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.
Fluid-structure coupling with Lattice-Boltzmann approach for the analysis of fast transient dynamics in the context of hydrogen risk
With a view to preparing for the future in the field of high-fidelity, high-performance simulation, the CEA is working with its academic and industrial partners to explore the potential of fluid-structure couplings involving Lattice Boltzmann Methods (LBM). The coupling is part of an open-source standard promoted by the CEA, and promising first steps have been taken for compressible flows interacting with structures undergoing large displacements and rupture. Significant obstacles remain to be overcome, particularly for more complex fluid representations that are representative of industrial needs, especially for the safety of carbon-free energy devices such as batteries and nuclear reactors.
This doctoral work therefore focuses on extending the available basic building blocks to the case of flame propagation in hydrogen/air mixtures, in deflagration and detonation regimes with possible transition between the two, and in interaction with flexible structures undergoing finite displacement. This presupposes, in particular, the consideration of compressible flows with high Mach numbers significantly exceeding those used to date, requiring an in-depth reanalysis of coupling schemes and fluid-structure interaction techniques.
The thesis will be part of a collaboration between the IRESNE Institute (CEA Cadarache) and the M2P2 laboratory (AMU). The work will be mostly localized at M2P2 with a close methodological supervision from IRESNE, especially in the field of coupling techniques.
DEM-LBM Coupling for simulating the ejection of immersed granular media in compressible Fluid under High Pressure Gradients
In Pressurized Water Reactors (PWRs), the fuel consists of uranium oxide (UO2) pellets stacked in metallic cladding. During a Loss of Coolant Accident (LOCA) scenario, the rapid temperature increase can cause deformation and sometimes rupture of these claddings. This phenomenon can potentially lead to the ejection of fuel fragments into the primary circuit. This phenomenon is known as FFRD (Fuel Fragmentation, Relocation, and Dispersal). Since the cladding is the first safety barrier, it is crucial to evaluate the amount of dispersed fuel. Experimental studies have shown that the size, shape of the fragments, shape of the breach, and internal pressure significantly influence the ejection. However, the speed of the initial depressurization phase makes direct measurements difficult. Numerical approaches, particularly through fluid-grain coupling (LBM-DEM), offer a promising alternative. The IRESNE Institute at CEA Cadarache, through the PLEIADES software platform, is developing these tools to model the behavior of fragments. However, the compressibility of the gas needs to be integrated to accurately reproduce the initial depressurization. In this context, the laboratory M2P2 of the CNRS, a specialist in modeling compressible flows with the LBM method and developer of the ProLB software, brings its expertise to integrate this effect. The thesis therefore aims to design and improve a compressible model in the LBM-DEM coupling, to conduct a parametric study, and to develop a 3D HPC demonstrator capable of leveraging modern supercomputers.
This CEA thesis will be conducted in close collaboration between the Fuels Research Department (DEC) of the IRESNE Institute at CEA Cadarache and the laboratory M2P2 (CNRS). You will be primarily located at M2P2 but will make regular visits to CEA within the Fuel Simulation Laboratory, to which you will be affiliated. The approaches developed in this thesis ensure a high scientific level with numerous potential industrial applications both within and outside the nuclear field.
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)
Constrained geometric optimization of immersed boundaries for thermal-hydraulic simulations of turbulent flow in a finite-volume approach
The technical issue underpinning this thesis topic is the mitigation of the consequences of a loss of primary coolant accident in a pressurized water reactor with loops. It is of the utmost importance to minimize the flow of water leaving the vessel and to manage the available cold water reserves for safety injections as effectively as possible, in order to prevent or delay core flooding, overheating, and possible core degradation. To this end, the use of passive devices operating on the principle of hydraulic diodes, such as vessel flow limiters or advanced accumulators, is being considered. The subject of this thesis is the geometric optimization of this type of device, described by an immersed boundary, in order to maximize its service efficiency.
Several recent theses have shown how to introduce the Penalized Direct Forcing (PDF) immersed boundary method into the TRUST/TrioCFD software, under various spatial discretizations and for laminar and turbulent regimes. Similarly, they have ruled on the possibilities of deterministic geometric optimization in the finite-element context during simulations, based on the use of the PDF method.
After a bibliographic study of this kind of method, we will focus on the possibilities of implementation in finite volume discretization, the consideration of constraints, and the comparison to reference calculations. The latter will be carried out on academic and industrial configurations (accumulators and flow limiters).
The doctoral student will work in a R&D unit on innovative nuclear system within the IRESNE Institute (CEA Cadarache. He will develop skills in fluid mechanics and numerical methods.
Development of an autonomous module for glass alteration modeling and its coupling with reactive transport codes
In the context of the sustainable and safe use of nuclear energy within a carbon-free energy mix that addresses the climate emergency, managing radioactive waste inventory is a priority concern. The alteration of nuclear glass therefore directly affects the long-term assessment of the safety of geological storage of this waste. Understanding and simulating these processes is therefore a major scientific, industrial, and societal challenge. Existing models, such as GRAAL2 [1] developed at the CEA, capture the passivation mechanisms governing glass alteration, bridging nanometric processes to mesoscopic scale through mesoscopic-scale kinetic laws used in reactive transport codes (RTC).
This PhD aims to develop an autonomous glass module (GM) based on the GRAAL2 model, capable of computing glass alteration kinetics and interfacing with different reactive transport codes (HYTEC, CRUNCH…). The main objectives are: (i) to design and implement a kinetic module, (ii) to develop a coupling interface managing information exchange with RTC, (iii) to define and carry out numerical validation campaigns on reference test cases for both the GM and the coupler, and (iv) to perform sensitivity and uncertainty analyses to identify the key parameters controlling glass behavior in a multi-material context (glass, iron, clay).
