Patent Number: 053496191
Section: claims

1. A fuel assembly for a light water reactor comprising a plurality of fuel rods which contain a primary fissile material, wherein at least one of water rods, in which cooling water flows therein when said fuel assembly is provided in the reactor, is provided at least in one of each corner position of an outermost layer of a polygonal arrangement of the fuel rods and a position in the outermost layer adjacent to said corner position in such a manner that said water rods are located in rotation symmetry, each of said water rods being filled with water over a length at least corresponding to a fuel effective length, and the fuel rods are provided at positions in an adjacent layer to the outermost layer which are adjacent to those positions at which said water rods are located. 2. A fuel assembly for a light water reactor according to claim 1, wherein said fissile material includes primarily plutonium when exposure is zero. 3. A fuel assembly for a light water reactor according to claim 2, wherein one of said water rods is located in one of each corner position of the arrangement of the fuel rods and the position in the outermost layer adjacent to said corner position in rotation symmetry. 4. A fuel assembly for a light water reactor according to claim 2, wherein two of said water rods are provided in each corner position of the arrangement of fuel rods and in a position in the outermost layer adjacent to said corner position in such a manner that said water rods are located in rotation symmetry. 5. A fuel assembly for a light water reactor according to claim 2, wherein two of said water rods are provided in both positions in the outermost layer adjacent to each corner position of the arrangement of the fuel rods in such a manner that said water rods are located in rotation symmetry. 6. A fuel assembly for a light water reactor according to claim 2, wherein said polygonal arrangement has substantially a square shape. 7. A fuel assembly for a light water reactor according to claim 1, wherein some of said plurality of the fuel rods contain primary plutonium as a fissile material when exposure is zero, and the others of the fuel rods only contain uranium as a fissile material when exposure is zero. 8. A fuel assembly for a light water reactor according to claim 7, wherein said polygonal arrangement has substantially a square shape. 9. A fuel assembly for a light water reactor according to claim 7, wherein one of said water rods is located in one of each corner position of the arrangement of the fuel rods and the position in the outermost layer adjacent to said corner position in rotation symmetry. 10. A fuel assembly for a light water reactor according to claim 7, wherein two of said water rods are provided in each corner position of the arrangement of fuel rods and in a position in the outermost layer adjacent to said corner position in such a manner that said water rods are located in rotation symmetry. 11. A fuel assembly for a light water reactor according to claim 7, wherein two of said water rods are provided in both positions in the outermost layer adjacent to each corner position of the arrangement of the fuel rods in such a manner that said water rods are located in rotation symmetry. 12. A fuel assembly disposed in a light water reactor, said fuel assembly comprising a plurality of fuel rods which contain a primary fissile material, wherein at least one of solid moderator rods is provided in one of each corner position of an outermost layer of a polygonal arrangement of the fuel rods and a position in the outermost layer adjacent to said corner position in such a manner that said solid moderator rods are located in rotation symmetry, each of said solid moderator rods being filled with a solid moderator over a length at least corresponding to a fuel effective length, and the fuel rods are provided at positions in an adjacent layer to the outermost layer which are adjacent to those positions at which said solid moderator rods are located. 13. A fuel assembly according to claim 12, wherein said fissile material includes primarily plutonium when exposure is zero. 14. A fuel assembly according to claim 13, wherein said polygonal arrangement has substantially a square shape. 15. A fuel assembly according to claim 13, wherein one of said solid moderator rods is located in one of each corner position of the arrangement of the fuel rods and the position in the outermost layer adjacent to said corner position in rotation symmetry. 16. A fuel assembly according to claim 13, wherein two of said solid moderator rods are provided in each corner position of the arrangement of fuel rods and in a position in the outermost layer adjacent to said corner position in such a manner that said solid moderator rods are located in rotation symmetry. 17. A fuel assembly according to claim 13, wherein two of said solid moderator rods are provided in both positions in the outermost layer adjacent to each corner position of the arrangement of the fuel rods in such a manner that said solid moderator rods are located in rotation symmetry. 18. A fuel assembly according to claim 12, wherein some of said plurality of the fuel rods contain plutonium as a primary fissile material when exposure is zero, and the others of the fuel rods only contain uranium as a fissile material when exposure is zero. 19. A fuel assembly according to claim 18, wherein said polygonal arrangement has substantially a square shape. 20. A fuel assembly according to claim 18, wherein one of said solid moderator rods is located in one of each corner position of the arrangement of the fuel rods and the position in the outermost layer adjacent to said corner position in rotation symmetry. 21. A fuel assembly according to claim 18, wherein two of said solid moderator rods are provided in each corner position of the arrangement of the fuel rods and in a position in the outermost layer adjacent to said corner position in such a manner that said solid moderator rods are located in rotation symmetry. 22. A fuel assembly according to claim 18, wherein two of said solid moderator rods are provided in both positions in the outermost layer adjacent to each corner position of the arrangement of the fuel rods in such a manner that said solid moderator rods are located in rotation symmetry. 23. A core of a light water reactor comprising first fuel assemblies each including a plurality of fuel rods which only contain uranium as a fissile material when exposure is zero, second fuel assemblies each including a plurality of fuel rods which contain plutonium as a primary fissile material when exposure is zero, in which in said second fuel assembly at least one of moderator rods is provided at least in one of each corner position of an outermost layer of a polygonal arrangement of the fuel rods and a position in the outermost layer adjacent to said corner position in such a manner that said moderator rods are located in rotation symmetry, each of said moderator rods being filled with a filling substance over a length at least corresponding to a fuel effective length, and the fuel rods are provided at positions in an adjacent layer to the outermost layer which are adjacent to those positions at which said moderator rods are located. 24. A core of a light water reactor according to claim 23, wherein said second fuel assemblies are each arranged such that one of said moderator rods is located in one of each corner position of the arrangement of the fuel rods and the position in the outermost layer adjacent to said corner position in rotation symmetry. 25. A core of a light water reactor according to claim 23, wherein said second fuel assemblies are each arranged such that two of said moderator rods are provided in each corner position of the arrangement of fuel rods and in a position in the outermost layer adjacent to said corner position in such a manner that said moderator rods are located in rotation symmetry. 26. A core of a light water reactor according to claim 23, wherein said second fuel assemblies are each arranged such that two of said moderator rods are provided in both positions in the outermost layer adjacent to each corner position of the arrangement of fuel rods in such a manner that said moderator rods are located in rotation symmetry. 27. A core of a light water reactor according to claim 23, wherein said second fuel assemblies have substantially the same shape and dimensions as said first fuel assemblies. 28. A core of a light water reactor according to claim 23, wherein said polygonal arrangement has substantially a square shape.