Most open-pool type research reactors, which are cooled by forced convection, have a downward flow for several advantages. Downward flow can minimize pool surface radiation, and makes a fuel assembly easy to be fixed on the grid plate of a reactor core. After LOFA (Loss of Flow Accident) or even normal shutdown, a reactor core has to go through a flow inversion from downward to upward owing to a natural circulation. As the pump is turned off, coolant flow in the core becomes slow and finally stagnant upon a flow inversion occurrence. During the flow inversion, it is a concern that the onset of nucleate boiling (ONB) or a departure from nucleate boiling (DNB) may occur. Recently, there have been several studies on predicting the flow inversion and temperature of a fuel plate to find whether an ONB occurs, using one-dimensional thermal hydraulic analysis codes and computational fluid dynamics (CFD) codes. These studies have concentrated on only a single channel but they did not consider other important effects such as the piping, valves, and pool. In the present study, a numerical simulation using a CFD code was carried out to determine the flow inversion phenomenon. The CFD model considers 21 thin plates of an in-line array which release heat uniformly. The 21 plates make 22 thin channels of which the inlets are connected to a large pool. The outlets of the thin channels are connected to the large pool through a flap valve. A CFD simulation was carried out during 80 seconds after the reactor shutdown. It shows the behaviors of a flow through the flap valve as well as the flow inversion in the channels. Another numerical simulation using the RELAP5 code was conducted to compare with the CFD simulation results. In the comparison, CFD predictions such as coolant temperature variation, heating plate temperature variation, and flow rate through the flap valve, show similar results with those of the RELAP5 analyses.

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