ML20151Q224
| ML20151Q224 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 08/01/1988 |
| From: | Cockfield D PORTLAND GENERAL ELECTRIC CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| NUDOCS 8808110076 | |
| Download: ML20151Q224 (56) | |
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Portland General ElectricCompany David W. Cockfield Vice President, Nuclear August 1, 1988 Trojan Nuclear Plant Docket 50-344 License NPF-1 U.S. Nuclear Regulatory Commission Document Control Desk Washington DC 20555
Dear Sir:
Pacific Northwest Seismicity As part of a continuing activity, PGE has b?en monitoring the results of research concerning Pacific Northwest seismicity and earthquake hazards associated with the Cascadis. Subduction Zone. Information presented at a workshop and symposium earlier this year was submitted with our letter of June 15, 1988. As described in that letter, PGE has responded to a number of questions from the Oregon Department of Energy (ODOE) concerning the status of Cascadia subduction Zone research, and seismic capability of the Trojan Nuclear Plant with respect to the possibility of a large magnitude subduction zone osrthquake. Copies of the ODOE questions and PGE responses, submitted to ODOE on June 6, 1988 are provided in the enclosure. Attachments B and C to our June 6 letter to ODOE, consisting of Trojan FSAR Sections 2.5 and 3.7, respectively, are not included in the enclosure since the FSAR is readily available at NRC offices. On June 16, 1988, PCE, the intervenor, and OBOE met with a subcommittee of the Energy Facility Siting Co mcil (EFSC) for the State of Oregon. Infor-mation provided in the enclosed question responses was sumnarized in a presentation by PGE, followed by presentations by the intervener and ODOE. Following that meeting, we received ODOE's conclusion that Trojan can do0I 8808110076 80080! in PDR ADOCK 05000344 P PNV , g{ 121 S W Sa:rron S::eet. Powd. Oregon 97204
4-f 4 1 W M M C Ovi @ rf l Document Control Desk August 1, 1988 Page 2 withstand the largest earthquakes now being postulated without danger to the public. That conclusion sas accepted at a full meeting of the EFSC on June 30 1988. We will keep you informed of significant developments on this topic. Sincerely,
/
Attachments c: Mr. John B. Martin Regional Administrator, Region V U.S. Nuclear Regulatory Commission Mr. Bill Dixon State of Oregon Department of Energy Mr. R. C. Bare NRC Resident Inspector Trojan Nuclear Plant , i l i
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n <3 June 6, 1988 PORTLAND GENERAL ELECTRIC COMPANY (PGE) RESPONSE TO OREGON DEPARTMENT OF ENERGY (ODOE) QUESTIONS CONCERNING THE POSTULATED CASCADIA SUBDUCTION ZONE EARTHOUAKE ODOE Ouestion 1 Describe what is meant by the Qascadia Subduction Zone. PGE Response The Cascadia Subduction Zone is the region of interface where the Juan de Fuca plate is being overridden by the North American plate . Discussion The old) Juan de Fuca plate is a geologically young (8 oceanic upwelling magma plate originacing from a mid-ocean spreading zone ofto 10 million years off the coasts of southwestern Canada,(molten rock from beneath the earth's cru located and northwestern California (Figure 1-1). western As Washington newly formedand Oregon, oceanic crustal it Ridge, material gradually (lithosphere) cools andspreads away from the Juan de Fuca thickens. The Juan de Fuca plate is converging with the continental North American plateatin an east-northeasterly direction at a rate that been estimated has convergence rate is unknown.three to four centimeters per year; the exact de Fuca and North American platesThe cor. vergence zone between the Juan begins approximately continental slope, and100 km (62 miles)(referred to as the trench axis) it offshore at the base of the in north-south orientation. extends approximately 1,000 km (620 miles) In the convergence zone, below and being overridden by the North American platethe Juan de Fuca p known as subduction. in a process The convergence zone extends to the east where the Juanangle. shallow de Fuca plate dips below the North American plate at a Approximately 250 km (155 miles) inland, the downward plate eventually assimilatesangle of subduction increases significantly, and the J the continental crust of (by melting) into the mantle beneath area of the convergence ofthe North American plate (Figure 1-2). The American plate, the Juan de Fuca plate with the North to the assimilation ofand extending from the base of the continental slope to as the Cascadia Subduction the Juan de Fuca plate, is generally referred Zone. Study of , earthquakes subduction typically zones occur worldwide has shown that subduction zone in two places (see Figure 1-2): 1) the "plate interface" (ie, the boundary) between the subducting and overriding ial within theplates, subducting and 2) the "intra-slab" region composed of mater-plate. The distinction between these
( <4 possiblo subduction zone earthquake sources is important to the f -
-Cascadia Question 3.Subduction Zone discussion provided in the response to 2-
. . _ . . . _ _ -~ .
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1 REPRESENTATIVE SUllDUCTION ZONE MODEL SPREADING ZONE OCEAN RIDGE OCEAN TRENCH Q :. . OCEAN COAST CONTINENT f - . . . . . . .\ . . . .. ... P> arse rtare.
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i '4 ODOE Ouestion 2 How zonesdoes the the around Cascadia world?Subduction Zone compare to other subduction PGE Response In general, duction zones.the Cascadia Subduction Zone is unlike most other sub-oceanic plates in the world andThe Juan de Fuca plate is one of the smallest ting slabs. is one of the youngest known subduc-ison with other subduction zones.The rates of convergence are also very low in comp In addition, of earthquakes observed during recorded history (the lastthe number150 andyears) size has been low compared to other zones. Despite these differences, between the postulated characteristics ofresearchers have made several comparisons Zone and those of other subduction zones aroundthe Cascadia the world. Subduction Early stages of viewpoints. research on the subject have provided a variety of Discussion the oceanic crustal materialsIn a comparative analysis of subduction zones con in the subducting plate, the rate of convergence between the plates, and historic earthquakes associated with the subduction zones, it has been proposed that the Cascadia Subduction Zone may have the potential for generation of large earthquakes (Heaton and Kanamori, 1984). Subsequent studies have proposed that tion zones in southern Chile,the Cascadia Subduction Zone may be similar to subduc have strong seismic coupling southwestern Japan and Columbia which (which can result in large earthquake events) Hartzell,and have 1986). generated large historic earthquakes (Heaton and Another researcher southwestern Japan,(Davis, southern1988) Chile,concludes that comparison with the southwestern Columbia, Rivera and northern Cocos (in Mexico) Cascadia potential. Subduction Zone may be very dissimilarsubduction zones suggests that the in seismogenic The Juan de Fuca plate is characterized as a much ) younger topographically smooth segment of octanic lithosphere. ; postulated convergence rate of 3 to 4 The I relatively slower than that of most centimeters per year, which is ! an anomalously large accumulation of other subduction zones, and that { temperature and high fluid pressure occurs attrench sediment with high tion Zone convergence boundary. (such sediment the Cascadia Subduc-the southern Chile Subduction Zone, but is also present in l These sediments have been postulated the volume is much smaller.) l to provide a lubricating medium to allow of buildup theplate converging interfaceplates to interact without large scale resistance. l region of weak seismic coupling, ie, This may provide for a occurring mainly in the form of aseismicinterplate motion could then be quakes) creep. If (not subject to earth-the interplate coupling is weak, a seismogenic
1 , .s l interface betwson tha Jucn de Fuca and North American plates may be very tudesnarrow andby exhibited incapable of generating other subduction earthquakes of the magni-zones. At this time, researchers have not reached consensus on what the 1 subduction zone comparisons mean with respect j generation potential of the cascadia Subduction Zone.to the earthquake 1 I l 1 l 1 l
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ODOE Outstion 3 (s Describe the recorded history of significant earthquakes in Oregon and Washington. associated with the Cascadia subduction Zone. Indicate the earthquakes that PGE Response The recorded history of earthquakes in the Pacific Northwest shows that most events have occurred offshore. The significant earth-quakes that centered have in the beenSound Puget recorded from onshore events were generally area. the subducting slab, not at the Juan Deep earthquakes occurred within plate interface. de Fuca plate-North American quake has been postulated (The very large Cascadia Subduction Zone earth-to occur at the interface.) The largest are those thatearthquakes associated with the Cascadia Subduction Zone
. 1965 (MB 6.5) inoccurred in 1949 (MBarea.
