Patent Number: 050680820
Section: claims

1. A fuel assembly for a nuclear reactor of the type in which a number of fuel rods, each constructed by filling a clad with a fuel material, are arranged in a bundle, comprising: a plurality of first fuel rods having a partial effective fuel area filled with a fuel material and having a portion in which enrichment of a fissile nuclide in the clad of the fuel rod is significantly reduced, or the fissile nuclide does not exist at all, positioned on an axial level including a portion at which subcriticality becomes small at a period in which maintenance of reactor shut-down margin is small during a reactor operation period; and  a plurality of second fuel rods having a total effective fuel area filled with a fuel material throughout an entire axial length of the clad of the fuel rod, wherein said first fuel rods are disposed in straight lines inside the bundle so as to horizontally separate said second fuel rods into a plurality of sub-bundles.  a plurality of first fuel rods having a partial effective fuel area filled with a fuel material and having a portion in which enrichment of a fissile nuclide in the clad of the fuel rod is significantly reduced, or the fissile nuclide does not exist at all, positioned on an axial level including a first section at which subcriticality becomes small at a period in which maintenance of reactor shut-down margin is small during a reactor operation period and a second section located between said first section and a lower end of the effective fuel area; and  a plurality of second fuel rods having at least one partial interposed zone in which enrichment of a fissile nuclide in the clad of the fuel rod is significantly reduced, or the fissile nuclide does not exist at all, positioned at said second section, wherein said first fuel rods are disposed inside the bundle so as to horizontally separate said second fuel rods into a plurality of sub-bundles.  a plurality of first fuel rods having a partial effective fuel area filled with a fuel material and having a portion in which enrichment of a fissile nuclide in the clad of the fuel rod is significantly reduced, or the fissile nuclide does not exist at all, positioned on an axial level including a first section at which subcriticality becomes small at a period in which maintenance of reactor shut-down margin is small during a reactor operation period and a second section located between said first portion and a lower end of the effective fuel area;  a plurality of second fuel rods having at least one partial interposed zone in which enrichment of a fissile nuclide in the clad of the fuel rod is significantly reduced, or the fissile nuclide does not exist at all, positioned at said second section; and  a tube through which a moderator passes disposed in the fuel assembly, wherein said second fuel rods are arranged adjacent to the tube. 2. The fuel assembly according to claim 1, wherein said portion in which enrichment of a fissile nuclide in the clad of the fuel rod is significantly reduced, or the fissile nuclide does not exist at all, comprises an interposed zone in which enrichment of the fissile nuclide is significantly reduced or the fissile nuclide does not exist at all. 3. The fuel assembly according to claim 2 wherein said interposed zone has an axial length substantially equal to or longer than a thermal neutron diffusion length during a reactor power operation period. 4. The fuel assembly according to claim 3 wherein the axial length of said interposed zone is shorter than one-third of an axial length of the effective fuel area of said second fuel rod. 5. The fuel assembly according to claim 2, wherein said interposed zone is located at a position on an axial level so that at least a portion of said interposed zone is at distance between two-thirds and five-sixths of the length of said total effective fuel area measured from the lower end of said total effective fuel area. 6. The fuel assembly according to claim 2 wherein said interposed zone is occupied with a solid moderating material. 7. The fuel assembly according to claim 2 wherein said interposed zone is occupied with a liquid moderating material. 8. The fuel assembly according to claim 7 wherein said liquid material is water. 9. The fuel assembly according to claim 2 wherein a gas plenum is formed in said interposed zone. 10. The fuel assembly according to claim 2 wherein a depleted uranium is charged in said interposed zone. 11. The fuel assembly according to claim 2 wherein a natural uranium is charged into said interposed zone. 12. The fuel assembly according to claim 2 wherein a ceramic material suppressing neutron absorption characteristics at a final stage of a reactor operation cycle is inserted into said interposed zone. 13. The fuel assembly according to claim 12 wherein said ceramic material is a porous heat-proof ceramic. 14. The fuel assembly according to claim 2, wherein a burnable poison having a concentration of an extent such that said burnable poison will vanish at a final stage of a reactor operation cycle is contained in said interposed zone. 15. The fuel assembly according to claim 2 wherein fuel materials disposed adjacent to said interposed zone contain a burnable poison having a concentration to an extent such that said burnable poison will vanish at the final stage of the reactor operation cycle. 16. The fuel assembly according to claim 2, wherein said first fuel rods having the interposed zone are arranged in a linearly crossing pattern. 17. The fuel assembly according to claim 1 wherein said first fuel rod comprises a short fuel rod having an axial length shorter than that of said second fuel rod, said short fuel rod being constructed by removing a portion of the fuel rod existing on an axial level including a portion at which subcriticality becomes small at a period in which maintenance of reactor shut-down margin is small during a reactor operation period. 18. The fuel assembly according to claim 17 wherein a plural number of said short fuel rods are provided with an effective fuel area having an axial length ranging between one-half to five-sixth of the length of said total effective fuel area said second fuel rod. 19. The fuel assembly according to claim 17 wherein a plural number of said short fuel rods are arranged in a linearly crossing pattern. 20. The fuel assembly according to claim 17 wherein the lower end portion of said short fuel rods are arranged substantially in the same plane as the lower end portion of said second fuel rod is arranged. 