Mass transfers and hydrodynamic coupling: experimental investigation and models validation and calibration
In the context of the energy transition and the crucial role of nuclear power in a low-carbon energy mix, understanding and then mitigating the consequences of any accident leading to a reactor core meltdown, even a partial meltdown, is an imperative research direction.
During a core meltdown accident, a pool of molten material, known as corium, can form at the bottom of the reactor vessel. The composition of the pool can change over time. The corium bath is not homogeneous and can stratify into several immiscible phases. As the overall composition of the corium changes, so do the properties of the different phases. The vertical stratification order of the phases may change, leading to a vertical rearrangement of the phases. During this rearrangement, one phase passes through the other in the form of drops. The order of the phases and their movements are of prime importance, as they have a major influence on the heat flows transmitted to the tank. A better understanding of these phenomena will enable us to improve the safety and design of both current and future reactors.
Initial models have already been produced, but they lack validation and calibration. Prototype experiments are difficult to set up and none are planned in the short term. This thesis proposes to fill this gap by carrying out an experimental study of the phenomenon using a water-based simulating system that allows local instrumentation and large-scale test campaigns. The aim is to validate and calibrate the existing models, and even develop new ones, with a view to capitalising on these results in the PROCOR software platform, which is used to estimate the probability of a reactor vessel breach. The experimental set-up would be built and operated at the LEMTA laboratory at the University of Lorraine, where the PhD student would be seconded. In terms of experiments, two cases will be studied, the single drop case, and the stratified case with drop formation via Rayleigh-Taylor instabilities.
The work will be mainly experimental, with a component involving the use of code for calibration and validation, and may include a modelling component. It will be carried out entirely at the LEMTA laboratory in Nancy. The PhD student will benefit from LEMTA's expertise in the development of simulating experimental devices, fluid transfers and metrology. They will be part of a dynamic environment made up of researchers and other PhD students. The candidate should have knowledge of transfer phenomena (mass transfer in particular), as well as a definite interest in experimental science.
Understanding the effect of doping on the lifespan of advanced Li-ion battery electrode materials
The development of new electrode materials for Li-ion batteries is primarily focused on two often contradictory objectives: increasing the energy density, and thus the range of vehicles, and reducing the cost of batteries. Disordered NaCl-structured materials, such as Li2MnO2F, thanks to the combination of their Mn-rich, low-cost composition and high Li-ion storage capacity, allow these two aspects to be reconciled. Unfortunately, these materials undergo rapid degradation during cycling, which limits their lifespan. It is therefore necessary to address this degradation to make these materials competitive. Recently, our group has developed a strategy for stabilizing the material by modifying its structure, which is the subject of a patent. The goal of this thesis is to deepen this strategy by improving the understanding of the stabilization mechanism by varying its parameters. The PhD student will have access to all synthesis tools to realize these new materials, as well as electrochemical characterization tools on our battery platform to evaluate their performance. The student will also be required to perform in-depth structural characterizations, notably via various X-ray diffraction methods (including synchrotron).
Assisted generation of complex computational kernels in solid mechanics
The behavior laws used in numerical simulations describe the physical characteristics of simulated materials. As our understanding of these materials evolves, the complexity of these laws increases. Integrating these laws is a critical step for the performance and robustness of scientific computations. Therefore, this step can lead to intrusive and complex developments in the code.
Many digital platforms, such as FEniCS, FireDrake, FreeFEM, and Comsol, offer Just-In-Time (JIT) code generation techniques to handle various physics. This JIT approach significantly reduces the time required to implement new simulations, providing great versatility to the user. Additionally, it allows for optimization specific to the cases being treated and facilitates porting to various architectures (CPU or GPU). Finally, this approach hides implementation details; any changes in these details are invisible to the user and absorbed by the code generation layer.
However, these techniques are generally limited to the assembly steps of the linear systems to be solved and do not include the crucial step of integrating behavior laws.
Inspired by the successful experience of the open-source project mgis.fenics [1], this thesis aims to develop a Just-In-Time code generation solution dedicated to the next-generation structural mechanics code Manta [2], developed by CEA. The objective is to enable strong coupling with behavior laws generated by MFront [3], thereby improving the flexibility, performance, and robustness of numerical simulations.
