This paper investigates the behavior of a small crack in thermal striping zone in pipings of a PWR nuclear power plant. To motivate the current research, this paper first surveys piping damages including pipe failure and crackings due to thermal stratification and thermal striping. Then the effect of several parameters which govern thermal stratification and thermal striping - such as the heat transfer coefficient of pipe surface, the temperature difference across density interface, the fluctuation amplitude and frequency of density interface, crack length, pipe thickness, fluid pressure, and material properties of pipe-on the crack behavior is investigated numerically. The numerical results are normalized in order to generalize them with wider applications in mind.
The current research has five major results. First, the results indicate that the heat transfer coefficient of pipe inner surface and the fluctuation frequency of density interface are governing parameters for $\triangle{K}_I$ at the crack tip. Second, for lower pressure, $(\triangle{K}_I)_{\mbox{eff}}$ decreases because of the contact of crack surfaces. Third, for a given crack ratio, $\triangle{K}_I$ increases as the fluctuation frequency increases. But it decreases as the fluctuation frequency increases further past a certain decreases as the fluctuation frequency increases further past a certain fluctuation frequency. In addition, $\triangle{K}_I$ keeps increasing as the heat transfer coefficient increases. Fourth, for a given fluctuation frequency, $\triangle{K}_I$ gains a peak value as the crack ratio changes. And the crack ratio at which $\triangle{K}_I$ gains the peak becomes larger as the fluctuation frequency of density interface increases. Lastly, based on the above mentioned results, the prediction of the behavior of a crack subjected to thermal striping is possible.
Thermal striping plays an important role in crack initiation and initial crack growth since the effect of thermal striping is limited to near pipe inner surface. However, for a large crack, the crack behavior is governed by bending moment due to thermal stratification and other mechanical loads rather than by the stress fluctuation due to thermal striping. Accordingly, a small crack in thermal striping zone should be evaluated to investigate the possibility of crack initiation and initial crack growth by thermal striping load, even though the small crack satisfies the acceptance criteria for cracks in ASME Code Sec. XI during in-service inspection.
