Safety by Design

When a major earthquake struck the northern coast of Japan in July, the Kashiwazaki-Kariwa nuclear power plant responded as designed, shutting down its operating reactors safely.

A “defense in depth” philosophy employed by nuclear plants in Japan, the United States and other nations uses robust plant design and construction and redundant, physically separated safety systems to ensure public health and safety even in severe circumstances like earthquakes.

Nuclear plant seismic design must meet national codes , standards and regulations.  Compliance with these standards and regulations ensures there is a substantial safety margin with respect to earthquakes.

Nuclear plants are designed to withstand earthquake-induced ground motions, focusing on systems and components most important to safety, such as critical buildings and systems involved in safely shutting down the plant and keeping it secured.  The design of noncritical buildings, such as offices, uses safety standards closer to typical commercial facilities.

This commitment to safe plant operation begins with a detailed evaluation of potential earthquake-induced ground motion at the site, followed by a thorough analysis and testing of plant structures, systems and equipment, using simulated earthquake-induced vibrations.  If a tremor above a specified level affects a plant, officials must perform extensive inspections before a company can restart a reactor.

All U.S. nuclear plants are designed to withstand earthquakes of a magnitude that is equivalent to or greater than the largest known tremor for the region where it is built.  Plant operators will shut down the reactor even if the seismic event is well within levels the design can accommodate.

Operators then perform extensive inspections prior to restarting the plant.  If an earthquake exceeds the maximum level, the plant cannot restart without U.S. Nuclear Regulatory Commission approval, following extensive inspections to determine if it is safe to resume power production.

Each nuclear plant has seismic instrumentation to record earthquake-induced motions at the site.  For instance, Kashiwazaki-Kariwa has 97 seismographs on its site.  Plant operators use the recordings to evaluate the level of earthquake vibrations at the site and determine if it must shut down.  Detailed physical inspections supplement the recordings to evaluate the impact of an earthquake at the site and the condition of the plant structures, systems and equipment.  In the event of an earthquake, employees analyze the recordings and the inspection results before restarting the reactor.

Engineers and scientists calculate the potential for earthquake-induced ground motion for a site using a wide range of data.  They also review the impacts of historical earthquakes up to 200 miles, with careful study given to those within 25 miles.  They use this research to determine the maximum potential earthquake that could affect the site.