Thermohydraulic modelisation of a steam generator and chemical species propagation
Steam generators are essential components of nuclear reactors whose main function is heat exchange. The chemical species present in steam generators are the cause of many parasitic phenomena (clogging, fouling, sludge deposition, etc.). Numerical simulation of species transport, taking into account the migration of chemical species and exchanges between species, both intra- and inter-phase, will allow a better understanding and better management of these problems. Numerical resolution of species transport systems presents real difficulties, in particular the management of the appearance and total disappearance of certain species, high void rates, as well as rapidly excessive calculation times.
While relying on the new code for nuclear components developed at STMF, the thesis will address the following three main scientific issues:
• Upstream, the analysis of numerical methods allowing in particular the management of evanescence, as mentioned above, and thermo-hydraulic modeling at high void rates. For this, we will rely on the PolyMAC and PolyVEF numerical schemes, already implemented in the component code.
• The physical modeling of a steam generator in the new component code, via the addition (in C++) of correlations specific to steam generators, the completion of the state laws already available, etc..
• The determination of the major chemical species to be transported, in order to be able to take into account both thermo-hydraulics and chemistry. The algorithmic coupling between thermo-hydraulics and chemistry, taking into account feedback, being the long-term objective.
While benefiting from the existing parallelization of the component code, the thermo-hydraulic and chemical modeling will be done taking into account the constraints on computation times.
Chemo-mechanical modeling of the coupling between carbonation, rebar corrosion and cracking in cementitious materials
Rebar corrosion is one of the main causes of premature degradation of concrete infrastructures, including in the nuclear sector, where concrete is extensively used in containment structures and waste storage facilities. Carbonation, caused by the penetration of CO2 into the concrete, lowers the pH of the pore solution, promoting rebar corrosion. This corrosion leads to the formation of expansive products that can cause cracking in the material. The proposed thesis work, developed as part of a European collaborative project between CEA Saclay, École des Mines de Paris - PSL, and IRSN, aims to develop a numerical model to simulate these phenomena. The model combines a reactive transport code (Hytec) and a finite element code (Cast3M) to study the local effects of carbonation-induced corrosion on concrete cracking. This project will benefit from parallel experimental work to gather data for parameter identification and model validation. The first part of the research will focus on modeling the carbonation of cementitious materials under unsaturated conditions, while the second part will address the corrosion of rebar caused by the pH drop induced by carbonation. The model will describe the growth of corrosion products and their expansion, inducing stress within the concrete and potential microcracking.
This research project is aimed at a PhD student wishing to develop their skills in materials science, with a strong focus on multi-physical and multi-scale modeling and numerical simulations. The thesis will be carried out principally at CEA Saclay and at École des Mines de Paris – PSL (Fontainebleau).
Reactive metals corrosion in innovative binders – Experimental study and hydro-chemo-mechanical modelling
Nuclear waste management requires the packaging of several kinds of metal wastes for long-term storage. These wastes, which can be very reactive metals, are prone to corrosion and commonly immobilised into containers with hydraulic binders as embedding matrices. Innovative binders (low carbon cements, alkali activated materials) are thus developed to increase the packaging performances. The main objective of the European project STREAM (in the frame of the Eurad-2 program) is to evaluate the interactions between these metal wastes and the selected cement matrices. The PhD thesis purpose is to investigate the reactive metal corrosion in the selected binder with electrochemical techniques. A generic experimental protocol will be developed in order to determine the impact of the corrosion products growth at the metal/binder interface on the global mechanical behaviour of the binder-waste composite and possible micro-cracks occurrence. A post-mortem characterisation will be performed on the metal/binder microstructure with mechanical properties measurements of the materials at the interface, especially the corrosion products. Afterwards, these results will feed a simplified Hydro-Chemo-Mechanical (HCM) model aiming the simulation of corrosion consequences on the composite material behaviour. Subsequently, this model will be used for long-term simulation at the waste package scale.
This research project is aimed at a PhD student wishing to improve his/her skills in materials science both in the experimental field and in the modelling/simulation of coupled physicochemical phenomena.
