Reactor core of FBR type reactor

A gas-sealed assembly disposed between fuel assemblies contains a substantially cylindrical cavity having a coolant flowing aperture. The liquid level of the coolants in the cavity is adapted so as to change from the upper portion containing a reactor top level to the lower portion by temperature change of coolants. When the amount of coolants is reduced, the sealed gas is thermally expanded due to elevation of coolant temperature at the periphery of the gas sealed assemblies, so that the inner liquid level is lowered from a position above the reactor core top level to the lower portion to form a cavity. At the same time, although temperature of coolants in the fuel assemblies is also elevated, the inside of the gas-sealed assembly contains cavity up to the level of the reactor core, accordingly, neutrons generated in the fuel assemblies leak upwardly from the gas sealed assembly toward the upper direction of the reactor core to provide a negative reactivity. Since the temperature of coolants is elevated also when the reactor power increases, the same effect as in a case that the coolant flow rate is reduced is provided. Then, inherent safety upon occurrence of reactor accident is improved. (N.H.)