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{{Short description|Device for controlled nuclear reactions}}
{{About|nuclear fission reactors|nuclear fusion reactors|Fusion power}}
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From top, left to right
# [[Chicago Pile-1]], the first nuclear reactor
# [[Shippingport Atomic Power Station]], the first peacetime reactor
# [[HTR-10]], a prototype to the first Generation IV reactor, [[HTR-PM]]
# The [[Convair NB-36H]], the first aircraft to test an onboard reactor
# [[Operation Sea Orbit]], the first nuclear-powered circumnavigation
# The [[Chernobyl sarcophagus]], built to contain the effects of the [[Chernobyl disaster|1986 disaster]]
}}
A '''nuclear reactor''' is a device used to initiate and control a fission [[nuclear chain reaction]]. Nuclear reactors are used at [[nuclear power plant]]s for [[electricity generation]] and in [[nuclear marine propulsion]]. When a fissile nucleus like [[uranium-235]] or [[plutonium-239]] absorbs a [[neutron]], it splits into lighter nuclei, releasing energy, gamma radiation, and free neutrons, which can induce further fission in a self-sustaining [[chain reaction]]. The process is carefully controlled using [[control rods]] and [[neutron moderator]]s to regulate the number of neutrons that continue the reaction, ensuring the reactor operates safely, although inherent control by means of [[delayed neutron]]s also plays an important role in reactor output control. The efficiency of nuclear fuel is much higher than fossil fuels; the [[Enriched uranium#Low-enriched uranium (LEU)|5% enriched uranium]] used in the newest reactors has an energy density 120,000 times higher than coal.<ref name= "r199" >{{cite web |date=2024-05-20 |title=Nuclear Fuel Cycle Overview |url=https://world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview |access-date=2024-11-04 |website=World Nuclear Association}}</ref><ref name= "y385" >{{cite web |author=Science and Mathematics Education Research Group, University of British Columbia |title=Physics Nuclear Physics: Nuclear Reactors |url=https://scienceres-edcp-educ.sites.olt.ubc.ca/files/2015/01/sec_phys_nuclear_reactors.pdf |access-date=2024-11-04}}</ref>

Nuclear reactors have their origins in the World War II Allied [[Manhattan Project]].{{NoteTag|Hungarian physicist [[Leo Szilard]] discovered the nuclear chain reaction and patented a design in 1934, preceding the discovery of nuclear fission.<ref>L. Szilárd, [http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=GB630726 "Improvements in or relating to the transmutation of chemical elements," ] {{Webarchive|url=https://web.archive.org/web/20080621120547/http://v3.espacenet.com/textdoc?DB=EPODOC |date=21 June 2008 }} British patent number: GB630726 (filed: 28 June 1934; published: 30 March 1936).</ref>|name=a}} The world's first artificial{{NoteTag|An extinct [[natural nuclear fission reactor]] was discovered in 1972 in Oklo, Gabon.<ref name= "DavisGould2014" >{{cite journal |last1=Davis |first1=E. D. |last2=Gould |first2=C. R. |last3=Sharapov|first3=E. I. |title=Oklo reactors and implications for nuclear science |journal=International Journal of Modern Physics E |volume=23 |issue=4 |year=2014 |pages=1430007–236 |issn=0218-3013 |doi=10.1142/S0218301314300070 |arxiv = 1404.4948 |bibcode = 2014IJMPE..2330007D |s2cid=118394767}}</ref>|name=b}} nuclear reactor, Chicago Pile-1, achieved [[Criticality (status)|criticality]] on 2 December 1942.<ref name= ":0" /> Early reactor designs sought to produce [[weapons-grade plutonium]] for [[Fission bomb|fission bombs]], later incorporating grid electricity production in addition. In 1957, [[Shippingport Atomic Power Station]] became the first reactor dedicated to peaceful use; in Russia, in 1954, the first small nuclear power reactor APS-1 OBNINSK reached criticality. Other countries followed suit.

Heat from [[nuclear fission]] is passed to a [[working fluid]] [[Nuclear reactor#By coolant|coolant]] (water or gas), which in turn runs through [[Turbine|turbines]]. In commercial reactors, turbines drive [[electrical generator]] shafts. The heat can also be used for [[district heating]], and industrial applications including [[desalination]] and [[Hydrogen production#Steam reforming – gray or blue|hydrogen production]]. Some reactors are used to produce [[isotopes]] for [[Nuclear medicine|medical]] and [[industrial radiography|industrial]] use. Reactors pose a [[nuclear proliferation]] risk as they can be configured to produce plutonium, as well as [[tritium]] gas used in [[Boosted fission weapon|boosted fission weapons]]. Reactor spent fuel can be [[Nuclear reprocessing|reprocessed]] to yield up to 25% more nuclear fuel, which can be used in reactors again. Reprocessing can also significantly reduce the volume of nuclear waste, and has been practiced in Europe, Russia, India and Japan. Due to concerns of proliferation risks, the United States does not engage in or encourage reprocessing.<ref name= "z699" >{{cite web |title=Spent Fuel Reprocessing Options |url=https://www-pub.iaea.org/MTCD/publications/PDF/te_1587_web.pdf |access-date=2024-08-30 |publisher=IAEA}}</ref>

Reactors are also used in [[nuclear propulsion]] of vehicles. [[Nuclear marine propulsion]] of ships and submarines is largely restricted to naval use. Reactors have also been tested for nuclear [[Nuclear-powered aircraft|aircraft propulsion]] and [[Nuclear thermal rocket|spacecraft propulsion]].

Reactor safety is maintained through various systems that control the rate of fission. The insertion of control rods, which absorb neutrons, can rapidly decrease the reactor's output, while other systems automatically shut down the reactor in the event of unsafe conditions. The buildup of neutron-absorbing fission products like [[xenon-135]] can influence reactor behavior, requiring careful management to prevent issues such as the [[iodine pit]], which can complicate reactor restarts. There have been two reactor accidents classed as an [[International Nuclear Event Scale]] Level 7 "major accident": the 1986 [[Chernobyl disaster]] and 2011 [[Fukushima nuclear accident|Fukushima disaster]].

{{As of|2022}}, the [[International Atomic Energy Agency]] reported there are 422 nuclear power reactors and 223 nuclear [[research reactors]] in operation around the world.<ref>{{cite web|url=https://pris.iaea.org/pris/|title=PRIS – Home|website=pris.iaea.org|access-date=10 April 2019|archive-date=11 February 2012|archive-url=https://web.archive.org/web/20120211095840/http://www.iaea.org/programmes/a2/|url-status=live}}</ref><ref>{{cite web|url=https://nucleus.iaea.org/RRDB/RR/ReactorSearch.aspx?rf=1|title=RRDB Search|website=nucleus.iaea.org|access-date=6 January 2019|archive-date=18 September 2010|archive-url=https://web.archive.org/web/20100918002503/https://nucleus.iaea.org/RRDB/RR/ReactorSearch.aspx?rf=1|url-status=live}}</ref><ref>{{Citation|last=Oldekop|first=W.|title=Electricity and Heat from Thermal Nuclear Reactors|date=1982|url=http://dx.doi.org/10.1007/978-3-642-68444-9_5|work=Primary Energy|pages=66–91|place=Berlin, Heidelberg|publisher=Springer Berlin Heidelberg|doi=10.1007/978-3-642-68444-9_5|isbn=978-3-540-11307-2|access-date=2021-02-02|archive-date=5 June 2018|archive-url=https://web.archive.org/web/20180605083309/https://link.springer.com/chapter/10.1007/978-3-642-68444-9_5|url-status=live}}</ref> The US [[United States Department of Energy|Department of Energy]] classes reactors into generations, with the majority of the global fleet being [[Generation II reactor|Generation II reactors]] constructed from the 1960s to 1990s, and [[Generation IV reactor|Generation IV reactors]] currently in development. Reactors can also be grouped by the choices of coolant and moderator. Almost 90% of global nuclear energy comes from [[pressurized water reactors]] and [[boiling water reactors]], which use it as a coolant and moderator.<ref name= "c847" >{{cite web |last=Region |first=CountryBy TypeBy |date=2024-08-29 |title=In Operation & Suspended Operation |url=https://pris.iaea.org/PRIS/WorldStatistics/OperationalReactorsByType.aspx |access-date=2024-08-30 |website=PRIS}}</ref> Other designs include [[Heavy-water reactor|heavy water reactors]], [[Gas-cooled reactor|gas-cooled reactors]], and [[Fast-breeder reactor|fast breeder reactors]], variously optimizing efficiency, safety, and [[Nuclear fuel|fuel type]], [[Enriched uranium|enrichment]], and [[burnup]]. [[Small modular reactor|Small modular reactors]] are also an area of current development. These reactors play a crucial role in generating large amounts of electricity with low carbon emissions, contributing significantly to the global energy mix.

==Operation==
{{Main|Nuclear reactor physics}}

[[File:Nuclear fission.svg|upright=1.15|thumb|An example of an induced nuclear fission event. A neutron is absorbed by the nucleus of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons. Though both reactors and [[nuclear weapons]] rely on nuclear chain reactions, the rate of reactions in a reactor is much slower than in a bomb.]]

Just as conventional [[thermal power station]]s generate electricity by harnessing the [[thermal energy]] released from burning [[fossil fuels]], nuclear reactors convert the energy released by controlled [[nuclear fission]] into thermal energy for further conversion to mechanical or electrical forms.

===Fission===
{{Main|Nuclear fission}}

When a large [[fissile]] [[atomic nucleus]] such as [[uranium-235]], [[uranium-233]], or [[plutonium-239]] absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, (the [[Nuclear fission product|fission products]]), releasing [[kinetic energy]], [[gamma rays|gamma radiation]], and [[neutron|free neutron]]s. A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a [[nuclear chain reaction]].

To control such a nuclear chain reaction, [[control rod]]s containing [[neutron poison]]s and [[neutron moderators]] are able to change the portion of neutrons that will go on to cause more fission.<ref name= "DOE HAND" >
{{cite web|url=http://www.hss.energy.gov/NuclearSafety/techstds/standard/hdbk1019/h1019v2.pdf|title=DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory|publisher=US Department of Energy|archive-url=https://web.archive.org/web/20080423194722/http://www.hss.energy.gov/NuclearSafety/techstds/standard/hdbk1019/h1019v2.pdf|archive-date=23 April 2008|url-status=dead|access-date=24 September 2008}}
</ref> Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring or instrumentation detects unsafe conditions.<ref>{{cite web |title=Reactor Protection & Engineered Safety Feature Systems |work=The Nuclear Tourist |url=http://www.nucleartourist.com/systems/rp.htm |access-date=25 September 2008 |archive-date=22 August 2018 |archive-url=https://web.archive.org/web/20180822051052/http://www.nucleartourist.com/systems/rp.htm |url-status=live }}</ref>

===Heat generation===
The reactor core generates heat in a number of ways:
* The [[kinetic energy]] of fission products is converted to [[thermal energy]] when these nuclei collide with nearby atoms.
* The reactor absorbs some of the [[gamma rays]] produced during fission and converts their energy into heat.
* Heat is produced by the [[radioactive decay]] of fission products and materials that have been activated by [[neutron absorption]]. This decay heat source will remain for some time even after the reactor is shut down.

A kilogram of [[uranium-235]] (U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2&nbsp;×&nbsp;10<sup>13</sup> [[joules]] per kilogram of uranium-235 versus 2.4&nbsp;×&nbsp;10<sup>7</sup> joules per kilogram of coal).<ref>{{cite web|url=http://bioenergy.ornl.gov/papers/misc/energy_conv.html |title=Bioenergy Conversion Factors |publisher=Bioenergy.ornl.gov |access-date=18 March 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110927181836/http://bioenergy.ornl.gov/papers/misc/energy_conv.html |archive-date=27 September 2011 }}</ref><ref>{{cite book |url=https://archive.org/details/nuclearweaponswh0000bern/page/312 |title=Nuclear Weapons: What You Need to Know |author=Bernstein, Jeremy |year=2008 |page=[https://archive.org/details/nuclearweaponswh0000bern/page/312 312] |isbn=978-0-521-88408-2 |publisher=[[Cambridge University Press]] |access-date=17 March 2011 }}</ref>{{Original research inline|date=March 2012}}<!--see this is more than [[WP:CALC]]. See [[Talk:Nuclear reactor#Energy efficiency?]]-->

The fission of one kilogram of [[uranium-235]] releases about 19 billion [[kilocalories]], so the energy released by 1&nbsp;kg of uranium-235 corresponds to that released by burning 2.7 million kg of coal.

