Several instances of thermal fatigue cracking in small-diameter PWR branch lines off reactor main loop piping led to an industry program to evaluate the loading mechanisms responsible for the cracking. It was found that swirl penetration of hot reactor coolant into the normally stagnant drain lines can result in cyclic thermal stratification in the horizontal run of the drain driving the thermal loading. Models were developed to predict the thermal conditions and cyclic behavior that resulted in the cracking. Thermal transient and stress analysis was conducted to test the model predictions and to assure that cracking could be predicted. Further review was conducted for the related piping events where leakage had occurred. These investigations provided considerable insight into how to evaluate the larger populations of lines in operating plants where there have been no indications of cracking. These investigations have shown that two cases of cracking were due to un-insulated configurations that resulted in high cycling temperature differentials in the region of cracking. In addition, the lines where cracking occurred had rigid vertical supports in the region of stratification, leading to high elbow bending moments as a result of the constraint.
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