the Seattle-Tacoma 7.1*) in Punet Sound and in Figure 3-1 shows the pattern of earthquake epicenters in the Pacific Northwest. Discussion The red majority of the earthquakes in the Pacific Northwest have occur-offshore and are associated with geologic structural features /' other than the Cascadia Subduction Zone. These features include oceanic zones, thetransform faults such Queen Charlotte as the Fault, the San Blanco and Mendocino Andreas fracture Fault, internal deformation within the Explorer and Gorda plates, and scattered shallow crustal seismicity. The pattern of onshore earthquakes shown in Figure 3-2 varies significantly in both north-south orientation and in depth. The majority of earthquake epicenters are located in the general area of Puget Sound. of Proceeding south from Puget Sound, the number and size recorded earthquakes diminishes; infrequent earthquakes that have been located in this southern region have recorded or estimated magnitudes in the low to moderate range. Cross-sections show that through the Puget Sound region (Figures 3-3 and 3-4) the earthquakes in this region are occurring in two distinct groups: (1) Shallow crustal earthquakes within the North American plate appear or less. in the projection at depths of approximately 30 km Body WaveRefer Magnitude. Magnitude (M B ) is approximately the same as Richter to the Glossary of Terms, Attachment A. l
,7 - 1
(2) A socond rcgion of earthquakes occurs within the subducted Juan do Fuca plate at depths of approximately 30 km or more. t The deeper set of earthquakes are "intra-slab" events and can be referred to as Cascadia Subduction Zone events. The more shallow set of earthquakes appears to be related to shallow faults in the Puget Sound region. The relationship of these more shallow events to the subduction process *< not clear. The largest earthquakes associated with the Cascadia Sub(action Zone are those that occurred in 1949 (MB 7.1) in Puget Sound end in 1965 (11B 6.5) in the Seattle-Tacoma area. Modeling of these events has confirmed that they occurred within the Juan de Fuca slab and not at the plate interface. No recorded earthquakes as low as magnitudes
- of 3 (on the Richter Scale) are known to have occurred on the plate interface. South of the Puget Sound region in northern and central Oregon, the shallow crustal earthquake activity decreases dramatically. In addition, recorded earthquakes within the subducted plate, which is estimated to be deeper in this region (Crosson-Owens, 1987; Wea ve r- Ba ke r , 1988), are also very sparse. It has been proposed that the greiter seismicity of the Puget Sound area, as compared with the Willamette Valley region to the south, is due to arching of the subducted Juan de Fuca plate beneath western Washington. The subducted plate bcneath the Willamette Valley is more planar in its configuration and much steeper in its eastward dip.
- Refer to the Glossary of Terms. Attachment A.
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i OpOE Ouestion 4 r I What are currently proposed tion Zone? What tectonicearthquake models for potential the Cascadia Subduc-with these models?is the postulated associated t PGE Response , Cascadia Subduction from a postulated Zone completely research has resulted in models
' locked' range that seismic potential of unspecified plate interface with large 1 recurrence intervals (repeat I times), to speculation results in continuous, that the plate interface is ' unlocked' which complex theories propose thatessentially aseismic, movement. Other more {
mented across its north-south extentthe Cascadia Subduction Zone is seg- { modes of deformation in terms of configuration, dip and rate ofand may have convergence, i and hence varying seismogenic potential. l Cascadia Subduction Zone could rupture in asimilar singleto reeventS the Chilean earthquake of 1960 which was the largest earthquake ever , recorded and Hartzell, 1987).had a seismic moment magnitude
- of Mw 9.5 (Heaton and There are different scales used to measure the size Mercali",
fled of earthquakes etc). (ie, Richter *, Seismic Moment Magnitude, Modi- i lated to each othor. This Theso various scales cannot be exactly corre- i magnitude of 9.5. Mw value is not equal to a Richter i I At present fundamental question of Cascadia Subduction Zone seismic A large body of evidence from the geoscientific record , which is . subject to consistent consensus interpretation, will be needed to approach a of expert opinion. Discussion Figure 4-1 illustrates the prominent i subduction process and the region of features 'of the oceanic plate l concerning the seismogenic potential interest for discussions This region, referred for large thrust earthquakes. of thrust to as the seismogenic earthquakes observed at subduction zones. interface, is the source The seismogenic interface begins in a the weaker accreted continental crust inmaterials and the more competentregion of transition between the convergence zone, (very strong) reg.on where the slab becomes more ductile (ie,and extends downward to a potential for earthquake The seismogenic interface, occurrence relates to the width ofless brittle). the relative movement the rate and character (gradual or abrupt) of strength of between the plates along the interface, and the the interface rock formations. Refer to the Glossary of Terms, Attachment A.
Invostigators in many areas of the geosciences are continuing their assessment Zone. Much of of the seismogenic potential of the Cascadia Subduction the geologic and seismologic data needed to further develop and calibrate various tectonic models of the Cascadia Sub-duction Zone remain to be obtained and consistently interpreted. A consensus of expert Subduction Zone is thus not yetopinion on a seismogenic model for the Cascadia additional studies. available and will require many This response, therefcre, does not address all of the Cascadia Subduction Zone tectonic models presently under development, but will instead characterize the range of models that have been published. The locked plate theory contends that the interface between the subducting Juan de Fuca plate plate is strongly coupled, such that and the overriding North American plates is resisted. relative motion between the , elastic strain energy across the interface.This resistance leads to the accumulation Eventual rupture of the interface large plate may result in abrupt energy release in the form of earthquakes. 1 I As previously noted, Subduction Zone with other subduction zones aroundsomethe researchers world, have compare{ particularly those : in southern Chile, southwestern Japan, and Columbia, historic earthquakes. exhibited strong seismic coupling and large which have ! oceanic plates at Parameters such as the age of the subducting ! the base of the continental slope (trench), the rate of convergence overriding continental between plates,theand underriding oceanic plates and the ; the magnitudes of historic earth- 3 quakes associated with these other subduction zones were selected ' for comparisons (Heaton and Kanamori, 1984). Based on the age of the Juan de Fuca plate at the interface of the Cascadia a postulated Subduction Zone, estimated rate of convergence of 3 to be 8 to 10 million years, and to 4 centimeters per year, the earthquske potential of the Cascadia subduction Zone has been char-acterized (Heaton-Xanamori, 1984) to be on the order of seismic moment magnitude Mw 8 to Mw 8.5. Figure 4-2 indicates a rela-tionship between maximum energy magnitude Mw, convergence rate, and age of the subducted lithosphere. Speculation has included the possibility that a Cascadia Subduction Zone could rupture in a single event, essentiallysimilethe er co the Chilean earthquake of 1960 which was the largest earthquake ever recorded and had a seismic moment magnitude of Mw 9.5 Heaton and Hartzell, 1987). Simulated ground motions were reopose(d that could occur during large subduction zone thrust earth.tukes. approach was required t This were greater available than 7.5.for earthquakes with seismic moment magnitudesbecause However, special very large Chilean earthquake (a convergence rate 3 geologic conditions favoring the times that of 14 -
Juan do Fuca/ North America, a very narrow accretionary wedge
- which equates to a very wide seismogenic plate interface) have not been observed in the Pacific Northwest.
Variations in convergence rate along the Cascadia Subduction Zone (slower in Oregon),in Oregon), dip of the subducted Juan de Fuca plate (steeper sible reasons why the entire Cascadia Zoneand thickness of sediments being sub would not rupture as a single segment. The(even if seismogenic) physical division of the subducted plate into a central arched slab beneath the Puget Sound area and neighboring, more planar slabs to the north and south, provides an additional basis for segmenting the Cascadia Subduction Zone in terms of its seismogenic behavior. All Pacific subduction zones show such segmentation and no single plate boundary such as the Juan de Fuca/ North American boundary has ever been ruptured in a great earthquake along its entire length. Despite its great length of rupture (approximately 1000 km), the 1960 Chilean earthquake broke only about one-fifth of the plate boundary along which it occurred. In contrast to the locked plate hypothesis. other researchers (Davis, 1988; Byrne, et al. 1987; Sykes, et al, 1987) have suggested that the Cascadia Subduction Zone is unlike the known seismogenic subduction zones to which it has been compared. The Juan de Fuca plate is e celatively young oceanic plate with a slower other subduction zones) convergence rate. Also, there is(relative a large to accumulation of trench sediment at the Cascadia convergence boundary. These sediments are up to 2 km thick Subduction Zone with temperatures estimated to be 200*C at their base, and with high internal fluid pressure; they are believed to enhance stable sliding behavior such that plate convergence can occur without large accumu-lation of strain across the interface. In this model, tude expected, energy releases in the form of earthquakes should notlargebemagni-and the model indicates that generalized comparisons of the valid. Cascadia Subduction Zone to other subduction zones are not For in southern Chile,example, comparison has been made to the subduction zone earthquake in 1960. which was the location of a magnitude My 9.5 However, several differences are apparent: The 1000-km-long rupture initiated in older oceanic crust (30-35 Juan demillion years as compared tc 8-10 million years for the Fuca plate). The (9 cm/ plate yearconvergence rates for southern Chile are much higher vs 3 cm/ year). Despite the fact that the southern Chile zone has a sediment-filled trench, Crystalline the Cenozoic rocks crop out on accretionary prism is very narrow. trench, suggesting subduction the sea floor erosion, within a very 100 back-close-in km of the stop, and a very wide seismogenic interface.
