Applicability of Two-fluid Model in Predicting Onset of Flow Instability in a Narrow Rectangular Cooling Channel under Low Pressure

Plate-type fuel assemblies have been widely used in the research reactors to enhance power density. This type fuel assembly consists of a number of fuel plates, supporting plates, cooling channels between the fuel plates, common inlet and outlet plenums. The cooling channels are completely isolated from each other so that the cross flow between the channels does not occur. Hence, if onset of flow instability (OFI) takes place in any one of cooling channels during the flow blockage accident in a plate-type fuel assembly, it can develop CHF and cause OFI in an adjacent cooling channel. This phenomenon is called flow instability propagation, one of the greatest concerns in safety analysis of research reactors. Therefore, it is necessary to precisely assess the two-phase flow instability in a narrow rectangular cooling channel and its propagation caused by the flow blockage in order to improve the safety of a research reactor. In the present work, CFD analysis was performed to assess the capability of the CFD code in predicting OFI in a narrow rectangular cooling channel under low pressure. It is found that the CFD code with the models implemented in this study can predict the tendency of two-phase pressure drop that the pressure drop increases after the initiation of OFI although the mass flow rate decreases. These results show that it is possible to predict OFI under low pressure in a narrow rectangular channel using a commercial CFD code with the models implemented in this study.