View source for Physics:Liquid metal cooled reactor

{{Short description|Nuclear reactor where the coolant is liquid metal}}
A '''liquid metal cooled nuclear reactor''', or '''LMR''' is a type of [[Physics:Nuclear reactor|nuclear reactor]] where the primary [[Chemistry:Nuclear reactor coolant|coolant]] is a [[Chemistry:Liquid metal|liquid metal]]. Liquid metal cooled reactors were first adapted for [[Engineering:Breeder reactor|breeder reactor]] power generation. They have also been used to power [[Engineering:Nuclear submarine|nuclear submarines]].

Due to their high thermal conductivity, metal coolants remove heat effectively, enabling high [[Physics:Power density|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 [[Physics:Boiling point|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 [[Engineering:Electric power conversion|power conversion cycles]] with high thermodynamic efficiency. This makes them attractive for improving power output, cost effectiveness, and [[Physics:Fuel efficiency|fuel efficiency]] in nuclear power plants.

Liquid metals, being electrically highly conductive, can be moved by [[Engineering:Electromagnetic pump|electromagnetic pump]]s.<ref name="bonin">{{cite book |last1=Bonin |first1=Bernhard |last2=Klein |first2=Etienne |date=2012 |title=Le nucléaire expliqué par des physiciens}}</ref> 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 [[Chemistry:Alkali metal|alkali metal]]s), corrosion and/or production of radioactive activation products may be an issue.

==Design==
Liquid metal coolant has been applied to both [[Engineering:Thermal-neutron reactor|thermal-]] and [[Physics:Fast-neutron reactor|fast-neutron reactor]]s. 

To date, most fast neutron reactors have been liquid metal cooled fast reactors (LMFRs). When configured as a breeder reactor (e.g. with a breeding blanket), such reactors are called liquid metal fast breeder reactors (LMFBRs).  


==Coolant properties==

Suitable liquid metal coolants must have a low neutron capture [[Physics:Neutron cross section|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 [[Physics:Operating temperature|operating temperature]].

Liquid metals generally have high [[Physics:Boiling point|boiling point]]s, reducing the probability that the coolant can boil, which could lead to a [[Physics:Loss-of-coolant accident|loss-of-coolant accident]]. Low [[Physics:Vapor pressure|vapor pressure]] enables operation at near-[[Physics:Ambient pressure|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.

{| class="wikitable sortable" align="right"
|+Liquid metal coolants
!Metal Coolant!![[Physics:Melting point|Melting point]]!![[Physics:Boiling point|Boiling point]]
|-
|[[Chemistry:Sodium|Sodium]]||97.72&nbsp;°C, (207.9&nbsp;°F)||883&nbsp;°C, (1621&nbsp;°F)
|-
|NaK||−11&nbsp;°C, (12&nbsp;°F)||785&nbsp;°C, (1445&nbsp;°F)
|-
|[[Chemistry:Mercury (element)|Mercury]]||−38.83&nbsp;°C, (−37.89&nbsp;°F)|| 356.73&nbsp;°C (674.11&nbsp;°F)
|-
|[[Chemistry:Lead|Lead]]||327.46&nbsp;°C, (621.43&nbsp;°F)||1749&nbsp;°C, (3180&nbsp;°F)
|-
|[[Chemistry:Lead-bismuth eutectic|Lead-bismuth eutectic]]||123.5&nbsp;°C, (254.3&nbsp;°F) ||1670&nbsp;°C, (3038&nbsp;°F)
|-
|[[Chemistry:Tin|Tin]]||231.9&nbsp;°C, (449.5&nbsp;°F) ||2602&nbsp;°C, (4716&nbsp;°F)
|}

===Mercury===
[[Physics:Clementine (nuclear reactor)|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,<ref>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</ref> and a high<ref>{{Cite web|url=https://www.ncnr.nist.gov/resources/n-lengths/elements/hg.html|title=Neutron Scattering Lengths and cross sections|website=www.ncnr.nist.gov}}</ref> neutron cross-section, it has fallen out of favor.

