Behavior of air bubbles injected into a water vessel of 30°sector model in half length scale of a nuclear reactor vessel is observed in a water pool and classified into characteristic zones according to bubble formation types. Released gas forms globule bubbles in the release zone above an air injection nozzle. In the split/coalescence zone, globule bubbles split into small bubbles of about 10 mm diameter and these small bubbles rise in the water pool to coalesce into large bubbles near the pool surface. Depletion of 0.3 - 2.7 μm dioctyl sebacate (DOS) aerosols injected into the water pool is determined under a stagnant flow condition of the water pool. Deposition of DOS aerosols in tubing from an aerosol generator to the water vessel and the water droplets generated by the breakage of bubbles at the pool surface are measured in error estimation. The decontamination factor (DF) of DOS aerosols rising through the water pool increases in an exponential function of the water head between 0 and 3 m, giving a value of over 100 at 3 m water head. Aerosol diameter has a weak influence on the value of DF.
