Anthropogenic iodine-129 in the environment

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Anthropogenic iodine-129 ( 129 I) in the environment , which is produced by releases from above-ground nuclear weapons tests and the processing of nuclear fuel , covers the natural production of 129 I by spallation on xenon in the atmosphere and by spontaneous nuclear fission . 129 I, with a half-life of 15.7 million years, is the longest-lived radioisotope of the element iodine .

Generation and Decay

Relevant nuclear reactions

Due to its long half-life, the high fission yield and its mobility in the environment, iodine-129 is of great importance for the long-term observation of deposits for spent nuclear fuel elements.

Natural production

The following sources contribute to the natural production of iodine-129:

  • Spontaneous fission of 238 U in the ocean
  • Leaching of fission products from rock
  • Release of fission products through volcanic activity
  • Production through cosmic radiation (spallation on xenon)
  • Entry through extraterrestrial matter (neutron capture at tellurium)

All natural sources together amount to 3.6 · 10 19 atoms per year. This production rate leads to a ratio of 129 I / I ≈ 10 −12 .

Anthropogenic production

The anthropogenic production of 129 I now exceeds natural production and will be discussed further in this article. Typical measured values ​​in the iodine of the thyroid show values ​​for Europe of 129 I / I ≈ 10 −8 .

Decay

129 I is converted 100% into 129 Xe by beta-minus decay . This emits partly promptly, partly as a core isomer with a half-life of 8.8 h, a gamma quantum with an energy of 40 keV.

Global iodine cycle

Global iodine cycle

The global iodine cycle can be shown for long-term effects as shown opposite. The number in the top left of each box indicates the mean length of time iodine has stayed in years. The numbers on the arrows show how the iodine exiting a box is distributed.

Because of its long service life, 129 I takes part in the entire iodine cycle. Since the transport of 129 I from the oceans to the land is very effective, this transport route over the river dominates cosmogenically produced iodine.

Evidence of 129 I.

Due to its long half-life and low gamma energy, detection methods that register the decay are very inefficient at 129 I. 129 I is therefore usually detected by mass spectrometry .

Data from the Fiescherhorn glacier

Sampling and preparation

In 1988 the University of Bern took a 76 m long ice core with a diameter of 75 mm from the Fiescherhorn glacier (Bernese Oberland) at an altitude of 3950 m. The aim of the 129 I measurements on this core was to increase the deposition rate of 129 I. determine and identify the sources of anthropogenic 129 I. A quarter of the cross-section (10.7 cm 2 ) was used to measure the radiocuclides 3 H, 10 Be, 36 Cl, 41 Ca, 129 I and 137 Cs. For dating, the maximum of the tritium values ​​was determined and thus the year 1963, the year with the greatest fallout from aboveground nuclear weapons tests , was identified. Assuming that the annual accumulation rates are roughly constant, annual samples were taken from 1963 onwards.

The entire year of ice was processed into a sample. For each sample between 3 mg and 7 mg of iodine in the form of I - resp. IO 3 - added and separated by means of an ion exchanger . By measuring the 129 I / I ratio from the sample cross-section and the assumption that the entire 129 I fallout is present in the sample, it was possible to calculate the deposition caused by precipitation, which can be seen in the adjacent figure.

I-129 precipitation on Fiescherhorn (Switzerland):
dot-dash "Calculated bomb
fallout" triangles "I-129 fallout"
dashed line " calculated from Cs-137 data, estimate from the increase in the atmospheric Kr-85 concentration"
circles " Tree ring data Karlsruhe "

Comparison with other data

With the help of the model of the global iodine cycle shown above, the following assumptions were made for the calculation:

  • The main source of the 129 I are the large bombs with an explosive power of over 1 Mt TNT equivalent (≅ 5 · 10 15 J ≅ 1.6 · 10 26 divisions).
  • Half of these bombs get the energy from nuclear fusion (no 129 I production) and the other half from the rapid fission of 238 U.
  • Only nuclear weapons tests in the northern hemisphere were taken into account.
  • Only half of the 129 I produced reaches the stratosphere and is distributed according to the model.
  • A list of the nuclear weapons tests considered can be found here.

