Kinetics of segregation and precipitation in Fe-Cr-C alloys under irradiation : coupling magnetic, chemical and elastic effects
Ferritic steels are being considered as structural materials in future fission and fusion nuclear reactors. These alloys have highly original properties, due to the coupling between chemical, magnetic and elastic interactions that affect their thermodynamic properties, the diffusion of chemical species and the diffusion of point defects in the crystal. The aim of the thesis will be to model all of these effects at the atomic scale and to integrate them into Monte Carlo simulations in order to model the segregation and precipitation kinetics under irradiation, phenomena that can degrade their properties in use. The atomic approach is essential for these materials, which are subjected to permanent irradiation and for which the laws of equilibrium thermodynamics no longer apply.
The candidate should have a good background in statistical physics or materials science, and be interested in numerical simulations and computer programming. The thesis will be carried out at CEA Saclay's physical metallurgy laboratory (SRMP), in a research environment with recognised experience in multi-scale modelling of materials, with around fifteen theses and post-doctoral contracts in progress on these topics.
A Master 2 internship on the same subject is proposed for spring 2025 and is highly recommended.
Purification of chloride salts for safe use in energy production systems: development of methods, understanding and optimization.
Chloride molten salts are of major interest as coolants of high temperature energy production systems (solar, nuclear). However, they suffer from the high corrosion rates on structural materials, which is mainly related to their chemical purity. The control of oxygen activity is of prime interest to limit the dissolution of a large number of elements. However, some salts of interest for the nuclear industry (ternary NaCl-MgCl2-PuCl3 and its surrogate NaCl-MgCl2-CeCl3) are particularly difficult to purify, due to their high affinity with water.
Therefore, the understanding of the nature and stability of species formed in non-purified system (chlorides, oxides, oxi-chlorides, hydroxi-chlorides) is mandatory to propose appropriate purification methods for industrial systems. The Ph D will have to purify and characterize different salt mixtures (from binary to quaternary systems) from available methods in the laboratory:
• For purification: electrolysis, precipitation, filtration, chlorinating gas bubbling
• For characterization: electrochemical technics, potentiometric O sensors, Raman spectroscopy, analytical chemistry, materials characterization…
The thesis will take place at the institute of Energy (IRESNE) of the CEA Cadarache (Provence, France). The main laboratory (LMCT) has a large experience of advanced coolants chemistry (in particular sodium). Some collaborations are engaged with other labs of the CEA (Marcoule) and with the LGC Toulouse, both having long experience in molten salt chemistry.
The student should be graduated in electrochemistry or materials science.
Sub-Grid modelling of interfacial heat and mass transfers applied to condensation of bubble swarms
To assess the safety of nuclear power plants, the CEA develops and uses multi-scale thermohydraulic simulation tools. The application of CFD to two-phase flows is limited because it requires many models that are difficult to determine. Among our other tools, direct numerical simulations (DNS) with resolved interfaces provide reference data inaccessible by experimental means. This is for example the case of bubble swarms, where heat and mass transfers are influenced by complex collective effects.
In order to reduce the cost of these DNS simulations, we recently developed an approach [1] which shows promising results: it consists of coupling a fine resolution of thermal transfers at the liquid-vapor interfaces to a far field calculated on a less resolved mesh. To broaden the application of this method to more industrial cases, it is necessary to take into account collisions between bubbles and to adapt the model to the phase change.
During this thesis, we propose to start with this physical modeling work and its implementation in C++ in our open-source simulation code TRUST/TrioCFD [2]. Next, we will use this new capacity to carry out a parametric study and an in-depth physical analysis of the phenomena which would ultimately lead to an improvement in heat transfer models in industrial codes.
