What Is the Function of a Moderator in a Nuclear Reactor?

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.