At the same time, plutonium is preferred over uranium as nuclear bomb fuel because much less is needed to make a blast of equal size. And while it is difficult to work with, it does not need to undergo the complex process of purification required for uranium.

The 43 tons of surplus plutonium in the American stockpile could fuel up to 10,000 nuclear weapons and even more “dirty bombs” — ordinary explosives that spew radioactive debris. Alternatively, they could fuel 43 large reactors for about a year.

After studying a range of options, the Clinton administration decided to build a mox fuel plant to dispose of a portion of the plutonium, awarding a contract to a consortium now called Shaw Areva Mox Services.

The rest of the plutonium was to be mixed with highly radioactive nuclear waste and immobilized in glass or ceramic blocks, making it difficult and dangerous for any thief to extract. The government judged the mox route to be more expensive, but the dual-track approach was seen as insurance should either fail.

That strategy also helped persuade Jim Hodges, the Democratic governor of South Carolina from 1999 to 2003, to sign off on plutonium shipments to the Savannah River Site. When the Bush administration canceled the glass-block disposal program, Mr. Hodges was furious.

His concern, he said in a recent interview, was that South Carolina would become a dumping ground if the mox program did not work out because of political or technical difficulties. “That site was never designed for long-term plutonium storage,” he said. “We were concerned about health and safety.” Now, he said, that dumping ground is in danger of coming to pass.

Mr. Abraham said that budget cuts had made it necessary to end one of the programs, and that with the Russians favoring mox, the administration had feared that going the other route would discourage Moscow from keeping its end of the bargain. (Only later, Mr. Abraham added, did he decide to join Areva in a largely advisory role.)