Radiative heat transfer: efficient numerical resolution of associated problems in Beerian or non-Beerian media for the validation of simplified models
This research proposal focuses on the study, through modeling and numerical simulation, of heat transfer within a heterogeneous medium composed of opaque solids and a transparent or semi-transparent fluid. The considered modes of transfer are radiation and conduction.
Depending on the scale of interest, the radiance is the solution of the Radiative Transfer Equation (RTE). In its classical form, the RTE describes heat transfer phenomena at the so-called local scale, where solids are explicitly represented in the domain. At the mesoscopic scale of an equivalent homogeneous medium, however, the radiance is governed by a generalized RTE (GRTE) when the medium no longer follows the Beer–Lambert law. In this work, we focus on the numerical resolution of the RTE in both configurations, ultimately coupled with the energy conservation equation for temperature.
In deterministic resolution of the RTE, a standard approach for handling the angular variable is the Discrete Ordinates Method (Sn), which relies on quadrature over the unit sphere. For non-Beerian media, solving the GRTE is a very active research topic, with Monte Carlo methods often receiving more attention. Nevertheless, the GRTE can be linked to the generalized transport equation, as formulated in the context of particle transport, and a spectral method can be applied for its deterministic Sn resolution. This is the direction pursued in this PhD project.
The direct application of this work is the numerical simulation of accidents in Light Water Reactors (LWR) with thermal neutrons. Modeling radiative heat transfer is crucial because, in the case of core uncovering and fuel rod drying, radiation becomes a major heat removal mechanism as temperatures rise, alongside gas convection (steam). This topic is also relevant in the context of the nuclear renaissance, with startups developing advanced High Temperature Reactors (HTR) cooled by gas.
The goal of this thesis is the analysis and development of an innovative and efficient numerical method for solving the GRTE (within a high-performance computing environment), coupled with thermal conduction. From an application standpoint, such a method would enable high-fidelity simulations, useful for validating and quantifying the bias of simplified models used in engineering calculations.
Successful completion of this thesis would prepare the student for a research career in high-performance numerical simulation of complex physical problems, beyond nuclear reactor physics alone.
Enhanced Quantum-Radiofrequency Sensor
Through the Carnot SpectroRF exploratory project, CEA Leti is involved in radio-frequency sensor systems based on atomic optical spectroscopy. The idea behind the development is that these systems offer exceptional detection performance. These include high sensitivity´ (~nV.cm-1.Hz-0.5), very wide bandwidths (MHz- THz), wavelength-independent size (~cm) and no coupling with the environment. These advantages surpass the capabilities of conventional antenna-based receivers for RF signal detection.
The aim of this thesis is to investigate a hybrid approach to the reception of radio-frequency signals, combining atomic spectroscopy measurement based on Rydberg atoms with the design of a close environment based on metal and/or charged material for shaping and local amplification of the field, whether through the use of resonant or non-resonant structures, or focusing structures.
In this work, the main scientific question is to determine the opportunities and limits of this type of approach, by analytically formulating the field limits that can be imposed on Rydberg atoms, whether in absolute value, frequency or space, for a given structure. The analytical approach will be complemented by EM simulations to design and model the structure associated with the optical atomic spectroscopy bench. Final characterization will be based on measurements in a controlled electromagnetic environment (anechoic chamber).
The results obtained will enable a model-measurement comparison to be made. Analytical modelling and the resulting theoretical limits will give rise to publications on subjects that have not yet been investigated in the state of the art. The structures developed as part of this thesis may be the subject of patents directly exploitable by CEA.
Exploring the Strategic Benefits of 0V Storage for Na-ion Batteries
Recently deployed on a commercial scale, the Na-ion battery technology demonstrates excellent behaviour during medium or long-term storage at zero voltage. This characteristic offers numerous safety advantages during the transport, assembly and storage of cells and modules, as well as during emergency shutdowns in the event of external issues. But are there no consequences for battery performance?
This research project aims to study and better understand the electrochemical mechanisms at play when the potential difference across the terminals is maintained at 0 V.
Initially, advanced dynamic characterisation techniques will be used to analyse and compare the electrochemical, thermal and mechanical properties of battery materials. The results will enrich calendar and cycling ageing models at the cell scale, thereby improving their accuracy and reliability. Subsequently, tests will be conducted on mini-battery modules assembled in various electrical architectures to study cell behaviour during cycling and ageing, particularly in response to the application of negative voltage. Specific battery management system (BMS) solutions could then be proposed to address these issues.
The scientific approach will involve implementing advanced characterisation and instrumentation techniques, conducting ageing and safety tests to identify mechanisms, and developing ageing models. This approach will draw on the expertise and testing facilities of CEA-Liten at the Bourget du Lac site in Savoie.
