Operators at a nuclear plant declared an emergency for several hours early Monday when instruments indicated a problem during a planned shutdown, but no damage was done, officials said.

There were no injuries, no radiation released and no equipment damaged at the LaSalle Generating Station in LaSalle County, officials said. The plant, which is owned by Chicago-based Exelon Corp., is about 55 miles (90 kilometers) southwest of Chicago.

The nature of the incident — a control rod not going into the reactor — automatically triggered the declaration of a "site area emergency," but "it never really progressed into being a danger," said Patti Thompson, spokeswoman for the Illinois Emergency Management Agency.

A site area emergency is the second-highest of the four categories in the federal Nuclear Regulatory Commission's emergency response system. Emergency operations centers were activated in LaSalle and Grundy counties, Thompson said.

The plant was scheduled to shut down early Monday for a refueling outage, but it did not shut down properly, officials said.

Company officials said instruments showed three of the 185 control rods failed to insert fully into the reactor core and operators declared a "site area emergency" at 12:28 a.m.

Operators reset the control rod position indication system and then found one rod was out of position, company officials said.

The investigation by Exelon Nuclear officials and NRC officials was continuing.