CO2 alarms forced workers to temporarily evacuate a turbine building at Pennsylvania's Beaver Valley nuclear plant Friday morning.

Unit 2 declared an unusual event, the Nuclear Regulatory Commission's lowest emergency designation, for about an hour and a half. In that time, according to the NRC, crews found the CO2 had come from a turbine fire protection system, which was soon isolated. The gas could no longer be detected by 8:51 a.m., and no fires or injuries were reported. The reactor continued to run at full power through the event.

An NRC spokesman told the Pittsburgh Tribune-Review that the turbine building was evacuated as a precaution, and the plant's fire brigade responded to the alarm.

FirstEnergy operates two Westinghouse pressurized water reactors at the plant in Shippingport, Pa., that together produce about 1,800 megawatts. Unit 2 at the plant had returned to service June 9 after unusual vibration in the turbine caused by loose wires prompted an outage 12 days earlier. The reactor underwent refueling and repairs last fall that included reactor pressure vessel head and turbine work.