heavy water

water in which hydrogen atoms have been replaced by deuterium, used chiefly as a coolant in nuclear reactors. Also called deuterium oxide.

[1930-35]

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or deuterium oxide

Water composed of two atoms of deuterium (D; a heavy isotope of hydrogen) and one atom of oxygen (O), chemical formula D₂O. Water from most natural sources contains about 0.

015% deuterium oxide; this can be enriched or purified by distillation, electrolysis, or chemical processing. Heavy water is used as a moderator in nuclear power plants, slowing down the fast neutrons so that they can react with the fuel in the reactor. Heavy water is also used in research as an isotopic tracer for chemical reactions and biochemical pathways. Water with tritium (T₂O) rather than deuterium may also be called heavy water.

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▪ chemical compound

(D2O),also called  Deuterium Oxide,  

       water composed of deuterium, the hydrogen (q.v.) isotope with a mass double that of ordinary hydrogen, and oxygen. (Ordinary water has a composition represented by H₂O.) Thus, heavy water has a molecular weight of about 20 (the sum of twice the atomic weight of deuterium, which is 2, plus the atomic weight of oxygen, which is 16), whereas ordinary water has a molecular weight of about 18 (twice the atomic weight of ordinary hydrogen, which is 1, plus oxygen, which is 16).

      Ordinary water as obtained from most natural sources contains about one deuterium atom for every 6,760 ordinary hydrogen atoms. If water is electrolyzed the gas produced at the cathode is mostly hydrogen, and thus the residual water is enriched in deuterium content. Continued electrolysis of hundreds of litres of water until only a few millilitres remain yields practically pure deuterium oxide. This operation, until 1943 the only large-scale method used, has been superseded by less expensive processes, such as fractional distillation (D₂O becomes concentrated in the liquid residue because it is less volatile than H₂O). The heavy water produced is used as a moderator of neutrons in nuclear power plants. In the laboratory heavy water is employed as an isotopic tracer in studies of chemical and biochemical processes.

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