Numerical simulation of dynamic flow structure and thermal stratification phenomena in LMFBR

The three-dimensional analysis of thermal stratification in the upper plenum of MONJU is conducted using the commercial CFD code, FLUENT ver. 12.0. Since the temperature gradient near the thermal stratification interface would cause thermal stress in the reactor components, it is important to understand the characteristics of thermal stratification. As the result of numerical analysis, it is understood that the interface of thermal stratification is influenced by the flow pattern in the upper plenum of MONJU. After the jet from the core outlet is impinged on the upper core structure, the hot fluid flows obliquely upward to the inner barrel under the steady-state condition. On the other hand, the jet from core outlet flows to the lower part of the upper plenum, and then cold fluid flows through the flow holes under the transient condition. Hence, the flow structure has changed from the steady-state condition as the flow rate and temperature of the core outlet decrease due to the turbine trip. It is considered that the flow path of flow holes comes to govern under the transient condition, since the hot sodium acts as the plug due to the buoyancy. (author)