Patent Number: 048572632
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The apparatus shown in the drawings is a rack 31 for storing spent fuel. The rack 31 consists of an array of storage locations 29, 33, 35 and 75 (FIG. 5) secured to a base 32. The rack 31 is supported on the floor 34 of a deep pool of water on studs 36 extending from pads 38. The pads 38 are of the type disclosed in application Ser. No. 282,991, filed July 14, 1981 to Machado et al. for Nuclear Reactor Spent Fuel Storage Rack and assigned to Westinghouse Electric Corporation (herein Machado). The pads 38 are secured to the underside of the base 32 and are provided with facilitates for levelling the rack as disclosed in Machado. Lugs 40 are provided intermediate the pads 38. The lugs 40 are engageable by lifting rods (not shown) extending from a crane (not shown) for raising and lowering the rack 31. The cells 29, and cells 75 at the corners of rack 91, are fabricated and are arranged so that the additional locations 33 and 35 are formed between the of the cells 29 and between the cells 29 and 75. Each additional storage location is abutted by a plurality of fabricated storage locations (FIG. 5). The general term "storage locations" is applied to the fuel-assembly-receiving members generally including fabricated members or storage locations such as 29, 75 as well as storage locations 33, and 35 formed between fabricated cells 29, 75 as boundaries. The storage locations 29, 33, 35 and 75 have substantially equal open transverse cross sectional areas and are so dimensioned as to accommodate the spent-fuel assemblies of the plant where the rack is installed. Each cell 29 is an elongated body 42 which may be of polygonal or circular or other transverse cross sectional shape. Typically, the body 42 is in the form generally of an elongated rectangular parallelopiped with open ends 37 and 39 at the top and bottom. The body is formed of long sheets 41 (FIGS. 12, 13), typically of type 304 stainless steel, pressed into a structure consisting of two sections 43 channel-shaped downwardly as seen in FIG. 13 joined by a center section 45 channel-shaped upwardly. The so-formed sheet is bent at right angles about the longitudinal center line 46 of the center section in the direction of the arrows 47 or 49 to form two sides or walls 48 or a half section 50 of the body (FIGS. 4, 4A). A pair of these halves are abutted along these edges 51 and welded along the abutted edges to form the body 42. The sides 48 of the body are reentrant along the channel sections 43. The welded joints along the edges 51 and the joint 53 at the right angle bends of the halves project outwardly forming the apices of the transverse cross section of the body. The sheets 41 which form the halves of the bodies each has a slot 55 near the end 56 which is just below the top 37. In a typical case the sheet 41; i.e., the body 42, may have a length or height of 162.00 inches. The width of the sheet 41 may be 20.883 inches and the thickness 0.062 inch. Another typical set of dimensions are length 188.87 inches, width 19.145 inches and thickness 0.075 inch. The cells 29 may be regarded as being joined in a group consisting of a central cell 29 (FIG. 5) from each of whose edges 51 and 53 a cell 29 extends radially. Each edge 51 or 53 of a central cell is contiguous to an edge 51 and 53 of a radially extending cell 29. The contiguous edges are joined by a series of welds 59 along their lengths forming by fusing a filler wire 61 (FIG. 7). The welds 59 are so spaced that the response of the rack to the anticipated seismic acceleration of the rack in the geographical region where the rack is installed is minimized. This feature can be understood from the graph shown in FIG. 11. The seismic vibration frequency of the rack, characteristic of the region of the plant, is plotted horizontally in Hertz and the acceleration in g, gravity constant, is plotted vertically. It is seen that the curve has a maximum at about 10 Hz. The welds 59 should be so spaced that the frequency is low. At low frequencies the acceleration is low. Seismic damage to the rack is precluded. The welds can also be spaced so that the seismic vibration frequency is high; i.e., the rack is stiff, where such a condition reduces acceleration. A wrapper plate 63 is welded externally to each side of the body 42. The wrapper plate 63 (FIGS. 14-16) is a thin sheet of type 304 stainless steel pressed so that it has the form of an elongated channel 65. The wrapper plate may have different dimensions for different purposes. Typical dimensions are shown in the following Table I in inches. TABLE I ______________________________________ Length or Height Perpendicular to Base 32 Width Thickness ______________________________________ 161.00 9.836 .062 187.87 9.050 .075 162.00 11.500 .062 188.87 