In a nuclear reactor the moderator is used to.....
decrease the speed of the neutrons
produce fission of the nuclei
produce fusion of the nuclei
The moderator, which is of importance in thermal reactors, is used to moderate, that is, to slow down, neutrons from fission to thermal energies. The probability that fission will occur depends on incident neutron energy. Physicists calculate with fission cross-section, which determines this probability.
Nuclei with low mass numbers are most effective for this purpose, so the moderator is always a low-mass-number material. In a fast reactor there is no moderator, only fuel and coolant. The moderation of neutrons is undesirable in fast reactors. Commonly used moderators include regular (light) water (roughly 75% of the world’s reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium and beryllium oxide (BeO) have been used occasionally, but they are very costly.
The probability of the fission U-235 becomes very large at the thermal energies of slow neutrons. This fact implies increase of multiplication factor of the reactor (i.e. lower fuel enrichment is needed to sustain chain reaction)
Fast reactors use fast neutrons to split uranium or plutonium nuclei. They use higher fuel enrichment to sustain chain reaction. The moderation of neutrons is undesirable in fast reactors.