(AP) -- Officials detected no release of radiation at the Hanford Nuclear Reservation and no workers were injured, said Randy Bradbury, a spokesman for the Washington state Department of Ecology.
No workers were inside the tunnel when it collapsed, causing soil on the surface above to sink up to 1.2 metres over a 38 square metre area, officials said.
The tunnels are hundreds of metres long, with about 2.5 metres of soil covering them, the US Department of Energy said.
The cause of the collapse was not immediately known.
It was discovered as part of a routine inspection and occurred during a massive cleanup that has been under way since the 1980s and costs more than US$2 billion a year. The work is expected to take until 2060 and cost more than US$100 billion.
Hanford officials said they were studying the area of the collapse to determine how to create a barrier between the contaminated equipment in the tunnel and the outside air.
Workers near the site were evacuated and hundreds of others farther away were told to remain indoors for several hours, the federal agency said.