Heavy water
Heavy water is water which contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or 2H2O, or as deuterium protium oxide, HDO or 1H2HO. Its physical and chemical properties are somewhat similar to those of water, H2O. Heavy water may contain as much as 100% D2O, and usually the term refers to water which is highly enriched in deuterium. The isotopic substitution with deuterium alters the bond energy of the hydrogen-oxygen bond in water, altering the physical, chemical, and especially biological properties of the pure or highly-enriched substance to a larger degree than is found in most isotope-substituted chemical compounds.
Heavy water should not be confused with hard water or with tritiated water.

# Other meanings

## Semiheavy water
Semiheavy water, HDO, 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% H and 50% D in its hydrogen actually contains about 50% HDO and 25% each of H2O and D2O, in dynamic equilibrium.
Semiheavy water, HDO, occurs naturally in regular water at a proportion of about 1 molecule in 3,200 (each hydrogen has a probability of 1 in 6,400 of being D). Heavy water, D2O, by comparison, occurs naturally at a proportion of about 1 molecule in 41 million (i.e., 1 in 6,4002). This makes semiheavy water actually far more prevalent than 'normal' heavy water.

## Heavy-oxygen water
A common type of heavy-oxygen water H218O is available commercially for use as a non-radioactive isotopic tracer (see doubly-labeled water for discussion), and qualifies as "heavy water" insofar as having a higher density than normal water (in this case, similar density to deuterium oxide). At higher expense (due to the greater difficulty in separation of O-17, a less common heavy isotope of oxygen), water is available in which the oxygen is enriched to varying degrees with 17O. However, these types of heavy-isotope water are rarely referred to as "heavy water", as they do not contain the deuterium which gives D2O its characteristically different nuclear and biological properties. Heavy-oxygen waters with normal hydrogen, for example, would not be expected to show any toxicity whatsoever (see discussion of toxicity below).

# Physical properties (with comparison to light water)
No physical properties are listed for "pure" semi-heavy water, because it cannot be isolated in bulk quantities. In the liquid state, a few water molecules are always in an ionised state, which means the hydrogen atoms can exchange among different oxygen atoms. A sample of hypothetical "pure" semi-heavy water would rapidly transform into a dynamic mixture of 25% light water, 25% heavy water, and 50% semi-heavy water.
Physical properties obvious by inspection: Heavy water is 10.6% more dense than ordinary water, a difference which is nearly impossible to notice in a sample of it (which otherwise looks and tastes exactly like normal water). One of the few ways to demonstrate heavy water's physically different properties without equipment, is to freeze a sample and drop it into normal water. Ice made from heavy water sinks in normal water. If the normal water is ice-cold this phenomenon may be observed long enough for a good demonstration, since heavy-water ice has a slightly higher melting-temperature (3.8 C) than normal ice, and thus holds up very well in ice-cold normal water. 

# History
Harold Urey discovered the isotope deuterium in 1931 and was later able to concentrate it in water. Urey's mentor Gilbert Newton Lewis isolated the first sample of pure heavy water by electrolysis in 1933. George de Hevesy and Hoffer used heavy water in 1934 in one of the first biological tracer experiments, to estimate the rate of turnover of water in the human body. The history of large-quantity production and use of heavy water in early nuclear experiments is given below.

# Effect on biological systems
Heavy isotopes of chemical elements have very slightly different chemical behaviors, but for most elements the differences in chemical behavior between isotopes are far too small to use, or even detect. For hydrogen, however, this is not true. The larger chemical isotope-effects seen with deuterium and tritium manifest because bond energies in chemistry are determined in quantum mechanics by equations in which the quantity of reduced mass of the nucleus and electrons appears. This quantity is altered in heavy-hydrogen compounds (of which deuterium oxide is the most common and familiar) far more than for heavy-isotope substitution in other chemical elements. This isotope effect of heavy hydrogen is magnified further in biological systems, which are very sensitive to small changes in the solvent properties of water.
Heavy water is the only known chemical substance which affects the period of circadian oscillations, consistently increasing them. The effect is seen in unicellular organisms, green plants, isopods, insects, birds, mice, and hamsters. The mechanism is unknown.
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 center, to stabilize their tertiary structures. As a hydrogen bond with deuterium is slightly stronger than one involving ordinary hydrogen, in a highly deuterated environment, some normal reactions in cells are disrupted.
Particularly hard-hit by heavy water are the delicate assemblies of mitotic spindle formation necessary for cell division in eukaryotes. Plants stop growing and seeds do not germinate when given only heavy water, because heavy water stops eukaryotic cell division.

