High-isolation power supply
With the rapid evolution of technologies and the growing challenges of miniaturization and resource management, power converters are facing ever more stringent performance requirements. To meet these needs, the use of wide-bandgap semiconductors such as SiC (silicon carbide) and GaN (gallium nitride) is becoming increasingly common. These materials significantly increase the switching speed of converters, reducing losses and improving efficiency.
However, this switching speed brings additional challenges: the steepness of the switching edges can cause stray currents that interfere with switch controls. To counter these undesirable effects, it is necessary to use switch drivers offering a high level of insulation. The traditional solution is based on high-frequency magnetic transformers, but these devices are expensive, take up a lot of space and offer limited insulation.
Thesis objective: the aim of this thesis is to design a new solution for powering wide-gap component drivers, by replacing magnetic transformers with piezoelectric transformers. This innovative approach aims to reduce costs, space requirements and improve the overall efficiency of power conversion systems.
Supervision and ressources: the selected candidate will work as part of a leading-edge research team, renowned for its expertise in the field of power conversion using piezoelectric resonators. The team has the resources and know-how to support the development and validation of this innovative technology.
Mesoscopic simulations and development of simplified models for the mechanical behaviour of irradiated concrete
In nuclear power plants, the concrete biological shield serves as a support for the reactor vessel and as a protective shield against radiation. Over the long term, prolonged exposure to neutron radiation can cause the concrete aggregates to expand through amorphisation, leading to micro-cracking and degradation of its mechanical properties. This is an important issue in studies aimed at extending the life of power plants. At the mesoscale, these phenomena can be modelled by separating the behaviour of the aggregates, the cementitious matrix and the interfacial transition zones. However, it is difficult to describe the initiation and propagation of microcracks in such complex heterogeneous multi-cracked systems. The aim of this thesis, carried out as part of a Franco-Czech ANR project, is to develop a high-performance numerical simulation tool for analysing the effects of neutron irradiation on concrete at the mesoscopic scale. A coupled thermo-hydro-mechanical approach will be used in which the behaviour of the matrix will take into account shrinkage, creep and micro-cracking. The simulations will be validated using experimental data obtained on tested samples, and the numerical tool will then be used to estimate the impact of various factors on the behaviour and performance of concrete subjected to neutron irradiation.
This research project is aimed at a PhD student wishing to develop their skills in materials science, with a strong focus on multiphysical and multiscale modelling and numerical simulations.
Cohesive powder simulation: link between atomic and grain scale
Nuclear fuel is produced through a powder metallurgy process that involves several stages of the granular medium preparation (grinding, mixing, pressing and sintering). The powders used during these stages exhibit strong cohesion between the grains, making their flow behavior complex. Predicting powder behavior is a critical industrial challenge to quickly adapt to raw material changes, optimize product quality, and enhance production rates.
This thesis aims to establish a link between the properties of powders and their behavior during flow and pressing. Grain cohesion is a key factor that influences both the flow and densification of granular materials. This cohesion is governed by several interparticle forces, such as van der Waals forces, capillary interactions, and electrostatic forces. Understanding these interactions at the atomic scale is essential for accurately predicting and modeling powder behavior. The thesis seeks to address two central questions: How do the surface properties of grains at the atomic level influence the cohesive forces at the grain scale? And how can we scale up from the atomic level to the grain scale to simulate powders more realistically?
Multi-scale simulation approaches are crucial for bridging the gap between microscopic phenomena and the macroscopic behavior of granular materials. Current Discrete Element Method (DEM) simulations rarely incorporate fundamental interactions, such as van der Waals forces, electrostatic forces, and capillary effects, into their contact models. Recent research (1) (2) has explored the impact of cohesion using a simplified approach, treating it as an attractive force or cohesive energy. Simulation methods like Molecular Dynamics (MD) or Coarse-graining enable the modeling of material behavior at finer scales, based on these local structural and chemical properties. A deeper understanding of cohesion at small scales will enhance the predictive capabilities of DEM simulations and clarify the relationship between powder properties and their overall behavior.The main goal of this thesis is to better understand the relationships between atomic-scale interactions and grain-scale cohesion and to assess the consequences for simulations of powder pressing and flow.
