According ABC, that activist group included Sister Megan Rice, an 82-year-old nun.
Rice, along with at least two other activists, cut through perimeter fences to reach the outer wall of a building where highly enriched uranium, a key nuclear bomb component, is stored, according to the Irish Times. Officials said the group, which goes by the name Transform Now Plowshares, painted slogans and threw what is believed to be human blood on the wall of the facility.
Ralph Hutchinson, coordinator for the Oak Ridge Environmental Peace Alliance, told Reuters the group's intention was not to point out the lack of security, but to take a stance against the making of nuclear weapons.
"It wasn't so they could show how easy it was to bust into this bomb plant, it was because the production of nuclear weapons violates everything that is moral and good. It is a war crime."
The U.S. Department of Energy hired the private film WSI Oak Ridge to provide security at the facility. WSI is a subsidiary of the international security firm G4S, the same firm who drew criticism for failing to provide adequate security for the 2012 London Olympic Games. The group insisted that the nuclear material was never compromised, according to Reuters.