Modeling and upscaling of sodium boiling flow within a 4th generation nuclear reactor core
The stabilized boiling in sodium is a subject that has been studied for many years at CEA in order to improve the validation of scientific calculation tools such as CATHARE3. Being able to reproduce properly this phenomena is a key safety related question for liquid metal liquid 4th generation reactors. When an unprotected loss of flow (ULOF) happens in the reactor and the safety measures are not deployed, the coolant can reach saturation, which can ultimately lead to a degradation of the subassembly. In order to avoid this situation, new fuel assembly designs provide negative neutronic feedback as the void fraction is generated. To understand how this void fraction evolves in the sub-assembly (within the rod bundle or the top plenum), the code requires a state of the art sodium modeling in terms of momentum, heat and mass transfer.
To improve the qualification of the CATHARE3 code for such situations, the doctoral student will implement CFD models allowing a better understanding of the boiling mechanisms in sodium-cooled subassemblies. New CFD models, such as large interface modelling, wall boiling, heat and mass exchange at the interface will be applied, yielding detailed information on local variables. Subsequently, this detailed information will be transferred to the 1D system code during an upscaling operation. Once this information is properly gathered and transferred, new models will be developed and implemented into the system code. Finally, these new models will be confronted to experimental data in a validation exercise over the CATHARE code validation database. Ultimately, the aim is to increase the confidence in the CATHARE3 1-D simulation tool for predicting the specific physics of sodium boiling during an unprotected loss of flow transient.
The doctoral student will be based in a research unit on innovative nuclear systems at CEA/IRESNE Cadarache, in a dynamic and international environment. Travel to CEA-Saclay and EDF-Chatou is planned during the thesis, as well as participation in international conferences.
Analysis of solid oxide cell degradation by transmission electron microscopy and atomic probe tomography
Nowadays, high-temperature electrolysis is considered as one of the most promising technology for producing green hydrogen. The electrolysis reaction takes place in a Solid Oxide Cell (SOC) composed of an oxygen electrode (made of LSCF or PrOx) and a hydrogen electrode (made of Ni-YSZ) separated by an electrolyte (made of YSZ). To accompany industrialization f SOCs, the durability still needs to be improved. The main performance losses are due to the degradation of the two electrodes. In order to propose an improvement, it is essential to gain a precise understanding of electrode degradation mechanisms. In this thesis, we thus propose to apply high-resolution transmission electron microscopy and atom probe tomography (SAT) to study electrode degradation after aging under current. On the one hand, advanced electron microscopy techniques coupled with energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) will be applied. In addition, analyses carried out on a SAT will provide three-dimensional information particularly suited to the complex structure of the electrodes.
This work should provide a better understanding of the degradation mechanisms of high-temperature electrolysis cells. Recommendations for their manufacture can then be made to improve their lifespan.
Predictive Diagnosis and Ageing Trajectory Estimation of New Generation Batteries through Multi-modalities Fusion and Physics-Informed Machine Learning
Context:
Lithium-ion and emerging Sodium-ion batteries are crucial for energy transition and transportation electrification. Ensuring battery longevity, performance, and safety requires understanding degradation mechanisms at multiple scales.
Research Objective:
Develop innovative battery diagnostic and prognostic methodologies by leveraging multi-sensor data fusion (acoustic sensors, strain gauge sensors, thermal sensors, electrical sensors, optical sensors) and Physics-Informed Machine Learning (PIML) approaches, combining physical battery models with deep learning algorithms.
Scientific Approach:
Establish correlations between multi-physical measurements and battery degradation mechanisms
Explore hybrid PIML approaches for multi-physical data fusion
Develop learning architectures integrating physical constraints while processing heterogeneous data
Extend methodologies to emerging Na-Ion battery technologies
Methodology:
The research will utilize an extensive multi-instrumented cell database, analyzing measurement signatures and developing innovative PIML algorithms that optimize multi-sensor data fusion and validate performance using real-world data.
Expected Outcomes:
The thesis aims to provide valuable recommendations for battery system instrumentation, develop advanced diagnostic algorithms, and contribute significantly to improving the reliability and sustainability of electrochemical storage systems, with potential academic and industrial impacts.
