IO access scheduling on magnetic tapes using machine learning
Numerical simulations are used to obtain responses to physical phenomena that
cannot be reproduced, either because they are too dangerous or too expensive.
The models used for these simulations are increasingly complex, in terms of
size and precision, and require access to increasingly large computing and
data storage capacities. To this end, and in order to optimize costs, the use
of mass storage technologies such as magnetic tapes is critical. However, to
ensure good overall system performance, the development of algorithms and
mechanisms related to data placement and tape access scheduling is essential.
The objective of the thesis is to study the technology of magnetic tapes, as
well as existing mechanisms such as RAO (Recommended Access Order) or request
retention; and to implement new strategies for the optimization of magnetic
tape performance.
Towards automated and reconfigurable microfluidic platforms for the study and development of nuclear fuel recycling processes
The main objective of this PhD project is the design and development of the first automatic and reconfigurable microfluidic platform dedicated to research and development on the nuclear fuel cycle. In a context where mastering nuclear processes remains a key challenge, both for energy production and for the sustainable management of nuclear materials, microfluidic devices represent a particularly promising approach. These autonomous laboratories on a chip have already demonstrated their potential in various fields, such as chemistry, materials science, and biology. Their application to nuclear processes would help reduce radiation exposure risks, minimize waste generation, and optimize resources by enabling a larger number of experiments to be performed safely, quickly, and reproducibly. For about a decade, the DMRC has been conducting phenomenological studies on the main stages of the nuclear process (dissolution, solvent extraction, precipitation, etc.) using microfluidic devices. In parallel, it has developed PhLoCs (Photonic-Lab-on-Chips), which allow the miniaturization of several analytical techniques (UV-Vis spectroscopy, LES, holography, etc.) and their integration for online monitoring of the investigated phenomena. Nevertheless, no truly autonomous and fully automated platform currently exists that combines process execution with integrated analytical monitoring.
The aim of this PhD work is therefore to make a decisive step by designing a modular device where several functional chips can be assembled, some dedicated to process operations (e.g., uranium/plutonium separation) and others to online measurements, within a flexible configuration adapted to nuclear environments. In addition, the research will focus on integrating new instrumental techniques directly on chips, such as FTIR and UV-Vis-NIR spectroscopies, which are crucial for studying critical process steps, including solvent degradation. This project thus aims to establish the foundations of next-generation microfluidic platforms that combine safety, modularity, and performance to advance nuclear fuel cycle research. At the end of the PhD, the candidate will have developed unique expertise in microfluidics applied to nuclear processes, combining optical instrumentation and automation. These skills will offer strong career opportunities in research and advanced process engineering.
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.
Representation of Cross Sections based on the Wavelet Expansion Method, and Development of a Dedicated Solver
The deterministic solution of the neutron transport equation traditionally relies on the use of the multigroup approximation to discretize the energy variable. The energy domain is divided using a one-dimensional mesh, where the volume elements are called "groups" in neutronics. Within each group, all physical quantities (neutron flux, cross sections, reaction rates, etc.) are projected using piecewise constant functions. This homogenization of cross sections, which are the input data of the transport equation, becomes particularly challenging in the presence of resonant nuclei, whose cross sections vary rapidly over several decades. Correcting for this requires computationally expensive on-the-fly treatments to improve the accuracy of the transport solution.
The goal of this thesis is to eliminate the need for the multigroup approximation in the resonant energy range by applying a Galerkin projection of the continuous energy equation onto an orthonormal wavelet basis. The candidate will develop a generic expansion method adapted to mixtures of resonant isotopes, including preprocessing of cross sections, selection of the wavelet basis, and determination of an efficient coefficient truncation strategy. A dedicated neutron transport solver will be developed, with a focus on efficient algorithmic implementation using advanced programming techniques suited to modern architectures (GPU, Kokkos). The results of this thesis research will be valorized through publications in peer-reviewed international journals and presentations at scientific conferences.
Thermal Barrier Coatings with enhanced mechanical properties performed by plasma spraying
Increasing the performance of aircraft gas turbines requires improvements in the materials used in the combustion chamber and on the parts at the outlet of the chamber. Widely used in the aerospace industry, plasma spraying enables the application of low-conductivity ceramic coatings that provide a thermal barrier protection for metal parts. The mechanical stress observed require coatings that are increasingly resistant in mechanical terms. As a result, the thesis will focus on developing plasma-sprayed thermal barrier coatings with increased mechanical strength while maintaining good thermal insulation compared to the state of the art yttria stabilized zirconia thermal barrier coating currently used in gas turbine engines. For example, particular attention will be paid to toughness, which is the ability of a material to resist fracture in the presence of a crack. Factors that can influence toughness include composition, microstructure, and the addition of reinforcements. The use of original solutions, such as bio-inspired ones, is also a possibility.
