| 摘要 |
PURPOSE: To achieve the target of high burn-up by disposing fuel rods adjacent to both of a plurality of water rods having the maximum average enrichment. CONSTITUTION: Two water rods 3a disposed in the direction of a central diagonal line in a channel box 1a have large diameters which occupy the area of approximately 3 or 5 fuel rods 2a provided in the 9×9 fuel rods and non-boiling water is allowed to flow therein. Here, if a degree of enrichment of the other fuel rods 4a adjacent to both of the rods 3a is set in another degree of the maximum enrichment, local output peaking should be large at the time point when burning advances, and the output of the fuel rods facing the water area of the rods 3a will increase. A margin for 1% plastic strain of gadolinia content fuel rods can be made that or more for the 1% plastic strain of the fuel rods 4a at the time point when the minimum gadolinia of the margin for the 1% plastic strain of the gadolinia content fuel rods burns up. Thereby the degree of the enrichment of the gadolinia content fuel rods can be increased and other degree of fuel average enrichment is easily increased.
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