All 16 workers exposed to radiation at the Idaho National Laboratory were allowed to go home following the incident, which officials Wednesday said likely resulted from decades-old plutonium powder that escaped its damaged stainless-steel shell.
After a follow-up lung scan Wednesday, one worker still tested positive for radioactive material in the lungs and was receiving extra attention at the lab's medical facility. Seven of the employees tested positive for external skin contamination, and six had positive nasal swipes.
All 16 workers will undergo weeks of testing, including urine analysis to evaluate their level of exposure. Plutonium, if it remains in the body, can cause cell damage.
Lab health director Sharon Dossett said none of the exposed workers was exhibiting outward symptoms of radiological exposure. They were allowed to go home because they posed no threat to others, she said.
"These isotopes are internal hazards; they're not external hazards," Dossett told reporters at a news conference Wednesday. "There's no hazard to their family members or anybody they would come into contact with."
INL officials declined to provide details about the workers, including their genders.
The lone employee who tested positive for radioactive material in the lungs Wednesday had breathed in Americium-241, an isotope commonly found in nuclear waste. While the lung scans aren't sensitive enough to detect plutonium, the presence of Americium-241 proves plutonium is there, too.
"It's for sure," Dossett said.
According to lab officials, it may be weeks before the extent of all the workers' exposure is known.
Idaho National Laboratory officials said filters meant to keep radioactive material from being released from exhaust systems inside the facility functioned properly. There was no risk to the public or the environment, the laboratory said in a statement.
The lab has designed and constructed 52 reactors since its founding in 1949 in the desert west of Idaho Falls.
One of those reactors, the Zero Power Physics Reactor, was dismantled last year, but 6-inch-long plutonium fuel plates that date back to research done in the 1970s remain inside the building.
Early Tuesday afternoon, workers wearing lab coats and gloves but not respiratory gear were recovering the fuel so it could be shipped to a U.S. Department of Energy facility in an undisclosed state. The lab originally reported it was bound for Nevada but later retracted the statement.
Lab officials suspect the stainless steel cladding that surrounded the plutonium was damaged years ago, beginning a slow-but-steady process of plutonium oxidation that led to the exposure. When workers opened an aluminum box that housed the fuel plates and cut away plastic wrapping, they noticed several grains of powder that escaped.
"At that point, the job was suspended. The personnel exited the area," said Phil Breidenbach, nuclear operations director of the Materials and Fuel Complex, where the accident occurred.
"However, as they were exiting, a constant air monitor went off, indicating contamination had gotten into the air," he said. "When they exited the area to their prearranged safe zone, they started surveying the individuals and found contamination."
Breidenbach said workers followed proper protocols, including asking managers for permission to remove the plastic wrapping from the fuel plate. But he said his employer, Battelle Energy Alliance, the contractor that runs the lab for the federal government, fell short of its foremost obligation: to protect the safety of employees.
"We didn't do that well yesterday," Breidenbach said.
As a consequence, Battelle and the Department of Energy suspended similar work at the Materials and Fuels Complex, with a resumption planned only after an investigation determines just how the accident happened and how it can be avoided in the future.
"We agree with Battelle Energy Alliance's immediate actions, to halt other similar work out there at the site, until we can best understand what happened, and put appropriate improvements in our protocols," said Rick Provencher, manager of the Energy Department's Idaho Operations Office. "That's the prudent thing for them to do."