Investigators are working to determine why one of the two reactors at the Prairie Island nuclear plant near Red Wing, Minn., shut down automatically Thursday after a turbine problem triggered fire alarms.

While no fire was reported, protocol requires the plant to declare an "unusual event" if it takes more than 15 minutes for operators to visually confirm no such event occurred.

Federal officials with the Nuclear Regulatory Commission were notified of the problem just after 1:15 p.m. Thursday.

Plant personnel terminated the declaration by 3 p.m.

An "unusual event" is the lowest of four emergency classifications for nuclear plants, said NRC spokeswoman Viktoria Mitlyng.

No employees were evacuated from the Xcel Energy-owned facility and no one was injured.

"There was no danger to the workers or the public," she said.

As of 5 p.m., Unit 2 remained offline, while Unit 1 was running at full power, she said.

Investigators remained on the site late Thursday to determine why the fire alarms sounded.

Unit 2 also shut down in June for repairs after a turbine trip caused by low oil pressure.

The two pressurized water reactors at Prairie Island generate 1,076 megawatts of electricity, which is enough to power nearly 1 million homes.