An emergency has been declared at the Hanford Nuclear Reservation in southeastern Washington state which for decades made plutonium for nuclear weapons.
Train carriages full of nuclear waste were in the section of tunnel which collapsed on Tuesday morning.
The sprawling site is now used to clean up the United States’ largest volume of radioactive defence waste.
Hundreds of workers were told to “take cover”, although none were inside the tunnel when it collapsed.
Workers were told to remain indoors and to not eat or drink anything.
Randy Bradbury, a spokesman for the Washington state Department of Ecology, said there has been no apparent release of radiation and no workers were injured.
A message to workers, said: “There are concerns about subsidence in the soil covering railroad tunnels near a former chemical processing facility.
“The tunnels contain contaminated materials.
“Actions taken to protect site employees include: faciility personnel have been evacuated.
“As a precaution, workers in potentially affected areas of the Hanford Site have gone indoors.
“Access to the 200 East Area of the Hanford Site, which is located in the centre of the Hanford Site, has been restricted to protect employees.”
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