ABSTRACT Pool scrubbing codes combining bubble dynamics and aerosol transport models are used to determine the pool scrubbing efficiency. However, the effect of bubble parameters, diameter and bubble rising velocity (v b), on retention efficiency was not yet analysed much in detail. In this experimental study, the effects of air content, S/P temperature, submergence, and downcomer diameter on bubble parameters at the pool surface are evaluated under wetwell venting conditions applying the backlit shadowgraphy technique. With increasing air content in the carrier gas, v b decreased while no significant effect was observed on Sauter mean diameter (SMD). Under the deeper submergence, the results of SMD and v b were less scattered. With increasing pool temperature, SMD and v b decreased. The thermal stratification of pool water did not affect bubble parameters. Downcomer diameter did not affect SMD, while slight increase in v b was observed. Measured SMD and the corresponding v b SMD have a fair agreement with the values adopted in the pool scrubbing codes; thus, the deposition velocities of gravitational sedimentation, inertial impaction, and Brownian diffusion demonstrate fair agreement with those based on the measured data. Steam condensation is the dominant retention mechanism except for low subcooling conditions. Therefore, modelling of steam condensation retention mechanism should be further carefully examined.