Iodine-129 (129I) is a radioisotope of iodine.

Formation and decay

129I 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 1950's and 1960's.

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 some by cosmic ray muons striking tellurium-130. [R.Edwards. Iodine-129: Its Occurrence in Nature and Its Utility as a Tracer. Science, Vol 137 (1962) pp.851-853] [ [ Radioactives Missing From The Earth ] ]

129I decays with a half-life of 15.7 million years, with low-energy beta and gamma emissions, to 129Xe.

Fission Product

129I is one of the 7 long-lived fission products that are produced in significant amounts. Its yield is 0.6576% per fission (U-235). Larger proportions of other iodine isotopes like 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 likely to be the radionuclide of most potential impact at long times.

Since 129I has a modest neutron absorption cross-section, 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 [J.A. Rawlins et al. Partitioning and transmutation of long-lived fission products. Proceedings International High-Level Radioactive Waste Management Conference. Las Vegas, USA (1992).] or by high-powered lasers. [J. Magill et al. Laser transmutation of iodine-129. Applied Physics B: Lasers and Optics. Vol. 77(4) (2003).]


Groundwater age dating

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, [ [ | Discovery Channel Canada's Web site ] ] as well as identifying younger groundwaters by the increased anthropogenic 129I levels since the 1960's. []

Meteorite age dating

In 1960 physicist John H. Reynolds discovered that certain meteorites contained an isotopic anomaly in the form of an overabundance of xenon-129. He inferred that this must be a decay product of long-decayed radioactive iodine-129. This isotope is produced in quantity in nature only in supernova explosions. As the half-life of 129I is comparatively short in astronomical terms, this demonstrated that only a short time had passed between the supernova and the time the meteorites had solidified and trapped the 129I. These two events (supernova and solidification of gas cloud) were inferred to have happened during the early history of the Solar System, as the 129I isotope was likely generated before the Solar System was formed, but not long before, and seeded the solar gas cloud isotopes with isotopes from a second source. This supernova source may also have caused collapse of the solar gas cloud. [cite book
first=Donald D. | last=Clayton | year=1983
title=Principles of Stellar Evolution and Nucleosynthesis
pages=p. 75 | edition=2nd edition
publisher=University of Chicago Press | id=ISBN 0226109534
] [cite web
author=Bolt, B. A.; Packard, R. E.; Price, P. B. | year=2007
title=John H. Reynolds, Physics: Berkeley
publisher=The University of California, Berkeley


ee also

* [ ANL factsheet]

* [ Monitoring iodine-129 in air and milk samples collected near the Hanford Site: an investigation of historical iodine monitoring data]
* [ Studies with natural and anthropogenic iodine isotopes: iodine distribution and cycling in the global environment]
* [ Some Publications using 129I Data from IsoTrace, 1997-2002]

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