Just days before Christmas, a secret flight took off from the Czech Republic heading for Russia.
The US is anxious to help Russia in its efforts to recover the material
Until it touched down amid tight security, the details of the flight were kept highly classified for fear of terrorists intercepting the cargo - four specialised transport canisters containing 6kg of highly enriched uranium which could be used for nuclear weapons.
The flight marked a further step in an increasingly aggressive programme to secure nuclear material by Russia and the US amid continuing fears that gaining nuclear material is a priority for al-Qaeda.
Meeting in London on 4 January were the two top officials involved in the US-Russian efforts - US Secretary of Energy Spencer Abraham and Director of the Russian Federal Atomy Energy Agency Aleksandr Rumyantsev.
They told the BBC news website that they were accelerating their protection programme and expanding the scope of co-operation between their two countries to try to ensure that no nuclear material could fall into the wrong hands.
"If terrorists somehow managed to get hold of fissile material then the consequences would be devastating," Mr Rumyantsev said.
And he warned that even if the number of casualties was low, the psychological impact of something like a dirty bomb would compare with the impact Chernobyl had on the Russian psyche.
After the end of the Cold War, the biggest concern was so-called "loose nukes" in the former Soviet Union where there were more than 27,000 nuclear weapons.
The fear was that poorly secured nuclear weapons could be stolen by criminals or terrorists.
Spencer Abraham was in London to meet Russian officials
Since then major efforts have been undertaken jointly by the US and Russia to try to prevent this by destroying weapons and improving security at sites.
But while securing such weapons remains a priority, there is now increased concern that nuclear materials rather than a fully developed weapon might become the target for terrorists.
Al-Qaeda's desire to get hold of nuclear material is longstanding and was recognised by British intelligence at least as early as 1998, although some of Osama Bin Laden's early attempts to secure such material proved amateurish and unsuccessful.
However, recent reports suggest Osama Bin Laden's desire to get hold of some kind of nuclear material is undimmed, and concern will only have been heightened by news that in 2003, he sought and received approval from a Saudi cleric for the use of a nuclear weapon against the US.
As well as the Czech flight, there have also been deals with Serbia, Bulgaria, Romania, Libya and Uzbekistan to return materials from reactors
However, most experts believe that a dirty bomb - involving the dispersal of radiological material by an explosion - is a far more plausible threat than the detonation of a nuclear warhead.
The former requires far less technical know-how, merely the combination of a traditional bomb with whatever material terrorists can lay their hands on.
To counter this, the US and Russia are placing a growing emphasis on a "global clearout" that reaches beyond the two nations and beyond just nuclear weapons by covering things like nuclear fuel held at research reactors in third countries.
So far, as well as the 22 December Czech flight, there have also been deals with Serbia, Bulgaria, Romania, Libya and Uzbekistan to return materials from reactors back to either the US or Russia where the technology was developed.
"The significance of this can't be overestimated," Spencer Abraham told the BBC news website.
The task, though, is huge - more than 100 research reactors around the world run on weapons grade highly enriched uranium and the hope is to convert many of them to use lower enriched uranium fuel which is less dangerous.
America's Global Threat Reduction Initiative aims to remove or secure all high risk nuclear and radiological materials around the world but one of the biggest tasks is simply trying to make an inventory of what materials are out there.
The close co-operation between the US and Russia and between Mr Abraham and Mr Rumyantsev has achieved much, but for those worried about nuclear proliferation and terrorism, the biggest challenge may come not from Russia, but from states which have more recently sought or achieved nuclear capability.
These would include Pakistan, where some scientists are thought to have been in contact with al-Qaeda, and also North Korea, where there are long-standing concerns about the passing on of technology.
As more states try to acquire nuclear weapons, the challenge to stop nuclear materials falling into the wrong hands is likely to grow more and more demanding.