===Cooling===
A [[nuclear reactor coolant]] – usually water but sometimes a gas or a liquid metal (like liquid sodium or lead) or [[molten salt]] – is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the [[turbines]], like the [[pressurized water reactor]]. However, in some reactors the water for the steam turbines is boiled directly by the [[reactor core]]; for example the [[boiling water reactor]].<ref name= "HSWCOOLANT" >{{cite web |title=How nuclear power works |date=9 October 2000 |publisher=HowStuffWorks.com |url=http://science.howstuffworks.com/nuclear-power3.htm |access-date=25 September 2008 |archive-date=22 October 2019 |archive-url=https://web.archive.org/web/20191022013236/https://science.howstuffworks.com/nuclear-power3.htm |url-status=live }}</ref>

===Reactivity control===
{{Main|Nuclear reactor physics|Passive nuclear safety|Delayed neutron|Iodine pit|SCRAM|Decay heat}}
The rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events. Nuclear reactors typically employ several methods of neutron control to adjust the reactor's power output. Some of these methods arise naturally from the physics of radioactive decay and are simply accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose.

The fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the [[control rod]]s. Control rods are made of [[neutron poison]]s and therefore absorb neutrons. When a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces – often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power.

The physics of radioactive decay also affects neutron populations in a reactor. One such process is [[delayed neutron]] emission by a number of neutron-rich fission isotopes. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder (termed "[[prompt neutron]]s" ) released immediately upon fission. The fission products which produce delayed neutrons have [[Half-life|half-lives]] for their [[Radioactive decay|decay]] by [[neutron emission]] that range from milliseconds to as long as several minutes, and so considerable time is required to determine exactly when a reactor reaches the [[critical mass (nuclear)|critical]] point. Keeping the reactor in the zone of chain reactivity where delayed neutrons are ''necessary'' to achieve a [[critical mass]] state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise the time between achievement of criticality and [[nuclear meltdown]] as a result of an exponential power surge from the normal nuclear chain reaction, would be too short to allow for intervention. This last stage, where delayed neutrons are no longer required to maintain criticality, is known as the [[prompt critical]] point. There is a scale for describing criticality in numerical form, in which bare criticality is known as ''zero [[dollar (reactivity)|dollars]]'' and the prompt critical point is ''one dollar'', and other points in the process interpolated in cents.

In some reactors, the [[coolant]] also acts as a [[neutron moderator]]. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. [[Thermal neutron]]s are more likely than [[fast neutron]]s to cause fission. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore a less effective moderator.

In other reactors, the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors, power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to [[scram]] the reactor in an emergency shut down. These systems insert large amounts of poison (often [[boron]] in the form of [[boric acid]]) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.<ref name= "TOURISTRP" >{{cite web |title=Reactor Protection & Engineered Safety Feature Systems |work=The Nuclear Tourist |url=http://www.nucleartourist.com/systems/rp.htm |access-date=25 September 2008 |archive-date=22 August 2018 |archive-url=https://web.archive.org/web/20180822051052/http://www.nucleartourist.com/systems/rp.htm |url-status=live }}</ref>

Most types of reactors are sensitive to a process variously known as xenon poisoning, or the [[iodine pit]]. The common [[fission product]] [[Xenon-135]] produced in the fission process acts as a neutron poison that absorbs neutrons and therefore tends to shut the reactor down. Xenon-135 accumulation can be controlled by keeping power levels high enough to destroy it by neutron absorption as fast as it is produced. Fission also produces [[iodine-135]], which in turn decays (with a half-life of 6.57 hours) to new xenon-135. When the reactor is shut down, iodine-135 continues to decay to xenon-135, making restarting the reactor more difficult for a day or two, as the xenon-135 decays into cesium-135, which is not nearly as poisonous as xenon-135, with a half-life of 9.2 hours. This temporary state is the "iodine pit." If the reactor has sufficient extra reactivity capacity, it can be restarted. As the extra xenon-135 is transmuted to xenon-136, which is much less a neutron poison, within a few hours the reactor experiences a "xenon burnoff (power) transient". Control rods must be further inserted to replace the neutron absorption of the lost xenon-135. Failure to properly follow such a procedure was a key step in the [[Chernobyl disaster]].<ref>{{cite web|url=http://www.eepublishers.co.za/images/upload/Meyer%20Chernobyl%205.pdf |title=Chernobyl: what happened and why? by CM Meyer, technical journalist. |url-status=dead |archive-url=https://web.archive.org/web/20131211073343/http://www.eepublishers.co.za/images/upload/Meyer%20Chernobyl%205.pdf |archive-date=11 December 2013 }}</ref>

Reactors used in [[nuclear marine propulsion]] (especially [[nuclear submarine]]s) often cannot be run at continuous power around the clock in the same way that land-based power reactors are normally run, and in addition often need to have a very long core life without [[Reactor refueling|refueling]]. For this reason many designs use highly enriched uranium but incorporate burnable neutron poison in the fuel rods.<ref>{{cite book|last1=Tsetkov|first1=Pavel|last2=Usman|first2=Shoaib|editor=Krivit, Steven|title=Nuclear Energy Encyclopedia: Science, Technology, and Applications|year=2011|publisher=Wiley|location=Hoboken, NJ|isbn=978-0-470-89439-2|pages=48; 85}}</ref> This allows the reactor to be constructed with an excess of fissionable material, which is nevertheless made relatively safe early in the reactor's fuel burn cycle by the presence of the neutron-absorbing material which is later replaced by normally produced long-lived neutron poisons (far longer-lived than xenon-135) which gradually accumulate over the fuel load's operating life.

===Electrical power generation===
The energy released in the fission process generates heat, some of which can be converted into usable energy. A common method of harnessing this [[thermal energy]] is to use it to boil water to produce pressurized steam which will then drive a [[steam turbine]] that turns an [[alternator]] and generates electricity.<ref name= "TOURISTRP" />

=== Life-times ===
Modern nuclear power plants are typically designed for a lifetime of 60 years, while older reactors were built with a planned typical lifetime of 30-40 years, though many of those have received renovations and life extensions of 15-20 years.<ref>{{cite web |url=https://pris.iaea.org/PRIS/WorldStatistics/OperationalByAge.aspx |website=IAEA Power Reactor Information System – operational by age |access-date=12 July 2024 |title=PRIS - Miscellaneous reports - Operational by Age }}</ref> Some believe nuclear power plants can operate for as long as 80 years or longer with proper maintenance and management. While most components of a nuclear power plant, such as steam generators, are replaced when they reach the end of their useful lifetime, the overall lifetime of the power plant is limited by the life of components that cannot be replaced when aged by wear and [[neutron embrittlement]], such as the reactor pressure vessel. <ref name= "dismantling_sci-am-2009" >[https://www.scientificamerican.com/article/nuclear-power-plant-aging-reactor-replacement-/ ''How Long Can a Nuclear Reactor Last?''] {{Webarchive|url=https://web.archive.org/web/20170202073144/http://www.scientificamerican.com/article/nuclear-power-plant-aging-reactor-replacement-/ |date=2 February 2017 }} Paul Voosen, Scientific American, 20 Nov 2009</ref> At the end of their planned life span, plants may get an extension of the operating license for some 20 years and in the US even a "subsequent license renewal" (SLR) for an additional 20 years.<ref>[https://www.nrc.gov/reactors/operating/licensing/renewal/subsequent-license-renewal.html ''Status of Subsequent License Renewal Applications.''] {{Webarchive|url=https://web.archive.org/web/20180121051705/https://www.nrc.gov/reactors/operating/licensing/renewal/subsequent-license-renewal.html |date=21 January 2018 }} NRC, 24 Feb 2022</ref><ref>[https://www.energy.gov/ne/articles/whats-lifespan-nuclear-reactor-much-longer-you-might-think ''What's the Lifespan for a Nuclear Reactor? Much Longer Than You Might Think''] {{Webarchive|url=https://web.archive.org/web/20200609230342/https://www.energy.gov/ne/articles/whats-lifespan-nuclear-reactor-much-longer-you-might-think |date=9 June 2020 }}. Office of Nuclear Energy, 16 Apr 2020</ref>

Even when a license is extended, it does not guarantee the reactor will continue to operate, particularly in the face of safety concerns or incident.<ref>{{Cite news |date=2006-08-05 |title=Swedish nuclear reactors shut down over safety concerns |url=https://en.wikinews.org/wiki/Swedish_nuclear_reactors_shut_down_over_safety_concerns |newspaper=Wikinews |access-date=16 May 2023 |archive-date=16 May 2023 |archive-url=https://web.archive.org/web/20230516123219/https://en.wikinews.org/wiki/Swedish_nuclear_reactors_shut_down_over_safety_concerns |url-status=live }}</ref> Many reactors are closed long before their license or design life expired and are [[Nuclear decommissioning|decommissioned]]. The costs for replacements or improvements required for continued safe operation may be so high that they are not cost-effective. Or they may be shut down due to technical failure.<ref name= "sapl-2017" >[https://saplnh.org/about-nuclear/nuclear-plant-lifespans/ ''The True Lifespan of a Nuclear Power Plant''] {{Webarchive|url=https://web.archive.org/web/20230219095448/https://saplnh.org/about-nuclear/nuclear-plant-lifespans/ |date=19 February 2023 }}. Seacoast Anti-Pollution League (SAPL), 2017</ref> Other ones have been shut down because the area was contaminated, like Fukushima, Three Mile Island, Sellafield, Chernobyl.<ref>{{Cite book |last=IAEA |title=Cleanup of Large Areas Contaminated as a Result of a Nuclear Accident}}</ref> The British branch of the French concern [[EDF Energy]], for example, extended the operating lives of its [[Advanced Gas-cooled Reactor]]s with only between 3 and 10 years.<ref name= "edf-lifetime" >[https://www.edfenergy.com/energy/nuclear-lifetime-management ''Extending the operating lives of Advanced Gas-cooled Reactors''] {{Webarchive|url=https://web.archive.org/web/20230219093947/https://www.edfenergy.com/energy/nuclear-lifetime-management |date=19 February 2023 }}. EDF Energy</ref>
All seven AGR plants are expected to be shut down in 2022 and in decommissioning by 2028.<ref>[https://www.edfenergy.com/about/nuclear/decommissioning ''Nuclear decommissioning''] {{Webarchive|url=https://web.archive.org/web/20230219093959/https://www.edfenergy.com/about/nuclear/decommissioning |date=19 February 2023 }}. EDF (accessed Feb 2023)</ref> [[Hinkley Point B nuclear power station|Hinkley Point B]] was extended from 40 to 46 years, and closed. The same happened with [[Hunterston B nuclear power station|Hunterston B]], also after 46 years.

