Ce travail a été réalisé en cotutelle entre l’Université Charles à Prague (République Tchèque) et l'Université de Strasbourg (France). Il a également profité d’une coopération entre l'Institut de Chimie Inorganique de l'Académie des Sciences de République Tchèque et le Commissariat à l'Énergie Atomique et aux Énergies Alternatives (CEA, Cadarache, France). Les résultats des travaux ont contribué au projet OCDE / AEN Serena 2 (Programme portant sur l’étude des effets d'une explosion de vapeur dans un réacteur nucléaire à eau). La thèse présentée se situe dans le domaine de la sûreté nucléaire et de la science des matériaux. Elle traite de l’Interaction Combustible-Réfrigérant (ICR, ou FCI en anglais pour Fuel-Coolant Interaction) susceptible d’intervenir lors d’un accident grave de réacteur nucléaire et actuellement à l’étude dans les programme de R&D. Au cours d’un accident de fusion d’un coeur de réacteur, les matériaux fondus peuvent interagir avec le liquide de refroidissement (eau légère), aussi appelé réfrigérant. Cette interaction peut se produire à l'intérieur de la cuve ou, en cas de rupture de celle-ci, à l'extérieur. Ces deux scénarios sont couramment appelés Interaction Combustible-Réfrigérant en- et hors- cuve et se distinguent de par les conditions du réacteur lors de l’accident : pression du système, degré de sous refroidissement de l’eau, etc. L'interaction entre le combustible fondu et le liquide de refroidissement peut évoluer vers une détonation thermique appelée «explosion de vapeur» qui peut endommager le réacteur, voire compromettre l'intégrité du confinement. Des expériences récentes ont montré que la composition du combustible a un effet majeur sur l’apparition et le rendement d’une telle explosion. En particulier, des comportements différents ont été observés entre un matériau simulant, l'alumine, qui explose très facilement, et diverses compositions de corium prototypique (80 m. % UO2, 20% m.% ZrO2). Cet «effet matériau» a suscité un intérêt nouveau pour les analyses post-expériences des débris issus de l’ICR afin de déterminer les mécanismes qui interviennent au cours de ces phénomènes extrêmement rapides. La thèse est organisée en neuf chapitres. Le chapitre 1 constitue une introduction générale et présente le contexte d’un accident grave d’un réacteur nucléaire. Quelques exemples d’accidents graves (Three Miles Island 1979, Tchernobyl 1986 et Fukushima 2011) sont brièvement abordés. Le chapitre 2 résume les aspects théoriques de l'interaction combustible-réfrigérant. Il est divisé en quatre parties correspondant aux quatre étapes généralement rencontrées lors du mécanisme d’ICR i) Prémélange - le combustible fondu, versé dans l'eau, se fragmente en gouttelettes grossières qui s’isolent d’un film de vapeur. ii) Déclenchement – le film de vapeur entourant les gouttes de combustible est déstabilisé, permettant ainsi la fragmentation fine du combustible. iii) Propagation - la fragmentation du combustible se propage à l’ensemble du prémélange, augmentant ainsi la surface de contact entre le combustible fondu et l’eau. Ceci conduit à une production intense de vapeur à grande échelle. iv) Expansion (explosion) - l'énergie thermique transférée du combustible à l'eau est transformée en travail mécanique de la vapeur.[...] / This work has been performed under co-tutelle supervision between Charles University in Prague (Czech Republic) and Strasbourg University (France). It also profited from the background and cooperation of Institute of Inorganic Chemistry Academy of Science of the Czech Republic and French Commission for Atomic and Alternative energies (CEA Cadarache). Results of the work contribute to the OECD/NEA project Serena 2 (Program on Steam Explosion Resolution for Nuclear Applications).Presented thesis can be classed in the scientific field of nuclear safety and material science. It is aimed on the socalled “molten nuclear Fuel – Coolant Interaction” (FCI) that belongs among the recent issues of the nuclear reactorsevere accident R&D. During the nuclear reactor melt down accident the melted reactor load can interact with