Iodine-129

Iodine-129 (129I) is produced when xenon in the atmosphere reacts with cosmic rays. It can also be produced in the earth's crust during the spontaneous fission of natural uranium. It has a half-life of 15.7 million years, which is longer than that of other radioactive iodine, but its low abundance makes it very difficult to measure.

The Fukushima Daiichi Nuclear Power Plant accident released a significant amount of iodine-131, a nuclide with a half-life of 8 days, which is currently undetectable, but attempts are being made to reproduce this behavior using iodine-129 (129I), which also originated from the accident.

Radioactive iodine in the ocean  ¹²⁹I

　¹²⁹I migrates to the ocean and mixes uniformly with stable iodine (¹²⁷I) in seawater, resulting in an atomic number ratio (¹²⁹I /¹²⁷I) of 1500 x 10^-15. By looking at the deviation of this ratio, it is possible to determine the amount and movement of ¹²⁹I originating from the Fukushima Daiichi Nuclear Power Plant accident or from reprocessing facilities, for example.