Japan’s Kashiwazaki-Kariwa is the world’s largest nuclear power station, with seven reactors that have the capacity to generate electricity to meet the needs of seven large cities.

In July 2007, an earthquake measuring 6.8 on the Richter scale struck about 10 miles offshore from the plant site. It was the strongest earthquake ever near a nuclear plant. All the reactors shut down safely and all the essential nuclear safety functions, such as automatic reactor shutdown, cooling and containment, worked as designed.

However, measurements showed that the quake caused ground motions exceeding the original specifications of the plant design. Therefore, the plant remains shut down while owner Tokyo Electric Power (TEPCO) continues inspections and safety evaluations of the plant. Included in this exhaustive evaluation is a thorough geological survey to determine new safety standards for ground motion at Kashiwazaki-Kariwa.

Using ground acceleration data from the 2007 earthquake, TEPCO has evaluated the integrity of safety equipment at the site through a combination of analyses and inspections. The company’s scientists have analyzed the seismic effects by calculating movement in the building floors and equipment response. They also have been conducting visual inspections and various functional, engineering and material tests on plant systems and components.

Although the ground acceleration during the 2007 earthquake exceeded the plant’s design for dynamic seismic force, the quake-generated forces applied to the plant’s safety-significant structures, systems and components were of about the same strength as the design standard. Therefore, the operation of these systems was not challenged.

Although inspection results found no significant damage occurred at the plant, the fact that the ground acceleration exceeded one of the reactor design conditions required that TEPCO reexamine its earthquake analysis. The company also reviewed the conventional method for estimating ground acceleration values.

After careful investigation and analysis of the earthquake data and a thorough geological survey, TEPCO identified geological structures at the Kashiwazaki-Kariwa site that contributed to the intense ground motion. These newly discovered factors have been incorporated into determining the plant’s new safety criteria for ground motion. To further enhance the safety of the plant, TEPCO is reinforcing plant safety-related structures and equipment so that they could withstand a quake exceeding the revised criteria for all seven units.

The company submitted a report detailing these analyses to Japan’s Nuclear and Industrial Safety Agency (NISA) in May. The revised ground motion criteria are being reviewed by NISA and Japan’s Nuclear Safety Commission. According to the site superintendent, the external inspections will be completed by the end of July. TEPCO will conti-nue to review and confirm the seismic safety of the Kashiwazaki-Kariwa plant based on the new ground motion criteria and on information gained from the NISA review process.

TEPCO also has enhanced emergency preparedness at the plant. The company has improved radioactive materials management and communication systems and has significantly improved its fire response system. A new technical support center that meets rigorous earthquake standards also was built. Through such a facility, TEPCO plans to keep community leaders and the public informed about the seismic improvements at the plant.

TEPCO is determined to strengthen its nuclear power stations with added seismic safety and emergency preparedness and is committed to sharing lessons learned with the nuclear energy community worldwide, believing this will contribute to the improvement of nuclear safety. For more information, visit http://www.tepco.co.jp/en/index-e.html.

—Nuclear Energy Insight, August/September 2008