Moderator Definition in Physics

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In physics, a moderator is a material that slows the speed of neutrons. It is also known as a neutron moderator. Using a moderator changes fast neutrons into thermal neutrons. Thermal neutrons increase the likelihood of interaction with another nucleus to initiate fission.

Moderator Examples

Water, graphite and heavy water are all commonly used moderators in nuclear reactors. The most common neutron moderator is "light water," which may be fresh water or may be deuterium-depleted water.


  • Kratz, Jens-Volker; Lieser, Karl Heinrich (2013). Nuclear and Radiochemistry: Fundamentals and Applications (3rd ed.). John Wiley & Sons. ISBN 9783527653355.
  • Stacey., Weston M. (2007). Nuclear Reactor Physics. Wiley-VCH. ISBN 3-527-40679-4.
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Helmenstine, Anne Marie, Ph.D. "Moderator Definition in Physics." ThoughtCo, Aug. 29, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 29). Moderator Definition in Physics. Retrieved from Helmenstine, Anne Marie, Ph.D. "Moderator Definition in Physics." ThoughtCo. (accessed January 29, 2023).