Small-sized FBR type reactor

A reactor core having nuclear fuels loaded therein is contained in a reactor container, then it is filled with coolants and closed by a shielding plug. A reflecting device is moved vertically while containing the reactor container to control a burning reactivity of nuclear fuels. A penetrating pipe is disposed to the reactor container passing through the reactor core from at least one of upward and downward directions, and the reactor power is controlled by charging/discharging the control rod to the penetrating pipe. Then, since the controllable burning reactivity can be increased, subcriticality can be ensured even if the small-sized FBR type reactor is submerged under water. Further, nuclear fuels having long-reactivity life can be used as the controllable burning reactivity is increased. (N.H.)