The state sent a letter to federal lawyers saying the Energy Department's March 31 proposal that would have eliminated many deadlines for Hanford cleanup "is not acceptable to Washington."
The Department of Energy, meanwhile, said a state proposal also issued March 31 that would have left many deadlines in place was unrealistic.
The state proposal does not "adequately account for the realities of technical issues resolution, project management requirements and budget constraints," the Energy Department said in a news release.
While the state warned that it might consider legal action, the Energy Department said it wanted to keep negotiating the issues.
Hanford for decades made plutonium for nuclear weapons, and now is engaged in cleaning up the nation's largest volume of radioactive wastes. The site is near Richland.
The cleanup is governed by a federal court consent decree reached in 2010 that sets strict milestones for the cleanup process. But the Energy Department has said it is in danger of missing many of those milestones because of the scientific complexity of the work.
The state has threatened to take the department back to court in an effort to get the decadeslong cleanup back on track. The cleanup costs taxpayers about $2 billion per year.
"The people of our region made a significant sacrifice for our nation when the U.S. selected the Hanford site to produce plutonium as part of the Manhattan Project during World War II," state Attorney General Bob Ferguson said.
"Today's announcement should serve as notice to Energy that we are considering taking the next legal step as early as next week," Ferguson said.
The next step would be triggering a dispute resolution process that includes a 40-day period of negotiations, the state said. If no agreement can be reached, the parties can return to federal court.
A major problem with the cleanup is that construction of a unique facility called the Waste Treatment Plant, which is designed to turn the most dangerous wastes into glasslike logs for eventual burial, has been indefinitely delayed by technical and safety concerns.
"Energy proposes to indefinitely extend most of the consent decree's WTP deadlines by trading current hard deadlines and specific tasks for future unspecified milestones to be set on an open-ended, rolling basis," the state complained in the letter.
The consent decree set deadlines for emptying some of Hanford's underground waste tanks and starting to treat up to 56 million gallons of nuclear waste at the Waste Treatment Plant. But the Energy Department has said most of the remaining deadlines are at risk of being missed, including having the plant fully operational by 2022.
The Energy Department wants to set deadlines only for retrieving some waste from leak-prone tanks and on parts of the Waste Treatment Plant not affected by technical and safety issues.
The department has said setting deadlines that likely will be missed creates false expectations in the community and with the state, and erodes confidence in the cleanup work.