( CNN ) -- When an elderly nun and two fellow peace activists walked undetected onto one of the nation 's most secure nuclear facilities last year , they wanted to call attention to the dangers of nuclear weapons . Yet their actions in Oak Ridge , Tennessee , triggered a very different concern : . If three older peaceniks can easily trespass onto the Y-12 National Security Complex -- once considered the `` Fort Knox '' for highly enriched uranium -- just how safe are the nation 's nuclear weapons facilities from terrorists ? Nuclear weapons interactive : Who has what ? Sister Megan Rice , 83 ; Greg Boertje-Obed , 57 ; and Michael Walli , 63 , will stand trial this week on federal charges of destroying U.S. government property , depredation against federal property exceeding $ 1,000 , trespassing and injuring national-defense premises . The last charge alone carries a maximum penalty of 20 years in prison . The trial began Tuesday at federal court in Knoxville , Tennessee , according to WATE , CNN 's affiliate in Knoxville . In the predawn hours of July 28 , 2012 , Rice , Boertje-Obed and Walli walked under the cover of darkness through the woods and up a hillside , approaching a chain-link fence surrounding the Oak Ridge nuclear facility . Armed with flashlights and a bolt cutter , they cut their way through the fence , fully expecting to be arrested on the spot . Instead , they walked nearly a mile , cutting through four fences in all , breaching what was supposed to be the most tightly secured uranium processing and storage facility in the country . `` When we got to the very high security fence where there 's a lethal force authorized ... I thought , maybe we should turn around , '' Boertje-Obed told CNN 's David Mattingly . But they did n't . Hours later , the three activists were finally confronted by a guard after hoisting banners , spray-painting messages and splattering human blood on a building that houses highly enriched uranium . Since the incident , Congress has held a series of hearings and issued security recommendations to the Department of Energy and its National Nuclear Security Administration , which runs Y-12 and seven other nuclear weapons sites . Most recently , Deputy Energy Secretary Daniel Poneman testified before the House Energy and Commerce Committee in March that the Department of Energy has taken `` several major actions ... to improve security '' since the Y-12 breach , including management changes and independent security reviews . Today , changes at Y-12 are noticeable . A new security contractor is in charge . New signs and security fences are going up . While last year 's security breach shed light on systematic weaknesses at Y-12 , a former nuclear reactor safety manager at the Sandia National Laboratories said he does n't think the nation 's nuclear weapons material were ever at any risk . `` What these people did was more like trespassing than gaining access to any weapons-grade material , '' said Michael Allen , who is now a vice provost and dean at Middle Tennessee State University . `` Once they got in , they could spray paint things , but it 's just like if you got into Fort Knox , you would n't know the combination to the locks . '' Allen said the country 's nuclear weapons material is spread across eight sites around the country that are `` usually out in the middle of nowhere '' to protect against threats . `` There 's really only one place where the weapons are fully assembled ; I 'm not going to tell you which one , '' Allen said . `` And the reason there are eight places , they do n't want people to have the full knowledge ( of what the other facilities do ) . `` We almost never see an assembled nuclear weapon . '' But another nuclear security expert said the Y-12 breach was more than just trespassing by a few environmental protesters . `` This was a very serious incident because they penetrated the protected area , and that 's when there was supposed to be an immediate security response , '' said Edwin Lyman , a senior scientist with Global Security Program , a watchdog group that monitors the Department of Energy and the Nuclear Regulatory Commission . `` If they had been trained , the kind of paramilitary group that the Department of Energy is supposed to be ensuring they can protect against , with hand-carried explosives , other breaching tools , physical access to the building structure ... the guards would have already lost . '' Lyman said the breach was not an isolated incident . `` It was a result of this reduced central oversight , giving contractors more responsibility for supervising themselves , and that 's an invitation of corner-cutting and complacency to set in , '' he said . The incident not only broke the public 's trust that the government is `` exercising good oversight '' of its nuclear weapons facilities , according to Lyman , but he said it also has `` global implications . '' `` If we ca n't even control our own nuclear weapons material , it shows what a major challenge it is around the world ... that have comparably dangerous materials but are even less protected . And , Lyman pointed out , if the United States appears to have vulnerabilities in protecting its nuclear weapons material , `` then that not only reduces our authority to criticize other countries , it raising questions about the integrity of our own security. `` CNN 's David Mattingly and Tristan Smith contributed to this report . Watch Erin Burnett weekdays 7pm ET . For the latest from Erin Burnett click here .
Three peace activists face trial this week for breaching a nuclear weapons site . The men were arrested in the northern city of Murmansk on December 7 after they entered an area where radioactive material was being stored, prosecutors said. If convicted , they could face 20 years in prison .