Development of a downward-facing nucleate boiling correlation for thermal hydraulics analysis

The concept of in-vessel retention (IVR) by passive external reactor vessel cooling (ERVC) in a flooded cavity during a severe accident has now been recognized as a viable approach to retain the radioactive core melt (i.e., corium) within the reactor vessel. During the process of long-term in-vessel cooling of core melt, the heat flux must remain below the critical heat flux (CHF) level to maintain a regime of nucleate boiling such that the integrity of the reactor pressure vessel (RPV) is not compromised. In this study, steady-state data for downward facing boiling (DFB) obtained under simulated IVR-ERVC conditions in the Subscale Boundary Layer Boiling (SBLB) test facility is critically analyzed based upon which a new DFB correlation is derived mathematically from theoretical considerations with the coefficients resulting from the scaling analysis being determined from the DFB data. The new correlation which adequately predicts the local variation of the nucleate boiling heat flux along the outer surface of the RPV under IVR-ERVC conditions can be used to describe the long-term cooling behavior of the corium within the RPV of advanced nuclear power plants.