Patent ID: 7822165

Claim:
A fuel assembly for a pressurized water 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 adapted to allow flow of fluid coolant therethrough and past said fuel rods when said fuel assembly is installed in the nuclear reactor and a plurality of guide thimbles extending along said fuel rods through and supporting said grid, a debris filter bottom nozzle disposed below said grid, below lower ends of said fuel rods, supporting said guide thimbles and adapted to allow flow of fluid coolant into said fuel assembly, said debris filter bottom nozzle comprising a substantially horizontal plate extending substantially 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 therethrough at least two different hole designs, the first hole design being a plurality of holes receiving lower ends of said guide thimbles where they are supported by said plate and the second hole design being a plurality of flow through holes extending completely through said plate for the passage of coolant fluid from a lower face of said plate to the upper face of said plate, each of said coolant flow through holes extending substantially in the axial direction of said fuel rods, in fluid communication with said unoccupied spaces, and in the extended direction at least some of said coolant flow through holes having a profile substantially of a venturi with flaring at both ends, wherein the flaring at the lower face of said plate comprises a series of a plurality of concentric countersinks of different included angles and depths into the coolant flow through hole.