Experimental and theoretical research on upper plenum entrainment with air-water and steam-water

Abstract

Upper plenum entrainment phenomenon occurs in the automatic depressurization process during the Small Break Loss of Coolant Accident (SBLOCA) in reactor pressure vessel, which may result in reactor disaster. The upper plenum entrainment experiments with and without reactor internals were carried out with air-water and steam-water as working mediums on the Automatic Depressurization and Entrainment Test Loop for Upper plenum entrainment (ADETEL-U) which scaled after AP1000 nuclear reactor. The experimental phenomena were observed by visualization method and the reliable data were collected and analyzed. The results indicate that the entrainment rate will increase with the increase of gas flow rate under the same $h_g^{\ast }$, and the entrainment rate will decrease significantly with the decrease of the mixed liquid level when the range of $h_g^{\ast }$ is low. The results confirm that a large number of liquid droplets will be deposited on the surface of the reactor internals, which greatly reduces the entrainment rate. Under the same conditions, the entrainment rate with the reactor internals is about 10% of that without the reactor internals. There is a huge discrepancy between the existing pool entrainment rate models and the experimental data, with the maximum deviation of 200 times. Based on the experimental results, new upper plenum entrainment models for near surface region and high gas flux region of momentum controlled region are proposed. The error decreases by orders of magnitude compared to existing models, which suggested that the new model can accurately predict upper plenum entrainment phenomenon in the pressure vessel.