Editing Neutron moderator (section)

== 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.