A fast neutron reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons. Such a reactor needs no neutron moderator, but must use fuel that is relatively rich in fissile material when compared to that required for a thermal reactor.

In practice, this means using relatively highly enriched uranium or plutonium. It is impossible to build a fast reactor to using only natural uranium fuel. However, it is possible to build a fast reactor that, after the initial fuel charge, will produce more fissile material than it consumes, and can from then on be refueled using natural uranium with no further enrichment required. This is the concept of the fast breeder reactor or FBR.

Because absorption in the moderator is a major loss of neutrons in a thermal reactor, a fast reactor has an inherently superior neutron economy; that is, there are excess neutrons not required to sustain the chain reaction. These neutrons can be used to produce extra fuel, as in the fast breeder reactor, or to transmute long-halflife waste to less troublesome isotopes, see Phenix, or some can be used for each purpose.

Early fast reactors used mercury cooling and plutonium metal fuel; later, NaK cooling was used, and molten lead cooling for naval propulsion units. The latest generation of power stations use MOX fuel and molten sodium cooling.

Fast reactors include:

• Small lead-cooled fast reactors used for naval propulsion, particularly by the Soviet Navy.
• CLEMENTINE, the first fast reactor, built in 1946 in New Mexico. Plutonium metal fuel, mercury coolant, power 25 kW thermal, used for research, especially as a fast neutron source.
• EBR-I at Idaho Falls, which in 1951 became the first reactor to generate significant amounts of electrical power.
• Dounreay Fast Reactor (DFR) at Dounreay, Scotland, which commenced operation in 1959 and produced 14MWe.
• Prototype Fast Reactor (PFR), 250MWe, also at Dounreay.
• Phénix, the first fast reactor built by France, currently used for experiments on transmutation of nuclear waste.
• Superphénix, in France, 1200MWe, closed in 1997 due to high costs of operation compared to PWRs.
• Monju reactor, 300MWe, in Japan, the only FBR power station still operating in 2004.
• BN-350 , constructed by the Soviet Union on the Caspian Sea, 130MWe plus 80,000 tons of fresh water per day.
• BN-600 , constructed by the Soviet Union, 600MWe.

As of 2004, new FBRs are planned or under construction in China and India.