Impact of increased temperatures on the geochemical behaviour of trivalent actinides in aquatic systems

The Safety Case for a radioactive waste repository in deep geological formations requires detailed chemical and thermodynamic information on the stored radionuclides in their relevant oxidation states. Although a comprehensive summary of critically evaluated thermodynamic data is available via the blue book series of the NEA-TDB (“Nuclear Energy Agency – Thermochemical Database”), the majority of this data is limited to ambient conditions (Grenthe et al., 2020). In the case of the disposal of high-active, heat-producing waste, however, the near-field of the repository will experience increased temperatures at early operative phases for several hundred or a few thousand years. Radionuclides may come into contact with aquatic solutions or brines at elevated temperatures in the case of early canister failure. Besides other factors of the overall disposal concept (e.g. the geometry of the repository, type and amount of stored radionuclide inventories), host rock characteristics themselves limit the extent of the allowable temperature increase. For example, in clay formations the maximum temperature should stay at around or below ∼100∘C in order to avoid an irreversible change in the host rock retention capacity, whereas rock salt allows much higher temperatures of up to 200 ∘C. Increased temperatures will have a distinct impact on the geochemical behaviour of radionuclides, potentially affecting their mobility and retention in the near field. Besides reactions at the solid–liquid interface (e.g. dissolution/precipitation reactions of the waste matrix, sorption reactions of the radionuclides to surfaces), complexation reactions with inorganic and organic ligands present in the aqueous phase potentially affect migration behaviour of the radionuclides. A quantitative thermodynamic description of these processes requires standard stability constants (log⁡βn0(T)), as well as standard reaction enthalpies and entropies (ΔrHm,n0, ΔrSm,n0). The precise experimental determination of these data for all relevant radionuclide/ligand reactions requires a vast amount of time and effort. In this regard, reliable extrapolation methods in particular for standard stability constants valid for 25 ∘C to higher temperatures are considered to support a comprehensive description. Recently, the German Federal Ministry of Education and Research (BMBF)-funded collaborative research project “Therm AC” focused on the experimental determination of new thermodynamic data at higher temperatures, as well as the comparison with the analogous results yielded by extrapolation methods. The Thermochemical Database Project of the OECD-NEA (NEA-TDB) is currently in the process of preparing a comprehensive state-of-the-art report on the high temperature thermodynamics of radionuclides, further emphasizing the particular relevance of this interesting topic. Within this contribution, a critical overview on the recent advances in the field of high temperature studies of radionuclides in aqueous solutions will be given. Besides summarizing information on key technical aspects relevant for high temperature studies, the effect of increased temperatures on the complexation of trivalent actinides with chloride will be discussed in more detail in order to illustrate newly derived in-depth understanding of the impact of increased temperatures on the (geo)chemical behaviour of trivalent actinides on the molecular scale (Skerencak-Frech et al., 2014).