Nuclear reactor core and fuel exchange method of nuclear reactor

In a BWR type reactor, first MOX fuel assemblies and uranium fuel assemblies are loaded, and at least one of the first MOX fuel assemblies and the uranium fuel assemblies is removed for every predetermined operation cycle. Second MOX fuel assemblies by the same number as that of the removed fuel assemblies are newly loaded. Four fuel assemblies disposed in adjacent with at least one of newly loaded second MOX fuel assemblies are either of the first MOX fuel assemblies or the second MOX fuel assemblies. Then, since the amount of thermal neutrons flowing from the uranium fuel assemblies to the MOX fuel assemblies due to the difference of their absorbing cross section can be reduced and the local power distribution is flattened, the linear power density of the entire reactor core can be flattened. (N.H.)