Experimental study of the two-phase natural convection and vaporization regimes in the cooling pool of a nuclear facility
Nuclear energy, with low CO2 emissions, is one of the major players in France's energy transition. In this context, the management of the cooling of irradiated fuel elements is a matter of utmost importance. This thesis focuses on two-phase natural convection flows and vaporization phenomena that can develop in the cooling pools of various nuclear facilities, particularly those having a significant vertical variation in the saturation temperature of the coolant due to their great depth. These pools are used to dissipate the residual heat from irradiated fuels in many types of nuclear reactors, both existing and planned. In an accident scenario with a significant heat release from the fuels, the water in these pools can vaporize, eventually limiting their cooling capability. Among the possible phase change mechanisms in deep pools is the gravity-driven flashing, a phenomenon found in various natural or industrial systems analogous to vertical channels heated from below. However, this phenomenon has been little studied in the specific configuration of a pool and was only recently observed in this context. Therefore, the objective of this thesis is to better understand the phenomenon, as well as the turbulence induced within the coolant by the bubbles it generates, in order to improve state-of-the-art thermal-hydraulic models for simulating such pools. The proposed research, of an experimental nature, will be conducted in collaboration with the Catholic University of Louvain (UCLouvain, Belgium) and the LEGI laboratory of CNRS Grenoble, with a significant portion of the research carried out at UCLouvain. The candidate will be affiliated to the Core and Circuit Thermal-hydraulics Laboratory (LTHC) of CEA IRESNE, specialized in the study of two-phase flows in nuclear facilities. During the thesis, finely resolved experimental data in both space and time will be acquired and interpreted, contributing to a better understanding of the phenomenon. To achieve this, advanced techniques such as stereo particle image velocimetry (3D PIV) in two-phase media, thermometry and shadowgraphy will be employed. During this thesis project, the PhD student will be able to develop skills in the field of experimental thermal-hydraulics through the definition, execution, and interpretation of tests, as well as the use of advanced two-phase flow measurement techniques.
Prediction of Soiling on PV modules/systems through Real-World Environment Modeling and Data Fusion
Photovoltaic (PV) systems, particularly those installed in regions prone to soiling such as arid areas, coastal sites, and agricultural zones, can experience energy losses of up to 20–30% annually. These losses translate to financial impacts exceeding €10 billion in 2023.
This thesis aims to develop a robust and comprehensive method to predict soiling accumulation on PV modules and systems by combining real-world environmental modeling with operational PV data (electrical, thermal, optical). The research will follow a bottom-up approach in three stages:
1. Component/Module Level: Reproduction and modeling of soiling accumulation in laboratory conditions, followed by experimental validation. This stage will leverage the CEA’s expertise in degradation modeling, including accelerated testing.
2. Module/System Level: Implementation of monitoring campaigns to collect meteorological, operational, and imaging data, combined with field soiling tests on a pilot site. The data will validate and enhance CEA diagnostic tools by introducing innovative features such as AI-driven soiling propagation prediction.
3. System/Operational Level: Validation of the proposed method on commercial PV modules in utility-scale PV plants, aiming to demonstrate scalability and real-world applicability.
The outcomes of this thesis will contribute to the development of an innovative tool/method for comprehensive soiling diagnostics and prognostics in PV installations, enabling the minimization of energy losses while anticipating and optimizing cleaning strategies for PV plants.
Multi-physical characterization of potassium hybrid supercapacitors for performance improvement
The PhD subject focuses on the optimization of potassium hybrid supercapacitors (KIC), which combine the properties of supercapacitors (power, cyclability) and batteries (energy). This system, developed at the CEA, represents a promising technology, low cost and without critical/strategic materials. However, performance optimization still requires overcoming various obstacles observed in previous work, in particular on the intercalation of potassium in graphite and the heating phenomena of cells during operation. In order to explore in depth the operating mechanisms of the KIC system, an essential part of the thesis project will include experiments conducted at the ESRF (European Synchrotron Radiation Facility), where advanced diffraction and imaging techniques will be used to analyze the structure of the materials and their behavior in real operating conditions. The processing of the data collected will also be crucial in order to establish correlations between the physicochemical properties of the materials and the overall performance of the system. This thesis will contribute to the fundamental understanding of the multi-physical mechanisms at stake in KIC to develop innovative design strategies and thus improve their capacity, energy efficiency and lifetime.
