Heavy water is water highly enriched in the hydrogen isotope deuterium.we can compare the neutron inatractions with billiard ball collision, where neutron collids wth nucleus of other automs & lose energey. If the colliding nucleus size is small like hydrogen nucleus it will lose maximum energey. If nucleus is hevy the neutron hits the nucleus just changes its direction but not that much chnage in the energy of neutron. So we use heavy water as moderator to slow down neutrons.