Understanding of corrosion mechanisms and means of mitigating corrosion in a NaCl-ThCl4-UCl3 salt. Application to future molten salt fuel and coolant reactors

The molten salt reactor concept is based on dissolving the fuel in a molten salt. This liquid fuel concept is highly innovative and in many respects represents a break with current reactors, which are all based on the use of a solid fuel and a fluid coolant. Recently, the emergence of American start-ups proposing this innovative concept and the major effort made in China have revived interest worldwide in studying this technology, which offers a number of advantages, both real and potential, over the use of solid fuel, particularly in terms of incineration and intrinsic safety. To build a feasibility demonstrator for this breakthrough concept, extensive research is needed to acquire data and justify the behaviour of the containment barriers, primarily the metal barrier in contact with the salt. In the case of molten salt reactors, the structural materials, nickel-based alloys, are chosen to optimise their behaviour in terms of corrosion and high temperature. Corrosion of the materials is one of the critical points to be overcome when building this reactor. A detailed understanding of the corrosion mechanisms of the alloy chosen as the structural material, on the one hand, and of the chemistry of the ternary salt NaCl-ThCl4-UCl3 envisaged, on the other hand, are necessary to predict the material corrosion rate over the lifetime of the demonstrator. These studies will enable several corrosion mitigation methods to be developed. Each of these processes will be tested and evaluated under nominal conditions and then aggravated.
The first part will be devoted to understanding the corrosion mechanisms of the alloy and the chemistry of the NaCl-ThCl4-UCl3 salt. To this end, tests will be carried out at the IPN in Orsay and the corrosion mechanisms and chemistry studies will be established using electrochemical techniques and microstructural characterisation of corroded samples (thermogravimetry, SEM, TEM, XPS, Raman, GD-OES, etc.). Secondly, material protection tests using different types of salt redox control will be carried out and then tested in nominal and aggravated environments.
This approach will make it possible to meet a major and ambitious corrosion control challenge for an innovative energy process.

Study of the behaviour of volatile fission products in nuclear fuels under temperature transients

The quantity of radioactive fission products, which can be released out of irradiated nuclear fuel in accidental conditions, is a key data for the design of safety components of a Nuclear power plant. This release was largely studied in the case of severe accidents but less is known for temperature below 1400°C. The aim of the Ph.D. Work will consist in using new in-situ characterization tools that are now available on MERARG furnace at the IRESNE institute in CEA-Cadarache in order to improve the characterization of volatiles PFs in this temperature range. The Ph.D. Student will have to design and run thermal treatment on irradiated nuclear fuel making use of in and ex-situ gamma spectrometry, in-situ optical sighting device and measurement of the PO2 of the furnace atmosphere. This work will be performed with the support of experimental team at IRESNE. The Ph.D. Student will also participate to the interpretation of the results in close collaboration with experimental and modeling teams at IRESNE. In order to test the proposed mechanism, the Ph.D. Student can also perform separate-effects experiments on simulating materials in collaboration with team at university. This work will be valued both in industrial context with technical notes and in academic context with paper in journals and attending scientific conferences.

3D chemical investigation of 10 nm FDSOI CMOS devices.

The development of 10 nm FDSOI (Fully Depleted Silicon On Insulator) technology leads to new constraints on the architecture of transistors. The gate width (10 nm) requires a specific integration of the gate that controls the threshold voltage. The variability of the threshold voltage depends on the concentration, spatial distribution and chemical nature of dopants in the source and drain area. Therefore, it is crucial to understand the impact of growth conditions of the metallic gate, source, drain and annealing temperature to activate the dopant. To master these new constraints, the use of characterization techniques that can identify the structural and chemical mechanisms (distribution and quantification) acting in the gate, source and drain will be essential. Among all the chemical characterization techniques, Atom Probe Tomography is the technique of choice that offers a 3D chemical and quantitative mapping of a sample with nanometre scale resolution.
The objectives of this PHD will be to: (i) develop 3D characterization methodologies (distribution and chemical composition of species within the gate and source-drain area) of transistors, (ii): investigate the impact of growth conditions, annealing temperature activating the dopants and implantation dose. The PHD student will try to model the formation mechanisms of the observed chemical compounds.

