The diffusion of a suite of trace solutes (radiotracers and stable isotope species) through Opalinus Clay was studied on rock material from two different boreholes (BDR-2, BMA-A1) of the Mont Terri Underground Research Laboratory in Switzerland. Most tests were carried out on samples from borehole BDR-2, where the DR long-term diffusion-retention experiment was conducted. For these samples, the radial through-diffusion method was applied in an orientation determining the diffusion component parallel to the bedding plane. The samples from borehole BMA-A1 were exposed to 226Ra2+ to determine diffusion behaviour both parallel and perpendicular to bedding. These tests were preceded by diffusion measurements of tritiated water in both orientations. The resulting experimental diffusion data were compiled and modelled using single species numerical simulations implemented in COMSOL Multiphysics® to obtain best-fit parameter values for the effective diffusion coefficients, the rock capacity factors, and their related uncertainties. The results obtained from the samples from borehole BDR-2 demonstrated good agreement with data reported in a previous study on Opalinus Clay from a different location in the Underground Research Laboratory after accounting for variations in mineralogical and pore water compositions. The measured diffusivity of 226Ra2+ was consistent with those of the other earth-alkaline tracers, such as Sr2+ and Ba2+. Geochemical simulations of the effective diffusion coefficients using an established electrostatical model produced predictions that closely matched the experimental data. These simulations implemented in PHREEQC used the total clay mineral content of the rock and the pore water composition as the main input variables. The broad range of test conditions of this study confirms that (i) diffusion in Opalinus Clay parallel to the bedding is ∼4–5 times faster than perpendicular to the bedding, (ii) the diffusion behaviour of chemically different species can be adequately predicted when taking into account exclusion of anionic species from and surface diffusion of cationic species at the negatively charged surface of the relevant clay minerals. In Opalinus Clay, sorption of Ba2+ and Ra2+ on 'high-affinity' surface sites is known to dominate uptake of these elements at trace concentrations, however, the present results indicated that these species do not undergo surface diffusion. Overall, the outcome confirms the robustness of the model approaches and provides a reliable basis for predicting diffusion parameters used in radionuclide transport modelling in safety analysis for deep geological repositories of radioactive waste in clay-rich sediments, particularly for tracers lacking experimental data, and for supporting the interpretation of diffusion measurements at field scales.
Contact
Prof. Dr. Sergey Churakov
PSI Center for Nuclear Engineering and Sciences
Paul Scherrer Institute PSI
+41 56 310 41 13
sergey.churakov@psi.ch
[English]