ML19275L113
| ML19275L113 | |
| Person / Time | |
|---|---|
| Issue date: | 01/07/1983 |
| From: | Advisory Committee on Reactor Safeguards |
| To: | Advisory Committee on Reactor Safeguards |
| References | |
| ACRS-2039A, NUDOCS 8303170669 | |
| Download: ML19275L113 (27) | |
Text
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MAR 0 7 N b Co w.
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r tiEt'0PAf!DtP1 FOR: Thonas !!. Fovak, Assistant Director for licensing, DL FP0f':
Janes P. Knight, Assistant Director for Corporents f. Structures Engineering, DE St!PJECT:
PRAFT SAFETY EVAL.UAT10t! PEPORT - GE010GY Af!D SEISMOLOGY - RIVER pet:0 STATI0t!, lit 11TS 1 At:D 2 Plant f'ane:
River Bend Station Docket !!unbers:
50-458 and 50-459 Licensing Stace: OL Peview Pesponsible Branch: Licensing Branch No. P Responsible Project fianeger:
E. Ueinkan III Enclosed are the neology ard seisnology sections for the River Pend draft SEP. This irput applies to the SFP sections 2.5.1, 2.5.2 and 2.5.3.
The report was prepared by llarold Lefevre, Geologist and Jeff Kinball, Seisnologist.
As stated in the attached draf t SEP, the staff concluded that there are no known cepable faults at the site or in the recion around the site and that the SSF and OBE are adeauate. The steff will be evenining the follcwing topic eroes between nrw and the final SER. These areas are:
- 1) renote-sensing imegery study of the site crea, 2) interpretation of seismic reflection surveys recently conducted in the innediate vicinity of the site, 3) identification of the type of trap from which the tho hydrocarbons are produced at a rearby connercial oil well, 4) updatinc the FSfR to conply with Standard review Plan (t!UREG-0800) ard, particularly for Section ?.5.2, in order to adequetely reflect state-of-the-art practice in seisnology.
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SURNAME) 8303160669 830307 ADOCK05000g8 CF DATE)
NHC FORM 216 (10-80) NRCM 024a OFFICIAL RECORD COPY usa m
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MAR 0 71983 At the conclusion of these studies we will be assessing the need for nodifying previous positions with respect to the River Bend site. At this tine, however, we see no need to modify the position taken for Piver Bend during the CP review.
Original signed 37:
ynn, n ? O Janes P. Knight, Assistant Director for Conponents & Structures Engineoring Division of Engineering Attachnent:
As steted cc: w/o attachment DISTRIBUTION:
R. Vollmer DOCUMENT CONTROL DESK' G. Lear GSB RDG cc: w/attachnent J. Knight A. Schwencer R. Jackson L. Peiter S. Brocoum H. Lefevre A. K. Ibrahin L. Heller J. Kinball E. Ucinkan III
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DRAFT GE0 LOGY AND SEISMOLOGY
SUMMARY
RIVER BEND STATION GULF STATES UTILITIES COMPANY DOCKET NOS. 50-458/50-459 2.5 Geology and Seismolooy In our review of the River Bend Station, Units 1 and 2 Final Safety Analysis Report, we have considered pertinent information gathered since our initial geologic and seismologic review (Construction Permit Safety Evaluation Report of September, 1974).
The U. S. Geological Survey (USGS) served as the NRC's geological and seismological advisor during the Construction Permit review. Our consultant, the Lawrence Livermore National Laboratory, is currently performing a probabilistic seismic hazard analyses of the site. The results of this study will be discussed on our final Safety Evaluation Report.