Learning Interpretable Models for Stress Corrosion of Stainless Steels Exposed in the Primary Environment of PWRs
Stress corrosion cracking (SCC) of austenitic alloys in water-cooled nuclear reactors is one of the most significant component degradation phenomena. SCC occurs due to the synergistic effects of tensile stresses, environment and material susceptibility. For reactor life extension, understanding this mechanism is essential. The methodology most frequently employed to investigate SCC cracking is an experimental one, requiring lengthy and costly tests of several thousand hours. Furthermore, the considerable number of critical parameters that influence susceptibility to SCC cracking and coupling effects have resulted in test grids increasing in length and complexity. This thesis proposes a novel approach based on the use of interpretable models that are driven by the artificial intelligence of fuzzy logic. The aim is to reduce the length and cost of research activities by focusing on relevant tests and parameters that can improve environmental performance. The key issues here will be to add the performance of artificial intelligence to the experimental approach, with the aim of defining susceptibility domains for the initiation of SCC cracks as a function of the critical parameters identified in the model, and providing data for the development of new materials by additive manufacturing. The thesis will develop a numerical model that can be used as guidance in decision-making regarding the stress corrosion mechanism. The future PhD student will also carry out experimental work to validate this new numerical approach.
Oxidation kinetics of U1-yPuyO2 mixed oxides: experimental study and modelling
The widespread use of MOX fuels (based on (U,Pu)O2 mixed oxides) in current nuclear reactors is an option being studied in France. Such a strategy could make it possible to stabilise plutonium stocks and conserve uranium resources. This scenario involves the multi-recycling of plutonium, which requires an upgrade of the existing plant to enable reprocessing of spent MOX fuel at industrial rates. The development of innovative processes and related basic research is then imperative.
Oxidation of MOX spent fuel by ad hoc thermal treatment could overcome one of the technological barriers identified, i.e. how to separate the fuel from its cladding prior to the dissolution step. The idea is to take advantage of the phase changes that occur as the fuel oxidises to collapse it into powder. However, data on the oxidation of (U,Pu)O2 oxides are currently scarce in the literature. The aim of this PhD thesis is to help fill this gap. The student in charge of this work will first have to characterise the nature of the phases formed during the oxidation of (U,Pu)O2 oxides, as well as the kinetics and mechanisms involved. These results will lead to the proposal of a phenomenological model linking the kinetics of (U,Pu)O2 oxidation with the Pu content, the O2 partial pressure, the temperature and the duration of the thermal treatment.
At the end of this PhD, the graduate student, with initial training in the physical chemistry of materials, will master a wide range of experimental techniques as well as advanced methods for modelling the reactivity of solids. These skills will open up many job opportunities in academic research or industrial R&D, both within and outside the nuclear sector.
Please note: A final internship is also offered in preparation for this PhD position. For further information, please contact the supervising team.
Impact of fission products and microstructure on the oxidation mechanisms of (U,Pu)O2 fuels
The widespread use of MOX fuels (based on (U,Pu)O2 mixed oxides) in nuclear reactors is an option currently being studied in France. Such a strategy could help to stabilise the plutonium inventory while conserving natural uranium resources. Such scenario involves the multi-recycling of plutonium, which requires an upgrade of the existing infrastructures to enable the reprocessing of spent MOX fuel at industrial scale. Consequently, the development of innovative processes and related basic research is then imperative.
The oxidation of MOX spent fuel by ad hoc thermal treatment could overcome a major technological barrier identified, i.e. how to separate the fuel from its cladding prior to the dissolution step. However, there is limited data available on the oxidation of (U,Pu)O2 and even less has investigated the impact of fission products and the fuel microstructure properties. The aim of this PhD thesis is to help fill this gap. The selected student will study (U,Pu)O2 samples with a microstructure identical to that of industrial MOX fuels, as well as (U,Pu)O2 doped with inactive fission products, thus simulating irradiated fuels, as shown in a thesis defended in the laboratory. The experimental work will focus on oxidation experiments coupled with in-situ and multi-scale analyses using laboratory technics and synchrotron radiation-based characterizations. These results will lead to the proposal of a phenomenological description linking the kinetics of (U,Pu)O2 oxidation with the fission products chemistry, the O2 partial pressure and the temperature and duration of the thermal treatment.
