Reactor core

A BWR reactor core having a two-stream constitution using two kinds of fuel assemblies with different enrichment degrees as exchange fuels is an effective means for improving the flexibility of cycle length. However, since the reactivity performance is quite different with each other and power mismatching is increased, it leads to the degradation of a minimum critical power ratio, in other words, degradation of an operation margin. In view of the above, the difference of the enrichment degree is defined within a range from 0.2 to 0.8 wt% of U-235 and most of fuel assemblies comprising fresh fuels and fuels for the second cycle with a high enrichment degree are disposed near control cells. In coexistence of the different kinds of fuels, the power of fuels with the high enrichment degree is relatively higher and that of fuels with a low enrichment degree is lower, thereby increasing radial power peaking coefficient. Since the power of the fuels at the vicinity of the control cell is relatively low, the increase of a radial peaking coefficient is improved by this fuel arrangement. The flexibility of the cycle length can be improved without deteriorating the minimum critical power ratio. (N.H.)