Suggestions for the Proper Treatment of Buoyancy Force Effect on Single Phase Thermal Stratification Phenomenon

In the operating nuclear power plants, piping and components installed in the in-service testing systems connected to the reactor coolant system are potentially exposed to fatigue caused by thermally stratified flow, possibly threatening its integrity. The emergency core cooling system (ECCS) is one of candidates for which the thermal stratification phenomenon can occur. Because temperature difference between the hot coolant in the cold leg and the cold cooling water injected from the ECCS line is 200 K or more, buoyancy due to density difference may have significant influence on thermal-hydraulic characteristics of the mixing flow. Therefore, in this study, the need to consider proper buoyancy models in governing equations, especially turbulence transport equations, for accurate prediction of single phase thermal stratification in the cold leg and downcomer by ECCS injection is numerically studied using ANSYS CFX R17.2.