CHARLESTON, S.C. (Reuters) - South Carolina is suing the U.S. government to recover $100 million in fines it says the Department of Energy owes the state for failing to remove one metric ton of plutonium stored there, state Attorney General Alan Wilson said on Tuesday.

The lawsuit, filed on Monday, is the state’s largest case ever against the U.S. government, Wilson said in a news release.

Congress approved fines of $1 million per day for the first 100 days of each year through 2021, beginning last year, if the weapons-grade plutonium was not removed from the Savannah River Site at the state’s border with Georgia, the attorney general’s office said.

The lawsuit seeks money owed this year. The state is still working on a claim for 2016 fines as well, Wilson said.

The federal government cannot break its obligations and “leave South Carolina as the permanent dumping ground for weapons-grade plutonium,” Wilson, a Republican, said in the complaint.

A Department of Energy spokeswoman said the agency does not comment on pending litigation.

Built in the 1950s, the U.S.-owned Savannah River Site processes and stores nuclear materials in support of national defense and U.S. nuclear nonproliferation efforts.

A U.S. treaty with Russia in 2000 required each country to dispose of 34 metric tons of weapons-grade plutonium, left over from the Cold War.

The United States began building a mixed oxide fuel fabrication facility, known as the MOX project, at the Savannah River Site to dispose of weapons-grade plutonium by mixing it with uranium to form safer fuel pellets for use in commercial nuclear reactors.

But the project is years overdue and billions over budget, and the technology for the new fuel fabrication is not fully developed. Russian President Vladimir Putin last October pulled out of the plutonium pact amid rising tensions over Ukraine and Syria.

The Trump administration proposed in the fiscal year 2018 budget to scrap the project and pursue diluting the plutonium and disposing it underground, an alternative called for by the Obama administration.