Hackers breached at least a dozen US power plants in attacks in May and June, US media report, citing intelligence officials.
The targets included the Wolf Creek nuclear facility in Kansas, according to several reports.
An urgent Department of Homeland Security (DHS) report indicated a foreign power, possibly Russia, was responsible, the New York Times said.
The DHS document carried the second-highest threat rating, the Times said.
Wolf Creek Nuclear Operating Corp declined to say if the plant was hacked but said there had been “no operational impact” at the plant.
“The reason that is true is because the operational computer systems are completely separate from the corporate network,” spokeswoman Jenny Hageman told Reuters.
In a joint statement with the FBI, a spokesman for the Department of Homeland Security said there was “no indication of a threat to public safety”.