Shape optimization for innovation in nuclear fuels
Nuclear industry is currently developping enhanced Accident Tolerant Fuels" (ATF) [1]. These fuels feature enhanced physical properties; in particular, thanks to the addition of thermal conductors inside the fuel, they tend to be colder in standard as well as in accident conditions.
This thesis aims at developping numerical strategies (that will be programmed into a semi-industrial code) in order to propose new "shapes" of fuels (by "shape", we mean internal structures or microstructures), and to optimze already existing concepts. It will take advantage of recent numerical and mathematical techniques related to the so-called "shape optimization" [2]. Based on the previous work [3], more and more complex physical phenomena will be taken into account : first, thermal conductivity and mechanical behaviour in standard conditions, then gaz diffusion... Discussion with experts and modelization will be necessay in order to reformulate these physical behaviours into forms amenable to numerical simulation.
This thesis will take place at the CEA center of Cadarache in the fuel research department, in a laboratory devoted to modelling and numerical methods. The latter is affiliated to the Institute IRESNE for the research low-carbon energy production.
This project will be in collaboration with Nice University offering so an environment both academic and connected to application.
It also takes part in the PEPR DIADEM called Fast-in-Fuel, a national research project.
We search for excellent candidates with a solid background in scientific computing, analysis and numerical analysis of partial differential equations, as well as in optimization. Skills in physics (mechanics and thermics) will also be considered. The proposed subject aims at a concrete application at the intersection of various scientific fields, and it is largely exploratory. Hence, curiosity and creativity will also be highly appreciated.
[1] Review of accident tolerant fuel concepts with implications to severe accident progression and radiological releases, 2020.
[2] G. Allaire. Shape optimization by the homogenization method, volume 146 of Applied Mathematical Sciences. Springer-Verlag, New York, 2002.
[3] T. Devictor. PhD Manuscript, 2025 (in preparation)
Experimental Investigation and DEM Simulation of Actinide Powder Segregation During Transfer Processes
The fabrication of nuclear fuels based on actinide oxides (UO2, PuO2) involves numerous powder-handling operations, during which segregation phenomena may occur. These phenomena—arising from differences in particle size, shape, density, or surface condition—directly affect the homogeneity of the mixtures and, consequently, the quality and consistency of the resulting fuel pellets. Controlling these effects is therefore a major industrial challenge to ensure both process robustness and final product conformity.
This PhD project aims to deepen the understanding of the mechanisms driving powder de-mixing of UO2 during transfer stages, particularly during vibratory conveyor transport and gravitational discharge. The main scientific objective is to establish the relationship between the physical and rheological properties of the powders, the process operating conditions, and the intensity of the observed segregation phenomena. The work will combine experimental studies and numerical simulations using the Discrete Element Method (DEM) to identify the material and process parameters influencing segregation. Experimental setups will be developed to characterize the powders and quantify the degree of de-mixing, while simulations will serve to validate and extrapolate the experimental observations.
Conducted at CEA Cadarache, within the Uranium Fuel Laboratory (LCU) of the Institute for Research on Nuclear Systems for Low-Carbon Energy Production (IRESNE), and in collaboration with the TIMR laboratory at UTC, this project will provide recommendations to limit segregation during industrial operations and improve the prediction of segregation tendencies in powder mixtures, particularly in cohesive actinide powders.
The PhD candidate will disseminate their findings through publications and conference presentations. They will also have the opportunity to learn and refine several transferable techniques applicable to a wide range of materials science and engineering contexts.In particular, the issues related to the physics of granular materials, which form the core of this thesis, are of significant industrial relevance and are shared by many other sectors handling powders, such as the pharmaceutical, food processing, and powder metallurgy industries.
Impact of the porosity on the MOX (U,Pu)O2 fuel
The nuclear fuel performances depend on their thermomecanical behaviors and, therefore, their thermal conductivity. This property varies significantly with high porsity levels especially in mixed oxided (composed of uranium and plutonium) used in fast ractors.
The aim of this thesis is to assess the impact of the pore qualities and shapes on the thermal conductivity on fissile materials and to propose a thermal conductivity law depending of the quantity, the length, the shape and the interconnectivity of its porosity. To reach this goal, recent measures on thermal properties are in progress by laser heating, allowing a better understanding of the fuel behavior in temperature ranges mostly unexplored like very high temperatures (until 2500°C), are in progress in the european research center (JRC) in Karlsruhe. These measures are performed on materials with different microstructures. These measures will be interpreted from thermograms and compared to simulation results (image analysis, converting 2D image in a 3D problem, TM-FFT)[1].
