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|Section2= |Section3= }} Heavy water (deuterium oxide ( )) is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium ( or D, also known as ''heavy hydrogen''), rather than the common hydrogen-1 isotope ( or H, also called protium) that makes up most of the hydrogen in regular water. ==Explanation== Deuterium is an isotope of hydrogen whose nucleus comprises both a neutron and a proton; the nucleus of a protium (regular 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 "light" water. The weight of a heavy water molecule, however, is not substantially different from that of a normal water molecule, because about 89% of the molecular weight of water comes from the single oxygen atom rather than the two hydrogen atoms. The colloquial term ''heavy water'' refers to a highly enriched water mixture that contains mostly deuterium oxide (D2O), but also some hydrogen-deuterium oxide (HDO) and a smaller number of ordinary hydrogen oxide (H2O) molecules. For instance, the 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. For comparison, ordinary water (the "ordinary water" used for a deuterium standard) contains 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 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 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.〔(Harold Clayton Urey (1893–1981) )〕 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 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 heavy water reactors have the advantage of being able to run on natural uranium without using graphite moderators that pose radiological〔http://www-pub.iaea.org/MTCD/publications/PDF/ngwm-cd/PDF-Files/paper%2017%20(Holt).pdf〕 and dust explosion〔http://cigr.ageng2012.org/images/fotosg/tabla_137_C0371.pdf〕 hazards in the decommissioning phase. Most modern reactors use enriched uranium with ordinary water as the moderator. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heavy water」の詳細全文を読む スポンサード リンク
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