Nucleate boiling within porous deposits: study of the coupling between coolant composition and capillary vaporization
In the search of the optimal combination of low-carbon energy sources to address the challenge of climate change, nuclear energy plays a crucial role alongside intermittent renewable energies. In this context, the performance and safety of Pressurized Water Reactors (PWRs), which make up the French nuclear fleet, remain an active and high-value research area.
In these reactors, a subcooled nucleate boiling regime can occur, particularly when the local temperature of the coolant exceeds its saturation temperature. This wall boiling promotes the formation of porous deposits of metallic oxides. Within the porosities of these deposits, gas nuclei can be trapped and lead to the onset of nucleate boiling on these surfaces. The vapor formed through a wick boiling or capillary vaporization mechanism then escapes through the chimneys of the deposit. The chemistry of the coolant affects not only the thermodynamic properties of the fluid (such as saturation temperature and latent heat) but, more importantly, its interfacial properties (surface tension and solid/liquid/gas contact angles). These interfacial properties directly control the capillary forces within the deposits, and thus the onset and dynamics of subcooled boiling. As of today, the influence of coolant chemistry on the initiation and development of subcooled nucleate boiling within porous heated surfaces remains poorly understood.
The objective of this PhD is therefore to systematically study the coupled influence of coolant composition and capillary vaporization on nucleate boiling within porous substrates heated by conduction.
The research will follow an experimental approach to investigate how coolant chemistry affects surface tension and contact angles, in order to characterize fluid wetting on idealized porous substrates. Subcooled convective boiling experiments will also be conducted, with the phenomena characterized by shadowgraphy and fiber-optic thermometry.
The PhD will take place within the Thermal Hydraulics of Core and Circuits Laboratory (LTHC) and the Contamination Control, Coolant Chemistry and Tritium Management Laboratory (LMCT) at CEA IRESNE (Cadarache, France). The work will be supervised by Prof. Benoît Stutz of the University of Savoie Mont Blanc. Throughout this project, the doctoral student will develop expertise in interfacial physico-chemistry and two-phase thermohydraulics through the observation, characterization, and modeling of complex multiphysics phenomena.
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.
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.
Modeling and dynamic studies of a space Nuclear Electric Propulsion system
Nuclear technology is key to enabling the establishment of scientific bases on the Moon or Mars, or for exploring deep space. Its use can take several forms (RTG, NTP among others), but this thesis focuses on Nuclear Electric Propulsion (NEP): heat produced by a nuclear reactor is converted into electricity to power an ionic propulsion engine. Various concepts have been studied in the past (PROMETHEUS, MEGAHIT and DEMOCRITOS, typically for Jupiter satellite exploration missions), while currently design studies are underway at CEA for a 100 kWe nuclear-electric NEP system.
The system of interest combines several specific design choices: uranium nitride fuel, direct gas cooling (helium-xenon mixture) and energy conversion system based on a Brayton cycle, as well as waste heat evacuation through thermal radiation. These choices address requirements to minimize mass and volume, and to ensure performance and reliability for the duration of the scientific mission. Analysis of the dynamic behavior of the nuclear-electric system is therefore crucial for project success. However, the issue of transient modeling of a complete spatial nuclear-electric system is very poorly addressed in the state of the art, especially for NEP.
The thesis objectives are therefore to research and develop physical models adapted to a NEP system, to propose an approach for their validation, and finally to implement them to analyze the dynamic behavior of the reactor and contribute to improving its design. Several mission phases will be studied: reactor startup in space, power variation transients for the ionic propulsion engine, reactor response in case of failure, and its potential shutdown with the problem of safe residual power evacuation.
The thesis will be conducted at IRESNE Institute (CEA Cadarache), in a stimulating scientific environment, and integrated into a team designing innovative nuclear reactors. CNES will also be involved in monitoring the work, particularly to define the ionic propulsion engine characteristics and exploration missions of interest for the nuclear-electric system. The thesis topic, combining modeling, fluid mechanics, thermodynamics, neutronics, and space mechanics, will lend itself to scientific communication and allow the development of key skills for an academic or industrial career.
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.
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.
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.