55 greenpeace activists who broke into France's oldest power plant at Fessenheim - near the border with Germany and Switzerland - go on trial on Thursday.
"We are breaking the rules in order to secure a better future" said the defendants, who said the action last March was to highlight security weaknesses at atomic installations.
"It was my duty to do this and I did it" said Eddy Varin, a French civil servant. "It was legitimate".
The activists include 20 nationalities, 21 Germans, seven Italians and others from France, Turkey, Austria, Hungary, Australia and Israel.
They are accused of trespassing and causing wilful damage to the plant.
The court in the eastern French city of Colmar will determine if the Greenpeace members smashed down a metal security gate with their truck to enter the plant or simply broke a lock to get in.
They could face up to five years in prison if convicted, but in the past protestors who have broken into nuclear installations have been given six-month suspended prison sentences.
In a deal with the French Green party before the 2012 presidential elections, President Hollande promised to close Fessenheim - which was commissioned in 1977 - by the end of 2016.
France is an international leading actor for atomic energy and ranks second after the United States with 58 operating reactors.