Molecular Dynamics study of deformation and phase transition mechanism in tin

Several pressure-induced phase transformations have been predicted and observed in tin; its phase diagram reflects its special position in group IV of the periodic table of elements, where the lighter elements (C, Si, Ge) tend to form covalent bonds. The most stable phase at 0K corresponds to a diamond phase similar to those found in lighter elements. However, pressure and temperature transitions are observed, associated with a change in the nature of the interatomic bonds. The thermodynamic and mechanical properties of the different phases of tin, as well as the structural transitions, are fairly well known today, but are still difficult to reproduce using electronic structure calculations.

For classical Molecular Dynamics (MD) simulations, a number of semi-empirical potentials have been proposed in the literature, which can reproduce certain parts of the phase diagram or certain properties, but which are limited in their ability to predict certain properties, in particular the elastic constants. Recently, Machine Learning Interatomic Potentials (MLIPs) have been developed to improve the description of the properties of the different crystalline phases. However, these potentials, trained on crystalline phases at temperature and liquid configurations, do not take into account the specific distortions of the lattices encountered during deformation of the material (dislocation formation, maculation).

Development of a chloride recovery process by precipitation – Application to molten salt reactors

Molten Salt Reactors (MSRs) represent an innovative option for safer and more sustainable nuclear energy.
They use liquid chloride salts containing actinides, enabling the closure of the nuclear fuel cycle.
During operation, these salts become enriched with fission products and impurities, making chemical treatment necessary.
Enrichment in chlorine-37 aims to limit the formation of chlorine-36, a long-lived radioactive isotope.
Controlling and recycling chloride ions is therefore a major challenge.
The CEA is developing a hydrometallurgical precipitation process to recover enriched chlorine in solid form.
This process is compatible with the La Hague reprocessing plant, in partnership with Orano.
The research focuses on the influence of actinides and fission products on the precipitation reaction and their retention in the solid.
The solubility and purity of the precipitate are studied using various physicochemical techniques.
Purification protocols are optimized when contamination is detected.
Once purified, the solid is recycled to produce reusable chlorine, notably through electrolysis or redox reactions.
This work contributes to the development of innovative reactors and benefits from strong scientific and industrial support.

Development of isotopic and elemental analysis methods on irradiated fuels for the reduction of sample quantities.

The objective of this postdoctoral research is to develop analytical methods for the overall reduction of sample quantities required for high-precision multi-element isotopic analysis (actinides and PF) of spent nuclear fuel, particularly through the use of novel "low-quantity" introduction methods on multi-collector ICPMS. These developments will notably reduce the amount of radioactive waste (consumables and effluents), the dose rate, and the exposure time of analysts/radioactive samples associated with this type of measurements.
To carry out this project, the candidate will conduct analytical developments in a controlled environment to minimize the quantities of elements required for analysis while maintaining or improving uncertainty levels compared to currently available methods.

Development of catalysts for high-temperature electrolysis applied to ammonia synthesis

The project aims to develop novel catalysts and electrodes for the electrochemical synthesis of ammonia (NH3) at high temperature (400–650 °C), using Solid Oxide Electrolysis Cells (SOEC) and Protonic Ceramic Electrolysis Cells (PCEC). Ammonia is not only a key chemical for fertilizers, but also a promising fuel and a hydrogen carrier. Unlike the energy-intensive and carbon-heavy Haber-Bosch process, the electrochemical route allows for more sustainable production with zero CO2 emissions if powered by clean electricity. The post-doctoral fellow will contribute to the identification and shaping of catalysts (e.g. Ru/YSZ, Fe/YSZ, Mn4N/BCZY), their integration into electrodes, and the evaluation of electrochemical performance. The work combines catalysis, electrochemistry, advanced characterization, and preliminary energy assessment, in close collaboration with CEA teams and academic partners.

Miniaturised analytical method dedicated to the screening of candidate molecules for the capture and removal of radionuclides

This project aims at developing a miniaturized multiplex device dedicated to the screening of the chelating ability of potential molecules for the decorporation of certain radionuclides (RN) from the nuclear power industry, for which current treatments are not satisfactory. The objective is to accelerate the identification of the most promising chelating molecules, while benefiting from the advantages of miniaturisation, such as the consumption of very small quantities of molecules and RN. In a previous project, a phosphated monolith of various lengths has been grafted in situ and characterised in capillaries of 100 µm internal diameter. The quantities of UO22+, Zr4+, Sr2+, Co2+, Cs+ and Ag+ immobilised on these monolithic phases have been measured online by coupling to an ICP-MS.Based on this work, the candidate will be responsible for developing and validating the miniaturised screening method with UO22+, for which data and chelating molecules are available, extending the approach primarily to Zr4+, Sr2+, Co2+, and to fabricate the microfluidic device incorporating parallel microchannels, in order to ultimately screen candidate molecules for distinct RNs in a single fluidic microsystem.