High energy density Li-Ion and Na-Ion Positive Electrodes with reduced critical material content

This PhD subject will aim at developing new positive electrode materials based on glasses for high Energy Density Li-Ion and Na-Ion cells with reduced critical material content. These developments will be held jointly between the laboratory of materials for batteries from CEA-Grenoble and LDMC lab from CEA-Marcoule that is specialized in the formulation and characterization of glass materials.
The work will be focused on the optimization of the complex formulation of the glass cathodes to solve the issues related to first cycle irreversible loss and low cycling performances. The main objective will be to obtain one composition without critical raw materials exhibiting more than 1000Wh/kg at active material level vs 700 for state-of-the-art materials. This target will be reached with the support of advanced characterization techniques such as X-Ray Diffraction and RAMAN and FTIR spectroscopies. A dedicated effort will concern the development of operando or in-situ measures to be able to explain the link between electrochemical performances and glass characteristics, what has never been reported in the litterature.
This thesis will allow the candidate to gain valuable professional experience in the glass and energy sectors. He or she will develop skills in materials science and electrochemistry. In addition, thanks to his work environment, he will be able to assimilate a culture of nuclear waste conditioning.

4D printing of thermo-magnetic composite materials using light-driven additive manufacturing techniques

This PhD research project explores the cutting-edge field of 4D printing, a field that integrates smart materials into additivemanufacturing processes. The aim is to create nanocomposite objects with multifunctional capabilities, enabling them to change shapeand properties in response to external stimuli.

In this PhD project, we will primarily focus on liquid crystal elastomers (LCEs) as the active matrix. LCEs are a versatile class ofprogrammable polymer materials that can undergo reversible deformation under various stimuli, such as light, heat, electric fields, andmagnetic fields, transitioning from disordered to oriented phases. Because of their actuation properties, LCEs are promising candidatesin applications like artificial muscles in medicine and soft robotics.

Consequently, the project's first objective is to devise a method for 3D printing LCE resins using light-driven printing processes, includingdigital light processing (DLP), direct ink writing (DIW), and two-photon polymerization. The project also explores the possibility of co-printing using two laser sources with different wavelengths. This will result in designed objects capable of programmed deformationsand reversibility. To further enhance the actuation capabilities of the LCE matrices, magnetic particles will be incorporated into thethermoresponsive LCE resin. Thus, the second objective of the project is to develop a strategy for self-assembling and spatiallyorienting embedded magnetic nanoparticles in LCE resins during light-driven printing processes (DLP, DIW, 2PP). Ultimately, the thirdobjective of this project is to combine these two strategies to create sophisticated multifunctional soft machines and devices suitable forcomplex environments. Experiments will follow an incremental trial-and-error research approach, with the aim of improving machinelearning models by designing purpose-built objects.

The envisioned research work can be summarized into the following macro-steps:
- Specification of target shape-changes depending on the multiple stimulation scenarios
- Selection of active particles, formulation of the LCE, and synthesis of the particles
- Development of hybrid additive manufacturing strategies with possible instrumentation
- Printing proofs-of concept and conducting mechanical and actuation tests
- Characterization of composite structures
- Development of simulation models
- Realization of a demonstrator (e.g., crawling robot, actuators for the automotive sector…)

Study of the production of martensitic stainless steel 13-4 by Laser Metal Deposition: influence of process parameters, powder characteristics and post-treatments on microstructure and mechanical properties at fracture

Additive manufacturing processes are now widely studied for numerous applications in the nuclear industry. The aim of the studies dedicated to optimising the Laser Metal Deposition (LMD) metal additive manufacturing process for the production and shaping of a 13-4 martensitic stainless steel is to obtain a material with mechanical properties at fracture, particularly in terms of impact strength, that comply with the specifications for use. This work explores the complex relationships between the microstructural characteristics (phase present, granular structure, texture, precipitation, etc.) induced by the process and the resulting mechanical performance.
Additive manufacturing, in particular the LMD process, offers multiple advantages in terms of design flexibility and customisation of metal components. However, obtaining mechanical properties at fracture that meet specifications is a major challenge, particularly for high-temperature applications in corrosive environments.
This thesis focuses on the optimisation of the LMD process to ensure that components manufactured from 13-4 martensitic stainless steel exhibit microstructural characteristics and mechanical performance appropriate to their intended applications, with particular emphasis on impact properties. Determining the optimum process parameters, including the characteristics of the powders and associated post-treatments, the analysis of the microstructure, and the correlation between the microstructure and the mechanical properties constitute a major challenge for the complete control of this process.