Information considered since issuance of the CP Safety Evaluation Report in September,1974 includes (1) data gathered from both site and near-site investigations, including geologic and geophysical information acquired as a result of extensive oil / gas exploration in the site region, (2) discussions with individuals knowledgeable of the geology of the site and region, (3) a staff review of recently-acquired literature, (4) a discussion, by the applicant,'of the potential for geothermal-geopressured energy resource development and seismicity that
. has been observed in areas where these wells exist, (5) new information, based upon recent as well as on-going studi~es, en trie extent of the subsurface oeologic structure associated with the 1811-1812 New Madrid earthquake. (6) a limited seismic hazard analysis assessing the probability of exceeding the Operating Basis Earthquake during the plant life, and (7) a compilation of observed seismicity in the vicinity of the site through 1978. As a result of our recent review and evaluation of the above geologic and seismologic information, we have determined that our Construction Permit Safety Evaluation Report conclusions regarding the safety of the River Bend Station from a geological and seismological standpoint remain valid. The staff is confident that the results of the applicants on-going efforts will confirm the adequacy of the seismic design for the site. The most relevant subjects presently being addressed by the applicant include: (1) a remote-sensing imagery
. study, (2) an interpretation uf geophysical seismic reflection profiles within 8 km (5 mi) of the plant, and (3) an identification of the type of trap constitutina an oil reservoir in the site vicinity.
The applicant's presentation in FSAR Sections 2.5.1, 2.5.2 and 2.5.3 has satisfied the requirements of 10 CFR Part 50, 10 CFR Part 100 and Appendix A to Part 100 by (1) performing post-Construction Permit site and near-site geological, seismological and geophysical investigations / studies as needed and by (2) cansulting with individuals knowledgcble of the local geology and seismology. The seismologic
. portion of Section 2.5,of the Final Safety Analysis Report however is somewhat outdated and should be revised to reflect state-of-the-art practice. This information will aid the staff in evaluating the applicant's conclusions regarding the favorable geologic and seismic characteristics of the River Bend Station site.
In addition, the applicant is in conformance with apolicable portions of the following documents:
(1) Regulatory Guide 1.60, " Design Response Spectra for Seismic Design of Nuclear Power Plants."
(2) Regulatory Guide 1.70, " Standard Format and Content of Safety Analysis Reports for Nuclear Power Plants." Revision 2.
(3) Regulatory Guide 1.132, " Site Investigations for Foundations of Nuclear Power Plants."
The newly-acquired information reviewed for the River Bend Station Units 1 and 2, is discussed in Sections 2.5.1, 2.5.2 and 2.5.3 below.
2.5.1 Geologic Information Since issuance of the Construction Permit Safety Evaluation Report in September,1974, the applicant has conducted geologic investigations both on and off-site and has evaluated geophysical data obtained by others. These investigations and analyses have resulted in an increased understanding and confirmation of the surface and subsurface conditions of the River Bend site and vicinity.
No new known information has yet
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been developed indicating that our earlier conclusions regarding the suitability of the River Bend site should now be modified.
With the exception of three topic areas, the applicant has responded to all staff geologic-related questions. Those areas incompletely addressed by the applicant include: (1) performance of a remote-sensing imagery study, (2) interpretation of seismic reflection surveys recently conducted in the immediate vicinity of the site, and (3) identification of the trap from which the hydrocarbons are produced from a commercial oil well 4.8 km (3 mi) southeast of the site.
The principal newly-developed geologic / geophysical information presented in the FSAR relative to the River Bend site and site vicinity (within S F.
km (5 mi) of River Bend) has been derived from the following " sources:
(1) Mapping and photographing of the excavation for the plant structures.
(2) Acquisition and intepretation of several of the seismic reflection surveys conducted since issuance of the Constructio'1 Permit.
(3) Structural contour maps of geologic horizons underlying the site and vicin.ity. These maps were prepared by the Geomap Company of Houston, Texas.
Geologic mapping of the plant excavation by the applicants, coupled with complete photographic coverage, have confirmed the geologic integrity of the plant foundations area. As predicted by the applicants' pre-Construction Permit investigations, the River Bend excavations as inspected, mapped, and photographed show no evidence of either faulting or through-going deformation. The staff inspected the excavations for Units 1 and 2 on Nov. 11, 1976, and concluded that no structural abnormalities were visible within either the mapped or currently visible portions of-the excavation.
