Study of fuel assembly creep in fluid-structure interaction

In the context of the energy transition and the low-carbon mix, controlling the performance and safety of the nuclear reactors in the fleet is an imperative that still opens up avenues for research and development with high added value. This is particularly true for the optimization of fuel elements.
Indeed, during its stay in the core of a power reactor, the fuel assembly is subjected to mechanical, thermal and hydraulic constraints. It undergoes a change in its geometry, in particular an elongation and a lateral deformation, due to the creep phenomenon linked jointly to the irradiation and the flow of water in the core. With the increase in the residence times of fuel assemblies in reactors and due to increasingly demanding conditions, the need to understand the phenomenon is necessary to improve the robustness of the design. This is in particular a problem of fluid-structure interaction where the flow plays a role in the creep behavior of the structure and where the deformation of the structure modifies the flow.
A previous study made it possible to implement an experimental device to obtain rapid creep on reduced-scale fuel assembly models. These tests were able to highlight a significant effect of the fluid inlet conditions on the creep behavior of the assemblies. The objective of the proposed thesis work is then to analyze the experimental results using simulation tools in order to understand and quantify the phenomenology of the coupling in fluid-structure interaction under creep. This analysis could lead to the realization of additional tests. Another important aspect will be the transposability of the results to real conditions. The thesis will be carried out at the IRESNE institute of the Cadarache center, in collaboration with the industrialist Framatome, bringing its operational vision in the monitoring and orientation of the research work. The proposed work therefore opens up solid perspectives at the end of the thesis both in research centers and in industrial environments.

Development of a lensless microfluidic instrument for in-situ measurement of facies-dependent dissolution kinetics

This thesis is part of an ambitious program designated as a priority research program. This project identifies the subsoil as a major reservoir of resources necessary for the energy transition.
One of the major issues is the dissolution of ores in the context of mining and extractive metallurgy. In particular, with the objective of process industrialization, the dissolution kinetics of ores must be compatible with the footprint of the installations, biocompatibility and the volume of reagents consumed.
The observation today is the very strong mismatch between the volume of experimental data produced and those necessary to model the chemical processes essential to demonstrate the viability of industrial processes.
This thesis proposes to develop a millifluidic prototype bench for mass kinetic data acquisition using lensless imaging techniques. This will make it possible to measure dissolution reaction kinetics using 3D reconstitution techniques, in-situ, under stable chemical conditions and with statistical representativeness allowing the original properties of the solid to be taken into account.
A large part of the research will be directed towards the development of the lensless optical technique in a millifluidic device and the mass production of chemical kinetic data for catalytic dissolution models.
The desired profile is that of a general physics and chemistry student, with a strong desire to learn in areas they are least familiar with, such as microfluidics or optics. At the end of this thesis, the student will acquire solid professional experience in applied research and will learn to evolve in a multithematic environment.

Development of very low carbon content martensitic stainless steels reinforced by a nano-oxides dispersion

This thesis aims to optimize the performance of future nuclear steels. Martensitic steels are particularly studied for the components of sodium-cooled fast reactor cores, as they exhibit lower swelling under irradiation compared to austenitic grades. To improve their creep properties, these steels are sometimes reinforced with a fine dispersion of stable nanometric oxides (Oxides Dispersed Strengthened). However, conventional martensitic ODS steels, with chromium content limited to 9-11 %Cr, often suffer from low toughness at room temperature.
Recent research indicates that the toughness of ODS steels could be significantly enhanced with very low carbon content. This thesis proposes an original approach that combines the exceptional toughness and corrosion resistance of Maraging steels with an ODS-type precipitation. Indeed, the Maraging stainless steels are rich in chromium (10-15 %) and nickel (4-9 %), with carbon content below 0.02 % by weight. After austenitization and quenching, these steels exhibit a martensitic structure, providing an outstanding balance between yield strength and toughness.
To evaluate the performance of these disruptive grades, compositions of interest will be selected, developed, and characterized at CEA with collaboration of academic partner teams.