An increasing number of reactors is reaching or crossing their design lifetimes of 30 or 40 years. In 2014, [[Greenpeace]] warned that the lifetime extension of ageing nuclear power plants amounts to entering a new era of risk. It estimated the current European nuclear liability coverage in average to be too low by a factor of between 100 and 1,000 to cover the likely costs, while at the same time, the likelihood of a serious accident happening in Europe continues to increase as the reactor fleet grows older.<ref name= "greenpeace-2014" >[https://www.greenpeace.org/static/planet4-netherlands-stateless/2018/06/Briefing-Lifetime-extension-of-ageing-nuclear-power-plants.pdf ''Lifetime extension of ageing nuclear power plants: Entering a new era of risk.''] {{Webarchive|url=https://web.archive.org/web/20230315082620/https://www.greenpeace.org/static/planet4-netherlands-stateless/2018/06/Briefing-Lifetime-extension-of-ageing-nuclear-power-plants.pdf |date=15 March 2023 }} Greenpeace, March, 2014 (2.6&nbsp;MB). [https://inis.iaea.org/search/search.aspx?orig_q=RN:46030160 In German]</ref>

==Early reactors==
{{See also|Nuclear fission#History}}
[[File:Stagg Field reactor.jpg|thumb|The [[Chicago Pile-1|Chicago Pile]], the first artificial nuclear reactor, built in secrecy at the University of Chicago in 1942 during World War II as part of the US's [[Manhattan project]]]]
[[File:Otto Hahn und Lise Meitner.jpg|thumb|upright=0.75|[[Lise Meitner]] and [[Otto Hahn]] in their laboratory]]
[[File:ChicagoPileTeam.png|right|thumb|Some of the [[Chicago Pile-1|Chicago Pile Team]], including [[Enrico Fermi]] and [[Leó Szilárd]]]]

The [[neutron]] was discovered in 1932 by British physicist [[James Chadwick]]. The concept of a nuclear chain reaction brought about by [[nuclear reaction]]s mediated by neutrons was first realized shortly thereafter, by [[Hungary|Hungarian]] scientist [[Leó Szilárd]], in 1933. He filed a patent for his idea of a simple reactor the following year while working at the [[British Admiralty|Admiralty]] in London.<ref>L. Szilárd, [http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=GB630726 "Improvements in or relating to the transmutation of chemical elements," ] {{Webarchive|url=https://web.archive.org/web/20080621120547/http://v3.espacenet.com/textdoc?DB=EPODOC |date=21 June 2008 }} British patent number: GB630726 (filed: 28 June 1934; published: 30 March 1936).</ref> However, Szilárd's idea did not incorporate the idea of nuclear fission as a neutron source, since that process was not yet discovered. Szilárd's ideas for nuclear reactors using neutron-mediated nuclear chain reactions in light elements proved unworkable.

Inspiration for a new type of reactor using uranium came from the discovery by [[Otto Hahn]], [[Lise Meitner]], [[Fritz Strassmann]] in 1938 that bombardment of uranium with neutrons (provided by an alpha-on-beryllium fusion reaction, a "[[neutron howitzer]]" ) produced a [[barium]] residue, which they reasoned was created by the fissioning of the uranium nuclei. In their second publication on nuclear fission in February 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a [[nuclear chain reaction]]. Subsequent studies in early 1939 (one of them by Szilárd and Fermi) revealed that several neutrons were indeed released during the fissioning, making available the opportunity for the nuclear chain reaction that Szilárd had envisioned six years previously.

On 2 August 1939, [[Albert Einstein]] signed a letter to President [[Franklin D. Roosevelt]] (written by Szilárd) suggesting that the discovery of uranium's fission could lead to the development of "extremely powerful bombs of a new type", giving impetus to the study of reactors and fission. Szilárd and Einstein knew each other well and had worked together years previously, but Einstein had never thought about this possibility for nuclear energy until Szilard reported it to him, at the beginning of his quest to produce the [[Einstein-Szilárd letter]] to alert the U.S. government.

Shortly after, [[Nazi Germany]] invaded Poland in 1939, starting [[World War II]] in Europe. The U.S. was not yet officially at war, but in October, when the Einstein-Szilárd letter was delivered to him, Roosevelt commented that the purpose of doing the research was to make sure "the Nazis don't blow us up." The U.S. nuclear project followed, although with some delay as there remained skepticism (some of it from Fermi) and also little action from the small number of officials in the government who were initially charged with moving the project forward.

The following year, the U.S. Government received the [[Frisch–Peierls memorandum]] from the UK, which stated that the amount of [[uranium]] needed for a [[chain reaction]] was far lower than had previously been thought. The memorandum was a product of the [[MAUD Committee]], which was working on the UK atomic bomb project, known as [[Tube Alloys]], later [[British contribution to the Manhattan Project|to be subsumed]] within the [[Manhattan Project]].

Eventually, the first artificial nuclear reactor, [[Chicago Pile-1]], was constructed at the [[University of Chicago]], by a team led by [[Italians|Italian]] physicist [[Enrico Fermi]], in late 1942. By this time, the program had been pressured for a year by U.S. entry into the war. The Chicago Pile achieved [[critical mass|criticality]] on 2 December 1942<ref name= ":0" >The First Reactor, U.S. Atomic Energy Commission, Division of Technical Information</ref> at 3:25&nbsp;PM. The reactor support structure was made of wood, which supported a pile (hence the name) of graphite blocks, embedded in which was natural uranium oxide 'pseudospheres' or 'briquettes'.

Soon after the Chicago Pile, the [[Metallurgical Laboratory]] developed a number of nuclear reactors for the [[Manhattan Project]] starting in 1943. The primary purpose for the largest reactors (located at the [[Hanford Site]] in [[Washington (state)|Washington]]), was the mass production of [[plutonium]] for nuclear weapons. Fermi and Szilard applied for a patent on reactors on 19 December 1944. Its issuance was delayed for 10&nbsp;years because of wartime secrecy.<ref>Enrico, Fermi and Leo, Szilard {{US Patent|2708656}} "Neutronic Reactor" issued 17 May 1955</ref>

"World's first nuclear power plant" is the claim made by signs at the site of the [[EBR-I]], which is now a museum near [[Arco, Idaho]]. Originally called "Chicago Pile-4", it was carried out under the direction of [[Walter Zinn]] for [[Argonne National Laboratory]].<ref>{{cite web|url=http://www.ne.anl.gov/About/hn/news960320.shtml|title=Chicago Pile reactors create enduring research legacy – Argonne's Historical News Releases|work=anl.gov|access-date=21 August 2013|archive-date=13 June 2022|archive-url=https://web.archive.org/web/20220613201103/http://www.ne.anl.gov/About/hn/news960320.shtml|url-status=live}}</ref> This experimental [[LMFBR]] operated by the [[U.S. Atomic Energy Commission]] produced 0.8&nbsp;kW in a test on 20 December 1951<ref>[https://inlportal.inl.gov/portal/server.pt/gateway/PTARGS_0_200_816_259_0_43/http%3B/inlpublisher%3B7087/publishedcontent/publish/communities/inl_gov/about_inl/home_page_fact_sheets/sheets/experimental_breeder_reactor___i_4.pdf Experimental Breeder Reactor 1 factsheet], Idaho National Laboratory {{webarchive |url=https://web.archive.org/web/20081029200744/https://inlportal.inl.gov/portal/server.pt/gateway/PTARGS_0_200_816_259_0_43/http%3B/inlpublisher%3B7087/publishedcontent/publish/communities/inl_gov/about_inl/home_page_fact_sheets/sheets/experimental_breeder_reactor___i_4.pdf |date=29 October 2008 }}</ref> and 100&nbsp;kW (electrical) the following day,<ref>{{cite web | url=http://www.ans.org/pubs/magazines/nn/docs/2001-11-2.pdf | title=Fifty years ago in December: Atomic reactor EBR-I produced first electricity | publisher=American Nuclear Society Nuclear news | date=November 2001 | access-date=18 June 2008 | archive-url=https://web.archive.org/web/20080625035749/http://www.ans.org/pubs/magazines/nn/docs/2001-11-2.pdf | archive-date=25 June 2008 | url-status=dead }}</ref> having a design output of 200&nbsp;kW (electrical).

Besides the military uses of nuclear reactors, there were political reasons to pursue civilian use of atomic energy. U.S. President [[Dwight Eisenhower]] made his famous [[Atoms for Peace]] speech to the [[UN General Assembly]] on 8 December 1953. This diplomacy led to the dissemination of reactor technology to U.S. institutions and worldwide.<ref>{{cite web|url=https://www.pbs.org/wgbh/nova/tech/the-nuclear-option.html|title=The Nuclear Option — NOVA {{!}} PBS|website=www.pbs.org|date=11 January 2017|access-date=2017-01-12|archive-date=3 September 2017|archive-url=https://web.archive.org/web/20170903030256/https://www.pbs.org/wgbh/nova/tech/the-nuclear-option.html|url-status=live}}</ref>

The first nuclear power plant built for civil purposes was the AM-1 [[Obninsk Nuclear Power Plant]], launched on 27 June 1954 in the [[Soviet Union]]. It produced around 5&nbsp;MW (electrical). It was built after the [[F-1 (nuclear reactor)]] which was the first reactor to go critical in Europe, and was also built by the Soviet Union.

After World War II, the U.S. military sought other uses for nuclear reactor technology. Research by the Army led to the power stations for Camp Century, Greenland and McMurdo Station, Antarctica [[Army Nuclear Power Program]]. The Air Force Nuclear Bomber project resulted in the [[Molten-Salt Reactor Experiment]]. The U.S. Navy succeeded when they steamed the [[USS Nautilus (SSN-571)|USS ''Nautilus'']] (SSN-571) on nuclear power 17 January 1955.

The first commercial nuclear power station, [[Calder Hall nuclear power station|Calder Hall]] in [[Sellafield]], England was opened in 1956 with an initial capacity of 50 MW (later 200 MW).<ref name=Kragh>{{cite book |last=Kragh|first=Helge |title=Quantum Generations: A History of Physics in the Twentieth Century |url=https://archive.org/details/quantumgeneratio0000krag|url-access=registration|publisher=Princeton University Press |location=Princeton NJ |year=1999 |page=[https://archive.org/details/quantumgeneratio0000krag/page/286 286] |isbn=0-691-09552-3}}</ref><ref name= "bbc17oct" >{{cite news |url=http://news.bbc.co.uk/onthisday/hi/dates/stories/october/17/newsid_3147000/3147145.stm |title=On This Day: 17&nbsp;October |access-date=9 November 2006 |work=BBC News |date=17 October 1956 |archive-date=27 October 2019 |archive-url=https://web.archive.org/web/20191027163058/http://news.bbc.co.uk/onthisday/hi/dates/stories/october/17/newsid_3147000/3147145.stm |url-status=live }}</ref>

The first portable nuclear reactor "Alco PM-2A" was used to generate electrical power (2 MW) for [[Camp Century]] from 1960 to 1963.<ref>{{cite web | url=http://gombessa.tripod.com/scienceleadstheway/id9.html | title=Science Leads the Way | publisher=Camp Century, Greenland | first=Frank J. | last=Leskovitz | access-date=9 September 2008 | archive-date=29 August 2010 | archive-url=https://web.archive.org/web/20100829201023/http://gombessa.tripod.com/scienceleadstheway/id9.html | url-status=live }}</ref>

[[File:HPR1000, reactor coolant system.png|thumb|Primary coolant system showing [[reactor pressure vessel]] (red), [[Steam generator (nuclear power)|steam generators]] (purple), [[pressurizer]] (blue), and pumps (green) in the three coolant loop [[Hualong One]] [[pressurized water reactor]] design]]

==Reactor types==
{{image frame
|width=210
|caption=Number of reactors by type (end 2014)<ref name= "IAEA_reactors_stats" >{{cite web|title=Nuclear Power Reactors in the World – 2015 Edition|url=http://www-pub.iaea.org/MTCD/Publications/PDF/rds2-35web-85937611.pdf|publisher=International Atomic Energy Agency (IAEA)|access-date=26 October 2017|archive-date=16 November 2020|archive-url=https://web.archive.org/web/20201116191727/https://www-pub.iaea.org/MTCD/Publications/PDF/rds2-35web-85937611.pdf|url-status=live}}</ref>
|content=<div style= "text-align:left" >
{{#invoke:Chart|pie chart
| radius = 100
| slices =
( 277: PWR:: [[Pressurized Water Reactor]])
( 80: BWR:: [[Boiling Water Reactor]] )
( 15: GCR:: [[Gas Cooled Reactor]] )
( 49: PHWR:: [[Pressurized Heavy Water Reactor]] )
( 15: LWGR:: [[LWGR]] )
( 2: FBR:: [[Fast Breeder Reactor]] )
| units suffix =
| percent = true
}}</div>
}}
{{image frame
|width=210
|caption=Net power capacity (GWe) by type (end 2014)<ref name= "IAEA_reactors_stats" />
|content=<div style= "text-align:left" >
{{#invoke:Chart|pie chart
| radius = 100
| slices =
( 257.2: PWR:: [[Pressurized Water Reactor]])
( 75.5: BWR:: [[Boiling Water Reactor]] )
( 8.2: GCR:: [[Gas Cooled Reactor]] )
( 24.6: PHWR:: [[Pressurized Heavy Water Reactor]] )
( 10.2: LWGR:: [[LWGR]] )
( 0.6: FBR:: [[Fast Breeder Reactor]] )
| units suffix =
| percent = true
}}</div>
}}

[[File:Pulstar2.jpg|thumb|upright|[[North Carolina State University|NC State]]'s PULSTAR Reactor is a 1&nbsp;MW pool-type [[research reactor]] with 4% enriched, pin-type fuel consisting of UO<sub>2</sub> pellets in [[zircaloy]] cladding.]]

