An expert at the Nuclear Regulatory Commission (NRC) has asked the agency to shut down California’s only operating nuclear plant while it studies whether the plant is safe from earthquake risks.

In a 42-page confidential report obtained both by The Associated Press and environmental group Friends of the Earth, Michael Peck, who served as the top on-site inspector for the Diablo Canyon Power Plant, asked the NRC to reconsider the plant’s safety rules.

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Peck did not assert that the plant was unsafe. Instead, he said that the NRC has not formally examined whether or not Diablo Canyon can withstand an earthquake from a nearby fault discovered in 2008.

Allowing the plant, on the shore of the Pacific Ocean about halfway between Los Angeles and San Francisco, to continue to run “challenges the presumption of nuclear safety,” Peck wrote.

Peck filed his report in July 2013 as part of an internal NRC process that allows employees to challenge decisions of their superiors. The NRC has not yet responded to the report, and the agency declined to comment to AP.

Pacific Gas and Electric Co., which operates the Diablo Canyon plant, told the AP the reactors are safe. Any concerns over seismic activity were resolved in the 1970s when it was being constructed, after discovery of another nearby fault prompted new retrofitting, the utility said.

Peck also declined to comment to the AP.

Earthquake safety at nuclear plants has received new attention since 2011, when Japan’s Fukushima Daiichi nuclear power plant suffered multiple meltdowns following an earthquake and tsunami.

Sen. Barbara Boxer Barbara Levy BoxerThe Hill's Morning Report - Presented by Facebook - Biden, Harris launch Trump offensive in first joint appearance Bottom line Polls show big bounce to Biden ahead of Super Tuesday MORE (D-Calif.), chairwoman of the Environment and Public Works Committee, said she was “alarmed” to hear of Peck’s report.

“When I have questioned NRC about seismic issues at Diablo Canyon in the past, the response has been evasive and insufficient,” Boxer said in a statement.

“Damaging earthquakes can occur at any time, and the NRC’s failure to act constitutes an abdication of its responsibility to protect public health and safety.”

Friends of the Earth said the report was troubling.

“Inspector Peck is the canary in the coal mine, warning us of a possible catastrophe at Diablo Canyon before it’s too late,” Damon Moglen, a strategic adviser with the group, said in a statement.

“We agree with him that Diablo Canyon is vulnerable to earthquakes and must be shut down immediately,” he said. “Rather than the NRC keeping this a secret, there must be a thorough investigation with public hearings to determine whether these reactors can operate safely.”