Study of plutonium oxalate formation mechanisms – Application to molten salt reactors
Molten salt reactors (MSRs) offer a promising alternative for sustainable nuclear energy production, thanks to their intrinsic safety and their ability to close the nuclear fuel cycle, notably through the use of a fast neutron spectrum. This type of reactor can use liquid chloride salts containing plutonium and other actinides as fuel. As part of the development of this nuclear pathway, the CEA supports the development of a PuCl3 production process. The synthesis of this chloride has already been carried out at small scale at the CEA and elsewhere in the world. Several starting materials can be used for the synthesis of the trichloride, notably plutonium metal, plutonium oxide and plutonium oxalate. The most industrially promising synthesis route is the oxalate route, because it can be transferred to the equipment already present at the La Hague site. This process consists of converting the oxalate into plutonium chloride via a gas–solid reaction with a chlorinating agent, such as HCl for example. However, the reaction mechanism and the decomposition of the oxalate in a chlorinated environment are still poorly understood. A detailed understanding of this transformation would make it possible to optimize operating conditions and facilitate the scale-up of this synthesis. The topic will initially focus on determining the reaction mechanism of Ce oxalate (a surrogate for Pu) to the chloride. Small-scale studies will be performed to identify the various reaction intermediates using analytical techniques such as X-ray diffraction (XRD), thermogravimetric analysis / differential thermal analysis (TGA/DTA) and analysis of the gases produced during the reaction. The kinetics as well as the enthalpy changes will also be studied in order to obtain key data for modelling a large-scale process. Subsequently, an optimization of the PuCl3 synthesis at the scale of a few tens of grams will be carried out. These studies will first be conducted under non-radioactive conditions on a surrogate to validate the experimental approach, before being transposed to radioactive conditions.
Effects of alpha decay on the alteration of nuclear glasses: simulation, understanding, and consideration in geochemical models
This Ph-D at the CEA on the alteration of nuclear glass is central to the challenges of sustainable radioactive waste management. The doctoral student will acquire expertise in materials and modeling, paving the way for exciting careers in research, engineering, or the nuclear industry. In deep geological storage, contact with groundwater can cause glass alteration, which is the main source of radionuclide release. The CEA is developing a multi-scale model that needs to be adapted to take into account the effects of glass self-irradiation. The aim of the thesis is to identify the mechanisms modified by irradiation and to parameterize the model. The doctoral student will conduct controlled irradiation experiments on non-radioactive glasses and compare them to ²44Cm-doped active glass. The structural and physicochemical changes induced will be characterized using various techniques (Raman, IR, NMR, SEM, TEM, DSC, etc.). Targeted alteration tests will be used to observe the impact of the level of damage on the kinetics of alteration. The results will be used to adjust and validate the predictive model under conditions representative of geological storage. The work will be carried out both in an active environment (shielded cells) and in an inactive laboratory. An M2 internship is available on the same subject. Profile: M2 or materials engineer, physical chemistry.
Study and Modelling of Tritium Speciation from the Outgassing of Tritiated Waste
Tritium, the radioactive isotope of hydrogen, is used as fuel for nuclear fusion, particularly in the ITER research reactor currently under construction in Cadarache (France). Its small size allows it to easily diffuse into materials, which will lead to the production of waste containing tritium after the operational phase of ITER.
To optimize the management of this tritiated waste, the CEA is developing technological solutions aimed at extracting and recycling tritium, as well as limiting its migration to the environment. The effectiveness of these solutions largely depends on the chemical form in which tritium is released. Experience from the outgassing of tritium from various types of waste indicates that it is released in two main chemical forms: tritiated hydrogen (HT) and tritiated water vapor (HTO), in varying proportions.
However, the mechanisms determining the distribution of tritium between these two species are not well understood. Several factors, such as oxygen and water concentrations, the nature and surface state of the waste, and the concentration of tritium, can influence this speciation.
The objectives of this thesis are as follows:
- To identify the phenomena affecting the speciation of tritium during the outgassing of tritiated waste.
- To conduct an experimental study to verify the proposed hypotheses.
- To develop a numerical model to predict the proportions of HT and HTO released, in order to optimize the management of this waste.
