The 1.6 gigawatt (GW) unit, known as OL3, was expected to reconnect on Monday at around 1000 GMT, an extension of the outage by 11 hours compared to the initial estimate, according to the regulatory statement.
Plagued by construction delays, OL3 began regular electricity output in April this year, some 14 years behind schedule.
Finland has said the nuclear reactor, Europe's first in 16 years, is expected to meet around 14% of the country's electricity demand, boosting energy security.
