Patent Number: 047708441
Section: summary

BACKGROUND OF THE INVENTION This invention generally relates to casks for transporting nuclear fuel rods, and is specifically concerned with a modular cell wall that provides an improved basket structure for use within such a transportation cask. Basket structures for use in conjunction with fuel rod transportation casks are known in the prior art. Generally, such baskets are formed from an "eggcrate" of stainless steel plates that are slotted and interfitted to define an array of square, elongated cells. Each of the cells holds an elongated fuel rod container that has a square cross-section. The cells of the eggcrate space the fuel rod containers from one another so as to maintain a subcritical state when the cells are completely loaded with nuclear fuel. As further assurance from subcriticality, the sides of each of the fuel rod containers are typically clad with a neutron-absorbing material, such as Boral.RTM.. The eggcrate of the basket structure is insertable into and removable from a cylindrical transportation casks that is typically formed from carbon steel which may be up to one foot thick. During the loading operation, the entire transportation cask is hoisted down into and completely submerged within a waterfilled cask-loading shaft on the premises of the nuclear reactor facility. Spent nuclear fuel rods are loaded into the containers held within the cells of the eggcrate of the basket structure. These spent fuel rods may either be held together in spent fuel assemblies or in consolidation canisters, either of which is receivable within the fuel rod container held in the cells of the eggcrate. After the loading operation has been completed, the entire transportation cask is hoisted upwardly by a crane above the surface of the water in the shaft, and allowed to drain. Thereafter, the cask is then lidded, and the fuel rods are transported to their final destination. Unfortunately, such prior art basket structures are not without shortcomings. One of the most serious of these shortcomings is the tendency of the stainless steel plates in eggcrate type designs to warp in response to thermal and mechanical stress, which may result in the mis-shaping of one or more of the containers in the basket cells. Even relatively small amounts of warping may render the individual container useless, as the inner dimensions of the container are designed to hold either a spent fuel assembly or a consolidated fuel container with an absolute minimum amount of lateral "slack" so that the fuel rod assembly or fuel rod container will not tend to rattle within the basket structure when the transportation cask is in motion. Still another shortcoming associated with eggcrate type basket designs is their relative difficulty of manufacture. The interfitting slots in the plates that form the basket must be cut with a great deal of precision if the resulting cells are to be highly uniform in size. Moreover, when two interfitting plates are welded together along their mutually interlocking slots, the resulting weld beads can contract the metal along the slots to such an extent that the square cross sections of the eggcrate cells become warped into rhombi. If one attempts to correct such weld warpage by the application of counterwelds, one sometimes straightens one row of cells at the expense of buckling another row. It has been suggested that the warpage problem might be solved by merely increasing the size of the cells. Such a solution, however, would significantly increase the size and weight of the basket structure and would not necessarily solve the problem. Moreover, the greater amount of slack space resulting from such larger cells would create other problems that would have to be dealt with. A final shortcoming of eggcrate type designs is the difficulty of removing the rods from the basket in the event the cask is exposed to a damaging amount of mechanical shock. The mechanical interdependency of the interlocking plates may cause the entire basket structure to undergo severe warpage in a case where the casks is dropped on its side with one set of corners of the eggcrate vertically aligned, which in turn would tend to misshape all of the cells into flattened rhombi. Such a flattened rhombus shape would in turn cause the walls of the cells to pinch the fuel rods contained therein, thereby impeding removal. Clearly, what is needed is an improved basket structure for use in a fuel rod transportation cask that is relatively simple and inexpensive to manufacture tosamll tolerances, but yet which will maintain such dimensions tolerances throughout a broad range of thermal and mechanical stress. Ideally, such a structure would be devoid of small crevices and recesses so that the cask is easily drainable, and have good heat transfer characteristics so that residual heat from the spent fuel rods will be readily dissipated into the ambient atmosphere. Finally, the cells in the basket structure should be easily repairable in the case of an accident. SUMMARY OF THE INVENTION Generally speaking, the invention is an improved basket structure for use within a nuclear fuel transportation cask that overcomes the shortcomings associated with the prior art. The improved basket structure comprises a plurality of cells for receiving fuel rods, wherein the sides of each of the cells are formed from a plurality of elongated side members, each of which terminates in a flange of enlarged thickness along one of its lengthwise edges. The flange includes a slot that is complementary in shape to its free edge so that cells may be formed by interfitting the free edge of one side member with the complementary slot in the flange of another. The enlarged flange of each side member serves to space its respective cell from adjacent cells in order to maintain subcriticality, and to further provide part of a mechanical means for interconnecting the cells. Each of the flanges may be integrally formed in its respective side member, and may include first and second legs that are orthogonal with respect to one another. The inside corner of the first and second legs of the flange may house the edge-receiving slot, and the outside corner is preferably beveled in order to define a recess for receiving a weld bead when flanges of different cells are abutted against one another. In the preferred embodiment, each of the side members and its respective flange is unitarily extruded from an extrudable metal, and clad with a neutron-absorbing material such as Boral.RTM. on its outside surface in order to further ensure subcriticality. The modular construction of the basket structure of the invention from a plurality of uniformly dimensioned side members results in a basket structure that is simple and inexpensive to manufacture to close tolerances but yet which is not prone to warping, and which further may be easily repaired in the event of shock damage. The cellular make-up of the basket structure makes it very easy to construct baskets having different numbers of cells without any significant amount of re-tooling. Hence the same manufacturing tooling may be used to construct basket structures small and light enough to be carried by truck, or heavy enough to be carried by train or barge. The modular basket structure is also devoid of small crevices and cavities that can retain water, which in turn allows the basket to be easily drained when hoisted out of a spent fuel pool. Finally, the mutually interconnected flange between the adjacent cells provide thermal conduction paths that facilitate the cooling of the cask.