Workers at the Hanford site in Central Washington have completed the demolition of a sprawling complex of buildings that produced most of the plutonium  for U.S. nuclear weapons.

The walls for the last major structure — the main processing facility of the Plutonium Finishing Plant — were knocked down in December, and the final rubble hauled away.

“Removal of this iconic building forever changes the landscape at the Hanford site,” said Tom Teynor, demolition project director for the Energy Department, who called the project completion a “historic accomplishment” in the decades-long clean-up of the once-secretive area established in 1943, during World War II, to produce plutonium.

The finishing plant, the last stop in the Hanford production process, operated from 1949 to 1989, and was a complex of more than 90 buildings. Workers who staffed two production lines produced hockey-puck size buttons of plutonium that would be shipped elsewhere to be incorporated into nuclear weapons.

Work on the cleanup and demolition has been underway for some 20 years. Most of that time has been spent removing equipment, piping, asbestos and other materials that had to be cleared out before it was safe to undertake demolition. Jason Casper, a vice president of the contractor CH2M Hill Plateau Remediation Company, called the project, “one of the most challenging risk-reduction efforts in the DOE complex.”

Demolition began in 2016, and since then the plant’s four main buildings have been taken down. But there have been problems along the way.

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In December 2017, demolition work was halted for nine months after radiological contamination was found to have spread to offices and onto vehicles. More than 40 workers were eventually found to have inhaled or ingested contaminated particles, and state Ecology Department testing found traces of plutonium and radioactive americium contamination had spread to other areas up to 10 miles from the demolition site.

Some of the demolition work was able to resume in September 2018, according to Dieter Bohrmann, a spokesman for Plateau Remediation Company.

The final work at the finishing plant involves packaging and disposal of some remaining rubble, taking core samples beneath building pads and stabilization of soil cover. It should be completed by this summer, according to a statement released by the Energy Department.