Browsing by Author "Dr. David McNelis, Committee Member"

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    Improvement of Photon Buildup Factors for Radiological Assessment
    (2006-04-27) Schirmers, Fritz Gordon; Dr. Man-Sung Yim, Committee Chair; Dr. David McNelis, Committee Member; Dr. H. Omar Wooten, Committee Member; Dr. Donald Dudziak, Committee Member; Dr. Peter Bloomfield, Committee Member
    Slant-path buildup factors for photons between 1 keV and 10 MeV for nine radiation shielding materials (air, aluminum, concrete, iron, lead, leaded glass, polyethylene, stainless steel, and water) are calculated with the most recent cross-section data available using Monte Carlo and discrete ordinates methods. Discrete ordinates calculations use a 244-group energy structure that is based on previous research at Los Alamos National Laboratory (LANL), but extended with the results of this thesis, and its focused studies on low-energy photon transport and the effects of group widths in multigroup calculations. Buildup factor calculations in discrete ordinates benefit from coupled photon/electron cross sections to account for secondary photon effects. Also, ambient dose equivalent (herein referred to as dose) buildup factors were analyzed at lower energies where corresponding response functions do not exist in literature. The results of these studies are directly applicable to radiation safety at LANL, where the dose modeling tool Pandemonium is used to estimate worker dose in plutonium handling facilities. Buildup factors determined in this thesis will be used to enhance the code's modeling capabilities, but should be of interest to the radiation shielding community.
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    Target and Core Optimization for an Electron Accelerator-Driven Transmutation Facility
    (2008-04-25) Swaney, Paul Michael; Dr. Ernie Stitzinger, Committee Member; Dr. David McNelis, Committee Member; Dr. Man-Sung Yim, Committee Chair
    The current statutory limit for Yucca Mountain is quickly being met by waste produced at the reactors operating in the United States. A possible method of modifying reactor waste for more efficient storage in Yucca Mountain is transmutation. This study analyzes the use of an electron accelerator targeting its beam on a neutron producing target within a subcritical reactor fueled with transuranic waste from power reactor operations. To maximize the transmutation effectiveness of the design, several loading patterns were analyzed for neutronics behavior and transmutation effectiveness. Designs utilizing multiple batches of fuel or multiple targets within the core were also analyzed. The loading pattern analysis showed no clear beneficial loading pattern for the neutronics of the core; however, the results indicated that placing Curium within the innermost fuel assemblies improved the transmutation effectiveness of the core. The most effective loading pattern in terms of neutronics and transmutation effectiveness utilized Curium assemblies in the innermost locations, Americium assemblies in the second ring, and Neptunium and Plutonium assemblies in the outermost locations. The use of multiple batches in the core layout demonstrated superior neutronics behavior but lacked in transmutation effectiveness. On the other hand, the use of multiple targets in the core did not exhibit the lower peaking factors expected and also performed poorly in the area of transmutation effectiveness. Most likely, these core designs should be combined with more effective loading patterns to maximize their benefits. When comparing electron accelerator based systems with proton accelerator based systems, the proton based systems have a significant advantage due to the higher neutron production efficiency within the target. However, economic and timeline considerations make the deployment of electron based systems attractive in specific scenarios.