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, andwherein said fuel assembly includes a fuel pin bundle having fuel pins filled with a fissile fuel and a non-fissile fuel placed in a wrapper tube and having an outermost fuel pin adjacent to the neutron reflector and another fuel pin placed at a farther distance away from the neutron reflector, and wherein, prior to neutron exposure in the reactor, the ratio of the fissile fuel amount in the outermost fuel pin to total amount of the fissile fuel and the non-fissile fuel in the outermost fuel pin is made smaller than the ratio of the fissile fuel amount in the another fuel pin to the total amount of the fissile fuel and the non-fissile fuel in another fuel pin. 2. The fast reactor according to claim 1, wherein said neutron reflector further has a low reflection region at a central portion thereof, and the low reflection region has a lower neutron reflection ability as compared to the reflection region. 3. The fast reactor according to claim 1, wherein said reflection region of the neutron reflector includes at least one of carbide and graphite, and the neutron reflector includes a region made of steel containing at least one of chromium and nickel. 4. The fast reactor according to claim 1, wherein said reflection region has a largest thickness of the neutron reflector. 5. The fast reactor according to claim 1, wherein said reflection region comprises a neutron reflector material having a highest effective density the neutron reflector. 6. The fast reactor according to claim 1, further comprising an external frame formed of steel which contains at least one of chromium and nickel, wherein said external frame encloses the neutron reflector. 7. The fast reactor according to claim 6, wherein said external frame including at least of a part which comprises austentic stainless steel having a high resistance against swelling, the stainless steel containing iron, chromium, and nickel as a primary component, and further containing titanium. 8. The fast reactor according to claim 1, wherein said neutron reflector includes at least of a part which comprises austentic stainless steel having a high resistance against swelling, the austentic stainless steel containing iron, chromium, and nickel as a primary component, and further containing titanium. 9. The fast reactor according to claim 1, wherein said neutron reflector is provided so as to surround the reactor core, and a position of a boundary in the axial direction between the reflection region and the remainder of the neutron reflector varies with a circumferential position. 10. The fast reactor according to claim 1, wherein said reactor core has a first region having a higher ratio of fissile material to total amount of fissile and non-fissile material than that in a second region of the reactor core, and said first region being a region other than said second region being located from a place between substantially one tenth and one fifth of the height of the reactor core from the bottom end thereof to a place at substantially one half of the height thereof. 11. The fast reactor according to claim 1, wherein said reactor core has a first region having a higher fuel smear density than that in a second region of the reactor core, and said first region being a region other than said second region being located from a place between substantially one tenth and one fifth of the height of the reactor core from the bottom end thereof to a place at substantially one half of the height thereof. 12. The fast reactor according to claim 1, wherein said reactor core has a first region having a larger fuel diameter than that in a second region of the reactor core so as to achieve a low differential reflector reactivity at the central portion in the axial direction, and said first region being a region other than said second region being located from a place between substantially one tenth and one fifth of the height of the reactor core from the bottom end thereof to a place at substantially one half of the height thereof. 13. The fast reactor according to claim 1, wherein said reactor core has a first region having one of a higher ratio of fissile material to total amount of fissile material and non-fissile material, a higher fuel smear density, and a larger fuel diameter than that in a second region of the reactor core, and said first region being a region other than said second region being located from a place between substantially one tenth and one fifth of the height of the reactor core from the bottom end thereof to a place at substantially one half of the height thereof. 14. The fast reactor according to claim 1, wherein said reactor core has a first region having one of a ratio of fissile material to total amount of fissile and non-fissile material, a fuel smear density, and a fuel diameter, which is gradually increased in an upward direction, and said first region being a region from a place between substantially one tenth and one fifth of the height of the reactor core from the bottom end thereof to the top of the reactor core. 15. The fast reactor according to claim 1, wherein said reactor core has a first region having one of a higher ratio of fissile material to total amount of fissile and non-fissile material, a higher fuel smear density, and a larger fuel diameter than that in a second region of the reactor core, and said first region being a region other than the second region and being located between the top of the reactor core and substantially one half of the height thereof. 16. The fast reactor according to claim 1, wherein said reactor core includes first regions only or both first and second regions with minor actinides in the fuel at a high content than that in the other region, the first region being located from a place between substantially one tenth and one fifth of the height of the reactor core from the bottom end thereof to a place at substantially one half of the height thereof, the second region being located from the bottom of the reactor core to substantially one half of the height thereof. 17. The fast reactor according to claim 16, wherein said minor actinide is one of neptunium and americium. 