Controlling the behavior of fission gases in nuclear fuel (uranium oxide) is an important industrial issue, as fission gas release or precipitation limit the use of fuels at extended burn-ups. The gas behavior is strongly influenced by the material’s microstructure evolution due to the aggregation of irradiation-induced defects (gas bubbles, dislocation loops and lines). Cluster dynamics (CD) (a kind of rate theory model) is relevant for modelling the nucleation/growth of the defect clusters, there gas content and the gas release. The current model has been parameterized following a multiscale approach, based on atomistic calculations (ab initio or empirical potentials). This model has been successfully applied to annealing experiments of UO2 samples implanted with rare gas atoms and has emphasized the impact of the irradiation damage on gas release. The aim of this PhD thesis is now to improve the model, particularly the damage parameterization, and to extend its validation domain through in depth comparison of simulation with a large set of recently obtained experimental results, such as gas release measurement by annealing of sample implanted in ion beam accelerator, bubble and loop observation by transmission electrons microscopy of implanted or in-pile irradiated samples. This global analysis will finally yield an improved parameterization of the CD model.
The research subject combines a “theoretical” dimension (improving the model) with an “experimental” one (interpreting existing experiments or designing some new ones). The variety of techniques will introduce you into the experimental world and thus broaden your scientific skills. You will be welcomed at the Fuel Behavior Modeling Laboratory (part of the Institute for Research on Nuclear Systems for Low-Carbon Energy Production, IRESNE, CEA Cadarache), where you will benefit from an open environment rich in academic collaborations. You also have to manage collaborations for the experiments analysis, for the model development and for the specification of additional atomistic calculations. You will be at the interface of atomistic techniques, large-scale simulation and various experimental techniques. Therefore, You will develop a broad view of irradiation effects in materials and of multi-scale modelling in solids in general.
This project is an opportunity to contribute to the overall development of numerical physics applied to multi-scale modeling of materials, occupying a pivotal position and adopting a global viewpoint. This will allow experiencing yourself the way computed fundamental microscopic data finally helps solving complex practical issues.

Further readings:
Skorek et al. (2012). Modelling Fission Gas Bubble Distribution in UO2. Defect and Diffusion Forum, 323–325, 209.
Bertolus et al. (2015). Linking atomic and mesoscopic scales for the modelling of the transport properties of uranium dioxide under irradiation. Journal of Nuclear Materials, 462, 475–495.

Uranium – molybdenum alloys UMo present excellent thermal properties and a good uranium density. For those reasons, they are considered as nuclear fuel candidates for research reactors. It is therefore crucial for the CEA to deploy new computational methodologies in order to investigate the evolution of their thermo-physical properties under irradiation conditions.

This project is centered on the application of atomistic methods in order to investigate the stability and diffusion of intra-granular xenon clusters within the metallic nuclear fuel UMo.
The first step of your work will involve continuing the development of atomic-scale computational models for UMo, as initiated within the host laboratory. These models use machine learning methods to develop interatomic potentials and will be validated by comparison with existing experimental data for this material. They will then be used to assess the temperature-dependent evolution and the impact of defect accumulation (both point and extended defects) on several thermophysical properties critical to fuel modeling, such as elastic properties, density, thermal expansion, as well as thermal properties like specific heat and thermal conductivity. In collaboration with other researchers in the department, you will format these results for integration into the Scientific Computing Tools used to simulate the behavior of nuclear fuels.

In a second phase, you will be responsible for extending the validity of your models to account for the formation of fission gases such as xenon within UMo single crystals. This will enable you to simulate the stability of xenon clusters in UMo crystals. These calculations, performed using classical molecular dynamics methods, will be systematically compared with experimental observations obtained via transmission electron microscopy.

The results obtained during the various stages of this project will be completely innovative and will be the subject of scientific publications as well as presentations at international scientific conferences. Besides, this work will enable you to complement your training by acquiring skills applicable to many areas of materials science, including ab initio calculations, machine learning-based interatomic potential fitting, classical molecular dynamics, use of CEA supercomputers, and key concepts in statistical physics and condensed matter physics—fields in which the supervising team members are recognized experts.

You will join the Fuel Behavior Modeling Laboratory at the Research Institute for Nuclear Systems for Low-Carbon Energy Production (IRESNE, CEA Cadarache), a dynamic research team where you will have regular opportunities to interact with fellow PhD students and researchers. This environment also provides extensive opportunities for national and international collaboration, including with:
• Developers and users of the MAIA fuel performance code (dedicated to research reactor fuel studies),
• Experimental researchers from the Nuclear Fuel Studies Department,
• Teams from other CEA centers (Saclay, CEA/DAM),
• International partners.
This rich and multidisciplinary context will enable you to fully engage with the scientific community focused on nuclear materials science.

[1] Dubois, E. T., Tranchida, J., Bouchet, J., & Maillet, J. B. (2024). Atomistic simulations of nuclear fuel UO2 with machine learning interatomic potentials. Physical Review Materials, 8(2), 025402.
[2] Chaney, D., Castellano, A., Bosak, A., Bouchet, J., Bottin, F., Dorado, B., ... & Lander, G. H. (2021). Tuneable correlated disorder in alloys. Physical Review Materials, 5(3), 035004.

With the emergence of new start-ups in the nuclear field, it is essential to extend the validation basis for Fast Neutron Reactor (FNR) fuel performance codes to lower linear power operating regimes, an area that has yet to be fully explored.
Given the lower temperatures reached in the fuel, the microstructure induced by irradiation is completely different from what is typically observed at higher linear power (formation of a central hole, columnar grains, etc.). These lower operating temperatures also lead to a decrease in fission gas release (FGR), which can cause significant gas swelling of the fuel. At the same time, low operating temperatures can also lead to an increase in the density of defects (dislocations) induced during irradiation (lower defect annealing efficiency), resulting in an indirect increase in fuel swelling.
It is therefore important to determine the density of dislocations in the fuel, as their ambivalent role shows that they can slow down the release of gases by trapping them and promoting their storage in intragranular bubbles, while also facilitating their migration if they form a connected network.
In order to improve our understanding of the phenomena involved and the models of fuel swelling under irradiation, it is essential to have experimental results such as the densities and sizes of Fission Gas (FG) bubbles and the densities of dislocations in these operating regimes.
The Laboratory for Fuel Characterization and Property Studies (LCPC) within the Research Institute for Nuclear Systems for Low-Carbon Energy Production (IRESNE), to which the PhD student will be affiliated, is equipped with state-of-the-art instruments recently acquired (TEM, SEM-FIB, SIMS, EPMA, XRD) for the study of irradiated materials allowing him to develop advanced experimental skills within the specific context of a Basic Nuclear Installation. This work will be carried out in close collaboration with the teams responsible for developing the multiphysics scientific computing tools of the PLEIADES software platform. It is clear that the skills acquired during the thesis will be valuable in a future career in both academia and industry. The doctoral student will also be able to promote their work to the international academic community and the industrial world through oral presentations and peer-reviewed articles.