JOB PROFIL: The applicant must be master-2 graduated with training in materials science and ideally metallurgy. The proposed subject is mainly experimental. A basic knowledge of electronic microscopy and/or XRay diffraction is required for this position. At the end of the PhD the applicant will be highly skilled in steel metallurgy and will have operated a large number of microstructural characterizations advanced tools (SEM, TEM, SAXS XRD, DSC). Naturally, he/she can pretend to a metallurgy researcher position in a large range of industries.

Hydrogen transport and trapping in austenitic alloys coupling experiments and simulations.

Molecular hydrogen H2 is an alternative energy carrier to traditional fossil fuels, gas or oil. It meet the current energy and environmental challenges, i.e. the need to store greenhouse gases free energy produced by intermittent means such as wind turbines or solar panel. Nevertheless, its safe storage and transportation is one of the keys to its use. The containers or pipes that carry the hydrogen must be leaktight and maintain their integrity over time, for both economical and safety reasons. Understanding and predicting the behavior of hydrogen in container/pipeline alloys and the associated mechanical degradation – such as embrittlement – is therefore crucial for the development of the hydrogen industry. These issues are also generic to all alloys exposed to a source of hydrogen, in corrosion or in the metallurgical industries where the hydrogen simply comes from contact with water, or in the oil&gas industry where hydrogen comes from hydrogen sulphides present in hydrocarbons.

If many experimental works have identified hydrogen embrittlement as the origin of the degradation of alloys exposed to hydrogen, large gray areas still remain on the mechanisms at work due to experimental difficulties and the great variability of the observed phenomena. In addition, the transport and trapping of hydrogen prior to mechanical degradation are poorly known and poorly documented at the nanoscale.

The objective of the thesis is to explore the mechanisms of hydrogen trapping / transport in austenitic materials, as well as its distribution in volume, prior to cracking in order to be able to report and explain the experimental observations.
To achieve this objective, the thesis work will be dedicated to the study of pure nickel, a model system for the austenite phase. The study will be carried out in three stages: (i) thermodesorption measurements and (ii) atomic scale simulations using molecular dynamics, both feeding chemical kinetics modeling coupled with Fick's law at the mesoscopic scale.

Smart materials for low-carbon applications

The topic of this thesis focuses on the design of smart materials for low-carbon applications, with an emphasis on metallic additive manufacturing. This technology has revolutionized industrial production methods by enabling the creation of complex, lightweight parts while ensuring increased precision and flexibility. This is particularly relevant in demanding sectors such as aerospace, automotive, and nuclear industries, where reliability is crucial. By integrating optical sensors into metallic structures through additive manufacturing processes, it becomes possible to perform real-time monitoring of critical parameters such as stress, temperature, and radiation doses. This enhances the safety and efficiency of operational and maintenance activities. The thesis aims to address the challenges related to the monitoring and control of infrastructure conditions, ensuring continuous monitoring of structures and precise control of environmental parameters. Additionally, the study examines the durability of materials and how embedded sensors can function in hostile environments. Finally, this research aspires to develop solutions for effective and secure remediation and decommissioning processes.

Experimental and numerical analysis of fluid-structure interactions in the propagation of rarefaction waves through complex structures in pressurized water reactors

Loss of coolant accident (LOCA) in pressurized water reactors (PWR) leads to fast transient phenomena, such as the propagation of rarefaction waves within the reactor's internal structures. These waves generate transient pressure loads between different areas, such as the reactor core and the bypass zone, which places stress on the baffle. The deformation of this critical structure can compromise the structural integrity of the reactor and complicate the handling of fuel assemblies, particularly their removal after the accident.

The main scientific objective is to develop, implement, and validate new numerical models that allow for a more accurate simulation of rarefaction wave propagation through complex obstacles. The current state of the art relies on simplified models, validated only for simple configurations such as single-orifice plates. However, there is a need to extend these models to more complex geometries, such as plates with multiple holes, using different numerical methods.
The development of a porosity model to represent fuel assemblies is also crucial. The expected results will be validated experimentally and have direct applications for industrial partners EDF and Framatome, enhancing the industrial relevance of this research.