the coolant (light water). This interaction can be located inside the vessel or outside in the case of vessel break-up. These two scenarios are commonly called in- and ex-vessel FCI and they differ in the conditions such as initial pressure of the system, water sub-cooling etc. The Molten fuel – coolant interaction can progress into thermal detonation called “steam explosion” that can challenge the reactor or containment integrity.Recent experiments have shown that the melt composition has a major effect on the occurrence and yield of such explosion. In particular, different behaviors have been observed between simulant material (alumina), which has important explosion efficiency, and some prototypic corium compositions (80 w. % UO2, 20% w. % ZrO2). This “material effect” has launched a new interest in the post-test analyses of FCI debris in order to estimate the processes occurring during these extremely rapid phenomena. The thesis is organized in nine chapters. The chapter 1 gives the general introduction and context of the nuclear reactor accident. Major nuclear accidents (Three Miles Island 1979, Chernobyl 1986 and Fukushima 2011) are briefly described. The chapter 2 summarizes the theoretical aspects of the fuel – coolant interaction. It is divided in four thematic fields according to the FCI progression. In general, FCI has four stages: i) Premixing – hot melt is poured in water and fragmented in coarse droplets surrounded by steam filmii) Triggering – steam film around melt droplets is destabilized allowing fine fragmentation iii) Propagation – the fine fragmentation propagate through the premixture increasing the melt – water interface area, which leads to large steam production iv) Expansion (explosion) – Thermal energy transferred from the melt to water is changed into mechanical workof the steam.The chapter 3 summarizes the research conducted in different experimental facilities using nonradioactive simulant or radioactive prototypic materials. The chapter 4 shows the results of thermodynamic calculations, by which thepossible chemici reactions between melts and water/steam at high temperatures were modeled. Second part presentsthe results of 1D calculations of radiation heat transfer from FCI materials to water/steam. The chapter 5 describes the material analyses of non-radioactive simulant debris coming from MISTEE experimental research program (KTH, Sweden) and PREMIX, ECO facilities (FZK, Germany). The chapters 6 to 8 describe the material analyses of radioactive prototypic debris coming from KROTOS research program (CEA, France). The KROTOS KS2 test used melt composition 70 w. % UO2 and 30 w. % ZrO2, the KS4 test 80 w. % UO2 and 20 w. % ZrO2, the last KS5 test used suboxidized melt 80.1 w. % UO2 and 11.4 w. % ZrO2 and 8.5 w. % metallic Zr. The chapter 9 concludes the work and presents future perspectives.

Der Magnesiabaustoff, dessen Bindemittelphasen basische Magnesiumsalz-Hydrate sind, wird aufgrund seiner Beständigkeit gegenüber salinaren Lösungszusammensetzungen zur Errichtung geotechnischer Barrieren im Salzgestein eingesetzt. Der Nachweis der Langzeitstabilität kann über das thermodynamische Gleichgewicht zwischen potentieller Zutrittslösung und vorliegender Bindemittelphase(n) im Baustoff erfolgen. Im Rahmen dieser Arbeit wurden zum einen Löslichkeitsdaten im quaternären System Mg2+ / Cl-, OH-, SO42- // H2O bei 25 °C und 40 °C experimentell ermittelt. Zum anderen wurde der Bindemittelphasenbestand nach Abschluss der Abbindereaktion bestimmt. Dazu wurde der Phasenbestand rezeptur- und temperaturabhängig (unterschiedliche Tmax) mittels Röntgenpulverdiffraktometrie über den gesamten Abbindeprozess hinweg analysiert, über die RIETVELD-Methode quantifiziert sowie die Gefüge- und Volumenentwicklung verfolgt. Sowohl die nunmehr verfügbaren Löslichkeitsdaten im quaternären System Mg2+ / Cl-, OH-, SO42- // H2O als auch der erhaltene Kenntnisstand zum rezepturtypbedingten Abbindeverhalten und resultierende(n) Bindemittelphase(n) erlauben eine sichere Bewertung der Langzeitbeständigkeit des Magnesiabaustoffes unter salinaren Bedingungen.

