Tritium, ³H or T, is the radioisotope of hydrogen. It is produced either naturally in the atmosphere by cosmic radiation on atmospheric gases or by CanDU reactors and other nuclear activities and released to the environment from tritium-handling facilities. Atmospheric testing of thermonuclear devices (hydrogen fusion bombs) during the 1950s and 60s also released considerable amounts of tritium to the atmosphere. Tritium decays to ³He with a relatively short half-life of 12.32 years or 388,789,632 seconds. This gives it a decay constant (ln2/T½) of 1.78 · 10^-9. Environmental tritium is generally expressed in tritium units, TU, where 1 TU = 1 T per 10¹⁸ H. Higher levels in tritium-contaminated regions are typically expressed by units of activity, or Becquerels, where 1 Bq = 1 decay event per second. In water, 1 TU = 0.119 Bq/L.