Numerical and experimental study of cryogenic refrigeration system for HTS-based nuclear fusion reactors
The challenge of climate change and the promise of CO2-free energy production are driving the development of new nuclear fusion reactor concepts that differ significantly from systems such as ITER or JT60-SA [R1]. These new fusion reactors push the technological boundaries by reducing investment and operating costs through the use of high-temperature magnets (HTS) to confine the plasma [R4]. These HTS promise to achieve high-intensity magnetic fields while operating at higher cooling temperatures, thereby reducing the complexity of cryogenic cooling, which is normally achieved by forced circulation of supercritical helium at approximately 4.5 K (see 1.8 K for WEST/Tore Supra) delivered by a dedicated cryogenic plant.
The pulsed operation of tokamaks induces a temporal variation in the thermal load absorbed by the cooling system. This operating scenario has led to the development of several load smoothing techniques to reduce the amplitude of these thermal load variations, thereby reducing the size and power of the cooling system, with beneficial effects on cost and environmental impact. These techniques use liquid helium baths (at approximately 4 K) to absorb and temporarily store some of the thermal energy released by the plasma pulse before transferring it to the cryogenic installation [R5].
The objective of this thesis is to contribute to the development of innovative concepts for the refrigeration of large HTS systems at temperatures between 5 and 20 K. It will include (1) the modeling of cryogenic system and cryodistribution architectures as a function of the heat transfer fluid temperature, and (2) the exploration of innovative load smoothing techniques in collaboration with the multidisciplinary "Fusion Plant" team of the PEPR SUPRAFUSION project. The first part will involve the development and improvement of 0D/1D numerical tools called Simcryogenics, based on Matlab/Simscape [R6], through the implementation of physical models (closure laws) and the selection of appropriate modeling techniques to analyze and compare suitable architectural solutions. The second part will be experimental and will involve conducting load smoothing experiments using an existing cryogenic loop operating between 8 and 15 K.
This activity will be at the forefront of the nuclear fusion revolution currently underway in Europe [R3, R7] and the United States [R4], addressing a wide range of cryogenic engineering fields such as refrigeration technologies, superfluid helium, thermo-hydraulics, materials properties, system and subsystem design, and the design and execution of cryogenic tests. It will thus be useful for the development of new generations of particle accelerators using HTS magnets.
[R1] Cryogenic requirements for the JT-60SA Tokamak https://doi.org/10.1063/1.4706907]
[R2] Analysis of Cryogenic Cooling of Toroidal Field Magnets for Nuclear Fusion Reactorshttps://hdl.handle.net/1721.1/144277
[R3] https://tokamakenergy.com/our-fusion-energy-and-hts-technology/fusion-energy-technology/
[R4] https://tokamakenergy.com/our-fusion-energy-and-hts-technology/hts-business/
[R5] “Forced flow cryogenic cooling in fusion devices: A review” https://doi.org/10.1016/j.heliyon.2021.e06053
[R6] “Simcryogenics: a Library to Simulate and Optimize Cryoplant and Cryodistribution Dynamics”, 10.1088/1757-899X/755/1/012076
[R7] https://renfusion.eu/
[R8] PEPR Suprafusion https://suprafusion.fr/
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)
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.
Three-Dimensional Fine Measurements of Boundary Layers in Turbulent Flows within PWR Fuel Assemblies
The production of electricity through nuclear energy is a key pillar of the energy transition due to its low carbon footprint. In a continuous effort to improve safety and performance, the development of new knowledge and tools is essential.
Fuel assemblies, which are components of a reactor core, face various challenges involving thermo-hydraulic phenomena. These include flow-induced vibrations, power transmission associated with critical fluxes, and fluid-structure interactions in cases of assembly deformation or seismic excitation. In all these situations, the behavior of the fluid near the wall plays a crucial role. The use of Computational Fluid Dynamics (CFD) allows for the simulation of these phenomena with the goal of obtaining predictive tools. The experimental validation needs required by today's simulations push classical measurement techniques to their limits. There is a strong need for refined experimental data in both time and space on complex geometries.
This doctoral project aims to address this need by leveraging the latest advancements in optical measurements for turbulent flows. By combining index matching techniques, panoramic cameras, and Particle Tracking Velocimetry (PTV), it is possible to measure the velocity field in a representative volume (approximately 1 cm³) with a spatial density of around 10 micrometers. This allows for the simultaneous measurement of flow in the boundary layer and the hydraulic channel.
The thesis will primarily be conducted at the Hydromechanics Laboratory (LETH) at the IRESNE Institute (CEA Cadarache) and will involve collaboration with the Thermo-Fluids Lab at George Washington University. Travel to the USA will be required.
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.
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.
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.