An ageing Belgian nuclear reactor shut down on Saturday afternoon, just three days after it was restarted, but there was no safety risk, the power utility that operates it said.

The Doel 1 reactor in northern Belgium was restarted on December 30, after originally being shut down in February in line with a law that called for the country's gradual exit from nuclear power.

"Doel 1 automatically shut down at 6:00 pm," a spokeswoman for power utility Electrabel told AFP, confirming a report published by the Belga news agency. "Everything went according to procedure. There was no impact on safety, and no impact on staff, local residents and the environment."