A failed electrical insulator was to blame for the power failure that caused a U.S. nuclear reactor to shut down, said Exelon Energy officials on Tuesday.

The insulator, a piece of protective equipment that helps regulate the flow of electricity in the plant's switchyard, failed on Monday morning and fell off the metal structure to which it was attached. That interrupted power and caused the Unit 2 reactor at the Byron Generating Station to shut down automatically as a precaution.

The U.S. Nuclear Regulatory Commission declared Monday's incident an “unusual event”, the lowest of four levels of emergency.

Officials hoped to replace the part by the end of the day, though it remained unclear how soon the reactor could return to service, spokesman Paul Dempsey said.

It was not immediately clear what caused the insulator to fail, Mr. Dempsey said.

Another reactor at the plant was operating normally.

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