A berm holding the flooded Missouri River back from a Nebraska nuclear power station collapsed early Sunday, but US federal regulators said they were monitoring the situation and there was no danger.

An aerial view of the Fort Calhoun Nuclear Power Plant in eastern Nebraska, surrounded by Missouri River flood waters June 24, 2011. [Agencies]

The Fort Calhoun Nuclear Station shut down in early April for refueling, and there is no water inside the plant, the US Nuclear Regulatory Commission said. Also, the river is not expected to rise higher than the level the plant was designed to handle. NRC spokesman Victor Dricks said the plant remains safe.

The federal commission had inspectors at the plant 20 miles (32 kilometers) north of Omaha when the 2,000-foot (600-meter) berm collapsed about 1:30 am Sunday. Water surrounded the auxiliary and containment buildings at the plant, it said in a statement.

The Omaha Public Power District has said the complex will not be reactivated until the flooding subsides. Its spokesman, Jeff Hanson, said the berm wasn't critical to protecting the plant but a crew will look at whether it can be patched.

"That was an additional layer of protection we put in," Hanson said.

The berm's collapse didn't affect the reactor shutdown cooling or the spent fuel pool cooling, but the power supply was cut after water surrounded the main electrical transformers, the NRC said. Emergency generators powered the plant until an off-site power supply was connected Sunday afternoon, according to OPPD.

NRC Chairman Gregory Jaczko will tour the plant Monday. His visit was scheduled last week. On Sunday, he was touring Nebraska's other nuclear power plant, which sits along the Missouri River near Brownville.

Both nuclear plants issued flooding alerts earlier this month, although they were routine as the river's rise has been expected. The Brownville plant has been operating at full capacity.

Flooding remains a concern all along the Missouri because of massive amounts of water the US Army Corps of Engineers has released from upstream reservoirs. The river is expected to rise as much as 5 to 7 feet (1.5 to 2.1 meters) above flood stage in much of Nebraska and Iowa and as much as 10 feet (3 meters) over flood stage in parts of Missouri.

The corps expects the river to remain high at least into August because of heavy spring rains in the upper Plains and substantial Rocky Mountain snowpack melting into the river basin.