Evaluation of Acoustic- and Flow-Induced Vibration of the BWR Main Steam Lines and Dryer

The boiling water reactor (BWR-3) steam dryer in the Quad Cities (QC) Unit 2 Nuclear Power Plant was 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 the safety relief valve (SRV) in the main steam lines (MSLs). The acoustic resonance was considered to be generated by the interaction between the sound field and an unstable shear layer across the closed side branches of SRVs. We have started a research program on BWR steam dryers to develop methods of evaluating the loading. Moreover, it is necessary to evaluate the dryer integrity of BWR-5 plants, which are the main type of BWR in Japan. In the present study, we conducted 1/10-scale BWR model tests and analysis to investigate the flowinduced acoustic resonance and acoustic characteristics in MSLs. The test apparatus consisted of a steam dryer, a steam dome, and 4 MSLs with 20 SRV stub pipes. Computational fluid dynamics (CFD) analysis was conducted to evaluate the acoustic source in MSLs. Finite element method (FEM) was applied to calculate the three-dimensional wave equations for acoustic analysis. We demonstrated that large fluctuating pressure occurred in the high- and low-frequency regions. The high-frequency fluctuating pressure was generated by the flow-induced acoustic resonance in the SRV stub pipes. We evaluated the acoustic source (that is, the fluctuating pressure) in MSLs by unsteady CFD calculations, and we evaluated the pressure propagation by acoustic analysis. These results were verified by comparison with the results of scale-model tests, and they showed good agreement with the experimental results. The effects of the difference between the properties of air and steam were numerically investigated, and it was found that the effects on the acoustic resonance in the SRV stub pipes were not significant.

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