The panda tests 9 and 9bis investigating gas mixing and stratification triggered by low momentum plumes

This paper presents the results of two tests, performed in the PANDA facility in the frame of the OECD SETH project. The Test 9 and Test 9bis were characterized by the same initial Froude number (Fro ∼ 1) and were performed at constant pressure in the vessels. The vessels were initially filled with air. During the tests the plumes were generated by injecting a constant amount of steam in Vessel 1 and continuous venting of a steam-air mixture from Vessel 2. In Test 9bis the temperature of the injected steam and the initial containment wall and fluid temperatures where chosen to reach later the conditions for (wall) condensation. The on-set of condensation occurred in Test 9bis at the top of Vessel 1 when, due to the steam injection the steam partial pressure increased enough to eliminate the steam superheating. The condensed droplets propagated toward the bottom of Vessel 1, where due to the lower steam partial pressure, a portion of condensate vaporized again. The evaporation had the effect of increasing the steam partial pressure and progressively new conditions for steam condensation were obtained also at the bottom of Vessel 1. The condensation in Test 9bis had the effect of mitigating the reduction of plume buoyancy (which is a function of the air-steam concentration in Vessel 1) and for this reason the overall plume trajectory, steam-air mixing, stratification and transport between the two compartments were affected.