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Liquid metal cooled reactor

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
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

 where the primary coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

 is a liquid metal
Liquid metal
Liquid Metal may refer to:* Metals in a liquid state** Metals which have a relatively low melting point, such as tin or lead.** A fusible alloy** A poor metal.* Liquid Metal , a radio channel* XM Liquid Metal, a radio channel...

. Liquid metal cooled reactors were first adapted for nuclear submarine
Nuclear submarine
A nuclear submarine is a submarine powered by a nuclear reactor . The performance advantages of nuclear submarines over "conventional" submarines are considerable: nuclear propulsion, being completely independent of air, frees the submarine from the need to surface frequently, as is necessary for...

 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
Power density
Power density is the amount of power per unit volume....

 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.

Design


In practice, all liquid metal cooled reactors are fast neutron reactor
Fast neutron reactor
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...

s, 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
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

 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
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...

.

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
Power density
Power density is the amount of power per unit volume....

 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.
Liquid metal coolants
CoolantMelting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...

Boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

Sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

97.72°C, (207.9°F) 883°C, (1621°F)
NaK
NaK
NaK, or sodium-potassium alloy, an alloy, of potassium , and sodium , is usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with water and may catch fire when exposed to air, so must be handled with special precautions...

785Cº, (1445Fº)
Mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

356.73°C (674.11°F)
Lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

327.46 °C, (621.43 °F) 1749 °C, (3180 °F)
Lead-bismuth eutectic
Lead-bismuth eutectic
Lead-Bismuth Eutectic or LBE is a eutectic alloy of lead and bismuth used as a coolant in some nuclear reactors, and is a proposed coolant for the lead-cooled fast reactor, part of the Generation IV reactor initiative....

123.5°C, (254.3°F) 1670°C, (3038°F)

Mercury


Clementine
Clementine (nuclear reactor)
Clementine was the code name for the world's first fast neutron nuclear reactor. It was an experimental scale reactor. The maximum output was 25 kW and was fueled by plutonium and cooled by liquid mercury. Clementine was located at Los Alamos National Laboratory in Los Alamos, New Mexico...

 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
Neutron cross-section
In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power...

, 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. Neutron activation of sodium also causes these liquids to become intensely radioactive during operation, though the half-life is short hence their radioactivity doesn't pose an additional disposal concern.

Lead



Lead has excellent neutron properties (reflection, low absorption) and is a very potent radiation shield against gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

s. 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
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...

, but lead-bismuth eutectic
Lead-bismuth eutectic
Lead-Bismuth Eutectic or LBE is a eutectic alloy of lead and bismuth used as a coolant in some nuclear reactors, and is a proposed coolant for the lead-cooled fast reactor, part of the Generation IV reactor initiative....

 is highly corrosive to most metals used for structural materials.

Submarines


The Soviet
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 November class submarine
November class submarine
The Project 627 class submarine was the Soviet Union's first class of nuclear-powered submarines. The North Atlantic Treaty Organization used the standard radio communication phonetic alphabet to denote submarine classes...

 K-27
Soviet submarine K-27
The K-27 was the only submarine of Projekt 645 in the Soviet Navy. Project 645 did not have or need its own NATO reporting name. That project produced just one test model nuclear submarine, one which incorporated a pair of experimental VT-1 nuclear reactors that used a liquid-metal coolant ,...

 and all seven Alfa class submarine
Alfa class submarine
The Soviet Union/Russian Navy Project 705 was a class of hunter/killer nuclear powered submarines. The class is also known by the NATO reporting name of Alfa...

s used reactors cooled by a lead-bismuth alloy (VT-1 reactor
VT-1 reactor
The VT-1 reactor was the nuclear fission reactor used in a pair to power Soviet submarine K-27 as part of the Soviet Navy's Project 645 Кит-ЖМТ. It is a liquid metal cooled reactor , using highly enriched uranium-235 fuel to produce 73 MWt of power....

s in K-27; BM-40A
BM-40A reactor
The BM-40A reactor is the nuclear fission reactor used to power four of the seven boats of the Soviet Navy's Project 705 Лира fourth generation submarines...

 and OK-550 reactor
OK-550 reactor
The OK-550 reactor is the nuclear fission reactor used to power three of the seven boats of the Soviet Navy's Project 705 Лира fourth generation submarines...

s in others). Both the Soviet and US navies had earlier constructed prototype attack submarines using LMFR power units.

USS Seawolf (SSN-575)
USS Seawolf (SSN-575)
USS Seawolf , a unique submarine, was the third ship of the United States Navy to be named for the seawolf, the second nuclear submarine, and the only U.S. submarine built with a liquid metal cooled nuclear reactor....

