Countercurrent gas-liquid flow in a PWR hot leg under reflux cooling (1). Air-water tests for 1/15-scale model of a PWR hot leg

In the case of loss of residual heat removal systems under mid-loop operation during shutdown of a PWR plant, reflux cooling by a steam generator (SG) is expected, and steam generated in the reactor core and water condensed in the SG form a countercurrent flow in a hot leg. The flow is highly complicated because the hot leg consists of a horizontal pipe, an elbow, and an inclined pipe. In this study, a scale model of the PWR hot leg (1/15th of the actual plant size) is used to investigate flow patterns and characteristics of countercurrent flow limitation (CCFL). The following conclusions are obtained. (1) The effects of gas volumetric flux JG and water volumetric flux JG on flow pattern in the hot leg were made clear. Flow patterns in the elbow and inclined section are strongly affected by those in the horizontal section. (2) When the flow rate of the supplied water is constant, the value of JG at the onset of transition from stratified flow to wavy flow obtained by increasing JG is larger than that at the onset of transition from wavy flow to stratified flow obtained by decreasing JG. (3) CCFL characteristics obtained by increasing JG differ from those obtained by decreasing JG. CCFL data obtained by decreasing JG agree well with available data. (4) The boundary between wavy flow and stratified flow in the horizontal section obtained by decreasing JG agree well with the CCFL characteristics. (author)