Evaluation of Effects of Ground Motion on Hamaoka Nuclear Power Station based on Results published by the Cabinet Office and Status of Review of Earthquake Countermeasures
April 26, 2013
Chubu Electric Power Co.,Inc.
On March 18, 2013, the Cabinet Office published a second report concerning estimates of damage resulting from a major earthquake in the Nankai Trough. The estimates of damage presented in the report are based on the results of estimation of the distribution of seismic intensity using source fault models for generating strong ground motions formulated by the Committee for Modeling a Nankai Trough Megaquake of the Cabinet Office, which assumes an earthquake of the greatest magnitude that could occur along the Nankai Trough. (These source fault models for generating strong ground motions were published in the first report issued in March 2012; it will be termed the "Cabinet Office model" below.)
Chubu Electric Power has conducted an evaluation of effects on the equipment and facilities essential for maintaining safety in Units 2-5 of Hamaoka Nuclear Power Station in their current state of shutdown based on this Cabinet Office model, and has verified that seismic safety will be maintained.
However, based on the amplification of vibration observed in Unit 5 during the Suruga Bay earthquake, it is judged to be necessary to further increase seismic resistance with a focus on safety-related equipment and facilities in Unit 5 and its surrounds, and we are proceeding with the necessary measures based on the status of review of long-period ground motion and other factors stipulated for consideration by the Cabinet Office and the standards incorporated in new regulations.
In addition, because it is also necessary to conduct a detailed examination of the seismic resistance of tsunami counter measure including the tsunami protections walls presently under construction, we have decided to revise the target for completion of these measures from December 2013 to the end of fiscal 2014.
1 Evaluation of effects of ground motion on Hamaoka Nuclear Power Station based on results published by the Cabinet Office
(Evaluation of ground motion based on the Cabinet Office model)
We have evaluated the ground motion at Hamaoka Nuclear Power Station based on the Cabinet Office model of an earthquake of maximum intensity (termed "Cabinet Office ground motion" below), and found it to be around 1,000 gals at its maximum.
We also evaluated ground motion reflecting the results of a survey of underground structures at the facility and an analysis of seismographic records undertaken based on the fact that the vibration experienced by Unit 5 during the Suruga Bay earthquake of August 2009 was stronger than that experienced by other reactors (these results will be termed "survey and analysis results" below).
The survey and analysis results are as follows: 1) The main cause of the amplification of vibration in Unit 5 was a low-velocity layer distributed at a shallow level around the peripheries of the reactor; 2) This low-velocity layer is not distributed elsewhere (i.e., is confined to the area around Unit 5; and 3) According to seismographic records, the amplification of vibration observed at Unit 5 was restricted to the direction of arrival of motion produced by the Suruga Bay earthquake, and no conspicuous amplification of vibration was recorded at any measurement points other than those surrounding Unit 5.
Reflecting these survey and analysis results, we therefore modified the Cabinet Office model of an earthquake of maximum intensity by concentrating a zone generating strong seismic waves in the direction of arrival of seismic waves observed in the amplification of vibration of Unit 5, in order to evaluate ground motion reflecting the amplification of vibration at Unit 5 (termed "hypothetical amplified ground motion using the Cabinet Office model" below). The maximum figure was found to be around 1,900 gals.
(Evaluation of effects on equipment and facilities)
Effects on equipment and facilities (reactor buildings, foundations, fuel racks) essential to maintaining the safety of Units 2-5 in their current state of shutdown were evaluated for the Cabinet Office ground motion for Units 2-4 (a maximum of around 1,000 gals) and the hypothetical amplified ground motion using the Cabinet Office model for Unit 5 (a maximum of around 1,900 gals). The results of these evaluations showed that the seismic safety of Hamaoka Nuclear Power Station Units 2-5 would be maintained under these conditions.
2 Status of review of earthquake countermeasures and future responses
The level of Cabinet Office ground motion is close to the current earthquake resistance level for equipment and facilities important to safety in Units 3-5 (approximately 1,000 gals), but the hypothetical amplified ground motion using the Cabinet Office model (a maximum of around 1,900 gals) exceeds the ground motion figure(1,454 gals*) used to verify the seismic resistance of Unit 5 based on the Suruga Bay earthquake, and it is therefore expected that in future it will be necessary to realize further increases in seismic resistance with a focus on safety-related equipment and facilities in Unit 5 and its surrounds.
We are also proceeding with a concrete examination of facilities to be subject to construction and reinforcement work, etc. and relevant designs in relation to all the facilities of Units 3-5 based on the status of review of long-period ground motion and other factors stipulated for consideration by the Cabinet Office and the standards incorporated in new regulations. We are seeking to formulate construction plans at the earliest possible stage within this fiscal year, and to commence construction work as soon as possible.
Because it will also be necessary to proceed with construction of tsunami countermeasures, including the tsunami protection walls currently being built, while conducting a detailed analysis of seismic resistance and modifying designs as necessary, we have revised our target for completion of the measures from December 2013 to the end of FY2014, which is also the target for completion of measures including prevention of overflow of intake ponds and other such facilities. The impact on performance for this quarter will be minimal.
Chubu Electric Power is committed to continuing to make steady progress with measures for further enhancement of the safety of Hamaoka Nuclear Power Station. At the same time, we will also do everything in our power to provide detailed explanations of the content of those measures so as to contribute to the peace of mind of the local community and of society at large.
*Study of the Effects on the Seismic Safety of Hamaoka Nuclear Power Station Unit 5 based on the Experience of the Suruga Bay Earthquake (A report compiling details reported to a government working group, published on December 15, 2010)