Patent Number: 047117583
Section: summary

BACKGROUND OF THE INVENTION The present invention is related to the long-term storage of spent fuel that has been removed from a nuclear reactor, and more particularly, to a spent fuel storage cask having a basket which supports the spent fuel and which dissipates heat generated by the spent fuel. The basket includes a plurality of grid assemblies which provide storage slots for the spent fuel and which conduct heat to the walls of the cask. 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 commercially 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. Commercially 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 transferred 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 having a basket with a plurality of grid assemblies for supporting spent fuel, either in the form of fuel assemblies or consolidated fuel, or both, and for conducting heat generated thereby to the walls of the cask. Another object of the present invention is to provide a storage cask having a basket with disk-shaped grid assemblies which are spaced apart from one another in a column and which expand, after the basket is inserted into a container to form the cask, so that they come into contact with corresponding rings on the interior walls of the container in order to transmit heat from the basket to the rings, the diameters of the grid assemblies in the column and their corresponding rings decreasing slightly from the top of the column to the bottom in order to facilitate insertion of the basket into the container. Another object of the present invention is to provide a basket having legs which support grid assemblies at spaced-apart positions, the grid assemblies being movably mounted on the legs by top and bottom rings which are affixed to the legs and which are spaced further apart than the thickness of the grid assemblies. Another object of the present invention is to provide heat-transmitting wedges for positioning cells within the basket while permitting differential expansion of the cells with respect to the basket, the cells in turn enclosing spent fuel assemblies. These and other objects can be attained by providing a container having a cavity defined by substantially cylindrical walls. A basket disposed in the cavity has a plurality of substantially disk-shaped grid assemblies which are mounted at different vertical positions above the container floor. Each grid assembly includes plates which are joined together to provide a matrix of apertures and metal elements which are affixed to the plates to provide a substantially circular periphery. The apertures provided by the matrixes of the grid assemblies are aligned to provide storage slots for accepting spent fuel. When the cask elements are fabricated at normal shop temperatures, the diameters of the grid assemblies are slightly less than the diameter of the inner cask wall, so that the basket can be inserted into the container. When spent fuel is loaded and stored, however, the temperature within the cask rises and the grid assemblies expand with respect to the container so that the peripheries of the grid assemblies come into heat-conducting contact with the container walls. In accordance with one aspect of the invention, the container walls include vertically spaced rings which project slightly into the cavity. Each ring corresponds to a grid assembly and is positioned adjacent the periphery of the corresponding grid assembly. At the time of fabrication the diameter of a ring is slightly (e.g., about 0.3 cm) greater than the diameter of the corresponding grid assembly, and in order to alleviate the risk that the basket might jam while it is being inserted into the container, the diameter of each grid assembly and the diameter and of its corresponding ring are slightly less than the diameters of the grid assembly and ring above it. The changing diameters not only afford a greater latitude for error during the initial stages of insertion, they also provide visual guides for correcting the alignment as the insertion process progresses. In accordance with another aspect of the invention, the basket includes legs to which the grid assemblies are mounted at spaced-apart positions. Four of the metal elements of each grid assembly are provided with holes through which the legs extend. The grid assemblies are confined between top and bottom rings affixed to the legs on either side of the holes. The distance between the rings is slightly greater than the thickness of the grid assemblies in order to permit differential expansion of the elements as the temperature within the cask rises. In accordance with another aspect of the invention, open-ended cells for enclosing fuel assemblies can be inserted into the storage slots of the basket. Each cell has four walls which have dimensions corresponding to those of the fuel assembly to be enclosed and which support "neutron poison." The cells are positioned in the storage slots by heat-conducting wedges which are welded to the plates of the grid assemblies and which are spaced apart slightly from two of the cell walls. This slight spacing allows differential expansion of the cells with respect to the basket but permits transfer of heat to the basket, particularly if the cask is flooded with helium. In accordance with yet another aspect of the invention, the rings on the container wall are wider than the thicknesses of the corresponding grid assemblies, and moreover beveled surfaces are provided on both the rings and the peripheries of the grid assemblies in order to facilitate insertion and/or removal of the basket.