Heavy water

• 1. Introduction

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  Heavy water is a form of water whose ice sinks in an ordinary glass of water. Chill that glass until it is ice-cold, and the sinking cube still refuses to melt, because heavy ice holds together until 3.7 degrees Celsius. Ordinary ice melts at zero, so the difference shows up without any instrument. Chemists call the liquid deuterium oxide. It has attracted commandos, bombers, spies and weapons inspectors, and it has also quietly helped scientists study the Sun. What makes a molecule of water heavy? Why did several nations scramble to make it before and during the Second World War? And why do animals die from drinking it while a detector deep underground depends on it?

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• 2. Anatomy Of A Molecule

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  A deuterium nucleus contains one proton and one neutron. The nucleus of ordinary hydrogen, called protium, contains a proton alone. That extra neutron makes a deuterium atom roughly twice as heavy as a protium atom. A heavy water molecule swaps both of its protium atoms for deuterium. Even so, the molecule gains less weight than you might expect, because about 89% of its mass belongs to the single oxygen atom.

  Vienna Standard Mean Ocean Water, the reference sample for ordinary water, carries about 156 deuterium atoms for every million hydrogen atoms. That works out to 0.0156% of the hydrogen. Chemists also use the term heavy water more loosely for any water with more deuterium than usual. The heavy water inside CANDU reactors is enriched to 99.75% by hydrogen atom-fraction. Most of it is true deuterium oxide, with some semiheavy water and a little ordinary water mixed in.

  Semiheavy water, written HDO, is far more common than the pure article. In normal water, about 1 molecule in 3,200 is HDO, while fully heavy molecules appear roughly once in 41 million. Hydrogen atoms hop rapidly between molecules, so any mix of light and heavy hydrogen settles into a dynamic equilibrium. Water that is half protium and half deuterium turns out to be about 50% HDO and 25% each of the other two forms. For that reason, semiheavy water cannot exist as a stable bulk liquid. If made in the gas phase and deposited directly as ice, though, it could hold together, since the rigid lattice stops molecules colliding.

  Pure heavy water is about 10.6% denser than the ordinary kind, and it is not radioactive. It is also less dissociated at a given temperature, and neutral heavy water at 25 degrees Celsius has a pD of 7.44. Its color differs too. Ordinary water is faintly blue because vibrations in its molecules weakly absorb red light. In heavy water those vibrations shift into the infrared, so red light passes and the blue tint disappears.

  Taste is less settled. A 1935 experiment reported not the "slightest difference" between ordinary and heavy water. A more recent study found that humans do taste a slight sweetness, mediated by the TAS1R2/TAS1R3 taste receptor. Rats offered a choice between distilled water and heavy water avoided the heavy kind by smell.

  Two other variants complete the family. Water enriched in heavier oxygen isotopes is denser than normal water, but it is rarely called heavy water because it lacks the extra deuterium behind the nuclear and biological oddities. It costs more than deuterium oxide because the oxygen isotopes are harder to separate. Tritiated water, built with tritium, is radioactive. Doubly labeled water, which carries both heavy oxygen and heavy hydrogen, serves as a non-radioactive tracer.

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• 3. Discovery And Demand

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  Harold Urey, an American Nobel laureate, discovered deuterium in 1931 and later learned to concentrate it in water. Heavy water was first produced in 1932, a few months after that discovery. In 1933, Urey's mentor Gilbert Newton Lewis isolated the first sample of pure heavy water by electrolysis.

  Biology borrowed it almost at once. In 1934, George de Hevesy and Erich Hofer used heavy water in one of the first biological tracer experiments, estimating how fast the human body turns over its water. That same year, Emilian Bratu and Otto Redlich studied how heavy water splits into ions.

  The discovery of nuclear fission in late 1938 gave the liquid a new job. Researchers needed a neutron moderator that captured very few neutrons, and heavy water fit. It soon became part of early nuclear energy research.

  During the Second World War, the company Fosfatbolaget in Ljungaverk, Sweden, produced 2,300 liters of heavy water per year. It sold the output to Germany at 1.40 SEK per gram. Elsewhere, the United States and Soviet Union suspected in the 1930s that Austrian chemist Fritz Johann Hansgirg had built a pilot plant for the Empire of Japan in Japanese-ruled northern Korea, using a new process he had invented.

