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.
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.
Experimental study of the behavior of fission gases in Fast Neutron Reactor fuels irradiated at low power.
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.
Acoustic imaging on irradiated fuel elements : from implementation to interpretation in terms of Metal/Ceramic interface
To improve the flexibility of civil nuclear reactors, many research programs are conducted by CEA in support of the French nuclear plants operator EDF and fuel fabricant FRAMATOME, specifically concerning the behaviour of fuel elements under irradiation.
The fuel elements consist of a metallic cladding and ceramic pellets. In case of power variations, the presence of a gap between the cladding and the pellets, and the bonding between them in case of contact, are fundamental for the mechanical strength of the fuel element.
To complete the current characterizations after irradiation of the pellet-cladding interface, the feasibility of a non-destructive method based on acoustic imaging has been studied and validated.
As a continuation of this first study, the objective of the thesis is to complete instrumentation of an operating measurement bench with the acoustic measurement chain. This bench is located in a cell which is dedicated to examinations of irradiated fuel elements.
The thesis work includes preparation and implementation of a qualification protocol of the acoustic chain, with images acquisitions on irradiated fuel elements. Acoustic signals processing will be developed to correct the effects of the external corrosion layer. Final expected results are images representing axial and azimuthal localization of the contact or gap between the cladding and the pellets, and the fraction of the bonding zones.
The PhD student will be based within IRESNE, an Institute of the CEA localized in Cadarache (in the south of France) which is specialized in the Research for Nuclear Systems for Low Carbon Energy Production. The work will take place in a nuclear installation which is equipped with various tools for multiscale characterization of the irradiated fuel from nuclear or experimental reactors.
This multidisciplinary work will be carried out in collaboration with a team from IES (Institute for Electronic and Systems - CNRS - Montpellier). This team is specialized in acoustic developments from probes to complete imaging systems.
Thanks to the resources and expertise of the two entities CEA and IES, the student will acquire solid skills in the fields of modeling, instrumentation and metrology. Results will be valorised in international publications and communications.
Mechanical behavior of fourth-generation Li-Ion cells, study at the microstructure scale
Competition to increase the energy density of Li-Ion batteries is leading to the consideration of batteries with solid rather than liquid electrolytes. In this regard, sulfur-based electrolytes such as argyrodites are of great interest due to their high ionic conductivity and mechanical properties allowing a simpler manufacturing. Under the effect of lithiation/delithiation cycles, the silicium active particles embedded within this solid electrolyte cause volume variations that can damage the electrode and reduced its lifetime. This is why batteries with solid sulfide electrolytes only cycle properly when kept under pressure. The objective of this thesis is therefore to model these charge-discharge phases pf the battery at the microstructure scale representative of these new solid electrolyte electrodes. At the silicon particle scale, the work will consist of formulating a lithiation-delithiation model based on previous theoretical work and by comparison with available experimental data. Then, 3D models of electrode microstructures consisting of an argyrodite-type solid electrolyte and silicon particles will be established based on existing characterizations (SEM images). Finally, the microscopic mechanical model of lithiation-delithiation will be integrated on these microstructure models, studying in particular the effects of external mechanical loading on the intensity of mechanical interactions at the microstructure scale and the potential locations of damage. These simulation results will be compared with available measurements (macroscopic and local deformation measurements).
These studies will be carried out at CEA Cadarache within the Institute for Research on Nuclear Systems for Low-Carbon Energy Production (IRESNE), in close collaboration with the teams of the Laboratory for Innovation in New Energy Technologies and Nanomaterials (LITEN) at CEA Grenoble.
This framework will allow the PhD student to evolve in a stimulating scientific environment and to promote their research work both in France and abroad through conferences and publications in peer-reviewed journals.
Modeling of water ingression in a severe accident by separate effect testing
Nuclear energy is one of the pillars of the energy transition due to its low carbon footprint. It requires advanced safety studies, particularly regarding hypothetical severe nuclear accidents. These scenarios involve core meltdown and the formation of corium (molten radioactive material magma). Understanding corium behavior is a key element of nuclear safety.
At IRESNE institute of CEA Cadarache, the MERELAVA facility studies accident mitigation strategies by spraying water onto corium from above. A prototypical corium bath (containing depleted uranium) is cooled by water spraying under realistic conditions. This setup allows the study of complex interactions between corium, water, and the sacrificial concrete beneath.
In this context, the water ingression phenomenon plays a central role in corium cooling. During spraying, the solidified crust cracks, water seeps into the cracks and evaporates, significantly increasing the extracted heat flux compared to conduction alone. However, current models poorly describe this mechanism and struggle to predict its impact, mainly due to its highly multi-physical nature.
This thesis aims to study ingression through dedicated experiments on MERELAVA, to characterize the crust and to measure the ingression flux using 3D-printed representative matrices. The goal is to improve the existing physical model, with results compared to more complex experimental data. The thesis will primarily take place in the Severe Accidents experimental laboratory of the IRESNE institute. The candidate should have expertise in fluid mechanics and heat and mass transfer.
What mechano-thermal coupling is necessary for fast transients? Evaluation of the contributions of thermodynamics to irreversible processes.
