Patent Number: 047708441
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIG. 1, wherein like numerals designate like components of the invention throughout all of the several figures, the transportation cask 1 of the invention includes a cylindrical vessel 2 for containing an improved basket structure 3. The basket structure 3 includes both a cell assembly 4 which may have twenty-four interconnected cells 5a-x for holding either fuel assemblies 6 or consolidated fuel cannisters (not shown), as well as a plurality of circular former plates 7a-j that circumscribe the cell assembly 4. It should be noted that the modular construction afforded by the use of individual cell structures allows the basket structure to have virtually any number of cells. The cylindrical vessel 2 of the transportation cask 1 further includes a closure lid 8 which may be detachably mounted around the upper edge of the vessel 2 in a gas-tight seal. The floor (not shown) of the cylindrical vessel 2 is provided with a plurality of symmetrically arranged drain holes which may be selectively opened for draining water out of the interior of the vessel 2. The side walls of the cylindrical vessel 2 of the transportation cask 1 may be made from carbon steel that is approximately twelve inches thick. In the alternative, these walls may be a composite of stainless steel, lead and a neutron-absorbing plastic of a type known in the art. On the balance, carbon steel is the preferred material due to its relatively low-cost, high strength, and favorable heat conduction qualities. With reference now to FIGS. 1 and 2, the former plates 7a-j each have a circular outer edge 9 whose diameter D2 is approximately the same as the inner diameter of the vessel 2, and a stepped inner edge 10 that is substantially complementary in shape to the perimeter of the cell assembly 4. In the preferred embodiment, both the cell assembly 4 and the former plates 7a-j are formed from aluminum, and interconnected at their respective corners by means of welds 11a-x which are applied on both the upper and lower surfaces of each of the plates 7a-j. To facilitate assembly, each of the former plates 7a-j is formed from two semicircular pieces that can be brought together much like the heat conductors 15a,b illustrated in FIG. 1 (and discussed in detail hereinafter), and interconnected by means of the welds 12a, b shown in FIG. 2. The minimum radial distance 14 between the outer corners of the cell assembly 4 and the outer diameter of the former plates 7a-j is at least one inch. Such dimensioning allows the former plates 7a-j to provide adequate support between the corners of the cell assembly 4 and the inner diameter of the vessel 2 without impeding the flow of heat from the spent fuel rods disposed within the cell assembly 4 to the outer walls of the vessel 2. If the cell assembly 4 is fabricated from aluminum, ten former plates 7a-j approximately two inches thick are uniformly spaced throughout the approximately 160-inch length of the assembly 4 as is illustrated in FIG. 1. If, however, the cell assembly 4 is formed from a stronger but less heat-conductive metal such as type 304 stainless steel, the former plates 7a-j should also be formed from a type 304 stainless steel in order to avoid weld joints between dissimilar metals. Stainless steel former plates 7a-j need only be approximately one inch thick to provide sufficient mechanical support strength between the corners of the cell assembly 4 and the inner diameter of the vessel 1. However, because of the lower thermal conductivity of stainless steel as opposed to aluminum, nine pairs of semi-circular aluminum heat conductors 15a,b (of which only one pair is shown) should be inserted between all the former plates 7a-j so that the internal temperature of the vessel 2 does not exceed 380.degree. C. Like the former plates 7a-j, the aluminum heat conductors 15a,b should have a circular outer edge 15.1, and a stepped inner edge 15.2 that is complementary in shape to the perimeter of the cell assembly 4. However, unlike the former plates 7a-j, the outer diameters D1 of these aluminum heat conductors 15a,b should be somewhat smaller than the inner diameter D2 of the cylindrical vessel 2 in order to compensate for the greater amount of thermal expansion that aluminum undergoes relative to stainless steel. Specifically, the outer diameters D1 of each of the nine pairs of heat conductors 15a,b should be dimensioned so that when these conductors are inserted between the former plates 7a-j and loaded into a vessel 2 along with a load of spent fuel rods, the resulting greater thermal expansion of the aluminum will cause it to differentially expand enough to come into a good thermal engagement between the inner walls of the vessel 2 and the outer walls of the cell assembly 4 without plastically deforming either the heat conductors 15a, b or the cell assembly 4. The provision of nine pairs of such aluminum heat conductors 15a, b between the parallel former plates 7a-j not only compensates for the relatively lower thermal conductivity of the stainless steel used in plates 7a-j in this embodiment, but also lends additional lateral support between the outer walls of the cell assembly 4 and the inner walls of the cylindrical vessel 2 in the event that the vessel 2 is dropped or otherwise exposed to an inadvertent mechanical shock. While it would be possible merely to thicken the former plates 7a-j made from stainless steel until their heat conductivity is the same as that of aluminum plates, the relatively large amounts of stainless steel required for this purpose would render the transportation cask 1 quite heavy. The improved basket structure 3 of the instant invention may be used in conjunction with the inventive, shock-absorbing and heat conducting aluminum former plates described and claimed in co-pending U.S. patent entitled "Improved Shock-Absorbing and Heat-Conductive Basket for Use in a Fuel Rod Transportation Cask" assigned to the Westinghouse Electric Corporation, the entire specification of which is incorporated hereby by reference. With reference now to FIGS. 3 and 4, each of the individual cells 5a-x that form the cell assembly 4 includes an elongated interior 16 having a square cross section that is approximately the same as the square cross section of a spent fuel assembly 6 (or consolidated spent fuel canister), as well as an exterior 18 that is preferably clad with sheets 20 