Heavy water

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3.82 °C, 38.88 °F (276.97 K)

101.4 °C, 214.56 °F (374.55 K)

Heavy water is water containing a higher-than-normal proportion of the hydrogen isotope deuterium, either as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO.[1] Physically and chemically, it resembles water, H2O; in water, the deuterium-to-hydrogen ratio is about 156ppm, (see Vienna Standard Mean Ocean Water). Heavy water is water that is highly enriched in deuterium, up to as much as 100% D2O. The isotopic substitution with deuterium alters the bond energy of the water's hydrogen-oxygen bond, altering the physical, chemical, and, especially, the biological properties of the pure, or highly-enriched, substance to a degree greater than is found in most isotope-substituted chemical compounds. Pure heavy water is not radioactive. It is about 11% denser than water, but otherwise, is physically very similar to water.

Heavy water exhibits dose and species-dependent chemical toxicity. The adult human body naturally contains deuterium equivalent to that in five grams of heavy water, which is thought to be harmless. Comparable laboratory doses are used as non-radioactive tracers in human and animal metabolic experimentation. However, larger concentrations of heavy water are toxic in eukaryotic organisms, when heavy water replaces about 25% to 50% of the body's water. At these levels, the substance interferes with cellular mitotic apparatus, preventing cell-division. Single-celled prokaryotic organisms such as bacteria, which do not have a mitotic apparatus, may survive and grow slowly in heavy water. However, eukaryotic organisms as simple as single-celled protozoa, and including all higher (multi-cellular) organisms, if given only heavy water, soon stop dividing and growing. For example, plant seeds will not germinate in heavy water. Mammals given heavy water fall ill from lack of needed blood-cell and intestinal-cell replacement, and die when about 50% of their body-water has been replaced with heavy water.

Relatively pure heavy water was produced in 1933, soon after the discovery of deuterium, the stable heavy isotope of hydrogen. With the discovery of nuclear fission in late 1938, and the need for a neutron moderator that captured few neutrons, heavy water became an important component of early nuclear energy programs during World War II (1939–1945). Partly because of Nazi Germany's (1933–1945) technological reliance upon scarce heavy water for nuclear reactor research, they failed to produce a functioning nuclear reactor for the duration of the war. Since then, heavy water is an essential component in the design of some nuclear reactors, either for generating electric power or for producing nuclear-weapons isotopes, such as plutonium-239. Most contemporary enriched-uranium nuclear reactors use normal "light water" (H2O) for neutron moderation.

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