The 500-megawatt Fort Calhoun Nuclear Generating Station, 19 miles from Omaha, "is still protected," Mike Jones, spokesman for plant owner-operator Omaha Public Power District, told CNN.
"This was an additional, a secondary, level of protection that we had put up," Jones said. "The plant remains protected to the level it would have been if the aqua berm had not been added."
More than 2 feet of water rushed in around buildings and electrical transformers as the swollen Missouri River overflowed its banks and rushed over an 8-foot-high, 2,000-foot-long berm that collapsed after being punctured by a piece of heavy equipment, Jones said.
The chances of floodwaters getting into the building where the nuclear reactor core is kept are almost zero, he said.
The core contains the nuclear fuel components where nuclear reactions take place.
The Omaha Public Power District was looking into the possibility of patching and refilling the temporary water dam, the Omaha World-Herald reported.
U.S. nuclear regulatory official toured the plant Monday with U.S. Rep. Jeff Fortenberry, R-Neb., and Rep. Lee Terry, also a Nebraska Republican.
"It's pretty jarring to see a boat tied up to the nuclear power plant. ... It's an intense operation going on there, particularly with water surrounding all the buildings," Fortenberry said. "There's no water inside; they have multiple, redundant systems in place."
The river, overflowing due to heavy rainfall and melting snow from the Rocky Mountains, is not expected to rise higher than the level the plant was designed to protect against, the Omaha power district said.
Nuclear Regulatory Commission Chairman Gregory Jaczko was scheduled to visit the plant Monday.
The plant has been shut down for refueling since April 7 and won't be restarted until floodwaters recede, the NRC said.
It was found last year to have deficiencies in procedures to protect against flooding, but those were corrected early this year, the NRC said.
The plant was running from an offsite power supply early Monday, the Omaha power district said. Its power supply was cut off and emergency generators temporarily powered the plant when the floodwaters surrounded the main electrical transformers, the district said.
Nuclear power plants need electricity, even when shut down, to keep key components cool to avoid any degradation or melting of the core that could result in the release of radiation.
Catastrophic flooding after Japan's March 11 earthquake and tsunami knocked out cooling systems at the Fukushima Daiichi nuclear power plant, resulting in three reactors melting down and producing the worst nuclear disaster since Chernobyl.