Patent Number: 056299648
Section: description

DETAILED DESCRIPTION OF THE INVENTION To provide a concise and appropriate description, alternative terminology may be employed depending on the context. In particular, a neutron absorbing plate may be alternatively referred to as neutron absorber, or simply as absorber. The perspective view of a preferred embodiment of the invention for use with BWR fuel assemblies illustrated in FIG. 1A shows the invention to comprise a neutron absorber plate 1, a right lower fuel assembly clip 6A, a left lower fuel assembly clip 6B, a mounting plate 15 with a plurality of flow holes, such as hole 15A, an aperture 16, and a latch 17. The neutron absorber plates are typically 13 to 14 feet in length, but only 5 inches in width. To conveniently illustrate these plates in FIG. 1A, their midsection has ben shown as broken away along the lines at drawing numeral 4. For this configuration, the neutron absorber plate 1 has been folded to form two plates a right plate 2 and a left plate 3. These plates are positioned generally orthogonal to one another to fit about two sides of the generally rectangular fuel assemblies. The upper ends of these plates are connected to two adjacent outer edges of the mounting plate, while the right and left clips, 6A and 6B are connected to lower ends of the right and left absorber plates respectively. These clips extend outward from the the absorber plates to grip the lower end of the fuel assembly. The neutron absorber plate is formed of thee layers, a backing plate 25, a neutron absorbing sheet, such as sheet 26B, and a cover plate, such as 27B. There is usually a separation 25A on the backing plate between the mounting plate 15 and the top of a cover plate, such as cover plate 27A. There is no need to extend the neutron absorbing sheet 26B to the top of the backing plate because there is no fuel in the fuel assembly in this area. The fabrication and assembly of the invention are described in greater detail in the description of FIG. 6. As has been noted, the configuration of the invention shown in FIG. 1A is referred to as the chevron configuration because of the orthogonal positioning of the absorber plate which has been designed to fit about a generally rectangular fuel assembly, such as the fuel assembly 7 shown in FIG. 2. The BWR fuel assembly shown in FIG. 2 can be seen to comprise a flow channel 8, an upper fuel tie plate 9 with a plurality of apertures such as 9A and 9B which are designed to accept the upper end of fuel rods, such as fuel rod 9C, a fuel rod interim spacer 11, a lower fuel rod tie plate 13, a bail 14, and a flow channel corner plate 10. Since this fuel assembly has an aspect ratio similar to that of the neutron absorber plate shown in FIG. 1, the midsection of the fuel assembly in FIG. 2 has been shown as broken away at drawing numeral 12 for convenience of illustration. The basic function of the fuel assembly is to provide a means of conveniently handling a plurality of fuel rods while maintaining a prescribed spacing between the rods. Typically, the fuel rods are oriented parallel to one another and are spaced apart by a distance of approximately 3/8 of an inch. This is accomplished by fastening the rods in apertures, such as 9A and 9B, in the upper tie plate and in similar apertures in the lower tie plate. The bail, which is mounted on the upper surface of the upper tie plate, is used as the principal means of lifting and transporting the fuel assembly about a power station. It is particularly useful in raising and lowering the fuel assembly in and out of the fuel storage rack. The flow channel, which is secured to both the upper an lower tie plates, is used to aid in directing the flow of water about the fuel rods while the fuel assembly is in the reactor core. The flow channel includes two spacer buttons 55 whose purpose is to provide the proper spacing between adjacent fuel assemblies in the reactor core. These buttons extend generally 0.316 inch outwardly from two sides of the flow channel as can be seen in FIG. 2. The outward extension of these buttons make it difficult to closely fit neutron absorbers about all four sides of the fuel assembly because of the mechanical interference which would occur on the two sides of the fuel assembly containing these spacer buttons. This is a problem with prior art neutron absorber that attempt to cover all four sides without making provision for these buttons. However, the chevron configuration of the present invention shown in FIG. 1A and 1B covers only two sides of the fuel assembly and can easily accommodate these buttons. The orthogonal cross section of the absorber plate in the chevron configuration can be seen in FIG. 1B. This Figure is a plan view of the present invention with the upper section removed to clearly show the orthogonal orientation of the right and left absorber plates. This cross sectional view shows the location of the neutron absorber sheets 26A and 26B, covering plates 27A and 27B, and backing plate 25. This Figure also shows lower end mounting clips 6A and 6B located at the lower end of the absorber plate. When the invention is in place about a fuel assembly, these clips extend about the lower end of the flow channel to hold the lower end of the