NRC defends Diablo Canyon earthquake study at meetings

The Nuclear Regulatory Commission on Wednesday defended a recent study finding that Diablo Canyon nuclear power plant could withstand an earthquake along the newly discovered Shoreline Fault.

Representatives of the agency held two open house meetings in San Luis Obispo to take public comment on the study and answer questions. Earthquake experts with the commission and PG&E were available to answer questions about the Shoreline and other faults around Diablo Canyon.

The NRC study concurs with PG&E’s conclusions that ground motion produced by a quake along the Shoreline Fault would be within the plant’s existing design limits, which are based on the larger Hosgri Fault farther offshore.

“The Shoreline Fault has a low slip rate that is estimated to be an order of magnitude less than the Hosgri Fault, which implies that its overall contribution to the hazard is relatively limited,” the report concluded.

The Shoreline Fault was discovered in 2008 just offshore of the plant. At its closest point, it is only 2,000 feet from Diablo Canyon’s reactors.

The NRC’s report is based on inspections of the plant by teams of seismic experts with the NRC and other agencies that went beyond what is typically done at other nuclear plants, said Annie Kammerer, a senior NRC seismologist.

Anti-nuclear groups including the San Luis Obispo Mothers for Peace take issue with the agency’s conclusions. They contend that the study underplays the possibility that the Shoreline Fault is connected to other faults and could produce a stronger quake than thought, a quake that key safety equipment at Diablo Canyon may not be able to withstand.

“Don’t let PG&E continue to get away with this,” Mothers for Peace spokeswoman Sherry Lewis told the NRC.

The group wants the agency to wait until an independent group of seismologists completes a comprehensive evaluation of the earthquake threat facing the plant due in March 2015. However, Kammerer said this seismic hazard re-evaluation would not change the findings of the Shoreline evaluation.

“If there is any nuclear plant in the U.S. that we seismically understand, it’s this plant,” she said. “And that’s appropriate because this plant has more active faults than any other plant.”

The issue of the seismic safety of Diablo Canyon took a dramatic turn earlier this month when the state Coastal Commission denied a request by PG&E to conduct high-energy earthquake mapping offshore of the plant. Those surveys would have answered some of the outstanding questions about the Hosgri Fault.

The surveys would have provided some useful information but were not required by the NRC, Kammerer said. Low-energy and onshore seismic surveys already conducted by PG&E will provide a wealth of new information about the faults surrounding the plant.

PG&E also recently completed “walkdown” inspections that looked at flooding and seismic protection features at the plant. These inspections were required by the NRC as a result of the Fukushima Dai-ichi nuclear accident in Japan in 2011.