4.0 Summary of Major Review Topics, DCISC 15th Annual Report - July 1, 2004 thru June 30, 2005
4.20 Earthquakes and Tsunamis
4.20.1 Overview and Previous Activities
This section of the report provides updates on recent seismic events, tsunamis or related matters that could affect DCPP.
In previous reports the DCISC has reviewed with PG&E earthquakes occurring in the DCPP vicinity of California as well as seismic designs, analyses, and activities related to DCPP. This has included updates to PG&E’s Long Term Seismic Program which is an NRC license condition requiring PG&E to monitor and evaluate seismic events world-wide which could potentially affect DCPP design.
In the previous period the DCISC reviewed the following:
- December 22, 2003 San Simeon Earthquake Report
- December 22, 2003 San Simeon Earthquake Plant Review
- Long Term Seismic Program Update and December 22, 2003 San Simeon Earthquake
The DCISC concluded last period that the DCPP response to the December 22, 2003 San Simeon Earthquake appeared satisfactory. There was no visible damage to the plant which operated normally through the event. One issue, an exceedence of the expected vertical seismic response at the top of the Unit 1 Containment, is to be evaluated by PG&E and reported to the NRC. The exceedence is considered to be insignificant by PG&E.
4.20.2 Current Period Activities
The DCISC reviewed the following seismic items during the current reporting period:
- December 22, 2003 San Simeon Earthquake Update
- Seismic Monitoring System
- Tsunami Threat at DCPP
December 22, 2003 San Simeon Earthquake Update
The DCISC Fact-finding Team met with the Civil Engineering Supervisor David Wong; Senior Civil Engineer Bill Horstman; and I&C Engineer Kevin O’Neil to review developments in the Seismic Program and the new Seismic Monitoring System (Volume II, Exhibit D.5, Section 3.6). The DCISC has been reviewing DCPP seismic items since its inception in 1990.
A review of the December 22, 2003 San Simeon Earthquake was provided. The Magnitude 6.5 earthquake, approximately 50 km NNW of DCPP, produced an acceleration of less than 0.05g, compared to the Design Earthquake acceleration of 0.2g and the DCPP ultimate [Hosgri] design basis acceleration of 0.75g and Long-Term Seismic Program design basis acceleration of 0.83g. The earthquake had no noticeable permanent effect on DCPP. The DCISC has performed several reviews of this event.
In its reports to NRC, DCPP identified one exceedence [or excursion] to its Design Earthquake design basis: the vertical Unit 1 Containment instrument response was slightly larger than predicted for one narrow frequency band. However, there was no excursion when compared to the Hosgri or LTSP predictions, which used a more sophisticated model for vertical amplification than the original design basis earthquake analysis. Due to its construction and geometry, the containment dome is expected to have more complex vertical amplification than other plant structures; however, the equipment affected by this vertical acceleration is quite robust (containment spray rings and the plant vent).
PG&E performed a Vertical Dynamic Amplification Study to determine the impact of the excursion on the Containment dome. PG&E reported to NRC at a May 27, 2004 meeting that the study demonstrated that structures and related commodities (piping, equipment, etc.) satisfy the applicable design basis acceptance criteria. PG&E stated that NRC was satisfied at that meeting and at a subsequent public meeting on June 9, 2004 that the San Simeon Earthquake was well within the DCPP design basis and did not require any further evaluation. PG&E is to provide a final report to NRC by December 31, 2004. PG&E also reviewed the Long-Term Seismic Program relative to the San Simeon Earthquake and reported to NRC that there was no effect.
The U.S. Geological Survey (USGS) had issued a preliminary report on the San Simeon Earthquake which showed a discrepancy with PG&E’s conclusions. PG&E reported that the USGS did not have the complete data set from earthquake monitoring instruments close-in to the plant. PG&E has now provided the data, and it expects a revised final USGS report but not for about two years.
It is noteworthy that the plant seismic response simulations do accurately predict the plant’s response to the San Simeon Earthquake, including accurately predicting the spectral response of the containment dome. In essence, this earthquake provides the best experimental confirmation to date that the models are accurate, and that the predicted plant response to the design basis earthquake is accurate.
There have been two additional earthquakes since San Simeon as follows:
- Parkfield Earthquake of September 28, 2004 – a Magnitude 6.0 earthquake occurring on the San Andreas Fault about 80 km from DCPP. It produced a maximum acceleration of 0.012g at DCPP as recorded by the Seismic Monitoring System. Walkdowns showed no damage.
- Hosgri Earthquake of November 6, 2004 – a Magnitude 2.5 earthquake occurring offshore about 7.5 km from DCPP. Its maximum acceleration was not measurable by the Seismic Monitoring System, thus acceleration was @lt; 0.01g, the minimum detection level.
