"We have resumed production," TVO spokeswoman Johanna Aho said.

The reactor went back online after midnight and was running at 1,400 MW at 1:30 pm (1130 GMT), expecting to reach full output of around 1,600 MW by the end of the day.

TVO said Monday the reactor, located on Finland's southwest coast, "was in normal production when a fault in the turbine plant automatically stopped the facility's electricity production."

"The cause of the fault was revealed to be a malfunctioning temperature measurement in the generator's cooling system," it said, adding that "the incident had no impact on nuclear safety."

The next-generation European pressurised water reactor (EPR), built by the French-led Areva-Siemens consortium, produces more than 10 percent of Finland's electricity.

After 14 years of delays, the reactor was put into regular service in April 2023.

Like several other EPR projects, it was plagued by repeated construction delays and billions of dollars in cost overruns.

Olkiluoto 3 is the single largest nuclear reactor in Europe, while Ukraine's Zaporizhzhia plant, with its six reactors, is the largest nuclear plant.