Employees at the Hanford Site plant, located about 170 miles (275 kilometers) southeast of downtown Seattle, were sent an alert by management in early morning telling them to "secure ventilation" and refrain from "eating or drinking."
The Hanford nuclear site was used to produce plutonium for the bomb that brought an end to World War II.
Its last reactor closed down in 1987 but millions of gallons of leftover waste are contained in tanks at the site.
A statement by the US Department of Energy said personnel at the facility were evacuated and workers in potentially affected areas had gone indoors.
"There are concerns about subsidence in the soil covering railroad tunnels near a former chemical processing facility," the statement said. "The tunnels contain contaminated materials."
It said it had activated an emergency operation at the plant at 8:26 am, shortly after an alert was declared.
There were no reports of injury and officials at the plant could not be immediately reached for comment.
There was speculation that road work near the tunnel caused it to collapse.
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