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.
Development of 4D-STEM with variable tilts
The development of 4D-STEM (Scanning Transmission Electron Microscopy) has profoundly transformed transmission electron microscopy (TEM) by enabling the simultaneous recording of spatial (2D) and diffraction (2D) information at each probe position. These so-called “4D” datasets make it possible to extract a wide variety of virtual contrasts (bright-field imaging, annular dark-field imaging, ptychography, strain and orientation mapping) with nanometer-scale spatial resolution.
In this context, 4D-STEM with variable beam tilts (4D-STEMiv) is an emerging approach that involves sequentially acquiring electron diffraction patterns for different incident beam tilts. Conceptually similar to precession electron diffraction (PED), this method offers greater flexibility and opens new possibilities: improved signal-to-noise ratio, faster two-dimensional imaging at higher spatial resolution, access to three-dimensional information (orientation, strain, phase), and optimized coupling with spectroscopic analyses (EELS, EDX).
The development of 4D-STEMiv thus represents a major methodological challenge for the structural and chemical characterization of advanced materials, particularly in the fields of nanostructures, two-dimensional materials, and ferroelectric systems.
Monitoring and modeling the evolution of microstructural properties during the fabrication of MOX fuel
The nuclear fuel MOX (Mixed OXide), a ceramic obtained from a mixture of uranium and plutonium oxides, represents a strategic alternative for the valorization of plutonium resulting from the reprocessing of spent fuel. MOX pellets are produced industrially using a powder metallurgy process combined with material densification through high-temperature sintering. The rejected products are reintroduced into the process in the form of "chamotte" powder. Yet, the influence of the content and nature of this chamotte on the microstructural stability of the material remains poorly understood, particularly during the pressing and sintering stages. This aspect is critical for both the mechanical integrity and the in-reactor behavior of MOX fuels. A better understanding of these phenomena, combined with refined modeling, would make it possible to optimize industrial processes and ultimately improve the reliability of these fuels.
The objective of this PhD project is to study and model the evolution of the microstructural properties of MOX fuel as a function of the proportion and nature of the chamotte added during fabrication. The thesis strategy will rely on an integrated approach combining experimental studies with numerical simulations. It will be based on multi-scale characterization of the microstructure, coupling imaging and spectroscopy techniques, as well as on the three-dimensional reconstruction of the microstructure from experimental 2D images. The ultimate goal is to establish a link between the elastic properties of the material and its microstructure. This work will build on a combined experimental and modeling approach, bringing together the expertise of the supervisory team for experiments on plutonium-bearing materials, and for numerical modeling (micromechanical modeling, FFT-based calculations).
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 numerical modeling methods applied to ceramic materials. These skills will open up many job opportunities in academic research or industrial R&D, both within and outside the nuclear sector.
Metallurgy under extreme conditions
The microstructure-properties relationship is a core concept of metallurgy, and of materials engineering in general. For instance, the hardness of quenched steels emerges from their martensitic microstructure, induced by a phase change in iron. Here we are concerned about metallurgy under extreme conditions in which metallic samples undergo pressurizations in the 100 GPa (=1 million atmospheres) range, making it possible to synthesise new crystalline phases with potentially interesting properties (hardness, magnetism, etc.).
Studied systems will include tin, then indium and cobalt. The three of them exhibit a rich polymorphism under high pressure and temperature. We will seek to elucidate the role of defects such as twinning and plasticity on the mechanism and kinetics of these transitions. This will be done by comparing experimental observations with microstructure predictions obtained through mesoscopic simulation. High pressure/ high temperature will be mainly generated by laser-heated diamond anvil cells, and characterisation tools will include in situ X-ray imaging by diffraction and tomography, as well as electron microscopy. The X-ray sources used will be synchrotron sources and the European free-electron X-ray laser.
Development of theoretical Raman spectra with application on minerals from the surface of Mars
As we push the boundaries of space exploration with new missions to nearby planets, improving our investigation tools is crucial. Mars rovers have revealed a surface mineralogy unlike anything on Earth, shaped by the planet’s former hydrosphere followed by an extended dry and cold environment. For example, this favors the formation of perchlorates, or mixed silicate–salts glassy phases — minerals that are difficult to synthesize and stabilize on Earth but remain surprisingly stable on Mars. Recent Raman spectrometry data confirms their presence, highlighting an opportunity for deeper research. Understanding these minerals could offer new insights into Martian chemistry and planetary evolution.
Here we want to calculate the theoretical Raman spectra of perchlorates and other Martian minerals using the density functional perturbation theory (DFPT) as implemented in the ABINIT package. We want to obtain not only the position and the intensity of the peaks, but also the peak widths. They are necessary to correctly identify between similar spectra and to assess, by integration, the actual intensity of the peaks, which are directly comparable to experimental values on the field. These allow us to choose the representative peaks that can be used in identification and to analyze the displacement patterns associated with the vibrations. The results of our simulations will be compared and interpreted in the light of measurements performed by the current rovers on the surface of Mars.
For this, we need to implement several third- and fourth-order derivatives of the energy. This will be done as a series of DFPT terms, where the perturbations can be atomic displacements or electric fields. We will use a combination of the 2n+1 theorem and finite differences. The implementation will be done within the "Projector Augmented-Wave" approach (PAW) to DFT. The entire development effort will be integrated into the ABINIT package and made available to the entire community. ABINIT (www.abinit.org) is a highly visible international collaborative project for ab initio simulations based on DFT and DFPT. The computed spectra will be made available to the community via the WURM database.
