Study of vibration effects on electrical cable diagnosis using reflectometry
This thesis focuses on the effect of vibrations on the diagnosis of electrical cables using reflectometry. Cable systems, which are present in many critical infrastructures such as aeronautics, railways, space systems, and nuclear facilities, are exposed to mechanical and environmental stresses that may lead to soft and intermittent faults. Under vibration, these faults may appear, disappear, or modify their electrical signature, making their detection particularly challenging.
One of the main challenges concerns No Fault Found situations, in which a fault observed during operation becomes non-reproducible once the vibration conditions disappear. Another important issue is the temporary masking of certain faults by vibrations, which may lead to false negatives during diagnosis and delay the detection of latent degradation.
The objective of this thesis is to better understand and model the electromechanical behavior of cable faults subjected to vibrational stress, in order to link vibration profiles, the mechanical and electrical evolution of the fault, and the signatures measured by reflectometry. The work will be based on experiments combining fast reflectometry and a high-speed camera, as well as on the development of models and analysis tools. Experimental and simulated data will then be used to improve the detection, characterization, and prediction of fault evolution, with a view to advanced diagnosis and predictive maintenance.
Investigation of Very High Cycle Fatigue Behavior of 13-4 Martensitic Stainless Steel Manufactured by Laser Metal Deposition: Influence of Microstructure, Post-treatments and Temperature Project
Recent research on 13-4 martensitic stainless steel manufactured by metal additive manufacturing, particularly using the Laser Metal Deposition (LMD) process, has made it possible to obtain materials with good mechanical properties. Following this optimization phase, current work is now focused on studying their Very High Cycle Fatigue (VHCF) behavior, which is a critical criterion for components subjected to repeated loading under severe operating conditions.
Fatigue is one of the main causes of failure in metallic components during service. This thesis therefore aims to understand and model the fatigue behavior of LMD-produced 13-4 steel. The work will investigate the influence of microstructure, thermomechanical treatments, and testing conditions on crack initiation and propagation during mechanical loading.
Experimental investigations will be carried out using ultrasonic fatigue testing devices. Failure mechanisms will be analyzed through multi-scale characterization techniques such as EBSD, SEM, and TEM. The final objective is to develop a predictive model capable of estimating the service life of components under operating conditions.
Characterisation of the physico-chemical properties of solid residues from biomass hydrothermal carbonisation
Hydrothermal carbonisation (HTC) is a thermochemical conversion process performed in pressurized water (2-6 MPa) between 180 and 260°C. The main product is a carbonaceous solid residue (hydrochar). Various applications are foreseen for hydrochar: combustion, gasification, adsorption, catalysis, soils amendment, hard carbon for Na-ion batteries, …, each of them requiring specific properties.
The objective of the thesis is to characterise and better understand the origin of several physico-chemical properties of biomass hydrochars. A special attention will be paid to hydrophobicity and drying capacity, to physical and textural characteristics of the particles (porosity, granulometry, specific surface), as well as to chemical characteristics (composition). The influence of biomass type and HTC conditions on these properties will be investigated.
The approach will consist in: experimentations in batch reactors on pre-selected biomass resources, together with use of different characterisation techniques for hydrochars; analysis of results aiming at determining links between the characteristics, elucidating the links between the resource and its hydrochar properties as a function of operational conditions.
Integrated waste treatment: design and optimisation of a multi-waste treatment scheme for a multi-purpose energy production
At the city scale, multiple waste streams such as household waste, compost, sewage sludge, yard waste, non-recyclable plastics, used oils, metals, glass, and others. All of these feedstocks exhibit variable seasonality and carbon content. Nowadays, the aforementioned streams are managed through recycling, and in some cases incineration or landfilling. Alternative treatment technologies, such as gasification, hydrothermal gasification, and anaerobic digestion, are being explored as potential pathways to improve the overall sustainability of waste management.
Existing scientific studies have largely focused on the conversion of individual waste types or on the application of a single technology to a specific waste stream, without accounting for regional integration, resource variability or systemic assessment. A city-scale analysis of waste streams could enable the identification of synergies between different waste types and the identification of optimal conversion pathways.
In this context, a key scientific challenge lies in the development of an integrated, multi-waste treatment framework capable of modelling, optimizing, and assessing a multi-waste, multi-product energy system at the city scale. The objective of this PhD project is to investigate waste treatment at the city scale, accounting for the seasonality of waste generation, waste stream composition, and local energy demand (heat, electricity, and gas). The work will consider local and European regulations (Waste Framework Directive, AGEC law, and RED III directive) as well as techno-economic and environmental aspects. The study will focus on one to three representative geographic areas and will establish a methodology that can be further applied to a broad range of territorial contexts.
Study of mechanical stress on Solid State Micro-batteries
CEA-Leti provides integrated microstorage solutions, including solid state (or solid electrolyte) microbatteries. Solid-state micro-batteries are among the most promising microstorage technologies for applications in several fields such as the internet of things and implantable devices for medical use. The objective of this thesis is to study the impact of mechanical stresses on microbatteries, particularly during microbattery charge/discharge cycles. To this end, two approaches will be considered: experimental study with the development of mechanical test benches and numerical simulation.
