The 1,600 megawatts (MW) unit, known as OL3, on Sunday suffered an unexpected outage due to a turbine problem, TVO and Nordic power bourse Nord Pool said.
"The cause of the fault was revealed to be a malfunctioning temperature measurement in the generator's cooling system," TVO said in a statement, adding that nuclear safety had not been impacted.
"According to the current estimate, the plant unit is expected to return to production at noon on Nov. 21."
Finding alternative power supply proved costly for Finland, where the price at the day-ahead auction soared to 287.87 euros per megawatt hour (MWh) for Tuesday, up from 116.09 euros/MWh for Monday, and much above prices in the rest of the Nordics.
Monday's price had been set before the reactor outage occurred.
TVO had initially predicted a return to full production capacity on Monday morning, but in a regulatory filing said it was instead aiming for a partial restart to take place on Tuesday.
Output capacity on Tuesday was predicted to reach 1,285 MW, while the full 1,600 MW could be seen on Wednesday, according to the filing.
Plagued by construction delays, OL3 began regular electricity output in April this year, some 14 years behind schedule.
Finland has said the nuclear reactor, the first to come on line in Europe in 16 years, is expected to meet around 14% of the country's electricity demand, boosting energy security.
