EAU LOURDE L'eau lourde est de l’oxyde de deutérium (formule : D2O ou ²H2O). Chimiquement, elle est identique à l’eau normale (H2O), mais les atomes d’hydrogène dont elle est composée en sont des isotopes lourds, du deutérium, dont le noyau contient un neutron en plus du proton présent dans chaque atome d’hydrogène. Il existe aussi de l’eau semi-lourde (HDO ou H²HO), d'ailleurs en proportion naturelle bien plus importante que l'eau lourde. Nota : de l'eau formée à partir d'oxygène 18 et d’hydrogène est sensiblement de même masse que l'oxyde de deutérium, et pourrait donc aussi être désignée sous le nom d'eau lourde ; en pratique c'est rarement le cas, sauf précision explicite. Il ne sera pas ici question de ce composé. De même, l'oxyde de tritium T2O ou 3H2O est parfois désigné sous le nom d’eau super-lourde ; voir l'article Eau tritiée. Gilbert Newton Lewis isola, en 1933, le premier échantillon d’eau lourde pure. Sa densité est plus élevée que celle de l'eau, d'où son nom d'eau « lourde ». Merci Magik _____________________________ HEAVY WATER Heavy water is water highly enriched in the hydrogen isotope deuterium;e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction (in ordinary water, the deuterium-to-hydrogen ratio is about 156 deuterium atoms per million hydrogen atoms). The term "heavy water" today is somewhat colloquial, and pure heavy water for research and commercial use is generally commercially offered, and referred to, as deuterium oxide. It is not radioactive. It is about 11% denser than water, but otherwise, is physically and chemically similar. However, the difference (especially the biological properties) is larger than in most other isotope-substituted compounds 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 bond, and this in turn is enough to cause differences that are important to some biochemical reactions. The human body naturally contains deuterium equivalent to five grams of heavy water, which is harmless. When a large fraction of water (> 50%) in higher organisms is replaced by heavy water, however, the result is cell dysfunction and death. Heavy water was first produced in 1933, 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 an important component of early nuclear energy programs during World War II (1939–1945). Partly because of Nazi Germany's overreliance on heavy water, it didn't produce a functioning reactor for the duration of the war. Since then, heavy water is an essential component in some types of reactor, either for power or for nuclear-weapons isotopes, such as plutonium-239. These heavy water reactors have the advantage of being able to run on natural uranium without the use of hazardous graphite moderators. Most modern reactors use enriched uranium and normal "light water" (H2O) moderator. Thanks Magik ___________________________