The United Nations nuclear watchdog has coordinated a multinational project carried out by Serbia to move two and half tonnes of highly radioactive nuclear spent fuel to a secure Russian facility, where the material arrived on Wednesday.

The International Atomic Energy Agency (IAEA) said it coordinated the removal of the material from a Serbian nuclear research reactor where it posed potential security and environmental threats.

It was the largest single shipment of spent nuclear fuel made under an international programme to repatriate such material to the nations that originally supplied it, the IAEA said in a press release.

The delivery of the spent fuel to the Russian facility ends the project to repatriate fuel from the Vinca Institute of Nuclear Sciences outside Belgrade, where the Soviet Union had built and fuelled a 6.5-megawatt nuclear research reactor in the 1950s.

The project began in 2002 when fresh highly enriched uranium (HEU) fuel was transferred to Russia. Today’s shipment consisted of over 8,000 spent fuel elements, including 13 kilogrammes of HEU.

“This was a very complicated project. We had to involve governments, contractors, and non-governmental organizations,” said Yukiya Amano, the IAEA Director General. “It was a great success. It was a success story and we are very happy to continue to cooperate with stakeholders to repatriate highly enriched uranium,” he added.

The latest fuel transfer began on 18 November, when 16 shipping containers holding the fuel were loaded onto heavy cargo trucks at the Vinca Institute.

Using trucks and trains, the convoy traversed Serbia, Hungary, and Slovenia under heavy security before arriving on 21 November at the Slovenian port of Koper. There, crews loaded the containers onto a cargo ship which then began a three-week journey to Russia’s arctic port at Murmansk.

Back on rails, the fuel moved to Russia’s reprocessing facility at Mayak, where technicians will separate the still-usable uranium from the spent fuel and store the remaining nuclear waste for future safe disposal.

The IAEA has actively participated in efforts to repatriate research reactor fuel, including transfers from the Czech Republic, Hungary, Libya, Romania, and Viet Nam. In addition, the IAEA is supporting efforts to help nations convert their research reactors to use low-enriched uranium fuel.