Modelling large-scale laboratory HTO and strontium diffusion experiments in Mont Terri and Bure clay rocks

Abstract

Clay formations are adequate host rocks for high-level radioactive waste repositories. Since in clay materials diffusion is the main radionuclide transport mechanism, the determination of diffusion coefficients under conditions as realistic as possible is fundamental for the performance assessment of deep geological repositories. Recently, the design of large-scale laboratory diffusion experiments in large cylindrical blocks that allows determining experimental diffusion profiles in 3D was developed [García-Gutiérrez, M., Cormenzana, J.L., Missana, T., Mingarro, M., Martín, P.L., 2006. Large-scale laboratory diffusion experiments in clay rocks. Physics and Chemistry of the Earth 31, 523–530]. In the present work, we will focus on the modelling of several new experiments carried out with this novel experimental set-up. These experiments were performed in Opalinus (OPA) clay samples from Mont Terri (Switzerland) and Callovo-Oxfordian (COx) clay samples from Bure (France), using both neutral (HTO) and weakly sorbing cationic ( 85Sr) tracers. In these new experiments, the clay bedding was roughly parallel to the cylinder bases, and this allowed following a new modelling approach, which represented a significant improvement in respect to previous experiments. After the diffusion period, 24 small cylinders were drilled in each clay sample at different distances to the cylinder axis. Then they were sliced and the tracer activity in each slice was measured to obtain 24 vertical concentration profiles. These experimental profiles were compared with those calculated for different values of the diffusion coefficients and sensitivity studies were carried out. Both numerical and graphical comparisons were done in order to establish the range of values for the diffusion coefficients that better adjusted the experimental results. Estimations of D p both parallel and perpendicular to the bedding planes were obtained for HTO in OPA and COx clays. Only D a parallel to bedding could be estimated for 85Sr in OPA and COx clays. It is interesting noticing that diffusion coefficients measured both in Opalinus clay and Callovo-Oxfordian clay were quite similar.