Bulgaria switched one of the reactors at its Kozloduy nuclear power plant back into the electricity grid on Tuesday after more than a month of repair and refuelling work.
The reactor — only one of two currently in operation at Kozloduy — was shut down for regular maintenance work and refuelling on April 24.
However, cracks were detected in the protective casings of 37 out of the reactor's 61 control rods, requiring the replacement of the casings of all control rods.
The plant's operators insisted the additional repair work did not lead to any delay to the maintenance timetable and the reactor was switched back into the national power grid on Monday evening.
It was the second such incident at Kozloduy within the past year, after similar cracks were found in 31 control rods at the other operational reactor last October.
Only two of Kozloduy's six are actually operational, after the other four were shut down as one of the conditions for Bulgaria's accession to the European Union in 2007.
Already in 2006, a similar problem was detected in the number-five reactor while it was in operation, leading to an emergency shutdown.
That incident was rated level two out of seven on the International Nuclear and Radiological Event Scale (INES).