- Refer to the "Glossary of Terms", Attachment A.
15 -
i Other investigators have suggested that stress buildup in the Cas- ! cadia Subduction Zone is very complex, and is dominated by the [ interaction well of the Juan de Fuca plate with neighboring plates as as extensional Spence (1988) for example, "slab-pull" stresses within the plate itself. 1 believes that ! the Juan de Fuca plate provides significantthe youth and buoyancy of ( particularly at resistance to subduction. { north-south compressive stresses. parts of the subduction zone perpendicular to the ' This model implies that various parts of the subduction zone may have different seismogenic potential, i l Several studies have shown that subduction has slowed considerably (aboutthe rate of Juan de Fuca plate from 6.5 million to about 500,000 years ago. 60%) during the period data on the convergence rate over the past 500.000 There is no direct years, but the i The Gorda and Explorer plate remnants may have ceas altogether. The Juan de Fuca plate subduction beneath Oregon is considered to have greatly Analysis showsslowed that and seismicity in this region is very sparse . in areas to the north.the rate of convergence is lower in Oregon than It has been speculated that the convergence zone resistance to continued subduction and progressive weakening of the subducted cessation of Juanslabdeextensional Fuca plate forces may be evidence of long-term subduction. Offshore occurringHolocene across thedeformation confirms that convergence is still Cascadia zone. gence is very slow, However, if the rate of conver-occurrence should be very low,then the likelihood or frequency of earthquake Subduction Zone off the coast particularly in the southern Cascadia of Oregon. i 15 - I
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ODOE Ouestion 5 # What evidence in there from the geologic record that has been inter-preted of to supportinquiry? the scientific any of these models, and what is the current status PGE Response Recent geologic investigations have identified evidence for recurrent subsidence of coastal estuary deposits (tidewater marsh burial events that appear to have recurred at irregular intervals) in southwestern Washington and northwestern Ore'gon. ,
"jerky" subsidence has been cited as evidence thatThis evidence or of abrupt large earthquakes may have occurred several times in the Cascadia Subduction Zone dur- ;
ing the past several thousand years (Atwater, Peterson, Grant 1987-1988). Sequences of intertidal bay mud layers overlaying layers of peat-like material that range in depth from 0.5 to 2.0 meters,ofwith possibility rapidrelatively subsidencesharp interface features, sugesst ' the events. Radiocarbon indicate that dating of these sequences has been interpreted to 1,000, 1,500, submergence events occurred in the vicinity of 300, 1,700, 2,500, 2,800, and 3,500 years ago. Sudden downdrops creating these burial sequences are speculated to have , been generated by large, prehistoric Cascadia Subduction-Zone earthquakes. i buried The presence of sand layers overlying some of the as evidence that formations low land in these sequences has been interpreted tsunamis may have resulted from the same events that caused the subsidences. These rences,subsidences but may be evidence of paleoseismic earthquake occur- i they remain to be spatially correlated along the coast ! and event-correlated ted that from site-to-site. It remains to be demonstra- I { be exhibited these in data are consistent the geologic record with other evidence that should Zone earthquakes have occurred. if large Cascadia Subduction For example, additional evidenc6 , that would be expected to be exhibited in the geologic record l
~
includes evidence for paleoliquifaction such as event-correlated vented uplifted sands underlying exposed subsided wetlands, subsided or wavecut ward uplifts, and benches along rocky headlands, sea floor and land-landslides for which rainfall and non-Cascadia Subduction Zone earthquakes can be excluded as probable causes. Until such observations are made,
- convincing evidence for large-magnitude the evidence Cascadia for subsidence Subduction is not Zone !
events. Research efforts response to Question 6. in this area are continuing. See the 2
- 19 -
ODOE Ouestion 6 What I in progress or planned by the scientific community? additional res PCR_ Response Much of the scientific inquiry concerning the Cascadia Subduction Zone 19P7) is being funded as part of a five-year program (which began in by the Ur.ited States Geological Survey (USGS) National Earthquake Hazards Reduction Program (NEHRP).through the Part of that program Northwest. is a regional earthquake hazards assessment for the Pacific Their draft work plan for fiscal years 1987 to 1989 is intended to definaactivity. ities for further research guidelines and to identify responsibil - Beginning with the recently concluded April 1992) 1986 workshop, a series of five annual workshops (1988 through are to be held in the Puget Sound area to review accomplish-ments and program directions, and to foster knowledge-base develop-ment and as published application. Proceedings USGS Open-File from the annual workshops will be reports. Typically, projects under the NEHRP Program are be involved in the future.funded annually and additional investigators may As described in the response to Question 11, PGE is monitoring the d evelo pme rit t from, and results of, this program. On-going fellowing:research presented at the 1988 workshop included the Affiliation Research Description Dr. Gary Rogers Geological Survey of Canada Pacific Northwest Earthquake Potential Dr. Brian Atwater USGS Coastal evidence of large prehistoric Pacific Northwest earthquakes.
)
Al Rogers USGS Factors affecting i i ground motion in , Pacific Northwest i earthquakes. Ken King USGS Puget Sound and ; Portland urban areas i earthquake hazards I assessment program. ) Tom Urban USGS Investigation of under-water evidence of geo-logic faulting in Puget Sound. I l
Affiliation Research Description Dr. Curt Peterson Oregon State University and ' Investigation of subsi-Alan Nelson USGS ded tidewater marshes on the coast of Oregon. Dr. William Spence USGS, National Earthquake Information Center Continuing investigation of the seismicity and tectonics of the Cascadia Plate System. Robert S. Crosson University of Washington Investigation of sub-coastal earthquakes in the Puget Sound region. S. T. Algermissen USGS Estimating anticipated ' ground motion due to earthquakes in the Pacific Northwest. Dr. John Tinsley USGS Investigation of geo-logic factors that influence propagation of earth motion. quake ground Dr. Robert Schuster USGS Investigation of earth-quake-induced ground f ailures 11. Western Washington. Margaret Hopper USGS Investigation of the effects of past earth-quakes in the Puget Sound area. Tom Heaton USGS Investigation of earth-quake hazards on the Cascadia Subduction ' Zone. A number held at of ongoing research activities were reported at a symposium ium was coordinated by Dr. Brian Atwaterthe 1988. University of Washington o The r.ympos. coastal evidence for the past occurrence o(USGS), whose report on , Dr. Atwater's research. earthquakes was published in Science Magazine in M i shorelines on the Washington coast, investigating subsided marshes and uplifted is continuing. coastal evidence of past In addition, Gary Carver (Humboldt State University), earthquake events is being investigated by Washington University), and Mary Reinhart Harvey Kelsey (Weatern (University of hashington). i l 4
Lotor this year. Atwater and Nelson will be investigating the geo-logic record of large magnitude earthquakes in southern Chile. Ongoing research reported at the May symposium included the follow-ing activities, primarily involving review of geologic and other physical records forzone, possibly subduction evidence supporting the theory that large, earthquakes may have occurred in the Pacific Northwest 17 0 '.) years ago. in the geologic past, most recently about 300 and Affiliation - Research Description Alan G. Hull University of California, Santa Barbara Radiocarbon dating of buried soil layers on the Washington coast. David K. Yamaguchi University of Colorado Identification, by dendrochronology, of the time when cedar trees died, possibly as a result of salt water intrusion when the land elevation changed rela-tive to sea level. Wendy C. Grant USGS Investigation of tidal marshes for evidence of i earthquake-related sub- ' sidence along the Salmon and Nehalem Rivers in northern oregon, ! Michael Lisowski USGS i Investigation of geo. l logic strain accumula- i tion in western Washington and south- t western British I Columbia. Paul Vincent University of Oregon Resea,ching elevation surveys for evidence of uplift and subsidence along the Oregon coast. Albert A. Eggers University of Puget Sound Investigation of defor-mation of Pleistocene sediments in the Tacoma , Narrows, Washington area. 4
ODOE Ouestion ? What is the seismic design basis for the Trojan Plant? PGE Response The Trojan Nuclear Plant. seismic design basis is represented by the design response spectra provided as Figures 7-1 and 7-2. These spectra Regulations Federal were established in accordance with Title 10 of the Code of Part 100, Appendix A, Siting Criteria for Nuclear Power Plants". "Seismic and Geologic These spectra, which establish the ground motions to be withstood as a function of and damping ratio peak ground acceleration represent the designperiod basis for Trojan, not the single-point peak ground acceleration levels of the(OBE). Earthquake Safe Shutdown Earthquake (SSE) and Operating Basis Discussion Investigations performed pursuant to establish Trojan's seismic design basisto 10 CFR 100 Appendix A criteria included: (1) Determination of the lithologic, stratigraphic, hydrologic, and structural geologic surrounding the site,conditions of the site and the region including its geologic history. (2) Identification and evaluation of tectonic structures underlying the site and the region surrounding the site, expressed at the surface. whether buried or (3) Evaluation of physical prior earthquakes evidence concerning the behavior during of the surficial geologic materials and the substrata underlying the site from the lithologic, strati-graphic, and structural geologic studies. (4) Determination of the materials of underlying the static and thedynamic site. engineering properties (5) Listing of all historically reported earthquakes which have ! affected or which could reasonably be expected to have affected the site, including the date of occurrence and the following measured or estimated data: magnitude or highest and a plot of the epicenter or location of highest iniensity,* intensity. (6) Correlation of epicenters or locations of highest intensity of historically reported earthquakes, where possible, with tectonic structures, the site. any part of which is located within 200 miles of cannot Epicenters or locationc of highest intensity which be reasonably correlated with tectonic structures were to be identified with tectonic provinces, any part of which is located within 200 miles of the site.