The selected PhD candidate should have a solid background in computational science and a strong interest in numerical simulation and C++ programming. They should be capable of working independently and demonstrate initiative. The doctoral student will benefit from guidance from the developers of MFront and Manta (CEA), as well as the developers of the A-Set code (a collaboration between Mines-Paris Tech, Onera, and Safran). This collaboration within a multidisciplinary team will provide a stimulating and enriching environment for the candidate.
Furthermore, the thesis work will be enhanced by the opportunity to participate in conferences and publish articles in peer-reviewed scientific journals, offering national and international visibility to the thesis results.
The PhD will take place at CEA Cadarache, in south-eastern France, in the Nuclear Fuel Studies Department of the Institute for Research on Nuclear Systems for Low-Carbon Energy Production (IRESNE)[4]. The host laboratory is the LMPC, whose role is to contribute to the development of the physical components of the PLEIADES digital platform [5], co-developed by CEA and EDF.
[1] https://thelfer.github.io/mgis/web/mgis_fenics.html
[2] MANTA : un code HPC généraliste pour la simulation de problèmes complexes en mécanique. https://hal.science/hal-03688160
[3] https://thelfer.github.io/tfel/web/index.html
[4] https://www.cea.fr/energies/iresne/Pages/Accueil.aspx
[5] PLEIADES: A numerical framework dedicated to the multiphysics and multiscale nuclear fuel behavior simulation https://www.sciencedirect.com/science/article/pii/S0306454924002408
Multiphysic modeling of sintering of nuclear fuel pellet: effect of atmosphere on shrinkage kinetics
Uranium dioxide (UO2) fuels used in nuclear power plants are ceramics, for which solid-phase sintering is a key manufacturing step. The sintering stage involves heat treatment under controlled partial O2 pressure that induces coarsening of UO2 grain and then consolidation and densification of the material. Grain growth induce material densification and macroscopic shrinkage of the pellet. If the green pellet (powder obtained by pressing, manufacturing step before sintering) admit a highly heterogeneous density, this gradient leading to differential shrinkage and the appearance of defects. Furthermore, the sintering atmosphere, i.e., the gas composition in the furnace, impacts grain growth kinetics and thus the shrinkage of the pellet. Advanced simulation is the key to improving understanding of the mechanisms observed as well as optimizing manufacturing cycles.
The PhD thesis aims at developing a Thermo-chemo-mechanical modeling of sintering to simulate the impact of the gas composition and properties on the pellet densification. This scale will enable us to take into account not only the density gradients resulting from pressing, but also the oxygen diffusion kinetics that have a local impact on the densification rate, which in turn impacts the transport process. Therefore, a multiphysics coupling phenomenon has to be modelled and simulated.
This thesis will be conducted within the MISTRAL joint laboratory (Aix-Marseille Université/CNRS/Centrale Marseille CEA-Cadarache IRESNE institute). The PhD student will leverage his results through publications and participation in conferences and will have gained strong skills and expertise in a wide range of academic and industrial sectors.
Potential of magnesium silicate binders for the solidification / stabilization of contaminated soil
Soil contamination by radioactive substances represents a major challenge in terms of public health and environmental protection. Among the various strategies considered for managing such polluted soils, the excavation of contaminated materials offers a pathway to the safe reuse of the site. The excavated soils, when characterized by low to intermediate activity and short-lived radionuclides, must be stabilized prior to disposal. In this context, cementation is widely used due to its moderate cost, ease of implementation, and capacity to confine numerous pollutants. However, its application to soils rich in swelling clays presents two major limitations: poor workability of the fresh material and volumetric instability of the hardened product. To address these issues, this thesis aims to evaluate the potential of magnesium silicate cements as an alternative to conventional calcium silicate cements. These emerging binders are currently attracting growing interest, particularly in the fields of earthen construction and the development of low-carbon materials.
