Abstract: In this paper,ANSYS CFX program is used to perform Computational Fluid Dynamic analyses of normally stagnant non-isolable reactor coolant system branch lines.Take Safety Injection,out-of-service charging and Residual heat removal suction lines as example to make models for calculation.The potential exists for thermal stratification is discussed in the paper.The effects of location of valve leakage,the in-leakage rate,the temperature of leakage,the length of horizontal or vertical section and other factors are studied.The results verify that the different generic configurations have different mechanisms for thermal stratification.Thermal stratification in up-horizontal/horizontal configurations is caused by the interaction of swirl flow with in-leakage of cold water from a leaking normally closed valve;in down-horizontal configurations,the thermal stratification occurs due to the cyclic penetration and retreat of the swirl flow in the branch line,combined with heat transfer to the environment.