Heavy water 

Semiheavy water, HDO, is more common than heavy water. Heavy water occurs naturally in regular water as a lot less than 115 parts per million, that is a lot less than about one part in 7,000. It may be separated from regular water by distillation 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 [1] (http://www.heavywaterboard.org/).

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 isn't 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 isn't 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