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| In [[nuclear engineering]], a '''neutron moderator''' is a medium that reduces the speed of [[fast neutron]]s, thereby turning them into [[thermal neutron]]s capable of sustaining a [[nuclear chain reaction]] involving [[uranium-235]].
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| Commonly used moderators include [[water|regular (light) water]] (roughly 75% of the world's reactors), solid [[graphite]] (20% of reactors) and [[heavy water]] (5% of reactors).<ref>{{cite book
| | If you would like use the '''MathML''' rendering mode, you need a wikipedia user account that can be registered here [[https://en.wikipedia.org/wiki/Special:UserLogin/signup]] |
| | last = Miller, Jr.
| | * Only registered users will be able to execute this rendering mode. |
| | first = George Tyler
| | * Note: you need not enter a email address (nor any other private information). Please do not use a password that you use elsewhere. |
| | authorlink =
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| | title = Living in the Environment: Principles, Connections, and Solutions (12th Edition)
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| | publisher = [[The Thomson Corporation]]
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| | year = 2002
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| | location = Belmont
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| | pages = 345
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| | url =
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| | isbn = 0-534-37697-5}}</ref>
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| [[Beryllium]] has also been used in some experimental types, and [[hydrocarbon]]s have been suggested as another possibility.
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| |+Currently operating [[nuclear power]] reactors by moderator
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| !Moderator!!Reactors!!Design!!Country
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| |none ([[fast neutron reactor|fast]])||1||[[BN-600]]||Russia (1)
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| |graphite||29||[[Advanced gas-cooled reactor|AGR]], [[Magnox]], [[RBMK]]|| United Kingdom (18), Russia (11)
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| |heavy water||29||[[CANDU]]||Canada (17), South Korea (4), Romania (2),<br /> China (2), India (2), Argentina, Pakistan
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| |light water||359||[[Pressurized water reactor|PWR]], [[Boiling water reactor|BWR]]||27 countries
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| == Moderation == | | '''MathML''' |
| 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 just under 15 minutes. 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.
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| Since the [[kinetic energy]], <math>E</math>, can be related to [[temperature]] via:
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| <math>E=\frac{1}{2}mv^2=\frac{3}{2}k_B T</math> | | '''source''' |
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| the characteristic [[neutron temperature]] of a several-MeV neutron is several tens of millions of degrees [[Celsius]].
| | <span style="color: red">Follow this [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering link] to change your Math rendering settings.</span> You can also add a [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering-skin Custom CSS] to force the MathML/SVG rendering or select different font families. See [https://www.mediawiki.org/wiki/Extension:Math#CSS_for_the_MathML_with_SVG_fallback_mode these examples]. |
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| Moderation is the process of the reduction of the initial high kinetic energy of the free neutron. Since energy is conserved, this reduction of the neutron kinetic energy takes place by transfer of energy to a material known as a moderator. It is also known as ''neutron slowing down'', since along with the reduction of energy comes a reduction in speed.
| | ==Demos== |
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| The probability of scattering of a neutron from a nucleus is given by the [[nuclear cross section|scattering cross section]]. The first couple of 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.
| | Here are some [https://commons.wikimedia.org/w/index.php?title=Special:ListFiles/Frederic.wang demos]: |
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| Given the [[Momentum#Special case: m1.3Dm2|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.
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| [[Image:Elastischer stoß.gif|frame|center|Elastic collision of equal masses]] | | * accessibility: |
| | ** Safari + VoiceOver: [https://commons.wikimedia.org/wiki/File:VoiceOver-Mac-Safari.ogv video only], [[File:Voiceover-mathml-example-1.wav|thumb|Voiceover-mathml-example-1]], [[File:Voiceover-mathml-example-2.wav|thumb|Voiceover-mathml-example-2]], [[File:Voiceover-mathml-example-3.wav|thumb|Voiceover-mathml-example-3]], [[File:Voiceover-mathml-example-4.wav|thumb|Voiceover-mathml-example-4]], [[File:Voiceover-mathml-example-5.wav|thumb|Voiceover-mathml-example-5]], [[File:Voiceover-mathml-example-6.wav|thumb|Voiceover-mathml-example-6]], [[File:Voiceover-mathml-example-7.wav|thumb|Voiceover-mathml-example-7]] |
| | ** [https://commons.wikimedia.org/wiki/File:MathPlayer-Audio-Windows7-InternetExplorer.ogg Internet Explorer + MathPlayer (audio)] |
| | ** [https://commons.wikimedia.org/wiki/File:MathPlayer-SynchronizedHighlighting-WIndows7-InternetExplorer.png Internet Explorer + MathPlayer (synchronized highlighting)] |
| | ** [https://commons.wikimedia.org/wiki/File:MathPlayer-Braille-Windows7-InternetExplorer.png Internet Explorer + MathPlayer (braille)] |
| | ** NVDA+MathPlayer: [[File:Nvda-mathml-example-1.wav|thumb|Nvda-mathml-example-1]], [[File:Nvda-mathml-example-2.wav|thumb|Nvda-mathml-example-2]], [[File:Nvda-mathml-example-3.wav|thumb|Nvda-mathml-example-3]], [[File:Nvda-mathml-example-4.wav|thumb|Nvda-mathml-example-4]], [[File:Nvda-mathml-example-5.wav|thumb|Nvda-mathml-example-5]], [[File:Nvda-mathml-example-6.wav|thumb|Nvda-mathml-example-6]], [[File:Nvda-mathml-example-7.wav|thumb|Nvda-mathml-example-7]]. |
| | ** Orca: There is ongoing work, but no support at all at the moment [[File:Orca-mathml-example-1.wav|thumb|Orca-mathml-example-1]], [[File:Orca-mathml-example-2.wav|thumb|Orca-mathml-example-2]], [[File:Orca-mathml-example-3.wav|thumb|Orca-mathml-example-3]], [[File:Orca-mathml-example-4.wav|thumb|Orca-mathml-example-4]], [[File:Orca-mathml-example-5.wav|thumb|Orca-mathml-example-5]], [[File:Orca-mathml-example-6.wav|thumb|Orca-mathml-example-6]], [[File:Orca-mathml-example-7.wav|thumb|Orca-mathml-example-7]]. |