The thesis will adopt a combined approach, both experimental and numerical. The use of the MADMAX platform will allow for the testing of various complex obstacles and the collection of detailed experimental data using specialized sensors. This data will be used to validate the numerical models developed in the EUROPLEXUS software. Additionally, the simulations will include innovative approaches such as a new porosity model for the internal structures of the reactors. Participation in international conferences and publication of results are planned to ensure the scientific dissemination of the findings.

The thesis will be conducted at the DYN laboratory of CEA Paris-Saclay, equipped with unique experimental facilities, such as the MADMAX platform, and has strong expertise in numerical modeling. Several industrial (EDF, Framatome) and academic collaborations will provide a rich environment for the doctoral candidate, with regular exchanges within international networks.

The ideal candidate should possess solid skills in fluid mechanics, structural dynamics, numerical modeling (finite element, finite volume), and programming. Previous experience with tools like EUROPLEXUS will be a plus. An M2 internship may be offered to familiarize the candidate with the methods and tools used in this thesis.

This thesis will enable the doctoral candidate to acquire highly specialized skills in fluid-structure interactions, numerical modeling, and experimentation in an industrial context. These skills are in high demand in the energy, aerospace, and advanced simulation technology sectors, paving the way for careers in applied research or engineering within the industry.

Multiphysics modeling of fission gas behavior and microstructure evolution of nuclear fuels

The climate crisis demands urgent action and a rapid shift towards carbon-free technologies. This requires the development of advanced materials for more efficient electricity production and storage, including innovation in nuclear reactor fuels. To enhance the safety and efficiency of both current and future nuclear power plants, it is crucial to understand and predict fuel behavior under operating and accidental conditions.

A critical issue is related to fission gases produced upon nuclear fissions. These gases have low solubility and form small bubbles that grow from nanoscale to microscale during fuel operation, significantly impacting the fuel's overall properties. While experimental characterization is essential, numerical simulations complement this work by modeling bubble formation and growth, as well as the consequences in terms of changes in fuel properties. This approach is key to the design of next-generation, high-performance nuclear fuels.

This PhD project aims to advance simulation models for fission gas behavior within the polycrystalline structure of nuclear fuels, with a particular focus on uranium oxides. The PhD student will develop a physical model using the phase-field method, compute necessary input parameters, and conduct numerical simulations that replicate irradiation experiments performed in our department. Direct comparison between simulation results and experimental data will enable deeper insights into the underlying physics of gas behavior, including bubble formation, gas release, and fuel swelling. Additionally, this project will serve as validation for the INFERNO scientific code that will be used for these simulations on national supercomputers.

The research will be conducted at the Nuclear Fuel Department (DEC) of the IRESNE Institute(CEA-Cadarache), in collaboration with CEA fuel modeling and experimental characterization experts. The PhD student will have opportunities to share their findings through scientific publications and presentations at international conferences. Throughout the project, they will develop expertise in multiphysics modeling, numerical simulations, and scientific computing. These highly transferable skills will prepare them for a successful career in academic research, industrial R&D, or materials engineering.

References :
https://doi.org/10.1063/5.0105072
https://doi.org/10.1016/j.commatsci.2019.01.019

Mass transfers and hydrodynamic coupling: experimental investigation and models validation and calibration

During a core meltdown accident, a pool of molten material, known as corium, can form at the bottom of the reactor vessel. The composition of the pool can change over time. The corium bath is not homogeneous and can stratify into several immiscible phases. As the overall composition of the corium changes, so do the properties of the different phases. The vertical stratification order of the phases may change, leading to a vertical rearrangement of the phases. During this rearrangement, one phase passes through the other in the form of drops. The order of the phases and their movements are of prime importance, as they have a major influence on the heat flows transmitted to the tank. A better understanding of these phenomena will enable us to improve the safety and design of both current and future reactors.

