As PG&E’s aging Diablo Canyon nuclear power plant is up for a 20-year federal license renewal, one scientist cites new GPS data to suggest there’s a looming seismic threat from an active thrust fault that extends under the plant. But other experts say he is overstating the risk.
“It’s deeply concerning,” said seismologist Peter Bird, an emeritus professor at UCLA, who was commissioned by the anti-Diablo group Mothers for Peace to analyze the seismic risk to plant on the San Luis Obispo county coastline.
Bird’s analysis – submitted as part of a petition to shut down the plant — highlights the risk from a portion of the San Luis Bay thrust fault that he says extends under the plant and along the coastline.
At the time the plant was built, no seismic fault had been identified. While several faults have been identified since, Bird concludes the San Luis fault poses the greatest risk of generating a quake large enough to overwhelm the plant’s seismic retrofit protections.
PG&E has analyzed all the faults around the plant – including the San Luis Bay fault – and concludes a quake big enough to damage the plant’s core may occur once in 33,000 years. But Bird cites his analysis of new GPS data – tracking minute changes in distance between fixed points and geologic features – as indicating significant strain building up the San Luis thrust fault in the Irish Hills surrounding the plant.
“Thrust faulting occurs when two blocks of crust are squeezed together and one slips up and over the other — unfortunately, this can happen very suddenly and very violently,” Bird said.
Using GPS data from the U.S. Geological Survey to estimate earth movement near the fault, Bird predicts a quake violent enough to overcome Diablo’s seismic protections may hit once every 2,000 years.
“Unfortunately, we don’t know when the previous earthquake is, so we have to accept, I’m afraid, that the risk every year is one in 2,000.”
At a recent hearing at the state’s Diablo Independent Safety Committee, the chairman of independent panel charged with reviewing Bird’s findings acknowledged the San Luis Bay fault’s location.
“We agree there is an active thrust fault beneath the plant — that seems clear. The question is what is its capability?” USC seismologist Tom Jordan said at a meeting of the panel in June. “What’s the recurrence interval for large earthquakes? How big can they be? And so forth — we have not done that assessment.”
But in an interview, Jordan said his review team concluded Bird’s GPS-based analysis is substantially overstating the risk of a major quake on that fault.
“You have to be very careful in interpreting the GPS data,” he said. Although Jordan’s team agrees the Irish Hills area poses the biggest seismic threat, the panel’s report concluded Bird uses incorrect assumptions and fails to fully consider the influence of nearby faults.
“We come up with numbers that are significantly less than the strains that Dr. Bird is talking about,” Jordan said.
While they disagree about the risk, the scientists agree that PG&E should fully incorporate GPS data when it assesses the risk from seismic strain and activity near the plant.
“We need to take the data we’ve got and make sure we are understanding it properly and make sure that we are drawing conclusions from that data that are scientifically correct and plausible,” Jordan said.
In a statement, PG&E says its “lasting commitment to seismic safety” is reflected in a long-term research program dating back to the 1980s. Research, it says, that recently found “no significant increase in seismic risk” at Diablo beyond the level regulators relied on to declare the plant seismically safe.
The utility stresses that the Nuclear Regulatory Commission – which is still evaluating PG&E’s license renewal — recently rejected the petition to shut down the plant based on Bird’s concerns. PG&E says it will follow many of the Jordan review team’s recommendations as it studies the seismic risk going forward.
Source: NBC Bay Area
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