Experimental and theoretical approach to select deep eutectic solvents for the delamination of photovoltaic panels
Multimaterial assemblies recycling such as photovoltaic (PV) panels is necessary to guarantee the circularity of materials. Today, PV panels recycling is mostly managed through pyrometallurgy which is limited in terms of materials recovery (polymers are incinerated and recovered silicium and silver yields are low) and emits noxious and greenhouse gases, such as HF generated by fluorinated polymer incineration.
Chemical delamination processes consist in the immersion of panels into a hot solvent to swell the polymeric tie-layers and separate the materials. These processes have the potential to largely improve recovery yields of all materials, but they are environmentally impacting due to the operating conditions and solvent use.
The solvent choice defines the impact of the process. In the current literature, this choice is mostly based on trial and error approaches that are tedious. Moreover, efficient solvents for PV delamination are generally toxic and flammable, which limits the environmental relevance of the processes.
Deep eutectic solvents (DES), which are the association of a hydrogen bond donor and acceptor (HBD and HBA), present interesting solvent properties for such processes. The use of organic hydrophobic molecules as HBA or HBD can impart the solvents with a dual hydrophobic and hydrophilic character that brings a capacity to interact with various molecules to the solvent, including polymers. As HBA/HBD combinations are numerous, it is important to develop a systematic methodology for their choice.
The present topic proposes a methodology based on a combination of theoretical modelling and experimental validation measurements to better understand the contributions of HBD/HBA structures on DES properties. The study will focus on the identification of the interactions between DES and EVA (ethylene vinyl acetate), a widely used polymer in PV panels, through the study of phenomenon of solubilization and solvolysis (covalent bond breakage) to identify the contributions to the delamination phenomenon. At the end of the thesis, the DES selection will be validated through the demonstration of the delamination of a PV panel and the analysis of the separated layers.
Design and implementation of innovative alloys for high-performance NdFeB magnets via the grain-boundary diffusion process
Permanent magnets based on rare earth elements (RE), particularly neodymium-iron-boron (Nd-Fe-B) magnets, are essential for high-performance motors and generators for use in electric vehicles and wind turbines. To increase their resistance to demagnetization, heavy rare earth elements (HRE) such as dysprosium (Dy) or terbium (Tb) are partially substituted for neodymium. However, these HREs, although crucial for magnetic properties, are critical materials with supply risks. Hence, the challenge is to reduce their content while maintaining performance, in order to meet the growing demand for high-performance magnets. In this context, the grain boundary diffusion (GBD) process of HREs emerges as the most effective solution. It enables the diffusion of HREs from the surface of sintered magnets toward grain boundaries and then into the magnetic phase grains. This localization of HREs at the grain peripheries improves resistance to demagnetization while preserving high remanence, thereby reducing the required amount of HREs.
The objective of this Ph.D is to provide a comprehensive understanding of the diffusion and localization mechanisms of HREs while limiting parasitic reactions that unnecessarily consume them. The doctoral work will combine a comprehensive experimental approach, including the development of HRE-RE-M (where M is a transition metal) alloys for GBD, the study of diffusion, the characterization of equilibrium phases, and detailed magnetic and thermodynamic analyses, particularly through an in-depth study of phase diagrams. This approach aims to define the optimal compositions and diffusion conditions to achieve the targeted magnetic performances.
Selective precipitation of zirconium: an innovative step for future nuclear reprocessing processes
The recycling of spent nuclear fuel is a strategic mission for the CEA aimed at improving the sustainability of the fuel cycle and reducing the impact of radioactive waste. In this context, zirconium has been identified as one of the most problematic fission products in the PUREX process due to its complex chemistry in nitric medium. It is involved in co-extraction phenomena, the formation of interfacial cruds, the degradation of contactor hydrodynamic performance, and the reduction of decontamination factors.
The objective of this PhD thesis is to develop a selective zirconium precipitation step directly within the dissolution liquor prior to liquid-liquid extraction. The central scientific challenge is to understand how to control the selectivity of this precipitation in a highly nitric and complex medium.
The proposed approach is based on the use of a precipitating agent adhering to the CHON principle. The work will first focus on single-metal systems and subsequently on mixtures representative of dissolved fuel, in order to identify competitive phenomena between elements.
