ML19309G968
| ML19309G968 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 04/14/1980 |
| From: | Reid R Office of Nuclear Reactor Regulation |
| To: | Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| References | |
| NUDOCS 8005080070 | |
| Download: ML19309G968 (10) | |
Text
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%u j April 14, 1980 Docket No. 50-312 Mr. J. J. Mattimoe Assistant General Manager and Chief Engineer Sacramento Municipal Utility District 6201 S Street P. O. Box 15830 Sacramento, California 95813
Dear Mr. Mattimoe:
RE: SEISMIC ASSESSMENT OF THE RANCHO SEC0 NUCLEAR GENERATING STATION We have recently undertaken a brief review of information relating to the seismology of the region around the Rancho Seco Nuclear Generating Station.
This review was initiated due to information that has come to light since the granting of the Rancho Seco operating license in 1974. Of particular interest is the area to the east of the site that includes the Foothills Fault System. The studies that you ccmpleted, as documented in the Final Safety Analysis Report (FSAR), acknowledged the existence of this fault system, but found that the last movement in the system occurred in the late Jurassic Age (more than 135 million years before the present).
With this finding, you concluded that the Foothills Fault System was inactive.
The nearest active faults considered in your studies were the Hayward and San Andreas Faults, 70 and 89 miles to the west of the plant respectively, and faults over 80 miles to the east of the site beyond the Sierra Nevada Range. However, more recent studies than your own have discovered active i
faults in the Foothills Fault System in the Oroville/ Auburn area which cause us to question the status of this fault system in the vicinity of Rancho Seco. These studies are highlighted in the enclosure which summarizes the results of our review.
Although we have concluded that there is no immediate seismic hazard at the site, we consider that it is appropriate that you undertake a review of the new information that has become available since the licensing of Rancho Seco concerning the seismology of the region.
Your attention is drawn to Item 9 of the enclosure which lists specific evaluations which should be made.
However, your review should not be limited to these items. For all. intents and purposes, we expect that you should meet the i
intent of Appendix A to 10 CFR Par? 100 for the site and the region east of Rancho Seco including the Foothills Fault System.
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Mr. J. J. Mattimoe April 14, 1980 You are requested to review the enclosure and provide us with a scope and schedule of your program that will be necessary to properly assess the fault movement potential in the Foothills Fault System and its impact on the operation of the Rancho Seco facility. Since we have already been in comunication with your staff on an informal basis concerning this issue, it is not unreasonable to expect that you can provide us with your scope and schedule in 30 days.
You are encouraged to make every effort to meet this date.
If for any reason you are unable to do'so, please inform this office in writing as soon as possible along with the reasons for the delay.
Sincerely, Robert W. Reid, Chief Operating Reactors Branch #4 Divisicn of Operating Reactors
Enclosure:
As stated cc w/ enclosure: See next page
Sacramento Municipal Utility District
!i cc w/ enclosure (s):
Christopher Ellison, Esq.
David S. Kaplan, Secretary and Dian Grueuich, Esq.
General Counsel California Energy Commission 6201 S Street 1111 Howe Avenue P. O. Box 15830 Sacramento, California 95825 Sacramento, California 95813 Ms. Eleanor Schwartz Sacramento County California State Office Board of Supervisors 600 Pennsylvania Avenue, S.E., Rm. 201 827 7th Street, Room 424 Washington, D.C.
20003 Sac,amento, California 95814 Docketing and Service Section Office of the Secretary Business and Municipal Department U. S. Nuclear Regulatory Commission Sacramento City-County Library Washington, D.C.
20555 828 1 Ctreet Sacramento, California 95814 Resident Inspector P. O. Box 48 D1 rector, Technical Assossment Fair Oaks, California 95628 Division Office of Radiation Programs Dr. Richard F. Cole (AW-459)
Atomic Safety and Licensing Board U. S. Environmental Protection Agency Panel Crystal Mcll #2 U. S. Nuclear Regulatory Commission Arlington, '!irginia 20460 Washington, D.C.
20555 U. S. Environmental Protection Agency Mr. Frederick J. Shon A
c Safety and Ucensing Boad i
S C0 INATOR hh 5
c sco a ifornia 94111 t
D.C Mr. Robert B. Borsum Elizabeth S. Bowers, Esq.
Babcock & Wilcox Chairman, Atomic Safety and t!uclear Power Generation Division Licensing Board Panel Suite 420, 7735 Old Georgetown Road U. S. Nuclear Regulatory Comission I.ethesda, Maryland 20014 Washington, DC 20555 Thomas Baxter, Esq.
Shaw, Pittman, Potts & Trowbridge 1800 M Street, TN Mashington, D.,C.
-2003G Herbert H. Brown, Esq.
Mr. Michael R. Eaton Energy Issues Coordinator "
Lawrence Coe Lanpher, Esq.
