Heavy water

Heavy water, formally deuterium oxide, is a molecule with a chemical composition equivalent to water, in which the two atoms of the most abundant isotope of hydrogen, protium, are replaced by two deuterium, a heavy isotope of hydrogen (also known as "heavy hydrogen"). Its chemical formula is: D₂O or ²H₂O.

Introduction

Heavy water is a form of water that contains a larger than normal amount of deuterium, an isotope of hydrogen, (also known as "heavy hydrogen") in instead of the common isotope of hydrogen-1, or protium, of which most normal water is composed. Therefore, some or most of the hydrogen atoms in heavy water contain a neutron, causing each hydrogen atom to hydrogen to be about twice as heavy as a normal hydrogen atom (although the weight of water molecules is only moderately affected, since about 89% of the molecular weight resides in the oxygen atom). The increased weight of hydrogen in water makes it a bit more dense.

The colloquial term heavy water is often also used to refer to a highly enriched mixture of water that contains mostly deuterium oxide, but also contains some ordinary water molecules. Thus, for example, the heavy water used in CANDU reactors is 99.75% enriched for each hydrogen atom, which means that 99.75% of the hydrogen atoms are of the heavy type (deuterium). By comparison, in ordinary water, sometimes called "light water," there are only about 156 deuterium atoms for every million hydrogen atoms.

Heavy water is not radioactive. In its pure form, it has a density about 11% greater than that of water, but is otherwise physically and chemically similar. However, the various differences between deuterium-containing waters (particularly affecting biological properties) are greater than in any other common isotopically substituted compound because deuterium is unique among stable isotopes in being twice as heavy as deuterium. the lightest isotope. This difference increases the strength of the hydrogen-oxygen bonds in the water, and this in turn is enough to cause differences that are important for some of the biochemical reactions. The human body naturally contains deuterium equivalent to approximately five grams of heavy water, which is harmless. When a large fraction of the water (>50%) in higher organisms is replaced by heavy water, the result is cell dysfunction and death.

Heavy water was first synthesized in 1932, a few months after the discovery of deuterium. With the discovery of nuclear fission in late 1938, and the need for a neutron moderator that captured few neutrons, heavy water heavy became a research component of early nuclear power. Since then, heavy water has been an essential component in some types of reactors, both those that generate power and those designed to produce isotopes for nuclear weapons. These heavy water reactors have the advantage of being able to use natural uranium without the use of graphite moderators (which can pose radiological or dust explosion hazards in the decommissioning phase). Most modern reactors use uranium enriched with " normal light water (H₂O) as moderator.

Other forms of heavy water

Semi-heavy water

Semi-heavy water, HDO, exists as long as there is water with light hydrogen (Protium, ¹H) and deuterium (D or ²H) in the mix. This is because hydrogen atoms (hydrogen-1 and deuterium) are rapidly exchanged between water molecules. Water that contains 50% H and 50% D in its hydrogen actually contains approximately 50% HDO and 25% each of H₂O and D₂ Or, in dynamic equilibrium. In normal water, about 1 molecule in 3,200 is HDO (one hydrogen in 6,400 is in the D form), and heavy water molecules (D₂O) only occur in one ratio of approximately 1 molecule in 41 million (ie, one in 6,400²). Therefore, semi-heavy water molecules are much more common than "pure" heavy water molecules; (homoisotopic).

Heavy water-oxygen

Water enriched in the heavier oxygen isotopes ¹⁷O and ¹⁸O is also commercially available, for example, for use as a non-radioactive isotopic tracer. This is "heavy water" as it is denser than normal water (H₂¹⁸O is about as dense as D₂O, and the H₂¹⁷O is halfway between the H₂O and D₂ O), but it is rarely called heavy water, since it does not contain the deuterium that gives D₂O its unusual nuclear and biological properties. It is more expensive than D₂O due to the greater difficulty in separating the ¹⁷O and ¹⁸O.

Tritiated water

Tritiated water contains tritium instead of protium or deuterium. The chemical formula for tritiated water, tritium oxide or super heavy water is: T₂O or ³H₂O. This form is radioactive.

