KNOXVILLE, Tenn. — An 84-year-old nun was sentenced Tuesday to nearly three years in prison for breaking into a nuclear weapons complex and defacing a bunker holding bomb-grade uranium, a demonstration that exposed serious security flaws at the Tennessee plant.

Two other peace activists who broke into the facility with Megan Rice were sentenced to more than five years. Although officials said there was never any danger of the protesters reaching materials that could be detonated or made into a bomb, the break-in raised questions about safekeeping at the National Security Complex in Oak Ridge.