Neutron moderator

A neutron moderator is a medium which reduces the velocity of fast neutrons and in which the fuel of a conventional nuclear reactor is immersed. Without moderator, such a reactor can not operate. The role of moderator is to slow down neutrons to promote fission chain reaction. The most commonly uses moderators are ordinary or heavy water (boiling or under pressure) and graphite. In the case of ordinary or heavy water, the medium is also used for cooling in order to remove the heat produced in the heart of the reactor.