Patent ID: 7889829

Claim:
A fuel assembly for a nuclear reactor including: a plurality of elongated nuclear fuel rods having an extended axial length; at least a lowermost grid supporting said fuel rods in an organized array and having unoccupied spaces defined therein to allow a flow of fluid coolant there-through and past said fuel rods when said fuel assembly is installed in the nuclear reactor; a plurality of guide thimbles extending along said fuel rods through and supporting said grid; a bottom nozzle disposed below said grid, below lower ends of said fuel rods, supporting said guide thimbles and permitting the flow of fluid coolant into said fuel assembly, said bottom nozzle comprising a substantially horizontal plate extending transverse to the axis of the fuel rods and having an upper face directed toward said lowermost grid, said upper face of said plate having defined there-through a plurality of flow through holes extending completely through said plate for the passage of the fluid coolant from a lower face of said plate to the upper face of said plate, each of said coolant flow through holes in fluid communication with said unoccupied spaces; and said lowermost grid comprising a first, spaced, parallel arrangement of elongated straps extending along a plane substantially orthogonal to the axis of the fuel assembly and a second, spaced, parallel arrangement of elongated straps extending along the plane substantially orthogonal to the axis of the fuel assembly and perpendicular to the first, spaced, parallel arrangement of elongated straps in an egg-crate lattice arrangement that defines a plurality of cells therein through which the fuel rods and guide thimbles pass, each of a first plurality of said cells, through which the fuel rods pass, having walls respectively with a cell height along the axial dimension of the fuel assembly equal to the width of one of the first and second, spaced, parallel arrangement of elongated straps and a cell width along the elongated dimension of the corresponding, elongated strap, and a wall of each of the first plurality of said cells having at least two distinct protrusions that separately extend from the wall inwardly into the cell respectively at different elevations along the height of the wall of the first plurality of cells respectively laterally offset from the center of the cell wall from which the protrusions extend with the protrusions on each of the cell walls from which the protrusions extend arranged in an asymmetric pattern about both a horizontal and a vertical plane and spaced from the nuclear fuel rods at least at the beginning of life of the fuel assembly.