[en] In view of the practical importance of the drift-flux model for two-phase flow analysis in general and in the analysis of nuclear-reactor transients and accidents in particular, the distribution parameter and the drift velocity have been studied for downward two-phase flows. The constitutive equation that specifies the distribution parameter in the downward flow has been derived by taking into account the effect of the downward mixture volumetric flux on the phase distribution. It has been assumed that the constitutive equation for the drift velocity developed by Ishii for a vertical upward churn-turbulent flow determines the drift velocity for the downward flow over all of flow regimes. The newly developed drift-flux model has been validated by 462 data sets obtained in various literatures under various experimental conditions. These data sets cover extensive flow and loop conditions such as flow system (air-water and steam-water), channel diameter (16 mm -102.3 mm), pressure (0.1 MPa - 1.5 MPa), and mixture volumetric flux (-0.45 m/s - -24.6 m/s). An excellent agreement has been obtained between the newly developed drift-flux model and the data within an average relative deviation of ±15.4%. (author)

[en] Optimizing melt spreading in the aftermath of a core disruptive accident is crucial for achieving sufficient melt cooling to maintain reactor containment integrity. Two approx. = 30 kg-scale experiments performed at the VULCANO facility explore the spreading of high-temperature molten corium-concrete mixtures over ceramic and sacrificial concrete substrates. Imaging of the melt front propagation revealed a 7% increase in spreading length and a 30% increase in maximum front velocity during spreading over sacrificial concrete, despite a reduced mass partaking in spreading due to increased holdup within the crucible. Infrared imaging of the melt indicated surface temperatures around 45℃ lower during spreading on sacrificial concrete, resulting in a roughly three-fold increase in melt viscosity. The enhanced viscosity and reduced mass during the VE-U9-concrete test imply an increased spreadability on sacrificial concrete greater than the observed 7% increase in spreading length. This enhanced spreadability on sacrificial concrete could be explained by the apparent gliding motion of the melt, consistent with reduced friction at the melt-substrate interface. Reduced friction at the melt-substrate interface is best explained by a diphasic film of molten concrete and gaseous concrete decomposition products acting as a lubricant between the melt and solid substrate. (author)