21. The fuel assembly according to claim 17 wherein a power spike suppressing material is disposed above the fuel material filled in said short fuel rod. 22. The fuel assembly according to claim 1 wherein a plurality of tubes through which a moderator passes are disposed in the fuel assembly and at least one of said first fuel rods is located at an area positioned between said tubes. 23. The fuel assembly according to claim 22 wherein said lubes comprise water rods. 24. A fuel assembly for a nuclear reactor of the type in which a number of fuel rods, each constructed by filling a clad with a fuel material, are arranged in a bundle, comprising: 25. The fuel assembly according to claim 24 wherein said first section exists in an area including at least a portion ranging between two-thirds and five-sixth of the length of the effective fuel area measured upwardly from the lower end portion of the effective fuel area. 26. The fuel assembly according to claim 24 wherein an interposed member made of a material which has a significantly reduced enrichment of a fissile nuclide or in which the fissile nuclide does not exist at all is inserted into said first fuel rod at the portion in which enrichment of a fissile nuclide is significantly reduced or the fissile nuclide does not exist at all. 27. The fuel assembly according to claim 26 wherein said interposed member has an axial length substantially equal to or longer than a thermal neutron diffusion length during a reactor output power operation period. 28. The fuel assembly according to claim 27 wherein the axial length of said interposed member is shorter than one-third of an axial length of the effective fuel area of said second fuel rod. 29. The fuel assembly according to claim 26 wherein said interposed member is occupied with a solid moderator. 30. The fuel assembly according to claim 26 wherein said interposed member is occupied with a liquid moderator. 31. The fuel assembly according to claim 30 wherein said liquid moderator is water. 32. The fuel assembly according to claim 26 wherein a gas plenum is formed in said interposed member. 33. The fuel assembly according to claim 26 wherein said interposed member is occupied with a depleted uranium. 34. The fuel assembly according to claim 26 wherein said interposed member is occupied with a natural uranium. 35. The fuel assembly according to claim 26 wherein said interposed member is occupied with a ceramic material suppressing neutron absorption characteristics at a final stage of a reactor operation cycle. 36. The fuel assembly according to claim 35 wherein said ceramic material is a porous heat-proof ceramic. 37. The fuel assembly according to claim 26 wherein a burnable poison having a concentration to an extent such that said burnable poison will vanish at a final stage of a reactor operation cycle is contained in said interposed member. 38. The fuel assembly according to claim 26 wherein fuel materials disposed adjacent to said interposed member contained a burnable poison having a concentration to an extent such that said burnable poison will vanish at the final stage of the reactor operation cycle. 39. The fuel assembly according to claim 26 wherein a plural number of said first fuel rods provided with the interposed member are arranged linearly. 40. The fuel assembly according to claim 26 wherein a plural number of said first fuel rods provided with the interposed member are arranged in a linearly crossing pattern. 41. The fuel assembly according to claim 24 wherein said first fuel rod is a short fuel rod having an axial length shorter than said second of the fuel rod provided with the interposed zone and comprises a fuel rod which is provided with a removed portion on an axial level including a portion at which subcriticality is made small at a period in which maintenance of reactor shut-down margin is made difficult during the reactor operation period. 42. The fuel assembly according to claim 41 wherein a plural number of said short fuel rods are arranged linearly. 43. The fuel assembly according to claim 41 wherein a plural number of said short fuel rods are arranged in a linearly crossing pattern. 44. The fuel assembly according to claim 41 wherein the lower end of said short fuel rod lies substantially in the same plane as the lower end of said second fuel rod provided with the interposed zone lies. 45. The fuel assembly according to claim 41 wherein a power spike suppressing member is disposed above a top portion of a fuel material filled in said short fuel rod. 46. The fuel assembly according to claim 2, wherein said interposed zone comprises an output power spike suppressing member comprising at least one of depleted uranium, natural uranium, a burnable poison, and a non-burnable oxide material. 47. The fuel assembly according to claim 46, wherein said interposed zone comprises a burnable poison. 48. The fuel assembly according to claim 46, wherein said interposed zone comprises a non-burnable oxide material. 49. The fuel assembly according to claim 24, wherein said interposed zone comprises an output power spike suppressing member selected from the group consisting of depleted uranium, natural uranium, a burnable poison, and a non-burnable oxide material. 50. The fuel assembly according to claim 49, wherein said interposed zone comprises a burnable poison. 51. The fuel assembly according to claim 49, wherein said interposed zone comprises a non-burnable oxide material. 52. A fuel assembly for a nuclear reactor of the type in which a number of fuel rods, each constructed by filling a clad with a fuel material, are arranged in a bundle, comprising: 53. The fuel assembly according to claim 1, further comprising a tube through which a moderator passes, said tube disposed in a center portion of the bundle. 54. The fuel assembly according to claim 16, further comprising a tube through which a moderator passes, said tube disposed at the intersection of said linearly crossing shape. 55. The fuel assembly according to claim 24, further comprising a tube through which a moderator passes, said tube disposed in a center portion of the bundle. 56. The fuel assembly according to claim 40, further comprising a tube through which a moderator passes, said tube disposed at the intersection of said linearly crossing shape. 57. The fuel assembly according to claim 55, wherein said tube comprises a water rod. 58. The fuel assembly according to claim 52, wherein said tube comprises a water rod. 59. The fuel assembly according to claim 2, wherein the central portion of said interposed zone is located at a distance of about three-fourths of the length of said total effective fuel area from the lower end of said total effective fuel area.