The Nuclear Regulatory Commission says Xcel Energy failed to maintain an adequate flood plan for its Monticello nuclear generating plant.
The Monticello plant has the same design as the plant in Fukushima, Japan, that melted down after an earthquake and tsunami.
The NRC has been urging American utilities with similar plants to upgrade their disaster preparations.
The agency notified Xcel that its flood plan did not assure flood protection measures could be completed within 12 days.
However, in February Xcel brought materials on the site to build a berm around the plant in case of a flood. The NRC acknowledges there is no current safety concern. The NRC says the earlier failure is likely to result in extra inspections and potentially other actions by the agency.
In a statement, Xcel wrote that safety and security at its nuclear plants are top priorities.