It will cost about $20 billion to dispose of 34 metric tons of surplus weapon-grade plutonium by burying it in the New Mexico desert, compared with almost $50 billion to convert the material into commercial reactor fuel using an unfinished facility in South Carolina, Energy Secretary Rick Perry told Congress on Thursday.

The preliminary life-cycle cost figures, offered without much further explanation in a letter to the leaders of the Senate Armed Services strategic forces subcommittee, were part of Perry’s official — and long expected — declaration that he is canceling the Mixed Oxide Fuel Fabrication Facility (MFFF) under construction at the Department of Energy’s Savannah River Site in Aiken, S.C.

The 2018 National Defense Authorization Act allowed Perry to waive a legal requirement to build the MFFF if, among other things, DoE certified an alternative method of eliminating surplus weapons plutonium was about half as expensive as simply finishing the plant.

“This letter constitutes my execution of that waiver authority,” Perry wrote in a letter to the Senate panel with jurisdiction over DoE’s nuclear-weapon programs.

The MOX plant, being built by CB&I AREVA MOX Services, was put under contract in 1999 to fulfill the terms of an arms control pact that requires the U.S. and Russia to each eliminate 34 metric tons of surplus plutonium left over from the Cold War arms race.

The project’s cost and schedule have since run off the rails. In 2015, DoE asked Congress to cancel the MFFF and instead allow the agency to dilute the plutonium at proposed Savannah River Site facilities, mix it with concrete-like grout, and bury it deep underground at the Waste Isolation Pilot Plant near Carlsbad, N.M.

Congress must still appropriate funds for this so-called dilute and dispose alternative. A draft appropriations bill due to be marked up by the House Appropriations Committee on Wednesday does not include such funding, but could be amended during the session.

The Energy Department is legally barred from halting MFFF construction until a month after Perry’s delivers the life-cycle cost estimate for dilute and dispose, according to the 2018 omnibus appropriations bill signed into law in March. 

Meanwhile, DoE last week formally announced plans to repurpose the MFFF to produce plutonium nuclear-warhead cores.