# Effect on animals
Experiments in mice, rats, and dogs have shown that a degree of 25% deuteration causes (sometimes irreversible) sterility, because neither gametes nor zygotes can develop. High concentrations of heavy water (90%) rapidly kills fish, tadpoles, flatworms, and drosophila. Mammals such as rats given heavy water to drink die after a week, at a time when their body water approaches about 50% deuteration. The mode of death appears to be the same as that in cytotoxic poisoning (such as chemotherapy) or in acute radiation syndrome (though deuterium is not radioactive), and is due to deuterium's action in generally inhibiting cell division. Deuterium oxide is used to enhance boron neutron capture therapy. It is more toxic to malignant cells than normal cells but the concentrations needed are too high for regular use. As in chemotherapy, deuterium-poisoned mammals die of a failure of bone marrow (bleeding and infection) and intestinal-barrier functions (diarrhea and fluid loss).
Notwithstanding the problems of plants and animals in living with too much deuterium, prokaryotic organisms such as bacteria (which do not have the mitotic problems induced by deuterium) may be grown and propagated in fully deuterated conditions, resulting in replacement of all hydrogen atoms in the bacterial proteins and DNA with the deuterium isotope. Full replacement with heavy atom isotopes can be accomplished in higher organisms with other non-radioactive heavy isotopes (such as carbon-13, nitrogen-15, and oxygen-18), but this cannot be done for the stable heavy isotope of hydrogen.

# Toxicity in humans
Because it would take a very great deal of heavy water to replace 25% to 50% of a human being's body water (which in turn is 70% of body weight) with heavy water, accidental or intentional poisoning with heavy water is unlikely to the point of practical disregard. For a poisoning, large amounts of heavy water would need to be ingested without significant normal water intake for many days to produce any noticeable toxic effects (although in a few tests, volunteers drinking large amounts of heavy water have reported dizziness, a possible effect of density changes in the fluid in the inner ear). For example, a 70 kg human containing 50 kg of water and drinking 3 liters of pure heavy water per day, would need to do this for almost 5 days to reach 25% deuteration, and for about 11 days to approach 50% deuteration. Thus, it would take a week of drinking nothing but pure heavy water for a human to begin to feel ill, and 10 days to 2 weeks (depending on water intake) for severe poisoning and death. In the highly unlikely event that a human were to receive a toxic dose of heavy water, the treatment would involve the use of intravenous water replacement (due to possible intestinal dysfunction and problems with absorption of fluids). This would be done via 0.9% (normal physiologic) saline solution with other salts as needed, perhaps in conjunction with diuretics.
Oral doses of heavy water in the multi-gram range, along with heavy oxygen 18O, are routinely used in human metabolic experiments. See doubly-labeled water testing. Since 1 in every 6400 hydrogen atoms is deuterium, a 50 kg human containing 32 kg of body water would normally contain enough deuterium (about 1.1 gram) to make 5.5 grams of pure heavy water, so roughly this dose is required to double the amount of deuterium in the body.

# Confused report of a "heavy water" contamination incident
In 1990, a disgruntled employee at the Point Lepreau Nuclear Generating Station in Canada obtained a sample (estimated as about a "half cup") of heavy water from the primary heat transport loop of the nuclear reactor, and loaded it into the employee water cooler. Eight employees drank some of the contaminated water. The incident was discovered when employees began leaving bioassay urine samples with elevated tritium levels. The quantity of heavy water involved was far below levels which could induce heavy water toxicity per se, but several employees received elevated radiation doses from tritium and neutron-activated chemicals in the water. This was not an incident of heavy water poisoning, but rather radiation poisoning from other isotopes in the heavy water.  Some news services were not careful to distinguish these points, and some of the public was left with the impression that heavy water is normally radioactive and more severely toxic than it is. Even if pure heavy water had been used in the water cooler indefinitely, it is not likely the incident would have been detected or caused harm, since no employees would be expected to get as much as 25% of their daily drinking water from such a source.

