Diffusion of strongly sorbing cations (60Co and 152Eu) in compacted FEBEX bentonite

Abstract Diffusion experiments in compacted FEBEX bentonite were performed with strongly sorbing radionuclides, 60 Co and 152 Eu. Diffusion experiments with these radionuclides present several difficulties: first of all these tests are very time consuming because of the high sorption on the clays, secondly these elements not only present high sorption onto clays but also on diffusion cells, tubing, filters and reservoirs, typically used in the classical through-diffusion or in-diffusion methods , which makes difficult the interpretation of the results. In this study, the experiments were performed using the instantaneous planar source method , where a paper filter tagged with a tracer is placed between two tablets of compacted bentonite. The apparent diffusion coefficient ( D a ) is obtained analysing the tracer concentration profile in the samples at the end of the experiment, both with an analytical and a numerical approach. The ranges of D a values obtained from these experiments in the FEBEX clay compacted at 1.65 g/cm 3 are (0.5–2.3) × 10 −13  m 2 /s for Co and (0.8–2.5) × 10 −14  m 2 /s for Eu. Results showed that the analytical solution is able to fit reasonably well the Eu concentration profiles, whereas Co concentration profiles show a different behavior, not straightforward to explain, which was also analyzed by numerical methods.