In nuclear power plants, high cycle thermal fatigue induced by temperature fluctuation of the coolant is one of frequent failure modes. To ensure the safety of nuclear power plant systems, it is important to prevent thermal fatigue failure.
Typical causes of high cycle thermal fatigue are thermal striping at Tee-junction and thermal stratification oscillation. In order to evaluate thermal stress caused by thermal striping, a frequency response function has been developed. This function was derived from a heat transfer and thermal elastic theories, and can adequately evaluate thermal stress induced by temperature gradient into wall-thickness direction.
However, this theoretical method cannot adequately evaluate thermal stress by thermal stratification oscillation, because this phenomenon has the fluid temperature distribution gradient along axial direction.
To investigate the mechanism of thermal stress generated by oscillation of thermal stratification, two types of models were studied. In the first type, fluid temperature oscillates with sinusoidal history at the same location, and in the second one, the boundary layer of hot and cold fluid temperature moves with sinusoidal velocity.
Through clarification of the stress generation mechanism, the frequency response function was improved to evaluate stress by the thermal stratification oscillation. Applicability of this function was verified through agreement with finite element simulations.