A posteriori estimates for the mixed finite element discretization of the multigroup diffusion criticality problem
A postdoctoral fellowship is proposed on the a posteriori estimates for the mixed finite element discretization of the multigroup diffusion criticality problem.
The objective is to develop efficient and reliable a posteriori estimates for a multigroup diffusion criticality problem with strong spatial heterogeneities, i.e. a model where the parameters, typically the coefficients of the equations, vary rapidly in space. Mathemically speaking, the criticality problem is a non-symmetric generalized eigenvalue problem.
At the reactor core scale, using simplified models is common in the nuclear industry. Precisely, the simplified models can be the neutron diffusion model or the simplified transport model. We derived rigorous em a posteriori error estimates for mixed finite
element discretizations of the neutron diffusion source problem, and proposed an adaptive mesh refinement strategy that preserves the Cartesian structure. A first application of this approach to the criticality problem was performed. Regarding the industrial context and specifically the numerical simulations, our approach is part of the development of a mixed finite element solver called MINOS in the APOLLO3 code. Further extensions of the a posteriori estimates were studied such the multigroup diffusion source problem and a Domain Decomposition decomposition denoted the DD+L2 jumps method. The enlisted approaches are based on the formulation of a source problem. The objective is to extend the a posteriori approach to a non-symmetric generalized eigenvalue problem.
Study of a low-cost K-ion storage system: electrolyte, safety and prototyping
The UPBEAT project (France 2030) aims to develop a low-cost potassium-ion technology that is free of critical materials and capable of providing the performance of LiFePO4-type Li-ion cells. The work proposed to the post-doc is in line with this objective: it will involve developing optimised organic liquid electrolytes for this new system (Prussian White vs. Graphite), by studying the most promising salts, solvents and additives, while maintaining the objectives of cost and durability. The proposed solutions (with and without fluorine) will be formulated, characterised and electrochemically tested in complete cells (coin cells and pouch cells including components optimisations) to measure their effectiveness in terms of lifetime and power response. Operando measurements and post-mortem characterisations will be used to understand the effects of the various components. The systems that best meet the project's requirements will also be subjected to abuse tests to assess the safety of the final system.
Development of an electromagnetic jet material characterization probe
The subject falls within the scope of non-destructive testing of the electromagnetic properties of materials.
The aim is to upgrade an existing experimental device based on the use of a radiofrequency probe that extracts the magnetic permeability of the material covering an object from the measured reflection coefficient. Solving the direct problem using numerical simulations allows us to establish charts that are used to solve the inverse problem. The sensitivity to material properties, spatial resolution, and measurement uncertainties of the current device are limited by the antenna. Recent studies have demonstrated the value of using an electromagnetic jet-based probe for characterization with sub-wavelength resolution. Based on this work, the objective is to design and build a new probe that meets the desired performance requirements. The candidate will be responsible for the design and simulation work and then for monitoring the production of the prototypes. He/she will also be responsible for testing these prototypes on reference objects to demonstrate their advantages over the current solution. The new probe will then be integrated into the current measurement system and process
The postdoctoral research will proceed in three main stages. The first will consist of studying the principle of electromagnetic jet antennas and proposing a probe design suitable for the measurement method. Commercial simulation software will be used for the design, followed by internal codes for the validation of the selected prototype. The second stage will involve the manufacture of the prototype, followed by tests with reference samples to validate the concept. Finally, the probe will be integrated into the test bench and the calculation and extraction chain..
Modeling and Analysis of Prospective Scenarios for Hydrogen in France and Germany H/F
The use of hydrogen produced by electrolysis, along with its derivatives (such as synthetic methanol and synthetic kerosene), is one of the solutions envisioned to decarbonise certain sectors such as the steel industry and sea and air transport. The development of a European hydrogen transport network is envisaged to support production, access to storage facilities, exchanges between regions with high renewable potential and industrial hubs, and to limit certain electricity grid reinforcement costs.
