By Laura Zuckerman
SALMON, Idaho (Reuters) - The radiation exposure of 16 workers at a nuclear research lab in Idaho stemmed from a failure to properly assess the risks posed by the handling of decades-old plutonium fuel cells, federal investigators concluded on Wednesday.
In its report on the November 8 mishap at the Idaho National Laboratory, the U.S. Energy Department's Office of Health, Safety and Security also found the lab erred in not activating its emergency plan sooner after the accident, a delay that may have compromised medical treatment of the workers.
The panel recommended the lab conduct a fresh assessment of "the likelihood, severity and risk of accidents," as well as the effectiveness of hazard controls at the deactivated reactor where the exposure occurred.
The Idaho National Laboratory, occupying 890 square miles in eastern Idaho, is the Energy Department's leading nuclear research center, employing some 6,000 government workers and contractors.
The decommissioned reactor involved in the accident is located within a complex of facilities used for remotely handling, processing and examining spent nuclear fuel, radioactive waste and other irradiated materials.
Sixteen workers were exposed to low-level plutonium radiation when a container holding a plutonium fuel plate was opened in the process of preparing the material for shipment to another facility. Subsequent inspections found that a layer of stainless steel cladding that envelopes the spent nuclear fuel inside the container was defective.
Thirteen of the workers tested positive for actual radioactive contamination, either on their clothing or from nasal swabs, and two of those were found to have inhaled radioactive particles, lab spokesman Ethan Huffman told Reuters.
None has shown any sign of radiation sickness or other ailments, and all 16 returned to the job the next day, though the two who tested positive in lung scans have stayed away from further radiological work while medical evaluations of them continue, Huffman said.
Still, the radiation doses were all believed to be minimal and "we don't believe there is anything that would be of concern in terms of long-term effects for them," he said.
The decommissioned reactor has remained closed since the accident amid continuing decontamination efforts, but officials said the radiation release posed no risk to the public.
The exposed workers are all employees of laboratory contractor Battelle Energy Alliance.
Lab director John Grossenbacher issued a statement on Wednesday saying officials there "deeply regret" the incident and promising to improve safety and training programs "and to better understand the hazards of our work."
The board of investigators said the accident could have been avoided had the lab and Battelle paid more attention to well-documented safety risks posed by the plutonium fuel plates and taken greater precautions.
"Through a review of records, the board found that the probability of encountering damaged plutonium fuel plates is higher than expressed" in existing safety protocols for the reactor, which was decommissioned in 1992, the panel said.
"As a result, workers were at increased risk of exposure to uncontrolled radioactive material," the safety board said.
Battelle, the report found, had rated the chance of an accident like the one that occurred as "extremely unlikely," and there was no evidence "that any drill was performed that would have prepared the workforce to respond to an event like the" November 8 mishap.
The panel also faulted the lab for failing to activate its emergency plan sooner to ensure a better-coordinated and timely response. That error "limited the effectiveness of the medical response" and delayed an assessment of how strong a dose of radiation the workers received, the panel said.
The effects of radiation worsen the longer radioactive material remains in the body.
Plutonium is considered more dangerous when inhaled than ingested because particles lodge in the lungs instead of being eliminated by the body, according to the U.S. Centers for Disease Control and Prevention.
(Editing by Steve Gorman and Dan Burns)