Numerical study on the thermal stratification characteristics of AP1000 pressurizer surge line

The surge line is implemented to connect the pressurizer with reactor cooling system in Pressurized Water Reactor (PWR). The integrity of surge line plays an important role for reactor safe operation. In the literatures, much work has been contributed to the thermal hydraulic characteristics and reliability research on traditional pressurizer surge line. For the advanced Generation-III PWR, the AP1000 adopts a new spiral-shaped pressurizer surge line concept. In this paper, the three-dimensional thermal hydraulic transient characteristics in AP1000 pressurizer surge line are studied and the formation mechanism of surge line thermal stratification phenomenon is investigated. The variation of maximum temperature difference with time in the whole surge line is achieved. Results show that the thermal stratification is becoming weaker along the flow direction and the most dangerous region is the entrance of horizontal position. The thermal stratification affected by the circumference effect is not significant during the surge line thermal fatigue analysis. In addition, through the comparison of temperature distribution in pressurizer surge line between the traditional PWR and AP1000, it can be seen that the AP1000 pressurizer surge line has smaller temperature difference at the corresponding position with shorter period. The new design concept of AP1000 surge line brings a great advantage to reduce the hazard causing by the thermal stratification. This work provides a valuable guideline for the AP1000 pressurizer surge line fatigue assessment.