In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used to have light nuclei. Heavy nuclei will not serve the purpose because (a) they will break up (b) elastic collision of neutrons with heavy nuclei will not slow them down (c) the net weight of the reactor would be unbearably high (d) substances with heavy nuclei do not occur in a liquid or gaseous state at room temperature
For elastic collision masses of both must be equal so that they can exchange the velocities. To slow down the speed of neutron substance should be made up of 1 proton for perfectly elastic ie., we need light nuclei not heavy. In heavy nuclei only direction will change not the speed
