ML19343C452

Comments on Proposed Characteristics of Strong Earthquake Ground Motion for Seismic Design Criteria.Discusses Aspects of Selecting Seismic Design Criteria for Sites & Adequacy of Proposed Design Levels.Certificate of Svc Encl

ML19343C452
Person / Time
Site:	Seabrook
Issue date:	03/20/1981
From:	Trifunac M SOUTHERN CALIFORNIA, UNIV. OF, LOS ANGELES, CA
To:	NRC ATOMIC SAFETY & LICENSING APPEAL PANEL (ASLAP)
Shared Package
ML19343C448	List: ML19343C447 ML19343C452
References
NUDOCS 8103240236
Download: ML19343C452 (18)

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, . . . --

.O UNITED STATES OF AMERICA NUCLEAR REGULATORY COMMISSION

'BEFORE THE-ATOMIC SAFETY AND LICENSING APPEAL BOARD

-In the Matter of )

-PUBLIC' SERVICE COMPANY ) Docket No. 50-443 0F NEW HAMPSHIRE, ET.-AL ) 50-449 (Seabrook Station, Units 1'& 2 -)

' COMMENTS by Prof. Mihailo D. Trifunac i

March 20,1981

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INTRODUCTION The proposed characteristics of strong earthquake ground motion for seismic design of the Seabrook nuclear i;wer plant may be summarized as follows:

1. Safe Shut Down Earthquake (SSE) will not exceed MI=VIII at the site.

2. Maximum. peak acceleration which will result from MI=VIII at the site is 0.256 (where g=981 cm/sec2 ),

3. Design Response Spectrum shapes are consistent with the Reg.

Guide 1.60 spectra recomended by the U.S.N.R.C.. At short periods _(high frequencies, f230 Hz) these spectra approach

^

the peak absolute ground acceleration equal to 0.25g.

4. _From geological view point the site can be classified as a rock site.

. In its recent ' order, CLI-80-30,12 NRC 295, 298 (1980), the Atomic Safety and Licensing Appeal Board states:

"The Appeal ' Board shall also reopen the record to take more evidence

^

, on the consistency of Appendix A and-staff's methodology for cor-relating vibratory motion with the SSE. In particular, the parties should provide a discussion of th relation between the mean of the maximum ground acceleration and the maximum effective ground acce-

'leration."

_In response to this order, in the following I will discuss some as-pects of selecting.the seismic design criteria at this site and will try 4

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2 to test the adequacy of tne above proposed desijn levels. As will be seen from the subsequent pages I will analyze the end product only by using the methods thich to c:e, at present, appear to be the most suit-able. In this respect I will employ my own interpretation of the r..ini-mum requirements contained in the Appendix A.

. 3 SELECTED COMMENTS ON A DETERMINISTIC INTERPRETATION OF APPENDIX A AND THE ASSOCIATED DIFFICULTIES.

By some, Appendix A (together with the associated Regulatory Guides) is interpreted to lead to detailed step by step instructions on how the future strong earthquake is to be estimated for the design of nuclear power plants. With this viewpoint, it is possible to compartmentalize all required operations and to develop " standard" procedures, which, if carefully documented, may precisely outline the needed tasks and their ultimate output, the design response spectra. From an administrative, licensing and design-viewpoints this approach at least formally, appears inviting, since, through precisely documented procedurer , it leads u. rect'y to the results. Such precisely outlined and executed procedures, however, may not always lead to an accurate description of the physical nature of the problem; if the uncertainties associated with various operations are not properly reflected in the distribution of the estimated output values.

-For Eastern U.S., for example, a typical analysis essentially begins by specifying "the largest credible" earthquake shaking at a site (usually measured in terms of the Modified Mercalli Intensity, MMI). The diffi-culties associated with this first step sometimes result from a naive expectation that the past seismicity, geology and tectonics in the area can-somehow be employed to compute the largest credible site intensity.

I believe that many licensing difficulties could be avoided by recognizing that it is' sufficient to evaluate the maximum intensities by means of a distribution function, rather than through a selection of a precise yet not necessarily an accurate point estimate.

