Workers at the Perry Nuclear Power Plant immediately evacuated it on April 22 when radiation levels rose while it was shutting down for a refuelling outage, the commission said yesterday. Plant officials don't believe workers were exposed to radiation levels "in excess of NRC limits," the commission said.

"The plant is in a safe condition and there has been no impact to workers at the plant or members of the public from this issue," the commission said in a statement.

Radiation levels rose while workers were removing a monitor that measures nuclear reactions during start-up, low-power operations and shutdown, the commission said.

The highest radiation exposure to any of the workers was 98 millirems, which is equivalent to two or three chest X-rays, a spokesman for the plant's owner said.

The NRC's limit for radiation exposure in a year is 5,000 millirems, he said.

The commission, which began inspecting the plant on Monday, did not say how high the radiation levels were or how often such inspections occur.

The nuclear plant, owned by Akron-based FirstEnergy Corp., is about 56 kilometres northeast of Cleveland and began operating in 1987. FirstEnergy spokesman Todd Schneider said the four workers involved were contractors hired to assist with the plant refuelling. He said the contractors were working in a containment building underneath the reactor at the time.

"The contractors did not use the proper method to remove this piece of equipment from underneath the reactor," Schneider said.

The plant refuelling has continued on schedule, Schneider said.

"It shouldn't have happened, but the bottom line was it did not impact the safety or health of the contractors or the public," he said.

In March 2010, a small fire broke out in a water pump's lubrication system at the plant. The fire burned for several hours, and two members of the plant's fire brigade were taken to a hospital for heat stress.