MOX fuel assembly for PWR

In a fuel assembly, MOX fuel rods containing fissionable Pu are bundled in substantially regular square matrix except for four corners and the outer side of control rod guide tubes at the outermost circumference. UO2 fuel rods to which burnable poisons comprising gadolinium (Gd2O3) and erbium (Er2O3) are mixed are disposed at the four corners and the outer side of the control rod guide tubes. Further, low enrichment degree MOX fuel rods are disposed to the outermost circumferential portion except for each of the corners and portions in diagonally adjacent with each of the corners. Highly enrichment degree MOX fuel rods are disposed to the remaining central portion of the fuel assembly. With such a constitution, power peaking at the outer circumferential portion of the fuel assembly is suppressed, and the number of division for the enrichment degree in the fuel assembly can be reduced, thereby enabling to reduce fuel forming cost. (I.N.)