Investigation of the effect of the microstructure upon the oxidative long-term dissolution of uranium dioxide: a fundamental approach including synthesis and corrosion of simplified model systems as well as electron microscopy and atomistic simulation

• Applicants: Dr. Felix Brandt; Professor Roger De Souza, Ph.D.; Professor Dr. Joachim Mayer
• Subject Area: Glass, Ceramics and Derived Composites ; Mechanical Properties of Metallic Materials and their Microstructural Origins ; Mineralogy, Petrology and Geochemistry ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 455439961

The aim of the proposed project is to obtain a deep fundamental understanding of the process of oxidative corrosion of UO2-based ceramics, materials that are widely considered as suitable model systems for spent nuclear fuel (SNF). Due to the corrosion of the container (typically stainless steel), the geochemical environment of a deep geological repository for nuclear waste is assumed to be reducing. However, at the surface of SNF in contact with water, radiolysis due to radioactive decay produces oxidizing species. H2O2 is considered to be the most relevant of these. Oxidation of the actinides transforms them from insoluble to highly soluble species. On a fundamental level, this oxidation process is similar to the corrosion of metals. Therefore, the working hypothesis of this project is that grain boundaries play a major role during this corrosion mechanism. A combined macroscopic and microscopic experimental approach will be followed including synthesis, characterization and corrosion of simple UO2 as a reference as well as more complex UO2 ceramics doped with rare earth elements, and in a further step, doped UO2 ceramics with additional metallic particles. These materials will be characterized by means of classical electron microscopy (SEM, EDX) and EBSD to understand how the grain boundaries and other weak spots of the ceramic’s structure contribute to the dissolution. Advanced transmission electron microscopy techniques will be employed for detailed high-resolution investigations of the fresh and corroded surfaces. Grain boundaries and grain orientations will be systematically characterized before and after being corroded. In addition, atomistic simulations of grain boundaries will be performed to determine the point defect thermodynamics. Based on these data, continuum simulations will be set up which can predict the defect concentrations in (and diffusion rates along) the space charge zones, since it is point defects that play a central role in solid-state reactions.Ultimately, the observations and findings from this combined experimental and theoretical approach will be used to derive a model of the corrosion behavior of UO2-based ceramics. Such a model could be used to mathematically describe the long-term corrosion behavior of UO2-based spent nuclear fuel and derive consequences for the long-term stability and release of radionuclides from such a ceramic.

• DFG Programme: Research Grants