Previous plant measurements and scale model tests have demonstrated that intense fluctuating pressure acted on the steam dryer in boiling water reactors (BWRs) at higher velocity flows than in normal operation. The cause of the dryer loading 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 the interaction between the sound field and the unstable shear layer across the opening of the side branches of the SRV stub pipes. Some air scale tests have been conducted and they are useful for evaluation of occurrence of acoustic resonance in SRV stub pipes and characteristics of fluctuating pressure in MSLs. However, it is possible that differences in pressure conditions and fluid properties may cause the dryer loading to be underestimated. In the present study, we conducted scale tests under actual steam conditions to evaluate the scale model test methods for BWR dryer loading estimation. The test apparatus consisted of a steam dryer, steam dome and 4 MSLs with 20 SRV stub pipes. We demonstrated that acoustic resonance occurred in the SRV stub pipes and the fluctuating pressure which propagated from the SRVs to the dryer caused fluctuating stress on the steam dryer at the SRV resonance frequency. Acoustic resonance started when Strouhal number decreased below 0.6 in both the scale model air and steam tests. The onset of resonance due to the single vortex mode was not influenced by pressure conditions and fluid properties. The increase of fluctuating pressure due to the double vortex mode which occurred at Strouhal number values from 0.8 to 0.9 could be clearly seen in the scale model steam tests unlike in the air tests. The results showed that the self-excited acoustic resonance was affected by the static pressure and fluid properties for the scale model air tests. However, no significant influence from steam pressure was seen at pressure higher than 3 MPa. Normalized fluctuating pressure was almost the same regardless of pressure. We verified that normalization by dynamic pressure in the main pipe was a reasonable approach for evaluation of fluctuating pressure in the SRV stub pipes. Increase of fluctuating pressure due to the double vortex mode was clearly distinguished in SRV stub pipes but not strong in the MSL pipes and had insignificant impact on the dryer loading.

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