Patent Number: 052326578
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear fuel storage and shipping and, more particularly, is concerned with an improved flux trap neutron absorber arrangement for a fuel storage body, such as a fuel shipping cask or a fuel storage pool of a fuel storage facility. 2. Description of the Prior Art A fuel storage facility provides for on-site storage of both new and spent fuel assemblies at nuclear power plants. The fuel storage facility includes a fuel pit or pool which is a reinforced concrete structure with a stainless steel liner, filled with borated reactor makeup water. Fuel storage containers or cans of square cross-section and standing upright in a spaced side-by-side array are provided under water in the fuel pool. The cans are designed to accommodate a large number of fuel assemblies, for example 850, at predetermined locations such that the fuel assemblies are maintained in a sub-critical array in the fuel pool. Neutron absorbers or poisons, such as boron carbide, in slab-like form are typically mounted in narrow pockets extending vertically along the sides of the cans, with the makeup water filling the remainder of the space between the cans, to assist in maintaining the fuel in a condition of subcriticality. Fast neutrons are emitted by the fuel and therefore it is desirable to be able to slow them so that they can be absorbed more effectively in the absorber material. The slabs of boron carbide and volume of borated makeup water between them serve as a flux trap neutron absorber arrangement in the storage pool between the stored fuel assemblies. The water provides a fast neutron slow-down region with the surrounding boron carbide, in the slab or plate form, providing a thermal neutron absorber. The fast neutrons enter into the water contained in the slow-down region between the boron carbide plates of thermal neutron absorber. The hydrogen atoms in the water slow the fast neutrons down between the plates so that they can be absorbed by the thermal neutron absorber of the plates. A plurality of such flux trap neutron absorber arrangements are located between the cans containing the fuel assemblies to assist in maintaining the fuel assembly array in a safe shutdown subcritical condition. Because the pool space is fixed at the nuclear power plants and the demand for more and higher enrichment fuel storage is becoming critical, there is a need for maximizing the amount of fuel that can be stored there. As a result the minimization of the storage cell structural volume in the pool is important. Dimensional changes as small as 0.1 inch are critical to the designer, in meeting the sub-criticality requirements, maximizing the storage capacity, and minimizing material requirements. Consequently, there is a pressing need for improvements in the design of the flux trap absorber arrangement for maximizing available fuel storage space. SUMMARY OF THE INVENTION The present invention provides an improved flux trap neutron absorber arrangement designed to satisfy the aforementioned needs. The arrangement of the present invention replaces some of the fast neutron moderating or slowing water used in a fuel storage body, such as a fuel storage pool or fuel shipping cask, with a metal hydride. Since the efficiency of the design of a flux trap neutron absorber arrangement depends upon the slow-down of fast neutrons by hydrogen atoms or other low mass elements in the trap region, the higher the hydrogen density, the more efficient is the design. Some metal hydrides have a higher hydrogen density than water and thus will increase the efficiency of the flux trap neutron absorber arrangement. Their use also allows storage of fuels of higher enrichment. Furthermore, the use of a metal hydride would eliminate the variation in hydrogen density in the water with water temperature. As the water temperature rises, the volumetric hydrogen density decreases which reduces the absorber efficiency. In some design cases, this effect can be important. Accordingly, the present invention is directed to a plurality of flux trap neutron absorber arrangements set forth in a nuclear fuel storage body. The fuel storage body includes a pool of fast neutron slowing fluid and a plurality of upright storage cans submerged in the fluid and disposed in a spaced side-by-side array. Each storage can is composed of a plurality of side walls connected together to receive and store a nuclear fuel assembly. The plurality of flux trap neutron absorber arrangements are disposed in the storage body between the storage cans. In one embodiment of the present invention, each flux trap neutron absorber arrangement comprises: (a) separate means extending vertically along and attached to the exterior of each of the adjacent side walls of adjacent spaced storage cans for forming respective pockets extending along the adjacent side walls and being spaced from one another; (b) an elongated flat plate of a thermal neutron absorber material mounted in each of the pockets, the plates of thermal neutron absorber material being likewise spaced from one another and defining a fast neutron slow-down region therebetween; (c) a slab of a metal hydride disposed in the fast neutron slow-down region between the plates of thermal neutron absorber material and the separate pocket forming means on the adjacent side walls; and (d) a canister containing the slab of the metal hydride being disposed in the fast neutron slow-down region, the canister being connected to at least one of the adjacent side walls of the adjacent storage cans. In another embodiment of the present invention, each flux trap neutron absorber arrangement comprises: (a) at least one elongated flat plate of a thermal neutron absorber material mounted between each adjacent pair of the spaced storage cans and adjacent one of the side walls of the storage cans, the plate of thermal neutron absorber material and the other of the side walls of the storage cans defining a fast neutron slow-down region therebetween; (b) a slab of a metal hydride disposed in the fast neutron slow-down region between the plate of thermal neutron absorber material and the other side wall of the storage cans; and (c) a canister containing the plate of thermal neutron absorber material and the slab of metal hydride, the canister being connected to at least one of the adjacent side walls of the adjacent storage cans. In one form, the means extending vertically along and attached to one of the adjacent side walls of the adjacent storage cans forms a pocket extending along the one side wall, and a second elongated flat plate of a thermal neutron absorber material is mounted in the pocket. In both embodiments of the arrangement, the thermal neutron absorber material of the plates preferably is boron carbide. Preferably, the metal hydride of the slabs is titanium hydride. Alternatively, the metal hydride can be gadolinium-titanium hydride or a rare earth hydride. These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.