(CNN) -- Tuesday's Virginia earthquake triggered the shutdown of a nearby nuclear power plant and spurred declarations of "unusual events" at plants as far away as Michigan, U.S. authorities reported.
Dominion Virginia Power said both reactors at its North Anna plant, less than 20 miles from the epicenter of the magnitude-5.8 quake, shut down automatically after the first tremors.
Amanda Reidelbach, an emergency management spokeswoman for Louisa County, said the plant vented steam, but there was no release of radioactive material.
David Heacock, the utility's chief nuclear officer, said the 1,800-megawatt plant was operating on emergency power and the two pressurized-water reactors were safely deactivated.
"The plants are designed for this kind of a seismic event," Heacock said. "There is no apparent damage to anything at the plant right now."
Crews were working to restore off-site power to the plant Tuesday evening, the utility said.
Dan Stoddard, senior vice president of nuclear operations for Dominion, said some power from the grid had been restored to the reactors and it was possible full power could be achieved late Tuesday. It was not clear if a 4.2-magnitude aftershock Tuesday evening would affect the operation.
Stoddard said there was no damage to the spent fuel pool.
The diesel generators supplying backup power have enough fuel to operate 30 days and more can be brought in, if needed, Stoddard told CNN
"The power plant is safe," Stoddard said.
One of those generators failed a few minutes after it kicked in, said Joey Ledford, a spokesman for the Nuclear Regulatory Commission's regional office in Atlanta. But only one generator is required to supply power to each reactor unit, leaving another unit in reserve, Ledford said.
"They're in good shape at this point," he said.
Stoddard said even with the failure of one generator, Dominion has four others in use or available.
The North Anna plant is about 50 miles northwest of Richmond and about 90 miles southwest of Washington. Operators declared an alert -- the second-lowest level of emergency reporting under U.S. nuclear regulations -- after the quake struck shortly before 2 p.m., the Nuclear Regulatory Commission said.
NRC inspectors went over the North Anna plant as part of a review of U.S. nuclear facilities after the disaster at Japan's Fukushima Daiichi, where three reactors melted down following the massive earthquake and tsunami that struck the plant March 11. They found that diesel generators needed to provide emergency power were functional and "readily available for short-term actions," and the utility moved quickly to replace one portable generator that was not working at the time, according to the NRC's inspection report.
At Fukushima Daiichi, the worst nuclear accident since Chernobyl, cooling systems and backup generators were knocked out by the 15-meter (48-foot) tsunami that swamped the plant shortly after the magnitude-9 earthquake.
Dominion Virginia said reactors at its other nuclear station, the Surry plant near Newport News, were still running.
Surry and 11 plants in five other states issued an "unusual event" declaration, the lowest level of emergency notice, according to the NRC. They are the Shearon Harris plant in North Carolina; the Calvert Cliffs plant in Maryland; Pennsylvania's Susquehanna, Three Mile Island, Limerick and Peach Bottom plants; the Oyster Creek, Hope Creek and Salem plants in New Jersey; and the D.C. Cook and Palisades plants in Michigan.
"All these plants continue to operate while plant personnel examine their sites," the NRC said.
CNN's Emily Smith, Brian Todd, Jeanne Meserve, Scott Bronstein, Shawn Nottingham and Matt Smith contributed to this report.