The reactor had to be shut down Wednesday due to problems in the cooling system at its electric generator, Nuclearelectrica said in a statement.
Nuclearelectrica operates two 700-megawatt reactors in Cernavoda, covering around 20% of Romania’s energy consumption.
The latest unscheduled shutdown in Cernavoda took place in November last year, when Unit 2 stopped automatically due to a computer malfunction.
Nucleareletrica is majority owned by the state, through the Economy Ministry.