Iodine-129

Iodine-129 (¹²⁹I) is a long-lived radioisotope of iodine that occurs naturally, but is of greater interest as a man-made nuclear fission product, where it is a potential radiological contaminant. The same contamination, though, together with its long half-life, make it serve as a tracer of environmental processes that have nothing to do with its creation.

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 have been released into the atmosphere by nuclear weapons testing in the 1950s and 1960s, by nuclear reactor accidents, and most of all by the (both military and civilian) reprocessing of spent nuclear fuel.

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 16.14 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 5/6 ¹²⁹I and 1/6 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 the radionuclide of most very-long-term concern as it may seep out of an undisturbed repository before it has decayed.

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 gamma irradiation.

A large fraction of the ¹²⁹I contained in spent fuel is released into the gas phase, when spent fuel is first chopped and then dissolved in boiling nitric acid during reprocessing. At least for civil reprocessing plants, special scrubbers are supposed to withhold 99.5% (or more) of the Iodine by adsorption, before exhaust air is released into the environment. However, the Northeastern Radiological Health Laboratory (NERHL) found, during their measurements at the first US civil reprocessing plant, which was operated by Nuclear Fuel Services, Inc. (NFS) in Western New York, that "between 5 and 10% of the total ¹²⁹I available from the dissolved fuel" was released into the exhaust stack. They further wrote that "these values are greater than predicted output (Table 1). This was expected since the iodine scrubbers were not operating during the dissolution cycles monitored."