The Laboratory for the Analysis of Radioelement Migration (LAMIR) at the Institute for Research on Nuclear Systems (IRESNE) of the CEA Cadarache has developed a set of measurement methods to characterize the release of fission products from nuclear fuel during transient thermal transients. For these transients, it is important to simulate the mechanical stresses associated with temperature changes that could lead to fracturing of the tested fuel samples . This thesis focuses on modeling hypothetical and very rapid accidental power transients. Its objective is to implement a new model based on the thermodynamics of irreversible processes (TIP).
The first part of this thesis will aim to validate the thermomechanical coupling model in TIP, which was proposed in our laboratory (https://www.mdpi.com/2813-4648/3/4/33). This will be an essentially analytical approach to establish the orders of magnitude of the various mechanisms involved. The second part will apply this formalism to experimental results obtained during rapid heating experiments using laser beams.
One of the main challenges of numerical simulation with TIP is calculating the temperature and stress fields simultaneously, rather than sequentially as in current models. We will start with a 1D program (in Python or another language) that will be progressively refined. Comparing the results obtained with TIP and with current models will help us identify situations in which TIP-specific couplings must be taken into account to achieve accurate predictions.
The PhD candidate will benefit from the support of experts in thermodynamics, mechanics, and programming. The research will lead to scientific publications and conference presentations. Owing to the diversity of the fields involved, this thesis topic offers excellent career prospects in both industry and academic research.
Lightweight and high-strength metamaterials with innovative architectures manufactured by additive manufacturing for constrained environments
Environmental constraints, rising raw material costs, and the need to reduce carbon footprints drive the development of more porous materials that combine lightness with mechanical strength. Such materials meet the requirements of strategic sectors including aerospace, space, transportation, energy, and high-performance physics instruments.
Mechanical metamaterials, composed of micro-lattice structures produced by 3D printing, offer a unique potential to address these challenges. By tailoring the topology of their internal networks, it becomes possible to achieve stiffness-to-density ratios higher than those of conventional materials and to adapt their architecture to target specific mechanical or functional properties.
This thesis is part of this wave of innovation. It aims to develop ultralight metallic metamaterials whose architecture is optimized to maximize mechanical performance while maintaining isotropy, ensuring predictable behavior using conventional engineering tools, including finite element analysis, numerical simulation, and multiscale approaches. The research builds on the recognized expertise of the CEA, particularly at IRAMIS and IRFU/DIS, in designing isotropic random metastructures and shaping them through metal additive manufacturing.
By combining numerical mechanics, advanced design, multi-process additive manufacturing, and in situ characterization, this thesis seeks to push the current limits of design and fabrication of complex metallic structures.
Multiscale modelling of twinning in tin
Twinning is a displacive deformation mechanism characterized by a continuous deformation of the material. Although widely studied for other industrial materials such as titanium alloys, this inelastic mechanism remains poorly understood and incompletely modeled for complex crystallographic structures. However, due to the reduced number of symmetries in these structures, dislocation slip is insufficient to accommodate deformation in certain loading directions, requiring the activation of twinning. This is the case for tin, which has a tetragonal structure. In particular, twinning contributes significantly to the mechanical response of tin at high strain rates and low temperatures. At intermediate temperatures and strain rates, a competition between dislocation plasticity and twinning plasticity can occur, making it crucial to describe the coupling between these two phenomena. Proposing a better description of this coupling will shed new light on the experimental data available at CEA DAM. The objective of the thesis is to develop a multiscale approach, from molecular dynamics to continuum mechanics, validated by experiments, to converge on a model that describes the behavior of tin over a wide range of temperatures and strain rates.
Development of injectable adhesive hydrogels for the treatment of retinal tears
Retinal tears then detachment, a serious eye condition (20–25 cases per 100,000 in France each year), requires urgent surgery. Current treatments involve removing the vitreous, using gas as a tamponade agent, and sealing tears with laser. However, this method presents drawbacks, including patient restrictions (e.g., prolonged lying down) and complications (e.g., cataracts). Injectable hydrogels are being explored as alternative tamponade agents, but they do not display adhesive properties to suture the tears and laser treatment is still required. Surgical glues have also been tested, but cyanoacrylate-based adhesives are toxic, fibrin-based sealants are hard to use in the eye, and current hyaluronan (HA)-based materials lack sufficient stability and adhesion.
This PhD project aims to develop a sterile, injectable HA-based hydrogel with strong adhesive properties to seal retinal tears. Key requirements include biocompatibility, injectability (30G needle), tissue adhesiveness (1.5–3.7 N), and rapid delivery (within 1 hour). Our group has previously developed an injectable HA hydrogel with dynamic crosslinking, offering long-term stability, biocompatibility, and optical transparency. To confer it with tissue-adhesion properties, two strategies will be tested: (1) addition of tissue-adhesive tannic acid in the hydrogel formulation, or (2) grafting tissue-adhesive groups onto the HA backbone. The hydrogel will be tested for its biocompatibility and adhesiveness in preclinical eye models.
This innovative hydrogel could simplify retinal surgery, reduce complications, lower costs, and improve recovery. Beyond retinal repair, it may have applications in cornea surgery and other medical fields.