ML20043D788
| ML20043D788 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 04/23/1990 |
| From: | Page B Advisory Committee on Reactor Safeguards, STANFORD UNIV., STANFORD, CA |
| To: | Seiss C ILLINOIS, UNIV. OF, URBANA, IL, Advisory Committee on Reactor Safeguards |
| References | |
| ACRS-CT-1970, NUDOCS 9006110173 | |
| Download: ML20043D788 (2) | |
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TTttk Mt602 ETANFRD STNU siamo.' ( w...,na 64 u 2 t B I AL M15172b21M 23 April 1990 Dr. Chester P. Siess University of Illinois %
3110 Civil Engineering
- Urbana, IL 61801 Dear Chet I attended the April 19 and 20 parts of the recent 4-day NRC/PGLE meeting in San Francisco, regarding the Diablo Canyon nuclear plant. The PG6E team addressed several (but not all) of the questions that had been put to them by the NRC staff last February (?). 'Ihe company representatives responded well, on the whole. Naturally, there are still dif ferences of opinion concerning some of their interpretations, but to me the dif ferences do not seem to be critical.
One of the best reports was by Paul Somerville, regarding new research on the location of the 1927 "Lompoc" earthquake which some experts have associated with the Hesgri f r.ilt. Using the S-P arrival times from a 1927 Netherlands record, an ar. of appropriate radius was drawn. The arc, which would ideally contain t',e epicenter, f alls well of f shore.- For confirmation, travel times for the Santa Lucia Banks and Coalinga earthquakes, as determined f ro: the same(?) inst rument in The Netherlands, were used for calibration, and two more arcs for the 1927 event were drawn. They are reasonably close to the first one.
7.> establish the latitude of the earthquake, an azimuth Irot pasadena was dctermined by vector analysis of P arrivals ou variously i
oriented instruments in Pasadena. The method was checked by comparing its results when applied to recent earthquakes of known locations, and it appears to be quite accurate. The azimuth for the 1927 event, taken in con-I junction with the arcs drawn f rom The Netherlands, gives an epicenter south-west of Pt. Conception for this M 7.0 earthquake. An ingenious suplemental study of the arrival times of the 1927 tsunami, et several stations including San Trancisco, San Diego, and Hilo, gave a fairly large locus west and southwest of Pt. Conception, and the seismically determined epicenter lies in the middle of this locus. Although this location does not agree very well with the on-land pattern of intensity ditribution, the latter is poorly constrained and I accept the new location. The new epicenter is in line with the Santa Lucia Bank f ault zone, and the 1927 focal mechanism and orientation of nodal planes fit this apparent relation. The PG&E team presented reasons for believing that the Santa Lucia Bank fault zone, if lengthened to include the new 1927 epicenter, could produce a 7.2 eat thquake opposite the Diablo Canyon site, but that both deterministic and probabilistic analyses show no significant increase in hazard at the site.
The only other subject I will mention here is the matter of a possible
" blind thrust" or detachment under the site, as postulated by Namson and g61 3 900423 DESICHATED ORIGINAL CT-1970 PDC
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and Davis. The PG6E group made two dif ferent models using the Namson and Davis method and assuming that there is indeed a sub-horizontal thrust. They tried using two different depths for the thrust, - say 5 km and 10 km (1 don't remember exactly). They showed that the published Natron and Davis model and their own two models do not produce the structures and strains that actually exist in the Diablo Canyon region. They maintain that if a low-angle blind f ault existed, it would not cont ribute significantly to the seismic hazard.
A n umbe r of other topics were discussed, in response to NRC questions.
To11 ovine the presentations and discussions. Bob Rothman held a private " wrap-up" caucus. Those present were Rothman, Richard McMullen (SRC). Robert Brown (USGS), Stephen Lewis (USGS). D.E.Slemmons (r. of Sevada). Douglas C. lark (U. of Nevada), and I. Rothman suggested i
that there is little point in conf ronting PG6E with more questior.s.
l Most of those present said they were. satis fied,- in fact, everyone except Steve Lewis. Lewis has reservations which include the followingt PGLE has not used its deep seismic reficetion data in constructing its structural model, and has not used the Tebruary 1990 M4.2 earthquake on the Queente st ructure of fshore. Lewis questions some of PG6E's seismic l
reflection interpretations. (We carefully went over the profiles and ' inter-pretations at a workshop meeting earlier this year, and everyone had a
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chance t o argue and try t o support different ideas). He has doubts about the n ew location of the 1927 earthquake. l'am sure that if Jim Crouch had
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been invited to the caucus, he too would have found fault with PG6E's vi(ws of of fshore seismotectonics.
When the PGLE representatives returned to the meeting room, Bob Rothman told them that probably there would be no more major questions.
Hou ever he rerinded them that some earlier questions have not yet been answered, and he urged them to respond as soon as possible to expedite the preparation of the staff's Supplemental Safety Evaluation Report. He conmended the PG6E team for doing a good job of acquiring data, but stated that there would always be dif ferences of opinion as to interpretations.
I sense that the geologic / geophysical / tectonic studies are finally tapering of f, to the satisf action of most of us. Of course, new ideas and information will have to be considered from time to time in the future.
1 Probably you will hear f rom George Thompson, who attended the sessions I rissed.
I With all good wishes.
- Yours, t
Benjamin M. Page ACRS Consultant i
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