to further enhance the excessive moderation of neutrons so as to reduce the reactivity. Thus, the use of the part length fuel rods also produces an effect to increase the reactor shut down margin. (3) Axial distribution of enrichment In general, the neutron flux densities are low at upper and lower end portions than at the middle portion of the reactor. Therefore, the upper and lower end portions of the reactor core is charged with natural uranium, while the middle portion is enriched with the uranium. With this arrangement, it is possible to decrease leakage of neutrons to the upper and lower end portions, while enhancing the ratio of use of thermal neutrons, thereby increasing reactivity at the reactor core. The fuel assembly embodying the present invention has an axial enrichment distribution as shown in FIG. 1A. It will be seen that the upper and lower end regions of the effective fuel zone are charged with natural uranium, while the region intermediate between these natural uranium regions is charged with enriched uranium. The enriched uranium region has three axial section, namely, upper, middle and lower sections having different levels of average enrichment. The upper end of the effective fuel zone of the part length fuel rod is positioned within the middle section of the effective fuel zone of the fuel assembly. A description will now be given of the axial lengths of the natural uranium regions which are the upper and lower regions of the fuel assembly. FIG. 8 illustrates the relationship between the rate of increase in the power peaking in the lower axial portion and the rate of improvement in the fuel economy, using, as a parameter, the axial length of the natural uranium regions which are the upper and lower end regions. The axial length is expressed in terms of the number of nodes, wherein one node corresponds to 1/24 of the effective fuel length H. An increase in the axial length of the natural uranium region in one hand improves fuel economy but on the other hand enhances the power peaking along the axis. It is necessary to decrease the number of nodes of the natural uranium region when it is desired to suppress the power peaking along the axis. The axial power peaking in terms of combination of radial and local peaking must be such that the linear power generation ratio does not exceed an operational limit value. In addition, there is a tendency that the effect of improving fuel economy is saturated when the above-mentioned parameters changed from (2/1) to (3/2). For these reasons, in the fuel assembly embodying the present invention, the upper end regions of a length corresponding to two nodes, extending downward from the top end of the effective fuel zone, and the lower end region of a length corresponding to one node, extending upward from the bottom end of the effective fuel zone, are used as the natural uranium regions. As stated above, the effective fuel length H corresponds to 24 nodes. In each value of the parameter, the numerator and the denominator in parenthesis () respectively represent the number n1 of nodes of the natural uranium region at the upper end region of the fuel assembly and the number n2 of the nodes of the natural uranium region at the lower end region of the fuel assembly. In the enriched uranium region of the fuel assembly embodying the present invention, the average enrichment is highest in the middle section, medium in the lower section and lowest in the upper section. The average enrichment of the portion of the middle section above the level of the upper end of the effective fuel zone of the part length fuel rod is smaller than that of the portion of the middle section below the level of the upper end of the effective fuel zone of the part length fuel rod. In the fuel assembly in accordance with the present invention, the average enrichment is higher in the middle section than in the lower section. According to this arrangement, a more uniform axial distribution of power is obtained and, at the same time, increase in the power peaking to increase in mis-match of power levels among fuel assemblies due to enhancement in the exposure is suppressed. The resultant margin of the power peaking can be used for other peaking which contribute to improvement in the fuel economy. In order to attain uniform or even power distribution along the axis of the fuel assembly, it is necessary that the boundary between the middle section having greater enrichment and the lower section having smaller enrichment has to be positioned within the range between 1/3 and 7/12 of the effective fuel length H as measured from the lower end of the effective fuel zone. The part length fuel rods are not disposed in the outermost portion of the matrix of fuel rods constituting the fuel assembly but in portions where the local power peaking are comparatively low. The part length fuel rod therefore has an enrichment level lower than that of the average enrichment over the whole fuel assembly. Therefore, as stated before, the average enrichment of the portion of the middle section above the level of the upper end of the effective fuel zone of the part length fuel rod is slightly smaller than that of the portion of the middle section below the level of the upper end of the effective fuel zone of the part length fuel rod. It is to be noted, however, the average enrichment of the portion of the middle section above the level of the upper end of the effective fuel zone of the part length fuel rod is greater than that of the lower section. The provision of the upper section which has the smallest average enrichment among the three sections of the enriched uranium region is intended to effectively lower the average enrichment over the entire fuel rod by making efficient use of a reactivity gain which is offered as a result of reduction in the burnable poison concentration in the upper section and also to improve the reactor shut down margin. That is to say, the neutron importance is smaller in the upper section and therearound than in other sections of the enriched uranium region, so that a greater reduction in the enrichment can be set in the upper section for a given reactivity gain than in other sections. This enables the total amount of uranium to be reduced. Furthermore, in the cold state of the reactor in which the reactor shut down margin is minimum, the neutron flux density is higher in the upper section than in other section. By setting the average enrichment in the upper section to a level lower than those in other sections of the enriched uranium region, it is possible to reduce the reactivity in the reactor core so as to improve the reactor shut down margin. In the fuel assembly in accordance with the present invention, the difference in the average enrichment between the middle section and the lower section is smaller than that between the middle section and the upper section. More specifically, the difference in the average enrichment between the portion of the middle section below the level of the upper end of the effective fuel zone of the part length fuel rod and the lower section is smaller than the difference in the average enrichment between the portion of the middle section above the level of the upper end of the effective fuel zone of the part length fuel rod and the upper section of the enriched uranium region. Thus, the fuel assembly in accordance with the present invention has such an average enrichment distribution that the average enrichment is high in the lower section as compared with the fuel assemblies shown in FIGS. 1 and 2 of Japanese Patent Publication No. 3-78954 in which the average enrichment in the lower section is equal to that in the upper section. The fuel assembly of the present invention has the 9.times.9 matrix arrangement of fuel rods, thus exhibiting greater thermal margin than the conventional 8.times.8 fuel assemblies. In the fuel assembly of the present invention, the increase in the axial power peaking within this surplus thermal margin is achieved by increasing the average enrichment in the lower section as compared with that in the conventional 8.times.8 fuel assembly. This difference will be clear also from the fact that the difference in the average enrichment between the middle section and the lower section in the fuel assembly of an embodiment of the invention, e.g., 0.16% as shown in FIGS. 3 and 4, is smaller than those of the known fuel assemblies disclosed in Japanese Patent Publication No. 3-78954 and U.S. Pat. No. 5,198,186 (in both cases, the difference is 0.20% in the fuel assemblies shown in FIGS. 1 and 2). Thus, the fuel assembly embodying the present invention offers a greater fuel economy over the known fuel assemblies. The lower section of the enriched uranium region inherently has a low void fraction and, hence, a large effect of neutron moderation by coolant. The increase in the content of the fissile material in this lower section provides an increase in the axial peaking of the power. Thus, the axial power distribution of the fuel assembly is such that a slight peak of the power appears in the lower section. The increase in the average enrichment in the lower section where the power peak exists serves to increase the neutrons utilization factor and, hence, an increase in the reactivity in this section, thus contributing to improvement in the fuel economy. The increase in the neutron utilization factor means that a comparatively greater power is obtained with a comparatively low amount of fissile material, which enables the average enrichment over the whole fuel assembly to be reduced, thus offering saving of uranium. Since the axial power distribution in the beginning period of the operation cycle is such that a power peak appears in the lower section, void fraction is increased in the upper part of the fuel assembly so as to cause accumulation of plutonium in upper part of the fuel assembly in the beginning of the operation cycle. In later half part of the operation cycle, the accumulated plutonium is burnt so that the axial power peak appears in the upper part of the fuel assembly. Thus, a greater spectrum shift effect is produced to improve the fuel economy. The above-described axial distribution of enrichment in the fuel assembly embodying the present invention is achieved by the use of the large-diameter water rods and part length fuel rods. Thus, the invention affords a greater fuel exposure with a reduced increment in the average enrichment. In the fuel assembly embodying the present invention, the degree of flatting of the axial power distribution is slightly inferior to those shown in Japanese Patent Publication No. 3-78954 and U.S. Pat. No. 5,198,186, because of the slight increase in the axial power peak appearing in the lower section due to the relatively large average enrichment in the lower section. Nevertheless, the required level of flattening of the axial power distribution is obtained thanks to the large average enrichment in the middle section. (4) Radial distribution of enrichment over cross-section of fuel assembly The neutron flux density distribution in a fuel assembly tends to be such that the neutron flux density is comparatively high in the portions adjacent the channel box due to presence of water gap around the channel box. In the fuel assembly embodying the present invention, fuel rods having comparatively high levels of average enrichment are disposed to face the channel box so as to increase the rate of utilization of neutrons and, hence, the reactivity of the whole fuel assembly. Thus, in the fuel assembly embodying the present invention, fuel rods having enrichment levels higher than that of the whole fuel assembly are arranged to form the outermost layer of fuel rods in the cross-section of the fuel assembly. (5) Axial distribution of burnable poison In general, some of fuel rods incorporated in a fuel assembly for a BWR contain burnable poison in order to adjust the surplus reactivity in the reactor core. In the upper part of the reactor core, void fraction is high and the neutron spectrum is hard, so that burning of the burnable poison takes place rather slowly. In the fuel assembly embodying the present invention, as shown in FIG. 1B, the concentration of the burnable poison is low in the upper part (upper section) so as to suppress any loss of reactivity due to presence of residual unburnt burnable poison. FIG. 9 shows the relationship between the axial length of the upper section where the concentration of the burnable poison is low and the rate of improvement in the fuel economy. The fuel economy can be increased as the axial length of the upper section is increased. This effect, however, is progressively saturated and, as this length exceeds three nodes, the rise in the fuel economy becomes dull. In one design form of the fuel assembly embodying the present invention, therefore, the axial length of the upper section is determined to be 2 nodes or so. As shown in FIG. 1B, in the fuel assembly embodying the present invention, the enriched uranium region has three sections having different levels of concentration of burnable poison. The natural uranium regions do not contain burnable poison. The average density level of the burnable poison is lowest in the lower section, medium in the middle section and highest in the upper section. These three sections coincide with the lower section, middle section and the upper section, respectively, of the enriched uranium region discussed before in paragraph (3) in regard to the axial enrichment distribution. The fuel assembly embodying the present invention exhibit at comparatively low levels of output at the natural uranium regions in the upper and lower parts thereof, due to leakage of neurons. In this fuel assembly, since the natural-uranium regions constituting the upper and ;lower end regions do not contain burnable poison, loss of reactivity due to presence of residual burnable poison can be suppressed in these upper and lower end regions. Thus, in the fuel assembly embodying the present invention, the concentration of the burnable poison is set to a high level in the lower section as compared with the middle section, in order to minimize the influence of the burning of the burnable poison on the axial power distribution due to difference in the void fraction along the axis of the fuel assembly, whereby the axial power distribution is rendered uniform. This suppresses increase in the power peaking caused by increase in the mismatch among fuel assemblies caused by increase in the fuel burn-up degree. The resultant margin for the power peaking can be used for creating other peaking which are intended to improve the fuel economy. According to the axial distribution of the burnable poison concentration, the burnable poison can be substantially fully burnt away by the end of the operation cycle. II. Nuclear reactor A description will now be given of the outline of a nuclear reactor of the invention having a reactor core loaded with the fuel assemblies embodying the present invention. As is the case of the fuel assembly embodying the present invention, the reactor core of the reactor in accordance with the present invention also is designed for higher fuel economy. (1) Loading of high-burn-up fuel in the radially outermost region in the core In the nuclear reactor embodying the present invention, fuel assemblies in which fuel has been burnt to a comparatively high degree of burn-up are disposed in the radially outermost region thereof, whereas, new fuel assemblies and fuel assemblies in which the fuel burn-up degrees are still low are charged in the central region of the reactor core. Thus, the amount of fissile material, e.g., U-235, can be increased in the central region where the neutron flux density is comparatively high. This effect, in combination with the effect to suppress the leakage of neutrons to the exterior of the reactor core, makes it possible to enhance the reactivity in the reactor core. Relationship between the fuel economy and the power peaking presented by a variety of patterns of loading of fuel assemblies in the radially outermost region in the core is shown in FIG. 11. (2) TWO different types of fuel assemblies having different levels of burnable poison concentration The core of the nuclear reactor embodying the present invention employs two types of fuel assemblies having different amounts of burnable poison per assembly. It is possible to adjust the reactivity in the reactor core by varying the ratio between the numbers of these to types of fuel assemblies in such a manner as to make a good use of the difference in the reactivities between two types of fuel assemblies exhibited until the burnable poison is burnt away. It is therefore possible to easily control the reactivity in the reactor core to cope with a demand for operating conditions such as the time period of operation of the nuclear reactor. The nuclear reactor of the present invention also permits an adequate control of the surplus reactivity by the control rods during operation of the reactor. At the same time, the number of the control rods employed in the running of the reactor can be minimized, requiring reduced renewal of the control rods. Gadolinia, which is one of burnable poisons, has a large thermal neutron absorption cross-sectional area. Most of the neutrons are absorbed by the surfaces of the gadolinia-containing fuel rods. It is therefore possible to control the rate of absorption of neutrons by changing the number of the gadolinia-containing fuel rods. At the same time, an effective period of absorption of neutrons by the gadolinia-containing fuel rods can be adjusted by changing the amount or concentration of gadolinia on such gadolinia-containing fuel rods. (3) Control cell In the reactor embodying the present invention, fuel assemblies of high burn-up, i.e., fuel assemblies in which the amounts of the fissile material are comparatively low, are used as the four fuel assemblies which surround each of the control rods which are inserted into the reactor core for the purpose of control of the reactor power. These four fuel assemblies together with the control rod form a control cell. The four fuel assemblies in each control cell have low levels of reactivity and, hence, low levels of assembly power. Therefore, even when the linear power generation ratio is increased due to extraction of the control rod after a long stay in the control cell, the level of the linear power generation ratio reached as a result of the extraction is still lower than the operational limit value. FIG. 10 illustrates the concept of the nuclear reactor embodying the present invention. Numeral 25 denotes fuel assemblies, 26 denotes fuel assemblies which have been burnt to certain degrees of burn-up and 27 denote control cells. An embodiment of the fuel assembly in accordance with the present invention will be described with reference to FIGS. 1 to 3. The fuel assembly of this embodiment, denoted by 16, is a 9.times.9 fuel assembly intended for use in a BWR. The fuel assembly 16 comprises upper and lower tie plates 18 and 19, a plurality of fuel rods 15 and a plurality of water rods 13, the fuel rods 15 and the water rods 13 being held at their upper and lower ends by the upper and lower tie plates 18, 19. A plurality of fuel spacers 20 arranged in the axial direction of the fuel assembly 16 hold the fuel rods so as to maintain the required spacing between adjacent fuel rods 15. Similarly, the water rods 13 are held and spaced from each other by the fuel spacers 20. The fuel rods 15 bundled by the fuel spacers 20 are surrounded by a channel box 12 which is secured to the upper tie plate 18. A channel fastener (not shown) is secured to the upper tie plate 18 at the same side as the control rod which is denoted by 14. Although not illustrated, the fuel rod 15 has a clad tube charged with a multiplicity of fuel pellets and closed at its upper and lower ends with plugs. The fuel pellet comprises UO2 as a fuel material and contains U-235 as the fissile material. Fuel pellets are compacted downward by the force of a spring charged in the gas plenum in the clad tube. There are two water rods 13 which are arranged in the central region of the cross-section of the fuel assembly substantially along a diagonal line of the cross-section. As stated in Paragraph 1-(1) before, these fuel rods occupies an area which can accommodate seven fuel rods. The outside diameter of the water rod 13 is greater than the pitch at which the fuel rods 15 are arranged, and the diameter of each water rod is so large as to occupy an area which can accommodate three-and-a-half fuel rods. The diameter of the water rod, however, is reduced at a lower end portion of the water rod below the lowermost fuel spacer 20, in order to prevent bending stress in the lower end portion of the water rod from becoming excessively large in the event of, for example, earthquake. Each water rod is hollow to define a passage for non-boiling water. By using these water rods 13, the water-to-fuel ratio is increased as compared with the conventional fuel assemblies, thereby preventing deterioration in the core characteristics while improving reactivity. The control rod 14 has a cross-shape cross-section. One control rod 14 is used in combination with every four fuel assemblies. There are two types of reactor core: namely, a reactor core referred to as "D lattice core" in which the width of the water gap formed on the side wall of the fuel assembly facing the inserted control rod is greater than the width of the water gap on the side wall of the fuel assembly opposite to the control rod; and a reactor core referred to as "C lattice core" in which the width of the water gap formed on the side wall of the fuel assembly facing the inserted control rod is equal to the width of the water gap on the side wall of the fuel assembly opposite to the control rod. The illustrated fuel assembly 16 is intended to be loaded in a C lattice core. The fuel assembly includes eight types of fuel rods 1 to 8 as indicated in FIG. 4. These fuel rods 1 to 8 are arranged in the cross-section of the fuel assembly within the channel box 12 in a manner shown in FIG. 3. The fuel rods indicated at 6 are part length fuel rods while other fuel rods are full length fuel rods. Each of the full length fuel rods designated at 1 to 5, 7 and 8 has regions charged with natural uranium, i.e., natural uranium blanket regions, at the upper and lower end regions of its effective fuel zone. The axial overall length of the effective fuel zone of each such long fuel rod is equal to the effective fuel length H mentioned before. The upper natural uranium blanket region extends downward from the top end of the effective fuel zone over a length of 2/24 the effective fuel length H, while the lower natural uranium blanket region extends upward from the bottom end of the effective fuel zone over a length corresponding to 1/24 the effective fuel length. Thus, the axial lengths of the natural uranium blanket regions in the illustrated embodiment provides the combination (2/1) of the axial lengths which, as explained before in connection with FIG. 8, provides optimum effect in improvement in the fuel economy. The part length fuel rod 6 does not have any natural uranium blanket region. The effective fuel zone of the part length fuel rod 6 is within the range of from 1/24 to 15/24 of the effective fuel length H as measured from the bottom end of the effective fuel zone of the full length fuel rod. In each of the full length fuel rods 1 to 5, 7 and 8, the region between 1/24 and 22/24 of the effective fuel length H as measured from the lower end of the effective fuel zone constitutes an enriched uranium region which is charged with fuel enriched in uranium. In each of the full length fuel rods 1, 3 to 5, 7 and 8, the enrichment is uniform over the entire axial length of the enriched uranium region, as shown in FIG. 4, whereas in the full length fuel rod 2,the enriched uranium region has three sections having different levels of enrichment, as shown in the same Figure. The enriched uranium region of the part length fuel rod has a uniform distribution of enrichment over the entire axial length thereof. The enrichment levels in the enriched uranium regions are: 4.8 wt % in the fuel rod 1, 3.9 wt % in the fuel rods 3, 7 and 8, 3.4 wt % in the fuel rod 4, 2.2 wt % in the fuel rod 5 and 4.3 wt % in the fuel rod 6. In the enriched uranium region of the fuel rod 2, the lower section extending from a point of 1/24 to a point of 8/24 of the effective fuel length H as measured from the bottom of the effective fuel zone has an enrichment of 4.3 wt %, the middle section between the point of 8/24 to the point of 20/24 has an enrichment of 4.8 wt %, and the upper section extending between the point of 20/24 to the point of 22/24 has enrichment of 3.9 wt %. Each of the fuel rods 7 and 8 contains gadolinia as the burnable poison, in its fuel pellets which are disposed in the enriched uranium region. The fuel rod 7 has such an axial gadolinia concentration distribution that the gadolinia content is 5.5 wt % in the section which extends from a point of 1/24 to the point of 8/24 of the effective fuel length H as measured from the bottom end of the effective fuel zone, 4.5 wt % in the section extending between the point of 8/24 and the point of 20/24 and 3.5 wt % in the section which extends from the point of 20/24 to the point of 22/24 . The fuel rod 8 has such an axial gadolinia concentration distribution that the gadolinia content is 4.5 wt % in the section which extends from a point of 1/24 to the point of 20/24 of the effective fuel length H as measured from the bottom end of the effective fuel zone and 3.5 wt % in the section extending between the point of 20/24 and the point of 22/24. Fuel rods 1 to 6 do not contain gadolinia. The fuel rods 1 to 8 having different axial enrichment distributions as described are arranged in the manner shown in FIG. 3, so that the whole fuel assembly 16 exhibits such an axial distribution of average enrichments across the cross-section that the average enrichment is 4.00 wt % in the lower section extending between a point of 1/24 to the point of 8/24 of the effective fuel length H as measured from the bottom end of the effective fuel zone of the fuel assembly 16, 4.16 wt % in the middle section which is between the point of 8/24 and the point of 20/24 and 3.82 wt % in the upper section which extends from the point of 20/24 to the point of 22/24. The middle section is divided into two portions: namely, a portion which is above the level of the top end of the effective fuel zone of the part length fuel rod 6 and a portion which is below the same. These portions therefore will be referred to as an upper middle section and a lower middle section, respectively. In the illustrated embodiment, the upper middle section has an average enrichment of 4.15 wt % across the cross-section of the fuel assembly, while the lower middle section has an average enrichment of 4.16 %. The value of 4.16 wt % mentioned above as the value of the average enrichment of the whole middle section has been obtained by rounding the value of the third decimal place. The average enrichment across the cross-section of the fuel assembly in each of the upper and lower natural uranium blanket regions is 0.71 wt %. The average enrichment over the entire fuel assembly 16 is 3.70 wt %. The fuel assembly having the described axial enrichment distribution provides the advantages discussed before in Paragraph 1-(3). In this embodiment, although the average enrichment is increased, the water-to-fuel ratio can adequately be increased by virtue of use of the pair of fuel rods, this realizing the advantage stated in the foregoing Paragraph 1-(1). Furthermore, the described embodiment employs only one type of such a kind of fuel rod that has upper, middle and lower sections having different levels of enrichment in the enriched uranium region, so that the number of the types of fuel rods is reduced to realize a simple reactor core construction while facilitating production and preparation of the fuel assemblies. In BWR, the void fraction is increased towards the upper end of the reactor core, so that the density of the coolant (water) as the moderator is smaller in the upper part of the reactor core. Therefore, loading of fuel assemblies having uniform axial enrichment distribution in the reactor core tends to crate such an axial power distribution that a peak of the power appears in a lower part of the fuel assemblies. This tendency is enhanced due to the use of the part length fuel rods in the fuel assembly 16 because the part length fuel rods provides a greater amount of charging of the fuel material in the lower part of the fuel assembly than in the upper part of the same. In order to overcome this problem, in the present invention, the enrichment is set to a higher level in the middle section than in the lower section, so as to flatten or uniformalize the axial distribution of the power. The aforesaid two conditions or requirements, namely, the difference in the average enrichment between the mid section and the lower section being 0.16 wt % and the boundary between the middle and lower sections being at the point of 8/24 of the effective fuel length H from the bottom of the effective fuel zone have been determined to provide an effect of flattening or uniformalizing the axial power distribution to a level which meet the design requirements. The fuel assembly 16 has fourteen gadolinia-containing fuel rods. In such fuel rods, the gadolinia concentration is lowest in the lower section, medium in the middle section and highest in the upper section. The difference in the gadolinia content between the lower and middle sections is about 0.5 wt % as a mean. As stated before, in a BWR, the void fraction increases towards the upper end of the reactor core so that the density of the coolant as the moderator is lower in the upper part of the reactor core than in the lower part of the same. Therefore, the neutron spectrum is softer in the lower part of the reactor core than in the upper part, so that the burning of gadolinia proceeds more rapidly in the lower part than in the upper part of the reactor core. This tends to accelerate the rise of the reactivity in the lower part of the reactor core, promoting the power peaking in this part of the core. In the described embodiment of the invention, therefore, the gadolinia concentration i increased in the lower section so as to achieve an adequate control of the rate of rise of the reactivity. The range of the difference in the gadolinia concentration has been determined so as to optimize the effect of flattening of the axial power distribution. The described embodiment employs two types of gadolinia-containing fuel rods, so as to realize a difference in the gadolinia content of about 1 wt % or greater within the same fuel rod, in order to meet a requirement from the view point of fabrication of the fuel. In the fuel assembly 16, the average enrichment across the cross-section is lower in the upper section of the enriched uranium region than in the middle and lower sections of the same. The upper section of the lowest average enrichment corresponds to the regions in the fuel rods 7 and 8 where the concentrations of the burnable poison are low, thus compensating for the reduction in the reactor shut down margin. The aforesaid axial length of the upper section, i.e., 3/24 the effective fuel length H, has been determined to optimize the effect of improving fuel economy produced, as will be described later, by a reduction in the amount of the burnable poison. As stated before, the fuel assembly 16 has fourteen gadolinia-containing fuel rods. In each of the fuel rods 7 and 8 in this fuel assembly 16, the gadolinia concentration per unit axial length is so valid along the axis that it is higher in the upper section of the enriched uranium region than in other underlying, i.e., the middle and lower sections, of the enriched uranium region, whereby improved fuel economy is achieved as shown in FIG. 9. The point indicated as "the point in accordance with the invention" shows the effect of improvement in the fuel economy as achieved by the reduction in the amount of gadolinia in the fuel assembly 16. The fuel assembly has, in the radially outermost region of its cross-section, fuel rods 2 and 3 having regions of enrichment levels higher than the average enrichment over the whole fuel assembly. This arrangement offers the advantage of reactivity gain as stated before in the foregoing Paragraph 1-(4). The fuel assembly 16 as described offers a remarkable improvement in the fuel economy. More specifically, the fuel assembly makes it possible to enhance the exposure, e.g., to a level of 45 GWd/t in terms of discharge exposure, with minimal increment of the average enrichment, while preserving sufficiently large values of the thermal margin in the reactor core and the reactor shut down margin. This remarkably reduced the fuel cycle cost, as well as the demand for disposal of used fuel assemblies. Another embodiment of the fuel assembly in accordance with the present invention will be described with reference to FIGS. 5 and 6. This embodiment of the fuel assembly, denoted by 16A, is basically the same in the construction as the fuel assembly 16, and is intended for use in a C lattice core as is the case of the fuel assembly 16. The types of the fuel rods 1 to 6 and 8 used in this fuel assembly 16A are the same as those in the fuel assembly 16, but the fuel assembly 16A employs a fuel rod 7A in place of the fuel rod 7 in the fuel assembly 16. The fuel rod 7A has a uniform axial enrichment distribution throughout its enriched uranium region. Namely, the enrichment is set substantially constant to a value of 3.9 wt % throughout the enriched uranium region. However, the gadolinia concentration in the enriched uranium region of this fuel rod 7A is so varied along the axis that the gadolinia concentration is 4.5 wt % within the range of from 1/24 to 8/24 of the effective fuel length H as measured from the bottom of the effective fuel zone and 3.5 wt % in the range of from 8/24 to 22/24 of the effective fuel length H. The axial enrichment distribution in the fuel assembly 16A is the same as that in the fuel assembly 16. The fuel assembly 16A has sixteen gadolinia-containing fuel rods. The average gadolinia concentration of the 16 gadolinia-containing fuel rods is so valid along the axis that the average concentration is lowest in the lower section, medium in the middle section and highest in the upper section. A difference in the average gadolinia concentration of about 0.5 wt % is set between the lower section and the middle section. The fuel assembly 16A offers the same advantages as those presented by the first embodiment 16 of the fuel assembly. The fuel assembly has fourteen gadolinia-containing fuel rods, while the fuel assembly 16A has sixteen. In each of these fuel assemblies, a difference in the gadolinia concentration is posed between the middle and lower section of the enriched uranium region: more specifically, the gadolinia concentrations are 4.5 wt % and 5.5 wt %, respectively, in the middle and lower sections of the fuel assembly 16, and 3.5 wt % and 4.5 wt %, respectively, in those sections in the fuel assembly 16A. By loading these two types of fuel assemblies 16, 16A in the core of the reactor, it is possible to adequately control the reactivity in the core to cope with demand for variation in the operating condition such as a change in the time period of operation and to obtain reactor core characteristics with sufficient safety margin. A boiling water reactor as an embodiment of the nuclear reactor of the invention will be described with reference to the drawings. The boiling water reactor has a reactor core 30 as shown in FIG. 7. The core 30 is of the C lattice type. Only a quarter part of the cross-section of the core is shown in FIG. 7. There are many squares shown in FIG. 7 each of which represents a unitary fuel assembly. Numerals 1 to 5 allocated to the squares show the periods of stay of the fuel assemblies in the BWR reactor core. Thus, the fuel assemblies indicated at numeral 1 are going to experience the first cycle of operation, while the fuel assemblies indicated at 2 are going to experience the second cycle of operation. Similarly, fuel assemblies indicated at numerals 3 and 4 are going to be subjected to the third and fourth cycles of operation. The term "operation cycle" or "fuel cycle" is used to mean a predetermined period between the startup of the BWR after a shuffling or renewal of the fuel assemblies and the shutoff of the BWR for the next shuffling or renewal of the fuel assemblies. Thus, the fuel assemblies which have experienced greater numbers of cycles have greater degrees of burn-up. The fuel assemblies 31, represented by squares indicated at "1", are fresh fuel assemblies. The fuel assemblies 16 having fewer gadolinia-containing fuel rods are used as these fresh fuel assemblies 31. The fuel assemblies 32, represented by squares indicated at "(1)", are also fresh fuel assemblies. The fuel assemblies 16A having greater number of gadolinia-containing fuel rods than the fuel assemblies 16 are used as these fresh fuel assemblies 32. The fuel assemblies 33 to 36 have experienced more than one cycle of operation and, hence, gadolinia in their fuel assemblies have been consumed away. Some of the fuel assemblies 35 are used as the fuel assemblies which, together with a control rod surrounded by them, form a control cell 37. The control cells 37 are provided for the purpose of facilitating the operations of the control rods which are conducted to control the power and the reactivity in the BWR during the operation of the latter. The fuel assemblies 36 which have been burnt up to a high degree and, hence, having low content of U-235, are disposed in the radially outermost region of the cross-section of the reactor core 30, whereas the fuel assemblies 31 And 32 which have high contents of U-235 are disposed in the radially central region of the cross-section of the reactor core. This type of reactor core 30 offers the advantage discussed before in Paragraph 2-(2). FIG. 11 shows the relationship between the proportion of the number of the fuel assemblies 36 of high-burn-up arranged in the radially outermost region of the cross-section of the reactor core and the extent of improvement achieved in the fuel economy. More specifically, in FIG. 11, a point "a" provides, as a reference, the level of the fuel economy as obtained when the reactor core 30 is fully loaded with the fuel assemblies 36 which are going to experience the fifth cycle, while a point "c" shows the level of the fuel economy as attained when the fuel assemblies are positioned only in the radially outermost region of the cross-section of the reactor core. A point "b" indicates an intermediate case, i.e., the case where the proportion of the number of the fuel assemblies 36 is between the cases indicated by the points "a" and "c". It will be seen that the rate of increase in the power peaking increases, but the rate of improvement in the fuel economy decreases, as the proportion of the number of the fuel assemblies disposed in the radially outermost region is increased. Therefore, in the illustrated embodiment of the invention, the radially outermost layer of the fuel assemblies in the cross-section of the reactor core 30 is constituted by the fuel assemblies 36, i.e., in conformity with the case "c" described above. In the illustrated BWR embodying the present invention, the advantages stated before in paragraph 2-(2) are achieved by virtue of the fact that the fuel assemblies 36 which have comparatively high degrees of burn-up are disposed near the outer peripheral portion of the reactor core 30. That is, an adaptability to change in the operation period of the BWR is enhanced and a reactivity gain is obtained. As will be understood from the foregoing description, according to the present invention, it is possible to enhance the exposure of the fuel assemblies with minimal increment of the average enrichment, while preserving sufficiently large values of safety margins such as the thermal margin of the reactor and the reactor shut down margin. This remarkably increases the energy extractable per fuel assembly, leading to a remarkable improvement in the fuel economy. |
abstract | A fast reactor including a reactivity control assembly including a reactor shutdown rod and neutron absorbers, a reactor shutdown rod drive mechanism, and units of neutron absorber drive mechanism. The reactor shutdown rod drive mechanism causes an inner extension tube to fall and release the reactor shutdown rod by a gripper section by turning off the power supply to a holding magnet at the time of scram. When grasping the neutron absorbers, an outer extension shaft is pulled up to allow both of the extension shafts to be inserted. After the outer extension tube gets to a handling head section of the neutron absorber, the outer extension shaft is pushed down to grasp the neutron absorber externally by latch fingers of the gripper section so that the neutron absorber can be moved up and down. |
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summary | ||
description | This application is a divisional of application Ser. No. 13/885,555, filed Sep. 5, 2014, now allowed, which is a National Stage of PCT/SE2011/051340, filed Nov. 9, 2011 and claims priority to Swedish Patent Application No. 1051277-0, filed Dec. 2, 2010, the disclosures of all of which are incorporated herein by reference in their entirety. The present invention refers to an electron exit window foil. More particularly, the present invention relates to an electron exit window foil for use in a corrosive environment and operating at a high performance. Electron beam devices may be used to irradiate objects with electrons, e.g. for surface treatment. Such devices are commonly used within the food packaging industry, where electron beams are providing efficient sterilization of packages, e.g. plastic bottles or packaging material to be later converted into a package. A main advantage with electron beam sterilization is that wet chemistry, using e.g. H2O2, may be avoided thus reducing the high number of components and equipment required for such wet environments. An electron beam device typically comprises a filament connected to a power supply, wherein the filament is emitting electrons. The filament is preferably arranged in high vacuum for increasing the mean free path of the emitted electrons and an accelerator is directing the emitted electrons towards an exit window. The electron exit window is provided for allowing the electrons to escape from the electron beam generator so they may travel outside the electron beam generator and thus collide with the object to be sterilized and release its energy at the surface of the object. The electron exit window typically consists of thin electron permeable foil that is sealed against the electron beam generator for maintaining the vacuum inside the electron beam generator. A cooled support plate in the form of a grid is further provided for preventing the foil to collapse due to the high vacuum. Ti is commonly used as the foil material due to its reasonable good match between high melting point and electron permeability, as well as the ability to provide thin films. A problem with a Ti film is that it may oxidize, leading to reduced lifetime and operational stability. In order to achieve a long lifetime of the exit window, a maximum temperature of approximately 250 C should not be exceeded during the operation of the electron beam device. Typically, a high performance electron beam device is designed to provide 22 kGy at up to 100 m/min at 80 keV when used for sterilizing a running web of material. A plain Ti foil may thus not be used with such high performance electron beam devices, since the amount of emitted electrons transmitted through the window may cause temperatures well above this critical value. In filling machines, i.e. machines designed to form, fill, and seal packages, sterilization is a crucial process not only for the packages, but for the machine itself. During such machine sterilization, which preferably is performed during start-up, the outside of the exit window will be exposed to the chemicals used for machine sterilization. A highly corrosive substance such as H2O2, which is commonly used for such applications, will affect the exit window by means of etching the Ti. Different solutions for improving the properties of the exit window have been proposed to overcome the above-mentioned drawbacks. EP0480732B describes a window exit foil consisting of a Ti foil, and a protective layer of Al that is forming an intermetallic compound by thermal diffusion treatment of the Ti/Al construction. This solution may be suitable for relatively thick exit windows, i.e. windows allowing a protective layer being thicker than 1 micron. However, an intermetallic compound is not acceptable on a thin Ti foil since it would reduce its physical strength. EP0622979A discloses a window exit foil consisting of a Ti foil and a protective layer of silicon oxide on the side of the exit foil facing the object to be irradiated. Although the Ti foil may be protected by such layer, silicon oxide is very brittle and may easily crack in the areas where the foil is allowed to flex, i.e. the areas between the grids of the supportive plate when vacuum is provided. This drawback is making the foil of EP0622979A unsuitable for applications where the exit foil is exhibiting local curvatures, such as electron beam devices using a grid-like cooling plate arranged in contact with the exit foil. An object of the present invention is to reduce or eliminate the above-mentioned drawbacks. A further object is to provide an electron exit foil that is able to decrease the heat load as well as the corrosion on the foil. An idea of the present invention is thus to provide an electron beam generator having a prolonged operating lifetime, requiring a reduced service, and being more cost-effective than prior art systems due to inexpensive coating processes and the appliance of well-established X-ray manufacturing processes. According to a first aspect of the invention, an electron exit window foil for use with a high performance electron beam generator operating in a corrosive environment is provided. The electron exit window foil comprises a sandwich structure having a film of Ti, a first layer of a material having a higher thermal conductivity than Ti, and a flexible second layer of a material being able to protect said film from said corrosive environment, wherein the second layer is facing the corrosive environment. The first layer may be arranged between the film and the second layer, or the film may be arranged between the first layer and the second layer. The second layer may comprise at least two layers of different materials, which is advantageous in that different mechanical and/or physical properties of the foil, such as erosion resistance and strength, may be tailor made for the particular application. The first layer may be selected from a group consisting of materials having a ratio between thermal conductivity and density being higher than of Ti. The first layer may be selected from the group consisting of Al, Cu, Ag, Au, or Mo, and the second layer may be selected from the group consisting of Al2O3, Zr, Ta, or Nb. The corrosive environment may comprise H2O2. Hence, the foil may be implemented in electron beam devices operating in machines being subject to corrosive sterilizing agents, such as for example filling machines within the food packaging industry. The electron exit window foil may further comprise at least one adhesive coating between the Ti film and first layer or the second layer. Said adhesive coating may be a layer of Al2O3 or ZrO2 having a thickness between 1 and 150 nm. This is advantageous in that any reaction or material diffusion is prevented at the film/layer interface or the adhesion between the Ti film and a layer or between two layers is improved. According to a second aspect, an electron beam generator configured to operate in a corrosive environment is provided. The electron beam generator comprises a body housing and protecting an assembly generating and shaping the electron beam, and a support carrying components relating to the output of the electron beam, said support comprising an electron exit window foil according to the first aspect of the invention. The advantages of the first aspect are also applicable for the second aspect of the invention. According to a third aspect of the invention, a method for providing an electron exit window foil for use with a high performance electron beam generator operating in a corrosive environment is provided. The method comprises the steps of providing a film of Ti, providing a first layer of a material having a higher thermal conductivity than Ti onto a first side of said film, and providing a flexible second layer of a material being able to protect said film from said corrosive environment, wherein the second layer is facing the corrosive environment. The step of providing a flexible second layer may comprise arranging said flexible second layer onto a second side of said film. The step of providing a flexible second layer may comprise arranging said flexible second layer onto said first layer. At least one of steps of providing a first layer or providing a flexible second layer may be preceded by a step of providing an adhesive coating onto said film. According to a fourth aspect of the invention, a method for providing a high performance electron beam device is provided. The method comprises the steps of attaching a film of Ti onto a frame, processing said film by providing a first layer of a material having a higher thermal conductivity than Ti onto a first side of said film, and providing a flexible second layer of a material being able to protect said film from said corrosive environment, wherein the second layer is facing the corrosive environment, and attaching said foil-frame subassembly to a tube housing of an electron beam device for sealing said electron beam device. The advantages of the first aspect of the invention are also applicable for the third and the fourth aspects of the invention. With reference to FIG. 1 an electron beam device is shown. The electron beam device 100 comprises two parts; a tube body 102 housing and protecting an assembly 103 generating and shaping the electron beam, and a supportive flange 104 carrying components relating to the output of the electron beam, such as a window foil 106 and a foil support plate 108 preventing the window foil 106 from collapsing as vacuum is established inside the device 100. Further, during operation of the electron beam device the foil 106 is subject to excessive heat. Thereby, the foil support plate 108 also serves the important purpose of conducting heat generated in the foil 106 during use away from the foil 106 of the device. By keeping the foil temperature moderate a sufficiently long lifetime of the foil 106 may be obtained. With reference to FIG. 2, an electron exit window is shown comprising the foil 106 and the foil support 108. The support 108 is arranged inside the electron beam device such that vacuum is maintained on the inside of the exit window. This is indicated by P1 and P2 in FIG. 2, where P1 denotes atmospheric pressure outside the exit window and P2 represents vacuum on the inside. During manufacturing, the foil support plate 108, being of copper, is preferably attached to the flange 104 forming a part of the tube body 102. The flange 104 is generally made of stainless steel. The window foil 106 is then bonded onto a separate frame thus forming a foil-frame sub assembly. The foil 106 is subsequently coated, in order to improve its properties regarding for instance heat transfer. The foil-frame subassembly is subsequently attached to the tube body 102 to form a sealed housing. In an alternative embodiment, the exit window foil 106 is attached directly to the flange, being attached to the support plate, before the flange is welded to the tube body. In this embodiment, the exit window foil is consequently coated prior to being attached to the tube body 102. With reference to FIG. 3a-f, different embodiments of an electron exit window foil 106a-f are shown. Starting with FIG. 3a, the foil 106a comprises a thin film of Ti 202. The Ti film 202 has a thickness of approximately 5 to 15 microns. On a first side of the Ti film 202, a thermally conductive layer 204 is arranged. The thermally conductive layer 204 is provided in order to transfer heat along the exit foil such that a reduced temperature is achieved across the entire foil 106. The thermally conductive layer 204 is provided by means of any suitable process, such as sputtering, thermal evaporation, etc, and should allow for a sufficient improvement in thermal conductivity for lowering the temperature of the electron exit window foil 106a while still allowing the foil to bend into the apertures of the support plate 108 when vacuum is applied. Preferably, the material of the thermally conductive layer 204 is chosen from the group consisting of Al, Cu, Ag, Au, and Mo. Although other materials, such as Be, may have a higher ratio between thermal conductivity and density they are considered as poisonous and hence not preferred, especially in applications in which the electron beam device is arranged to process consumer goods. On the other side of the Ti film a protective layer 206 is arranged. The protective layer 206 is provided by means of any suitable coating process, such as sputtering, thermal evaporation, etc. Preferably, the material of the protective layer is chosen from the group consisting of Al2O3, Zr, Ta, and Nb due to their resistance against hydrogen peroxide containing environments. It should thus be understood that the protective layer 206 is facing the atmospheric environment, i.e. the objects to be sterilized. The thickness of the thermally conductive layer 204 is preferably between 1 and 5 microns and the thickness of the protective layer 206 is substantially less than 1 micron. Preferably, the thickness of the protective layer 206 is approximately 200 nm. By keeping the window foil 106 as thin as possible, the electron output is maximized. The thickness of the protective layer 206 should thus be designed such that it is capable of protecting the Ti film from a) corrosion by hydrogen peroxide or other aggressive chemical agents which may be provided in the particular application, and b) corrosion caused by the plasma created by the electrons in the air. Further, the thickness of the protective layer 206 should ensure tightness and physical strength, such that the second layer 206 is flexible in order to allow the entire foil to bend and conform to the apertures of the support plate 108 when vacuum is applied. A yet further parameter may be the density, for allowing electron transmittance through the protective layer 206. By arranging the thermally conductive layer 204 and the protective layer 206 on opposite sides of the Ti foil, stress in the layers may be reduced. For example, if using Al as the thermally conductive layer and Zr as the protective layer, the Ti foil arranged in between those layers will reduce some of the stress induced upon heating. This is due to the fact that the coefficient of thermal expansion of Ti lies between the corresponding value of Al and Zr. FIG. 3b shows another embodiment of a foil 106b. Here, the thermally conductive layer 204 and the protective layer 206 are provided on the same side of the Ti film 202 such that the protective layer 206 is coated directly on the thermally conductive layer 204. This structure may be advantageous for electron beam devices, for which the electron exit window foil must be mounted to the tube housing before coating, i.e. not allowing coating of the side of the Ti foil facing the interior of the electron beam device. FIGS. 3c and 3d show two different embodiments similar to what has been previously described with reference to FIGS. 3a and b. However, in FIGS. 3c and 3d the protective layer 206 comprises at least two layers of different materials; a first layer 208 and a second layer 209. The first layer 208 and the second layer 209 of the protective layer 206 are both selected from the group consisting of Al2O3, Zr, Ta, and Nb, or alloys thereof. It should however be understood that each one of the layers 208, 209 could per se be a sandwich of two or more protective layers. For example, the corrosion protection layer 206 itself could be a multilayer structure comprising an oxide, a metal, an oxide, a metal, etc. According to a specific embodiment such multilayer structure may be formed by a first layer of ZrO2, a second layer of Zr, a third layer of ZrO2, and a fourth layer of Zr. This is advantageous in that a potential disruption in one of the sub layers does not induce a significant reduction of the overall corrosion protection of the protective layer 206. In order to achieve good adherence between the different layers/films of the electron exit window foil, adhesive barrier coatings may be provided at the interface. Such coatings may be a thin layer of Al2O3 or ZrO2, having a thickness between 1 to 150 nm, preferably between 50 and 100 nm. The use of such coatings is advantageous in that they prevent any reaction or diffusion of material at the interface between Ti and the thermally conductive layer and/or the protective layer. Reaction or diffusion may result in the formation of an intermetallic compound which negatively changes the characteristics of the materials involved. In the case of a thin Ti foil it may get reduced physical strength. Further, the presence of intermetallic compounds may reduce the thermal conductivity and the corrosion protective ability of the thermally conductive layer 204 and the protective layer 206 respectively. FIG. 3e describes a further embodiment similar to that of FIG. 3a but provided with barrier coatings of the kind described above. The electron exit window foil 106e comprises a sandwich structure having a film of Ti, a first layer 204 of Al having a higher thermal conductivity than Ti, and a flexible second layer 206 of Zr being able to protect said film 202 from a corrosive environment, wherein the second layer 206 is facing the corrosive environment. The thermally conductive layer 204 of aluminium (Al) is arranged on a first side of the titanium (Ti) foil. A first barrier coating 210a of zirconium oxide (ZrO2) is provided in between said Ti film 202 and said Al layer 204. On the other side of the Ti film 202 the protective layer 206 of zirconium (Zr) is arranged. A second barrier coating 210b of zirconium oxide (ZrO2) is provided in between the Ti film and the Zr layer 206. This embodiment is advantageous in that the Ti foil 202 is surrounded on one side by Al as the thermally conductive layer and on the other side by Zr as the protective layer. Since the coefficient of thermal expansion of Ti lies between the corresponding values of Al and Zr, some of the stress induced during heating of the foil will be reduced. As an alternative one or both of the barrier coatings 210a, 210b may instead be made of aluminium oxide (Al2O3). It is an advantage if the barrier coatings are based on a material provided in either the thermally conductive layer or in the protective layer. For example, if the protective layer is zirconium and the thermally conductive layer is aluminium, it is preferred that either aluminium oxide or zirconium oxide are used for the barrier coatings. This is due to the fact that the layers are applied by a sputtering machine. In a such machine sputter targets are used, one for each material that should be deposited. One and the same target can be used for both e.g. zirconium and zirconium oxide. The same applies for aluminium and aluminium oxide. Hence, it is preferred if the barrier coating is an oxide of a material used in either the corrosion protection or the thermal conductivity layer. FIG. 3f shows an embodiment similar to that of FIG. 3b but provided with barrier coatings. The electron exit window foil 106f comprises a sandwich structure having a film of Ti, a first layer 204 of Al having a higher thermal conductivity than Ti, and a flexible second layer 206 of Zr being able to protect said film 202 from a corrosive environment, wherein the second layer 206 is facing the corrosive environment. The thermally conductive layer 204 and the protective layer 206 are provided on the same side of the Ti film 202. On top of the Ti film 202 a first barrier coating 210a is coated. The barrier coating 210a is made of aluminium oxide (Al2O3). The thermally conductive layer 204 of aluminium (Al) is coated on said first barrier coating 210a. On the thermally conductive layer 204 there is in turn coated a second barrier coating 210b. Said barrier coating 210b is also made of aluminium oxide (Al2O3). Finally, the protective layer 206, being made of zirconium (Zr), is coated on said second barrier coating 210b. As an alternative one or both of the barrier coatings 210a, 210b may instead be made of zirconium oxide (ZrO2). In both the embodiments of FIGS. 3e and 3f the protective layer 206 may be a multilayer structure as described in relation to FIGS. 3c and 3d. The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. |
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description | The present invention relates to the delivery of reactor monoliths, and in particular to methods and related apparatus for the delivery of reactor monoliths into an interior space of a reactor. Reactor monoliths are utilized in a wide variety of applications including large scale trickle bed reactors for chemical and petroleum refining processes. These reactors typically include housings or structures of significantly large dimension, e.g., twelve feet in diameter and fifty feet in height, having a single opening allowing entry into the reactor. A catalyst is placed within the reactor in the form of beads, and the like, which is typically loaded into the interior of the reactor by extending a “sock” through the single opening and “blowing” the catalyst along the sock. A gravity forced loading procedure is also used. An operator located within the interior space of the reactor directs a free end of the sock to spread the catalyst about the reactor. Another known method for loading the catalyst into the reactor includes use of large cranes to lower wire baskets filled with the beads. As is known, preformed reactor monoliths 10 (FIG. 1) provide significant hydrodynamic advantages to blown in or dumped in beads. These reactor monoliths come in a wide variety of geometrical configurations and sizes, some of which allow the interlocking placement thereof, as best illustrated in FIG. 2. However, as many of these preformed monoliths are constructed of ceramics or other similarly fragile materials, it is difficult to use current methods of delivery to deliver the monoliths into an associated reactor without imparting significant damage thereto. A method/apparatus is desired that allows efficient and economical delivery of preformed reactor monoliths into the interior space of a reactor that simultaneously eliminates or reduces damage thereto. One aspect of the present invention is a method for delivering a plurality of reactor monoliths into an interior of a reactor that includes providing at least one tubular member having an outer wall defining an interior space adapted to allow sliding movement of a reactor monolith therethrough, and providing a plurality of engagement members located within the interior space of the at least one tubular member and spaced along a length thereof, wherein each engagement member is actuable between an extended position and a retracted position. The at least one tubular member is extended into an interior of a reactor such that a first end of the at least one tubular member is located near an opening in the reactor and a second end of the at least one tubular member is inserted into the interior of the reactor. The plurality of engagement members is controlled such that each engagement member actuates between the extended position, thereby preventing sliding movement of the monolith through the tube, and a retracted position, thereby allowing the monolith to slide past the engagement member within the tube. Another aspect of the present invention is an apparatus for delivery of reactor monoliths into an interior space of a reactor that includes at least one tubular member having an outer wall defining an interior space adapted to allow sliding movement of a reactor monolith therethrough, a first end adapted to be located near an opening in a reactor, and a second end adapted to be inserted into an interior space of the reactor. A plurality of engagement members are located within the interior space of the at least one tubular member and spaced along a length thereof, each engagement member being actuable between an extended position, wherein the engagement member extends into the interior space and is adapted to prevent sliding movement of the monolith through the tube, and a retracted position, wherein the engagement member is retracted towards the outer wall, thereby allowing the monolith to slide past the engagement member within the tube. A controller is operably coupled to the engagement members for controlling the members in a sequential manner, thereby allowing the monolith to be slowly moved along the length of the tube and preventing a continuous free-fall descent. Yet another aspect of the present invention is a method of packaging reactor monoliths to facilitate the delivery thereof into an interior space of a reactor including packaging at least one reactor monolith into a container, and connecting a plurality of containers in a linear fashion, thereby creating a chain of linked-together containers and allowing the chain of containers to be lowered into an interior space of a reactor. The methods and related apparatus of the present invention allows efficient and economical delivery of preformed reactor monoliths into the interior space of a reactor while reducing or eliminating damage to the monoliths. Specifically, the present inventive methods reduce costs associated with the installation and removal of the monoliths, while the apparatus may be operated by even unskilled workers, can be easily and quickly adjusted, are capable of a long operating life, and are particularly well adapted for the proposed use. These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings. For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in FIGS. 3-5, 8 and 11. However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. The reference numeral 20 (FIGS. 2 and 3) generally designates a reactor monolith delivery apparatus embodying the present invention. In the illustrated example, the apparatus includes a tubular member 22 having a cylindrically-shaped outer wall 24 defining an interior space 26, a first end 28 and a second end 30. The delivery apparatus 20 further includes a plurality of engagement members 32 located within the interior space 26 of the tubular member 22 and spaced along a length of the tubular member 22. Each engagement member 32 is actuable between an extended position (A), wherein the engagement member 32 extends into the interior space 26 of the tubular member 22 and prevents sliding movement of the monolith 10 through the tubular member 22 in a direction as indicated by directional arrow 34, and a retracted position (B), wherein the engagement member 32 is retracted towards the outer wall 24, thereby allowing the monolith 10 to slide past the engagement member 32 within the tubular member 22. The delivery apparatus 20 further includes a controller 35 operably coupled to the engagement members 32 for controlling the engagement members in a sequential manner, thereby allowing the monolith 10 to be slowly moved along the length of the tubular member 22 and preventing a continuous free-fall decent thereof, as discussed below. In the illustrated example, each engagement member 32 includes an expandable bladder 36 that extends across a substantial portion of the interior space 26 within the tubular member 22 to prevent movement of the monolith 10 along the length of the tubular member 22. In this configuration, an underside of the monolith 10 rests on the bladder 36 when the bladder 36 is in the inflated condition. Alternatively, the delivery apparatus 20 (FIG. 4) is configured such that the bladder 36 abuts a side of the monolith 10 when in the inflated condition, thereby trapping the monolith 10 between the tubular member 22 and the inflated bladder 36. Similar to as described above, when in the deflated condition, the bladder 36 collapses against the outer wall 24 of the tubular member 22, thereby allowing passage of the monolith 10 along the tubular member 22. However, the bladder 36 can be of other configurations, for example, those which surround the monolith 10 entirely or almost entirely, as long as the bladder configuration is successful in trapping the monolith 10. The reference numeral 20a (FIG. 5) generally designates another embodiment of the reactor monolith delivery apparatus of the present invention. Since the delivery apparatus 20a is similar to the previously-described delivery apparatus 20, similar parts appearing in FIG. 5 and FIG. 3 respectively are represented by the same, corresponding reference numeral, except for the suffix “a” in the numerals of the latter. In the illustrated example, the engagement member 32a of the delivery apparatus 20a includes an iris-type escapement assembly 38. Each escapement assembly 38 includes an actuator 40 having a plurality of teeth 42 actuable between an extended position (FIG. 6), wherein at least a portion of each of the teeth 42 extend into the interior space 26a of the tubular member 22a a sufficient amount so as to prevent passage of the monolith 10 beyond the escapement assembly 38 in the downward direction 34a, and a retracted position (FIG. 7), wherein the teeth 42 are retracted in close proximity to or withdrawn through apertures in the outer wall 24a of the tubular member 22a, thereby allowing passage of the monolith 10 past the escapement assembly 38. The reference numeral 20b (FIG. 8) generally designates another embodiment of the reactor monolith delivery apparatus of the present invention. Since the delivery apparatus 20b is similar to the previously-described delivery apparatus 20, similar parts appearing in FIG. 8 and FIG. 3, respectively, are represented by the same, corresponding reference numeral, except for the suffix “b” in the numerals of the latter. In the illustrated example, the tubular member 22b is provided with a rectangular cross-sectional configuration. The engagement member 32b of the delivery apparatus 20b includes a plurality of escapement assemblies 44, each of which include a pair of actuator mechanisms 46 each actuating a pin 48 between an extended position (A), wherein the pin 48 extends into the interior space 26b of the tubular member 22b, thereby preventing the monolith 10 from traveling past the escapement assembly 44 in the direction 34b, and a retracted position (B), wherein the pin 48 is retracted from within the interior space 26, thereby allowing the monolith 10 to slide past the escapement assembly 44. In operation, the controller 35, 35a, 35b is preferably provided as an air logic circuit that is operably connected to the associated engagement member 32, 32a, 32b. However, the controllers described herein are not limited to air logic circuits, and can instead be any form of controller capable of controlling descent of the monolith. An example of a preferred alternative controller is a gas containing logic controller similar to an air logic controller but wherein at least a portion, and preferably substantially all of the air is replaced by an inert gas such as nitrogen. Preferably, the controller 34, 34a, 34b is programmed so as to provide a controlled descent of the monolith 10 within the interior space 26, 26a, 26b of the tubular member 22, 22a, 22b and prevent a free-fall descent of the monolith, while simultaneously preventing the monoliths 10 from colliding with one another when being delivered through the delivery apparatus 20, 20a, 20b. As the application of each delivery apparatus 20, 20a, 20b is similar, the description of delivery apparatus 20 should be considered illustrative of each of the delivery apparatus 20, 20a, 20b. As best illustrated in FIG. 2, the delivery apparatus 20 is extended into an interior space 50 of a large-scale trickle bed reactor 52 typically used in a chemical and/or petroleum refinery process, such that the first end 28 of the tubular member 22 of the delivery apparatus 20 is accessible through an opening 54 in the outer housing 55 of the reactor 52, and the second end 30 of the tubular member 22 of the delivery apparatus 20 is located within the interior space 50 of the outer housing 55. Preferably, a plurality of delivery apparatus 20 may be linked together in an end-to-end configuration, thereby allowing the total length of the delivery assembly to be reconfigured as the level of monoliths 10 contained within the reactor 52 changes. During delivery, monoliths 10 are loaded into the delivery apparatus 20 at the first end 28 accessible from the opening 54 of the reactor 52, and removed from within the delivery apparatus 20 at the second end 30 located within the interior space 50 of the reactor 52. The plurality of engagement members 32 prevent a free-fall descent of the monoliths 10 within the delivery apparatus 20, and further prevent contact between the monoliths 10 during the delivery thereof. The reference numeral 60 (FIG. 9) generally designates another embodiment of the reactor monolith delivery apparatus. In the illustrated example, the delivery apparatus 60 includes a plurality of packaged containers 62 each containing a monolith, preferably heat-sealed or otherwise hermetically sealed therein. The delivery apparatus 60 further includes a flexible member 64 such as the length of rope or cable and along which the containers 62 are attached. However, the invention is not limited to such packaged containers, and the monolith could be attached to the flexible member 64 using other means, for example by adhering the monolith to the flexible member with adhesive, or ultrasonic welding of the flexible member to the monolith, or by using a plastic sleeve that is twisted tightly to hold the monolith in place. Preferably, each container 62 is connected with the flexible member 64 during the heat sealing process by heat sealing each container 62 to the flexible member 64, thereby preventing the container 62 from sliding along the flexible member 64. It is foreseeable that the monolith 10 may be preassembled into groups 66 prior to the packaging thereof into containers 62, thereby eliminating the necessity to assemble each and every individual monolith 10 after delivery into the reactor 52. The monoliths 10 are delivered into the interior space 50 of the reactor 52 by extending one end of the delivery apparatus 60 through the opening 54 in the outer housing 55 of the reactor 52, and slowing lowering the delivery apparatus 60 into the interior space 50. An operator located within the interior space 50 of the reactor 52 receives each of the containers 62 of the delivery apparatus 60 as the flexible member 64 is slowly lowered into the reactor 52, where each container 62 is opened and the monolith 10, or assembly of monoliths 66, is placed within the reactor 52. The delivery apparatus 60 may be manually lowered into the interior space 50 of the reactor 52, or may be lowered via a suitable mechanical system, such as a winch, and/or pulley system. It is foreseeable that the materials as used to construct the containers 62 would be constructed of a biodegradable material, or alternatively a material that dissolves within the chemical or petroleum product to be refined without adversely effecting the same. In an alternative embodiment, a delivery apparatus 65 comprises a plurality of heat sealed containers 67, wherein each monolith 10 is separated by a divider 68 such as a heat seal, thereby defining each of the containers 67. The monoliths 10 are delivered to within the interior space 50 of the reactor 52 by lowering the delivery apparatus 65 into the reactor 52 in a manner similar to that described above with respect to the delivery apparatus 60. The reference numeral 70 (FIG. 11) generally designates another embodiment of the reactor monolith delivery apparatus embodying the present invention. In the illustrated example, the delivery apparatus 20 includes a tubular member 72 having an outer wall 74 defining an interior space 76 therein. The tubular member has a first end 78 and a second end 80. In a first embodiment, the delivery apparatus 70 includes a vacuum source 82 operably coupled with the first end 78 of the tubular member 72 and providing a vacuum pressure in a direction 84 as exerted on the monolith 10, thereby preventing a free-fall of the monolith 10 within the interior space 76 of the tubular member 72. In a second embodiment, the delivery apparatus 70 is provided with a pressure source 84 operably coupled with the second end 80 of the tubular member 72 and providing an air pressure in a direction as indicated by directional arrow 86 that acts on the monolith 10, again preventing a free-fall of the monolith 10. The pressure as exerted by the vacuum source 82 and/or the pressure source 84 may be adjusted so as to regulate the rate of decent of the monolith 10 within the delivery apparatus 70. The methods and related apparatus of the present invention allows efficient and economical delivery of preformed reactor monoliths into the interior space of a reactor while reducing or eliminating damage to the monoliths. Specifically, the present inventive methods reduce costs associated with the installation and removal of the monoliths, while the apparatus are operable by even unskilled workers, can be easily and quickly adjusted, are capable of a long operating life, and are particularly well adapted for the proposed use. In the foregoing description, it will be readily appreciated by those skilled the art that modifications made be made to the invention without departing from concepts disclosed herein. Such modifications are to be considered included in the following claims, unless these claims by their language expressly state otherwise. |
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claims | 1. A method of managing gas production in a nuclear fission reactor, the method comprising: providing a sacrificial metal in direct physical contact with a molten salt fissile fuel, and preventing, with the sacrificial metal, accumulation of volatile iodine compounds released from the molten salt, wherein the sacrificial metal is one of or a combination of any of zirconium, vanadium, chromium and silver. 2. The method according to claim 1 wherein the molten salt fuel comprises actinide halides. 3. A method of managing gas production in a fission reactor comprising fuel tubes containing a molten salt fissile fuel, the method comprising: bringing into direct physical contact the molten salt with a sacrificial metal, and preventing, with the sacrificial metal, accumulation of volatile iodine compounds released from the molten salt, wherein the sacrificial metal is one of or a combination of any of zirconium, vanadium, chromium and silver. 4. The method according to claim 3, further comprising: permitting gasses produced during fission of the molten salt fissile fuel to pass out from the fuel tubes into a coolant surrounding the fuel tube or into a gas space in contact with the coolant. 5. The method according to claim 3, wherein the sacrificial metal is provided as a plating in the fuel tubes. 6. The method according to claim 3, wherein the sacrificial metal is provided as particles or as a coating on particles in the molten salt. 7. The method according to claim 3, wherein the sacrificial metal is provided as an insert immersed in the molten salt or as a coating on an insert immersed in the molten salt. 8. The method according to claim 3, wherein the molten salt fuel comprises actinide halides. 9. A fuel tube for a nuclear fission reactor, the fuel tube comprising: a molten salt fissile fuel and a sacrificial metal in direct physical contact with the molten salt, wherein the sacrificial metal is configured to prevent accumulation of volatile iodine compounds released from the molten salt, and wherein the sacrificial metal is one of or a combination of any of zirconium, titanium, vanadium, chromium and silver. 10. A fuel tube according to claim 9, wherein the fuel tube is configured to permit gasses to pass out from the fuel tube when the fuel tube is inserted into a nuclear fission reactor, the gasses passing out into coolant of the nuclear fission reactor or into gas space above the coolant. 11. The fuel tube according to claim 10, wherein an opening of the fuel tube is closed with a sintered plug, the sintered plug being configured to allow passage of gasses and not to allow passage of liquids. 12. A fuel tube according to claim 10, wherein the fuel tube extends vertically into the gas space when the fuel tube is inserted into the nuclear fission reactor, and comprises an opening within the gas space. 13. A fuel tube according to claim 12, wherein the fuel tube further comprises a capillary tube extending vertically into the gas space, and the opening is at an upper end of the capillary tube. 14. A fuel tube according to claim 10, wherein the fuel tube further comprises a diving bell assembly with an outer opening immersed in the coolant when the fuel tube is inserted into the nuclear fission reactor. 15. The fuel tube according to claim 9, wherein the sacrificial metal is provided as plating on a surface of the fuel tube. 16. The fuel tube according to claim 9, wherein the sacrificial metal is provided as particles or as a coating on particles in the fuel tube. |
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description | This patent application claims the benefit of U.S. provisional patent application Ser. No. 60/475,027, filed Jun. 2, 2003, the entirety of which is incorporated by reference herein. The work leading to the disclosed invention was funded in whole or in part with Federal funds from the National Institutes of Health and the Health Resources and Services Administration. The Government may have certain rights in the invention under NIH contract number CA78331 and HRSA Grant No. 4C76HF00691-01-01. The present invention is related to the field of devices and methods for generating high energy ion beams. The present invention is also related to uses of high energy ion beams for radiation therapy. In addition, the present invention is related to the field of treating patients in cancer treatment centers using high energy ion beams. Radiation therapy is one of the most effective tools for cancer treatment. It is well known that the use of proton beams provides the possibility of superior dose conformity to the treatment target as well as providing a better normal tissue sparing, as a result of the Bragg peak effect, compared to photons (e.g., X-rays) and electrons. See, e.g. T. Bortfeld, “An analytical approximation of the Bragg curve for therapeutic proton beams”, Med. Phys., 2024–2033 (1997). While photons show high entrance dose and slow attenuation with depth, protons have a very sharp peak of energy deposition as a function of beam penetration. As a consequence, it is possible for a larger portion of the incident proton energy to be deposited within or very near the 3D tumor volume, thus avoiding radiation-induced injury to surrounding normal tissues that commonly occurs with x-rays and electrons. Despite the dosimetric superiority characterized by the sharp proton Bragg peak, utilization of proton therapy has lagged behind that of photon therapy. This lag is apparently due to the operating regime (the total operating cost for accelerator maintenance, energy consumption, and technical support) for proton accelerators being at least an order of magnitude higher compared to electron/X-ray medical accelerators. Currently, proton therapy centers utilize cyclotrons and synchrotrons. See, e.g., Y. A. Jongen et al., “Proton therapy system for MGH's NPTC: equipment description and progress report”, Cyclotrons and their Applications, J. C. Cornell (ed) (New Jersey: World Scientific) 606–609 (1996); “Initial equipment commissioning of the North Proton Therapy Center”, Proc. of the 1998 Cyclotron Conference (1998); and F. T. Cole, “Accelerator Considerations in the Design of a Proton Therapy Facility”, Particle Acceleration Corp. Rep (1991). Despite a somewhat limited number of clinical cases from these facilities, treatment records have shown encouraging results particularly for well localized radio-resistant lesions. See, e.g., M. Fuss et al., “Proton radiation therapy (PRT) for pediatric optic pathway gliomas: Comparison with 3D planned conventional photons and a standard photon technique”, Int. J. Radiation Oncology Biol. Phys., 1117–1126 (1999); J. Slater et al., “Conformal proton therapy for prostate carcinoma” Int. J. Radiation Oncology Biol. Phys., 299–304 (1998); W. Shipley et al., “Advanced prostate cancer: the results of a randomized comparative trial of high dose irradiation boosting with conformal protons compared with conventional dose irradiation using photons alone”, Int. J. Radiation Oncology Biol. Phys., 3–12 (1995); and R. N. Kjellberg, “Stereotactic Bragg Peak Proton Radiosurgery for Cerebral Arteriovenous Malformations” Ann Clin. Res., Supp. 47, 17–25 (1986). This situation could be greatly improved by the availability of a compact, flexible, and cost effective proton therapy system, which would enable the widespread use of this superior beam modality and therefore bring significant advances in the management of cancer. Thus, there remains the problem of providing a practical solution for a compact, flexible and cost-effective proton therapy system. See, e.g., C.-M. Ma et al., “Laser accelerated proton beams for radiation therapy”, Med. Phys., 1236 (2001); and E. Fourkal et al., “Particle in cell simulation of laser-accelerated proton beams for radiation therapy”, Med. Phys., 2788–2798 (2002). Such a proton therapy system will require three technological developments: (1) laser-acceleration of high-energy protons, (2) compact system design for ion selection and beam collimation, and (3) the associated treatment optimization software to utilize laser-accelerated proton beams. U.S. Patent Application Pub. No. US 2002/0090194 A1 (Tajima) discloses a system and method of accelerating ions in an accelerator to optimize the energy produced by a light source. It is disclosed that several parameters may be controlled in constructing a target used in the accelerator system to adjust performance of the accelerator system. Simulations of the laser acceleration of protons reported by Fourkal et al., showed that, due to their broad energy spectrum, it is unlikely that laser accelerated protons can be used for therapeutic treatments without prior proton energy selection. If such an energy distribution is achieved, however, it should be possible to provide a homogeneous dose distribution through the so-called Spread Out Bragg's Peak (“SOBP”). Using multiple beams (beamlets) it should also be possible to conform the dose distribution to the target laterally (intensity modulation). Intensity-modulated radiation therapy (“IMRT”) using photon beams could deliver more conformal dose distribution to the target while minimizing the dose to surrounding organs compared to conventional photon treatments. In “On the role of intensity-modulated radiation therapy in radiation oncology”, Med. Phys., 1473–1482 (2002), R. J. Shultz, et al. addressed the role of the intensity-modulated radiation therapy in treatments of specific disease sites. This topic of research is still in its latent stage requiring accumulation and analysis of more data, but the findings of Shultz et al. suggest that at least there could be an advantage of using IMRT for treatments of such sites as the digestive system (colorectal, esophagus, stomach), bladder and kidney. Giving a homogeneous dose distribution in the target's depth direction may be possible; see, e.g., C. Yeboah et al., “Intensity and energy modulated radiotherapy with proton beams: Variables affecting optimal prostate plan”, Med. Phys., 176–189 (2002); and A. Lomax, “Intensity modulation methods for proton radiotherapy”, Phys. Med. Biol., 185–205 (1999). Accordingly, Energy- and Intensity-Modulated Proton Therapy (“EIMPT”) should further improve target coverage and normal tissue sparing effects. In recent years, the planning and delivery of X-rays has improved considerably so that the gap between the conventional proton techniques and X-ray methods has decreased dramatically. The main pathway of research has been toward the optimization of individual beamlets and the calculation of optimal intensity distributions (for each beamlet) for intensity modulated treatments. See, e.g., E. Pedroni, “Therapy planning system for the SIN-pion therapy facility”, in Treatment Planning for External Beam Therapy with Neutrons, ed. G. Burger, A. Breit and J. J. Broerse (Munich: Urban and Schwarzenberg); and T. Bortfeld et al., “Methods of image reconstruction from projections applied to conformation radiotherapy”, Phys. Med. Biol., 1423–1434 (1990). Unfortunately, the implementation of intensity modulation for proton beams has lagged behind that of photons due to the design limitations of conventional beam delivery methods in proton therapy. See, e.g., M. Moyers “Proton Therapy”, The Modern Technology of Radiation Oncology, ed. J. Van Dyk (Medical Physics Publishing, Madison, 1999). Thus, there remains the problem of providing a combination of a compact proton selection and collimation device and treatment optimization algorithm to make EIMPT possible using laser-accelerated proton beams. Laser acceleration was first suggested in 1979 for electrons (T. Tajima and J. M. Dawson, “Laser electron accelerator”, Phys. Rev. Lett., 267–270 (1979)), and rapid progress in laser-electron acceleration began in the 1990's after Chirped Pulse Amplification (“CPA”) was invented (D. Strickland, G. Mourou, “Compression of amplified chirped optical pulses,” Opt. Comm., 219–221 (1985)) and convenient high fluence solid-state laser materials such as Ti:sapphire were discovered and developed. The first experiment that has observed protons generated with energy levels much beyond several MeV (58 MeV) is based on the Petawatt Laser at Lawrence Livermore National Laboratory (“LLNL”). See, e.g., M. H. Key et al., “Studies of the Relativistic Electron Source and related Phenomena in Petawatt Laser Matter Interactions”, in First International Conference on Inertial Fusion Sciences and Applications (Bordeaux, France, 1999); and R. A. Snavely et al., “Intense high energy proton beams from Petawatt Laser irradiation of solids”, Phys. Rev. Lett., 2945–2948 (2000). Until then, there had been several experiments that observed protons of energy levels up to 1 or 2 MeV. See, e.g., A. Maksimchuk et al., “Forward Ion acceleration in thin films driven by a high intensity laser”, Phys. Rev. Lett. 4108–4111, (2000). Another experiment at the Rutherford-Appleton Laboratory in the U.K. has been reported recently with proton energy levels of up to 30 MeV. See, e.g., E. L. Clark et al., “Energetic heavy ion and proton generation from ultraintense laser-plasma interactions with solids”, Phys. Rev. Lett., 1654–1657 (2000). It has long been understood that ion acceleration in laser-produced plasma relates to the hot electrons. See, e.g., S. J. Gitomer et al., “Fast ions and hot electrons in the laser-plasma interaction”, Phys. Fluids, 2679–2686 (1986). A laser pulse interacting with the high density hydrogen-rich material (plastic) ionizes it and subsequently interacts with the created plasma (collection of free electrons and ions). The commonly recognized effect responsible for ion acceleration is a charge separation in the plasma due to high-energy electrons, driven by the laser inside the target (see, e.g., A. Maksimchuk et al., Id., and W. Yu et al., “Electron Acceleration by a Short Relativistic Laser Pulse at the Front of Solid Targets”, Phys Rev. Lett., 570–573(2000)) or/and an inductive electric field as a result of the self-generated magnetic field (see, e.g., Y. Sentoku et al., “Bursts of Superreflected Laser Light from Inhomogeneous Plasmas due to the Generation of Relativistic Solitary Waves”, Phys. Rev. Lett., 3434–3437 (1999)), although a direct laser-ion interaction has been discussed for extremely high laser intensities, on the order of 1022 W/cm2; see, e.g., S. V. Bulanov et al, “Generation of Collimated Beams of Relativistic Ions in Laser-Plasma Interactions”, JETP Letters, 407–411 (2000). These electrons can be accelerated up to multi-MeV energy levels (depending on laser intensity) due to several processes, such as ponderomotive acceleration by propagating laser pulse (W. Yu et al., Id.); resonant absorption in which a part of laser energy goes into creation of a plasma wave which subsequently accelerates electrons (S. C. Wilks and W. L. Kruer, “Absorption of Ultrashort, ultra-intense laser light by solids and overdense plasmas” IEEE J. Quantum Electron., 1954–1968 (1997)); and “vacuum heating” due to the v×B component of the Lorentz force (W. L. Kruer and K. Estabrook, “J×B heating by very intense laser light,” Phys. Fluids, 430–432 (1985)). Because of the number of mechanisms for electron acceleration and the corresponding electric field generation, different regimes of ion acceleration are possible. Understanding the mechanisms of ion acceleration in the interaction of laser pulse with a solid target and quantification of the ion yield in terms of the dependencies on the laser pulse and the plasma parameters are useful for designing laser proton therapy systems. Having the quantified ion yield of a laser-accelerated proton ion beam alone is typically insufficient for preparing a therapeutically-suitable proton ion dose. Such proton ion beams have a wide energy distribution that further require energy distribution shaping (i.e., the resulting high energy polyenergetic ion beam) to be therapeutically suitable. In addition to needing to shape the polyenergetic beam's energy distribution, beam size, direction and overall intensity need to be controlled to provide proton beams that are therapeutically sufficient for irradiating a target in a patient. Lower-energy protons typically treat shallower regions in a patient's body, whereas higher-energy protons treat deeper regions. Thus, there remains the problem of providing systems and methods for forming therapeutically-suitable polyenergetic ion beams from sources of laser-accelerated high energy protons that are capable of treating a predetermined three dimensional conformal region within a body. Such ion selection systems are presently needed to provide low-cost, compact, ion therapy systems to enable the greater availability of positive ion beam therapy to society. The present inventor has now designed ion selection systems for forming therapeutically-suitable polyenergetic ion beams. In a first aspect of the present invention there are provided ion selection systems, having a collimation device capable of collimating a laser-accelerated high energy polyenergetic ion beam, the laser-accelerated high energy polyenergetic ion beam including a plurality of high energy polyenergetic positive ions; a first magnetic field source capable of spatially separating the high energy polyenergetic positive ions according to their energy levels; an aperture capable of modulating the spatially separated high energy polyenergetic positive ions; and a second magnetic field source capable of recombining the modulated high energy polyenergetic positive ions. The present inventor has also designed methods of forming high energy polyenergetic positive ion beams from laser-accelerated high-energy polyenergetic ion beam sources that are suitable for ion beam therapy. Thus, in a second aspect of the present invention there are provided methods of forming a high energy polyenergetic positive ion beam, including the steps of forming a laser-accelerated high energy polyenergetic ion beam including a plurality of high energy polyenergetic positive ions, the high energy polyenergetic positive ions characterized as having a distribution of energy levels; collimating the laser-accelerated ion beam using a collimation device; spatially separating the high energy positive ions according to their energy levels using a first magnetic field; modulating the spatially separated high energy positive ions using an aperture; and recombining the modulated high energy polyenergetic positive ions using a second magnetic field. Within additional aspects of the invention there are provided laser-accelerated high energy polyenergetic positive ion therapy systems that are capable of delivering therapeutic polyenergetic beams to a three-dimensional conformal target in a body. In these aspects of the invention there are provided laser-accelerated high energy polyenergetic positive ion therapy systems, including: a laser-targeting system, the laser-targeting system having a laser and a targeting system capable of producing a high energy polyenergetic ion beam, the high energy polyenergetic ion beam including high energy positive ions having energy levels of at least about 50 MeV; an ion selection system capable of producing a therapeutically suitable high energy polyenergetic positive ion beam from a portion of the high energy positive ions; and an ion beam monitoring and control system. In another aspect of the invention, there are provided methods of treating patients with a laser-accelerated high energy polyenergetic positive ion therapy system, including the steps of identifying the position of a targeted region in a patient; determining the treatment strategy of the targeted region, the treatment strategy including determining the dose distributions of a plurality of therapeutically suitable high energy polyenergetic positive ion beams for irradiating the targeted region; forming the plurality of therapeutically suitable high energy polyenergetic positive ion beams from a plurality of high energy polyenergetic positive ions, the high energy polyenergetic positive ions being spatially separated based on energy level; and delivering the plurality of therapeutically suitable high energy polyenergetic positive ion beams to the targeted region according to the treatment strategy. In a related aspect of the invention, there are provided laser-accelerated ion beam treatment centers, including: a location for securing a patient; a laser-accelerated high energy polyenergetic positive ion therapy system capable of delivering a therapeutically suitable polyenergetic positive ion beam to a patient at the location, the ion therapy system having a laser-targeting system, the laser-targeting system having a laser and at least one target assembly capable of producing a high energy polyenergetic ion beam, the high energy polyenergetic ion beam including high energy polyenergetic positive ions having energy levels of at least about 50 MeV; an ion selection system capable of producing a therapeutically suitable high energy polyenergetic positive ion beam using the high energy polyenergetic positive ions, the high energy polyenergetic positive ions being spatially separated based on energy level; and a monitoring and control system for the therapeutically suitable high energy polyenergetic positive ion beam. In additional aspects of the present invention there are provided methods of producing radioisotopes using the laser-accelerated high energy polyenergetic ion beams provided herein. In these aspects of the present invention there are provided methods of producing radioisotopes, including the steps of forming a high energy polyenergetic positive ion beam, including forming a laser-accelerated ion beam having a plurality of high energy positive ions, the high energy polyenergetic positive ions characterized as having an energy distribution; collimating the laser-accelerated high energy polyenergetic ion beam using at least one collimation device; spatially separating the high energy polyenergetic positive ions according to energy using a first magnetic field; modulating the spatially separated high energy polyenergetic positive ions using an aperture; recombining the spatially separated high energy polyenergetic positive ions using a second magnetic field; and irradiating a radioisotope precursor with the recombined spatially separated high energy polyenergetic positive ions. Other aspects of the present invention will be apparent to those skilled in the art in view of the detailed description of the invention as provided herein. The following abbreviations and acronyms are used herein: CORVUS a treatment optimization system for photon IMRT from NOMOS CPA chirped pulse amplification CT computer-aided tomography DICOM Digital Imaging and Communications in Medicine DICOM RT DICOM Radiation Therapy Supplement DVH dose-volume histogram EIMPT energy- and intensity-modulated proton therapy EGS4 Electron Gamma Shower (version 4) Monte Carlo code system GEANT(3) a Monte Carlo system for radiation (proton, neutron, etc) simulation IMRT intensity-modulated (photon) radiation therapy JanUSP a high power (1019–1021 W/cm2) laser at LLNL LLNL Lawrence Livermore National Laboratory LLUMC Loma Linda University Medical Center, Loma Linda, Calif. MCDOSE an EGS4 user-code for dose calculation in a 3-D geometry MGH Massachusetts General Hospital, Boston, Mass. MLC multileaf collimator NOMOS NOMOS Corp., Sewickley, Pa. NTCP normal tissue complication probability PC personal computer PIC particle-in-cell (simulation technique for laser plasma physics) PMC primary monitor chamber PSA prostate-specific antigen PTV planning target volume PTRAN a Monte Carlo code system for proton transport simulation RTP radiotherapy treatment planning SMC secondary monitor chamber SOBP spread out Bragg peak (for proton/ion beams) SSD source-surface distance TCP tumor control probability MeV million electron volts GeV billion electron volts T Tesla As used herein, the term “protons” refers to the atomic nuclei of hydrogen (H1) having a charge of +1. As used herein, the term “positive ions” refers to atoms and atomic nuclei having a net positive charge. As used herein, the term “polyenergetic” refers to a state of matter being characterized as having more than one energy level. As used herein, the term “high energy” refers to a state of matter being characterized as having an energy level greater than 1 MeV. As used herein, the term “beamlet” refers to a portion of a high energy polyenergetic positive ion beam that is spatially separated, or energetically separated, or both spatially and energetically separated. The terms “primary collimator”, “primary collimation device”, “initial collimator”, and “initial collimation device” are used interchangeably herein. The terms “energy modulation system” and “aperture” are used interchangeably when it is apparent that the aperture referred to is capable of modulating a spatially separated high energy polyenergetic positive ion beam. All ranges disclosed herein are inclusive and combinable. In one embodiment of the present invention there is provided a laser-accelerated polyenergetic ion selection system for radiation therapy. The design of this system typically includes a magnetic field source that is provided to spatially separate protons of different energy levels. A magnetic field source is also provided to separate out plasma electrons that initially travel with the protons. While these two magnetic field sources are typically provided by the same magnetic field source, two or more separate magnetic field sources may be provided to carry out these functions. After the protons have been spatially separated, one or more apertures are typically provided to select an energy distribution needed to cover the treatment target in the depth direction for a given beamlet. The form of an aperture is dictated by the location as well as the depth dimension of the target, as described more fully below. Once the spatial position and the target size are known, the proton energy spectrum needed to cover the target for a given beamlet in the depth direction is calculated by combining the depth dose curves of different proton energy levels, as described more fully below. Due to the angular distribution of protons, a primary collimation device is typically employed to reduce spatial mixing of different energy protons. The primary collimation device is typically employed to collimate the positive ions into a magnetic field that separates the ions by energy levels. As a result of this spatial mixing, the proton energy spectrum in a given spatial location typically has a small spread that depends on the energy of the protons. The depth dose curves are typically calculated using the spread out (i.e., polyenergetic) proton spectrum. In this regard, the depth dose curves for the proton energy modulation are typically modified to account for this polyenergetic spreading effect, as described more fully below. Description of a proton selection and collimation system: In one embodiment of the present invention there is provided an ion selection and collimation device needed for proton energy modulation. Using the 2D particle in cell simulation code (PIC), described by C. K. Birdsall and A. B. Langdon in Plasma Physics via Computer Simulation (McGraw-Hill Book Company, Singapore 1985), the interaction of a petawatt laser pulse with a thin dense foil (hydrogen rich) was simulated, yielding protons with energy well beyond 200 MeV and maximum energy reaching 440 MeV. The simulations were performed for a 3.6 μm (in the radial direction) full width at half-maximum (FWHM, 14 femtosecond (fs) linearly polarized laser pulse with a wavelength, λ=0.8 μm and intensity I=1.9×1022 W/cm2, normally incident onto a thin dense plasma slab (ionized foil) with a density thirty times higher than the critical density ncr=4π2mec2ε0/(e2λ2) and thickness d≈1 μm. Such la reach of the recent technological developments, as described by G. A. Mourou et al., in “Ultrahigh-Intensity Lasers: Physics of the Extreme on a Tabletop”, Physics Today, 22–28 (1998). The basic configuration of such as laser light source system is described in U.S. Pat. No. 5,235,606, issued Aug. 10, 1993 to Mourou et al., which is incorporated by reference herein. U.S. patent application Ser. No. 09/757,150 filed by Tajima on Jan. 8, 2001, Pub. No. U.S. 2002/0090194 A1, Pub. Date Jul. 11, 2002, “Laser Driven Ion Accelerator” discloses a system and method of accelerating ions in an accelerator using such a laser light source system, the details of which are incorporated by reference herein in their entirety. The protons coming from a thin foil are typically accelerated in the forward direction by the electrostatic field of charge separation induced by the high intensity laser. Further details of this process are described by V. Yu. Bychenkov et al., in “High energy ion generation in interaction of short laser pulse with solid density plasma”, Appl. Phys. B, 207–215 (2002). Over a period of several tens of plasma frequency ωp=√{square root over (ne2/meε0)} cycles, protons are typically accelerated to relativistic energy levels. The maximum value of the proton energy levels typically depend on several factors, including laser pulse length and intensity, and plasma foil thickness. The late time dynamics can be discerned by PIC code, which shows that protons reach a stationary distribution (energy, angular) and move in a formation together with the electrons. This reassures the preservation of the low proton emittance, shielding proton space charge, which otherwise could be detrimental to the emittance. The angular distribution of protons exhibits the spread which depends on the energy. Typically, the general trend is such that the higher the energy of the accelerated protons, the more they are emitted in the forward direction. The depth dose distribution calculated using the laser-accelerated proton spectrum shows that the polyenergetic positive ion spectrum emitted from the target typically cannot be readily used for radiation treatments. A high energy deposition to the area beyond the effective Bragg peak typically arises from the high entrance dose to the superficial structures and the long tails in the polyenergetic dose distributions. Thus, in one embodiment of the present invention, one delivers a homogeneous dose to the tumor volume to minimize the dose to the surrounding healthy tissues. This is achieved by providing an ion (e.g., proton) selection and collimation device that generates the desired polyenergetic proton energy distribution. This device separates polyenergetic positive ions (e.g., protons) into spatial regions according to their energy. The spatially separated regions of the positive ions are subsequently controlled using at least one magnetic field. The spatially separated positive ions are controllably modulated using an aperture to provide the desired dose. Optionally, the device also includes a magnetic field source for generating a magnetic field to eliminate the plasma electrons that travel with the positive ions. This optional magnetic field source can be the same or a different magnetic field as the one spatially separating the polyenergetic positive ions. This magnetic field is also useful for eliminating plasma electrons traveling together with the laser-accelerated positive ions. A schematic diagram of one embodiment of the ion selection system (100) is provided in FIG. 1. Referring to this figure, there is provided a series of magnetic field sources that produce a magnetic field pattern B=B(z)ez, the z-direction being perpendicular to the page. A first magnetic field source provides a first magnetic field (102), listed as “5.0 T into page”, at a distance from 5 cm to 20 cm from a plasma target (104) located at 0 cm along the x (primary beam) axis (114). High energy polyenergetic positive ions (110) are generated by the interaction between the plasma target (104) with a suitable laser pulse (not shown). A beam of high energy polyenergetic positive ions (e.g., protons) (106) enter the first magnetic field (102) after exiting an initial collimation device (108). The protons are shown exiting the initial collimation device (108) into the first magnetic field (102), the protons being characterized as having an angular spread. A second magnetic field (112) source provides a second magnetic field listed as “5.0 T into page” at a distance from 60 cm to 75 cm from the plasma target (104) along the x (primary beam) axis (114). High energy polyenergetic positive ions (116) (protons in certain embodiments) enter the second magnetic field (112) after exiting an aperture (118). Also shown in FIG. 1 is a third magnetic field source providing a third magnetic field (120), which is listed as “5.0 T out of page” at a distance from 25 cm to 55 cm from the plasma target (104) located at 0 cm along the x axis (114). The x axis as drawn is parallel to the beam axis (114) of the laser in this embodiment. Other coordinate orientations and coordinate systems, such as cylindrical and spherical coordinate systems, can be suitably used. High energy polyenergetic positive ions (126) enter the third magnetic field (120) after exiting the first magnetic field (102). The first magnetic field (102) is shown spatially separating the trajectories (128) of the high energy polyenergetic positive ions by energy level. The third magnetic field (120) is shown bending the trajectories of spatially separated ions (130) towards the aperture (118). The aperture modulates the ion beam by controllably selecting a portion of the spatially separated ions, as described further herein. The third magnetic field (120) is also shown bending the trajectories of the spatially separated polyenergetic positive ions (132) towards the beam axis and towards the second magnetic field (112). The second magnetic field (112) recombines the spatially separated and modulated ions (134) to form a recombined ion beam (136). The recombined ion beam (136) is shown entering a secondary collimation device (138). Upon exiting the secondary collimation device (138), a high energy polyenergetic positive ion beam is provided that is suitable for use in high energy polyenergetic positive ion radiation therapy. Suitable magnetic field sources for this and various embodiments of the present invention typically have a magnetic field strength in the range of from about 0.1 to about 30 Tesla, and more typically in the range of from about 0.5 to about 5 Tesla. The Lorentz force of the magnetic field typically spreads out the polyenergetic protons. The lower energy protons (140) typically are deflected more from their original trajectories exiting the initial collimation device 108) (“initial collimator”) than are the high energy protons (142). As described herein, many of the embodiments of the present invention use magnetic field sources to provide magnetic fields for manipulating the positive ion beams. In additional embodiments of the present invention one or more of the magnetic field sources are replaced by, or combined with, one or more electrostatic field sources for manipulating the positive ion beams. The initial collimator (108) typically defines the angular spread of the incoming beam (106) entering the first magnetic field (102). The tangent of the angle of the beam spread of the beam (106) exiting the initial collimator (108) is typically about the ratio of one half the distance of the initial collimator exit opening (144) where the beam exits the collimator to the distance of the collimator exit opening (144) to the proton beam source (i.e., the plasma target, 104). Typically, this angle is less than about 1 radian. The emitting angle is the angle of the initial energy distribution exiting the target system (i.e., target, 104 and initial collimation device, 108). Electrons (146) are typically deflected in the opposite direction from the positive ions by the first magnetic field and absorbed by a suitable electron beam stopper (148). Suitable electron stoppers (148) include tungsten, lead, copper or any material of sufficient thickness to attenuate the electrons and any particles they generate to a desired level. The aperture (118) is typically used to select the desired energy components, and the matching magnetic field setup (in this embodiment, the second magnetic field, 112) is selected that is capable of recombining the selected protons (134) into a polyenergetic positive ion beam. Suitable apertures typically can be made from tungsten, copper or any other materials of sufficient thickness that are capable of reducing the energy levels of positive ions. This energy level reduction is typically carried out to such a degree that the positive ions can be differentiated from those ions that do not go through the aperture. In various embodiments of the present invention, the aperture geometry can be a circular, rectangular, or irregular-shaped opening (150)(or openings) on a plate (152)(or slab), which when placed in a spatially separated polyenergetic ion beam, is capable of fluidically communicating a portion of the ion beam therethrough. In other embodiments, the aperture (118) can be made from a plate that has multiple openings that are controllably selected, such as by physical translation or rotation into the separated ion beam to spatially select the desirable energy level or energy levels to modulate the separated ion beam. The modulation of the ion beam gives rise to a therapeutically suitable high energy polyenergetic positive ion beam (136) as described herein. Suitable apertures include multi-leaf collimators. In addition to controllably selecting the spatial position of the openings that fluidically communicate the spatially separated ion beams, the aperture openings may also be controllably shaped or multiply shaped, using regular or irregular shapes. Various combinations of openings in the aperture (118) are thus used to modulate the spatially separated ion beam (130). The spatially separated positive ions (132) are subsequently recombined using the second magnetic field (134). The high and low energy positive ion (e.g., proton beam) stoppers (154 and 156, respectively) typically eliminate unwanted low-energy particles (140) and high-energy particles (not shown). Because of the broad angular distribution of the accelerated protons (which depends on a given energy range), there is typically a spatial mixing of different energy positive ions after they pass through the first magnetic field. For example, a portion of the low energy protons may go to regions where the high energy particles reside, and vice versa. Reducing the spatial mixing of protons is typically carried out by introducing a primary collimation device, such as the initial collimation device 108 of the embodiment depicted in FIG. 1. A primary collimation device is typically used to collimate protons to the desired angular distribution. As described further below, proton spatial differentiation is typically carried out by passing the positive ions through a small collimator opening prior to their entering the first magnetic field. An example of a small collimator opening is depicted in FIG. 1 as the initial collimator opening (144). Typically, the collimator exit opening (144) is not arbitrarily small, since smaller openings typically lower the dose rate and increase the treatment time. As a result of the finite size of the collimator opening (144), the protons are typically spatially mixed. Accordingly, any given spatial location for a collimator opening (however small) typically provides a polyenergetic proton energy distribution. While not being bound by any particular theory of operation, the energy modulation calculations take into account the polyenergetic characteristics of the positive ions entering the ion selection device to provide the needed depth dose curves. The polyenergetic characteristics of these positive ions is understood through the influence of the magnetic field on the dynamics of the positive ions. The following description is directed to the dynamics of protons, as one illustrative embodiment. Additional embodiments to other positive ions in addition to protons are also envisioned. To describe the proton's dynamics in the magnetic field, a numerical code is written which solves the following equation of motion, ⅆ p i ⅆ t = e v i × B ( 1 ) where p=mpv/√{square root over (1−v2/c2)}, B is the magnetic induction vector, mp is the proton rest mass and i signifies the particle number. For one embodiment of the present invention, this equation was solved using a symplectic integration algorithm developed by J. Candy and W. Rozmus in “A Symplectic Integration Algorithm for Separable Hamiltonian Functions “, J. Comp. Phys. 230–239 (1991). The initial conditions [(r0i, v0i)] were obtained from the PIC simulation data, which provided the phase-space distribution for protons. The contribution of the self-consistent fields on the proton dynamics were neglected, since the Lorentz force created by the external magnetic field to separate the electrons from the protons is greater for the magnetic field induction used in the calculations than the Coulomb force in the region beyond the initial collimation device. Using the equation of balance between the Lorentz and the inter-particle Coulomb forces, one arrives at a condition for particles spatial separation distance for which the magnetic force prevails over the Coulomb force, r > ( e 4 π ɛ 0 Bv ) 1 / 2 ( 2 ) where B is the magnitude of the magnetic field, v is the particle velocity and e is an elementary charge. The average inter-particle distance r can be obtained from the particle density r=n−1/3, thus the inequality (2) can be rewritten in the form: n < ( 4 π ɛ 0 Bv e ) 3 / 2 ( 3 ) Providing the lowest therapeutic energy protons of about 50 MeV, which corresponds to proton velocity of v=0.3c, and the magnetic field induction B=5 T, the condition (3) gives, n<2*1020 cm−3. The particle density in the region beyond the initial collimation device can be estimated using the arguments presented by E. Fourkal et al. in “Particle in cell simulation of laser-accelerated proton beams for radiation therapy”, Id. (2002). In this region the particle density is n=4*1013 cm−3, which is far below the estimated threshold value of 2*1020 cm−3. This estimate validates the assumption of the insignificant contribution of the self-consistent electrostatic field on the proton dynamics in the external magnetic field. The calculations of the proton dynamics in the magnetic field have also neglected such boundary effects as edge focusing due to the influence of the fringing field patterns at the edge of a sector field. These effects are expected to be small in the bulk of the selection system due to the canceling action of alternating magnetic field patterns (with the same absolute value of the field induction). As the positive ions (e.g., protons) leave the final field section, the boundary fringe field can introduce some focusing effect. This effect can be accounted for by using the magnetic field distribution at the boundary. Monte Carlo calculations: While not being bound by any particular theory of operation, the GEANT3 Monte Carlo radiation transport code is used for dose calculations. GEANT3 is used to simulate the transport and interactions of different radiation particles in different geometries. The code can run on different platforms. A detailed description of the operation and usage of GEANT3 has been given by R. Brun et al., in GEANT3—Detector description and simulation tool Reference Manual (1994). GEANT3 is equipped with different user selectable particle transport modes. Being more versatile than most Monte Carlo codes concerning the production of secondaries, GEANT3 has three options to deal with these rays. An important user controlled variable for these options is DCUTE below which the secondary particle energy losses are simulated as continuous energy loss by the incident particle, and above it they are explicitly generated. In the first option, the secondary particles are produced over the entire energy range of the incident particle. This mode is termed as “no fluctuations”. The second mode of energy loss is “full fluctuations”, in which secondaries are not generated, and the energy loss straggling is sampled from a Landau (“On the energy loss of fast particles by ionization”, J. Phys. USSR, 201–210 (1944)), Vavilov (“Ionisation losses of high energy heavy particles”, Soviet Physics JETP, 749–758 (1957)) or Gaussian distribution each according to its validity limits (R. Brun et al., Id.). The third is “restricted fluctuations”, with generation of secondaries above DCUTE and restricted Landau fluctuations below DCUTE. In principle, choosing energy loss fluctuations typically carries an advantage if energy deposited is scored in voxel sizes larger than the range of secondaries. This results in great savings of computation time and avoids tracking a large number of secondaries generated below DCUTE. Typically, a continuous energy loss by the incident particle is assumed according to the Berger-Seltzer formulae. Moliere multiple scattering theory is used by default in GEANT3. Multiple scattering is well described by Moliere theory. See, e.g., G. Z. Moliere, “Theorie der Streuung schneller geladener Teilchen I: Einzelstreuung am abgeschirmten Coulomb-Feld”, Z. Naturforsch., a, 133–145 (1947); and G. Z. Moliere, “Theorie der Streuung schneller geladener Teilchen II: Mehrfach-und Vielfachstreuung”, Z. Naturforsch., a, 78–85 (1948). A limiting factor in the Moliere theory is the average number of Coulomb scatters Ω0 for a charged particle in a step. When Ω0<20, the Moliere theory is typically not applicable. According to E. Keil et al. in “Zur Eifach-und Mehrfachstreuung geladener Teilchen”, Z. Naturforsch, a, 1031–1048 (1960), the range 1<Ω0≦20 is called the plural scattering regime. In this range a direct simulation method is used for the scattering angle in GEANT3 (R. Brun et al., Id.). A simplification of the Moliere theory by a Gaussian form is also implemented in GEANT3. The Gaussian multiple scattering represents Moliere scattering to better than 2% for 10<Ω0≦108. The hadronic interactions in matter (elastic, inelastic, nuclear fission, neutron nuclear capture) are described by two software routines, GHEISHA and FLUKA, which are available to users of GEANT. The GHEISHA code generates hadronic interactions with the nuclei of the current tracking medium, evaluating cross-sections and sampling the final state kinematics and multiplicity, while the GEANT philosophy is preserved for the tracking purposes. A number of routines that exist in GHEISHA are responsible for generating the total cross-sections for hadronic interactions, calculating the distance to the next hadronic interaction according to the total cross-sections and finally the main steering routine for the type of occurred hadronic interaction. FLUKA is a simulation program, which as a standalone code contains transport and the physical processes for hadrons and leptons and tools for geometrical description. In GEANT, only the hadronic interaction part is included. As with the GHEISHA package, the FLUKA routines can compute the total cross-sections for hadronic processes, and perform the sampling between elastic and inelastic processes. The cross-sections for both types of interactions are computed at the same time as the total cross-section. Subsequently, a particle is sent to the elastic or inelastic interaction routines. After the interaction, the eventual secondary particles are written to the GEANT stack. The following control parameters were used to calculate the depth dose distributions for proton beams in the example presented herein: The cutoff energy for particles was 20 keV, the Rayleigh effect was considered, δ-ray production was turned on, continuous energy loss for particles below cutoff energy levels sampled directly from the tables, Compton scattering was turned on, pair production with generation of e−/e+ was considered, photoelectric effect was turned on, and positron annihilation with generation of photons was considered. Results and Discussion: The PIC simulations show that the maximum proton energy of the polyenergetic proton beam is a function of many variables including the laser pulse intensity and duration, as well as the target density and its thickness. The quantitative dependence of the maximum proton energy on laser/plasma target parameters can be found in Fourkal et al. The overall results of this study showed that the maximum proton energy increases with decreasing thickness of the plasma target reaching the plateau for the target thicknesses on the order of the hot electron Debye length (for a given laser intensity). In the same time, the proton energy is a non-monotonous function of the laser pulse length, reaching the maximum value for the laser-pulse length of the order of 50 femtoseconds. Thus, depending on the simulation parameters, one can obtain a broad spectrum of energy distributions for the accelerated protons. FIGS. 2(a) and 2(b) show the energy and angular distributions for the protons accelerated by the laser pulse described above. For the laser/plasma parameters chosen in the simulation, the maximum proton energy reaches the value of 440 MeV, which is much higher than typically needed for radiotherapy applications. To reduce the unwanted protons, as well as to collimate them to a specific angular distribution, a primary collimation device is provided. Its geometrical size and shape is typically tailored to the energy and angular proton distributions. For example, in one embodiment of the present invention there is provided a 5 cm long tungsten collimator that absorbs the unwanted energy components. Because of its density and the requirement for the compactness of the selection system, tungsten is a favorable choice for collimation purposes. A suitable primary collimator opening provides a 1×1 cm2 field size defined at 100 cm SSD. Protons that move into an angle larger than this are typically blocked. With the magnetic field configuration shown in FIG. 1, for example, the solution to the equation of motion (1) with the initial conditions given by the proton phase space spectra obtained from the PIC simulations, yields the proton spatial distributions N═N(y) at the plane x=40 cm, z=0 cm, as shown in FIG. 3. This shows that the magnetic field spreads the polyenergetic protons into spatial regions according to their energy and angular distributions. Their spatial distribution is such that the lower energy particles are deflected at greater distances away from the central axis, and as the proton energy increases the spatial deflection decreases. Therefore, the contribution of both the magnetic field and the primary collimator (with a specific collimator opening) creates such a spatial proton distribution that allows the energy selection or proton energy spectrum reformation, using an aperture. The geometric shape of an aperture typically determines the energy distribution of the therapeutic protons. As mentioned above, due to the presence of the angular spread, there is typically a spatial mixing of different energy protons. As a result of this mixing, the proton energy distribution in a given spatial location is typically no longer monochromatic, but has a spread around its peak. FIG. 4 shows the proton energy distributions at different spatial locations. These distributions were calculated by counting the number of protons in the given spatial location of width Δy=3 mm as a function of energy. This figure shows that the lower energy particles have a much smaller spread than the high energy particles. Without being bound to a particular theory of operation, this result is apparently due to the higher energy protons not being deflected as much in the magnetic field as are the lower energy particles. Because of the energy spread effect, the depth dose curves needed for the energy modulation calculations typically are modified to include the effect of the energy spread in the calculations, since mono-energetic protons are not typically for the depth dose calculations. Using the GEANT3 Monte Carlo transport code the dose distributions for the proton energy spectra shown in FIG. 4 for a 4×4 cm2 field size was calculated. The results of the simulation are shown in FIG. 5. The presence of an energy spread in the proton spectra leads to the broadening of the dose distributions, which leads to a less sharp falloff of the energy-modulated Bragg peak as compared to the case of mono-energetic beams. See, e.g., T. Bortfeld. The broadening is typically most profound for the higher energy protons. FIGS. 6(a) and 6(b) show the spatial distribution of protons N=N(y) at the plane x-40 cm, z=0 cm for the magnetic field configuration shown in FIG. 1, using a primary collimator opening of 5×5 cm2 defined at 100 cm SSD and the proton energy distributions Ni=Ni(E), where index i denotes the energy levels of the polyenergetic proton beams. Comparing FIG. 5, 6(a) and 6(b) to FIGS. 3 and 4 the spatial separation of protons at larger openings is less effective leading to the higher order of spatial mixing and the larger spread in the energy distributions. The energy spread as used herein is defined as the difference between the maximum and the minimum energy in the distribution. FIG. 7 shows the energy spread as a function of a collimator opening for several proton energy levels; the energy spread increases with increasing aperture opening and is more profound for higher energy particles. As a result of the energy spread effect, the depth dose curves will typically have less sharp falloff beyond the effective Bragg peak region for wider apertures as compared to the cases of narrower collimator openings. FIG. 8 shows the dose distributions for the proton energy spectra shown in FIG. 6(b), which corresponds to a primary collimator of 5×5 cm2 defined at 100 cm SSD, normalized to the incident proton fluence. Comparing FIG. 5 with FIG. 8 shows that desirable dosimetric characteristics from the laser accelerated protons are typically obtained for smaller primary collimator openings. Suitable primary collimator openings are typically smaller than about 2000 cm2, more typically smaller than about 100 cm2, and even more typically smaller than about 1 cm2, when defined at 100 cm SSD. Typically there is a lower limit on the size of the collimator opening, which is suitably determined by the field size, dose rate, or both, that the system can yield after beam collimation. The geometry of the collimator opening typically influences the treatment time. Once the depth dose distributions for polyenergetic proton beamlets are determined, a proton energy distribution that provides a homogeneous dose along the target's depth direction is calculated using the target location and volume. In one embodiment, the following steps are carried out to calculate the desired proton energy distribution: 1. The geometrical size of the target (in the depth direction) determines the proton energy range for radiating the target. Using the depth dose distributions for a given energy range, the weights for the individual polyenergetic beamlet are computed, with the assumption that the weight for the beamlet with the energy distribution, which gives the effective Bragg peak at the distal edge of the target, is set to one. The weights Wi=Wi(E) are computed based on the requirement of the constancy of the dose along the depth direction of the target. 