Liquid metal cooled reactor
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A liquid metal cooled nuclear reactor, liquid metal fast reactor or LMFR is an advanced type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation applications. They have safety advantages because the reactor doesn't need to be kept under pressure, and they allow a much higher power density than traditional coolants. Disadvantages include difficulties associated with inspection and repair of a reactor immersed in opaque molten metal, and depending on the choice of metal, corrosion and/or production of radioactive activation products may be an issue.
In practice, all liquid metal cooled reactors are fast neutron reactors, and to date most fast neutron reactors have been liquid metal cooled fast breeder reactors (LMFBRs), or naval propulsion units. The liquid metals used typically need good heat transfer characteristics. Fast neutron reactor cores tend to generate a lot of heat in a small space when compared to reactors of other classes. A low neutron absorption is desirable in any reactor coolant, but especially important for a fast reactor, as the good neutron economy of a fast reactor is one of its main advantages. Since slower neutrons are more easily absorbed, the coolant should ideally have a low moderation of neutrons. It is also important that the coolant does not cause excessive corrosion of the structural materials, and that its melting and boiling points are suitable for the reactor's operating temperature.
Ideally the coolant should never boil as that would make it more likely to leak out of the system, resulting in a loss-of-coolant accident. Conversely, if the coolant can be prevented from boiling this allows the pressure in the cooling system to remain at neutral levels, and this dramatically reduces the probability of an accident. Some designs immerse the entire reactor and heat exchangers into a pool of coolant, virtually eliminating the risk that inner-loop cooling will be lost.
Coolant properties 
While pressurised water could theoretically be used for a fast reactor, it tends to slow down neutrons and absorb them. This limits the amount of water that can be allowed to flow through the reactor core, and since fast reactors have a high power density most designs use molten metals instead. Water's boiling point is also much lower than most metals demanding that the cooling system be kept at high pressure to effectively cool the core.
|Coolant||Melting point||Boiling point|
|Sodium||97.72°C, (207.9°F)||883°C, (1621°F)|
|NaK||-11°C, (12°F)||785°C, (1445°F)|
|Mercury||-38.83°C, (-37.89°F)||356.73°C (674.11°F)|
|Lead||327.46 °C, (621.43 °F)||1749 °C, (3180 °F)|
|Lead-bismuth eutectic||123.5°C, (254.3°F)||1670°C, (3038°F)|
Clementine was the very first liquid metal cooled nuclear reactor and used mercury coolant, thought to be the obvious choice since it is liquid at room temperature. However, because of disadvantages including high toxicity, high vapor pressure even at room temperature, low boiling point, producing noxious fumes when heated, relatively low thermal conductivity, and a high neutron cross-section, it has fallen out of favor.
Sodium and NaK 
Sodium and NaK don't corrode steel to any significant degree and are compatible with many nuclear fuels, allowing for a wide choice of structural materials. They do, however, ignite spontaneously on contact with air and react violently with water, producing hydrogen gas. This was the case at the Monju Nuclear Power Plant in a 1995 accident and fire. Neutron activation of sodium also causes these liquids to become intensely radioactive during operation, though the half-life is short and therefore their radioactivity doesn't pose an additional disposal concern.
Lead has excellent neutron properties (reflection, low absorption) and is a very potent radiation shield against gamma rays. The higher boiling point of lead provides safety advantages as it can cool the reactor efficiently even if it reaches several hundred degrees Celsius above normal operating conditions. However, because lead has a high melting point and a high vapor pressure, it is tricky to refuel and service a lead cooled reactor. The melting point can be lowered by alloying the lead with bismuth, but lead-bismuth eutectic is highly corrosive to most metals used for structural materials.
The Soviet November-class submarine K-27 and all seven Alfa-class submarines used reactors cooled by a lead-bismuth alloy (VT-1 reactors in K-27; BM-40A and OK-550 reactors in others). Both the Soviet and US navies had earlier constructed prototype attack submarines using LMFR power units.
USS Seawolf was the second nuclear submarine, and the only U.S. submarine to have a sodium-cooled nuclear power plant. It was commissioned in 1957, but it had leaks in its superheaters, which were bypassed. In order to standardize the reactors in the fleet, the submarine's sodium-cooled reactor was removed starting in 1958 and replaced with a pressurized water reactor.
Nuclear aircraft 
Power generation 
The Sodium Reactor Experiment was an experimental sodium-cooled nuclear reactor sited in a section of the Santa Susana Field Laboratory then operated by the Atomics International division of North American Aviation. In July 1959, the Sodium Reactor Experiment suffered a serious incident involving the partial melting of 13 of 43 fuel elements and a significant release of radioactive gases. The reactor was repaired and returned to service in September 1960 and ended operation in 1964. The reactor produced a total of 37 GW-h of electricity.
Fermi 1 in Monroe County, Michigan was an experimental, liquid sodium-cooled fast breeder reactor that operated from 1963 to 1972. It suffered a partial nuclear meltdown in 1963 and was decommissioned in 1975.
At Dounreay in Caithness, in the far north of Scotland, the United Kingdom Atomic Energy Authority (UKAEA) operated the Dounreay Fast Reactor (DFR), using NaK as a coolant, from 1959 to 1977, exporting 600 GW-h of electricity to the grid over that period. It was succeeded at the same site by PFR, the Prototype Fast Reactor, which operated from 1974 to 1994 and used liquid sodium as its coolant.
The Soviet BN-600 and BN-350 and U.S. EBR-II nuclear power plants were sodium cooled. EBR-I used a liquid metal alloy, NaK, for cooling. NaK is liquid at room temperature. Liquid metal cooling is also used in most fast neutron reactors including fast breeder reactors such as the Integral Fast Reactor.
Many Generation IV reactor studies are liquid metal cooled:
- Bunker, Merle E. "Early Reactors From Fermi’s Water Boiler to Novel Power Prototypes" a chapter in Los Alamos Science - Winter/ Spring 1983 Edition Page 128. Published by Los Alamos National Laboratory and available here: http://library.lanl.gov/cgi-bin/getfile?00416628.pdf
- The Decay of the Atomic Powered Aircraft Program
- Ashley, R.L.; et. al. (1961). SRE Fuel Element Damage, Final Report of the Atomics International Ad Hoc Committee. NAA-SR-4488-supl.