Nuclear reactor core

If an enrichment degree of plutonium is increased, a void coefficient is more increased toward negative side, and the reactivity response is too large relative to small output change to worsen the operation controllability of the reactor core. Then in the present invention, a plurality kinds of fuel assemblies having different U-235 concentration in uranium to be mixed with plutonium are loaded. The burnup degree is attained by controlling the enrichment degree of uranium to be mixed with plutonium and, since the total amount of plutonium is identical even if the isotope composition of plutonium is different, the characteristics of the void coefficient are substantially the same. Further, even if plutonium of any isotope composition is supplied, the enrichment degree of plutonium is not changed and, accordingly, it is not necessary to change the production steps for MOX fuel rods, and the enrichment degree of uranium to be supplied may be controlled in an external plant. With such procedures, substantial Pu-thermal utilization is possible, to provide a great effect of effective resource utilization. (N.H.)