UPDATE 2/8: BYRON (WIFR) -- 23 News has learned federal investigators at the Byron Nuclear Power Plant are looking at a possible design problem which could affect safety at the plant and may have been partly responsible for last weeks emergency shutdown.

You may remember last Monday when reactor two at the plant went offline. At the time Exelon said there was an electrical power fluxuation and the plant went offline automatically.

Now Nuclear Regulatory Commission spokesperson Viktoria Mitlyng tells us investigators are concerned that the automatic safety switch was actually thrown manually. Tonight the NRC is questioning if there is a design problem with Byron and its sister plant at Braidwood.

Initial reports say a worker fortunately noticed the power fluxuation and cut power--a job which was designed to have been done automatically. Exelon also is investigating and said they expect to respond tomorrow.

BYRON, Ill. (AP) -- A northern Illinois nuclear plant is back up and running after a reactor lost power and shut down last week.

Exelon Nuclear says the Unit 2 reactor at the Byron Generating Station began producing electricity at about noon Tuesday after it was reconnected to the electrical grid.

An outage on Jan. 30 started when an electrical insulator failed and fell off the metal structure it was attached to. That interrupted power and caused the reactor to automatically shut down as a precaution.

Exelon says technical experts did hundreds of maintenance and inspection tasks while the unit was offline.

The Byron Generating Station is about 95 miles northwest of Chicago.