There are also other isotopic varieties such as an unnamed form that would correspond to a "semi-superheavy water", whose chemical formula is HTO, THO or ¹H³ HO. This form is radioactive.

Properties

This difference in the elements of the nucleus modifies some of its physical properties, such as density or boiling point. Heavy water is present, in small quantities, mixed with normal water, and can be separated from it by fractional distillation. It can also be separated from water by absorption with ammonia containing deuterium.

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 water turnover in the human body. Emilian Bratu and Otto Redlich studied the self-dissociation of heavy water in 1934. From the late 1930s and during World War II, great advances were made in the production and use of heavy water in large quantities in early nuclear experiments. Many of these experiments were kept secret due to military importance.

Production

On Earth, deuterated water, HDO, occurs naturally in normal water at a ratio of about 1 molecule in 3,200. This means that 1 in 6,400 hydrogen atoms is deuterium, which is 1 part in 3200 by weight (weight of hydrogen). HDO can be separated from normal water by distillation or electrolysis and also by various chemical exchange processes, which take advantage of the kinetic isotope effect.

The difference in mass between the two isotopes of hydrogen translates into a difference in zero point energy and therefore a slight difference in the rate at which the reaction proceeds. Once HDO becomes a significant fraction of water, heavy water becomes more prevalent as water molecules exchange hydrogen atoms very frequently. Production of pure heavy water by distillation or electrolysis requires a large cascade of stills or electrolysis chambers and consumes large amounts of energy, so chemical methods are generally preferred.

The most cost-effective process for producing heavy water is the double-temperature exchange sulfide process (known as the Girdler sulfide process) developed in parallel by Karl-Hermann Geib and Jerome S. Spevack in 1943.

An alternative process, patented by Graham M. Keyser, uses lasers to selectively cleave deuterated hydrofluorocarbons to form deuterium fluoride, which can then be separated by physical media. Although the energy consumption for this process is much lower than for the Girdler sulfide process, this method is currently uneconomical due to the expense of obtaining the necessary hydrofluorocarbons.

As noted modern commercial heavy water is almost universally known and sold as deuterium oxide. It is most often sold in various purity grades, from 98% enrichment to 99.75-99.98% deuterium enrichment (nuclear reactor grade) and occasionally even higher isotopic purity.

Producing plants

• The Soviet Union: started production in 1934 in Dnepropetrovsk, but was interrupted due to Operation Barbecue. After 1946 five plants were built with a total annual production of 20 tons.
• Argentina: is the main producer and exporter, producing in a plant with capacity of 200 t/year; it is located in Arroyito (provincia del Neuquén), operated by the state company ENSI.
• America: produced heavy water until the 1980s. In 1953, the United States began to use heavy water in the plutonium production reactors of Savannah River Site (SRS). The first of the five heavy water reactors came online in 1953, and the last one was put in a cold stop in 1996. SRS reactors were heavy water reactors that allowed the production of both plutonium and tritium for the U.S. nuclear weapons program.

The United States developed the Geib-Spevack method based on chemical exchange, which was used for the first time on a large scale in a plant built in Dana, Indiana, in 1945 and on the Savannah River Plant, South Carolina in 1952. The SRP was operated by DuPont for USDOE until April 1, 1989 when it was acquired by Westinghouse.

• Canada: was the world's largest producer until the closure of the heavy water plant in 1997. As part of its contribution to the Manhattan Project, Canada built and operated an electrolytic plant with a production capacity of 6 tons per year of heavy water in Trail, British Columbia, which began operating in 1943.

The power reactor designed by Atomic Energy of Canada Limited (AECL) requires large quantities of heavy water to act as a neutron and refrigerant moderator. AECL requested two heavy water plants that were built and operated in the Canadian Atlantic one at Glace Bay (by Deuterio of Canada Limited) and the other at Port Hawkesbury, Nova Scotia (by General Electric Canada). These plants proved to have important design, construction and production problems and so AECL built the Bruce Heavy Water Plant, which later sold to Ontario Hydro, to ensure a reliable supply of heavy water for future plants. The two plants in Nova Scotia were closed in 1985, when their production turned out to be unnecessary.

