A fuel assembly includes a thimble screw connects each control rod guide tube to a bottom nozzle. A drain hole is formed extending through the thimble screw. A dashpot has a large-diameter portion having the same diameter as that of each control rod guide tube. A fuel assembly comprises a bottom nozzle (4) set on a lower core plate of a nuclear reactor. A top nozzle (3) has a hold down spring to urge the bottom nozzle against the lower core plate. Control rod guide tubes for guiding control rods have passed through the top nozzle toward the lower core plate. Grids (5-7) are mounted on the control rod guide tubes. Fuel rods (8) are held by the grids to be parallel to the control rod guide tubes. A thin tube-like dashpot is formed on each control rod guide tubes to reduce a fall velocity of the control rods. A thimble screw connects each control rod guide tube to the bottom nozzle. A drain hole is formed extending through the thimble screw. It has a large-diameter portion at its lower portion with the same diameter as that of each control rod guide tubes. The diameter of the drain hole is (0.04-0.08)D, where D is an inner diameter of the large-diameter portion. An independent claim is included for a thimble screw.
