United States Nuclear Regulatory Commission - Protecting People and the Environment

Flooding and Seismic Defense

Addressed Potential Impacts from Natural Disaster

Flooding and Seismic Defense icon, consisting of a solid round, olive green cirle, with a white outline of seismic wave representationIn light of the Fukushima plant damage from the extreme earthquake and tsunami, the NRC requested U.S. nuclear power plants operators to perform detailed "walkdown" inspections of their installed seismic and flooding protection features. The operators ensured the features met current requirements, and identified, corrected, and reported any degraded conditions. NRC inspectors performed follow-up reviews.

Flooding and Seismic Defense icon, consisting of a solid round, blue-green cirle, with the bottom third, a white outline of water with pointy little wavesBased on advances in the knowledge and understanding of seismic and flooding hazards and given the severity of the event at Fukushima Dai-ichi, the NRC requested the licensees of operating reactors to reanalyze potential flooding and seismic effects. These reevaluations used updated information and methodologies to inform plant operators of potential impacts to their sites. As a result, several nuclear power plant owners modified the protection of certain plant structures, systems, and components, or they identified alternative strategies to maintain the safety of the reactors in the event of a flooding or seismic event.

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Image of generic nuclear plant that highlights Flood Protection features including: sand bags, inflatable berms, pump, site drainage, permanent flood walls, diesel generators, plant elevation and watertight doors.

Examples of Flooding Protection Features

  1. Sandbags and/or Inflatable Berms: Temporary watertight barriers that would be installed in advance of a flood, designed to keep the site dry.
  2. Pumps: Temporary and permanent pumps can alleviate the effects of flooding by removing water from the site.
  3. Site Drainage: A site's design can reduce or even prevent flooding by directing floodwaters away from vital power plant areas. Drainage systems can be either man-made (drains) or natural (grading).
  4. Permanent Flood Walls: Man-made structures primarily designed to prevent floodwaters from entering a power plant site. These walls, built from engineered and/or natural materials, also reduce flood effects associated with erosion, wave action, and debris flow.
  5. Plant Elevation: Permanently increasing the elevation of a nuclear power plant's site during construction helps to ensure potential flooding sources are less likely to affect the site.
  6. Watertight Doors: Specially-designed doors, similar to those found on ships, keep floodwaters away from vital reactor systems and equipment, such as emergency diesel generators.

Examples of Seismic Protection Features

  1. Anchorages and Restraints: Mechanically attaching equipment to a surface to prevent movement during an earthquake. This minimizes potential damage to the restrained equipment as well as nearby equipment.
  2. Spatial Separation: Physical separation of equipment or structures to minimize their interactions during an earthquake.
  3. Isolation Systems and Dampers: Structural elements that substantially decouple equipment or structures from the base substructure impacted by an earthquake. This protects the structural integrity of the isolated equipment or building.

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Page Last Reviewed/Updated Friday, January 04, 2019