CTC electrolyte pour LiS system
Lithium-Sulfur (Li-S) Batteries are among the most promising energy storage technologies for the fifth generation of batteries, often referred to as post-Li-ion. With a theoretical energy density five times higher than that of conventional Li-ion batteries and an abundant availability of sulfur, the Li-S system offers a unique potential to meet the growing demand for sustainable energy storage. However, current technology is limited by major challenges related to the dissolution of polysulfides in the electrolyte, leading to active sulfur loss, poor cycle life, and insufficient electrochemical performance. These limitations currently hinder the market deployment of this technology.
This thesis aims to explore an alternative approach based on an all-solid electrochemical sulfur conversion mechanism. To achieve this, a next-generation organic solid electrolyte developed in the laboratory will be implemented. This electrolyte features a unique lithium-ion conduction mechanism within a crystalline lattice, preventing polysulfide solubilization. The main objectives are:
1. To understand and control the ionic conduction mechanisms in these electrolytes.
2. To integrate this solid electrolyte into an innovative Li-S system.
3. To optimize the cathode structure for the solid-state mechanism and evaluate the electrochemical performance on a representative prototype scale.
The PhD candidate will use a wide range of characterization and analysis techniques to carry out this project:
• Formulation and characterization of the organic solid electrolyte: Techniques such as FT-IR and NMR to analyze chemical structures and identify the properties of synthesized materials (DSC, TGA, XRD, etc.).
• Electrochemical characterization: Analyses using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and symmetric cell cycling tests to study ionic conduction properties and electrolyte stability.
• Formulation and performance study of the cathode: Formulation of carbon/sulfur composites and sulfur cathodes integrating the solid electrolyte; galvanostatic cycling tests and advanced interface analyses to understand and optimize solid-state sulfur conversion.
The research will progress in three main phases:
1. Development and characterization of the solid electrolyte: Material development, analysis of conduction mechanisms, and optimization of ionic and mechanical properties.
2. Design and optimization of the cathode structure: Improving electrolyte/cathode interfaces for solid-state sulfur conversion.
3. Electrochemical performance evaluation: Experimental validation of prototypes through in-depth tests, including cyclability and power performance.
Development of a predictive power model for a photovoltaic device under spatial constraints
CEA is developing new cell and module architectures and simulation tools to assess the electrical performance of photovoltaic (PV) systems in their operating environment. One of these models, called CTMod (Cell To Module), takes into account not only the different materials making up the module, but also the different cell architectures. For space applications, the community wants to use terrestrial silicon-based technologies that can be integrated on flexible PVAs (Photovoltaic Assembly). The space environment imposes severe constraints. A relevant evaluation of performance at the start and end of a mission is therefore essential for their dimensioning.
The aim of this thesis is to correlate physical models of radiation-matter degradation in space with electrical models of photovoltaic cells. Performance degradations linked to the various electron, proton and ultraviolet (UV) irradiations of the space environment will be evaluated and validated experimentally. Linked to the CTMod Model, this new approach, never seen in the literature, will able to get a more accurate understanding of interactions between radiations and PVAs. These degradations result from non-ionizing energy deposition phenomena, quantified by the defect dose per displacement, and ionizing ones quantified by the total ionizing dose for protons and electrons. In the case of UV, the excitation of electrons in matter generates chain breaks in organic materials and colored centers in inorganic materials. Initially, the solar cell used in the model will be a Silicon cell, but the model can be extended to include other types of solar cell under development, such as perovskite-based cells.
Multiphe hydrogen injection at anode side of PEMFC
The alternating feeding architecture (known as Ping-Pong) was developed by the CEA. This architecture emerged in 2013 and has been implemented in several fuel cell systems. Following the latest tests on this architecture, questions remained unanswered. First, it is a question of understanding how species (hydrogen, nitrogen, liquid and gaseous water) move in cells operating with alternating feeding. Control laws influences these movements, it will be necessary to identify the levers to make the most out of it and then to propose methods to promote the evacuation of water and nitrogen while avoiding the evacuation of hydrogen.
The thesis work will aim to optimize the anode architecture with alternating feeding and to bring this architecture to maturity. The key points are the search for an optimum control of this architecture, the achievement of a hydrogen rejection rate of less than 1%. Finally, this optimization will also have to maximize the durability of the stack.
