Heavy water

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Heavy water is a loose term which usually refers to deuterium oxide, D₂O or ²H₂O. Its physical and chemical properties are somewhat similar to those of water, H₂O. 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 the hydrogen atom. This isotopic substitution alters the bond energy of the hydrogen-oxygen bond in water, altering the physical, chemical, and especially biological properties of the 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; however, heavy water can be used to create tritium, a principal source of energy release in a thermonuclear weapon.

[edit] Other meanings

[edit] Semiheavy water

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% H and 50% D 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 3200 (each hydrogen has a probability of 1 in 6400 of being D). Heavy water, D₂O, by comparison, occurs naturally at a proportion of about 1 molecule in 41 million (i.e., 1 in 6400²).

[edit] 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). Even more expensively, water is available in which the oxygen is 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).

[edit] Physical properties (with comparison to light water)

Property	D₂O (Heavy water)	H₂O (Light water)
Melting point (°C)	3.82	0.0
Boiling point (°C)	101.4	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, mPa·s)	1.25	1.005
Surface tension (at 25°C, μJ)	7.193	7.197
Heat of fusion (cal/mol)	1,515	1,436
Heat of vaporization (cal/mol)	10,864	10,515
pH (at 25°C)	7.41 (sometimes "pD")	7.00

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 ionized 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 than normal ice (3.8°C), and thus holds up very well in ice-cold normal water. [1]

[edit] History

Harold Urey discovered the isotope deuterium in 1931 and was later able to concentrate it in water. For further history see deuterium. Urey's mentor Gilbert Newton Lewis isolated the first sample of pure heavy water by electrolysis in 1933. 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.

[edit] 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 greatly 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.

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, 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. Because eukaryotic cell division stops in heavy water, seeds therefore do not germinate in heavy water, and plants stop growing when given only heavy water.

[edit] Effect on animals

Experiments in mice, rats, and dogs [2] 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, of course, deuterium is not radioactive), and is due to deuterium's action in generally inhibiting cell division. Deuterium oxide has even been tested as a chemotherapeutic agent, but it seems to offer no advantages. As in chemotherapy, deuterium-poisoned mammals die of a failure of bone marrow (bleeding and infection) and intestinal-barrier functions (diarrhea and fluid loss).

Not withstanding 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 (see reference in previous paragraph). Full replacement with heavy atom isotopes can be accomplished in higher organisms with other non-radioactive heavy isotopes (such as carbon-13 and nitrogen-15), but this cannot be done for the stable heavy isotope of hydrogen.

[edit] 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 (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.

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.

[edit] Confused report of a "heavy water" contamination incident

In 1990, a disgruntled employee at the Point Lepreau Nuclear Generating Station 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.[3]. Though 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.]

[edit] 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 (see Vienna Standard Mean Ocean Water, or VSMOW), 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.

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.

[edit] 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, 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.

[edit] 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 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. 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 16 November 1943, the allied air forces dropped more than 400 bombs on the site. In the night of February 27-28 Operation Gunnerside succeeded. Commandos managed to demolish small but key bits of the electolytic cells, dumping the accumulated heavy water down the factory drains. Arguably (see below) this prevented Germany from building a nuclear reactor (German nuclear weapons would not have automatically followed the reactor for many reasons). The Norsk operation is one of the great commando/sabotage operations of WW2.

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. 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. 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.

[edit] 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.

[edit] India

India is the world's second largest producer of heavy water through its Heavy Water Board [4].

[edit] Iran

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. *"Iranian Nuclear Chief Mohammad Sa'idi Explains Why Iran Produces Heavy Water: Drinking It Helps Fight Cancer and AIDS"aired on the Iranian News Channel (IRINN) on August 27, 2006

[edit] 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.

[edit] Applications

[edit] 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.

[edit] 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 or Uranium-233 than a comparable light-water reactor, leading them to be of greater concern in regards to nuclear proliferation. The breeding and extraction of plutonium or U-233 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 U235 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. On August 26, 2006 Iran announced the launch of a new phase in its Arak heavy-water reactor project. [5]

There is no evidence that civillian 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 from the world's major producer Ontario Hydro, without special license. Current (2006) cost of a kg 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.

It is worth noting that Plutonium or Uranium-233 can be produced as a consequence of the operation of any nuclear reactor; heavy water is not a pre-requisite. In fact, 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 functioned with neither enriched uranium nor heavy water.

[edit] 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, 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 might be able to turn into other types of neutrinos on the way to Earth. SNO detects the Cherenkov radiation in the water from high-energy electrons produced from electron-type neutrinos as they undergo reactions with neutrons in deuterium. SNO also detects the same radiation from neutrino< — >electron scattering events. The use of deuterium is critical to the SNO function, because all three "flavors" (types) of neutrinos [6] 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 in NaCl 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 Cherenkov radiation radiation, but it also provides deuterium to detect exotic mu type (μ) and tau (τ) 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.

[edit] 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.

[edit] 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 appllications, 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 No. 5246061 [7]. Such a system has not yet been tested.

[edit] Tritium breeding starting material

Tritium is produced in heavy water-moderated reactors when deuterium captures a neutron. While this reaction has a small cross-section and produces useful amounts of tritium only in reactors with very high neutron fluxes, this method of tritium production requires less technological sophistication than the usual production of tritium by neutron transmutation of lithium-6. Tritium is an important material in nuclear weapons programs, since it is useful for boosting the yields of fission weapons, as well as for constructing thermonuclear devices.

[edit] See also

Water Portal

[edit] External links

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Categories: Forms of water | Nuclear materials