Understanding the mechanisms of oxidative dissolution of (U,Pu)O2 in the presence of platinum group metals
The treatment of MOx fuel, composed of a mixed uranium and plutonium oxide (U,Pu)O2, is aimed at recycling plutonium. Plutonium dioxide (PuO2) is notably difficult to dissolve in concentrated nitric acid. However, by introducing a highly oxidizing agent, such as Ag(II), into the nitric acid, plutonium can be solubilized with fast dissolution kinetics—a process known as oxidative dissolution. The fission products present in irradiated MOx, particularly platinum group metals, can potentially impair the effectiveness of plutonium’s oxidative dissolution through side reactions. For the industrial deployment of this method, it is therefore crucial to understand how platinum group metals influence the dissolution kinetics. Yet, there is currently very limited data on this subject.
This thesis aims to address this knowledge gap. The proposed research involves a parametric experimental study of increasing complexity: initially, the impact of platinum group metals on Ag(II) consumption will be investigated separately, followed by their effect during the dissolution of (U,Pu)O2. These findings will enable the development of a kinetic model for the dissolution process based on the studied parameters.
By the end of this thesis, the candidate, with a strong background in physical or inorganic chemistry, will have gained expertise in a wide range of experimental techniques and advanced modeling methods. This dual competence will open up numerous career opportunities in academic research or industrial R&D, both within and beyond the nuclear sector.
Oxidation kinetics of U1-yPuyO2 mixed oxides: experimental study and modelling
The widespread use of MOX fuels (based on (U,Pu)O2 mixed oxides) in current nuclear reactors is an option being studied in France. Such a strategy could make it possible to stabilise plutonium stocks and conserve uranium resources. This scenario involves the multi-recycling of plutonium, which requires an upgrade of the existing plant to enable reprocessing of spent MOX fuel at industrial rates. The development of innovative processes and related basic research is then imperative.
Oxidation of MOX spent fuel by ad hoc thermal treatment could overcome one of the technological barriers identified, i.e. how to separate the fuel from its cladding prior to the dissolution step. The idea is to take advantage of the phase changes that occur as the fuel oxidises to collapse it into powder. However, data on the oxidation of (U,Pu)O2 oxides are currently scarce in the literature. The aim of this PhD thesis is to help fill this gap. The student in charge of this work will first have to characterise the nature of the phases formed during the oxidation of (U,Pu)O2 oxides, as well as the kinetics and mechanisms involved. These results will lead to the proposal of a phenomenological model linking the kinetics of (U,Pu)O2 oxidation with the Pu content, the O2 partial pressure, the temperature and the duration of the thermal treatment.
At the end of this PhD, the graduate student, with initial training in the physical chemistry of materials, will master a wide range of experimental techniques as well as advanced methods for modelling the reactivity of solids. These skills will open up many job opportunities in academic research or industrial R&D, both within and outside the nuclear sector.
Please note: A final internship is also offered in preparation for this PhD position. For further information, please contact the supervising team.
Impact of fission products and microstructure on the oxidation mechanisms of (U,Pu)O2 fuels
The widespread use of MOX fuels (based on (U,Pu)O2 mixed oxides) in nuclear reactors is an option currently being studied in France. Such a strategy could help to stabilise the plutonium inventory while conserving natural uranium resources. Such scenario involves the multi-recycling of plutonium, which requires an upgrade of the existing infrastructures to enable the reprocessing of spent MOX fuel at industrial scale. Consequently, the development of innovative processes and related basic research is then imperative.
The oxidation of MOX spent fuel by ad hoc thermal treatment could overcome a major technological barrier identified, i.e. how to separate the fuel from its cladding prior to the dissolution step. However, there is limited data available on the oxidation of (U,Pu)O2 and even less has investigated the impact of fission products and the fuel microstructure properties. The aim of this PhD thesis is to help fill this gap. The selected student will study (U,Pu)O2 samples with a microstructure identical to that of industrial MOX fuels, as well as (U,Pu)O2 doped with inactive fission products, thus simulating irradiated fuels, as shown in a thesis defended in the laboratory. The experimental work will focus on oxidation experiments coupled with in-situ and multi-scale analyses using laboratory technics and synchrotron radiation-based characterizations. These results will lead to the proposal of a phenomenological description linking the kinetics of (U,Pu)O2 oxidation with the fission products chemistry, the O2 partial pressure and the temperature and duration of the thermal treatment.
By the end of this PhD, the graduate student with a background in physical chemistry of materials will have developed expertise in a broad range of experimental techniques. These skills will open up many career opportunities in academic research or industrial R&D, both within and beyond the nuclear sector.
Tri-axial cell investigations and consideration of the influence of the behaviour of the agglomerates (U-Pu)O2 microstructure on the simulation of fuel shaping
The research topic concerns the influence of the behaviour of the (U-Pu)O2 agglomerate microstructure on the simulation of fuel shaping through triaxial cell investigations. It is based on multi-scale experimental and numerical studies in order to propose simulations of the shaping of actinide fuels, taking into account the breakage and rearrangement of agglomerates in the behaviour laws VER on homogenised VER. To this end, investigations in triaxial cells are envisaged, on the one hand on VER using X-CT tomography on simulating inactive model powders and on the other hand on industrial-sized samples on real active powders. Fracture tests using X-ray tomography will also be carried out on inactive materials and without tomography on active materials, in order to compare experimental and numerical results in the case of damage to pre-sintered fuels. A comparison will also be planned to take into account the impact of the proposed approach on the parameters of the models currently used for macroscopic simulations of fuel shaping on an industrial scale.
Freeze-Casting: ice texturing
The thesis topic focuses on MOX fuels with controlled porosity. The student will have to develop a concentrated aqueous suspension in solid phase, dispersed and stable over time with respect to sedimentation. This suspension will be optimized using an experimental design. The tests to be carried out will typically be zeta potential and rheology measurements. The parameters to be taken into account will be the dry matter content as well as the nature and concentration of certain additives (dispersants, surfactants, organic binders) that can be incorporated into the formulation.
In a second step, the texturing conditions by the controlled growth of ice crystals will be explored, again using an experimental design.
After freeze-drying and sintering, the objective is to obtain a residual porosity controlled in size, morphology and interconnection. The sintered microstructures will be characterized by ceramography, scanning electron microscopy, image analysis and X-ray tomography on a line capable of accommodating radioactive materials.
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.