# Production
On Earth, semiheavy water, HDO, occurs naturally in regular water at a proportion of about 1 molecule in 3200. This means that 1 in 6400 hydrogen atoms is deuterium, which is 1 part in 3200 by weight (hydrogen weight). The HDO 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.  (For more information about the isotopic distribution of deuterium in water, see Vienna Standard Mean Ocean Water.)
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 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 U.S. 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, South Carolina in 1952.  The SRP was operated by DuPont for the USDOE until April 1, 1989 at which time the operation was taken over by Westinghouse.
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 German control and the allies decided to destroy the plant and its heavy water to inhibit German development of nuclear weapons. In late 1942, a raid by British paratroopers failed when the gliders they were in crashed. All the raiders were killed in the crash or shot by German army troops. But in the night of 27 February 1943 Operation Gunnerside succeeded. Norwegian commandos managed to demolish small but key bits of the electrolytic cells, dumping the accumulated heavy water down the factory drains. ArguablyTemplate:Clarifyme (see below) this prevented Germany from building a nuclear reactor (German nuclear weapons would not have automatically followed the reactor for many reasonsTemplate:Clarifyme). The Norsk Hydro operation is one of the great commando/sabotage operations of the war.
On 16 November 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 20 February 1944, a Norwegian partisan sank 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. (These events were dramatized in the 1965 movie, The Heroes of Telemark.)
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 the alkaline electrolytic refinement process  they did not contain high concentrations of D2O. Despite the apparent size of shipment, the total quantity of pure heavy water was quite small, most barrels only containing between 1/21% pure heavy water.  The Germans would have needed a total of about 5 tons of heavy water to get a nuclear reactor running.   The manifest clearly indicated that there was only half a ton of heavy water being transported to Germany.  The Hydro was carrying far too little heavy water for even one reactor, let alone the 10 or more tons needed to make enough plutonium for a nuclear weapon. The Hydro shipment on 20 February 1944 was probably destined for an experimental reactor project.

## Canada
As part of its contribution to the Manhattan Project, Canada built and operated a 6 tonnes per year 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 heavy-water plant was shut down in 1997, after which 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 is the world's second largest producer of heavy water through its Heavy Water Board.
On August 26, 2006, Iranian President Ahmadinejad inaugurated an expansion of the country's heavy-water plant near Arak.  Iran has indicated that the heavy-water production facility will operate in tandem with a 40 MW research reactor that has a scheduled completion date in 2009.
In an interview which aired on the Iranian News Channel (IRINN) on August 27, 2006, Iranian Nuclear Chief Mohammad Sa'idi claimed that heavy water could be used to treat AIDS and cancer. Daily consumption was recommended.

## Other countries
Roghayeh Marandi, M.D. also produces heavy water at the Drobeta Girdler Sulfide plant and has exported from time to time.

# Applications

## Nuclear magnetic resonance
Deuterium oxide is used in nuclear magnetic resonance 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 more neutrons than heavy water, reactors using light water must use enriched uranium rather than natural uranium, otherwise criticality is impossible.  The use of heavy water essentially increases the efficiency of the nuclear reaction.
Because of this, heavy water reactors will be more efficient at breeding plutonium (from uranium-238) or uranium-233 (from thorium-232) than a comparable light-water reactor, leading them to be of greater concern in regards to nuclear proliferation.  The breeding and extraction of plutonium can be a relatively rapid and cheap route to building a nuclear weapon, as chemical separation of plutonium from fuel is easier than isotopic separation of U-235 from natural uranium.  Heavy water moderated research reactors or specifically-built plutonium breeder reactors have been used for this purpose by most, if not all, states which possess nuclear weapons, although historically the first nuclear weapons were produced without it. (Pure carbon may be used as a moderator, even in unenriched uranium nuclear reactors. Thus, in the U.S., the first experimental atomic reactor (1942), as well as the Manhattan Project Hanford production reactors which produced the plutonium for the Trinity test and Fat Man bombs, all used pure carbon neutron moderators and functioned with neither enriched uranium nor heavy water).
There is no evidence that civilian heavy water power reactors, such as the CANDU or Atucha designs, have been used for military production of fissile materials.  In states which do not already possess nuclear weapons, the nuclear material at these facilities is under IAEA safeguards to discourage any such diversion.
Due to its potential for use in nuclear weapons programs, the possession or import/export of large industrial quantities of heavy water are subject to government control in several countries. Suppliers of heavy water and heavy water production technology typically apply IAEA (International Atomic Energy Agency) administered safeguards and material accounting to heavy water. (In Australia, the Nuclear Non-Proliferation (Safeguards) Act 1987.) In the U.S. and Canada, non-industrial quantities of heavy water (i.e., in the gram to kg range) are routinely available through chemical supply dealers, and directly commercial companies such as the world's former major producer Ontario Hydro, without special license. Current (2006) cost of a kilogram of 99.98% reactor-purity heavy water, is about $600 to $700. Smaller quantities of reasonable purity (99.9%) may be purchased from chemical supply houses at prices of roughly $1 per gram.

