Pacific Gas and Electric Co. did not use the most conservative analysis when it determined that Diablo Canyon nuclear power plant can withstand the most powerful earthquake that is likely to occur near the plant.
That is the conclusion of a state-appointed independent panel of scientists tasked with reviewing PG&E’s seismic safety assessments and backed by two prominent local seismologists who have played key roles in evaluating the threat that earthquakes pose to the nuclear plant.
San Luis Obispo County Supervisor Bruce Gibson and former state Sen. Sam Blakeslee said the utility consistently evaluates seismic data it has collected around the plant in a way that reinforces its claim that the plant is safe despite considerable uncertainty in the data.
“Every time PG&E identifies a threat, they sharpen their pencils and determine that the plant can withstand that threat,” said Gibson, who sits on the independent panel of scientists appointed by the state to evaluate PG&E’s seismic studies.
“We will not come out and say the plant is unsafe, but we will continue to raise concerns about the uncertainty, and there are some serious outstanding concerns,” he added.
PG&E denied the charge, saying that it has used the most advanced seismic experts and analytical techniques to evaluate the earthquake threat facing Diablo Canyon. This included analyzing the possibility of multiple simultaneous ruptures along several faults that would produce greater shaking at the plant.
As a result, more is known about how Diablo Canyon would respond in an earthquake than any other nuclear plant in the nation, PG&E spokesman Blair Jones said.
“The results of this analysis demonstrate the plant’s earthquake design is appropriate and safe, and it will be independently reviewed by the Nuclear Regulatory Commission,” Jones said.
Post-Fukushima studiessubmitted a safety evaluation
In September, the utility published the results of those surveys and concluded that the plant could withstand the most powerful quake the faults around Diablo Canyon are likely to produce. The report PG&E filed with the NRC this month reinforces those conclusions.
The agency required that those studies be undertaken at all nuclear plants in the nation following the 2011 Fukushima Dai-ichi nuclear disaster in Japan.
PG&E determined that the plant could safely withstand the ground motions that would be caused by the most powerful earthquake that has a 1-in-10,000 chance of occurring within a given year. The study looked at key safety equipment and the plant’s ability to safely shut down in a large quake.
In December, Blakeslee told a U.S. Senate committee that PG&E has consistently downplayed the seismic threat to Diablo Canyon. Echoing Gibson’s criticisms, Blakeslee told The Tribune that the seismic data regarding Diablo Canyon contains a variety of gaps and uncertainties.
He said PG&E has a pattern of concluding “serendipitously” that the plant can withstand even powerful earthquakes, despite those uncertainties.
“A more conservative approach to the problem would have come up with significantly larger shaking calculations,” he said. “When I think of a nuclear power plant, the more conservative approach should be taken.”
The NRC has appointed a task force to handle Diablo Canyon’s and the nation’s other nuclear plants’ seismic plans, said Lara Uselding, NRC spokeswoman. That group will now begin a detail review of the report.
“The staff is currently processing the receipt of those documents,” she said. “They will be performing a screening and prioritization review in 30 to 60 days, which will include reviewing the licensee’s reevaluated hazard and any required interim actions.”
Peer review targets 3 concerns
The utility conducted a series of onshore and offshore surveys using boats that emitted loud noises that penetrated into the Earth’s crust and trucks that transmitted vibrations into the ground.
The California Public Utilities Commission appointed a panel of scientists, including Gibson, called the Independent Peer Review Panel to double-check the accuracy of PG&E’s conclusions.
Since it was formed in 2011, the IPRP has published nine reports evaluating Diablo Canyon’s seismic studies and found a series of deficiencies, which PG&E has largely ignored, Gibson said.
In February, PG&E abruptly canceled a meeting with the panel to discuss the findings of its most recent report.
“PG&E has tried to ignore us and has been unresponsive to our requests,” he said. “PG&E is not happy about us being around.”
Jones said that PG&E did take into account some of the panel’s recommendations in its report filed with the NRC in September.
"The IPRP had direct feedback on the advanced seismic studies plan, and it was altered based on their feedback," he said.
Gibson highlighted three areas of PG&E’s analysis that the panel has questioned. These show a pattern of PG&E using what Gibson called a fudge factor to downplay the seismic hazard:
- The slip-rate of the Hosgri Fault, which sits offshore from Diablo Canyon and is the dominant earthquake fault in the area.PG&E determined the annual slip-rate to be 1.8 millimeters while the panel concluded that the rate was 2.2 millimeters annually. While the difference may seem small, the slip-rate is important because it shows the potential energy of the fault, Gibson said. The slip-rate measures the distance each side of the fault moves in relation to the other.
- Gaps in the seismic knowledge of the Irish Hills surrounding the plant.Because the terrain around the plant is so rugged and steep, the data the utility gathered was weak. Gibson said the weak data, particularly for the deeper portions of the faults, and PG&E’s inconsistencies in interpreting the data do not rule out a stronger quake originating in the Irish Hills. For example, PG&E identified faults in some areas when the same data could indicate that they could also extend into other areas.
- Characterizations of the geology surrounding the plant to determine whether it would increase or reduce the shaking caused by an earthquake, called site amplification modeling.Blakeslee said the geology around the plant is highly varied and likened it to “tapioca pudding with lots of raisins and ice cream swirls.” Again, PG&E used complicated data and the least conservative interpretations to make its determinations, Blakeslee and Gibson said. For example, it was based on data from two large quakes. The panel recommended that PG&E also include data from smaller quakes to make its determination.
“PG&E and the IPRP have collaborated exactly as the CPUC prescribed,” Jones said. “We value the IPRP’s feedback on the advanced seismic studies and have met with them on multiple occasions to share information and solicit their feedback.”
Rochelle Becker with the San Luis Obispo-based antinuclear group Alliance for Nuclear Responsibility said PG&E must address the panel’s concerns in order to make the seismic data regarding the plant as complete as possible.
“We don’t have the answers to our questions regarding these studies,” she said. “Until we do, they are absolutely incomplete.”
Gaps in data
Chris Wills, a geological engineer with the California Geological Survey, chaired the IPRP. He said the IPRP concluded that PG&E’s data has gaps and some of its conclusions are not fully supported by the seismic data. It will take several months for the IPRP to review the voluminous report PG&E filed with the NRC this month to determine if those shortcomings have been corrected.
“We are just starting now to look at this final report to see whether we agree with its findings or not,” Wills said. “We will need a few months to review it at the same time the NRC is doing its review.”
Gibson acknowledges that neither the NRC nor PG&E are under any legal obligation to follow the panel’s recommendations. However, a thorough response to its recommendations would go a long way to assuaging fears in the community about the seismic safety of Diablo Canyon, he said.
“PG&E has a trust problem within certain segments of the community,” Gibson said. “They could have used us to their benefit.”