PURPOSE: To prevent the expansion and deformation of the lower part of a channel box in a lower tie plate constituting fuel assembly of a reactor, by means of inserting a metal tube between a channel box covering the fuel rods and the lower tie plate.

CONSTITUTION: Many fuel rods 1 are holded by an upper tie plate 2 and a lower tie plate 3. Fuel rod 2 are penetrated through a spacer 4 and are inserted into a channel box 5. A metal tube 32 is set up at the lower tie plate by means of embracing the lower end of the fuel rods. A space 33 is generated between the metal tube 32 and the channel box 5, and one part of coolant leaks by becoming to LF, but, the pressure at an end part of the channel box 51 becomes to the pressure reduced by the loss of the pressure due to the leeaked water flowing the space 33 from the pressure at the upper end part of the metal tube, and the pressure at the end part of channel box is reduced.