and plutonium oxidized, turning into power. The cladding was made of stainless steel.
One worker tested positive for americium-241 in the lungs, which is taken as a sure sign that he has inhaled plutonium.
From CBS News (11/10/2011):
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."
..... (full article at the link)