PG&E minimizes quake risks at nuclear plant, critics say

The Diablo Canyon Nuclear Power plant at the edge of the Pacific Ocean in San Luis Obispo, Calif., as seen on Tues. March 31, 2015. The Diablo Canyon Nuclear Power plant at the edge of the Pacific Ocean in San Luis Obispo, Calif., as seen on Tues. March 31, 2015. Photo: Michael Macor / The Chronicle Photo: Michael Macor / The Chronicle Image 1 of / 9 Caption Close PG&E minimizes quake risks at nuclear plant, critics say 1 / 9 Back to Gallery

Critics call it “sharpening the pencil.”

Since the Diablo Canyon nuclear power plant opened on a rocky stretch of California coast in 1985, researchers have discovered three nearby fault lines capable of stronger quakes than the one that struck Napa last year.

And yet the plant’s owner, Pacific Gas and Electric Co., insists that Diablo isn’t in greater danger than previously thought. If anything, it’s in less.

PG&E has, at several times in Diablo’s complicated history, changed the way the company assesses the amount of shaking nearby faults can produce, as well as the plant’s ability to survive big quakes.

To Diablo’s critics, PG&E keeps tweaking the math to make California’s last nuclear plant look safer than it really is. If PG&E’s seismic studies showed that nearby faults could produce more shaking than the plant was designed to handle, Diablo could be forced to close.

“The company has been claiming that the plant is stronger and stronger as more faults have been discovered,” said former state Sen. Sam Blakeslee, who has a doctorate in geophysics and lives nearby. “The utility has been moving the goal posts.”

PG&E insists that years of seismic studies at the plant near San Luis Obispo have given the company a more accurate picture than before. The biggest neighboring fault line — the Hosgri, discovered 3 miles offshore while Diablo was under construction — can’t shake the plant nearly as much as initially thought, according to PG&E. And the methods PG&E has developed to assess seismic threats at Diablo are far more precise than the ones used when the plant was designed, the company says.

“It is a gold standard of how to look at seismicity and the geology surrounding any infrastructure, not just nuclear power plants,” said Ed Halpin, PG&E senior vice president and chief nuclear officer. “In my opinion it should be held up and applauded.”

Instead, PG&E’s methodology has ended up in court.

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Lawsuit last year

Environmentalists pushing to close Diablo filed a lawsuit last year claiming the the Nuclear Regulatory Commission — the federal agency overseeing the nation’s nuclear plants — illegally let PG&E amend the seismic safety portion of Diablo’s operating license without public hearings. The suit came after one of the commission’s former inspectors at Diablo argued that the plant was no longer operating within the terms of its license and should be shut down.

His objections, and the commission’s handling of them, are now under investigation by the commission’s own internal watchdog office.

The inspector, Michael Peck, insists that if PG&E wants to change the methods it uses to measure earthquake threats at Diablo Canyon, the company needs a formal amendment to the plant’s license — a process that requires public testimony. Any new seismic methodology that produces “less conservative” results than the original — meaning, less predicted shaking — requires a license amendment, he said.

Without an amendment, PG&E must employ the same methodology used when the plant was designed, according to Peck. And under those methods, the newly discovered faults can produce more shaking than the plant was licensed to handle, he said.

'It’s not allowed’

“I’ve been an inspector 25 years,” Peck told The Chronicle. “I compare what PG&E does against the license, and I can tell you that it’s not allowed. I can’t tell you it’s unsafe, because I’m not a seismologist. But I can tell you it’s not allowed.”

The commission rejected his argument, saying “the older analytical techniques were overly conservative and no longer technically justified.” Though Peck considers that opinion irrelevant to the letter of the plant’s license, the commission stands behind it, and PG&E’s new methods.

“The NRC remains satisfied that PG&E’s past submittals have appropriately analyzed emerging information regarding the seismic makeup of the area surrounding Diablo Canyon,” said commission spokeswoman Lara Uselding.

But the differences are substantial.

An internal PG&E memo, obtained by some of Diablo’s opponents, said that if the company had to assess one of the recently discovered faults by the old methodology, the plant would need a waiver from the commission to stay open. “That would almost certainly result in exceeding code allowable limits that would require us to get NRC approval to continue to operate both units,” the December 2011 memo said.

The seismic safety of America’s nuclear plants has come under intense scrutiny after a quake off Japan’s eastern coast in 2011 triggered a tsunami that swamped the Fukushima Daiichi plant, knocking out its power supply and causing three of its reactors to melt down.

Earthquake safety concerns, however, have shadowed Diablo Canyon for more than 40 years.

When construction on the plant began, in 1968, PG&E considered the location free of active faults. But in 1971, geologists working for Shell Oil found the Hosgri just offshore.

Ground motion is key

For a power plant, the most important gauge of an earthquake’s severity isn’t the magnitude — it’s the amount of ground motion an earthquake will create at the plant itself. Diablo had originally been designed to withstand a specific level of shaking — .4g, or .4 times the force of gravity. An earthquake on the Hosgri Fault, researchers decided, could produce more violent shaking than that, with a peak ground motion of .75g.

By then, much of the plant had already been built. So PG&E had to retrofit Diablo before it ever opened.

The utility added concrete buttresses, blockier versions of the structures used to support cathedrals. Cinder block walls were reinforced with steel plates and screws the width of hands. Open metal grating that was supposed to cover the floor of the turbine hall was replaced with steel plates welded into place. A maze of support beams were threaded between pipes and ducts.

“It was quite an engineering exercise, figuring it out, making it work,” said Jearl Strickland, director of projects at the plant.