  In October 1939, Soviet physicists Yakov Borisovich Zel'dovich and Yulii Borisovich Khariton concluded that heavy water and carbon were the only workable moderators for a natural uranium reactor. In August 1940, together with Georgy Flyorov, they submitted a plan to the Russian Academy of Sciences. It calculated that a reactor would need 15 tons of heavy water. The country had no uranium mines at the time, so young Academy workers went to photographic shops in Leningrad to buy uranium nitrate. The whole project stopped in 1941, when German forces invaded under Operation Barbarossa.

  By 1943, Soviet scientists noticed that all Western scientific literature on heavy water had vanished. Flyorov warned Joseph Stalin in a letter. At that moment, the entire country held only 2-3 kg of the substance. In late 1943, the Soviet purchasing commission in the United States obtained 1 kg, and it acquired a further 100 kg in February 1945. When the war ended, the NKVD took over the project.

  In October 1946, as part of what was called the Russian Alsos, the NKVD deported German scientists who had made heavy water during the war. Among them was Karl-Hermann Geib. They worked under physical chemist Max Volmer at the Institute of Physical Chemistry in Moscow, and the plant they built was producing large quantities by 1948.

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• 4. Raid On Vemork

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  In 1934, the company Norsk Hydro built the first commercial heavy water plant at Vemork, in Tinn, Norway. It eventually produced 4 kg per day. From 1940 and throughout the war, Germany controlled the site, and the Allies decided to destroy both the plant and its heavy water to slow any German nuclear weapons effort.

  The first attempt came in late 1942. Operation Freshman, a raid by British airborne troops, failed when both gliders crashed. The raiders died in the crashes or were executed by the Germans afterward.

  Operation Gunnerside succeeded on the night of the 27th of February 1943. Norwegian commandos and local resistance demolished small but key parts of the electrolytic cells and sent the accumulated heavy water down the factory drains.

  On the 16th of November 1943, Allied air forces dropped more than 400 bombs on the site. The raid prompted the Nazi government to move all available heavy water to Germany for safekeeping. On the 20th of February 1944, a Norwegian partisan sank the ferry M/F Hydro as it carried the shipment across Lake Tinn. Fourteen Norwegian civilians died. Most of the heavy water was presumably lost, though a few barrels were only half full, so they floated and may have been recovered and taken to Germany.

  A barrel salvaged in 2004, together with production records from Norsk Hydro, changed the picture. The barrels held water of pH 14, which fits the alkaline electrolytic refining process. Yet most contained only 0.5-1% pure heavy water. The manifest showed about half a ton in total. Germany would have needed about 5 tons to start a reactor, and 10 or more tons to make enough plutonium for a bomb. The German program was far less advanced than the Manhattan Project, and no reactor built in Nazi Germany came close to criticality. No amount of heavy water would have changed that.

  Norwegian heavy water kept turning up later. Israel admitted that it ran the Dimona reactor with Norwegian heavy water sold to it in 1959. India probably used Norwegian heavy water as well, re-exported through Romania and Germany.

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• 5. Slowing The Neutron

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  The CANDU reactor runs on natural uranium, and heavy water is the reason. A moderator slows neutrons so they are more likely to split uranium-235 than to be swallowed by uranium-238, which captures neutrons without fissioning. Light water absorbs more neutrons than heavy water does. Reactors moderated with ordinary water must therefore use enriched uranium, or criticality is impossible. Heavy water reactors also avoid graphite moderators, which pose radiological and dust explosion hazards when a plant is decommissioned.

  The Soviet RBMK design took another route. It used graphite as moderator and ordinary water as coolant, avoiding both enriched uranium and heavy water. That combination produced a positive void coefficient, one of a series of design flaws that led to the Chernobyl disaster. Graphite fires also played a part in that accident. Graphite has since disappeared from standard reactor designs, and most modern reactors use enriched uranium with ordinary water as the moderator.

  Heavy water also opens a path to weapons. Chemically separating plutonium from spent fuel is easier than isotopically separating uranium-235 from natural uranium, so breeding plutonium can be a relatively rapid and cheap route to a bomb. Israel, India and North Korea first used plutonium from heavy water reactors burning natural uranium. China, South Africa and Pakistan first built weapons with highly enriched uranium.

  The American effort went another way. The first experimental atomic reactor of 1942 used graphite and normal water cooling. So did the Hanford production reactors that made the plutonium for the Trinity test and the Fat Man bombs. Russian and British plutonium production also used graphite.

