She has been arrested 40 or 50 times for acts of civil disobedience and once served six months in prison. In the Nevada desert, she and other peace activists knelt down to block a truck rumbling across the government’s nuclear test site, prompting the authorities to take her into custody.

She gained so much attention that the Energy Department, which maintains the nation’s nuclear arsenal, helped pay for an oral history in which she described her upbringing and the development of her antinuclear views.

Now, Sister Megan Rice, 82, a Roman Catholic nun of the Society of the Holy Child Jesus, and two male accomplices have carried out what nuclear experts call the biggest security breach in the history of the nation’s atomic complex, making their way to the inner sanctum of the site where the United States keeps crucial nuclear bomb parts and fuel.

“Deadly force is authorized,” signs there read. “Halt!” Images of skulls emphasize the lethal danger.

With flashlights and bolt cutters, the three pacifists defied barbed wire as well as armed guards, video cameras and motion sensors at the Oak Ridge nuclear reservation in Tennessee early on July 28, a Saturday. They splashed blood on the Highly Enriched Uranium Materials Facility — a new windowless, half-billion-dollar plant encircled by enormous guard towers — and hung banners outside its walls.