Fuel assembly

Purpose: To flatten the power distribution by varying the degree of enrichment in the vertical direction of a fuel assembly for use in BWR type reactor, in which the reactivity is lowered in the upper portion than in the lower portion of at least one of corner rods to control the control rod hysteresis effect, increase the fuel safety and improve the reactor operation efficiency. Constitution: The reactor core is divided into an upper region and a lower region at a position between the upper end and 1/3 - 7/12 of the entire length therebelow along the axial direction of a fuel rod, in which the average degree of enrichment for uranium of the fuel assembly is made higher in the upper portion and lower in the lower portion, while on the other hand, degree of enrichment of the corner rod on the side of the control rod is made lower in the upper portion than in the lower portion. As the result, increase in the power peaking due to the generation of voids can be reduced to thereby effectively suppress the control rod hysteresis effect. While the uranium enrichment degree in one of the rods is 1.2% in the upper portion and 1.4% in the lower portion, the enrichment degree in the other of the rods is 1.7 - 2.5% in the upper portion and 1.2 - 2.1% in the lower portion. (Nakamoto, H.)