Patent Number: 051768771
Section: claims

1. A nuclear fuel assembly for a nuclear reactor, having a plurality of vertically extending fuel rods arranged side by side in a square array and containing fissile material, said array having two adjacent first sides which, when the fuel assembly is in said reactor core, are next to a control rod region of said core and two adjacent second sides which, when the fuel assembly is in said reactor core, are next to a non-control rod region of the core, wherein, when said array as seen in horizontal cross section is divided into four regions a-d and excluding from any said regions the rods lying on the diagonal line joining opposite corners of the array at which a said first side meets a second side, said four regions being a: the rods in the row and column of said array adjoining said first sides;  b: the rods lying between said region a and said diagonal line;  c: the rods lying between said region d and said diagonal line,  d: the rods in the row and column of said array adjoining said second sides and;  a: the rods in the row and column of said array adjoining said first sides;  b: the rods lying between said region a and said diagonal line;  c: the rods lying between said region d and said diagonal line,  d: the rods in the row and column of said array adjoining said second sides and;  a: the rods in the row and column of said array adjoining said first sides;  b: the rods lying between said region a and said diagonal line;  d: the rods in the row and column of said array adjoining said second side and;  c: the rods lying between said region d and said diagonal line, 2. A fuel assembly according to claim 1 wherein said average concentration of fissile material per fuel rod is higher in said region b than in said region c by an amount in the range of at least 5-10%. 3. A fuel assembly according to claim 1 wherein a plurality of said fuel rods contain burnable poison, there being more of said rods containing said burnable poison in said region c than in said region b. 4. A fuel assembly according to claim 1 having at least one water rod occupying a central position in said array symmetrical with respect to said diagonal line and with respect to the other diagonal line joining opposite corners of the array at which respectively said first sides meet each other and said second sides meet each other. 5. A fuel assembly according to claim 1 having at least one water rod located wholly within said region c, there being no water rod in said region b. 6. A fuel assembly according to claim 1 wherein the number of said fuel rods in said region b is larger than the number thereof in said region c. 7. A fuel assembly according to claim 1 wherein said fissile material is present in fuel pellets in said fuel rods, the average outer diameter of the fuel pellets being larger in said region b than in said region c. 8. A fuel assembly according to claim 1 wherein said fissile material is present in pellets in said fuel rods, and the average pellet density per fuel rod is higher in said region b than in said region c. 9. A fuel assembly according to claim 1 having at least one water rod located in said array, wherein at least one of (i) the number of water rods and (ii) the total flow passage area of said at least one water rod is larger in said region c than in said region b. 10. A fuel assembly according to claim 1 wherein said fissile material comprises at least one of U-235, Pu-239 and Pu-241. 11. A nuclear fuel assembly for a nuclear reactor, having a plurality of vertically extending fuel rods arranged side by side in a square array and containing fissile material, said array having two adjacent first sides which, when the fuel assembly is in said reactor core, are next to a control rod region of said core and two adjacent second sides which, when the fuel assembly is in said reactor core, are next to a non-control rod region of the core, wherein, when said array as seen in horizontal cross section is divided into four regions a-d and excluding from any said regions the rods lying on the diagonal line joining opposite corners of the array at which a said first side meets a said second side, said four regions being 12. A nuclear fuel assembly for a nuclear reactor, having a plurality of vertically extending fuel rods arranged side by side in a square array and containing fissile material, said array having two adjacent first sides which, when the fuel assembly is in said reactor core, are next to a control rod region of said core and two adjacent second sides which, when the fuel assembly is in said reactor core, are next to a non-control rod region of the core, wherein, when said array as seen in horizontal cross section is divided into four regions a-d and excluding from any said regions the rods lying on the diagonal line joining opposite corners of the array at which a said first side meets a said second side, said four regions being 13. A nuclear reactor core containing at least one fuel assembly according to claim 1. 14. A nuclear reactor core containing at least one nuclear fuel assembly according to claim 1, said core having a first water gap constituting a control rod region adjacent said two first sides of said fuel rod assembly and a second water gap comprising a non-control rod region adjacent said two second sides of said fuel assembly, said first water gap being wider than said second water gap. 15. A nuclear reactor core containing at least one nuclear fuel assembly according to claim 11. 16. A nuclear reactor core containing at least one nuclear fuel assembly according to claim 11 said core having a first water gap constituting a control rod region adjacent said two first sides of said fuel rod assembly and a second water gap comprising a non-control rod region adjacent said two second sides of said fuel assembly, said first water gap being wider than said second water gap. 17. A nuclear reactor core containing at least one nuclear fuel assembly according to claim 12. 18. A nuclear reactor core containing at least one nuclear fuel assembly according to claim 12 said core having a first water gap constituting a control rod region adjacent said two first sides of said fuel rod assembly and a second water gap comprising a non-control rod region adjacent said two second sides of said fuel assembly, said first water gap being wider than said second water gap.