In the Spanish concept of a high level radioactive waste (HLRW) repository, the canister with the spent fuel would be surrounded by a layer of FEBEX bentonite (Ca-Mg smectite clay) compacted to a dry density of 1.65 g/cm3. Since the permeability of the compacted bentonite is very low, the main mechanism for radionuclide transport in the clay barrier is expected to be diffusion, retarded by the sorption on the solid. Since the mayor component of the radioactive waste is uranium it is very important to study its diffusion behaviour in the compacted clay.

Generally, effective (De) and apparent (Da) diffusion coefficients are obtained from "through-diffusion" and "in-diffusion" experiments, respectively. The obtention of both parameters in the same sample is a very interesting issue because the former includes the information on the porous medium structure (connectivity, constrictivity, tortuosity) and the latter takes into account the sorption on the solid phase. In this work, through-diffusion experiments were carried out for studying uranium diffusion and both effective and apparent diffusion coefficients were estimated, from the same experiment, by using different theoretical approaches.