Evaluation of the success of in-vessel corium retention using external reactor vessel cooling mechanism for a small PWR

Among the several study fields of severe accidents, In-Vessel Retention (IVR) via External Reactor Vessel Cooling (ERVC) referred to as IVR-ERVC is a critical subject being explored for various reactor designs. This study aims to investigate and assess the effectiveness of IVR of molten corium in a PWR utilizing the ERVC mechanism. A RELAP5/SCDAP model developed for the assessment of IVR-ERVC was applied to a small PWR after its successful validation. The boundary conditions applied to the model are evaluated through simulation of severe accidents of the small reactor (998 MWth). Thermal hydraulic studies of sensitive parameters have been carried out to determine the safety margin and effectiveness of the IVR-ERVC method under a variety of process conditions. The results indicate that a high mass flux is required to avoid boiling crises at low degrees of subcooling (around saturation conditions). A mass flux of 186.44 kg/m2-s or higher is required to provide a sufficient safety margin for inlet water with degree of subcooling of 8 K. The minimum Critical Heat Flux (CHF) is found to be 2.5 MW/m2 at a mass flux of 186.0 kg/m2-s for this case. Critical Heat Flux Ratio (CHFR) is found in the range of 6.0–17.0 with the cooling water flow rate of 20 kg/s (mass flux of 34.48 kg/m2-s) and 65 K subcooling, which provides a sufficient safety margin to avoid boiling crisis. It is concluded that the ERVC is an effective mechanism for the prevention of RPV rupture under active and passive conditions of the reference reactor.