Characterisation of the gaseous leak at the contact interface between rough surfaces during loading and unloading - application to the case of metal seals
In various industrial applications, fully metallic seals are employed to guarantee a high level of sealing of mechanical assemblies under severe thermodynamics conditions. Their performance is entirely controlled by the mechanical behaviour of the contacting interface between the facing rough surfaces of the seal and the flange, similar to a fracture, anisotropic and multi-scale by nature. The objective of the thesis is to improve our comprehension and predicting capabilities of the sealing mechanisms of gases in a rough fracture using a numerical approach coupled with experiments.
The work takes place in the continuity of previous studies performed at the laboratory. It will focus first on the conception of an experimental apparatus that will be used to press two metallic rough surfaces against each other with a given force, having the possibility to measure the corresponding leakage rate as well. The experiments will be performed during loading and unloading of the contact to characterise the hysteresis phenomenon brought by the permanent deformation of the sealing material at first loading. The results obtained will be compared to numerical ones in various configurations using models developed at the laboratory, in order to validate these latter. By experience, it is known that the flow simulation gives satisfactory results, but discrepancies persist in the contact mechanics model. Thus, it should be improved regarding the plastic effects specifically encountered in contact, considering the finite thickness of the sealing liner and optimizing the computational cost. Afterwards, the preceding results will be transposed to the industrial case of the HELICOFLEX metal seal, using a two-scale modelling strategy, coupling the macroscopic information at the seal scale to the microscopic one at the roughness scale.
Sub-Grid modelling of interfacial heat and mass transfers applied to condensation of bubble swarms
To assess the safety of nuclear power plants, the CEA develops and uses multi-scale thermohydraulic simulation tools. The application of CFD to two-phase flows is limited because it requires many models that are difficult to determine. Among our other tools, direct numerical simulations (DNS) with resolved interfaces provide reference data inaccessible by experimental means. This is for example the case of bubble swarms, where heat and mass transfers are influenced by complex collective effects.
In order to reduce the cost of these DNS simulations, we recently developed an approach [1] which shows promising results: it consists of coupling a fine resolution of thermal transfers at the liquid-vapor interfaces to a far field calculated on a less resolved mesh. To broaden the application of this method to more industrial cases, it is necessary to take into account collisions between bubbles and to adapt the model to the phase change.
During this thesis, we propose to start with this physical modeling work and its implementation in C++ in our open-source simulation code TRUST/TrioCFD [2]. Next, we will use this new capacity to carry out a parametric study and an in-depth physical analysis of the phenomena which would ultimately lead to an improvement in heat transfer models in industrial codes.
[1] M. Grosso, G. Bois, A. Toutant, Thermal boundary layer modelling for heat flux prediction of bubbles at saturation: A priori analysis based on fully-resolved simulations, International Journal of Heat and Mass Transfer, Vol 222, 2024, https://doi.org/10.1016/j.ijheatmasstransfer.2023.124980
[2] Trio_CFD webpage : http://triocfd.cea.fr/recherche/modelisation-physique/two-phase-flows
A macroscale approach to evaluate the long-term degradation of concrete structures under irradiation
In nuclear power plants, the concrete biological shield (CBS) is designed to be very close of the reactor vessel. It is expected to absorb radiation and acts as a load-bearing structure. It is thus exposed during the lifetime of the plant to high level of radiations that can have consequences on the long term. These radiations may result especially in a decrease of the material and structural mechanical properties. Given its key role, it is thus necessary to develop tools and models, to predict the behaviors of such structures at the macroscopic scale.
Based on the results obtained at a lower scale - mesoscopic simulations, from which a better understanding of the irradiation effect can be achieved and experimental results which are expected to feed the simulation (material properties especially), it is thus proposed to develop a macroscopic methodology to be applied to the concrete biological shield. This approach will include different phenomena, among which radiation-induced volumetric expansion, induced creep, thermal defromations and Mechanical loading.
These physical phenomena will be developed within the frame of continuum damage mechanics to evaluate the mechanical degradation at the macroscopic scale in terms of displacements and damage especially. The main challenges of the numerical developments will be the proposition of adapted evolution laws, and particularly the coupling between microstructural damage and damage at the structural level due to the stresses applied on the structure.
Multi-physical characterization of potassium hybrid supercapacitors for performance improvement
The PhD subject focuses on the optimization of potassium hybrid supercapacitors (KIC), which combine the properties of supercapacitors (power, cyclability) and batteries (energy). This system, developed at the CEA, represents a promising technology, low cost and without critical/strategic materials. However, performance optimization still requires overcoming various obstacles observed in previous work, in particular on the intercalation of potassium in graphite and the heating phenomena of cells during operation. In order to explore in depth the operating mechanisms of the KIC system, an essential part of the thesis project will include experiments conducted at the ESRF (European Synchrotron Radiation Facility), where advanced diffraction and imaging techniques will be used to analyze the structure of the materials and their behavior in real operating conditions. The processing of the data collected will also be crucial in order to establish correlations between the physicochemical properties of the materials and the overall performance of the system. This thesis will contribute to the fundamental understanding of the multi-physical mechanisms at stake in KIC to develop innovative design strategies and thus improve their capacity, energy efficiency and lifetime.
