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Shevchenko BN350 nuclear fast reactor and desalination plant situated on the shore of the Caspian Sea. The plant generates 135 MW_e and provides steam for an associated desalination plant. View of the interior of the reactor hall.

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.

On average, more neutrons per fission are produced from fissions caused by fast neutrons than from those caused by thermal neutrons. Therefore, there is a much larger excess of neutrons not required to sustain the chain reaction. These neutrons can be used to produce extra fuel, or to transmute long-halflife waste to less troublesome isotopes, such as the Phénix reactor near Cadarache in France, or some can be used for each purpose. Though conventional thermal reactors also produce excess neutrons, fast reactors can produce enough of them to breed more fuel than they consume. Such designs are known as fast breeder reactors.

Water, the most common coolant in thermal reactors, is generally not a feasible coolant for a fast reactor, because it acts as a neutron moderator. However some variants of the Generation IV reactor known as the supercritical water reactor may technically be considered fast neutron reactors.

All current fast reactors are liquid metal cooled. Early reactors used mercury cooling and plutonium metal fuel. NaK cooling is popular in test reactors due to its low melting point. Molten lead cooling has been used in naval propulsion units as well as some other prototype reactors. Some of the newer generation of power stations use molten sodium cooling.

Gas-cooled fast reactors have been researched as well.

In practice sustaining a fission chain reaction with fast neutrons means using relatively highly enriched uranium or plutonium. It is impossible to build a fast reactor using only natural uranium fuel. However, it is possible to build a fast reactor that will breed fuel by producing more fissile material than it consumes. After the initial fuel charge such a reactor can be refueled by reprocessing. Fission products can be replaced by adding natural or even depleted uranium with no further enrichment required. This is the concept of the fast breeder reactor or FBR.

So far, all fast neutron reactors have used either MOX or metal alloy fuel.

Like thermal reactors, fast neutron reactors are controlled by keeping the criticality of the reactor reliant on delayed neutrons, allowing for control utilizing control rods/blades.

Shevchenko BN350 desalination unit. View of the only nuclear-heated desalination unit in the world

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 at Los Alamos, 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.
• EBR-II Prototype for the Integral Fast Reactor.
• The Dounreay fast reactors, DFR (Doureay Fast Reactor) and PFR (Prototype Fast Reactor), in Caithness, in the Highland area of Scotland. DFR commenced operation in 1959 and produced 14MWe. PFR produced 250MWe.
• SEFOR in Arkansas, a 20MWt research reactor which operated from 1969 to 1972.
• Rhapsodie in Cadarache (20 then 40 MW) between 1967 and 1982.
• Phénix, a fast reactor built by France, currently used for power generation and experiments on transmutation of nuclear waste.
• Superphénix, in France, 1200MWe, closed in 1997 due to a political decision and very high costs of operation.
• Integral Fast Reactor, a design of fast rector with an integral fuel cycle, developed and cancelled in the USA in the 1990's.
• Monju reactor, 300MWe, in Japan.
• BN-350, constructed by the Soviet Union in Shevchenko (today's Aqtau) on the Caspian Sea, 130MWe plus 80,000 tons of fresh water per day.
• BN-600, constructed by the Soviet Union, 600MWe.
• Fast Flux Test Facility, 400MWt, Operated flawlessly from 1982 to 1992, at Hanford Washington, now deactivated, liquid sodium is drained with argon backfill under care and maintenance.

As of 2006, new FBRs are planned or under construction in China and India, and construction had resumed on the BN-800 in Russia.

• Fast breeder reactor
• Sodium-cooled fast reactor
• Integral Fast Reactor
• Lead-cooled fast reactor
• Gas-cooled fast reactor
• Generation IV reactor

• ANL report on EARLY SOVIET FAST REACTORS
• Article on recent work on fast neutron reactors in Scientific American, December, 2005