Why is heavy water used as a moderator?
Heavy water is a type of water highly enriched in the hydrogen isotope deuterium. We can conclude the neutron interactions with billiard ball collision, where neutron attracts with nucleus of other atoms & lose energy. If the colliding nucleus size is minute like hydrogen nucleus it will lose maximum energy. If nucleus is heavy the neutron hits the nucleus just changes its direction but not that much alter in the energy of neutron. So we use heavy water as moderator to slow down neutrons.