[en] During one cycle operation period from fuel exchange to next fuel exchange, a reactor is shut down temporarily at least for once. At the initial stage of the cycle before the shut down, the reactor is operated without loading fuels at fuel loading positions at the outer circumferential portion of the reactor core and, at the latter stage of the cycle after the shutdown, fresh fuels are loaded at the fuel loading positions at the outer circumference of the reactor core and the reactor is operated to the final stage of the cycle. In view of the above, it is not necessary to control the difference of the surplus reactivity between the former half and the latter half of the cycle by control rods or burnable poisons, so that the amount of the burnable poisons are reduced, the amount of unburnt fuels is reduced and the surplus reactivity at the latter half of the cycle can be increased. (N.H.)