AbstractNumerical analyses were carried out to predict quantitatively the condensation characteristics between the water and vapor inside a suppression tank in the International Thermonuclear Experimental Reactor (ITER). The suppression tank is one of the ITER safety devices. The pressure rise in an ITER vacuum vessel during an ingress-of-coolant event (ICE) is suppressed by the effect of condensation of vapor in the suppression tank. The water–vapor two-phase flow characteristics with condensation under the ITER pressure conditions were clarified numerically. Moreover, flow visualization experiments were carried out to understand the condensation behavior under low pressure and cold water. The predicted flow configurations agreed well with the experimental results.