Evaluation of a hybrid in-vessel retention strategy with ex-vessel cooling for APR1400 under extended station blackout conditions

The purpose of this study is to examine the success window of a hybrid in-vessel retention (IVR) strategy coupled with ex-vessel cooling (ERVC) under an extended Station Blackout (SBO). The high-power-density reactor, APR-1400, is selected and modelled using the computer code ASYST, to examine the thermal–hydraulic response and evaluate the efficacy of a hybrid IVR-ERVC strategy as the accident progresses. Specifically, the hybrid IVR-ERVC strategy refers to combining in-vessel injection as well as ex-vessel cooling to maintain the vessel integrity. Naturally, depressurization of the pressure vessel, which is a precursor to the in-vessel injection, is also applied. The hybrid IVR-ERVC strategy is meant to mitigate the accident and prevent a vessel breach using a set of operator actions within the framework of severe accident management guidelines (SAMG), capitalizing on the portable equipment of the Diverse and Flexible (FLEX) strategy. Three high level candidate actions (HLCAs), namely primary-side depressurization and in-vessel injection along with ex-vessel cooling via cavity flooding are systematically implemented to assess their effectiveness in maintaining the vessel’s integrity for a mission time of 72 h. By combining those high level actions, the corium can be cooled both internally as well as externally to avoid the critical heat flux bottleneck.