A nuclear that produces (breeds) the same kind of fissile material as it burns is called a breeder reactor. Breeder reactors while using plutonium as a fuel can produce more plutonium (Pu-239) than it consumes, by converting non-fissionable U-238, that
predominates in natural uranium ore. These reactors do not use moderator to slow down the neutrons so are Fast Breeder Reactors (FBR