A large fraction of the I contained in spent fuel is released into the gas phase, when spent fuel is first chopped and then dissolved in boiling nitric acid during reprocessing. At least for civil reprocessing plants, special scrubbers are supposed to withhold 99.5% (or more) of the Iodine by adsorption, before exhaust air is released into the environment. However, the Northeastern Radiological Health Laboratory (NERHL) found, during their measurements at the first US civil reprocessing plant, which was operated by Nuclear Fuel Services, Inc. (NFS) in Western New York, that "between 5 and 10% of the total I available from the dissolved fuel" was released into the exhaust stack. They further wrote that "these values are greater than predicted output (Table 1). This was expected since the iodine scrubbers were not operating during the dissolution cycles monitored."
The Northeastern Radiological Health Laboratory further states that, due to limitations of their measuring systems, the actual release of I may have even been higher, "since [I] losses [by adsorption] probably occurred in the piping and ductwork between the stack and the sampler". Furthermore, the sample taking system used by the NERHL had a bubbler trap for measuring the tritium content of the gas samples before the iodine trap. The NERHL found out only after taking the samples that "the bubbler trap retained 60 to 90% of the I sampled". They concluded: "The bubblers located upstream of the ion exchangers removed a major portion of the gaseous I before it reached the ion exchange sampler. The iodine removal ability of the bubbler was anticipated, but not in the magnitude that it occurred." The documented release of "between 5 and 10% of the total I available from the dissolved fuel" is not corrected for those two measurement deficiencies.
Military isolation of plutonium from spent fuel has also released I to the atmosphere: "More than 685,000 curies of iodine 131 spewed from the stacks of Hanford's separation plants in the first three years of operation." As I and I have very similar physical and chemical properties, and no isotope separation was performed at Hanford, I must have also been released there in large quantities during the Manhattan project. As Hanford reprocessed "hot" fuel, that had been irradiated in a reactor only a few months earlier, the activity of the released short-lived I, with a half-life time of just 8 days, was much higher than that of the long-lived I. However, while all of the I released during the times of the Manhattan project has decayed by now, over 99.999% of the I is still in the environment.
Ice borehole data obtained from the university of Bern at the Fiescherhorn glacier in the Alpian mountains at a height of 3950 m show a somewhat steady increase in the I deposit rate (shown in the image as a solid line) with time. In particular, the highest values obtained in 1983 and 1984 are about six times as high as the maximum that was measured during the period of the atmospheric bomb testing in 1961. This strong increase following the conclusion of the atmospheric bomb testing indicates that nuclear fuel reprocessing has been the primary source of atmospheric iodine-129 since then. These measurements lasted until 1986.