The French Alternative Energies and Atomic Energy Commission (CEA) is a major player in research and innovation in the field of low-carbon energy. In collaboration with EDF and Framatome, it conducts research combining simulation and experimentation to improve the safety of French reactors. The CEA is developing the PLEIADES simulation platform, which models the behavior of nuclear fuel using a multiphysical and multiscale approach.
This thesis focuses on studying the behavior of fuel rods during a hypothetical Loss Of Coolant Accident (LOCA) scenario in a pressurized water reactor. These rods consist of fuel pellets stacked within a zircaloy cladding, which serves as the first containment barrier for radioactive fission products. During a LOCA, the cladding temperature rises, while the internal pressure of the fuel rod exceeds that of the primary coolant system. Due to heat and oxidation, the cladding is weakened, this can lead locally to the cladding ballooning or even its rupture.
The resulting sudden depressurization can lead to the fragmentation of the fuel, which has already been fragmented into several pieces during its operation in the reactor. The fragments may then disperse into the primary coolant loop, altering the core cooling conditions—which constitutes a fundamental safety concern. The objective of this thesis is therefore to quantify the impact of cladding ballooning on fuel fragmentation and relocation, in order to predict its dispersion in the event of cladding rupture and thereby strengthen safety demonstrations.
This thesis work aims to develop a three-dimensional simulation approach for the release of fission products and the fine fragmentation of fuel observed under thermomechanical loads. Particular attention will be paid to the influence of the confinement exerted by the shell on fragmentation mechanisms. To this end, the Discrete Element Method (DEM) will be coupled with Cohesive Zone Models (CZM) to simulate crack propagation in an elementary volume composed of a few grains, under various loading, confinement, and geometric conditions. A multiscale approach will then be used to extend the simulations to fuel pellet scale. Volumes representative of the pellet’s different zones will be studied to gradually build a macroscopic-scale description, using cellular automata and/or full three-dimensional calculations. The models will be evaluated by comparing them with representative experimental data.
The student will be based at the IRESNE Institute (CEA-Cadarache) in the Laboratory of multi-type fuels expertise and performance code validation (LEVA) within the Fuel Study Department (DEC). This doctoral research will be conducted jointly at : 1) LEVA and 2) the Laboratory of Mechanics and Civil Engineering (LMGC, University of Montpellier). The doctoral student will build upon the findings of previous doctoral research as well as the high-performance computational tools ExaDEM and the proven LMGC90 tool. The doctoral student will develop skills at the intersection of mechanics, materials science, and nuclear engineering, within a research environment that combines scientific expertise, academic collaboration, and industrial partnerships. The work may be showcased through publications and presentations at international conferences.

A. Nusslé « Modélisation de la fragmentation fine du combustible UO2 lors d’un Accident de Perte de réfrigérant Primaire », Thèse de doctorat, Université de Montpellier, 2026
V. Richefeu & al., Advanced strategies for discrete simulations with three-dimensional R-shapes in rockable framework. Computer Physics Communications, 320, 109997, 2026.