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
1
they will break up.
2
elastic collision of neutrons with heavy nuclei will not slow them down.
3
the net weight of the reactor would be unbearably high.
4
substances with heavy nuclei do not occur in liquid or gaseous state at room temperature.