Status of a two-phase CFD approach to the PTS issue

A two-phase CFD modelling approach of the Pressurized Thermal Shock (PTS) problem has been developed and is being validated in the context of PWR life time safety studies. The cold water injection results in strong condensation and complex 3D two-phase phenomena. Direct Contact Condensation (DCC) occurs on the jet and on the free surface of the stratified flow in the leg. These surfaces are much larger than the cells size used in the computational domain, in this sense they can be called large interfaces. DCC depends strongly on the liquid side heat transfer, which is modelled as a function of turbulence, which itself depends on momentum exchange between gas and liquid. A statistical model is used to represent turbulence in each phase. The large interfaces require a special modelling. It has been recently developed and implemented in the NEPTUNE-CFD code which is based on an Eulerian two-fluid model. The present status of this large interface modelling is presented. The validation relies on separate effects experiments such as air-water or steam-water stratified flows and on a more integral experiment, COSI, which represents a cold leg scaled 1/100 for volume and power from a PWR under SBLOCA conditions. The interest of the considered experimental data for PTS CFD is discussed. (authors)