Electrabel, the Belgian arm of French group Engie, shut down the Doel 3 nuclear reactor in the early hours of Friday because of a water leak, only days after it resumed output after a nearly 2-year halt, a spokeswoman said.
The reactor was put into "a hot shutdown" at 1 am (0000 GMT), meaning it has not been cooled down completely and can restart relatively quickly, spokeswoman Anne-Sophie Huge said.
She said it was too soon to say when it would restart, however, because analysis of the cause of the leak had only just begun.
The 1,006 megawatt Doel 3 was put back into operation early on Monday after a 21-month shut-down prompted by micro-cracks found in casing.
Technical experts said there was no safety risk from the micro-cracks, which were also found in another Belgian reactor Tihange 2. Huge said the latest shutdown was a separate issue.
Electrabel operates seven Belgian nuclear reactors, four in Doel and three in Tihange, with a total capacity of some 6,000 megawatts, covering about 55 percent of
electricity consumption in Belgium.
Belgium plans to decommission all its nuclear plants by 2025.
(Reporting by Barbara Lewis; Editing by Ruth Pitchford)