By the end of this PhD, the graduate student with a background in physical chemistry of materials will have developed expertise in a broad range of experimental techniques. These skills will open up many career opportunities in academic research or industrial R&D, both within and beyond the nuclear sector.
Eco-designed materials for encapsulating new-generation flexible photovoltaic modules
The lifetime of thin-film devices such as Organic Photovoltaic (OPV) devices or new-generation lightweight and/or flexible Silicon (Si) photovoltaic modules is critical to their commercialization. In particular, it is crucial to encapsulate them with highly gas-barrier materials to avoid degradation through various water/oxygen insertion mechanisms that can be coupled to illumination. This objective is all the more complex when the device and its encapsulation need to be flexible. Moreover, the eco-design of this new generation of flexible modules raises the question of the nature of the encapsulation materials used, as well as that of the end-of-life of the materials making up the modules. For example, the current use of fluorinated polymers for encapsulation generates toxic products at end-of-life, and could be replaced by the use of eco-designed materials, potentially bio-sourced, if the performance is adapted to the photovoltaic technology employed and the use.
The aim of this thesis will be to study the physico-chemical properties (gas barriers, mechanical, thermal, etc.) of bio-sourced encapsulants developed as part of a national PEPR BioflexPV project. These studies will cover both sealing materials and flexible caps. In addition, these materials will be used to encapsulate real OPV and flexible Si devices, in order to study their degradation under different illumination, temperature and humidity conditions. These studies will help define the degradation mechanisms involved, depending on the photovoltaic technology used (OPV or Si), and thus define the desired properties for bio-sourced encapsulants.
Development of high-halogen argyrodites for all-solid all-sulfide battery
All-solid-state batteries have been enjoying renewed interest in recent years, as this technology offers the prospect of higher energy densities due to the use of lithium as a negative electrode, as well as increased battery safety compared with Li-ion technology. The use of sulfides as positive electrode materials coupled with argyrodite as solid electrolyte are interesting systems to develop. The argyrodites achieve ionic conductivities close to those of liquid electrolytes. Moreover, the electrochemical stability window of sulfides is close to that of argyrodite, making all-sulfide technology a promising one for the development of all-solid batteries.
In order to improve the conduction properties of argyrodites, recent studies have shown that ionic conductivity is highly dependent on their local structure. Solid-state NMR thus appears to be a promising technique for probing the local environments of the nuclei mentioned, and in particular for quantifying the variety of different local environments favoring an increase in ionic conductivity. Some compositions enriched in halides appear to promote ionic conduction, and the synthesis of corresponding materials and their structure will be studied.
The thesis will focus on two main areas: the study of all-sulfide batteries and the fine characterization of argyrodite with controlled local structures. Halogen-rich argyrodites will be developed and studied to determine the influence of different local environments on conduction properties.
Synthesis, characterization and modeling of (Mn,Co)3O4 thin films applied to corrosion layers and spintronics
Spinel-type transition metal oxides (AB2O4) appear spontaneously during the generalized corrosion of steels or alloys in aqueous or gaseous environments at high temperatures. This spinel phase forms a continuous corrosion layer and thus regulates corrosion processes by controlling conductivity and material transport between the material and the oxidizing medium. They are also applied voluntarily as protective coatings against degradation phenomena. In particular, the Mn-Co-O spinel system is very promising as protective conductive layers on ferritic stainless steel used to fabricate interconnects in solid oxide fuel cells for green hydrogen production. The composition of the spinel phase determines the protective performance of the coatings. This feature is particularly delicate for materials used in high-temperature electrolyzers, as electronic transport must be optimal (high electrolysis), but must not be accompanied by material transport (low cation diffusion).
