New seismic design spectra for nuclear power plants

Nuclear Engineering and Design 203 (2001) 249–257

New seismic design spectra for nuclear power plants

Robin K. McGuire a,*, Walter J. Silva b, Roger Kenneally c

a Risk Engineering, Inc., Boulder, CO 80305, USAb Pacific Engineering and Analysis, El Cerrito, CA 94530, USA

c US Nuclear Regulatory Commission, Rock6ille, MD 20852, USA

Received 2 February 2000; accepted 1 July 2000

Abstract

Under a US Nuclear Regulatory Commission-sponsored project recommendations for seismic design groundmotions for nuclear facilities are being developed. These recommendations will take several forms. Spectral shapeswill be developed empirically and augmented as necessary by analytical models. Alternative methods of scaling therecommended shapes will be included that use a procedure that integrates over fragility curves to obtain approxi-mately consistent risk at all sites. Site-specific soil effects will be taken into account by recommending site-specificanalyses that can be used to modify rock hazard curves at a site. Also, a database of strong motion records will bearchived for the project, along with recommendations on the development of artificial motions. This will aid thegeneration of motions for detailed soil- and structural-response studies. © 2001 Elsevier Science B.V. All rightsreserved.

www.elsevier.com/locate/nucengdes

1. Introduction

In 1996 the Nuclear Regulatory Commission(NRC) amended its regulations to update thecriteria used in decisions regarding nuclear powerplant siting, including geologic, seismic, andearthquake engineering considerations for futureapplications; USNRC (1996). As a follow-on tothe revised siting regulations, it is necessary todevelop state-of-the-art recommendations on thedesign ground motions commensurate with seis-mological knowledge and engineering needs. The

current design spectra in Regulatory Guide 1.60(USNRC, 1993) were based on limited, princi-pally western United States earthquake strong-motion records available at that time. Since 1996the NRC has funded a project to develop up-to-date seismic design spectra for the US. The workcombines empirical and analytical approaches,supplementing data where they are sparse usingtheoretical methods to develop the recommendedspectra for a range of earthquake magnitudes anddistances. Soil conditions necessarily involve site-specific parameters, and we demonstrate and rec-ommend procedures to account for local soileffects on earthquake motions. A Review Panelconsisting of Carl Stepp (Chair), David Boore,Allin Cornell, I.M. Idriss, and Robert P. Kennedy

* Corresponding author. Present address: Risk EngineeringInc., Suite A, 4155 Darley Avenue, Boulder, CO 80303, USA.Tel.: +1-303-4993000; fax: +1-303-4994850.

E-mail address: [email protected] (R.K. McGuire).

0029-5493/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0 029 -5493 (00 )00345 -9

R.K. McGuire et al. / Nuclear Engineering and Design 203 (2001) 249–257250

review the work and offer guidance on proce-dures. The prime contractor is Risk Engineering,Inc., with Pacific Engineering and Analysis devel-oping databases, spectral shapes, site responseprocedures, and spectral matching criteria. Thispaper reviews the scope of the work, indicates thedirection that recommendations are taking, andpresents preliminary results. Final results will beavailable in the project report.

2. Spectral shapes

Databases for the western US are available inthe form of strong motion accelerograms for mo-ment magnitudes M in the range 5.0–7.6 andsource-to-site distances R of 1–200 km. Rockconditions in California are generally soft, withnear-surface shear wave velocities of 200–450 m/s(700–1500 ft/s).

The databases of strong motion records andempirical attenuation relations form the basis forrecommended spectral shapes on rock for definedM and R bins, augmented as necessary by analyt-ically derived shapes. In the application of thesespectral shapes for design, the M–R combinationis defined by the dominant earthquake as deter-

mined from a probabilistic seismic hazard analysis(PSHA). Examples of procedures for defining thedominant earthquake are described in McGuire(1995) and USNRC (1997).

A summary of rock and soil records from thewestern US is shown in Table 1, in terms of Mand R bins. Also shown are preliminary summarystatistics for mean peak ground acceleration(PGA), mean peak ground velocity (PGV), andmean peak ground displacement (PGD). Thissummary indicates the usual trends in strong mo-tion data, i.e. that data are abundant for moder-ate magnitudes at moderate source-to-sitedistances, but are sparse for large magnitudes andshort distances and small magnitudes at long dis-tances. The former category is more troublesomefrom a design perspective and requires modelingfor confirmation.

