Patent Number: 
Section: claims

1. A nuclear power generation system comprisinga nuclear reactor,a heat exchanging system for exchanging heat between a primary coolant for cooling the nuclear reactor and a secondary coolant comprised of carbon dioxide or light water, anda turbine power generation system for generating power using heat of the secondary coolant;the nuclear reactor comprising:a reactor core having fuel assemblies comprising a plurality of fuel rods, the fuel rods comprising cladding tubes containing metallic fuel including at least one selected from uranium-235, uranium-238 and plutonium-239;a reactor vessel containing the reactor core;the primary coolant being one of metallic sodium, lead, and lead-bismuth loaded into the reactor vessel and heated by the reactor core; andat least one neutron reflector disposed around the reactor core,wherein the at least one neutron reflector disposed around the reactor core has neutron reflection efficiency for achieving criticality in the reactor core with an effective multiplication factor of neutrons emitted from the reactor core being maintained at or above about 1, andwherein the at least one neutron reflector is coupled to metallic members having a coefficient of thermal expansion greater than a coefficient of thermal expansion of the at least one neutron reflector, and the neutron reflection efficiency of the at least one neutron reflector is changeable utilizing displacement thereof due to dimensional change of the metallic members coupled thereto in accordance with changes in the temperature in the reactor vessel, thereby achieving load following control. 2. The nuclear power generation system according to claim 1, wherein the at least one neutron reflector disposed around the reactor core has a height lower than a height of the reactor core, and is movable upwardly or downwardly along the reactor core with a movement mechanism. 3. The nuclear power generation system according to claim 1, wherein the at least one neutron reflector provided around the reactor core has substantially the same, or smaller, length compared with a full length of the fuel assemblies. 4. The nuclear power generation system according to claim 1, wherein the metallic members comprise at least one of elastic spring or spiral members extending around or above the fuel assemblies. 5. The nuclear power generation system according to claim 1,wherein the at least one neutron reflector comprises a first neutron reflector and a second neutron reflector each extending along a different concentric circle extending around a center of the reactor core, each of the first and second neutron reflectors divided into two or more sections extending along their respective concentric circles,wherein the sections of the first neutron reflector are coupled to a first spiral metallic member provided on a concentric circle about the reactor core, andwherein sections of the first neutron reflector and the sections of the second neutron reflector are moveable, with respect to one another, in a circumferential direction about the reactor core,the movement in the circumferential direction forming slits therebetween in a circumferential direction, wherein the widths of the slits in the circumferential direction are changeable as a result of dimensional change of the first spiral metallic member based on the temperature in the reactor vessel. 6. The nuclear power generation system according to claim 5, wherein each of the first and second neutron reflectors are further radially divided into two or more sections. 7. The nuclear power generation system according to claim 5, wherein the sections of the second neutron reflector are coupled to a second spiral metallic member disposed along a concentric circle of the reactor core, and the spiral directions of the first spiral metallic member and the second spiral metallic member are in opposite directions. 8. The nuclear power generation system according to claim 1, wherein a material of the at least one neutron reflector is selected from beryllium, beryllium oxide, graphite, carbon, and stainless steel. 9. The nuclear power generation system according to claim 5, wherein carbon is provided as a lubricant between the sections of the first neutron reflector and the sections of the second neutron reflectors of the second group. 10. The nuclear power generation system according to claim 5, wherein the sections of the first neutron reflector and the sections of the second neutron reflector overlap in the radial direction from the center of the reactor core, and the widths of the radial overlaps define the temperature at which the slits open and at which the criticality reaches 1. 11. The nuclear power generation system according to claim 1,wherein the metallic members compriseadjustment springs having opposed first and second ends, anda fixation cylinder, against which the first end of the adjustment springs contact, is provided circumferentially outside of the circumferential location of the at least one neutron reflector;the at least one neutron reflector is divided into two or more arcuate sections extending along a concentric circle extending around the reactor core; anda plurality of reflector moving jigs corresponding in number to the number of sections of the neutron reflector, each reflector jig comprising an adjustment spring support plate and a neutron reflector adjusting rod; andeach adjustment spring contacts, at its second end, the adjustment spring support plate provided outside the fixation cylinder,wherein each neutron reflector adjusting rod is coupled to a corresponding neutron reflector at one end thereof and fixed to the adjustment spring support plate at an opposite end thereof, and the reflector adjusting rod moves the neutron reflector with respect to the location of the fuel assemblies upon changes in the temperature of the adjustment springs whereby load following control for the energy output from the nuclear reactor is enabled. 12. The nuclear power generation system according to claim 1,wherein the at least one neutron reflector comprises a multi-layer plurality of rings, each ring comprising a plurality of ring segments extending circumferentially around the fuel rods; and,the metallic members are disposed radially outwardly of and around the multi-layer neutron reflectors rings, wherein different ring segments of the multi-layer plurality of rings are coupled to different portions of the metallic members,wherein, upon a change in temperature of the metallic members, slits are formed between the neutron reflector rings in the circumferential direction and the widths of the slits are dependent upon thermally induced dimensional change of the metallic members to adjust the neutron leakage, whereby load following control for output from the nuclear reactor is enabled. 13. The nuclear power generation system according to claim 1,wherein the at least one neutron reflector comprises a plurality of neutron reflector sections, each of the neutron reflector sections comprising a first end coupled to a supporting rod and a second end distal to the first end, the supporting rods lying on a circular path extending around the reactor core,wherein the first end of each neutron reflector section is rotatable about the supporting rod and thereby form a slit extending in the circumferential direction about the reactor core, and each of the metallic members comprise spiral metallic members connected at a first end thereof to one of the supporting rods and at a second end thereof to the neutron reflector section associate to the rod to which it is connected,wherein changes in the temperature of the spiral metallic members causes dimensional change in the spiral metallic members, thereby causing the first end of the neutron reflector section connected thereto to rotate about the supporting rod to cause the second end of that neutron reflector section to move away from first end of an adjacent neutron reflector section to form a slit therebetween, wherein the span of the slit between the second end of the neutron reflector section and the adjacent first end of the adjacent neutron reflector section varies based on the temperature of the spiral metallic member, whereby load following control for output from the nuclear reactor is enabled. 14. The nuclear power generation system according to claim 4,wherein the at least one of an elastic spring or a spiral metallic members comprise at least one of stainless steel, a nickel based superalloy, and a nickel-cobalt based superalloy, or a bimetal. 15. The nuclear power generation system according to claim 1, wherein a neutron absorber is provided outside the neutron reflector. 16. The nuclear power generation system according to claim 15, wherein the neutron absorber comprises actinoids. 17. The nuclear power generation system according to claim 1,wherein the reactor core has a plurality of fuel rods comprise cladding tubes comprising at least one of ferritic stainless steel or chromium-molybdenum steel containing metallic fuel selected from at least one of an alloy of zirconium, uranium-235, uranium-238, and plutonium-239 or an alloy of zirconium and one selected from uranium-235, uranium-238 and plutonium-239. 18. The nuclear power generation system according to claim 1, wherein the heat exchanging system comprises a main heat exchanger supplied with the primary coolant heated by the nuclear reactor through a conduit, the main heat exchanger including a circulating secondary coolant heated by heat exchange with the primary coolant. 19. The nuclear power generation system according claim 1, wherein the primary coolant comprises lead or lead-bismuth and the secondary coolant comprises light water heated by heat exchange with the primary coolant.