Role of surface properties of UO2 powder particles on their agglomeration suitability and rheological behaviour

This study aims to predict the powder flow behavior in the context of nuclear fuel fabrication. This issue is common to many industrial fields because poor powder flow can lead to process problems such as pipe clogging, reduced rates, or the presence of heterogeneities in the final product. The first objective of this PhD thesis is, on the one hand, to provide a more accurate description of the powder agglomerates and, on the other hand, to characterize their surface. Based on these surface and structural data of UO2 powder particles, the second objective of this work is to achieve a better understanding of the agglomeration/desagglomeration properties in order to correlate them with the flow properties.
The future PhD student will need to use and develop experimental methods (particle characterization tools, surface characterization analyzers, phenomenological modeling) at the IRESNE institute (CEA-Cadarache) in the fuel study department (DEC), specifically within a team dedicated to experiments on nuclear fuel.
This study, applied to UO2 powders, has a generic nature because it is suitable for the study of all granular media. At the end of the PhD, the doctoral candidate will communicate the results through publications and conference presentations. An expertise in granular media will be acquired, which is an attractive and valuable skill in many industrial fields such as agri-food, pharmaceutical industries, metallurgy, or building materials.

Impact of irradiation parameters on the alpha’ phase formation in oxide dispersion strengthened steels

Ferritic-martensitic oxide dispersion strengthened steels (ODS steels) are materials of great interest in the nuclear industry. Predominantly composed of iron and chromium, these materials can become brittle due to the precipitation of a chromium-rich phase, called a', under irradiation. This phase, known to be sensitive to irradiation conditions, provides an ideal topic for a deeper exploration of the capability to emulate neutron irradiation with ions. Indeed, while ion irradiations are frequently used to understand phenomena observed during neutron irradiations, the question of their representativeness is often raised.

In this thesis, we aim to understand how the irradiation parameters can affect the characteristics of the a' phase in ODS steels. To do so, various ODS steels will be irradiated under different conditions (flux, dose, temperature, and type of particles, such as ions, neutrons, electrons), and subsequently analyzed at the nanoscale. The a' phase (size, chromium content) obtained for each ion irradiation condition will be compared to the one after neutron irradiation.

Resistance of an austenitic stainless steel obtained by hot isostatic pressing to stress corrosion cracking in nuclear pressurized water reactor

Due to their good mechanical properties and good corrosion resistance, AISI type 316L austenitic stainless steels (ASS) are used in a wide variety of industrial fields. Several components of the primary circuit of pressurized water nuclear reactors are made of 316L ASS. However, the susceptibility of these steels to stress corrosion cracking in this environment is an important issue. Innovative manufacturing processes to obtain better properties, more complex geometries or to reduce supply times and costs are currently in development. Hot isostatic pressing of metal powders is one of them. The objective of the thesis is precisely to evaluate the resistance to stress corrosion cracking of an ASS obtained by this innovative process and to establish the relations between its microstructure and its properties.
The process requires enclosing the powder in a container before compaction treatment and heat treatment. The surface is then machined to eliminate the container and the affected material. Given the local strain resulting from this machining and the primary importance of the pre-strain level on the resistance of 316L ASS obtained by conventional means to stress corrosion cracking, particular attention will be paid to the effect of this parameter on the resistance to stress corrosion of the studied material.
This thesis constitutes for the candidate the opportunity to address a problem of durability of metallic materials in their environment following a multidisciplinary scientific approach combining metallurgy, mechanics and physico-chemistry and based on the use of various cutting-edge techniques available at the CEA and at the Ecole des Mines. The skills that he will thus acquire can therefore be valued during the rest of his career in the industry (including non-nuclear) or in academic institutions.

Structural evolution under electron irradiation of lamellar hydroxydes and hydrates

The societal context of the study is the optimization of cementitious matrices for nuclear waste conditioning. These cementitious matrices are composed of hydrated minerals, some of which are lamellar (portlandite Ca(OH)2, brucite Mg(OH)2, brushite CaHPO4.2H2O, gibbsite Al(OH)3...). Very few data exist in the literature on the structural damage of these hydrated lamellar minerals under electron irradiation. The aim of the proposed thesis is to experimentally investigate irradiation-induced structural modifications in various types of compounds, with a view to gaining a better understanding of the damage mechanisms of these compounds under irradiation, and to identify irradiation sensitivity criteria in order to ultimately optimize the chemical and mineralogical composition of the materials.

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