Hundreds of workers at the Hanford nuclear waste site in Washington state have been ordered to "take cover" after a portion of a tunnel appeared to collapse.

The US Department of Energy said it activated its emergency operations protocol after reports of a "cave-in" at the 200 East Area in Hanford, home to numerous solid waste sites about 200 miles from Seattle. Energy Department officials said in a statement they were "responding to reports of a cave-in of a 20 foot section of a tunnel that is hundreds of feet long that is used to store contaminated materials."

Cleaning up radioactive materials at the Hanford site, a federal facility, has been one of the Energy Department's priorities for years. Reactors located at Hanford produced plutonium for America's nuclear weapons program and uranium metal fuel for commercial reactors. Plutonium production ended in 1980 and the cleanup program began in 1989.

Former Energy Department official Robert Alvarez said that the rail cars carry spent fuel from a reactor area along the river to the chemical processing facility, which then extracts dangerous plutonium and uranium. He said the plant lies near the middle of the sprawling 580-square mile Hanford site and was "a very high hazard operation." Many contaminated pieces of equipment, including the rail cars, have been left in the tunnels, he said.

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Workers were told to evacuate and, "as a precaution, workers in potentially affected areas of the Hanford Site have gone indoors," according to the statement. "Access to the 200 East Area of the Hanford Site, which is located in the center of the Hanford Site, has been restricted to protect employees."

In an update, the Energy Department said that responders were on the scene and were reporting that the soil had "subsided" in an area approximately 4 feet by 4 feet over one of the tunnels next to the Plutonium Uranium Extraction Facility, also known as PUREX.

"There is no indication of a release of contamination at this point," the department said in a statement. "The subsidence of soil was discovered during a routine surveillance of the area by workers."

The tunnels are hundreds of feet long, with approximately eight feet of soil covering them, the Energy Department added. The depth of the subsidence of soil appears to be two to four feet deep.

Alvarez cited a 1997 report, which said that the tunnel was 109.1 metres long, 6.7 metres high and 5.9 metres wide. First constructed decades ago, the walls are 14 inches thick and held up by pressure-treated Douglas fir timbers arranged side by side, the report said.They rest on reinforced concrete footings.

An unnamed source told NBC affiliate KING that workers may have created a vibration that caused a nearby tunnel filled with highly contaminated material to collapse.

Although the Trump administration has vowed to slash the budgets of most Energy Department programs, the administration does not plan to skimp on the department's programme charged with the Hanford cleanup and with other nuclear sites. It has requested $6.5 billion for agency's environmental management program for 2018.

The budget for Hanford alone is about $2.3 billion in the current fiscal year, about $1.5 billion of that going to the management and treatment of approximately 56 million gallons of radioactive liquid waste currently stored in underground storage tanks.

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Trump has been slow to fill science related positions, and he has not yet named a new assistant secretary for environmental management. It is currently being headed by a career department employee serving in an acting capacity.

During his recent confirmation hearing, Energy Secretary Rick Perry was asked by Washington Senator Maria Cantwell about the Hanford site.

"So are you committed to funding Hanford cleanup and what it takes and getting the waste treatment plant finished?" Cantwell asked.

"Senator, I'm committed to working with you and prioritising what is one of the most dangerous, most polluted sites that we have in this country," Perry replied. He vowed to visit Hanford and said he looked forward to "walking that site with you."