Patent Number: 042808740
Section: claims

1. In a structure of a nuclear reactor core having axially upper and lower regions and radially central and peripheral regions and comprising predetermined fuel assemblies utilized with other fuel assemblies, each of said predetermined fuel assemblies including a plurality of first fuel rods each containing a fissile material and a burnable poison, and a plurality of second fuel rods each containing a fissile material without burnable poison, said first and second rods extending in the axial direction of said nuclear reactor core when said predetermined fuel assembly is loaded in said nuclear reactor core, the improvement comprising said predetermined fuel assembly having a plurality of said first fuel rods including one group of first fuel rods having respectively an upper part having an upper end portion and a lower part having a lower end portion, said upper and lower parts being separated at the axial middle thereof, said lower part being arranged to be within the axially lower region of said nuclear reactor core and packed with the fissile material of a uniform enrichment, and said upper part being arranged to be within the axially upper region of said nuclear reactor core and packed with the fissile material of a uniform enrichment more abundant than that packed in said lower part, and said one group of said first fuel rods containing the burnable poison having a concentration decreasing from the axial middle toward said upper and lower end portions, and the remaining first fuel rods of said predetermined fuel assembly having respectively and upper end portion, a lower end portion and a middle portion, said upper end portion being arranged to be within the axially upper region of said nuclear reactor core and packed with the fissile material without burnable poison, said lower end portion being arranged to be within the axially lower region of said nuclear reactor core and packed with the fissile material without burnable poison, and said middle portion being packed with the fissile material containing the burnable poison, a plurality of said predetermined fuel assemblies being distributed over said nuclear reactor core in such a way that the concentration of the burnable poison in said first fuel rods and the number of said first fuel rods decrease from said radially central region towards said peripheral region. 2. A structure of a nuclear reactor core as claimed in claim 1, wherein said fissile material is uranium-235. 3. A structure of a nuclear reactor core as claimed in claim 2, wherein said burnable poison is selected from the group consisting of gadolinium, hafnium, europium and compounds thereof. 4. A structure of a nuclear reactor core as claimed in claim 1, further comprising control rods each surrounded by a group of four fuel assemblies formed of a combination of said first and second fuel assemblies, said first fuel assembly comprising said first fuel rods and said second fuel rods, said second fuel assembly comprising only said second fuel rods, and the ratio of the number of said first fuel assemblies to that of the second fuel assemblies in the group surrounding each said control rod being 1:3. 5. A structure of a nuclear reactor core as claimed in claim 4, wherein said fissile material is uranium235. 6. A structure of a nuclear reactor core as claimed in claim 5, wherein said burnable poison is selected from the group consisting of gadolinium, hafnium, europium and compounds thereof. 7. A structure of a nuclear reactor core as claimed in claim 4, wherein said first fuel assemblies have an exposure degree less than that of said second fuel assemblies. 8. A structure of a nuclear reactor core as claimed in claim 7, wherein said second fuel assemblies in the group surrounding each said control rod have different exposure degrees from one another. 9. A structure of a nuclear reactor core as claimed in claim 4, wherein said fissile material is uranium-235. 10. A structure of a nuclear reactor core as claimed in claim 9, wherein said burnable poison is selected from the group consisting of gadolinium, hafnium, europium and compounds thereof. 11. A structure of a nuclear reactor core as claimed in claim 1 or 4, wherein in each of said first fuel assemblies having said first fuel rods, the concentration of said burnable poison contained in said first fuel rods and the number of said first fuel rods, at the beginning of an exposure cycle, is so selected that said burnable poison is completely diminished at the end of the same exposure cycle. 12. A structure of a nuclear reactor core as claimed in claim 1 or 4, wherein the nuclear reactor core is the core of a boiling water reactor. 13. In a structure of a nuclear reactor core having an axis, and axially upper and lower regions and radially central and peripheral regions, and loaded with fuel assemblies each extending substantially parallel with said axis, said fuel assemblies comprising a plurality of first fuel assemblies which respectively have a plurality of first fuel rods each packed with a fissile material containing a burnable poison and a plurality of second fuel rods each packed with the fissile material without burnable poison, and a plurality of second fuel assemblies which respectively have a plurality of said second fuel rods and no first fuel rods, the improvement comprising both said first and second fuel assemblies each having an upper part which are separated at the axial middle thereof, said lower part being arranged to be within said axially lower region and having therein the fissile material of a uniform enrichment, and said upper part being arranged to be within said axially upper region and having the fissile material of a uniform enrichment more abundant than that in said lower part, each said first fuel assembly having a plurality of said first fuel rods including one group of said first fuel rods having respectively axially upper and lower ends and an axial middle, and being packed with the fissile material containing the burnable poison having concentration decreasing from said axial middle toward said upper and lower ends, and the remaining first fuel rods of said first fuel assemblies having respectively axially upper and lower end portions and an axially middle portion, said axially upper and lower end portions being packed with the fissile material without burnable poison, and said axially middle portion being packed with the fissile material containing the burnable poison, said first fuel assemblies being distributed over said nuclear reactor core in such a way that the concentration of a burnable poison and the number of said first fuel rods in said first fuel assembly are larger in said radially central region than in said peripheral region. 14. A structure of a nuclear reactor core as claimed in claim 13, further comprising control rods each surrounded by a group of four fuel assemblies formed of a combination of said first and second fuel assemblies, and the ratio of the number of said first fuel assemblies to that of the second fuel assemblies in the group surrounding each said control rod being 1:3. 15. A structure of a nuclear reactor core as claimed in claim 13 or 14, wherein the nuclear reactor core is the core of a boiling water reactor.