Analyses of liquid entrainment through ADS-4 in AP1000 during a typical small break LOCA transient

Abstract The Westinghouse-designed AP1000 is a Generation III + reactor to receive Design Certification from the US Nuclear Regulatory Commission (NRC). It is an advanced passive nuclear power plant which adopts the forces of nature and simplicity of design to enhance plant safety and operations and reduce construction costs. A typical SBLOCA transient in AP1000 can be divided into four different phases: the blow-down phase, the natural circulation phase, the ADS (automatic depressurization system) blow-down phase and the IRWST injection phase. Late in the ADS blow-down phase, the fourth stage ADS (ADS-4) valves attached to two hot legs open and vent directly to the containment. If a critical vapor velocity is reached in the two hot legs, liquid can be entrained out the ADS-4. The liquid entrainment phenomenon is important for safety evaluation and licensing of AP1000 and advanced passive reactors with larger powers in the future. Although plenty of work has been done around liquid entrainment in the past few decades, the proposed liquid entrainment models rely on data from experiments far from AP600/AP1000 prototypic conditions. In the present study, the RELAP5 code is modified with entrainment onset and entrainment rate correlations obtained from the ATLATS test facility which was constructed using proper scaling criterion to preserve the phenomena in the AP600/AP1000 prototype reactor. Furthermore, liquid entrainment behavior through ADS-4 during a 2-in. cold leg SBLOCA in AP1000 is analyzed.