Thermodynamic study of photoactive materials for solar cells

The development of solar photovoltaic electricity generation requires the development of new materials for converting solar radiation into electron-hole pairs. Organic-inorganic hybrid perovskites (HOIPs) of the CsPbI3 type, with substitutions of Cs by formamidinium (FA) and/or methylammonium (MA) ions, have emerged as very promising materials in terms of performance and manufacturing. Substitutions of Cs with elements such as Rb, Pb with Sn, and I with Br are also being considered to improve stability or performance. The synthesis and optimization of the composition of layers of such materials require a better understanding of their thermodynamic equilibrium properties and stability. The objective is to construct a thermodynamic model of the Cs-Rb-FA-Pb-Sn-I-Br system. The project began with the ternary Cs-Pb-I system, which resulted in a paper [1]. The next step will focus on the ternary Cs-Pb-Br system, followed by the quaternary Cs-Pb-I-Br system. The approach uses the CALPHAD method, which focuses on building a database and an analytical formulation of the phases Gibbs energy, capable of reproducing thermodynamic and phase diagram data. A critical review of the data in the literature will enable this database to be initialized and the missing data will be evaluated by experiments and/or DFT calculations.

Microfluidics applied to the separation of actinides in nuclear samples by chromatography

The main objective of this post-doctoral position is to develop a method for separating the fission products, plutonium and uranium in nuclear samples by chromatography with a resin volume of 200 µL or less. This project is structured around three research axes.

The first one consists in optimising the miniaturised separation method. The resin packing protocol, the pressure applied during the separation and the eluant compositions will be studied by comparing the chromatograms and by calculating the associated decontamination factors. These developments will be carried out using simulated samples first, and then with plutonium-containing samples. Control over the redox adjustment step will be necessary to maximize the decontamination factors. A second development axis will focus on the conception of a user-friendly system, minimizing interventions in the glovebox in order to reduce the user's exposition to ionizing radiation. The experience of the laboratory in terms of experimental setup miniaturisation and micro-fabrication will be useful for this post-doctoral position. The third research axis consists in applying the developments of the first two axes to the determination of isotopic composition of nuclear samples by TIMS or MC-ICP-MS with a per-mil level of uncertainty in a radiation-controlled laboratory.

Spin-lattice interactions in Machine Learning assisted ab initio simulations

The scientific field addressed by this postdoctoral project lies at the intersection of ab initio molecular dynamics, machine learning, and the thermodynamic characterization of materials under extreme conditions. Traditional AIMD simulations are a powerful tool to study temperature- and pressure-dependent properties from first principles, but their high computational cost limits their widespread use. By developing and applying machine learning-assisted sampling techniques like MLACS, this postdoc aims to drastically reduce the computational burden while retaining ab initio accuracy. This enables the efficient exploration of phase diagrams in high-pressure and high-temperature conditions. This research contributes to both fundamental understanding and practical modeling of materials, offering high-impact tools for the scientific community.

Bayesian inference-based ab initio phase diagrams

Experimental and Thermodynamic Modeling of Corium Phases Formed During Severe Nuclear Accidents (24 months)

During severe accidents in pressurized water reactors, uranium dioxide (UO2) fuel reacts with zirconium alloy cladding and the steel vessel, forming a mixture of liquid and solid phases known as "in-vessel corium". If the vessel ruptures, this corium interacts with the concrete raft, forming "ex-vessel corium". This phenomenon occurred in the Chernobyl and Fukushima severe accidents. To simulate these stages, multi-physics codes require accurate thermodynamic and thermophysical data for the various phases of corium. This project aims to fill the data gap through experimental measurements and modeling. The work will involve synthesizing samples, measuring liquidus/solidus temperatures and liquid phase densities, and characterizing samples using advanced techniques. Moreover, the laser heating setup combined with aerodynamic levitation (ATTILHA) used to acquire data will be improved. Experimental results will be compared with thermodynamic models (TAF-ID database), and discrepancies will be resolved using the CALPHAD method. Thermophysical data will also be validated using atomistic simulations and other measurement techniques.