2. Once the weights are known, the proton energy distribution N(E) for providing a suitable dose along the target's depth dimension are calculated by convolving the weights Wi(E) with the energy distributions Ni(E) of polyenergetic proton beamlets to give N ( E ) = ∑ i W i ( E ) N i ( E ) ( 4 ) where index i runs through energy levels of the polyenergetic proton beamlets for radiating the area of interest (in depth direction). A suitable energy modulation prescription for protons is provided by the formulation of the absorbed dose distribution for electrons introduced by Gustafsson, A., et al., in “A generalized pencil beam algorithm for optimization of radiation therapy”, Med. Phys., 343–356 (1994), in which the incident particle differential energy fluence integrated over the surface and solid angle corresponds to the energy distribution defined in Eq. (4). As an example, a hypothetical target with spatial dimensions 4×4×5 cm3, located at depth lying between 9 cm and 14 cm is considered. The energy range of polyenergetic protons required to cover this target is 110 MeV<E<152 MeV. Using both the depth dose distributions for polyenergetic proton beamlets with the spread out energy spectra discussed earlier and the condition of a constancy of the resultant dose along the target's depth direction, the weights Wi for each individual beamlet, that are indicated in Table 1 are readily obtained. TABLE 1W1521.00W1490.25W1460.15W1430.12W1400.10W1370.095W1340.09W1310.085W1280.08W1250.07W1220.06W1190.05W1160.04W1130.035W1100.03 Distribution of weights corresponding to protons with a different characteristic energy: In one embodiment of the present invention, a procedure for finding the weights is provided. This procedure is mathematically similar to minimizing the following functional Γ ( z ) = ∑ i W i D i ( z ) - D 0 , for 9 cm ≤ z ≤ 14 cm ( 5 ) where i denotes energy bins, Di is the depth-dose distribution corresponding to the ith polyenergetic energy bin and D0 is a constant corresponding to a specific dose level (typically larger than the distant Bragg peak in view of the contribution from the adjacent depth-dose distributions). The physical meaning of the weights are described further. The absolute value of each individual weight is correlated to the physical method associated with the actual energy modulation process in the selection system. The design of the energy modulation system (i.e., the aperture) is achieved by either using an aperture whose geometric shape is correlated to the weights or by using a slit, which can move along the y-axis in the region where the protons are spread according to their energy levels, and the time spent in a given region will be proportional to the value of the weight for the given energy. Convolving the weights of the Table (1) with the energy distributions for each individual beamlet according to equation (4), one obtains the actual modulated energy distribution that will deliver the SOBP for the given target's depth dimension. This energy distribution differs from that calculated using monoenergetic proton beams (for which the weights themselves represent the actual energy distribution) because of the presence of particles with energy levels beyond the ones associated with the weights, which typically arises from a consequence of a finite primary collimator. The presence of these “extra particles” typically makes the dose distribution beyond the SOBP fall off less sharply than that obtained using mono-energetic beams. FIG. 9 shows the proton energy spectrum (a) and the corresponding dose distribution (normalized to the incident proton fluence) (b) for a target considered in the calculations. The resultant dose distribution shows the quick fall off of the dose beyond the distal edge of the target although not as dramatic as for an ideal case of convolving mono-energetic protons shown also in FIG. 9(b). The entrance dose is still significant compared to the dose to the target. In order to reduce the entrance dose, several proton beams coming from different directions but converging at the target could be used, so that the target receives the prescribed dose and the surrounding healthy tissue receives much less dose. Therefore, the energy and intensity modulated proton therapy is expected to further improve target coverage and normal tissue sparing. Dose Rate Determination: As mentioned earlier, it is important to determine the absolute dose rate that the ion selection system can yield. This quantity is closely related to the absolute number of accelerated protons. From the PIC simulations it was determined that for a laser intensity of about I=1.9×1022 W/cm2 and pulse length of about 14 fs, the number of protons accelerated to energy levels higher than about 9 MeV is about 4.4×105 when the total number of protons used in PIC simulation is 1048576. Without being bound by a particular theory of operation, not all of the protons in the plasma slab are believed to interact with the laser. Only those protons that are located in the laser's propagation path typically experience the strongest interaction. In simulation studies, the laser occupies an area of about ⅗ of the total size of the simulation box (in a direction perpendicular to the propagation), which provides about 6.3×105 protons (out of 1048576) that will “sample” the laser. This means that about 70% of the effective number of protons are accelerated to energy levels higher than about 9 MeV. On the other hand, the total number of protons in a plasma slab that subtends the laser pulse can be estimated using the proton density of the foil nf as well as the laser focal area S and the thickness of the foil d to give N=S×nf×d≈2×1012. Finally this gives about N=0.7*2×1012=1.4×1012 protons that will be typically accelerated to energy levels greater than about 9 MeV. With the above in mind, the absolute dose delivered to the target is estimated in the following way. The polyenergetic beams needed to cover the target in depth direction (9 cm≦z≦14 cm) will typically have an energy range of about 110–152 MeV. The number of protons in the energy range of about 147 MeV<E<157 MeV moving into the angle of 0.01 radian (approximately 2.6% of the total number of protons in the energy range 147 MeV<E<157 MeV) is N=2.6×108, which corresponds to Φ0=2.6×108 l/cm2 (1×1 cm2 field size) per laser pulse for the initial fluence of protons at a distance of about 100 cm from the source. FIG. 9(b) shows that the dose deposited by protons in the Monte Carlo simulations (normalized to the initial fluence) at depths 9 cm≦d≦14 cm is about D0=1.6×10−9 Gy*cm2. This gives D=D0*Φ0≈0.43 Gy per laser shot. Typical lasers operating in a 10 Hz repetition rate yield D≈256 Gy per minute for the pencil beam of 1×1 cm2. The dose rate is typically not only a function of laser-plasma parameters but also depends on the location and volume of the target. This leads to D≈64 Gy/min for the target located at depth z=25 cm (the distal edge of the target) with volume of 1×1×5 cM3. While not being bound to any particular theory of operation, the reduction of the dose rate in this case is apparently due to both the smaller number of protons in the energy range needed to cover the deeply seated target, as well as the less energy deposited within the target (the height of the Bragg peak gets smaller as the proton energy increases). The calculation presented above estimates the absolute dose rate for 1×1 cm2 pencil beam. More typically, the cross-section of the treatment volume is larger in area than 1×1 cm2 and the “effective” dose rate becomes smaller and comparable to that of conventional linear accelerators. Larger targets can be effectively treated by scanning the high energy polyenergetic positive ion beam over the target. In an alternative embodiment, treatment target volumes larger than the cross section of the beam is irradiated by varying the field size to cover the cross sectional depth at the field volume using different proton energy levels in individual beams. Multiple beams varying in energy, area, location and shape can be combined to conform to the targeted volume. For example, for the hypothetical target considered in the energy-modulation calculations with spatial dimensions of 4×4×5 cM3, the dose rate becomes D=256/16=16 Gy/min. The same estimations would give D=4 Gy/min for a target located at depth z-25 cm and a volume of 4×4×5 cm3. The calculations presented above can also be used to estimate the treatment time needed for a given target. Assuming the 2 Gy treatment regiment, the time needed to deliver this dose to a target with a volume of 4×4×5 cm3 located at depth of 14 cm is t=2/16=0.125 minute. This is carried out using a laser-accelerated high energy polyenergetic positive ion beam treatment center (200), such as the one described in FIG. 17. Referring to the laser-accelerated high energy polyenergetic positive ion beam treatment center (200) in FIG. 17, there is provided a main laser beam line (202) that is reflectively transported using a series of beam reflectors, e.g., mirrors (204, a–f), to a target and ion selection system (100). The target and ion selection system (100) includes the target system for generating high energy polyenergetic ions and an ion separation system, such as depicted schematically in FIG. 1 (with target) and 18 (without target). The proton beam exiting the target and ion selection system includes therapeutically suitable high energy polyenergetic positive ions that are generated as described above. As shown, the proton beam exiting the target and ion selection system are directed in the direction parallel to the direction of the laser beam entering the target and ion selection system. The proton beam (206) is shown directed towards a couch (208), which locates the patient and the patient's target. The mirrors (204a–f) and target and ion selection system (100) are capable of being rotated (here shown capable of being rotated in the x-z plane, the z direction being perpendicular to the x-y plane) around the axis of the main laser beam line using a gantry. Typically, the final mirror (204, f) from which the laser beam is reflected into the target and ion selection system (100) is fixed to the target and ion selection system. The distance between the final mirror (204, f) and mirror (204, e) and ion selection system is shown adjustable along the y direction to permit scanning of the proton beam (206) along the y direction. The distance between mirror (e) and mirror (d) is shown adjustable along the x direction to permit scanning of the proton beam along the x direction. Suitable target and ion selection systems (100) are compact (i.e., less than about 100 to 200 kg in total mass, and less than about 1 meter in dimension). The compactness of the target and ion selection systems permit their positioning with robotically-controlled systems to provide rapid scanning of the proton beam (206) up to about 10 cm/s. One embodiment of the high energy polyenergetic positive ion beam radiation treatment centers of the present invention includes the components as shown in FIG. 17, along with a suitable laser (such as described with respect to FIG. 12 below) and a system for monitoring and controlling the therapeutically suitable high energy polyenergetic positive ions. Suitable lasers are typically housed in a building, such as in the same building as the positive ion beam treatment center, or possibly in a nearby building connected by a conduit for containing the laser beam. The main laser beam line (202) is typically transported through the building within shielded vacuum conduit using a series of mirrors (e.g., 204) to direct the laser beam (202) to the target and ion selection system (100). The target and ion selection system (100) is typically mounted on a gantry, which is placed in a treatment room. In additional embodiments of the present invention, the main laser beam (202) is split using a beam splitter into a plurality of laser beams emanating from a single laser. Each of the laser beams emanating from the beam splitter is directed to an individual target and ion selection system (100) for treating a patient. In this fashion, high energy polyenergetic positive ion radiation treatment centers are provided using one laser source and a plurality of ion therapy systems to treat a plurality of patients. In certain embodiments of the high energy polyenergetic positive ion radiation treatment centers of the present invention, there are provided a plurality of treatment rooms, each treatment room having an individual target and ion selection system, a location for a patient, and a proton beam monitoring and controlling system. A plurality of treatment rooms equipped this way enables a greater number of patients that can be treated with the investment of one high power laser for providing therapeutically suitable high energy polyenergetic positive ions. Laser-accelerated proton beams also typically generate neutrons, which may contaminate the ion beam. The energy modulation process leads to a large portion of proton energy being deposited within the beam stoppers as well as the aperture and collimators. As described earlier, N=1.4×1012 protons have energy levels higher than 9 MeV. In this regard, these protons can be accelerated by the laser, and only 0.02% of the total proton energy is allowed to go through the final collimator and be deposited within the target. Proper shielding is typically provided to prevent the “waste” protons and unselected particles and their descendants from leaking out of the treatment unit. There is a finite probability that some of the contaminant particles may pass through the final (or secondary) collimating device (138) or leak out through the shielding. Determining the number of contaminant particles is typically considered in the shielding calculations. Coulomb Expansion of Proton beam: Without being bound by a particular theory of operation and referring to FIG. 1, it is believed that as the protons go through the aperture (118), the subsequent recombining magnetic field configuration (112), and through the secondary collimation device (138), the protons (134) form a non-neutral proton plasma with uncompensated charge, which typically tends to spread apart due to a repulsive force arising from the Coulomb interaction among the protons. This repulsive force typically introduces an extra divergence to the proton beam in addition to the initial divergence. The initial divergence is typically due to the angular spread of the laser-accelerated protons, which is typically controlled by the geometry of the primary collimation device. The magnitude of the repulsive force depends on the proton density at the exit region. Both the theoretical description as well as the particle in cell simulations can be used to estimate the rate at which the given distribution of protons will expand. For simplicity, a spherically symmetrical distribution of protons with a given initial density and size is assumed to correspond to the size and density of the proton cloud at the exit region. Due to the spherical symmetry of the problem considered, the subsequent time evolution of the system typically maintains its symmetry. The equation of motion for the outer most protons, which can approximate the size of a proton cloud, is, in the non-relativistic limit, m ⅆ 2 r ⅆ t 2 = eQ 4 π ɛ 0 r r 3 ( 6 ) where m is the proton mass and Q is the charge of the proton cloud. It is convenient to introduce the dimensionless units τ=tωpi, r=RR0, where R0 is the initial radius of the proton cloud, ωpi=√{square root over (ne2/mpε0)} is the proton plasma frequency and n is the initial proton density. In these units, the equation governing the evolution of the outer part of the proton cloud is, ⅆ 2 R ⅆ 2 τ = R 3 R 3 ( 7 ) The numerical solution to this equation with the initial conditions R=1, dR/dτ=0 when τ=0 is plotted in FIG. 10. To convert these results to the real space-time variables, the value for the proton plasma frequency ωpi is used, which in turn typically requires the knowledge of the initial proton density in a cloud. The total number of protons in a cloud can be estimated using the arguments presented earlier. Through suitable calculations, the number of protons accelerated to energy levels higher than about 9 MeV is determined to be about N≈1.4*1012. A small fraction (≈0.03) of these protons typically pass through the initial collimation device, giving N≈4*1010. In one embodiment of the present invention described in FIG. 1, where an exit point of the particle selection system is at 70 cm away from the source, the volume that the accelerated protons occupy is determined as the product V=ΔLxΔLyΔz, where ΔLx, ΔLy and ΔLz are the spatial dimensions of the proton cloud. For a 0.7×0.7 cm2 field size, ΔLy=0.7 cm, ΔLz=0.7 cm. ΔLx can be found by calculating the spatial extent, at the exit point, between the fastest and the slowest particles used for the therapeutic purposes (typically about 50 MeV<E<about 500 MeV; and more typically about 80 MeV<E<about 250 MeV). For these energy levels, ΔLx=L*(1−vs/vf)≈25 cm. With that in mind, the average proton density and the proton plasma frequency are n=N/V≈3.5×1010 cm−3, ωpi≈6×107 s−1. Providing a patient location 1 meter (“m”) away from the secondary collimation device, the average time required for a proton beam to reach a patient is t≈7*10−9 s, giving τ=ωpit=0.4. FIG. 10 shows that at τ=0.4, a two to three percent increase in the size of the proton cloud is expected to arise primarily from the electrostatic repulsion. FIG. 10 also shows the results of PIC simulations of the non-neutral proton plasma dynamics with the initial conditions corresponding to those used in this description. As shown here, there is a good agreement between the two approaches. The calculations shown above represent an upper limit for the rate of proton divergence due to the electrostatic repulsion. Typically, due to the energy modulation process, the total number of particles will be less than that used in the calculations (since many of the initial protons will be discarded), thus a lower beam divergence rate due to the electrostatic repulsion typically results. In one embodiment of the present invention there is provided a proton selection system. The calculations provided herein show that ion selection systems of the present invention that utilize a magnetic field along with a collimation device can generate proton beams with energy spectra suitable for radiation treatment. Due to the broad energy and angular distributions of the laser-accelerated protons, the ion selection system provides polyenergetic positive ion (e.g., proton) beams with energy distributions that have an energy spread in them, leading to broader dose distributions as compared to the case of monoenergetic protons. A design of this embodiment provides for a collimator opening of about 1×1 cm2 defined at about 100 cm SSD, the energy spread for about 80 MeV proton beam is about 9 MeV, and the energy spread for about 250 MeV proton beam is about 50 MeV. In this system, as the primary aperture opening increases, the spread in proton energy distributions increases as well. The calculated depth-dose distributions for collimator openings of about 1×1 cm2, about 5×5 cm2 and about 10×10 cm2 show the preference of using narrower apertures. The aperture opening cannot be arbitrarily small, since it would decrease the effective dose rate for larger targets. A collimator opening of about 1×1 cm2 defined at about 100 cm SSD typically provides an adequate treatment time as well as typically provides satisfactory depth-dose distributions for energy-modulated proton beams. The proton selection systems provided by the various embodiments of the present invention open up a way for generating small beamlets of polyenergetic protons that can be used for inverse treatment planning. Due to the dosimetric characteristics of protons, the energy and intensity modulated proton therapy can significantly improve the conformity of the dose to the treatment volume. In addition, healthy tissues are spared using the methods of the present invention compared to conventional treatments. Overall results suggest that the laser accelerated protons together with the ion selection system for radiation treatments will bring significant advances in the management of cancer. Radiation therapy is one of the most effective treatment modalities for prostate cancer. In external beam radiation therapy, the use of proton beams provides the possibility of superior dose conformity to the treatment target and normal tissue sparing as a result of the Bragg peak effect. FIG. 11 shows the energy deposition (or dose) as a function of the penetration depth for protons, photons (X-rays), electrons, and neutrons. While neutrons and photons (X-rays) show high entrance dose and slow attenuation with depth, monoenergetic protons have a very sharp peak of energy deposition as a function of the beam penetration just before propagation through tissue stops. As a consequence, it is possible for almost all of the incident proton energy to be deposited within or very near the 3D tumor volume, avoiding radiation-induced injury to surrounding normal tissues. Protons have a higher linear energy transfer component near the end of their range, and are expected to be more effective biologically for radiotherapy of deep-seated tumors than conventional medical accelerator beams or cobalt-60 sources. In spite of the dosimetric superiority characterized by the sharp Bragg peak, utilization of proton therapy has lagged far behind that of photons for prostate treatment. This is because the operating regime for proton accelerators is at least an order of magnitude higher in cost and complexity, which results in their being too expensive for widespread clinical use compared to electron/photon medical accelerators. Conventional proton accelerators are cyclotrons and synchrotrons, of which only two such medical facilities exist in the U.S., those of Massachusetts General Hospital (MGH) (Jongen 1996, Flanz et al. 1998) and Loma Linda University Medical Center (LLUMC) (Cole 1991). Both occupy a very large space (entire floor or building). Although they are growing in number, only several such clinical facilities exist in the world (Sisterson 1999). Despite a somewhat limited number of clinical cases from these facilities, treatment records have shown encouraging results particularly for well-localized radio resistant lesions (Sisterson 1989, 1996; Austin-Seymour et al., Duggan and Morgan 1997; Seddon et al. 1990; Kjellberg 1986). The degree of clinical effectiveness for a wide variety of malignancies has not been quantified due to limited treatment experience with this beam modality. This situation will be greatly improved by the availability of a compact, flexible, and cost-effective proton therapy system, as provided by the present invention. The present invention enables the widespread use of this superior beam modality and therefore bring significant advances in the management of cancers, such as brain, lung, breast and prostate cancers. In one embodiment of the present invention there is provided a compact, flexible and cost-effective proton therapy system. This embodiment relies on three technological breakthroughs: (1) laser-acceleration of high-energy polyenergetic protons, (2) compact system design for ion selection and beam collimation, and (3) treatment optimization software to utilize laser-accelerated proton beams. As described above, laser-proton sources have been developed to accelerate protons using laser-induced plasmas. U.S. patent application Ser. No. 09/757,150 filed Jan. 8, 2001, Pub. No. U.S. 2002/0090194 A1, Pub. Date Jul. 11, 2002, “Laser Driven Ion Accelerator”, discloses a system and method of accelerating ions in an accelerator using such a laser light source system, the details of which are incorporated by reference herein in their entirety. Such laser-proton sources are compact for the reason that the accelerating gradient induced by the laser is far greater, and the beam emittance is far smaller, than current radio-frequency and magnet technology based cyclotrons and synchrotrons (Umstadter et al. 1996). One embodiment of the present invention provides an ion-selection system in which a magnetic field is used to spread the laser-accelerated protons spatially based on their energy levels and emitting angles, and apertures of different shapes are used to select protons within a therapeutic window of energy and angle. Such a compact device eliminates the need for the massive beam transportation and collimating equipment that is common in conventional proton therapy systems. The laser-proton source and the ion selection and collimating device of the present invention are typically installed on a treatment gantry (such as provided by a conventional clinical accelerator) to form a compact treatment unit, which can be installed in a conventional radiotherapy treatment room. A treatment optimization algorithm is also provided to utilize the small pencil beams of protons generated with ion selection systems of the present invention to obtain conformal dose distributions for cancer therapy, such as for prostate treatment. In various embodiments of the present invention there are provided optimal target configurations for laser-proton acceleration and methods for ion selection and beam collimation. In this embodiment of the present invention, dose distributions of laser-accelerated protons for cancer treatment are typically determined by dose calculation of proton beamlets, optimization of beamlet weights and delivery of beamlets using efficient scan sequence. Commercial software is available for carrying out intensity modulation of photon beams for targeting. Such software can be adapted for use with laser-accelerated proton beams by the following steps: calculating dose needed; optimizing the weights of the beam; and determining the sequence of the therapeutically suitable high energy polyenergetic positive ion beams. As a specific example, the treatment of prostate cancer is carried out by selecting beam incident angles based on the target volume and its relationship with the critical structures (rectum, bladder and femurs), preparing positive ion beams with different shapes, sizes and/or energies, optimizing the weights of individual beamlets, generating a scan sequence based on the beam weights, and verifying the final dose distribution by Monte Carlo calculations or by measuring with a suitable monitoring device. Laser acceleration was first suggested in 1979 for electrons (Tajima and Dawson 1979) and rapid progress in laser-electron acceleration began in the 1990's after chirped pulse amplification (CPA) was invented (Strickland et al. 1985) and convenient high fluence solid-state laser materials such as Ti:sapphire were discovered and developed. The first experiment that has observed protons generated with energy levels much beyond several MeV is based on the Petawatt Laser at the Lawrence Livermore National Laboratory (LLNL) (Key et al. 1999, Snavely et al. 2000). Until then there had been several experiments that observed protons of energy levels up to 1 or 2 MeV, which were considered to be ‘standard’ (Maximchuck et al. 2000). Another experiment at the Rutherford-Appleton Laboratory in the U.K. has been reported recently with proton energy levels of up to 30 MeV (Clark et al. 2000). The Petawatt Laser is a specially modified arm of large NOVA Laser at LLNL. The pulse is shortened by the CPA technique (Strickland et al. 1985) into several hundred fs (femtosecond, fs=10−15 sec), but it is not ultrashort (i.e. in the range of tens of fs). In the latest Petawatt Laser experiments, high-energy protons of 58 MeV were observed (Key et al. 1999, Snavely et al. 2000). A surprisingly large fraction of laser energy (of the order of 10%) was converted into proton energy in these experiments. Without being bound by a particular theory of operation, the electrostatic field generated by electrons driven by the laser is generally considered to be the main initiator (Wilks et al. 1999). Hydrogen atoms and thus protons, which are quickly generated from ionization of hydrogen, are typically accelerated from the back surface of the metal due to the electronic space charge to high energy levels. There are several relevant theoretical and computational studies of proton acceleration at high laser intensities (Rau et al. 1998; Bulanov et al. 1999; Wilks et al. 1999; Ueshima et al. 1999, Fourkal et al. 2002a). Experimental investigations on laser-proton acceleration using a short pulsed CPA intense Ti:sapphire laser (JanUSP) have been carried out. This technology is different from that of the Petawatt Laser (based on a glass laser). The short-pulsed Ti:sapphire laser can be much more compact and have higher repetition than the glass laser. This is particularly useful for radiotherapy applications as multiple shorts are typically needed for one treatment. The JanUSP laser system is shown in FIG. 12. A continuous train of 800 nm sub-100 fs pulses is emitted from a commercial mode locked oscillator pumped by 8 Watts of 530 nm light. The time-frequency transform limited oscillator output is stretched in a folded diffraction grating pulse stretcher to approximately 250 ps. The stretched 4 nJ pulse is then amplified in a regenerative amplifier to 8 mJ and then to 220 mJ in a 5 pass amplifier in a bow-tie configuration. Isolation from amplified spontaneous emission and pre-pulse leakage from the regenerative amplifier is provided by three stages of glan polarizer Pockel cell pulse slicers. The portion of the laser operates at 10 Hz and 90 mJ energy, allowing both rapid setup and timing of diagnostics at intensities up to 1019 W/cm2. Two additional stages of amplification are pumped by a frequency doubled Nd:Silicate glass amplifier. These final amplifiers raise the stretched beam energy to greater than 21 J. A vacuum compressor employing two 40 cm diameter gratings is used for pulse recompression to 80 fs. The 200 TW compressed pulse is routed in vacuum to the target chamber, where it is focused onto the target by a 15 cm diameter F/2 off-axis parabola to provide focal intensities on target of >2×1021 W/cm2. The Gaussian focal spot is approximately 2 μm in diameter. Because of its high focal intensity, the JanUSP laser is a suitable laser that is coupled to a targeting system for generating high energy polyenergetic ion beams in accordance with the invention. A facility for a laser-accelerated ion therapy system can be designed using previous neutron treatment suites in existing cancer treatment facilities, which provide adequate space and shielding. A typical laser useful in the ion therapy system has a similar construction as the JanUSP laser. The laser pulse repetition rate is typically designed at a rate of from 1–100 Hz, but typically is about 2 to 50 Hz, and most typically about 10 Hz. Laser intensity is typically in the range of from about 1017 W/cm2 to about 1024 W/cm2, more typically in the range of from about 1019 W/cm2 to about 1023 W/cm2, and even more typically in the range of from about 1020 W/cm2 to about 1022 W/cm2, and most typically about 1021 W/cm2, which is commercially available. It has been found that the target configuration plays an important role in laser-proton acceleration. At an intensity of 1021 W/cm2, recent theoretical and computational results (Tajima 1999; Ueshima et al. 1999) show that under favorable conditions protons can be accelerated up to about 400 MeV (Table 2). It was found (Tajima 1999) that the innovation of the target and judicious choice of laser and target parameters can yield a large number of protons with energy levels>100 MeV. Depending on the details of the target preparation and geometry, as well as the pulse length and shape, the average and maximum energy levels of protons (and other ions) vary. In Case 3, with the most sophisticated target, the average proton energy is in excess of 100 MeV and the maximum is 400 MeV. The energy converted into ions amounts to 14% of the incoming laser energy. This efficiency is consistent with the Petawatt Laser, where about 10% conversion efficiency into protons was observed although parameters and preparations differed from Case 3. TABLE 2Particle-in-cell (PIC) Results (Ueshima et al. 1999) on proton andelectron acceleration by laser irradiation on three thin targets. A laserintensity of 1021 W/cm2on the target surface is applied.Case 1Case 2Case 3Energy conversion50%24%31%Ion 4% 8%14%Electron48%16%17%Peak energy H+200 MeV400 MeV400 MeVPeak energy Al10+ 1 GeV 2 GeV 2 GeVPeak energy electron 25 MeV 15 MeV 20 MeVAverage energy H+ 58 MeV 95 MeV115 MeVAverage energy Al10+130 MeV500 MeV500 MeV Without being bound to a particular theory of operation, a high laser intensity in the range of from about 1017 W/cm2 to about 1024 W/cm2 is believed to be an important parameter in the generation and acceleration of positive ions to energy levels suitable for radiation therapy. An other important parameter is the design of suitable targets that generate polyenergetic protons. Various suitable targets for generating high energy polyenergetic positive ions are known. Suitable targets have been designed using various materials, dimensions, and geometry. Laser irradiation fashion, e.g., intensity and spot size, is also known to influence the generation of positive ions. According to preliminary PIC simulations of the optimized laser target interaction (Ueshima et al. 1999; Tajima 1999, Fourkal et al. 2002a), the charge separation distance of a few microns with the electrostatic field on the order of 100 GeV/mm is expected to develop upon the irradiation of high Z materials (electron density of about 1024/cm3). With this field over this distance, protons can be accelerated to energy levels greater than 100 MeV. With proper geometry and dimensions of the target, the average proton energy levels may be increased by several times over a simple target. U.S. patent application Ser. No. 09/757,150 filed Jan. 8, 2001, Pub. No. U.S. 2002/0090194 A1, Pub. Date Jul. 11, 2002, “Laser Driven Ion Accelerator”, is incorporated by reference herein for the disclosures pertaining to target construction used in a laser-proton accelerator systems. Such targets are suitably used in various embodiments of the present invention. In Table 2, Case 3, with a particular target shape, an average proton energy greater than 100 MeV and the maximum energy at 400 MeV are provided. Various target configurations are readily tested for higher energy proton generation. Based on these laser specifications, particle-in-cell (PIC) simulations have also been performed to investigate the effect of target shape, material and laser pulse length on the energy of laser-accelerated protons (Fourkal et al. 2002a). These results show that using a laser intensity of 1021 W/cm2 and a pulse length of 50 fs, protons can be accelerated to 310 MeV. FIG. 13 shows the angular distributions of these protons and the maximum proton energy as a function of the laser pulse length for the same laser intensity. The raw proton beams from a laser-driven proton accelerator have a broad energy spectrum and variable beam profiles for different energy levels; they typically cannot be used directly for therapeutic applications. One solution to this problem is to design a compact ion selection and collimation device in order to deliver small pencil beams (beamlets) of protons with desired energy spectra to cover the treatment depth range, as described earlier above and further below. As shown in FIG. 14, 1 cm×1 cm beamlet depth dose curves are provided for different polyenergetic protons, described above. By combining the depth dose curves of different spectra, a spread out Bragg peak (SOBP) is achieved that covers the treatment target in the depth direction (FIG. 15). This process is termed herein, “energy modulation”. Although the spectrum-based (polyenergetic) SOBP is not as clean as the monoenergetic SOBP, the weights of individual proton beamlets can be varied through an optimization routine to conform the dose distribution to the target laterally. As used herein, this process is termed “intensity modulation”, which is commonly used for photon beam treatments. The estimated dose rate for the laser proton beams shown in FIGS. 14 and 15 is 1–20 Gy per minute for field sizes from 1 cm×1 cm to 20 cm×20 cm. Intensity-modulated radiation therapy (IMRT) using photon beams typically can deliver more conformal dose distributions to the prostate target (and the associated nodes) compared to conventional 4–6 photon field treatments. Modulation of the dose distribution of photon beams in the depth direction is essentially impossible, however, this is not the case with proton beams (Verhey and Munzenrider 1982). Accordingly, energy- and intensity-modulated proton therapy (EIMPT) further improves target coverage and normal tissue sparing for radiation treatments, such as for the treatment of prostate cancer. The combination of a compact ion selection and collimation device and an associated treatment optimization algorithm typically makes EIMPT possible using laser-accelerated proton beams. Without being bound to a particular theory of operation, the polyenergetic nature of a laser proton beam makes it ideal for EIMPT since it is convenient for both energy modulation (using a spectrum) and intensity modulation (through beam scanning). To demonstrate the superiority of EIMPT for prostate treatment, dose distributions of prostate plans using different treatment modalities were compared (Ma et al. 2001a, Shahine et al. 2001). FIG. 16 shows dose volume histograms (DVH) of the target and the rectum for a prostate treatment. The proton isodose distribution is also shown. The photon IMRT plan was derived from a commercial treatment optimization system, CORVUS (NOMOS Corp., Sewickley, Pa.) using eight 15 MeV photon beams. The gantry angles were 45, 85, 115, 145, 215, 245, 275, and 315 degrees. The 8-field conventional proton plan included energy modulation but did not have intensity modulation. The proton beams were incident at the same gantry angles as the photon IMRT plan. The 8-field EIMPT included both energy modulation and intensity modulation with the same gantry angles. The 4-field conventional proton plan was derived using only 45, 115, 245, and 315 degrees ports. This shows that target coverage can be significantly improved using both energy- and intensity-modulation in a proton treatment. The rectum dose is much lower with the 8 field EIMPT compared to other beam modalities. The 8-field conventional proton plan is better than the 4-field proton plan and the latter is better than the 8-field photon IMRT plan in terms of the rectum dose. The results of Ma et al. 2001a are consistent with the findings of Cella et al. (2001), who compared 5-field intensity-modulated proton beams with 5-field IMRT (the Memorial Sloan-Kettering Cancer Center technique, Burman et al. 1997), 2-field conventional protons (the LLUMC technique, Slater et al. 1998), and the conventional 6-field photon treatment for prostate. EIMPT plans are consistently superior to conventional treatments and IMRT plans in target coverage and normal tissue sparing (lower doses to rectum, bladder and femoral heads). The results of Ma et al. 2001a described above assumed ideal energy selection and beam collimation for the proton beamlets. The actual beamlet dose distributions of realistic proton spectra generated by the ion radiation system of the present invention will typically not be the same as the ideal dose distributions used in the preliminary calculations of Ma et al. 2001 a, which also used a 2D patient geometry to generate these plans. The present inventor has demonstrated that different beamlet dose distributions can be combined through beamlet optimization to obtain ideal dose distributions. In one embodiment of the present invention, PIC simulations are performed to derive optimal target configurations and laser parameters and then use the simulated proton beam data to design an efficient ion selection and beam collimation device. The simulated proton phase space data is used for the Monte Carlo simulations to obtain accurate dose distributions using the proton beamlets from the proton therapy unit to achieve optimal target coverage and normal tissue sparing. Through energy- and intensity-modulation, high-energy protons generated by a laser-accelerated proton source are developed into an effective modality for radiation therapy. The positive ion therapy systems of the present invention are comparable to conventional photon clinical accelerators both in size and in cost. Therefore, the widespread use of this compact, flexible and low-cost proton source will result in significant benefits for cancer patients. Methods System Design: As described above, the raw proton beams accelerated by laser induced plasmas typically cannot be used directly for radiotherapy treatment. An important component of a laser proton radiotherapy system is a compact ion selection and beam collimation device, which is coupled to a compact laser-proton source to deliver small pencil beams of protons of different energy levels and intensities. In one embodiment of the present invention there is provided an overall design of a laser-proton therapy system, which includes system structure and layout, mechanisms of the major components and research strategies for the experiment work (Ma 2000). FIG. 17 shows a schematic diagram of one embodiment of a laser-accelerated positive ion beam treatment center (e.g., laser-proton therapy unit, the laser not shown). The laser and the treatment unit are typically placed on the same suspension bench to ensure laser beam alignment (negligible energy loss due to the small distance). This also keeps the whole system compact. The target assembly and the ion selection device are placed on a rotating gantry and the laser beam is transported to the final focusing mirror 204(f) through a series of mirrors 204(a–e). The distances between mirrors 204(d) and 204(e) and mirrors 204(e) and 204(f) are adjusted to scan the proton beam along x- and y-axis, respectively, which generates a parallel scanned beam. An alternative method is to swing the target and ion selection device about the laser beam axis defined by mirrors 204(d) and 204(e) and that defined by 204(e) and 204(f), respectively, to achieve a scan pattern. This generates a divergent scan beam. The treatment couch is adjusted to perform coplanar and noncoplanar, isocentric and SSD (source-to-surface distance) treatments. PIC study of proton acceleration: PIC simulations of target configurations and laser parameters are carried out for optimizing laser proton acceleration. The PIC simulation method computes the motions of a collection of charged particles (e.g., ions) interacting with each other and with externally applied fields. Charged plasma species are modeled as individual macroparticles (each macroparticle represents a large number of real particles). Since the spatial resolution is limited by the size of the particle, the spatial grid (cell) is introduced across the simulation box. The size of the grid is approximately equal to the size of the macroparticle. The charge densities as well as the electric currents are calculated at each grid position by assigning particles to the grid according to their position employing a weighting scheme. Once the charge density and the current density at the grid positions are known, the electric and magnetic fields at the same grid points are calculated using Poisson's and Maxwell's equations. These equations are typically solved using Fast Fourier Transforms (FFT). Fields at the particle positions are subsequently determined using an inverse weighting scheme in which the fields at the grid points are interpolated to the points of particle locations to yield the fields at particle locations. Particles are then moved via Newton's equations, using a leap-frog finite differencing method (positions and fields are calculated at integer time-steps, velocities at half time-steps). This procedure is repeated to give the time evolution of the system. A two-dimensional, electromagnetic relativistic PIC code is typically used for carrying out these optimization experiments. At each time step, the coordinates and momenta of the particles and electromagnetic field are calculated for the given initial and boundary conditions. All the variables to be calculated are functions of time and two spatial coordinates x and y. Different laser parameters and target geometry are simulated. Further details of our PIC simulations are described further herein and in Fourkal et al., 2002a. PIC simulations are performed using the codes developed by Tajima (1989). These one to two-and-one-half dimensional, first-principle, full dynamics physics tools are particularly effective for ultrafast intense laser matter interaction. Those skilled in the art are experienced with high field science analyses (for example, Tajima et al. 2000) and with PIC simulations in plasma physics (Fourkal et al. 2002a). These skills can be applied to simulate previous experiments and the experimental setups currently used to confirm the experimental laser-proton acceleration results. The experimental situations are analyzed and the configurations and parameters are optimized to guide further experiments. Suitable targets used are typically simple freestanding planar foils and composite planar foils of plastic and other materials. Dense gas targets are also suitable targets. PIC simulations of these target configurations using different laser intensities, focal spot sizes and pulse lengths can be performed of the ion radiation facility of the present invention. An optimal set of laser parameters is found using these simulations that can produce protons of energy levels up to at least 250 MeV with small angular distribution and high dose rate. These PIC simulation results are used for further analytical studies on the ion selection and beam collimation system. Characterization of laser-accelerated proton beams: Accurate determination of the characteristics of all the particle components in a laser-accelerated proton beam is particularly important. This knowledge assists the design and operation of the ion selection and beam collimation system. The energy, angular and spatial distributions of laser-accelerated protons are evaluated from the PIC simulations. Beam characterization studies are carried out for source modeling and beam commissioning for further dosimetric studies. Several Monte Carlo codes have been installed, expanded and extensively used for radiation therapy dose calculation including EGS4 (Nelson et al. 1985), PENELOPE (Salvat et al. 1996), PTRAN (Berger 1993), and GEANT (Goosens et al. 1993). The codes typically run on a PC network consisting of 16 Pentium III (866 MHz) microprocessors. Magnetic field distributions are simulated using commercial software, which is suitable for 3-dimensional field simulation and the results are compared with measurements of an ion radiation system of the present invention. Radiation transport in a magnetic field has been extensively simulated for electron beams (Ma et al. 2001b, Lee and Ma 2000). Software is implemented and verified for protons to obtain proton energy, angular and spatial distributions at the exit window of the laser-proton device. The geometry of an ion radiation system of the present invention is used in the simulations. The characteristics of the anticipated beams are studied to evaluate their advantages and disadvantages for radiation oncology application. Analytical study of ion selection and beam collimation: To use the proton beams for treatment, one typically removes the contaminant photons, neutrons and electrons from the beam using any of a variety of beam stopping and shielding materials. In preferred embodiments of the ion selection systems of the present invention, low-field magnets are used to separate the four major radiation components. As shown schematically in FIG. 18, several 3 Tesla magnetic fields (220, 222, 224) are used to deflect protons a small angle. A photon beam stopper (228) is placed on the beam axis (230). Suitable beam stoppers (228, 234) are used to remove unwanted low- and high-energy protons. The matching magnetic field setup in this embodiment assists the recombining of the selected protons, and the final beam is collimated by the primary and secondary collimators 242 and 240, respectively. The opening of the collimator is typically small (about 0.5 cm×0.5 cm), and the collimators are typically greater than about 10 cm in total thickness. Scattered protons from the beam stoppers 228, 234 and the protons missing the opening of the aperture are not transmitted through the collimator opening. As the bremsstrahlung photons and neutrons are also forward directed, a 1–2 cm wide, 10 cm thick tungsten stopper typically stops all the direct particles and the scattered particles are terminated by the shielding materials (not shown). Electrons typically are deflected downward by the magnetic field (220) and absorbed by an electron stopper. FIG. 19(a) shows the proton energy and angular distributions before and after ion selection. Lower energy protons (140) typically have larger angular spread compared to higher energy protons (142). In FIG. 19(b), lower energy protons (140) they typically spread over a larger area (244) spatially after going through the magnets compared to the spatial spread (246) of higher energy protons. An aperture (238) typically is used to select the desired energy components. FIG. 19(c) shows the energy spectrum of raw protons (solid line) and that of the resulting selected protons (dashed line). FIG. 19(d) shows the depth dose curve of raw protons solid line) and that of the resulting selected protons (dashed line). A secondary monitor chamber (240) (“SMC” in FIG. 18) measures the intensity of each energy component. A primary monitor chamber (242) (“PMC” in FIG. 18) is also provided. Various ways of monitoring ion beams and control systems are disclosed in U.S. patent application Ser. No. 09/757,150 filed Jan. 8, 2001, Pub. No. U.S. 2002/0090194 A1, Pub. Date Jul. 11, 2002, “Laser Driven Ion Accelerator”, the portion of which pertaining to monitoring ion beams and control systems is incorporated by reference herein. A suitable laser-proton beam, as selected by the ion selection system (100) of the present invention, typically has an energy spectrum suitable for a desired treatment depth range (uniform dose over that range). By using a plurality of beams, a conformal and uniform dose coverage in the beam direction is achieved for essentially any target shape and depth. The design parameters for the ion selection and collimating system using the experimental setup described above can be optimized by those skilled in the art. Because the proton beams are very small in cross-section, suitable magnetic field (“B-field”) sources for providing high magnetic fields within a small space are used. Suitable magnets for providing such magnetic fields are readily available to those skilled in the art. The ion selection system of the present invention does not require strict B-field spatial distribution, for example, the fields may have a slow gradient or a fast gradient. Likewise, the opposing B-fields may be matched or mismatched. One skilled in the art can perform theoretical optimization studies on different magnets to determine various compact geometries. A suitable compact geometry is illustrated in FIG. 18, which provides dimensions of less than 50 cm in length and less than 40 cm in diameter. The properties of the primary beam for treatment and the leakage through the collimating system together with other contaminant particles can be investigated using a numerical simulation program for further treatment planning dose calculations. Criteria for proton spectra and beamlet dose distributions are determined based on the minimum requirements for beam penumbra laterally and in the depth direction for treatment optimization. The results are used to guide further optimization work on collimator design and proton energy selection and modulation studies. Source models for the proton beams are also investigated so that for patient simulation, the phase-space information can be reconstructed from the source models rather than using large phase-space data files (inefficient for simulation and large disk space, Ma 1998, Ma et al. 1997) or simulating the laser proton device every time. Beam commissioning procedures are also established by one skilled in the art for validating the source model parameters and the beam reconstruction accuracy. FIG. 20 illustrates one set of design principles of the present invention of the ion selection mechanism. Since different laser-protons have different angular distributions (three energy levels are shown in FIG. 20(a)), a collimator (e.g. 108, FIG. 1) is typically used (i.e., positioned at the distance along beam axis 0 cm in FIG. 18) to define the field size. When the initial collimator (108) has a square opening, and the polyenergetic collimated protons of different energy levels have passed through the magnet fields, the collimated protons will reach different transverse locations (250) (as shown at the distance 30 cm in FIG. 18). FIG. 20 (a and b) shows the square fields of 50, 150 and 250 MeV protons, which are well separated spatially. The transverse plane is referred to as “the energy space (plane)” as different proton energy levels typically occupy different transverse locations. Because of the finite size of the initial collimator there typically is some overlap of proton energy levels, which typically depends on the size of the initial collimator, the magnetic field strength and the distance from the energy plane to the initial collimator. For selecting the desired energy of this embodiment, a second collimator is typically used, which is typically positioned at the corresponding transverse location. As shown in FIG. 20(b), a square aperture (248) (on the right hand side) is used to select either the 50, 150 or the 250 MeV field. A differential transmission chamber (the secondary monitor chamber, SMC in FIG. 18) is used to measure the intensity of each energy component. Multiple laser pulses are typically provided to produce a combination of protons to provide a desired spectrum. The desired proton energy spectrum is used to produce a therapeutically high energy polyenergetic positive ion beam, which provides uniform dose distributions over a desired depth range. Another embodiment of the ion selection system of the present invention is to use variable aperture sizes at the energy space (plane) to select both an energy and the total number of protons of that energy (intensity) simultaneously. This embodiment typically requires fewer laser pulses to achieve a desired proton spectrum compared to the preceding embodiment. This variable aperture size embodiment preferably uses an elongated aperture at the energy space with variable widths at different transverse (energy) locations. Without being bound by a particular theory of operation, this design allows for energy and intensity selection simultaneously from the same laser pulse. This appears to be a highly efficient way to use a polyenergetic laser-proton beam to achieve a uniform dose over a depth range for radiation therapy. A variable energy aperture size typically uses a subsequent differential magnetic system to recombine the fields of different proton energy levels to a similar field size. In certain embodiments, a secondary collimation device (138) (FIG. 1) is typically provided to define the final field size and shape of the positive ions that form the therapeutically suitable high energy polyenergetic positive ion beam. Small shaped beams (e.g., squares, circles, rectangles, and combinations thereof) are provided in to modulate the intensity of individual beamlets so that a conformal dose distribution to the target volume can be achieved. Since the individual proton beams can have variable energy spectra for providing a uniform dose distribution over the depth range of the target volume, EIMPT can be used to produce a more uniform proton dose distribution in the target than photon IMRT (Lomax 1999, Ma et al. 2001). Another method of modulating the spatially separated high energy polyenergetic positive ion beam is to deliver EIMRT using a plurality of individual narrow energy polyenergetic proton beams at a time with a relatively large field that covers at least a portion of the cross-section of the target volume at the corresponding depth (i.e., the depth of the Bragg Peak). In this embodiment, there is provided a modulatable secondary collimation device that is capable of modulating the spatially separated beam. The modulatable secondary collimation device may have a variable shape, which can be realized using an aperture, as described earlier, such as a multileaf collimator (MLC). A number of laser pulses are typically provided using this embodiment to treat a target volume. While the aperture that modulates the energy levels typically moves in the transverse direction to select a desired energy spectrum to cover the depth range of at least a portion of the entire target volume, the modulatable secondary collimation devices (e.g., the MLC) are capable of changing the field shape of the recombined beam to enclose at least a portion of the cross-section of the target volume at the corresponding depths. The methods described herein for the ion selection systems (100) of the present invention may suitably be performed using the devices and instrumentalities described herein. Because the proton beams are typically small in cross-section, it is possible to establish a high magnetic field within a small space. Certain embodiments of the present invention do not require strict B-field spatial distribution, rather, the magnetic fields may have a slow gradient, they may be spatially overlapping, or both. Suitable embodiments of the present invention will include at least two magnetic field sources that have matching, opposite, B-fields. For example, the ion selection system geometry provided in FIG. 18, which is less than 50 cm in length and less than 40 cm in diameter, includes a first magnetic field source (220) of 3.0 T into the page, a second magnet field source (224) of 3.0 T into the page, and a third magnetic field source (222) of 3.0 T out of the page. The geometry may be further reduced in the beam direction by using higher magnetic fields, smaller photon beam stoppers, or both. Improvement of Monte Carlo dose calculation tools: Dose calculation tools for EIMPT are also provided in accordance with the invention. Dose calculation is performed in treatment optimization for laser accelerated proton beam therapy because the dose distributions of small proton beamlets are significantly affected by the beam size and heterogeneous patient anatomy. Patient dose calculations are estimated using the GEANT3 system. The code is designed as a general purpose Monte Carlo simulation. The dose distributions shown in FIG. 16 (a–d) took about 100 hours of CPU time on a Pentium III 450 MHz PC. Much faster computers that are currently available should be able to reduce this computation time by at least about one or two orders of magnitude. For accelerating dose calculation, a fast proton dose calculation algorithm has been developed based on conventional photon and electron Monte Carlo dose calculation algorithms (Ma et al. 1999a–b, 2000ab, Deng et al. 2000ab, Jiang et al. 2000a, 2001, Li et al. 2000, 2001). Various variance reduction techniques have been implemented in the code to speed up the Monte Carlo simulation. These include “deterministic sampling” and “particle track repeating” (Ma et al. 2000b, Li et al. 2000), which are very efficient for charged particle simulations. The implementation of this fast Monte Carlo code is tested using the GEANT3 code. The source models are also implemented to reconstruct the phase-space parameters (energy, charge, direction and location) for the proton pencil beams emerging from the laser proton therapy device during a Monte Carlo dose calculation. Suitable software is available (Moyers et al 1992, Ma et al. 1999b) that can be adapted for use in treating patients with laser-accelerated polyenergetic positive ions. Such software first converts the patient CT data into a simulation phantom consisting of air, tissue, lung and bone. Based on the contours of the target volume and critical structures, the software computes the dose distributions for all the beamlets of different spectra, incident angles (e.g., gantry angles specified by the planner), and incident locations (e.g., within a treatment port/field). The final dose array for all the beamlets is provided to the treatment optimization algorithm, as described further below. Improvement of treatment optimization tools: In certain embodiments, improved treatment optimization tools for EIMPT are also provided. A treatment optimization algorithm has been developed based on typical polyenergetic proton beams generated from a typical laser proton accelerator and actual patient anatomy. Commonly used “inverse-planning” techniques include computer simulated annealing (Webb 1990, 1994), iterative methods (Holmes and Mackie 1994a, Xing and Chen 1996), filtered back projection and direct Fourier transformation (Brahme 1988, Holmes and Mackie 1994b). Considering the calculation time and the possible complexity with proton beams, the iterative optimization approach (based on a gradient search) is suitably adopted. This is based on iterative optimization algorithms for photon and electron energy- and intensity-modulation (Pawlicki et al. 1999; Jiang 1998, Ma et al. 2000b, Jiang et al. 2000b). Improved algorithms for energy- and intensity-modulated proton beams are tested. Further improvements of the algorithm is carried out in view of the special features of the realistic proton beams. The “optimizer” performs the following tasks: (1) takes the beamlet dose distributions from the dose calculation algorithm (see above), (2) adjusts the beamlet weights (intensities) to produce the best possible treatment plan based on the target/critical structure dose prescriptions, and (3) outputs the intensity maps (beamlet weighting factors) for all the beam ports and gantry angles for beam delivery sequence studies. Treatment plan comparison: The present invention has been evaluated for the treatment modality for prostate cancer. Comparisons are made of treatment plans generated by EIMPT using laser-accelerated proton beams with those generated by existing beam modalities such as conventional photon and proton beams and photon IMRT. A group of 20 clinical cases for prostate alone, prostate+seminal vesicles, and prostate+seminal vesicles+lymph nodes have been performed using EIMPT under the same conditions as for conventional radiotherapy treatments using conventional photons and protons and photon IMRT. The treatment plans are compared with those using a commercial RTP system for conventional photon beams with 4 or 6 photon fields (the FOCUS system) and a commercial treatment optimization system for IMRT with 5–9 intensity modulated photon fields (the CORVUS system). These cases are also planned using the proton treatment planning module in the FOCUS system, for conventional proton treatments with 2–6 fields. The plans are evaluated using isodose distributions, DVHs, TCP, NTCP and other biological indices with emphasis on target coverage, target dose homogeneity and normal tissue sparing. The same objective (penalty) functions are used for both proton EIMPT and photon IMRT, under similar conditions. The “goodness” of a treatment plan is judged based on the appearance of the isodose distributions and on DVH, TCP, NTCP and other biological indices. A significantly improved plan is considered to possess one or more of the following: (a) more uniform (5–10%) dose within the target volume, much less (moderate vs. high or low vs. moderate) dose to the immediately adjacent normal structures, (b) a significantly reduced exit/scatter dose (by a factor of two or more) to remote organs, and (d) an unambiguously improved dose distribution. Furthermore, a physician typically makes a clinical judgment as to whether a particular plan would be used and provide reasons justifying this decision. Production of Radioisotopes. The present invention also provides methods of producing radioisotopes using the laser-accelerated high energy polyenergetic ion beams provided herein. The production of 2-deoxy-2-18F fluoro-D-glucose (“[18F]FDG”) is carried out by proton bombardment of the chemical precursors leading to the radioisotopes. These processes use proton beams generated using traditional cyclotron and synchrotron sources. For example, J. Medema, et al. [http://www.kvi.n1/˜agorcalc/ecpm31/abstracts/medema2.html] have reported on the production of [18F] Fluoride and [18F] FDG by first preparing [18F] fluoride via the 18O(p, n) [18F] nuclear reaction in 18O enriched water, and producing the [18F]FDG by recovering the [18F]fluoride via the resin method and the cryptate drying process. The present invention provides high energy polyenergetic ion beams suitable for use in this process of preparing radioisotopes. Thus, the process of producing radioisotopes includes the steps of forming a high energy polyenergetic proton beam as described herein to provide an appropriate particle, target and beam current. A target precursor is filled with H218O. The high energy polyenergetic proton beam irradiates the target precursor until a preselected integrated beam current or time is reached. The target pressure is typically monitored by a pressure transducer. When the integrated beam current or the time is reached the [18F]fluoride is used for chemically synthesizing [18F] FDG. The final product is isotonic, colorless, sterile, and pyrogen free and is suitable for clinical use. Various alternate embodiments of the present invention are further depicted in FIGS. 21–44, in which the ion tracks are illustrated to provide a general position and orientation of the ions. For example, FIGS. 21, 23 (schematic cross sections) and 22 (perspective) depicts an embodiment of an ion selection system (100) composed of a collimation device (408) capable of collimating a laser-accelerated high energy polyenergetic positive ion beam, the laser-accelerated high energy polyenergetic ion beam having a plurality of high energy polyenergetic positive ions; a first magnetic field source (magnet 202) capable of spatially separating the high energy polyenergetic positive ions according to their energy levels; an aperture (418) capable of modulating the spatially separated high energy polyenergetic positive ions; and a second magnetic field source (magnet 412) capable of recombining the modulated high energy polyenergetic positive ions. FIG. 24 depicts a schematic of an embodiment of an ion selection system similar to that provided in FIG. 21 that further includes a third magnetic field source (magnet 420), the third magnetic field source capable of bending the trajectories (428) of the spatially separated high energy polyenergetic positive ions towards the aperture (418). FIG. 25 depicts a schematic of an embodiment of an ion selection system similar to that provided in FIG. 24 that shows the aperture (418) being placed inside the magnetic field of the third magnetic field source (magnet 420). FIG. 26 depicts a schematic of an embodiment of an ion selection system similar to that provided in FIG. 24 that shows the aperture (418) being placed outside of the magnetic field of the third magnetic field source (magnet 420), where the third magnetic field source is separated into two portions. FIG. 27 depicts a schematic of an embodiment of an ion selection system in which the magnetic field of the third magnetic field source (magnet 420) is capable of bending the trajectories (428) of the modulated high energy polyenergetic positive ions towards the second magnetic field source (magnet 412). FIG. 28 depicts a schematic of an embodiment of an ion selection system in which the second magnetic field source (magnet 412) is capable of bending the trajectories (428) of the modulated high energy polyenergetic positive ions towards a direction that is not parallel to the direction of the laser-accelerated high energy polyenergetic ion beam. FIG. 29 depicts a schematic of an embodiment of an ion selection system in which the second magnetic field source (magnet 412) is capable of bending the trajectories (428) of the modulated high energy polyenergetic positive ions towards a direction that is parallel to the direction of the laser-accelerated high energy polyenergetic ion beam. FIG. 30 depicts a schematic of an embodiment of an ion selection system that further shows a secondary collimation device (430) capable of fluidically communicating a portion of the recombined high energy polyenergetic positive ions therethrough. FIG. 31 depicts an embodiment of an ion selection system that shows a secondary collimation device (430) that is capable of modulating the beam shape of the recombined high energy polyenergetic positive ions. FIG. 32 depicts details of a rotatable wheel (440) with an aperture (418) having a plurality of openings (442, 444), each of the openings capable of fluidically communicating high energy polyenergetic positive ions therethrough. FIG. 33 depicts details of an aperture that is a multileaf collimator (408) having openings (444, 442) that are capable of passing low energy ions, high energy ions, respectively, or a combination thereof. FIG. 34 depicts how an ion selection system in accordance with the invention manipulates ion beams. This figure depicts the forming of a laser-accelerated high energy polyenergetic ion beam including a plurality of high energy polyenergetic positive ions (110), the high energy polyenergetic positive ions (110) characterized as having a distribution of energy levels. The collimating of the laser-accelerated ion beam (110) is performed using a collimation device (collimator 408), and the positive ions (140, 142) are spatially separated according to their energy levels using a first magnetic field (magnet 402). The spatially separated high energy polyenergetic positive ions are modulated using an energy selection aperture (418) and the modulated high energy polyenergetic positive ions are recombined (428) using a second magnetic field (magnet 412). In this embodiment, a portion of the positive ions are transmitted through the aperture, e.g., having energy levels in the range of from about 50 MeV to about 250 MeV, and other portions are blocked by the energy selection aperture (418). FIG. 35 depicts the bending of the trajectories of the positive ions (140, 142) in a direction away from the beam axis of the laser-accelerated high energy polyenergetic ion beam (110) using the first magnetic field (magnet 402). FIG. 36 depicts the bending of the trajectories of the spatially separated positive ions (140, 142) in a direction towards aperture (444) using the third magnetic field (magnet 420). FIGS. 37 and 38 depict the spatially separated high energy positive ions being modulated by energy level (low energy (140) and high energy (142), respectively) using a location-controllable opening in aperture (442, 444). FIG. 39 depicts an embodiment of an ion selection system in which the third magnetic field (magnet 420) is capable of bending the selected positive ions towards the second magnetic field (magnet 412), as in FIG. 28. FIG. 40 depicts an embodiment of an ion selection system in which the high energy polyenergetic positive ions are spatially separated over distances up to about 50 cm. FIG. 41 depicts an embodiment of an ion therapy system that includes a laser-targeting system, the laser-targeting comprising a laser and a targeting system (104) capable of producing a high energy polyenergetic ion beam (110), the high energy polyenergetic ion beam including high energy polyenergetic positive ions having energy levels of at least about 50 MeV. The high energy polyenergetic positive ions are spatially separated (428) based on energy level (140, 142), and an ion selection system capable of producing a therapeutically suitable high energy polyenergetic positive ion beam from a portion of the high energy polyenergetic positive ions is provided. Also provided is a differential chamber (448) and an integration chamber (446). Positive ions of different energies will typically pass through different parts of the differential chamber (448) that measures the differences in energies of the ions, which monitors the energy of the selected ions. Typically, the differential chamber (448) does not control the energy selection aperture, The integration chamber is provided to generate a signal that is analyzed (e.g., by a computer or suitable data processor, not shown) to determine the position of the aperture (418) and the aperture openings. FIGS. 42(a–d) depicts perspective diagrams of a variety of laser-accelerated high energy polyenergetic positive ion beam treatment centers (200), that each suitably include at least one of the ion therapy systems depicted in FIGS. 21–41 and a location for securing a patient (i.e., a couch, 208). For example, FIG. 42(a) depicts a suitable treatment center of the type described above with respect to FIG. 17 in which the laser beam (202) is reflectively transported to the target assembly (100) using a plurality of mirrors (204). FIG. 42(b) depicts a suitable treatment center that includes an optical monitoring and control system (450) for the laser beam (202). FIG. 42(c) depicts a suitable treatment center in which at least one beam splitter or mirror (452) is provided to split the laser beam (202) into split or reflected laser beams 454 to each of at least two target assemblies (100) or to reflect the laser beam to one of the target assemblies (100). Depicted is a suitable treatment center that shows the laser-targeting system having two target assemblies and two ion selection systems each capable of individually producing a therapeutically suitable high energy polyenergetic positive ion beam from each of the individual high energy polyenergetic positive ion beams. An individual polyenergetic ion beam monitoring and control system is also provided for each of the therapeutically suitable high energy polyenergetic positive ion beams. This embodiment depicts a mirror (452) that is capable of being positioned in and out of the main laser beam to direct the beam to one of the ion therapy systems. Alternatively, a beam splitter can be used when a sufficiently powerful laser beam is provided so that split beams can be used simultaneously by two or more ion therapy systems. For providing patient privacy, typical ion therapy centers having two or more ion therapy systems will have an individual treatment room for each of the ion therapy systems. In such embodiments, the laser beam source is suitably located in a separate room or building. FIG. 42(d) depicts an embodiment of the treatment center that further includes an optical monitoring system (450). In this embodiment, the optical monitoring system (450) permits the operator to know, and control, which of the ion therapy systems is being activated. FIG. 43 is a flow-chart (500) of a method of treating a patient in accordance with the invention. This method includes the steps (502–508) of identifying the position of a targeted region in a patient, determining the treatment strategy of the targeted region, the treatment strategy comprising determining the dose distributions of a plurality of therapeutically suitable high energy polyenergetic positive ion beams for irradiating the targeted region (e.g., determining the energy distribution, intensity and direction of a plurality of therapeutically suitable high energy polyenergetic positive ion beams); forming the plurality of therapeutically suitable high energy polyenergetic positive ion beams from a plurality of high energy polyenergetic positive ions, the high energy polyenergetic positive ions being spatially separated based on energy level; and delivering the plurality of therapeutically suitable polyenergetic positive ion beams to the targeted region according to the treatment strategy. Thus, methods and systems providing high energy polyenergetic positive ion radiation therapy have been provided. While the present invention has been described in connection with the exemplary embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. For example, one skilled in the art will recognize that the present invention as described in the present application may apply to any configuration of magnets, apertures and collimators that selects positive ions based on energy from a source of laser-accelerated high energy polyenergetic positive ions. Therefore, the present invention should not be limited to any single embodiment, but rather should be construed in breadth and scope in accordance with the appended claims. M. 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Dawson, ‘Laser electron accelerator’, Phys. Rev. Lett. 43, 267 (1979). T. Tajima, K. Mima, and H. Baldis, Eds. “High Field Science” (Plenum, New York, 2000). Y. Ueshima, Y. Sentoku, Y. Kishimoto, and T. Tajima, ‘Simulation on interaction of a relativistically intense short pulse laser with solid thin film’, in Proc. JIFT Workshop, K. Mima and T. Tajima, Eds. (JIFT, Tokai, 1999). Umstadter, S. Y. Chen, A. Maksimchuk, G. Mourou, and G. Mourou, ‘Nonlinear Optics in Relativistic Plasmas and Laser Wakefield Acceleration of Electrons’, Science 273, 472(1996). D. Umstadter, S. Y. Chen, A. Maksimchuk, G. Mourou, and R. Wagner, Science 273, 606 (1996). L. J. Verhey and J. E. Munzenrider, Proton beam therapy, Ann Rev. Biophys. Bioeng. 11:331–57 (1982). Webb S., Optimization of conformal radiotherapy dose distributions by simulated annealing. Phys. Med. Biol., 34:1349–1370 (1990). Webb S. Optimizing the planning of intensity-modulated radiotherapy. Phys. Med. Biol., 39:2229–2246 (1994). S. C. Wilks, W. L. Kruer, T. Cowan, S. Hatchett, M. Key, A. B. Langdon, B. Lasinski, A. McKinnon, P. Patel, T. Phillips, M. Roth, P. Springer, R. Snavely, Bull. Amer. Phys. Soc. 44, 229 (1999). Xing L and Chen G. T. Y. Iterative methods for inverse treatment planning. Phys. Med. Biol, 41:2107–2123, (1996). |
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043436820 | abstract | A plant having feed water heating means for heating feed water supplied to a nuclear steam supply unit during plant start up and/or shutdown. High pressure heaters are positioned in a feed water delivery line between a feed water pump and a steam generator forming part of the nuclear steam supply unit. Steam is admitted into the high pressure feed water heaters from a main steam delivery line extending from the steam generator during start up to heat the feed water. |
abstract | An apparatus for an ion trap includes an electrically conductive substrate having top and bottom surfaces and having vias that cross from the top surface to the bottom surface. The apparatus includes a pair of planar first electrodes supported over said top surface and second electrodes having planar surfaces. The planar surfaces are located over said top surface, and portions of the planar surfaces are located laterally adjacent to said planar first electrodes. One of the second electrodes includes a portion that is located in one of the vias and traverses the substrate. |
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claims | 1. A method of generating a single photon, comprising:preparing an optical resonator including a resonator mode of a resonance angular frequency ωc;preparing a material contained in the optical resonator, including a low energy state |g> and a high energy state |e>, and including a transition angular frequency ωa between |g>−|e> that is varied by an external field;applying, to the material, light of an angular frequency ωl different from the resonance angular frequency ωc; andapplying a first external field to the material to vary the transition angular frequency ωa to resonate with the angular frequency ωl, such that a state of the material is changed to the high energy state |e>, and then applying a second external field to the material to vary the transition angular frequency ωa to resonate with the resonance angular frequency ωc, such that the state of the material is restored to the low energy state |g>. 2. The method according to claim 1, wherein the first external field is applied to make the transition angular frequency ωa equal to the angular frequency ωl for a period of π/Ω to change the state of the material to the high energy state |e>, and then the second external field is applied to make the transition angular frequency ωa equal to the resonance angular frequency ωc for a period of π/g to restore the state of the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system. 3. The method according to claim 1, wherein the first external field is applied to vary the transition angular frequency ωa to cross an angular frequency domain between ωl−Δ/2 and ωl+Δ/2 in a period longer than 1/Ω and shorter than T to change the state of the material to the high energy state |e>, and then the second external field is applied to vary the transition angular frequency ωa to cross an angular frequency domain between ωc−Δ/2 and ωc+Δ/2 in a period longer than 1/g and shorter than T to restore the state of the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 4. The method according to claim 1, wherein the first external field is applied to make the transition angular frequency ωa equal to the angular frequency ωl for a period of π/Ω to change the state of the material to the high energy state |e>, and then the second external field is applied to vary the transition angular frequency ωa to cross an angular frequency domain between ωc−Δ/2 and ωc+Δ/2 in a period longer than 1/g and shorter than T to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 5. The method according to claim 1, wherein the first external field is applied to vary the transition angular frequency ωa to cross an angular frequency domain between ωl−Δ/2 and ωl+Δ/2 in a period longer than 1/Ω and shorter than T to change the state of the material to the high energy state |e>, and then the second external field is applied to make the transition angular frequency ωa equal to the resonance angular frequency ωc for a period of π/g to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 6. The method according to claim 1, wherein the optical resonator is a one-sided Fabry-Perot resonator. 7. The method according to claim 1, wherein the material is a rare-earth ion contained in crystal, a transition between |g>−|e> corresponds to an f-f transition of the rare-earth ion, and the external field is an electric field or a magnetic field. 8. A method of reading a quantum bit, comprising:preparing an optical resonator including a resonator mode of a resonance angular frequency ωc;preparing a material contained in the optical resonator, including a low energy state |g>, a high energy state |e>, and two states |0> and |1>, and including a transition angular frequency ωa between |g>−|e> that is varied by an external field;generating a first pulse beam and a second pulse beam that resonate a transition between |g>−|e> and a transition between |l>−|e>, respectively;controlling the first pulse beam and the second pulse beam to temporally overlap each other to shift a first state in which a first intensity of the first pulse beam is higher than a second intensity of the second pulse beam, to a second state in which the second intensity is higher than the first intensity, to generate a third pulse beam;applying the third pulse beam to the material; andapplying a first external field to the material after applying the third pulse beam thereto, to vary the transition angular frequency ωa to resonate with the angular frequency ωl, then applying a second external field to the material to vary the transition angular frequency ωa to resonate with the resonance angular frequency ωc, and reading a quantum bit depending upon whether a photon ejected from the optical resonator is detected. 9. A single-photon generation apparatus comprising:an optical resonator including a resonator mode of a resonance angular frequency ωc;a material contained in the optical resonator, including a low energy state |g> and a high energy state |e>, and including a transition angular frequency ωa between |g>−|e> that is varied by an external field;a light source configured to apply, to the material, light of an angular frequency ωl different from the resonance angular frequency ωc;an external-field generation unit configured to apply external fields to the material to vary the transition angular frequency ωa to resonate with one of the angular frequency ωl and the resonance angular frequency ωc; anda controller configured to control the light source to apply the light of the angular frequency ωl to the material, and to control the external-field generation unit to make the transition angular frequency ωa resonate with the resonance angular frequency ωc to change a state of the material to the high energy state |e>, and then to control the external-field generation unit to make the transition angular frequency ωa resonate with the resonance angular frequency ωc to restore the state of the material to the low energy state |g>. 10. The apparatus according to claim 9, wherein the controller controls the external-field generation unit to make the transition angular frequency ωa equal to the angular frequency ωl for a period of π/Ω to change the state of the material to the high energy state |e>, and then controls the external-field generation unit to make the transition angular frequency ωa equal to the resonance angular frequency ωc for a period of π/g to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system. 11. The apparatus according to claim 9, wherein the controller controls the external-field generation unit to vary the transition angular frequency ωa to cross an angular frequency domain between ωl−Δ/2 and ωl+Δ/2 in a period longer than 1/Ω and shorter than T to change the state of the material to the high energy state |e>, and then controls the external-field generation unit to vary the transition angular frequency ωa to cross an angular frequency domain between ωc−Δ/2 and ωc+Δ/2 in a period longer than 1/g and shorter than T to restore the state of the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 12. The apparatus according to claim 9, wherein the controller controls the external-field generation unit to make the transition angular frequency ωa equal to the angular frequency ωl for a period of π/Ω to change the material to the high energy state |e>, and then controls the external-field generation unit to vary the transition angular frequency ωa to cross an angular frequency domain between ωc−Δ/2 and ωc+Δ/2 in a period longer than 1/g and shorter than T to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 13. The apparatus according to claim 9, wherein the controller controls the external-field generation unit to vary the transition angular frequency ωa to cross an angular frequency domain between ωl−Δ/2 and ωl−Δ/2 in a period longer than 1/Ω and shorter than T to change the state of the material to the high energy state |e>, and then controls the external-field generation unit to make the transition angular frequency ωa equal to the resonance angular frequency ωc for a period of π/g to restore the material to the low energy state |g>, Ω being a Rabi angular frequency that indicates a magnitude of coupling of the light of the angular frequency ωl and a two-state physical system, g being a coupling constant indicating a magnitude of coupling of the resonator mode and the two-state physical system, T being a longitudinal relaxation time of a transition between |g>−|e>, Δ being a homogeneous broadening. 14. The apparatus according to claim 9, wherein the optical resonator is a one-sided Fabry-Perot resonator. 15. The apparatus according to claim 9, wherein the material is a rare-earth ion contained in crystal, a transition between |g>−|e> corresponds to an f-f transition of the rare-earth ion, and the external-field generation unit applies an external field, such as an electric field or a magnetic field, to the material. 16. A quantum-bit-reading apparatus, comprising:the single-photon generation apparatus as claimed in claim 9, which employs a material including two states |0> and |1>, as well as the low energy state |g> and the high energy state |e>;a generation unit configured to generate a first pulse beam and a second pulse beam that resonate a transition between |g>−|e> and a transition between |1>−|e>, respectively;a controller configured to control the first pulse beam and second pulse beam to temporally overlap each other to shift a first state in which a first intensity of the first pulse beam is higher than a second intensity of the second pulse beam, to a second state in which the second intensity is higher than the first intensity, to generate a third pulse beam;an applying unit configured to apply the third pulse beam to the material; anda controller configured to control the external-field generation unit, after the applying unit applies the third pulse beam, to make the transition angular frequency ωa resonate with the angular frequency ωl, then to control the external-field generation unit to make the transition angular frequency ωa resonate with the resonance angular frequency ωc, and to read a quantum bit depending upon whether a photon ejected from the optical resonator is detected. |
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claims | 1. A radiation detector comprising:a photoelectric conversion substrate converting light to an electrical signal; anda scintillator layer being in contact with the photoelectric conversion substrate and converting externally incident radiation to light,the scintillator layer being made of a phosphor containing Tl as an activator in CsI, which is a halide, a concentration of the activator in the phosphor being 1.6 mass %±0.4 mass %, a concentration distribution of the activator in an in-plane direction being within ±15%, and a concentration distribution of the activator in a film thickness direction being within ±15%. 2. The radiation detector according to claim 1, wherein in the scintillator layer, the concentration distribution of the activator in the in-plane direction is ±15% or less in a region of a unit film thickness of 200 nm or less and the concentration distribution of the activator in the film thickness direction is ±15% or less in the region of the unit film thickness of 200 nm or less. 3. The radiation detector according to claim 1, wherein the scintillator layer has a columnar crystal structure. 4. A method for manufacturing a radiation detector including a photoelectric conversion substrate converting light to an electrical signal and a scintillator layer being in contact with the photoelectric conversion substrate and converting externally incident radiation to light,the scintillator layer being made of a phosphor containing Tl as an activator in CsI, which is a halide,the method comprising:forming the scintillator layer by a vapor phase growth technique using a material source of CsI and Tl, a concentration of the activator in the phosphor being 1.6 mass %±0.4 mass %, a concentration distribution of the activator in an in-plane direction being within ±15, and a concentration distribution of the activator in a film thickness direction being within ±15%. 5. A scintillator panel comprising:a support substrate transmissive to radiation; anda scintillator layer being in contact with the support substrate and converting externally incident radiation to light,the scintillator layer being made of a phosphor containing Tl as an activator in CsI, which is a halide, a concentration of the activator in the phosphor being 1.6 mass %±0.4 mass %, a concentration distribution of the activator in an in-plane direction being within ±15%, and a concentration distribution of the activator in a film thickness direction being within ±15%. 6. The scintillator panel according to claim 5, wherein in the scintillator layer, the concentration distribution of the activator in the in-plane direction is ±15% or less in a region of a unit film thickness of 200 nm or less and the concentration distribution of the activator in the film thickness direction is ±15% or less in the region of the unit film thickness of 200 nm or less. 7. The scintillator panel according to claim 5, wherein the scintillator layer has a columnar crystal structure. 8. The scintillator panel according to claim 5, wherein the support substrate is formed from a material composed primarily of a light element rather than a transition metal element. 9. A method for manufacturing a scintillator panel including a support substrate transmissive to radiation and a scintillator layer being in contact with the support substrate and converting externally incident radiation to light,the scintillator layer being made of a phosphor containing Tl as an activator in CsI, which is a halide,the method comprising:forming the scintillator layer by a vapor phase growth technique using a material source of CsI and Tl, a concentration of the activator in the phosphor is 1.6 mass %±0.4 mass %, a concentration distribution of the activator in an in-plane direction and a film thickness direction being within ±15%, and a concentration distribution of the activator in a film thickness direction being within ±15%. |
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description | This application is a continuation of U.S. patent application Ser. No. 11/422,092 filed on Jun. 5, 2005 which claims priority of provisional application 60/743,022 filed on Dec. 9, 2005. The present invention relates to the field of semiconductor fabrication tooling; more specifically, it relates to a method of improving the performance of charged particle beam fabrication tooling and apparatus for improving the performance of charged particle beam fabrication tooling. Ion implantation tools and other charged particle beam tools, are used extensively in the semiconductor industry. An ongoing problem is the deposition of foreign material on the wafers being processed. Existing methods of mitigating foreign material require extensive manual cleaning of tools after the loss of product to foreign material becomes excessive. Therefore, there is an ongoing need in the industry for a method of mitigating foreign material related product loss on wafers processed in ion implantation tools and other charged particle beam tools. A first aspect of the present invention is a chamber having an interior surface; a pump port for evacuating the chambers; a substrate holder within the chamber; a charged particle beam within the chamber, the charged beam generated by a source and the charged particle beam striking the substrate; and one or more liners in contact with one or more different regions of the interior surface of the chamber, the liners preventing material generated by interaction of the charged beam and the substrate from coating the one or more different regions of the interior surface of the chamber. A second aspect of the present invention is the first aspect, wherein each of the one or more liners is removable from the chamber. A third aspect of the present invention is the first aspect, further including one or more access ports in the chamber, the one or more access ports having corresponding access port covers and wherein each of the one or more liners is removable through at least one of the one or more access ports. A fourth aspect of the present invention is the first aspect, further including one or more access ports in the chamber, the one or more access ports having corresponding access port covers and wherein each of the one or more liners is removeably attached to one of the access port covers. A fifth aspect of the present invention is the first aspect, wherein each of the one or more liners has a first surface and a opposite second surface, the first surface in contact with a region of the interior surface of the chamber and the second surface facing the charged particle beam. A sixth aspect of the present invention is the fifth aspect, wherein the second surface of at least one of the one or more liners is textured. A seventh aspect of the present invention is the first aspect, wherein each of the one or more liners has a surface contour designed to mate with a corresponding contour of a region of the interior surface of the chamber. An eighth aspect of the present invention is the first aspect, wherein at least one of the one or more liners is compression fitted to a corresponding region of the interior surface of the chamber. A ninth aspect of the present invention is the first aspect, wherein at least one of the one or more liners is removeably fastened to a corresponding region of the interior surface of the chamber. A tenth aspect of the present invention is the first aspect, wherein at least one of the liners has a thickness of between about 0.05 inches and about 0.20 inches. An eleventh aspect of the present invention is the first aspect, wherein the liners comprise aluminum or graphite. A twelfth aspect of the present invention is the first aspect, wherein the liners are essentially free of iron, nickel, chrome, cobalt, molybdenum, beryllium, tungsten, titanium, tantalum, copper, magnesium, tin, indium, antimony, phosphorous, boron and arsenic. The term “charged particle beam tool or system” is defined to be any tool that generates a beam of charged atoms or molecules or other particles and is capable of directing that charged species to the surface of or into the body of a wafer or substrate. Examples of charged particle beam systems include but is not limited to ion implantation tools, ion milling tools and electron beam tools and other plasmas tools such as reactive ion etch (RIE) tools. A wafer is one type of semiconductor substrate. FIG. 1 a schematic side view of an exemplary ion implantation system according to an embodiment of the present invention. In FIG. 1, an ion implantation system 100 includes a beam generation chamber 105, an analyzer chamber 110, a pumping chamber 115, a resolving chamber 120 and a wafer chamber 125 connected to resolving chamber 120 by a flexible bellows 130. The sidewalls of beam generation chamber 105, analyzer chamber 110, pumping chamber 115, resolving chamber 120 and wafer chamber 125 are illustrated in sectional view, all other structures are illustrated in plan view. Beam generation chamber 105 includes an ion/plasma source 135, an extractor 140 and a beam defining aperture 145. Analyzer chamber 110 includes pole ends 150 of an electromagnet (not shown), an exit tube 155 an access port 160 and a access port cover 165. Pumping chamber 115 includes a pumping port 170, a deflector aperture 175, an access port 180 and an access port cover 185. Resolving chamber 120 includes a selectable aperture 190, a beam sampler 195, an electromagnetic aperture 200, an electron shower aperture 205, an electron shower tube 210, a first access port 215, a first access port cover 220, a second access port 225, a second access port cover 230, a third access port 235 and a third access port cover 240. Wafer chamber 125 includes a slideable and rotatable-stage 245. Beam generation chamber 105, analyzer chamber 110, pumping chamber 115, resolving chamber 120 and a wafer chamber 125 are all connected together by vacuum tight seals and evacuated through pump port 170. Additional pump ports may be provided, for example in beam generation chamber 105. Wafer chamber 125 can be tilted relative to resolving chamber 120. Beam generation chamber 105, analyzer chamber 110, pumping chamber 115, resolving chamber 120 and a wafer chamber 125 are fabricated from solid or hollow cast blocks of aluminum that are bored out. Electromagnetic pole end 150 comprises iron. Electron shower tube 210 comprises graphite and is negatively charged. In operation, an ion plasma is generated within ion source 135 and ions extracted from the ion source by extractor 140 to generate an ion beam that is projected along a beam path 250 by the electromagnet. After being passing through defining aperture 145, the ion beam is passed through analyzer chamber 110 where only ions of a predetermined charge to mass ratio exit through exit aperture 155. After passing through pumping chamber 135, selectable aperture 190, beam sampler 195, electromagnetic aperture 200, electron shower aperture 205, and electron shower tube 210, the ion beam strikes a substrate on stage 245. The exact locations and thicknesses of unwanted material layer formation is a function of the specific interior geometry and arrangement of components and the fabrication process being run, but in an example of one type of ion implantation tool these location occur in the analyzer, pumping and resolving chambers. These layers are formed by ions striking the walls and depositing there, materials (including photoresists) from the wafers vaporizing or being physically or chemically removed from the wafer as well as reaction of the ion/plasma beam with trace gases in the various chamber. When these layers become thick enough flakes break off and are swept down to the wafer chamber where they land on the wafers being processed. These flakes can have dimensions in the sub-micron regime. There are several locations on the interior surfaces of analyzing chamber 110, pumping chamber 115 and resolving chamber 120 that layers of material my build up on. These regions are discernable by buildup of layers of material after operation of implanter over extended periods of time. In analyzing chamber 110, the top bottom and sidewalls in a region “A” partially defined by the dashed lines is a region of particularly heavy material deposition. In pumping chamber 115, virtually all surfaces in a region “B” partially defined by the dashed lines is a region of particularly heavy material deposition. In resolving chamber 120, lower surfaces in a region “C” partially defined by the dashed lines is a region of particularly heavy material deposition. FIG. 2 is a schematic top view of the analyzer chamber 110 of FIG. 1 with removable liners in place. The sidewalls of analyzer chamber 110 are illustrated in sectional view, all other structures are illustrated in plan view. In FIG. 2, an analyzer inner foreign material shield 260 and an analyzer outer foreign material shield 265 are removeably attached to the respective sidewalls 270 and 275 of analyzer chamber 110. Removeably attached to access port cover 165 is an analyzer striker plate 280. Removeably attached to outer foreign material shield 260 and striker plate 280 are an analyzer upper liner 285A and an identical analyzer lower liner 285B illustrated by heavy lines for clarity. In one example, liners 2885A and 285B comprise aluminum. In one example liners 285A and 285B are between about 0.05 inches and about 0.20 inches thick. In one example, outer foreign material shield 260, inner foreign material shield 265 and striker plate 280 are comprised of graphite or aluminum. Outer foreign material shield 260, inner foreign material shield 265 and striker plate 280 roughened or textured by, for example, by machining, bead blasting, sand blasting, or etching. It is advantageous from a contamination point of view that outer foreign material shield 260, inner foreign material shield 265, striker plate 280 and liners 285A and 285B not contain significant amounts (are essentially free) of iron, nickel, chrome, cobalt, molybdenum, beryllium, tungsten, titanium, tantalum, copper, magnesium, tin, indium, antimony, phosphorous, boron or arsenic. A feature of liners 285A and 285B is that they do not overlap electromagnetic pole end 150 so as not to interfere with the magnetic flux lines of the electromagnet. FIG. 3 is a side view through line 2B-2B of FIG. 2 of an 290 analyzer liner assembly of FIG. 1. Analyzing chamber 110 (see FIG. 2) is rectangular in cross-section so analyzer assembly 290 comprising, striker plate 280 and liners 285A and 285B just fits in between a top wall 295A and a bottom wall 295B of analyzing chamber 110. Striker plate 280 has a height “H1” Inside surfaces 300A and 300B of respective liners 285A and 285B are advantageously roughened or textured by, for example, by machining, bead blasting, sand blasting, or etching. FIG. 4 is an isometric view of the inner shield of FIG. 1 and FIG. 5 is an isometric view of the outer shield of FIG. 1. In FIG. 4, a region 305 of inner shield 260 has a height “H1” and in FIG. 5, a region 310 of inner shield 260 also has a height “H1.” Returning to FIG. 2, it can be seen that inner and outer shields 260 and 265 and striker plate 280 have a first function of collecting ionized species that do not have the required mass/charge ratio and as a consequence get coated with a layer of unwanted material. Thus inner and outer shields 260 and 265 and striker plate 280 serve a second function of preventing portions of the top and bottom walls of analyzer chamber from becoming coated with unwanted material. Liners 285A and 285B also become coated with unwanted layers of material. By removing access port cover 160, liners 285A and 285B as well as outer foreign material shield 260, inner foreign material shield 265, striker plate 280 may be periodically removed for cleaning, clean and then reinstalled or a previously cleaned replacement set of liners, shields and striker plate installed in the machine while the removed liners and shields are cleaned. In either case tool down time is significantly less than cleaning the chamber surfaces themselves and the cleaning is more thorough. FIG. 6 is a schematic top view of pumping chamber 115 of FIG. 1 with removable liners in place. The sidewalls of pumping chamber 115 are illustrated in sectional view, all other structures are illustrated in plan view. In FIG. 6, a first aperture liner 315, a second aperture liner 320, a pump chamber liner 325, a pump port liner 330 and an access port liner 335 (illustrated by heavy lines for clarity) are removeably positioned in contact with interior surfaces of pumping chamber 115. L liners 315, 320, 325, 330 and 335 are removed and installed through access port 180. By removing access port cover 185, liners 315, 320, 325, 330 and 335 may be periodically removed for cleaning, clean and then reinstalled or a previously cleaned replacement set of liners installed in the machine while the removed liners are cleaned. In either case tool down time is significantly less than cleaning the chamber surfaces themselves and the cleaning is more thorough. While gaps are illustrated between liners 315, 320, 325, 330 and 335, these gaps are advantageously designed to be zero (liners touching) or as close to zero as practical without interfering with easy install and removal of the liners. In one example, liners 315, 320, 325, 330 and 335 comprise aluminum. In one example liners 315, 320, 325, 330 and 335 are between about 0.05 inches and about 0.20 inches thick. Liners 315, 320, 325, 330 and 335 are roughened or textured by, for example, by machining, bead blasting, sand blasting, or etching blasting. It is advantageous from a contamination point of view that liners 315, 320, 325, 330 and 335 not contain significant amounts of iron, nickel, chrome, cobalt, molybdenum, beryllium, tungsten, titanium, tantalum, copper, magnesium, tin, indium, antimony, phosphorous, boron or arsenic. FIG. 7A is a top view and FIG. 7B is a side view of pumping chamber liner 325 of FIG. 6. Pumping chamber liner 325 is comprised of two identical liners, a lower liner 325A and an upper liner 325B, which are curved along beam path 250 to fit the main bore of pumping chamber 115 (see FIG. 6) along the beam path direction. Notches 340A and 340B are curved to match the bore of an access port bore and a pump bore respectively. FIG. 8A is a side view and FIG. 8B is a front view of first aperture liner 315 of FIG. 6. First aperture liner 315 is comprised of two identical liners, a lower liner 315A and an upper liner 315B with corresponding bores 345A and 345B centered along beam path 250. FIG. 9A is a side view and FIG. 9B is a front view of second aperture liner 320 of FIG. 6. Second aperture liner 320 includes a circular bore 350 centered along beam path 250. FIG. 10A is a top view, FIG. 10B is a front view and FIG. 10C is a flat projection view of pump port liner 330 of FIG. 6. In FIG. 10C, an outside edge 355A will face pump port 170 (see FIG. 6) and an inside edge 355B will face the interior of pumping chamber 115 (see FIG. 6). In FIG. 10B, the curves of inside edge 355B are shaped to match intersection of the pump port bore and the main bore of pumping chamber 115 (see FIG. 6) when rolled to form a ring having a gap 360 where edges 365A and 365B are proximate to each other. Gap 360 allows access port liner to “spring” or compression fit inside pumping chamber 115 (see FIG. 6). FIG. 11A is a top view, FIG. 11B is a front view and FIG. 11C is a flat projection view of access port liner 335 of FIG. 6. In FIG. 11C, an outside edge 370A will face access port 170 (see FIG. 6) and an inside edge 370B will face the interior of pumping chamber 115 (see FIG. 6). In FIG. 11B, the curves of inside edge 370B are shaped to match intersection of the access port bore and the main bore of pumping chamber 115 (see FIG. 6) when rolled to form a ring having a gap 375 where edges 380A and 380B are proximate to each other. Gap 375 allows pump port liner to “spring” fit inside pumping chamber 115 (see FIG. 6). Returning to FIG. 6, liners 320 and 325 are held in place by liner 315 which in turn is held in place by liners 330 and 335. Thus liners 315, 320, 325, 330 and 335 are can be easily removed for cleaning and clean liners easily installed. FIG. 12 is a schematic top view of resolving chamber 120 of FIG. 1 with removable liners in place. The sidewalls of resolving chamber 120 are illustrated in sectional view, all other structures are illustrated in plan view. In FIG. 12, a third aperture liner 385, a first lower pump chamber liner 390, and a second lower pump chamber liner 395 are removeably positioned in contact with interior surfaces of resolving chamber 120. Liners 385, 390 and 395 are installed and removed through access port 215. By removing access port cover 220, liners 385, 390 and 395 may be periodically removed for cleaning, clean and then reinstalled or a previously cleaned replacement set of liners installed in the machine while the removed liners are cleaned. In either case tool down time is significantly less than cleaning the chamber surfaces themselves and the cleaning is more thorough. While gaps are illustrated between liners 385, 390 and 395, these gaps are advantageously designed to be zero (liners just touching) or as close to zero as practical without interfering with easy install and removal of the liners. In one example, liners 385, 390 and 395 comprise aluminum. In one example liners 385, 390 and 395 are between about 0.05 inches and about 0.20 inches thick. Liners 385, 390 and 395 are roughened or textured by, for example, by machining, bead blasting, sand blasting, or etching. It is advantageous from a contamination point of view that liners 385, 390 and 395 not contain significant amounts of iron, nickel, chrome, cobalt, molybdenum, beryllium, tungsten, titanium, tantalum, copper, magnesium, tin, indium, antimony, phosphorous, boron or arsenic. FIG. 13A is a side view and FIG. 13B is a front view of third aperture liner 385 of FIG. 12. Third aperture liner 385 includes a circular bore 400 centered along beam path 250. Also illustrated in FIG. 13A, (in cross-section) is second aperture liner 325 and a portion of resolving chamber 120. It can be seen that second aperture liner 325 fits into bore 400 to prevent foreign material from being trapped between third aperture liner 385 and walls of resolving chamber 120. FIG. 14A is a top view and FIG. 14B is a edge view of first resolving chamber liner 390 of FIG. 12. Liner 390 is curved along beam path 250 to fit the main bore of resolving chamber 120 (see FIG. 12) along the beam path direction. A key 405 is provided on one side of liner 390. Liner 390 is positioned on the bottom surfaces of resolving chamber 120 under selectable aperture 190, and beam sampler 195 (see FIG. 12). FIG. 15A is a top view and FIG. 15B is a edge view of second resolving chamber liner 395 of FIG. 12. Liner 395 is curved along beam path 250 to fit the main bore of resolving chamber 120 (see FIG. 12) along the beam path direction. A keyhole 410 is provided on one side of liner 395. Liner 395 is positioned on the bottom surfaces of resolving chamber 120 under selectable aperture 190, and beam sampler 195 (see FIG. 12). Key 405 of liner 390 (see FIG. 14A) engages keyhole 410 of liner 395 when the liners are in place. Returning to FIG. 12, there is no liner under electromagnetic aperture 200 and electron shower aperture 205 or on the upper surfaces of resolving chamber 120, because buildup of material in these locations is not significant. There are two options designing liners. The first option is to place liners over as many interior surfaces of the charged particle beam tools as possible without interfering with the operation of the tool. The second option is to place liners over only those interior surfaces of the charged particle beam tools where significant material buildup is expected (for example, cooler surfaces) or has been found to occur. FIG. 16 is a schematic top view of an exemplary charged particle beam tool 420 according to a embodiment of the present invention. In FIG. 16, charged particle beam system 420 comprises a source chamber 425, a pumping chamber 430, a beam alignment/defection chamber 435 and a target chamber 440. The arrangement of chambers can vary from tool to tool and some chambers may be combined into a single chamber. Pumping chamber 430 includes replaceable aperture liners 445A and 445B and replaceable pump chamber liners 450A, 450B and 450C which may be installed and removed through an access port 455. Beam alignment/defection chamber 435 includes replaceable aperture liners 460A and 460B and replaceable pump chamber liners 465A, 465B, 465C and 465D which may be installed and removed through an access ports 470A and 470B. A charged particle beam 475 is generated in source chamber 420 by a beam source 480, passes through pump chamber 430, beam alignment/defection 435 and strikes a target 485 in target chamber 440. In one example, beam 475 comprises a species selected from the group consisting of phosphorus containing species ions, boron containing species ions, arsenic containing species ions, germanium containing species ions, carbon containing species ions, nitrogen containing species ions, helium ions, electrons, protons, or combinations thereof. All liners 445A, 445B, 450A, 450B, 450C, 460A, 460B, 465A, 465B, 465C and 465D are formed of material selected to not contain chemical elements detrimental to the operation of or process being performed by tool 420. Liners 445A, 445B, 450A, 450B, 450C, 460A, 460B, 465A, 465B, 465C and 465D may be held in place by compression, fasteners or gravity. There may be more or less liners than the number shown in FIG. 16. The surfaces of liners 445A, 445B, 450A, 450B, 450C, 460A, 460B, 465A, 465B, 465C and 465D away from the inside surfaces of the various chambers may be advantageously roughened or textured by machining, bead blasting, sand blasting, or etching. Liners 445A, 445B, 450A, 450B, 450C, 460A, 460B, 465A, 465B, 465C and 465D may be cleanable or disposable. Thus, the embodiments of the present invention provide an apparatus and a method of mitigating foreign material related product loss on wafers processed in ion implantation tools and other charged particle beam tools. The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention. |
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description | The invention relates to a package for storing and/or transporting radioactive materials, such as fresh or irradiated nuclear fuel assemblies. It is concerned in particular with attaching a cooling element to an outer wall element of the package. A package for transporting and/or storing radioactive materials comprises a side body closed at its two longitudinal ends by a bottom and a lid respectively. The side body comprises a plurality of cooling elements which project from outer wall elements outwardly from the package. These cooling elements are often very close to each other along a circumferential direction of the package. The cooling elements are in particular attached by welding to the wall elements. But, some welding methods generate a risk of damaging/severing the cooling elements, in particular in proximity of their base. Other welding methods have the risk of deteriorating the outer wall elements. The invention aims at solving at least partially the problems met in solutions of prior art. In this regard, one object of the invention is a package for transporting and/or storing radioactive materials such as nuclear fuel assemblies. The package comprises a wall element and a cooling element attached to the wall element. The cooling element projects from the wall element outwardly from the package. The cooling element comprises a base and at least one fin which is integral with or fixed to the base. According to the invention, the base extends on either side of the fin respectively towards two opposite side ends of the base, each of the side ends being attached to the wall element via a weld. The base enables the weld metal zone to be moved away from the fin base, upon attaching the cooling element to the wall element. Thus, the risk of damaging/severing the fin is reduced, when using known welding methods with an intensive and localised heat input such as a laser beam or electron beam welding. In addition, such welding methods limit the rise in temperature of the wall element upon welding, as well as the risks of irreversibly deforming the wall element resulting therefrom by plastic shrinkage. Consequently, the invention reduces the risks of damaging the cooling element, while limiting deformations of the wall element upon attaching the cooling element to the wall element. The invention can optionally include one or more of the following characteristics combined to each other or not. Advantageously, the wall element comprises a housing in which the base is arranged, at least one of the side ends of the base being connected to at least one side edge of the housing via a weld. The heat conduction between the base and the wall element is thereby facilitated. According to a particular embodiment, the base is housed in the housing so as to be flush with the surface of the wall element at least at one edge of the housing. As a result, there is a better attachment of the cooling element and a better heat conduction between the cooling element and the wall element. Advantageously, the base height along at least one of the side ends is higher than or equal to half the mean thickness of the fin. The heat conduction between the base and the wall element is thereby improved. According to an advantageous embodiment, the wall element, the base and/or the fin comprise copper. The copper allows a suitable heat conduction but the wall element, the base and/or the fin are further likely to be plastically deformed. Advantageously, the base and the fin are integrally formed as a single piece. Thereby, they are rather easy to manufacture, while better discharging heat off the package. According to another advantageous embodiment, the package comprises a neutron shield block and at least one heat conduction inner element, the wall element being rigidly integral with or fixed to the heat conducting inner element, the heat conducting inner element being in contact with a ferrule of the package, the ferrule, the heat conducting inner element and the wall element surrounding at least partially the neutron shield block. According to another particular embodiment, the cooling element comprises at least two fins and a base common to the fins, the base extending on either side of the fin towards two opposite side ends of the base, each of the ends being attached to the wall element via a weld. Thereby, the manufacture of the package is easier, because of the common base. According to another advantageous embodiment, the package comprises a second cooling element attached to the wall element, the second cooling element comprising at least one fin which is integral with or fixed to the base, the distance between the cooling elements being lower than the height of at least one of the fins. The number and density of the cooling elements make the package difficult to make, while offering a proper heat discharge. The invention is also concerned with a heat conduction element for a package for transporting and/or storing radioactive materials, comprising: a heat conducting inner element, at least one cooling element, and a wall element for forming an outer shell portion of the package, the heat conducting inner element and the cooling element being located on either side of the wall element which mechanically and thermally connects them, the cooling element comprising a base and at least one fin which is integral with or fixed to the base. According to the invention, the base extends on either side of the fin respectively towards two side ends opposite to the base, each of the side ends being attached to the wall element via a weld. The invention also relates to a method for manufacturing a package for transporting and/or storing radioactive materials such as defined above. According to the invention, the method comprises a step of welding each of both opposite side ends of the base to the wall element. Advantageously, the width of the base between its two side ends is higher than or equal to twice the mean thickness of the fin. The risk of severing the fins is further reduced by further moving the weld metal zone from the fin base, upon attaching the cooling element to the wall element. According to a particular embodiment, the base is welded to the wall element by electron beam or laser beam welding. According to another particular embodiment, the base is arranged in a housing provided in the wall element, the base being welded in the housing along a thermal contact interface, the thermal contact interface being tilted at an angle between 0° to 30° with respect to the height direction of the fin. Attaching the cooling element to the base is improved, while promoting heat exchanges between the base and the cooling element. According to an advantageous embodiment, the manufacturing method comprises a step of making notches in a plate to form fins of the cooling element, the notches being spaced from each other along the longitudinal direction of the plate. Advantageously, the plate comprises the base of the cooling element. The step of welding the side ends of the base along the longitudinal direction of the plate occurs after the step of making the notches. Preferably, the method comprises a step of twisting the fins about their longitudinal axis after the welding step. Manufacturing a package with many fins is thereby facilitated. Identical, similar or equivalent parts of the different figures bear the same reference numerals so as to facilitate switching from one figure to the other. FIG. 1 represents a package 2 for transporting and/or storing radioactive materials such as nuclear fuel assemblies. The package 2 comprises a side body 20 delimited radially inwardly by a steel ferrule 21 and radially outwardly by heat conductive elements 22. The side body 20 extends along a longitudinal axis X-X of the package. The package is closed on either side of the side body 20 along the longitudinal direction X-X by a lid 4 and by a bottom 6. In the present document and unless otherwise indicated, the adjective “longitudinal” means substantially parallel to the longitudinal axis X-X, the adjective “radial” means oriented along a direction substantially orthogonal to this axis and the adjective “transversal” means along a plane substantially orthogonal to the longitudinal axis X-X. The term “circumferential” designates a direction about the longitudinal axis X-X. The ferrule 21 delimits an inner cavity 5 of the package 2 inside which a basket 7 is housed to store nuclear fuel assemblies inside the package 2. The package 2 and the basket 7 housed in the package 2 define a container 1 for transporting and/or storing radioactive materials. The heat conducting elements 22 each comprise a wall element 26 and at least one cooling element 30 which is welded to an outer surface S1 of the wall element 26. The heat conductive element 22 is made of copper or one of its alloys because of its high heat conductivity. In FIG. 2, each heat conductive element 22 is represented along a circumferential direction with two cooling elements 30 for the sake of visibility. In FIG. 3, the heat conductive element 22 is represented with three cooling elements 30 including a first cooling element 31 and a second cooling element 32. The heat conductive elements 22 thus each include a heat conducting inner element 28 which is bent at its first end 28a and which is connected to the inner surface S2 of the wall element 26 at its second end 28b opposite to the first end 28a. The first ends 28a are attached to the ferrule 21, for example by welding. As can be seen in FIG. 2, the wall elements 26 are connected to each other by welds 29, so as to form an outer wall of the package 2. The cooling elements 30 radially project from the wall element 26 outwardly from the package 2, whereas the heat conducting inner element 28 projects from the wall element 26 inwardly from the package 2. In other words, the cooling elements 30 and the heat conducting inner elements 28 are radially located on either side of the wall elements 26. The heat conducting elements 22 each enclose a neutron shield block 24 inside the side body 20. This neutron shield block 34 is located, along the circumferential direction, between two consecutive heat conducting inner elements 28. It is located along the radial direction between the ferrule 21 on the one hand and the wall element 26 of the heat conductive element 22 on the other hand. In reference more specifically to FIG. 3 and to the first embodiment, the cooling elements 30 each comprise a plurality of fins 34 which are spaced from each other along the longitudinal direction X-X of the package 2 and which extend along a radial direction Y-Y. On the other hand, the fins 34 are each twisted about their longitudinal axis Y-Y. Each of the cooling elements 30 comprises a single base 40 which is common to its fins 34 and from which the fins 34 radially extend. The base 40 extends substantially continuously along the longitudinal direction X-X over the entire length of the cooling element 30. More generally, the distance “d” between two consecutive cooling elements 31, 32 is lower than the height h1 of the fins 34, which allows heat to be properly discharged off the package 2. However, the low distance “d” between the cooling elements 30 relative to their height h1 tends to make the package 2 more difficult to manufacture. The heat is discharged off the container 1 from the ferrule 21, successively through the heat conducting inner elements 28, the wall elements 26 and possibly the cooling elements 30. FIGS. 4 and 5 represent a container 1 for transporting and/or storing radioactive materials which is distinguished from the container 1 according to the first embodiment by the structure of the cooling elements 30. In the second embodiment, the cooling elements 30 each comprise a single plate-shaped fin 35. This fin 35 substantially extends over the entire length of the package 2 along the longitudinal direction X-X. The base 40 extends substantially continuously along the longitudinal direction X-X over the entire length of the cooling element 30. The distance “d” between two consecutive fins 31, 32 is substantially identical to that between two consecutive fins 34 of the first embodiment. The number of cooling elements 30 of the package 2 according to the second embodiment is substantially identical to the number of cooling elements 30 of the package 2 according to the first embodiment. Only two cooling elements 30 have been represented in FIG. 5 for the sake of clarity. In reference both to the first and the second embodiment, each of the cooling elements 30 includes at least one fin 34, 35 and the base 40. The cooling element 30 is connected to the wall element 26 at the base 40. In reference to FIG. 6, the base 40 comprises a first side end 44 and a second side end 45 opposite to the first side end 44. The fin 34, 35 is located between the side ends 44, 45. In the first and the second embodiment, the side surfaces delimiting the side ends 44, 45 are substantially orthogonal to the bottom 42 of the base. The bottom 42 of the base is substantially orthogonal to the axis Y-Y of the fin 34, 35. The width of the base I2, taken between its side ends 44, 45 is about twice the mean thickness e1 of the fin 34, 35. The width I4 of the first end 44 is substantially equal to the width I5 of the second end 45. The height h2 of the base is higher than or equal to half the thickness e1 of the fin 34, 35. The base 40 is housed in a groove 50 made in the wall element 26. This groove 50 substantially extends along the longitudinal direction X-X of the package 2. It forms a recess made in the outer surface S1 of the wall element 26. The base 40 is housed in the groove 50 so as to be flush with the surface of the wall element 26. In other words, the height h2 of the base 40 is substantially equal to the height h3 of the groove 50, which promotes heat exchanges between the bottom of the groove 52 and the bottom 42 of the base 40 by contact. As can be seen in FIG. 8, the groove 50 is delimited sideways by a first side edge 54 and by a second side edge 55 opposite to the first side edge 54. The first side edge 54 is intended to be in mechanical contact with the first side end 44 of the base along a first heat exchange interface S4 via a first weld. The first side edge 54 has a shape substantially complementary to that of the first side end 44. The first weld extends throughout the first side edge 54, which promotes heat exchanges between the wall element 26 and the fin 34, 35. The second side edge 55 is intended to be in mechanical contact with the second side end 45 of the base along a second heat exchange surface S5 via a second weld. The second side edge 55 has a shape substantially complementary to that of the second side end 45. The second weld extends substantially throughout the second side edge 55, which promotes heat exchanges between the wall element 26 and the fin 34, 35 which are represented by the arrow F. The first weld and the second weld are the only welds of the base 40 to the wall element 26. They are made without filler material. The groove 50 has a width I3 between its side edges 54, 55 which is substantially equal to the width I2 of the base 40 by taking the first weld and the second weld into account, in order to promote heat exchanges between the wall element 26 and the cooling element 30. FIG. 7 represents an alternative embodiment in which the first side end 44 has a frustoconical shape forming an angle α1 with the axis Y-Y of the fin 34, 35 and the second side end 45 has a frustoconical shape forming an angle α2 with the axis Y-Y of the fin 34, 35. The angle α1 is substantially equal to the angle α2 and about 30° at most, in order to facilitate attaching the base 40 to the side edges 54, 55 of the groove, while ensuring a suitable heat conduction between the wall element 26 and the cooling element 30. The method for manufacturing the package 2 according to the first or the second embodiment is illustrated in reference to FIG. 8. The cooling elements 30 are attached to the wall element 26 by welding with an intensive and localised heat input such as a laser beam or electron beam welding. These welding methods are in particular favoured with respect to the arc welding which requires to pre-heat the wall element 26 and/or the cooling element 30 with the risk of irreversibly deforming them by plastic shrinkage. The cooling element 30 is first placed into the groove 50 of the wall element 26 so as to be flush with the surface of the wall element 26, as illustrated by the arrow A. Then, the cooling element 30 is attached at its first side end 44 to the first side edge 54 by a first welding beam 61, so as to make the first weld along the first heat contact interface S4. Concomitantly or after making the first weld, the cooling element 30 is attached at its second side end 45 to the second side edge 55 by a second welding beam 62, so as to make the second weld along the second heat contact interface S5. Welding the cooling element 30 at the side ends 44, 45 of the base enables the welding beams 61, 62 to be moved away from the fin 34, 35, which limits the risks of severing it upon welding. The welding beams 61, 62 are in particular substantially parallel to the axis Y-Y of the fin 34, 35. The cooling elements 30 are welded one after the other on the wall element 26 along the circumferential direction of the package 2. A second cooling element 32 is only attached to the wall element 26 when the first cooling element 31 immediately adjacent thereto has been attached to the corresponding wall element 26 over the entire length of this cooling element 31. FIG. 9 illustrates the manufacture of a package 2 for transporting and/or storing radioactive materials according to a third embodiment. The cooling elements 30 of this package each include either a plurality of fins 34 spaced along the longitudinal direction X-X, according to the first embodiment, or a single plate-shaped fin 35, as in the second embodiment. The manufacturing method according to FIG. 9 is distinguished from the manufacturing method according to FIG. 8 in that the base 40 of each cooling element 30 is not housed in a groove 50. On the contrary, the base 40 is welded to the uniformly planar surface of the wall element 26. The cooling element 30 is attached at its first side end 44 by the first welding beam 61, along a third heat contact interface S6. The cooling element 30 is attached at its second side end 45 by a second welding beam 62, along a fourth heat contact interface S7. The third heat contact interface S6 and the fourth heat contact interface S7 result from the side ends 44, 45 of the base 40 molten on the wall element 26. The welding beams are preferably substantially parallel with respect to the axis Y-Y of the at least one fin 34, 35 of the cooling element 30. The angle θ1 of the first welding beam 61 with the axis Y-Y is in particular between 0° and 30°. The angle θ2 of the second welding beam 62 with the axis Y-Y is in particular between 0° and 30°. The angle θ1 is substantially identical to the angle θ2. FIG. 10 illustrates the manufacture of the fins 34 of one of the cooling elements 30 of the package 2 according to the first embodiment. First, the manufacturing method comprises a step of making notches 33 in a plate 36 so as to form fins 34. This plate 36 is of an analogous shape as that of the cooling element 30 according to the second embodiment. The notches 33 are evenly spaced from each other along the longitudinal direction X-X. In this step, the fins 34 each have a rectangular shape. The notched cooling element 30 is then welded to the wall element 26 substantially over the entire length of the cooling element 30. The fins 34 are then twisted along the arrow B, preferably each in the same direction, about their longitudinal axis Y-Y, so as to increase the spacing between two consecutive fins 34. The heat conducting element 22 is then substantially identical to that represented in FIG. 3 in reference to the first embodiment. Of course, various modifications can be made by those skilled in the art to the invention just described without departing from the scope of the disclosure of the invention. The packages 2 have a substantially cylindrical side body 20. However, the side body 20 can assume other suitable shapes, such as a hexagonal shape. The heat conductive elements 22 represented can comprise a single cooling element 30, two cooling elements 30 to three cooling elements 30 or more. However, the distance “d” between the cooling elements 30 is preferably lower than the height h1 of the fins 34, 35. In the embodiments represented, the cooling elements 30 each extend substantially along the longitudinal direction X-X of the package 2 but it is intended that they can be tilted with respect to the longitudinal axis X-X. The cooling elements 30 remain preferably substantially parallel to each other. In the embodiments represented, the base 40 is substantially symmetrical through planar symmetry passing through the axis Y-Y of the fin 34, 35 of the cooling element. However, it could be dissymmetrical with respect to this plane. In reference to FIG. 10 and to manufacturing the package according to the first embodiment, the notches 33 can be made only once the cooling element 30 has been welded to the wall element 26. In reference to FIG. 6, the bottom 42 of the base 40 can also be welded to the bottom 52 of the groove. The welds can possibly be made with a filler material. |
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claims | 1. An environmentally sequestered nuclear spent fuel pool system comprising:a base slab;a plurality of vertical sidewalls extending upwards from and adjoining the base slab, the sidewalls comprising an inner surface and a top surface;a cavity that holds pool water defined by the inner surface of the sidewalls and the base slab, the cavity having an open top end;a pool liner system comprising an outer liner having an outer surface adjacent to the inner surface of the sidewalls and an inner surface that faces the cavity, an inner liner having an outer surface adjacent to the inner surface of the outer liner and an inner surface that faces the cavity and is wetted by the pool water, and an interstitial space formed between the outer surface of the inner liner and the inner surface of the outer liner, the interstitial space forming an air gap between the inner and outer liners that extends uninterrupted from the inner surface of the outer liner to the outer surface of the inner liner, each of the inner and outer liners terminating at a top terminal end;a top embedment plate circumscribing the cavity at a top surface of the sidewalls and adjoining the cavity without closing the open top end of the cavity, the top embedment plate comprising an inner vertical side facing the cavity and an opposite outer vertical side embedded within the sidewalls;wherein the top terminal end of the inner liner is sealably attached to the inner vertical side of the top embedment plate at a first seal location and the top terminal end of the outer liner is sealably attached to the inner vertical side of the top embedment plate at a second seal location such that a hermetically sealed top flow plenum is formed between the first and second seal locations; andwherein the top flow plenum is in fluid communication with the interstitial space. 2. The spent fuel pool system according to claim 1, wherein a horizontal portion of the inner and outer liners extend across and covers the base slab between opposing sidewalls, the horizontal portions of the inner and outer liners and portions covering the sidewalls forming a continuous barrier encapsulating the pool water. 3. The spent fuel pool system according to claim 1, wherein one vertical side of the top flow plenum is bounded by a portion of the outer surface of the inner liner and an opposing vertical side of the top flow plenum is bounded by a portion of the inner vertical side of the top embedment plate. 4. The spent fuel pool system according to claim 1, wherein the top flow plenum extends around an entire perimeter of the spent fuel pool. 5. The spent fuel pool system according to claim 1, further comprising a flow passageway formed through the top embedment plate that is in fluid communication with the top flow plenum, the flow passageway having an outlet end penetrating a top surface of the top embedment plate. 6. The spent fuel pool system according to claim 1, wherein the top embedment plate has a horizontal thickness greater than a thickness of the inner and outer liners combined. 7. The spent fuel pool system according to claim 1, wherein the top terminal ends of the inner and outer liners are welded separately and directly to the top embedment plate. 8. The spent fuel pool system according to claim 1, wherein the inner liner, the outer liner, and the top embedment plate are made of the same metallic material. 9. The spent fuel pool system according to claim 1, further comprising at least one fuel storage rack disposed on the base slab, the storage rack having a plurality of cells each configured for holding a spent nuclear fuel assembly containing nuclear fuel rods. 10. The spent fuel pool system according to claim 1 wherein the outer surface of the inner liner and the inner surface of the outer liner are both exposed directly to the air gap. 11. The spent fuel pool system according to claim 1 wherein the outer vertical side of the top embedment plate is directly adjacent to a recessed portion of the inner surface of the sidewalls. 12. The spent fuel pool system according to claim 11 wherein a top surface of the embedment plate is substantially flush with the top surface of the sidewalls. 13. The spent fuel pool system according to claim 1 wherein a height of the inner liner measured from a top surface of the base slab to the top terminal end of the inner liner is greater than a height of the outer liner measured from the top surface of the base slab to the top terminal end of the outer liner. 14. A nuclear spent fuel pool system comprising:a base slab having a top surface;a plurality of vertical sidewalls extending upwards from the top surface of the base slab, each of the sidewalls having an inner surface and an outer surface opposite the inner surface;a cavity that holds pool water collectively defined by the inner surfaces of the sidewalls and the top surface of the base slab;a pool liner system comprising:an outer liner having a first surface disposed against the inner surface of the sidewalls and an opposite second surface that faces the cavity; andan inner liner having a first surface facing the second surface of the outer liner and an opposite second surface that faces the cavity and is wetted by the pool water, wherein the first surface of the inner liner is spaced apart from the second surface of the outer liner thereby forming an interstitial space between the inner and outer liners;an embedment plate embedded into the sidewalls and having a top surface; andthe inner liner terminating at a top terminal edge that is spaced a first distance from the top surface of the embedment plate and the outer liner terminating at a top terminal edge that is spaced a second distance from the top surface of the embedment plate, the second distance being greater than the first distance;a first seal weld coupling the top terminal edge of the inner liner to the embedment plate; a second seal weld coupling the top terminal edge of the outer liner to the embedment plate; anda flow plenum defined between the first and second seal welds and between the embedment plate and a portion of the inner liner that protrudes above the top terminal end of the outer liner. 15. The spent fuel pool system according to claim 14 wherein the embedment plate extends from a bottom surface to the top surface, and wherein the bottom surface of the embedment plate is located between the top terminal edges of the inner and outer liners and the top surface of the base slab, each of the inner and outer liners coupled to the embedment plate along a vertical surface of the embedment plate that faces the cavity and extends between the top and bottom surfaces of the embedment plate. 16. The spent fuel pool system according to claim 14 wherein the top surface of the embedment plate is substantially flush with a top surface of the sidewalls, and wherein the embedment plate comprises a first surface facing the cavity and an opposite second surface, and wherein the first surface of the embedment plate is substantially flush with the inner surfaces of the sidewalls. 17. The spent fuel pool system according to claim 14 wherein the interstitial space forms an air gap that extends uninterrupted between the first surface of the inner liner and the second surface of the outer liner. 18. An environmentally sequestered nuclear spent fuel pool system comprising:a base slab;at least one sidewall extending upwards from the base slab, the sidewall having an inner surface;a cavity defined by the inner surface of the sidewall and the base slab, the cavity filled with pool water and having an open top end;a pool liner system comprising an outer liner having an inner surface and an outer surface and an inner liner having an inner surface and an outer surface, the outer surfaces of each of the inner and outer liners facing the inner surface of the sidewall and the inner surfaces of each of the inner and outer liners facing away from the inner surface of the sidewall, the outer liner being positioned between the inner liner and the sidewall so that the outer surface of the inner liner faces the inner surface of the outer liner, each of the inner and outer liners being located in the cavity such that the inner surface of the inner liner is wetted by the pool water;the outer surface of the inner liner spaced apart from the inner surface of the outer liner to form an air gap between the inner and outer liners;and a top embedment plate coupled to the sidewall along a top end of the sidewall, the top embedment plate comprising an inner vertical side facing the cavity, each of the inner and outer liners sealed to the inner vertical side of the top embedment plate at or near top terminal ends of the inner and outer liners; anda portion of the inner liner extending beyond the top terminal end of the outer liner so that a top flow plenum is formed between the outer surface of the portion of the inner liner and the inner vertical side of the top embedment plate; wherein the top flow plenum is in fluid communication with the air gap between the inner and outer liners; and wherein the top embedment plate comprises a passageway that is in fluid communication with the top flow plenum and an ambient environment. |
