The in-containment refueling water storage tank (IRWST) is an advanced design concept that has been adopted for the APR-1400 (a reactor type recently developed by Korea). It condenses high-temperature high-pressure fluid discharged from the reactor cooling system through pressurizer relief valves during transients. The condensation of high enthalpy fluid increases the temperature of the coolant in the IRWST. If the temperature of the water storage tank exceeds the temperature limit of 93.3°C (bubble escape temperature), oscillatory vibration occurs, and part of the steam does not condense and instead rises until it is discharged to the air inside the water storage tank. This phenomenon burdens a mechanical load upon the water storage tank structure and thereby compromises structural integrity of the IRWST. In particular, as the IRWST spargers are installed asymmetrically, they cause an uneven temperature distribution and then raise the topical temperature to the bubble escape temperature so prematurely that the cooling efficiency of the IRWST water storage tank may deteriorate.

To improve the previous experiment [KSME Int. J., 18, 820 (2004)] and simulate these conditions, a cylindrical water storage tank was fabricated with a height and volume ratio identical to the actual IRWST, that is, 1:1 and 1:400, respectively; then, the steam condensation pattern and temperature distribution inside the water storage tank were observed and measured. The result of the experiment revealed that the horizontal temperature distribution was quite uniform and that the temperature was the highest on the surface of the water except near the sparger; in particular, the temperature of the surface of the water between the two spargers was the highest. And, a relatively uniform vertical temperature rise was observed. However, in the lower part of the tank (lower than 40 cm from the end of the bottom hole), the distribution revealed many interesting things related to the natural convection flow patterns. Also, when bubbles escaped at the temperature limit, a severe vibration and an attendant noise were observed.