ML20213E102
| ML20213E102 | |
| Person / Time | |
|---|---|
| Site: | Columbia  |
| Issue date: | 03/17/1982 |
| From: | Kimball J Office of Nuclear Reactor Regulation |
| To: | Rolonda Jackson Office of Nuclear Reactor Regulation |
| References | |
| CON-WNP-0499, CON-WNP-499 NUDOCS 8203310033 | |
| Download: ML20213E102 (4) | |
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A 3-DISTRIBUTION:
iCENTRAL FILE MAR 171982
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Robert E. Jackson, Chief 0
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Leon Reiter, Leader
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JeffKimball,Seismolodist
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SUBJECT:
REVIEW OF WNP-2 SEISMIC EXPOSURE At:ALYSIS The WMP-2 applicant has subnitted a seismic exposure study as part of amendment 18 to the W!iF-2 FSAR (Appendix 2.5K), and in the recent nemorandum of February 26, 1982 from R. L. Ferguson to H. R. Denton the applicant indicated that more reliance may need to be placed on the probability study submitted. The exposure study has been intensly reviewed, and we have found that before this analysis can be used to supplement deterministic judgement, sensitivity studies and clarification of several items needs to be accomplished. These issues are discussed in the attachment to this meno.
Jeff Kimball, Seismologist Seistmlogy Section Geosciences Branch, DE f
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WtlP-2 Attachment Review of WilP-2 Seismic Exposure Analysis An important part of completing a seismic hazard analysis involves the selection of an approach to incorporate the uncertainty of all input parameters into the analysis. Difficulty in accounting for this uncertainty is one of the reasons the staff has used probability studies in a limited sense. In the WitP-2 seismic exposure analysis earthquakes are modelled as occurring on known geologic structure.
flo events are assumed to have the potential for occurring randemly within the site region and only geologic structures within 50 kilometers of the site are thought to contribute to the probability of the OBE and SSE. Other key elements of the applicants methodology to estimate the seismic hazard involves the rate of activity i
along each source zone segment, including the largest earthquake on each segment and ground. motion attenuation model.
The applicants approach allows for the incorporation of uncertainties and alternative hypothesis of some of the input parameters through the use of fault trees.
These parameters include the tectonic model, fault geometry, i
poteiitial source segmentation, source capability and maximum magnitude on each source.
Subjective consensus weights were assigned by the applicant l
and their consultants,to each branch of the fault tree.
The majority of the l
l subjective weights are based upon geologic knowledge of the Columbia Plateau, with the Gable and Rattlesnake sources being the most important contributors to the exposure at the WilP-2 site.
Because there are so l
many possible fault trees, changing tne subjective probabilities by a small amount would probably have only a small impact on the final results.
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In addition to the subjective weights the review has concentrated on the attenuation relationship, the recurrence relationship, and how the event probabilities (defined as the product of segmentation, tectonic model, capability, source geometry and maximum magnitude subjective probabilities) have been combined with the probability of exceeding the SSE.
Specific areas of parameter variation that we have been able to test include the upper magnitude cutoff, the sensitivity of individual source segments 'eing correct and comparison of assumed recurrence with historical o
seismicity.
Listed below are comments which could be used as meeting agenda topics along with requests for specific sensitivity runs on the exposure analysis.
It would be extremely helpful to also have each one of these queries responded to in a report form.
1.
Do the subjective weights span the range of geologic and seismologic possibilities for the structures modelled in the exposure analysis? Have you tested the sensitivity of changing the subjective probabilities? How would you take into account the possibility of differing opinions (numerical values) on the subjective wsights that you have assigned?
2.
As shown in figures 2.5J-20 and 2.5J-21 of Amendment 18 of the FSAR, deeper microearthquakes (non-swarm) are occurring in a somewhat scattered fashion throughout the Columbia Plateau.
These events do not appear to be occurring on the seismogenic structures assumed for the exposure analysis.
What is your position on the character of faults that these events are occurring on, including recurrence and maximum magnitude?
It is particularly important to discuss how you would limit the maximum magnitude considering the lack of knowledge of the lower crust.
Quantitatively, how much do these events contribute to the exposure (hazard} at the WNP-2 site?
Summary:
The staff has used probability studies increasingly in decision-making particularly when we can compare the site in question in a relatihe sense with other sites.
It is not presently clear how the WNP-2 analysis could be used in this relative sense!
Based on the experience that the staff has hadinothercaserehiewsitisherylikelythatwewilllookatanumber of items in reaching a final judgement.
This judgement will likely encompassconsiderationsofprobabilityanddeterministicratesofactihity and ground motion.
If the Southeast Anticline is assumed to be capable, the applicantshouldberequestedtoprohidetheirpositiononwhatdeterministic ground motion elements should be considered and how they should be integrated with the probability study.
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