Finland's nuclear safety regulator Friday gave the country's four nuclear reactors a clean bill of health after stress tests commissioned by the European Council.

But nuclear authority STUK's chief Jukka Laakson said: "There will have to be some tightening of the security regulations that would also affect exisiting plants."

"Plant security must take into consideration situations in which security systems suffer power outages of 72 hours," Laaksonen said.

Finland's four operating nuclear reactors are run by power companies Fortum and Teollisuuden Voima (TVO), who both participated in the stress tests ordered in the wake of the March Fukushima disaster.