===Classifications===

====By type of nuclear reaction====
All commercial power reactors are based on [[nuclear fission]]. They generally use [[uranium]] and its product [[plutonium]] as [[nuclear fuel]], though a [[thorium fuel cycle]] is also possible. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that sustain the fission [[chain reaction]]:
* [[Thermal reactor|Thermal-neutron reactor]]s use slowed or [[thermal neutron]]s to keep up the fission of their fuel. Almost all current reactors are of this type. These contain [[neutron moderator]] materials that slow neutrons until their [[neutron temperature]] is ''thermalized'', that is, until their [[kinetic energy]] approaches the average kinetic energy of the surrounding particles. Thermal neutrons have a far higher [[Nuclear cross section|cross section]] (probability) of fissioning the [[fissile]] nuclei [[uranium-235]], [[plutonium-239]], and [[plutonium-241]], and a relatively lower probability of [[neutron capture]] by [[uranium-238]] (U-238) compared to the faster neutrons that originally result from fission, allowing use of [[low-enriched uranium]] or even [[natural uranium]] fuel. The moderator is often also the [[coolant]], usually water under high pressure to increase the [[boiling point]]. These are surrounded by a [[reactor vessel]], instrumentation to monitor and control the reactor, [[radiation shielding]], and a [[containment building]].
* [[Fast-neutron reactor]]s use [[fast neutron]]s to cause fission in their fuel. They do not have a [[neutron moderator]], and use less-moderating coolants. Maintaining a chain reaction requires the fuel to be more highly [[isotope separation|enriched]] in [[fissile]] material (about 20% or more) due to the relatively lower probability of fission versus capture by U-238. Fast reactors have the potential to produce less [[transuranic]] waste because all [[actinides]] are fissionable with fast neutrons,<ref>{{Cite journal | doi = 10.1007/BF00750983| title = Fast-reactor actinoid transmutation| journal = Atomic Energy| volume = 74| page = 83| year = 1993| last1 = Golubev | first1 = V. I.| last2 = Dolgov | first2 = V. V.| last3 = Dulin | first3 = V. A.| last4 = Zvonarev | first4 = A. V.| last5 = Smetanin | first5 = É. Y. | last6 = Kochetkov | first6 = L. A.| last7 = Korobeinikov | first7 = V. V.| last8 = Liforov | first8 = V. G.| last9 = Manturov | first9 = G. N.| last10 = Matveenko | first10 = I. P.| last11 = Tsibulya | first11 = A. M.| s2cid = 95704617}}</ref> but they are more difficult to build and more expensive to operate. Overall, fast reactors are less common than thermal reactors in most applications. Some early power stations were fast reactors, as are some Russian naval propulsion units. Construction of prototypes is continuing (see [[fast breeder]] or [[Generation IV reactor#Fast reactors|generation IV reactors]]).

In principle, [[fusion power]] could be produced by [[nuclear fusion]] of elements such as the [[deuterium]] isotope of [[hydrogen]]. While an ongoing rich research topic since at least the 1940s, no self-sustaining fusion reactor for any purpose has ever been built.

====By moderator material====
Used by thermal reactors:
* [[Graphite-moderated reactor]]s
** Mostly early reactors such as the Chicago pile, Obninsk am 1, Windscale piles, RBMK, Magnox, and others such as AGR use graphite as a moderator.
* Water moderated reactors
**[[Heavy-water reactor]]s (Used in Canada,<ref name= "hyperphysics" >{{cite web|last1=Nave|first1=R|title=Light Water Nuclear Reactors|url=http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/ligwat.html|website=Hyperphysics|publisher=Georgia State University|access-date=5 March 2018|archive-date=3 December 2017|archive-url=https://web.archive.org/web/20171203053318/http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/ligwat.html|url-status=live}}</ref> India, Argentina, China, Pakistan, Romania and South Korea).<ref>{{Cite book|last=Joyce|first=Malcolm|date=2018|title=Nuclear Engineering|publisher=Elsevier|chapter=10.6|doi=10.1016/c2015-0-05557-5|isbn=9780081009628}}</ref>
** [[Light-water reactor|Light-water-moderated reactors]] (LWRs). Light-water reactors (the most common type of thermal reactor) use ordinary water to moderate and cool the reactors.<ref name= "hyperphysics" /> Because the light hydrogen isotope is a slight neutron poison these reactors need artificially enriched fuels. When at [[operating temperature]], if the temperature of the water increases, its density drops, and fewer neutrons passing through it are slowed enough to trigger further reactions. That [[negative feedback]] stabilizes the reaction rate. Graphite and heavy-water reactors tend to be more thoroughly thermalized than light water reactors. Due to the extra thermalization, and the absence of the light hydrogen poisoning effects these types can use [[natural uranium]]/unenriched fuel.
* Light-element-moderated reactors.
** [[Molten-salt reactor]]s (MSRs) are moderated by light elements such as lithium or beryllium, which are constituents of the coolant/fuel matrix salts [[Lithium fluoride| "LiF" ]] and [[Beryllium fluoride| "BeF<sub>2</sub>]]", [[Lithium chloride| "LiCl" ]] and [[Beryllium chloride| "BeCl<sub>2</sub>]]" and other light element containing salts can all cause a moderating effect.
** [[Liquid metal cooled reactor]]s, such as those whose coolant is a mixture of lead and bismuth, may use BeO as a moderator.
* [[Organic nuclear reactor|Organically moderated reactors]] (OMR) use [[biphenyl]] and [[terphenyl]] as moderator and coolant.

====By coolant====
[[File:RIAN archive 450312 Treatment of interior part of reactor frame.jpg|thumb|Treatment of the interior part of a [[VVER|VVER-1000]] reactor frame at [[Atommash]] ]]
[[File:Thermal reactor diagram.png|thumb|In thermal nuclear reactors (LWRs in specific), the coolant acts as a moderator that must slow down the neutrons before they can be efficiently absorbed by the fuel.]]
* Water cooled reactor. These constitute the great majority of operational nuclear reactors: as of 2014, 93% of the world's nuclear reactors are water cooled, providing about 95% of the world's total nuclear generation capacity.<ref name= "IAEA_reactors_stats" />
** [[Pressurized water reactor]] (PWR) Pressurized water reactors constitute the large majority of all Western nuclear power plants.
*** A primary characteristic of PWRs is a pressurizer, a specialized [[pressure vessel]]. Most commercial PWRs and naval reactors use pressurizers. During normal operation, a pressurizer is partially filled with water, and a steam bubble is maintained above it by heating the water with submerged heaters. During normal operation, the pressurizer is connected to the primary reactor pressure vessel (RPV) and the pressurizer "bubble" provides an expansion space for changes in water volume in the reactor. This arrangement also provides a means of pressure control for the reactor by increasing or decreasing the steam pressure in the pressurizer using the pressurizer heaters.
*** [[Pressurized heavy water reactor]]s are a subset of pressurized water reactors, sharing the use of a pressurized, isolated heat transport loop, but using [[heavy water]] as coolant and moderator for the greater neutron economies it offers.
** [[Boiling water reactor]] (BWR)
*** BWRs are characterized by boiling water around the fuel rods in the lower portion of a primary reactor pressure vessel. A boiling water reactor uses <sup>235</sup>U, enriched as uranium dioxide, as its fuel. The fuel is assembled into rods housed in a steel vessel that is submerged in water. The nuclear fission causes the water to boil, generating steam. This steam flows through pipes into turbines. The turbines are driven by the steam, and this process generates electricity.<ref name= "nuclear_energy" >{{cite web |last1=Lipper |first1=Ilan |first2=Jon |last2=Stone |url=http://www.umich.edu/~gs265/society/nuclear.htm |title=Nuclear Energy and Society |publisher=University of Michigan |access-date=3 October 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090401172451/http://www.umich.edu/~gs265/society/nuclear.htm |archive-date=1 April 2009 }}</ref> During normal operation, pressure is controlled by the amount of steam flowing from the reactor pressure vessel to the turbine.
** [[Supercritical water reactor]] (SCWR)
*** SCWRs are a [[Generation IV reactor]] concept where the reactor is operated at supercritical pressures and water is heated to a supercritical fluid, which never undergoes a transition to steam yet behaves like saturated steam, to power a [[Steam generator (boiler)|steam generator]].
** [[Reduced moderation water reactor]] [RMWR] which use more highly enriched fuel with the fuel elements set closer together to allow a faster neutron spectrum sometimes called an [[Epithermal neutron]] Spectrum.
** Pool-type reactor can refer to unpressurized water cooled [[open pool reactor]]s,<ref>{{cite web|url=https://www.ans.org/news/article-2066/pool-reactors-1-an-introduction/|title=Pool Reactors 1: An Introduction -- ANS / Nuclear Newswire|access-date=6 November 2021|archive-date=6 November 2021|archive-url=https://web.archive.org/web/20211106161715/https://www.ans.org/news/article-2066/pool-reactors-1-an-introduction/|url-status=live}}</ref> but not to be confused with [[pool type LMFBR]]s which are sodium cooled
** Some reactors have been cooled by [[heavy water]] which also served as a moderator. Examples include:
***Early [[CANDU]] reactors (later ones use heavy water moderator but light water coolant)
***[[DIDO (nuclear reactor)|DIDO]] class research reactors
* [[Liquid metal cooled reactor]]. Since water is a moderator, it cannot be used as a coolant in a fast reactor. Liquid metal coolants have included [[sodium]], [[NaK]], lead, [[lead-bismuth eutectic]], and in early reactors, [[mercury (element)|mercury]].
** [[Sodium-cooled fast reactor]]
** [[Lead-cooled fast reactor]]
* [[Gas cooled reactor]]s are cooled by a circulating gas. In commercial nuclear power plants carbon dioxide has usually been used, for example in current British AGR nuclear power plants and formerly in a number of first generation British, French, Italian, & Japanese plants. [[Nitrogen]]<ref>{{cite journal |title=Emergency and Back-Up Cooling of Nuclear Fuel and Reactors and Fire-Extinguishing, Explosion Prevention Using Liquid Nitrogen. |journal=USPTO Patent Applications |date=2018-05-24 |volume=Document number 20180144836 }}</ref> and helium have also been used, helium being considered particularly suitable for high temperature designs. Utilization of the heat varies, depending on the reactor. Commercial nuclear power plants run the gas through a [[heat exchanger]] to make steam for a steam turbine. Some experimental designs run hot enough that the gas can directly power a gas turbine.
* [[Molten-salt reactor]]s (MSRs) are cooled by circulating a molten salt, typically a eutectic mixture of fluoride salts, such as [[FLiBe]]. In a typical MSR, the coolant is also used as a matrix in which the fissile material is dissolved. Other eutectic salt combinations used include [[Zirconium tetrafluoride| "ZrF<sub>4</sub>" ]] with [[Sodium Fluoride| "NaF" ]] and [[Lithium chloride| "LiCl" ]] with [[Beryllium chloride| "BeCl<sub>2</sub>" ]].
* [[Organic nuclear reactor]]s use organic fluids such as biphenyl and terphenyl as coolant rather than water.

