Light water reactors can be refueled with natural uranium and plutonium. When the plutonium provides less than one-half of the fissions, the plutonium fissile inventory ratio will be greater than unity. Therefore, a self-sustained system is obtained. The burnup that can be obtained by this refueling method is ∼4000 MWd/ton. A doubling of the fuel lifetime is obtained (∼8000 MWd/ton) when the spectral shift control method is adopted.