  Germany's choice of heavy water came from mistakes and quarrels. Its program, run on more modest means than the Manhattan Project, wrongly dismissed graphite because it did not recognize the effect of impurities. Uranium isotope separation seemed too big a hurdle, which left heavy water. Many leading scientists had been driven into exile, and many ended up working for the Manhattan Project. Propaganda sneered at "Jewish physics," and mistrust divided early enthusiastic Nazis from the Mitläufer, who went along or kept a low profile.

  There is no evidence that civilian heavy water power reactors, such as the CANDU or Atucha designs, have been used to make military fissile material. In countries without nuclear weapons, nuclear material at these facilities falls under IAEA safeguards. Several countries also control large industrial quantities of heavy water. Yet in the United States and Canada, gram-to-kilogram amounts are routinely sold without a special license by chemical supply dealers.

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• 6. Factories Worldwide

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  Making one tonne of heavy water at the Bruce Heavy Water Plant in Ontario took 340,000 tonnes of feed water. The plant used the Girdler sulfide process, the most cost-effective method, a dual-temperature system developed in parallel by Karl-Hermann Geib and Jerome S. Spevack in 1943. Modern heavy water is sold as deuterium oxide in grades from 98% enrichment up to 99.75-99.98% for nuclear reactor use.

  Bruce was part of a complex with eight CANDU reactors that supplied its heat and power, near Tiverton on Lake Huron. At its peak it held two fully operational plants of 800 tonnes per year each, making it the largest in the world at 1600 tonnes per year. Atomic Energy of Canada Limited issued the construction contract in 1969. The first unit entered service on the 28th of June 1973 and reached design output in April 1974. A second unit followed in 1979, but the planned third and fourth were cancelled because fewer CANDU stations were built than expected and the first two ran far better than planned. The first unit closed in 1984, and in 1993 half of the second was demolished. The rest served exports until it shut for good in 1997. In the 1970s and 1980s, heavy water made up 15-20% of the capital cost of each CANDU reactor.

  Canada's earlier efforts were rougher. A Trail, British Columbia plant began operating in 1943 as Canada's Manhattan Project contribution, with a design capacity of 1000 to 1200 lb per month. Two Nova Scotia plants, at Glace Bay and Point Tupper, suffered significant design, construction and production problems. Glace Bay reached full production in 1984, and Point Tupper in 1974. Both closed in 1985 when their output proved unnecessary.

  During the Manhattan Project, the United States built three plants under the P-9 Project, at Morgantown, West Virginia, near Dana and Newport, Indiana, and near Childersburg and Sylacauga, Alabama. The experimental reactor Chicago Pile-3 used heavy water as moderator and went critical in 1944. The three plants shut in 1945 after producing around 81,470 lb. The Wabash plant resumed work in 1952. In 1953, the Savannah River Site began using heavy water in plutonium production reactors that also made tritium. The first of five came online in 1953, and the last went into cold shutdown in 1996.

  Argentina's plant at Arroyito, supplied by the Swiss company Sulzer, was the world's largest. It made 200 ST in 2015 but has been idle since 2017. India is now the world's largest producer through its Heavy Water Board, with exports to the Republic of Korea, China and the United States. Romania once made it at the Drobeta plant, and France ran a small plant in the 1950s and 1960s.

  Iran has been building a plant at Khondab near Arak since 1996, and on the 26th of August 2006 President Ahmadinejad inaugurated its expansion. Under the Joint Comprehensive Plan of Action, Iran may store only 130 t, and it exports the surplus as the world's third largest exporter. In 2023, customers proposed prices above 1,000 dollars per liter.

  Pakistan has two production sites in Punjab. The Khushab Nuclear Complex, commissioned in 1995-96, is central to its plutonium, deuterium and tritium supply, and a Multan facility sells to nuclear power plants in Karachi and Chashma. Unlike India and Iran, Pakistan exports none of its heavy water.

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• 7. Living With Deuterium

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  Rats given heavy water to drink die after about a week, once their body water nears 50% deuteration. The cause is deuterium's general blocking of cell division. Death resembles chemotherapy poisoning or acute radiation syndrome, even though deuterium is not radioactive. The bone marrow fails, causing bleeding and infection, and the intestinal barrier fails, causing diarrhea and fluid loss.

  The root lies in hydrogen bonds. A hydrogen bond with deuterium is slightly stronger than one with ordinary hydrogen, and enzymes depend on finely tuned networks of these bonds. The delicate mitotic spindles needed for cell division in eukaryotes are particularly hard-hit. Plants stop growing and seeds fail to germinate on heavy water alone. Tobacco does not germinate, though wheat does. A 1972 demonstration showed that more deuterium means less plant growth.

