How much will it cost to build what could well be the first machine to achieve a self-sustaining nuclear fusion reaction? Apparently no one really knows, and that information vacuum hasn’t been sitting well with some influential US lawmakers.

Construction of the international prototype fusion reactor is under way at Cadarache in southern France, but the project is proceeding without a formal cost baseline. In May, four senators who hold the purse strings for the US contribution to ITER ordered an investigation into ITER’s cost; they asked the Government Accountability Office to figure out how much the project will cost and what the US will have to pay.

During a hearing of the Appropriations Committee’s energy and water development subcommittee, which she chairs, Senator Dianne Feinstein (D-CA) asserted that the US share of ITER will reach $3 billion. The Department of Energy’s fiscal year 2014 budget request, released in April,...

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