The Fort Calhoun Nuclear Station turned to diesel-powered generators Sunday after disconnecting from the main grid because of rising floodwaters.
That move came after water surrounded several buildings when a water-filled floodwall collapsed.
The plant, about 19 miles north of Omaha, remains safe, Omaha Public Power District officials said Sunday afternoon.
Sunday's event offers even more evidence that the relentlessly rising Missouri River is testing the flood worthiness of an American nuclear power plant like never before. The now-idle plant has become an island. And unlike other plants in the past, Fort Calhoun faces months of flooding.
Floodwater surrounded the nuclear plant's main electrical transformers after the Aqua Dam, a water-filled tubular levee, collapsed, and power was transferred to emergency diesel generators.
OPPD officials said the transfer was precautionary because of water leaking around the concrete berm surrounding the main transformers.
Plant operators later reconnected to off-site power once all safety checks had been completed.
Water now surrounds the auxiliary and containment buildings, which are designed to handle flooding up to 1,014 feet above sea level. The river is at 1,006.3 feet and isn't forecast to exceed 1,008 feet.
The U.S. Nuclear Regulatory Commission is monitoring the Missouri River at the plant, which has been shut down since early April for refueling. The Fort Calhoun plant will remain surrounded at least through August as the U.S. Army Corps of Engineers continues dumping unprecedented amounts of water from upstream dams.
The 2,000-foot berm collapsed about 1:25 a.m. Sunday due to “onsite activities,” OPPD officials said. The Aqua Dam provided supplemental flood protection and was not required under NRC regulations.
“We put up the aqua-berm as additional protection,” said OPPD spokesman Mike Jones. “(The plant) is in the same situation it would have been in if the berm had not been added. We're still within NRC regulations.”
The NRC says its inspectors were at the plant when the berm failed and have confirmed that the flooding has had no impact on the reactor shutdown cooling or the spent fuel pool cooling.
The NRC said there is a separate, earthen berm to protect the electrical switchyard and a concrete barrier surrounding electrical transformers.
Last week, the NRC augmented its inspection staff at Fort Calhoun. In addition to the two resident inspectors, three more inspectors and a branch chief were added to provide around the clock coverage of plant activities.
Both the plant's operator, the Omaha Public Power District, and federal regulators say the plant remains safe.