No description available.

In this thesis, defect formation by a radiation event in the normal MgAl2O4 was investigated using molecular dynamics (MD) simulations. The mechanisms and activation barriers for point defects to diffuse were determined using temperature accelerated dynamics (TAD). The role of cation inversion on defect formation and defect diffusion in spinel-structured oxides was analysed by performing the simulations in three spinels: the normal MgAl2O4, the half-inverse MgGa2O4 and the fully inverse MgIn2O4. The methodology employed in this thesis can be utilised for simulations in other ionic materials.

No description available.

No description available.

Dans le cadre des recherches sur le traitement-recyclage des combustibles nucléaires du futur, la conversion oxalique, éprouvée industriellement pour le plutonium (IV), est le procédé de référence envisagé pour la production finale d’oxydes d’actinides. L’optimisation de ce procédé, basé sur une étape de précipitation oxalique suivie de la calcination en oxyde de l’oxalate précipité, nécessite de maîtriser les propriétés des solides formés dès l’étape initiale de précipitation. La morphologie de l’oxalate, conservée après calcination en oxyde, impacte toutes les étapes du procédé. Dans ce contexte, cette étude s’intéresse à la modulation de la morphologie des oxalates d’actinides tétravalents, thorium (IV) et plutonium (IV), grâce à l’ajout d’additifs organiques dans le milieu de précipitation.Les molécules sélectionnées, choisies parmi les composés dits « CHON » possédant des capacités de complexation et/ou de précipitation des actinides, présentent différents impacts sur la structure cristalline des oxalates précipités, la morphologie des particules, leur taille et/ou leur agglomération. L’analyse du solide, combinée aux mesures de solubilité, de cinétiques de nucléation, de croissance et de noyaux d’agglomération, conduit à proposer des mécanismes pour chaque modulation, sur la base desquels les différences entre oxalate de thorium (IV) et oxalate de plutonium (IV) sont discutées. La méthodologie multidisciplinaire développée dans le cadre de ce travail conduit à des propositions de renforcement de l’impact des additifs utiles dans une perspective de transposition à d’autres systèmes chimiques et notamment d’autres oxalates d’actinides ou de mélanges d’actinides. / Considering the treatment and recycling of future nuclear fuels, the oxalic conversion, which is used at an industrial scale for plutonium (IV) reprocessing, is the synthesis route considered as a reference for the future actinides oxides elaboration. In order to improve this process, based on the actinide oxalate precipitation and the calcination of the precipitate into oxide, a control of the solid properties at the precipitation step is necessary. This is particularly due to the impact of the oxalate morphology at all the stages of the process, up to the calcination at which the oxalate morphology is kept. This study deals with morphology modulations of tetravalent actinides oxalates, thorium (IV) and plutonium (IV), by addition of organic compounds in the precipitation medium. Several impacts of selected “CHON” additives were identified. The molecules, chosen among the actinides complexing and precipitating agents, can act on the crystal structure of the precipitates, the particles morphology, their size and/or their agglomeration. Solid-state analyses combined with the measurements of solubility, nucleation rates, crystal growth rates and agglomeration kernels, allow to shed light on the mechanisms underlying the morphology modulations. This work shows some differences between the behavior of thorium (IV) oxalate and plutonium (IV) oxalate systems. Their origins are discussed in the manuscript. The multidisciplinary methodology developed during this work leads to suggest how to re inforce the additives morphological impacts which can be useful in view of a transposition to other chemical systems, especially other actinides or mixed actinides oxalates.

Depuis 1993, un phénomène de précipitation mettant en jeu le molybdène et le zirconium (produits de fission) conduit à l’encrassement des équipements de dissolution des usines d’AREVA-La Hague. Le précipité formé, ZrMo2O7(OH)2(H2O)2, tend à inclure du plutonium(IV) ce qui impose une gestion de la criticité. La thèse a pour double objectif (i) d’étudier la composition du précipité dans des domaines plus riches en plutonium en prévision du recyclage de combustibles de type MOX ou RNR (ii) d’approfondir la connaissance des mécanismes d’inhibition de la précipitation. L’approche a consisté à caractériser les phases formées lorsque la teneur en plutonium du milieu de dissolution augmente et à déterminer l’évolution de la composition du solide en fonction du temps de séjour des solutions dans les équipements de dissolution. Les travaux ont d’abord été menés en système simulant (molybdate mixte de zirconium et de cérium) puis en actif. L’étude d’une solution visant à inhiber le phénomène d’encrassement et d’inclusion du plutonium dans le solide par ajout de TeVI en solution a montré qu’un tel ajout impacte très fortement le domaine de précipitation des composés de référence. Une nouvelle phase, amorphe aux rayons X, précipite en très faible quantité sur un large domaine de précipitation. Cette phase inclue peu, voire pas, d’élément IV et il convient de parler d’effet « retard » du tellure VI sur la précipitation des molybdates mixtes de zirconium et d’éléments IV : la présence de tellure VI retarde l’atteinte de l’équilibre thermodynamique via la formation d’une phase cinétique amorphe, métastable. / Since 1993, a precipitation phenomenon involving molybdenum and zirconium (produced by fission) leads to the fouling of the Areva La Hague factory dissolution equipments. The formed compound, ZrMo2O7(OH)2(H2O)2, tends to include plutonium (IV) which imposes a management of the criticality. In this way, the objective of these thesis is first to study the composition of the precipitate proceeding form higher plutonium contents in expectation of the recycling of MOX or RNR fuels. Secondly, these work aims at increasing the knowledge of the precipitation inhibition mechanisms. The approach consisted in characterizing the formed crystallographic phases when the plutonium amount in solution increases and determining the solutions residence time in dissolution equipment dependence of the solid composition. At first, the researches were realized in feigning system (zirconium and cerium mixed molybdates) then in radioactive environment. The study of a preventive solution to inhibit the fouling phenomenon and the plutonium inclusion in the solid by addition of TeVI in solution showed that such an addition impacts very strongly the precipitation domains of reference compounds. A new phase, amorphous to the X-rays, precipitates in very small quantity on a wide precipitation domain. This phase includes very low or no amount of tetravalent element and we must talk about a “delay” effect of the tellurium on the mixed zirconium and tetravalent elements molybdates precipitation: the presence of tellurium VI delays the thermodynamic equilibrium achievement through the precipitation of an amorphous kinetic metastable phase.

