Fueling method

Purpose: To enable calculation for the reactor power from each of fuels upon successive replace of fresh fuels with long fuel effective length, without changing the reactor performance calculating device. Method: In a operation transfer cycle of replacing fuels charged in the reactor core, for example, each by 1/3 on every operation cycle of the reactor core, 1/3 of the fuels with the effective fuel length A is replaced with fresh fuels with the effective fuel length B. In this case, the fresh fuels constitute such an arrangement that a layer of natural uranium or depleted uranium is added to the upper portion of the reactor core containing the fuels with the effective fuel length A. The neutron reactivity of the natural uranium or depleted uranium is very much low and the reactor power from this portion is negligibly small. Accordingly, the reactor core performance calculation for the reactor core can be made as that for the reactor core with the effective fuel length A. (Seki, T.)