Influence of chromium doping of UO2 fuel on fission product speciation under accidental conditions
The development of nuclear reactors is part of a drive to improve safety, with, for example, the deployment of nuclear fuels with improved properties in terms of their behavior under accident conditions, the so-called E-ATF (Enhanced Accident Tolerant Fuel). Industrial operator FRAMATOME is developing the Cr2O3-doped UO2 fuel as E-ATF. However, very little data is available on the behavior of fission products from Cr-doped fuel under accident conditions.
This thesis proposes to develop a synthesis process for Cr-doped UO2 fuel that simulates irradiated fuel, in order to study the behavior of the elements (Cr and fission products) at different temperatures and under different oxygen partial pressures. The methodology is based on an experimental approach combining synthesis of model materials and in-depth chemical characterization, complemented by a theoretical approach (thermodynamic calculations) enabling thermal sequences to be dimensioned and the proposed reaction mechanisms to be confirmed.
The thesis will be carried out at CEA Cadarache (France), within IRESNE (Research Institute on Nuclear Systems for Low-Carbon Energy Production). The PhD student will be hosted in a laboratory dedicated to the study of uranium-based compounds in the Department of Fuel Studies (DEC). Depending on the densification processes chosen, experiments of various duration may be carried out in other laboratories in France or Europe.
The PhD student will have the opportunity to learn advanced characterization techniques for ceramic materials, gain access to experiments on large-scale instruments (synchrotron) and take part in exchanges with the academic community(CNRS, Universities, JRC). He or she will be able to promote his work through publications and participation in conferences.
At the end of this thesis, the PhD student will have acquired skills in materials science and solid state characterization that he/she will be able to use in various materials fields, as well as experience in the nuclear environment of interest to the nuclear industry.
Phenomenological study of the coupled effects of iodine and oxygen on Iodine induced Stress-Corrosion-Cracking (I-SCC) of zirconium alloys
The Pressurized Water Reactor (PWR) core is composed of fuel assemblies, for which the fuel cladding is the first barrier for the confinement of the fuel and the fission products. Pellet Cladding Interaction (PCI) occurs during increases in the reactor power and results in the expansion of the pellets that produces a thermomechanical loading on the fuel cladding. In conjunction with iodine expelled from the pellet, it can, in theory, lead to the failure of the cladding by Iodine Stress Corrosion Cracking (I-SCC).
The objective of this PhD is to study the phenomenology of I-SCC, in mechanical and chemical conditions as close as possible to the conditions seen by the PWR cladding in PCI (in terms of the oxygen and iodine partial pressures).
The PhD has three main parts. The first part will focus on the study of the effect of the stress on the I-SCC susceptibility of zirconium, at different partial pressures of iodine and oxygen. The tests will be simulated and analyzed using numerical models of the I-SCC process. The second part will focus on the effect of the temperature on I-SCC as a function of the stress, and the partial pressures of iodine and oxygen. The third part will focus on the effect of a thick zirconia layer, at the inner wall of the cladding, on the I-SCC susceptibility of the cladding.
Development of a digital twin of industrial equipment: coupling chemistry / thermo-hydraulics / corrosion
This PhD subject is part of CEA R&D aimed at developing and improving decarbonized technologies for energy production, in response to climate issues. More specifically, it is part of the spent fuel reprocessing stage used in current nuclear reactors. The simulation of the operation and aging of this equipment is a major challenge for the sustainability of the activities of fuel reprocessing plants.
The objective of the thesis is to respond to these challenges, by developing a modeling of the corrosion of one or more equipments in the plants based on their operation. This will require coupling chemical reaction models (in solution and corrosion) with thermo-hydraulic models. These developments will be carried out using modeling tools developed by the CEA.
By making it possible to simulate the corrosion of equipment, the development of such a model will make it possible to optimize its lifespan (by seeking to optimize its operation, for example) or to accurately estimate (and therefore anticipate) the time needed for its replacement.
Brittle fracture of low alloy steels: sensitivity of mesosegregation regions to quenching and tempering conditions
The pressure vessels of the primary circuit of French nuclear power plants are made by assembling low-alloy steel components, forged from high-tonnage ingots (> 100t) that solidify in a non-uniform manner. The high thickness of the component also implies that the evolution of temperature during post-forging heat treatments vary significantly depending on the position in the thickness of the component. These two effects contribute to producing heterogeneous microstructures that can significantly weaken the material.
The scientific objective of this thesis is to evaluate which elements within the microstructure are responsible, and in what proportion, for increased embrittlement of the material for certain unfavorable heat treatment conditions. Conversely, better identifying the range of heat treatment conditions for which this embrittlement of the material remains contained, for a given initial microstructure, is an objective with high industrial stakes. Several heat treatments have already been applied to coupons from a rejected industrial component before subjecting them to Charpy impact toughness tests, in the field of the brittle to ductile transition of the material. Instrumented mechanical tests will be conducted as well as advanced fractographic and microstructural analyses in order to identify the evolution of the nature of the initiation sites according to the heat treatment conditions. These elements will then be integrated into a local approach to fracture model developed specifically to account for the effects of microstructural variations on the resistance to brittle fracture of low-alloy steels.
