Editing Neutron moderator

{{Short description|Substance that slows down particles with no electric charge}}
{{Science with neutrons}}
In [[nuclear engineering]], a '''neutron moderator''' is a medium that reduces the speed of [[Neutron temperature#Fast|fast neutrons]], ideally without [[neutron capture|capturing]] any, leaving them as [[Neutron temperature#Thermal|thermal neutrons]] with only [[Thermal energy|minimal (thermal) kinetic energy]]. These '''thermal neutrons''' are immensely more susceptible than '''fast neutrons''' to propagate a [[nuclear chain reaction]] of [[uranium-235]] or other [[Fissile material|fissile]] [[Radionuclide|isotope]] by colliding with their [[atomic nucleus]].

[[Water]] (sometimes called "light water" in this context) is the most commonly used moderator (roughly 75% of the world's reactors). Solid [[graphite]] (20% of reactors) and [[heavy water]] (5% of reactors) are the main alternatives.<ref>{{cite book
  | last = Miller, Jr.
  | first = George Tyler
  | title = Living in the Environment: Principles, Connections, and Solutions
  | publisher = [[The Thomson Corporation]]
  | year = 2002
  | location = Belmont
  | pages = 345
  | isbn = 0-534-37697-5| edition = 12th
 }}</ref> [[Beryllium]] has also been used in some experimental types, and [[hydrocarbon]]s have been suggested as another possibility.

== Moderation ==
[[Neutron diffraction|Neutrons]] are normally bound into an [[atomic nucleus]] and do not exist free for long in nature. The unbound neutron has a [[half-life]] of [[Free neutron decay|10 minutes and 11 seconds]]. The release of neutrons from the nucleus requires exceeding the [[binding energy]] of the neutron, which is typically 7-9 [[MeV]] for most [[isotopes]]. [[Neutron source]]s generate free neutrons by a variety of nuclear reactions, including [[nuclear fission]] and [[nuclear fusion]]. Whatever the source of neutrons, they are released with energies of several MeV.

According to the [[equipartition theorem]], the average [[kinetic energy]], <math>\bar{E}</math>, can be related to [[temperature]], <math>T</math>, via:
:<math>\bar{E}=\frac{1}{2}m_n \langle v^2 \rangle=\frac{3}{2}k_B T</math>, 
where <math>m_n</math> is the neutron mass, <math>\langle v^2 \rangle</math> is the average squared neutron speed, and <math>k_B</math> is the [[Boltzmann constant]].<ref>{{cite book|last1=Kratz|first1=Jens-Volker|last2=Lieser|first2=Karl Heinrich|title=Nuclear and Radiochemistry: Fundamentals and Applications|date=2013|publisher=John Wiley & Sons|isbn=9783527653355|edition=3|url=https://books.google.com/books?id=gXNwAAAAQBAJ&q=neutron+moderator+kinetic+energy+%22boltzmann+constant%22&pg=PT346|access-date=27 April 2018}}</ref><ref>{{cite book|last1=De Graef|first1=Marc|last2=McHenry|first2=Michael E.|title=Structure of Materials: An Introduction to Crystallography, Diffraction and Symmetry|date=2012|publisher=Cambridge University Press|isbn=9781139560474|page=324|url=https://books.google.com/books?id=NMUgAwAAQBAJ&q=neutron+moderator+kinetic+energy+%22boltzmann+constant%22&pg=PA324|access-date=27 April 2018}}</ref> The characteristic [[neutron temperature]] of several-MeV neutrons is several tens of billions [[kelvin]].

Moderation is the process of the reduction of the initial high speed (high kinetic energy) of the free neutron. Since energy is conserved, this reduction of the neutron speed takes place by transfer of energy to a material called a ''moderator''.

The probability of scattering of a neutron from a nucleus is given by the [[nuclear cross section|scattering cross section]]. The first few collisions with the moderator may be of sufficiently high energy to excite the nucleus of the moderator. Such a collision is [[inelastic collision|inelastic]], since some of the kinetic energy is transformed to [[potential energy]] by exciting some of the internal [[Degrees of freedom (physics and chemistry)|degrees of freedom]] of the nucleus to form an [[Nuclear isomer|excited state]]. As the energy of the neutron is lowered, the collisions become predominantly [[elastic collision|elastic]], i.e., the total kinetic energy and momentum of the system (that of the neutron and the nucleus) is conserved.

