The Belene nuclear power plant is the largest bilateral project involving the two countries. Two power units with Russian VVER-1000 pressurized water reactors are to be built.
Atomstroyexport, Russia's nuclear power equipment and service export monopoly, won the tender in 2005 for the Belene contract, worth around 4 billion euros ($5.8 billion).
The French-German consortium Areva-Siemens is also involved in the project.
"This project is both Bulgarian and European," Bulgarian PM Sergei Stanishev said, adding that it had undergone a thorough inspection by the European Commission.
Atomstroyexport commissions over 20% of global orders. It is currently involved in the construction of two power units for India's Kudankulam power plant, and is building a power unit for Iran's Bushehr NPP.
The Russian company also took part in the construction of the Tianwan nuclear power plant in China, where two power units have been built and are now being tested.