SPOKANE, Wash. (AP) — A gigantic nuclear waste treatment plant in eastern Washington state that has been under construction for 18 years is largely completed and soon will be ready to start processing radioactive wastes left over from the construction of the nation's nuclear arsenal, the U.S. Department of Energy said Wednesday.

The so-called vitrification plant is a key component in cleaning up the legacy of wastes left at the Hanford Nuclear Reservation from decades of making plutonium for nuclear weapons.

The $17 billion plant is designed to treat the bulk of the 56 million gallons of the most toxic radioactive waste stored in 177 underground tanks. Hanford produced about two-thirds of the nation’s plutonium from World War II through the Cold War.

"We are one step closer to processing 90% of the waste stored in underground tanks at Hanford,'' U.S. Sen. Maria Cantwell, D-Washington, said in taped remarks at a ceremony to mark the milestone in Richland, Washington.

Cantwell called it the world's largest radioactive waste treatment plant, and she said it was on track to begin processing the waste in 2023.

"It's an unprecedented step in cleaning up the most toxic site in the United States,'' Cantwell said.

The sprawling vitrification plant is designed to take in the mostly liquid nuclear waste and convert it into a glasslike substance for eventual burial in an underground repository that has yet to be built. Efforts to construct the repository in Nevada have run into intense political opposition.

The U.S. Department of Energy 30 years ago signed a legal agreement with the U.S. Environmental Protection Agency and the state of Washington that sets timelines for cleaning up the Hanford site, which is about 200 miles southeast of Seattle. Many of the timelines have been pushed back because of design and construction delays with the plant.

Officials for the Department of Energy lauded the efforts of 15,000 agency employees on the Hanford site to complete major construction work on the plant, especially given the problems caused by COVID-19 in 2020.

"Cleaning up Hanford is a legal, moral and ethical imperative,'' Deputy Energy Secretary Mark Menezes said.

There have been suggestions over the decades to leave the waste, which dates from the Manhattan Project in World War II, in the ground and move on. But Menezes said spending the more than $3 billion per year on cleaning up Hanford waste was the right thing to do.

"It is on the precipice of actual tank waste treatment,'' Menezes said of the plant.

Even when the plant is fully operational, the Department of Energy has estimated it will cost tens of billions of dollars and decades of work to treat all of the waste at Hanford.