Flotation for Li-ion active materials recycling : limitations and influence of hydrodynamics and interfacial physico-chemistry on their selective separation
Battery recycling is now a major geopolitical, economic and environmental issue for the EU. Graphite, which makes up the anode of Li-ion batteries, is very rarely recycled. It is concentrated in a fraction called blackmass, where it is mixed with metal oxides of high commercial value. This graphite is then considered as an impurity and causes oversizing of hydrometallurgical operations. Since natural graphite is considered critical by EU and in order to reduce the operating and investment costs of hydrometallurgical processes, it is proposed to carry out a pre-treatment step on the blackmass in order to valorize the graphite directly. This stage is carried out by flotation. This process for separating solids suspended in water uses gas in the form of air bubbles to separate the particles according to their difference in wettability and therefore their attachment to the air bubbles. The complexity of the flotation process, linked to the dependence on both the nature of the interfaces and the hydrodynamic conditions, requires in-depth understanding of the mechanisms involved.
The aim of the proposed project, which follows on from two internal projects, is to identify the mechanisms at work during flotation, using methods of interfaces characterization, stability and rheology of foam fraction, imaging, etc., with a view to improving the performance of the flotation stage and extending it to other recycling challenges.
The PhD thesis work will be carried out at the Laboratoire des technologies de Valorisation des procédés et des Matériaux pour les ENR (LVME) at CEA Grenoble and in close collaboration with the Laboratoire de Caractérisations Avancées pour l'Energie (LCAE) at CEA Grenoble, the Laboratoire des Procédés Supercritiques et décontamination (LPSD) and the Laboratoire de développement des procédés de recyclage et valorisation pour les systèmes énergétiques décarbonnés (LRVE) at CEA Marcoule (30). In parallel with the experimental work, the models and mechanisms involved and the associated technical solutions will have to be proposed.
The scientific and industrial interest of the subject guarantees that the work will be promoted through international communications. After the PhD, you can join one of the best academic or applied research teams, or pursue an R&D career directly in industry.
Numerical simulation of the impact between immersed structures in a compressible liquid using immersed boundary type approaches.
Many industrial systems involve structures immersed in dense fluids. Examples include the submarine industry, or, more specifically, certain 4th generation nuclear reactors using coolant fluids such as sodium or salt mixtures. The effect of the interaction of the surrounding fluid on the contact forces between structures is a phenomenon of primary importance, particularly during accidental transient scenarios that can generate large displacements of structures whose residual integrity must be demonstrated for safety purposes.
In the context of this thesis, we are particularly interested in modeling the rapid impact of a structural fragment immersed in a fluid against a wall, resulting, for example, from an explosive phenomenon in a nuclear reactor vessel cooled by sodium. In this context, the sodium, modeled as a compressible fluid, is treated numerically using a volume-finite approach. The reactor's internal structures are treated using a finite-element approach. In order to deal with large structural displacements and possible fracturing, “immersed boundary” techniques are used for fluid-structure interaction.
The aim of this thesis is to define an innovative numerical method to better simulate the fluid film between two structures that come into contact in this context. Initially, we will focus on identifying the physical characteristics of the flow at the level of the fluid film (compressibility, viscosity, etc.) that have the greatest influence on the kinematics of the structures. Secondly, the main challenge of this thesis will be to improve current numerical methods in order to represent the flow characteristics of the fluid film as accurately as possible.
The proposed thesis will be carried out at CEA Saclay, in close collaboration with the EM2C laboratory at CentraleSupélec, within the environment of the Université Paris-Saclay. The PhD student will be immersed in a team with recognized expertise in transient simulations of fluid-structure interaction.
Impact of solvent nanostructure on uranium precipitation: a physicochemical approach for nuclear recycling
Recycling nuclear fuel is a major challenge to ensure a sustainable energy future. The CEA, in partnership with Orano and EDF, has been developing a new process for separating plutonium-rich fuels for several years. The goal is to replace the current TBP/TPH system with a redox-free process, better suited for the reprocessing of MOX or fast neutron reactors (FNR).
In this context, this thesis proposes to study the behavior of organic solvents loaded with uranium to understand and prevent the formation of precipitates, a phenomenon that could impact the performance of industrial processes. The scientific approach will focus on the supramolecular scale and compare different monoamides to evaluate the effect of alkyl chains on the physicochemical properties and nanostructure of the solutions.
