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==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 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 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 |title=Pool Reactors 1: An Introduction – ANS / Nuclear Newswire |url=https://www.ans.org/news/article-2066/pool-reactors-1-an-introduction/ |url-status=live |archive-url=https://web.archive.org/web/20211106161715/https://www.ans.org/news/article-2066/pool-reactors-1-an-introduction/ |archive-date=6 November 2021 |access-date=6 November 2021}}</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, and 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. Use 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 type of fuel ==== * Uranium * Plutonium * [[Mixed oxide (MOX) fuel]] * Uranium-plutonium alloy{{Citation needed|date=March 2025}} * Transuranium element mix ([[neptunium]], [[plutonium]], [[americium]], [[curium]]){{Citation needed|date=March 2025}} ====By phase of fuel==== * Solid fueled ** Ceramic *** Oxide *** Carbide *** Nitride ** Metal * Fluid fueled ** [[Aqueous homogeneous reactor]] ** [[Molten-salt reactor]] ** Molten metal reactor (e.g. [[Los Alamos Molten Plutonium Reactor Experiment|LAMPRE]]){{Citation needed|date=March 2025}} * [[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 }} {{small|(4.33 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 countries. Some operate with high-enriched uranium fuel, and international efforts are underway to substitute low-enriched fuel.<ref>{{cite web |title=World Nuclear Association Information Brief – Research Reactors |url=http://www.world-nuclear.org/info/inf61.htm |url-status=dead |archive-url=https://web.archive.org/web/20061231105602/http://www.world-nuclear.org/info/inf61.htm |archive-date=31 December 2006 |access-date=3 May 2007 |df=dmy-all}}</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 can be refueled during power operation and employ a pressure tube design instead of a PWR-style pressure vessel. However, unlike CANDU they are 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 U.S. Department of Energy. 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 years. This is a thermal-neutron reactor design. Decommissioning costs can be high due to the large volume of the 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, and 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 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 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 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.
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