MINNEAPOLIS (AP) - The Nuclear Regulatory Commission says one of the two reactors at Xcel Energy's Prairie Island nuclear plant shut down automatically after a problem developed in the unit's turbine.

NRC spokeswoman Viktoria Mitlyng says officials and NRC inspectors at the plant are looking into why the turbine went offline Thursday. When that happened, she says, fire alarms went off in the control room.

Because it took more than 15 minutes for plant operators to confirm that there was no fire, she says, they had to report it to the NRC as an "unusual event," which they did around 1:15 p.m.

Mitlyng says they're investigating why the fire alarms went off. But she says the plant is safe.

An Xcel Energy spokeswoman says the company is preparing a statement.