| | ** From our testing, ChromeVox and JAWS are not able to read the formulas generated by the MathML mode. |
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| 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:
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| <math>\xi= \ln\frac{E_0}{E}=1+\frac{(A-1)^2}{2A}\ln\left(\frac{A-1}{A+1}\right)</math>.<ref name="
| | To test the '''MathML''', '''PNG''', and '''source''' rendering modes, please go to one of the following test pages: |
| Weston">{{cite book
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| | last = Stacey.
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| | first = Weston M
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| | *[[Linebreaking]] |
| | title = Nuclear reactor physics
| | *[[Unique Ids]] |
| | publisher = [[Wiley-VCH]]
| | *[[Help:Formula]] |
| | year = 2007
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| | location =
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| | pages = 29–31
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| | url = http://books.google.com/books?id=iolyNyJYEaYC
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| | isbn = 3-527-40679-4}}</ref>
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| This can be reasonably approximated to the very simple form <math>\xi\simeq \frac{2}{A+1}</math>.<ref name="
| | *[[Inputtypes|Inputtypes (private Wikis only)]] |
| DB">{{cite book |last= Dobrzynski |first= L. |coauthors= 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</math>:<math> n=\frac{1}{\xi}(\ln E_0-\ln E)</math>.<ref name="DB" />
| | *[[Url2Image|Url2Image (private Wikis only)]] |
| | | ==Bug reporting== |
| [[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]].]] | | If you find any bugs, please report them at [https://bugzilla.wikimedia.org/enter_bug.cgi?product=MediaWiki%20extensions&component=Math&version=master&short_desc=Math-preview%20rendering%20problem Bugzilla], or write an email to math_bugs (at) ckurs (dot) de . |
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| ===Choice of moderator materials===
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| Some nuclei have larger [[absorption cross section]]s 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" />
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| ''The ideal moderator is of low mass, high scattering cross section, and low absorption cross section''.
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| {| class="table"
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| !
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| ![[Hydrogen]]
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| ![[Deuterium]]
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| ![[Beryllium]]
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| ![[Carbon]]
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| ![[Oxygen]]
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| ![[Uranium]]
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| |-
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| |Mass of kernels [[atomic mass unit|u]]
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| |1
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| |2
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| |9
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| |12
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| |16
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| |238
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| |Energy decrement <math>\xi</math>
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| |1
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| |0,7261
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| |0,2078
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| |0,1589
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| |0,1209
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| |0,0084
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| |-
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| |Number of Collisions
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| |18
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| |25
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| |86
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| |114
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| |150
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| |2172
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| |}
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| ===Distribution of neutron velocities once moderated===
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| 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.
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| ==Reactor moderators==
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| In a [[thermal reactor|thermal nuclear reactor]], the nucleus of a heavy fuel element such as [[uranium]] absorbs a [[thermal neutron|slow-moving free neutron]], becomes unstable, and then splits ("[[Nuclear fission|fission]]s") into two smaller atoms ("[[fission product]]s"). The fission process for [[uranium-235|<sup>235</sup>U]] nuclei yields two fission products: two to three [[fast neutron|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 neutron]]s are released from a uranium fission event than thermal neutrons are required to initiate the event, the reaction can become self-sustaining — a [[chain reaction]] — under controlled conditions, thus liberating a tremendous amount of energy (see article [[nuclear fission]]).
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| [[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> 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.]]
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| The probability of further fission events is determined by the [[nuclear cross section|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 to cause further fission. (Note: It is not ''impossible'' for fast neutrons to cause fission, just much less likely.) 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 [[uranium-235|<sup>235</sup>U]] nuclei and hence continue the chain reaction. This speed happens to be equivalent to temperatures in the few hundred celsius range.
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| 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]] (SCWR), the proportion of fast fissions may exceed 50%, making it technically a [[fast neutron reactor]].
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| 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]] at all with thermal neutrons.
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| Moderators are also used in non-reactor neutron sources, such as [[plutonium]]-[[beryllium]] and [[spallation]] sources.
