Patent Number: 052710548
Section: summary

BACKGROUND OF THE INVENTION The invention relates to the use of stress relieving slots in the forming of perimeter strip grid corner-pieces of increased flatness. The grids are elements of nuclear fuel assemblies and particularly are useful as nuclear fuel assembly support grids designed to avoid hang-up of grid corners during insertion into the core of a nuclear reactor vessel. In the past, nuclear fuel grids were constructed with non-formed beveled corners. When loading or unloading nuclear reactors with individual fuel assemblies, assemblies diagonally adjacent to each other could become caught at the outer corner of the grid. This type of hooking has in the past lead to destruction of the perimeter strips of grids, so that the assemblies could not be re-inserted into the core. This is especially true for fuel assemblies which have been bowed during operation. To overcome this problem, it has been proposed to use formed corners. See U.S. Pat. No. 4,705,663 to J. Steven. The corner would be formed by pushing the strip material inward toward the corner fuel rod at the top and bottom strip edges, thus causing the corner to curve inward forming a beveled corner and providing varying radii in the transverse direction with its outer longitudinal portions having a greater radius than its inner portion. Since the corner has a larger bevel the problem of grid hooking less likely. However, these corners have not been used in grids with small cut-out features in the perimeter strips because flexing deformation occurred which destroyed the flatness of the flat side sections on either side of the transverse corner bend line. These deformations were caused by corner forming. This occurrence was denoted "oil canning" and was caused by attempts to design "camming-corner" perimeter strips. SUMMARY OF THE INVENTION To eliminate the problem of flexed deformation or oil canning, additional cut-outs of different sizes, shapes and types were made in the perimeter strip. A slot located next to the support features, i.e., the arches and the springs, if necessary, was determined to provide the best flatness characteristics. It was positioned to eliminate most of the corner forming stresses, thus eliminating the flexed deformation oil canning of the strip. The size of the slot was designed to allow the grid to maintain its overall mechanical strength while only slightly changing the grid's corner cell arch stiffness. Without the addition of these slots the formed beveled corner could not be produced on commercial Inconel grids for nuclear fuel assemblies with small cut-outs for arches and pilot holes. The location and type of slot is not obvious. In fact, two metal forming experts consulted, who together have over 80 years of metal forming experience, did not believe the described slot would solve the problem. However, the addition of the slots eliminated the instability and flexed deformation in the strip and prevented oil canning to provide an improved perimeter strip grid corner-piece of increased flatness on either side of the transverse bend line.