====By generation====
* Generation I reactor (early prototypes such as [[Shippingport Atomic Power Station]], research reactors, non-commercial power producing reactors)
* [[Generation II reactor]] (most current [[nuclear power plant]]s, 1965–1996)
* [[Generation III reactor]] (evolutionary improvements of existing designs, 1996–2016)
* [[Generation III reactor#Lists of Generation III+ reactors|Generation III+ reactor]] (evolutionary development of Gen III reactors, offering improvements in safety over Gen III reactor designs, 2017–2021)<ref>{{cite web|url=https://analysis.nuclearenergyinsider.com/russia-completes-worlds-first-gen-iii-reactor-china-start-five-reactors-2017|title=Russia completes world's first Gen III+ reactor; China to start up five reactors in 2017|date=8 February 2017|website=Nuclear Energy Insider|access-date=10 July 2019|archive-date=13 August 2020|archive-url=https://web.archive.org/web/20200813174111/https://analysis.nuclearenergyinsider.com/russia-completes-worlds-first-gen-iii-reactor-china-start-five-reactors-2017|url-status=live}}</ref>
* [[Generation IV reactor]] (technologies still under development; unknown start date, see below)<ref name= "gen-iv_wna-2020" />
* Generation V reactor (designs which are theoretically possible, but which are not being actively considered or researched at present).
In 2003, the French [[Commissariat à l'Énergie Atomique]] (CEA) was the first to refer to "Gen II" types in ''Nucleonics Week''.<ref>''Nucleonics Week'', Vol. 44, No. 39; p. 7, 25 September 2003 Quote: "Etienne Pochon, CEA director of nuclear industry support, outlined EPR's improved performance and enhanced safety features compared to the advanced Generation II designs on which it was based." </ref>

The first mention of "Gen III" was in 2000, in conjunction with the launch of the [[Generation IV International Forum]] (GIF) plans.

"Gen IV" was named in 2000, by the [[United States Department of Energy]] (DOE), for developing new plant types.<ref>{{cite web |url=http://www.euronuclear.org/info/generation-IV.htm |title=Generation IV |publisher=Euronuclear.org |access-date=18 March 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110317125012/http://www.euronuclear.org/info/generation-IV.htm |archive-date=17 March 2011 }}</ref>

====By phase of fuel====
* Solid fueled
* Fluid fueled
** [[Aqueous homogeneous reactor]]
** [[Molten-salt reactor]]
* [[Gaseous fission reactor|Gas fueled]] (theoretical)

====By shape of the core====
* Cubical
* Cylindrical
* Octagonal
* Spherical
* Slab
* Annulus

====By use====
* Electricity
** [[Nuclear power plant]]s including [[small modular reactor]]s
* Propulsion, see [[nuclear propulsion]]
** [[Nuclear marine propulsion]]
** Various proposed forms of [[rocket propulsion]]
* Other uses of heat
** [[Desalination]]
** Heat for domestic and industrial heating
** [[Hydrogen production]] for use in a [[hydrogen economy]]
* Production reactors for [[Nuclear transmutation|transmutation]] of elements
** [[Breeder reactor]]s are capable of producing more [[fissile material]] than they consume during the fission chain reaction (by converting [[Fertile material|fertile]] U-238 to Pu-239, or Th-232 to U-233). Thus, a uranium breeder reactor, once running, can be refueled with [[natural uranium|natural]] or even [[depleted uranium]], and a thorium breeder reactor can be refueled with [[thorium]]; however, an initial stock of fissile material is required.<ref name= "Gen4" >{{cite web |url= http://www.gen-4.org/PDFs/GenIVRoadmap.pdf |title= A Technology Roadmap for Generation IV Nuclear Energy Systems |url-status= dead |archive-url= https://web.archive.org/web/20061005211316/http://www.gen-4.org/PDFs/GenIVRoadmap.pdf |archive-date= 5 October 2006 |df= dmy-all |access-date= 5 March 2007 }}&nbsp;{{small|(4.33&nbsp;MB)}}; see "Fuel Cycles and Sustainability" </ref>
** Creating various [[radiation|radioactive]] [[isotope]]s, such as [[americium]] for use in [[smoke detector]]s, and cobalt-60, molybdenum-99 and others, used for imaging and medical treatment.
** Production of materials for [[nuclear weapon]]s such as [[weapons-grade]] [[plutonium]]
* Providing a source of [[neutron radiation]] (for example with the pulsed [[Godiva device]]) and [[positron radiation]]{{Clarify|date=March 2008|reason=Neither linked article mentions reactors used to generate positrons. Needs explanation.}} (e.g. [[neutron activation analysis]] and [[potassium-argon dating]]{{Clarify|date=March 2008}}<!-- how are reactors used for dating? Linked article makes no mention of positron sources -->)
* [[Research reactor]]: Typically reactors used for research and training, materials testing, or the production of radioisotopes for medicine and industry. These are much smaller than power reactors or those propelling ships, and many are on university campuses. There are about 280 such reactors operating, in 56&nbsp;countries. Some operate with high-enriched uranium fuel, and international efforts are underway to substitute low-enriched fuel.<ref>{{cite web | url=http://www.world-nuclear.org/info/inf61.htm | title=World Nuclear Association Information Brief -Research Reactors | url-status=dead | archive-url=https://web.archive.org/web/20061231105602/http://www.world-nuclear.org/info/inf61.htm | archive-date=31 December 2006 | df=dmy-all | access-date=3 May 2007 }}</ref>

===Current technologies===
{{unreferenced section|date=June 2015}}
[[File:Diablo canyon nuclear power plant.jpg|thumb|[[Diablo Canyon Power Plant|Diablo Canyon]] – a PWR]]
* [[Pressurized water reactor]]s (PWR) [moderator: high-pressure water; coolant: high-pressure water]

:: These reactors use a pressure vessel to contain the nuclear fuel, control rods, moderator, and coolant. The hot radioactive water that leaves the pressure vessel is looped through a steam generator, which in turn heats a secondary (nonradioactive) loop of water to steam that can run turbines. They represent the majority (around 80%) of current reactors. This is a [[thermal neutron]] reactor design, the newest of which are the Russian [[VVER-1200]], Japanese [[Advanced Pressurized Water Reactor]], American [[AP1000]], Chinese [[Hualong One|Hualong Pressurized Reactor]] and the Franco-German [[European Pressurized Reactor]]. All the [[United States Naval reactor]]s are of this type.
* [[Boiling water reactor]]s (BWR) [moderator: low-pressure water; coolant: low-pressure water]

:: A BWR is like a PWR without the steam generator. The lower pressure of its cooling water allows it to boil inside the pressure vessel, producing the steam that runs the turbines. Unlike a PWR, there is no primary and secondary loop. The [[thermal efficiency]] of these reactors can be higher, and they can be simpler, and even potentially more stable and safe. This is a thermal-neutron reactor design, the newest of which are the [[Advanced Boiling Water Reactor]] and the [[Economic Simplified Boiling Water Reactor]].
[[File:CANDU at Qinshan.jpg|thumb|The [[CANDU]] [[Qinshan Nuclear Power Plant]]]]
* [[Pressurised heavy water reactor|Pressurized Heavy Water Reactor]] (PHWR) [moderator: high-pressure heavy water; coolant: high-pressure heavy water]

:: A Canadian design (known as [[CANDU]]), very similar to PWRs but using [[heavy water]]. While heavy water is significantly more expensive than ordinary water, it has greater [[neutron economy]] (creates a higher number of thermal neutrons), allowing the reactor to operate without [[Isotope separation|fuel enrichment facilities]]. Instead of using a single large pressure vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. These reactors are fueled with natural [[uranium]] and are thermal-neutron reactor designs. PHWRs can be refueled while at full power, ([[online refueling]]) which makes them very efficient in their use of uranium (it allows for precise flux control in the core). CANDU PHWRs have been built in Canada, [[Argentina]], China, [[India]], [[Pakistan]], [[Romania]], and [[South Korea]]. India also operates a number of PHWRs, often termed 'CANDU derivatives', built after the Government of Canada halted nuclear dealings with India following the 1974 [[Smiling Buddha]] nuclear weapon test.
:[[File:Elektrownia Ignalina.jpg|thumb|The [[Ignalina Nuclear Power Plant]] – a RBMK type (closed 2009)]]
* Reaktor Bolshoy Moschnosti Kanalniy (High Power Channel Reactor) ([[RBMK]]) (also known as a Light-Water Graphite-moderated Reactor—LWGR) [moderator: graphite; coolant: high-pressure water]

:: A Soviet design, RBMKs are in some respects similar to CANDU in that they are refuelable during power operation and employ a pressure tube design instead of a PWR-style pressure vessel. However, unlike CANDU they are very unstable and large, making [[containment building]]s for them expensive. A series of critical safety flaws have also been identified with the RBMK design, though some of these were corrected following the [[Chernobyl disaster]]. Their main attraction is their use of light water and unenriched uranium. As of 2024, 7 remain open, mostly due to safety improvements and help from international safety agencies such as the DOE. Despite these safety improvements, RBMK reactors are still considered one of the most dangerous reactor designs in use. RBMK reactors were deployed only in the former [[Soviet Union]].
[[File:Sizewell A.jpg|thumb|The [[Magnox]] [[Sizewell A]] nuclear power station]]
[[File:Torness Nuclear Power Station, Scotland.JPG|thumb|The [[Torness nuclear power station]] – an AGR]]
* [[Gas-cooled reactor]] (GCR) and [[advanced gas-cooled reactor]] (AGR) [moderator: graphite; coolant: carbon dioxide]

:: These designs have a high thermal efficiency compared with PWRs due to higher operating temperatures. There are a number of operating reactors of this design, mostly in the United Kingdom, where the concept was developed. Older designs (i.e. [[Magnox]] stations) are either shut down or will be in the near future. However, the AGRs have an anticipated life of a further 10 to 20&nbsp;years. This is a thermal-neutron reactor design. Decommissioning costs can be high due to large volume of reactor core.
* [[Breeder reactor|Liquid metal]] [[Fast breeder reactor#Fast breeder reactor|fast-breeder reactor]] (LMFBR) [moderator: none; coolant: liquid metal]
[[File:Topaz nuclear reactor.jpg|thumb|right|Scaled-down model of [[TOPAZ nuclear reactor]]]]

:: This totally unmoderated reactor design produces more fuel than it consumes. They are said to "breed" fuel, because they produce fissionable fuel during operation because of [[neutron capture]]. These reactors can function much like a PWR in terms of efficiency, and do not require much high-pressure containment, as the liquid metal does not need to be kept at high pressure, even at very high temperatures. These reactors are [[fast neutron]], not thermal neutron designs. These reactors come in two types:

[[File:Superphénix.jpg|thumb|The [[Superphénix]], closed in 1998, was one of the few FBRs.]]

