Federal investigators have launched a probe into whether the Nuclear Regulatory Commission erred when it let Pacific Gas and Electric Co. change earthquake safety standards at the Diablo Canyon power plant without public hearings, The Chronicle has learned.
The regulatory agency’s own internal watchdog — the Office of the Inspector General — has been delving into the issue, which is the subject of a lawsuit filed in the fall by environmentalists trying to close Diablo Canyon, California’s last nuclear plant.
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The plant was originally designed to safely shut down after an earthquake producing ground movement as intense as 0.4 times the force of gravity. But in 1971, geologists working for Shell Oil Co. identified a fault just 3 miles offshore, the Hosgri Fault, capable of creating ground motions at the plant up to 0.75 the force of gravity.
PG&E concluded that Diablo Canyon could survive the extra shaking. But the methods PG&E used to measure the threat posed by the Hosgri were different from — and less conservative than — the methods used to set the plant’s initial seismic safety standards.
In 2011, after other faults had been found nearby, PG&E asked the commission to amend its license to stipulate that the plant could safely shut down after ground motions of 0.75 the force of gravity, based on that less conservative Hosgri Fault methodology. The commission refused and told PG&E to withdraw its request. Still, PG&E in 2013 changed the plant’s final safety analysis report — a document required by Diablo’s license — to say it can withstand such a quake using the Hosgri Fault methodology.
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Appearance of collusion
Both Friends of the Earth and the alliance complained last year about the appearance of collusion on the Sept. 10 documents. At the time, a commission spokeswoman denied the accusations and said the matter would be forwarded to the inspector general’s office for review. Becker said Wednesday she was pleased to see the investigators follow through.
“We are heartened that the Nuclear Regulatory Commission is taking the time to take our concerns seriously,” she said. “We are skeptical that the commission will act on what they find. But we’re always hoping to be pleasantly surprised.”
Read more at Feds probe PG&E report on California nuclear plant safety
