A federal grand jury has toughened the charges against three anti-war protesters who authorities say cut their way through three security fences and spray-painted slogans on the walls of a nuclear weapons plant in Tennessee.

The indictment released Thursday in Knoxville charges the three, including an 82-year-old Roman Catholic nun, with "depredation" of the Y-12 National Security Complex in Oak Ridge. The charge carries a maximum penalty of 10 years in prison.

The indictment includes previous charges of trespassing and malicious destruction of property.

The 811-acre complex stores the nation's supply of weapons-grade uranium, makes nuclear warhead parts and provides nuclear fuel for the Navy and research reactors worldwide.

The Department of Energy and defense attorneys didn't immediately return calls seeking comment.

Y-12 has added security training and replaced top security managers since the breach.