- Refer to the Glossary of Terms, Attachment A.
- 23 -
I i
(7) For faults, 1 any part of which is within 200 miles of the site ( and which may be of significance in establishing the Safe Shut-down Earthquake (SSE), determination of whether these faults are to be considered as capable faults. A "capable fault" is a fault which has exhibited one or more of the following characteristics: Movement at or near the ground surface at least once within the past 35,000 years, or movement of a recurring nature within the past 500,000 years.
. Macro-seismicity instrumentally determined with records of sufficient with precision to demonstrate a direct relationship a fault.
. A structural relationship to a capable fault according to the above characteristics reasonably expected to be such that movement accompanied by movementon one could be ;
other. on the ' Desailed information developed from these siting studies is presented in Sections 2.5 and 3.7 of the Trojan Final Safety Analysis Report (FSAR), provided as Attachments B and C, respectively. In summary, the Trojan Nuclear Plant is located in an area that experiences l moderate to low seismic activity, and safety-related structures are located on Question bedrock of ancient volcanic origin (see the response to 12). j Significant historic earthquake epicenters and their intensities within 150 miles of the site are shown in Figure 7-3. Records show that ofthe maximum intensity reported at Rainier, Oregon, four miles north i intensity occurred on alluvial deposits,the site, Since this it was Modified Mercali Vill. ! at V11.the Trojan site the intensity for this same event did notis probableexceed that on rock 1 l eration of approximately 0.12 g.Ir. tensity V11 correlates with a peak horizo The Safe Shutdown Earthquake (SSE), also referred to as the Design Basis Earthquake, is based on an evaluation of the maximum earth-( mologypotential quake considering and specific regionalofand local geology and seis-characteristics l local subsurface material. ' 1 The Operating Basis Earthquake (OBE) is that earthquake which, considering the regional and local geology and seismology and l' specific ably characteristics be expected of local subsurface material, could reason-to affect the Plant site during the operating life of the Plant. i For the Safe Shutdown Earthquake (SSE), a conservative intensity of Modified Mercali Vill was selected, since it is probable that an 1 intensity during of Vill has recorded never been experienced at the Trojan site history. ' horizontal ground acceleration of 0.25 g.An intensity Vill corresponds Therefore, a horizontal to a peak acceleration Plant of 0.25 g was used for the design of the Trojan Nuclear for the SSE. Although intensity horizontal ground acceleration Vll correlates with a peak of 0.12 g, of 0.15 g was used for the Trojan OBE. a more conservative value
D0Volop2cnt of tho soisaic design basis for Trojan was reviewed by the United States Geological Survey (USGS), serving as a consultant l, to the Nuclear Regttlatory Commission (NRC). Their findings were provided to the Advisory Committee for Reactor Safeguards (ACHS), an advisory group for the NRC Commissioners composed of recognized experts in various engineering and technical fields. The ACRS con-cluded that the 0.15 g OBE and 0.25 g SSE are adequate, to be used in conjunction with conserva-tively derived response spectra. The Trojan seismic design response spectra are shown in Figures 7-1 and 7-2 for the OBE and SSE, respectively. Conservatism applied in developing the ground accelerations for the OBE and the SSE and in developing design the seismic design response spectra provides significant margin. These spectra are developed for close-in, medium range and distant earthquakes, and thus are "broad band" to account for a wide range of ground motions. These spectra, which establish the ground motions to be withstood as a function of period and damping ratio, represent the design basis for Trojan, not the single-point peak ground acceleration levels of the SSE and OBE. i
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\lshk gERENCES:
1 Bose mcp is token from Internoteonel Mop of the World N L-10 32 Coscode Range g 2ag i* 2. Faults shown are from. v -47' _ _ . .Y} 7p._ 7.r, P 'Ne , Geologic Mon of Woshington,1961, secle 1:500,000 rF Wi 8ader 2 7 fir,cu.ao.nej Geol)gic Map of Oregon west of the 121Meridion,t96f, g ggglg g 500,000 m a 3. U S Eerthquake5 1928 - 196B" U.S.C. B G.S. rk g *
"Earthquake Histo,y of the United States" Port 1. U.S.C. 8 G,S
' o YmW A E.S S A.Hypotenter Octa Cords,1968 - Sept.1969. U.S.C.8 G.S.
y 8 i r 'r ;' "Abstrccis of Earthqucke Reports' 1967 - Sept.1968. U.S.C.8 G.S. I # E g "BuHetin Seismological Society of Americo" j Jon.1939. Desenptive Cotolog of Earthquokes of the Pacific Coast of the United States 1769-1928. Jort 1953- Washingtort Eorthquokes (930 - 1951. A Jcn 1963 Earthquakes in Oregon 1841-1958. Junel967- Washington State Earthquokes 1840-1965. 46 4 The Tectorne Mop of North Americo,19695 shows o possible I I fouff c!ong the Columbio River between Portiond and Kelsci Evoluoton of this possibility indicctes o fault should not be [~ inferred there. . < g 7 2 , e Y.S [ 25 g }: } I g ,...e (I }
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ODOE Ou@stion 8 Compare the Trojan design criteria with design requirements used nonnuclear construction in this area. for PcE Response In general, the Uniform Building Code (UBC area (Figure 8-1) a r e a'. is used to design non-nuc)for Trojan is designed a Seismic lear structures Zone 2 in this are considerably more stringentin accordance with NRC requirements, which in the response to Question 7. than UBC requirements, as described Figure and 8-2 compares the seismic design response spectrum for an Troj the corresponding lateral force coefficients from the UBC The earthquake design criteria for the Trojan Plant, in all ranges of . natural requirement period, are a large factor above for conventional structures. the UBC earthquake design Discussion The most the Trojandramatic Plant differences between the seismic design basis for for ings,other dams,classes of nonnuclear structures such as high rise bu bridges, etc. - inelastic earthquake structuralareresponse, in the design concepts of elastic vs analysis methods. for the Trojan Plant,In an elastic seismic design,andsuch static asversus that useddynamic structures are required respondwithout motions to the amplitude and frequency content to be considered to significant of earthquake ground would otherwise reduce the earthquake nonlinear structural lateral performance which Stated differently, in an elastic design, load design demand. to resonate at their structural frequenciesstructures are considered earthquake ground motion. Thus, without in response to the effects, the structures experience large amplificationsconsideration over of 'detuning' earthquake ground motion input. the result is a substantial As illustrated in Figure 8-2, the ; demand, approaching 0.50 g, increase in the earthquake design load 0.25 g. over the input ground motion level of 1.n the more realistic inelastic seismic design approach is given to the experience based performance capabilitiesrecognition , of struc-tures to deform in a nonlinear required structural integrity and thus, fashion while maintaining their formance, not because of inelastic per-motions. tionally The seismic design of conventional structures has tra taken this more realistic approach, - the inherent with the result that become "detuned" and response is acknowledged. dissipate energy based onural Thus, inelastic str exhibit increased amplitudes. the UBC curves in Figure 8-2 do not
1 Another najor difference in these seismic design concepts is the equivalent static lateral load method used in the UBC design i (. approach versus the dynamic analysis method used in nuclear plant design. In the equivalent static load approach, for the particular UBC seismic zone in which the structure is located, earthquake lateral load coefficients are determined from categories of site i conditions (rock, stiff soil and medium to soft soil) and categories l' of lateralresisting moment load resisting frame, systems etc). (shear wall or braced frame, develop equivalent static lateral These loadscoefficients based on the are structure then used to ' weight tributary to the various stories in a building, for example, to determine the earthquake lateral force demands. Structural res-ponse modes higher accounted for. than the fundamental This static equivalent lateral load approach is are only approximately 1 simplified and rational but of earthquake structural demands.provides only a general characterization ! In the dynamic analysis method, such as used the site seismic response characteristics are firstin nuclear plant design, determined and l then translated base, elastic into foundation springs, non-linear models finite for the structures element, etc.) to (fixed-determine their dynamic response parameters. Seismic input motions are then applied eration time-histories (accelerograms)to these complex models either in th 1 l or spectrum model inputs (spectral accelerations). The seismic loading design seismic response ; demands are then determined by structural response solutions which account for all characteristics of structural behav37r of interest (translation, modes rocking, torsion, etc.) and all inertLa load response of significance. The dynamic analysis method used in nuclear plant design provides a much better representation of the actual seismic characteristics of structures. is time consuming and costly,Being a much more datailed analysis however,method, it and may classes of conventional structures. Even in areas of high seismic not be practical for certain exposure, require that such as Los Angeles and San Francisco, local codes do not ; l in the structural design ofsite specific studies and dynamic analysis be performed buildings, except under specific suchconditions. structures as high-rise office ; The seismic design i criteria for these classes of conventional structures in high seis-mic it isrisk forzones is seldom, if ever, the design of nuclear plants. required to be as conservative as A visual Trojan Nuclear Plantrepresentation of the dramatic differences between the requirements is and Uniform illustrated in Figure 8-2. Building Code seismic design In that figure, the maximum and minimum UBC required earthquake lateral design coeffi- ! cients for plotted conventional structures located in Seismic Zone 2 are against ! the Trojan safe shutdown earthquake (SSE) design response spectrum. Trojan plant, in all As shown, the earthquake design criteria for the above the UBC earthquake ranges design of natural period, is a large factor requirement l structures. for conventional l l 30 - '
