Neutron moderator

medium that reduces the speed of fast neutrons, turning them into thermal neutrons that can sustain nuclear chain reactions; e.g. water, graphite, heavy water, beryllium

A neutron moderator is a material that slows down neutrons. This is done in nuclear power stations so the neutrons keep a steady nuclear chain reaction to provide heat.

Neutron moderators are an essential part of most nuclear reactors. Common neutron moderators are ordinary water (called light water in the nuclear industry), heavy water, and graphite.