Heavy water

Heavy water is dideuterium oxide, or D₂O or ²H₂O. 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.

Table of contents

1 Uses 1.1 Neutron Moderator 1.2 Neutrino Detector 2 Toxicity 3 Production 3.3 Norway 3.4 Canada 3.5 India 3.6 Other Countries 4 Data 5 External Links

Uses

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. The CANDU reactor uses this design. 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.

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.

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.

Norway

In 1934, Norsk Hydro built the first commercial heavy water plant with a capacity of 12 tonnes per year at Vemork. During World War II, the allies decided to destroy the plant and its heavy water in order to inhibit the German development of nuclear weapons. In late 1942, a raid by British paratroopers failed when the gliders crashed. All the raiders were killed in the crash or shot by the Germans. In February 1943, a group of 12 Norwegian infiltrators, trained in Britain and dropped by parachute into Norway, managed to disrupt production for two months by dynamiting the facilities. On November 16, 1943, the allied air forces dropped over 400 bombs on the site.

The allied air raid prompted the German government to move all available heavy water to Germany for safekeeping. However, on February 20 1944, a Norwegian partisan was able to sink the ferry carrying the heavy water across Lake Tinnsjoe at the cost of 14 Norwegian civilians.

The story was used for a 1966 Hollywood movie, The Heroes of Telemark, starring Kirk Douglas.

Canada

The Bruce Heavy Water Plant in Ontario was the world's largest heavy water production plant with a capacity of 700 tonnes per year. It used the Girdler Sulfide process to produce heavy water, and required 340,000 tonnes of feed water to produce one tonne of heavy water. It was part of a complex that included 8 CANDU reactors which provided heat and power for the heavy water plant. The site was located on the Bruce Peninsula between Lake Huron and Georgian Bay where it had access to all the waters of the Great Lakes.

The Bruce plant was commissioned in 1979 to provide heavy water for an expected large increase in Ontario's nuclear power generation. It turned out that the nuclear power division of Ontario Hydro, the provincially owned electric power system, was planning to build generators for which there was no demand. Improved efficiency in the use and recycling of heavy water plus the over-production at Bruce left Canada with far more heavy water than it needed for its future needs. Also, the Girdler process released large amounts of hydrogen sulfide as a byproduct, raising environmental concerns. The Bruce plant was shut down in 1997. In 2003, the new owners of the site asked for permission to decommission and disassemble the plant.

Atomic Energy of Canada Limited (AECL) is currently researching other more efficient and environmentally benign processes for creating heavy water. This is essential for the future of the CANDU reactors since heavy water represents about 20% of the capital cost of each reactor.

India

India is the world's second largest producer of heavy water through its Heavy Water Board [1].

Other Countries

Argentina is another declared producer of heavy water. Presumably, all the countries with nuclear weapons can also produce heavy water.

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

External Links

• Federation of American Scientists article on the production of heavy water
• Heavy Water: A Manufacturers Guide for the Hydrogen Century (PDF file)