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.