Disposal of high-level nuclear waste (HLW), either as spent fuel or as vitrified waste, is foreseen in deep geological repositories in metallic canisters, surrounded by  multiple technical and geotechnical barriers. During the (geo)chemical evolution of this multi barrier system, it needs to be considered that ground water may migrate through the barriers and reach the canisters. In aqueous environments, components of the multi-barrier system will undergo alteration and degradation, the metallic canisters will corrode and upon failure the waste matrix itself will start to alter, thereby forming secondary phases via dissolution and subsequent re-precipitation. For example, iron (hydr)oxides will form upon steel canister corrosion, sheet silicates, calcium molybdate or barium sulfate are expected to form upon waste glass alteration. Calcite is expected to form as alteration product of concrete based materials. Such phases have the potential to scavenge previously released radionuclides (RN) and to retard their migration to the biosphere. Various molecular scale retention mechanisms, from surface adsorption to structural incorporation can result in an effective RN immobilization. Among these processes, structural incorporation by formation of solid-solutions is argued to lead to a sustainable retention in the near field. However, despite the abundance of solid-solutions in natural systems, reliable kinetic and thermodynamic models to predict their formation are hardly available.

The INE-Secondary Phases group aims at molecular scale process understanding of secondary phase formation in environments typical for HLW repositories, and develops thermodynamic and kinetic models for the prediction of RN retention by solid-solution formation. Investigations rely on molecular scale information gained by application of advanced spectroscopy-, diffraction-, and microscopy techniques (X-ray absorption and X-ray diffraction based methods, electron microscopies, atomic force microscopy, laser fluorescence spectroscopy…) in combination with computational efforts. The aim of the group is to provide thermodynamic and kinetic data,  which allow to improve the accuracy and precision of performance assessment calculations for deep geological HLW repositories.

The investigations are carried out largely in the frame of national and international projects funded by waste management agencies, the European Commission, or by German Federal Ministries.

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