UO2 Powders: Morphological Characterization of Aggregates and Study of Their Interactions Using a Combined Experimental and numerical Approach
This PhD thesis is part of the optimization of nuclear fuel fabrication processes, which rely on the powder metallurgy of uranium dioxide (UO2) and plutonium dioxide (PuO2). These powders exhibit a hierarchical microstructure composed of crystallites forming rigid aggregates, themselves agglomerated into larger structures. The morphology and interactions between aggregates play a key role in the macroscopic behavior of the powders—particularly their flowability, compressibility, and agglomeration capacity—and directly influence the quality of the fuel pellets obtained after pressing and sintering. However, the experimental characterization of these aggregates remains complex and does not yet allow for the establishment of a predictive link between synthesis processes and morphological properties.
The objective of this thesis is to combine experimental and numerical approaches to achieve a detailed characterization of the aggregates in a reference powder. Experimentally, techniques such as Scanning Electron Microscopy (SEM), specific surface area measurement (BET), and laser granulometry will be used to determine particle size, roughness, and size distribution. In parallel, numerical simulations based on the Discrete Element Method (DEM) will be employed to construct a granular digital twin consistent with the experimentally measured properties. This digital twin will allow the reconstruction of the internal structure of the aggregates, the evaluation of interparticle adhesion forces, and the analysis of agglomeration and densification phenomena under controlled conditions.
The PhD will take place at CEA Cadarache within the Institute for Research on Nuclear Systems for Low-Carbon Energy Production (IRESNE). The student will be assigned to the PLEIADES Fuel Development Laboratory (LDOP), which specializes in simulating nuclear fuel behavior (from fabrication to in-reactor performance) and in multi-scale numerical methods. The work will be carried out in collaboration with the CNRS/LMGC in Montpellier, internationally recognized for its research on granular materials, and with the Uranium Fuel Laboratory (LCU – CEA Cadarache), which has extensive experience in the experimental characterization of uranium powders.
The PhD candidate is expected to demonstrate strong skills in numerical simulation and in the physical analysis of results. He will share its results through publications and conference presentations and will have the opportunity to learn or further develop various experimental and numerical techniques that can be applied in other contexts.In particular, the issues related to the physics of granular media — which constitute the core of this PhD — are of significant industrial relevance and are common to many other sectors handling powders, such as pharmaceuticals, agri-food, and powder metallurgy.
[Hebrard2004] S.Hebrard, Etude des mécanismes d’évolution morphologique de la structure des poudres d’UO2 en voie sèche, thèse de doctorat, CEA-LSG2M-COGEMA), 2004.
[Pizette2010] P. Pizette, C.L. Martin a, G. Delette, P. Sornay, F. Sans, Compaction of aggregated ceramic powders: From contact laws to fracture and yield surfaces, Powder Technology, 198, 240-250, 2010.
[Tran2025] T.-D. Tran , S. Nezamabadi , J.-P. Bayle, L. Amarsid, F. Radjai , Effect of interlocking on the compressive strength of agglomerates composed of cohesive nonconvex particles, Advanced Powder Technology 36, 2025.
Atomistic investigation of the thermophysical properties of metallic nuclear fuel UMo
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.
Blended positive electrodes in solid-state batteries: Effect of the electrode fabrication process on electrochemistry
The development of cost-effective, high-energy-density solid-state batteries (SSBs) is essential for the large-scale adoption of next-generation energy storage technologies. Among various cathode candidates, LiFePO4 (LFP) and LiFe1??Mn?PO4 (LFMP) offer safety and cost advantages but suffer from low working voltages and limited kinetics compared to Ni-rich layered oxides such as LiNi0.85Mn0.05Co0.1O2 (NMC85). To balance energy density, rate capability, and stability, this PhD project aims to develop blended cathodes combining LFMP and NMC85 in optimized ratios for solid-state configurations employing sulfide electrolytes (Li6PS5Cl). We will investigate how fabrication methods- including slurry-based electrode processing and binder-solvent optimization- affect the electrochemical and structural performance. In-depth operando and in situ characterizations (XRD, Raman, and NMR) will be conducted to elucidate lithium diffusion, phase transition mechanisms, and redox behavior within the blended systems. Electrochemical impedance spectroscopy (EIS) and titration methods will quantify lithium kinetics across various states of charge. By correlating processing conditions, microstructure, and electrochemical behavior, this research seeks to identify optimal cathode compositions and manufacturing strategies for scalable, high-performance SSBs. Ultimately, the project aims to deliver a comprehensive understanding of structure–property relationships in blended cathodes, paving the way for practical solid-state battery technologies with enhanced safety, stability, and cost efficiency.