Hill, Christopher and Phillips, P. C.
Sierra Club Legislative Office 1900 M St., NW 1107 9th St., Roofn 1020 Washington, D. C.
20036 Sacramento, CA 95814 Helen Hubbard P. O. Box 63
.Sunol, California 94586
Sacramento Municipal Utility 3
J.
District cc w/ enclosure (s):
Atomic Safety and Licensing Board Panel U. S. Nuclear Regulatory Commission Washington, D.C.
20555 Atomic Safety and Licensing Appeal Board Panel U. S. Nuclear Regulatory Commission Washington, D.C.
20555 California Department of Health ATTN: Chief, Environmental Radiation Control Unit Radiological Health Section 714 P Street, Room 498 Sacramento, California 95814.
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Enclosure I
REVIEW 0F THE SEISMIC ASSESSMENT OF THE RANCHO SEC0 NUCLEAR GENERATING STATION 1.
The Rancho Seco Unit 1 Final Safety Analysis Report (1968) presented the following information in support of the seismic design of the
- station, a.
The faulting nearest the site (approximately 10 miles to the east) is the Bear Mountain Fault Zone which is the western branch of the Sierra Foothills Fault System.
b.
TheFoothillsFaultSystemhasbeeninactihesincetheJurassic (135 million years before the present (nybp)).
c.
The highest earthquake intensity--in the site area from. historic data is Modified Mercalli V.
d.
Thenearestactihefaultingalongwhichhistoriclargeearthquakes haYe originated are the Haywarc' and San Andreas Faults, 70 and 89 miles tothewestrespectihely,andfaultsoYer80milestotheeastbeyond theSierraNehadaRange.
e.
Based on the distances to these faults,it was estimated that the maximum earthquake intensity which would be experienced at the site is Modified Mercalli VI.
f.
A peak horizontal acceleration no greater than 0.05g during the life of the plant was postulated.
2.
Housner response spectra using 0.25g as an anchor for the SSE and 0.139 for the OBE were used for the design of the facility and were considered conservatihe.
1 i 3.
TheU..S.GeologicalSurhey(USGS)andtheU.S..CoastandGeodeticSurhey (USC&GS)rehiewedthegeologicandseismicsecti6nsoftheFSARas consultants to the AEC. Their reports stated the following:
a.
TheFoothillsFaultSystemispre-Cenozoic (atleast60 mybp).
Howeher,indihidualfaultsinthesystemshaYemorerecentmohement. The youngest displaced rocks are probably late Pliocene or early Ple'stocene (1-2 mybp).
b.
Geophysical data suggest that structures similar to the Foo'thills Fault System appear to underlie the site in the crystalline rocks.
c.
Based on the historic record, the accelerations proposed by tne applicant intheFSARwereconsideredtobeconserhatihe.
The SER accepted the conclusions reached in the FSAR and by the USGS and USC&GS.
4.
On August 1, 1975, a magnitude 5.7 earthquake occurred on one of the faults oftheSierraFoothillsFaultSystemnearOrohille. The epicenter of this ehentisapproximately85milesfromtheRanchoSecosite. As a result of this earthquake the USGS and the California Dihision of Mines.and Geology (CDMG) performed extensihe seismological and geological inhestigations of the FoothillsFault System.
In addition, Woodward Clyde Consultants (WCC) performed studies under a contract with the U. S.,ureau of Reclamation in support of the Auburn Dam Construction, which is approximately 40 miles from the Rancho Seco site. Most of these inhestigations were perfomed in the FoothillsFaultSystembetweenOrohilleandAuburn.
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S.
The following are some of the findings of these :;tudies.
a.
A prominent northwest-trending lineament system in the Western Foothills, which is controlled by the structural grain of the underlying Paleozoic and: Mesozoic metamorphic rocks, continues for the length of the Foothills and controls the i; rend of late Cenozoic faulting (both the Bear Mountain Fault Zone and the Melones Fault Zone) which follows pre-existing planes of weakness.
b.
High-precisionlehelingdataindicatecontemporaryhertical deformation occurring within the Western' Sierra Nehada. More particularly,this is closely associated with faults within each of the two main branches of the Foothills Fault System.
TrenchesintheFoothillsFaultSysteminthehicinityofthe c.
AuburnDansiteyieldevidenceofactihefaulting. Atleastsehen localities have experienced displacement within the past 100,000 years.
d.
For a distance of at least ten miles west of the Bear Mountain Fault Zone, the linear pattern asscciated with the Western Foothills is re-flected in the Tertiary rocks of the Great Valley which overlie the Mesozoic metamorphic rocks, as mapped by WCC from aerial photography.
Two of these linear features drawn on the map by WCC geologists e.
trend along lines that project toward the Rancho Seco site. However, USGS geologists did not find justification for these linear features in their field studies of the Tertiary and later deposits in the Sacramento area.
f.
