Heavy water
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Heavy water is deuterium oxide, or D2O or 2H2O. Its physical and chemical properties are similar to those of 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. This isotopic substitution alters the bond energy (chemistry) of the hydrogen-oxygen bond in water, altering the physical and chemical properties of the substance. Gilbert Newton Lewis isolated the first sample of pure heavy water in 1933.
Semiheavy water, HDO, also exists whenever there is water with hydrogen-1 (or protium) and deuterium present in the mixture. This is because hydrogen atoms (hydrogen-1 and deuterium) are rapidly exchanged between water molecules. Water containing 50 percent H and 50 percent D actually contains about 50 percent HDO and 25 percent each of H2O and D2O, in dynamic equilibrium.
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Uses
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 from hydrogen and therefore does not contribute to the NMR signal at the hydrogen resonance frequency.
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. Heavy water production reactors can be designed to turn uranium into bomb-usable plutonium without requiring enrichment facilities. Heavy water production reactors have been used for this purpose by India, Israel, Pakistan, North Korea, Russia and USA. There is no evidence that heavy water power reactors, such as the CANDU design, have been used for military plutonium production.
Due to its potential for use in nuclear weapons programs, heavy water is subject to government control in several countries. Suppliers of heavy water and heavy water production technology typically apply IAEA administered safeguards and material accounting to heavy water. (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
To perform their tasks, enzymes rely on their finely tuned networks of hydrogen bonds, both in the active center with their substrates, and outside the active centre, to stabilize their tertiary structures. As a hydrogen bond with deuterium is slightly stronger than one involving ordinary hydrogen, in a highly deuterated environment, the normal reactions in the cell are disrupted. Experiments in mice, rats, and dogs [1] have shown that a degree of 25 % deuteration causes sterility. High concentrations (90 %) rapidly kills fish, tadpoles, flatworms, and drosophila.
Nonetheless, accidental or intentional poisoning is unlikely, as large amounts of heavy water would have to be ingested to produce any noticeable effects.
Production
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, all of which exploit a kinetic isotope effect. In short, the difference in mass between the two hydrogen isotopes translates into a difference in the zero-point energy and thus into 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. The most important chemical method is the Girdler Sulfide process.
United States
In 1953, the United States began using heavy water in plutonium production reactors at the Savannah River Site. The first of the five heavy water reactors came online in 1953, and the last was placed in cold shutdown in 1996. The SRS reactors were heavy water reactors so that they could produce both plutonium, and tritium for the US nuclear weapons program.
The US developed the Girdler Sulfide chemical exchange production process which was first demonstrated on a large scale at the Dana, Indiana plant in 1945 and at the Savannah River Plant, Georgia in 1949. The SRP was operated by DuPont for the USDOE until about 1980.
Norway
In 1934, Norsk Hydro built the first commercial heavy water plant at Vemork, Tinn, with a capacity of 12 tonnes per year. From 1940 and throughout World War II the plant was under Nazi German control, and the allies decided to destroy the plant and its heavy water in order to inhibit 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 German army troops. 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 more than 400 bombs on the site.
The allied air raid prompted the Nazi government to move all available heavy water to Germany for safekeeping. On February 20 1944, a Norwegian partisan sunk the ferry M/F Hydro carrying the heavy water across Lake Tinn at the cost of 14 Norwegian civilians, and most of the heavy water was presumably lost. A few of the barrels were only half full, and therefore could float, and may have been salvaged and transported to Germany. However, recent investigation of production records at Norsk Hydro and analysis of an intact barrel that was salvaged in 2004 revealed that although the barrels in this shipment contained water of pH 14 – indicative of heavy water refinement – the barrels did not contain high concentrations of D2O. Hence, the shipment on February 20 1944 may not have been essential, and the history of the heavy water transport from Norway to Germany may not be fully known after all.
Canada
As part of its contribution to the Manhattan Project, Canada built and operated a 6 T/a electrolytic heavy water plant at Trail, BC, which started operation in 1943.
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 and operated in Atlantic Canada at Glace Bay (by Deuterium of Canada Limited) and Port Hawkesbury, Nova Scotia (by General Electric Canada). These plants proved to have significant design, construction and production problems and so AECL built the Bruce Heavy Water Plant, which it later sold to Ontario Hydro, to ensure a reliable supply of heavy water for future power plants. The two Nova Scotia plants were shut down 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.
India
India is the world's second largest producer of heavy water through its Heavy Water Board [2].
Other countries
Argentina is another declared producer of heavy water, using an ammonia/hydrogen exchange based plant supplied by Switzerland's Sulzer company. Romania also produces heavy water at the Drobeta Girdler Sulfide plant and has exported from time to time. France operated a small plant during the 1950's and 60's. Also, in 1958, Britain allegedly sold 20 tons of heavy water to Israel.
Physical Properties (with comparison to light water)
| Property | D2O (Heavy water) | H2O (Light water) |
|---|---|---|
| Melting point (°C) | 3.82 | 0.0 |
| Boiling point (°C) | 101.72 | 100.0 |
| Density (at 20°C, g/mL) | 1.1056 | 0.9982 |
| Temp. of maximum density (°C) | 11.6 | 4.0 |
| Viscosity (at 20°C, centipoise) | 1.25 | 1.005 |
| Surface tension (at 25°C, dyn·cm) | 71.93 | 71.97 |
| Heat of fusion (cal/mol) | 1,515 | 1,436 |
| Heat of vaporization (cal/mol) | 10,864 | 10,515 |
See also
External links
- Federation of American Scientists article on the production of heavy water
- Heavy Water: A Manufacturer’s Guide for the Hydrogen Century (PDF file)
- Straight Dope Staff Report: Is "heavy water" dangerous?ar:ماء ثقيل
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