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abstract | An X-ray tube device includes a main body that incorporates a bulb, which generates X-rays, a collimator that is provided to protrude from the main body in an irradiation direction of the X-rays in a part of a front surface (first surface), which is a surface of the main body, and has an irradiation window for irradiating the X-rays with an adjusted irradiation range, and connectors that are provided for connecting a guard unit for keeping a distance from a test object, between the front surface (first surface) of the main body and a front surface (second surface), which is a surface of the collimator where the irradiation window is provided. |
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abstract | The invention relates to a printed value document having at least one luminescent substance. |
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description | The present invention relates generally to detection of gamma-ray and X-ray radiation, and specifically to systems and methods of radiation detection for medical diagnosis. In a typical nuclear medicine diagnostic procedure, a radiopharmaceutical material comprising a radioisotope tracer is administered to a patient. An example of a radioisotope tracer is Technetium-99m, which is a gamma ray emitter. Radiation subsequently emitted by the radiopharmaceutical material inside the body indicates sites at which the tracer has been absorbed. A detector for measuring the emitted radiation is generally positioned at several locations around the body, and a collimator is placed between the body and the detector so that the approximate direction from which radiation is emitted may be determined. The collimator is made of a material that is opaque to gamma-rays and X-rays, such as lead or tungsten. Channels through the collimator allow radiation emitted from a narrow solid angle to pass through the opaque material. In an embodiment of the present invention, a collimator comprises a plurality of substantially similar adjustable collimator channels, each collimator channel typically being arranged in a two-dimensional matrix. The collimator is typically positioned, in a camera head, before a detector mounting that provides a detector for each collimator channel. Each collimator channel directs radiation from radioisotopes injected into a region of the body of a patient to its respective detector. Each collimator channel in the collimator has multiple dimensional configurations. In a given dimensional configuration each collimator channel, collects radiation from a given volume of the region. For each of the multiple configurations the respective multiple volumes for a given channel have a different size. Furthermore, the multiple volumes for each given collimator channel typically enclose each other, in, a manner that is generally similar to Russian nested dolls. The detector for each channel measures respective multiple radiation levels received in the multiple configurations and, as described below, generates one or more images having intensities proportional to the concentration of the radioisotopes and an absorption coefficient in the region. By using collimator channel dimensional configurations that collect radiation from relatively large volumes, embodiments of the present invention reduce the acquisition time required for generation of images representing the region. The region being imaged may be divided into a number of similarly shaped virtual volume elements, herein termed voxels, each voxel having a respective radioisotope concentration. There is a dependency between the number of adjustable collimator channels, the number of configurations of each channel, and a possible number of voxels. Typically, on the basis of the dependency and the number of configurations, the processor sets the number and size of the voxels, and thus a resolution for the one or more images. Each of the image representations corresponds to a respective set of the voxels, and an operator may select the sets of voxels as desired, for example, as horizontal/vertical plane slices, and/or as one or more non-planar slices. In some embodiments of the present invention the configurations of a given collimator channel are implemented by changing an effective length and/or a cross-section of the channel. In a disclosed embodiment, the effective length of a collimator channel is changed by stacking one or more cylinders on each other. Alternatively or additionally, the effective length is changed by changing the separation between two or more cylinders. In some embodiments, each adjustable collimator channel is configured so that the multiple volumes of the region subtended at a given detector are sections of cones or pyramids having a common vertex and a common axis of symmetry, but different semi-angles. In one embodiment, at least part of the collimator channel comprises a cavity which can be filled with liquid that is opaque to the radiation. The channel may be adjusted by filling or partly filling the cavity with the liquid, which changes the volume of the region subtended at the detector associated with the collimator channel. In an alternate embodiment, two or more camera-heads are employed to measure radiation intensity, each camera head having a respective collimator. Each collimator has collimator channels in a different configuration. The camera heads are mounted so that they, and the detectors they contain, can be repositioned sequentially to the same position with respect to the region of the patient's body, and radiation measurements are made for each camera head. When a given camera head is in the position, a processor operates the camera. The signals received from the different camera heads (each in the same position) correspond to the signals received by one camera head having adjustable collimator channels. In a further alternate embodiment, rather than the two or more complete camera heads being repositioned, only the collimators are repositioned, the camera heads and the detectors they contain remaining fixed in position. In a yet further alternate embodiment, at least one of the collimators of the two or more camera heads comprises collimator channels that have multiple dimensional configurations. There is therefore provided, according to an embodiment of the present invention, apparatus for detecting radiation emitted from a number of volume elements of a body, the apparatus including: a first plurality of detector elements, each detector element being configured to output signals indicative of an intensity of radiation that is incident thereon; a first plurality of adjustable collimator channels, each adjustable collimator channel being associated with and being positioned between a respective detector element and the body, each adjustable collimator channel having a second plurality of dimensional configurations defining respective different sets of the volume-elements from which emitted radiation impinges on the respective detector element; and a processor coupled to compute a radiation intensity from at least a portion of the volume elements in response to the signals output by the detector elements in at least two of the dimensional configurations of the adjustable collimator channels. Each adjustable collimator channel may include a first collimator channel aligned with a second collimator channel and separated therefrom by an adjustable gap. The first collimator channel may be aligned with the respective detector element and may be separated therefrom by a variable gap. Typically, the processor is coupled to adjust at least one of the variable gap and the adjustable gap. The first and second collimator channels may have different cross-sectional areas. In one embodiment, each adjustable collimator channel includes a third plurality of collimator channels, and the processor is coupled to align one or more of the third plurality of collimator channels with the respective detector element. Alternatively, each adjustable collimator channel includes a third plurality of collimator channels each having different lengths. In a disclosed embodiment each adjustable collimator channel includes a cavity which is configured to receive a liquid opaque to the radiation. The liquid may include mercury. The cavity may alter a length of the adjustable collimator channel on receipt of the liquid. Alternatively, the cavity alters a cross-section of the adjustable collimator channel on receipt of the liquid. Typically, the emitted radiation includes gamma rays. The processor may be configured to generate a representation of radioisotopes in the body in response to the radiation intensity. In some embodiments, the dimensional configurations include a first configuration defining a first set of the volume elements and a second configuration defining a second set of the volume elements, wherein the first set includes the second set. Typically, the first set includes a first section of a first cone, and the second set includes a second section of a second cone, the first and the second cones having a common axis of symmetry. The processor may be coupled to compute the number of the volume elements in response to the value of the first plurality, the value of the second plurality, and the signals. Typically, the processor may be coupled to compute the number of the volume elements iteratively, so as to determine a largest number of the volume elements. The number may be a product of the value of the first plurality and the value of the second plurality. The portion may include a group of the volume elements selected by an operator of the apparatus. There is further provided, according to an embodiment of the present invention, apparatus for detecting radiation emitted from a body, the apparatus including: a first camera head, including a first detector element and a first collimator channel, the first detector element operative to output first signals indicative of a first radiation intensity, the first collimator channel being positioned between the first detector element and the body so as to define a first volume of the body from which emitted radiation impinges on the first detector element; a second camera head, including a second detector element and a second collimator channel, the second detector element operative to output second signals indicative of a second radiation intensity, the second collimator channel being positioned between the second detector element and the body so as to define a second volume of the body from which emitted radiation impinges on the detector element, the second volume being smaller than and included in the first volume; and a processor coupled to compute a radiation intensity from at least a portion of the body in response to the first signals and the second signals. The apparatus may include a positioning mount operative to set the first camera head in a given position and orientation to measure the first signals and to set the second camera head in the given position and orientation to measure the second signals. Typically, the first volume includes a first conic volume, and the second volume includes a second conic volume concentric with the first conic volume. The apparatus may include a positioning mount operative to set the first collimator channel in a first position and orientation with respect to the first detector element so as to measure the first signals and to set the second collimator channel in a second position, and orientation with respect to the second detector element so as to measure the second signals. Typically, the first collimator channel is fixedly coupled to the second collimator channel, and the first collimator channel and the second collimator channel are included in a common configurable collimator of the first and second camera heads. In an embodiment, at least one of the first and second collimator channels has a plurality of dimensional configurations defining respective different sets of volume, elements of the body from which the radiation is emitted. There is further provided, according to an embodiment of the present invention, a method for detecting radiation emitted from a number of volume elements of a body, including: providing a first plurality of detector elements, each detector element being configured to output signals indicative of an intensity of radiation that is incident thereon; positioning a first plurality of adjustable collimator channels between a respective detector element and the body, each adjustable collimator channel having a second plurality of dimensional configurations defining respective different sets of the volume elements from which emitted radiation impinges on the respective detector element; and computing a radiation intensity from at least a portion of the volume elements in response to the signals output by the detector elements in at least two of the dimensional configurations of the adjustable collimator channels. There is further provided, according to an embodiment of the present invention, a; method for detecting radiation emitted from a body, including: positioning a first collimator channel between, a first detector element and the body so as to define a first volume of the body from which emitted radiation impinges on the first detector element, the first detector element being operative to output first signals indicative of a first radiation intensity; positioning a second collimator channel between a second detector element and the body so as to define a second volume of the body from which emitted radiation impinges on the second detector element, the second detector element being operative to output second signals indicative of a second radiation intensity, the second volume being smaller than and included in the first volume; and computing a radiation intensity from, at least a portion of the body in response to the first signals and the second signals. There is further provided, according to an embodiment of the present invention, apparatus for detecting radiation emitted from a body, the apparatus including: a detector element, which is operative to output signals indicative of an intensity of radiation that is incident thereon; an adjustable collimator channel, positioned between the detector element and the body so as to define a volume of the body from which emitted radiation impinges on the detector element, and having at least a first configuration in which the emitted radiation impinges on the detector element from a first volume and a second configuration in which the emitted radiation-impinges on the detector element from a second volume smaller than and included in the first volume; and a processor coupled to compute a radiation intensity from at least a portion of the volume in response to the signals output by the detector element in at least the first and second configurations of the adjustable collimator channel. There is further provided, according to an embodiment of the present invention, a method for detecting radiation emitted from a body, including: outputting, from a detector element, signals indicative of an intensity of radiation that is incident on the detector element; positioning an adjustable collimator channel between the detector element and the body so as to define a volume of the body from which emitted radiation impinges on the detector element, the adjustable collimator channel having at least a first configuration in which the emitted radiation impinges on the detector element from a first volume and a second configuration in which the emitted radiation impinges on the detector element from a second volume smaller than and included in the first volume; and computing a radiation intensity from at least a portion of the volume in response to the signals output by the detector element in at least the first and second configurations of the adjustable collimator channel. The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: FIG. 1 is a schematic, pictorial illustration of a radiation detection system 20, according to an embodiment of the present invention. A radiopharmaceutical having a radioisotope tracer is administered to a patient's body 22. A radiation detecting device 21, typically an X-ray camera or a gamma-ray camera, senses radiation emitted from sites in a region 23 of body 22 that have absorbed the radioisotope tracer. Depending on the procedure being followed in using system 20, region 23 may comprise a part or all of body 22. Device 21 is hereinbelow, by way of example, assumed to comprise a gamma-ray camera head 24. Camera head 24 comprises a collimator 26 and detector elements 40, herein also referred to as detectors 40. Elements 40 typically comprise electrodes coupled to a semiconducting material such as Cadmium Zinc Telluride. Such detector elements are known in the art, and an example of a detector having such detector elements is described in U.S. Pat. No. 5,847,398 to Shahar, et al., which is incorporated herein by reference. Alternatively, detectors 40 may be formed from scintillators. Detectors 40 may be used for measuring X-rays and may operate by photon counting or current integration. Unless stated otherwise, in the description hereinbelow detectors 40 are assumed to comprise electrodes coupled to a semiconducting material. A cross-section of the collimator and the detectors is shown in more detail in FIG. 2. Camera head 24 transmits to a processor 28 signals indicative of the radiation from region 23 reaching detectors 40. Processor 28 typically processes the signals to determine radioisotope absorption sites, as well as concentrations of the radioisotope at the sites. Processor 28 may also be coupled to a display 30 or to other image generating means, such as a printer, which may provide a map or image of the absorption sites and concentrations therein for analysis by an operator 32 of system 20. To perform its operations processor 28 uses a memory 29 to store the signals from the camera. Memory 29 also stores software for analysis of the signals, and results of the analysis, as is described in more detail below. FIG. 2 is a schematic view of a cross-section of camera head 24, in a plane perpendicular to a virtual line II (FIG. 1), according to an embodiment of the present invention. The cross-section shows collimator 26, which is positioned between the patient's body and detectors 40. Detectors 40 comprise a multiplicity, of similar electrodes mounted together, typically as a rectangular or hexagonal array, on a semiconducting material 51, which acts as a detector mounting 54. Collimator 26 comprises two collimation plates, a collimation plate 42 adjacent the detector mounting, and a collimation plate 44, which faces region 23. Plate 42 is configured to cover detector elements 40, and plate 44 is generally similar in form to plate 42. Plates 42 and 44 are coupled to each other by a set of, brackets 46, which are adjustable so as to vary the width of a gap 48 between the plates. Alternatively or additionally, plates 42 and 44 may be coupled together by any other convenient adjustable coupling system known in the art, such as clamps and/or braces. Brackets 46 may set the width of gap 48 based on external automated control, such as control by processor 28, or, alternatively, by, manual control. Additionally, a sensor (not shown) may sense the gap width and transmit the value to processor 28 so that the processor may control the gap. Furthermore, the distance between mounting 54 and plate 42 may also be, varied, typically by a coupling system similar to that described above, and which for reasons of clarity is not shown in FIG. 2. It is to be understood that the horizontal orientation of the two plates and mounting 54 indicated in FIG. 2 is merely for the purpose of elucidation and that collimator 26 comprising the two joined plates may be oriented in any suitable direction vis-à-vis region 23. The distance of collimator 26 from region 23 may also be adjusted by processor 28. Plates 42 and 44 are made of a material which is selected to be opaque to the radiation emitted by the radioisotope. Such materials typically comprise lead and/or tungsten, although other materials for forming collimator channels are known in the art. Holes through plate 42, indicated as collimator channels 50, are aligned with detector elements 40 and with holes in plate 44, indicated as collimator channels 52. Channels 50 and 52 typically have circular, rectangular, or hexagonal cross-sections, although embodiments of the present invention are not limited to a particular cross-sectional shape for the channels. In some embodiments the cross-section of the collimator channels has the same shape as detector elements 40. Alternatively, the channel cross-section and detector element shape may be different. Herein, plates 42 and 44, and their associated collimator channels, are assumed, by way of example, to be formed by drilling holes in a solid sheet. However, other methods for forming the plates and their channels will be apparent to those having ordinary skill in the art, such as by using “honeycombs” of the opaque material, and/or by casting the material. All such methods are assumed to be comprised within the scope of the present invention. Hereinbelow, except where otherwise stated, detectors 40 are assumed to be circular, and channels 50 and 52 are assumed to have circular cross-sections so that they are cylindrical prisms. Those with ordinary skill in the art will be able to adapt the description herein for detectors that are not circular, and/or for collimator channels that have non-circular cross-sections, thus forming non-circular prisms. Collimator channels 50 and 52 permit some of the emitted radiation from region 23 to pass through collimator 26 so as to impinge on detectors 40. Inter alia, the amount of radiation passing through collimator 26 may be varied by adjusting gap 48, and/or by varying the distance of detectors 40 from plate 42. Examples of such variations are described further hereinbelow with reference to FIGS. 3A, 3B, and 3C. FIGS. 3A, 3B, and 3C are schematic diagrams of dimensional configurations of a pair 47 of collimator channels, according to an embodiment of the present invention. Pair 47 is also herein termed adjustable collimator channel 47. Adjustable collimator channel 47 is formed from a given collimator channel 50 and an associated collimator channel 52 within collimator 26 (FIG. 2). A respective detector 40 and adjustable collimator channel 47 define an axis of symmetry 67 of the adjustable collimator channel. In a first dimensional configuration of collimator 26, as illustrated in FIG. 3A, gap 48 between plates 42 and 44 is a relatively small distance H1, and a distance 41 between detector 40 and plate 42 is approximately 0. An effective length H1 of collimator 26, comprising the widths of plates 42 and 44 and the width of gap 48, is therefore relatively short. In this configuration, adjustable collimator channel 47 defines a volume 66 of region 23 (FIG. 1) from which emitted radiation may be received by the detector 40 associated with the channel. Radiation from this volume subtends a solid angle 64. Volume 66 is approximately in the shape of a frustum, although the base and the upper surface of the volume are bounded by the surface, of region 23 or of body 22, and are typically not parallel planes. In a second dimensional configuration of collimator 26, illustrated in FIG. 3B, gap 48 between plates 42 and 44 is increased from that of the first configuration, and distance 41 remains at approximately 0. An effective length H2 of collimator 26 is longer than H1. In the second configuration, adjustable collimator channel 47 defines a volume 76 for received emitted radiation, volume 76 being smaller than, and included in, volume 66. Radiation from volume 76 subtends a solid angle 74, which is smaller than, and which is included in, solid angle 64. In, a third dimensional configuration of, collimator 26, illustrated in FIG. 3C, gap 48 between plates 42 and 44 is the same as for the second configuration. Distance 41 has been changed so that it is greater than 0. An effective length H3 of collimator 26 is longer than H2. In the third configuration, adjustable collimator channel 47 defines a volume 77 for received emitted radiation, volume 77 being smaller than, and included in, volume 76. Radiation from volume 77 subtends a solid angle 75, which is smaller than, and which is included in, solid angle 74. The configurations illustrated in FIGS. 3A, 3B, and 3C are implemented by changing dimensions of collimator channel 47, and/or by changing dimensions between the collimator channel and its associated detector 40. By varying these dimensions, a given collimator channel 47 may be arranged into a multiplicity of configurations, each configuration being selected to receive radiation from a different volume, such as are exemplified by volumes 66, 76, and 77. Embodiments of the present invention use a multiplicity of configurations of collimator 26 and detectors 40, as explained in more detail below, to determine concentrations of the radioisotope in different volume elements of region 23. The description above exemplifies that volumes 77, 76, and 66 enclose each other. Other configurations of plates, 42 and 44, such as may be generated by plates 42 and 44 being translated horizontally with respect to each other, may generate a volume for each configuration that may not completely enclose each other. Such configurations will be apparent to those having ordinary skill in the art, and all such configurations are assumed to be comprised within the scope of the present invention. FIGS. 4A, 4B, and 4C are schematic diagrams illustrating a method of partitioning region 23 (FIG. 1), according to an embodiment of the present invention. In FIG. 4A region 23 is assumed to be enclosed in a volume 80, which is divided into a set of similarly shaped volume elements 82. Volume elements 82 are also herein termed voxels 82. Typically, voxels 82 are parallelepipeds, although voxels 82 may comprise any other shapes, such as triangular prisms, which can be arranged to completely fill the volume they enclose. Hereinbelow, except where otherwise stated, voxels 82 are assumed to comprise rectangular parallelepipeds, and volume 80 is also assumed to comprise a rectangular parallelepiped. Detectors 40, mounted on detector mounting 54, are herein assumed to be rectangular in shape. Collimator channels 47 are assumed to be generally similar to those described with respect to FIGS. 3A, 3B, and 3C. In the following description, detectors 40 and collimator channels 47 are differentiated using letter suffixes, e.g., detectors 40A, 40B, . . . and collimator channels 47A, 47B, . . . . For clarity only two collimator channels 47A and 47B are depicted in FIG. 4A, and the channels are shown as cylinders. Channels 47A and 47B are assumed to be respectively associated with detectors 40A and 40B. Although in operation of system 20 channels 47A and 47B typically have the same effective lengths, herein, for the purpose of explanation, channel 47A is assumed to have an effective length longer than that of channel 47B. Detector 40A receives radiation from a set 84 of voxels and parts of voxels. The voxels and parts in set 84 are comprised of those elements that are included in a cone 88 defined by the dimensions of channel 47A, the dimensions of detector 40A, and the relative orientations and spacing between the collimator channel and the detector. Similarly, detector 40B receives radiation from a set 86 of voxels and parts of voxels. The elements in set 86 are comprised of those that are included in a cone 90 defined by the dimensions of channel 47B, the dimensions of detector 40B, and the relative orientations and spacing of the collimator channel and detector. Assuming that the only difference between the cone definitions is the difference in height of the two collimator channels, cone 86 encloses more voxels and parts of voxels than cone 88. As is apparent from the diagram, some voxels and parts of voxels are included in both cones. FIG. 4B is a cross-section of voxels 82, detectors 40, and mounting 54, as shown in FIG. 4A. In FIG. 4B, cross-sections of cones 91, 92, 93, 94, 95, 96 generated by two collimator channels 47C and 47D, for respective detectors 40C and 40D, are shown. The collimator channels are shown in FIG. 4B as having their shortest effective height, and each channel, by way of example, has two other longer effective heights. Each collimator channel and detector combination thus generates three cones, so that detector 40C receives radiation from voxels and parts of voxels in cones 91, 92 and 93, and detector 40D receives radiation from elements in cones 94, 95, and 96. Bases of the cones are also shown in FIG. 4B. FIG. 4C is a schematic exploded view of voxels 82 and parts of voxels included in a typical cone generated by a given detector 40 and associated collimator channel 47 not shown in FIG. 4C). FIG. 4C illustrates that complete voxels may be included in the cone, as well as parts of voxels that typically have differing shapes and volumes from each other. As explained in more detail below, embodiments of the present invention use signals derived from different multiple sets of voxels and parts of voxels, such as, sets described in the examples above, to determine concentrations of radioisotopes in the voxels. FIG. 5 is a schematic diagram illustrating a method for analysis of results obtained in operation of system 20, according to an embodiment of the present invention. FIG. 5 shows an exemplary detector 40E and its associated collimator channel 47E, which have multiple different dimensional configurations, as have been described above. Detector 40E is also herein-referred to as detector d, d acting as an identifying index, or pixel number, of the detector. In a given one of the configurations, herein termed confn, detector d and collimator channel 47E define a generally conical or pyramidal structure 100, corresponding to a “viewing solid angle” for the detector. The actual dimensions and shape of structure 100 depend on the dimensions of detector d and collimator channel 47E, as well as on their relative positions, as is described above. In system 20 it is assumed that detector d and collimator channel 47E can be reconfigured to N different configurations, defining different structures generally similar to structure 100, referred to herein as conf1, . . . , confn, . . . , confN, where N is a positive integer and n is any integer between 1 and N. Region 23 is assumed to be enclosed in rectilinear volume 80, which is partitioned into a total of M=I·J·K voxels 82, as described above (FIG. 4A). The M voxels are constructed on mutually orthogonal i, j, and k axes, and volume 80 has edges (in terms of numbers of voxels) I, J, and K. In the following description each voxel 82 may be uniquely identified by an ordered triple (i,j,k), where i, j, k are positive integers, or by a positive integer m, where 1<m<M=I·J·K. During operation of system 20, there is an average concentration Ci,j,k of radioisotope in each voxel (i,j,k), and the radiation intensity emitted by voxel (i,j,k) is linearly dependent on Ci,j,k. The intensity of radiation Ii,j,kd,n received by detector 40E from each voxel (i,j,k) is linearly dependent on a solid angle θi,j,kd,n and a volume fraction Vi,j,kd,n, both of which are subtended by the voxel (i,j,k) at detector 40E. When a channel such as channel 47E is associated with detector 40E and has configuration confn, then Vi,j,k=1 if voxel (i,j,k) is completely enclosed by virtual structure 100 and 0<Vi,j,k<1 if voxel (i,j,k) is partly enclosed by virtual structure 100. The intensity Ii,j,k emitted from a voxel (i,j,k) is given by:Ii,j,k=Ci,j,k·Vi,j,k (1) where Ci,j,k is the average radioisotope concentration in voxel (i,j,k) and Vi,j,k is the volume or partial volume of voxel (i,j,k). Defining Ii,j,k as the radiation intensity emitted from a complete voxel (i,j,k) when Vi,j,k=1 and then Ii,j,k=Ci,j,k. According to this definition, when only a fraction Vi,j,k<1 of voxel (i,j,k) is enclosed in virtual structure 100, then the radiation intensity emitted from such an incomplete voxel (i,j,k) is Ii,j,k=Ci,j,k·Vi,j,k. The intensity of radiation Ii,j,kd,n received by detector d from voxel (i,j,k) when the associated collimator is configured in confn, is given by:Ii,j,kd,n=αi,j,kd,n·Ci,j,kθi,j,kd,n·Vi,j,kd,n=βi,j,kd,n·Ii,j,k (2) where αi,j,kd,n is a linearizing constant of voxel (i,j,k) also known as the absorption/attenuation factor between voxel (i,j,k) and detector d associated with a collimator in configuration confn. In a situation without absorption and scattering αi,j,kd,n is equal to 1. Expression (2) may also be written in the form:Ii,j,kd,n=βi,j,kd,n·Ii,j,k (2a) where βi,j,kd,n=αi,j,kd,n·θi,j,kd,n·Vi,j,kd,n is the proportional coefficient between the radiation intensity Ii,j,kd,n received by a detector d from voxel (i,j,k) and the radiation Ii,j,k emitted from voxel (i,j,k) when the associated collimator is configured in confn. From expressions (2) and (2a), the total intensity Sd,n received by detector d from all the voxels (i,j,k) defined by virtual structure 100 corresponding to configuration confn of the collimator is given by: S d , n = ∑ i , j , k conf n I d , n = ∑ i , j , k conf n α i , j , k d , n · C i , j , k · θ i , j , k d , n · V i , j , k d , n = ∑ i , j , k conf n β i , j , k d , n · I i , j , k ( 3 ) where the sum in expression (3) is taken over all voxels (i,j,k) wholly or partly included in configuration confn, αi,j,kd,n is the attenuation factor between voxel (i,j,k), and detector d associated with collimator channel in configuration confn, θi,j,kd,n is the solid angle in which voxel (i,j,k) is viewed from detector d associated with collimator channel in configuration confn, Vi,j,kd,n is the volume fraction of voxel (i,j,k) enclosed by structure 100 as viewed from detector d associated with a collimator channel in configuration confn, and βi,j,kd,n=αi,j,kd,n·θi,j,kd,n·Vi,j,kd,n is the proportional coefficient between the intensities of the radiation emitted from voxel (i,j,k) and the radiation received from voxel (i,j,k) at detector d associated with collimator channel in configuration confn. In expression (3) the values of i,j, and k are chosen to correspond to the same voxel. Similarly, the values of d and n are chosen to correspond to the same detector and collimator configuration, respectively. Expression (3) may also be written in the formSd,n=βI,I,Id,n·II,I,I+ . . . +βI,J,Kd,n·Ii,j,k (4) where I,J,K are the maximum values of integers i,j,k. It will be understood that for any specific value of i,j,k, a number of values of βi,j,kd,n are 0, corresponding to those voxels and parts of voxels which are not included in the structure equivalent to virtual structure 100. Also, the products θi,j,kd,n·Vi,j,kd,n are functions of the geometry of the detector configurations, and of the locations and dimensions of voxels (i,j,k). Thus, the values of θi,j,kd,n may be pre-calculated from the detector configurations and voxel parameters. Similarly, the attenuation coefficients αi,j,kd,n may be found by attenuation mapping, also known as an attenuation correction method, which is known in the X-ray imaging art. Alternatively, αi,j,kd,n may be assumed to be equal to 1. Accordingly, the proportional coefficients βi,j,kd,n=αi,j,kd,n·θi,j,kd,n·Vi,j,kd,n may be pre-calculated from the detector configurations and voxel parameters as well. Herein it is assumed that there are D detectors (pixels) in camera head 24, each detector and its associated collimator channel having N configurations. Accordingly, D and N are the maximum values for integers d,n, respectively. Thus, in total, there are D·N expressions similar to expression (4): S 1 , 1 = β 1 , 1 , 1 1 , 1 · I 1 , 1 , 1 + … + β I , J , K 1 , 1 · I I , J , K ⋮ · S D , N = β 1 , 1 , 1 D , N · I 1 , 1 , 1 + … + β I , J , K D , N · I I , J , K ( 5 ) Expressions (5) are D·N simultaneous linear equations where the coefficients βi,j,kd,n=αi,j,kd,n·θi,j,kd,n·Vi,j,kd,n are known and their values may be pre-calculated. The values of βi,j,kd,n may be calculated from the geometrical relations between the positions of the measured object, the detector and the configuration of the collimator. For example, as illustrated by FIGS. 3A-3C, the coefficients of βi,j,kd,n can be calculated from the size of detector 40, the distance H1, gap 48 between channels 42 and 44, the distance of detector 40 from channel 42 and the distance between volume 66 and channel 44. The values of these geometrical parameters may be provided by an operator into processor 28 which in turn calculates coefficients βi,j,kd,n. Alternatively, the imaging system may include position sensors (not shown in FIGS. 3A-3C) to measure these geometrical parameters, so that they may be provided to processor 28 to calculate coefficients βi,j,kd,n. There are M=I·J·K unknown intensities Ii,j,k emitted from voxels i,j,k. Accordingly, ifD·N=M=I·J·K (6) i.e., if the number M of voxels i,j,k is equal to the product of the number D of detectors and the number N of configurations of the detectors, then, as is known in the mathematical art, expressions (5) may be uniquely solved for all the intensities Ii,j,k emitted from voxels i,j,k. The existence of a solution depends on the values of coefficients βi,j,kd,n. Methods for evaluating whether expressions (5) are solvable are well known in the art, and are explained, for example, in “A First Course in Numerical Analysis” by Ralston et al., published by McGraw-Hill. In an, embodiment of the present inventions processor 28 sets the D detectors 40 and their associated collimator channels 47, of camera head 24, to have N configurations. For each configuration, the processor receives a signal from each detector, so that in total processor 28 receives D·N signals, corresponding to the values SI,I . . . SD,N of expressions (5). As is described below with reference to FIG. 6, processor 28 uses the signals received from the position sensors, indicating the value of the geometrical parameters mentioned above, to set a number of voxels within a region being imaged, and to find the radiation intensity Ii,j,k or number of photons emitted by the radioisotope within each voxel. The method of image-reconstruction described above has the following advantages: High resolution. High sensitivity. Cross-section presentations. As explained above, the size of voxels i,j,k, may be chosen as desired. Thus the size of these voxels may even be chosen to be smaller than the size of the detector (pixel) to achieve sub pixel resolution. The smaller the size of voxels i,j,k, the larger is their number M and thus the number of the linear equation in expression (5) should be larger and equal to M (D·N=I·J·K). This means that for increasing the resolution, the number of configurations N of the collimator associated with the detector should be increased as well to fulfill the requirement of expression (6). In embodiments of the present invention the sensitivity of the camera head is increased-significantly in comparison with a camera head having a prior art collimator. The configurable collimator according to embodiments of the present invention has multiple configurations corresponding to multiple solid angles, of a collimator channel associated with a detector, through which the measured object is viewed and measured. Most of these angles are much larger than the solid angle of (a prior art collimator channel associated with a detector, through which the measured object is viewed and measured. This results in better collection efficiency of the radiation emitted from the measured object and collected by the detectors and leads to a higher sensitivity in the embodiments according to the present invention. The method described above is highly flexible for image display and presentation. Solving the system of equations written in expression (5) gives the values of the radiation intensity Ii,j,k, or number of photons, emitted by the radioisotope within each voxel i,j,k. Thus, as described below in relation to FIG. 7, the imaged volume of the object may be displayed by any desired cross-section or slice of the imaged object. FIG. 6 is a flowchart 120 showing steps performed by processor 28 in operating camera head 24, according to an embodiment of the present invention. In a first step 122, operator 32 delineates volume 80 (FIG. 5) enclosing region 23, and inputs the dimensions and location of the volume to processor 28. The operator also provides processor 28 with the number of detectors D, and the number of configurations N, that may be assumed by each detector. In a second step 124, processor 28, under the supervision of the operator, divides volume 80 into M similarly shaped voxels, where the value of M is set, to be equal to or less than the product D·N. The actual value of M depends on the numbers of voxels in each edge of volume 80. Typically, the value of M is set to be as large as possible, given the constraints above. Voxels 82 may be cubes, or alternatively, the lengths of edges of voxels 82 may be set to be unequal. In a third step 126, processor 28 computes the values of coefficients βi,j,kd,n, given the parameters determined in the first and second steps. The values are computed for all D·N combinations of d and n, and processor 28 stores the computed values. In a fourth step 128, operator 32 injects the patient so that region 23 absorbs the radioisotope. In a fifth step 130, system 20 is operated in its N configurations. In each of the configurations processor 28 receives and stores in memory 29 signals from each of the D detectors, so that for the N configurations the processor stores D·N signal values corresponding to SI,I . . . Sd,n. In a sixth step 132, given the values of βi,j,kd,n previously stored in step 126, and the signal values corresponding to SI,I . . . Sd,n found in step 130, processor 28 determines if expressions (5) are solvable. If, in step 132, processor 28 determines that expressions (5) are not solvable, in a step 136 the number of voxels into which volume 80 is divided is reduced, by increasing the dimensions of the voxels. The change in voxel dimension, and the corresponding reduction of number of voxels in volume 80, is performed by processor 28, typically under supervision of operator 32′. In one embodiment of the present invention, the number of voxels is reduced by decrementing by 1 the value of edge I, J, or K. In a step 138, processor 28 recomputes the values of coefficients βi,j,kd,n for all D·N combinations of d and n, and processor 28 stores the recomputed values. Flowchart 120 then returns to step 132. If, in step 132, processor 28 determines that expressions (5) are solvable, the processor, in an intensity evaluation step 134, computes the intensities Ii,j,k, by solving the expressions, and stores the values of the intensities in association with identities (i,j,k) of respective voxels. Flowchart 120 then ends. Consideration of flowchart 120 shows that processor 28 applies an iterative process to the D·N results it receives from camera head 24, and that the process generates a largest number of voxels into which region 23 may be divided. Forming the largest number of voxels corresponds to generating images of region 23, described in more detail with respect to FIG. 7, with a highest resolution. FIG. 7 illustrates sections of region 23 that may be generated from the intensities derived from flowchart 120, according to an embodiment of the present invention. Consideration of flowchart 120 shows that on completion of the flowchart, processor 28 has calculated the intensities Ii,j,k, of every voxel (i,j,k) in volume 80. Operator 32 uses processor 28 to display the resulting intensities on display 30, typically by selecting the complete set of voxels and displaying them in a perspective view, or alternatively by selecting subsets of the voxels, such as one or more slices 152 of voxels. Slices 152 may comprise planes normal to one of axes i, j, or k. Alternatively slices 152 may comprise planes which are non-normal to the axes. Further alternatively, processor 28 may select and display subsets of voxels that are comprised in one or more non-planar surfaces, such as sets of voxels of one or more surfaces similar to sections of an onion. In an alternative image presentation, the intensities of the voxels (i,j,k) may be added to form an image projection. For example all the intensities of voxels (i,j,k) may be summed along the columns of volume 80 to produced a two dimensional image projection similar to the image display received by a conventional collimator. Those having ordinary skill in the art will appreciate that methods other than that exemplified by flowchart 120 may be used to determine, values of intensities Ii,j,k from signals generated by detectors 40. For example, since for each of the N configurations a different volume of region 23 is subtended at each given detector 40, processor 28 may be configured to determine one or more differential signals, corresponding to respective one or more differential volumes of region 23. The differential volumes are typically in the form of annular volumes, such as are generated by taking a difference between volume 66 and volume 76 (FIGS. 3A and 3B). Processor 28 may be arranged to compute intensities Ii,j,k from the overlap of differential volumes generated by different detectors 40. Other methods for generating, the intensities will be familiar to those having ordinary skill in the art, and all such methods are included in the scope of the present invention. FIGS. 8A-14 described below illustrate alternative collimators to collimator 26, according to embodiments of the present invention. Each alternative collimator performs generally the same function as collimator 26, enabling each detector 40 in camera head 24 to receive radiation from different volumes of region 23. As appropriate, mounting 54 and/or detectors 40 are shown in each illustration of the alternative configurations. FIGS. 8A and 8B are schematic cross-sectional views of a collimator 200, and FIG. 8C is a view of, detectors and collimator channels of the collimator, according to an embodiment of the present invention. Apart from the differences described below, the operation of collimator 200 is generally similar to that of collimator 26 (FIG. 2), such that elements indicated by the same reference numerals in both collimators 26 and 200 are generally identical in construction and in operation. Collimator 200, comprises three plates 42, 204, and 206, which processor 28 may move vertically with respect to each other. Processor 28 may also move detectors 40 on mounting 54 vertically with respect to the plates. As described above with reference to FIG. 2, plate 42 comprises collimator channels 50, each of which is aligned with a respective detector 40. In collimator 200, by, way of example detectors 40 are assumed to have rectangular cross-sections. Also, detectors 40 are assumed to be distributed in a two-dimensional rectangular array defined by two orthogonal repetition vectors. Each collimator channel 50 is also assumed to have a rectangular cross-section, which encloses a vertical projection of its associated detector 40. Collimator channels 50 are distributed in a two-dimensional rectangular array having substantially the same repetition vectors as those which define the two-dimensional array in which detectors 40 are distributed. FIG. 8C shows a view of detectors 40 and one collimator channel 50. Plate 204 is generally similar to plate 42, but has collimator channels 210 instead of channels 50. Each collimator channel 210 has a rectangular cross-section, so, that each channel aligns with four detectors 40 arranged as a 2×2 pattern. Collimator channels 210 are distributed in a two-dimensional rectangular array defined by two orthogonal repetition vectors which are double the repetition vectors defining the two-dimensional array in which detectors 40 are distributed. Plate 206 is also generally similar to plate 42, but has collimator channels 212 instead of channels 50. Each collimator channel 212 has a rectangular cross-section, so that each channel aligns with 16 detectors 40 arranged as a 4×4 pattern. Collimator channels 212 are distributed in a two-dimensional rectangular array defined by two orthogonal repetition vectors which are four times the repetition vectors defining the two-dimensional array of detectors 40. FIG. 8C illustrates the relation between detectors 40, and collimator channels 50, 210 and 212. By positioning plates 42, 204, and 206 differently with respect to detectors 40, as illustrated by arrows 214, processor 28 may set different configurations for the detectors of collimator 200. Each set of positions for the plates and detectors corresponds to a different configuration for collimator 200. For each different configuration, each detector 40 receives radiation from a set of voxels or parts of voxels in region 23, the sets typically being different for each different configuration. Each of the sets is defined by a generally pyramidal shape which encloses region 23, the pyramidal shape being set by the positions of plates 42, 204, and 206, and the position of detectors 40. Unlike the generally conical shapes defined by the collimator plates of collimator 26, the generally pyramidal shapes generated by collimator 200 typically do not have common axes of symmetry. Typically, for a given detector 40, the sets of voxels or parts of voxels defined by the different configurations at least partly include each other. FIGS. 9A and 9B are schematic views of a collimator 220, according to an embodiment of the present invention. Collimator 220 comprises a plurality of generally similar plates 222. Plates 222 are generally similar to plate 42 (FIG. 2), each plate 222 having collimator channels 226, which are generally similar to channels 50. Collimator channels 226 may be aligned with detectors 40, by processor 28 moving each plate 222 into alignment with detectors 40, or the processor may move each plate so that its collimator channels are completely out of alignment. Processor 28 moves the plates into and out of alignment by sliding the plates horizontally on tracks 228, using a plate alignment mechanism 224. Alignment mechanism 224 comprises a rod 230, which is configured to push the plates horizontally into alignment, or to pull them out of alignment. In one embodiment, plates 222 are pushed/pulled into/out of alignment by rod 230 having a magnet edge 231 that allows rod 230 to pull plates 222 out of alignment by magnetizing the ferromagnetic frame of plates 222 with rod 230. Mechanism 224 also comprises a vertical translator 232, which is able to position rod 230 against any of plates 222. Processor 28, operates mechanism 224. Typically, processor 28 may also move detectors 40 vertically with respect to plates 222, as indicated by double-headed arrow 221, so as to maintain the distance from the detectors to an uppermost plate 222 approximately constant. By way of example, FIG. 9A shows one lower plate 222 in alignment with the detectors, and seven upper plates moved out of alignment. FIG. 9B shows five lower plates 222 in alignment with the detectors, and three upper plates moved out of alignment. It will be apparent that collimator 220 may be configured into seven configurations, each configuration allowing each given detector 40 to receive radiation from seven different volumes of region 23. FIGS. 10A and 10B are schematic views of a collimator 240, according to an embodiment of the present invention. Apart from the differences described below, the operation of collimator 240 is generally similar to that of collimator 220 (FIGS. 9A and 9B), such that elements indicated by the same reference numerals in both collimators 220 and 240 are generally identical in construction and in operation. In collimator 240, each plate 222 is attached to a respective shaft 242, each shaft being rotatable by processor 28 so as to place its respective plate into, or out of, alignment with detectors 40. In one embodiment of the present invention, shafts 242 may be configured as a concentric set of shafts 244. By way of example, collimator 240 comprises three plates 222, and may be configured into three configurations. FIG. 10A illustrates a configuration with three plates 222 in alignment with detectors 40. FIG. 10B illustrates a configuration with one plate in alignment with the detectors, and two plates out of alignment. FIGS. 11A, 11B and 11C are schematic views of a collimator 260, according to an embodiment of the present invention. Apart from the differences described below, the operation of collimator 260 is generally similar to that of collimator 220 (FIGS. 9A and 9B), such that elements indicated by the same reference numerals in both collimators 220 and 260 are generally identical in construction and in operation. Collimator 260 comprises a plurality of collimator plates having different heights, rather than the same height collimator plates 222 in collimator 220. By way of example, collimator 260 comprises plates 262, 264, 266, and 268, herein referred to collectively as plates 270. Plates 270 have collimator channels 226 within the plates. Plates 270 are attached to a rotatable shaft 272, which is operated by processor 28. Processor 28 may rotate each of the plates so that channels 226 within one of the plates are in alignment with detectors 40, and so that the other channels are not aligned with the detectors. FIG. 11A is a cross-sectional side view of a first configuration of collimator 260 showing plate 262 in alignment, and plate 264 out of alignment, with detectors 40. FIG. 11B is a cross-sectional side view of a second configuration of collimator 260 showing plate 264 in alignment, and plate 262 out of alignment, with detectors 40. In both configurations plates 266 and 268 (not shown in FIGS. 11A and 11B) are also out of alignment with detectors 40. Typically, detectors 40 may be raised or lowered, as explained above, so as to be at a substantially constant distance from the plate with which they are aligned with. FIG. 11C shows a top view of collimator 260. For clarity, detectors 40, with which channels of one of plates 270 are aligned by processor 28, are not shown. Collimator 260 may be positioned in four different configurations, corresponding to the four different heights of plates 270. Other embodiments similar to collimator 260, using different numbers of plates, each having a different height, will be apparent to those skilled in the art. All such embodiments are to be considered as being within the scope of the present invention. FIGS. 12A and 12B are schematic views of a radiation detection system 280, generally similar to a system used in Single Photon Counting Tomography (SPECT), according to an embodiment of the present invention. Unlike embodiments described above, system 280 comprises a plurality of generally similar camera heads 282, each camera head 282, except for the differences described below, being generally similar to camera head 24. Herein camera heads 282 are differentiated from each other with a letter suffix. By way of example, system 280 is assumed to comprise four camera heads 282A, 282B, 282C, and 282D. Each camera head 282 comprises a substantially similar detector mounting 54, each detector mounting 54 being associated with a set of detectors 40 to form a detector assembly 284. The detector mountings, detectors, and detector assemblies are differentiated from each other by having the same letter suffix as their respective camera heads. Each set of detectors 40A, 40B, 40C, 40D is respectively associated with a different collimator 292, 294, 296, and 298, herein also referred to collectively as collimators 290. Each collimator 292, 294, 296, and 298 has respective channels 293, 295, 297 and 299 for their respective detectors. Collimators 290 are generally similar to plate 42 (FIG. 2). However, each collimator and its channels have a different length. Also, each collimator 290 is fixed with respect to its respective detectors. Camera heads 282 are attached to a track 302, which acts as a positioning mount for the camera heads and is designed to allow all of the camera heads to be relocated in space to the same location 304. The camera heads and track are configured so that when the camera heads are in location 304, detectors 40A, 40B, 40C, and 40D are sequentially positioned in registration with each other. Also, when each camera head is in location 304, each collimator 290 is positioned with respect to its associated detectors 40 so that radiation from region 23 is directed by the channels of the collimator to the detectors. When a given camera head 282 is in location 304, processor 28 is configured to operate the camera head so as to receive signals from detectors 40. System 280 thus effectively has a number of different configurations equal to the number of different camera heads 282 in the system. In the exemplary system illustrated in FIGS. 12A and 12B, system 280 has four configurations. In some embodiments of system 280, processor 28 is configured to operate camera heads in one or more other locations defined by track 302. In some embodiments, camera heads in the multiple locations may be operated simultaneously. In each of the other locations, detectors 40 are in registration with each other and are positioned to receive radiation from region 23. For example, camera heads 282 and track 302 may be configured so that camera heads 282 may be positioned in registration, and operated by processor 28, in location 304 and in a second location 306. In general, for systems such as system 280 comprising a plurality of camera heads, the number N of configurations of the system (where N is as defined above with respect to FIG. 5), is given by:N=NC·NL (7) where NC is the number of camera heads in the system, and NL is the number of locations in which each camera head may be positioned. In an alternative arrangement of system 280, collimators 292, 294, 296, and 298 are not associated with a specific detector 40A, 40B, 40C, and 40D. In the alternative arrangement, detectors 40A, 40B, 40C, and 40D are typically fixed, and collimators 290 are movable into registration with the detectors using track 302. In a further alternative arrangement, collimators 292, 294, 296, and 298 may be coupled together to form, a common configurable collimator for detectors 40A, 40B, 40C, and 40D. Typically, in this further alternative configuration, track 302 rotates in steps into registration positions. The common configurable collimator is statically maintained in each, registration position for a time at least equal to a signal acquisition time for detectors 40A, 40B, 40C, and 40D. Thus track 302 moves in a rotational step motion between the acquisition static positions. In each rotational acquisition position the collimator between one of the detectors 40A-40D and the measured object has a different height. Accordingly, each detector 40A-40D has multiple acquisitions, each acquisition being performed by a configurable collimator that has different height for each of the different rotational acquisition positions. In a yet further alternative arrangement of system 280, collimators 290 are each separately configurable with a plurality of dimensional configurations. For example, collimators 290 may be generally similar to collimator 26 (FIG. 2), but each collimator may be arranged to have different ranges of dimensional configurations. In another alternative arrangement of system 280, each collimator 290 may be generally similar to different types of collimators. For example, collimator 292 may be generally similar to collimator 26, and collimator 294 may be generally similar to a collimator 320 (described below). FIGS. 13A, 13B, 13C and 13D are schematic diagrams of an alternate adjustable collimator 320, according to an embodiment of the present invention. Apart from the differences described below, the operation of collimator 320 is generally similar to that of collimator 26 (FIGS. 1 and 2), such that elements indicated by the same reference numerals in both collimators 26 and 320 are generally identical in construction and in operation. Collimator 320 comprises top plate 42 and a bottom plate 321. For clarity, in FIG. 13A only a section 323 of plate 42 and a section 324 of plate 321 are shown. In plate 42 channels 50 and detectors 40 are in alignment, as described with reference to collimator 26. FIG. 13B is a schematic perspective view, of bottom plate 321. An outline 331 corresponds to section 324 illustrated in FIG. 13A. Bottom plate 321 faces region 23, and the plate comprises a cavity 322 and cylindrical channels 327 through the cavity. In contrast to plate 42, plate 321 is constructed from material, typically sheet material, which is substantially transparent to the radiation emitted from the radioisotopes in region 23. Cavity 322 may be dynamically filled and/or emptied from a reservoir 328 with a liquid which is, relatively opaque to the radiation, such as mercury. Typically cavity 322 is divided internally with partitions 333 which guide the flow of the liquid, as shown by arrows in FIG. 13B. Reservoir 328 typically comprises a filling and discharge pump, and plate 321 comprises a vent 329 allowing air to leave/enter the cavity as it is filled/emptied. When empty, cavity 322 is transparent to the emitted radiation, and radiation reaches detector elements 40 through collimator channels 50. When cavity 322 is filled, the effective length of collimator 320 is extended to a bottom side 325 of plate 321, and collimator channels 327 act to direct radiation. Thus, when cavity 322 is filled, radiation reaches detectors 40 through channels 50 extended by corresponding channels 327 of bottom plate 321. In a disclosed embodiment, multiple plates generally similar to plate 321 may be stacked one on top of the other to form a collimator in which each plate can be filled/empty individually to produce a similar effect as that illustrated by FIGS. 9A and 9B, wherein plates 226 are aligned with/removed from detectors 40. FIG. 13C is a cross-section through one pair of channels 50 and 327, which form an adjustable channel 319, and shows the effect that filling cavity 322 has on the radiation reaching detector 40. When cavity 322 is empty, corresponding to a first configuration, radiation passing through adjustable channel 319 subtends a large solid angle 329 at detector 40. When cavity 322 is filled, corresponding to a second configuration, radiation passing through adjustable channel 319 subtends a small solid angle 326 at the detector. In some embodiments, cavity 322 may be partially filled so as to create different height channels 327, each height corresponding to a different configuration of collimator 320. Processor 28 may vary the heights in steps, or substantially continuously. In some embodiments, only one filling point is used to fill cavity 322 from reservoir 328, although more filling points may be provided so that the cavity may be filled quickly. Provided that cavity 322 is substantially completely filled, collimator 320 may be used in substantially any orientation. FIG. 13D illustrates a cross-section of an alternative embodiment of collimator 320 wherein cavity 322 is subdivided, by way of example, into three isolated sub-compartments 322A, 322B, and 322C. Typically, the sub-compartments may be filled independently. Configuring cavity 322 to have isolated sub-compartments allows the cavity to be partially filled, while allowing collimator 320 to be used in non-horizontal orientations. Providing sub-compartments, and configuring the sub-compartments appropriately, also allows operator 32 to select different values for the solid angles and volumes of region 23 that are subtended by the different configurations, as well as increasing the number of configurations available in collimator 320. In collimator 320 the number of configurations depends on the number of sub-compartments, and the number of sub-compartments may be set to be any convenient number. In the example illustrated in FIG. 13D, there are three sub-compartments, and thus there are four configurations for collimator 320. FIG. 14 is a schematic diagram of an adjustable collimator channel 430 formed in a cavity, according to an embodiment of the present invention. Apart from the differences described below, the operation, of adjustable collimator channel 430 is generally similar to that of adjustable collimator channel 47 (FIGS. 3A, 3B, 3C), such that elements indicated by the same reference numerals in collimator channels 47 and 430 are generally identical in construction and in operation. As opposed to adjustable collimator channel 47 described hereinabove, multiplicities of which are formed in two separate plates, multiplicities of adjustable collimator channels similar to adjustable collimator channel 430 may be formed in one plate 42. For clarity, FIG. 14 shows a section of plate 42 having only one adjustable collimator channel 430. Channel 430 comprises a closed tubular cavity 432 formed in plate 42, the cavity having as its inner surface an inner cylinder 434 and as its outer surface an outer cylinder 435. Cylinder 434 is closed at its upper end by a given detector 40. An annulus 433, generally coplanar with detector 40, surrounds the detector and closes an upper end of cavity 432. Cavity 432 is closed at its lower end by an annulus 439. Cavity 432 has its outer surface an outer cylinder 436. Cavity 432 may be filled/emptied with a liquid which is opaque to radiation emitted by radioisotopes. The liquid is filled/emptied using a reservoir 428, which is generally similar to reservoir 328 (FIG. 13A). Cavity 432 has an opening (not shown) for inlet/outlet of air to/from the cavity. Inner cylinder 434 and annulus 439 are selected from material substantially transparent to radiation. Thus, when cavity 432 is unfilled, a large solid angle defined by a lower edge 437 of cylinder 436 is subtended at detector 40. When cavity 432 is filled with liquid opaque to radiation, a smaller solid angle, defined by a lower edge 438 of cylinder 434, is subtended at detector 40. Cavity 432 being unfilled corresponds to a first dimensional configuration of collimator channel 430, cavity 432 being filled corresponds to a second dimensional configuration of the channel. It will be understood that adjustments to collimator channel 430 are by changing a cross-sectional area of a channel forming the collimator and may be controlled by processor 28. Although the embodiments described above typically relate to gamma ray detection in medical applications, detection of additional types of radiation may be performed, and may likewise be applied in the context of medical and/or non-medical applications, according to the principles of the present invention. Such types of radiation include electromagnetic radiation other than gamma rays, charged and uncharged particle radiation such as is generated from decay of radioisotopes, and radiation, such as ultrasound, transmitted as a wave motion by a material. It will be appreciated that combinations of systems described above may be used to form collimators having multiple different configurations. For example, different configurations may be generated by varying an effective length and/or an effective cross-section of a channel. It will thus be appreciated that, embodiments described above are cited by way of example, and that the present invention is not limited to what, has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. |
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051695944 | abstract | A method of remotely installing or removing and assembling and disassembling nozzle dams (10) from outside a steam generator manway (14) using a tool set. The tools are a bifurcated segment lifting (torquing and push-pull) tool 60 to unweight and faciltate the operation of sliding dam segments (30,32) edgewise along sliding brackets (40) and bars (44) in an assembly and disassembly operation; a cam-lock component (36) operating T-shaped tool (80) with a nozzle dam rib avoiding offset (88), transverse quick-connect body (50b) straddling tool (90) pull a sleeve (50c) in order to disassembly quick-connects; and, a gasket installing, tensioning and removing hook tool (102). |
047138335 | claims | 1. An X-ray source apparatus comprising, in an evacuated chamber, an X-ray target of a selected material which emits X-rays when bombarded with electrons of at least a predetermined energy; a source of electrons; means for accelerating electrons from said source to at least the predetermined energy; means for generating a magnetic field with curved lines of magnetic flux interlinking said target and said electron source and with the magnetic field having sufficient strength over the whole of the interlinking magnetic flux lines between said target and said electron source such that electrons of the energies of those accelerated from said source with components at angles to the magnetic field are constrained by the field to execute helical paths adjacent said source and to travel generally in the direction of the lines of flux, said target being sized and positioned to intercept substantially all of those lines of flux that intercept said electron source so that substantially all the electrons from said source bombard said target to cause said target to emit X-rays; and aperture means blocking straight line paths between said source and said target but permitting passage of substantially all the electrons travelling along the flux lines from said source to said target. 2. Apparatus as claimed in claim 1 wherein said target is at earth potential and the means for accelerating comprises an earthed grid or iris along the lines of flux interlinking said source and said target and means for producing an electron accelerating electrical potential gradient between the source and the grid or iris. 3. Apparatus as claimed in claim 1 further comprising means defining a specimen region; and wherein said target is located adjacent the specimen region in the magnetic field to irradiate a specimen in the specimen region. 4. Apparatus as claimed in claim 3 wherein said specimen region defining means includes means for evacuating said specimen region to a pressure different from the pressure in the evacuated chamber. 5. Apparatus as claimed in claim 1 wherein said means for accelerating comprises an earthed grid or iris along the lines of flux interlinking said source and said target and means for producing an electron accelerating electrical potential gradient between said source and said grid or iris. 6. Apparatus as claimed in claim 1 wherein said electron source comprises a wire filament arranged to extend in a line at an acute angle to the lines of flux at said source, and a DC voltage source to heat said filament. 7. Apparatus as claimed in claim 1 wherein said electron source comprises a wire filament arranged to extend in a circle in a plane perpendicular to the lines of flux at said source, and a DC voltage source connected to heat said filament with a DC current directed about said filament to cause a force on said filament directed radially outwards thereof. 8. Apparatus as claimed in claim 1 wherein said magnetic field generating means generates a magnetic field in the order of about 7 Tesla. |
claims | 1. An image receptive phosphor screen, comprising: (a) an infrared-absorbing substrate; and (b) a phosphor layer coated on said substrate, wherein said phosphor layer comprises a phosphor powder composition comprising a dopant accepting base comprising at least one element selected from amongst those of groups IIA to VIA of the Periodic Table, about 0.0025 to 0.1 weight percent of a first dopant comprising a source of samarium, and about 0.0025 to 0.2 weight percent of a second dopant comprising a source of cerium; wherein said phosphor powder has a narrow particle size distribution and an average particle size, measured in its long dimension, of greater than 0 up to less than about 5 microns; and wherein said image receptive screen is fast scannable as a result of said phosphor powder being of small particle size, having low dopant proportions and having been reactivated by mild heating at a temperature of less than about 550xc2x0 C. after a sinter of said phosphor powder composition has been comminuted under conditions sufficient to at least partially deactivate the phosphor. 2. The phosphor screen according to claim 1 , wherein said base comprises strontium, and said screen further comprises an infrared absorbing layer between said substrate and said phosphor layer. claim 1 3. The phosphor screen according to claim 2 , further comprising a protective, transparent overcoat that covers substantially all of said phosphor layer. claim 2 4. An image receptive phosphor screen, comprises: (a) a substrate; (b) an infrared-absorbing layer, comprising at least one infrared-absorbing compound, coated on said substrate; and (c) a phosphor layer, comprising a phosphor powder composition comprising a dopant accepting base comprising at least strontium, about 0.0025 to 0.1 weight percent of a first dopant comprising a source of samarium, and about 0.0025 to 0.2 weight percent of a second dopant comprising a source of cerium; wherein said phosphor powder has a narrow particle size distribution and an average particle size, measured in its long dimension of greater than 0 up to less than about 5 microns; and wherein said image receptive screen is fast scannable, with infra red radiation, as a result of said phosphor layer having been made by sintering said composition, size reducing said phosphor powder under conditions sufficient to at least partially deactivate said phosphor, reactivating said deactivated, size reduced phosphor by mild heating at a temperature of less than about 550xc2x0 C. whereby causing at least some of said reactivated, comminuted phosphor particles to agglomerate, and deagglomerating said reactivated phosphor under conditions sufficient to at most minimally deactivate said phosphor. 5. An image receptive phosphor screen, comprising: (a) an infrared-absorbing substrate; and (b) a phosphor layer coated on said substrate, wherein said phosphor layer comprises a phosphor powder composition comprising: a dopant accepting base comprising at least one element selected from amongst those of groups IIA to VIA of the Periodic Table; about 0.0025 to 0.1 weight percent, based on the weight of said phosphor, of a first dopant comprising a source of a rare earth element having the ability to trap electrons in said phosphor; and about 0.0025 to 0.2 weight percent, based on the weight of said phosphor, of a second dopant comprising a source of a rare earth element, that is different from said electron trapping element, having the ability to be a luminescent center in said phosphor; wherein said phosphor powder has a narrow particle size distribution and an average particle size, measured in its long dimension, of greater than 0 up to less than about 5 microns; and wherein said image receptive screen is fast scannable as a result of said phosphor powder having a small particle size, having low dopant proportions and having been made by a method comprising: sintering said composition under conditions sufficient to form a phosphor sinter; comminuting said phosphor sinter, under conditions sufficient to at least partially deactivate said phosphor, whereby forming at least partially deactivated very small phosphor particles; reactivating said at least partially deactivated phosphor particles by mild heating at a temperature of less than about 550xc2x0 C., whereby forming at least partially agglomerated, mildly deactivated phosphor particles; and deagglomerating said at least partially agglomerated phosphor particles under very mild conditions sufficient to break up said agglomerated particles into said powder phosphor without substantially deactivating said phosphor. 6. The phosphor screen according to claim 5 , wherein said base comprises strontium, and said screen further comprises an infrared absorbing layer between said substrate and said phosphor layer. claim 5 7. The phosphor screen according to claim 6 , further comprising a protective, transparent overcoat that covers substantially all of said phosphor layer. claim 6 |
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summary | ||
description | This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/EP2013/057902 which has an International filing date of Apr. 16, 2013, which designated the United States of America, and which claims priority to German patent application number DE 102012206546.6 filed Apr. 20, 2012, the entire contents of each of which are hereby incorporated herein by reference. At least one embodiment of the invention generally relates to a scattered radiation grid of a CT detector comprising a plurality of detector elements arranged in multiple rows in the phi direction and z direction of a CT system, the grid having a plurality of free passage channels arranged to correspond to the detector elements and the free passage channels being fully enclosed by walls at their longitudinal sides. Scattered radiation grids, also referred to as collimators, for detectors in CT systems are generally known. Until now stacks of thin tungsten plates bonded in a support mechanism were used in CT detectors. These allow the suppression of scattered radiation in the phi direction, in other words in the gantry rotation direction. Until now there was no collimation in the z direction or system axis direction. However it is also known that scattered beam correction is much more effective with a collimator acting in the phi direction and the z direction than with a simple phi collimator, particularly in dual source CT systems. This can be demonstrated by a significant dose reduction for a given contrast/noise ratio or improved artifact reduction. Also when greater detector z coverage is required, it becomes increasingly difficult to manufacture the support mechanism with enough accuracy to hold the plates in position. If such a phi/z collimator is built in the conventional manner, in other words with individual plates, there is a further problem in that the plates have to be aligned with the focus of the x-ray tube in both directions. As it is difficult to produce single-piece collimators that extend over the entire detector surface, a modular structure is often used here. One problem with scattered radiation grids of modular structure with a number of adjacent grid modules is that artifacts are produced in the projections recorded therewith in the region of the joining points of two grid modules, having a negative effect on the image quality of a tomographic image data record reconstructed from such projections or producing visible artifacts in the tomographic representation. It is therefore necessary with such collimator modules for different wall thicknesses to be implemented at different positions on the component, for example on the beam exit side or at the edges adjoining adjacent modules. A collimator structure, in which plates embodied in the manner of combs are intermeshed, is also known from the publication DE 10 2005 044 650 B4. This method is complex and is also made problematic in that the plates should be aligned with the focus. It is also known from the publication US 2008/0023636 A1 that polymers filled with metal particles can be made to harden in a grid-type form. The disadvantage of this method is the limited fill level of the compound at around 50%, which significantly reduces the collimation effect due to the reduced absorption capacity. It is further proposed in publication DE 10 2010 011 581 A1 that the walls in both directions should be produced by selective laser melting SLM. SLM is a method in which metallic components can be produced in almost any complex geometries directly from 3D CAD data. It involves many layers of powdered metal being melted selectively one above the other using a laser beam based on the calculated surfaces, until the desired structure is produced. The method is such that the structures produced in this manner have very rough surfaces, which have to be further processed by way of a series of subsequent processes. Also it is not always possible to achieve all the desired wall thicknesses. At least one embodiment of the invention is directed to an improved scattered radiation grid of a CT detector. Advantageous developments of the invention are the subject matter of subordinate claims. The inventors have identified that it is advantageous to produce scattered radiation grids that are effective in two dimensions and can in some instances be combined in a modular manner to form a larger unit using a three-dimensional screen-printing method. In this process a suspension of a highly absorbent material, preferably powdered metal, for example lead, copper, molybdenum, tantalum, tungsten or another element with a high absorption coefficient, and a binder are printed on top of one another layer by layer using a screen, thereby achieving a three-dimensional structure. By changing the screen in such a manner that the opening raster becomes narrower or wider, or generally changes, as the number of layers increases, it is possible to align the walls of the collimator with the focus and to configure the channels in the shape of truncated pyramids. With such a method such layering first produces a blank which does not yet have the final strength. For the final hardening process the resulting blank is hardened by means of a concluding sintering process after the ultimate component height has been achieved. The inventors have identified that it is advantageous to produce scattered radiation grids that are effective in two dimensions and can in some instances be combined in a modular manner to form a larger unit using a three-dimensional screen-printing method. In this process a suspension of a highly absorbent material, preferably powdered metal, for example lead, copper, molybdenum, tantalum, tungsten or another element with a high absorption coefficient, and a binder are printed on top of one another layer by layer using a screen, thereby achieving a three-dimensional structure. By changing the screen in such a manner that the opening raster becomes narrower or wider, or generally changes, as the number of layers increases, it is possible to align the walls of the collimator with the focus and to configure the channels in the shape of truncated pyramids. With such a method such layering first produces a blank which does not yet have the final strength. For the final hardening process the resulting blank is hardened by means of a concluding sintering process after the ultimate component height has been achieved. Advantages of at least one embodiment of the proposed method are that much greater component accuracy can be achieved with screen-printing than with the known methods. Wall roughness and wall position differences in particular are greatly reduced compared with those achieved using SLM methods, in particular thinner walls can also be achieved than when using the SLM method. It is therefore also possible to design the outer walls of the component just with half the wall thickness D so that when a number of grid modules are lined up to form an overall grid, the effective wall thickness Dx2 at the joining surfaces of the grid modules is the same as the wall thickness in the interior of the component. Also wall regions on the lower face of the collimator, in other words on the beam exit side, can also be configured thicker with the proposed method, simply by changing the screen. This embodiment prevents a variable shadow being thrown due to instability of the x-ray focus onto the active pixel surface. A further advantage of the screen-printing method is that the process surface of typical screen-printing machines is much larger than the process surface of SLM machines. Therefore many more components can be produced at the same time in one pass. Also larger collimators can be produced, which span a number of sensor boards or even a number of detector modules. Specific structures for assembling the collimators in the detector mechanism can also be produced easily. The particular two-step screen-printing production method, in which a relatively stable but not yet finally hardened blank is produced by multiple screen-printing in the first step, the blank only achieving its final strength in the second step, means that the as yet not finally hardened blank can undergo additional shaping after screen-printing and before the final hardening. It is therefore possible to produce a blank with an initially simple rectangular outer structure and then to act on the blank with a shaping effect so that an alignment of the walls and the radiation-conducting passage channels is brought about in the direction of a common focus. A blank shaped in this manner can then be made to achieve the desired final strength by sintering in the second step. Based on these basic concepts the inventors propose a scattered radiation grid of a CT detector comprising a plurality of detector elements arranged in multiple rows in the phi direction and z direction of a CT system, the scattered radiation grid having a plurality of free passage channels arranged to correspond to the detector elements and the free passage channels being fully enclosed by walls at their longitudinal sides. According to the invention the walls of the scattered radiation grid are produced using a three-dimensional screen-printing method. To structure the walls during the course of the screen-printing method a suspension of powdered metal and binder can preferably be used but it is also possible to use a suspension of another material with a high x-ray absorption coefficient, for example an element with an atomic number greater than 19, in other words an element from the fourth group, preferably from the fifth group, of the periodic system. In principle here, the greater the effective active cross section of the wall material, the more efficiently the unwanted scattered radiation is absorbed. During the course of the printing method passage channels in the shape of truncated pyramids can be produced by replacing the screens used at least once, in other words passage channels, the through passage surface of which is larger at one end of the channel than at the other end, their longitudinal axes being aligned respectively with a common focus. Also during the course of the printing method passage channels with a cross section that varies with height or passage channels in the shape of truncated pyramids can be produced by replacing the screen used at least once or a number of times with a successively changing, preferably narrowing covered region in the screen. The scattered radiation grid in a not yet finally hardened state—and without alignment of the passage channels with a common focus—can generally be shaped in such a manner that the passage channels in the shape of truncated pyramids are shaped in such a manner that their longitudinal axes are respectively aligned with a common focus. Similarly the passage channels and/or walls can be produced so that they are aligned parallel to one another in a first production phase and a mechanical shaping process can be applied to them before a final hardening, bringing about the alignment of the passage channels or the walls with a common focus. During such a mechanical shaping process a scattered radiation grid that is originally cuboid in respect of its external dimensions can be pressed into the shape of a truncated cone. Alternatively the radiation entry side and/or radiation exit side can be pressed onto a cylindrical or spherical surface, so that the radiation entry side and/or radiation exit side is molded to the cylindrical or spherical surface and the walls and passage channels are therefore aligned with a focus. It can also be advantageous for the passage channels to be embodied as narrowed in the region of the beam exit side of the scattered radiation grid. This prevents or at least reduces shadowing due to slight variations in the focus position relative to the detector. At least one embodiment of the inventively produced scattered radiation grid can on the one hand be a complete scattered radiation grid. The scattered radiation grid can also be made up of a number of individually produced grid modules, the grid modules then having the features of the scattered radiation grid as described above. In particular it is advantageous here if the walls of the grid modules forming an outer face of the grid modules are configured thinner than, preferably half as thin as, the remaining walls of the grid modules. This means that the same effective wall thickness results at the joining surfaces of the grid modules as at the other walls within the grid modules. This broadly standardizes compensation for scattered radiation over the entire detector. The outer walls of the grid modules can also be embodied in such a manner that the grid modules engage in one another with a form fit. Some of the walls of the grid modules can also have elongations on the beam exit side, serving for alignment at the detector. Finally during the production of the scattered radiation grid or the grid modules the change between differently dimensioned screens can be embodied in such a manner that the walls of the grid or the grid modules are configured so that they taper in steps. The advantage of such an embodiment is that fewer different screens have to be provided and the fact that there are fewer screen changes means that there is also less calibration outlay, in other words the production method is generally more economical. FIG. 1 shows a dual source CT system 1 with two emitter/detector systems consisting of a first x-ray tube (emitter) 2 with a first detector system 3 positioned opposite and a second x-ray tube 4 offset by an angle of 90° with a detector system 5 positioned opposite, said emitter/detector systems being arranged on a gantry in a gantry housing 6. Both detector systems 3 and 5 each have a scattered radiation grid G in order primarily to intercept the scattered radiation produced in each instance by the other emitter/detector system. The scattered radiation grids (only shown schematically here) are produced as described above, are modular in structure and bring about a reduction of scattered radiation in both the phi direction and the z direction. The z direction here is considered to be the coordinate axis lying in the direction of the system axis 9 and the phi direction is considered to be the rotational direction of the gantry, in other words the direction of the detector rows. FIGS. 2 to 4 are intended to show how a grid is produced from a blank produced in an initially rectangular shape using the 3D screen-printing method by shaping it on a cylindrical casing, the passage channels of the grid being aligned with a common focus. FIG. 2 shows a perspective 3D diagram of a grid or grid module 10, the structure of which is essentially rectangular. Therefore all the walls W and passage channels K produced there are aligned parallel to one another. FIG. 3 shows the scattered radiation grid or grid module 10 again from the side. In FIG. 4—again shown in a side view—the grid 10 from FIG. 3 is shaped with the aid of two cylindrical casings 11 or two spheres 11 so that the grid structure and the passage channels in the grid structure are aligned with a common focus—which should correspond to the focus of the x-ray tube present in the emitter/detector system. Although the invention has been illustrated and described in detail using the preferred exemplary embodiment, the invention is not restricted by the disclosed examples and other variations can be derived therefrom by the person skilled in the art without departing from the scope of protection of the invention. |
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055132340 | abstract | The present invention relates to a structural member for nuclear reactor pressure tubes. More particularly, the present invention relates to a new structural member that is used as a beam designed to support the loads and stresses of multiple transversely disposed nuclear reactor fuel channel pressure tubes. |
abstract | A multi-leaf collimator for a radiotherapy apparatus comprises a plurality of elongate leaves mounted in a carriage, the carriage being mounted on a substrate, wherein the leaves are independently moveable relative to the carriage in a longitudinal direction, and the carriage is moveable in that direction relative to the substrate, and a control apparatus is arranged to receive a signal representing leaf positions relative to the substrate and to control the leaf positions relative to the carriage and the carriage positions relative to the substrate so as to achieve those leaf positions relative to the substrate. Most MLCs sense the current positions of the leaves relative to the substrate. The control apparatus can therefore compare the current leaf positions to the signaled leaf positions, and move the leaves and the carriage accordingly. A corresponding method is also disclosed. |
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summary | ||
abstract | An optical grating (8) includes a substrate (9), on the surface (9a) of which a periodic structure (10) is formed that is embodied to diffract incident radiation (11), in particular incident EUV radiation, with a specified wavelength (λτ) into a predetermined order of diffraction, in particular into the first order of diffraction (m=+1). The optical grating also has a coating (12) applied onto the periodic structure with at least one layer (13, 14) that is embodied to suppress the diffraction of the incident radiation into at least one higher order of diffraction (m=+2, . . . ) than the predetermined order of diffraction. |
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abstract | A collimator for a computer tomograph includes a number of collimator plates and a holder for the collimator plates. The collimator plates include, at the edge, a number of tabs spaced apart from one another. In a fashion corresponding to this, the holder has receptacles that are assigned to the tabs and that are spaced apart from one another, into which the tabs of the collimator plates engage. This configuration ensures highly accurate positioning of even long collimator plates. |
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059094753 | claims | 1. A spent nuclear fuel container comprising a basket having a plurality of compartments for storing the spent fuel, and a surrounding concrete cylinder enclosing the basket, the invention characterized in that portions of the said compartments of said basket are constructed of thermally conducting material and said portions include thermally conducting extensions integral with the thermally conducting portions of said basket and extending through said surrounding concrete cylinder so that a continuous thermally conducting heat sink runs from the interior of the basket to the exterior of the container. 2. The spent nuclear fuel container of claim 1 in which the plurality of compartments are formed from a stack of compartmented grids, and the compartmented grids are cast metal or metal alloy grids. 3. A method for storing spent nuclear fuel in a concrete storage cask the method comprising loading the spent nuclear fuel into storage compartments, and sealing the concrete storage cask, the invention characterized in that at the walls of the storage compartments are thermally conducting members at least a substantial portion of which extend through the concrete of the cask to the outside of the cask so that heat flows uninterruptedly from the storage compartments to the exterior of the cask. |
claims | 1. A boiling water nuclear power plant comprising:a nuclear reactor building including a reactor containment vessel and a reactor pressure vessel;an external building which is installed independently outside the nuclear reactor building, which includes a power source and an operating panel independent of the nuclear reactor building, and which has an anti-hazard property;a water injection pump installed inside the external building;a water injection pipe configured to perform water injection on at least the reactor pressure vessel or the reactor containment vessel in the nuclear reactor building from the water injection pump; anda valve connected to the water injection pipe;wherein the valve connected to the water injection pipe includes valves arranged inside the nuclear reactor building and valves arranged outside the nuclear reactor building, and the valves arranged inside the nuclear reactor building include a valve operated to be kept constantly open and a check valve which are arranged outside the reactor containment vessel; andwherein the check valve allows water injection from the outside into the nuclear reactor building and prevents backward flow. 2. The boiling water nuclear power plant according to claim 1, wherein a branching-off pipe is provided at one of the water injection pipes; and a hose connection portion allowing connection of a hose of a pumper vehicle is provided at an end of the branching-off pipe. 3. The boiling water nuclear power plant according to claim 2, wherein the branching-off pipe is provided inside the external building; and the hose connection portion is arranged inside the external building. 4. The boiling water nuclear power plant according to claim 2, wherein the branching-off pipe is provided inside the external building; and the hose connection portion is arranged outside the external building. 5. The boiling water nuclear power plant according to claim 2, wherein the branching-off pipe is provided inside the nuclear reactor building; and the hose connection portion is arranged outside the nuclear reactor building. 6. The boiling water nuclear power plant according to claim 2, wherein the external building includes a garage storing the pumper vehicle. 7. The boiling water nuclear power plant according to claim 1, wherein:the plurality of water injection pipes includes a first water injection pipe performing water injection on the reactor pressure vessel, and a second water injection pipe performing water injection on other portions of the boiling water nuclear power plant; andthe boiling water nuclear power plant includes a bypass pipe performing water injection on the first water injection pipe from the second water injection pipe, and a valve installed at the bypass pipe. 8. The boiling water nuclear power plant according to claim 2, comprising, at a position different from the external building, a garage storing the pumper vehicle. 9. A boiling water nuclear power plant comprising:a nuclear reactor building including a reactor containment vessel and a reactor pressure vessel;an external building which is installed independently outside the nuclear reactor building, which includes a power source and an operating panel independent of the nuclear reactor building, and which has an anti-hazard property;a water injection pump installed inside the external building;a water injection pipe configured to perform water injection on at least the reactor pressure vessel or the reactor containment vessel in the nuclear reactor building from the water injection pump; anda valve connected to the water injection pipe;wherein the valve connected to the water injection pipe includes valves arranged inside the nuclear reactor building and valves arranged outside the nuclear reactor building, the valves arranged inside the nuclear reactor building including a valve operated to be kept constantly open and a check valve which are arranged outside the reactor containment vessel, and the valves arranged inside the nuclear reactor building are connected to a water injection port inside the nuclear reactor building via an up-grade water injection pipe;wherein the check valve allows water injection from the outside into the nuclear reactor building and prevents backward flow; anda water discharge mechanism including a flow rate restrictor is provided at the lowermost point of the up-grade water injection pipe in the nuclear reactor building. 10. The boiling water nuclear power plant according to claim 9, wherein a branching-off pipe is provided at the water injection pipe; and a hose connection portion allowing connection of a hose of a pumper vehicle is provided at an end of the branching-off pipe. 11. The boiling water nuclear power plant according to claim 10, wherein the branching-off pipe is provided inside the external building; and the hose connection portion is arranged inside the external building. 12. The boiling water nuclear power plant according to claim 10, wherein the branching-off pipe is provided inside the external building; and the hose connection portion is arranged outside the external building. 13. The boiling water nuclear power plant according to claim 10, wherein the branching-off pipe is provided inside the nuclear reactor building; and the hose connection portion is arranged outside the nuclear reactor building. 14. The boiling water nuclear power plant according to claim 10, wherein the external building includes a garage storing the pumper vehicle. 15. The boiling water nuclear power plant according to claim 9, wherein:the water injection pipe includes a first water injection pipe performing water injection on the reactor pressure vessel, and a second water injection pipe performing water injection on other portions of the boiling water nuclear power plant; andthe boiling water nuclear power plant includes a bypass pipe performing water injection on the first water injection pipe from the second water injection pipe, and a valve installed at the bypass pipe. 16. The boiling water nuclear power plant according to claim 10, comprising, at a position different from the external building, a garage storing the pumper vehicle. |
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claims | 1. A radiation shield for an eluant vial in combination with a radiation shielding lid for use with a radiopharmaceutical elution system, the radiation shield comprising:a shield body having a closed top, an open bottom, and defining a cavity extending from the bottom toward the top, wherein the cavity is designed to accommodate at least a bottom portion of an eluant vial; anda pair of shielding wings extending downward from the bottom and partially surrounding the cavity,wherein the shield body and the shielding wings comprise at least one of depleted uranium, tungsten, and tungsten impregnated plastic;the radiation shielding lid comprising a body having an upper surface and an opposing lower surface; a first opening defined in the body, the first opening having a lower end at the lower surface of the body and an upper end intermediate the upper and lower surfaces of the body; a pair of finger recesses defined in the body, the recesses having an upper end and a lower end, wherein at least portions of the upper ends of the recesses are located at the upper surface of the body, and wherein at least portions of the lower ends of the recesses are located at the upper end of the first opening; and first and second wings, each of which extends upward from the upper end of the first opening and only partially about a circumference of the upper end of the first opening such that gaps are defined between the first wing and the second wing. 2. The radiation shield set forth in claim 1, wherein the body comprises at least one of depleted uranium, tungsten, tungsten impregnated plastic, and lead. 3. The radiation shield set forth in claim 1, wherein the shielding wings of the radiation shield are receivable in the finger recesses such that the shielding wings are in opposing relationship with the gaps defined between the first and second wings of the lid and the top of the shield body covers the first and second wings. 4. The radiation shield set forth in claim 1, wherein the finger recesses of the radiation shielding lid have a generally ellipsoidal shape, and wherein each shielding wing of the radiation shield includes an exterior surface having a curved portion sized and shaped to engage the finger recesses of the radiation shielding lid. 5. The radiation shield set forth in claim 4, wherein each shielding wing further includes a flat bottom surface, wherein the bottom surface is substantially parallel to the bottom of the shield body. 6. The radiation shield set forth in claim 1, wherein the finger recesses are diametrically opposed to one another with respect to the first opening, and wherein the shielding wings are diametrically opposed to one another about the cavity in the shield body. 7. The radiation shield set forth in claim 1 wherein the radiation shielding lid further comprises a second opening defined in the body, the second opening having a lower end at the lower surface of the body and an upper end at the upper surface of the body, the second opening being spaced apart and separate from the first opening. 8. The radiation shield set forth in claim 1, wherein the shield body and the shielding wings further comprise at least one of polypropylene and polycarbonate. 9. The radiation shield set forth in claim 1, further comprising a radiation shielding core comprising at least one of depleted uranium, tungsten, tungsten impregnated plastic, and lead, wherein the shielding body and shielding wings are at least partially constructed from an overmolded thermoplastic material. 10. The radiation shield set forth in claim 1, wherein the cavity has a generally cylindrical shape, and the shielding wings extend circumferentially about the cavity. 11. A radiation shield for an eluant vial in combination with a radiation shielding lid for use with a radiopharmaceutical elution system, the radiation shield comprising:a shield body having a closed top, an open bottom, and defining a cavity extending from the bottom toward the top, wherein the cavity is designed to accommodate at least a bottom portion of an eluant vial; anda pair of shielding wings extending downward from the bottom and partially surrounding the cavity,wherein the shield body and the shielding wings comprise at least one of depleted uranium, tungsten, and tungsten impregnated plastic;the radiation shielding lid comprising a body having an upper surface and an opposing lower surface, wherein the body comprises at least one of depleted uranium, tungsten, tungsten impregnated plastic, and lead; a first opening defined in the body, the first opening having a lower end at the lower surface of the body and an upper end intermediate the upper and lower surfaces of the body; and a recess defined in the body, the recess having an upper end and a lower end, wherein at least a portion of the upper end of the recess is located at the upper surface of the body, and wherein at least a portion of the lower end of the recess is located at the upper end of the first opening; and first and second wings, each of which extends upward from the upper end of the first opening and only partially about a circumference of the upper end of the first opening such that gaps are defined between the first wing and the second wing. 12. The radiation shield set forth in claim 11, wherein the body comprises at least one of depleted uranium, tungsten, tungsten impregnated plastic, and lead. 13. The radiation shield set forth in claim 11, wherein the radiation shielding lid further comprises a second opening defined in the body, the second opening having a lower end at the lower surface of the body and an upper end at the upper surface of the body, the second opening being spaced apart and separate from the first opening. |
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043748015 | description | DESCRIPTION OF AN EMBODIMENT In FIG. 1, there can be seen the part of the reactor housing closed by the sealed casing 1 in communication with the housing 2 of the fuel separated from the reactor housing by the sealed casing 1 and by a double casing 4. The reactor housing contains the vessel 3 of the reactor resting on the concrete well 5. The vessel 3 of the reactor contains the core 6 inside which the assemblies 7 are disposed parallel to each other in a regular lattice. The vessel 3 is surrounded by the swimming pool 8 which, on reloading of the reactor, is filled with water up to the level 9, this water also filling the vessel 3 which is open on reloading. Reloading is carried out at the vessel by a reactor-loading machine 10 movable above the upper level of the swimming pool on rails 11 extending over the whole length of the swimming pool. The loading machine 10 comprises at least one loading mast 12 allowing taking up of the assemblies 7 in the vessel of the reactor and their conveying inside the swimming pool 8 under the water of this swimming pool at a depth which prevents the risk of irradiation of personnel carrying out reloading. A machine for carrying out permutation of clusters 14 of absorbent material is also disposed in a fixed position in this swimming pool to change clusters of absorbent material associated with some of the fuel assemblies involved in transfers carried out in the core. Instead of a machine with a single loading mast 12 as represented in FIG. 1, a loading machine with three masts can be used. This assures which ensures not only the taking up of the assembly but also the permutations of the clusters and plugs associated with the assemblies when permutations in position of the assemblies in the core are carried out. In this case, there is clearly no advantage in having a machine for permutation of the clusters, such as the machine 14 in the swimming pool of the reactor. A rocker 15 is also disposed in this swimming pool 8, alongside the fuel housing, allowing vertical or horizontal positioning of a container 16 mounted articulated on a conveyor 17 allowing the container to be moved between the swimming pool 8 of the reactor and the swimming pool 18 for the fuel disposed inside the fuel housing 2. To move from the swimming pool of the reactor to the swimming pool for the fuel and back, the conveyor 17 and the container 16 connected to it pass inside a transfer pipe 20 closable at the end facing the swimming pool of the reactor and horizontally disposed. When the container 16 is vertically disposed with the aid of the rocker 15, inside the swimming pool of the reactor, it is possible to put an assembly 7 inside this container with the aid of the loading machine 10 which moves into position 10'. The rocker also allows the container to be returned to the horizontal position on the conveyor 17 and the conveyor and the container in this position can pass inside the pipe 20 to transfer the container and the assembly which is disposed inside into the swimming pool for the fuel. The set of apparatuses located inside the swimming pool of the reactor which has just been described forms part of the state of the known art, the loading operations carried out currently in nuclear power stations making use of such apparatuses which are well-known to users of nuclear power stations. The conveyor 17 also allows the container to be brought inside the swimming pool 18 for the fuel at the level of a new type of rocker 21 which allows the container with an assembly therein to be put in one or two vertical positions, in which the assembly is in its normal position or conversely in the position inverted relative to that which it occupies in the core reactor, i.e., with its lower cap uppermost. The rocker 21 will be described in more detail with reference to FIG. 2. The swimming pool for the fuel can include, in a manner known per se, one or several chutes such as 23 which allow a fuel assembly, for example a new assembly brought by the rolling bridge 24, to be placed at the bottom of the swimming pool in a vertical position. The swimming pool for the fuel presents several parts such as 18a, 18b, 18c separated by dams such as 19a and 19b. Thanks to these dams, the handling means of the swimming pool for the fuel allows fuel assemblies or rods to be conveyed from one to the other of these regions of the swimming pool for the fuel. Various tracks such as 26 for moving the machines for handling fuel elements are disposed above swimming pool 18. In particular, a machine 30 for handling rods which will be described in more detail with reference to FIGS. 4 to 8 can move above the swimming pool for the fuel to handle rods. The region for storing the used fuel of the swimming pool 18 also includes a unit 31 for storing used or defective fuel rods which will be described in more detail with reference to FIG. 10. Finally, in the region beyond the swimming pool for the fuel, i.e., in a region outside the water, a manual tool 32 for handling new fuel rods can move. This tool, which will be described in more detail with reference to FIGS. 8 and 9, allows new fuel rods to allows new fuel rods to be conveyed to a framework at the bottom of the swimming pool for fuel by a chute whose loading is completed with the aid of the tool 32, each time a set of rods is taken from this framework to re-equip a fuel assembly from the core of the reactor. Reference will now be made to FIG. 2 in order to describe the rocker apparatus 21 allowing the container 16 and the fuel assembly inside it to be put into the swimming pool for the fuel in a vertical position. The conveyor 17 includes two beams such as 17a separated by a distance slightly greater than the diameter of the container 16 with an assembly inside it, this container including two connection pieces such as 35 which can engage two pivots such as 36 solid with the beams 17a and 17b of the conveyor, respectively. In this way, the container 16 can be connected to the conveyor 17 for rotary movements with respect to the conveyor about the pivots 36. The connection pieces 35 also allow the container of the conveyor to be disengaged. The rocker 21 comprises a chassis 39 fixed at the bottom of the swimming pool for the fuel to which is fixed in the direction of the conveyor 17 an endless screw 40 which can be driven in rotation about its axis in the framework by a rod 41 driven by a crank disposed in the upper part of the rocker above the level of the swimming pool. This endless screw is in engagement with a yoke 42 for lifting the container, by movement of this lifting yoke between a position 42a and a position 42b. The position 42a of the yoke allows the container 16 to be put in a vertical position so that the assembly which is disposed inside the container is in the position it occupies in the core of the reactor, i.e., with its upper cap in the high position. The position 42b, termed position for taking up rods, allows the container 16 to be swung with the assembly contained therein into a vertical position such that the assembly is in the inverted position relative to the one it occupies in the core of the reactor, i.e., it is then disposed with its lower cap uppermost. The assemblies are in fact designed so that the lower cap can be easily removed, while the upper cap has members which it is best not to remove. To gain access to the rods, it is therefor preferable to remove the lower cap of the assembly. In the two positions 42a and 42b, the lifting yoke is engaged on the container 16 so that the latter can be driven in rotation about the axis 35 by traction on the lifting yoke 42 in one of its positions 42a and 42b by means of a traction cable 43 driven by a winch 44. The winch 44 is itself movable on an endless screw 45 which allows it to be moved between a position 44a and a position 44b corresponding respectively to the position 42a and the position 42b of the lifting yoke, by means of a crank and a control rod. The unit for driving and guiding the winch 44a can be moved aside to allow access to the container and to the assembly in vertical position of the machine 30 for handling rods. When traction is effected on the cable 43 by means of the winch 44 in one of its positions 44a and 44b, the lifting yoke 42, the container and the assembly are driven in a rotary movement about the axis 36 until the container and the assembly are in the normal vertical position 16m or in the inverted vertical position 16i, the lifting yoke then coming into position 42m or 42i. As the apparatus 45 for moving the winch 44 is in retracted position, it is possible to bring one of the masts 47 of the machine 30 for handling fuel rods over the container and the assembly in normal position or in inverted position. A general view in elevation of the machine 30 for handling the fuel is shown in FIG. 3 while FIG. 4 shows a view in section of this machine in which the position of the gripping masts and the detection apparatus is clear. The machine for handling the fuel has a chassis 50 provided with wheels 51 moving on the rails 26 above the swimming pool for the fuel. This handling machine also has a cylinder 52 bearing three masts which can be positioned by rotation successively above an assembly disposed at the bottom of the swimming pool for the fuel. As shown in FIGS. 4 and 5, the machine for handling rods has three fixed masts 53, 54 and 55 and a detector apparatus 56 vertically movable along a mast 57 bearing a camera for viewing the handling of the rods. The mast 53 allows gripping of the assembly and removal of the lower cap of this assembly when it is inverted vertical position. The masts 54 and 55 allow gripping of the whole of the rods of an assembly, the mast 54 allowing gripping of a sub-assembly comprising half the rods and the mast 55 gripping of the other sub-assembly. FIG. 5 shows the end of the mast 53 allowing approach to and gripping of the assembly 7 disposed inside the container 16 under the mast 53 in inverted vertical position and removal of its lower cap. The fixed mast 53 of the machine 30 for handling the fuel rods has an assembly of rollers 60 for guiding the telescopic shaft 61 bearing the tooling allowing taking up of and approach to the assembly and unscrewing of the lower cap 64 connected to the support tubes 65 of this assembly by screws. At its upper part the container 16 includes centering fingers 66 on which the base plate 67 of the tool 68 for lifting the assembly is centered. This tool is constituted by a grab having gripping fingers 69 maneuverable by rods 70 connected to pneumatic actuators 71 for opening and closing the fingers, allowing the assembly to be seized by its cap, in particular by its upper cap, when the assembly is in normal position. The tool 68 also includes a device 72 for locking the fingers which is movable by means of a spring 73, unlocking being possible only when the assembly rests at its lower part on a support and when the grab 68 is able to move vertically and downwards with respect to the assembly. Taking up by the upper cap of the assembly in normal position is necessary for complete removal of the elements from the core of the reactor. A tooling allowing unscrewing of the lower cap 64 of the assembly 7 is also disposed in the center of the grab 68, the lower part of the grab having centering pins 74 which engage in the lower cap 64 so as to position the unscrewing tool above the lower cap of the assembly in a precisely centered position. When the unscrewing tooling is in position on the lower plate of the assembly, an assembly of turnscrews 75 mounted on a plate 76 will be in position on each of the connecting screws of the lower cap of the assemblies and the support tubes 65. The plate 76 bearing the turnscrews is guided into the tool 68 and can be moved in the vertical direction by means of an actuator 77. The turnscrews 75 are mounted to rotate on the plate 76 and can be actuated to unscrew or to screw on the lower plate of the assembly by rods 78 solid with a plate 79 with a threading of very large pitch engaged inside the rods of the turnscrews, which have a screw-threaded bore corresponding to the threading of the rods 78. Maneuvering of the turnscrews can be effected by an actuator 80 causing traction on the plate 79. The machine for handling rods also includes a detector-bearing carrier 56 vertically movable on the mast 57, this detector-bearing carrier being congrous in section which the square section of the assembly, so that a centering apparatus can allow the base of the detector bearer to be precisely centered on the assembly when the lower plate has been removed by means of the tooling disposed in the mast 53. To achieve this, the cylinder 52 of the handling machine simply has to be turned to bring the detector-bearing carrier vertically above the assembly, and this detector-bearing carrier is then lowered by means of a winch to the level of the assembly. When the detector bearer is exactly centered on the assembly, the detectors are each in position against the end of a rod so that each of these detectors can determine whether the rod with which it is in contact can be retained in the assembly or must be removed. Detectors, for example ultrasonic ones, are in fact known which allows cracks to be detected in the cladding of a fuel rod, these cracks making the rod unfit to be re-used in the assembly. Detectors are also known which are capable of determining the depletion ratio of the fuel in the rod with which they are in contact. The detector apparatus is connected to means allowing recording of the position of the fuel rods to be retained and those to be replaced in the assembly. The tool for gripping the rods disposed inside the mast 54 (or 55) of the handling machine 30 is shown in FIGS. 6 and 7. The fixed mast 54 has guiding rollers 81 to guide, in the vertical direction, a telescopic mast 82 of square section which is movable inside the fixed mast 54 by means of a winch apparatus. This telescopic mast has guiding rollers 83 which allow guiding and movement inside the telescopic mast of a gripping tool 84 whose end with gripping tongs is represented on a larger scale in FIG. 7. The tool 84 is of square section and has dimensions similar to those of the assembly. The telescopic mast 82 bears combs for guiding the fuel rods 85 and 86 articulated on the telescopic mast 82 so as to be placeable in working position (comb 85) or in non-working position (comb 86) by means of apparatuses with cams and small rods 87 and 88 controlled by the movement of the handling tool 84 inside the telescopic mast 82. The combs 85 and 86 are retracted at the passing of the tool 84 and are returned to position to guide the rods after the passing of the tool 84. The telescopic mast 82 has centering pins 91 at its base which are positioned in recesses at the upper part of the container 16 in order to center the tool for handling the rods above the assembly 7. When the telescopic mast is in place on the container in centered position, the handling tool 84 is lowered until the central rods 92 of the tongs 93 fixed on the plate 90 solid with the base of the tool 84 come into contact with the plugs 95 of the fuel rods 96 of the assembly 7. The tongs assembly 93 is then in opening position and a mounting which is flexible thanks to springs 98 allows compensation for small displacements in the position of the plugs of the rods of the assembly in the vertical direction. The handling tool also includes a plate 99 on which pneumatic actuators 100 are mounted whose rod is connected to sleeves 101 which come to engage on the outer part of the tongs so as to close them when the corresponding actuator 100 is actuated downwards. The handling apparatuses associated with the masts 54 and 55 of the machine 30 are identical, except that one of the toolings associated with one of the masts acts on a first assembly of fuel rods comprising 132 fuel rods in the case of fuel assemblies with 264 rods used in pressurized water nuclear reactors, while the tooling associated with the other mast of the gripping machine acts on the other 132 fuel rods. It is not in fact possible to seize simultaneously all of the rods disposed in the assembly in a square-meshed structure, because the spaces between the different plugs of the fuel rods are not sufficient for the tongs to be inserted thereinto. It is therefore necessary to operate in two stages, one being carried out by the tool associated with one of the masts and the other by the tool associated with the other mast. The tool associated with the first mast is capable of taking up the rods disposed on the diagonals of the assembly disposed in a first direction, by taking up only every other diagonal, in while the other tooling is capable of taking up the rods disposed on the diagonals in the other direction, also by passing over every other diagonal. The two toolings are therefore jointly capable of taking up all the rods of the assembly. The combs for guiding the rods will therefore be arranged with their spaces in the directions of the diagonals concerned. With reference to FIGS. 8 and 9, the tool 32 for handling new rods which operates in the region of the housing of the fuel outside the swimming pool is seen to include a body 103 whose upper part bears a handle 104 which is necessary for connecting the tool for handling new rods to a handling bridge 105 (shown in FIG. 1). A tong-bearing tool 106 is guided by rollers 107 inside the body 103 of the tool so that it moves in the vertical direction inside this body 103. At its base, the tool has a positioning foot 108 with two positions, each of these positions corresponding to the take-up position in a rack for storing new rods, in which the rods are arranged as in an assembly, in two sub-assemblies arranged one diagonal out of two in the assembly. When the handling tool is in one of these two positions, the tongs 110 mounted on a plate 109 are in contact at their lower part with the plugs of the new fuel rods disposed in the storage rack. The tool also includes a transverse plate 112 on which are mounted sliders 113, on the ends of which are fixed sleeves 114 which, in their low position, can come to engage on the tongs so as to close them on the plugs of the fuel rods. The sleeves are pushed downwards by springs 115 which keep them in the closed-tongs position. When the tooling is brought to the storage rack for new rods, the tongs are kept open by means of a pin which prevents movement downwards of the slider 113 solid with the sleeve 114. To cause the tongs to close on the plugs of the selected fuel rods, the pin is simply unlocked by effecting a quarter turn of the sleeve while grasping it by the ring 116. When the ring 116 is released, the spring 115 brings the sleeve into the closed-tongs position. The tongs assembly corresponding to the assembly of rods required to be extracted being closed, the gripping tool inside the body 103 is simply lifted by means of a crank acting on a pulley 117 driving the gripping tool by means of a cable 118 and an opposing pulley 119. The whole of the system for storing used or defective rods taken from the assembly will now be described with reference to FIG. 10. This apparatus 31 is composed of two combs 121 and 122 with a space 123 between them in which a rod pusher 124 can move towards a removal container 125 having recesses for disposing fuel rods against each other. Each of the combs 121 and 122 includes a set of recesses for rods brought into vertical position above the combs 127 and 128. A stop is associated with each of the combs 121 and 122 allowing the rods to be advanced up to the space 123 between the two combs. A stop 131 is associated with the comb 121 and a stop 132 is associated with the comb 122. Each of the stops 131 and 132 is mounted on two carriers with rollers, movable in the longitudinal direction of rod recesses in the combs by means of endless screws such as 133 driving the carrier 134 of the stop 131. The container 125 includes recesses 135 perpendicular to the recesses of combs 121 and 122 for collecting a whole row of fuel rods pressed one against the other originating from the space 123 between the tools and pushed by the pusher 124. The removal container 125 is mounted on wheels allowing it to move on the rails 136. The container 125 can advance in the longitudinal direction of the combs so as to present each of its recesses 135 opposite the space 123 for filling with used or defective fuel rods. The fuel rods are kept together inside the recesses of the combs and the removal container by springs allowing holding of the rods and their removal by pressure. An operation will now be described which is carried out on an assembly removed by the loading machine of the reactor from the core 6 of this reactor at a region in which, in loading procedures as practised to date, assemblies are replaced by new assemblies. The removed assembly is brought by the loading machine 10 into the container 16 which is placed in the vertical position by means of the rocker 15, which returns the container to the horizontal position when the assembly has been deposited. The conveyor 17 is then pulled to the swimming pool for the fuel 18a and the container is brought into position on the rocker 21 disposed at the bottom of the swimming pool for the fuel. To position it on the rocker 21, the end of the container comes to abut on one projecting end of the lifting yoke 42 and movement of the conveyor 17 is then stopped. The lifting yoke 42 is then put in its position 42b so as to put the assembly inside the container 16 into inverted vertical position. During rotation of the container, the cap of this container, kept closed by contact of the end of the container with one rail occupying the lower part of the rocker, and semi-circular in shape, has been opened under the effect of springs pushing this cap when contact between the cap and the guiding rail is no longer assured. When the container is in inverted vertical position, the assembly is accessible from the top, and the apparatus for moving the winch for lifting the container is then retracted, enabling bringing up by the top of the rocker of the machine for handling the rods whose mast 53 is positioned vertically above the container. After centering of the tool for removing the lower plate of the fuel assembly on this assembly, the plate is unscrewed and taken up by the grab. All that is then required is to turn the cylinder 52 of the machine for handling the rods so as to bring the detector-bearing carrier vertically above the open assembly and to lower and center the detector bearer on the assembly so as to effect the detection and location of rods to be replaced in the assembly, either because of wear or because of faults prohibiting their retention in the assembly. This detection and location can also be carried out in the swimming pool of the reactor or in the reactor independently of the operations in the swimming pool for the fuel. Data concerning the rods to be replaced supplied by the detectors are used both to program the operation of the tongs of the tools for gripping used rods in the assembly, and also to determine reloading with new rods taken from the storage racks of new rods to re-equip a storage assembly of new rods disposed in the swimming pool and positioned in the chute of the swimming pool. All that is then required is to turn the cylinder of the machine for handling fuel rods so as to bring one of the two masts 54 or 55 vertically above the container containing the assembly taken from the core of the reactor and whose lower end is removed. One of the two masts is then used to remove all the fuel rods to be replaced contained in the sub-assembly which is accessible with the aid of this gripping mast, by means of the tongs assembly whose closing has been programmed as a function of the data supplied by the detectors. The first assembly of rods is then raised inside the mast of the machine for handling the rods, and the second removal, corresponding to the second sub-assembly, is carried out with the aid of the mast 55 brought into position above the assembly. The whole of the machine 30 for handling the fuel is then brought above the system for storing the used rods to dispose the removed rods in the two combs of this storage system. The machine is then brought above a framework completely filled with new rods disposed with its fuel rods accessible from above in a chute or in a storage recess at any location in the swimming pool for the fuel. The masts used for removing the used rods are then used to remove the exactly identical assemblies of new or recycled rods and convey them to the assembly disposed in the container, into which these new or recycled rods are inserted in place of the used rods removed by the machine. During these operations carried out with the handling machine, the manual tool for handling new rods is used to re-equip the assembly for storing new rods with assemblies of rods exactly identical to the assemblies of new rods which have been removed by the handling machine to replenish the assembly disposed in the conveying container. To do this, this assembly for storing new rods is brought to the upper part of the chute, i.e., to the upper level of the swimming pool, so that the manual tool can deposit its load of new rods there. Thus the framework containing the new rods remaining permanently in the chute is constantly filled with rods, since removal by the machine for handling the rods is immediately followed by replenishment with the aid of the manual tool for conveying the new rods. In the case of recycling partially used rods, a second framework in fixed position placed in a rack for storing used fuel is used and equipped with rods to be recycled by a manual tool similar in design to that for new rods. When the assembly from the core of the reactor which was placed in the conveying container has been entirely re-equipped, the tool associated with the mast 53 of the handling machine is used to screw the lower cap of the assembly back on again. Swinging of the container and the assembly into the horizontal position can then be effected so that the carrier can return this assembly onto the rocker 15 of the swimming pool for the reactor. Return to the vertical position of the container with the aid of the rocker 15 allows the assembly to be removed with the aid of the machine 10 for loading the reactor. This machine is then brought to the level of the machine for permutation of the clusters and the assembly is re-equipped with the control cluster, necessary for the new location in which the assembly will be replaced. The assembly is then put back in its new location in the core of the reactor. It can therefore be seen that the principal advantages of the invention are that it allows re-equipping of an assembly like that removed from the core of the reactor, while retaining the fuel rods of this assembly which are neither defective nor completely used up, and while keeping the framework of this assembly, making it unnecessary to handle and transport it to the swimming pool for used fuel and then to a reprocessing plant. All the handling carried out in the swimming pool for the used fuel involves the rods only. This avoids the handling of extremely heavy assemblies and allows the best re-use of rods which can be used again. The fact that the frameworks of the assemblies and some of the rods are retained allows reprocessing time to be gained and lessens the area necessary for storing irradiated materials before and after reprocessing. The method according to the invention therefore allows both economies in fuel material since greater depletion of fissile material is obtained, and economies in handling, conveying and storing operations carried out on the fuel assemblies. The invention is not limited to the embodiment just described, but also includes various modifications. Thus, in the example described, the fuel assembly remains in the conveying container on the rocker during all the rod-changing operations carried out in the swimming pool for the fuel, but it is possible to put this assembly in another device, for example a chute or any storage region within the swimming pool for the fuel. The exemplifying embodiment described by way of example is used as used with a pressurized water reactor, but it is also possible to use the method according to the invention for other types of reactor, provided that their fuel elements are in the form of assemblies of parallel rods disposed vertically in the reactor, with the possibility of access to the rods by de-mounting of one of the plates of the assembly. Testing of the rods of the assembly before partially changing them can be effected with detectors of any type, provided that these detectors allow detection of cracks in the cladding material, depletion of the fuel or geometric deformations. Testing and identification of the rods can also be carried out in the swimming pool of the reactor or in the reactor itself. |