10.650 .075 ______________________________________ The wrapper plate 63 is welded at its extending edges 67 to each outer wall 48 (FIG. 4) of the body 42 with the cavity formed by the channel 65 facing the reentrant cavity of the wall so that each wall 48 and each wrapper plate form a pocket 69. The pocket 69 may serve for the deposit of neutron-absorbing material should the need arise. The pockets 69 provide the option for readily including neutron poison in the rack 31 either before or after the rack is installed in the pool. The storage locations 33 are formed between each set of four cells 29. The four cells include a central cell, for example 29a (FIG. 5), two radial cells for example, 29b and 29c, welded to the central cell and a radial or central cell, for example 29d, of an adjacent group of welded cells 29. The walls of the storage locations 33 are the wrapper plates 63 welded to a wall 48 each of the adjacent cells 29. The storage location 33 has pockets 69 in common with each of the adjacent four cells 29 as shown in FIG. 5. Cell 29b is along the periphery of the rack 31. A number, usually most of the cells are formed between four internal cells 29a, 29b, 29c, 29d. There are four pockets 69 in each storage location 29 and 33. The pockets are interposed in each storage location on each side between a stored spent-fuel assembly (not shown) and the fuel assemblies in adjacent storage locations which have walls in common with the storage location. The pockets 69 maintain the required CTC spacing between assemblies without the need of spacers, such as grids, between storage locations. The pockets 69 also maintain the fuel-assembly-to-fuel-assembly separation. This permits closer spacing of assemblies limited by criticality concerns. The cells or storage locations 35 are along the periphery of the rack. Each cell 35 is formed between three adjacent cells, for example 29a, 29b, 75 (or an internal fabricated cell 29). The remaining side of each cell 35 is formed by a cover plate 71. The cover plate 71 is joined to the ends (corners) of the contiguous cells by welds 73 along the longitudinal joints between the cover plate 71 and the edges 53 (FIG. 9). The welds 73 may be spaced in the same way as the welds 59. The cells 75 in the corners (FIGS. 5, 8) of the rack are closed by half angle sections 77 (FIG. 8) similar to the half sections 50 which are combined to form the cells 29. The ends of the half sections are welded to the ends of the joints 51 of the adjacent cells by spaced welds 79 (FIG. 10). The storage locations 29, 33, 35 and 75 are secured to the base 32 by fillet welds 81 (FIGS. 4, 8) joining the lower ends of the walls 48 to the base. To the extent practicable the four walls 48 of each cell are welded to the base as shown in FIGS. 4 and 5. In cases in which one of the walls is not accessible, only three of the walls are welded to the base 32 as shown in FIGS. 4A and 5. Water is circulated through holes 83 and 85 in the base 32, upwardly along the cells, and through the pockets 69. The fuel storage rack 31 is subdivided into Region 1, a smaller region, and Region 2, a larger region. Region 1 accommodates temporarily off-core spent-fuel assemblies. Region 2 accommodates assemblies with a specified minimum burn-up. The assemblies in Region 1 must be spaced substantial distances to avoid a critical condition. For this purpose, certain cells in Region 1 are provided with caps 101 (FIG. 17) to prevent inadvertent deposit of assemblies at too close a spacing. The caps 101 close the top openings of cells in Region 1 in a checkerboard or honeycomb pattern. However, instead of uncapped cells being encircled by single capped cells, an uncapped cell may be encircled by groups of several capped cells. Each cap 101 has a head 103 and a stem 105. The head 103 is of generally rectangular transverse cross section and longitudinally tapers from an opening 107 at the top to a flat frame-like surface 109. The stem 105 is of generally rectangular transverse cross section. From opposite faces 111 of the stem, fingers 113 extend inwardly at an angle. The cap 101 is placed on a cell 29, 33, 35 or 75 with the stem penetrating into the cell and the flat surface 109 spanning the pockets 69 and in engagement with the upper rims of the walls forming the cell and the upper rims of the wrapper plate 65. The cap 101 is composed of stainless steel or other suitable metal so that the fingers 113 are resilient. When the cap 101 is thrust into the opening in a cell, the tips of the fingers 113 are sprung and penetrate into the slots 55 and lock the cap 101 in the cell. The cap can only be removed by a special tool which penetrates into the pocket 69 and retracts the fingers 113. While preferred embodiments of this invention have been disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.