Downward facing boiling and steam venting under simulated ERVC conditions

This study investigates the enhancement of external reactor vessel cooling (ERVC) under severe accident conditions. One feasible method involves the use of an enhanced vessel/insulation design to facilitate steam through the bottleneck of the annular channel between the reactor pressure vessel (RPV) and the surrounding insulation structure. To assess the viability of the enhanced vessel/insulation design, steady-state boiling experiments were performed using test vessels surrounded by an enhanced ERVC insulation structure. When compared to the flow behavior predicted by a steam venting model, good agreement is obtained. Improvement of the steam venting process through the narrow gap location is observed, which in turn leads to the enhancement of local critical heat flux values when compared to the case without using an enhanced vessel/insulation design. Detailed analysis of the experimental video record provides further physical evidence for the buoyancy-driven two-phase flow inside the flow channel as well as the bubble dynamics near the local CHF limit. (author)