The primary goal of this thesis is to make connections between the atomic-scale interactions and grain-scale cohesion and to simulate the powder flow and compaction processes.
One of the main challenges in this project is the development of DEM contact laws that incorporate complex atomic-scale interactions. This will require close collaboration between experts in atomic-level simulations and those working on DEM modeling. Additionally, validating these models through experimental comparisons is essential to ensure their accuracy and relevance for industrial applications.
The PhD candidate will be based at the IRESNE Institute (CEA-Cadarache) within the Laboratory of Numerical Methods and Physical Components on the PLEIADES platform, part of the Department of Fuel Studies. They will collaborate with the Fuel Behavior Modeling Laboratory and will have access to state-of-the-art modeling and simulation tools, as well as a collaborative environment with the Mechanics and Civil Engineering Laboratory at the University of Montpellier.
Bibliography
1. Sonzogni, Max. Modélisation du calandrage des électrodes Li-ion en tant que matériau granulaire cohésif : des propriétés des grains aux performances de l'électrode. s.l. : Thèse, 2023.
2. Tran, Trieu-Duy. Cohesive strength and bonding structure of agglomerates composed. 2023.
Hydrogen and ammonia combustion within porous media: experiments and modelling
- Context
Current energy prospects suggest the use of hydrogen (H2) and ammonia (NH3) as carbon-free energy carriers to achieve neutrality by 2050. NH3 offers advantages like high energy density and safe storage but faces combustion challenges such as narrow flammability and high NOx emissions. Interestingly, some H2 can be obtained by partial cracking of NH3 to create blends of more favourable combustion properties, with open questions regarding pollutant emissions and unburnt NH3 content.
- Challenges
Porous burners show promise for safe and low-pollutant combustion of NH3/H2 blends. However, material durability issues and the complexity of flame stabilization pose significant hurdles. Fortunately, recent advances in additive manufacturing enable the precise tailoring of porous matrices, but the experimental characterization remains difficult due to the opacity of the solid matrix.
- Research objectives
The PhD candidate will operate an experimental bench at CEA Saclay to conduct combustion experiments with NH3/H2/N2+air mixtures in various porous burners. Key tasks will include designing new burner geometries, comparing experimental results with numerical simulations, and advancing the modelling of porous burners using 1D Volume-Averaged Models and asymptotic theory. Experimental measurements will include hotwire anemometry, infrared thermometry, output gas composition analysis, chemiluminescence, and laser diagnostics. The porous burners will be manufactured using 3D printing techniques with materials such as stainless steel, inconel, alumina, zirconia, and silicon carbide.
The research aims to develop more robust and efficient porous burners for NH3/H2 combustion, enhancing their practical application in achieving carbon neutrality. The candidate will contribute to advancing the field through experimental data, innovative designs, and improved modelling techniques.
Study of of the thermodynamic of K2CO3-CO2-H2O for the development of NET and SAF technologies
.Bioenergy with Carbon Capture and Storage (BECCS) uses biomass energy while capturing the carbon dioxide released by the process, resulting in negative emissions into the atmosphere. The reference process in Europe uses potassium carbonate but at atmospheric pressure [1], whereas its sequestration or hydrogenation into sustainable molecules requires high pressures.
The thesis consists in acquiring new thermodynamic and thermo-chemical data at high temperature/pressure [2] required for the energy optimization of such a process, and integrating them into a thermodynamic model.
The overall process will then be reassembled in order to quantify the expected energy gain.
The thesis will be carried out at the Thermodynamic Modeling and Thermochemistry Laboratory (LM2T), in collaboration with LC2R (DRMP/SPC) for the experimental part.
References :
[1]K. Gustafsson, R. Sadegh-Vaziri, S. Grönkvist, F. Levihn et C. Sundberg, «BECCS with combined heat and power: assessing the energy penalty,» Int. J. Greenhouse Gas Control, vol. 110, p. 103434, 2021.
