In a breeder nuclear reactor as one fuel is depleted, another fuel is formed.

A breeder reactor is a nuclear reactor that creates more fissile material than it consumes. It requires an initial charge of fissile material, such as highly enriched uranium or plutonium, and a supply of fertile material, such as natural uranium, depleted uranium or thorium. Breeder reactors can utilize nearly 100% of the energy contained in uranium and thorium ores, while the reactors currently used for nuclear power generation can use at most 1%.