SPOKANE, Wash. (AP) — A portion of an underground tunnel containing rail cars full of 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 (half to 1.2 meters) over a 400 square foot (37.1 square meters) area, officials said.

The tunnels are hundreds of feet long, with about eight feet (2.4 meters) of soil covering them, the U.S. Department of Energy said.

The cause of the collapse was not immediately known. It was discovered during 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.

Workers near the site of the collapse Tuesday were evacuated and hundreds of others farther away were told to remain indoors for several hours, the agency said. Some of the 9,000 workers at the site were sent home early along a safe access route.

“No action is currently required for residents of Benton and Franklin counties,” the U.S. 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.”

U.S. Energy Secretary Rick Perry was briefed on the incident that Washington Gov. Jay Inslee called it a serious situation.

“Ensuring the safety of the workers and the community is the top priority,” said Inslee, a Democrat who previously represented the Hanford region in Congress.

The anti-nuclear group Beyond Nuclear said the incident helped show “radioactive waste management is out of control.”

The accident occurred at a plant known as the Plutonium Uranium Extraction Facility, or PUREX, located in the middle of the 500-square-mile Hanford site — half the size of Rhode Island.

Hanford for decades made plutonium for nuclear weapons and is now the nation’s largest depository of radioactive defense waste, with about 56 million gallons of waste, most of it in 177 underground tanks.

As part of a huge, ongoing cleanup, rail cars full of radioactive waste were driven into tunnels and buried, Bradbury said.

The Hanford site was built during World War II and made plutonium for most of the U.S. nuclear arsenal, including the bomb dropped on Nagasaki, Japan, at the end of the war.

Worker safety has long been a concern at Hanford.

Washington Attorney General Bob Ferguson filed a lawsuit last fall against the Energy Department and its contractor, Washington River Protection Solutions, contending vapors released from underground nuclear waste tanks posed a serious risk to workers.

Ferguson said that since the early 1980s, hundreds of workers have been exposed to vapors escaping from the tanks and that those breathing the vapors developed nosebleeds, chest and lung pain, headaches, coughing, sore throats, irritated eyes and difficulty breathing.

Lawyers for the Energy Department have said no evidence has been provided showing workers have been harmed by vapors.

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Washington Gov. Jay Inslee issued this statement:

“The Department of Energy informed us this morning that a tunnel was breached that was used to bury radioactive waste from the production of plutonium at the Hanford Nuclear Reservation. I appreciate that the White House reached out to my office to alert us to the situation as well.

“This is a serious situation, and ensuring the safety of the workers and the community is the top priority. Our understanding is that the site went into immediate lockdown, in which workers were told to seek shelter, and all access to the area has been closed.

“Federal, state and local officials are coordinating closely on the response, and the state Department of Ecology is in close communication with the U.S. Department of Energy Richland Office. My office is in close communication with these agencies and directly with Department of Energy headquarters in DC. We will continue to monitor this situation and assist the federal government in its response.”

Oregon US Sen. Ron Wyden issued this statement:

“Today’s cave-in at the PUREX plant should remind everyone that the temporary solutions DOE has used for decades to contain radioactive waste at Hanford have limited lifespans, whether they are underground tunnels for storing contaminated equipment or aging steel tanks filled with high-level radioactive waste,” Wyden said. “The longer it takes to clean up Hanford, the higher the risk will be to workers, the public and the environment.”

KOIN 6 News contributed to this report

Copyright 2017 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.