Numerical Analysis of Thermal Stratification for Upper Plenum in Pool-Type Sodium Fast Reactor

Abstract During the scram transient of China demonstration sodium fast reactor, thermal stratification phenomenon is observed in the upper plenum. The paper simulates the thermal stratification which occurs when hotter sodium from the core outlet enters the upper plenum along with colder sodium from the DHX entering the bottom of upper plenum using ANSYS commercial code. The results illustrate three transient stages of thermal stratification: In the first stage, hotter sodium accumulate in the top of the plenum while interface continues descending; simultaneously colder sodium from the DHX enter into plenum forming another thermal stratification phenomenon with rising interface until both interfaces meet forming a stable interface near the core outlet; In the second stage, as the flow rate and temperature at the core outlet gradually decrease, the hotter sodium cannot fully reach the top of the upper plenum, and another interface is formed at the top. In the third stage, the thermal stratification interface at the top gradually disappears, and the final thermal stratification interface is formed near the core outlet. Furthermore, it is also concluded that the interface descend rate decreases with the increase of Richardson number.