Editing Nuclear reactor (section)

===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 }}&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 |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>