Study of an innovative cleaning process dedicated to the treatment of residual sodium in facilities using liquid sodium as a coolant
Sodium is used as a heat transfer fluid in fast neutron nuclear reactors. Given the operating temperatures of these facilities, all surfaces in contact with liquid sodium remain wetted with residual sodium once the circuits have been drained. The treatment of this residual sodium is required to ensure the safety of interventions on components and structures in a dismantling process. The reference method for this action is cleaning with water in a dedicated cleaning pit. This process involves a reaction of sodium with water in different forms, by controlling the reaction kinetics, which is instantaneous and highly exothermic without controlling the contacting of the reagents.
An exploratory study was carried out at CEA (PhD thesis defended in 2014) on the use of salts to mitigate reaction kinetics. The Sodium and advanced coolant technology laboratory (DES/IRESNE/DTN/STCP/LESC) thus has R&D facilities, instrumented and dedicated to the study of sodium cleaning processes and equipped with the functionalities of an industrial cleaning pit , such as spray nozzles, atomizing nozzles and an immersion device.
The main scientific objective of this new PhD is now to identify, understand and model the physicochemical mechanisms involved in the sodium-water reaction kinetics involving salts. This work will make it possible to limit or avoid pressure wave phenomena or of explosion during the treatment of residual sodium from fast neutron nuclear reactor circuits during their decommissioning and dismantling. The PhD student's mission will be to define the experimental design, to actively participate in carrying out the test campaigns, to analyse the results and to propose an interpretation of the observed phenomena (kinetics, pressure peak, local temperature rise, etc.). The aim of the experimental campaign will be to acquire reliable thermodynamic and reaction kinetic data, such as reaction times, variation of dynamic pressure, temperature rise, composition of the gas and liquid phases, speciation in liquid phase and visualization of the phenomenology via high-speed camera. Modelling tools will be used to establish and simulate a reaction kinetic model. Ultimately, the proposed work will make it possible to qualify the process for industrial application in the field of decommissioning/dismantling, which is a major challenge for the French nuclear industry.
In addition to the experience acquired in the field of nuclear systems dismantling, the proposed work opens up professional prospects, particularly towards research centers and R&D departments in industry.
A master internship is proposed by the team in addition to the thesis.
Uncertainty quantification and sensitivity analysis for vibrations of thin structures under axial flow
Fluid-structure interaction (FSI) phenomena are omnipresent in industrial installations where structures are in contact with a flowing fluid that exerts a mechanical load. In the case of slender flexible structures, IFS can induce vibratory phenomena and mechanical instabilities, resulting in large displacement amplitudes. The nuclear industry is confronted with this problem, particularly concerning piping, fuel assemblies, and steam generators. Computation codes are an essential tool that, based on several input parameters, provide access to quantities of interest (output variables) that are often inaccessible experimentally for the prevention and control of vibrations. However, knowledge of input parameters is sometimes limited by a lack of characterization (measurement error or lack of data) or simply by the intrinsically random nature of these parameters.
In this context, this thesis aims to analyze the vibratory response of a thin structure with uncertain geometric characteristics (structure with a curvature defect, localized or global). In particular, we aim to understand how geometric uncertainties affect the stability of the flexible structure.
This characterization will be carried out both theoretically and numerically. As the work progresses, the effect of different uncertainties (linked, for example, to the material characteristics of the structure or the properties of the incident flow) may be considered. Ultimately, the work carried out as part of this thesis will enable us to improve the prediction and control of vibrations of thin structures under axial flow.
Fluid-structure interactions and associated instabilities are present in many fields, whether in aeronautics with the phenomena of wing flutter, in nuclear power with the vibrations of components under flow, in biology for the understanding of underwater animal locomotion, in botany for the understanding of plant growth, in sport for performance optimization, in energy recovery from fluid-excited flexible structures. The thesis will enable the student to acquire a wide range of skills in mathematics, numerical simulation, fluid mechanics and solid mechanics, and to train for research in the field of fluid and solid mechanics, leading ultimately to a career in this field, whether in academia or in applied research and development in numerous fields of interest to scientists and society in general. A 6-month internship subject is also offered as a preamble to the thesis (optional).
Education level: Master 2 / Final year of engineering school.
Required training: continuum mechanics, strength of materials (beam theory)
fluid mechanics, fluid-structure interaction, numerical simulation (finite elements).
Multiscale dynamics of a slender structure with frictional singularities: application to a fuel assembly
The dynamic modeling of complex structures may require to take into account phenomena occurring at very different scales. However, a full refined modeling of this type of structure generally leads to prohibitive calculation costs. Multiscale modeling then presents an alternative solution to this problem, taking into account each phenomenon at the most appropriate scale.
We are interested here in slender structures subjected to mechanical stresses with frictional contacts between the structure and the retaining elements. The behavior of slender structures is in general represented by beam models, but accurately taking into account all the local contact/friction requires massive 3D models.