The Scandinavian Caledonides form a mountain range comprising nappe stacks of numerous far-travelled thrust sheets. The thrust sheets consist of diverse lithologies representing pre- and synorogenic sedimentary and igneous rocks subsequently metamorphosed to various degrees, from the Late Neoproterozoic to Middle Devonian. In particular, (ultra)-high-grade metamorphic rocks have been recorded in the Seve Nappe Complex (SNC), extending >1000 km along strike of the Scandinavian Caledonides. Included in the SNC of northern Sweden is the Vássačorru Igneous Complex (VIC), consisting of bimodal magmatic suites, that formed c. 845 Ma. Fieldwork was conducted in the Kebnekaise mountains of northern Sweden, focusing on the high-grade Aurek metagabbro within the VIC of the SNC. Aurek is a key locality representing both initial stages of Iapetus Ocean formation in the Ediacaran and later stage Caledonian subduction affinities, from the collision between Laurentia and Baltica. In this study, petrological description, zircon U-Pb geochronology, mineral chemistry analysis, whole rock composition, and thermodynamic modeling was performed. Zircon U-Pb geochronology yielded protolith ages of 609±2.5 Ma, and 614±2.3 Ma, suggesting the Aurek metagabbro to not be part of the VIC, as has previously been described. The age of Aurek can instead be correlated to the Kebnekaise Dyke Swarms at c. 607 Ma, in the Kebnekaise mountains. Whole rock major and trace element data of e.g., Al2O3 (15.0 – 25.0 ppm) versus SiO2 (46.0 – 53.0 ppm), Rb (2.0 – 18.0 ppm), Zr (8.0 – 58.0 ppm) versus Y (2.7 – 18.0 ppm), Th/Yb ratio 0.25 – 2.0 and Nb/Yb ratio 1.30 – 5.14, indicate assimilation of continental crust. These major and trace element signatures show that the protolith of the Aurek metagabbro probably was emplaced in a continental rift setting in the Ediacaran. Semi-quantitative thermodynamic modeling from this study present blueschist to amphibolite facies conditions for the Aurek metagabbro at 11.8 – 12.6 kbar and 480 – 565 oC, confirming the unit experienced subduction, possibly in the Late Cambrian to Early Ordovician. The metamorphic grade and protolith age show similar features to correlative rock sequences in the Tsäkkok Lens, south of Aurek, in Norrbotten. Consequently, this study concludes that subduction, exhumation and subsequent deformation for Aurek, probably was equivalent to those of the Tsäkkok Lens, extending the HP affinities of the SNC further north in the Swedish Caledonides. / Skandinaviska Kaledoniderna utgör en bergskedja bestående av olika skollor som transporterats hundratals kilometer från sin ursprungskälla. Skollorna består av varierande bergarter som representerar olika utvecklingsskeden i formationen av Kaledoniderna under senare Neoproterozoikum och mellan Devon. Utmärkande har höggradiga metamorfiska bergarter återfunnits i Seveskollan som sträcker sig >1000 km längs med strykningsriktningen av de Skandinaviska Kaledoniderna. I norra Sverige inkluderar Seveskollan det magmatiska Vássačorru-komplexet, bestående av bimodal magmatism som bildats ca 845 Ma. Fältarbete utfördes kring Kebnekaisebergen i norra Sverige, med fokus på höggradig metagabbro från Aurek, ett område inom det magmatiska Vássačorru-komplexet. Aurek är ett viktigt område som representerar både initiala stadier av Iapetushavets bildande och efterföljande formationer från kollisionen mellan Laurentia och Baltica plattorna. I denna studie utfördes petrologisk beskrivning av mineral, U-Pb geokronologi av zirkon, kemisk analys av mineral och bulkkomposition av bergarter, samt termodynamisk modellering. U-Pb dateringen av zirkon resulterade i en ursprungsålder på 609±2,5 Ma och 614±2,3 Ma för metagabbro från Aurek. Detta indikerar att metagabbro i Aurek inte är en del av det magmatiska Vássačorru-komplexet, något som tidigare antagits. Åldern kan istället korreleras till Kebnekaise-gångkomplexet med en ålder på ca 607 Ma. Huvud- och spårelement i Aureks metagabbro tyder på assimilering av kontinentalskorpa, vilket föreslår att ursprungsbergarten till metagabbro i Aurek bildades i en kontinental spridningszon. Den termodynamiska modelleringen resulterade i metamorfiska förhållanden på mellan 11,8 – 12,6 kbar och 480 – 565 oC för bergarterna, vilket påvisar att den tektoniska miljön som senare präglat bergarterna förmodligen var associerad med en subduktionszon.