137 Cs activities were measured in the same ice core . These were converted using the ratio of the corresponding yields in the rapid cleavage.

Like 129 I, 85 Kr is a fission product. As a noble gas, it mixes very quickly with the atmosphere. On the Schauinsland in the Black Forest (1240 m, Baden-Württemberg, D), the 85 Kr concentrations in the air have been measured since 1955 . Short-term fluctuations are seen as local effects, i.e. H. interpreted as emissions from European reprocessing plants. From these values, conclusions were drawn about the emission of 129 I, which is also released during reprocessing.

At a black locust (Robinia pseudoacacia), the km in distance from the exhaust chimney 1 of the experimental reprocessing plant Karlsruhe stand, which were 129 I / I ratios measured in the tree rings. Plants get their iodine mainly from the soil water. The iodine in the soil water comes from the soil itself and the input from the troposphere. Since the "bottom" reservoir is very sluggish with a residence time of 1000 years, annual variations, such as those observed in the tree rings, are due to the entry from the atmosphere. The values ​​from Hausschild and Aumann were scaled in such a way that there was good agreement for the years 1950–1970. As of 1970, the values ​​are no longer comparable with the Fiescherhorn data due to the local entry through the commissioning of the WAA Karlsruhe .

Discussion and evaluation

The anthropogenic sources of 129 I are discussed below :

  • 129 I from bomb tests

The 129 I-curves measured between 1960 and 1965 do not show the structure of the calculated bomb peak. In particular, the main peak from 1963 is missing in the iodine data. A possible explanation for this deviation could be that the distribution of 129 I in the stratosphere and its duration are different from those of the fission products 137 Cs and 90 Sr, for which the result of this model calculation is sufficient good right.

  • 129 I from military reprocessing

The 85 Kr curve shows a strong increase around 1955. The reason for this is the military processing of nuclear fuels for plutonium production, which was carried out mainly in the USA and the USSR (Asian part). During reprocessing, noble gases such as 85 Kr are completely released and volatile substances such as 129 I are partially released into the atmosphere. In the years up to 1970, the 129 I and 85 Kr curves do not match either in shape or in height. From this it can be concluded that 129 I is not emitted like 85 Kr and is distributed globally in the atmosphere.

  • 129 I from civil reprocessing

The increase in the 129 I from 1965 onwards is due to emissions from the reprocessing plants (WAA) in La Hague (F) and Sellafield (GB). Since fuel rods with a significantly higher burnup and thus a correspondingly higher 129 I content are processed for civil purposes , the 129 I output from these plants has increased significantly.

Radiological evaluation

The levies from the European reprocessing plants lead to a 129 I fallout of 2 · 10 7 atoms per cm 2 and year. According to the above model, this leads to a 129 I / I concentration of 10 −8 in the biosphere. Values ​​of this magnitude are currently also measured in thyroid glands. The radiological limit value for 129 I / I is 10 −3 , i.e. a factor of 100,000 higher.

Investigation of further environmental reserves

Investigation of groundwater

Iodine and chlorine are dissolved in water as salts and can be detected in groundwater flows. With iodine-129 and chlorine-36 (half-life 301,300 years), these elements have two long-lived radioisotopes that can be used to determine the age of groundwater flows. A fundamental problem here is the formation of mixed water if it must be assumed that "young" water has entered several places. In the same way, the effect of adding brine must be taken into account. In particular, the 129 I and 36 Cl signatures differ depending on whether old brine with old water is added or old brine with young water. Evidence of this kind is used to investigate hydrocarbon deposits, for example the Fruitland Formation (USA).

Investigation of deep water formation in the North Atlantic

Iodine-129 has been and is being released into the sea in large quantities from the reprocessing plants at Sellafield and La Hague. At Sellafield this is around 20 kg per year. At La Hague, the value was roughly in this range until around 1990. After that, the value rose to around tenfold due to the increased use of civil reprocessing.

The levies from Sellafield and La Hague combine in the North Sea and move northward together with the North Atlantic Current. On the coast of Norway, for example, decreasing concentrations in the ratio 129 I / I can be measured from south to north . Since this could also be a consequence of the dilution by salty water of the North Atlantic Current, the 129 I concentrations were also measured in relation to other fission products such as 137 Cs and 99 Tc. The various measurements are well tolerated by a significant increase in total emissions around 1990.