Central and eastern US (CEUS) strong motionrecords are sparse. Thus it is necessary to aug-ment the CEUS empirical motions with analyti-cally derived spectral shapes. This analysis uses apoint- and finite-source representation of theearthquake rupture, attenuates both body andsurface waves, accounts for near-surface attenua-tion of high frequencies, and assumes that groundmotion is a band-limited, white noise process.

Table 1Characteristics of WUS records in M–R b ins (preliminary)

MSite R (km) Mean PGA (g)No. of spectra Mean PGV (cm/s) Mean PGD (cm)

Rock 0.808.140.18300–105–5.96–6.9 0–10 32 0.44 32.7 6.227+ 0–10 6 0.93 81.7 47.4

18.60.2624 3.110–105–5.9Soil6–6.9 46.9 14.80–10 77 0.387+ 44.5 21.30–10 4 0.40

0.545.080.11180Rock 10–505–5.96–6.9 8.81 1.9610–50 238 0.137+ 10–50 6 0.17 8.80 2.50

0.876.630.11378Soil 10–505–5.96–6.9 10–50 542 0.14 10.8 2.25

0.16 22.4 10.510–50 567+5–5.9 50–100 32Rock 0.05 2.22 0.216–6.9 50–100 102 0.06 3.87 0.797+ 2.645.160.061050–100

0.383.110.0642Soil 50–1005–5.96–6.9 50–100 158 0.07 6.23 1.267+ 50–100 14 0.10 11.2 5.42

R.K. McGuire et al. / Nuclear Engineering and Design 203 (2001) 249–257 251

Fig. 1. Normalized rock spectral shapes for WUS and CEUS 1-corner and 2-corner models, M=6.5 and R=25 km.

Calibration of the model with available recordsconfirms the underlying assumptions and providesestimates of the model parameters. One outstand-ing issue, however, is whether the seismic energyat the source has a ‘‘single-corner’’ or ‘‘double-corner’’ spectrum; this is the focus of independentresearch, and the current project will include eachmodel as an alternative. Rock conditions in theCEUS are generally hard, with near-surface shearwave velocities generally exceeding 3000 m/s(10 000 ft/s).

Fig. 1 indicates the difference in spectral shapesbetween the single- and double-corner models, forboth the WUS and CEUS. The shapes are pre-sented as ratios of spectral acceleration dividedby PGA. CEUS shapes typically have morehigh-frequency content but lower SA at interme-diate periods, when normalized by PGA. The

double-corner model has the largest influence forCEUS spectral shapes at periods longer than 0.5s.

3. Choice of spectral level

In addition to the spectral shape, the overalllevel of the spectrum must be specified. Thischoice may be made from a PSHA by defining atarget annual frequency of exceedence for thespectrum. Alternatively the level could be definedusing an acceptable annual frequency of failure Pf

at the component level and convoluting the seis-mic hazard results with component fragilitycurves to relate component performance to seis-mic hazard. The failure frequency Pf can be repre-sented as:


PF=&�

0

H(a)dPF/a

dada (1)

where H(a) is the hazard curve and PF/a is theprobability of failure (the ‘‘fragility’’) givenground motion amplitude ‘‘a ’’, which capturesboth response and capacity uncertainties.

With some realistic assumptions on the shapeof the hazard curve and the fragility curve, it ispossible to derive a simple expression for PF. Firstwe assume that the hazard curve H(a) is linear onlog-log scale, i.e.

H(a)=ka−KH (2)

where a is spectral acceleration level, k is a con-stant, and KH is the slope of the hazard curve inlog–log space. Actual hazard curves tend to getsteeper at higher amplitudes, but over the impor-tant range of amplitudes for PF calculations theycan be approximated as linear on log–log scale.

Second we assume that component fragilitiesare lognormally distributed. This means that

PF/a=& a

0

1

y2pbexp

!−

(ln y− ln y)2

2b2

"dy (3)

where ln y= ln CAP50, the median component ca-pacity, and b is the logarithmic standard deviationof capacity.

Substituting Eqs. (Eq. (2)) and (Eq. (3)) into(Eq. (1)) gives

PF=&�

0

ka−Kn1

a2pbexp

!−

(ln y− ln y)2

2b2

"da.

