BWR type reactor with natural convection system

Solid moderators are formed to the outside of a channel box for a fuel assembly constituting a reactor core. Solid moderators are disposed such that they are present from the upper end and at least for about 1/3 of the effective fuel portion of the fuel assembly. This can ensure neutron moderating effect in the out channel portion to thereby flatten the axial power distribution. Further, even when a pressure is increased upon transient state, only the voids in the inner channel portion are crushed to reduce the change of void coefficient. Accordingly, since the amount of the neutral fluxes increased is small and the power fluctuation range can be reduced, the operation margin is increased and the transient characteristics can be improved. Further, since the axial power distribution and the transient characteristics can be improved, nuclear fuel assemblies at high reactivity can be disposed to the inside of the reactor core, by which the burning efficiency can be increased to improve the fuel economy. (T.M.)