Some workers at the plant have been evacuated and about 3,000 others near the area at the center of the Hanford Site were directed to take shelter indoors.
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.
Tom Carpenter, the executive director with watchdog organization Hanford Challenge who has spoken with workers at the site since the incident, called the tunnel collapse worrisome and said the evacuation was the correct call.
All workers in the region are accounted for, the Department of Energy says.
ORIGINAL: 11:30 a.m.
A portion of a tunnel at a troubled nuclear site in Washington state collapsed on rail cars full of radioactive waste, prompting authorities to declare an emergency and evacuate workers.
"There is no indication of a release of contamination at this point", a statement by the US Department of Energy said. The cause of the collapse has not been determined.
The Waste Treatment Plant, shown here in May 2015, is used to stabilize the large inventory of high-level nuclear waste from Hanford's tank farms. Hanford made the plutonium for the atomic bomb that was dropped on Nagasaki, Japan, and much of the plutonium for the nation's nuclear arsenal.
Over the course of its operating lifetime, the facility produced almost 56 million gallons of radioactive and chemical waste that is now stored in 177 underground tanks.
A tunnel near the site contains several rail cars that have been temporarily buried because they were used to transport irradiated fuel rods and remain contaminated.
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Officials are working on how to address the cave-in that was discovered at the Hanford Nuclear Reservation Tuesday.
Access to the affected area, which is located in the center of the Hanford Site, has been restricted to protect employees.
The PUREX facility, once used to extract plutonium from spent nuclear fuel, has been idle for years but remains "highly contaminated", the agency said.
And Energy Secretary Rick Perry said during his confirmation hearing he'd prioritize the Hanford cleanup.
Eight rail cars are in the tunnel that collapsed, but it feeds into a longer tunnel that contains 28 loaded rail cars. It's estimated that almost two-thirds of the plutonium used by the US government during Hanford's lifespan - and a staggering amount of related waste - was produced at this one site.
The robot isn't the only machine to have worked on the 500-square-mile site.
It is located about 200 miles southeast of Seattle and is half the size of Rhode Island. A complete cleanup of the nuclear waste site could cost $107 billion and take until 2060 to complete, according to the latest government estimate.
The Energy Department had previously said that the soil sunk by 4 to 6 feet, but later removed that estimate and a spokeswoman was unsure as to the depth of the breach.
Geranios reported from Spokane, Washington.
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