The geologic and geophysical information derived from the pre-and post-Construction Permit hydrocarbon test wells and several of the seismic reflection profiling surveys within 8km (5 mi) of the River Bend picnt have been obtained and evaluated by the applicant. These data, as interpreted by the applicant, confirm the earlier (CP) conclusions relative to the near site area that neither faulting nor through-going deformation appear to be present.
As of August, 1982, seven post-CP test holes (one of which has been commercially successful) and approximately 66 km (41 mi) of geophysical prospecting (seismic reflection profiling) have been completed within 8 km (5 mi) of the River Bend plant. This commercial well is located 4.8 km (3 mi) southeast of the plant. Because of the great depth (in excess of 5,181 m (17,000 ft) and consequent low hydrocarbon reservoir porosity, potential geologic hazards (such as possible differential subsidence) which may be associated with the near-site development of shallow hydrocarbons do not exist.
Copies of the proprietary seismic reflection data have recently (January 12,1983) been received from the applicant. The staff is currently
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- evaluating these data in order to confirm the Applicants' finding (not yet in~cluded by amendment into the FSAR) that no faulting exists within 8 km (5 miles) of the River Bend plant.
Contouring of several Cretaceous horizons by a private firm (not affiliated with the applicant), the Geomap Company of Houston, Texas and construction, by the applicant of a 20.9 km (13 mi) long northwest trending geologic cross-section extending through the plant area to depths in excess of 5,181 m (17,000 ft), have confirmed the geologic integrity of the site (8 km (5 mi) radius) area.
It should be noted, however, that the Geomap Company's structural contour maos are not totally comprehensive in that the maps portray an evaluation of only geophysically-logged test holes, and do not reflect the company's interpretation of the seismic surveys conducted within the area.
2.5.2 l'ibratory Ground Motion In our review of the seismological aspects of the plant site we have considered pertinent information gathered since our initial seismologic review which was made in conjunction with the issuance of the Construction Permit (September,1974). The conclusion of the Construction Permit review was that 0.10g and 0.05g are adequate values for the peak horizontal accelerations for the Safe Shutdown Earthquake (SSE) and Operating Basis Earthquake (OBE), respectively (CP SER, September,1974). The acceleration levels (0.109 and 0.05g) proposed for the SSE and OBE have been used to anchor a Regulatory Guide 1.60
. response spectra. Since the c6nclusion of the Construction Permit review no new information has become available to change the staff's original conclusions.
The requirements of Appendix A to 10 CFR Part 100 in the determining the vibratory ground motion corresponding to the SSE have been met.
2.5.2.1 Seismicity In FSAR Section 2.5.2.1 the applicant presents the results of a thorough study of historical seismicity for the Gulf Coast up to December 1978.
The study shows that the Gulf Coast is an area of infrequent and low seismicity. This is substantiated by the work of Barstow et al. (1981),
who have produced an earthquake frequency map of the United States east of the Rocky Mountains. This map shows that the area within 160 kilometers (100 miles) of the River Bend site has 0 to 3 earthquakes (Modified Mercalli Intensity greater than cr equal to III or magnitude greater than or equal to 2.0), in 11,689 square kilometers. Details of the significant events utilized to evaluate the River Bend _ site are discussed in SER section 2.5.2.3.
2.5.2.1.1 Seismicity Associated with Growth Faults Recently, the staff became aware of detailed seismic monitoring in areas associated with the development of Gulf Coast geopressured-geothermal energy wells. These studies (Mauk,1981; Statton et al.1981) have documented small magnitude earthquakes (less than M = 2.0) in areas
where growth faults exist. Growth faults are faults in sedimentary rock that forms contemporaneous 1y with deposition, with normal fault motion due to downslope mass buildup of sediments. As described in FSAR section 2.5.1 growth faults are'known to exist from about 11.5 to 35 kilometers south of the site. Because of this, the staff requested that the applicant discuss the potential for geopressure-geothermal energy development in the near vicinity of the River Bend site, and also discuss the seismicity that has been observed in areas where these wells exist.