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N 9 e. 9 ~ o o o E 6 o 0 -D S ' NOl1VW37300V 7VW103dS ODOE Ouestion 9 How do the consequences of a postulated subduction zone earthquake differ from earthquake? the anticipated effect of a Trojan design basis PGE Response Figure 9-1 compares the Trojan seismic design basis with the synthe-sized lated large subduction by Heaton zone earthquake and Hartzell, 1987. ground motion spec *c.ra postu-seismic design basis with spectra generated by recorded groundFigure 9-2 c motions from actual large subduction zone earthquakes. comparisons show that the Trojan Plant can accommodate the These ground motions resulting zone earthquakes. from postulated extraordinarily large subduction Discussion In a hazard assessment of earthquake ground motion exposure, the proper context of comparison between the postulated occurrence of large Cascadia Subduction Zone earthquakes and the Trojan seismic design basis probabilistic (or any other evaluation. seismic design criteria) would be a ! At present, however, consensus data on the hazard due to Cascadia Subduction Zone earthquake ground motions at l any location in the Pacific Northwest are not available. ) The following, however, is a comparison of the Trojan seismic design basis with (1) synthesized motion spectra, and (2) withlarge subduction zone earthquake ground the spectra generated from recorded ground motions from large subduction zone earthquakes that have occurred elsewhere in the world. the actual earthquake resistance ofIn these comparisons, note that systems above the seismic design basis thedue Trojan Plant structures and to inherent tisms, which is a significant increase (see the response conserva-to Question 10), is not represented. l j It should tion. be recognized that Trojan is located on a rock founda-The rock reduces the effect of earthquakes since there is ; less amplification of the ground motion, as seen when structures are I located on alluvium (soil). Several rupture zone models and types of grorni motion from some of the largest subduction zone earthquakes ever recorded (Hyuganda, Tokachi-Oki, been used and Miyagi-Oki in Japan and St. Elias in Alaska) have to synthesize earthquakes by Heaton-Hartzell ground motions projected for postulated (1987). A rupture zone model used in this synthetic (600 miles) earthquake ground motion generation extends 950 km east to west in(Figure north-south 9-3). orientation This rupture and 200 km (125 miles) from extending from about would encompass an area northern California to southern Canada and from the Cascadia Subduction Zone convergence region (approximately 135 km or 85 miles off the coast of Oregon and Washington) 200 km (125 miles) inland to about the longitude of Puget Sound (this would be a rupture zone area of approximately 73,400 square miles).
- 33 -
Ground motion was simulated for the model by adding the seismic energies of the recorded subduction zone earthquakes described above, to equal the earthquake energy corresponding to that assumed if the Chilean earthquake of 1960 were transposed to the Pacific Northwest. quake to represent (PGE considers this transposition of the Chilean earth-effect of this assumed a highly improbable worst-case scenario.) The energy release, in the form of ground motion response Figure 9-4.spectra for various size earthquakes, is shown in The spectra are represented as average (expected) ground ratio) motions (using a 5 percent damping or energy dissipation as a approximately 50 function of seismic moment magnitude for locations km (30 miles) inland from the coast [the Trojan Nuclear Plant is located approximately 75 km (45 miles) inland from the coastenergy seismic and about 115 for release km this (70 miles) from synthetic the assumed center of model). earthquakes, For the largest motions are postulated to be about 25 percent larger than as shown large in Puget in the FigureSound 9-4 at area. coastal locations and about 67 percent Average peak horizontal ground motion 0.60 accelerations are characterized as being in the range of times the force of gravity (referred sites and 0.26 g for Puget Sound sites (approximately to as 0.60 g) for coastal longitude as Trojan). the same (SSE) (The Trojan Plant Safe Shutdown Earthquake is based on a peak horizontal acceleration of 0.25 (; however, the Operating Basis Earthquake (OBE) of 0.15 g (with the attendant assumptions) SSE greater than 0.25 g.)octually controlled the Trojan design and equates to an The synthesized (67 percent of ground motion spectra adjusted for inland locations Heaton and Hartzell,the spectra shown in Figure 9-4'as suggested by 1987) are plotted in Figure 9-1 together with the Trojan seismic design basis Safe Shutdown Earthquake (SSE) spectrum (5 percent damping ratio). The synthesized spectra have not this been adjusted for the the Trojan rock sitewould response characteristics; is conservative since adjustment computed ing Figure ground 9-1, it motion levels for the synthesizedtend to reduce the curve. is important to note that In analyz-periods of Trojan's safety-related structures,thepiping natural andresponse equipment are typically less than 0.30 seconds, have natural periods approaching 1.0 second.and noThus, structures or systems the relevant portion of the figure is shown in the boxed area. Comparison indi-cates that the response spectrum used for the seismic design of the Trojan plant will accommodate postulated ground motions Figure 9-2 shows the relationship of velocity spectrum with response velocity spectrathe Trojan(adjusted distance, SSE design forresponse as explained below) generated from the largest subduction zone earthquakes ded. The subduction forzone which ground motions have been reliably recor-earthquakes magnitudes Mw - 8, represented had seismic moment which is in the range of postulated magnitudes for large Cascadia Subduction Zone earthquakes. All of the record-ing locations were classified as rock sites. Distances from the subduction zone earthquake rupture surface to the location of the ground motion recording stations were as follows:
Valparaiso, Chilo - 39 km l La Villita, Mexico - 19 km La Union, Mexico - 23 km [- Caleta de Campo, Mexico - 15 km Zihuatenejo, Mexico - 28 km i I I The distance between the Trojan site and the hypothesized Cascadia j Subduction Zone seismogenic interface is presently characterized as being 60 km or more, which is greater than the distances at which the ground motions shown in Figure 9-2 were recorded. The recorded ! spectra shown in Figure 9-2 were adjusted for the difference in distance from earthquake rupture using appropriate ground motion l attenuation relationships (Young, et al, 1988). ; As was the case with Figure 9-1, in analysis of Figure 9-2 it is l important to note that ! the natural response periods of Trojan's I safety-related structures, piping, and equipment are typically less than 0.30 seconds, ' periods approaching 1.0 second.and no such structures or systems have natural As shown in Figure 9-2, the Trojan seismic design basis spectrum i l envelopes the spectra based on actual measured subduction zonc earthquake data from This comparison againChile and Mexico for magnitude Mg 8 events. indicates that the Trojan seismic design can accommodate earthquakes. ground motions resulting from very large subduction zone i' In Figure 9-1, motions at periodsthe comparisons greater thanshowaboutvery good agreement except for 0.70 seconds. In this long-period trum andrange, the divergence in shape between the Trojan SSE spec-the hypothesized mostly attributable to the subduction effect zone earthquake spectra is 1 I to amplify the longer period motions, of soil site rosponse, which tends ponse where such amplifications do notas compared to rock site res-appear. Figure 9-2, As can be seen in where the spectra were developed from earthquake motions recorded ally on result. rock sites, amplified long-period motions do not gener-A discussion on the significant design margins available above the Trojan seismic design basis capability is presented in the response to Question 10.