Design and test of a PLL in FD-SOI 28nm technology
The goal of this PhD thesis is to design a Phase Locked Loop for generic use at 5 GHz. This PLL will also include a study regarding each building bloc sensitivity to radiation and thermal sensitivity regarding space environment. This is the main point of this PhD thesis because integrating a PLL in harsh environment requires an accurate knowledge of the circuit's parameters. The candidate will begin its work by analysing existing works on the FD-SOI technology (structure characteristics and impact on radiation hardening) to serve as a base for its work and design a Phase Locked Loop architecture. He will also study how to characterise each PLL building bloc variations in harsh environment (radiation and temperature).
Superconducting Silicon and detection in the far Infrared Universe
Silicon technologies occupy a central position in today’s digital landscape, both for the fabrication of semiconductor devices and for the development of advanced sensors. In 2006, the discovery of superconductivity in silicon heavily doped with boron opened a new field of research. Since then, several laboratories, including CEA, have been investigating its electronic properties and potential applications. This emerging material exhibits particularly attractive characteristics for systems operating at sub-Kelvin cryogenic temperatures, especially in the fields of quantum electronics and ultra-sensitive detectors used in fundamental physics and astrophysics.
Despite these advances, the understanding of superconducting silicon remains incomplete, particularly regarding its thermal, mechanical, and optical properties at the micrometric scale. The proposed PhD aims to address these gaps by combining modelling, design, technological fabrication, and cryogenic characterization of prototype devices, within a close collaboration between CEA-Léti and CEA-Irfu. The main objective will be to develop a new generation of detectors based on this superconducting material and to demonstrate their relevance for the detection of electromagnetic radiation in the terahertz and far-infrared ranges.
Hybrid CPU-GPU Preconditioning Strategies for Exascale Finite Element Simulations
Exascale supercomputers are based on heterogeneous architectures that combine CPUs and GPUs, making it necessary to redesign numerical algorithms to fully exploit all available resources. In large-scale finite element simulations, the solution of linear systems using iterative solvers and algebraic multigrid (AMG) preconditioners remains a major performance bottleneck.
The objective of this PhD is to study and develop hybrid preconditioning strategies adapted to such heterogeneous systems. The work will investigate how multilevel and AMG techniques can be structured to efficiently use both CPUs and GPUs, without restricting computations to a single type of processor. Particular attention will be paid to data distribution, task placement, and CPU–GPU interactions within multilevel solvers.
From a numerical point of view, the research will focus on the analysis and construction of multilevel operators, including grid hierarchies, intergrid transfer operators, and smoothing procedures on avalible GPU's and CPU's. The impact of these choices on convergence, spectral properties, and robustness of preconditioned iterative methods will be studied. Mathematical criteria guiding the design of efficient hybrid preconditioners will be investigated and validated on representative finite element problems, e.g., regional-scale earthquake analysis.
These developments will be coupled with domain decomposition and parallelization strategies adapted to heterogeneous architectures. Particular attention will be paid to CPU–GPU data transfers, memory usage, and the balance between compute-bound and memory-bound kernels. The interaction between numerical choices and hardware constraints, such as CPU and GPU memory hierarchies, will be designed and developed to ensure scalable and efficient implementations.
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.
Introduction of innovative materials for sub-10nm contact realization
As part of the FAMES project and the European ChipACT initiative, which aim to ensure France’s and Europe’s sovereignty and competitiveness in the field of electronic nano-components, CEA-LETI has launched the design of new FD-SOI chips. Among the various modules being developed, the fabrication of electrical contacts is one of the most critical modules in the success of advanced node development.
For sub-10 nm node, the contact realization is facing a lot of challenges like punchthrough (due to low etch selectivity during contact etching), voids during metal deposition, self-alignment, and parasitic capacitance. New breakthrough approach has recently been proposed consisting in the deposition of new dielectric films with chemical gradient. This thesis focuses on the development (deposition an etching processes) of new gradient compounds incorporated into SiO2 to address the current issues.
Advanced electrode materials by ALD for ionic devices
This work aims to develop Advanced ultrathin cunductive layers (<10nm) by ALD (Atomic Layer Deposition)for électrodes use(resistivity 100). The other challenge aims to reduce the ALD-based electrode layer thickness less than 5nm while still maintaining the advanced electric properties (resistivity in the mOhm range).
This work covers multiple aspects including inter alia ALD process, ALD precursors, Elementary characterization of intrinsec properties (physico-chemical, morphological and electrochemical) as well as integration on short loop 3D devices.
Development of a new numerical scheme, based on T-coercivity, for discretizing the Navier-Stokes equations.
In the TrioCFD code, the discretization of the Navier-Stokes equations leads to a three-step algorithm (see Chorin'67, Temam'68): velocity prediction, pressure solution, velocity correction. If an implicit time discretization scheme is to be used, the pressure solution step is particularly costly. Thus, most simulations are performed using an explicit time scheme, for which the time step depends on the mesh size, which can be very restrictive. We would like to develop an implicit time discretization scheme using a stabilized formulation of the Navier-Stokes problem based on explicit T-coercivity (see Ciarlet-Jamelot'25). It would then be possible to solve an implicit scheme directly without a correction step, which could significantly improve the performance of the calculations. This would also allow the use of the P1-P0 finite element pair, which is frugal in terms of degrees of freedom but unstable for a classical formulation.