Editing Heavy water (section)

== Composition ==

The deuterium [[Atomic nucleus|nucleus]] consists of a [[neutron]] and a [[proton]]; the nucleus of a protium (normal hydrogen) atom consists of just a proton. The additional neutron makes a deuterium atom roughly twice as heavy as a protium atom.

A molecule of heavy water has two deuterium atoms in place of the two protium atoms of ordinary water. The term heavy water as defined by the [[International Union of Pure and Applied Chemistry|IUPAC]] Gold Book<ref>{{GoldBookRef|title=heavy water|file=H02758}}</ref> can also refer to water in which a higher than usual proportion of hydrogen atoms are deuterium. For comparison, [[Vienna Standard Mean Ocean Water]] (the "ordinary water" used for a deuterium standard) contains about 156 deuterium atoms per million hydrogen atoms; that is, 0.0156% of the hydrogen atoms are {{sup|2}}H. Thus heavy water as defined by the Gold Book includes [[semiheavy water]] (hydrogen-deuterium oxide, HDO) and other mixtures of {{chem|D|2|O}}, {{chem|H|2|O}}, and HDO in which the proportion of deuterium is greater than usual. For instance, the heavy water used in [[CANDU reactor]]s is a highly enriched water mixture that is mostly deuterium oxide {{chem|D|2|O}}, but also some hydrogen-deuterium oxide and a smaller amount of ordinary water {{chem|H|2|O}}. It is 99.75% enriched by hydrogen atom-fraction; that is, 99.75% of the hydrogen atoms are of the heavy type; however, heavy water in the Gold Book sense need not be so highly enriched. The weight of a heavy water molecule, however, is not very different from that of a normal water molecule, because about 89% of the mass of the molecule comes from the single [[oxygen]] atom rather than the two hydrogen atoms.

Heavy water is not [[Radioactive decay|radioactive]]. In its pure form, it has a density about 11% greater than water but is otherwise 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 [[isotopologue|isotope-substituted compound]] because deuterium is unique among heavy [[stable isotope]]s in being twice as heavy as the lightest isotope. This difference increases the [[bond energy|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 (biology)|cell]] dysfunction and death.<ref>{{Cite journal|pmid=10535697 |title=Pharmacological uses and perspectives of heavy water and deuterated compounds |author1=D. J. Kushner |author2=Alison Baker |author3=T. G. Dunstall |journal=Can. J. Physiol. Pharmacol. |volume=77 |issue=2 |pages=79–88 |date=1999 |doi=10.1139/cjpp-77-2-79}}</ref>

Heavy water was first produced in 1932, a few months after the discovery of deuterium.<ref>{{cite web| url = http://www.columbia.edu/cu/chemistry/fac-bios/brus/group/pages/urey.html| title = Harold Clayton Urey (1893–1981)|website = [[Columbia University]]}}</ref> 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 power|nuclear energy]] research. 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 [[Pressurized heavy-water reactor|heavy water reactors]] have the advantage of being able to run on natural [[uranium]] without using [[graphite]] moderators that pose radiological<ref>{{cite web|url=http://www-pub.iaea.org/MTCD/publications/PDF/ngwm-cd/PDF-Files/paper%2017%20(Holt).pdf |title=Radioactive Graphite Management at UK Magnox Nuclear Power Stations |website=Pub-iaea.org |access-date=11 January 2017}}</ref> and [[dust explosion]]<ref>{{cite web |url=http://cigr.ageng2012.org/images/fotosg/tabla_137_C0371.pdf |title=Archived copy |access-date=25 August 2012 |url-status=dead |archive-url=https://web.archive.org/web/20140422132744/http://cigr.ageng2012.org/images/fotosg/tabla_137_C0371.pdf |archive-date=22 April 2014 }}</ref> hazards in the decommissioning phase. The graphite moderated Soviet [[RBMK]] design tried to avoid using either [[enriched uranium]] or heavy water (being cooled with ordinary water instead) which produced the positive [[void coefficient]] that was one of a series of flaws in reactor design leading to the [[Chernobyl disaster]]. Most modern reactors use enriched uranium with ordinary water as the moderator.