Activists from the environmental group Greenpeace have broken into a nuclear power station outside of Paris in an attempt to draw attention to the vulnerability of France's atomic energy sites.

Greenpeace activists tried to enter several nuclear sites around the country Monday, but had the most success at the Nogent-sur-Seine plant, where nine activists were able to get past the initial layer of security. Some of them were able to scale a domed containment building and plant a banner that read “Facile” — or easy — before being arrested.

The head of Greenpeace policies says it was not difficult to get into the facility.

French President Nicolas Sarkozy denounced the activists as “irresponsible” for risking lives. But French authorities promised a thorough investigation, saying that the activists had exposed lapses in the security system.

Officials of the French power company that operates the nuclear site said they had been aware of the incursion but chose not to do anything because the activists were unarmed and had peaceful intentions. They added that at no time was the facility at risk.

But speaking on French radio, a Greenpeace nuclear expert said the incursion underscores the weakness not only of the security systems around French nuclear power plants, but also of stress tests the government is conducting on the nuclear plants. She said the tests do not take into account the possibility of human threats like terrorist attacks.