Impact of a sodium nitrate saline plume on the radionuclide confinement properties of cementitious matrices
Using modelling to predict the migration of radioactive species through a well-known porous matrix, such as concrete, is a major challenge for society, particularly in the context of studies linked to the radioactive waste management. Demonstrating that the proposed model is robust through targeted laboratory experiments under extreme chemico-physical conditions is one of the scientific challenges proposed by the CEA as part of this PhD research project.
The young reseacher will be responsible for designing, carrying out and modelling experimental lab-tests on the retention and diffusion of radionuclides of interest in controlled cementitious conditions or under perturbation due to the nitrate plume leading to very high concentrations in the pore solution. The main expected result is to propose a predictive model coupling chemistry under extreme ionic strenght conditions and transport through complex cementitious matrices, validated by experimental data acquired on simple systems.
Surrounded by a team of experts in the field of measuring and modelling radionuclides migration in porous media, the PhD student will be able to develop or extend his/her skills in the following areas: chemistry, analytical chemistry, physico-chemistry, radiochemistry and modelling.
Improving the predictivity of large eddy simulations using machine learning guided by high-fidelity simulations
This thesis aims to explore the application of machine learning techniques to improve turbulence modeling and numerical simulations in fluid mechanics. More specifically, we are interested in the application of artificial neural networks (ANNs) for large eddy simulation. The latter is a modeling approach that focuses on the direct resolution of large turbulent structures, while modeling small scales by a subgrid-scale model. It requires a certain ratio of total kinetic energy to be resolved. However, this ratio may be difficult to achieve for industrial simulations due to the high computational cost, leading to under-resolved simulations. We aim to improve the latter by focusing work along two main axes: 1) Using ANNs to build generic sub-mesh models that outperform analytical models and compensate for coarse spatial discretization; 2) Training ANNs to learn wall models. One of the main challenges is the ability of the new models to generalize correctly in configurations different from those used during training. Thus, taking into account the different sources and quantification of uncertainties plays a vital role in improving the reliability and robustness of machine-learned models.
Thermally conductive yet electrically insulating polymer nanocomposite based on core-shell (nano)fillers oriented by magnetic field
Advances in power electronics, electric motors and batteries, for example, are leading to a significant increase in heat production during operation. This increase in power density combined with reduced heat exchange surfaces amplifies the challenges associated with heat dissipation. The absence of adequate dissipation leads to overheating of electronic components, impacting on their performance, durability and reliability. It is therefore essential to develop a new generation of heat dissipating materials incorporating a structure dedicated to this structure.
The objective and innovation of the PhD student's work will lie in the use of highly thermally conductive (nano)fillers that can be oriented in an epoxy resin in a magnetic field. The first area of work will therefore be to electrically isolate the thermally conductive (nano)charges with a high form factor (1D and 2D). The electrical insulation of these charges of interest will be achieved by a sol-gel process. The synthesis will be controlled and optimised with a view to correlating the homogeneity and thickness of the coating with the dielectric and thermal performance of the (nano)composite. The second part will focus on the grafting of magnetic nanoparticles (NPM) onto thermally conductive (nano)fillers. Commercial NPMs will be evaluated as well as grades synthesised in the laboratory. The (nano)composites must have a rheology compatible with the resin infusion process.
Modeling and experimental validation of a catalytic reactor and optimization of the process for the production of e-Biofuels
During the past 20 years, « Biomass-to-liquid » processes have considerably grown. They aim at producing a large range of fuels (gasoline, kerozene, diesel, marine diesel oil) by coupling a biomass gazéification into syngaz unit (CO+CO2+H2 mixture) and a Fischer-Tropsch (FT) synthesis unit. Many demonstration pilots have been operated within Europe. Nevertheless, the low H/C ratio of bio-based syngaz from gasification requires the recycling of a huge quantity of CO2 at the inlet of gaseification process, which implies complex separation and has a negative impact on the overall valorization of biobased carbon. Moreover, the possibility to realize, in the same reactor, the Reverse Water Gas Shift (RWGS) and Fischer-Tropsch (FT) reaction in the same reactor with promoted iron supported catalysts has been proved (Riedel et al. 1999) and validated in the frame of a CEA project (Panzone, 2019).
Therefore, this concept coupled with the production of hydrogen from renewable electricity opens new opportunities to better valorize the carbon content of biomass.
