Simplified Model for Rotary Tube Calcination
Since the vitrification lines at La Hague began operation in 1989, ORANO (formerly AREVA) has faced difficulties in controlling the calciner. Actions taken to significantly reduce these problems have considerably eased them, but without completely eliminating them. Most of the recommended actions are based on expert opinions, which themselves are based on inactive test results that don't cover all situations encountered by ORANO. To definitively resolve these control difficulties, it was decided to launch a more theoretical modeling study, while simultaneously investigating new calciner control instrumentation.
Microfluidics for biomimetic detection of airbone pathogens
Air represents a complex contamination pathway that is difficult to control and through which numerous biological, biochemical, or chemical agents can affect populations and healthcare workers. Standard detection approaches, whether qPCR, antigen tests, or ELISA tests, rely on reagents specific to known and targeted agents. These approaches are therefore unsuitable for detecting an unknown pathogen that could result in a new pandemic. To face such unknown agents, new biosensors will be needed to distinguish between pathogenic and non-pathogenic agents. Also, these sensors will have to be miniature for deployment.
With a new microfluidic system the present project aims to explore original approaches for conducting such detection without preconceived notions. Based on the laboratory's experience and developments, the PhD will include :
- developing new materials and designs to optimize and to enable multiple bioaerosol sampling;
- developing a biomimetic biochip and optimize molecular interactions using microflows controlled at the micro/milliscale.
You will design a microfluidic card integrating new detection strategies and study them experimentally using prototypes already developed in the laboratory.
Optimization of transports in the Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells: Artificial Intelligent as a support to define optimal porous structures and usage
The design and manufacturing of innovative materials with required properties is a key objective for developing advanced technologies in the field of energy, such as Hydrogen and Alcaline fuel cells and Electrolysers. These improvements will contribute to propose even more attractive low-carbon electrical energy systems, with reduced pollution and green-house effects.
This thesis focuses on the Gas Diffusion Layer (GDL) which plays a crucial role on the performance and durability of Proton Exchange Membrane Fuel Cell (PEMFC).
Your main aim will be to set-up a numerical approach so as to propose improved porous structures to optimize the different transports inside a GDL, for given targets and constraints. To do so, you will make the bridge between advanced modeling of (electrical, heat, liquid, gas) transports in 3D porous media and artificial intelligence. You will then analyze the influence of the operating conditions on such optimal structures and propose design recommendations.
This work will be conducted in close relationship between world-renowned scientific actors : the fuel cells and the modelling teams of CEA/LITEN (Grenoble), the specialists of transports in porous media at CNRS/IMFT (Toulouse), and the specialists of GDL, modeling and AI at FZJ (Juelich, https://www.fz-juelich.de/en).
Scientific publications are expected and patents could also be proposed.
Dynamics of a very high temperature heat pump coupled to a thermal storage system. Experimental and numerical study.
In the context of an electricity mix with a high proportion of intermittent renewable energy sources, massive energy storage solutions will be of major interest. For the vast majority of these solutions, electricity is converted into energy that can be stored on a large scale (e.g. pressure energy, chemical or electrochemical energy, etc.), then converted back into electricity. Losses occur during each of these stages (conversion, storage), so the efficiency of the complete system is an important issue and requires a good understanding of each conversion and storage stage.
The innovative system that we want to study is a Carnot battery, i.e. a thermal battery associated with thermodynamic conversion cycles (electrical energy to thermal energy to electrical energy). The anticipated advantages are numerous: the possibility of integrating thermal flows, the absence of geographical constraints, a degree of freedom in the choice of temperatures and storage materials, the use of alternators for inertia, etc. The identified challenges are reactivity and overall efficiency.
The research will focus on the charging cycle (very high temperature heat pump) and its coupling with thermal storage, initially from a static and then a dynamic perspective. Unsteady numerical modelling will be developed and used to design the Carnot battery system. Tests carried out on an experimental installation at the CEA will be used to validate and enhance the modelling results.
Etude du comportement d'un composite CMC en température par essais in situ en tomographie X
The proposed topic concerns the study of the mechanical behavior of an oxide/oxide ceramic matrix composite material at temperature (up to 1000°C). The originality of the subject lies in the use of in situ X-ray tomography to access, on the one hand, the macroscopic deformation of the tested specimens and, on the other hand, the microscopic damage mechanisms that characterize this type of so-called "damageable" material.
This technique was developed at room temperature during a previous thesis: the aim here is to apply it at higher temperature and to more complex stresses (e.g., traction-torsion). The aim will also be to propose developments to the existing volumetric image correlation analysis protocol.
Implicit/explicit transition for numerical simulation of Fluid-Structure Interaction problems treated by immersed boundary techniques
In many industrial sectors, rapid transient phenomena are involved in accident scenarios. An example in the nuclear industry is the Loss of Primary Coolant Accident, in which an expansion wave propagates through the primary circuit of a Pressurized Water Reactor, potentially vaporizing the primary fluid and causing structural damage. Nowadays, the simulation of these fast transient phenomena relies mainly on "explicit" time integration algorithms, as they enable robust and efficient treatment of these problems, which are generally highly non-linear. Unfortunately, because of the stability constraints imposed on time steps, these approaches struggle to calculate steady-state regimes. Faced with this difficulty, in many cases, the kinematic quantities and internal stresses of the steady state of the system under consideration at the time of occurrence of the simulated transient phenomenon are neglected.
