Numerical study on thermal stratification phenomena in upper plenum of LMFBR “MONJU”

The three-dimensional analysis of thermal stratification in the upper plenum of MONJU is conducted using the commercial CFD code, FLUENT ver.12.1. 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 for evaluating the structural integrity in the upper plenum. 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. After the jet from the core outlet impinges on the upper core structure, the hot fluid flows obliquely upward to the inner barrel under the 40% electric output and flow condition. On the other hand, the jet from the core outlet flows to the lower part of the upper plenum, and then cold fluid flows through the flow holes after the turbine trip. Hence, the flow structure has changed from the initial 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 the flow holes has been maintained for a long duration since the thermal stratification changes the distribution of buoyancy forces.

► Three-dimensional analysis of thermal stratification in the upper plenum of MONJU is conducted using a commercial CFD code, FLUENT.
► A comparison between the numerical simulation and the trip test of MONJU is made.
► This study shows that the interface of thermal stratification is influenced by the flow pattern in the upper plenum of MONJU.