| 摘要 |
PURPOSE: To increase the burn-up of fuel assembly while sufficiently keeping the safety margin such as thermal margin, furnace shotdown margin or the like of a reactor, with low increase of the average enrichment. CONSTITUTION: Two water rods of large diameter are installed in adjacent to each other in region where seven fuel rods can be installed. A short fuel rod and a low fuel rod are used as the fuel rod. The natural uranium regions are formed at the upper and lower edges of a fuel effective length region of the fuel assembly. An enriched uranium region located between the natural uranium region, has the upper, centeral and lower regions, and the average enrichment is successively lowered by the order of the central region, the lower region and the upper region. The difference of the average enrichment between the centeral region and the lower region is smaller than that between the central region and the upper region. The density of the burnable position of the upper region is lower than that of the other region in the enriched uranium region.
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