Heavy Water (D2O) is a compound of an isotope of hydrogen called heavy hydrogen or Deuterium (D) and oxygen.
This is also known as Deuterium Oxide. Deuterium has an atomic mass of 2, as against 1 for normal hydrogen (H) due to presence of an extra neutron in the nucleus. Deuterium is present in hydrogen and hydrogen bearing compounds like water, hydrocarbons, etc. and has a small natural occurrence (D/D+H) of about 140 to
160 ppm. So it is necessary to process large quantities of the low concentration feed stock to produce the final product which is enriched to the reactor grade i.e. 99.8 mole %. Heavy Water has great similarity in its physical and chemical properties to ordinary water. But its nuclear properties display a significant variation which makes it an extremely efficient material for use as moderator in a nuclear reactor.