Study of the amorphous intermediate states during the precipitation of actinides oxalate
Growing energy needs and the climate emergency require a rapid transition to completely carbon-free energy, by mixing renewable energies and sustainable nuclear power. In this context, the precipitation of plutonium and uranium in the form of oxalate constitutes a key step in the industrial process of recycling spent fuel. A detailed understanding of the crystallization mechanisms of these oxalates thus constitutes a major challenge for better management of these operations.
However, it is now widely accepted that ions in solution assemble into crystals via a series of non-crystalline transient states, which fundamentally contradicts all classical nucleation theories used in precipitation models. In particular, we have demonstrated in recent years that rare earth oxalate crystals (Eu, Nd, Ce, Tb), some used to experimentally simulate the recycling of uranium and plutonium, form via liquid, reagent-rich nanodroplets which separate from the aqueous solvent. This behavior modifies the view hitherto retained for the precipitation of these oxalates and leads us to question the behavior of actinide oxalates.
The aim of this thesis is to confirm or refute that transient mineral droplets also form during the formation of uranium and plutonium oxalates, and to determine whether crystallization transients impact the precipitation models used to calibrate the recycling process of nuclear fuel. This study will not only impact precipitation processes used in recycling, but will also advance a fundamental question about long-debated “non-classical” crystallization.
Dislocation glide in body-centered-cubic high-entropy alloys
High entropy alloys are single-phase multi-component solid solutions, all elements being present in high concentrations. This class of materials has significant improvements in mechanical properties over "conventional" alloys, particularly their high strength at high temperature. It is commonly accepted that good mechanical performance comes from the interactions of dislocations with the alloying elements and that at high temperature interstitial impurities or interstitial doping, such as oxygen, carbon or nitrogen, play a preponderant role. The study of plasticity in concentrated alloys with a body-centered cubic crystal structure in the high temperature range therefore constitutes the objective of this PhD thesis. The associated technological challenges are important, these alloys being promising structural materials, notably for nuclear applications where operating temperatures above room temperature are targeted.
This work aims to understand and model the physical mechanisms controlling the mechanical strength of these alloys at high temperature, by considering different concentrated alloys of increasing complexity and by using atomistic simulations, in particular ab initio electronic structure calculations. We will first focus on the binary alloy MoNb before extending to the ternary alloys MoNbTi and MoNbTa and studying the impact of oxygen impurities on plastic behavior of these alloys. We will model the dislocation cores and analyze their interaction with interstitial and substitutional elements in order to determine the energy barriers controlling their mobility. Based on these ab initio results, we will develop strengthening models notably allowing us to predict the yield strength as a function of temperature and alloy composition.
This work will be carried out within the framework of the DisMecHTRA project funded by the French National Research Agency, allowing in particular to compare our strengthening models with the data from the experiments which are planned in the project (mechanical tests and transmission electron microscopy), and which will be carried out by the other partners (CNRS Toulouse and Thiais). The PhD thesis, hosted at CEA Saclay, will be co-supervised by a team from CEA Saclay and MatéIS (CNRS Lyon).
Perovskite ferroelectric oxynitride thin films with tunable properties
N-doped oxides and/or oxinitrides constitute a booming class of compounds with a broad spectrum of useable properties and in particular for novel technologies of carbon-free energy production. Indeed, the insertion of nitrogen into the crystal lattice of a semiconductor oxide allows, in principle, to modulate the value of its band gap or to introduce additional electronic states and thus to obtain new functionalities and optical properties. The production of oxynitride single crystalline thin films is highly challenging. In this essentially experimental thesis work, thin films of oxynitrides will be developed by atomic plasma-assisted molecular beam epitaxy. We will start from BaTiO3, which synthesis is well mastered in the laboratory, to realize co-dopings with nitrogen and compensating metals in order to preserve the neutrality of the elementary unit cell. The resulting structures will be studied for their chemical compositions, crystalline structures and ferroelectric characteristics. These observations will be correlated with their performance for the photo-electrolysis of water, which allows the virtuous production of hydrogen. Finally, the corrosion resistance of these new materials will also be studied.