This thesis will take place in the French Institute for Research on Nuclear Systems for Low-Carbon Energy production (IRESNE) in the Expertise and Validation on multi-fuel Applications Laboratory (LEVA). LEVA is part of the Fuel Study and Simulation Department and its missions consist of :
- Answer to industrial demands by providing studies ;
- Validation of the Scientific Calculation Tools (OCS) of the PLEIADES plateform ;
- Enhance the fuel behavior understanding ;
- Manage the Fuel databases.
Finally, the collaboration with JRC Karlsuhe will be a chance to work within an international framework which also is a strenght of LEVA.
This work will be valorized through conferences participations and publications in peer-reviewed journals. Furthermore, the PhD student will have the possibility to acquire or strengthen some technical skills (experimental data interpretation, modelling) applicable in various fields of material science and engineering.
[1] This work forms a natural extension of the PHD thesis "The Thermal conductivity of mixed oxide fuel (MOX) : effect of temperature, elementary chemical composition, microstructure and burn-up in reactor" - TEL - Thèses en ligne.
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.
Radiological signatures in Antarctica: development and validation of analytical methodologies
Hosted by the IRESNE Institute at the CEA-Cadarache center, the PhD student will contribute to the analytical development of the Laboratoire d’Analyses Radiochimiques et Chimiques (LARC), which has provided expert analytical support for over 60 years in the fields of nuclear reactors, fuel cycle, waste management, and decommissioning. The main objective of the project is to develop and optimize analytical methods for detecting radiological markers through collaborations with internal (LANIE, LEXAN) and external (CSIC, CIEMAT) partners. The analyses will focus on 137Cs and 210Pb using gamma spectrometry, uranium and plutonium isotopes using MC-ICPMS, and overall alpha/beta activity using liquid scintillation. In a second phase, these methods will be applied to a variety of samples, including those collected in Antarctica as part of the GEOCHEM project [1], in order to investigate the spatial distribution and origin of these radiological markers [2].
By the end of this multidisciplinary PhD project, the student will have gained solid experience in measuring gamma, alpha, and beta radiation. Additionally, interpreting the analytical results in connection with environmental parameters will develop critical thinking skills and foster scientific curiosity.
[1] Maestro, A. et al. Fracturation pattern and morphostructure of the Deception Island volcano, South Shetland Islands, Antarctica. Antarct. Sci. 37, 176–200 (2025).
[2] Xu-Yang, Y. et al. Radioactive contamination transported to Western Europe with Saharan dust. Sci. Adv. 11, eadr9192 (2025).
Towards a new iterative approach for the efficient modeling of mechanical contact
As part of the modeling and simulation of nuclear fuel behavior across different reactor types, the Institute for Research on Nuclear Energy Systems for Low-Carbon Energy Production (IRESNE) at CEA Cadarache, in partnership with various industrial and academic stakeholders, is developing the PLEIADES software platform for fuel behavior simulation. In this context, the interaction between the fuel and its cladding, the first containment barrier, is a key phenomenon for understanding and predicting the behavior of fuel elements.
The modeling and numerical simulation of mechanical contact phenomena represent a major scientific and technological challenge in solid mechanics, due to the intrinsic complexity of the problem, characterized by its highly nonlinear and non-smooth nature.
To overcome the limitations of classical approaches, such as the penalty or Lagrange multiplier methods, new contact resolution strategies based on iterative fixed-point schemes are currently being explored at the CEA. These approaches offer several advantages: they avoid the direct solution of complex and ill-conditioned systems, significantly improve numerical efficiency, and exhibit very low sensitivity to algorithmic parameters, making them particularly well suited for high-performance computing (HPC) environments.
The objective of this PhD work is to extend these strategies to more complex and realistic situations, by taking into account nonlinear material behaviors and incorporating more sophisticated contact laws, such as friction. Depending on the progress of the work, the final phase will focus on transferring the developments to a high-performance computing (HPC) environment, using a parallel finite element solver.
The project will benefit from internationally recognized expertise in mechanics, applied mathematics, and nuclear fuel simulation, with supervision from CEA researchers and additional academic collaborations (CNRS).
[1] P. Wriggers, "Computational Contact Mechanics", Springer, 2006. doi:10.1007/978-3-540-32609-0.
[2] V. Yastrebov, "Numerical Methods in Contact Mechanics", ISTE Ltd and John Wiley & Sons, 2013. doi: 10.1002/9781118647974
[3] I. Ramière and T. Helfer, “Iterative residual-based vector methods to accelerate fixed point iterations”, Computers & Mathematics with Applications, vol. 70, no. 9, pp. 2210–2226, 2015. doi: 10.1016/j.camwa.2015.08.025.
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.
Investigation of Fuel Damage under Reactivity-Initiated Accident Conditions Using Laser Heating: Correlation with Fission Gas Release
High-power laser heating is an experimental technique developed within the Fuel Study Department that allows the induction of thermal transients on nuclear ceramic samples. It notably makes it possible to reproduce, at the laboratory scale, the thermomechanical conditions representative of an incidental or accidental sequence, in order to study fundamental mechanisms such as fuel cracking or fragmentation.
