Letters to the Editor

PG&E slights public with Diablo seismic study based on shaky data

Bruce Gibson
Bruce Gibson dmiddlecamp@thetribunenews.com

The fourth anniversary of the Fukushima earthquake passed last week with hardly a footnote in the press. The historic scale of that natural force — so nearly beyond human comprehension — overwhelmed a human construction we thought was indestructible: the Fukushima Dai-ichi nuclear power plant.

Notably, last week’s local press was filled with assurances from PG&E, owner of the Diablo Canyon nuclear power plant, that something like that couldn’t happen here.

Beneath those comforting words, however, some important questions remain unanswered. Unfortunately, PG&E has tried to ignore or dismiss some important issues raised by scientific peer reviewers, and these issues remain unresolved. Thus, our understanding of earthquake risks remains uncertain.

Figuratively, Fukushima and its aftershocks have been felt intensely in San Luis Obispo County, where residents have long struggled to understand the earthquake risk at Diablo Canyon. Four years ago, a re-examination of seismic risk at the plant was already planned, as a result of legislation carried through by former state Sen. Sam Blakeslee.

Because this costly and crucial study was to be conducted by PG&E and funded by its ratepayers, the California Public Utilities Commission (CPUC) required an unprecedented level of public scientific review. The Independent Peer Review Panel (IPRP) is composed of scientists from seven public agencies (including me, representing our county) and was chartered by the CPUC to bring transparency and public trust to this important investigation.

I have written previously about the IPRP process and PG&E’s disregard for it, especially concerning PG&E’s poorly designed offshore seismic proposal (ultimately denied) and the surprising public release of its 1,800-page seismic investigation. Now, with the IPRP review of that report, the pattern repeats, and questions remain unanswered.

While assessing earthquake risks is a specialized technical undertaking, the concepts are straightforward: We evaluate geologic structures to estimate where, how big and how often significant earthquakes might occur. Then, for a large earthquake of concern, we evaluate how strong the ground shaking at the plant site might be.

In both steps, the IPRP has clearly stated that PG&E has drawn conclusions that are not supported by its own data. Thus, it’s premature to conclude that the plant would be unaffected during a large nearby earthquake.

In the first step, for example, PG&E did numerous seismic reflection surveys along winding roads in the Irish Hills east of Diablo. While the data quality was poor, PG&E’s technical team interpreted a series of faults that it concluded posed little seismic threat to the plant.

However, the IPRP found the data so poor that “even if all reflectors shown in the seismic sections are images of geologic features, the interpretations of various faults are inconsistent and not unique,” and “the IPRP is not confident that the tectonic model described as being developed from these surveys is well constrained.”

Basically, PG&E’s team reached a conclusion that doesn’t justify its ruling out more problematic faults.

In the second study step, regarding ground shaking, PG&E estimated certain physical properties of the rocks directly around and beneath Diablo Canyon. These “shear wave velocity” measurements are important in determining expected ground shaking. PG&E did not discuss the uncertainty of these estimates, and the IPRP found the values didn’t match previously measured values.

PG&E also reduced the calculated ground shaking by a factor called a “site term,” which the IPRP noted was “based on (only) two recorded earthquakes (and thus) may represent other factors, rather than site conditions. IPRP is not convinced that this factor is adequately constrained for use in ground motion calculations.”

I’ve called this a “fudge factor.”

In any case, significant questions remain to be addressed.

In reviewing and commenting on the entire PG&E study, the IPRP was willing and able to discuss its concerns with PG&E. Our offer was declined and, in fact, PG&E has yet to respond to issues raised in any of our last three reports.

So what’s next?

In a further slight to public process, PG&E has submitted its seismic hazard assessment to the Nuclear Regulatory Commission, without bothering to give the IPRP so much as a courtesy copy.

Panel members have downloaded documents from the PG&E website to begin an evaluation. The Nuclear Regulatory Commission will make the final call on plant safety and the seismic hazard, but the IPRP will continue to evaluate whether our issues have been addressed.

PG&E’s attitude toward this important public process is disturbing. While it fails to resolve, or even engage on important technical issues, the company proffers a soothing press release assuring us that all is well, followed by a rather condescending op-ed repeating its “commitment to plant safety.”

Assessing this earthquake risk is serious business. At Fukushima, the experts got it wrong by an order of magnitude.

PG&E’s commitment to our community should include a real and transparent engagement on these important technical matters. We deserve no less.

Related stories from San Luis Obispo Tribune

  Comments