Separate-effect experiment simulating steam condensation on emergency core cooling system (ECCS) water in PWR cold legs during reflood phase of large-break loss-of-coolant accident (LBLOCA) was conducted in OECD/NEA ROSA Project using the Large Scale Test Facility (LSTF). A test section was furnished in the downstream of the LSTF break unit horizontally attached to the cold leg. The boundary test conditions were defined based on PWR LBLOCA analysis by RELAP5/MOD3.2.1.2 code considering typical Japanese safety analysis conditions. Significant condensation of steam appeared in a short distance from the simulated ECCS injection point, and the steam temperature in the test section decreased immediately after the initiation of the ECCS water injection. Fluid temperature distribution at 50 mm downstream from the ECCS injection point was significantly non-uniform, but became almost uniform in less than 350 mm. Total steam condensation rate estimated from the difference between steam flow rates at the test section inlet and outlet was in proportion to the simulated ECCS water mass flux until the complete condensation of steam. Inlet steam was completely condensed if inlet steam mass flux is less than 195 kg/(m2s) when the simulated ECCS water mass flux is 148 kg/(m2s); equivalent to full high-pressure injection with single-failure low-pressure injection conditions. Clear images of high-speed video camera were obtained on droplet behaviors through the viewer at 200 mm downstream from the ECCS injection point, especially for annular mist flow. The number of flowing droplets decreased with increasing distance from the ECCS injection point.

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