It has been demonstrated that intense fluctuating pressures acted on the steam dryer in a boiling water reactor (BWR) at higher velocity flows than in normal operation through actual BWR plant tests [1] and scale model air tests [2]. 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 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. Some air scale tests were conducted and those are useful for evaluation of occurrence of acoustic resonance in SRV stub pipes and characteristics of fluctuating pressure in MSLs. However, it is a possibility that differences in pressure conditions and fluid proprieties caused 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 Hitachi Utility Steam Test Leading facility (HUSTLE) was used for the scale model steam tests of MSLs and a steam dryer. Steam which was at the same pressure and temperature conditions as an actual BWR was used in the tests. The test apparatus consisted of the steam dryer, steam dome and 4 MSLs with 20 SRV stub pipes.

We demonstrated that acoustic resonance occurred in the SRV stub pipes and fluctuating pressure which propagated from the SRVs to the dryer caused fluctuating stress on the steam dryer at the SRV resonance frequency. The increasing of fluctuating pressure due to the double vortex mode which occurred at a Strouhal number (St) from 0.8 to 0.9 could be clearly seen in the scale model steam tests unlike in the air tests. It was possible that self excited acoustic resonance was affected by the low pressure conditions and different fluid proprieties for the scale model air tests. However, no significant influence from steam pressure was seen at more than 3MPa. 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. Increasing of fluctuating pressure due to the double vortex mode was clearly distinguished for SRV stub pipes but became smaller for MS pipes and had insignificant impact on the dryer loading.

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