Potential for In-Vessel Retention through Ex-Vessel Flooding

External reactor vessel cooling (ERVC) is a severe accident management strategy that involves flooding the reactor cavity to submerge the reactor vessel in an attempt to cool core debris that has relocated to the lower head. Advanced and existing light water reactors (LWRs) are considering ERVC as an accident management strategy for in-vessel retention (IVR) of relocated debris. In the probabilistic risk assessment (PRA) for the AP600 design, Westinghouse credits ERVC for preventing vessel failure during postulated severe accidents with successful reactor coolant system (RCS) depressurization and reactor cavity flooding. To support Westinghouse's position on IVR, the Department of Energy (DOE) contracted the University of California - Santa Barbara (UCSB) to produce a peer - reviewed report, DOE/ID-10460. As part of their evaluation of IVR for the AP600, the Nuclear Regulatory Commission (NRC) tasked the Idaho National Engineering and Environmental Laboratory (INEEL) to perform an in-depth review of the UCSB study and calculational model, its peer review comments, and UCSB's resolution method to identify any areas where technical concerns weren't addressed. In addition, INEEL was tasked to perform an independent analysis effort to investigate the impact of residual concerns and parameter uncertainties on the margins to failure and conclusions presented in the UCSB study. This paper summarizes results from INEEL's review, and presents independent calculations to resolve residual peer reviewer concerns (overestimated margins, sensitivity calculations, etc.)