La filière de l’énergie nucléaire française repose principalement sur les réacteurs à eau pressurisée, utilisant UO2 comme combustible. Si les procédés actuellement utilisés sont suffisamment robustes pour obtenir des pastilles de UO2 de grade suffisant pour une utilisation en réacteur à partir d’une grande variété de concentrés uranifères, des voies alternatives sont développées afin d’anticiper d’éventuelles contraintes économiques et environnementales. L’une d’entre elles repose sur la conversion des oxydes d’uranium en provenance des mines en peroxyde d’uranium UO4∙4H2O (studtite) par ajout de peroxyde d’hydrogène. Cependant, des différences de cinétique de conversion en studtite sont observées entre U3O8 et UO3, ce dernier étant plus réactif que U3O8. Une première étape conduisant à l’oxydation de U3O8 en UO3 dans des conditions industriellement acceptables nécessite d’être explorée. Trois procédés d’oxydation novateurs ont été investigués au cours de cette étude. Un premier axe de recherche s’est focalisé sur l’emploi de l’ozone, un puissant oxydant. Après optimisation du montage permettant l’amélioration du bullage de ce gaz dans une suspension aqueuse de U3O8, il a été montré que cet oxyde se transforme en métaschoepite (UO3∙2H2O). Le couplage de l’ozone et du peroxyde d’hydrogène (procédé peroxone) constitue une deuxième voie permettant d’améliorer la cinétique de conversion de l’oxyde en peroxyde d’uranium UO4∙4H2O. Enfin, l’utilisation conjointe de peroxyde d’hydrogène et des ultrasons, qui provoquent des transformations physiques et chimiques en générant des radicaux dans le milieu, permet d’accélérer significativement la cinétique de conversion en studtite. / French nuclear power industry mainly relies on pressurized water reactors using UO2 fuel. Even if the currently used processes are sturdy enough to obtain UO2 fuel pellets of sufficient grade from a wide variety of uranium concentrates, alternative ways are developed in order to anticipate possible economic and environmental constraints. One of them is based on the conversion of uranium oxides from the mines into uranium peroxide UO4∙4H2O (studtite) by hydrogen peroxide addition. However, studtite conversion kinetics differences are observed when starting from U3O8 or UO3, the latter is more reactive than U3O8. Thus, a first step leading to the oxidation of U3O8 to UO3 under industrially acceptable conditions needs to be explored. Three innovative oxidation processes were investigated in this study. A first axis of research is focused on the use of ozone, a powerful oxidant, alone. After optimization of experimental setup allowing the improvement of the gas bubbling in an aqueous suspension of U3O8, it has been shown that this oxide is transformed into metaschoepite (UO3∙2H2O). The coupling of ozone with hydrogen peroxide (peroxone process) is a second way to improve the conversion kinetics of oxide to uranium peroxide UO4∙4H2O. Finally, the use of ultrasound, which causes physical and chemical transformations by generating radicals in the medium, coupled with hydrogen peroxide allows the conversion kinetics into studtite to be significantly accelerated.