:::[[Lead-cooled fast reactor|Lead-cooled]]
:::: Using lead as the liquid metal provides excellent radiation shielding, and allows for operation at very high temperatures. Also, lead is (mostly) transparent to neutrons, so fewer neutrons are lost in the coolant, and the coolant does not become radioactive. Unlike sodium, lead is mostly inert, so there is less risk of explosion or accident, but such large quantities of lead may be problematic from toxicology and disposal points of view. Often a reactor of this type would use a [[lead-bismuth eutectic]] mixture. In this case, the bismuth would present some minor radiation problems, as it is not quite as transparent to neutrons, and can be transmuted to a radioactive isotope more readily than lead. The Russian [[Alfa class submarine]] uses a lead-bismuth-cooled fast reactor as its main power plant.
::: [[Sodium-cooled fast reactor|Sodium-cooled]]
:::: Most LMFBRs are of this type. The [[TOPAZ nuclear reactor|TOPAZ]], [[BN-350]] and [[BN-600]] in USSR; [[Superphénix]] in France; and [[Enrico Fermi Nuclear Generating Station|Fermi-I]] in the United States were reactors of this type. The sodium is relatively easy to obtain and work with, and it also manages to actually prevent corrosion on the various reactor parts immersed in it. However, sodium explodes violently when exposed to water, so care must be taken, but such explosions would not be more violent than (for example) a leak of superheated fluid from a pressurized-water reactor. The [[Monju Nuclear Power Plant|Monju reactor]] in Japan suffered a sodium leak in 1995 and could not be [[Monju Nuclear Power Plant#2010 Restart|restarted]] until May 2010. The [[EBR-I]], the first reactor to have a core meltdown, in 1955, was also a sodium-cooled reactor.
* [[Pebble-bed reactor]]s (PBR) [moderator: graphite; coolant: helium]
:: These use fuel molded into ceramic balls, and then circulate gas through the balls. The result is an efficient, low-maintenance, very safe reactor with inexpensive, standardized fuel. The prototypes were the [[AVR reactor|AVR]] and the [[THTR-300]] in Germany, which produced up to 308MW of electricity between 1985 and 1989 until it was shut down after experiencing a series of incidents and technical difficulties. The [[HTR-10]] is operating in China, where the [[HTR-PM]] is being developed. The HTR-PM is expected to be the first generation IV reactor to enter operation.<ref name= "WNN2018" >{{cite news|url=https://www.neimagazine.com/features/featurehtr-pm-making-dreams-come-true-7009889/|title=HTR-PM: Making dreams come true|work=Nuclear Engineering International|access-date=12 December 2019|archive-date=28 March 2022|archive-url=https://web.archive.org/web/20220328064002/https://www.neimagazine.com/features/featurehtr-pm-making-dreams-come-true-7009889/|url-status=dead}}</ref>
* [[Molten-salt reactor]]s (MSR) [moderator: graphite, or none for fast spectrum MSRs; coolant: molten salt mixture]
::These dissolve the fuels in [[fluoride]] or [[chloride]] salts, or use such salts for coolant. MSRs potentially have many safety features, including the absence of high pressures or highly flammable components in the core. They were initially designed for aircraft propulsion due to their high efficiency and high power density. One prototype, the [[Molten-Salt Reactor Experiment]], was built to confirm the feasibility of the [[Liquid fluoride thorium reactor]], a thermal spectrum reactor which would breed fissile uranium-233 fuel from thorium.
* [[Aqueous homogeneous reactor]] (AHR) [moderator: high-pressure light or heavy water; coolant: high-pressure light or heavy water]

:: These reactors use as fuel soluble nuclear salts (usually [[uranium sulfate]] or [[uranium nitrate]]) dissolved in water and mixed with the coolant and the moderator. As of April 2006, only five AHRs were in operation.<ref>{{cite web|url=https://nucleus.iaea.org/RRDB/RR/ReactorSearch.aspx|title=RRDB Search|website=nucleus.iaea.org|access-date=6 January 2019|archive-date=12 May 2019|archive-url=https://web.archive.org/web/20190512142147/https://nucleus.iaea.org/RRDB/RR/ReactorSearch.aspx|url-status=live}}</ref>

===Future and developing technologies===

====Advanced reactors====
More than a dozen advanced reactor designs are in various stages of development.<ref name= "UIC" >{{cite web |title=Advanced Nuclear Power Reactors |publisher=[[World Nuclear Association]] |url=http://world-nuclear.org/info/inf08.html |access-date=29 January 2010 |archive-date=6 February 2010 |archive-url=https://web.archive.org/web/20100206181830/http://www.world-nuclear.org/info/inf08.html |url-status=dead }}</ref> Some are evolutionary from the [[pressurized water reactor|PWR]], [[boiling water reactor|BWR]] and [[Pressurised Heavy Water Reactor|PHWR]] designs above, some are more radical departures. The former include the [[advanced boiling water reactor]] (ABWR), two of which are now operating with others under construction, and the planned [[passively safe]] [[Economic Simplified Boiling Water Reactor]] (ESBWR) and [[AP1000]] units (see [[Nuclear Power 2010 Program]]).
* The [[integral fast reactor]] (IFR) was built, tested and evaluated during the 1980s and then retired under the Clinton administration in the 1990s due to nuclear non-proliferation policies of the administration. Recycling spent fuel is the core of its design and it therefore produces only a fraction of the waste of current reactors.<ref name= "pbs" >{{cite web |url=https://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html |title=Nuclear Reaction: Why Do Americans Fear Nuclear Power? |access-date=9 November 2006 |publisher=Public Broadcasting Service (PBS) |author=Till, Charles |archive-date=17 April 2018 |archive-url=https://web.archive.org/web/20180417094454/https://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html |url-status=live }}</ref>
* The [[pebble-bed reactor]], a [[high-temperature gas-cooled reactor]] (HTGCR), is designed so high temperatures reduce power output by [[Doppler broadening]] of the fuel's neutron cross-section. It uses ceramic fuels so its safe operating temperatures exceed the power-reduction temperature range. Most designs are cooled by inert helium. Helium is not subject to steam explosions, resists neutron absorption leading to radioactivity, and does not dissolve contaminants that can become radioactive. Typical designs have more layers (up to 7) of passive containment than light water reactors (usually&nbsp;3). A unique feature that may aid safety is that the fuel balls actually form the core's mechanism, and are replaced one by one as they age. The design of the fuel makes fuel reprocessing expensive.
* The [[small, sealed, transportable, autonomous reactor]] (SSTAR) is being primarily researched and developed in the US, intended as a fast breeder reactor that is passively safe and could be remotely shut down in case the suspicion arises that it is being tampered with.
* The [[Clean and Environmentally Safe Advanced Reactor]] (CAESAR) is a nuclear reactor concept that uses steam as a moderator – this design is still in development.
* The [[reduced moderation water reactor]] builds upon the [[Advanced boiling water reactor]] ABWR) that is presently in use, it is not a complete fast reactor instead using mostly [[epithermal neutron]]s, which are between thermal and fast neutrons in speed.
* The [[hydrogen-moderated self-regulating nuclear power module]] (HPM) is a reactor design emanating from the [[Los Alamos National Laboratory]] that uses [[uranium hydride]] as fuel.
* [[Subcritical reactor]]s are designed to be safer and more stable, but pose a number of engineering and economic difficulties. One example is the [[energy amplifier]].
* Thorium-based reactors — It is possible to convert Thorium-232 into U-233 in reactors specially designed for the purpose. In this way, thorium, which is four times more abundant than uranium, can be used to breed U-233 nuclear fuel.<ref name=NASA>{{cite journal|last1=Juhasz|first1=Albert J.|last2=Rarick|first2=Richard A.|last3=Rangarajan|first3=Rajmohan|title=High Efficiency Nuclear Power Plants Using Liquid Fluoride Thorium Reactor Technology|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090038711.pdf|website=NASA|date=October 2009|access-date=27 October 2014|archive-date=28 April 2021|archive-url=https://web.archive.org/web/20210428205700/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090038711.pdf|url-status=live}}</ref> U-233 is also believed to have favourable nuclear properties as compared to traditionally used U-235, including better neutron economy and lower production of long lived transuranic waste.
** [[Advanced heavy-water reactor]] (AHWR) — A proposed heavy water moderated nuclear power reactor that will be the next generation design of the PHWR type. Under development in the [[Bhabha Atomic Research Centre]] (BARC), India.
** [[KAMINI]] – A unique reactor using Uranium-233 isotope for fuel. Built in India by [[Bhabha Atomic Research Centre|BARC]] and Indira Gandhi Center for Atomic Research ([[IGCAR]]).
** India is also planning to build fast breeder reactors using the thorium – Uranium-233 fuel cycle. The FBTR (Fast Breeder Test Reactor) in operation at [[Kalpakkam]] (India) uses Plutonium as a fuel and liquid sodium as a coolant.
** China, which has control of the [[Cerro Impacto]] deposit, has a reactor and hopes to replace [[coal energy]] with nuclear energy.<ref name=sch>{{cite web|url=https://supchina.com/2019/01/14/venezuela-china-explained-2/|title=The Venezuela-China relationship, explained: Belt and Road {{!}} Part 2 of 4|date=14 January 2019|website=SupChina|language=en-US|access-date=24 June 2019|archive-url=https://web.archive.org/web/20190624005848/https://supchina.com/2019/01/14/venezuela-china-explained-2/|archive-date=24 June 2019|url-status=dead}}</ref>

Rolls-Royce aims to sell nuclear reactors for the production of [[synfuel]] for aircraft.<ref>{{cite web |url=https://www.bloomberg.com/amp/news/articles/2019-12-06/rolls-royce-pitches-nuclear-reactors-as-key-to-clean-jet-fuel |title=Rolls-Royce Touts Nuclear Reactors as Key to Clean Jet Fuel |website=[[Bloomberg News]] |access-date=19 December 2019 |archive-date=19 December 2019 |archive-url=https://web.archive.org/web/20191219210954/https://www.bloomberg.com/amp/news/articles/2019-12-06/rolls-royce-pitches-nuclear-reactors-as-key-to-clean-jet-fuel |url-status=dead }}</ref>

====Generation IV reactors====
[[Generation IV reactor]]s are a set of theoretical nuclear reactor designs. These are generally not expected to be available for commercial use before 2040–2050,<ref name= "sa-2014" >{{Cite web |last=De Clercq |first=Geert |date=October 13, 2014 |title=Can Sodium Save Nuclear Power? |url=https://www.scientificamerican.com/article/can-sodium-save-nuclear-power/ |access-date=2022-08-10 |website=Scientific American |language=en |archive-date=29 July 2021 |archive-url=https://web.archive.org/web/20210729090905/https://www.scientificamerican.com/article/can-sodium-save-nuclear-power/ |url-status=live }}</ref> although the World Nuclear Association suggested that some might enter commercial operation before 2030.<ref name= "gen-iv_wna-2020" >[https://world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/generation-iv-nuclear-reactors.aspx ''Generation IV Nuclear Reactors''] {{Webarchive|url=https://web.archive.org/web/20230330074852/https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/generation-iv-nuclear-reactors.aspx |date=30 March 2023 }}. World Nuclear Association, update Dec 2020</ref> Current reactors in operation around the world are generally considered second- or third-generation systems, with the first-generation systems having been retired some time ago. Research into these reactor types was officially started by the Generation&nbsp;IV International Forum (GIF) based on eight technology goals. The primary goals being to improve nuclear safety, improve proliferation resistance, minimize waste and natural resource utilization, and to decrease the cost to build and run such plants.<ref name= "UIC1" >{{cite web |title=Generation IV Nuclear Reactors |publisher=[[World Nuclear Association]] |url=http://world-nuclear.org/info/inf77.html |access-date=29 January 2010 |archive-date=23 January 2010 |archive-url=https://web.archive.org/web/20100123063413/http://www.world-nuclear.org/info/inf77.html |url-status=dead }}</ref>
* [[Gas-cooled fast reactor]]
* [[Lead-cooled fast reactor]]
* [[Molten-salt reactor]]
* [[Sodium-cooled fast reactor]]
* [[Supercritical water reactor]]
* [[Very-high-temperature reactor]]

