Aliquid metal cooled nuclear reactor (LMR) is a type ofnuclear reactor where the primarycoolant is aliquid metal. Liquid metal cooled reactors were first adapted forbreeder reactor power generation. They have also been used to powernuclear submarines.
Due to their high thermal conductivity, metal coolants remove heat effectively, enabling highpower 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 theboiling 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 drivepower conversion cycles with high thermodynamic efficiency. This makes them attractive for improving power output, cost effectiveness, andfuel efficiency in nuclear power plants.
Liquid metals, being electrically highly conductive, can be moved byelectromagnetic 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 (foralkali metals), corrosion and/or production of radioactive activation products may be an issue.
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Liquid metal coolant has been applied to boththermal- andfast-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 surrounding material to breed fissile material; i.e. a breeding blanket), such reactors are called liquid metal fast breeder reactors (LMFBRs).
Suitable liquid metal coolants must have a low neutron capturecross section, must not cause excessive corrosion of the structural materials, and must have melting and boiling points that are suitable for the reactor'soperating temperature.
Liquid metals generally have highboiling points, reducing the probability that the coolant can boil, which could lead to aloss-of-coolant accident. Lowvapor 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 |
|---|---|---|
| Mercury | −38.83 °C, (−37.894 °F) | 356.73 °C (674.114 °F) |
| Sodium | 97.72 °C, (207.896 °F) | 883 °C, (1621.4 °F) |
| NaK | −11 °C, (12.2 °F) | 785 °C, (1445 °F) |
| Lead | 327.46 °C, (621.428 °F) | 1749 °C, (3180.2 °F) |
| Lead-bismuth eutectic | 123.5 °C, (254.3 °F) | 1670 °C, (3038 °F) |
| Tin | 231.9 °C, (449.42 °F) | 2602 °C, (4715.6 °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 (aeutectic 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, theExperimental 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 theMonju Nuclear Power Plant in a 1995 accident and fire. Sodium was the coolant used inExperimental Breeder Reactor II. It was demonstrated in 1986, to an invited international audience, to be "walk-away" safe. Sodium is also the coolant used in the Russian BN reactor series and the Chinese CFR series in commercial operation today.[4][5]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 cooledGen IV LMFR, one based on oxide fuel, the other on the metal-fueledintegral fast reactor.
A sodium cooledGE/HitachiPRISM reactor will be part of the Natrium system being built byTerraPower atKemmerer, Wyoming.
Lead has excellent neutron properties (reflection, low absorption) and is a very potent radiation shield againstgamma rays. The high boiling point of lead provides safety advantages as it can cool the reactor efficiently even if it reaches several hundreddegrees 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 withbismuth, butlead-bismuth eutectic is highly corrosive to most metals[6][7] used for structural materials. The high density and viscosity of lead results in large pumping expense, which reduces the overall efficiency of the system. Neutron absorption in lead and bismuth produce radioactive isotopes with relatively long half lives. The most dangerous of these is polonium-210, which was used topoisonAlexander Litvinenko on November 1, 2006,
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).[6][8]
Beside its highly corrosive character,[6][7] its main disadvantage is the formation byneutron activation of209
Bi (and subsequentbeta decay) of210
Po (T1⁄2 = 138.38 day), a volatilealpha-emitter highlyradiotoxic (the highest knownradiotoxicity, above that ofplutonium).
Althoughtin today is not used as a coolant for working reactors because it builds a crust,[9] it can be a useful additional or replacement coolant atnuclear disasters orloss-of-coolant accidents.[10]
TheSovietNovember-classsubmarineK-27 and all sevenAlfa-class submarines used reactors cooled by lead-bismuth eutectic andmoderated withberyllium as their propulsion plants. (VT-1 reactors inK-27;BM-40A andOK-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 itssuperheaters, 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 apressurized water reactor.
Liquid metal cooled reactors were studied byPratt & Whitney for use innuclear aircraft as part of theAircraft Nuclear Propulsion program.[11]
TheSodium Reactor Experiment was an experimental sodium-cooledgraphite-moderated nuclear reactor (A Sodium-Graphite Reactor, or SGR) sited in a section of theSanta Susana Field Laboratory then operated by the Atomics International division ofNorth 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 ofradioactive gases.[12] 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 theHallam Nuclear Power Facility, another sodium-cooled graphite-moderated SGR that operated inNebraska.
Fermi 1 inMonroe County, Michigan was an experimental, liquid sodium-cooledfast breeder reactor that operated from 1963 to 1972. As a result of an ill-advised design change insisted by the AEC, to which Argonne National Laboratory objected, itsuffered a partial nuclear meltdown in 1963 and was decommissioned in 1975.
The 20 MWeExperimental Breeder Reactor II entered service in 1964, and operated flawlessly until it and the research program being conducted there were unwisely terminated by the Clinton Administration in 1994. Nobel Physics LaureateHans Bethe had described it as "the best research reactor ever built." It powered the entireIdaho National Laboratory facility nearIdaho Falls and sold excess power to the local grid.
AtDounreay inCaithness, in the far north ofScotland, theUnited Kingdom Atomic Energy Authority (UKAEA) operated theDounreay 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, thePrototype Fast Reactor, which operated from 1974 to 1994 and used liquid sodium as its coolant.
The SovietBN-600 is sodium cooled. TheBN-350.EBR-I used a liquid metal alloy,NaK, for cooling. NaK is liquid at room temperature. Liquid metal cooling is also used in mostfast neutron reactors includingfast breeder reactors such as theIntegral Fast Reactor.
ManyGeneration IV reactors studied are liquid metal cooled:
