Employees at Washington state's Hanford nuclear waste site were asked to "take cover" Friday morning because steam was coming from a small building in one of the site's tunnels, officials at the complex said.
Details about the cause of the steam weren't immediately available, but "there is no indication of a release of a radiological release," a message to employees on the site's official website said.
The workers' instructions to take cover was precautionary, the site said.
"Crews wearing protective equipment and breathing filtered air are preparing to approach (the tunnel), at which time they will take radiological surveys and start the generators to power lights and cameras inside the tunnel and air sampling equipment to further assess conditions inside and outside of the tunnel," the site said.
The Hanford Site once produced plutonium for atomic weapons.
The building where the steam was spotted Friday houses equipment designed to move a massive door used to access the tunnel, the employee statement said. Railcars containing contaminated equipment were last placed here in 1996, Hanford officials said.