The objective of this communication is to present some preliminary applications to pressurized thermal shock (PTS) investigations of the CFD two-phase flow solver of the new NEPTUNE thermal-hydraulics platform, being jointly developed by EDF (Electricite´ De France) and CEA (Commissariat a` l’Energie Atomique) and also supported by IRSN (Institut de Radioprotection et Suˆrete´ Nucle´aire) and FRAMATOME-ANP. In the framework of plant life extension, the Reactor Pressure Vessel (RPV) integrity is a major concern, and an important part of RPV integrity assessment is related to PTS analysis. In the case where the cold legs are partially filled with steam, it becomes a two-phase problem and new important effects occur, such as condensation due to the Emergency Core Cooling (ECC) injections of sub-cooled water. Thus, an advanced prediction of RPV thermal loading during these transients requires sophisticated two-phase, local scale, 3D codes. In that purpose, a program has been set up to extend the capabilities of the NEPTUNE two-phase CFD solver. The major challenge is to develop new physical models to take into account the complex two-phase phenomena: free surface, jet of cold water through steam, entrainment of droplets by steam, entrainment of bubbles by water, and, above all direct contact condensation. Turbulent mixing in the liquid layer plays a central role by controlling at the same time the thermal mixing in the water and the condensation rate at the interface between liquid and vapor. A sustained effort in this area has started both at CEA and EDF. New models are developed and validated with the help of available experimental data sets, which are unfortunately very few. Beyond analytical single-effect cases, it is important to compute instrumented cases close to the life-size one, including major physical effects. This is the case of the COSI experimental facility, representing a cold leg with ECC injection, where temperature profiles have been measured. Numerical results on a steady-state COSI test are presented and analyzed, showing encouraging results. However, research work is still required to achieve a better modeling of the heat transfer and turbulence at the steam-water interface.

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