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l 4 ODOE Ouention 1_0 Put couldaside occurquestions of whether an and theifrequency event of the postulated. magnitude of its recurrence. What margins exist in the seismic safety at design of the Trojan Plant that would provide for ground motion levels significantly greater than those represented by the seismic design basis? PGE Response There are Nuclear numerous conservatisms in the seismic design of the Trojan Plant. The conservatisms result in an additional margin of safety for large earthquakes even beyond that shown in the response to Question 9, which show that Trojan can withstand the ground motions resulting These margins provide from postulated Trojan withlarge the subduction zone earthquakes. motions two or more times the design basis apability to withstand ground ground motions. The increased Figure 10-1. seismic capability from these conservatisms is shown in Discussion . There are numerous different conservatisms inherent in the seismic (earthquake) design of Trojan that result in substantial margins for structures, components and equipment. Trojaa structures, components and equipment I can withstand ground motion levels significantly greater than the seismic design basis earthquake without endangering the public health and safety. Independent studies concerning earth- ! quake margins and conservatisms are described on the following pages. Seismic margins in nuclear plant design and construction have been investigated in the nuclear industry for the past decade. Seismic { margin research Program sponsoredprograms include the Seismic Safety Margin Research 3 rent by the Nuclear Regulatory Comn.ission (NRC), cut-research by the Seismic Qualification Utility Group (an indus-try group representing essentially all nuclear utilities organized to verify seismic qualifications of nuclear plant equipment), on-going seismic margin identification programs sponsored by the l Electric Power Research Institute, and plant-specific seismic probabilistic risk assessments 20 such assessments performed). (there have been approximately The conclusion of these detailed programs and studies is that are substantial margins with respect there nuclear plants in the United States. toThis the uniform seismic industry design basis exper-of ience demonstrates that similar margins exist at the Trojan Plant. Seismic margins specific to Trojan provide further confidence that the Trojan Plant has margins similar to those observed at other plants of where these detailed margin studies have been conducted. One these characteristics of Trojan is its relatively recent design vintage. (As described in the response to Question 7, seismic design criteria 10 CFR 100, Appendix for theA.Trojan Plant are in accordance Uith Prior to Appendix A, regulations with respect to seismic criteria were not clearly defined.) i l
l l There are other specific conservatisms in the Trojan design criteria, such as lower-than-usual damping (energy absorption) values and the l [ relatively high ratio (0.60) of Operating Basis Earthquake (OBE) to Safe Shutdown margins Earthquake with respect (SSE), that create greater-than-usual to the SSE. l In addition, factors that ! studies to be of the most have been identified significance in early in several of themargins lower sels-mic margin nuclear plants (such as mechanical and electrical equip-ment anchorages) have been the subject of conscientious seismic evaluations licensed. at the Trojan Plant in the years since it was originally All of these factors support the conclusion that industrywide seis-mic margin evaluation experience and Trojan-specific factors provide assurance that the Trojan Plant could safely sustain ground motions two or more times the seismic design basis ground motions. This mergin above the design basis seismic event provides additional assurance that Trojan would safely withstand the subduction zone event postulated in Question 9 without endangering the public health and safety. The seismic margin assessments that have been conducted by the NRC, the based are Electric on Power Research Institute, and by individual utilities three approaches: engineering analysis, testing of structures and equipment to beyond their design basis levels, and the actual performance of structures. piping, and equipment in conventional motions. power plants that were subjected to very strong ground These bases and the resulting conservatisms and margins are explained in further detail below.
- 1. Conservatisms in Desion The engineering design process uses assumptions and meth<dology which contain conservatisms with respect to the actual perfor-mance of the structures and systems. Examples of these areas of conservatism are:
i Actual Versus Desion Minimum Material Properties - In the orig-inal design, before actual material properties are known, con-servative, minimum material strengths are assumed. As construc- ! tion progresses, actual samples are tested to determine representative material strengths. these tests are typically 10 to 30 percentMean value material greater strengths than those from used in the design. As an example of this, review of represen-tative concrete test cylinder data for the Trojan Containment s concrete (47 samples) showed that, for a concrete design compressive strength of 5000 psi, the mean value from the test cylinders was 6876 psi, about 38 percent greater than the design value. In another comparison for concrete in the Fuel Building (60 samples), 3000 psi, which had a design compressive strength of the mean value from the test cylinders was 5094 psi, about 70 percent greator than the design value. Representative l test samples for the reinforcing steel used in the Containment i l l 41 -
structure 104,400' psi,alsowhich showed c usen is about tensile strength value of a 16 percent increase over the required minimum value of 90,000 psi. In typical nuclear plant l structures, i determination.mean strength is appropriate for actual capacity Seismic Analysis analysis was usedMethod - The response spectrum method of in the seismic Plant structures, systems, design of most of the Trojan and components. i This method of I analysis,methods modeling in comparison that to the time-history and finite element ' could be used in a seismic analysis, has been recognized or more. to provide a margin on the order of 10 percent Operatina Basis Earthouake Versus Safe Shutdown Earthouake Design Criteria - For Trojan, the OBE is larger than the typical one-half the SSE value for nuclear plants. The Trojan Plant OBE peak horizontal that for the SSEground (0.25 g).acceleration (0.15 g) is 60 percent of ; Because of the low damping ratios i and allowable stresres associated with the OBE design criteria, the design for most structural elements is determined by the OBE requirements. This results in an additional margin beyond the design requirement for the SSE of 20 percent or more. ; ConservativeinDampine dissipation Ratios a structure - Damping or system asisitaresponds measure of energy motion. to vibratory As more energy is dissipated, the overall response l (vibration) of the structure or system is reduced. Thus, the ! higher the damping ratio is, the lower the response becomes. i Duch loweredind to structures response systems. in turn reduces the potential for damage I Very conservative damping values, low as 0.5 percent for piping analysis, 2 percent for OBE, and as ) 5 percent Trojan Plantfordesign. SSE structural analysis have been used in the i j Much higher damping values, ie, 5 percent ' for piping and 10 percent for structures, can realistically be expected. Thus, a design margin that can range from 25 percent to more than 100 percent can result due to the difference { between ing values. the conservative design and realistically expected damp- l Broadened Floor Response Spectra
- For Trojan, response spectra that are used to define the seismic input for a component or structure were broadened to account for potential variations in structural properties (conservatively assumed in the original design sented by stage) that could the spectral affect the location of resonance repre-peaks. The spectra peaks are broadened I by amounts of plus and minus 10 percent or (usually) more. A percentage of conservatism around 15 percent attributable to broadened spectra is usually reported in the literature.
Desian design areCriteria very -conservative. Loading combinations used in nuclear plant In actuality, the combined loads are very unlikely to exist simultaneously. Mcre importantly.
