The study is done in the context of Pressurized Thermal Shock (PTS) CFD investigations related to PWR life duration safety studies. In the simulations of such situations direct contact condensation on free surfaces much larger than cells size is a key phenomenon. Those large interfaces require a special full set of models which has been recently implemented in the NEPTUNE_CFD code. Out of large interfaces regions, some dispersed or churn flows can take place. Both situations can be taken into account in the computational domain. The approach includes an interface recognition but not an interface reconstruction. It is evaluated on COSI experiment. COSI facility is scaled 1/100 for volume and power from a 900 MW PWR under LOCA thermal hydraulic conditions. Measurements include temperature profiles at various axial positions in the pipe. The paper focuses on one test corresponding to a situation with a low water level in the leg. It is a demanding case for two-phase CFD because -a- the liquid layer which is of prime importance in PTS studies can be quite thin in comparison with the whole computational domain -b- the emergency core cooling jet plays an important role. Results are within a reasonable range even with a coarse mesh. Calculations with finer meshes quantify the effect of a better simulation of the jet fall and impact on the wall.

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