Diffusion of 226Radium through Opalinus Clay: combined diffusion and sorption study

226Ra is considered to be relevant not only as important contributor to naturally occurring radioactive materials (NORM) and the environmental hazards associated with NORM but also as a critical radionuclide that needs to be considered in safety cases for the deep geological disposal of spent nuclear fuel. In support of such safety case studies, the structural uptake of 226Ra into solid solutions within the ternary solid-solution aqueous solution system (Ba,Sr,Ra)SO4 has been studied extensively and thermodynamic data describing this retention mechanism are now available [1]. These solid solutions may form in the case of canister failure close to the surface of spent nuclear fuel due to the reaction of Ba released by spent fuel corrosion with sulphate present in the ground water. An additional retention mechanism, which has so far not been studied in detail, is the adsorption of 226Ra to the host rock. Especially clays are known to have a significant retention potential for bivalent cations such as 226Ra. The sorption and diffusion of radium in clay rocks is so far not quantitatively understood but derived from known values of Ba and Sr. In addition, the solid-solution formation between Ra and BaSO4 present in host rocks like Opalinus clay (OPA) may add an additional retention potential. Here, we report new experimental data to improve the quantitative understanding of the retention of 226Ra in clay rocks relevant for nuclear waste management. OPA samples originating from the Mont Terri Underground Rock Laboratory (bore hole BMA-A1) and synthetic OPA pore water [2] were used, both for diffusion and batch sorption experiments. Batch sorption experiments were carried out on the OPA clay grain size fraction < 63 µm with an S/L ratio of 117 g/L. These suspensions were spiked with a 226Ra tracer resulting in an initial Ra-concentration of 1.44 x 10-8 mol/L. Ra-concentrations of solution samples taken after 7 days were analysed by γ-spectrometry. Through-diffusion experiments were carried out with a conservative tracer (HTO) as well as with 226Ra, perpendicular to the bedding (planar) and parallel to the bedding (radial). The starting 226Ra concentration was 1.4 x 10-9 mol/L. Before the diffusion experiments were started, the OPA core samples were re-saturated with synthetic OPA pore water and loaded to a confining pressure of 7 MPa to make them comparable to earlier experiments [3,4]. The radiotracer concentrations were determined by LSC-measurements of samples taken from both, the high and the low concentration reservoirs. From the batch sorption experiments, the distribution coefficient for 226Ra was determined to be Rd = 36.6 L/kg. Sorption of the tracer on container walls was negligible. The HTO through diffusion experiments resulted in a rock capacity factor = 0.17 for the radial and = 0.20 for the planar core sample. The values of the effective diffusion coefficient De for HTO of 4.17 x 10-11 m2s-1 for the radial core sample and 1.34 x 10-11 m2s-1 are in good agreement with previous studies on similar samples [5]. The breakthrough of 226Ra was monitored after 100 days (planar) and 120 days (radial), respectively, whereas the steady state occurs after about 400 days in both cases. First modelling results obtained with a combination of the PHREEQC and COMSOL geochemical and reactive transport codes indicate that the retention within the consolidated OPA clay core samples is higher than predicted based on the batch sorption experiments. This could be due to enhanced sorption at frayed clay edges and/or to solid-solution formation of 226Ra with sulphates present in the OPA core samples.[1] V.L. Vinograd, D.A. Kulik, F. Brandt, M. Klinkenberg, J. Weber, B. Winkler, D. Bosbach, Appl. Geochemistry, 89, 59 (2018).[2] Pearson, F.J., PSI Internal Report TM-44-98- 07, Paul Scherrer Institut, Villigen PSI, Switzerland (1998).[3] L.R. Van Loon, J.M. Soler, M.H. Bradbury, J. Contam. Hydrol., 61, 73 (2003).[4] L.R. Van Loon, J.M. Soler, W. Müller, M.H. Bradbury, Environ. Sci. Technol., 38, 5721 (2004). [5] L.R. Van Loon & J.M. Soler, Nagra NTB 03-07, Switzerland (2004)