Iodine-129

Iodine-129 (129I) is a long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission products, where it serves as both tracer and potential radiological contaminant. 129I is one of seven long-lived fission products. It is primarily formed from the fission of uranium and plutonium in nuclear reactors. Significant amounts were released into the atmosphere as a result of nuclear weapons testing in the 1950s and 1960s. It is also naturally produced in small quantities, due to the spontaneous fission of natural uranium, by cosmic ray spallation of trace levels of xenon in the atmosphere, and by cosmic ray muons striking tellurium-130. 129I decays with a half-life of 15.7 million years, with low-energy beta and gamma emissions, to stable xenon-129 (129Xe). 129I is one of the seven long-lived fission products that are produced in significant amounts. Its yield is 0.706% per fission of 235U. Larger proportions of other iodine isotopes such as 131I are produced, but because these all have short half-lives, iodine in cooled spent nuclear fuel consists of about 129I and the only stable iodine isotope, 127I. Because 129I is long-lived and relatively mobile in the environment, it is of particular importance in long-term management of spent nuclear fuel. In a deep geological repository for unreprocessed used fuel, 129I is likely to be the radionuclide of most potential impact at long times. Since 129I has a modest neutron absorption cross-section of 30 barns, and is relatively undiluted by other isotopes of the same element, it is being studied for disposal by nuclear transmutation by re-irradiation with neutrons or by high-powered lasers. 129I is not deliberately produced for any practical purposes. However, its long half-life and its relative mobility in the environment have made it useful for a variety of dating applications. These include identifying very old waters based on the amount of natural 129I or its 129Xe decay product, as well as identifying younger groundwaters by the increased anthropogenic 129I levels since the 1960s.