The Bruce Heavy Water Plant in Ontario was the largest heavy water production plant in the world, with a capacity of 700 tonnes per year. He used the Girdler sulfur process to produce heavy water, and required 340 000 tons of power water to produce a ton of heavy water. It was part of a complex that included eight CANDU reactors that provided heat and energy for the heavy water plant. The site was located at the Douglas Point/Bruce Nuclear Generation Station near Tiverton, Ontario, at Lake Huron, where it had access to the waters of the Great Lakes.

Bruce's plant was inaugurated in 1979 to supply heavy water to a large increase in nuclear power generation in Ontario. The plant proved to be much more efficient than expected and only three of the four planned units were finally built. In addition, the nuclear power programme slowed down and finally stopped because an excess in the supply of electricity was appreciated. Improved efficiency in the use and recycling of heavy water, plus excess production in Bruce, made Canada have enough heavy water for its anticipated future needs. In addition, the Girdler process involves large amounts of hydrogen sulphide, which contradicted increased environmental concerns. The Bruce heavy water plant was closed in 1997, after which it gradually dismantled and the site was cleared.

Atomic Energy of Canada Limited (AECL) is investigating other more efficient and environmentally friendly processes for the manufacture of heavy water. This is essential for the future of CANDU reactors as heavy water represents approximately 20% of the capital cost of each reactor.

• Norway: in 1934, the company Norsk Hydro inaugurated the first commercial production plant in the world, in the hydroelectric plant of Vemork, Tinn, with a capacity of 12 tons per year. During the Second World War, from 1940, the plant fell under German control and the Allies decided to destroy the plant to inhibit the German development of nuclear weapons (see Battle of Heavy Water).
• India: is one of the largest global heavy water producers through Heavy Water Board and also exports to countries such as the Republic of Korea and the United States. The development of the heavy water process in India took place in three phases: The first phase (from the end of 1950 to the mid-1980s) was a technology development period, the second phase was the deployment of technology and the stabilization process (meaning from 1980 to the beginning of 1990) and the third phase was consolidation and change towards improvement in energy production and conservation.
• Iran: On 26 August 2006, Iranian President Ahmadineyad inaugurated the expansion of the country's heavy water plant near Arak. Iran has indicated that the heavy water production plant will operate in tandem with a research reactor of 40 MW (megawatts) that had an expected completion date in 2009. In January 2020, Iran announced that it produced so much heavy water that it already exported to eight countries.
• Romania: produces heavy water at the Drobeta Girdler Sulfide plant and exports occasionally.
• France: operated a small plant until 1970.
• United Kingdom: In 1958, it exported 20 tons to Israel.

Applications

Nuclear Magnetic Resonance

Deuterium oxide is used in nuclear magnetic resonance spectroscopy when water is used as the solvent if the nuclide of interest is hydrogen. The reason is that the signal of the light water solvent molecules (¹H₂O) interferes in the observation of the signal of the molecule of interest dissolved in it. Deuterium has a different magnetic moment and therefore does not contribute to the 1H-NMR signal at the hydrogen -1 resonance frequency.

Organic Chemistry

Deuterium oxide is often used as the deuterium source for the preparation of specifically labeled isotopes of organic compounds. For example, CH bonds adjacent to ketone carbonyl groups can be replaced by CD bonds, using acid or base catalysis. Trimethylsulfoxonium iodide, made from dimethyl sulfoxide and methyl iodide, can be recrystallized from deuterium oxide, and then dissociated to regenerate deuterium-labelled methyl iodide and dimethyl sulfoxide. In cases where specific double labeling with deuterium and tritium is contemplated, the investigator should be aware that the deuterium oxide, depending on age and origin, may contain some tritium.

Fourier Transform Infrared Spectroscopy

Deuterium oxide is often used instead of water in FTIR collection. The spectra of the proteins in H₂O solution create a strong band that overlaps with the amide I region of the proteins. The D₂O band is shifted away from the I region of the amide.

