to continue a chain reaction (this is why natural uranium will not work in a bomb).

²³⁸U can be used as a source material for creating plutonium-239, which can in turn be used as nuclear fuel. Breeder reactors carry out such a process of transmutation to convert the fertile isotope ²³⁸U into fissile ²³⁹Pu. It has been estimated that there is anywhere from 10,000 to five billion