Iodine-129 (¹²⁹I) is a long-lived

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

that occurs naturally but is also of special interest in the monitoring and effects of man-made nuclear

fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the releas ...

s, where it serves as both a tracer and a potential radiological contaminant.

Formation and decay

¹²⁹I is one of seven

long-lived fission product Long-lived fission products (LLFPs) are radioactive materials with a long half-life (more than 200,000 years) produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity, it is necessary to isolate them from hum ...

s. It is primarily formed from the fission of

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

and

plutonium Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is a silvery-gray actinide metal that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four ...

nuclear reactors A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction. They are used for commercial electricity, marine propulsion, weapons production and research. Fissile nuclei (primarily uranium-235 or plutonium-2 ...

. Significant amounts were released into the

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

nuclear weapons testing Nuclear weapons tests are experiments carried out to determine the performance of nuclear weapons and the effects of Nuclear explosion, their explosion. Nuclear testing is a sensitive political issue. Governments have often performed tests to si ...

in the 1950s and 1960s, by nuclear reactor accidents and by both military and civil reprocessing of spent nuclear fuel. It is also naturally produced in small quantities, due to the

spontaneous fission Spontaneous fission (SF) is a form of radioactive decay in which a heavy atomic nucleus splits into two or more lighter nuclei. In contrast to induced fission, there is no inciting particle to trigger the decay; it is a purely probabilistic proc ...

natural uranium Natural uranium (NU or Unat) is uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes from ura ...

, by

cosmic ray spallation Cosmic ray spallation, also known as the x-process, is a set of naturally occurring nuclear reactions causing nucleosynthesis; it refers to the formation of chemical elements from the impact of cosmic rays on an object. Cosmic rays are highly ene ...

of trace levels of

xenon Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...

in the atmosphere, and by

cosmic ray Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the ...

muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of ''ħ'', but with a much greater mass. It is classified as a ...

s striking

tellurium Tellurium is a chemical element; it has symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...

-130. ¹²⁹I decays with a

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

of 16.14 million years, with low-energy

beta Beta (, ; uppercase , lowercase , or cursive ; or ) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Ancient Greek, beta represented the voiced bilabial plosive . In Modern Greek, it represe ...

and

gamma Gamma (; uppercase , lowercase ; ) is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter normally repr ...

emissions, to stable

xenon-129 Naturally occurring xenon (54Xe) consists of seven stable isotopes and two very long-lived isotopes. Double electron capture has been observed in 124Xe (half-life ) and double beta decay in 136Xe (half-life ), which are among the longest measure ...

(¹²⁹Xe).

Long-lived fission product

¹²⁹I is one of the seven

s that are produced in significant amounts. Its yield is 0.706% per fission of ²³⁵U. Larger proportions of other iodine isotopes such as ¹³¹I are produced, but because these all have short half-lives, iodine in cooled

spent nuclear fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...

consists of about 5/6 ¹²⁹I and 1/6 the only stable iodine isotope, ¹²⁷I. Because ¹²⁹I is long-lived and relatively mobile in the environment, it is of particular importance in long-term management of spent nuclear fuel. In a

deep geological repository A deep geological repository is a way of storing hazardous or radioactive waste within a stable geologic environment, typically 200–1,000 m below the surface of the earth. It entails a combination of waste form, waste package, engineered seals ...

for unreprocessed used fuel, ¹²⁹I is likely to be the radionuclide of most potential impact at long times. Since ¹²⁹I has a modest

neutron absorption Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, wh ...

cross-section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture and engineering 3D * Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) ...

of 30

barns A barn is an agricultural building usually on farms and used for various purposes. In North America, a barn refers to structures that house livestock, including cattle and horses, as well as equipment and fodder, and often grain.Allen G. ...

, and is relatively undiluted by other isotopes of the same element, it is being studied for disposal by

nuclear transmutation Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed. A transmutat ...

by re-irradiation with

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

s or gamma irradiation.

Release by nuclear fuel reprocessing

A large fraction of the ¹²⁹I contained in spent fuel is released into the gas phase, when spent fuel is first chopped and then dissolved in boiling

nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...

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 ¹²⁹I 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." I129Fiescherhorn

The Northeastern Radiological Health Laboratory further states that, due to limitations of their measuring systems, the actual release of ¹²⁹I may have even been higher, "since sup>129Ilosses y adsorptionprobably 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 Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...

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 ¹²⁹I sampled". NERHL concluded: "The bubblers located upstream of the ion exchangers removed a major portion of the gaseous ¹²⁹I 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 ¹²⁹I available from the dissolved fuel" is not corrected for those two measurement deficiencies. Military isolation of plutonium from spent fuel has also released ¹²⁹I 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 ¹²⁹I and ¹³¹I have very similar physical and chemical properties, and no isotope separation was performed at Hanford, ¹²⁹I 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 ¹³¹I, with a half-life time of just 8 days, was much higher than that of the long-lived ¹²⁹I. However, while all of the ¹³¹I released during the times of the Manhattan project has decayed by now, over 99.999% of the ¹²⁹I 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 ¹²⁹I 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.

Applications

Groundwater age dating

¹²⁹I is not deliberately produced for any practical purposes. However, its long half-life and its relative mobility in the environment have made it useful for a variety of dating applications. These include identifying older groundwaters based on the amount of natural ¹²⁹I (or its ¹²⁹Xe decay product) present, as well as identifying younger groundwaters by the increased anthropogenic ¹²⁹I levels since the 1960s.

Meteorite age dating

In 1960, physicist John H. Reynolds discovered that certain

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

s contained an isotopic anomaly in the form of an overabundance of ¹²⁹Xe. He inferred that this must be a

decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps ( d ...

of long-decayed radioactive ¹²⁹I. This isotope is produced in quantity in nature only in

supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...

explosions. As the half-life of ¹²⁹I is comparatively short in astronomical terms, this demonstrated that only a short time had passed between the supernova and the time the meteorites had solidified and trapped the ¹²⁹I. These two events (supernova and solidification of gas cloud) were inferred to have happened during the early history of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

, as the ¹²⁹I isotope was likely generated before the Solar System was formed, but not long before, and seeded the solar gas cloud isotopes with isotopes from a second source. This supernova source may also have caused collapse of the solar gas cloud.

References

External links

ANL factsheetMonitoring iodine-129 in air and milk samples collected near the Hanford Site: an investigation of historical iodine monitoring dataStudies with natural and anthropogenic iodine isotopes: iodine distribution and cycling in the global environment
{{Webarchive, url=https://web.archive.org/web/20140119053310/http://www.physics.utoronto.ca/~isotrace/isowhe.html , date=2014-01-19 Isotopes of iodine Fission products Radionuclides used in radiometric dating