The long-term safety assessment of a nuclear waste repository in deep geological formations requires a well-founded thermodynamic description of the processes relevant for the migration and retention of the actinides in the near- and far-field of the repository. The migration of the actinides is limited by their low solubility and interaction with inorganic material (i.e. interaction with mineral surfaces (sorption, incorporation). On the other hand complexation with dissolved inorganic ligands (OH−, CO32−, SO42−, Cl−, F−, H2PO4−, H3SiO4−and HCO3−) can increase their mobility. Besides rock salt and granite, clay formations are considered as potential host rocks for a nuclear waste repository.Small organic ligands (e.g. acetate (≤ 1865 μM), propionate (≤ 127 μM), etc.) are present in the pore water of clay rocks. Additionally, the pore waters of different origins vary considerably in their ionic strength and electrolyte composition (NaCl, CaCl2, MgCl2, etc.).
In the past, a broad variety of complexation reactions of actinides in their most important oxidation states were studied and the resulting stability constants (log β0n) and the thermodynamic data (ΔrH0m, ΔrS0m) are listed in thermodynamic databases. However, the majority of this data is valid only for 25°C. Due to the radioactive decay, temperatures in the near field of a nuclear waste repository will reach up to 200°C, depending on the disposed waste and the surrounding technical and geological barrier. This will have a distinct impact on the geochemistry of the actinides. Thus, thermodynamic data for the actinides at T ≥ 25°C is of major importance for a comprehensive long-term safety assessment.
We study complexation reactions of actinides with various organic and inorganic ligands at elevated temperatures. Time-resolved laser fluorescence spectroscopy (TRLFS) is applied to determine thermodynamic data (log K0n(T), ei,k(T), DrH0m, DrS0m, DrC0p,m), whereas extended X-ray absorption fine structure spectroscopy (EXAFS) is used to obtain structural data of the formed actinide complexes. The experiments are performed in a custom-built high-temperature cell which is modified for the respective spectroscopic technique.