The combination of low temperatures and substantial irradiation damage may dramatically change the as-fabricated microstructure of nuclear fuels. Specifically, the original micrometric grains are gradually replaced by nanometric, pristine grains alongside micrometric pores of roughly spherical shape. This novel microstructure, which is classically referred to as high burnup structure (HBS), affects the performance of nuclear fuel under irradiation in commercial reactors, since it modifies important fuel properties, such as thermal conductivity and elastic properties. Moreover, the associated porosity is responsible for additional fuel swelling worsening the pellet-cladding mechanical interaction, and may release its stored quantity of fission gas in accident scenarios, e.g., loss of coolant accidents (LOCAs) or reactivity-initiated accidents (RIAs), potentially mining cladding integrity.
This thesis work aims at identifying the leading mechanisms for uranium dioxide restructuring, by a combination of analysis of experimental data, theoretical and computational investigations, with the ultimate goal of developing a mechanistic model for the formation of the HBS to be integrated in the framework of a fuel performance code.
The candidate will:
• Carry out an extensive bibliography on the open and internal CEA literature, analyzing and interpreting the experimental data selected for the present work.
• Develop and include into a computational software of the PLEIADES platform a model for the evolution of point defects in uranium dioxide, which would describe the evolution under irradiation of relevant quantities (e.g., dislocation loops and lines) by physics-based evolution equations.
• Use (and if, necessary, improve) computational codes conceived to study the restructuring of materials to investigate the uranium dioxide behavior at high burnup and low temperature, possibly using inputs from the lower-scale model developed in the thesis.
Each computational part of the work will be accompanied by a critical comparison to the experimental data and an analysis of the associated modeling uncertainties. Finally, the model will be applied to analyze the fuel performance of Small Modular Reactors for heat generation currently under study at CEA.

The results achieved in the PhD thesis will be presented in conferences and workshop on the international stage (e.g., NuMat – The Nuclear Materials Conference, MMM – Multiscale Materials Modeling Conference, MiNES – Materials in Nuclear Energy Systems), to give the candidate the opportunity of interact with international experts on the subject. Moreover, the candidate will publish his results on international scientific journals (Computational Material Science, Journal of Nuclear Materials, Journal of the European Ceramic Society).

Within the IRESNE institut (CEA Cadarache), the candidate will join a multidisciplinary scientific community on materials modeling (solid state physics, thermal, mechanics, applied math, thermochemistry, thermodynamics) open to both international research and industrial world, and will have the possibility to interact with the different scientists and engineers involved in the different steps of design a novel nuclear reactor. The candidate will integrate the group of the thesis director for a certain period of time during the thesis, where the experience gained at CEA will be complemented by glimpses of more academic scientific research.