Neutron Moderator

The main technological application of heavy water has been as a moderator in nuclear fission processes. It is used in certain types of nuclear reactors as a neutron moderator to slow down neutrons so they are more likely to react with uranium-235, the fissile isotope, rather than uranium-238, which captures neutrons without fission. The CANDU reactor uses this design. Light water also acts as a moderator, but because light water absorbs more neutrons than heavy water, reactors with a light water moderator must use enriched uranium instead of natural uranium, otherwise the critical mass is not reached.

Because they do not require uranium enrichment, heavy water reactors are a concern in terms of nuclear proliferation, since the production and extraction of plutonium (which appears as a byproduct of the process) can be a relatively quick and cheap way to build a nuclear weapon, as chemical separation of plutonium from fuel is easier than isotopic separation of U-235 from natural uranium. This possibility made its use seriously considered in the development of the first nuclear reactors, which is why it became a strategic substance. During World War II, the Allies undertook a series of direct actions to prevent Nazi access to heavy water (see Battle of Heavy Water). However, in the United States, the first experimental atomic reactor (1942), as well as the Hanford production reactors of the Manhattan Project that produced the plutonium for the Trinity test and the Fat Man bomb, used pure carbon (graphite) as a moderator. of neutrons combined with tap water in the cooling pipes, and it worked without enriched uranium or heavy water. Russian and British plutonium production also uses graphite-moderated reactors. Today it has lost part of its importance, when other materials are used as moderators in nuclear power plants, mainly light water or graphite, although this has also lost its usefulness because it can burn.

Among the nuclear weapon states, Israel, India and North Korea created their first weapons using plutonium generated in heavy water moderated reactors and natural uranium fuel, while China, South Africa and Pakistan built their first weapons with highly enriched.

There is no evidence that civilian power heavy water reactors, such as the CANDU or Atucha designs, have been used for military production of fissile materials. In non-nuclear-weapon states, nuclear material at these facilities is under IAEA (International Atomic Energy Agency) safeguards to prevent any diversion.

Because of its potential use in nuclear weapons, the programs, possession or import and export of large industrial quantities of heavy water are subject to government control in several countries. Heavy water suppliers and heavy water production technology generally apply IAEA-administered controls to heavy water material accounting. In the United States and Canada, non-industrial quantities of heavy water (i.e., on the multi-kilogram schedule) are routinely available without a special license through chemical supply distributors and trading companies, such as the former major global producer Ontario Hydro. The current (2006) cost of one kilogram of 99.98% heavy water (reactor purity), is approximately $600 to $700. Small amounts of reasonable purity (99.9%) can be purchased from chemical supply houses for prices of about $1 per gram.

Neutrino detector

The Sudbury Neutrino Observatory (SNO) in Sudbury, Ontario uses 1,000 tons of heavy water on loan from Atomic Energy of Canada Limited. The neutrino detector is located 2,100 meters underground in a mine, to protect it from muons produced by cosmic rays. The SNO was built to answer the question of whether or not it is possible that electron-type neutrinos produced by fusion in the Sun (theoretically the only type that the Sun should produce directly) might be capable of transforming into other types of electrons. neutrinos on the way to Earth. The SNO detects Cherenkov radiation in water from high-energy electrons produced from electron neutrinos undergoing reactions with deuterium neutrons, converting them into protons and electrons (only electrons move fast enough to be detected in this way). The SNO also detects the same radiation in neutrino ↔ electron 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 function of the SNO, because all three "flavors" (types) of neutrinos can be detected in a third type of reaction, neutrino-decay, in which a neutrino of any type (electron, muon, or tau) scatters from a deuterium (deuteron) nucleus, transferring sufficient energy to break the deuteron loosely bound to a neutron-proton bond. This event is detected when the free neutron is absorbed by the ³⁵Cl^- present, as NaCl has been deliberately dissolved in the heavy water, causing the emission of gamma rays characteristic of The capture. Therefore, in this experiment, heavy water not only provides the transparent medium necessary to produce and visualize Cherenkov radiation, but also provides deuterium to detect an exotic type of mu (μ) and tau (τ) neutrinos, as well as a non-absorbing moderator medium to preserve free neutrons from this reaction, until they can be absorbed by an easily detectable activated neutron isotope.

