As for the fast reactor, the temperature fluctuation in core outlet zone in normal operation was an important and typical thermal hydraulic phenomenon. In certain conditions, these temperature fluctuations can lead to thermal-mechanical damage to the Upper Core Structures (UCS). In this paper, the temperature fluctuation in Core Outlet Region of China Experimental Fast Reactor (CEFR) was numerically simulated and researched using the CFD software FLUENT. In the simulation process, LES turbulence method was used for the turbulent model selection. For the mesh generation, 1/4 sector model was built considering the layout of the symmetry and very small-size cell was used for satisfied with the Corant number. At the same time the core outlet temperatures and flows of the coolant in each subassembly under reactor rated power was used as the boundary conditions. The simulation shows that the temperature fluctuation was mainly concentrated in the zone between the fuel subassembly and control rod assembly for the large temperature difference of these subassemblies CEFR was operated in normal conditions. The largest fluctuation amplitude was 19°C and the remarkable frequency was below 5Hz. It belongs to typically low frequency fluctuation. The conclusion was useful for further experimental work and reactor operation.

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