Large-Scale Experimental Analysis of Steam Sub-Atmospheric Condensation for ITER Vacuum Vessel Pressure Suppression System During LOCA Event

The loss of coolant accidents (LOCA) postulated to occur in the vacuum vessel (VV) of the International Thermonuclear Experimental Reactor (ITER) entails dangerous VV pressurization, which is foreseen to be mitigated by steam discharge into the Vacuum Vessel Pressure Suppression System (VVPSS). This safety system realizes steam direct contact condensation (DCC) in four Vapour Suppression Tanks (VSTs, 100 m3 each) at a very characteristic initial condition: subatmospheric cover gas pressure, for receiving steam from the VV at low pressure (0.2 ÷ 1.5 bar abs). This upper limit was defined to preserve the VV structural integrity. The scenario of steam DCC at vacuum condition in a VVPSS tank was extensively experimentally investigated at University of Pisa (UNIPI) Laboratory Guerrini, in a suitable Large-Scale Test Facility (LSTF), assuming a wide range of water pool temperature, injected steam mass flow rate and water pool pressure, characteristic for ITER VV postulated LOCA event. In addition, both a scaled and mock-up configuration of the sparger adopted in the VVPSS were implemented and tested in the LSTF. This activity continues and builds up the UNIPI research commitment, started with Small-Scale Test Facility projects (SSTF, condensation vessel of 4.5 m3), increasing the condensation tank volume and steam mass flow rate to 92 m3 and 500 g/s, respectively. This effort aimed to contribute to fill the existing literature gap regarding the steam DCC at subatmospheric pressure. An overall number of 174 steam DCC experimental tests, subdivided in four campaigns, was carried out in the LSTF for water pool temperature ranging between 30 and 100°C, steam mass flow rate 50 ÷ 500 g/s and water pool cover gas pressure 20 ÷ 100 kPa abs. Steady state pure steam and steam plus noncondensable (air) tests were carried out with both sparger system A (scaled) and B (mock-up). In addition, pure steam transient tests were also performed adopting both spargers. The experimental results were plotted in the available condensation map, previously elaborated during SSTF activities at subatmospheric condition, verifying the instability regime domains with the support of the LSTF video acquisition system. In addition, the analysis of pressure drops across both spargers was carried out providing a relation with the injected steam mass flow rate. Finally, the condensation efficiency was evaluated for the whole test campaign showing a value next to 100% (complete condensation), with the exception of tests performed with water pool at saturation conditions (92%). This research activity permitted an extensive characterization of steam DCC regimes in vacuum conditions, occurring in ITER VVPSS during postulated LOCA scenarios, and contributed to evaluate and confirm the VVPSS safe functioning.