claims | 1. A system for processing at least one section of a thin film sample on a substrate, comprising: a processing arrangement, which when executing a computer program, is configured to perform the following steps:(a) controlling an irradiation beam generator to emit successive irradiation beam pulses at a predetermined repetition rate,(b) controlling a shaping of each of the irradiation beam pulses to define at least one line-type beam pulse, wherein a profile of each of the line-type beam pulses includes a top portion, a leading portion and a trailing portion, the at least one line-type beam pulse being provided for impinging the film sample;(c) controlling an irradiation of a first portion of the film sample with at least the top portion of a first one of the line-type beam pulses, to at least partially melt the first portion, the irradiated first portion being allowed to resolidify and crystallize to form approximately uniform areas therein, and(d) after step (c), controlling an irradiation of a second portion of the film sample with at least the top portion of a second one of the line-type beam pulses to at least partially melt the second portion, the irradiated second portion being allowed to resolidify and crystallize to form approximately uniform areas therein,wherein the processing arrangement controls an emission of the second one of the line-type beam pulses to immediately follow an emission of the first one of the line-type beam pulses,wherein at least one section of the first portion of the film sample is prevented from being irradiated by the trailing portion of the second one of the line-type beam pulses. 2. The method according to claim 1, wherein the top portion of each of the line-type beam pulses has an energy density which is above a complete melting threshold. 3. The method according to claim 1, wherein the top portion of each of the line-type beam pulses has an energy density which is below a complete melting threshold. 4. The system according to claim 1, wherein the processing arrangement, when executing the compute program, controls each of the leading and trailing portions of the first one of the line-type beam pulses to irradiate a part of the first portion, and wherein each of the leading and trailing portions of the second one of the line-type beam pulses irradiates a part of the second portion. 5. The system according to claim 1, wherein each of leading and trailing portions of the first and second ones of the line-type beam pulses has first and second sections, wherein each of the first sections of the leading and trailing portions of the first and second ones of the line-type beam pulses has an energy density which is sufficient to at least partially melt at least one of the respective first portion and the respective second portion, and wherein each of the second sections of the leading and trailing portions of the first and second ones of the line-type beam pulses has an energy density lower than a threshold level which is sufficient to at least partially melt at least one of the respective first portion and the respective second portion. 6. The system according to claim 1, wherein the processing arrangement, when executing the computer program, performs step (d) after step (c) is completed and after the film sample is translated for a particular distance with respect to an impingement by the beam pulses of the first portion. 7. The system according to claim 1, wherein the leading portion of the first one of the line-type beam pulses has a first length, wherein the top hat portion of the first one of the line-type pulses has a second length, and the trailing portion of the second one of the line-type beam pulses has a third length, and wherein the particular distance is greater than the sum of the first, second and third lengths. 8. The system according to claim 1, wherein a section of the leading portion of the first one of the line-type beam pulses that has energy density that is between a complete melting threshold and a crystallization threshold and has a first length, wherein the top portion of the first one of the line-type pulses has energy density that is above the complete melting threshold and has a second length, and the trailing portion of the second one of the line-type beam pulses has energy density that is below the complete melting threshold and has a third length, and wherein the particular distance at least a larger of the sum of the first and second lengths and the sum of the second and third lengths. 9. The system according to claim 1, wherein the beam pulse has a Gaussian shape. 10. The system according to claim 1, wherein the first portion of the film sample is irradiated by the top portion of the first one of the line-type beam pulses, wherein the second portion of the film sample is irradiated by the top portion of the second one of the line-type beam pulses, wherein the top portion of each of the first and second ones of the line-type beam pulses has an approximately constant energy density, and wherein the first and second irradiated areas are partially melted by the respective first and second ones of the line-type beam pulses, wherein the top portion of each of the first and second ones of the line-type beam pulses has an approximately constant energy density, and wherein the first and second irradiated areas are partially melted by the respective first and second ones of the line-type beam pulses. 11. The system according to claim 1, wherein no portion of the second one of the beam pulses irradiates any section of the first irradiated and resolidified area. 12. The system according to claim 1, wherein the at least one section of the first portion of the film sample that is prevented from being irradiated by the trailing portion of the second one of the line-type beam pulses includes an active region. 13. The system according to claim 1, wherein the first and second portions of the film sample include active regions of a thin film device. 14. The system according to claim 1, wherein the irradiation beam pulses are shaped by a mask to define the line-type beam pulses. 15. The system according to claim 1 wherein the processing arrangement, when executing the computer program, is further configured to perform the following steps:(g) after step (c) and before step (d), translating the film sample for a particular distance with respect to an impingement by the beam pulses in a periodic manner and based on a frequency of the irradiation of the irradiation beam generator. 16. The system according to claim 1, wherein the first and second portions of the film sample include pixel areas. 17. The system according to claim 1, wherein the first and second portions include areas which are configured to situate thereon an active region of at least one thin-film transistor “TFT” device. 18. A system for processing at least one section of a thin film sample on a substrate, comprising: a processing arrangement, which when executing a computer program, is configured to perform the following steps:(a) controlling an irradiation beam generator to emit successive irradiation beam pulses at a predetermined repetition rate,(b) controlling a shaping of each of the irradiation beam pulses to define at least one line-type beam pulse, wherein a profile of each of the line-type beam pulses includes a top portion, a leading portion and a trailing portion, the at least one line-type beam pulse being provided for impinging the film sample,(c) controlling an irradiation of a first portion of the film sample with at least the top portion of a first one of the line-type beam pulses, to at least partially melt the first portion, the irradiated first portion being allowed to resolidify and crystallize to form approximately uniform areas therein,(d) after step (c), controlling an irradiation of a second portion of the film sample with at least the top portion of a second one of the line-type beam pulses to at least partially melt the second portion, the irradiated second portion being allowed to resolidify and crystallize to form approximately uniform areas therein,wherein the processing arrangement controls an emission of the second one of the line-type beam pulses to immediately follow an emission of the first one of the line-type beam pulses,wherein at least one section of the first portion of the film sample is prevented from being irradiated by the trailing portion of the second one of the line-type beam pulses(e) receiving data associated with locations on the film sample to be irradiated, and(f) after step (c) and before step (d), translating the film sample for a particular distance with respect to an impingement by the beam pulses based on the received data. 19. The system according to claim 18, wherein the top portion of each of the line-type beam pulses has an energy density which is above a complete melting threshold. 20. The system according to claim 18, wherein the top portion of each of the line-type beam pulses has an energy density which is below a complete melting threshold. |
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description | The present invention claims benefit of and is a conversion of U.S. Provisional Patent Application No. 61/056,372 filed 27 May 2008 to inventor Sievers, the contents of which are incorporated herein by reference. The present disclosure relates generally to preparing printing plates. This disclosure describes a method and an apparatus for improving light exposure, e.g., ultraviolet exposure of photo-curable printing plates, e.g., photopolymer flexographic printing plates, letterpress plates and other polymer printing plates, as well as polymer sleeves and polymer coated printing cylinders. Photo-curable, of course, means curable by photons, e.g., light, e.g., light in the ultraviolet range or some other range. Photopolymer plates have found a broad range of applications. A variety of different methods can be applied for transferring an image for printing, e.g., in the form of imaging data, to a polymer plate. For example, an image mask, which can be a film applied to the surface of the plate while the plate is exposed, or a layer directly on top of the polymer surface is laser ablated layer directly on top of the polymer surface, is placed on top of a polymer sheet. By a digital plate is meant a plate that is exposed to imaging data by ablating a mask material that is on the plate, e.g., by exposure to laser radiation in an imaging device. The process of producing a digital plate is called a digital process herein. By a conventional analog plate is meant a plate that is exposed to imaging data by exposing photographic film according to the imaging data, and then using the film to form a mask during exposure to curing radiation. The process of producing a conventional analog plate is called an analog process herein. Irrespective of the way imaging data is transferred to the plate, the plate needs light, e.g., UV light for curing. Such UV curing is currently done by one of several different methods. After curing, the non-cured portions of the polymer are removed, either using solvents, or by melting the non cured material through heat treatment and absorption with a web. Polymer printing plates are three dimensional, that is, include a depth dimension from the printing surface. Small printing details on the plate's surface carry ink for printing. Analog plates, i.e., plates produced using a conventional analog process can produce small features that have printing features that are substantially flat. Such a feature is called a flat top herein. It is much more difficult to produce such flat tops on digital plates, i.e., using a digital process. Features on digital plates tend to have rounded surfaces that extend down in depth. Such a feature is called a round top herein. It would be advantageous to have a method and apparatus that allows the shape of the three dimensional printing features, such as halftone dots and other structures on the printing plate to be controlled, e.g., so that round tops or flat tops can be produced on a digital plate as an operator choice. Overview Described herein is a method and apparatus, and a plate cured using the method. The method and apparatus allow the shape of the three dimensional printing features, such as halftone dots and other structures on the printing plate to be controlled. This can be applied with digital flexography, digital letterpress printing, and other digital printing plates, as well as polymer sleeves and polymer coated printing cylinders. One embodiment includes an apparatus for curing a printing plate made of or having photo-curable material e.g., UV-curable material thereon. The apparatus comprises a light exposure unit including a light source, e.g., a UV source to produce light energy, e.g., UV energy, the light exposure unit capable of generating at least a first illumination intensity and a second illumination intensity, and a power supply coupled to and configured to control the light exposure unit. The light exposure unit is capable of generating at least a first illumination intensity and a second illumination intensity such that curing can produce printing features that have flat tops or round tops on a part of the plate according to the illumination intensity output by the light exposure unit. One embodiment includes an apparatus for curing a printing plate made of or having photo-curable material e.g., UV-curable material thereon. The apparatus comprises a light exposure unit including a light source, e.g., a UV source to produce light energy, e.g., UV energy, the light exposure unit capable of generating at least a first illumination intensity and a second illumination intensity. The apparatus further comprises a drive mechanism to produce relative motion between the light exposure unit and the plate during curing of the plate, and a control system coupled to and configured to control the drive mechanism and light exposure unit. The elements are arranged such that curing can produce printing features that have flat tops or round tops on a part of the plate according to the illumination intensity output by the light exposure unit during an initial time period of the total time period that light energy illuminates the photo-curable material on the part of the plate. One embodiment includes a method of curing a printing plate made of or having photo-curable material thereon. The method includes producing light energy on part of the printing plate using a light exposure unit capable of generating at least a first illumination intensity and a second illumination intensity, such that curing can produce printing features on the plate that can be switched to have either flat tops or round tops according to the illumination intensity output by the light exposure unit. One embodiment includes a method of curing a printing plate made of or having photo-curable material thereon. The method comprises producing light energy on part of the printing plate using a light exposure unit capable of generating at least a first illumination intensity and a second illumination intensity. The method further comprises producing relative motion between the light exposure unit and the plate during curing of the plate, and coordinating the relative motion and the illumination intensity from the light exposure unit. The steps are configured such that curing can produce printing features on the plate that can be switched to have either flat tops or round tops according to the illumination intensity output by the light exposure unit during an initial time period that light energy illuminates the photo-curable material. One embodiment includes a photo-curable printing plate, cured according to a method of curing a printing plate made of or having photo-curable material thereon. The method comprises producing light energy on part of the printing plate using a light exposure unit capable of generating at least a first illumination intensity and a second illumination intensity. The method further comprises producing relative motion between the light exposure unit and the plate during curing of the plate, and coordinating the relative motion and the illumination intensity from the light exposure unit. The steps are configured such that curing can produce printing features on the plate that can be switched to have either flat tops or round tops according to the illumination intensity output by the light exposure unit during an initial time period that light energy illuminates the photo-curable material. Particular embodiments may provide all, some, or none of these aspects, features, or advantages. Particular embodiments may provide one or more other aspects, features, or advantages, one or more of which may be readily apparent to a person skilled in the art from the figures, descriptions, and claims herein. Curing Polymer Plates: The term photopolymer plate, or in its shortened form, polymer plate is used herein to refer to any printing plate, cylinder or sleeve that is cured by application of light, such as ultraviolet (UV) radiation, i.e., that is made of or has thereon a photo-curable material such as a photopolymer. While today, the UV curable material is typically made of a polymer, hence the term, in this disclosure including the claims, a photopolymer plate, or a polymer plate for short means a plate, cylinder or sleeve made of or with any UV curable material thereon. UV curing will briefly be described by way of background. The present invention, however, does not depend on any particular theory. It is believed that the absence or presence of oxygen during the curing process plays an important role in the shape of the three-dimensional structure in the plate. It is believed that oxygen acts as an inhibitor to the polymerization: oxygen molecules end the chain reaction of polymerization and restrict the length of polymer chains formed by the polymerization. A certain amount of oxygen already exists inside the plate materials when the UV curing starts. Additional oxygen from the surrounding air can enter the plate during the curing process once the oxygen concentration inside the plate drops. FIG. 1 shows a simple cross-section of an example halftone dot that results from UV exposure through a mask by UV light from a UV source. The solid line shows the halftone dot profile when there is no or relatively little oxygen during curing, while the broken line shows a simple example of the sort of dot profile that results when there is a lot of oxygen during curing, i.e., during the polymerization process. The presence of oxygen is believed to cause shorter polymer chains than with less oxygen, resulting in a kind of melting of the halftone dots as shown in the broken line profile in FIG. 1. In an analog plate process, e.g., in which a film with the image thereon is placed on top of the polymer plate, and the polymer plate is cured by UV light, the UV light enters via the film. The film is believed to act as a barrier for the oxygen from the environment. This makes the polymer grow until the top of the surface like shown by the solid line in FIG. 1, and also as in FIG. 2 which shows a simple cross-section of a simple example printing pattern with flat tops that results from UV exposure through a film mask by UV light from a UV source. Such a shape is called a flat top shape herein. In a digital printing process, e.g., in which an ablatable layer is ablated with a laser beam, the plate material underneath is cured by UV light entering the plate through the revealed areas. Oxygen can also readily enter the plate through these ablated areas. It is observed that the halftone dots do not grow flat, and may not reach up to the original surface level of the polymer plate, but instead build round shaped structures which stay at slightly below the original surface level, as shown, for example by the dotted line of FIG. 1. FIG. 3 shows a simple cross-section of a simple example printing pattern with round tops that results from UV exposure through a laser ablated film by UV light from a UV source. Such a rounded shape is called a round top shape herein. Digital processes offer the advantage that no film or film processing equipment or the related chemicals are necessary. Digital processes are also believed to be more precise and capable of smaller dot sizes and higher line count resolutions. It is desired to obtain flat tops with a digital process. One known method includes placing a film over the ablated material during curing to simulate a conventional analog process. However, such a process is cumbersome, and furthermore, choosing between a flat top and a round top result requires more equipment and more workflow methods. Described herein is an apparatus and a method of curing plates made of or having photo-curable material thereon that enables an operator to choose having features with flat tops or with round tops, and using the same digital workflow equipment for both cases. Some Embodiments Embodiments of the invention include a method of curing plates and an apparatus for curing printing plates. The plates are made of or have thereon photo-curable material, e.g., UV curable material such as photopolymer. The methods and apparatuses allow the shape of three dimensional printing features, such as halftone dots and other structures on the printing plate to be controlled. Embodiments of the method and the apparatus are applicable to digital flexography, digital letterpress printing, and/or to making other digital printing plates, as well as for curing polymer sleeves and polymer coated printing cylinders. The apparatus includes a light exposure unit including a light source, e.g., a UV source. The light exposure unit is capable of being adjusted to at least two different illumination intensity outputs, a first intensity level that is configured to produce round top features, e.g., round top halftone dots on the printing plate, and a second intensity level that is configured to produce flat top features, e.g., flat top halftone dots on the printing plate. The second intensity level is higher than the first intensity level. In one embodiment, either the selected first or the second intensity level is applied to the total curing time, denoted T. In an alternate embodiment, the second intensity level is applied first and for a significantly less than the total curing time T. In one embodiment, the time is approximately T/4, and the remaining time 3T/4 is applied at the first intensity level. One embodiment includes a drive mechanism to produce relative motion between the light exposure unit and the plate during curing of the plate, and a control system configured to control the drive mechanism and the light exposure unit. The control system in one embodiment is configured to coordinate the relative motion and control the illumination intensity from the light exposure unit during the curing. FIG. 5 shows a simplified block diagram of one embodiment of the invention that includes a rotating drum 501 with a polymer plate 503 thereon, the plate 503 being an imaged plate, that is, having with the image mask thereon. An exposure unit with light source 505, e.g., a UV source moves parallel to the drum axis in what is called the longitudinal direction. A control system 507 is connected to a drive mechanism 509, a power supply 511 and the exposure unit 505. Some embodiment of producing more than one intensity level uses LED arrays, e.g., UV LED arrays. Such arrays are made, for example, by Nichia Corporation of Tokyo Japan. One feature of such LED arrays is the ability to adjust their UV output. The UV output intensity can simply be changed by changing the drive current supplied to the LEDs. Such LED arrays also feature having a relatively small amount of waste energy. In one embodiment, the light source of exposure unit 505 includes a plurality of LEDs. The power supply 511 under control of the control system 507 is capable of switching the exposure unit with light source 505 between providing at least two different output levels including the first intensity level and the second intensity level, e.g., by switching some of the LEDs in the exposure unit on or off depending on the desired output intensity level. The first intensity level is configured to generate standard round top printing features such as halftone dots. The second intensity level is higher than the first intensity level and is configured to generate flat top printing features such as halftone dots. Note that using LEDs that each are capable of the same intensity, the light exposure unit needs to include a higher number of LEDs compared to an exposure unit that is only capable of producing one illumination level for generating round tops. For example, the inventor discovered that for many typical plates, a ratio of 2 produces the desired result. For example, for common polymer plates such as Cyrel DPI™ from E.I. Dupont de Nemours and Company, Wilmington, Del. (DuPont), the time for the entire curing process is between 12 and 15 minutes at an illumination intensity (power per unit area) of 19-20 mW/cm2. Doubling the intensity from 20 mW/cm2 to 40 mW/cm2 gives a significant change from a convex round tops to flat tops. In such an example, twice as many LEDs are needed compared to a curing system for curing in the specified standard way for such plates that would, for digital plates, typically result in round tops. LED exposure units today are expensive. FIG. 6 shows a block diagram of one embodiment of a curing apparatus that uses an exposure unit 605 with a light source that includes two sections, a first section 621 and a second section 622. A power supply 611 in some embodiments is able to control at least the light output of one section independent of the other. In particular, in some embodiments, the illumination intensity of each section can be controlled independently. The first section 621 includes the same number of LEDs as required for standard illumination at the first intensity level, which is sufficient to cure round tops. The second section 622 is smaller than the first section 621, and is configured to illuminate any area of the plate ahead of the first section 621, e.g., it travels ahead of the first section 621 in the longitudinal direction. The second section 622 is equipped with more LEDs per area unit than the first section 621 so that the second section can illuminate at the higher second intensity level, which is sufficient to cure flat tops. The control system is configured so that the first section 621 operates the whole curing time at the first intensity level. In one embodiment, the second section 622 can be switched to illuminate at either the first intensity level or at the higher second intensity level, so that an operator can select whether features are cured to have flat tops or round tops. The inventor has discovered that the surface area illuminated by the second section 622 can be significantly smaller than the area of the first section 621. This may be because the significant change in the shape of the halftone dots from round top to flat top occurs in the part of the halftone dot closest to the printing surface, while the inner part remains the same for round tops or flat tops. See FIG. 1. Curing of the curable material, e.g., the polymer, appears to start at the surface and propagate with time into the depth of the material. The inventor has found that it is sufficient to cure only the upper parts during the first minutes with the higher second intensity level, while the deeper parts of features such as halftone dots can be cured at the lower first intensity level. The inventor also has found that curing for approximately a quarter of the total curing time at the higher second intensity level is sufficient to produce the desired flat tops. Therefore, in one embodiment in which the footprint of the exposure unit 605 is approximately of constant width in the transverse direction perpendicular to the longitudinal direction, the length of the second section 622 in the longitudinal direction is one quarter of the entire length of the light source of the exposure unit 605, that of the sum of the lengths of first section 621 and the second section 622 in the longitudinal direction. Because only one quarter of the area needs double the number of LEDs (assuming each LED has constant output), the number of LEDs is 25% more than a standard exposure unit. That is, if the embodiment of FIG. 5 requires 2N LEDs to expose at the second intensity level, then one embodiment with the two sections 621 and 622 needs 1.25N LEDs. While one embodiment includes a light exposure unit that includes LED arrays, alternate embodiments may use different ways of achieving the (at least two) output illumination intensities. In one alternate embodiment, the light source includes a plurality of arc lamps, arranged such that some can be switched on and off, or that a shutter or other mechanism is included to achieve two controllable output levels. In another embodiment, the light source includes a combination of fluorescent light tubes and LEDs. In one version, one form of illumination—e.g., the using of LEDs occurs at the start of curing of any region to start the curing process, and the second form of illumination, e.g., the using of the fluorescent illumination, occurs for the remainder of the curing. In another embodiment, both illumination forms are used for completion of the curing. In another embodiment, the light source includes a combination of fluorescent light tubes and arc lamps. One form of illumination is used to start the curing process, and in one embodiment, the other used for completion of the curing, or, in another embodiment, both are used for completion of the curing. Thus, in one embodiment, the light source is divided into two sections that illuminate different sized areas, and wherein at least one section's light output intensity can be controlled independently of the other section's light output intensity. In some embodiments, the illumination intensity of each section can be controlled independently. In some embodiments, the smaller area section is capable of generating higher output intensity than the larger area section. In some embodiments, the illumination from the smaller area section hits a region in plate's photo-curable material before the radiation from the larger area section hits the region. FIG. 6 for example, shows the power supply as including two power supply units. Of course, those in the art will understand that there are many different ways of achieving the two illumination levels, and any method of achieving the different illumination levels can be used in different embodiments. Note that the control system in one embodiment is such that the first region is switchable between either at the first or the second illumination intensity. In some embodiments, the second illumination intensity can be increased continuously to a level being sufficient to cure a desired shape between round top and flat top in the polymer. Thus, one embodiment has been described in which the drive mechanism causes the light source to move along the direction of the axis or rotation of a rotatable drum having a plate thereon while the drum is rotated by the drive mechanism in order to cure the curable material of or on the plate. In another one embodiment, the plate is on a flatbed, e.g., of a flatbed scanner, also called an x-y table, and a drive mechanism is configured to produce relative motion between the exposure unit and the plate. In one such embodiment, the light source moves during curing above the plate placed on a flatbed table. FIG. 7 shows a simplified block diagram of one embodiment of such a flatbed 701 in which a plate 703 with a mask thereon is cured. A control system 705 is coupled to a power supply 611 and configured to control the output of an illumination unit 605 that includes a first illumination section 621 and a second illumination section 622. The control system 705 also is coupled to and configured to control a drive mechanism 707 that is configured to move the illumination unit 605 back and forth in a transverse direction while also moving the illumination unit 605 in a longitudinal direction. In another embodiment, the light source moves during curing in a transverse direction and the polymer plate moves during curing in a longitudinal direction perpendicular to the transverse axis. FIG. 8 shows a simplified block diagram of one embodiment of such a flatbed 801 in which a plate 703 with a mask thereon is cured. A control system 805 is coupled to a power supply 611 and configured to control the output of an illumination unit 605 that includes a first illumination section 621 and a second illumination section 622. The control system 805 also is coupled to and configured to control a drive mechanism 807 that is configured to move the illumination unit 605 in a transverse direction while also moving the plate in a longitudinal direction. One advantage of the embodiments shown is that inline curing combined with imaging is possible. That is, the imaging and curing may be combined in the one apparatus. In some versions of the apparatuses shown in FIGS. 6, 7 and 8, a laser imaging unit including imaging elements is included and configured to transfer imaging data to the part of the plate precedes the curing illuminating part. In such a case, the plate is at some stage partially imaged. In another embodiment, the imaging occurs separately prior to the plate being loaded for exposure to UV for curing. In such a case, the plate is fully imaged. FIG. 15 shows a simple drawing of yet another embodiment of a rotating drum arrangement. In this arrangement, an illumination unit 1509 is configured to illuminate at two levels, a first illumination level and a second illumination level. In this embodiment, the illumination unit 1509 extends to cover one dimension of the plate. In the example shown, this is the longitudinal direction. Relative motion in only one direction is then necessary, in this example, the circumferential, i.e., transverse direction. One advantage of the embodiment shown is that with it, inline curing is possible. FIG. 16 shows another simple drawing of yet another embodiment, this one a flatbed arrangement, in which an illumination unit 1609 extends to cover one dimension of the plate. In the example shown, this is the transverse direction. Relative motion in only one direction is again used, in this example, the longitudinal direction. In this example, the exposure unit moves in the longitudinal direction over the plate. FIG. 17 shows another embodiment of a flatbed arrangement in which an illumination unit 1709 extends to cover one dimension of the plate, in this case with the relative motion provided by the plate moving in the longitudinal direction. FIG. 18 shows a simple drawing of yet another embodiment of a rotating drum arrangement in which the illumination unit 1809 extends to cover one dimension of the plate, in this case the circumferential direction. In the example shown, the illumination unit wraps around the drum, e.g., is in the form of a toroid around the drum. Relative motion is provided in the longitudinal direction. No rotation of drum is necessary for curing, but of course might still occur, e.g., as the drum is slowing down, in another embodiment, in line with the imaging. That is, a laser imaging apparatus including imaging elements is included and configured to transfer imaging data to the part precedes the illuminating part. Note that details of the imaging are not shown in order not to obscure the curing operation of the apparatus. In FIGS. 6 to 8 and 15 to 18, some elements such as the illuminating source have different reference numbers but may be similar or identical in structure. Similarly the plate and other shown elements may be similar or identical even if different reference numerals are used. Note that some of the arrangements above may include an imaging unit to enable inline imaging and curing. Other arrangements do not include the imaging unit. The invention is not limited to combining of the imaging and curing in one exposure apparatus, and in some arrangements, the imaging and curing can be carried out separately each in its own apparatus. Therefore, in some arrangements, there is a separate imaging apparatus, e.g., a rotating drum imaging apparatus as is known in the art, or a flatbed imaging apparatus as is known in the art, and also a separate curing apparatus, e.g., a flatbed arrangement that include one or more features of the present invention or a rotating drum arrangement that include one or more features of the present invention. Other variations also are possible. An LED Source In one embodiment, the light exposure unit includes a light tunnel of light reflective, e.g., mirrored walls and has a polygonal cross-section like a kaleidoscope. One such light exposure unit is briefly described herein. Other such light exposure units are described in more detail in U.S. Provisional Patent Application No. 61/055,910 filed 23 May 2008 to inventor Sievers, the contents of which are incorporated herein by reference. FIG. 9A shows a simplified drawing of one example of a light tunnel light exposure unit. The light tunnel includes four side walls 903 each having a reflective inner surface, e.g., by each wall being mirrored or having a mirror attached on the inner surface, so that the light tunnel has a rectangular cross-section. Other embodiments have cross sections that are other than rectangular. In FIG. 9A, the dimensions for a rectangular cross-section are shown as the width of the long side, denoted a, the width of the wide side, denoted b, and the light tube length, also called its height, denoted c. A UV source 901 located at or near one end, called the source end of the light tunnel and arranges in operation to produce light radiation, e.g., UV radiation to the inside of the light tunnel towards the other end of the light tunnel, called the plate end, including towards the reflective inner surfaces of the walls. The plate end is placed near an exposed mask 905 that is on the surface of photocurable plate material, e.g. a polymer plate. Typically, the mask 905 is an exposed part of a digital plate. The dimensions a, b, and c are configured according to parameters such as the plate size in the case of illuminating a whole plate, the power output(s) of the light source, that is, the power output of the light tube and how much of the power is directed towards the inside of the tube. FIG. 9B shows a simplified cross-section of light tunnel and light source, e.g., UV source and also the cross-section of an example halftone dot 907 exposed by a light exposure unit that includes a light tube and a light source capable of a first intensity level and a second intensity level, including pair of opposite mirrors of a “kaleidoscope” light tube made of pairs of opposite walls 903 with reflective inner surfaces, e.g., mirrors in accordance with an embodiment of the present invention. The support shoulders of the halftone dot 907 that result by exposure by such a “kaleidoscope” source is broader than if the same UV source 901 was used without the kaleidoscope light tunnel. FIG. 9B shows singly reflected ray such as ray 913 and a multiply reflected ray of light 915, e.g., of UV. Having such multiple wall reflections in a kaleidoscope broadens the shoulders that support small features such as a halftone dot. The ray 915 that is so-reflected multiple times is shown by a dotted line. FIG. 9C shows a simplified cross-section of an example cured halftone dot exposed by a light source 901, using a light tube that includes, in addition to the side walls 903 with respective inner reflective surfaces, an additional another element 921 with an inner reflective surface 923, e.g., a flat mirror located behind the light source, e.g., UV source and parallel to the polymer plate. In FIG. 9C, rays that are reflected off the flat mirror behind the light source, e.g., rays 917 are shown as broken lines. In different embodiments of the kaleidoscope illuminator such as FIGS. 9A, 9B or FIG. 9C, the reflective surface can be made of different materials. In general, the reflectors of embodiment can be built of flat mirrors or of materials with reflective surfaces, so these walls having reflective inner surfaces can be inexpensive and easy to build. In some embodiments, the reflective surfaces include metal coated glass plates such as conventional glass mirrors. In other embodiments, the light source is a UV source, and the walls are made of UV reflective material coated aluminum sheets, with the inner surfaces coated with the reflective material. In some version of such aluminum sheets, the respective reflective surface of at least some of the walls is embossed with UV light diffusing structures. In some embodiments, the light source is a UV source comprising a fluorescent tube. In other embodiments, the light source is a UV source comprising a mercury lamp. In some embodiments, the light exposure unit is in a fixed position above a flat lying photopolymer printing plate positioned to cure the plate by exposure for a sufficient exposure time. Consider for example, the illumination arrangement of FIGS. 9A, 9B or FIG. 9C. Denote the power density, i.e., the intensity, e.g., in W/cm2 by H. For a given UV power the intensity of the illumination H is:H=P/(a*b);where P denotes the power out of the aperture and * denotes multiplication. The exposure, denoted E to the plate is then E=H*T, e.g., in J/cm2 if P is in W/cm2, where T denotes the exposure time in s. A sufficient exposure time is required for curing in order to ensure that the chain reactions that cause curing have sufficient time. Thus, the requirements for curing are energy, e.g., as an intensity in J/cm2 applied over a particular curing time T. The light tube length c affects the degree of commingling the angle of incidence. The inventors have noticed that the longer the light tube length, the more homogeneous is the angle distribution. In some embodiments, the light source is a UV source comprising an array of UV LEDs. Such UV LEDs are available, for example, from Nichia Corporation, Tokyo Japan. Such UV LEDs have several desirable (albeit not necessary) properties, e.g., they exhibit high spectral purity with 100% of their output power at 365 nm. The wavelength has almost no temperature dependence. They produce relatively little heat, they do not overly heat a plate's polymer material. They age slowly, e.g., about 5% power reduction over 1000 h of use, so provide long life with low maintenance requirement. The power output is proportional to the applied electric power. Nichia Corporation makes for the assignee of the present invention an array module that has between 100,000 and 200,000 UV LEDs. With 21A input at 4.4V, such an array module is capable of about 10 W of UV output at a wavelength of 365 nm. The array module includes microchannels for water cooling, and typically uses water that is purified, e.g., free of ions or any particles larger than about 50 μm. One embodiment of the invention uses a light source made up of such LED array modules. One embodiment of such a UV light source includes 20 such array modules combined to produce a UV light source for a light tube that has a rectangular cross section of about 8 cm by 42 cm. The light source embodiment produces in operation light output of up to 200 W of 365 nm UV radiation. The light source is used, for example, with an external drum exposure system as described further below. One example of such an array, using 15 LED array modules, is now described in more details. FIG. 10 shows a line drawing made from an actual photograph of an LED array light source. The light source includes two rows 1003 and 1005 of LED array modules, a first row 1003 of 7 modules shown parallel to a second row 1005 of 8 array modules. Each row is offset ½ of the separation of array modules—the array module pitch—relative to the other row. The water cooling connections, e.g., pipes 1007 and electrical connections, e.g., wires 1009 for connecting power to the modules can clearly be seen. FIG. 11 shows a line drawing of a perspective view of an individual LED array module 1103, and is from an actual photograph. The heat sink and water input/output can clearly be seen. A1 Euro coin (23.25 mm in diameter) is also shown to give a sense of the size. The source itself includes a 4×4 matrix 1105 of sets of LEDs. As will be seen in the following photographs, each set includes a 4×4 array of LED units, and each LED unit includes 22×19 LEDs. In such an arrangement, there are therefore 16×16×22×19=107008 UV LEDs. FIG. 12 shows a line drawing made from a photograph of an enlarged view of an array module and shows the 4×4 matrix 1105 of sets 1203 of LEDs and leads thereto. Each set 1203 can be seen to include a 4×4 array of LED units. FIG. 13 shows a line drawing made from a photograph of an even more enlarged view of an array module, showing individual sets 1203 of LEDs, including one complete set. The sets 1203 of LEDs each include a 4×4 matrix of LED units 1303, each LED unit 1303 made up of 22×19 LEDs. FIG. 14 shows a line drawing made from a photograph of a yet even more enlarged view of an array module, showing individual units 1303 of LEDs, including one complete unit 1303 of 22×19 LEDs. Possible Theory of Operation In order to enhance understanding of features of the present invention, a theory of operation is presented. Any such theory or mechanism of operation or finding presented herein is not intended to make the present invention in any way dependent upon such theory, mechanism of operation or finding. The inventor has noticed several properties of curing at different intensity levels, i.e., different levels of power per unit area. Using a very simplified view of polymerization process, polymerization can be broken up into three reaction steps: 1) Activation of the starter radicals by UV light. 2) Chain growth of the polymer. 3) Chain ending through oxygen. Each reaction has a certain time constant and total time. For common polymer plates such as Cyrel DPI™ from E.I. Dupont de Nemours and Company, Wilmington, Del. (DuPont), the time for the entire curing process is between 12 and 15 minutes at an illumination intensity (power per unit area) of 19-20 mW/cm2. This results in an energy per unit area of 14.4 to 18 Joules/cm2 to get all radicals starting chains. The activating the radicals step occurs relatively fast when a starter radical interacts with a light photon. A long exposure time is required because the polymer material is not fully packed with starter radicals, and not every photon hits a starter radical. Thus it takes a certain amount of photons and consequently energy, to activate all starter radicals and in principle they could be activated all at the same time. Polymers include a certain amount of oxygen. However, because conventional processes still enable flat tops, the amount of oxygen inside the plate in itself is not sufficient to get round tops. For round tops, it is believed additional oxygen has to diffuse into the plate. Diffusion in dense matter is a slow process. If the intensity is increased, that is, there are more photons per unit area, more polymer chains can be started, while the number of chains finished by oxygen remains the same as with the lower intensity. The inventor has discovered that increasing the intensity can lead to more activation of the starter radicals. The inventor ran some experiments, and discovered that, staring with the specified curing for a plate, e.g., for certain common polymer types, e.g., DuPont's Cyrel DPI of exposure to around 19-20 mW/cm2 for about 15 minutes, and then doubling the intensity from 20 mW/cm2 to 40 mW/cm2 gives a significant change from a convex round top to flat top. Indeed, if the power level is increased even further the top of the halftone dots will take a concave shape. FIG. 4A shows simple cross-section of a round top halftone dot, e.g., as obtained in a digital process at a first intensity level, e.g., 20 mW/cm2. FIG. 4B shows simple cross-section of a flat top halftone dot, e.g., as obtained in a digital process at a second intensity level, e.g., 40 mW/cm2. FIG. 4C shows simple cross-section of a concave top halftone dot, e.g., as obtained in a digital process at an intensity higher than the second intensity level. Thus, changing power levels from a first level to a higher second level thus changes the dot shape. Conventional UV exposing units, however, do not lend themselves to operating at more than one power level, and typically are only capable of power sufficient for obtaining round tops. Conventional off-the-shelf fluorescent tube-based units, for example, do not readily lend themselves to increasing the UV power because there ore no tubes with higher output power available and as the tubes are already arranged very close together there is no space to double the number of tubes. The Plates By the terms “polymer plate” and “photopolymer plate” herein is meant a plate with any type of photo-curable material thereon, whether made of polymer or not. One example is UV-curable material. Another example is material cured by light of different wavelength, not necessarily UV. While today, such curing is typically carried with UV, and such materials are typically photopolymers, use of the term “photopolymer” herein is not meant to be limiting to a polymer composition. The inventor anticipates that in the future, there may be new materials and compositions that also are curable UV radiation of a desired wavelength, and the invention is equally applicable to plates having such material thereon. The method and apparatuses described herein are used for curing many types of plates. The plates can be flexographic plates, flexographic imaging cylinders, flexographic sleeves, and so forth. The plate also can be letterpress plates having UV curable material thereon. Furthermore, the plates can be imaged using a conventional analog process, e.g., film, so that the curing is with the developed film over the plate material. Furthermore, the plates can be imaged using a digital process, e.g., by laser ablating an abatable surface on the plate material prior to final curing, so that curing is with the mask on the plate material after ablation of some of the mask material according to imaging data. Therefore, in some embodiments, the plate is a photopolymer printing plate that is a digital plate. In some embodiments, a photopolymer printing plate that is a conventional analog plate. In some embodiments, the plate is a photopolymer printing plate that is a sleeve. In some embodiments, the plate is a photopolymer printing plate that is a polymer coated cylinder. Furthermore, some embodiments of the invention are in the form of a plate—any of the plates described above—that has been cured, the curing according to a method as described herein. Furthermore, some embodiments of the invention are in the form of a plate—any of the plates described above—that has been imaged then cured, the curing according to a method as described herein. General Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining” or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulate and/or transform data represented as physical, such as electronic, quantities into other data similarly represented as physical quantities. In a similar manner, the term “processor” may refer to any device or portion of a device that processes electronic data, e.g., from registers and/or memory to transform that electronic data into other electronic data that, e.g., may be stored in registers and/or memory. A “computer” or a “computing machine” or a “computing platform” may include one or more processors. Note that when a method is described that includes several elements, e.g., several steps, no ordering of such elements, e.g., of steps is implied, unless specifically stated. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. Similarly, it should be appreciated that in the above description of example embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the DESCRIPTION OF EXAMPLE EMBODIMENTS are hereby expressly incorporated into this DESCRIPTION OF EXAMPLE EMBODIMENTS, with each claim standing on its own as a separate embodiment of this invention. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination. In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description. As used herein, unless otherwise specified the use of the ordinal adjectives “first”, “second”, “third”, etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner. All U.S. patent publications, U.S. patents, and U.S. patent applications cited herein are hereby incorporated by reference. In the case the Patent Rules or Statutes do not permit incorporation by reference of material that itself incorporates information by reference, the incorporation by reference of the material herein excludes any information incorporated by reference in such incorporated by reference material, unless such information is explicitly incorporated herein by reference. Any discussion of prior art in this specification should in no way be considered an admission that such prior art is widely known, is publicly known, or forms part of the general knowledge in the field. In the claims below and the description herein, any one of the terms comprising, comprised of or which comprises is an open term that means including at least the elements/features that follow, but not excluding others. Thus, the term comprising, when used in the claims, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A and B. Any one of the terms including or which includes or that includes as used herein is also an open term that also means including at least the elements/features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising. Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limitative to direct connections only. The terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. “Coupled” may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other. Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention. |
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abstract | The method and system herein pertain to an EUV photon source which includes a plasma chamber filled with a gas mixture, multiple electrodes within the plasma chamber defining a plasma region and a central axis, a power supply circuit connected to the electrodes for delivering a main pulse to the electrodes for energizing the plasma around the central axis to produce an EUV beam. The system can also include a preionizer for ionizing the gas mixture in preparing to form a dense plasma around the central axis upon application of the main pulse from the power supply circuit to the electrodes. A set of baffles may be disposed along the beam path outside of the pinch region to diffuse gaseous and contaminant particulate flow emanating from the pinch region and to absorb or reflect acoustic waves emanating from the pinch region away from the pinch region. |
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044141777 | claims | 1. A phase monitoring apparatus for coolant within a pressurized nuclear reactor vessel, comprising: 2. An apparatus as set out in claim 1 wherein the temperature sensing means comprises a thermocouple fixed to the open end of the length of tubing. 3. An apparatus as set out in claim 1 wherein the temperature sensing means comprises a thermocouple fixed to the open end of the length of tubing; 4. An apparatus as set out in claim 1 wherein the pressure sensing means is located exterior to the vessel. 5. An apparatus as set out in claim 1 wherein the pressure sensing means is located exterior to the vessel and the length of tubing is thermally insulated from the vessel to said pressure sensing means. |
abstract | The present invention is to easily associate X-ray projection data and scanning table z-direction coordinate information with each other. Using set parameters of the operations of a scanning gantry and a scanning table, the association of the X-ray projection data and scanning table z-direction coordinate information with each other is executed. Thereafter, image reconstruction is carried out based on the X-ray projection data to obtain a tomographic image. The operation set parameters are stored as part of the X-ray projection data. Alternatively, they are collectively stored even in the case of files separate from the X-ray projection data. |
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052895130 | abstract | Method of making a fuel assembly lattice member and the lattice member made by such method. The method includes placing a plurality of elongate metal straps on a computer controlled conveyor which successively conveys the straps into alignment with each of a plurality of computer controlled piercing and drawing dies belonging to a progressive die machine. The dies are selectively actuated by the computer to form such elements as curved deflector vanes and spring members on each strap member. After the piercing and drawing operations are completed, the straps are joined by welding to form a lattice member of hexagonal cross section, the lattice member defining a plurality of rhombic-shaped fuel rod cells and a plurality of generally rhombic-shaped guide tube thimble cells therethrough. The rod cells are capable of receiving respective ones of a plurality of fuel rods and the thimble cells are capable of receiving respective ones of a plurality of thimble tubes. The rhombic shape of the rod cells cooperate with the deflector vanes to deflect a component of a fluid stream about the longitudinal center axis of each fuel rod for maintaining liquid substantially single-phase fluid flow over the surface of each fuel rod in order to avoid Departure from Nucleate Boiling (DNB) on the surface of the fuel rods. |
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abstract | A molten metal reactor (10) quickly entrains a feed material in the molten reactant metal (16) and provides the necessary contact between the molten reactant metal and the feed material to effect the desired chemical reduction of the feed material. The reactor (10) includes a unique feed structure (24) adapted to quickly entrain the feed material into the molten reactant metal (16) and then transfer the molten reactant metal, feed material, and initial reaction products into a treatment chamber (12). A majority of the desired reactions occur in the treatment chamber (12). Reaction products and unspent reactant metal are directed from the treatment chamber (12) to an output chamber (14) where reaction products are removed from the reactor. Unspent reactant metal (16) is then transferred to a heating chamber (15) where it is reheated for recycling through the system. |
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description | The present application claims priority to U.S. Provisional Patent Application No. 62/098,984, entitled “Molten Salt Nuclear Reactor and Method of Controlling the Same” and filed on Dec. 31, 2014, and U.S. Provisional Patent Application No. 62/234,889, entitled “Molten Chloride Fast Reactor and Fuel” and filed on Sep. 30, 2015, both of which are specifically incorporated herein for all that they disclose and teach. The present application also claims priority to U.S. Provisional Patent Application No. 62/097,235, entitled “Targetry Coupled Separations” and filed on Dec. 29, 2014, which is specifically incorporated herein for all that it discloses and teaches. The present application is also related to U.S. patent application Ser. No. 14/981,512, entitled “Molten Nuclear Fuel Salts and Related Systems and Methods” and filed on Dec. 28, 2015, which is specifically incorporated herein for all that it discloses and teaches. Molten salt reactors (MSRs) identify a class of nuclear fission reactors in which the fuel and coolant are in the form of a molten salt mixture containing solid or dissolved nuclear fuel, such as uranium or other fissionable elements. One class of MSR is a molten chloride fast reactor (MCFR), which uses a chloride-based fuel salt mixture that offers a high uranium/transuranic solubility to allow a more compact system design than other classes of MSRs. The design and operating parameters (e.g., compact design, low pressures, high temperatures, high power density) of an MCFR offer the potential for a cost-effective, globally-scalable solution to zero carbon energy. The described technology provides a molten salt reactor including a nuclear reactor core configured to contain a nuclear fission reaction fueled by a molten fuel salt. A molten fuel salt control system coupled to the nuclear reactor core is configured to remove a selected volume of the molten fuel salt from the nuclear reactor core to maintain a parameter indicative of reactivity of the molten salt reactor within a selected range of nominal reactivity. In one implementation, a molten salt reactor including a nuclear reactor core configured to sustain a nuclear fission reaction fueled by a molten fuel salt. The molten fuel salt control system includes a molten fuel salt exchange system that fluidically couples to the nuclear reactor core and is configured to exchange a selected volume of the molten fuel salt with a selected volume of a feed material containing a mixture of a selected fertile material and a carrier salt. In another implementation, the molten fuel salt control system includes a volumetric displacement control system having one or more volumetric displacement bodies insertable into the nuclear reactor core. Each volumetric displacement body is configured to volumetrically displace a selected volume of molten fuel salt from the nuclear reactor core when inserted into the nuclear reactor core. In one implementation, the volumetric displacement body removes the selected volume of molten fuel salt from the nuclear reactor core, such as via a spill-over system. This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other implementations are also described and recited herein. A molten salt fast reactor system employs a molten fuel salt in a fast neutron spectrum fission reactor. One type of molten salt reactor includes a fluoride salt as the carrier salt for the fissile fuel. Another type of molten salt reactor is a molten chloride fast reactor (MCFR) with a chloride salt as the carrier salt for the fissile fuel. Although the below description is written with respect to a molten salt chloride reactor, it is to be appreciated that the description, components, and methods described herein may be applicable to any molten fuel salt reactor. In an MCFR system, the fast neutron spectrum provided by chloride salts enables good breed-and-burn performance using the uranium-plutonium fuel cycle. The fast neutron spectrum also mitigates fission product poisoning to provide exceptional performance without online reprocessing and the attendant proliferation risks. During operation of an MCFR system, a molten fuel salt control system allows maintenance of fuel reactivity and/or fuel composition within desired operational bounds. In one implementation, the molten fuel salt control system includes a molten fuel salt exchange system that removes molten fuel salt from the nuclear reactor core, such as to maintain a parameter indicative of reactivity within a selected range of a nominal reactivity. In an additional or alternative implementation, a molten fuel salt control system includes a volumetric displacement control assembly to remove molten fuel salt from a nuclear reactor to control the fission reaction in the MCFR system (e.g., to maintain a parameter indicative of reactivity within a selected range of a nominal reactivity). The volumetric displacement control assembly may contain or be formed of non-neutron absorbing materials, neutron absorbing materials, and/or moderators. FIG. 1 schematically illustrates an example molten chloride fast reactor (MCFR) fuel cycle 100 with a MCFR parent reactor 102 and a MCFR daughter reactor 104. A particular classification of fast nuclear reactor, referred to as a “breed-and-burn” fast reactor, is a nuclear reactor capable of generating more fissile nuclear fuel than it consumes. For example, the neutron economy is high enough to breed more fissile nuclear fuel (e.g., plutonium-239) from fertile nuclear reactor fuel (e.g., uranium-238) than it burns. The “burning” is referred to as “burn-up” or “fuel utilization” and represents a measure of how much energy is extracted from the nuclear fuel. Higher burn-up typically reduces the amount of nuclear waste remaining after the nuclear fission reaction terminates. The example MCFR fuel cycle 100 is designed to use molten salt as a carrier for the fissile fuel in the reactor(s). In one example, this carrier salt may include one or more of a sodium salt, a chloride salt, a fluoride salt, or any other appropriate molten fluid to carry the fissile fuel through the reactor core. In one example, the molten chloride salt includes a ternary chloride fuel salt, although other chloride salts may be employed alternative to or in addition to the ternary chloride salt, including without limitation binary, ternary and quaternary chloride fuel salts of uranium and various fissionable materials. Various compositions have been explored through modelling and testing with a focus on high actinide concentrations and a resulting compact reactor size. For example, bred plutonium can exist as PuCl3 within the MCFR fuel cycle 100, and reduction-oxidation control can be maintained by adjusting the ratio of the oxidation states of the chloride salt used as fertile feed material. The example MCFR fuel cycle 100 enables an open breed-and-burn fuel cycle (e.g., exhibiting equilibrium, quasi-equilibrium, and/or non-equilibrium breed-and-burn behavior) employing a uranium-plutonium fuel cycle and resulting in significantly lower volumes of waste than a conventional open fuel cycle. Various implementations of the described technology provide for a molten fuel salt having a uranium tetrachloride (UCl4) content level above 5% by molar fraction, which aids in establishing high heavy metal content in the molten fuel salt (e.g., above 61% by weight). Uranium tetrachloride implementations may be accomplished through a mixture of UCl4 and uranium trichloride (UCL3) and/or an additional metal chloride (e.g., NaCl), such that desirable heavy metal content levels and melting temperatures (e.g., 330°-800° C.) are achieved. In one implementation, the MCFR parent reactor 102 includes a reactor vessel designed to hold the molten fuel salt as a reactor core section, one or more heat exchangers, control systems, etc. In one implementation, the reactor vessel may have a circular cross-section when cut along a vertical or Z-axis (i.e., yielding a circular cross-section in the XY plane), although other cross-sectional shapes are contemplated including without limitation ellipsoidal cross-sections and polygonal cross-sections. The MCFR parent reactor 102 is started with a loading into the reactor vessel an enriched fuel charge of initial molten fuel 106, such as using uranium-235 as a startup fuel, such as in the form of UCl4 and/or UCl3, along with a carrier salt (e.g., NaCl). In one example, the initial molten fuel 106 mixture contains enriched uranium at 12.5 w %, although other compositions may be employed. The initial molten fuel 106 circulates through a reactor core section in the reactor vessel of the MCFR parent reactor 102. In one implementation of the MCFR parent reactor 102, the molten fuel salt flows in an upward direction as it is heated by the fission reaction in the internal central reactor core section and downward around the internal periphery of the reactor vessel as it cools. It is to be appreciated that other additional or alternative molten fuel flows may also be employed (such as the primary coolant loop 313 of FIG. 3) that are designed to use the convention flows of a heated fluid and gravity, and/or assisted fluid flows through values, pumps, and the like. The constituent components of the molten fuel are well-mixed by the fast fuel circulation flow (e.g., one full circulation loop per second). In one implementation, one or more heat exchangers are positioned at the internal periphery of the reactor vessel to extract heat from the molten fuel flow, further cooling the downward flow, although heat exchangers may additionally or alternatively be positioned outside the reactor vessel. After initial startup, the MCFR parent reactor 102 reaches criticality in nuclear fission and the initial fissile fuel (e.g., enriched uranium) converts the fertile fuel to fissile fuel (breeds up). In the example of initial fissile fuel including enriched uranium, this fissile enriched uranium can breed depleted and/or natural uranium up to another fissile fuel, e.g., plutonium. This breed-and-burn cycle can breed enough plutonium-239 fissile nuclear fuel (e.g., in the form of PuCl3) to not only operate for decades but to also supply fuel for the MCFR daughter reactor 104 and other daughter and granddaughter reactors. Although other daughter and/or granddaughter reactors are not shown, it is to be appreciated that multiple reactors may be fed by the removed used fuel from the parent reactor 102 to one or more daughter reactors, which may then feed start up material to one or more granddaughter reactors, and on and on. In one implementation, the MCFR parent reactor 102 operates at 1000 MWt, which corresponds to a natural fuel circulation point design, although other operating outputs are achievable under different operating conditions, including forced fuel circulation to achieve higher thermal power levels. Other fertile fuels may include without limitation used nuclear fuel or thorium. As previously suggested, during normal operations, the MCFR parent reactor 102 breeds with sufficient efficiency to support a gradually increasing reactivity. The MCFR parent reactor 102 can be maintained at critical (e.g., barely critical) by removing molten fuel salt 108 (which may contain fissile fuel, fertile fuel, carrier salt, and or fission products) from the MCFR parent reactor 102 and replacing the removed molten fuel salt 108 with fertile fuel salt at a slow rate. In this manner, reactivity can be controlled by periodic removal of a volume of fully mixed molten fuel salt that circulates within the reactor vessel, depicted as removed molten fuel 108, and periodic replacement of the removed molten fuel 108 with depleted uranium chloride salt, depicted as fertile molten fuel feed 110. Other fertile fuels may include without limitation natural uranium, used nuclear fuel or thorium. In one implementation, the removed molten fuel 108 can be prepared for disposal as waste or it can be stored until sufficient material is available to start a new MCFR plant (e.g., the MCFR daughter reactor 104). In some cases, the removed molten fuel 108 can be used to start or initiate the MCFR daughter plant without reprocessing the removed molten fuel 108. In the latter scenario, it may be possible for nearly all actinides to move to the next MCFR plant for additional burn-up, thus avoiding proliferation risks associated with nuclear waste. Furthermore, the molten fuel salt exhibits a large negative temperature coefficient, very low excess reactivity, and passive decay heat removal, which combine to stabilize the fission reaction. The MCFR parent reactor 102 outputs certain waste components, illustrated as waste 112. In one implementation, the waste 112 does not contain actinides. Instead, the waste 112 includes gaseous and possibly volatile chloride fission products 114 and solid fission products 116, such as noble metals. The waste 112 can be captured through mechanical filtering and/or light gas sparging or any other appropriate technique to filter waste 112 from the molten fuel salt while the MCFR parent reactor 102 is in operation or the removed molten fuel 108 may be separated, treated, and re-introduced to the reactor. The mechanical filtering captures the solid fission products 116 and other particulates that are less soluble in the molten fuel salt. Similarly, noble fission product gasses are captured and allowed to decay in holding tanks. The filters containing the insoluble and longer lived solid fission products 116 form a portion of the waste stream. In one implementation, the waste 112 also reduces or eliminates criticality concerns as the waste 112 does not contain fissile isotopes separated from the fuel salt. The waste 112 components may include any one or more of transmutation products of the nuclear fission or any one of its decay products, chemical reaction products of the fuel salt with other fission products, corrosion products, etc. The elemental components of the waste 112 (also generally called fission products herein) are based upon the elemental components of the fuel salt, carrier salt, components and coatings, etc. For a molten chloride salt, fission products may include any one or more of noble gases and/or other gases including Iodine, Cesium, Strontium, halogens, tritium, noble and semi-noble metals in aerosol form, and the like. Solid waste fission products may include noble metals, semi-noble metals, alkali elements, alkali earth elements, rare earth elements, etc. and molecular combinations and thereof. FIG. 2 illustrates example MCFR reactivity control resulting from periodic molten fuel removal of molten fuel salt and replacement with a fertile molten fuel feed material, referred to as molten fuel salt exchange. Molten fuel salt exchange systems represent a type of molten fuel salt control system. The X-axis 200 represents time in effective full power years, and the Y-axis represents reactivity in terms of modeled k-effective 202. The parameter, k-effective, represents the multiplication factor, which indicates the total number of fission events during successive cycles of the fission chain reaction. Each drop in k-effective, such as drops 204, 206, and 208, represents a molten fuel salt exchange event. By replacing bred up or fissile molten fuel salt within the reactor with a fertile molten fuel feed, the MCFR can be maintained within a threshold level of a nominal reactivity. In some cases, the nominal reactivity is at an average near-zero excess reactivity operating condition with an upper threshold defining a maximum reactivity of that fuel cycle to trigger a molten fuel exchange, and the lower threshold defining the minimum reactivity to be achieved after the molten fuel exchange. The nominal, upper threshold, and/or lower threshold reactivity levels may stay the same or change over the lifetime of the MCFR based upon design, operation, and/or safety parameters. These parameters, which are indicative of reactivity, may include, without limitation, thermal energy desired to be generated by the reactor, safety levels, component design and lifetime constraints, maintenance requirements, etc. It should be understood that other reactivity control techniques may be employed in combination with molten fuel salt exchange, including without limitation use of a volumetric displacement assembly, neutron-absorbing control assemblies, etc. Furthermore, other molten salt reactors may employ a similar molten fuel exchange feature. As illustrated in FIG. 2, the periodic replacement of molten fuel salt with the fertile molten fuel feed may be used to limit reactivity and maintain ongoing breed-and-burn behavior within the reactor. Chronologically, the initial enriched fuel charge of molten fuel salt and fertile molten fuel salt can breed up, thereby increasing the reactivity within the reactor. After the reactor breads up, the periodic removal of fissile material acts to periodically (whether with uniform or non-uniform periods over time) reduce or control the reactivity of the reactor, returning the reactivity of the molten fuel salt back to an acceptable and pre-selected threshold level which may be a critical condition 210 (e.g., a barely critical condition) at each molten fuel salt exchange operation to approximate an average near-zero excess reactivity operating condition. This exchange operation can be repeated over time, resulting in the “saw tooth” reactivity curve, such as that shown in the MCFR reactivity control graph of FIG. 2. In some implementations, periodic exchange operations can allow the reactor to operate indefinitely without adding supplemental enriched fuel material. While molten fuel salt exchange is described as periodic, it should be understood that such exchange may be performed in a batch-wise, continuous, semi-continuous (e.g., drip) manner, etc. It is to be appreciated that increasing the frequency (which may be paired with smaller volumes of removed bred up fuel) can tighten the control or thresholds around the nominal reactivity to which the MCFR is controlled. FIG. 3 illustrates an example MCFR system 300 equipped with a molten fuel salt exchange assembly 301. In one implementation, the MCFR system 300 includes a reactor core section 302. The reactor core section 302 (which may also be referred to as a “reactor vessel”) includes a molten fuel salt input 304 and a molten fuel salt output 306. The molten fuel salt input 304 and the molten fuel salt output 306 are arranged such that, during operation, a flow of molten fuel salt 308 may form or include conical sections acting as converging and diverging nozzles, respectively. In this regard, the molten fuel salt 308 is fluidically transported through the volume of the reactor core section 302 from the molten fuel salt input 304 to the molten fuel salt output 306. The reactor core section 302 may take on any shape suitable for establishing criticality within the molten fuel salt 308 within the reactor core section 302. As shown in FIG. 3, the reactor core section 302 may be in the form of an elongated core section and may having a circular cross-section when cut along a vertical or Z-axis (i.e., a circular cross-section in the XY plane), although other cross-sectional shapes are contemplated including without limitation ellipsoidal cross-sections and polygonal cross-sections. The dimensions of the reactor core section 302 are selected such that criticality is achieved within the molten fuel salt 308 when flowing through the reactor core section 302. Criticality refers to a state of operation in which the nuclear fuel sustains a fission chain reaction, i.e., the rate of production of neutrons in the fuel is at least equal to rate at which neutrons are consumed (or lost). For example, in the case of an elongated core section, the length and cross-sectional area of the elongated core section may be selected in order to establish criticality within the reactor core section 302. It is noted that the specific dimensions necessary to establish criticality are at least a function of the type of fissile material, fertile material and/or carrier salt contained within the example MCFR system 300. As part of the reactor startup operation, the example MCFR system 300 is loaded with an initial enriched fuel charge of molten fuel salt. The reactor startup operation initiates a fission reaction with a breed-and-burn fuel cycle. The reactivity of the fission reaction of the example MCFR system 300 increases over time (see FIG. 2.). When reactivity fails to satisfy an acceptable reactivity condition (e.g., k-effective meets or exceeds a threshold, such as an upper threshold of 1.005, as indicated in the example shown in FIG. 2), also referred to as an “exchange condition” or a “control condition,” a selected volume of molten fuel salt 308 is removed from the reactor core section 302 and a selected volume and composition of fertile molten fuel feed 310 (e.g., a salt loaded with fertile material, such as depleted and/or natural uranium, used nuclear fuel or thorium.) is loaded into the reactor core section 302 in place of the removed molten fuel salt 308. The removed molten fuel salt 308 may include without limitation one or more of the following: lanthanides, other fission products, fissile material, fertile material and/or carrier salt. It is noted that a non-specific removal of lanthanides reduces the fission product inventory reactor core section 302 and the associated poisoning but also removes some of the fissile material from the reactor core section 302. In FIG. 3, the molten fuel salt exchange assembly 301 is operably coupled to the reactor core section 302 (or another portion of the example MCFR system 300) and is configured to periodically replace a selected volume of the molten fuel salt 308 with a selected volume and composition of the feed material 310. In this regard, the molten fuel salt exchange assembly 301 can control the reactivity and/or composition of the molten fuel salt 308 within the example MCFR system 300. The composition of the molten fuel salt 308 influences the oxidation states of the molten fuel salt 308. In one implementation, it is noted that the molten fuel salt 308 removed from the reactor core section 302 (shown as removed molten fuel 312) includes at least some fissile material, while the feed material 310 includes at least some fertile material. In another implementation, the removed molten fuel 312 includes one or more fission products. For example, the removed molten fuel 312 may include without limitation one or more lanthanides generated via fission within the molten fuel salt 308. In yet another implementation, the removed molten fuel 312 may include without limitation a mixture of fissionable material (e.g., UCl4), one or more fission products (e.g., one or more lanthanides and/or a carrier salt (e.g., NaCl). While molten fuel salt exchange is described as periodic, it should be understood that such exchange may be performed in a batch-wise, continuous, semi-continuous (e.g., drip) manner, etc. As the molten fuel salt 308 within the reactor core section 302 breeds up, converting fertile material to fissile material, the molten fuel salt exchange assembly 301 removes some of the molten fuel salt 308 as the removed molten fuel 312, which contains some volume of fissile material, and replaces the removed molten fuel 312 with the feed material 310, which includes at least some fertile material. In another implementation, the removed molten fuel 312 includes one or more fission products. Accordingly, the molten fuel salt exchange assembly 301 may act as a control mechanism on the reactivity within the example MCFR system 300 and may serve to return the reactivity of the molten fuel salt 308 to a critical condition (e.g., a barely critical condition). Thus, in one implementation, the molten fuel salt exchange assembly 301 of the example MCFR system 300 can allow operation of the example MCFR system 300 indefinitely without adding further enrichment. The molten fuel salt of the feed material 310 may include without limitation one or more fertile fuel salts, such as a salt containing at least one of depleted uranium, natural uranium, thorium, or used nuclear fuel. For example, in the case of a chloride-based fuel, one or more fertile fuel salts may include a chloride salt containing at least one of depleted uranium, natural uranium, thorium, or a used nuclear fuel. In some cases, the feed material 310 may contain fissile fuel, such as enriched uranium, which can be fed into the example MCFR system 300 at a rate or molecular volume less than the initial volume (e.g., 12.5%). This inclusion of fissile fuel in the feed fuel may be used throughout the lifetime of the example MCFR system 300, or alternatively, may be occasionally used to speed up or enrich the molten fuel salt within the example MCFR system 300 to enhance later removed fuel in future molten fuel salt exchanges for placement in daughter reactors. Furthermore, the molten fuel salt of the feed material 310 may include without limitation one or more fissile and/or fertile fuel salts mixed with a carrier salt, such as NaCl, although other carrier salts may be employed. The reactor core section 302 may be formed from any material suitable for use in molten salt nuclear reactors. For example, the bulk portion of the reactor core section 302 may be formed from one or more molybdenum alloys, one or more zirconium alloys (e.g., Zircaloy), one or more niobium alloys, one or more nickel alloys (e.g., Hastelloy N), ceramics, high temperature steel and/or other appropriate materials. The internal surface of the reactor core section 302 may be coated, plated or lined with one or more additional material in order to provide resistance to corrosion and/or radiation damage. In one example, the reactor core section 302 may be constructed wholly or substantially from a corrosion and/or radiation resistant material. In one implementation, the reactor core section 302 includes a primary coolant system 311, which may include one or more primary coolant loops 313 formed from piping 315. The primary coolant system 311 may include any primary coolant system suitable for implementation in a molten fuel salt context. In the illustrated implementation, the primary coolant system 311 circulates molten fuel salt 308 through one or more pipes 315 and/or fluid transfer assemblies of the one or more of the primary coolant loops 313 in order to transfer heat generated by the reactor core section 302 via one or more heat exchangers 354 to downstream thermally driven electrical generation devices and system or other heat storage and/or uses. It should be understood that an implementation of the example MCFR system 300 may include multiple parallel primary coolant loops (e.g., 2-5 parallel loops), each carrying a selected volume of the molten fuel salt inventory through the primary coolant system 311. In the implementation illustrated in FIG. 3, the molten fuel salt 308 is used as the primary coolant. Cooling is achieved by flowing molten fuel salt 308 heated by the ongoing chain reaction from the reactor core section 302, and flowing cooler molten fuel salt 308 into the reactor core section 302, at the rate maintaining the temperature of the reactor core section 302 within its operational range. In this implementation, the primary coolant system 311 is adapted to maintain the molten fuel salt 308 in a subcritical condition when outside of the reactor core section 302. It is further noted that, while not depicted in FIG. 3, the example MCFR system 300 may include any number of additional or intermediate heating/cooling systems and/or heat transfer circuits. Such additional heating/cooling systems may be provided for various purposes in addition to maintaining the reactor core section 302 within its operational temperature range. For example, a tertiary heating system may be provided for the reactor core section 302 and primary coolant system 311 to allow a cold reactor containing solidified fuel salt to be heated to an operational temperature in which the salt is molten and flowable. Other ancillary components may also be utilized in the primary coolant loop 313. Such ancillary components may be include one or more filters or drop out boxes for removing particulates that precipitate from the primary coolant during operation. To remove unwanted liquids from the primary coolant, the ancillary components may include any suitable liquid-liquid extraction system such as one or more co-current or counter-current mixer/settler stages, an ion exchange technology, or a gas absorption system. For gas removal, the ancillary components may include any suitable gas-liquid extraction technology such as a flash vaporization chamber, distillation system, or a gas stripper. Some additional implementations of ancillary components are discussed in greater detail below. It is noted herein that the utilization of various metal salts, such as metal chloride salts, in example MCFR system 300 may cause corrosion and/or radiation degradation over time. A variety of measures may be taken in order to mitigate the impact of corrosion and/or radiation degradation on the integrity of the various salt-facing components (e.g., reactor core section 302, primary coolant piping 315, heat exchanger 354 and the like) of the example MCFR system 300 that come into direct or indirect contact with the fuel salt or its radiation. In one implementation, the velocity of fuel flow through one or more components of the example MCFR system 300 is limited to a selected fuel salt velocity. For example, the one or more pumps 350 may drive the molten fuel salt 308 through the primary coolant loop 313 of the example MCFR system 300 at a selected fuel salt velocity. It is noted that in some instances a flow velocity below a certain level may have a detrimental impact on reactor performance, including the breeding process and reactor control. By way of non-limiting example, the total fuel salt inventory in the primary loop 313 (and other portions of the primary coolant system 311) may exceed desirable levels in the case of lower velocity limits since the cross-sectional area of the corresponding piping of the primary loop 313 scales upward as flow velocity is reduced in order to maintain adequate volumetric flow through the primary loop 313. As such, very low velocity limits (e.g., 1 m/s) result in large out-of-core volumes of fuel salt and can negatively impact the breeding process of the example MCFR system 300 and reactor control. In addition, a flow velocity above a certain level may detrimentally impact reactor performance and longevity due to erosion and/or corrosion of the internal surfaces of the primary loop 313 and/or reactor core section 302. As such, suitable operational fuel salt velocities may provide a balance between velocity limits required to minimize erosion/corrosion and velocity limits required to manage out-of-core fuel salt inventory. For example, in the case of a molten chloride fuel salt, the fuel salt velocity may be controlled from 2-20 m/s, such as, but not limited to, 7 m/s. In the example implementation illustrated in FIG. 3, the molten fuel salt exchange assembly 301 (a “molten fuel salt exchange system”) includes a used-fuel transfer unit 316 and a feed-fuel supply unit 314. In one implementation, the used-fuel transfer unit 316 includes a reservoir 318 for receiving and storing used-fuel 312 (e.g., burned fuel) from one or more portions of the MCFR system 300. As previously noted, the used-fuel 312 transferred to and stored in reservoir 318 represents a portion of the molten fuel salt mixture 308 previously used fission reaction within the MCFR system 300 and may include initial fissile material, bred up fissile material, fertile material and/or fission products, such as lanthanides. In another implementation, the used-fuel transfer unit 316 includes one or more fluid transfer elements for transferring molten fuel salt 308 from one or more portions of the MCFR system 300 to the reservoir 318. The used-fuel transfer unit 316 may include any fluid transfer element or device suitable for molten salt transfer. By way of non-limiting example, the used-fuel transfer unit 316 may include one or more pipes 320, one or more valves 322, one or more pumps (not shown) and the like. In another implementation, the used-fuel transfer unit 316 may transfer molten fuel salt 308 from any portion of the MCFR system 300 fluidically coupled to the reactor core section 302. By way of nonlimiting example, the used-fuel transfer unit 316 may transfer molten fuel salt 308 from any portion of the primary circuit, such as, but not limited to, the reactor core section 302, the primary coolant system 311 (e.g., primary coolant loop 313) and the like, to the reservoir 318. In one implementation, the feed-fuel supply unit 314 includes a feed material source 317 for storing feed material 310 (e.g., mixture of fertile material and carrier salt). In one implementation, the feed material 310 may include a mixture of a selected fertile material (e.g., depleted uranium, natural uranium, used nuclear fuel, thorium and the like) and a carrier salt (e.g., NaCl) mixed such that the concentration of the molten feed material has a concentration of fertile material compatible with the molten fuel salt 308 remaining in the primary circuit of the MCFR system 300. In another implementation, the fertile material may include a fertile salt, such as uranium chloride, thorium chloride and the like. In this regard, the particular components of the feed material may be selected so as to at least approximately maintain or adjust the stoichiometry and/or chemistry (e.g., the chemical composition and/or reactivity) present in the molten fuel salt 308 contained within the MCFR system 300. In one implementation, the molten fuel salt exchange assembly 301 is capable of transferring the used fuel 312 out of the one or more portions of the MCFR system 300 while concurrently or sequentially transferring the feed material (e.g., which can include a mixture of a selected fertile material and a carrier salt) into the one or more portions of the MCFR system 300. In another implementation, the transfers may be performed synchronously or asynchronously. In another implementation, the feed-fuel supply unit 314 includes one or more fluid transfer elements for transferring feed material 310 from the feed material source 317 to one or more portions of the MCFR system 300. The feed-fuel supply unit 314 may include any fluid transfer element or device. By way of non-limiting example, the feed-fuel transfer unit 314 may include one or more pipes 324, one or more valves 326, one or more pumps (not shown) and the like. In another implementation, the feed-fuel supply unit 314 may transfer feed material 310 from the feed material source 317 to any portion of the MCFR system 300 fluidically coupled to the reactor core section 302. By way of non-limiting example, the feed-fuel supply unit 314 may transfer feed material 310 from the feed material source 317 to any portion of the primary circuit, such as, but not limited to, the reactor core section 302, the primary coolant system 311 (e.g., primary coolant loop 315) and the like. In one implementation, the feed material 310 is continuously transferred by the feed-fuel supply unit 314 to the reactor core section 302. By way of non-limiting example, the feed material 310 is continuously transferred at a selected flow rate by the feed-fuel supply unit 314 to the reactor core section 302. It is to be appreciated that the method of molten fuel salt removal may be continuous, semi-continuous, or in batches, and may be the same as or different from the method or timing of the fuel replacement. In another implementation, the feed material 310 is transferred batch-wise (i.e., in discrete volume units) by the feed-fuel supply unit 314 to the reactor core section 302. By way of example, the feed material 310 is transferred to the reactor core section 302 at a selected frequency (or at non-regular time intervals), a selected volume transfer size, and a selected composition for each batch transfer. The selected frequency, volume transfer size, and composition can vary over time. In another implementation, the feed material 310 is transferred by the feed-supply unit 314 to the reactor core section 302 in a semi-continuous matter. By way of non-limiting example, the feed material 310 is transferred to the reactor core section 302 via drip delivery. Such a semi-continuous feed of material (and simultaneous removal of utilized fuel from the reactor core section 302) may allow for limiting reactivity swings to less than 100 pcm (percent mille or change in keff of less than 0.01). In another implementation, the feed-fuel supply unit 314 may include multiple feed material sources and associated fluid transfer elements (e.g., valves and piping) to allow an exchange of multiple variations of feed materials, so as to maintain the oxidation state of the reactor core section 302. For example, individual feed material sources, each containing one of UCl3, UCl4, or NaCl, may be used to selectively adjust the chemical composition of the molten fuel salt 308. See FIG. 8 for an explanation of the ternary phase diagram for UCl3—UCl4—NaCl (in mole %), wherein the oxidation states and stoichiometry of the molten fuel salt 308 may be controlled by adding selected volumes of UCl3, UCl4, or NaCl. In one implementation, the reservoir 318 includes one or more storage reservoirs suitable for receiving and storing the molten fuel salt from the reactor core section 302. The reservoir 318 may be sized and or designed to limit reactivity of the used fuel salt 312 to reduce or limit reactivity below criticality. The reservoir 318 may include any one or more of neutron absorbers, moderating materials, heat transfer devices, etc. to ensure any ongoing nuclear fission reactions within the used fuel salt 312 do not exceed some specified threshold of design and/or safety. In another implementation, the reservoir 318 may include a second generation (“daughter”) fast spectrum molten salt reactor. It should be understood that used-fuel removal and feed material supply are coordinated to maintain the reactivity and/or composition of the molten fuel salt 308 within the reactor core section 302. Accordingly, in one implementation, the molten fuel salt exchange assembly 301 includes an exchange controller 328. In one implementation, the exchange controller 328 may control one or more active fluid control elements in order to control the flow of feed material 310 from the feed material source 317 and the flow of used fuel salt 312 from the reactor core section 302 to the reservoir 318. In one implementation, the valves 322 and 326 are active valves controllable via electronic signal from the exchange controller 328. By way of non-limiting example, the valves 322 and 326 may include, but are not limited to, electronically-controlled two-way valves. In this regard, the exchange controller 328 may transmit a control signal to one of or both of the valves 322 and 326 (or other active flow control mechanisms) to control the flow of feed material 310 from the feed material source 317 and the flow of used fuel salt 312 from the reactor core section 302 to the reservoir 318. It is noted herein that the present implementation is not limited to the electronically controlled valves, as depicted in FIG. 3, which are provide merely for illustrative purposes. It is recognized herein that there are a number of flow control devices and configurations applicable to molten salt transfer that may be implemented to control the flow of feed material 310 from the feed material source 317 and the flow of used fuel salt 312 from the reactor core section 302 to the reservoir 318. In one implementation, the molten fuel salt exchange assembly 301 includes one or more reactivity parameter sensors 330, as discussed above. As previously noted, the one or more reactivity parameter sensors 330 may include any one or more sensors for measuring or monitoring one or more parameters indicative of reactivity or a change in reactivity of the fuel salt 308 of the reactor core section 302. The reactivity parameter sensor 330 may include, but is not limited to, any one or more capable of sensing and/or monitoring one or more of neutron fluence, neutron flux, neutron fissions, fission products, radioactive decay events, temperature, pressure, power, isotropic concentration, burn-up and/or neutron spectrum. By way of non-limiting example, as discussed above, the one or more reactivity parameter sensors 330 may include, but are not limited to, a fission detector (e.g., micro-pocket fission detector), a neutron flux monitor (e.g., a fission chamber or an ion chamber), a neutron fluence sensor (e.g., an integrating diamond sensor), a fission product sensor (e.g., a gas detector, a β detector or a γ detector) or a fission product detector configured to measure a ratio of isotope types in a fission product gas. By way of another non-limiting example, as discussed above, the one or more reactivity parameter sensors 330 may include, but are not limited to, a temperature sensor, a pressure sensor or a power sensor (e.g., power range nuclear instrument). In another implementation, the reactivity is determined with one or more of the measured reactivity parameters (discussed above). In one implementation, the reactivity of the reactor core section 302 is determined by the controller 328 using a look-up table. In another implementation, the reactivity of the reactor core section 302 is determined by the controller 328 using one or more models. In another implementation, the reactivity parameter may be determined by an operator and entered directly into the controller 328 via an operator interface. It is noted herein that, while the reactivity parameter sensor 330 is depicted as being located within the fuel salt 308 in the reactor core section 302 of the MCFR system 300, this configuration is not a limitation on the present implementation, as noted previously herein. In one implementation, the determined reactivity parameter (whether measured or modeled), or a parameter indicative of reactivity, is compared with a predetermined reactivity threshold. If the determined reactivity parameter, or a parameter indicative of reactivity, satisfies a control condition (e.g., exceeds a high threshold or falls below a low threshold), a control system (e.g., a molten fuel salt exchange system, a volumetric displacement system, and/or other control systems) may be actuated to adjust the reactivity of the reactor core section 302 back into a nominal reactivity range. In another implementation, the one or more reactivity parameter sensors 330 are communicatively coupled to exchange controller 328. The one or more reactivity parameter sensors 330 are communicatively coupled to the exchange controller 328. For example, the one or more reactivity parameter sensors 330 may be communicatively coupled to the exchange controller 328 via a wireline connection (e.g., electrical cable or optical fiber) or wireless connection (e.g., RF transmission or optical transmission). In one implementation, the exchange controller 328 includes one or more processors and memory. In one implementation, the memory maintains one or more sets of program instructions configured to carry out one or more operational steps of the molten fuel salt exchange assembly 301. In one implementation, the one or more program instructions of the exchange controller 328, in response to the determined reactivity parameter exceeding the upper reactivity threshold, may cause the exchange controller 328 to direct the molten fuel salt exchange assembly 301 to replace a selected and determined volume of the molten fuel salt 308 of the MCFR system 300 with a selected and determined volume and composition of feed material 310 in order to control the reactivity and/or composition of the molten fuel salt 308 within the reactor core section 302. In another implementation, the one or more program instructions are configured to correlate a determined reactivity of the molten fuel salt 308 of the reactor core section 302 with a selected replacement volume and composition to compensate for the measured excess reactivity of the reactor core section 302, as well as other molten fuel salt compositional considerations. By way of non-limiting example, the reactivity parameter sensor 330 may acquire a reactivity parameter associated with the molten fuel salt 308 within the reactivity core section 302 (or another portion of the MCFR system 300). In settings where the reactivity parameter is indicative of a reactivity larger than a selected upper threshold, the exchange controller 328 may determine the replacement volume and composition to compensate for the elevated reactivity and direct the molten fuel salt exchange assembly 301 to remove the determined volume of molten fuel salt 308 from the reactor core section 302 (e.g., removed by used-fuel transfer unit 316) and replace the removed fuel salt with a substantially equal volume of feed material 310 (e.g., replaced by the feed-fuel supply unit 314). The amount of used-fuel 312 to be removed from the reactor core section 302 may be determined based upon the determined reactivity (measured or modeled) of the reactor core section 302, the determined amount of fissile and/or fertile fuel (measured or modeled), the waste (including fission products and other possible neutron absorbers) in the molten fuel salt 308, etc. The determined core reactivity, exceeding the upper threshold, may be compared to a lower threshold to determine an amount of change in reactivity needed to maintain the core reactivity within the bounds of the selected nominal reactivity. This amount of required change in reactivity can then be used with the existing fuel to determine the amount of used-fuel 312 to be removed to maintain core reactivity within the bounds of the upper and lower thresholds of reactivity. For example, the worth of a determined volume of removed used-fuel 312 may be determined (based upon the burn up of fissile fuel, the available fissile fuel, the remaining fertile fuel, and other components, e.g., fission products and carrier salts) of the existing fuel composition, and compared if sufficient to reduce reactivity of the reactor core to the lower threshold. Based upon the determined core reactivity after fuel removal, the worth, volume and components of the feed fuel may be determined to maintain reactivity for continued breeding of fuel, fuel volume requirements for the system, and maintain or adjust stoichiometry of the fuel overall. These determinations can be based upon computational models of reactivity and reactions, look up tables based on empirical and/or modeled data, etc. As noted above, any one or more (or combination of) the nominal reactivity level, the upper threshold reactivity level, and/or the lower reactivity threshold may dynamically change over the lifetime of the reactor for various operational and/or safety reasons. In another implementation, in settings where the frequency, volume, and composition of the replacement of molten fuel salt 308 with feed material 310 is predetermined, the exchange controller 328 may carry out a pre-determined scheduled exchange process via the control of active elements (e.g., valves 322 and 326, pumps and the like) of the molten fuel salt exchange assembly 301, based on time since last exchange cycle and/or determined reactivity of the reactor core section 302, as discussed herein. In alternative implementations, exchange may be performed at dynamically determined frequencies and/or volumes, based on results from reactivity parameter sensors 330 and other sensors, monitoring techniques, and computations. In one implementation, the selected volume and/or composition of feed-material added to the reactor core section 302 has a predetermined “worth” that can be adjusted up or down in volume and/or composition to match a target reactivity removal from a selected volume of used fuel removed from the reactor core section 302. In another implementation, the exchange controller 328 may direct the molten fuel salt exchange assembly 301 to perform a continuous exchange of molten fuel salt 308 with feed material 310, with feed material 310 being continuously fed to the reactor core section 302 and used-fuel 312 being continuously removed from the reactor core section 302 at a selected rate (e.g., 0.1-10 liters/day). In another implementation, the exchange controller 328 may direct the molten fuel salt exchange assembly 301 to perform semi-continuous exchange (e.g., drip) of molten fuel salt 308 with feed material 310. By way of example, the exchange controller 328 may direct the molten fuel salt exchange assembly 301 to perform drip exchange of molten fuel salt 308 with feed material 310, with feed material 310 being drip fed to the reactor core section 302 and discrete amounts of used-fuel 312 being simultaneously removed from the reactor core section 302. In another implementation, the exchange controller 328 may direct the molten fuel salt exchange assembly 301 to perform a batch-wise exchange of molten fuel salt 308 with feed material 310. By way of example, the exchange controller 328 may direct the molten fuel salt exchange assembly 301 to perform a series of discrete, or batch-wise, exchanges of molten fuel salt 308 with feed material 310, with discrete amounts of feed material 310 being fed to the reactor core section 302 and discrete amounts (equal in volume to the feed material) of used-fuel 310 being concurrently or sequentially removed from the reactor core section 302 at selected time intervals. By way of another non-limiting example, the exchange controller 328 may direct the molten fuel salt exchange assembly 301 to perform a single discrete, or batch-wise, exchange of molten fuel salt 308 with feed material 310, with a discrete amount of feed material 310 being fed to the reactor core section 302 and an equal amount of used-fuel 312 being concurrently or sequentially removed from the reactor core section 302 at the selected time. In another implementation, the MCFR system 300 includes one or more gas sparging units. The one or more gas sparging units are operably coupled to the reactor core section 302 and configured to continuously remove one or more waste gases (such as gaseous fission products like noble gases) from the molten fuel salt 308 of the reactor core section 302. By way of non-limiting example, the one or more gas sparging units include a helium and/or hydrogen gas sparging unit. It is noted that the noble gases include He, Ne, Ar, Kr and Xe. It is further noted that the gaseous waste absorbed in the molten fuel salt 308 may diffuse out of the molten fuel salt 308 of the reactor core section 302, allowing for them to be pumped out of the reactor via an associated gas pump. In another implementation, the reactor includes one or more filtering units. The one or more filtering units are operably coupled to the reactor core section 302 and configured to continuously remove one or more solid waste components, e.g., solid fission products such as noble and/or semi-noble metals or other particulate waste. By way of non-limiting example, the one or more filtering units may include one or more filters located in a bypass flow of the reactor core section 302 arranged to collect the one or more components of the solid waste, which precipitate and/or plate (depending on the design geometry) out of the molten fuel salt 308. It is noted that the noble and semi-noble metals include Nb, Mo, Tc, Ru, Rh, Pd, Ag, Sb and Te. In another implementation, the primary coolant system 311 includes one or more pumps 350. For example, one or more pumps 350 may be fluidically coupled to the primary coolant system 311 such that the one or more pumps 350 drive the molten fuel salt 308 through the primary coolant/reactor core section circuit. The one or more pumps 350 may include any coolant/fuel pump applicable to molten fuel salt 308. For example, the one or more fluid pumps 350 may include, but are not limited to, one or more mechanical pumps fluidically coupled to the primary coolant loop 313. By way of another example, the one or more fluid pumps 350 may include, but are not limited to, one or more electromagnetic (EM) and/or mechanical pumps fluidically coupled to the primary coolant loop 313. In another implementation, the MCFR system 300 includes a secondary coolant system 352 thermally coupled to the primary coolant system 311 via one or more heat exchangers 354. The secondary coolant system 352 may include one or more secondary coolant loops 356 formed from pipes 358. The secondary coolant system 352 may include any secondary coolant system arrangement suitable for implementation in a molten fuel salt context. The secondary coolant system 352 may circulate a secondary coolant through one or more pipes 358 and/or fluid transfer assemblies of the one or more secondary coolant loops 356 in order to transfer heat generated by the reactor core section 302 and received via the primary heat exchanger 354 to downstream thermally driven electrical generation devices and systems. For purposes of simplicity, a single secondary coolant loop 360 is depicted in FIG. 3. It is recognized herein, however, that the secondary coolant system 352 may include multiple parallel secondary coolant loops (e.g., 2-5 parallel loops), each carrying a selected portion of the secondary coolant through the secondary coolant circuit. It is noted that the secondary coolant may include any second coolant suitable for implementation in a molten fuel salt context. By way of example, the secondary coolant may include, but is not limited to, liquid sodium. It is further noted that, while not depicted in FIG. 3, the MCFR system 300 may include any number of additional or intermediate coolant systems and/or heat transfer circuits. It is noted herein that the utilization of various metal salts, such as metal chloride salts, in MCFR system 300 may cause corrosion and/or radiation degradation over time. A variety of measures may be taken in order to mitigate the impact of corrosion and/or radiation degradation on the integrity of the various salt-facing components (e.g., reactor core section 302, primary coolant piping 315, heat exchanger 354 and the like) of the MCFR system 300. In one implementation, using a noble metal as a cladding for various salt-facing components can mitigate the impact of corrosion of such components. In one implementation, the use of molybdenum cladding on the sodium-exposed surfaces can mitigate the impact of corrosion on such surfaces. In another implementation, the molten fuel salt may be maintained (e.g., via molten fuel salt exchange) in a redox (chemical reduction oxidation) state that is less corrosive. Certain additives may also be employed to mitigate the corrosive impact of the molten fuel salt on such components. FIG. 4 illustrates a graph 400 of modeled keff values (curve 402) of a reactor core and the total percentage of burn up of heavy metal (HM) fuel (curve 404) over time for a molten salt reactor controlled by the periodic exchange of molten fuel salt of the reactor with a fertile fuel salt. As also noted with regard to FIG. 2, the periodic exchange of molten fuel salt of the reactor with a fertile fuel salt may be used to limit reactivity and maintain ongoing breed-and-burn behavior within the molten salt reactor. In another implementation, the molten fuel salt exchange assembly may feed the molten salt reactor with salt loaded with fertile material (e.g., depleted uranium) at a rate that matches the rate at which fissile material is burned by the molten salt reactor, as discussed with regard to FIG. 5. Alternatively, the fertile material may be added at a different rate and/or time than the fissile fuel is removed. FIG. 5 illustrates a graph 500 of keff (curve 502) versus time for a modeled molten salt reactor with a depleted uranium feed provided at a rate that matches the reactor burn rate. It is noted that, in this implementation, the exchange assembly does not or need not specifically target lanthanides for removal from the molten salt reactor but rather removes them via bulk volume removal of the molten fuel salt within the molten salt reactor. The removed material may include without limitation one or more of the following: lanthanides, other fission products, fissile material, fertile material and/or the carrier salt. As shown in FIG. 5, the molten salt reactor breeds up and reaches a peak in keff of approximately 1.03 at around 10-15 years. The molten salt reactor thereafter experiences a loss in reactivity as the actinide inventory, including fissile material, falls while the fission product inventories increase. It is noted that such a configuration may operate for over 20 years and burn greater than 36% of the heavy metal fuel initially loaded into the reactor and later fed to the molten salt reactor during the molten salt reactor's lifetime. Example keff ranges that may be employed can include without limitation 1.0 as a low threshold and 1.035 as a high threshold, defining an example nominal reactivity range. Another example of keff can include without limitation 1.001 as a low threshold and 1.005 as a high threshold, defining another example nominal reactivity range. Yet another example nominal reactivity range may extend from just over 1.0 to about 1.01. Other nominal ranges and thresholds may be employed. Furthermore, other control systems may be employed, including without limitation control rods or control drums, moderators, etc. FIG. 6 illustrates a graph 600 depicting keff as function of time for a molten salt reactor with no addition of feed material and no removal of lanthanides. Curve 602 depicts keff for the case where waste fission products, such as noble gases and noble/semi-noble metals, are removed from the reactor core section 302. In such a scenario, calculations indicate that 30% burn-up may be achieved, with a lifetime of approximately 9 years. Curve 604 depicts keff as a function of time for the cases where nothing is removed from the reactor core section 302. In such a scenario, calculations indicate that a 10% burn-up may be achieved, with a lifetime of approximately 3 years. FIG. 7 illustrates an alternative example MCFR system 700 equipped with a molten fuel salt exchange assembly 701. The primary coolant system is configured such that a primary coolant 740 includes the molten fuel salt that circulates within the reactor vessel 742 of the reactor core section 702 (e.g., main vessel core). In this regard, the molten fuel salt does not flow out of the reactor core section 702 as part of the primary coolant circuit but rather the molten fuel salt is flowed as the primary coolant through the reactor core section 702. It is noted that in this implementation, the MCFR system 700 may include one or more heat exchangers 746 in the primary coolant circuit for the reactor core section 702, such that the molten fuel salt flows as the primary coolant 740 through the one or more heat exchangers 746, through the reactor core section 702, does not flow out of the reactor core section 702, and back through the one or more heat exchangers 746, as part of the primary coolant circuit. As such, heat from the reactor core section 702 is transferred from the molten fuel salt via one or more heat exchangers 746 to a secondary coolant system (not shown). In FIG. 7, the molten fuel salt exchange assembly 701 is operably coupled to the reactor core section 702 (or another portion of the example MCFR system 700) and is configured to periodically replace a selected volume of the molten fuel salt 708 with a selected volume and composition of the feed material 710. In this regard, the molten fuel salt exchange assembly 701 can control the reactivity and/or composition of the molten fuel salt 708 within the example MCFR system 700. In one implementation, it is noted that the molten fuel salt 708 removed from the reactor core section 702 (shown as removed molten fuel 712 in a reservoir 718) includes at least some fissile material, while the feed material 710 includes at least some fertile material. In another implementation, the removed molten fuel 712 includes waste that can include one or more fission products. For example, the removed molten fuel 712 may include without limitation one or more lanthanides generated via fission within the molten fuel salt 708. In yet another implementation, the removed molten fuel 712 may include without limitation a mixture of fissionable material (e.g., UCl4), one or more fission products (e.g., one or more lanthanides and/or a carrier salt (e.g., NaCl). While molten fuel salt exchange is described as periodic, it should be understood that such exchange may be performed in a batch-wise, continuous, or semi-continuous (e.g., drip) manner and may be periodic, sporadic or vary in timing from one fuel exchange to the next. In the example implementation illustrated in FIG. 7, the molten fuel salt exchange assembly 701 (a “molten fuel salt exchange system”) includes a used-fuel transfer unit 716 and a feed-fuel supply unit 714. The molten fuel salt exchange assembly 701 may include the same or similar elements and operate the same or in a similar manner as the molten fuel salt exchange assembly 301 of FIG. 3, although alternative structures and operations may also be employed. As shown in FIG. 7, an exchange controller 728 may control one or more active fluid control elements in order to control the flow of feed material 710 from the feed material source 717 and the flow of used fuel salt 712 from the reactor core section 702 to the reservoir 718. As the molten fuel salt 708 within the reactor core section 702 breeds up, converting fertile material to fissile material, the molten fuel salt exchange assembly 701 removes some of the molten fuel salt 708 as the removed molten fuel 712 in a feed material source 717, and replaces the removed molten fuel 712 with the feed material 710, which includes at least some fertile material. In another implementation, the removed molten fuel 712 includes one or more fission products. Accordingly, the molten fuel salt exchange assembly 701, removing not only fissile fuel but also lanthanides and other neutron absorbers, may act as a control mechanism on the reactivity and lifetime extender of the molten fuel salt 708 within the example MCFR system 700. The control advantage of the fuel exchange may serve to return the reactivity of the molten fuel salt 708 (monitored by a reactivity sensor 730 as discussed above with reference to reactivity sensor 330 of FIG. 3) to a critical condition (e.g., a barely critical condition) and may also increase the effectiveness of the reactor by removing neutron absorbers and/or modifiers. Thus, in one implementation, the molten fuel salt exchange assembly 701 of the example MCFR system 700 can allow operation of the example MCFR system 700 indefinitely without adding further enrichment. It should be understood that molten fuel salt exchange may occur during operation of the nuclear reactor and/or during maintenance shut-down periods. The molten fuel salt of the feed material 710 may include without limitation one or more fertile fuel salts, such as a salt containing at least one of depleted uranium, natural uranium, thorium, or used nuclear fuel. For example, in the case of a chloride-based fuel, one or more fertile fuel salts may include a chloride salt containing at least one of depleted uranium, natural uranium, thorium, or a used nuclear fuel. Furthermore, the molten fuel salt of the feed material 710 may include without limitation one or more fertile fuel salts mixed with a carrier salt, such as NaCl, although other carrier salts may be employed. FIG. 8 illustrates an example ternary phase diagram 800 for UCl3—UCl4—NaCl (in mole %). In one implementation, an MCFR system, as modelled, uses a salt mixture composed of various sodium chloride and uranium chloride components. One example of such compositions may include one more components of NaCl, UCl3, and/or UCl4, as shown in the ternary phase diagram 800 of FIG. 8. The shaded region 802 shows the extent of a 500° C. melting point envelope. Multiple fuel salt compositions have been considered and have been shown to be capable of net breed and burn behavior. Selection of the final composition depends on a variety of factors including oxidation state/corrosion, solubility, viscosity and reactor size. Modelling has investigated different specific salts in the ternary diagram 800 with melting points suitable for use in the MCFR implementations, including without limitation 82UCl4-18UCl3, 17UCl3-71UCl4-12NaCl and 50UCl4-50NaCl. Results of the modelling indicate that such fuel salt implementations will sustain breed and burn behavior and could be used in reactor implementations described herein. As mentioned, the ternary phase diagram 800 shows the expected melting temperature for any mixture of UCl3-UCl4-NaCl. Of particular interest are mixtures having a melting point less than about 500° C., which mixtures are illustrated in the shaded region 802 of the ternary phase diagram 800. The eutectic point 804 has a melt temperature of 338° C. and a composition of 17UCl3-40.5UCl4-42.5NaCl (i.e., 17 mol % UCL3, 40.5 mol % UCL4 and 42.5 mol % NaCl). The shaded region 802 indicates a melting point envelope of 500° C. Moving to the far-right of this shaded region 802 provides an example implementation 806, 17UCl3-71UCl4-12NaCl, but it should be understood that many possible compositions exist within the melting point envelope of the shaded area 802 as various fuel salt mixtures having a melting point below 500° C. Furthermore, if the melting temperature limit is slightly extended to 508° C., a composition of 34UCl3-66NaCl provides an option that is free of UCl4. Likewise, the ternary diagram 800 allows a range of specific UCl3-UCl4-NaCl fuel salt implementations to be identified for any given melting point limit between about 800° C. and 338° C. For example, ternary salts with melting points between 300-550° C., 338-500° C., and 338-450° C. may be easily identified. Example methods of detecting composition changes may include without limitation: 1) measurements of redox (chemical reduction oxidation) 2) online glow discharge mass spectrometry of a sample 3) reactivity changes in the core 4) offline sample analysis including GDMS (glass discharge mass spectroscopy) 5) gamma spectroscopy The specific composition of the mixture may include any formulation including two or more of UCl4, UCl3 or NaCl, such that the resulting uranium content level and melting temperature achieve desired levels. By way of non-limiting example, the specific composition may be selected so that the corresponding melting temperature falls between 330 and 800° C. By way of another non-limiting example, the specific composition may be selected so that the overall uranium content level is at or above 61% by weight. In addition to selecting the overall uranium content level the fuel composition may also be determined to meet a selected amount of fissile uranium (as opposed to fertile). For example, the specific composition of the molten fuel salt may be selected such that the U-235 content of the molten fuel salt is below 20%. The following discussion will identify particular implementations of interest, however the following discussion does not limit the scope of the invention as claimed to only the implementations described below, but rather, that any implementations identifiable from FIG. 8 are contemplated, as well as any implementations having different metal chlorides other than NaCl. Examples of additional, non-fissile metal chlorides include NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3, AmCl3, LaCl3, CeCl3, PrCl3 and/or NdCl3. Liquid fuels have an inherent advantage over solid fuels in that the heat is “born” within the fuel coolant. A solid fuel may (1) conduct heat to the outer surface of the fuel element, (2) conduct heat through the cladding (including past a physical gap or through a bond material), (3) convect the heat from the cladding surface to the primary coolant, and (4) advect the heat out of the core. By comparison, a liquid fuel provides acceptable thermal transfer with step (4) and transport the fuel salt/primary coolant out of the core and to the primary heat exchanger. Additionally, the liquid salts under consideration have volumetric heat capacities that are nearly twice that of liquid sodium at similar temperatures. Another key advantage provided by a molten fuel salt is the strong negative temperature coefficient—hot salt is less reactive than cold salt. As a result, transients that result in overheating (e.g., loss of heat sink) are limited in severity by the expansion of the fuel salt. For example, in a molten chloride fast reactor (MCFR), as the selected chloride salt composition is heated from 600 to 800° C., its density drops by more than 12%, providing a negative reactivity feedback that is approximately 50× stronger than that provided by the Doppler effect. Fuel salts with similar ratios of the number of mono-chlorides, tri-chlorides, and tetra-chlorides behave similarly. The oxidation state within reactor core section of a molten chloride fast reactor (MCFR), for example, may be defined as the ratio of the molecules grouped by the number of attached chlorine molecules. The oxidation state of the reactor core section can be controlled by exchanging a selected amount of fuel salt in the reactor core section with a similar amount of makeup salt or feed material, where the composition of the feed material is designed to bring the oxidation state of the reactor core section toward a target oxidation state. In one implementation, the feed material contains a mixture of a selected fertile material and a carrier salt. In one implementation, the fuel salt in the reactor core section is initially at an oxidation state that is mostly composed of mono-chlorides, tri-chlorides, and tetra-chlorides. This initial fuel salt composition (prior to removal a selected volume of the fuel salt and addition of feed material) is represented by the initial fuel salt vector (f), where the subscript x represents the number of chloride ions present in each molecule of the fuel salt. Molecules with 2, 5 and 6 chloride atoms can exist within the reactor core section in very small quantities, so they can be ignored the bulk properties of the molten chloride fuel are dominated by the mono-chlorides, tri-chlorides, and tetra-chlorides (see Equation (1), which indicates a simplified fuel salt vector in which the molten chloride fuel is dominated by mono-chlorides (f1), tri-chlorides (f3), and tetra-chlorides (f4)). As such, if the target salt mixture is PuCl2—UCl3—UCl4, one would control on di-chlorides, tri-chlorides, and tetra-chlorides. Note: the fuel salt vector may be generalized to other chloride salts and fluoride salts. Accordingly, a similar control approach may be applied to fluoride salts, where the subscript x represents the number of fluoride ions in each molecule of the fuel salt. ( f 1 f 2 f 3 f 4 f 5 f 6 ) ~ ( f 1 f 3 f 4 ) = ( f ) ( 1 ) As such, the initial fuel salt vector (f) may be represented by the simplified fuel salt vector given in Equation (1). Removal of a selected volume (r) of the initial fuel salt over a period of time (either as a large batch, a set or sequence of smaller batches, or a continuous or partially continuous stream) normalized to the amount of initial fuel salt present in the reactor at the start of that period of time (e.g., about 1% per year for a specific MCFR system) yields an adjusted fuel salt vector (f′), which is shown by Equation (2), representing the fuel salt remaining in the reactor after removal of a selected volume of the initial fuel salt. ( f 1 f 3 f 4 ) ~ C * ( f 1 f 3 f 4 ) -> ( f 1 ′ f 3 ′ f 4 ′ ) = ( f ′ ) ( 2 ) A target fuel salt composition within the reactor, represented by a target fuel salt vector (t), may be set to achieve a particular oxidation state and/or stoichiometry from the adjusted fuel salt composition (adjusted fuel salt composition (f′) by adding a selected volume and composition of feed material, which is represented by a feed fuel salt vector (m). This relationship is represented by Equations (3) and (4), where (r) ˜C*(f).(f)−(r)=(f′) (3)(f′)+(m)=(t) (4) In an alternative notation, this relationship is represented by Equations (5) and (6). ( f 1 f 3 f 4 ) - ( r 1 r 3 r 4 ) = ( f 1 ′ f 3 ′ f 4 ′ ) ( 5 ) ( f 1 ′ f 3 ′ f 4 ′ ) + ( m 1 m 3 m 4 ) = ( t 1 t 3 t 4 ) ( 6 ) Given Equations (3)-(6), the volume and composition of the feed material to be added to the reactor to achieved the target oxidation state and/or stoichiometry may be determined (e.g., (m)). For each molecule type, the makeup fuel salt vector (mx) may be represented by Equation (7), where the subscript x represents the number of fluoride ions in each molecule of the fuel salt and C represents the normalized amount removed in a given period of time.