|moderator (mŏd'ə-rā'tər) Pronunciation Key
A substance, such as graphite, water, or heavy water, placed in a nuclear reactor to slow neutrons down to speeds at which they are more likely to be captured by fissionable components of a fuel (such as uranium-235) and less likely to be absorbed by nonfissionable components of a fuel (such as uranium-238). Also called neutron moderator. See also slow neutron.
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