Unit 5, one of the NPP's two operating Soviet-made reactors, was switched back on following an inspection and positive assessment by the country's Nuclear Regulatory Agency, Kozloduy NPP said in a statement.
A total of 43 cartridges of fresh nuclear fuel delivered by U.S.-based Westinghouse Electric were loaded into the reactor, marking the start of a four-year phased transition to the Robust Westinghouse Fuel Assembly (RWFA) type at Unit 5.
The nuclear power plant operator also carried out preventive maintenance of the facilities, inspections and tests to ensure the reactor's long-term operation and increased safety.
Last month, Kozloduy NPP CEO Valentin Nikolov said the plant will terminate its contract with Russia for the supply of fresh nuclear fuel, having switched to Westinghouse's fuel for Unit 5.
In January, the plant received a permit from Bulgaria's nuclear energy regulator to store the RWFA fuel type on-site. The Nuclear Regulatory Agency said in April that it granted Kozloduy NPP a permit to switch to the new fuel type.