In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used; have light nuclei. Heavy nuclei will not serve the purpose because

Moderator is a substance which helps in nuclear chain reaction by reducing the speed of the free neutrons () and turning them into thermal neutrons (). During a thermonuclear reaction the nucleus of a heavy fuel such as Uranium absorbs these thermo neutrons and become unstable thereby splitting into lesser atoms or fission products. Now for a moderator we only use light nuclei because according to conservation of momentum and energy the fast particle hitting the moderator (light nuclei) acts as elastic collision with the particles of moderator thus transferring kinetic energy from the fast neutron to the moderator thereby helping in splitting of atom, in case of a heavy nuclei it will not split any atom but will absorb all the kinetic energy of the fast neutron thereby rendering no fission reaction. Therefore, for a continuous fission chain reaction we use a light nuclei moderator in the nuclear reactor.