The incident caused no radioactive leakage and was rated at zero on the International Nuclear Event Scale (INES), management said.

The problem occurred at one of the plant's two 1,000 megawatt reactors and was discovered during a routine safety check, it added.

The Kozloduy plant provides 47 percent of Bulgaria's electricity and has two older Soviet-era 440-megawatt reactors.

Both are to be closed down in 2006 under an agreement with the European Union, which has expressed safety concerns over the plant.

Two other 440-megawatt reactors were shut down in 2002 under pressure from the EU, which Bulgaria hopes to join in 2007.

WAR.WIRE