Properties of water water isotopes. Naturally occurring water is almost completely composed of the neutron-less hydrogen isotope protium. Only 155 ppm include deuterium ( or D), a hydrogen isotope with one neutron, and fewer than 20 parts per quintillion include tritium ( or T), which has two neutrons. Oxygen also has three stable isotopes, with present in 99.76%, in 0.04%, and in 0.2% of water molecules. Deuterium oxide, , is also known as heavy water because of its higher density. It is used in nuclear reactors as a neutron moderator. Tritium is radioactive, decaying with a half-life of 4500 days; exists in nature
Tritium the ocean floor over 10 to 20 year time-scales. Also evident in the Atlantic Ocean is the tritium profile near Bermuda between the late 1960s and late 1980s. There is a downward propagation of the tritium maximum from the surface (1960s) to 400 meters (1980s), which corresponds to a deepening rate of approximately 18 meters per year. There are also tritium increases at 1,500 meters depth in the late 1970s and 2,500 meters in the middle of the 1980s, both of which correspond to cooling events in the deep water and associated deep water ventilation. From a study in 1991,