Belgium on Monday restarted a reactor that was shut down at the weekend just three days after returning to service amid concern in neighbouring countries over its nuclear safety.

Doel No. 1 reactor -- Belgium's oldest -- had been shuttered in February after four decades in operation, in line with a law calling for the country's gradual exit from nuclear power.

It was restarted on December 30 under a deal to extend its working life -- and that of a companion reactor, Doel No. 2 -- until 2025 to help cope with power shortages.

But a generator problem on Saturday triggered a shutdown at the plant, located near the Dutch border about 20 kilometres (12 miles) from the port city of Antwerp.

"There was no safety risk," a spokeswoman for the nuclear facility's operator Electrabel told AFP.

It was restarted at 1000 GMT on Monday, a spokeswoman for the power plant told the Belga news agency.

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.

While the number of unplanned shutdowns at Belgian reactors has been no higher than average in recent years, Saturday's stoppage at Doel No. 1 followed two similar interruptions.

The first was on September 16 at the No. 1 reactor at Tihange, near the eastern city of Liege, which had been restarted after maintenance.

The second was Doel's No. 3 reactor, which shut down on Christmas Day, four days after a restart.

Tihange No. 2 and Doel No. 3 reactors had been restarted following a two-year pause after microscopic cracks were discovered in the reactor vessels.

Neighbouring countries have expressed concern despite assurances from experts that the facilities are safe.

Luxembourg's sustainable development minister, Camille Gira, is set to meet with Belgian Interior Minister Jan Jambon over the nuclear issue on January 18.

Dutch Environment and Infrastructure Minister Melanie Schultz will visit Doel with Jambon for a joint inspection on January 20.