Fuel assemblies

Purpose: To decrease the required amount of uranium in BWR type reactors by improving the conversion ratio and effective burning of nuclear fission materials. Constitution: The reactor core and the fuel assembly are equally divided into upper and lower two stages in the present invention. The fuel assembly is placed in the upper stage of the reactor core for the first one-half period and then placed at the lower reactor core in the remaining one-half period. Since the burn-up degree is smaller at the upper stage of the reactor core and greater at the lower stage of the reactor core, the reactivity is greater in the upper stage than in the lower stage, thereby causing the tendency that the power distribution is distorted greatly upwardly in the reactor core. However, the reactivity is suppressed by the high void ratio at the upper stage portion to thereby attain a burner reactor core having high conversion ratio. (Kamimura, M.)