===Sodium and NaK===
Sodium and NaK (a [[Physics:Eutectic|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 [[Engineering:Monju Nuclear Power Plant|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|date=June 2022}} [[Physics:Neutron activation|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 [[Engineering:Integral fast reactor|integral fast reactor]].

===Lead===
Lead has excellent neutron properties (reflection, low absorption) and is a very potent radiation shield against [[Physics:Gamma ray|gamma ray]]s. 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 [[Chemistry:Bismuth|bismuth]], but [[Chemistry:Lead-bismuth eutectic|lead-bismuth eutectic]] is highly corrosive to most metals<ref name="Weeks_1969">{{Cite journal |title=Liquidus curves and corrosion of Fe, Ti, Zr, and Cu in liquid Bi–Pb alloys.|first1=J. R.|last1=Weeks|first2=A. J.|last2=Romano|year=1969|journal=Corrosion |volume=25 |pages=131–136 |issue=3 |doi=10.5006/0010-9312-25.3.131|osti=4803122 }}</ref><ref name="Gosse_2014">{{cite journal | last1 = Gossé | first1 = Stéphane | title = Thermodynamic assessment of solubility and activity of iron, chromium, and nickel in lead bismuth eutectic | journal = Journal of Nuclear Materials | date = June 2014 | volume = 449 | issue = 1–3 | pages = 122–131 | issn = 0022-3115 | doi = 10.1016/j.jnucmat.2014.03.011 | pmid = | bibcode = 2014JNuM..449..122G | url = }}</ref> used for structural materials.

===Lead-bismuth eutectic===
{{Main|Chemistry:Lead-bismuth eutectic}}
[[Chemistry:Lead-bismuth eutectic|Lead-bismuth eutectic]] allows operation at lower temperatures while preventing the freezing of the metal coolant in a lower temperature range (eutectic point: {{nobr|123.5 °C / 255.3 °F)}}.<ref name="Weeks_1969" /><ref name="NEA_Handbook_2005">{{Cite book| last1 = Fazio| first1 = Concetta| last2 = Li| first2 = Ning| last3 = Na| first3 = Byung-Chan| title = Handbook on heavy liquid metal technology. Prepared in the frame of the OECD/NEA working party of the fuel cycle| accessdate = 2022-06-05| date = 2005-07-01| url = https://www.osti.gov/etdeweb/biblio/20750745}}</ref> 

Beside its highly corrosive character,<ref name="Weeks_1969" /><ref name="Gosse_2014" /> its main disadvantage is the formation by [[Physics:Neutron activation|neutron activation]] of [[Physics:Bismuth-209|{{Chem|209|Bi}}]] (and subsequent [[Physics:Beta decay|beta decay]]) of [[Physics:Polonium-210|{{Chem|210|Po}}]] ([[Physics:Half-life|T{{sub|{{frac|1|2}}}}]] = 138.38 day), a volatile [[Physics:Alpha decay|alpha-emitter]] highly radiotoxic (the highest known radiotoxicity, above that of [[Chemistry:Plutonium|plutonium]]).

===Tin===
Although [[Chemistry:Tin|tin]] today is not used as a coolant for working reactors because it builds a crust,<ref>[https://www.itri.co.uk/index.php?option=com_mtree&task=att_download&link_id=26774&cf_id=24 Atmospheric corrosion of tin and tin alloy]{{Dead link|date=March 2018}}</ref> it can be a useful additional or replacement coolant at nuclear disasters or [[Physics:Loss-of-coolant accident|loss-of-coolant accident]]s. 

