Browsing by Subject "uranium"
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Item Atomic Diffusion in the Uranium-50wt% Zirconium Nuclear Fuel System(2013-06-17) Eichel, DanielAtomic diffusion phenomena were examined in a metal-alloy nuclear fuel system composed of ?-phase U-50wt%Zr fuel in contact with either Zr-10wt%Gd or Zr-10wt%Er. Each alloy was fabricated from elemental feed material via melt-casting, and diffusion samples of nominal 1.5 mm thickness were prepared from the resulting alloy slugs. The samples were assembled into diffusion couples and annealed for periods of 14, 28, and 56 days at temperatures of 550?C, 600?C, and 650?C. Thus, the U-50Zr/Zr-10Er system and the U-50Zr/Zr-10Gd system were each annealed for three different time periods at each of three different temperatures, for an initial total of 18 diffusion interfaces that were to be studied. In practice, data was collected from only 12 of the 18 interfaces. At 650?C, the U-50wt%Zr alloy exists in the ?-phase region, which enabled the comparison of diffusion behavior between the ? phase and ? phase. Diffusion samples were examined by collecting composition profiles across the diffusion interface for each element via electron probe microanalysis. From the resulting experimental data diffusion coefficients were evaluated. Diffusion coefficients were found to be on the order of 10^-19 m2/s in the ?-phase systems, and 10^-17 m^2/s in the ?-phase systems. It was observed that atomic mobility of all diffusing species was generally greater in the U-50Zr/Zr-10Gd system than in the U-50Zr/Zr-10Er system; furthermore, it was found that diffusion rates were considerably higher above the phase transformation temperature into the ? phase, as indeed would be expected in the more open structure of the body-centered cubic ? phase, as compared to the hexagonal ?-phase U-Zr. However, values for diffusion coefficients measured in this study were considerably smaller than those found in past studies of ?-phase U-Zr, which are on the order of 10^-17 m^2/s. It is likely that diffusion was inhibited by the formation of stable metal oxides resulting from oxygen contamination; it is also possible that diffusion was suppressed by the presence of the erbium and gadolinium.Item Methods for Investigating Gas Bubble Formation in Uranium-Zirconium Alloys(2013-05-06) Mews, Kathryn Ann WrightUranium-zirconium alloy nuclear fuels have many advantages as compared with ceramic fuels, especially for fast reactor systems. However, metallic fuels aren?t currently used in commercial power production due in part to issues with fuel swelling during irradiation. A major contributor to this expansion issue, the formation of fission gasses into bubbles, is examined here. Methods to evaluate evolution of fission gas bubbles within a U-Zr alloy are discussed and refined. Specifically, transmission electron microscopy (TEM) for viewing bubbles within the interior of the alloy is investigated. One constraint on the use of the TEM is the lack of literature pertaining to what thinning techniques are successful for U-10Zr alloy. Both initial and final thinning techniques were investigated. After thinning was complete, the specimens were viewed via TEM to determine their suitability. In addition, samples of U-Zr alloy were irradiated with gas atoms in an accelerator to simulate bubble initiation and formation and viewed via TEM. Only preliminary investigations were completed. Evaluated electrolyte solutions included one part phosphoric acid to two parts sulfuric acid and two parts water (A), one part phosphoric acid to one part ethanol and one part glycerol (B), one part hydrochloric acid to one part water (C), two parts methanol to 13 parts phosphoric acid (D), and one part perchloric acid to nine parts acetic acid (G). Positive responses were received from the solutions A, C, and G which generated electron transparent areas with few to no process induced artifacts. TEM trials with each electrolyte were performed on un-irradiated U-10Zr alloy. These trials indicated that the polishing methods would work but are not yet optimal. In addition, U-10Zr alloy was irradiated and viewed in the TEM where those polished with electrolyte A consistently included the presence of large circular features that could indicate bubbles or voids. The foundation was laid for further study to be done on this topic through the use of electropolishing solutions as final thinning techniques. Additional work that is recommended includes: electropolishing parameter refinement; implantation of heavier gas atoms or at elevated temperatures; annealing of the implanted alloy; and computer modeling of resulting first principles phenomena.Item The investigation of spices by use of instrumental neutron activation analysis(Texas A&M University, 2008-10-10) Wise, Jatara RobThe spices consumed in the U.S. diet contain many elements other than the pure spice that many assume they eat. In particular, most of these spices contain radionuclides that are absorbed from the ground soil and water that contains trace contaminants. For this research, instrumental neutron-activation analysis (INAA) was used to determine the activities of U-235 fission products in common spices. Using this information, the concentrations of natural uranium in these spices and the doses to individuals consuming the spices were calculated. Nine spices and two standard reference materials were selected for analysis. The spices chosen were cinnamon, cumin, turmeric, oregano, thyme, cayenne, ginger, chili powder, and paprika. For comparison, NIST-certified "orchard leaves" and "spinach leaves" were used. The spices and standards were placed in polyethylene vials and heat-sealed. The samples were divided into irradiation groups of 30 seconds, 12 hours, and long irradiations of 10 to 12.8 hours. After irradiation, all samples were counted on an HPGe detector for time periods ranging between 10 minutes to 65 hours. After counting, the results were analyzed using Genie 2000 software. The Genie 2000 analysis revealed no detectable fission products for samples irradiated for 30 seconds or counted for short times. However, long counts revealed the high-yield U-235 fission products molybdenum-99 and what appeared to be cerium-144. However, after comparing the experimental values with the calculated values, it was determined that the experimental values of Ce-144 were not credible and the focus shifted solely toward Mo-99. From Mo-99 activities, uranium content could be calculated. Using this information, the committed dose equivalent (CDE) and the committed effective dose equivalent (CEDE) for ingestion of uranium was calculated. The CEDE values were based on an assumed ingestion of 6.5 grams of each spice per year. The doses from ingesting these spices ranged from CDE and CEDE doses of 4.31E-05 mSv and 3.08E-06 mSv, respectively. Based on these measurements consumption of these spices, even when combined, would not result in annual CDE or CEDE doses approaching the limits for the public of 50 mSv and 1 mSv, respectively, for a year of chronic ingestion.