The U.S. Nuclear Regulatory Commission says it has started a special inspection at a nuclear reactor in northeast Ohio where high radiation levels were reported in a work area.
Four workers at the Perry Nuclear Power Plant, which is about 35 miles northeast of Cleveland, immediately evacuated on April 22 when radiation levels rose while the plant was in the process of shutting down for a refueling outage, according to Todd Schneider, spokesman for the reactor's owner, FirstEnergy Corp.
The commission said the plant is safe and that officials do not believe workers were exposed to radiation levels "in excess of NRC limits."
The commission said radiation levels rose when workers were removing a monitor that measures nuclear reactions during start-up and shutdown.
"The special inspection team began work on Monday and will review the circumstances surrounding the higher-than-expected radiation levels in the work area," the NRC said in a statement.
"The team will gather data to establish a sequence of events, review the utility’s work planning and engineering actions, determine if there were human performance factors that may have contributed to the event and evaluate the actual radiological consequences including exposure to the workers."
FirstEnergy's Schneider said one workers were exposed to relatively low levels of radiation, equal to "two or three X-rays' worth."
"We take (the incident) seriously and don’t want it to happen again," he said. "But we want to put it into perspective."
The plant experienced numerous safety problems several years ago, causing the NRC to monitor its safety operations every three months in 2005, when the plant was forced to shut down briefly because of problems with pumps that circulate coolant through the reactor's core.
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.
In addition to the safety problems, regulators have identified concerns over poor procedures and "human performance" issues" at the plant during the past two years, NRC public affairs officer Viktoria Mitlyng told msnbc.com.
"Basically, we're holding the company accountable for addressing these concerns," Mitlyng said. "(They) can be at the bottom of issues that are minor today but may become serious if they are not addressed."
FirstEnergy's Schneider acknowledged that the plant, which has around 700 workers, had seen "human performance" problems in the past but was working hard to address them.
"We are improving that trend and will continue to improve that trend in the future," he said. "We've been working directly with our employees."
Mitlyng and Schneider did not provide details on exactly what these human performance issues were.
The 2010 U.S. population within 10 miles of the plant was 83,410, according to U.S. Census data analyzed for msnbc.com. The U.S. population within 50 miles was 2.2 million, with additional population in Canada.