Given the [[elastic collision|mathematics of elastic collisions]], as neutrons are very light compared to most nuclei, the most efficient way of removing kinetic energy from the neutron is by choosing a moderating nucleus that has near identical mass.

[[Image:Elastischer stoß.gif|frame|center|Elastic collision of equal masses]]

A collision of a neutron which has mass of 1 with a <sup>1</sup>H nucleus (a [[proton]]) could result in the neutron losing virtually all of its energy in a single head-on collision. More generally, it is necessary to take into account both glancing and head-on collisions. The ''mean logarithmic reduction of neutron energy per collision'', <math>\xi</math>, depends only on the atomic mass, <math>A</math>, of the nucleus and is given by:

<math>\xi= \ln\frac{E_0}{E}=1-\frac{(A-1)^2}{2A}\ln\left(\frac{A+1}{A-1}\right)</math>.<ref name="
Weston">{{cite book
  | last = Stacey.
  | first = Weston M
  | title = Nuclear reactor physics
  | publisher = [[Wiley-VCH]]
  | year = 2007
  | pages = 29–31
  | url = https://books.google.com/books?id=9gLXk-LRvZwC
  | isbn = 978-3-527-40679-1}}</ref>

This can be reasonably approximated to the very simple form <math>\xi\simeq \frac{2}{A+2/3}</math>.<ref name="
DB">{{cite book |last= Dobrzynski |first= L. |author2=K. Blinowski  |title= Neutrons and Solid State Physics|publisher= Ellis Horwood Limited |year= 1994 |isbn= 0-13-617192-3}}</ref> From this one can deduce <math>n</math>, the expected number of collisions of the neutron with nuclei of a given type that is required to reduce the kinetic energy of a neutron from <math>E_0</math> to <math>E_1</math>
:<math> n=\frac{1}{\xi}(\ln E_0-\ln E_1)</math>.<ref name="DB" />

[[Image:Translational motion.gif|frame|right|In a system at thermal equilibrium, neutrons (red) are elastically scattered by a hypothetical moderator of free hydrogen nuclei (blue), undergoing thermally activated motion. Kinetic energy is transferred between particles. As the neutrons have essentially the same mass as [[protons]] and there is no absorption, the velocity distributions of both particles types would be well-described by a single [[Maxwell–Boltzmann distribution]].]]

===Materials===
Some nuclei have larger [[Neutron cross section#Absorption cross section|absorption cross sections]] than others, which removes free neutrons from the [[flux]]. Therefore, a further criterion for an efficient moderator is one for which this parameter is small. The ''moderating efficiency'' gives the ratio of the [[Nuclear cross section#Macroscopic cross section|macroscopic cross sections]] of scattering, <math>\Sigma_s</math>, weighted by <math>\xi</math> divided by that of absorption, <math>\Sigma_a</math>: i.e., <math>\frac{\xi\Sigma_s}{\Sigma_a}</math>.<ref name="Weston" /> For a compound moderator composed of more than one element, such as light or heavy water, it is necessary to take into account the moderating and absorbing effect of both the hydrogen isotope and oxygen atom to calculate <math>\xi</math>. To bring a neutron from the fission energy of <math>E_0</math> 2 MeV to an <math>E</math> of 1 eV takes an expected <math>n</math> of 16 and 29 collisions for H<sub>2</sub>O and D<sub>2</sub>O, respectively. Therefore, neutrons are more rapidly moderated by light water, as H has a far higher <math>\Sigma_s</math>. However, it also has a far higher <math>\Sigma_a</math>, so that the moderating efficiency is nearly 80 times higher for heavy water than for light water.<ref name="Weston" />

''The ideal moderator is of low mass, high scattering cross section, and low absorption cross section''.
{| class="table" 
 !
 ![[Hydrogen]]
 ![[Deuterium]]
 ![[Beryllium]]
 ![[Carbon]]
 ![[Oxygen]]
 ![[Uranium]]
 |-
 |Mass of nuclei [[atomic mass unit|u]]
 |1
 |2
 |9
 |12
 |16
 |238
 |-
 |Energy decrement <math>\xi</math>
 |1
 |0.7261
 |0.2078
 |0.1589
 |0.1209
 |0.0084
 |-
 |Number of Collisions
 |18
 |25
 |86
 |114
 |150
 |2172
 |}

