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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.
Heavy water reactors offer some advantages over those that use normal, or light, water as a neutron moderator. Heavy water allows the use of natural uranium as fuel, while light water reactors require enriched uranium, which is much more difficult to produce.