Officials detected no release of radiation and no workers were injured, said Randy Bradbury, a spokesman for the Washington state department of ecology.
There were no workers inside the tunnel when it collapsed. But nearby Hanford workers were evacuated and others who were farther away were told to remain indoors, the US Department of Energy said.
The accident occurred at a facility known as Purex, located in the middle of the sprawling Hanford site, which is half the size of Rhode Island, Bradbury said.