Suppression pool testing at the SIET labs (2) Steam chugging investigation under the presence of non-condensable gas in a vertical opened pipe

Chugging of steam direct contact condensation is a well-known phenomenon and one of the most studied in connection to steam direct contact condensation in the suppression chamber of nuclear power plants. Chugging occurs at small mass flux, low water temperatures and large pipe diameters and is characterized by a strong bubble implosion and cyclic behavior. In the past, chugging has been mainly studied in relation to the high pressure load it exerts on the pool structures, in particular during the late stage of a loss of coolant accident. The focus of the present experimental investigation is to comprehend the effects of air on the bubble growth and implosion. Such conditions are of main interest after the reactor pressure vessel failure during a severe accident progression. In the performed experiment a relatively large diameter steel pipe (0.2 m) was employed with two different steam mass flow rates, respectively 30 and 100 g/s and air mass flow rate varying up to 8 g/s. The water temperature has been measured at sixteen locations in the pool at four different elevations and a high speed camera has been employed to visualize the bubble growth and implosion. It has been found that the presence of non- condensable gas, also in a relatively small amount, can significantly decrease the strength of the bubble implosion and the subsequent effects on the structures. Nevertheless the presence of non- condensable maintains the oscillatory behavior of the steam-water interface, which characterizes the chugging phenomenon, so that large water mixing occurs within the pool. The present study aims to drive the creation of generic physical models to be used in both lumped parameters severe accident codes and detailed CFD simulations.