Finnish energy company Fortum says both of the units at its Loviisa nuclear plant are offline.
Unit 2 was turned off on Saturday following a leak. The plant's other reactor, Unit 1, is meanwhile down at the same time owing to annual servicing. This means that neither of the plant's two reactors will produce electricity for a few days.
On Saturday, Fortum said it had detected a small leak in Unit 2's primary circuit. Fixing this problem required the unit be turned off and cooled.
Sasu Valkamo, director of the Loviisa plant, said the leak was very small and did not pose any danger to people or the environment.
Unit 1 will undergo maintenance until Friday, while Unit 2 is scheduled to be plugged back into the grid on Tuesday, according to Fortum.
The Loviisa nuclear power plant, Finland's first, accounts for approximately ten percent of Finland's annual electricity production. At full capacity, the combined output of its units is 500 megawatts.
In comparison, the new Olkiluoto 3 reactor can reach a maximum output of 1,600 megawatts. That new reactor, which is still in the testing phase, reached full output for the first time last Thursday.