Workers at the Beaver Valley Nuclear Power Station in Shippingport, Pa., about 35 miles northwest of Pittsburgh, were alerted to a problem in the turbine-generator area. A second alarm sounded shortly after signaling a CO2 discharge, prompting the evacuation, the Pittsburgh Tribune-Review reported.
The plant's fire brigade responded in accordance with federal regulations and cleared the scene about 9:55 a.m. Monday, a little more than an hour after the alarms went off.
Regulators from the Nuclear Regulatory Commission classified the incident as an "unusual event" -- the lowest of four classifications for technical malfunctions at a nuclear power plant.
The cause of the discharge remained under investigation.