Former U.S. Energy Secretary Bill Richardson wants the Obama administration to keep alive a South Carolina project to turn weapons-grade materials into commercial nuclear reactor fuel.

In a letter dated Aug. 21 to Senate Democratic leader Harry Reid and obtained by The Associated Press this week, Richardson said the mixed-oxide fuel project under construction at the Savannah River Site is the best way to fulfill an agreement with Russia. Richardson oversaw the deal as energy secretary in 2000.

"I write today to express my profound concern over determined attempts by some to use the budget process to unravel our progress in meeting key international nuclear non-proliferation commitments," Richardson wrote.

Richardson was secretary when the countries agreed to dispose of 34 metric tons each of weapons-grade plutonium, an amount officials say is equal to 17,000 warheads.

The project known as MOX is two-thirds complete but is billions of dollars over budget and years behind schedule. The Obama administration said last year it wanted to put the project on hold, citing billions in cost overruns and project delays, prompting a lawsuit from South Carolina. That suit was dropped when the administration agreed to temporarily fund the project.

Richardson says several reports proposing an alternative method to dealing with the plutonium were "made-to-order" by project opponents. According to one study by an Air Force contractor "with absolutely no nuclear construction experience or expertise," Richardson wrote, the project would cost more than $50 billion over its life cycle and would take nearly 30 more years to complete. Both figures, Richardson said, "vastly defy" government estimates.

Another report released about a week ago estimated that the project would need up to $800 million annually to be viable, contrasted to the roughly $400 million currently allocated to the effort.

Critics of the MOX project often mention a method known as downblending — a process that involves mixing the plutonium with another material to stabilize it, then storing it at a repository, like one in Richardson's home state of New Mexico — as a less costly alternative.

But in his letter, Richardson says the facility has been closed since 2014 due to several accidents there and that any method but MOX would violate the agreement with Russia.

"It is time to finish what we start," Richardson wrote to Reid, asking him to push the Energy Department to come up with updated cost and time estimates for MOX. "I hope you'll join me in showing the Department of Energy and others that abandoning our non-proliferation commitments is a very bad idea."

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