Finland's long-delayed Olkiluoto-3 nuclear reactor went online for the first time on Saturday, the plant's operator said.

The EPR reactor, built by the French-led Areva-Siemens consortium on Finland's southwest coast, was started up in December for testing, 12 years behind schedule.

"Today on Saturday, March 12, 2022, at 12.01 pm (10 a.m. GMT), the plant unit has been connected to the national grid at a power output of 103 MW," TVO wrote in a statement.

Once regular energy production begins, now expected in July 2022, it will supply about 14% of Finland's energy at 1,650 megawatts (MW), it said.

The plant is to become Europe's most powerful reactor. Olkiluoto 3 will run alongside two existing reactors at Eurajoki on Finland's west coast.

The French-developed EPR reactor model was the first nuclear power station to be procured in Europe after the 1986 Chernobyl disaster, designed to relaunch nuclear power.

It was touted as offering higher power and better safety.

But EPR builds in Finland, France and the U.K. have been plagued by delays and cost overruns.

In Olkiluoto's case, the delays led to bitter compensation disputes between TVO and Areva.

In the U.K., Hinkley Point in southwest England has pushed back its planned electricity production by half a year to mid-2026.