The applicant,'in FSAR section 2.5.1 and 2.5.2, has presented a thorough discussion on geothermal-geopressured energy potential and observed seismicity in these areas. Presently, the closest area of observed seismicity and energy well development is approximately 100 kilometers southwest of R.iver Bend. Although the possibility exists that geothermal-geopressed energy development may become feasible within 35 kilometers in the future, presently there are no plans for this activity to be initiated.
Based upon staff analyses and review of the applicant's analysis, we conclude that there is no concern that strong ground motion will be produced by movement along growth faults in the River Bend site vicinity.
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2.5.2.2 Tectonic Province In the Safety Evaluation Report of the construction-permit review (September,1974) the staff concluded that the River Bend site is located in the Gulf Coastal Plain physiographic province. Current staff practice (outlined in Appendix A to 10 CFR 100) locates the River Bend site in the Gulf Coastal Plain Tectonic Province, which is characterized by thousands of feet of unmetamorphosed sediments of the Gulf Coast geosyncline. This is consistent with other Safety Evaluation Reports for sites in the Gulf Coastal region (Allens Creek,1974; South Texas, 1975;BlueHills,k976,YellowCreek,1977;ComanchePeak;1980, Waterford,1981, Grand Gulf,1981).
King (1969) defines the ' Atlantic and Gulf Coastal Plains as platform deposits (Mesozoic age and younger) that were laid over the deformed basement (Paleozoic and older rocks of the Appalachian and Ouachita orogenic belts). The platform deposits thicken and slope seaward from the exposed parts of these orogenic belts, with the basement descending beneath them.
From New Jersey, south and then west to the Llano Uplif t -in central Texas, the landward border of the platform deposits (Coastal Plain) is drawn at the contact of the Cretaceous and/or Tertiary deposits with the underlying and overlapped basement rocks. The River Bend site is located in the Gulf Coastal Plain Tectonic Province, which is part of the Coastal Plain extending from Florida westward and southward into Mexico.
The Applicant has proposed tectonic provinces for the River Bend region based on crustal patterns, geologic history and earthquake history. We have reviewed this, and although we establish somewhat different boundaries, the applicants analysis of the maximum earthquake for the a
. River Bend site, in accordance with Appendix A to 10 CFR Part 100, is equivalent to the staff's.
2.5.2.3 Maximum Earthquake Potential Within the Gulf Coastal Plain Tectonic Province, the staff recognizes that different regions of this large province exhibit vastly different levels of seismicity.
In particular, to arrive at the appropriate choice of the Safe Shutdown Earthquake for the River Bend site, we recognize three areas of seismicity within or intersecting this province: (1) the Mississippi Embayment (New Madrid) Earthquake Zone (2) the Ouachita-Wichita belt of seismicity, and (3) seismicity in the
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remainder of the Gulf Coastal Plain Tectonic Province.
Each of these zones of seismicity are described below.
The staff is also currently evaluating seismic reflection profile data, and through Lawrence Livermore Laboratory performing a probabilistic seismic hazard analysis for the River Bend site. The reflection profile data will aid the staff in determining if any geologic structures or faults exist in the near site region. The probabilistic hazard analysis will aid the staff in assessing the conservatism of the SSE. Both of these topics will be discussed in more detail in the SER, although it is not anticipated that either topic will alter past staff conclusion regarding the adequacy of the River Bend site.
2.5.2.3.1 Mississippi Embayment Earthquake Zone The Mississippi Embayment Earthquake Zone is a region of much higher seismic activity than the remainder of the eastern United Sta.,_s.
It has also been the source region of the largest earthquakes in the eastern United States, the 1811-1812 New M'adrid earthquakes. The largest event in this series, February 7,1812 had a Modified Mercalli (MM) intensity of XI-XII (Nuttli,1973), and a magnitude (M ) of 7.3 b
(Nuttli and Brill, 1981). During the construction permit review of the River Bend site, the staff concluded that the closest approach of earthquakes similar to the 1811-1812 series in near Memphis, Tennessee, about 500 kilometers (310 miles) from the site.
