Patent Number: 
Section: claims

1. 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 less than d less than 0.08 D where D is an inner diameter of the large-diameter portion. 2. A thimble screw providing a drain hole formed to extend through the thimble screw for connecting a control rod guide tube of a fuel assembly to a bottom nozzle, the 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 the bottom nozzle against the lower core plate, a plurality of control rod guide tubes which guide control rods, having passed through the top nozzle, toward the lower core plate, a plurality of grids mounted on the control rod guide tubes, a plurality of fuel rods held by the grids to be substantially parallel to the control rod guide tubes, and a thin tube-like dashpot formed on each of the control rod guide tubes to reduce a fall velocity of a corresponding one of the control rods, wherein an inner diameter D, with substantially the same diameter as that of each of the control rod guide tubes, of a large-diameter portion at a lower portion of the dashpot, and a diameter d of the drain hole satisfy the following equation 0.04 D less than d less than 0.08 D.