(mx)=(tx)−(1−C)*(fx) (7) Nuclear fission reactors operate at zero or approximately zero excess reactivity to operate at a constant power. In addition to controlling the oxidation state of the molten fuel salt in the reactor, the reactivity of the described molten salt reactor implementations can be adjusted in situ by swapping fuel salt for a feed material. In a burner molten salt reactor, fissile material is burned so reactivity tends to decrease with time. As such, the feed material is designed to contain a significant quantity of high reactivity fuel salt rich in fissile material, such as enriched uranium or reprocessed transuranics. In a breeder molten salt reactor, fissile material is produced faster than it is consumed by the fission reaction, so the reactivity tends to increase with time. As such, the feed material is designed to contain low reactivity fuel salt that is rich in fertile material, such as natural uranium, depleted uranium, used nuclear fuel, or thorium. The rate at which feed material is introduced to the reactor core is selected to maintain the reactivity within certain design limits, such as nominal reactivity (e.g. keff equaling 1 or slightly greater than 1, an upper reactivity threshold, and/or a lower reactivity threshold). FIG. 9 illustrates example operations 900 for a molten fuel salt exchange process. A system provisioning operation 902 provides a molten chloride fast reactor (which is an example molten salt reactor) with a molten fuel salt exchange system. A monitoring operation 904 monitors for an exchange condition for the molten fuel salt. For example, one or more reactivity parameter sensors may monitor the reactivity within the molten chloride fast reactor, and/or chemical composition sensors, such as Raman spectroscopy may monitor the composition of the molten fuel salt within the molten chloride fast reactor. In an implementation, the monitoring may be performed in real-time using Raman spectroscopy. Raman spectroscopy provides information about molecular vibrations that can be used for sample identification and quantitation. The technique involves shining a monochromatic light source (i.e. laser) on a sample and detecting the scattered light. Some amount of fuel may be removed from the reactor core, such as in a side stream, and passed through a monitoring cell that includes a ‘window’ through with the spectroscopy can be performed. Examples of Raman windows materials are fused quartz, fused silica, sapphire, diamond, and some glasses. Any material may be used as long as it can meet the operational parameters of the reactor and monitoring system. An exchange condition may be set for monitored reactivity, composition, or some other operating parameter to trigger a molten fuel salt exchange event. If the exchange condition has not been satisfied, then a decision operation 906 returns processing to the monitoring operation 904. If the exchange condition has been satisfied, then the decision operation 906 progresses processing to a removal operation 908, which removes a selected volume of molten fuel salt from the molten chloride fast reactor. A replacement operation 910 replaces the removed volume of the molten fuel salt with a selected volume and/or composition of feed material into the molten chloride fast reactor. Processing returns to the monitoring operation 904. FIG. 10 illustrates a molten salt reactor 1000 equipped with a volumetric displacement element assembly 1002. Volumetric displacement systems represent a type of molten fuel salt control system. In one implementation, the volumetric displacement assembly 1002 is operably coupled to the reactor core section 1004 containing a molten fuel salt 1006. The volumetric displacement assembly 1002 is arranged so as to selectively displace a volume of the molten fuel salt 1006. In this regard, the volumetric displacement assembly 1002 may displace a volume of the fuel salt 108 in order to control reactivity within the molten fuel salt 1006. The volumetric displacement element assembly 1002 may control reactivity of the molten salt reactor 1000 by controlling the volume of molten fuel salt 1006, and thus the fissile material, displaced in the reactor core section 1004 (e.g., center region of the core section). By way of a non-limiting example, in settings where the reactor core section 1004 possesses excess reactivity, a sufficient volume (e.g., 0.1 to 10.0 m3) of molten fuel salt 1006 may be displaced by the volumetric displacement assembly 1002 such that the reactivity decreases to a lower reactivity threshold, such as critical or sub-critical levels. It should be appreciated that multiple volumetric displacement assemblies may be used in various configurations within the molten salt reactor 1000. In one implementation, the volumetric displacement assembly 1002 includes a volumetric displacement element 1010, an actuator 1012 and an actuator controller 206. In one implementation, the volumetric displacement element 1010 is formed from a non-neutron-absorbing material. In this regard, the volumetric displacement element 1010 controls reactivity in the molten salt reactor 1000 via the volumetric fluid displacement of the molten fuel salt 1006 (and fissile material) and not through a neutron absorption process. It is noted that the utilization of a non-neutron-absorbing material is particularly advantageous in the molten salt reactor 1000 as it avoids large impacts on reactivity, which may occur with the introduction of neutron-absorbing materials into the reactor core section 1004. A non-neutron-absorbing volumetric displacement element, which operates based on volumetric fluid displacement of the molten salt, may provide subtler reactivity control than neutron-absorbing control elements. It should be understood, however, that the volumetric displacement element 1010 (e.g., displacement rod) may be formed from any non-neutron absorbing material, although neutron absorbing and/or moderating materials may additionally or alternatively be employed in such elements. As such, the volumetric displacement element 1010 may alternatively include a neutron transparent material or a neutron reflector material. For example, the volumetric displacement element 1010 may be formed, but is not required to be formed, from zirconium, steel, iron, graphite, beryllium, molybdenum, lead, tungsten, boron, cadmium, one or more molybdenum alloys (e.g., TZM alloy), one or more tungsten alloys (e.g., tungsten carbide), one or more tantalum alloys, one or more niobium alloys, one or more rhenium alloys, one or more nickel alloys, silicon carbide and the like. In such implementations, the volumetric displacement element 1010 may limit reactivity through the volumetric fluid displacement of fuel and through the absorption of neutrons. In one implementation, the volumetric displacement element 1010 includes a rod 1016, as shown in FIG. 10. For example, the volumetric displacement element 1010 includes a solid rod or a hollow rod. It is noted herein that the displacement rod 1016 may take on any type of rod shape. For example, a displacement rod of the volumetric displacement assembly 1010 may take on a cylindrical shape, a square or rectangular prism shape, a triangular prism shape, a polygonal prism shape and the like. In another implementation, the volumetric displacement element 1010 may include a set of rods (not shown). For example, the set of rods may be arranged in an array or spoke pattern. In one implementation, the actuator 1012 is operably coupled to the volumetric displacement element 1010, such that the actuator 1012 may selectively translate the volumetric displacement element 1012. The actuator 1012 may include any actuation device. For example, the actuator 1012 may include, but is not limited to, a displacement rod drive mechanism. In one implementation, the actuator 1012 is configured to drive the volumetric displacement element 1010 bidirectionally. In this regard, the actuator 1012 may drive the volumetric displacement element 1010 into and/or out of the reactor core section 1004 as desired. In another implementation, the actuator 1012 is configured to stop driving the volumetric displacement element 1010 at one or more intermediate positions between a first stop position and a second stop position. In this regard, the actuator 1012 may translate the volumetric displacement element 1010 along a selected direction (e.g., axial direction) so as to insert a selected amount of the volumetric displacement element 1010 into the molten fuel salt 1006 of the reactor core section 1004. For example, in the case of a rod-shaped volumetric displacement element 1010, the actuator 1012 may insert a selected volume of the volumetric displacement element 1010 by controlling the length L of the rod-shaped volumetric displacement element 1010 inserted into the molten fuel salt 1006. It is noted that the volumetric displacement assembly 1002 may displace any amount of volume of the molten fuel salt 1006 within the reactor core section 1004 necessary to reduce the reactivity of the molten fuel salt 1006 within the reactor core section 1004 as desired. By way of non-limiting example, the volume of molten fuel salt 1006 within the reactor core section 1004 may range from 10 to 100 m3, depending on the particular fuel formulation and operation context of the molten salt reactor 1000. In this setting, a displacement volume of only a fraction of a cubic meter may supply sufficient volumetric salt displacement to significantly reduce reactivity within the reactor core section 1004 and, in some cases, shutdown the reactor. For example, in marginal control or non-shutdown operations, the displacement volume imparted by the volumetric displacement element 1010 may include, but is not limited to, a displacement volume between 0.1 to 10 m3. In one implementation, as shown in FIG. 10, the volumetric displacement assembly 1010 may insert the volumetric displacement element 1010 into a central region of the reactor core section 1004. In this regard, the actuator 1012 may translate the volumetric displacement element 1010 along the axial direction of the reactor core section 1004, as shown in FIG. 10. It is noted that given a rotationally symmetric core section, as that depicted in FIG. 10, the greatest reactivity worth associated with the volumetric displacement element 1010 may be realized by positioning the volumetric displacement element 1010 at the cross-sectional center of the reactor core section 1004. It is noted that a centered volumetric displacement element 1010 is not a limitation on the molten salt reactor 1000 of the present disclosure and is provided merely for illustrative purposes. In another implementation, the actuator controller 1010 is configured to selectively direct the actuator 1012 to insert a selected volume of the volumetric displacement element 1010 a selected distance into a volume of the molten fuel salt 1006 contained within the reactor core section 1004. For example, the actuator controller 1014 may direct the actuator 1012 to translate the volumetric displacement element 1010 such that the volumetric displacement element 1010 partially or entirely submerses in the molten fuel salt 1006. The actuator controller 1014 is communicatively coupled to the actuator 1012. For example, the actuator controller 1014 may be communicatively coupled to the actuator 1012 via a wireline connection (e.g., electrical cable or optical fiber) or wireless connection (e.g., RF transmission or optical transmission). In one implementation, the actuator controller 1012 includes an operator interface configured to receive volumetric displacement actuation instructions from an operator. In this regard, an operator may selectively direct the control the actuation state of the volumetric displacement element 1010. In another implementation, the actuation controller 1014 may automatically direct the actuation of the volumetric displacement element 1010 in response to one or more sensed or monitored parameters of the molten salt reactor 1000, as discussed below. In another implementation, the molten salt reactor 1000 includes a reactivity parameter sensor 1030. The reactivity parameter sensor 1030 includes any one or more sensors capable of measuring or monitoring one or more parameters indicative of reactivity or a change in reactivity of the molten fuel salt 1006 of the molten salt reactor 1000. For example, the reactivity parameter sensor 1030 may include, but is not limited to, any one or more sensors capable of sensing and/or monitoring one or more of neutron fluence, neutron flux, neutron fissions, fission products, radioactive decay events, temperature, pressure, power, isotropic concentration, burn-up and/or neutron spectrum. In one implementation, the reactivity parameter sensor 1030 includes a fission detector. For example, the reactivity parameter sensor 1030 may include, but is not limited to, a micro-pocket fission detector. In another implementation, the reactivity parameter sensor 1030 includes a neutron flux monitor. For example, the reactivity parameter sensor 1030 may include, but is not limited to, a fission chamber or an ion chamber. In another implementation, the reactivity parameter sensor 1030 includes a neutron fluence sensor. For example, the reactivity parameter sensor 1030 may include, but is not limited to, an integrating diamond sensor. In another implementation, the reactivity parameter sensor 1030 includes a fission product sensor. For example, the reactivity parameter sensor 1030 may include, but is not limited to, a gas detector, a β detector or a γ detector. In another implementation, the reactivity parameter sensor 1030 includes a fission product detector configured to measure a ratio of isotope types in a fission product gas. In another implementation, the reactivity parameter sensor 1030 includes a temperature sensor. In another implementation, the reactivity parameter sensor 1030 includes a pressure sensor. In another example, the reactivity parameter sensor 1030 includes a power sensor. For example, the reactivity parameter sensor 1030 may include, but is not limited to, a power range nuclear instrument. In another implementation, the reactivity is determined with one or more of the measured reactivity parameters (discussed above). In one implementation, the reactivity of the reactor core section 1004 is determined by the actuator controller 1012 using a look-up table. For example, measured values for temperature, pressure, power level and the like may be used in conjunction with one or more look up tables to determine the reactivity of the reactor core section 1004. In another implementation, the reactivity of the reactor core section 1004 is determined by the actuator controller 1014 using one or more models. For example, the one or more models may include, but are not limited to, a neutronics modeling software package executed by the one or more processors of the actuator controller 1014. For instance, a suitable neutronics software package may include, but is not limited to, MCNP, CINDER, REBUS and the like. In another implementation, the reactivity parameter may be determined by an operator and entered directly into the actuator controller 1014 via an operator interface. It is noted herein that, while the reactivity parameter sensor 1030 is depicted as being located within the molten fuel salt 1006 in the reactor core section 1004 of the molten salt reactor 1000, this configuration is not a limitation on the present implementation and is provided merely for illustrative purposes. Rather, it is noted that one or more reactivity parameter sensors 1030 may be located at various positions of the molten salt reactor 1000 including, but not limited to, at a position within the reactor core section, at a position external to the reactor core section 1004 (e.g., at external surface of reactor core section 1004), in or along one or more pipes of a primary coolant system, in or near a primary heat exchanger, in or along one or more pipes of a secondary coolant system and the like. In another implementation, the one or more reactivity parameter sensors 1030 are communicatively coupled to actuator controller 1014. The one or more reactivity parameter sensors 1030 are communicatively coupled to the actuator controller 1014. For example, the one or more reactivity parameter sensors 1030 may be communicatively coupled to the actuator controller 1014 via a wireline connection (e.g., electrical cable or optical fiber) or wireless connection (e.g., RF transmission or optical transmission). In one implementation, the actuation controller 1014 may direct the actuator 1012 to adjust the position of the volumetric displacement element 1010 (and, thus, the reactivity of the molten fuel salt 1006) based on the measured reactivity parameter. In one implementation, the actuation controller 1014 includes one or more processing units and memory. In one implementation, the memory maintains one or more sets of program instructions configured to carry out one or more operational steps of the volumetric displacement assembly 1010. In one implementation, the one or more program instructions of the actuation controller 1014 may cause the actuator controller 1014 to direct the actuator 1012 to drive the volumetric displacement assembly 1010 into the reactor core section 1004 to displace a selected volume of the molten fuel salt 1006 within the reactor core section 1004. In another implementation, the one or more program instructions are configured to correlate a determined reactivity of the reactor core section 1004 with a displacement volume necessary to compensate for the measured reactivity of the reactor core section 1004. For example, as discussed above, the reactivity parameter sensor 1030 may acquire a reactivity parameter associated with the molten fuel salt 1006 within the reactivity core section 1004. In settings where the reactivity parameter is indicative of a reactivity larger than a selected tolerance level, the actuator controller 1014 may determine the displacement volume to compensate for the elevated reactivity and direct the actuator 1012 to insert enough of the volumetric displacement element 1010 to achieve at least this level of volumetric salt displacement. In another implementation, in settings where complete reactor shutdown is required, the actuator controller 1014 may direct the actuator 1012 to insert the entire volumetric displacement element 1010 into the reactor core section 1004 in order to achieve maximum volumetric salt displacement. FIG. 11 illustrates a molten salt reactor 1100 equipped with a volumetric displacement element assembly 1102 and a molten fuel salt spill-over system 1130 with a volumetric displacement element 1110 not submerged in molten fuel salt. In one implementation, the molten fuel salt spill-over system 1130 includes one or more fuel salt uptakes 1132 and one or more spill-over reservoirs 1134. It is noted that in some cases the volumetric displacement of the molten fuel salt 1106 by the volumetric displacement element 1110 may cause a rise in the fuel salt level above a desired level. In one implementation, the molten fuel salt spill-over system 1130 is configured to transport molten fuel salt 1106 that is displaced above the maximum tolerated fill level of the reactor core section 1104, as shown in FIG. 12. By way of non-limiting example, the fuel salt uptake 1132 may be placed approximately 10 cm above a nominal fuel salt level. In this regard, when the volumetric displacement element 1110 is engaged, it may, in some cases, cause the molten fuel salt level to rise above normal salt level. Molten salt that reaches the fuel salt uptake 1132 is then transported to the spill-over reservoir 1134. It should be appreciated that multiple volumetric displacement assemblies may be used in various configurations within the molten salt reactor 1100. FIG. 12 illustrates a molten salt reactor 1200 equipped with a volumetric displacement element assembly 1202 and a molten fuel salt spill-over system 1230 with a volumetric displacement element 1210 submerged in molten fuel salt. While the molten fuel salt spill-over system 1230 depicted of FIG. 12 is depicted in the context of the volumetric displacement element assembly 1202 and volumetric displacement element 1210, this is not a requirement on the molten fuel salt spill-over system 1230. In this regard, the molten fuel salt spill-over system 1230 of the present disclosure may be implemented in a context that does not include the volumetric displacement assembly 1202 and volumetric displacement element 1202. In one implementation, the molten fuel salt spill-over system 1230 may be implemented in order to account for thermal expansion of the molten fuel salt 1206. By way of non-limiting example, in the case where the fuel salt uptake 1232 is place at 10 cm above the normal salt level a mere 50° C. increase in temperature of the fuel salt 108 may cause the molten fuel salt 1206 to reach the fuel salt uptake 1232. By way of another non-limiting example, approximate increase of 200° C. in temperature of the molten fuel salt 1206 may cause the molten fuel salt 1206 to spill over through the fuel salt uptake 1232 and lead to 1-5 m3 of fuel salt to spill into one or more spill-over reservoirs 1234. Spilled-over fuel salt 1236 is shown in the one or more spill-over reservoirs 1234. It is recognized herein that the combination of very low excess reactivity and the strong thermal feedback of the molten fuel salt 1206 may allow for nearly passive operation. In this sense, use of the displacement element 1210 may be limited. As the demand on the turbine (not shown) of the nuclear reactor plant varies, the temperature(s) associated with the primary cooling loop will vary slightly. This, in turn, will vary the temperature of the molten fuel salt 1206. As a result, the molten fuel salt 1206 will obtain a new average temperature, and thus, density, causing the fluid level of the molten fuel salt 1206 to increase or decrease. By way of non-limiting example, in the event that demand for electricity increase, the steam of the turbine comes out at a reduced temperature. As a result, temperatures throughout the nuclear reactor system are reduced, causing the molten fuel salt 1206 to decrease in temperature and increase in density. This increase in density results in an increase in reactivity. In addition, the fluid level of the molten fuel salt 1206 is decreases, while increased reactivity causes the power of the molten salt reactor 1200 to increase, thereby meeting the increased demand on the turbine. In turn, increase in power causes the temperature of the molten fuel salt 1206 to increase and the fluid level of the molten fuel salt 1206 to return to (or near) its original level. It is further recognized that, in the event of a loss of heat sink or a turbine trip, temperatures throughout the molten salt reactor 1200 would increase. As a result of increased temperatures in the molten fuel salt 1206, the molten fuel salt 1206 would decrease in density, causing the molten fuel salt 1206 to become less reactive. The decrease in density would cause the fluid level to rise and, in some instances (e.g., +50° C. temperature rise) the fluid level of the molten fuel salt 1206 reach the level of the fuel salt uptake 208. Such a rise in fluid level may then cause some molten fuel salt 1206 to spill over into the one or more spill-over reservoirs 1234, which would serve to further reduce reactivity in the reactor core section 1204. As a result, the molten salt reactor 1200 may go into a sub-critical state and remain in that state, even upon cooling. In another implementation, the molten fuel salt spill-over system 1230 may include a return pathway (e.g., one or more pipes, one or more pumps and one or more valves), where fuel salt stored in the one or more spill-over reservoirs 1234 may be actively pumped out of the one or more spill-over reservoirs 1234 and back into the reactor core section 1204 in order to reestablish a critical state. In another implementation, the displacement element 1210 may be used to accelerate the above process as well as control or shape changes in reactivity/density/temperature during normal operation. It should also be understood that various structural modifications to the displacement element 1210 may be employed to enhance control performance and manage influence that molten fuel salt turbulence may have on the placement and stability of the displacement element 1210 within the reactor core section 1204. Such structural modifications may include without limitation different shapes, sizes, and numbers of displacement elements 1210, dynamic shape change features in displacement element 1210, baffles and/or nozzles in the displacement element 1210, and other flow-friendly features to the displacement element 1210. It should be appreciated that multiple volumetric displacement assemblies may be used in various configurations within the reactor core section 1204. FIG. 13 illustrates various example stages of a fuel displacement cycle 1300. In stage 1302, the displacement element 1301 includes a hollow or solid displacement rod 1303 inserted through rod inlet 1305 and a displacement body 1307 having a width w that is wider than both the displacement rod 1303 and the rod inlet 1305 and a height h that is less than the height y of the reactor core section 1311. As a result, the maximum volume of displacement can be vertically selected/located within the reactor core section 1311 by raising or lowering the displacement body 1307 to a desired height in the molten fuel salt 1309 within the reactor core section 1311. The dashed line 1320 indicates the molten fuel salt level when the displacement element has not yet been lowered into the molten fuel salt 1309. It should be understood that the displacement rod 1303 and/or the displacement body 1307 may be formed of or filled with various materials, including non-neutron absorbing materials and neutron absorbing materials. In stage 1302, the displacement element has been partially lowered into the molten fuel salt, resulting in a raising of the molten fuel salt level. The subsequent stages 1304, 1306, 1308, 1310, and 1312 show progressively lower insertions of the displacement body 1307 into the molten fuel salt 1309, resulting in increasingly higher levels of the molten fuel salt 1309, although such increasing levels of molten fuel salt 1309 may be mitigated by a spill-over system. Stage 1312 illustrates a fully immersed displacement body 1307. By displacing the volume of molten fuel salt 1309 at a particular location within the reactor core section, the reactivity within the reactor core section 1311 can be controlled. Even after the displacement body 1307 is fully immersed within the molten fuel salt 1309, the vertical location within the reactor core section 1311 can further influence the reactivity (e.g., the lower the displacement body 1307, the more negative influence on reactivity) in the illustrated implementations. See FIG. 14 and the associated discussion. It should be appreciated that multiple volumetric displacement assemblies may be used in various configurations within the reactor core section 1311. FIG. 14 illustrates two example stages 1402 and 1404 of a fuel displacement cycle 1400. In stage 1402, the displacement element 1401 includes a hollow or solid displacement rod 1403 and a displacement body 1407 inserted deep into molten fuel salt 1409 within a reactor core section 1411. In stage 1404, the displacement body 1407 inserted less deeply into the molten fuel salt 1409 within the reactor core section 1411. As a result, the maximum volume of displacement can be vertically selected/located within the reactor core section 1411 by raising or lowering the displacement body 1407 to a desired height in the molten fuel salt 1409 within the reactor core section 1411. It should be understood that the displacement rod 1403 and/or the displacement body 1407 may be formed of or filled with various materials, including non-neutron absorbing materials and neutron absorbing materials. Accordingly, in one implementation, the reactivity control may be characterized as more negative in the stage 1402 than in the stage 1404 because the displacement body 1407 is inserted more deeply into the reactor core section 1411, displaying more fuel at an input region of the reactor core section 1411, where the molten fuel salt 1409 first enters the active fission reaction region at each circulation cycle. It should be appreciated that multiple volumetric displacement assemblies may be used in various configurations within the reactor core section 1411. FIG. 15 illustrates example operations 1500 for a molten fuel salt displacement process. A system provisioning operation 1502 provides a molten chloride fast reactor (which is an example molten salt reactor) with a molten fuel salt exchange system. A monitoring operation 1504 monitors for a control condition for the molten fuel salt (e.g., k-effective meets or exceeds a threshold, such as 1.005). For example, one or more reactivity parameter sensors may monitor the reactivity within the molten chloride fast reactor. The control condition may be set for monitored reactivity or some other operating parameter to trigger a molten fuel salt displacement event. If the control condition has not been satisfied, then a decision operation 1506 returns processing to the monitoring operation 1504. If the control condition has been satisfied, then the decision operation 1506 progresses processing to an insertion operation 1508, which inserts a displacement body into molten fuel salt within a reactor core section. A positioning operation 1510 positions the displacement body into the molten fuel salt of the molten chloride fast reactor to remove a selected volume of molten fuel salt from the reactor core section to obtain desired reactivity parameters in the molten chloride fast reactor. Processing returns to the monitoring operation 1504. In one implementation, an example molten salt reactor includes a nuclear reactor core configured to contain a nuclear fission reaction fueled by a molten fuel salt. A molten fuel salt control system is coupled to the nuclear reactor core and is configured to remove a selected volume of the molten fuel salt from the nuclear reactor core to maintain a parameter indicative of reactivity of the molten salt reactor within a selected range of nominal reactivity. Another example molten salt reactor of any preceding reactor provides a molten fuel salt control system that includes a molten fuel salt exchange system fluidically coupled to the nuclear reactor core and configured to exchange a selected volume of the molten fuel salt with a selected volume of a feed material containing a mixture of a selected fertile material and a carrier salt. Another example molten salt reactor of any preceding reactor provides a molten fuel salt exchange system that includes a feed-fuel supply unit configured to transfer the feed material into the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides a molten fuel salt exchange system that a feed-fuel supply unit configured to transfer a selected volume of the feed material into the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides a molten fuel salt exchange system that the molten fuel salt exchange system that includes a feed-fuel supply unit configured to transfer a selected composition of the feed material into the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides a molten fuel salt exchange system that includes a used-fuel transfer unit configured to transfer the selected volume of the molten fuel salt as used-fuel from the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides a molten fuel salt exchange system that is configured to transfer concurrently the selected volume of the molten fuel salt from the nuclear reactor core and the feed material into the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides a molten fuel salt exchange system that controls reactivity of the nuclear fission reaction by exchanging the feed material with the selected volume of the molten fuel salt in the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides a molten fuel salt exchange system that controls composition of the molten fuel salt in the nuclear fission reaction by exchanging the feed material with the selected volume of the molten fuel salt in the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides a fast spectrum fission reactor and the molten fuel salt includes a chloride salt. Another example molten salt reactor of any preceding reactor provides a molten fuel salt exchange system controls a composition of UCl3-UCl4-NaCl in the spectrum fission reaction by exchanging the feed material with the selected volume of the molten fuel salt in the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides a molten fuel salt exchange system is configured to exchange repeatedly a selected volume of the molten fuel salt with a selected volume of the feed material to maintain the parameter indicative of reactivity of the molten salt reactor within a selected range of nominal reactivity over time. Another example molten salt reactor of any preceding reactor further includes a reactivity parameter sensor positioned proximate the nuclear reactor core. The nuclear parameter sensor is configured to monitor one or more parameters indicative of reactivity of the nuclear reactor core. A controller communicatively couples to the reactivity parameter sensor to receive the one or more parameters indicative of reactivity of the nuclear reactor core. The controller is configured to control exchange of the selected volume of the molten fuel salt with the selected volume of a feed material containing a mixture of a selected fertile material and a carrier salt based on the one or more parameters. Another example molten salt reactor of any preceding reactor provides the molten fuel salt control system to further include a volumetric displacement control system having one or more volumetric displacement assemblies insertable into the nuclear reactor core. Each volumetric displacement assembly is configured to volumetrically displace a selected volume molten fuel salt from the nuclear reactor core when inserted into the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides the molten fuel salt control system to further include a volumetric displacement control system having one or more volumetric displacement bodies insertable into the nuclear reactor core, each volumetric displacement body being configured to volumetrically displace a selected volume of molten fuel salt from the nuclear reactor core when inserted into the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides the molten fuel salt control system to further include a volumetric displacement control system having one or more volumetric displacement bodies insertable into the nuclear reactor core, each volumetric displacement body being configured to volumetrically displace a selected volume of molten fuel salt from the nuclear reactor core when inserted into the nuclear reactor core, the volumetric displacement control system further having molten fuel salt spill-over system configured to transport molten fuel salt that is displaced by the volumetric displacement body above a tolerated fill level of the nuclear reactor core. Another example molten salt reactor of any preceding reactor provides the molten fuel salt control system to further include a volumetric displacement control system having one or more volumetric displacement bodies insertable into the nuclear reactor core, each volumetric displacement body being configured to volumetrically displace a selected volume of molten fuel salt from the nuclear reactor core when inserted into the nuclear reactor core, the volumetric displacement control system being insertable at multiple insertion depths into the nuclear reactor core to maintain the parameter indicative of reactivity of the molten salt reactor within a selected range of nominal reactivity over time. Another molten salt nuclear reactor includes a nuclear reactor core configured to sustain a nuclear fission reaction fueled by a molten fuel salt and means for exchanging a selected volume of the molten fuel salt with a selected volume of a feed material containing a mixture of a selected fertile material and a carrier salt. Another molten salt nuclear reactor includes a nuclear reactor core configured to sustain a nuclear fission reaction fueled by a molten fuel salt and means for removing a selected volume of the molten fuel salt from the nuclear reactor core to maintain a parameter indicative of reactivity of the molten salt reactor within a selected range of nominal reactivity. An example method includes sustaining a nuclear fission reaction fueled by a molten fuel salt within a nuclear reactor core and removing a selected volume of the molten fuel salt from the nuclear reactor core to maintain a parameter indicative of reactivity of the molten salt reactor within a selected range of nominal reactivity. Another example method of any preceding method further includes replacing the selected volume of the molten fuel salt with a selected volume of a feed material containing a mixture of a selected fertile material and a carrier salt. Another example method of any preceding method wherein the replacing operation includes transferring the feed material into the nuclear reactor core. Another example method of any preceding method wherein the replacing operation includes transferring a selected volume of the feed material into the nuclear reactor core. Another example method of any preceding method wherein the replacing operation includes transferring a selected composition of the feed material into the nuclear reactor core. Another example method of any preceding method wherein the replacing operation includes controlling the reactivity of the nuclear reactor core based on the selected volume of the feed material. Another example method of any preceding method wherein the replacing operation includes controlling the composition of the molten fuel salt fueling the nuclear fission reaction within the nuclear reactor core based on the selected composition of the feed material. Another example method of any preceding method wherein the replacing operation includes controlling the composition of the UCl3-UCl4-NaCl fueling the nuclear fission reaction within the nuclear reactor core based on the selected composition of the feed material. Another example method of any preceding method wherein the method further includes monitoring satisfaction of an exchange condition by the molten fuel salt and controlling exchange of the selected volume of the molten fuel salt with the selected volume of a feed material containing a mixture of a selected fertile material and a carrier salt responsive to satisfaction of the exchange condition. Another example method of any preceding method wherein the method further includes monitoring one or more reactivity parameters indicative of reactivity of the nuclear reactor core and controlling exchange of the selected volume of the molten fuel salt with the selected volume of a feed material containing a mixture of a selected fertile material and a carrier salt based on the one or more reactivity parameters. Another example method of any preceding method wherein the method further includes monitoring one or more composition parameters indicative of composition of the molten fuel salt of the nuclear reactor core and controlling exchange of the selected volume of the molten fuel salt with the selected volume of a feed material containing a mixture of a selected fertile material and a carrier salt based on the one or more composition parameters. Another example method of any preceding method wherein the removing operation includes volumetrically displacing the selected volume molten fuel salt from the nuclear reactor core by inserting one or more volumetric displacement bodies into molten fuel salt within the nuclear reactor core. Another example method of any preceding method wherein the removing operation includes transporting the volumetrically displaced volume of molten fuel salt from the nuclear reactor core via a molten fuel salt spill-over system when the volumetrically displaced volume of molten fuel salt is displaced by the volumetric displacement body above a tolerated fill level of the nuclear reactor core. Another example method of any preceding method provides a method wherein each volumetric displacement body is configured to volumetrically displace a selected volume of molten fuel salt from the nuclear reactor core when inserted into the nuclear reactor core, the volumetric displacement control system being insertable at multiple insertion depths into the nuclear reactor core to maintain the parameter indicative of reactivity of the molten salt reactor within a selected range of nominal reactivity over time. An example fast spectrum molten salt nuclear reactor includes a reactor core section including a fuel input and a fuel output, the fuel input and the fuel output arranged to flow a molten chloride salt nuclear fuel through the reactor core section. The molten chloride salt nuclear fuel including a mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt, the mixture of UCl4 and at least one additional metal chloride salt having a UCl4 content greater than 5% by molar fraction. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the uranium concentration in the mixture of UCl4 and at least one additional metal chloride salt is greater than 61% by weight. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the additional uranium chloride salt including UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt has a composition of 82UCl4-18UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt has a composition of 17UCl3-71UCl4-12NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt has a composition of 50 UCl4-50NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the additional metal chloride including at least one of NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3, AmCl3, LaCl3, CeCl3, PrCl3 or NdCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt has an additional metal chloride salt concentration at or below the precipitation concentration for the an additional metal chloride salt. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt having a melting temperature below a temperature of 800 degrees Celsius. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt having the selected melting temperature above a temperature of 330 degrees Celsius. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides breed-and-burn behavior established within the molten chloride salt nuclear fuel with a uranium-plutonium cycle. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the fuel input located on a first side of the reactor core section and the fuel output located on a second side of the reactor core section opposite to the fuel input. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides a protective layer disposed on at least one surface facing the molten chloride salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides that the at least one surface exposed to the molten chloride salt nuclear includes an internal surface of the reactor core section. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the protective layer that is substantially resistant to at least one of corrosion or radiation. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the protective layer including at least one of a refractory alloy, a nickel alloy, a refractory metal or silicon carbide. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a reflector assembly configured to reflect at least a portion of neutrons emanating from the reactor core section back to the molten chloride salt nuclear fuel within the reactor core section, the reflector assembly including a plurality of reflector modules, at least some of the reflector modules containing a liquid reflector material. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides at least one of the reflector modules formed from at least one of a molybdenum alloy, a nickel alloy or a carbide. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the liquid reflector material including at least one of liquid lead or liquid lead-bismuth. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a displacement assembly operably coupled to the reactor core section and configured to selectively displace a volume of the molten salt nuclear fuel in order to control reactivity within the molten salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the displacement assembly configured to displace a volume of the molten salt nuclear fuel in order to reduce reactivity within the molten salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the displacement assembly that includes a displacement element, an actuator operably coupled to the displacement element, and a controller. The controller is configured to selectively direct the actuator to control a position of the displacement element in order to control the reactivity within the molten salt nuclear fuel contained within the reactor core section. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the displacement element that is formed from a substantially non-neutron-absorbing material. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a molten salt transfer assembly. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt transfer assembly to include a molten salt transfer unit fluidically coupled to the reactor core section and configured to transfer a selected portion of the molten chloride salt fuel from a portion of the fast spectrum molten salt nuclear reactor to a reservoir. The molten salt transfer unit is further configured to transfer a feed material including at least some fertile material from a feed material supply to a portion of the fast spectrum molten salt nuclear reactor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least some fertile material of the feed material that includes at least one fertile fuel salt. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least one fertile fuel salt in include a salt containing at least one of depleted uranium, natural uranium or thorium. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least one fertile fuel salt to include a salt containing at least one metal from a used nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a fission product removal unit configured to remove at least one fission product from the molten chloride salt fuel. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a primary coolant loop fluidically coupled to the input of the nuclear core section and the output of the nuclear core section. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a primary heat exchanger and a secondary coolant loop, the primary coolant loop and the secondary coolant loop thermally coupled via the primary heat exchanger. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes at least one pump disposed along the primary coolant loop to circulate the molten chloride salt nuclear fuel through the primary coolant loop. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least pump that circulates the molten chloride salt nuclear fuel through the primary coolant loop at or below a selected flow velocity limit. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a gas sparging unit configured to remove one or more noble gases from the molten chloride salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a filter unit configured to remove at least one of a noble metal or a semi-noble metal from the molten salt nuclear fuel. A example method of fueling a fast spectrum molten salt nuclear reactor includes providing a volume of UCl4, providing a volume of at least one of an additional uranium chloride salt or an additional metal chloride salt, mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction, and supplying the molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction to at least a reactor core section of the fast spectrum molten salt nuclear reactor. Another example method of any preceding method includes providing a volume of at least one of an additional uranium chloride salt or an additional metal chloride salt by providing a volume of UCl3. Another example method of any preceding method includes providing a volume of at least one of an additional uranium chloride salt or an additional metal chloride salt by providing a volume of at least one of NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3, AmCl3, LaCl3, CeCl3, PrCl3 or NdCl3. Another example method of any preceding method includes providing the mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction by mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction and a melting temperature between 330 and 800° C. Another example method of any preceding method includes providing the mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction by mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a composition of 82UCl4-18UCl3. Another example method of any preceding method includes providing the mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction by mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a composition of 17UCl3-71UCl4-12NaCl. Another example method of any preceding method includes providing the mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction by mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a composition of 50 UCl4-50NaCl. Another example method of any preceding method includes providing the mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt by mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt inside of the fast spectrum molten salt nuclear reactor. Another example method of any preceding method includes providing the mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt by mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt outside of the fast spectrum molten salt nuclear reactor. An example molten chloride salt fuel for use in a fast spectrum molten salt nuclear reactor prepared by a process including providing a volume of UCl4, providing a volume of at least one of an additional uranium chloride salt or an additional metal chloride salt, and mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction. An example fast spectrum molten salt nuclear reactor includes a reactor core section including a fuel input and a fuel output. The fuel input and the fuel output are arranged to flow a mixture of molten salt nuclear fuel and at least one lanthanide through the reactor core section at start-up of the fast spectrum molten salt nuclear reactor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least one lanthanide that includes at least one of La, Ce, Pr or Nd. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of molten salt nuclear fuel and at least one lanthanide that includes a mixture of molten salt nuclear fuel and at least one lanthanide formed by mixing the molten salt nuclear fuel with at least one lanthanide chloride. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least one lanthanide chloride that includes at least one of LaCl3, CeCl3, PrCl3 or NdCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of molten salt nuclear fuel and at least one lanthanide that includes a mixture of molten salt nuclear fuel and at least one lanthanide having a lanthanide concentration between 0.1 and 10% by weight. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of molten salt nuclear fuel and at least one lanthanide having a lanthanide concentration between 0.1 and 10% by weight that includes a mixture of molten salt nuclear fuel and at least one lanthanide having a lanthanide concentration between 4 and 8% by weight. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of molten salt nuclear fuel and the at least one lanthanide that is formed outside of the fast spectrum molten salt nuclear reactor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of molten salt nuclear fuel and the at least one lanthanide that is formed inside of the fast spectrum molten salt nuclear reactor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the fuel input and the fuel output that are arranged to flow a mixture of molten salt nuclear fuel and at least one lanthanide through the reactor core section prior to achieving a selected reactivity threshold in the fast spectrum molten salt nuclear reactor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the fuel input and the fuel output that are arranged to flow a mixture of molten salt nuclear fuel and at least one lanthanide through the reactor core section prior to achieving criticality in the fast spectrum molten salt nuclear reactor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the fuel input and the fuel output that are arranged to flow a mixture of molten salt nuclear fuel and at least one lanthanide through the reactor core section prior to generation of a selected amount of plutonium in the fast spectrum molten salt nuclear reactor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that includes a mixture of at least two of a first uranium chloride, a second uranium chloride or an additional metal chloride. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the additional metal chloride that includes at least one of NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3 or AmCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides at least one of the first uranium chloride or the second uranium chloride that includes at least one of UCl4 or UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that has a composition of 82UCl4-18UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that has a composition of 17UCl3-71UCl4-12NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that has a composition of 50 UCl4-50NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that has a composition of 34 UCl3-66NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride that includes at least 5% by molar fraction UCl4. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride that has a uranium concentration of greater than 61% by weight. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride that has a melting point between 330 and 800 degrees Celsius. An example method of fueling a fast spectrum molten salt nuclear reactor includes providing a molten salt nuclear fuel and providing at least one lanthanide. Prior to start-up of the fast spectrum molten salt nuclear reactor, the molten salt nuclear fuel is mixed with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel. The lanthanide-loaded molten salt nuclear fuel is supplied to at least a reactor core section of the fast spectrum molten salt nuclear reactor. Another example method of any preceding method provides a molten salt nuclear fuel by providing a mixture of at least two of a first uranium chloride, an additional uranium chloride and an additional metal chloride. Another example method of any preceding method provides a molten salt nuclear fuel by providing a mixture of at least two of UCl4, UCl3 and an additional metal chloride. Another example method of any preceding method provides the additional metal chloride to include at least one of NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3 or AmCl3. Another example method of any preceding method provides a molten salt nuclear fuel by providing a molten salt nuclear fuel having at least 5% by molar fraction UCl4. Another example method of any preceding method provides a molten salt nuclear fuel by providing a molten salt nuclear fuel having a uranium concentration of greater than 61% by weight. Another example method of any preceding method provides a molten salt nuclear fuel by providing a molten salt nuclear fuel having a melting point between 330 and 800 degrees Celsius. Another example method of any preceding method provides at least one lanthanide by providing at least one of La, Ce, Pr or Nd. Another example method of any preceding method provides at least one lanthanide by providing at least one lanthanide in the form of a lanthanide chloride. Another example method of any preceding method provides at least one lanthanide in the form of a lanthanide chloride by providing at least one of LaCl3, CeCl3, PrCl3 or NdCl3. Another example method of any preceding method provides mixing of the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel by mixing the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel having a lanthanide concentration between 0.1 and 10% by weight. Another example method of any preceding method provides mixing of the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel having a lanthanide concentration between 0.1 and 10% by weight by mixing the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel having a lanthanide concentration between 4 and 8% by weight. Another example method of any preceding method provides mixing of the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel by mixing the molten salt nuclear fuel with the at least one lanthanide outside of the fast spectrum molten salt nuclear reactor. Another example method of any preceding method provides mixing of the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel by mixing the molten salt nuclear fuel with the at least one lanthanide inside of the fast spectrum molten salt nuclear reactor. Another example method of any preceding method provides, prior to start-up of the fast spectrum molten salt nuclear reactor, the mixing of the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel by, prior to achieving a selected reactivity threshold in the fast spectrum molten salt nuclear reactor, mixing the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel. Another example method of any preceding method provides, prior to start-up of the fast spectrum molten salt nuclear reactor, mixing of the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel by, prior to achieving criticality in the fast spectrum molten salt nuclear reactor, mixing the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel. Another example method of any preceding method provides, prior to start-up of the fast spectrum molten salt nuclear reactor, mixing of the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel by, prior to generation of a selected amount of plutonium in the fast spectrum molten salt nuclear reactor, mixing the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel. An example molten salt fuel for use in a fast spectrum molten salt nuclear reactor prepared by a processing that includes providing a molten salt nuclear fuel, providing at least one lanthanide, and prior to start-up of the fast spectrum molten salt nuclear reactor, mixing the molten salt nuclear fuel with the at least one lanthanide to form a lanthanide-loaded molten salt nuclear fuel. An example fast spectrum molten salt nuclear reactor includes a reactor core section including a fuel input and a fuel output. The fuel input and the fuel output are arranged to flow a molten salt nuclear fuel through the reactor core section. A displacement assembly is operably coupled to the reactor core section and configured to selectively displace a volume of the molten salt nuclear fuel in order to control reactivity within the molten salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the displacement assembly as configured to selectively displace a volume of the molten salt nuclear fuel at a central region of the reactor core section. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the displacement assembly as configured to displace a volume of the molten salt nuclear fuel in order to reduce reactivity within the molten salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the displacement assembly to include a displacement element, an actuator operably coupled to the displacement element, and a controller. The controller is configured to selectively direct the actuator to control a position of the displacement element in order to control the reactivity within the molten salt nuclear fuel contained within the reactor core section. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the displacement element and the reactor section to be centered along a common axis. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the actuator as configured to drive the displacement assembly into the reactor core section in order to reduce the reactivity within the molten salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the actuator as configured to withdraw the displacement assembly from the reactor core section in order to increase the reactivity within the molten salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding claim includes a reactivity parameter sensor configured to sense at least one reactivity parameter of the molten chloride salt nuclear fuel, wherein the reactivity parameter sensor is communicatively coupled to the controller. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the reactivity parameter sensor that includes at least one of a fission detector, a neutron flux monitor, a neutron fluence sensor, a fission product sensor, a temperature sensor, a pressure sensor or a power sensor. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the controller as configured to selectively direct the actuator to control the position of the displacement element within the reactor core section in response to at least one sensed reactivity parameter of the molten chloride salt nuclear fuel from the reactivity parameter sensor. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the displacement element that includes a displacement rod. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the displacement element that includes a plurality of displacement rods. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the displacement element as formed from a substantially non-neutron-absorbing material. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the displacement element as formed from at least one of a substantially neutron-transparent material or a substantially neutron-reflective material. Another example fast spectrum molten salt nuclear reactor of any preceding claim includes a spill-over system configured to transport excess molten salt nuclear fuel out of the reactor core section. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the spill-over system that includes a fuel salt uptake. The fuel salt uptake is positioned above a selected maximum molten salt nuclear fuel fill level of the reactor core section and configured to transport excess molten salt nuclear fuel out of the reactor core section. At least one fluid transport element and a spill-over reservoir are also included. The at least one fluid transport element fluidically couples the fuel salt uptake and the spill-over reservoir. The spill-over reservoir is configured to store excess molten salt nuclear fuel received from the at least one fluid transport element. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the molten salt nuclear fuel that includes a mixture of at least two of a first uranium chloride, a second uranium chloride or an additional metal chloride. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the additional metal chloride that includes at least one of NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3, AmCl3, LaCl3, CeCl3, PrCl3 or NdCl3. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides at least one of the first uranium chloride or the second uranium chloride that includes at least one of UCl4 or UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the molten salt nuclear fuel that has a composition of 82UCl4-18UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the molten salt nuclear fuel that has a composition of 17UCl3-71UCl4-12NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the molten salt nuclear fuel that has a composition of 50 UCl4-50NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the molten salt nuclear fuel that has a composition of 34 UCl3-66NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride that includes at least 5% by molar fraction UCl4. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride that has a uranium concentration of greater than 61% by weight. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride that has a melting point between 330 and 800 degrees Celsius. Another example fast spectrum molten salt nuclear reactor of any preceding claim provides the molten salt nuclear fuel that includes a mixture of at least one uranium fluoride and an additional metal fluoride. An example method includes determining a reactivity parameter in a molten salt nuclear fuel of a molten salt nuclear reactor and, responsive to the reactivity parameter in the molten salt nuclear fuel, displacing a selected volume of the molten salt nuclear fuel with at least one displacement element to control the reactivity of the molten salt nuclear fuel. Another example method of any preceding method provides the determining a reactivity parameter in a molten salt nuclear fuel of a molten salt nuclear reactor by acquiring at least one of a neutron production rate, a neutron absorption rate, a neutron flux, a neutron fluence, a temperature, a pressure, a power or a fission product production rate of the molten salt nuclear fuel, and determining a reactivity parameter in the molten salt nuclear fuel of a molten salt nuclear reactor based on the at least one of a neutron production rate, a neutron absorption rate, a neutron flux, a neutron fluence, a temperature, a pressure, a power or a fission product production rate. Another example method of any preceding method provides, responsive to a reactivity parameter in the molten salt nuclear fuel, displacing a selected volume of the molten salt nuclear fuel with at least one displacement element to adjust the reactivity of the molten salt nuclear fuel by responsive to a reactivity parameter indicative of excess reactivity in the molten salt nuclear reactor, displacing a selected volume of the molten salt nuclear fuel with at least one displacement element to reduce the reactivity of the molten salt nuclear reactor. Another example method of any preceding method provides displacing a selected volume of the molten salt nuclear fuel with at least one displacement element by displacing a selected volume of the molten salt nuclear fuel by driving at least a portion of at least one displacement element into the molten salt nuclear fuel to reduce the reactivity of the molten salt nuclear reactor. Another example method of any preceding method provides displacing a selected volume of the molten salt nuclear fuel with at least one displacement element by displacing a selected volume of the molten salt nuclear fuel by withdrawing at least a portion of at least one displacement element from the molten salt nuclear fuel to increase the reactivity of the molten salt nuclear reactor. Another example method of any preceding method provides displacing a selected volume of the molten salt nuclear fuel by driving at least a portion of at least one displacement element into the molten salt nuclear fuel by displacing a selected volume of the molten salt nuclear fuel by driving a selected amount of at least one displacement element into the molten salt nuclear fuel, wherein the selected amount is based on the determined reactivity parameter. Another example method of any preceding method provides displacing a selected volume of the molten salt nuclear fuel by driving at least a portion of at least one displacement element into the molten salt nuclear fuel by displacing a selected volume of the molten salt nuclear fuel by driving at least a portion of at least one displacement element into a volume of the molten salt nuclear fuel within a reactor core section of the molten salt nuclear reactor. Another example method of any preceding method provides displacing a selected volume of the molten salt nuclear fuel by driving at least a portion of at least one displacement element into a volume of the molten salt nuclear fuel within a reactor core section of the molten salt nuclear reactor by displacing a selected volume of the molten salt nuclear fuel by driving at least a portion of at least one displacement element into a volume of the molten salt nuclear fuel at a central region of the reactor core section of the molten salt nuclear reactor. Another example method of any preceding method provides displacing a selected volume of the molten salt nuclear fuel with at least one displacement element by displacing a selected volume of the molten salt nuclear fuel with at least one displacement rod. Another example method of any preceding method provides displacing a selected volume of the molten salt nuclear fuel with at least one displacement rod by displacing a selected volume of the molten salt nuclear fuel with at least one hollow displacement rod. Another example method of any preceding method provides displacing a selected volume of the molten salt nuclear fuel with at least one displacement rod by displacing a selected volume of the molten salt nuclear fuel with at least one solid displacement rod. Another example method of any preceding method provides displacing a selected volume of the molten salt nuclear fuel with at least one displacement rod by displacing a selected volume of the molten salt nuclear fuel with a plurality of displacement rods. Another example method of any preceding method provides the at least one displacement rod that is formed from at least one of lead or tungsten. Another example method of any preceding method provides the displacing a selected volume of the molten salt nuclear fuel with at least one displacement element by displacing a selected volume of the molten salt nuclear fuel with at least one displacement rod formed from a substantially non-neutron-absorbing material. Another example method of any preceding method provides the displacing a selected volume of the molten salt nuclear fuel with at least one displacement element by displacing between 0.1 and 10 cubic meters of the molten salt nuclear fuel with at least one displacement element. Another example method of any preceding method provides determining a reactivity parameter in a molten salt nuclear fuel of a molten salt nuclear reactor by determining a reactivity parameter in a molten salt nuclear fuel including a mixture of at least two of a first uranium chloride, an additional uranium chloride or an additional metal chloride. Another example method of any preceding method provides determining a reactivity parameter in a molten salt nuclear fuel including a mixture of at least two of a first uranium chloride, an additional uranium chloride or an additional metal chloride by determining a reactivity parameter in a molten salt nuclear fuel including a mixture of at least two of a first uranium chloride, an additional uranium chloride or an additional metal chloride a mixture of at least two of UCl4, UCl3 and an additional metal chloride. Another example method of any preceding method provides the additional metal chloride that includes at least one of NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3, AmCl3, LaCl3, CeCl3, PrCl3 or NdCl3. Another example method of any preceding method provides determining a reactivity parameter in a molten salt nuclear fuel by determining a reactivity parameter in a molten salt nuclear fuel having at least 5% by molar fraction UCl4. Another example method of any preceding method provides the determining a reactivity parameter in a molten salt nuclear fuel by determining a reactivity parameter in a molten salt nuclear fuel having a uranium concentration of greater than 61% by weight. Another example method of any preceding method provides determining a reactivity parameter in a molten salt nuclear fuel by determining a reactivity parameter in a molten salt nuclear fuel having a melting point between 330 and 800 degrees Celsius. An example fast spectrum molten salt nuclear reactor includes a reactor core section including a fuel input and a fuel output, the fuel input and the fuel output arranged to flow a molten salt nuclear fuel through the reactor core section and a molten fuel salt exchange assembly operably coupled to the reaction core section and configured to replace a selected volume of the molten salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor. The molten salt nuclear fuel includes at least some fissile material. The feed material includes at least some fertile material. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the selected volume of feed material that is substantially equal in volume to the selected volume of the molten salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the replaced selected volume of the molten salt nuclear fuel that includes at least some fission products. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least some fission products that includes one or more lanthanides. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the replaced selected volume of the molten salt nuclear fuel that includes a carrier salt. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten fuel salt exchange assembly that includes a used-fuel transfer unit fluidically coupled to the reactor core section and configured to transfer a selected volume of the molten salt fuel from the reactor core section to a reservoir and a feed-fuel supply unit fluidically coupled to the reactor core section and configured to transfer a selected volume of feed material including at least some fertile material from a feed material source to the reactor core section. Another example fast spectrum molten salt nuclear reactor of any preceding reactor that includes a controller is configured to selectively direct the used-fuel unit to transfer a selected volume of the molten salt fuel from the reactor core section to a reservoir and to selectively direct the feed-fuel supply unit to transfer a feed material including at least some fertile material from a feed material source to a portion of the reactor core section. Another example fast spectrum molten salt nuclear reactor of any preceding reactor that includes a reactivity parameter sensor configured to sense at least one reactivity parameter of the molten salt nuclear fuel, wherein the reactivity parameter sensor is communicatively coupled to the controller. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the controller as configured to selectively direct the used-fuel transfer unit to transfer a selected volume of the molten salt fuel from the reactor core section to a reservoir and the controller is further configured to selectively direct the feed-fuel supply unit to transfer a feed material including at least some fertile material from a feed material source to a portion of the reactor core section in response to at least one sensed reactivity parameter of the molten salt nuclear fuel from the reactivity parameter sensor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the reactivity parameter sensor that includes at least one of a fission detector, a neutron flux monitor, a neutron fluence sensor, a fission product sensor, a temperature sensor, a pressure sensor or a power sensor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the reservoir that includes at least one of a storage reservoir. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the reservoir that includes at least one second generation molten salt reactor. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least some fertile material of the feed material that includes at least one fertile fuel salt. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least one fertile fuel salt that includes a salt containing at least one of depleted uranium, natural uranium or thorium. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the at least one fertile fuel salt that includes a salt containing at least one metal from a used nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that includes a mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the additional metal chloride that includes at least one of NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3, AmCl3, LaCl3, CeCl3, PrCl3 or NdCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides at least one of the first uranium chloride or the second uranium chloride that includes at least one of UCl4 or UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that has a composition of 82UCl4-18UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that has a composition of 17UCl3-71UCl4-12NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that has a composition of 50 UCl4-50NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that has a composition of 34 UCl3-66NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride that includes at least 5% by molar fraction UCl4. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride that has a uranium concentration of greater than 61% by weight. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of at least a first uranium chloride, a second uranium chloride and an additional metal chloride that has a melting point between 330 and 800 degrees Celsius. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the molten salt nuclear fuel that includes a mixture of at least one uranium fluoride and an additional metal fluoride. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a gas sparging unit configured to remove a noble gas from the molten salt nuclear fuel. Another example fast spectrum molten salt nuclear reactor of any preceding reactor includes a filter unit configured to remove at least one of a noble metal or a semi-noble metal from the molten salt nuclear fuel. An example method includes operating a molten salt fast spectrum nuclear reactor including a molten salt nuclear fuel and replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor. The molten salt nuclear fuel includes at least some fissile material. The feed material includes at least some fertile material. Another example method of any preceding method provides replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material by replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material equal in volume to the selected volume of the molten salt nuclear reactor. Another example method of any preceding method provides replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material by replacing a selected volume of the molten salt nuclear fuel including at least some fission products with a selected volume of feed material. Another example method of any preceding method provides replacing a selected volume of the molten salt nuclear fuel including at least some fission products with a selected volume of feed material by replacing a selected volume of the molten salt nuclear fuel including one or more lanthanides with a selected volume of feed material. Another example method of any preceding method provides replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material by replacing a selected volume of the molten salt nuclear fuel including a carrier salt with a selected volume of feed material. Another example method of any preceding method provides replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor by replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material to maintain the reactivity of the molten salt nuclear fuel of molten salt nuclear reactor. Another example method of any preceding method includes measuring a reactivity parameter of the molten salt nuclear fuel of the molten salt fast spectrum nuclear reactor. Another example method of any preceding method provides replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor by, responsive to the measured reactivity parameter of the molten salt nuclear fuel, replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor. Another example method of any preceding method provides measuring a reactivity parameter of the molten salt nuclear fuel of the molten salt fast spectrum nuclear reactor by measuring at least one of a neutron production rate, a neutron absorption rate, a neutron flux, a neutron fluence, a temperature, a pressure, a power or a fission product production rate of the molten salt nuclear fuel of the molten salt fast spectrum nuclear reactor. Another example method of any preceding method provides replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor by continuously replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor. Another example method of any preceding method provides replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor by repeatedly replacing a selected batch volume of the molten chloride salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor. Another example method of any preceding method provides replacing a selected volume of the molten salt nuclear fuel with a selected volume of feed material to control the reactivity of the molten salt nuclear reactor, the molten salt nuclear fuel including at least some fissile material, the feed material including at least some fertile material by removing a selected volume of the molten salt nuclear fuel from the fast spectrum molten salt nuclear reactor, the removed selected volume of molten salt nuclear fuel including at least some fissile material, and supplying a selected volume of feed material to the fast spectrum molten salt nuclear reactor, the supplied selected volume of feed material including at least some fertile material. Another example method of any preceding method provides a rate of supply of the selected volume of feed material that is selected to match a rate of addition of fertile material into the molten salt nuclear reactor to a rate of burning of fissile material within the molten salt nuclear reactor. Another example method of any preceding method provides the removed selected volume of the molten salt nuclear fuel that further includes at least one of a fission product, a fertile material or a carrier salt. Another example method of any preceding method provides the at least some fertile material of the feed material that includes at least one fertile fuel salt. Another example method of any preceding method provides the at least one fertile fuel salt that includes a salt containing at least one of depleted uranium, natural uranium or thorium. Another example method of any preceding method provides the at least one fertile fuel salt that includes a salt containing at least one metal from a used nuclear fuel. Another example method of any preceding method provides the at least one fertile fuel salt that maintains a chemical composition of the molten salt reactor fuel. Another example method of any preceding method includes removing a noble gas from the molten salt nuclear fuel via a gas sparging process. Another example method of any preceding method includes removing at least one of a noble metal or a semi-noble metal from the molten salt nuclear fuel via a plating process. An example system includes at least one first generation molten salt nuclear reactor including a molten salt nuclear fuel, at least one second generation molten salt nuclear reactor, and a molten salt transfer unit configured to transfer a volume of molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor to at least one second generation molten salt nuclear reactor. The volume of the molten salt nuclear fuel includes at least some fissile material enriched in the at least one first generation molten salt nuclear reactor. Another example system of any preceding system provides the volume of the molten salt nuclear fuel including at least some fissile material that is enriched in the at least one first generation molten salt nuclear reactor to so as to achieve criticality in the at least one second generation molten nuclear reactor. Another example system of any preceding system provides the volume of the molten salt nuclear fuel including at least some fissile material that is enriched in the at least one first generation molten salt nuclear reactor to so as to achieve criticality in the at least one second generation molten nuclear reactor without enrichment of the volume of the molten salt nuclear fuel in the at least one second generation molten nuclear reactor. Another example system of any preceding system provides operation of the at least one first generation molten salt nuclear reactor to enrich at least some uranium to generate Pu-239 within the at least one first generation molten salt nuclear reactor. Another example system of any preceding system provides the volume of molten salt nuclear fuel transferred from the at least one first generation molten salt nuclear reactor to the at least one second generation molten salt nuclear reactor that includes Pu-239 generated within the at least one first generation molten salt nuclear reactor. Another example system of any preceding system provides the molten salt transfer unit that includes a fission product removal system configured to remove one or more fission products from the volume of molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor. Another example system of any preceding system provides the at least one first generation molten salt nuclear reactor that includes: a plurality of first generation molten salt nuclear reactors. Another example system of any preceding system provides the at least one second generation molten salt nuclear reactor that includes a plurality of second generation molten salt nuclear reactors. Another example system of any preceding system provides the molten salt nuclear fuel of the at least one first generation molten salt nuclear reactor that includes a mixture of at least two of a first uranium chloride, a second uranium chloride or an additional metal chloride. Another example system of any preceding system provides the additional metal chloride that includes at least one of NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3, AmCl3, LaCl3, CeCl3, PrCl3 or NdCl3. Another example system of any preceding system provides at least one of the first uranium chloride or the second uranium chloride that includes at least one of UCl4 or UCl3. Another example system of any preceding system provides the molten salt nuclear fuel that has a composition of 82UCl4-18UCl3. Another example system of any preceding system provides the molten salt nuclear fuel that has a composition of 17UCl3-71UCl4-12NaCl. Another example system of any preceding system provides the molten salt nuclear fuel that has a composition of 50 UCl4-50NaCl. Another example system of any preceding system provides the molten salt nuclear fuel that has a composition of 34 UCl3-66NaCl. Another example system of any preceding system provides the mixture of at least two of a first uranium chloride, a second uranium chloride or an additional metal chloride that includes at least 5% by molar fraction UCl4. Another example system of any preceding system provides the mixture of at least two of a first uranium chloride, a second uranium chloride or an additional metal chloride that has a uranium concentration of greater than 61% by weight. Another example system of any preceding system provides the mixture of at least two of a first uranium chloride, a second uranium chloride or an additional metal chloride that has a melting point between 330 and 800 degrees Celsius. Another example system of any preceding system provides the molten salt nuclear fuel of the at least one first generation molten salt nuclear reactor that includes a mixture of at least one uranium fluoride and an additional metal fluoride. An example method includes enriching at least a portion of a molten salt nuclear fuel in at least one first generation molten salt nuclear reactor, removing a volume of the enriched molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor, and supplying at least a portion of the removed volume of molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor to at least one second generation molten salt nuclear reactor. Another example method of any preceding method provides enriching at least a portion of a molten salt nuclear fuel in at least one first generation molten salt nuclear reactor by enriching at least a portion of a molten salt nuclear fuel in at least one first generation molten salt nuclear reactor so as to achieve criticality in the at least one second generation molten nuclear reactor. Another example method of any preceding method provides enriching at least a portion of a molten salt nuclear fuel in at least one first generation molten salt nuclear reactor so as to achieve criticality in the at least one second generation molten nuclear reactor by enriching at least a portion of a molten salt nuclear fuel in at least one first generation molten salt nuclear reactor so as to achieve criticality in the at least one second generation molten nuclear reactor without enrichment of the volume of the molten salt nuclear fuel in the at least one second generation molten nuclear reactor. Another example method of any preceding method provides enriching at least a portion of a molten salt nuclear fuel in at least one first generation molten salt nuclear reactor by enriching at least some uranium within a volume of the molten salt nuclear fuel of the at least one first generation molten salt nuclear reactor to generate Pu-239. Another example method of any preceding method includes removing one or more fission products from the at least a portion of the volume of molten salt nuclear fuel removed from the at least one first generation molten salt nuclear reactor. Another example method of any preceding method provides supplying at least a portion of the removed volume of molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor to at least one second generation molten salt nuclear reactor by supplying a portion of the removed volume of molten salt nuclear fuel from the at least one first generation molten fast spectrum salt nuclear reactor to a first second generation molten salt nuclear reactor and supplying at least one additional portion of the removed volume of molten salt nuclear fuel from the at least one first generation fast spectrum molten salt nuclear reactor to at least one additional second generation molten salt nuclear reactor. Another example method of any preceding method provides removing a volume of the enriched molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor by removing a volume of molten salt nuclear fuel from at least one first generation molten salt nuclear reactor to control reactivity of the at least one first generation molten salt nuclear reactor. Another example method of any preceding method provides removing a volume of the enriched molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor by continuously removing a volume of the enriched molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor. Another example method of any preceding method provides removing a volume of the enriched molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor by repeatedly removing a selected batch of a volume of the enriched molten salt nuclear fuel from the at least one first generation molten salt nuclear reactor. Another example method of any preceding method that includes supplying a selected volume of feed material to the at least one first generation molten salt nuclear reactor, the feed material including at least some fertile material. Another example method of any preceding method provides the at least some fertile material of the feed material that includes at least one fertile fuel salt. Another example method of any preceding method provides the at least one fertile fuel salt that includes a salt containing at least one of depleted uranium, natural uranium or thorium. Another example method of any preceding method provides the at least one fertile fuel salt that includes a salt containing at least one metal from a used nuclear fuel. Another example method of any preceding method provides the at least one fertile fuel salt that maintains a chemical composition of the molten salt reactor fuel. Another example method of any preceding method includes supplying a selected volume of feed material to the at least one second generation molten salt nuclear reactor, the feed material including at least some fertile material. An example fast spectrum molten salt nuclear reactor includes a reactor core section including a fuel input and a fuel output. The fuel input and the fuel output are arranged to flow a molten chloride salt nuclear fuel through the reactor core section. The molten chloride salt nuclear fuel includes a mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt, the mixture of UCl4 and at least one additional metal chloride salt having a UCl4 content greater than 5% by molar fraction. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the uranium concentration in the mixture of UCl4 and at least one additional metal chloride salt that is greater than 61% by weight. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the additional uranium chloride salt that includes UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt has a composition of 82UCl4-18UCl3. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt that has a composition of 17UCl3-71UCl4-12NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt that has a composition of 50 UCl4-50NaCl. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt that has an additional metal chloride salt concentration at or below the precipitation concentration for the additional metal chloride salt. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the mixture of UCl4 and at least one of an additional uranium chloride salt or an additional metal chloride salt that has a melting temperature below a temperature of 800 degrees Celsius. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides the selected melting temperature that is above a temperature of 330 degrees Celsius. Another example fast spectrum molten salt nuclear reactor of any preceding reactor provides breed-and-burn behavior that is established within the molten chloride salt nuclear fuel with a uranium-plutonium cycle. An example method of fueling a fast spectrum molten salt nuclear reactor includes providing a volume of UCl4, providing a volume of at least one of an additional uranium chloride salt or an additional metal chloride salt, mixing the volume of UCl4 with the volume of the at least one of an additional uranium chloride salt or an additional metal chloride salt to form a molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction, and supplying the molten chloride salt nuclear fuel having a UCl4 content greater than 5% by molar fraction to at least a reactor core section of the fast spectrum molten salt nuclear reactor. Another example method of any preceding method provides a volume of at least one of an additional uranium chloride salt or an additional metal chloride salt by providing a volume of UCl3. Another example method of any preceding method provides the chlorine in the UCl4 that is enriched with 37Cl. Another example method of any preceding method provides the chlorine in the salt that is enriched to at least 75% 37Cl. The above specification, examples, and data provide a complete description of the structure and use of exemplary implementations of the invention. Since many implementations of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. Furthermore, structural features of the different implementations may be combined in yet another implementation without departing from the recited claims. |
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059463651 | description | Shown in FIG. 1 is a guide tube of a fuel assembly for a pressurized-water nuclear reactor designated by the general reference numeral 1. The guide tube 1 is of zirconium alloy and has a cylindrical external surface whose diameter is constant along the length of the tube, except for the upper part 2 of the tube which is bell-mouthed and has for example a tapped inner bore. This upper part of the tube permits fixing the tube in the upper terminal element of the fuel assembly, optionally by the use of detachable fixing means. The guide tube 1 further comprises an internally tapped lower end by means of which the tube may be fixed to the lower terminal element of the fuel assembly. The guide tube 1 comprises a main or body part 1a and a lower end part 1b which differ from each other in that the main part 1a has a first wall thickness e1 and the lower end part 1b a second thickness e2 exceeding the thickness e1. Consequently, the part 1b constitutes a reinforced part of the tube. In some cases, the guide tube may comprise a reinforced part of increased thickness between two main parts which have a thickness less than the reinforced part, instead of a reinforced end part. The outside diameter of the tube is constant and identical in the main part of the tube and in the reinforced part 1b. The inside diameter of the tube in the main part 1a is therefore larger than the inside diameter of the tube in the part 1b and the guide tube 1 has a transition region 1c between its parts 1a and 1b. In the transition region 1c, the internal surface of the tube is formed by a conical or tapered chamfer whose vertex angle is around 10.degree.. The wall of the lower end part of the tube may have through openings, such as 3, which permit limiting the overpressure of the cooling liquid of the reactor in the lower part of the guide tube upon the dropping of the absorber rod guided by the guide tube rendering more progressive the braking of the absorber rod upon the dropping of a cluster. The increased thickness e2 of the wall of the guide tube in the lower end part 1b of the guide tube permits reinforcing the lower part of the guide tube and avoiding deterioration of this lower part by the effect of the overpressure upon the dropping of the absorber rod of the control cluster and in the course of the transitional periods of the nuclear reactor. However, the presence of an intermediate region 1c whose internal wall has the shape of a conical or tapered chamfer creates a discontinuity as concerns the guiding of the absorber rod in the guide tube. Further, the intermediate region may be a weak region of the tube. Further, to produce the tube shown in FIG. 1, there must be employed a forming process, such as a rotary hammering, for reducing the thickness of the wall in the main part of the guide tube, i.e. along the major part of the length of the tube, then for inwardly upsetting the thick wall of the lower part of the tube. Such a forming process is delicate to carry out and requires a relatively long operating time. The process according to the invention permits producing a tube which is in a single piece and has a reinforced part, by a rolling technique on a pilgrim or pilger rolling mill. Shown diagrammatically in FIG. 2 are the main elements of a pilgrim rolling mill for forming a tube from a tubular blank. The pilgrim rolling mill designated by the general reference numeral 5 mainly comprises a first die 6a and a second die 6b in the form of splined cylinders mounted to be rotatable about their axes, and a mandrel 7 having a symmetrical shape of revolution. The dies 6a and 6b are rotatively mounted by means of their respective shafts 8a and 8b in a movably mounted cage associated with driving means so as to be capable of travelling in the axial direction of the mandrel 7 in one direction or the other with a constant amplitude, as diagrammatically shown by the double arrow 9. Each of the dies 6a and 6b comprises a respective peripheral groove 10a or 10b, named spline, which has a cross-sectional shape in the radial direction of the die which is close to a semi-circular shape. The cross section of the grooves 10a and 10b of the dies 6a and 6b has a dimension which varies continuously along the periphery of the groove, the section having a maximum dimension in an entrance part and a minimum section in the exit part of the groove. The dies 6a and 6b are driven in rotation about their respective axis in one direction or the other owing to the displacement of the cage in one direction or the other during the reciprocating displacement diagrammatically represented by the double arrow 9. The pilgrim rolling mill shown in FIG. 2 permits effecting the rolling of the wall of a tubular blank 11 engaged on the mandrel 7 in such manner as to progressively reduce the diameter and the thickness of the wall of the blank and obtain, at the ouput end of the mill, a tube 12 whose diameter and wall thickness are less than the diameter and wall thickness of the blank 11. Owing to the rolling, the blank 11 undergoes an elongation which may be considerable in the axial direction. The mandrel 7 on which the blank 11 is engaged is connected to a rod 13 which permits moving the mandrel 7 in translation and in rotation about its axis. The pilgrim rolling mill 5 further comprises a carriage (not shown) which may be fixed to the blank 11 by clamps. The carriage permits advancing the blank in the rolling direction after each of the steps effected by the pilgrim rolling mill. The device for advancing the blank also permits rotating it about its axis at the end of each of the rolling steps. The mandrel 7 comprises a first cylindrical part 7a whose diameter is less than the inside diameter of the blank 11, a second symmetrical part 7b of revolution whose meridian curves have substantially the shape of parabolas and a slightly conical or tapered end part 7c whose diameter is the final inside diameter of the tube 12 to be produced or close to said final inside diameter. The dies 6a and 6b are disposed on opposite sides of the mandrel 7 on which the blank 11 and the tube 12 in the course of rolling are engaged, in such manner that the grooves 10a and 10b constitute, during the travel in the axial direction and the rotation of the dies, a tube-forming surface having a roughly circular section. Owing to the fact that the dimension of the cross sections of the grooves 10a and 10b vary in a continuous manner along the periphery of the dies, the dimensions of the cylindrical forming surface of the tube themselves vary between a maximum dimension and a minimum dimension during the displacements of the cage and dies. The cage travels in the axial direction with an amplitude substantially corresponding to the length of the mandrel, along the region 7b of reduction of the blank and the region 7c of the calibration of the tube 12; the diameter and the thickness of the blank 11 are progressively reduced to the values of the diameter and wall thickness of the tube 12. At the end of each of the displacements of the rolling cage, the blank is advanced along the mandrel with a certain amplitude in the axial direction, and the blank is made to turn about its axis through a certain angle. Simultaneously, the mandrel 7 is made to turn about its axis by the rod 13. The rolling can be effected in a substantially continuous manner by engaging blanks one after the other on the rod 13 and the mandrel 7 and collecting the tubes 12 at the output end of the rolling mill. The pilgrim rolling method just described can be applied to the production of guide tubes comprising a lower end part having a wall of increased thickness relative to the wall thickness of the main part of the tube. To carry out the process for producing guide tubes according to the invention, there is employed a pilgrim or pilger rolling mill comprising a mandrel of a special shape, such as that shown in FIG. 3. The mandrel 14 having a symmetrical shape of revolution comprises a screw-threaded end 15 for connecting the mandrel to a holding and actuating rod. Following on the threaded part in the axial direction 16, the mandrel comprises a first cylindrical part 17 whose diameter is less than the inside diameter of the starting blank used for forming the guide tube, a first part 18 having a symmetrical decreasing section of revolution and a meridian in the shape of a parabola or a shape which approaches a parabola, and a third slightly conical or tapered part 21 whose diameter is equal to roughly the inside diameter of the reinforced lower end part of the guide tube to be produced. The mandrel 14 therefore comprises a plurality of successive forming sections in the axial direction constituting different guide tube-forming stages. The stepped forming mandrel 14 is used for producing guide tubes according to the invention in the course of two successive stages respectively represented in FIGS. 4A and 4B effected in this order or in the opposite order which may possibly be carried out on successive sections of a tube blank so as to produce in a single operation a rolled product from which it is possible to obtain, by cutting off, a plurality of guide tubes having a reinforced part. FIG. 4A shows the mandrel 14 in the course of a first pilgrim rolling stage of a tubular blank 22 of which both the diameter and the wall thickness exceed the diameter and the wall thickness of the guide tube to be produced. The pilgrim rolling mill comprises two dies 10a and 10b which are similar to the dies described in the case of the pilgrim rolling mill 5 shown in FIG. 2. The pilgrim rolling mill used for carrying out the process of the invention and shown in FIGS. 4A and 4B differs from the pilgrim rolling mill of the conventional type shown in FIG. 2 only in respect of the use of the stepped mandrel 14 and the mechanization of the axial displacement of the mandrel. The dies 10a and 10b freely rotatively mounted in a cage which may be displaced in the axial direction 16 of the mandrel 14, permit reducing the diameter and the thickness of the blank 22. The blank 22 was obtained by prior production and shaping operations which may themselves include pilgrim rolling operations. The blank 22 has an inside diameter slightly larger than the outside diameter of the cylindrical part 17 of the mandrel 14 and a wall thickness exceeding the wall thickness of the guide tube in its lower end part where the wall thickness is maximum. As can be seen in FIG. 4A, the pilgrim rolling is effected during the first rolling stage with the mandrel 14 placed in such manner that the dies 10a and 10b rotatively mounted in the cage of the rolling mill are displaced in a reciprocating manner along the parts 18 and 19 of the mandrel 14. In this way, the inside diameter and the outside diameter of the tubular product 24 obtained at the output end of the rolling mill, i.e. on the downstream side of the region 19 of the mandrel 14, are identical to the inside diameter and the outside diameter of the main part of the guide tube to be produced. In particular, the part 19 of the mandrel 14 constitutes a part for calibrating the product 24 so that its inside diameter has for precise dimension the required inside diameter for producing the guide tube. The dimension and the arrangement of the dies 10a and 10b are such that the outside diameter of the product 24 at the output end of the rolling mill precisely corresponds to the required outside diameter for producing the guide tube. The first rolling stage is carried out by effecting a certain number of successive rolling steps between which the blank 22 is advanced and rotated about its axis, the mandrel 14 being also rotated through a certain angle between the successive rolling steps. A coding device associated with the pilgrim rolling mill permits determining in a very precise manner the length of the product 24 obtained at the output end of the rolling mill. When a predetermined length of the product has been obtained at the output end of the rolling mill, the coding device delivers a signal for displacing the mandrel 14 in the axial direction and possibly stopping the rolling mill. The second rolling stage, shown in FIG. 4B, is indeed effected after a displacement of the mandrel 14 in the axial direction toward the upstream end of the pilgrim rolling mill so as to place the parts 20 and 21 of the mandrel in the working region of the rolling mill, i.e. in the region of the displacement of the dies 10a and 10b. The displacement of the mandrel 14 from its position shown in FIG. 4A for carrying out the first rolling stage to its position shown in FIG. 4B for carrying out the second rolling stage, can be achieved after having stopped the pilgrim rolling operation, the cage in which the dies 10a and 10b are mounted being stationary, or without stopping the rolling, in which case the cage in which the dies are mounted remains in motion. In the second stage of the rolling, the rotative dies 10a and 10b displaced in the axial direction by the cage of the rolling mill, reduce the dimensions of the blank in such manner as to obtain at the output end of the rolling mill, i.e. on the downstream side of the part 21 of the mandrel, a rolled tubular product 25 whose inside diameter calibrated by the part 21 of the mandrel is equal to the required inside diameter for producing the guide tube, in its reinforced lower end region. The outside diameter of the product 25 is identical to the outside diameter of the product 24 obtained in the first rolling stage owing to the fact that the dies 10a and 10b employed are the same as those used in the first stage of the rolling. The mandrel 14 must comprise, generally, a first part whose cross-sectional diameter diminishes in the axial direction from a value less than the inside diameter of the blank to a value equal to the inside diameter of the main part of the tube to be produced, and a second part whose cross-sectional diameter diminishes, in the axial direction of the mandrel, from a value equal to the inside diameter of the main part of the tube to be produced to a value equal to the inside diameter of the lower end part of the tube to be produced. The wall thickness of the product 25 at the output end of the rolling mill therefore very precisely corresponds to the wall thickness to be obtained in the lower end part of the guide tube. The coding device permits, as before, stopping the second rolling stage as soon as a predetermined length of the product 25 has been obtained at the output end of the rolling mill. The mandrel is then displaced from its second position to its first position. The first rolling stage, the first displacement of the mandrel, the second rolling stage and the second displacement of the mandrel may be affected repeatedly as many times as necessary for completely rolling a blank 22. Also, it is obviously possible to invert the first and second rolling stages. In this way there is obtained at the output end of the pilgrim rolling mill a rolled tubular product comprising successive sections 24 whose inside diameter and wall thickness correspond to the inside diameter and wall thickness of the main part of a guide tube 25 to be produced, and whose inside diameter and wall thickness correspond to the diameter and wall thickness of the lower end parts of the guide tubes to be produced. Such a product obtained at the output end of the rolling mill is shown in FIG. 5 in which the wall thickness differences have been greatly exaggerated. In the case of the production of guide tubes for fuel assemblies of a pressurized water nuclear reactor, the main or body part 24 of the guide tubes has a thickness e1 which may be 0.5 mm. The reinforced lower end parts of the guide tubes have a thickness e2 of the order of 1.2 mm. The production process according to the invention on a pilgrim rolling mill permits obtaining a transition region between the main parts and the reinforced parts of the guide tubes of a length l of the order of 180 mm. In all cases, this length of the transition region exceeds 100 mm. As the guide tube itself has a diameter of the order of 12.5 mm, the result is that the change in the inside diameter of the guide tube between the main part and the reinforced part is very progressive. The transition region between the parts of different diameters of the internal surface of the tube is different from a chamfer, which constitutes the difference between the guide tubes according to the invention and guide tubes of the prior art. Further, the continuous tube-forming process permits obtaining transition regions in which the metal is faultless and which therefore do not constitute weak regions of the tube. As seen in FIG. 6, the tubular product 26 obtained at the output end of the rolling mill and comprising successive regions 24 and 25 of different wall thicknesses has a constant outside diameter. Further, the length of the regions 24 and 25 was chosen when rolling, in such manner as to provide, by a cutting or sectioning of the tube 26 in given regions, a plurality of guide tubes 27 each comprising a main or body part 27a and a reinforced lower end part 27b having the required length and wall thickness. Preferably, the regions 24 and 25 have a length which is roughly equal respectively to double the length of the main part and double the length of the reinforced part of the guide tube to be produced. The location of the position of the cutting lines 28 along which a tool 29 must effect in succession the cutting of the rolled tubular product 26, is obtained by the use of a device for precisely locating the transition regions 27c between the parts of the tubular product 26 of different thicknesses. The end of the regions 25 of the tubular product 26 connected to the transition regions 27c, of length 180 mm, along which the wall changes from the first thickness e1 to the second thickness e2 is very precisely located. The precise location of the end of the regions 25 and 27c may be obtained with the use of an air gauge 30 comprising a pipe 31 which is axially engaged inside the tubular product 26 and includes a nozzle 32 at its end. The flow characteristics of the air through the nozzle 32 which can be ascertained by the air gauge 30 permit very precisely determining the end of the regions 25 of thickness e2 and the transition regions 27c between the parts of the tubular product having different thicknesses. The cutting tool 29, formed by a cutting disc, is placed at a definite distance from the position which had been located for effecting the cutting of the tube along the line 28. The cutting tool 29 may be a tool of a flying cutting device which is displaced in synchronism with the tubular product 26 at the output end of the rolling mill. It is also possible to determine the position of the end of the regions 25 and of the transition regions 27 with the use of a coil 33 surrounding the tube and constituting an eddy current sensor. By locating the ends of the successive regions 25 and transition regions 27c of the rolled tubular product 26, it is possible to cut off guide tubes 27, 27', 27" formed by successive sections of the tubular product 26. It is possible to envisage blanks and pilgrim rolling operations which permit obtaining four to five guide tubes having a length of the order of 4 m, from each of the blanks 22 rolled in the pilgrim rolling mill. It is also possible to employ a flying or mobile cutting device which is actuated by the advance or the retraction of the mandrel between the different rolling stages, with a certain time delay, or by devices for locating the ends of the transition regions such as those described hereinbefore, for cutting the rolled product in the form of the guide tube at the output end of the rolling mill and during the rolling operation. By achieving a precise location of the transition regions and effecting the cutting of the guide tubes by a flying cutting device at the output end of the rolling mill, it is possible to avoid undesired variations in the length of the successive regions of different wall thicknesses 24 and 25 of the rolled product and obtain guide tubes by a simple cutting. In the usual manner, the length of the lower end region of the tube whose wall thickness exceeds the wall thickness of the main part of the tube, represents 10 to 30% of the total length of the guide tube. The invention therefore permits obtaining guide tubes in one piece, reinforced for example in their lower part, by means of a rolling process which may easily be rendered automatic and results in a very high productivity. It must be understood that the scope of the invention is not intended to be limited to the described embodiment. Thus the mandrel on which the pilgrim rolling is carried out may have a form different from that described. The process may be used for producing guide tubes composed of a material different from a zirconium alloy. Generally, the invention is applicable to the production of guide tubes of fuel assemblies of any type in which the motion of the control rods is slowed down by a dash-pot effect. |
description | This invention relates to a luminescent substance, a value document, a security element and a security paper having at least one luminescent substance as an authenticity feature. The invention also relates to different methods for checking the authenticity of such value document, security element or security paper, and to methods for production thereof. Security paper will hereinafter be understood to be paper that is e.g. already equipped with security elements, such as a watermark, security thread, hologram patch, etc., but is not yet fit for circulation and is an intermediate product in production of the value document. Value document will refer to the product fit for circulation. The designation “value document” will refer in the context of the invention to bank notes, checks, shares, tokens, identity cards, credit cards, passports and other documents as well as labels, seals, packages or other elements for product protection. The protection of value documents against forgery by means of luminescent substances has been known for some time. EP 0 052 624 B2 uses for example luminescent substances based on host lattices doped with rare earth metals. Preferably, substances are used in which either the absorption or the emission is outside the visible spectral range. If the emissions are at wavelengths between approx. 400 nm and approx. 700 nm, the luminescent substances are detectable with the eye upon suitable excitation. For some applications this is desired, e.g. in authenticity testing by illumination with UV light. For other applications, however, it is of advantage if the emission is outside the visible spectral range, since special detectors are then necessary for detecting the substances. However, luminescent substances with characteristic properties that are suitable for protecting value documents and in particular for automatic authenticity recognition are restricted in number. Most inorganic and organic luminescent substances have uncharacteristic, wide spectra, insufficient emission intensity and are moreover often commercially available. This impedes their identification and makes it impracticable to use several of said substances simultaneously. Starting out from this prior art, the invention is based on the problem of increasing the number of luminescent substances suitable as authenticity features for value documents and providing in particular value documents and security papers having authenticity features in the form of luminescent substances that differ from value documents and security papers having hitherto known luminescent substances by a characteristically altered excitation and/or emission spectrum. The solution to this problem can be found in the independent claims. Developments are the subject matter of the subclaims. According to the invention, at least one luminescent substance is used for protection. Preferably, its emission spectrum is in the visible or infrared spectral range (VIS, IR). Particularly preferably, the luminescent substance emits in the near infrared (NIR). The excitation is preferably also effected in the near infrared. Depending on the inventive luminescent substance used, the emission band can follow the Stokes or anti-Stokes rule or quasi-resonance can be observed. The substances suitable for the inventive authenticity protection are luminescent substances with the general formulaXZO4 whereX stands for Sca Yb Lac Ced Pre Ndf Smg Euh Gdi Tbk Dyl HOm Ern Tmo Ybp Luq Sb(III)r Bis Crt Mn(III)u F(III)v [Baw Mn(II)x Fe(II)y Caz Sn(II)α Srβ Coγ Niδ Cuε]3/2 [Naη Kλ]3 [U(IV)μ Pbπ Thσ]3/4 U(VI)φ/2 andZ stands for Nbza Tazb Vzc Pzd [Tize Zrzf Sn(IV)zg]5/4 Wzh5/6 Fe(III)Zi5/3 anda+b+c+d+e+f+g+h+i+k+l+m+n+o+p+q+r+s+t+u+v+3/2(w+x+y+z+α+β+γ+δ+ε)+3(η+λ)+3/4(μ+π+σ)+φ/2=1, anda, b, c, d, e, f; g, h, i, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z, α, β, γ, δ, ε, η, λ, μ, π, σ and φ each range from 0 to 1, andza+zb+zc+zd+5/4(ze+zf+zg)+5/6zh+5/3zi=1, andza, zb, zc, zd, ze, zf, zg, zh and zi each range from 0 to 1. The symbols listed for X and Z correspond to the symbols in the periodic system of the elements, O stands for oxygen. The X elements Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sb, Bi, and Cr have the oxidation state 3 here. The X elements Ba, Ca, Sn, Sr, Co, Ni and Cu have the oxidation state 2 here. The X elements Na and K have the oxidation state 1 here. The X elements Mn and Fe have the oxidation state 2 and/or 3 here. The X elements Pb and Th have the oxidation state 4 here. The X element U has the oxidation states 4 and/or 6 here. The Z elements Nb, Ta, V and P have the oxidation state 5 here. The Z elements Ti, Zr and Sn have the oxidation state 4 here. The Z element W has the oxidation state 6 here. The Z element Fe has the oxidation state 3 here. It is of course possible to replace individual X and/or Z elements by further elements, such as indium (In), aluminum (Al), magnesium (Mg) and chromium (Cr), as long as the stoichiometric ratios are taken into account and the crystal lattice allows. In particular, dopings with chromium and aluminum or with magnesium and aluminum are preferred. In a preferred embodiment, the X element present is at least La or Y or both, i.e. it holds that b≠0 and/or c≠0 in the general formula. All other X elements or only selected ones thereof can of course also be present additionally. In particular, k≠0 and/or o≠0 holds here. In a further preferred embodiment, X stands for Yb Lac and b+c=1 and 0<b<1 and 0<c<1. In this case, only the two elements La and Y are present in the luminescent substance as X elements. It is further preferred that X stands for La or for Y, i.e. that only one X element is present in the general formula. In another preferred embodiment, X stands for Yb Lac Pre Ndf Dyl HOm Ern Ybp Crt Mn(III)u Fe(III)v [Mn(II)x Fe(II)y Coγ Niδ Cuε]3/2 and it holds that b+c+e+f+l+m+n+p+t+u+v+3/2(x+y+γ+δ+ε)=1, and b, c, e, f, l, m, n, p, t, u, v, x, y, γ, δ, ε each range from 0 to 1. Further it is preferred if the inventive luminescent substance contains at least one, at least two or at least three rare earth elements as X elements, i.e. that c≠0 and/or d≠0 and/or e≠0 and/or f≠0 and/or g≠0 and/or h≠0 and/or i≠0 and/or k≠0 and/or l≠0 and/or m≠0 and/or n≠0 and/or o≠0 and/or p≠0 and/or q≠0. In particular, the rare earth elements are selected from the group of Ho, Er, Yb and Nd, i.e. f≠0 and/or m≠0 and/or n≠0 and/or p≠0. Particularly preferably, Y is also present in the presence of Ho, Er, Yb and/or Nd. If at least two rare earth elements are present, these are particularly preferably the combinations Er and Yb with n≠0 and p≠0, Nd and Yb with f≠0 and p≠0 or Er and Nd with n≠0 and f≠0. If Ho, Er or Yb is present according to the general formula, i.e. if m≠0 or n≠0 or p≠0, then Pr, Dy, Nd, Cr, Mn, Fe, Co, Ni and/or Cu are preferably also present, so that it holds that e≠0 and/or l≠0 and/or f≠0 and/or t≠0 and/or u≠0 and/or v≠0 and/or x≠0 and/or y≠0 and/or γ≠0 and/or δ≠0 and/or ε≠0. If Nd is present according to the general formula, i.e. if f≠0, then Pr, Dy, Cr, Mn, Fe, Co, Ni and/or Cu are preferably also present, so that it holds that e≠0 and/or l≠0 and/or t≠0 and/or u≠0 and/or v≠0 and/or x≠0 and/or y≠0 and/or γ≠0 and/or δ≠0 and/or ε≠0. In a further preferred embodiment, X stands for Yb Lac Pre Ndf Ern Ybp Fe(III)v, and b+c+e+f+n+p+v=1, and b, c, e, f, n, p and v each range from 0 to 1. Further preferably, X stands for Yb Ybp Pre, b+e+p=1, b, e and p each range from 0 to 1, it preferably holding that 0<b<1 and 0<e<1 and 0<p<1. Further preferably, X stands for Yb Ndf Fe(III)v, b+f+v=1, b, f and v each range from 0 to 1, it preferably holding that 0<b<1 and 0<f<1 and 0<v<1. Further preferably, X stands for Yb Ern, b+n=1, b and n each range from 0 to 1, it preferably holding that 0<b<1 and 0<n<1. Further preferably, X stands for Yb Ndf Ern, b+f+n=1, b, f and n each range from 0 to 1, it preferably holding that 0<b<1 and 0<f<1 and 0<n<1. Further preferably, X stands for Yb Ybp Ndf, b+p+f=1, b, p and f each range from 0 to 1, it preferably holding that 0<b<1, 0<p<1 and 0<f<1. Further preferably, X stands for Yb Ndf, b+f=1, b and f each range from 0 to 1, it preferably holding that 0<b<1 and 0<f<1. Further preferably, X stands for Yb Ybp, b+p=1, b and p each range from 0 to 1, it preferably holding that 0<b<1 and 0<p<1. Further preferably, X stands for Y. In a further preferred embodiment, the Z element present is at least Ta, Nb, P, Ti or W. At least all five elements can also be present. Thus it holds that za≠0 and/or zb≠0 and/or zd≠0 and/or ze≠0 and/or zh≠0. Further preferably, the Z elements present are only elements selected from the group of Ta, Nb and P. Thus Z stands for Nbza Tazb Pzd, za+zb+zd=1, and za, zb and zd each range from 0 to 1, it preferably holding that 0<za<1, 0<zb<1 and 0<zd<1. In particular, Z stands for Nb, i.e. niobates are present, or Z stands for Ta, i.e. tantalates are present. Further preferably, Z stands for Nbza Tazb, za+zb=1, and za and zb each range from 0 to 1, it preferably holding that 0<za<1 and 0<zb<1. In this embodiment, niobate-tantalate mixtures are present. Compounds with the following formulae are also preferred: Yb Lac Nbza TaZb O4, where b+c=1, za+zb=1, and b, c, za and zb each range from 0 to 1. Yb Ybp NbO4, where b and p range from 0 to 1, b+p=1. Particularly preferably it holds that p>0.5. Yb Ybp Ndf NbO4, where b, p and f each range from 0 to 1, and b+p+f=1. Particularly preferred compounds are YNbO4; Yb Ndf NbO4 with b+f=1 and 0<b<1 and 0<f<1; YbYbp NbO4 with b+p=1 and 0<b<1 and 0<p<1; YbYbpNdfNbO4 with b+p+f=1 and 0<b<1 and 0<p<1 and 0<f<1; YbYbp Ndf NbO4: (Mg, Al) with 0<b<1 and 0<p<1 and 0<f<1, an additional doping with magnesium and aluminum being present; Yb Ndf NbO4: (Cr, Al) with 0<b<1 and 0<f<1, an additional doping with chromium and aluminum being present; Yb YbpPreNbO4 with b+e+p=1 and 0<b<1 and 0<e<1 and 0<p<1; YbNdf Fev NbO4 with b+f+v=1 and 0<b<1 and 0<f<1 and 0<v<1; Yb Ern NbO4 with b+n=1 and 0<b<1 and 0<n<1; YbNdfErf NbO4 with b+f+n=1 and 0<b<1 and 0<f<1 and 0<n<0 It preferably holds for all compounds in which Y and Yb are present that the proportion of Yb is greater than the proportion of Y. The positions and forms (intensity, width, etc.) of the excitation and/or emission bands are dependent on the quantity ratios of the elements involved, the type of elements and the type and quantity of the dopants. For protection of value documents it is possible to use both broadband and narrowband luminescence, but for reasons of selectivity the narrowband luminescence is preferred. One normally speaks of narrowband emission when the bands occurring in the emission spectrum show an average half-value width of less than 50 nm. However, this does not mean that bands having a half-value width outside this range do not also solve the inventive problem. Variation and combination of the inventive luminescent substances open up numerous possibilities for influencing the excitation and emission spectra of the inventive luminescent substances and thus producing a multiplicity of security features. Besides the evaluation of the excitation and/or emission spectra, the lifetime of luminescence or decay time can likewise be used for distinction. Evaluation can take account of not only the wavelengths of the excitation or emission lines but also their number and/or form and/or intensities, so that any desired coding can be represented. Likewise it is possible to obtain an energy transfer between like and/or unlike elements in certain inventive luminescent substances, i.e. produce a quasi-resonance, and to use it for identification. If the value document is marked, not with one, but with several, of the inventive luminescent substances, the number of distinguishable combinations can be increased further. If different mixing ratios are moreover distinguished from each other, the number of combinations can be increased again. Marking can be done either at different places on the value document or at the same place. If the luminescent sub-stance is applied or incorporated at different places on the value document, it is possible to produce a spatial code, in the simplest case e.g. a bar code. Further, the falsification security of the value document can be increased if the special selected luminescent substance e.g. in a value document is linked with other information of the value document, permitting a check by means of a suitable algorithm. The value document can of course have further additional authenticity features, such as classic fluorescence and/or magnetism, besides the inventive luminescent substance. The luminescent substances can be incorporated into the value document in a great variety of ways according to the invention. For example, the luminescent substances can be incorporated into a printing ink. It is also possible to admix the luminescent substance to the paper pulp or plastic mass during production of a value document based on paper or plastic. The luminescent substances can likewise be provided on or in a plastic supporting material, which for example can in turn be at least partly embedded in the paper pulp. The supporting material, which is based on a suitable polymer, such as PMMA, and in which the inventive luminescent substance is embedded, can have the form of a security thread, a mottling fiber or a planchet. Likewise, for product protection the luminescent substance can be e.g. incorporated directly into the material of the object to be protected, e.g. in housings and plastic bottles. However, the plastic or paper supporting material can also be fastened to any other desired object e.g. for product protection. In this case the supporting material preferably has the form of a label. If the supporting material is part of the product to be protected, as is the case e.g. with tear threads, any other design is of course also possible. In certain application cases it can be expedient to provide the luminescent substance as an invisible coating on the value document. It can be present here all over or also in the form of certain patterns, such as stripes, lines, circles, or also in the form of alphanumeric characters. To ensure the invisibility of the luminescent substance, it is possible according to the invention to use either a colorless luminescent substance in the printing ink or coating lacquer, or a colored luminescent substance in such a low concentration that the transparency of the coating is just given. Alternatively or in addition, the supporting material can also already be suitably colored, so that colored luminescent substances are not perceived due to their inherent color. The inventive luminescent substances are normally processed in the form of pigments. For better processing or to increase their stability, the pigments can in particular be present as individually encapsulated pigment particles or be covered with an inorganic or organic coating. For example, the individual pigment particles are surrounded by a silicate cover and can thus be more easily dispersed in media. Likewise, different pigment particles of a combination can be encapsulated jointly, e.g. in fibers, threads, silicate covers. It is thus e.g. no longer possible to change the “code” of the combination subsequently. “Encapsulation” refers here to complete encasing of the pigment particles, while “coating” also refers to partial encasing or coating of the pigment particles. The inventive luminescent substances are characterized in particular by their high intensity in the emission spectrum and the simple production. Furthermore, the inventive luminescent substances have the advantage that the location of the emission bands is already influenced by simple variation in the elementary composition, thereby providing a multiplicity of distinguishable feature substances. In the following, some examples of the inventive luminescent substance will be explained in more detail. For preparation, the initial substances in oxidic form or substances that can be converted to oxides are mixed in a suitable ratio, then annealed, crushed, washed (e.g. with water), dried and ground. ComponentQuantitySubstancePurity1419.55 gY2O35 N2 50.76 gEr2O34 N3105.83 gNb2O54 N4423.86 gFe2O3Carbonyl iron5500.00 gNa2SO4(ultrapure) Components 1 to 5 are mixed intimately at high turbulence. The mix is poured into crucibles (sintered ceramics based on Al2O3) and annealed at 1150° C. between 6 to 24 hours. The material cooled over 1 to 2 days after the oven is switched off is washed free of sulfate (detection limit<1 mg/ltr) and finely ground with a suitable mill. Homogeneous incorporation into the paper, plastic or a suitable printing ink is favored depending on the degree of fineness of grinding. The thus produced compound has the totals formula Y2.8Er0.2Nb0.60Fe4O12. Further embodiments and advantages of the invention will be explained in the following with reference to the FIGURE. The proportions shown in the FIGURE do not necessarily correspond to the relations existing in reality and serve mainly to improve clarity. FIG. 1 shows an embodiment of the inventive security element. The security element consists in this case of a label 2 composed of a paper or plastic layer 3, a transparent cover layer 4 and an adhesive layer 5. Said label 2 is connected via the adhesive layer 5 to any desired substrate 1. Said substrate 1 can be a value document, identity card, passport, certificate or the like, but also another object to be protected, such as a CD, package or the like. The luminescent substance 6 is contained within the volume of the layer 3 in this embodiment. Alternatively, the luminescent substance could also be contained in a printing ink (not shown) which is printed on one of the label layers, preferably on the surface of the layer 3. Instead of providing the luminescent substance in or on a supporting material which is then fastened to an object as a security element, it is also possible according to the invention to provide the luminescent substance directly in the value document to be protected or on the surface thereof in the form of a coating. |
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048511854 | summary | BACKGROUND OF THE INVENTION This invention relates to the shielding of radiation emitted by nuclear reactor components during the removal thereof from the nuclear reactor, and particularly, to a method and apparatus for absorbing radiation when such components are removed from the reactor during maintenance and refueling operations. A nuclear reactor includes a number of components within the reactor shell, such as fuel rods, extension or lifter rods supported by a grid, etc. From time-to-time, it is necessary to remove some or all of such components from the reactor for maintenance and/or refueling purposes. As is known, such components are radioactive, and it is both necessary and desirable to protect maintenance personnel with respect to radiation energy emitted by said components. For example, the federal standard for exposure of an individual to such radiation is 3000 millirems in a quarter of a year, with 5000 millirems as a maximum yearly dose, but some nuclear reactor operating companies have a requirement that the exposure not exceed 1250 millirems in a quarter of a year. It is known to use water as a shield in nuclear reactor installations during the opening of the reactor and the removal of components therefrom. For example, it is known to provide a tank (refueling pool) with concrete walls around and spaced from the reactor but having its walls extending above the top of the reactor and to fill the tank close to the top thereof with water with additives, such as boric acid, while the reactor is open to absorb most of the radiation from the reactor and its components during the time that it is open. A crane is provided above the tank for the removal of the reactor cover and such components. It is sometimes found that in some of the older nuclear reactor installations having such a tank, the depth of the tank is such that when the tank is substantially filled with water, the water does not completely cover the reactor components after they are removed from the reactor and placed in the water resting either directly on the bottom of the tank or on a stand resting on the bottom of the tank. For example, one known installation includes a component known as the "upper internals", which consists of a plurality of lifter rods, e.g. 53 rods, detachably connected to the control rods during the operation of the reactor to control the nuclear chain reaction. The height of such upper internals is such that when they are removed from the reactor and placed in the water-filled tank, they are not completely covered by the water. Thus, the upper end of the upper internals may project to a greater or lesser extent above the upper surface of the water, which means in some cases that as much as 16 inches of the upper ends of such internals will not be shielded. The radiation level of the exposed portion of the upper internals may be of the order of 100-200 millirems per hour which means that personnel exposed to such radiation may reach their radiation quota within only about six hours of exposure. Of course, if the normal refueling time of several days applies, this could mean serious reduction in the number of skilled supervisors or foremen who are continuously available. Therefore, it is desirable to use some type of shielding between the maintenance personnel and the parts of the upper internals protruding from the pool. In the past, radiation exposure of workers has been reduced by the use of lead blankets on the conventional manipulator crane which is positioned adjacent to the exposed tops of the upper internals as they rest in their stored position in the refueling pool. However, due to weight restrictions imposed by crane manufacturers, crane indexing problems caused by the weight of the lead blanket, and the difficulties encountered in attempting to produce effective shielding by placing lead blankets over such large equipment as a crane, the use of lead blankets for shielding is not entirely satisfactory. Because of other structure in the reactor containment vessel, such as cranes and their equipment, it is practically impossible to increase the depth of the tank, and hence, the depth of the water, and it would be expensive to increase the depth of the tank, bearing in mind that the tank may have concrete walls two feet thick and spaced about 20 feet apart, and the walls may be over 100 feet in length. Furthermore, an increase in the depth of the entire tank would be unnecessary. One object of the invention is to provide complete shielding for a radioactive reactor component, such as the upper internals, which is removed from a reactor and which has a height greater than the depth of the water in the water filled refueling tank when the latter is filled to capacity. SUMMARY OF THE INVENTION In accordance with the invention, the portion of a radioactive reactor component, such as the upper internals, which extends above the upper surface of he water in the existing water-filled refueling tank, is covered by an inverted vessel which is open at its bottom, but which is otherwise gas tight, so that such portion of the component is within the vessel and the bottom wall portion of the inverted vessel is immersed in the water. With the vessel supported in such position, air is evacuated from the interior of the upper portion of the vessel which causes the water level in the vessel to rise above the upper level of the surrounding water in the tank and causes the portion of the component which would otherwise project above the water level in the tank to be completely covered by the higher level of water within the inverted vessel. In this way, the radiation level outside the vessel and from the component may be reduced by as much as 30 times, or more. The invention also provides apparatus suitable for such use and includes a reinforced vessel capable of withstanding the pressures encountered due to the evacuation of air from the vessel and the forces encountered when lifting and moving the vessel. The apparatus of the invention may also include air evacuation apparatus mounted on the vessel structure, or separately mounted, and means for maintaining, lowering, and determining the water level within the vessel. |
