Initially loaded reactor core

In an initially loaded reactor core formed by combining various kinds of fuel assemblies having different average enrichment degrees, MOX fuel rods are filled into a portion of fuels of low enrichment degree fuels or a portion of fuels of a low enrichment degree and a medium enrichment degree, and the fuels are loaded to a portion of the inner side of the reactor core except for the outermost circumference. Since a thermal neutron absorbing cross sectional area of plutonium 239 or plutonium 241 as fission products is greater than that of uranium 235, and neutron absorption of plutonium 240 is greater than that of uranium 238, neutron spectrum thereof is made harder than that of uranium fuel assemblies, to reduce a neutron moderating effect. Then, fuel assemblies of low enrichment degree approaches from a state of excess moderation to an optimum moderating state. In addition, since plutonium is accumulated from an initially loaded reactor core, the void reactivity coefficient can be increased. Economy of initially loaded reactor core and transient characteristic can be improved. (N.H.)