Patent ID: 7773716

Claim:
A fast reactor having a reflector control system, comprising: a reactor vessel which is filled with a liquid metal coolant; a reactor core including a fuel assembly, which is placed at a central position of the reactor vessel; and a neutron reflector including a region, which is placed at an upper side thereof and at which a neutron absorber or a neutron transmitting material having a neutron reflection ability lower than that of the liquid metal coolant is placed and a reflection region which is placed at a lower side thereof and provided outside the reactor core so as to be moved in a vertical direction in an installed state of the fast reactor for adjusting leakage of neutrons from the reactor core so as to control a reactivity thereof, wherein said neutron reflector is moved in an upward direction in accordance with a change in reactivity caused by burn-up of a fuel, and the reflection region includes an upper sub-region and a lower sub-region both with higher fast-neutron reflection ability than that of the liquid metal coolant, the lower sub-region contains a material with a higher fast-neutron reflection ability than that of the upper sub-region.