Experimental Study of Acoustic and Flow-Induced Vibrations in BWR Main Steam Lines and Steam Dryers

The boiling water reactor (BWR-3) in the Quad Cities (QC) Unit 2 Nuclear Power Plant experienced a significant increase in steam moisture under extended power up-rate (EPU) conditions. Inspection of the stem dryer showed it was likely that BWR steam dryer had been damaged by high cycle fatigue due to acoustic-induced vibration. 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 (MSLs). Acoustic resonance was considered to be generated by interaction between the sound field and an unstable shear layer across the closed side branches of the SRV stub pipes. We have started a research program on BWR dryers to develop their loading evaluation methods. Moreover, it has been necessary to evaluate the dryer integrity of BWR-5 plants which are the main type of BWR in Japan. In the present study, we used 1/10-scale BWR tests and analysis to investigate the flow-induced acoustic resonance in SRV stub pipes and propagation of fluctuating pressure from SRVs to the dryer through the MSLs. The test apparatus consisted of a steam dryer, a steam dome and 4 MSLs with 20 SRV stub pipes. Acoustic analysis was also applied to evaluate the propagation of fluctuating pressure from MSLs to the dryer. We demonstrated that acoustic resonance occurred in SRV stub pipes at higher velocity flows than in the normal operation and fluctuating pressure propagated from SRVs to the dryer. The amplitude of the fluctuating pressure due to several stub pipes was much larger than that in one stub pipe because of interaction between them. The fluctuating pressure generated in stub pipes was propagated to the dryer through the MSLs with decreasing amplitude. The amplitude of the fluctuating pressure observed at the top of stub pipes was reduced significantly in the MSLs and maintained fairly well in the steam dome. The flow-induced acoustic resonance did not occur for Strouhal number above 0.6 in the MSLs system of the BWR-5. Results of the tests were compared with those of acoustic analysis. The acoustic analysis could well predict the dryer loading.