Patent Number: 
Section: claims

1. A light water reactor core comprising fuel which is enriched by adding plutonium or plutonium and an actinide to a uranium containing at least one of a depleted uranium, natural uranium, a degraded uranium and a low enriched uranium, which further comprises: fuel assemblies having fuel rods arranged in a triangular lattice configuration; and  large-diameter control rods to be inserted into said fuel assemblies, said large-diameter control rod comprising at least one absorption rod and guide tubes in which said large diameter control rods are inserted, each said guide tube being disposed in a region having an area equivalent to at least 7 fuel rod unit lattice cells, wherein  a breeding ratio of the reactor core is near 1.0 or larger than 1.0, and a void coefficient of the reactor core is negative. 2. A light water reactor core comprising fuel which is enriched by adding plutonium or plutonium and an actinide to a uranium containing at least one of a depleted uranium, natural uranium, a degraded uranium and a low enriched uranium, which further comprises; fuel assemblies having fuel rods arranged in a triangular lattice configuration; and  large-diameter control rods to be inserted into said fuel assemblies, said large-diameter control rod comprising at least one absorption rod and guide tubes in which said large diameter control rods are inserted, each said guide tube being disposed in a region having an area equivalent to at least 7 fuel rod unit lattice cells, wherein  a breeding ratio of the reactor core is near 1.0 or larger than 1.0, and a power coefficient of the reactor core is negative, and a void coefficient of the reactor core is positive or zero. 3. A light water reactor core according to  claim 1 , wherein said breeding ratio is within the range of 1.0 to 1.15. claim 1 4. A fuel assembly comprising fuel which is enriched by adding plutonium or plutonium and an actinide to a uranium containing at least one of a depleted uranium, natural uranium, a degraded uranium and a low enriched uranium, which further comprises: fuel assemblies having fuel rods arranged in a triangular lattice configuration; and  at least one guide tube to insert a large-diameter control rod thereinto, said guide tube disposed in a region having an area equivalent to at least 7 fuel rod unit lattice cells, wherein  a breeding ratio of the fuel assembly is near 1.0 or larger than 1.0. 5. A fuel assembly according to  claim 4 , which comprises a water-excluding region on a surface of said guide tube, said water-excluding region being formed of a substance having a slowing down power smaller than a slowing down power of light water. claim 4 6. A fuel assembly according to  claim 4 , which comprises fuel rods closely arranged in a triangular lattice configuration, and a gap between said rods is within the range of 0.7 to 2.0 mm. claim 4 7. A light water reactor core, which is composed of the fuel assemblies according to  claim 6 . claim 6 8. A fuel assembly according to  claim 4 , which comprises fuel rods closely arranged in a triangular lattice configuration, and an effective water-to-fuel volume ratio of the fuel assembly is within the range of 0.1 to 0.6. claim 4 9. A light water reactor core, which comprises the fuel assemblies according to  claim 8 . claim 8 10. A light water reactor core according to  claim 1 , wherein an average fissionable plutonium enrichment in the reactor core except for an outer peripheral portion and blanket portions of a top and a bottom end portions is within the range of 6 to 20 wt %. claim 1 11. A fuel assembly according to  claim 4 , wherein an average fissionable plutonium enrichment in a fuel region except for blanket portions of a top and a bottom end portions is within the range of 6 to 20 wt %. claim 4 12. A light water reactor core according to  claim 1 , wherein a core-average void fraction under operation of an output power higher than 50% of a rated output power is within the range of 45 to 70%. claim 1 13. A light water reactor core according to  claim 1 , wherein all the large-diameter control rods connected to one control drive mechanism are inserted into one fuel assembly, and said large-diameter control rods are inserted into all of the fuel assemblies loaded in a region except for an outermost peripheral of the reactor core. claim 1 14. A light water reactor core according to  claim 13 , wherein a shape of a transverse plane of said fuel assembly is hexagonal or square. claim 13 15. A light water reactor core according to  claim 1 , wherein the plurality of large-diameter control rods connected to one control drive mechanism are inserted into three hexagonal fuel assemblies, and said large-diameter control rods are inserted into all of the fuel assemblies loaded in a region except for an outermost peripheral of the reactor core. claim 1 16. A light water reactor core according to  claim 1 , wherein the plurality of lare-diameter control rods connected to one control drive mechanism are inserted into four square fuel assemblies, and said large-diameter control rods are inserted into all of the fuel assemblies loaded in a region except for an outermost peripheral of the reactor core. claim 1 17. A light water reactor core according to  claim 1 , wherein said large-diameter control rod comprises a follower portion in a top end portion, said follower