COLUMBIA, S.C. (AP) — Gov. Nikki Haley on Tuesday moved forward with her threat to sue the U.S. Department of Energy for the agency's failure to meet a Jan. 1 nuclear fuel deadline, asking South Carolina's top prosecutor to pursue litigation and collection of $1 million daily fine.

"The federal government has, once again, failed to keep its promise to the people of our state," Haley wrote to Attorney General Alan Wilson in a letter obtained by The Associated Press. "South Carolina will not sit idly by while DOE continues — in violation of federal law — to ignore its commitment to the people of South Carolina."

The Savannah River Site's mixed-oxide project — known as MOX — is intended to turn weapons-grade plutonium into commercial nuclear reactor fuel. It is years behind schedule, and billions over its original budget.

Officials with the Department of Energy did not immediately return a message seeking comment Tuesday.

MOX is intended to help the United States fulfill an agreement with Russia to dispose of at least 34 metric tons apiece of weapons-grade plutonium. Government officials say that's enough material for about 17,000 nuclear warheads.

Construction began in 2007. The General Accountability Office has said the project is more than three years behind its completion deadline — and, at $8 billion, at least $3 billion over budget.

Since MOX is not operating, by law the federal government was supposed to remove 1 metric ton of plutonium from South Carolina by Jan. 1 or pay $1 million a day for "economic and impact assistance," up to $100 million yearly, until either the facility meets production goals or the plutonium is taken out of state for storage or disposal elsewhere.

A clause in the law, however, makes the fine "subject to the availability of appropriations."

Haley warned Energy Secretary Ernest Moniz in December that the state would be forced to sue if his agency didn't start making payments. In writing to Wilson, Haley also enclosed a letter she received this month from Moniz, who said the administration remains committed to removing the surplus plutonium from the Savannah River Site and reiterated South Carolina's "critical role" in the country's nonproliferation efforts.

Haley copied Tuesday's letter to the state's congressional delegation. U.S. Sens. Lindsey Graham and Tim Scott and U.S. Rep. Joe Wilson — father of the state's attorney general — have previously encouraged the governor to sue to keep the project going.

South Carolina did sue the Obama administration in 2014 after officials said they wanted to shutter the project, citing cost overruns and delays. In that lawsuit, the state said the federal government had made a commitment to South Carolina and shouldn't mothball the effort, noting it legally couldn't use money intended to build the plant to shut it down.

The state dropped the suit several months later, when the administration committed to funding the project through that fiscal year. But the administration has since said it's searching for a less expensive way to dispose of the plutonium, such as immobilizing it in glass or processing it in different kinds of reactors.

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Kinnard can be reached at http://twitter.com/MegKinnardAP . Read more of her work at http://bigstory.ap.org/content/meg-kinnard/