18. The fast reactor according to claim 1, wherein said reactor core includes a minor actinide, and the content of the minor actinide is increased toward the bottom end of the reactor core except for a region from the bottom thereof to a place between substantially one tenth and one fifth of the height of the reactor core. 19. The fast reactor according to claim 18, wherein said minor actinide is one of neptunium and americium. 20. 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, andwherein said fuel assembly includes a fuel pin bundle having fuel pins each having a fuel smear density placed in a wrapper tube and having an outermost fuel pin adjacent to the neutron reflector and a fuel pin placed at a further distance away from the neutron reflector, and, prior to neutron exposure in the reactor, the fuel smear density of the outermost fuel pin is smaller than the fuel smear density of the fuel pin placed at a farther distance away from the neutron reflector. 21. The fast reactor according to claim 1, wherein said fuel assembly includes a fuel pin facing the neutron reflector and a fuel pin placed at a distance from the neutron reflector, and a ratio of the fissile fuel to amount of fissile fuel and non-fissile fuel filled in the fuel pin facing the neutron reflector is changed in the axial direction. 22. The fast reactor according to claim 1, wherein said fuel assembly containing fuel pins, and the fuel pins each have a low fissile material region, an intermediate fissile material region, and a high fissile material region in that order from a coolant inlet side to a coolant outlet side. 23. The fast reactor according to claim 1, wherein said fuel assembly includes a fuel pin facing the neutron reflector and a fuel pin placed at a distance from the neutron reflector, and a fuel smear density of the fuel pin facing the neutron reflector is increased in the axial direction from a coolant inlet side to a coolant outlet side. 24. The fast reactor according to claim 1, wherein said neutron reflector comprises a structural member at a side facing the fuel assembly and a moderator at another side. 25. The fast reactor according to claim 1, wherein said neutron reflector comprises a structural member at an upper side in a lifting direction and a moderator at another side. 26. The fast reactor according to claim 1, wherein said reactor core further includes a neutron absorption assembly which is placed among the fuel assembly. 27. The fast reactor according to claim 26, wherein said neutron absorption assembly is disposed at the central position among the fuel assembly. 28. The fast reactor according to claim 26, wherein said neutron absorption assembly comprises a case member, a core shutdown rod placed at the central position of the case member, and a fixed absorber disposed outside of the core shutdown rod. 29. The fast reactor according to claim 26, wherein said neutron absorption assembly comprises a case member, a core shutdown rod placed at a central position of the case member, and a fixed absorber disposed outside of the core shutdown rod, and the solid absorber has a tube-shaped structure formed of a plurality of segments. 30. The fast reactor according to claim 29, wherein said segments include at least one comprising a structural member so as to adjust the reactivity of the fuel assembly. 31. The fast reactor according to claim 26, wherein said neutron absorption assembly comprises a case member, a core shutdown rod placed at the central position of the case member, and a fixed absorber disposed outside of the core shutdown rod, and the case member has a polygonal structure. 32. The fast reactor according to claim 1, wherein said reactor core further comprises a minor actinide annihilation assembly provided outside the fuel assembly. 33. The fast reactor according to claim 32, wherein said minor actinide annihilation assembly comprises a mixed portion of a high concentration moderator and a minor actinide, a mixed portion of a low concentration moderator and a minor actinide, and a minor actinide portion provided in that order from a side facing the fuel assembly to an external side. 34. The fast reactor according to claim 1, wherein:said fuel pin bundle has a first plurality of said fuel pins adjacent to the neutron reflector, a second plurality of fuel pins located at a first distance further away from said neutron reflector than said first plurality of fuel pins, and a third plurality of fuel pins located a second distance further away from said neutron reflector than said first distance;the ratio of the fissile fuel to amount of fissile fuel and non-fissile fuel filled in the first plurality of fuel pins is smaller than the ratio of the fissile fuel to amount of fissile fuel and non-fissile fuel filled in the second plurality of fuel pins; andthe ratio of the fissile fuel to total amount of the fissile fuel and the non-fissile fuel in the second plurality of fuel pins is smaller than the ratio of the fissile fuel to total amount of the fissile fuel and the non-fissile fuel in the third plurality of fuel pins. 35. The fast reactor according to claim 34, comprising:said second plurality of fuel pins being located adjacent to said first plurality of fuel pins, and said third plurality of fuel pins being located adjacent to said second plurality of fuel pins. 36. The fast reactor according to claim 20, wherein:said fuel pins are filled with a fissile fuel and a non-fissile fuel;said fuel pin bundle has a first plurality of said fuel pins adjacent to the neutron reflector, a second plurality of fuel pins located at a first distance further away from said neutron reflector than said first plurality of fuel pins, and a third plurality of fuel pins located a second distance further away from said neutron reflector than said first distance;the fuel smear density of the first plurality of fuel pins is smaller than the fuel smear density of the second plurality of fuel pins; andthe fuel smear density of the second plurality of fuel pins is smaller than the fuel smear density of the third plurality of fuel pins. 37. The fast reactor according to claim 36, comprising:said second plurality of fuel pins being located adjacent to said first plurality of fuel pins, and said third plurality of fuel pins being located adjacent to said second plurality of fuel pins.