Physico-chemical multi-scale modelling of coalescence

The recycling of metals by hydrometallurgy is necessary to ensure both the development of new energy technologies and the sustainability of the nuclear power cycle. The industrial processes used here, such as liquid-liquid extraction, involve the flow of two fluids under agitation whose interfaces form and deform. The coalescence of drops thus plays a very important role. The modelling of these complex two-phase systems involved in the extraction process must therefore take this phenomenon into account in the best possible way. In this thesis, we propose to describe for the first time the coalescence of drops in a realistic way by a multi-scale approach to take into account the physicochemical aspects of the phenomena. Firstly, molecular dynamics simulations will allow us to propose a stochastic model where the probability of coalescence will be expressed as a function of geometric parameters (distance and size of drops). Then the role of physico-chemical quantities in the phenomenon will be systematically described (role of surface tension, presence of surfactants, viscosities, etc.). The ultimate goal is to arrive at computational fluid dynamics (CFD) simulations in which coalescence, and in particular its random nature, will have been developed and validated by molecular dynamics calculations performed at the atomic level, taking into account the physicochemistry of the interface. Such a model would be a major step towards predicting the kinetics of liquid two-phase media, not only in the context of optimising recycling processes, but also for many other applications.
Candidate profile: Master of Physics - Master of Chemistry - Engineering School - ENS. After the thesis, the candidate can either continue in the academic field, with a high level of competence in modelling, or move towards industry by developing his/her dual competence in modelling and recycling.

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.).

Study of the formation of phases of interest for corium: chernolylite (U,Zr)SiO4 and mixed oxide (U,Zr)O2

In the event of a severe accident in a nuclear reactor, the loss of cooling leads to high-temperature heating of the UO2 fuel and surrounding materials. The fuel can then react with the zirconium alloy cladding and steel vessel to form a complex mixture of solid and liquid phases, known as in-vessel corium. In a second step of the accident, if the vessel is breached, this corium can flow out and react with concrete. The silicate (U,Zr)SiO4 known as chernobylite (present in the Chernobyl corium) can then form by reaction between the mixed oxide (U,Zr)O2 and silica SiO2.
To predict the heat exchanges as well as the phases formed in the corium, severe accident codes are coupled with thermodynamic calculation software using databases describing the thermodynamic properties of the corium. These calculations are performed using the CALPHAD method, by minimizing the free enthalpy of the system. In these databases, the UO2-ZrO2-SiO2 pseudo-ternary diagram is one of the important to be well described in order to predict the phases formed in the corium. However, the solubility limit of uranium in zircon (ZrSiO4) and the thermodynamic properties of chernobylite (Zr,U)SiO4 are poorly known. Moreover, the mixed oxide (U,Zr)O2 remains poorly characterized from a thermodynamic point of view. We therefore propose to study the stability of these phases to overcome the lack of data and to improve thermodynamic models of the UO2-SiO2-ZrO2 and U-Zr-O systems.
After optimizing the conditions for synthesis by wet chemistry route and purification of the two solid solutions, the samples will be characterized from a physico-chemical and thermodynamic point of view. All these experimental results will serve as input data for modeling the UO2-ZrO2-SiO2 and U-Zr-O systems.

The candidate will have a Master's degree or an engineering diploma in radiochemistry, separative chemistry or materials science. During his/her work, he/she will be required to master a wide range of techniques related to the solution chemistry particularly that of uranium and zirconium), the solid-state chemistry, the physical chemistry and the characterization of materials, including their thermodynamic properties. This will allow him/her to leverage his/her skills not only in the nuclear field, but also more broadly in that of materials elaboration and characterization.

