ML13317A703
| ML13317A703 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 10/19/1981 |
| From: | Moody W SOUTHERN CALIFORNIA EDISON CO. |
| To: | Crutchfield D Office of Nuclear Reactor Regulation |
| References | |
| TASK-03-06, TASK-3-6, TASK-RR NUDOCS 8110220182 | |
| Download: ML13317A703 (10) | |
Text
Southern California Edison Company P. 0.
BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD, CALIFORNIA 91770 W. C. MOODY TELEPHONES MANAGER, NUCLEAR LICENSING O
o 19 (213) 572-1817 Octobuer 19, I981 (213) 572-1806 Director, Office of Nuclear Reactor Regulation Attention:
D. M. Crutchfield, Chief Operating Reactors Branch No. 5 Division of Licensing U. S. Nuclear Regulatory Commission Washington, D. C. 20555 Gentlemen:
Subject:
Docket No. 50-206 SEP Topic 111-6 Seismic Design Considerations San Onofre Nuclear Generating Station Unit 1 By letter dated April 28, 1980, SCE provided information regarding the probability of-exceedance of various earthquake levels at the San Onofre Unit 1 site. In a meeting with the NRC Staff on October 15, 1981, and in telephone conversations on October 16 and 19, 1981, SCE and its consultants provided updated information on this subject.
The enclosed report documents information presented at the October 15 meeting and in the subsequent telephone discussions. This information relates to estimating the probability of exceeding a O.5g Housner earth quake for use in a risk evaluation 6f the seismic withstand capability of the north turbine building extension. The conclusion of this report is that for the period range of interest for the north turbine building extension (0.5 to 0.6 seconds), a probability of exceedance of the 0.5g Housner spectrum of about 10 or less is appropriate for the eight-month period between now and June 1, 1982.
(As indicated in SCE's letter dated September 28, 1981, SCE will modify the north turbine building extension by June 1, 1982 or shutdown the plant.)
In addition, even an extremely conservatively postulated magnitude 7 earthquake on the Hypothesized Offshore Zone of Deformation opposite the site, represents no more than about a 15-to-25% chance that ground motions at the site would equal or exceed 0.5g for the specified velocities defined by 0.5g Housner in the period range of interest.
If you have any additional questions on this matter, please let me know.
F0110220182-811o0194o
.PDR ADOCk 0000
PROBABILITY OF EXCEEDANCE OF O.5g HOUSNER RESPONSE SPECTRUM
1.0 INTRODUCTION
This report supplements the results of the WCC 18 April 80 report entitled, "Development of Instrumental Response Spectra with Equal Probability of Exceedance for Unit 1" which was submitted to the NRC by letter dated April 28, 1980. The results presented herein result primarily from modifications in the mean attenuation relationships for peak acceleration and spectral velocity with distance used in the 18 April 80 report. These modifications result from earthquake data analyzed subsequent to the 18 April 80 report.
These analyses provide the probability of exceedance of a peak accelera tion of 0.5g or spectral velocity as defined by the Housner spectrum anchored at 0.5g in the period range of 0.5 to 0.6 seconds. Specifically, Section 2 briefly describes the analysis procedure and results documented in the 18 April 80 report. The basis for modifications made in the current analysis and results are presented in Section 3. Sensitivity.studies covering a weighting scheme developed to equalize the damage potential of various magnitude earthquakes, a discussion of deterministic results compared to the 1/2g Housner spectra, and a discussion of the probability of exceeding the Housner spectrum given a M7 earthquake on the Hypothesized Offshore Zone of Deformation opposite the site are presented in Section 4.
Finally, Section 5 presents a summary of conclusions based on the results of the current analyses.
2.0 ANALYSIS AND RESULTS OF 18 APRIL,80 REPORT The analysis procedure and results of the seismic exposure analysis completed for the site were presented in the 18 April 80 report. The analysis procedure is shown schematically on Figure 1. The input parameters include a source seismicity model, an attenuation model and the exposure evaluation criteria. The source seismicity model provides for the characterization of the SONGS site and vicinity within about 200 km of the site in terms of the 'activity of the various sources (major faults) and their proximity to the site. The recurrence relation ships define the seismicity and are constrained by the instrumental historic activity, moment rate and the maximum estimated magnitude as described in the 18 April 80 report. The attenuation model for the 18 April 80 report was essentially based on the SONGS 2 and 3 instrumental attenuation relationships as presented in the report. The exposure evaluation criteria was simply the annual probability of exceedance.
The results of these analyses are presented in Figure 2 in terms of equal probability of exceedance spectra at 2% damping. The Housner spectrum anchored to 0.5g is also shown in Figure 2 and shows an annual probability of exceedance of 6 x 10-4 for the peak acceleration and for spectral values for periods of approximately 0.5 seconds and longer.
3.0 MODIFICATION TO INPUT PARAMETERS AND RESULTS OF CURRENT ANALYSIS Subsequent to the 18 April 80 report, analyses of the data from recent earthquakes (most notably IV79) showed the attenuation input parameters for the previous analyses to be conservative. Therefore, to provide input to a seismic exposure analysis for the north turbine building extension with a characteristic period of from 0.5 to 0.6 seconds and a damping of
4% a revised analysis was completed based on a current attenuation relation ship. This relationship uses the previous data set supplemented by IV79 data and allows for an increase in near field peak acceleration (10 to 15%
over IV79) for the more stiff SONGS site conditions. It also uses the near field spectral values from IV79 in the period range of 0.5 to 0.6 seconds directly for SONGS. These changes reduce the calculated probabil ities. Other modifications to the earlier analysis include the use of larger dispersion for small earthquakes and lower contribution of low magnitude earthquakes in the long period range. The former of these changes increases the probability of exceedance and the latter reduces it slightly. The recurrence relationship was also modified by changing the maximum magnitude from M6-1/2 to M7.
