The leakage struck on Friday during the reactor's annual outage for maintenance "in connection with the filling of the reactor pool," TVO said in a statement.

"The coolant flowed into containment rooms closed to the environment and into the floor drain system of the containment. The incident did not pose any risk to the personnel, the environment, or nuclear safety," TVO said.

According to TVO, the leak was deemed to have been the result of "a human error," as a hatch to the pool had not been closed properly.

"The significance of the event to radiation safety was low, in the end, owing to the safety actions taken," TVO said.

TVO added that the incident would not affect the planned duration of the annual outage, which is scheduled to last until May.

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.

Like several other EPR projects, the reactor, located on Finland's southwest coast, 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.