l the design ally criteria no yielding parformance allowed during, isorone where there is essenti- ' 1 - permanent deformation j [ allowed after, a design basis earthquake. This is a very large conservatism important since in actual performance, one of the single l is its ability to yield, characteristic decoupleof a structure from large in resisting earthquakes structurbi resoninces, and absorb energy. Such limited yielding and limitad of permanent deformation does not impair the functionality a rtructure. characteristics are In the designallowed implicitly of conventional structures, these in the design process, but they are not allowed to be accounted in the design for Trojan's structures, systems or components. For Troian, the full effect of the earthquake was designed to be resisted without reliance on this very large reserve energy capacity. design basis, In the unlikely event of an earthquake exceeding the was not relied this reserve capacity is available even though it upon during the original design. There have been a number of studies performed for nuclear plant structures at other sites and (such as probabilistic risk assessments for the Zion Indian Point Nuclear Plants) that evaluates the margin attributable inelastically, to and theitinherent ability of structures to perform 500 percent (a factor of ranges from approximately 200 percent to motions) two to five times the design ground depending upon the type of design and construction. 2. Conservatisms Demonstrated From Tests. The major safety-related electrical equipment installed in the ! Trojan after anNuclear Plantincludes: earthquake which must remain functional during and motor control centers, 480-V meta)-clad switchgear, transformers, teries and battery racks. This equipment control panels, and bat- ; qualified by actual testing on a shake table. has been seismically I The expected largest seismic motion at the location of the equipment in the plant is simulated, and the equipment is tested for its cap-abilitythe after to design perform required basis safety-related earthquake event. functions during and This equipment is ment installed in most typically very similar or identical to equip-nuclear power plants where it was seis-mically plants. qualified 'or earthquake levels specific to those These items have, therefore, different levels of vibratory motion. been tested for many plants, At several other nuclear the same equipment has been than those that result from the Trojan Plant seismic qualified to levels higher criteria. design These successful test. results indicate that electri-cal equipment is very rugged and capable of withstanding earth-quake motions larger than the Trojan SSE. The Electrical Power Research Institute (EPRI) has compiled industry-wide of test and experience-based information in the form generic equipment response spectra, which provide a data bank
- 43 -
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of the seismic capabilities of such equipment. This data demon-strates that seismic demand and equipment capacities.there is substantial margin ava EPRI data, In addition to the the nuclear industry has made an extensive effort to assess These the seismic studies, capabilities of generic types of equipment. to-date, in the seismic capacity of generic equipmenthave also shown large margins those used in the Trojan Plant. types similar to In terms there is of structural dynamic response of cable tray systems, a significant ! expected performance. difference between design criteria and damping ratios The most dramatic difference is betwoon which are usually (energy dissipation) used in design criteria, low, and , demonstrated by cable tray system dthe high values which have been I actual seismic response experience.ynamic tests and evidenced in systems, For Trojan cable tray which included the SSE,thewas design damping ratio used for load combinations, 5 percent. ing percent 10 ratios demonstrated by generic tests are on the order ofCable tray system da to 30 percent. In terms of seismic response, the reduct. ion in structural demand between 5 and 20 percent damping can approach a margin of 100 percent or more. system cable trays at Trojan have significant margin above sented by the seismic design bases. thatTherefore, repre-
- 3. .
Experience-Based Marcin Demonstration The safety-related structures at Trojan have an inherent to resist s those associated with the seismic design basis. earthquake grou Such margins can be deduced by reviewing actual building performance in res-ponse to ground the seismic motions design experienced basis ground during the earthquake with motion. I This comparison demon-strates conventional structures can survive ground motions well ; above their design basis with only minor, acceptable damage. Since the structures at Trojan are designed to criteria signif-icantly more conservative than conventional structures, they can 4 be expected to survive earthquake motions significantly above l their no design loss basis, also with only minor, acceptable damage and of function. The performance of has been observed for centuries, structures subjected to strong ground motions reviews of this performance been conductedbut only recently have detailed performance of and documented. structures and systems designed to seismic The criteria recent less than those used at Trojan have been reported in a and Industrial Facilities andpublication entitled "The Effects of Earthquakes on P i i Plant the Implications for Nuclear Power ' neers Design" published by the American Society of Civil Engi-(ASCE), 1987. quakes in the world, This publication covers 15 major earth-earthquake in 1952. starting with the Kern County, California uakes included conventional power plants,The facilities affected by these earth substations, transmis-sion and distribution facilities, industrial plants, paper mills 44 -
and rofinery facilities. Thsso fccilities include structures and systems those at Trojan. similar in function, but not design and quality, to The conventional power plants and other indus-trial facilities were designed to commercial standards. These standards used are far less for a nuclear plant.conservative than those that would be An example of inherent Plant. reserve strength is the El Centre Steam As stated in the ASCE report. the structural frame and equipment was designed to accommodate specified loads corres-pondingthe During to Imperial a peak ground acceleration range of 0.10 to 0.20 g. Valley earthquake (1979), the El Centre Steam Plant acceleration; experienced ground motion considerably above 0.20 g less than 1 km from the plant the measured ground. acceleration in the other. was 0.35 g in one horizontal direction and 0.49 g I No significant structural damage was observed, even though the peak ground acceleration experienced by the Plant was at least twice that assumed for its design basis. These observations at the El Centre Steam Plont are typical and are borne out by the general observations following the 15 earthquakes reported in this publication, which are: i i Power plants experienced only local damage in earthquakes. Engineered structures and facilities designed and constructed to withstand design levels. seismic loads survive seismic events beyond their l i ( As the another past example of experience-based demonstrations, during few years the Seismic Qualification Utilities Group carried out a comprehensive survey of non-nuclear facilities which have experienced strong motion earthquakes with peak ground acceleration as high as 0.60 g. The facilities surveyed had equipment similar to that used in Trojan. The results of the survey indicated that such equipment properly performed its functions during and after the earthquakes provided their , anchorages were properly designed. In nuclear plant design, ! special attention is focused on provision of reliable equipment anchorages. confidence that These findings from non-nuclear facilities provido equipment and piping used in nuclear plants, with their more rigorous design requirements, would also properlythan severe function following their design basis.earthquakes significantly more A fourth basis for PGE's conclusions regarding conservatisms in Trojan's design comes from knowledge gained since performance of the original Plant design and upgrades. from additional conservatisms introduced by A detailed review and strengthening of the Control.
- 45
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Auxiliary and Fuel Building complox* and review and modifications to equipment ledge and added and piping conservatisms. supports are examples of such increased know-l, Summary For Trojan Plant structures and systems, taking the aggregate of the margin factors described above, and including additional conserva-tisms and knowledge of the seismic response o' Trojan, the struc-tures and systems can be expected to withstand earthquake ground motions two or basis seismic design more without times larger than those associated with the safety-related systems. impairing the safety function of any The Atomic Building Safety matter concludedand Licensing Board which reviewed.the Control of that even though 30 percent to 50 percent the intended margin was lacking, sufficient capability remained to withstand 0.25 SSE. The an event at leas: 50 percent greater than the intended margins were all restored by PGE in 1981 by the strengthening the structures. 46 - _ _ .. _, - . . ~ - - _ _ _ . . _ _ __._.._-___ _ . _ _ _ _ _ _ _ . - - - _ _ _ ,
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4 ODOE Ouestion 11 What that programs a exist at PGE major earthquake to monitor in the development of the possibility Pacific Northwest subduction zone event? could result from a PGE Response For many years PGE has had an active program for monitoring activ-ities relating to the seismicity of the Pacific Northwest and the Cascadia Subduction Zone. PGE will continue monitoring the results of ongoing research related to Cascadia Subduction Zone earthquake issues, as well as other geo-science Plant. Nuclear research that may have a bearing on operation of the Trojan This review is being conducted by PGE's technical staff and management, using consultants as appropriate. Representa-tives of PGE have been active in attending seminars, workshops, and conferences dealing with seismic issues, and they routinely review information received from the Earthquake Engineering Research Insti-tute.
