A neutron flux on the order of 2·10²° neutrons/cm² at 0.18 MeV induces formation of point defects (vacancies and interstitials) in single crystal 3C-SiC causing a volume lattice expansion (swelling) of over 3% that can be measured by X-Ray diffraction. The crystal lattice can be completely restored with an annealing temperature equal to or higher than the irradiation temperature. This phenomenon serves as a basis for temperature measurements and allows the determination of the maximum temperature, if the exposure time is known. The single crystal 3C-SiC sensor is applicable to small, rotating and hard to access parts due to its size of 300-500 microns, wide temperature range of 100-1450 °C, "no-lead" installation, inert chemical properties and high accuracy of temperature measurements. These features make it possible to use the sensor in gas turbine blades, automotive engines, valves, pistons, space shuttle ceramic tiles, thermal protection system design, etc. This work describes the mechanism of neutron irradiation of single crystal 3C-SiC, the formation of point defects and their concentration, the different temperature measurement techniques, and the application of Maximum Temperature Crystal Sensors (MTCS) for maximum temperature measurements in both stationary and non-stationary regimes.

Zirconium carbide (ZrC) powders were produced by carbothermal reduction reactions using fine-scale carbon/metal oxide mixtures as the starting materials. The reactant mixtures were prepared by pyrolytic decomposition of solution-derived precursors. The latter precursors were synthesized via hydrolysis/condensation of metal-organic compounds. The first step in the solution process involved refluxing zirconium alkoxide with 2,4 pentanedione ("acacH") in order to partially or fully convert the zirconium alkoxy groups to a chelated zirconium diketonate structure ("zirconium acac"). This was followed by the addition of water (under acidic conditions) in order to promote hydrolysis/condensation reactions. Precursors with variable carbon/metal ratios were produced by varying the concentrations of the solution reactants (i.e., the zirconium alkoxide, "acacH," water, and acid concentrations.) It was necessary to add a secondary soluble carbon source (i.e., phenolic resin or glycerol) during solution processing in order to obtain a C/Zr molar ratio close to 3 (as required for stoichiometry) in the pyrolyzed powders. The phase development during carbothermal reduction was investigated for oxide-rich carbon-deficient and slightly carbon-rich compositions. The reaction was substantially completed after heat treatments in the range of ~1400-1500oC. The crystallite sizes were in the range of ~100-130 nm. However, some oxygen dissolved in the lattice and some free carbon was present. Heat treatment at temperatures >1600oC was required to complete the reaction. The dry-pressed powder compacts, with varying C/Zr molar ratios, were pressureless sintered to relative densities in the range of ~98-100% at 1950oC.

Apatite group of minerals have been widely used in applications like detoxification of wastes, disposal of nuclear wastes and energy applications in addition to biomedical applications like bone repair, substitution, and coatings for metal implants due to its resemblance to the mineral part of the bone and teeth. X-ray diffraction patterns of bone are similar to mineral apatites such as hydroxyapatite and fluorapatite. Formation and physicochemical properties of apatites can be understood better by computer modeling. For this reason, lattice parameters of possible apatite compounds (A10(BO4)6C2), constituted by A: Na+, Ca2+, Ba2+, Cd2+, Pb2+, Sr2+, Mn2+, Zn2+, Eu2+, Nd3+, La3+, Y3+ / B: As+5, Cr+5, P5+, V5+, Si+4 / and C: F-, Cl-, OH-, Br-1 were predicted from their elemental ionic radii by artificial neural networks techniques. Using artificial neural network techniques, prediction models of lattice parameters a, c and hexagonal lattice volumes were developed. Various learning methods, neuron numbers and activation functions were used to predict lattice parameters of apatites. Best results were obtained with Bayesian regularization method with four neurons in the hidden layer with &lsquo / tansig&rsquo / activation function and one neuron in the output layer with &lsquo / purelin&rsquo / function. Accuracy of prediction was higher than 98% for the training dataset and average errors for outputs were less than 1% for dataset with multiple substitutions and different ionic charges at each site. Non-stoichiometric apatites were predicted with decreased accuracy. Formulas were derived by using ionic radii of apatites for lattice parameters a and c.

No description available.

