Study of the stratification phenomena occuring in the injection line of the ECCS during LOCA and fluid-dynamics of an inclined water jet. The ECCS, devoted to the reflooding of the core in case of loss of coolant accident, injects cold water that causes the condensation of the steam; condensation magnitude must be studied to asses temperature and pressure changes and the effect on the pressure thermal shock phenomena (PTS). It has been observed that, for certain values of mass flow rate, the injection pipe on the cold leg is no longer fully filled but it shows a free surface flow. The amount of pipe filled by the water is likely to be linked to the mass flow rate and to the inclination of the pipe.