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
Effect of water radiolysis on the hydrogen absorption flux by austenitic stainless steels in the core of a nuclear pressurized water reactor
In pressurized water nuclear reactors, the core components are exposed to both corrosion in the primary medium, pressurized water at around 150 bar and 300°C, and to neutron flux. The stainless steels in the core are damaged by a combination of neutron bombardment and corrosion. In addition, radiolysis of the water can have an impact on the mechanisms and kinetics of corrosion, the reactivity of the medium and, a priori, the mechanisms and kinetics of hydrogen absorption by these materials. This last point, which has not yet been studied, may prove problematic, as hydrogen in solid solution in steel can lead to changes in (and degradation of) the mechanical properties of the steel and induce premature cracking of the part. This highly experimental thesis will focus on the study of the impact of radiolysis phenomena on the corrosion and hydrogen uptake mechanisms of a 316L stainless steel exposed to the primary medium under irradiation. Hydrogen will be traced by deuterium, and neutron irradiation simulated by electron irradiation on particle accelerators. An existing permeation cell will be modified to allow in operando measurement by mass spectrometry of the deuterium permeation flux through a sample exposed to the simulated primary water under radiolysis conditions. The distribution of hydrogen in the material, as well as the nature of the oxide layers formed, will be analysed in detail using state-of-the-art techniques available at the CEA and in partner laboratories. The doctoral student will ultimately be required to (i) identify the mechanisms involved (corrosion and hydrogen entry), (ii) estimate their kinetics and (iii) model the evolution of hydrogen flux in the steel in connection with radiolysis activity.
Polycrystalline numerical simulations of the mechanical behavior of fuel rod cladding used in pressurized water reactors
The fuel rods of pressurized water nuclear reactors are made of uranium oxide pellets stacked in zirconium alloy tubes. In reactor, these materials undergo mechanical loading that lead to their irreversible deformation. In order to guarantee the safety and increase the performance of nuclear reactors, this deformation must be modeled and predicted as precisely as possible. In order to further improve the predictivity of the models, the polycrystalline nature of these materials as well as the physical deformation mechanisms must be taken into account. This is the objective of this study, which consists of developing a physically based multi-scale numerical model of the fuel rod cladding.
The mechanical behavior of metallic materials is usually modeled by considering the material as homogeneous. In fact metallic materials are made of many crystalline grains clustered together. The behavior of the material is therefore the result of the deformation of individual grains but also their interactions between each other. In order to take into account the polycrystalline nature of the material, mean-field self-consistent polycrystalline models have been developed for many years. These models are based on the theory of homogenization of heterogeneous materials. Recently, a polycrystalline model, developed in a linear and isothermal framework, has been coupled with an axisymmetric 1D finite element calculation to simulate the in-reactor deformation of cladding tubes. A complex mechanical loading history, mimicking the stresses and strains experienced by the cladding has been simulated.
The objective of this PhD work is to extend the field of application of this model in particular by applying it to a non-linear framework in order to simulate high stress loadings, to extend it to anisothermal conditions but also to carry out 3D finite element simulations with at each element and each time step a simulation using the polycrystalline model. These theoretical and numerical developments will finally be applied to the simulation of the behavior of fuel rods in a power ramp situation thanks to its integration into a software platform used for industrial applications. This approach will allow to better assess the margins available to operate the reactor in a more flexible manner, allowing it to adapt to changes in the energy mix in complete safety.
Chemo-mechanical modeling of the coupling between carbonation, rebar corrosion and cracking in cementitious materials
Rebar corrosion is one of the main causes of premature degradation of concrete infrastructures, including in the nuclear sector, where concrete is extensively used in containment structures and waste storage facilities. Carbonation, caused by the penetration of CO2 into the concrete, lowers the pH of the pore solution, promoting rebar corrosion. This corrosion leads to the formation of expansive products that can cause cracking in the material. The proposed thesis work, developed as part of a European collaborative project between CEA Saclay, École des Mines de Paris - PSL, and IRSN, aims to develop a numerical model to simulate these phenomena. The model combines a reactive transport code (Hytec) and a finite element code (Cast3M) to study the local effects of carbonation-induced corrosion on concrete cracking. This project will benefit from parallel experimental work to gather data for parameter identification and model validation. The first part of the research will focus on modeling the carbonation of cementitious materials under unsaturated conditions, while the second part will address the corrosion of rebar caused by the pH drop induced by carbonation. The model will describe the growth of corrosion products and their expansion, inducing stress within the concrete and potential microcracking.
This research project is aimed at a PhD student wishing to develop their skills in materials science, with a strong focus on multi-physical and multi-scale modeling and numerical simulations. The thesis will be carried out principally at CEA Saclay and at École des Mines de Paris – PSL (Fontainebleau).
Physics of perovskite materials for medical radiography: experimental study of photoconduction gain
X-rays is the most widely used medical imaging modality. It is used to establish diagnostics, monitor the evolution of pathologies, and guide surgical procedures.
The objective of this thesis is to study a perovskite type semiconductor material for its use as a direct X-ray sensor. Perovskite-based matrix imagers demonstrate improved spatial resolution and increased signal, and can thus help improve patient treatment. Prototype X-ray imagers manufactured at the CEA already provide radiographic images but their performances are limited by the instability of the sensor material.
