The research activities on high-level radioactive waste (HLW) at IEK-6 are focused on materials science aspects of the behaviour of waste forms under repository relevant conditions with a focus on the interactions of spent nuclear fuel (SNF) with groundwater. This focus comprises research into the fast or "instant" release of some volatile radionuclides (e.g. fission gases, and parts of the Cs and I inventories) from SNF immediately after contact with water (so-called instant release fraction IRF) as well as into long-term SNF matrix corrosion processes. These activities aim at an enhanced mechanistic understanding of processes that govern the radionuclide release from SNF in the repository environment (i.e. the radionuclide source term) on a molecular level to reduce uncertainties and conservatisms in performance assessments (PA), and to improve the confidence in system understanding beyond a simple phenomenological description.
Due to the chemical and structural complexities of SNF and its high radiation field, studies on SNF samples cannot document all the mechanisms and processes contributing to the long-term oxidative matrix corrosion of SNF in the repository environment. Therefore, we also investigate simplified UO2-based model systems in a tiered bottom-up approach. These single-effect studies contribute to quantifying the contributions of diverse processes to the overall long-term SNF matrix corrosion. Complementary to the various experimental approaches, atomistic simulations on the chemical states of fission and activation products in SNF aim at providing further insight into the behaviour of these materials. These innovative approaches pursued at IEK-6 refine the understanding of SNF behaviour in the repository environment. The findings contribute to the scientific basis of the safety case for deep geological disposal by reducing uncertainties of matrix dissolution of SNF and radionuclide release from SNF.