The successful candidate will be co-advised between the IPGP (Paris) and the CEA (Bruyères-le-Chatel, S of Paris) groups. IPGP is a world-renowned geosciences research institute founded in 1921, associated with the CNRS, a component of the Université Paris Cité and employing more than 500 people. The group of Razvan Caracas is highly active in computational mineralogy, study of matter at extreme conditions, and planetology. The Quantum simulation of Matter group at CEA Bruyères-le-Chatel led by Marc Torrent is a main developer of the ABINIT package, highly active in density functional theory, projector augmented-wave, and high-performance computing.
Magnetic Tunnel Junctions at Boundaries
Spin electronics, thanks to the additional degree of freedom provided by electron spin, enables the deployment of a rich physics of magnetism on a small scale, but also provides breakthrough technological solutions in the field of microelectronics (storage, memory, logic, etc.) as well as for magnetic field measurement.
In the field of life sciences and health, giant magnetoresistance (GMR) devices have demonstrated the possibility of measuring the very weak fields produced by excitable cells on a local scale (Caruso et al, Neuron 2017, Klein et al, Journal of Neurophysiology 2025).
Measuring the information contained in the magnetic component associated with neural currents (or magnetophysiology) can, in principle, provide a description of the dynamic, directional and differentiating neural landscape. It could pave the way for new types of implants, thanks to their immunity to gliosis and their longevity.
The current bottleneck is the very small amplitude of the signal produced (<1nT), which requires averaging the signal in order to detect it.
Tunnel magnetoresistances (TMR), in which a spin-polarised tunnel current is measured, offer sensitivity performance that is more than an order of magnitude higher than GMR. However, they currently have too high a level of low-frequency noise to be fully beneficial, particularly in the context of measuring biological signals.
The aim of this thesis is to push back the current limits of TMRs by reducing low-frequency noise, positioning them as break sensors for measuring very weak signals and exploiting their potential as amplifiers for small signals.
To achieve this objective, an initial approach based on exploring the materials composing the tunnel junction, in particular those of the so-called free magnetic layer, or on improving the crystallinity of the tunnel barrier, will be deployed. A second approach, consisting of studying the intrinsic properties of low-frequency noise, particularly in previously unexplored limits, at very low temperatures where intrinsic mechanisms are reached, will guide the most promising solutions.
Finally, the most advanced structures and approaches at the state of the art thus obtained will be integrated into devices that will provide the building blocks for going beyond the state of the art and offering new possibilities for spin electronics applications. These elements will also be integrated into systems for 2D (or even 3D) mapping of the activity of a global biological system (neural network) and for evaluating capabilities for clinical cases (such as epilepsy or motor rehabilitation).
It should be noted that these improved TMRs may have other applications in the fields of physical instrumentation, non-destructive testing, and magnetic imaging.
Electronic excitations in unidimensional nano-objects: an ab initio description and connection with quantum entanglement
Understanding the electronic properties of valence electrons in nano-objects is not only of fundamental interest but also essential for the design of next-generation optoelectronic devices. In such systems, electron confinement in low-dimensional structures gives rise to unique properties.
These properties are inherently linked to fundamental characteristics of matter and the associated quantum fluctuations. More recently, concepts such as quantum entanglement and Fisher quantum information have been connected to spectroscopic properties. On the other hand, these spectroscopic properties can be probed through experimental techniques, including absorption, photoemission, and inelastic X-ray scattering.
Recently, we demonstrated that the widely used formalism to study isolated nano-objects was not adapted, and that it affected the calculated optical properties. We evidenced, theoretically and experimentally, that for the two-dimensional objects, the optical response contained, beyond the transverse contribution, a resonance coming from the plasmon, which corresponds to a longitudinal response. The role of the interfaces revealed to be determinant. The project of this year is to have a critical analysis of the optical properties of unidimensional objects.
Beyond the fundamental characterization of the 1D dielectric function, this research will explore its connection to quantum entanglement and Fisher quantum information—concepts that, to date, have not been investigated in low-dimensional systems.
Origins and consequences of the heterogeneous alteration of nuclear glasses
This PhD project focuses on the long-term behavior of nuclear glasses used to confine long-lived radioactive waste.
In aqueous environments, these glasses generally undergo homogeneous alteration: the transformation into alteration products occurs at a uniform rate across the entire surface.
However, cases of heterogeneous alteration also exist, where the glass/gel interface becomes irregular, forming pits or cavities.
Two key questions arise: what are the mechanisms responsible for that behavior, and what are the consequences for the long-term durability of the glass?
Several hypotheses have been suggested in the literature, such as local fluctuations in solution composition or mechanical stress at the glass surface, but no definitive explanation has yet been established.
The proposed approach combines accelerated experiments with chemical, mechanical, and structural characterizations, as well as modeling (e.g., mesoscopic and Monte Carlo models).
Experiments will be conducted on glasses with various surface states (polished, irradiated, fractured, etc.) using analytical tools such as SEM, TEM, and nanoSIMS.
Once the mechanisms are identified, the long-term impact of these heterogeneous alteration patterns can be assessed.
The project seeks candidates with a background in chemistry or materials science, strong interest in modeling, and solid theoretical knowledge.
Influence of a nano-antenna on the intersystem crossing rate of a single molecule
As part of the continuation of the ANR JCJC PlasmonISC project, we propose a thesis subject mainly experimental in nano-photonics. The objective of the thesis is to study the influence of a nano-antenna (plasmonic, magnetic or dielectric) on the rate governing the photophysics of fluorescence emission from a single molecule, with a particular interest in the intersystem crossing rate. We have developed a dedicated optical bench combining optical and atomic force microscopy, an experimental procedure, as well as signal processing tools, showing encouraging first results with a dielectric tip. We wish to continue to explore the single molecule/nano-antenna interaction with other types of tips generating other physical effects. The ability to control the transition to the triplet state is of great interest for single photon sources, organic light emitting diodes, and in chemistry.