Free outflow from the end of a horizontal circular pipe related to flow from the PWR cold leg to the downcomer

During a loss-of-coolant accident in a pressurized water reactor (PWR), there is a risk that pressurized thermal shock (PTS) may occur due to the rapid cooling of the downcomer wall caused by the emergency core cooling (ECC) water injected into the cold leg that flows into the downcomer. To predict the PTS, it is necessary to accurately predict the temperature of the ECC water, the collision of the water jet on the downcomer wall, the velocity, thickness, and the spread of the liquid film. Therefore, to help understand this flow phenomenon, we reviewed studies on free outflow from a circular pipe. In many previous studies, experimental findings on the flow regimes, transition conditions between flow regimes, characteristics of the flow shape have been obtained in a form that was almost consistent with each other. And, through theoretical analysis, it was also possible to predict the flow regime and flow surface shape to a certain extent quantitatively. In contrast, when considering the flow from the cold leg to the downcomer, it is necessary to deal with the flow field in a specific situation, such as the flow into a narrow gap rather than a free space, the existence of rounded corners at the outlet of the circular pipe, and the influence of steam flow flowing from the core to the cold leg. However, few previous studies consider these factors, so we summarized them as knowledge that needs to be accumulated in the future. In this review article, 30 references are included. • Studies on the outflow from circular pipes related to PTS were reviewed. • Consistent experimental and theoretical results have already been obtained. • There has been insufficient knowledge of flow into narrow gaps such as downcomers. • We summarized the issues with insufficient knowledge to understand PTS occurrence.