Influence of initial iodine concentration on the iodine uptake by the argilite of Tournemire

Titre du congrès :TRACER 4th International Conference on Tracers and Tracing Methods Ville du congrès :Autrans Date du congrès :03/10/2006

Among the radionuclides that can migrate from a radioactive waste repository to the geosphere, 129I poses a particular problem due to its high mobility in geomedia and its very long half-life of 1.6.107 years. For this reason, we studied iodide migration through the Tournemire argillite for a better understanding of its behaviour, especially its low affinity for argillite. Previous studies showed an irreversible iodide sorption (or an uptake with a kinetically slow desorption) on the Tournemire argillite with radial diffusion experiments [1]. This sorption was also observed on the Callovo-Oxfordian argillite by through-diffusion experiments. In this case, the influence of the initial iodide concentration on the I- retention was demonstrated [2]. In the present study, radial diffusion experiments have been carried out to quantify such an effect on iodide uptake by the argillite of Tournemire. We used eight radial diffusion cells [1] which were associated with different iodide concentrations (10-5 to 10-2 M) using 125I- as a radiotracer. All the reservoir cells were filled with a synthetic solution having a chemical composition as close as possible to that prevailing in Tournemire shale pore water. Reducing conditions for iodide were ensured both by an addition of thiosulfate to the synthetic solution and by carrying out all the handling and storing cells in a glove box under nitrogen atmosphere. At first, an equilibration stage was applied to the cells in order to limit chemical and osmotic disruptions. Study of the in-diffusion of non-reactive tracers (Br-, Cl-) allowed the determination of their diffusion coefficients and their diffusion-accessible porosities. The comparison between the halide-accessible porosity and the iodide-rock capacity factor (obtained from iodide radial in-diffusion) led to an estimate of the iodide distribution ratio as a function of initial iodide concentrations. Out-diffusion of iodide permitted to quantify the extent of the reversibility of I-uptake. Results show the influence of the initial iodide concentration on iodide retention. In-diffusion behaviour of I- and Br- is similar at high initial iodide concentration (no significant iodide sorption) while, at low initial iodide concentration, the relative quantity of iodide that is removed from the reservoir is higher than that of bromide, suggesting the occurrence of a preferential iodide uptake. Moreover, out-diffusion experiments suggest an irreversible sorption phenomenon, or a kinetically very slow desorption.