COLUMBIA - Federal officials overseeing a South Carolina project to turn weapons-grade plutonium into commercial nuclear reactor fuel should have a better handle on why costs have ballooned by billions of dollars, according the General Accountability Office.

In a report released Thursday, GAO officials wrote that the U.S. Department of Energy needs to analyze cost increases for the program to produce mixed-oxide fuel, or MOX.

The MOX plant is being built at the Savannah River Site, a former nuclear bomb plant along the South Carolina-Georgia border where reactors have been shuttered for years. It would be the first of its kind in the United States to create fuel from weapons-grade plutonium and represents an international nonproliferation effort.

Both the United States and Russia have committed to disposing of at least 34 metric tons apiece of weapons-grade plutonium - an amount, according to the National Nuclear Security Administration, that is enough material for about 17,000 nuclear warheads.

Construction on the MOX plant began in 2007, but the project has undergone years of delays as its costs have continued to rise. Last year, the GAO said the project was more than three years behind its 2016 completion deadline and at least $3 billion over budget.

The NNSA, a semi-autonomous agency within the Energy Department, manages the MOX project. In its report issued Thursday, the General Accountability Office criticized the agency for not analyzing the origins of cost increases.

"Without a root cause analysis, it is uncertain whether NNSA will be able to accurately identify underlying causes of the cost increases for the MOX facility," auditors wrote. Because NNSA hasn't done such work, the report went on, the agency "cannot provide assurance that it has correctly identified the underlying causes to ensure that they will not lead to further cost increases as the projects move forward."

Officials have said that rising projects costs are due in part to some contractors being hired before designs were complete. As a result, the NNSA has said, some costs have averaged 60 percent higher than estimated.

The Obama administration has slowed funding on the project, asking Congress last year for $320 million in its 2014 budget - down more than 25 percent from 2012. In its budget request, the administration wrote that its high costs "may make the project unaffordable" and pledged to look for different ways to dispose of plutonium.

Last summer, project contractors received warnings that could be the precursors for hundreds of layoffs, job losses the signatories note could have wide-reaching economic effects on the surrounding area. Citing newspaper reports projecting that 500 layoffs at MOX would cost five surrounding counties about $42 million in payroll, a bipartisan group of South Carolina's Washington representatives wrote to Energy Secretary Ernest Moniz, inviting him to come see firsthand what is happening at the site.

NNSA officials did not immediately return messages seeking comment. In an email, Sierra Club adviser Tom Clements, a longtime critic of the MOX project, said Thursday that more transparency is needed on the costly project.

"There is ample blame to go around for this shocking situation," Clements said. "It's time for the cover-up of MOX costs to end and DOE's National Nuclear Security Administration must immediately release their cost estimate for the mismanaged MOX program so that it can be subjected to public scrutiny."

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Kinnard can be reached at http://twitter.com/MegKinnardAP