Fort Calhoun Nuclear Station, owned by the Omaha Public Power District, is one of three U.S. nuclear power plants that will get increased oversight from regulators because of safety problems and unplanned shutdowns. The primary concern at the station 20 miles north of Omaha appears to be possible flooding.

The other two are the H.B. Robinson nuclear plant in South Carolina and Wolf Creek in Kansas, the Nuclear Regulatory Commission said.

Commission Chairman Gregory Jaczko said the three "are the plants we are most concerned about."

The United States has 104 nuclear reactors at 65 nuclear power plants.

The NRC said all 104 reactors operate safely, and the heightened review of the three plants is routine.

"The NRC felt the three required significant additional oversight but continue to operate safely," said Scott Burnell, an agency spokesman.

Fort Calhoun, on the Missouri River, is one of two nuclear plants in Nebraska. The other is Cooper Nuclear Station near Brownville, which is owned by the Nebraska Public Power District.

OPPD spokesman Jeff Hanson said Fort Calhoun has been on the NRC's "degraded cornerstone" watch list for almost a year because of a potential flooding issue.

He said the utility has procedures in place to protect a water-pumping station near the Missouri River in case of a 100-year flood using sandbags, but they were not in writing.

"We did not have specific instructions on how to sandbag," Hanson said.

During the past year, he said, OPPD installed floodgates to protect the pumping station.

Last year, the plant experienced some minor flooding when high water from the Missouri River caused a drain to back up. Hanson said the plant was never in any danger.

The NRC plans to inspect the plant soon, Hanson said, adding that Fort Calhoun will remain on the agency's watch list for a year after the inspection.

All U.S. nuclear plants are inspected frequently. If enough minor problems or issues are identified, a plant moves to a second level of inspection, Burnell said.

Items that aren't resolved in a reasonable time -- or new items of higher significance -- can move a plant to a third level of closer inspection and oversight. That is where the three plants in South Carolina, Kansas and Nebraska are listed, Burnell said.

The Associated Press contributed to this story.