The PhD is based on the coupled RWGS+FT synthesis in the same catalytic reactor. On the one hand a kinetic model will be developed and implemented in a multi-scale reactor model together with hydrodynamic and thermal phenomena. The model will be validated against experimental data and innovative design will be proposed and simulated. On the other hand, the overall PBtL process will be optimized in order to assess the potential of such a process.
Understanding the mechanisms of direct CO2 hydrogenation using (Na,K)FeOx catalysts via theoretical-experimental coupling
In the context of climate change, we need to reduce our CO2 emissions by using less energy. Another approach is to capture, store and use CO2, with the aim of moving towards a circular carbon economy and, ultimately, defossilization. With this in mind, the direct hydrogenation of CO2 enables it to be transformed into molecules of interest such as hydrocarbons, via the coupling of the reverse water gas shift (RWGS) reaction and Fischer-Tropsch synthesis (FTS).
Computational operando catalysis has recently emerged as a reasoned alternative to the development of new catalysts, thanks to a multi-scale approach from the atom down to the active particle, to model catalyst selectivity and activity. New tools combining ab initio simulations (DFT) and molecular dynamics (MD) via machine learning algorithms bridge the gap between the precision of DFT calculations and the power of atomistic simulations. Current bifunctional catalysts (active for RWGS, and FTS) for direct CO2 hydrogenation are based on doped iron oxides (metal promoters).
The aim of this project is the theoretical study of Na-FeOx and K-FeOx catalysts doped with Cu, Mn, Zn and Co, in 4 stages: DFT simulations (adsorption energies, density of states, energy barriers, transition states), microkinetic modeling (reaction constants, TOF), construction of interatomic potentials by DFT/machine learning coupling, simulation of whole particles (selectivity, activity, microscopic quantities).
This theoretical study will go hand in hand with the synthesis and experimental measurements of the studied catalysts, and optimized catalysts emerging from the computational results. All the accumulated data (DFT, MD, catalytic properties) will be fed into a database, which can eventually be exploited to identify descriptors of interest for CO2 hydrogenation.
Development of an advanced grade of nano-reinforced austenitic steel for use under intense flux
Recent work has shown that it is possible to obtain ODS (Oxide Dispersion Strengthened) austenitic steels for use under intense flux. These new grades are beginning to be studied for nuclear applications around the world. They should have remarkable properties, particularly in terms of resistance to swelling under irradiation and creep, thanks to the addition of nano-reinforcements in exceptional density (10^23 to 10^24 m-3). These ODS steels are obtained by powder metallurgy, by co-grinding a metal powder with an oxide powder. The aim of this work is to succeed in manufacturing, using an innovative process, ODS austenitic steel cladding tubes. It will be necessary to master the recrystallization of these tubes, to propose a first critical evaluation by specifying the property/microstructure relationships and by evaluating, by irradiation with charged particles, the behavior under irradaition of this new material. The student will be trained in Scanning Electron Microscopy and the techniques that result from it (X-ray analysis, EBSD, etc.), in SAXS, in the performance and exploitation of mechanical tests. He will have to acquire good notions in Transmission Electron Microscopy and in Tomographic Atomic Probe. The understanding of the behavior under irradiation will be guided by simulations by cluster dynamics.
Development of solid porous siliceous supports for actinide sorption - Behaviour under irradiation
The aim of this research project is to study the densification of a mesoporous structure under the effect of irradiation damage produced by the presence of an actinide (238Pu) in the porous structure. To achieve this, siliceous materials based on mesoporous silicas modified by the addition of additive elements (B, Al, etc.) will be used. The purpose of adding these elements is to weaken the mesoporous structure in order to promote densification. The characteristics of the mesoporous structure (pore diameter, wall size, symmetry of the pore network) will be other parameters of the study. These materials will be functionalised with phosphonate ligands for actinide adsorption: thorium as a simulant in a preliminary stage, followed by plutonium. The final part of this work, which will continue beyond the thesis, will involve using various techniques (SAXS, BET, microscopy, etc.) to study the evolution of the mesoporous structure under the effect of irradiation damage as the material ages. This fundamental research work could have spin-offs in the field of nuclear waste conditioning materials: ageing of gels on the surface of nuclear glass, support material for decontaminating radioactive effluents. Part of the work will be carried out at CEA Marcoule's Atalante facility.