PERRY -- Workers refueling the Perry nuclear power plant were exposed briefly to unexpectedly high radiation, federal regulators said Tuesday.

The Nuclear Regulatory Commission on Monday sent a team of radiation safety inspectors to the Lake County power plant owned by FirstEnergy Corp.

The inspectors want to to figure out what happened and whether mistakes were made.

The incident occurred Friday as four contract workers pulled a radiation monitor from the reactor core. The plant shut down April 18 for refueling and maintenance.

While removing the monitor, workers identified an increase in radiation, the NRC said in a statement late Tuesday. "The workers stopped and immediately left the area when the higher-than-expected levels were identified," NRC spokeswoman Viktoria Mitlyng said.

Company spokesman Todd Schneider said the contractors "did not use the proper method to remove the monitor. We need to improve our oversight. "

He said the highest dose of radiation received by one of the workers was equal to about three x-rays.

In March, the agency notified FirstEnergy that Perry had not improved its three-year record as a power plant with "human performance issues," meaning workers make too many simple mistakes.