The U.S. government’s lone site for handling and processing weapons-grade uranium has been temporarily shut down after anti-nuclear activists infiltrated the premises. Three activists — including an 82-year old nun — reportedly cut through fences to paint slogans and throw blood on the wall of the Y12 nuclear weapons plant in Oak Ridge, Tennessee. The Y12 facility processes uranium for new hydrogen bombs. Calling themselves the “Transform Now Plowshares,” the three activists appeared before a U.S. magistrate judge in Knoxville on Thursday. The facility will remain shut down at least until next week. U.S. officials have maintained no nuclear materials were jeopardized, but experts have marveled at how a small group could have infiltrated the high-risk site. One former congressional investigator and security consultant called the breach the “worst we’ve ever seen.”