IT WAS the stuff of modern nightmares: nearly 10st (63 kilograms) of weapons-grade uranium — enough to make 2.5 nuclear bombs — sitting in a poorly guarded research facility in the former Soviet republic of Uzbekistan.

For six years American, Russian and Uzbek officials wrangled over how to dispose safely — and secretly — of the stockpile near the Uzbek capital of Tashkent.

This month they finally pulled it off.

In an unprecedented operation, the highly enriched uranium was taken on a train to a reprocessing plant in Russia, according to the International Atomic Energy Agency. The mission was the first of its kind since the Soviet collapse in 1991, and could serve as a model for dozens of other sites around the region, the IAEA said.

“This may be a small quantity, but it is a very important step forwards — it’s a pathfinder,” Pablo Adelfang, an IAEA official, told The Times. “With the lessons learnt from this one, the others will come easier.”

Environmental groups welcomed the operation, but gave warning that Russia was creating similar problems for the future by exporting nuclear fuel to other countries, including India and Iran. “It’s important that Uzbekistan does not have this kind of material,” said Jan Vande Butte, the head of Greenpeace’s nuclear campaign. “But overall this is not going to reduce the risks because Russia’s policy is to increase exports of such materials.”

Moscow first sent the fuel to Uzbekistan in the Soviet era for use in a ten-megawatt research reactor at the Institute of Nuclear Physics of Uzbekistan, 19 miles from Tashkent.

When the Soviet Union collapsed it was among hundreds of nuclear facilities that were left vulnerable to theft, terrorist attacks or accidents because of poor security and funding.

Concerns about such sites grew after September 11, 2001, and the US set up the Global Threat Reduction Initiative in 2004 to try to stop terrorists from obtaining nuclear material.

The Uzbek reactor was of particular concern because of its large stockpile of spent fuel, Mr Adelfang said. “In other words, it would no longer injure anyone who handled it for a short period of time and therefore would not deter potential thieves.”

The IAEA, the US, Russia, Uzbekistan and Kazakhstan took six years to agree on a plan to transport the fuel by train through Kazakhstan to the Mayak nuclear processing plant in the Russian city of Chelyabinsk. The mission involved about 50 people, but fewer than 10 knew all its details because of security concerns.

Russia is still fighting Chechen rebels who have carried out a string of recent terrorist attacks. This month Kazakh border guards found a bomb near the Aksu railway station on the Russian border.

Uzbekistan is dealing with the aftermath of last year’s uprising in the eastern city of Andijan which security forces crushed, killing dozens of people. US criticism of the massacre soured relations between Washington and Tashkent, complicating the nuclear mission.

Brian Wilkes, a spokesman for America’s National Nuclear Security Administration, said that the US had funded the $11 million (£6 million) operation and provided equipment and training.

The fuel, which was stored under water, was placed in Russian casks in reinforced containers. There were four shipments between January and April 19. For each one, three trucks transported the fuel six miles to a station where they were loaded on a train.

The IAEA is now helping to convert the Uzbek reactor to run on fuel that cannot make a nuclear weapon; 33 such research reactors have been converted. But more than 100 others — many in the former Soviet Union — still run on weapons-grade uranium.

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