A liquid metal cooled nuclear reactor, or LMR is a type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for breeder reactor power generation. They have also been used to power nuclear submarines.
Due to their high thermal conductivity, metal coolants remove heat effectively, enabling high power density. This makes them attractive in situations where size and weight are at a premium, like on ships and submarines. Most water-based reactor designs are highly pressurized to raise the boiling point (thereby improving cooling capabilities), which presents safety and maintenance issues that liquid metal designs lack. Additionally, the high temperature of the liquid metal can be used to drive power conversion cycles with high thermodynamic efficiency. This makes them attractive for improving power output, cost effectiveness, and fuel efficiency in nuclear power plants.
Liquid metals, being electrically highly conductive, can be moved by electromagnetic pumps. [1] Disadvantages include difficulties associated with inspection and repair of a reactor immersed in opaque molten metal, and depending on the choice of metal, fire hazard risk (for alkali metals), corrosion and/or production of radioactive activation products may be an issue.
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Liquid metal coolant has been applied to both thermal- and fast-neutron reactors.
To date, most fast neutron reactors have been liquid metal cooled and so are called liquid metal cooled fast reactors (LMFRs). When configured as a breeder reactor (e.g. with a breeding blanket[ definition needed ]), such reactors are called liquid metal fast breeder reactors (LMFBRs).
Suitable liquid metal coolants must have a low neutron capture cross section, must not cause excessive corrosion of the structural materials, and must have melting and boiling points that are suitable for the reactor's operating temperature.
Liquid metals generally have high boiling points, reducing the probability that the coolant can boil, which could lead to a loss-of-coolant accident. Low vapor pressure enables operation at near-ambient pressure, further dramatically reducing the probability of an accident. Some designs immerse the entire core and heat exchangers into a pool of coolant, virtually eliminating the risk that inner-loop cooling will be lost.
| Metal 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) |
| Tin | 231.9 °C, (449.5 °F) | 2602 °C, (4716 °F) |
Clementine was the 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, [2] and a high [3] neutron cross-section, it has fallen out of favor.
Sodium and NaK (a eutectic sodium-potassium alloy) do not corrode steel to any significant degree and are compatible with many nuclear fuels, allowing for a wide choice of structural materials. NaK was used as the coolant in the first breeder reactor prototype, the Experimental Breeder Reactor-1, in 1951.
Sodium and NaK 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. Sodium is also the coolant used in the Russian BN reactor series and the Chinese CFR series in commercial operation today.[ citation needed ] Neutron activation of sodium also causes these liquids to become intensely radioactive during operation, though the half-life is short and therefore their radioactivity does not pose an additional disposal concern.
There are two proposals for a sodium cooled Gen IV LMFR, one based on oxide fuel, the other on the metal-fueled integral fast reactor.
Lead has excellent neutron properties (reflection, low absorption) and is a very potent radiation shield against gamma rays. The high 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 [4] [5] used for structural materials.
Lead-bismuth eutectic allows operation at lower temperatures while preventing the freezing of the metal coolant in a lower temperature range (eutectic point: 123.5 °C / 255.3 °F). [4] [6]
Beside its highly corrosive character, [4] [5] its main disadvantage is the formation by neutron activation of 209
Bi (and subsequent beta decay) of 210
Po (T1⁄2 = 138.38 day), a volatile alpha-emitter highly radiotoxic (the highest known radiotoxicity, above that of plutonium).
Although tin today is not used as a coolant for working reactors because it builds a crust, [7] it can be a useful additional or replacement coolant at nuclear disasters or loss-of-coolant accidents.
Further advantages of tin are the high boiling point and the ability to build a crust even over liquid tin helps to cover poisonous leaks and keeps the coolant in and at the reactor. It has been tested by Ukrainian researchers and was proposed to convert the boiling water reactors at the Fukushima Daiichi nuclear disaster into liquid tin cooled reactors. [8]
The Soviet November-class submarine K-27 and all seven Alfa-class submarines used reactors cooled by lead-bismuth eutectic and moderated with beryllium as their propulsion plants. (VT-1 reactors in K-27; BM-40A and OK-550 reactors in others).
The second nuclear submarine, USS Seawolf was the only U.S. submarine to have a sodium-cooled, beryllium-moderated 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,[ citation needed ] the submarine's sodium-cooled, beryllium-moderated reactor was removed starting in 1958 and replaced with a pressurized water reactor.
Liquid metal cooled reactors were studied by Pratt & Whitney for use in nuclear aircraft as part of the Aircraft Nuclear Propulsion program. [9]
The Sodium Reactor Experiment was an experimental sodium-cooled graphite-moderated nuclear reactor (A Sodium-Graphite Reactor, or SGR) 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. [10] 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.
SRE was the prototype for the Hallam Nuclear Power Facility, another sodium-cooled graphite-moderated SGR that operated in Nebraska.
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 is sodium cooled. The 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 reactors studied are liquid metal cooled:
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. As of 2022, the International Atomic Energy Agency reports there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world.
A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants. In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated, high pressure water then flows to a steam generator, where it transfers its thermal energy to lower pressure water of a secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator. Most use anywhere from two to four vertically mounted steam generators; VVER reactors use horizontal steam generators.
A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons, as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. Around 20 land based fast reactors have been built, accumulating over 400 reactor years of operation globally. The largest of this was the Superphénix Sodium cooled fast reactor in France that was designed to deliver 1,242 MWe. Fast reactors have been intensely studied since the 1950s, as they provide certain advantages over the existing fleet of water cooled and water moderated reactors. These are:
In nuclear engineering, the void coefficient is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids form in the reactor moderator or coolant. Net reactivity in a reactor is the sum total of multiple contributions, of which the void coefficient is but one. Reactors in which either the moderator or the coolant is a liquid typically will have a void coefficient value that is either negative or positive. Reactors in which neither the moderator nor the coolant is a liquid will have a void coefficient value equal to zero. It is unclear how the definition of "void" coefficient applies to reactors in which the moderator/coolant is neither liquid nor gas.
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reactors are the most common type of nuclear reactor, and light-water reactors are the most common type of thermal-neutron reactor.
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A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a mixture of molten salt with a fissionable material.
A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.
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The lead-cooled fast reactor is a nuclear reactor design that use molten lead or lead-bismuth eutectic coolant. These materials can be used as the primary coolant because they have low neutron absorption and relatively low melting points. Neutrons are slowed less by interaction with these heavy nuclei so these reactors operate with fast neutrons.
A sodium-cooled fast reactor is a fast neutron reactor cooled by liquid sodium.
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. It has a melting point of 123.5 °C/254.3 °F and a boiling point of 1,670 °C/3,038 °F.
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FLiBe is the name of a molten salt made from a mixture of lithium fluoride (LiF) and beryllium fluoride. It is both a nuclear reactor coolant and solvent for fertile or fissile material. It served both purposes in the Molten-Salt Reactor Experiment (MSRE) at the Oak Ridge National Laboratory.
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Types of nuclear fission reactor | |||||||||||||||
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| Light water | |||||||||||||||
| Heavy water by coolant |
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None (fast-neutron) |
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