====Generation V+ reactors====
Generation V reactors are designs which are theoretically possible, but which are not being actively considered or researched at present. Though some generation V reactors could potentially be built with current or near term technology, they trigger little interest for reasons of economics, practicality, or safety.
* Liquid-core reactor. A closed loop [[Nuclear thermal rocket#Liquid core|liquid-core nuclear reactor]], where the fissile material is molten uranium or uranium solution cooled by a working gas pumped in through holes in the base of the containment vessel.
* [[Gaseous fission reactor|Gas-core reactor]]. A closed loop version of the [[Nuclear lightbulb|nuclear lightbulb rocket]], where the fissile material is gaseous uranium hexafluoride contained in a fused silica vessel. A working gas (such as hydrogen) would flow around this vessel and absorb the UV light produced by the reaction. This reactor design could also function [[Gas core reactor rocket|as a rocket engine]], as featured in Harry Harrison's 1976 science-fiction novel ''Skyfall''. In theory, using UF<sub>6</sub> as a working fuel directly (rather than as a stage to one, as is done now) would mean lower processing costs, and very small reactors. In practice, running a reactor at such high power densities would probably produce unmanageable [[neutron flux]], weakening most [[IFMIF|reactor materials]], and therefore as the flux would be similar to that expected in fusion reactors, it would require similar materials to those selected by the [[IFMIF|International Fusion Materials Irradiation Facility]].
** Gas core EM reactor. As in the gas core reactor, but with [[photovoltaic]] arrays converting the [[UV light]] directly to electricity.<ref>{{cite web |url=http://isjaee.hydrogen.ru/pdf/AEE04-07_Prelas.pdf |title=International Scientific Journal for Alternative Energy and Ecology, DIRECT CONVERSION OF NUCLEAR ENERGY TO ELECTRICITY, Mark A. Prelas |url-status=dead |archive-url=https://web.archive.org/web/20160304024833/http://isjaee.hydrogen.ru/pdf/AEE04-07_Prelas.pdf |archive-date=4 March 2016 |access-date=7 December 2013 }}</ref> This approach is similar to the experimentally proved [[photoelectric effect]] that would convert the X-rays generated from [[aneutronic fusion]] into electricity, by passing the high energy photons through an array of conducting foils to transfer some of their energy to electrons, the energy of the photon is captured electrostatically, similar to a [[capacitor]]. Since X-rays can go through far greater material thickness than electrons, many hundreds or thousands of layers are needed to absorb the X-rays.<ref>Quimby, D.C., High Thermal Efficiency X-ray energy conversion scheme for advanced fusion reactors, ASTM Special technical Publication, v.2, 1977, pp. 1161–1165</ref>
* [[Fission fragment reactor]]. A fission fragment reactor is a nuclear reactor that generates electricity by decelerating an ion beam of fission byproducts instead of using nuclear reactions to generate heat. By doing so, it bypasses the [[Carnot cycle]] and can achieve efficiencies of up to 90% instead of 40–45% attainable by efficient turbine-driven thermal reactors. The fission fragment ion beam would be passed through a [[magnetohydrodynamic generator]] to produce electricity.
* [[Hybrid nuclear fusion]]. Would use the neutrons emitted by fusion to fission a [[breeder reactor|blanket]] of [[fertile material]], like [[Uranium-238|U-238]] or [[thorium|Th-232]] and [[Nuclear transmutation|transmute]] other reactor's [[spent nuclear fuel]]/nuclear waste into relatively more benign isotopes.

====Fusion reactors====
{{Main|Fusion power}}
Controlled [[nuclear fusion]] could in principle be used in [[fusion power]] plants to produce power without the complexities of handling [[actinides]], but significant scientific and technical obstacles remain. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050. The [[ITER]] project is currently leading the effort to harness fusion power.

==Nuclear fuel cycle==
{{Main|Nuclear fuel cycle}}
Thermal reactors generally depend on refined and [[enriched uranium]]. Some nuclear reactors can operate with a mixture of plutonium and uranium (see [[MOX]]). The process by which uranium ore is mined, processed, enriched, used, possibly [[nuclear reprocessing|reprocessed]] and disposed of is known as the [[nuclear fuel cycle]].

Under 1% of the uranium found in nature is the easily fissionable U-235 [[isotope]] and as a result most reactor designs require enriched fuel.
Enrichment involves increasing the percentage of U-235 and is usually done by means of [[gaseous diffusion]] or [[gas centrifuge]]. The enriched result is then converted into [[uranium dioxide]] powder, which is pressed and fired into pellet form. These pellets are stacked into tubes which are then sealed and called [[Nuclear fuel|fuel rods]]. Many of these fuel rods are used in each nuclear reactor.

Most BWR and PWR commercial reactors use uranium enriched to about 4% U-235, and some commercial reactors with a high [[neutron economy]] do not require the fuel to be enriched at all (that is, they can use natural uranium). According to the [[International Atomic Energy Agency]] there are at least 100 [[research reactor]]s in the world fueled by highly enriched (weapons-grade/90% enrichment) uranium. Theft risk of this fuel (potentially used in the production of a nuclear weapon) has led to campaigns advocating conversion of this type of reactor to low-enrichment uranium (which poses less threat of proliferation).<ref>{{cite web | work=IAEA | url=http://www.iaea.org/NewsCenter/News/2006/heu_symposium.html | title=Improving Security at World's Nuclear Research Reactors: Technical and Other Issues Focus of June Symposium in Norway | date=7 June 2006 | access-date=3 August 2007 | archive-date=14 August 2007 | archive-url=https://web.archive.org/web/20070814090210/http://www.iaea.org/NewsCenter/News/2006/heu_symposium.html | url-status=live }}</ref>

[[Fissile]] U-235 and non-fissile but [[fissionable]] and [[Fertile material|fertile]] U-238 are both used in the fission process. U-235 is fissionable by thermal (i.e. slow-moving) neutrons. A thermal neutron is one which is moving about the same speed as the atoms around it. Since all atoms vibrate proportionally to their absolute temperature, a thermal neutron has the best opportunity to fission U-235 when it is moving at this same vibrational speed. On the other hand, U-238 is more likely to capture a neutron when the neutron is moving very fast. This U-239 atom will soon decay into plutonium-239, which is another fuel. Pu-239 is a viable fuel and must be accounted for even when a highly enriched uranium fuel is used. Plutonium fissions will dominate the U-235 fissions in some reactors, especially after the initial loading of U-235 is spent. Plutonium is fissionable with both fast and thermal neutrons, which make it ideal for either nuclear reactors or nuclear bombs.

Most reactor designs in existence are thermal reactors and typically use water as a neutron moderator (moderator means that it slows down the neutron to a thermal speed) and as a coolant. But in a [[fast breeder reactor]], some other kind of coolant is used which will not moderate or slow the neutrons down much. This enables fast neutrons to dominate, which can effectively be used to constantly replenish the fuel supply. By merely placing cheap unenriched uranium into such a core, the non-fissionable U-238 will be turned into Pu-239, "breeding" fuel.

In [[thorium fuel cycle]] [[thorium-232]] absorbs a [[neutron]] in either a fast or thermal reactor. The thorium-233 [[beta decay]]s to [[protactinium]]-233 and then to [[uranium-233]], which in turn is used as fuel. Hence, like [[uranium-238]], thorium-232 is a [[fertile material]].

===Fueling of nuclear reactors===
The amount of energy in the reservoir of [[nuclear fuel]] is frequently expressed in terms of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full-power days in a reactor's operating cycle (between refueling outage times) is related to the amount of [[fissile]] [[uranium-235]] (U-235) contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U-235 in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full-power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent", having spent four to six years in the reactor producing power. This spent fuel is discharged and replaced with new (fresh) fuel assemblies.{{citation needed|date=March 2019}} Though considered "spent," these fuel assemblies contain a large quantity of fuel.{{citation needed|date=March 2019}} In practice it is economics that determines the lifetime of nuclear fuel in a reactor. Long before all possible fission has taken place, the reactor is unable to maintain 100%, full output power, and therefore, income for the utility lowers as plant output power lowers. Most nuclear plants operate at a very low profit margin due to operating overhead, mainly regulatory costs, so operating below 100% power is not economically viable for very long.{{citation needed|date=March 2019}} The fraction of the reactor's fuel core replaced during refueling is typically one-third, but depends on how long the plant operates between refueling. Plants typically operate on 18 month refueling cycles, or 24 month refueling cycles. This means that one refueling, replacing only one-third of the fuel, can keep a nuclear reactor at full power for nearly two years.{{citation needed|date=March 2019}}

The disposition and storage of this spent fuel is one of the most challenging aspects of the operation of a commercial nuclear power plant. This nuclear waste is highly radioactive and its toxicity presents a danger for thousands of years.<ref name= "nuclear_energy" /> After being discharged from the reactor, spent nuclear fuel is transferred to the on-site [[spent fuel pool]]. The spent fuel pool is a large pool of water that provides cooling and shielding of the spent nuclear fuel as well as limit radiation exposure to on-site personnel. Once the energy has decayed somewhat (approximately five years), the fuel can be transferred from the fuel pool to dry shielded casks, that can be safely stored for thousands of years. After loading into dry shielded casks, the casks are stored on-site in a specially guarded facility in impervious concrete bunkers. On-site fuel storage facilities are designed to withstand the impact of commercial airliners, with little to no damage to the spent fuel. An average on-site fuel storage facility can hold 30 years of spent fuel in a space smaller than a football field.{{citation needed|date=March 2019}}

Not all reactors need to be shut down for refueling; for example, [[pebble bed reactor]]s, [[RBMK|RBMK reactors]], [[molten-salt reactor]]s, [[Magnox]], [[Advanced gas-cooled reactor|AGR]] and [[CANDU]] reactors allow fuel to be shifted through the reactor while it is running. In a CANDU reactor, this also allows individual fuel elements to be situated within the reactor core that are best suited to the amount of U-235 in the fuel element.

The amount of energy extracted from nuclear fuel is called its [[burnup]], which is expressed in terms of the heat energy produced per initial unit of fuel weight. Burnup is commonly expressed as megawatt days thermal per metric ton of initial heavy metal.

==Nuclear safety==
{{Main|Nuclear safety}}
{{See also|Nuclear reactor safety system}}
Nuclear safety covers the actions taken to prevent [[nuclear and radiation accidents and incidents]] or to limit their consequences. The nuclear power industry has improved the safety and performance of reactors, and has proposed new, safer (but generally untested) reactor designs but there is no guarantee that the reactors will be designed, built and operated correctly.<ref name=globen/> Mistakes do occur and the designers of reactors at [[Timeline of the Fukushima nuclear accidents|Fukushima]] in Japan did not anticipate that a tsunami generated by an earthquake would disable the backup systems that were supposed to stabilize the reactor after the earthquake,<ref>{{cite web |url=http://www.thebulletin.org/web-edition/columnists/hugh-gusterson/the-lessons-of-fukushima |title=The lessons of Fukushima |author=Gusterson, Hugh |date=16 March 2011 |work=Bulletin of the Atomic Scientists |url-status=dead |archive-url=https://web.archive.org/web/20130606023005/http://www.thebulletin.org/web-edition/columnists/hugh-gusterson/the-lessons-of-fukushima |archive-date=6 June 2013 }}</ref> despite multiple warnings by the NRG and the Japanese nuclear safety administration.{{citation needed|date=April 2016}} According to [[UBS]] AG, the [[Fukushima I nuclear accidents]] have cast doubt on whether even an advanced economy like Japan can master nuclear safety.<ref>{{cite web|url=http://www.businessweek.com/news/2011-04-04/fukushima-crisis-worse-for-atomic-power-than-chernobyl-ubs-says.html |title=Fukushima Crisis Worse for Atomic Power Than Chernobyl, UBS Says |author=Paton, James |date=4 April 2011 |work=Bloomberg Businessweek |url-status=dead |archive-url=https://web.archive.org/web/20110515064305/http://www.businessweek.com/news/2011-04-04/fukushima-crisis-worse-for-atomic-power-than-chernobyl-ubs-says.html |archive-date=15 May 2011 }}</ref> Catastrophic scenarios involving terrorist attacks are also conceivable.<ref name=globen>{{cite web |url=http://www.stanford.edu/group/efmh/jacobson/Articles/I/WWSEnergyPolicyPtI.pdf |title=Providing all Global Energy with Wind, Water, and Solar Power, Part I: Technologies, Energy Resources, Quantities and Areas of Infrastructure, and Materials |author1=Jacobson, Mark Z. |author2=Delucchi, Mark A. |name-list-style=amp |year=2010 |work=Energy Policy |page=6 }}{{dead link|date=December 2015}}</ref> An interdisciplinary team from [[MIT]] has estimated that given the expected growth of nuclear power from 2005 to 2055, at least four serious nuclear accidents would be expected in that period.<ref>{{cite web |url=http://web.mit.edu/nuclearpower/pdf/nuclearpower-full.pdf |title=The Future of Nuclear Power |author=Massachusetts Institute of Technology |year=2003 |page=48 |access-date=15 June 2011 |archive-date=12 April 2019 |archive-url=https://web.archive.org/web/20190412094517/http://web.mit.edu/nuclearpower/pdf/nuclearpower-full.pdf |url-status=live }}</ref>

