The paper presents new, transient simulation results of condensing steam in the event of emergency core cooling where water is injected into the cold leg during a postulated loss-of-coolant-accident (LOCA) of a pressurized water reactor (PWR), a phenomenon known as Pressurized Thermal Shock (PTS). The model is first validated for the Lim et al. (1984) experiment involving a smooth to wavy turbulent, stratified steam-water flow in a 2D channel, then in 3D to predict the experimental data from COSI (EDF-CEA-AREVA). The computational framework is based on interface tracking, combined with large-scale prediction of turbulence, a new methodology known as LEIS (Large-Eddy & Interface Simulation) where super-grid or super-scale turbulence and interfaces are directly solved whereas the sub-grid or sub-scale parts are modelled. Because ergodic steady-state flow conditions are difficult to attain, recourse is made of the V-LES (instead of LES), where the flow-dependent cutoff filter is larger and independent from the grid. The computational approach is completed by a DNSbased interfacial phase-change heat transfer model built within the surface divergence (SD) theory. The original SD model is found to return better results when modified to account for scale separation, i.e. to segregate low-Re from high-Re number flow portions in the same flow. While the 2D validation results are excellent, the 3D PTS results are also good, but need to be run further before reaching statistically steady-state conditions.