As discussed in Section 2.5.1 of the Grand Gulf Safety Evaluation Report (September, 1981), recent geological and geophysical data suggest that the subsurface geologic structure in the New Madrid region extends southwest of Memphis, to the Ouachita orogenic belt and within about 370 kilometers (230 miles), of the site. This is consistent with the information presented by Hamilton and Zietz (1982). The staff has not completed its review of the ongoing geophysical and seismological research regarding the New Madrid extension and its large magnitude earthquake potential as far as establishing a final staff position on this issue. For the purposes of this draft SER, we have conservatively assumed for the seismic design of River Bend that on 1811-1812 type earthquake could occur at a distance of 370 kilometers (230 miles) from the site (as opposed to about 500 kilometers established at the construction permit stage). As discussed in.SER Section 2.5.2.4.1 the ground motion from this assumed event does not exceed the SSE.
In
addition, listed below are factors, which argue for terminating the New Madrid Seismic Zone near Memphis, 500 kilometers from the River Bend site.
As part of the Systematic Evaluation Program a group of 1.
seismol'ogists were asked to give their expert opinion on input parameters for a seismic hazard analysis (TERA,1980). One of these parameters was the configuration of seismic source zones.
Results compiled by TERA demonstrate that there are varying opinions among experts on earthquake zonation and correlation with structure in this region of the United States. Most experts (SEP panels, TERA,1980, pp C-1 to C-14) terminated the New Madrid source zone at or near Memphis.
2.
Both historic and recent microearthquake activity have been confined to the region north of Memphis. Detailed microearthquake monitoring in the New Madrid region has revealed that the vast majority of seismicity is occurring North of Memphis (see for example Quarterly Bulletin No. 26, St. Louis University, Central,
Mississippi Valley Earthquake Bulletin, 1980).
3.
Seismic reflection profiling in the New Madrid region demonstrate that faulting cuts Cretaceous material (although no single fault has been found which could be associated with the 1811-1812 earthquake sequence, McKeown, 1981). Seismic profiling just south of Memphis (done at the CP stage) shows no faulting of Cretaceous material, suggesting there has been no faulting in the last 65 million years. These reflections lines, however, show little
. information at deeper depths so no definitive statements can be made about deeper faulting at the present time.
4.
Results of Zoback et al. (1980), using reflection data in the more seismically active areas of north of Mempris, demonstrates that faulting is more abundant in the area north of Ridgely, Tennessee, than it is to the southwest, toward Memphis.
5.
A recent report by Nuttli and Brill (1981) on Earthquake source zones in the central United States determined from historical seismicity shows the New Madrid source zone terminating near Memphis.
2.5.2.3.2 Ouachita-Wichita Belt of Seismicity The boundaries of the east-west Quachita-Wichita belt of seismicity are subject to interpretation by various investigators. Nuttli and Brill (1981) shows this east-west trending area overlapping the Nemaha Ridge
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Seismic Zone while the applicants' tectonic features map (River Bend FSAR Figure 2.5-36) shows the buried Appalachian front. Both maps though, show the Ouachita-Wichita belt of seismicity and tectonic features to intersect and overlap the Gulf Coastal Plain Tectonic Province. While the boundaries are somewhat ambiguous the largest historically-recognized earthquake in the Ouachita-Wichita belt of seismicity is the October 22, 1882 MM intensity VII earthquake which occurred near Paris, Texas. Estimates of its intensity range from MMI=VI-VII, Coffman and Von Hake, (1973), who placed the epicenter neer Fort Smith, Arkansas and estimated that it approached or possibly even reached MMI=VIII (Docekal, 1970). Docekal, (1970), who upon reevaluation
of the data, relocated the epicenter near Paris, Texas. Based on the available data it is the staff's view that the earthquake occurred near Paris, Texas, and that the maximum intensity was no higher than VII (Comanche Peak, CP review SER, 1974). A recent evaluation of this event by the United' States Geological Survey (Stover et al.,1982) supports this conslusion. Because this event has not been associated with known geologic structure, we assume its occurrence at the closest approach of the Ouachita-Wichita belt of seismicity (150 miles) to the River Bend site. Assuming the occurrence of a MM intensity VII earthquake at 240 kilometers (240 km, 150 miles) distance from the site and using the intensity-att'enuation relationship of Gupta and Nuttli (1976) results in a MM intensity IV at the site. Modified Mercalli (MM) intensity = IV is well below that of the SSE.
