Seismic shift? 

If sunlight truly is the best disinfectant, PG&E must feel like it got a mouthful of bug spray from the light beam focused on its Diablo Canyon “ground motion” modeling by an obscure state body known as the Independent Peer Review Panel, or IPRP.

The IPRP advises the California Public Utilities Commission on Diablo Canyon earthquake analyses. It is comprised of seismic hazard specialists from the California Geological Survey, California Coastal Commission, California Governor’s Office of Emergency Services, California Energy Commission, California Seismic Safety Commission, and San Luis Obispo County.

Though dripping with pious professional courtesy and laden with technical jargon, the recently circulated IPRP Report No. 6 (which can be downloaded at cpuc.ca.gov/NR/rdonlyres/B882E69C-758D-476E-A62D-6FCEC63BD277/0/IPRPReportno6.pdf) pulls the lid off PG&E’s black box computer model to reveal the type of slithering analytic shortcuts that only survive in the dark.

The IPRP was established to review the various offshore and onshore seismic surveys being performed by PG&E under its $62-plus million ratepayer-funded program. The high energy, three-dimensional, offshore studies proposed by PG&E were criticized by many as carpet bombing the seabed to produce information that PG&E would likely ignore anyway. The permit was denied last December by the Coastal Commission after a storm of controversy.

PG&E’s ground motion modeling, however, is a much cheaper effort (about $2.5 million of ratepayer money) but with a significantly larger impact on the projections of earthquake hazard at Diablo Canyon. Although its other reports have all been in reaction to PG&E proposals, the IPRP decided on its own initiative to look into the ground motion model because the model’s assumptions about the plant site conditions influence the shaking from every earthquake fault studied.

In evaluating the targets PG&E had proposed for the offshore and onshore studies, the IPRP asked PG&E in 2011 to prioritize seismic uncertainties on the basis of their impact on the hazard calculations in PG&E’s probabilistic model. Far and away the top ranked was uncertainty about the slip rate of the Hosgri Fault. Clearing up this uncertainty became a primary objective of PG&E’s seismic studies.

According to the IPRP, “Uncertainty in Hosgri slip rate may lead to calculated ground motion hazard that varies by a factor of nearly 2.”

What makes IPRP Report No. 6 so significant is that the thumb-on-the-scale tweaks, which it identifies in PG&E’s ground motion modeling, have a much larger impact! The threshold question of whether Diablo should be modeled as a “hard rock” site or as a more typical “soft rock” site, in the IPRP’s words, “increases the hazard by more than a factor of 3.” That’s more than 50 percent greater than the uncertainty about the Hosgri slip rate!

IPRP Report No. 6 aims its harshest criticism at PG&E’s failure to adequately verify its optimistic assumptions with hard data. Drilling and properly instrumenting a few boreholes at the plant site may not be as high tech glamorous as rapid-firing cannons into the ocean for several weeks 24-7, but you’d think PG&E would have recognized the obvious scientific benefit.

Then again, as the IPRP points out, “Comp-ared to traditional approaches, the PG&E method resulted in lower ground motion hazard estimates, particularly in the spectral period range important to [Diablo Canyon].”

Of course.

More chillingly, the adjustment of PG&E’s tweaks made by the IPRP to conform to a more generic assumption about sub-surface velocities “brings the estimated ground motion hazards beyond the original design level when used in typical, state-of-the-practice seismic hazard analysis ... .”

That marks the second time this year that it’s come to light that Diablo Canyon may be out of compliance with its seismic design standard. The first was the still-unresolved scandal surrounding the thwarted attempt by the NRC’s Senior Resident Inspector, Dr. Michael Peck, to cite PG&E for violation of Diablo’s “Safe Shutdown Earthquake” requirement (first reported in this newspaper’s June 27 edition: ‘Calculational garbage: Did the NRC allow PG&E to dodge Diablo Canyon seismic licensing requirements?”).

Dr. Peck was troubled by PG&E’s unwillingness to subject information about the Shoreline Fault to the most demanding elements of the Diablo seismic design basis: the damping and soil-structure interaction assumptions embedded in the so-called Double Design Earthquake or “DDE” test. Using PG&E’s own calculations, Dr. Peck determined that the Shoreline Fault, the Los Osos Fault, and the San Luis Bay Fault all are capable of producing peak ground motion some 70 percent greater than Diablo Canyon’s “Safe Shutdown Earthquake” design basis.

After more than two years of pushing, Peck was eventually overruled by his superiors at the NRC, but not before PG&E took the extraordinary step—just seven months after Fukushima—of trying to amend the Diablo license to weaken its seismic requirements. As PG&E admitted in its November 2011 10-Q filing:

“If the NRC does not approve the request the Utility could be required to perform additional analyses of Diablo Canyon’s seismic design which could indicate that modifications to Diablo Canyon would be required to address seismic design issues. The NRC could order the Utility to cease operations until the modifications were made or the Utility could voluntarily cease operations if it determined that the modifications were not economical or feasible.”

 Based on PG&E’s internal report of a December 2011 conference call obtained by the Alliance for Nuclear Responsibility, Dr. Peck’s direct supervisor, Neil O’Keefe, provided some peculiar coaching to the utility. By PG&E’s account,

“Neil then asked us if there is any technical reason for leaving the DDE in the design basis ... his advice is that we eliminate the DDE as our safe shutdown earthquake for our licensing basis. His opinion is that by leaving it in, it appears as if we are covering something up. We need to be able to tell a simple story for people to be able to understand, and the simple story won’t stand on its own if we leave the DDE in.”

PG&E ultimately negotiated an agreement with the NRC to defer applying the DDE analysis until 2015, and withdrew its license amendment request. The NRC made clear that it wouldn’t change Diablo Canyon’s seismic design basis, and expressed doubt that either the Shoreline Fault or the Hosgri Fault will be able to satisfy the “Safe Shutdown Earthquake” requirement when the 2015 analysis is performed. NRC staff reiterated this latter conclusion in testifying at a California Energy Commission workshop this past June.

Having never previously encountered a request to redefine a plant’s “Safe Shutdown Earthquake,” the NRC asked PG&E to create a table comparing its existing seismic review analyses to the “applicable provisions” of the NRC’s Standard Review Plan, which has been applied to new plants since 1997. PG&E provided a staggering 331-page list of deviations! The NRC hasn’t even mentioned whatever new seismic standards might arise from the Fukushima experience.

PG&E has relied upon political string-pulling and the docile nature of its regulatory lapdogs to evade Diablo Canyon’s seismic requirements for a long time. As it becomes increasingly harder to conceal analytic corner-cutting in a post-Fukushima world, the durability of PG&E’s commitment to keep the plant operating will face growing disbelief.

Rochelle Becker is the executive director of the Alliance for Nuclear Responsibility. Send comments to the executive editor at [email protected].