Experimental investigation of liquid entrainment in ADS-4 depressurization line with steam-water

The fourth stage Automatic Depressurization System (ADS-4) is an important part in Westinghouse AP600/AP100 which enables controlled depressurization of reactor coolant system during small break LOCA. However, the coolant may be entrained into the containment through the ADS-4 branch line simultaneously, which poses great threats to reactor core uncovering and melting. To investigate the entrainment phenomenon of ADS-4 in AP1000, ADS-4 Depressurization and Entrainment TEst Loop (ADETEL) with a scaling ratio of 1:5.6 was constructed and visualization experiments were conducted with steam-water. The experiment were conducted at atmospheric pressure with the maximum steam flow rate of 700 kg/h. The entrainment processes were recorded by a high speed camera and analyzed in detail. Onset of entrainment and entrainment rate data were obtained and compared with existing test data and correlations. Empirical correlation of ADS-4 branch entrainment onset has been developed. A clear difference exists between ADETEL entrainment onset data and available correlations, which may caused by the differences in fluids physical properties, horizontal pipe flow conditions, determination of entrainment onset and d/D ratios. The entrainment onset is more likely to be reached in small d/D ratios conditions due to the stronger Bernoulli effect. Besides, AP1000 has a higher branch quality than AP600 at the same liquid level in the hot leg.