Patent Number: 
Section: claims

1. 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; anda 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. 2. The fast reactor according to claim 1, wherein said lower sub-region is a region from a bottom end of the neutron reflector to a place substantially two-fifths of the height thereof. 3. The fast reactor according to claim 1, wherein said lower sub-region comprises a material having a larger neutron scattering cross-section for fast neutrons having an energy of substantially 0.1 to 1 MeV as compared to that of the upper sub-region. 4. The fast reactor according to claim 1, wherein said neutron reflector is composed of steel containing at least one of chromium and nickel, and the lower sub-region includes at least one of chromium and nickel at a higher content as compared to that of the upper sub-region of the neutron reflector. 5. The fast reactor according to claim 1, wherein said upper sub-region is made of a material different from that of said lower sub-region. 6. The fast reactor according to claim 5, wherein said upper sub-region is composed of steel containing at least one of chromium and nickel, and the lower sub-region region includes at least one of chromium and nickel at a higher content as compared to that of the upper sub-region. 7. The fast reactor according to claim 1, wherein the lower sub-region has a higher fast-neutron reflection ability as compared with that of the upper sub-region prior to neutron exposure of said reflector. 8. A fast reactor having a reflector control system, comprising:a reactor vessel which is filled with a liquid metal coolant;a reactor core located in the reactor vessel including a fuel assembly; anda neutron reflector provided outside the reactor core so as to be moved in a vertical direction and which includes a first neutron absorbing or transmitting material located at an upper side thereof having a neutron reflection ability lower than that of the liquid metal coolant, a first neutron reflection material located below said first absorbing or transmitting material, and a second neutron reflection material located below said first neutron reflection material,wherein the first and second neutron reflection materials both have a higher fast-neutron reflection ability than that of the liquid metal coolant, and the first neutron reflection material is different from the second neutron reflection material. 9. The fast reactor according to claim 8, wherein said first and second materials are located in a region between a bottom end of the neutron reflector and substantially two-fifths of the height thereof. 10. The fast reactor according to claim 8, wherein each of said first and second materials comprises a material having a large neutron scattering cross-section for fast neutrons having an energy of substantially 0.1 to 1 MeV. 11. The fast reactor according to claim 8, wherein said first material is composed of steel containing at least one of chromium and nickel, and the second neutron reflection material includes at least one of chromium and nickel at a higher content as compared to that of the first neutron reflection material. 12. The fast reactor according to claim 8, wherein the second material has a higher fast-neutron reflection ability as compared with that of the first material prior to neutron exposure of said reflector.