Spectroscopic investigations of the retention of Np and Pu on ZrO2 and influence of inorganic ligands
Corrosion of the zircaloy cladding surrounding spent nuclear fuel (SNF) rods is a crucial process to be considered during a safety assessment of a repository for high radioactive waste. In a worst-case scenario, the interactions of dissolved actinide ions with solid corrosion products such as zirconium oxide phases, representing the first barrier during the dissemination of the radionuclides, are in the focus of safety research in the near field of a repository. Such processes are furthermore of concern in early-failure scenarios of the extended interim storage of SNF rods in containers in the vicinity of nuclear power plants. The impact of common inorganic ligands on these processes is less investigated yet. In particular, the retention capacity of the solid phases might significantly be changed in the presence of such ligands.
This project will provide a thorough characterization of selected actinides, namely Np and Pu at oxidation states (+V/+VI), and their retention on zirconium phases (corrosion products) in the presence of inorganic sulfate. Sulfate as model ligand shares the same structure with other oxyanions, such as phosphate, but with a lower tendency to form poorly soluble compounds. Following the comprehensive characterization of the sulfate system further studies with phosphate ligands can be conducted under suitable conditions. The main goal is a detailed understanding of the interface processes (formation of surface species, their stoichiometry, structure and binding modes) at a molecular level and the derivation of thermodynamic parameter (enthalpy and entropy values) of the surface reactions.
For this, following a thorough characterization of the solid phase, a combined approach of batch experiments performed under different chemical conditions (pH, ionic strength, initial concentration of Np/Pu) and spectroscopic investigations at a molecular level (IR, XAS, CTR-RAXR) will be applied, both in the presence and absence of inorganic ligands. The knowledge generated will be used during Surface Complexation Modeling (SCM) studies to constrain the number of surface species as well as their denticity. The derived surface complexation constants will be implemented in thermodynamic databases (RES³T, THEREDA). Eventually, Isothermal Titration Calorimetry (ITC) will enable the derivation of the enthalpy and entropy of reactions as well as heat capacity of reactions by performing experiments at elevated temperature.