Diffusion of U(VI) in Opalinus Clay Studied down to Ultratrace Levels

Natural clay rocks, such as Opalinus Clay (OPA), are considered as potential host rocks for the disposal of high-level nuclear waste due to their ability to limit radionuclide transport to molecular diffusion. Laboratory studies related to the investigation of radionuclide diffusion behavior are usually performed at relatively high concentrations. For instance, U diffusion in clay rocks has been investigated at concentrations as low as approximately 10^–4 mol·m^–3 clay. This study addresses ²³³U(VI) diffusion in OPA down to ultratrace concentrations of 10^–9 mol·m^–3 using accelerator mass spectrometry. Fitting of the experimental data with a one-dimensional pore diffusion model revealed no significant changes in the U(VI) effective diffusion coefficient (D_e) over the investigated concentration range of 10^–3 to 10^–9 mol·m^–3. Such concentration-independent behavior constitutes valuable information for the prediction of long-term U(VI) migration in geological repositories with OPA as host rock.