The next task'then consists of relating the assigned site intens 3

4 of shaking to an amplitude of peak ground acceleration. Here one finds a considerable spread of the recorded peak acceleration for the same site intensity,shown in Figure 1. This figure illustrates how the distrioutions of peaks P(as ag ) might look like. The dashed straight lines, approximating this unknown distribution, have been plotted here by using the Table VI in Trifunac (1976). It is seen, for example, that the 80% confidence interval for. recorded peak accelerations for the site intensity MPI=VIII is from

'0.15g to '0.70g. The point to be noted here is that when a peak accelera-tion is presented it must be viewed with this wide distribution in mind.

In Figure 1 and elsewhere in this discussion " peak acceleration" will mean the largest absolute amplitude of acceleration function versus time recorded during the complete strong earthquake motion. For linear response analysis using response superpositicq technique this is the only correct use'of peak acceleration, since the computed response spectra asymptotically approach this peak value as the period of the single degree of freedom oscillator approaches zero. I believe that the term " effective peak acce-1eration" should not be considered at all, since, so far, no one has pre-cisely defined what is meant by this expression. From the past experience, I .found that it is usually smaller than the recorded acceleration, that its interpretation varies from one experts to the next, and that it some-how reconciles the past analyses with recent or new recorded data. From several cases of its usage that I have seen it appears that it avoids the

. physical basis of the problem and allows unwarranted freedom for expert judgement.

The third and often the last step in specifying the design earth-s-

quake motians at a site is to use-the appropriate peak acceleration to

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5 scale the amplitudes of the standard shape of the response spectrum given by the Regulatory Guide 1.60. Unfortunately the shape of this response spectrum does not represent the correct average plus one standard deviation or average spectrum envelope, since, prior to analysis for spectral shapes, all spectral a-nlitudes have been normalized by i ' the ' corresponding peak acceleration. 'his normalization leads to the zero standard deviation of spectral amplitudes at the short period end.

When multiplied by the average peak ground acceleration the resulting spectrum amplitudes are close to the average spectral amplitudes at the short period end, and as T increases approach the average plus one standard deviation spectrum from below. If multiplied by the average plus one standard deviation the resulting spectra overestimate the actual average plus one standard deviation of spectral amplitudes at all T. In spite of these difficulties, the above procedures can yield an adequate

_ seismic design _ basis in_ the range of intermediate and small response amplitudes where the detailed and more refined analysis is not essential.

ItL is further noted _that in the critical re-evaluation of the existing design spectra, the detailed review and justification of the tasks involved in the above step by step methodology cannot be expected to resolve all difficulties, since this methodology. itself represents an approximation.

EA PROBABILISTIC INTERPRETATION OF APPENDIX A To formulate an_ independent basis for evaluation of the seismic design response spectra at Seabrook site (Reg. _ Guide 1.60 spectra with short period amplitudes at 0.25g)Lthe following procedure is considered:

.l.- Describe seismicity at and surrounding the plant site by a

6 stationary uniform Poisson sequence of earthquakes in time and space, such that the number, N , of earthquakes of given maximum (epicentral) intensity, I, is given by log 10 N = a -bI where e and b are constants that can be evaluated for the site.

2. Compute the Uniform Risk Spectra (URS) at the site for the seismicity given by 1 above and for the local site specific geologic conditions, following the procedures presented by Anderson and Trifunac (1977).

3. Compare the derived URS amplitudes with the proposed Seabrook design spectra, evaluate the differences and discuss the adequacy of the proposed design spectra.

Figure 2 presents the data on the expected number of earthquakes (per year) with different epicentral intensities I on MMI scale, in the 2

area of about 27000 km . identified as Boston-New Hampshire region (Chinnery,1979). -The.outside (left) scale on the vertical axis gives

-the logarithm of the number of earthquakes per year for the entire region 2

-of 2700L km . The right (inside) scale on the y-axis gives the logarithm 2

'of. the nt mber of earthquakes per year per 1000 km assuming the total area of 27000 km . Open circles represent Nc (number of earthquakes greater than and equal to.I). The full circles represent N the number of earth-quakes of. intensity which corresponds to the x-coordinate of the point.

-The seismicity model

' log 10 cN = 2.15 - 0.59I (1)

• Number o'f earthquakes per year per 1000 km 2 4

7 corresponds to the one discussed by Chinnery (1979). The model log 10 "c = 1.52 - 0.481 (2) represents an example of a " pessimistic" interpretation of 109 10 Ic versus MMI data, available so far, in that it mcst probably overestimates the frequency of earthquake occurrence for earthquakes with MMI2 VII.

For calculation of URS following Anderson and Trifunac (1977), equa-tion-(1) is transformed to log jg N = 0.59 - 0.591 . (3)

Perusal of figure 5 on the page 763. of Chinnery (1979) shows that the distribution of- epicenters within the zone corresponding to the Boston-New Hampshire region is not uniform. To further add to the " pessimistic" nature of the model-(2) I will assume, arbitrarily, that all these 2

everts'have occurred'in a small area A = 13500 km . With this, equation (2) leads- to log 10 N =. 0.22 - 0.481 (4)

To apply the method of Anderson and Trifunac (1977), it is convenient to choose the maximum intensity which can occur in each source region.

By perfonning'a series of calculations for different maximum intensities it-is possible to show how.this parameter influences the end result.

To compute the URS at the site I assumed that the whole region 1

surrounding the site can be represented by a unifona diffused zone as '

defined by Anderson and Trifunac-(1977). In the case of model (3) this is. equivalent to a" redistribution of the past seismicity into a uniform seismicity:per area surrounding the site. For' the model (4) this h equivalent 'to postulating a future ' seismicity per unit area'which is,

8 a

comparable to the most active sub areas of the Boston-New Hampshire region during the past 50 to 150 years. Since the model (4) assumes that this will occur uniformly at the Seabrook site and all surrounding areas, this model represents a " pessimistic" prediction of future-seismicity, which would be considerably higher than what has been observed there so far.

Figures 3 and 4 summarize the results of computed URS of Pseudo Relative Velocity (PSV) at a rock site (h=0 km, where h represents the depth of sediments beneath the site), for horizontal ground motion and j for structural damping of c =0.05. Each figure shows two groups (each consisting of three URS spectra) of curves for p=0.05 and p=0.30. Here p represents the probability that these spectra will be exceeded at least once during the next 50 years. Each group contains three URS curves corresponding to the three cases analyzed and for the maximum intensity in_the area assumed to be VIII, X or XII. Both figures also show the SSE spectra proposed for the Seabrook site and corresponding to 0.25g peak acceleration and G =0.05.

Figure 3 shows that these calculations suggest the probability of exceecing the proposed Seabrook design spectra during the next 50 years is less than 0.05, assuniing no limit on the maximum epicentral intensity in the region. _ Assuming that the largest possible intensities are X and.then VIII the probability of exceeding the SSE spectra for Seabrook is-further reduced. This also means that for the seismicity represented by the model i n equation _(3),' during the next 50 years the probability that the peak acceleration equal to 0.25g will be exceeded is less than (Imax.= VIII and X) or equal to'0.05 (if I max =XII).

Results in Figure 4 show that-if. the site seismicity is equal to

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[ ' that described by equation (4) that the probabilities of exceeding the proposed SSE. spectra at Seabrook Site are as follows:

Maximum area intensity Probability of exceeding Seabrook Spectra I max < 0.05

( = VIII L =I =X < 0.15 max 4

I max

=.XII < 0.30 The differences between the shapes of URS for this site and the pro-T1 posed SSE spectra exemplify the ability of the URS method to reflect the site specific conditions which in this case are primarily influenced by

.the nature of the rock site (this tends to increase the high frequency

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URS _ amplitudes relative to the fi, sed shape of Reg. Guide 1.60 spectrum) and the nature of the log 10 N versus'MMI.

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10 CONCLUSIONS

-The above probabilistic calculations suggest that the pr; posed SSE design spectra for Seabrook site (corresponding to 0.259 peak acceleration) may be acceptablL. However,.before I can finalize this

conclusion,-I would have to' carry out additional-and more detailed

. calculations to fina whether the above model of seismicity in equation

- (4)'is indeed a "sufficiently pessimistic" representation of possible 1 seismicity during .the next 50 years.

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2 REFERENCES Ande son, J.G. and M.D. Trifunac (1977). On Uniform Risk Functionals which Describe Strong Earthquake-Ground Motion: Definition, Nume-rical Estimation and an Application to the Fourier Amplitudes of Acceleration, Dept. of Civil Engineering Report 77-02, U.S.C. ,

Los Angeles (also NUREG-0405 Vol. 2, p. E1 - E100).

