Science Fair Project Encyclopedia
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. Gilbert Newton Lewis isolated the first sample of pure heavy water in 1933.
Semiheavy water, HDO, also exists, although not in pure form: a sample of water of average composition HDO actually contains 50 percent HDO and 25 percent each H2O and D2O.
Nuclear magnetic resonance
Deuterium oxide is used in nuclear magnetic resonance (NMR) spectroscopy when the solvent of interest is water and the nuclide of interest is hydrogen. This is because the signal from the water solvent would interfere with the signal from the molecule of interest. Deuterium has a different magnetic moment than hydrogen and therefore does not contribute (significantly) to the NMR signal at the hydrogen resonance frequency
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.
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.
Heavy water is not considered toxic, but some metabolic reactions require normal ("light") water, so consumption of exclusively heavy water can cause illness. This can be thought of as similar to inhalation of pure nitrogen, the primary component of air—it is not the nitrogen that is dangerous, but rather the lack of oxygen. Poisoning is unlikely except in unusual industrial and scientific situations.
Experiments with mice have shown that the main effect of heavy water's 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 percent heavy water. At this point symptoms begin to appear, 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 believed to be pronounced enough to make this a useful therapy.
On Earth, semiheavy water, HDO, occurs naturally in regular water at a proportion of 1 part per 3200. It may be separated from regular water by distillation or electrolysis and also by various chemical exchange processes which exploit the differing affinities of deuterium and hydrogen for various compounds. In each case the slight difference in molecular weight produces a slight difference in the speed at which the reaction proceeds. Once HDO becomes a significant fraction of the water, heavy water will become more prevalent as well as water molecules trade hydrogen atoms very frequently. To produce pure heavy water by distillation or electrolysis requires a large cascade of stills or electrolysis chambers, and consumes large amounts of power, so the chemical methods are generally preferred.
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 by the Special Operations Executive 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.
See also: Norwegian heavy water sabotage
The Atomic Energy of Canada Limited (AECL) design of power reactor requires large quantities of heavy water to act as a neutron moderator and coolant. AECL ordered two heavy water plants which were built in Atlantic Canada at Glace Bay and Port Hawkesbury, Nova Scotia. These plants proved to have significant design, construction and production problems and so Ontario Hydro ordered the Bruce Heavy Water Plant so that it would have a reliable in-house supply of heavy water for current and future power plants. The two AECL plants were shutdown in 1985 when their production proved to be unnecessary.
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 at Douglas Point in Bruce County on Lake Huron where it had access to the waters of the Great Lakes.
The Bruce plant was commissioned in 1979 to provide heavy water for a large increase in Ontario's nuclear power generation. The plants proved to be significantly more efficient than planned and only three of the planned four units were eventually commissioned. In addition, the nuclear power programme was slowed down and effectively stopped due to a perceived oversupply of electricity (later shown to be temporary!), in 1993. Improved efficiency in the use and recycling of heavy water plus the over-production at Bruce left Canada with enough heavy water for its anticipated future needs. Also, the Girdler process involves large amounts of hydrogen sulfide, raising environmental concerns if there should be a release. The Bruce plant was finally shut down in 1997. The plant was gradually dismantled and the site cleared.
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.
- 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
- Federation of American Scientists article on the production of heavy water
- Heavy Water: A Manufacturers Guide for the Hydrogen Century (PDF file)
- Straight Dope Staff Report: Is "heavy water" dangerous?
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