The PhD student's work will begin with the development of test benches, the first of which will apply variable pressure to the surface of a microbattery during charge/discharge cycles. He/she will be required to develop the pressure measurement equipment. Once the mechanical test bench is operational, other characterizations, such as measuring anode deformations, will be considered. In parallel with this experimental work, a mechanical model will be developed. This model will be progressively refined using the experimental results obtained with the mechanical test bench, and new characterizations may be implemented in order to obtain the mechanical properties of the different materials used. Ultimately, the objective will be to propose the integration of new layers to improve the mechanical performance of microbatteries during cycling.
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/
Code Development and Numerical Simulation of Gas Entrainment in Sodium-Cooled Fast Reactors
In sodium-cooled fast reactors (SFRs), the circulation of liquid sodium is ensured by immersed centrifugal pumps. Under certain conditions, vortices can develop in recirculation zones, promoting the entrainment of inert gas bubbles (typically argon) located above the free surface. If these bubbles are drawn into the primary circuit, they can damage pump components and compromise the safety of the installation. This phenomenon remains difficult to predict, particularly during the design phase, as it depends on numerous physical, geometrical, and numerical parameters.
The objective of this PhD work is to contribute to a better understanding and modeling of gas entrainment in free-surface flows typical of SFRs, through Computational Fluid Dynamics (CFD) simulations using the open-source code TrioCFD, developed by the CEA. This code includes an interface-tracking module (Front Tracking) that is particularly well-suited for simulating two-phase phenomena involving a deformable free interface.
Reduction of reinforcement in reinforced concrete structures through nonlinear calculations and topological and evolutionary optimizations
Reinforcing steel plays a major role in the behavior of reinforced concrete structures. Nevertheless, significant conservatisms may sometimes be imposed by design codes, raising questions about the feasibility of construction or the viability of the structure (economic, environmental, etc.). It is within this context that the doctoral research takes place. Building on recent developments, the work aims to propose an innovative design approach relying on the use of nonlinear finite element calculations, combined with topological optimization algorithms (defining reinforcement directions and bar cross-sections) and evolutionary optimization algorithms (determining the placement of bars with fixed cross-sections).
The method should, through an iterative process, yield solutions that meet an optimal design configuration. Considering the multiple, potentially conflicting objectives to minimize (such as cost, feasibility, strength, and carbon footprint), the approach will guide the configuration of input parameters based on an analysis of the relevant output results.
Applying the method to complex, practice-based case studies (for example, beam-column junctions) will demonstrate its relevance compared with more conventional design methods. By the end of the thesis, the doctoral candidate will have developed advanced skills in the use and development of state-of-the-art tools, ranging from nonlinear finite element simulation to modern optimization techniques based on artificial intelligence.
Fluid-structure interaction in a network of slender solids in a confined environment
As part of its study of progressive deformations in fuel assemblies within PWR cores, the CEA has developed two simulation tools. The first, Phorcys [1], calculates the flow of coolant in and around slightly deformed assemblies using a network of parametric pressure drops, then deduces the fluid forces acting on the structures. The second, DACC [2], uses finite element simulation to analyze thermomechanical behavior under irradiation and the interaction between assemblies during power cycles. Finally, fluid-structure interaction is analyzed using numerical coupling of these two tools, within which uncertainties can be propagated and analyzed [3].
The nuclear revival program (SMR, 4th generation reactors, PN, etc.) is providing new technologies and new core and fuel assembly topologies that need to be analyzed in terms of the risks associated with quasi-static deformations of core assemblies. With a view to both capitalizing on and extending the possibilities of simulation, the aim is to enable these two tools to handle the flow and deformation of slender structures in a more generic way in order to cover a wide range of nuclear technologies efficiently and quickly.
To do this, it will be necessary to identify, classify, and then model in a reduced but predictive manner the main flow structures that may occur within a fluid volume cluttered with slender structures with a large exchange surface area. The complete hydraulic model of the core will thus be created by concatenating elementary models that comply with strict interfacing conditions. A method for analyzing the overall flow obtained will then enable the quantification of the force field contributing to the deformations. A similar logic of classification and scaling would also be implemented with regard to the evaluation of reversible and irreversible deformations of a slender structure subjected to external stresses and severe irradiation. One difficulty is that the fine topology of a fuel assembly can exhibit nonlinearities at small scales that propagate in part to the macroscopic scale. Ultimately, a robust, cost-effective partitioned coupling will have to be implemented between the coolant flow and these individual structures, which deform and interact in a constrained environment.
The modeling framework thus constructed will make it possible to study the progressive deformations of assemblies and the associated risks for a wide range of nuclear reactor technologies.
Fluid–structure interaction in mixtures: theory, numerical simulations and experiments
This PhD project is part of research on fluid–structure interactions (FSI) in complex media, particularly fluid mixtures involving multiple phases (liquid/liquid or liquid/gas) and/or suspended particles. The objective is to develop a thorough, multi-scale understanding of the coupled mechanisms between deformable structures (such as droplets, interfaces, or flexible walls) and the flows of complex mixtures, by combining theoretical modelling, advanced numerical simulations, and comparison with experimental data.