Study on Depressurized Loss of Coolant Accident and its Mitigation Method Framework at Very High Temperature Gas Cooled Reactor Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pz50gz64w

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  • To understand how a Depressurized Loss of Forced Convection (D-LOFC) initiated from a double-ended guillotine break may affect further operation of a High Temperature Gas Cooled Rector (HTGR), thorough understanding of each specific stage of this event is required. Key considerations that need to be determined is the amount of air that will ingress into the reactor vessel during a Depressurized Conduction Cooldown (DCC) accident as well as the time scale of each events stage. Reactor components constructed of graphite will, at sufficiently high temperatures produce exothermic reactions in the presence of air while undergoing oxidation reaction. There is a danger that it may cause loss of core structural integrity via oxidation or surface corrosion. Thus, without any mitigation systems, this accident might result in exothermic chemical reactions of graphite and oxygen depending on the accident scenario and the design. Having the knowledge on the amount of air inside the vessel one can investigate how to reduce air concentration via specifically designed mitigation system. The main idea of the applied mitigation method is to replace air in the core with buoyancy force by insertion of secondary gas: helium, nitrogen or argon into the reactor's lower plenum. This can help to mitigate graphite oxidation inside the reactor core by reducing air volume mass fraction in the reactor components and by lowering the core and lower plenum temperatures. This dissertation presents design framework for a secondary gas insertion system that will mitigate the potential core/structure damage during DCC event. Obtained results will serve as a guidelines for future design engineers while applying the concept to existing design.
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