NES events

The different departments of the Institute of Resource Ecology work in the fields of "radioactive waste management", "radioecology" and "reactor safety". The interdisciplinary approach is focused on the elucidation of the processes and behavior of actinides and fission products at the molecular level and the associated parameterization. Part of the work and focus of this presentation is the work on thermodynamic databases for both solubility (THEREDA) and sorption (RES³T) of radionuclides.

Prediction of realistic diffusion parameters of radionuclides in clay-rich host rocks and bentohite is relevant in the context of deep geological disposal of radioactive waste. The negative permanent charge of clay minerals (illite and smectite) enhances porewater concentration of aqueous cations and depletes that of anions near clay surfaces. This leads to greater diffusion rates of cations and to anion exclusion and retardation in compacted clayrock systems. The ClaySorDif model accounts for the effective diffusion coefficients De and distribution ratios Rd of elements and their aqueous species in argillaceous porous media. It extends the ClaySor model, which is a GEMS reincarnation of the widely known 2SPNE SC/CE multi-site sorption model by Bradbury and Baeyens (B&B), now made consistent with the PSI TDB 2020 chemical thermodynamic database. To help estimate De, Rd, and element-accessible porosity, the simple and efficient MPDL (mean potential in Donnan layer) model was implemented in Python within the ClaySorDif extension of pyGEMS/xGEMS codes. Verification of ClaySorDif against the PHREEQC results and the recent experimental data for Swiss argillaceous rocks shows that the simultaneous use of ClaySor and MPDL models is possible without any loss of accuracy. The modelling tool, running on the LES JupyterLab server, is currently in use for on-going research projects and applications.

For the Swiss repository performance assessment concept, a state-of-the-art sorption database (SDB) is needed for different siting regions under investigation. The GEM-Selektor geochemical code is used to interpret the data and compile the SDB for the safety analysis. The Kd values are calculated directly by GEMS, including aqueous speciation, solubility, and concentration-dependent adsorption of each radionuclide for each argillaceous rock/porewater system defined for the different siting regions. The PSI TBD2020 for Nagra, extended with an internally consistent thermodynamic dataset for surface complexation and ion exchange (the updated "ClaySor" version), is also used in the ClaySorDif model extension for the diffusion coefficients of relevant elements