==Nuclear accidents==
{{See also|Lists of nuclear disasters and radioactive incidents}}
[[File:Fukushima I by Digital Globe.jpg|thumb|Three of the reactors at [[Fukushima I nuclear accidents|Fukushima I]] overheated, causing the coolant water to [[Water splitting|dissociate]] and led to the hydrogen explosions. This along with fuel [[nuclear meltdown|meltdowns]] released large amounts of [[radioactive]] material into the air.<ref>{{cite news |url=https://www.nytimes.com/2011/06/02/world/asia/02japan.html?_r=1&ref=world |title=Report Finds Japan Underestimated Tsunami Danger |author=Fackler, Martin |date=1 June 2011 |work=The New York Times }}</ref>]]

Serious, though rare, [[Nuclear and radiation accidents and incidents|nuclear and radiation accidents]] have occurred. These include the [[Windscale fire]] (October 1957), the [[SL-1]] accident (1961), the [[Three Mile Island accident]] (1979), [[Chernobyl disaster]] (April 1986), and the [[Fukushima Daiichi nuclear disaster]] (March 2011).<ref name=timenuke/> [[Nuclear-powered submarine]] mishaps include the [[Soviet submarine K-19|K-19]] reactor accident (1961),<ref name=rad>[http://www.iaea.org/Publications/Magazines/Bulletin/Bull413/article1.pdf Strengthening the Safety of Radiation Sources] {{Webarchive|url=https://web.archive.org/web/20150111213912/http://www.iaea.org/Publications/Magazines/Bulletin/Bull413/article1.pdf |date=11 January 2015 }} p. 14.</ref> the [[Soviet submarine K-27|K-27]] reactor accident (1968),<ref name=johnston2007>{{cite web |url=http://www.johnstonsarchive.net/nuclear/radevents/radevents1.html |title=Deadliest radiation accidents and other events causing radiation casualties |author=Johnston, Robert |date=23 September 2007 |publisher=Database of Radiological Incidents and Related Events |access-date=27 June 2011 |archive-date=23 October 2007 |archive-url=https://web.archive.org/web/20071023104305/http://www.johnstonsarchive.net/nuclear/radevents/radevents1.html |url-status=live }}</ref> and the [[Soviet submarine K-431|K-431]] reactor accident (1985).<ref name=timenuke>[https://web.archive.org/web/20090328130544/http://www.time.com/time/photogallery/0,29307,1887705,00.html The Worst Nuclear Disasters]. ''Time''.</ref>

Nuclear reactors have been launched into Earth orbit at least 34 times. A number of incidents connected with the unmanned nuclear-reactor-powered Soviet [[RORSAT]] especially [[Kosmos 954]] radar satellite which resulted in nuclear fuel reentering the Earth's atmosphere from orbit and being dispersed in northern Canada (January 1978).

==Natural nuclear reactors==
{{Main|Natural nuclear fission reactor}}
Almost two billion years ago a series of self-sustaining nuclear fission "reactors" self-assembled in the area now known as [[Oklo]] in [[Gabon]], West Africa. The conditions at that place and time allowed a [[natural nuclear fission reactor|natural nuclear fission]] to occur with circumstances that are similar to the conditions in a constructed nuclear reactor.<ref>[http://video.google.com/videoplay?docid=-2334857802602777622 Video of physics lecture] – at Google Video; a natural nuclear reactor is mentioned at 42:40 mins into the video {{webarchive |url=https://web.archive.org/web/20060804021811/http://video.google.com/videoplay?docid=-2334857802602777622 |date=4 August 2006 }}</ref> Fifteen fossil natural fission reactors have so far been found in three separate ore deposits at the Oklo uranium mine in Gabon. First discovered in 1972 by French physicist [[Francis Perrin (physicist)|Francis Perrin]], they are collectively known as the [[Natural nuclear fission reactor|Oklo Fossil Reactors]]. Self-sustaining [[nuclear fission]] reactions took place in these reactors approximately 1.5&nbsp;billion years ago, and ran for a few hundred thousand years, averaging 100&nbsp;kW of power output during that time.<ref>Meshik, Alex P. (November 2005) [http://www.scientificamerican.com/article/ancient-nuclear-reactor/ "The Workings of an Ancient Nuclear Reactor." ] {{Webarchive|url=https://web.archive.org/web/20150315120003/http://www.scientificamerican.com/article/ancient-nuclear-reactor/ |date=15 March 2015 }} ''Scientific American.'' p. 82.</ref> The concept of a natural nuclear reactor was theorized as early as 1956 by [[Paul Kuroda]] at the [[University of Arkansas]].<ref name= "OCRWM" >{{cite web|title=Oklo: Natural Nuclear Reactors |work=Office of Civilian Radioactive Waste Management |url=http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml |access-date=28 June 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060316101947/http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml |archive-date=16 March 2006 }}</ref><ref name= "ANS1" >{{cite web |title=Oklo's Natural Fission Reactors |work=[[American Nuclear Society]] |url=http://www.ans.org/pi/np/oklo |access-date=28 June 2006 |archive-date=30 March 2021 |archive-url=https://web.archive.org/web/20210330044447/https://ans.org/pi/np/oklo/ |url-status=live }}</ref>

Such reactors can no longer form on Earth in its present geologic period. Radioactive decay of formerly more abundant uranium-235 over the time span of hundreds of millions of years has reduced the proportion of this naturally occurring fissile isotope to below the amount required to sustain a chain reaction with only plain water as a moderator.

The natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years, cycling on the order of hours to a few days.

These natural reactors are extensively studied by scientists interested in geologic [[radioactive waste]] disposal. They offer a case study of how radioactive isotopes migrate through the Earth's crust. This is a significant area of controversy as opponents of geologic waste disposal fear that isotopes from stored waste could end up in water supplies or be carried into the environment.

==Emissions==
Nuclear reactors produce [[tritium]] as part of normal operations, which is eventually released into the environment in trace quantities.

As an [[isotope]] of [[hydrogen]], tritium (T) frequently binds to oxygen and forms [[tritiated water|T<sub>2</sub>O]]. This molecule is chemically identical to [[water|H<sub>2</sub>O]] and so is both colorless and odorless, however the additional neutrons in the hydrogen nuclei cause the tritium to undergo [[beta decay]] with a [[half-life]] of 12.3 years. Despite being measurable, the tritium released by nuclear power plants is minimal. The United States [[Nuclear Regulatory Commission|NRC]] estimates that a person drinking water for one year out of a well contaminated by what they would consider to be a significant tritiated water spill would receive a radiation dose of 0.3 millirem.<ref name=NRC_Tritium_Backgrounder>{{cite report|date=February 2016|title=Backgrounder: Tritium, Radiation Protection Limits, and Drinking Water Standards|url=https://www.nrc.gov/docs/ML0620/ML062020079.pdf|publisher=United States Nuclear Regulatory Commission|access-date=17 August 2017|archive-date=18 August 2017|archive-url=https://web.archive.org/web/20170818090910/https://www.nrc.gov/docs/ML0620/ML062020079.pdf|url-status=live}}</ref> For comparison, this is an order of magnitude less than the 4 millirem a person receives on a round trip flight from Washington, D.C. to Los Angeles, a consequence of less atmospheric protection against highly energetic [[cosmic ray]]s at high altitudes.<ref name=NRC_Tritium_Backgrounder />

The amounts of [[strontium-90]] released from nuclear power plants under normal operations is so low as to be undetectable above natural background radiation. Detectable strontium-90 in ground water and the general environment can be traced to weapons testing that occurred during the mid-20th century (accounting for 99% of the Strontium-90 in the environment) and the Chernobyl accident (accounting for the remaining 1%).<ref>{{cite web|title=Radionuclides in Groundwater|url=https://www.nrc.gov/reactors/operating/ops-experience/tritium/rn-groundwater.html|website=U.S. NRC|publisher=nrc.gov|access-date=2 October 2017|archive-date=2 October 2017|archive-url=https://web.archive.org/web/20171002215404/https://www.nrc.gov/reactors/operating/ops-experience/tritium/rn-groundwater.html|url-status=live}}</ref>

==See also==
{{div col}}
* [[List of nuclear reactors]]
* [[List of small modular reactor designs]]
* [[List of United States Naval reactors]]
* [[Neutron transport]]
* [[Nuclear decommissioning]]
* [[Nuclear power by country]]
* [[Nuclear power in space]]
* [[One Less Nuclear Power Plant]]
* [[Radioisotope thermoelectric generator]]
* [[Safety engineering]]
* [[Sayonara Nuclear Power Plants]]
* [[Small modular reactor]]
* [[Thorium-based nuclear power]]
* [[Traveling wave reactor|Traveling-wave reactor (TWR)]]
* ''[[World Nuclear Industry Status Report]]''
* [[Nuclear microreactor]]
{{div col end}}

== Notes ==
{{reflist|group=note}}

==References==
{{reflist}}

==External links==
{{Commons category|Nuclear reactors}}
* [http://www.iaea.org/pris/ The Database on Nuclear Power Reactors – IAEA] {{Webarchive|url=https://web.archive.org/web/20130602010449/http://www.iaea.org/pris/ |date=2 June 2013 }}
* [https://web.archive.org/web/20110427113431/http://www.nmt.edu/nmt-news/336-2011/4101-uranium-conference-adds-session-reviewing-implications-of-japan-accident Uranium Conference adds discussion of Japan accident]
* [https://web.archive.org/web/20040929083915/https://www.democracynow.org/article.pl?sid=04%2F09%2F24%2F1359225 A Debate: Is Nuclear Power The Solution to Global Warming?]
* [https://web.archive.org/web/20050826111231/http://www.ucsusa.org/clean_energy/nuclear_safety/page.cfm?pageID=1408 Union of Concerned Scientists, Concerns re: US nuclear reactor program]
* [http://www.vega.org.uk/video/programme/67 Freeview Video 'Nuclear Power Plants — What's the Problem' A Royal Institution Lecture by John Collier by the Vega Science Trust.] {{Webarchive|url=https://web.archive.org/web/20111103151240/http://www.vega.org.uk/video/programme/67 |date=3 November 2011 }}
* [http://www.nei.org/howitworks/electricpowergeneration/ Nuclear Energy Institute — How it Works: Electric Power Generation] {{Webarchive|url=https://web.archive.org/web/20100130212938/http://www.nei.org/howitworks/electricpowergeneration |date=30 January 2010 }}
* [https://web.archive.org/web/20061010034414/http://alsos.wlu.edu/qsearch.aspx?browse=science%2FNuclear+Reactors Annotated bibliography of nuclear reactor technology from the Alsos Digital Library]
**{{in lang|ja}} [https://drive.google.com/file/d/1zui6mGN5fT0C34rMCQMtG0ygCAu1MEWF/view ソヴィエト liên bang における vũ trụ dụng nguyên tử lô の khai phát とその thật dụng ] {{Webarchive|url=https://web.archive.org/web/20190603155840/https://drive.google.com/file/d/1zui6mGN5fT0C34rMCQMtG0ygCAu1MEWF/view |date=3 June 2019 }}

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