Dear Straight Dope: What is the deal with heavy water? Is it harmful to humans, and if so, in what way? I've read a little bit about it and its use in nuclear reactors, but the politics of that aspect of it make it hard to find out simple facts about the substance itself. Larry Nagel, Shelby Township, Michigan

Q.E.D. replies:

Heavy water (D2O) is one of several commonly-used moderators found in nuclear reactors (others include graphite, beryllium and light–i.e., ordinary–water). A moderator slows down fast-moving neutrons released by nuclear fission so they have more time to react with the nuclear fuel. That permits a sustained, controlled chain reaction using unenriched uranium. Reactors using enriched uranium don’t require a moderator.

Heavy water is chemically identical to the ordinary water we know and love. The difference is that heavy water is made with a hydrogen isotope that has a neutron in addition to the proton in its nucleus. This isotope is called deuterium and occurs naturally at the rate of about 1 deuterium atom for every 6,700 normal hydrogen or protium atoms, which have just one proton and no neutrons in the nucleus. Deuterium is not radioactive, unlike the even heavier and rarer hydrogen isotope tritium, which is made primarily in nuclear reactors and has two neutrons in the nucleus. Tritium, which has a half-life of about 12.5 years, finds use in luminescent paints for watches and various displays, chemical tracers, and hydrogen bombs. If you’re organizing the chemicals in your closet, don’t get the deuterium and the tritium mixed up.

There are physical differences between light and heavy water. Heavy water is (duh) heavier, having a density of 1.108 g/cm3. Heavy water ice will actually sink in light liquid water. The freezing and boiling points are also elevated somewhat, with heavy water freezing at 3.81°C (38.86°F) and boiling at 101.42°C (214.56°F) at standard atmospheric pressure.

Despite the fact the light water and heavy water are chemically identical, heavy water is mildly toxic. How can this be? Since heavy water is heavier than normal water, the speed of chemical reactions involving it is altered somewhat, as is the strength of some types of bonds it forms. This affects certain cellular processes, notably mitosis, or cell division, due to the difference in binding energy in the hydrogen bonds needed to make certain proteins. Mouse studies have shown that drinking only heavy water along with normal feed eventually causes degeneration of tissues that need to replenish themselves frequently, and leads to cumulative damage from injuries that don’t heal as quickly. One study likens the effects to those suffered by chemotherapy patients. Heavy water toxicity manifests itself when about 50% of the water in the body has been replaced by D2O. Prolonged heavy water consumption can cause death.

Don’t get any funny ideas about using heavy water as a virtually untraceable and undetectable murder weapon, though. Given its role in breeder reactors for producing weapons-grade plutonium, production and distribution of heavy water is closely monitored and controlled. Obtaining a significant amount is damn near impossible for the average Joe, and you’d need a LOT of it to kill anyone. It’s also expensive–one estimate puts the price at about $300 per kilogram. Hit ’em over the head with a bottle of Poland Spring and save yourself some grief.

Q.E.D.

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