 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 superheater
Superheater
A superheater is a device used to convert saturated steam or wet steam into dry steam used for power generation or processes. There are three types of superheaters namely: radiant, convection, and separately fired...

s, 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
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

.

Nuclear aircraft


Liquid metal cooled reactors were studied by Pratt & Whitney
Pratt & Whitney
Pratt & Whitney is a U.S.-based aerospace manufacturer with global service operations. It is a subsidiary of United Technologies Corporation . Pratt & Whitney's aircraft engines are widely used in both civil aviation and military aviation. Its headquarters are in East Hartford, Connecticut, USA...

 for use in nuclear aircraft
Nuclear aircraft
A nuclear aircraft is an aircraft powered by nuclear energy. Research into them was pursued during the Cold War by the United States and the Soviet Union as they would presumably allow a country to keep nuclear bombers in the air for extremely long periods of time, a useful tactic for nuclear...

 as part of the Aircraft Nuclear Propulsion‎ program.

Power generation


The Sodium Reactor Experiment
Sodium Reactor Experiment
The Sodium Reactor Experiment was a pioneering nuclear power plant built by Atomics International at the Santa Susana Field Laboratory, nearby Simi Valley, California. The reactor operated from 1957 to 1964...

 was an experimental sodium-cooled nuclear reactor sited in a section of the Santa Susana Field Laboratory
Santa Susana Field Laboratory
The Santa Susana Field Laboratory is a complex of industrial research and development facilities located on a 2,668 acre portion of the Southern California Simi Hills in Simi Valley, California, used mainly for the testing and development of Liquid-propellant rocket engines for the United States...

 then operated by the Atomics International division of North American Aviation
North American Aviation
North American Aviation was a major US aerospace manufacturer, responsible for a number of historic aircraft, including the T-6 Texan trainer, the P-51 Mustang fighter, the B-25 Mitchell bomber, the F-86 Sabre jet fighter, the X-15 rocket plane, and the XB-70, as well as Apollo Command and Service...

. 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
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 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
Enrico Fermi Nuclear Generating Station
The Enrico Fermi Nuclear Generating Station is a nuclear power plant on the shore of Lake Erie near Monroe, in Frenchtown Charter Township, Michigan. It is approximately halfway between Detroit, Michigan, and Toledo, Ohio. It is also visible from parts of Amherstburg, Ontario. Two units have been...

 in Monroe County, Michigan
Monroe County, Michigan
Monroe County is a county in the U.S. state of Michigan. According to the U.S. Census Bureau, the 2010 population is 152,021. The largest city and county seat is Monroe. The U.S. Census Bureau defines all of Monroe County as conterminous with the Monroe Metropolitan Area...

 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
Dounreay
Dounreay is the site of several nuclear research establishments located on the north coast of Caithness, in the Highland area of Scotland...

 in Caithness, in the far north of Scotland, the United Kingdom Atomic Energy Authority
United Kingdom Atomic Energy Authority
The United Kingdom Atomic Energy Authority is a UK government research organisation responsible for the development of nuclear fusion power. It is an executive non-departmental public body of the Department for Business, Innovation and Skills and was formerly chaired by Lady Barbara Judge CBE...

 (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
BN-600 reactor
The BN-600 reactor is a sodium-cooled fast breeder reactor, built at the Beloyarsk Nuclear Power Station, in Zarechny, Sverdlovsk Oblast, Russia. Designed to generate electrical power of 600 MW in total, the plant dispatches 560 MW to the Middle Urals power grid...

 and BN-350
BN-350 reactor
The BN-350 was a sodium-cooled fast reactor nuclear power plant located at Aktau , Kazakhstan, situated on the shore of the Caspian Sea. Construction of the BN-350 Fast breeder reactor began in 1964, and the plant first produced electricity in 1973...

 and U.S. EBR-II nuclear power plant
Nuclear power plant
A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...

s were sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 cooled. EBR-I used a liquid metal alloy, NaK
NaK
NaK, or sodium-potassium alloy, an alloy, of potassium , and sodium , is usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with water and may catch fire when exposed to air, so must be handled with special precautions...

, for cooling. NaK is liquid at room temperature. Liquid metal cooling is also used in most fast neutron reactor
Fast neutron reactor
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...

s including fast breeder reactors.

Many Generation IV reactor
Generation IV reactor
Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...

 studies are liquid metal cooled:
  • Sodium-Cooled Fast Reactor (SFR)
  • Lead cooled fast reactor
    Lead cooled fast reactor
    The lead-cooled fast reactor is a nuclear power Generation IV reactor that features a fast neutron spectrum, molten lead or lead-bismuth eutectic coolant. Options include a range of plant ratings, including a number of 50 to 150 MWe units featuring long-life, pre-manufactured cores...