With power level of 1,600 megawatts, the plant located on the Nordic country's southwestern coast is also now the third most powerful electricity production facility globally, the company said.

OL3's production is being closely followed in Finland, where the hope is that the plant could ease the coming winter's challenges as European energy prices have soared following Russia's invasion of Ukraine.

"The plant unit is now the most powerful electricity production facility in Europe," TVO said in a statement, adding that regular operation expected to start in December 2022.

Around 40 percent of Finland's electricity production now comes from Olkiluoto, as the OL1 and OL2 reactors combined produce approximately 21 percent and the new OL3 alone around 19 percent.

The reactor, built by the French-led Areva-Siemens consortium, went online in March -- 12 years behind schedule -- after suffering a long string of setbacks.

The plant's regular production was expected to start this summer but was postponed to December, after "foreign material" was observed in the turbine's steam reheater.

Operator TVO said that the ten remaining sets of tests will impact the power levels in the coming months.

"In some of the upcoming tests, the plant unit's production is either intentionally interrupted or the power level is lowered," the company said.

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