Role of surface properties of UO2 powder particles on their agglomeration suitability and rheological behaviour

This study aims to predict the powder flow behavior in the context of nuclear fuel fabrication. This issue is common to many industrial fields because poor powder flow can lead to process problems such as pipe clogging, reduced rates, or the presence of heterogeneities in the final product. The first objective of this PhD thesis is, on the one hand, to provide a more accurate description of the powder agglomerates and, on the other hand, to characterize their surface. Based on these surface and structural data of UO2 powder particles, the second objective of this work is to achieve a better understanding of the agglomeration/desagglomeration properties in order to correlate them with the flow properties.
The future PhD student will need to use and develop experimental methods (particle characterization tools, surface characterization analyzers, phenomenological modeling) at the IRESNE institute (CEA-Cadarache) in the fuel study department (DEC), specifically within a team dedicated to experiments on nuclear fuel.
This study, applied to UO2 powders, has a generic nature because it is suitable for the study of all granular media. At the end of the PhD, the doctoral candidate will communicate the results through publications and conference presentations. An expertise in granular media will be acquired, which is an attractive and valuable skill in many industrial fields such as agri-food, pharmaceutical industries, metallurgy, or building materials.

Impact of irradiation parameters on the alpha’ phase formation in oxide dispersion strengthened steels

Ferritic-martensitic oxide dispersion strengthened steels (ODS steels) are materials of great interest in the nuclear industry. Predominantly composed of iron and chromium, these materials can become brittle due to the precipitation of a chromium-rich phase, called a', under irradiation. This phase, known to be sensitive to irradiation conditions, provides an ideal topic for a deeper exploration of the capability to emulate neutron irradiation with ions. Indeed, while ion irradiations are frequently used to understand phenomena observed during neutron irradiations, the question of their representativeness is often raised.

In this thesis, we aim to understand how the irradiation parameters can affect the characteristics of the a' phase in ODS steels. To do so, various ODS steels will be irradiated under different conditions (flux, dose, temperature, and type of particles, such as ions, neutrons, electrons), and subsequently analyzed at the nanoscale. The a' phase (size, chromium content) obtained for each ion irradiation condition will be compared to the one after neutron irradiation.

Resistance of an austenitic stainless steel obtained by hot isostatic pressing to stress corrosion cracking in nuclear pressurized water reactor

Due to their good mechanical properties and good corrosion resistance, AISI type 316L austenitic stainless steels (ASS) are used in a wide variety of industrial fields. Several components of the primary circuit of pressurized water nuclear reactors are made of 316L ASS. However, the susceptibility of these steels to stress corrosion cracking in this environment is an important issue. Innovative manufacturing processes to obtain better properties, more complex geometries or to reduce supply times and costs are currently in development. Hot isostatic pressing of metal powders is one of them. The objective of the thesis is precisely to evaluate the resistance to stress corrosion cracking of an ASS obtained by this innovative process and to establish the relations between its microstructure and its properties.
The process requires enclosing the powder in a container before compaction treatment and heat treatment. The surface is then machined to eliminate the container and the affected material. Given the local strain resulting from this machining and the primary importance of the pre-strain level on the resistance of 316L ASS obtained by conventional means to stress corrosion cracking, particular attention will be paid to the effect of this parameter on the resistance to stress corrosion of the studied material.
This thesis constitutes for the candidate the opportunity to address a problem of durability of metallic materials in their environment following a multidisciplinary scientific approach combining metallurgy, mechanics and physico-chemistry and based on the use of various cutting-edge techniques available at the CEA and at the Ecole des Mines. The skills that he will thus acquire can therefore be valued during the rest of his career in the industry (including non-nuclear) or in academic institutions.

Structural evolution under electron irradiation of lamellar hydroxydes and hydrates

The societal context of the study is the optimization of cementitious matrices for nuclear waste conditioning. These cementitious matrices are composed of hydrated minerals, some of which are lamellar (portlandite Ca(OH)2, brucite Mg(OH)2, brushite CaHPO4.2H2O, gibbsite Al(OH)3...). Very few data exist in the literature on the structural damage of these hydrated lamellar minerals under electron irradiation. The aim of the proposed thesis is to experimentally investigate irradiation-induced structural modifications in various types of compounds, with a view to gaining a better understanding of the damage mechanisms of these compounds under irradiation, and to identify irradiation sensitivity criteria in order to ultimately optimize the chemical and mineralogical composition of the materials.

Custom synthesis of diamond nanoparticles for photocatalytic hydrogen production

Our recent results show that nanodiamond can also act as a photocatalyst, enabling the production of hydrogen under solar illumination [1]. Despite its wide band gap, its band structure is adaptable according to its nature and surface chemistry [2]. Moreover, the controlled incorporation of dopants or sp2 carbon leads to the generation of additional bandgap states that enhance the absorption of visible light, as shown in a recent study involving our group [3]. The photocatalytic performance of nanodiamonds is therefore highly dependent on their size, shape and concentration of chemical impurities. It is therefore essential to develop a "tailor-made" nanodiamond synthesis method, in which these different parameters can be finely controlled, in order to provide a supply of "controlled" nanodiamonds, which is currently lacking.

