Effect of gravity on agitation within a turbulent bubbly flow in a channel
Understanding two-phase flows and the boiling phenomenon is a major challenge for the CEA, for both the design and safety of nuclear power plants. In a Pressurized Water Reactor (PWR), the heat generated by the nuclear fuel is transferred to the water in the primary circuit. Under accident conditions, the water in the primary circuit can enter a nucleate boiling regime, or even evolve to a boiling crisis. While the phenomenon of boiling is the subject of numerous studies, the dynamics of the generated bubbles also receive special attention at the CEA. This thesis will focus on the coupling between the turbulence generated by a shear flow and the agitation induced by the bubbles. Its originality lies in the study of the effect of gravity, achieved by tilting the channel, a parameter that can generate complex flow regimes.
This experimental work will be based on the new CARIBE facility at CEA Saclay. The PhD student's mission will be to characterize the different flow regimes and then to conduct a detailed study of the flow by implementing specific metrology (including Particle Image Velocimetry (PIV), hot-film anemometry, and optical probes). Conducted within the LE2H laboratory, the project will benefit from a close collaboration with the LDEL (CEA Saclay) and the IMFT (Toulouse). The PhD student will work in a dynamic environment with other PhD students and will present their work at national and international conferences.
We are looking for a candidate with a background in fluid mechanics and a strong interest in experimental work (a Master's thesis internship is possible). This PhD offers the opportunity to develop expertise in instrumentation, data analysis, and turbulent two-phase flows—skills that are highly valued in the energy, industrial, and academic research sectors.
Localised solidifications in Molten Salt Reactors
In a Molten Salt Reactor (MSR), the nuclear fuel is a liquid, high-temperature salt which acts as its own coolant. Some accidental transients (over-cooling of the fuel, leak) may cause localised solidifications of the fuel salt in the core. These solidifications will have in turn an impact on the salt flow in the core, as well as its neutronic behavior, and could lead to a localised over-heating of the core vessel. Such transients are not well studied, although they have a major impact on the safety and design of an MSR.
The objective of the PhD is to study different accidental transients that would lead to localised solidifications, and to study their impact on the neutronics and thermal-hydraulics of the core. These analyses will require the use of multiphysics, MSR-adapted numerical tools, such as the CFD code TrioCFD and its extensions TRUST-NK (neutronics) and Scorpio (reactive transport), as well as the deterministic neutronic code APOLLO3. In order to balance precision and computation time, different models will be tested, depending on the transient studied: 1D/ turbulent 3D (RANS, LES) models for thermal-hydraulics ; diffusion / SPn transport / Sn transport for neutronics.
Constrained geometric optimization of immersed boundaries for thermal-hydraulic simulations of turbulent flow in a finite-volume approach
The technical issue underpinning this thesis topic is the mitigation of the consequences of a loss of primary coolant accident in a pressurized water reactor with loops. It is of the utmost importance to minimize the flow of water leaving the vessel and to manage the available cold water reserves for safety injections as effectively as possible, in order to prevent or delay core flooding, overheating, and possible core degradation. To this end, the use of passive devices operating on the principle of hydraulic diodes, such as vessel flow limiters or advanced accumulators, is being considered. The subject of this thesis is the geometric optimization of this type of device, described by an immersed boundary, in order to maximize its service efficiency.
Several recent theses have shown how to introduce the Penalized Direct Forcing (PDF) immersed boundary method into the TRUST/TrioCFD software, under various spatial discretizations and for laminar and turbulent regimes. Similarly, they have ruled on the possibilities of deterministic geometric optimization in the finite-element context during simulations, based on the use of the PDF method.
After a bibliographic study of this kind of method, we will focus on the possibilities of implementation in finite volume discretization, the consideration of constraints, and the comparison to reference calculations. The latter will be carried out on academic and industrial configurations (accumulators and flow limiters).
The doctoral student will work in a R&D unit on innovative nuclear system within the IRESNE Institute (CEA Cadarache. He will develop skills in fluid mechanics and numerical methods.
Nucleate boiling within porous deposits: study of the coupling between coolant composition and capillary vaporization
In the search of the optimal combination of low-carbon energy sources to address the challenge of climate change, nuclear energy plays a crucial role alongside intermittent renewable energies. In this context, the performance and safety of Pressurized Water Reactors (PWRs), which make up the French nuclear fleet, remain an active and high-value research area.
