Diffusion coefficient measurements in consolidated clay by RBS micro-scale profiling

Abstract

The determination of radionuclide diffusion coefficients is necessary for performance assessment of clay formations as a geological barrier for deep geological repositories for high-level radioactive waste. Diffusion coefficients of elements that are retained by the clay and have short diffusion lengths are quite difficult to measure over reasonable time spans, by conventional techniques. This paper presents a novel application of the Rutherford Backscattering Spectrometry (RBS) technique to determine solute diffusion coefficients in consolidated clays. The RBS technique was selected because it allows the measurement of concentration profiles at short range distances (μm). The proposed methodology was tested on the Opalinus clay (OPA) from the Swiss Jura, where the Mont Terri underground laboratory is located. The diffusion of four elements with different adsorption behaviors onto the clay was analyzed: Eu and U as adsorbing elements, Sr as low adsorbing, and Re as a non-adsorbing element with anionic behavior. Apparent diffusion coefficients ( D a) could be experimentally determined for Re, Eu and U after a few days of diffusion time, while for Sr it was only possible to estimate a lower limit for the diffusion coefficient. The uncertainties of the proposed methodology are discussed on the basis of sensitivity studies.