The US Department of Energy is starting construction of a controversial facility to recycle nuclear warheads.

On 1 August, work finally began on a US$4.8 billion mixed-oxide fuel-fabrication facility at the Savannah River Site in South Carolina. The plant will convert surplus, weapons-grade plutonium into fuel for sale to commercial power reactors in North and South Carolina.

Supporters claim that the facility will help to dispose of dangerous weapons material safely. But sceptics, including some key members of Congress, balk at its enormous cost and question whether it will improve the nation's nuclear security.

Since the 1990s, the department has touted mixed-oxide fuel as the best way to dispose of 34 tonnes of highly enriched plutonium sitting in the US stockpile, to fulfil its disarmament agreement with Russia. The technique blends plutonium with uranium so that it can be burned in conventional light-water reactors. Dozens of European reactors run on mixed-oxide fuel made from old commercial fuel, according to Matthew Bunn, a non-proliferation expert at Harvard University in Cambridge, Massachusetts.

The US plant design is actually a smaller version of the commercial French Melox reprocessing facility at Marcoule. Engineers have modified the design slightly to handle weaponized plutonium alloys, and they have reduced shielding because weapons-grade isotopes are less radioactive than spent commercial fuel.

The programme gained political momentum in part because Russia was planning to build a sister facility to convert an equal amount of its own plutonium surplus, according to Daryl Kimball, executive director of the Arms Control Association in Washington DC. But the Russians have backed away from the plan in recent years, Kimball says.

The US plant has also been beset by policy reviews, a lengthy licensing process and congressional opposition. It is now more than a decade behind schedule, and construction costs have ballooned by nearly 400% since 2002.

Nevertheless, department officials stand by the programme. In addition to fulfilling their disarmament obligations, the plan will be cheaper than long-term storage, Secretary of Energy Samuel Bodman told Congress.

Bunn disagrees, in part because the department will be storing other plutonium anyway. “The cost of having a few extra bunkers is actually very modest,” he says.

Which side Congress will take remains to be seen. The Senate is proposing roughly $390 million for construction in fiscal year 2008, but House appropriators want to give it just $168 million. A final compromise is expected to be reached later this autumn.