Experimental investigation of entrainment effect on the countercurrent flow in the Hot Leg and Pressurizer Surge Line assembly of third-generation passive nuclear reactors

The effect of entrainment from the Hot Leg (HL) to the Pressurizer Surge Line (PZR-SL) on the characteristics of Countercurrent Flow (CCF), is experimentally studied using air-water in a transparent test section, 1/4 scaled-down model of the HL and PZR-SL assembly of AP1000. The typical CCF behaviors are recorded with video cameras for understanding of mechanisms of Countercurrent Flow Limitation (CCFL) affected by entrainment. Two entrainment flow regimes, stratified-flow entrainment and slug-flow entrainment, are found to occur under certain HL pipe water levels and air velocity conditions. The entrainment effect on the characteristics of CCF in the test section is analyzed in detail based on the two-phase air and water Kutateladze numbers, the transient hydraulic parameters and the flow behaviors. The stratified-flow entrainment does not always influence CCFL extent, unless the entrainment is strong enough and reverse flow of water occurs in the PZR-SL pipe. However, the slug-flow entrainment markedly influences CCFL extent. Two empirical correlations are developed respectively for high HL pipe water level situation with slugging entrainment and for medium and low HL pipe water level situation without slugging entrainment. The present data are compared and verified with the Small-Break Loss-of-Coolant Accidents (SB-LOCAs) tests data on AP600 Scaled Integral Test Facility (APEX). The present experimental research provide insight on the CCF flow phenomena in the HL and PZR-SL assembly for the third-generation passive nuclear reactors during the Fourth Stage Automatic Depressurization Stage (ADS-4) of SB-LOCAs and support theoretical model development and the validation of computational codes.