Modeling and characterization of CFET transistors for enhanced electrical performance
Complementary Field Effect Transistors (CFETs) represent a new generation of vertically stacked CMOS devices, offering a promising path to continue transistor miniaturization and to meet the requirements of high-performance computing.
The objective of this PhD work is to study and optimize the strain engineering of the transistor channel in order to enhance carrier mobility and improve the overall electrical performance of CFET devices. The work will combine numerical modeling of technological processes using finite element methods with experimental characterization of crystalline deformation through transmission electron microscopy coupled with precession electron diffraction (TEM-PED).
The modeling activity will focus on predicting strain distributions and their impact on electrical properties, while accurately accounting for the complexity of the technological stacks and critical fabrication steps such as epitaxy. In parallel, the experimental work will aim to quantify strain fields using TEM-PED and to compare these results with simulation outputs.
This research will contribute to the development of dedicated modeling tools and advanced characterization methodologies adapted to CFET architectures, with the goal of improving spatial resolution, measurement reproducibility, and the overall understanding of strain mechanisms in next-generation transistors.
Electron beam probing of integrated circuits
The security of numerical systems relies on cryptographic chains of trust starting from the hardware up to end-user applications. The root of chain of trust is called a “root of trust” and takes the form a dedicated Integrated Circuit (IC), which stores and manipulates secrets. Thanks to countermeasures, those secrets are kept safe from extraction and tampering from attackers.
Scanning Electron Microscope (SEM) probing is a well-known technique in failure analysis that allows extracting such sensitive information. Indeed, thanks to a phenomenon known as voltage contrast, SEM probing allows reading levels of transistors or metal lines. This technique was widely used in the 90s on ICs frontside, but progressively became impractical with the advance of manufacturing technologies, in particular the increasing number of metal layers. Recent research work (2023) showed that SEM-based probing was possible from the backside of the IC instead of frontside. The experiments were carried-out on a quite old manufacturing technology (135 µm). Therefore, it is now essential to characterize this threat on recent technologies, as it could compromise future root of trusts and the whole chains of trust build on top of them.
The first challenge of this PhD is to build a reliable sample preparation process allowing backside access to active regions while maintaining the device functional. The second challenge is to characterize the voltage contrast phenomenon and instrument the SEM for probing active areas. Once the technique will be mature, we will compare the effect of the manufacturing technology against those threats. The FD-SOI will be specifically analyzed for potential intrinsic benefits against SEM probing.
Scaling Up Dislocation Dynamics Simulations for the Study of Nuclear Material Aging
Materials used in nuclear energy production systems are subjected to mechanical, thermal, and irradiation condition, leading to a progressive evolution of their mechanical properties. Understanding and modeling the underlying physical mechanisms involved is a significant challenge.
Dislocation Dynamics simulation aims to understand the behavior of the material at the crystal scale by explicitly simulating the interactions between dislocations, microstructure, and crystal defects induced by irradiation. The CEA, CNRS, and INRIA have been developing the NUMODIS calculation code for this purpose since 2007 (Etcheverry 2015, Blanchard 2017, Durocher 2018).
More specific work on zirconium alloys (Drouet 2014, Gaumé 2017, Noirot 2025) has allowed the validation and enhancement of NUMODIS's ability to handle these individual physical mechanisms by directly comparing them with experiments, through in situ tensile tests under a transmission electron microscope. However, these studies are limited by the current inability of the NUMODIS code to handle a sufficiently high and representative number of defects, and thus to obtain the mechanical behavior of the grain (~10 microns).
The objective of the proposed work is to implement new algorithms to extend the functionalities of the code, propose and test new numerical algorithms, parallelize certain parts still processed sequentially, and ultimately demonstrate the code's ability to simulate the deformation channeling mechanism in an irradiated zirconium grain.
The work will focus primarily on algorithms for calculating velocities, junction formation, and time integration, requiring both mastery of dislocation physics and the corresponding numerical methods. Algorithms for integration recently proposed by Stanford University and LLNL will be implemented and tested for this purpose.
