Experimental and theoretical research on liquid entrainment in AP1000 ADS blow-down phase of SBLOCA

The fourth stage Automatic Depressurization System (ADS-4) 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 reduces the coolant inventory in the primary loop and poses great threats to reactor core uncovering and melting. To make up for the deficiency of liquid entrainment database in advanced passive safety PWR, the Automatic Depressurization and Entrainment TEst Loop (ADETEL) modeled after API000 with a scaling ratio 1:5.6 was constructed, and the test data of ADS-4 entrainment and upper plenum entrainment were collected and analyzed. The experiments were conducted at atmospheric pressure with the maximum steam flow rate of 700kg/h. The entrainment processes were recorded by a high speed camera and interesting details were captured. A clear difference exists between ADETEL ADS-4 entrainment onset data and available correlations, which may caused by the differences in fluids physical properties, horizontal pipe flow conditions, determination methods of entrainment onset and geometries (e.g., diameter ratio between branch and hot leg, d/D). The upper plenum entrainment test data shows that the entrainment deposition effect of the reactor internals is smaller than the entrainment promotion effect due to the decrease of flow area and acceleration of steam in die upper plenum. Using the visualization information gained from experiments, the authors develop a model for predicting the entrainment onset in upward tee branch. The model is compared against the experimental data and shows good agreement.