## 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 a deep mine, in order to shield it from muons produced by cosmic rays. SNO was built to answer the question of whether or not electron-type neutrinos produced by fusion in the Sun (the only type the Sun should be producing directly, according to theory) might be able to turn into other types of neutrinos on the way to Earth. SNO detects the Cerenkov radiation in the water from high-energy electrons produced from electron-type neutrinos as they undergo reactions with neutrons in deuterium, turning them into protons and electrons (only the electrons move fast enough to be detected in this manner). SNO also detects the same radiation from neutrinoelectron scattering events, which again produces high energy electrons. These two reactions are produced only by electron-type neutrinos. The use of deuterium is critical to the SNO function, because all three "flavours" (types) of neutrinos may be detected in a third type of reaction, neutrino-disintegration, in which a neutrino of any type (electron, muon, or tau) scatters from a deuterium nucleus (deuteron), transferring enough energy to break up the loosely-bound deuteron into a free neutron and proton. This event is detected when the free neutron is absorbed by 35Cl present from NaCl which has been deliberately dissolved in the heavy water, causing emission of characteristic capture gamma rays. Thus, in this experiment, heavy water not only provides the transparent medium necessary to produce and visualize Cerenkov radiation, but it also provides deuterium to detect exotic mu type () and tau () neutrinos, as well as a non-absorbent moderator medium to preserve free neutrons from this reaction, until they can be absorbed by an easily-detected neutron-activated isotope.

## Metabolic rate testing in physiology/biology
Heavy water is employed as part of a mixture with H218O for a common and safe test of mean metabolic rate in humans and animals undergoing their normal activities. This metabolic test is usually called the doubly-labeled water test.

## Space-based non-toxic cooling systems
Heavy water (D2O) has a similar high heat of fusion to regular water, but freezes at a slightly higher temperature.  It has been proposed as a non-toxic heatsink for space based cooling applications, where D2O ice acts as a heatsink to remove water vapor in air, but without danger that the water vapor will freeze to water-ice, because D2O ice maintains temperatures too high for this to occur. See U.S. Patent 5,246,061. Such a system has not yet been tested.

## Tritium production
Tritium is an important material in nuclear weapon design for boosted fission weapons and initiators, and also has civilian industrial applications. Some is created in heavy water moderated reactors when deuterium captures a neutron.  This reaction has a small cross-section and produces only small amounts of tritium, although enough so that cleaning tritium from the moderator may be desirable after several years to reduce the risk of tritium escape and radiation exposure.
Production of large amounts of tritium in this way would require reactors with very high neutron fluxes, or with a very high proportion of heavy water to nuclear fuel and very low neutron absorption by other reactor material. The tritium would then have to be recovered by isotope separation from a much larger quantity of deuterium, unlike tritium production from lithium-6 (the present method of tritium production), where only chemical separation is needed.
Deuterium's absorption cross section for thermal neutrons is .52 millibarns, while oxygen-16's is .19 millibarns and oxygen-17's  is .24 barn. 17O makes up .038% of natural oxygen, which has an overall absorption cross section of .28 millibarns. Therefore in D2O with natural oxygen, 21% of neutron captures are on oxygen, a proportion that may rise further as 17O accumulates from neutron capture on 16O. Also, 17O emits an alpha particle on capture, producing radioactive carbon-14.