Science, Tech, Math › Science Moderator Definition in Physics Share Flipboard Email Print Boris SV / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated January 13, 2020 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. Sources 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.