Stabilization of secondary phases in nanoreinforced ferritic steels: High-throughput screening approach of chemical compositions
Ferritic steels reinforced by oxide dispersion strengthening (ODS) are considered for use in 4th Generation and fusion nuclear reactors due to their excellent thermomechanical properties and stability under irradiation. However, these steels are weakened by secondary phases resulting from complex interactions between alloying elements and interstitials (C, N, O) introduced during their processing. Some alloying elements (such as Nb, V, Zr, Hf) could potentially stabilize these undesirable phases and mitigate their detrimental effects on the mechanical behavior of ODS steels. This thesis aims to develop a high-throughput screening method to identify optimal alloy compositions by combining rapid fabrication and characterization techniques. The PhD student will synthesize various compositions of ODS steels through powder metallurgy and carry out chemical, microstructural, and mechanical characterizations. This work will enhance the understanding of interstitial stabilization mechanisms and propose effective methodologies for characterizing new materials. The PhD student will gain in-depth knowledge in metallurgy and data processing, providing opportunities in industry, nuclear start-ups, and research.
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.
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.
Deciphering the roles of surface chemistry and multi-scale structuration in controlling the storage performances of graphene-based supercapacitors
Summary of the project: The project’s objective is to advance fundamental research by elucidating the intrinsic relationship between the properties of graphene-based material and their electrochemical storage performances in supercapacitor cells, thanks to the combination of basic and advanced characterization techniques, particularly adapted to the investigation of the evolutions of the surface chemistry and multi-scale structure upon cycling. These findings will enable to provide a multi-scale understanding of storage mechanism and will help to further design materials with enhanced storage properties.
Study of the corrosion behaviour in NaCl-MgCl2-CeCl3 of a nickel-based alloy in the presence of fission products (Te, S) for molten salt reactor
Access to clean and affordable energy seems more crucial than ever in the current context of climate emergency. Several avenues have been explored for years, but many technological barriers remain to be overcome in order to realise them, as they represent significant technological breakthroughs. Whether it's for energy storage or 4th generation nuclear reactors, the molten salt medium used as a heat transfer fluid and/or fuel is highly corrosive, making the choice of structural materials very complex.
The objective of the proposed PhD project within the Service of Corrosion and Material Behaviour (S2CM) is the comprehensive study of the behaviour of promising nickel-based alloys in the NaCl-MgCl2-CeCl3 ternary system, representative of the salt used in the French molten salt reactor concept, at 600°C. By "comprehensive", this refers to everything from specimen preparation to the multi-scale and multi-technique characterisation of corrosion products. This topic has therefore a strong experimental character and focuses on understanding corrosion mechanisms. The influence of fission products, such as tellurium or sulphur, on corrosion mechanisms will be specifically studied.
Towards an understanding of the expansive behavior of certain cement-based evaporator concentrates: experimental approach and simplified chemistry-transport-mechanics coupled modeling
In the nuclear industry, evaporation is a commonly used process to reduce the volume of low- or intermediate-level radioactive waste before its conditioning. This results in evaporator concentrates, high-salinity solutions that can contain a wide range of ionic species. These concentrates are then stabilized and solidified in a cement-based matrix, a material with many intrinsic qualities (low cost, availability, ease of implementation, good mechanical resistance, stability under irradiation, etc.). However, the acceptance of cemented waste packages in a repository depends on meeting a number of specifications. For instance, it is necessary to demonstrate the absence of expansion that could damage the matrix when stored in a humid environment.
The thesis will aim to understand the mechanisms governing the volumetric changes of cement matrices when stored underwater. The study will be conducted on synthetic waste, simulated by dissolving salts in water at the desired concentrations. It will begin with an experimental phase that will provide the input data for the building of a simplified physico-chemical model of the cement wasteforms to estimate their macroscopic mechanical behaviour as well as the main leached fluxes.
This research project is aimed at a PhD candidate wishing to develop skills in materials science and open new perspectives for the conditioning of radioactive waste. It will be carried out in collaboration with ONDRAF, the Belgian National Agency for Radioactive Waste Management, and will rely on the expertise of two CEA laboratories: the Laboratory of Formulation and Characterization of Mineral Materials (CEA Marcoule) and the Laboratory for the Study of the Behaviour of Concrete and Clays (CEA Saclay).