Patent Number: 043426204
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear fuel storage and in particular to an apparatus for storing fuel assemblies in a pool. U.S. Pat. No. 4,177,385 issued on Dec. 4, 1979 to Frank Bevilacqua for "Nuclear Fuel Storage" discloses an apparatus and method for storage of fuel in a stainless steel, egg-crate frame within a storage pool. Fuel is initially stored in a checkerboard pattern or in each opening if the fuel is of low enrichment. Additional fuel (or fuel of higher enrichment) is later stored by adding box inserts within each opening in the frame, thereby forming flux-traps between the openings. Still higher enrichment fuel is later stored by adding poison material around the boxes. The method and apparatus described in the Bevilacqua patent can be significantly improved by simplifying the structure of the box inserts and the manner in which poison is added thereto. SUMMARY OF THE INVENTION It is an object of the invention to provide such a simplified box insert which can be shipped to the reactor plant in a form that is convenient and easy to handle, and which may thereafter be fabricated quickly and inserted into the frame. Another advantage of the invention is that the box inserts may be formed by very thin plates of stainless steel without sacrificing structural rigidity. In the preferred embodiment the poison material is provided directly on the box inserts. This permits a thinner overall box dimension so that a larger water gap between poison boxes can be maintained in the frame. The increased water gap enhances the flux-trap effect and accordingly provides a less reactive, safer storage facility. The inventive box insert comprises a plurality of vertically extending plates arranged as a open-ended polygonal container having a smaller cross-sectional area than the opening in the frame. Each plate has a flat portion forming a respective side of the container and an integral tab portion rigidly projecting outwardly from the longitudinal edge of the container. The adjacent tabs of each plate are connected, thereby giving the container rigidity and providing the container with a plurality of outwardly projecting ribs. The box may then be slidingly inserted into the frame so that the ribs fit into the corners of the frame defining the opening. Thereafter a fuel assembly may be lowered into the box portion of the container, which is rigidly maintained in spaced relation from the frame by the ribbed portions of the box.