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 {{wipe|Ukraine}} researchers and was proposed to convert the [[Engineering:Boiling water reactor|boiling water reactor]]s at the Fukushima Daiichi nuclear disaster into liquid tin cooled reactors.<ref>[http://www.kyivpost.com/content/ukraine/ukraine-advises-japan-to-use-tin-to-cool-fukushima-100116.html Ukraine advises Japan to use tin to cool Fukushima reactor] Kyivpost</ref>

==Propulsion==

===Submarines===
The [[Place:Soviet Union|Soviet]] {{sclass2|November|submarine}} {{ship|Soviet submarine|K-27||2}} and all seven {{sclass2|Alfa|submarine|2}}s used reactors cooled by lead-bismuth eutectic and [[Physics:Neutron moderator|moderated]] with [[Chemistry:Beryllium|beryllium]] as their propulsion plants. ([[Engineering:VT-1 reactor|VT-1 reactor]]s in ''K-27''; [[Engineering:BM-40A reactor|BM-40A]] and [[Engineering:OK-550 reactor|OK-550 reactor]]s in others). 

The second nuclear submarine, {{USS|Seawolf|SSN-575|6}} was the only U.S. submarine to have a sodium-cooled, [[Chemistry:Beryllium|beryllium]]-[[Physics:Neutron moderator|moderated]] nuclear power plant. It was commissioned in 1957, but it had leaks in its [[Engineering:Superheater|superheater]]s, which were bypassed. In order to standardize the reactors in the fleet,{{Citation needed|date=November 2019}} the submarine's sodium-cooled, beryllium-moderated reactor was removed starting in 1958 and replaced with a [[Engineering:Pressurized water reactor|pressurized water reactor]].

=== Nuclear aircraft ===
Liquid metal cooled reactors were studied by Pratt & Whitney for use in nuclear aircraft as part of the [[Engineering:Aircraft Nuclear Propulsion|Aircraft Nuclear Propulsion]] program.<ref>{{Cite web|url=https://facts.net/nuclear-energy-facts/|title=40 Curious Nuclear Energy Facts You Should Know|date=December 9, 2019}}</ref>

==Power generation==
The [[Physics:Sodium Reactor Experiment|Sodium Reactor Experiment]] was an experimental sodium-cooled [[Chemistry:Graphite|graphite]]-moderated nuclear reactor (A Sodium-Graphite Reactor, or SGR) sited in a section of the [[Organization:Santa Susana Field Laboratory|Santa Susana Field Laboratory]] then operated by the Atomics International division of [[Company:North American Aviation|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 [[Physics:Radioactive decay|radioactive]] gases.<ref>{{cite book| last = Ashley| first = R.L.| title = SRE Fuel Element Damage, Final Report of the Atomics International Ad Hoc Committee| year = 1961| url = http://www.etec.energy.gov/Health-and-Safety/Documents/SSFLPanelFiles/NAA-SR-4488-Final.pdf| id = NAA-SR-4488-supl| display-authors = etal| url-status = dead| archive-url = https://web.archive.org/web/20090410205724/http://www.etec.energy.gov/Health%2Dand%2DSafety/Documents/SSFLPanelFiles/NAA-SR-4488-Final.pdf| archive-date = 2009-04-10}}</ref> 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 [[Physics:List of civilian nuclear accidents#1960s|suffered a partial nuclear meltdown]] in 1963 and was decommissioned in 1975.

At [[Engineering:Dounreay|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 [[Engineering:BN-600 reactor|BN-600]] is sodium cooled. The [[Engineering:BN-350 reactor|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 [[Engineering:Generation IV reactor|Generation IV reactor]]s studied are liquid metal cooled:
* [[Engineering:Sodium-cooled fast reactor|Sodium-cooled fast reactor]] (SFR)
* [[Chemistry:Lead-cooled fast reactor|Lead-cooled fast reactor]]

== References ==
{{reflist}}

{{Nuclear fission reactors}}

{{DEFAULTSORT:Liquid Metal Cooled Reactor}}
[[Category:Liquid metal fast reactors| ]]
[[Category:Nuclear physics]]

{{Sourceattribution|Liquid metal cooled reactor}}