[1] M. Grosso, G. Bois, A. Toutant, Thermal boundary layer modelling for heat flux prediction of bubbles at saturation: A priori analysis based on fully-resolved simulations, International Journal of Heat and Mass Transfer, Vol 222, 2024, https://doi.org/10.1016/j.ijheatmasstransfer.2023.124980
[2] Trio_CFD webpage : http://triocfd.cea.fr/recherche/modelisation-physique/two-phase-flows
Study of MOx and model compounds leaching in underwater storage conditions
This thesis deals with nuclear fuel recycling in France, with a focus on the multi-recycling of uranium and plutonium from MOX fuels, planned for 2040. Spent fuel is stored underwater in pools, where a cladding defect could lead to water contamination and complicate reprocessing. This thesis proposes to study the leaching of these fuels and the appearance of secondary phases under conditions simulating storage. The work is divided into three parts: preparation of model compounds, study of chemical durability of model and industrial materials, and analysis of secondary phases forming on the surface of irradiated fuels. The aim is to gain a better understanding of the stability of these phases as a function of chemical and irradiation conditions, as well as their transformation mechanisms. The results will enable us to develop models for the behavior of defective rods over several decades, contributing to safer and more efficient management of irradiated fuels.
Investigation of autocatalysis phenomena occurring in nitric acid dissolution through electrochemical methods
The nuclear fuel recycling process, used at the La Hague plant in France, begins with the nitric dissolution of spent fuel, mainly composed of uranium and plutonium oxides. In a context of plant renewal and widespread of MOX fuel recycling, innovative new dissolution equipment are currently studied. The sizing of such devices is currently limited by the absence of a fully comprehensive model for the dissolution of mixed oxides, which is a highly complex reaction (three-phase involved, self-catalytic, heterogeneous attack, etc.). Despite substantial progress made in previous studies, a number of questions remain unanswered, particularly concerning the reaction mechanisms involved and the nature of the catalyst.
Electrochemical methods (cyclic voltammetry, electrochemical impedance spectroscopy, rotating electrode, etc.) have never been used to understand dissolution, yet they should prove relevant as already demonstrated by the studies carried out on this subject by CEA Saclay in the field of corrosion. Therefore, the aim of this thesis is to apply these experimental methods for the first time to the dissolution of nuclear fuels, through a phenomenological approach. To achieve this, the student will be able to rely on the teams and facilities of Saclay and Marcoule centers, specialized respectively in electrochemical methods for the corrosion studies and the physico-chemical modeling of dissolution.
This cross-disciplinary study, involving materials science, electrochemistry and chemical engineering, will follow a stimulating fundamental research approach, but will also take place in a highly dynamic industrial context. Initially, the work will be carried out on inactive model and noble materials (at the Saclay center), then on real materials containing uranium and/or plutonium (at the Marcoule center).
Understanding the mechanisms of oxidative dissolution of (U,Pu)O2 in the presence of platinum group metals
The treatment of MOx fuel, composed of a mixed uranium and plutonium oxide (U,Pu)O2, is aimed at recycling plutonium. Plutonium dioxide (PuO2) is notably difficult to dissolve in concentrated nitric acid. However, by introducing a highly oxidizing agent, such as Ag(II), into the nitric acid, plutonium can be solubilized with fast dissolution kinetics—a process known as oxidative dissolution. The fission products present in irradiated MOx, particularly platinum group metals, can potentially impair the effectiveness of plutonium’s oxidative dissolution through side reactions. For the industrial deployment of this method, it is therefore crucial to understand how platinum group metals influence the dissolution kinetics. Yet, there is currently very limited data on this subject.
This thesis aims to address this knowledge gap. The proposed research involves a parametric experimental study of increasing complexity: initially, the impact of platinum group metals on Ag(II) consumption will be investigated separately, followed by their effect during the dissolution of (U,Pu)O2. These findings will enable the development of a kinetic model for the dissolution process based on the studied parameters.
By the end of this thesis, the candidate, with a strong background in physical or inorganic chemistry, will have gained expertise in a wide range of experimental techniques and advanced modeling methods. This dual competence will open up numerous career opportunities in academic research or industrial R&D, both within and beyond the nuclear sector.
Towards a Method for characterizing the electrokinetic Properties of Particles in water at high Temperatures
In the field of industry and particularly energy, liquid water circuits are omnipresent. Fluids, by interacting with pipes made from metal alloys, inevitably lead to the formation of corrosion products.
