Some 100 cubic metres of reactor coolant leaked into so-called "containment rooms" at the Olkiluoto 3 nuclear power plant on Friday, the plant's operator Teollisuuden Voima (TVO) has announced in a press release.
The release explained that the coolant flowed into the containment rooms as well as a floor drain system as a result of human error, as a hatch had not been closed properly.
"The incident did not pose any risk to the personnel, the environment, or nuclear safety," TVO's statement said, adding that the significance of the leak to radiation safety was low, "due to the safety actions taken".
Olkiluoto 3 is Finland's newest and biggest nuclear reactor, but the project has been beset by problems and delays since construction first began 20 years ago.
An investigation into Friday's incident has been launched, TVO noted, and steps will be taken to prevent any possible repeat.
The containment rooms and floor drain system are designed to collect and drain any reactor coolant leakages.
Work on the nuclear reactor's annual outage began on 1 March and will continue as planned, despite Friday's leak, and will last until the beginning of May.
Last year, similar outage work dragged on five weeks longer than expected, pushing electricity prices up in Finland.