What are the uses of heavy water?

Answer 1

Heavy water (²H₂O) is used in some nuclear reactors and is sometimes used as a chemical tag to follow water during a chemical or biochemical event.

Answer 2

Heavy water (deuterium oxide, D₂O) is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium (D, also known as heavy hydrogen), rather than the common hydrogen-1 isotope (¹H) that makes up most of the hydrogen in normal water.

Heavy water is used in certain types of nuclear reactors, where it acts as a neutron moderator to slow down neutrons so that they are more likely to react with the fissile uranium-235 than with uranium-238.