* Bulgaria would have to pay for one nuclear reactor: PM

* Sofia, Moscow to decide whether to build or resell unit (Adds detail, background)

SOFIA, June 16 (Reuters) - An international court has ruled that Bulgarian state energy firm NEK should pay nearly 550 million euros ($620 million) in compensation to Russia’s Atomstroyexport for a cancelled nuclear power project, Bulgarian officials said on Thursday.

Prime Minister Boiko Borisov said the arbitration court ruling meant NEK would have to pay for one nuclear reactor and needed to decide together with Atomstroyexport what to do with it.

Russian state company Rosatom said in a statement it is satisfied with the decision.

The ruling was made at a court in Geneva under the auspices of the Paris-based International Chamber of Commerce.

Bulgaria, a European Union country, had been under pressure from its Western allies about its energy dependence on Moscow.

It contracted Atomstroyexport in 2006 to build two 1,000 megawatt reactors at Belene on the Danube River, but abandoned the project six years later, after failing to attract western investors for the 10 billion euros ($11 billion) scheme.

Options now include approaching the European Commission for advice on whether to install the reactor, or to seek an agreement with the Russian state nuclear company to resell it to a third party, Borisov told reporters.

“The arbitration ruling in its essence says that NEK and Atomstroyexport should sit together and decide whether they will build that reactor, whether they will sell it and be done with it,” Borisov said.

“I am personally very content with the arbitration court’s objectivity. I was fearing heavy sanctions or fines, but nothing of that sort happened,” he said.

Energy Minister Temenuzhka Petkova said Bulgaria received the decision, which ran to over 700 pages, late last night and that NEK will approach Atomstroyexport in the coming days to seek a solution.

Petkova said the court has ruled that NEK should cover the funds the Russian state company had spent to produce equipment for the project, but has rejected claims for additional works and damages from lost profits.

Atomstroyexport had sought more than 1 billion euros in compensation from NEK over the cancellation of the Belene plant.

Bulgaria is currently extending the lifespan of its two 1,000 Soviet-made reactors at its Kozloduy nuclear power plant and is looking for an investor to build one more reactor at the same site. ($1 = 0.8887 euros) (Reporting by Angel Krasimirov and Tsvetelia Tsolova; Editing by Ruth Pitchford)