JP2856728B

PROBLEM TO BE SOLVED: To raise average uranium enrichment and raise average take-out burnup to 45GWd/t without arranging fuel rods with gadolinia (Gd rods) in the outermost of fuel assemblies in the case a restriction of maximum uranium enrichment of 5.0wt.% is given to fuel assemblies for a boiling, water reactor. SOLUTION: The average uranium enrichment of fuel rods 20 to 23 is made 4wt.% or more, the number fraction of Gd rods to the whole fuel rods is made 20 to 30%. The enrichment of them is made in between the pellet minimum uranium enrichment and the pellet maximum uranium enrichment and to be 0.84 to 0.96 of the maximum uranium enrichment. The average uranium enrichment in the fuel assembly cross section in enriched fuel part excluding the blanket region at the upper and lower ends is made 4.5wt.% and more or 4.3wt.% and more. Or the ratio of the average uranium enrichment in the fuel assembly cross section in the particular fuel part and the fuel pellet maximum uranium enrichment is made 1.11 or less or 1.16 or less.