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 decay products, where it serves as both tracer and potential radiological contaminant.

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 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 some 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 xenon-129 (¹²⁹Xe).

Fission product

¹²⁹I is one of the 7 long-lived fission products that are produced in significant amounts. Its yield is 0.706% 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.