The expected results pertain to the understanding of precipitation mechanisms, the identification of parameters governing process selectivity, and the demonstration of quantitative zirconium removal without significant actinide loss. This approach could lead to a more robust PUREX process, better suited to high-burnup fuels. The PhD candidate will develop expertise at the interface of coordination chemistry, separation chemistry, and nuclear process engineering, within a research environment of high scientific and industrial impact featuring an extensive array of characterization techniques.
Development of Monte Carlo methods for reactor kinetics analysis: application to the ZOE pile
The Monte Carlo code TRIPOLI-5 is under development with the mid-term aim of simulating nuclear reactor behavior under both steady-state and transient conditions. This is achieved by leveraging high-performance computing (HPC) architectures and natively integrating physical feedback effects. Methods for addressing non-stationary transport problems are categorized into three main approaches: the point kinetic approximation, the determination of the asymptotic behaviour, and solving the exact kinetic equations which allows for a comprehensive characterization of core behavior. In this PhD thesis, we propose to evaluate the performance of these three approaches within the HPC context of TRIPOLI-5 and subsequently implement them by leveraging and generalizing the strategies derived from previous work. In particular, an additional challenge in this thesis will involve addressing photonuclear reactions, which induce a coupling between neutron and photon tranport equations. The developments achieved during this thesis will then be applied to the ZOE pile, France's first nuclear reactor, built by the CEA at the Fontenay-aux-Roses site, which achieved criticality on December 15, 1948. A set of experimental measurements available in historical reports will provide a validation basis for the reactor kinetics analysis methods developed in TRIPOLI-5. Since the ZOE pile was heavy-water moderated, photonuclear reactions on deuterium must be explicitly accounted for to accurately determine the evolution of reactor kinetics.
Monte Carlo Methods for Generating Diffusion Data for Light Water Reactor Core Calculations
Neutronic calculations for light water reactors (LWRs) require determining the power distribution in the core, a computationally expensive task. To address this challenge, deterministic simulations are traditionally performed in two steps. First, isolated 2D calculations are carried out: fuel assembly simulations using transport theory with reflective boundary conditions. Then, full 3D simulations are performed: these use homogenized few-group cross sections generated in 2D to model the entire core via diffusion theory.
However, the deterministic codes used for the first step rely on approximations (e.g., energy dependence of cross sections, treatment of anisotropic scattering), which limit the accuracy of the results. In contrast, Monte Carlo methods offer an approximation-free alternative capable of producing reference-level results.
This thesis aims to develop and qualify Monte Carlo methods for generating diffusion coefficients and multi-group cross sections in the TRIPOLI-5 code (co-developed by the CEA and the ASNR). These data will then be used as input for deterministic codes to perform full-core neutronic simulations.
Few-group diffusion coefficients, which are essential for diffusion solvers, are not available in nuclear data libraries. Current deterministic methods rely on approximations, introducing uncertainties. This thesis will propose solutions to compute these coefficients with high fidelity in a continuous-energy Monte Carlo context.
The thesis will explore various approaches, including methods for calculating diffusion coefficients (e.g., the Cumulative Migration Area method) and leakage models. For each approach, the accuracy of the results will be evaluated with the goal of industrial implementation in TRIPOLI-5.
The implementation of these methods in TRIPOLI-5 will improve the accuracy of diffusion data for downstream solvers and overcome the limitations of current deterministic toolchains. Ultimately, this thesis will contribute to establishing TRIPOLI-5 as a reference platform for generating high-fidelity data for light water reactor core calculations, while paving the way for industrial integration of these methods.
Modeling of flow regime transitions in horizontal and inclined pipes: dynamic aspects and scalability
Understanding fluid flows in the piping systems of industrial facilities is of crucial importance for their safety and operation. In accidental scenarios studied for the primary circuit of a pressurized water reactor (PWR), two-phase flows within pipes may occur, exhibiting a wide variety of regimes depending on the encountered conditions. The transitions between these different regimes (stratified, annular, slug, bubbly, etc.) under such conditions are the subject of extensive experimental and numerical research. In particular, system-scale thermohydraulic simulation codes, such as CATHARE, must accurately compute these regime transitions.
This thesis aims to improve the prediction at the system scale of two-phase flow regime transitions in horizontal or inclined pipes. The project is structured around three main objectives. The first objective seeks to better account for scaling and fluid transposition issues by investigating models that influence the stratified/intermittent and stratified/annular transitions, including interfacial friction. The second objective focuses on predicting the transition between bubbly and intermittent regimes by revisiting the modeling of bubble segregation and mixing, and by incorporating new experimental data. The third objective, directly related to the previous one, addresses the transient effects of regime transitions, particularly by exploring the use of interfacial area transport equations.
