Direct Contact Steam Condensation in unstable regimes at sub-atmospheric pressure and at low subcooling: Experimental and Theoretical Analyses

The main safety system of ITER (International Thermonuclear Experimental Reactor), named Vacuum Vessel Pressure Suppression System (VVPSS) utilizes Direct Contact Condensation (DCC) to prevent over pressurization of the Vacuum Vessel during Loss of Coolant Accident (LOCA) or Loss of Vacuum Accident (LOVA). The VVPSS directs the steam at sub-atmospheric pressure through a multi-hole sparger into four Vapour Suppression Tanks (VSTs). One of the most critical aspects of DCC is the onset of unstable condensation steam regimes under certain thermohydraulic conditions, which can generate significant pressure loads on the pool structure. Specifically, the use of a multi-hole sparger can cause neighbouring steam jet plumes to coalesce, leading to potentially dangerous unstable condensation regimes. This doctoral thesis investigates these unstable condensation regimes caused by the coalescence of the steam jet plumes under low steam mass flux, low pool subcooling temperatures and sub-atmospheric pressure, using a multi-hole sparger. During these regimes, large steam bubbles form due to coalescence. These bubbles detach from the main steam flow, after passing the last row of holes, collapse and generate strong pressure loads, which cause a reverse flow of subcooled water into the sparger. The entry of subcooled water triggers Condensation Induced Water Hammer (CIWH) inside the sparger. The thesis includes also a preliminary CFD analysis to simulate these phenomena and proposes a methodology, based on the sparger design correlation to prevent them. In addition, the thesis investigates the influence of non-condensable gases and dust on unstable condensation regimes. Understanding the role of these components is essential, as they significantly affect steam condensation efficiency and the pressure loads generated during such regimes. Specifically, the injection of non-condensable gases reduces the heat transfer coefficient between steam and water, thereby lowering the fraction of steam energy absorbed by the water. This reduction helps mitigate pressure spikes associated with unstable condensation. Conversely, the presence of dust enhances the energy absorbed by the water at elevated temperatures, promoting more efficient heat transfer.