About 20 members of the Greenpeace environmental group managed to climb on top of the dome of a nuclear reactor in Fessenheim, near the border with Germany and Switzerland, to highlight weaknesses at atomic installations.
The activists, mostly from Germany but also from Italy, France, Turkey, Austria, Hungary, Australia and Israel, were all convicted of trespassing and causing wilful damage.
Prosecutor Bernard Lebeau said that, while debate on nuclear energy was "perfectly legitimate", their actions were completely "illegal."
Only three of the 55 defendants – two Frenchmen and a German - turned up in court in the Alsace city of Colmar to face trial.
Eddy Varin, a 41-year-old French civil servant, said he felt he had a duty to do what he did, while computer specialist Jean-Michel Vourgère said the media stunt was to raise public awareness.
France, the world's most nuclear-dependent country, operates 58 reactors but President François Hollande has promised to cut reliance on nuclear energy from more than 75 per cent to 50 per cent by shutting 24 reactors by 2025.
Hollande has pledged to close Fessenheim, which was commissioned in 1977, by the end of 2016.