In the North Atlantic, the North Atlantic Current continues to cool, resulting in the formation of the North Atlantic deep water . It flows over the Greenland-Scotland Ridge towards North America, then further south to reach Antarctica after about 1000 years. Iodine-129 can now be used as a tracer along this path.

Individual evidence

  1. ↑ http://www-nds.iaea.org/sgnucdat/c3.htm Cumulative Fission Yields, IAEA
  2. ↑ Michael JM Wagner: Medium-heavy radionuclides: New detection methods and applications of nickel-59, tin-126 and iodine-129. Doctoral thesis, Zurich 1995. (a) Appendix F. Download
  3. ↑ U. Fehn, GR Holdren, D. Elmore, T. Brunelle, R. Teng, PW Kubik: Determination of natural and anthropogenic 129 I in marine sediments. In: Geophys. Res. Letters. Volume 13, 1986, p. 137.
  4. ↑ a b J. Handl, E. Oliver, D. Jacob, KJ Johanson, P. Schuler: Biospheric 129 I Concentrations in the pre-nuclear and nuclear age. In: Health Physics. Volume 65, 1993, p. 265.
  5. ↑ G. Pfennig, H. Klewe-Nebenius, W. Seelmann-Eggebert: Nuklidkarte. Research Center Karlsruhe, 1998.
  6. ↑ MJM Wagner, B. Dittrich-Hannen, H.-A. Synal, M. Suter, U. Schotterer: Increase of 129 I in the environment. In: Nuclear Instruments and Methods in Physics Research B. Volume 113, 1996, p. 490.
  7. ↑ F. Stampfli: Ion chromatographic analyzes on ice samples from a high alpine glacier. Licentiate thesis, Inst. Anorg. anal. and phys. chemistry, University of Bern, 1989.
  8. ↑ a b c d Michael JM Wagner: Moderately heavy radioniclides: New detection methods and applications of nickel-59, tin-126 and iodine-129. Doctoral thesis, Zurich 1995. (b) Chapter 4.4. Download
  9. ↑ Ch. Schiffmann: Measurement and interpretation of Kr-85 activities in hydrology and soil gas samples. Licentiate thesis, Physics Institute, University of Bern, 1993.
  10. ↑ J. Hausschild, DC Aumann: Iodine-129 and natural iodine in tree rings in the vicinity of a small nuclear fuels reprocessing plant. In: Natural Sciences. Volume 72, 1985, p. 270.
  11. ↑ Chlorine-36
  12. ↑ G. Snyder, J. Fabryka-Martin: I-129 and Cl-36 in dilute hydrocarbon waters: Marine-cosmogenic, in situ, and anthropogenic sources. Applied Geochemistry, 22 (3), 2007, 692-714.
  13. ^ F. Yiou, GM Raisbeck, ZQ Zhou, LR Kilius, PJ Kershaw: Improved Estimates of Oceanic Discharges of 129 I from Dellafield and La Hague . International Conference on Environmental Radiochemistry, Oslo, Norway 1995
  14. ^ GM Raisbeck, F. Yiou, ZQ Zhou, LR Kilius: Marine discharges of I-129 by the nuclear reprocessing facilities of La Hague and Sellafield . Radioprotection 32, 1997, p. 91.
  15. ↑ GM Raisbeck, F. Yiou, ZQ Zhou, LR Kilius: 129 I as a Tracer of Reprocessing Discharges in the Arctic Oceans . International Conference on Environmental Radiochemistry, Oslo, Norway 1995
  16. ^ F. Yiou, GM Raisbeck, GC Christensen: 129 I / 127 I, 129 I / 137 Cs and 129 I / 99 Tc in the Norwegian coastal current from 1980 to 1998 . Journal of Environmental Radioactivity 60, 2002, p. 61.
  17. ↑ HN Edmonds, ZQ Zhou, GM Raisbeck, F. Yiou, JM Edmond: Distribution and behavior of anthropogenic 129 I in water masses ventilating the North Atlantic Ocean . Journal of Geophysical Research Atmospheres 106, 2001, p. 6881.