The aim of this PhD is to develop a bottom-up approach to nanodiamond synthesis using a sacrificial template (silica beads or fibers) to which diamond seeds < 10 nm are attached by electrostatic interaction. The growth of diamond nanoparticles from these seeds will be achieved by exposing these objects to a microwave-enhanced chemical vapor deposition (MPCVD) growth plasma, allowing very fine control of (i) the incorporation of impurities into the material (ii) its crystalline quality (sp2/sp3 ratio) (iii) its size. This growth facility, which exists at the CEA NIMBE, is used for the synthesis of boron-doped diamond core-shells [4]. In the second part of the thesis, an innovative process (patent pending) is implemented to achieve MPCVD growth of diamond nanoparticles by circulating the sacrificial templates in a gas stream. During this work, different types of nanodiamonds will be synthesized: intrinsic nanoparticles (without intentional doping) and nanoparticles doped with boron or nitrogen.

After growth, the nanoparticles will be collected after dissolution of the template. Their crystal structure, morphology and surface chemistry will be studied at CEA NIMBE by scanning electron microscopy, X-ray diffraction and Raman, infrared and photoelectron spectroscopy. A detailed analysis of the crystallographic structure and structural defects will be carried out by high-resolution transmission electron microscopy.

Nanodiamonds will then be surface-modified to give them colloidal stability in water. Their photocatalytic performance for hydrogen production will be evaluated in collaboration with ICPEES (Strasbourg University).

[1] Patent, Procédé de production de dihydrogène utilisant des nanodiamants comme photocatalyseurs, CEA/CNRS, N° FR/40698, juillet 2022.
[2] Miliaieva et al., Nanoscale Adv. 2023.
[3] Buchner et al., Nanoscale (2022)
[4] Henni et al., Diam. Relat. mater. (under review)

Multiscale metamaterials based on 3D-printed biosourced polymer composites

Reducing the density of materials is one of the best ways to diminish our energy footprint. One solution is to replace massive materials by microlattices. Among these, random architecture structures inspired by bird bone structure offer the best advantages, with isotropic mechanical behavior and increased mechanical resistance, while meeting the challenges of the circular economy. These material-saving metamaterials are manufactured by 3D printing and can be compacted at the end of their life cycle. Among manufacturing technologies, UV polymerization of liquid organic resin or composite is the most promising. It produces mechanically resistant materials without generating manufacturing waste. It is also possible to include large quantities of bio-sourced fillers, reducing even further their environmental impact.

The PhD-thesis proposed here focuses on the development of polymeric nanocomposite microlattice structures from resin formulation to mechanical properties study (viscoelasticity, yield stress, fracture resistance) through printing and post-processing stages. From a more fundamental point of view, the aim is to study the link between the composition, shape and surface properties of the fillers on one hand, and the imprimability of the composite resine and the mechanical properties of the resulting metamaterial on the other hand. The thesis will focus on the study of cellulose-type fillers in nanoparticle, microparticle or fiber form. This multidisciplinary study bridges technology to science while producing data for a digital twin.

Characterizations and modeling of metal tritide aging: application to palladium tritide

Using alternative energy sources such as fusion requires the storage and use of great amount of hydrogen.
This thesis work is about the storage of hydrogen isotopes by palladium hydrides at low equilibrium pressure.
This solid state storage, which ensures both safety and compactness, is particularly interesting for tritium, the radioactive isotope of hydrogen which decay produces helium-3. Helium-3 tends to form nanobubbles which modify the physico-chemical properties of palladium tritide. This phenomenon is called aging. When helium-3 concentration reaches a critical value it is released in the gas phase which can lead to an increase in the storage facility.
In order to better understand and predict the aging phenomenon, material which are aged under tritium during several years are characterized. Studying microstructure, nanobubbles architecture, chemical composition and mechanical behavior evolutions. The acquired data are then used as inputs and outputs of the aging mechanisms modeling.

Exploring the reactivity of oxide based catalysts by radiolysis

In the context of the search for processes that are less polluting and more energy-efficient than current processes, it is interesting to produce high-stake molecules such as C2H4 by developing alternative synthesis routes to steam cracking, which is used in the majority of cases, but is energy-intensive and based on fossil resources. Processes such as photocatalysis, which relies on the use of light energy, seem an attractive way of generating these molecules of interest. In this context, we have already shown that the use of TiO2-based photocatalysts decorated with copper particles enables the production of ethylene from an aqueous solution of propionic acid, with a selectivity (C2H4/other carbonaceous products) of up to 85%.

