Bulgaria's sole nuclear power plant Kozloduy closed one of its two 1,000- megawatt units for a scheduled annual overhaul at 2 a.m. on Saturday.
The repairs, during which the reactor will be refueled, are scheduled to be completed at the end of May, the plant said in a statement.
Kozloduy's other 1,0000 MW unit was working at full capacity, the statement added.
On Friday the government said the plant had signed a contract with a consortium led by France's EDF and Russia's Rosenergoatom for preparatory works for extending the lifespan of its two units by up to 20 years. No details about the financial parametres of the contract or its timeframe were disclosed.
Kozloduy remained with just two operational reactors after the government closed down four units of 440 MW each to address nuclear safety concerns of the European Union prior to the country's accession to the bloc. Bulgaria joined the EU in 2007.
Earlier this month the government in Sofia decided to add a new unit to the Kozloduy plant, after dropping a project to build a second nuclear plant in the country.