Heavy Water

Heavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D₂O or ²H₂O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H₂O.

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 which captured few neutrons, heavy water soon achieved importance in relation to early nuclear programs during World War II. Due in part to German reliance on scarce heavy water for reactor research in this war, Germany did not succeed in producing a functioning reactor during World War II. Since this war, heavy water has played a part in a number of reactor designs, both in designs for power and for nuclear weapon-making. Reactors which use enriched uranium, however, are able to use normal “light water” for neutron moderation, and remain the most common type of reactor in use today.

Heavy water itself is not radioactive, and has physical properties similar to water save for being about 11% more dense. However, as commercially made, heavy water contains whatever tritium was present in the water from which it was isolated. When the water in eukaryotic organisms is replaced by more than about 25 to 50% heavy water, they experience toxicity due to interference by the deuterium with the mitotic apparatus of these cells. Higher organisms, including mammals, if given only heavy water, soon become ill and die at the point that about half their body water has been replaced. Bacteria, however, are able to grow slowly in pure heavy water.

Small concentrations of heavy water are nontoxic. The adult human body naturally contains deuterium equivalent to the amount in about 5 grams of heavy water, and comparable doses of heavy water are still used as safe non-radioactive tracers for metabolic experiments in humans and other animals.