To provide a structure avoiding wasteful flow of a coolant in gaps among fuel assemblies and allowing the fuel assemblies to be drawn out or inserted easily when a nuclear reactor is stopped, for use at the core of a fast breeder reactor.
In grid spacers 1, mounted on at least one part or more along the direction of the axis of each fuel assembly placed in a fast breeder reactor core, and restraining the movement of each fuel assembly in the direction of its axis and in the vertical direction, plates made of a bimetal 2 or shape memory alloy, in which a metal on the outside has a higher coefficient of thermal expansion than the fuel- assembly-side metal, are arranged circumferentially on the outer peripheries of at least those grid spacers 1 which are located at the top of a heating part. During power generation, the plates made of the bimetal 2 or shapememory alloy are each most deformed at its center in such a way as to project outwards, and the expanded plates made of the bimetal 2 or shape-memory alloy, mounted on the grid spacers 1 of adjacent fuel assemblies, make contact with one another to stop circulation of a coolant in gaps among the fuel assemblies, and are restored to their flat shapes during the stop of the fast breeder reactor, allowing the fuel assemblies to be freely drawn out or inserted.
MINAMI RYOGO
SHIMIZU KATSUHIKO