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description | This application is related to U.S. application Ser. No. 12/568,619, filed Sep. 28, 2009 and entitled “Beam Filter Positioning Device,” the disclosure of which is incorporated herein by reference in its entirety. This disclosure relates generally to radiation apparatuses and methods and in particular to beam filter positioning devices and radiation apparatuses and systems incorporating the beam filter positioning devices, which are useful in radiation therapy including radiosurgery such as stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Radiosurgery is a highly precise, intensified form of radiation therapy. Stereotactic radiosurgery (SRS) has been used to treat brain disorders such as brain tumors and lesions. Conventionally, SRS cones are used with linear accelerators to help achieve precise delivery of high dose radiation. SRS cones are typically made from tungsten and have a conical hole through which radiation passes creating a focused treatment beam. In the prior art, SRS cones are installed on a mount assembly, which is externally attached to an interface mount on a linear accelerator. The conventional scheme for using externally mounted SRS cones is time consuming and labor intensive. Further, externally mounted SRS cones may present a potential collision hazard with the treatment couch or the patent. Beam filter positioning devices or assemblies including one or more collimators such as SRS collimators are provided. Radiation apparatuses and systems incorporating the beam filter positioning devices or assemblies are also provided. Other embodiments are described further herein. Various embodiments of beam filter positioning devices and apparatuses and systems incorporating the devices are described. While various embodiments are described in connection with stereotactic radiosurgery (SRS), it will be appreciated that the assemblies, apparatuses and systems can also be used to perform other forms of radiation therapy. The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting since the scope of the invention will be defined by the appended claims, along with the full scope of equivalents to which such claims are entitled. Various relative terms such as “above,” “under,” “upper,” “over,” “on,” top,” “bottom,” “higher,” and “lower” etc. may be used to facilitate description of various embodiments. The relative terms are defined with respect to a conventional orientation of a structure, which may not necessarily represent an actual orientation of the structure in manufacture or use. The following detailed description is, therefore, not to be taken in a limiting sense. As used in the description and appended claims, the singular forms of “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Thus, for example, reference to “an SRS collimator” may include one or more SRS collimators, and reference to “the beam filter” may include one or more beam filters described herein. As used herein, the term “axis” refers to a mechanism that is operable to move an object in a direction. For example, a “linear axis” refers to a mechanism that is operable to move an object in a linear direction. A “rotational axis” refers to a mechanism that is operable to rotate an object in an angular direction. An axis may preferably include a servo motor and one or more feedback devices that are electrically coupled to a control mechanism operable with user interface software. A close loop control can be used to control the axis and automatically adjust the position of an object in a system. As used herein the term “beam filter” refers to a member that modulates one or more parameters of a particle beam such as the energy, intensity, shape, direction, dose distribution, or other beam parameters. A particle beam includes but is not limited to a beam of electrons, photons, protons, heavy ions, or other particles. By way of example, a beam filter includes but is not limited to a photon flattening filter, an electron scattering foil, a proton scattering foil, and a collimator such as an SRS collimator. As used herein the term “radiosurgery” refers to an intensified form of radiation therapy in which focused high energy radiation is precisely delivered to a target. Radiosurgery is therefore generally performed in fewer sessions than conventional radiation therapy. Stereotactic radiosurgery (SRS) refers to the treatment of tumors or lesions or abnormalities in the brain or spinal column. Stereotactic body radiation therapy (SBRT) is typically used for targets that are outside the brain and the spine. SBRT is most commonly used for targets in the lung, liver, pancreas, breast, prostate, and kidney. The targets of radiosurgery are generally small compared to radiotherapy targets, such as targets with a largest dimension less than about 30 mm. This disclosure provides an assembly which includes a body, one or more collimators supported by the body, and one or more axes operable to move the body. Each of the collimators may have a hole configured to define the shape of a treatment beam useful e.g. in radiotherapy, radiosurgery, or stereotactic radiosurgery. The hole may be in conical, cylindrical or other suitable shape. Advantageously, the collimators can be optimized and supported by a body that optionally carries other beam filters such as photon flattening filters, electron scattering foils, etc. Therefore, the collimators can be advantageously placed in the treatment head of a radiation apparatus. Motion axes and control systems that are used to move and control positions of the other beam filters can be used to move and control the positions of collimators. One or more, and in some embodiments, two or more motion axes can be used to move the body supporting the collimators. The motion axes can be linear axes or rotational axes, or any combination of linear and rotational axes. For example, the motion axes may include a linear axis operable to translate the body and a rotational axis operable to rotate the body. In some embodiments, the assembly can be designed such that the linear axis is operable to support and translate the rotational axis which operates to rotate the body and the collimators. In other words, the rotational axis that operates to rotate the body and the collimators can be further moved by the linear axis in a linear direction. While it is not intended to limit the scope of the appended claims, in some embodiments, the assembly including a body supporting or carrying the collimators and optionally other beam filters, may be analogous to a rotatable carousel, which is further movable in a linear direction. Alternatively, the assembly can be designed such that the rotational axis is operable to support and rotate the linear axis which operates to translate the body and the collimators. That is, the linear axis which operates to translate the body and the collimators in a linear direction can be supported and rotated by the rotational axis. In alternative embodiments, two or more linear axes can be used to move the body supporting the collimators. In some embodiments, the body supporting the collimators may support and/or carry additional beam filters such as one or more photon flattening filters, one or more electron scattering foils, and elements for field light simulation such as a mirror. The collimators, photon flattening filters, electron scattering foils, and other elements can be arranged in any suitable configurations on the supporting body. By way of example, one or more collimators and one or more photon flattening filters can be arranged in a circular or an arc configuration having a first radius. One or more electron scattering foils can be arranged in a circular or an arc configuration having a second radius different from the first radius. The electron scattering foils can be arranged outer of the collimators and/or photon flattening filters. Alternatively, the collimators and/or photon flattening filters can be arranged outer of the electron scattering foils. In alternative embodiments, the collimators, photon flattening filters, electron scattering foils, and other elements can be arranged in other suitable configurations such as a running track configuration with two semi-circles connected with straight lines. The assembly may further include a target assembly and an axis operable to move a target to a desired location. An ion chamber assembly may also be included in the assembly and an axis can be used to move an ion chamber to a desired location. Therefore, in a preferred embodiment, the assembly may include two axes for moving a body supporting collimators and/or other beam filters, one axis for moving a target assembly, and one axis for moving an ion chamber assembly. Such an assembly can be supported by a support body as a modular structure, which can be disposed as a unit and secured in a treatment head. Each of the four axes may include a servo motor coupled to a control system operable with user interface software. The control system may control the motion axes to simultaneously or sequentially move the collimators, photon flattening filters, electron scattering foils, field light simulation elements, targets, and ion chambers to their desired locations in a coordinated manner. In some embodiments the disclosure provides a radiation apparatus such as a linear accelerator which includes a treatment head and one or more SRS collimators placed in the treatment head. In particular, the radiation apparatus includes a radiation source having a target configured to produce radiation when impinged by electrons, and one or more SRS collimators residing in the treatment head. The one or more SRS collimators are configured to collimate the radiation to provide a treatment beam suitable for stereotactic radiosurgery. The target may reside in the treatment head. Alternatively, the target may reside outside the treatment head. The SRS collimators may have a through hole configured to define the shape of a treatment beam at the isocenter. The through hole in the SRS collimators may be in conical, cylindrical or other suitable shape. The SRS collimators may be moved relative to the radiation source so that an SRS collimator with a particularly sized hole can be selected for a planned SRS operation. In some embodiments, the SRS cones may be positioned adjacent to the patient. The SRS collimators can be supported or carried by any suitable body member and moved by any suitable motion axis or axes. In general, any positioning device and motion axes can be used to position the SRS collimators with sufficient accuracy. In some preferred embodiments, the SRS collimators can be placed in a beam filter positioning device or assembly as described above, which supports or carries photon flattening filters, electron scattering foils, and field light mirror, etc. Therefore, in some embodiments, a body member may support SRS collimators, photon flattening filters, electron foils, and a field light mirror. The SRS collimators and photon flattening filters may be arranged in a circular or an arc configuration having a first radius, and the electron scattering foils may be arranged in a circular or an arc configuration having a second radius. The electron scattering foils can be arranged at a different radius from that of the SRS collimators and/or photon flattening filters. The body member supporting SRS collimators and various other beam filters can be moved by one or more, or two or more motion axes. The motion axes can be linear axes or rotational axes, or any combination of linear and rotational axes. For example, the body member can be moved by a combination of a linear axis and a rotational axis, in which the rotational axis may be supported and further translated by the linear axis. Alternatively, the body member can be moved by a combination of a linear axis and a rotational axis, in which the linear axis can be supported and further rotated by the rotational axis. A combination of two or more linear axes can also be used to move the body member. The disclosure further provides a radiation system. The radiation system may include a control system which is operable with user interface software and programmed to control motion axes which are designed to move various devices or components such as SRS collimators, photon flattening filters, electron scattering foils, a field light mirror, targets, and an ion chamber etc. The motion axes may include servo motors and feedback devices which are coupled to the control system. The control system may be programmed to command the servo motors such that the motion axes may move devices or components to desired locations. The control system may also receive signals from the feedback devices and command the servo motors based on the feedback signals so that the positions of SRS collimators or other beam filters, targets, or ion chamber etc. can be automatically adjusted. Exemplary embodiments are now described with reference to the figures. It should be noted that the figures are not drawn to scale, and are only intended to facilitate the description of specific embodiments. They are not intended as an exhaustive description or as a limitation on the scope of the invention. FIGS. 1 and 2 illustrate an exemplary linear axis 100 and rotational axis 200 respectively, which can be used in the assemblies or apparatuses of the disclosure. The linear axis 100 and rotational axis 200 may respectively include a motor 102, 202, a load 104, 204 coupled to the motor 102, 202, and one or more feedback devices 106, 108, 206, 208. The load 104, 204 may include a body supporting one or more collimators such as SRS collimators according to some embodiments. The load 104, 204 may also be an energy switch assembly, a target assembly, a beam filter assembly, an ion chamber assembly, a collimation assembly, an MLC assembly, or a treatment couch. The load 104, 204 may further be various other devices, units, components, or a body supporting one or more of the above described devices, units, components or assemblies. The feedback device 106, 206 may be coupled to the motor shaft to provide feedback signals which may be used to measure the position and/or velocity of the motor. The feedback device 108, 208 may be coupled to the load 104, 204 to provide feedback which may be used to measure the position and/or velocity of the load. One or more feedback devices may be coupled to the motor 102, 202 and/or one or more feedback devices coupled to the load 104, 204 respectively to provide feedback on the position and/or velocity of the motor and the load respectively. One or more feedback devices may also be coupled to the load 104, 204 each of which may independently provide feedback on the position and/or velocity of the load. The motor 102, 202 and feedback devices 106, 108, 206, 208 may be electrically coupled to a controller 114, 214. Structural features 110a, 110b, 210a, 210b define the end-of-travel of the axis 100, 200 and the range of travel of the axis. The structural features 110a, 110b, 210a, 210b can be fixed structures or hardstops the locations of which will not be changed for the life of the system. As used herein, the structural features 110a, 110b can be two independent or separate hardstops (e.g. in FIG. 1), or the structure features 210a, 210b can be one hardstop with two hard contact surfaces (e.g. in FIG. 2). The linear axis 100 and rotational axis 200 may optionally include limit switches or sensors 112a, 112b (shown in FIG. 1 not in FIG. 2) located near the hardstops. When triggered, the limit switches or sensors may signal the controller 114, 214 that the axis is approaching an end of travel. The controller 114, 214 may then reduce the axis speed and lower the peak torque capacity of the motor to avoid collision damage to the axis and increase positioning accuracy. The motors 102, 202 are preferably a servo motor such as brush servo motors, brush-less servo motors, stepper motors, linear motors, servo-controlled dual-cylinder pneumatic/hydraulic drives, or any other suitable motors. Various motors are known in the art and their detail construction is omitted herein for clarity of the description of the disclosure. The motors 102, 202 may be coupled to the load via various suitable means. For example in FIG. 1, a ball screw 103 may engage with the load 104 and be coupled to the motor 102 via a coupler. In operation the motor 102 may rotate the ball screw 103, which in turn translates the load 104 in a linear direction. In FIG. 2, a shaft 203 may engage the load 204 e.g. via a bearing assembly and be coupled to the motor 202 via a coupler. The motor 202 rotates the shaft 203, which in turn rotates the load 204 in an angular direction. Any other means of coupling known in the art may be used to transmit the driving force from the motor to the load. For example, timing belt, pulleys, rollers, nuts, guides and various other units may be used to drivingly couple the load to the motor. The feedback devices 106, 108, 206, 208 may be relative, incremental, or absolute feedback devices. The feedback devices and the controller keep track of the overall absolute positions throughout the range of motion of the axis. For example, the feedback devices can be optical encoders, magnetic encoders, transducer encoders such as resolvers or linear varying differential transducers (LVDT), and capacitive encoders. The feedback devices can be linear or rotary encoders, absolute or incremental encoders. Various encoders, resolvers, Hall sensors, tachometers and potentiometers are known in the art and commercially available and thus their detail construction is not described herein. In general, a rotary encoder is a position feedback device that sends a digital pulse as exact angular increments about a single revolution. An incremental encoder can also send an index pulse at every revolution at the same rotational angle of the device. A resolver is a rotary position feedback device that gives absolute position through one full revolution. The voltage value generated when a resolver is rotated to exactly 0 degree is called null voltage. A series of Hall sensors may be used e.g. in a brushless electric motor to detect the position of the permanent magnet. Hall sensors are typically used for motor commutation, but a system of Hall sensors, for the purpose of homing, can be considered a positioning device since it gives distinct position information of the motor per revolution. A tachometer is an analog device which returns an electrical signal (voltage) as proportional to rotation speed. A tachometer is a feedback for shaft rotational velocity. A potentiometer is an analog device which returns an electrical signal (resistance) as a function of rotation angle. A potentiometer is an angular positioning sensor. By way of example, when a feedback device such as an encoder or a resolver is coupled to a motor, the position of the rotating motor shaft can be ascertained and the position of the load connected to the motor shaft calculated by counting pulses or reading the voltages in the direction of rotation and tracking the revolutions of the encoder or resolver. For example, when a home or reference position has been established for an axis, the controller may capture the angle of the feedback device and resets the device at the reference position. As the axis is commanded to move from the reference position, the controller receives subsequent pulses or voltage signals from the feedback device each of which corresponds to a predetermined unit change in angular or rotational position of the motor. As the axis includes a mechanism that translates the motor shaft rotation into linear or angular movement of the load, the current position of the load can be calculated based on the current angle of the feedback device and the total revolutions of the device tracked and recorded by the controller. It should be noted that a linear motor and a linear encoder or any combination of suitable motors and feedback devices can be used. The controller 114, 214 may include a memory, a processor, and an input and output (I/O) device. The memory stores programs or algorithms including servo loop control algorithms and other programs for operation of various motion axes. Dimensional data of fixed structural features or hardstops in the radiation system may be provided to the controller and stored in the memory. For example, the value of distance between the hardstops for a linear axis, or the value of angle between hardstops for a rotational axis may be provided to and stored in the controller's memory. The processor executes the programs and generates commands for operation of the motion axes. The controller receives signals from the feedback devices and sensors and sends signals such as voltage and current output to command the motor via the input and output (I/O) device or system. The controller 114, 214 may be programmed to execute a servo loop algorithm such as a torque control, velocity control or position control etc., and modify the current or voltage output to the motor based on the feedback from the feedback devices. For instance, based on the actual feedback position and the desired position of the motor or the load, the controller may produce a power output required to drive the motor or the load to a desired position. The controller may be programmed to monitor the magnitude change pattern of the motor's electrical parameters such as the motor current and back EMF etc. during the motion of the axis toward a hardstop, and compare the monitored value with a predetermined value stored in the controller. The controller may monitor current using electronic circuitry designed to allow direct reading of the current sent to the motor. The controller may also be programmed to monitor the motor feedback device or load feedback device during the motion of the axis toward a hardstop. The velocity of the motor or the load may be measured by monitoring back EMF or the feedback devices coupled to the motor or load and compared with a predetermined value stored in the controller. Various methods are known by which the controller can determine velocity from feedback devices. For example, when position-based feedback devices such as encoders, resolvers, a series of Hall sensors, or potentiometers are used, the controller may compute velocity from the position difference over a given time period. With velocity-based feedback devices such as a tachometer, the controller may compute velocity from the direct feedback value times a given proportionality constant. The controller can also determine velocity using the motor's electronic characteristic of back EMF. The controller may include electronic circuitry for determining both the voltage supplied to the motor and the return voltage. By comparing these voltages along with known motor constants, the controller can compute the motor velocity. In cases where the system includes limit switches which signal the controller that the axis is approaching its end of travel, the controller may also be programmed to reduce the axis speed and lower the peak torque capacity of the motor to avoid collision damage to the axis and improve the accuracy of measurement. The controller may be programmed to execute a homing routine to establish a home position for an axis and record the home position. The controller may capture signals from the feedback devices or sensors which are indicative of the current position of the motor or the load, and calculate the current position of the motor or the load with reference to the home position that has been established. The controller may be programmed to generate alert or warning messages if it determines that certain faults occur. FIG. 3 is a schematic representation of an exemplary radiation system 300 that can embody the principle of the disclosure. In general, the radiation system 300 may include a linear accelerator system 310, a treatment head 320, a patient support system 330, and a control system 340. The linear accelerator system 310 and the treatment head 320 may be supported and enclosed in a gantry 302, which may be rotatably supported by a stand 304. The radiation system may optionally include various devices for image acquisition 352, 354, 356. The linear accelerator system 310 may include an electron gun 312 configured to produce and inject electrons into an accelerator guide 314, which may have a plurality of accelerating cavities coupled with pulsed microwave energies. An energy switch assembly 316 may be mounted to the accelerator guide 314 operable to assist in modulating the energy levels of output electron beams 316. The output electron beam 317 may be directed to the treatment head 320 which may house various device assemblies configured to produce, shape, or monitor treatment beams. A target assembly 322 may include one or more targets configured to produce X-rays upon impingement by electrons. The target assembly 322 may be moved with a linear and/or a rotational axis to position a target relative to a beam line 306. In a photon or an SRS mode operation, a target may be positioned in the beam path for producing X-ray radiation. In an electron mode operation, the target may be moved out of the beam path to allow an electron beam to pass unimpeded. In alternative embodiments, the target assembly 322 may reside outside the treatment head 320. A beam filter assembly 324 may support one or more collimators, and optionally one or more photon flattening filters and one or more electron scattering foils. The beam filter assembly 324 may also support other devices such as field light mirror etc., as will be described in greater detail below. The collimators supported by the beam filter assembly may have a conically or cylindrically shaped hole configured to define a treatment beam useful e.g. in radiosurgery, stereotactic radiosurgery, or any other forms of radiation therapy. The photon flattening filter may shape radiation to provide a uniform dose distribution across the radiation field. The electron scattering foil may scatter incident electrons to provide a broadened, uniform profile of a treatment beam. The beam filter assembly 324 can be moved for positioning a collimator, a photon flattening filter, or an electron scattering foil relative to the beam path. The beam filter assembly can be moved by one or more and in some instances two or more motion axes. For example, the beam filter assembly may be moved by two linear axes in orthogonal directions (e.g. in X-Y). The beam filter assembly may also be moved by a combination of a linear axis and a rotational axis. By way of example, the beam filter assembly may be moved by a linear axis and a rotation axis, in which the rotational axis may be supported and further translated by the linear axis. Alternatively, the beam filter assembly may be moved by a linear axis and a rotational axis, in which the linear axis may be supported and further rotated by the rotational axis. Ion chamber assembly 326 may include ion chambers configured to measure the parameters of a treatment beam such as beam energy, dose distribution, and dose rate etc. The ion chamber assembly 326 may be moved with a linear axis and/or a rotational axis relative to the beam path. In an SRS or a photon mode operation, the ion chambers 326 may be positioned under an SRS collimator or a photon flattening filter for measuring the parameters of a radiation beam. In an electron mode operation, the ion chambers may be positioned under an electron scattering foil in the beam centerline for detecting the parameters of an electron beam. Collimation assembly 328 may include upper collimator jaws and lower collimator jaws each of which may be moved by a linear or rotational axis to provide secondary collimation. The linear or rotational axes for the lower or upper collimator jaws may be independently controlled. The upper and lower collimator jaws may be housed in an enclosure and rotated by a rotational axis. Multileaf collimator (MLC) 329 may include a plurality of individual leaves each of which may be moved with a linear axis. By moving individual leaves to selected positions in a controlled manner, the size and shape of the treatment beam can be controlled. Patient support system 330 may include a base 332 and a couch top 334. Linear axes may move the couch top 334 in the lateral (x-axis) and/or longitudinal (y-axis) directions. Linear axis may also move the base 332 vertically so that the couch top 334 may be moved in the vertical directions (z-axis). Rotational axes may rotate the couch 334 about an isocenter to provide a different couch angle relative to the radiation source, or rotate the couch top 334 to provide pitch, yaw, and/or roll rotation. The radiation system 300 may optionally include devices for imaging such as imaging source 352, image acquisition devices 354 and 356 for use with keV or MV sources. Various linear and/or rotational axes may be used to move the sources and image acquisition devices in linear and/or angular directions. Control system 340 controls the operation of the radiation system 300, preferably with a computer user interface 342. The control system 340 may include a processor 344 such as a digital signal processor, a field programmable gate array, a central processing unit, or a microprocessor. The processor 344 may execute programs and generate signals for operation of the motion axes and other devices or assemblies of the accelerator system. In some embodiments, the control system 340 may include a main control unit 346 which may supervise or regulate a plurality of controllers or nodes or sub-nodes 348a-348f. Each controller or node 348a-348f may be configured to control one or more motion axes for moving or positioning one or more devices. Responsive to the commands from a controller, one or more motion axes may move one or more devices or assemblies such as an energy switch, a target, an SRS collimator, a photon filter, an electron scattering foil, field light units, a treatment couch, imaging units etc. in a controlled and automatic manner based on a plan or routine, or based on the input from a user. The controller 348a-348f may receive signals from feedback devices, sensors, or from other devices such as the ion chambers, and generate commands for adjustment when necessary. For example, based on the beam parameter signals provided by the ion chamber 326, the control system 340 may recalculate and generate commands for adjustment to various motion axes. The motion axes may respond and adjust automatically the positions e.g. of the energy switch, target, SRS collimators, photon filters, or electron foils etc. FIGS. 4, 5, and 6 illustrate an assembly 400 of this disclosure. FIG. 4 is a cut-away view of the assembly 400 showing various components or devices. FIG. 5 is a bottom view of the assembly 400 showing additional components or devices. FIG. 6 is bottom view of the assembly 400, with some devices or components not shown for clarity of showing additional devices or components. In general, the assembly 400 includes a beam filter assembly 440 including a body member 442 supporting various components such as SRS collimators 444, photon flattening filters 446, electron scattering foils 448, and field light mirror 450, etc. The assembly 400 may also include a target assembly 460 including a substrate 462 supporting one or more targets 464. As shown in FIG. 5, the assembly 400 may further include an ion chamber assembly 470, a field light projector 480, and a backscatter filter 490. The assembly may be supported by a supporting structure 402 and mounted in a treatment head of a radiation system illustrated in FIG. 3. The beam filter assembly 440 may be moved in an angular direction as indicated by arrow A-A. The beam filter assembly 440 may also be moved in a linear direction as indicated by arrow B-B. The angular movement of the beam filter assembly 440 can be accomplished by the rotational axis 404, which may be supported by a stage 408. The linear movement of the beam filter assembly 440 can be accomplished by a linear axis 406 (FIG. 6), which may be supported by the support body 402. In this embodiment, the rotational axis 404, which is supported by the stage 408, may be further moved by the linear axis 406. By moving the beam filter assembly 440 using a combination of the rotational axis 404 and the linear axis 406, a collimator 444, a photon flattening filter 446, an electron scattering foil 448 or a mirror 450 can be positioned relative to a beam centerline based on a selected mode operation. While a combination of a linear axis and a rotational axis is shown and described for illustration purpose, it should be noted that the beam filter assembly 440 can also be moved by a combination of two or more linear axes. The beam filter assembly 440 may be secured to the stage 408 via a body member 410. The body member 410 may be fixedly attached to the stage 408 via any suitable means such as e.g. pins, screws etc. The rigid attachment to the stage 408 by the body member 410 allows the beam filter assembly 440 to move with the stage 408. The body member 410 can be further rotatably coupled to the beam filter assembly 440 via a bearing assembly 412. The bearing assembly 412 allows the beam filter assembly 440 to rotate in an angular direction. FIG. 6 shows the exemplary stage 408 and linear axis 406 in greater detail. For clarity, the beam filter assembly 440, which is shown in FIG. 4, is not shown in FIG. 6. The linear axis 406, which may include a motor 406a and a ball screw 406b, can be secured to the supporting structure 402 via a mount 406c. The motor 406a operates to drive the ball screw 406b, which engages and thus moves the stage 408 and the beam filter assembly 440 attached to the stage in a linear direction. Guide rails and other mechanisms may be used to define the linear movement of the stage 408. A feedback device 406d may be coupled to the motor 406a to provide primary feedback on the position of the ball screw 406b. A separate feedback device 406e may be used to provide redundant or secondary feedback. The motor 406a can be a servo motor electrically connected to a controller which is operable with user interface software. FIG. 6 also shows the rotational axis 404 in greater detail. The rotational axis 404, which may include a motor 404a, a pulley 404b, and roller guides 404c etc., may be supported by a support body, which may be secured to the stage 408 by any suitable means. A timing belt (not shown) may be wound around the pulley 440b and the beam filter assembly structure 442b. When actuated, the motor 404a drives the pulley 404b to turn, which transmits the rotation force to the timing belt. The timing belt engages beam filter assembly 442b and rotates it in an angular direction. The roller guides 404c can be adjusted to control the timing belt tension. A feedback device 404d coupled to the motor 404a may provide primary control feedback. A second housed resolver 404e may be used to provide redundant or secondary feedback. While a specific motor, roller guides, and feedbacks are described in detail for illustrative purposes, other types of drive mechanisms or feedback devices can also be used in the motion axes. Returning to FIG. 4, the stage 408 may be configured to support a primary collimator 414. The stage 408 may have an opening for receiving the primary collimator 414, which may have a shape such as having a step at the bottom to fit in the opening and held in place. Suitable means such as pins, screws, etc. can be used to secure the primary collimator 414 to the stage 408. The primary collimator 414 can be made from tungsten or other suitable high density metals. Passageway 416 e.g. in a conical shape may be provided in the primary collimator 414 to generally define the field of radiation. The stage 408 may also support shielding 420. Shielding 420 may be located under the stage 408 and can be attached to the stage 408 by any suitable means such as pins, screws etc. Shielding 420 may be provided with a passageway 422 e.g. in a conical shape that may extend from and align with the passageway 416 in the primary collimator 414. Therefore, the primary collimator 414 and shielding 420 may be moved with the stage 408, for example, away from the beam centerline in electron mode or field light simulation operation. Referring to FIG. 6, the shielding 420 may be provided with a channel 424 on the bottom side to provide a travel path or clearance for SRS collimators 444 and photon filters 446 while rotating in an angular direction. Opening 426 in the shielding 420 allows the body member 410 to pass through to secure the beam filter assembly 440 to the stage 408. FIG. 7 shows a perspective view of a beam filter assembly 440 of this disclosure. For clarity, collimators and photon flattening filters are not shown in FIG. 7. The beam filter assembly 440 may have one or more ports or openings 452 configured to receive one or more SRS collimators 444 and/or one or more photon flattening filters 446. The beam filter assembly 440 may also support electron scattering foils 448. The SRS collimators 444, photon flattening filters 446 and electron scattering foils 448 can be arranged in any suitable configurations. In some preferred embodiments, the SRS collimators 444 and photon flattening filters 446 may be arranged in a circular or an arc configuration having a first radius. Six ports are shown in FIG. 7 for positioning the collimators 444 and/or photon flattening filters 446. It will be appreciated that a different number of ports can be provided. The scattering foils 448 may be arranged in a circular or an arc configuration having a second radius. The second radius is preferably different from the first radius. For example, the electron scattering foils 448 may be arranged outer of the SRS collimators 444 and photon flattening filters 446. Alternatively, the SRS collimators 444 and photon flattening filters 446 may be arranged outer of the electron scattering foils 448. The photon flattening filters 446 can be in various forms including e.g. conical form. The conical photon filters can be held in the ports 452 by any suitable means such as pins, screws etc. The conical filters 446 may point upwards towards the radiation source or downwards. The materials, forms and/or configuration of the photon flattening filters 446 can be chosen to match the energy of the X-rays produced based on specific applications. The electron scattering foils 448 may include primary scattering foils 448a and secondary scattering foils 448b. The combination of primary and secondary scattering foil pairs 448a, 448b may provide a broadened, uniform profile of a treatment beam. Nine pairs of electron scattering foils are shown in FIG. 7, six grouped together on one side and three grouped together on the opposite side. It will be appreciated that a different number of electron scattering foils can be provided. The primary foils 448a and secondary foils 448b may be arranged at different elevations by a structure 454 mounted to the body member 442. The structure 454 may raise the primary foils 448a above the secondary foils 448b and vertically aligns a primary foil 448a with a secondary foil 448b. The increased distance between the primary and secondary scattering foils allows the primary scattering foils to be higher in the treatment head and closer to the same elevation or location where the photon source (the target) is located. Having the source of electrons and the source of photons at an about same location is desirable since treatment planning and other design aspects of the treatment head are generally optimized around the location of the photon source. The increased separation between the primary and secondary electron foils also makes electron beam performance less sensitive to small machining variations in the thickness of the secondary foils and in the separation distance. An electron foil assembly with small separation between the upper and lower foils requires tighter tolerances on spacing and thickness of the lower foils to achieve uniform electron beam performance. FIG. 8 is a cross-sectional view of a SRS collimator 444 that can be supported or carried by the beam filter assembly 440 of this disclosure. The SRS collimator 444 can be made from tungsten or any other suitable high density materials. The SRS collimator 444 can be provided with a passageway 456 for shaping the radiation passing there through. In general, the collimator passageway 456 can be in any configurations for providing any desired shape of treatment beams. By way of example, the SRS collimator 444 may have a conically-, cylindrically-, or trapezoidally-shaped hole. In some embodiments, the size of the collimator passageway can be machined such that the provided treatment beams are suitable for radiosurgery such as stereotactic radiosurgery. Precision electrical discharge machining (EDM) or other suitable techniques known in the art can be used in manufacturing the collimators. By way of example, collimators may have a passageway 456 sized to provide a treatment beam with a projected opening with a largest dimension of 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, or 20, 25, or 30 mm. The outer dimension of the SRS collimators 444 can be in any shape such as conical, hemi-spherical, cylindrical, trapezoidal, or rectangular etc. Preferably the collimator 444 may have an outer shape and dimension that help provide shielding coverage. Separate members 458 may be used to provide additional shielding and secure the collimator in the ports on the beam filter assembly. A combination of the linear axis and rotational axis or other combination of motion axes allow for automated adjustments of the position of SRS collimators, photon flattening filters, and electron scattering foils. Motorized axes may be controlled by a computer and adjustments can be made using a software interface rather than manual adjustment as in the prior art. With a suitable 2D radiation sensor (such as a grid ion chamber array) and an automated tuning software application, these adjustments can be made without human intervention. This eliminates the need for medical physicists or radiation therapist to manually load or unload SRS cones or cone mount, which in turn minimize the amount of manual quality assurance (QA). Furthermore, since the SRS collimators can be placed within a beam filter assembly mounted in the treatment head, a potential source of collision is removed. In conventional radiation machines, SRS cones are externally mounted, which may present a potential hazard of collision with the treatment couch or the patient. It will be appreciated by one of ordinary skill in the art that the SRS collimators described herein can be moved and positioned by any number of mechanisms. The specific devices and mechanisms described above are provided for illustration purpose and therefore the present claims are not so limited. The use of two or more motion axes such as a rotational axis and a linear axis to adjust the position of SRS collimators 444, photon flattening filters 446, and electron scattering foils 448 makes it practical to place the collimators, photon filters, and electron scattering foils at a different radius of a beam filter assembly 440. To position the SRS collimators, photon filters and electron scattering foils at two or more different radii allows for a greater number of collimators, filters or foils available at two or more radii, as compared to confining the collimators, filters and foils at a same radius. A greater selection of collimators, filters, and foils may allow for a greater selection of X-ray and electron energies and radiotherapy applications. The two-radius design also allows for a smaller inner radius for the SRS collimators 444 and photon flattening filters 446. A smaller inner radius of a travel path would introduce a greater curvature in the shielding gaps, hence substantially reducing the direct radiation leakage paths which would otherwise require heavy and expensive shielding plugs. The use of a separate inner radius for SRS collimators and photon filters allows for a large, simple and effective primary collimator 414. Prior art designs have significant compromises to the primary collimator below the target. In most prior designs, the primary collimator is fixed and chopped up in complex and inefficient ways to allow motorized filters and foils to penetrate it. Earlier designs place primary collimator shielding further from the radiation target requiring significantly greater mass, complexity and cost of shielding components. Returning to FIG. 4, the assembly 400 may further include a target assembly 460. The target assembly 460 positions a target in the beam path for generation of X-rays in an SRS mode or a photon mode, or moves a target out of the beam path in an electron mode. The target assembly 460 includes a substrate 462 and one or more target buttons 464 supported by the substrate. The target assembly 460 can be fixedly attached to the base plate 402 via a mount 466. The target assembly 460 can be driven by a linear axis 468. Alternatively, the target assembly may be driven by a rotational axis. The linear axis 468 may be similar to the linear axis 406 described above in connection with the stage 408, and thus detail description of its construction and operation is omitted. The target assembly 460 may include one or more targets each being optimized for the energy of an incident electron beam. For example, the target assembly 460 may include a first target adapted for an SRS mode, a second target for a photon mode, a third target for a second SRS or second photon mode, etc. The material of a target can be chosen and/or the thickness of a target be optimized for an incident electron beam with a particular energy level. In operation, the linear axis 468 moves or positions one of the targets in the beam path for an SRS mode or a photon mode operation. In an electron mode, the linear axis 468 removes the targets 464 out of the beam path to allow an electron beam passes unimpeded. FIG. 9 illustrates an exemplary assembly 400 in a photon mode operation in accordance with some embodiments of the disclosure. The primary collimator 408, shielding 420, ion chamber 470, and backscatter filter 490 have been positioned in the beam centerline using linear axis 406 and 474 respectively. Rotational axis 404 is actuated to move the filter-foil assembly 440 in an angular direction to align one of the photon flattening filters 446 in the beam centerline. Sequentially or simultaneously, the linear axis 468 is actuated to position a target button 464 in the beam centerline. An electron beam 902 impinges the target button 464 and X-rays 904 are produced. The field of X-rays 904 is shaped as the X-rays pass through the passageways in the primary collimator 408 and shielding 420. A radiation beam with a uniform dose distribution is obtained as the X-rays pass through a photon flattening filter 446. The parameters of the treatment beam are detected as the beam passes through the ion chamber 470. Backscatter filter 490 located under the ion chamber 470 blocks backscatter radiation from entering the ion chamber 470 to ensure accurate measurement of the radiation beam parameters. Because the mirror 450 is installed on the filter-foil assembly 440 and is off the beam centerline in the photon mode, the treatment beam generated pass downstream unimpeded by the mirror. Depending on the energy of an incident electron beam 902 for a particular application, the linear axis 468 may move the target assembly 460 to position a target button 464 that is optimized for such beam energy in the beam path for optimized performance of the target. Similarly, depending on the energy of an incident electron beam, the rotation axis 404 may rotate to position a flattening filter 446 that is optimized for such beam energy in the beam centerline for optimized performance of the filter. FIG. 10 illustrates an exemplary assembly 400 in an electron mode in accordance with some embodiments of the disclosure. In an electron mode, linear axis 468 moves the target assembly 460 to retrieve the target from the beam centerline. Linear axis 406 drives the stage 408 to move the primary collimator 408, shielding 420, and backscatter filter 490 away from the beam centerline. Because the electron scattering foils 408 have a different or greater radius than the photon flattening filters 446 on the filter-foil assembly 440, driving the filter-foil assembly 440 to move the flattening filters 446 away from the beam centerline would bring the scattering foils 448 to the beam centerline. Rotational axis 404 moves the filter-foil assembly 440 in an angular direction to align one of the electron scattering foils 448 with the beam centerline. The primary and secondary scattering foils 448 scatter the electron beam to produce a broadened, uniform profile of a treatment beam 1004. Depending on the energy of an incident electron beam for a particular application, the rotational axis 404 may rotate the filter-foil assembly 440 to align a scattering foil that is optimized for such beam energy in the beam path for optimized performance of the foil. The parameters of the treatment beam are detected as the beam passes through the ion chamber 470. FIG. 11 illustrates an exemplary assembly 400 in a field light simulation mode in accordance with some embodiments of the disclosure. Linear axis 474 drives the ion chamber assembly 470 to move the ion chamber away from the beam centerline. Linear axis 406 drives the stage 408 to move the primary collimator, shielding, and backscatter filter away from the beam centerline. Because the mirror member 450 has a greater radius than the photon flattening filters 446 on the beam filter assembly 440, driving the beam filter assembly 440 to move the flattening filters 446 away from the beam centerline would bring the mirror member 450 to the beam centerline. Rotational axis 404 moves the beam filter assembly 440 in an angular direction to position the mirror 450 in the beam centerline. The ion chamber axis 474 moves and adjusts the position of a light source 480 to project the source to a virtual radiation source position 1102. Mirror 450 reflects light projected from the light source 480 to illuminate an area e.g. on the surface of a patient's skin for simulation. FIG. 12 illustrates an exemplary assembly 400 in an SRS mode operation in accordance with some embodiments of this disclosure. The primary collimator 408 and shielding 420 are positioned and aligned in the beam centerline. The ion chamber 470 and the backscatter filter 490 are also positioned in the beam centerline. Rotational axis 404 moves the beam filter assembly 440 in an angular direction to align an SRS collimator 444 in the beam centerline. Sequentially or simultaneously, the linear axis 468 is actuated to position a target button 464 in the beam centerline. An electron beam 1202 impinges the target button 464 and X-rays are produced. The field of X-rays is generally shaped as the X-rays pass through the primary collimator 408 and shielding 420. The field of radiation is further defined by the SRS collimator 444 to provide a focused treatment beam 1204, with a projected size that is suitable for radiosurgery or stereotactic radiosurgery. Additional collimation devices such as collimator jaws or MLC leaves may offer additional collimation of the treatment beam outside of the area shielded by the SRS collimator 444. FIG. 13A is a partial, cross-sectional view of the assembly 400 showing the collimation of the beam 1302 by the primary collimator 408, shielding 420, and SRS collimator 444. Additional collimator jaws (not shown in FIG. 13A) may offer additional collimation, providing a projected beam at the isocenter shown in FIG. 13B. In FIG. 13B, the four rectangular regions 1304a, 1304b, 1304c, and 1304d represent shielding provided by the collimator jaws. The annular region 1306 represents the shielding provided by the SRS collimator 444. The central circular region 1308 represents the focused treatment beam passing through the hole in the SRS collimator 444 and unimpeded by the collimator jaws. The parameters of the treatment beam are detected as the beam passes through the ion chamber 470. FIG. 14A shows that the SRS collimator 444 can be shaped, mounted and/or supported in a way that allows projection of the treatment beam 1402 onto the ion chamber 470, allowing the ion chamber to be effectively used as a feedback and safety device. FIG. 14B illustrates the beam projection at the center of the ion chamber 470 at a cross-section taken along line A-A in FIG. 14A. The inner annular region 1404 represents beam attenuation by the SRS collimator 444 (e.g. 95% attenuation). The outer annular region 1406 represents the beam attenuation outside the area of the SRS collimator 444 (e.g. 0% attenuation). The central circular region 1408 represents the beam attenuation along the path of the hole in the SRS collimator 444 (e.g. 0% attenuation). The beam passing through the hole in the SRS collimator (1408) and outside the area of the SRS collimator (1406) is projected onto the ion chamber 470, providing sufficient beam for detection by the ion chamber 470. Backscatter filter 490 located under the ion chamber blocks backscatter radiation from entering the ion chamber to ensure accurate measurement of the radiation beam parameters. One of the advantages of the assembly of the disclosure is that it can be configured to automatically adjust the position of collimators, beam filters, field light assembly, or other device components. The automatic adjustment can be accomplished by a control system operable by a computer software interface such as a Graphical User Interface (GUI). The control system may include a processor such as for example, a digital signal processor (DSL), a central processing unit (CPU), or a microprocessor (μP), and a memory coupled to the processor. The memory serves to store programs for the operation of the beam filter positioning device and other programs. The processor executes the program and generates signals for operation of the motion axes or other components of the assembly. Responsive to the signals from the control system, the assembly operates in which one or more motion axes move the collimators, beam filters, field light source, mirror, or other device components in a controlled and automatic manner based on a plan or routine, or based on a demand input from a user. The control system also receives feedback signals from sensors or resolvers in the motion axes, or from other device components such as the ion chamber, and generates signals for adjustment when necessary. Exemplary embodiments of beam filter assemblies, radiation apparatuses, and radiation systems have been described. Those skilled in the art will appreciate that various modifications may be made within the spirit and scope of the invention. All these or other variations and modifications are contemplated by the inventors and within the scope of the invention. |
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description | The invention relates to a method for thinning a sample, the method comprising: providing a sample attached to a probe, providing a sample carrier, said sample carrier showing a rigid structure, said rigid structure showing an boundary to which the sample can be attached, attaching the sample to the boundary of the rigid structure, and exposing the sample to a milling process or an etching process so as to at least partially thin the sample. The invention further relates to a sample carrier equipped to perform the method according to the invention. Such a method is known from “The total release method for FIB in-situ TEM sample preparation”, T. M. Moore, Microscopy Today, Vol. 13, No. 4, pages 40-42, more specifically page 40, column 2 as “The total release method for in-situ lift-out”. Such a method is used in e.g. the semiconductor industry, where samples are taken from semiconductors for inspection/analysis in e.g. a Transmission Electron Microscope (TEM) by irradiating the wafers with a particle beam, such as a beam of gallium ions. The impinging beam causes removal of material, also known as milling or sputtering of the material. As known to the person skilled in the art, a semiconductor sample to be inspected in a TEM must be extremely thin, preferably 50 nm or less. As the sample taken out of a semiconductor wafer is often much thicker, the sample needs to be prepared by thinning it after its extraction from the wafer. To obtain a sample with the correct orientation (e.g. along certain crystal orientations of the wafer of which the sample was a part), the sample orientation must be controlled to within e.g. 1 degree during the milling process. Incorrect alignment may result in warping of the sample during milling, but may e.g. also result in problems during the subsequent inspection/analysis. In the known method a sample is cut from the semiconductor wafer. The sample may be cut in e.g. a Focused Ion Beam apparatus (FIB) in two ion milling steps, thereby freeing a wedge from the wafer. After separation the sample is then welded to a probe using e.g. Ion Beam Induced Deposition (IBID) and transported to a TEM sample carrier (the so-named lift-out grid). The sample is then welded to the sample carrier (using e.g. IBID) and the probe is detached from the sample by cutting the probe tip. The sample is then exposed to a milling process in the form of FIB milling to thin it to the required thickness. A disadvantage of the known method is that welding the probe to the sample and severing the sample from the probe takes time. This reduces throughput, which obviously is very important in the industrial environment of the semiconductor industry. It is remarked that a method is known in which the sample is picked-up with an electrically charged glass needle without making a weld, after which the sample is laid on a conductive membrane (the so-named ex-situ method described in the same article “The total release method for FIB in-situ TEM sample preparation”, T. M. Moore, Microscopy Today, Vol. 13, No. 4, pages 40-42, more specifically page 40, column 2). However, it is then not possible to thin the sample after extraction of the sample from the wafer. Ex-situ lift-out also makes it more difficult to do so-named end-pointing. End-pointing is the process performed to determine when the thinning must be stopped and is preferably done by observing e.g. the electron transparency of the sample during the thinning process (either continuously or by temporary interrupting the thinning process). End-pointing then requires free access to the sample from both sides: one side to direct an electron beam to and the other side to detect transmitted electrons from. Using the ex-situ method, both sides of the sample are formed between the two surfaces of the wafer, and are thus not freely accessible. Another disadvantage of the known method is that, after attaching the sample to the sample carrier, the orientation of e.g. the crystallographic axes in the sample with respect to the sample carrier is insufficiently reproducible. As a result human intervention is needed to determine the orientation and align the sample during the milling process. This hinders automation of the thinning process. The invention aims to provide a method with higher throughput and improved parallelism between sample and sample carrier. The invention describes a sample carrier for thinning a sample taken from e.g. a semiconductor wafer. In one embodiment, the sample carrier comprises a rigid structure of, for example, copper with an outer boundary and a supporting film, e.g., made of carbon, extending beyond the outer boundary. By placing the sample on the supporting film, the sample can be attached to the rigid structure using Electron Beam Induced Deposition (EBID), Ion Beam Induced Deposition (IBID), Laser Beam Induced Deposition (LBID), or by placing a small droplet of glue or resin between the sample and the rigid structure. The supporting film aligns the samples when they are placed onto it. After attaching the sample to the rigid structure the sample can be thinned with an ion beam, during which thinning the supporting film is locally removed as well. The invention results in better alignment of the sample to the sample carrier, and also in more freedom how the sample is transported from the wafer to the sample carrier, e.g. with the help of an electrically charged glass needle. The latter eliminates the attaching/severing steps that are normally associated with the transport of a sample to a sample carrier. To that end the method according to the invention is characterized in that the sample carrier is provided with a supporting film adhered to said rigid structure, said supporting film at least partially extending beyond the boundary to which the sample can be attached, and before attaching the sample to the rigid structure the sample is placed on the supporting film. The sample carrier used in the method according to the invention comprises a rigid structure to which the sample is attached, and a supporting film on which the sample is placed before attaching the sample to the rigid structure of the sample carrier. By placing the sample on the supporting film, the sample can then be attached to the rigid structure with e.g. Electron Beam Induced Deposition (EBID), Ion Beam Induced Deposition (IBID), Laser Beam Induced Deposition (LBID), or e.g. by placing a small droplet of glue or resin between the sample and the rigid structure. It is remarked that the sample can be placed on either side of the supporting film. The supporting film can be very thin and thereby sufficiently transparent to a beam of light or e.g. electrons to allow good positioning with respect to the rigid structure. Welding the sample to the sample carrier can be done by e.g. glue, in which case the sample is attached to the supporting film, but also to the rigid structure. The latter is possible with e.g. EBID or IBID, as the supporting film is while performing EBID or IBID locally removed, so that a weld to the rigid structure is formed. The method according to the invention allows the sample to be picked up with e.g. an electrically charged glass needle and deposited on the film, thereby eliminating the time consuming steps of attaching the sample to the probe and severing the sample from the probe. As the sample is placed on the supporting film, the sample will also align itself with the plane of the supporting film. This enables thinning the sample to a uniform thickness along a well defined plane, and thereby enables further automating the process of sample preparation. In an embodiment of the method according to the invention the sample is placed on the surface of the film opposite to the rigid structure. Placing the sample on the side of the supporting film opposite to the rigid structure allows easier positioning of the thin sample. It is remarked that the sample often shows a thickness comparable to or smaller than the thickness of the rigid structure. The side of the supporting film is however a flat surface, allowing said easier positioning. In a further embodiment of the method according to the invention a part of the supporting film is, after placing the sample on the supporting film, situated between a part of the sample and the rigid structure. By positioning the sample such that a small part of the film is sandwiched between a part of the sample and the rigid structure, still easier positioning is achieved. A part of the sample is in this embodiment overlapping with the rigid structure, so that a weld can easily be formed between the sample and the rigid structure, while another part of the sample does not show such an overlap, so that imaging that part of the sample in transmission mode is possible. It is remarked that the supporting film does not hinder the thinning of the sample as it is easily removed by the milling process or the etching process that is also used for the thinning of the sample. In another embodiment of the method according to the invention the boundary is an outer boundary, so that the supporting film is not completely surrounded by the rigid structure. Milling the sample to the required thickness is e.g. done by irradiating the sample with a beam of particles, in which the beam irradiates the sample at an angle almost parallel to the surface. Therefore sideway access to the sample is required. To make this possible the sample is preferably attached to an outer portion of the rigid structure. This implies that the supporting film at least locally extends outward of the outer boundary of the rigid structure, and is thus not completely surrounded by the rigid structure. In yet another embodiment of the method according to the invention milling the sample comprises exposing the sample to a particle beam. Milling a sample with a beam of particles, such as a beam of ions or electrons, is a process known to the person skilled in the art. Milling may be performed by a particle beam alone (e.g. using an ion beam), but may also be performed in the presence of certain etchants. The etching effect of these etchants may be triggered or greatly enhanced by the particle beam. In still another embodiment of the method according to the invention attaching the sample is performed with EBID or IBID or LBID. Connecting two work pieces with EBID, IBID or LBID is a process known to the person skilled in the art. In another embodiment of the method according to the invention during the milling process the supporting film is at least partly removed. During milling material is often removed from both sides of the sample. This implies that the supporting film is removed at the location where—before attaching the sample to the rigid structure—it supported the sample. In a further embodiment of the method according to the invention the milling process completely removes the supporting film from at least the thinned part of the sample. By completely removing the supporting film from at least the thinned part of the sample not only milling of both sides of the sample is possible, but it also avoids contamination of the sample or at least the presence of supporting film material on the thinned part of the sample, which could hinder further analysis of the sample. In an aspect of the invention a sample carrier for carrying a sample in a particle-optical apparatus, said sample carrier provided with a rigid structure showing an outer boundary to which a sample can be attached, is characterized in that the sample carrier comprises a supporting film for supporting the sample, said supporting film adhered to said rigid structure and extending from the outer boundary, so that the supporting film is not completely surrounded by the rigid structure. It is remarked that a sample carrier provided with a rigid structure in the form of e.g. a copper gauze and a supporting film of e.g. carbon. Such sample carriers are routinely used in TEM microscopy. In such grids the supporting film is completely surrounded by the outer boundary of the rigid structure, contrary to the sample carrier according to the invention, in which the support film extends beyond the outer boundary of the rigid structure. In an embodiment of the sample carrier according to the invention the supporting film is a carbon film or a polymer film. It is a well known technique to produce flat film of carbon or polymer to form supporting films on TEM grids. This same technique can be used for producing the sample carriers according to the invention. In another embodiment of the sample carrier according to the invention the rigid structure comprises a metal. The production of sample carriers from a metal, such as copper, stainless steel, iron, molybdenum, aluminium, titanium, silver, platinum, or alloys comprising one or more of these materials, is known to the person skilled in the art. FIG. 1A schematically depicts a sample carrier according to the invention and a probe with the sample approaching the sample carrier. FIG. 1A shows a sample 1 attached to probe 2. The sample carrier 3 comprises two parts, the supporting film 4 and the rigid part 5. The rigid structure shows an outer boundary 6. The supporting film partially extends beyond this outer boundary 6 of the rigid structure 5. The rigid part of the sample carrier may be a copper foil. Also other materials may be used, preferable electrically conductive. Also the supporting film is preferably electrically conductive, e.g. a carbon film or a film of a conductive polymer. However, also the use of supporting films made from silicon or e.g. nitride can be envisaged. The sample can be attached to the probe by electrostatic forces, e.g. to a glass electrode that is electrically charged. Also other means of attachment may be used, such as based on glue, Beam Induced Deposition (laser, ion or electron), a mechanical gripper or e.g. using vacuum force (suction). The supporting film is preferably a very thin film with a thickness of e.g. 100 nm or less. The supporting film is therefore transparent to observation methods using e.g. light or Scanning Electron Microscopy, so that the sample is easily aligned to the rigid structure at the other side of the supporting film. However, much thicker films of several hundreds of nanometers have been used successfully. An advantage of such thicker films is that it is easier to produce a flat film in the same plane as the rigid part of the sample carrier. FIG. 1B schematically depicts the sample carrier where the probe positions the sample on the supporting film. FIG. 1B can be thought to be derived from FIG. 1A. Probe 2 and sample 1 are moved to the sample carrier 3 and positioned such, that the sample may be placed on supporting film 4 and close to or overlapping the rigid structure 5. The sample is placed on the side of the supporting film opposite to the rigid structure. FIG. 1C schematically depicts the sample carrier and the sample attached to it. FIG. 1C can be thought to be derived from FIG. 1B. Probe 2 is now removed, and sample 1 is attached to the rigid structure 5 with a weld 7. This weld can be a weld made with EBID, IBID or LBID, or it can e.g. be a weld made by the deposition of a droplet of glue. It is remarked that this weld needs to be present at the moment the thinning starts, but that it is not necessary to do so at the moment that the probe it removed. Experiments show that the sample sticks to the sample carrier sufficiently to keep its position on the sample carrier, and that it can even be transported unwelded to another (dedicated) apparatus where the thinning takes place. It is further remarked that the weld will often be formed to the rigid structure, as the supporting film is too fragile to withstand e.g. Beam Induced Deposition (EBID, IBID or LBID). FIG. 1D schematically depicts a cross section of the sample carrier and the sample attached to it. FIG. 1D shows a cross-sectional view of the sample carrier depicted in FIG. 1C along line AA′ can be thought to be derived from FIG. 1C. The supporting film 4 is shown to adhere to the rigid structure 5. As can be seen, sample 1 is supported by the supporting film 4 and attached to the outer edge 6 of rigid structure 5. FIG. 1E schematically depicts a detail from FIG. 1D after thinning of the sample. FIG. 1E schematically shows area B from FIG. 1D after thinning the sample. Supporting film 4 is not a continuous film anymore, but a cut-out is made in it during the thinning of the sample (as will be shown in FIG. 1F). Rigid structure 5 and sample 1 are connected by weld 7. In the thinning process sample 1 is at side 8A thinned, while at side 8B it retained its original thickness so as not to damage the weld, what could lead to loss of the sample. The sample is thus not completely thinned to a uniform thickness, which would be difficult in the direct proximity of the weld and rigid structure. However, the part of interest of the sample is thinned to a uniform thickness and no material from the supporting film is left on the thinned part of the sample. In this figure it is also shown that the sample and the sample carrier are not in direct contact with each other, but that a small gap exists between them: gap 9. Such a gap may be present as long as weld 7 can bridge the gap, but such a gap need not be present. FIG. 1F schematically depicts the sample carrier and the thinned sample attached to it. FIG. 1F can be thought to be derived from FIG. 1C. As was already shown in FIG. 1E the sample is thinned, which caused a cut-out 10 to be formed in the supporting film 4. The sample is now thinned and ready for inspection/analysis in e.g a TEM or a Scanning Transmission Electron Microscope (STEM), or another instrument in which the sample needs to show a defined surface or thickness. It is remarked that it is envisaged that a sample is first placed on the supporting film, then attached to the rigid structure, after which the supporting film is removed, by e.g. milling, etching or dissolving the supporting film. FIG. 2 schematically depicts an alternative sample carrier according to the invention. FIG. 2 can be thought to be derived from FIG. 1A. The sample carrier 3 depicted here not only comprises an outer boundary 6 to which samples can be attached, but also an area with a gauze or mesh, that is: an area with a multitude of cut-outs 10. The supporting film 4 extends over the area of the gauze, thereby creating multiple locations where a sample can be placed on the supporting film. By forming the supporting film sufficiently thin to be transparent to e.g. an electron beam of a TEM, the sample carrier can also be used as a conventional TEM grid. As known to the person skilled in the art, such supporting films are known from conventional TEM grids. It is remarked that it is also envisaged that the supporting film does not cover or does not completely cover the gauze, and that the sample is completely supported to or solely attached to the rigid structure. It is further envisaged that a sample is first placed on the supporting film, then attached to the rigid structure, after which the supporting grid is removed, by e.g. milling, etching or dissolving the supporting film. This enables the use of a supporting film that in itself is not sufficiently thin to be penetrated by an electron beam, or at least so thick that they would hinder proper analysis of the sample. It is remarked that inspection/analysis of the sample can take place in another apparatus than in the apparatus in which the thinning takes place, but that it is also envisaged that it takes place in the same apparatus. |
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claims | 1. A method for exposing an object to X-rays comprising the steps of:providing an X-ray machine including an X-ray tube equipped for emitting X-rays with an energy lower than or equal to 70 keV and a phototimer coupled to said X-ray tube for switching said tube on and off in accordance with an X-ray dose in the range from 0.75 up to 0.85 mR reaching said phototimer,placing an object between said X-ray tube and said phototimer,placing a cassette with a binderless storage phosphor panel or screen between said object and said phototimer andactivating said X-ray tube for exposing said object, said cassette and said phototimer until said phototimer switches said X-ray tube off, wherein said binderless storage phosphor panel comprises a vacuum deposited phosphor layer (1) on a support (2), and wherein said support includes a layer of amorphous carbon (23) having a thickness between 500 μm and 2000 μm. 2. Method according to claim 1, wherein said support further includes a reflective auxiliary aluminum layer (22) with a thickness between 0.2 μm and 200 μm. 3. Method according to claim 2, wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 4. Method according to claim 3, wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 5. Method according to claim 4, wherein said phosphor layer comprises a needle shaped CsX:Eu phosphor, wherein X represents a halide selected from the group consisting of Br and Cl. 6. Method according to claim 5, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 7. Method according to claim 4, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 8. Method according to claim 3, wherein said phosphor layer comprises a needle shaped CsX:Eu phosphor, wherein X represents a halide selected from the group consisting of Br and Cl. 9. Method according to claim 8, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 10. Method according to claim 3, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 11. Method according to claim 2, wherein said phosphor layer comprises a needle shaped CsX:Eu phosphor, wherein X represents a halide selected from the group consisting of Br and Cl. 12. Method according to claim 11, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 13. Method according to claim 2, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 14. Method according to claim 1, wherein said support further includes a protective auxiliary layer (21) between said reflective auxiliary layer and said phosphor layer. 15. Method according to claim 14, wherein said protective auxiliary layer (21) is a layer of parylene wherein said parylene is selected from the group consisting of parylene C, parylene D and parylene HT. 16. Method according to claim 15, wherein said phosphor layer comprises a needle shaped CsX:Eu phosphor, wherein X represents a halide selected from the group consisting of Br and Cl. 17. Method according to claim 16, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 18. Method according to claim 15, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 19. Method according to claim 14, wherein said phosphor layer comprises a needle shaped CsX:Eu phosphor, wherein X represents a halide selected from the group consisting of Br and Cl. 20. Method according to claim 19, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 21. Method according to claim 14, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 22. Method according to claim 1, wherein said phosphor layer comprises a needle shaped CsX:Eu phosphor, wherein X represents a halide selected from the group consisting of Br and Cl. 23. Method according to claim 22, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 24. Method according to claim 1, wherein said support further includes a polymeric auxiliary layer (24) farther away from said phosphor layer than said layer of amorphous carbon. 25. Method according to claim 1, wherein said method is a mammographic application method. |
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abstract | A movable device for measuring physical quantities of nuclear materials contained in a shielded cell, which device can be brought up against the shielded cell and can be retracted therefrom, the device configured to carry out the measurement in the position in which it is against the shielded cell. The device includes a carriage, a support member placed on the carriage, and a shielded container placed on the support member. The shielded container includes a transfer container configured to store the nuclear material to be measured, and an opening configured to be aligned with an opening in one wall of the shielded cell. The support member is made of graphite and includes a housing accommodating a neutron emission module, a casing covering the shielded container, the casing being made of graphite, and a neutron measurement mechanism fastened to the casing. |
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claims | 1. A detector module for a modular x-ray detector comprising:a plurality of x-ray detector substrates and associated anti-scatter collimators,wherein each x-ray detector substrate has a plurality of detector diodes, and each x-ray detector substrate has an associated anti-scatter collimator,the x-ray detector substrates are tiled with respect to each other to form the detector module,the anti-scatter collimators are anti-scatter foils or plates that are interleaved between the x-ray detector substrates,the anti-scatter foils or plates are thinner than the x-ray detector substrates,the diodes of the x-ray detector substrates define an active detector volume,the x-ray detector substrates and the anti-scatter foils or plates having a length direction, the anti-scatter foils or plates and the x-ray detector substrates being aligned to point an x-ray source in the length direction,each x-ray detector substrate has an integrated circuit configured to collect x-ray signals from the diodes, the integrated circuit being attached to the x-ray detector substrate at the bottom of the x-ray detector substrate in the length direction of the x-ray detector substrate, the top of the x-ray detector substrate being where the x-rays enter, andthe associated anti-scatter collimator is disposed above the integrated circuit to protect the integrated circuit from radiation. 2. The detector module of claim 1, wherein the integrated circuits are Application Specific Integrated Circuits (ASICs). 3. The detector module of claim 2, wherein the ASIC is extending over the edge of the x-ray detector substrate so that part of the ASIC is outside of the silicon detector substrate to enable connection of power and data transfer to the ASIC without having to route the connection of the power and data transfer to the ASIC on the silicon detector substrate. 4. The detector module of claim 2, wherein signals are routed from the individual diodes to inputs of the ASIC. 5. The detector module of claim 2, wherein power lines and data transfer lines are wire-bonded to power and data transfer pads at the ASIC outside the substrate, or a redistribution layer on the substrate is used to connect to power, data transfer pads and to input signal pads and redistribute the input signals from the x-ray detector substrate to the ASIC. 6. The detector module of claim 2, wherein a heat conductor is attached to the ASIC to provide cooling. 7. The detector module of claim 1, wherein the anti-scatter foils are made of a heavy material. 8. The detector module of claim 7, wherein the heavy material is Tungsten. 9. The detector module of claim 1, wherein the integrated circuits are Application Specific Integrated Circuits (ASICs), the anti-scatter collimators are Tungsten foils, and the ASICs are placed on the x-ray detector substrate under the Tungsten foils to minimize dead area in the detector and so the ASICs are protected from direct radiation. 10. The detector module of claim 1, wherein a tapered geometry in which, for each x-ray detector substrate, the x-ray detector substrate and the associated anti-scatter collimator are pointing back to the source is provided by a spacer placed at the silicon detector substrate or at the anti-scatter collimator. 11. The detector module of claim 1, wherein each x-ray detector substrate and corresponding integrated circuit is formed as a sensor multi-chip module (MCM) assembly, and a plurality of sensor MCM assemblies are connected into the detector module. 12. The detector module of claim 11, wherein the detector module is sub-divided into a plurality of detector tiles, where each detector tile includes a plurality of sensor MCM assemblies. 13. The detector module of claim 12, wherein each detector tile includes a circuit configured to demultiplex commands from the corresponding detector module to the detector tile to reduce the number of connections between the detector tile and the detector module. 14. The detector module of claim 13, wherein the commands are control commands directed to the sensor MCM assemblies. 15. The detector module of claim 12, wherein the detector module comprises a plurality of data storage circuits and data processing circuits, wherein each detector tile is managed by a data processing circuit. 16. The detector module of claim 15, wherein the detector module includes a control and communication circuit configured to distribute control commands for the sensor MCM assemblies and control readout of stored scanning data from the data storage circuits. 17. The detector module of claim 1, wherein the x-ray detector substrates are Silicon detector substrates. 18. A modular x-ray detector comprising:a plurality of the detector modules of claim 1. |
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claims | 1. A system for storing hazardous waste material, the system comprising:a container configured to sealingly contain hazardous waste material;a first cell, the first cell comprising a filling station configured to transfer hazardous waste material into the container;a second cell, the second cell comprising an evacuation system configured to evacuate the container that contains the hazardous waste material, the second cell being isolated from the first cell, the first cell held at a first pressure and the second cell held at a second pressure, the first pressure being less than the second pressure; andan interlock, the interlock coupling the first cell to the second cell;wherein the first cell, second cell and interlock are configured to transfer the container that contains the hazardous waste material from the first cell to the second cell while maintaining at least one seal between the first cell and the second cell. 2. The system of claim 1, wherein the filling station includes:a blender configured to mix the hazardous waste material with additives;a hopper coupled to the blender; anda fill nozzle coupled to the hopper and configured to removably and sealingly couple to the container and transfer the hazardous waste material and additive mixture into the container. 3. The system claim 1, wherein the hazardous waste material includes calcined material. 4. The system of claim 1, wherein the first cell does not exchange air with the second cell while hazardous material is being transferred into the container by the filling station. 5. The system of claim 1, wherein the filling station includes:an off-gas sub-system having a vacuum nozzle configured to removably and sealingly couple to the container. 6. The system of claim 1, wherein the second cell comprises a baking and sealing station. 7. The system of claim 6, wherein the baking and sealing station is configured to seal a filling port of the container. 8. The system of claim 6, wherein the baking and sealing station includes an orbital welder. 9. The system of claim 6, wherein the baking and sealing station includes a welding station, a bake-out furnace and the evacuation system, the evacuation system having a vacuum nozzle configured to removably and sealingly couple to the container. 10. The system of claim 1 further comprising:a third cell, the third cell being isolated from the first cell and the second cell, the second cell and third cell configured to transfer the container from the second cell to the third cell. 11. The system of claim 10, wherein the third cell comprises a hot isostatic pressing station. 12. The system of claim 10, wherein the third cell is held at a third pressure, the third pressure being greater than the second pressure. 13. The system of claim 10 further comprising:a fourth cell, the fourth cell being isolated from the first cell, the second cell and the third cell, the third cell and fourth cell configured to allow the container to be transferred from the third cell to the fourth cell. 14. The system of claim 13, wherein the fourth cell comprises a cooling and packing station. 15. The system of claim 13, wherein the fourth cell is held at a fourth pressure, the fourth pressure being greater than the third pressure. 16. The system of claim 1, wherein the interlock includes decontamination equipment. 17. The system of claim 1, further comprising:a recycle line configured to add secondary hazardous waste into the container, and the secondary hazardous waste includes an evacuation filter used during evacuation of previous containers. 18. The system of claim 17, wherein the secondary hazardous waste includes mercury evacuated from previous containers. |
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043022898 | claims | 1. Method of refuelling in a light water boiling nuclear-reactor having a core containing a plurality of fuel rod bundles which are built up from a plurality of fuel rods, comprising the steps of replacing at least one burnt-up fuel rod bundle with a fuel rod bundle which is at least partly composed of fuel rods from burnt-up fuel rod bundles from said reactor, and of selecting, when composing said composed fuel rod bundle for said light-water boiling reactor having uranium dioxide and any plutonium dioxide as fuel, said burnt-up fuel rod bundles having a maximum content of fissile material in the form of U 235, Pu 239 and Pu 241 of 1.75% of the initial weight of uranium and any plutonium in the fuel the mean content of fissile material in the fuel rod bundle thus composed being higher than the mean content of fissile material in the fuel rod bundle which is replaced by said composed fuel rod bundle. 2. Method according to claim 1, wherein said replacing step is carried out by removing a number of fuel rods from a first burnt-up fuel rod bundle and inserting a number of fuel rods from at least one second burnt-up fuel rod bundle into said first burnt-up fuel rod bundle, said fuel rods thus inserted having a higher average content of fissile material than said fuel rods which have been removed from the first fuel rod bundle. 3. Method according to claim 2, wherein said removing and inserting steps include the step of retaining fuel rods in the first fuel rod bundle which function as supporting elements in the first fuel rod bundle. 4. Method according to claim 2, wherein said removing and inserting steps include the step of retaining spacers, spacer holder rods and top and bottom plates in the first fuel rod bundle. 5. Method according to claim 1, wherein said composed fuel rod bundle includes water-filled tubes placed in selected positions thereof instead of fuel rods. 6. Method according to claim 1, wherein said composed fuel rod bundle includes vertical fuel rods at least several of said vertical fuel rods being arranged with ends thereof facing upwardly which had been facing downwardly in said burnt-up bundles. 7. Method according to claim 1, wherein said composed fuel rod bundle includes selected fuel rod positions thereof being maintained empty. |
042242584 | description | The apparatus illustrated in FIG. 1 has a container 1, closed at the top and of funnel-shape below, which has an opening at the side near the top and opposite a horizontally directed nozzle 2 provided for projecting a stream of liquid. The nozzle 2 is connected with a vibrator 3 and with another container 15, only partly or symbolically shown in the drawing, for holding a supply of the liquid solution. The stream of liquid 4, subdivided into drops by the action of vibrator and nozzle is directed into the ammonia gas phase 6 which is provided in the space above the surface of the aqueous ammonia solution 5 in the container 1 and is deflected by the force of gravity while passing through the ammonia gas phase. In order to prevent that through projection or dropping, there should be formed particles that are not sufficiently uniform, either because no stationary liquid stream has yet been established or because this stream is in a state of collapse. A vertical diaphragm or baffle 7 is provided that can be shifted vertically in position between appropriate limits, so that in one of its two end positions the entrance opening for the container 1 is covered. Furthermore, the diaphragm is so connected with a funnel 8 that when the diaphragm 7 is closed, the liquid coming out of the nozzle 2 is drained by the funnel 8. In order to prevent the gaseous ammonia phase from reaching the nozzle 2, the suction hood 10 is provided above the entrance opening of the container 1. The particles of nuclear fuel or breeder material formed by hardening in the container 1 are drawn off at the bottom through an outlet 9. As is further shown in FIG. 1, nozzles 11, illustrated in cross-section, for introduction of ammonia gas are provided in the form of supply tubes with lateral slits so disposed with respect to the nozzle 2 that produces the liquid drop stream so that one of them is above and the other underneath the succession of drops that make up the steam 4. The slits are so directed that the ammonia impinges on the droplet stream 4 in a flow that forms an angle of about 40.degree. with the tangent to the path of the liquid drops. By this arrangement of the nozzles 11 a passing gas flow on all sides of the drops is provided, resulting in uniform solidification of the drops even at a drop formation rate of more than 3,000 drops per minute. The apparatus shown in FIG. 2 has a container 12 that is open on top and, again, funnel-shaped below for the aqueous solution of ammonia 5 and the ammonia phase gas 6 above it. The nozzle 2' that is connected with a vibrator 3' and a container 16 for the nitrate solution is, in this case, disposed above the container 12, and a suction system 10 to prevent upward flow of the ammonia gas is provided around the rim of the container 12. Nozzles 13 are provided for the supply of the ammonia gas to the ammonia phase 6 and, as shown in FIG. 2, are directed at an angle of about 45.degree. to the vertical stream of drops and slightly offset vertically with respect to each other. Although the invention has been described with respect to particular illustrative embodiments, it will be understood that modifications and variations are possible within the inventive concept; for example, a horizontal row of nozzles 2 could be arranged one behind the other in the aspect of FIG. 1, with either individual or common vibrator(s) and supply container(s) 15 to produce parallel streams of drops against which ammonia flows from elongated slits parallel to the axes of the supply pipes that with their slits provide the gas supply nozzles 11, rather than a single nozzle 2 providing a single stream 4 of droplets. |
claims | 1. A test system having at least one test disc (1, 1′), an isolator and an evaluation unit (21) for testing the seal of a glove, which is installed in a particular port of the isolator, wherein the test disc (1, 1′) is connected to the port in a hermetically sealed manner, wherein the glove and the test disc (1, 1′) define a glove volume, which is placed under excess pressure by the test disc (1, 1′), wherein the test disc (1, 1′) has a pressure-measuring device with a microprocessor (9) and a memory for recording and storing a glove volume pressure profile and a data interface (14), wherein the test disc (1, 1′) is configured by means of a reading device (15) to determine both the identity of the glove by reading a first identification element, which is arranged on the glove, and the identity of the port by reading a second identification element, which is arranged on the port. 2. The test system according to claim 1, further including a radially-expanding sealing device (2, 2′) and a first micro-air pump (12) to expand the sealing device (2, 2′). 3. The test system according to claim 2, further including a second micro-air pump (13) with a pre-filter (6) to fill the glove volume. 4. The test system according to claim 1, further including an electrical energy source in the form of an accumulator. 5. The test system according to claim 1, wherein the reading device (15) has an RFID module, CCD sensors or laser sensors. 6. The test system according to claim 1, wherein the test disc (1, 1′) has a control device for automatically setting a pressure in the glove volume. 7. The test according to claim 1, wherein the pressure profile and information regarding the identification element can be in particular wirelessly transmitted, in an encrypted format where appropriate, via the data interface (14) to an evaluation unit of a test system, wherein the data interface has in particular a WiFi module, a WLAN module, a Bluetooth module or another radio-based transceiver module. 8. The test system according to claim 1, wherein the evaluation unit (21) comprises a storage unit and an output unit and can be connected to a user database (22), wherein the test system is configured to assign the pressure profile with the identification data precisely to one glove and one port, and assess a status and/or estimate a residual period of use of the glove. 9. The test system according to claim 1, wherein the evaluation unit (21) has WiFi module, WLAN module, Bluetooth module or other radio-based transceiver module. 10. The test system according to claim 1, wherein process-related data about the use of the glove can be stored in the evaluation unit (21) and taken into account in the evaluation. 11. The test system according to claim 1, further including a plurality of test discs for simultaneously testing a plurality of gloves, wherein the test discs communicate with the evaluation unit. 12. A method for assessing the seal of a glove using a test system according to claim 1,wherein the test disc (1, 1′) is connected to the port in a hermetically sealed manner such that the glove and the test disc (1, 1′) define a glove volume, which is then placed under excess pressure by the test disc (1, 1′), wherein the pressure profile is recorded by the test disc (1, 1′) over a predefinable period and identification data of the glove and the port is assigned to the pressure profile by reading a first identification element arranged on the glove and a second identification element arranged on the port, wherein a pressure drop, which is compared with a limiting value, is determined from the pressure profile. 13. A method according to claim 12, wherein historical data, in particular process data, is taken into account during the evaluation of the state. 14. A method according to claim 12, wherein the pressure profiles for a plurality of gloves are received simultaneously from a plurality of test discs (1, 1′) and processed, wherein the respective pressure profiles are assigned unambiguously to a corresponding glove and port. 15. A method according to claim 12, wherein removal of the test disc (1, 1′) from the port is prevented if a defect in the glove is detected. 16. A method according to claim 12, wherein a pressure profile, which is recorded for a specific glove at an earlier point in time, is compared with a pressure profile, which is recorded for said glove at a later point in time, wherein said comparison is taken into account for the estimation of a residual period of use. |
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041475909 | abstract | 1. Nuclear propulsion apparatus comprising:. A. means for compressing incoming air; PA1 B. nuclear fission reactor means for heating said air; PA1 C. means for expanding a portion of the heated air to drive said compressing means; PA1 D. said nuclear fission reactor means being divided into a plurality of radially extending segments; PA1 E. means for directing a portion of the compressed air for heating through alternate segments of said reactor means and another portion of the compressed air for heating through the remaining segments of said reactor means; and PA1 F. means for further expanding the heated air from said drive means and the remaining heated air from said reactor means through nozzle means to effect reactive thrust on said apparatus. |
042017386 | description | DESCRIPTION OF A PREFERRED EMBODIMENT The starting solution for the process of the present invention is uranyl nitrate preferably in a concentration range of between about 200 and 300 gU/l at about 2M HNO.sub.3. The concentrated uranyl nitrate is batch denitrated using conventional apparatus and procedures by the controlled addition of formic acid to the hot uranyl nitrate solution. One conventional technique is described in Denitration of Nitric Acid Solution by Formic Acid, USAEC Report DP-1299, 1972. While the formic acid concentration by itself is not critical, the rate of addition of formic acid should be in the range of between about 0.30 to about 0.70 moles/(min)(liter of uranyl nitrate feed). The preferred concentration for the use of this feed rate is a concentration at or in excess of 77.5% or 19.9 M formic acid. Formic acid concentrations as low as 50% and as high as 90% may be used provided appropriate adjustment is made for the feed rate, the solution temperature, and the off gas removal system. As in conventional denitration, the solution temperature is maintained at about 90.degree. C. for a period of about one to two hours in the presence of excess formic acid to assure complete denitration. Although excess formic acid is added during denitration, the solution acid concentration must be controlled to about 0.3 moles per 100 grams of solution to prevent uranyl formate monohydrate precipitation during the denitration step. Such uncontrolled or premature precipitation could adversely effect the particle size distribution of the precipitated salt. Thus, the carefully controlled denitration step produces an unsaturated solution of uranyl formate containing all of the uranium from the feed uranyl nitrate solution. Unsaturated uranyl formate solution from the denitration step and at the denitration temperature of about 90.degree. C. is then contacted with additional formic acid to precipitate uranyl formate monohydrate. The formic acid is added in sufficient stoichiometric excess and at a rate selected to control the nucleation and growth of crystalline particles during precipitation and to yield the desired particle size distribution for powder metallurgical requirements. Formic acid preferably at or in excess of 19.9 M concentration is added at a rate of between about 0.40 and 1.27 moles/(min)(liter of uranyl nitrate feed) to raise the solution acid concentration from about 0.3 to about 1.4 moles/100 g of solution. A higher concentration of formic acid (about 1.6 to 1.7 moles/100 g solution) is achieved by conventional volume reduction (evaporation) and controlled addition of small quantities of 90% formic acid. At this concentration the solution and precipitate are cooled to ambient room temperature (about 25.degree. C.). The precipitate is then recovered by filtration. The precipitate is washed during this step with 90% formic acid to remove residual contaminants. The filter cake, consisting essentially of substantially pure uranyl formate monohydrate (UO.sub.2 (HCOO).sub.2.H.sub.2 O), is then dried at 110.degree. to 120.degree. C. for one to four hours to provide a crystalline uranyl formate monohydrate salt. At this stage the crystalline formate salt, which has a uniform particle size distribution, is suitable for calcining to U.sub.3 O.sub.8. Calcining is conducted in a conventional manner in a static bed by heating the salt in air at about 10.degree. C. per minute to a final temperature of about 800.degree. C. to produce a free-flowing crystalline U.sub.3 O.sub.8 powder having a controlled particle size distribution. The calcination temperature is maintained for 4 to 8 hours to ensure complete conversion to U.sub.3 O.sub.8 product. For the purpose of this specification, the word "controlled" when used in connection with "particle size" may be defined as having a "uniform narrow range" of particle size compatible with powder metallurgical grade aluminum powder and suitable for powder metallurgical use. The U.sub.3 O.sub.8 product may be further characterized as a U.sub.3 O.sub.8 powder consisting essentially of discrete crystalline U.sub.3 O.sub.8 particles with a minimum of particle agglomeration. The bulk morphology of the calcined U.sub.3 O.sub.8 powder is very similar to the crystalline uranyl formate monohydrate prior to calcination. Also, because of the crystalline morphology of the uranyl formate monohydrate, calcination to U.sub.3 O.sub.8 does not result in significantly reduced particle size. Having described a preferred embodiment, the following specific examples will serve to further illustrate the present method of preparing U.sub.3 O.sub.8 for powder metallurgical use: EXAMPLE I To demonstrate the feasibility of preparing U.sub.3 O.sub.8 having a controlled particle size distribution, an exemplary feed solution (85 ml) was prepared of uranyl nitrate having a concentration of 250 gU/l at 2 M HNO.sub.3. Feed solution was contacted in suitable denitration apparatus with 70 ml of 19.9 M formic acid at a feed rate of 0.70 moles formic acid/(min)(liter of uranyl nitrate feed) solution at a temperature of 90.degree. C. Sufficient formic acid was added to achieve a concentration of 0.3 moles /100 grams of solution at the completion of denitration. The solution was digested for one hour after completion of the formic acid addition to assure complete denitration and formation of an unsaturated solution of uranyl formate. An additional 365 ml of 19.9 M formic acid was added to the unsaturated uranyl formate solution at a feed rate of 1.27 moles/(min)(liter of uranyl nitrate feed) at 90.degree. C. to precipitate uranyl formate monohydrate. To maximize uranium precipitation the formic acid was added until the solution acid concentration reached 1.4 moles/100 g of solution. The solution was evaporated to reduce the solution volume by 60% and adjusted with 75 ml of 90% formic acid. The slurry was cooled to ambient temperature and the precipitate recovered by filtration at a formic acid concentration of 1.7 moles/100 g solution. The precipitate was washed with 90% formic acid and the resulting filter cake was dried at 110.degree. C. for 2 hours to provide crystalline uranyl formate monohydrate salt. This salt was then calcined in a static bed in an air atmosphere at 800.degree. C. for 6 hours. The resulting product was a crystalline free-flowing U.sub.3 O.sub.8 powder having the particle size distribution shown in the FIGURE of the attached drawing. EXAMPLE II Using the same feed solution, concentrations and procedure as in Example I, and with volumes adjusted to reflect the larger quantity of feed, U.sub.3 O.sub.8 was prepared from a 170 ml uranyl nitrate sample. In this example, formic acid was added to the feed solution for denitration at a rate of 0.36 moles/(min)(liter of uranyl nitrate feed) and to the precipitation step at a rate of 0.63 moles/(min)(liter of uranyl nitrate feed). Calcination of the resulting precipitate produced a crystalline U.sub.3 O.sub.8 powder having the particle size distribution shown in the attached FIGURE. EXAMPLE III Another feed solution containing uranyl nitrate at a concentration of 207 gU/l was processed using 90% formic acid at denitration and precipitation feed rates of 0.35 and 0.63 moles/(min)(liter of uranyl nitrate feed), respectively. The crystalline U.sub.3 O.sub.8 powder resulting from calcination of the uranyl formate monohydrate precipitation had a particle size distribution as shown in the attached FIGURE. EXAMPLE IV A uranyl nitrate solution having 227 gU/l was processed using 90% formic acid at denitration and precipitation feed rates of 0.24 and 0.42 moles/(min)(liter of uranyl nitrate feed), respectively. The U.sub.3 O.sub.8 powder particle size distribution resulting from this example is also shown in the attached FIGURE. It will be noted from an examination of the accompanying FIGURE that the particle size distribution of U.sub.3 O.sub.8 prepared by the process described in Examples I through IV compare favorable with powder metallurgical grade aluminum powder. In all cases the U.sub.3 O.sub.8 powder prepared from the controlled precipitation of uranyl formate monohydrate displayed the desirable physical characteristics of (1) no large particles, (2) crystalline particle morphology, and (3) a narrow uniform particle size distribution that matched that of the aluminum powder. |
summary | ||
054955114 | abstract | A device for passively inerting the gas mixture forming in the reactor containment of a nuclear power plant in an accident situation is proposed, which device is based on the use of chemical substances which react or disintegrate, releasing an inerting gas or gas mixture when a certain temperature of reaction is reached. This device is especially suitable for use in connection with catalytic recombiners for removing hydrogen through oxidation with the oxygen present. The heat resulting from this exothermic process of recombination can be put to use for heating up chemical substances to the required temperature, these having temperatures of reaction that lie above the temperature (approximately 100.degree. C.) that develops in the reactor containment in an accident situation. |
description | The present application claims the benefit of priority to U.S. Provisional Application No. 62/355,057 filed Jun. 27, 2016, the entirety of which is incorporated herein by reference. The present invention generally relates to storage of nuclear fuel, and more particularly to an improved nuclear fuel storage rack system for use in a fuel pool in a nuclear generation plant. A conventional free-standing, high density nuclear fuel storage rack is a cellular structure typically supported on a set of pedestals from the floor or bottom slab of the water-filled spent fuel pool. The bottom extremity of each fuel storage cell is welded to a common baseplate which serves to provide the support surface for the upwardly extending vertical storage cells and stored nuclear fuel therein. The cellular region comprises an array of narrow prismatic cavities formed by the cells which are each sized to accept a single nuclear fuel assembly comprising a plurality of new or spent nuclear fuel rods. The term “active fuel region” denotes the vertical space above the baseplate within the rack where the enriched uranium is located. High density fuel racks used to store used nuclear fuel employ a neutron absorber material to control reactivity. The commercially available neutron absorbers are typically in a plate or sheet form and are either metal or polymer based. The polymeric neutron absorbers commonly used in the industry were sold under trade names Boraflex and Tetrabor, with the former being the most widely used material in the 1980s. The neutron absorber panels have been typically installed on the four walls of the storage cells encased in an enveloping sheathing made of thin gage stainless steel attached to the cell walls in the active fuel region. Unfortunately, the polymeric neutron absorbers have not performed well in service. Widespread splitting and erosion of Boraflex and similar degradation of Tetrabor have been reported in the industry, forcing the plant owners to resort to reducing the density of storage (such as a checkered board storage arrangement) thereby causing an operational hardship to the plant. A neutron absorber apparatus is desired which can be retrofit in existing fuel racks suffering from neutron absorber material degradation in order to fully restore reactivity reduction capacity of the storage cells. Embodiments of the present invention provide a neutron absorber insert system which can be readily added in situ to existing storage cells of the fuel rack having degraded neutron absorbers and reduced reactivity reduction capacity. The system comprises a plurality of neutron absorber apparatuses which may be in the form of absorber inserts configured for direct insertion into and securement to the fuel storage cells. The inserts have a low-profile small and thin cross sectional footprint which does not significantly reduce the storage capacity of each storage cell. A fuel assembly may be inserted into a central longitudinally-extending cavity of the insert and removed therefrom without first removing the insert. The inserts include a locking feature which is automatically deployed and secures the insert in the cell, as further described herein. Advantageously, the absorber insert may utilize an available edge surface on an existing storage tube of the fuel rack which can be engaged by the locking feature of the absorber tube. This eliminates the need for modifying the existing fuel rack in order to accommodate the insert, thereby saving time and expense. In one embodiment, the edge surface may be part of an existing neutron absorber sheathing structure on the fuel storage tube. The inserts may advantageously be deployed in the existing fuel rack storage cells via remote handling equipment such as cranes while the rack remains submerged underwater in the spent fuel pool. In one aspect, a neutron absorber apparatus for a nuclear fuel storage system includes: a fuel rack comprising a vertical longitudinal axis and plurality of longitudinally-extending storage cells, each cell comprising a plurality of cell sidewalls defining a cell cavity configured for storing nuclear fuel therein; a sheath integrally attached to a first cell sidewall of a first cell and defining a sheathing cavity configured for holding a neutron absorber material; an absorber insert comprising plural longitudinally-extending neutron absorber plates each comprising a neutron absorber material, the insert disposed in the first cell; and an elastically deformable locking protrusion disposed on one of the absorber plates, the locking protrusion resiliently movable between an outward extended position and an inward retracted position; the locking protrusion lockingly engaging the sheath to axially restrain the insert and prevent removal of the insert from the first cell. In another aspect, a neutron absorber apparatus for a nuclear fuel storage system includes: a fuel rack comprising a vertical longitudinal axis and plurality of longitudinally-extending storage tubes each defining a cell, each storage tube comprising a plurality of tube sidewalls defining a primary cavity; an absorber insert insertably disposed in the primary cavity of a first storage tube, the absorber insert comprising a plurality of absorber plates arranged to form a longitudinally-extending neutron absorber tube having an exterior and an interior defining a secondary cavity configured for storing a nuclear fuel assembly therein, each absorber plate formed of a neutron absorber material; an upper stiffening band extending perimetrically around an upper end of the absorber tube, the upper stiffening band attached to the exterior of the absorber tube and protruding laterally outwards beyond the absorber plates to engage the tube sidewalls of the first storage tube; a lower stiffening band extending perimetrically around a lower end of the absorber tube and disposed at least partially inside the secondary cavity, the lower stiffening band attached to the interior of the absorber tube; wherein the absorber plates of the insert assembly are spaced laterally apart from the tube sidewalls of the first storage tube by the upper stiffening band forming a clearance gap therebetween. In another aspect, a neutron absorber apparatus for a nuclear fuel storage system includes: a fuel rack comprising a plurality of longitudinally-extending storage cells, each cell comprising a plurality of cell walls defining a cell cavity for storing nuclear fuel; a longitudinally-extending absorber tube insertably disposed in a first cell of the fuel rack and having an exterior and an interior, the absorber tube comprising: an elongated chevron-shaped first absorber plate comprising a first section and a second section angularly bent to the first section along a bend line of the first absorber plate; an elongated chevron-shaped second absorber plate comprising a third section and a fourth section angularly bent to the third section along a bend line of the second absorber plate; an upper stiffening band extending perimetrically around upper ends of the first and second absorber plates and coupling the first and second absorber plates together. Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. All drawings are schematic and not necessarily to scale. Parts shown and/or given a reference numerical designation in one figure may be considered to be the same parts where they appear in other figures without a numerical designation for brevity unless specifically labeled with a different part number and described herein. The features and benefits of the invention are illustrated and described herein by reference to exemplary embodiments. This description of exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. Accordingly, the disclosure expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features. Furthermore, all features and designs disclosed herein may be used in combination even if not explicitly described as such. In the description of embodiments disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation. Terms such as “attached,” “affixed,” “connected,” “coupled,” “interconnected,” and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. It will be appreciated that any numerical ranges that may be described herein shall be understood to include the lower and upper numerical terminus values or limits of the cited range, and any numerical values included in the cited range may serve as the terminus values. Referring to FIGS. 1-5, a nuclear facility which may be a nuclear generating plant includes a fuel pool 40 according to the present disclosure configured for storing a plurality of nuclear fuel racks 100. The fuel pool 40 may comprise a plurality of vertical sidewalls 41 rising upwards from an adjoining substantially horizontal bottom base wall or slab 42 (recognizing that some slope may intentionally be provided in the upper surface of the base slab for drainage toward a low point if the pool is to be emptied and rinsed/decontaminated at some time and due to installation tolerances). The base slab 42 and sidewalls 41 may be formed of reinforced concrete in one non-limiting embodiment. The fuel pool base slab 42 may be formed in and rest on the soil sub-grade 26, the top surface of which defines grade G. In this embodiment illustrated in the present application, the sidewalls are elevated above grade. The base slab 42 may be located at grade G as illustrated, below grade, or elevated above grade. In other possible embodiments contemplated, the base slab 42 and sidewalls 41 may alternatively be buried in sub-grade 26 which surrounds the outer surfaces of the sidewalls. Any of the foregoing arrangements or others may be used depending on the layout of the nuclear facility and does not limit of the invention. In one embodiment, the fuel pool 40 may have a rectilinear shape in top plan view. Four sidewalls 41 may be provided in which the pool has an elongated rectangular shape (in top plan view) with two longer opposing sidewalls and two shorter opposing sidewalls (e.g. end walls). Other configurations of the fuel pool 40 are possible such as square shapes, other polygonal shapes, and non-polygonal shapes. The sidewalls 41 and base slab 42 of the fuel pool 40 define an upwardly open well or cavity 43 configured to hold cooling pool water W and the plurality of submerged nuclear fuel racks 100 each holding multiple nuclear fuel bundles or assemblies 28 (a typical one shown in phantom view seated in a fuel rack cell in FIG. 5). Each fuel assembly 28 contains multiple individual new or spent uranium fuel rods. Fuel assemblies are further described in commonly assigned U.S. patent application Ser. No. 14/413,807 filed Jul. 9, 2013, which is incorporated herein by reference in its entirety. Typical fuel assemblies 28 for a pressurized water reactor (PWR) may each hold over 150 fuel rods in 10×10 to 17×17 fuel rod grid arrays per assembly. The assemblies may typically be on the order of approximately 14 feet high weighing about 1400-1500 pounds each. The fuel racks 100 storing the fuel assemblies are emplaced on the base slab 42 in a high-density arrangement in the horizontally-abutting manner as further described herein. The fuel pool 40 extends from an operating deck 22 surrounding the fuel pool 40 downwards to a sufficient vertical depth D1 to submerge the fuel assemblies 28 in the fuel rack (see, e.g. FIG. 6) beneath the surface level S of the pool water W for proper radiation shielding purposes. The substantially horizontal operating deck 22 that circumscribes the sidewalls 41 and pool 40 on all sides in one embodiment may be formed of steel and/or reinforced concrete. In one implementation, the fuel pool may have a depth such that at least 10 feet of water is present above the top of the fuel assembly. Other suitable depths for the pool and water may be used of course. The surface level of pool water W (i.e. liquid coolant) in the pool 40 may be spaced below the operating deck 22 by a sufficient amount to prevent spillage onto the deck during fuel assembly loading or unloading operations and to account to seismic event. In one non-limiting embodiment, for example, the surface of the operating deck 22 may be at least 5 feet above the maximum 100 year flood level for the site in one embodiment. The fuel pool 40 extending below the operating deck level may be approximately 40 feet or more deep (e.g. 42 feet in one embodiment). The fuel pool is long and wide enough to accommodate as many fuel racks 100 and fuel assemblies 28 stored therein as required. There is sufficient operating deck space around the pool to provide space for the work crew and for staging necessary tools and equipment for the facility's maintenance. There may be no penetrations in the fuel pool 40 within the bottom 30 feet of depth to prevent accidental draining of water and uncovering of the fuel. In some embodiments, a nuclear fuel pool liner system may be provided to minimize the risk of pool water leakage to the environment. The liner system may include cooling water leakage collection and detection/monitoring to indicate a leakage condition caused by a breach in the integrity of the liner system. Liner systems are further described in commonly owned U.S. patent application Ser. No. 14/877,217 filed Oct. 7, 2015, which is incorporated herein by reference in its entirety. The liner system in one embodiment may comprise one or more liners 60 attached to the inner surfaces 63 of the fuel pool sidewalls 41 and the base slab 42. The inside surface 61 of liner is contacted and wetted by the fuel pool water W. The liner 60 may be made of any suitable metal of suitable thickness T2 which is preferably resistant to corrosion, including for example without limitation stainless steel, aluminum, or other. Typical liner thicknesses T2 may range from about and including 3/16 inch to 5/16 inch thick. Typical stainless steel liner plates include ASTM 240-304 or 304L. In some embodiments, the liner 60 may be comprised of multiple substantially flat metal plates or sections which are hermetically seal welded together via seal welds along their contiguous peripheral edges to form a continuous liner system completely encapsulating the sidewalls 41 and base slab 42 of the fuel pool 40 and impervious to the egress of pool water W. The liner 60 extends around and along the vertical sidewalls 41 of the fuel pool 40 and completely across the horizontal base slab 42 to completely cover the wetted surface area of the pool. This forms horizontal sections 60b and vertical sections 60a of the liner to provide an impervious barrier to out-leakage of pool water W from fuel pool 40. The horizontal sections of liners 60b on the base slab 42 may be joined to the vertical sections 60a along perimeter corner seams therebetween by hermetic seal welding. The liner 60 may be fixedly secured to the base slab 42 and sidewalls 41 of the fuel pool 40 by any suitable method such as fasteners. With continuing reference to FIGS. 1-5, the fuel rack 100 is a cellular upright module or unit. Fuel rack 100 may be a high density, tightly packed non-flux type rack as illustrated which is designed to be used with fuel assemblies that do not require the presence of a neutron flux trap between adjacent cells 110. Thus, the inclusion of neutron flux traps (e.g. gaps) in fuel racks when not needed is undesirable because valuable fuel pool floor area is unnecessarily wasted. Of course, both non-flux and flux fuel rack types may be stored side by side in the same pool using the seismic-resistant fuel storage system according to the present disclosure. The invention is therefore not limited to use of any particular type of rack. Fuel rack 100 defines a vertical longitudinal axis LA and comprises a grid array of closely packed open cells 110 formed by a plurality of adjacent elongated storage tubes 120 arranged in parallel axial relationship to each other. The rack comprises peripherally arranged outboard tubes 120A which define a perimeter of the fuel rack and inboard tubes 120B located between the outboard tubes. Tubes 120 are coupled at their bottom ends 114 to a planar top surface of a baseplate 102 and extend upwards in a substantially vertical orientation therefrom. In this embodiment, the vertical or central axis of each tube 120 is not only substantially vertical, but also substantially perpendicular to the top surface of the baseplate 102. In one embodiment, tubes 120 may be fastened to baseplate 102 by welding and/or mechanical coupling such as bolting, clamping, threading, etc. Tubes 120 include an open top end 112 for insertion of fuel assemblies, bottom end 114, and a plurality of longitudinally extending vertical sidewalls 116 (“cell walls”) between the ends and defining a tube or cell height H1. Each tube 120 defines an internal cell cavity 118 extending longitudinally between the top and bottom ends 112, 114. In the embodiment shown in FIG. 2A-B, four tube sidewalls 116 arranged in rectilinear polygonal relationship are provided forming either a square or rectangular tube 120 in lateral or transverse cross section (i.e. transverse or orthogonal to longitudinal axis LA) in plan or horizontal view (see also FIG. 3). Cells 110 and internal cavities 118 accordingly have a corresponding rectangular configuration in lateral cross section. The top ends of the tubes 120 are open so that a fuel assembly can be slid down into the internal cavity 118 formed by the inner surfaces of the tube sidewalls 116. Each cell 110 and its cavity 118 are configured for holding only a single nuclear fuel assembly 28. Tubes 120 may be made of any suitable preferably corrosion resistant metal, such as without limitation stainless steel or others. Baseplate 102 may be made of a similar or different corrosion resistant metal. It will be appreciated that each tube 120 can be formed as a single unitary structural component that extends the entire desired height H1 or can be constructed of multiple partial height tubes that are vertically stacked and connected together such as by welding or mechanical means which collectively add up to the desired height H1. It is preferred that the height H1 of the tubes 120 be sufficient so that the entire height of a fuel assembly may be contained within the tube when the fuel assembly is inserted into the tube. The top ends 112 of tubes 120 may preferably but not necessarily terminate in substantially the same horizontal plane (defined perpendicular to longitudinal axis LA) so that the tops of the tube are level with each other. The baseplate 102 at the bottom ends 114 of the tubes defines a second horizontal reference plane HR. As best shown in FIGS. 2A-B, tubes 120 are geometrically arranged atop the baseplate 102 in rows and columns along the Z-axis and X-axis respectively. Any suitable array size including equal or unequal numbers of tubes in each row and column may be provided depending on the horizontal length and width of the pool base slab 42 and number of fuel racks 100 to be provided. In some arrangements, some or all of the fuel racks 100 may have unequal lateral width and lateral length as to best make use of a maximum amount of available slab surface area as possible for each installation. For convenience of reference, the outward facing sidewalls 116 of the outboard tubes 120A may be considered to collectively define a plurality of lateral sides 130 of the fuel rack 100 extending around the rack's perimeter as shown in FIGS. 2A-B. Referring to FIGS. 1-5, each fuel rack 100 comprises a plurality of legs or pedestals 200 which support rack from the base slab 42 of the fuel pool 40. Pedestals 200 each have a preferably flat bottom end 204 to engage the pool base slab 42 and a top end 202 fixedly attached to the bottom of the baseplate 102. The pedestals 200 protrude downwards from baseplate 102. This elevates the baseplates 102 of the rack off the base slab 42, thereby forming a gap therebetween which defines a bottom flow plenum P beneath rack 100. The plenum P allows cooling water W in the pool to create a natural convective circulation flow path through each of the fuel storage tubes 120 (see e.g. flow directional arrows in FIG. 5). A plurality of flow holes 115 are formed in the rack through baseplate 102 in a conventional manner to allow cooling water to flow upwards through the cavity 118 of each tube 120 and outward through the open top ends 112 of the tubes. Commonly owned U.S. patent application Ser. No. 14/367,705 filed Jun. 20, 2014 shows fuel rack baseplates with flow holes, and is incorporated herein by reference in its entirety. The pool water W flowing through the tubes is heated by the nuclear fuel in fuel assemblies, thereby creating the motive force driving the natural thermal convective flow scheme. Referring now then to FIGS. 3 and 5, flow holes 115 create passageways from below the base plate 102 into the cells 110 formed by the tubes 120. Preferably, a single flow hole 115 is provided for each cell 110, however, more may be used as needed to create sufficient flow through the tubes. The flow holes 115 are provided as inlets to facilitate natural thermosiphon flow of pool water through the cells 110 when fuel assemblies having a heat load are positioned therein. More specifically, when heated fuel assemblies are positioned in the cells 110 in a submerged environment, the water within the cells 110 surrounding the fuel assemblies becomes heated, thereby rising due to decrease in density and increased buoyancy creating a natural upflow pattern. As this heated water rises and exits the cells 110 via the tube open top ends 112 (see FIG. 1), cooler water is drawn into the bottom of the cells through the flow holes 115. This heat induced water flow and circulation pattern along the fuel assemblies then continues naturally to dissipate heat generated by the fuel assemblies. Pedestals 200 may therefore have a height selected to form a bottom flow plenum P of generally commensurate height to ensure that sufficient thermally-induced circulation is created to adequately cool the fuel assembly. In one non-limiting example, the height of the plenum P may be about 2 to 2.5 inches (including the listed values and those therebetween of this range). To facilitate lateral cross flow of cooling water between cells 110 in the fuel rack 100, a minimum of two lateral flow holes 115A may be provided proximate to the lower or bottom end 114 of each tube 120 (see, e.g. FIGS. 4 and 5). Each hole defines top, bottom, and side edges in tube material. In one embodiment, the flow holes 115A may be formed by a punching operation. Pedestals 200 may have any suitable configuration or shape and be of any suitable type. Each fuel rack 100 may include a plurality of peripheral pedestals 200 spaced apart and arranged along the peripheral edges and perimeter of the baseplate 102, and optionally one or more interior pedestals if required to provide supplemental support for the inboard fuel assemblies and tubes 120B. In one non-limiting embodiment, four peripheral pedestals 200 may be provided each of which is located proximate to one of the four corners 206 of the baseplate. Additional peripheral pedestals may of course be provided as necessary between the corner pedestals on the perimeter of the baseplate. The pedestals are preferably located as outboard as possible proximate to the peripheral edges 208 of the baseplates 102 of each fuel rack or module to give maximum rotational stability to the modules. With continuing reference to FIGS. 1-5, each fuel rack storage tube 120 in some embodiments may include a longitudinally-extending absorber sheath 300 disposed on one or more tube sidewalls 116. The sheath 300 extends at least over the active zone or height of the fuel rack tubes 120 where the fuel is positioned in the fuel rack 100 (see, e.g. FIG. 5). Sheath 300 has a raised profile or projection from the tube sidewall 116. Sheath 300 has a vertically elongated and generally flat body including top end 310 defining a top lip or edge, bottom end 311 defining a bottom lip or edge 436, and a sidewall 312 extending axially between the top and bottom ends. The top and bottom ends 310, 311 terminate at a point spaced apart from the top and bottom ends 112, 114 of the storage tube 120 as shown. The sheath 300 may be attached to the tube sidewall 116 via welding or another suitable technique. Sheath sidewall 312 is spaced laterally apart from the sidewall 116 of the tube 120 such that each “picture frame” sheath 300 forms an envelope defining a sheathing cavity 301 between the sheath and tube sidewall which is configured for receiving neutron absorber material 302 therein (e.g. in sheet or panel form as represented in FIGS. 4 and 5). The sheath body is therefore configured and laterally offset from the tube sidewall 116 by a distance commensurate with the dimensions and thickness of the absorber sheet or panel inserted therein. The boron-containing material or “poison” may be Boraflex, Tetrabor, (both previously mentioned) or another. In some existing used fuel rack installations, the absorber material 302 may be in a degraded condition thereby requiring augmentation with a neutron absorber apparatus disclosed herein to restore fuel neutron reactivity control to the fuel rack. FIGS. 6-13 show a neutron absorber apparatus according to the present disclosure. The apparatus may be in the form of a shaped neutron absorber insert 400 configured to be slidably insertable into one of the tubes 120 and cells 110 of the fuel rack 110 shown in FIGS. 1-5 discussed above. Absorber insert 400 includes a plurality of longitudinally-extending neutron absorber walls or plates 402 each comprising a neutron absorber material operable to control reactivity of the fuel stored in the fuel rack cells. The absorber plates 402 may be made of a suitable boron-containing metallic poison material such as without limitation borated aluminum. In some embodiments, without limitation, the absorber plates 402 may be formed of a metal-matrix composite material, and preferably a discontinuously reinforced aluminum/boron carbide metal matrix composite material, and more preferably a boron impregnated aluminum. One such suitable material is sold under the tradename METAMIC™. Other suitable borated metallic materials however may be used. The boron carbide aluminum matrix composite material of which the absorption plates 402 are constructed includes a sufficient amount of boron carbide so that the absorption sheets can effectively absorb neutron radiation emitted from a spent fuel assembly, and thereby shield adjacent spent fuel assemblies in a fuel rack from one another. The absorption plates may be constructed of an aluminum boron carbide metal matrix composite material that is about 20% to about 40% by volume boron carbide. Of course, other percentages may also be used. The exact percentage of neutron absorbing particulate reinforcement which is in the metal matrix composite material, in order to make an effective neutron absorber for an intended application, will depend on a number of factors, including the thickness (i.e., gauge) of the absorption plates 402, the spacing between adjacent cells within the fuel rack, and the radiation levels of the spent fuel assemblies. In one configuration, absorber insert 400 may comprise an assembly formed by two bent and chevron-shaped angled plates (designated 402A and 402B for convenience of reference), which are held together by metallic upper and lower stiffening bands 404, 406. Each plate 402A, 402B has the shape of a common structural angle sized to fit within the interior dimensions of each fuel rack storage tube 120/cell 110. Absorber plates 402A, 402B may each be formed of a generally flat or planar plate or sheet of neutron absorber material which is mechanically bent along a linear longitudinal bend line BL extending the plate's length L2 to form first and second half-sections 408, 410. The bend line BL may be located midway between the two side edges 412 of the plates 402A or 402B so that each half-section 408, 410 has an equal width W2. In other possible embodiments, the half-sections may have unequal widths. Half-sections 408 and 410 may be arranged mutually perpendicular to each other at a 90-degree angle around the bend line BL in one embodiment as shown. When the absorber plates 402A, 402B are fastened together via the stiffening bands 404, 406, they collectively form a tubular box frame comprising a four-sided rectilinear absorber tube 424 having a vertical centerline IC and defining an exterior surface 418 and interior surface 420. Interior surface 420 in turn defines a longitudinally-extending and completely open central cavity 422 configured for insertably receiving and holding a nuclear fuel assembly 28 therein (typical fuel assembly shown in FIG. 5). Cavity 422 extends from upper end 414 to lower end 416 of the absorber tube 424. The ends 414 and 416 of the tube are open. Absorber tube 424 and concomitantly cavity 422 may have a square cross sectional shape in one embodiment as shown. Rectangular or other cross sectional tube and cavity shapes may be used in some embodiments depending on the cross sectional shape of the fuel storage tubes 120. The mating longitudinal edges 426 of the absorber tube plates 402A and 402B may laterally spaced apart in some embodiments forming an axially extending slot 412 for the entire length of the absorber tube assembly (see, e.g. FIG. 6). The slot width is fixed by the upper and lower stiffening bands 404, 406 to which the absorber plates are fastened. In other embodiments, the longitudinal edges 426 of the absorber plates 402A, 402B may be abutted without any appreciable gap. Upper and lower stiffening bands 404, 406 may be annular ring-like structures having a complementary configuration to the absorber tube 424. Stiffening bands 404, 406 may have a square configuration in the non-limiting illustrated embodiment. The upper and lower bands are attached to the upper and lower extremities of the absorber tube plates 402A, 402B, respectively. Methods used to secure the bands 404, 406 to the upper and lower ends 414, 416 of the plates include for example without limitation welding, riveting, threaded fasteners, or other techniques. The stiffening bands may be made of a corrosion resistant metal, such as stainless steel in one embodiment. Referring to FIGS. 6-10, the upper stiffening band 404 extends perimetrically around the upper end 414 of the absorber tube 424. The upper stiffening band 404 is sized to closely fit inside the upper region of the fuel storage cell 110/tube 120 with a very small clearance between interior surfaces of the fuel rack storage tube sidewalls 116 and the band, thereby giving the absorber tube 424 structural rigidity and rotational fixity of position in the storage cell at the upper end of the absorber tube. In one embodiment, the upper stiffening band is preferably attached to the exterior surfaces 418 of the absorber tube plates 402A, 402B at the upper end 414 of absorber tube 424. The upper stiffening band may be disposed precisely at the upper end 414 of absorber tube 424 as illustrated, or in other embodiments may be proximate to but spaced vertically downwards apart from the upper end 414. In either case, upper stiffening band 404 is preferably located at an elevation at least above the top end 310 of the absorber sheath 300 on storage tube 120 to prevent interference with the sheath when inserting the absorber tube into the fuel storage cell 110. Upper stiffening band 404 projects laterally and transversely outwards from and beyond the exterior of the absorber tube 424 to engage the sidewalls 116 of the storage tube. When the absorber tube 424 is installed in one of the fuel rack cells 110 as shown in FIG. 5, the outwards projection of upper stiffening band 404 laterally spaces the absorber tube 424 apart from the interior cell side walls 116. This creates a clearance gap G1 between the exterior surfaces 418 of the absorber tube 424 (formed by tube absorber plates 402A, 402B) and interior surfaces of the cells 110 (formed by the sidewalls 116 of the fuel storage tubes 120). Gap G1 is preferably sized commensurate to the lateral projection depth D2 of the sheaths 300 on the fuel storage tubes 120 to receive the sheaths in the gap when installing the absorber tube 424 in the fuel storage cell 110. This allows the absorber tube 424 to be slideably inserted into the fuel storage cell 110 without interference from the projection of the sheaths 300 outwards from the sidewalls 116 of the storage tube 120 (see, e.g. FIG. 5). Because the sheaths 300 have a longitudinal length which terminates short of the upper and lower ends of the fuel storage tubes 120 as shown in FIG. 4, the upper stiffening band 404 may be fully seated inside the upper end of the storage tube without interference from the sheath (see, e.g. FIG. 9). To further avoid interference with the sheaths 300 when the absorber tube 424 is slid into the fuel storage tube 120 through the open top end 112 of the storage tube, the lower stiffening band 406 is instead mounted in the interior or cavity 422 of the absorber tube in one embodiment as best shown in FIG. 10. Lower stiffening band 406 extends perimetrically around the lower end 416 of the absorber tube 424 in cavity 422. The lower stiffening band provides structure rigidity and rotationally fixity in position to the lower end portion of the absorber tube 424 when seated in the fuel storage cell 110. Lower stiffening band 406 may be completely recessed inside the absorber tube 424 within central cavity 422 wherein the lower end of the tube 424 engages the baseplate 102 of the fuel rack when the absorber insert is fully inserted therein. In alternative embodiments, the lower stiffening band may have an extended length and protrude downwards beyond the lower end 416 of the absorber tube 424 to engage the baseplate 102. If the storage tube 120 has optional lateral flow holes 115A as shown in FIGS. 4 and 5, matching flow holes (not shown) may be provided at corresponding locations in the lower stiffening band 406. When the absorber tube 424 is fully seated in the storage tube 120, the flow holes in absorber tube would become concentrically aligned with the lateral flow holes 115A of the storage tube to preserve fuel pool cooling water cross flow between cells 110. According to another aspect, the absorber tube 424 may include one or more axial restraints to lock and axially fixate the tube in longitudinal position within the storage cell 110 of the fuel rack 100. Referring to FIGS. 6-11, the axial restraints in one non-limiting embodiment may be formed by elastically deformable locking protrusions comprised of metal leaf spring clips 430. Spring clips 430 each have an elongated body formed of corrosion resistant spring steel. Clips 430 include a lower fixed end portion 432 rigidly attached to the exterior surface 418 of the absorber tube 424 and an opposite resiliently movable cantilevered upper free-end locking portion 434. Fixed end portion 432 may be substantially flat and fixedly attached to absorber tube plates 402A, 402B by any suitable means, such as without limitation welding, riveting, or fasteners in some embodiments. Locking portion 434 extends upwardly from fixed end portion 432 and is obliquely angled thereto forming a space between the locking portion and the absorber tube 424. Locking portion 434 thus projects laterally outwards from the absorber tube 424 (i.e. absorber plates 402A, 402B). When the absorber tube 424 is installed in the fuel rack storage tube 120, locking portion 434 is also obliquely angled to the vertical longitudinal axis LA of the fuel rack (identified in FIG. 2). The locking spring clips 430 are positioned on the lower half of absorber tube 424 and arranged to engage an available edge disposed on the lower half of the fuel storage tubes 120. In one embodiment, the spring clips may be positioned to engage a free bottom edge 436 of the sheaths 300 which is laterally spaced away from sidewall 116 of the storage tube 120, (see, e.g. FIGS. 4, 5, and 11). The free bottom edges 436 are often formed near the lateral end portions 438 of the bottom end 430 of the sheath 330 where the sheath is not welded or otherwise attached to the storage tube 120. In such configurations, the spring clips 430 may be disposed proximate to the corners 428 of the lower half of the absorber tubes 424 to engage the bottom edges 436 of the sheaths 300. Any suitable number of spring clips 430 may be provided. In one embodiment, at least two spring clips 430 may be provided preferably on different sides of the absorber tube 424. In other embodiments, each of the four sides of the absorber tube may have at least one spring clip. Preferably, at least one spring clip 430 is located to engage one available bottom edge 436 of a sheath 300 of the storage cell 110 in which the absorber tube is installed to lock the absorber tube axially in place in the cell. It bears noting that at least one of the four storage tube sidewalls 116 inside of each fuel storage cell 110 includes a sheath 300 for engagement by a locking spring clip 430. This single engagement is sufficient to lock the absorber tube 424 in position within the storage cell. The locking protrusion or spring clip 430 is resiliently movable between an outward an inward deflected and retracted position for sliding the absorber tube 424 into the fuel storage tube 120 or cell 110, and an outward undeflected and extended position for engaging the sheath 300 and locking the absorber tube in position in the fuel rack 100. Operation of the locking protrusion or spring clip 430 will become evident by describing a method for installing a tubular neutron absorber insert 400 in a storage cell 110 of a fuel rack. A suitable cell 110 may first be selected having at least one available absorber sheath 300 for locking the insert in the fuel rack 100. In one example, cell 110A identified in FIG. 3 may be selected. The fuel rack 100 may be still submerged in the fuel pool 40 and radioactively active. Preferably, a fuel assembly 28 if already present in cell 110A may be removed first. An absorber insert 400 which may be in the form of absorber tube 424 described above is then positioned over and axially aligned with cell 110A. The locking spring clip or clips 430 are initially in their outward undeflected and extended position (see, e.g. FIG. 11). An overhead hoist or crane may be used to deploy the absorber insert 400. The insert 400 is then slowly lowered into the cell 110A through open top end 112 of the cell. After the lower end 416 of the absorber insert 400 passes through the cell top end 112, at least one of the locking spring clips 430 slideably engages the top end 310 of at least one absorber sheath 300. The spring clip 430 compresses and folds inward to the deflected and retracted position against the absorber tube 424. As the absorber insert 400 continues to be lowered farther into the cell 110A, the locking portion 434 of the spring clip 430 slides along the sidewall 312 of the sheath 300 and remains in the compressed retracted position. When the spring clip 430 eventually passes beneath and reaches a lower elevation in cell 110A below the bottom end 311 of the sheath, the spring clip 430 will snap open via its elastic memory returning to the initial extended position of the spring clip thereby catching and lockingly engaging the bottom edge 436 of sheath 300 (see, e.g. FIGS. 5 and 11). This locking engagement between the sheath 300 and locking portion 434 of spring clip 430 prevents the absorber insert 400 from being axially withdrawn from the fuel rack cell 110A, thereby locking the insert in axial position in the fuel rack. Advantageously, reactivity control to cell 110A is fully restored despite the degraded original boron-containing neutron absorber material which may still be present in the sheath. The open cavity 422 of the low profile absorber insert 400 is configured to allow a fuel assembly 28 to be inserted into cell 110A following the absorber restoration process, and to be removed from the storage cell without requiring removal of the insert. It bears noting that while the upper stiffening band 404 rotationally and laterally stabilizes the upper portion of the absorber insert 400 in the storage tube 120, the sheath 300 on the tube sidewall and the spring clips 430 act to rotationally and laterally stabilize lower portions of the insert by preventing excessive movement even during a seismic event. The absorber insert 400 may also be used in some embodiments with a fuel storage tube 120 that does not include an absorber sheath 300 on at least one sidewall 116 for engagement by the spring clip 430, but instead includes an optional flow hole 115A as shown in FIG. 4. In such a case, the spring clip 430 may be configured and arranged on the absorber insert 400 to engage a top edge of the flow hole 115A for locking the insert axially in place in the tube. The insertion process and action of the spring clip 430 is the same as described above, except that the surface of the storage tube sidewall 116 engages the spring clip 430 to fold the clip inwards in the retracted position until it passes below the flow hole 115A. At that elevation, the clip springs or snaps back to the outward undeflected and extended position to lockingly engage the hole. FIG. 12 shows an alternative construction of an absorber insert 400 according to the present disclosure. In lieu of the upper and lower stiffening bands 404, 406 coupling two chevron-shaped or angled absorber plates 402A, 402B together as shown in FIG. 6, each absorber plate 402C, 402D may be shaped as a structural channel. A longitudinal slot 412 may be formed between mating edges 426 of the plates 402C and 402D as shown in FIG. 12. All other element of construction including spring clips 430 and stiffening bands 404, 406 may otherwise be the same as absorber plates 402A, 402B described herein. While the foregoing description and drawings represent exemplary embodiments of the present disclosure, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope and range of equivalents of the accompanying claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other forms, structures, arrangements, proportions, sizes, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. In addition, numerous variations in the methods/processes described herein may be made within the scope of the present disclosure. One skilled in the art will further appreciate that the embodiments may be used with many modifications of structure, arrangement, proportions, sizes, materials, and components and otherwise, used in the practice of the disclosure, which are particularly adapted to specific environments and operative requirements without departing from the principles described herein. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive. The appended claims should be construed broadly, to include other variants and embodiments of the disclosure, which may be made by those skilled in the art without departing from the scope and range of equivalents. |
