Patent Number: 047818832
Section: summary

BACKGROUND OF THE INVENTION The present invention is related to the longterm storage of spent fuel that has been removed from a nuclear reactor, and more particularly, to a spent fuel storage cask having a continuous grid basket assembly which supports the spent fuel and which dissipates heat generated by the spent fuel. FIG. 1 illustrates a typical fuel assembly 20 for supplying nuclear fuel to a reactor. Assembly 20 includes a bottom nozzle 22 and a top nozzle 24, between which are disposed elongated fuel rods 26. Each fuel rod 26 includes a cylindrical housing made of a zirconium alloy such as commerically available "Zircalloy-4", and is filled with pellets of fissionable fuel enriched with U-235. Within the assembly of fuel rods 26, tubular guides (not shown) are disposed between nozzles 22 and 24 to accommodate movably mounted control rods (not illustrated) and measuring instruments (not illustrated). The ends of these tubular guides are attached to nozzles 22 and 24 to form a skeletal support for fuel rods 26, which are not permanently attached to nozzles 22 and 24. Grid members 28 have apertures through which fuel rods 26 and the tubular guides extend to bundle these elements together. Commerically available fuel assemblies for pressurized water reactors include between 179 and 264 fuel rods, depending upon the particular design. A typical fuel assembly is about 4.1 meters long, about 19.7 cm wide, and has a mass of about 585 kg., but it will be understood that the precise dimensions vary from one fuel assembly design to another. After a service life of about 3 years in a pressurized water reactor, the U-235 enrichment of a fuel assembly 20 is depleted. Furthermore, a variety of fission products, having various half-lives, are present in rods 26. These fission products generate intense radioactivity and heat when assemblies 20 are removed from the reactor, and accordingly the assemblies 20 are moved to a pool containing boron salts dissolved in water (hereinafter "borated water") for short-term storage. Such a pool is designated by reference number 30 in FIG. 2. Pool 30 is typically 12.2 meters deep. A number of spent fuel racks 32 positioned at the bottom of pool 30 are provided with storage slots 34 to vertically accommodate fuel assemblies 20. A cask pad 36 is located at the bottom of pool 30. During the period when fuel assemblies 20 are stored in pool 30, the composition of the spent fuel in rods 26 changes. Isotopes with short half-lives decay, and consequently the proportion of fission products having relatively long half-lives increases. Accordingly, the level of radioactivity and heat generated by a fuel assembly 20 decreases relatively rapidly for a period and eventually reaches a state wherein the heat and radioactivity decrease very slowly. Even at this reduced level, however, rods 26 must be reliably isolated from the environment for the indefinite future. Dry storage casks provide one form of long-term storage for the spent fuel. After the heat generated by each fuel assembly 20 falls to a predetermined amount--such as 0.5 to 1.0 kilowatt per assembly, after perhaps 10 years of storage in pool 30--an opened cask is lowered to pad 36. By remote control the spent fuel is tranferred to the cask, which is then sealed and drained of borated water. The cask can then be removed from pool 30 and transported to an above-ground storage area for long-term storage. The requirements which must be imposed on such a cask are rather severe. The cask must be immune from chemical attack during long-term storage. Furthermore, it must be sufficiently rugged mechanically to avoid even tiny ruptures or fractures during long-term storage and during transportation, when the cask might be subjected to rough treatment or accidents such as drops. Moreover, the cask must be able to transmit heat generated by the spent fuel to the environment while nevertheless shielding the environment from radiation generated by the spent fuel. The temperature of the rods 26 must be kept below a maximum temperature, such as 375.degree. C., to prevent deterioration of the zirconium alloy housing. Provisions must also be made to ensure that a chain reaction cannot be sustained within the cask; that is, that the effective criticality factor K.sub.eff remains less than one so that a self-sustaining reaction does not occur. These requirements impose stringent demands upon the cask, which must fulfill its storage function in an utterly reliable manner. SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a mechanically rugged storage cask which prevents fission products and radiation from escaping into the environment while dissipating heat generated by spent fuel. Another object of the present invention is to provide a storage cask which is sufficiently versatile to accomodate spent fuel in various different forms, including fuel assemblies having different dimensions and fuel in consolidated form, and to store different forms of spent fuel simultaneously. Another object of the present invention is to provide a storage cask having a grid basket assembly for supporting spent fuel and for conducting heat generated thereby to the walls of the cask. Another object of the present invention is to provide cell assemblies for use in cooperation with the grid basket assembly when spent fuel assemblies are stored. Another object of the present invention is to provide a storage cask wherein heat is transmitted from the grid basket assembly to channel sections affixed to the inner walls of the cask via thin gaps between the channel sections and the edges of the elements forming the grid basket assembly, the channel sections additionally permitting movement of the grid basket assembly with respect to the walls of the cask during temperature variations. These and other objects can be attained by providing a container having a grid basket assembly which includes a plurality of metal plates joined together to provide a matrix of storage slots for the spent fuel, and elongated channel sections which are affixed to the inside walls of the container and which accommodate the edges of the plates in order to transmit heat from the grid basket assembly to the walls of the container without preventing relative movement between the grid basket assembly and the walls. In accordance with one aspect of the invention, the metal plates of the grid basket assembly provide elongated storage slots having generally rectangular cross sections, and consolidation canisters or cells having walls which include neutron moderating material are mounted in the storage slots. Such cells accommodate spent fuel assemblies and can be individually configured in accordance with the dimensions of the fuel assemblies which they are to receive. Each cell can include four panel portions which are affixed to the walls of the storage slots by tabs. Alternatively, the cells may include shell elements which have wall portions that are spaced apart from the walls of the storage slots and corner poritons that project outward to contact the walls of the storage slots. In other embodiments, the cells may include shell elements having walls which provide substantially rectangular cross sections, with spacer elements being provided in the form of dimples in the shells or spacer members affixed to the shells. Regardless of the cell embodiment, at least some of the walls of the cells include sheets of boron carbide or other "neutron poison" supported by wrapper elements, which may include apertures to permit visual confirmation that neutron poison is present and to facilitate drainage of borated water when spent fuel is being loaded into the cask. A consolidation canister containing fuel rods, instead of a cell for receiving a fuel assembly, is deposited in a storage slot if the slot is to be used for storage of fuel in consolidated form. In accordance with another aspect of the invention, the grid basket assembly is supported above the floor of the container in order to facilitate drainage of borated water. This may be accomplished by providing cut-outs at the lower ends of the plates forming the grid basket assembly, by terminating the channels of the channel sections above the floor of the container in order to support the plates above the floor, or by providing support elements disposed between the floor of the container and the lower ends of the plates of the grid basket assembly. These support elements can have flanges which are positioned to permit consolidation canisters to rest on the cask floor but to support the lower ends of the cells. Alternatively, the cells can be mechanically attached to the grid assembly at one region so that the cells can move with respect to the grid basket assembly at other places as temperature changes, or hooks can be provided at the upper ends of the cells in order to hang the cells from the plates of the grid basket assembly. The grid basket assembly is preferably fabricated by making slots from the bottom to the middle of a first set of plates, making corresponding slots from the top to the middle of a second set of plates, and interdigitating the plates to provide a matrix of elongated storage slots having rectangular cross sections.