Heavy water can serve as a neutron moderator in a nuclear reactor, which slows down fast particles and encourages the fission reaction. Neutrons in a fission reaction travel either fast or slowly. The fast neutrons tend to be absorbed by certain isotopes of uranium rather than continuing the fission reaction, but the larger deuterium atoms in heavy water help slow down these fast neutrons, ensuring they continue the chain reaction.
Heavy water molecules are heavy because they contain deuterium. Ordinarily, the nucleus of a hydrogen atom consists of a single proton, but deuterium is an isotope of hydrogen that contains one neutron as well as one proton, vastly increasing the weight of the element and altering its atomic properties.
Heavy water reactors offer some advantages over those that use normal, or light, water as a neutron moderator. Heavy water allows the use of natural uranium as fuel, while light water reactors require enriched uranium, which is much more difficult to produce. Additionally, heavy water reactors can be used to convert natural uranium into weapons-grade material as part of the fission process, a potential threat to nuclear nonproliferation. For this reason, the import and export of large quantities of heavy water is regulated in many countries, and the international trade in heavy water is open to scrutiny.