The move follows a legal complaint filed by a whistleblower, a former engineer at the Tricastin power station in the southeast of the country.

In his complaint to police in October 2021 targeting nuclear plant operator EDF, the engineer, whose identity was not given, said he had repeatedly alerted the company to the incidents and also written to the environment minister.

Events that the nuclear operator failed to declare to the national safety agency ASN, or played down, include an unexplained power surge at one of the reactors in 2017 and flooding inside the station the following year, according to the engineer.

An investigating magistrate in the southern port city of Marseille is now probing the power station for fraud and "endangering the lives of others", the legal source said.

Other suspected violations include damage to the environment by leakage of toxic substances, obstructing checks by nuclear inspectors and workplace harassment of the engineer, who says he was sidelined after sounding the alarm.

France, which derives around 70 percent of its electricity from nuclear power, has been exploring a possible extension of the lifetime of its ageing stations, several of which have come up against their 40-year limit.

That includes Tricastin, built in 1980-81 -- making it one of France's oldest nuclear stations.

It is on a list of installations that the ASN agency said last year could be renovated to extend their lifespan.

Currently operations at 12 of France's 56 nuclear reactors are shut down because of corrosion issues, EDF said last month.

In February, President Emmanuel Macron called for a "rebirth" of France's nuclear industry, with 14 new plants, as part of efforts to move away from fossil fuels.

The launch of the Tricastin probe was evidence of the accusations' "extreme gravity", the whistleblower's high-profile lawyers Vincent Brengarth and William Bourdon said in a statement to AFP.

Contacted by AFP, both EDF and ASN declined to comment.

Last November, however, ASN chief inspector Christophe Quintin told AFP that routine checks at Tricastin had not revealed any incidents that might have gone unreported.

Independent radioactivity research association CRIIRAD welcomed the investigation, saying it raised important issues such as nuclear safety and transparency on nuclear issues.

"The judiciary is sending a strong signal, but will it be able to get to the bottom of this?" asked the association's spokesman, Roland Desbordes.

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EDF - ELECTRICITE DE FRANCE