Approximately 17kg of highly enriched uranium (HEU) was transported from the Russian-supplied 2MWt IRT Sofia research reactor in Bulgaria to Russia on 23 December 2003.

The move is part of the Russian Reactor Fuel Return Initiative led by the USA and the IAEA and saw the weapons-usable uranium, enriched to 36% U-235, airlifted from Gorna Oryahovista to Dmitrovgrad, in Russia, to be refabricated into low-enriched uranium fuel. The cost of the removal and airlift – some $400,000 – was met by the USA.