Federal officials said there was no sign so far that any radioactive material had leaked following the collapse of part of the tunnel.
Employees at the sprawling Hanford Site plant, located about 170 miles (275 kilometers) southeast of Seattle, were sent an early morning alert by management telling them to "secure ventilation" and refrain from "eating or drinking."
Federal officials said there was no sign so far that any radioactive material had leaked following the collapse of part of the tunnel, which contained rail cars filled with nuclear waste.
"There is no indication of a release of contamination at this point," a statement by the US Department of Energy said. "Responders are getting closer to the area where the soil has subsided for further visual inspection."
It said responders on the scene were reporting that soil had slid on top of the tunnel in an area 20 feet by 20 feet (six meters by six meters).
There were no reports of injury and officials at the plant could not be immediately reached for comment.
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.
The Department of Energy said the affected tunnel was located next to the Plutonium Uranium Extraction Facility known as PUREX.
"The subsidence of soil was discovered during a routine surveillance of the area by workers," it said. "The tunnels are hundreds of feet long, with approximately eight feet of soil covering them."
The department said personnel at the facility were evacuated and workers in potentially affected areas had gone indoors.
It said it had activated an emergency operation at the plant at 8:26 am, shortly after an alert was declared.
There was speculation that road work near the tunnel caused it to collapse.