  Concentrations over 50% are lethal to multicellular organisms, with a few exceptions. Switchgrass grows on 50% heavy water, Arabidopsis thaliana tolerates 70%, and Vesicularia dubyana 85%. Funaria hygrometrica handles 90%, and the nematode Panagrolaimus superbus lives in nearly 100%. Bacteria survive in 98%, and prokaryotes can have every hydrogen in their proteins and DNA replaced with deuterium. Fully deuterated Escherichia coli and Torula were raised this way, but molds like Aspergillus could not replicate.

  In animals, 25% deuteration prevents gametes or zygotes from developing, sometimes causing irreversible sterility. Mice raised from birth on 30% heavy water have 25% deuteration in body fluid and 10% in the brain. They are normal except for sterility. Concentrations of 90% rapidly kill fish, tadpoles, flatworms and Drosophila. Heavy water is more toxic to malignant cells than normal ones, but the doses needed are too high for regular use. Even so, 2021 evidence showed that 30% heavy water in drinking water suppressed tumor growth in a standard mouse model of human melanoma.

  Yeast gives odd results. Fission yeast Schizosaccharomyces pombe grew slowly and showed altered glucose metabolism in strong heavy water, and it activated its heat-shock response. Yet heavy water has extended the lifespan of certain yeasts by up to 85%, possibly by reducing reactive oxygen species turnover. It also lengthens circadian cycles in organisms from unicellular ones to green plants, isopods, insects, birds, mice and hamsters. The mechanism is unknown.

  In people, heavy water acts a little like ethanol. It temporarily changes the density of the cupula relative to the endolymph in the vestibular organ, causing dizziness, nausea and positional nystagmus. The eyes drift in the direction opposite to alcohol, since heavy water is denser than water, not lighter. A human body naturally holds deuterium equal to about five grams of heavy water, which is harmless. A 50 kg person with 32 kg of body water would need roughly 5.5 g to double that amount. Real poisoning would require large amounts, with little normal water for many days.

  In 1990, an employee at the Point Lepreau Nuclear Generating Station in Canada took about a half cup of heavy water from the reactor's primary heat transport loop and put it into a cafeteria drink dispenser. Eight employees drank some. Elevated tritium in their bioassay urine samples exposed the incident. The tritium and neutron-activated chemicals, not the heavy water itself, gave several workers elevated radiation doses. Some news services blurred this, leaving the impression that heavy water is normally radioactive and more toxic than it is.

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• 8. Detectors And Labs

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  The Sudbury Neutrino Observatory in Ontario holds 1,000 tonnes of heavy water on loan from Atomic Energy of Canada Limited. It sits 6800 ft underground in a mine, shielded from muons produced by cosmic rays. It was built to ask whether electron-type neutrinos made by fusion in the Sun might turn into other types on the way to Earth.

  The detector watches for Cherenkov radiation from fast electrons. Electron-type neutrinos striking neutrons in deuterium create protons and electrons. Neutrinos of any type can also scatter off electrons, and a third reaction lets neutrinos of any type, including muon and tau, break a deuteron into a free neutron and proton. Salt dissolved in the heavy water absorbs that neutron on chlorine-35 and emits gamma rays. So heavy water supplies a transparent medium, the deuterium itself, and a moderator that keeps the neutron alive.

  In chemistry labs, nuclear magnetic resonance spectroscopy uses deuterium oxide as a solvent when hydrogen is the nucleus under study. Ordinary water's signal would overwhelm the dissolved molecule, while deuterium has a different magnetic moment and stays silent. Adding deuterium oxide, called a D2O shake, swaps out labile hydrogens, which then vanish from the spectrum. Organic chemists also use it as a deuterium source, for example to replace C-H bonds beside ketone carbonyl groups. In infrared spectroscopy of proteins, ordinary water makes a strong band that overlaps the amide I region, while the heavy water band shifts away.

  Physiologists give heavy water, mixed with a heavy-oxygen form, to measure mean metabolic rate in people and animals going about their normal activities. The elimination rate of deuterium alone shows body water turnover, which varies with environment, size, sex, age and physical activity. Deuterium oxide also improves boron neutron capture therapy, simply by slowing neutrons without capturing them.

  Tritium is the last product. Some forms in heavy water reactors when deuterium captures a neutron, a rare event that still yields enough to justify cleaning tritium from the moderator every few years. Tritium decays into helium-3, a stable but rare isotope that is highly sought after. Yet helium-3 is a strong neutron poison, so any buildup in a heavy water moderator must be kept as low as possible.

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