The thesis will be conducted within the IRESNE Institute (Institute for Research on Nuclear Systems for Low Carbon Energy Production) at the CEA site in Cadarache, in a laboratory specialised in tritium studies. The PhD candidate will work in a stimulating scientific environment and will have the opportunity to showcase their research work. The candidate must hold an engineering degree or a master’s degree in Chemical Engineering, Process Engineering, or Chemistry.
Bottom-up synthesis of nanographene and study of their optical and electronic properties
This project is part of an ANR project, which aims to synthesize perfectly soluble and individualized graphene nanoparticles in solution and incorporate them into spin electronics devices. To do this, we will draw on the laboratory's experience in synthesizing and studying the optical properties of graphene nanoparticles to propose original structures to several groups of physicists who will be responsible for studying the optical and electronic properties and manufacturing spin valve-type devices.
Tailored Peptide Ligands for Actinide Complexation: From Structure to Selectivity
The processes involved in the nuclear fuel cycle, such as the PUREX process designed to separate uranium and plutonium from fission products, rely on ligands capable of selectively complexing actinide cations to enable their extraction. The chemical functions carried by these ligands play a key role in determining both their affinity and selectivity toward metal cations. Studying the influence of these functional groups, such as carboxylic acids and phosphates, is therefore essential for the design of new extracting molecules, as well as for the development of decorporation strategies.
Over the past decade, cyclic peptides have been developed for their ability to complex uranyl ions with high selectivity over calcium. Organized in ß-sheet conformations, these peptides display a functional face bearing complexing groups (carboxylates, phosphates). Their amino acid composition can be tuned to finely adjust the chemical nature of the coordination site, making these cyclic peptides tailor-made molecular architectures for probing cation complexation. However, while their interaction with uranium is now well characterized, their ability to bind transuranic elements remains largely unexplored.
This PhD project aims to study the complexation of actinides such as plutonium and neptunium by various cyclic peptides. The combination of NMR spectroscopy and classical molecular dynamics simulations will provide detailed structural information on the formed complexes. Complementary techniques, including UV-Vis-nIR and EXAFS spectroscopies, ESI-MS mass spectrometry, and fluorescence spectroscopy, will deepen the characterization. By combining experimental and computational approaches, this work will enhance our understanding of ligand–actinide interactions while paving the way for the design of innovative extracting and decorporating molecules.
Investigation of autocatalysis phenomena occurring in nitric acid dissolution through electrochemical methods
The nuclear fuel recycling process, used at the La Hague plant in France, begins with the nitric dissolution of spent fuel, mainly composed of uranium and plutonium oxides. In a context of plant renewal and widespread of MOX fuel recycling, innovative new dissolution equipment are currently studied. The sizing of such devices is currently limited by the absence of a fully comprehensive model for the dissolution of mixed oxides, which is a highly complex reaction (three-phase involved, self-catalytic, heterogeneous attack, etc.). Despite substantial progress made in previous studies, a number of questions remain unanswered, particularly concerning the reaction mechanisms involved and the nature of the catalyst.
Electrochemical methods (cyclic voltammetry, electrochemical impedance spectroscopy, rotating electrode, etc.) have never been used to understand dissolution, yet they should prove relevant as already demonstrated by the studies carried out on this subject by CEA Saclay in the field of corrosion. Therefore, the aim of this thesis is to apply these experimental methods for the first time to the dissolution of nuclear fuels, through a phenomenological approach. To achieve this, the student will be able to rely on the teams and facilities of Saclay and Marcoule centers, specialized respectively in electrochemical methods for the corrosion studies and the physico-chemical modeling of dissolution.
This cross-disciplinary study, involving materials science, electrochemistry and chemical engineering, will follow a stimulating fundamental research approach, but will also take place in a highly dynamic industrial context. Initially, the work will be carried out on inactive model and noble materials (at the Saclay center), then on real materials containing uranium and/or plutonium (at the Marcoule center).
Understanding the mechanisms of oxidative dissolution of (U,Pu)O2 in the presence of Ag(II) generated by ozonation
The recycling of plutonium contained in MOx fuels, composed of mixed uranium and plutonium oxides (U,Pu)O2, relies on a key step: the complete dissolution of plutonium dioxide (PuO2). However, PuO2 is known to dissolve only with great difficulty in the concentrated nitric acid used in industrial processes. The addition of a strongly oxidizing species such as silver(II) significantly enhances this dissolution step—this is the principle of oxidative dissolution. Ozone (O3) is used to continuously regenerate the Ag(II) oxidant in solution.