Indeed, in certain situations, such as a thermal transient of the Reactivity-Initiated Accident (RIA) type, fuel fragmentation (or over-fragmentation) can lead to the release of fission gases and ultimately result in the rupture of the fuel rod cladding.
This type of transient is particularly characterized by a complex spatiotemporal evolution of temperature within the fuel, which is difficult to reproduce at the laboratory scale. To date, only high-power laser heating techniques make it possible to replicate the heating rates reached during such transients and to reproduce the thermomechanical conditions of an RIA at the scale of a manipulable sample in the laboratory.
In this context, the PhD project aims to provide experimental data related to fuel fragmentation and over-fragmentation under Reactivity-Initiated Accident conditions. To achieve this, the student will be required to improve and develop the existing experimental setup and perform experiments aimed at reproducing the thermomechanical conditions leading to fuel fragmentation. A combined experimental/modeling approach will be necessary to optimally design and interpret the experiments. The data obtained will be used to validate the fragmentation models developed at CEA and should also allow projections for integrating these experimental techniques into shielded cells.
The PhD will be conducted within a collaborative framework (CHAIRE MATLASE) between LAMIR (Laboratory for the Analysis of Radionuclide Migration) within the Institute for Research on Nuclear Systems for Low-Carbon Energy Production (IRESNE) at CEA Cadarache, and the ILM team (Laser-Matter Interaction) at the Institut Fresnel in Marseille. The latter will provide expertise in high-power laser/material interactions and optical instrumentation for the development of the system and complex optical diagnostics.
This environment will allow the doctoral student to work in a stimulating scientific setting and to disseminate their research both in France and internationally, through conferences and publications in peer-reviewed journals.
[1]M. Reymond, J. Sercombe, L. Gallais, T. Doualle, and Y. Pontillon, ‘Thermo-mechanical simulations of laser heating experiments on UO2’, Journal of Nuclear Materials, vol. 557, 2021, doi: 10.1016/J.JNUCMAT.2021.153220.
[2]M. Reymond et al., ‘High power laser heating of nuclear ceramics for the generation of controlled spatiotemporal gradients’, J Appl Phys, vol. 134, no. 3, p. 33101, Jul. 2023, doi: 10.1063/5.0146541.
[3]Hugo Fuentes et al., ‘Numerical and experimental simulation of nuclear fuel fragmentation via laser heating of ceramics’, TopFuel 2024. Accessed: Oct. 02, 2025. [Online]. Available: https://www.researchgate.net/publication/386167297_Numerical and experimental simulation of nuclear fuel via laser heating of ceramics
Simulation of crack initiation and propagation in random heterogeneous materials
This PhD thesis is concerned with cracking in nuclear fuels at the microstructure level, a phenomenon that is essential to understand in order to model the behavior of materials under irradiation. Indeed, crack initiation and propagation can lead to the release of fission gases and the formation of fragments inducing fissile matter displacement. Current industrials models are based on simplified representations of the porous microstructure and empirical fracture criteria, which limits their physical accuracy and validation by separate effects.
To overcome these limitations, the proposed thesis work consists of using multi-scale approaches and high-performance computing (HPC) finite element simulations. The main objectives are to define a Representative Volume Element (RVE) for crack initiation in materials with random porosity, improve the failure criteria used in legacy codes and define their uncertainties, and finally establish the domain of validity for analyzing crack propagation in the RVE.
The first line of research consists of rigorously defining the size of the RVE based on local physical variables such as the maximum principal stress. Variance reduction methods will be used to optimize the number of calculations required and estimate the associated errors.
In a second step, simulations performed to determine the RVE size will be used to improve industrial models. The approach will seek to separate the mechanical effects of an isolated bubble from those resulting from interactions between neighboring bubbles. Machine learning techniques may be used to develop this new model. Validation will be based on indirect measurements of cracking, such as gas release observed during thermal annealing, particularly for high burn-up structure (HBS) fuels, where legacy models fail to predict the kinetics of cracking.
Finally, crack propagation within the RVE will be studied using 3D phase field simulations, which allow for detailed representation of the various stages after the crack initiation. The influence of boundary conditions on the RVE will be examined by comparison with simulations on larger domains.
The thesis will be carried out at the Institute for Research on Nuclear Systems for Low-Carbon Energy Production (IRESNE) of the CEA Cadarache, within the PLEIADES platform development team, which is specialized in fuel behavior simulation and multiscale numerical methods. It will be conducted in collaboration with the CNRS/LMA as part of the MISTRAL joint laboratory, notably on aspects relating to the analysis of random medium representativeness and micromechanical simulation of crack propagation.