In an open letter to Chancellor Angela Merkel released on Saturday, Greens co-leader Simone Peter wrote, "The growing threat to a large percentage of the population due to old, failure-prone nuclear power stations at Fessenheim, Cattenom, Tihange, Doel, Beznau and Temelin fills us with great concern."

Rhineland-Palatinate Premier Malu Dreyer also weighed in, calling on French President Francois Hollande to immediately shut down the border nuclear plants of Fessenheim and Cattenom. Polls ahead of state elections in Rhineland-Palatinate show Dreyer of the Social Democratic Party (SPD) in a close race with Julia Klöckner of Merkel's Christian Democratic Union (CDU).

German daily "Süddeutsche Zeitung" and regional broadcaster WDR reported Friday that France's nuclear watchdog ASN downplayed an incident two years ago to the International Atomic Energy Agency at the Fessenheim plant, located near the German and Swiss borders, where a water leak damaged the electrical system, leading to one of its reactors being shut down.

At the time of the incident, a crisis management team decided to shut down the reactor as a precautionary measure by introducing boron to the cooling water.

The joint report said operators temporarily lost complete control over the reactor after water incapacitated one of the two parallel reactor security systems.

ASN's report, the media outlets reported, did not mention the use of boron.

"I am not aware of any other case where a power rector here in western Europe suffered an incident in which it had to be shut down with the use of boron," nuclear safety expert Manfred Mertins was quoted in the report as saying.

Following Friday's report, German Environment Minister Barbara Hendricks repeated her call for France to terminate operations at the Fessenheim plant.

French President Francois Hollande promised during his election campaign to shut Fessenheim by the end of his current five-year term in 2017.

Hollande has since announced plans to close the plant by the end of 2016, but that has been put on hold because a new-generation reactor in Flamanville will not be completed until 2018.

Fessenheim houses two 900 megawatt reactors and has been operational since 1977, making it France's oldest operating reactor.

jlw/sms (dpa, AFP)