FILE - In this June 14, 2011 file photo, the Fort Calhoun nuclear power plant is surrounded by the rising waters from the Missouri River in Fort Calhoun, Neb. (AP Photo/Nati Harnik, File)

In a letter submitted Friday afternoon to internal investigators at the Nuclear Regulatory Commission, a whistleblower engineer within the agency accused regulators of deliberately covering up information relating to the vulnerability of U.S. nuclear power facilities that sit downstream from large dams and reservoirs.

The letter also accuses the agency of failing to act to correct these vulnerabilities despite being aware of the risks for years.

These charges were echoed in separate conversations with another risk engineer inside the agency who suggested that the vulnerability at one plant in particular -- the three-reactor Oconee Nuclear Station near Seneca, S.C. -- put it at risk of a flood and subsequent systems failure, should an upstream dam completely fail, that would be similar to the tsunami that hobbled the Fukushima Daiichi nuclear facility in Japan last year. That event caused multiple reactor meltdowns.

In the letter, a copy of which was obtained by The Huffington Post, Richard H. Perkins, a reliability and risk engineer with the agency's division of risk analysis, alleged that NRC officials falsely invoked security concerns in redacting large portions of a report detailing the agency's preliminary investigation into the potential for dangerous and damaging flooding at U.S. nuclear power plants due to upstream dam failure.

Perkins, along with at least one other employee inside NRC, also an engineer, suggested that the real motive for redacting certain information was to prevent the public from learning the full extent of these vulnerabilities, and to obscure just how much the NRC has known about the problem, and for how long.

"What I've seen," Perkins said in a phone call, "is that the NRC is really struggling to come up with logic that allows this information to be withheld."

Perkins was the lead author of the report, which was completed in July of 2011 -- roughly four months after an earthquake and subsequent tsunami flooded the Fukushima Daiichi nuclear power plant in Japan, cut off electric power to the facility and disabled all of its backup power systems, eliminating the ability to keep the reactors cool and leading to a meltdown.

In addition to the Oconee facility, the report examined similar vulnerabilities at the Ft. Calhoun station in Nebraska, the Prairie Island facility in Minnesota and the Watts Bar plant in Tennessee, among others.

The report concluded, among other things, that the failure of one or more dams sitting upstream from several nuclear power plants "may result in flood levels at a site that render essential safety systems inoperable." High floodwaters could conceivably undermine all available power sources, the report found, including grid power, emergency diesel backup generators, and ultimately battery backups. The risk of these things happening, the report said, is higher than acceptable.

"The totality of information analyzed in this report suggests that external flooding due to upstream dam failure poses a larger than expected risk to plants and public safety," Perkins's report concluded, adding that the evidence warranted a more formal investigation.

In response to the report, the NRC launched an expanded investigation, which is ongoing. It also folded the dam failure issue into the slate of post-Fukushima improvements recommended by a special task force formed in the aftermath of that disaster. But in a press release dated March 6 of this year, the agency said the report "did not identify any immediate safety concerns."

The NRC made a heavily redacted copy of the report publicly available on the NRC website the same day.

"Nuclear power plant designs include protection against serious but very rare flooding events, including flooding from dam failure scenarios," the agency release noted. "Dam failures can occur as a consequence of earthquakes, overflow, and other mechanisms such as internal erosion and operational failures. A dam failure could potentially cause flooding at a nuclear power plant site depending on a number of factors including the location of the dam, reservoir volume, dam properties, flood routing, and site characteristics."

At the time of the report's public release, the agency was also weighing a response to a Freedom of Information Act request for related documents from a reporter with the Cascadia Times of Portland, Ore. Two months ago, the agency responded in part to that request by issuing another, similarly redacted version of the report, which was also published at the NRC website.

In justifying the redactions, the agency argued, among other things, that the report contained propriety commercial information, that release of the redacted information would "harm an identifiable private or governmental interest," and that "disclosure could reasonably be expected to endanger the life or physical safety of an individual."

In his letter to the NRC's inspector general, however, Perkins argued that these justifications were invalid.

"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 wrote in his letter. "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. Concurrently, the NRC concealed the information from the public."

In a conversation with The Huffington Post, Perkins elaborated on the redacted material. "My estimation is that if people saw the information that we have, and if they knew for how long we've had it, some might be disappointed at how long it's taken to act, and some might be disappointed that, to date, we haven't really acted at all."

The agency acknowledged in its March 6 release that the issue relating to upstream dams "came to the staff’s attention long before the current interest in natural disasters raised by the Japan earthquake/tsunami and reactor accident." But, the release added, "new sources of information on this issue have accumulated over the past few years. This information includes inspections of flood protection and related procedures, as well as recent re-evaluations of dam failure frequencies and possible flood heights at some U.S. nuclear power plants, suggesting that flooding effects in some cases may be greater than previously expected."

As for the redactions and Perkins's formal complaint, Eliot Brenner, a spokesman for the NRC, said he could not comment on complaints submitted to, or investigations undertaken by the agency's inspector general. But in an email message, Brenner pointed out that "the NRC coordinated with the Department of Homeland Security, Federal Energy Regulatory Commission, and U.S Army Corps of Engineers on necessary redactions" in the report.

A second risk engineer at NRC who is familiar with the unredacted report, and who requested anonymity for concern over reprisals at the agency, told The Huffington Post that the Department of Homeland Security, for its part, had signed-off on releasing the report without redactions. This, the engineer said, undermines arguments made by some agency officials that certain information should be withheld because the upstream dam vulnerability could be exploited by terrorists, who might target dams in order to precipitate a flood and subsequent meltdown at a downstream nuclear facility.

If this were truly such a security concern, however, it would be incumbent on the agency to act swiftly to eliminate that threat, the engineer stated. As it is, the engineer suggested, no increased security actions have been undertaken.

Meanwhile, the engineer is among several nuclear experts who remain particularly concerned about the Oconee plant in South Carolina, which sits on Lake Keowee, 11 miles downstream from the Jocassee Reservoir. Among the redacted findings in the July 2011 report -- and what has been known at the NRC for years, the engineer said -- is that the Oconee facility, which is operated by Duke Energy, would suffer almost certain core damage if the Jocassee dam were to fail. And the odds of it failing sometime over the next 20 years, the engineer said, are far greater than the odds of a freak tsunami taking out the defenses of a nuclear plant in Japan.

"The probability of Jocassee Dam catastrophically failing is hundreds of times greater than a 51 foot wall of water hitting Fukushima Daiichi," the engineer said. "And, like the tsunami in Japan, the man‐made 'tsunami' resulting from the failure of the Jocassee Dam will –- with absolute certainty –- result in the failure of three reactor plants along with their containment structures.

"Although it is not a given that Jocassee Dam will fail in the next 20 years," the engineer added, "it is a given that if it does fail, the three reactor plants will melt down and release their radionuclides into the environment."