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

Diffusion experiments in compacted FEBEX bentonite were performed with strongly sorbing radionuclides, 60Co and 152Eu. 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 (Da) 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 Da values obtained from these experiments in the FEBEX clay compacted at 1.65 g/cm3 are (0.5–2.3) × 10−13 m2/s for Co and (0.8–2.5) × 10−14 m2/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.

► Diffusion experiments in compacted bentonite with strongly sorbing radionuclides as 60Co and 152Eu are difficult to perform.
► Analytical and numerical approaches are not enough to explain the actual diffusion behavior.
► All the experiments show a common trend: Da values tend to increase when the distance from the tracer source increases.
► The shape of concentration profiles shows that not only diffusion but other processes are involved.