Why can't they enrich uranium in peace?

"Sister Megan Rice, 83, and two fellow peace activists staged a non-violent protest to symbolically disarm the Oak Ridge Y-12 nuclear weapons facility, home to the nation's main supply of highly enriched uranium, in July. They were initially charged with trespassing, a misdemeanor punishable by up to a year in prison, but felony charges quickly followed. They were eventually convicted of interfering with national security and damage to federal property."