However, photocatalysis kinetics can be slow, and it can take a long time to identify the best catalysts or catalyst/reagent pairs for a given reaction. So, in order to determine whether radiolysis, which relies on the use of radiation to ionize matter, can be an effective method of screening catalysts, initial experiments have already been carried out on catalyst (TiO2 or Cu TiO2)/reagent (propionic acid more or less concentrated) pairs, previously studied in photocatalysis. Initial results obtained by radiolysis are encouraging. In these experiments, only dihydrogen production was measured. A significant difference was observed in this production depending on the system: it was high during radiolysis of propionic acid with TiO2 nanoparticles, and significantly lower in the presence of Cu TiO2 nanoparticles, suggesting a different reaction path in the latter case, in line with observations made during photocatalysis experiments.

The aim of this thesis work will be to extend these initial results by synthesizing nanoparticles (catalysts), preparing reagent/catalyst mixtures, then irradiating them and measuring the various gases produced by gas-phase micro-chromatography, with special attention on ethylene. Particular attention will be paid to determining the species formed, especially transient ones, in order to ultimately propose reaction mechanisms accounting for the differences observed for the different reagent/catalyst pairs. Comparisons will also be made with results obtained by photocatalysis.

Sitinakite materials for continuous treatment of Sr-contaminated effluents

The aim of this thesis is to develop sitinakite-type materials compatible with a continuous treatment process for strontium-contaminated effluents.
Sitinakite is a poorly crystalline silico-titanate phase with ion exchange properties. In particular, the sodium atoms present in the channels of this structure are mobile and can exchange selectively with strontium ions. This means that exchange with strontium will take priority even in the presence of other competing cations from the family of alkaline earth elements, such as calcium.
However, for sorption materials to be suitable for continuous effluent treatment involving a high flow rate through the filter element, they need to be shaped. In fact, fine powders are not suitable for such continuous processes because of the clogging phenomena of the filtering elements.
Consequently, the research teams proposing this thesis topic have developed a protocol for shaping millimetre-sized sitinakite granules. This involves converting millimetre-sized TiO2 granules into sitinakite via a hydrothermal pseudomorphic transformation reaction. However, if the phase conversion works, it leads to a loss of efficiency with a consequent slowdown in the rate of exchange between sodium and strontium compared with sitinakite in powder form.
This thesis therefore proposes to adapt the transformation protocol so that the sodium - strontium exchange rates are faster, equivalent to the powder system. This will involve treating sitinkaite granules or pre-treating precursor TiO2 granules prior to processing in order to increase the specific surface area of the final materials and thus improve the accessibility of the exchange sites.
The PhD student will also analyse the effects of irradiation on the sorption properties of materials induced by the presence of radioactive Sr in the materials. In particular, this will involve finding out whether irradiation can lead to the release of strontium, for example. To this end, material irradiation campaigns will be carried out on electronic irradiators (LSI, Polytechnique), the aim of which will be to simulate the presence of a beta emitting element such as 90Sr.
The applicant profile we're looking for is based on a Master 2 and/or school of engineering student with a specialization in solid-state chemistry, particularly materials. Ideally, the candidate will have notions of the physical chemistry of interfaces. The PhD student will benefit from the expertise of the two host laboratories in the field of porous materials and nuclear decontamination. These two aspects will help the candidate in his or her post-doctoral job search and enable him or her to apply for offers in the field of decontamination, whether nuclear or in other sectors (water treatment, soil decontamination, etc.).

Incorporation of U and Pu in nuclear glasses and impact on their long term behavior under water

In order to stabilize its final high-activity waste, the Atalante facility plans to vitrify it on the VESPA platform implemented on the C18 shielded line. The glasses produced will be stored in the short term at the ATALANTE facility and are intended to be sent in the medium term to the French disposal facility, at the Bure site.
The calcination vitrification process of the VESPA platform will aim to produce sodium borosilicate glasses as close as possible to nuclear glasses used at the La Hague plant by Orano. These glasses will have to incorporate higher contents of uranium and plutonium (U and Pu) into their vitreous network than the glasses used industrially until now.
Therefore, to validate the proposed domain of chemical composition of the so-called “Atalante” glasses, the candidate will first have to study the limits of incorporation of U and Pu in UOX type glasses.
Once the chemical domain has been determined, a second part of the PhD thesis will be devoted to the study of the kinetics of alteration of glasses of interest under water and thus examine the impact of U and Pu concentrations on their long-term behavior properties.
The profile of the desired candidate is Master 2 level or equivalent in materials science. He/she must be rigorous, organized and present a strong interest in characterization techniques of solids or liquids as well as for the synthesis of materials. Experience in the glass and/or nuclear field would be a plus. This thesis will offer him/her the opportunity to develop skills in numerous characterization techniques and also experience of work in nuclear facility.
Master 2 materials science.