The decision covers the nation's biggest collection of radioactive waste, held in 177 underground tanks at the sprawling reservation near Richland that has been engaged in environmental cleanup for the past two decades. The material is left over from the production of plutonium for nuclear weapons.
The document said the Energy Department intends to retrieve 99 percent of the stored waste and close up the tanks. Some of the tanks date back to World War II and have leaked.
It's necessary to remove the radioactive material to avoid future leaks into groundwater and other safety concerns, the decision says.
The dangerous waste will eventually be converted into a glass-like substance at a $12 billion plant whose construction on the Hanford site is stalled by safety concerns. The glassy logs are intended to be buried in a national repository, the location of which is still undetermined.
The tanks will be "landfill closed," which means they will be filled with grout, stabilized and left in place. It was deemed too expensive and dangerous to have workers actually dismantle the highly radioactive tanks, said Suzanne Dahl, tank waste manager for the state Department of Ecology.
"We cannot have people up there with blowtorches," she said.
The watchdog group Hanford Challenge criticized the decision to leave some 1 percent of the highly radioactive waste in the tanks.
"That's 500,000 gallons or more," said executive director Tom Carpenter. "We expect all of the waste from the tanks to be removed."
The Energy Department said it also intends to dismantle the aboveground portions of the Fast Flux Test Facility reactor and "entomb" the underground portions of the closed research reactor.
The decision is a significant step toward achieving the cleanup mission at Hanford, said Ken Picha, Energy Department's deputy assistant secretary. He added that the agency received support from the state Department of Ecology and Environmental Protection Agency on the environmental impact statement.
The state agency and the EPA have a legal agreement with the Energy Department that sets deadlines to clean up the Hanford site, which is half the size of Rhode Island.
Dahl applauded completion of the document, "which will guide cleanup and waste management activities at Hanford for decades to come."
Hanford from the 1940s to the 1980s made plutonium for nuclear weapons.
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