The activists got into the four-reactor plant at Tricastin near Avignon before dawn, and displayed banners bearing President Francois Hollande's image and the slogan: "Tricastin nuclear accident. President of catastrophe?"
They also projected the slogan "Tricastin nuclear accident" and a forked line representing a fissure in green letters on the wall of a building. The interior ministry said all 29 activists involved in the protest had been detained.
None had been able to gain access to sensitive areas "namely the control rooms," the ministry said.