(4)

Transforming the integration variable a to vari-able x = ln a gives

PF=k

2pb

&�−�

exp{−KHx}

× exp!

−(x− ln y)2

2b2

"dx. (5)

The integrand above is in the form

exp{cx}Z(x) (6)

where c is a constant and Z(x) is the normaldensity function. The definite integral (Eq. (5))can be solved by expansion or by published meth-ods of integrating functions of normal probabilitydistribution (e.g. Owen, 1980), yielding

PF=kCAP−KH

50exp

!12

(KHb)2". (7)

This form, designated the ‘‘risk equation,’’ wasfirst derived by G. Toro and published in Sewellet al. (1991, 1996). Expressing the hazard Hs at aground motion level a* corresponding to a safe-shutdown ground motion (SSGM), using (Eq. (2))gives:

Hs=k(a�)−KH. (8)

Solving for k and substituting into Eq. (7) gives:

PF=Hs(a�)KHCAP−KH

50 exp!1

2(KHb)2". (9)

We can now derive a probability ratio Rp as theprobability Hs that a* will be exceeded, dividedby the probability of failure PF:

Rp=Hs/PF. (10)

This ratio is usually much greater than unitybecause PF is much less than the hazard at a*. Rp

can be expressed as:

Rp=�CAP50

a��KH

exp!

−12

(KHb)2". (11)

Instead of using the median capacity CAP50 todesignate capacity, we can use the ‘‘high confi-dence of low probability of failure’’ value, orHCLPF, where for a lognormal distribution thetwo are related by

HCLPF=CAP50exp {−xpb} (12)

where xp is the number of standard deviates corre-sponding to the frequency of failure at theHCLPF, which is 2.326 for 1% frequency of fail-ure. Also, we can express the required HCLPF interms of a* times a factor of safety FR :

HCLPF=SSGM·FR. (13)

Solving these last two equations for CAP50 andSSGM, and substituting into Eq. (Eq. (11)) gives:

Rp=FRKH exp

!xpKHb−

12

(KHb)2". (14)

This gives a simple means to calculate PF, giventhat the hazard associated with the SSGM isknown. The probability ratio Rp depends on thefactor of safety FR, the hazard curve slope KH,


and b of the fragility function; for the HCLPFdefined at the 1% frequency of failure point,xp=2.326 as explained above.

Eq. (14) also gives an easy way to compute theeffect of hazard curve slope and fragility b on PF

for a specified hazard corresponding to a selectedUHS. Stated another way, if we pick a UHS ateach site with the same annual probability ofexceedence, and define the HCLPF in terms ofEq. (13), Eq. (14) allows us to examine the riskconsistency across sites for different hazard curveslopes KH and fragility uncertainties b. The use ofEq. (14) in this way is demonstrated below.

A couple of points about the distributions ofH(a*) and PF are important. H(a*) is uncertainbecause of lack of knowledge in the earth sciencesabout earthquake sources, ground motions, etc.This uncertainty has been quantified by EPRI andLLNL at CEUS plant sites and by utilities atseveral WUS plant sites. If we use the mean of thisdistribution we will achieve a mean PF for any setof design rules. The mean has the advantage thatwe can compute (and control) the mean PF formultiple plants. That is, we have n plants and atotal acceptable probability of component failureat these plants, we can achieve that by specifyinga mean PF at each plant. The disadvantage is thatthe mean is sensitive to low probability, highconsequence assumptions in the seismic hazardanalysis and is not as stable (from study to study)as the median.

If we use the median H(a*) we will achieve anapproximate median PF. The median has the ad-vantage that it is more stable than the mean, buta target mean or median PF over n plants cannotreadily be translated to a required median PF ateach plant. So use of the median H(a*) leads toill-constrained limits on PF over multiple plants.For this reason the use of the mean H(a) curve isrecommended.

A final point is that Rp can be controlled by‘‘deterministic acceptance criteria’’ associated withdesign codes and guides, and by a ‘‘scale factor’’that moves the capacity up or down as a functionof the hazard curve slope KH, the desired PF, orthe desired Rp for a given H(a). This scale factoris conveniently thought of as a scaling of the UHSto specify an SSGM spectrum. The total factor of

safety FR, is then a times SF, where a is theconservatism achieved by design procedures (e.g.1.67 on the HCLPF) and SF is the scale factor.The SSGM is then the UHS scaled by SF. It isappropriate to define SF to scale the UHS toaccount for the site-specific (and natural period-specific) slope of the hazard curve. R.P. Kennedy(personal communication, 1997) has suggested thefollowing scale factor:

SF=max{0.7,0.35AR1.2} (15)

where AR= [logKH]−1. Thus AR increases as thehazard curves become more shallow, so SF in-creases, i.e. the design values become higher forshallow hazard curves. With this definition, theSSGM can be thought of as:

SSGM=UHS×SF (16)

i.e. the SSGM is the UHS ‘‘corrected’’ for theslope of the hazard curve. For AR=2.40 (whichcorresponds to slope KH=2.63), SF=1, i.e. theSSGM equals the UHS.

Another way to look at the design is throughthe total factor of safety FR (see Eq. (13)). If theamount of conservatism in design codes andguides (sometimes referred to as the ‘‘determinis-tic acceptance criterion’’) is 1.67, then the totalfactor of safety FR is:

FR=1.67SF. (17)

The advantage of using a slope-dependent scalefactor SF as defined in Eq. (15) is demonstrated inthe next section.

4. Results for example sites

To test several methods for risk-consistent spec-tra, we examined eleven sites and three groundmotion measures at each site (shown in Table 2).

For the first 27 sets of results we used theLLNL hazard curves calculated for the USNRC(Sobel, 1994). For the ‘‘California’’ site, we calcu-lated hazard at a site located near Santa Maria,California (120.5°W, 35.0°N), which has high fre-quencies dominated by nearby faults and longperiods dominated by the more distant San An-dreas fault. (A repeat of the 1857 earthquake


Table 2Sites and ground motion measures used for testing procedures

Measure No. SiteSite MeasureNo.

Arkansas plant1 PGA 17 Shearon Harris SV 1 HzSV 1Hz 18Arkansas plant Shearon Harris2 SV 10 Hz

Arkansas plant3 SV 10 Hz 19 Susquehanna PGAPGA 20 Susquehanna4 SV 1 HzBrowns FerrySV 1 Hz 21Browns Ferry Susquehanna5 SV 10 HzSV 10 Hz 226 VogtleBrowns Ferry PGAPGA 23Davis Besse Vogtle7 SV 1 Hz

Davis Besse8 SV 1 Hz 24 Vogtle SV 10 HzSV 10 Hz 25Davis Besse Zion9 PGA

Maine Yankee10 PGA 26 Zion SV 1 Hz11 Maine Yankee SV 1 Hz 27 Zion SV 10 Hz

SV 10 Hz 28Maine Yankee California12 PGASeabrook13 PGA 29 California SV 1 Hz

SV 1 Hz 3014 CaliforniaSeabrook SV 10 HzSV 10 Hz 31Seabrook Washington15 PGA

Shearon Harris16 PGA 32 Washington SV 1 Hz33 Washington SV 10 Hz

dominates the long period hazard at this site.) Forground motion estimation the attenuation equa-tion of Abrahamson and Silva (1995) wasselected.

The last site examined was in Washington, lo-cated at 121°W and 46°N. This is in south-centralWashington and also has high frequencies domi-nated by local earthquakes and low frequenciesdominated by a large earthquake. In this case alarge subduction zone earthquake controls thelong-period hazard. We model this event using theassumptions of the US Geological Survey for thenational seismic hazard maps. That is, an earth-quake of M#9 occurs in the subduction zonewith rate 1/500 per year (credibility 1/3), or earth-quakes of M#8 to 9 occur with rate 1/110 peryear (credibility 2/3). For both the California andWashington sites we model local earthquakes withthe US Geological Survey gridded seismicity, aswell as local faults for the California site.

Calculations were made of the probability ratioRp for the 33 site-parameter combinations listedabove. This is an appropriate parameter to usebecause, if we start with the same hazard levelH(a*) at all sites and all natural periods, andachieve a consistent Rp with our procedure, wewill achieve a consistent probability of failure PF.

Fig. 2. Ratio Rp=Hs/PF for 11 sites and three ground motionmeasures. Top: without slope-dependent scale factor. Bottom:with slope-dependent scale factor.