The structural and mechanical properties of s-I methane hydrate have been investigated by first principles calculations. For the first time, the fully elastic constant tensor of s-I methane hydrate is obtained entirely ab-initio. The calculated lattice parameter, bulk modulus, and elastic constants were found to be in good agreement with experimental data at ambient pressure. The Young modulus, Poisson ratio and bulk sound velocities are estimated from the calculated elastic constants and compared with wave speed measurements available.

No description available.

No description available.

This thesis work reports in depth analyses of measured µ-XRD and µ-XAS data from standard UO2, chromia (Cr2O3) doped UO2 and MOX fuels, and interpretation of the results considering the role of chromium as a dopant as well as several fission product elements. The lattice parameters of UO2 in fresh and irradiated samples and elastic strain energy densities in the irradiated UO2 samples have been measured and quantified. The µ-XRD patterns have further allowed the evaluation of the crystalline domain size and sub-grain formation at different locations of the irradiated fuel pellets. Attempts have been made to determine lattice parameter and next neighbor atomic environment in chromia-precipitates found in fresh chromia-doped fuel pellets. The local structure around Cr in as-fabricated chromia-doped UO2 matrix and the influence of irradiation on the state of chromium in irradiated fuel matrix have been addressed. Finally, for a comparative understanding of fission gases behavior and irradiation induced re-solution phenomenon in standard and chromia-doped UO2, the last part of the present work tries to clarify the fission gas Kr atomic environment in these irradiated fuels. The work performed on Kr, by micro-beam XAS, comprises the determination of Kr next neighbor distances, an estimation of gas atom densities in the aggregates, and apparent internal pressures in the gas bubbles.

Cette thèse présente une analyse approfondie et comparative des résultats de mesures µ-XRD et µ-XAS sur des combustibles UO2 standard, dopé au sesquioxyde de chrome (Cr2O3) et MOX, irradiés ou non. Elle présente également l'interprétation des résultats en regard des effets induits par le chrome en tant que dopant ainsi que par la présence de plusieurs produits de fission. Les paramètres de maille de l’UO2 et les paramètres de densité d'énergie de déformation élastique dans les matériaux irradiés ou non ont été mesurés et quantifiés. Les données de µ-XRD ont en outre permis l'évaluation de la taille des domaines cristallins, ainsi que l’étude de la formation de sous-grains à différentes positions au sein des pastilles de combustibles irradiés. Le paramètre de maille et l'environnement atomique local du chrome dans des précipités d’oxyde de chrome présents dans les pastilles de combustible non-irradié ont également été déterminés. La structure locale du Cr dans la matrice du combustible dopé et l'influence de l'irradiation sur l'état du chrome dans la matrice de combustible ont été étudiées. Enfin, pour une comparaison du comportement des gaz de fission et du phénomène de re-solution induite par l'irradiation dans l’UO2 standard ou dopé, la dernière partie de ce travail propose une tentative d'analyse de l’environnement atomique du Kr dans ces deux combustibles irradiés. Le travail effectué par micro-faisceau XAS sur ce gaz de fission a permis la détermination des distances du Kr avec ses proches voisins, une estimation des densités atomiques des gaz de fission dans les agrégats et des pressions internes apparentes dans ces nano-phases de gaz inertes. / This thesis work reports in depth analyses of measured µ-XRD and µ-XAS data from standard UO2, chromia (Cr2O3) doped UO2 and MOX fuels, and interpretation of the results considering the role of chromium as a dopant as well as several fission product elements. The lattice parameters of UO2 in fresh and irradiated samples and elastic strain energy densities in the irradiated UO2 samples have been measured and quantified. The µ-XRD patterns have further allowed the evaluation of the crystalline domain size and sub-grain formation at different locations of the irradiated fuel pellets. Attempts have been made to determine lattice parameter and next neighbor atomic environment in chromia-precipitates found in fresh chromia-doped fuel pellets. The local structure around Cr in as-fabricated chromia-doped UO2 matrix and the influence of irradiation on the state of chromium in irradiated fuel matrix have been addressed. Finally, for a comparative understanding of fission gases behavior and irradiation induced re-solution phenomenon in standard and chromia-doped UO2, the last part of the present work tries to clarify the fission gas Kr atomic environment in these irradiated fuels. The work performed on Kr, by micro-beam XAS, comprises the determination of Kr next neighbor distances, an estimation of gas atom densities in the aggregates, and apparent internal pressures in the gas bubbles.