Diablo Danger Underestimated?

New studies, using state-of-the-art seismic mapping technology, show that fault lines threatening the Diablo Canyon nuclear power plant are more complex and interconnected than Diablo’s designers could have known. This complexity negates the seismic predictions used to justify the plant’s location. Unfortunately, we now know that the network of earthquake faults nearby and underneath PG&E’s nuclear facility could be activated by a mega-thrust earthquake far to the north, at the Cascadia Subduction Zone. Subduction-zone earthquakes are the most powerful quakes in the world and can exceed magnitude 9.0.

The Cascadia, which begins near Vancouver Island, is a 620-mile-long fault line that intersects the San Andreas Fault just off Cape Mendocino in Northern California. This region of powerful and unpredictable earthquakes connects directly to the Diablo Canyon site. After evaluating the U.S. nuclear power facilities in the wake of the Fukushima disaster, the Nuclear Regulatory Commission has recently ranked the aging Diablo Canyon nuclear plant as uniquely vulnerable to unanticipated seismic activity, a “Group One … hazard.”

In the early ’60s, Pacific Gas & Electric first chose Bodega Bay as the site for its proposed nuclear plant. The company began excavating the foundation, but then a fault line was discovered on-site, and that nixed the plan for Bodega Bay. PG&E then proposed a spot fairly close to where the Diablo Canyon plant sits today. This second location also got crossed off the list because of faults. When, finally, construction of the Diablo Canyon nuclear facility began near Avila Beach, PG&E maintained that there were no active faults within 30 miles of the facility. The plant was originally designed to withstand a magnitude 6.75 earthquake but was later upgraded to weather a magnitude 7.5 shaker. Unknown at the time, the plants’ two reactors were situated near undiscovered faults.

For more than 30 years, seismologists have argued that the utility companies have underestimated the seismic threat to their nuclear facilities, especially Diablo Canyon’s redesigned structural supports. In 2011, the Nuclear Regulatory Commission (NRC) ranked Diablo Canyon as the nation’s nuclear plant most vulnerable to earthquakes. In 2012, Michael Peck, who for five years was lead NRC inspector at Diablo Canyon, argued that the plant was no longer operating within its license and that it should be shut down until PG&E demonstrated that the reactors and other equipment could survive earthquakes on the newly discovered faults. In a letter sent to PG&E on May 13, 2015, the NRC revealed that Diablo Canyon is classified as one of the nation’s two “Group One” nuclear facilities, “that have the highest re-evaluated hazard relative to the original plant seismic design.”

Recognized only recently, the Diablo Cove Fault Line runs east to west directly under the Unit One Reactor and turbine building! The fault underneath the facility is connected to the entire network of faults. About a quarter mile west of the facility, the Diablo Cove Fault cuts across the seismically active Shoreline Fault, itself only recently discovered. The Shoreline is connected to the feared Hosgri Fault, a component of the San Andreas Fault System. Because this location is so tectonically active, and the system of faults is so complex, no one can predict safety with confidence. The Diablo Cove Fault, the Shoreline Fault, the Hosgri Fault, the San Andreas Fault, and the Cascadia Subduction Zone are all seismically linked, and the power stored within the combined network of fault systems could create an earthquake sufficient to exceed Diablo Canyon’s safeguards.