[en] In a horizontal cross section in the vicinity of the upper end portion of short fuel rods, the enrichment degree of fuel rods at the outermost circumferential corner is decreased to less than 60% relative to the average enrichment degree in the horizontal cross section, to reduce a cold temperature power peaking in the outermost circumferential corners. Next, in the second layer from the outer circumference in a 9 x 9 arrangement, gadolinia-incorporated fuel rods are disposed each at a position in adjacent with the corner, to reduce the cold temperature power peaking for the fuel rods other than the corners. Then, the cold temperature power peaking is made substantially the same value as that during power operation, so that local increase of neutron fluxes is suppressed to suppress increase of the maximum fuel rod enthalpy upon control rod dropping accidents at reactor startup. As a result, a margin relative to a threshold value causing a fuel rod failure can be increased. (N.H.)