Can you drink Heavy Water?
Heavy water, formally called deuterium oxide or 2H2O or D2O, is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium, (also known as “heavy hydrogen“) rather than the common hydrogen-1 isotope that makes up most of the hydrogen in normal water. Therefore, some or most of the hydrogen atoms in heavy water contain a neutron, causing each hydrogen atom to be about twice as heavy as a normal hydrogen atom (although the weight of the water molecules is not as substantially affected, since about 89% of the molecular weight resides in the unaffected oxygen atom). The increased weight of the hydrogen in the water thus makes it slightly more dense. The colloquial term heavy water is often also used to refer a highly enriched water mixture that contains mostly deuterium oxide but also contains some ordinary water molecules as well: for instance heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction, meaning that 99.75% of the hydrogen atoms are of the heavy type. In comparison, in ordinary water, which is the “ordinary water” used for a deuterium standard on Earth, there are only about 156 deuterium atoms per million hydrogen atoms.
Heavy water is not radioactive. In its pure form, it has a density about 11% greater than water, but otherwise, is physically and chemically similar. Nevertheless, the various differences in deuterium-containing water (especially affecting the biological properties) are larger than in any other commonly occurring isotope-substituted compound because deuterium is unique among heavy stable isotopes in being twice as heavy as the lightest isotope. This difference increases the strength of water’s hydrogen-oxygen bonds, and this in turn is enough to cause differences that are important to some biochemical reactions. The human body naturally contains deuterium equivalent to about five grams of heavy water, which is harmless. When a large fraction of water (> 50%) in higher organisms is replaced by heavy water, the result is cell dysfunction and death.
Heavy water was first produced 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 became a component of early nuclear energy. Since then, heavy water has been an essential component in some types of reactor, both those that generate power and those designed to produce isotopes for nuclear weapons, such as plutonium-239. These heavy water reactors have the advantage of being able to run on natural uranium without the use of graphite moderators which can pose radiological and dust explosion hazards in the decommissioning phase. Most modern reactors use enriched uranium with normal “light water” (H2O) as the moderator.
Production
On Earth, deuterated 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, 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. Production of 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.
An alternative process, patented by Graham M. Keyser, uses lasers to selectively dissociate deuterated hydrofluorocarbons to form deuterium fluoride, which can then be separated by physical means. Although the energy consumption for this process is much less than for the Girdler sulfide process, this method is currently uneconomical due to the expense of procuring the necessary hydrofluorocarbons.
As noted, modern commercial heavy water is almost universally referred to, and sold as, deuterium oxide. It is most often sold in various grades of purity, from 98% enrichment to 99.75–99.98% deuterium enrichment (nuclear reactor grade) and occasionally even higher isotopic purity.
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