[en] Fuel assemblies to be loaded in the inner region of a reactor core tank comprise plutonium/uranium mixed oxide fuels of low fission product mass, and fuel assemblies to be loaded in the outer region comprise plutonium/uranium mixed oxide fuels of high fission product mass. Further, the fission product mass of the fuel assemblies loaded in four central channels is identical with fuel assemblies in the inner region but the fuel assemblies are mixed with gadolinia to the outer layer fuel rods as burnable poison. This can decrease power peaking at the center of the reactor when a fuel exchange pattern of 1/4 reactor core rotational symmetry is adopted without increasing the mass of other fissionable products. Accordingly, a reactor core of the 1/4 rotational symmetry capable of flattening a power distribution can be attained. (I.N.)