The first objective will be to study the influence of various formulation parameters on the reactivity and properties of magnesium silicate cements. This will be followed by an in-depth investigation of the interactions between the cement phases and the main constituents of contaminated soils. Finally, the long-term durability of the formulated materials will be assessed through leaching tests, which will serve as input for reactive transport modelling, with the aim of gaining a better understanding of the degradation mechanisms and the long-term behaviour of the materials.
This research project is intended for a PhD candidate interested in advancing his/her expertise in materials physical chemistry and contributing to the development of innovative solutions for contaminated soil management and low-impact binder technologies.
Elaboration and durability evaluation of water-permselective multilayer membranes for the CO2 conversion into e-fuels
The catalytic hydrogenation of CO2 into e-fuels is considered to decarbonize certain modes of transport that are difficult to electrify. However, some of the considered reactions are thermodynamically balanced (limited CO2 conversion efficiencies) and catalyst degradation by the produced water is observed. The use of membrane reactors, allowing water separation, is envisaged. For this, the development of water-permselective membranes, without defects and resistant to synthesis conditions, is necessary. Previous studies have targeted LTA and SOD zeolite membranes for this application. However, the presence of defects reduces their selectivity, and their performance deteriorates during operation. The objective of this thesis is therefore to study the sealing of membrane defects and the deposition of protective layers on their surface to improve their performance and durability. To achieve this, the deposition of permselective zeolite layers will first be carried out hydrothermally on suitable porous supports. The sealing of defects by impregnation/conversion of silica precursors in a supercritical CO2 environment will then be studied. Finally, different protective layers (zeolite, ceramic oxide, etc.) will be deposited on the membranes (sol-gel, supercritical CO2, hydrothermal methods). The coatings will be characterized (XRD, SEM, porosimetry, elipsometry, etc.) to ensure their chemical nature, thickness/homogeneity, and porosity. Gas permeation performance will be evaluated at the various stages of preparation, and the durability of the membranes will be studied in the presence of water vapor at different temperatures.
The candidate will work within the Supercritical Processes and Decontamination Laboratory (Marcoule), and will benefit from the laboratory's expertise in ceramic membranes. The student will interact with the laboratory's technicians, engineers, doctoral students and post-doctoral fellows and will exchange with the collaborators of the Reactors and Processes Laboratory (Grenoble). The doctoral student will be involved in the different stages of the project, the publication of results and the presentation of their work at conferences. They will develop solid scientific knowledge in the fields of environment and energy, as well as in project management.
Systemic conditions for the development of the battery industry in Europe: public policies, industrial ecosystem, and geoeconomics.
As a global leader in carbon neutrality, Europe bases its development model on energy transition and has developed decarbonised technological solutions in many areas. However, this political lead has not always translated into industrial competitiveness in the global market, despite efforts to innovate. An industrial decline has been observed, leaving Europe in a weak position in international markets.
The European Union’s objective of achieving carbon neutrality by 2050 requires a profound overhaul of the energy system, which will mobilize a range of technologies. This transition will bring technical, economic, and social challenges.
Recent geopolitical upheavals, such as trade tensions and supply chain volatility, have increased uncertainty in the global geo-economic landscape. Faced with these challenges, decision-makers are seeking to broaden their strategic vision. The EU has recognized the need for strategic autonomy in a fragmented world, where access to certain resources and equipment is becoming more difficult and could be used as a geopolitical weapon.
Gaining control over European supply chains to ensure stable access to energy and critical resources in a context of global competition has now become a political priority. This includes establishing production capacities for low-carbon technologies within Europe. All of these objectives can only be met by combining a wide range of policy measures, striking a balance between energy, environmental and industrial policies. However, some of these measures could come into conflict with the policies implemented over the last few decades to build the European energy market, as well as those underpinning trade and investment relations.
In this context, this thesis proposes a theoretical framework for analysing the systemic conditions for the development of the European battery industry, integrating the dimensions of public policy, industrial sovereignty and geo-economic issues. It will be carried out within the Energy Markets Regulation and Organization (ROME) research unit of the CEA's Institute for Research and Studies in Energy Economics (I-Tésé), in academic partnership with the University of Paris Dauphine-PSL.