Study of the CHON-UNEX process for the removal of high heat-emitters from spent fuel

The removal of high heat emitters from spent fuels allows to reduce the volume necessary for a safe storage in deep repository. Some of these isotopes can also be recycled, for instance 241Am as fuel for fast-breeder reactors and 137Cs as a gamma source (radiotherapy, sterilization). A new process called CHON-UNEX has been recently proposed, whose ligands and diluents are composed of only C, H, O and N elements so that the effluents can be eventually vaporized and do not form secondary wastes. It consists in a co-extraction of all high heat emitters followed by successive stripping steps to enable the recycling of these elements. The Sr, Am and Ln extractant is a diglycolamide (DGA) that also serves as modifier for the Cs extractant, a calixarene crown-ether (CC) insoluble in kerosene.
At first, the Ph.D. student will study several commercially available DGAs in combination with 2 CCs to assess their extraction efficiency and selectivity, followed by the determination of the speciation and the supramolecular assemblies of these systems by various analytical tools. Then, the student will perform the selective stripping of Am by phenanthroline-based hydrosoluble ligands previously tested on Lns by an other student.
The student will eventually gain knowledge on the nuclear fuel cycle, hydrometallurgy and analytical tools that can lead to pursue academic work or to a job in the hydrometallurgy industry.
The candidate is M.Sc. with chemistry as major. A solid basis in analytical chemistry is recommended.

Kinetics and mechanisms of corium leaching

During a severe nuclear accident, significant quantities of "volatile" fission products are released during core degradation. A further significant proportion of radionuclides (RN) from degraded nuclear fuels are incorporated into solidified materials (corium and fuel debris), and their medium- to long-term stability is of major importance for accident site safety. Depending on the progress of the accident and subsequent interactions with different materials (cladding, internal structures, vessel materials and concrete), corium may have different structures and compositions. Generally speaking, it is a highly heterogeneous, multiphase material within which the distribution of radionuclides is poorly understood. In this context, the aim of this thesis is to improve our understanding of the mechanisms of corium leaching and radionuclide release into water, through parametric studies on model materials. This work, carried out with a view to increasing the complexity of the materials studied, will enable us to prioritize the influence of different parameters (temperature, solution composition, presence of oxidizing radiolytic species) on the transfer of RNs into solution. Taken together, these results will improve our understanding of the evolution of the RN inventory contained in a corium in an underwater cooling scenario, and our knowledge of the chemical durability of the different phases it contains.

Candidate profile:
Master II or engineering degree with a specialization in nuclear cycle chemistry or materials chemistry.

Professional value for the candidate:
At the end of this thesis work, the candidate will be able to enhance several technical skills (i) speciation calculations using geochemical calculation software (ii) analysis of solutions by various techniques (ICP-OES, ICP-MS, ion chromatography, UV-vis spectroscopy), (iii) structural and morphological characterizations of materials (DRX, IR and Raman spectroscopies, optical and electron microscopies, X-ray absorption). In addition to the knowledge acquired in the field of materials leaching, this work will provide the candidate with skills in scientific project management (managing different tasks involving several partners, meeting deadlines, production of deliverables, etc.) and scientific communication (oral and written). This thesis work will also enable the candidate to build up a professional network in the nuclear field, and make a name for himself/herself within the CEA and the national and international scientific community.

Direct precipitation of mixed oxides by reductive hydrothermal conversion

The development of new technologies of nuclear reactors implies to consider innovative methods for manufacturing (U,Pu)O2 MOx fuels. In this context, recent works focused on the hydrothermal conversion of tetravalent actinide carboxylates, in particular oxalates. This process enables hydrated actinide oxides to be obtained directly by using "mild" conditions. The characteristics of the powders obtained can also be controlled by adjusting the experimental conditions. However, no study involving the uranyl cation UO22+ has been reported, although several studies have highlighted the reduction of U(VI) by organic matter in geological environments.
The aim of this PhD thesis is therefore to address the direct precipitation, under reductive hydrothermal conditions, of UO2+x and associated solid solutions from uranyl-based solutions. The study of simple systems containing only U(VI) will first be undertaken by considering different sources of organic matter. A multiparametric study will specify the experimental conditions for the reduction of U(VI) to U(IV) and the quantitative formation of UO2+x, while the reduction mechanism will be studied using in situ XANES analyses on ESRF FAME and FAME-UHD beamlines. The second part of the work will concern the study of mixed systems initially containing uranium (VI) and a tetravalent cation. The U(VI)-Th and U(VI)-Ce(IV) systems will be studied as a first approach, in order to progressively increase the complexity of the redox behaviour of the samples. Finally, the study will be transposed to the U(VI)-Pu(IV) system at CEA Marcoule's ATALANTE facility, in collaboration with the DES/DMRC/SPTC/LSEM. Regardless of the chemical system studied, a complete physico-chemical characterisation of the solids obtained will be undertaken. The sintering of the powders prepared will also be studied.
The aim of this thesis work is therefore to propose an alternative route for manufacturing/remanufacturing future-generation nuclear fuels, by offering the original possibility of reducing uranium(VI) in situ in the reactor, which constitutes a direct route from ions in solution to the final solid. The successful candidate will have a master's or engineering degree in radiochemistry, separative chemistry or materials chemistry. In the course of his/her work, he/she will be required to master numerous techniques relating to materials chemistry, microscopy and solution chemistry, which will enable him/her to develop skills not only in the nuclear field, but also more broadly in the field of ceramic materials development.

