Bulgaria's Kozloduy nuclear power plant said Monday it had shut down one of its two reactors following a problem in the cooling system, but insisted this had no impact on radioactivity levels.
The shutdown occurred late Sunday.
"The reason for the stoppage was a hydrogen leak in the cooling system of the turbo-generator, which is located in the conventional, non-radioactive section of the unit," the plant said in a statement.
It added that work was underway to fix the problem and get the unit up and running again.
Bulgaria's sole nuclear plant, Kozloduy has only two 1,000-megawatt reactors left, after four smaller units had to be shut for safety reasons ahead of the country's accession to the European Union in 2007.
The plant generates about a third of all electricity produced in the country.