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.
Study and Modelling of Tritium Speciation from the Outgassing of Tritiated Waste
Tritium, the radioactive isotope of hydrogen, is used as fuel for nuclear fusion, particularly in the ITER research reactor currently under construction in Cadarache (France). Its small size allows it to easily diffuse into materials, which will lead to the production of waste containing tritium after the operational phase of ITER.
To optimize the management of this tritiated waste, the CEA is developing technological solutions aimed at extracting and recycling tritium, as well as limiting its migration to the environment. The effectiveness of these solutions largely depends on the chemical form in which tritium is released. Experience from the outgassing of tritium from various types of waste indicates that it is released in two main chemical forms: tritiated hydrogen (HT) and tritiated water vapor (HTO), in varying proportions.
However, the mechanisms determining the distribution of tritium between these two species are not well understood. Several factors, such as oxygen and water concentrations, the nature and surface state of the waste, and the concentration of tritium, can influence this speciation.
The objectives of this thesis are as follows:
- To identify the phenomena affecting the speciation of tritium during the outgassing of tritiated waste.
- To conduct an experimental study to verify the proposed hypotheses.
- To develop a numerical model to predict the proportions of HT and HTO released, in order to optimize the management of this waste.
The thesis will be conducted within the IRESNE Institute (Institute for Research on Nuclear Systems for Low Carbon Energy Production) at the CEA site in Cadarache, in a laboratory specialised in tritium studies. The PhD candidate will work in a stimulating scientific environment and will have the opportunity to showcase their research work. The candidate must hold an engineering degree or a master’s degree in Chemical Engineering, Process Engineering, or Chemistry.
Modeling of water ingression in a severe accident by separate effect testing
Nuclear energy is one of the pillars of the energy transition due to its low carbon footprint. It requires advanced safety studies, particularly regarding hypothetical severe nuclear accidents. These scenarios involve core meltdown and the formation of corium (molten radioactive material magma). Understanding corium behavior is a key element of nuclear safety.
At IRESNE institute of CEA Cadarache, the MERELAVA facility studies accident mitigation strategies by spraying water onto corium from above. A prototypical corium bath (containing depleted uranium) is cooled by water spraying under realistic conditions. This setup allows the study of complex interactions between corium, water, and the sacrificial concrete beneath.
In this context, the water ingression phenomenon plays a central role in corium cooling. During spraying, the solidified crust cracks, water seeps into the cracks and evaporates, significantly increasing the extracted heat flux compared to conduction alone. However, current models poorly describe this mechanism and struggle to predict its impact, mainly due to its highly multi-physical nature.
This thesis aims to study ingression through dedicated experiments on MERELAVA, to characterize the crust and to measure the ingression flux using 3D-printed representative matrices. The goal is to improve the existing physical model, with results compared to more complex experimental data. The thesis will primarily take place in the Severe Accidents experimental laboratory of the IRESNE institute. The candidate should have expertise in fluid mechanics and heat and mass transfer.
Effects of structural heterogeneities on air flow through reinforced concrete walls
The containment building represents the third barrier to confinement in nuclear power plants. Its role is to protect the environment in the event of a hypothetical accident by limiting releases to the outside. Its function is therefore closely linked to its tightness. Traditionally, the estimation of the leakage rate is based on a sound knowledge of transfer properties (such as permeability), combined with a chained (thermo-)hydro-mechanical simulation approach. While the mechanical behavior of the structure is now broadly well understood, progress is still needed in the comprehension and quantification of fluid flow. This is particularly true in the presence of heterogeneities (cracks, honeycombs, construction joints, reinforcements, cables, etc.), which represent situations that can locally disturb permeability. This is the context of the present PhD topic.
The work will consist, through a methodology combining experimental testing and numerical simulation, in improving the representation of fluid flow by explicitly accounting for the impact of heterogeneities. An initial analysis will define an experimental plan, which will then be carried out. The results will be analyzed in order to empirically characterize the influence of each type of heterogeneity tested on transfer properties. A simulation approach, exploiting the experimental findings, will then be developed using finite element and discrete methods. Finally, the applicability of the methodology to a real-scale structure will be assessed, while explicitly accounting for uncertainties regarding the presence and impact of such heterogeneities (probabilistic approach).The PhD will therefore rely on state-of-the-art experimental and numerical tools and methods, and will be conducted in a rich collaborative context (CEA, ASNR, EDF).
Mass transfers and hydrodynamic coupling: experimental investigation and models validation and calibration
With the energy transition and the paramount importance of the nuclear energy in this context, it is pivotal to understand the consequences of potential accident with core meltdown, as well as thinking about mitigation strategy.
During a nuclear severe accident with core meltdown a magma called corium can form a pool in the reactor lower head. The pool is not homogeneous and can stratify into multiple immiscible layers. The composition of the pool may evolve in time, due to progressive material assimilation. With the evolution of the global composition of the corium, the properties of the layers evolve. The vertical position of these layer may then change. This change comes with the creation of droplets from a layer which then cross the other one. The vertical order of the different layers as well as their movements have a significant impact on the heat fluxes imposed on the reactor vessel. A better understanding of these phenomena improves safety of both nowadays and future nuclear reactors.
Modelling work has been done, but it lacks validation and need calibration. Prototypical experiments (with actual materials present inside a reactor) are difficult to carry and are not foreseen in the near future. This PhD aims at experimentally studying the mass transfer between a droplet and its surrounding as well as the droplet creation. The planned experimental setup will use a water-based system which allow for local measurement. The goal is to validate, calibrate the existing model, and potentially create new ones. The final goal being to capitalize the work into PROCOR software platform. The experimental setup will be constructed and operated in LEMTA laboratory in University of Lorraine, where the student will work.
The PhD work will be mainly experimental but will also require software use for calibration, validation and for the design of the experimental setup. This work will be conducted in close collaboration between the laboratories LMAG in CEA/IRESNE (Cadarache) and LEMTA in University of Lorraine (Nancy). The student will work in LEMTA, where the experiments will be conducted, while being part of the CEA. The student will benefit from LEMTA’s expertise in building of experimental setup, transport phenomena in fluids and metrology, and from LMAG’s expertise in mass transfer, physical modeling and simulation in the scope of nuclear severe accidents. The student will regularly interact with CEA team which will follow the work closely. The student will therefore have to regularly go to CEA Cadarache.
The PhD student will be integrated to a dynamic environment comprised of researchers and other PhD students. The PhD candidate needs to be knowledgeable in transport phenomena, and needs to have a taste for experimental sciences.