The 1,000-megawatt unit 6 in Kozloduy on the Danube was built in 1991 with its 30-year lifespan ending in 2021.

The decision to extend the plant's lifespan followed checks by experts of Russia's Rosatom and France's EDF, according to the Bulgarian nuclear regulatory agency (BNRA).

"I can declare responsibly that unit 6 can operate without any problems over the next 30 years," Kozloduy's chief executive Nasko Mihov told journalists.

The plant would, however, have to renew the licence every 10 years.

Already in 2017, the BNRA greenlighted a similar 10-year license of Kozloduy's other 1,000-megawatt unit 5.

Previously, Bulgaria had to shut four smaller 440-megawatt units at Kozloduy for safety reasons in order to ensure the country's 2007 EU accession.

The government had been keen to extend the lifespan of the state-owned plant's two remaining units, which produce about 34 percent of the country's electricity, Energy Minister Temenuzhka Petkova said.

To compensate for Kozloduy's partial closure, Bulgaria has long planned to build a second twin-reactor plant at Belene, also on the Danube.

In August, seven companies and consortiums, including Russia's Rosatom and China's CNNC, filed offers to become strategic investors in the new 2,000-megawatt nuclear power plant.

The energy ministry is expected to announce the shortlisted candidates in December, then hold talks before inviting them to submit binding offers.

Plans to build the second plant have met with opposition.

Some experts say it would not be economically viable as neither Bulgaria -- under US and EU pressure to limit its energy dependence on Russia -- nor its neighbours are expected to need new energy sources before 2035.

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EDF - ELECTRICITE DE FRANCE