Lining Up New Protections with New Flood Info

Lauren K. Gibson
Project Manager
Japan Lessons Learned Division

Walkdowns (3)The NRC is moving forward on connecting two important lessons we learned from the Fukushima nuclear accident in Japan: protecting key safety functions and reevaluating flood hazards. The agency’s ongoing work would require U.S. nuclear power plants to ensure their protection strategies account for updated flood levels.

The Commission has approved the staff’s plan for completing the reevaluated flooding hazards review. The staff’s plan also covers how U.S. plants must account for the new hazards in their mitigation strategies for beyond-design-basis events. The plan requires U.S. plants to determine which flood hazard data could affect their strategies. We believe this approach is the quickest way to provide the most significant flood protection improvements.

The NRC assesses plants’ re-evaluated flood hazards to see whether the re-evaluated hazards were properly calculated. Plants need these assessments to evaluate their strategies against the re-evaluated hazard. We’re still reviewing some plants’ work; we’re issuing interim letters so those plants know how to follow the rest of the staff’s plan.

The plants examine whether their strategies work under the new hazard conditions and make any appropriate adjustments. For example, a strategy might require a pump in a location submerged by the new possible flood level. The plant would then consider options such as relocating the pump. These assessments and adjustments would be substantially complete by 2016.

The second part of completing the flooding hazard work involves either a focused evaluation or a broader integrated assessment of the plant’s protection capabilities. The specific work depends on:

  1. Which hazards, if any, cause flood levels higher than the plant’s original level.
  2. Whether the plant’s flood protections have available physical margin. (For example, if the new flood hazard level is six feet and a plant’s existing wall is seven feet tall, the wall has available physical margin to handle the new flood level.)
  3. Whether the higher flooding levels disable the plant’s ability to cool the reactor core or spent fuel pool, or protect containment.

If the local intense precipitation hazard is the only cause of a higher level, then the plant performs a focused evaluation. If other flooding hazards are involved, but the plant has available physical margin and can maintain safety functions, then the plant only needs a focused evaluation. The focused evaluation would identify any physical or procedure changes needed to address the new flood level. We would review and inspect these changes to ensure they resolve the issue.

The remaining plants would perform an integrated assessment, looking at all flooding hazards and identifying any changes needed to protect the plant from the new hazard. We’ll review these assessments and decide if voluntary plant actions would be effective or if the NRC must order plant changes.

You can find out more information about Recommendation 2.1—Flooding on the Japan Lessons Learned portion of the NRC website.