Mitigation of a Loss of Coolant Accident in ITER Vacuum Vessel by Means of Steam Pressure Suppression

A Loss of Coolant Accident (called ICE cat. IV) is postulated to occur in the ITER Vacuum Vessel due to a rupture of a shielding blanket cooling piping. The mitigation of this accident is realized by means of a Pressure Suppression System, constituted by 4 Tanks of 100 m3 of volume each. The Pressure Suppression Tanks (PSTs) system operates at sub-atmospheric pressure. No experimental results of steam condensation at sub atmospheric pressure conditions have been found in the technical literature. Therefore, both analytical-numerical analyses and experimental tests were carried out at the University of Pisa on a 1:22 scale apparatus, simulating the PST. Scaling studies were performed in order to calculate the effectiveness and functional performance of PST, simulating in the reduced scale apparatus the transient of steam mass flow rate occurring during the ICE IV event. The accidental scenarios have been determined considering the results of previous thermal hydraulic studies performed at ITER. Subsequently, a quite extensive experimental campaign has been carried out on the reduced scale apparatus in order to study the influence of the main thermal hydraulic parameters that characterize the steam condensation efficiency in sub-atmospheric conditions. This paper illustrates the results of analytical and numerical analyses simulating the PST behaviour during the transient of steam mass flow rate due to the Ingress of Coolant Event. The transient has been simulated experimentally applying the elaborated scale laws. The designed tanks of the PST have matched the safety goal to reduce the system pressurization condensing the injected steam at sub-atmospheric pressure.