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

The diffusion of (134)Cs(+) and (22)Na(+) 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 gamma-spectrometry. By choosing the appropriate abrasive paper, a resolution down to 15 microm 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 microm 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 (22)Na(+), effective diffusion coefficients of D(e)=2.0 x 10(-11)m(2)s(-1) and D(e)=1.5 x 10(-11)m(2)s(-1) for Benken (Zurcher Weinland) OPA and Mont Terri OPA, respectively, were derived. The distribution coefficients were K(d)=3.1 x 10(-4) and 0.9 x 10(-4)m(3)kg(-1), respectively. For (134)Cs(+) the effective diffusion coefficients were higher, i.e. D(e)=3.1 x 10(-11)m(2)s(-1) for OPA from Benken and D(e)=3.0 x 10(-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 (22)Na(+) with those from earlier through-diffusion experiments showed that there is good agreement between the two methods. In the case of (134)Cs(+) 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.