Both the French nuclear authority, ASN, and the company operating the two Fessenheim nuclear reactors, French energy giant EDF, allegedly did not divulge the gravity of the incident on April 9, 2014, when one of the reactors had to be shut down after water was found leaking from several places.

Researchers from German daily "Süddeutsche Zeitung" and public broadcaster WDR claim the incident at Fessenheim, which is in Alsace near the border with Germany, could turn out to be one of "most dramatic nuclear accidents ever in Western Europe."

They are basing the claim on a document they say they have obtained, sent by ASN to the then-head of the facility on April 24, 2014.

The letter and subsequent reply reveal that the reactor could not be shut down in an ordinary fashion due to control rods being jammed. The reactor had to be shut down by adding boron to the pressure vessel, an unprecedented procedure in Western Europe, according to an expert.

"I don't know of any reactor here in Western Europe that had to be shut down after an accident by adding boron," Manfred Mertins, expert and government advisor on nuclear reactor safety, told WDR and Süddeutsche Zeitung.

The reports say the official report ASN released did not contain information on adding boron nor the jammed control rods. It was also not reported in that way to the International Atomic Energy Agency (IAEA).

The Fessenheim reactors went online in 1977 and 1978, making them France's oldest. The government has repeatedly said it would shut down the facility after fierce criticism from politicians at home, as well as from neighboring Germany and Switzerland.

On Friday, Eveline Lemke, environment minister for the German state of Rhineland-Palatinate, which borders Alsace, called for Fessenheim to be shut down immediately. She said she was "dismayed to hear about yet another incident involving a French reactor," adding that France's nuclear watchdog was "evidently failing."

Germany has also been at loggerheads with Belgium over the country's Tihange nuclear reactor near their shared border. It was shut down in March 2014, but went back online in December last year, despite concerns over cracks in its pressure vessels.

Nuclear power still provides three-quarters of France's energy needs, but the government passed legislation last summer to cut the country's dependence on atomic energy.