In a nuclear reactor, a moderator is mixed with the uranium fuel to slow down the fast neutrons to the speed of thermal neutrons, which enables fission to occur in a controlled nuclear chain reaction. Most of the nuclear reactors in the United States use water as a moderator. The elastic collisions occurring between the fast neutrons and the moderator slow down the neutrons without absorbing them.
Because the hydrogen nuclei in water molecules possess about the same mass as the fast neutrons, a collision between a neutron and a proton in a hydrogen nucleus will result in the neutron losing almost all of its kinetic energy. In addition to water, about 20 percent of the nuclear reactors worldwide use solid graphite as a neutron moderator. In about five percent of the reactors worldwide, the moderator used is heavy water. In some experimental reactor types, the element beryllium has been used as a moderator.
In a nuclear electric power plant, the water used as a neutron moderator is also used to heat the water that drives the steam turbines that produce the electric power. The moderator water, however, remains completely sealed off from the water that drives the turbines. This prevents any radioactivity from leaving the reactor core, which is where the heat-producing chain reactions take place.