Fuel assembly and thimble screw of the same

A fuel assembly comprising:

• a bottom nozzle set on a lower core plate of a nuclear reactor;
• a top nozzle with a hold down spring to urge said bottom nozzle against the lower core plate;
• a plurality of control rod guide tubes which guide control rods, having passed through said top nozzle, toward the lower core plate;
• a plurality of grids mounted on said control rod guide tubes;
• a plurality of fuel rods held by said grids to be substantially parallel to said control rod guide tubes;
• a thin tube-like dashpot formed on each of said control rod guide tubes to reduce a fall velocity of a corresponding one of the control rods;
• a thimble screw which connects each of said control rod guide tubes to said bottom nozzle;
• and
• a drain hole formed to extend through said thimble screw, wherein
• said dashpot has a large-diameter portion, at a lower portion thereof, with substantially the same diameter as that of each of said control rod guide tubes, and a diameter d of said drain hole falls within a range of 0.04 D<d<0.08 D where D is an inner diameter of the large-diameter portion.