===Distribution of neutron velocities===
After sufficient impacts, the speed of the neutron will be comparable to the speed of the nuclei given by thermal motion; this neutron is then called a [[thermal neutron]], and the process may also be termed ''thermalization''. Once at equilibrium at a given temperature the distribution of speeds (energies) expected of rigid spheres scattering elastically is given by the [[Maxwell–Boltzmann distribution]]. This is only slightly modified in a real moderator due to the speed (energy) dependence of the absorption cross-section of most materials, so that low-speed neutrons are preferentially absorbed,<ref name="DB" /><ref>[http://www.ncnr.nist.gov/resources/n-lengths/ Neutron scattering lengths and cross sections] V.F. Sears, ''Neutron News'' 3, No. 3, 26-37 (1992)</ref> so that the true neutron velocity distribution in the core would be slightly hotter than predicted.

==Reactor moderators==
{{See also|Nuclear fission}}
In a [[thermal-neutron reactor]], the nucleus of a heavy fuel element such as [[uranium]] absorbs a slow-moving free neutron, becomes unstable, and then splits into two smaller atoms ([[Nuclear fission product|fission products]]). The fission process for [[uranium-235|<sup>235</sup>U]] nuclei yields two fission products, two to three fast-moving free neutrons, plus an amount of energy primarily manifested in the kinetic energy of the recoiling fission products. The free neutrons are emitted with a kinetic energy of ~2 MeV each. Because more free neutrons are released from a uranium fission event than thermal neutrons are required to initiate the event, the reaction can become a self-sustaining [[nuclear chain reaction]] under controlled conditions, thus liberating a tremendous amount of energy.

[[Image:U235 Fission cross section.png|thumb|left|450px|[[Fission cross section]], measured in [[barn (unit)|barns]] (a unit equal to 10<sup>−28</sup>&nbsp;m<sup>2</sup>), is a function of the energy (so-called [[excitation function]]) of the neutron colliding with a <sup>235</sup>U nucleus. Fission probability decreases as neutron energy (and speed) increases. This explains why most reactors fueled with <sup>235</sup>U need a moderator to sustain a chain reaction and why removing a moderator can shut down a reactor.]]

The probability of further fission events is determined by the fission cross section, which is dependent upon the speed (energy) of the incident neutrons. For thermal reactors, high-energy neutrons in the MeV-range are much less likely (though not unable) to cause further fission. The newly released fast neutrons, moving at roughly 10% of the [[speed of light]], must be slowed down or "moderated", typically to speeds of a few kilometres per second, if they are to be likely to cause further fission in neighbouring <sup>235</sup>U nuclei and hence continue the chain reaction. This speed occurs at temperatures in the few hundred Celsius range.

In all moderated reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more fully ''thermalised'' than others; for example, in a [[CANDU reactor]] nearly all fission reactions are produced by thermal neutrons, while in a [[pressurized water reactor]] (PWR) a considerable portion of the fissions are produced by higher-energy neutrons. In the proposed water-cooled [[supercritical water reactor]], the proportion of fast fissions may exceed 50%, making it technically a [[fast-neutron reactor]].

A fast reactor uses no moderator but relies on fission produced by unmoderated fast neutrons to sustain the chain reaction. In some fast reactor designs, up to 20% of fissions can come from direct fast neutron fission of [[uranium-238]], an isotope which is not [[Fissile material|fissile]] at all with thermal neutrons.

Moderators are also used in non-reactor [[neutron source]]s, such as [[plutonium]]-[[beryllium]] (using the {{chem|9|Be}}([[alpha particle|α]],n){{chem|12|C}} reaction) and [[spallation]] sources (using ([[proton|p]],xn) reactions with neutron rich heavy elements as targets).

