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The Vacuum Vessel Pressure Suppression System (VVPSS), a key safety system of the ITER plant, is designed to protect the Vacuum Vessel (VV) from over pressure occurring in the case of LOCA (Loss Of Coolant Accident) or other pressurizing accidents such as LOVA (Loss Of Vacuum Accident). The steam condensation in the Suppression Tanks (main elements of the VVPSS system), occurs at sub-atmospheric pressure. The steam condensation, at pressures equal or greater than the atmospheric, has been numerically analyzed and experimentally investigated in the past in order to optimize the design of the pressure suppression system of boiling water nuclear reactors. However, very limited experimental data is available concerning the steam condensation in a water tank at sub-atmospheric pressure. In order to analyze the steam condensation in these operating conditions, an experimental study, funded by ITER Organization, is conducted at the Department of Civil and Industrial Engineering (DICI) of University of Pisa. The tests analyze the condensation of saturated or superheated steam at sub-atmospheric pressures (4.2 kPa and slightly above the water vapour saturation pressure), and pool temperature up to 50°C at several heights of water head. The experimental facility, to perform this study, has been set up with a significant scaling factor regarding the full size installation at ITER. In this paper the experimental rig, the conditions of the experiments, and the test matrix are presented. The temperature and pressure measurements with details of the data acquisition system are described. The tests were performed at different patterns of the sparger exit holes (1, 3 and 9) and for three steam mass flow rates per one hole. The results show very high efficiency of condensation for all examined conditions. Finally, a comparison between the condensation regimen at sub-atmospheric and at atmospheric pressure is discussed.

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