The software tools used will include the CATHARE code, while experimental data will be provided from CEA facilities and the literature.
Effects of turbulence and mixing on the interactions between a hydrogen-air flame and a spray
This project studies the interactions between a hydrogen-air flame and a water spray in the context of a severe accident (e.g., hydrogen release in a nuclear containment). The goal is to analyze how sprays influence flame propagation and generated overpressures, which can either inhibit combustion or accelerate it through turbulence and obstruction mechanisms.
The experimental program is divided into:
1/ Characterization of spray-induced turbulence (1 or 2 nozzles, in open air or confined vessel) using PIV (Particle Image Velocimetry).
2/ Study of combustion impact: comparison of flame dynamics (H2/air) with and without spray, measuring propagation speeds and overpressures.
3/ Numerical modeling: adaptation of CFD codes (e.g., WALL-HY) to simulate flame-droplet interactions, building on previous work (G. Gai’s thesis).
This work aims to provide reference data and a predictive model to assess hydrogen-related risks in the presence of spray systems.
Fundamental study of crystallization mechanisms in borosilicate glasses: toward an understanding of the synergies between Gd and Rare Earth Elements
In France, nuclear power plants generates approximately 68% of the country’s electricity, with a very low carbon footprint. However, it produces various types of radioactive waste throughout its life cycle, including so-called high-level waste resulting from spent fuel reprocessing. These wastes are confined within a borosilicate glass matrix whose structure allows for the incorporation of a large number of chemical elements at the atomic level, ensuring excellent long-term performance properties. Future challenges facing the industry may lead to changes in nuclear fuels and their burn-up rates, as well as adaptations to processing and recycling methods, which could alter the nature and chemical composition of waste solutions intended for vitrification.
Among these elements, gadolinium (Gd) — used as a neutron absorber in nuclear fuels — is of particular interest due to its complex behavior in solid solution and its potential to form crystalline phases (e.g., silicate apatites) when interacting with other rare earth elements (La, Nd) or actinides (Am). These crystalline phases could profoundly alter the structure of the glassy matrix and affect the material’s properties (chemical durability, radiation resistance. This thesis therefore aims to understand and quantify the synergistic effects between gadolinium and other rare earth elements in simplified borosilicate glasses, in order to better understand the crystallization affinities among the various elements. It will thus enable the identification of the nature, structure, size, and concentration of the various phases that may form. Their structural changes induced by alpha irradiation will also be studied to simulate the effects of actinides, particularly the swelling of crystalline phases and its impact on the material.
The PhD student will benefit from the recognised skills of the host laboratory in glass formulation as well as the study of their physico-chemical properties. All of the resources made available will enable a global approach to the subject, working on a fast-growing topic with major industrial and societal implications. The experience acquired during this interdisciplinary work will be useful in the field of materials.
Development and characterization of bio-based photoresists for lithography applications
Context :
CEA-Leti designs the microelectronic components of tomorrow. This involves developing new materials for devices, as well as essential materials for integrated circuit manufacturing, such as photoresists. These innovations also aim to reduce environmental impact by creating products without PFAS or toxic substances. In this context, the Advanced Lithography Processes Laboratory (LALP) collaborates with the Materials for Sensors and Delivery Laboratory (LMCD) at CEA to co-develop new bio-based lithography resists containing methacrylate functions. The goal of the PhD is to study the performances of these new photoresists in lithography and to evaluated their environmental and economical impact. To do so, an emphasis will be made on the influence of the chemical synthesis on the optical properties of the resists, their polymerization mechanisms, and the mechanical characteristics of the resists after exposure. Finally, tools will be given to conduct life cycle analysis on the different products.
Required skills :
The study will examine the entire photolithography process: from the synthesis of the base product and its formulation, to the final structuring. An initial study will analyze the rheological properties of the material after synthesis. Next, its behavior will be evaluated during the spin-coating step (ellipsometry and TGA/DSC). An analysis will focus on the product's sensitivity to a wavelength of 365 nm. Characterization of the product will be carried out by examining the shape of the obtained patterns (SEM) and the physical properties of the resist (ellipso-porosimetry and AFM). Etching tests will also be conducted.