Beyond Design Basis Event Considerations
There is a set of accident conditions that are more severe than those included in the plant design basis. These are called beyond design basis accidents (BDBA) in the United States (design extension conditions or DEC in Europe) and include accidents that start within the design basis but progress to more severe conditions due to additional failures. There are two distinct set of procedures and guidance to provide assistance to the plant staff in responding to these BDBAs. The first set of procedures and guidance is applicable to accidents that are progressing toward significant core damage and are beyond the scope of traditional emergency operating procedures, such as an extreme external event. For these types of events, additional guidance beyond the emergency operating procedures is needed. The goal of this BDBA guidance is to arrest the accident progression before significant core damage occurs. The second set of procedures and guidance is applicable to accidents that have already progressed to significant core fuel damage. The goal of this severe accident guidance is to prevent or minimize challenges to the remaining fission product barriers. Both sets of procedures and guidance are explored in this article.