Micromechanical Modeling of the Behavior of Polycristals with Imperfect Interfaces: Application to Irradiated UO2 Fuel
This thesis aims to analyze the thermomechanical properties of UO2 fuel used in pressurized water reactors (PWRs), accounting for the effects of microscopic defects. It focuses particularly on intergranular decohesion phenomena observed at various stages of fuel evolution, notably prior to crack initiation and propagation. The objective of this thesis is to clarify the impact of decohesion on both the local and effective properties of UO2 during irradiation. To this end, intergranular decohesion is modeled at the local scale by means of imperfect interface models, which ensure traction continuity while allowing for a displacement jump at grain boundaries. This modeling approach enables the development of homogenization models incorporating innovative theoretical and numerical advances, capable of capturing the behavior of the fuel at very high temperatures, under off-normal and accidental conditions. This work will be conducted at CEA Cadarache,in the Institute for Research on Nuclear Systems for Low-Carbon Energy Production (IRESNE), in close collaboration with national and international research teams. The tools developed will contribute to improving our understanding of the fuel's properties and to enhancing the accuracy and reliability of existing models, particularly those implemented in the PLEIADES simulation platform developed by the CEA in collaboration with French nuclear industry partners.
TRANSIENT LIQUID PHASE SINTERING OF UOX AND MOX FUEL PELLETS
The subject is related to the manufacture of UOX and MOX fuels. The main objective is to identify dopant pairs that allow the formation of a transient liquid phase during the fuel sintering step. For this, phase diagram calculations using the CALPHAD method will have to be carried out, also taking into account the requirements related to the irradiation phase once the fuel is loaded into the reactor. The most promising pairs will then be evaluated in the context of the manufacture of a UOX fuel and a MOX fuel. The experiments to be carried out will essentially be: the preparation of a powdery material, the shaping by pressing of this material in the form of cylinders representative of fuel pellets and the study of the high-temperature sintering of these UOX and MOX formulation cylinders. After sintering, a very important step will be the characterization at the macroscopic and microscopic scales of these pellets. The first year of the thesis will take place at the CEA center in Cadarache. The next two will take place at the CEA site in Marcoule. The first year of the PhD will take place at the CEA Cadarache center within the ICPE Uranium Fuel Laboratory. The following two years will be spent at the INB Atalante facility on the CEA Marcoule site. The candidate will work in two facilities unique in Europe and will be able to develop experience working in a nuclear environment with a highly innovative approach that will lead to the publication of original scientific results.
Nuclear fuel fragmentation under thermal gradient of fuel during laser heating: correlation, numerical simulation and and adaptation of the experimental setup.
The aim of this thesis is to simulate the cracking of nuclear fuel, which consists of a brittle ceramic material, uranium dioxide, during laser heating experiments. The objective is to compare the numerical results with experimental data through image correlation. This comparison will make it possible to optimize the experimental setup, improve the quality of the experimental results, and move toward a quantitative validation of the gradient damage models used in the simulations.
The starting point of this work is a campaign of uranium dioxide pellet fragmentation by laser heating, carried out as part of the PhD of Hugo Fuentes [1] in one of the experimental laboratories of the Institute for Research on Nuclear Energy Systems for Low-Carbon Energy Production (IRESNE) at CEA Cadarache (DEC/SA3E/LAMIR). This heating technique reproduces temperature gradients representative of reactor conditions. For each test, films showing the evolution of cracks and surface temperature changes in the pellet are available.
These films will be analyzed by digital image correlation (DIC) [3] using an in-house software tool to determine optimal boundary conditions for the numerical simulations and extract relevant data for model validation. The experiments will then be modeled using gradient damage models developed in the PhD theses of David Siedel and Pedro Nava Soto [2]. Based on the results obtained, the PhD candidate will be able to optimize and/or adapt the setup to study other operating conditions and conduct a new experimental campaign.
The PhD student will work in close collaboration between a simulation laboratory and an experimental laboratory within the IRESNE Institute at CEA Cadarache. The proposed work is open-ended and may be promoted through participation in national or international conferences and the publication of scientific articles in high-impact journals.
References
[1] Fuentes, Hugo, Doualle, Thomas, Colin, Christian, Socié, Adrien, Helfer, Thomas, Gallais, Laurent, and Lebon, Frédéric. Numerical and experimental simulation of nuclear fuel fragmentation via laser heating of ceramics. In: Proceedings of Top Fuel 2024, Grenoble, 29 September 2024.
