Global environmental transfer of 129 I Faby Sunny, R.N. Nair, Manish Chopra ⇑ , V.D. Puranik Environmental Assessment Division, Bhabha Atomic Research Centre, Mumbai 400085, India article info Article history: Received 25 March 2013 Received in revised form 16 November 2013 Accepted 19 November 2013 Available online 8 December 2013 Keywords: 129 I Global environmental transfer Compartment model Effective dose abstract 129 I is released to the environment via natural production due to cosmic interaction in the upper atmo- sphere; past nuclear weapon tests and routine releases from nuclear power plants (NPP) and fuel repro- cessing plants (FRP). In this study, a compartmental model is used to estimate the transfer of 129 I through various environmental segments like ocean atmosphere, land atmosphere, terrestrial biosphere, ocean mixed layer, surface soil, deep ocean, ocean sediment, shallow subsurface soil and deep subsurface soil due to its release in any one or more compartments. Due to NPP and FRP releases into the land atmo- sphere for a period of 1000 y, the highest inventory of 129 I is observed in the surface soil up to a period of 3000 y; afterwards the deep ocean shows the highest inventory. The lowest inventory is found in the ocean sediment up to a period of 200 y; followed by the ocean atmosphere up to a period of 1250 y; after- wards the land atmosphere shows the lowest inventory. The maximum annual effective dose to the world population due to releases of 129 I from NPP and FRP for a period of 100 y is estimated as 4.14  10 6 mSv/ y. If the release period is 1000 y, the annual effective dose increases to 1.05  10 5 mSv/y and for an infi- nite period of release, it is estimated as 1.5  10 4 mSv/y. The model results are verified by comparing the effective dose per TBq release of 129 I at different time periods with those reported by different interna- tional agencies and good matching is observed among the values. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Some radionuclides upon release may become globally dis- persed and act as a long term source of radiation for the world pop- ulation due to their long half-lives and chemical nature. This is in addition to the radiation exposure during the initial dispersion of these radionuclides from their points of discharge. The radionuc- lides which are important in this context are 85 Kr, 3 H, 14 C and 129 I with half-lives of 10.72 y, 12.35 y, 5730 y and 1.7  10 7 y respectively. Once these radionuclides become globally dispersed, members of the world population will be irradiated at the same le- vel. The longest lived out of these radionuclides, 129 I, is produced in nature by cosmic-ray induced (n, p) spallation of xenon in the atmosphere. A large percentage of total 129 I in the environment is produced by the fission of uranium atoms during operation of nuclear reactors and by fission of plutonium or uranium during the detonation of nuclear weapons. The natural production of 129 I is estimated as 5.29  10 5 Bq/y, whatever amount produced due to natural processes will be released into the environment (IAEA, 1985). The corresponding inventory of 129 I is 1.48 TBq in the envi- ronment on a global basis (IAEA, 1985). The nuclear power plants (NPP) and fuel reprocessing plants (FRP) globally produce 129 I at a rate of 4.81  10 4 Bq/MWtd. This production rate corresponds to 1.86  10 13 Bq/y (UNSCEAR, 2000). About 2.8% of 129 I produced in NPP and FRP is released into the environment, which works out to be 5.21  10 11 Bq/y. The rate of 129 I production in NPP and FRP is assumed to be constant as approximately the rate of NPP and FRP closures would be the same as the emergence of new NPP and FRP globally. The release of 129 I due to past nuclear weap- on tests in the atmosphere is estimated as 3.7  10 11 Bq (IAEA, 1985). Once the 129 I is released in any sector of the environment, due to its very long half-life, it will transfer to many other sectors of the environment. The transfer of these nuclides through the envi- ronment can be described adequately using global circulation models or compartment models. Each compartment in a global cir- culation model may represent a large part of an environmental medium in which the radionuclide is assumed to be homoge- neously mixed. The transfer between compartments is described using linear, first-order kinetics. A model of the global iodine cycle was developed by Kocher assuming the atmosphere, hydrosphere, lithosphere and terrestrial biosphere as the compartments (Kocher, 1979). It is estimated that the mean residence time of iodine in surface soil is of the order of 10,000 y and that the mixing of iodine thorough the ocean would require 1000 y or more. Therefore, the most important parameter for determining dose rates and cumulative doses following the re- lease of 129 I is the mean residence time of iodine in the surface soil region. Thus, for a realistic long-term population dose assessment, 0306-4549/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2013.11.036 ⇑ Corresponding author. Tel.: +91 22 25590403. E-mail address: mchopra@barc.gov.in (M. Chopra). Annals of Nuclear Energy 65 (2014) 320–324 Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene