Fort Calhoun Nuclear Power Plant flooded by Missouri river.

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The Fort Calhoun nuclear power plant north of Omaha, Neb., is encircled by the swollen waters of the Missouri River, which reached a height of nearly 1,007 feet above sea level at the plant yesterday.

The NRC says floodwaters could reach 1,014 feet if the Gavins Point Dam upriver on the Missouri in South Dakota were breached.

http://www.nytimes.com/cwire/2011/06/24/24climatewire-a-nuclear-plants-f...

The NRC responded in its October 2010 letter that once flooding reached 1,004 feet, water would have entered the plant and the ability of emergency workers to move around the site would "significantly degrade."

If levels exceeded 1,004 feet, water would reach the lower floodgates, hampering the welding of plates to door frames, the NRC said. At 1,008.5 feet, the technical support center used by emergency technicians would have been inundated. At 1,010 feet, water would begin to enter the auxiliary building, "shorting power and submerging pumps. The plant could then experience a station blackout with core damage estimated within 15 to 18 hours," under a worst-case scenario, the NRC said.

The NRC concluded that the use of the fire truck for emergency pumping would fail, after it determined the truck could not draw floodwaters successfully from the turbine building. Moreover, it was not clear how workers could operate a crane to lift the fire truck into position if outside power were lost, the NRC staff added.

NRC inspectors concluded that at flooding levels above 1,008 feet, the plant "would experience a loss of offsite power and loss of intake structure" and water pumps providing essential cooling water to the plant. In that case, "the plant would be incapable of reaching cold shutdown" with normal operations -- a fundamental safety requirement imposed by the NRC. The commission's Region IV office in Arlington, Texas, issued a notice of violation against the plant on Oct. 6 last year, finding that the issues were of "substantial importance" to the plant's safety.

The plant's defenses include new steel gates and other hard barriers protecting an auxiliary building with vital reactor controls, and a water-filled berm 8 feet tall that encircles other parts of the plant. Both systems are designed to hold back floodwaters reaching 1,014 feet above sea level. Additional concrete barriers and permanent berms, more sandbags and another power line into the plant have been added. The plant was shut down in April for refueling and will remain so until the flood threat is passed.

But a year ago, those new defenses were not in place, and the plant's hard barriers could have failed against a 1,010-foot flood, the Nuclear Regulatory Commission contends in a yearlong inspection and enforcement action against the plant's operator, the Omaha Public Power District (OPPD).

The NRC has not completed its evaluation of the new defenses installed at Fort Calhoun, nor has it resolved how OPPD handled the new information about flood threat that the NRC says the utility received via the Corps of Engineers.