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

Abstract

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 D e = 2.0 × 1 0 - 11 m 2 s - 1 and D e = 1.5 × 1 0 - 11 m 2 s - 1 for Benken (Zürcher Weinland) OPA and Mont Terri OPA, respectively, were derived. The distribution coefficients were K d = 3.1 × 1 0 - 4 and 0.9×10 −4 m 3 kg −1, respectively. For 134Cs + the effective diffusion coefficients were higher, i.e. D e = 3.1 × 1 0 - 11 m 2 s - 1 for OPA from Benken and D e = 3.0 × 1 0 - 11 m 2 s - 1 for OPA from Mont Terri. The distribution coefficients determined were K d = 0.16 m 3 kg - 1 for Benken and 0.23 m 3 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.