Excited state intramolecular proton transfer (ESIPT) reactions have aroused considerable interest in the last 10-15 years. The ESIPT reaction is normally extremely rapid and yields excited species of considerably lower energy than the initial Franck-Condon excited state. ESIPT has therefore been used for the rapid dissipation of energy (e. g. for polymer protection against UV degradation) and to produce fluorophores with large Stokes shifts. Approximately half of the compounds studied to date involve ESIPT to nitrogen but there has been no real attempt at a coherent study of ESIPT to nitrogen. In this work we have tried to remedy this deficiency by studying a range of compounds of increasing complexity in order to characterise ESIPT and subsequent reactions and to evaluate how these properties vary with molecular structure. The molecules selected for this studyf all into two categories: - The azole group includes the 2-(2'-hydroxyphenyl)-oxazole (HPO) and -thiazole (HPT) whose study complements their well known benzo counterparts (HPBT and HPBO) and the related imidazoles. - The compounds of the pyridine group are related to the basic structure of 2-(2'-hydroxyphenyl)-pyridine (HPP). Structural variations involve benzo fusion (quinolines) and addition of a 3,3' bridge. A complementary compound is the well studied 2,2'-bipyridyl-3,3'-diol (BP(OH)2) which undergoes double proton transfer. Whenever possible, the methoxy counterparts were prepared in order to study the photochemistry of these compounds when ESIPT is not possible. The absorption and fluorescence properties of these systems were studied as a function of solvent, temperature and pH conditions. Lifetimes, quantum yields and pKa data were determined under these various conditions and semi-empirical quantum chemical calculations were performed on each system.

Les verres de phosphates de molybdène sont des matériaux d’intérêts pour le stockage des déchets nucléaires et pour le développement de batterie à électrolyte solide. Les propriétés thermiques et électriques des molybdophosphates vitreux dépendent fortement de la structure locale des atomes de molybdène mais la coordination de ce métal de transition fait toujours l’objet de débat. La spectroscopie RMN 95Mo est une technique de caractérisation attractive pour sonder l’environnement local car cet isotope possède une large gamme de déplacement chimique (~8000 ppm). De plus le molybdène est un noyau quadripolaire. Il existe peu de données reliant les paramètres RMN à la structure des molybdophosphates donc nous avons étudié la structure locale de composés modèles cristallisés. Notre méthodologie est de combiner la RMN expérimentale avec des calculs à l’aide de l’algorithme GIPAW (approche périodique) pour mieux comprendre l’influence de l’environnement local sur l’évolution des paramètres RMN (déplacement chimique, anisotropie de déplacement chimique et constante de couplage quadripolaire). Nous avons établi des corrélations entre la coordination du molybdène et les paramètres RMN anisotropes. Au cours de ce travail, des calculs DFT avec la méthode GIAO (approche moléculaire) ont permis de démontrer que la constante de couplage quadripolaire est un paramètre semi-local. L’étude des composés cristallisés a permis de définir des relations entre la structure des verres de molybdophosphates et les propriétés thermiques, électriques et les mécanismes de cristallisation. / Molybdenum phosphate glasses are materials of interest for nuclear waste storage and solid-state electrolytes. Thermal and electrical properties of vitreous molybdophosphates are highly dependent on molybdenum local environment. 95Mo is an attractive NMR-active isotope with a wide chemical shift range (over 7000 ppm). 95Mo is a quadrupolar nucleus (I=5/2). This interaction is known to be an invaluable probe to local changes in materials. Data linking 95Mo NMR parameters to the structure of molybdophosphates are scarce, so we studied model compounds. Our methodology is to combine experimental NMR and DFT calculations using GIPAW algorithm (periodic approach) to obtain a better knowledge of the evolution of NMR parameters (chemical shift, chemical shift anisotropy and quadrupolar coupling constant). We have established correlations between molybdenum coordination number and anisotropic NMR parameters. During this work, DFT calculations using GIAO method (molecular approach) demonstrated that quadrupolar coupling constant is a semi-local parameter. Crystal compounds study allowed us to define relationships between the structure of molybdophosphate glasses and thermal, electrical and crystallization mechanisms.

In an effort to be able to use photoinitiators to their maximum potential, the sequence of events that occurs in an ink formulation during the UV curing process has have been studied and information presented to allow more effective formulation. Emphasis has been placed on highlighting the variables that have the greatest impact both on photoinitiator efficiency and on the suitability of individual photoinitiators and synergists for use in particular applications. These variables were found to be photoinitiator thermal stability, UV light utilisation, reaction mechanisms and cure reactivity. A wide range of photoinitiators and synergists were investigated using thermogravimetric analysis (TGA) and thermogravimetric analysis-mass spectroscopy (TGA-MS) to define both their thermal stability and whether under heating they thermally decompose or merely evaporate. Differential photocalorimetry (DPC) was used to determine which wavelengths from a typical medium pressure mercury curing lamp are the most important for providing cure, with both theoretical and practical methods being used to define the extent to which these wavelengths penetrate into pigmented and non-pigmented coatings. A procedure was devised and validated for this purpose. The reaction mechanism and photodecomposition products of a range of photoinitiators were investigated using gas chromatography-mass spectroscopy (GC-MS) and radical trapping experiments. The reaction mechanisms are discussed in terms of available literature knowledge. Evidence is also presented suggesting that, with only particular exceptions, cleavage photoinitiators can also react by a hydrogen abstraction mechanism in the presence of an amine synergist. A real time infrared spectrometer (RTIR) was set up and a method validated for following the UV curing reaction through changes in the acrylate double bond concentration. The advantages and disadvantages of this instrument are discussed in terms of other similar instruments reported in the literature, and the technique subsequently used to measure the reaction rates of a wide range of photoinitiators. Other factors such as photoinitiator concentration, amine synergist type I level and formulation viscosity were also investigated to determine their influence on the cure process.