== Form and location ==

The form and location of the moderator can greatly influence the cost and safety of a reactor. Classically, moderators were precision-machined blocks of [[Nuclear graphite|high-purity graphite]]<ref name="arregui16b">{{cite journal | last1 = Arregui Mena | first1 = J.D. | display-authors = etal   | year = 2016 | title = Spatial variability in the mechanical properties of Gilsocarbon | url = https://www.researchgate.net/publication/308515387 | journal = Carbon | volume = 110| pages = 497–517| doi = 10.1016/j.carbon.2016.09.051| bibcode = 2016Carbo.110..497A | s2cid = 137890948 }}</ref><ref name="arregui18">{{cite journal | last1 = Arregui Mena | first1 = J.D. | display-authors = etal   | year = 2018 | title = Characterisation of the spatial variability of material properties of Gilsocarbon and NBG-18 using random fields | url = https://www.researchgate.net/publication/327537624 | journal = Journal of Nuclear Materials | volume = 511 | pages = 91–108| doi = 10.1016/j.jnucmat.2018.09.008| bibcode = 2018JNuM..511...91A | s2cid = 105291655 | doi-access = free }}</ref> with embedded ducting to carry away heat.  They were in the hottest part of the reactor and therefore subject to [[corrosion]] and [[ablation]].  In some materials, including graphite, the impact of the neutrons with the moderator can cause the moderator to accumulate dangerous amounts of [[Wigner effect|Wigner energy]]. This problem led to the infamous [[Windscale fire]] at the Windscale Piles, a nuclear reactor complex in the United Kingdom, in 1957. In a carbon dioxide cooled graphite moderated reactor where coolant and moderator are in contact with one another, the [[Boudouard reaction]] needs to be taken into account. This is also the case if fuel elements have an outer layer of carbon—as in some [[TRISO]] fuels—or if an inner carbon layer becomes exposed by failure of one or several outer layers.

In [[pebble-bed reactor]]s, the nuclear fuel is embedded in spheres of reactor-grade [[pyrolytic carbon]], roughly of the size of [[Pebble|pebbles]]. The spaces between the spheres serve as ducting. The reactor is operated above the Wigner annealing temperature so that the graphite does not accumulate dangerous amounts of Wigner energy.

In CANDU and PWR reactors, the moderator is liquid water (heavy water for CANDU, light water for PWR). In the event of a [[loss-of-coolant accident]] in a PWR, the moderator is also lost and the reaction will stop. This negative [[void coefficient]] is an important safety feature of these reactors. In CANDU the moderator is located in a separate heavy-water circuit, surrounding the pressurized heavy-water coolant channels. The heavy water will slow down a significant portion of neutrons to the resonance integral of {{chem|238|U}} increasing the neutron capture in this isotope that makes up over 99% of the uranium in CANDU fuel thus decreasing the amount of neutrons available for fission. As a consequence, removing some of the heavy water will increase reactivity until so much is removed that too little moderation is provided to keep the reaction going. This design gives CANDU reactors a positive void coefficient, although the slower neutron kinetics of heavy-water moderated systems compensates for this, leading to comparable safety with PWRs.<ref>[http://www.nuclearfaq.ca/Meneley_Muzumbdar_reactivity_review_CNS2009.pdf D.A. Meneley and A.P. Muzumdar, "Power Reactor Safety Comparison - a Limited Review", Proceedings of the CNS Annual Conference, June 2009]</ref> 

In the light-water-cooled, graphite-moderated [[RBMK]], a reactor type originally envisioned to allow both production of [[weapons grade plutonium]] and large amounts of usable heat while using natural uranium and foregoing the use of heavy water, the light water coolant acts primarily as a neutron absorber and thus its removal in a loss-of-coolant accident or by conversion of water into steam will ''increase'' the amount of thermal neutrons available for fission. Following the [[Chernobyl nuclear accident]] the issue was remedied so that all still operating RBMK type reactors have a slightly negative void coefficient, but they require a higher degree of [[uranium enrichment]] in their fuel.

==Impurities==

Good moderators are free of neutron-absorbing impurities such as [[boron]]. In commercial nuclear power plants the moderator typically contains dissolved boron. The boron concentration of the reactor coolant can be changed by the operators by adding boric acid or by diluting with water to manipulate reactor power. The [[German nuclear weapon project|Nazi Nuclear Program]] suffered a substantial setback when its inexpensive graphite moderators failed to function. At that time, most graphites were deposited onto boron electrodes, and the German commercial graphite contained too much boron. Since the war-time German program never discovered this problem, they were forced to use far more expensive heavy water moderators. This problem was discovered by physicist [[Leó Szilárd]].<ref>{{Citation |last=Weinberg |first=Alvin |title=Memorial Tributes |volume=7 |pages=143–147 |year=1994b |chapter=Herbert G. MacPherson |chapter-url=http://www.nap.edu/openbook.php?record_id=4779&page=142 |publisher=National Academy of Engineering Press |doi=10.17226/4779 |isbn=978-0-309-05146-0}}</ref>

== Non-graphite moderators ==

Some moderators are quite expensive, for example [[beryllium]], and reactor-grade heavy water. Reactor-grade heavy water must be 99.75% pure to enable reactions with unenriched uranium. This is difficult to prepare because heavy water and regular water form the same [[chemical bond]]s in almost the same ways, at only [[kinetic isotope effect|slightly different speeds]].