invention to the fuel assembly. The upper end of the invention for BWR fuel assemblies is secured to the fuel assembly by means of the latch 17 which can be seen in FIG. 1A secured to the upper surface of the mounting plate. The latch is shown in greater detail in FIGS. 3 and 4A. FIG. 3 is a plan view of the mounting plate 15, showing the latch 17 and its principal components, a captured screw 24, a hinge 19 and a tongue 20. The hinge rotatably connects the tongue to the mounting plate. The hinge includes trunnions 19A and 19B. The point of rotation is shown more clearly in the side view of the latch provided by FIG. 4A. In this Figure, the latch tongue is cross hatched from upper left to lower right to distinguish it from the mounting plate, which is cross hatched in the opposite direction, upper right to lower left. The tongue passes through an aperture 15B in the mounting plate where the hinge connects the tongue to the cover plate. FIG. 4A shows the details of one embodiment of the hinge. In this embodiment, the hinge comprises a cylindrical port 19C passing through the tongue to accept the two trunnions 19A and 19B, only one of which, 19A, is shown in this Figure. The trunnions are disposed on either side of the tongue and are attached to the mounting plate 15. Cylindrical projections on the trunnions extend into the cylindrical port in the tongue, rotatably supporting it and providing it with the ability to rotate with respect to the mounting plate. The tongue's cross section is in the form of a reversed "S" which allows the upper portion of the tongue 20A to be located above the mounting plate, while the lower portion 20B is located below the mounting plate. The lower portion of the tongue, when in its latched position as shown in FIG. 4A, extends beneath the corner plate 10 of the flow channel, securing the invention to the fuel assembly. The tongue is maintained in its latching position by means of a captured screw 24 and a nut 18. The captured screw 24 is attached to the upper portion of the tongue 20A, while the nut 18 is attached to the bottom of the mounting plate in a position to receive screw 24. The screw 24 is threaded into the nut, enabling it to hold the upper portion of the tongue down on the mounting plate and thereby preventing the lower portion of the tongue from releasing the corner plate. The lower tip 24A of screw 24 is conical to aid in guiding it into the nut 18. The head of the screw 24B is also conical to aid in guiding a socket wrench on to the screw head. Typically, a socket wrench attached to a long shaft is guided onto the screw head and the screw is tightened down onto the cover plate manually from the top of the storage pool. The use of specialized personnel or tools is completely avoided. Prior to lowering the the invention down onto a fuel assembly, the tongue must be oriented generally vertically to enable it to later close on the corner plate. The tongue is maintained in this vertical position by means of a slight compression occurring between the tongue and the edge of aperture 15, as indicated by path of rotation 20C of the lower portion 20B of the tongue. This path results in a slight interference fit for the tongue as it rotates to a vertical position through aperture 15B. Once the invention is in position on the fuel assembly, the socket wrench can be moved manually by way of the attached shaft to tap the latch past this interference point and into position where the captured screw may be tightened down, forcing the tongue down flush onto the mounting plate. A captured lock washer 24C is located just below the screw head and above the upper portion of the tongue. As the screw is tightened down, it is locked in place by this washer. There is rarely any vibration occurring in the storage pools, but this feature is an added safety factor designed to provide an extra margin of safety to protect against such event, even though they may be rare. The configuration of a PWR fuel assembly differs from that of a BWR fuel assembly and accordingly requires a different embodiment of the present invention to provide a convenient means of attachment to the PWR fuel assembly. The differences in the two types of fuel assemblies lies primarily in the components forming their upper and lower end fittings and the absence of a flow channel in the PWR fuel assembly. A PWR fuel assembly 49 is shown in FIG. 4B. Since the means for attachment of the invention describe herein is made to the upper end of the PWR fuel assembly, only the the upper end of the PWR fuel assembly is shown in this Figure. The lower portion of this fuel assembly is deleted below point 50. The primary components of the upper end of the PRW fuel assembly comprise a flow nozzle 47, a plurality of fuel rods, such as rod 9D, a plurality of guide tubes, such as guide tube 54, and spacer grids, such as spacer grid 53. The guide tubes are attached to the nozzle 47 and also attached to the spacer grid 53. The connection between the spacer grid and the guide tubes occurs within the fuel assembly. The spacer grid extends inward from the periphery of the fuel assembly where it is shown in FIG. 4B, passing about the fuel rods to reach the guide tubes. The fuel rods are connected to and supported in position by the