A high-resolution abrasive method for determining diffusion profiles of sorbing radionuclides in dense argillaceous rocks

The diffusion of 134Cs+ and 22Na+ in Opalinus Clay (OPA) was studied by in-diffusion laboratory experiments. The diffusive tracer profiles in the rock were determined using a high-resolution abrasive peeling method. The radionuclide activities in the grinding swarf were measured directly via γ-spectrometry. By choosing the appropriate abrasive paper, a resolution down to 15 μm can be achieved. This is important when analysing strongly sorbing radionuclides such as tri- and tetravalent actinides that show steep, shallow diffusion profiles. In this study, a resolution between 20 and 90 μm was obtained which was sufficient for a good spatial resolution of the diffusion profiles. Both the effective diffusion coefficients and the distribution coefficients of the radionuclides could be determined by applying a single reservoir with decreasing source concentration analysis for a semi-infinite case. In the case of 22Na+, effective diffusion coefficients of and for Benken (Zürcher Weinland) OPA and Mont Terri OPA, respectively, were derived. The distribution coefficients were Kd=3.1×10-4 and 0.9×10−4 m3 kg−1, respectively. For 134Cs+ the effective diffusion coefficients were higher, i.e. for OPA from Benken and for OPA from Mont Terri. The distribution coefficients determined were for Benken and 0.23 m3 kg−1 for Mont Terri. Comparison of the data obtained for the weakly sorbing 22Na+ with those from earlier through-diffusion experiments showed that there is good agreement between the two methods. In the case of 134Cs+ such a comparison was not possible because through-diffusion data are not available. Because through-diffusion methods cannot be applied to strongly sorbing tracers in reasonable time periods, in-diffusion combined with high-resolution abrasive peeling offers an excellent alternative for measuring the diffusion properties of strongly sorbing tracers in dense argillaceous rocks.