The reactor near Liege is one of seven operated by Electrabel, a subsidiary of France's GDF Suez.
The fire "happened outside the nuclear zone," said mayor Alexis Housiaux of the adjoining town of Huy.
Electrabel said the fire at the Tihange 3 reactor was brought under control around midday, and that there had been "no impact on nuclear safety and the safety of the workers and the residents near the site."
A preliminary investigation ruled out sabotage -- the cause of a shutdown at another plant in August -- concluding that the fire had a "technical cause".
Both had previously halted operations between June 2012 and June 2013.
Some 55 per cent of Belgium's electricity is nuclear.