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.
Study of an innovative cleaning process dedicated to the treatment of residual sodium in facilities using liquid sodium as a coolant
Sodium is used as a heat transfer fluid in fast neutron nuclear reactors. Given the operating temperatures of these facilities, all surfaces in contact with liquid sodium remain wetted with residual sodium once the circuits have been drained. The treatment of this residual sodium is required to ensure the safety of interventions on components and structures in a dismantling process. The reference method for this action is cleaning with water in a dedicated cleaning pit. This process involves a reaction of sodium with water in different forms, by controlling the reaction kinetics, which is instantaneous and highly exothermic without controlling the contacting of the reagents.
An exploratory study was carried out at CEA (PhD thesis defended in 2014) on the use of salts to mitigate reaction kinetics. The Sodium and advanced coolant technology laboratory (DES/IRESNE/DTN/STCP/LESC) thus has R&D facilities, instrumented and dedicated to the study of sodium cleaning processes and equipped with the functionalities of an industrial cleaning pit , such as spray nozzles, atomizing nozzles and an immersion device.
The main scientific objective of this new PhD is now to identify, understand and model the physicochemical mechanisms involved in the sodium-water reaction kinetics involving salts. This work will make it possible to limit or avoid pressure wave phenomena or of explosion during the treatment of residual sodium from fast neutron nuclear reactor circuits during their decommissioning and dismantling. The PhD student's mission will be to define the experimental design, to actively participate in carrying out the test campaigns, to analyse the results and to propose an interpretation of the observed phenomena (kinetics, pressure peak, local temperature rise, etc.). The aim of the experimental campaign will be to acquire reliable thermodynamic and reaction kinetic data, such as reaction times, variation of dynamic pressure, temperature rise, composition of the gas and liquid phases, speciation in liquid phase and visualization of the phenomenology via high-speed camera. Modelling tools will be used to establish and simulate a reaction kinetic model. Ultimately, the proposed work will make it possible to qualify the process for industrial application in the field of decommissioning/dismantling, which is a major challenge for the French nuclear industry.
In addition to the experience acquired in the field of nuclear systems dismantling, the proposed work opens up professional prospects, particularly towards research centers and R&D departments in industry.
A master internship is proposed by the team in addition to the thesis.
Simulation of the evolution of dislocation microstructures in UO2: impact of dislocation climbing at high temperature
Carbon neutrality requires the development of low-carbon energy production systems, including nuclear power. The safety analysis of nuclear reactors requires the containment of fission products in all operating conditions, including the integrity of the first barrier made up of the fuel elements. For rod-type designs, which consist of a stack of fuel pellets in a metallic cladding, the mechanical behavior of uranium dioxide (UO2), pellet material, plays an important role in the cladding integrity assessment. During power transients, fuel-cladding contact increases mechanical stresses on the cladding, and fuel creep can accommodate swelling deformations, thereby reducing the stresses induced the cladding. One of the challenges is to understand and predict this phenomenon of UO2 creep, and in particular the mechanisms that drive it at the polycrystalline microstructural scale.
The main objective of the thesis will be to provide simulation methods and reference results in support of multi-scale modeling of the mechanical behavior of fuel at high-temperature, which is highly dependent on dislocation climbing mechanisms. To this end, a computational scheme will be developed, based on the coupling of a dislocation dynamics code (NUMODIS) and a code for solving nonlinear partial differential equations by FFT (AMITEX-FFTP), in order to describe the evolution of a dislocation microstructure (NUMODIS) under the effect of dislocation climbing induced by vacancy diffusion (AMITEX-FFTP). Simulations based on this approach will then be used to quantify the recovery of stored dislocation density with the effect of climbing mechanisms in different configurations (temperatures, stresses, etc.). Ultimately, this work will improve and validate the existing micromechanical modeling implemented in the CEA's PLEIADES simulation platform.
This thesis will be carried out under the joint supervision of the Département d'Etude des Combustibles (Institut IRESNE, CEA Cadarache) and the Département de Recherche sur les Matériaux et la Physico-chimie (Institut ISAS, CEA Saclay), and in collaboration with IM2NP at Aix Marseille Université. The thesis work will be carried out at the LM2C (Cadarache) and LC2M (Saclay) laboratories, in an environment that provides access to extensive expertise in multi-scale materials modeling. The research work will be promoted through publications and participation in international conferences in the materials field.
