Flow Visualization of Acoustic Resonance for Safety Relief Valves in Power Uprated BWRs

The boiling water reactor (BWR-3) in the Quad Cities Unit 2 Nuclear Power Plant (QC-2) experienced a significant increase in steam moisture under extended power uprate conditions (EPU). Inspection of the steam dryer showed it was likely to have been damaged by high cycle fatigue carry over, while the cause of the dryer failure was considered as flow-induced acoustic resonance at the stub pipes of safety relief valves (SRVs) in the main steam lines. Acoustic resonance was considered to be generated by interaction between the sound field and unstable shear layers across the closed side branches of the SRV stub pipes. From the dryer failure experience in QC-2, it became apparent that quantitative evaluation of the dryer loading was important for BWRs which were undergoing EPU planning. Acoustic resonance frequency can be evaluated using the geometry of the SRV stub pipes and the speed of sound. To predict the fluctuation pressure, it is necessary to consider a feedback mechanism between unsteady vorticity fluctuation and aerodynamic sound. Although theoretical approaches to sound propagation have been reported, experiments focused on vorticity are quite limited. In this study, in order to clarify this mechanism, a particle image velocimetry system (PIV) is used to visualize the flow field first, and then its effect on the wall pressure fluctuation is estimated by measuring the pressure fluctuation at the top of the stub pipe with a pressure sensor. Vortex growth from the leading edge of the cavity is confirmed and its shedding frequency is close to the frequency of the fluctuating pressure at the top of the stub pipe. Additionally, the pattern of maximum vorticity versus Strouhal number corresponds to the fluctuating pressure at the top of the stub duct. Thus, detected vorticity near the trailing edge of the cavity is the aerodynamic sound source.