In these reactors, a subcooled nucleate boiling regime can occur, particularly when the local temperature of the coolant exceeds its saturation temperature. This wall boiling promotes the formation of porous deposits of metallic oxides. Within the porosities of these deposits, gas nuclei can be trapped and lead to the onset of nucleate boiling on these surfaces. The vapor formed through a wick boiling or capillary vaporization mechanism then escapes through the chimneys of the deposit. The chemistry of the coolant affects not only the thermodynamic properties of the fluid (such as saturation temperature and latent heat) but, more importantly, its interfacial properties (surface tension and solid/liquid/gas contact angles). These interfacial properties directly control the capillary forces within the deposits, and thus the onset and dynamics of subcooled boiling. As of today, the influence of coolant chemistry on the initiation and development of subcooled nucleate boiling within porous heated surfaces remains poorly understood.
The objective of this PhD is therefore to systematically study the coupled influence of coolant composition and capillary vaporization on nucleate boiling within porous substrates heated by conduction.
The research will follow an experimental approach to investigate how coolant chemistry affects surface tension and contact angles, in order to characterize fluid wetting on idealized porous substrates. Subcooled convective boiling experiments will also be conducted, with the phenomena characterized by shadowgraphy and fiber-optic thermometry.
The PhD will take place within the Thermal Hydraulics of Core and Circuits Laboratory (LTHC) and the Contamination Control, Coolant Chemistry and Tritium Management Laboratory (LMCT) at CEA IRESNE (Cadarache, France). The work will be supervised by Prof. Benoît Stutz of the University of Savoie Mont Blanc. Throughout this project, the doctoral student will develop expertise in interfacial physico-chemistry and two-phase thermohydraulics through the observation, characterization, and modeling of complex multiphysics phenomena.
Detailed Numerical investigations on highly-concentrated bubbly flows
To assess the safety of industrial facilities, the CEA develops, validates, and uses thermohydraulic simulation tools. Its research focuses on modelling two-phase flows using various approaches, from the most detailed to the largest system-scale. In order to better understand two-phase flows, Service of Thermal-hydraulic and Fluid Mechanics (STMF) is working on implementing a multi-scale approach in which high-fidelity simulations (DNS, Direct Numerical Simulation of two-phase flows) are used as “numerical experiments” to produce reference data. This data is then averaged to be compared with models used on a larger scale. This approach is applied to high-pressure flows where the bubbly flow regime is maintained even at very high void fractions. The Laboratory of Development at Local Scales (LDEL) belonging to STMF has developed a DNS method (Front-Tracking) implemented in its open-source thermo-hydraulics code: TRUST/TrioCFD [1] (object-oriented code, C++). In several PhDs, it has been used to perform massively parallel simulations to describe interfaces in detail without resorting to models, for example in groups of bubbles (called swarms) [2][3][4].
Currently applied to low-concentration two-phase bubbly flows (volume fraction less than 12%), the objective of this thesis will be to evaluate and use the method at higher void fractions. Reference HPC simulations of bubble swarms will be conducted on national supercomputers up to gas fractions of 40%. The quality of the results will be evaluated before extracting physical models of bubble interactions under these conditions. The objective of these models is to recover the overall dynamics of the bubble swarm at much lower resolutions, thereby enabling the study of larger systems in disequilibrium (external forcing of imposed turbulence generation, imposed average velocity gradient, etc.).
This work is funded by the French ANR, in collaboration with IMFT and LMFL, in parallel with two other theses with which there will be strong interactions. It will be performed at CEA-Saclay, in the STMF/LDEL laboratory. It includes numerical aspects (validation), computer developments (C++), and a physical analysis of the flows obtained.
Diphasic thermoregulation system for ultra wide bandgap diamond semiconductors
The objective of this thesis is to study a diphasic thermoregulation system for ultra wide bandgap diamond semiconductors. One of the specific behavior of diamond semiconductors is the negative temperature coefficient of is on-state resistance. The thermoregulation proposed in this thesis aims to optimize the global losses of the system and to insure both temperature and electrical constraints between several diamond semiconductors in parallel.
Based on specifications that will be defined at the beginning of this theses (calories to dissipate, temperature range to control), the PhD candidate will have to:
- Define a temperature control strategy
- Define most appropriate materials and fluid of this system
- Design the thermoregulation system
- Realize and validate experimentally the proposed system
This thesis will tackle numerical simulation (component and thermoregulation system modelling) and experimental tests through the realization of a TRL 3-4 prototype of power converter system integrating diamond Schottky diodes.
The global objective to achieve is to put forward an innovative system modeled and experimentally demonstrated, where control strategy, dimensional and operative elements will be investigated and optimized.