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050080695 | description | DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The nuclear reactor in FIG. 1 comprises a pressure vessel 1 which is made of prestressed concrete and is provided with overpressure valves 2. A reactor core 3 included in the reactor is placed in its entirety in a reactor vessel 4. The core 3 is arranged in the lower half, usually in the lower fourth of the vessel space 5. The liquid in which the reactor vessel 4 is immersed is neutron-absorbing and in the exemplified case consists of an aqueous solution of boric acid. During normal operation the core 3 is cooled by a light water flow continuously flowing through the reactor vessel 4, the heat contents of the flow being utilized in a steam generator (not shown in FIG. 1). Water heated in the reactor departs via the conduit 6 to the steam generator and is pumped, after giving off its heat contents, via the conduit 7 back to the reactor. From the conduit 7 the water is brought via the gap 8 down to and from below through the core 3 and reaches the conduit 6 in heated condition after passage of the upper, tubular section 9 of the reactor vessel 4. During this procedure a boundary 10 is maintained between the coolant flowing through the reactor and the neutron-absorbing liquid in the upper part of the reactor, and a boundary 11 is maintained between these liquids in the lower part of the reactor. (The positions of these boundaries during normal operation are shown by dotted lines.) FIG. 1 shows the state of the reactor when a pressure equilibrium between the neutron-absorbing liquid and the reactor coolant, established during normal operation, has been disrupted and the neutron-absorbing liquid has flown into the reactor so that the power of the reactor core 3 has been reduced to the decay power. Above the liquid surface in the pressure vessel 1 a space 12 with gas and/or steam is then often provided. The pressure vessel 1 is adapted to form part of a circulation system for self-circulation of the liquid from the pressure vessel 1 with any contents of steam of the liquid and of uncondensable gas. The self-circulation is brought about by utilizing the property of the liquid to have a higher density at a lower temperature than at a higher temperature. The circulation system comprises an evaporator 14 arranged in an evaporation pool 13, a supply conduit 15 for conducting liquid from a point of connection 15a in the upper part of the pressure vessel 1 to the evaporator 14, and a discharge conduit 16 for conducting liquid from the evaporator 14 to a point of connection 16a, located below the point of connection 15a, on the pressure vessel 1. The evaporator 14, which may consist of a tubular coil, is located at a higher level than the points of connection 15a and 16a. The evaporation pool 13 may consist of a pool with water. Heat from the evaporation pool 13 is discharged passively by free departure of steam via a conduit 17 to a heat sink 18, which, for example, may consist of the surrounding open air. If the conduit 17 is closed, the heat, as will be described later on, may be discharged via the conduit 19. In certain operating situations, when the liquid level in the pressure vessel 1 may become low and lie below the point of connection 15a, only steam with any contents of uncondensable gas escapes from the vessel 1 via the conduit 15. In order to ensure, in all or practically all operating situations, that the heat transfer on the primary side of the evaporator 14, i.e., on that side which is part of the circulation system previously described, is good, the discharge conduit 16, at a level above the bottom 16b (FIG. 2), is connected to a discharge vessel (flash tank) 21 by a connecting conduit 20, which only allows a considerably smaller flow than the flow in the circulation system, the discharge vessel 21 being provided with one or more outlets 22a, 23a for gas and steam for maintaining a lower pressure in the discharge vessel 21 than in the evaporator 14. In an analogous manner, the supply conduit 15 may be connected, at a level above the bottom 15b (FIG. 2), to the discharge vessel 21, by means of a conduit 20a. By connecting the conduits 20 and 20a above the bottom of the conduits 15 and 16, a heat transport to the discharge vessel 21 is counteracted when the liquid flow in the discharge conduit 16 fills up a minor part of the tube area. Preferably, less than 1% of the flow is diverted through the evaporator 14 to the discharge vessel 21. As will be clear from the above description, the primary medium through the evaporator 14 may consist of different contributions of water and steam and any uncondensable gas occurring. The flow, which is passively discharged through the discharge vessel 21, prevents any major accumulation of uncondensable gas on the primary side of the evaporator 14 deteriorating the heat transfer to the evaporator 14. Passive discharge of the flow may take place by connecting the conduit 22 to a heat sink 24, which may, for example, consist of the surrounding open air, whereby the steam and the gas, before departing by themselves, are suitably brought to pass through a gas cleaning system. Alternatively, the discharge of the flow may take place through the conduit 23 to a heat sink 25 placed at a lower level than the discharge vessel 21, for example in the form of a pool of water. Via the previously mentioned tube 19, this heat sink 25 may communicate with the evaporation pool 13 in order to take up medium from this pool if the tube 17 is closed and overboiling occurs in the pool 13. Any condensed liquid which is collected in the discharge vessel 21 is discharged through the conduit 26 or through the conduit 23 if the conduit 26 is closed. The pressure vessel 1, the evaporation vessel 13 with the evaporator 14, the conduits 15 and 16, the discharge vessel 21 and the heat sink 25 are enclosed, in the illustrated case, in a common prestressed concrete containment 27. Within the concrete containment 27 is a space 28 which is filled with a gas, preferably air or nitrogen. This space is arranged in communication with the heat sink 25 by means of a liquid seal 29. The heat sink 25 is preferably connected to a heat sink (not shown) arranged outside the concrete containment 27, for example the surrounding open atmosphere (via a gas cleaning system) and can thus be said to serve as a temporary heat sink. Irrespective of whether the reason for the operating situation with decay power cooling of the core 3 is that a leakage has occurred in the pressure vessel 1 or in some other pressurized part within the concrete containment 27, or there is another reason, gas and steam will at least substantially be discharged from the discharge vessel 21 via the conduit 22. If the conduit 22 is closed to keep the discharged gas within the concrete containment 27, the gas and the steam will instead be discharged through the conduit 23. This will be the case even if a leakage of the kind mentioned has occurred to the space 28. The necessary pressure difference for this procedure arises because the conduits 20 and 20a in their entirety, or at least in some part, have a cross section which is small in relation to the cross section of the conduits 15 and 16 and will therefore serve as efficient throttle means for gas and steam transport. During normal operation of the device, i.e., when the reactor produces power which is utilized in the steam generator, gas and steam are preferably discharged out through the conduit 22 while at the same time water is discharged out through the conduit 26. The conduit 23 is then closed with the aid of a water seal at the outlet of the conduit into the liquid in the pool 25. During normal operation the liquid level in the pressure vessel 1 always lies above the point of connection 15a. |
claims | 1. A cask configured to receive a canister containing radioactive material, said cask having an inner surface delimiting a cask housing to receive the canister, said cask also comprising a canister extraction/insertion assembly bearing against said inner surface, the canister extraction/insertion assembly configured to move the canister inside the cask housing in a longitudinal direction of the cask, in a carried position in which this canister is free of any contact with said inner surface,characterized in that said canister extraction/insertion assembly is provided with a carriage bearing against said inner surface and with a canister support structure carried by said carriage, said carriage and support structure configured to take up a drawn-together position in a radial direction of the cask allowing contact between the canister and said inner surface, and a drawn-apart position in the radial direction of the cask in which the canister carried by the support structure takes up its carried position,and in that one of the two elements from among said carriage and said support structure is provided with at least one guide ramp cooperating with a ramp follower provided on the other of said two elements, said ramp configured so that-application of a relative translational movement between the carriage and canister support structure, in the longitudinal direction of the cask, causes a changeover of said two elements from said drawn-together position to said drawn-apart position, or conversely. 2. A cask according to claim 1, characterized in that said carriage is a travelling carriage, and in that said ramp follower is a roller. 3. A cask according to claim 1, characterized in that said canister extraction/insertion assembly is arranged in a cavity opened towards the cask housing, and defined by said inner surface of the cask. 4. A cask according to claim 3, characterized in that it comprises two canister supports partly defining said inner surface and being spaced at an angle around a longitudinal axis of the cask, so as to partly delimit said open cavity between them. 5. A package of radioactive material, characterized in that it comprises:a cask configured to receive a canister containing radioactive material, said cask having an inner surface delimiting a cask housing to receive the canister, said cask also comprising a canister extraction/insertion assembly bearing against said inner surface, the canister extraction/insertion assembly configured to move the canister inside the cask housing in a longitudinal direction of the cask, in a carried position in which this canister is free of any contact with said inner surface,said canister extraction/insertion assembly is provided with a carriage bearing against said inner surface and with the canister support structure carried by said carriage, said carriage and support structure configured to take up a drawn-together position in a radial direction of the cask allowing contact between the canister and said inner surface, and a drawn-apart position in the radial direction of the cask in which the canister carried by the support structure takes up its carried position,and in that one of the two elements from among said carriage and said support structure is provided with at least one guide ramp cooperating with a ramp follower provided on the other of said two elements, said ramp configured so that-application of a relative translational movement between the carriage and canister support structure, in the longitudinal direction of the cask, causes a changeover of said two elements from said drawn-together position to said drawn-apart position, or conversely, andthe canister containing said radioactive material and arranged inside said cask housing. 6. A package of radioactive material according to claim 5, characterized in that the canister is of cylindrical shape and has a circular section, and contains at least one of irradiated nuclear fuel assemblies and nuclear waste. 7. An extraction/insertion system for a canister containing radioactive material, the canister configured to be extracted from and inserted in a cask housing delimited by an inner surface of a cask, characterized in that said system comprises a canister extraction/insertion assembly configured to bear against said inner surface of the cask and being provided with a carriage and canister support structure carried by said carriage, said carriage and support structure configured to take up a drawn-together position in a radial direction of the cask allowing contact between the canister and said inner surface, and a drawn-apart position in the radial direction of the cask in which the canister carried by the support structure takes up a carried position in which this canister is free of any contact with said inner surface, one of the two elements from among said carriage and said support structure being provided with at least one guide ramp cooperating with a ramp follower provided on the other of said two elements, said ramp configured so that the application of a relative translational movement between the carriage and canister support structure, in a longitudinal direction of the cask, causes a changeover of said two elements from said drawn-together position to said drawn-apart position, or conversely, said extraction system also comprising mobilizing means in the longitudinal direction of the cask connected to one of said two elements, and retractable abutment means cooperating with the other of said two elements. 8. An extraction/insertion system according to claim 7, characterized in that said mobilizing means are connected to the carriage. 9. A method to transfer a canister containing radioactive material, from a first to a second entity chosen from among the group consisting of a cask and a receiver housing delimited by an inner surface, characterized in that it comprises the following successive steps consisting of:bringing the canister located inside the first entity to a carried position in which this canister is free of any contact with said inner surface associated with said first entity;mobilizing said extraction/insertion assembly carrying the canister so as to cause this extraction assembly and the canister to enter inside said second entity. 10. A method according to claim 9, characterized in that said first entity consists of the cask. 11. A method according to claim 10, characterized in that said step consisting of setting in movement said extraction/insertion assembly is conducted in a direction of extraction in the longitudinal direction of the cask, and in that it is followed by the following successive steps consisting of:bringing the canister to a deposited position inside the receiver housing in which this canister is free of any contact with said extraction assembly; andsetting in movement said extraction assembly in an opposite direction to the direction of extraction, so as to re-insert this extraction assembly inside said cask. 12. A method according to claim 11, characterized in that said canister extraction assembly belongs to an extraction system for a canister containing radioactive material, this canister configured to be extracted from and inserted in a cask housing delimited by an inner surface of a cask, said system comprising a canister extraction/insertion assembly configured to bear against said inner surface of the cask and being provided with a carriage and canister support structure carried by said carriage, said carriage and support structure configured to take up a drawn-together position in a radial direction of the cask allowing contact between the canister and said inner surface, and a drawn-apart position in the radial direction of the cask in which the canister carried by the support structure takes up a carried position in which this canister is free of any contact with said inner surface, one of the two elements from among said carriage and said support structure being provided with at least one guide ramp cooperating with a ramp follower provided on the other of said two elements, said ramp configured so that the application of a relative translational movement between the carriage and canister support structure, in a longitudinal direction of the cask, causes the changeover of said two elements from said drawn-together position to said drawn-apart position, or conversely, said extraction system also comprising mobilizing means in the longitudinal direction of the cask connected to one of said two elements, and retractable abutment means cooperating with the other of said two elements, the extraction system being used to implement said transfer method. 13. A method according to claim 12, characterized in that said step consisting of bringing the canister to its carried position inside the cask is conducted by implementing the following successive operations:connecting said mobilizing means of the extraction system to one of said two elements from among said carriage and said support structure of the extraction assembly;actuating a first abutment belonging to said retractable abutment means so as to bring this first abutment from a retracted position to an abutting position allowing the locking in translation of the other of said two elements in the direction of extraction of said longitudinal direction of the cask;actuating mobilizing means in the direction of extraction so as to cause movement of the carriage and support structure from said drawn-together position to said drawn-apart position; andactuating said first abutment so as to bring it from said abutting position to said retracted position. 14. A method according to claim 12, characterized in that said step consisting of bringing the canister to a deposited position inside the receiver housing is conducted by implementing the following successive operations:actuating a second abutment belonging to said retractable abutment means so as to bring this second abutment from a retracted position to an abutting position allowing the locking in translation of the other of said two elements in the opposite direction to the direction of extraction;actuating mobilizing means in said opposite direction to cause displacement of said carriage and support structure from said drawn-apart position to said drawn-together position; andactuating said second abutment so as to bring it from said abutting position to said retracted position. |
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claims | 1. A probe, comprising:an optical fiber having a tapered end forming an apex;an electronically conductive transparent film formed on a surface of the tapered end; anda first metal film formed on a first surface of said optical fiber other than the surface of the tapered end and electrically connected to said electrically conductive transparent film,wherein said probe is configured for use with a probe control apparatus that controls a distance between said probe and a sample via a shear force gap control method. 2. The probe as claimed in claim 1, wherein the length of said first metal film is no less than 5 mm and the thickness of said first metal film is from 0.2 μm to 10 μm. 3. The probe as claimed in claim 2, further comprising:a second metal film formed on a second surface of said optical fiber other than the surface of the tapered end, said second metal film being no less than 10 μm thick; anda transitional metal film formed on a third surface of said optical fiber other than the surface of the tapered end and connecting said first metal film with said second metal film, said transitional metal film having a thickness that continuously increases along a direction from said first metal film to said second metal film. 4. The probe as claimed in claim 1, further comprising:a material configured to prevent transmission of light from said electrically conductive transparent film to portions of the tapered end other than the apex. |
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summary | ||
047327308 | claims | 1. A stuck fuel rod capping sleeve comprising: an inner cylindrical sleeve made of low work hardening, highly ductile material and having tapered and split end portions; an outer cylindrical sleeve made of moderately ductile material and mounted on the inner cylindrical sleeve, the outer cylindrical sleeve having threads located at each end thereof; and a locking sleeve threaded on each end of the outer cylindrical sleeve, each locking sleeve having threads located on its exterior surface. an inner cylindrical sleeve made of low work hardening, highly ductile material and having tapered and split end portions; an outer cylindrical sleeve made of moderately ductile material and mounted on the inner cylindrical sleeve, the outer cylindrical sleeve having threads located at each end thereof; and a locking sleeve threaded on each end of the outer cylindrical sleeve, each locking sleeve having a tapered portion formed to cooperate with the tapered and split end portions of the inner cylindrical sleeve. 2. A stuck fuel rod capping sleeve as recited in claim 1, wherein a cap is threaded on each said locking sleeve. 3. A stuck fuel rod capping sleeve comprising: |
description | The principle of the emission noise reduction technique is shown in FIG. 1. The primary electrons are extracted from the Schottky emitter 10, focused by the source lens 30, accelerated to a final beam voltage of 1 keV and refocused with the final lens 70 onto the sample 90. As is known in the art, the electron-optical lenses may be either electrostatic lenses, magnetic lenses, or combination of the two. When a periodic voltage is applied to the deflection plates 60, the focused beam is swept across the sample 90 and generates secondary electrons. (As is known in the art, deflection coils could be used in place of the deflection plates.) Secondary electrons which escape from the sample surface strike the detector 80 and contribute to the signal Id which is used to create a secondary electron image. However, only a small fraction of the emitted electrons hit the sample. The majority of the emitted electron current Ie, typically 50-200 xcexcA, is collected by the extraction electrode 20 (which has an extraction electrode aperture extending through it). A small portion of the electron current, typically 100-300 nA, passes through the first lens 30. In a conventional set-up, the majority of this current is collected by the beam-limiting element 50 (having a beam-limiting aperture extending through it), and only a small fraction Ib, typically 1-50 nA, is utilized for imaging. The novel approach of the present invention utilizes a screening element 40 (having a screening aperture extending through it) located between the emitter 10 and the beam-limiting aperture 50, which screening aperture 40 collects most of the current transmitted by the first lens 30. Only a small fraction Iba of the electron current, approximately 1-10%, is collected by the beam-limiting aperture 50. (As used herein, references to the xe2x80x9cbeam-limiting aperturexe2x80x9d and xe2x80x9cscreening aperturexe2x80x9d should be understood to encompass the blocking or truncating structure that defines the aperture.) In order to achieve good noise suppression, the screening aperture 40 should let through only a portion of the beam in which the electrons are correlated. For electron emission along the axis of a Schottky emitter, the electrons are correlated within an emission half cone angle a given approximately by α = 2 π kT Φ where T is the tip temperature, k is Boltzmann""s constant and "PHgr" is the electron energy. At 1800K, xcex1 is 14 mrad for 1 kV electrons, which is more than typically used in the microcolumn operation (5-10 mrad). The current Iba collected by the beam-limiting aperture 50 is then used as a reference signal in the image processing. Specifically, current measuring circuitry coupled to the beam-limiting aperture 50 measures the portion of the electron beam that is blocked. An implementation of the noise suppressing scheme is illustrated also in FIG. 2. The current Iba (top graph) collected by the screened beam-limiting aperture 50 shows an emission noise peak 100 made while the electron beam Ib is scanned (for example along the x-axis) across the sample 90. The secondary electron signal Id (middle graph) includes an emission noise peak 110, superimposed on the imaging signal representing useful substrate information. This additional peak, due to the fluctuation in the emission current, could be interpreted as a substrate defect. The spurious emission noise peak 110 can then be suppressed or eliminated from consideration by processing the secondary electron signal Id data using the current Iba data collected by the screened beam-limiting aperture 50 (bottom graph). For example, the secondary electron signal Id may be divided by the current Iba collected by the beam-limiting aperture 50 or, alternatively, the current Iba collected by the beam-limiting aperture 50 may be subtracted from the secondary electron signal Id. (If needed, prior to such subtraction or division, either or both of the electron signal Id data or the current Iba data may linearly transformed with a shift of the origin or multiplication by a scaling factor.) The correction of the secondary electron signal Id data to account for emission noise by using the current Iba data collected by the screened beam-limiting aperture 50 can be suitably carried out by a processor. The elimination of the effect of the emission noise increases the detection sensitivity of an inspection tool, in particular to defects smaller than the beam spot size. This allows the use of a larger spot size and the imaging of the substrate on a more coarse pixel grid. Such imaging in turn reduces the total number of required pixels and therefore increases the throughput of the tool. The role of the screening aperture is crucial. If, for example, the current from the extractor electrode aperture or an un-screened beam-limiting aperture were used as a reference signal, the probability of noise suppression would be significantly reduced. This is due to the fact that the electron emission from the tip is strongly localized, and varies on a microscopic scale. Consequently, the electron beam varies spatially such that the noise in one part of the beam may be quite independent from the noise in a different part of the beam. The majority of the emitted electron current Ie, collected by the extraction electrode, includes thermal emission from the shank of the emitting tip, and is therefore not a sensitive measure of emission noise near the tip apex. Similarly, the current collected by an un-screened beam-limiting aperture contains emission from emitting regions that do not contribute to the beam current Ib. The electron current collected by the extraction electrode or an unscreened beam-limiting aperture has been used before as a means of trying to stabilize the emitted electron current using a direct feedback loop. This earlier approach did not prove practical, for the reasons described above. The use of a feedback loop to control the electrostatic field applied to the emitter has the further disadvantage of disturbing the dynamic equilibrium between electrostatic forces, surface migration and electron emission at the tip, which results in varying electron emission conditions and electron-optical properties. The scope of the present invention is meant to be that set forth in the claims that follow and equivalents thereof, and is not limited to any of the specific embodiments described above. |
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description | FIG. 5 is a perspective view of a double strip mixing grid for nuclear reactor fuel assemblies having a 5xc3x975 array in accordance with the primary embodiment of the present invention. FIG. 6 is a plan view of the double strip mixing grid of FIG. 5, with fuel rods removed from the grid. FIG. 7 is a plan view of the double strip mixing grid of FIG. 5, showing the operational function of mixing blades of the grid. FIG. 8 is a plan view of the double strip mixing grid of FIG. 5, showing the coolant currents formed by the mixing blades of the grid. FIG. 9 is a perspective view of a double strip mixing grid for nuclear reactor fuel assemblies having a 5xc3x975 array in accordance with the second embodiment of the present invention. FIG. 10 is a plan view of the double strip mixing grid of FIG. 9, with only one fuel rod set within the central cell of the rid. FIG. 11 is a plan view, showing the construction of one intersection of the grid of FIG. 10. FIG. 12 is a top perspective view of the intersection of FIG. 11. FIG. 13 is a bottom perspective view of the intersection of FIG. 11. FIG. 14 is a perspective view of a nozzle sheet of each, double strip according to this invention. FIG. 15 is a perspective view of a bladed sheet of the double strip according to this invention. FIG. 16 is a perspective view of a double strip having a coolant channel according to this invention, with the coolant channel, being cut along its central axis. As shown in the drawings, the double strip mixing grid 310 or 410 according to the preferred embodiments of this invention is used for placing and supporting a plurality of elongated nuclear fuel rods 325 within a nuclear reactor fuel assembly, and comprises two sets of intersecting double inner strips, which are arranged while intersecting each other at right angles prior to being encircled with four perimeter strips, thus forming an egg-crate pattern. Each of the two sets of grid strips is fabricated by integrating two thin sheets together into a single structure while defining a plurality of coolant channels in each of the two strips. In such a case, the two thin sheets of each of the strips are preferably continuously welded together at their junctions. In the double strip mixing grid 310 or 410 of this invention, the outlet of each coolant channel is formed by cutting a predetermined portion of one sheet of each of the two strips at the top edges of the strips. In addition, the cross-sectional area of each of said coolant channels is gradually enlarged in a direction from the inlet to the outlet of the channel, or varies such that it is maximized at a middle portion of the channel supporting a fuel rod within each, four-walled cell. In the double strip mixing grid 310 or 410 of this invention, each of the intersecting strips has a thickness preferably ranging from 0.25 mm to 0.40 mm, while each of the coolant channels has a width preferably ranging from 7 mm to 10 mm. In the double strip mixing grid 310 of FIG. 5, the two intersecting double strips are each fabricated by integrating two stamped thin sheets together into a single structure while forming a plurality of regularly spaced coolant channels between the two sheets. In the-present invention, each of the thin sheets of the double strip is preferably made of zircaloy, or the alloy of tin, iron, chrome and zirconium. However, it should be understood that the sheets of the strips may be preferably made of inconel that has been typically used as a material of such grid strips in the prior art. In the mixing grid 310 of FIG. 5, four first grid strips, each having a plurality of swirling flow blades 330 and a plurality of lateral-flow blades 331, regularly intersect four second grid strips, having the same construction as that of the first strips, at right angles prior to being encircled with four perimeter strips, thus forming a double strip mixing grid having a 5xc3x975 array with twenty five cells. In FIG. 5, the perimeter strips are not shown, and so the sixteen outside cells defined by the two sets of intersecting inner strips and the four perimeter strips are not-completely formed. Only nine fuel rods 325 are set in the nine inside cells formed by the intersecting inner strips. In the double strip mixing grid 310 of FIG. 5, the coolants flowing in the coolant channels of the strips are mixed with the coolants flowing outside the channels at positions around the mixing blades provided at the nozzles of the channels, thus forming swirling flow currents at positions around the swirling flow blades 330 and lateral flow currents at positions around the lateral flow blades 331. The coolants within the nuclear fuel assembly are thus actively and effectively mixed together. In such a case, the swirling flow currents are formed at positions around the intersections of the strips included in the grid, while the lateral flow currents are created between the four-walled cells of the grid. In such a case, the lateral flow currents of coolants flow toward the swirling flow blades 330, and so the lateral flow currents promote the formation of a swirling flow of coolants, in addition to forcing the coolants to be actively and effectively mixed together. FIG. 6 is a top plan view, showing the configuration of the double strip mixing grid 310 of FIG. 5 having both the swirling flow blades 330 and the lateral flow blades 331. The arrangement of both the swirling flow blades 330 and the lateral flow blades 331 on the intersecting double strips of the grid is shown in FIG. 6 in more detail. In the double strip mixing grid of FIG. 6, a plurality of first inner double strips 315, having only the swirling flow blades, intersect a plurality of second inner double strips 316, having both the swirling flow blades and the lateral flow blades, at right angles to form a desired egg-crate pattern. In the present invention, it is possible to fabricate a desired double strip mixing grid 310 of FIGS. 5 and 6 by intersecting the two types of double strips 315 and 316 together at right angles prior to encircling the intersected strip structure with the four perimeter strips. FIGS. 7 and 8 show the blade shape of the double strip mixing grid 310 of FIG. 6, and the coolant currents 340 formed by the swirling flow blades 330 and the coolant current 341 formed by the lateral flow blades 331 of the grid 310. It is possible for those skilled in the art to more clearly understand the style of the coolant currents formed by the two types of blades 330 and 331, in addition to understanding the forming positions of the coolant currents within the grid in more detail. FIG. 9 shows a double strip mixing grid 410 for nuclear reactor fuel assemblies having a 5xc3x975 array in accordance with the second embodiment of the present invention, with the double strips of the grid 410 having only the swirling flow blades 330 without having the lateral flow blades 331, different from the double strip mixing grid 310 of the primary embodiment. In this drawing, only one fuel rod is set in the central cell of the mixing grid 410 for ease of description. In the present invention, it is possible to fabricate a double strip mixing grid 410 of the second embodiment using two types of strips 318 and 319. However, it should be understood that the grid 410 of the second embodiment can be formed using one of either type of strips 318 or 319 without affecting the functioning of this invention. FIG. 10 is a plan view of the double strip mixing grid 410 of FIG. 9, showing both the shape of the mixing blades including the swirling flow blades 330 and the shape of the intersecting double strips in more detail. FIG. 11 is a plan view, showing the construction of one intersection of the double strip mixing grid 410 of FIG. 10. The shape of the swirling flow blades 330 formed on the grid 410 of the second embodiment is shown in more detail in FIG. 11. In the present invention, the two sets of inner strips are intersected together at right angles, and are welded together at their intersections through a TIG (Tungsten Inert Gas) welding process or a laser beam welding process, thus forming the welded intersections 317 as shown in FIG. 11 and a desired number of four-walled cells for the fuel rods. FIGS. 12 and 13 are top and bottom perspective views of the intersection of FIG. 11, respectively. In a nuclear fuel assembly fabricated with the double strip mixing grids of this invention, coolants flow from the bottom edge of each grid to the top edge of each grid while passing through the inside and outside of the coolant channels defined in the two sheets of each of the intersecting double strips, and are mixed together at positions around the nozzles of the coolant channels, provided along the top edge of the grid and having the swirling flow blades, thus forming desired swirling motion of coolants. Such a swirling motion of coolants is shown by the arrows designated by the reference numeral 340 in FIG. 12. In order to prevent the fuel rod positioning springs of the intersecting double strips included in the mixing grid of this invention from being excessively increased in their strength, a vertical slot 350 is formed on each sheet of the strips at a position around each coolant channel. When each thin sheet of the intersecting double strips, having a thickness of 0.35 mm, is provided with a slot 350, having a width of 0.5 mm and a length of 16 mm, at each of the spring portions, it is possible to decrease the stiffness of the positioning springs by ⅓ times, and increase the elastic range of the springs two times in comparison with a grid not having such slots. FIG. 14 is a perspective view of a nozzle sheet of each double strip included in the mixing grid of this invention. FIG. 15 is a perspective view of a bladed sheet of the double strip included in the mixing grid of this invention. When the two sheets of FIGS. 14 and 15 are integrated together into a single structure, it is possible to fabricate a double strip having a plurality of coolant channels of FIGS. 11, 12 and 13. In In the present invention; a plurality of first portions having coolant nozzles 361 and a plurality of second portions having swirling flow blades 330 are alternately arranged along each sheet of the double strips of the double strip mixing grid of this invention. The two sheets may be integrated into a first double strip 318 or a second double strip 319 of FIGS. 11, 12 and 13 by arranging the nozzles 361 and the blades 330 at alternating positions. A plurality of first and second double strips 318 and 319 intersect each other at right angles prior to being welded at their intersections, thus forming a desired double strip mixing grid of FIG. 8. FIG. 16 is a perspective view of a double strip, fabricated by integrating the two sheets of FIGS. 14 and 15 together to form a plurality of regularly spaced coolant channels between the two sheets. In the drawing of FIG. 16, the coolant channel is cut along its central axis. As shown in FIG. 16, coolant flows into the channel through the inlet 360 and flows out of the channel from the outlet 361 during an operation of the nuclear reactor. In such a case, the coolant current from the outlet 361 restricts a formation of vortexes in the coolant, which flows outside the channel and collides against the mixing blades to make such vortexes if the grid does not form such a coolant current discharged from the outlet 361. The coolant current from the outlet 361 also makes a smooth flow of coolant currents formed by the mixing blades. In the double strip mixing grid of this invention, the coolant mixing blades are provided along the top edges of the intersecting double strips having coolant channels. Therefore, it is possible to fabricate a double strip mixing grid 410 having only the swirling flow blades, a double strip mixing grid 310 having both the swirling flow blades and the lateral flow blades, or a double strip mixing grid having only the lateral mixing blades. As described above, the present invention provides a double strip mixing grid for nuclear reactor fuel assemblies. In the present invention, a plurality of inner double strips, each fabricated by integrating two thin sheets together into a single structure having a plurality of coolant channels, are intersected at right angles to form a desired mixing grid. Due to the coolant channels, the mixing grid of this invention effectively mixes the low temperature coolant with the high temperature coolant within a nuclear fuel assembly during an operation of a nuclear reactor, thus improving the thermal efficiency of the nuclear fuel assemblies. This mixing grid also effectively prevents the coolant from being partially overheated, and so it is possible to improve the soundness of nuclear reactors. In the double strip mixing grid of this invention, the coolant currents discharged from the nozzles of the channels restrict a formation of vortexes in the coolant, which flows outside the channels and collides against the mixing blades of the top edges of the strips to make such vortexes if the grid does not form such coolant currents discharged from the nozzles of the channels. Therefore, the mixing grid of this invention further enhances its coolant mixing function in comparison with conventional mixing grids only having the mixing blades without such channels. This mixing grid thus further improves the thermal efficiency of the nuclear fuel assemblies. In addition, the double strip mixing grid of this invention is designed to elastically support an elongated fuel rod by the sheets of the double strips collaterally acting as positioning springs. Therefore, each fuel rod set within a cell of the grid of this invention is elastically supported by four positioning springs. This means that the double strip mixing grid of this invention effectively supports a displacement of each fuel rod by two positioning springs in the same manner as that of a conventional grid having both positioning springs and dimples. The mixing grid of this invention thus stably supports each fuel rod during an operation of a nuclear reactor, different from a conventional grid supporting the fuel rod by one positioning spring. In addition, the double strips of the mixing grid of this invention is provided with a vertical slot at a position, where the strip comes into contact with a fuel rod while supporting the fuel rod. The elastic range of the positioning springs of the mixing grid according to this invention is preferably enlarged. Due to the slots, each positioning spring of the double strip is desirably and elastically opened at a position around each slot to support a fuel rod at two support surfaces when the spring supports the fuel rod. Therefore, each positioning spring provides two support surfaces for the fuel rod, thus enlarging the fuel rod contact area of the grid and effectively protecting the fuel rod from a fretting corrosion. In addition, the intersecting inner strips of the double strip mixing grid of this invention may be preferably and continuously welded together at their intersections through a continuous welding process, in addition to a conventionally performed alternate spot welding process. Therefore, it is possible to improve the mechanical strength of the mixing grid for nuclear fuel assemblies. This finally improves the mechanical strength of the nuclear fuel assemblies. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. |
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048470066 | claims | 1. A solid bitumen product for long term storage of radioactive waste and having encapsulated therein a granular ion-exchange resin, said ion-exchange resin being at least partially saturated with radioactive ions the product characterized in that the ion-exchange resin is in a swollen aqueous form. 2. A method for producing a solid bitumen product for long term storage of radioactive waste having encapsulated therein a granular ion-exchange resin including radioactive ions, the method characterized by mixing the ion-exchange resin with a bitumen and water emulsion, and by adding the ion-exchange resin to the emulsion in an amount at which the break point of the emulsion is reached and the mixture is transformed to a solid product in which the ion-exchange resinis present in a swollen aqueous form. 3. The method according to claim 2 characterized further is that the mixing is carried out with the ion-exchange resin in a moist swollen condition. 4. The method according to claim 2 characterized further in that the mixing is initiated with the ion-exchange resin in dry condition. 5. The method according to claim 2 characterized further by being carried out at a temperature of from 5-60% C. 6. The method according to claim 2 characterized further by inclusion in the mixing of a cationic emulsion having a pH value lower than 4. 7. The method according to claim 2 characterized further in that the emulsion has a water content of from 15-50% by weight. 8. The method according to claim 2 characterized further in that the emulsion has a water content of from 15-30% by weight. |
description | 1. Field of the Invention The present invention relates to a method of manufacturing a radiological image conversion panel and also relates to a radiological image conversion panel. 2. Background of the Related Art This kind of radiological image conversion panel is used in an indirect type (X-ray indirect conversion system) of radiation detector together with a sensor panel having a plurality of photoelectric conversion elements. This kind of radiation detector is known, e.g., in JP-A-2012-159305. The radiation detector has a phosphor layer (scintillator) made up of a group of columnar structures formed on one of surfaces of a substrate with fluorescent crystals being respectively formed in the form of columns. In this arrangement, the light generated inside each of the columnar structures through radiation exposure is caused to be propagated while being confined within the columnar structures by taking advantage of the difference between the refractive index of the columnar structures and the refractive index of the gaps (air) between the columnar structures (optical confinement effect). The light is caused to be incident on the photoelectrical conversion elements respectively disposed opposite to the columnar structures. The light is thus converted into electrical signals (image signals) depending on the amount of light. If the light generated in the columnar structures gets leaked outside without being confined to the columnar structures, the leaked light will be incident on columnar structures other than the photoelectrical conversion elements on which the light is originally intended to be incident. The amount of light will then become insufficient in the photoelectrical conversion elements on which the light is originally intended to be incident. On the other hand, the amount of light increases in the photoelectrical conversion elements into which additional light is caused to be incident. As a result, the quality of image is deteriorated. Therefore, in order to obtain clear images in the radiation detector, it becomes important how the leaking light to the outside of the columnar structures can be suppressed. As a solution, in the above-described conventional example, the following proposal is made. Let that side of the columnar structures which lie on the side of the substrate be defined as a base end and let that side of the columnar structures which lie on the side of the photoelectric conversion elements be defined as a front end. Then, the front end of the group of columnar structures is covered with a reflection film which is made of a metal or a metal alloy. However, if the reflection film is formed in a manner to fill the gaps between the front ends of the adjoining columnar structures, the light will be reflected only on the interface between the columnar structures and the reflection film. Further, since there is no reflection film on the side of the base end of the substrate, there is a limit in effectively suppressing the light from leaking to the outside of the columnar structures. In view of the above-described points, this invention has an advantage of providing a method of manufacturing a radiological image conversion panel as well as a radiological image conversion panel in which leaking of light out of columnar structures of a phosphor layer can be effectively suppressed. In order to solve the above problems, the present invention is a method of manufacturing a radiological image conversion panel comprising a phosphor layer containing therein a fluorescent substance which emits light through radiation exposure. The method comprises the step of forming the fluorescent substance into respective columnar structures on one of surfaces of a substrate to thereby obtain a phosphor layer made up of a group of columnar structures. The method further comprises the step of forming reflection films by respectively covering an outer surface of each of the columnar structures with a reflection film while leaving a gap between respective adjoining columnar structures. The reflection film is arranged to reflect light of a predetermined wavelength. In case a refractive index of the gap is lower than a refractive index of the columnar structures, the reflection films are formed of an inorganic material having a higher refractive index than the refractive index of the columnar structures. In this invention, the term “gap” (or clearance) refers not only to the case where the atmosphere (refractive index 1) is present between the respective adjoining columnar structures, but also to the case where a material having a lower refractive index than the refractive index of the columnar structures is buried in the gap. Further, according to this invention, the feature of forming the reflection films of an inorganic material refers not only to a case where the reflection film is formed of a single-layer film of an inorganic material, but also to a case where the reflection film is formed of a laminated film of different inorganic materials. According to this invention, the light generated in the columnar structures through radiation exposure will be propagated along the inside of the columnar structures while reflecting over the entire length of the columnar structures not only along the interfaces between the columnar structures and the reflection films, but also along the outer surfaces (the surfaces on the side of the gaps) of the reflection films. Therefore, the leaking of light out of the columnar structures can be effectively suppressed. As a result, when the radiological image conversion panel obtained by this invention is applied to a radiation detector, the light propagated along the inside of the columnar structures can be made to be incident on the photoelectric conversion elements on which the light is originally intended to be incident. Therefore, the quality of the image can be improved. In this invention, in case the columnar structures have deliquescent characteristics, preferably the method further comprises, prior to the step of forming the reflection films, the step of forming a moisture-proof film in a manner to cover the outer surface of each of the columnar structures. The moisture-proof film is formed of an inorganic material having a lower refractive index than the refractive index of the reflection films. Therefore, the above-described effect of suppressing the light from leaking out of the columnar structures is not impaired. Further, even in case the moisture-proof film is deteriorated through reaction with water, since the reflection film is present on an outside of the moisture-proof film, the shape of the columnar structures as well as the shape of the radiological image conversion panel can be maintained. In this case, aluminum oxide film is preferably used as the moisture-proof film, and zinc oxide film is preferably used as the reflection film. By the way, the feature in this invention in that the moisture-proof film is made of an inorganic material applies not only to the case where the moisture-proof film is formed of a single-layer film of an inorganic material, but also to the case where the moisture-proof film is formed of a laminated film of different inorganic materials. Further, in order to solve the above-described problems, this invention is a method of manufacturing a radiological image conversion panel comprising a phosphor layer containing therein a fluorescent substance which emits light through radiation exposure. The method comprises the step of forming the fluorescent substance into respective columnar structures on one of surfaces of a substrate to thereby obtain a phosphor layer made up of a group of columnar structures. The method further comprises the step of forming reflection films by respectively covering an outer surface of each of the columnar structures with a reflection film while leaving a gap between respective adjoining columnar structures. The reflection film is arranged to reflect light of a predetermined wavelength. In case a refractive index of the gap is higher than a refractive index of the columnar structures, the reflection films are formed of an inorganic material having a lower refractive index than the refractive index of the columnar structures. According to the above-described invention, the light generated in the columnar structures through radiation exposure will be propagated along the inside of the columnar structures while reflecting over the entire length of the columnar structures not only on the interfaces between the columnar structures and the reflection films, but also on the outer surfaces (the surfaces on the side of the gaps) of the reflection films. Therefore, the leaking of light out of the columnar structures can be effectively suppressed. As a result, when the radiological image conversion panel obtained by this invention is applied to a radiation detector, the light propagated along the inside of the columnar structures can be made to be incident on the photoelectric conversion elements on which the light is originally intended to be incident. Therefore, the quality of the image can be improved. The method of manufacturing a radiological image conversion panel according to this invention preferably further comprises the step of filling the gaps with a reflection material having a higher refractive index than the refractive index of the fluorescent substance. In this invention in case the columnar structures have deliquescent characteristics, the method preferably further comprises, prior to the step of forming the reflection films, the step of forming a moisture-proof film in a manner to cover the outer surface of each of the columnar structures. Then, the deliquescence of the columnar structures can advantageously be suppressed. Furthermore, by arranging that the moisture-proof film is made of an inorganic material having a higher refractive index than the refractive index of the reflection films, effect of suppressing the light from leaking out of the columnar structures is not impaired. Further, even in case the moisture-proof film is deteriorated through reaction with water, since the reflection film is present on the outside of the moisture-proof film, the shape of the columnar structures as well as the shape of the radiological image conversion panel can be maintained. The reflection films are preferably formed by atomic layer deposition method. According to this arrangement, the outer surface of the respective columnar structures can be covered by the thin reflection film that is formed of an inorganic material. Therefore, the gaps can surely be secured between the columnar structures. In order to solve the above problems, there is provided a radiological image conversion panel comprising a substrate and a phosphor layer which is made up of a group of columnar structures of the fluorescent substance formed into respective columnar structures on one of surfaces of the substrate. The radiological image conversion panel further comprises a reflection film which reflects light of a predetermined wavelength, and the reflection film covers an outer surface of each of the columnar structures. In case a refractive index of the columnar structures is higher than a refractive index of a gap between respective adjoining columnar structures, the reflection film is formed of an inorganic material having a higher refractive index than the refractive index of the columnar structures. In this invention, in case the columnar structures have deliquescent characteristics, the radiological image conversion panel preferably further comprises a moisture-proof film covering an outer surface of each of the columnar structures, between the outer surface of each of the columnar structures and the reflection film. The moisture-proof film is preferably formed of an inorganic material having a lower refractive index than the refractive index of the reflection films. In order to solve the above problems, there is provided a radiological image conversion panel comprising a substrate and a phosphor layer which is made up of a group of columnar structures of the fluorescent substance formed into respective columnar structures on one of surfaces of the substrate. The radiological image conversion panel further comprises a reflection film which reflects light of a predetermined wavelength, and the reflection film is arranged to cover an outer surface of each of the columnar structures. In case a refractive index of the columnar structures is lower than a refractive index of a gap between respective adjoining columnar structures, the reflection film is formed of an inorganic material having a lower refractive index than the refractive index of the columnar structures. This invention includes a case in which a reflection material with a higher refractive index than a refractive index of the columnar structures, is filled into the gaps. In this invention, in case the columnar structures have deliquescent characteristics, the radiological image conversion panel preferably further comprises a moisture-proof film which covers the outer surface of each of the columnar structures, between the outer surface of each of the columnar structures and the reflective film. The moisture-proof film is further formed of an inorganic material having a higher refractive index than the refractive index of the reflection films. With reference to the accompanying drawings, a description will now be made of a radiological image conversion panel according to an embodiment of this invention by taking as an example in which this invention is applied to an X-ray indirect conversion system of detector. In each of the drawings the elements common to all are referenced with the same reference numerals and alphabets, so that repeated explanations are omitted. With reference to FIG. 1, alphabetical reference mark RD denotes a radiation detector, which is made up of a radiation image conversion panel 1 and a sensor panel 2. The radiation image conversion panel 1 is provided with a substrate 11 and a phosphor layer (scintillator) 12 which is formed on one of surfaces of the substrate 11. As the substrate 11 there may be used a carbon plate, glass plate, quartz substrate, sapphire substrate, and the like but, without being limited to the above, there may also be used a substrate that is capable of forming thereon columnar structures 12a as described hereinbelow. The phosphor layer 12 is made up of a group of columnar structures 12a in which the fluorescent substance is respectively formed into columnar structures. The columnar structures 12a are made up of a group of columnar structures 12a each being obtained by forming fluorescent substance into the shape of respective columns. These columnar structures 12a can be made up of columnar crystals that can be obtained, e.g., as a result of crystal growth. As the material for making up the columnar structures 12a, there may be used one which is selected from one of CsI:Tl, NaI:Tl, GOS (Gd2O2S), and the like. A description will now be made of an example in which there was used as the fluorescent substance making up the columnar structure 12a, CsI:Tl which has a luminous wavelength of 540 nm, a refractive index of 1.79, and deliquescent characteristics. The above-described radiological image conversion panel 1 is arranged such that an outer surface of each of the columnar structures 12a made of CsI:Tl is covered with a moisture-proof film 13 having moisture-proof characteristics (or water vapor barrier properties), while leaving or maintaining a gap 12b between the respectively adjoining columnar structures 12a. In addition, the surface of the moisture-proof film 13 is covered with a reflection film 14 which reflects the light of predetermined wavelengths (visible light), while leaving a gap 12b between the respectively adjoining columnar structures 12a. In this embodiment, since the gaps 12b between the columnar structures 12a are filled with air the refractive index of which is 1, the refractive index of the gaps 12b is arranged to be lower than the refractive index of the columnar structures 12a. The reflection film 14 is formed of an inorganic material such as zinc oxide, silicon nitride, titanium oxide, zinc sulfide, niobium oxide, and the like which has a higher refractive index than the refractive index of the columnar structures 12a. The moisture-proof film 13 is formed of an inorganic material such as aluminum oxide, silicon oxide and the like which has a lower refractive index than the refractive index of the reflection film 14. The moisture-proof film 13 and the reflection film 14 may be formed not only by single-layer films of the above-described inorganic material but also by laminated films of different inorganic materials. As a method of forming these moisture-proof film 13 and the reflection film 14, it is preferable to use an atomic layer deposition method (ALD method), but other forming methods such as CVD and the like may also be used. In case the columnar structures 12a are made of CsI:Tl, the reflection film 14 shall preferably be formed of a zinc oxide film having a refractive index of 1.9-2.0, and the moisture-proof film 13 shall preferably be formed of an aluminum oxide film having a refractive index of 1.63. According to this arrangement, by covering the aluminum oxide film 13 with the zinc oxide film 14 that does not react with moisture, the aluminum oxide film 13 can be prevented from deteriorating through reaction thereof with moisture. Even if the aluminum oxide film 13 is deteriorated, the shape of the columnar structures 12a and consequently the shape of the radiological image conversion panel 1 can be maintained. The above-described sensor panel 2 is provided with a substrate 21, a plurality of photoelectric conversion elements 22 formed on the surface of the substrate 21, and a protective film 23 that covers these photoelectric conversion elements 22. The photoelectric conversion elements 22 are respectively disposed opposite to the columnar structures 12a such that, once the light propagated along the inside of the columnar structures 12a gets incident on the photoelectric conversion elements 22, the light is converted to an electrical signal (image signal) dependent on the amount of the incident light. As the sensor panel 2 there may be used one having a known construction. Therefore, detailed explanation thereof is omitted here. Next, a description will now be made of an apparatus for manufacturing a radiological image conversion panel (hereinafter simply referred to as a “manufacturing apparatus”) RM which is used in manufacturing the above-described radiological image conversion panel 1. The manufacturing apparatus RM illustrated in FIG. 2 is provided with a transfer chamber T in the center. This transfer chamber T has disposed therein a transfer robot R which transfers the substrate 11. As the transfer robot R there may be used a so-called frog-leg type of robot as illustrated, as well as other types of known robots. Therefore, detailed explanation thereof is omitted here. To the transfer chamber T there is connected vacuum exhaust means (not illustrated) so that the transfer chamber T can be maintained in a predetermined vacuum degree. The transfer chamber T is formed into the shape of a square in plan view (i.e., as seen from top) and has connected to the circumference thereof a load-lock chamber L and each of processing chambers A-C with a gate valve GV being interposed therebetween. In the processing chamber A phosphor layer 12 is formed by a vacuum vapor deposition method, in the processing chamber B moisture-proof film 13 is formed by atomic layer deposition method, and in the processing chamber C reflection film 14 is formed by atomic layer deposition method. Alternatively, the moisture-proof film 13 and the reflection film 14 may be formed in the same processing chamber. As the processing chamber A in which the phosphor layer 12 is formed by a vacuum vapor deposition method, there may be used one having a known construction. Therefore, detailed explanation thereof is omitted here. With reference to FIG. 3 a description will be made in concrete of the above-described processing chamber B. Since the above-described processing chamber C has a construction that is similar to that of processing chamber B, explanation thereof will be omitted here. The above-described manufacturing apparatus RM is provided with a vacuum chamber 31 which defines the processing chamber B. On a ceiling portion of the vacuum chamber 31 there is mounted a top plate 31a. In the following description, the direction looking toward the ceiling portion of the vacuum chamber 31 is defined as “up (or upper side)” and the direction looking toward the bottom side thereof is defined as “down (or lower side).” At the bottom portion of the vacuum chamber 31 there is provided a support member (stage) 32 which contains therein heating means such as a heater and the like (not illustrated), and an upper plate 33 which is movable up and down is mounted on the support member 32 in a manner to lie opposite to each other. To an upper surface of the upper plate 33 there is connected a driving shaft 34 of driving means (not illustrated). It is thus so arranged, by moving the driving shaft 34 in the up or down direction, that the upper plate 33 can be moved between a processing position illustrated in thick lines and a transferring position illustrated in imaginary lines. The support member 32 is provided with a lift pin 32a which is movable up and down. By lifting the lift pin 32a in a state in which the upper plate 33 has been lifted to the transferring position, the substrate 11 can be transferred. A side wall 32b is vertically disposed along the periphery portion of the support member 32b. When the lower surface of the upper plate 33 that has been lowered to the processing position comes into contact with the upper surface of the substrate 11, there is defined a reaction space Sp of a smaller volume inside the processing chamber B. The above-described manufacturing apparatus RM is provided with a gas nozzle 35 which faces the reaction space Sp. The gas nozzle 35 has connected thereto two gas pipes 36a, 36b which are in communication with different gas sources so that the first and the second raw gases can be supplied to the reaction space Sp in a pulsed manner. For example, as the first raw gas to be supplied from the gas pipe 36a, aluminum trimethyl gas can be used. As the second raw gas to be supplied from the gas pipe 36b, H2O gas, oxygen or ozone can be used. As the carrier gas for the raw gases, inert gas such as argon gas or nitrogen gas can be used. At the bottom of the vacuum chamber 31 there is provided an exhaust gas pipe 37 which is in communication with the vacuum exhaust means such as vacuum pump, and the like (not illustrated). It is thus possible to evacuate the processing chamber B and also to control the pressure in the reaction space Sp to a predetermined pressure. Although not illustrated, the above-described manufacturing apparatus RM is provided with a known control means having a microcomputer, a sequencer, and the like. It is thus so arranged that the control means performs an overall control over the operation of the transfer robot R, the operation of the lift pin 32a and the upper plate 33, the supply of raw gas, the operation of the vacuum exhaust means, and the like. A description will now be made of a method of manufacturing the above-described radiological image conversion panel RP by using the above-described manufacturing apparatus RM. First, the substrate 11 is housed into the load lock chamber L and the load lock chamber L is evacuated. Then, after setting in position the substrate 11 by the transfer robot R onto the stage inside the processing chamber A, the evacuating means is operated to evacuate the processing chamber A to a predetermined vacuum degree (e.g., 1×10−5 Pa). Thereafter, by means of the vacuum deposition method, a phosphor layer 12 is formed on the surface of the substrate 11 by forming a group of columnar structures 12a made of CsI:Tl to a length of 100-1000 μm (see FIG. 4(a)). The substrate 11 on which the above-described phosphor layer 12 has been formed is set in position by the transfer robot R onto the support member 32 in the processing chamber B. Thereafter, the upper plate 33 is lowered to define the reaction space Sp. Then, the substrate 11 is heated to a temperature of 80-150° C. and the reaction space Sp is supplied with aluminum trimethyl gas (carrier gas: N2 gas) and H2O gas in a pulsed state. In this manner, as illustrated in FIG. 4(b), the outer surfaces of the columnar structures 12a are respectively coated with an aluminum oxide film, which serves as a moisture-proof film 13, while leaving a gap 12b between the respectively adjoining columnar structures 12a (step of forming moisture-proof film). The thickness of the moisture-proof film 13 can be set within a range of 10-100 nm. After having formed the aluminum oxide film 13, the introduction of the gas into the reaction space Sp is stopped. The upper plate 33 is lifted and the lift pin 32a is also lifted. The substrate 11 on which is formed the above-described aluminum oxide film 13 is set in position by the transfer robot R onto the support member 32 in the processing chamber C. Thereafter, in a manner similar to the film deposition in the processing chamber B, the upper plate 33 is lowered to define the reaction space Sp. Then, the reaction space Sp is alternately supplied in a pulsed manner with diethylzinc gas (carrier gas: N2 gas) and H2O gas. In this manner, as illustrated in FIG. 4(b), the surface of the aluminum oxide film 13 is respectively covered with a zinc oxide film serving as a reflection film 14 while leaving a gap 12b between the respectively adjoining columnar structures 12a (reflection film forming step). The thickness of the reflection film 14 may be set to a range of 200-300 nm. Let us define that side of the columnar structures 12a which lies on the side of the substrate 11 as a base end portion, and let us define that side of the columnar structures 12a toward which they grow as a front end portion. Then, by removing the front end portions of the columnar structures 12a by chemical mechanical grinding and the like, there can be obtained a radiological image conversion panel 1 having the construction as illustrated in FIG. 4(c). Instead of forming the moisture-proof film 13 and the reflection film 14 of single-layer films, they may be respectively formed of laminated films. In this case, the film thickness of each of the films constituting the laminated film may be appropriately adjusted to suitable ones. As explained so far, according to this embodiment, the outer surfaces of the columnar structures 12a are covered with the moisture-proof film 13 and the reflection film 14 while leaving a gap 12b between the respective adjoining columnar structures 12a. As a result, the light generated within the columnar structures 12a through radiation exposure is propagated inside the columnar structures 12a while getting reflected not only on the interface between the columnar structures 12a and the moisture-proof film 13, and the interface between the moisture-proof film 13 and the reflection film 14, but also on the outer surface (surface on the side of the gap 12b) of the reflection film 14. Therefore, the leaking of the light from the columnar structures 12a can be effectively suppressed. As a result, when the radiological image conversion panel 1 obtained by this invention is applied to the radiation detector RD, the light that has propagated along the inside of the columnar structures 12a can be caused to be incident onto the photoelectrical conversion element 22 on which the light is originally intended to be incident. Therefore, the image quality can be improved. Further, according to this embodiment, since the columnar structures 12a having deliquescent characteristics are covered with the moisture-proof film 13, the columnar structures 12a can be prevented from getting deliquescent. Still furthermore, since the moisture-proof film 13 is covered with the reflection film 14, the moisture-proof film 13 can be prevented from getting deteriorated through reaction with the moisture. Should the moisture-proof film 13 be deteriorated, the shape of the columnar structures 12a and consequently the shape of the radiological image conversion panel 1 can still be maintained. In order to confirm the above-described effects, the following experiments were made. In these experiments, as the columnar fluorescent substances (columnar structures) 12a, Cs1 was formed on a glass substrate to a thickness of 600 nm by vacuum vapor deposition method. Then, the surface of the fluorescent substance 12a was covered with an aluminum oxide film 13 having a thickness of 50 nm by using the ALD method, and a zinc oxide film 14 was formed, by using the ALD method, on the surface of the aluminum oxide film 13 to a thickness of 300 nm. When the structures thus obtained were placed under the atmospheric tests of 60° C. and 90% relative humidity (RH), it has been confirmed that, even after the lapse of more than 24 hours, the fluorescent substance 12a was not deliquescent. On the other hand, when the radiological image conversion panel that was manufactured in a similar method as that of the above-described method, except for the fact that the zinc oxide film 14 was not formed, was placed under the same atmospheric tests, it has been confirmed that the fluorescent substance 12a was deliquescent at the point of time of 1 hour elapsing. According to the above, it has been confirmed that, by forming the zinc oxide film 14, the radiological image conversion panel can be prevented from getting deteriorated through reaction of the aluminum oxide film 13 with moisture. Explanation has so far been made of the embodiment of this invention, but this invention shall not be limited to the above. In the above-described embodiment an explanation has been made of an example in which the moisture-proof film 13 was interposed between the columnar structures 12a and the reflection films 14. However, in case the columnar structures 12a do not have deliquescent characteristics, the moisture-proof film 13 may be omitted. In this arrangement, the outer surfaces of the columnar structures 12a are covered by the reflection films 14 while leaving a gap 12b between the respectively adjoining columnar structures 12a. According to this arrangement, the light generated in the columnar structures 12a through radiation exposure will be propagated along the inside of the columnar structures while reflecting over the entire length of the columnar structures not only along the interface between the columnar structures 12a and the reflection films 14, but also along the outer surfaces (the surfaces on the side of the gaps 12b) of the reflection film 14. Therefore, the leaking of light out of the columnar structures can be effectively suppressed. Further, in the above-described embodiment a description was made of an example in which the refractive index of the gaps 12b is lower than the refractive index of the columnar structures 12a. This invention can, however, be applied to a case in which the gaps 12b are filled with titanium oxide having a refractive index of 2.2-2.6 so as to make the columnar structures 12a of Cs1 having a refractive index of 1.79, as illustrated in FIG. 5. In this case, as illustrated in FIG. 6(a), by coating the outer surface of the columnar structures 12a with a reflection film 14 formed of an inorganic material (e.g., silicon oxide) having a lower refractive index than the refractive index of the columnar structures 12a, it is possible, like in the above-described embodiment, to obtain a higher refractive index at the interface between the columnar structures and the reflection film 14, thereby preventing the light from leaking out of the columnar structures 12a. Further, in case the columnar structures 12a have deliquescent characteristics, in a manner similar to the above-described embodiment, it is preferable to interpose the moisture-proof film 13 which covers the outer wall surface of the columnar structures 12a and which is formed of an inorganic material having a higher refractive index than the refractive index of the reflection film 14. After formation of the reflection film 14, the gaps 12b are filled with titanium oxide. Then, like in the above-described embodiment, by removing the front end portions of the columnar structures 12a by chemical mechanical grinding and the like, there can be obtained a radiological image conversion panel 1 having the construction as illustrated in FIG. 6(b). |
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description | This application is a continuation-in-part of U.S. application Ser. No. 11/257,607, filed Oct. 24, 2005 now abandoned, incorporated herein by reference, which claims the benefit of U.S. Provisional Application No. 60/621,105, filed Oct. 22, 2004, incorporated herein by reference. The United States Government has rights in this invention pursuant to Contact No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC. 1. Field of the Invention The present invention relates neutron multiplicity counting techniques, and more specifically, it relates to such a neutron multiplicity counting technique that reduces pulse pile up dead time. 2. Description of Related Art The standard approach to neutron multiplicity counting is through the use of a “shift register” sliding word that is gated and counted repeatedly. Usually this gives data for one gate width. The shift register is a one input device where pulses can pile up and be lost. Another approach is a list mode data acquisition system. Every pulse is assigned a time fiducial and stored as a word. The volume of data that accumulates is many gigabytes if the objective is a non-destructive assay. A large quantity of data is required to minimize statistical errors. Neutron detection is the effective detection of neutrons entering a well-positioned detector. There are two key aspects to effect In this step lies the crucial point of the analysis: the extracted ionization values are plotted. Specifically, the graph plots energy deposition in the tail against energy deposition in the entire signal for a range of neutron energies. Typically, for a given energy, there are many events with the same tail-energy value. In this case, plotted points are simply made denser with more overlapping dots on the two-dimensional plot, and can thus be used to eyeball the number of events corresponding to each energy-deposition. A considerable random fraction ( 1/30) of all events is plotted on the graph. If the tail size extracted is a fixed proportion of the total pulse, then there will be two lines on the plot, having different slopes. The line with the greater slope will correspond to photon events and the line with the lesser slope to neutron events. This is precisely because the photon energy deposition current, plotted against time, leaves a longer “tail” than does the neutron deposition plot, giving the photon tail more proportion of the total energy than neutron tails. The effectiveness of any detection analysis can be seen by its ability to accurately count and separate the number of neutrons and photons striking the detector. Also, the effectiveness of the second and third steps reveals whether event rates in the experiment are manageable. If clear plots can be obtained in the above steps, allowing for easy neutron-photon separation, the detection can be termed effective and the rates manageable. On the other hand, smudging and indistinguishability of data points will not allow for easy separation of events. Detection rates can be kept low in many ways. Sampling of events can be used to choose only a few events for analysis. If the rates are so high that one event cannot be distinguished from another, physical experimental parameters (shielding, detector-target distance, solid-angle, etc.) can be manipulated to give the lowest rates possible and thus distinguishable events. It is important here to observe precisely those variables that matter, since there may be false indicators along the way. For example, ionization currents might get periodic high surges, which do not imply high rates but just high energy depositions for stray events. These surges will be tabulated and viewed with cynicism if unjustifiable, especially since there is so much background noise in the setup. One might ask how experimenters can be sure that every current pulse in the oscilloscope corresponds to exactly one event. This is true because the pulse lasts about 50 ns, allowing for a maximum of 2×107 events every second. This number is much higher than the actual typical rate, which is usually an order of magnitude less, as mentioned above. This means that is it highly unlikely for there to be two particles generating one current pulse. The current pulses last 50 ns each, and start to register the next event after a gap from the previous event. Although sometimes facilitated by higher incoming neutron energies, neutron detection is generally a difficult task, for all the reasons stated earlier. Thus, better scintillator design is also in the foreground and has been the topic of pursuit ever since the invention of scintillation detectors. Scintillation detectors were invented in 1903 by Crookes but were not very efficient until the PMT (photomultiplier tube) was developed by Curran and Baker in 1944. The PMT gives a reliable and efficient method of detection since it multiplies the detection signal tenfold. Even so, scintillation design has room for improvement as do other options for neutron detection besides scintillation. Detection hardware refers to the kind of neutron detector used (the most common today is the scintillation detector) and to the electronics used in the detection setup. Further, the hardware setup also defines key experimental parameters, such as source-detector distance, solid angle and detector shielding. Detection software consists of analysis tools that perform tasks such as graphical analysis to measure the number and energies of neutrons striking the detector. Experiments that make use of this science are typically scattering experiments whose scattered particles of interest are neutrons. Perhaps the most noteworthy among these experiments is the trademark experiment of the European Muon Collaboration, first performed at CERN and now termed the “EMC experiment.” The same experiment is performed today with more sophisticated equipment to obtain more definite results related to the original EMC effect. Neutron detection is also used at nuclear reactors and where californium-252 is used as a neutron source. Neutrons are a fundamental part of any experiment or technique involving nuclear fission, and thus detection of neutrons is an important part of the radiation protection strategy of such establishments. Neutron detection is used for varying purposes. Each application has different requirements for the detection system. For reactor instrumentation, neutron flux is an important measure of power in nuclear power and research reactors. Boiling water reactors may have dozens of neutron detectors, one per fuel assembly. Most neutron detectors used in nuclear reactors are optimized to detect thermal neutrons. In particle physics, neutron detection has been proposed as a method of enhancing neutrino detectors. Neutron radiation is a hazard in nuclear reactors. Neutron detectors used for radiation safety must take into account the way damage caused by neutrons varies with energy. Secondary neutrons are one component of particle showers produced in Earth's atmosphere by cosmic rays. Dedicated ground-level neutron detectors, namely neutron monitors, are employed to monitor variations in cosmic ray flux. Neutron detection is not an easy science. The major challenges faced by modern-day neutron detection include background noise, high detection rates, neutron neutrality, and low neutron energies. The main components of background noise in neutron detection are high-energy photons, which aren't easily eliminated by physical barriers. The other sources of noise, such as alpha and beta particles, can be eliminated by various shielding materials, such as lead, plastic, thermo-coal, etc. Thus, photons cause major interference in neutron detection, since it is uncertain if neutrons or photons are being detected by the neutron detector. Both register similar energies after scattering into the detector from the target or ambient light, and are thus hard to distinguish. Coincidence detection can also be used to discriminate real neutron events from photons and other radiation. Since the detector lies in a region of high beam activity, it is hit continuously by neutrons and background noise at overwhelmingly high rates. This obfuscates collected data, since there is extreme overlap in measurement, and separate events are not easily distinguished from each other. Thus, part of the challenge lies in keeping detection rates as low as possible and in designing a detector that can keep up with the high rates to yield coherent data. Neutrality of Neutrons Neutrons are neutral and thus do not respond to electric fields. This makes it hard to direct their course towards a detector to facilitate detection. Neutrons also do not ionize atoms except by direct collision, so gaseous ionization detectors are ineffective. Detectors relying on neutron absorption are generally more sensitive to low-energy thermal neutrons, and are orders of magnitude less sensitive to high-energy neutrons. Scintillation detectors, on the other hand, have trouble registering the impacts of low-energy neutrons. Gaseous ionization detectors can be adapted to detect neutrons. While neutrons do not typically cause ionization, the addition of a nuclide with high neutron cross-section allows the detector to respond to neutrons. Nuclides commonly used for this purpose are boron-10, uranium-235 and helium-3. Further refinements are usually necessary to isolate the neutron signal from the effects of other types of radiation. As elemental boron is not gaseous, neutron detectors containing boron use boron trifluoride (BF3) enriched to 96% boron-10 (natural boron is 20% B-10, 80% B-11). In a typical setup of a neutron detection unit, the incoming particles, comprising neutrons and photons, strike the neutron detector; this is typically a scintillation detector consisting of scintillating material, a waveguide, and a photomultiplier tube (PMT), and will be connected to a data acquisition (DAQ) system to register detection details. The detection signal from the neutron detector is connected to the scaler unit, gated delay unit, trigger unit and the oscilloscope. The scaler unit is merely used to count the number of incoming particles or events. It does so by incrementing its tally of particles every time it detects a surge in the detector signal from the zero-point. There is very little dead time in this unit, implying that no matter how fast particles are coming in, it is very unlikely for this unit to fail to count an event (e.g., incoming particle). The low dead time is due to sophisticated electronics in this unit, which take little time to recover from the relatively easy task of registering a logical high every time an event occurs. The trigger unit coordinates all the electronics of the system and gives a logical high to these units when the whole setup is ready to record an event run. The oscilloscope registers a current pulse with every event. The pulse is merely the ionization current in the detector caused by this event plotted against time. The total energy of the incident particle can be found by integrating this current pulse with respect to time to yield the total charge deposited at the end of the PMT. This integration is carried out in an analog-digital converter (ADC). The total deposited charge is a direct measure of the energy of the ionizing particle (neutron or photon) entering the neutron detector. This signal integration technique is an established method for measuring ionization in the detector in nuclear physics. The ADC has a higher dead time than the oscilloscope, which has limited memory and needs to transfer events quickly to the ADC. Thus, the ADC samples out approximately one in every 30 events from the oscilloscope for analysis. Since the typical event rate is around 106 neutrons every second, this sampling will still accumulate thousands of events every second. The ADC sends its data to a DAQ unit that sorts the data in presentable form for analysis. The key to further analysis lies in the difference between the shape of the photon ionization-current pulse and that of the neutron. The photon pulse is longer at the ends (or “tails”) whereas the neutron pulse is well-centered. This fact can be used to identify incoming neutrons and to count the total rate of incoming neutrons. The steps leading to this separation are gated pulse extraction and plotting-the-difference. Ionization current signals are all pulses with a local peak in between. Using a logical AND gate in continuous time (having a stream of “1” and “0” pulses as one input and the current signal as the other), the tail portion of every current pulse signal is extracted. This gated discrimination method is used on a regular basis on liquid scintillators. The gated delay unit is precisely to this end, and makes a delayed copy of the original signal in such a way that its tail section is seen alongside its main section on the oscilloscope screen. After extracting the tail, the usual current integration is carried out on both the tail section and the complete signal. This yields two ionization values for each event, which are stored in the event table in the DAQ system. In this step lies the crucial point of the analysis: the extracted ionization values are plotted. Specifically, the graph plots energy deposition in the tail against energy deposition in the entire signal for a range of neutron energies. Typically, for a given energy, there are many events with the same tail-energy value. In this case, plotted points are simply made denser with more overlapping dots on the two-dimensional plot, and can thus be used to eyeball the number of events corresponding to each energy-deposition. A considerable random fraction ( 1/30) of all events is plotted on the graph. If the tail size extracted is a fixed proportion of the total pulse, then there will be two lines on the plot, having different slopes. The line with the greater slope will correspond to photon events and the line with the lesser slope to neutron events. This is precisely because the photon energy deposition current, plotted against time, leaves a longer “tail” than does the neutron deposition plot, giving the photon tail more proportion of the total energy than neutron tails. The effectiveness of any detection analysis can be seen by its ability to accurately count and separate the number of neutrons and photons striking the detector. Also, the effectiveness of the second and third steps reveals whether event rates in the experiment are manageable. If clear plots can be obtained in the above steps, allowing for easy neutron-photon separation, the detection can be termed effective and the rates manageable. On the other hand, smudging and indistinguishability of data points will not allow for easy separation of events. Detection rates can be kept low in many ways. Sampling of events can be used to choose only a few events for analysis. If the rates are so high that one event cannot be distinguished from another, physical experimental parameters (shielding, detector-target distance, solid-angle, etc.) can be manipulated to give the lowest rates possible and thus distinguishable events. It is important here to observe precisely those variables that matter, since there may be false indicators along the way. For example, ionization currents might get periodic high surges, which do not imply high rates but just high energy depositions for stray events. These surges will be tabulated and viewed with cynicism if unjustifiable, especially since there is so much background noise in the setup. One might ask how experimenters can be sure that every current pulse in the oscilloscope corresponds to exactly one event. This is true because the pulse lasts about 50 ns, allowing for a maximum of 2×107 events every second. This number is much higher than the actual typical rate, which is usually an order of magnitude less, as mentioned above. This means that is it highly unlikely for there to be two particles generating one current pulse. The current pulses last 50 ns each, and start to register the next event after a gap from the previous event. Although sometimes facilitated by higher incoming neutron energies, neutron detection is generally a difficult task, for all the reasons stated earlier. Thus, better scintillator design is also in the foreground and has been the topic of pursuit ever since the invention of scintillation detectors. Scintillation detectors were invented in 1903 by Crookes but were not very efficient until the PMT (photomultiplier tube) was developed by Curran and Baker in 1944. The PMT gives a reliable and efficient method of detection since it multiplies the detection signal tenfold. Even so, scintillation design has room for improvement as do other options for neutron detection besides scintillation. It is desirable to provide neutron multiplicity counting utilizing multiple gates, with different definitions of the gate and counting approach, in a parallel analogy designed to reduce pulse pile up dead time. A system is desired that preprocesses neutron data into small files in real time, and reduces processing time required for gigabytes of list mode data. It is an object of the present invention to provide a digital data acquisition unit that collects data (e.g., neutron multiplicity data) at high rate and in real-time preprocesses large volumes of data into directly useable forms. This and other objects will be apparent to those skilled in the art based on the disclosure herein. Pulses from a multi-detector array are fed in parallel to individual inputs that are tied to individual bits in a digital word. Data is collected by loading a word at the individual bit level in parallel, so that there is no latency such as in a technique that uses a shift register. The word is read at regular intervals, all bits simultaneously, with no manipulation, to minimize latency. The electronics then pass the word to a number of storage locations for subsequent processing, thereby removing the front-end problem of pulse pileup. Latency is therefore limited to the latch time in the counter. The word is used simultaneously in several internal processing schemes that assemble the data in a number of more directly useable forms. The technique is useful generally for high-speed processing of digital data, and specifically for non-destructive assaying of nuclear material and assemblies for, typically, mass and multiplication of special nuclear material (SNM). The invention is a digital data acquisition method and apparatus that collects data at high rate and in real-time preprocesses large volumes of data into directly useable forms. To explain the invention, an exemplary neutron detector system is provided for making measurements on samples that contain fissile material. As shown in the neutron counting requirements matrix of FIG. 1, the system operates in two different modes and performs several classes of measurements. FIG. 16 is a flow chart of an embodiment the present method of event counting comprising: inputting edge triggered input signals into parallel input circuits observing each event to be counted; creating a clock to control a minimum summing interval wherein data is collected (counted), for use by a parallel set of means for adding, wherein each input circuit is operatively connected to multiple private (independent) means for adding of said parallel set; reading a sum in each said means for adding during said minimum summing interval to produce a sum read; zeroing each said means for adding at the end of the minimum summing interval; storing said sum read into multiple arrays; and constructing summed sections from said array to build data structures comprising multiple superset interval sizes, interval sizing after an external trigger, event totals in a fixed interval, event totals in a fixed interval after an external trigger, time intervals between events, time intervals between events after an external trigger, and arrival time of certain clump sizes after an external trigger. A second embodiment of the present method of event counting comprises: inputting input signals into parallel input circuits observing each event to be counted; controlling a minimum summing interval in which data is counted for use by a parallel set of means for adding; producing a sum read; zeroing each said means for adding; storing said sum read; and building data structures. In this second method, wherein said input signals may be edge triggered. The minimum summing interval is controlled with a clock. Each input circuit is operatively connected to multiple independent means for adding of said parallel set. The sum read is produced by reading a sum in each said means for adding during said minimum summing interval. The means for adding are zeroed at the end of a minimum summing interval. The sum read may be stored into multiple arrays. Data structures are built by constructing summed sections from said array and may comprise data selected from the group consisting of multiple superset interval sizes, interval sizing after an external trigger, event totals in a fixed interval, event totals in a fixed interval after an external trigger, time intervals between events, time intervals between events after an external trigger, and arrival time of certain clump sizes after an external trigger. The data structures may comprise multiple superset interval sizes, interval sizing after an external trigger, event totals in a fixed interval, event totals in a fixed interval after an external trigger, time intervals between events, time intervals between events after an external trigger, and arrival time of certain clump sizes after an external trigger. An apparatus for event counting according to the present invention comprises: means for inputting input signals into parallel input circuits observing each event to be counted; means for controlling a minimum summing interval in which data is counted for use by a parallel set of means for adding; means for producing a sum read; means for zeroing each said means for adding; means for storing said sum read; and means for building data structures. One may also describe the two modes as three modes: self triggered mode I, self triggered mode II and externally triggered mode II. Mode II counting when self-triggered is internally triggered like mode I. Mode II external trigger is typically called the neutron generator triggered counting. Mode I will be used for making measurements of neutrons generated by the natural radioactivity of the sample material. In this mode the detector system will employ internally generated, periodic triggers to detect neutrons in data acquisition gates (DAGs). DAG nomenclature is defined in FIG. 2. In this mode, the DAGs are uncorrelated with the neutron emission times. See FIGS. 1 and 2. Mode II will be required for measurements on samples with very low natural neutron activity; it may also be useful for measurements on some samples with higher natural activity. Most of the neutrons detected in this mode will be generated by interactions (mainly induced fission) initiated by pulses of 14-MeV neutrons injected into the sample material by an ion-tube (D,T) neutron generator. The periodic triggers for the detector, in this mode, are provided by the neutron generator, at a fixed time relative to the 14-MeV neutron pulses. The DAGs and the induced-fission neutrons emitted by the sample are thus highly correlated in time. See FIGS. 1 and 3. In both Mode I and Mode II, two classes of measurements (Class A and Class B) are required, and a third class (Class C) can provide valuable information in Mode II, but is not applicable to Mode I. For each class of measurement the neutrons detected within the DAGs must be sorted in different ways. In order to minimize overall data collection time, it is necessary to carry out the various classes of measurements (i.e., implement the different data sorting algorithms) simultaneously. (There may be cases, in Mode II, in which different Beam Delays are required for different measurement classes, which would require separate measurements.) Class A: In this class of measurement data will be sorted to record statistics on neutron multiplicities detected within temporal sub-gates with different widths. A Feynman Variance type of analysis can be carried out with these data. Although the same data sorting algorithm (the “Inefficient Implementation”) can be used for both Mode I and Mode II measurements, other sorting algorithms can greatly improve data collection efficiency in Mode I. It is feasible to implement at least one of these (the “Efficient Implementation”). FIGS. 4 and 5 are an example of the Mode 1A. FIGS. 10 and 11 are examples of Mode 2A. Class B: In this class of measurement data will be sorted to record statistics on the time intervals between successive neutrons detected within the DAGs. A Rossi-Alpha type of analysis can be carried out with these data. The same data sorting algorithm applies for both Mode I and Mode II. FIGS. 12 and 13 are examples of Mode 2B counting. FIGS. 6, 7, 8 and 9 are examples of Mode 1B type of counting. Class C: In this class of measurement data are sorted according to the number of multiplets in each time bin within the data acquisition gate. These data allow one to measure the neutron die-away following the injection of the e.g., 14-MeV neutron pulse into the sample. FIGS. 14 and 15 are examples of Mode 2C counting. In summary, four different data sorting algorithms (depending on how you choose to categorize the counting modes) must be implemented in order to carry out all of the classes of analysis that are necessary for both Mode I and Mode II measurements, although only two are applicable in Mode I and only three are applicable in Mode II. It is desirable to implement simultaneous sorting of data by all four algorithms for all measurements, in order to simplify field operation of the detector system. Analyses will be carried out, of course, only on the data sets applicable for a particular mode. The current neutron detectors consist of several (typically 14) 3He proportional-counter tubes embedded in a polyethylene moderator. The tubes may be in a single pod or in a pair of pods. The output pulses from the tubes are fed to an electronic module containing amplifiers and pulse-sorting circuitry. The electronics module has four principal functions: 1) It supplies the high-voltage to the 3He tubes and power for the electronic counting circuitry from a self-contained battery pack. 2) It permits user selection of a) one of the two triggering modes, internal (Mode I) or external (Mode II), b) a “Start Delay,” Δ1, for Mode II (set to the minimum value, 1-μs, for Mode I), c) the width, τo, of the fundamental data-sorting time bins (minimum value currently restricted to I μs), and d) the number of Data Acquisition Cycles (DACs) for the measurement (typically 105-108). 3) It amplifies and shapes the analog output signals from each tube (separate amplifier and discriminator for each tube) and feeds the signals to a data collection and sorting system. 4) It sorts the data collected on each DAC into the four data matrices required for the different modes and analysis types, and appropriately increments the cumulative data matrices at the end of each DAC. It outputs the cumulative data matrices at the end of each measurement. The electronics module will also display and/or print the average total counting rate in units of neutrons/DAG to allow the operator to adjust the length of the DAG and/or the sample-to-detector distance to achieve good data collection efficiency. It may also print a reminder to the operator that the number of neutrons/DAG needs to be large. (Since the number of counting bins will be fixed at 256, the length of the DAG is determined by the value of τo that is set). The schematic representations of the neutron beam and the Beam Delay (Δo) shown in FIG. 3 apply only to Mode II. When wanting data from Mode L the 14-MeV neutron generator (i.e., external trigger input) is not used. The start pulse for the DAC is generated internally. The delay, Δ2, is essentially zero, and Δ1 is kept at the minimum value consistent with the triggering and data sorting requirements for the cycle (approximately 1-μs). The user-selected value, τo, of the fundamental counting bin width, therefore, determines LG (the number of bins is fixed at 256), and (together with the fixed value of Δ1) the length of the DAC (LC) and, of course, its inverse, the pulse repetition frequency (PRF). In Mode II, the user selects the values of τo. Δ1, and the PRF of the neutron generator (within the operational limits of approximately 500-5000 Hz). The neutron generator control module provides a TTL output pulse that serves as the DAC start pulse. The neutron output from the generator occurs at a delay, Δ0, approximately 20-40 us after the start of the TTL pulse. The duration of the neutron beam pulse is determined by the selected PRF and the neutron generator duty factor (nominally fixed by the manufacturer at some value in the 5-10% range, but, in practice, somewhat PRF dependent). FIGS. 3, 10, 12 and 14 show timing marks. The number of time bins in the DAG will be fixed at 256. Each bin has the same width, τo, which can be selected by the user to adjust the length of the DAG as required by the measurement to be made. The minimum value of τo is fixed at one microsecond by the current electronics in the system. The sum of neutron counts from all of the 3He tubes in the detector is recorded in each time bin. See FIGS. 2 and 3. Δ1 is kept to its minimum value and Δ2 is set to zero in Mode I, in order to maximize data acquisition efficiency. In Mode II, LG, Δ1, and LC can all be set by the user. If these choices are not made judiciously [i.e., if LC<(LG+Δ1)], one could get a negative value of Δ2! See FIG. 3. In Mode II, the measurement requirements may require the neutron “beam” to be positioned entirely prior to the start of the DAG, more or less coincident with the DAG, or overlapping part of the DAG. Variability of the PRF, Δ1, and τo allows such flexibility in beam position. Note that the beginning and end of the neutron “beam” is not well defined in time. Also, the term “beam” is used loosely, here; the 14-MeV neutrons are emitted isotropically by generator, and do not form a spatial beam in the usual sense of the word. See FIG. 3. FIGS. 4 and 10 show examples of subgate detail. FIG. 12 illustrates another type of subgate counting. The Level-1 subgates shown are equivalent to the fundamental Time Bins. In principle, each Level-1 subgate could comprise 2 or more bins. If longer Level-1 subgates are required, this can be achieved, in the implementation shown, by increasing the size of τo. It is possible, in principle, to implement a data-sorting algorithm that contains more subgates of Level-2 and higher. There are possible modifications of the current implementation (containing the same numbers of subgates of each level) in which some of the longer subgates could comprise different groupings of time bins than the ones indicated in the figure. On any given DAC, the neutron multiplicities in some of those subgates would generally differ from the multiplicities in the illustrated set of subgates. The total multiplicity count in all subgates of a given length would, over a measurement of many DACs, be statistically equivalent for all such variations of the implementation shown. Referring now to FIGS. 6, 8, 12 and 14: (a) The average number of neutrons per DAG needs to be large. Any data acquisition cycles on which only zero or one neutron is detected provide no useful data for the Rossi-Alpha analysis. In order to collect data efficiently, it is necessary that an average of several (say ≧10) neutrons be detected on each cycle. (b) If two neutrons are counted in a single bin, we consider the earlier of the two to be the second member of a neutron pair with the nearest preceding neutron; the later neutron is the first member of a pair with the next succeeding neutron; and the two neutrons, themselves, constitute a pair separated by a time interval smaller than τo. We arbitrarily define this to be a time interval of “zero” width. If three neutrons occur in a single bin, we have two intervals of zero width, etc. The foregoing description of the invention has been presented for purposes of illustration and description and 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. The embodiments disclosed were meant only to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims. |
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claims | 1. Process for dissolving nuclear fuel, comprising immersion of the nuclear fuel in a nitric acid solution, wherein it comprises mechanical milling of the nuclear fuel, this mechanical milling being performed in the nitric acid solution during said immersion. 2. The dissolution process according to claim 1, wherein the nitric acid solution is heated to between 90° C. and 105° C. 3. The dissolution process according to claim 1, wherein the molar concentration of the nitric acid solution is between 1 mol/L and 10 mol/L. 4. The dissolution process according to claim 1, wherein the nitric acid solution also comprises a neutron poison such as gadolinium. 5. The dissolution process according to claim 1, wherein mechanical milling is performed throughout the entire duration of immersion. 6. The dissolution process according to claim 1, wherein the nuclear fuel comprises at least one plutonium oxide and/or at least one mixed oxide of plutonium and of at least one second metal selected from among uranium, thorium, neptunium, americium and curium. 7. The dissolution process according to claim 6, wherein, the second metal being uranium, the nuclear fuel containing at least one mixed oxide of uranium and plutonium is a MOX fuel. 8. The dissolution process according to claim 1, wherein the nuclear fuel is irradiated nuclear fuel. 9. The dissolution process according to claim 1, wherein the nuclear fuel comprises fabrication rejects of non-irradiated nuclear fuel. 10. The dissolution process according to claim 1, further comprising, when the nuclear fuel is confined within a cladding, a step to de-clad the nuclear fuel, this decladding step being prior to immersion. 11. The dissolution process according to claim 1, further comprising the implementation of a mill equipped with mechanical milling means. 12. The dissolution process according to claim 11, wherein the mill is a bead or pebble mill. 13. The dissolution process according to claim 3, wherein the molar concentration of the nitric acid solution is between 3 mol/L and 8 mol/L. 14. The dissolution process according to claim 12, wherein the beads or pebbles are in zirconium dioxide. 15. Process for dissolving irradiated nuclear fuel comprising the following successive steps, taken in this order:(a) dissolving irradiated nuclear fuel by immersion in nitric acid solution, after which a nitric dissolution solution is obtained containing dissolution fines;(b) separating the dissolution fines from the nitric dissolution solution; and(c) dissolving the dissolution fines separated at step (b),wherein the dissolving step (c) comprises immersion of the dissolution fines in a nitric acid solution and mechanical milling of the dissolution fines, the mechanical milling being performed in the nitric acid solution during said immersion. 16. The process according to claim 15 further comprising, when the irradiated nuclear fuel is confined within a cladding, a step to de-clad the irradiated nuclear fuel, this decladding step preceding step (a). 17. The dissolution process according to claim 15, wherein the irradiated nuclear fuel comprises at least one plutonium oxide and/or at least one mixed oxide of plutonium and of at least one second metal selected from among uranium, thorium, neptunium, americium and curium. 18. The dissolution process according to claim 17, wherein, the second metal being uranium, the nuclear fuel comprising at least one mixed oxide of uranium and plutonium is MOX fuel. |
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description | This application is the U.S. national phase of the International Patent Application No. PCT/FR2011/050775 filed Apr. 6, 2011, which claims the benefit of French Application No. 1052651 filed Apr. 8, 2010, the entire content of which is incorporated herein by reference. The invention relates to the heater tubes for a pressurizer of a primary cooling system of a pressurized water nuclear reactor. A heater tube, for such a pressurizer of a primary cooling system, normally comprises an outer metal casing that has an elongated cylindrical shape (for example 22 mm diameter by 2 m long approximately) called a “sheath”, and a heater mounted inside the sheath. Such tubes are mounted on a lower portion of the pressurizer, as explained in detail in document FR-2 895 206, and are submerged in the water of the primary cooling system that contains the pressurizer. They are used to raise the operating pressure of the primary cooling system. It will thus be understood that the tubes are under load when in use and undergo, in particular, thermal stress which, when combined with work-hardening stress as described below, potentially causes stress corrosion. Incidents have shown that leaks could occur on the heater tubes of the state of the art. In particular, the sheath of a tube can crack, such that the inside of the tube is open to the water present in the pressurizer. There follows a possible deterioration of the heater of the tube, loss of operation of the tube, or even the leaking of the pressurized water to the outside of the pressurizer, through the inner space of the tube. As a result, a solution is sought for limiting the risks of cracking of the sheath, due in particular to the stress corrosion that the sheath undergoes. A solution was proposed in the above-mentioned document FR-2 895 206 that aimed to deposit a protective nickel coating onto the external surface of the sheath, using electrolysis. However, the solution of adding material leads to an alteration in the geometry of the tube, in particular by increasing the diameter thereof. Moreover, the solution is not completely secure, as the risk of removal of the nickel layer under the effect of impacts or friction cannot be ruled out. Given the increase in diameter, this solution cannot be used with heater tubes that are already manufactured, as they may no longer match the dimensions of the supports. Furthermore, it is lengthy to implement. The present invention aims to improve the situation. To this end the invention proposes the treatment of the tubes with a view to reducing the above-mentioned cracking risks. The treatment provided in the context of the invention is, generally, the heat treatment of the tubes in order to recrystallize at least the external surface of the sheath. Thus, the present invention relates to a method for the treatment of a heater tube intended for use in a pressurizer of the primary cooling system of a nuclear reactor. The heater tube includes a heater housed in a substantially cylindrical sheath. The sheath includes an external surface that is liable to undergo stress corrosion, at least partially, while the tube is in use. In particular, as the sheath includes a steel-type material, for example of the work-hardened austenitic stainless steel type, the method in the context of the invention includes a heat treatment step of at least the external surface of the sheath, in order to recrystallize the material of the sheath, at least on the surface thereof. The material thus recrystallized is not subject to the phenomenon of stress corrosion by comparison with the tubes of the state of the art, without deterioration, which eliminates the risks of cracking and ultimately extends the life of the tube. Preferably, the heat treatment uses induction heating applied starting from the external surface of the sheath. In particular, a heat treatment is envisaged that includes a rise in temperature on the external surface of the sheath comprised within a range of 800° C. to 1,100° C. and preferably between 900° C. and 1,050° C. or between 950° C. and 1,050° C., for example 960° C., 970° C. or even 1,000° C. By applying a heat treatment using induction heating, the rise in temperature of the heater resulting from the heat treatment is advantageously limited to a maximum value of 900° C., allowing the electrical resistance and isolation properties of the heater to be retained. In an embodiment described in detail hereinafter, the heat treatment using induction heating consists of applying an alternating current in the windings of an inductance coil surrounding the external surface of the sheath. The frequency of the alternating current can be chosen and is preferably at least 100 kHz. The higher the frequency, the more the energy transmitted to the sheath using induction heating is concentrated on a small thickness of the sheath according to a so-called “skin” effect. Said frequency value is given in a context where the induction winding has a diameter of 30 to 50 mm and for a sheath the diameter of which is of the order of 20 to 25 mm. The inductor is arranged around the tube and, in particular, a relative displacement of the inductor with respect to the tube is preferably applied, at least in translation along the tube. In one embodiment, the speed of the translational displacement is comprised between 100 and 900 mm per minute, for a power supplied by induction comprised between 1 and 50 kW. Preferably, the inductor is of the solenoid type. In one embodiment, a supply of inert gas can moreover be provided onto the external surface of the sheath in order to avoid oxidation following the heat treatment. After the heat treatment, it is also possible to apply cooling by blowing a fluid (for example air) onto the external surface of the sheath. The present invention also relates to a heater tube, as such, obtained by the method in the context of the invention. In particular, the sheath of the tube includes at least on its external surface a thickness of recrystallized material. The thickness is preferably greater than or of the order of 1 mm. The thickness is advantageously comprised between approximately 1 mm and a total thickness of the sheath of the tube, and more particularly comprised between approximately 1.5 mm and approximately 3 mm, for example approximately 2 mm. By “recrystallized material” is meant the fact that the heat treatment applied contributes to regenerating severely deformed grains having high hardness, into grains with equal axes having high or medium hardness. Thus, a trace of the method of the invention on the tube consists in that the hardness of the sheath on its external surface is lower than for a standard tube of the state of the art. Typically, a hardness equivalent to a value less than or equal to approximately 240 Vickers or even less than approximately 200 Vickers can be measured on the external surface of the sheath of a treated tube in the context of the invention. These hardness values represent respectively recrystallized material thicknesses greater than or of the order of 1 mm or approximately 1.5 mm to 2 mm. As explained hereinafter, initially, the heater is mounted in the sheath of the tube by crimping, the external surface of the sheath being swaged. Work-hardening of the external surface of the sheath results. As will be seen hereinafter, there is a synergic effect between the work-hardening and the heat treatment in the context of the invention. It is then possible to observe on a tube, before the heat treatment in the context of the invention, traces of work-hardening by swaging, in particular on the external surface of the sheath. Advantageously, the consequences of the work-hardening (in particular in terms of stress corrosion resistance) disappear overall after the treatment of the invention. Thus, the heat treatment chosen in the context of the invention is preferably a treatment using induction heating, aiming to promote recrystallization of the material from which the sheath is made, in particular on the external surface of the sheath. By way of non-limitative example, the material of the sheath can typically be an austenitic steel (containing essentially iron, 16 to 20% chrome and 8 to 14% nickel, as well as carbon (less than 1%) and optionally molybdenum, niobium or titanium). It has in fact been observed that the risk of corrosion of the sheath of a tube can be linked to its method of manufacture by swaging, causing the substantial work-hardening of the metal, in particular on the external surface of the sheath. FIG. 3 represents an enlarged view of the surface SUR of the sheath of a tube, showing in particular very work-hardened grains close to the external surface SUR of the sheath. For this first reason, heat treatment using induction heating is advantageous since, in principle, firstly it promotes a rise in temperature in particular on the external surface of the material treated using induction heating. Treatment using induction heating is also advantageous at least for a second reason: it is suspected that overall heat treatment (at approximately 1,050° C. for recrystallizing the sheath of a tube) might cause deterioration of the electrical properties of the tube and in particular of the heater mounted inside the sheath. As a result, surface heat treatment of the tube only, and in particular of the sheath, selectively, is preferred in one embodiment of the invention. Heat treatment using induction heating is therefore suitable. When the temperature of the heater is above 900° C., it is in fact suspected that deterioration of the electrical properties may occur. Thus, treatment using induction heating, advantageously of the surface of the sheath, makes it possible to improve the morphological defects (significant plasticization, dislocations and local stresses) on the surface of the sheath, linked in particular to the work-hardening of the sheath during the manufacture of the tube. Moreover, when the heat treatment is carried out by means of a solenoid surrounding the tube, the recrystallization heat treatment can be implemented without creating any heat treatment discontinuities. Axially, continuous and regular heat treatment can be obtained by continuous and regular displacement of the tube in the inductor, or vice-versa. Radially, heat treatment takes place simultaneously over the whole circumference of the sheath with substantially equal intensity. The risks of forming radial stress non-uniformity during the recrystallization treatment are therefore low. In particular, the stresses due to the work-hardening of the sheath during manufacture of the tube are absorbed uniformly over the circumference of the tube. Stress non-uniformity could occur if, during the surface heat treatment, certain areas of the sheath that are more significantly work-hardened undergo recrystallization treatment to a lesser extent than other areas of the tube sheath that are less significantly work-hardened. Radial stress non-uniformity creates areas of high stress on one side of the tube and areas of low stress on another side of the tube, which could contribute to bending the tube. Moreover, the energy (therefore the temperature) required for recrystallizing a work-hardened steel is less than for a steel that is not work-hardened. For example, while a non work-hardened steel starts recrystallization at 1,050° C., the same steel superficially work-hardened needs only a smaller rise in temperature, for example 960° C., considering moreover that not all of the surface of said steel is work-hardened and that the work-hardening is not homogeneous over the whole thickness of the sheath. This observation makes it possible to reduce the temperature to be applied to the sheath for its recrystallization and therefore also to reduce the temperature that the heater must undergo inside the sheath. Use of a surface temperature comprised between 900° C. and 1,050° C. or more particularly between 950° C. and 1,050° C., for example 960° C., 970° C. or even 1,000° C. makes it possible to ensure surface recrystallization when the surface of the sheath includes areas that are less significantly work-hardened than other areas. In particular, these surface temperatures make it possible to recrystallize portions of the sheath that are less work-hardened than the external surface, for example areas closer to the centre. As mentioned previously, there is a synergic effect between the work-hardening and the heat treatment in the context of the invention. In particular, the work-hardening initially present makes it possible to reduce the temperature of the treatment. Moreover, the treatment according to the invention makes it possible to overcome defects from the manufacturing of the tubes by work-hardening. The heat treatment according to the invention allows the majority of the stresses present in the sheath to be absorbed, including residual stresses caused by the work-hardening and present deep within the sheath, below the external surface. When the recrystallization treatment is carried out over a thickness of the order of those mentioned above, in particular approximately 1.5 mm or approximately 2 mm, the majority of the thickness of the sheath is treated. The majority of the stresses induced in the sheath by work-hardening during the manufacture of the tube are then absorbed. The external surface of the sheath thus undergoes only minimal stress on the part of layers that are further inside the sheath. By absorbing the stresses due to the work-hardening of the sheath, the method according to the invention makes it possible to reduce the stresses that are present overall in the tube to values less than approximately 100 MPa, or even less than approximately 80 MPa. Thus, the stresses present overall in the tube are markedly less than the limit stresses above which stress corrosion can take place in use, i.e. for tubes having a sheath made from austenitic steels, stresses of the order of 300 MPa to 400 MPa. Firstly, reference is made to FIG. 1, in which the portion of the tube intended to be submerged in a pressurizer is shown. In this case, it includes a cylindrical-shaped sheath 5 made from stainless steel. It will thus be understood that the method can be applied to any tube the sheath of which is produced from the general family of “stainless steels” (without particular limitation of the proportion of alloys forming said steel). The central core of the tube includes a mandrel 2, usually made from copper, inside the sheath 5, along the central axis of the sheath, as well as a heating wire 1 coiled around the mandrel 2 in a spiral and interposed between the mandrel 2 and the sheath 5. The heating wire constitutes the heater mentioned above in the general presentation of the invention. The heating wire 1 comprises an electrically conductive resistive metal core 3, for example made from copper or nickel-chrome alloy. A protective metal coating made from steel 6 (see in particular the detail in FIG. 2) surrounds the core 3. The coating 6 is electrically isolated from the core 3 by an insulator 4 for example magnesia (MgO). The heating wire 1, wound around the mandrel 2 forming contiguous turns, is intended to be connected to a connector electrically connected to an electricity generator making an electric current flow in the conductor wire 1. Details on the connection of such a heater tube and its use in the primary cooling system of a nuclear reactor are described in publication FR-2 895 206. Referring now to FIG. 2, the thickness of the sheath 5 (between points A and C), in an embodiment that is in no way limitative, is 2.45 mm. The thickness of the protective coating 6 of the heating wire 1 is 0.5 mm (between points C and D in FIG. 2). The thickness of the magnesia lining 4 is 0.4 mm (between points D and E in FIG. 2). Thus it will be understood that the representation in FIGS. 1 and 2 is not necessarily to scale. Finally, the diameter of the conductive core 3 of the heating wire is approximately 1.5 mm (between points E and F). Furthermore, the elements surrounded by the sheath 5 are crimped into the sheath according to a step of shrinking the sheath by swaging, which moreover generates the mechanical stress that is liable to affect the stress corrosion resistance. After shrinking, the sheath 5 is in close contact with the coils 1 of the heating element, as shown in particular in FIG. 2. According to a first series of tests carried out, a rise in temperature of the external surface of the sheath 5 of approximately 1,050° C. was sought, for the purpose of its recrystallization. With reference to FIG. 4, it was estimated that the external surface of the sheath (curve A) exhibited a temperature rise peak of 1,050° C., promoting recrystallization. At point J, corresponding to approximately 83% of the power received by induction (“skin effect” known in treatment using induction heating), the rise in temperature is approximately 1,000° C. In particular, curve B shows the temperature profile at 1.5 mm from the external surface of the sheath (at point B in FIG. 2). It became apparent that a rise in temperature to only 900° C. already allowed recrystallization of the material of the sheath. Thus, said first series of tests made it possible to recrystallize practically the whole of the sheath, including its volume. It will be observed however, on the curve marked E, that the temperature of the core 3 of the heating wire does not exceed 800° C., making it possible to retain the conductive properties of the core 3 of the heating wire, thus ensuring that the treatment in the context of the invention does not produce any deterioration of the content of the tube. Overall, a rise in temperature of the external surface of the sheath is sought within a range of 800° C. to 1,100° C., and preferably 900° C. to 1,050° C., a temperature range sufficient to recrystallize the material of the sheath. To said constraint is added a maximum rise in temperature of the magnesia 4 that is limited to 850° C. (at point D in FIG. 2), in order to ensure a smaller rise in temperature of the core 3 of the heating wire. In order to respect these constraints, advantageously a set of induction parameters is chosen from at least: the frequency f(Hz) of the alternating current flowing in the coils of the inductor (reference IND in FIG. 6), it being understood that the higher said frequency, the more the energy received by induction is confined to the surface of the sheath 5 (by skin effect), the power P (W) or as an equivalent the amperage of the current for the chosen frequency, the duration of application of the heat treatment, shown in the example in FIG. 6 by a speed V (mm/min) of relative displacement of the inductor IND with respect to the sheath 5 of the tube. Of course, the lower the speed of the inductor with respect to the tube, the greater the rise in temperature. These different effects are thus shown in FIG. 5, which represents an estimate of the temperature rises for a higher speed of travel, but with a higher power density. It will be noted here that the interface between the protective coating of the heating wire and the magnesia (point D) undergoes a rise in temperature of less than 750° C. According to the set of tests carried out, it transpires that the frequency of the alternating current to be provided is preferably greater than 150 kHz, so as to protect the magnesia 4 and/or the conductive core 3 of the heating wire 1, while limiting the rise in temperature to a threshold value of the order of 800 to 900° C. The power supplied can be within a range of 1 to 50 kW. The relative speed of movement of the inductor IND with respect to the tube can be comprised within a range of 100 to 900 mm/min. Under these conditions, it is preferable to provide a solenoid inductor having an inside diameter of 30 to 50 mm, it being understood that the diameter of the tube, in a given embodiment, is 22 mm. Preferably, as shown in FIG. 6, the tube is rotated during heat treatment (arrow R) about its central axis, in order to homogenize the heat treatment applied to the sheath. Of course, the parameters of the treatment using induction heating such as, in particular, the frequency, the power and the speed of travel are adjustable in the treatment installation shown in FIG. 6 according to the precise dimensions of the elements constituting the tube, according to their material, or other constraints. It will be understood generally that the effect sought in the treatment using induction heating is to create an alternating magnetic field (using alternating currents flowing in the inductor) in order to generate induced currents on the external surface of the sheath of the tube. Said induced currents instantly heat the area where they occur. On the other hand, the inner elements of the tube such as the inner surface of the sheath, and in particular the heating wire 1 and the mandrel 2 are, in principle, only heated by thermal conduction (as clearly shown by curves E to I in FIGS. 4 and 5). It will thus be understood that the treatment thickness is ultimately a function of the chosen frequency value (for the skin effect) and of the treatment time, or in an equivalent manner, of the speed of travel of the inductor with respect to the tube (by thermal conduction). Recrystallization of at least the external surface of the sheath 5 of the tube then occurs. The recrystallization is seen in particular by the fact that the material becomes softer when recrystallized. Typically it is possible to measure a hardness of less than or equal to approximately 240 Vickers by a penetration measurement using a conical diamond at a pressure of 5 kg on the external surface of the sheath 5 of a tube treated using the method in the context of the invention. The thickness of the recrystallized sheath is at least 1 mm. Thus it will be understood that tracing the method in the context of the invention on the treated tube consists of measuring a hardness less than or equal to approximately 240 Vickers, for example over at least 1 mm thickness from the external surface of the sheath 5 of the tube. FIG. 6 shows the blowing B of a fluid onto the tube, immediately after the treatment using induction heating. Indeed a cooling effect can be provided (for example by air) in order to reduce the temperature of the elements constituting the tube, after recrystallization of the sheath. In this way the temperature is reduced at the ends of the curve, as shown in FIGS. 4 and 5. The tube can also be protected from oxidation (after rise in temperature) by installing a muffle (quartz sleeve around the tube) for supplying an inert gas (for example argon, helium or possibly nitrogen). Said muffle supplying an inert gas (not shown in FIG. 6) can operate between the inductor IND and the air blower B in the diagram shown. In a variant, the heat treatment can be carried out in a cabinet under an inert gas atmosphere in order to avoid superficial oxidation of the sheath. More generally, the present invention is not restricted to the embodiments given above; it extends to other variants. Thus, the air blower B shown in FIG. 6 for cooling the tube can simply be removed. Moreover, the application of inert gas onto the sheath is also optional. Due to the short duration of treatment, the possible oxidation of the tube remains limited. At most, a slight blueing of the external surface of the sheath 5 is noted. Said oxidation can simply be removed by a final pickling step (a step already planned and implemented in the general manufacturing method of the tubes). During said pickling step, the thin oxidation layer formed by the treatment using induction heating is removed, making it possible to avoid providing for the blowing of inert gas or applying the heat treatment in an inert gas chamber such as described above. Moreover, as stated above, the temperature rise values given in the examples in FIGS. 4 and 5 allow numerous variants. Generally, it can be assumed that as the recrystallization of the sheath can take place between 800 and 1100° C., the conditions of treatment using induction heating aim to raise the temperature of the external surface of the sheath accordingly, while seeking to limit the rise in temperature of the heating wire to approximately 900° C. at most. Moreover, it is also preferable that the rise in temperature of the external surface of the sheath does not exceed a threshold value, for example above 1,100° C., or that the duration of the heat treatment is also limited to a threshold value, in order not to promote so-called “secondary recrystallization” which is seen overall through a lack of homogeneity in the size of the crystalline grains, weakening the material. Moreover, as explained above, if the external surface of the sheath is work-hardened overall, the maximum temperature rise at the surface of the sheath (peak of curve A of FIG. 4 or 5) can be reduced below 1000° C., for example to 960° C. More generally, heat treatment using induction heating has been described above by way of example, but the invention can be applied to any type of heat treatment capable of selectively restricting the rise in temperature mainly to the sheath of the tube. For example, heating by laser scanning or by annular torch on the surface of the sheath can be envisaged. The treatment by annular torch, reproducing heat treatment having similar advantages to those of treatment by a cylindrical solenoid, is particularly advantageous. |
