Water composed of two atoms of deuterium (D; a heavy isotope of hydrogen) and one atom of oxygen (O), chemical formula D₂O. Water from most natural sources contains about 0.015percnt deuterium oxide; this can be enriched or purified by distillation, electrolysis, or chemical processing. Heavy water is used as a moderator in nuclear power plants, slowing down the fast neutrons so that they can react with the fuel in the reactor. Heavy water is also used in research as an isotopic tracer for chemical reactions and biochemical pathways. Water with tritium (T₂O) rather than deuterium may also be called heavy water.

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