Experimental study on the effects of fine bubbles on polydisperse submicron aerosol removal efficiency during pool scrubbing

Radioactive aerosols are strongly diffusive and migratory and thus have presented one of the greatest challenges during the decommissioning of the Fukushima Daiichi Nuclear Power Plant (NPP). Although cutting through debris underwater can suppress the generation of radioactive aerosols from pool scrubbing to some extent, the removal efficiency of bubble columns can be influenced by many factors. In this study, fine bubbles (microbubbles and nanobubbles) with large specific surface areas were introduced into a simple scrubber; nanobubbles, in particular, are known to have long residence times in water. The effects of fine bubbles on the aerosol removal efficiency during pool scrubbing were studied for TiO 2 (around 100 nm) and ZrO 2 (around 100 nm) aerosols. Due to the fact that TiO 2 (4.23g/cm 3 ) has similar density with CsOH (3.68g/cm 3 ) and CsI (4.51g/cm 3 ). On the other hand, ZrO 2 was found in the fuel debris (Zirconium-Water Reaction). To clarify the effects of fine bubbles, three kinds of water were prepared (i.e., distilled water, nanobubble water, and microbubble water). As a result, the removal efficiency of fine bubbles for TiO 2 aerosols decreased, while that observed for ZrO 2 aerosols improved in some cases. The improved removal efficiency achieved using fine bubbles may provide a new method for suppressing the generation of radioactive aerosols in the decommissioning of the Fukushima Daiichi NPP. the submicron radioactive aerosols due to pool scrubbing the removal efficiency of bubble columns by many In the of aerosol by described by Fuchs (1964) and Ghiaasiaan (1997), the internal circulation of gas inside a rising gas bubble is equivalent to Hill’s vortex and the absorption of aerosols at the interfaces of the rising bubbles can be described by inertial deposition, sedimentation, and diffusion. These three processes are related to particle size, bubble size, and the velocity at which bubbles rise. Many methods have been proposed to increase the amount of aerosols removed by the water phase, including air bubble subdividing devices (Cadavid-Rodriguez, 2014) and surfactants (Koch, 2012). Additionally, it has been shown that the strength of the internal circulation within bubbles is an important parameter affecting its particle removal rate. Slower internal circulation due to the presence of surfactants at the water–bubble interface will greatly reduce the particle removal rate (Friedlander, 2000). However, by adding surfactants, many microbubbles (MBs) can be generated at the nozzle inlet (Koch, 2012) and more particles can be removed from the resultant MBs due to their small size. Therefore, to achieve the highest removal efficiency with surfactants, the trade-off between internal circulation and bubble size is unavoidable.