The much cheaper light water moderator (essentially very pure regular water) absorbs too many neutrons to be used with unenriched natural uranium, and therefore [[uranium enrichment]] or [[nuclear reprocessing]] becomes necessary to operate such reactors, increasing overall costs. Both enrichment and reprocessing are expensive and technologically challenging processes, and additionally both enrichment and several types of reprocessing can be used to create weapons-usable material, causing [[Nuclear proliferation|proliferation]] concerns.

The CANDU reactor's moderator doubles as a safety feature. A large tank of low-temperature, low-pressure heavy water moderates the neutrons and also acts as a heat sink in extreme loss-of-coolant accident conditions. It is separated from the fuel rods that actually generate the heat.  Heavy water is very effective at slowing down (moderating) neutrons, giving CANDU reactors their important and defining characteristic of high "[[neutron economy]]". Unlike a light water reactor where adding water to the core in an accident might provide enough moderation to make a subcritical assembly go [[Criticality (status)|critical]] again, heavy water reactors will decrease their reactivity if light water is added to the core, which provides another important safety feature in the case of certain accident scenarios. However, any heavy water that becomes mixed with the emergency coolant light water will become too diluted to be useful without isotope separation.

== Nuclear weapon design ==
{{Main|Uranium hydride bomb}}
Early speculation about [[nuclear weapon]]s assumed that an "atom bomb" would be a large amount of fissile material moderated by a neutron moderator, similar in structure to a [[nuclear reactor]] or "pile".<ref>[http://nuclearweaponarchive.org/Nwfaq/Nfaq8.html#nfaq8.2.1 Nuclear Weapons Frequently Asked Questions - 8.2.1 Early Research on Fusion Weapons]</ref> Only the [[Manhattan Project]] embraced the idea of a chain reaction of fast neutrons in pure metallic uranium or plutonium. Other moderated designs were also considered by the Americans; proposals included [[Uranium hydride bomb|using uranium deuteride]] as the fissile material.<ref name="upshot">[http://www.nuclearweaponarchive.org/Usa/Tests/Upshotk.html Operation Upshot–Knothole]</ref><ref name="globalsecurity">[http://www.globalsecurity.org/wmd/systems/w48.htm W48] - globalsecurity.org</ref> 

In 1943 [[Robert Oppenheimer]] and [[Niels Bohr]] considered the possibility of using a "pile" as a weapon.<ref>{{Cite web |url=http://www.ask.ne.jp/~hankaku/english/np5y.html |title=Atomic Bomb Chronology: 1942-1944 |access-date=2008-12-16 |archive-url=https://web.archive.org/web/20080528074940/http://www.ask.ne.jp/~hankaku/english/np5y.html |archive-date=2008-05-28 |url-status=dead }}</ref> The motivation was that with a graphite moderator it would be possible to achieve the chain reaction without the use of any isotope separation. However, plutonium can be produced ("[[Breeder reactor|bred]]") sufficiently isotopically pure as to be usable in a bomb and then has to be "only" separated chemically, a much easier processes than isotope separation, albeit still a challenging one. In August 1945, when information of the [[Atomic bombings of Hiroshima and Nagasaki|atomic bombing of Hiroshima]] was relayed to the scientists of the German nuclear program who were interred at Farm Hall in England, chief scientist [[Werner Heisenberg]] hypothesized that the device must have been "something like a nuclear reactor, with the neutrons slowed by many collisions with a moderator".<ref>[[Hans Bethe]] in ''[[Physics Today]]'' Vol 53 (2001) [http://www.nd.edu/~nsl/Lectures/phys205/pdf/Nuclear_warfare_3.pdf]</ref> The German program, which had been much less advanced, had never even considered the plutonium option and did not discover a feasible method of large scale isotope separation in uranium.

