Accident tolerant fuel rod failure under low stress: A case study of BWR under station blackout using Bison

Accident tolerant fuels (ATFs) such as FeCrAl cladding have been designed to have improved performance in normal operation and provide increased coping time during design basis accidents (DBA) and beyond DBA. A key aspect in the evaluation of ATF designs is to determine potential increases in the coping time with respect to advancements in fuel and cladding materials. The station blackout (SBO) event has been widely recognized as one of the most severe conditions that a nuclear power plant can experience. In this comparative study, the fuel performance code Bison is utilized to determine the coping time of FeCrAl and Zircaloy claddings under a Short-Term SBO (ST-SBO) accident without any mitigation measures. In particular, four fuel rod burst criteria for Zircaloy cladding and two for FeCrAl cladding are used. It is found that none of the cladding burst criteria are fulfilled, and hoop stress is quite low in this BWR SBO design for both claddings, a stark difference from LOCA or PWR SBO events where high stress burst is common and has been demonstrated widely. Specifically, a new cladding failure mechanism for high temperature and low stress is identified in this study. The origin of the low stress at burst is also investigated.