A Preliminary Severe Accident Analysis of a Generic Small Boiling Water Reactor

Abstract

Canadian Nuclear Laboratories (CNL) has been conducting research to support the deployment of the various types of small modular reactors (SMRs) in Canada. Among the different SMR technologies, Canada is poised to start deploying several small, natural-circulation-cooled boiling water reactors (BWRs). These types of small-BWRs utilize two major systems that are natural-circulation-driven: the isolation condenser system (ICS) for decay heat removal, and the passive containment cooling system (PCCS). While the use of such passive safety systems is likely to reduce the overall likelihood and severity of accidents, it is still critical to understand how these passive systems could be impaired, and where in accident space that is possible. These then help to inform what mitigating equipment, accident management guidelines, and emergency preparedness plans would need to be in place to address the lower frequency extreme events. These efforts include a thorough understanding of the severe accident behavior of the small BWR and a good estimation of the associated radionuclide source terms. This paper summarizes CNL’s initial efforts on the severe accident analysis of a generic small BWR. The analysis was performed using the MELCOR severe accident analysis code with a focus on two major types of events, 1) a station blackout accident (SBO) with impaired ICS, and 2) a small-break loss-of-coolant accident (LOCA). The effectiveness of ICS and PCCS were assessed with particular interest in the impact of released hydrogen on the performance of these systems.