Employees at the sprawling Hanford Site plant, located about 170 miles (275 kilometers) southeast of Seattle.
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 at midday 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.
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.
"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 emergency crews 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).
"The subsidence of soil was discovered during a routine surveillance of the area by workers," it said.
There were no reports of injury and officials at the plant could not be immediately reached for comment.
The Department of Energy said the affected tunnel was one of two located next to a largely decommissioned Plutonium Uranium Extraction Facility known as PUREX.
The tunnels, which measure hundreds of feet, were used at the beginning of the 1950s to store contaminated equipment and the cave-in apparently took place in an area where the two join together. Both tunnels are covered with approximately eight feet of soil.
The department said personnel at the facility were evacuated and workers in potentially affected areas had gone indoors.
The emergency was declared at 8:26 am following an alert from the "200 East Area" containing the PUREX plant, which is no longer in use.
"Crews are using hand surveying techniques in the outer areas around the PUREX facility," a statement said.
"At and near the area of subsidence crews have deployed a TALON, which is a remote operated surveying device that is capable of radiological and industrial hygiene monitoring as well as capturing video footage."
The TALON device allows crews to survey a potentially contaminated area from a distance of up to a half mile.