After the success of the Manhattan Project, all major [[:Category:Nuclear weapons programs|nuclear weapons programs]] have relied on fast neutrons in their weapons designs. The notable exception is the ''Ruth'' and ''Ray'' test explosions of [[Operation Upshot–Knothole]]. The aim of the [[University of California Radiation Laboratory]] (UCRL) designs was the exploration of deuterated polyethylene charge containing uranium<ref name="herk">{{cite book |author-link=Gregg Herken |first=Gregg |last=Herken |title=Brotherhood of the Bomb |url=https://archive.org/details/brotherhoodofbom0000herk |url-access=registration |date=2003}}</ref>{{refpage|chapter 15}} as a candidate thermonuclear fuel,<ref name="swordsoarIII">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=III |date=1995 |url=http://www.uscoldwar.com |access-date=2016-12-28}}</ref>{{refpage|203}} hoping that [[deuterium]] would fuse (becoming an active medium) if compressed appropriately. If successful, the devices could also lead to a compact primary containing minimal amount of fissile material, and powerful enough to ignite RAMROD<ref name="swordsoarIII" />{{refpage|149}} a [[thermonuclear weapon]] designed by UCRL at the time. For a "hydride" primary, the degree of compression would not make deuterium to fuse, but the design could be subjected to boosting, raising the yield considerably.<ref name="swordsoarI">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=I |date=1995 |url=http://www.uscoldwar.com |access-date=2016-12-28}}</ref>{{refpage|258}} The [[Pit (nuclear weapon)|cores]] consisted of a mix of [[uranium deuteride]] (UD<sub>3</sub>),<ref name="swordsoarIII" />{{refpage|202}} and deuterated polyethylene. The core tested in ''Ray'' used uranium low enriched in U<sup>235</sup>, and in both shots deuterium acted as the neutron moderator.<ref name="swordsoarI" />{{refpage|260}} The predicted [[Nuclear weapon yield|yield]] was 1.5 to 3 kt for ''Ruth'' (with a maximum potential yield of 20 kt<ref name="swordsoarVII">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=VII |date=1995 |url=http://www.uscoldwar.com |access-date=2016-12-28}}</ref>{{refpage|96}}) and 0.5-1 kt for ''Ray''. The tests produced yields of 200 [[tons of TNT]] each; both tests were considered to be [[Fizzle (nuclear explosion)|fizzles]].<ref name="upshot" /><ref name="globalsecurity" />

A side effect of using a moderator in a nuclear explosive is that as the chain reaction progresses, the moderator will be heated, thus losing its ability to cool the neutrons. Another effect of moderation is that the time between subsequent neutron generations is increased, slowing down the reaction. This makes the containment of the explosion a problem; the [[inertia]] that is used to confine [[Nuclear weapon design#Implosion-type weapon|implosion type]] bombs will not be able to confine the reaction. The result may be a fizzle. The explosive power of a fully moderated explosion is thus limited; at worst it may be equal to a chemical explosive of similar mass. According to Heisenberg: "One can never make an explosive with slow neutrons, not even with the heavy water machine, as then the neutrons only go with thermal speed, with the result that the reaction is so slow that the thing explodes sooner, before the reaction is complete."<ref name="Rose1998">{{cite book|author=Paul Lawrence Rose|author-link=Paul Lawrence Rose|title=Heisenberg and the Nazi Atomic Bomb Project: A Study in German Culture|url=https://archive.org/details/isbn_9780520229266|url-access=registration|access-date=6 May 2017|year=1998|publisher=[[University of California Press]]|isbn=978-0-520-21077-6|page=[https://archive.org/details/isbn_9780520229266/page/211 211]}}</ref> While a nuclear bomb working on thermal neutrons may be impractical, modern weapons designs may still benefit from some level of moderation. A beryllium tamper used as a [[neutron reflector]] will act as a moderator.<ref>[http://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.7.3 Nuclear Weapons Frequently Asked Questions - 4.1.7.3.2 Reflectors]</ref><ref name="killus">[http://unintentional-irony.blogspot.com/2007/07/n-moderation.html N Moderation]</ref>

