WASHINGTON (Reuters) - Switching the disposal of weapons-grade plutonium to New Mexico from an existing South Carolina site could cost the U.S. government more than $20 billion and raise safety concerns, according to a new study commissioned by the firms building the current site.

The report by High Bridge Associates, the consulting firm hired by the joint venture building the South Carolina site, is the latest of several dueling studies. Others say a shift to a New Mexico facility could save billions of dollars.

High Bridge, a consultant to the nuclear industry, in its report said disposal of significant quantities of plutonium at the Department of Energy's Waste Isolation Pilot Project in New Mexico would pose serious risks, including the potential for an uncontrolled nuclear chain reaction.

High Bridge was hired to produce the report by CBI-Areva MOX Services, a joint venture owned by France's Areva SA and Chicago Bridge and Iron NV.

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CBI-Areva is building a facility at DOE's Savannah River site in South Carolina that will mix weapons-grade plutonium with uranium to form safer mix-oxide (MOX) fuel pellets for use in commercial nuclear reactors.

The report said it would cost $3 billion to complete work at the MOX site, which could begin operations in 2022. Moving the work to New Mexico could entail more than $20 billion in costs arising from safety measures and environmental impact studies, it said.

The High Bridge report focused attention on the planned use of 55-gallon drums as packaging for the plutonium, a use not currently approved at the WIPP site. Growth in salt chambers there could crush the drums and trigger a nuclear chain reaction, the report said. A recent report by Stanford University published in the journal Nature raised similar concerns.

A 2000 agreement between Russia and the United States called for both sides to dispose of 34 metric tons of plutonium, the equivalent of about 17,000 nuclear weapons. Changing course would violate the agreement, the report said.

The DOE's National Nuclear Security Administration has noted that several analyses show diluting the plutonium and disposing of it at the WIPP site would cost less than half of the MOX approach.

The DOE had no immediate comment on the High Bridge report.

Congress has blocked prior DOE efforts to reduce funding for the MOX facility and shift disposal to New Mexico. The DOE's fiscal 2017 budget request, set for release next week, is expected to envisage a shift to the WIPP site.

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(Reporting by Andrea Shalal; Editing by Bill Trott and Leslie Adler)