Diablo’s cost ballooned, especially after PG&E realized that some of the new seismic supports had been installed backward. Estimated at $600 million before the Hosgri Fault was discovered, the cost swelled to $2.2 billion with the modifications, then $5.5 billion after the modifications were redone, Strickland said.

In the end, PG&E persuaded a divided commission that the refortified plant could survive a major quake on the Hosgri, with peak ground motions of .75g.

But PG&E’s methods for assessing shaking from the Hosgri Fault — and the plant’s ability to resist it — were different from those used in the original design.

One controversial addition was the “tau effect,” purported to show how the plant’s large foundation would limit shaking inside. Some commissioners expressed reservations it would reduce shaking as much as PG&E claimed, but they were overruled.

A unique twist

Diablo’s operating license contained an added twist not found at any other nuclear plant. Along with the original seismic safety standard of .4g, the license also said the plant must safely shut down its reactors after a quake on the Hosgri Fault with .75g of ground motion.

Two very different levels of shaking, derived from two different methodologies.

Which standard applies? To Peck and Diablo’s critics, the .4g standard is the only one that matters, because it formed the design basis for the plant and its license. The Hosgri earthquake, they say, was tacked on to the license as a kind of appendix, illustrating how the plant would respond to a specific earthquake on a specific fault. To PG&E, the .75g standard is the one that counts, because it shows that the retrofitted plant is stronger than originally designed.

“It doesn’t matter the name of the fault — if you’re built to .75, you’re built to .75,” said PG&E spokesman Blair Jones.

It may sound like a technicality. But this basic disagreement took on ever-greater importance as other faults were found near Diablo, almost circling the plant. The biggest of those, the Shoreline Fault, was spotted just a few hundred yards offshore in 2008. Its discovery prompted a whole new round of questions and studies.

PG&E says the Shoreline can produce a magnitude 6.7 earthquake with ground motions of .57g at the plant. Another nearby fault, the San Luis Bay Fault, can shake the plant even more, with .63g of ground motion. Since both of those shaking estimates fall below .75g, Diablo remains within its operating license, according to the company. But to critics who back the .4g standard, those estimates prove that the plant is already violating its license and should be shut down.

How estimates are made

And then there’s the question of how PG&E makes its estimates. The company has repeatedly changed — or from its standpoint, refined — its formulas for assessing earthquake danger, while adding new data from seismic studies at the site.

The changes are so dramatic that PG&E now claims the Hosgri Fault can only generate .46g of shaking at the plant, far below the old estimate of .75g.

“When the Hosgri Fault was first characterized, the tools that they had in place at the time were not as sophisticated as the tools we have today,” Strickland said. “So as a result, they had to make some very rough assumptions.”

Techniques questioned

Blakeslee and others question some of PG&E’s techniques.

For example, the company’s recent ground-motion estimates try to take into account how the rocks beneath Diablo behaved during previous earthquakes. But the model only includes measurements from two significant quakes — the 6.6-magnitude San Simeon earthquake of 2003, and the 6.0 Parkfield quake of 2004. Neither struck within 40 miles of Diablo, and both lay in the opposite direction of the Hosgri and Shoreline faults.

Again, the technicalities matter. If PG&E hadn’t used site-specific data in its equations, all of the newly discovered faults would produce shaking greater than .75g, according to an independent panel set up by the California Public Utilities Commission to review Diablo’s seismic studies. That small change in the math kept PG&E from violating the terms of its license.

“They looked at the shaking at the plant, and that’s not an unreasonable way to do it,” said Bruce Gibson, a San Luis Obispo County supervisor and geophysicist who serves on the state’s Independent Peer Review Panel. “But you can’t draw conclusions from just two earthquakes. You need dozens or hundreds of earthquakes from different directions.”

Strickland says that the site-specific factor doesn’t just include the San Simeon and Parkfield quakes — it also includes “micro-quakes” recorded by seismometers planted around Diablo. In addition, the company used specially designed trucks to shake the ground, with the seismometers recording how the shock waves traveled through the rocks.

PG&E’s supporters

PG&E’s approach does have supporters, including William Lettis, a geologist who belongs to a separate panel, assembled by the Nuclear Regulatory Commission, to come up with its own seismic model for Diablo.

“Where earthquake science is moving is to use site-specific information where it’s available,” Lettis said. “To ignore site-specific information where it’s available would be flat-out wrong.”

In addition, PG&E says the retrofitted plant was built with enough of a safety margin in all its structures and equipment that it can actually withstand even stronger earthquakes than the .75g standard would suggest.

By the company’s assessment, Diablo can endure .83g of ground motion before its weakest components risk failure. PG&E last month submitted to the Nuclear Regulatory Commission a report saying the plant can survive the strongest quakes likely to strike in 10,000 years, even quakes that erupt on multiple faults at once.

“The conservative assumptions that were made when the plant was retrofitted for Hosgri still hold true, and in turn they provide a significant level of margin,” Strickland said.

Too many questions

The environmental group that filed last year’s lawsuit, Friends of the Earth, wants to force PG&E into pursuing a formal license amendment for Diablo, subjecting all of the company’s seismic assessments to public scrutiny. Meanwhile, Gibson says too many questions remain about PG&E’s methods to give an accurate sense of the plant’s safety.

“The important question is, 'Is their sharpening of the pencil legitimate?’” he said. “Did they really figure out something worthwhile? We don’t know.”

David R. Baker is a San Francisco Chronicle staff writer. E-mail: dbaker@sfchronicle.com Twitter: @DavidBakerSF