Seismic Monitoring System
PG&E has replaced its 1970s analog and 1980s digital technology Seismic Monitoring System (SMS) because parts are no longer manufactured or supported, calibration and service required vendor support, and data retrieval and digitization were cumbersome and time consuming. The Reactor Seismic Trip Instrumentation remains.
The new digital SMS was installed, checked-out and turned over to Operations in November 2004. The SMS ties-into the Control Room Computer and Plant Data Network Interface. It fully meets the requirements of NRC RG 1.12, Rev. 2. The 19 recorders are decentralized and able to record and store multiple events and display peak acceleration. The process rack is powered by vital power. The Control Room Computer provides automatic event analysis reports. Although not yet turned over to Operations, the new system was functional and recorded the September 28, 2004 Parkfield Earthquake.
PG&E has completed its analyses of the December 22, 2003 San Simeon Earthquake and showed that the earthquake response is properly predicted by models and that the earthquake response exceedence at the top of containment satisfied applicable acceptance criteria. The earthquake response at DCPP was well within the plant design basis.
The new Seismic Monitoring System has been installed and is fully operational. It replaces an outdated system and provides substantially improved data retrieval and analysis.
Tsunami Threats at DCPP
The DCISC met with Dr. Lloyd Cluff, Director of PG&E Geosciences Department Earthquake Risk Management, to review the threat of tsunamis on DCPP at an April 6-7, 2005 Fact-finding meeting (Volume II, Exhibit D.8, Section 3.4) and heard his presentation at the June 1-2, 2005 DCISC Public Meeting (Volume II, Exhibit B.9). Although aware of the tsunami design basis for DCPP, this is the first in-depth review the DCISC has made of tsunamis. The review was requested due to the December 26, 2004 Sumatra-Andaman Islands Earthquake and resulting tsunami.
The Sumatra Earthquake was magnitude 9.0, a very large earthquake. It prompted PG&E to review its tsunami design basis and take a new look at similar-sized earthquakes that have caused tsunamis, especially those that had the potential to affect DCPP. Dr. Cluff discussed the mechanics of tsunami generation from undersea earthquakes and described the December 26, 2004 event from initiation of the earthquake to the conclusion of the resulting tsunami. Wave run-up heights in various locations were reviewed. In Banda Aceh waves were believed to be as high as about 35 meters (115 feet), and in Phuket, Thailand waves reached a maximum height of almost 11 meters (36 feet).
A review of other worldwide tsunamis resulting from earthquakes at or above magnitude 9.0 revealed five which initially had the potential to affect DCPP:
- 1700 Cascadia magnitude 9.0 earthquake off the coast of Oregon and Washington at Willapa Bay
- 1946 Aleutians magnitude 9.3 earthquake
- 1952 Kamchatka, Japan magnitude 9.0 earthquake
- 1960 Chilean magnitude 9.5 earthquake off the western coast of Chile near Valdive
- 1964 Alaska magnitude 9.2 earthquake
DCPP is designed for storm surge waves of 36 feet and tsunami waves of 20 feet. In 1981, DCPP experienced a 31-foot storm surge. Because of the location and relative geometry of DCPP and the Cascadia (Washington-Oregon) earthquake, there would be no significant tsunami wave action at DCPP, particularly compared to the storm surge that has already been experienced at the plant. Waves from Alaska and Chile could be expected to reach DCPP in five and 13 hours, respectively.
The range of wave heights from these earthquake-generated tsunamis recorded at Avila Beach tide gauges was 2.9 feet (Chile) to 5.3 feet (Alaska). PG&E concluded that none of these events would pose a threat to DCPP. PG&E is collecting additional tide-gauge data for storm surges and tsunamis at several Pacific coast sites for NRC; this is scheduled for completion in early 2006.
PG&E’s analysis of significant earthquake-caused tsunamis, including the December 2004 Sumatra event super-imposed on Alaska , indicates that none represents a threat that would exceed the maximum tsunami design basis events at DCPP.
4.20.3 Conclusions and Recommendations
- Conclusion:
- The DCPP response to the December 22, 2003 San Simeon Earthquake appeared satisfactory. There was no damage to the plant which operated normally through the event. One lingering issue, an exceedence of the expected vertical seismic response at the top of the Unit 1 Containment, has been evaluated by PG&E and reported to the NRC. The exceedence was well within the design basis of the plant. Although subject to tsunamis, they are included in the design basis, and a recent evaluation shows that they are not a threat that would exceed the design basis.
- Recommendations:
- None