The originality of the work proposed here is to build an efficient multiscale and multimodel approach between beam and massive models which makes it possible to locally take into account the friction contact of slender structures. We are therefore moving towards the use of local multigrid (or multilevel) methods which naturally allow a non-intrusive multiscale coupling. The accuracy of these methods depends on the choice of transfer operators between scales, which must be carefully defined. It will also be necessary to take into account the incompatibility of the meshes supporting the models on the various relevant scales. Hence, the final model will consist in an enriched beam model taking into account local contact phenomena.
The developed model will be compared with experimental results obtained during test campaigns already carried out, and with reference numerical solutions, of increasing complexity, intended to finely validate the relevance of the proposed multiscale approach.
The strong potential of the targeted multiscale approaches, applied in this subject to the nuclear field, could be exploited by the candidate for other industrial issues such as those of aeronautics or the automotive industry.
This thesis will take place within the framework of the joint MISTRAL laboratory between the CEA and the LMA (Laboratoire de Mécanique et d’Acoustique) in Marseille. The PhD student will carry out the major part of his thesis within the CEA (IRESNE institut, Cadarache) in teams specialized in numerical methods and dynamic modeling of complex structures. The doctoral student will travel regularly to Marseille to discuss with the university supervisors.
Kinetics of the Melting Front in a Phase Change Material Used for Decay Heat Removal in an Innovative Nuclear Reactor
In the context of developing innovative sodium-cooled fast reactors (SFR), this PhD aims to explore the use of a phase change material (PCM) to remove residual power. The PCM studied in this project is Zamak, a metallic alloy that presents advantageous properties for such thermal applications. Some SFR designs incorporate passive safety systems intended to ensure the removal of residual power, which refers to the heat generated by delayed fission and radioactive decay of fuel isotopes after reactor shutdown. The use of PCM is a promising option, as they can absorb and store heat through a melting process and subsequently release it gradually during a solidification process.
The core of this PhD focuses on Computational Fluid Dynamics (CFD) modeling of the Zamak melting process and the scaling of this model for use in a system-size calculation tool. The main challenge lies in predicting the behavior of the melting front, its stability, and its impact on the kinetics of residual power removal. This melting front is influenced by numerous factors such as the wetting angle and the physico-chemical properties of the PCM-wall or PCM-surrounding gas interface, which will be examined throughout the thesis. The research will thus involve developing a CFD model that integrates these aspects, using a porous enthalpy approach, allowing predictive simulations of the PCM's behavior in the residual power removal system. A scaling analysis will then be conducted.
The PhD candidate will be part of a research team on innovative reactors at the IRESNE institute located at the CEA Cadarache site. Career opportunities after the thesis include academic research, R&D, and the nuclear industry, as well as sectors utilizing PCM technologies.
High-isolation power supply
With the rapid evolution of technologies and the growing challenges of miniaturization and resource management, power converters are facing ever more stringent performance requirements. To meet these needs, the use of wide-bandgap semiconductors such as SiC (silicon carbide) and GaN (gallium nitride) is becoming increasingly common. These materials significantly increase the switching speed of converters, reducing losses and improving efficiency.
However, this switching speed brings additional challenges: the steepness of the switching edges can cause stray currents that interfere with switch controls. To counter these undesirable effects, it is necessary to use switch drivers offering a high level of insulation. The traditional solution is based on high-frequency magnetic transformers, but these devices are expensive, take up a lot of space and offer limited insulation.
Thesis objective: the aim of this thesis is to design a new solution for powering wide-gap component drivers, by replacing magnetic transformers with piezoelectric transformers. This innovative approach aims to reduce costs, space requirements and improve the overall efficiency of power conversion systems.
Supervision and ressources: the selected candidate will work as part of a leading-edge research team, renowned for its expertise in the field of power conversion using piezoelectric resonators. The team has the resources and know-how to support the development and validation of this innovative technology.
Mesoscopic simulations and development of simplified models for the mechanical behaviour of irradiated concrete
In nuclear power plants, the concrete biological shield serves as a support for the reactor vessel and as a protective shield against radiation. Over the long term, prolonged exposure to neutron radiation can cause the concrete aggregates to expand through amorphisation, leading to micro-cracking and degradation of its mechanical properties. This is an important issue in studies aimed at extending the life of power plants. At the mesoscale, these phenomena can be modelled by separating the behaviour of the aggregates, the cementitious matrix and the interfacial transition zones. However, it is difficult to describe the initiation and propagation of microcracks in such complex heterogeneous multi-cracked systems. The aim of this thesis, carried out as part of a Franco-Czech ANR project, is to develop a high-performance numerical simulation tool for analysing the effects of neutron irradiation on concrete at the mesoscopic scale. A coupled thermo-hydro-mechanical approach will be used in which the behaviour of the matrix will take into account shrinkage, creep and micro-cracking. The simulations will be validated using experimental data obtained on tested samples, and the numerical tool will then be used to estimate the impact of various factors on the behaviour and performance of concrete subjected to neutron irradiation.
This research project is aimed at a PhD student wishing to develop their skills in materials science, with a strong focus on multiphysical and multiscale modelling and numerical simulations.