Diphasic thermoregulation system for ultra wide bandgap diamond semiconductors
The objective of this thesis is to study a diphasic thermoregulation system for ultra wide bandgap diamond semiconductors. One of the specific behavior of diamond semiconductors is the negative temperature coefficient of is on-state resistance. The thermoregulation proposed in this thesis aims to optimize the global losses of the system and to insure both temperature and electrical constraints between several diamond semiconductors in parallel.
Based on specifications that will be defined at the beginning of this theses (calories to dissipate, temperature range to control), the PhD candidate will have to:
- Define a temperature control strategy
- Define most appropriate materials and fluid of this system
- Design the thermoregulation system
- Realize and validate experimentally the proposed system
This thesis will tackle numerical simulation (component and thermoregulation system modelling) and experimental tests through the realization of a TRL 3-4 prototype of power converter system integrating diamond Schottky diodes.
The global objective to achieve is to put forward an innovative system modeled and experimentally demonstrated, where control strategy, dimensional and operative elements will be investigated and optimized.
Study of Zn, Cr, Fe, Ni synergies on crystallisation in simplified glasses of nuclear interest
In France, nuclear power-plants used for electricity production generate high-level long-lived radioactive wastes through spent fuel reprocessing. These wastes are confined within a borosilicate glass matrix, whose structure allows for the incorporation at the atomic scale of a large number of elements, and which displays excellent long-term properties. The industry challenges are leading to changes in the nuclear fuel composition, which can thus induce a modification of the spent-fuel composition to be vitrified.
Chromium is as such an element of interest: its relatively low solubility in borosilicate glasses as well as its tendency to crystallise with other elements, such as iron, nickel and zinc, needs to be further investigated. This thesis aims to study the synergetic effect of Cr, Ni, Fe and Zn on crystallisation in simplified peralkaline glasses of nuclear interest in order to better comprehend affinities between these elements, thus identifying both the nature and quantity of the several crystalline phases which may form.
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.
Tailored Peptide Ligands for Actinide Complexation: From Structure to Selectivity
The processes involved in the nuclear fuel cycle, such as the PUREX process designed to separate uranium and plutonium from fission products, rely on ligands capable of selectively complexing actinide cations to enable their extraction. The chemical functions carried by these ligands play a key role in determining both their affinity and selectivity toward metal cations. Studying the influence of these functional groups, such as carboxylic acids and phosphates, is therefore essential for the design of new extracting molecules, as well as for the development of decorporation strategies.
Over the past decade, cyclic peptides have been developed for their ability to complex uranyl ions with high selectivity over calcium. Organized in ß-sheet conformations, these peptides display a functional face bearing complexing groups (carboxylates, phosphates). Their amino acid composition can be tuned to finely adjust the chemical nature of the coordination site, making these cyclic peptides tailor-made molecular architectures for probing cation complexation. However, while their interaction with uranium is now well characterized, their ability to bind transuranic elements remains largely unexplored.
This PhD project aims to study the complexation of actinides such as plutonium and neptunium by various cyclic peptides. The combination of NMR spectroscopy and classical molecular dynamics simulations will provide detailed structural information on the formed complexes. Complementary techniques, including UV-Vis-nIR and EXAFS spectroscopies, ESI-MS mass spectrometry, and fluorescence spectroscopy, will deepen the characterization. By combining experimental and computational approaches, this work will enhance our understanding of ligand–actinide interactions while paving the way for the design of innovative extracting and decorporating molecules.
Investigation of autocatalysis phenomena occurring in nitric acid dissolution through electrochemical methods
The nuclear fuel recycling process, used at the La Hague plant in France, begins with the nitric dissolution of spent fuel, mainly composed of uranium and plutonium oxides. In a context of plant renewal and widespread of MOX fuel recycling, innovative new dissolution equipment are currently studied. The sizing of such devices is currently limited by the absence of a fully comprehensive model for the dissolution of mixed oxides, which is a highly complex reaction (three-phase involved, self-catalytic, heterogeneous attack, etc.). Despite substantial progress made in previous studies, a number of questions remain unanswered, particularly concerning the reaction mechanisms involved and the nature of the catalyst.
