iodine129

Iodine-129 (I-129) is a radionuclide of the element iodine. It has mass number 129 and atomic number 53. It decays by beta minus emission to xenon-129 and has a half-life of about 15.7 million years.

Natural occurrence and production: I-129 is produced in nuclear fission of uranium-235 and plutonium-239 in reactors and during reprocessing. Small amounts are also generated by spontaneous fission of uranium-238 and by cosmic ray spallation in the atmosphere. In the environment, I-129 occurs only in trace amounts and its presence is largely linked to human activities such as nuclear fuel reprocessing, atmospheric testing in the mid-20th century, and certain nuclear accidents.

Chemical behavior and mobility: I-129 behaves chemically like stable iodine, forming iodide (I−) and organoiodine species. As such, it follows natural iodine cycles and can be mobilized in air, water, and soil. When ingested, it can accumulate in the thyroid gland, which is a key consideration for radiological protection.

Uses and measurement: Due to its long half-life and environmental mobility, I-129 is used as a tracer in hydrology and studies of iodine transport in ecosystems. Detection typically relies on accelerator mass spectrometry (AMS) to measure the 129I/127I ratio, with supplementary radiometric techniques in some contexts.

Safety and regulation: Although I-129 has a low specific activity because of its long half-life, ingestion or inhalation can still pose thyroid radiation dose risks. Releases are monitored in nuclear facilities and waste management programs, with regulatory controls designed to minimize environmental and public exposure.