Patent Number: 
Section: claims

1. A fast reactor having a reactivity control reflector comprising:a reactor vessel in which a coolant is accommodated;a reactor core in the reactor vessel dipped with the coolant;a core barrel surrounding the outside of the reactor core;a partition wall spaced apart from and parallel to the core barrel; anda reflector interposed between the core barrel and the partition wall, outside the reactor core, and surrounding only a portion of the reactor core so that the reflector moves around a periphery of the reactor core in a vertical direction to control reactivity of the reactor core, the reflector being cylindrically shaped,wherein the reflector has a lower neutron reflecting portion having a neutron reflection capability higher than that of the coolant and an upper cavity portion located above the lower neutron reflecting portion and having a neutron reflection capability lower than that of the coolant, and the upper cavity portion is composed of a plurality of hermetically-sealed vessels, andwherein the hermetically-sealed vessels are cylindrically shaped,wherein the upper cavity portion includes a frame assembly that surrounds peripheries of the hermetically-sealed vessels, supports the lower neutron reflecting portion and restrains the upper cavity portion in a horizontal direction,wherein the hermetically-sealed vessels are surrounded by supporting rods and are arranged side by side, stacked and accommodated in the frame assembly where the frame assembly includes a coupling member and the supporting rods. 2. The fast reactor having the reactivity control reflector according to claim 1, wherein the lower neutron reflecting portion comprises a plurality of laminated metal plates, and the laminated metal plates have a plurality of coolant flow paths in which the coolant flows. 3. The fast reactor having the reactivity control reflector according to claim 1, wherein the lower neutron reflecting portion comprises a plurality of laminated metal plates or SiC plates, and each of the laminated metal plates includes metal mainly composed of chromium-molybdenum steel, nickel steel, nickel or includes Inconel. 4. The fast reactor having the reactivity control reflector according to claim 1, wherein the lower neutron reflecting portion comprises a plurality of laminated metal plates or SiC plates to which a plurality of coolant flow paths, in which the coolant flows, are formed so as to communicate with each other. 5. The fast reactor having the reactivity control reflector according to claim 4, wherein a number of the plurality of coolant flow paths is larger on a reactor core side than on a reactor vessel side. 6. The fast reactor having the reactivity control reflector according to claim 1, wherein the reflector is arranged such that a plurality of pads are disposed around the upper and lower ends of the lower neutron reflecting portion and the upper cavity portion. 7. The fast reactor having the reactivity control reflector according to claim 1, wherein the hermetically-sealed vessels comprise convex portions or recessed portions formed to the upper and lower ends thereof, and horizontal movement of the hermetically-sealed vessels is configured to be restrained by connecting the hermetically-sealed vessels to each other in a columnar state by concave/convex engagement or engagement coupling through a framework. 8. The fast reactor having the reactivity control reflector according to claim 1, wherein upper and lower end plates are disposed to the hermetically-sealed vessels disposed to the upper cavity portion, the upper and lower end plates are integrally welded to the hermetically-sealed vessels, and the upper cavity portion is divided into a plurality of segments. 9. The fast reactor having the reactivity control reflector according to claim 1, wherein the reflector is arranged such that the upper cavity portion is formed on an extending line above a region of the lower neutron reflecting portion, and the hermetically-sealed vessels having a maximum diameter are disposed in a region of the upper cavity portion. 10. The fast reactor having the reactivity control reflector according to claim 1, wherein the reflector is arranged such that the hermetically-sealed vessels having a plurality of different diameters are disposed so that a volume occupied by the space in the hermetically-sealed vessels is 80% or more of an entire volume of the upper cavity portion. 11. The fast reactor having the reactivity control reflector according to claim 1, wherein the upper cavity portion is arranged such that the hermetically-sealed vessels are disposed in a horizontal direction so that influence on the core reactivity is reduced at a time of breaking any one of the hermetically-sealed vessels. 12. The fast reactor having the reactivity control reflector according to claim 1, further comprising a joint, configured to absorb thermal expansion, deformation and vibration, that couples the lower neutron reflecting portion to the upper cavity portion. 13. The fast reactor having the reactivity control reflector according to claim 1, further comprising a thermal expansion absorption element, attached to at least one of an upper portion and a lower portion of the upper cavity portion, configured to absorb thermal expansion in a vertical direction and absorb displacement of the hermetically-sealed vessels in the vertical direction. 14. The fast reactor having the reactivity control reflector according to claim 13, wherein the thermal expansion absorption element comprises one of a coil spring, a disk spring and a sheet spring.