The PhD will take place at the Laboratory for Environmental Transfer Modeling (LMTE), within the IRESNE Institute (CEA, Cadarache site, Saint-Paul-lès-Durance). The project will provide the PhD candidate with cross-disciplinary skills in geochemistry, multiphysics coupling, and scientific software development, opening career opportunities in both academic research and nuclear/environmental engineering.
References:
[1] M. Delcroix, P. Frugier, E. Geiger, C. Noiriel, The GRAAL2 glass alteration model: initial qualification on a simple chemical system, Npj Mater Degrad 9 (2025) 38. https://doi.org/10.1038/s41529-025-00589-4.
Proximal primal-dual method for joint estimation of the object and of unknown acquisition parameters in Computed Tomography.
As part of the sustainable and safe use of nuclear energy in the transition to a carbon-free energy future, the Jules Horowitz research reactor, currently under construction at the CEA Cadarache site, is a key tool for studying the behaviour of materials under irradiation. A tomographic imaging system will be exploited in support of experimental measures to obtain real-time images of sample degradation. This imaging system has extraordinary characteristics due to its geometry and to the size of the objects to be characterized. As a result, some acquisition parameters, which are essential to obtain a sufficient image reconstruction quality, are not known with precision. This can lead to a significant degradation of the final image.
The objective of this PhD thesis is to propose methods for the joint estimation of the object under study and of the unknown acquisition parameters. These methods will be based on modern convex optimization tools. This thesis will also explore machine learning methods in order to automate and optimize the choice of hyperparameters for the problem.
The thesis will be carried out in collaboration between the Marseille Institute of Mathematics (I2M CNRS UMR 7373, Aix-Marseille University, Saint Charles campus) and the Nuclear Measurement Laboratory of the IRESNE institute of the French Alternative Energies and Atomic Energy Commission (CEA Cadarache, Saint Paul les Durance). The doctoral student will work in a stimulating research environment focused on strategic questions related to non-destructive testing. He or she will also have the opportunity to promote his or her research work in France and abroad.
Modeling of Critical Heat Flux Using Lattice Boltzmann Methods: Application to the Experimental Devices of the RJH
LBM (Lattice Boltzmann Methods) are numerical techniques used to simulate transport phenomena in complex systems. They allow modeling fluid behavior in terms of particles moving on a discrete grid (a "lattice"). Unlike classical methods, which solve the differential equations of fluids directly, LBM simulate the evolution of the fluid particle distribution functions in a discrete space using propagation and collision rules.
The choice of lattice in LBM is a crucial step, as it directly affects the accuracy, efficiency, and stability of the simulations. The lattice determines how fluid particles interact and move through space, as well as how the discretization of space and time is performed.
LBM methods exhibit a natural parallelism because the computations at each grid point are relatively independent. Compared to classical CFD methods, LBM can better capture certain complex phenomena (such as multiphase, turbulent, or porous media flows) because they rely on a mesoscopic modeling of the fluid, directly derived from particle kinetics, rather than on a macroscopic resolution of the Navier–Stokes equations. This approach allows for a finer representation of interfaces, nonlinear effects, and local interactions, which are often difficult to model accurately using classical CFD methods. LBM therefore enables the capture of complex phenomena at a lower computational cost. Recent studies have notably shown that LBM can reproduce the Nukiyama boiling curve (pool boiling) and, consequently, accurately calculate the critical heat flux. This flux corresponds to a bulk boiling, known as a boiling crisis, which results in a sudden degradation of heat transfer.
The critical heat flux is a crucial issue for the experimental devices (DEX) of the Jules Horowitz Reactor, as they are cooled by water either via natural convection (fuel capsule-type devices) or forced convection (loop-type devices). Thus, to ensure the proper cooling of the DEX and reactor safety, it is essential to verify that the critical heat flux is not reached within the studied parameter range. It must therefore be determined with precision. Previous studies conducted on a fuel-capsule-type DEX using the NEPTUNE-CFD code (classical CFD methods) have shown that modeling is limited to regions far from the critical heat flux. In general, flows with high void fractions (greater than 10%) cannot be easily resolved using classical CFD approaches.
The student will first define a lattice to apply LBM to a RJH device under natural convection. They will consolidate the results obtained for the critical heat flux on this configuration by comparing them with available data. Finally, exploratory calculations under forced convection (laminar to turbulent regime) will be conducted.
The student will be hosted at the IRESNE institute.
Electrical impédanceTomography for the Study of Two-Phase Liquid Metal/Gas Flows
As part of the sustainable use of nuclear energy within a carbon-free energy mix in combination with renewable energies, fourth-generation fast neutron reactors are crucial for closing the fuel cycle and controlling uranium resources. Ensuring the safety of such a sodium-cooled reactor relies for a significant part on the early detection of gas voids in their circuits. In these opaque and metallic environments, optical imaging methods are ineffective, making it necessary to develop innovative techniques.
This PhD project is part of the development of Electrical Impedance Tomography (EIT) applied to liquid metals, a non-intrusive approach enabling the imaging of local conductivity distributions within a flow.
The work will focus on the study of electromagnetic phenomena in two-phase metal/gas systems, in particular the skin effect and eddy currents generated by oscillating fields.
Artificial-intelligence approaches, such as Physics-Informed Neural Networks (PINNs), will be explored to combine numerical learning with physical constraints and will be compared with purely numerical simulations.
The objective is to establish refined physical models adapted to metallic environments and to design inversion methods robust against measurement noise.
Experiments on Galinstan will be conducted to validate the models and demonstrate the feasibility of detecting gas inclusions in a liquid metal.
This research, carried out at IRESNE Institute of CEA Cadarache, will open new perspectives in electromagnetic imaging for opaque, highly conductive media.
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.