Generally, nuclear fuel cycle operations are of concern to the immediate locale only, with relatively minor contributions to more general environmental tritium levels. Since the partial atmospheric nuclear test ban treaty in 1963, the worldwide levels of tritium in the environment have been decreasing at a rate approximately equal to its half-life (Okada and Momoshima, 1993).

Present-day tritium has three major sources of origin: (1) natural production in the upper atmosphere through cosmic ray-induced spallations of nuclides and particle capture reactions of nitrogen and oxygen ( Geyh and Schleicher, 1990), (2) residual activities from atmospheric nuclear weapons testing ( Libby, 1963), and (3) on-going nuclear fuel cycle operations. Tritium is a pure, low-energy beta emitter ( E max = 18.6 keV) with a half-life of 12.43 years.

BRIAN CARTER, in Handbook of Radioactivity Analysis (Second Edition), 2003 1.