Thesis for the Degree of Doctor of Philosophy - DTU Orbit

Among iodine isotopes, 129 I has a half-life of 15,7 My and it is naturally formed by cosmic-
ray-induced spallation of xenon in the upper atmosphere, spontaneous fission of 238 U in the
geosphere and in minor quantities, by neutron bombardment of tellurium. Thermal neutron
induced fission of 235 U is another minor natural source in the lithosphere. The reported natural
isotopic ratio of 129 I/ 127 I is about10 -12 in terrestrial and marine environment (Hou et al., 2009;
Fehn et al., 2007). Presently, the source of additional 129 I in the environment is mainly from
human nuclear activity such as nuclear reprocessing facilities, nuclear weapons testing and
accidents associated with nuclear power plants. Over the past decade, liquid and gaseous
releases of 129 I from reprocessing facilities such as La Hague (France) and Sellafield (UK) has
increased the natural environmental concentration by several orders of magnitude (Alfimov et
al., 2004; Fehn et al., 2007; Aldahan et al., 2007). In the period 1999 - 2009, respectively 14.6
and 4.6 TBq of 129 I have been discharged into the English Channel from the La Hague
reprocessing plant and to the Irish Sea from the Sellafield reprocessing plant (www.ospar.org).
It is well known that seaweed accumulates iodine from seawater and that the process depends
strongly on seaweed type/species (Leblanc et al., 2006) as well as the region in which they are
found (Shah et al., 2005). Furthermore marine algae play an important role (Leblanc et al.,
2006) in the global cycle of iodine in the environment in the sense that they accumulate iodine
as iodide (Kupper et al., 1998) from seawater and transform a part of it into volatile organic
iodine (VOI), such as methyl iodide (CH3I) or diiodomethane (CH2I2; Carpenter et al. 2007).
Once released from the seawater surface the volatile organic iodine are broken down by
photolysis and reactions with ozone (O3) (Jones & Carpenter, 2005; Martino et al. 2006)
forming a reactive pool of iodine species which afterwards contribute to the ozone depletion,
particulates formation and cloud condensation nuclei formation (Küpper et al., 2008; O’Dowd
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