[en] A shroud is disposed coaxially at the inner side of a pressure vessel, and a reactor core having substantially circular cross section is disposed in the shroud. The reactor core is constituted with a large number of fuel assemblies each having a square cross section. A cross-like control rod is inserted to the central gap of the fuel assembly at the center or a portion of the outer circumference of the reactor core. A natural uranium with less emission of fast neutrons, reprocessed and recovered uranium having not more than 1.5wt% of uranium 235, thorium or depleted uranium generated upon enrichment of uranium is used for fuels of fuel assemblies in adjacent with the pressure vessel and the shroud among fuel assemblies at the outer circumference of the reactor core. With such a constitution, the maximum value of localized fast neutron flux irradiation to the shroud and the pressure vessel is reduced. Accordingly, safety and extension of the service life of the reactor can be attained. (I.N.)