The present invention relates to long term or semipermanent storage of spent nuclear fuel assemblies, and more particularly to a storage rack for spent fuel assemblies and a method of storage of such fuel assemblies to maximize the capacity of the rack while preventing criticality.
Nuclear reactors consist of an array of fuel rods containing the nuclear fuel. The fuel rods are metal tubes, typically from 8 to 15 feet in length and about 1/2 inch in diameter, and are supported in groups in fuel assemblies which may comprise a considerable number of rods. The large reactors utilized for power generation contain a large number of these fuel assemblies arranged in a suitable configuration.
After an extended period of operation, the irradiated or spent fuel assemblies must be removed from the reactor and replaced. The spent fuel rods contain residual amounts of the original fuel material, and varying amounts of numerous fission products resulting from fission of the nuclei of the original fuel and other nuclear reactions, as well as from radioactive decay of initially-formed fission products. Certain of these materials are themselves fissionable. Many of the fission products are highly radioactive, at least initially, and thus produce considerable heat, while the entire fuel assembly is dangerously radioactive. The fuel rods can be reprocessed by chemically separating the fissionable material for reuse as fuel and recovering various other fission products, such as certain rare earth elements, for example, which have substantial commercial value.
Suitable facilities must be provided for the storage of these highly radioactive fuel assemblies after removal from the reactor until they can be reprocessed or otherwise disposed of. Such storage presents serious problems since the fuel assemblies are initially highly radioactive and generate a great deal of heat. They must, therefore, be kept submerged in water which serves as a coolant to prevent overheating as well as a radiation shield and moderator for the fast neutrons which are still being emitted. It is also necessary to be sure that the assemblies are stored in a manner that will prevent criticality of the collection of fuel assemblies while keeping the space required to a minimum. After some period of time, the heat generated and the radioactivity of the fuel assemblies decline, since many of the fission products have relatively short halflives, and the nature of the storage problem changes as both the heat to be dissipated and the radiation hazard decrease.
In our prior U.S. Pat. No. 4,010,375, there is disclosed a storage rack for spent nuclear fuel assemblies which is primarily intended for temporary storage of fuel assemblies in a water-filled pit. The rack consists of a checkerboard array of storage cells with the spent fuel assemblies placed in alternate cells. The intervening cells are filled with water which functions as a moderator and as a coolant, and also include a poison or neutron-absorbing material. This arrangement prevents criticality of the collection of fuel assemblies while maximizing the capacity of the rack for relatively short term storage. We have also proposed, in our copending application Ser. No. 851,038, filed Nov. 14, 1977, to use a similar rack structure for very long term or permanent storage, and to maximize the capacity by completely filling all the cells in the array after sufficient irradiation of the fuel and subsequent radioactive decay has occurred to make this safely possible. Concrete shielding may be provided for permanent storage. There is also a need, however, for relatively long term or semipermanent storage of spent nuclear fuel assemblies in a manner which will permit the storage of a maximum number of such assemblies in a given space, with complete safety, for a relatively long period until they can be disposed of by reprocessing, placing them in permanent storage, or otherwise.
In the conventional design of spent fuel storage facilities, criticality is prevented by means of the spacing, or pitch, between adjacent fuel assemblies in the storage rack. There is a possibility that it may at some time be necessary to completely unload the reactor to make repairs or inspections inside the reactor pressure vessel, and it is, therefore, assumed that it may be necessary to place nearly fresh or unirradiated fuel assemblies in the storage rack during such repairs or inspection. In conventional designs, therefore, the pitch between adjacent fuel storage locations has been determined on the basis of the reactivity of fresh or unirradiated fuel assemblies. This, of course, requires a much larger pitch than is needed to prevent criticality with spent fuel assemblies after discharge from the reactor, and results in reduced storage capacity of a given space. Furthermore, once the pitch has been determined in a conventional design, it cannot be changed without rebuilding the storage rack. The arrangement and dimensions of such a rack are such that additional spent fuel assemblies cannot be placed between the initial storage positions both because the usual designs do not provide for accommodating such additional fuel assemblies and because the dimensions are generally too small.