Reactor core for FBR type reactor

Fuel assemblies are disposed in an annular shape and neutron absorption assemblies filled with neutron absorbents are disposed to the inside of the annular shape. Further, the reactor also comprises special fuel assemblies for ensuring neutron leakage paths from the reactor core when coolants in the fuel assemblies are in a void state, by disposing one or more regions not provided with fuel elements. By disposing the neutron absorbent assemblies, neutron leakage can be increased also in the reactor core with a relatively small surface are of the reactor core thereby enabling to suppress the increase of the reactor core size. Further, by using the special fuel assem lies, neutron leakage from the reactor core can be promoted if the coolants in the special fuel assemblies are in the void stated, to reduce the void reactivity. (N.H.)