spacer grid. The nozzle 47, which is located on the top of the fuel assembly, has a rectangular outer perimeter, conforming generally to the outer perimeter of the fuel assembly. This nozzle also includes a rectangular inner aperture 51, which is centrally located and is symmetrically positioned with respect to the outer perimeter. This aperture produces a nozzle that is in the form of a rectangular wall which is approximately one-half inch thick. The means for attachment to the PWR fuel assembly is illustrated in the embodiment of the invention shown in the upper portion of FIG. 4B. This embodiment comprises an absorber plate 1, a rectangular sleeve 43, and four upper attachment clips 44. The sleeve is formed about a rectangular aperture 52 which is sufficiently wide to fit closely about the fuel assembly nozzle 47. The upper attachment clips are shown in detail in FIG. 4C. These clips comprise arms 45 and retaining tips 46. The arms are connected to the top of the sleeve and extend over the top of the nozzle and then downward inside the aperture 51 to a depth where the nozzle contains a recess 48. At this depth, the arms of the clips terminate in the retaining tips 46. As can be seen in FIG. 4C, the retaining tips have a triangular cross section, with one of the orthogonal sides aligned with the clip arm and the other extending beneath the collar recess to secure the invention to the fuel assembly. This means of attachment can also be applied while the fuel assembly is in storage. As the invention is lowered about the fuel assembly, the hypotenuse of the retaining clips acts as a cam to force the arms of the clips sufficiently towards the center of aperture 52 to allow the the clips to pass down into the aperture 51 until the recess 48 has been reached. The arms of the clips 44 are resilient and cause the tips to extend under the recess once they have reach the depth of the recess. The clips may be released by use of a special tool to force them back into the aperture and out from under the recess. As shown in FIG. 7, a fuel assembly 7 with the invention attached is typically stored in a storage rack 37 that is normally located in a 40 foot deep pool. The fuel storage rack consists of a generally rectangular outer wall 32 within which the internal volume is divided into a series of small rectangular cells such as cells 33. When placing a fuel assembly into storage, the fuel assembly, such as assembly 7 is lowered down into a cell such as 33A to occupy the volume in the fuel rack showed by projection lines 38. In FIG. 7, the fuel assembly 7 has been foreshortened for illustrative purposes. The height of the fuel assembly is approximately the same as that of the fuel rack. The fuel assembly essentially occupies the total volume of the cell, with the exception of a clearance space between the fuel assembly and the cell walls which is used to to permit the insertion of the present invention about the fuel assembly while it remains in storage in a fuel rack. In most currently used fuel racks, all four walls of the fuel cell contained neutron absorber plates. FIG. 8 shows a plan view representation of the cells in a fuel rack 37. This rack consists of the outer wall 32, a plurality of cells 33 with all four walls of the cells containing absorber plates. For illustrative purposes, the location of the absorber plates in the cell walls is shown in this Figure by the darkened peripheral area in cell 34. These are the absorber plates that are now corroding and are losing their effectiveness as neutron absorbers, requiring replacement by means such as the present invention. Cell 34 is shown more clearly in the larger drawing of this cell presented in FIG. 8B. This cell is formed of four walls, such as wall 34A, containing the existing, but often depleted neutron absorbing material. This cell also includes a cross sectional view of the chevron configuration of the invention, 34B as it would normally to positioned in such a cell. The chevron configuration of the present invention, which ordinarily protect only two sides of the fuel assembly, can be oriented to protect all four sides of the fuel assembly by using the proper orientation of the absorber plates in the fuel rack. This orientation is illustrated in FIG. 9 where a fuel storage rack 37 is shown holding a number of fuel assemblies with the present invention secured to each fuel assembly. The orientation and general location of the absorber plates of the invention is represented by a chevron, such as chevron 35. A chevron is drawn in each cell containing the invention. In this Figure, each chevron is oriented to protect the upper and the left wall of its respective cell. Three contiguous cells containing three chevron configured absorber plates 35A, 35B, and 35C with the same orientation, completely protect all four sides of the cell containing absorber plates 35A. The upper and left wall of the cell containing absorber plates 35A are protected by absorber plates 35A. The left plate from chevron plates 35B protects the right wall of the cell containing 35A and the upper plates from chevron plates 35C protects the lower wall of this cell. Continuing the same orientation through out the entire fuel rack will provide protection of all four wall of all cells