WASHINGTON — The federal government and the state have settled a decades-old dispute over how to split the cost of the cleanup of the West Valley Demonstration Project, a massive task that could cost as much as $9 billion.

The deal, filed in federal court in Buffalo on Tuesday, sets percentages that the federal government and the state will pay for particular elements of the cleanup of the Cattaraugus County nuclear site.

Because the scope of the cleanup has not yet been determined, it’s impossible to predict how much either government will pay, said Bryan Bower, the U. S. Department of Energy’s director of the West Valley project.

Under the deal, the federal government will pay 90 percent of the cost of the cleanup of the high-level nuclear waste tanks at West Valley, with the state paying 10 percent, Bower said. The same percentages will apply to the cleanup of the original processing building at the site.

Meanwhile, the state and federal governments will evenly split the cleanup cost of much of the ground water contamination stemming from the site, along with the cleanup cost of the federally licensed disposal area. The state will pay 70 percent of the cost of the cleanup of the state-licensed disposal area, with the federal government picking up the rest.

Exactly what all that means won’t be known until the spring, when a final environmental impact statement and cleanup decision will be released. The overall cost of the cleanup could be as low as $1 billion or as high as $9 billion, depending on how much waste is removed from the site.

Nevertheless, both Bower and Gov. David A. Paterson expressed relief that an agreement had finally been reached.

“After nearly 30 years of struggling with this issue, we are happy to see that all parties were able to put their differences aside and work toward a solution that is best for both the federal and state taxpayer,” Bower said. “With the cost responsibility issues behind us, we can move forward, hand in hand, with the ongoing cleanup of the site.”

Meanwhile, Paterson said: “This resolution is a significant milestone in our ongoing work to decontaminate the West Valley site and ensure the area’s long-term environmental health and safety. With this agreement, our state and federal governments may proceed with a clear understanding of their roles and responsibilities.”

Some $2.4 billion already has been spent on the cleanup, with the federal government paying most of it and the state paying about $270 million.

West Valley was the site of the nation’s only privately operated commercial nuclear fuel reprocessing facility. The operation separated reusable uranium and plutonium from spent fuel, which came from both commercial and federal nuclear reactors.

The facility shut down in 1972, and the state and federal governments have been arguing about the liability for its cleanup since then.