A breeder reactor is a nuclear fission reactor that has the defining characteristic that it produces more fissionable isotopes than it consumes. A breeder reactor uses the nonfissionable uranium isotope U-238 with small amounts of the fissionable U-235, and produces the radioactive, fissionable plutonium isotope Pu-239. The principle behind a breeder reactor is that the U-238 is bombarded with neutrons produced by the U-235 fission, which converts it into the highly unstable and radioactive uranium isotope U-239, with a half-life of only 23.5 minutes. U-239 decays by beta radiaton and is transmuted into the unstable neptunium isotope Np-239, which has a half-life of around two and a half days. This decays by beta radiation into the desired plutonium, which is far more stable (with a half-life of 2.44 x 10^4 years).
Breeder reactors are usually used for scientific experiments and nuclear weapons production, as well as for producing nuclear fuel to be used by energy-producing nuclear reactors. The world's largest and most efficient breeder reactor is France's Super-Phenix reactor, which entered service in 1984, and produces approximately 20% more fuel than it consumes. This beast is cooled by liquid sodium.