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Natural iodine in a clay formation: Implications for iodine fate in geological disposals |
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| Authors: | F Claret C Lerouge M Bizi JP Ghestem T Sato E Giffaut |
| Institution: | a BRGM, 3 Avenue Claude Guillemin, 45060 Orléans Cedex 2, France b Laboratory of Environmental Geology, Research Group of Geoenvironmental/Engineering Division of Solid Waste, Resources and Geoenvironmental/Engineering Graduate School of Engineering, Hokkaido University Kita 13 Nishi 8, Sapporo 060-8628, Japan c Andra, 1 rue Jean-Monnet, Châtenay-Malabry 92298, France |
| Abstract: | Iodine is one of the most problematic radioisotopes in the context of nuclear waste geological disposal due to its high mobility. Considerable effort has been dedicated to the measurement of its potential retardation during diffusive transport leading to conflicting results, from no retardation to significant retardation, leading in turn to considerable debate. The present study aims at providing new insights into this aspect of the iodine problem by careful quantification of iodine reservoirs in the Callovian-Oxfordian (COx) clay rock taken here as model material for these studies. The present study confirmed the ubiquitous presence of iodine at 1-5 mg kg−1 level in the COx clayey formation. The iodide concentration level in the porewater is also confirmed at a value in the range ∼20-40 μmol L−1, i.e. higher than the expected range of radio-iodine concentration in the far-field of the storage. Surprisingly, most of the iodine was found not to be associated with organic matter but rather in an inorganic form associated with carbonate minerals. This result has potentially significant implications for the fate of radio-iodine. In undisturbed far-field conditions, most natural iodine would not be accessible for isotopic exchange with radioactive iodine, reducing the effective Kd to negligible values. During laboratory experiments, good monitoring of the geochemical parameters (at least the Eh, pH, PCO2, Ca] and Mg]) is mandatory to avoid iodine-bearing carbonate precipitation and to enable rigorous interpretation of the iodide diffusion/retention experiments. |
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