Storage rack for nuclear fuel assemblies

A storage rack for spent nuclear fuel rods is provided having multiple parallel tubes of polygon cross-section with their confronting surfaces having embossed buttons in engagement and having at least one pad of neutron poison material interposed between the confronting surfaces. The tube cells are secured together such that the neutron poison material effectively establishes each individual cell as a neutron isolation chamber and further functions to dampen mechanical vibrations which may be applied to the assembled storage rack.

BACKGROUND OF THE INVENTION 
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. 
STATEMENT OF THE INVENTION 
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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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.