<p>Ni-based superalloys continue to exert themselves as the industry standards in high stress and highly corrosive/oxidizing environments, such as are present in a gas turbine engine, due to their excellent high temperature strengths, thermal and microstructural stabilities, and oxidation and creep resistances. Gas turbine engines are essential components for energy generation and propulsion in the modern age. However, Ni-based superalloys are reaching their limits in the operating conditions of these engines due to their melting onset temperatures, which is approximately 1300 °C. Therefore, a new class of materials must be formulated to surpass the capabilities Ni-based superalloys, as increasing the operating temperature leads to increased efficiency and reductions in fuel consumption and greenhouse gas emissions. One of the proposed classes of materials is termed refractory complex concentrated alloys, or RCCAs, which consist of 4 or more refractory elements (in this study, selected from: Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) in equimolar or near-equimolar proportions. So far, there have been highly promising results with these alloys, including far higher melting points than Ni-based superalloys and outstanding high-temperature strengths in non-oxidizing environments. However, improvements in room temperature ductility and high-temperature oxidation resistance are still needed for RCCAs. Also, given the millions of possible alloy compositions spanning various combinations and concentrations of refractory elements, more efficient methods than just serial experimental trials are needed for identifying RCCAs with desired properties. A coupled computational and experimental approach for exploring a wide range of alloy systems and compositions is crucial for accelerating the discovery of RCCAs that may be capable of replacing Ni-based superalloys. </p> <p>In this thesis, the CALPHAD method was utilized to generate basic thermodynamic properties of approximately 67,000 Al-bearing RCCAs. The alloys were then down-selected on the basis of certain criteria, including solidus temperature, volume percent BCC phase, and aluminum activity. Machine learning models with physics-based descriptors were used to select several BCC-based alloys for fabrication and characterization, and an active learning loop was employed to aid in rapid alloy discovery for high hardness and strength. This method resulted in rapid identification of 15 BCC-based, four component, Al-bearing RCCAs exhibiting room-temperature Vickers hardness from 1% to 35% above previously reported alloys. This work exemplifies the advantages of utilizing Integrated Computational Materials Engineering- and Materials Genome Initiative-driven approaches for the discovery and design of new materials with attractive properties.</p> <p> </p> <p><br></p>

[pt] Resíduos de biomassas são de grande interesse por serem matérias primas para a geração de energia renovável. Neste trabalho foram estudados os processos de conversão térmica de pirólise lenta e de carbonização hidrotermal, empregando os resíduos de bagaço de cana-de-açúcar, bagaço de malte e casca de coco. A cinética e a termodinâmica das reações foram determinadas experimentalmente pelo cálculo da energia de ativação, fator de frequência, energia livre de Gibbs, entalpia e entropia do complexo ativado. A pirólise foi avaliada empregando a termogravimetria em atmosfera inerte, para uma faixa de temperatura do ambiente até 1.000 graus Celsius e aplicando diferentes taxas de aquecimento. A carbonização hidrotermal procedeu em um reator tipo autoclave Parr modelo 452HC2, em diferentes temperaturas e tempos de operação. Modelos cinéticos da pirólise para métodos Model-free, denominados Kissinger, Flynn- WallOzawa (FWO) e Kissinger-Akahira-Sunose (KAS), e Model-fitting, intitulado Coats-Redfern, foram investigados. Para os modelos KAS, FWO e Coats-Redfern foram analisados 19 tipos de mecanismos reacionais e respectivos coeficientes de determinação (R2) foram determinados. O bagaço de cana-de-açúcar se ajustou ao método de Kissinger (R2 de 0,9973) e ao Coats-Redfern, entretanto os métodos FWO e KAS não se adequam a este material. Os testes com bagaço de malte e casca de coco se ajustaram a todos os métodos aplicados, apresentando valores de R2 elevados (0,9 a 0,9999). O modelo para a cinética da carbonização hidrotermal foi aplicado, onde as três biomassas apresentaram a ordem variando de 3 a 3,49, valor compatível com a literatura. Adicionalmente foram feitas caracterizações físico-químicas nos resíduos de biomassa e nos biocarvões produzidos na carbonização hidrotermal, incluindo análise elementar e imediata, microscopia eletrônica de varredura, espectroscopia de infravermelho, densidade aparente, pH, condutividade e poder calorífico. / [en] Biomass residues are of great interest because they are raw materials for the generation of renewable energy. In this work, the processes of thermal conversion of slow pyrolysis and hydrothermal carbonization were studied using the residues of sugarcane bagasse, malt bagasse and green coconut shell. The kinetics and thermodynamics of the reactions were determined experimentally by calculating the activation energy, frequency factor, Gibbs free energy, enthalpy and entropy of the activated complex. Pyrolysis was evaluated using thermogravimetry in an inert atmosphere, from a room temperature up to 1,000 Celsius and applying different heating rates. Hydrothermal carbonization was carried out in a Parr autoclave type 452HC2 reactor, at different temperatures and times of operation. Kinetic models of pyrolysis for Model-free methods, called Kissinger, Flynn-WallOzawa (FWO) and Kissinger-Akahira-Sunose (KAS), and Model-fitting, entitled Coats-Redfern, were investigated. For the KAS, FWO and Coats-Redfern models, 19 types of reaction mechanisms were analyzed and their determination coefficients (R2) were evaluated. Sugarcane bagasse was adjusted to the Kissinger method (R2 equal to 0.9973) and Coats-Redfern, however the FWO and KAS methods are not suitable for this material. The tests with malt bagasse and coconut fiber were adjusted to all applied methods, showing high R2 values (0.9 to 0.9999). The model for the kinetics of hydrothermal carbonization was applied, where the three biomasses presented the order varying from 3 to 3.49, a value compatible with the literature. In addition, physical-chemical characterizations were carried out for the biomass residues and biochar produced by hydrothermal carbonization, including elementary and immediate analysis, scanning electron microscopy, infrared spectroscopy, apparent density, pH, conductivity and calorific value.