Upper plenum entrainment plays an important role for the coolant inventory in small break LOCA. When the mixture level in the pressure vessel is lower than the height of the hot leg, the coolant will still be entrained to the hot leg in the form of upper plenum entrainment with further decrease of the liquid level in the pressure vessel, posing threat to core uncovering and melting. ADETEL (Automatic Depressurization and Entrainment Test Loop) was scaled after the automatic depressurization lines and the pressure vessel in AP1000 to investigate the upper plenum entrainment. The main droplet entrainment mechanisms from the two-phase interface to the hot leg has been obtain, including soaring, splashing, fragmentation, hitting and fusion. The dimensionless analysis was applied obtain the significant dimensionless number based on the mass conservation and energy conservation. It can be found that the predict data was different from the experiment data with an error of ±50%.

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