During the operation of nuclear power plants (NPPs), the thermal fatigue is one of the most severe aging factors for the components. Therefore, it is significant to monitor the fatigue degradation of components caused by the thermal variation. Moreover, the fatigue monitor result is widely used in the life extension of NPPs. From this viewpoint, the Nuclear Power Institute of China (NPIC) has developed an advanced fatigue monitor system which can classify the operational transient and calculate the cumulative fatigue usage. In classical fatigue monitor system, two methods are widely used to calculate the cumulative fatigue usage. They are the cycle based method and the stress based method. In the cycle based method, the operational transient is classified into a conservative design transient, and then the remaining times of design transient is used to describe the fatigue condition. Obviously, the cycle based method is fast, but conservative. In the stress base method, the monitored thermal data is used to calculate stress and then do the fatigue analysis. This method is more accurate than the cycle based one, but this method is very time-consuming. In present work, a hybrid fatigue analysis of cycle based and stress based method is proposed. In this method, an off-line stress based fatigue database is developed using finite element method. The cycle based method is adopted to determine the transient effects. However, according to the off-line database, the consumed transient time is no longer an integer, but is a fraction to describe the difference between the cumulative fatigue usage of design transient and the one of operational transient.

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