The candidate should hold a Master's degree (Master 2) in chemistry, physical chemistry, or materials science. Skills in analytical chemistry, spectroscopy (NMR, FTIR), and scattering techniques (SANS, SAXS) will be highly valued. By joining this project, you will become part of the CEA's cutting-edge laboratories (ICSM/LTSM and DMRC/SPTC/LILA), equipped with world-class facilities for studying radioactive samples. You will benefit from multidisciplinary supervision, including opportunities for international collaborations. This thesis represents a major scientific challenge with direct industrial applications, offering you valuable experience in the field of nuclear separation and processing technologies.
Monte Carlo methods for sensitivity to geometry parameters in reactor physics
The Monte Carlo method is considered to be the most accurate approach for simulating neutron transport in a reactor core, since it requires no or very few approximations and can easily handle complex geometric shapes (no discretisation is involved). A particular challenge for Monte Carlo simulation in reactor physics applications is to calculate the impact of a small model change: formally, this involves calculating the derivative of an observable with respect to a given parameter. In a Monte-Carlo code, the statistical uncertainty is considerably amplified when calculating a difference between similar values. Consequently, several Monte Carlo techniques have been developed to estimate perturbations directly. However, the question of calculating perturbations induced by a change in reactor geometry remains fundamentally an open problem. The aim of this thesis is to investigate the advantages and shortcomings of existing geometric perturbation methods and to propose new ways of calculating the derivatives of reactor parameters with respect to changes in its geometry. The challenge is twofold. Firstly, it will be necessary to design algorithms that can efficiently calculate the geometric perturbation itself. Secondly, the proposed approaches will have to be adapted to high-performance computing environments.
Influence of delayed neutron precursors losses resulting from fission gas evacuation on molten salt reactors dynamics
Over the past twenty years, molten salt reactors (MSRs) have been the focus of renewed interest in the international nuclear community (national programs, start-ups, including one from the CEA). Modern MSR concepts feature a system for evacuating fission gases, which accumulate in the expansion tank. Some of these gases will consist of radionuclides that are delayed neutron precursors, which will therefore be lost for the fission chain reaction. This should further reduce the effective fraction of delayed neutrons in these reactors, already reduced by the circulation of the fuel salt outside the critical zone. The aim of this thesis is to assess the extent of this reduction, and its influence on reactor dynamics.
Such an assessment may involve numerical simulations that take into account 1) a differentiation of delayed neutron precursor groups into “liquid phase groups” and “gas phase groups”, and 2) two-phase flow models (where each type of group joins its corresponding phase). In order to differentiate the groups, we need to evaluate the “liquid” and “gas” fractions for each of them, based for example on the branching ratios of the nuclear evaluations and knowledge of the chemical elements joining each of the phases. Once this has been done, simulations can be carried out with the CATHARE “system” code (already able to use two-phase models) and the TRUST-NK “core” code (whose two-phase calculation functions may require further development) to assess the influence of precursor loss on reactor dynamics.
Methodology for studying the deployment of a fleet of innovative nuclear reactors driven by grid needs and constraints
Power grids are to a society what the blood system is to the human body: the providers of electrical energy essential to the daily life of all the organs of society. They are highly complex systems that have to ensure balance at all times between consumer demand and the power injected onto its lines, via mechanisms on different spatial and temporal scales.
The aim of this thesis is to develop a methodology for optimizing the deployment of innovative nuclear reactors in power grids, adapted to their specific needs and constraints. This approach should be applicable to a wide variety of grids, from island to continental scale, and to various levels of penetration and technologies of Variable Renewable Energies (VREs). Network constraints will need to reflect stability requirements in the short term (location and capacity of inertial reserves, participation in ancillary services), medium term (controllability and load following), and long term (seasonal availability and load factor of generation resources). Innovative nuclear reactors can be of any technology, and are characterized by macroscopic parameters such as load ramp-up/down kinetics, partial power levels, time before restart, cogeneration capacities, etc., as well as the technical and economic data required for dispatching. The aim is then to be able to draw up a profile (i.e. location, power, kinetics) of nuclear reactor fleets guaranteeing stabilized operation of power grids despite a high VREs penetration rate. Two main contributions are expected:
- Academic contribution: to propose an innovative methodology for optimizing the deployment of large-scale energy systems comprising innovative nuclear reactors, by integrating both the physics of power grids and their operational constraints;
- Industrial contribution: develop recommendations for the optimal deployment of innovative nuclear reactors in power systems incorporating VREs, taking into account aspects such as reactor power and inertia, location, reserve requirements for system services, load-following capability and availability.
The PhD student will be based in an innovative nuclear systems research unit. At the intersection of the study of nuclear reactor dynamics, power system physics and optimization, this energetics thesis will offer the PhD student the opportunity to develop in-depth knowledge of tomorrow's energy systems and the issues associated with them.