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collected Also, PGE routinely reviews the summary reports of data and northern from the seismic Oregon monitoring (see Figure 11-1).network located in Washington sponsored by the U.S. This network is jointly Geological Survey and U.S. Depar' .ent of Energy, and operated by monitoring stations, including the University of Washington. twenty-two : two in the Portland area, have been operating in Oregon since approximately 1980. ' I Programs in place at PGE are responsive and adequate for the evalu-ation of seismic research and developments in the Pacif'c Northwest that may potentially affect facilities owned and operated by PGE. l l l 48 -
EEISMOGRAPH STATIONS OPERATING DURING THE lst QUARTER 198 125.00 49.50 117.00 59 5n
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' ' ' 200 KM from l University of k'ashington l Geophysics Program I Quarterly Network Report 88-A i January 1 - March 31, 1988 FIGURE 11-1
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ODOE Ousstion 12 What of theisareas PGE's nearaction plan Trojan to obtain data on the geological structure and to assess the effects of earthquakes on Trojan? PGE Response Data on the geologic structure at the Trojan Plant site and in the surrounding plant siting area were collected and evaluated during the original studies. determined from geological and geophysicalProperties of the rock structure which w Trojan FSAR Section 2.5, Attachment B) are summarized evaluationsbelow: (described in The site is underlain by bedrock, which is a part of the Goble series of Upper Eocene age. The bedrock is exposed on the ground surface alongRiver. the Columbia a narrow, elongated ridge bordering the west bank of All Seismic Category I structures and Seismic Category II structures housing Seismic Category I equipment are founded on the rock which forms the ridge. The lesser with bedrock is volcanic amounts in origin and consists principally of tuffs of flow and basalt flows. breccias, tuff breccias, agglomerates, ground surface since they are more resistantBasalt and agglomerate often are ex tuff: however, tuffs and flow breccias are the to erosion than is the predominant rock type in thethe Thus ridge and they generally provide the structural foundations capacity ofstrength of the tuffs generally determine the bearing the foundation rocks. The lowest and highest unconfined compressive strength of 41 samples of tuff which were tested 360 psi (26 ton /ft2) and 2,790 psi (200 tons /ft2). are is 1,225 psi or 88 tons /ft2 The geophysical surveys showed the The average compression wave velocities in the bedrock to be 8,200 to 10,600 fps, which indicates good foundation conditions. Shear-wave velocities of the foundation rock ranged from 4,500 to 5,000 fps. Site response studies pursuant to were performed as part of the development ofthe applicable federal regulations the Trojan seismic design criteria (see the response to Question 7). Trojan structures are founded on very strong rock, as described above, and the seismic design response spectra are broad band to account for ground motion inputs over a wide range of frequencies. characteristics of the Plant site have beenThus, the seismic response evaluated, and were enveloped in the seismic design response spectra. In addition, seismic inal plantmonitoring design, instrumentation, installed as part of the orig-ing both ground motions andprovides the capability of recording and analyz-the actual structural response of Trojan Plant safety-related structures and systems (for seismic motions of significance that may be received at the site). The Trojan Plant ent categories of seismic monitoring system consists of three differ-instrumentation: an array of five triaxial strong-motion magnetic accelerograph tape recording system,sensors with its initiating trigger and an array of six peak acceleration recorders, and a 36 channel multielement seismoscope. 50 -
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The function of the strong-motion accelerograph system is to provid / location of the sensors. triaxial acceleration time-history data ofthe seismic e res reference, To obtain a free-field ground motion the sensors)the is initiating trigger located on rock adjacent for this system (and for one of Fuel Building complex. (1 each on the base slab and wall),Two sensors are locatedto the C in the Containment I Fuel Building and Control Building. and sensors are located in the seismic trigger is 0.01 g. The threshold setting of the 1 to a central main multichannel magnetic tape data recorder locatThe control room. ed in the accele j rechargeable batteries which provide sufficientThe entire system is powere) event of a-c power failure. reserve power in the
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The system seismic is operated trigger. in a standby mode until activated by the Upon activation, ational statusrange adjustable withinof 0.10 second and continuesthe system achieves fully oper-to operate for an falls below the trigger threshold 10 to 60level. seconds after the last seismic shock continue to run each time the The system will restart or mic data on each sensor channel threshold is exceeded. Recorded sels-tape playback system. is available through the magnetic i be recorded on a paperAny stripchannel chart. may be selected individually to i
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In addition to the triaxial accelerographs, i recorders are installed at various locations. peak acceleration l emergency diesel generator, ger, These are on the ' the top of the Control Building, top of component cooling water heat exchan-the intake structure, and top and bottom ofthe Fuel Building, top of the Containment. The spectrummulti-element recorder seismoscope is a 36 channel triaxial response of 12 peak acceleration response recorders in each slab. the Itthree axes)(which is rigidly attached to the Containm ent base power source. is a mechanical spectra for the eventFollowing arecorder seismic and does not rely on an external event, instrument also has presetcan be plotted from the recorded data.the This ground m j SSE response accelerations and frequencies whiche would annunci-bindicators ! vides ated in the control room if such indication to the control roomevents were operating <>to occur . This pro- ; initiate the appropriate planned actions in response lC which to th would e event. PGE recently consulted their collective judgmentgeologists and seismologists to det ermine means to obtain, supplemental geophysical data thatconcerning the need for, a characterization siting studies . of the Trojan site response would enhance monitoring prog) ram beyondOpinions receivedawere that (based continuous site on the original the level of installed of the very would lowbelevel unlikelyof to provide significantinstrumentation currently years of monitoring would likely regional seismic activity. results because be required Thus, tens of data for evaluation, to collect sufficient site response wouldand a change be unlikely toinresult. the characterization of the 51
. "es In summary, geologie and seismologic evaluations of the Trojan Plant site and siting surrounding area were performed during the original plant studies. l Instrumentation is currently in place to provide additional data on response characteristics of the site and safety-related intensitystructures were to beand systems if a seismic event of significant experienced.
are not l considered to be of appreciable value. Additional monitoringPGE Therefore, programs does not feel there monitoring is value added by engaging in supplemental seismic or geophysical investigation programs at this time. j Relevant data from existing seismic monitoring networks (Figure 11- 1) and ongoing seismic site response studies Northwest will continue to be evaluated, (King et al) in the Pacific i As stated in the responses to Questions 6 and 11, PGE is actively l monitoring the progress and results of ongoing geoscience research , programs focused on the evaluation of seismic hazards in the Pacific Northwest. l I l 1 l l l 1 1 i 52 -
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ODOE Ouestion 13 7'" s What will PGE do to assure independent review and resolution of future developments which might impact Trojan earthquake resistance? PGE Response The United States responsibility of reviewing Regulatoryissues Commission (NRC) is charged with the the Trojan Nuclear Plant. relating to the safe operation of ications, if deemed necessary, andTheytohave the authority to require modif-restrict Plant operation if the public health and safety is at risk. In 1984, the NRC reviewed the matter of subduction of the Juan de Fuca plate in the Pacific Northwest tion interface.and the potential for a great earthquake along the subduc-stated In their safety evaluation for Trojan, the NRC that Foundation, and "the United States Geological Survey, National Science seismologic whether or notand a great geologic investigations to assess the possibility of earthquake is likely or even credible. The (NRC) staff is well informed as to the progress of research and will continue to aaintain this awareness.this .ongoing conclude that there is no reason to alter the seismic design . . We basis , for Trojan . . ating license reviews.". approved during the construction permit and oper-Review has been of a research continuing findings effortconcerning the Cascadia Subduction Zone (USGS), which serves as a consultantby the to United States Geological Survey the NRC. Investigators affiliated with the USGS (Atwater, Heaton, Spence, Weaver, et al) are responsible for the m&jority of Cascadia Subduction Zone research currently in progress. The NRC contributes to the funding toward this research. Results of USGS research are published as :
"Open File Reports" which, if relevant to their activities, are l reviewed by the NRC's Structural and Geosciences Branch of the Office of Nuclear Reactor Regulation. i i
4 As requested by the March 1984 letter, PGE Will continue to keep the NRC informed of significant findings relative to the Trojan site, in particular with respect to the Cascadia subduction Zone. In addition, PGE will obtain the services of a consulting firm to review research (with respect toresults safe operation concerning of the Cascadia subduction Zone consultant will advise PGE of significantthe Trojan Nuclear Plant). and The research developments new disveveries of evidence concerning Cascadia Subduction 7.ono charac*stictics. 1
- 53 -
d k %O s ! ATTACRMENT A GLOSSARY OF TERMS Accretionary Wedce refers to the low-strength sediment deposit subduction process, composed of material sheared off from the oceanic peak during the t of deformations where the stiffness of the lateralElastic load-carrying Response! system remains essentially linear. Inelastic Response refers to the response of a structure in the range of deformations load-carrying s where the stiffness of the structure's members occurs,ystem becomes and energy non-linear. is thereby Yielding of structural absorbed. Intensity structures.is a measure of an earthquake's offect on people and well as the duration and depth, Intensity depends on the magnitude of an as earthqua geology, frequency of ground motion, distance fromand the type the quality epicenter, of construction, and on accurate observations of damage. usually reported using the Modified Mercali (MM) scale. Intensity is , earthquake with MM intensity of II will be noticed by onlyAna few people, an earthquake causes destruction of structures,and landslides and may cause If susp observable cracks in the ground, it would be classified as Intensity X. Macnitude source, refers to the amount of energy released at the earthquake or the "strength" of an earthquake based on seismographic observations. the Richter magnitude scale. Earthquake magnitudes are frequently reported using Generally, an earthquake of Richter magnitude 3 will cause no damage and be noticed by only a few people, while a Richter magnitude of 7 may cause considerable damage to buildings energy and other structures located close to the zone of release. Other magnitude scales are:
. Body Wave Macnitude (M B ) is derived from the characteristics of body waves recorded from an earthquake. l i
Seismic Moment Macnitude (Mw) is a measure of earthquake energy in terms of rigidity of the rock formation through which the slipthe rupture occurs, the area of the rupture surface, and displacement. The largest this century was of Mw = 9.5 in southern Chiledocumented earthquake of 1960). (May 22, Plate - segments of the earth's outer shell (lithosphere) 50-150 km 1 thickanother. which move ( one Most horizontally across the earth's surface relative to ! mountain chains, trenchesof the earth's major tectonic patterns, including { j f to plate tectonic processe,s and interaccions atand ocean basins, may be directly linke plate boundaries, _.,w ..& g-,mm,, ,,., - y,-,,,,,,.oq,* -}}