Patent Number: 062467417
Section: claims

1. A fuel assembly for a pressurized water reactor, comprising a lower nozzle to be disposed on a lower core plate of said pressurized water reactor;  an upper nozzle having hold-down spring means for holding down said lower nozzle against said lower core plate;  a plurality of control rod guide thimbles for guiding control rods extending through said upper nozzle toward said lower core plate;  a plurality of supporting grids mounted on said control rod guide thimbles;  a number of fuel rods held by said supporting grids in parallel with said control rod guide thimbles;  each of said control rod guide thimbles shaped to form a thin tubular dashpot at a lower portion of said control rod for reducing falling speed of said control rods;  wherein said dashpot includes a large diameter section having a diameter substantially equal to that of said control rod guide thimble formed at a lower portion of said dashpot, and a small diameter section having a diameter smaller than that of said large diameter section formed at an upper portion of said dashpot, and  wherein said small diameter section has an effective length dimensioned so as to fall within a range of 0.03 L to 0.1 L, where L represents an entire length of said control rod guide thimble. 2. A fuel assembly according to claim 1, wherein the effective length of said small diameter section ranges from 0.04 L to 0.06 L. 3. A fuel assembly according to claim 1, wherein the length of said dashpot is dimensioned so as to fall within a range of 0.16 L to 0.18 L.