Heavy water is a form of water that contains a higher proportion of the isotope deuterium, which has one proton and one neutron in its nucleus, compared to regular water that contains only protons. This gives heavy water a greater molecular weight, making it denser than ordinary water. Its chemical formula is D2O, where "D" represents deuterium. Heavy water is primarily used in nuclear reactors, particularly in CANDU reactors, as a neutron moderator. This means it helps slow down neutrons, making it easier for them to sustain a nuclear chain reaction. Additionally, heavy water has applications in scientific research and various industrial processes.