English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

. The Safety Case for a radioactive waste repository in deep geologicalformations requires detailed chemical and thermodynamic information onthe stored radionuclides in their relevant oxidation states. Althougha comprehensive summary of critically evaluated thermodynamic data isavailable via the blue book series of the NEA-TDB (“Nuclear Energy Agency – Thermochemical Database”), themajority 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 geochemicalbehaviour of radionuclides, potentially affecting their mobility andretention in the near field. Besides reactions at the solid–liquidinterface (e.g. dissolution/precipitation reactions of the wastematrix, sorption reactions of the radionuclides to surfaces),complexation reactions with inorganic and organic ligands present inthe aqueous phase potentially affect migration behaviour of theradionuclides. A quantitative thermodynamic description of theseprocesses requires standard stability constants (log⁡βn0(T)), as well as standard reaction enthalpies andentropies (ΔrHm,n0,ΔrSm,n0). The precise experimental determinationof these data for all relevant radionuclide/ligand reactions requiresa vast amount of time and effort. In this regard, reliableextrapolation methods in particular for standard stability constantsvalid for 25 ∘C to higher temperatures are consideredto support a comprehensive description. Recently, the German FederalMinistry of Education and Research (BMBF)-funded collaborativeresearch 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).

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