The Nordic country's nuclear safety authority, STUK, gave permission on Thursday "for making the reactor critical and conducting low power tests," Finnish electricity producer TVO said in a statement.

The EPR reactor, built by the French-led Areva-Siemens consortium, is almost twelve years behind its initial scheduled completion date of 2009.

Once regular energy production begins, now expected in June 2022, it will supply almost 15 percent of Finland's energy at 1,650 megawatts.

"We are now moving step by step with a safety-first attitude towards the moment we have waited for [for] a long time," TVO senior vice president Marjo Mustonen said in the statement.

Olkiluoto 3, which will run alongside two existing reactors at Eurajoki on Finland's west coast, was the first nuclear power station to be procured in Europe after the 1986 Chernobyl disaster.

But the French-developed EPR reactor model, touted as offering higher power and better safety, has been plagued by delays and cost overruns, leading to bitter compensation disputes between TVO and Areva.

Other EPR builds in France and the UK have also been beset with delays, with Hinkley Point in southwest England pushing back its planned electricity production by half a year to mid-2026.

Costs have swelled by around pound500 million ($705 million, 580 million euros) to as much as pound23 billion.

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