Carbon neutrality requires the development of low-carbon energy production systems, including nuclear power. The safety analysis of nuclear reactors requires the containment of fission products in all operating conditions, including the integrity of the first barrier made up of the fuel elements. For rod-type designs, which consist of a stack of fuel pellets in a metallic cladding, the mechanical behavior of uranium dioxide (UO2), pellet material, plays an important role in the cladding integrity assessment. During power transients, fuel-cladding contact increases mechanical stresses on the cladding, and fuel creep can accommodate swelling deformations, thereby reducing the stresses induced the cladding. One of the challenges is to understand and predict this phenomenon of UO2 creep, and in particular the mechanisms that drive it at the polycrystalline microstructural scale.
The main objective of the thesis will be to provide simulation methods and reference results in support of multi-scale modeling of the mechanical behavior of fuel at high-temperature, which is highly dependent on dislocation climbing mechanisms. To this end, a computational scheme will be developed, based on the coupling of a dislocation dynamics code (NUMODIS) and a code for solving nonlinear partial differential equations by FFT (AMITEX-FFTP), in order to describe the evolution of a dislocation microstructure (NUMODIS) under the effect of dislocation climbing induced by vacancy diffusion (AMITEX-FFTP). Simulations based on this approach will then be used to quantify the recovery of stored dislocation density with the effect of climbing mechanisms in different configurations (temperatures, stresses, etc.). Ultimately, this work will improve and validate the existing micromechanical modeling implemented in the CEA's PLEIADES simulation platform.
This thesis will be carried out under the joint supervision of the Département d'Etude des Combustibles (Institut IRESNE, CEA Cadarache) and the Département de Recherche sur les Matériaux et la Physico-chimie (Institut ISAS, CEA Saclay), and in collaboration with IM2NP at Aix Marseille Université. The thesis work will be carried out at the LM2C (Cadarache) and LC2M (Saclay) laboratories, in an environment that provides access to extensive expertise in multi-scale materials modeling. The research work will be promoted through publications and participation in international conferences in the materials field.