Top 10 Stories of 2011: No. 5 — Diablo Canyon nuclear power plant reacts to Japan’s Fukushima disaster

For many, 2011 will be remembered as the year of Fukushima.

On March 11, a magnitude-9.0 earthquake struck Japan. The powerful quake coupled with a large tsunami that followed overwhelmed the Fukushima Dai-ichi nuclear power plant, crippling four of its six reactors, causing radiation releases and leading to the evacuation of 80,000 people near the plant.

The accident left many residents of San Luis Obispo County wondering whether a similar accident would be possible at Diablo Canyon nuclear power plant, located along California’s earthquake-prone Central Coast. It also greatly intensified calls for PG&E to suspend plans to renew the plant’s two operating licenses until a thorough seismic investigation of the earthquake faults around the plant is completed.

In June, the utility and the Nuclear Regulatory Commission bowed to the pressure, and the relicensing request was suspended, pending completion of the seismic studies. Now, December 2015 is the earliest Diablo Canyon could be relicensed.

Similarly, the state Public Utilities Commission has closed a request by PG&E to recoup $85 million from ratepayers to pay for relicensing.

“We can now focus on making sure the seismic studies are well designed and independently peer-reviewed at every step of the way,” said Rochelle Becker of the Alliance for Nuclear Responsibility, which urged the Public Utilities Commission to close the request rather than just suspend it.

PG&E has finished two of three years of intense seismic fieldwork focusing on the four major faults around the plant — the Hosgri, Shoreline, San Luis Bay and Los Osos faults. Two-dimensional onshore and offshore surveys have been completed.

Late next year, PG&E will conduct three-dimensional surveys of a large area of ocean from Point San Luis north to Cambria. Because they can potentially harm sea life, these high-energy studies require a special permit from the State Lands Commission.

In all, these studies will cost PG&E ratepayers $64 million. An independent panel of geologists, including county Supervisor Bruce Gibson, will peer review the studies.

In addition to postponing relicensing, other safety steps have been taken at Diablo Canyon in the aftermath of the Fukushima disaster. The NRC conducted an inspection at Diablo Canyon and all other nuclear plants in the nation looking at preparedness for extreme events, such as the one in Japan.

The agency maintains that a similar event in the United States is highly unlikely and all plants, including Diablo Canyon, are designed to withstand the type of earthquakes nearby faults are likely to produce.

The nuclear industry has determined that the tsunami, not the earthquake itself, was responsible for most of the damage sustained by the Fukushima plant. Specifically, the tsunami inundated the plant’s backup safety equipment, leaving the facility without power.

For this reason, much of the emphasis since March 11 has been on improving flood protection. For example, watertight seals on cooling water pumps at Diablo Canyon have been improved to better withstand flooding.

Perhaps Diablo Canyon’s best defense against a disastrous tsunami is the geography around the plant. Faults near the plant are strike-slip faults in which tectonic plates move horizontally past one another.

This type of ground movement does not displace large amounts of ocean water, the cause of a tsunami. Additionally, the plant sits atop an 85-foot-tall bluff, an elevation above where a tsunami is likely to reach.

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