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] Heavy water is dideuterium oxide, or D₂O. It is chemically the same as normal water, H₂O, but the hydrogen atoms are of the heavy isotope deuterium, in which the nucleus contains a neutron in addition to the proton found in the nucleus of any hydrogen atom.

Heavy water occurs naturally in regular water as 115 parts per million, that is slightly more than one part in 10,000. It may be separated from regular water by distilation or by electrolysis. In each case the slight difference in molecular weight produces a slight difference in the speed at which the reaction proceeds. To produce pure heavy water a large cascade of stills or electrolysis chambers is required, and large amounts of electric power are consumed.

Heavy water is used in certain types of nuclear reactors where it acts as a neutron moderator to slow down neutrons so that they can react with the uranium in the reactor. Light water also acts as a moderator but because light water absorbs neutrons, reactors using light water must use enriched uranium rather than natural uranium otherwise crticality is impossible. The CANDU reactor uses this design.

Because heavy water reactors can use natural uranium, it is of concern in efforts to prevent nuclear proliferation. A nation with a sufficiently powerful heavy water reactor can use it to turn uranium into bomb-usable plutonium without requiring enrichment facilities. Heavy water reactors have been used for this purpose by India, Israel, Pakistan and North Korea. North Korea also possesses graphite-moderated reactors, as used by the USA, UK, USSR and France for their bomb programs (in fact it was stated in British Parliament that one of these had been built to the declassified blueprints for Calder Hall). A major part of the negotiations involving North Korean nuclear reactors have been to attempt to shut down all of these reactors.

Due to its usefulness in nuclear weapons programs, heavy water is subject to government control in several countries. (In Australia, the Nuclear Non-Proliferation (Safeguards) Act 1987).

India is the world's second largest producer of heavy water through its Heavy Water Board .

Toxicity

Heavy water is claimed by some to be toxic. However, someone falling into a tank of pure heavy water would drown with no evidence of deuterium poisoning.

It is speculated that continuous ingestion of heavy water would eventually cause symptoms similar to chemotherapy or radiation poisoning. The slightly different reaction rate of heavy water, which is its main chemical difference to light water, inhibits cell division slightly. After several days of ingesting only heavy water, the body fluids would contain about 50% heavy water, and at this point symptoms would begin owing to the decrease in cell division rates of rapidly dividing tissues, such as hair roots and stomach linings. Aggressive cancers would also go into remission, but the effect is not predicted to be great enough to make this a useful therapy.

Whether this is true poisoning is doubtful. It depends for its effect on the patient taking only heavy water. An intake of say 25% heavy water in ordinary water would produce no symptoms indefinitely. So it is not so much that heavy water is damaging to health, but rather that light water is necessary for it.

Similarly, if one were to breath only nitrogen, death would result in a few minutes. Helium would have the same effect. But neither of these gases is regarded as toxic.

Possibly, those who claim toxicity are confusing deuterium poisoning with tritium poisoning. Deuterium used in reactors quickly becomes contaminated with tritium owing to neutron capture. However the symptoms of tritium poisoning are very different.

Data

• boiling point: 101.42° C (214.56°F) at standard pressure.
• freezing point: 3.81° C (38.86° F).
• relative density: 1.1079 at standard temperature and pressure