The incident occurred at about 10 a.m. in the 200 West area of the site, where workers have been retrieving contaminated waste and removing contaminated equipment.

At the time, workers were removing a container containing radioactive waste from the Plutonium Finishing Plant. The container slid off a forklift about 1½ feet to the ground and rolled on its side.

Workers verified through a visual inspection that the container was not breached, and radiological monitoring determined that no contaminants had been released, the U.S. Department of Energy said in a statement.

The take cover order was lifted by 11 a.m., and workers returned to work. No one was injured.

Exactly how many workers were forced to take cover was unknown. About 1,500 people are assigned to work in the 200 West area, said Geoff Tyree, spokesman for Fluor Hanford, the contractor handling cleanup in that part of the nuclear site.

Beginning in 1949, the Plutonium Finishing Plant was the last step in converting plutonium nitrate solutions into pure plutonium "buttons" about the size of hockey pucks, which were sent to other Energy Department sites to make atomic bombs. The work stopped in 1989 at the end of the Cold War.

Work is now focused on dismantling and tearing apart the plutonium plant's contaminated equipment, which will be packaged and sent to a nuclear waste repository in New Mexico. The deadline for the plant to be demolished is 2016 under the Tri-Party Agreement, the cleanup pact signed by the state, Energy Department and the Environmental Protection Agency.