PURPOSE: To enhance fuel economy by setting the average concentration of combustible toxicants in fuel rod to be arranged in a region contiguous to a moderator rod higher than that in the region other than the outermost layer facing a channel box and the corner parts of second outer layer.
CONSTITUTION: The average concentration of combustible toxicants in a first group fuel rods 4 arranged contiguously to a thick water rod 2 arranged in the center of a fuel assembly is set higher than that of a second group fuel rods 5 arranged in the region other than the region contiguous to a channel box 1 and the corner parts of inner layer. The hatched region 6 represents a region contiguous to a water rod 2 where the thermal neutron flux is high and the hatched region 7 represents a region where the thermal neutron flux is relatively low. Since the combustible toxicants in the region 7 are substantially burnt up simultaneously with those in the region 6, the reactivity is improved resulting in the enhancement of economy.
AOYAMA TADAO
KOYAMA JUNICHI
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