The French-developed European Pressurised Reactor model was designed to relaunch nuclear power in Europe after the Chernobyl catastrophe of 1986, and was touted as offering higher power and better safety.

But EPR projects in Finland, France and the UK have been plagued by delays and cost overruns.

The reactor built by the French-led Areva-Siemens consortium on Finland's southwest coast went online in March, 12 years behind schedule.

Energy production had been expected to start this summer, with the aim of supplying about 15 percent of Finland's electricity consumption.

"Inspection and repair needs concerning the cooling system of OL3's generator will last approximately three weeks", the operator Teollisuuden Voima said in a statement.

Unlike the reactor, the generator is located with the huge turbine in the non-nuclear part of the plant, a TVO spokeswoman confirmed to AFP.

A separate problem also occurred on Tuesday during the ongoing test phase.

During a planned shutdown of the reactor, a safety system controlling the chain reaction with boron was triggered "unexpectedly", but did not pose any danger.

TVO said Friday the repairs of the cooling systems "are not related" to the incident concerning the boron pumps.