Reactor core

In a BWR-type reactor, a moderator region having a smaller neutron moderating performance than that of moderators is disposed at least to one of fuel assemblies loaded in (at) a first cycle. That is, since the moderator region having a smaller neutron moderating performance than that of moderators is present in an initially loaded reactor core of the first cycle, neutrons are less moderated than in the fuel assemblies loaded in other cycles. Accordingly, an average neutron energy is increased and the amount of absorbed neutrons due to resonance absorption by U-238 is increased when a void coefficient is increased to increase a negative absolute value of a void reactivity coefficient. Since the negative absolute value of the void reactivity coefficient is increased, a power changing ratio occurred when negative reactivity is applied to the reactor core is made identical to that in other cycles even in the first cycle having relatively large delayed neutron ratio, so that a reactor core with less fluctuation in lowering of the power upon increase of the void coefficient can be constituted throughout the plant service life. (N.H.)