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Heavy water is dideuterium oxide, or D₂O or H₂O. 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. Semiheavy water, HDO, also exists.

Production

On Earth, heavy water occurs naturally in regular water at a proportion of roughly one part in 6,000. It may be separated from regular water by distillation or 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.

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

Use

Neutron Moderator

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 criticality 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).

Neutrino Detector

The Sudbury Neutrino Observatory (SNO) in Sudbury, Ontario uses 1000 tonnes of heavy water on loan from Atomic Energy of Canada Limited. The neutrino detector is 6800 feet underground in an old mine to shield it from cosmic rays. SNO detects the Cherenkov radiation as neutrinos pass through the heavy water.

Toxicity

Heavy water is mildly toxic. Experiments with mice showed that the main effect of the slightly different reaction rate is to inhibit mitosis, causing progressive damage of tissues that need rapid regeneration. After several days of ingesting only heavy water, the body fluids contain about 50% heavy water, and at this point symptoms begin owing to the decrease in cell division rates of rapidly dividing tissues, such as hair roots and stomach linings. Aggressive cancers might also go into remission, but the effect is not predicted to be great enough to make this a useful therapy.

Poisoning is doubtful except in unusual industrial and scientific situations. It depends on the patient taking only heavy water. An intake of, say, 25% heavy water in ordinary water would produce no symptoms. So it is not so much that heavy water is damaging to health, but rather that light water is necessary for it.

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