PURPOSE: To prevent the leakage of coolant in a fuel assembly of a reactor using a liquid metal coolant, by means of setting up a O-ring made of a material with a larger thermal expansion coefficient than that of said supporting structure of entrance nozzle at the outer circumferent end part of the entrance nozzle.

CONSTITUTION: A nuclear fuel assembly 20 has a plurality of fuel pins 20a in its inner part, and has an entrance nozzle 21 at the underpart. A supporting structure 1 supporting a fuel assembly 20 is composed of an upper supporting plate 2, a lower supporting plate 3, and a supporting tube 4, and holes are bored at the wall surface of the supporting tube 4, thereby coolant is entered via entrance nozzles. A groove 25 is set up at the end part of the entrance nozzle, and a metal made of an O-ring 26 composed of a material with a larger thermal expansion coefficient than that of the supporting structure 1, is set up in said groove 25. If the reactor is operated, the leakage of the coolant is prevented by means of contacting to the supporting structure 1 by the O-ring 26 by being raised by temp.