Heavy water is like regular water except that instead of two hydrogen atoms and an oxygen atom (H20) it has two deuterium atoms and an oxygen atom (D20). Deuterium is a hydrogen atom with an extra neutron.\nWater is used in nuclear reactors to moderate the reaction speed, and to absorb heat. Nuclear fission starts by giving off neutrons that create a chain reaction. Any water around it absorbs those neutrons (becoming heavy water) and slows the reaction. If heavy water is used instead of regular hydrogen water, the neutrons are not absorbed. This means that less pure nuclear material can be used to sustain a reaction. For example unrefined uranium, which is much less expensive, can be used instead of enriched uranium.
Heavy water