Officials report that they've just observed a sunken area in the soil and it's unclear whether the tunnel has collapsed.
"This is sort of a forgotten legacy of the nuclear age", said Paul Carroll, the director of programs for the nuclear nonproliferation group Ploughshares Fund and who previously worked on nuclear cleanup programs for the Department of Energy.
The Hanford Site, about 150 miles southeast of Seattle, is a former nuclear production complex and home to a long-running, challenging and sometimes troubled clean-up operation. Energy Department Secretary Rick Perry has been briefed on the incident.
"There is no indication of a release of contamination at this point", the alert says. The Energy Department in recent years has spent about $2 billion a year on cleanup work.
But never, he said, has there been a tunnel collapse.
Tom Carpenter, executive director of Hanford Challenge, a non-profit that monitors the nuclear reservation said plutonium has already been detected in local fish stocks and worries that if changes don't come soon, wild salmon stocks could be next.
Hanford produced plutonium fuel for the U.S. military for four decades, according to the U.S. Department of Energy.
Hanford for decades made plutonium for the nation's nuclear weapons arsenal.
Washington Gov. Jay Inslee (D) called the incident a "serious situation" in a statement.
"The subsidence of soil was discovered during a routine surveillance of the area by workers", it said.
The agency also says there is no sign that any workers were exposed to radioactivity.
"We don't know exactly how the soil caved in, it's too early", Heeter said.
The Hanford site was built during World War II and made plutonium for most of the USA nuclear arsenal, including the bomb dropped on Nagasaki, Japan, at the end of the war. Many contaminated pieces of equipment, including the rail cars, have been left in the tunnels, he said.
This photo shows a 20 foot by 20 foot area of collapsed ground near the site. Material from the Hanford Site was used in the bomb dropped on Nagasaki, Japan, during the final days of World War II.
Hundreds of workers at a nuclear site in the USA state of Washington were ordered to take cover on Tuesday after a tunnel filled with contaminated material collapsed near the facility, federal officials said.
All personnel have been accounted for and there are no reports of injuries, federal Department of Energy officials said.
Officials say there was no release of radiation and no workers were injured; no workers were inside the tunnel when it collapsed, but others were evacuated or told to remain indoors.