By Tsvetelia Tsolova

SOFIA (Reuters) - Bulgarians are expected to vote in favor of building a new nuclear power plant in their first referendum in the post-communist era on Sunday, challenging the government's decision to abandon the multi-billion-dollar project.

The plebiscite will be seen as a test of public support for the policies of rightist Prime Minister Boiko Borisov ahead of a July election, even though high turnout requirements mean the result of the nuclear referendum is not likely to be binding.

Only 9.34 percent of the voters in the Balkan country had cast a ballot by 1 p.m. (6.00 a.m. ET), six hours before the polling stations were to close, official data showed. Analysts said the apparent voter apathy was mainly due to lack of expert information on the cost, benefits and need for a new plant.

Borisov, already struggling to revive a lackluster economy, canceled the construction of the 2,000-megawatt plant at Belene in March, saying the Balkan country could not afford to foot the bill, estimated at more than 10 billion euros ($13.5 billion).

Bulgaria's allies in Brussels and Washington also opposed the project, fearing it would deepen the country's economic and political dependence on Russia. Moscow offered to finance the plant, which would have been built by its Atomstroyexport.

But opinion polls suggested two thirds of people who said they intended to vote would choose to press on with the plant, many of them hoping it would rein in electricity prices, create jobs and help make Bulgaria an energy hub for southeast Europe.

"Bulgaria needs a new nuclear power plant. I do not want my kids to pay high electricity bills and that's what will happen if we give up the construction of the Belene plant," engineer Georgi Avramov, 49, told Reuters ahead of the vote.

FRUSTRATED VOTERS

While the economy has emerged from a deep recession, it is growing only slowly and many voters are frustrated Bulgaria still trails other ex-communist members of the European Union, with wealth per capita less than half the bloc's average.

Analysts said a strong vote in favor of Belene would be an embarrassment for Borisov, a former bodyguard who has made little progress in his promises to root out corruption since he came to power in 2009.

Unpopular austerity measures imposed by Borisov's ruling center-right GERB party have already narrowed its lead over opposition Socialists - who called for the referendum - ahead of parliamentary elections in July.

"If we get a strong 'pro-Belene' vote as expected, even though the results are likely to be invalid, we would already have a serious political issue on which Borisov will have to act very carefully," said Gallup International political analyst Kantcho Stoichev.

The result of the referendum, which asked whether new nuclear power plant should be built and not about the use of nuclear power in general, will only be valid if 4.35 million out of the 6.9 million eligible voters take part - a figure analysts say will be almost impossible to reach.

If 20 percent of voters participate and half of them vote in favor of Belene, the issue will go to parliament for a final decision.

Bulgaria has an operational 2,000 megawatt nuclear power plant at Kozloduy and has hired U.S. firm Westinghouse to draw up plans to add another 1,000 MW unit at the site.

The new plant at Belene, on the River Danube, was also opposed by environmentalists, who said it would be built near an earthquake-prone area, and by rights groups who said the high-cost project would encourage corruption in the EU's poorest member state. ($1 = 0.7421 euros)

(Additional reporting by Angel Krasimirov; Editing by Mark Heinrich)