An upgrade to a Soviet-era reactor in Bulgaria's sole nuclear power plant has been completed, extending its lifespan until 2051, the Energy Ministry said on Wednesday.

The Kozloduy plant, located next to the river Danube on the border with Romania, has two 1,000 megawatt reactors that produce about 33 percent of Bulgaria's electricity.

"The results of the completed works indicate safe operation of the plant's equipment, buildings and systems of Unit 6, until 2051," the ministry said in a statement

Bulgaria's Nuclear Regulatory Agency is expected to grant a 10-year extension to operating licence for Unit 6, whose licence is due to expire in 2019. The regulator offers licences for a maximum of 10 years.

The upgrade to a second reactor in the plant, Unit 5, was completed in 2016. At the time of completion, the ministry said the upgrade would keep the reactor working for 30 more years.

The cost of the project to upgrade both reactors, launched in 2015, was 360 million euros ($420 million).

Electricity prices are a sensitive issue, as power bills particularly in winter months account for a large part of household spending. Protests about high energy bills toppled the centre-right government of Prime Minister Boyko Borisov in 2013.