Moderator

A material used in many reactors to slow down, or moderate, the fast neutrons released by fission, because uranium-235 is far more likely to fission when struck by a slow neutron. Ordinary water slows neutrons in the fewest collisions but absorbs some of them; heavy water absorbs far fewer, which is why a heavy-water reactor can run on unenriched uranium; graphite absorbs few neutrons but needs more collisions to slow them, so graphite cores are large. A reactor with no moderator at all runs on fast neutrons instead, and is called a fast reactor.

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