File photo — The Defense Waste Processing Facility at the Savannah River Site near Aiken, S.C. in 2013. The Energy Department began to cure an environmental nightmare at the site in 1996 by opening a factory to turn liquid radioactive wastes into a safer form. (Stephen B. Morton/AP)

A group of more than a dozen prominent former arms negotiators and senior diplomats has sent a letter to Energy Secretary Ernest Moniz urging an end to the U.S. nuclear fuel program at the government’s Savannah River complex that they say is too costly and a threat to non-proliferation efforts.

The project at the Energy Department’s sprawling Savannah River site in South Carolina, which over 15 years has cost U.S. taxpayers nearly $5 billion, was designed to take plutonium no longer needed for nuclear weapons and turn it into fuel for commercial nuclear reactors. The goal was to dispose of 34 metric tons of excess weapons grade plutonium by mixing it with uranium oxide and creating a mixed-oxide fuel, or MOX, for commercial nuclear reactors.

The letter stressing national security and earlier reports on the soaring cost estimates for the MOX plan have put mounting pressure on the Obama administration to scrap the program at the 310-square-mile Savannah River site near Aiken, S.C., despite strong support from the Nuclear Energy Institute and South Carolina’s powerful congressional delegation.

Critics say the MOX project, which has ballooned far beyond original estimates, is a political boondoggle that should be shut down. They say there are less expensive options for disposal.

The letter sent Tuesday zeroes in on other countries — including South Korea and China — that have shown interest in pursuing civilian reprocessing, which would separate fissionable plutonium from nuclear waste that can then be recycled in a MOX process. Japan is on the verge of starting up a reprocessing facility.

File photo -- Dressed in contamination suits, three workers are watched by a crane specialist on a video monitor in the control room of the Defense Waste Processing Facility at the Savannah River Site in 2013. (Stephen B. Morton/AP)

The United States has discouraged these projects for fear they would generate and spread plutonium that could be used for nuclear weapons.

“In addition to saving money, ending the current MOX program would be in the nation’s national security interest,” the letter said, noting that the United States would have more credibility with other nations if it ended its own MOX program and treated the plutonium as waste.

In a key part of the Iran nuclear deal, Tehran agreed to reconfigure its reactor near Arak to produce medical isotopes instead of plutonium.

For the Obama administration, the domestic politics of the MOX project might be the most important factor. Sen. Lindsey O. Graham (R-S.C.), generally a budget hawk, in 2013 threatened to put a hold on Moniz’s nomination for fear the administration might abandon the Savannah River project. Former House Democratic majority whip James E. Clyburn (D-S.C.) has also supported the facility; his district includes part of the Savannah River site.

On Sept. 4, the South Carolina Attorney General Alan Wilson wrote a letter to Moniz warning that the Energy Department had “legally binding requirements” to make the MOX project work. He said that dropping the MOX project would cost more than 1,500 jobs and breach an agreement with Russia. The Energy Department had earlier discussed freezing work on the MOX plant while it reviewed options.

The letter sent to Moniz on Tuesday urging cancellation of the project was signed by a group of prominent people from across the political spectrum.

The signatories included former nuclear arms negotiators Robert Einhorn and Robert Gallucci; former ambassadors Thomas Pickering and Joseph Nye; former White House director for arms control, former Pentagon and intelligence official Henry S. Rowen; former head of the Carnegie Endowment for International Peace Jessica Matthews; former Nuclear Regulation Commission members Peter Bradford and Victor Gilinsky; National Medal of Science winner and a designer of the first hydrogen bomb Richard Garwin; and nuclear policy experts Henry Sokolski, Frank von Hippel, S. David Freeman and Ploughshares Fund president Joseph Cirincione.

“Continuing with the MOX program and recycling work helps plutonium recycling advocates in Japan, China, South Korea, and other states maintain the illusion that plutonium separation and recycle are activities that responsible non-weapon states engage in,” the letter said. “The United States has for four decades consistently opposed the spread of such activities because of the obvious proliferation danger of putting nuclear-weapons explosive materials into commercial channels.”

By abandoning its own program, the United States would gain credibility in dealing with those countries, the letter said. “None of these activities make economic sense,” the letter says. A U.S. renunciation of reprocessing would be timely with an international nuclear summit scheduled to take place next spring. Japan is close to opening a large reprocessing plant at Rokkasho.

Congress has been weighing the renewal of civilian nuclear cooperation with China; the current text of that agreement would allow China to launch a reprocessing facility using U.S. technology or material without approval from the U.S. government.

The civilian nuclear agreement with South Korea barred that country from developing its own reprocessing facility.

“So far the debate has been whether or not the U.S. should continue its MOX program at Savannah River,” said Sokolski, an organizer of the letter. “This letter ups the ante by turning this budgetary issue into an occasion to get others, starting with Japan, China and South Korea, to defer the commercial production and stockpiling of nuclear explosive plutonium in the name of international security.”

With the new fiscal year approaching, cost concerns are key, too. The letter comes on the heels of an August Energy Department report — dubbed the “red team” report. It reviewed an earlier cost estimate by the Aerospace Corp. In the end, both reports concluded that it would be far cheaper to blend excess weapons plutonium down to waste rather than using the mixed oxide, or MOX, process.

The red team report, written largely by experts at the department’s national laboratories, said that the reprocessing option would require an increase in funding from the current nearly $345 million a year. With twice that, reprocessing could possibly begin after 15 years of construction and about three years of commissioning at a total cost of about $30 billion.

The president’s fiscal year 2016 budget proposes no change in funding. But in a June interview with the Aiken Standard, Moniz said that it would take “north of $1 billion a year” for decades to complete. In what was likely a preview of other budget debates, Moniz said that the determination of Republicans to stick to mandatory non-defense spending cuts known as sequester would make it unlikely that Congress could increase funds.

The red team report said it would cost less than half that much to dilute the plutonium and ship it to a deep underground storage facility called the Waste Isolation Pilot Plant in New Mexico.

The red team report also said that the lack of sustained funding had caused “project inefficiencies and strained relationships between DOE and the contractor. This uncertainty has in turn led to a lack of workforce confidence in program stability, resulting in low levels of staff retention (exacerbated by loss of the most qualified workers), and low morale in the remaining workforce.” It said that “the downward performance spiral is accompanied by an upward cost escalation spiral.”

The earlier Aerospace report reached similar conclusions on costs. Those conclusions were first publicized by the Union of Concerned Scientists. The Aerospace report put life cycle costs at $47.5 billion — without decommissioning and demolition costs, UCS said. The UCS says that the reprocessing plant ultimately would cost tens of billions of dollars more to operate.

The National Defense Authorization Act requires a second Aerospace study that will look at five different options for disposing of the plutonium. An senior Energy Department official said they would make a decision after receiving that report later this month.

Moreover, there aren’t any utilities rushing to buy the MOX fuel. Edwin Lyman, a senior scientist in global security at the Union of Concerned Scientists, said that the MOX fuel is more expensive than alternative nuclear fuels. He said utilities want incentive fees to take the MOX fuel because it would force nuclear reactor operators to make plant modifications, such as in the control rods, and take new security precautions.

“It is unlikely a utility will want to take on this risk without being compensated,” Lyman said.