You job will be to study the mechanisms responsible for the photoconduction gain and photocurrent drift of thick perovskite layers from both a theoretical and an experimental standpoint. To this end, you will adapt the electro-optical characterization benches of the laboratory, conduct experiments and analyze the data collected. You will also have the opportunity to perform advanced characterizations with specialized laboratories within the framework of national and international collaborations. The results of this thesis will provide a better understanding of the material properties and guide its ellaboration to produce high-performance X-ray imagers.
Role of microstructure in the performance of sintered SmFe12-xMx magnets
The TR-Fe12 type and the ThMn12 quadratic structure have very interesting intrinsic properties (magnetocrystalline anisotropy HA, saturation magnetization Ms and Curie temperature TC). They are considered as the best alternative to NdFeB magnets, offering TR savings of around 35% by weight.
For Sm-based compounds of the Sm(Fe,Ti,V)12 type, it has been demonstrated that the formation of a phase at grain boundaries by the addition of additive elements can significantly increase coercivity up to 1.4 T. However, remanence remains low, below 1 T, which limits the field of application of these magnets. In this thesis, we propose a study of addition elements likely to increase both the remanence and coercivity of TR-Fe12 type magnets.
For Sm-Fe12 compounds, the additions studied will aim to increase the remanence of these magnets by reducing the rate of Fe substitution by stabilizing elements. They will also promote the formation of paramagnetic phases at grain boundaries to enhance coercivity.
Development of an advanced grade of nano-reinforced austenitic steel for use under intense flux
Recent work has shown that it is possible to obtain ODS (Oxide Dispersion Strengthened) austenitic steels for use under intense flux. These new grades are beginning to be studied for nuclear applications around the world. They should have remarkable properties, particularly in terms of resistance to swelling under irradiation and creep, thanks to the addition of nano-reinforcements in exceptional density (10^23 to 10^24 m-3). These ODS steels are obtained by powder metallurgy, by co-grinding a metal powder with an oxide powder. The aim of this work is to succeed in manufacturing, using an innovative process, ODS austenitic steel cladding tubes. It will be necessary to master the recrystallization of these tubes, to propose a first critical evaluation by specifying the property/microstructure relationships and by evaluating, by irradiation with charged particles, the behavior under irradaition of this new material. The student will be trained in Scanning Electron Microscopy and the techniques that result from it (X-ray analysis, EBSD, etc.), in SAXS, in the performance and exploitation of mechanical tests. He will have to acquire good notions in Transmission Electron Microscopy and in Tomographic Atomic Probe. The understanding of the behavior under irradiation will be guided by simulations by cluster dynamics.
Influence of chromium doping of UO2 fuel on fission product speciation under accidental conditions
The development of nuclear reactors is part of a drive to improve safety, with, for example, the deployment of nuclear fuels with improved properties in terms of their behavior under accident conditions, the so-called E-ATF (Enhanced Accident Tolerant Fuel). Industrial operator FRAMATOME is developing the Cr2O3-doped UO2 fuel as E-ATF. However, very little data is available on the behavior of fission products from Cr-doped fuel under accident conditions.
This thesis proposes to develop a synthesis process for Cr-doped UO2 fuel that simulates irradiated fuel, in order to study the behavior of the elements (Cr and fission products) at different temperatures and under different oxygen partial pressures. The methodology is based on an experimental approach combining synthesis of model materials and in-depth chemical characterization, complemented by a theoretical approach (thermodynamic calculations) enabling thermal sequences to be dimensioned and the proposed reaction mechanisms to be confirmed.
The thesis will be carried out at CEA Cadarache (France), within IRESNE (Research Institute on Nuclear Systems for Low-Carbon Energy Production). The PhD student will be hosted in a laboratory dedicated to the study of uranium-based compounds in the Department of Fuel Studies (DEC). Depending on the densification processes chosen, experiments of various duration may be carried out in other laboratories in France or Europe.
The PhD student will have the opportunity to learn advanced characterization techniques for ceramic materials, gain access to experiments on large-scale instruments (synchrotron) and take part in exchanges with the academic community(CNRS, Universities, JRC). He or she will be able to promote his work through publications and participation in conferences.
At the end of this thesis, the PhD student will have acquired skills in materials science and solid state characterization that he/she will be able to use in various materials fields, as well as experience in the nuclear environment of interest to the nuclear industry.
Phenomenological study of the coupled effects of iodine and oxygen on Iodine induced Stress-Corrosion-Cracking (I-SCC) of zirconium alloys
The Pressurized Water Reactor (PWR) core is composed of fuel assemblies, for which the fuel cladding is the first barrier for the confinement of the fuel and the fission products. Pellet Cladding Interaction (PCI) occurs during increases in the reactor power and results in the expansion of the pellets that produces a thermomechanical loading on the fuel cladding. In conjunction with iodine expelled from the pellet, it can, in theory, lead to the failure of the cladding by Iodine Stress Corrosion Cracking (I-SCC).
The objective of this PhD is to study the phenomenology of I-SCC, in mechanical and chemical conditions as close as possible to the conditions seen by the PWR cladding in PCI (in terms of the oxygen and iodine partial pressures).
The PhD has three main parts. The first part will focus on the study of the effect of the stress on the I-SCC susceptibility of zirconium, at different partial pressures of iodine and oxygen. The tests will be simulated and analyzed using numerical models of the I-SCC process. The second part will focus on the effect of the temperature on I-SCC as a function of the stress, and the partial pressures of iodine and oxygen. The third part will focus on the effect of a thick zirconia layer, at the inner wall of the cladding, on the I-SCC susceptibility of the cladding.