with the exception of the the right and lower outside walls of the fuel storage rack. Neutron absorber plates can be generally omitted from these walls because they face open water which is itself a neutron absorber. However, if additional protection is desired, additional neutron absorbing plates can be added to these outside walls in several ways. A first way is to add a series of absorber plates along the outside wall such as plates 36 which are shown positioned along the right outside wall in FIG. 9. A second way is to add a single, large absorber plate, such as plate 40, to protect one or two walls of the storage rack. A third way is to use two of the chevron absorber plates per fuel cell to protect all four walls within a cell as shown in cell 39. This arrangement may be applied to any cell as necessary to increase neutron absorption, but in most instances it can be restricted to use only along the right and lower wall in FIG. 8 for cost savings purposes. The relatively thin wall of the present invention makes this more feasible than is possible with prior art thick wall absorbers. In order to describe other alternate ways of providing neutron absorber protection that can be use along the outside wall of the fuel rack, it is necessary to briefly consider alternative configurations of the invention. Although the chevron configuration is the preferred configuration of the present invention, as will be shown in greater detail below, the invention may be fabricated as a single plate absorber, such as shown in FIG. 5B, as a double plate absorber, such as shown in FIG. 6, as a triple plate absorber, such as shown in cell 41 of FIG. 9, as a quadruple plate absorber, such as shown in cell 42, and as a large plate absorber such as plate 40 of FIG. 9. These various configurations of the invention may be applied in different ways to achieve additional neutron absorber protection for the outside walls of the fuel rack. For example, further cost savings may be obtained by using a fourth way of protecting this wall in which the single absorber plate form of the present invention is attached to the fuel assembly and positioned within a cell against an outside wall as shown by the location of the single plate absorber 36A in FIG. 9. The addition of a single plate to the chevron configuration in effect produces a triple plate absorber. A triple plate absorber may be fabricated directly and installed as shown in cell 41 of FIG. 9 to produce the same results. In cell 42 of FIG. 9 outside protection of all four walls of the cell is required if in addition to the upper and left walls both the right and lower outside walls of the fuel rack are to be protected. If the protection is to be obtained from absorber plates within the cell, either the quadruple plate configuration shown in this cell or a combination of the previously described plate configurations can be used. An advantage of using absorber plates within the cell is they are relatively easily installed. These plates can be secured to the mounting plate of the invention in any desired configuration and installed in the same manner as previously described for the chevron configuration. Applying absorber plates to the outside of fuel storage racks can be somewhat more difficult than installing plates within the cell as described above, but it is feasible and constitutes a fifth way to protect the fuel rack wall. This way is feasible because the outside walls are relatively accessible. They face open water within the storage pool, providing ample room to maneuver absorber plates and tools required for the installation of the absorber plates. The absorber plates may be installed on a fuel storage rack without removal of either the fuel assemblies or the fuel storage rack. The edges of these absorber plates incorporate projections that are designed to grip the the walls of the fuel storage rack, as shown in FIG. 9 by edge projection 40A on plate 40. This edge projection extends about the corner contour of the fuel rack to provide a lip that grips the fuel rack. Such edge projections reduce the required number of conventional securing means, such as screws and tapped holes, that are required to be placed in the fuel rack, thereby reducing the amount of under water machining operations. The internal features of the absorber plates used in the present invention are shown in FIG. 5A. In this Figure, the plates are shown as being flat because they are initially fabricated from flat plate material which is later folded into the chevron configuration shown in FIG. 6. In FIG. 5A, an absorber plate assembly 23A comprises a backing plate 25, a first neutron absorbing sheet 26A, a second neutron absorbing sheet 26B, a first cover plate 27A, and a second cover plate 27B. The backing and cover plates are made of a material such as stainless steel or aluminum having a thickness ranging between 0.020 and 0.050 inches. These materials are selected to prevent corrosion of the neutron absorbing sheet which would otherwise occur because of contact with the water in the storage pool The neutron absorbing sheet is made of a neutron absorbing material such as cadmium having a thickness ranging between 0.004 and 0.040 inches. The cover plates are separated from each other to expose an