A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Development of Approaches to Common Cause Dependencies with Applications to Multi-Unit Nuclear Power Plant(2018) Zhou, Taotao; Modarres, Mohammad; Droguett, Enrique López; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The term “common cause dependencies” encompasses the possible mechanisms that directly compromise components performances and ultimately cause degradation or failure of multiple components, referred to as common cause failure (CCF) events. The CCF events have been a major contributor to the risk posed by the nuclear power plants and considerable research efforts have been devoted to model the impacts of CCF based on historical observations and engineering judgment, referred to as CCF models. However, most current probabilistic risk assessment (PRA) studies are restricted to single reactor units and could not appropriately consider the common cause dependencies across reactor units. Recently, the common cause dependencies across reactor units have attracted a lot of attention, especially following the 2011 Fukushima accident in Japan that involved multiple reactor unit damages and radioactive source term releases. To gain an accurate view of a site's risk profile, a site-based risk metric representing the entire site rather than single reactor unit should be considered and evaluated through a multi-unit PRA (MUPRA). However, the multi-unit risk is neither formally nor adequately addressed in either the regulatory or the commercial nuclear environments and there are still gaps in the PRA methods to model such multi-unit events. In particular, external events, especially seismic events, are expected to be very important in the assessment of risks related to multi-unit nuclear plant sites. The objective of this dissertation is to develop three inter-related approaches to address important issues in both external events and internal events in the MUPRA. 1) Develop a general MUPRA framework to identify and characterize the multi-unit events, and ultimately to assess the risk profile of multi-unit sites. 2) Develop an improved approach to seismic MUPRA through identifying and addressing the issues in the current methods for seismic dependency modeling. The proposed approach can also be extended to address other external events involved in the MUPRA. 3) Develop a novel CCF model for components undergoing age-related degradation by superimposing the maintenance impacts on the component degradation evolutions inferred from condition monitoring data. This approach advances the state-of-the-art CCF analysis in general and assists in the studies of internal events of the MUPRA.Item FUNDAMENTAL UNDERSTANDING OF SOFC CATHODE DURABILITY; A KINETICS AND CATALYSIS STUDY(2015) Huang, Yi-Lin; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid oxide fuel cells (SOFC) have been demonstrated as great prospects for electrochemical conversion of fuels, providing both high efficiency and high power density. Understanding the fundamentals of the oxygen reduction reaction (ORR) mechanisms is necessary to further improve cathode performance. Two different testing systems, gas phase isotopic oxygen exchange and electrical conductivity relaxation, were built to study the kinetics of cathode powders and bulk samples, respectively. A robust strategy was established to extract kinetic parameters from transient response curves for a variety of materials and conditions using numerical solutions. In-situ gas phase isotopic oxygen exchange, which provides real-time information about cathode surface kinetics, was used to determine the ORR mechanisms and the interactions of other gaseous species with the solid surface for two cathode materials: La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and (La0.8Sr0.2)0.95MnO3±x (LSM). LSCF has a faster dissociation reaction than LSM, and the limiting step is the surface exchange. Additionally, LSM likely contains different vacancy concentrations in the near surface region and in the bulk. A mathematic model is further established to unify surface exchange rates from different experiments and link solid-state diffusion to surface heterogeneous catalysis. In addition, the long-term durability of these materials is a major challenge. A novel technique called isotope saturated temperature programmed exchange (ISTPX) has been developed to determine the temperature and PO2 range that is preferable for the exchange of water and CO2 on LSM and LSCF. The presence of CO2 and water indicates blocking effects on the LSCF surface from 300°C to 600°C, possibly resulting in two separate degradation mechanisms. On the other hand, CO2 and water exchange with LSM through homoexchange mechanism with a relatively minor impact. Based on isotope exchange results, surface modified LSCF cathodes were fabricated. The surface modification of LSCF through Mn ion implantation enhances the chemical surface exchange coefficient (kchem) from 4.4x10-4 cm/s to 1.9x10-3 cm/s at 800°C. The aims of this study are to increase knowledge and information about the ORR. The results allow us to further investigate the ORR mechanisms as well as to engineer new cathode materials/structures that can improve cathode performance and durability.