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."

The company said the shutdown was caused by a "normal safety mechanism", in order to restart it safely.

Belgium agreed last month to extend the life of Doel 1 and Doel 2, both of which are 40 years old, until 2025 under a deal that ended years of battles that threatened the country with power blackouts.

When running, the two reactors can produce 866 megawatts, equivalent to around six percent of Belgium's total electricity capacity.

Environmentalists, especially in neighbouring Germany which is phasing out nuclear power, have criticised extending the life of Belgium's reactors which they say are too old to be considered safe.

Belgium has suffered a series of nuclear mishaps in recent years, with three of the country's seven reactors closed at one point, in two cases due to the discovery of micro-cracks in the reactor vessels.