Russia will build two new nuclear blocks at Hungary's Paks nuclear power plant.
Russian President Vladimir Putin and Hungarian Prime Minister Viktor Orban reached the agreement during talks outside Moscow on January 14.
Putin said the new units at the Paks plant south of Budapest would double the capacity of the Soviet-designed power station, which is currently two gigawatts.
The head of Russia's state-run Rosatom company, Sergei Kiriyenko, said Russia was offering Hungary up to 10 billion euros (more than $13 billion) for construction of the new units.
Besides Rosatom, France's Areva and the U.S. company Westinghouse had expressed interest in the contract.
Paks is run by Hungary's state-owned MVW, a group that also imports natural gas from Russia's Gazprom and reportedly wants to negotiate a price cut.
Based on reporting by Interfax, ITAR-TASS, and Reuters