Damage has been detected in Finland's Olkiluoto 3 nuclear reactor, which after years of delays had been expected to come fully online in December, operator TVO said Tuesday.

"Damage has been detected in the internals of the feedwater pumps located in Olkiluoto 3's turbine island during maintenance and inspection work," TVO said in a statement.

"This will most likely have an effect on the continuation of Olkiluoto 3's nuclear commissioning and the start of regular electricity production," TVO said, adding that an estimate of the schedule would be made in the coming days.

The damage detected did not affect nuclear safety, the operator said.

The new problems come just ahead of winter descending on Europe, with fears of energy shortages after Russia's invasion of Ukraine.

Finland is also expecting to rely on Olkiluoto's new EPR reactor after a halt in electricity imports from its neighbor Russia.

Olkiluoto 3, first announced in 2003, is now the most powerful reactor in Europe and the third in the world.

On September 30 it reached its full capacity for the first time.

At the time, it alone produced about 20 percent of Finland's electricity -- 40 percent when the two existing Olkiluoto 1 and 2 reactors were added.

At the beginning of October, there were about 10 tests at full power on Olkiluoto 3.