Studies of the extent of surface diffusion in the migration of radionuclides through geological materials

Surface diffusion has been invoked previously to explain rates of radionuclide migration which were greater than those predicted. Results were generally open to interpretation but the possible existence of surface diffusion, whereby sorbed radionuclides could potentially migrate at much enhanced rates, necessitated investigation. In the present work, results from through-diffusion experiments have indicated that for strontium and americium, surface diffusion accounted for 50-80% and 5-60% of the total diffusion respectively, depending on the geological material. However, for caesium, no surface diffusion contribution was observed. Evidence for anion exclusion of iodide was found. Therefore, although some radionuclides do appear to partake in surface diffusion, the magnitude of the phenomenon is probably not suffient to affect repository safety assessment modelling. (orig.)