Tritium biological, chemical, and physical paths throughout the world oceans because of its evolving distribution. Tritium has thus been used as a tool to examine ocean circulation and ventilation and, for such purposes, is usually measured in Tritium Units where 1 TU is defined as the ratio of 1 tritium atom to 10 hydrogen atoms. As noted earlier, nuclear weapons testing, primarily in the high-latitude regions of the Northern Hemisphere, throughout the late 1950s and early 1960s introduced large amounts of tritium into the atmosphere, especially the stratosphere. Before these nuclear tests, there were only about 3 to 4 kilograms of
Tritium in small quantities by some nuclear power plants. In January 2014 it was made public that a total of 875 TBq (2.45 g) of tritium are on the site of Fukushima Daiichi, and the amount of tritium contained in the contaminated water is increasing by approximately 230 TBq (0.64 g) per year. According to a report by Tepco ""Tritium could be separated theoretically, but there is no practical separation technology on an industrial scale"." Tritium's decay product helium-3 has a very large cross section for reacting with thermal neutrons, expelling a proton, hence it is rapidly converted back to tritium