Fig. 2 shows Rp values for the 33 site-parame-ter combinations, calculated using the mean haz-ard curve for each site. For this plot Rp wascalculated from Eq. (14) using the derivationfrom the risk equation. The top plot in Fig. 2shows Rp when the SSGM is taken to be equalto the UHS at the natural period of the parame-ter; the bottom plot shows Rp when theSSGM=UHS×SF, as in Eq. (16). The scalefactor SF really helps the consistency across sitesand across parameters; results without SF varyfrom about 6 to 48 (a factor of 8), but with SFthey vary from about 18 to 45 (a factor of 2.5).This remaining factor of 2.5 is the effect of b. Itwould be inappropriate to define the SSGM onthe basis of component response, since thatwould require multiple design spectra for a sin-gle facility.

5. Procedures for soil sites

Results presented above assume that facilitydesign is for a rock site. If soil conditions existat a site, modifications will be necessary toderive the appropriate design level. Several op-tions are being considered; these follow ideas ex-pressed by Cornell (1996) and Cornell andBazzurro (personal communication, 1997).

5.1. Option 1: direct approach

This approach models soil response directly asa function of M and R (through a site specificattenuation relation), to calculate soil hazardcurves HS(a) as

Hs(a)=&

Ps[A\a �M,R ]fM,R dmdr. (18)

This has the advantage of directness and consis-tency with the derivation of rock hazard curves.The disadvantages, however, make this approachunworkable for most sites. First, the seismichazard analysis cannot be conducted prior toobtaining detailed soil-specific information (shearwave velocities, modulus and damping curves)for each location where facilities are to be de-

signed at the site. This procedure couples thedesign criteria process with the collection of site-specific information, and preliminary designsbased on approximate amplification factorswould be awkward. Second, if preliminary siteinformation were obtained and later refined, theentire seismic hazard analysis must be repeatedand documented to incorporate the new infor-mation. Finally, this option has not been used inthe past, at least for site-specific soil properties,and implementing it would require addressing is-sues of consistency and accuracy in representingsoil response with a generic attenuation equationform. These disadvantages are not insurmount-able, but they imply that a different approachwould be more efficient.

5.2. Option 2: simple scale factor

In this approach, the hazard curve H(a) forrock is represented as spectral acceleration aR(h)and is simply scaled up (or down) at each fre-quency by a soil-dependent shape:

as(h)=ar(h)S(aR, f) (19)

where S(aR, f) is a mean scale factor that de-pends on aR and frequency f, and is developedthrough site specific response analyses. S(aR, f)can accommodate variabilities in site amplifica-tion caused by uncertainties in soil properties,for example. This approach has the advantageof simplicity, but the disadvantage of inaccuracy.S(aR, f ) is a function not only of aR and f butalso of M (but probably not of distance R),because soil amplification depends on character-istics of the ground motion such as duration andfrequency content (not just at frequency f ). Thedominant contribution by M changes as a func-tion of aR level, source contribution to hazard,and other factors.

5.3. Option 3: more detailed scale factor

The third option is to develop a more detailedscale factor that accounts for additional featuresof ground shaking, but allows the simplicity ofusing hazard curves developed on rock:


P [As\as]

=&&

faR(aR,M,…)P

�S\

as

ar

�ar, f,M,…n

dmdar…

(20)

where faR(aR,M,…) is the rock hazard curve in

density form, and the integral is over all factorsM… that are used to develop the scale factor. Thehazard results faR

(aR,M,…) can be obtained froma standard seismic hazard analysis where resultshave been deaggregated by the contributions bymagnitude, distance, etc.

The advantage of Eq. (20) is that additionaluncertainties, most importantly on soil character-istics and their effects on amplification, can beincorporated into

P�

S\as

ar

�ar, f,M,…n

.

The project is currently examining alternativedefinitions of scale factors under this option andtheir accuracies by comparing with Option 1 (thedirect approach) for several sites and sets of soilproperties. Recommendations will be made basedon the most workable option that can be imple-mented to give accurate estimates of soil hazard.

6. Conclusions

To calibrate design spectral shapes, strong mo-tion records are available for the WUS over arange of magnitudes and distances. There are stillcomparatively few records for large magnitudesand short source-to-site distances, however. Forthe CEUS, all magnitudes and distances lacksufficient empirical strong motion data, and theserecords will be generated with ground motionmodels calibrated to replicate available groundmotion characteristics at smaller magnitudes andlonger distances.

An outstanding issue is the use of the single-corner or double-corner model of the seismicsource spectrum. Results will be presented in thisproject for both models, anticipating that severalyears will be required to achieve resolution of thisissue.

