Heavy Water

Heavy water, formally called deuterium oxide or 2H₂O or D₂O, is a form of water that contains the hydrogen isotope deuterium, rather than the common protium isotope. The colloquial term heavy water is often also used to refer a highly enriched water mixture that contains mostly deuterium oxide but also contains some ordinary water molecules as well: for instance heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction. In comparison, in ordinary water, there are only about 156 deuterium atoms per million hydrogen atoms).

Heavy water is not radioactive. In its pure form, it has a density about 11% greater than water, but otherwise, is physically and chemically similar. This difference in density (especially affecting the biological properties) is larger than in any other commonly occurring isotope-substituted compound because deuterium is unique among heavy stable isotopes in being twice as heavy as the lightest isotope. This difference increases the strength of water's hydrogen-oxygen bonds, and this in turn is enough to cause differences that are important to some biochemical reactions. The human body naturally contains deuterium equivalent to five grams of heavy water, which is harmless. When a large fraction of water (> 50%) in higher organisms is replaced by heavy water, the result is cell dysfunction and death.

Heavy water was first produced in 1932, a few months after the discovery of deuterium. With the discovery of nuclear fission in late 1938, and the need for a neutron moderator that captured few neutrons, heavy water became a component of early nuclear energy. Partly because of Nazi Germany's over-reliance on heavy water, not realizing that pure carbon could be used instead, it did not produce a functioning reactor for the duration of World War II. Since then, heavy water has been an essential component in some types of reactor, both those that generate power and those designed to produce isotopes for nuclear weapons, such as plutonium-239. These heavy water reactors have the advantage of being able to run on natural uranium without the use of graphite moderators which can pose radiological and dust explosion hazards in the decommissioning phase. Most modern reactors use enriched uranium and normal "light water" (H₂O) moderator.