Sr Megan Rice, a Sister of the Heart of the Child Jesus, is part of a group called Transform Now Plowshares, who broke into the Highly-Enriched Uranium Manufacturing Facility (HEUMF) in Oak Ridge Tennessee last week.
The facility is the American Government's only facility for handling, processing and storing weapons-grade uranium.
Sr Megan and two other activists allegedly broke through four perimeter fences and walked for two hours through a "fatal force zone" patrolled by guards and attached two banners to the pillars of the building housing the HEUMF saying: "Transform Now Plowshares" and "Swords into Plowshares Spears into Pruning Hooks - Isaiah".
According to the group's online blog they read a statement to the guard who confronted them.
They offered guards bread and carried a bible, candles and white roses.