The power uprating of the nuclear power plant is conducted in United States, EU countries and so on, and also is planned in Japan. However, the degradation phenomena such as flow-induced vibration and wall thinning may increase or expose in the power uprate condition. In U.S. NPP, the dryer had been damaged by high cycle fatigue due to acoustic-induced vibration. This is caused by acoustic resonance at the stub pipes of safety relief valves (SRVs) in the main steam lines (MSL). Increased velocity by uprating excites the pressure fluctuations and makes large amplitude resonance. We have started to develop the evaluation methods of pressure fluctuations at the dryer in BWR-5, which are the main type of BWR in Japan. In this study, to investigate the fluctuating pressure at rated power condition, we conducted steam flow simulations in actual main steam piping geometry and conditions. As a result, small amplitude resonance was observed in rated power condition.

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