SPOKANE, Wash.— A portion of an underground tunnel containing rail cars filled with radioactive waste collapsed Tuesday at a sprawling storage facility in a remote area of Washington state, forcing an evacuation of some workers at the site that made plutonium for nuclear weapons for decades after World War II.

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 two to four feet over a 400 square foot area, officials said.

The tunnels are hundreds of feet long, with about eight feet of soil covering them, the U.S. 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 $2 billion a year. The work is expected to take until 2060 and cost more than $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.

Non-essential workers among the labor force of 9,000 at the site were sent home early along a safe route.

"No action is currently required for residents of Benton and Franklin counties," the Energy Department said, referring to the nearly 300,000 residents near the site about 200 miles southeast of Seattle. "There is no indication of a release of contamination at this point."