Fennovoima said on Wednesday it had ended negotiations with Japan's Toshiba on supplying a 1,600 megawatt reactor for the facility in northern Finland. In February it dropped France's Areva from the process.
The consortium's CEO, Juha Nurmi, said Rosatom's medium-sized reactor turned out to be more suitable for its owners' needs than Toshiba's larger one and said Rosatom's interest in taking a stake in the project also played a role.
Rosatom, however, was not listed as a possible supplier in Fennovoima's original permit, granted by the Finnish government and approved by the parliament in 2010, creating a judicial dilemma on whether the permit needs fresh approval.
Economic Affairs Minister Jan Vapaavuori said the government would decide if new approval was needed after the matter was thoroughly reviewed. He estimated that would take about a year.
Nurmi said the consortium was in talks with the ministry and would deliver any information they requested.
Fennovoima will now hold talks with Rosatom on a 1,200 megawatt reactor order, and its owners will hold negotiations with the Russian company about it taking a 33 percent stake in the consortium. The parties aim to reach deals by the end of this year.
Rosatom, owned by the Russian state, has recently been stepping up its foreign operations and has won a deal to supply Turkey's first nuclear reactor.
The Fennovoima consortium was founded to provide cheap energy to a group of around 60 shareholders, including stainless steel maker Outokumpu, retailer Kesko and metals firm Boliden.
Power production at the plant in Pyhajoki is due to start in the 2020s.
German utility E.ON exited the consortium last year, raising doubts over plans to build a large reactor in Pyhajoki, originally estimated to cost around 4-6 billion euros ($5.2-$7.8 billion).
E.ON's departure prompted Fennovoima to consider the possibility of a smaller reactor because it not need to produce as much electricity as originally thought and also wanted to keep costs down.