A vote on whether to build a second nuclear power plant in Bulgaria, the EU member's first referendum since the end of communism in 1989, looked set to fail Sunday because of insufficient turnout.

Final exit poll counts showed that between 20.5 and 21.8 percent of voters cast ballots, about a third of the 4.35 million people needed for the result to be valid.

"Should Bulgaria develop nuclear energy by constructing a new nuclear power plant?" the referendum asked.

Although it did not mention it explicitly, at stake was the 2,000-megawatt Belene power plant project on the Danube river in northern Bulgaria, first launched back in 1987.

Severe financial constraints and a lack of foreign investors after the 2009 withdrawal of German energy giant RWE prompted Prime Minister Boyko Borisov's rightwing government to abandon the project last March.

The opposition Socialists initiated the referendum after Atomstroyexport, the Russian state-owned firm set to build Belene, filed a compensation claim for one billion euros ($1.3 billion).

The result of the vote, although likely invalid because of the low turnout, was in support of reviving the project, with between 60.3 and 61 percent voting "yes" and between 39 and 39.7 percent voting "no", the exit polls showed.

Partial official results with 36.2 of the ballots counted showed 58.4 percent "yes" votes compared to 40.4 percent "no", with almost 1.2 percent of ballots invalid, the central electoral committee said.

The majority of voters in otherwise pro-nuclear Bulgaria were either apathetic or did not feel qualified to make a decision amid confusion about the costs and Belene's economic viability, analysts said.

"You cannot ask people to give an expert opinion on a business project.... This makes the poll absurd," political analyst Ivan Krastev said before the vote.

Alpha Research analyst Boryana Dimitrova said the "badly formulated, ambiguous question" had puzzled voters, who could not decide if it was only asking about Belene or nuclear power in general.

This has not stopped the former communist Socialist party and Borisov's GERB party from casting the referendum as an important popularity test before July general elections.

A recent Alpha Research poll showed waning support for GERB at about 21 percent, still ahead of the Socialists on 18.5 percent -- although the former communists' popularity has been rising.

Both sides claimed victory in the referendum.

"The Bulgarians once again demonstrated common sense, understanding that this issue does not even deserve their attention," Borisov commented.

His main opponent, Socialist party leader and former premier Sergey Stanishev, countered: "This result is a personal defeat for Borisov.... It is clear for what and against whom the people voted."

"With turnout of about 20 percent, I do not think that anyone should speak of victory or defeat," political analyst Ognyan Minchev said.

The government has already announced plans to extend the operational life of the two 1,000-megawatt reactors at Kozloduy, Bulgaria's only nuclear plant, beyond their 2017 and 2021 deadlines.

It is also considering adding a third reactor to the Soviet-built plant, which was partially mothballed under pressure from Brussels on the eve of Bulgaria's accession to the European Union in 2007.

Kozloduy still generates 34 percent of all electricity produced in the country, allowing Bulgaria to keep its position as a top electricity exporter in the Balkans.

The Belene project was a priority for Atomstroyexport as its first in an EU member state.

Critics however have warned that it would strengthen Moscow's energy grasp on the small former Soviet-bloc state, which gets almost all its oil and gas from Russia.

The central electoral committee has yet to announce its official turnout figures. Final results are expected within three days.

If participation is confirmed to be over 20 percent and over half of voters join the "yes" camp, parliament will have to review the issue within three months -- though Borisov has vowed to again strike it down.

His party has a near majority in the 240-seat legislature and can rely on the support of a small rightwing formation and some independent lawmakers to overrule the project again.