2.5.2.3.3 Gulf Coastal Plain Seismicity Seismicity in the remainder of the Gulf Coastal Plain Tectonic Province is relatively uniform. The largest historical intensity earthquake which has not been associated with known geologic structure is the MM intensity VI event which was centered near Donaldsonville, Louisiana, on October 19, 1930. The largest instrumentally recorded earthquake, which has not been associated with known geologic structure is the m = 4.8 b
event which occurred offshore in the Gulf of Mexico on November 5, 1963.
Based on the lack of structural association, we assume that an earthquake similar to that which occurred in Donaldsonville or the Gulf of Mexico, could occur near to or at the plant site. Although the January 8,1891 Rusk, Texas earthquake is reported as MM intensity
. VII (Coffman and Von Hake,1973), the felt area gf this event is very small (felt only at Rusk). This had led Nuttli and Brill (1981) to interpret this event as being a very small magnitude shallow earthquake.
The staff concurs with Nuttli and Brill (1981) and has concluded that th'is event should be considered as a very shallow small magnitude earthquake (Comanche Peak SER, 1981, Waterford, SER, 1981). Ground motion estimates from the above events are discussed in SER section 2.5.2.4.2.
2.5.2.4 Safe Shutdown Earthauake In determining the Safe Shutdown Earthquake we have followed the Tectonic Province approach described in Appendix A to 10 CFR Part 100.
The applicants' proposed Safe Shutdown Earthquake acceleration level of
-.m 0.10g is used as the high frequency anchor point for a Regulatory Guide 1.60 response spectra. As discussed below, this is a conservative representation of the Safe Shutdown Earthquake response spectra.
2.5.2.4.1 Ground Motion From New Madrid Type Earthquake In the operating license review, the staff has estimated the ground moticn of the postulated New Madrid type earthquake at a distance of 370 kilometers (230 miles), from the site. Because of the low attenuation of seismic energy in the central United States the duration of motion is longer than is typical for earthquakes of equal size in the western United States at large distances. Nuttli (1973) believes that damage from larger earthquakes in the Midcontinent of the United States, can be attributed to long duration ground motion produced by surface waves in
the frequency range.33 Hz to 1 Hz. We concur that the long duration ground motion is due to attenuation differences and is a reasonable explanation for the phenomena observed at large distances from the New Madrid earthquakes. To account for this attenuation difference between the eastern and western United States, the staff has used a recent ground motion relationship developed by Tera-Lawrence Livermore National Laboratories and the NRC staff for the Systematic Evaluation Program (Jackson, 1980) and relationships developed by Campbell (1981) and Nuttli and Herrmann (1981). The staff places more confidence in the Nutt11 and Herrmann (1981) relationships because it allows for specific use of regional Gulf Coastal attenuation factors (developed by Singh, 1981).
Ground motion estimates have been calculated assuming an mb = 7.3 earthquake occurred 370 kilometers (230 miles) from the site.
In the higher frequency range (frequencies above about 2 Hz) the staff estimates for peak acceleration range from.02 to.06g (Nuttli and Herrmann, 1981, value =.02 ).
For lower frequencies the staff 9
estimates for peak velocity range from 5.2 to 9.8 cm/sec.
The staff has compared the' River Bend SSE, at 5% damping to the postulated New Madrid earthquake using the above estimated peak ground motion values and the spectral amplification factors (84th percentile) of Newmark and Hall (1978). The River Bend SSE spectra would exceed the New Madrid spectra at all frequencies.
