FUEL ASSEMBLY FOR BOILING WATER REACTOR

PROBLEM TO BE SOLVED: To optimize H/U ratio of a highly enriched fuel assembly and improve fuel economy by properly controlling the number of second fuel rods biased to the opposite to control rod side region. SOLUTION: When fuel assembly inside is separated by a boundary plane 110 in vertical direction into control rod side region 111 and the opposite to the control rod side region 112, the number of second fuel rods 101b arranged in the opposite to the control rod side region 112 is made larger than the number of second fuel rods 101b arranged in the control rod side region 111. By this constitution, the fuel quantity can be reduced by the shorter length of the second fuel rods 101b, and the H/U ratio can be increasing by increasing the water amount in the opposite to the control rod side region 112 owing to the saturated water region produced in the upper part of the second fuel rods 101b. While in the control rod side region 111, the H/U ratio can be suppressed by suppressing the decrease of fuel quantity and the increase of water amount. Even at high enrichment of at least 3.9 wt.% in average enrichment, the H/U ratio can be optimized.