Suppression pool testing at the SIET labs (3) Experiments on Steam Direct Contact Condensation in a Vertical Multi-holes Sparger

During the first phase of the Fukushima Daiichi severe accident, the reactor core isolation cooling system (RCIC), installed in unit 2 (1F2) and unit 3 (1F3), was operated in order to maintain high core water levels. Even though the RCIC systems in the two units have quite similar design, the pressure increase in the two suppression chambers was significantly different. The main RCIC design is identical in the unit 2 and 3, except for the RCIC sparger. While in the 1F2 it is a vertical pipe open at its bottom, in the 1F3 it is a vertical multi-hole sparger, with 432 holes with 1 cm diameter along its top region, and 252 holes with 2.5 cm diameter in its bottom region. The very different configuration is believed to play an important role in the condensation methods affecting the water temperature and the creation of hot spots in the pool. In the present activity experiments are carried out on a scaled down transparent pipe with steam mass flow rate 24 times smaller than the real RCIC value. Given the large sensitivity that the holes diameter, steam mass flux and water head, have on the condensation methods, scaling has been performed in order to maintain almost 1:1 ratio for such values. The water temperature has been measured at 12 locations in the water pool and a high-speed camera has been employed to visualize the bubble formation through the holes and the steam behavior within the pipe. High speed imaging has shown that the 2.5 cm holes were sufficiently large to trigger chugging phenomena, transferring a large amount of water inside the sparger so that a large depressurization is created and the liquid water is sucked backwards within the pipe. These phenomena are repeated in a nearly cyclic behavior and, at every cycle, a large amount of water is transferred from the pipe to the pool with large velocity thus enhancing the water mixing. Once the pool temperature reaches around 45 degrees Celsius, the large oscillations cease and bubbling occurs only from the top region of the pipe (i.e. the small holes region). Such sparger design leads to the creation of stable stratification and to the most unwanted condition in case of severe accidents, because it drastically decreases the efficiency of the suppression chamber as heat sink.