==Materials used==
* Hydrogen, as in ordinary "light water". Because [[Hydrogen-1|protium]] also has a significant cross section for neutron capture only limited moderation is possible without losing too many neutrons. The less-moderated neutrons are relatively more likely to be captured by uranium-238 and less likely to fission uranium-235, so light-water reactors require enriched uranium to operate. 
** There are also proposals to use the compound formed by the chemical reaction of metallic uranium and hydrogen ([[uranium hydride]]—UH<sub>3</sub>) as a combination fuel and moderator in [[Hydrogen Moderated Self-regulating Nuclear Power Module|a new type of reactor]].
** Hydrogen is also used in the form of cryogenic liquid [[methane]] and sometimes [[liquid hydrogen]] as a [[cold neutron]] source in some [[research reactor]]s: yielding a [[Maxwell–Boltzmann distribution]] for the neutrons whose maximum is shifted to much lower energies.
** Hydrogen combined with carbon as in [[paraffin wax]] was used in some early German experiments.
* Deuterium, in the form of [[heavy water]], in heavy water reactors, e.g. CANDU. Reactors moderated with heavy water can use unenriched [[natural uranium]].
* Carbon, in the form of reactor-grade graphite<ref name="arregui16b"/> or pyrolytic carbon, used in e.g. RBMK and pebble-bed reactors, or in compounds, e.g. [[carbon dioxide]]. As carbon dioxide contains twice as many oxygen atoms as it does carbon atoms and both have moderating and neutron absorbing effects in a similar range (see above), a significant share of the moderation in a (yet to be built) carbon dioxide moderated reactor would actually come from the oxygen. Lower-temperature reactors are susceptible to buildup of Wigner energy in the material. Like deuterium-moderated reactors, some of these reactors can use unenriched natural uranium.
** Graphite is also deliberately allowed to be heated to around 2000 K or higher in some [[research reactor]]s to produce a hot neutron source: giving a Maxwell–Boltzmann distribution whose maximum is spread out to generate higher energy neutrons.
* Beryllium, in the form of metal. Beryllium is expensive and toxic, so its use is limited. Beryllium was used in the [[S2G reactor]].<ref>{{cite web |url=https://lynceans.org/wp-content/uploads/2020/02/Marine-Nuclear-Power-1939-2018_Part-2A_USA_submarines.pdf |first=Peter |last=Lobner 
|date=July 2018 |website=lynceans.org
|title=Marine Nuclear Power: 1939 – 2018, Part 2A: United States - Submarines
 |access-date=11 Sep 2024}}</ref><ref>{{cite web | url=https://books.google.com/books?id=9TRUAAAAMAAJ&dq=s2g+reactor+beryllium&pg=SA25-PA6 | title=Naval Reactors Physics Handbook: The physics of intermediate spectrum ractors, edited by J.R. Stehn | date=1964 }}</ref>
* [[Lithium]]-7, in the form of a [[lithium fluoride]] salt, typically in conjunction with [[beryllium fluoride]] salt ([[FLiBe]]). This is the most common type of moderator in a [[molten-salt reactor]].

Other light-nuclei materials are unsuitable for various reasons. [[Helium]] is a gas and it requires special design to achieve sufficient density; [[lithium]]-6 and [[boron]]-10 absorb neutrons.

{| class="wikitable"
|+Currently operating [[nuclear power]] reactors by moderator
|-
!Moderator!!Reactors!!Design!!Country
|-
|none ([[fast-neutron reactor|fast]])||2||[[BN-600]], [[BN-800 reactor|BN-800]]||Russia (2)
|-
|graphite||25||[[Advanced gas-cooled reactor|AGR]], [[Magnox]], [[RBMK]]|| United Kingdom (14), Russia (9)
|-
|heavy water||29||[[CANDU]], [[Pressurized heavy-water reactor|PHWR]] ||Canada (17), South Korea (4), Romania (2),<br /> China (2), India (18), Argentina, Pakistan
|-
|light water||359||[[Pressurized water reactor|PWR]], [[Boiling water reactor|BWR]]||27 countries
|}{{Portal|Nuclear technology}}

==Notes==
{{Reflist|30em}}

==References==
* {{cite book | title = DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory. Vol. 2 (DOE-HDBK-1019/2-93) | date = January 1993 | publisher = [[U.S. Department of Energy]] | url = http://energy.gov/sites/prod/files/2013/06/f2/h1019v2.pdf | access-date = November 29, 2013 | archive-url = https://web.archive.org/web/20131203041437/http://energy.gov/sites/prod/files/2013/06/f2/h1019v2.pdf | archive-date = December 3, 2013 | url-status = dead }}

{{Nuclear technology}}

{{DEFAULTSORT:Neutron Moderator}}
[[Category:Nuclear technology]]
[[Category:Neutron instrumentation|Moderator]]
[[Category:Neutron moderators| ]]