Bulgarians vote Sunday in a referendum on a second nuclear power plant, which is being viewed as a political popularity contest ahead of July elections, even if the low turnout expected would invalidate the result.

The EU member's first referendum since the end of communism in 1989 asks Bulgaria's 6.9 million voters the question: "Should Bulgaria develop nuclear energy by constructing a new nuclear power plant?"

Even if stopping short of explicitly mentioning it, at stake is the revival of an on-again, off-again project first conceived in 1987 at Belene on Bulgaria's northern Danube river border with Romania.

Last March, severe financial constraints, difficulties attracting investors and safety worries following the Fukushima disaster in Japan prompted the government to pull the plug.

But the opposition Socialists initiated a referendum, hoping that the generally pro-nuclear Bulgarians would help revive the deal.

Atomstroyexport, the state-owned Russian firm that had been set to construct the plant, also filed a one-billion-euro ($1.3 billion) compensation claim at the Paris-based International Court of Arbitration.

In order to be valid the referendum has to attract participation from as many people as those who voted in the last general elections, or 4.35 million voters, and over half of them must support the "yes" or "no" stance.

Surveys however indicate this will be a tall order, with uncertainties about the project's cost and economic feasibility confusing the majority of voters and increasing apathy.

People "don't know what's being asked," political analyst Ivan Krastev said.

"You cannot ask people to give an expert opinion on a business project.... This makes the poll absurd," he added.

This however has not stopped the ruling right-wing GERB party of Prime Minister Boyko Borisov and the Socialists from perceiving the contest as an important popularity test before the next general elections in July.

If more than 20 percent of the 6.9 million voters take part, and over half of them say "yes", parliament will have to review the issue within three months. But it is not obliged to rule in favour of reviving the deal.

"We do not have money to build Belene whatever the referendum result," Borisov told private Nova television on Friday.

Instead, the government envisages extending the operational life of the two 1,000-megawatt reactors at its only nuclear power plant at Kozloduy beyond their 2017 and 2021 deadlines, Borisov added.

Bulgaria agreed to shut four smaller units at the Soviet-built Kozloduy to secure its European Union accession in 2007, which most Bulgarians had slammed as "treason".

Surveys on the eve of the ballot showed that 60 to 62 percent of an expected 1.6 to 2.1 million people who turn up to vote Sunday will back Belene compared with 38 to 40 percent opposed.

Polls open at 6:00 am (0400 GMT) and close at 7:00 pm (1700 GMT). Exit polls are expected shortly afterwards and preliminary results later on Sunday evening.