Study of Effects on the Seismic Safety of Hamaoka Nuclear Power Station Reactor No. 5 Based on the Experience of the Suruga Bay Earthquake
December 15, 2010
Chubu Electric Power Co.,Inc.
Chubu Electric Power has analyzed the reasons for the fact that Hamaoka Nuclear Power Station Reactor No. 5 experienced more severe tremors than other reactors during the Suruga Bay earthquake which occurred in August last year, and has studied effects on the seismic safety of the reactor based on the experience of the Suruga Bay earthquake. The findings have been reported to a government Working Group.*1
The details reported to the Working Group are summarized below.
Outline of details reported to government Working Group:
1. It was determined that the main cause for the more intense tremors recorded at Reactor No. 5 in comparison with other reactors was a shallow low-velocity layer*2 identified as extending from underneath Reactor No. 5 in an approximately easterly direction.
2. With regard to the determination of effects on the seismic safety of Reactor No. 5 based on the experience of the Suruga Bay earthquake, and taking into consideration the items that have been verified to the present as a result of analyses of seismographic records, even when the effect of site amplification in the event of the predicted Tokai Earthquake*3 was tentatively reflected in the study, no impediments were determined to the maintainance of function in the main facilities important to seismic design in Reactor No. 5.
In addition, in order to further increase accountability in relation to the seismic safety of Reactor No. 5, a study of the seismic safety margins of the main structures important to seismic design in Reactor No. 5 in the event of a hypothetical Tokai Earthquake*4 was conducted. No impediments to the maintenance of function in the main facilities important to seismic design were determined.
On December 03, the government systematized the status of review of these details.
We presented an explanation of the situation to the Omaezaki city assembly today. Reactor No. 5 will be restarted following the presentation of further explanations to local residents.
Evaluation of the seismic safety of Reactor No. 5 in light of the new Seismic Design Review Guide will proceed when the knowledge obtained from the experience of the Suruga Bay earthquake has been further clarified on the basis of the present survey of subterranean structures, etc.
*1 The government Working Group refers to the Joint Working Group on Earthquake, Tsunami, Geology, and Ground Foundation and Structural Working Group under the Seismic and Structural Design Subcommittee, Nuclear and Industrial Safety Subcommittee, Advisory Committee for Natural Resources and Energy.
*2 The low-velocity layer is an area of bedrock in which seismic waves (S-waves) travel approximately 30% slower than in the surrounding bedrock.
*3 Ocean trench earthquakes occur periodically in the Suruga Trough. The predicted Tokai Earthquake is an earthquake with its epicenter within the epicentral area of the Ansei-Tokai Earthquake of 1854, but not within the epicentral area of the Tonankai Earthquake of 1944, which forms part of the former.
*4 The hypothetical Tokai Earthquake assumes, in the case of the predicted Tokai Earthquake, hypothetical asperities (regions of the epicentral area in which seismic motion is particularly intense) directly underneath the grounds of the Hamaoka Nuclear Power Station. This scenario was established as a more severe condition than standard seismic motion.