Furthermore, the applications in question involve solid structures interacting with the fluid, undergoing large-scale deformation and possibly fragmenting. A immersed boundary technique known as MBM (Mediating Body Method [1]) recently developed at the CEA enables structures with complex geometries and/or undergoing large deformations to be processed efficiently and robustly. However, this coupling between fluid and solid structure has only been considered in the context of "fast" transient phenomena treated by "explicit" time integrators.
The final objective of the proposed thesis is to carry out a nominal regime calculation followed by a transient calculation in a context of fluid/immersed-structure interaction. The transient phase of the calculation is necessarily based on "explicit" time integration and involves the MBM fluid/structure interaction technique. In order to minimize numerical disturbances during the transition between nominal and transient regimes, the calculation of the nominal regime should be based on the same numerical model as the transient calculation, and therefore also rely on an adaptation of the MBM method.
Recent work defined an efficient and robust strategy for calculating steady states for compressible flows, based on "implicit" time integration. However, although generic, this approach has so far only been tested in the case of perfect gases, and in the absence of viscosity.
On the basis of this initial work, the main technical challenges of this thesis are 1) to validate and possibly adapt the methodology for more complex fluids (in particular water), 2) to introduce and adapt the MBM method for fluid-structure interaction in this steady-state calculation strategy, 3) to introduce fluid viscosity, in particular within the framework of the MBM method initially developed for non-viscous fluids. At the end of this work, implicit/explicit transition demonstration calculations with fluid-structure interaction will be implemented and analyzed.
An internship can be arranged in preparation for thesis work, depending on the candidate's wishes.
[1] Jamond, O., & Beccantini, A. (2019). An embedded boundary method for an inviscid compressible flow coupled to deformable thin structures: The mediating body method. International Journal for Numerical Methods in Engineering, 119(5), 305-333.
Turbulence synthesization methods for hybrid URANS/LES CFD approaches in multi-scale simulation of nuclear cores
Problem description: Fluid-structure interactions in nuclear reactor cores are a result of mechanisms occurring at different space scales. The component scale represents the global flow inside the core and is generally simulated though porous media methods. The local scale represents the fuel assembly: it requires CFD scale-resolving methods to calculate consistent fluid forces on the structures, and it features a certain degree of fluid-structure coupling. With the goal of performing multi-scale simulations of a core, the local scale requires the generation of boundary conditions from the component scale. This can be achieved only by a synthetic generation of turbulence, based on the flow results at the component scale. However, the porous media approach used at the component scale does not contain details on the turbulent quantities: the development of new numerical methods is required for generating consistent synthetic turbulence in this configuration.
Objectives:
1. Identify proper hybrid URANS/LES approaches for fuel assembly vibration related issues
2. Identify available turbulence parameters in porous media methods and explore bottom-up scaling approaches
3. Develop a turbulence synthesization method applicable to any fuel array inside a core
Expected results:
1. A novel approach for fluid-induced vibration analysis based on a multi-scale method
2. Clarify the key parameters to generate proper turbulence-resolved boundary conditions in the specific configuration studied
3. Validate the new methods on available experimental configurations
In situ 3D visualization and modeling of grain growth during solidification of 316L steel in welding and additive manufacturing processes
CEA is currently carrying out R&D studies to assess the potential of Additive Manufacturing (AM) processes using wire deposition (WAAM and WLAM) for 316L steel, a material used in the manufacture of a large number of components. These processes are similar to the welding techniques currently used in the manufacture and repair of parts for the nuclear industry. Microstructures with a strong crystallographic texture are often obtained after welding or additive manufacturing, leading to highly anisotropic mechanical behaviors, and the prediction of these microstructures is also a key element in ensuring the reliability of non-destructive testing of parts manufactured in this way.
The aim of the thesis, which will be based on a coupled experimental/simulation approach, is to gain a better understanding of the main physical phenomena involved in solidification, in particular grain growth.
To this end, an original approach to characterizing these phenomena will be conducted on the basis of an innovative instrumented test, with the aim of obtaining a high-resolution quasi-3D view of the molten zone during solidification. The results of the experimental approach will enrich the physical models of solidification, already implemented in a 3D CA-FE (Cellular Automaton-Finite Element) model, combining a Cellular Automata (CA) approach and thermal or multiphysics modeling (FE) of the molten bath, to simulate the solidification microstructures resulting from additive manufacturing and welding processes.
Study of the influence of the microstructure of a 316L steel produced by the L-PBF process on its mechanical properties: characterization and modeling of creep and fatigue behavior
Research into additive manufacturing for the nuclear industry shows that the production of 316L austenitic steel components using laser powder bed fusion (L-PBF) presents technical challenges, including process control, material properties, qualification and prediction of mechanical behaviour under service conditions. The final properties differ from traditional processes, often exhibiting anisotropy that challenges existing design standards.
These differences are linked to the unique microstructure resulting from the L-PBF process. Controlling the manufacturing chain, from consolidation to qualification, requires an understanding of the interactions between process parameters, microstructure and mechanical properties.
The aim of this thesis is to study the relationships between the microstructure, texture and mechanical properties of 316L steel manufactured by the L-PBF process, under static or cyclic loading. This includes the influence on creep and fatigue properties, and the development of a model to predict mechanical behaviour. Using samples of 316L steel with specific microstructures consolidated by L-PBF, the proposed study aims to establish links between microstructure and mechanical properties to better predict in-service behaviour.