Abstract
In nuclear power plants, a lot of failure cases by high cycle thermal fatigue were reported, which was induced by temperature fluctuation of the coolant. To ensure the safety of nuclear power plants, thermal fatigue damage should be evaluated adequately. For evaluation of thermal stress due to fluid temperature change, frequency response function was developed by Kasahara et al.. This function can evaluate thermal stress caused by thermal stratification oscillation, which is one of typical causes of high cycle thermal fatigue. However, the function was proposed under the ideal condition where the thickness of stratification could be ignored. Thus temperature distribution in direction of vertical to thermal stratification is not considered enough. Consequently, calculated thermal stress could be too conservative. Therefore influence of the thickness on occurrence mechanism of thermal stress was investigated by conducting finite element simulation. Based on the clarified mechanism, the frequency response function was improved for adequate evaluation of thermal stratification. By using the improved function, stress time series can be obtained, from which thermal fatigue damage was evaluated.
