Nuclear fuel assembly and nuclear reactor core containing said assembly

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

• 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;
• over at least part of the height region of said assembly containing fissile material, the average concentration of fissile material per fuel rod is higher in said region b than said region c by at least 5% and, among all said regions a, b, c, d, is a minimum in said region a, and said fuel assembly further including at least one water rod occupying in said array a central position 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.

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

• 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,
• over at least part of the height region of said assembly containing fissile material, the average concentration of fissile material per fuel rod is higher in said region b than said region c by at least 5% and, among all said regions a, b, c, d, is a minimum in said region a, and said fuel assembly further including at least one water rod arranged in said array wholly within said region c, there being no water rod in said region b.