Electrochemical methods (cyclic voltammetry, electrochemical impedance spectroscopy, rotating electrode, etc.) have never been used to understand dissolution, yet they should prove relevant as already demonstrated by the studies carried out on this subject by CEA Saclay in the field of corrosion. Therefore, the aim of this thesis is to apply these experimental methods for the first time to the dissolution of nuclear fuels, through a phenomenological approach. To achieve this, the student will be able to rely on the teams and facilities of Saclay and Marcoule centers, specialized respectively in electrochemical methods for the corrosion studies and the physico-chemical modeling of dissolution.
This cross-disciplinary study, involving materials science, electrochemistry and chemical engineering, will follow a stimulating fundamental research approach, but will also take place in a highly dynamic industrial context. Initially, the work will be carried out on inactive model and noble materials (at the Saclay center), then on real materials containing uranium and/or plutonium (at the Marcoule center).
Study of homogeneous SIMMOX synthesis and dissolution based on hydroxide pathway
The dissolution of spent nuclear fuel is an essential first step in its reprocessing. The kinetics of irradiated (U,Pu)O2 (MOX) dissolution currently hinders industrial-scale reprocessing and therefore requires a better understanding of the mechanisms involved in order to overcome this industrial obstacle. However, studying the dissolution of irradiated MOX fuel in order to identify and model the various stages and mechanisms involved is hampered by the high radiotoxicity of such material and the representativeness of the available samples. In order to simplify these studies and establish representative models, numerous tests have been carried out on model compounds (e.g., non-irradiated UO2 and MOX). Among these, SIMfuel (U,Pu)O2 compounds doped with up to 11 fission products aim to represent the chemical complexity of irradiated fuels. The conventional approach to manufacturing SIMfuel by mixing solid-phase reagents requires sintering of fuel pellets at high temperatures (>1600°C). In order to reproduce the behavior of fission products (reduction-oxidation, distribution, etc.) for irradiated fuels at lower temperatures, an alternative approach has been developed based on the synthesis of oxides via the hydroxide route. This method allows for the simultaneous and homogeneous precipitation of numerous metal cations and significantly lowers the sintering temperature. This approach has already enabled the study of SIMfuel incorporating rare earths, platinoids, and molybdenum under representative conditions. However, this approach has never been implemented for the synthesis of SIMfuel containing both plutonium and all fission products relevant to the study of dissolution.
The objective of this thesis is to implement such syntheses, based on recent results obtained concerning the synthesis of MOx by the hydroxide route. To this end, SIMfuels will be synthesized to represent spent MOx-type fuels (SIMMOx). To represent the different zones present in spent fuel, SIMMOx with different Pu/(U+Pu) ratios will be considered. These SIMMOx will undergo dissolution tests to characterize their behavior during this stage.
TRANSIENT LIQUID PHASE SINTERING OF UOX AND MOX FUEL PELLETS
The subject is related to the manufacture of UOX and MOX fuels. The main objective is to identify dopant pairs that allow the formation of a transient liquid phase during the fuel sintering step. For this, phase diagram calculations using the CALPHAD method will have to be carried out, also taking into account the requirements related to the irradiation phase once the fuel is loaded into the reactor. The most promising pairs will then be evaluated in the context of the manufacture of a UOX fuel and a MOX fuel. The experiments to be carried out will essentially be: the preparation of a powdery material, the shaping by pressing of this material in the form of cylinders representative of fuel pellets and the study of the high-temperature sintering of these UOX and MOX formulation cylinders. After sintering, a very important step will be the characterization at the macroscopic and microscopic scales of these pellets. The first year of the thesis will take place at the CEA center in Cadarache. The next two will take place at the CEA site in Marcoule. The first year of the PhD will take place at the CEA Cadarache center within the ICPE Uranium Fuel Laboratory. The following two years will be spent at the INB Atalante facility on the CEA Marcoule site. The candidate will work in two facilities unique in Europe and will be able to develop experience working in a nuclear environment with a highly innovative approach that will lead to the publication of original scientific results.
Study of new concepts for miniaturizable and parallelizable liquid-liquid extractors
In the process of developing procedures, their miniaturization represents a major challenge for upstream research and development (R&D). Indeed, the miniaturization of procedures offers numerous advantages in terms of reducing the volume of raw materials, waste management, screening possibilities, automation, and safety for personnel.
