"There was a legal obligation to close down the reactors for safety reasons," Piebalgs told a press conference in Brussels.

"To come back to the issue now is like driving down a one-way street in the opposite direction. The (European) Commission is willing to reopen the issue but not in the sense of reopening a closed reactor."

Bulgaria closed down the two nuclear reactors at its only plant at Kozloduy, in the northwest, on December 31 as part of the requirements to join the EU -- which it duly did the following day -- and received 570 million euros in compensation.

But Bulgarian Energy Minister Rumen Ovcharov said on Friday that Sofia would ask the European Union for permission to restart the nuclear reactors to ease energy demands in the region.

Bulgaria has been one of the Balkans' main exporters of energy, exporting some 7.8 billion kilowatt-hours of electricity in 2006.

It mothballed its two oldest 440-megawatt reactors, 1 and 2, in 2002 but these have yet to be dismantled. Sofia then shut down two other old but revamped 440-megawatt blocs, 3 and 4, in order to secure EU accession in 2007.

Only the two most modern 1,000-megawatt reactors remain in operation at Kozloduy and there are no plans to close them down. Bulgaria plans to build a second nuclear plant in Belene in the north of the country in 2013.

The EU has cited studies conducted in 1992 by the G7 group of industrialised nations stating that reactors of the Kozloduy type cannot be modernised at a reasonable price and should be shut down due to security concerns.

The problems caused by the reactor shut-downs is expected to be on the agenda at a meeting of EU energy ministers in Brussels on February 15-16.

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