The transfer of around 700kg of highly enriched uranium (HEU) from Dounreay in Scotland to the US has been completed.
The HEU will be down-blended in the US to be used as fuel in civil nuclear reactors.
The transfer was initially announced by the UK Government as part of its commitment to the 2016 Nuclear Security Summit in Washington DC.
David Peattie, CEO of the Nuclear Decommissioning Authority (NDA) said: “The successful completion of the complex work to transfer HEU is an important milestone in the programme to decommission and clean up Dounreay site.”
The US is sending a different form of the material to Europe, where it will be used as research reactor fuel and in production of medical isotopes.