| 摘要 |
PURPOSE:To enable suppression of down-grading of a thermal margin by making a horizontal cross-section of a fuel assembly to be a region a of a group of fuel assemblies which face a wide gap water region and by increasing a reference amount of a U-235 to be contained in a region b of the group of the fuel assemblies, other than the region a in the wide gap water region side. CONSTITUTION:Fuel rods 2 are arranged in a square grid shape and a large diameter water rod 3 is arranged at the center part. At that situation, a fuel region is divided into 4 fuel groups, that is, a fuel group (region a4) which faces a wide gap water region, a fuel group (region b5) in the wide gap water region side except the region a, a fuel group (region c6) which faces a narrow gap water region and a fuel group (region d7) in the narrow gap water region side except the region c. And then, an amount of U-235 contained in each region 4 (region a), region 5 (region b), region 6 (region c) and region 7 (region d) is controlled respectively, according to an enrichment distribution, and, by increasing the the enrichment of a fuel not adjoining to the rod 3, a relative amount of the U-235 in the region b is made to be the maximum, and, on the other hand, the relative amount of the U-235 in the region a is made to be the minimum in each region.
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