area located generally in the middle of the backing plate about dashed line 31 along which the backing plate is later folded into the chevron configuration shown in FIG. 6. Cadmium is the preferred material for the neutron absorbing sheet because it provides a high level of neutron absorption even when used in sheets that are only 0.004 to 0.040 inch thick. However, the invention is not limited to the use of cadmium. Other materials such as borated stainless steel and boron in a ceramic matrix may serve as the neutron absorbing material in this invention. The first and second neutron absorbing sheets are smaller than the cover plates an are positioned so that they are centered beneath the first and second cover plates, respectively. This positioning of the neutron absorbing sheets provides an area along the edges of the cover plates, indicated by dashed line 29, for sealing the neutron absorbing sheets between the cover plates and the backing plate. Along these edges, the cover plates lie directly over the backing plate without an intervening neutron absorbing sheet. The cover plates are joined to the backing plate in this area by means of a process, such as seam welding, which provides a seal that protects the neutron absorber sheets from the corrosive action of the water in the storage pool. As shown in FIG. 5A there are two distinct absorber plates spaced apart and supported in this position by a single backing plate which is folded after seam welding of the cover plates to form the chevron configuration. Typically, there is a space 25A between the top of the cover plate and the top of the backing plate because there is no fuel in this area of the fuel assembly. There are a number of distinct advantages to this method of fabrication. The first advantage is this method avoids a major problem encountered with prior art processes where there is no area free of a cover late or neutron absorber sheet. Unless special, costly precautions are taken, the folding of the backing plate in the area where it is not free of a cover plate would cause buckling and possibly cracks which would in turn leave the absorber sheet exposed. In the present invention, there is no cover plate nor neutron absorber sheet in the area in which the backing plate is folded, permitting the backing plate to be folded easily without concern for damaging the seal about the neutron absorber sheet. This feature provides a significant cost saving during fabrication and increases the reliability of the neutron absorber plate. Another significant cost saving advantage is gained by this method of fabrication because a fast, low cost process such as seam welding can be used for sealing the neutron absorber sheet. The invention is first fabricated using readily available flat sheet material. While the invention is still in flat form it is seam welded at low cost with standard, readily available equipment. It is then folded without damage, again by means of standard low cost readily available equipment. In some previously used processes, tubes with circular cross sections or rectangular cross sections were required to be welded. Such cross sections made it difficult or impossible to use low cost processes such as seam welding. FIG. 5B shows a single plate absorber 28B in accordance with the present invention. This absorber plate is fabricated in the same way as the invention shown in FIG. 5A with the exception that it is designed to cover only one side of a fuel assembly. This absorber plate can be used in such applications as the single plate absorber 36A shown in FIG. 9 or wherever additional neutron absorption is required, such as on the side of a fuel assembly. The absorber plate shown in FIG. 5B has two additional features, not shown in FIG. 5A. The first is the neutron absorber sheet is divided into two or more segments such as segments 26C and 26D as in the cover plate, such as segments 27C and 27D. There is space between these segments that exposes the backing plate 25. The segments of the cover plate are sealed to protect the corresponding segments of the neutron absorbing. The segmenting and individual sealing of the segmented portions of the neutron absorbing sheet in this way permits the absorbing sheet to be cut between the welded seals of the individual segments to provide various lengths of absorber plate in the field. The second feature is the angled lower end of the absorber plate leading to a tip 28. Although there is usually room to fit within a cell of a fuel storage rack the present invention attached to a fuel assembly, there are occasions when the fuel assembly may be warped. A single absorber plate can be maneuvered more easily than a two plate assembly and may be able to be pressed by a tight fitting area with the aid of the angled lower end. As noted above, it is usually possible to provide protection in four radial directions with the chevron configuration and it is also possible in most instances to insert the chevron configuration into a cell of a fuel storage rack while attached to a fuel assembly. This is possible because the chevron configuration covers only two sides of the fuel assembly, leaving ample room for insertion by avoiding the use of all the clearance space on all four sides of the fuel assembly. These