"DOE considers these events to be of high safety significance," John Boulden, a director of enforcement and oversight for the Energy Department in Washington, D.C., wrote in the violation notice issued to the Idaho National Laboratory in Idaho Falls.

He was referring to two incidents in 2011 that exposed employees to radiation at the nation's leading nuclear research and development center, which is operated by contractor Battelle Energy Alliance.

The lab was fined for failure to prevent contamination of a worker's hand in August 2011. In November 2011, 16 workers were also exposed to low-level plutonium radiation while preparing fuel plates for shipping suffered no adverse health effects.

Workers who tested positive for radioactive contamination, including two who inhaled radioactive particles, showed dosages below Energy Department occupational limits and federal regulatory limits, said lab health director Sharon Dossett.

"As it turned out, the doses were fairly small - and that was fortuitous," she said.

Dossett said several of the workers have sought counseling for anxiety triggered by the incident.

"The magnitude and duration of the uncontrolled plutonium release presented a high potential for an adverse impact on worker safety," Boulden wrote about the accident.

An Energy Department investigation released this year faulted the lab for failing to properly assess risks posed by the handling of decades-old plutonium fuel cells and for not activating its emergency plan sooner after the exposure.

Battelle had rated the chance of a mishap like the one that occurred as "extremely unlikely," the report found.

In the violation notice issued on Thursday, federal regulators found Battelle failed to perform real-time air monitoring in November 2011 "to detect and provide early warning to individuals of events that could lead to substantial unplanned exposures to airborne radioactivity."

Idaho National Laboratory has implemented 59 of 79 corrective actions - which include upgraded air monitoring. It intends to make the rest of the changes by the end of the year, spokesman Ethan Huffman said.

The laboratory would not appeal the $412,500 fine, which will be paid by Battelle, he said.

The Energy Department has cited the lab for safety or other violations at least three times since 2007, records show.

(Editing by Cynthia Johnston and Lisa Shumaker)

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