Abstract: A fast reactor comprises a reactor vessel to be arranged vertically in a reactor building, a reactor vessel upper structure disposed on an upper portion in the reactor vessel, a drum structure suspended from the reactor vessel structure into a central portion in the reactor vessel so as to define an annular portion between an outer periphery of the drum structure and an inner periphery of the reactor vessel, and a reactor core disposed in the drum structure. A reflector is disposed in the reactor vessel and is vertically movable along an outer periphery of the reactor core. The reflector may be composed of grain materials movable by gravity. An intermediate heat exchanger is further disposed at a portion above the reactor core and an electromagnetic pump is disposed in the reactor vessel for circulating coolant. The reactivity of the reactor core can be controlled by the reflector to thereby reduce the neutron irradiation amount to the reactor vessel.