The 883-megawatt plant, which was at 100 percent power at the time, shut down safely at 10:24 a.m. While the Nuclear Regulatory Commission was notified, there was no emergency, FirstEnergy spokesman Todd Schneider said.

It has been about four months since the plant restarted after being shut down for more than two years after the discovery of serious safety problems caused by corrosion on top of the reactor. Davis-Besse probably will be able to restart by the end of the week, Schneider said.

"It's a minor issue we believe at this point," he said. "It appears to be an equipment issue."

Employees were doing a quarterly test involving the control rod mechanisms when the plant shut down automatically, he said. The control rods are used to regulate the nuclear fission inside the reactor. When the carbon rods drop into the reactor, the fission process stops.

The plant, in Oak Harbor about 25 miles east of Toledo along Lake Erie, had operated 131 straight days without a shutdown, Schneider said.

Federal regulators allowed the troubled plant to restart in March. FirstEnergy spent more than $650 million in repairs and to buy replacement power over that time.

The utility will buy electricity from other companies to make up for the loss of Davis-Besse this week, Schneider said.

The plant will not need NRC permission to restart unless employees discover a major safety equipment problem, NRC spokeswoman Viktoria Mitlyng said. NRC inspectors are on-site monitoring the situation, she said.