[Carl Vehse]

"The powdery substances had been encased in a double-wrapped plastic bag placed inside the container, whose lid was fastened with six bolts. When one of the workers opened the lid, the black powder sprayed out under pressure, exposing the men to radiation."

Sounds like there may have been some residual moisture and/or nitric acid left in the powder, and radiolysis of the moisture by the plutonium's alpha radiation resulted in the production of hydrogen or nitrogen oxide gas. The gas pressure in the sealed container caused some of the radioactive powder to blow out when the lid of the container was unbolted. The main danger is worker inhalation of the plutonium dust as blew out of the container. External radiation is minor for that amount of plutonium or uranium.

[chajin]

That makes more sense than the explanation in the article.