Counter-current flow limitation in a scale-down model of a PWR hot leg

In the case of a hypothetical failure of a residual heat removal system under mid-loop operation during shutdown of a pressurized water reactor (PWR), steam generated in a reactor core and water condensed in a steam generator (SG) form a counter-current flow in a hot leg. Though several studies have been carried out to understand counter-current flow limitation (CCFL) in the hot leg, our knowledge on CCFL is still rudimentary. In this study, a scale-down model of the hot leg (one-fifteenth of the actual plant) is used to investigate the effects of fluid viscosity on CCFL in the hot leg. Air and water or air and glycerol-water solutions are used for the gas and liquid phases, respectively. Water levels in the hot leg are also measured using conductivity probes. The gas and liquid flow rates at CCFL are estimated by applying a one-dimensional momentum equation to the flow in the hot leg. As a result, the following conclusions are obtained. (1) CCFL causes a drastic change in the distribution of water level in the hot leg. (2) The liquid viscosity scarcely affects the interfacial force. The interfacial force is mainly due to form drag caused by the presence of large-amplitude waves.