The Nuclear Regulatory Commission is withholding information from the public about the threat of flooding at U.S. reactors downstream from large dams and reservoirs, an NRC engineer is accusing.

Richard Perkins, with NRC's Division of Risk Analysis, said the commission pulled information from a key study that outlined the danger of flooding at certain plants to cover up the agency's failure to adequately address the safety issue. Perkins' accusations come in a Sept. 14 letter first reported by The Huffington Post.

Perkins told NRC's inspector general the commission issued a heavily redacted version of the July 2011 report in response to a Freedom of Information Act request, citing "security sensitivities." Perkins said text pulled from the study, emails, internal NRC documents and emails with other agencies show NRC's reasoning was not sound.

"The Nuclear Regulatory Commission staff may be motivated to prevent the disclosure of this safety information to the public because it will embarrass the agency," Perkins said. "The redacted information includes discussion of, and excerpts from, NRC official agency records that 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 also said that the issue is "not related to a technical opinion or distinction" and that NRC violated the law.

NRC's concerns about flooding have mainly focused on the Oconee Nuclear Station in South Carolina and the Fort Calhoun Station in Nebraska. The Fort Calhoun plant has been shut down since April, when flooding along the Missouri River forced its closure (Greenwire, Dec. 14, 2011).

Eliot Brenner, a spokesman for NRC, said he could not discuss why the information was redacted from the report but said the commission coordinated with the Department of Homeland Security, Federal Energy Regulatory Commission and U.S Army Corps of Engineers on the sensitivity of redacted information in the report.

Language in the report also explained that "due to the sensitive nature of some information in this analysis, redactions are necessary in this public version."

In March, NRC announced it would begin evaluating dangers associated with the failure of dams upstream from U.S. reactors, noting that the commission believed the issue deserved a closer look. Nuclear plants are designed to withstand rare flooding events, including those from dam failure, that can occur after an earthquake, internal erosion or operational failure, according to NRC.

Although the commission had been looking at the problem for years and had identified no "immediate safety concerns," the agency said it was reviewing the threat following the magnitude-9 earthquake and tsunami at Japan's Fukushima Daiichi plant in March 2011.

The agency also said that new information from plant inspections and re-evaluations had shed additional light on the issue and that "flooding effects in some cases may be greater than previously expected."

Former Republican NRC Chairman Dale Klein said the commission wouldn't cover up information but instead needs to be sensitive about what information is leaked to terrorists. "The NRC doesn't withhold information, but they certainly wouldn't want to give others hints of what they might do to cause hypothetical problems," Klein said.

But David Lochbaum, director of the Union of Concerned Scientists' Nuclear Safety Project, said Perkins and other NRC officials had been working on the report since 2006 and thought the agency needed to address the issue after the Fukushima crisis. NRC's motivation for pulling information from the report is questionable, Lochbaum said, and a spotlight needs to be shined on the issue to ensure the dams are secure and reactors are safe.

Lochbaum also said NRC's decision to address dam failures through a "generic issue" could take decades to complete, even though the threat is more immediate. "We'd like to get a spotlight on this; otherwise, it could take a decade to get fixed," he said.