The propose post doc subject is part of the Franco-German research project CrossHy focuses on analyzing possible deployment pathways for hydrogen transport infrastructure in France and Germany. Post dow work will rely on two complementary modeling tools (REMix, ANTARES)for optimizing development and operation of hydrogen infrstructures at European Level. The project involves regular exchanges between French and German research teams.
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.
Condensation of Humid Air during Loss of Insulation Accident in a LH2 tank (CHALIA Project)
Liquid hydrogen is increasingly becoming the key energy vector for industrial decarbonization in heavy mobility. It is stored at 20K in a double-walled tank with an insulating vacuum. Any compromise to the integrity of the outer wall will allow hot air to enter the insulating vacuum. Nitrogen, oxygen, and water vapor will condense or even desublimate on the cold wall of the inner tank, thereby transferring heat to the cryogen, which will begin to boil. This boiling causes a pressure increase, leading to the opening of safety valves to prevent tank rupture. To better understand these complex phenomena, the CEA, Fenex Collaborative Research Center, and the University of Western Australia have submitted the CHALIA project to the Franco-Australian Center for Energy Transition. This project was approved in October. The post-doctoral position offered by the CEA involves setting up an analytical experiment using an existing glass cryostat to study in detail the various phenomena and measure the heat fluxes transmitted to the cryogen during the different phases of the accident. A gradual approach is proposed, starting with nitrogen entry before progressing to a binary mixture (synthetic air) or a ternary mixture (humid air). The project also aims to identify and quantify the phases involved in the process using various optical methods. The work will be conducted in close collaboration with researchers from the University of Western Australia, who will focus on scaling up the findings.
In-situ 4D tracking of microstructural evolution in atomistic simulations
The exponential growth of high-performance computing has enabled atomistic simulations involving billions or even trillions of particles, offering unprecedented insight into complex physical phenomena. However, these simulations generate massive amounts of data, making storage and post-processing increasingly restrictive. To overcome this limitation, on-the-fly (in-situ) analysis has emerged as a key approach for reducing stored data by extracting and compressing relevant information during runtime without significantly affecting simulation performance.
In this context, tracking the four-dimensional (space and time) microstructural evolution of materials under extreme conditions is a major scientific challenge. Atomistic simulations provide a unique spatial resolution to analyze crystalline defects such as dislocations, twinning, vacancies and pores, which govern dynamic phase transformations, melting, damage and mechanical behavior. By capturing their spatio-temporal evolution, it becomes possible to study their statistics, correlations and collective effects in out-of-equilibrium regimes, leading to more accurate predictive material models.
This project builds on advances of the exaNBody high-performance computing platform and a recently developed in-situ clustering method in the ExaStamp molecular dynamics code at CEA. This method projects atomic information onto a 3D Eulerian grid to perform real-time clustering. The objective is to extend this approach to full 4D tracking, enabling the time-resolved monitoring of clusters. This will allow dynamic graph-based analysis of their evolution, including changes in size, shape and temporal behavior, providing new insights into microstructural dynamics at the atomic scale.
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.
Formal Explanations for Artificial Intelligence
The candidate will contribute to the PyRAT formal analyzer, developped in the lab. This state-of-the-art analyzer is both used as a research sandbox and as an industrial-grade tool. As such, the candidate will work at the boundary of academia and industry.
The candidate missions are the following:
- actively build, update and deliver a state of the art on formal verification, in particular formal verification of machine learning and formal explanations
- contribute to scientific and technical discussions on PyRAT's design and implementations, and pursue said implementations
- investigate and apply the uses of PyRAT for formal explanations
- contribute to funded projects, either national or international, both by institutional and industrial actors, in particular by helping writing deliverables on such projects
- contribute to publications and/or technical reports around PyRAT
- help the dissemination of PyRAT, in particular by contributing to tutorials, courses and presentations and presenting them at scientific and industrial venues