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Heavy water is a loose term which usually refers to '''deuterium oxide''', or D2O or 2H2O. Its physical and chemical properties are somewhat similar to those of Light 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. 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 . Heavy water should not be confused with Hard Water or Tritiated 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). 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). 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. 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 Reactor s can easily use natural (unenriched) uranium, heavy water becomes a material 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 (although this is not the only route for using natural uranium, see below). 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, but in theory any working nuclear reactor can be used for making weapons-grade plutonium, by simply running the reactor for a shorter-than-normal time after fueling, and then by reprocessing the fuel. Due to its potential for use in Nuclear Weapons programs, 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 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. It is worth noting that nuclear reactors for the production of plutonium can be made without either enriched uranium or heavy water, if they use very highly purified carbon as the moderator (the Nazis, due to a mistake in dealing with impure carbon, did not know this). 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 and Fat Man bombs, all functioned with neither enriched uranium nor heavy water. Neutrino detector The ) into a free Neutron and Proton . This event is then detected when the free neutron is absorbed by 35Cl present in NaCl dissolved in the heavy water, causing emission of characteristic gamma rays. In this experiment, heavy water not only provides the transparent medium necessary to produce and visualize Cherenkov Radiation radiation, but it also provides both a target for exotic mu and tau neutrinos, as well as a non-absorbent medium to preserve free neutrons until they can be absorbed by the proper 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 . TOXICITY 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 unsually sensitive to small changes in the solvent properties of water. To perform their tasks, Enzyme s rely on their finely tuned networks of Hydrogen Bond s, both in the active center with their substrates, and outside the active centre, to stabilize their Tertiary Structure s. 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 , Tadpole s, Flatworm s, 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, Procaryotic organisms like 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. 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 Poison ing 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 O-18, are routinely used in human metabolic experiments. See Doubly-labeled Water testing. In 1990, a disgruntled employee at the Point Lepreau Nuclear Generating Station took a sample of heavy water from the primary heat transport loop of the reactor and loaded it into a water cooler. 8 employees drank some of the contaminated water. The incident was discovered when employees began leaving Bioassay urine samples with elevated Tritium levels. The quantities involved were well below levels which could induce heavy water toxicity, but several employees received elevated radiation doses from tritium and activated chemicals in the water. {Link without Title} . As such, this was not really an incident of heavy water poisoning so much as radiation poisoning from other isotopes in the heavy water. Had pure heavy water been used in the water cooler, even indefinitely, it is unlikely that the incident would ever have been detected, since no employees would be expected to get more than 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 (see 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 . 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 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, South Carolina in 1952. 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 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 Paratrooper s 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. The allied air raid prompted the Nazi government to move all available heavy water to Germany for safekeeping. On 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. It was of no military significance, which is why it was only lightly guarded. 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 Reactor s 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 {Link without Title} . 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. PHYSICAL PROPERTIES (WITH COMPARISON TO LIGHT WATER) Note for teachers: 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 (do make sure the normal water is ice-cold if you want to play with this phenomenon for long-- heavy water ice has a slightly higher melting-temperature than normal ice (3.8 C), so it holds up very well in ice-cold normal water). SEE ALSO EXTERNAL LINKS
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