Five miles west of the Foothills / Great Valley front, and 10 miles' southeast of the Rancho Seco site, two small faults, paralleling the Foothills Fault Zone trend, offset Tertiary strata. Theamountofoffsetisbeliehed to be cnly a few feet and the last movement it thought by the USGS to be pre-Quaternary (2 mybp)..
l g.
RecentearthquakeactihityintheWesternFdothillsnorth(Chico and Orchille) and south (Mariposa) of the Rancho Seco site has occurred along faults parallel with this strong NW linear trend. Other than the sequenceofeventsassociatedwiththeOrohilleearthquakeandtwo smaller events in the Mariposa area (85 miles from Rancho Seco) there has been no significant earthquake activity reported on the Sierra Fcothills Fault Zone.
h.
Microsarthquake actihity has beev found along.the Foothills Fault System between Folsom and Oroville. Most of the micro-seismic events located in the Auburn area were in or near the Rocklin-Penryn Pluton.
composite focal mechanisms calculated for these ehents resulted in fault planes striking N10"W and dipping 55 to 70 to the east with normal slip motion.
i.. It is postulated that the Rocklin-Penryn Pluton is being deformed bythesameregionalstresspatternthatcausedCenozoicmohementelse-where en the Foothills Fault System.
- j. WCC in their report en the Auburn dam site postulated a maximum credible earthquake in the magnitude range 6.0 to 6.5.
The USGS in a rehiew of the report stated that the maximum credible earthquake was under-estimated by WCC and should be 6.5 to '7.0: in magnitude.
6.
WeknowofnoseismicityorfaultmohementstudiesintheRanchoSecoarea on which to base any definitive conclusions as to the seismic hazard.
The seismic hazard, hcweher, at the Rancho Seco site is judged to be less than at the Auburn Dam site. This judgement is based on the fact that Rancho Seco is 10 to 13 miles from the western most surface trace of the Foothills Fault Zone, and there is no eartnquake inducer such as a deep reservoir near l
Rancho Seco.
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l 7.
To estimate the peak horizontal acceleration which might be expected at the Rancho Seco site, an earthquake of approximately magnitude 6.5 was assumed at a distance of approximately 10 miles. Peak acceleration calculations result in a range of mean halues which are dependent on the attenuation-acceleration relationship used.
I Author Mean Peak Acceleration Magnitude Boore et al, 1978 0.25g 6.0-6.4 Schnabel and Seed, 1973 0.28g 6.6 Donohan,1973 0.20g 6.6 Trifunac, 1976 0.40g 6.6 In the recent well-recorded Imperial Valley earthquake of magnitude 6.4 the peak acceTerations at a distance of 10 miles from the fault ranged from 0.1 to 0.4gs 8.
Asprehiouslynotedtheresponsespectrausedinthedesignofthestation were Housner spectra based oi. 0.25g for the SSE and 0.13g for the OBE.
These spectra haYe halues which are in general about half of those obtained under current practices using Reg. Guide 1.60.
9.
Basedonthislimitedehaluation,theseismicdesignoftheRanchoSeco stationdoesnotappeartobeascan'serhatiheaswasassumedwhenthe original rehiew was completed approximately ten years ago. Further studies should be undertaken by the licensee to up-date and rehiew the vast amount of infonnatiors available on the geology and seismology of the general site region and a complete evaluation should be made of the impact of this infonnation upon the original siting considerations. The follow-ing types of questions should be considered as part of that
' eYaluation:
j a.
Is the Bear Moentain Fault Zone actihe in the Rancho Seco area?
Are there any actihe parts of the Bear Mountain Fault System closer s.
than 10 miles to the site?
c.
What is the maximum magnitude earthquake which should be considered on the Bear Mountaint Fault and what effect can be expected at the sitefromthisehent?
- 10. We conclude based on our present limited eYaluation,that the licensee be requested to undertake these studies as soon as possible and that seriousconsiderationbegiYentoincludingthisfacilityintheSeismic Review Group re-review considerations. We feel that there is no imediate seismic hazard at Rancho Seco since empirical data indicate that the design acceleration for the SSE (0.25g) is a reasonable representatihe of the mean peak acceleration that would result at the site from the occurrence of a magnitude 6.0 to 6.5 earthquake. No eartrauake of this size or larger has occurred anywhere along the entire Foothills Fault System during historical time,, nor is there a deep reserhoir in the hicinity of the site that could serve to increase the chance of this or any other size earthquake occurring.
With respect to the'imediate consequence we recomend that 00R engineers consider the acceptability of the differences between the utilization of RG 1.60 spectra and the Housner spectra. We also recomend that, during the saggested reassessment of the new seismological and geological infomation,
, spectral characteristics be considered in a joint seismological-engineering evaluation.
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