Duke Energy’s Oconee nuclear plant in South Carolina faces more flood risk from an upstream dam than earlier estimated, the Nuclear Regulatory Commission says.

Estimates of flooding if the 385-foot-high Jocassee Dam failed “have increased substantially,” says a 2011 NRC report, and could plunge Duke’s biggest nuclear plant into the dire, powerless condition called station blackout.

The NRC removed large portions from the public version of the report, saying it involves critical infrastructure that could become a security target. That led the report’s lead author, an NRC risk analysis engineer, to claim that the agency is hiding embarrassing information.

The blacked-out parts of the report, engineer Richard Perkins wrote the NRC’s inspector general last month, “show the NRC has been in possession of relevant, notable, and derogatory safety information for an extended period but failed to properly act on it.”

Perkins said Thursday he couldn’t talk about the redacted material. The Huffington Post first reported his charge.

Duke says Oconee is safe and has taken steps to reduce flood risks.

NRC spokesman Scott Burnell said the agency stands by its editing.

“We think we took the appropriate approach,” he said. “The report was meant to lay out current understanding about how flooding hazards had been looked at in the past and how they could be considered at this time. The conclusion was that we do need to look at that.”

The NRC began a formal study of the safety implications of dam failure on nuclear plants in March. The issue will also be evaluated in the wake of last year’s nuclear crisis in Japan, which included flooding from a tsunami.

NRC has worked with Duke on flood risks at Oconee for several years, Burnell said. He said the 2011 report focused on Oconee and a second nuclear plant, Fort Calhoun in Nebraska, because recent information on them was available.

In 1995, the report says, Duke estimated the odds that Oconee’s reactor cores could be damaged by flooding after a dam failure at seven in 1,000,000. The report says the agency’s own estimate is an order of magnitude larger, but doesn’t specify the risk.

The Jocassee Dam, just below the state line in South Carolina’s northwestern corner, about 120 miles west of Charlotte, impounds an 8,000-acre lake. The water level at the dam is 300 feet higher than the level of Lake Keowee, where Oconee sits about 11 miles downstream.

Duke spokeswoman Sandra Magee said the utility began updating flood analyses at Oconee several years ago.

Duke added flood structures to the site in 1984 to increase its safety margin, she said. After updated analysis, she said, Duke added and raised flood barriers in 2009 and 2010.

She said the NRC did not mandate the changes, which protected critical structures such as tanks that hold backup cooling water and standby equipment to safely shut the plant down.

“We think we have reduced our risk margin due to flooding,” Magee said.

Dam failure was not evaluated in licensing the three-reactor plant, which opened in 1973. Since then, the NRC report says, analytic methods have improved as development grew in the watersheds around dams.

Oconee County emergency management director Scott Krein said Duke notified him about the NRC report. The county, which had separate plans to deal with a dam failure and a nuclear accident, now prepares for both events simultaneously.

Jocassee Dam failing “would be a catastrophic event for our county, but we have plans to deal with it,” Krein said.