The Hanford Nuclear Reservation says a 20-foot section of a tunnel system where it stores contaminated material and equipment collapsed today, sparking an emergency alert and restrictions on workers’ movements. No injuries were reported.

A remotely operated TALON robot surveyed the scene and detected no release of contamination, Hanford said in its online update on the emergency.

Hanford said workers conducting routine surveillance this morning discovered the 20-by-20-foot hole in the roof of one of the two storage tunnels at the Plutonium Uranium Extraction Plant, or PUREX Plant, in the site’s central 200 East Area.

The tunnels were constructed of wood and concrete during the Cold War, and covered with about 8 feet of soil. They’ve been used for decades to store contaminated equipment from plutonium production operations at the site in southeastern Washington state.

The cave-in occurred in the 200 East Area, around a spot where the two tunnels join together, Hanford said.

The workers on the surveillance team were evacuated, and thousands of employees sheltered in place for hours. As of this afternoon, all non-essential employees at the Hanford site have been released, officials said in a tweet.

“Officials continue to monitor the air and are working on how they will fix the hole in the tunnel roof,” Hanford said in its afternoon update. “They are looking at options that would provide a barrier between the contaminated equipment in the tunnel and the outside air that would not cause the hole in the tunnel’s roof to widen.”

The roof collapse doesn’t appear to have any potential to affect personnel beyond the boundary of the Hanford site, officials said.

“It’s too early to know what caused the roof to cave in,” Destry Henderson, a spokesman for Hanford’s emergency operations center, said in a Facebook video. “We may not know that for some time.”

Members of the public can consult Hanford.gov or call 509-376-8116 for updates.