This change had essentially no effect on the results. The final results are presented in Figure 3 for 4% damping. As shown in Figure 3 the probability of exceedance of the Housner spectra anchored at 0.5g is about 1.5 x 10-4 for zero period acceleration and in the period range of 0.5 to 0.6 seconds. This value is about a factor of 4 lower than the 18 April 1980 results. For the exposure period of 8 months, the probability of exceedance would reduce to a range of 1 to 4 x 10 for the current and the 18 April 1980 analyses.
The above range is considered to provide reasonable bounds to the estimated probability of exceedance for the 0.5g Housner spectrum in the period range of interest at the SONGS site based on all available data and taking into account the sensitivities of the various parameters.
4.0 EQUAL DAMAGE CONSIDERATIONS AND COMPARISON WITH DETERMINISTIC RESULTS To put the above-described results in the context of damage potential, some modification to input parameters should be incorporated into the analysis to reflect the fact that low magnitude earthquakes have a lower damage
potential than high magnitude earthquakes. To accomplish this, guidance was derived from seismic induced liquefaction potential criteria and observed damage during earthquakes. Based on conservative interpretation of such information, M4-1/2 and 6 earthquakes were assumed to be 50 and 80%, respectively, as damaging as a M7 earthquake for the same intensity of ground motion. In the exposure analysis, therefore, small magnitudes were given lower weights than larger magnitudes with M4-1/2, 6 and > 7 given weights of 0.5, 0.8 and 1.0, respectively. It is noted that no attempt was made to further reduce the ground motion from the instrumental values to those that would be effective in causing structural damage.
The result of incorporating the weighting factors given above in the seismic exposure analysis yields lower probabilities of exceedance by a factor of at least 3 below the unweighted case. This would reduce the range of probability of exceedance of the 1/2g Housner spectrum in the period range of interest developed in Section 3 to slightly in excess of 10-4 to the mid to low 10- range for an 8-month exposure period.
It was noted in inspecting the attenuation relationships, in both the 18 April 80 analysis and the current analysis, that the value of 0.5g.
(or corresponding 0.5g Housner spectral velocities between 0.5 and 0.6 seconds) is between 1/2cr and 1 above the median value. This means that even if a M7 earthquake were to occur on the OZD opposite the site the probability of exceedance of 0.5g or the corresponding 0.5g Housner spectral values of interest would only be in the range of 15 to 25%. This is consistent with the results of various deterministic analyses completed for the site including: (1) the Tera-Delta earthquake simulation spectra that fall below the 1/2g Housner spectrum; (2) the IV79 spectral data in the period range of interest at 8 km which fall below the 1/2g Housner
spectrum; (3) the mean of-the Units 2 and 3 WCC instrumental spectrum which falls below the 1/2g Housner spectrum in the period range of interest, and (4) the peak acceleration from the Tera empirical analysis which is near 0.5g.
5.0 CONCLUSION
S Based on the foregoing discussion and results of analyses, the following conclusions are made:
- 1. The probability of exceedance of O.5g peak acceleration and spectral velocities defined by the Housner spectrum andhored at O.5g in the period range of 0.5 to 0.6 seconds is small, ranging between 1-1/2 and 6 x 10-4 (figures 2 and 3) annually. For an 8-month exposure period, this range reduces to about 1 to 4 x 10-4
- 2. For application to an evaluation of structural damage potential, the above probabilities should be reduced by at least a factor of
- 3. This reduction assumes that M 4-1/2 and 6 earthquakes have 50%
and.80%, respectively, of the damage potential of a M7 for the same intensity of shaking. These reductions are considered conservative based on a comparison of observed damage during large and small earthquakes. The factor of 3 or more reduction in probability of exceedance would yield a probability of exceedance of about 10-4 or less for the 0.5g Housner spectrum in the period range of interest for the evaluation of the structural capacity of the north turbine building extension.
- 3. Assuming the occurrence of a M7 earthquake on the HOZD opposite the site, there is only about a 15 to 25% chance that ground motions at
the site would equal or exceed 0.5g or the spectral velocities defined by the 0.5g Housner spectrum in the period range of 0.5 to 0.6 sec. These observations are consistent with deterministic ground motion analyses developed from earthquake simulation studies by Tera Delta and by empirical ground motion studies completed by WCC and Tera.
INPUTS ANALYSIS RESULTS Source Seismicity Model Location Recurrence Magnitude Range Obtain probability distribution of spectral ordinates by combining over probabilities clate peified Attenuation Model of occurrence of different of exceeding specified magnitude earthquakes, distance response spectral Site Conditions to fault, and spectral acceleration ordinates Transmission Path Conditions for a given distance Magnitude and Distance Exposure Evaluation Criteria*
Period of Interest Probability of Exceedence Figure 1 Schematic Representation of Approach
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