"During the test, the plant was supposed to produce electricity normally," said Johanna Aho, head of communication at TVO.

Before production was interrupted at 1:35 pm (1135 GMT), Olkiluoto 3 was undergoing a fault ride-through test where a short circuit is created near the power plant.

"As a result of the test, the production at Olkiluoto 3 was interrupted," TVO said.

The company added that "the cause is being investigated in more detail."

The reactor only resumed production last Wednesday following a shutdown due to a malfunctioning temperature measurement in the generator's cooling system.

The next-generation European pressurised water reactor (EPR), built by the French-led Areva-Siemens consortium, produces more than 10 percent of Finland's electricity.

Like several other EPR projects, the reactor, located on Finland's southwest coast, was plagued by repeated construction delays and billions of dollars in cost overruns.

Olkiluoto 3 is the single largest nuclear reactor in Europe, while Ukraine's Zaporizhzhia plant, with its six reactors, is the largest nuclear plant.

ehu/jll/imm

AREVA