The fast-neutron reactor, commonly referred to as fast reactor, is a nuclear reactor design that uses high-energy or fast spectrum neutrons to produce energy from nuclear fission. In contrast to thermal spectrum reactors such as the light water reactor, fast reactors are able to fission most actinides as opposed to only specific isotopes.
Because the neutron capture cross section is much smaller than in thermal spectrum, fast reactors must expose the fuel to higher neutron flux, which means the fuel has a higher enrichment than in light water reactors. This means that the fuel is more susceptible to meltdown than in thermal reactors. Also, because the fuel of choice, plutonium-239, performs relatively poorly in thermal spectrum, there are no neutron moderating materials such as graphite or liquid water in a fast reactor. This makes the reactor design more complex, as materials and mechanisms that would otherwise control the neutron flux in thermal spectrum must be addressed with other techniques.
The most well-known fast reactor design is the liquid metal fast breeder reactor, and its derivatives, the Integral Fast Reactor and Traveling Wave reactor. These reactors are cooled by liquid sodium or a liquid eutectic mixture of lead and bismuth.