Development of Zinc Sodium-ion batteries for stationary storage of renewable energy
In the global context of massive renewable energy deployment, production and storage are becoming increasingly intertwined. Battery electrochemical energy storage systems (BESS) are currently experiencing strong market growth. These systems differ radically from electric mobility solutions due to their specific characteristics (cost, safety, durability). Faced with the limitations of Li-ion batteries (fire risks, the criticality of lithium and cobalt, production costs), aqueous zinc/sodium-ion technology presents a disruptive alternative. Based on abundant, non-toxic, and inherently safe materials, it offers unique potential for long-term storage with a low environmental impact. The zinc battery sector faces scientific challenges that limit reversibility and cycle life, notably the formation of zinc dendrites and cathode instability. This doctoral thesis project proposes to overcome these obstacles through a research and development strategy for innovative electrodes based on the reversible transformation of zinc into zinc phosphate in an aqueous sodium phosphate medium. This choice of electrolyte allows the use of sodium-ion positive electrodes as well as AGM (absorptive glass mat) separators, developed notably for lead-acid batteries.
The thesis work will focus on experimental electrochemical studies combined with multiphysics modeling of the system at the cell scale, taking into account the thermodynamics and kinetics of the included reactions. This approach will allow for the rapid exploration of a vast design space to identify the conditions enabling scaling up and transfer to industry, meeting the imperatives of energy sovereignty and the circular economy.
Innovative techniques for evaluating critical steps and limiting factors for batteries formation
The battery manufacturing sector in Europe is currently experiencing strong growth. The electrical formation step that follows battery assembly and precedes delivery has received little academic attention, despite being crucial for battery performance (lifespan, internal resistance, defects, etc.). It is an essential time-consuming and costly step in the process (>30% of the cell manufacturing cost, and 25% of the equipment cost in a Gigafactory) that would greatly benefit from optimization.
In this thesis, we propose studying battery formation using innovative, complementary, operando non-intrusive techniques. The goal is to identify the limiting mechanisms of the electrolyte impregnation step (filling electrode pores) and of the initial charge. The candidate will implement experimental methods to monitor and analyze these mechanisms. He will also establish a methodology and protocols for studying these steps, combining electrochemical measurements with non-intrusive physical characterizations under operating conditions. The research will focus on optimizing formation time and quality control during this stage.
Adjoint sensitivity method applied to industrial modeling of nuclear reactor cores
The objective of this thesis is to lay the foundations for applying the adjoint sensitivity method to industrial modeling of solid fuel nuclear reactor cores. The main topic will be the consideration of the coupling between neutronics, thermohydraulics, heat diffusion in fuel rods, and evolution.
Modeling and dynamic studies of a space Nuclear Electric Propulsion system
Nuclear technology is key to enabling the establishment of scientific bases on the Moon or Mars, or for exploring deep space. Its use can take several forms (RTG, NTP among others), but this thesis focuses on Nuclear Electric Propulsion (NEP): heat produced by a nuclear reactor is converted into electricity to power an ionic propulsion engine. Various concepts have been studied in the past (PROMETHEUS, MEGAHIT and DEMOCRITOS, typically for Jupiter satellite exploration missions), while currently design studies are underway at CEA for a 100 kWe nuclear-electric NEP system.
The system of interest combines several specific design choices: uranium nitride fuel, direct gas cooling (helium-xenon mixture) and energy conversion system based on a Brayton cycle, as well as waste heat evacuation through thermal radiation. These choices address requirements to minimize mass and volume, and to ensure performance and reliability for the duration of the scientific mission. Analysis of the dynamic behavior of the nuclear-electric system is therefore crucial for project success. However, the issue of transient modeling of a complete spatial nuclear-electric system is very poorly addressed in the state of the art, especially for NEP.
The thesis objectives are therefore to research and develop physical models adapted to a NEP system, to propose an approach for their validation, and finally to implement them to analyze the dynamic behavior of the reactor and contribute to improving its design. Several mission phases will be studied: reactor startup in space, power variation transients for the ionic propulsion engine, reactor response in case of failure, and its potential shutdown with the problem of safe residual power evacuation.
The thesis will be conducted at IRESNE Institute (CEA Cadarache), in a stimulating scientific environment, and integrated into a team designing innovative nuclear reactors. CNES will also be involved in monitoring the work, particularly to define the ionic propulsion engine characteristics and exploration missions of interest for the nuclear-electric system. The thesis topic, combining modeling, fluid mechanics, thermodynamics, neutronics, and space mechanics, will lend itself to scientific communication and allow the development of key skills for an academic or industrial career.