A method has been developed and tested todetermine the amplitude of ground motion fordesign, as well as the shape. Results using testsites in the CEUS and on the west coast, andusing typical component fragility characteristics,indicate that the annual frequency of componentfailure is about 15–45 times less than the annualfrequency of exceedence of the design spectrum,using realistic design procedures. This means that,for example if the median frequency of exceedenceof a site’s design spectrum is 1×10−4, the mediancomponent frequency of failure is about 3×10−6. The ultimate choice of a recommendedspectral level must be made with a combination ofanalysis to determine acceptable failure frequen-cies, calibration to accepted existing design proce-dures, and judgment.

Recommending spectral shapes for soil sitesrequires additional procedures. One straightfor-ward method is to conduct the PSHA with site-specific soil attenuation equations, to obtainseismic hazard curves and uniform hazard spectra(UHS) for the soil surface. However, as recom-mended in USNRC (1997), it is often more practi-cal to conduct the PSHA for rock outcropconditions and later translate these to soil surfacemotions, because various facilities may be locatedon different soils, or detailed site-specific datamay not be available early in the project. In thiscase a site’s rock UHS at a target annual fre-quency of exceedence can be translated to a soilUHS at the same or similar frequency of excee-dence, accounting for uncertainties in the soilproperties. Procedures to accomplish this will bedemonstrated in the project.

In addition to recommended spectral shapes,the project will archive a database of strong mo-tion records for the recommended M and R bins,for both rock and soil conditions. These will beempirical records for bins where data are abun-dant, augmented by artificial motions derived tohave the correct frequency content for bins wheredata are sparse or non-existent.

A final set of recommendations concerns crite-ria to match artificial motions to recommendedspectral shapes and levels. Such motions might beused for input to detailed dynamic analyses ofbuilding response, for example. The recommended


procedures for developing artificial motions con-centrate on matching response spectral amplitudesat multiple frequencies and dampings, and putless emphasis on matching power spectral densityfunctions.

This NRC-sponsored project will offer a num-ber of recommendations on choosing spectralshapes, selecting design levels, and generatingtime histories of motion for the design of nuclearfacilities. The objective is to achieve consistentdesign levels across the country for a range ofseismic environments and site conditions. Proce-dures developed in this project to define groundmotion for a risk-consistent, performance-baseddesign are an integral part of the recommenda-tions. A second objective is to make the proce-dures easy-to-understand and technically justified,so that they will be readily accepted. There is aneed to strike a balance between the engineeringconservatism required to achieve the safe design,seismological knowledge, and preservation of im-portant earthquake ground motion characteris-tics, such that realistic responses are considered.The results from this research will also providetools for the seismic design of non-reactor facili-ties and will influence the design of non-nuclearfacilities.

DisclaimerThis paper was prepared in part by an em-

ployee of the United States Nuclear RegulatoryCommission. It presents information that doesnot currently represent an agreed-upon staff posi-

tion. NRC has neither approved nor disapprovedits technical content.

References

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Cornell, C.A. (1996). ‘‘Calculating building seismic perfor-mance reliability: a basis for multi-level design norms,’’Paper 2122, 11th World Conference on Earthquake Engi-neering., Acapulco.

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Sewell, R.T., G.R. Toro, R.K. McGuire (1991). ‘‘Impact ofground motion characterization on conser6atism and 6ari-ability in seismic risk estimates.’’ Risk Eng., Inc., Report toUSNRC, May.

Sewell, R.T., G.R. Toro, R.K. McGuire (1996). ‘‘Impact ofground motion characterization on conser6atism and 6ari-ability in seismic risk estimates.’’ US Nuc. Reg. Comm.,Report NUREG/CR- 6467, July.

Sobel, P. (1994). Re6ised Li6ermore seismic hazard estimatesfor sixty-nine nuclear power plant sites east of the RockyMountains. US Nuclear Regulatory Commission, ReportNUREG-1488, April.

USNRC (1993). ‘‘Design response spectra for seismic design ofnuclear power plants.’’ Regulatory Guide 1.60, Revision 1,December.

USNRC (1996). ‘‘Reactor site criteria including seismic andearthquake engineering criteria for nuclear power plants.’’Federal Register, Vol. 61, p. 65157, 11 December.

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New seismic design spectra for nuclear power plants