Patent Number: 043893694
Section: summary

BACKGROUND OF THE INVENTION The invention described herein relates to nuclear reactor fuel assemblies and more particularly to a fuel assembly grid designed to improve reactor performance and to be manufactured at a cost less than conventional grids. Commercial nuclear reactors used for generating electric power include a core composed of a multitude of fuel assemblies which generate heat used for electric power generation purposes. Each fuel assembly includes an array of fuel rods and control rod guide tubes held in spaced relationship with each other by grids of egg-crate configuration spaced along the fuel assembly length. The fuel rods may be approximately 0.5 inch in diameter and about 14 feet long thus requiring a number of supporting grids along their length. Each grid includes interwoven Inconel or Zircaloy straps which form multiple cells, each cell having springs on two adjacent walls and a pair of projections or dimples on each of the other two walls forming a cell. The springs laterally impress resistive forces on each fuel rod in the assembly. Although this fuel assembly design performs exceptionally well in a nuclear reactor, one disadvantage inherent in the design is that the the inwardly projecting springs and dimples occasionally mar or score the surface of fuel rods during the time they are being pulled into the fuel assembly grids. In carrying out this fuel rod loading operation, the grids are held immovably in position while a longitudinal steel rod attached to the end of a fuel rod pulls it axially through the aligned openings or cells in the grids. As the rod engages the springs and dimples in the grid cells, their edges engage the exposed relatively soft surface of the moving fuel rod and, in some cases, score its surface sufficiently deep as to cause the rod to fall outside established fuel rod surface specifications. Also, the grid strap material from which the springs and dimples are formed has a high neutron capture cross section, particularly when made of Inconel. Although annealed Zircaloy is not as deleterious, to some extent it adversely affects reactor performance and efficiency. The best balance between material stiffness and low neutron capture cross section should be reached for efficiency purposes. SUMMARY OF THE INVENTION Briefly stated, in accordance with the teachings of this invention, the above disadvantages are overcome by providing an improved design of fuel assembly grid wherein the grid body comprises two separate sets of interwoven straps, each formed to egg-crate configuration and positioned vertically with respect to each other. One set of interwoven straps is placed on top the other and both sets are held against longitudinal or radial movement by a peripheral strap to which the ends of the straps are welded. The square openings thus formed in the juxtaposed sets of straps are aligned with each other to receive fuel rods which extend axially therethrough. Each cell which comprises the aligned square openings on the strap sets, contains a pair of spaced dimples on each of two adjacent straps facing each cell and are arranged to provide lateral support to a fuel rod. The force applied laterally to a fuel rod in each cell is provided by a spring arranged to extend diagonally across the grid with a portion in each cell designed to engage a side of a fuel rod opposite from the dimples. It will occur to those skilled in the art that the height of the grids can be reduced by virtue of the elimination of spring members heretofore found in the middle of each grid strap.