PURPOSE: To lengthen a cycle life by making poisonous concentration of an upper range of fuel rods containing combustible poison lower than that of a center and a lower ranges and making the number of poisonous fuel rods of the center range less than that of the lower range.

CONSTITUTION: 22 nodes except for natural uranium ranges of the 24 nodes in the axial direction of a fuel assembly are divided into an upper, a center and a lower ranges. Gadolinium concentration and number of fuel rods containing gadolinium in each range are set as follows: upper range: 4.0wt.%, the number of 12, center range: 5.5wt.%, the number of 11, lower range: 5.5wt.% the number of 12. The infinite multiplication factor (K∞) of the center range is enlarged and that (K∞) of the lower range is lessened to prevent the output distribution in the axial direction of the fuel assembly from being made a lower peak. Thus, axial output distribution is flattened and a cycle life can be lengthened.