PG&E has completed a four-year assessment of the earthquake danger facing Diablo Canyon nuclear power plant and concluded that the plant can withstand a quake from any of the five individual faults near the plant — as well as what could occur if any of them are connected.
“The conclusions are that Diablo Canyon is seismically safe,” said Ed Halpin, PG&E’s chief nuclear officer. “These studies are unprecedented and unlike anything that has been done at any other nuclear power plant in the nation and likely the world.”
Released Wednesday, the study will be peer-reviewed by two independent scientific panels, one appointed by the California Public Utilities Commission and the other appointed by the federal Nuclear Regulatory Commission. This peer-review process is expected to be complete in 2015, and all the information contained in the study will be available to the public.
Using low-energy offshore and high-energy onshore seismic survey equipment to map the faults surrounding the plant and their movements over time, the studies reveal new details about the Hosgri, Shoreline, Los Osos, San Simeon and San Luis Bay faults.
Specifically, the study gives updated estimates of the maximum amount of shaking or ground acceleration an earthquake along those faults, or along a combination of the faults, could cause at the plant, expressed in a factor called G-force. Diablo Canyon and its equipment are designed to withstand a G-force of 0.75.
The G-force measurements used in the study are different than the most commonly used yardstick for measuring the plant’s earthquake safety, which is that the plant can withstand a magnitude-7.5 quake on the Hosgri Fault. Magnitude measures the force of an earthquake at the fault itself, not at the plant, which is why G-force is preferred in this case.
The assessment that the plant is seismically safe is based on the fact that none of the faults could deliver a jolt of more than 0.75 G-force, Halpin said.
“Nothing exceeds the engineering and other designs of the plant,” he said.
The study lists the updated G-force capability of the faults compared to the estimate before the seismic surveys were completed. For example, the estimate for the Shoreline Fault was increased to 0.57 from 0.56 G-force; the Los Osos Fault was decreased to 0.50 from 0.54; the San Luis Bay Fault was increased to 0.63 from 0.61; and the Hosgri Fault was significantly reduced to 0.46 from 0.75.
The study also looked at the G-force potential if several faults were linked. For example, if the Hosgri and San Simeon faults were linked, the G-force potential would be 0.55, and if the Shoreline Fault were linked to the Hosgri and San Simeon faults, the G-force potential would increase to 0.60.
San Luis Obispo County Supervisor Bruce Gibson is a seismologist who serves on the Independent Peer Review Panel, a group of scientists convened by the California Public Utilities Commission to do an independent assessment of the study.
He received a copy of the study Wednesday but has not had a chance to review it. He said it is unfortunate that PG&E chose to release the study to the public before releasing it to the panel.
“It was our expectation that the data sets would be released to the IPRP, so we would have a chance to analyze them before releasing them to the public, but I guess that was not PG&E’s plan,” he said.
PG&E, however, said its commitment was to release the report to the public.
Neal Driscoll is a member of a second peer-review committee that will be examining the PG&E study. He is a member of the NRC-mandated Senior Seismic Hazard Analysis Committee and is a professor of geology and geophysics at the Scripps Institution of Oceanography in San Diego.
He said PG&E marshaled many state-of-the-art tools for the study to better understand the faults around Diablo Canyon and reduce uncertainty. He looks forward to analyzing the study.
“I think releasing these reports to the public so that they can be vetted and peer-reviewed is a great step forward,” he said.
Rep. Lois Capps, D-Santa Barbara, said she is pleased that the study is moving forward.
“As always, my top priority is ensuring that Diablo Canyon is safe,” she said. “This report, which I have just received and will be examining closely, is an important step forward in fully understanding the seismic situation near Diablo Canyon and taking appropriate precautions.”
Jane Swanson, spokeswoman for the anti-nuclear group San Luis Obispo Mothers for Peace, said the group will have a detailed response when it has had a chance to read the report.
“San Luis Obispo Mothers for Peace will study this report with the assistance of our attorney and expert consultants,” she said. “It is one more example of PG&E’s history of trying to rationalize away the extreme earthquake hazards to the Diablo Canyon reactors.”
The seismic assessment began in 2010, prompted by the discovery of the Shoreline Fault in 2008, which runs just offshore of the plant. The tsunami and nuclear meltdown at the Fukushima plant in Japan in 2011 added urgency to the studies.
The NRC has required all nuclear plants in the nation to conduct additional seismic hazard evaluations in the aftermath of the Fukushima disaster. The information generated by the seismic surveys will be included in PG&E’s post-Fukushima report that is due to the NRC in 2015.
The state Public Utilities Commission authorized PG&E to spend as much as $64 million mapping and analyzing the network of faults surrounding the plant.
As originally planned, the mapping had three major components. The first was onshore mapping on PG&E lands surrounding the plant and along 120 miles of roads in the county. Huge trucks emitted vibrations into the ground, the reflections of which were picked up by an array of ground sensors.
This was followed by offshore, low-energy surveys, which involved emitting loud sounds into the ocean that penetrated into Earth’s crust and bounced back to be picked up by an array of sensors towed behind the survey ship.
The main purpose of the low-energy surveys was to map how much movement has taken place over time along the area’s faults. This slip rate and other information can be used to estimate how frequently earthquakes take place along a fault and their strength.
PG&E had planned to conduct high-energy offshore surveys in 2013 but was denied a necessary permit by the state Coastal Commission. That agency said the utility had not done enough to protect marine wildlife from the extremely loud blasts of sound.
The high-energy tests would have allowed seismologists to map the quake faults at depths of 6 to 9 miles underground where the quakes occur and determine whether they are connected or not.
The fact that the high-energy surveys were not conducted did not diminish the overall conclusions of the study, Halpin said. Seismologists were able to fill in the gaps from the missing high-energy data using data from the low-energy surveys was well as data from the U.S. Geological Survey and other sources, he said.