The operator TVO announced the latest delay amid fears of energy shortages following Russia's invasion of Ukraine for the facility, already more than 12 years behind schedule.

It had been expected finally to come fully online in December but that date was pushed back to February after TVO had reported in October detecting damage in the feedwater pumps located in Olkiluoto 3's turbine island.

A detailed investigation into the cracks is now ongoing and this, the company admitted, had proved "especially challenging."

"The electricity production of Olkiluoto 3 will be continued on Tuesday, 27 December 2022. During test production, approximately 1.3 terawatt hours of electricity will be produced," TVO said in a statement.

"Around ten significant tests still remain. Regular electricity production is to start on 8 March 2023.

"The investigation into the damage in Olkiluoto 3's feedwater pumps has mainly been completed," the company added, saying cracks had most likely been caused by the test production, where the pumps are used in abnormal circumstances.

"The feedwater pumps have been operated outside the range of normal use during production tests, which has led to a higher-than-normal strain on the pumps," it explained.

Finland had been expecting to rely on the new reactor for its electricity needs this winter, given fears of energy shortages after Russia, a major supplier to Europe, invaded Ukraine.

But the regular production date has been pushed back multiple times this year alone.

The most powerful nuclear plant unit in Europe at 1,600 megawatts, Olkiluoto 3 reached full power in late September for the first time since construction began in 2005.

Even during test production, the plant unit produces about 15 percent of Finland's electricity demand at full power, according to TVO, while the existing Olkiluoto 1 and 2 reactors take the share to 40 percent.

The French-developed European Pressurised Reactor model was designed to relaunch nuclear power in Europe after the Chernobyl catastrophe of 1986, and was touted as offering higher power and better safety.

But EPR projects in Finland, France and the UK have been plagued by delays and cost overruns.

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AREVA