Patent Number: 052992454
Section: summary

BACKGROUND OF THE INVENTION Field of the Invention The present invention is generally related to nuclear fuel assembly spacer grids and more particularly to mixing vanes in the spacer grids. General Background Fuel assemblies for nuclear reactors are formed from a number of fuel rods held in position by spacer grids located along the length of the fuel rods. The spacer grids, interleaved or crisscrossing metal strips on edge, provide an individual cell for each fuel rod. Some spacer grids are provided with a series of projections, grid mixing vanes, that serve to cause a predetermined turbulence and mixing of coolant as it flows through the spacer grids and around the fuel rods. During grid and fuel assembly fabrication, some of the mixing vanes can be accidentally bent. This can have at least two adverse effects. One is that the bent vane then has a different effect on coolant flow and may cause a greater pressure drop in coolant flow. This can degrade the critical heat flux. Another effect is that a mixing vane bent closer to a fuel rod creates the potential for fuel rod fretting and mixing vane breakage. Known grid assemblies are not directed to such problems. SUMMARY OF THE INVENTION The present invention addresses the above problems in a straightforward manner. What is provided is a grid assembly having grid mixing vanes that are internal to the overall envelope of the grid assembly. Multiple metal strips are interleaved together on edge to define a plurality of individual cells each sized to receive a single nuclear fuel rod. The metal strips are each provided with a series of projections that extend from one edge. The projections are bent relative to the metal strips in an alternating side to side fashion such that four of the projections, which form mixing vanes to direct coolant flow toward the interior of the cells, preferably have a maximum bend angle of 90 degrees relative to the metal strips.