[2] Nava Soto, Pedro, Fandeur, Olivier, Siedel, David, Helfer, Thomas, and Besson, Jacques. Description of thermal shocks using micromorphic damage gradient models. European Solid Mechanics Conference, Lyon, 2025.
[3] Castelier Etienne, Rohmer E., Martin E., Humez B. Utilisation de la dimension temporelle pour ameliorer la
correlation d'images. 20 eme Congres Francais de Mecanique, 2011.
Chemical and mechanical properties of N-A-S-H aluminosilicates of geopolymer
Management of low- and medium-level nuclear waste relies primarily on cements, but their limitations with regard to certain types of waste (reactive metals, oil) require the exploration of new, more effective materials. Geopolymers, particularly those composed of hydrated sodium aluminosilicates (Na2O–Al2O3–SiO2–H2O, or N–A–S–H), appear to be a promising alternative thanks to their chemical compatibility with certain types of waste.
However, despite the growing interest in geopolymers, scientific obstacles remain: 1) The available thermodynamic data on N-A-S-H is still incomplete, making it difficult to predict their long-term stability via modeling, 2) The role of their atomic structure in regard to their reactivity remains unclear, and 3) The links between chemical composition (in terms of Si/Al ratio) and mechanical properties are not established, limiting the representativeness of the models created.
By combining experimentation and modeling in order to link atomic structure and properties, this thesis aims to obtain robust and novel data on the chemical and mechanical properties of N-A-S-H. The thesis is organized around three major objectives: 1) determining the impact of N-A-S-H composition on dissolution and establishing thermodynamic solubility constants, 2) characterizing their atomic structure (aluminols, silanols, and hydrated environments) using advanced NMR spectroscopy, and 3) linking their mechanical properties, measured by nanoindentation, to their structure and composition using molecular dynamics modeling.
Surface technologies for enhanced superconducting Qubits lifetimes
Materials imperfections in superconducting quantum circuits—in particular, two-level-system (TLS) defects—are a major source of decoherence, ultimately limiting the performance of qubits. Thus, identifying the microscopic origin of possible TLS defects in these devices and developing strategies to eliminate them is key to superconducting qubit performance improvement. This project proposes an original approach that combines the passivation of the superconductor’s surface with films deposited by Atomic Layer Deposition (ALD), which inherently have lower densities of TLS defects, and thermal treatments designed to dissolve the initially present native oxides. These passivating layers will be tested on 3D Nb resonators than implemented in 2D resonators and Qubits and tested to measure their coherence time. The project will also perform systematic material studies with complementary characterization techniques in order to correlate improvements in qubit performances with the chemical and crystalline alteration of the surface.
Numerical modelling of large ductile crack progagation and assessment of margins comparing to engineering approach
Predicting failure modes in metal structures is an essential step in analyzing the performance of industrial components where mechanical elements are subjected to significant stress (e.g., nuclear power plant components, pipelines, aircraft structural elements, etc.). To perform such analyses, it is essential to correctly simulate the behavior of a defect in ductile conditions, i.e., in the presence of significant plastic deformation before and during propagation.
Predictive numerical simulation of ductile tearing remains an open scientific and technical issue despite significant progress made in recent years. The so-called local approach to fracture, notably the Gurson model (and its modified version GTN), is widely used to model ductile tearing. However, its use has limitations: significant computation time, simulation stoppage due to the presence of completely damaged elements in the model, and non-convergence of the result when the mesh size is reduced.
The aim of this thesis is to develop the ductile tear simulation model used at LISN so that it can be applied to large crack propagation on complex structures. It also aims to compare the results obtained with engineering methods that are simpler to implement.
Thermal Barrier Coatings with enhanced mechanical properties performed by plasma spraying
Increasing the performance of aircraft gas turbines requires improvements in the materials used in the combustion chamber and on the parts at the outlet of the chamber. Widely used in the aerospace industry, plasma spraying enables the application of low-conductivity ceramic coatings that provide a thermal barrier protection for metal parts. The mechanical stress observed require coatings that are increasingly resistant in mechanical terms. As a result, the thesis will focus on developing plasma-sprayed thermal barrier coatings with increased mechanical strength while maintaining good thermal insulation compared to the state of the art yttria stabilized zirconia thermal barrier coating currently used in gas turbine engines. For example, particular attention will be paid to toughness, which is the ability of a material to resist fracture in the presence of a crack. Factors that can influence toughness include composition, microstructure, and the addition of reinforcements. The use of original solutions, such as bio-inspired ones, is also a possibility.