LChinnery, M.A. (1979). A Comparison of the Seismicity of Three Regions

- of the Eastern United States, Bull . Seism. Soc. Amer. , 6][,757-772.

- Trifunac, M.D. (1976). A Note on the Range of Peak Amplitudes of Re-l' corded Accelerations Velocities and Displacements with respect to I

theLModified Mercalli Intensity Scale, Earthquake Notes, Vol. 47, No. 1, 9 - 24.

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. UNITED STATES OF A" ERICA l NUCLEAR REGULATORY C0" MISSION

$ BEFORF THE ATOMIC'SAFFTY AND LICENSING APPEAL, BOARD

. In the Matter of- )

)

PilBLIC SERVICE C0"PANY OF ) Docket Nos. 50-443 NEW HAMPSHIRE, et al. 50-444 (Seabrook Station, Units 1 and 2) )

CERTIFICATE OF SERVICE I hereby certify that copies of Letter from M.D. Trifunac to R.P. Lessy enclosing COMMENTS BY PROF. . MIHAILO D. TRIFUNAC" in the above-captioned proceeding have been served on the following by deposit in the United States mail, first class, or, as indicated by an asterisk, throuch deposit in the-Nuclear Regulatory Commission's internal mail system, this 23rd

day of March,1981:

Alin S. Rosenthal, Esq. , Chai rman* Joseph F. Tubridy, Esq.

Atomic Safety and Licensing 4100 Cathedral Avenue, N.W.

Appeal Board l Uashington, DC 20016 U.S. Nuclear Regulatory Commission Washington, DC 20555 Dr. Ernest O. Salo Professor of Fisheries Research

'Dr. John H. Buck *' Institute

Atomic -Safety and Licensing College of Fisheries Appeal Board University of Wash'ngton U.S. Nuclear Regulatory Commission Seattle, Washingtoa 98195 Washington, DC 20555 Dr. Kenneth A. McCollom Dr. W.: Reed Johnson

• 1107 West Knapp Street Atomic Safety and Licensing Stillwa tar, Oklahoma 74074

- Appeal Board '

'J.S. Nuclear Regulatory Commission Robert A. Backus, Esq.

Washington, DC -20555 0'Neill, Backus, Spielcan, Little 116 Lowell Street '

Ivan W. Smith, Esq.* Manc' ester, NH 03101 Atomic Safety 'and Licensing Board Panel . Ellyn -R. Weiss, Esq.

. U.S. Nu'elear Regulatory Commission Harmon & Veiss-l Washington, DC 20555 1725.1 Street, N.W.

Suite'506 Hashington, DC -20005

- ~ = .r- -- , , ,,. . - . . -*

• r 'r y---- -

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• Thomas G. Dignan, Jr., Esq. Ms. Elizabeth H. Weinhold John A. Ritsher, Esq. 3 Godfrey Avenue Ropes & Gray Hampton, NH 03842 225 Franklin Street Boston, MA 02110 D. Pierre G. Cameron, Jr., Esq.

General Counsel Norman Ross, Esq. Public Service Company of 30 Francis Street New Hampshire Brookline, MA 02146 1000 Elm Street l'anchester, NH 03105 E. Tupper Kinder, Esq.

Assistant Attorney General Francis S. Wright, Asst. Atty. Gen.

Office of Attorney General Laurie Burt, Esq. , Asst. Atty. Gen.

State House Annex Commonwealth of Massachusetts Room 208 Environmental Protection Divis'.an Concord, NH 03301 One Ashburton Place,19th Floor Boston, MA 02108 William C. Tallman Chairman and Chief Executive Atoc.tc Safety and Licensing Officer Board Panel

• Public Service Company of U.S. Nuclear Regulatory Commission Washington, DC

~

New Hainpshire 20555 1000' Elm Street Manchester, NH 03105 Atomic Safety and Licensin._

i Appeal Board

• Docketing and Service Section* U.S. Nuclear Regulatory Commission Office of the Secretary Washington, DC 20555 U.S. Nuclear Regulatory Commission Washington, DC 20555

/

Roy P. Lessy Deputy Assistc @/ Chief Hearing Counsel

.- . _ ,-_ .