The referendum on the fate of nuclear energy in Bulgaria is a personal slap on the face for Bulgarian Prime Minister, Boyko Borisov.

The statement was made by former Energy Minister, and one of the key people in the opposition, left-wing Bulgarian Socialist Party, BSP, Rumen Ovcharov, minutes after the first exit polls results emerged.

"The results show that this is the beginning of Borisov's end because many of the members of his ruling Citizens for European Development of Bulgaria party, GERB, have voted yes on the question, yes for building a second Nuclear Power Plant in the Danube town of Belene," said Ovcharov – one of the strongest proponents of the Belene project.

He further said he would wait for final results in order to declare victory for the Socialists.

A turnout of 20.3% was announced Bulgaria's Alfa Research polling agency minutes before the closing of the polling sections at 7 pm in Bulgaria's first national referendum since 1989.

The referendum asks the question "Should nuclear energy be developed in the Republic of Bulgaria through the construction of a new nuclear power plant?"

The vote will be binding if the number of votes equals that of votes at the last parliamentary elections, 4,225,124, and if more than half of them vote "yes".

If less people vote on the referendum, but if the turnout is over 20%, and more than half of the votes are positive, then the question is tabled for debate to parliament.

At 10 am, the Central Electoral Commission had reported a turnout of 2.1%, at 1 pm it rose to 9.34%, and to 17.4% at 5 pm.

The referendum was sponsored by the Bulgarian Socialist Party, which gathered a petition of more than the 0.5 million legally needed signatures.

Originally, Belene NPP had to be built by Russian state company Atomstroyexport, but the Bulgarian cabinet froze the project in the early spring of 2012 due to perceived lack of economic effectiveness.