A beyond design basis event resulted in severe core damage at 3 units at Fukushima Dai-iche. In response the International community identified lessons learned, conducted initial stress tests, and requested nuclear power plant operators to conduct additional evaluations and act on the results of these evaluations. In the US actions have included development of enhanced strategies to deal with an extended loss of offsite power or the ultimate heat sink, design and procurement of portable equipment to support core cooling indefinitely, and external hazard evaluations. The key external hazard evaluations are related to external flooding and seismic hazards.
In addition to directed evaluations, NRC, ASME and NEI are pursuing refinements to the risk informed approach to decision making. The use of PRA in a risk informed decision making process, combined with defense in depth and safety margin considerations, has expanded considerably in the past two decades. So the questions addressed in this paper are: Why expand this approach further? How are PRA and traditional practices balanced? And what is the outcome if we do not expand this approach?
The insights are based on the ongoing activity by NRC to address Near Term Task Form (NTTF) Recommendation 1 [1,2] NUREG-2150 [3] which proposes a more risk informed regulatory process, ASME [4] and Industry activity lead by the Nuclear Energy Institute (NEI) [5]; and past and ongoing uses of PRA.
In summary, the paper concludes that an improved and increased use of a risk informed process is appropriate so as to better address lessons learned from the events at Fukushima, to better understand and act on limiting plant features (“vulnerabilities”), and have a framework in place to address any future beyond design basis events or new information more coherently.
A key lesson learned from Fukushima Dai-ichi is that PRA can identify vulnerabilities which can be acted on to reduce the potential for loss of another plant. But this requires the International community to embrace PRA, and to date the International community has not. This is because most in the Industry rely on checklists and are challenged to envision low frequency high consequence events.