Although this process has demonstrated its efficiency, the mechanisms involved remain poorly understood and scarcely documented. A deeper understanding of these mechanisms is essential for any potential industrial implementation.
The aim of this PhD work is to gain insight into the interaction mechanisms within the HNO3/Ag/O3/(U,Pu)O2 system. The research will be based on a parametric experimental study of increasing complexity. First, the mechanisms of generation and consumption of Ag(II) will be investigated in the simpler HNO3/Ag/O3 system. In a second phase, the influence of various parameters on the oxidative dissolution of (U,Pu)O2 will be examined. The results will lead to the development of a kinetic model describing the dissolution process as a function of the studied parameters.
At the end of this PhD, the candidate—preferably with a background in physical chemistry—will have acquired advanced expertise in experimental techniques and kinetic modeling, providing a strong foundation for a career in academic research or industrial R&D, both within and beyond the nuclear sector.
Exploration of VACNTs in Anode-less Batteries: Mechanism and Cell Optimization
Anode-less or anode-free batteries are getting increasing attention owing to their excellent energy density, cost efficiency, and ease of process upscaling. Exploring anode-less battery will offer a breakthrough in energy storage devices by using the lithium reserve already present in the NMC cathode to reversibly cycle after an initial formation process, which will reduce the overall thickness, processing steps, and cost of materials, and provide excellent energy density. Vertically aligned CNTs (VACNTs) on metal substrates can be an interesting choice for this application due to their low thickness, reproducible synthesis process, and uniform surface properties, which have already proven their applicability in supercapacitors. In this PhD project, we will investigate their newer avenue of applications- anode-less batteries, where VACNTs act as the lithium or sodium plating substrate. We will study the electrochemistry of VACNT in lithium anode-less batteries (in liquid and solid electrolytes) and in sodium anode-less batteries in a liquid electrolyte. The PhD student will work on the synthesis optimizations of VACNT to tune the thickness and density to match their electrochemistry. Post-cycling studies (Raman and SEM) will be carried out to study the effect of cycling and the electrolytes on the VACNT layers. The project aims to explore the possibility of the application of VACNTs in various energy storage systems, which can open up new application possibilities and valorization
Photo- and thermocatalytic cross-coupling of esters for the synthesis of biosourced alkenes
The easy access to energy and carbon-based raw materials offered by the fossil feedstock allowed a rapid growth of our society. Nevertheless, the expected depletion of fossil resources and climate change, require changing for a more sustainable model. Bio-based feedstock is a promising source of carbon to substitute petrochemicals but require a drastic change of the actual model. While the current paradigm relies on the production of energy and high-value molecules through oxidation steps, a model based on Carbon Circular Economy, i.e. the transformation of CO2 and biomass feedstock that are already highly oxidized materials demands the development of new methodologies for reduction, deoxygenation, and the direct use of oxygenated bonds to access functionalized and useful organic molecules.
In organic chemistry, cross-coupling reactions represent one of the major tools to create C–C bonds. However, they are still based mainly on the use of organic halides as electrophiles. In this project, the PhD candidate will demonstrate that readily available and abundant alkyl esters can serve as electrophilic coupling partners in catalyzed cross-coupling reactions with alkenes. Esters can indeed be directly biosourced or easily synthesized from alkyl carboxylic acids and alcohols, thereby diminishing the environmental impact of the carbon-carbon bond formation.
Synthetic methodologies towards functionalized azaheterocycles and application to energetic molecules
The objective of the PhD is to develop new synthesis and/or functionalization methods to obtain functionalized heterocyclic molecules. These molecules are based on 5- or 6-member azaheteroaromatic rings (diazines, triazines, triazoles, tetrazoles, etc.). The targeted structures make it possible to envisage high densities and enthalpies of formation, while maintaining low sensitivity (thermal, mechanical, etc.). They find applications in the energy field, notably propulsion, explosives and gas generators (airbags). In addition, these heterocyclic compounds as well as the intermediates are also structurally close to families of biologically active products and/or likely to exhibit fluorescence properties, as already shown in a previous PhD in the laboratory.