Numerical study on temperature fluctuation of upper plenum in FBR with a more realistic model

Highlights: • Temperature fluctuation of upper plenum in FBR with a more realistic model was investigated. • Temperature fluctuation of sodium around center column was obtained. • Spatial distribution of velocity and temperature were analyzed. • Distribution of temperature fluctuation intensity were analyzed. • Amplitude and frequency of temperature fluctuation were presented. - Abstract: The thermal fatigue caused by the temperature fluctuation is more serious in FBR than that in PWR due to thermal characteristic. Limited by the ability of the computer, the simplified jet models were used to study the temperature fluctuation in published literature, which can not fully reflect the real characteristic of temperature fluctuation of upper plenum in FBR. In present study, the temperature fluctuation of upper plenum with a more realistic model (i.e. three heads of assemblies and the part of central column) in FBR was simulated with LES method. By comparison, the velocity and temperature fields are more complex in a more realistic model than that of the simplified jet model. The characteristic of temperature fluctuation was obtained around the central column. In a more realistic model, the fluid produces intense mixing near interface of the adjacent different temperature regions, which lead to severe temperature fluctuation. The distributions of temperature and temperature fluctuation intensity are asymmetric. The temperature field is divided into three regions with high temperature range θ = 195–285° for the three kinds of temperature inlet conditions, while that is divided into two regions with high temperature range θ = 75–285° for two kinds of temperature inlet conditions. The changes of temperature and temperature fluctuation intensity are complicated by the influence of the head of assembly in the coordinate directions. The position of the most intense oscillation appears in the upper position of the head outlet. However, near the bottom of the central column, the thermal stratification and temperature fluctuation are still relatively obvious with the temperature difference about 30–40 K and the amplitude about 0.1–0.15 times of the maximum inlet temperature difference. Meanwhile, the obvious dominant frequency does not appear. The energy of temperature fluctuation is distributed in a wide frequency range f < 20 Hz.