A Russian firm, Atomstroyexport, has won a contract to build a nuclear power plant by the River Danube in Bulgaria.
The two 1,000-megawatt reactors will cost 3.9bn euros (£2.6bn; $5.1bn).
The Russian firm, in which Russian energy giant Gazprom has a 49% stake, will work jointly with France's Areva and Germany's Siemens, Reuters reports.
The first unit at Belene is planned to be ready in six-and-a-half years. An earlier Belene project was frozen in 1991 amid environmental concerns.
Both the Russian firm and a rival Czech consortium led by Skoda JS, had offered to use an existing 1,000 MW pressurised water reactor that Bulgaria had bought in the earlier Belene project.
But Bulgaria decided the older unit would not be used, Reuters reports.
An official at Russia's atomic power agency Rosatom told Russia's Itar-Tass news agency that "all characteristics of the project offered to Bulgaria are consistent with the norms and requirements of the International Atomic Energy Agency".
The Soviet-built Kozloduy plant began operating in 1974
The Belene plant will use two third-generation water-cooled VVER-1000 reactors.
Bulgaria, set to join the European Union in January, agreed to close four Soviet-designed reactors at a nuclear plant in Kozloduy. The two oldest reactors there have already been shut down.
Decommissioning of the old nuclear reactors was a condition for EU accession talks.
Rosatom chief Sergei Kiriyenko said the Belene deal marked "Russia's return to the European market of nuclear technologies".