Significant work will also be devoted to adapting the current code (hybrid MPI-OpenMP parallelism) to new computing machines that favor GPU processors, through the adoption of the Kokkos programming model.
Building on both previous experimental and numerical work, this study will conclude with the demonstration of NUMODIS's ability to simulate the channeling mechanism in an irradiated zirconium grain and to identify or even model the main physical and mechanical parameters involved.
At the interface between several fields, the candidate must have a good foundation in physics and/or mechanics, while being comfortable with programming and numerical analysis.
References:
1. Etcheverry Arnaud, Simulation de la dynamique des dislocations à très grande échelle, Université Bordeaux I (2015).
2. Blanchard, Pierre, Algorithmes hiérarchiques rapides pour l’approximation de rang faible des matrices, applications à la physique des matériaux, la géostatistique et l’analyse de données, Université Bordeaux I (2017).
3. Durocher, Arnaud, Simulations massives de dynamique des dislocations : fiabilité et performances sur architectures parallèles et distribuées (2018).
4. Drouet, Julie, Étude expérimentale et modélisation numérique du comportement plastique
des alliages de zirconium sous et après irradiation (2014).
5. Gaumé, Marine, Étude des mécanismes de déformation des alliages de zirconium
après et sous irradiation (2017).
6. Noirot, Pascal, Etude expérimentale et simulation numérique, à l'échelle nanométrique et en temps réel, des mécanismes de déformation des alliages de zirconium après irradiation (2025).
Fatigue crack growth modelling with residual stress - Improvement of the Gtheta method
Residual stresses are self-balanced stress fields found in certain mechanical components in the absence of external loading. Caused by welding, for example, these stresses can potentially affect the behaviour of the structure and its resistance to fracture. When demonstrating the integrity of a mechanical component, particularly in the context of nuclear safety, it is crucial to precisely understand the role of these stress fields on the component's resistance. In the case of fatigue crack propagation, to accurately model all the phenomena involved (stress redistribution, evolution of plasticity, closure effect), it will be necessary to improve numerical tools, such as meshing and crack propagation methods (AMR, X-FEM...) and the J-integral interpolation in the case of through-cracks (Gtheta method). The thesis will consist of two complementary parts: (a) numerical development aimed at improving the Gtheta method in Castem FE code, associated with a 3D crack propagation modelling using AMR, and (b) continuation of component scale tests on fatigue crack propagation in different configurations of residual stress fields.
Detailed Numerical investigations on highly-concentrated bubbly flows
To assess the safety of industrial facilities, the CEA develops, validates, and uses thermohydraulic simulation tools. Its research focuses on modelling two-phase flows using various approaches, from the most detailed to the largest system-scale. In order to better understand two-phase flows, Service of Thermal-hydraulic and Fluid Mechanics (STMF) is working on implementing a multi-scale approach in which high-fidelity simulations (DNS, Direct Numerical Simulation of two-phase flows) are used as “numerical experiments” to produce reference data. This data is then averaged to be compared with models used on a larger scale. This approach is applied to high-pressure flows where the bubbly flow regime is maintained even at very high void fractions. The Laboratory of Development at Local Scales (LDEL) belonging to STMF has developed a DNS method (Front-Tracking) implemented in its open-source thermo-hydraulics code: TRUST/TrioCFD [1] (object-oriented code, C++). In several PhDs, it has been used to perform massively parallel simulations to describe interfaces in detail without resorting to models, for example in groups of bubbles (called swarms) [2][3][4].
Currently applied to low-concentration two-phase bubbly flows (volume fraction less than 12%), the objective of this thesis will be to evaluate and use the method at higher void fractions. Reference HPC simulations of bubble swarms will be conducted on national supercomputers up to gas fractions of 40%. The quality of the results will be evaluated before extracting physical models of bubble interactions under these conditions. The objective of these models is to recover the overall dynamics of the bubble swarm at much lower resolutions, thereby enabling the study of larger systems in disequilibrium (external forcing of imposed turbulence generation, imposed average velocity gradient, etc.).
This work is funded by the French ANR, in collaboration with IMFT and LMFL, in parallel with two other theses with which there will be strong interactions. It will be performed at CEA-Saclay, in the STMF/LDEL laboratory. It includes numerical aspects (validation), computer developments (C++), and a physical analysis of the flows obtained.