Iodine-129

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Template:Long-lived fission products Iodine-129 (¹²⁹I) is a radioisotope of iodine. It decays with a half-life of 15.7 million years with low-energy beta and gamma emissions, decaying to ¹²⁹Xe.

¹²⁹I has the longest halflife of any fission product, and is one of only 7 long-lived fission products. Its yield of 0.6576% per fission is about 10% as great as the yield of Tc-99, Zr-93, or Cs-135, but much larger than the yield of Pd-107, Se-79, or Sn-126.

Larger proportions of heavier 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 difficult to immobilize in the environment, has a modest but sufficient neutron absorption cross section, and is relatively undiluted by other isotopes of the same element, ¹²⁹I and ⁹⁹Tc are the leading candidates among fission products for disposal by nuclear transmutation by re-irradiation with neutrons. (Actinide wastes, which are not fission products, are good candidates for disposal by fission in a fast reactor, accelerator-driven subcritical reactor, or fusion-fission reactor.)

See also

• ANL factsheet