Metabolic Rate Tests in Physiology/Biology

Heavy water is used as part of a mixture with H₂¹⁸O for a common and reliable test of mean metabolic rate in humans and animals subjected to to their normal activities. This metabolic test is generally called the double labeled water test.

Tritium production

Tritium is the active substance in autogenic lighting; other uses include autoradiography and radioactive labeling. It is also used in the design of nuclear weapons for powered fission weapons and initiators. At a theoretical level it should play an important role in the development of controlled nuclear fusion.

Some tritium is generated in heavy water moderated reactors, when deuterium captures a neutron. This reaction has a small cross section (the imaginary area of neutron capture around the nucleus) and produces only trace amounts of tritium, though enough to justify tritium cleaning of the moderator every few years to reduce the environmental risk from tritium in a leak.. For the production of a large amount of tritium in this way, reactors with very high neutron fluxes would be needed, or with a very high proportion of heavy water to nuclear fuel and very low neutron absorption by other reactor material. The tritium would have to be recovered by isotope separation of a much larger amount of deuterium, as opposed to lithium-6 production (the current procedure), where only chemical separation is needed. The absorption cross section of deuterium for thermal neutrons is 0.52 millibarn (barn = 10⁻²⁸ m², milli = 1/1000), while that of oxygen-16 is 0.19 millibarns and oxygen-17 is 0.24 barn. ¹⁷O offsets 0.038% of natural oxygen, so the total cross section is 0.28 millibarns. Thus, in D₂O with natural oxygen, 21% of the neutron captures are in oxygen, rising higher and higher as ¹⁷O accumulates from the neutron capture of ¹⁶O. In addition, ¹⁷O can emit an alpha particle upon neutron capture, producing radioactive carbon-14.

Effect on biological systems

Different isotopes of chemical elements have slightly different chemical behaviors, but for most elements the differences are too small to be used, or even detected. For hydrogen, however, this is not true. The greatest effects observed between protium (light hydrogen) versus deuterium and tritium because the binding energies in chemistry are determined in quantum mechanics by equations in which the amount of mass reduction appears from the nucleus and electrons. This amount is altered in high-hydrogen compounds (of which deuterium oxide is the most common and familiar) rather than by heavy isotope substitution in other chemical elements. This heavy hydrogen isotope effect is further magnified in biological systems, which are highly sensitive to small changes in the solvent properties of water.

Heavy water is the only chemical known to affect the period of circadian oscillations, constantly increasing the length of each cycle. The effect is seen in single-celled organisms, green plants, isopods, insects, birds, mice, and hamsters. The mechanism is unknown.

To carry out their tasks, enzymes rely on their finely tuned networks of hydrogen bonds, both in the active site with their substrates and outside the active site, to stabilize their tertiary structures. Since a hydrogen bond with deuterium is slightly stronger than that with ordinary hydrogen, in a highly deuterated environment, some cellular reactions break down.

Particularly affected by heavy water are the delicate mitotic spindle assemblies, a formation necessary for cell division in eukaryotes. Plants stop growing and seeds do not germinate when heavy water alone is administered, because heavy water stops eukaryotic cell division. The deuterium cell is larger and causes a change in the direction of division. cell membrane also changes, and reacts first with heavy water. In 1972 it was shown that an increase in the percentage content of deuterium in water reduces plant growth. Research carried out on the growth of prokaryotic microorganisms in a heavy hydrogen environment showed that under these artificial conditions, all hydrogen atoms in water molecules could be replaced by deuterium. Experiments showed that bacteria can live in 98% heavy water. However, all concentrations of more than 50% deuterium were found to in the water molecules killed the plants.

It has been proposed that low doses of heavy water may slow down the aging process by helping the body resist oxidative damage through the kinetic isotope effect. A team from the Institute for the Biology of Aging, located in Moscow, conducted an experiment to determine the effect of heavy water on longevity with fruit flies and found that while large amounts were deadly, smaller amounts increased lifespan by up to 30%.