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An internal document quietly released this week by the agency, however, suggests that some NRC employees with extensive knowledge of the threat believe the agency is violating its own regulatory mission by allowing at least one reactor to stay online, given the potential for flooding that would likely hobble core cooling equipment and possibly trigger a meltdown.

The odds of this happening, of course, are small. But the NRC is reviewing a variety of risks at United States nuclear power plants in the aftermath of the 2011 meltdown at the Fukushima-Daiichi facility in Japan, which succumbed to a rare but devastating earthquake and subsequent tsunami. Among the risks being considered by the agency is the potential at several U.S. plants on inland waterways to suffer damage if upstream dams are breached or otherwise fail.

Concerns that the NRC was not acting swiftly or decisively enough on the upstream dam risk were raised late last year by two other engineers inside the agency. Steps are being taken to shore up flood defenses at several plants, but no facility has been shuttered by regulators as a result of the heightened scrutiny.

In a memo dated March 19 and posted to the NRC website on Monday, Carl F. Lyon and Patrick D. Milano, both members of the NRC’s Office of Nuclear Reactor Regulation, argue that a rare but consequential rainfall is all that would be needed to disable cooling systems for a reactor at the Watts Bar nuclear station — referred to as “WBN Unit 1” — near Spring City, Tenn.

“Only the probable maximum precipitation event, without a concurrent earthquake or tsunami or any other accident,” the two employees wrote, “is needed at WBN Unit 1 to produce a loss of all decay heat removal systems.”…… http://www.huffingtonpost.com/2013/03/29/nuclear-power-flood-risk_n_2979098.html