Initial models have already been produced, but they lack validation and calibration. Prototype experiments are difficult to set up and none are planned in the short term. This thesis proposes to fill this gap by carrying out an experimental study of the phenomenon using a water-based simulating system that allows local instrumentation and large-scale test campaigns. The aim is to validate and calibrate the existing models, and even develop new ones, with a view to capitalising on these results in the PROCOR software platform, which is used to estimate the probability of a reactor vessel breach. The experimental set-up would be built and operated at the LEMTA laboratory at the University of Lorraine, where the PhD student would be seconded. In terms of experiments, two cases will be studied, the single drop case, and the stratified case with drop formation via Rayleigh-Taylor instabilities.

The work will be mainly experimental, with a component involving the use of code for calibration and validation, and may include a modelling component. It will be carried out entirely at the LEMTA laboratory in Nancy. The PhD student will benefit from LEMTA's expertise in the development of simulating experimental devices, fluid transfers and metrology. They will be part of a dynamic environment made up of researchers and other PhD students. The candidate should have knowledge of transfer phenomena (mass transfer in particular), as well as a definite interest in experimental science.

Local understanding of the corium-concrete interface through experimentation

In the context of the energy transition and the crucial role of nuclear power in a low-carbon energy mix, understanding and then mitigating the consequences of any accident leading to even partial core meltdown is an imperative research direction.
In the event of a severe core meltdown accident, the amalgam of materials produced by core meltdown, or corium, may interact with the concrete of the plant's floor. The lack of understanding of local and interfacial physical phenomena during corium-concrete interaction (ICB) has led to the development of various international simulation tools. None of them has been able to explain the recent observations at the Fukushima Daiichi accident site. It is therefore crucial to improve the ICB simulation tools.
The aim of this thesis is to carry out a detailed, local experimental study of the corium/concrete interface with prototypical corium (depleted uranium). To this end, the candidate will design a test device to be introduced into the VITI inductive furnace of the PLINIUS platform dedicated to the study of severe accidents at the Cadarache center. After qualification of the experimental set-up, local corium/concrete interaction tests in VITI will be carried out on different types of concrete (including a sample from Fukushima) and with different coriums, enabling an incremental approach using separate effects. Ablation will be characterized via mass loss and hydrogen release. The interface will also be characterized after rapid corium removal. Samples will also be X-rayed (e.g. tomography). As the work progresses and the phenomenology of the Molten Corium Concrete Interaction is understood, a model may be developed and integrated into a simulation tool.
The thesis work will be carried out jointly in the experimental and severe accident modeling laboratories of the IRESNE institute at Cadarache, in a research environment of the highest international standard for the study of multiphysical phenomena at very high temperatures. This work will also be enriched by research carried out within the framework of the ANR IMMOC, in partnership with academics (CNRS Laboratoire Navier, AMU-CNRS Madirel...).

Experimental characterization and numerical simulation of brittle fracture of intergranular oxides : Application to Irradiation-Assisted Stress Corrosion Cracking

Metal alloys used in industrial applications can form oxide layers in the presence of a corrosive environment. These oxides may be uniformly distributed on the surface and/or localized at the grain boundaries. In the latter case, the oxidized grain boundaries may experience brittle fracture under mechanical loading, potentially leading to intergranular cracking of the material. This mechanism is, for example, a possible scenario for the failure of austenitic stainless steel bolts used in the internals structure of Pressurized Water Reactors (PWRs). Under the effect of mechanical loading,
neutron irradiation and the presence of a corrosive environment, these bolts fail through a phenomenon known as irradiation-assisted stress corrosion cracking. To model this phenomenon, we need to determine the fracture properties of intergranular oxides, and to take into account the coupling between cracking, oxidation and irradiation. In this thesis, experimental and numerical work will be combined. Firstly numerical simulations based on the variational approach to fracture approach will be assessed in order to design micro-beam micromechanics experiments aimed at reliably determining the fracture properties of oxides, and also to study the couplings between cracking, oxidation and irradiation. In particular, the cracking-oxidation coupling that prefigures the transition between initiation and propagation will be investigated in detail. These experiments will then be carried out on model and industry-relevant steels, and interpreted using numerical simulations. Finally, all the results obtained in this work will be incorporated into simulations of polycrystalline aggregates, in order to assess the possibility of quantitatively predicting intergranular cracking in the context of irradiation-assisted stress corrosion.