To date, the counter-current liquid-liquid extraction process does not have a convincing miniaturization solution, although the applications are numerous: in pharmacy, chemical synthesis, nuclear, or nuclear medicine.
The CEA-ISEC in Marcoule has developed new microfluidic tools to perform these operations in a simple and operational manner, based on a fine understanding of the instabilities of two-phase flows in capillaries.
This 3-year study topic proposes:
- To experiment, understand, and finely model the flows and mass transfers;
- To optimize and then transpose the phenomena to industrially significant volumes;
- To publish and participate in international conferences.
The doctoral student will benefit from learning about the world of research in a team that values quality in the supervision and future of its doctoral students, in a multidisciplinary team ranging from process engineering to instrumentation, with projects ranging from research to industry.
General competencies in chemical engineering and mass transfer are required. Competencies in collaborating with our academic partners will be essential to the success of the study project.
INFLUENCE OF THE DRY GRANULATION ON THE MANUFACTURING OF SFR MOX FUELS
The subject is related to the manufacture of MOX U,Pu)O2 fuel for Fast Neutron Reactors. The current process integrates a co-grinding step of uranium and plutonium dioxides to generate a powder medium which is then shaped by uniaxial pressing to generate cylindrical fuel pellets which are then sintered at high temperature. The collected powder medium has poor flowability which limits the rates of shaping by pressing. The objective of the thesis is therefore to evaluate the impact of mechanical granulation of the powder medium on the flowability, the pressing step and the microstructure obtained after sintering. Dissolution tests in nitric acid will also be carried out on certain very specific microstructures. The thesis will be based on a formal experimental plan developed using specific software (JMP). The PhD will take place at the INB Atalante facility on the CEA Marcoule site. The candidate will work in a unique facility in Europe and will be able to develop expertise in working in a nuclear environment with a highly innovative approach that will lead to the publication of original scientific results.
Mechanical degradation of Solid Oxide Cells: impact of operating and failure modes on the performances
Solid oxide cells (SOCs) are electrochemical devices operating at high temperature that can directly convert fuel into electricity (fuel cell mode – SOFC) or electricity into fuel (electrolysis mode – SOEC). In recent years, the interest on SOCs has grown significantly thanks to their wide range of technological applications that could offer innovative solutions for the transition toward a renewable energy market. However, despite of all their advantages, the large-scale industrialization of this technology is still hindered by the durability of SOCs. Indeed, the SOCs remain limited by various degradation phenomena including mechanical damage in the electrodes. For instance, the formation of micro-cracks in the so-called ‘hydrogen’ electrode is a major source of degradation. However, the precise mechanism and the full impact of the micro-cracks on the electrode performances are still unknown. By a multi-physic modelling approach, it is proposed in this thesis (i) to simulate the damage in the microstructure of the electrode and (ii) to calculate its impact on the loss of performances. Once the model validated on dedicated experiments, a sensitivity analysis will be conducted to provide relevant guidelines for the manufacturing of improved robust and performant electrodes.
Thermodynamic and experimental approach of the reactivity in multi-constituted Silicon-Metal-Carbon systems for ceramic brazing
The development of ceramic-based material assemblies plays a fundamental role in technological innovation in many engineering fields. The choice of materials and joining process must ensure a functional, reliable and durable assembly, whose properties comply with the specifications of the application.
The PhD thesis is part of the development of brazing alloys optimized for the joining of ceramics (primarily silicon carbide) considered for various applications in harsh environments, particularly in the field of energy. Indeed, the design of these materials requires a good knowledge of the reactivity at the liquid alloy / ceramic interface. In this context, the thesis will contribute to the development of a thermodynamic and experimental approach to predict and understand the reactivity in multi-constituted Si-Metal-Carbon systems. This work includes a study of the wetting and interfacial reactivity of selected alloys (wetting and brazing experiments, fine characterization of the interfaces by different techniques such as FEG-SEM, X-ray diffraction, TEM, XPS) with the support of thermodynamic modelling using the CALPHAD method. This highly experimental work will be carried out in a dynamic and collaborative environment.