A breeder reactor is a type of nuclear reactor. It is unique in that it produces more fissile material (the material used to sustain fission chain reactions) than it consumes while producing energy.
Breeder reactors achieve these results by using a "fertile material" that is converted into fissile material by irradiation or neutron capture. Some examples of fertile materials include thorium-242, uranium-234, uranium-238, plutonium-238 and plutonium-240.
The first breeder reactor was created in the United States in 1951. They have also been used in France and India. While breeder reactors initially appear to be an almost perfect energy source, the fact that they generate plutonium as a byproduct creates serious complications. Plutonium is extremely radioactive and stays dangerously potent for up to 100,000 years, leading toa disposal problem. Plutonium is also much easier to refine than uranium, making it ideal for the manufacture of nuclear weapons. Security of stored and disposed-of plutonium therefore comes with added expense and danger. These issues can be mitigated somewhat by adding certain elements such as neptunium and curium to the plutonium, which make it much more difficult to refine for weaponry while still allowing it to be used for fuel. However, breeder reactors are still widely considered to be too dangerous and are rarely used.