[2] S. Zhang, X. Ye et Y. Lu, «Development of a Potassium Carbonate-based Absorption Process with Crystallization-enabled High-pressure Stripping for CO2 Capture: Vapor–liquid Equilibrium Behavior and CO2 Stripping Performance of Carbonate/Bicarbonate,» Energy Procedia, 2014
Study of wire additive manufacturing of a nuclear component with complex geometry
The general aim of the thesis is to study the feasibility of a component for the DEMO fusion reactor using Wire Additive Manufacturing (WAM). To achieve this, the PhD student will first design and manufacture demonstration parts representative of different sub-parts of the component in the laboratory's additive manufacturing cells. He or she will use CAD/CAM software to manufacture parts of increasing size and complexity, while ensuring repeatability.
These parts will be subjected to characterization work, firstly dimensional, to check their geometric conformity with the project specifications; but also microstructural and metallurgical, to guarantee manufacturing quality, in particular the absence of defects within the material (porosity, inclusions...) or metallurgical phases detrimental to its mechanical strength.
Finally, the PhD student will also be required to simulate the manufacture of certain parts using the finite element method, in order to analyze the evolution of parameters of interest, such as temperature, during manufacture, and to estimate the state of deformation and stress after manufacture. These simulations can be used to correct certain discrepancies between expected and actual results, within the framework of a calculation-test dialogue that will see the implementation of instrumentation also serving to validate the models. These simulations will be carried out using the Cast3M finite element code developed at CEA.
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Effect of plastic strain on brittle fracture: Decoupling of deformation induced dislocation structures and deformation induced microtexture evolution
In the nuclear field, the integrity of components must be ensured throughout their operating life, even in the event of an accident. The demand for justification of component resistance to the risk of sudden rupture is growing, and is being applied to a wide range of piping lines and equipment. The demonstration principle consists in showing that, even in the presence of a defect, the equipment is capable of withstanding the loads it is likely to be subjected to.
Particular attention is paid to brittle fracture by cleavage, because of its unstable and catastrophic nature, which immediately leads to the ruin of the component. Brittle fracture is sensitive to the level of plasticity and triaxiality at the crack tip, which explains the beneficial structural effect often observed on real components compared to laboratory specimens. The industrial challenge is to better understand the role of plasticity in relation to microtexture on brittle fracture, in order to improve current prediction criteria.
In the course of this thesis, the brittle fracture toughness of a ferritic steel will be evaluated after various types of mechanical pre-strain. By the end of the thesis, the candidate will have acquired solid skills in mechanical testing, microscopic analysis and numerical simulation. The work will be carried out between the LISN laboratory of the CEA and the materials center of the Ecole des Mines de Paris.
Development of a multiscale / multimodel boundary condition
In the context of thermalhydraulics, Computational Fluid Dynamics (CFD) codes are widely used for design and safety analysis. CFD codes solve the Navier-Stokes equations in three dimensions. They mostly rely on the Reynolds-averaged formulation of the Navier-Stokes equations. This approach allows for a detailed representation of the flow while requiring a limited numbers of hypotheses (turbulence models, law of the wall). A fine spatial discretisation is needed in order to achieve good prediction capabilities. This implies a large number of control volumes. The computational resources necessary to carry out a calculation at the industrial scale, such as a two-phase flow transient on the entire primary side of a nuclear reactor, are often prohibitive by present-day standards.
In order to cut the computational cost, a coarser spatial discretisation can be retained. Depending on the case of interest, the best practise guidelines of the RANS approach might not all be respected. Further hypotheses need to be added in order to maintain the quality of the model’s predictions. Such models may include pressure drops, heat transfer correlations or mixing terms. This approach is often referred to as a porous media approach.
Regardless of the method, the system of interest is often restricted to an open-loop model, which requires boundary conditions for the equation system to be solved.
Multi-scale coupling methods aim at using each approach where it best suited. The rationale is to reduce the computational burden while capturing the relevant physical phenomena.
Multi-scale coupling can be either one-way or two-way. In a one-way coupling, boundary conditions obtained from a first calculation are used as boundary conditions for another calculation. There is no feedback from the second calculation on the first one. In a two-way coupling, the coupled codes exchange data in the form of boundary conditions, usually at each time step. There is feedback between the two codes. Two-way is the method that is selected in the following.
The boundary conditions used in the standard approach are developed for cases were only macroscopic data are available, flow rate and temperature at the inlet, pressure at the outlet. In the context of a multi-scale coupling, data that are more detailed can be available such as velocity and pressure fields. This thesis work aims at developing boundary conditions, which can take benefit of all the available data in order to make the coupling as seamless as possible.
