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Thermal-Hydraulic Simulation and Analysis of Two-Phase Thermal Shock in Pressurized Light Water Power Plants

Ghafari, Mohsen | 2018

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 51190 (46)
  4. University: Sharif University of Technology
  5. Department: Energy Engineering
  6. Advisor(s): Ghofrani, Mohammad Bagher
  7. Abstract:
  8. As a result of fission reaction in a nuclear reactor, the produced high neutron flux would affect the material of Reactor Pressure Vessel (RPV). This neutron radiation has a detrimental impact on the mechanical properties of the RPV material such as hardening (or embrittlement) while neutrons are absorbed by the material. A major concern in embrittled RPVs is propagation of critical flaw causing through-wall cracks. Some transients leading to overcooling of RPV intensify the propagation of theses cracks and result in thermal load on RPV, known as Pressurized Thermal Shock (PTS). Such situation could be created in case of Emergency Core Cooling System (ECCS) actuation which leads to injection of cold water into the primary loop in some accidents, e.g. Loss Of Coolant Accident (LOCA). A complete analysis of PTS has three main steps including Probabilistic Safety Assessment (PSA), thermal-hydraulics analysis and fracture mechanics analysis.The objectives of thermal-hydraulic step of PTS analysis are the prediction of plant response to LOCA and production necessary data for fracture mechanics analysis. As the injection of ECCS water, the primary coolant may either be in single-phase or two-phase condition, depending on the leak size, its location, and the operating conditions of the nuclear power plant. So, the PTS investigation would be considered for single phase and two-phase condition. Due to 3D complex phenomena in two-phase PTS, the current CFD (Computational Fluid Dynamics) methods and traditional codes need a considerable improvement to bridge the research gaps.
    The current research focuses on impinging and stratified flow zone and proposes new modeling strategy based on PTS requirements. According to obtained results, employment of polydisperse model in impinging zone and consideration of different mean diameters for dispersed bubbles leads to better prediction for gas phase entrainment near the impinging point. Also, in stratified flow zone, the modification of turbulence boundary condition at the interface improves the evaluation of heat transfer coefficient, temperature gradient in water section and condensation rate based on surface renewal theory. In the other section of this research, two different criteria are proposed for prediction of flow regime in the cold leg based on Kelvin-Helmholtz instability theory and propagation of bubbly flow near steam/water interface. based on these criteria and modification of interfacial turbulence modeling, two-phase PTS between injection point and downcomer of RPV is simulated and used for thermal-hydraulic steps of PTS in Bushehr nuclear power plant
  9. Keywords:
  10. Cooling System Accident Loss ; Temperature Gradient ; Turbulence Flow Modeling ; Interfacial Heat Transfer Coefficient ; Thermal Shock ; Pressure Vessel ; Two-Phase Stratified Flow

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