of Boral.RTM. (or some other neutron-absorbing material) on all sides of the individual cell that face an adjacent cell. In the case of cell 5b shown enlarged in FIG. 3, only the lower and left-hand sides need to be clad with Boral.RTM., since these are the only sides of the cell 5b that are adjacent to other cells. However, in the case of a cell such as 5d, all four sides need to be clad as is indicated in FIG. 2. In all cases, Boral.RTM. sheet material is laminated over the outside surface of the walls of the cells 5a-x. As is best seen with respect to FIGS. 3A, 3B and 4, each of the cells 5a-x may be formed from either four side members 21 or two side members orthogonally welded together depending upon whether the side member 21 forms one or two sides of the cells 5a-x. In the FIG. 3A Embodiment, each of these side members 21 is in turn formed from a rectangular plate 22 of aluminum or other extrudable metal having a free edge 24 along one of its lengths, and an enlarged flange 26 along its other length. Each of the enlarged flanges 26 is formed from a pair of short, orthogonally disposed legs 28a, 28b. Disposed between the inner surfaces 31a, 31b of the legs 28a, 28b is a short recess or slot 33. Slot 33 is complementary in shape to the free edge 24 of the rectangular plate 22 that forms each of the side members 21 so that the free edge 24 of one side member 21 may be inserted into the slot 33 of another side member 22 in the configuration illustrated in FIGS. 3 and 4. Four side members 21 may be permanently interconnected to form a single cell 5b by means of welds 34a-d which interconnect the enlarged flange 26 of one side member 21 to the outside of the rectangular plate 22 of another side member 21. When each side member 21 forms two sides of the cell the structure of the cell 5b in the FIG. 3B embodiment is exactly the same as the FIG. 3A embodiment, the only difference being that the upper right and lower left hand corners thereof are formed as shown. The outer surfaces 35a, 35b of the legs 28a, 28b that form each of the enlarged flanges 26 are likewise orthogonally disposed with respect to one another, as is best seen in FIG. 4. These two outer surfaces 35a, 35b converge into a beveled portion 37 as indicated. The beveled portion 37 of each of the enlarged flanges 26 provide room for weld beads 45, 48, etc. that interconnect the flanges 26 disposed on the corners of each of the cells 5a to connect the cells 5a-x into the cell assembly 4 illustrated in FIG. 1. The precise manner in which this may be done is best seen with respect to FIG. 5. Specifically, flange corners 43 and 44 of cells 5c and 5d are interconnected by means of weld bead 45 that extends the entire 160-inch length of these cells. Next, flange corner 46 of cell 5d is welded to flange 47 of cell 5e by means of weld 48, thereby resulting in three, interconnected cells 5c, 5 d and 5e. The flange corner 49 of cell 5f is then connected to flange corner 43 of cell 5c by means of weld 50. Next, cell 5g is placed in the position shown. Flange 51 of cell 5g is not welded onto adjacent flange corner 44 of cell 5d and corner 49 of cell 5f for two reasons. First, the provision of such a weld in this location is not necessary for either the structural or the thermal integrity of the basket structure 3 since all the other flange corners 52, 56 and 58 of cell 5g can be connected to adjacent flange corners by means of other welds. Secondly, it is desirable to avoid the application of unnecessary welds on the basket structure 3 in order to expedite fabrication, and to minimize the chances of any weld-induced warpage. In the next step of assembling the cell assembly 4, cell 5h is placed in the position shown. For the same reasons given with respect to flange corner 51 of cell 5g, flange corner 54 of cell 5h is not welded to adjacent flange corners 52 and 47 of cells 5g and 5e. However, flange corner 55 of cell 5h can be welded to flange corner 56 of cell 5g by means of weld 57. Moreover, flange corner 58 of cell 5g can further be welded onto flange corner 59 of cell 5f by means of weld 60. Similarly, cells 5i, 5j and 5k are interconnected to the cell assembly 4 by means of welds 61, 62 and other welds (not shown) which follow the same pattern. The end result is that each of the cells 5a-x is welded at at least three of its corners to either adjacent cells, or to the stepped interiors 10 of the previously discussed former plates 7a-j. As is best seen in FIG. 3, the welding together of four of the enlarged flanges 26 of adjacent cells 5a-5x (such as cells 5c-5k) creates spacing blocks 65 which securely interconnect the cells 5a-x into a single cell assembly 4, and further serve the purpose of spacing the interiors 16 of each of the cells 5a-x a distance D3 so that subcriticality is maintained between the fuel rods loaded into each of the cells 5a-x. The legs 28a, 28b of each of the flanges 26 of the side members 21 are thick enough so that the flange 26 will not warp when welded to an adjacent flange, but yet which are short enough to allow for cladding Boral.RTM. sheet 20 of sufficient width W1 to cover the side member 21 so neutrons radiating out of the fuel rods disposed in the cells 5a-x will generally have to penetrate two layers of Boral.RTM. before penetrating into the rods held in an adjacent cell. Such a cladding configuration provides further assurance that the fuel rods loaded into the improved basket structure 3 will remain subcritical. Finally, it should be noted that in the event that the cask 1 receives a severe mechanical shock on its side, most of the shock would tend to be absorbed by the cells 5a-x closest to the point of impact. The generally cellular structure of the cell assembly 4 would help to minimize the amount of warpage and buckling experienced by the cells 5a-x farthest away from the point of impact. The end result is that in all but the most severe accidents, the shape of some if not most of the cells 5a-x would be preserved to the extent that many of the rods disposed in the basket structure 3 would not be pinched or otherwise bound within their respective cells, hence allowing for easy rod recovery. While the term "side member" has been used herein to refer to member that forms only one side of a cell, it should be noted that angular side members may be formed which create two sides of a four-sided cell, and that the term "side member" is to be read in such a liberal sense.