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abstract | A radiation protection material (10) is arranged between at least two layers (21, 22) of at least one plastic-containing element in order to produce a radiation protection element. At least part of the gas present between the at least two layers (21, 22) is removed. The at least two layers (21, 22) are connected with each other. |
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062020382 | abstract | A method and apparatus for monitoring a source of data for determining an operating state of a working system. The method includes determining a sensor (or source of data) arrangement associated with monitoring the source of data for a system, activating a method for performing a sequential probability ratio test if the data source includes a single data (sensor) source, activating a second method for performing a regression sequential possibility ratio testing procedure if the arrangement includes a pair of sensors (data sources) with signals which are linearly or non-linearly related; activating a third method for performing a bounded angle ratio test procedure if the sensor arrangement includes multiple sensors and utilizing at least one of the first, second and third methods to accumulate sensor signals and determining the operating state of the system. |
045086419 | claims | 1. A process for the decontamination of steel surfaces by removal of the contaminated surface layer with an aqueous decontaminating solution in a recirculation loop and for final treating the used aqueous solution after removal of the surface layer for waste disposal, which process comprises: (a.sup.1) contacting the steel surfaces with an aqueous decontaminating solution comprising at least one acid selected from the group consisting of formic acid and acetic acid, and at least one reducing agent selected from the group consisting of formaldehyde and acetaldehyde, in a concentration to hold dissolved Fe.sup.2+ -ions stably in the solution; (b.sup.1) monitoring the concentration of dissolved Fe.sup.2+ -ions, acid and aldehyde of the decontaminating solution during the dissolution process; (c.sup.1) treating the used decontaminating solution to precipitate iron values dissolved therein in the form of iron hydroxide or in the form of water-insoluble iron (III) compounds, and separating precipitated iron compounds from the liquid by filtering; and (d.sup.1) treating the aqueous solution remaining after said precipitation to obtain a regenerated decontaminating solution having the desired content of acid and aldehyde, and recircuate it for a new dissolution cycle; and (e.sup.1) treating the used decontaminating solution to precipitate iron values dissolved therein in the form of iron hydroxide or in the form of water-insoluble iron (III)-compounds and separating precipitated iron compounds from the liquid by filtering; (f.sup.1) decomposing the precipitated iron compounds of steps (c.sup.1) and (e.sup.1) thermally and/or catalytically into iron oxide-containing radioactive materials- and into radioactivity-free gaseous decomposition products, and subjecting the iron oxide to nuclear waste disposal by mixing it with cement; and (g.sup.1) oxidizing the radioactivity-free solution of step (e.sup.1) with an oxidizing agent and decomposing therein dissolved formate or acetate salts. (a.sup.2) contacting the steel surface with an aqueous decontaminating solution comprising formic acid and formaldehyde as a reducing agent in a concentration to hold dissolved Fe.sup.2+ -ions stably in the solution; (b.sup.2) monitoring the concentration of dissolved Fe.sup.2+ -ions, formic acid, and formaldehyde of the decontaminating solution during the dissolution process; (c.sup.2) treating the used decontaminating solution by electrolysis to precipitate iron values dissolved in the solution as metallic iron for waste disposal and to oxidize acid-ions to formic acid; and (d.sup.2) treating the liquid of the electrolytic process to obtain a regenerated decontaminating solution having the desired content of formic acid and formaldehyde and recirculate it for a new dissolution cycle; and (e.sup.2) treating the used decontaminating solution to precipitate iron values dissolved therein in the form of iron hydroxide or in the form of water-insoluble iron (III)-compounds and separating precipitated iron compounds from the liquid by filtering; (f.sup.2) decomposing the precipitated iron compounds of steps (c.sup.2) and (e.sup.2) thermally and/or catalytically into iron oxide--containing radioactive materials--and into radioactivity-free gaseous decomposition products, (g.sup.2) oxidizing the radioactivity-free solution of step (e.sup.1) with an oxidizing agent and decomposing therein dissolved formate or acetate salts. (a.sup.3) contacting the steel surfaces with an aqueous decontaminating solution comprising formic acid and formaldehyde as a reducing agent in a concentration to hold dissolved Fe.sup.2+ -ions stably in the solution; (b.sup.3) monitoring the concentration of Fe.sup.2+ -ions, formic acid and formaldehyde of the decontaminating solution during the dissolution process; (c.sup.3) treating the used decontaminating solution by electrolysis to precipitate iron values dissolved in the solution as metallic iron--containing radioactive materials--and to decomposite the aqueous solution to gaseous decomposition products; and (d.sup.3) treating the precipitated iron for waste disposal. 2. A process according to claim 1, wherein before precipitation of the iron in the used decontaminating solution, dissolved iron (II) compounds are oxidized to iron (III) compounds by the addition of an oxidizing agent and are precipitated as water-insoluble iron (III) compounds. 3. A process according to claim 1, wherein, to precipitate iron hydroxide or iron (III) compounds from the used decontaminating solution, alkali metal hydroxide or carbonate is added and after separation of the precipitate from the liquid the alkali metal salt present therein is oxidatively decomposed into alkali metal hydroxide, alkali metal carbonate, carbon dioxide and water. 4. A process according to claim 3, wherein the precipitation of water-insoluble iron compounds from the used decontaminating solution is carried out in a batch process wherein after the precipitation of a first batch of decontaminating solution and the oxidizing treatment of the separated liquid the thus treated liquid is used for precipitation of the iron compounds from a second batch of decontaminating liquid and the process is repeated until all the iron is precipitated from the whole of the decontaminating solution. 5. A process according to claim 1, wherein before filtering the precipitate of a preceding precipitation process is added to the used decontaminating solution as a flocculating agent. 6. A process according to claim 1, wherein the mixing of the precipitate with cement is such that a ferrocement-like product is produced. 7. A process for the decontamination of steel surfaces by removal of the contaminated surface layer with an aqueous decontaminating solution in a recirculation loop and for final treating the aqueous solution after removal of the surface layer for waste disposal, which process comprises: 8. A process as claimed in claim 7, wherein the electrolysis is conducted with an iron cathode. 9. A process according to claim 7 wherein before precipitation of the iron in the used decontaminating solution, dissolved iron (II) compounds are oxidized to iron (III) compounds by the addition of an oxidizing agent. 10. A process according to claim 7 wherein, to precipitate iron hydroxide or iron (III) compounds from the used decontaminating solution, alkali metal hydroxide or carbonate is added and after separation of the precipitate from the liquid the alkali metal salt present therein is oxidatively decomposed into alkali metal hydroxide, alakali metal carbonate, carbon dioxide and water. 11. A process according to claim 10, wherein the precipitation of water-insoluble iron compounds from the used decontaminating solution is carried out in a batch process wherein after the precipitation of a first batch of decontaminating solution and the oxidizing treatment of the separated liquid the thus treated liquid is used for precipitation of the iron compounds from a second batch of decontaminating liquid and the process is repeated until all the iron is precipitated from the whole of the decontaminating solution. 12. A process according to claim 7, wherein before filtering the precipitate of a preceding precipitation process is added to the used decontaminating solution as a flocculating agent. 13. A process according to claim 7, wherein before the mixing of the precipitate with cement is such that a ferrocement-like product is produced. 14. A process for the decontamination of steel surfaces with an aqueous decontaminating solution and for treating the aqueous solution after decontamination of the surfaces for waste disposal, which process comprises the steps: 15. A process according to claim 14, wherein the aqueous decontaminating solution is recirculated in a loop for the treatment of the contaminated steel surfaces, wherein during the removal of the contaminated surface layer the used decontaminating solution is treated by electrolysis to precipitate the dissolved iron and to oxidize acid-ions to formic acid, and liquid of the electrolysis process is regenerated to decontaminating solution having the desired content of formic acid and formaldehyde and is recirculated for a new dissolution cycle. 16. A process according to claim 14, wherein the electrolysis is conducted with an iron cathode. |
062883003 | summary | FIELD OF THE INVENTION The present invention relates to the thermal processing of organic materials and, more particularly, the thermal processing of organic materials, such as ion exchange resins and polymeric sorbents, in the presence of metal oxides, such as hydrated ferric oxide. The invention also relates to processes for immobilizing organic materials, including hazardous wastes. BACKGROUND OF THE INVENTION Ion exchange resins are synthetic, porous organic solids, typically having a polystyrene matrix, acidic or basic groups bonded to the matrix and hydrogen or sodium ions bonded to the acidic or basic groups. They are effective chemical filters for hazardous wastes in contaminated water, for example, which may include radioactive and non-radioactive materials. Polymeric sorbents, such as charcoal, have a charged surface and are also effective chemical filters of such wastes. However, the use of these materials to absorb hazardous wastes presents the problem of the effective disposal of the contaminated ion exchange resins and polymeric sorbents. U.S. Ser. No. 08/713,243, filed on Sep. 12, 1996, assigned to the assignee of the present invention and incorporated by reference, herein, discloses a process for the immobilization of radioactive and other hazardous wastes with ferric oxides such as ferrihydrite. It was demonstrated that ion exchange resins and polymeric sorbents, and other contaminated materials, may be effectively immobilized by mixing the contaminated resin or sorbent with hydrated ferric oxide comprising at least 20% Fe.sub.2 O.sub.3, by dry weight of the total weight of the mixture. The mixture was pressed at temperatures of about 260.degree. C. A large part of the water was removed while the mixture was under pressure of 70,000 psi for a period of time to produce a solid composition containing the contaminated material. Such a mixture was successfully consolidated at a pressure of 25,000 psi, with the addition of additives such as metallic fines. Prior to mixing with the hydrated ferric oxide, the ion exchange resin or polymeric sorbent was dried by heating, as well as ground to reduce its particle size. In the Examples, the cation ion exchange resin was dried at 120.degree. C. while the polymeric sorbent was dried at 118.degree. C. Volume reduction is an important economic consideration in hazardous waste disposal because the volume of the waste to be disposed is a significant factor in the burial cost. The pressure and temperature of a disposal process are also important economic considerations because of their impact on processing costs. In U.S. Ser. No. 08/713,243, volume reductions of up to 10 times for ion exchange resins were achieved by pressing at 70,000 psi and 260.degree. C. The volume of the ion exchange resins and polymeric sorbents immobilized for disposal in accordance with U.S. Ser. No. 08/713,243 could be further decreased by preheating the resin or sorbents at higher temperatures. However, thermal processing of organic solids tends to proceed in an uneven manner, resulting in local hot spots of material. Such hot spots can cause local eruptions and popping in the sample, and enhance the emission of hazardous organic compounds, such as the products of incomplete combustion ("PICs"). Such emissions create serious health risks. Heating ion exchange resins in particular causes the loss of sorbed volatiles and moisture followed by partial decomposition of the resin itself and further volatilization. Cation exchange resins are stable up to about 120.degree. C., while anion exchange resins are stable only up to about 60.degree. C. Incineration and thermal decomposition have also been proposed for the immobilization and volume reduction of hazardous wastes. Incineration, for example, is discussed in "Incineration of Ion-Exchange Resins in a Fluidized Bed", Valkiainen, et al., Nuclear Technology, Vol. 58, August 1982, pp. 248-255; and "Incineration of Ion-Exchange Resins Using a Cocentric Burner", Chino et al., Transactions of the American Nuclear Society, 44, (1983), pp. 434-435. However, when processed at sufficiently high temperatures to cause decomposition in an aerated or oxygenated environment, which is typical in incineration and thermal decomposition processes, ion exchange resins and polymer sorbents undergo the same thermal instabilities discussed above. Similarly, disposal procedures including heat treatments for other organic polymers and plastics, which form a large part of the solid wastes generated by human activity, present such problems. Vitrification is another disposal technique, wherein the waste material is mixed with metal oxide, such as sodium oxide, calcium oxide or boron oxide, and silica at temperatures over 800.degree. C. to form a glass for immobilizing the residues of the waste material. Because of the high temperatures involved, vitrification is an expensive, complex procedure. It would be advantageous to avoid the thermal instabilities caused by high temperature processing of organic materials in aerated or oxygenated environments. It would also be advantageous to decrease the pressures and temperatures used in waste disposal processes. SUMMARY OF THE INVENTION It has been found that the presence of metal oxides, such as hydrated metal oxides, during the thermal decomposition of organic materials in air or oxygen enables their thermal decomposition to proceed more smoothly and minimizes the evolution of undesirable and hazardous vapors and fumes. In accordance with one embodiment of the present invention, prior to or concurrent with a heat treatment, the organic materials are mixed with the hydrated metal oxide. The heat treatment can be the incineration or thermal decomposition of the organic materials. Preferably, the hydrated metal oxide is added to the organic materials prior to the start of thermal decomposition. Thermal treatments at temperatures above that which cause the onset of decomposition for that material can then proceed with more even heating of the organic materials, resulting in fewer local eruptions, less popping and decreased emission of volatile, hazardous organic compounds, such as the products of incomplete combustion ("PICs"), up to about 500.degree. C. Preferably, the heat treatment is conducted between about 300.degree. C. to about 450.degree. C. In addition to the ion exchange resins, the organic materials which may be heat treated in accordance with the present invention include ion exchange resins, polymeric sorbents, other polymers and plastics. The organic materials may be waste material such as hazardous wastes, radioactive wastes or municipal wastes, for example. The hydrated metal oxide may be hydrated ferric oxide, hydrated aluminum oxide or hydrated titanium oxide, for example. Ferrihydrite is preferred. In accordance with another embodiment of the present invention, the thermal treatment of a mixture of organic materials and a metal oxide is part of a process for immobilizing the organic materials in a matrix of ferric oxide. Either the metal oxide mixed with the organic materials is hydrated ferric oxide or hydrated ferric oxide is added to the heat treated mixture. The heat treated mixture is pressed for a period of time to remove a large part of the water content to produce a solid composition. Preferably, the pressing step is performed at room temperature. Higher temperatures may also be used. The pressing step can take place at about 70,000 psi, for example. The required pressure can be lowered by the addition of certain additives, such as magnesium oxide and ammonium dihydrogen phosphate, prior to pressing. Preferably, with the addition of additives, the pressure of the pressing step is less than about 30,000 psi. More preferably, the pressure is less than about 15,000 psi. In accordance with another embodiment of the invention, contaminated materials are consolidated in a matrix of ferric oxide by 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 and gradually removing a large part of the water present in the mixture at room temperature for a period of time to produce a solid composition. Pressures less than 30,000 psi and more preferably less than 15,000 psi may be used with the addition of additives. |
description | The present disclosure relates to binary multileaf collimator (MLC) delivery with a per-leaf field width in a radiation treatment system. In radiation treatment, doses of radiation delivered via a radiation treatment beam from a source outside a patient's body are delivered to a target region in the body, in order to destroy tumorous cells. Care must be taken to minimize the amount of radiation that is delivered to non-treatment regions while maximizing the amount of radiation delivered to the intended treatment regions. In radiation treatment, a radiation treatment beam aperture shapes the radiation treatment beam to conform, as closely as possible, to the intended target region. The radiation treatment beam aperture is commonly defined by an MLC. Described herein are embodiments of methods and apparatus for binary MLC radiation treatment delivery with per-leaf field width. In radiation treatment systems, opposing banks of leaves of an MLC may be used to create one or more patterns that shape a radiation treatment beam to conform to a target region. For target regions with non-uniform shapes, IMRT can be utilized to deliver more complicated radiation treatment doses. Intensity modulated radiotherapy (IMRT) includes a variety of radiation treatment techniques that, essentially, vary the radiation treatment beam intensity that is directed at the target region. In IMRT, rather than having the MLC shape the radiation treatment beam to match a particular outline, the MLC is instead used to create an array of beam shapes that generate a desired intensity modulation and a desired 3D dose distribution via overlapping radiation fields of (possibly) different intensities. In some embodiments, binary MLCs include a plurality of leaf pairs, arranged in two opposing banks. Each bank of leaves is used to form a treatment slice by positioning the leaf in a closed position or open position with respect to the beam. In some embodiments, the superior-inferior (sup-inf) field width (e.g., the width formed by the openings of the leaf pairs in the MLC) is constant across all leaves of the MLC. Disadvantageously, this means that such systems are incapable of conforming the field of the radiation treatment beam to a target profile along the length of the target. Due to this limitation, field sizes in binary MLCs are generally limited to less than 5 cm. Larger field sizes would generally be undesirable for treating a majority number of target regions, due to the amount of radiation exposure to non-target regions. One solution to the above problems is to use dynamic jaws to better conform the field to the target region on the superior and inferior ends. Such a technique does not conform the field to the edges of the target along its length, however, because the field size is defined by the jaws and is constant across the entire MLC. Another solution is to use a non-binary, shape-conforming MLC. Such MLCs can be slow, however, which may negatively impact treatment time. Another solution is provided herein. Advantageously, the embodiments described herein allow an MLC to conform treatment beam fields to target regions, while minimizing radiation exposure to non-treatment regions. Furthermore, the embodiments described herein allow for larger field sizes (e.g., larger than 5 cm), which may increase the speed of treatments. Furthermore, the embodiments described herein allow an MLC to modulate fluence field, not just in the IEC-Xb direction, but also in the IEC-Yb direction, as described herein. Furthermore, the embodiments described herein allow for more opportunities for modulation in the longitudinal direction. This may allow treatment plans to have a looser pitch (e.g., close to 1), where sup-inf modulation is handled by longitudinal modulation of MLC leaves. Alternatively, a tighter pitch may be maintained with additional opportunities to modulate the treatment beam over the same sup-inf region. The systems and methods described herein accomplish the above advantages via the use of a high-speed MLC. One example, of such a high-speed MLC is an electromagnetic MILC (eMLC), as described herein. It should be noted, however, that alternative variations of a high-speed MLC may be used to perform the operations described herein. For the purposes of the present disclosure, a high-speed MLC may be any MLC that is capable of very fast leaf motion (e.g., approximately able to cross a 5 cm field in less than 100 ms). It should be noted that although “eMLC” is used throughout the present disclosure, the systems and methods described herein are equally compatible with any other form of high-speed MLC. Furthermore, for the purposes of this description, the terms “fluence,” “intensity,” and “dose” are used as follows. Fluence is the number of photons or x-rays that crosses a unit of area perpendicular to a radiation beam. Fluence rate is the fluence per unit time. Intensity is the energy that crosses a unit area per unit time. Fluence and intensity are independent of what occurs in a patient, and more specifically are not dose. Dose is the amount of energy absorbed by tissue by virtue of radiation impacting the tissue. Radiation dose is measured in units of gray (Gy), where each Gy corresponds to a fixed amount of energy absorbed in a unit mass of tissue (e.g., 1 joule/kg). Dose is not the same as fluence, but increases/decreases as fluence increases/decreases. The terms “target” and “target region” may refer to one or more fiducials near (within some defined proximity to) a treatment area (e.g., a tumor). In another embodiment, a target may be a bony structure. In yet another embodiment a target may refer to soft tissue of a patient. A target may be any defined structure or area capable of being identified and tracked, as described herein. FIG. 1A illustrates a helical radiation delivery system 800 in accordance with embodiments of the present disclosure. The helical radiation delivery system 800 may include a linear accelerator (LINAC) 850 mounted to a ring gantry 820. The LINAC 850 may be used to generate a radiation beam (i.e., treatment beam) by directing an electron beam towards an x-ray emitting target. The treatment beam may deliver radiation to a target region (i.e., a tumor). The treatment system further includes a multileaf collimator (MLC) 860 coupled with the distal end of the LINAC 850. The MLC 860 may be an eMLC, as described herein. The MLC includes a housing that houses multiple leaves that are movable to adjust an aperture of the MLC to enable shaping of the treatment beam. The ring gantry 820 has a toroidal shape in which the patient 830 extends through a bore of the ring/toroid and the LINAC 850 is mounted on the perimeter of the ring and rotates about the axis passing through the center to irradiate a target region with beams delivered from one or more angles around the patient. During treatment, the patient 830 may be simultaneously moved through the bore of the gantry on a treatment couch 840. The helical radiation delivery system 800 includes an imaging system, comprising the LINAC 850 as an imaging source and an x-ray detector 870. The LINAC 850 may be used to generate a mega-voltage x-ray image (MVCT) of a region of interest (ROI) of patient 830 by directing a sequence of x-ray beams at the ROI which are incident on the x-ray detector 870 opposite the LINAC 850 to image the patient 830 for setup and generate pre-treatment images. In one embodiment, the helical radiation delivery system 800 may also include a secondary imaging system consisting of a kV imaging source 810 mounted orthogonally relative to the LINAC 850 (e.g., separated by 90 degrees) on the ring gantry 820 and may be aligned to project an imaging x-ray beam at a target region and to illuminate an imaging plane of a detector after passing through the patient 130. FIG. 1B illustrates a radiation treatment system 1200 that may be used in accordance with alternative embodiments described herein. As shown, FIG. 1B illustrates a configuration of a radiation treatment system 1200. In the illustrated embodiments, the radiation treatment system 1200 includes a linear accelerator (LINAC) 1201 that acts as a radiation treatment source and an MLC 1205 (e.g., an eMLC) coupled with the distal end of the LINAC 1201 to shape the treatment beam. In one embodiment, the LINAC 1201 is mounted on the end of a robotic arm 1202 having multiple (e.g., 5 or more) degrees of freedom in order to position the LINAC 1201 to irradiate a pathological anatomy (e.g., target) with beams delivered from many angles, in many planes, in an operating volume around a patient. Treatment may involve beam paths with a single isocenter, multiple isocenters, or with a non-isocentric approach. LINAC 1201 may be positioned at multiple different nodes (predefined positions at which the LINAC 1201 is stopped and radiation may be delivered) during treatment by moving the robotic arm 1202. At the nodes, the LINAC 1201 can deliver one or more radiation treatment beams to a target, where the radiation beam shape is determined by the leaf positions in the MLC 1205. The nodes may be arranged in an approximately spherical distribution about a patient. The particular number of nodes and the number of treatment beams applied at each node may vary as a function of the location and type of pathological anatomy to be treated. In another embodiment, the robotic arm 1202 and LINAC 1201 at its end may be in continuous motion between nodes while radiation is being delivered. The radiation beam shape and 2-D intensity map is determined by rapid motion of the leaves in the MLC 1205 during the continuous motion of the LINAC 1201. The radiation treatment system 1200 includes an imaging system 1210 having a processing device 1230 connected with x-ray sources 1203A and 1203B (i.e., imaging sources) and fixed x-ray detectors 1204A and 1204B. Alternatively, the x-ray sources 1203A, 1203B and/or x-ray detectors 1204A, 1204B may be mobile, in which case they may be repositioned to maintain alignment with the target, or alternatively to image the target from different orientations or to acquire many x-ray images and reconstruct a three-dimensional (3D) cone-beam CT. In one embodiment, the x-ray sources are not point sources, but rather x-ray source arrays, as would be appreciated by the skilled artisan. In one embodiment, LINAC 1201 serves as an imaging source, where the LINAC power level is reduced to acceptable levels for imaging. Imaging system 1210 may perform computed tomography (CT) such as cone beam CT or helical megavoltage computed tomography (MVCT), and images generated by imaging system 1210 may be two-dimensional (2D) or three-dimensional (3D). The two x-ray sources 1203A and 1203B may be mounted in fixed positions on the ceiling of an operating room and may be aligned to project x-ray imaging beams from two different angular positions (e.g., separated by 90 degrees) to intersect at a machine isocenter (referred to herein as a treatment center, which provides a reference point for positioning the patient on a treatment couch 1206 during treatment) and to illuminate imaging planes of respective detectors 1204A and 1204B after passing through the patient. In one embodiment, imaging system 1210 provides stereoscopic imaging of a target and the surrounding volume of interest (VOI). In other embodiments, imaging system 1210 may include more or less than two x-ray sources and more or less than two detectors, and any of the detectors may be movable rather than fixed. In yet other embodiments, the positions of the x-ray sources and the detectors may be interchanged. Detectors 1204A and 1204B may be fabricated from a scintillating material that converts the x-rays to visible light (e.g., amorphous silicon), and an array of CMOS (complementary metal oxide silicon) or CCD (charge-coupled device) imaging cells that convert the light to a digital image that can be compared with a reference image during an image registration process that transforms a coordinate system of the digital image to a coordinate system of the reference image, as is well known to the skilled artisan. The reference image may be, for example, a digitally reconstructed radiograph (DRR), which is a virtual x-ray image that is generated from a 3D CT image based on simulating the x-ray image formation process by casting rays through the CT image. In one embodiment, IGRT delivery system 1200 also includes a secondary imaging system 1239. Imaging system 1239 is a Cone Beam Computed Tomography (CBCT) imaging system, for example, the medPhoton ImagingRing System. Alternatively, other types of volumetric imaging systems may be used. The secondary imaging system 1239 includes a rotatable gantry 1240 (e.g., a ring) attached to an arm and rail system (not shown) that move the rotatable gantry 1240 along one or more axes (e.g., along an axis that extends from a head to a foot of the treatment couch 1206. An imaging source 1245 and a detector 1250 are mounted to the rotatable gantry 1240. The rotatable gantry 1240 may rotate 360 degrees about the axis that extends from the head to the foot of the treatment couch. Accordingly, the imaging source 1245 and detector 1250 may be positioned at numerous different angles. In one embodiment, the imaging source 1245 is an x-ray source and the detector 1250 is an x-ray detector. In one embodiment, the secondary imaging system 1239 includes two rings that are separately rotatable. The imaging source 1245 may be mounted to a first ring and the detector 1250 may be mounted to a second ring. In one embodiment, the rotatable gantry 1240 rests at a foot of the treatment couch during radiation treatment delivery to avoid collisions with the robotic arm 1202. As shown in FIG. 1B, the image-guided radiation treatment system 1200 may further be associated with a treatment delivery workstation 150. The treatment delivery workstation may be remotely located from the radiation treatment system 1200 in a different room than the treatment room in which the radiation treatment system 1200 and patient are located. The treatment delivery workstation 150 may include a processing device (which may be processing device 1230 or another processing device) and memory that modify a treatment delivery to the patient 1225 based on a detection of a target motion that is based on one or more image registrations, as described herein. FIG. 1C. Illustrates a C-arm radiation delivery system 1400. In one embodiment, in the C-arm system 1400 the beam energy of a LINAC may be adjusted during treatment and may allow the LINAC to be used for both x-ray imaging and radiation treatment. In another embodiment, the system 1400 may include an onboard kV imaging system to generate x-ray images and a separate LINAC to generate the higher energy therapeutic radiation beams. The system 1400 includes a gantry 1410, a LINAC 1420, an MLC 1470 (e.g., an eMLC) coupled with the distal end of the LINAC 1420 to shape the beam, and a portal imaging detector 1450. The gantry 1410 may be rotated to an angle corresponding to a selected projection and used to acquire an x-ray image of a VOI of a patient 1430 on a treatment couch 1440. In embodiments that include a portal imaging system, the LINAC 1420 may generate an x-ray beam that passes through the target of the patient 1430 and are incident on the portal imaging detector 1450, creating an x-ray image of the target. After the x-ray image of the target has been generated, the beam energy of the LINAC 1420 may be increased so the LINAC 1420 may generate a radiation beam to treat a target region of the patient 1430. In another embodiment, the kV imaging system may generate an x-ray beam that passes through the target of the patient 1430, creating an x-ray image of the target. In some embodiments, the portal imaging system may acquire portal images during the delivery of a treatment. The portal imaging detector 1450 may measure the exit radiation fluence after the beam passes through the patient 1430. This may enable internal or external fiducials or pieces of anatomy (e.g., a tumor or bone) to be localized within the portal images. Alternatively, the kV imaging source or portal imager and methods of operations described herein may be used with yet other types of gantry-based systems. In some gantry-based systems, the gantry rotates the kV imaging source and LINAC around an axis passing through the isocenter. Gantry-based systems include ring gantries having generally toroidal shapes in which the patient's body extends through the bore of the ring/toroid, and the kV imaging source and LINAC are mounted on the perimeter of the ring and rotates about the axis passing through the isocenter. Gantry-based systems may further include C-arm gantries, in which the kV imaging source and LINAC are mounted, in a cantilever-like manner, over and rotates about the axis passing through the isocenter. In another embodiment, the kV imaging source and LINAC may be used in a robotic arm-based system, which includes a robotic arm to which the kV imaging source and LINAC are mounted as discussed above. Aspects of the present disclosure may further be used in other such systems such as a gantry-based LINAC system, static imaging systems associated with radiation therapy and radiosurgery, proton therapy systems using an integrated image guidance, interventional radiology and intraoperative x-ray imaging systems, etc. FIG. 2A illustrates a multileaf collimator (MLC) 31 to provide a radiation treatment dose to a target region, in accordance with embodiments described herein. MLC 31 includes two banks of opposing leaves 33, where each leaf 37 may be positioned continuously across the radiation field. The two banks of leaves 33 are positioned so as to collimate the beam 30 in the desired shape. In one embodiment, each leaf 37 may travel beyond the midpoint of the collimator in order to provide flexibility when achieving the desired collimation. The configuration illustrates fully open (41), partially open (43) and closed (45) leaf states. In an example of radiation therapy, each gantry angle has one beam associated with that particular gantry angle, which beam 30 is then collimated into multiple shapes by an MLC. Treatment beam 30 passes through the shaped aperture 47 formed by the leaves 37. The resulting collimated beam continues onto a target 14 within the patient 38. FIG. 2A also illustrates how the treatment beam may be visualized or conceptualized as many different beamlets 49. Leaves 37 of the MLC 31 are moved into various positions to achieve desired shapes or apertures for specified periods of time to achieve fluence map 51 for that particular beam. Modulation of the conceptualized beamlets occurs by sequentially and monotonically moving the leaves into desired positions to achieve desired shapes or apertures such that the time a conceptualized beamlet is exposed controls the intensity of that beamlet. In one embodiment, “monotonic,” as herein, means an ordered sequence of apertures where the sequence is dictated by a continuum from one aperture to a subsequent aperture, or where individual leaves increment in one direction during a given series of apertures. In other words, a sequence of apertures would be dictated by mechanical limitations of the MLC, not so much by what may achieve the more optimal treatment delivery. In one embodiment, a sequence would go from aperture 1, then 2 then 3 and so on, and not from 1 to 3 then to 5 then back to 2. Rather than use a single conformal shape, the MLC may deliver a sequence of shapes. The net amount of radiation received at any given gantry position is based upon the extent to which the different shapes permit the passage or blockage of radiation. As seen in FIG. 2A, the shape of MLC 31 shown does not directly correspond to the beamlet intensities of the fluence map 51. As will be appreciated, the depicted fluence map shows the accumulation of intensities for multiple shapes the MLC has taken for that particular gantry angle. A common limitation of conventional shaping MLCs is that the leaves defining the shapes move relatively slowly. Using a large numbers of shapes, or shapes that require large leaf motions, can result in longer patient treatments. Likewise, the speed of the leaves can limit the ability of conventional shaping-MLC's to deliver time-sensitive treatments, such as utilizing synchronized motion of delivery components (e.g., gantry, couch, x-ray energy etc.). In part for these reasons, prior 2-D intensity map delivery techniques have been limited to beams delivered from static positions. Alternately, prior systems that do allow continuous motion of the radiation source generally only allow single aperture shapes or morphing from one aperture shape to another as the radiation source moves, and do not allowed a 2-D intensity map to be delivered from each radiation source position. FIG. 2B illustrates a bottom view of a multileaf MLC 61, in accordance with embodiments described herein. The binary MLC 61 has a plurality of leaves 63 arranged in two banks 65, 67. Each bank of leaves is used to form a treatment slice by positioning the leaf in a closed position or open position with respect to the beam. As shown in FIG. 2B, the leaves may work in concert to be both open (A), both closed (B), or where only one leaf is open/closed (C). In a conventional binary MLC, the leaves 63 open (A) to the same, uniform width during an entire single positional section. In a conventional shaping MLC, the leaves 63 may open (A) to different, various widths during an entire single positional section. A common limitation of conventional binary MLCs is that the leaves 63 may not be open to a variety of different widths for any fraction of time during each positional section. Thus, shaping radiation beams to a target area while simultaneously minimizing radiation exposure to non-target areas may be difficult. Advantageously, the methods and systems described herein, allow for the benefits of a shaping MLC (e.g., the leaves 63 may be open to a variety of different widths for any fraction of time during each positional section), while maintaining the speed of a binary MLC. FIG. 2C illustrates a perspective view of a multileaf high-speed MLC 62, in accordance with embodiments described herein. In one embodiment, a radiation modulation device 34 includes an electromagnetically actuated MLC (eMLC) 62, which includes a plurality of leaves 66 operable to move from position to position, to provide intensity modulation. Leaves 66 can move to any position between a minimally and maximally-open position, with sufficient speed such that leaf sequencing or positioning will not be significantly influenced by any previous or future positions of any individual leaf Stated another way, leaf speed is sufficient such that the mechanics of the MLC do not unduly influence the determination of leaf position at any given time for the delivery of a radiation therapy treatment or fraction. Each leaf 66 is independently controlled by an actuator (not shown, but more fully described below), such as a motor, or magnetic drive in order that leaves 66 are controllably moved from fully open, fully closed or to any position between open and closed as described in greater detail below. The actuators can be suitably controlled by computer 74 and/or a controller. In one embodiment, the MLC 62 is coupled with the distal end of the LINAC of a radiation treatment delivery system. A processing device of the computer 74 may control the plurality of leaf pairs 66 of the MLC 62 such that for each of a plurality of radiation beam delivery positional sections corresponding to a range of radiation beam positions over a discrete time interval, each leaf pair of the plurality of opposing leaf pairs 66 is open to a fixed opening for a fraction of time in the discrete time interval and closed for the remaining fraction of time in the discrete time interval, while a radiation beam of the radiation treatment system is active. In one embodiment, the fixed opening and the fraction of time form overlapping radiation fields of different intensities that combine to result in an intensity modulated fluence field delivered to a treatment target. In one embodiment, the fixed opening conforms to the outline of a treatment target, projected back along the radiation beam to the MLC, and within a maximum range of travel of plurality of leaf pairs within the MLC. This concept is further described with respect to FIGS. 4A-C and FIG. 5B. In one embodiment, the processing device of the computer 74 may control the MLC 62 to modulate a sub-beam intensity of the radiation beam across a plurality of sub-beams that subdivide a fluence field into a 2D grid, and wherein a plurality of independent 2D sub-beam intensity patterns are delivered from a plurality of gantry angles while the gantry moves continuously. This concept is further described with respect to FIGS. 3A-F and FIG. 5A. In one embodiment, the LINAC including the MLC 62 is mounted on a rotating gantry, wherein radiation beams delivered from the range of radiation beam positions rotate around a treatment target. The treatment target may be moved axially through a bore of the rotating gantry, and the radiation beams delivered from the range of radiation beam positions may follow a helical path about the treatment target. In another embodiment, the LINAC and the MLC 62 are mounted on a robotic arm, and the radiation beam delivered from the range of radiation beam positions is non-coplanar. FIG. 2D illustrates a top view of the leaves of a multileaf high-speed MLC 240, in accordance with embodiments described herein. Central portion 302 of MLC 240 includes inner leaf guides 301, aperture 1050 and 14 leaf pairs (1010-1039) in various positions between leaf guide inner supports 301. While 14 leaf pairs are shown, more or fewer leaf pairs may be provided according to the design requirements of a particular system. In one embodiment, there are 64 leaf pairs. In another embodiment, there are 96 leaf pairs. In still another embodiment, there are 32 leaf pairs. As will be apparent, radiation is collimated through this section 302 of the collimator. In FIG. 2D, each leaf is positioned in a particular position to define a particular aperture or shape 1050, through which radiation may pass, also referred to herein as a state. Leaf pairs 1010 and 1011 through 1038 and 1039 are controlled using the control schemes and drivers described herein to enable simultaneous volume and intensity modulation. In alternative aspects, one or more controllable jaws are used to provide primary collimation of the beam, defined by the inner edges 301 i and the frames 97A, 97B (i.e., the jaws will block the open space between support frame B and leaf pair 1010/1011 and between support frame A and leaf pair 1038/1039). Additionally or alternatively, the one or more pairs of jaws may be adjusted to reduce the size of the primary collimated beam to smaller than the frame size. FIG. 2E illustrates an exemplary leaf arrangement for a multileaf high-speed MLC, in accordance with embodiments described herein. A collimated field 1040 having a center line 1030 is provided by a pair of jaws or other collimator device. In the illustrative embodiment, two leaves form a complementary leaf pair for shaping and modulation of the collimated field 1040. For example, leaves 1010 and 1011 are one leaf pair. Leaves 1018, 1019 are another and leaves 1024, 1025 still another. Each leaf in each pair may be positioned anywhere within field 1040. The inner edges of each leaf within a leaf pair face each other and may create an opening, the collection of openings formed by each leaf pair forms aperture 1050. Aperture 1050 corresponds to an aperture of FIG. 2D previously described and is set according to a treatment plan. In one embodiment, an aperture 1050 is determined prior to administering radiation therapy to a patient in the treatment planning process, and occurs at a particular point during delivery of the treatment plan. Aperture 1050 may change according to a number of factors, such as for example, the three dimensional shape of the treatment area, intensity modulation, fluence, and beamlets within a treatment volume, as described herein. Embodiments of the high-speed MLCs described herein achieve volume and intensity modulation alone, or in simultaneous combination by providing snap state control. FIGS. 3A-C illustrate exemplary leaf open-time profiles, in accordance with embodiments described herein. Unlike using traditional MLCs, by using an eMLC (or some other suitable high-speed MLC) the amount of fluence transmitted at each point along the leaf-pair's direction of travel (e.g., the IEC-Yb direction) may be precisely controlled, while at the same time the constantly moving radiation source (e.g., the LINAC) traverses an arc short enough to be considered as one position. To generate a plan for such a high-speed MLC, leaf-open-time profiles may be determined. In one embodiment, a leaf-open-time profile indicates the open time for leaf pairs for a discrete time interval. To generate an eMLC plan, a leaf-open-time profile for each leaf pair in each positional section may be divided into discrete beamlets. An optimizer may determine an ideal leaf-open-time for each beamlet, as described below. From the leaf-open-time profile, front and back leaf-motion profiles for each leaf pair may be generated, only allowing leaves to move in a single direction in each positional section. In one embodiment, leaves alternate between moving back-to-front and front-to-back in successive positional sections, so as a leaf-pair finishes its travel in one positional section it will be in position to begin its travel in the next positional section. FIG. 3A illustrates an example leaf-open-time profile. As shown, the leaf-pair open-times, represented by the bars sitting on the IEC-Yb axis, may be different for each leaf pair. Note that in the leaf motion profile algorithm described above, the total time needed to deliver all the leaf-pair open-times is: w ( 1 ) + ∑ i = 2 n max { w ( i ) - w ( i - 1 ) , 0 } Or, equivalently: w ( n ) + ∑ i = 1 n - 1 max { w ( i ) - w ( i + 1 ) , 0 } . If this total time is less than the discrete time interval for the positional section, then the leaf motion profiles may be centered in the positional section, as shown in FIG. 3B. FIG. 3C illustrates the leading leaf motion 302 and the trailing leaf motion 304, centered in the positional section. FIGS. 3D-F illustrate exemplary optimized leaf open-time profiles, in accordance with embodiments described herein. In one embodiment, a modulation factor constraint may be applied to the leaf-open-time profiles so that the leaf open times are not excessively large (and thus delay treatment time). To generate the modulation factors, the average beamlet open-time is calculated for all non-zero beamlets across all leaves and positional sections. The discrete time interval is then determined to be the average open-time, multiplied by the desired modulation factor. For each positional section and leaf; beamlet open-times are then adjusted such that the total leaf-open time is no greater than the discrete time interval corresponding to the positional section. There are several ways to make this adjustment. In one embodiment, individual beamlet open-times that are greater than the discrete time interval may be decreased to be equal to the projection time, and then the smallest beamlet open-times may be increased until the total leaf open-time is less than or equal to the discrete time interval, as illustrated in FIGS. 3D-F. FIG. 3G illustrates an exemplary leaf open-time profile that incorporates a maximum velocity, in accordance with embodiments described herein. Notably, the leaves of a high-speed MLC move quickly, but not instantaneously. To generate a plan that is actually deliverable, leaf motion profiles may account for the finite leaf speed of the MLC. In one embodiment, this can be done by incorporating a maximum leaf velocity (and possibly leaf acceleration) into the generated leaf profiles. In one embodiment, the algorithm may strive to keep the area in each column constant. In the example profile 306, instantaneous leaf motion has been changed to leaf motion with a finite velocity. Note that the leaf motion in each segment may begin a little early, so that the integral open time for the beamlets remains unaffected. In one embodiment, if the start of travel in the next projection overlaps with the end of travel in the current projection, then the leaf may change direction before reaching the end of its travel, resulting in slightly less fluence being delivered to the target area. The operations of FIGS. 3A-G are described further with respect to FIG. 5A. FIGS. 4A-C illustrate a variety of exemplary leaf arrangements conforming to a target region 401, in accordance with embodiments described herein. Each leaf pair is positioned in a particular way such that the leaf pair openings define a particular aperture or shape to conform to target region 401. During treatment, radiation passes through the aperture defined by the combined leaf pairs, and strikes the target region below. A high-speed MLC (e.g., an eMLC), such as that described herein, opens and closes each leaf pair to an open state or a closed state during a discrete time interval. Each leaf pair may be open for a different fiction of time (e.g., an open-time fraction) during the discrete time interval, and each leaf pair may be open to a different width during a corresponding open-time fraction. Furthermore, the treatment beam may be active during the entire discrete time interval. Advantageously, by activating the treatment beam during the entire discrete time interval, and allowing each leaf pair to be open for only a fraction of the discrete time interval to a specified width (which may be different than widths of other leaf pairs), precise doses of radiation may be delivered to a variety of complicated target region shapes. The operations of FIGS. 4A-C are described further with respect to FIG. 5B. FIG. 5A is a flowchart illustrating a method 500 for fast sliding window with a high-speed MLC, in accordance with embodiments described herein. In general, the method 500 may be performed by processing logic that may include hardware (e.g., processing device, circuitry, dedicated logic, programmable logic, microcode, hardware of a device, etc.), software (e.g., instructions run or executed on a processing device), or a combination thereof. In some embodiments, the method 500 may be performed by processing logic of the radiation treatment system 800 of FIG. 1A. As shown in FIG. 5A, the method 500 may begin at block 502 with the processing logic shaping, via a collimator mounted to a gantry of a radiation treatment system, a radiation beam directed at a target. In one embodiment, the collimator is a multileaf collimator (MLC) comprising a plurality of leaf pairs. The MLC may be a high-speed MLC (e.g., an eMLC), as described herein. Processing logic at block 504 may modulate, by a processing device, a sub-beam intensity of the radiation beam across a plurality of sub-beams that subdivide a fluence field into a 2D grid. In one embodiment, the sub-beam intensity is modulated by independently modulating a rate of travel of front and back leaves in each leaf pair, as each leaf pair moves in a single pass from one end of a corresponding line of travel to another end of the corresponding line or travel. Additional details corresponding to the modulation of sub-beam intensities are provided with respect to FIGS. 3A-G. In a variety of embodiments, the 2D grid described herein is a rectangular grid. In other embodiments, the 2D grid is any shape. In one embodiment, a first axis of the 2D sub-beam grid is determined by an index of the leaf pairs along one axis of the MLC, and a second axis of the 2D grid is along a line of travel of the leaf pairs. Processing logic at block 506 may deliver a plurality of independent 2D sub-beam intensity patterns from a plurality of gantry angles. In one embodiment, the plurality of independent 2D sub-beam intensity patterns may be delivered from a plurality of gantry angles while the gantry moves continuously. In one embodiment, each of the plurality of leaf pairs changes direction when delivering the fluence pattern for each subsequent gantry angle. In one embodiment, processing logic may constrain the motion of the plurality of leaf pairs used to deliver the 2D fluence pattern from a particular gantry angle to occur in less than a pre-selected time period (as described with respect to FIG. 3D-F). In one embodiment, processing logic may center leaf pair motion for a particular gantry angle that takes less than the pre-selected time period within the pre-selected time period (as described with respect to FIG. 3A-F). In one embodiment, a first leaf in a leaf pair that follows a second leaf in the leaf pair when delivering the intensity pattern from a particular gantry angle does not reach the end of its travel before reversing direction to deliver the intensity pattern for a subsequent gantry angle (e.g., leaf pairs to not need to wait for other leaf pairs to end their travel before reversing direction). In one embodiment, the continuous motion of the gantry may be a helical motion. For example, processing logic may rotate the gantry continuously, wherein the 2D sub-beam intensity pattern for a particular gantry angle is approximated by delivering the intensity pattern over a small arc. As used herein, “small arc” may refer to a sub-arc of the total gantry rotation that is small enough that it may be treated as a single gantry angle for the purposes of planning the intended dose distribution. In one embodiment, “small arc” may refer to approximately 7 degrees around the total gantry rotation. In other embodiments, other arc sizes may be used to deliver one-dimensional beamlet intensity patterns. In one embodiment, processing logic moves the target axially through a center of the gantry via an axial support (e.g., a treatment couch), wherein the gantry and axial support move simultaneously during irradiation of the target to perform a helical delivery. In one embodiment, the helical pitch during the irradiation of the target is greater than or equal to 0.5. In other embodiments, other helical pitches greater than or less than 0.5 may be used. FIG. 5B is a flowchart illustrating a method 501 for binary MLC delivery with a per-leaf field width, according to embodiments. In general, the method 501 may be performed by processing logic that may include hardware (e.g., processing device, circuitry, dedicated logic, programmable logic, microcode, hardware of a device, etc.), software (e.g., instructions run or executed on a processing device), or a combination thereof. In some embodiments, the method 501 may be performed by processing logic of the radiation treatment system 800 of FIG. 1A. As shown in FIG. 5B, the method 501 may begin at block 503 with the processing logic determining a plurality of radiation beam delivery positional sections to contain MLC leaf control instructions while a radiation beam is active. In one embodiment, as described herein, each of the plurality of radiation beam delivery positional sections corresponds to a range of radiation beam positions over a discrete time interval (e.g., along an arc of a gantry of the radiation treatment system). For example, the radiation beam delivery positional sections (e.g., projections) may correspond to the radiation beam being delivered from at least one of a different position or a different direction. In other words, the positional sections may be thought of as positional nodes, from which a LINAC may deliver a radiation treatment beam in a particular direction. The positional sections may include a range of positions (e.g. a zone). For example, an arc in a helical treatment delivery system may be divided into a plurality of discrete positional sections (e.g., where each positional section includes some number of degrees around the arc). In one embodiment, a positional section may include approximately seven degrees around an arc (e.g., seven degrees of gantry rotation). In other, non-helical embodiments, positional sections may be defined in terms of three-dimensional spaces. In one embodiment, the different direction remains constant while the different position follows a linear trajectory that sweeps the radiation beam over a length of a treatment target. In one embodiment, the different directions are non-coplanar. At block 505, processing logic generates a plurality of openings for each of the plurality of radiation beam delivery positional sections, each of the plurality of openings corresponding to one of a plurality of leaf pairs of the MLC. Advantageously, each of the plurality of openings for each of the plurality of positional sections may correspond to a different width. For example, in one embodiment, two or more of the plurality of openings correspond to different widths in the same positional section. In one embodiment, the plurality of openings conform to an outline of a treatment target, projected back along the radiation beam to the MLC, and within a maximum range of travel of the plurality of leaf pairs within the MLC. At block 507, processing logic generates a plurality of leaf open-time fractions for each of the plurality of radiation beam delivery positional sections, each of the plurality of leaf open-time fractions corresponding to one of the plurality of leaf pairs of the MLC. In one embodiment, leaf open-time fractions are discrete amounts of time that are less than the discrete time interval. In another embodiment, a leaf open-time fraction may be a discrete amount of time that is equal to the discrete time interval. Advantageously, leaf open-time fractions allow each of the plurality of leaf pairs to be open for a different amount of time during the discrete time interval. For example, in the present embodiment, two or more of the plurality of leaf open-time fractions during the discrete time interval may be different. At block 509, processing logic controls, by a processing device, the plurality of leaf pairs of the MLC such that each leaf pair of the plurality of leaf pairs is opened to a corresponding opening of the plurality of openings for a corresponding leaf open-time fraction of the plurality of leaf open-time fractions during the discrete time interval corresponding to the range of radiation beam positions, while the radiation beam of the radiation treatment system is active. In one embodiment, the treatment beam is active during the entire discrete time interval (e.g., as the LINAC travels through the positional section). In one embodiment, the plurality of leaf open-time fractions form overlapping radiation fields of different intensities that combine to result in an intensity modulated fluence field delivered to a treatment target. Advantageously, the above operations allow a radiation treatment delivery system to effectively time-modulate a radiation treatment beam while precisely conforming to the outline of a target area. FIG. 6 illustrates examples of different systems 600 within which a set of instructions, for causing the systems to perform any one or more of the methodologies discussed herein, may be executed. In alternative implementations, the machine may be connected (e.g., networked) to other machines in a LAN, an intranet, an extranet, and/or the Internet. Each of the systems may operate in the capacity of a server or a client machine in client-server network environment, as a peer machine in a peer-to-peer (or distributed) network environment, or as a server or a client machine in a cloud computing infrastructure or environment. The systems are machines capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. As described below and illustrated in FIG. 6, a system 600 may include a diagnostic imaging system 605, a treatment planning system 610, and a treatment delivery system 615. Diagnostic imaging system 605 may be any system capable of producing medical diagnostic images of a patient that may be used for subsequent medical diagnosis, treatment planning, treatment simulation and/or treatment delivery. For example, diagnostic imaging system 605 may be a computed tomography (CT) system, a magnetic resonance imaging (MRI) system, a positron emission tomography (PET) system, a combination of such systems, or the like. For ease of discussion, diagnostic imaging system 605 may be discussed below at times in relation to an x-ray imaging modality. In other embodiments, other imaging modalities such as those discussed above may also be used. In one embodiment, diagnostic imaging system 605 includes an imaging source 620 to generate an imaging beam (e.g., x-rays) and an imaging detector 630 to detect and receive the beam generated by imaging source 620, or a secondary beam or emission stimulated by the beam from the imaging source (e.g., in an MRI or PET scan). In one embodiment, imaging source 620 and imaging detector 630 may be coupled to a digital processing system 625 to control the imaging operation and process image data. In one embodiment, diagnostic imaging system 605 may receive imaging commands from treatment delivery system 615 and/or treatment planning system 610. Diagnostic imaging system 605 includes a bus or other means 680 for transferring data and commands among digital processing system 625, imaging source 620 and imaging detector 630. Digital processing system 625 may include one or more general-purpose processors (e.g., a microprocessor), special purpose processor such as a digital signal processor (DSP) or other type of processing device such as a controller or field programmable gate array (FPGA). Digital processing system 625 may also include other components (not shown) such as memory, storage devices, network adapters and the like. Digital processing system 625 may be configured to generate digital diagnostic images in a standard format, such as the Digital Imaging and Communications in Medicine (DICOM) format, for example. In other embodiments, digital processing system 625 may generate other standard or non-standard digital image formats. Digital processing system 625 may transmit diagnostic image files (e.g., the aforementioned DICOM formatted files) to treatment delivery system 615 over a data link 683, which may be, for example, a direct link, a local area network (LAN) link or a wide area network (WAN) link such as the Internet. In addition, the information transferred between systems may either be pulled or pushed across the communication medium connecting the systems, such as in a remote diagnosis or treatment planning configuration. In remote diagnosis or treatment planning, a user may utilize embodiments of the present disclosure to diagnose or treat a patient despite the existence of a physical separation between the system user and the patient. In one embodiment, treatment delivery system 615 includes a therapeutic and/or surgical radiation source 660 to administer a prescribed radiation dose to a target volume in conformance with a treatment plan. Treatment delivery system 615 may also include imaging system 665 to perform computed tomography (CT) such as cone beam CT, and images generated by imaging system 665 may be two-dimensional (2D) or three-dimensional (3D). Treatment delivery system 615 may also include a digital processing system 670 to control radiation source 660, receive and process data from diagnostic imaging system 605 and/or treatment planning system 610, and control a patient support device such as a treatment couch 675. Digital processing system 670 may be connected to or a part of a camera feedback system. Digital processing system 670 may be configured to perform any of the operations described herein. Digital processing system 670 may include a processing device that represents one or more general-purpose processors (e.g., a microprocessor), special purpose processor such as a digital signal processor (DSP) or other type of device such as a controller or field programmable gate array (FPGA). The processing device of digital processing system 670 may be configured to execute instructions to perform the operations described herein. In one embodiment, digital processing system 670 includes system memory that may include a random access memory (RAM), or other dynamic storage devices, coupled to a processing device, for storing information and instructions to be executed by the processing device. The system memory also may be used for storing temporary variables or other intermediate information during execution of instructions by the processing device. The system memory may also include a read only memory (ROM) and/or other static storage device for storing static information and instructions for the processing device. Digital processing system 670 may also include a storage device, representing one or more storage devices (e.g., a magnetic disk drive or optical disk drive) for storing information and instructions. The storage device may be used for storing instructions for performing the treatment delivery steps discussed herein. Digital processing system 670 may be coupled to radiation source 660 and treatment couch 675 by a bus 692 or other type of control and communication interface. In one embodiment, the treatment delivery system 615 includes an input device 678 and a display 677 connected with digital processing system 670 via bus 692. The display 677 can show trend data that identifies a rate of target movement (e.g., a rate of movement of a target volume that is under treatment). The display can also show a current radiation exposure of a patient and a projected radiation exposure for the patient. The input device 678 can enable a clinician to adjust parameters of a treatment delivery plan during treatment. Treatment planning system 610 includes a processing device 640 to generate and modify treatment plans and/or simulation plans. Processing device 640 may represent one or more general-purpose processors (e.g., a microprocessor), special purpose processor such as a digital signal processor (DSP) or other type of device such as a controller or field programmable gate array (FPGA). Processing device 640 may be configured to execute instructions for performing simulation generating operations and/or treatment planning operations discussed herein. Treatment planning system 610 may also include system memory 635 that may include a random access memory (RAM), or other dynamic storage devices, coupled to processing device 640 by bus 686, for storing information and instructions to be executed by processing device 640. System memory 635 also may be used for storing temporary variables or other intermediate information during execution of instructions by processing device 640. System memory 635 may also include a read only memory (ROM) and/or other static storage device coupled to bus 686 for storing static information and instructions for processing device 640. Treatment planning system 610 may also include storage device 645, representing one or more storage devices (e.g., a magnetic disk drive or optical disk drive) coupled to bus 686 for storing information and instructions. Storage device 645 may be used for storing instructions for performing the treatment planning steps discussed herein. Processing device 640 may also be coupled to a display device 650, such as a cathode ray tube (CRT) or liquid crystal display (LCD), for displaying information (e.g., a 2D or 3D representation of the VOI) to the user. An input device 655, such as a keyboard, may be coupled to processing device 640 for communicating information and/or command selections to processing device 640. One or more other user input devices (e.g., a mouse, a trackball or cursor direction keys) may also be used to communicate directional information, to select commands for processing device 640 and to control cursor movements on display 650. Treatment planning system 610 may share its database (e.g., data stored in storage 645) with a treatment delivery system, such as treatment delivery system 615, so that it may not be necessary to export from the treatment planning system prior to treatment delivery. Treatment planning system 610 may be linked to treatment delivery system 615 via a data link 690, which in one embodiment may be a direct link, a LAN link or a WAN link. It should be noted that when data links 683, 686, and 690 are implemented as LAN or WAN connections, any of diagnostic imaging system 605, treatment planning system 610 and/or treatment delivery system 615 may be in decentralized locations such that the systems may be physically remote from each other. Alternatively, any of diagnostic imaging system 605, treatment planning system 610, and/or treatment delivery system 615 may be integrated with each other in one or more systems. It will be apparent from the foregoing description that aspects of the present disclosure may be embodied, at least in part, in software. That is, the techniques may be carried out in a computer system or other data processing system in response to a processing device 625, 640, or 670 (see FIG. 6), for example, executing sequences of instructions contained in a memory. In various implementations, hardware circuitry may be used in combination with software instructions to implement the present disclosure. Thus, the techniques are not limited to any specific combination of hardware circuitry and software or to any particular source for the instructions executed by the data processing system. In addition, throughout this description, various functions and operations may be described as being performed by or caused by software code to simplify description. However, those skilled in the art will recognize what is meant by such expressions is that the functions result from execution of the code by processing device 625, 640, or 670. A machine-readable medium can be used to store software and data which when executed by a general purpose or special purpose data processing system causes the system to perform various methods of the present disclosure. This executable software and data may be stored in various places including, for example, system memory and storage or any other device that is capable of storing at least one of software programs or data. Thus, a machine-readable medium includes any mechanism that provides (i.e., stores) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-readable medium includes recordable/non-recordable media such as read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc. The machine-readable medium may be a non-transitory computer readable storage medium. Unless stated otherwise as apparent from the foregoing discussion, it will be appreciated that terms such as “receiving,” “positioning,” “performing,” “emitting,” “causing,” or the like may refer to the actions and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (e.g., electronic) quantities within the computer system's registers and memories into other data similarly represented as physical within the computer system memories or registers or other such information storage or display devices. Implementations of the methods described herein may be implemented using computer software. If written in a programming language conforming to a recognized standard, sequences of instructions designed to implement the methods can be compiled for execution on a variety of hardware platforms and for interface to a variety of operating systems. In addition, implementations of the present disclosure are not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement implementations of the present disclosure. It should be noted that the methods and apparatus described herein are not limited to use only with medical diagnostic imaging and treatment. In alternative implementations, the methods and apparatus herein may be used in applications outside of the medical technology field, such as industrial imaging and non-destructive testing of materials. In such applications, for example, “treatment” may refer generally to the effectuation of an operation controlled by the treatment planning system, such as the application of a beam (e.g., radiation, acoustic, etc.) and “target” may refer to a non-anatomical object or area. In the foregoing specification, the disclosure has been described with reference to specific exemplary implementations thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the disclosure as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. |
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048470376 | claims | 1. Apparatus for the inspection of nuclear reactor fuel rods combined in fuel rod clusters with spaces therebetween in a fuel assembly, comprising test probes each being disposed at a different respective level along the fuel assembly, fingers each being part of a respective one of said test probes, ultrasonic test head each being disposed on a respective one of said fingers, means for inserting said test heads into the spaces between the fuel rods, and means for correcting the insertion position of each of said test probes independently of the insertion position of the others of said test probes, before insertion of said test probes. 2. Apparatus according to claim 1, including a rack holding said probes assigned to said levels, said inserting means moving said rack together with said probes in a given insertion direction of said probes, and a common drive moving said probes at all of said levels transverse to said given insertion direction. 3. Apparatus according to claim 2, wherein said rack has a lower surface, and including a support plate having brackets, at least two mutually parallel spindle nuts fixed on said lower surface of said rack, and spindles mounted in said brackets and engaging in said spindle nuts for moving said rack and said support plate. 4. Apparatus according to claim 2, wherein said rack has bearing points at each of said levels, shafts disposed at said bearing points at each of said levels, said shafts each having a central region in the form of a spindle, probe carriers in the form of spindle nuts each being disposed on a respective one of said central regions, and pair of bars each bordering a respective one of said central regions and being fixed relative to a respective one of said shafts, said bars jutting out from said shafts in the same direction as said fingers of said probe by a distance at least equal to the diameter of a fuel rod, said bars of said pairs being spaced apart by a distance equal to the nominal width of a fuel assembly, and said shafts being displaceable relative to said bearing points. 5. Apparatus according to claim 4, wherein said rack has opposite side walls at which said bearing points are disposed, said shafts pass through said bearing points and said side walls and have free ends protruding from said side walls, and including abutments disposed on said free ends of said shafts, and compression springs each being disposed between one of said abutments and a respective one of said side walls. 6. Apparatus according to claim 4, including worm wheels each being disposed on a respective one of said shafts, tongue and groove connections securing said worm wheels on said shafts, and a worm shaft extending transversely to said shafts and simultaneously engaging all of said worm wheels. 7. Apparatus according to claim 6, including hubs each being disposed on said rack for limiting axial movement of a respective one of worm wheels. 8. Apparatus according to claim 1, including a base plate for holding a fuel assembly, two struts jutting out from said base plate parallel to each other and to the fuel assembly, spindle drive mechanisms each being disposed transverse to said struts at a respective one of said levels, spindle nuts each being disposed on a respective one of said spindle drive mechanisms, probe carriers each being disposed on a respective one of said spindle nuts, and drive elements each being disposed on a respective one of said probe carriers for inserting said probes into the spaces between the fuel rods. 9. Apparatus according to claim 8, wherein said correcting means include a strip guiding said probe fingers, and an ultrasonic transducer disposed on said strip between said probe fingers, said ultrasonic transducer emitting sound waves in a given insertion direction of said probes and receiving returning echoes providing positional determination of said probes relative to a fuel rod. 10. Apparatus according to claim 8, including other struts offset by 90.degree. with respect to said first-mentioned struts, and other probes disposed on said other struts at levels different from said first-mentioned levels. |
description | This is a Continuation of International Application PCT/EP2017/050835, which has an international filing date of Jan. 16, 2017, and which claims the priority of the German Patent Application No. 102016200814.5, filed Jan. 21, 2016. The disclosures of both applications are incorporated in their respective entireties into the present application by reference. The present invention relates to a reflective optical element for the extreme ultraviolet wavelength range, having a multilayer system that extends over a surface on a substrate, wherein the multilayer system has layers from at least two different materials with different real part of the refractive index at a wavelength in the extreme ultraviolet wavelength range which alternate, wherein a layer of one of the at least two materials forms a stack with the layer or layers arranged between the former and the layer of the same material which is closest at an increasing distance from the substrate. In addition, the present invention relates to an optical system for EUV lithography and to an EUV lithography apparatus having such a reflective optical element. The present application claims the priority of the German patent application 10 2016 200 184.5 of Jan. 21, 2016, the disclosure of which is hereby incorporated into the present application by reference in its entirety. In EUV lithography apparatuses, reflective optical elements for the extreme ultraviolet (EUV) wavelength range (e.g. wavelengths between approximately 5 nm and 20 nm) such as photomasks or mirrors on the basis of multilayer systems are used for the lithography of semiconductor devices. Since EUV lithography apparatuses generally have a plurality of reflective optical elements, they must have as high a reflectivity as possible to ensure sufficiently high overall reflectivity. In order to ensure, among other things, as high a reflectivity as possible of the individual reflective optical elements, the aim is an ability to reflect all rays of the local beam uniformly well at high local incidence angle bandwidths. To this end, the number and the thicknesses of the individual stacks of the multilayer system are optimized. In the simplest case, these are periodic multilayer systems, in which the number of stacks or periods is reduced to the extent that the reflectivity curve has the desired width. In this multilayer system, however, the reflectivity still strongly varies in dependence on the angle of incidence and the wavelength. In a further step, it is also possible to provide in the multilayer system two or more sections, in which the respective total stack thickness and the layer ratio within the stack are different. Furthermore, said two sections can also have different numbers of stacks. In variants, the stack thicknesses and/or the layer thickness ratios within the stacks can also be varied continuously over the entire layer sequence of the multilayer system. In US 2003/0222225 A1, the wavelength band, over which sufficient reflectivity of the EUV radiation is achieved, is widened by varying, in adaptation to the angle of incidence distribution over the surface of the multilayer system, the ratio Γ of the layer that is made from material with a lower real part of the refractive index to the total thickness of the respective stack over the surface of the multilayer system. This is based on the observation that for a specific wavelength with a lower Γ, the angle of incidence of maximum reflectivity is shifted to angles of greater than 0°, that is to say a maximum reflectivity that deviates from normal incidence is achieved. The stack thickness is preferably kept constant and at the same time the thickness of the layers made from material with a higher real part of the refractive index and from material with a lower real part of the refractive index is varied. This can be done in steps or continuously. Depending on the angle of incidence distribution over the surface, the stack thickness can alternatively or additionally also be varied in a surface region. It is an object of the present invention to provide a further reflective optical element that can make possible a higher reflectivity over greater angle of incidence ranges. This object is achieved by a reflective optical element for the extreme ultraviolet wavelength range, having a multilayer system that extends over a surface on a substrate, wherein the multilayer system has layers from at least two different materials with different real part of the refractive index at a wavelength in the extreme ultraviolet wavelength range which alternate, wherein a layer of one of the at least two materials forms a stack with the layer or layers arranged between the former and the layer of the same material which is closest at an increasing distance from the substrate, wherein in at least one stack the material of the layer with the lower real part and/or the material of the layer with the higher real part of the refractive index over at least one partial surface is different than it is over the remaining surface of the multilayer system. It has been found that it is possible to exert influence on the angle dependence of the reflectivity to the effect that, for a fixed wavelength, widening of the angle of incidence range with a higher reflectivity as compared to an unchanged multilayer system can be observed not only by varying the layer thicknesses or the layer thickness ratio within a stack over the surface of a multilayer system, but also due to lateral material variations within at least one stack layer. It has been found in particular that it is also possible to exert influence on the angle dependence of the reflectivity to the effect that, for a fixed wavelength, widening of the angle of incidence range with a higher reflectivity as compared to an unchanged multilayer system can be observed by way of the combination of two or more substances within a layer with a lower real part of the refractive index or within a layer with a higher real part of the refractive index. The angle dependence of the reflectivity can be influenced already by providing a combined layer in only one stack. It is also possible for a plurality of such modified layers with higher and/or lower real part of the refractive index to be provided in the multilayer system. If the layers made from material with a lower and/or higher real part of the refractive index are not modified in all stacks, said one or more modified stack layers are advantageously situated rather in a substrate-remote section of the multilayer system. It is possible to modify only one layer made from material with a lower real part or only one layer from material with a higher real part of the refractive index or to modify both layers of a stack as described. If a stack has more than one layer made from material with a higher real part of the refractive index and one made from material with a lower real part of the refractive index, it is also possible for one or possibly more of the additional stack layers in at least one stack to be modified laterally with respect to the material. It should be pointed out that in the case of a fixed angle of incidence, a widening of the wavelength range with a higher reflectivity can be observed analogously. A widening of the angle of incidence range in which higher reflectivities are achieved can be realized with one or more modified layers with lower or higher real part of the refractive index, which extend over only one part of or over the entire surface of the multilayer system. With particular preference, the reflective optical element is designed for different angles of incidence of extreme ultraviolet radiation over the surface of the multilayer system and the proportion of the at least two substances in the modified layer or layers with lower or higher real part of the refractive index varies in dependence on the angle of incidence. Due to the correlation of the lateral material variation with the angle of incidence distribution over the surface of the multilayer system, it is also possible to achieve the highest possible reflectivity values over greater angle of incidence ranges. Investigations have shown that it is possible to exert influence on the angle of incidence dependence of the reflectivity at a specific wavelength by way of different proportions of the at least two substances in the modified layer in a manner similar to that by way of the layer thickness ratio Γ. It is therefore possible to optimize the reflectivity very precisely for different angle of incidence distribution over the surface of the multilayer system by way of varying the proportions. The proportion of the at least two substances can be varied in steps. Preferably, the proportion varies continuously so as to image, with as precise a fitting as possible, the continuous profile of the angle of incidence distributions that exists in EUV lithography apparatuses over the lit surface of the multilayer system. For manufacturing reasons, it is of particular advantage if in the at least one modified stack the material of the layer with a lower or higher real part of the refractive index is a combination of exactly two substances. In a first preferred embodiment, the layer with the lower and/or higher real part of the refractive index in the at least one stack is made from sub-layers of the two or more substances, wherein the respective sub-layer thicknesses of which over at least one partial surface are different than they are over the remaining surface of the multilayer system. Such modified layers can be produced with conventional coating methods by successively applying the individual sub-layers. In a second preferred embodiment, the material of the layers with the lower and/or higher real part of the refractive index in the at least one stack has a mixture ratio of the at least two substances, wherein the mixture ratio over at least one partial surface is different than it is over the remaining surface of the multilayer system. With typical coating methods, such modified layers can be produced by simultaneous application of the two or more substances, wherein the concentration of the individual starting materials during the coating process is set in accordance with the mixture ratio to be attained. This can be, among other things, doping the layer material with lower and/or higher real part of the refractive index with a particle proportion that varies over the surface. In a further variant, the material of the modified layer can also be substances that correspond to a chemical base compound with different stoichiometric ratios. The total thickness and/or the ratio of the thickness of the layer with the lower real part of the refractive index to the total thickness of at least one stack, that is to say the layer thickness ratio Γ, over at least one partial surface is advantageously different than it is over the remaining surface of the multilayer system. Both measures can likewise serve to exert influence on the reflectivity for specific angles of incidence. The material of at least one layer advantageously varies in terms of its density. Lateral density changes within a layer can be achieved for example by way of ion polishing. Depending on the duration, intensity and ion energy, a varying degree of material compaction can be achieved locally. Depending on how focused the ion beam is, the material compaction can be attained in a highly targeted fashion and with a high lateral resolution. This is advantageous in particular in the production of reflective optical elements for use in more complex angle of incidence distributions. It should be pointed out that, analogously, the wavelength bandwidths of the reflective optical element for a fixed angle of incidence can be increased. This correspondingly applies to wavelength distributions of the incident radiation. In preferred embodiments, the reflective optical element has silicon as the material with a higher real part of the refractive index, molybdenum as the material with a lower real part of the refractive index, and two or more of the group of molybdenum, ruthenium, niobium, scandium, titanium, carbon, carbide as the at least two substances, or has ruthenium as the material with a lower real part of the refractive index, silicon as the material with a higher real part of the refractive index, and two or more of the group of silicon, boron carbide, beryllium, boron, carbon as the at least two substances. Such reflective optical elements are suitable in particular for wavelengths in the range between 12.5 nm and 15.0 nm. By such a specific selection of the materials, it is possible to attain a high reflectivity via a high contrast between layers with lower and higher real part of the refractive index. This is because the difference between the imaginary parts of for example molybdenum and ruthenium in said wavelength range on the one hand and silicon on the other is sufficiently high. At the same time, the real part and imaginary part of the refractive index of molybdenum and ruthenium are sufficiently different to be able to exert a considerable influence on the angle of incidence distribution of the reflectivities. This likewise applies if additionally lateral density variations are provided. The total thickness and/or the ratio of the thickness of the layer with the lower real part of the refractive index to the total thickness of at least one stack, that is to say the layer thickness ratio Γ, over at least one partial surface is advantageously different than it is over the remaining surface of the multilayer system. Both measures can likewise serve to exert influence on the reflectivity for specific angles of incidence. The partial surfaces with a different layer thickness ratio and with material variation can but do not have to be congruent. This analogously applies to specific wavelength ranges of the incident radiation in the case of a fixed angle of incidence. The object is furthermore achieved by an optical system for EUV lithography or by an EUV lithography apparatus having at least one reflective optical element as described above. FIG. 1 schematically shows an EUV lithography apparatus 10. Essential components are the illumination system 14, the photomask 17 and the projection system 20. The EUV lithography apparatus 10 is operated under vacuum conditions so that the EUV radiation in the interior thereof is absorbed as little as possible. A plasma source or a synchrotron can serve for example as the radiation source 12. In the example illustrated here, a plasma source is used. The emitted radiation in the wavelength range of approximately 5 nm to 20 nm is firstly focused by a collector mirror 13. The operating beam is then introduced into the illumination system 14. In the example illustrated in FIG. 1, the illumination system 14 has two mirrors 15, 16. The mirrors 15, 16 guide the beam onto the photomask 17 having the structure, which is intended to be imaged onto the wafer 21. The photomask 17 is likewise a reflective optical element for the EUV wavelength range, which is exchanged depending on the production process. With the aid of the projection system 20, the beam reflected from the photomask 17 is projected onto the wafer 21 and the structure of the photomask is thereby imaged onto said wafer. In the example illustrated, the projection system 20 has two mirrors 18, 19. It should be pointed out that both the projection system 20 and the illumination system 14 can have in each case only one or three, four, five or more mirrors. In order to ensure the highest possible and constant reflectivity over angles of incidence and angle of incidence ranges that are as great as possible, one or more of the mirrors or the photomask have a special multilayer system, wherein the multilayer system has layers from at least two different materials with different real part of the refractive index at a wavelength in the extreme ultraviolet wavelength range which alternate, wherein a layer of one of the at least two materials forms a stack with the layer or layers arranged between the former and the layer of the same material which is closest at an increasing distance from the substrate, wherein in at least one stack the material of the layer with the lower or the higher real part of the refractive index is a combination of at least two substances, the respective proportion of which in this layer over at least one partial surface is different than it is over the remaining surface of the multilayer system. FIG. 2 schematically illustrates the structure of a reflective optical element 50. The illustrated example shows a reflective optical element based on a multilayer system 51. The multilayer system here substantially comprises alternatingly applied layers of a material with a higher real part of the refractive index at the operating wavelength at which for example the lithographic exposure is carried out (also called spacer 54) and of a material with a lower real part of the refractive index at the operating wavelength (also called absorber 55), wherein in the example shown here, an absorber-spacer pair forms a stack 53 which corresponds to a period in the case of periodic multilayer systems. In certain respects a crystal is thereby simulated whose lattice planes correspond to the absorber layers at which Bragg reflection takes place. The thicknesses of the individual layers 54, 55 and also of the repeating stacks 53 can be constant over the entire multilayer system 51 or vary, depending on what spectral or angle-dependent reflection profile is intended to be achieved. The reflection profile can also be influenced in a targeted manner by the basic structure composed of absorber 55 and spacer 54 being supplemented by further absorber or spacer materials in order to increase the possible maximum reflectivity at the respective operating wavelength. To that end, in some stacks absorber and/or spacer materials can be mutually interchanged or the stacks can be constructed from more than one absorber and/or spacer material or have additional layers made of further materials. The absorber and spacer materials can have constant or varying thicknesses over all the stacks in order to optimize the reflectivity. Furthermore, it is also possible to provide in individual or all stacks additional layers for example as diffusion barriers between spacer and absorber layers 54, 55. The multilayer system 51 is applied on a substrate 52 and forms a reflective surface 60. Materials having a low coefficient of thermal expansion are preferably chosen as substrate materials. The first layer adjoining the substrate 52 can be an absorber layer, a spacer layer or an additional layer. A protective layer 56 can be provided on the multilayer system 51, said protective layer protecting the reflective optical element 50 against contamination, inter alia. FIG. 3 schematically shows the construction of a first exemplary variant of a modified stack 53′ comprising a spacer layer 54 and a combined absorber layer 55′. In the example illustrated here, the combined absorber layer 55′ consists of exactly two sub-layers 551 and 552 made from respectively different substances. The proportion of the two substances in the combined absorber layer 55′ varies in the example illustrated in FIG. 3 by way of their respective sub-layer thickness varying in the lateral extent over different partial sections 61, 61′, 61″ of the multilayer system. Thus, in the example illustrated in FIG. 3, the thickness of the sub-layer 551 on the left-hand side 61 is at a maximum, and the thickness of the sub-layer 552 on the right-hand side 61″ is at a maximum. Inbetween 61′, the thickness of the sub-layer 551 continuously decreases and the thickness of the sub-layer 552 continuously increases. What is achieved hereby is that, in the region 61 of the multilayer system, which is on the left-hand side in FIG. 3, the reflectivity is at a maximum for other angles of incidence than for the region 61″ of the multilayer system, which is on the right-hand side in FIG. 3. The actual angles of incidence and reflectivities in the individual case are dependent, among others, on the material or substance selection and the layer and sub-layer thicknesses. The sub-layer thickness change can extend in a direction of the surface of the multilayer system and remain constant in the other direction perpendicular with respect thereto in the surface plane. Sub-layer thickness changes, however, can also be effected in two dimensions over the surface. Depending on which angle of incidence regions are intended to have as high a reflectivity as possible, one or more stacks 53 can be modified in the manner described here. In a modification of the embodiment illustrated in FIG. 3, the modified absorber layer 55′ can also be made not from sub-layers but from a mixture of two substances. Rather than the sub-layer thickness, the mixture ratio or the concentration gradient of the two substances can be locally adapted in accordance with the desired angle of incidence with maximum reflectivity. This can refer to, among others, a varying doping of the layer material. The profile of the mixture ratio or of the concentration gradient can correspond to the profile of the sub-layer thickness change. FIG. 4A illustrates a further exemplary variant of a modified stack 53″, in which the spacer layer 54′ is embodied as a combination of two substances. They are applied as sub-layers 541, 542, the thickness of which continuously varies over the partial surfaces 61, 61′, 61″ of the multilayer system—preferably in dependence on the distribution of the angles of incidence over the surface of a mirror or a photomask or of another reflective optical element that is based on said multilayer system—such that the respective proportion of the individual substance in the spacer layer 54′ varies continuously. FIG. 4B schematically illustrates the stack from above. In the present example, the entire surface, which corresponds to the surface of the reflective optical element or the multilayer system thereof, is divided into three partial surfaces 61, 61′, 61″. The material of the spacer layer 54′ is unchanged in the partial surface region 61, while it varies in the partial surface regions 61′, 61″. In further embodiments, there can also be two, four, five, six or more partial surfaces. Even in modifications of this variant, the modified spacer layer 54′ can also be made not from sub-layers but from a mixture of two substances. Rather than the sub-layer thickness, the mixture ratio or the concentration gradient of the two substances over the surface of the multilayer system can be adapted in accordance with the desired angle of incidence with maximum reflectivity. This can refer to, among others, a varying doping of the layer material. In the previously described examples, the ratio of the thickness of the layer with the lower real part of the refractive index to the total thickness of at least one stack, that is to say the layer thickness ratio Γ, over at least one partial surface is different than it is over the remaining surface of the multilayer system. In modifications, the total thickness over at least one partial surface can alternatively or additionally be different than it is over the remaining surface of the multilayer system. In further modifications, the layer density can additionally vary laterally over the surface. It is also possible to combine the individual measures for lateral material variation. These measures, and those mentioned above, can likewise serve to exert influence on the reflectivity for specific angles of incidence. When selecting the materials for spacer and absorber layers and in particular the substances for the combined layer, it is advantageous if the two or more substances for a wavelength in the extreme ultraviolet wavelength range have real parts of the refractive index that differ as much as possible so as to be able to exert a measurable influence on the angle of incidence distribution of the reflectivity by way of changes in the proportions thereof. Especially for the EUV wavelength range, for example combinations of molybdenum, ruthenium, niobium, scandium, carbon and/or titanium are suitable for the modified absorber layer, wherein the respective combination can also be present in the form of an alloy or a compound such as carbide, e.g. molybdenum carbide, in variable stoichiometric compositions. For the modified spacer layer, for example combinations of silicon, boron carbide, beryllium, boron and/or carbon are suitable for the EUV wavelength range. In a first preferred embodiment for reflective optical elements for the EUV wavelength range between 12.5 nm and 15.0 nm, two mirrors were investigated, the multilayer system of which in each case had fifteen stacks of in each case silicon as a spacer and a combination of molybdenum and ruthenium as combined absorber layers. The ratio of combined absorber layer thickness to stack thickness in the case of both mirrors was constant at 0.37. The modified absorber layers were produced by co-sputtering, with the result that a mixture with locally different concentrations of molybdenum and ruthenium can be obtained. In FIG. 5, the reflectivity R in dependence on the angle of incidence for different ratios (see also FIG. 6) of ruthenium to molybdenum for the first mirror is plotted with a dashed line and for the second mirror with a dash-dotted line. The wavelength of the incident beam was 13.5 nm. For comparison, the solid line illustrates the reflectivity curves for a first comparative mirror with conventional multilayer system, i.e. with a multilayer system of fifteen stacks with silicon spacer layers and molybdenum absorber layers. In FIG. 6, the ratios V of ruthenium to molybdenum in the fifteen absorber layers with respect to the angle of incidence for the first mirror are plotted by way of rectangles, for the second mirror by way of triangles, and for the comparative mirror by way of diamonds. The ratio V is here defined such that V=1 corresponds to equal proportions of molybdenum and ruthenium, and V=0 corresponds to a pure molybdenum layer. The angle of incidence was always stated in degrees and relative to the surface normal. In order to further optimize the reflectivity for each angle of incidence, the thickness of the stacks was increased by a factor F as compared to the normal incidence. This factor F in FIG. 7 is likewise plotted with respect to the angle of incidence with rectangles for the first mirror, with triangles for the second mirror, and with diamonds for the comparative mirror. In the case of the first comparative mirror, the reflectivity in the maximum decreases as the angle of incidence increases despite a variable stack thickness factor F. Due to the variation of the ratio V of ruthenium to molybdenum between approximately 0.25 for angles of incidence of about 30° to 0 for angles of incidence of about 32.5° in the case of the first mirror, the reflectivity can be kept substantially constant over an angle interval of approximately 2.5°. Here, the stack thickness factor F deviates slightly from that of the first comparative mirror only toward smaller angles of incidence. By varying the ratio V between approximately 0.25 for an angle of incidence of approximately 30° to approximately 0.85 for an angle of incidence of slightly over 32° in the case of the second mirror, it is even possible to achieve an increase of the maximum reflectivity. Due to the high proportion of ruthenium, stack thicknesses, which are slightly higher are involved than in the case of the first mirror and in the case of the first comparative mirror. In optical systems or EUV lithography apparatuses, it is possible using the second mirror to compensate reflectivity gradients caused by other reflective optical elements. A further mirror in accordance with a further preferred embodiment was investigated. Said mirror had a multilayer system of fifteen stacks with absorber layers made from ruthenium and combined spacer layers, which comprised a combination of silicon and boron carbide with a variable proportion over the mirror surface. The ratio of absorber layer thickness to stack thickness was constant at 0.37. In FIG. 8, the reflectivity R in dependence on the angle of incidence is plotted with a dashed line for different ratios of boron carbide to silicon for the further mirrors. The wavelength of the incident beam was again 13.5 nm. For comparison, a solid line illustrates the reflectivity curves for a second comparative mirror with a multilayer system comprising silicon spacer layers with boron carbide intermediate layers of constant thickness as diffusion barriers and ruthenium absorber layers. In FIG. 9, the ratios V (see also FIG. 9) of silicon to boron carbide in the fifteen spacer layers are plotted with respect to the angle of incidence for said further mirrors with diamonds and for the second comparative mirror with rectangles. The ratio V is here defined such that V=1 corresponds to a pure silicon layer, and V=0 corresponds to a pure boron carbide layer. For the second comparative mirror, the spacer layers were considered to be a unit with the diffusion barriers made from boron carbide, such that a constant ratio of approximately 0.75 of silicon to boron carbide resulted. The angle of incidence was again always stated in degrees and relative to the surface normal. In order to optimize the reflectivity for each angle of incidence, the thickness of the stacks was increased by a factor F as compared to the normal incidence. This factor F in FIG. 10 is likewise plotted with respect to the angle of incidence with diamonds for the further mirror and with rectangles for the second comparative mirror. In the case of the second comparative mirror, the reflectivity in the maximum again decreases as the angle of incidence increases despite a variable stack thickness factor F. Due to the variation of the ratio V of silicon to boron carbide between approximately 0.55 for angles of incidence of about 21° to approximately 0.75 for angles of incidence of about 29° in the case of the further mirror, the reflectivity can be kept substantially constant over an angle interval of approximately 8°. Here, the stack thickness factor F deviates slightly from that of the second comparative mirror only toward smaller angles of incidence. The multilayer system of the further mirror can be produced particularly easily by changing the production method of the second comparative mirror such that the thickness of the boron carbide layers is locally varied over the surface in dependence on the expected angle of incidence during use as an optical reflective element. It should be pointed out that the maximum reflectivity remains constant over larger angle intervals toward smaller angles of incidence. It should also be pointed out that the angle dependence of the maximum reflectivity can additionally be influenced by way of the variation of the ratio of the absorber layer thickness to stack thickness over the surface of the multilayer system. Due to the strongly reduced angle of incidence dependence of the maximum reflectivity, the reflective optical elements introduced here can be used particularly well in optical systems for EUV lithography or in EUV lithography apparatuses in which generally greater angle of incidence variations over the surface of a lit reflective optical element should be expected in particular due to the presence of a plurality of reflective optical elements and the attempt to arrange them so as to save as much space as possible. 10 EUV lithography apparatus 12 EUV radiation source 13 collector mirror 14 illumination system 15 first mirror 16 second mirror 17 mask 18 third mirror 19 fourth mirror 20 projection system 21 wafer 50 reflective optical element 51 multilayer system 52 substrate 53, 53′, 53″ stack 54, 54′ spacer 55, 55′ absorber 56 protective layer 551, 552 absorber sub-layer 541, 542 spacer sub-layer 60 reflective surface 61, 61′, 61″ partial surfaces |
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description | This application claims the benefit of Korean Patent Application No. 10-2012-0012218, filed on Feb. 7, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 1. Field of the Invention The present invention relates to a water-spray residual heat removal system for a nuclear power plant, and more particularly, to a residual heat removal system corresponding to a hybrid of a water-spray system and an air cooling system that greatly extends an operating time by spraying a cooling water to a heat exchanger or submerging the heat exchanger in water so that a relatively great amount of heat is removed using a latent heat of water evaporation at an early phase of a nuclear accident at which time a relatively great amount of residual heat is released, and by removing a core residual heat using an air cooling at a late phase of the nuclear accident at which time a supply of cooling water is exhausted or a heat load is significantly decreased. 2. Description of the Related Art A pressurized water reactor (PWR) may use a steam generator (SG) secondary heat removal system to cool a decay heat. A conventional steam generator (SG) secondary heat removal system may not provide an auxiliary feedwater to a steam generator when a power supply of a water feeding pump is lost, or a water source is depleted due to internal or external causes. When a large disaster such as the Fukushima nuclear accident occurs, it may be difficult to approach an accident site and thus, it may be difficult to separately provide a power source for a cooling system of a nuclear power plant, and it may be difficult to replenish a cooling water to a water tank containing the heat exchanger. In this instance, a core residual heat may not be removed for a relatively long period of time, for example, a period of several days. That is, a conventional PWR passive auxiliary feedwater system is available to cool a reactor system when a heat exchanger is imbedded with water. Thus, an available cooling time may be limited by a duration of power supplying time and a water volume of cooling tank. When a relatively severe accident occurs in a nuclear power plant due to internal or external circumstances, reproviding a power source or a cooling water may be difficult due to a damaged road or radioactive fallout and thus, a residual heat removal may be stopped, thereby leaving a nuclear reactor susceptible to a critical risk. Accordingly, a research needs to resolve the issues described above. An aspect of the present invention provides a residual heat removal system for a nuclear power plant that may cool a heat exchanger without a restriction on an operating time for cooling since the heat exchanger may be cooled by circulating air in an atmosphere in addition to water. Another aspect of the present invention also provides a residual heat removal system for a nuclear power plant that may remove heat through use of a latent heat of water evaporation by spraying a cooling water to a heat exchanger or in a state in which the heat exchanger is submerged in a water tank at an early phase of a nuclear accident, and may be converted to an air cooling system at a later phase of the nuclear accident after eight hours from the accident initiation, that is, a residual heat level is relatively at low. Still another aspect of the present invention also provides a residual heat removal system for a nuclear power plant that may cool a heat exchanger for a relatively long period of time without a restriction on time and without replenishing a supply of cooling water since the heat exchanger may be cooled through use of air in the atmosphere, and use of an alternating current (AC) power supply may be omitted. Yet another aspect of the present invention also provides a residual heat removal system for a nuclear power plant that may be readily converted between an air cooling and a water cooling, and may have an enhanced safety through use of a method of blocking a leakage of a radioactive substance by closing valves at both ends of an air duct, and releasing the radioactive substance inside the air duct into a reactor containment building when a radiation leakage accident occurs due to a damaged heat exchanger tube. According to an aspect of the present invention, there is provided a residual heat removal system for a nuclear power plant, the residual heat removal system including an air duct provided on an outside of a reactor containment building, a heat exchanger disposed on an inside of the air duct, a first pipe to transfer, to the heat exchanger, a steam generated in a steam generator disposed on an inside of the reactor containment building, and a second pipe to transfer, to the steam generator, condensation water that is cooled and condensed in the heat exchanger, wherein the heat exchanger is air-cooled using outside air flowing inside of the air duct. The residual heat removal system may further include a cooling water supply module to supply a cooling water-to the heat exchanger, wherein the heat exchanger is water-cooled by spraying the cooling water on the heat exchanger. The air duct may include a first shut-off valve to selectively open and close one end of the air duct, wherein the heat exchanger is water-cooled by being submerged in a cooling water supplied from the cooling water supply module when the first shut-off valve is closed. The cooling water supply module may be located at a higher elevation when compared to the heat exchanger so as to supply the cooling water to the heat exchanger through use of a water head differential. The cooling water supply module may include a water pipe to supply the cooling water, and a water pipe shut-off electric valve to open and close the water pipe, wherein a plurality of water pipe shut-off electric valves are provided. The air duct may include a first shut-off valve and a second shut-off valve to selectively open and close both ends of the air duct, the first pipe and the second pipe may include a first opening and closing valve unit and a second opening and closing valve unit to selectively open and close the first pipe and the second pipe, respectively, and the air duct may be cut off from the outside air and the reactor containment building when the first shut-off valve, the second shut-off valve, the first opening and closing valve unit, and the second opening and closing valve unit are closed. The residual heat removal system may further include a connection pipe disposed between the air duct and the reactor containment building, wherein the connection pipe may include a check valve and a safety valve so that a fluid inside the air duct may be released into the reactor containment building when the first shut-off valve, the second shut-off valve, the first opening and closing valve unit, and the second opening and closing valve unit are closed. The first pipe and the second pipe may have a downward slope in a direction of the steam generator. According to an embodiment of the present invention, it is possible to cool a heat exchanger without a restriction on an operating time for cooling since the heat exchanger may be cooled by circulating air in an atmosphere in addition to water. According to another embodiment of the present invention, it is possible to remove heat through use of a latent heat of evaporated water by spraying a cooling water to a heat exchanger or in a state in which the heat exchanger is submerged in a water tank at an early phase of the nuclear accident after eight hours from the accident initiation, that is, a residual heat level is relatively at low. According to still another embodiment of the present invention, it is possible to cool a heat exchanger for a relatively long period of time without a restriction on time and without replenishing a supply of cooling water since the heat exchanger may be cooled through use of an air, and an AC power supply may not be used for operation. According to yet embodiment of the present invention, it is possible to readily convert between an air cooling system and a water cooling system, and have an enhanced safety through use of a method of blocking a linkage of a radioactive substance by closing valves at both ends of an air duct, and releasing the radioactive substance inside the air duct into a reactor containment building when a radiation leakage accident occurs due to a damaged heat exchanger tube. Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Exemplary embodiments are described below to explain the present invention by referring to the figures. FIG. 1 is a diagram illustrating a residual heat removal system according to an embodiment of the present invention. Referring to FIG. 1, a residual heat removal system 100 may include an air duct 110, a heat exchanger 120, a first pipe 130, and a second pipe 140. A nuclear power generation system 10 may operate using a thermal cycle through a device such as a steam generator 12 disposed on an inside of a reactor containment building 11. In this instance, the heat exchanger 120 corresponding to an axis of the thermal cycle may be located on an inside of the air duct 110 that is disposed on an outside of the reactor containment building 11. The steam generator 12 and the heat exchanger 120 may be connected through the first pipe 130 and the second pipe 140, and a refrigerant may be circulated to form the thermal cycle. The first pipe 130 and the second pipe 140 may include a first opening and closing valve unit 135 and a second opening and closing valve unit 145. When an error occurs in a nuclear power generation, the first pipe 130 and the second pipe 140 may be shut off by the first opening and closing valve unit 135 and the second opening and closing valve unit 145 so that a refrigerant may not flow freely between an inside and an outside of the reactor containment building 11. This arrangement will be described later with reference to FIG. 4. In this instance, the first pipe 130 through which a fluid such as a refrigerant gas moves from the steam generator 12 to the heat exchanger 120 may be constructed to form an upward slope in a direction of the steam generator 12 so that the fluid may flow naturally. The second pipe 140 through which a fluid moves from the heat exchanger 120 to the steam generator 12 may be constructed to form a downward slope in a direction of the steam generator 120. That is, the first pipe 130 and the second pipe 140 may be disposed to be parallel to one another, and may be disposed so that the steam generator 120 may be located at a relatively higher elevation, and the fluid may naturally flow. The air duct 110 may have a shape of a pipe, and may be formed in a shape of a Venturi tube in which a velocity of an air-flow increases at a portion of the steam generator 120 due to a reduced internal flow area. A first shut-off valve 118 and a second shut-off valve 116 may be provided on both sides of the air duct 110, and the first shut-off valve 118 and the second shut-off valve 116 may allow a flow of air in the air duct 110, and aid in performing water cooling by submerging the heat exchanger 120 in water 220 by closing one side of the air duct 110. Closing of the air duct 110 will be further described with reference to FIG. 2. Each of operating motors 117 and 115 may be attached to each of the first shut-off valve 118 and the second shut-off valve 116 to operate the first shut-off valve 118 and the second shut-off valve 116, respectively. A cooling water supply module 200 may include a cooling water tank 210 and a cooling water 220 contained in the cooling water tank 210, and the cooling water 220 may spray the cooling water 220 to the heat exchanger 120 through being connected to the cooling water tank 210 via a water pipe 230, thereby cooling the heat exchanger 120 by using the cooling water 220. In this instance, a cooling effect may be doubled by the sprayed cooling water 220 and outside air flowing into the air duct 110. The air duct 110 may be filled with the cooling water 220 through the water pipe 230 to perform a cooling by way of a water cooling scheme. The water pipe 230 may include a water pipe shutting-off electric valve 235 to control a flow of the cooling water 220. In this instance, a power source that provides power to the water pipe shut-off electric valve 235 may be constructed by a dedicated direct current (DC) battery, and a plurality of power sources may be constructed to secure safety by operating a spare power source when one of the plurality of power sources fails. A plurality of the water pipe shut-off electric valves may be provided in anticipation of a failure. A connection pipe 155 that connects an inside of the air duct 110 and an inside of the reactor containment building 11 may be provided. The connection pipe 155 may be used to release a high-pressure fluid inside of the air duct 110 into the reactor containment building 11. The releasing of the high-pressure fluid will be further described with reference to FIG. 5. The connection pipe 155 may include a valve, and the valve may correspond to a check valve. A plurality of valves may be provided. The connection pipe 155 may include a safety valve 150. Hereinafter, an operation of the residual heat removal system according to embodiments of the present invention will be described. Referring to FIG. 1, the heat exchanger 120 may be air-cooled by opening both ends of the air duct 110 to allow outside air to flow. A water cooling scheme may be selectively used in addition to an air cooling scheme to spray the cooling water 220 to the heat exchanger 120 through use of the cooling water supply module 200. FIG. 2 is a diagram illustrating a residual heat removal system according to another embodiment of the present invention. Referring to FIG. 2, the first shut-off valve 118 located at a bottom of the air duct 110 may be closed, and the cooling water 220 may be provided into the air duct 110 through use of the cooling water supply module 200. The heat exchanger 120 may be submerged in the cooling water 220, and be cooled in a water cooling scheme. In this instance, the second shut-off valve 116 may be opened so that steam of the cooling water 220 may be released. FIG. 3 is provided to describe usefulness of each cooling scheme. FIG. 3 illustrates a residual heat when a cooling scheme is converted from a water spray cooling scheme that removes a relatively large amount of heat to an air cooling scheme that removes a relatively small amount of heat. An appropriate scheme may be selected based on intent, urgency, efficiency, and the like of a cooling based on an eight hour period. A scheme of water-cooling a heat exchanger having a relatively large amount of heat removal by submerging the heat exchanger in a cooling water may be used selectively. A restriction on a cooling time may be excluded by removing heat through use of an evaporation heat of water by spraying a cooling water to a heat exchanger or in a state in which the heat exchanger is submerged in a water tank in an early phase of an accident, and being converted to an air cooling system in a late phase of the accident after eight hours from the accident initiation, that is, a residual heat level is relatively at low. A system corresponding to a hybrid of a water-spray system and an air cooling system may be provided to greatly extend an operating time by spraying a cooling water to a heat exchanger or submerging the heat exchanger in water so that a relatively great amount of heat is removed using an evaporation heat at an early phase of a nuclear accident during which a relatively great amount of residual heat is released, and by removing a core residual heat using an air cooling at a later phase of the nuclear accident during which a supply of cooling water is exhausted or a heat load significantly decreases. By applying the system, a period of time for removing a residual heat may be extended without replenishing a supply of cooling water to a heat exchanger pool and thus, a safety system prepared in anticipation of a large natural disaster or a safety system prepared in anticipation of a Station Black Out (SBO) may be applied. FIG. 4 is a diagram illustrating a residual heat removal system according to still another embodiment of the present invention. A flow in the first pipe 130 and the second pipe 140 may be blocked through use of the first opening and closing valve unit 135 and the second opening and closing valve unit 145 provided in the first pipe 130 and the second pipe 140. A flow of a refrigerant may be blocked in a pipe when an error occurs in the heat exchanger 120, and the like. FIG. 5 is a diagram illustrating a residual heat removal system according to yet another embodiment of the present invention. When the heat exchanger 120 is damaged, the air duct 110 may be closed using the first shut-off valve 118 and the second shut-off valve 116 provided on both sides of the air duct 110, and a linkage of a radioactive substance may be blocked by closing the first pipe 130 and the second pipe 140 through use of the first opening and closing valve unit 135 and the second opening and closing valve unit 145 provided in the first pipe 130 and the second pipe 140, respectively as illustrated in FIG. 4. The connection pipe 155 may be opened so that a high-pressure radioactive substance on an inside of the air duct 110 may flow into the reactor containment building 11. In this instance, safety may be enhanced through extension of a threshold of a pressure. That is, a check valve provided in the connection pipe 155 may operate so as to prevent a backflow of a radioactive substance from the reactor containment building 11 to the heat exchanger 120, and the safety valve 150 that opens at a lower pressure than a design pressure of the reactor containment building 11 may be provided to operate so as to prevent a linkage of a radioactive substance by releasing a pressurized radioactive substance inside of the air duct 110 into the reactor containment building 11 when a pressure of the air duct 110 increases in response to the first shut-off valve 118 and the second shut-off valve 116 at both ends of the air duct being closed. After a spray cooling water is exhausted, the first shut-off valve 118 and the second shut-off valve 116 at a top and a bottom of the air duct 110 may be opened, thereby removing heat from the heat exchanger 120 by air naturally circulating from a top portion to a bottom portion of the air duct 110. In this instance, heat may continue to be removed without replenishing a supply of cooling water since heat released from the heat exchanger 120 may be removed without the cooling water. Although a few exemplary embodiments of the present invention have been shown and described, the present invention is not limited to the described exemplary embodiments. Instead, it would be appreciated by those skilled in the art that changes may be made to these exemplary embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents. |
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051851237 | abstract | Apparatus suitable for annealing the interior walls of a nuclear reactor which comprises a nuclear reactor shell, a cap securely attached to the nuclear reactor shell, wherein the cap has an inlet line for the introduction of a hot gas under high pressure and an outlet line for removing low pressure gas from the interior of the apparatus, means, such as a cylindrical shell, that helps define an annular space adjacent to the vertical walls of the reactor through which gas can flow and means for directing low pressure gas to the outlet line. |
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