Development of lipophilic aminopoly(carboxylic) or hydroxamic acid ligands for ion separation

The subject of this thesis has been defined with the aim of proposing new liquid-liquid extraction (LLE) processes applicable to the recycling of lanthanides and actinides at downstream of the nuclear fuel cycle. The proposed methodology relies essentially on the availability of lipophilic ligands derived from highly polar chelating agents such as poly(aminocarboxylic) acids (PACA) or hydroxamic acids (HA). As a first step, the PhD student will be involved in a major organic synthesis project, building up a library of several of these different derivatives. During the first phase of the thesis, the candidate will be allowed to develop his activity based on knowledge and results already acquired in this field at LTSM. Concomitantly to this, the candidate will be asked to characterize the extraction properties (affinity and selectivity) of the yielded ligands towards valuable lanthanides and actinides. This will be carried out using well proven approaches, enabling quantitative and qualitative monitoring of ELL performed on a scale of a few milliliters. The candidate will also study the behavior of ligands in organic solution (before and after extraction) by small-angle X-ray scattering (SAXS). During the second phase of the project, he or she will be asked to develop new, more efficient and, in some cases, stereoselective (enantio- and/or diastereoselective) synthesis strategies when several stereoisomers can be obtained. Characterization of both the extraction properties and organic phase behavior of each stereoisomer will allow to describe the extent of the influence of stereoisomery on these properties. Papers will also be written during this second phase. In the third and final phase, the candidate will focus on writing his/her thesis and related articles.The PhD student will be supervised throughout his thesis by two co-directors and a supervisor, who will provide him with multidisciplinary expertise. The candidate's profile should correspond to that of a PhD student already possessing a strong background in organic synthesis and notions of separative and coordination chemistry. Prone to take an interest in liquid-liquid extraction and the colloidal phenomena that accompany the process, the candidate will have acquired skills that will enable him/her to apply for engineering or research positions in the fields of organic chemistry and/or hydrometallurgy.

Eutectic solvents for leaching and separation of lanthanides and actinides

The separation of chemical elements is at the heart of the mining, the recycling of nuclear and electronic waste and water decontamination. However, hydrometallurgical leaching and liquid-liquid extraction processes use considerable volumes of water and solvents, which are often polluting and volatile. Environmental issues therefore represent a key challenge for future hydrometallurgical processes.
Hydrophobic eutectic solvents have recently been considered for metal extraction. They offer many advantages, such as lower volatility, higher flash point and improved extraction performance, combined with different extraction mechanisms compared with conventional diluents.
The aim of this thesis will therefore be to prepare and test hydrophobic eutectic solvents (HES) based on extractant molecules known in the laboratory, and to study their liquid-liquid extraction and leaching properties with respect to different metals, with particular interest in the extraction of uranium from mines. Extraction mechanisms in these unconventional solvents will be addressed, with close observation of complexation and structuration effects, in order to optimize performance. The extraction properties of uranium and various metals will be evaluated by ICP-OES and compared with the physico-chemical and structural properties of the systems (density and viscosity measurements, acid-base and Karl Fisher titration, NMR and IR spectroscopies, tensiometry, small-angle X-ray and neutron scattering (SAXS, SANS)...).
Requested skills: chemistry, analytical chemistry, physical chemistry, and knowledge of hydrometallurgy.
The applicant will gain experience in separative chemistry, management of scientific projects and will be eligible for engineering and/or research positions in the fields of chemistry and hydrometallurgy.