The doctoral student will have to model the movements of species at different time scales (10ms to 10 minutes), understand the mechanisms, adapt the control laws and validate the new control laws on a test bench.
This work will identify solutions to efficiently evacuate liquid water and nitrogen and minimize H2 rejection and then obtain superior performance compared to conventional architectures.
Dynamic clamping of hygrogen fuel cells: experimental and numerical simulation approach
The impact of the clamping of PEMFC stacks has been demonstrated by the publication of numerous experimental measurements. Passive clamping systems were developped to garantee the minimum elasticity necessary notably during temperature changes or to improve the stress distribution. The new components are finer and finer presenting a reduced elasticity range, moreover latest publications demonstrate the impact of clamping on the deformation and performance of few microns thick active layers and it should be a major improvement to integrate an accurate dynamic clamping.
The first aim of the phD is to study experimetally the impact of the dynamic control of the clamping on the performances of stacks. These tests will be performed with stacks integrating either stamped metallic bipolar plates: the reference technology, or printed cells: the new technology in development at CEA. In parallel, the candidate will learn the model, actually under development thanks to a phD, simulating stresses and deformations, and the associated multiphysic parameters such as porosity or electric resistance, in function of clamping.
Thanks to the synthesis of these experimental and numerical results the candidate will improve the undertanding of the impact of the clamping and will propose solutions to improve notably the durability which is a critical point for our ongoing european or industrial projects.
In function of the phD progress, vibratory tests could be performed to evaluate the potential input of mechanical spectroscopy, notably for diagnosis.
For high-performance, safe, and long-lasting batteries: understanding the role of an additive in liquid electrolytes
The trade-off between performance, aging, and safety remains a major challenge for Li-ion batteries [1]. Indeed, the incorporation of certain additives into the 3rd-generation electrolyte aims to delay or reduce the consequences of thermal runaway, thus reducing the risk of fire or explosion. However, this approach can have negative effects on other key parameters, such as ionic conductivity [2,3]. Therefore, this thesis proposes to study the coupled effects of these additives in order to better understand and potentially predict their impact on each of these indicators.
At the beginning of this work, an additive will be selected to study its role in an NMC 811/Gr-Si chemistry and a 3rd-generation liquid electrolyte, in terms of performance, long-term stability, and safety. The additive will be chosen based on the state of the art and post-mortem analysis of commercial cells representative of the current market. In parallel, new commercial cells of a few Ah will be used. These will be equipped with a reference electrode, internal temperature measurement, and ionic conductivity monitoring. The cells will then be activated with the selected electrolyte at different additive concentrations. Electrochemical performance, along with chemical and morphological characterization of the materials present, will be studied. Key safety parameters (thermal stability, release of reducing gases, O2, released energy, flammability of the electrolyte) for these new cells will be measured at different additive concentrations. The internal instrumentation, including the reference electrode, will also be used innovatively to study the onset of thermal runaway under these conditions.
A full aging campaign will be conducted over a maximum period of one year. At regular intervals, a sample of cells will be studied to characterize the impact of aging on chemical, electrochemical, and morphological changes, as well as on key safety parameters. The most important mechanisms, along with simplified laws governing safety as a function of additive quantity and aging, will be proposed.
[1] Batteries Open Access Volume 9, Issue 8, August 2023, Article number 427
[2] Journal of Energy Storage 72 (2023) 108493
[3] Energy Storage Materials 65 (2024) 10313
Numerical optimisation of internal safety devices of batterry cells depending on chemistry
Thermal runaway (TR) of a battery pack's elementary accumulator is a key factor that can lead to various safety issues, such as fires or explosions, involving both property and people. Several safety devices can prevent and/or mitigate the consequences of thermal runaway, including the PTC (Positive Temperature Coefficient) to limit short-circuit current, the CID (Current Interrupt Device) to disconnect the external electrical terminals from the internal active elements, and the Safety Vent for cell depressurization. Internal gas pressure is the main triggering factor. However, since the gas quantity strongly depends on the chemistry involved, these safety devices should be optimized for future battery generations.
In this PhD thesis, we will develop a methodology for sizing these safety devices through numerical simulations, incorporating all characterizations from the material scale to abusive cell testing. This research will therefore focus on both numerical and experimental aspects in parallel, in collaboration with other laboratories in our department