Editing Neutron moderator (section)

==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}}