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| == Form and location == | |
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| The form and location of the moderator can greatly influence the cost and safety of a reactor. Classically, moderators were precision-machined blocks of high purity graphite 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 energy]]. This problem led to the infamous [[Windscale fire]] at the Windscale Piles, a nuclear reactor complex in the United Kingdom, in 1957.
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| Some [[pebble-bed reactor]]s' moderators are not only simple, but also inexpensive{{Citation needed|date=September 2009}}: the nuclear fuel is embedded in spheres of reactor-grade [[pyrolytic carbon]], roughly of the size of [[tennis ball]]s. The spaces between the balls serve as ducting. The reactor is operated above the Wigner annealing temperature so that the graphite does not accumulate dangerous amounts of [[Wigner energy]].
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| In [[CANDU]] and [[pressurized water reactor|PWR]] reactors, the moderator is liquid water ([[heavy water]] for CANDU, [[Water|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. 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>
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| ==Moderator impurities== | |
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| Good moderators are also 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 World War II nuclear program suffered a substantial setback when its inexpensive graphite moderators failed to work. At that time, most graphites were deposited on 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. In the U.S., [[Leó Szilárd]], a former chemical engineer, discovered the problem.
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| == Non-graphite moderators ==
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| 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 slightly different speeds.
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| 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 proliferation concerns. Reprocessing schemes that are more resistant to proliferation are currently under development.
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| 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|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."
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| == Nuclear weapon design ==
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| {{Main|Uranium hydride bomb}}
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| 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 neutron]]s in pure metallic [[uranium]] or [[plutonium]]. Other moderated designs were also considered by the Americans; proposals included [[Uranium hydride bomb|using uranium hydride]] 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>[http://www.ask.ne.jp/~hankaku/english/np5y.html Atomic Bomb Chronology: 1942-1944]</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]]. 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]], interned 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>
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| 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 ''[[Upshot-Knothole Ruth|Ruth]]'' and ''[[Upshot-Knothole Ray|Ray]]'' test explosions of [[Operation Upshot-Knothole]]. The aim of the [[University of California Radiation Laboratory]] design was to produce an explosion powerful enough to ignite a [[thermonuclear weapon]] with the minimal amount of fissile material. The [[Nuclear reactor core|core]] consisted of [[uranium hydride]], with [[hydrogen]], or in the case of ''Ray'', [[deuterium]] acting as the neutron moderator. The predicted [[Nuclear weapon yield|yield]] was 1.5 to 3 kt for ''Ruth'' 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 test)|fizzles]].<ref name="upshot" /><ref name="globalsecurity" />
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| The main benefit of using a moderator in a nuclear explosive is that the amount of fissile material needed to reach [[Criticality (status)|criticality]] may be greatly reduced. Slowing of fast neutrons will increase the [[Nuclear cross section|cross section]] for [[neutron absorption]], reducing the [[critical mass]]. A side effect is however that as the chain reaction progresses, the moderator will be heated, thus losing its ability to cool the neutrons.
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| 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 end result may be a fizzle instead of a bang.
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| The explosive power of a fully moderated explosion is thus limited, at worst it may be equal to a chemical explosive of similar mass. Again quoting 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."''
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| While a nuclear bomb working on [[thermal neutron]]s may be impractical, modern weapons designs may still benefit from some level of moderation. A [[beryllium]] tamper used as a [[neutron reflector]] will also 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>
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| ==Materials used==
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| * [[Hydrogen]], as in ordinary "[[Water|light water]]." Because [[Hydrogen-1|protium]] also has a significant [[Neutron cross section|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 reactor]]s require [[enriched uranium]] to operate.
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| ** 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]].
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| ** 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.
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| ** Hydrogen combined with carbon as in [[paraffin wax]] was used in some early [[German nuclear energy project|German experiments]].
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| * [[Deuterium]], in the form of [[heavy water]], in [[heavy water reactor]]s, e.g. [[CANDU]]. Reactors moderated with heavy water can use unenriched [[natural uranium]].
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| * [[Carbon]], in the form of reactor-grade [[graphite]] or [[pyrolytic carbon]], used in e.g. [[RBMK]] and [[pebble-bed reactor]]s, or in compounds, e.g. [[carbon dioxide]] [http://www.bookrags.com/research/carbon-dioxide-chmc]. 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.
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| ** Graphite is also deliberately allowed to be heated to around 2000 K or higher in some [[research reactor]]s to produce a [[neutron temperature|hot neutron]] source: giving a [[Maxwell–Boltzmann distribution]] whose maximum is spread out to generate higher energy neutrons.
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| * [[Beryllium]], in the form of metal. Beryllium is expensive and toxic, so its use is limited.
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| * [[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]].
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| 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.
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| ==References==
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| * {{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 | accessdate = November 29, 2013}}
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| === Notes ===
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| {{reflist}}
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| ==See also==
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| *[[Nuclear cross section]]
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| *[[Neutron reflector]]
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| {{Nuclear technology}}
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| {{DEFAULTSORT:Neutron Moderator}}
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| [[Category:Nuclear technology]]
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| [[Category:Neutron instrumentation|Moderator]]
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| [[Category:Neutron moderators| ]]
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