The student will acquire skills in a wide range of ultra-high vacuum techniques, molecular beam epitaxy growth, clean room lithography, ferroelectric measurements and photo-electrolysis of water, as well as in state-of-the-art synchrotron radiation techniques.
The Pd-Rh-Ru-Te-O system in nuclear glasses and its impact on the glass melt conductivity
In France, high-level nuclear waste is vitrified. The components of the waste are integrated in a homogeneous vitreous matrix. However, platinum group metals (PGM) Pd, Rh and Ru are very poorly soluble in the glass melt and they form particles, combined or not with oxygen or tellurium.
Ru and Rh may reduce in their metallic state during glass processing. They are then more electrically conductive and their effect on the physical properties of the glass melt may affect the vitrification process control. Hence, the knowledge of the speciation and the morphology of the PGM elements is essential for the control of the process.
Thereby, this PhD will be split in 2 interdependent approaches: the first one by thermodynamic Calphad calculations and the other one by experimentations. First, the experimental approache will aim to understand and quantify the reduction of (Ru,Rh)O2 and the solubilisation of Ru and Rh in Pd-Te thanks to elaborations and characterizations (SEM and XRD mainly) of glasses with PGM particles. The results will complete a Calphad database. Calculations will help to discuss experimental results and will enable to predict the PGM state in the glass melt during the industrial vitrification. Secondly, electrical conductivity measurements at high temperature will be implemented on the glasses previously made to determine the impact of Ru and Rh speciation on the global conductivity of the melt.
The applicants must be rigorous, autonomous and have good communication and writing skills. Knowledge and experience in the field of glass or thermodynamics would be a plus.
Online analysis of actinides surrogates in solution by LIBS and AI for nuclear fuel reprocessing processes
The construction of new nuclear reactors in the coming years will require an increase in fuel reprocessing capacity. This evolution requires scientific and technological developments to update process monitoring equipment. One of the parameters to be continuously monitored is the actinide content in solution, which is essential for process control and is currently measured using obsolete technologies. We therefore propose to develop LIBS (laser-induced breakdown spectroscopy) for this application, a technique well suited for quantitative online elemental analysis. As actinide spectra are particularly complex, we shall use multivariate data processing approaches, such as several artificial intelligence (AI) techniques, to extract quantitative information from LIBS data and characterize measurement uncertainty.
The aim of this thesis is therefore to evaluate the performance of online analysis of actinides in solution using LIBS and AI. In particular, we aim to improve the characterisation of uncertainties using machine learning techniques, in order to strongly reduce them and to meet the monitoring needs of the future reprocessing plant.
Experimental work will be carried out on non-radioactive actinide simulants, using a commercial LIBS equipment. The spectroscopic data will drive the data processing part of the thesis, and the determination of the uncertainty obtained by different quantification models.
The results obtained will enable publishing at least 2-3 articles in peer-reviewed journals, and even to file patents. The prospects of the thesis are to increase the maturity level of the method and instrumentation, and gradually move towards implementation on a pilot line representative of a reprocessing process.
Perovskite devices for solar hydrogen production
Project Overview:
The PhD thesis is part of the ICARUS European project, aiming to develop efficient solar energy conversion systems for a carbon-neutral future. The project focuses on integrating photoelectrochemical (PEC) water splitting and photovoltaic (PV) power generation.
Key Objectives:
•Develop innovative metal halide perovskite solar cells with tunable bandgaps for broader light absorption.
•Optimize printed carbon-based solar cells and scaffolds for improved conductivity, mechanical resistance, and durability.
•Incorporate innovative carbon counter electrodes into perovskite devices.
•Upscale and manufacture solar modules.
•Integrate the developed modules into a final PEC prototype.
Research Focus:
The PhD candidate will primarily focus on:
•Printed carbon-based solar cells: Optimizing ink properties, investigating the behavior of printed conductive ink under various conditions, and characterizing conductivity and mechanical resistance.
•Perovskite devices: Incorporating innovative carbon counter electrodes and evaluating their performance and stability.
