A failed piece of equipment at a switchyard at the Byron Generating Station caused the Exelon plant to shut down Monday morning.
Company officials say steam containing low levels of radioactivity was vented to reduce pressure after the power loss. Diesel generators were supplying the reactor with electricity.
State officials say they do not expect to find hazardous levels of radioactive tritium, but they are checking the area.
The incident has been declared an 'unusual event' by officials. That is the lowest of four levels of emergency.