As an example, in case of two code instances, each one solving a portion of a physical domain relying on the same discretisation and modelling options, the results obtained from these two instances should be identical to that of a single code instance relying on the same discretisation and modelling options solving the entire domain.
Study of fuel assembly creep in fluid-structure interaction
In the context of the energy transition and the low-carbon mix, controlling the performance and safety of the nuclear reactors in the fleet is an imperative that still opens up avenues for research and development with high added value. This is particularly true for the optimization of fuel elements.
Indeed, during its stay in the core of a power reactor, the fuel assembly is subjected to mechanical, thermal and hydraulic constraints. It undergoes a change in its geometry, in particular an elongation and a lateral deformation, due to the creep phenomenon linked jointly to the irradiation and the flow of water in the core. With the increase in the residence times of fuel assemblies in reactors and due to increasingly demanding conditions, the need to understand the phenomenon is necessary to improve the robustness of the design. This is in particular a problem of fluid-structure interaction where the flow plays a role in the creep behavior of the structure and where the deformation of the structure modifies the flow.
A previous study made it possible to implement an experimental device to obtain rapid creep on reduced-scale fuel assembly models. These tests were able to highlight a significant effect of the fluid inlet conditions on the creep behavior of the assemblies. The objective of the proposed thesis work is then to analyze the experimental results using simulation tools in order to understand and quantify the phenomenology of the coupling in fluid-structure interaction under creep. This analysis could lead to the realization of additional tests. Another important aspect will be the transposability of the results to real conditions. The thesis will be carried out at the IRESNE institute of the Cadarache center, in collaboration with the industrialist Framatome, bringing its operational vision in the monitoring and orientation of the research work. The proposed work therefore opens up solid perspectives at the end of the thesis both in research centers and in industrial environments.
Experimental and numerical analysis of fluid-structure interactions in the propagation of rarefaction waves through complex structures in pressurized water reactors
Loss of coolant accident (LOCA) in pressurized water reactors (PWR) leads to fast transient phenomena, such as the propagation of rarefaction waves within the reactor's internal structures. These waves generate transient pressure loads between different areas, such as the reactor core and the bypass zone, which places stress on the baffle. The deformation of this critical structure can compromise the structural integrity of the reactor and complicate the handling of fuel assemblies, particularly their removal after the accident.
The main scientific objective is to develop, implement, and validate new numerical models that allow for a more accurate simulation of rarefaction wave propagation through complex obstacles. The current state of the art relies on simplified models, validated only for simple configurations such as single-orifice plates. However, there is a need to extend these models to more complex geometries, such as plates with multiple holes, using different numerical methods.
The development of a porosity model to represent fuel assemblies is also crucial. The expected results will be validated experimentally and have direct applications for industrial partners EDF and Framatome, enhancing the industrial relevance of this research.
The thesis will adopt a combined approach, both experimental and numerical. The use of the MADMAX platform will allow for the testing of various complex obstacles and the collection of detailed experimental data using specialized sensors. This data will be used to validate the numerical models developed in the EUROPLEXUS software. Additionally, the simulations will include innovative approaches such as a new porosity model for the internal structures of the reactors. Participation in international conferences and publication of results are planned to ensure the scientific dissemination of the findings.
The thesis will be conducted at the DYN laboratory of CEA Paris-Saclay, equipped with unique experimental facilities, such as the MADMAX platform, and has strong expertise in numerical modeling. Several industrial (EDF, Framatome) and academic collaborations will provide a rich environment for the doctoral candidate, with regular exchanges within international networks.
The ideal candidate should possess solid skills in fluid mechanics, structural dynamics, numerical modeling (finite element, finite volume), and programming. Previous experience with tools like EUROPLEXUS will be a plus. An M2 internship may be offered to familiarize the candidate with the methods and tools used in this thesis.
This thesis will enable the doctoral candidate to acquire highly specialized skills in fluid-structure interactions, numerical modeling, and experimentation in an industrial context. These skills are in high demand in the energy, aerospace, and advanced simulation technology sectors, paving the way for careers in applied research or engineering within the industry.