Transient Behaviors of Thermo-Hydraulic and Thermal Stratification in the Pressurizer Surgeline for the Nuclear Power Plant

In pressurized water reactor (PWR) system, the surgeline plays an important role in bonding the pressurizer and the primary circle. Some considerable problems, including the thermo-hydraulics, the thermal stratification and the accompanying thermal stress under transient conditions, pose risks to the surgeline integrity. Herein, a fully-coupled flow-heat-thermo-elasticity model was developed to investigate the transient behavior of thermo-hydraulic parameters and the thermal stratification phenomenon in PWR. To evaluate the nonuniformity of the stratified flow, a stratification degree indicator ζ is introduced. It is found that during the outsurge flow, the increase of temperature variation will enlarge the temperature gradient on the wall, corresponding to a more serious deformation. In the cases of positive temperature variation, the higher temperature variation causes higher stratification degree ζ, and vice versa. The mass flow rate ṁ and the stratification degree are in negative correlation. The local deformation can reach 1.802 cm under a 50 K temperature variation, while its location varies from case to case. More attention should be paid to the regulation between the highest deformation location and the surgeline thermo-hydraulic parameters.