Results 1 - 10 of 1557
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[en] Uranium, plutonium, and several radionuclide surrogates have been examined in a range of glass wasteform materials by x-ray absorption fine structure (XAFS) to improve the understanding of specific effects of glass melt processing conditions on the speciation of the metal ion. The glass melts have been subject to systematic compositional, radionuclide loading, and oxidation reduction potential variations during processing. The speciation of both uranium and plutonium responds to glass melt processing redox conditions and shows trends with increased radionuclide loading. A comparative investigation of the redox properties of plutonium and cerium, a plutonium surrogate, has been completed in a series of related glass wasteforms. The observed difference in oxidation states between the two metal ions in these wasteform materials has been successfully interpreted from consideration of the melt glass thermodynamics
[en] Depleted uranium nitride (UN) kernels with diameters ranging from 420 to 858 microns and theoretical densities (TD) between 87 and 91 percent were postprocessed using a hot isostatic press (HIP) in an argon gas media. This treatment was shown to increase the TD up to above 97%. Uranium nitride is highly reactive with oxygen. Therefore, a novel crucible design was implemented to remove impurities in the argon gas via in situ gettering to avoid oxidation of the UN kernels. The density before and after each HIP procedure was calculated from average weight, volume, and ellipticity determined with established characterization techniques for particle. Furthermore, micrographs confirmed the nearly full densification of the particles using the gettering approach and HIP processing parameters investigated in this work.
[en] A two-step processing was developed to prepare Yb2Si2O7-SiC nanocomposites. Yb2Si2O7-Yb2SiO5-SiC composites were first fabricated by a solid state reaction/hot-pressing method. The composites were then annealed at 1250°C in air for 2 h to activate the oxidation of SiC, which effectively transformed the Yb2SiO5 into Yb2Si2O7. The surface cracks purposely induced can be fully healed during the oxidation treatment. The treated composites have improved flexural strength compared to their pristine composites. As a result, the mechanism for crack-healing and silicate transformation have been proposed and discussed in detail.
[en] Large, hard ZrO2 agglomerates remained in an Al2O3/ZrO2 composite suspension after inefficient ball-milling. The ZrO2 agglomerates shrank away from the consolidated Al2O3/ZrO2 powder matrix during sintering, producing cracklike voids which were responsible for strength degradation
[en] The growth of alpha-Al2O3 from a planar specimen of thermally grown gamma-alumina on a molybdenum transmission electron microscope grid was studied. The alpha-Al2O3 grows into the transition alumina matrix and then thickens via a ledge growth mechanism. Faceted Mo crystallites cause pinning of alpha-Al2O3 ledges and are larger on alpha-Al2O3 than on the transition alumina matrix. 8 references
[en] Stress created by differential sintering, due to differences in initial bulk density, were determined, to an order of magnitude, by an experiment which estimated the differential sintering phenomenon on a macroscopic level. The experiments entailed determining the shrinkage rates of a powder isostatically pressed to two bulk densities. Using this information, stresses were determined by forcing the slower-densifying compact to shrink at the same rate as the faster-densifying compact and measuring the resulting forces with a load cell. Maximum stresses (between 1 and 3 MPa) were observed to occur in the intermediate stage of densification. Despite larger differential strains at higher temperatures, stresses decreased to zero at the latter stage of densification. Viscoelastic experiments, of the stress-relaxation type, were performed. Results showed that the sintering specimen was more rigid at lower temperatures and more fluid-like at higher temperatures. This explains the development of maximum stresses at intermediate temperatures
[en] Structural integrity of SiC layers on TRISO-coated HTGR fuel particles is a critical property necessary to ensure fission-product retention. Characterization and detection of microcracks in SiC layers by high-pressure (69 MPa) mercury intrusion is discussed. Particular emphasis is placed on relating metallic and gaseous fission-product release to the level of defective SiC layers. Gaseous fission-product release during irradiation is shown to be more accurately predicted when the fraction of defective SiC layers as determined by mercury intrusion is taken into account. 11 refs
[en] Simple thermodynamic means are described for understanding and predicting the influence of temperature changes, in various environments, on electronic properties of ceramics. Thermal gradients, thermal cycling, and vacuum annealing are discussed, as well as the variations of ctivities and solubilities with temperature. 7 refs
[en] The migration of gas-filled pores in KCl subjected to a temperature gradient occurs by a vaporization-condensation mechanism. In many cases the vaporization step, rather than gas-phase diffusion, can limit the rate of migration. Movement of large (>100 /mu/m), argon-filled pores in pure and Ba-doped KCl was observed using hot-stage microscopy. Velocities of large pores in pure KCl were limited by the diffusion rates of KCl monomers and dimers in argon. This observation is consistent with a model in which the vaporization rate is moderately reduced due to widely spaced surface ledges, but is still rapid with respect to gas-phase diffusion. The velocities of large pores in 10 and 40 ppm Ba-doped samples were significantly lower than those in pure samples. A model was developed which explains these decreased migration rates in terms of impurity poisoning of surface kink sites. The model couples detailed expressions for vaporization kinetics with those for gas-phase diffusion rates. 37 refs