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

摘要

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