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.)