important advantages make the chevron configuration the preferred embodiment of the present invention. Although the three plate or four plate configurations may on occasion be more difficult to insert into a fuel storage cell, they can be employed to serve as necessary in the special applications noted above. Three and four plate absorber can be fabricated using the same process described above. The number of sides dictates the width of the backing plate, the number of absorber sheets and the number of cover plates. Once sealed by welding or other appropriate means, the flat plate is folded in three to form a three sided or "U" cross section or into four to form a rectangular cross section, as required to produce the desired cross section. Prior art absorber plates in many instances tried to completely encircle the fuel assembly with the absorber medium because it was not known what discontinuities in the coverage of the absorber medium could be tolerated and still avoid criticallity. Computers with large storage capacities are more readily available now and have been used to determine this information. It has been determined that the small discontinuities in the absorber plates at the fold in the backing plate and at the outside edges of the right and left absorber plates can be easily tolerated, permitting the use of the present invention to provide the desired neutron absorption protection as shown and described. A special configuration of the present invention that essentially eliminates any loss of nuclear absorber coverage at the fold in the backing plate, and at the same time simplifies the fabrication process is shown in FIGS. 10A, 10B, 11, 12A and 12B. In this embodiment of the invention, a special backing plate made of borated aluminum or borated stainless steel replaces the aluminum or stainless steel backing plate of the embodiments described herein previously. This special backing plate is itself a nuclear absorber and needs no additional absorber added to provide sufficient nuclear absorption of this application. Borated aluminum or borated stainless steel contains natural or enriched boron. The enriched boron contains more Boron--10 atoms than natural Boron. As can be seen in FIGS. 12A and 12B, which are front and top views of this special configuration of the present invention, it has not been necessary to include a neutron absorbing sheet, such as sheet 26B or a cover plate, such as plate 27B, that are contained in the configuration shown in FIG. 1A and 1B. Virtually all the drawing numerals in FIGS. 12A and 12B are the same as were used in FIGS. 1A and 1B and refer to the same elements. In addition to the special backing plate and the absence of the cover plate and the neutron absorbing sheet, there are two additional minor differences between FIGS. 1A and 1B and FIGS. 12A and 12B. The first of these two minor differences is the two plates forming the chevron configuration in FIGS. 12A and 12B are welded together at their juncture, 64, whereas the plates forming the chevron configuration in FIGS. 1A and 1B are simply folded at their juncture. The second minor difference is the special backing plates in FIGS. 12A and 12B extend all the way upward to the mounting plate 15. Therefore, there is no omission of the neutron absorber such as there is in the area designated by drawing numeral 25A in FIG. 1A. FIGS. 10A, 10B and 11 are similar to FIGS. 5A, 5B and 6, with the exception that the multiplicity of drawing numerals in 5A, 5B and 6 necessary to identify the cover plates and neutron absorber sheets are absent in FIGS. 10A, 10B and 11. These drawing numbers can be eliminated because the cover plates and neutron absorber sheets are no longer necessary for the special configuration. FIG. 10 is a view of two separate borated backing plates, 60 and 61, used to provide the chevron formation for the special configuration. Unlike FIG. 5A, where one backing plate is folded to form the two plates of the chevron formation of FIG. 6, two individual backing plates are used instead. The two individual plates are necessary because the borated backing plates cannot be folded, but must welded. In FIG. 11, the two borated backing plates of FIG. 10A are welded together along seam 64 to provide the chevron formation necessary to produce plates 2 and 3 of the complete invention, shown in FIG. 12A. FIG. 10B shows a single plate absorber 62 using the special configuration and having a pointed tip 28 similar to the single plate absorber 28B of FIG. 5B. Again, the multiplicity of drawing numerals in FIG. 5B are absent in FIG. 10B because the cover plate and absorber sheet are no longer necessary for the special configuration. The single plate absorber of FIG. 10B can be cut as necessary at any point to fit a specific requirement at the storage site. The single blade absorber of FIG. 10B has all the advantages of the blade shown in FIG. 5B with less complication in manufacturer and in use in the field. The embodiment of the invention shown in FIG. 12A is designed to be used with a BWR fuel assembly. This special configuration is equally applicable to a PWR fuel assembly. For use with a PWR fuel assembly, the special backing plates are simply connected to the attachment means for a PWR shown in FIG. 4B.