Uncertainty quantification and sensitivity analysis for vibrations of thin structures under axial flow
Fluid-structure interaction (FSI) phenomena are omnipresent in industrial installations where structures are in contact with a flowing fluid that exerts a mechanical load. In the case of slender flexible structures, IFS can induce vibratory phenomena and mechanical instabilities, resulting in large displacement amplitudes. The nuclear industry is confronted with this problem, particularly concerning piping, fuel assemblies, and steam generators. Computation codes are an essential tool that, based on several input parameters, provide access to quantities of interest (output variables) that are often inaccessible experimentally for the prevention and control of vibrations. However, knowledge of input parameters is sometimes limited by a lack of characterization (measurement error or lack of data) or simply by the intrinsically random nature of these parameters.
In this context, this thesis aims to analyze the vibratory response of a thin structure with uncertain geometric characteristics (structure with a curvature defect, localized or global). In particular, we aim to understand how geometric uncertainties affect the stability of the flexible structure.
This characterization will be carried out both theoretically and numerically. As the work progresses, the effect of different uncertainties (linked, for example, to the material characteristics of the structure or the properties of the incident flow) may be considered. Ultimately, the work carried out as part of this thesis will enable us to improve the prediction and control of vibrations of thin structures under axial flow.
Fluid-structure interactions and associated instabilities are present in many fields, whether in aeronautics with the phenomena of wing flutter, in nuclear power with the vibrations of components under flow, in biology for the understanding of underwater animal locomotion, in botany for the understanding of plant growth, in sport for performance optimization, in energy recovery from fluid-excited flexible structures. The thesis will enable the student to acquire a wide range of skills in mathematics, numerical simulation, fluid mechanics and solid mechanics, and to train for research in the field of fluid and solid mechanics, leading ultimately to a career in this field, whether in academia or in applied research and development in numerous fields of interest to scientists and society in general. A 6-month internship subject is also offered as a preamble to the thesis (optional).
Education level: Master 2 / Final year of engineering school.
Required training: continuum mechanics, strength of materials (beam theory)
fluid mechanics, fluid-structure interaction, numerical simulation (finite elements).
HPC Parallel Integrodifferential Solver for Dislocation Dynamics
Context : Understanding the behavior of metals at high deformation rate [4] (between 104 and 108 s-1) is a huge scientific and technologic challenge. This irreversible (plastic) deformation is caused by linear defects in the crystal lattice : these are called dislocations, which interact via a long-range elastic field and contacts.
Nowadays, the behavior of metals at high deformation rate can only be studied experimentally by laser shocks. Thus, simulation is of paramount importance. Two approaches can be used : molecular dynamics and elastodynamics simulations. This thesis follows the second approache, based on our recent works [1, 2], thanks to which the first complete numerical simulations of the Peierls-Nabarro Equation (PND) [5] was performed. The latter equation describes phenomena at the scale of the dislocation.
PND is a nonlinear integrodifferential equation, with two main difficulties : the non-locality in time and space of the involved operators. We simulated it thanks to an efficient numerical strategy [1] based on [6]. Nevertheless, the current implementation is limited to one CPU –thus forbidding thorough investigations on large-scale systems and on long-term behaviors.
Thesis subject : There are two main objectives :
- Numerics. Based on the algorithmic method of [1], implement a HPC solver (High Performance Computing) for the PND equation, parallel in time and space, with distributed memory.
- Physics. Using the solver developped, investigate crucial points regarding the phenomenology of dislocations in dynamic regime. For exploiting the numerical results, advanced data-processing techniques will be employed, potentially enhanced by resorting to AI techniques.
Depending on the time remaining, the solver might be employed for investigating dynamic fractures [3].
Candidate profile : The proposed subject is multidisciplinary, between scientific computing, mechanics, and data-processing. The candidate shall have a solid background in scientific computing applied to Partial Differential Equations. Mastering C++ with OpenMP and MPI is recommended. Moreover, interest and knowledge in physics –especially continuum mechanics- will be a plus.
The PhD will take place at the CEA/DES/IRESNE/DEC in Cadarache (France), with regular journeys to Paris, for collaboration with CEA/DAM and CEA/DRF.
[1] Pellegrini, Josien, Shock-driven motion and self-organization of dislocations in the dynamical Peierls model, submitted.
[2] Josien, Etude mathématique et numérique de quelques modèles multi-échelles issus de la mécanique des matériaux. Thèse. (2018).
[3] Geubelle, Rice. J. of the Mech. and Phys. of Sol., 43(11), 1791-1824. (1995).
[4] Remington et coll., Metall. Mat. Trans. A 35, 2587 (2004).
[5] Pellegrini, Phys. Rev. B, 81, 2, 024101, (2010).
[6] Lubich & Schädle. SIAM J. on Sci. Comp. 24(1), 161-182. (2002).