Storage Rack for Nuclear Fuel Assemblies

1. Field of the Invention

This invention relates to a storage rack for spent nuclear fuel assemblies.

2. Description of the Prior Art

Storage racks for nuclear fuel assemblies are described in U.S. Pat. Nos. 4,010,375, 4,177,385, 3,859,533, 4,039,842, 4,029,968, 4,004,154. The objective of each of these prior art storage racks is to provide a safe facility for confinement of spent nuclear fuel rods in water pools for extended periods until the residual radioactive emissions are reduced to a level at which the fuel assemblies can be safely removed. To prevent unwanted concentrations of neutrons, it is a common practice to position neutron poison materials within the storage racks to isolate neutrons into restricted regions of the storage rack, thereby permitting greater numbers of fuel rod assemblies to be confined safely within an established aqueous storage pool.

The storage racks typically include square or rectangular cross-section tubes which are positioned vertically in the aqueous pool, parallel to each other. The individual tubes are secured into the storage rack. Typical neutron poison materials are boron carbide, aluminum-boron, cadmium, gadolinium. A particularly useful neutron poison material, Boroflex, is a silicone rubber pad having boron carbide particles homogeneously dispersed therein.

Typically the supporting racks will contain up to about 300 individual spent nuclear fuel rod assemblies. Typical fuel rod assemblies are from about 8 to 15 feet, long and about 1 to 9 inches square, each containing from 49 to 39 fuel rods. A typical aqueous storage pool has concrete walls and may be 40 feet deep.

There is a continuing need to increase the number of spent fuel rod assemblies which can be accumulated in one location, i.e. to increase the number of spent fuel rods in the volume of an available aqueous storage pool. Similarly there is a need to produce storage racks which reliably resist mechanical vibrations which may be applied to the storage rack from whatever source, but particularly to resist seismic vibrations due to local earthquake conditions.

According to the present invention an improved storage rack for nuclear fuel assemblies not only permits an increase in the number of fuel rod assemblies which can be confined in an established volume of aqueous storage pool, but also uniquely resists mechanical vibrations. The improved storage rack includes an array of individual metal storage cells having a polygon cross-section, preferably square or rectangular, which are spaced apart by means of at least one layer of neutron poison material. The individual cells are rigidly connected to each other with one or more layers of neutron poison material interposed therebetween.

The individual storage cells are steel tubes with embossed buttons in an array. Tubes are aligned and are secured together by welding engaged buttons. The neutron-poison material covers one or both of the confronting surfaces of the tubes except in the region of the buttons.

The layer or layers of neutron posion material covers a major portion, but less than all of each surface of each tube. The layer of neutron-poison material preferably is a resilient, flexible ribbon of rubber-like material which can be compressed between abutting tubes.

FIG. 1 is a perspective view of a portion of a typical aqueous storage pool for spent nuclear fuel assemblies showing a typical storage rack.

FIG. 2 is a fragmentary perspective view of a partial assembly of the storage rack of FIG. 1.

FIG. 3 is a horizontal fragmentary cross-section view of the assembled storage rack.

As shown in FIG. 1, a water storage pool 10 is usually recessed in the ground adjacent to a nuclear power generating facility. A storage rack 11 for spent nuclear fuel assemblies is secured in the pool 10 by appropriate mounting brackets (not shown). The storage rack 11 correspondingly is from 2.4 to 4.5 meters deep to accommodate the fuel rod assemblies which are to be confined within the storage pool 10.

In a preferred embodiment of the invention as shown in FIG. 2, the individual storage cells are tubes 12 having a square cross-section including cell walls 13, 14, 15, 16. The cells are usually fabricated from steel, for example, steel sheets of about 2.3 millimeters thickness. The cell walls 13, 14, 15, 16 are about 15 centimeters wide. The cell walls are provided with an array of embossed buttons 17, each of which is pressed outwardly from the tube 12. Each button surface is about 10 to 20 millimeters diameter. Each button 17 is embossed about 0.5 to 2.0 millimeters from the cell wall. The array of embossed buttons is such that the buttons of one tube will engage the buttons of the confronting, aligned tube so that the two cells can be secured together by welding through the engaged buttons. The outside surfaces of the confronting cell walls are thus spaced-apart by the thickness of the two engaged buttons, i.e., by about 1 to 4 millimeters.

The space between abutting tubes 12 is filled with neutron-poison material 18 which may be applied as a preformed ribbon 19 as shown in FIG. 2. The individual ribbons 19 of neutron poison material have an area which exceeds a major portion but is less than all of the surface area of the tube 12. Preferably the neutron poison material is a Boroflex composition ribbon which is a silicone rubber having boron carbide homogenously dispersed therein. The ribbon 19 has edge scallops 20 to accomodate the embossed buttons 17. The ribbon 19 is less that 4 millimeters thick, preferably about 2 to 3 millimeters thick in its uncompressed state.

After a number of slab-like structures 20 of tubes 12 as shown in FIG. 2 are prepared, they are aligned and secured together by means of fusion welding the engaged buttons 17. Pads 19 of neutron-poison material are positioned between the confronting tubes 12 of each slab of tubes before the welding is carried out. The resulting structure is a multicellular storage rack which optimizes the utilization of available volume in the aqueous storage pool 10. It will be observed that each individual tube 12 as shown in FIG. 3 is essentially completely surrounded with neutron poison material whereby each individual tube 12 can accommodate nuclear fuel assemblies. It will further be observed from an inspection of FIG. 3 that the overall assembly is a rigid three-dimensional network wherein the pads 18 of neutron poison material function as a damping ingredient for mechanical vibrations such as seismic vibrations which may be transmitted through the water storage pool 10 to the storage racks.

The mechanical damping is enhanced if the neutron-poison pads 18 are compressed between confronting cell walls as the tubes are welded together through the buttons 17.

The neutron-poison material may be an adherent coating applied directly to the tubes 12 instead of a pre-formed pad 19 of material. Such coatings may be spray-on, roll-on, trowel-on and other similar adherent coatings.

While square cross-section tubes are illustrated herein, other nestable polygon cross-section tubes may be used with this invention, e.g., rectangular, triangular, hexagonal, and other polygon shapes.