Fast neutron reactor

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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.

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, such as the Phénix reactor near Cadarache in France, or some can be used for each purpose.

Nuclear reactor design

Coolant

Most fast reactors are liquid metal cooled. Early 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.

Nuclear fuel

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 adding natural or even depleted uranium with no further enrichment required. This is the concept of the fast breeder reactor or FBR.

Control

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

Examples and Uses

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 high costs of operation compared to PWRs.
• 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 on the Caspian Sea, 130MWe plus 80,000 tons of fresh water per day.
• BN-600, constructed by the Soviet Union, 600MWe.

See also

• Fast breeder reactor (FBR).

• Breeder reactor.

External links and references

[ANL report on EARLY SOVIET FAST REACTORS]

[Article on recent work on fast neutron reactors in Scientific American, December, 2005]

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