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. 2.5.2.4.2 Ground Motion from Gulf Coastal Plain Earthquake As discussed in SER section 2.5.2.3.3 the largest historical earthquake which has not been associated with a geologic structure, within the Gulf Coastal Plain, is MM Intensity = VI or mb = 4.8.
The applicants proposed SSE acceleration level of 0.10g is used as the high frequency input to a Regulatory Guide 1.60 response spectra. An acceleration level of 0.10. roughly corresponds to a MM Intensity VI-VII occurring 9
near the River Bend site using the intensity-acceleration relationship of Trifuuac and Brady (1975). MM Intensity of VI-VII is more conservative then the largest historical intensity earthquake (MM Intensity - VI) in the Gulf Coastal Pla~in. The ground motion from the largest historical magnitude earthquake (m = 4.8) has been estimated as b
part of the Waterford Safety Evaluation Report (1981).
In addition the staff has also used recent ground relationships of Joyner and Boore.
(1981), Campbell (1981) and Nuttli and Herrmann (1981).
It should be noted that great care should be exercised when using the relationships of Joyner and Boore (1981) and Campbell (1981) for a mb = 4.8 where little data exists. Based on staff analysis and review of results discussed in the Vaterford SER (1981), we find that a Regulatory Guide 1.60 response spectra utilizing a peak acceleration of 0.10g accommodates the ground motion for a m = 4.8 earthquake. Thus, the b
design response spectra for the River Bend site would accommodate the largest earthquakes not associated with known geologic structures (MM Intensity = VI or mb = 4.8) if events similar to these were to occur near the site.
v 2.'5.2.5 Operating Basis Earthquake The applicant has proposed 0.05g for the acceleration level corresponding to the Operating Basis Earthquake.
The design vibratory ground acceleration for the Operating Basis Earthquake is taken to be one-half of the design vibratory ground acceleration for the Safe Shutdown Earthquake, consistent with Appendix A to 10 CFR 100.
The applicant has also provided the staff with results from a limited seismic hazard analysis, completed to assess the probability of exceeding the Operating Basis Earthquake during the plant life.
Input into this analysis included three circular source zones, each with one recurrence relationship and upper intensity cutoff. Aisousedwere three attenuation relationships, each with an uncertainty term.
Uncertainty in source zonation, recurrence relationship and upper intensity cutoff were not included. An important part of completing a seismic hazard analysis involves the selection of an approach to incorporate the uncertainty of all input parameters into the analysis.
Difficulty in accounting for this uncertainty is one of the reasons the staff has used probability studies in a limited sense.
In the River Bend study, it is the staffs opinion that the results should only be used in a qualitative sense due to the rudimentary source zones and relatively low upper intensity cutoff. Results of the River Bend study show that the Operating Basis Earthquake has a return period on the order of 100 to 1000 years.
l Consid,erir.g the low seismicity of the Gulf Coast and the compliance with Appendix A to 10 CFR Part 100, we find that proposed acceleration value for the Operating Basis Earthquake is conservative for the River Bend site.
2.5.3 Surf. ace Faulting Post Construction Permit site and regional subsurface information presented to date by the applicants reinforces the previous NRC staff opinion that there is no known evidence either at the River Bend site or within 8 km (5 mi) of the plant site to indicate surface taulting or that the potential exists for such faulting. The new information consists of (1) the applicants' site mapping efforts, (2) examination of site excavations by a NRC staff geologist, and (3) structural contour maps of deep geologic horizons [repared by an independent consul, ting firm.
We have asked the applicants to acquire, examine, and interpret post CP geophysical and bore hole data existing within 8 km (5 mi) of the River Bend Station. The applicants have acquired this information and are currently preparing a FSAR amendment addressing the results of their interpretation of these data. The Staff has been provided, and is currently evaluating the. proprietary seismic reflection data. The results of our interpretatirn as well as that of the applicants' will be addressed in the Final Safety Evaluation Report.