Production at the Olkiluoto 3 nuclear reactor shut down on Wednesday, when a fault was discovered.
"In a fault ride-through test, a momentary short circuit is created near the power plant in the network, causing a dip in voltage," said the plant's operator Teollisuuden Voima (TVO).
The shutdown comes just hours after a reactor at Sweden's Ringhals plant also shut down, taking more power out of the Nordic energy market.
Electricity spot prices have fluctuated wildly in recent weeks, with a bidding error forcing the price into negative territory last Friday.
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