portion being made of a substance having a slowing down power smaller than a slowing down power of light water. claim 1 18. A fuel assembly according to  claim 4 , wherein an average value of fissionable plutonium enrichments of a fuel rod arranged in a region adjacent to said guide tube and a fuel rod arranged in a region most distant from a center of said fuel assembly is smaller than an average value of fissionable plutonium enrichments of fuel rods arranged in the other positions. claim 4 19. A light water reactor core according to  claim 1 , wherein an average output power density of in a reactor core region except for an outer peripheral portion and blanket portions of a top and a bottom end portions is within the range of 100 to 300 kW/l. claim 1 20. A light water reactor core according to  claim 1 , wherein in regard to a height direction of the core except for blanket portions of a top and a bottom end portions, a length having an average fissionable plutonium enrichment with respect to a horizontal cross section of the fuel assembly higher than 6 wt % is within the range of 40 cm to 140 cm. claim 1 21. A fuel assembly according to  claim 4 , wherein in regard to a height direction of the reactor core except for blanket portions of a top and a bottom end portions, a length having an average fissionable plutonium enrichment with respect to a horizontal cross section of the fuel assembly higher than 6 wt % is within the range of 40 cm to 140 cm. claim 4 22. A fuel assembly according to  claim 4 , wherein an average value of fissionable plutonium enrichment in an upper half of the fuel assembly except for blanket portions of a top and a bottom end portions is lower than an average value of fissionable plutonium enrichment in an lower half. claim 4 23. A fuel assembly according to  claim 4 , wherein in regard to a height direction of the fuel assembly except for blanket portions of a top and a bottom end portions, the fuel assembly comprises regions having an average fissionable plutonium enrichment higher than 6 wt % in an upper and a lower parts of the fuel assembly; and a region having an average fissionable plutonium enrichment lower than 6 wt % in a region near a middle portion between the upper and the lower regions. claim 4 24. A fuel assembly according to  claim 23 , wherein in regard to a height direction of the fuel assembly except for blanket portions of the top and the bottom end portions, the average fissionable plutonium enrichments in the upper and the lower regions sandwiching the portion having the average fissionable plutonium enrichment lower than 6 wt % in the region near the middle portion between the upper and the lower regions are different from each other. claim 23 25. A light water reactor core according to  claim 1 , wherein a steam weight ratio at an exit of the reactor core under operation of an output power higher than 50% of a rated output power is within the range of 20 wt % to 40 wt %. claim 1 26. A light water reactor core according to  claim 1 , wherein the reactor core except for an outermost periphery of the reactor core is radially divided into two equal-area regions, and fuel assemblies are loaded so that an average value of number of cycles staying in the reactor core of the fuel assemblies loaded in the outer reactor core region is smaller than an average value of number of cycles staying in the reactor core of the fuel assemblies loaded in the inner reactor core region. claim 1 27. A light water reactor core according to  claim 1 , wherein an average value of orifice pressure drop coefficient of fuel assemblies in an outermost periphery of the reactor core and adjacent to the outermost periphery is larger than an average value of orifice pressure drop coefficient of fuel assemblies in the other regions. claim 1 28. A fuel assembly according to  claim 4 , wherein plutonium and uranium extracted from used fuel as loaded into the fuel assembly to be recycled together. claim 4 29. A light water reactor core composed of the fuel assemblies according to  claim 28 . claim 28 30. A fuel assembly according to  claim 4 , wherein plutonium, uranium and actinides extracted from used fuel are loaded at a time to be recycled together. claim 4 31. A light water reactor core composed of the fuel assemblies according to  claim 30 . claim 30 32. A light water rector core comprising fuel which is enriched by adding plutonium or plutonium and an actinide to a uranium containing at least one of a depleted uranium, natural uranium, a degraded uranium and a low enriched uranium, which further comprises: fuel assemblies having fuel rods arranged in a triangular lattice configuration; and  large-diameter control rods to be inserted into said fuel assemblies, said large-diameter control rods each comprising at least one absorption rod having a transverse cross-sectional area larger than a cross-sectional area of a unit lattice cell of the fuel rod; and  guide tubes in which said large-diameter control rods are inserted, said guide tubes each having water excluding rods outside for excluding light water between said guide tube and fuel rods adjacent to said guide tube and disposed in a region having an area equivalent to at least 7 fuel rod unit lattice cells, wherein  a breeding ratio of the reactor core is near 1.0 or larger than 1.0, and a void coefficient of the reactor core is negative.