(CNN) Employees at Washington state's Hanford nuclear waste site were asked to "take cover" for several hours Friday because steam was coming from a small building in one of the site's tunnels, officials at the complex said.

The US Department of Energy later said inspections at a tunnel containing waste confirmed there was no radiological release.

"Lights and cameras placed inside the tunnel to support stabilization efforts showed steam in the tunnel, resulting from the curing of the engineered grout recently placed in the tunnel to stabilize it," the agency said in a statement. "The curing process generates heat and moisture. When the warm moist air left the tunnel and interacted with the cool early morning atmosphere, steam was visible."

The workers' instructions to take cover was precautionary, the site said earlier.

The Hanford Site once produced plutonium for atomic weapons.

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