ADS-IRWST transient evaluation model for AP1000 SBLOCA analysis

The AP1000 nuclear power plant safety system is characterized by its unique passive Emergency Core Cooling System (ECCS), including Core Makeup Tanks (CMTs), Automatic Depressurization System (ADS), and In-containment Refueling Water Tank (IRWST), to mitigate SBLOCA accident. The most challenging period of AP1000 SBLOCA transient is the transition from CMT injection to IRWST injection after the actuation of the fourth stage ADS (ADS4). This paper is to report a transient evaluation model for AP1000 SBLOCA analysis during CMT to IRWST transition. The model simulates ADS4, CMT injection, IRWST injection, reactor core mixture level and upper plenum entrainment to hotlegs. Both critical and non-critical flow models are used to calculate mass flow loss through ADS1-4 vents. The most important feature of this model is that it constantly traces core/upper plenum two-phase mixture level, which demonstrates cooling capability of ECCS during the transient. A baseline case is analyzed to study the ADS-IRWST transient using AP1000 prototypic information. In addition, a sensitivity analysis is performed to further study the robustness of AP1000 ECCS during SBLOCA transient.