Study and simulation of phase entrainment in mixer-settler batteries
As part of the development of new liquid-liquid extraction separation processes, experimental tests are implemented to demonstrate the recovery of valuable elements sufficiently decontaminated from impurities. These tests are commonly carried out in mixer-settler batteries. However, depending on the operating conditions, these finished products may be contaminated by impurities. This contamination results from the combination of several factors:
-Hydrodynamic: Entrainment in the solvent of non-decanted aqueous drops containing impurities
-Chemical: the impurity separation factor is low (less than 10-3)
-Process: the entrainment of drops is amplified with the increase in the rate (reduction of the residence time of the drops)
This thesis aims to increase the understanding of the different phenomena responsible for these phase entrainments in order to estimate optimal operating parameters and to guarantee a contamination of the finished products below a fixed threshold. The aim will be to develop a macroscopic model to predict the flow rate of non-decanted droplets as a function of the operating conditions in the mixer-settler batteries. It will have to be based on hydrodynamic simulations coupling the resolution of a droplet population balance to a continuous phase flow. A coupling will be carried out between this hydrodynamic model and the PAREX or PAREX+ code to size the process diagrams. The qualification of the proposed models will have to be done by comparisons with experimental measurements (based on previous or future test campaigns).
Microemulsion model: Towards the prediction of liquid-liquid extraction processes
This multi-scale modeling thesis aims to develop innovative theoretical approaches and numerical tools to revolutionize strategic metal extraction processes, such as liquid-liquid extraction, whose underlying mechanisms remain poorly understood. To address these challenges, solvent phases will be represented as microemulsions through a synergy of mesoscopic and molecular modeling approaches.
The mesoscopic approach will involve the development of a code based on microemulsion theory using a random wavelet basis. This code will enable the characterization of the structural and thermodynamic properties of the solutions. The molecular approach will rely on classical molecular dynamics simulations to evaluate the curvature properties of the extractants, which are essential for bridging the two scales.
The new high-performance computational code may integrate artificial intelligence techniques to accelerate the minimization of the system’s free energy while accounting for all chemical species present with a minimal number of parameters. This will pave the way for new research directions, such as predicting speciation and calculating thermodynamic instabilities in ternary phase diagrams, thereby identifying unexplored experimental conditions.
This PhD thesis, conducted at the Mesoscopic Modeling and Theoretical Chemistry Laboratory at the Marcoule Institute for Separation Chemistry, will have applications in the recycling domain and extend to the broader field of nanoscience, thereby expanding the impact of this work.
The PhD candidate, with an academic background in physical chemistry, theoretical chemistry, or physics, and a strong interest in programming, will be encouraged to disseminate his/her scientific results through publications and presentations at national and international conferences. Upon completion of the thesis, the candidate will have acquired a wide range of skills in modeling and physical chemistry, opening numerous professional opportunities in both academic research and industrial R&D.
study of lithium isotopes separation by laser
This thesis concerns the study of different ways of separating lithium isotopes by laser. The work will be conducted both theoretically and experimentally. The objective is to determine an optimal way as well as its performances.
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.
Development of an automated and miniaturised system for the isotopic analysis of nuclear samples
Miniaturisation, which is the process of reducing an object’s, a method’s or a function’s dimensions while preserving or even upgrading its performances as compared to the classical scale, has a particular interest in the field of analytical chemistry for nuclear applications. Indeed, most of the analyses are performed in gloveboxes where miniaturisation and automation are a direct solution to the need for reduced doses and waste volumes. This PhD aims at developing a miniaturised and automated system, in a glove box, for performing high-precision isotopic measurements. This system will use capillary electrophoresis (CE) hyphenated with a nuclearised multicollector ICP-MS (MC-ICP-MS). During this PhD, the student will make use of micro-machining machines and 3D printers to develop an ergonomic system which will then be coupled to last generation MC-ICP-MS instruments available in our laboratory. The project will be focused on the conception of the automated system and its integration in the glove box, and on the further development of the existing CE method in order to perform isotopic and elemental analyses with nuclear samples. This PhD is hosted in a laboratory internationally recognized for its ability to carry out high precision isotopic measurements. An analytical chemistry curriculum is expected and a Master 2 internship is available before this PhD.
Redox behavior of technetium in the innovative PUMAS process: kinetic and speciation study
Technetium (Tc), an artificial radioactive element, makes up about 6% of the fission products in spent nuclear fuel. The PUREX process is used to separate uranium and plutonium from other fission products. However, Tc is co-extracted with these actinides, requiring an additional stripping step. In this stage, a stabilizing agent, hydrazinium nitrate (NH), is used, but due to its toxicity and CMR classification (Carcinogenic, Mutagenic, Reprotoxic), it is being replaced by less toxic alternatives such as oximes. Although promising, oximes exhibit slower stripping kinetics compared to NH. In the context of the PUMAS process, this thesis aims to understand the complex redox mechanisms of Tc and the kinetic differences between oximes and NH. The PhD student will study the reduced forms of Tc and analyze the reduction kinetics in the presence of U(IV) and anti-nitrous agents. A methodology will be developed to characterize the oxidation states of Tc, and reaction rate constants will be determined as a function of temperature and reactant concentrations.
The candidate will work closely with the supervising team to develop autonomy, adaptability, and the ability to propose innovative ideas. By the end of this journey, the candidate will have gained not only advanced technical skills but also abilities in project management, collaborative work, and scientific writing and communication. These competencies will provide strong prospects for a career in academic research or industry.
Molecular Dynamics Simulation of Plutonium(IV) in Solution
With the revival of nuclear power in France, the CEA is playing a key role in the nuclear industry of the future. In this context, engineers and researchers are mobilised to meet the growing needs of this industry. Plutonium is a key element in the nuclear fuel cycle. Acquiring molecular data is crucial to optimising and rationalising the mechanisms involved in separating this element.
Plutonium(IV) is one of the most common cationic forms in the nuclear fuel cycle. Its study by theoretical chemistry presents difficulties both in ab initio modelling (occupied f-block orbitals) and in classical atomistic simulations. In classical molecular dynamics simulation, the models necessarily require the addition of the polarisation effect, and sometimes even the addition of charge transfer, in order to reproduce the system's behaviour correctly. As a result, classical simulations containing plutonium (IV) are almost completely absent from the scientific literature. In addition, the speciation of this cation is sensitive to the acidity of the medium, which has to be taken into account in the simulations, adding a further difficulty.
The aim of this thesis is to use molecular dynamics (classical and ab initio) to simulate solutions containing plutonium, taking into account the effect of acidity. The PhD student will be faced with two main issues: the choice or development of a force field for the Pu4+ cation, and the design of a method for including acidity in the solutions. A crucial step in the process will be to compare the results with the available experimental data in order to conclude on the ability of the models to reproduce the experimental data. This thesis will be carried out in a multidisciplinary laboratory, combining experimental chemistry and theoretical modelling, while conducting both applied and fundamental research.
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.