Predicting the behavior of small particles (order of magnitude of µm) is therefore of particular interest. Indeed, due to their size, the behavior of the latter is governed by forces of electrical origin responsible for their adhesion to the surfaces. The electrokinetic properties and in particular the surface potential thus control the fate of the particle and can be defined using the zeta potential. This quantity characterizes a solid/solution couple and takes into consideration both the particle and its surface chemical properties as well as the solution where the particle is located.
If the characterization of the zeta potential at room temperature is quite widespread, its determination at high temperature is today confined to a few examples (theses by C. Cherpin 2022 [1] and M. Barale 2006 [2], studies of VTT [3] and EDF with the University of Besançon 2002 [4] and the EPRI patent 1994 [5]). The CEA (LC2R) has developed an innovative measurement method currently being patented to explore poorly developed experimental techniques based on theoretical hypotheses to be confirmed.
Through multi-physics (flow, temperature, chemistry, electrochemistry, etc.) and multi-scale (microscopic particles influencing a macroscopic state) approaches, the objective of the thesis is therefore to carry out measurements of the surface properties of particles in water at high temperature depending on the physicochemical conditions (pH, RedOx and temperature), to adapt existing models or propose new ones then validate them with experimental data.
The data thus obtained is intended to feed the simulation codes in order to better understand and control the aging of the circuits.
[1] C. Cherpin, PhD, 2022, Modelling the behaviour of colloidal corrosion products in the primary circuit of Pressurized Water Reactors
[2] M. Barale, PhD, 2006, Etude du comportement des particules colloïdales dans les conditions physico-chimiques du circuit primaire des réacteurs à eau sous pression
[3] E. Velin, Master’s Thesis, 2013, The effect of Temperature on the Zeta Potential of Magnetite Particles in Ammonia, Morpholine and Ethanolamine Solutions
Superconducting Devices in Silicon
The project focuses on the study of superconducting devices with silicon as a semiconductor. Those include standard silicon transistors with superconducting source and drain contacts and superconducting resonators. The common properties is the superconducting material which is elaborated with the constrain of being compatible with the silicon CMOS technology.
In the actual situation of the project, devices with CoSi2, PtSi and Si:B superconducting contacts have been fabricated using the 300 mm clean room facility at the LETI and in collaboration with our partners at Uppsala university and C2N Paris Saclay. The main issue is now to characterize the electronic transport properties at very low temperature.
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.
High-throughput experimentation applied to battery materials
High throughput screening, which has been used for many years in the pharmaceutical field, is emerging as an effective method for accelerating materials discovery and as a new tool for elucidating composition-structure-functional property relationships. It is based on the rapid combinatorial synthesis of a large number of samples of different compositions, combined with rapid and automated physico-chemical characterisation using a variety of techniques. It is usefully complemented by appropriate data processing.
Such a methodology, adapted to lithium battery materials, has recently been developed at CEA Tech. It is based, on the one hand, on the combinatorial synthesis of materials synthesised in the form of thin films by magnetron cathode co-sputtering and, on the other hand, on the mapping of the thickness (profilometry), elemental composition (EDS, LIBS), structure (µ-DRX, Raman) and electr(ochim)ical properties of libraries of materials (~100) deposited on a wafer. In the first phase, the main tools were established through the study of Li(Si,P)ON amorphous solid electrolytes for solid state batteries.
The aim of this thesis is to further develop the method so as to enable the study of new classes of battery materials: crystalline electrolytes or glass-ceramics for Li or Na, oxide, sulphides or metal alloys electrode materials. In particular, this will involve taking advantage of our new equipment for mapping physical-chemical properties (X-ray µ-diffraction, Laser-Induced Breakdown Spectroscopy) and establishing a methodology for manufacturing and characterising libraries of thin-film all-solid-state batteries. This tool will be used to establish correlations between process parameters, composition, structure, and electrochemical properties of systems of interest. Part of this work may also involve data processing and programming the characterisation tools.
This work will be carried out in collaboration with researchers from the ICMCB and the CENBG