In contrast, electronic transport properties of spinel-type transition metal oxides are generally not well understood. Measurements are made on complex corrosion layers (or coatings) of variable composition, low crystallinity, complex microstructure and low thickness. Furthermore, spinel oxides exhibit magnetic properties and composition-dependent cationic disorder that are usually ignored, even though they have a strong impact on electronic transport. The properties highlighted here are the ones that also hold significant importance within the field of spintronics. Thus, tuning the chemical composition of these spinel-structured oxides (normal, inverse or mixed) offers a wide range of magnetic (ferrimagnetic, antiferromagnetic) and electronic (semimetallic, semiconductor, insulator) properties. In particular, CoMn2O4 is expected to exhibit a complex magnetic configuration [1], mainly related to the arrangement of Co2+ and Mn3+ cations in interstitial sites, which needs to be analyzed in detail. Unlike corrosion layers, these physical studies require the synthesis of thin films of well-controlled composition and high crystallinity.
The aim of the thesis is to build up knowledge of physicochemical and structural properties of (Mn,Co)3O4 in order to contribute to the elaboration of Mn-Co-O phase diagrams and electronic transport models based on the relationship between order/disorder, magnetic properties and resistivity of (Mn,Co)3O4. Eventually, the whole (Fe,Cr,Mn,Co)3O4 system will be also considered. The study will be carried out on thin films of perfectly controlled composition and high crystallinity, and will be enhanced by numerical simulations. The experimental and theoretical work will be based on the results of previous studies on (Ni,Fe,Cr)3O4 epitaxial thin films [2,3].
The thesis will be divided as follows:
- Growth of thin films and multilayers by MBE (Molecular Beam Epitaxy) (J.-B. Moussy)
- Spectroscopic characterization using XPS (X-ray photoemission spectroscopy) (F. Miserque)
- Fine structure characterization by DRX and X-ray absorption (XMCD) (P. Vasconcelos)
- Modeling of core-level spectra (XPS, XAS and XMCD) and atomistic modeling (A. Chartier)
- Magnetic characterization by SQUID/VSM magnetometry and electric transport characterization (J.-B. Moussy)
[1] Systematic analysis of structural and magnetic properties of spinel CoB2O4 (B= Cr, Mn and Fe) compounds from their electronic structures, Debashish Das, Rajkumar Biswas and Subhradip Ghosh, Journal of Physics: Condensed Matter 28 (2016) 446001.
[2] Stoichiometry driven tuning of physical properties in epitaxial Fe3-xCrxO4 thin films, Pâmella Vasconcelos Borges Pinho, Alain Chartier, Denis Menut, Antoine Barbier, Myrtille O.J.Y. Hunault, Philippe Ohresser, Cécile Marcelot, Bénédicte Warot-Fonrose, Frédéric Miserque, Jean-Baptiste Moussy, Applied Surface Science 615 (2023) 156354.
[3] Elaboration, caractérisation et modélisation de films minces et multicouches à base d’oxydes (Ni,Fe,Cr)3O4 appliquées à la corrosion et à la spintronique, A. Simonnot, thèse en cours.
Investigation and use of uranium glasses for optical neutron detection
The Dosimetry, Sensors and Instrumentation Laboratory of the CEA/IRESNE Cadarache develops, manufactures and operates neutron flux detectors used in the vicinity of and inside nuclear reactor cores. In addition to conventional detectors (fission chambers, collectrons, etc.), the laboratory is working on innovative measurement methods such as optical detectors, semiconductors, fiber scintillators, etc. As part of this PhD thesis, the laboratory wants to explore the potential of Uranium-doped glasses. These glasses are known to show bright fluorescence under various types of radiations. The main idea of this thesis is to try to exploit this fluorescence to detect the fission reactions induced when the glass is exposed to a neutron flux. This could enable the development of a new generation of optical neutron detectors halfway between a fission chamber and a scintillator.
The thesis will focus on two main topics:
- firstly, a detailed understanding of fluorescence mechanisms, and the synthesis of uranium glass with properties optimized for our needs (sensitivity, emission spectrum, isotopic vector, etc.). Synthesis will be carried out in partner laboratories;
- secondly, the development of a dedicated instrumentation, probably in the form of optical fibers, to test these prototypes in a reactor.