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Heavy water (deuterium oxide (2H 2O) or D 2O) is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium (2H or D, also known as heavy hydrogen), rather than the common hydrogen-1 isotope (1H or H, also called protium) that makes up most of the hydrogen in regular water.
By 1942 the Allied leaders were certain that the heavy water was a critical component in Hitlerâs effort to produce an atomic weapon. Such neutron moderators were not necessary in atomic bombs, but the German physicists hoped to use heavy-water to moderate a sustained reaction within their stash of rare uranium-235.