•Module manufacturing: Upscaling and manufacturing solar modules based on the developed technologies.
•PEC prototype integration: Contributing to the final integration of the PEC prototype.
Expected Outcomes:
The research is expected to contribute to the development of highly efficient and sustainable solar energy conversion systems, supporting the transition to a carbon-neutral future. The findings will have implications for both academic research and industrial applications.
Giant magnetoresistance resistors for local characterization of surface magnetic state: towards Non-Destructive Testing (NDT) applications
CIFRE thesis in the field of non-destructive testing using magnetic sensors in collaboration with 3 partners:
Laboratoire de Nanomagnétisme et Oxyde (SPEC/LNO) du CEA Paris-Saclay
Laboratoire de Génie Electrique et Ferroélectricité (LGEF) de l’INSA Lyon
Entreprise CmPhy
Rheology of concentrated mineral-filled suspensions
As a research organization in the nuclear field and alternative energies, the CEA participates in fundamental studies involving dense suspensions. Inorganic particles (glass, zeolite, sludge, salts, or cement/sand) suspended in fluids, sometimes with very high viscosity like bitumen, are part of the systems under study for various applications. These include optimizing the filling of glass packages (Dem N' Melt process) or cement packages, where flow properties need to be optimized to ensure homogeneity of waste drums. Besides to addressing the recovery (historical sludges), treatment, and conditioning of waste in glass or bituminous matrices, concentrated suspensions of glass grains are being studied for high-temperature electrolysis production of dihydrogen.
In this optic, the research will initially focus on model concentrated suspensions, characterizing their flow properties under shear and compression. This latter type of mechanical test can trigger the appearance of frictional regimes, liquid/solid phase separation, and various non-linear responses that will need to be modeled. After this first stage, the topology, particle size distribution, and polydispersity of the solid particles will be varied to be as close as possible to the suspensions encountered in industry.
Activated conductive materials for energy conversion and energy storage through capacitive effect
Energy production from renewable sources requires efficient storage systems to address imbalances between supply and demand. This project aims to develop cost-effective supercapacitors using composite electrodes derived from industrial by-products. Mineral binders, such as geopolymers or Alkali Activated Materials (AAM), made conductive by dispersing carbon black, are being studied for energy storage or heat generation applications. Based on a recently filed patent, we propose a detailed study of these conductive composites. Their performance will be evaluated depending on formulation and shaping parameters. Additionally, the porous network and the dispersion of conductive charges in the material will be thoroughly characterized. Finally, material shaping tests will be conducted, and supercapacitors will be assembled to study the impact of the process (3D printing) and geometries.
Study of the synthesis and thermodynamic properties of the (An,Zr)O2 and (Zr,An)SiO4 compounds
In the event of a serious nuclear accident, the fuel in the reactor core may melt, resulting in the formation of a compound known as corium. Cases of major accidents and prototypical corium formation experiments have identified the formation of key compounds such as mixed oxides (U,Zr)O2 formed by interaction of the fuel with the zircaloy cladding and silicates (Zr,U)SiO4 formed by interaction of the corium with structural materials. In the case of MOx, (U,Pu)O2 fuels, corium formation could lead to the formation of equivalent phases with significant plutonium contents. However, experimental thermodynamic data on such compounds, which would enable their behaviour to be assessed, are currently non-existent. In this context, determining the conditions for synthesising such compounds with a good degree of purity is essential for acquiring such data. The synthesis of (Zr,Pu)O2 and (Zr,Pu)SiO4 solid solutions is therefore an essential first step before studying (Zr,U,Pu)O2 and (Zr,U,Pu)SiO4 systems.
The aim of this PhD thesis will be to determine the conditions suitable for the synthesis of these compounds, to carry out a series of characterisations enabling their purity to be assessed and their thermodynamic properties to be established. To achieve this, experiments will be carried out on the ATALANTE facility and a multi-technique characterisation approach will be chosen (XRD, Raman and infrared spectroscopies, SEM, synchrotron characterisation techniques, etc.). Solubility tests in a controlled environment will then be set up and calorimetric measurements carried out as part of international collaborations.